Exoemission from Processed Solid Surfaces and Gas Adsorption 9811969477, 9789811969478

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Springer Series in Surface Sciences 73

Yoshihiro Momose

Exoemission from Processed Solid Surfaces and Gas Adsorption

Springer Series in Surface Sciences Volume 73

Series Editors Gerhard Ertl, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany Hans Lüth, Peter Grünberg Institute, Forschungszentrum Jülich GmbH, Jülich, Germany Roberto Car, Department of Chemistry, Princeton University, Princeton, NJ, USA Mario Agostino Rocca, Dipartimento di Fisica, Università degli Studi di Genova, Genova, Italy Hans-Joachim Freund, Fritz-Haber-Institut der Max-Planck-Gesellschaft, Berlin, Germany Shuji Hasegawa, Department of Physics, University of Tokyo, Bunkyo-ku, Tokyo, Japan

This series covers the whole spectrum of surface sciences, including structure and dynamics of clean and adsorbate-covered surfaces, thin films, basic surface effects, analytical methods and also the physics and chemistry of interfaces. Written by leading researchers in the field, the books are intended primarily for researchers in academia and industry and for graduate students.

Yoshihiro Momose

Exoemission from Processed Solid Surfaces and Gas Adsorption

Yoshihiro Momose Department of Materials Science Ibaraki University Hitachi, Japan

ISSN 0931-5195 ISSN 2198-4743 (electronic) Springer Series in Surface Sciences ISBN 978-981-19-6947-8 ISBN 978-981-19-6948-5 (eBook) https://doi.org/10.1007/978-981-19-6948-5 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

The purpose of this book is to introduce readers to the study of electron emission phenomena of metals and semiconductors, in the context of surfaces excited by various methods, and corresponding applications to surface chemical technology. This electron emission substantially differs from well-known types of thermionic emission (high temperature), photoemission, and field emission (high electric field) from clean surfaces. Such electron emission has been widely known by nomenclature such as the Kramer effect, exoelectron emission, and exoemission. In this book, we used exoemission. Exoemission occurs from solid surfaces that are subjected to various processes and has received substantial attention in industrial fields such as tribology (friction, wear, and lubrication). Exoemission occurs at low temperatures, as measured with a Geiger–Müller (GM) counter. Professor Y. Tamai of Tohoku University has emphasized the importance of the Kramer effect in the field of tribology. I have been inspired by his guidance. He was the adviser of my thesis and a long-term collaborator; we have continued corresponding research. The contents of this book are based mainly on papers published by my group and collaborators. This book is an introductory text that is suitable for senior undergraduate and postgraduate students in physics, chemistry, engineering, and metallurgy. Furthermore, I have attempted to render the book suitable for scientists and researchers in industrial laboratories, who are interested in the surface properties of solid materials. The general level of presentation has been kept elementary, with emphasis on the physical and chemical properties of solids, as well as adsorbed or deposited materials. Controlling the properties of the surfaces and interfaces of solid materials is pertinent to the functionality as well as the reliability of the surfaces of solid materials. Processes at solid surfaces are important in industrial fields such as adhesion, tribology, catalysis, chemical processes, and corrosion. To achieve such functionality and reliability, a solid surface must first be subjected to pretreatments—i.e., activated—by various processes. One often performs surface processing of solid materials (such as metals and semiconductors) by external processes such as (1) mechanical treatment, (2) thin-film coating, and (3) exposure to ionizing irradiation (such as ultraviolet light, plasmas, and ion as well as electron beams). These surface processes are termed excitation. Consequently, one introduces functional groups v

vi

Preface

to the surface, and various types of lattice as well as electronic defects form in the vicinity of the surface. Furthermore, when placed under production sites after surface processing, oxygen, water vapor, and other environmental materials adsorb onto the surface. Engineering researchers must perform careful work to reproducibly impart the same properties to a surface; further efforts are required to develop technologies for monitoring surfaces and interfaces that are applicable for industrial use. After the aforementioned excitation, one usually stimulates the surface of solid materials by temperature and/or light irradiation and further applies an applied or accelerating voltage to the surface as well as rubbing the surface with a polymer rod. Such stimulations are conventionally termed thermally (temperature), optically or photo (light) stimulated electron emission, and the effect of applied voltage. Furthermore, we have developed two stimulation methods for photoelectron emission: thermally assisted and temperature-programmed photoelectron emission; as well as the method of tribo-stimulated (rubbing) electron emission. Thus, by using these stimulation methods, we imparted a difference to the electron emission characteristics of the surfaces. In particular, this book reveals the electronic properties of the processed surfaces under the influence of adsorbed foreign materials. This book is composed of four parts and 12 chapters: Part I, Introduction, Chap. 1; Part II, EE Mechanism of Metals Subjected to Adsorption, Chaps. 2–4; Part III, Outline of Development of EE Research, Chaps. 5–7; and Part IV, TAPE, TPPE, TriboEE, and XPS Characteristics of Processed Surfaces, Chaps. 8–12. I hope that those who study the physics and chemistry of metal surfaces, as well as those who are engaged in science and technology research that is related to metal surfaces, will use or refer to the results described in this document from various perspectives. It is a pleasure to dedicate this book to the late Professor Y. Tamai, who inspired my interest in the subject and with whom I had the privilege of working. I thank my postgraduate and senior undergraduate students of Department of Materials Science of Ibaraki University (Hitachi campus), who have assisted me in performing studies and experiments. I wish to thank Dr. K. Nakayama and Dr. T. Sakurai, with whom I have collaboratively performed research that is related to exoemission and have given continued support and valuable comments over many years. I acknowledge Dr. I. Ohshima, with whom I have conducted research in the field of mechanical treatment of metallic solids. I also thank Dr. M. Takeuchi and Professor H. Mase, who have made many useful suggestions about applied physics and electronics and provided encouragement. I also thank Professor Y. Nishiyama and Professor T. Matsunaga, with whom I studied surface chemistry in Tamai’s laboratory, and for Dr. S. Mori, who has given me a lot of information about tribology research. Finally, I thank my wife Takako Momose for her support and encouragement for this research. Hitachi, Japan

Yoshihiro Momose

Contents

Part I 1

Surface Phenomena and Exoemission . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Pertinence of Exoemission to Surface Phenomena . . . . . . . . . . . . . 1.1.1 Involvement in Surface Chemical Technology . . . . . . . . . 1.1.2 Work Function and Analysis Methods of Electron Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Exoemission Phenomena of Processed Surfaces . . . . . . . . . . . . . . 1.2.1 Importance of Exoemission Studies for Processed Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Historical Background . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3 Exoemission Measurements of Processed Surfaces . . . . . . . . . . . . 1.3.1 External Treatments and Terminology of Exoemission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Trend of Current Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.3 Measurement Apparatus and Surface Cleanliness . . . . . . 1.3.4 Origin of Exoemission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Part II 2

3

Introduction 3 3 3 6 9 9 10 11 11 12 14 14 15

EE Mechanism of Metals Subjected to Adsorption

EE of Clean Metals: Adsorption of Mainly O2 and H2 O in the UHV and HV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Specification of Practical Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Development of Chemiemission . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3 EE Attributable to Adsorption of Electronegative Gases . . . . . . . . 2.3.1 EE During Oxidation of Cs Films Deposited on Ru . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

21 21 22 26 31 33

EE from Metal Surfaces Covered with Oxide: Adsorption of Mainly O2 and H2 O and Oxide-Film Thickness . . . . . . . . . . . . . . . . 3.1 OSEE Observed in the UHV and HV for Al2 O3 /Al . . . . . . . . . . . .

35 35

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3.2 3.3

EE Observed in Air for MgO/Mg, Al2 O3 /Al, and NiO/Ni . . . . . . EE Observed in Counter Gas for Oxide-Covered Metal Surfaces of Sn, Al, Fe, Ni, and Cu . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Effects of Organic Adsorption, Applied Voltage, Light Irradiation, and Catalytic Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Effect of Adsorption on OSEE from Al . . . . . . . . . . . . . . . . . . . . . . 4.2 Effect of AV and Light Intensity on OSEE from Al . . . . . . . . . . . . 4.3 Relation Between EE and Catalytic Activity of Ag, Cu, and Pt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

42 46 52 53 53 56 62 64

Part III Outline of Development of EE Research 5

6

Materials, EE Measurement, and EE Characteristics . . . . . . . . . . . . . 5.1 EE Measurement Methods and EE Data Analysis Methods . . . . . 5.2 Stimulation by Thermal, Optical, and Tribological Methods After Excitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Nomenclature of EE Categorized by Stimulation Methods . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

69 69 70 72 73

TSEE Related to Plasma Treatment and Adsorption . . . . . . . . . . . . . . 75 6.1 Outline of TSEE of Metal Surfaces After Plasma Treatment . . . . 75 6.2 Effect of Discharge, Adsorption, and Heat Treatment on TSEE from Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 6.2.1 TSEE from Spark-Discharged Fe Surfaces and Adsorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78 6.2.2 TSEE from Oxidized and Plasma-Treated Ni Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80 6.2.3 TSEE After Electric Discharge Treatment and Chemical Reduction of Cu Surfaces . . . . . . . . . . . . . . 85 6.3 TSEE from Glass on Au Surfaces, Au, Ni, Si, and Graphite Subjected to Plasma Exposure; and XPS Analysis . . . . . . . . . . . . . 89 6.3.1 TSEE from Glass Deposited on Au Metal Surfaces . . . . 89 6.3.2 TSEE from Au and Ni Metal Surfaces to Exposed to Ar and O2 Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92 6.3.3 TSEE from Ni Metal Surfaces Exposed to Ar and O2 Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 6.3.4 TSEE from Si Wafer Powder Exposed to Ar Plasma . . . 97 6.3.5 TSEE from Graphite Exposed to CF4 , Ar, and O2 Plasma . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107

Contents

7

Effects of Blasting and Grinding Agents as Well as Cutting Fluids on TSEE from Mechanically Deformed Surfaces . . . . . . . . . . . 7.1 TSEE from Sandblasted Mild Steel and Ground Sand . . . . . . . . . . 7.1.1 TSEE from Sandblasted Mild Steel and Adsorption of Organic Vapors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1.2 TSEE from Ground Sand Granules (Aluminosilicate) and Adsorption of Organic Vapors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 EE from Metals and Plastics Blasted or Ground with Abrasive Agents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.1 TSEE from Metals Blasted with Silicon Carbide (SiC) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2.2 EE from Plastics Abraded with Al2 O3 and SiC . . . . . . . . 7.3 TSEE from Metal Surfaces Subjected to Cutting and Grinding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.1 TSEE from Al Surfaces Cut with a Tool Steel and Effect of Cutting Fluids . . . . . . . . . . . . . . . . . . . . . . . . 7.3.2 EE from Metals During Cutting with WC and Friction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.3.3 TSEE Under Light Illumination from Low-Carbon Steel Surfaces Ground with Al2 O3 . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

ix

111 111 111

112 115 115 117 117 117 126 127 132

Part IV TAPE, TPPE, TriboEE, and XPS Characteristics of Processed Surfaces 8

9

TAPE of Rolled and Scratched Fe Metal Surfaces . . . . . . . . . . . . . . . . 8.1 Temperature Dependence of PE from Rolled Fe Surfaces . . . . . . . 8.2 Wavelength Dependence of PE from Rolled Fe Surfaces . . . . . . . 8.3 PE from Practical Fe Surfaces Scratched in Air, Water, and Organic Liquids . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.1 PE in Temperature Scans of Scratched Fe Surfaces and XPS Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.2 Activation Energy of PE from Scratched Fe Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8.3.3 PE in Wavelength Scans of Scratched Fe Surfaces . . . . . 8.4 Temperature Analysis of PE and XPS Data of Scratched Fe Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

135 135 141

TAPE of Si Wafers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9.1 Effect of Adsorption of O2 and H2 O on EE from Si . . . . . . . . . . . 9.2 PE from Si Wafers and Activation Energy . . . . . . . . . . . . . . . . . . . . 9.3 PE from Si Wafer Surfaces Implanted with H, Si, and Ar Ions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

163 163 165

148 148 148 153 156 161

168 172

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10 TPPE Characteristics of Various Metal Surfaces . . . . . . . . . . . . . . . . . 10.1 Outline of TPPE for Metal Surface Analysis . . . . . . . . . . . . . . . . . 10.2 TPPE Characteristics and XPS Analysis . . . . . . . . . . . . . . . . . . . . . 10.2.1 Temperature Dependence of PE Total Count . . . . . . . . . . 10.2.2 XPS Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.2.3 TPPE Characteristics and Gas Adsorption Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10.3 TPPE Characteristics of Metals and Surface Pretreatment Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 TriboEE Occurring from Metal Surfaces During Sliding Contact with a Polymer Rod . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Outline of TriboEE from Metal Surfaces . . . . . . . . . . . . . . . . . . . . . 11.1.1 Electron Emission During Sliding Contact Between Metals and Polymers . . . . . . . . . . . . . . . . . . . . . . 11.1.2 Effect of Surface Pretreatments of Metals on TriboEE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1.3 Effect on TriboEE of Plasma-Polymerized Films Formed on Metal Surfaces . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Dependence of TriboEE Intensity on Elemental Metals . . . . . . . . 11.2.1 TriboEE Intensity of Elemental Metals . . . . . . . . . . . . . . . 11.2.2 Relationship of TriboEE Intensity of Metal Surfaces to the Work Function and Surface Potential . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2.3 TriboEE from Metal Surfaces Covered with an Oxide Film and Its Relationship to the Heat of Formation of Metal Oxides . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Relationship of the EE Intensity of Metal Surfaces to Their Chemical Activity and Electrostatic Attractive Force . . . . . . . . . . . . . 12.1 Application of TPPE to Cu Surfaces . . . . . . . . . . . . . . . . . . . . . . . . 12.1.1 Electrochemical Reduction of CO2 on Cu Electrodes and TPPE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12.1.2 TPPE Characteristics of Cu Subjected to Cleaning and Abrasion in Air, Water, and Alcohols . . . . . . . . . . . . 12.2 Corrosion Protection of Al Surfaces by Plasma-Polymerized Coatings and TPPE . . . . . . . . . . . . . . . . . . . .

173 173 176 176 178 181 184 189 191 191 191 192 196 198 198

200

200 205 207 207 207 210 211

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12.3 Corrosion Protection of Fe, Ni, and Cu Metal Surfaces by Plasma-Polymerized Coatings, and Its Relationship to the Electronic Properties of Metals . . . . . . . . . . . . . . . . . . . . . . . 214 12.3.1 Effect of TPPE on Electrostatic Attractive Force Between Metals, Semiconductors, and Tribocharged Polymers . . . . . . . . . . . . . . . . . . . . . . . . 221 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227

Part I

Introduction

Chapter 1

Surface Phenomena and Exoemission

Abstract The subject breadth of this book is attributable to the wide-ranging applications of surface science and technology. Transfer of electrons at the surface of metals and semiconductors, as well as the influence of foreign adsorbed materials on the transfer, is included under the following themes in this section: involvement in surface chemistry technology, classification of electron emission and the work function, exoemission phenomena of processed surfaces, historical background of exoemission, and measurement as well as the origin of exoemission.

1.1 Pertinence of Exoemission to Surface Phenomena 1.1.1 Involvement in Surface Chemical Technology Improving the properties of surfaces and interfaces of practical products plays an important role in many surface phenomena, such as adhesion, heterogeneous catalysis, corrosion, triboelectric charging, contact electric potential, corrosion, and friction. (Fig. 1.1). Upon encountering any technically difficult problems in industry, in many cases the surface is pertinent. Tamai [53] emphasized that surface chemical technology plays a fundamental or important role in corresponding surface phenomena in various industrial fields; such research is invaluable, given the broad application of surface chemical technology to tribology (friction, wear, and lubrication) and possible future developments. The frictional resistance that arises from the sliding of two solids in contact is because of surface adhesion, and there is wear. Therefore, by selecting a material that has been subjected to surface chemical treatment (or has a low-adhesive property in the context of surface chemical technology), it is possible to reduce friction and prevent abrasion. This reduced friction and abrasion are directly linked to energy and resource conservation; in accordance with some estimates, such improvements will result in a 5 trillion yen annual savings in Japan. One can measure the cause of adhesion between surfaces that corresponds to friction by measuring the exoemission from the surface. Thus, measurement of exoemission is pertinent to many surface chemical technologies and is a useful method for analyzing processed surfaces. To improve the surface functionality of metallic © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 Y. Momose, Exoemission from Processed Solid Surfaces and Gas Adsorption, Springer Series in Surface Sciences 73, https://doi.org/10.1007/978-981-19-6948-5_1

3

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1 Surface Phenomena and Exoemission

Fig. 1.1 Surface phenomena [32]

materials, one must process the surface by various methods such as mechanical machining, thin-film coating, irradiation with ultraviolet light, plasma treatment, or irradiation with an ion/electron beam. For this reason, developing technologies for monitoring or analyzing the effects of surface processing has been an active field of research [4]. We recently published two reports on thermally assisted photoelectron emission (TAPE) from iron surfaces that we scratched in various environments [34, 35, 37] and triboelectron emission (TriboEE) from various metals during rubbing with polytetrafluoroethylene (PTFE) [33]. Regarding the former (TAPE), Fig. 1.2 shows the experimental setup for measuring TAPE glow curves for iron surfaces scratched in various environments [34, 37]. The glow curve represents the curve of the intensity of the electron emission versus the temperature of a sample surface. Figure 1.3 shows the TAPE glow curves for iron surfaces scratched in various environments. Clearly, the environment strongly influences the glow curves [34, 37]. Regarding the latter (TribeEE), Fig. 1.4 shows the change in the intensity (cpm, counts/min) of TriboEE with rubbing time for the metals of groups 10 (Ni, Pd, and Pt) and 11 (Cu, Ag, and Au) of the periodic table [33]. Figure 1.5 shows the relationship between the TriboEE intensity and the heat of the formation of metal oxides, D(M–O) [33]. The electrons from metals with small D(M–O) values predominantly tunnel the surface oxide layer as a surface barrier, whereas regarding large D(M–O) values, the electron passes over the top of the barrier. Thus, the information obtained from measurements of exoemission from processed surfaces facilitates surface chemical technology. We explain these reports in detail in subsequent paragraphs.

1.1 Pertinence of Exoemission to Surface Phenomena

5

Fig. 1.2 Outline of experimental setup for measuring thermally assisted photoelectron emission from real surfaces [34, 37]. Reprinted with the permission from authors licensed under CC By 4.0, Tsinghua Springer, Momose et al. [34, 37]. Copyright© 2018

Fig. 1.3 Photoelectron emission glow curves observed during the Up1 scan for the different scratching environments [34, 37]. Reprinted with the permission from authors licensed under CC By 4.0, Tsinghua Springer, Momose et al. [34, 37]. Copyright© 2018

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1 Surface Phenomena and Exoemission

Fig. 1.4 The change of the intensity (cpm: counts/min) of TriboEE with rubbing time for the groups 10 (Ni, Pd, and Pt) and 11 (Cu, Ag, and Au) giving the median value of the TriboEE intensity [33]. Reprinted with the permission from authors licensed under CC By 4.0, MDPI, Momose [33]. Copyright© 2021

1.1.2 Work Function and Analysis Methods of Electron Emission Electron emission is a liberation of electrons from the surface of metallic materials. To escape from the solid surface, electrons must be given sufficient kinetic energy from an external energy source (which can be supplied by high temperature, irradiation with light, or application of a high electric field) to overcome the surface barrier. These typical electron emissions are termed (1) thermionic emission, (2) photoemission, and (3) field emission, respectively. We review the effects of temperature, photon energy, and field on the electron emission in accordance with refs. [1, 10–12, 14, 15, 24, 56]. Figure 1.6 shows the free-electron model for an electron in a metal, by

1.1 Pertinence of Exoemission to Surface Phenomena

7

Fig. 1.5 The relation between the TriboEE intensity and the D(M–O) values for metals in the groups 4, 5, 6, 10, and 11 [33]. Reprinted with the permission from authors licensed under CC By 4.0, MDPI, Momose [33]. Copyright© 2021

Sommerfeld [10]. It is assumed that the free electrons (i.e., those which correspond to the conductivity) find themselves in a potential, which is constant everywhere inside the metal. The potential energy of an electron at rest inside the metal is lower than that of an electron at rest outside the metal. The interior of the metal is represented by a potential energy box of depth E s , which is the energy difference between an electron at rest inside the metal and an electron at rest in vacuum. In general, E s is on the order of 10 eV. At temperature T = 0 K, all energy levels up to E F (the Fermi energy) are filled; all higher-energy levels are empty. One uses the energy of an electron at rest outside the metal as a reference, commonly termed the vacuum level. The work function is defined as φ = E s − E F . The thermionic emission is represented by the Richardson–Dushmann (1.1) [10, 24, 56], I = A(1 − r)T 2 e−φ/kBT ,

(1.1)

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1 Surface Phenomena and Exoemission

Fig. 1.6 Free electron model by Sommerfeld. E s is the energy difference between an electron at rest inside the metal and one at rest in vacuum. At temperature T = 0 K, all energy levels up to E F , which is called Fermi energy, are filled, all higher ones are empty. The energy of an electron at rest outside the metal is used as a reference and commonly referred to as the vacuum level. The work function is defined φ = E s − E F [10]. Reproduced with the permission from Dekker [10]. Copyright© 1957

where I is the density of the saturation emission current; the universal constant, termed Richardson’s constant, A = 4πmk 2 Be/h3 = 120 Amp/cm2 /deg2 (m and e denote the mass and charge, respectively, of the electron); h is Planck’s constant and k B is Boltzmann’s constants; T is temperature in Kelvin; and r is a suitable average of the reflection coefficient. Equation (1.1) indicates that I sharply increases with temperature and is larger with a smaller work function. Application of (1.1) to the surfaces of processed solid materials is difficult because of measurements at high temperature; thus, we did not use (1.1). Fowler [14] and DuBridge [11] substantially advanced methods of analyzing photoemission. One can measure the photoemission as functions of temperature at a constant wavelength of incident light as well as of wavelength (photon energy) at a constant temperature. This analysis method is termed the Fowler–DuBridge (FD) technique [12]. We have thoroughly applied this method to analysis of photoemission from processed surfaces. We term the former method temperature scan and the latter method wavelength scan. We introduce a report by [1]. They measured photoemission from a nickel ribbon at temperatures from 25 to 760 °C, at wavelengths from 225 to 253 nm, and analyzed the data by the FD method. We explain this method from their paper. One can represent the photoelectric current per unit area for a unit intensity of radiation, I, from a metal surface excited by radiation of frequency ν at temperature T (in Kelvin), by (1.2): ( I = αAT 2 Φ

) hν − φ , kB T

(1.2)

where φ is the photothreshold of the metal, α is a proportionality factor related to the probability of an electron absorbing a quantum of incident light, A is Richardson’s constant (regarding thermionic emission), and Φ(x) refers to the universal function given by (1.3) Φ(x) = π 2 /6 + x2 /2 − (e−x − e−2x /22 + e−3x /32 − . . .) for x ≥ 0.

(1.3)

1.2 Exoemission Phenomena of Processed Surfaces

9

The parameter x is (hν − φ)/k B T, and hν is the photon energy of incident light. Using the notation for the quantum yield Y FD and E p (instead of I and hν, respectively) in (1.2), to a good approximation, replacing (1.2) by x 2 /2, we may approximate Y FD by (1.4): YFD =

αA(Ep − φ)2 . 2kB2

(1.4)

Equation (1.4) indicates that one can determine the values of (αA/2k B 2 )1/2 and φ by using a straight line, obtained from the plot of the square root of Y FD against E p . In Chap. 8, we describe in detail the characteristics of TAPE from rolled and scratched Fe metal surfaces by the FD technique [36]. Field emission originates upon applying an intense electric field to a metal surface; i.e., the electrons tunnel through the surface potential barrier. In accordance with the Fowler–Nordheim equation [15], used for this type of emission, the emission current (I in units of amperes per square centimeter) is represented by (1.5) by using the units of volts per centimeter for the field strength (F) and electron volts for the work function (φ), respectively [56] I ∼ 1.6 × 10−6 (F 2 /φ) exp(−6.9 × 107 φ 3/2 /F)

(1.5)

Equation (1.5) indicates that as the work function of the metal becomes lower and the field strength becomes larger, the current increases. Usually, the values of φ for metals are several electron volts, such that an electric field that is stronger than 107 – 108 V/cm is required to produce field emission. One can explain the TriboEE that occurs upon rubbing a polymer rod on a metal surface by a field-enhanced electron emission model. Chapter 11 describes this emission.

1.2 Exoemission Phenomena of Processed Surfaces 1.2.1 Importance of Exoemission Studies for Processed Surfaces Exoemission occurs from the surface of solid materials that one mechanically deforms or exposes to ionizing radiation and further covers with foreign materials such as an oxide layer. This book focuses on exoemission. Such electron emission is because of a surface-dependent effect rather than clean surfaces [16]. Exoemission can provide useful information on the properties of electrons in the neighborhood of the surface of the solids, for fields such as adhesion, coating, corrosion, and catalysis. Therefore, many interesting reviews and original research papers on exoemission have been reported in various fields [3, 5, 7, 16, 19, 26, 42, 43, 47, 48]. Furthermore,

10

1 Surface Phenomena and Exoemission

there are interesting reviews on the electronic excitation or charge transfer in chemical reactions on metal surfaces [17, 30, 40]. In the field of tribochemistry (defined as “a branch of chemistry dealing with the chemical and physicochemical changes of solids due to the influence of mechanical energy”), electron emission has been a strong focus [8, 20, 28, 51], because exoemission is an elemental physical process for mechanically activating the surface of solids—in addition to high temperature, photon emission, electrostatic charge, and release of lattice components. Furthermore, there is an excellent article regarding the development of monitors that can examine the properties of surfaces and interfaces under ambient production environments, as well as enable real-time analysis for feedback control [2, 22]. Measurement of photoelectron emission will become a useful monitoring method. There is also an interesting article on triboelectric energy production methods [29]. This text discusses various materials [such as metallic materials, semiconductors, abrasive materials, carbon materials (graphite), and polymers] and environments (such as air, oxygen, water, organics, and plasma gases). We describe in detail the relationship between the exoemission characteristics to the (1) XPS data of the surfaces and (2) chemical properties of the adsorption materials.

1.2.2 Historical Background In accordance with a review by [42], we introduce the historical background of electron emission. The details of the articles are given in References. (a) Curie [9]: Observation of decayed and decaying emission from a substance placed in the vicinity of a radium salt, (b) Elster and Geitel [13]: Observation of induced photoemission from alkali halides colored by cathodic radiation, (c) Russell [45]: Observation that scratched Zn blackens photographic plates; (d) Rutherford [46]: Observation of decayed and decaying emission from materials placed near a radium salt, (e) Villard [57]: Observation of photographic plate blackening even upon X-ray exposure, (f) McLennan [31]: Observation of the release of low-energy, negatively charged particles from chlorides and sulfates subjected to cathode-ray bombardment, (g) Lewis and Burcham [27]: Observation of emission when one uses mechanically treated metals (e.g., Al, Cu, Ni, and brass) in a GM counter. (h) ~1950: Observation of high induced electron emission immediately after fabrication, involving cylinders outside the GM counter in 1950. This finding was difficult to explain. The term exoelectron is used for electrons in the electron emission. Kramer [26] proposed this term, published in “Der Metallische Zustand”. This electron emission from metals under various conditions occurs at room and elevated temperature. Such emission occurs from wood alloys during solidification. Kramer considered an energy

1.3 Exoemission Measurements of Processed Surfaces

11

source that causes electron emission as follows. When the pristine metal surface is abraded, the oxide layer is removed, leaving a bare metal surface. The new surface then begins to oxidize again, releasing a substantial quantity of chemical energy in the form of heat. When the liquid metal is cooled and solidified, the latent heat of fusion is released in the form of heat. Kramer hypothesized that this thermal energy causes electron emission and because of such an exothermal process, termed the emitted electrons (exoelectrons). In dedication of his pioneering investigations of exoelectron emission (frequently termed EEE), the phenomenon has often been termed the Kramer effect. However, the explanation of EEE by this mechanism is not considered valid today. We describe the terminology for the electron emission in the next section. Subsequently, regarding EEE from metallic materials, semiconductors, and insulators, International Symposiums on EEE have been continuously held in many countries: first (1956) Innsbruck (Austria), second (1966) Liblice (CSSR), third (1970) Braunschweig (FRG), fourth (1973) Liblice (CSSR), fifth (1976) Zvikov (CSSR), sixth (1979) Ahrenshoop (GDR), seventh (1983) Strasbourg (France), eighth (1985) Osaka (Japan), ninth (1988) Wroclaw (Poland), tenth (1992) Ekaterinburg (Russia), 11th (1994) Glucolazy (Poland), and 13th (2000) Jurmala (Latvia). Many researchers have participated in the symposiums and discussed EEE. Even today, the EEE phenomenon has been of substantial interest in the field of tribochemistry (friction, wear, and lubrication). Thus, the tribochemistry forum organized by K. Nakayama has been held as the satellite forum of the International Tribology Conference by the Tribochemistry Technical Committee, Japanese Society of Tribologists, in 1995 (Tokyo), 2000 (Tsukuba), 2005 (Nara), 2009 (Kyoto), 2011 (Hagi), 2013 (Lyon), 2015 (Nikko), and 2019 (Hakodate).

1.3 Exoemission Measurements of Processed Surfaces 1.3.1 External Treatments and Terminology of Exoemission In accordance with an excellent review by [48] in the report of the 4th International Symposium on Exoelectron Emission and Dosimetry in 1973, we discuss the excitation and stimulation methods that cause electron emission. The exoemission temporarily occurs even at a low temperature when one subjects the surface of a solid to the following external treatments: (a) Solid-state excitation of electromagnetic waves, electrons, protons, and ions by ionizing radiation; (b) Adsorption; (c) Mechanical treatments such as scratching, polishing, and strain; (d) Chemical reactions; (e) Phase transitions (e.g., solidification of metals and changes in crystal structure);

12

1 Surface Phenomena and Exoemission

(f) Polarization effects of pyroelectric materials {e.g., heating LiNbO3 crystals results in electron emission, hypothesized to be because of an electric field (106 V/cm) that is caused by a change in spontaneous polarization by heat [44]}. These external treatments are termed excitation. If during or after the excitation, additional energy by irradiation with light or by heating is necessary to produce the electron emission, the phenomena are termed optically (photo) stimulated exoelectron emission (OSEE or PSEE, respectively) and thermally (thermo-) stimulated exoelectron emission (TSEE). The incident light is usually ultraviolet-visible light, the wavelength of which is longer than the wavelength of the conventional photoelectric effect. The electron emission for only external treatments is termed the Kramer effect, cold emission, dark emission, and chemiemission. Initially, exoelectron emission referred only to electrons, but now includes cases of emission of ions; recently, the term exoemission was said to be more appropriate and therefore is used for electron and ion emission [16]. The relationship between the intensity of the emission and the temperature is termed the electron emission glow curve. Himmel and Kelly [21] categorized exoemission from metals as follows: (1) chemically stimulated electron emission from initially clean metal surfaces, (2) photostimulated emission from deformed metal surfaces, (3) roughening-enhanced photoemission from clean Al surfaces, (4) deformation-enhanced photoemission from oxidecovered Al in ultra-high vacuum (UHV), (5) environmental effects, (6) triboelectron emission from oxide-covered metal surfaces, and (7) thermally stimulated emission from oxide-covered metals. Therefore, use of excitation and stimulation is not clearly divided: nomenclature such as chemically stimulated, tribo-stimulated, and environmental effects has been introduced; and tribo-stimulated emission has been recently included. The terminology and abbreviations for electron emission from solid surfaces that are subjected to various excitations and stimulations are confusing. Hereafter, we use the following terminology (Table 1.1).

1.3.2 Trend of Current Studies The study of EE, particularly for metallic materials, has followed two trends. (1) For practical surfaces, EE measurements have been performed to characterize the surface of solids. Our studies focus on this area. For example, we examined the adhesive strength of a paint film on a sandblasted steel sample [54], and the quantity of chemical reduction of an oxide film on a copper metal sample by ethanol vapor [38], as well as their relationships to the number of electrons emitted from these metal surfaces. (2) For clean metal surfaces, much work has focused on clarifying the EE mechanism. For example, [49] reported that upon abrading Al surfaces at 7.0 × 10−9 Torr and exposure to air for 2 min at various pressures, the OSEE increased with increasing pressure, reached a broad maximum, and then rapidly decreased.

1.3 Exoemission Measurements of Processed Surfaces

13

Table 1.1 Abbreviations for technical words Abbreviation

Technical words

EE

Exoemission [Kramer effect and exoelectron emission (EEE) have been used frequently before.]

PE

Photoelectron emission

TSEE

Thermally stimulated electron emission

OSEE

Optically stimulated electron emission

PSEE

Photostimulated electron emission

TAPE

Thermally assisted photoelectron emission

TPPE

Temperature programmed photoelectron emission

TriboEE

Triboelectron emission

XPS

X-ray photoelectron spectroscopy (This is one of the surface analysis methods for measuring surface elements and electronic states.)

UHV

Ultra-high vacuum

HV

High vacuum

L

1 L = 10−6 Torr s (One Langmuir corresponds to an exposure of 10−6 Torr during one second.)

AN

Acceptor number (a measure of the electrophilic behavior of a solvent)

GM counter

Geiger-Müller counter

AV

Applied voltage or accelerating voltage (this causes a negative electric potential to a sample)

Regarding oxidation reactions of metals, it has long been unclear why one does not observe electron emission from clean surfaces, but such emission does result from pre-oxidized surfaces. The reason for these divergent experimental results has been explained [18]. The EE assembles all types of electron emission that occur from solid surfaces. In our studies, the main factors that determine and influence EE behavior are as follows: (1) Materials: metals, semiconductors, ionic crystals, carbon materials, and polymers. (2) External treatments for excitation: mechanical deformation, ionizing radiation, and exposure to various plasmas. (3) Stimulation methods: temperature (heating and subsequent cooling), irradiation of light (photon energy and corresponding intensity), and setting up applied voltage (AV) between a sample holder and the earthed grid of a Geiger counter. (4) Chemical species adsorbed on solid surfaces: oxygen, water vapor, and organic molecules. (5) Chemical properties of chemical species (e.g., acid–base properties, proton attracting power, electron affinity, and ionization potential): to examine the effect of the species on the EE intensity.

14

1 Surface Phenomena and Exoemission

(6) Electronic properties of surfaces (work function, electron affinity, band bending, dipole layer, surface potential, space charge, electron trap depth, and activation energy): for transfer of electrons through the overlayer. In this book, we discuss the characteristics of EE in relation to the aforementioned factors.

1.3.3 Measurement Apparatus and Surface Cleanliness Grunberg [18] reported instruments made for measuring EE: GM counters of vertical and horizontal types and a secondary electron multiplier. These instruments have been used because of the low observed current (10−8 –10−17 A) of the emitted electrons and the relatively low energy of the electrons (≈1 eV). Furthermore, [23] used a picoammeter. For GM counter tubes, problems include the action of a counter gas on solid surfaces and the presence of a nonuniform electric field. However, for the former problem, in our experiments, we used a mixing gas that consists of Ar and a quenching gas composed of various types of organic vapor to examine the effect of the adsorption of organic vapor on the surface during the EE measurements. Furthermore, researchers have used a commercial gas (termed Q gas, 99% He and 1% iso-C4 H10 ) for a flow-type GM counter; in which the gas flows at atmospheric pressure, such that the gas environment of the GM counter is continuously maintained during the EE measurements. Regarding the secondary electron multiplier tube, the irradiation of low-energy electrons does not substantially cause secondary electron emission [22]. Interestingly, when comparing the behavior of EE from X-ray-irradiated solids measured with a GM counter (counter gas: mixture gas of 30 Torr of H2 and 10 Torr of C2 H5 OH) with the behavior measured with a secondary electron multiplier tube, there was no distinction between the EE behaviors of metals (Al, Cu, and Ag), but those for ionic crystals (MgSO4 and NiSO4 ) were different [25]. EE occurs from various materials such as metals, semiconductors, and insulators regardless of the cleanliness of the surfaces. Solid surfaces have ranged from those placed in contact with the environment or adsorbed with foreign materials, to those that are well defined and cleaned. The gas atmosphere under EE measurements is a counter gas of a GM counter and the atmosphere of a secondary electron multiplier tube (the pressure of which ranges from the HV to UHV).

1.3.4 Origin of Exoemission It is difficult to explain the general mechanism of EE. Regarding ionic crystals such as alkali halides, ionizing radiation excitation creates traps filled with electrons in the forbidden band of the energy band near the solid surface [48]. The electrons are brought into the conduction band by light or thermal stimulation and can exit

References

15

the solid surface if the electrons’ energy is greater than the electron affinity. The electron affinity is the energy required to drive the electrons from the bottom of the conduction band into the vacuum level. The basic idea of this model is hypothesized to be correct. Although the basic process after release from the trap has not yet been clarified, regarding TSEE, the Maxwell energy distribution of the electrons in the conduction band is assumed, and only the electron with a highest energy of the Maxwell-tail (higher than the electron affinity) is released. The simplest of these trapping centers is one in which electrons are trapped in negative ion vacancies (e.g., F-colored centers) [5]. For NaCl and KCl crystals that contain F centers, researchers have observed OSEE that is concomitant with light absorption [52]. The figure of the intensity of electron emission and the absorption of light as a function of wavelength for the crystals were shown [47]. Another type of electron capture center is on a further outward surface, e.g., an adsorption layer. Ramsey [41] classifies the EE of the metal into the following: (1) intrinsic EE, attributable to a process that is inherent to the metal and (2) extrinsic EE, in which the presence of an adsorption or oxide layer contributes substantially. Regarding the EE classified as the latter type, the importance of an adsorption layer is implied by the experimental result that EE becomes large when Ni that is overlaid with an oxide film is subjected to electron bombardment [55] and also when Cu or Al with an oxide film is subjected to X-ray irradiation [39]. A decrease of the work function leads to the possibility that electrons can leave the surface. As an example, regarding an abraded metal surface (in relation to the origin of the electrons in an OSEE measurement experiment), the pertinent depth is ca. 2–3 nm [42]. Glaefeke [16] has reported the correlation of EE with sorption and transport phenomena. We obtained activation energies during temperature and wavelength scans [35, 38] and discussed their role in TAPE of scratched iron samples, based on the three-step model of [50]. Chapter 8 discusses these findings in detail.

References 1. I. Ames, R.L. Christensen, Anomalous photoelectric emission from nickel. IBM J. Res. Develop. 7(1), 34–39 (1963) 2. W.J. Baxter, Exoelectron Emission from Metals, in Research Technique in Non-destructive Testing, vol. 3, ed. by R.S. Sharp (Academic Press, London, 1977), pp.395–428 3. K. Becker, J.S. Cheka, R.B. Gammage, in Progress in Exoelectron Dosimetry. Proceedings of the 3rd International Symposium on Exoelectrons, vol. 80 (PTB–Mitteilungen, Braunschweig, 1970), pp. 318–354 4. F.J. Boerio, G.D. Davis, J.E. DeVries, C.E. Miller, K.L. Mittal, R.L. Opila, H.K. Yasuda, Polymer-metal (oxide) interfaces. Crit. Rev. Surface Chem. 3(1), 81–99 (1993) 5. A. Bohun, in The Physics of Exoelectron Emission on Ionic Crystals. Proceedings of the 3rd International Symposium on Exoelectrons, vol. 80 (PTB–Mitteilungen, Braunschweig, 1970), pp. 318–354 6. A. Böttcher, R. Grobecker, R. Imbeck, A. Morgante, G. Ertl, Exoelectron emission during oxidation of Cs films. J. Chem. Phys. 95(5), 3756–3766 (1991) 7. F.R. Brotzen, Emission of exoelectron from metallic materials. Phys. Stat. Sol. 22, 9–30 (1967)

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8. A. Ciniero, J. Le Rouzic, T. Baikie, Reddyhoff, The origins of triboemission-correlating wear damage with electron emission. Wear 374–375, 113–119 (2017) 9. H. Curie, Comptes Rendus 129, 174 (1899) 10. A.J. Dekker, Chapter 9 Free Electron Theory of Metals, in Solid State Physics. (Prentice-Hall, New York, 1957), pp.211–235 11. L.A. DuBridge, A further experimental test of Fowler’s theory of photoelectric emission. Phys. Rev. 39, 108 (1932) 12. L.A. DuBridge, in New Theories of the Photoelectric Effect (Hermann & Cie, Paris, France, 1935), pp. 5–57 13. J. Elster, H. Geitel, Ueber eine lichtelectrische Nachwirkung der Kathodenstrahlen. Ann. Phys. 295, 487–496 (1896) 14. R.H. Fowler, The analysis of photoelectric sensitivity curves for clean metals at various temperatures. Phys. Rev. 38, 45–56 (1931) 15. R.H. Fowler, L.W. Nordheim, Field emission from metallic surfaces. Proc. Roy. Soc. London Ser. A 119, 173–181 (1928) 16. H. Glaefeke, Exoemission, in Thermally Stimulated Relaxation in Solids. ed. by P. Bräunlich (Springer-Verlag, Berlin/Heidelberg, 1979), pp.225–273 17. T. Greber, Charge-transfer induced particle emission in gas surface reactions. Surf. Sci. Rep. 28, 1–64 (1997) 18. L. Grunberg, The study of freshly deformed metal surfaces with the aid of exo-electron emission. Wear 1, 142–154 (1957/1958) 19. L. Grunberg, A survey of exo-electron emission phenomena. Brit. J. Appl. Phys. 9, 85–93 (1958) 20. G. Heinicke, in 3.3 Physical Elementary Processes in the Mechanical Activation of Solids. Tribochemistry (Akademie–Verlag, Berlin, 1948), pp. 42–96 21. L. Himmel, P. Kelly, Exoemission from metals. Comments Sol. State Phys. 7(4), 81–90 (1976) 22. S.A. Hoenig, C.A. Savitz, W.A. Ott, T.A. Russel, M.T. Ali, in Applications of Exoelectron Emission to Nondestructive Evaluation of Alloying, Crack Growth, Fatigue, Annealing, and Grinding Processes (ASTM special technical publication 515, New York, 1972), pp. 107–125 23. S.A. Hoenig, F.J. Tamjidi, Exo-electron emission during heterogeneous catalysis (the effect of external electric potentials). Catalysis 28, 200–208 (1973) 24. J. Hölzl, F.K. Schulte, Work Function of Metals, in Solid Surface Physics. (Springer, Amsterdam, 1979), pp.1–150 25. G. Kralik, Vergleichende Untersuchung der Exoelektronenemission mit Zählrohr und Sekundäreelektronenvervielfacher. Acta Phys. Austriaca 16, 137–143 (1963) 26. J. Kramer, in Der Metallische Zustand (Vandenhoeck and Ruprecht, Göttingen, 1950) 27. W.B. Lewis, W.E. Burcham, An attempt to produce artificial radioactivity by an electron beam, with some notes on the behaviour of newly made Geiger-Muller counters. Math. Proc. Cambridge Phil. Soc. 32, 503–505 (1936) 28. E. Linke, in Electron Emission Upon Mechanical Treatment, eds. by H. Glaefeke, W. Wild, Proceedings of the 6th International Symposium on Exoelectron Emission and Applications (Wilhelm Pieck University, Ahrenshoop, 1979), pp. 133–136 29. C.X. Lu, C.B. Han, G.Q. Gu et al., Temperature effect on performance of triboelectric nanogenerator. Adv. Eng. Mater. 19, 1700275–1700283 (2017) 30. R.I. Masel, Adsorption I: The Binding of Molecules to Surfaces, in Principles of Adsorption and Reaction on Solid Surfaces. (Wiley, New York, 1996), pp.108–234 31. J.C. McLennan, XX. On a kind of radioactivity imparted to certain salts by cathode rays. Phil. Mag. 3, 195–203 (1902) 32. Y. Momose, Development of Surface Analysis Technique using Thermally and Optically Stimulated Electron Emission: a Lecture in Ibaraki University Evening Seminar (2004) 33. Y. Momose, Electron transfer through a natural oxide layer on real metal surfaces occurring during sliding with polytetrafluoroethylene. Coatings 11, 109 (2021). https://doi.org/10.3390/ coatings11010109

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34. Y. Momose, T. Sakurai, K. Nakayama, Photoelectron emission characteristics of iron surfaces scratched in different environments: Dependence on photon energy irradiation methods. Surf. Interf. Anal. 50, 1319–1335 (2018) 35. Y. Momose, T. Sakurai, K. Nakayama, Thermal analysis of photoelectron emission (PE) and X-ray photoelectron spectroscopy (XPS) data for iron surfaces scratched in air, water, and liquid organics. Appl. Sci. 10(6), 2111 (2020). https://doi.org/10.3390/app10062111 36. Y. Momose, D. Suzuki, T. Sakurai, K. Nakayama, Photoemission from real iron surfaces and its relationship to light penetration of the overlayer. Appl. Phys. A 118, 637–647 (2015) 37. Y. Momose, D. Suzuki, K. Tsuruya, T. Sakurai, K. Nakayama, Transfer of electrons on scratched iron surfaces: Photoelectron emission and X-ray photoelectron spectroscopy studies. Friction 6(1), 98–115 (2018) 38. Y. Momose, Y. Tamai, Relationship between exoelectron phenomena and reduction of copper oxide by ethanol vapour. Inst. Metals. 98, 110–113 (1970) 39. H. Müller, Untersuchungen an Kupferoxydul mittels Exoelektronen. Acta Phys. Austriaca 10, 474–480 (1957) 40. H. Nienhaus, Electronic excitations by chemical reactions on metal surfaces. Surf. Sci. Rep. 45, 1–78 (2002) 41. J.A. Ramsey, in Emission Phenomena in Metals and Semiconductors. Proceedings of the 4th International Symposium on Exoelectron Emission and Dosimetry, Liblice, 1973, pp. 193–208 42. J.A. Ramsey, Exoelectric Emission, in Progress in Surface and Membrane Science, vol. 11, ed. by D.A. Cadenhead, J.F. Danielli (Academic Press, London, 1976), pp.117–180 43. A.J.B. Robertson, Exoelectron emission from solids. Int. J. Electron. 51(5), 607–619 (1981) 44. B. Rosenblum, P. Bräunlich, J.P. Carrico, in Thermally Stimulated Field Emission of Exoelectron from Lithium Niobate. Proceedings of the 4th International Symposium on Exoelectron Emission and Dosimetry, Liblice, 1973, pp. 100–101 45. J.W. Russel, Proc. Roy. Soc. 61, 424 (1897) 46. E. Rutherford, Phil. Mag. 49, 161 (1900) 47. N. Sato, Exoelectron emission from metals. Bull. Jpn Inst. Metals 7(6), 313–322 (1968) 48. A. Scharmann, in Exoelectron Emission, Phenomena, and Parameters. Proceedings of the 4th International Symposium on Exoelectron Emission and Dosimetry, Liblice, October 1973, pp. 12–29 49. A. Scharmann, W. Kriegseis, J. Seibert, in Surface Effects of Exoelectron Emitting Solids. Proceedings of the 3rd International Symposium on Exoelectrons, vol. 80 (PTB–Mitteilungen, Braunschweig, 1970), pp. 318–354 50. W.E. Spicer, Surface Analysis by Means of Photoemission and Other Photon-Stimulated Processes, in Chemistry and Physics of Solid Surfaces. ed. by R. Vaselow, S.Y. Tong (CRC Press, Cleveland, 1977), pp.235–254 51. H.A. Spikes, Triboelectrochemistry: Influence of applied electrical potentials on friction and wear of lubricated contacts. Tribol. Lett. 68, 1–27 (2020) 52. B. Sujak, Measurements of the external photoelectric effect of polycrystalline layers of alkaline halides by means of a G.-M. counter. Acta Phys. Polon. 12, 241–243 (1953) 53. Y. Tamai, Surface chemical technology. Kagaku Kogyo (Chem. Ind.) 31(1) (1980) 54. Y. Tamai, Y. Sato, Y. Momose, in Exoelectron Phenomena and Adhesion of Plastic Film. Proceedings of the 5th International Congress on Metallic Corrosion, Tokyo, 1974, pp. 640–644 55. M. Tanaka, On an after effect of metal bombarded by electrons. Proc. Phys. Math. Soc. Jpn. (3rd Ser.) 22, 899–924 (1940) 56. M. Tsukada, in 3 Electron Emission from Metal Surfaces. Shigoto kansu (Work Function). Kyoritsu–Shuppan, Tokyo, 1989, pp. 42–56 57. P. Villard, Seances de la Societe Francaise de Physique 197 (1900)

Part II

EE Mechanism of Metals Subjected to Adsorption

Chapter 2

EE of Clean Metals: Adsorption of Mainly O2 and H2 O in the UHV and HV

Abstract Initial EE research largely focused on the mechanism of electron emission from metals. We describe the specification of practical surfaces, the historical background of EE—leading to chemiemission—and its relationship to the adsorption of oxygen as well as water vapor on various metals, the mechanism of EE that is attributable to the adsorption of electronegative gases, and EE during oxidation of Cs films deposited on Ru.

