X-ray photoelectron spectroscopy 9781617282409 1617282405

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CHEMICAL ENGINEERING METHODS AND TECHNOLOGY

X-RAY PHOTOELECTRON SPECTROSCOPY No part of this digital document may be reproduced, stored in a retrieval system or transmitted in any form or by any means. The publisher has taken reasonable care in the preparation of this digital document, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained herein. This digital document is sold with the clear understanding that the publisher is not engaged in rendering legal, medical or any other professional services.

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CHEMICAL ENGINEERING METHODS AND TECHNOLOGY

X-RAY PHOTOELECTRON SPECTROSCOPY

JOHANNA M. WAGNER EDITOR

Nova Science Publishers, Inc. New York

Copyright © 2011 by Nova Science Publishers, Inc. All rights reserved. No part of this book may be reproduced, stored in a retrieval system or transmitted in any form or by any means: electronic, electrostatic, magnetic, tape, mechanical photocopying, recording or otherwise without the written permission of the Publisher. For permission to use material from this book please contact us: Telephone 631-231-7269; Fax 631-231-8175 Web Site: http://www.novapublishers.com

NOTICE TO THE READER The Publisher has taken reasonable care in the preparation of this book, but makes no expressed or implied warranty of any kind and assumes no responsibility for any errors or omissions. No liability is assumed for incidental or consequential damages in connection with or arising out of information contained in this book. The Publisher shall not be liable for any special, consequential, or exemplary damages resulting, in whole or in part, from the readers’ use of, or reliance upon, this material. Any parts of this book based on government reports are so indicated and copyright is claimed for those parts to the extent applicable to compilations of such works. Independent verification should be sought for any data, advice or recommendations contained in this book. In addition, no responsibility is assumed by the publisher for any injury and/or damage to persons or property arising from any methods, products, instructions, ideas or otherwise contained in this publication. This publication is designed to provide accurate and authoritative information with regard to the subject matter covered herein. It is sold with the clear understanding that the Publisher is not engaged in rendering legal or any other professional services. If legal or any other expert assistance is required, the services of a competent person should be sought. FROM A DECLARATION OF PARTICIPANTS JOINTLY ADOPTED BY A COMMITTEE OF THE AMERICAN BAR ASSOCIATION AND A COMMITTEE OF PUBLISHERS. Additional color graphics may be available in the e-book version of this book. LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA X-ray photoelectron spectroscopy / editor, Johanna M. Wagner. p. cm. Includes index. ISBN 978-1-61728-240-9 (eBook) 1. X-ray photoelectron spectroscopy. I. Wagner, Johanna M. QC454.P48X22 2011 543'.62--dc22 2010014098

Published by Nova Science Publishers, Inc. † New York

CONTENTS Preface Chapter 1

vii X-ray Photoelectron Spectroscopy: Studies from Industrial & Bioactive Glass to Biomaterials A.W. Wren, F.R. Laffir, N.P. Mellot and M.R. Towler

1

Chapter 2

XPS for Investigating Cultural Heritage Materials Claudia Altavilla and Enrico Ciliberto

Chapter 3

X-Ray Photoelectron Spectroscopy Applied to Atomic Structure Analysis of Silicate Glasses Thin Layers Olga Kanunnikova

63

Influence of NO2 on Atmospheric Corrosion of Zinc Exposed in a Climate Chamber G. Juan Castaño

89

Chapter 4

Chapter 5

Chapter 6

Chapter 7

Chapter 8

Chapter 9

31

X-Ray Photoelectron Spectroscopy as a Tool in the Study of Nanostructured Titanium and Commercial PET Surfaces in Biotechnological Applications Hayden Webb, Vi Khanh Truong, T. Robert Jones, J. Russell Crawfor and Elena Ivanova

111

XPS as a powerful tool to investigate the surface properties of simple, doped and mixed metal oxides Silvia Gross and Lidia Armelao

125

X-Ray Photoelectron Spectroscopy Investigation of Iron Oxides in the Xuancheng Laterite Profile and Their Palaeoclimate Interpretations Hanlie Hong New Developments in X-Ray Photoelectron Spectroscopy Applied to Nanostructured Materials Ilaria Fratoddi, Chiara Battocchio and Giovanni Polzonett Mechanisms of Anhydrite Hydration Xinya Yan, Jinhua Wang and Hanlie Hong

187

201 209

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Chapter 10

XPS Analysis of Cotton Fibres N. Rjiba, P. Fioux, J.Y. Drean, R. Frydrych and M. Nardin

Chapter 11

The Capability of X-Ray Photoelectron Spectroscopy in the Characterization of Membranes: Correlation between Surface Chemical and Transport Properties in Polymeric Membranes María Jesús Ariza, Juana Benavente and Enrique Rodríguez-Castellón