2.1 Specification of Practical Surfaces A metal surface to be treated in reality appears with a particular process or history applied to that surface. Before proceeding to the chemiemission due to O2 and H2 O, we point out problems related to the behavior of organic compounds on metal surfaces in fields such degreasing and cleaning, antirust oils and pickling inhibitors, lubrication, cutting and grinding oils, painting, adhesion, etc. It is necessary to specify a surface with characteristics capable of capturing the state of the surface and discussing various behaviors of organic matter on the surface. To specify such surface, it is said that there is no way other than the means that can be used in production site. In this case, some of the surface physical and chemical properties are examined, and attempts are made to characterize the surface. At the same time, it is necessary to pay due attention to the change in the characteristics of the surface with time. Problems in production site arise from a pretreatment of metal surfaces for such as plating, coating, and painting. The term pretreatment includes degreasing and rust removal to clean the surface, and various surface treatments to provide conditions that match subsequent processing. The first step is to create a clean surface by a mechanical method, a chemical method, or the like, which give a different feature. Degreasing is intended to remove organic contaminants adhering to the surface. The problem in degreasing is the determination of degreasing effect. On the other hand, the main role of mechanical cleaning of the surface is not degreasing but rust removal. Sandblasting is commonly used in various pretreatments at any plant, but it is problematic how to determine and define the final surface condition. Thus, it becomes very important to specify and evaluate the state of surfaces finished by various methods (such as © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 Y. Momose, Exoemission from Processed Solid Surfaces and Gas Adsorption, Springer Series in Surface Sciences 73, https://doi.org/10.1007/978-981-19-6948-5_2

21

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2 EE of Clean Metals: Adsorption of Mainly O2 and H2 O …

sandblasting, abrasion with emery paper, degreasing and pickling, as well as plasma treatment). We describe EE from clean metal surfaces related to the adsorption of O2 and H2 O as the first step to specify processed solid surfaces. In accordance with [5], surfaces have been classified based on the cleanness: (a) technical; (b) clean, in which the surface is degreased and perhaps heated in ordinary vacuum; and (c) pure, in which the surface is prepared under stringent conditions such as UHV. Practical metal surfaces are in groups (a) and (b). A pure surface of group (c) of [5] is necessary to obtain information on the EE mechanism. However, EE might originate from not only pure surface but also might be associated with defects that are present in the surface layer or the inside bulk. Therefore, the effect of the adsorption of environments—such as O2 , H2 O, and organics—has been an active area of research regarding surfaces classified as groups (a) and (b).

2.2 Development of Chemiemission Ramsey [18], Greber [9], and Nienhaus [16] have published excellent reviews of chemiemission. We first describe the effect of gas adsorption on EE. First, in Fig. 2.1, we introduce EE measurement, conducted in an elegant manner by [14]. Lohff used an electron multiplier in the presence of oxygen for measuring EE from Zn. He scratched the surface with a steel brush in vacuo (>7 × 10−5 Torr). Upon applying oxygen, Lohff observed the emission after scratching as a function of time at two temperatures: 22 and 100 °C. The emission rapidly increased, passed through a maximum, and then slowly decayed or became almost constant. The intensity of the emission became greater, and the time interval for reaching the maximum was shorter, with increasing oxygen pressures. Furthermore, the intensity of the emission was much greater at 22 °C than at 100 °C. These findings indicate that the oxygen that was incorporated onto the surface had two effects on EE: increasing EE in the initial stage and attenuation of EE in the latter stage. Next, we describe historical studies of EE measured in the UHV. Delchar [4] observed that upon chemisorption of O2 onto the surface of a Ni film (evaporated at 10−10 Torr in small quantities), each oxygen addition was accompanied by a burst of electrons. The number of emitted electrons per adsorbed oxygen molecule was ca. 10−9 electrons/molecule at 77 K. Upon adding O2 at room temperature, the yield of electrons initially dropped to a low value and then increased, which caused a second peak. This behavior was related to the incorporation of oxygen into the Ni lattice. Regarding Cu and W, there was also EE during oxygen chemisorption, and the yield of electrons per adsorbed molecule was approximately 10−9 electrons/molecule. Gesell et al. [8] measured EE during chemisorption of H2 O vapor or O2 (pressure range: 5 × 10−10 to 1 × 10−6 Torr)—on fresh Mg surfaces, in complete darkness, at room temperature—with a picoammeter. They prepared the sample surface either by vacuum evaporation or abrasion with a stainless steel wire brush. Figure 2.2 shows the time development of EE from abraded Mg during exposure to O2 at various partial pressures. Two maxima for the intermediate pressure indicate that the process was not

2.2 Development of Chemiemission

23

Fig. 2.1 EE from Zn after scratching with a steel brush at different O2 pressures [14]. Reproduced with the permission from Springer Berlin Heidelberg, Lohff [14]. Copyright© 1956

a simple one. The photoelectric work function dropped to ~1.9 eV during the course of the emission and rose as the emission decreased. The maxima of EE occurred at times that are close to that of monolayer formation, and the second maximum is probably because of contamination of the water vapor that remained in the vacuum system. When the O2 pressure was 1.1 × 10−8 Torr, the maximum yield was 1.2 × 10−6 electrons/adsorbed molecule. Figure 2.3 shows the time development of EE during adsorption of H2 O vapor on abraded Mg. One maximum was evident after a substantial period of time. No double maxima were evident with H2 O vapor. The EE is hypothesized to be caused by the heat of chemisorption of O2 or H2 O. In the experiment regarding Al, performed under the same conditions, no measurable EE was evident. Upon exposure of Mg to C2 H5 OH, N2 , (CH3 )2 CO, (CH3 )2 CHOH, or H2 , no measurable EE was evident, but a small EE was evident in the case of CH3 OH. Regarding the work function [7], Fig. 2.4 shows the work function, EE, and photoemission (photon energy, hν = 1.93 ± 0.05 eV) from abraded Mg as a function of time, at an O2 pressure of approximately 7 × 10−9 Torr. The photoelectric work function was determined by the onset of photoemission. The photoelectric work function varied between 3.3 and 1.8 eV. The photoelectric work function proceeded through two minima, corresponding directly to the two maxima in the photoemission

24

2 EE of Clean Metals: Adsorption of Mainly O2 and H2 O …

and closely connected to the two maxima in the EE indicated in Fig. 2.2. The first maximum in the EE versus time after abrasion (Fig. 2.4) is close to the monolayer formation time during oxygen chemisorption. Moucharafieh and Olmsted III [15] (1971) observed EE—termed chemiemission—when Cs films (deposited on a gold substrate and annealed at −63.5 °C) reacted with O2 within a pressure range of 0.8 × 10−8 –8.0 × 10−8 Torr, with an

Fig. 2.2 Time development of EE for O2 on Mg at various pressures of O2 [8]. Reprinted with the permission from Elsevier, Gesell et al. [8]. Copyright© 1970

Fig. 2.3 Time development of EE for H2 O on Mg at various pressures of H2 O [8]. Reprinted Elsevier, Gesell et al. [8]. Copyright© 1970

2.2 Development of Chemiemission

25

Fig. 2.4 Time development of work function, EE, and photoemission from a fresh Mg surface [7]. Reprinted Elsevier, Gesell and Arakawa [7]. Copyright© 1972

electrometer in the dark. As an example, regarding a thick Cs film which previously reacted with O2 and was then annealed at a base pressure of 10−9 Torr for 6 min, the variation of the emission current with time under a constant O2 pressure of 3.7 × 10−8 Torr exhibited a delay of the onset of emission. This was followed by a relatively rapid increase in the emission to a maximum, after which the emission decayed approximately exponentially. The researchers conducted the analysis of the chemiemission curves in the following manner. From the value of the maximum emission current (I max , A), they determined the number of emitted electrons (Y DM ) per adsorbed oxygen molecule at the maximum current by the following equation: I max = (spAY DM e)/(2π mkT )1/2 , where s is the sticking probability (fraction of collisions that result in adsorption), p is the pressure of O2 , m is the molecular weight of O2 divided by Avogadro’s constant, k is Boltzmann’s constant, T is the temperature in Kelvin, sp/(2π mkT )1/2 is the number of adsorbed molecules per square centimeter per second, A is the area of the Cs surface, and e is the charge of an electron. When T is set to room temperature, p is represented by Torr, and when A is set to 5 cm2 , the following equation is obtained for O2 gas at room temperature: sY DM = I max /287p. The plot of sY DM versus p resulted in a linear increase of sY DM with pressure although

26

2 EE of Clean Metals: Adsorption of Mainly O2 and H2 O …

Table 2.1 Electron yields for some chemiemission processes References

System

[4]

Ni–O2

Pressure (Pa)

Maximum emission probability (electrons per molecule striking the surface) 10−9 ···10−10

[8]

Mg–H2 O–O2 Al–H2 O–O2

10−8 ···10−4 10−8 ···10−4

10−6 No emission

[15]

Cs–O2

10−6

10−2

[20]

K–O2 Al–O2 Mg–O2

>10−3

10−6 ···10−3

10−5 No emission No emission

[11]

Na–Cl2

10−4

5 × 10−8

Al–O2 –H2 O

10−4 ···10−2

10−9 ···10−10

[12] [6]

Mg–O2 Mg–Cl2 Al–O2 Al–Cl2

8×

10−5

10−8 10−8 No emission No emission

there was scatter in the data. Therefore, this relationship indicates that I max is proportional to the square of the pressure of O2 . The value of Y DM was on the order of 10−2 , and the values of s were in the range of 0.02–0.13. Regarding the emission yields, the Y DM in this case was substantially high; whereas as indicated in Table 2.1 in a subsequent paragraph, in most cases, the emission yields were small—within the range of 10−9 –10−5 . Furthermore, no emission was evident within the sensitivity limits (10−13 A) of the measurement apparatus for the following O2 , H2 O, and O2 –H2 O systems: Li–O2 , Mg–O2 , and Al–O2 (gas pressure: ca. 10−8 –10−5 Torr); as well as Cs–H2 O and Cs–H2 O–O2 (gas pressure: 10−8 –10−7 Torr). In the system of K–O2 , there was no chemiemission in O2 pressure ranges 10−5 Torr, chemiemission was similar to that of the system of Cs–O2 . These results indicate that the K–O2 system required three orders of magnitude higher pressures of O2 to produce chemiemission than the Cs–O2 system. Regarding K–O2 , the value of Y DM was 5 × 10−6 , and s was 0.002. Furthermore, in the Cs–H2 O–O2 system, a report indicates the poisoning effect of H2 O vapor, i.e., H2 O vapor did not lead to chemiemission and inhibited O2 chemiemission [20].

2.3 EE Attributable to Adsorption of Electronegative Gases Ramsey [19] reported interesting effects of the adsorption of electronegative gases such as O2 and Cl2 on electron emission from clean metal surfaces. This review discussed two subjects in detail: (1) the effects of decreasing the work function because of chemisorption and the formation of low work function patches on EE in the dark as well as OSEE [1], and (2) the effects of increasing the electron affinity of the electronegative gases on dissociative chemisorption, which can facilitate electron

2.3 EE Attributable to Adsorption of Electronegative Gases

27

transfer by tunneling from the substrate conduction band, which helps clarify the mechanism of EE by the Auger process [3, 17]. In particular, effect 2 pertains to energy transfer during chemisorption on metals. One can explain this effect in terms of electron transfer from the metal to the incoming species. The energy released might be dissipated either by a radiative process or by a nonradiative Auger transition, resulting in an excited electron inside the metal. If this aforementioned electron has sufficient energy and a suitable momentum, it might be able to escape the solid and be detected in vacuum as an exoelectron. Effect 1, found by [1], is as follows. Exoelectron emission and long-wavelength photoemission occur during oxidation of clean Mg, Al, and MgAl alloys by decreasing the work function. Allen et al. proposed the idea of developing low work function patches, termed exopatches. They determined work functions with a photoelectron spectrometer (XPS and exoelectron energy spectra) and the diode technique (surface potentials and volumetric adsorption of O2 ). The research group of [1] distinguished three types of electron emission by work function changes of the specimen surface and used the following terminology, as rewritten by [19]. (a) Exoelectrons: Electrons emitted because of oxygen interaction with a surface in complete darkness. (b) Photo/exoelectrons: Electrons emitted because of oxygen interaction with the surface while irradiated with photons of a wavelength substantially greater than that of the normal photoelectric threshold of the surface. This emission is termed PSEE or OSEE. (c) Photoelectrons: Electrons emitted because of photon irradiation but without simultaneous oxygen interaction, i.e., with a UHV system pumped down to its base pressure of 2φ, and (b) the electron yield should be proportional to the cube of the excess energy ΔE, over a range of energy up to several electron volts. Experimental exoemission studies [17] were performed, and the results were compared with the foregoing model. The Zr that was initially predosed with Na decreased the work function by 0.3 eV. These results are consistent with the model. The experiments suggest that E 1 A ≈ 8.1 eV for chlorine/Zr adsorption; the value a = 0.3 eV−1 that was used in this model is consistent with a purely Coulombic shift of the affinity level of the adsorbate species. Thus, only, electrons within ca. 1 eV of the Fermi edge are active in the electron emission. Regarding the adsorption of chlorine on clean surfaces of Y, Ti, Zr, and Hf, if E 1 A is independent of the substrate and has the value of 8 eV (as determined for Zr), (2.1) suggests that exoelectrons should be observed only for Y (φ = 3.1 eV) and Hf (φ = 3.9 eV), whereas no emission should be measurable for Ti (φ = 4.33 eV) and Zr (φ = 4.05 eV)—which indeed turned out to be the case [3, 19]. Varying the alkali predose enabled the work function to be adjusted and the consequent effect on the electron yield to be determined. As an example, when Cl2 was incident on a Zr surface that was predosed with Na, the electron yield decreased monotonically with increasing coverage of Cl2 and increased with initial Na predosing. Regarding particular substrate metals (Y, Zr, Ti, and Hf), the data points from alkali atoms (Na and Rb) and adsorbates (Cl2 , Br2 , and O2 ) lie on the same straight line in the logarithmic plot of log10 γ /X 2 against log10 (E 1 A − 2φ) in (2.1), regardless of the alkali or halogen adsorbed. These results were interpreted to be because of an atomic (X) electron affinity level rather than on a molecular (X 2 ) level. Regarding the O2 adsorbate, for clean Y, the electron yield was 10−7 electrons per incident O2 molecule. The yield of electrons exponentially decreased with time as the surface was covered with O2 ; whereas regarding Y predosed with Rb, the initial yield increased to 2 × 10−6 [3].

2.3.1 EE During Oxidation of Cs Films Deposited on Ru Böttcher et al. [2] studied in detail the emission of electrons during oxidation of Cs films deposited on Ru (001) and interpreted the EE mechanism. Oxidation of Cs thin films proceeds through various stages. Initially, suboxides that contain the structural unit Cs11 O3 are formed. Here, the O2− ions are incorporated below the surface, which remains in its zerovalent state. A peroxide Cs2 O2 is then formed. Here, all Cs atoms manifest as Cs+ and O2 2− . Upon O2 exposure, the work function φ decreases, reaches a minimum around completion of this step, and then continuously increases with formation of the superoxide species (CsO2 , which contains O2 − ions). Figure 2.8 [2] show the change in the work function and exoelectron emission intensity upon stepwise exposure of a Cs thin film (ca. 3 monolayers thickness, at 220 K) to an O2 pressure of 4 × 10−9 mbar. The first 0.3-L (1 L = 10−6 Torr s) exposure of O2 causes almost no change in the work function because of bulk diffusion of the O atoms.

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2 EE of Clean Metals: Adsorption of Mainly O2 and H2 O …

Then, the work function decreases because of the formation of Cs11 O3 units at the surface. A clean Cs surface is completely inactive to electron emission although the largest quantity of energy is released in the initial stage of oxidation to Cs11 O3 . Figure 2.8 indicates that the emission of electrons reaches its maximum only beyond the minimum of the work function. Thus, the electron emission is essentially confined to the stage of the surfaces in which the work function increases again, i.e., to the range of transformation Cs2 O2 → CsO2 . Electrons are emitted with a total probability on the order of 10−6 per reacting O2 molecule. Electron emission is explained by the Auger de-excitation mechanism (Fig. 2.7) [3]. This electron emission is caused by a nonadiabatic reaction that involves a transition of electrons (as provided by Cs0 atoms) of the substrate near the Fermi level (E F ) to the empty state (εA ), derived from the O2 affinity level, and located at ca. 2.5 eV below the E F . This affinity level (which for the free O2 molecule is ca. 0.4 eV below the vacuum level) decreases with decreasing distance of the impinging O2 molecule from the surface; the resonance ionization of this O2 molecule upon crossing E F can only be efficiently suppressed because of the absence of metallic states near E F . Furthermore, Fig. 2.9 [2] (regarding Cs films with the same thickness) shows the variation of the exoelectron current as a function of time at various O2 pressures. The time T max after which the maximum exoelectron emission current I max is reached varies approximately in proportion to 1/PO2 . These results suggest that exoemission is a first-order kinetic process with respect to PO2 . Finally, in the connection with this study, a similar observation made in a previous study with an Mg surface [7, 8] suggests that the two minima in the work function

Fig. 2.8 Variation of the current of exoelectrons [in arbitrary units, curvae (a)] and of the wrok function φ [curve (b)] upon stepwise exposure of a Cs film with about 3 ML thickness at 220 K to an O2 pressure of 4 × 10−9 mbar [2]. Reprinted AIP Publishing, Böttcher et al. [2]. Copyright© 1991

References

33

Fig. 2.9 Variation of the exoelectron current with time at various O2 pressures for Cs films with the same thickness of 3.4 ML, and at the same temperature T = 190 K [2]. Reprinted AIP Publishing, Böttcher et al. [2]. Copyright© 1991

(Fig. 2.4) closely correspond to the two maxima in the exoelectron emission intensity (Fig. 2.2). Although the reason for these results remains unclear, this difference might be ascribed to the surface oxidized films of Cs and Mg. Finally, Table 2.1 shows the electron yields of some chemiemission processes [10, 13].

References 1. G.C. Allen, P.M. Tucker, B.E. Hayden, D.F. Klemperer, Early stages in the oxidation of magnesium, aluminium and magnesium/aluminium alloys: I. Exoelectron emission and long wavelength photoemission. Surf. Sci. 102, 207–226 (1981) 2. A. Böttcher, R. Grobecker, R. Imbeck, A. Morgante, G. Ertl, Exoelectron emission during oxidation of Cs films. J Chem Phys 95(5), 3756–3766 (1991) 3. M.P. Cox, J.S. Foord, R.M. Lambert, R.H. Prince, Chemisorptive emission and luminescence: II. Electron and ion emission from chlorine and bromine reactions with yttrium, titanium, zirconium and hafnium surfaces. Surf. Sci. 129, 399–418 (1983) 4. T.A. Delchar, Exo-electron emission during oxygen chemisorption at clean nickel surfaces. J. Appl. Phys. 38, 2403–2404 (1967) 5. P.P. Ewald, Structure and Properties of Solid Surfaces; the Report of a Conference Organized by the Committee on Solids of the National Research Council (University of Chicago Press, Chicago, 1953), p.1 6. J. Ferrante, Exoelectron emission from a clean, annealed magnesium single crystal during oxygen adsorption. ASLE Trans. 20(4), 328–332 (1976) 7. T.F. Gesell, E.T. Arakawa, Work function changes during oxygen chemisorption on fresh magnesium surfaces. Surf. Sci. 33, 419–421 (1972) 8. T.F. Gesell, E.T. Arakawa, T.A. Callcott, Exo-electron emission during oxygen and water chemisorption on fresh magnesium surfaces. Surf. Sci. 20, 174–178 (1970)

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2 EE of Clean Metals: Adsorption of Mainly O2 and H2 O …

9. T. Greber, Charge-transfer induced particle emission in gas surface reactions. Surf. Sci. Rep 28, 1–64 (1997) 10. G. Heinicke, in 3.3 Physical Elementary Processes in the Mechanical Activation of Solids. Tribochemistry (Akademie–Verlag, Berlin, 1984), pp. 42–96 11. B. Kasemo, L. Wallden, Spontaneous emission of photons and electrons during chemisorption of chlorine on sodium. Solid State Comm. 15(3), 571–574 (1974) 12. E. Linke, G. Grabo, in Spontaneous Emission Of Electrons from Aluminium During Continuous Mechanical Treatment. Proceedings of the 5th International Symposium on Exoelectron Emission and Dosimetry, Zvikov, 1976, pp. 255–258 13. E. Linke, in Electron Emission Upon Mechanical Treatment, eds. by H. Glaefeke, W. Wild, Proceedings of the 6th International Symposium on Exoelectron Emission and Applications (Wilhelm Pieck University, Ahrenshoop, 1979), pp. 133–136 14. J. Lohff, Die Elektronenemission bei der Oxydation mechanisch bearbeiteter Metalloberflächen. Z Physik. 146, 436–446 (1956) 15. N. Moucharafieh, J. Olmsted III., Chemiemission from the cesium-oxygen surface reaction. J Phys Chem 75, 1928–1936 (1971) 16. H. Nienhaus, Electronic excitations by chemical reactions on metal surfaces. Surf. Sci. Rep. 45, 1–78 (2002) 17. R.H. Prince, R.M. Lambert, J.S. Foord, Chemisorptive emission and luminescence: I. Chlorine/zirconium. Surf. Sci. 107, 605–624 (1981) 18. J.A. Ramsey, Exoelectric emission. Prog. Surf. Membrane Sci. 11, 117–180 (1976) 19. J.A. Ramsey, The adsorption of gases on clean metal surfaces and exoelectron emission. Jpn. J. Appl. Phys. 24–4, 32–37 (1985) 20. N.K. Saadeh, J. Olmsted III., Chemiemission from metal-oxygen surface reactions. J. Phys. Chem. 79, 1325–1326 (1975)

Chapter 3

EE from Metal Surfaces Covered with Oxide: Adsorption of Mainly O2 and H2 O and Oxide-Film Thickness

Abstract Regarding practical surfaces, there has been a substantial focus on the electronic properties of the outer surface layers, where there are extrinsic electron capture centers or factors that influence the intensity of EE. The oxide film on metals also facilitates EE from the surface. Metals used in this context include Al, Mg, Ni, Sn, Fe, and Cu. We describe the effects of the oxide-film thickness, gas adsorption, temperature, light irradiation, and accelerating field strength that one applies to the sample.

3.1 OSEE Observed in the UHV and HV for Al2 O3 /Al Regarding initial EE studies, Brotzen [3], Scharmann [17], and Baxter [2] have published excellent reviews of EE, as well as micrographs of metal surfaces after subjection to mechanical deformation and fatigue. Mechanical treatments of oxidecovered metal surfaces produce EE that pertains to the creation and diffusion of vacancies in the covering oxide. We focus on the effect of the adsorption of O2 and H2 O vapor on EE from mechanically deformed surfaces. Ramsey [16] has explained the effect of adsorption of O2 and H2 O vapor—on OSEE from abraded Al—by measuring the change of the work function. Ramsey measured EE from high-purity Al (abraded with a stainless-steel brush) during exposure to air, O2 , N2 , and H2 O vapor; at various pressures between 10−5 and 10−8 Torr. A clean surface of Al after abrasion exhibits OSEE, but the OSEE corresponds to a shift of the photoelectric threshold, which depends on the pressure of the residual gas to which the surface was exposed. The wavelength of light used for stimulation was >345 nm, although the typical PE threshold of Al is 4.2 eV (limiting wavelength: 290 nm). In this case, the primary function of mechanical abrasion is to expose metal to the ambient gas. Changes in the work function that occur during adsorption of H2 O vapor and O2 vary the critical wavelength and increase or decrease the number of emitted electrons. These effects are interpreted in terms of the basic oxidation of metals. Initially, the nascent surface is covered with a monolayer of O2 . Presumably, at this stage, the O2 molecule dissociates into two atoms and exchanges its position with one metal atom. This layer adsorbs H2 O or OH and reduces the work function by © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 Y. Momose, Exoemission from Processed Solid Surfaces and Gas Adsorption, Springer Series in Surface Sciences 73, https://doi.org/10.1007/978-981-19-6948-5_3

35

36

3 EE from Metal Surfaces Covered with Oxide: Adsorption of Mainly O2 …

ca. 1.2 eV, because of the dipole moments formed as positive outward polar groups. The two adsorption steps take place in approximately 1 min under a pressure of 10−7 – 10−8 Torr. At the end of this process, the OSEE intensity increases considerably. The work function is then increased by reaction with O2 . The latter process is the reason why the OSEE decays over time under constant residual pressure in the vacuum system. Furthermore, one does not observe OSEE by illumination with visible light; even after strong polishing of Al with steel needles under UHV (10−10 Torr), or even after introducing O2 (295 nm) was used as a light source. The EE measurements were conducted by applying an accelerating voltage of 95 V (Fe, Ni, and Cu) and 24 V (Al) between the earthed grid and the sample. All of the metals during friction in the Geiger counter gas (mainly C2 H5 OH–Ar) gave two distinct types of EE (discussed in the next paragraph): dark emission that was stronger at the start of friction; and subsequent OSEE that gradually increased, then stabilized in the subsequent stage of friction.

3.3 EE Observed in Counter Gas for Oxide-Covered Metal …

49

Fig. 3.14 Geiger counter for measuring exoelectron emission from metal surface during friction process [14]. Reprinted with the permission from American Chemical Society, Momose and Namekawa [14]. Copyright© 1978

We next discuss the results for Fe. Figure 3.15 shows a typical example of Fe [14]. The former type of EE occurred only during rubbing, with and without optical stimulation. The latter type of EE occurred under optical stimulation, during friction and even after cessation of friction; although the intensity decayed gradually with time, and upon restarting the friction the OSEE rapidly recovered to the original level. The dark emission strongly depended on the presence of the oxide film on the metal surface that was formed by the pretreatment. The emission intensity increased in the following order: untreated specimen (100 counts/s) < specimen annealed in vacuum for 2 h at 350 °C (450 counts/s) < specimen annealed in vacuum for 1 h at 955 °C (820 counts/s) < specimen oxidized in air for 1 h at 300 °C (2200 counts/s). Regarding Fe after cessation of friction, the effect of exposure to O2 , H2 O vapor, and some other gases over the course of the decay on the recovery of OSEE was examined. Figure 3.16 shows the effect of exposure to Ar–O2 mixtures [14]. The higher the partial pressure of O2 , the weaker the OSEE intensity that one recovers. Figure 3.17 shows the relationship between the following: (1) The quantity of iron particles produced by friction for 120 min in the presence of various gases and (2) the total count of OSEE obtained for 20 min after admitting the counter gas, after removing these gases for 5 min [14]. The experimental data points give a nearly linear plot on a double-logarithmic scale, with the exception of the data points for H2 O vapor and O2 , which are located on both sides of the straight line. The quantity (median value) of the iron particles for each gas decreases in the following order: Ar (10,570 μg) >> O2 (2510 μg) > H2 O (500 μg) > counter gas (C2 H5 OH–Ar) (100 μg) > vacuum (50 μg) = C2 H5 OH (50 μg). Furthermore, the total count of OSEE, expressed in units per microgram of the quantity of iron particles, varies substantially and depends on the gas. This value decreased in the following order:

50

3 EE from Metal Surfaces Covered with Oxide: Adsorption of Mainly O2 …

H2 O (1130 counts/μg) > Ar (360 counts/μg) > vacuum (90 counts/μg) > counter gas (C2 H5 OH–Ar) (65 counts/μg) = C2 H5 OH (65 counts/μg) > O2 (zero counts/μg). A striking observation is that the adsorbed water tends to increase the activity of OSEE, whereas the adsorption of oxygen completely destroys the OSEE active sites, despite the greater quantity of wear particles.

Fig. 3.15 Exoelectron emission from Fe during friction at a speed at 510 rpm: (I, I′ ) without optical stimulation; (II) rubbing interrupted [14]. Reprinted with the permission from American Chemical Society, Momose and Namekawa [14]. Copyright© 1978

Fig. 3.16 Effect of exposure (E, 0.5 min) to Ar–O2 mixtures on the change of OSEE from Fe after cessation of friction: a without exposure; b exposure under vacuum for 5 min; c 0.35 N/m2 O2 (510 N/m2 ); d 0.76 N/m2 O2 (1100 N/m2 ); e 4.7 N/m2 O2 (6900 N/m2 ). The values in parentheses indicate the total pressure [14]. Reprinted with the permission from American Chemical Society, Momose and Namekawa [14]. Copyright© 1978

3.3 EE Observed in Counter Gas for Oxide-Covered Metal …

51

Fig. 3.17 Relationship between the total count of OSEE from Fe after friction in various environments and the amount of Fe particles produced in this process: (⬜) Ar (10,700 N/m2 ); (▲) O2 (1300 N/m2 ); (●) H2 O (2700 N/m2 ); (Δ) counter gas (C2 H5 OH 2700 N/m2 + Ar 11,200 N/m2 ); (◯) vacuum; (∎) C2 H5 OH (2700 N/m2 ). The values in parentheses indicate the pressure of the environment [14]. Reprinted with the permission from American Chemical Society, Momose and Namekawa [14]. Copyright© 1978

We summarize the EE characteristics for other metals as follows. Regarding the Ni samples, a striking feature is that Ni samples—after exposure to O2 under reduced pressures during the decay—corresponded to increasing OSEE (in contrast with the Fe samples), but this recovery of OSEE tended to decrease with increasing pressure of O2 . The exposure to H2 O vapor produced a low OSEE recovery, which was similar to the exposure under vacuum. Furthermore, regarding the Ni samples, the exposure to H2 gave much higher activity in the context of recovery of OSEE compared with adsorption of O2 . Regarding Al and Cu samples, there was some metal–metal welding between the wear particles and the metal sheet during friction. Regarding the Al samples, there was continual dark emission, which increased to a maximum at the beginning of friction, subsequently decreased, and then stabilized. Al samples yielded a dark emission that exhibited a considerably stronger intensity compared with the other metals. Furthermore, there is a strong OSEE; on termination of the friction, the OSEE increases steadily over a period of >2 h, reaches a maximum, and then decays slowly. The activity of OSEE from the wear particles (produced over the course of friction) was more strongly increased by the interactions with H2 O vapor compared with O2 . Regarding the Cu samples, upon termination of the friction, the OSEE decayed more rapidly compared with the Fe and Ni samples. The recovery of OSEE after exposure to O2 , H2 O vapor, and vacuum was low (ca. 35%) in all cases.

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3 EE from Metal Surfaces Covered with Oxide: Adsorption of Mainly O2 …

References 1. D.R. Arnott, J.A. Ramsey, Electron emission from anodically oxidised aluminium due to tensile deformation. Surf. Sci. 28, 1–18 (1971) 2. W.J. Baxter, Exoelectron Emission from Metals, in Research Techniques in Non-destructive Testing, vol. 3, ed. by R.S. Sharpe (Academic Press, London, 1977), pp.395–428 3. F.R. Brotzen, Emission of exoelectron from metallic materials. Phys. Stat. Sol. 22, 9–30 (1967) 4. H. Drost, D. Lange, U. Timm, H. Pupke, Thermostimulierte Ladungsträger-Nachemission von wasser-behandelten Metall- und Ge-Oberflächen. Acta Phys. Austriaca 25, 148–154 (1967) 5. A.G. Gel’man, A.I. Fainshtein, Possible mechanism for the two stages of exoelectron emission during oxidation of a metal. Sov. Phys. Solid State 14, 1752–1755 (1973) 6. A.G. Gel’man, I.L. Roikh, Relation of exoelectron emission of magnesium with oxidation and deformation excitation. Sov. Phys. Solid State 12, 2763–2765 (1971) 7. A. Gieroszynski, B. Sujak, Exoelectron emission in vacuum in the absence of light during plastic deformation of aluminium thickly coated with oxide. Acta Phys. Polon. 28, 311–327 (1965) 8. A. Gieroszynski, B. Sujak, Effect of oxide and measuring parameters on the decay of photostimulated exoelectron emission in vacuum from plastically deformed alumnium (II). Acta Phys. Polon. 28, 337–342 (1965) 9. A. Gieroszynski, B. Sujak, Spectral investigations of photostimulated emission of exoelectrons in vacuum from plastically deforemed aluminium covered with oxide layer. Part I Initial deformation. Acta Phys. Polon. 29, 275–282 (1966) 10. A. Gieroszynski, B. Sujak, Spectral investigations of photostimulated emission of exoelectrons in vacuum from plastically deforemed aluminium covered with oxide layer. Part III Slope of the decay curve in its initial stage. Acta Phys. Polon. 29, 533–547 (1966) 11. H. Kahlert, G. Kralik, Zur Elektronenemssion beim Schmelzen und Erstarren von Metallen. Acta Phys. Austriaca 23, 303–311 (1966) 12. E. Linke, D. Born, in Triboinduced Electron Emission And Work Function Measurement in the System Aluminium O2 -H 2 O Under UHV Conditions. Proceedings of the 4th International Symposium on Exoelectron Emission and Dosimetry, Liblice, 1973, pp. 159–161 13. E. Linke, K. Meyer, Über den zusammenhang zwischen triboinduzierter elektronenemission und adsorptionsprozessen im system Al–O2 –H2 O. Surf. Sci. 20, 304–312 (1970) 14. Y. Momose, T. Namekawa, Exoelectron emission from metals subjected to friction and wear, and its relationship to the adsorption of oxygen, water vapor, and some other gases. J. Phys. Chem. 82, 1509–1515 (1978) 15. W. Pong, Photoemission from Al–Al2 O3 films in the vacuum ultraviolet region. J. Appl. Phys. 40, 1733–1739 (1969) 16. J.A. Ramsey, The emission of electrons from aluminium abraded in atmospheres of air, oxygen, nitrogen and water vapour. Surf. Sci. 8, 313–322 (1967) 17. A. Scharmann, in Exoelectron Emission, Phenomena, and Parameters. Proceedings of the 4th International Symposium on Exoelectron Emission and Dosimetry, Liblice, 1973, pp. 12–29 18. T. Smith, Photoelectron emission from aluminum and nickel measured in air. J. Appl. Phys. 46, 1553–1558 (1975) 19. T. Smith, D.O. Thompson, in Characterizing of Al Fatigue Damage by Photoemission, Ellipsometry and Auger Spectroscopy. Proceedings of the 4th International Symposium on Exoelectron Emission And Dosimetry, Liblice, 1973, pp. 212–214 20. B. Sujak, A. Gieroszynski, Spectral investigations of photostimulated emission of exoelectrons in vacuum from plastically deformed aluminium covered with oxide layer. Part II limiting oxide thichckness. Acta Phys Polon 29, 523–531 (1966) 21. Surface Science Society of Japan, in 6.1.1 Inelastic Scattering and Definition of Terms Related to it in: X-ray Photoelectron Spectroscopy (Maruzen, Tokyo, 1998), pp. 99–103 22. D.O. Thompson, R. Young, G.A. Alers, T. Smith, Fatigue-induced photoelectron enhancement (exo-electron) from aluminum. J. Appl. Phys. 47, 3846–3857 (1976)

Chapter 4

Effects of Organic Adsorption, Applied Voltage, Light Irradiation, and Catalytic Activity

Abstract Stimuli such as the adsorption of organic molecules; an applied voltage or electric potential between a sample and the earthed grid of the counter; the intensity of irradiated light; and catalytic activity substantially influence the characterization of exoemission (EE), in the same manner as thermal stimulation.

4.1 Effect of Adsorption on OSEE from Al We describe the effect of three types of stimuli on OSEE from mechanically deformed Al metal surfaces [7, 8]: (1) adsorption of organic vapor, which was used as a quenching gas in a Geiger counter gas [7], (2an applied voltage, termed AV, between a sample and the grid of the counter, which causes negative electric potential to a sample; [8]; and (3) the illuminating light intensity (I p ) on the sample [8]. First, we examine the effect of the type of organic vapor on the glow curves of OSEE (which occurs under illumination by light) from ground Al powder [7]. Al powder (purity 99.5%, 0.50 g) with a natural oxide film was introduced into a glass vessel with an Al plate laid at the bottom and then ground between an iron bar and the plate at room temperature in air, usually for 10 min with a magnetic stirrer. The EE measurement was performed under irradiation with a weak fluorescent light (wavelength of light > 295 nm) by using a modified Geiger counter; with a counter gas consisting of a mixture of Ar (84 Torr) and organic vapor (20 Torr), except that of Ar (90 Torr)–(n-C3 H7 )2 NH (14 Torr). Eight counter gases were used. Figure 4.1 shows the effect of grinding time on the glow curves for the counter gas of Ar– C2 H5 OH. The glow curve exhibited a broad peak and the emission intensity increased with increasing time of grinding. The glow curve behavior in various counter gases differed widely despite the identical mechanical treatment. An emission peak was evident except for the counter gas of Ar–CH3 COOC2 H5 . Table 4.1 [7] shows the glow curve characteristics of the Al power that was ground in air for 10 min. The intensities and temperatures of the emission peak, the intensities at 25 and 239 °C, and the total count of the emitted electrons (temperature range of 25–239 °C) strongly depended on the organic vapor. The organic vapors, in descending order of the total count, were as follows: (n-C3 H7 )2 NH > n-C3 H7 NH2 > CH3 COOC2 H5 > C6 H6 > © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 Y. Momose, Exoemission from Processed Solid Surfaces and Gas Adsorption, Springer Series in Surface Sciences 73, https://doi.org/10.1007/978-981-19-6948-5_4

53

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4 Effects of Organic Adsorption, Applied Voltage, Light …

C2 H5 OH > (CH3 )2 CO > CH3 CN > n-C4 H9 Cl. Figure 4.2 [7] shows the relationship between the total count shown in Table 4.1 and the reciprocal dielectric constant (1/ε) of each organic compound. The total count of emitted electrons closely corresponds to the dielectric constant, except for n-C4 H9 Cl and C6 H6 . This result suggests that the emission was perhaps under the influence of the electric field (E) that was produced by the surface hydroxyl groups (–Al–OH); as a result of introducing organic substances to the surface, the electric field strength decreased from the value (E) to the value (E/ε). One can explain the OSEE in terms of electron emitting centers in the oxide film and the surface hydroxyl groups. In the experiments, the oxide film on the Al powder was broken—even with small loads—by grinding in a manner that revealed the bare metal surface; perhaps because the substrate metal was softer than its oxide, such that the latter cracked easily. The freshly exposed Al might interact with O2 , H2 O vapor, and other gases that are present in air immediately after grinding; and an oxide film might form again. Thus, the oxide film is hypothesized to contain many defects, notably vacancies; which aid in diffusion of metal ions, growth of the oxide layer, and partial hydroxylation. Ramsey [10] and Linke and Meyer [4] reported that the emission from abraded Al resulted from the adsorption of H2 O molecules as a positive hydrogen-outward orientation of either H2 O or OH, which decreased the work function. Furthermore, one can explain the OSEE based on acid-base interactions. The organic molecules on the straight line shown in Fig. 4.2 might be adsorbed by hydrogen bonding with surface hydroxyl groups. The order of the dielectric constant of the organic compounds used corresponds to the negative logarithm of the acidity constant, pK BH+ = −log K BH+ ; where K BH+ = ([B] × [H+ ])/[BH+ ] is the dissociation constant of an acid, for conjugate acids (BH+ ) of organic compounds as conjugate bases (B), as shown in Table 4.2 [7]. These results suggest that hydrogen bond formation facilitates emission and that there might have been emitting centers in the vicinity of Brönsted acid sites on the disturbed surfaces. Fig. 4.1 Effect of grinding time on the glow curve of ground Al powder (air exposure time, 1–1.5 min): a 60 min, b 10 min, c untreated powder [7]. Reprinted with the permission from American Chemical Society, Momose et al. [7]. Copyright© 1976

4.1 Effect of Adsorption on OSEE from Al

55

Table 4.1 Dependence of glow-curve characteristics of ground aluminum powder on organic vapor Organic vapor

Intensity at 25 °C, counts/s

Peak intensity, counts/s (temp. °C)

Intensity at 239 °C, counts/s

Total count (25–239 °C)

(n-C3 H7 )2 NH

50

150(88)

30

61,900

n-C3 H7 NH2

50

100(108)

25

45,400

CH3 COOC2 H5

12

No peak

83

29,200

C6 H6

7

34(108)

16

15,800

C2 H5 OH

10

19(72)

11

8300

(CH3 )2 CO

9

19(66)

1

5700

CH3 CN

2

9(80)

1

2700

n-C4 H9 Cl

0.5

2.5(110)

ca.0 (230 °C)

900 (25–230 °C)

Grinding time, 10 min; air exposure time, 1–1.5 min

Fig. 4.2 Total count of emitted electrons as a function of reciprocal dielectric constant (1/ε) of organic compounds [7]. Reprinted with the permission from American Chemical Society, Momose et al. [7]. Copyright© 1976

Table 4.2 Dielectric constant and negative logarithm of the acidity constant of conjugate acids of organic compounds used in the counter gases Organic compound as conjugate base, B (n-C3 H7 )2 NH

Dielectric constant/ε 3.068

−Logarithm of the acidity constant of conjugate acid/pK BH+ 11.00

n-C3 H7 NH2

5.31

10.69

CH3 COOC2 H5

6.02

−6.2

(CH3 )2 CO

20.70

−7.2

CH3 CN

37.5

−10.12

Furthermore, when a sample after grinding was kept in dry or wet ambient air, the moisture content of the air substantially influenced the EE glow curve. Al powder that was ground for 10 min in air was kept in dry or wet air for a longer time. In this case, the EE was measured with a counter gas of Ar–C2 H5 OH. Figure 4.3 [7] shows a plot of the total count (25–239 °C) versus exposure time under each ambient air

56

4 Effects of Organic Adsorption, Applied Voltage, Light …

Fig. 4.3 Plots of total count of ground Al powder versus time of exposure to air (22 °C): (◯) 0% RH, (Δ) 50%RH, (⬜) 100% RH [7]. Reprinted with the permission from American Chemical Society, Momose et al. [7]. Copyright© 1976

condition on a semi-logarithmic scale. The data points are on a straight line. The emission increased with decreasing humidity and then decayed with exposure time, with a steeper slope for dry air compared with wet air.

4.2 Effect of AV and Light Intensity on OSEE from Al Little is known about the effect of the electronic properties of the surface oxide film on OSEE. We have considered that one can expect the changes in the surface concentration of electrons (by negative electrical potential under AV and by stimulating light intensity I p ) to influence the interaction of C2 H5 OH in the counter gas with an oxide film on the Al metal surface. The OSEE experiment was performed as follows: (1) OSEE versus AV relationship after friction and subsequent exposure to the environment (vacuum, H2 O vapor, and O2 ), as well as the dark emission that occurs only during friction [9] versus AV; (2) OSEE versus AV after heat treatment during exposure to O2 ; and (3) OSEE versus I p at a value of the AV set at the start of the measurement, as well as its relationship to the AV [8]. Commercial rolled Al sheets (purity, >99.5%; thickness, 0.3 mm) were used. The samples were degreased with benzene solvent and then annealed in vacuo for 1 h at 300 °C before use. The OSEE measuring apparatus was basically a modified Geiger counter (counter gas: mixture of Ar, 84 Torr; and C2 H5 OH vapor, 20 Torr) with a friction device and a small electric bulb as the light source (Fig. 3.14). A heater was attached to the sample holder of the counter. To apply a variable negative electric potential to the sample holder relative to the earthed grid of the counter, a handmade dc-regulated supply was set up as an electric source of the AV. The experimental procedure was as follows. A flat metal surface was rubbed at 25 °C for 5 min during evacuation with a rotator, which consisted of an iron bar that was partly wound around with two narrow sheets of the same material. This iron bar was rotated with an external magnetic stirrer at a speed of 200 rpm. After cessation of friction, the sample was exposed to the environment [vacuum (0.005 Torr), H2 O vapor (15 Torr), and O2 (100 Torr)] for varying times at a certain temperature (usually 25 °C). After exposure to the environment, the counter gas was admitted into the counter and then the OSEE measurements were started at the same time as the bulb

4.2 Effect of AV and Light Intensity on OSEE from Al

57

was switched on. Regarding the OSEE versus AV measurements, under irradiation with light from a bulb at 2.07 W (5.9 V × 350 mA; wavelength, >295 nm), the AV was increased from 0 to 150 V at the rate of 5 V/min, and then immediately decreased to the original value at the same rate. With the measurement of OSEE versus I p , the electric power (W) that was expended in the bulb was changed—by increasing and subsequently decreasing (in small increments) the current that flowed through the bulb at a chosen value of the AV. Here, it was confirmed with a photocathode that the photo-current that was caused by the light at wavelengths of 350.5, 447.5, and 551 nm from the bulb increased in similar proportion to the increasing power expended by the bulb. Therefore, one can relate the expended power to an increase in the number of photons emitted. Figure 4.4 shows the relationship between OSEE from Al metal surfaces after friction and subsequent exposure to gaseous environments (vacuum, H2 O vapor, and O2 ) and the AV [8]. Each curve in Fig. 4.4 is a typical example, and shows the median value of AVmax of three or more measurements under the same conditions. In all cases, when the AV was increased, the OSEE rapidly increased and then passed through a maximum (this emission intensity is termed EImax, AV ) at a particular AV (AVmax ), followed by a rather slow decrease. Subsequently, when the AV was reduced, a maximum emission with a larger intensity was also evident at a higher AVmax ; but at less than this value, the emission sharply decreased and was no longer evident. Apparently, the OSEE versus AV relationship exhibited a hysteresis effect; the values of AVmax and EImax, AV strongly depended on the environment and exposure time. Figure 4.5 shows plots of all the data regarding AVmax against the time of exposure to each environment [8]. The following features are evident in Figs. 4.4 and 4.5: (1) In each environment, the longer the exposure time, the higher the value of AVmax . (2) The values of AVmax that were obtained during increasing AV are much lower than those during the subsequent decrease in AV, although for a broader maximum emission at a longer exposure time it is difficult to assign the position of AVmax . (3) Especially regarding O2 exposure, the maximum emission more rapidly shifted to a higher AVmax . (4) The value of EImax, AV was highest at a particular exposure time, i.e., 10 h for vacuum, 5 h for water vapor, and 1 h for oxygen. (5) The emission intensity for the same exposure time decreased in the order H2 O > vacuum > O2 at a lower AV, but O2 > H2 O > vacuum at a higher AV. Furthermore, the dark emission versus AV curve was measured. When the AV was increased at the same time as the friction was started, the emission gradually increased without any peak; stabilized at an intensity of 3.5 × 105 counts/min at 150 V; and subsequently—when the AV was decreased—the emission gradually decreased, with a sharp drop near AV = 50 V. Furthermore, the effect of heat treatment during exposure to O2 on OSEE was examined. After friction, O2 was introduced into the counter at a pressure of 100 Torr at 25 °C, and then heat-treated at various temperatures (25, 30, 40, 50, 60, and 100 °C) for 5 h. With increasing temperature, the OSEE became weaker with a faster decay on the lower AV side, and tended to output no peaks. Thus, the suppression of OSEE resulted from formation of a thicker oxide film. Figure 4.6 shows a typical example of an OSEE versus I P curve for the sample immediately after friction [8]. The measurement was performed in the range of

58

4 Effects of Organic Adsorption, Applied Voltage, Light …

Fig. 4.4 The relationship between OSEE from Al metal surfaces after friction and subsequent exposure to gaseous environments and AV: a vacuum (0.005 Torr); b H2 O vapor (15 Torr); c O2 (100 Torr). The numerical value in the right-hand represents the exposure time. Arrows (→ and ←) on the curves indicate that AV is increased and subsequently decreased, respectively [8]. Reprinted with the permission from American Chemical Society, Momose et al. [8]. Copyright© 1980

Fig. 4.5 Plots of AVmax giving a maximum emission versus time of exposure to the environments: a vacuum; b H2 O vapor; c O2 . The symbols (◯ and Δ) refer to the values of AVmax obtained during the increase in AV and during the subsequent decrease in AV, respectively [8]. Reprinted with the permission from American Chemical Society, Momose et al. [8]. Copyright© 1980

4.2 Effect of AV and Light Intensity on OSEE from Al

59

the electric power spent from 0.43 to 2.96 W at AV = 20 V. The behavior of the OSEE versus I P is similar to that versus AV; that is, a maximum emission (EImax, Ip ) was evident at a particular light intensity (I p, max ) and there was a hysteresis effect. Figure 4.7 [8] shows the OSEE versus I P curves during increasing I P , obtained at various values of the AV. Initially, when the OSEE was measured at 2.07 W, the AV increased; then the electric power changed from 0.28 to 8.57 W. Regarding the samples immediately after friction, with increasing AV (4 V → 5.6 V → 20 V), a sharper maximum emission with a larger intensity was evident at a lower value of I p, max . This finding clearly indicates that the values of I p, max during the increase in I p at different values of AV (set at the start of the measurement) exhibited a reciprocal relationship between the magnitude of AV and I p . Fig. 4.6 Effect of stimulating-light intensity (I p ) on OSEE immediately after friction: (◯) during in the increase in I p ; (●) during the subsequent decrease in I p . The value of AV was 20 V [8]. Reprinted with the permission American Chemical Society, Momose et al. [8]. Copyright© 1980

Fig. 4.7 OSEE versus stimulating-light intensity at different values of AV: a–c immediately after friction; d after friction and subsequent 50-h exposure to O2 (100 Torr, 25 °C). The value of AV = 5.6 V was AVmax in this experiment [8]. Reprinted with the permission from American Chemical Society, Momose et al. [8]. Copyright© 1980

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4 Effects of Organic Adsorption, Applied Voltage, Light …

The fact that there was a maximum emission in the OSEE curves as a function of AV or I p is attributable to two interaction modes of the adsorbed C2 H5 OH molecule (used in the counter gas) with the oxide layer adsorbed onto the metal surface. The amphoteric property of the surface hydroxyl group (–AlOH) is hypothesized to facilitate the maximum emission. The amphoteric property depends on the electron density of the oxygen atom of −AlOH. When polar organic compounds interact with oxide surfaces, either the surface hydroxyl group (–AlOH) or the organic functional group (–OH) of the ROH (R = C2 H5 ) can act as an acid (proton donor or electron pair acceptor), or as a base (proton acceptor or electron pair donor). Figure 4.8 [8] shows the interaction modes of the C2 H5 OH molecule with the hydroxylated oxide layer. Upon applying no external electric field, the surface hydroxyl group interacts with the C2 H5 OH molecule in accordance with the mode shown in Fig. 4.8a; i.e., the surface hydroxyl group preferentially acts as a proton donor to the −OH group of the ROH, forming a dipole such as ROH2 + −O− Al− by hydrogen bonding [7]. Furthermore, by applying a negative electric potential to the sample relative to the earthed grid of the counter, one forms the dipole layer, which causes a decrease of the apparent work function of the surface (the Schottky effect, [2]). At the same time, the emitted electrons are directed toward the grid and efficiently collected by the anode. Furthermore, electrons are transferred from the metal substrate to the thin oxide film. In the initial stage of applying the electric field, the surface layer forms a positively charged adsorbed layer of ROH2 + (because of the presence of the adsorbates binding to the polarized hydrogen atom) and a negatively charged space–charge region near the surface of O− Al− (because of the negatively polarized oxygen atom of the surface hydroxyl groups). This interaction mode contributes to the increase in the OSEE. A maximum emission in the OSEE versus AV curves occurs as a result of the alteration of the dipole orientation from the mode of the ROH2 + −O− Al-, leading to a decrease of the surface potential barrier by the Schottky effect relative to that of RO− − H2 + OAl-; thus increasing the potential barrier, formed by accumulation of negative charge on the surface hydroxyl group of the oxide film in the latter stage of applying the electric field (Fig. 4.8b). In this context, [1] reported that the attractive forces (attributable to the surface hydroxyl groups) strongly depend on the electron density of the oxygen atom in the group MOH (M: metal atom). If the electron density is low, the ability of the oxygen atom to accept a pair of electrons or donate a proton increases, and the surface hydroxyl group interacts as an acid (Fig. 4.8a). However, a high electron density on the surface oxygen atom—which causes the oxygen atom to act as a base—increases its tendency to bind to a proton; thus, the more electronegative oxygen atom is hydrogen-bonded to the hydrogen atom of the −OH group of the organic molecule (Fig. 4.8b). Because the oxide surface is more negatively charged with increasing external electric field, the surface hydroxyl group tends to act as a proton acceptor. Therefore, the C2 H5 OH molecule becomes a proton donor; thus, a dipole layer that is opposite in orientation preferentially results (Fig. 4.8b). Next, we describe a change in the OSEE versus I p curves (Figs. 4.6 and 4.7). This phenomenon is attributable to an internal photoeffect; i.e., with an increasingly negative charge of the surface layer, the larger the value of I p . Thus, one can explain

4.2 Effect of AV and Light Intensity on OSEE from Al

61

Fig. 4.8 Interaction modes of C2 H5 OH molecules adsorbed on the hydroxylated oxide layer built up on the metal surface during friction and subsequent exposure: a without any external electric fields or with a lower AV the C2 H5 OH molecule is oriented with the oxygen atom toward the hydrogen atom of the hydroxyl group: b with a higher AV the C2 H5 OH molecule is oriented with the hydrogen atom toward the oxygen atom of the hydroxyl group. The dashed lines indicate hydrogen bonds. The vector (+ →– ) represents a normal dipole moment due to the hydrogen bond [8]. Reprinted with the permission from American Chemical Society, Momose et al. [8]. Copyright© 1980

the OSEE versus I p behavior on the basis of the difference in the adsorption modes of C2 H5 OH on the surface hydroxyl group (−AlOH) (Fig. 4.8a, b) in the same manner as the OSEE versus AV relationship. Figure 4.7 indicates that as the AV increases, the I p, max decreases. This result indicates that accumulation of electrons at the surface corresponds to the synergistic action of AV and I p ; a certain number of electrons are necessary at the surface to reverse the orientation of the dipole layer because of the adsorbed C2 H5 OH molecule. The EImax versus AV relationship indicates the ability of organic molecules to interact with the oxide layer on the Al metal surface. Table 4.3 [5] shows the intensity of OSEE as a function of the AV for Al metal surfaces after mechanical rubbing under vacuum and ambient air, measured by using the counter gases that contain various organic vapors: CH3 OH, C2 H5 OH, n-C3 H7 OH, (CH3 )2 CO, and CH3 CN [5]. The AVmax is the median value from two to seven measurements. Counter gases that contained alcohol vapor corresponded to a maximum emission during the increase and subsequent decrease in the AV, but no peak was evident with counter gases that contained (CH3 )2 CO (acetone) and CH3 CN (acetonitrile). Regarding the alcohol vapor, an emission peak was also evident in the EI versus Ip curves. One can explain the peak in terms of the interaction modes between the alcohols (ROH) and surface hydroxyl group (−AlOH) that are bound to the thin Al oxide layer on the metal surface, as described in [8]. Regarding (CH3 )2 CO and CH3 CN, the change of the dipole orientation is more difficult compared with that for alcohols. The large values of EImax for the alcohols correspond to the fact that the acidity constants of the conjugate acids (ROH2 + ) of the alcohols (pKROH2+ = −2) are much larger than those for RCNH+ (−10) and R2 COH+ (−7). Here, R represents an alkyl group. In Table 4.3, the value of AVmax for the alcohols increases in the order n-C3 H7 OH ≤ C2 H5 OH < CH3 OH.

62

4 Effects of Organic Adsorption, Applied Voltage, Light …

Table 4.3 Characteristics of OSEE intensity (EI) versus applied voltage (AV) curves for rubbed Al metal surfaces Samplea

Organic vapor in counter gasb

EImax (105 cpm)c

AVmax c

EI at AV = 150 V (105 cpm)

I

CH3 OH CH3 OH

4.50 4.75

34 40

0.75

C2 H5 OH C2 H5 OH

2.30 2.55

8 19

1.00

n-C3 H7 OH n-C3 H7 OH

3.25 3.65

7 8

1.35

(CH3 )2 CO

No peak

0.55

CH3 CN

No peak

0.76

CH3 OH CH3 OH

3.45 4.95

0.90

C2 H5 OH C2 H5 OH

2.35 3.70

0.65

CH3 CN

No peak

0.15

II

a

I: rubbed under vacuum for 5 min; II: rubbed in ambient air (16 °C, 50% relative humidity) for 5 min b The composition of the counter gases was the same as in [7]: a mixture of Ar (84 Torr) and organic vapor (20 Torr). c The upper and lower values for the alcohols indicate the EI max obtained during the increase and subsequent decrease in AV, respectively

4.3 Relation Between EE and Catalytic Activity of Ag, Cu, and Pt Regarding catalysis or other chemical reactions on metal surfaces, there is a close relationship between the reaction rate and the EE rate, which indicates that the two phenomena occur simultaneously. Sato and Seo [11] measured EE with a Geiger counter. In the case of a counter gas of Ar + ethylene (C2 H4 ), an Ag2 O/Ag catalyst at 250 °C gives no EE, but in the case of a counter gas of Ar + C2 H4 + O2 one can observe EE continuously; whereas the chemical reaction proceeds in a steady state. The EE rate on Ag2 O/Ag catalyst was well-correlated to the rate of ethylene oxide (C2 H4 O) formation. Furthermore, a similar experimental result for Ag wires was obtained [12]. When in the latter counter gas, materials such as metallic Cu, NiO, and Fe2 O3 were used instead of Ag catalyst; there was no EE and C2 H4 underwent complete oxidation, producing CO2 and H2 O. The oxidation reaction on Ag surfaces is as follows: for incomplete oxidation, 2C2 H4 + O2 → 2 C2 H4 O; and for complete oxidation, C2 H4 + 3O2 → 2CO2 + 2H2 O. Momose and Tamai [6] reported a close relationship between the EE and chemical reduction of oxide-covered Cu metal. The number of electrons that were emitted by TSEE from the surfaces increased with decreasing mass of the oxide layer, caused by use of C2 H5 OH vapor as the quenching gas of a Geiger counter. These two chemical processes are associated

4.3 Relation Between EE and Catalytic Activity of Ag, Cu, and Pt

63

with the EE, concomitant with the oxidation of the organic molecules (C2 H4 and C2 H5 OH) on the oxide-covered metal surfaces. As will be discussed in Sect. 11.2.3, metals such as Ag, Cu, and Pt (considered here) are metals with weak or specific adsorption ability for various gases, including O2 . Hoenig and Tamjidi [3] have substantial interest in whether the EE current from Pt catalysts suppresses or enhances the reaction rate, and they tested this hypothesis by applying an external electric field to the catalyst. They observed EE during catalytic oxidation of CO, H2 , or NH3 on hot Pt. The catalyst was in the form of three wires, each 0.25 mm in diameter, twisted into a rope-like configuration. All of the oxidation experiments were run at a total pressure of 6 × 10−6 Torr. The partial pressures of CO, H2 , or NH3 were held at ca. 1 × 10−6 Torr. The catalyst wires were biased at any value from 0 (ground) to ±900 V with a power supply. The EE currents were collected with a stainless-steel collector that was biased at +24 V, and measured with a picoammeter. The product was measured with a mass spectrometer. For each gas, the Pt filament was slowly heated from 20 to 800 °C. The EE current from Pt at 800 °C for various gases (only at 2 × 10−6 Torr) decreased in the order: H2 > vacuum (8.1 × 10−8 Torr) ≫ NH3 > CO ≫ O2 . Figure 4.9 shows the rate of catalysis [CO + (1/2) O2 → CO2 ] and the EE current on a Pt filament catalyst for a system of CO and O2 . Here, the height of the CO2 peak (K, reaction rate constant, in arbitrary units) and the EE current (I e , in amperes) are represented as a function of time. The rate of catalysis was small at room temperature, but increased rapidly upon heating the catalyst at an operating temperature of 775 °C. The EE current followed the rate of the catalysis and was somewhat erratic until K was constant, at which time I e decreased to a slightly lower level. In Fig. 4.9, whenever the catalyst cools quickly to 20 °C from the operating temperature, one observes a surge. Other experiments on oxidation of H2 and NH3 yielded qualitatively similar results.

Fig. 4.9 Height of CO2 peak (reaction rate, K) and EE current (I e ) versus time for oxidation of CO on Pt catalyst [3]. [3] Reproduced with permission from Academic Press, Hoenig and Tamjidi. Copyright© 1973

64

4 Effects of Organic Adsorption, Applied Voltage, Light …

Fig. 4.10 Effect of various negative voltages of Pt filament on catalytic oxidation of CO [3]. Reproduced with the permission from Academic Press, Hoenig and Tamjidi [3]. Copyright© 1973

Figure 4.10 shows the effects of external electric fields on the catalysis and EE for oxidation of CO. Here, the bias was changed each time the filament was cooled to 20 °C; then the catalyst was heated to 770 °C and held at that temperature until K was effectively constant. The higher negative voltages yielded higher K values. Furthermore, surprisingly, the K value increased upon heating and decreased upon cooling. At high voltages (−83 V) the increase was rapid and the decrease was preceded by a steep pulse. At −30 V, the increase occurred more slowly and the decay became longer. The surge might be because of relaxation of the active surface state during catalysis. Thus, the EE might be used as a measure of the rate of certain catalytic reactions. One can follow the induction phase and the approach to steady state in catalysis in terms of the EE, as well as partially modulate further catalysis by the external electric potential.

References 1. J.C. Bolger, A.S. Michaels, Molecular Structure and Electrostatic Interactions at Polymer−Solid Interfaces, in Interface Conversion for Polymer Coatings. ed. by P. Weiss, G.D. Cheever (Elsevier, New York, 1968), pp.3–60 2. A.J. Dekker, Solid State Physics (Prentice-Hall, Tokyo, 1957), p.225 3. S.A. Hoenig, F. Tamjidi, Exo-electron emission during heterogeneous catalysis (the effect of external electric potentials). J. Catal. 28, 200–208 (1973) 4. E. Linke, K. Meyer, Über den zusammenhang zwischen triboinduzierter elektronenemission und adsorptionsprozessen IM system Al–O2 –H2 O. Surf. Sci. 20, 304–312 (1970) 5. Y. Momose, M. Ohkubo, J. Gunji, Interaction of mechanically rubbed aluminum with polar organic vapors found by measurements of optically stimulated exoelectron emission under the influence of applied electric potential. Phys. Stat. Sol. 77, K45–K48 (1983)

References

65

6. Y. Momose, Y. Tamai, Relationship between exo-electron phenomena and reduction of copper oxide by ethanol vapour. J. Inst. Metals. 98, 110–113 (1970) 7. Y. Momose, Y. Iguchi, S. Ishii, K. Komatsuzaki, Exoelectron emission from ground aluminum powder and its relation to the adsorption of oxygen, water, and some organic compounds. J. Phys. Chem. 80, 1329–1335 (1976) 8. Y. Momose, T. Ishii, T. Namekawa, Exoelectron emission from aluminum under the influence of applied electric potential and stimulating light, and its dependence on surface-polar organic interaction. J. Phys. Chem. 84, 2906–2913 (1980) 9. Y. Momose, T. Namekawa, Exoelectron emission from metals subjected to friction and wear, and its relationship to the adsorption of oxygen, water vapor, and some other gases. J. Phys. Chem. 82, 1509–1515 (1978) 10. J.A. Ramsey, The emission of electrons from aluminium abraded in atmospheres of air, oxygen, nitrogen and water vapour. Surf. Sci. 8, 313–322 (1967) 11. N. Sato, M. Seo, Chemically stimulated exo-electron emission from silver catalyst during partial oxidation of ethylene. J. Catal. 24, 224–232 (1972) 12. N. Sato, M. Seo, Chemically stimulated exo-emission from a silver catalyst. Nature 216(5113), 361–362 (1967)

Part III

Outline of Development of EE Research

Chapter 5

Materials, EE Measurement, and EE Characteristics

Abstract We outline the development of EE research conducted in the author’s laboratory. The topics under discussion are the materials, methods of excitation and stimulation, EE measurement apparatus, and the relationship of the EE data to the interactions with the environment (such as gases and organics). Chapters 6–12, based on published papers, describe these topics in detail.

5.1 EE Measurement Methods and EE Data Analysis Methods We outline the development of EE research using the following topics: materials, methods of excitation and stimulation, EE measurement apparatus, analytical equations to obtain EE characteristics, and application of EE data to interactions with the environment (such as adhesion). (1) Materials: metals (Fe, Cu, Al, Ni), semiconductor (silicon wafer), abrasive agents (Al2 O3 , SiO2 , SiC), polymers, and organics (2) Excitation methods (treatments before EE measurement): (a) Mechanical processing (rolling, abrasion, sandblasting, scratching, grinding, and cutting) (b) Adsorption and surface reaction with oxygen, water, and organics (c) Plasma treatments in oxygen and argon, which one usually uses to improve deposition and coating (d) Irradiation with UV light. (3) Methods to excite and stimulate material surfaces, which one usually performs at the same time as the EE measurements: (a) increase or decrease of temperature, (b) light irradiation (photon energies less or greater than the work function of the materials), (c) gas adsorption (Geiger counter gases that contain an organic vapor as a quenching gas), (d) applying an AV or electric field between the sample and the earthed grid of the counter, and (e) tribological friction on a metal surface with a polymer rotator. We categorize the latter three methods as the stimulating method. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 Y. Momose, Exoemission from Processed Solid Surfaces and Gas Adsorption, Springer Series in Surface Sciences 73, https://doi.org/10.1007/978-981-19-6948-5_5

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5 Materials, EE Measurement, and EE Characteristics

(4) We have made and used several modified Geiger counters that one can connect to a vacuum line. The counter gases are a batch type of a mixture that consists of Ar and organic vapor as a quenching gas and a flow type of Q gas [a mixture of 99% He (purity, 99.999%) and 1% iso-C4 H10 (iso-butane)]. Furthermore, we equipped the counters with a heater, light source, and friction device. We applied a negative potential of AV = −95 V (relative to the earthed grid of the counter) to the sample with a battery. Section 4.2 describes the effect of the AV on the EE. Regarding the counter gases, unless otherwise stated, we used a counter gas that consisted of a mixing gas of Ar [11,200 N/m2 (84 Torr)] and C2 H5 OH vapor [2700 N/m2 (20 Torr)]. (5) We used analytical equations as well as physical and chemical properties of materials to obtain the EE characteristics: (1) Analytical equations to quantitatively estimate the EE characteristics: (a) Arrhenius-type equation, including the activation energy and preexponential factor [13]. (b) Photoelectron emission (Fowler and DuBridge’s equation [12]). (c) Spicer’s three-step model [12]. (d) Analysis of TSEE glow curve (Sakurai’s equation [17]). (e) Thermal analysis of PE quantum yields as a function of photon energy [10]. (2) Schottky effect and electric field emission (tunnel effect [5]). (3) Relationship of the EE results to properties of adsorbed species and XPS results of solid surfaces. (4) Properties of adsorbed species: ionization energy, proton affinity, electron affinity [11], and acceptor number [11]. (5) Electronic properties of solid surfaces: photothreshold, work function [5], band bending, dipole layer, and surface field. (6) Metal-oxygen bond energies [5]. (6) Effect of EE intensity on the attractive force of the surface to charged polymers [15].

5.2 Stimulation by Thermal, Optical, and Tribological Methods After Excitation In our laboratory, we have used thermal and optical methods as important stimulation techniques in the EE measurements. We performed the TSEE measurements thermally in the dark and the OSEE measurements optically at a fixed wavelength of illuminating light at a given temperature. However, to examine the electronic properties of surfaces in more detail, we performed the OSEE measurements in combination with TSEE measurements. Recently, Momose et al. [10] provided a review of the EE of scratched Fe surfaces (obtained by using combined TSEE-OSEE measurements, which we hereafter represent by TPPE or TAPE).

5.2 Stimulation by Thermal, Optical, and Tribological Methods …

71

The EE apparatus consisted of a Geiger counter with a counter gas of Q gas, which we equipped with a heating system and connected to an optical system. We set the optical instruments (such as a light source and a grating monochromator of excellent performance) in the light irradiation system, such that one can precisely determine the wavelength and power of the irradiation light. We set a small deuterium lamp (power: nanowatt order) as the light source. One can scan the wavelength of light in the range of 200–350 nm; the size of the sample area exposed to the incident light was 0.5 mm × 3 mm. Furthermore, one can change the temperature of the sample in the range of 25–350 °C. We next describe the nomenclature of TPPE and TAPE. Regarding photoelectron emission or photoemission (PE), when one irradiates metals or certain other materials with ultraviolet radiation of the proper wavelength, the surfaces emit electrons [2]. Furthermore, Nienhaus [18] and Wodtke et al. [20] published excellent reviews on charge transfer in chemical reactions at metal surfaces. Therefore, the behavior of electrons at surfaces has been an active area of research. The PE facilitates a powerful method for investigating electronic properties of solid surfaces. Fowler [4] theoretically connected the effect of the temperature on PE from metals to the underlying statics, and DuBridge [3] developed a useful method for determining the photoelectric threshold (photoelectric work function). Spicer [19] introduced a three-step model that provides a general understanding of the theory of PE. This model divides the emission into three successive steps: (1) optical excitation, (2) transport of the excited electrons to the surface, and (3) escape of the excited electrons into vacuum. To measure the photoelectric current and photoelectric threshold carefully, cleaned metal surfaces have been used. However, the OSEE technique has been useful as the method for observing practical metal surfaces [1]. In general, the sensitivity of OSEE has focused on the magnitude of the photothreshold. In surface analysis methods by using electron emission, the development of nonvacuum-based methods for monitoring surfaces under industrial conditions in the field (such as tribology, adhesion, and fatigue, particularly at elevated temperatures) has been an active area of research. Detailed understanding of electron emission from surfaces that have been subjected to mechanical treatment—such as abrasion and friction, chemical action, or irradiation—has remained inaccessible because of the difficulty of properly controlling surface conditions during experiments [18]. However, applying OSEE measurements with a gas-flow Geiger counter to practical surfaces can provide useful information, such as the ability to emit electrons— which is related to the photothreshold and the number of electrons emitted during scanning the wavelength, although one cannot measure the energy distribution of the emitted electrons [6]. We have previously examined the temperature dependence of the number of emitted photoelectrons N T and the photothreshold φ of practical metal surfaces by conducting the OSEE measurements at various elevated temperatures. This method is temperature-programmed photoelectron emission (TPPE) [16]. TPPE provides interesting information regarding the temperature dependence of N T and φ, obtained by scanning the wavelength of the incident light at several temperatures, maintained during the process of the increase and subsequent decrease in the temperature (in the range 25–350 °C). A plot of N T against temperature substantially

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depends on the metal [6]. Furthermore, application of TPPE to many practical metal surfaces indicates that a plot of N T against φ (obtained at various temperatures) also produced strikingly divergent behavior between metals: For Pt, Cu, Ag, Au, Ni, Zn, and Sn, N T tends to decrease with increasing φ; whereas for Al, Pb, Ta, Pd, W, Ti, Mo, Nb, Co, and Fe, N T changes substantially despite little change in φ [16]—this result remains unexplained. Furthermore, we have reported the temperature dependence of the photoelectric emission from silicon wafers covered with a native oxide film by using a refined method, termed thermo- and photostimulated electron emission. Section 9.1 described this method [14]. We refer to the nomenclature of thermo- and photostimulated electron emission in this paper as thermally assisted photoelectron emission or photoemission (TAPE). A weakness of photoemission spectroscopy is its inability to work at pressures of 133 Pa or greater [19]. Development of simple, useful experimental techniques for monitoring practical surfaces that are based on a nonvacuum method remains an unmet need. One performs TAPE measurements in situ in a gas-flow counter at normal atmospheric pressure, by using a counter gas that consists of 99% He and 1% iso-C4 H10 vapor; but in our opinion, TAPE can sufficiently distinguish the differences in the electron emission between metals that one subjects to various surface treatments [10]. We introduce another type of EE, termed triboelectron emission or tribological electron emission (TriboEE). We have reported the usefulness of a gas-flow counter for measuring the electron emission from many types of rolled metal sheets, which occurs when one brings the surfaces into sliding contact against a polymer rotator of polytetrafluoroethylene and polyimide [5, 7–9]. Chapter 11 discusses this electron emission.

5.3 Nomenclature of EE Categorized by Stimulation Methods After the excitation, one performs EE measurements under various stimuli: elevated temperature, light irradiation, thermally assisted light irradiation, light irradiation at elevated temperatures, and a tribological process during friction with polymers. Table 5.1 classifies the EE measurement techniques used in our experiments, in terms of materials, excitations, and stimuli.

References

73

Table 5.1 Classification of EE measurement techniques used for processed solid surfaces Materials

Excitation methods

Acronyms of EE classified by stimulation methods

Clean metals

Adsorption (Chap. 2)

TSEEa , PSEEb

Metals

Adsorption (Chap. 3)

TSEE, PSEE

Metals

Adsorption and catalysis (Chap. 4)

PSEE, effectc of AV and I p

Metals, Si

Plasma, gas discharge (Chap. 6)

TSEE

Metals

Adsorption, chemical reactions (Chap. 6) TSEE

Abrasives

Abrasion, grinding (Chap. 7)

TSEE

Polymers

Abrasion (Chap. 7)

EE

Fe

Rolled real Fe (Chap. 8)

TAPEd

Fe

Mechanically scratched Fe (Chap. 8)

TAPE

Si

Ion implanted Si (Chap. 9)

TAPE

Metals

Adsorption (Chap. 10)

TPPEe

Metals

Friction with polymers (Chap. 11)

TriboEEf

Metals

Effect of EE on adhesive strength and chemical reaction (Chap. 12)

TPPE, TSEE

a

Thermally stimulated electron emission (TSEE); b Optically stimulated electron emission (OSEE) or photostimulated electron emission (PSEE); c Effect of applied voltage (AV) and light intensity (I p ); d Thermally assisted photoelectron emission or thermally assisted photoemission (TAPE); e Thermally programmed photoelectron emission (TPPE); f Triboelectron emission or tribological electron emission (TriboEE)

References 1. W.J. Baxter, Exoelectron Emission from Metals, in Research Techniques in Nondestructive Testing, vol. 3, ed. by R.S. Sharpe (Academic Press, London, 1977), pp.395–428 2. M. Cardona, L. Ley, Introduction, in Photoemission in Solids I. ed. by M. Cardona, L. Ley (Springer-Verlag, Berlin, 1978), pp.1–104 3. L.A. DuBridge, A further experimental test of Fowler’s theory of photoelectric emission. Phys. Rev. 39, 108–118 (1932) 4. R.H. Fowler, The analysis of photoelectric sensitivity curves for clean metals at various temperatures. Phys. Rev. 38, 45–56 (1931) 5. Y. Momose, Electron transfer through a natural oxide layer on real metal surfaces occurring during sliding with polytetrafluoroethylene: Dependence on heat of formation of metal oxides. Coatings 11, 109 (2021). https://doi.org/10.3390/coatings11010109 6. Y. Momose, M. Honma, T. Kamosawa, Temperature-programmed photoelectron emission technique for metal surface analysis. Surf. Interface Anal. 30, 364–367 (2000) 7. Y. Momose, K. Kubo, Observation of triboelectron emission from real copper surfaces in sliding contact with polytetrafluoroethylene and polyimide. Tribol. Int. 47, 212–220 (2012) 8. Y. Momose, Y. Yamashita, Triboelectron emission from metal surfaces in sliding contact with polytetrafluoroethylene: Relevance to work function and surface potential. Tribol. Int. 48, 232–236 (2012) 9. Y. Momose, Y. Yamashita, M. Honma, Observation of real metal surfaces by tribostimulated electron emission and its relationship to the analyses by XPS and photoemission. Tribol. Lett. 29, 75–84 (2008)

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10. Y. Momose, T. Sakurai, K. Nakayama, Thermal analysis of photoelectron emission (PE) and X-ray photoelectron spectroscopy (XPS) data for iron surfaces scratched in air, water, and liquid organics. Appl. Sci. 10(6), 2111 (2020). https://doi.org/10.3390/app10062111 11. Y. Momose, D. Suzuki, K. Tsuruya, T. Sakurai, K. Nakayama, Transfer of electrons on scratched iron surfaces: Photoelectron emission and X-ray photoelectron spectroscopy studies. Friction 6(1), 98–115 (2018) 12. Y. Momose, D. Suzuki, T. Sakurai, K. Nakayama, Photoemission from real iron surfaces and its relationship to light penetration of the overlayer. Appl. Phys. A 118, 637–647 (2015) 13. Y. Momose, D. Suzuki, T. Sakurai, K. Nakayama, Influence of temperature and photon energy on quantum yield of photoemission from real iron surface. Appl. Phys. A 117, 1525–1534 (2014) 14. Y. Momose, A. Satou, T. Sakurai, K. Nakayama, Characteristics of thermo- and photostimulated electron emission from silicon wafers. Surf. Interface Anal. 40, 620–622 (2008) 15. Y. Momose, M. Umeki, D. Suzuki, K. Nakayama, Surface electronic states and electrostatic attractive forces between metals or semiconductor and tribocharged polymers. MRS Symp. Proc. 872, J21.1.1–J21.1.6 (2005) 16. Y. Momose, T. Kamosawa, M. Homma, M. Takeuchi, Surface analysis of real metals by temperature programmed photoelectron emission technique relationship between TPPE characteristics and surface pretreatment methods. Surf. Finish. Soc. Jpn. 53, 675–682 (2002) 17. Y. Momose, T. Yamamoto, M. Takeuchi, T. Sakurai, Thermally stimulated exoelectron emission from silicon subjected to argon plasma treatment. J. Appl. Phys. 73(11), 7482–7486 (1993) 18. H. Nienhaus, Electronic excitations by chemical reactions on metal surfaces. Surf. Sci. Rep. 45, 1–78 (2002) 19. W.E. Spicer, Surface Analysis by Means of Photoemission and Other Photon-Stimulated Processes, in Chemistry and Physics of Solid Surfaces. ed. by R. Vanselow, S.Y. Tong (CRC Press, Cleveland, 1977), pp.235–254 20. A.M. Wodtke, D. Matsiev, D.J. Auerbach, Energy transfer and chemical dynamics at solid surfaces: The special role of charge transfer. Prog. Surf. Sci. 83, 167–214 (2008)

Chapter 6

TSEE Related to Plasma Treatment and Adsorption

Abstract Plasma treatment can improve the surface properties—such as wettability and adhesion—of various materials, but little is known about the electronic properties of material surfaces. We describe the relationship between the plasma treatment and TSEE glow curve of metals (Fe, Ni, Cu, and Au) and materials (glass/Au, silicon wafer powder, and graphite) after exposure to plasma. We indicate that the oxygen that is incorporated onto the surface layer is pertinent to TSEE.