Index

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239

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PREFACE X-ray photoelectron spectroscopy (XPS) is a quantitative spectroscopic technique that measures the elemental composition, empirical formula, chemical state and electronic state of the elements that exist within a material. XPS spectra are obtained by irradiating a material with a beam of X-rays while simultaneously measuring the kinetic energy (KE) and number of electrons that escape from the top 1 to 10 nm of the material being analyzed. This book reviews research in the field of X-ray photoelectron spectroscopy including: XPS studies from industrial and bioactive glass to biomaterials and applied to the atomic structure analysis of silicate glasses thin layers; XPS as a tool in the study of nanostructured titanium and in commercial PET surfaces in biotechnological applications and others. Chapter 1 – X-ray Photoelectron Spectroscopy (XPS) is a powerful research tool for the study of chemical and physical phenomenon occurring at surfaces of a wide range of materials. It was developed at the University of Uppsala, Sweden in the 1940-50s and works on the principle of the photoelectric effect in which soft X-rays (Mg and Al Kα x-ray) are used as the exciting photon source. The emitted photoelectrons have a kinetic energy distribution consisting of discrete peaks related to the binding energies of the photoionized atom and provide information on the electronic structure of a surface and its chemical composition. XPS is also a sensitive probe of the chemical environment or oxidation state of surface species. Since its development XPS has been applied for analysing many types of materials including metals, polymers, ceramics, composites, glasses, semiconductors and biological samples. More recently there has been an increase in the use of XPS in the characterisation of biomaterials where surface interactions play a key role in the success and longevity of an implanted biomaterial. The development of oxide glasses is one key area that benefits from the use of XPS where information relating to the composition and bulk structural properties can be determined. This can aid in better design of these materials and the ability to tailor towards specific applications. The growing use of XPS is reflected in the increase in publications. The number of XPS papers published in 2007 (5469) is almost double that published in 1997 (2830). This growth can be attributed to the fact that XPS can be successfully applied to a large variety of materials. Chapter 2 - The continuous demand for new analytical methods in studies relating to art and archaeology has prompted researchers to investigate the possible applications of a variety of non-conventional and innovative analytical techniques. In this chapter, the authors describe several interesting applications of X-ray photoelectron spectroscopy in order to undertand in greater detail the chemical phenomena which are present on the surface of archeological and

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artistic objects that they have been studied over the last ten years. After a preliminary discussion on the potentialities and limitations of XPS in this fascinating research field, a series of case studies on different kinds of art crafts is reported. The use of XPS, in monitoring the efficiency and the damage of restoring techniques on the surfaces of several masterpieces such as Michelangelo’s David and the S. Marco Basilica (Venice) mosaics, is also critically discussed. Finally, some innovative techniques, such as Self Assembled Monolayers, used for the protection of material surfaces are tested by the use of XPS. Chapter 3 - X-ray photoelectron spectroscopy studies changes in the chemical state of atoms within a material and traditionally used to analyze content and chemical state of elements in thin surface layer. An attempt has been made to expand the application area of Xray photoelectron spectroscopy to the analysis of the atomic structure of silicate glasses thin surface layers. The technique, based on the photoelectron spectra analysis, which allows one to infer information about content and statistics of the intermediate-range order structural units and cross-linking degree of glass structure was developed Chapter 4 - Nitrogen oxides are among the main pollutants of the atmosphere. In many places, the content of nitrogen dioxide exceeds that of sulfur dioxide, which is the main aggressive agent in atmospheric corrosion of zinc and galvanized steel. However, studies on the effect of NO2 on zinc corrosion are scarce and their results are variable and at times seemingly contradictory. The dry deposition on zinc surfaces of nitrogen dioxide (NO2), alone and in combination with sulfur dioxide (SO2), at temperatures of 35 and 25°C and relative humidity (RH) of 90%, was simulated in a climatic chamber. Pollutant concentrations belonged to highly polluted industrial atmosphere levels. From the gravimetric results obtained and from the characterization of the corrosion products by X-ray Photoelectron Spectroscopy (XPS) and Ion Exchange Chromatography (IC), we have verified an accelerating effect when NO2 acts in conjunction with SO2 at 25°C and 90% RH. At 35°C and 90% RH, the accelerating effect is smaller, which suggests it may be favoured by the presence of moisture. In those cases where an accelerating effect has been observed, highresolution XPS spectra showed a greater proportion of sulfates and the lack of nitrogen compounds in the corrosion products, indicating that NO2 participates indirectly as a catalyst of the oxidation of SO2 to sulfate. However, the corrosive action of NO2 alone is negligible compared with SO2. Chapter 5 - In this Chapter, two case studies are presented. In the first study, XPS was used to identify the oxidation states of titanium in nanostructured titanium films on titanium surfaces fabricated by different polishing (mechanical and chemical) and sputtering techniques. A layer of titanium dioxide was detected on both substrates, forming a protective passive film. The physiochemical and dielectric properties of this film determine the biocompatibility of titanium when used in the manufacture of medical implants. When formed in aqueous solution, air, or other oxygen-containing environments, the oxide layer is normally only a few nanometres thick and consists essentially of titanium dioxide. However, by varying the sputtering conditions, other chemical states of titanium oxide can be formed, such as Ti2O3 and TiO. Carbon was detected on all surfaces, at various levels ranging from 11% to 30%. Carbon has been shown to be a commonly encountered contaminant in similar studies. In the second study, XPS was used to investigate the bacterial modification of the surfaces of poly(ethylene terephthalate) (PET) bottles exposed to bacterial enrichment culture over a nine month period. Four elements, two carbon species, two oxygen species, and one nitrogen species were detected on the modified surface that were not detected on the reference

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surface, and two elements present on the reference surface were not detected on the modified surface. As was found in the analysis of the titanium surfaces, some carbon contamination was detected, however as carbon is the primary element in the polymer chains, it is difficult to evaluate the exact nature and extent of contamination. Chapter 6 - XPS is a well established surface sensitive method for the analysis of solid materials, allowing the determination of surface composition and the assessment of oxidation state and chemical environment of the different species. In fact, core level XPS is highly sensitive to the chemical environment, i.e. the binding energy may be strongly dependent on the oxidation state of the investigated ion, as well as on the neighbouring atoms. Oxide-based materials (simple, multicomponent and composites), are a fascinating and technologically appealing class of materials, exhibiting a huge variety of chemical, structural, optical, magnetic, electric and electronic properties. Since in oxides the functional properties are mainly related to the complex interplay among composition, microstructure, shape, characteristic size, charge density, chemical environment, presence of defects etc., the understanding of their actual chemical nature and electronic structure is a primary concern to fully exploit their functionalities in different fields. In this chapter, after a general overview on metal oxides and on their electronic structure and peculiarities, the application of XPS to their investigation will be introduced and discussed and some selected application fields will be considered. The effect of ion sputtering in oxides will be moreover shortly examined. Furthermore, in this contribution, the issues related to the determination of the actual chemical state of metal species in mixed oxides will be also extensively addressed and discussed. The possibility of XPS to evidence the presence of M-O-M’ mixed bonds will be highlighted. In the second part of the chapter, several different selected case studies will be analysed in detail, which deal with different oxide systems: simple oxides, also in mixed-valence state, (ZnO, WO3, CuxOy, VxOy), mixed oxides (e.g. SiO2-HfO2, SiO2-ZrO2, SiO2-TiO2, ZrO2TiO2), doped oxides (FexOy:TiO2, M(Pt, Ag):TiO2, M (Au, Pt, Pd, Rh):SiO2). In particular, the use of this method for the investigation of the chemical composition of different typologies of surfaces (thin films, powders) will be discussed. A further aspect to be dealt with is the perspective combination of XPS, also at variable pressure, with other site-specific spectroscopic methods (XAS, XEOL) and theoretical tools (e.g. DFT), which represents a valuable tool to unravel and predict issues related to the reactivity and to other functional properties of these materials Chapter 7 - X-ray photoelectron spectroscopy (XPS) has been applied to assess the mineral composition of iron oxides in a laterite soil profile at Xuancheng, southeastern China, the profile is 10.4 meter thick and is subdivided into 18 lithological layers. Since the analysis is restricted to the particle surface, this technique is uniquely suitable to sensitively determine the fine particle iron components in soils directly, especially those coating on clay mineral surfaces, and avoiding dissolving and recrystallizing of iron oxides during the treating processes of selective dissolution techniques. The results show that Fe 2p peaks of the soil samples can be decomposed to three components, the binding energy (BE) values of the three components of the 18 soil samples are in the ranges of 709.87 to 710.48, 710.88 to 711.37, and 712.08 to 713.14 eV, which are due to Fe 2p in hematite, goethite, and amorphous coating Fe3+ phases, respectively. The higher goethite content and the smaller hematite/goethite ratio indicate a warm and humid climate condition, while the smaller goethite content and the higher hematite/goethite ratio are indicative of a cold and dry climate