6.1 Outline of TSEE of Metal Surfaces After Plasma Treatment The plasma treatment described here is low-temperature plasma. This treatment, also termed glow discharge, uses a gas discharge under a vacuum of ca. 1 Torr. The plasma contains ions (positive and negative), electrons, metastable species, atoms, free radicals, and electromagnetic radiation (UV radiations) and is generated in a highly chemically active state. Researchers commonly modify the surface of materials with low-temperature plasma to impart high functionality (e.g., adhesiveness) to the surfaces of polymer materials, by exposing the surfaces to plasma and thus changing their characteristics [1]. Subjecting polytetrafluoroethylene surfaces to Ar plasma causes a chemical reaction with atmospheric oxygen (production of peroxy radicals [26]). Furthermore, the surface oxygen of iron oxide desorbs because of argon ion irradiation, even in inorganic substances [10]. Heinicke [9] describes in detail the relationship between chemical reactions, charging and discharging, and electron emission that occur when one applies mechanical energy to a solid. Plasma treatment has an effect that is similar to mechanical treatment. One can perform plasma treatment in various gases—such as oxygen, rare gases, and fluoroalkanes—which leads to various effects. Therefore, plasma treatment imparts differences to EE behavior. To use the characteristics of EE to evaluate active sites, studying the EE phenomena of solid surfaces upon exposure to ionizing radiation and treatment with low-temperature plasma will be insightful. In metals, atomically, clean surfaces do not emit exoelectrons, even when irradiated with ionizing radiation, but the presence of an oxide film or adsorbed film on the © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 Y. Momose, Exoemission from Processed Solid Surfaces and Gas Adsorption, Springer Series in Surface Sciences 73, https://doi.org/10.1007/978-981-19-6948-5_6

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surface contributes substantially to EE [6]. Regarding electron-irradiated Ni [36], the EE intensity increases most in a 230–240 nm oxide film. Regarding X-ray-irradiated Cu [30], the EE intensity reaches a maximum at a ca. 1000 nm oxide film. Wild et al. [37] reported interesting experimental results from 32 samples (Fig. 6.1). They measured TSEE glow curves upon exposure of a 400 nm SiO2 layer on a Si surface to radiation such as electrons, corona discharge, UV light (60 min), visible laser light (2 min), X-rays (104 min), and gamma rays. The intensity of the TSEE and the peak position differed substantially, depending on various excitations by ionizing radiation (Fig. 6.1). These TSEE glow curves were no longer evident in the second heating. In other words, the TSEE glow curves were transient. With electron bombardment in the energy range of 1–6 keV, they obtained a well-known glow curve with a single peak at 150 °C. This is termed the 160 °C maximum. Discharge under high vacuum (10−5 Torr) acted in the same manner as electron bombardment, whereas discharge at a vacuum of 10−1 Torr corresponds to a completely different TSEE glow curve. UV and laser irradiation required 60 and 2 min to obtain the same TSEE intensity, respectively. With X-ray irradiation, there was TSEE after 104 min of irradiation. However, there was no TSEE after neutron bombardment, ion implantation, or 65-keV electron bombardment. Fig. 6.1 TSEE glow curves of SiO2 layers after different kinds of excitation [37]. Reprinted with permission from Wild et al. [37]. Copyright© 1979

6.1 Outline of TSEE of Metal Surfaces After Plasma Treatment

77

Glaefeke [6] reported that TSEE glow curves that correspond to electron bombardment with low-energy electrons (0.2–30 eV) were somewhat similar to curves that correspond to UV light, and moderate-energy electrons (several 102 –103 eV) generated TSEE glow curves that resembled curves obtained after X-ray irradiation. Especially, low-energy electrons excited only adsorbates or surface states and deposited in the upper surface layers—which negatively charged the solid. Furthermore, moderate-energy electrons excited the volume states. Fitting et al. [5] reported the relationship between the TSEE glow curves of SiO2 and the latter’s excitation electron energy (E 0 ) (Fig. 6.2). With decreasing E 0 , the two main peaks at 165 °C and 285 °C—and the complex peak between 500 and 600 °C—decreased, whereas the low-temperature peak at 110 °C increased steadily. Because slow primary electrons can only penetrate into solids up to depths of 99.6%; thickness: 0.1 mm; size: 20 mm × 20 mm were used. The metal surface was cleaned with ethanol or benzene to remove the rust-preventing oil on the surface. Immediately afterward, the surface was spark discharged in air (16–30 °C, 28–66% relative humidity) for typically 2 min with a Tesla coil under a current density of 20 mA. The glow curves were measured in the temperature range 25–282 °C at a heating rate of 19 °C/min. For each set of experiments, the glow curves of three to 12 samples were measured to check the reproducibility of the results. Furthermore, the effects of the discharge duration (0.5–5 min), type of discharge atmosphere (air, N2 , or Ar), thickness of the oxide film, and humidity of the ambient air on the TSEE were examined. Figure 6.3 [22] shows typical glow curves for various organic vapors upon discharge in air for 2 min. Several emission peaks were evident in the vicinity of 55, 95, 135, and 160 °C; these peaks are termed peaks I–IV. Peak IV at ca. 160 °C was always observed, at the most substantial intensity. Figure 6.4 [22] indicates that the TSEE intensity of peak IV strongly depended on the organic vapor. The organic vapors in descending order of the intensity of the peak IV were as follows: n-C3 H7 NH2 (npropyl amine) > (n-C3 H7 )2 NH (di-n-propyl amine) ≥ CH3 COOC2 H5 (ethyl acetate) > (CH3 )2 CO (acetone) ≥ C6 H6 (benzene) ≥ C2 H5 OH (ethanol) ≫ CH3 CN (acetonitrile) > (CH3 )2 CHOH (isopropyl alcohol) > n-C4 H9 Cl (n-butyl chloride). Regarding the influence of the organics, Table 6.1 [22] summarizes the median values of the glow-curve characteristics of Fe sheets upon exposure to spark discharge in air for 2 min. The emitting centers were present at the surface spots that were formed by

6.2 Effect of Discharge, Adsorption, and Heat Treatment …

79

the discharge. Kaelble [14] proposed that the dependence of the TSEE intensity on the organics strongly corresponds to the protons’ attractive interactions between the functional groups of the adsorbed organic compounds and the surface hydroxyl groups (–FeOH) on the damaged metal surface. Fig. 6.3 Typical exoelectron glow curve for the Fe exposed to the spark discharge: a n-C3 H7 NH2 , b C2 H5 OH, c CH3 CN, d (CH3 )2 CHOH [22]. Reprinted with permission from Momose and Okaxzaki [22]. Copyright© 1973

Fig. 6.4 Dependence of the emission intensity of peak IV on the organic compounds (● denotes the median value. The peak-temperature range is also quoted) [22]. Reprinted with permission from Momose and Okaxzaki [22]. Copyright© 1973

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6 TSEE Related to Plasma Treatment and Adsorption

Table 6.1 Influence of organic vapor on the glow-curve characteristics of Fe upon exposure to spark discharge a Kind of organic vapor

Intensity of peak I Intensity of peak (counts/sec) II (counts/sec) (temp., °C) (temp., °C)

Intensity of peak IV (counts/sec) (temp., °C)

Total counts (25 − 282 °C)

n-C3 H7 NH2

55(50) (3/8) b

110(97) (6/8)

330(165)

93,500

(n-C3 H7 )2 NH

89(55) (6/6)

170(101) (2/6)

280(172)

126,000

CH3 COOC2 H5

41(55) (6/7)

–

270(168)

88,200

(CH3 )2 CO

19(55) (7/12)

–

230(170)

55,000

C6 H6

14(55) (7/9)

89(95) (1/9)

215(170)

56,100

C2 H5 OH

28(53) (6/11)

44(94) (3/11)

190(168)

57,500

CH3 CN

20(57) (2/7)

–

73(169)

19,700

(CH3 )2 CHOH

15(57) ((2/4)

27(98) (2/4)

37(152)

12,800 (25–210 °C)

n-C4 H9 Cl

5(53) (2/5)

–

15(173)

4400 (25–226 °C)

a Discharge

in air for 2 min denotes (the number of experiments in which the peak appeared/the total number of the experiments)

b (/)

6.2.2 TSEE from Oxidized and Plasma-Treated Ni Surfaces We reveal the behavior of the TSEE that one observes from oxidized and plasmatreated Ni metal surfaces [19]. The motivation of this study was to clarify, in more detail, the relationship between the emission during the chemical reaction on oxidecovered metal surfaces and the corresponding emitting surface characteristics. The surfaces were characterized by TSEE, XPS, X-ray diffraction (XRD), and the surface potential (SP). A Geiger counter with an Ar-C2 H5 OH counting gas was used for TSEE. Three metal samples were prepared. First, Ni sheets (purity, >99.7%; thickness, 0.2 mm) were oxidized by heating in air at temperatures of 25 °C (room temperature), 200, 400, 600, and 800 °C for 60 min, and subsequently, these were subjected to plasma treatment under an atmosphere of Ar or O2 . The plasma treatment was performed with a 13.56-MHz radiofrequency generator in a glass chamber that was equipped with two capacitively coupled external electrodes. The conditions of the plasma treatment were as follows: gas pressure, 0.2 Torr; power, 40 W; and exposure time, 2, 5, and 10 min. After a sample was mounted onto the sample holder in the counter, the TSEE glow curve was measured from 25 to 325 °C at a heating rate of 20 °C/min. The TSEE was observed not only for the samples exposed to Ar or O2 plasma, but also for samples that were only oxidized. Figure 6.5 shows TSEE glow curves for samples that were only oxidized at 25–800 °C [19]. The TSEE glow curves exhibited an emission peak at ca. 190–260 °C. The intensity of the peak increased with increasing oxidation temperature, reached a maximum at 400 °C, and there was no emission

6.2 Effect of Discharge, Adsorption, and Heat Treatment …

81

peak at 800 °C. Figure 6.6 shows typical glow curves for samples that were oxidized at 600 °C and then exposed to the following: Ar plasma for 10 min or O2 plasma for 2 min [19]. The intensity of TSEE after the plasma treatment became much greater than that for the samples that were only oxidized, with noteworthy increases at 200 °C (several emission peaks in each region). Regarding Ar plasma, four emission peaks were evident: at ca. 60, 100, 150, and 270 °C. Regarding O2 plasma, five emission peaks were evident: at ca. 50, 90, 130, 250, and 290 °C. The total count of electrons that were emitted in the glow curve for the samples that were exposed to both plasmas exhibited a maximum at an oxidation temperature of 400–600 °C. Furthermore, the total counts for 2-min exposure to O2 plasma were much greater than the corresponding counts regarding Ar plasma. The XPS and XRD data are as follows: (1) In the Ni 2p and O 1s spectra for the sheets that were only oxidized, before the TSEE measurements, as the oxidation temperature increased, metallic nickel (Ni 2p 853 eV) decreased. Furthermore, NiO (Ni 2p ~ 855 eV and O 1s ~ 530 eV) as well as Ni2 O3 (Ni 2p 856 eV and O 1s 532 eV) were evident. The metal surface was nearly completely covered with NiO at >600 °C. The Ni2 O3 is termed a defective, oxygen-rich surface layer. The C 1s peak decreased with increasing oxidation temperature. (2) Regarding the sheets that were only oxidized, after the TSEE measurement, metallic Ni 2p was clearly evident along with O 1s, assigned to Ni2 O3 at the oxidation temperatures of 25 and 200 °C. However, at 400 and 600 °C, the Ni 2p and the O 1s peaks substantially decreased along with a marked increase of the C 1s peak. The latter finding suggests that the metal-oxide film underwent reduction by the C2 H5 OH vapor in the counter gas (carbon materials were deposited from the C2 H5 OH). Thus, the maximum emission in the TSEE at an oxidation temperature of 400–600 °C pertains to the electron emission that corresponds to the reduction of the oxide film or the deposition of carbon materials. The metal surface that was oxidized at 800 °C was remarkably stable to chemical reduction, based on the similarity of the spectra before and after the TSEE measurements. Fig. 6.5 TSEE glow curves for Ni sheets oxidized only at 25–800 °C [19]. Reprinted with permission from AIP Publishing, Momose et al. [19]. Copyright© 1996

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6 TSEE Related to Plasma Treatment and Adsorption

Fig. 6.6 Typical TSEE glow curves for Ni sheets oxidized in air at 600 °C and then exposed to Ar plasma for 10 min or O2 plasma for 2 min [19]. Reprinted with permission from AIP Publishing, Momose et al. [19]. Copyright© 1996

(3) Regarding samples exposed to Ar plasma after oxidation in air, in the Ni 2p and O 1s spectra before TSEE measurements, metallic nickel was clearly evident at 853 eV, and the Ni2 O3 component increased with decreasing NiO component. This result is attributable to the action of etching of the metal surface by Ar ions that were present in the plasma. The XPS surface composition indicates that the O content of the surface tended to increase, but the Ni content decreased, compared with the sheets that were only oxidized, which resulted in an increased ratio of O 1s/Ni 2p. The characteristics of the XPS spectra for O2 plasma were similar to the corresponding characteristics for Ar plasma. (4) The XRD data for the sheets that were only oxidized indicated that only Ni metal was evident; an oxide film was not detected up to 400 °C; and at 600 and 800 °C, NiO was evident. The thickness of the nickel oxide film (formed by heating in air at 400 °C for 60 min) was ca. ~50 nm. No effect of plasma exposure was observed. Next, we show the relationship between the SP and TSEE. The SP for the unoxidized sheets was +300–400 mV. After the oxidation, with increasing oxidation temperature, the SP changed from positive to negative, reached a minimum (ca. − 200 mV) at 400–600 °C and then slightly increased at 800 °C. The decrease of the SP was mainly because of the removal of a contaminant with a positive charge (containing carbon and oxygen, which then became negative) and the growth of NiO. Regarding oxidized Ni metal, the SP was −0.2 V at an oxide film thickness of ca. 100 nm [34]. After exposure to O2 plasma, the SP became more negative (minimum value: ca. −700 to −800 mV), regardless of the oxidation temperature. These results suggest that the metal surface adsorbed negatively charged oxygen because of the

6.2 Effect of Discharge, Adsorption, and Heat Treatment …

83

plasma exposure. The SP after TSEE measurements substantially changed from negative to positive, up to +200 to +500 mV, which resulted in a substantial difference in the SP before and after TSEE measurements. This finding is attributable to desorption of the negatively charged oxygen during the TSEE measurements and deposition of carbon materials on the surface. The SP for Ar plasma was less negative (minimum value: −600 mV) than that for O2 plasma. We consider the chemical reaction of an alcohol (ROH) on oxides, in accordance with [11]. ROH might undergo molecular adsorption through the oxygen lone pair or dissociate by deprotonation. On some surfaces, there is dehydronation; primary alcohols afford aldehydes. In this experiment, the reaction of an alcohol RCH2 OH to an aldehyde RCHO is essentially a dehydrogenation and can be written RCH2 OH + 2O2− → RCHO + 2OH− + 2e− .

(6.1)

However, the reaction is often accompanied by loss of water, with lattice oxygen involved: RCH2 OH + O2− → RCHO + H2 O + 2e− .

(6.2)

In these reactions, there is a release of electrons. In accordance with [16], the forms of adsorbed reactive oxygen in heterogeneous catalytic oxidation are represented as follows: − 2− O2 → O− 2 (ads) → O (ads) → O (ads).

(6.3)

A substantially negative value of the SP for Ni samples that were oxidized and then exposed to O2 plasma, as previously described, is hypothesized to be attributable to the species indicated in (6.3). Taking such species into consideration, the electron source of TSEE in the present system is hypothesized to result from the reduction of the negatively charged defective oxygen-rich surface by C2 H5 OH, which causes deposition of carbon materials. We summarize the relationship between the results of XPS, SP, and TSEE as follows: (1) Figure 6.7 [19] shows a plot of the SP values against the atomic ratio of the O 1s/Ni 2p for the Ni samples that were only oxidized and subsequently subjected to Ar and O2 plasma exposure. The SP increased in a negative direction with decreasing O 1s/Ni 2p ratio in the order oxidized only 99.99%; thickness 0.1 mm) in air (at temperatures of 298, 463, 663, 863, and 1063 K) for oxidation times of 10 and 20 min with an electric furnace; afterward, the oxidized sheets were stored in dry air for 0, 2, 4, and 6 h before use. TSEE glow curves were measured with a Geiger counter {counter gas: Ar (11,200 Pa) and C2 H5 OH vapor (2600 Pa)}. Figure 6.12 shows a typical glow curve [21]. Two emission peaks (I and II) were evident at two temperature ranges (peak I: 413–493 K; peak II: 493–598 K). The intensities of the peaks substantially depended on the oxidation temperature and oxidation time. The intensity of peak I was much greater than that of peak II. The intensity of peak I at a 10-min oxidation was much higher than that at a 20-min oxidation time. Furthermore, the surface potential of the oxidized surfaces as a function of storage time was most negative (−50 to −63 mV) at 0-h storage time, slowly shifted to a positive direction, and reached a saturated level (ca. −40 mV) at 6 h, except for that at 663 K oxidation. Peaks I and II might originate from the release of negative charges that remained in the coexisting phases of CuO–Cu2 O and Cu2 O–Cu, respectively, at the surface layer during the chemical reduction with C2 H5 OH vapor. Furthermore, we investigated the effect of several organic vapors on the TSEE that corresponds to chemical reduction of Cu oxide films and examined the relationship to the chemical structure of the oxide films by XPS and XRD measurements [20, 21].

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6 TSEE Related to Plasma Treatment and Adsorption

Fig. 6.12 A typical TSEE glow curve for a Cu sample oxidized in air at 10-min 873 K [21]. Reprinted with the permission from Momose et al. [21]. Copyright© 2000

The organics were used as the quenching gas of the counter gas. The TSEE glow curves were measured in the temperature range from 25 to 325 °C with a Geiger counter {counter gas: Ar (84 Torr) and organic vapor (20 Torr)}. The organic vapors were CH3 OH, C2 H5 OH, (CH3 )2 CHOH) (2-propanol), and (CH3 )2 CO (acetone). Cu metal sheets (purity, 99.99%; thickness 0.1 mm) were used. The oxide film was prepared by heating the sheets in air for 20 min at a temperature of 25, 200, 400, 600, and 800 °C. Three specimens were used in each TSEE measurement. The glow curves for the aforementioned alcohols exhibited one emission peak, whereas there was no emission peak for acetone. The analysis of XPS and XRD—as a function of temperature during TSEE measurements—indicates that the alcohols could completely reduce the oxide film and in so doing expose the bare metal; however, reduction of the film with acetone was incomplete. The glow-curve characteristics strongly depended on the alcohol; the commencement temperature of the emission peak increased in the following order: (CH3 )2 CHOH (150 °C) < C2 H5 OH (165 °C) < CH3 OH (200 °C). The total number of electrons that were emitted in the glow curve for each alcohol was largest at the oxidation temperature of 600 °C and increased in the following order: (CH3 )2 CHOH < C2 H5 OH < CH3 OH. Here, we consider the mechanism of the reduction of the oxidized surface layer by alcohols. Table 4.3 of Sect. 4.2 indicated that the value of AV (AVmax ) giving a maximum emission in the curve of the OSEE intensity versus applied voltage (AV) for Al metal surfaces increased in the following order: n-C3 H7 OH ≤ C2 H5 OH < CH3 OH. The AVmax corresponds to the high values of the acidity constant (pKa ) of the conjugate acids of the alcohols that were used as the quenching gas of the counter gas. In the present experiments, the total number of emitted electrons for the alcohols corresponds to the acceptor number (AN) as per [8]: C3 H7 OH (AN = 33.5) < C2 H5 OH (37.1) < CH3 OH (41.3). The AN is a measure of the electrophilic behavior of a solvent. Here, the AN for C3 H7 OH is given instead of that for (CH3 )2 CHOH. Therefore, in accordance with [2] in the beginning of the chemical reduction of the oxide layer on the metal surface, the acid–base interactions between the alcohols (ROH) and surface hydroxyl group (–CuOH) on the oxidecovered metal surface occurred through the actions of the H atom of ROH on the unshared electron pair of the O atom of −CuOH (Fig. 4.8, right-hand side) because

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of the negatively charged oxide layer. CH3 OH was most strongly adsorbed on the surface compared with the other alcohols (C2 H5 OH and (CH3 )2 CHOH). However, the interaction mode produced an electric dipole, with its negatively charged end oriented toward the outside (represented by RO− –H2 + OCu-). This orientation of the electric dipole increased the potential barrier for the electron emission. Although the orientation hindered the electron emission, adsorption of ROH preceded the initiation of the chemical reduction. The electron-emitting centers were inside the oxide layer.

6.3 TSEE from Glass on Au Surfaces, Au, Ni, Si, and Graphite Subjected to Plasma Exposure; and XPS Analysis 6.3.1 TSEE from Glass Deposited on Au Metal Surfaces Plasma treatment of metals in an inert gas generally exhibits improved adhesion, but it is not certain whether the improved adhesion is because of surface cleaning, deposition of other materials, or metal surface modification [1]. Regarding the effect of mechanical treatment on metal surfaces, the observed TSEE represented surface activation [35], based on a study of the correlation between the EE activity of sandblasted steel surfaces and the increase in the adhesive strength as well as the protective activity of the painted film on the surface. In this study, the TSEE originated from the embedded sand particles. The TSEE activity of Au metal surfaces that were exposed to an Ar plasma in a glass chamber is attributable to SiO2 that was deposited onto the metal surface, which we ascribed to sputtering of the glass substance by Ar ions that were generated in the plasma [23]. We describe these results in detail. We examined the effect of the operational parameters of radio frequency (rf) plasma treatment on the TSEE for a plasma-treated Au metal surface and the relationship between the TSEE characteristics and the chemical as well as structural nature of SiO2 (obtained by XPS). Figure 6.13 shows the reactor, made of a Hario glass chamber (main component: SiO2 , 80.8%) for plasma treatment [23]. Two capacitively coupled Al electrodes were placed outside the chamber. Two types of Au samples (purity, >99.99%; thickness, 0.1 mm) for XPS and TSEE were mounted onto the glass sheet in the reactor and then plasma-treated in flow or batch systems with a 13.56-MHz rf generator. Ar was used as the plasma gas. Here, flow system refers to treatment during continuous streaming of the gas, and batch system refers to treatment in the gas while enclosed in the chamber. The operational conditions were set in two systems as follows: (1) flow system: pressure, 0.5 Torr; power, 50 W; treatment time, 10–600 s; and (2) batch system: pressure, 0.1–1.0 Torr; power, 10–50 W; treatment time, 10–600 s. The TSEE measurements were performed from 25 to 300 °C with a Geiger counter {counter gas: Ar (84 Torr) and C2 H5 OH vapor (20 Torr)}. In the XPS spectra for the

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6 TSEE Related to Plasma Treatment and Adsorption

flow system, signals for Si 2p (two components at binding energies of ~99.6 eV (Si metal) and ~103.5 eV (SiO2 ) were observed; their intensities periodically increased and decreased with increasing treatment time. The TSEE glow curves for 10-s treatment time in the flow system exhibited three emission peaks at ~120 °C (peak I), 190 °C (peak II), and 240 °C (peak III), but their peak intensities were weak. In contrast, the glow curves for 180-s treatment time resulted in only one emission peak with a remarkably enhanced intensity at ~160 °C. Figure 6.14 shows the change of the TSEE glow curve with treatment time (10–300 s) that was obtained in the batch system [23]. In the same manner as in the flow system, peaks I–III were evident with decreased intensities over short treatment times; however, with increasing treatment time, only, peak II became greater because peak II began to overlap with peaks I and III. Furthermore, one can attribute the sharp increase in the emission at ~300 °C in Fig. 6.14 to the Au metal. Furthermore, in the same manner, the TSEE glow curves changed with increasing pressure and increasing power in the batch system. Figure 6.15 shows plots of the Si 2p/O 1s ratio and the emission intensity of peak II against the treatment time in the batch system [23]. The Si 2p/O 1s ratio and the emission intensity of peak II both linearly increased with increasing treatment time although the increase in the Si 2p/O 1s ratio was less substantial at 300-s treatment time. Therefore, the intensity of peak II closely corresponds to the Si 2p/O 1s ratio below 300-s treatment time. Figure 6.16 [23] shows the relationship between the emission intensity of peak II and the corresponding ratio of Si 2p/O 1s that was obtained in the present experiments, except for 600-s treatment time in Fig. 6.15. The emission intensity of peak II increased in the vicinity of the ratio of Si 2p/O 1s~1, and then, the emission increased with increasing ratio of Si 2p/O 1s. In the Si 2p and O 1s spectra, as a function of treatment time in the batch system, the intensity of the O 1s peak decreased with increasing treatment time, whereas the intensity ratio of the two Si components remains almost unchanged (Fig. 6.17) [23]. Therefore, the phase of SiO2 that was deposited onto the metal surface became nonstoichiometric and contained oxygen vacancies. Consequently, the increase in the Si 2p/O 1s ratio that is evident in Fig. 6.15 indicates an increase in the quantity of oxygen-deficient silicon atoms in the 4 + oxidation state. In the context of peak II in the TSEE glow curves, Fig. 6.13 Reactor for plasma treatment using a RF generator [23]. Reprinted with permission from AIP Publishing, Momose and Takahashi [23]. Copyright© 1990

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Fig. 6.14 Change of the TSEE glow curve with treatment time in the batch system (0.5 Torr and 40 W): a 10 s, b 30 s, c 60 s, d 120 s, e 180 s, f 300 s [23]. Reprinted with permission from AIP Publishing, Momose and Takahashi [23]. Copyright© 1990

regarding peak II for the 60-s treatment time (Fig. 6.14c), Sakurai and coworkers [32, 33] applied an analytical function to obtain the depth of the trap E and the electron affinity χ of the nonstoichiometric SiO2 . The values were as follows: E = 0.68 eV and χ = 0.54 eV. Thus, one can attribute the emission mechanism for peak II to a bulk process. The values of E and χ were obtained for four peaks of TSEE from the Si wafer powder that was exposed to Ar plasma (Sect. 6.3.4).

Fig. 6.15 Relationship between the composition of Si 2p/O 1s (◯) and the emission intensity of peak II (Δ) as a function of treatment time in the batch system (0.5 Torr and 40 W) [23]. Reprinted with permission from AIP Publishing, Momose and Takahashi [23]. Copyright© 1990

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6 TSEE Related to Plasma Treatment and Adsorption

Fig. 6.16 Plots of the emission intensity of peak II against the compsition ratio of Si 2p/O 1s obtained in the flow and batch systems [23]. Reprinted with permission from AIP Publishing, Momose and Takahashi [23]. Copyright© 1990

Fig. 6.17 Change of Si 2p and O 1s XPS spectra with the treatment time in the batch sysem (0.5 Torr and 40 W): a untreated, b 60 s, c 180 s, d 300 s, e 600 s [23]. Reprinted with permission from AIP Publishing, Momose and Takahashi [23]. Copyright© 1990

6.3.2 TSEE from Au and Ni Metal Surfaces to Exposed to Ar and O2 Plasma In plasma treatment, the electrons are localized at the surface layer because of the effects of UV radiation and chemically active species that the plasma produces. We have studied the effect of various plasmas on the TSEE and XPS regarding metals in the following pairings: Ar and O2 plasmas for Au (impurity, 25 ppm>); and O2 , N2 , and C2 H5 OH vapor plasmas for Ni (purity, >99.6%) [29]. The metal samples were rolled sheets (thickness: 0.1 mm). Ni sheets were degreased with benzene and annealed in vacuo at 350 °C for 2 h. Both metal sheets were ultrasonically cleaned in ethanol solvent. The sizes of a sample were 10 mm × 10 mm (Au and Ni) and 15 mm × 15 mm (Au) for TSEE, and 3 mm × 3 mm for XPS. The plasmas were excited with 3- or 6-kV neon-sign transformers in a Pyrex bell-jar type reactor. This reactor was constructed with two internal Al electrodes and a sample holder (stainless steel) that was placed between the electrodes. The Au sheets were exposed to plasmas of Ar (40 Pa) and O2 (40 Pa) and the Ni sheets to plasma of O2 (40 Pa), N2 (40 Pa), and C2 H5 OH vapor (660 Pa). The TSEE measurement was performed in the dark

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Table 6.3 TSEE glow-curve characteristics for Au sheets (10 mm × 10 mm) that were exposed to Ar and O2 plasma (40 Pa) Gas

Voltage (kV)

Ar

3

6

O2

3

Time of exposure (s)

Emission intensity of peak (count/min) (temp., °C)

10

4100(170)

12,600

20

12,800(168)

29,600

300

9800(167)

35,300

10

2900(170)

7300

20

7500(163)

18,800

300

1900(176)

8100

2600(187)

7600

10

500(120)

20

400(124)

300 6

Total counts (25–300 °C)

10

1000(119)

20

500(121)

300

1400(186)

5000

300(190)

1700

1900(179)

6400

2300(183)

7400

600(178)

2400

The average of the data obtained in two or three measurements is listed

with a Geiger counter {counter gas: Ar (11,200 Pa) and C2 H5 OH (2600 Pa)}. Table 6.3 shows the glow-curve characteristics for Au sheets that were exposed to both plasmas [29]. Regarding the Au samples, Ar plasma produced only one TSEE peak in the vicinity of 170 °C, whereas for O2 plasma, two peaks were evident at ca. 180– 190 °C and also at 120 °C; the Ar plasma resulted in a much stronger emission than the O2 plasma. Furthermore, the plasma-treatment time corresponded strongly to TSEE. The XPS spectra after plasma treatment gave the following features. The Ar plasma produced a striking increase in the Si 2p peak and a pronounced increase in the O 1s peak. The Si 2p spectrum had two components: Si metal (99.4 eV) and SiO2 (103.5 eV). However, the O2 plasma treatment resulted in a more pronounced increase in the O 1s peak; the Si 2p peak was no longer evident, and there was a small peak that corresponded to Al 2s (at the plasma-treatment conditions of 40 Pa, 3 kV, and 300 s.) Incorporation of Si and Al was attributable to the materials that composed the plasma reactor. On the basis of the data for Au metal for both plasma treatments (Table 6.3), we hypothesize that the TSEE in this experiment corresponds to the Si oxide that was deposited onto the metal surface. Figure 6.18 shows a plot of the total count of emitted electrons in the TSEE glow curves against the composition ratio of O/Si for both plasmas at the following plasma-treatment conditions: pressure, 40 Pa; exposure time, 10, 20, and 300 s; and voltage, 3 kV [29]. The data points on the left-hand side refer to the Ar-plasma treatment, whereas the data points on the right-hand side refer to the O2 plasma treatment. The total count for the Ar plasma sharply increased with increasing ratio and reached a maximum at O 1s/Si 2p = 0.4. However, regarding the O2 -plasma treatment, the total count remarkably decreased

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Fig. 6.18 TSEE total counts plotted against O 1s/Si 2p XPS intensity ratio for Au sheets exposed to Ar (◯) and (●) plasmas (40 Pa, 3 kV) (1985 The Japan Society of Applied Physics) [29]. Reprinted with permission from The Japan Society of Applied Physics [29]. Copyright© 1985

with increasing ratio of O/Si and reached a minimum value at a larger O 1s value (O 1s/Si 2p → ∞). These findings suggest that the electron-emitting centers were localized in the silicon–oxygen layer rather than in the gold–oxygen layer. Next, we examined the change of TSEE from the Au sheets that were repeatedly exposed to Ar and O2 plasma. Figure 6.19 shows plots of the intensity and peak temperature of the peak emission against the total-plasma-treatment time for an Au sample (size, 15 mm × 15 mm), which was repeatedly exposed to Ar as well as O2 plasmas at 40 Pa and 3 kV [29]. During the first Ar-plasma treatment, the emission intensity immediately increased, passed through a maximum, and then rapidly decreased with increasing treatment time; conversely, the peak temperature rapidly increased but then remained at a constant level. In the subsequent O2 -plasma treatment, the emission substantially decreased, but the peak temperature increased somewhat; the subsequent Ar-plasma treatment did not bring about any recovery of the decreased emission. These findings indicate that a thickly oxygen-covered surface imparts difficulties to recovering the emission, even after exposure to Ar plasma. We next describe the results for Ni samples. Figure 6.20 [29] shows the TSEE glow curves for Ni samples that were exposed to O2 , N2 , and C2 H5 OH vapor plasma. All of the TSEE glow curves exhibited one main peak at ca. 177 °C, independent of the plasma atmosphere (O2 , N2 , and C2 H5 OH vapor) and an additional small peak in the vicinity of 300 °C for N2 . Regarding the XPS results, the O2 -plasma treatment produced a striking increase in the O 1s peak (532.8 eV) along with Ni 2p3/2 peaks that exhibited higher binding energies (857.0 and 863.4 eV). In this treatment, the C 1s peak was evident at 285.8 eV. The oxygen that was incorporated as a result of the O2 -plasma treatment consisted of Ni2 O3 or adsorbed oxygen. The N2 -plasma treatment did not result in incorporation of nitrogen into the surface. In the case of plasmas of N2 and C2 H5 OH vapor, there was no incorporation of Si. We hypothesize that TSEE corresponds to either an interaction of the incorporated oxygen or adsorbed

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Fig. 6.19 Emission intensity and temperature of the TSEE peak plotted against the total time of the plasma treatment for an Au sheet (15 × 15 mm2 ). The sample was repeatedly exposed to Ar (◯, ⬜ ) and O2 (●, ∎ ) plasmas (40 Pa, 3 kV) (1985 The Japan Society of Applied Physics) [29]. Reprinted with permission from The Japan Society of Applied Physics [29]. Copyright© 1985

carbon material with Ni metal. Figure 6.21 [29] shows a plot of the emission intensity of the TSEE peak against the XPS composition ratios of O 1s/Ni 2p3/2 and O 1s/C 1s (on a semi-logarithmic scale) for all of the plasma treatments with N2 , O2 , and C2 H5 OH vapor. Although the data points exhibited scatter, the intensity of the peak emission sharply increased, reached a broad maximum, and then slowly decreased with increasing ratio of O 1s/Ni 2p3/2 . The maximum was located in the vicinity of O 1s/Ni 2p3/2 = 8. In the case of the plot of the peak intensity versus O 1s/C 1s, the intensity of the peak emission resulted in a broad maximum in the vicinity of O 1s/C 1s = 1.0. Finally, the origin of the TSEE corresponds to the surface elemental composition at the adsorption layer, such as O/Si (Au), O/Ni, and O/C. We will describe the effect of Ar and O2 plasma on TSEE for Ni sheets in more detail in Sect. 6.3.3.

6.3.3 TSEE from Ni Metal Surfaces Exposed to Ar and O2 Plasma We compared the effect of adsorbed oxygen on TSEE from Ni metal, exposed to Ar and O2 plasma, by XPS [24]. The sheets (thickness: 0.1 mm) were degreased with benzene, annealed in vacuum at 300 °C for 3 h and then ultrasonically cleaned in C2 H5 OH solvent. The size of a sample was 10 mm × 10 mm (TSEE) and 3 mm × 3 mm (XPS). The plasma was excited in Ar and O2 (at the pressure of 0.5 or 0.3 Torr) for various times in a batch-type plasma reactor with a 6-kV neon-sign transformer. The TSEE measurements were performed with a Geiger counter {counter gas: Ar (84 Torr) and C2 H5 OH vapor (20 Torr)}. The glow curves were measured from 25 to 312 °C at a rate of 20 °C/min under an applied −92-V potential to the sample.

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Fig. 6.20 TSEE glow curves for Ni sheets (10 × 10 mm2 ) exposed to O2 , N2 , and C2 H5 OH vapor plasmas (6 kV, 300 s) (1985 The Japan Society of Applied Physics) [29]. Reprinted with permission from The Japan Society of Applied Physics [29]. Copyright© 1985

Fig. 6.21 TSEE peak intensity plotted against the O 1s/Ni 2p3/2 XPS intensity ratio for Ni sheets (10 × 10 and 3 × 3 mm2 ) exposed to O2 (●, ◯), N2 (▲, Δ), and C2 H5 OH vapoer ( ∎ , ⬜ ). The symbols (●, ●, ∎ ) and (◯, Δ, ⬜ ) refer to samples kept in dry air and in room air before plasma treatment, respectively (1985 The Japan Society of Applied Physics) [29]. Reprinted with permission from The Japan Society of Applied Physics [29]. Copyright© 1985

Figure 6.22 shows a typical glow curve for O2 plasma. Three emission peaks (I–III) were evident at ca. 100 °C, 190 °C, and 260 °C, respectively. These three peaks were evident in almost all cases. The emission intensity of peak III tended to be the largest. The glow curve exhibited a considerably different shape from that shown in Fig. 6.20 although we will comment on the manifestation of the TSEE peaks in a subsequent paragraph. In the case of Ar plasma, only two peaks (peaks I and II) were evident at the same temperatures as those for O2 plasma. The emission

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intensity of peak II was much greater than that of peak I. Figures 6.23 and 6.24 show the XPS spectra for Ar and O2 plasma, respectively, as a function of plasmatreatment time. The O 1s and Ni 2p spectra for both plasmas exhibited considerably different change with the plasma-treatment time. Furthermore, incorporation of Al and Si was almost negligible. Therefore, the origin of TSEE corresponds to the incorporated oxygen or carbon contaminant on the surface. Here, we explain the relationship between the results of TSEE and XPS for Ni that was exposed to Ar and O2 plasma. Figures 6.25 and 6.26 show plots of the total counts and intensities of the peaks in TSEE versus the ratio of O 1s/Ni 2p3/2 for Ar and O2 plasma, respectively. Regarding Ar plasma (Fig. 6.25), the magnitude of TSEE rapidly increased, reached a maximum at ca. O 1s/Ni 2p3/2 = 3–4, and then slowly decreased with increasing O 1s/Ni 2p3/2 . Regarding O2 plasma (Fig. 6.26), the magnitude of TSEE was much high in the initial stage and then sharply decreased with increasing ratio of O 1s/Ni 2p3/2 . The curve resolution indicates that one can separate the O 1s spectra into three components (Fig. 6.27): NiOads (peak A, binding energy = 532.1 eV), Ni2 O3 (peak B, 530.5 eV), and NiO (peak C, 528.6 eV). Regarding the Ar and O2 -plasma treatments at 0.3 Torr for exposure times of 5–300 s, we performed measurements of the curve resolution of the O 1s spectra and TSEE. The characteristics of the curve resolution and TSEE were as follows: (1) For both plasmas, the intensity (%) of Ni2 O3 predominated; the intensity (%) of the oxygen components decreased in the order Ni2 O3 > NiO > NiOads for Ar plasma, and Ni2 O3 ≫ NiO ≈ NiOads for O2 plasma. (2) For Ar plasma, the intensity of Ni2 O3 increased with increasing exposure time, whereas the intensity of NiO decreased with exposure time. (3) For O2 plasma, the intensity of Ni2 O3 decreased with the exposure time, whereas the intensity of NiO increased with exposure time. This behavior is opposite to that for Ar plasma. (4) Ar plasma produced only two TSEE peaks (I and II). (5) For O2 plasma, TSEE peak III began to manifest at exposure time longer than 20 s. Furthermore, in the case of O2 -plasma treatment for 30 s at various pressures of O2 (0.1–2.0 Torr), the NiO content strongly increased and TSEE peak III was no longer evident at pressures >0.5 Torr, concomitant with an increase in the emission intensities of peaks I and II. These findings suggest that the manifestation of the emission peaks in the TSEE glow curves strongly corresponded to the content ratio of Ni2 O3 /NiO. In other words, the TSEE peak II tended to be predominantly enhanced at a small value of the Ni2 O3 /NiO ratio.

6.3.4 TSEE from Si Wafer Powder Exposed to Ar Plasma An active area of research involves clarifying in more detail the nature of the emitting centers in the nonstoichiometric SiO2 deposited onto Au surfaces, as described in Sects. 6.3.1 and 6.3.2. We investigated the mechanism of TSEE from Ar plasmatreated Si wafer powder by XPS and with a newly developed theoretical equation [25]. A silicon wafer (purity, 99.999%) was ground into a powder with an agate mortar, and powder from 150–200 mesh (size: 0.064–0.105 mm) was used. The plasma reactor,

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6 TSEE Related to Plasma Treatment and Adsorption

Fig. 6.22 A typical TSEE glow curve for a Ni sheet exposed to an O2 plasma (0.5 Torr) for 5 s [24]. Reprinted with permission from Momose and Yasukawa [24]. Copyright© 1988

Fig. 6.23 Dependence of XPS spectra of Ni sheets exposed to Ar plasma (0.5 Torr) on plasma treatment time: a 5 s, b 10 s, c 20 s, d 30 s, e 60 s, f 180 s, g 300 s [24]. Reprinted Momose and Yasukawa [24]. Copyright© 1988

which was similar to that previously reported [23], consisted of a glass chamber that was equipped with two capacitively coupled external electrodes. Ar plasma (purity, >99.999%) was ignited in the reactor with a 13.56-MHz rf generator. Samples for the TSEE and XPS measurements were mounted onto a stainless-steel holder in the reactor and then exposed to the plasma in the batch system, which refers to the treatment of the gas that was enclosed in the chamber. The plasma-treatment conditions were as follows: gas pressure, 0.1 Torr, rf power, 40 W; and treatment time, 1–15 min. TSEE measurements were performed with Geiger counter {counter

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Fig. 6.24 Dependence of XPS spectra of Ni sheets exposed to O2 plasma (0.5 Torr) on plasma treatment time: a 5 s, b 10 s, c 20 s, d 30 s, e 60 s, f 180 s, g 300 s [24]. Reprinted with permission from Momose and Yasukawa [24]. Copyright© 1988

Fig. 6.25 TSEE peak intensities and total counts plotted against O 1s/Ni 2p3/2 XPS intensity ratio for Ni sheets exposed to an Ar plasma (0.5 Torr) [24]. Reprinted with permission from Momose and Yasukawa [24]. Copyright© 1988

gas: Ar (84 Torr) and C2 H5 OH vapor (20 Torr)}. After the treatment, the powder (mass: 0.0115–0.0424 g) was mounted onto a gold sample holder in the counter and then heated from 298 to 623 K at a rate of 20 K/min. The untreated sample produced no TSEE. The TSEE glow curve for the treated samples exhibited two broad emission peaks at ~363 and 463 K and then an increase in the intensity at >563 K. The broadening of the emission peaks might be associated with the powdered Si sample. The former emission peak increased more rapidly than the latter peak with increasing plasma-treatment time. The TSEE curves (after subtracting the increased emission at >563 K) were used for curve resolution. Sakurai

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6 TSEE Related to Plasma Treatment and Adsorption

Fig. 6.26 TSEE peak intensities and total counts plotted against O 1s/Ni 2p3/2 XPS intensity ratio for Ni sheets exposed to an O2 plasma (0.5 Torr) [24]. Reprinted with permission from Momose and Yasukawa [24]. Copyright© 1988

Fig. 6.27 Curve resolution of an O 1s spectrum for a Ni sheet exposed to Ar plasma (0.3 Torr) for 5 s [24]. Reprinted with permission from Momose and Yasukawa [24]. Copyright© 1988

et al. [33] proposed an analytically exact solution for an equation based on the EE model, in which an electron that is localized in a trap level is emitted to the vacuum through the conduction band. The emission intensity I(T ) at the temperature T in kelvin in the glow curve is represented with (6.4) and (6.5), depending on the trap depth (E) and the electron affinity (χ ) [25]: T

I (T ) = I0 H (T ) exp[(−χ /kT ) − (s/B) ∫ exp(χ /kT )dT ] T0

(6.4)

6.3 TSEE from Glass on Au Surfaces, Au, Ni, Si, and Graphite …

101

where T

T

T0

T0

H (T ) = ∫ exp[−E/kT − ( f /B) ∫ exp(−E/kT )dT + (s/B) T

∫ exp(−χ /kT )dT ]dT .

(6.5)

T0

Here, I 0 is a constant; k is the Boltzmann constant; s is the effective frequency factor for escape from the conduction band; B is the heating rate, and f is the frequency factor for the escape from the trap levels. Equations (6.4) and (6.5) were applied to the resolution of the observed glow curves. Figure 6.28 shows the curve resolutions of the glow curves for 10- and 15-min treatments [25]. The glow curves were separated into four peaks (I–IV). Table 6.4 [25] show the values of E, χ, s, and f that were obtained for the resolved four peaks. These results indicate that there were four trap levels at the SiO2 /Si near the interface. As peak I shifted to peak IV (i.e., the peak temperature increased), the values of E and f increased, but the χ value for each peak remained nearly equal although the value of s decreased. We consider the effect of adsorbed oxygen on TSEE. The XPS results are as follows: (1) The intensity of the O 1s peak for the plasma-treated samples increased compared with that for the untreated sample, and the ratio of the O 1s/Si 2p increased with increasing treatment time. (2) Fig. 6.29 indicates that the intensities of the resolved two peaks (I and II) that were in the lower-temperature region more substantially increased—compared with those of the peaks III and IV in the highertemperature region—with increasing O 1s/Si 2p ratio. We also observed this tendency for the plot against the ratio of Si 2p (oxide)/Si 2p (substrate) of the two components that were obtained from the curve resolution of the Si 2p spectra. Figure 6.30 shows the relationship between the trap depth and the electron affinity to the ratio of O 1s/Si 2p. The E values for peaks IV and III tended to slope more sharply upward Table 6.4 Characteristics for the resolved peaks of TSEE glow curves of Si exposed to Ar plasma Treatment time*

Peak

Trap depth, E/eV

Electron affinity, χ /eV

Frequency factor from conduction band, s/s−1

Frequency factor from trap depth, f /s−1

10 mina

Peak I

0.44

0.36

1.6 × 103

7.8 × 104 3.2 × 105

15

*

minb

Peak II

0.56

0.36

3.2 ×

Peak III

0.70

0.34

7.4 × 10

7.4 × 105

Peak IV

0.86

0.40

6.4 × 10

6.4 × 106

102

6.4 × 102

102

Peak I

0.32

0.36

6.4 ×

Peak II

0.42

0.34

2.0 × 102

2.0 × 103 1.3 × 104

Peak III

0.56

0.36

1.3 ×

Peak IV

0.72

0.38

5.9 × 10

102

The data correspond to the glow curves (a) and (b) given in Fig. 6.28

1.5 × 105

102

6 TSEE Related to Plasma Treatment and Adsorption

Fig. 6.28 a Curve resolution of the TSEE glow curve (10 min treatment time): (◯) experimental values; (…) resolved peaks; (–) total curve. b Curve resolution of the TSEE glow curve (15 min treatment time): (◯) experimental values; (…) resolved peaks; (–) total curve [25]. Reprinted with permission from AIP Publishing, Momose et al. [25]. Copyright© 1993

with increasing ratio than those of peaks I and II. These results suggest that the trap depth for peaks IV and III was more strongly influenced by the surface oxygen content. However, the values of χ were almost independent of the O 1s/Si 2p ratio. These results might suggest that the electrons that were lifted to the conduction band from the traps were able to move around freely at the surface, in a manner that yielded the average value of χ. Figure 6.31 shows the effect of the adsorbed oxygen on the binding energy difference between the two components of SiO2 and Si in the Si 2p spectra. The binding energy difference decreased with decreasing ratio of O 1s/Si 2p. This trend was also confirmed in the plot against the component ratio of Si 2p (oxide)/Si 2p (substrate) of the Si 2p spectra. In accordance with [7], it has been suggested that the O 2s contribution to the Si–O–Si bond increases, and the Si–O bond is shorter and stronger in the near-interfacial SiO2 region than the bond with less s-character that is present in the outer surface region [13]. Finally, a clean Si surface emits no EE after electron irradiation [4]. Therefore, the presence of oxide or adsorbed species facilitates the emission [6]. Regarding the present experiments, there might be four types of oxygen vacancies that are capable of trapping electrons around the Si atoms [15].