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condition. Climate changes suggested by the content of goethite and ratio of hematite to goethite are in good agreement with those inferred from clay mineralogy along the soil profile. The contents of goethite and hematite are closely related to that of kaolinite and illitesmectite. Total iron content of the laterite soils is not closely related to climate conditions. Our findings suggest that the content of goethite and the hematite/goethite ratio instead of total iron content can be used to probe the palaeoclimate conditions, and XPS method is efficient to detect the iron components in laterite soils. Chapter 8 - The study and characterization of nanostructured materials is growing very fast and potential applications of new materials at the nanoscale are envisaged, based on the general observation that the performance of matter at molecular scales will exhibit unexpected features, depending on size, shape and composition. The phenomena occurring at the nanoscale are of fundamental interest and involve synthesis and chemical-physical studies, optoelectronic and biotechnological characterization, for applications in several field of technology. One of the main goals to the study of nanostructured systems is to deeply understand the behavior of materials when the sizes are close to molecular dimensions. To address this purpose, different techniques have been developed and among surface sensitive techniques, X-ray Photoelectron Spectroscopy (XPS) is particularly suited and largely employed. In fact, XPS provides fundamental information on sample surface, elemental composition and chemical state of the elements of the material under analysis. By the analysis of the Binding Energy (BE) of core electrons, this technique allows qualitative elemental identification. Little BE variations, due to the chemical environment of the selected atom, determine the oxidation state and its perturbation due to the formation of new chemical bonds with interacting species. XPS surface sensitivity to the outmost layers of the investigated materials is an important peculiarity when the surface plays a fundamental role, as in nanostructured materials, in which the chemical nature of the surface, the surface reactions, the interface characteristics and molecular adhesion have a primary function. In this commentary the research developments and future perspectives of XPS characterization of nanostructured materials will be reviewed, with particular attention to surface and interface effects for nanoparticles of different sizes and shapes. Reports on these extremely important topics will be addressed. Chapter 9 - Previous studies on hydration of anhydrite were summarized and discrepancy in the mechanism of anhydrite hydration between different researchers was analyzed. In our present study, the double salt mechanism by Budnikov and the z/r ratio law by Murat et al were examined based on investigation of the formation and hydration of sulfate calcium double salt. Our results showed that most of the soluble sulfate compounds can excite the hydration of anhydrite. However, only part of the sulfate reacted with anhydrite and formed double salts at ambient temperature, and the rate of reaction between sulfate and anhydrite was less than that between the sulfate and gypsum, this argues against the double salt mechanism by Budnikov. During the hydration process, crystallization of hydrate sulfate exerts influence on hydration of sulfate calcium instead of the z/r ratio of excitation agent. Hydration of anhydrite in fact follows the path of dissolution-nucleation-crystallization. Soluble sulfate compounds decrease the surface potential barrier of nucleation and act as the catalytic agent of nucleation Chapter 10 - The chemical surface composition of three types of cotton fibers, from different origins and exhibiting different characteristics, is analyzed by X-ray photoelectron spectroscopy (XPS). It is shown that the surface properties of raw fibers are determined, to a

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large extent, by the presence of waxes, which can be considered as high molecular weight alkanes. Even after extraction in hot ethanol for 6 hrs, waxes and other non-cellulosic entities are still present on the cotton surface. These results are in good agreement with those obtained previously by a technique based on gas probe adsorption (inverse gas chromatography [1,2] Chapter 11 - Fundamentals of chemical surface analysis by X-ray photoelectron spectroscopy (XPS), a basic technique for determination of relative amount or atomic concentration percentages of chemical elements (except hydrogen) and their chemical state, is presented in this Chapter and applied to the characterization of several types of membranes, from different materials and diverse structures. Membrane surfaces XPS analysis is used to obtain information associated with: i) membrane stability or modifications under physical or chemical actions (γ and X-ray radiation, acid and base treatments, aging, ...); ii) identification of the type of monomer and functional groups among membranes of the same type of polymeric material (Vitrex vs. Udel polysulfone, aromatic vs. linear polyamides, ...); and iii) chemical differences in the outer surface of the same type of membranes (polymer hydration, polymer oxidations, percentage of charged groups on the polymer, presence of free or linked acid molecules, type of additives or impurities, ...), which ultimately determine the membrane performance. Regenerated cellulose, dense and porous polysulfone and commercial and experimental composite polyamide/polysulfone membranes are studied. All these membranes are commonly used for typical membrane separation processes such as dialysis, microfiltration, nanofiltration or gas separation. XPS on experimentally activated membranes (AMs) for heavy metal separation, obtained by trapping di-(2-ethyl hexyl)dithiophosphoric acid as a carrier on the polyamide top layer of a composite polyamide/polysulfone membrane, is also presented. On these membranes, important information such as the type of carrierpolymer interaction, carrier loss (AMs stability), aging of the carrier in the membrane or material oxidation and hydration can be also obtained from XSP analysis. Moreover, in order to correlate the chemical nature of membrane surfaces and the membrane behavior in the transport of electrolyte solutions, other surface and transport parameters—such as roughness, electrochemical surface charge density or ion transport numbers— were also determined for these membranes.

In: X-Ray Photoelectron Spectroscopy Editor: Johanna M. Wagner

ISBN: 978-1-61668-915-5 © 2011 Nova Science Publishers, Inc.

Chapter 1

X-RAY PHOTOELECTRON SPECTROSCOPY: STUDIES FROM INDUSTRIAL & BIOACTIVE GLASS TO BIOMATERIALS A.W. Wren1, F.R. Laffir2, N.P. Mellot1 and M.R. Towler1 1

2

Inamori School of Engineering, Alfred University, Alfred, New York, USA Materials and Surface Science Institute, University of Limerick, Limerick Ireland

ABSTRACT X-ray Photoelectron Spectroscopy (XPS) is a powerful research tool for the study of chemical and physical phenomenon occurring at surfaces of a wide range of materials. It was developed at the University of Uppsala, Sweden in the 1940-50s and works on the principle of the photoelectric effect in which soft X-rays (Mg and Al Kα x-ray) are used as the exciting photon source. The emitted photoelectrons have a kinetic energy distribution consisting of discrete peaks related to the binding energies of the photoionized atom and provide information on the electronic structure of a surface and its chemical composition. XPS is also a sensitive probe of the chemical environment or oxidation state of surface species. Since its development XPS has been applied for analysing many types of materials including metals, polymers, ceramics, composites, glasses, semiconductors and biological samples. More recently there has been an increase in the use of XPS in the characterisation of biomaterials where surface interactions play a key role in the success and longevity of an implanted biomaterial. The development of oxide glasses is one key area that benefits from the use of XPS where information relating to the composition and bulk structural properties can be determined. This can aid in better design of these materials and the ability to tailor towards specific applications. The growing use of XPS is reflected in the increase in publications. The number of XPS papers published in 2007 (5469) is almost double that published in 1997 (2830). This growth can be attributed to the fact that XPS can be successfully applied to a large variety of materials.

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A.W W. Wren, F.R R. Laffir, N.P. Mellot et al.

1. X-RAY Y PHOTOEL LECTRON SPECTROSC COPY: CONCEPT T & PRINC CIPLES X-ray Phottoelectron Speectroscopy (XP PS) is a powerful research tool t for surfacce chemical annalysis. It is a versatile tecchnique to stuudy physical and a chemical phenomena p occurring at suurfaces of materials m ranging from meetals, oxides, semiconducttors, glasses, ceramics, poolymers, comp posites and biiomaterials. XPS X was form merly known as a Electron Sppectroscopy foor Chemical Analysis A (ESC CA). It was devveloped by Kai K Siegbahn and a co-workerrs [1] at the U University of Uppsala, U Swedden in the 19400-1950s and laater won the Nobel N prize in 1981. XPS w works on the principle p of photoelectric p e effect [2, 3] in i which soft X-rays are used u as the exxciting photon n source. Mg Kα K (1253.6 eV V) and Al Kα α (1486.6 eV) X-rays are ussually used. Energy and lin ne width of thhe X-rays aree chosen to maximise m phottoemission annd optimise sppectral resolution. An X-raay photon interacts with a core level electron transsferring its phhoton energy and causing electron emiission by the photoelectricc effect as illustrated in figure 1[4]. The kineticc energy of thhe photoelectroon (Ek) is the difference beetween the X--ray photon fr andd the binding energy e (Eb) ennergy hυ (h is the Plank’s coonstant and υ is the X-ray frequency) off the core-lev vel electron. Since S the X-raay energy is known k and thhe kinetic eneergy of the phhotoelectron can c be experim mentally determ mined, the binnding energy of o the emittedd electron is giiven by the following equation:

where w is the instrumennt’s spectrom meter work fuunction. Sincee binding eneergy of an ellectron in a particular p shell of an atom is unique to each elementt, elements prresent on a m material’s surfaace other thann Hydrogen (H H) and Helium m (He) can be b identified and a relative coomposition deetermined. XP PS spectral linnes are identiffied by the coore shell from m which the ellectron was em mitted (1s, 2s, 2p, 3s, 3p, 3dd, etc).

Fiigure 1. Schemaatic of the photooemission proceess

X-Ray Photoelectron Spectroscopy: Studies from Industrial & Bioactive Glass…

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Figure 2. Schematic of the Auger process

In addition to the photoelectric process, relaxation of the excited ion may occur by the emission of auger electrons or fluorescent X-ray photons (figure 2). For elements with low atomic numbers (Z < 30), Auger emission is dominant. When an electron from a core level is emitted, another electron from a higher level falls to occupy the core level vacancy; this causes emission of a third electron known as the Auger electron to conserve energy released in the previous step [5]. Auger emission is independent of the photon energy and therefore the X-ray source. Following a photoelectric process, relaxation of the photoionized atom can also occur by the shake-up or the shake-down process in which electrons in the valence levels are excited to bound and unbound states respectively. In addition, multiplet splitting of peaks can occur when ions that have a remaining unpaired electron couple with other unpaired electrons in the atoms giving rise to several possible final state configurations and peaks at higher binding energy to the principle peak. Less intense plasmon loss peaks also appear at higher binding energies from the principle peak at equal spacings due to specific amounts of energy losses when the photoelectron excites free electrons in a material.

Figure 3. Universal curve showing the attenuation of an element’s path in a solid as a function of its KE

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A.W. Wren, F.R. Laffir, N.P. Mellot et al.

Figure 4. Spectrum from an Au surface

Figure 5. C 1s spectrum from poly-vinyltrifluoroacetate

XPS is surface sensitive probing a depth of ~ 10 nm from the top of the surface. Sampling depth is defined as the depth from which 95 % of all photoelectrons are scattered when they reach the surface (3 ), where ranges from 1 to 3.5 nm. While the path length of the X-ray photons is in the order of few micrometers, the inelastic mean free path of electrons is in the order of tens of angstroms. Electrons are generated from the entire X-ray probe volume, however kinetic energy losses due to inelastic scattering (plasmon and phonon excitations, electron hole pair formation, etc) allows only those electrons a few atomic layers deep beneath the surface to escape without energy loss to be detected, thus giving its useful property of surface sensitivity. Figure 3 shows the “Universal curve” of attenuation length or inelastic mean free path ( ) as a function of electron kinetic energy (KE) [6]. Electrons generated in XPS correspond to ~ 10 monolayers. A typical spectrum from a gold (Au) sample is shown in figure 4 [7]. The series of peaks correspond to photoemissions from the different core shells s, p d and f. Non-s levels are present as doublets due to spin-orbit coupling. For example, 4d level has a doublet designated by 4d3/2 and 4d5/2. The spin-orbit splitting ratio is 1:2 for p shells, 2:3 for d shells and 3:4 for f