6.3 TSEE from Glass on Au Surfaces, Au, Ni, Si, and Graphite …

103

Fig. 6.29 Relationship between the area ratio of curve-resolved TSEE peaks I and II to peaks III and IV and the atomic ratio of O 1s/Si 2p [25]. Reprinted with permission from AIP Publishing, Momose et al. [25]. Copyright© 1993

Fig. 6.30 Dependence of the trap depth (E) and the electron affinity (χ ) for each TSEE peak on the atomic ratio of O 1s/Si 2p:(⬜ ) peak I; (◯) peak II; (♦) peak III; (Δ) peak IV [25]. Reprinted with permission from AIP Publishing, Momose et al. [25]. Copyright© 1993

Fig. 6.31 The plot of the binding energy difference between Si 2p (oxide) and Si 2p (substrate) versus the ratio of O 1s/Si 2p. The data (●) is for the untreated sample [25]. Reprinted with permission from AIP Publishing, Momose et al. [25]. Copyright© 1993

6.3.5 TSEE from Graphite Exposed to CF4 , Ar, and O2 Plasma Expanded graphite sheets have been widely used as materials with high flexibility. The material is featured by its sp2 hybridization orbital in the basal plane of the graphite. Therefore, our focus has been on the TSEE of EGS subjected to plasma exposure and on its comparison with that for other materials exposed to plasma as described previously (Sects. 6.3.1, 6.3.2, 6.3.3 and 6.3.4). We reported TSEE from

104

6 TSEE Related to Plasma Treatment and Adsorption

EGS after plasma treatments [27]. Three types of gases {CF4 (purity, >99.999%), Ar (>99.999%), and O2 (99.95%)} were used as plasma gases. Prior to plasma exposure, EGS sheets (thickness, ~0.4 mm) were preheated at 150 °C for 15 min in vacuo. Plasma treatment was carried out in a reactor made of a Hario glass chamber with internal Al electrodes for CF4 and Ar plasma and with external Al electrodes for O2 plasma. The plasma was generated with a 13.56-MHz rf power source under the following conditions: pressure, 0.05–1.0 Torr (with a Pirani gauge); power, 20 W; and treatment time, 30–900 s. After that the samples were removed into ambient air; measurements of TSEE, XPS, and surface potential were performed. TSEE was measured with a Geiger counter that was connected to a vacuum system. Regarding the counter gas, a mixing gas of Ar (84 Torr) and C2 H5 OH vapor (20 Torr) was used. The TSEE glow curve of a sample was measured from 25 °C to 300 °C at a rate of 20 °C/min and a negative potential of −95 V relative to the earthed grid of the counter. A vibrating capacitive surface electrometer and an XPS spectrometer (Shimadzu ESCA750) were used. The measurements of SP and XPS were conducted immediately after plasma treatment and also after TSEE measurements following plasma treatment. Figure 6.32 shows the TSEE glow curves after plasma treatments and the plasma reactor (on the left-hand side; [27]). For all plasma treatments, with increasing temperature, the glow curve exhibited a sharp emission peak at ca. 98 °C. In CF4 gas, another broad peak was evident at ca. 250 °C. These two aforementioned peaks are termed peak I (98 °C) and peak II (250 °C). Figure 6.33 [27] shows the dependence of the intensities of these peaks on the plasma-treatment time. The treatment-time dependence of the intensities substantially differed between the plasmas. The O2 plasma continued to yield peak I at the highest intensity with plasma-treatment time, whereas in the CF4 plasma, peak I slowly decreased. Instead, peak II gradually increased and then became constant. The Ar plasma resulted in peak I with a lower intensity although the intensity did not change substantially. The intensity of peak I at the short treatment time was as follows: O2 treatment, as high as 10,000 count/min (cpm); CF4 treatment, 5000 cpm; and Ar treatment, 1000 cpm. Figure 6.34 [17] shows the XPS spectra for untreated and plasma-treated samples. Regarding the CF4 and Ar treatments, the Al 2p spectrum was evident; therefore, this spectrum originated from the internal Al electrodes. Regarding the Ar treatment, based on the peak position of the Al 2p spectrum, the Al component was introduced in the form of an oxide (alumina). However, regarding the CF4 treatment, the Al spectrum shifted to a higher binding energy; species such as AlF3 might form because the F 1s peak was evident. For all of the plasma treatments, the O 1s peak clearly increased (with the largest intensity of O 1s for the O2 -plasma treatment) compared with the untreated sample. Therefore, the O component might be bound to the surface of the EGS by the plasma treatment although the O component regarding CF4 and Ar plasma partially originated from the Al oxide. Table 6.5 [27] shows representative values of the TSEE peak I intensities, SP, and O 1s/C 1s (XPS intensity ratio) obtained at the plasma-treatment conditions, where Al 2p and Si 2p peaks (corresponding to the electrodes and glass chamber, respectively) were negligibly detected. Values before and after the TSEE measurement are given by

6.3 TSEE from Glass on Au Surfaces, Au, Ni, Si, and Graphite …

105

Fig. 6.32 TSEE glow curves for graphite surfaces exposed to O2 (a), CF4 (b), and Ar (c) plasmas. The exposure conditions: 0.05 Torr, 20 W, 900 s. The plasma formed by 13.56 MHz RF generator is shown on left side [27]. Reprinted with permission from AIP Publishing, Momose et al. [27]. Copyright© 1992

Fig. 6.33 Plots of the intensity of TSEE peak I (◯) and peak II (●) of graphite surfaces for O2 , CF4 , and Ar plasmas versus exposure time. The exposure conditions: 0.05 Torr, 20 W [27]. Reprinted with permission from AIP Publishing, Momose et al. [27]. Copyright© 1992

P and PT, respectively. The intensity of peak I decreased in the order O2 > CF4 > Ar, whereas both SP (P) and SP (PT ) decreased in a negative direction in the order Ar > O2 > CF4 . The negative SP (P) is hypothesized to be because of introducing fluorine and oxygen on the surface. The O 1s/C 1s (P) values immediately after plasma exposure exhibited small differences between plasma gases compared with peaks I and SP(P) and decreased in the order O2 ≈ Ar > CF4 . After the TSEE measurements, the SP (PT ) almost returned to its original value (the SP of the untreated sample was ~+200 mV). Furthermore, based on the O 1s/C 1s (PT ), the introduced oxygen was considerably desorbed. This result is because of thermal desorption during TSEE measurements.

106

6 TSEE Related to Plasma Treatment and Adsorption

Fig. 6.34 Dependence of the XPS spectra for graphite surfaces exposed to O2 , CF4 , and Ar plasmas: a untreated; b CF4 ; c Ar; d O2 . The exposure conditions: 7 Pa, 20 W, 900 s. The values of kcps (103 count/s) for the length of the lines indicated for each spectrum are represented [17]. Reproduced with permission from [17]. Copyright© 1992

Table 6.5 Comparison of the intensity of TSEE peak I, surface potential (SP), and XPS intensity ratio of O 1s/C 1s for graphite surfaces exposed to O2 , CF4 , and Ar plasmaa ΔSPd /mV

Peak I intensity/cpm

SP/mV Pb

PT c

O2

10,930

−703

+10

CF4

3940

−1155

−204

Ar

1250

−441

+19

Plasma gas

ΔO1s/C1se

O1s/C1s Pb

PT c

+713

0.081

0.041

0.040

+951

0.032

0.035

−0.003

+460

0.079

0.040

0.039

a

Plasma conditions: 0.05 Torr, 20 W, and 60 s. The SP of the untreated surface was ~+200 mV b Immediately after the plasma exposure c After the plasma exposure and subsequent TSEE measurement d Value of [SP(PT )–SP(P)] e Value of [O1s/C1s(P)–O1s/C1s(PT )]

We consider the role of adsorbed oxygen in TSEE and the SP on the graphite surface. In Table 6.5, the extent of recovery of SP is represented by ΔSP {= SP (PT ) – SP (P)}, and the extent of desorption of adsorbed oxygen is represented by ΔO 1s/C 1s {= O 1s/C 1s (P) – O 1s/C 1s (PT )}. Regarding the relationship of the TSEE peak I intensity and ΔSP against ΔO 1s/C 1s, we reported that upon O2 -plasma treatment of graphite, the ΔSP tended to increase with increasing ΔO 1s/C 1s, whereas the TSEE peak I intensity tended to decrease with increasing ΔO 1s/C 1s although the data points had some scatter [28]. In the present experiments, Fig. 6.35 [27] indicates that during the Ar-plasma treatment, the peak I intensity decreased with increasing ΔSP, which was nearly equal to the absolute value of SP (P). Therefore, these findings contradict the expectation that the trapping sites for TSEE peak I and the origin of ΔSP might be the same. Because oxygen that is incorporated on the graphite surface is pertinent to TSEE peak I and ΔSP, it is reasonable to hypothesize that the origin of the TSEE peak I considerably differs from that responsible for the change in SP (P). On the basis of the finding that the surface exhibited high values of the O 1s/C 1s ratio even after TSEE measurements,

References

107

Fig. 6.35 A plot of the intensity of TSEE peak I versus the ΔSP obtained for Ar plasma exposure at various pressures for graphite surfaces. The exposure condition: 20 W, 60 s [27]. Reprinted with permission from AIP Publishing, Momose et al. [27]. Copyright© 1992

the TSEE peak I might originate from the electron trapping sites that formed in the oxygens, which are more negatively charged and strongly bonded to the surface. Because the action of Ar and O2 plasma on graphite had a completely opposite effect on the TSEE for Au (Table 6.3), the action of O2 and Ar plasma in a manner that produced TSEE active centers completely differed between the expanded graphite and the Au surfaces. The O2 plasma acted much more strongly on the graphite, whereas the Ar plasma acted more strongly on the Au surface. Regarding the TSEE peak II observed only with the CF4 treatment, the peak II intensity increased with increasing exposure time in the same manner as the atomic ratios of F 1s/C 1s and Al 2p/C 1s. The fluorine-containing components that were deposited onto the surface were pertinent to the origin of peak II. This behavior is analogous to accumulation of electrons by a polytetrafluoroethylene–polymer rotator on metal surfaces in TriboEE, described in Chap. 11.

References 1. H.V. Boenig, Fundamentals of Plasma Chemistry and Technology (Technomic Pub. Co., Lancaster, 1988), p. 167 2. J.C. Bolger, A.S. Michaels, Molecular Structure and Electrostatic Interactions at Polymer−Solid Interfaces, in Interface Conversion for Polymer Coatings. ed. by P. Weiss, G.D. Cheever (Elsevier, New York, 1968), pp.3–60 3. D.T. Clark, A.J. Dilks, ESCA applied to polymers. XVIII. RF glow discharge modification of polymers in helium, neon, argon, and krypton. Polym. Sci. Polym. Chem. Ed. 16, 911–936 (1978) 4. J. Drenckhan, H. Gross, H. Glaefeke, Investigation of exo-electron emission of clean semiconductor surfaces. Phys. Status Solidi. A 2, K201 (1970) 5. H.J. Fitting, H. Glaefeke, W. Wild, W. Müller, R. Kreimann, in Electron Beam Excited Exoelectron Emission from Al 2 O3 and SiO2 . Proceedings of the 6th International Symposium of Exoelectron Emission and Applications (Ahrenshoop, 1979), pp. 71–73 6. H. Glaefeke, Exoemission, in Thermally Stimulated Relaxation in Solids. ed. by P. Bräunlich (Springer-Verlag, Berlin, 1979), pp.225–273

108

6 TSEE Related to Plasma Treatment and Adsorption

7. F.J. Grunthaner, P.J. Grunthaner, Chemical and electronic structure of the SiO2 /Si interface. Mater. Sci. Rep. 1, 65–160 (1986) 8. V. Gutmann, Ion pairing and outer sphere effect. Chimia 31, 1–7 (1977) 9. G. Heinicke, Tribochemistry (Carl Hanser Verlag, München, 1984), pp.203–214 10. M. Hendewerk, M. Salmeron, G.A. Somorjai, Water adsorption on the (001) plane of Fe2 O3 : An XPS, UPS, Auger, and TPD study. Surf. Sci. 172, 544–556 (1986) 11. V.E. Henrich, P.A. Cox, Organic Molecules. The surface Science of Metal Oxides (Cambridge University Press, Cambridge, 1994), pp. 276–283 12. J.P. Hiernaut, R.P. Forier, J. Van Cakenberghe, Influence of oxygen on electron-trapping by the surfaces of metal oxides. Vacuum 22, 471–473 (1972) 13. J.E. Huheey, Hybridization and Overlap, in Inorganic Chemistry, 3rd edn. (Harper and Row, New York, 1983), pp.118–119 14. D.H. Kaelble, 2 Intermolecular Forces and Structures, in Physical Chemistry of Adhesion. (Wiley-Interscience, New York, 1971), pp.45–83 15. V.S. Kortov, Role of non-stoichiometry in exoelectron oxide emission II. Exoemission activity. Jpn. J. Appl. Phys. 24, 69–71 (1985) 16. I.V. Krylova, The chemical aspect of exoemission. Russ. Chem. Rev. 45, 1101–1118 (1976) 17. Y. Momose, Plasma treatment and thermally stimulated exoelectron emission. J. Soc. Powder Technol. Jpn. 29(3), 206–210 (1992) 18. Y. Momose, K. Ikawa, T. Sato, S. Okazaki, XPS and ESR studies of the photodegradation of polyamidoimide and polyimide in O2 , O2 + N2 , Air, N2 , and vacuum atmosphere. J. Appl. Polym. Sci. 33(8), 2715–2729 (1987) 19. Y. Momose, K. Iwanami, J. Seki, Exoelectron emission from nickel oxide film and its relationship to the reduction by ethanol vapor. J. Vac. Sci. Technol. A 14, 104–109 (1996) 20. Y. Momose, N. Kokuba, O. Abe, in Analysis of Interaction of Copper Oxide with Alcohols Using Thermally Stimulated Electron Emission. Proceedings of the ITC Kobe, Satellite Forum, Tribochemistry (Nara, Japan, 2005), pp. 24–25 21. Y. Momose, S. Kojima, O. Abe, in Thermally Stimulated Exoemission Technique for Analysis of Oxidized Copper Surfaces Subjected to Reduction by Ethanol Vapour. Latvian Journal of Physics and Technical Sciences, Proceedings of the 13th International Symposium on Exoemission and Related Relaxation Phenomena (Jurmala, 2000), pp. 133–140 22. Y. Momose, H. Okazaki, Influence of organic vapours on exo-electron emission from iron surface exposed to discharge. Jpn. J. Appl. Phys. 12, 1890–1895 (1973) 23. Y. Momose, H. Takahashi, Thermally stimulated exoelectron emission from glass deposited on metal by argon plasma treatment. J. Vac. Sci. Technol. A 8(6), 3948–3953 (1990) 24. Y. Momose, T. Yasukawa, in Exoelectron Emission Excited by Mechanical Treatment or Ionizing Radiation and Adsorption of Gases, eds. by G. Jimbo, M. Senna, Y. Kuwahara. Proceedings of the 2nd Japan–Soviet Symposium on Mechanochemistry, Society of Powder Technology of Japan (Tokyo, 1988), pp. 79–88 25. Y. Momose, T. Yamamoto, M. Takeuchi, T. Sakurai, Thermally stimulated exoelectron emission from silicon subjected to argon plasma treatment. J. Appl. Phys. 73(11), 7482–7486 (1993) 26. Y. Momose, Y. Tamura, M. Ogino, S. Okazaki, M. Hirayama, Chemical reactivity between Teflon surfaces subjected to argon plasma treatment and atmospheric oxygen. J. Vac. Sci. Technol. A 10(1), 229–238 (1992) 27. Y. Momose, T. Ohaku, S. Okazaki, Y. Fujii, Thermally stimulated exoelectron emission from graphite surfaces exposed to oxygen, tetrafluoromethane, and argon plasmas. J. Vac. Sci. Technol. A 10(2), 353–361 (1992) 28. Y. Momose, T. Ohaku, H. Chuma, S. Okazaki, T. Saruta, M. Masui, M. Takeuchi, Electrical properties of O2 plasma treated solid surfaces. J. Appl. Polym. Sci. Appl. Polym. Symp. 46, 153–172 (1990) 29. Y. Momose, M. Noguchi, T. Kumada, A. Sakai, Thermally stimulated exoelectron emission (TSEE) and ESCA study of Au and Ni exposed to Ar, O2 , N2 and C2 H5 OH vapor plasmas. Jpn. J. Appl. Phys. Suppl. 24–4, 48–52 (1985)

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30. H. Müller, Untersuchungen an Kupferoxydul mittels Exoelektronen. Acta Phys. Austriaca 10, 474–480 (1957) 31. Ohshima I, Tajima S, Momose Y, in TSEE from Copper Surfaces Exposed to Electric Discharge and Its Relationship to Pretreatments. Proceedings of the 10th International Symposium on Exoelectron Emission and Applications (Moscow, 1993), pp. 105–109 32. T. Sakurai, Y. Momose, Spectral analysis of thermally stimulated exoelectron emission from glass deposited on metal by argon plasma treatment. Phys. Lett. A. 169, 55–57 (1993) 33. T. Sakurai, Y. Momose, M. Takeuchi, New method for analysis of thermally stimulated exoelectron emission glow curves. Phys. Stat. Sol. A. 135, 245–251 (1993) 34. T. Smith, Photoelectron emission from aluminum and nickel measured in air. J. Appl. Phys. 46, 1553–1558 (1975) 35. Y. Tamai, Y. Sato, Y. Momose, in Exoelectron Phenomena and Adhesion of Plastic Film. Proceedings of the 5th International Congress on Metallic Corrosion (Tokyo, 1972), pp. 640– 644 36. M. Tanaka, On an after effect of metal bombarded by electrons. Proc. Phys. Math. Soc. Jpn. 22, 899–924 (1940) 37. W. Wild, H. Glaefeke, H.J. Fitting, J. Neutzling, in Influence of Different Kinds of Excitation Radiation on TSEE Spectrum. Proceedings of the 6th International Symposium on Exoelectron Emission and Applications (Ahrenshoop, 1979), pp. 41–42

Chapter 7

Effects of Blasting and Grinding Agents as Well as Cutting Fluids on TSEE from Mechanically Deformed Surfaces

Abstract Prior to surface modification of practical metal surfaces, one performs various mechanical treatments. In fields such as tribochemistry and coatings, TSEE from mechanically deformed surfaces is of substantial interest because of its special chemical activity. Here, we introduce the effects of abrasives and cutting fluids; deformation methods such as blasting, grinding, and cutting; as well as adsorption and interactions of organics and cutting fluids on TSEE.

7.1 TSEE from Sandblasted Mild Steel and Ground Sand 7.1.1 TSEE from Sandblasted Mild Steel and Adsorption of Organic Vapors Momose [8] examined the influence of various organic vapors on the EE glow curves for sandblasted mild steel specimens. Prior to blasting, commercial mild steel specimens were annealed in air for 2 h at 400 °C. Commercial standard sand (ca. 60-mesh; components: SiO2 92.4%, Al2 O3 4.1%, Fe2 O3 0.6%, CaO 0.4%, MgO 0.2%) was blasted with compressed air. TSEE was measured with a modified Geiger counter. The counter gas consisted of a mixture gas of a total pressure of 104 Torr {with a typical composition of Ar (84 Torr) and organic vapor (20 Torr)} that was used as a quenching gas. The counter was connected to a vacuum line made of glass, where the pressures of Ar and organic vapor were adjusted. The measurement of the TSEE glow curve was performed from 22 to 280 °C. Table 7.1 shows the glow-curve characteristics. The TSEE glow curves exhibited one main emission peak in a narrow temperature range of 51–66 °C, and further (n-C3 H7 )2 NH gave another peak with a much weaker intensity at 235 °C; but the intensity of the main peak was strongly influenced by the type of organic vapor. The intensity of the peak increased in the following order: n-C4 H9 Cl ≪ CH3 CN < C6 H6 < CH3 OH < C6 H5 CH3 ≈ (CH3 )2 CO ≈ C2 H5 OH ≤ (CH3 )2 CHOH < CH3 COOC2 H5 < (n-C3 H7 )2 NH < C3 H7 NH2 . This order closely resembles that of the proton-attracting power of the functional group of the organics. On the basis of this finding, it was hypothesized that the emission of electrons that originate in the trapping centers in © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 Y. Momose, Exoemission from Processed Solid Surfaces and Gas Adsorption, Springer Series in Surface Sciences 73, https://doi.org/10.1007/978-981-19-6948-5_7

111

112

7 Effects of Blasting and Grinding Agents as Well as Cutting …

Table 7.1 Dependence of the glow-curve characteristics of sandblasted mild steel on organic vapor Organic vapor

Pressure of organic vapor, Torr

Average values Intensity at 22 °C, counts/s

Peak intensity, counts/s (temp. °C)

Intensity at 280 °C, counts/s

Total count (22–280 °C)

n-C3 H7 NH2

20

20

123(53)

32

23,200

n-C3 H7 NH2

14

18

109(52)

12

20,400

(n-C3 H7 )2 NH

14

14

82(56), 16(235)

12

21,400

CH3 COOC2 H5

20

11

39(57)

1

8200

(CH3 )2 CHOH

20

8

34(55)

ca.0

7300

C2 H5 OH

20

10

32(55)

ca.0

6900

(CH3 )2 CO

20

11

31(55)

1

6200

C6 H5 CH3

16

6

31(57)

27

9200

CH3 OH

20

7

26(51)

ca.0

6000

C6 H6

20

5

19(61)

11

5000

CH3 CN

20

3

11(58)

3

2500

n-C4 H9 Cl

20

0.3

1.5(66)

–

430 (22–223 °C)

Average values are given. The values do not contain the background count rate obtained from the glow curves without the specimens

deformed sand (such as SiO2 embedded on the metal surface) might be facilitated by the formation of hydrogen bonds between the functional group of the organic and the hydroxyl group (–SiOH) of the sand surface.

7.1.2 TSEE from Ground Sand Granules (Aluminosilicate) and Adsorption of Organic Vapors We have investigated the relationship between TSEE from ground sand and the effects of the adsorption of organics as well as the sand particle size [9]. Sand was mechanically ground in a grinding chamber of stainless steel that was equipped with a modified Geiger counter. The organic vapor was a quenching gas for the counter gas. Grinding of the sand was carried out in the chamber for 15 min by placing sand grains (0.50 g) together with a mild steel rod, degassing for 15 min, and then rotating the rod with an externally placed magnetic stirrer. The grinding was performed under evacuation, in a vacuum, and in environments of various organic vapors and water vapor. After degassing the environment gas, the counter gas [which was usually composed of Ar (11,200 Pa) and organic vapor (2600 Pa), as well as other gases of Ar (12,500 Pa)–C6 H5 CH3 (1300 Pa) and Ar (12,200 Pa)–(n-C3 H7 )2 NH (1600 Pa)]

7.1 TSEE from Sandblasted Mild Steel and Ground Sand

113

was introduced into the counter. The TSEE glow curve (measured from 25 to 194 °C) strongly depended on the grinding environment and organic vapors. In all cases, the glow curve exhibited one emission peak in the vicinity of 40–60 °C, although the sand that was ground in C3 H7 NH2 vapor also sometimes produced two maxima (a sharp peak in the vicinity of 30 °C and a broad peak at ca. 60 °C), leading to a substantial increase in the intensity of electron emission. The TSEE characteristics were featured by the number of electrons that were emitted during the TSEE measurements (termed total count) and the peak temperature. The TSEE characteristics widely varied with the organic vapor in the counter gases. The total count of the glow curve with one emission peak for sand that was ground during evacuation increased in the following order: n-C4 H9 C1 ≪ CH3 CN < C6 H6 ≤ CH3 OH < C6 H5 CH3 ≈ (CH3 )2 CO ≈ C2 H5 OH < (CH3 )2 CHOH < CH3 COOC2 H5 ≪ (n-C3 H7 )2 NH < n-C3 H7 NH2 . A similar order was also obtained for the sand that was ground in the same type of organic vapor as the organics in the counter gases. Figure 7.1 shows the relationship between the intensity of the peak emission and the peak temperature in the glow curve for sand that was ground during evacuation. The organics represented in Fig. 7.1 denote the quenching gas in the counter gas for the TSEE measurements. The intensity of the peak emission tended to decrease with increasing peak temperature; although the data points for n-C3 H7 NH2 , (n-C3 H7 )2 NH, and CH3 COOC2 H5 considerably deviated. The aforementioned order closely resembles that of the proton-attracting power of the functional group of the organics. In accordance with [5], the proton affinity of the organic functional group increased in the following order: chloride < nitrile < ester < ketone < hydroxyl group < amine. This order of proton affinity is in good agreement with the aforementioned order of the total count. Thus, the EE is hypothesized to be closely related to the strength of hydrogen bonding formed between the functional groups of the organics and the surface hydroxyl groups of damaged sand. For example, the unshared electron pair of the N atom of the amine (R–NH2 ) abstracts a hydrogen ion from the surface hydroxyl group (–Si–OH) in a manner that forms a positively charged cation (R–NH3 + ; whereas the O atom of the surface hydroxyl group becomes negatively charged (–Si–O– ) by losing a hydrogen ion. The electric dipole (with a positive pole toward the outside) formed by the adsorbed organic is hypothesized to lower the surface potential barrier, leading to facilitated electron emission from the electron capture center that is present in the damaged sand. One can explain the dependence of the peak emission intensity on the peak temperature (Fig. 7.1) as follows. An organic with a high proton affinity for the surface hydroxyl group is strongly adsorbed and reduces the surface potential barrier for electron emission; one observes an emission peak with an increased intensity at lower temperature. Next, the effect of the particle size of ground sand on the TSEE was of substantial interest. The degree of particle size (S) of the ground sand was determined as follows. The ground sand was divided into three sizes (100-mesh) with two sieves. Then the weight percentage of the ground sand (divided to each size) was obtained. The opening of the sieves was 0.25 mm (60mesh), 0.149 mm (100-mesh), and 0.20 mm (average) for the intermediate of 60– 100-mesh. The value of S was defined as follows: S (units: millimeters) = (ax +

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Fig. 7.1 Relationship between the intensity of the peak emission and its temperature in the glow curves for organic vapors contained in the counter gases for ground sand during evacuation [9]. Reprinted with the permission from CSJ, Momose and Miyata [9]. Copyright© 1982

by + cz)/100; where a, b, and c represent the opening of the sieves: 0.25, 0.20, and 0.149 mm, respectively; and x, y, and z the respective weight percentages. The total count of the TSEE linearly decreased with increasing S. Thus, as the sand grain became finer by grinding, more electron-trapping centers accumulated on the particle surface. However, [3] found that when 40-mesh quartz sand was ground, the EE continued to increase and be saturated until the weighted average size (Tyler sieves) of the sand was ca. 80-mesh but gradually decreased and became constant as the sand became finer. Therefore, the effect of the grinding in our experiments might have been confined to a stage of increasing EE. Furthermore, regarding the effect of the particle size of ground sand, two experimental results were performed. (1) The total count of TSEE for the sand ground after sieving of 48–60-mesh was almost equal to that for the nonsieved sand (48–115mesh), whereas the total count for the sand ground after sieving of 100–115-mesh was reduced to ca. 2/3 of the value for the nonsieved sand. In the latter case, because we used a magnet to confirm that the ground sand contained a large quantity of iron, we attributed the reduction of the total count to the presence of iron with a small EE activity. (2) The TSEE glow curves for sand that was sieved after grinding were measured in the temperature range from 25 to 280 °C with another Geiger counter (counter gas of n-C3 H7 NH2 –Ar) after standing in air (28 °C and 60% relative humidity) following grinding. The sand (40 g) after grinding in a vacuum for 30 min after evacuation for 15 min was sieved to a larger grain (48–65-mesh) and a smaller grain (>100-mesh). Figure 7.2 shows the glow curves for the sand (1 g; the iron was removed) of two particle sizes together with the nonsieved sand [9]. The glow curves exhibited two broad peaks in the lower- and higher-temperature

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Fig. 7.2 Dependence of the glow curves on particle size of ground sand [9]. (1) 65 mesh > (202,000), (2) without sieving (169,000), (3) > 100 mesh (132,000). The detail of the experiment is referred to in the text [9]. Reprinted with the permission from CSJ, Momose and Miyata [9]. Copyright© 1982

regions. The particle size of ground sand ranged widely (from 48 to 250-mesh) and exhibited a reduced proportion of larger-grain sand compared with sand that was not ground. In the low-temperature region, the intensity of the emission peak for three particle sizes was close, but in the high-temperature region the intensity of the peak for the sand with a larger particle size became much more substantial than that with a smaller grain size. This trend is opposite to that of the effect of particle size of the sand on the TSEE with one emission peak described previously. This finding (i.e., sand that is rich in larger-size particles substantially contributed to the increase in the peak emission in the higher-temperature region) corresponds to the fact that the electron-trapping centers might be formed in a deeper area of the mechanically damaged SiO2 lattice. Finally, the proton-attracting power of the functional groups of organics that were adsorbed onto the surface hydroxyl groups (i.e., those that were present on the surfaces that were mechanically deformed or exposed to a discharge) explains the increase in the intensity of EE; as described for ground Al (Sect. 4.1), electrically discharged Fe (Sect. 6.2.1), sandblasted mild steel (Sect. 7.1.1), and ground sand (Sect. 7.1.2). Furthermore, we describe the effect of the particle size of abrasives on TSEE in Sects. 7.2.2 and 7.3.3.

7.2 EE from Metals and Plastics Blasted or Ground with Abrasive Agents 7.2.1 TSEE from Metals Blasted with Silicon Carbide (SiC) When we sandblasted metals (Ni and Cu) with carborundum grain (silicon carbide, SiC) in compressed N2 gas, and then maintained the metals for a given time in wet or dry air, a clear difference was evident in the TSEE glow curves [7]. Ni (purity, 99.9%) and Cu (99.98%) specimens were used. Prior to the experiments, the specimen was

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annealed in air for 30 min at ca. 400 °C. The glow curves were measured with a Geiger counter (counter gas: a mixture of Ar, 83 Torr; and C2 H5 OH, 20 Torr). The glow curves for Ni and Cu immediately after the blasting in room air (23 °C and 38% relative humidity) exhibited nearly the same broad peak, with an emission intensity as follows: 15–20 counts/s at ca. 100 °C. In Fig. 7.3, after the blasting the glow curves for the Ni samples changed with time spent in air (known as the keeping time) that was saturated with water vapor, termed wet air. A new emission peak (assigned to Ni) gradually increased near 180 °C at 1-h keeping time, and reached a maximum at 220 °C after 3 h; whereas a wide emission with a maximum that was initially observed at ca. 100 °C decreased with keeping time, to a reduced level of several counts/s. For the blasted Cu sample after keeping for 3 h in wet air, there was no new peak in the glow curve. SiC that was crushed into a fine powder with an agate mortar in air (22 °C, 40% relative humidity) and then kept in air for 3 h resulted in a broad peak of several counts/s at ca. 130 °C, and a total count of 2000 in the glow curve. Therefore, the emission that was evident at ca. 130 °C in Fig. 7.3 is hypothesized to originate from the deformed Carborundum that was embedded in the Ni metal surface after the blasting. A Ni sample that was kept for 3 h in air (dried with P2 O5 ) after the blasting produced a weaker peak at a lower temperature of 180 °C, with no emission peak. Regarding formation of H2 O2 during cutting of metals (Zn, Al, Mg, and Ni) in water that contains O2 , [1] indicates that these metals produced a considerable quantity of H2 O2 ; whereas Cu afforded little or no H2 O2 . These findings suggest that compared with Cu, freshly deformed Ni had a stronger affinity for water regarding formation of H2 O2 . Similarly, the Ni that was deformed by SiC-blasting resulted in stronger emission intensity in the glow curve than Cu. Therefore, the electronic properties on the blasted Ni surface might correspond to the formation of H2 O2 , and furthermore the thermal decomposition of the hydroxide that was formed on Fig. 7.3 Dependence of the glow curves for Ni metal surfaces blasted with SiC on keeping time in wet air after blasting [7]. Reprinted with the permission from AIP Publishing, Momose [7]. Copyright© 1969

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the Ni surface. When one substantially heats Ni(OH)2 and it undergoes dehydration, the Ni(OH)2 completely converts into NiO at 230 °C. Regarding Ni surfaces, the humidity of the air where the blasted sample is kept had a substantial influence on the TSEE.

7.2.2 EE from Plastics Abraded with Al2 O3 and SiC Recently, wear powder of plastics that are dispersed in the environment has been an active area of research. Researchers have abraded plastic sheets of poly(vinyl chloride) (PVC), polyethylene (PE), and poly(methyl methacrylate) (PMMA) with several types of sandpapers: AA (Al2 O3 ) and CC (SiC) [6]. EE measurements indicate that mechanically abraded PE and PMMA sheets produced a negatively charged surface. Figure 7.4 [6] shows the time change of the intensity of EE from plastics (size: 30 mm × 30 mm × 2 mm) at room temperature after abrasion with SiC (CC240) paper. The EE was measured with a Geiger counter (counter gas: a mixing gas of Ar and C2 H5 OH). EE with a considerably strong intensity was observed for PE and PMMA, but regarding PVC the intensity was almost the same as the natural count rate. A small quantity of the abrasive particle was attached to the plastic surfaces. Table 7.2 [6] shows the intensity of EE from PMMA sheets—3 min after abrasion— with various abrasives and a steel brush. The number of abrasives refers to the size of the abrasive particle, in which the particles are increasingly fine with increasing number. In both abrasives the particle with larger particle size exhibited a higher intensity. This tendency is similar to that obtained for the ground sand indicated in Fig. 7.2. The origin of EE might be attributable to the damaged plastics in addition to the abrasive particles that become embedded on the surface, because abrasion with a steel brush results in EE. All of the plastics (PVC, PE, and PMMA) that were exposed to a spark discharge from a Tesla coil in air produced little EE; the emission level was nearly the same as the natural count rate.

7.3 TSEE from Metal Surfaces Subjected to Cutting and Grinding 7.3.1 TSEE from Al Surfaces Cut with a Tool Steel and Effect of Cutting Fluids The effect of commercial cutting fluids on TSEE from Al surfaces that one produces by orthogonal cutting is an active area of research. We describe the dependence of TSEE on the fluids from the interactions between the fluid molecules and the surface hydroxyl groups (–AlOH) on the surface [13]. A commercial metal plate (purity, >99.5%; size: 10 mm × 10 mm × 10.5 mm) was used. The specimens were

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Fig. 7.4 Time change of the intensity of EE from plastics sheets after abrasion with sandpaper of SiC (CC-240) [6] with the permission from Momose [6]. Copyright© 1970

Table 7.2 Intensity of EE from PMMA abraded with various abrasives and a steel brush

Abrasives

Intensity of EE at 3 min after the abrasion

AA-80

>10 count/s

AA-240

3.6

CC-80

6.4

CC-240

4.5

Steel brush

3.8

Natural count rate

0.8

degreased with benzene and then annealed under vacuum for 1 h at 350 °C before use. Cutting fluids of five types were prepared: (S) spindle oil (#60), (P) phosphoric ester, (S + P) a mixture of these fluids (98% the former and 2% the latter by volume), (E) emulsifiable oil (a mixture of 95% water and 5% mineral oil by volume), and (W) water. We will refer to the action of P on the surface hydroxyl groups on the metal surface in a subsequent paragraph. Orthogonal cutting was performed with a horizontal milling machine in room air. Cutting conditions were as follows: tool, high-speed tool steel with a square nose; tool rake angle (α), 0 °C to 30° at a fixed relief angle of 10°; cutting speed, 30–2800 mm/min; depth of cut (d), 0.05–0.20 mm. The cutting fluids were thinly coated onto the metal surfaces with a brush immediately before and after the cutting. TSEE glow curves of the metal surfaces—in the presence or absence of cutting fluids—were measured from 25 to 300 °C in the dark with a

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modified gas-flow Geiger counter [counter gas: Q gas, composed of He (99%) and isobutane (1%)]. For each cutting experiment, five specimens were used to check the reproducibility. A voltage of AV = 100 V was applied between the earthed grid of the counter and the specimen during TSEE measurement. Figures 7.5 and 7.6 show typical glow curves and the total counts of emitted electrons, respectively, for cutting in the absence of fluids [13]. One broad emission peak was evident near 150 °C, and the values of the total counts of the emitted electrons increased with increasing cutting speed. The values of the total counts increased and then became constant with increasing depth of the cut. Regarding cutting in room air at high humidity, the emission was markedly low. The TSEE glow curves for the specimens that were completely coated with the fluids differed from those without fluids. The TSEE glow curves for each fluid were as follows: (1) For (S), the emission increased with increasing temperature near 190–240 °C for the oil-coated specimen before cutting, and near 230–260 °C for the oilcoated specimen after cutting. The total counts of emitted electrons for the former surface were slightly more substantial than the total counts for the latter surface. (2) For (P), with increasing temperature the emission for the oil-coated specimen before the cutting rapidly increased and resulted in two emission peaks (near 65–80 °C and 125–130 °C), followed by a rapid decrease; emission was no longer evident near 155–165 °C. The oil-coated specimen after cutting resulted in one peak with a much weaker intensity, near 65–90 °C (Fig. 7.7).

Fig. 7.5 Typical TSEE glow curves for two Al surfaces cut without fluid at different depth of cut (d) at the fixed values of α (tool rake angle) = 20° and V (cutting speed) = 300 mm/min:: (1) d = 0.05 mm, (2) d = 0.15 mm [13]. Reprinted with the permission from JSLE (Jpn. Soc. Tribologists), Ohshima et al. [13]. Copyright© 1985

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Fig. 7.6 A plot of total counts versus cutting speed for Al surfaces cut without fluids [13]. Reprinted with the permission from JSLE (Jpn. Soc. Tribologists), Ohshima et al. [13]. Copyright© 1985

Fig. 7.7 TSEE glow curves for the Al surfaces coated with phosphoric ester (P) before cutting (a) and after cutting (b). The cutting conditions: V = 300 mm/min, d = 0.10 mm, and α = 0° [13]. Reprinted with the permission from JSLE (Jpn. Soc. Tribologists), Ohshima et al. [13]. Copyright© 1985

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(3) For (S + P), this mixed fluid resulted in a striking observation (Fig. 7.8). The mixed-oil-coated specimens before the cutting substantially exhibited enhanced emission, several tens of times larger than the specimens coated simply with the fluid of S and P, and resulted in a strong peak near 165 °C. This peak was only located in the temperature region where there was no emission for the fluids of simply S and P. Regarding the mixed-oil-coated specimens after cutting, there was an emission peak with a decreased intensity near 110 °C, but this decrease was followed by a rapid increase in the emission at >230–260 °C. Furthermore, regarding cutting of the specimens that were coated with the fluid (P) and mixture (S + P), the cutting force was markedly reduced and the surfaces became smooth, compared with specimens that were not coated with fluids or simply coated with the fluid (S). (4) For (E), the oil-coated specimens before cutting resulted in one emission peak with a strong intensity near 200–250 °C, whereas the specimens after cutting exhibited an emission that increased with increasing temperature near 150 °C. (5) For (W), the emission from the coated specimens before cutting started to slowly increase with increasing temperature near 100 °C, whereas specimens after cutting resulted in an emission that rapidly increased with increasing temperature near 200 °C. Finally, it has been concluded that the value of the total counts of emitted electrons for the specimens that were coated with fluids before cutting decreased in the following order: (S + P) >> (E) > (P) > (W) > (S). The presence of the phosphorus element on the cut surface was confirmed by XPS measurements. The fact that a polar compound (such as a phosphoric ester in the hydrocarbon oil, such as spindle

Fig. 7.8 TSEE glow curves for the Al surfaces coated with a mixed fluid (98% spindle oil (S) and 2% phosphoric ester (P) by volume) before cutting (a) and after cutting (b). The cutting conditions: V = 300 mm/min, d = 0.10 mm, and α = 0° [13]. Reprinted with the permission from JSLE (Jpn. Soc. Tribologists), Ohshima et al. [13]. Copyright© 1985

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oil) corresponded to the most enhanced emission strongly suggests that electrons can accumulate—or become abundantly trapped—in the vicinity of the oxide layer on the Al metal surface. Furthermore, regarding phosphate and phosphite, we will confirm in more detail the effect of various cutting fluids on the TSEE glow curves of cut Al metal surfaces [15]. Table 7.3 lists cutting fluids consisting of naphthene-base oil (#1); and cutting fluids mixed with Cl, S, and P-based extreme-pressure additives (#2–#7). Al specimens (purity, >99.5%) were degreased in benzene and then annealed under vacuum for 1 h at 350 °C. These cutting fluids were thinly spread onto the metal surface and the cutting tool with a brush before cutting. Orthogonal cutting was performed with a high-speed steel tool. The TSEE was measured in the dark from 25 to 300 °C with a modified gas-flow type Geiger counter (counter gas: Q gas). Figures 7.9, 7.10, 7.11 and 7.12 show the TSEE glow curves for each fluid. Figure 7.9 indicates that the surfaces without fluid (dry cutting) and simply the base oil (#1) resulted in completely different glow curves. Regarding the former, the emission started to slowly increase at ca. 50 °C and increased with increasing temperature; a broad peak was reached at ca. 200 °C, followed by a decrease. Regarding the latter, the commencement temperature of the emission substantially increased (ca. 120 °C), and the emission gradually increased with increasing temperature; a peak was reached at ca. 300 °C. Figure 7.10, shows the glow curves for fluids of #2 and #3 (containing a Cl-based additive). These glow curves were reproducibly observed. The emission started near 130 °C for #2 fluid and near 210–245 °C for #3 fluid. There was no emission peak; the emission simply increased with temperature. The total count of emitted electrons for #2 fluid was much more substantial than that for #3 fluid. This might be attributable to the difference in the chlorinated materials: paraffin and fat. Figure 7.11 shows the glow curves for fluids #4 and #5 (containing an S-based additive). The emission for #4 fluid started to increase at a slightly lower temperature than that for #5 fluid, and the total count for #4 fluid was more substantial than that for #5 fluid. Figure 7.12 shows the glow curves for fluids #6 and #7 (containing a P-based additive). The intensity of the emission was drastically increased; more strongly than that for fluids containing Cl- and S-based additives. The glow curves for #7 fluid exhibited a clear peak with a strong intensity at ca. 200 °C, followed by a rapid decrease. Figures 7.13 and 7.14 show the values of the total counts of emitted electrons during the heating of the glow curves and the commencement temperature of the emission, respectively. These characteristics were strongly influenced by the type of fluids used. In Fig. 7.13, the value of the total counts increased in the following order: −Cl (chlorinated fat) < –S–(dialkyl polysulfide) < O = P (OC6 H4 CH3 )3 (tricresyl phosphate) < [:P (–OC8 H16 )3 ] (trioctyl phosphite). Here, the Cl, S, and P atoms in these compounds have an unshared electron pair. The unshared electron pair is represented in the chemical formula of trioctyl phosphite. In particular, with the fluid containing [:P (–OC8 H16 )3 ], the value of the total counts of emitted electrons was strongly increased compared with the other fluids, and an emission peak with a strong intensity was distinct near 200 °C. The enhanced emission has been hypothesized to be associated with the proton-attracting power of the P atom of the additive to the

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Table 7.3 Cutting fluids (including Cl-, S-, and P-based additives) for Al surfaces subjected to cutting Fluid number

Component

Content

#1

Base oil [naphthenic mineral oil, 8 cSt (40 °C)]

#2

#1 + 3 vol.% Chlorinated paraffin

(Cl = 50 wt.%)

#3

#1 + 3% Chlorinated fat

(Cl = 35%)

#4

#1 + 3% Dialkyl polysulfide

(Inactive, S = 20%)

#5

#1 + 3% Dialkyl polysulfide

(Active, S = 32%)

#6

#1 + 3% Tricresyl phosphate

(P = 8.4%)

#7

#1 + 3% Trioctyl phosphite

(P = 7.4%)

Fig. 7.9 Typical TSEE glow curves for Al surfaces cut without fluid (dry) and with a base oil (#1) [15]. Reprinted with the permission from Ohshima et al. [15]. Copyright© 1988

Fig. 7.10 TSEE glow curves for Al surfaces cut with fluids containing Cl-based additive (#2 and #3) [15]. Reprinted with the permission from Ohshima et al. [15]. Copyright© 1988

hydrogen atom of the surface hydroxyl groups (–AlOH) that are present on the cut Al surfaces; i.e., this interaction forms a dipole (with positive outward pole), represented by −AlO– + H:P(–OC8 H16 )3 . This orientation of the dipole enhances the emission of electrons from the metal substrate. Furthermore, Fig. 7.14 shows that the commencement temperature of the emission in the glow curves substantially differed between

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Fig. 7.11 TSEE glow curves for Al surfaces cut with fluids containing S-based additive (#4 and #5) [15]. Reprinted with the permission from Ohshima et al. [15]. Copyright© 1988

Fig. 7.12 TSEE glow curves for Al surfaces cut with fluids containing P-based additive (#6 and #7) [15]. Reprinted with the permission from Ohshima et al. [15]. Copyright© 1988

the additives in the fluids: the temperature was reduced in the following order: – Cl (chlorinated fat) > –S–(dialkyl polysulfide) > O = P (OC6 H4 CH3 )3 (tricresyl phosphate) ≈:P (–OC7 H16 )3 (trioctyl phosphite). This order corresponds with the increasing order of the proton affinity of the functional groups of the additives. The values of the proton affinity and adsorption activity of related compounds are as follows [2, 4, 11]. In accordance with [4], the proton affinity for molecules with an unshared electron pair on the Cl, S, and P atoms increases in the following order: HCl (576 kJ/mol) < H2 S (639 kJ/mol) < (CH3 )3 P (944 kJ/mol). This is considered to be applied to the order of the proton-attracting interaction of the functional groups of the present extreme-pressure additives with the hydrogen atom of the surface hydroxyl groups on the cut metal surface. [2] reported that the proton affinity of

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trimethyl phosphite {P(OCH3 )3 } (= 222.9 kcal/mol) is more substantial than that of trimethyl phosphate {O = P(OCH3 )3 } (= 214.2 kcal/mol) {relative to PA(NH3 ) = 206.0 kcal/mol). Furthermore, [10] and [11] reported that one can represent the ability of a compound to adsorb onto a metal surface by an experimental value termed the adsorption activity (m × 102 s−1 ; the value of {(CH3 O)3 P} (= 2.8 s−1 ) is larger than that of {(CH3 O)3 PO}(= 0.8 s−1 ). These results indicate that the difference in the total counts of emitted electrons for trioctyl phosphite and tricresyl phosphate in Fig. 7.13 might be explained by their different proton affinities and adsorption activities. It is concluded that a fluid containing a compound with a higher proton affinity increases the total counts of EE and lowers the commencement temperature of EE from cut Al surfaces; although regarding trioctyl phosphite and tricresyl phosphate, the commencement temperature is close to each other. Spikes [16] and Wan et al. [17] have published excellent overviews of lubricating oil additives, zinc dialkyldithiophosphates (ZDDPs, ZnDTP), and phosphate-based lubricants. ZDDPs are well-known and widely used because of their antiwear, corrosion inhibition, and antioxidant properties. Although phosphorus compounds can actively react with steel surfaces in a manner that provides lubrication, the working mechanism of polyphosphate-lubricated interfaces remains unclear. In Sects. 6.2.1 and 8.3.1, we emphasize that one can explain the interactions of metal surfaces with molecules in the environment based on the following: the bond formation of the surface hydroxyl group on the Fe metal surface with the functional group of the adsorbed molecules. The interaction mode that researchers propose for the EE mechanism might be applied to the interactions between the lone electron pair of the O atom of the polyphosphate and the H+ ion of the surface hydroxyl group on the metal surface, which generates a dipole such as −Fe−O− H+ [: O = P≡]. Thus, the polyphosphate can adsorb strongly and tightly onto the metal surface (through the Fig. 7.13 Relation between the total counts of emitted electrons and the cutting fluids for cut Al surfaces [15]. Reproduced with the permission from Ohshima et al. [15]. Copyright© 1988

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Fig. 7.14 Relation between the commencement temperature of the emission and the cutting fluids for cut Al surfaces [15]. Reproduced with the permission from Ohshima et al. [15]. Copyright© 1988

bond formation between the surface hydroxyl group and the unshared electron pair of the O atom of the phosphate), which results in an excellent stable layer.