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shells. The relative intensities of the different peaks are dependent on the photoemission cross section or the probability of excitation of a single electron from a particular core shell. The binding energy of the core electrons are essentially dependent on the species to which it is bonded and resulting charge transfer determines the magnitude of the “chemical shift”. Thus, information on bonding environment (chemical state) can be determined. Chemical shifts can arise in several ways including differences in formal oxidation states, electronegativity, molecular environment, lattice sites, etc. Figure 5 illustrates the chemical shift effect of C 1s in poly-vinyltrifluoroacetate which has four different carbon (C) environments [8]. The core level binding energy of the carbon atom increases as the electronegativity of the neighbouring atoms, in this case oxygen and fluorine increases resulting in their respective chemical shifts. Surface analysis by XPS requires irradiating the sample in an ultra-high vacuum environment (< 10-9 torr) which is essential to increase the mean free path of the photons and electrons, remove adsorbed gases from the sample and prevent adsorption of contaminants on the surface. Therefore, samples need to be solids and vacuum compatible. For insulating samples, photoemission leads to build up of positive charges on the surface which can result in peak shifts and complex spectra. In such cases, a charge neutraliser is used to flood the surface of sample with low energy electrons which can compensate excess charge. Basic XPS instrumentation consists of an X-ray source, an electron analyser to measure the kinetic energy of the photoelctrons, a detector to count the number of electrons, and a data acquisition and processing system. Al and Mg are commonly used as X-ray target materials. In addition, a continuous bremsstrahlung (braking radiation) is also produced which increases the photoelectron background. With such X-ray sources static charging problem present in insulating samples is less severe since low energy electrons generated by the bremsstrahlung partially neutralise the built up charges. However, spectra obtained from a regular anode can sometimes be complicated by the presence of satellites and bremsstrahlung and therefore the use of a monochromator is useful. A monochromatic X-ray source increases spectral resolution and is important for resolving chemical states. A dual Al/Mg X-ray source used in conjunction can help in distinguishing between photoelectric and Auger peaks. Synchrotron X-rays are increasingly used with increasing ease of access to such facilities for in-depth surface analysis, as they allow the X-ray energy to be tuned, have higher brilliance; higher resolution and can have optimum surface sensitivity.

Figure 6. Angle resolved XPS

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Depth profiling or distribution of elements as a function of depth in XPS can be carried out by non-destructive or destructive methods. Non destructive method is based on varying the emission angle which varies the effective escape depth (d) making it more surface sensitive as shown in figure 6. For deeper depth profiling, surface layers are removed by sputtering with inert gas ions and XPS analysis is carried out on the new surface; the procedure is repeated to get information at a series of depths. More recently, there has been an increase in the use of XPS in the characterisation of glasses and glass based biomaterials where surface interactions play a key role in the success of the implanted material. Compositional and structural information of novel glass formulations generating new desirable properties can be studied using XPS. The O 1s spectrum of oxide glasses and the related chemical shifts provide useful information on the local environment of the oxygens. Glass samples can be shaped into structures that can be fractured in vacuum to expose pristine surfaces with minimum contamination for XPS analysis.

2. RELATING GLASS TO XPS A glass is an amorphous solid lacking long-range, periodic atomic structure and exhibiting a region of glass transformation behaviour [9]. While inorganic glasses can be produced in a wide range of compositions, often categorized by the predominant network forming element(s); including silicate glasses, phosphate glasses, germanate glasses, and fluoride glasses, silicate glasses are by far the most common in engineering applications and in nature. In particular, silicate glasses are used in a wide range of technologically important applications including a variety of optical components, flat panel displays, substrates/superstrates for a variety of photovoltaic related products, and throughout the biomedical field, as will be discussed in detail throughout the next section of this chapter. The desired performance properties of these glasses are often controlled by their surface properties, including; (1) their optical, chemical, and biological functionality, (2) mechanical and chemical durability, and (3) aesthetic integrity. Historically, XPS has been used to determine very specific aspects of glass surface (and bulk) chemical properties, in particular; (1) elemental composition (both qualitative and quantitative), (2) oxidation state of cations, (3) non-bridging oxygen (NBO) concentration, (4) organics on glass, (5) composition as a function of depth (depth profiling), and (6) various chemical aspects of coatings on glass. Specific examples of each have previously been provided in detail in a number of recent review articles [10-12]. XPS is a particularly flexible technique in that both surface and bulk composition and chemistry can be investigated through advances in non-destructive cleaning and in-situ fracture practices [10]. Furthermore, through careful experimental methods and sample preparation a wide variety of glass “forms” can be examined including fibers, powders, melt surfaces, and polished plates [10].