7.3.2 EE from Metals During Cutting with WC and Friction One requires data on EE from metal surfaces that were mechanically deformed without fluids under various conditions to characterize the deformed metal surface [12, 14]. Here, we consider aluminum (Al) (purity, >99.5%), brass (Bs (60% Cu, and carbon steel (S45C (0.45% C specimens that were deformed, by (1) cutting and (2) dry-friction, under various conditions [14]. The TSEE measurements were performed with a gas-flow type Geiger counter (counter gas: Q gas; which was assembled onto a lathe-like, peripheral-cutting device. Regarding cutting, a cemented tungsten carbide (WC) tool was used. Friction was conducted by pressing a rectangular metal tool against a rotating metal bar along the line of contact at an applied load. The cutting and friction speed (V ) ranged between 5 and 45 m/min, the depth of the cut (D) between 0.05 and 0.2 mm, the feed (F) between 0.05 and 0.2 mm/rev, and the applied load (L) between 4.9 and 19.6 N/cm. The metal specimens (S) were cut and rubbed in a Q gas atmosphere, in the dark at room temperature. Regarding the emission during cutting, the emission intensity (EI) considerably fluctuated with cutting time. Therefore, the EI values are expressed as the average for the total cutting time (13–113 s). For all specimens, the EI increased with increasing cutting speed, but the magnitude of the EI strongly depended on the metals (Fig. 7.15). The EI at the same cutting speed decreased in the order Al >> Bs >> S45C. The EI value also increased with increasing depth of cut for Bs.

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Fig. 7.15 Plots of emission intensity (EI) from metals during cutting with WC versus cutting speed: a Al, b brass, c S45C (1985 The Japan Society of Applied Physics) [14]. Reproduced with the permission from The Japan Society of Applied Physics [14]. Copyright© 1985

Regarding the friction, a rotating metal bar (Bs and S45C) was pressed with a rectangular metal tool (Al, Bs, and S45C) at various applied loads. The EI with a Bs (bar)–Al (tool) couple gradually increased at the initial stage and subsequently varied more irregularly with increasing friction time. The EI values were obtained from the average of the total counts, for a friction time of 6 min. Figure 7.16 shows the effect of friction speed and applied load on the EI for two couples: Bs (bar)–Al (tool) and Bs (bar)–S45C (tool). The EI increased and subsequently tended to become saturated with increasing friction speed. The EI also had a tendency to increase slightly with increasing applied load at the same friction speed. Figure 7.17 shows the effect of the applied load on the EI for two couples: (1) S45C (bar)–Al (tool) and (2) S45C (bar)–Bs (tool). The EI considerably increased with increasing applied load, with a strongly increased EI for couple 1 compared with couple 2. These results will facilitate characterization of mechanical deformation in the context of EI data from metals that are subjected to cutting and friction.

7.3.3 TSEE Under Light Illumination from Low-Carbon Steel Surfaces Ground with Al2 O3 We have reported the effect of the grinding operational conditions on the TSEE glow curves for ground low-carbon steel [12]. Two types of steel specimens (A and B) were used as work. The chemical compositions (mass percent) of work A were Fe (bal.), C (0.019), Si (0.001), Mn (0.34), P (0.006), and S (0.012); and those of work B were Fe (bal.), C (0.029), Si (0.001), Mn (0.29), P (0.009), and S (0.023). The contents of S and P for work A were smaller than those of work B. Prior to use, the specimens were degreased with benzene and then annealed under vacuum for 1 h at

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Fig. 7.16 Effect of friction speed and applied load on emission intensity (EI) during friction process for two couples: Bs (bar)–Al tool (left side) and Bs (bar)–S45C tool (right side) (1985 The Japan Society of Applied Physics) [14]. Reproduced with the permission from The Japan Society of Applied Physics [14]. Copyright© 1985

Fig. 7.17 Effect of applied load on emission intensity (EI) during friction process for two couples: S45C (bar)–Al tool (left side) and S45C bar–Bs tool (right side) (1985 The Japan Society of Applied Physics) [14]. Reproduced with the permission from The Japan Society of Applied Physics [14]. Copyright© 1985

920 °C. Grinding wheels composed of white fused alumina were used. The grinding was performed at the following conditions: grinding methods, up-cut or down-cut; grain size (#) of wheel, #46, #54, #60, #80, and #120; peripheral wheel speed (V), 1400–2200 m/min; work speed (v), 6–18 min/min; depth of cut (d), 10–30 µm. The surface of the grinding wheel was renewed with a diamond dresser before each grinding. The glow curves were measured from 25 to 285 °C {under light illumination with a tungsten lamp (100 V, 20 W)} with a modified Geiger counter (counter gas: Ar and C2 H5 OH vapor). The glow-curve characteristics were as follows: (1) In Fig. 7.18, in all cases the glow curves exhibited two emission peaks at ca. 100 and 200 °C. The second peak corresponded to the alumina grain that was

7.3 TSEE from Metal Surfaces Subjected to Cutting and Grinding

129

Fig. 7.18 Effect of grinding method on the EE glow curves for rolled carbon steel sheet ground with WA54I7V grinding wheel under the conditions (work: A, V: 1800 m/min, v: 9 m/min. d: 20 µm): (1) 20 µDown-0 µUp, (2) 20 µUp, (3) 20 µUp–0 µDown, (4) 20 µDown [12]. Reproduced with the permission from Ohshima and Momose [12]. Copyright© 1988

embedded in the surface, because the abrasive grain resulted in an emission peak at ca. 200 °C. (2) In Fig. 7.19, for work A the values of the total counts for the down-cut surfaces were much larger than those for the up-cut surfaces. This finding suggests that during grinding, the down-cut surfaces underwent substantial damage; the production of defects and strain were highly enhanced. (3) In Fig. 7.20, for work A the values of the total counts for the up-cut grinding increased with increasing number of the grain size of the grinding wheel. The first peak intensity substantially increased with decreasing grain size, whereas the second peak intensity negligibly changed. (4) In Figs. 7.21 and 7.22, for work B the values of the total counts for the up-cut grinding decreased with an increase in the peripheral wheel speed and the work speed. Further, regarding the effect of the depth of the cut, the values of the total counts were almost the same at the depths of the cut of 10 and 20 µm, but decreased at the depth of cut of 30 µm. The comparison of grinding conditions suggests that the grinding at heavier conditions tended to weaken the emission intensity. Finally, it is concluded that for mechanically deformed surfaces, abrasives {SiO2 /Al2 O3 (Sects. 7.1.1), SiC (Sect. 7.2.1),WC (Sect. 7.3.2), and Al2 O3 (Sect. 7.3.3)} and mechanical deformation conditions—as well as the adsorption of environmental molecules (Sects. 7.1.2 and 7.3.1)—of particles that are embedded in metal surfaces as well as in the metal had a substantial influence on the EE.

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7 Effects of Blasting and Grinding Agents as Well as Cutting …

Fig. 7.19 Relation between grinding method and total count of emitted electrons for work A ground under the conditions (grinding wheel: WA54I7V, V: 1800 m/min, v: 9 m/min, and d: 20 µm) [12]. Reproduced with the permission from Ohshima and Momose [12]. Copyright© 1988

Fig. 7.20 Relation between grain size of grinding wheel (WA-I7V) and total count of emitted electrons for work A ground under the conditions (V: 1800 m/min, v: 9 m/min, and d: 20 µm) [12]. Reproduced with the permission from Ohshima and Momose [12]. Copyright© 1988

7.3 TSEE from Metal Surfaces Subjected to Cutting and Grinding Fig. 7.21 Relation between grain size of wheel speed and total count of emitted electrons for work B ground under the conditions (grinding wheel: WA54I7V, v: 9 m/min, and d: 20 µm) [12]. Reproduced with the permission from Ohshima and Momose [12]. Copyright© 1988

Fig. 7.22 Relation between grain size of work speed and total count of emitted electrons for work B ground under the conditions (grinding wheel: WA54I7V, V: 1800 m/min, and d: 20 µm) [12]. Reproduced with the permission from Ohshima and Momose [12]. Copyright© 1988

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References 1. L. Grunberg, The formation of hydrogen peroxide on fresh metal surfaces. Proc. Phys. Soc. Sect. B 66, 153–161 (1953) 2. R.V. Hodges, T.J. McDonnel, J.L. Beauchamp, Properties and reactions of trimethyl phosphite, trimethyl phosphate, triethyl phosphate, and trimethyl phosphorothionate by ion cyclotron resonance spectroscopy. J. Am. Chem. Soc. 102(4), 1327–1332 (1980) 3. S.A. Hoenig, Monitoring the ball milling process by means of exo-electron emission. Mining Congr. J. 58(11), 34–35 (1972) 4. J.E. Huheey, in Inorganic Chemistry, Principles of Structure and Reactivity, 3rd edn. Harper & Row, New York (1983). Japanese translation by G. Kodama, H. Nakazawa, Tokyo Kagaku Dojin (Tokyo, 1984), p. 300 5. D.H. Kaelble, 2 Intermolecular Forces and Structures, in Physical Chemistry of Adhesion. (Wiley-Interscience, New York, 1971), pp.45–83 6. Y. Momose (1970) Dissertation Tohoku University 7. Y. Momose, Exo-electron emission due to interaction of water vapor with copper and nickel sandblasted with carborundum. J. Appl. Phys. 40(10), 4215–4217 (1969) 8. Y. Momose, Influence of organic vapours on exo-electron emission from sandblasted mild steel. Zeitschrift Physik A Hadrons Nuclei 250(3), 198–206 (1972) 9. Y. Momose, K. Miyata, Thermally stimulated exoelectron emission from sand (aluminosilicate) subjected to grinding and its relationship to adsorption of polar organic vapors. Nippon Kagaku Kaishi 9, 1435–1442 (1982) 10. S. Mori, Y. Imaizumi, Adsorption of model compounds of lubricant on nascent surfaces of mild and stainless steels under dynamic conditions. STLE Trans. 31(4), 449–453 (1988) 11. S. Mori, M. Sugimoto, A. Hareyama, Adsorption of lubricating oil components on the cut steel surface. Hyomen Kagaku 5(1), 22–27 (1984) 12. I. Ohshima, Y. Momose, in Characterization of Ground Low-Carbon Steel Surfaces by Means of TSEE. Proceedings of the 9th International Symposium on Exoelectron Emission and Applications, vol. 1 (Wroclaw, 1988), pp. 270–277 13. I. Ohshima, Y. Momose, K. Kawafune, in Characterization of Aluminium Surface Produced by Cutting by Means of Exoelectron Emission Phenomena. Proceedings of the JSLE International Tribology Conference (Tokyo, 1985), pp. 99–104 14. I. Ohshima, Y. Momose, K. Kawafune, Exoelectron emission from metals in the process of cutting and friction. Proceedings 8th international symposium on exoelectron emission and applications, Osaka. Jpn. J. Appl. Phys. 24(S4), 190–193 (1985) 15. I. Ohshima, Y. Momose, R. Murata, in Influence of Cutting Fluids on TSEE Glow-Curve Characteristics of Cut Aluminium Surface. Proceedings of the 9th International Symposium on Exoelectron Emission and Applications, vol. 1 (Wroclaw, 1988), pp. 246–253 16. H. Spikes, The history and mechanisms of ZDDP. Tribol. Lett. 17(3), 469–489 (2004) 17. S. Wan, A. Kiet Tieu, Y. Xia, H. Zhu, B.H. Tran, S. Cui, An overview of inorganic polymer as potential lubricant additive for high temperature tribology. Tribol. Int. 102, 620–635 (2016)

Part IV

TAPE, TPPE, TriboEE, and XPS Characteristics of Processed Surfaces

Chapter 8

TAPE of Rolled and Scratched Fe Metal Surfaces

Abstract We analyze the results of TAPE and XPS of rolled as well as scratched Fe surfaces and corresponding Arrhenius plot analyzes. We obtained the experimental data as a function of three variables: temperature, photon energy of incident light, and scratching environment. This study will facilitate understanding of electron transfer across practical metal surfaces in fields such as tribology, adhesion, catalysis, and corrosion.

8.1 Temperature Dependence of PE from Rolled Fe Surfaces We describe several studies of thermally assisted photoelectron emission (TAPE) from Fe metal surfaces that we performed with a refined measurement apparatus. Momose et al. [12] investigated the effect of temperature and photon energy on the quantum yield Y s (defined as the emitted electrons per incident photon) for practical iron metal surfaces. Here, practical surfaces refer to commercial rolled and then ambient air-exposed surfaces. The temperature dependence of the Y s values was analyzed with the Arrhenius equation. Furthermore, after TAPE measurements at various temperatures, the surface chemical composition—as well as the surface potential change—was examined by X-ray photoelectron spectroscopy (XPS) and surface potential difference (SPD) measurements. Thermal desorption spectroscopy (TDS) measurements were also carried out for a sample that had not undergone TAPE measurements. The temperature and photon energy dependence of the Y s values were related to the XPS, SP, and TDS results. The experiments were conducted as follows. Prior to use, Fe samples (purity 99.5%; thickness 0.1 mm) were ultrasonically cleaned in acetone, followed by drying under vacuum and then storage in a desiccator. The TAPE measurements were carried out with a gas-flow Geiger counter under a mixing gas of 99% He and 1% isobutane (iso-C4 H10 ) vapor as a quenching gas, at normal atmospheric pressure in the temperature range 25–353 °C, under UV irradiation at various wavelengths (λ). The mixing gas is termed Q gas. The heating rate was 20 °C/min. The values of λ were 200, 210, 220, and 230 nm. Samples were irradiated with light at a certain λ value while © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 Y. Momose, Exoemission from Processed Solid Surfaces and Gas Adsorption, Springer Series in Surface Sciences 73, https://doi.org/10.1007/978-981-19-6948-5_8

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8 TAPE of Rolled and Scratched Fe Metal Surfaces

they were heated to a chosen maximum temperature T max and then maintained at this temperature for ca. 5 min. The chosen T max values were 25, 108, 152, 203, 254, 303, and 353 °C. Figure 8.1 shows the experimental setup for measuring TAPE, which consisted of the following: (1) electron measuring chamber filled with flowing Q gas, (2) light illumination system, (3) electron counting system, and (4) heating system. Figure 8.2 shows examples of the electron emission intensity as a function of time during scanning under UV irradiation at two wavelengths. The Y s values at the T max values increased with increasing T max , particularly more rapidly with wavelengths of greater photon energy. The Arrhenius equation outputs the temperature dependence of the rate constant of a chemical reaction. In the present work, we applied the Arrhenius equation to the Y s values. The Arrhenius-type equation for the sets of Y s and T max under irradiation with a given λ is represented by (8.1): Ys = Y0 exp(−ΔE a /kB T ),

(8.1)

where Y 0 is the preexponential factor, ΔE a is the activation energy, k B is Boltzmann’s constant, and T is the temperature of T max in kelvin. Figure 8.3 shows Arrhenius plots of Y s against T max at various λ values. The linearity was considerably good over the entire temperature range from 25 to 353 °C for all of the λ values, which indicates that the quantum yield obeys the Arrhenius-type equation. Table 8.1 shows the Arrhenius plot parameters and the corresponding characteristics of the incident light. ΔE a increased from 0.040 to 0.112 eV, and Y 0 increased from 0.036 × 10–8 to 1.47 × 10–8 emitted electrons/photon, with increasing photon

Fig. 8.1 Experimental setup for measuring thermally assited photoemission [12]. Reprinted with permission from Springer Nature, Momose et al. [12]. Copyright© 2014

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Fig. 8.2 Examples of the electron emission intensity as a function of time during temperature scanning under UV irradiation with λ = 200 nm (A) and λ = 210 nm (B). A sample was heated to T max at the constant heating rate and then maintained at this temperature. The arrows on each curve denotes the time taken to reach T max , which were a 353, b 254, c 152, and d 108 °C, and the curve e 25 °C was for no heating [12]. Reprinted with permission from Springer Nature, Momose et al. [12]. Copyright© 2014

energy from 5.391 to 6.199 eV. In Table 8.1, the mean square errors were added to the values of the Arrhenius plot parameters. The results of XPS, SP, and TDS are given next. Figure 8.4 shows the temperature dependence of the N 1 s, Fe 3p, O 1 s, and C 1s compositions, averaged for all of the λ values. The temperature dependence of the surface compositions was as follows: (1) The C 1s composition predominated at lower temperatures 254 °C; opposite to the relationship in terms of the C 1s composition. (3) The Fe 3p composition gradually increased with increasing temperature and became nearly constant at >303 °C. (4) The N 1s composition tended to predominate at temperatures 254 °C. The sign of the SPD values clearly changed from positive to

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8 TAPE of Rolled and Scratched Fe Metal Surfaces

Fig. 8.3 Arrhenius plots for the quantum yield, Y s , at the T max values obtained for different λ values: a 200 nm, b 210 nm, c 220 nm, and d 230 nm [12]. Reprinted with permission from Springer Nature, Momose et al. [12]. Copyright© 2014

Table 8.1 Dependence of Arrhenius plot parameters (activation energy ΔE a and preexponential factor Y 0 ) on the quantum yield Y s, in terms of the incident light wavelength λ, photon energy hν, and photon intensity I hν λ/nm

hν/eV

I hν /1011 photons/s

ΔE a /eV

Y 0 /10−8 emitted electrons/photon

200

6.199

1.07

0.112 ± 0.001

1.47 ± 0.04

210

5.904

1.28

0.100 ± 0.002

0.57 ± 0.03

220

5.636

1.25

0.073 ± 0.003

0.15 ± 0.02

230

5.391

1.34

0.040 ± 0.003

0.036 ± 0.003

negative between 203 and 254 °C. Figure 8.6 shows the TDS spectra. The species that predominated at lower temperatures were m/z = 18 and 16, assigned to H2 O and OH, respectively; m/z = 2, assigned to H2; and m/z = 28. The m/z = 28 was relatively weak at lower temperatures; but sharply increased at >350 °C, reached a strong peak at 430 °C, and then rapidly decreased with increasing temperature. The m/z = 18 (H2O) curve exhibited three peaks at 120, 210, and 290 °C: α, β, and γ , respectively. The intensity of these peaks decreased in the order: α > γ > β. The m/z (OH) curve resulted in a similar pattern to that for m/z = 18 (H2O), but with a much weaker intensity. Therefore, the OH originated from the decomposition of H2O. The m/z for H2 corresponded to a broad peak over the temperature range 100 °C to 300 °C and again resulted in a small peak near 430 °C. The origin of the H2 species remains unknown. Regarding the m/z = 28 species (corresponding species include CO, C2 H4 , and N2 ), based on the temperature dependence of the N 1s composition (Fig. 8.4) and

8.1 Temperature Dependence of PE from Rolled Fe Surfaces

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Fig. 8.4 Change of the surface cmpositions of N 1s, Fe 2p, O 1s, and C 1s with temperature. Temperatures given in the figure are the T max values. XPS measurements were carried out after TAPE measurements under temperature scan to the T max values under light irradiation with fourwavelengths and subsequent wavelength scan at the same temperatures. The compositions averaged for the four wavelengths [12]. Reprinted with permission from Springer Nature, Momose et al. [12]. Copyright© 2014

the fact that the carbon species that was assigned to the CO component (which was observed at higher binding energy in the C 1s spectra) decreased with temperature, the m/z = 28 species originated from a carbon-containing material (such as CO and C2H4), but not N2.

Fig. 8.5 Change of the surface potential difference, SPD, with temperature. Temperatures given in the figure are the T max values. Surface potential (SP) measurements were carried out after TAPE measurements under under temperature scan to the T max values under light irradiation at λ = a 200 nm, b 210 nm, c 220 nm, and d 230 nm, and subsequent wavelength scan. The SPD values are the SP compared to that before TAPE measurements [12]. Reprinted with permission from Springer Nature, Momose et al. [12]. Copyright© 2014

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8 TAPE of Rolled and Scratched Fe Metal Surfaces

Fig. 8.6 The intensity of removed main species as a function of temperature by thermal desorption spectroscopy (TDS). The species (m/z) are a H2 O (18), b H2 (2), c OH (17), and d CO, C2 H4 (28). An Fe sample after only ultrasonical cleaning in acetone, followed by drying under vacuum, was heated at the heating rate of 20 °C/min under UHV of the TDS apparatus [12]. Reprinted with permission from Springer Nature, Momose et al. [12]. Copyright© 2014

The following two processes are considered to be related to the photoemission: penetration of incident light through the surface overlayer to electrons that are present inside the metal before its optical excitation and transfer of electrons that were excited in the metal through the surface overlayer to the surface. At present, although which process predominates remains unclear, we considered the former process from the XPS and TDS results. Removal of the aforementioned species with increasing temperature enables the incident light to penetrate through the surface layer into the metal, contributing to the increase in Y s . The surface overlayer (which contains water and weakly bound carbon materials) controls the penetration of the incident light and thus corresponds to the rate-determining step for the photoemission. Therefore, light penetration through the overlayer is an important preceding step to Spicer’s three-step model (mentioned in a subsequent paragraph). Regarding the effect of temperature on the electron emission intensity and the Y s values, thermal annealing is another important effect at the surface between the base metal and the overlayer [13]. Figure 8.7 indicates that as the temperature increased from 25 to 353 °C, the Y s value under irradiation with λ = 200 nm increased 9.3-fold, whereas the thermal energy of k B T increased from 0.025 to 0.054 eV by a factor of only 2.2 from the minimum to maximum temperature. These results indicate that the Y s value at λ = 200 nm at the maximum temperature was remarkably enhanced compared with the increase of the thermal energy. Regarding the dependence of the activation energy on the photon energy, although further research is necessary, we hypothesize the dependence to correspond to the

8.2 Wavelength Dependence of PE from Rolled Fe Surfaces

141

Fig. 8.7 Plots of the quantum yield, Y s , at the T max values against wavelength of incident light. T max = a 353 °C, b 303 °C, c 254 °C, d 203 °C, e 152 °C, f 108 °C, and g 25 °C [12]. Reprinted with permission from Springer Nature, Momose et al. [12]. Copyright© 2014

penetration of UV light through the surface layer (which contained water). We attribute the enhancement of the Y s with temperature to the accumulation of electrons in the vicinity of the metal surface together with the growth of iron cations (Fe3+ ), which corresponds to positive holes that are produced in the surface oxide layer by UV light [7].

8.2 Wavelength Dependence of PE from Rolled Fe Surfaces The temperature dependence of the photoemission quantum yield Y s of practical Fe surfaces under irradiation at wavelengths of 200, 210, 220, and 230 nm well-obeyed Arrhenius-type behavior and corresponded to activation energies ΔE a from 0.040 to 0.112 eV (increasing with increasing incident photon energy) [12]. This section investigates the following problems: Why the electron photoemission intensity gradually increased with increasing temperature, and how one can relate the activation energies reported in the previous paragraphs to the mechanism of the photoemission [11]. The measurement of TAPE during wavelength scanning of the incident light was added to the experimental procedure described in Sect. 8.1. A Fe sample was initially heated to temperatures ranging from 25 to 353 °C under light irradiation at wavelengths of 200, 210, 220, and 230 nm; and then, the wavelength

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8 TAPE of Rolled and Scratched Fe Metal Surfaces

was scanned from 300 to 160 nm at the final temperature. However, we used the Fowler–DuBridge theory [2–4] to analyze the photoelectric effect of the metals, and in so doing obtained a constant that pertains to the electron photoemission probability αA and photothreshold (photoelectric work function) φ. Regarding αA, A is Richardson’s constant (in the equation of thermionic emission); and α is the fraction of electrons that arrive at a unit area of the surface in unit time, absorb a quantum of energy, and escape when the unit intensity of the incident radiation falls onto the surface. From a plot of the square root of the electron photoemission intensity against the photon energy during scanning of the light wavelength, the values of αA and φ were obtained. Here, αA is the electron photoemission probability. The αA values increased with increasing temperature, and the φ values also increased with temperature. The intensity of TAPE was more substantially influenced by the temperature dependence of αA than the temperature dependence of φ. In applications of photoelectric effect theory, there has been much focus on determining φ, but the αA value has received less focus. Regarding the value of α, [2] remarked that it would be of substantial interest to measure experimentally the value of α for various metals with varying surface conditions. On the basis of the Arrhenius plots, αA had an activation energy ΔE αA = 0.096 eV. Furthermore, the XPS measurements were carried out after initial temperature and subsequent wavelength scans. The activation energies of the surface oxygen component ratio Z = O2– /(OH + O2– ) and the surface elemental composition ratio X = Fe/(O + N + C + Fe), obtained from the XPS data, were also determined based on their corresponding Arrhenius plots: ΔE Z = 0.113 eV and ΔE X = 0.039 eV, respectively. The values of ΔE Z , ΔE X , ΔE αA , and ΔE a = 0.112 − 0.040 eV closely corresponded to their respective quantum yields [12]. In other words, the increase in the emission intensity of TAPE with increasing temperature strongly corresponded to the increasing values of Z and X with increasing temperature. Thus, passage of the incident light through the surface overlayer was a rate-determining step for initiation of the photoemission, which was well-controlled by the temperature-dependent surface oxygen components and surface elemental compositions. Figure 8.8 shows the electron emission intensity during the wavelength-scanning process at various T max values. The emission intensity, expressed in units of cpm (counts per minute), increased with increasing photon energy E p , reached a maximum at ca. 6.33 eV, and then decreased. One can attribute the decrease to a reduction of the number of incident photons that arrive at the metal surface (attributable to the ambient air that is present in the light path of the illumination system). Integration of the curves in Fig. 8.8 results in the total number N T of electrons that are emitted during the wavelength scanning. In accordance with [4] and [2, 3], the principle of photoemission as a function of the incident photon energy is described. The photoelectric current per unit area for unit intensity of radiation I from a metal surface that is excited by radiation of frequency ν at temperature T (in kelvin) can be represented by the following (8.2) [1, 2]: I = α AT 2 ϕ[(hv − φ)/kB T )],

(8.2)

8.2 Wavelength Dependence of PE from Rolled Fe Surfaces

143

Fig. 8.8 Electrn emission intensity as function of incident photon energy, E p , during wavelength scanning at T max values of a 108 °C, b 203 °C, and c 353 °C after initial irradiation at λ = 200 nm [11]. Reprinted with permission from Springer Nature, Momose et al. [11]. Copyright© 2015

where h is Planck’s constant, φ is the photothreshold of the metal, k B is Boltzmann’s constant, α is a proportionality factor that pertains to the probability that an electron absorbs a quantum of incident light, A is the Richardson constant, and ϕ(x) refers to the universal function given by (8.3) ϕ(x) = π 2 /6 + x 2 /2 − (e−x − e−2x /22 + e−3x /32 − · · · ) for x ≥ 0.

(8.3)

The parameter x is (hν − φ)/k B T, and hν is the photon energy of an incident wavelength. Using the notation for the quantum yield Y FD and E p (instead of I and hν, respectively) in (8.2), and also (to a good approximation) replacing (8.3) with x 2 /2, one can use (8.4) as the approximate expression of Y FD : YFD = α A(hv − φ)2 /2kB2 .

(8.4)

Equation (8.4) indicates that one can determine the values of (αA/2k B 2 )1/2 and φ by using a straight line, obtained from the plot of the square root of Y FD against E p . In the present work, the values of αA were obtained by fitting (8.4) to the electron emission intensity data as a function of E p (Fig. 8.8). Figure 8.9 shows an example of a plot of the square root of the electron emission intensity against the incident photon energy. The plot clearly increased with increasing E p in the range 5.23–6.3 eV and thus yielded a straight line (a denoted in Fig. 8.9). One calculates the αA value, which has units of counts/(photon kelvin squared), from the slope β of this straight line, which has units of (counts per

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8 TAPE of Rolled and Scratched Fe Metal Surfaces

Fig. 8.9 A plot of the square root of electron emission intensity against incident photon energy, E p , during wavelength scanning from 300 to 170 nm at T max = 353 °C after initial irradiation at λ = 200 nm. The slope, β, necessary for obtaining the αA and the photothreshold, φ, were determined within the E p range of approximately 5.0–6.3 eV [11]. Reprinted with permission from Springer Nature, Momose et al. [11]. Copyright© 2015

minute)1/2 /electron volt, and the average photon intensity (photons/second) N over the aforementioned photon energy range by (8.5): α A = 2k 2B β 2 (1/60N )

(8.5)

The β increased approximately linearly, with a slightly deviated data point at 152 °C, with increasing temperature. The Y FD value in the present case is the quantum yield for the entire range of the photon energies that span the straight line, whereas the Y s given in [12] refers to the quantum yield at a certain wavelength. The N value was 1.24 × 1011 photons/s from the photon intensities for the λ values given in Table 8.1. The photothreshold φ was determined from the intercept of the straight line at the flat baseline (b denoted in Fig. 8.9). Figure 8.10 shows the XPS spectra of Fe 2p, O 1s, N 1s, C 1s, and Fe 3p; measured at various T max after initial irradiation at λ = 230 nm. In particular, the O 1s spectra greatly changed shape with temperature. The Fe 3p spectra also exhibited a change of shape with temperature, whereas there was little change in the Fe 2p spectra. Therefore, the Fe 3p composition was used for calculating the surface compositions of X. Figure 8.11 shows the temperature dependence of the O 1s spectra measured at various T max after initial irradiation at λ = 200 nm, and their resolved curves of OH and O2– . Arrhenius plots are given for αA (Fig. 8.12a), N T (Fig. 8.12b), Z = O2− /(OH + O2− ) (Fig. 8.13a), and X = Fe/(O + N + C + Fe) (Fig. 8.13b); which were measured at various T max after initial irradiation at λ = 200 nm. In both Figs. 8.12 and 8.13, the data points fell approximately on a straight line; except for the data points at 25 and 353 °C for Z, and at 25 °C for X (Fig. 8.13). The outlier data points at 25 °C in Fig. 8.13a, b were evident because the compositions of C 1s and O 1s changed little with increasing temperature from 25 to 108 °C (Fig. 8.4; [12]), whereas

8.2 Wavelength Dependence of PE from Rolled Fe Surfaces

145

Fig. 8.10 XPS spectra at different T max values after initial irradiation at λ = 230 nm: T max = a 25 °C, b 108 °C, c 152 °C, d 203 °C, e 254 °C, f 303 °C, g 353 °C [11]. Reprinted with permission from Springer Nature, Momose et al. [11]. Copyright© 2015

the values of αA (Fig. 8.12a) and N T (Fig. 8.12b) clearly increased with increasing temperature in this temperature range. This result indicates that the αA and N T more sensitively corresponded to increasing temperature than the compositions of O 1s and C 1s. However, this discrepancy remains unclear. The αA values apparently included temperature dependence and followed Arrhenius-type behavior. Table 8.2 shows the average values of the activation energies of αA, N T , Z, and X. The ΔE αA value was nearly twice as large as the ΔE NT . The ΔE Z value was 0.113 eV, obtained only for λ = 200 nm; and the ΔE X value was 0.039 eV, ca. one-third of the ΔE Z value. It is of much interest to examine how the activation energies of the photoemission pertain to those of the surface chemical structure. We compared the values of ΔE a (Table 8.1; [12]) and of ΔE αA , ΔE NT , ΔE Z , and ΔE X (Table 8.2, [11]). The fact that ΔE a = 0.112 eV at λ = 200 nm is in good agreement with the ΔE Z value and was slightly larger than ΔE αA ; furthermore, the values of ΔE NT and ΔE a = 0.040 eV at λ = 230 nm were close to the ΔE X . Because of the dehydration, the OH component decreased with increasing temperature, concomitant with the increasing O2− component. A close similarity between the activation energy values indicates that the photoemission in the temperature scan at 200 nm might correspond to the increasing Fe content, whereas the photoemission in the wavelength scan and in the temperature scan at 230 nm was strongly determined by the increase in the O2– component.

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8 TAPE of Rolled and Scratched Fe Metal Surfaces

Fig. 8.11 Typical curve resolution of the O 1s spectra at different T max after initial irradiation at λ = 200 nm into OH and O2− components. The open circles denote the obtained O 1s spectra, while the thick solid line and the two thin lines represent the fitting curve and the resolved curves of the two components [11]. Reprinted with permission from Springer Nature, Momose et al. [11]. Copyright© 2015 Fig. 8.12 Arrhenius plots for the electron photoemission probabilty, aA, obtained using (8.5) (a) and the total number, N T , of emitted electrons during the wavelength scanning (b) at different T max after initial irradiation at λ = 200 nm [11]. Reprinted with permission from Springer Nature, Momose et al. [11]. Copyright© 2015

8.2 Wavelength Dependence of PE from Rolled Fe Surfaces

147

Fig. 8.13 Arrhenius plots for Z = O2– /(OH + O2– ) (a) and X = Fe/( O + N + C + Fe) (b) at different T max after initial irradiation at λ = 200 nm. Z values were determined from OH and O2– intensities obtained from Fig. 9.11, while X values came from the surface compositions vey similar to those shown in Fig. 6 in [9.1]. The data points at T max = 25 and 353 °C (a) and that at 25 °C (b) have been excluded from the straight lines [11]. Reprinted with permission from Springer Nature, Momose et al. [11]. Copyright© 2015 Table 8.2 Activation energies of the electron photoemission probability αA, total number N T of emitted electrons during wavelength scanning, oxygen component ratio Z = O2− / (OH + O2− ), and surface composition ratio X = Fe/ (O + N + C + Fe), after initial irradiation at λ = 200, 210, 220, and 230 nm Activation energy of αA (ΔE αA /eV)

Activation energy of N T (ΔN T /eV)

Activation energy of Z (ΔE Z /eV)

Activation energy of X (ΔE X /eV)

0.096 ± 0.011

0.052 ± 0.005

0.113 ± 0.004

0.039 ± 0.005

The ΔE αA , ΔE NT , and ΔE X were the average for all the λ values, respectively, and the ΔE Z value was obtained only for λ = 200 nm

Regarding the two processes of the penetration of incident light and the transfer of electrons that are excited through the surface overlayer, as described in Sect. 8.1, in the context of the activation energy ΔE a , we consider the following: scattering or trapping action of the surface overlayer when the optically excited metallic electrons move toward and escape from the surface; as well as absorption, reflection, and transmission of the incident light during the propagation of incident light. However, the overall effect of the surface overlayer on the photoemission remains unclear. The next section uses the three-step model of photoemission developed by [14].

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8 TAPE of Rolled and Scratched Fe Metal Surfaces

8.3 PE from Practical Fe Surfaces Scratched in Air, Water, and Organic Liquids 8.3.1 PE in Temperature Scans of Scratched Fe Surfaces and XPS Analysis Sections 8.3 and 8.4 extend the PE study to Fe samples (scratched in various environments: air, water, methanol, cyclohexane, ethanol, benzene, and acetone). We describe the influence of the temperature and surface overlayer on the photoelectron emission from scratched, practical Fe surfaces [10]. The scratching was conducted with a diamond cutter in seven environments. The glow curves were measured in the range 25–339 °C with a Geiger counter, under UV irradiation at 210 nm. The temperature was scanned in two cycles (Up1 and Down1 and Up2 and Down2). The XPS measurements were carried out at 25 °C and after TAPE measurements at 200 and 339 °C. Figure 8.14 shows the glow curves for Fe samples, scratched in seven environments and unscratched. The PE at 40 °C in the Up1 scan for the scratched samples increased in the order of air < water ≈ methanol ≈ cyclohexane < ethanol < benzene < acetone, and then, each glow curve exhibited a gradual increase with increasing temperature through a broad peak. The glow curve in the Up1 scan was completely different from that in the other scans. Figures 8.15 and 8.16 show the Fe 2p and Fe 3p spectra at 25, 200, and 339 °C for the scratched Fe samples. The Fe 3p spectra substantially differed, depending on the environment. The emission intensity in the Up1 scan at the three temperatures increased with increasing oxygen component ratio, ZO = O2− /(OH− + O2− ). The dependence of the intensity of PE at 40 °C (and the total count of emitted electrons in the Up1 scan) on the environment was explained by the acid–base interactions of the surface hydroxyl groups (−MOH, M = Fe) on the surface of scratched Fe samples with the molecules within the liquid environment.

8.3.2 Activation Energy of PE from Scratched Fe Surfaces We evaluated TAPE in the context of Fe surfaces (scratched in seven environments) by the activation energy ΔEa, regarding the quantum yield in TAPE, under 210-nmwavelength irradiation [9]. Figure 8.17 shows PE glow curves for the environment (PE unit: 103 counts/min) observed during Up1 scans for the environment, which substantially depended on the environment. On the basis of the glow curves, the quantum yield Y (units: emitted electrons/photon) was obtained. A simple method was proposed for determining ΔE a by using Y, termed Y GC , at seven temperatures (up to 353 °C) for the Y glow curve. For the samples cleaned in acetone without scratching, the ΔE a obtained by using the Y GC values was almost the same as that obtained from Y for seven stationary temperatures (Y ST ). For scratched samples, TAPE was measured over two cycles of temperature increase and subsequent decrease

8.3 PE from Practical Fe Surfaces Scratched in Air …

149

(Up1, Down1 and Up2, Down2 scans) in the 25–339 °C range, and ΔE a was obtained from Y GC . The Arrhenius plot was approximated by a straight line, although a convex swelling peak was evident in the Up1 scan. Table 8.3 shows the activation energies for the Up1 and Up2 scans [9]. The values of ΔE aUp1 varied over a wide range of 0.035–0.212 eV, whereas the values of ΔE aUp2 were reduced in the 0.012–0.038-eV range. The effect of the environment on the activation energy was clear in the first scan. ΔE aUp1 for water was

Fig. 8.14 Glow curves during temperature-increasing (Up1 and Up2) and temperature-decreasing (Down1 and Down2) scans for the Fe samples scratched in a air, b water, c methanol, d ethanol, e acetone, f benzene, g cyclohexane, and for unscrathed sample h. The notations Up1 and Up2 represent the temperature-increasing scans from 25 to 339 °C, and the notations Down1 and Down2 the temperature-decreasing scans from 339 to 25 °C [10]. Reprinted John Wiely, Momose et al. [10]. Copyright© 2016

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8 TAPE of Rolled and Scratched Fe Metal Surfaces

Fig. 8.14 (continued)

much higher than that for acetone. This result can be explained in terms of the mode of the acid–base interactions between the liquid molecules and surface hydroxyl groups of Fe–OH. Figure 8.18 shows examples of the acid–base interaction modes that form two electric dipoles. In the case of H2 O, the H atom of H2 O is attracted to the unshared electron pair of the O atom of FeOH, which results in an electric dipole that is oriented with the negatively charged end toward the outside, represented by FeOH2 δ+ …OHδ− . In the case of (CH3 )2 CO, the H atom of FeOH was attracted to the unshared electron pair of the O atom of (CH3 )2 CO; which resulted in an electric dipole that was oriented with the positively charged end toward the outside, represented by FeOδ− …δ+ HO = C(CH3 )2 . The former orientation of the electric

8.3 PE from Practical Fe Surfaces Scratched in Air …

151

Fig. 8.15 Fe 2p spectra stacked with the increasing temperatures for the Fe samples screatched in the atmospheres shown at the top: a the XPS measurement at 25 °C, b after the TAPE measurement at 200 °C, and c after the TAPE measurement at 339 °C [10]. Reprinted John Wiely, Momose et al. [10]. Copyright© 2016

Fig. 8.16 Fe 3p spectra stacked with the increasing temperatures for the Fe samples screatched in the atmospheres shown at the top: a the XPS measurement at 25 °C, b after the TAPE measurement at 200 °C, and c after the TAPE measurement at 339 °C [10]. Reprinted John Wiely, Momose et al. [10]. Copyright© 2016

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8 TAPE of Rolled and Scratched Fe Metal Surfaces

Fig. 8.17 Photoelectron emission glow curves for schratched Fe samples observed during the Up1 scans, for the different environments [9]. Reprinted Authors licensed under CC By 4.0 Tsinghua Springer, Momose et al. [9]. Copyright© 2018 Table 8.3 Activation energies and preexponential factors of thermally assisted PE yield Y GC , obtained from the glow curves of Up1 and Up2 scans as well as activation energies of the oxygen component ratio Z O = O2− /(OH− + O2− ) for Fe samples scratched in various environments Scratching Activation environments energy in Up1 scan (ΔE aUp1 scan /eV) Air

0.212

Activation Preexponential factor in Activation energy in Up2 Up1 scan (Y 0UP1 /(10−8 energy of Z O emitted electrons/photon)) (ΔE ZO /eV) scan (ΔE aUp2 scan /eV) 0.028

5.77

Water

0.145

0.038

2.13

Methanol

0.159

0.020

3.41

0.023

Ethanol

0.079

0.026

0.56

0.032

Acetone

0.035

0.021

0.26

Benzene

0.039

0.035

0.22

Cyclohexane 0.111

0.033

0.89

0.062

0.033

0.38

Unscratched

The irradiation light wavelength: λ = 210 nm

0.043

8.3 PE from Practical Fe Surfaces Scratched in Air …

153

Fig. 8.18 Examples of orientations of electric dipoles formed by the acid-base interactions modes between the molecules of (CH3 )2 CO, C2 H5 OH, and H2 O and the surface hydroxyl groups (Fe– OH) based on: a the acceptor number, which represents the strength of the acidity, and b the proton affinity, which quantifies the tendency of a sample molecule to accept a proton or the gas-phase basicity. The former mode increases ΔE aUp1 and the latter decreases ΔE aUp1 . The dotted lines in the figure indicate the hydrogen bond, although the interaction mode for acetone as an acid is unclear [9]. Reprinted Authors licensed under CC By 4.0 Tsinghua Springer, Momose et al. [9]. Copyright© 2018

dipole contributed to a decrease in ΔE aUp1 , whereas the latter orientation contributed to an increase in ΔE aUp1 . Thus, formation of electric dipoles substantially influenced the PE intensity. The total count of electrons that were emitted during the Up1 and Up2 scans decreased with increasing ΔE aUp1 and ΔE aUp2 , respectively. The convex swelling peak in Fig. 8.17 was attributed to the removal of carbon materials from the scratched surface and the effect of the increased electron density of the O atom of the surface hydroxyl group of FeOH under light irradiation. Regarding the key step of the photoemission, we hypothesize that ΔE aUp1 for the scratched samples might pertain to the transfer of electrons that are photoexcited in the metal through the overlayer and reach the outer surface, rather than to the penetration of incident light; because in Table 8.3, there is little correspondence between the orders of ΔE aUp1 and ΔE ZO . The penetration of incident light was important for the cleaned Fe sample only in Sect. 8.2.