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3. SILICATE GLASS CORROSION Throughout the processing and service lifetime of the majority of glasses they are exposed to a wide variety of corrosive environments, many of which are aqueous based. Therefore, their surface properties as a function of aqueous corrosion are of both importance and significant interest. Glass corrosion induces transformations in the following three broad, albeit related categories: • • •

Surface composition, Surface structure/bonding, Surface morphology.

Surface chemistry and composition changes of silicate glasses with aqueous corrosion has been studied and mechanisms have been proposed for many years [13-28]. It is not the purpose here to review silicate glass corrosion, however, it is worthwhile to discuss some general points in order to gain an appreciation in how and why XPS has been extremely useful in the development of silicate glass corrosion models. It is accepted that the corrosion mechanisms of soda-lime-silicate and borosilicate glasses are one, or a combination of the following; (1) ion exchange, (2) hydration, and (3) network hydrolysis [29-38]. The mechanism(s) responsible in any given system relies primarily on the glass structure/composition and corrosive solution pH and temperature [39]. These mechanisms result in two separate dissolution “modes”; leaching and network dissolution. In a limited number of cases either the glass exhibits complete solubility (network dissolution) or will be highly insoluble (significant leaching). However, in the vast majority of cases a combination of mechanisms are present throughout the dissolution process, and hence some combination of dissolution modes are present as well [30, 33, 35, 37, 40]. This results in the formation of a surface layer/reaction zone which, in turn, affects, the mechanisms and kinetics of corrosion. Therefore, the composition, structure, and chemistry of the glass surface with corrosion, and hence XPS analyses of these surfaces, cannot be understated in the overall understanding of the mechanisms of glass corrosion.

4. APPLICATION OF XPS TO GLASS CORROSION As expected, given the above review, one of the principal applications of XPS in the study of glass corrosion, is the evolution of surface composition throughout the corrosion process. In particular, it has been shown experimentally, an alkali and/or alkaline-earth depleted leached layer forms on the surface of a glass reacted in a variety of aqueous and/or humid environments [13-15, 25-28]. For example, figure 7 shows the surface composition of a calcium-sodium aluminosilicate glass reacted in an acidic solution as a function of time. The release of mobile, alkali and alkaline-earth elements from the glass surface into the corrosion media has important implications in a variety of glass applications ranging from the biocompatibility of glasses used in biocements as a function of time to evolving optical properties of glasses under humid conditions. Additionally, when reacted under specific, static conditions secondary precipitates often form on the surface of glasses [41]. XPS has

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been used to determine both the chemical composition, and in a limited number of cases, bonding environments of these precipitates. XPS depth profiling can be utilized to determine the concentration of elements as a function of depth on glass samples reacted in various conditions. In fact, the application of depth profiling affords the determination of relative leaching rates of different elements within a given glass and “leachability” of a given element as a function of glass composition and corrosion environment. For example, figure 8 shows the depth profiles of (a) an unreacted calcium-sodium aluminosilicate glass and (b) the same calcium aluminosilicate glass reacted in an acidic solution for 10 hours. Note, after reaction for 10 hours all cations are leached, with respect to silica.

Figure 7. Surface composition by XPS of a calcium-sodium-aluminosilicate glass reacted in an acidic solution for various lengths of time

Figure 8. Depth profile (XPS) of a calcium-sodium-aluminosilicate glass unreacted and reacted for 10 hours in an acidic solution

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Through utilization of XPS in addition to complementary characterization techniques including; (1) vibrational spectroscopy for structural analysis, (2) solution analysis for quantitative dissolution and leaching rates, (3) secondary ion mass spectrometry and/or nuclear reaction analysis for quantitative hydrogen depth profiling, and (4) scanning electron microscopy and/or atomic force microscopy for morphological imaging, a broad understanding of silicate glass corrosion mechanisms has been developed. However, it is worth noting, although glass corrosion has been studied for moreover 50 years many questions remain and a predictive model of glass corrosion has yet to be developed. One hindrance in further understanding of glass corrosion mechanisms is the lack of ability for insitu analyses of glass surfaces and their chemical evolution with corrosion. A recent advance in XPS technology may prove to help alleviate this, and is briefly reviewed below. A recent, extensive review article summarizing future directions of XPS has recently been published [42]. In this article the author describes a list of advanced techniques which may prove to impact the understanding of glass corrosion including x-ray excited photoemission for probing buried interfaces, the use of soft and hard x-rays for advanced depth profiling, and time-resolved measurements at close to ambient pressures [42]. The application of ambient pressure x-ray photoelectron spectroscopy (APXPS), under humid conditions, is particularly promising for further advances in glass corrosion. A recent review article on ARXPS and analyses in the presence of water vapor was published in late 2009 [43]. This technique is capable of analyzing samples at greater than 5 Torr, thus opening the possibility of XPS analyses of samples to a variety of relative humidity as well as liquid water [43]. Use of ARXPS in the study of glass corrosion could lend insight into topics, which at present, we understand little including but not limited to water adsorption, preferential hydroxylation, and initial stages of leaching.