8.3.3 PE in Wavelength Scans of Scratched Fe Surfaces We describe the characteristics of the PE, activation energy, and photothreshold, derived from wavelength-scan data of incident light for the scratched Fe samples [8]. The PE quantum yields (emitted electrons/photon), Y FD (8.4) and Y s (8.1), were obtained by two means of irradiating the surface with photon energy: during the

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8 TAPE of Rolled and Scratched Fe Metal Surfaces

light wavelength scan in the range of 200–300 nm at 25, 200, and 339 °C; and during the temperature-increase and subsequent temperature-decrease scans in the range of 25–339 °C under 210-nm light irradiation. The equation Y FD = αA (E p − φ)2 /2k B (where αA is the electron photoemission probability, A is identical to the Richardson’s constant, E p is the incident photon energy, and φ is the photothreshold) was applied to the wavelength-scan data to obtain αA and φ. Figure 8.19 shows plots of the square root of the electron emission intensity against the incident photon energy for scratched Fe samples. Additionally, the emission intensity at 210-nm wavelength (I 210 ) and the total number of emitted electrons (N T ) were obtained during the wavelength scan. Table 8.4 [8] shows the characteristics of Arrhenius plots for αA, I 210 , and N T . The Arrhenius activation energies (obtained for αA, I 210 , and N T ) were ΔE αA = 0.074– 0.113 eV, ΔE 210 = 0.054–0.085 eV, and ΔE NT = 0.053–0.088 eV, respectively. The values of ΔE αA considerably depended on the environment. The values of φ slightly increased with increasing temperature for all of the environments but were close: the

Fig. 8.19 Plots of the square root of the electron emission intensity against the incident photon energy, E p , during the first wavelength scan of an Fe sample scratched in air from 300 to 200 nm at 3 temperatures: a 25 °C, b 200 °C, and 339 °C. Only the part of the plot giving the approximately straight line is represented by the equation and R2 in the figure. The values of the slope of the equation, β, necessary for obtaining the electron photoemission probability, αA and the photothreshold, φ, were determined from (9.5) [8]. Reprinted with permission from John Wiley, Momose et al. [8]. Copyright© 2018

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155

average was 4.81 eV (25 °C), 5.00 eV (200 °C), and 5.02 eV (339 °C). Furthermore, we focused on the relationship of the values of ΔE αA with the other activation energies: ΔE 210 , ΔE NT , and ΔE aUp1 . Figure 8.20 shows plots of various activation energies against ΔE αA . The values of ΔE 210 and ΔE NT increased with increasing ΔE αA ; whereas in the case of ΔE aUp1 {which was reported to be in the 0.212–0.035eV range (Table 8.3)}, the ΔE aUp1 values exhibited a completely different trend: exponentially decreasing with ΔE αA . This disagreement might be explained by the difference in times needed for temperature and wavelength scans. ΔE aUp1 might be attributable to the formation of surface barrier, dependent on the environment caused during the Up1 scan. For example, a decrease of the surface barrier for acetone was attributable to the electric dipole that was formed by acid-base interactions between the functional group of the liquid molecule and the O atom of the surface hydroxyl group of Fe–OH; such as FeOδ− –Hδ+ …O = C(CH3 )2 (Fig. 8.18). However, the surface hydroxyl group in the case of the wavelength scan had less activity in terms of interactions with the environmental molecules, because of the briefer light irradiation time in the wavelength scan (250 s) than in the temperature scan (950 s). In Fig. 8.21, for several scratching environments, we compared the dependence of ΔE 210 and ΔE aUp1 (obtained during the wavelength and Up1 temperature scans, respectively) on the acceptor number [5] and proton affinity [6] of the liquids. ΔE 210 tended to only slightly change with increasing acceptor number and proton affinity of the liquid molecules (Fig. 8.21a, b), whereas ΔE aUp1 increased with acceptor number, but tended to decrease with increasing proton affinity; although the data points were rather scattered (Fig. 8.21c, d). Thus, the TAPE results suggest that the electron density of the O atom of the surface hydroxyl group at the metal surface determined the influence of adsorbed molecules. Table 8.4 Activation energies and preexponential factors of the electron photoemission probability αA, intensity of electron emission at 210 nm I 210 , and total number of emitted electrons N T, obtained from the wavelength scans for Fe samples scratched in various environments Scratching environments

Activation energy of αA, (ΔE αA /eV)

Air

0.074 ± 0.014

Preexponential factor of αA, (αA)0 , 10−16 emitted electrons/photon 6.09 ± 1.27

Activation energy of I 210 , ΔE 210 /eV

Activation of energy of N T , ΔE NT /eV

0.057 ± 0.018

0.054 ± 0.018

Water

0.084 ± 0.001

6.94 ± 0.57

0.061 ± 0.005

0.064 ± 0.005

Methanol

0.113 ± 0.008

14.06 ± 0.62

0.083 ± 0.005

0.088 ± 0.008

Ethanol

0.081 ± 0.018

8.35 ± 3.75

0.054 ± 0.023

0.053 ± 0.025

Acetone

0.105 ± 0.031

13.23 ± 5.47

0.085 ± 0.022

0.069 ± 0.004

Benzene

0.092 ± 0.014

8.81 ± 1.97

0.071 ± 0.018

0.079 ± 0.021

Cyclohexane

0.081 ± 0.002

6.72 ± 0.44

0.058 ± 0.000

0.059 ± 0.006

Average

0.090

0.067

0.067

The mean values and mean deviations from the mean values of the data of 2 wavelength scan are given

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8 TAPE of Rolled and Scratched Fe Metal Surfaces

Fig. 8.20 Plots of the various activation energies against ΔE αA : a ΔE 210 for the intensity of the electron emission at 210 nm; b ΔE NT for the total number of emitted electrons, except for the first scan of acetone; and c ΔE aUp1 obtained from the Up1 temperature scan under 210-nmlight irradiation, except for methanol. In a and b, the values of ΔE 210 , ΔE NT , and ΔE αA from 2 wavelength scan measurements are used. In c, the values of ΔE aUp1 and ΔE αA come from Tables 8.3 and 8.4, respectively. The equations and R2 values of the dotted lines are given [8, 9]. Reprinted with permission from John Wiley, Momose et al. [8, 9]. Copyright© 2018

8.4 Temperature Analysis of PE and XPS Data of Scratched Fe Surfaces We describe the relationship between the PE data (Sect. 8.3) and the XPS data for the seven scratching environments [7]. The environments were the same as in the preceding section: air, benzene, cyclohexane, water, methanol, ethanol, and acetone. The standard XPS measurements of Fe 2p, Fe 3p, O 1s, and C 1s spectra were performed after wavelength scans at 25, 200, and 339 °C. The total number of electrons that were counted in the core spectra of the XPS measurements, termed the XPS intensity, was used as the XPS characteristics of the overlayer on the surfaces. In Fig. 8.22, the environments on the abscissa are arranged in increasing order of XPS intensity: air < benzene < cyclohexane < water < methanol < ethanol < acetone. Figure 8.22 indicates the following features: (1) The XPS intensities of Fe

8.4 Temperature Analysis of PE and XPS Data of Scratched Fe Surfaces

157

Fig. 8.21 Dependence of ΔE 210 obtained from the wavelength scans (a, b) and ΔE aUp1 obtained from Up1 temperature scan (c, d) on the acceptor number and the proton affinity of the molecules of the liquid environments, respectively. The values of ΔE 210 were taken from Table 8.4, and those of ΔE aUp1 from Table 8.3 [8, 9]. Reprinted with permission from John Wiley, Momose et al. [8, 9]. Copyright© 2018

2p, Fe 3p, and O 1s at 25, 200, and 339 °C tended to increase as the environments shifted rightward and increased with increasing temperature. (2) As the temperature increased from 25 to 200 °C and 300 °C, the O 1s intensity (Fig. 8.22c) at 25 °C gradually increased compared with the XPS intensities of Fe 2p (Fig. 8.22a) and Fe 3p (Fig. 8.22b). (3) The C 1s intensity (Fig. 8.22d) was almost independent of the temperature; although in the case of benzene and cyclohexane, the C 1s intensity fluctuated, increasing at 25 °C and then decreasing at 200 °C, before increasing at 339 °C (which led to a level equal to that for the other environments). One can attribute these results to the deposition of carbon materials by decomposition of these molecules, resulting from scratching and its removal by the temperature increase. Figure 8.23 shows the dependence of the O2− intensity on the environment at the three temperatures. The intensity at 25 °C gradually increased as the environment shifted rightward; whereas the intensity at 200 °C decreased for water, increased for methanol and ethanol, and decreased for acetone. The dependence of the O2− intensity on the environment was similar (in particular, at 200 °C) to the dependence of Fe 2p, Fe 3p, and O 1s (Fig. 8.22a–c, respectively (which exhibited a similar environment dependence). Figure 8.24a, b shows plots of the XPS intensity ratio of O2− /Fe 2p and of O2− /Fe 3p at 25, 200, and 339 °C, respectively, against the environment. Figure 8.24 indicates that generally, the XPS intensity ratios of O2− /Fe 2p and of O2− /Fe 3p were nearly constant at each temperature in the various environments, but the intensity ratios increased substantially with increasing temperature.

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8 TAPE of Rolled and Scratched Fe Metal Surfaces

Fig. 8.22 Dependence of the total number of electrons counted, termed XPS intensity, in the Fe 2p (a), Fe 3p (b), O 1s (c), and C 1s (d) spectra in the XPS measurements after the photoelectron emission (PE) measurement at 25, 200, and 339 °C for the real Fe surfaces on the scratching environments in the following order: (1) air, (2) benzene, (3) cyclohexane, (4) water, (5) methanol, (6) ethanol, and (7) acetone [7]. Reprinted Authors licensed under CC By 4.0 MDPI, Momose et al. [7]. Copyright© 2020

8.4 Temperature Analysis of PE and XPS Data of Scratched Fe Surfaces

159

Fig. 8.23 Dependence of the total number of electrons counted, XPS intensity of the O2– component in the O 1s spectra after the PE measurement at 25, 200, and 339 °C for the real Fe surfaces on the scratching environments: (1) air, (2) benzene, (3) cyclohexane, (4) water, (5) methanol, (6) ethanol, and (7) acetone [7]. Reprinted Authors licensed under CC By 4.0 MDPI, Momose et al. [7]. Copyright© 2020

The XPS results in Figs. 8.22, 8.23 and 8.24 indicate that the temperature dependence of the chemical structure (i.e., XPS intensity) of the overlayers on the scratched Fe surfaces determined the increase in the PE intensity with increasing temperature. In accordance with Table 4 of ref. [7], the average Arrhenius activation energies of the XPS intensities of O 1s and the O2− component of O 1s had larger values (ΔE O1s = 0.032 eV and ΔE O2 − = 0.048 eV, respectively), which were substantially larger than those of Fe 2p and Fe 3p (ΔE Fe2p = 0.017 eV and ΔE Fe3p = 0.018 eV, respectively). However, the latter data were limited for several environments, and on the basis of the temperature dependence of the XPS intensity ratio of O2− /Fe 3p and O2− /Fe 2p, a nonstoichiometric Fex O (which has p-type semiconductor character) accumulated at the interface. Figure 8.25 shows the actions of the two types of chemical species that were present at the overlayer on the intensity of PE in the Up1 temperature scan: (1) acid–base interactions of the environmental molecules with the surface hydroxyl group (FeOH) and (2) growth of an Fex O p-type semiconductor layer as a result of the transfer of electrons, iron ions, and oxygen species from the metal to the oxide film.

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8 TAPE of Rolled and Scratched Fe Metal Surfaces

Fig. 8.24 Dependence of a the ratio of XPS O2– intensity to XPS Fe 2p intensity and b the ratio of XPS O2– intensity to XPS Fe 3p intensity obtained from the O 1s, Fe 2p, and Fe 3p XPS spectra for 25, 200, and 339 °C for the real Fe surfaces on the scratching environments [7]. Reprinted Authors licensed under CC By 4.0 MDPI, Momose et al. [7]. Copyright© 2020

References

161

Fig. 8.25 Growth of FexO p-type semiconductor and acid-base interaction of the scratching environment molecule at the scratched surface as a result of the transfer of electrons, iron ions, and oxygen species from the metal during Up1 temperature scan. The electron density of the oxygen atom of Fe–OH is considered to become greater during the temperature scan [7]. Reprinted Authors licensed under CC By 4.0 MDPI, Momose et al. [7]. Copyright© 2020

References 1. I. Ames, R.L. Christensen, Anomalous photoelectric emission from nickel. IBM J. Res. Develop. 7(1), 34–39 (1963) 2. L.A. DuBridge, New Theories of the Photoelectric Effect (Hermann and Cie, Paris, 1935) 3. L.A. DuBridge, A further experimental test of Fowler’s theory of photoelectric emission. Phys. Rev. 39, 108–118 (1932) 4. R.H. Fowler, The analysis of photoelectric sensitivity curves for clean metals at various temperatures. Phys. Rev. 38, 45–56 (1931) 5. V. Gutmann, Ion pairing and outer sphere effect. Chimia 31, 1–7 (1977) 6. Jolly WL (1991) http://en.wikipedia.org/wiki/Protonaffinity (data page). Accessed 12 June 2015 7. Y. Momose, T. Sakurai, K. Nakayama, Thermal analysis of photoelectron emission (PE) and X-ray photoelectron spectroscopy (XPS) data for iron surfaces scratched in air, water, and liquid organics. Appl. Sci. 10(6), 2111 (2020). https://doi.org/10.3390/app10062111 8. Y. Momose, T. Sakurai, K. Nakayama, Photoelectron emission characteristics of iron surfaces scratched in different environments: dependence on photon energy irradiation methods. Surf. Interface Anal. 50, 1319–1335 (2018) 9. Y. Momose, D. Suzuki, K. Tsuruya, T. Sakurai, K. Nakayama, Transfer of electrons on scratched iron surfaces: photoelectron emission and X-ray photoelectron spectroscopy studies. Friction 6(1), 98–115 (2018) 10. Y. Momose, K. Tsuruya, T. Sakurai, K. Nakayama, Photoelectron emission and XPS studies of real iron surfaces subjected to scratching in air, water, and organic liquids. Surf. Interface Anal. 48, 202–211 (2016) 11. Y. Momose, D. Suzuki, T. Sakurai, K. Nakayama, Photoemission from real iron surfaces and its relationship to light penetration of the overlayer. Appl. Phys. A 118, 637–647 (2015) 12. Y. Momose, D. Suzuki, T. Sakurai, K. Nakayama, Influence of temperature and photon energy on quantum yield of photoemission from real iron surfaces. Appl. Phys. A 117, 1525–1534 (2014)

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13. S.J. Roosendaal, A.M. Vredenberg, F.H.P.M. Habraken, Oxidation of iron: the relation between oxidation kinetics and oxide electronic structure. Phys. Rev. Lett. 84, 3366–3369 (2000) 14. W.E. Spicer, Surface Analysis by Means of Photoemission and Other Photon-Stimulated Processes, in Chemistry and Physics of Solid Surfaces. ed. by R. Vaneslow, S.Y. Tong (CRC Press, Cleveland, 1977), pp.235–254

Chapter 9

TAPE of Si Wafers

Abstract We first outline the effect of the excitation method and gas adsorption (such as O2 and H2 O) on the TSEE glow curves of the SiO2 layer. We then describe TAPE of Si wafers obtained under temperature scans and incident light wavelength scans. We also report the characteristics (activation energy, photothreshold, and total counts of emitted electrons) of Si samples with an oxide layer and Si samples implanted with H, Si, and Ar ions.

9.1 Effect of Adsorption of O2 and H2 O on EE from Si Many studies have been conducted regarding EE from oxide films on Si. In Chap. 6, Figs. 6.1 and 6.2 showed TSEE glow curves for SiO2 layers. The samples in these cases were excited by different types of methods (Fig. 6.1) and under bombardment by electrons with various energies (Fig. 6.2). In Sect. 6.3.4 (for grounded Si powder exposed to Ar plasma), based on the TSEE glow curves, the trap depth and the electron affinity were obtained. In accordance with [14, 15] we will outline the studies reported previously. The EE of Si and Ge excited by X-ray irradiation or electron bombardment is attributable to the oxide layer [8, 14, 18, 19]. Drenckhan et al. [3] obtained TSEE glow curves after electron bombardment of an oxide layer of Si with a secondary electron multiplier. The experiment was reproducible over a pressure range of 10−9 –10−7 Torr. The shape of the glow curve varied depending on the energy of the excited electrons. In the case of 2-eV electrons, the electrons did not pass through the oxide layer and were reflected, which indicates that the electrons were captured in the trapping center on the surface. The glow curve was remarkably affected by the formation conditions of the oxide layer on the surface of a Si single crystal. Furthermore, Drenckhan et al. [2] observed a typical TSEE curve of the SiO2 layer when a Si single crystal was etched with HF (an acid) and then placed under UHV to produce an oxide film of ca. 2 nm on the surface. Afterward, the surface was excited with 1500-eV electrons. Thereafter, when the thin oxide film was first removed by Ar-ion bombardment at 7 × 10−4 Torr after placement under vacuum at 1 × 10−9 Torr, and then placement under vacuum at 8 × 10−9 Torr, no TSEE was observed after excitation by electron bombardment. Thus, in the case of © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 Y. Momose, Exoemission from Processed Solid Surfaces and Gas Adsorption, Springer Series in Surface Sciences 73, https://doi.org/10.1007/978-981-19-6948-5_9

163

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9 TAPE of Si Wafers

a clean surface of Ge, EE did not occur because of the similarity with Si [14]. The author of ref. 40 of [14], Klein [9] observed weak TSEE upon electron bombardment of Si single crystals cleaved under vacuum at 10−9 Torr. This might be attributable to monolayer adsorption of O2 during bombardment. Scharmann [14] introduced the mechanism for EE of an insulator that was excited by electron bombardment, proposed by Drenckhan et al. [4]. The average energy of emitted electrons reported to date ranges from ca. 0.1 eV to ca. 90 eV, which one cannot explain by the Maxwell-tail theory, because an energy of 0.1 eV corresponds to a temperature of 1160 K. Drenckhan et al. [4] accounted for the high electron energy, assuming the curvature of the energy band at the surface and the electric field. The surface of the insulator is positively charged during electron bombardment if its secondary yield δ (commonly defined as the number of emitted electrons per incident primary electron) is >1. The primary electrons lose most of their energy at the end of the transmission distance and are captured at the capture center, which creates a negative space-charge region. In this manner, one forms a strong electric field. During stimulation with light or heat, electrons in the conduction band are accelerated toward the surface. By this acceleration, one generates energy that exceeds the loss of energy because of the electron–electron scattering in the path and the electron affinity, causing EE. Here, regarding the surface of a semiconductor, the electron affinity is defined as the energy one obtains by moving an electron from the vacuum (just outside the semiconductor) to the bottom of the conduction band (just inside the semiconductor), typically denoted by E EA or χ. They measured an energy of ca. 90 eV after electron bombardment of the SiO2 layer. This model is supported by the following observations [3, 16]: (a) The SiO2 layer that is deposited onto the underlying surface of Al2 O3 exhibits a TSEE of SiO2 when the transmission distance of the primary electrons is shorter than the transmission distance of the deposited layer. If the transmission distance exceeds a certain value, one obtains the characteristic TSEE of Al2 O3 . Thus, the source depth of the exoelectrons corresponds to the transmission distance of the primary electrons. A typical value is 80 nm. (b) If the SiO2 layer is excited with high-energy electrons (1.5 keV) followed by bombardment with 2-eV electrons, the entire TSEE curve is no longer evident. In this case, the condition of δ < 1 applies, such that that the positive charge on the surface is canceled and the accelerating electric field becomes weak. Krylova and Svitov [10] investigated the effect of adsorption of O2 and H2 O on TSEE from an etched Si single crystal after excitation by electron bombardment. The TSEE glow curves were measured with a secondary electron multiplier under HV. First, even in the case of no excitation emission, maxima at 130 and 215 °C as well as an increase in emission at >400 °C were observed. The emissive particles were OH− ions. The second heating of this sample did not exhibit emission maxima. However, when excited with 1.5-keV electrons, the maxima were evident at the same temperatures as previously; in particular, the maximum at 130 °C was much larger. This maximum was reduced by vacuum heat treatment, but was increased by chemisorption of H2 O and O2 (4 Torr, 300 °C).

9.2 PE from Si Wafers and Activation Energy

165

9.2 PE from Si Wafers and Activation Energy We investigated the characteristics of TAPE from Si (100) wafer surfaces with a native oxide film as a function of temperature, with a gas-flow Geiger counter that was equipped with Q gas {He + iso-C4 H10 (~1%)} [12]. Commercial polished Si wafers {(100) plane, resistivity >1000 Ω cm, thickness 525 μm, and size 20 mm × 30 mm} were used. The TAPE apparatus shown in Fig. 8.1 was used. The sample surfaces were used as-received. The spot area of the incident light was 0.5 mm × 3 mm, and the power of the incident light on this spot was 1000 Ω m, thickness 525 μm, size 10 mm × 10 mm) were implanted with H, Si, and Ar ions at four concentration levels: 1016 , 1018 , 1019 , and 1020 atoms/cm3 . The depth of the implanted regions was a maximum of 0.25 μm, which was obtained by secondary-ion mass spectrometry for H and Ar ions and by Rutherford backscattering spectrometry for Si ions. The photoemission was measured with the same apparatus, with a counter gas of Q gas, as described in Sect. 9.2. One set of the TAPE measurements for one sample was as follows: Measurement of photoemission in the wavelength scan process from 300 to 200 nm was successively performed at three temperatures during the increase and subsequent decrease of the temperature scans under irradiation with 210-nm light: first at 25 °C, next after heating to 340 °C, and subsequently after cooling to 40 °C. The spot area of the incident light was 0.5 mm × 3 mm, the power of the light on this spot was Cu > Au > Ag (Sect. 10.2.1). This finding indicates the magnitude of the ability of these metals to interact with the O2− oxygen. However, the maximum PE total count of group B (Table 10.3) indicates that the value for Ta was largest, but the other metals resulted in considerably smaller values than the metals in group A. In the O 1s spectra of group B in Table 10.3, regarding the oxygen type before TPPE measurements, the O2− oxygen is preferred except in the case of Co and Zn; however, after TPPE measurements, the Oad oxygen decreased, and the O2− oxygen increased, especially regarding metals B (V). In Table 10.3, all of the metals of group B indicate that after the TPPE measurements, the metal core peak increased as the carbon peak decreased. In accordance with [1], we consider a simple qualitative indication of chemisorption on metals. Researchers have investigated chemisorption of a number of gases

184

10 TPPE Characteristics of Various Metal Surfaces

of practical interest for many metals. With few exceptions, the strength with which metals are chemisorbed is in the following sequence: O2 > C2 H2 > C2 H4 > CO > H2 > CO2 > N2 . The reactivity of metal surfaces toward these molecules differs substantially. Table 10.4 is a modified table from Bond (1987). Metals are divided into seven groups (A, B1, B2, B3, C, D, and E) by the chemisorption power of the seven gases. In Table 10.4, the metals that were used in the TPPE measurements are represented by bold and italic type, which represent the temperature independence and temperature dependence, respectively, of the PE total count. The chemisorption power of the gases is shown by the following symbols: (+) strong chemisorption, (±) weak chemisorption, and (−) no chemisorption. Furthermore, the number of the gases that strongly chemisorb was added in the rightmost column. The number of gases that exhibit a strong chemisorption force tends to decrease when one goes down from group A to group E. This result indicates that the ability of the metals to interact the gases becomes weak; thus, adsorption of gases on the metal surfaces becomes unstable with increasing temperature. In Table 10.4, the italic-type metals are concentrated in the lower part, whereas the bold-type metals are concentrated in the upper part. Therefore, the finding that the italic-type metals caused the variation of the PE total count with temperature is attributable to the weakness of the interaction strength of the metals to the gases, although the TPPE experiments were in the context of gases such as O2 and H2 O. Regarding the interactions of metals with adsorption gases, [2] and [6] proposed a classification of metals. This classification is based on whether the molecule adsorbs onto the surface of transition metals in a nondissociative (molecular) manner or in a dissociative process. When the metals in Table 10.1 are applied to this classification, group A favors molecular adsorption, whereas the group B has a strong tendency to cause dissociative adsorption. Thus, the metals that favor molecular adsorption cause the temperature dependence of the PE total count.

10.3 TPPE Characteristics of Metals and Surface Pretreatment Methods Our attention has focused on the effect of various surface pretreatments of metals on the TPPE results. Fifteen types of practical metal surfaces were subjected to plasma treatment after ultrasonic cleaning in acetone [10]. Commercial metal sheets (Al, Ti, Fe, Co, Ni, Cu, Nb, Mo, Ag, Sn, Ta, W, Pt, Au, and Pb) were used. In the case of ultrasonic cleaning, two more metals (Zn and Pd) were used [5, 8]. Ar plasma treatment was performed with a radiofrequency generator (13.56 MHz) under the following conditions: pressure, 40 Pa; power, 30 W; and treatment time, 5 min. The TPPE measurements were performed in the same manner as described in Sect. 10.1. Here, we term the treatments T1 (ultrasonic cleaning in acetone) and T2 (Ar plasma treatment).

+ + + + + −

+ + + + + +

Ni, Co

Rh, Pd, Pt, Ir

Mn, Cu,

Al, Au

Li, Na, K

Mg, Ag, Zn, Cd, In, Si, Ge, Sn, Pb, As, Sb, Bi

B2

B3

C

D

E

−

−

+

+

+

+

+

C2 H4

−

−

+

+

+

+

+

CO

−

−

−

±

+

+

+

H2

−

−

−

−

−

−

+

CO2

−

−

−

−

−

−

−

N2

Bold-type elements: the metals of the group B in the text; Italic-type elements: the metals of the group A in the text (+) means that strong chemisorption occurs; (±) means that it is weak; (−) means unobservable. ** Bond, G. C, Heterogeneous Catalysis: Principles and Applications, p. 29, (Clarendon Press, Oxford, 1987)

*

+

B1

C2 H2

+

Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ru, Os

A

Gases O2

Metals*

Group

Table 10.4 Classification of metals in accordance with their chemisorption abilities**

1

2

4

4

5

5

6

Number of chemisorbed gases

10.3 TPPE Characteristics of Metals and Surface Pretreatment Methods 185

186

10 TPPE Characteristics of Various Metal Surfaces

Table 10.5 shows the values of the PE total count and photoelectric threshold, obtained for the metal groups for both pretreatments. The maximum and minimum of nine data points obtained at the temperatures of 25 °C → 100 °C → 200 °C → 300 °C → 350 °C → 300 °C → 200 °C → 100 °C → 25 °C in the temperatureincrease process and subsequent temperature-decrease process are listed; however, in the cases of Zn, Sn, Au, and Pb, the data points are less than nine. The difference between the maximum and minimum, which represents the deviation range of the values, is represented as Δ. The deviation values indicate the degree of susceptibility to change of the PE total count and photoelectric threshold by temperature. On the basis of the dependence of the TPPE characteristics on both treatments of ultrasonic cleaning (T1) and exposure to Ar plasma (T2), one can approximately classify the metals as follows: (1) The metals that resulted in the PE total count in the order T1 > T2 were Pt, Cu, Ag, Au, Ta, Nb, and Co; those in almost the same level (T1 ≈ T2) were Al, Pb, W, Ti, and Mo; and those in the order T1 < T2) were Ni, Fe, and Sn. Furthermore, for Pt, Al, Ni, and Cu, exposure to Ar plasma substantially increased the PE total count in the lower-temperature region. (2) The metal that resulted in the photothreshold in the order T1 > T2 was Sn; those in almost the same level (T1 ≈T2) were Al, Pb, Au, Ni, Ta, W, Mo, Nb, Co, and Fe; and those in the order T1 < T2 were Pt, Cu, Ag, and Ti. (3) The deviation range (Δ) of the values of the PE total count and photothreshold— obtained for both treatments of T1 and T2—increased substantially for the metals in group A compared with those in group B (with some exceptions). We hypothesize that the deviation of the values of the PE total count and the photothreshold corresponds to the presence of the Oad oxygen, which was susceptible to thermal desorption and the change in the O2− oxygen during the temperature-increase process. We observed an interesting relationship between the PE total count and photothreshold for both pretreatments. Figure 10.8 shows the relationship between the TPPE characteristics in the temperature-increase process and subsequent temperature-decrease process for Pt, Al, and Ni that were subjected to ultrasonic cleaning in acetone as well as Ar plasma treatment. The PE total count was plotted against the photothresholds during the temperature-increase process and subsequent temperature-decrease process for Pt, Al, and Ni samples. In the case of Pt in Fig. 10.8a and Ni in Fig. 10.8c, there were considerable changes not only in the PE total count but also in the photothreshold. In other words, there was a tendency for the PE total count to decrease as the photothreshold increased. The metals with such a tendency were Pt, Cu, Ag, Au, Ni, Zn, and Sn. However, in the case of Al in Fig. 10.8b the photothreshold negligibly changed, but the PE total count deviated over a considerably large range in the vertical direction. On the basis of the XPS results for Al, there was an oxide layer of several nanometers or more on both surfaces of T1 and T2. Furthermore, on the basis of the changes of the XPS spectra by Ar-ion etching of the metal surfaces, the binding energy and shape of the O 1 s peak changed negligibly, although the peak decreased with increasing etching time. Shigekawa and Hyodo [13]

10.3 TPPE Characteristics of Metals and Surface Pretreatment Methods

187

Table 10.5 Variation of the PE total count and photoelectric threshold values obtained for the metal groups for both pretreatments from TPPE Metal treatment

PE total count/count

Metal group

Max

Metal

Min

Photothreshold/eV Δ (max−min)

Max

Min

Δ (max−min)

Ultrasonically cleaned in acetone (T1) A-I

A-II

B-III B-IV

B-V

Al

47,400

8900

38,500

4.30

4.13

0.17

Pt

46,400

5800

40,600

5.09

4.18

0.91

Pb

12,500

2500

10,000

5.17

4.71

0.46

Cu

14,500

9300

5200

4.69

4.45

0.24

Ag

20,100

4100

16,000

5.02

4.26

0.76

Au

15,900

5800

10,100

4.75

4.32

0.43

Ni

7500

2300

5200

5.11

4.73

0.38

Ta

19,900

16,700

3200

4.66

4.47

0.19

Pd

7500

5400

2100

5.01

4.81

0.20

W

6900

4900

2000

4.84

4.56

0.28

Ti

6700

5200

1500

4.71

4.59

0.12

Mo

6400

4900

1500

5.02

4.85

0.17

Zn

3900

2800

1100

4.67

4.34

0.33

Nb

4000

3200

800

5.14

5.00

0.14

Co

4400

2400

2000

5.17

4.95

0.22

Fe

3100

2400

700

5.15

4.87

0.28

Sn

2000

1700

300

4.83

4.41

0.42

Exposed to Ar plasma (T2) A-I

Pb

33,500

3400

30,100

4.99

4.55

0.44

A-II

Al

48,700

20,600

28,100

4.29

4.15

0.14

B-III B-IV

B-V

Pt

28,900

13,900

15,000

4.89

4.65

0.24

Cu

11,800

2100

9700

5.11

4.43

0.68

Ag

9200

3200

6000

5.14

4.47

0.67

Au

6100

3348

2752

4.81

4.37

0.44

Ni

14,700

3800

10,900

5.09

4.38

0.71

Ta

11,200

8700

2500

4.68

4.46

0.22

W

7100

4600

2500

5.06

4.64

0.42

Ti

6000

4700

1300

4.79

4.68

0.11

Mo

7100

4900

2200

5.05

4.89

0.16

Nb

3600

2000

1600

5.25

4.98

0.27

Co

2600

1600

1000

5.13

4.82

0.31

Fe

6600

2400

4200

5.14

4.84

0.30

Sn

5500

4600

900

4.49

4.16

0.33

188

10 TPPE Characteristics of Various Metal Surfaces

Fig. 10.8 Relationship between the PE total count and the phototreshold value for Pt (a), Al (b), and Ni (c): (◯) ultrasonically cleaned in acetone (T1); (∎) exposed to Ar plasma (T2) [10]. Reprinted with the permission Momose et al. [10]. Copyright© 2002

investigated in detail the PE of scratched Al under light irradiation of wavelength of 180–400 nm and explained the mechanism by a two-process model: Electrons are stored in the surface oxide layer from the metal inside by photon energy, and then, the electrons are emitted from the surface of the oxide layer to the surrounding environment. We hypothesize that one can apply a similar mechanism to the Al surface in our experiments: Because the photothreshold value of Al was almost unchanged, the rate-determining step of photoemission was not the electron emission from the outermost surface, but the electron transport from inside the metal to the electron trapping sites in the surface oxide layer (subsurface). The metals in which PE total count tended to decrease with increasing photothreshold value were Pt, Cu, Ag, Au, Ni, Zn, and Sn. The metals with a tendency to change negligibly in the photothresholds but to vary the PE total count over a considerably wide range were Al, Pb, Ta, Pd, W, Ti, Mo, Nb, Co, and Fe.

References

189

References 1. G.C. Bond, 3.4 Descriptive chemistry of chemisorption on metals, in Heterogeneous Catalysis: Principles and Applications, 2nd edn. (Clarendon Press, Oxford, 1987), pp. 28–30 2. G. Broden, T.N. Rhodin, C. Brucker, R. Benbow, Z. Hurych, Synchrotron radiation study of chemisorptive bonding of CO on transition metals—polarization effect on Ir(100). Surf. Sci. 59, 593–611 (1976) 3. S. Evans, Oxidation of the group IB metals studied by X-ray and ultraviolet photoelectron spectroscopy. Part 1. The surface oxidation of polycrystalline copper. J. Chem. Soc. Faraday Trans. II Mol. Chem. Phys. 71, 1044–1057 (1975) 4. H. Glaefeke, Exoemission, in Thermally Stimulated Relaxation in Solids. ed. by P. Bräunlich (Springer, Berlin, 1979), pp.225–273 5. T. Kamosawa, M. Homma, Y. Momose, Observation of real metal surfaces by temperature programmed photoelectron emission technique temperature dependence of the quantity of emitted electrons and its relationship to XPS analysis. J. Surf. Finishing Soc. Jpn. 51(8), 836–843 (2000) 6. R.I. Masel, Adsorption I: the binding of molecules to surfaces, in Principles of Adsorption and Reaction on Solid Surfaces. (Wiley, New York, 1996), pp.108–234 7. N.S. McIntyre, S. Sunder, D.W. Shoesmith, F.W. Stanchell, Chemical information from XPS— applications to the analysis of electrode surfaces. J. Vac. Sci. Technol. 18, 714–721 (1981) 8. Y. Momose, M. Honma, T. Kamosawa, Temperature-programmed photoelectron emission technique for metal surface analysis. Surf. Interface Anal. 30, 364–367 (2000) 9. Y. Momose, K. Sato, O. Ohno, Electrochemical reduction of CO2 at copper electrodes and its relationship to the metal surface characteristics. Surf. Interface Anal. 34, 615–618 (2002) 10. Y. Momose, T. Kamosawa, M. Honma, M. Takeuchi, Surface analysis of real metals by temperature programmed photoelectron emission technique relationship between TPPE characteristics and surface pretreatment methods. J. Surf. Finishing Soc. Jpn. 53(10), 675–682 (2002) 11. Y. Momose, S. Kohno, M. Honma., T. Kamosawa, B. Mishra, C. Yamauchi (eds.), Temperature programmed photoelectron emission analysis of copper surfaces subjected to cleaning and abrasion in organic liquids, in Proceedings of the 2nd International Conference in Processing Materials for Properties (TMS, Warrendale, Pa, 2000b), pp. 285–290 12. P.R. Norton, An investigation of the adsorption of oxygen and oxygen containing species on platinum by photoelectron spectroscopy. Surf. Sci. 47, 98–114 (1975) 13. H. Shigekawa, S. Hyodo, Two-process model for a comprehensive interpretation of photostimulated exoelectron emission. Jpn. J. Appl. Phys. 24–4, 21–26 (1985) 14. Y. Terunuma, K. Takahashi, T. Yoshizawa, Y. Momose, Temperature dependence of the photoelectron emission from intentionally oxidized copper. Appl. Surf. Sci. 115, 317–325 (1997) 15. Y. Terunuma, M. Honma, K. Takahashi, Y. Momose, Photoelectron emission from copper freshly abraded in organic liquids. J. Surf. Finishing Soc. Jpn. 47(12), 1075–1081 (1996) 16. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder, G.E. Muilenberg, Handbook of X-ray Photoelectron Spectroscopy (Perkin–Elmer, Minnesota, 1979) 17. K. Wandelt, Photoemission studies of adsorbed oxygen and oxide layers. Surf. Sci. Rep. 2, 1–121 (1982)

Chapter 11

TriboEE Occurring from Metal Surfaces During Sliding Contact with a Polymer Rod

Abstract First, we outline triboelectron emission (TriboEE), which occurs during rubbing between a metal and a polymer rod. Next, we describe the relationship between TriboEE characteristics and the XPS results of rubbed surfaces, the electronic properties of metals, and the thermodynamics data of metal-oxide formation— by arranging metals in groups and periods as per the periodic table. Furthermore, we note the effects of the chemical structure of the polymer rods.

11.1 Outline of TriboEE from Metal Surfaces 11.1.1 Electron Emission During Sliding Contact Between Metals and Polymers In fields such as adhesion, coatings, corrosion, and catalysis, the electron or charge transfer at metal surfaces (such transfer pertains to interactions with the environment) is an active area of research [2, 3, 13]. In Sect. 3.3, we described the electron emission from metal surfaces during rubbing with the same metal [9]. However, little is known about the electron emission from metals during sliding contact with different materials, such as polymers. The electron emission that occurs in the dynamic process of sliding contact plays an important role in tribochemistry, but little is known about the TriboEE effect. We have examined the effects of various factors, such as the type of metal and polymer, the metal surface oxide, the friction velocity, the metal specimens that are electrically insulated, and the metal specimens during heating and subsequent cooling [10]. Metal specimens used in this context are commercial sheets of Fe, Ni, and Cu (purity >99.6%, thickness 0.1 mm, size 30 mm × 30 mm). Prior to use, the specimens were degreased with benzene and then annealed for 2 h at 350 °C under vacuum. The polymer rotators were commercial polymer tubes such as polyethylene (PE), polytetrafluoroethylene (PTFE), polymethylmethacrylate (PMMA), and nylon-6, into the center of which an iron bar was inserted. The length of the rotator was 18 mm, and both its cylindrical ends (3 mm in length and 4.9 mm in diameter) were placed in sliding contact with the metal specimen. The TriboEE measurements were performed with a Geiger counter that was equipped with a friction device, in © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 Y. Momose, Exoemission from Processed Solid Surfaces and Gas Adsorption, Springer Series in Surface Sciences 73, https://doi.org/10.1007/978-981-19-6948-5_11

191

192

11 TriboEE Occurring from Metal Surfaces …

which the rotator was magnetically rotated on the metal sheet, usually at the rate of 120 rotations/min. In these experiments, a mixing gas {consisting of Ar (11,200 Pa) and C2 H5 OH vapor (2600 Pa)} was used as the counter gas. An accelerating voltage (AV) of −96 V was also applied to the sample holder (relative to the earthed grid of the counter) with a small battery. All metal sheets (Fe, Ni, and Cu) output similar behavior in the context of TriboEE: with PE and PTFE rods, the emission intensity increased and then was almost constant with friction time, whereas with PMMA and nylon-6, the emission was close to the background level. The XPS results indicate that an increase in TriboEE for Ni during rubbing with a PE rotator corresponds to the manifestation of metallic Ni in the Ni 2p3/2 spectrum (a result of the reduction of the oxide layer on the metal surface). The effect of friction velocity on the TriboEE for the pair Ni–PE was examined. The emission intensity increased proportionally with increasing friction velocity and subsequently decreased with decreasing friction velocity. When polymers such as gutta-percha and cellulose nitrate were separated from glass at various rates, an increase in the detachment rate resulted in an abrupt increase in electron emission; additionally, with high rates of detachment, strong fields were evident in the gap [1]. Therefore, in this case the effect of friction velocity might be associated with the strength of the fields that were repeatedly generated at the contact and subsequent detachment points during sliding. When specimens (Fe, Ni, and Cu) that were insulated by placing a 2-mm-thick PE sheet between the metal specimen and the sample holder were rubbed against a PE rotator, the emission became much weaker. Thus, the emission depended on the provision of electrons from the battery to the metal specimen. It has also been of substantial interest to study the temperature dependence of TriboEE. Figure 11.1 shows the variation of the TriboEE with temperature for Ni–PTFE. When a Ni specimen during rubbing with a PTFE rod was heated from 25 to 100 °C at a rate of 20 °C/min in the counter and was then spontaneously cooled, the intensity of the emission decreased with increasing temperature but subsequently increased with decreasing temperature. A similar temperature dependence was also observed for the Fe and Cu specimens. The decrease in the emission with increasing temperature might be associated with a gradual reduction of the quantity of adsorbed C2 H5 OH molecules in the counter gas on the specimen surface, but the mechanism remains unclear.

11.1.2 Effect of Surface Pretreatments of Metals on TriboEE We studied TriboEE from metal surfaces that we subjected to various pretreatments [7]. Commercial rolled metal sheets, Fe (purity >99.5%) and Cu (>99.9%), as well as PTFE were used. The polymer rotator was a PTFE tube (inner and outer diameters 0.5 and 1.5 mm, respectively), into which an iron wire (diameter 0.5 mm) was inserted, and cut off to a length of 10 mm. The metal surfaces were pretreated by the following three techniques: ultrasonic cleaning in acetone for 30 min, followed by drying in a vacuum (acetone cleaning); abrasion with #3000 emery paper, followed by cleaning

11.1 Outline of TriboEE from Metal Surfaces

193

Fig. 11.1 Temperature dependence of electron emission during friction of a Ni specimen against PTFE: (◯) with increasing temperature, (●) with decreasing temperature [10]. Reprinted Wiley–VCH Verlag, Momose and Noguchi [10]. Copyright© 1984

in acetone and then drying under vacuum (emery abrasion); and oxidation by heating in air for 30 min at 463 K (heating oxidation). Figure 11.2 shows the TriboEE experimental setup [7]. In these experiments, Q gas (He+ iso-C4 H10 , ca. 1%) was continuously introduced into the apparatus at a flow rate of ca. 100 bubbles/min under atmospheric pressure, and then the anode voltage was set at 1350 V. Friction was applied at a rotation rate of 400 rpm at 298 K for 60 min. The number of emitted electrons over 60 min, termed the TriboEE total count, was used to estimate the activity of TriboEE. Although the TriboEE total count data for each surface preparation exhibited substantial deviation, the median value for each preparation was markedly different. Figure 11.3a, b shows the XPS spectra for emery abrasion and heating oxidation of Fe samples (before and after TriboEE measurements, respectively). The XPS spectra before and after TriboEE measurements were measured for three values of TriboEE total count: minimum, median, and maximum. The F 1s peak after TriboEE measurements tended to increase with increasing TriboEE total count. A striking feature is that the F 1s peak intensity for emery abrasion (Fig. 11.3a) was much weaker than that for heating oxidation (Fig. 11.3b). A strongly enhanced F 1s peak for heating oxidation was attributable to the Fe2 O3 oxide film, which was confirmed by the increase in a lower binding energy component at 530 eV in the O 1s peak. Therefore, the quantity of adsorption of PTFE debris on the metal surface depended on the presence of the oxide film and thus had an important role in the TriboEE total count. Another striking feature is that for Fe samples with greater TriboEE total count (Fig. 11.3b) (rows c and d)), a new strong component was evident: at a higher binding energy of 693 eV in the F 1s spectra and at 295 eV in the C 1s spectra. These peaks suggest adsorption of positively charged PTFE debris onto the metal surface. Figure 11.4 shows the change of TriboEE total

194

11 TriboEE Occurring from Metal Surfaces …

count versus F 1s surface composition for Fe. The values of the TriboEE total count for acetone cleaning and heating oxidation were distributed at nearly the same level, but that for emery abrasion was close to zero. The latter finding indicates that the ability of abraded Fe samples to cause TriboEE was low because of the reduction in the F 1s component. Furthermore, the total count for acetone cleaning and heating oxidation tended to increase with increasing F 1s composition, which originated from PTFE debris that were adsorbed onto the surface. Furthermore, the F 1s composition (which contributed to the total count) became greater for heating oxidation than for acetone cleaning and emery abrasion. Regarding the Cu samples, the data points of the TriboEE total count considerably overlapped between the preparations, but the total count had a tendency to increase in the order: acetone cleaning ≈ emery abrasion < heating oxidation. The Cu samples that were abraded with emery paper had the ability to produce TriboEE. This finding clearly differs from the abraded Fe samples. Furthermore, the total count for each preparation for Cu tended to increase with increasing F 1s composition. Thus, the PTFE used for rubbing facilitated electron emission from the metals.

Fig. 11.2 The TriboEE measurement apparatus [7]. Reprinted John Wiley, Momose and Iwashita [7]. Copyright© 2004

11.1 Outline of TriboEE from Metal Surfaces

195

Fig. 11.3 A The XPS spectra for emery abrasion Fe: a before TriboEE measurement; b–d after TriboEE measurement; TriboE total count: b minimum, c median, d maximum. B The XPS spectra for heating oxidation Fe: a before TriboEE measurement: b–d after TriboEE measurement; TriboEE total count: b minimum, c median, d maximum [7]. Reprinted John Wiley, Momose and Iwashita [7]. Copyright© 2004

196

11 TriboEE Occurring from Metal Surfaces …

Fig. 11.4 Change of TriboEE total count versus F 1s surface composition for Fe: (◯) acetone cleaning, (∎) emery abrasion, (Δ) heating oxidation [7]. Reprinted John Wiley, Momose and Iwashita [7]. Copyright© 2004

11.1.3 Effect on TriboEE of Plasma-Polymerized Films Formed on Metal Surfaces We are interested in the effects of thin hydrocarbon film coatings on the TriboEE intensity for metals [6]. The metal surfaces were coated with a plasma-polymerized film (PPF). The metals were Fe (purity 99.5%), Ni (99.9%), and Cu (99.6%), and the polymer rotator was PTFE. In this experiment, TriboEE was termed frictionally induced exoelectron emission. The rotator was made in the same manner as in Sect. 11.1.2. The metals and PTFE rotators were pretreated by ultrasonic cleaning in acetone for 30 min. A PPF of cyclohexane (C6 H12 vapor was deposited onto the metal surfaces with a 13.56-MHz radiofrequency source at the following deposition conditions: (a) power 30 W; C6 H12 vapor pressure 0.3 Torr; plasma-polymerization times of 1, 5, 10, and 15 min; and (b) 30 W, 5 min, C6 H12 vapor pressures of 0.1 and 0.3 Torr. For each metal, more than three samples were prepared at each condition. The TriboEE measurements were performed with an apparatus that was equipped with Q gas (Fig. 11.2). The median values of the TriboEE total count for 60-min rubbing time were compared between metals. When the PPF-coated metal samples were rubbed with a PTFE rotator, TriboEE was consistently observed. The TriboEE total count considerably depended on the metals and PPF preparation conditions. Regarding the effect of plasmapolymerization time condition (a), the total count for each metal increased with time and then attained a maximum at 5 min, followed by a decrease. The total count at

11.1 Outline of TriboEE from Metal Surfaces

197

5 min decreased in the order: Ni > Cu > Fe. Furthermore, for plasma-polymerization pressure condition (b), for each metal the total count at 0.3 Torr was greater than that at 0.1 Torr. The total count at 0.3 Torr decreased in the order: Ni > Cu > Fe. On the basis of the C 1 s compositions in the XPS measurements, the PPFs thickened over time and were thicker at 0.3 Torr than at 0.1 Torr. These findings suggest that a certain thickness of the PPFs can contribute to an increase in the TriboEE total count. Furthermore, Fig. 11.5 shows a plot of the values of TriboEE total count against the atomic composition ratio of F 1 s/C 1 s, obtained by XPS measurements after the TriboEE measurements. The data points were from the metals only after ultrasonically cleaning in acetone and with PPFs deposited at the conditions of 30 W, 5 min, and vapor pressure 0.3 and 0.1 Torr. The data points increased with increasing ratio of F 1 s/C 1 s, with the exception of data points for Fe. In Fig. 11.5, the data points for Ni and Cu (not Fe) were represented by approximate straight lines. When PTFE and PE sheets were rubbed with a PTFE rotator, there was only weak emission. Therefore, the electrons of TriboEE originated from the metal substrate and came through the thin PPF to the surface. PTFE was placed at the most negative side in the triboelectric series. Because PPF consists of a hydrocarbon film, a PPF that is in sliding contact with a PTFE rotator might be positively charged and abstract electrons from the base metal. Consequently, the hydrocarbon film that formed on the metal surfaces strongly influenced the magnitude of TriboEE, as a result of capturing the fluorine component from PTFE. Fig. 11.5 Plot of the TriboEE total count against the composition ratio of F 1s/C 1s for metals ultrasonically cleaned in acetone only and coated with plasma-polymerized films: (◯) Fe; (Δ) Ni; (Cu (∎). The films were deposited at the conditions of 30 W, 5 min, and C6 H12 vapor pressure 0.1 and 0.3 Torr. [6]. Reprinted JAST, Momose et al., [6]. Copyright© 1999

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11.2 Dependence of TriboEE Intensity on Elemental Metals 11.2.1 TriboEE Intensity of Elemental Metals We investigated the dependence of the TriboEE total count and F 1 s composition on elemental metals [12]. The metal samples were commercial rolled sheets (18 types, described in the following paragraph), the thickness and purity of which were in the ranges of 0.1–0.3 mm and 99.2%–99.999%, respectively. The polymer rod rotator was PTFE. The metal samples and the rotator were only ultrasonically cleaned in a mixture of acetone (30 mL) and petroleum benzene (30 mL) for 15 min, followed by drying under vacuum for 15 min. A gas-flow Geiger counter with Q gas as a counter gas was used. The TriboEE measurements were performed for a rubbing time of 60 min at a rotation rate of 400 rpm at 298 K. For each metal, three samples were used. The TriboEE total count was represented as the TriboEE intensity. The metals were classified in accordance with their corresponding groups in the periodic table as follows: Ti and Zr (group 4); V, Nb, and Ta (group 5); Mo and W (group 6); Fe (group 8); Co (group 9); Ni, Pd, and Pt (group 10); Cu, Ag, and Au (group 11); Zn (group 12); Al (group 13); and Sn (group 14). Figure 11.6 shows the dependence of the TriboEE total count on the metals, arranged in accordance with their corresponding groups of the periodic table. For metals in the same groups of 4, 5, 6, 10, and 11, moving down the groups corresponded to an increased TriboEE total count. Figure 11.7 shows the intensity of the metals’ XPS spectra after the TriboEE measurements. We focus on the F 1 s composition for the metals in the groups 4, 10, and 11. When moving down the groups, the F 1 s content (which originated from the PTFE debris) that was attached to the metal surfaces increased. Here, by comparing the TriboEE intensity with the F 1 s intensity, for metals in groups 4, 10, and 11, the order of the TriboEE total count well corresponded to that of the F 1 s content in accordance with XPS. The effect of other electronic properties of the metal surface was examined. The TriboEE total count was almost independent of the photoelectric thresholds {determined in photoemission (PE) experiment} regardless of the metal, although the number of emitted electrons (PE total count) that were obtained in the PE measurements tended to decrease with increasing photothreshold at 298 K [12]. The surface potential (SP) was also almost unchanged before and after the TriboEE measurements. On the basis of the relationships between the XPS, PE, and SP results, the TriboEE corresponds to the release of electrons (caused by a surface electric field) that were transported to the attached PTFE debris from the metal substrate. We will show a schematic model of TriboEE in a subsequent paragraph.

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Fig. 11.6 The dependence of the TriboE total count on metals (three samples) arranged in the groups of the periodic table. The figure in front of each metal symbol denotes the period for the metal [12]. Reprinted Springer, Momose et al., [12] and Kubo. Copyright© 2008

Fig. 11.7 The intensities of the XPS spectra for metals after TriboEE measurement: (◇) F1s, (◆) O1s, (●) C1s, (⬜) metal core [12]. Reprinted Springer, Momose et al., [12]. Copyright© 2008

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11.2.2 Relationship of TriboEE Intensity of Metal Surfaces to the Work Function and Surface Potential The study of factors that influence TriboEE as a result of sliding contact between metals and PTFE was continued, by mapping the TriboEE intensity for 16 types of metals (described in Sect. 11.2.1, except Al and Sn) onto corresponding periods or groups in the periodic table, where the work function or photothreshold of the metals changed in order [11]. Figure 11.8 shows the TriboEE intensity for the metals arranged in periods. The TriboEE intensity shown in Figs. 11.6 and 11.8 corresponds to the work function [4] or photothreshold [12] in certain periods and groups: the order of the TriboEE intensity between two adjoining metals in the same periods (4–6) and groups (4–6) corresponds to the work function and photothreshold of the metal samples. This is a striking feature for metals that correspond to such periods or groups: as the work function or photothreshod of the metal decreased, the TriboEE intensity increased. However, the TriboEE intensity of metals in groups 10–11 cannot be attributable solely to the work function or photothreshod. Regarding the SP values of metal surfaces and mated PTFE riders between two adjoining metals after the TriboEE measurements, as the SP value of the metal surfaces shifted to a morepositive direction and that of the PTFE rotators became more negative, the TriboEE intensity increased, although there are a few adjoining pairs that are exceptions. Based on these findings, a model of the TriboEE consisting of two steps was proposed. The first step is electron transfer from a metal substrate to the empty states of a PTFE rotator surface (because of an electric field between the metal substrate and the PTFE rotator in the sliding contact), and the second step is generation of microplasma by a separation-induced electric field in the gap between the PTFE rotator (with occupied traps) and the metal surface (covered with an oxide film and deposited PTFE debris in the mechanical separation), which leads to TriboEE.

11.2.3 TriboEE from Metal Surfaces Covered with an Oxide Film and Its Relationship to the Heat of Formation of Metal Oxides In Sect. 11.2.2, we reported that in groups 4 (Ti and Zr), 5 (V, Nb, and Ta), and 6 (Mo and W) in the periodic table, moving down a group, the TriboEE intensity increased (ascribed to a decrease in the work function or photothreshold), whereas in groups 10 (Ni, Pd, and Pt) and 11 (Cu, Ag, and Au), moving down the group, the TriboEE intensity substantially increased, independent of the work function or photothreshold. This difference has been of substantial interest, and we considered that the TriboEE intensity might be influenced by the properties of the oxide layer on the metal surfaces [5]. Therefore, we examined the relationship of the TriboEE intensity to the heat of formation of the metal oxide per oxygen atom {termed the metal–oxygen bond energy, D(M–O)} with the XPS intensity ratio of the oxygen/metal on the surface,

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Fig. 11.8 The plot of the TriboEE intensity against metals arranged on the periodic table. The figures below the element symbol is the atomic number [11]. Reprinted Elsevier, Momose and Yamashita [11]. Copyright© 2012

for 18 types of metals. Corresponding metal oxides in the groups were as follows: groups 4 (TiO and ZrO2 ), 5 (VO, NbO, and Ta2 O5 ), 6 (MoO2 and WO2 ), 8 (Fe3 O4 ), 9 (CoO), 10 (NiO, PdO, and Pt3 O4 ), 11 (Cu2 O, Ag2 O, and Au2 O3 ), 12 (ZnO), 13 (Al2 O3 ), and 14 (SnO2 ). Figure 11.9 shows the relationship between the TriboEE intensity and the D(M–O) values for metals in groups 4–6, 10, and 11. On the basis of Fig. 11.9, we drew the following conclusions: (1) In groups 4 (Ti and Zr), 5 (V, Nb, and Ta), and 6 (Mo and W), the D(M–O) values were at approximately at the same level and were considerably higher than those in groups 10 and 11; however, moving down these groups, the TriboEE intensity progressively increased. (2) When the metals in groups 4–6 were rearranged in periods 4–6, moving to the left of periods 4 (Ti and V), 5 (Zr, Nb, and Mo), and 6 (Ta and W), the TriboEE intensity increased with increasing D(M–O) value. (3) In groups 10 (Ni, Pd, and Pt) and 11 (Cu, Ag, and Au), moving down the groups, the D(M– O) value slowly decreased, whereas the TriboEE intensity tended to increase. (4) When the metals in groups 10 and 11 were rearranged in periods 4–6, moving to the right of periods 4 (Ni and Cu), 5 (Pd and Ag), and 6 (Pt and Au), the TriboEE intensity increased with decreasing D(M–O) value. Previously, [11] reported that the increasing TriboEE intensity for groups 4–6 is attributable to the decrease in the work function [4] or photothreshold. Moving down groups 10 and 11, the increase in TriboEE corresponds to a decreasing D(M– O) value. This is a new observation. One can explain these facts with a schematic for TriboEE (Fig. 11.10), which represents the Schottky tunneling effects. For the

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Fig. 11.9 The relationship between the TriboEE intensity and the D(M–O) values for metals in groups 4, 5, 6, 10, and 11 [5]. Reprinted Authors licensed under CC By 4.0. MDPI, Momose [5]. Copyright© 2021

metals in groups 4–6, the electrons predominantly pass over the top of the surface barrier (consists of a surface oxide layer), whereas for metals in groups 10 and 11, the electrons preferentially tunnel through the surface barrier because of the reduction of the thickness of the surface barrier. The values of D(M–O) become a key factor in dividing the TriboEE into two routes, because of the surface oxide layer. The metals in the groups 4–6 complete the former route, whereas those in the groups 10 and 11 complete the latter route. Figure 11.11 shows the dependence of the D(M–O) (all metals) and XPS metal core intensity (groups 4–12) on the O 1s intensity. In Fig. 11.11, top, the D(M–O) indicates the ability of the metal to bind to the oxygen on the metal surface. Overall, the D(M–O) values increased with increasing O 1s intensity. However, there are some irregular data points: the D(M–O) values within groups 4 (Ti and Zr), 5 (V, Nb, and Ta), and 6 (Mo and W) were high and almost the same. Furthermore, moving to the left in period 4 (Ti and V), the D(M–O) value increased with the O 1s intensity,

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Fig. 11.10 Model of scheme for TriboEE from a metal sample covered with surface oxide layer consisting of large (a) and small (b) D(M–O) values during sliding contact with PTFE rider, where an external electric field is caused: a Electrons pass over the top of the barrier (Schottky effect), where the change in work function occurs due to well-known image force. b Electrons tunnel through the barrier (tunnel effect) [5]. Reprinted Authors licensed under CC By 4.0. MDPI, Momose [5]. Copyright© 2021

whereas in periods 5 (Zr and Nb) and 6 (Ta and W), the D(M–O) values increased despite the decrease in the O 1s intensity. On the basis of these findings, the D(M–O) values of the metals in groups 4–6 do not simply correspond to the O 1s intensity. However, regarding the metals in periods 4 (Ni and Cu), 5 (Pd and Ag), and 6 (Pt and Au), the D(M–O) values decreased with decreasing O 1s intensity despite the difference in the oxidation number of the metals. The D(M–O) values of these metals were substantially influenced by the O 1s intensity. In Fig. 11.11, bottom, regarding the metals in groups 4–6 and 8, which are located on the left-hand side of the periodic table, the metal core intensity tended to slowly increase with increasing O 1s intensity, although the data points were scattered. Regarding the metals in groups 9–12, which are present on the right-hand side of the periodic table, the metal core intensity rapidly increased with decreasing O 1s intensity. These behaviors exhibit a striking contrast; in particular, for Ag, Au, Pt, and Pd, the number of metal atoms that are bound to one adsorbed oxygen atom is much greater than that of the other metals. Furthermore, the XPS intensity ratio of the O 1s/metal core results in an opposite trend between groups 4–6 and groups 10 and 11. Thus, under the electric field on practical metal surfaces (caused by rubbing with PTFE), with large D(M–O) values, the electron from the metal passes over the top of the barrier; whereas with small D(M–O) values, the electron from the metals predominantly tunnels through the surface oxide layer as a surface barrier. Finally, Fig. 11.12 shows a schematic of TriboEE in the vicinity of a metal PTFE interface, reported in [8].

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Fig. 11.11 The plots of D(M–O) values for all metals used (above) and XPS intensities of metal core (below) for metals in groups 4, 5, 6, and 8 and in groups 9, 10, 11, and 12 against O 1s intensities after TriboEE measurement [5]. Reprinted Authors licensed under CC By 4.0. MDPI, Momose [5]. Copyright© 2021

References

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Fig. 11.12 The relationship between Triboelectron emission from a metal sample occurring during sliding process by PTFE rider and a natural oxide layer with D(M–O) [5]. Reprinted Authors licensed under CC By 4.0. MDPI, Momose [5]. Copyright© 2021

References 1. B.V. Deryagin, N.A. Krotova, V.P. Smilga, Chapter III § 8 Emission of fast electrons in adhesive contact breaking or crystal cleavage, in Adhesion of solids. Consultants Bureau (New York, 1978), pp. 77–92§ 2. T. Greber, Charge-transfer induced particle emission in gas surface reactions. Surf. Sci. Rep. 28, 1–64 (1997) 3. R.I. Masel, Adsorption I: The binding of molecules to surfaces, in Principles of Adsorption and Reaction on Solid Surfaces. (Wiley, New York, 1996), pp.109–234 4. H.B. Michaelson, The work function of the elements and its periodicity. J. Appl. Phys. 48, 4729–4733 (1977) 5. Y. Momose, Electron transfer through a natural oxide layer on real metal surfaces occurring during sliding with polytetrafluoroethylene: dependence on heat of formation of metal oxides. Coatings 11,109 (2021). https://doi.org/10.3390/coatings11010109 6. Y. Momose, T. Hayashi, K. Kiuchi, Formation of plasma-polymerized film on the metal surfaces and its relationship to frictionally induced exoelectron emission, in Proceedings of the JAST Tribological Conference (Tokyo, 1999), pp. 333–334 7. Y. Momose, M. Iwashita, Surface analysis of metals using tribostimulated electron emission. Surf. Interface Anal. 36, 1241–1245 (2004) 8. Y. Momose, K. Kubo, Observation of triboelectron emission from real copper surfaces in sliding contact with polytetrafluoroethylene and polyimide. Tribol. Int. 48, 212–220 (2012) 9. Y. Momose, T. Namekawa, Exoelectron emission from metals subjected to friction and wear, and its relationship to the adsorption of oxygen, water vapor, and some other gases. J. Phys. Chem. 82, 1509–1515 (1978) 10. Y. Momose, M. Noguchi, Electron emission during frictional contact between metals and polymers. Phys. Stat. Sol. A 82, K83–K86 (1984) 11. Y. Momose, Y. Yamashita, Triboelectron emission from metal surfaces in sliding contact with polytetrafluoroethylene: relevance to work function and surface potential. Tribol. Int. 48, 232– 236 (2012)

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12. Y. Momose, Y. Yamashita, M. Honma, Observation of real metal surfaces by tribostimulated electron emission and its relationship to the analyses by XPS and photoemission. Tribol. Lett. 29(1), 75–84 (2008) 13. H. Nienhaus, Electronic excitations by chemical reactions on metal surfaces. Surf. Sci. Rep. 45, 1–78 (2002)

Chapter 12

Relationship of the EE Intensity of Metal Surfaces to Their Chemical Activity and Electrostatic Attractive Force

Abstract The TPPE results of metal surfaces pertain to the adsorption, chemical activity, and attractive force of metal surfaces. Examples include electrochemical reduction of CO2 at Cu electrodes; adsorption of water and alcohols onto Cu; coatings of plasma-polymerized (PP) films on Al, Fe, Ni, and Cu; corrosion protection of Al surfaces with PP coatings; and the electrostatic attractive force between metals and semiconductor substrates as well as charged polymer surfaces.

12.1 Application of TPPE to Cu Surfaces 12.1.1 Electrochemical Reduction of CO2 on Cu Electrodes and TPPE We are interested in the relationship of the chemical activity of Cu electrode surfaces (in the context of electrochemical reduction of CO2 ) to its TPPE characteristics. We have focused on the selectivity of H2 , CO, CH4 , and C2 H4 that one produces in the electrolysis of CO2 in aqueous solution at Cu electrodes (commercial rolled sheets; purity >99.9%). The electrode surfaces after electrolysis were analyzed by XPS, TPPE, and surface potential (SP) measurements. Three types of the Cu electrode were prepared, as follows: (A) only ultrasonic cleaning in acetone, (B) oxidation at 473 K in air for 30 min, and (C) exposure to Ar plasma (generator: 13.56-MHz radiofrequency, exposure time 1−30 min, pressure 40 Pa, power 30 W). Figure 12.1 shows the reaction cell used for CO2 electrochemical reduction [14]. The electrolysis was conducted in aqueous 0.1 M KHCO3 , saturated with CO2 at room temperature, for 3 h. The TPPE measurement after the electrolysis was performed with a Geiger counter that was equipped with a counter gas of Q gas (He + 1% iso-C4 H10 ) [7]. The sample surface was irradiated with UV light (D2 lamp, 30 W), the wavelength of which was scanned from 300 to 160 nm; additionally, the number of photoelectrons emitted (PE total count) under the wavelength scan was measured. The photoelectron emission (PE) total count was measured at various temperatures from 298 to 623 K. A TPPE plot is the PE total count as a function of measurement temperature. The TPPE plot was performed after the electrolysis at various applied potentials. © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 Y. Momose, Exoemission from Processed Solid Surfaces and Gas Adsorption, Springer Series in Surface Sciences 73, https://doi.org/10.1007/978-981-19-6948-5_12

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Fig. 12.1 The reaction cell for CO2 electrochemical reduction. Unpublished figure, Momose et al. [14]. Copyright© 1997

First, we describe the results for electrode A. Figure 12.2 shows the gas production (percentage) versus electrolysis potential at 3 h of electrolysis time for electrode A. As the potential of the sample increased in a negative direction in the range from −1.6 to −2.2 V vs. saturated calomel electrode, the quantity of CH4 increased, concomitant with a decrease in the quantities of H2 and CO, whereas the quantity of C2 H4 increased in the intermediate range of the potential. The production at the potential of −2.2 V decreased in the order: CH4 > H2 > C2 H4 > CO; however, the SP value became positive and shifted in a more positive direction with increasingly negative potential. Figure 12.3 shows the corresponding TPPE plot. With increasing temperature, the PE total count increased, reached a maximum, and then decreased; the level of the maximum increased remarkably and shifted to higher temperatures with increasingly negative potential. The production of CH4 (Fig. 12.2) well-corresponded to the increase of the PE total count with increasingly negative potential (Fig. 12.3). On the basis of the XPS measurements, the O 1s peak (assigned to adsorbed oxygen) was evident with increasingly negative potential; additionally, in the potential range where the production of C2 H4 increased, both metallic Cu (335 eV) and Cu2 O (337 eV) were evident in the CuLMM spectra. We next discuss the other electrodes. Regarding electrode B, the main product was H2 , but the production of C2 H4 , CH4 , and CO was small. After the electrolysis the SP value became negative and the PE total count was much smaller. In the O 1s spectra, the peak that corresponds to oxide oxygen was much greater than the peak that corresponds to adsorbed oxygen. In the CuLMM spectra, Cu2 O was present to a larger extent than metallic Cu. Regarding electrode C, the surface underwent

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Fig. 12.2 Plots of the production ratio of gases vs. electrolysis potential at 3 h of electrolysis for electrode A: (◇) CH4 ; (⬜) C2 H4 ; (▲) CO; (●) H2 [7]. Reprinted John Wiley, Momose et al. [7]. Copyright© 2002

Fig. 12.3 Plots of PE total count versus measurement temperature for electrode A after 3 h electrolysis as a function of electrolysis potential vs. SCE: (◯)—1.6 V; (⬜)—1.7 V; (◇)—1.8 V; (●)—1.9 V [7]. Reprinted John Wiley, Momose et al. [7]. Copyright© 2002

oxidation easily upon immersion in the solution. With increasing plasma treatment time, the SP value decreased and then reached a constant level. The PE total count became considerably smaller than that for electrodes A and B. With increasing plasma treatment time, the production of H2 and CO gradually increased, but the hydrocarbon production gradually decreased, resulting in the following order of products at the potential of −1.8 V for 30-min plasma treatment: H2 > C2 H4 > CH4 > CO. Regarding all of the samples after the electrolysis, the oxide oxygen was present to a larger extent in the O 1s spectra, and both Cu and Cu2 O were evident in the CuLMM spectra. The decrease in the SP and PE total count with increasing plasma treatment time corresponds to the reduction of hydrocarbon production. The presence of the adsorbed oxygen at the electrode surface can lead to an increase in the number of electrons that are available at the metal surface. The adsorbed OH groups can contribute to the interactions with the carbonyl groups of CO2 . After the attachment of the H atoms of the OH groups to the oxygen atoms of CO2 , there is electron transfer from the metal to the carbon atom of CO2 , which

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results in formation of an intermediate compound—such as HO−CH2 −OH [13]. This intermediate compound leads to production of CH4 and C2 H4 . Thus, the feasibility of electron transfer from the metal surface to CO2 , as well as the adsorption site of CO2 at the surface, are important factors in hydrocarbon production (in terms of electrochemical reduction of CO2 ).

12.1.2 TPPE Characteristics of Cu Subjected to Cleaning and Abrasion in Air, Water, and Alcohols We describe in detail the change of the TPPE plots of Cu metal surfaces by adsorption of air, water, and organics [6, 12]. The Cu surfaces were ultrasonically cleaned and subsequently abraded with a screw in water, organic liquids, and ambient air. The TPPE apparatus consisted of a Geiger counter with Q gas (He+ iso-C4 H10 ) (Fig. 10.1). The curve of the intensity of PE vs. wavelength from 300 to 160 nm, termed the PE stimulation spectrum, was repeatedly measured at various temperatures in the temperature-increase and subsequent temperature-decrease process from 25 to 350 °C. Figure 12.4 shows typical PE spectra in the temperature-increase process in the first temperature scan. Figure 12.5 shows TPPE plots {PE total count (number of emitted electrons in a PE spectrum) vs. measurement temperature} in the first and second processes. The plot in the temperature-increase process in the first temperature scan, which was usually used, strikingly differed from that in the second temperature scan. In this case, the PE total count increased, reached a maximum at ca. 250 °C, and then decreased with increasing temperature. The TPPE plots were substantially influenced by the environment where the metal surface was abraded. Figure 12.6a–f shows TPPE plots for Cu surfaces abraded in H2 O, CH3 OH, C2 H5 OH, C3 H7 OH, (CH3 )2 CHOH, and ambient air, respectively, as a function of the abrasion period. Figure 12.7 shows TPPE plots of the Cu surfaces after only ultrasonic cleaning in the liquids, whereas Fig. 12.8 shows analogous plots after 10-min abrasion in the liquids and ambient air. Regarding the cleaned surfaces, the maximum PE total count decreased in the following order: C3 H7 OH > (CH3 )2 CHOH > C2 H5 OH > CH3 OH > H2 O. Regarding the mechanically abraded surfaces, the maximum tended to be evident at higher temperatures with increasing abrasion period; the level of the PE total count at ca. 250 °C, 10-min abrasion decreased in the following order: H2 O > CH3 OH > C2 H5 OH > (CH3 )2 CHOH > n-C3 H7 OH. The order of the organics and water was reversed as a result of the abrasion. The results in Fig. 12.7 correspond to a contaminant overlayer that remained on the steady-state metal surface. The formation of PE active centers in the contaminant overlayer was more strongly enhanced by adsorption of the alcohols, such as C3 H7 OH and (CH3 )2 CHOH, compared with adsorption of water. Regarding the results in Fig. 12.8, the liquids that were used in the present experiments were saturated with air, such that a metallic surface that one exposes by abrasion can react with not only the liquid molecule but also oxygen

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Fig. 12.4 Typical PE stimulation spectra at 250 °C for Cu abraded in CH3OH: abrasion period: a 5 min; b 10 min; c 30 min [12]. Reprinted Momose et al. [12]. Copyright© 2000

Fig. 12.5 Typical plots of PE total count versus measurement temperature for Cu abraded in C2 H5 OH for 10 min: a, b the 1st temperature increase and subsequent decrease process, respectively; c, d the 2nd temperature increase and subsequent decrease process, respectively [12]. Reprinted Momose et al. [12]. Copyright© 2000

that is dissolved in the liquid. The heat of wetting for copper powder in oxygensaturated water was greater than that in dissolved-oxygen-free water. Therefore, in the present experiments, the acid–base interactions of the water and alcohols during abrading occurred through oxygen that was previously adsorbed onto the freshly deformed metal surface. This process results in the formation of PE active centers in the resulting thin overlayer.

12.2 Corrosion Protection of Al Surfaces by Plasma-Polymerized Coatings and TPPE Organic coatings that one applies to metal surfaces provide corrosion protection by introducing a barrier to ionic transport and electrical conduction. Application of PP films for metal pretreatment or corrosion protection is an active area of

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Fig. 12.6 (a) TPPE plots (PE total count vs. measurement temperature) for Cu abraded in H2 O: abrasion period: a 5 min; b 10 min; c 30 min; d ultrasonic cleaning alone in H2 O [12]. Reprinted [12]. Copyright© 2000. (b) TPPE plots (PE total count vs. measurement temperature) for Cu abraded in CH3 OH: abrasion period: a 5 min; b 10 min; c 30 min; d ultrasonic cleaning alone in CH3 OH [12]. Reprinted [12]. Copyright© 2000. (c) TPPE plots (PE total count vs. measurement temperature) for Cu abraded in C2 H5 OH: abrasion period: a 5 min; b 10 min; c 30 min; d ultrasonic cleaning alone in C2 H5 OH [12]. Reprinted [12]. Copyright© 2000. (d) TPPE plots (PE total count vs. measurement temperature) for Cu abraded in C3 H7 OH: abrasion period: a 5 min; b 10 min; c 30 min; d ultrasonic cleaning alone in C3 H7 OH [12]. Reprinted [12]. Copyright© 2000. (e) TPPE plots (PE total count vs. measurement temperature) for Cu abraded in (CH3 )2 CHOH: abrasion period: a 5 min; b 10 min; c 30 min; d ultrasonic cleaning alone in (CH3 )2 CHOH [12]. Reprinted [12]. Copyright© 2000. (f) TPPE plots (PE total count vs. measurement temperature) for Cu abraded in ambient air: abrasion period: a 5 min; b 10 min; c 30 min [12]. Reprinted [12]. Copyright© 2000

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Fig. 12.7 a Dependence of TPPE plots for ultrasonic cleaning alone on the liquids: a H2 O; b CH3 OH; c C2 H5 OH; d (CH3 )2 CHOH; e C3 H7 OH [12]. Reprinted [12]. Copyright© 2000

Fig. 12.8 b Dependence of TPPE plots for 10-min abrasion on the abrasion liquids: a H2 O; b CH3 OH; c C2 H5 OH; d (CH3 )2 CHOH; e C3 H7 OH; f ambient air [12]. Reprinted [12]. Copyright© 2000

research because plasma polymerization is solvent-free and therefore environmentally friendly. We examined the relationship between the corrosion-protective performance of PP coatings on Al surfaces and the corresponding TPPE results [10]. Two mixtures {tetraethoxysilane [Si (OC2 H5 )4 ; i.e., TEOS] and O2 , and hexamethyldisiloxane [(CH3 )3 SiOSi(CH3 )3 ; i.e., HMDSO] and O2 } as well as only cyclohexane vapor (C6 H12 , CH) were used to form PP films. The Al surfaces were previously exposed to Ar plasma that was generated with a 13.56-MHz radiofrequency generator, and then the PP films were deposited. Measurements of the potentiodynamic (cathodic) polarization curves of the samples that were coated with the PP films was performed in 3 wt.% aqueous NaCl, in which the sample was used as the working electrode. On the basis of the curves, the weight loss rate (W; milligrams per square decimeter per day) of the electrodes was obtained to evaluate the corrosion-protective performance. Figure 12.9 shows the W values of the films against the pressure of O2 that was used to form the PP films. The level of the W values of the films decreased

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Fig. 12.9 Weight loss rate for the PP films as a function of O2 pressure and C6 H12 pressure: A TEOS film: (●) 5-min deposition time, (◯) 10-min deposition time; B HMDSO film: (▲) 6.7-Pa HMDSO pressure, (Δ) 13-Pa HMDSO pressure; C CH film [10]. Reprinted Elsevier [10]. Copyright© 2003

in the following order: HMDSO/O2 > CH > TEOS/O2 . It is evident that the PP film of the TEOS/O2 mixtures resulted in the best protective performance. The value of W for the uncoated sample was approximately 5 mg dm−2 d−1 . The XPS analysis indicates that the TEOS films had a chemical structure that was similar to that of silica (SiO2 ). Figure 12.10 shows the TPPE results before and after Ar-plasma treatment. The TPPE analysis indicates that the Ar-plasma pretreatment substantially increased the ability of the surface to emit electrons. Thus, the electronic activation and the surface cleaning can be pertinent to adhesion of a deposited film to a metal surface. Furthermore, the TEOS films suppressed the ability of the surface to emit electrons in the TPPE measurements.

12.3 Corrosion Protection of Fe, Ni, and Cu Metal Surfaces by Plasma-Polymerized Coatings, and Its Relationship to the Electronic Properties of Metals Many aspects of the degradation of the polymer/metal (oxide) interface are not yet well-understood [1]. If a metal is discontinuously covered with an organic coating and subsequently exposed to an aqueous corrosive environment, the coated metal

12.3 Corrosion Protection of Fe, Ni, and Cu Metal …

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Fig. 12.10 Photoelectron emission (PE) total count for Al substrates as a function of the PE measurement temperature: A only ultrasonically clean in acetone; B exposed to Ar plasma [10]. Reprinted Elsevier [10]. Copyright© 2003

acts as a cathode (i.e., an electron-accepting site), and the exposed metal acts as an anode (i.e., electron-generating site). The cathodic reaction affects the bond between the organic coating and the metal. Regarding the cathodic delamination of organic coatings from a metal surface, [4] emphasized that at cathodic potentials in which the reaction represented by Eq. (12.1) dominates there is no substantial delamination in the absence of O2 . An essential reactant in the cathodic reaction is the electron and the cathodic reaction occurs most easily on oxidized metals. H2 O + (1/2) O2 + 2e− = 2OH− ,

(12.1)

We are interested in evaluating PP films as corrosion-protective barriers by cyclic voltammetry (CV) (i.e., the current−voltage relationships upon application of a potential to a working electrode), and furthermore in the relationship of the CV results to the electronic properties of metals that one obtains in measurements of TPPE (Chap. 10) as well as TriboEE (Chap. 11). First, we introduce the corrosion-protective performance of PP films on metal substrates by CV [2]. A PP film from C6 H12 (cyclohexane) was deposited onto Fe, Ni, and Cu substrates with a radiofrequency generator (13.56-MHz). Rolled metal sheets (purity 99.5%, thickness 0.1 mm) were used as the metal substrates and were ultrasonically cleaned in acetone for 30 min prior to use. Sheets that were only cleaned were used as a control for CV measurements. After cleaning with Ar plasma at the conditions of 5 min, 13 Pa, and 30 W, the metal surfaces were exposed to plasma of C6 H12 vapor, which was generated at a flow rate of 5.0 × 10−3 mol/h (pressure: 40 Pa) under the following conditions: power of 30 W and deposition times of 1, 5, 10, and 15 min. Two or five samples were prepared under the same PP film deposition conditions to check the reproducibility of the experimental data. Cleaned and subsequently PP film-coated metal substrates were used as the

216

12 Relationship of the EE Intensity of Metal Surfaces …

working electrode in aerated 3 wt.% aqueous NaCl, and the cyclic voltammogram was measured repeatedly 10 times upon application of a triangular potential between −1 and 1 V versus the initial corrosion potential of the sample. Figures 12.11, 12.12 and 12.13 show representative cyclic voltammograms for the control and PP films (the latter were the coated Fe, Ni, and Cu samples). The cyclic voltammogram depended strongly on the metal substrate. Regarding Fe and Ni, only an anodic current was observed; however, regarding Cu, anodic and subsequently cathodic currents were evident. Here, the voltammograms are shown for only three potential sweep numbers of 1, 5, and 10. The characteristics of the voltammograms are as follows: (1) Regarding the anodic current level, regarding Fe, the level for sweep numbers 5 and 10 became higher than that for sweep number 1, whereas regarding Ni, the current decreased with increasing sweep number and decreased with increasing film deposition time. Regarding Cu, the relationship of the current level to the sweep number depended on the deposition time. For the control and 1-min deposition time, the current level for sweep numbers 5 and 10 became lower than that for sweep number 1, whereas for 5-, 10-, and 15-min deposition times, the current level for sweep numbers 5 and 10 became higher than that for sweep number 1. (2) As denoted by two arrows (→) in Figs. 12.11a, 12.12a, and 12.13a, the direction of the anodic current for the control Fe and Ni samples differed from that for the control Cu sample. This might be associated with the oxidation and subsequent reduction of the overlayer on the Cu metal surface. (3) The anodic current level for the metal substrates decreased in the order Fe > Ni > Cu. (4) The cathodic current that was observed for Cu decreased with increasing deposition time, and also decreased with increasing sweep number. The manifestation of the two types of current for Cu suggests that the Cu metal surface easily underwent oxidation and reduction. Let us consider the relationship of the electronic data to the two types of current that were only evident for Cu (Fig. 12.13). Table 12.1 shows the values of the photothreshold and PE total count of TPPE, the heat of the metal oxide per oxygen atom (Δf H 0 ), the metal–oxygen bond energy [D(M–O), calculated from Δf H 0 ], and the TriboEE intensity for the three metals. The Cu metal exhibited the largest values in the PE total count and the TriboEE intensity, and the lowest values of the photothreshold, Δf H 0 , and D(M–O). Therefore, in the vicinity of the overlayer of the Cu surface, there is an abundance of electrons that are able to move around freely, which facilitates the cathodic reaction represented by Eq. (12.1). Furthermore, in the same manner as the accelerating voltage (AV) of −94 V relative to the earthed grid (applied in the measurements of PE and TriboEE), the cathodic potential facilitates accumulation of electrons at the surface.

12.3 Corrosion Protection of Fe, Ni, and Cu Metal … Fig. 12.11 Dependence of the cyclic voltammogram for a Fe substrate coated with PP films at various deposition times on the potential sweep number (1, 5, and 10): a control; b 1 min; c 5 min; d 10 min; e 15 min [2]. Reprinted VSP [2] Copyright© 2001

217

218 Fig. 12.12 Dependence of the cyclic voltammogram for a Ni substrate coated with PP films at various deposition times on the potential sweep number (1, 5, and 10): a control; b 1 min; c 5 min; d 10 min; e 15 min [2]. Reprinted VSP [2] Copyright© 2001

12 Relationship of the EE Intensity of Metal Surfaces …

12.3 Corrosion Protection of Fe, Ni, and Cu Metal … Fig. 12.13 Dependence of the cyclic voltammogram for a Cu substrate coated with PP films at various deposition times on the potential sweep number (1, 5, and 10): a control; b 1 min; c 5 min; d 10 min; e 15 min [2]. Reprinted VSP [2] Copyright© 2001

219

c

b

a

14,500 ~ 9300

Anodic and cathodic currents

Cu

The data at 25 °C for the metals ultrasonically cleaned in acetone [8] From Table 10.5 [11] [5]

4.54

7500 ~ 2300

Anodic current 5.10

Ni

3100 ~ 2400

Anodic current 5.13

Fe

Max and Min of PE total count(countsb )

Types of current in CV

Metal

Photothresholda (eV)

TriboEE intensityc (105 counts)

6.59 ± 2.94 3.33 ± 1.15 6.83 ± 2.61

Heat of formation for metal oxide per oxygen atomc) /kJmol−1 (Metal –oxygen bond energy, D(M–O) (102 kJ mol−1 ) −279.6(Fe3 O4 ) (D(M–O) = 5.28) −239.9(NiO) (D(M–O) = 4.89) −168.6(Cu2 O) (D(M–O) = 4.18)

Table 12.1 Two types of current in the cyclic voltammetry (CV); and electronic properties for Fe, Ni, and Cu metals that were ultrasonically cleaned in acetone

220 12 Relationship of the EE Intensity of Metal Surfaces …

12.3 Corrosion Protection of Fe, Ni, and Cu Metal …

221

12.3.1 Effect of TPPE on Electrostatic Attractive Force Between Metals, Semiconductors, and Tribocharged Polymers We deloped a new, simple apparatus and method for measuring long-range electrostatic attractive forces between substrate (metal and silicon wafer) surfaces and tribocharged polymer surfaces, to estimate the effect of the electronic properties of substrate surfaces on polymer sheets [9]. The electronic properties were examined by TPPE (Chap. 11). Commercial rolled Ni and Ti sheets (purity 99.5%, thickness 0.1 mm) and polished Si wafers [(100) plane, resistance >1000 Ω·cm, thickness 525 µm] were used as the substrates. Sheets of polystyrene foam (PS) (thickness 5 mm, size 30 mm × 60 mm) and polytetrafluoroethylene (PTFE) (thickness 2 mm, size 30 mm × 70 mm) were used as tribocharged polymers. The size of the substrates was 20 mm × 20 mm (for attractive force measurements), 20 mm × 30 mm (for TPPE), and 3 mm × 3 mm (for XPS). Metal surfaces were prepared by the following three pretreatments: successive abrasion with three emery papers of 80-, 20-, and 5µm grain size (emery abrasion); ultrasonic cleaning in acetone, followed by drying under vacuum (acetone cleaning); and thermal oxidation in air at 863 K for 1 h (thermal oxidation). Regarding the Si wafer, two types of samples were used: asreceived, and thermally oxidized in air at 873 K for 3 h. Immediately before the attractive force measurements, the polymer surfaces were tribocharged by rubbing the surfaces on cellulose paper at a constant rotation rate. The polymer surfaces became positively and negatively charged after the rubbing: the surface potential of PS was >+2000 V and that of PTFE was