5. XPS ROLE IN GLASS FABRICATION In addition to surface corrosion, factors affecting the properties of glasses include the addition of transition metals. Studies on the inclusion of transition metals in oxide glasses have employed XPS to provide information on a number of structural properties. XPS has been used to distinguish between the Bridging Oxygen (BO) and Non-Bridging Oxygen (NBO) content of the transition metal doped glasses and has also been utilized to identify the various oxidation states of the transition metals and their concentration in oxide glasses [44]. This information can be useful in glass design as the inclusion of transition metals to oxides glasses have applications in colouring of glass [44], and also electrical and magnetic properties which they have as a result of their variable valence states [45]. XPS is useful in this instance as it can provide information on the chemical state of an atom in different bonding configurations as the chemical shift is a reflection of the change of the binding energy of the core electrons with changes in the chemical environment [45]. Mekki et al investigated 0.3Na2O-0.6SiO2-0.1TMO, where TMO=Fe2O3, [44] and found using XPS that both Fe2+ and Fe3+ can exist in sodium silicate glasses. Figure 9 shows the raw data of the Fe 3p spectrum. This spectrum shows a peak binding energy of 55 eV with a shoulder at a binding energy of 53 eV. The charge difference of one that exists between Fe2+ and Fe3+ will be reflected in an energy shift of the Fe2+ transition to a lower binding energy.

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Figure 9. Fe 3P core level photoelectron spectrum for Fe doped glasses

The peak at a binding energy of 53 eV with a full width half maxima (FWHM) of 2.4 eV is attributed to Fe2+ ions while the second transition at a binding energy of 55.5 eV with a FWHM of 2.4 eV is due to Fe3+ ions. The proportions of both Fe2+ and Fe3+ ions in the glass was also calculated from the area under the curve and was found to be 24% and 76% respectively[44]. Further studies on Fe-doped glasses revealed the role of Fe in sodiumsilicate glasses. Figure 10 shows a.) the base glass, 0.3Na2O-0.7SiO2 and b.) Fe doped glass, 0.3Na2O-0.6SiO2-0.1Fe2O3. Figure 10 highlights two distinct observations. Firstly that the NBOs are assigned to a lower binding energy, approximately 2 eV lower than the BO sites. Also, the addition of Fe to the glass results in an increased concentration of NBOs in the glass (figure 10b). This indicates that Fe2O3 plays a similar role as Na2O in the glass structure where it breaks BO sites and replaces them with NBO sites [44]. In a separate study by Mekki et al the surface structure of copper oxide doped sodium silicate glasses ((0.70-x)SiO2-0.30Na20-xCuO (x in the range 0-0.2) was investigated with XPS. This study was performed in order to determine the chemical state of the copper (Cu) atoms in these glasses, i.e. to what extent they occur as monovalent, divalent or trivalent. The reported binding energy for the Cu 2p photoelectron core level in Cu2O and in CuO polycrystalline compounds is 932.6 eV and 933.5 eV respectively. It is also been shown that copper compounds containing Cu2+ ions are exhibit strong satellites at binding energies of about 6 to 10 eV above the main core level lines, which are absent in compounds containing only Cu+ [45] . The satellites have been attributed to shake up transitions by ligand-metal 3d charge transfer associated with only Cu2+, given Cu+ contains a completely filled 3d shell. This makes it possible to qualitatively distinguish between the two oxidation states of copper through analysis of the Cu 2p photoelectron lines. The spectra for the Cu 2p region is presented in figure 11.

Figure 10. High resolution O 1s core level photoelectron spectra for a.) base glass and b.) Fe doped glass

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Figure 11. (a) Cu 2p3/2 spectrum where x=0.05 fitted with contribution from Cu+ ions, (b) Cu 2p3/2 spectrum for x=0.18 glass fitted with contributions from Cu+ and Cu2+ ions

Figure 12. (a) two peak fit for O 1s spectrum for x=0.18 glass, (b) same composition showing all possible contributions to O 1s transition

Figure 11 shows the Cu 2p spectrum where it was determined that Cu exists in two oxidation states, as monovalent and divalent ions. The Cu 2p peak was therefore resolved into two separate components with the charge difference between Cu+ and Cu2+ reflected in the energy shift between the two transitions. The addition of valence electron (increasing electron density) effectively screens the core electrons and so decreases the measured binding energies. Therefore Cu+ should have a lower binding energy than Cu2+. From figure 11a it was not possible to fit the spectrum with two peaks, however for figure 11b, with additions of x=0.18 CuO, it was possible to fit with two peaks with the higher binding energy corresponding to Cu2+, and also the presence of a small satellite contribution. The results of the best peak fit to the experimental data are Cu+ = 89.7% and Cu2+ = 10.3% [45]. Figure 12 shows the O 1s photoelectron spectrum with additions of x=0.18 CuO. It has previously been determined that the BO appears at a binding energy of 532 eV where it is covalently bound to two silica (Si) atoms (Si-O-Si), and at lower binding energies of 529 eV, NBOs appear where there exists a covalently bound oxygen to a Si and ionically bound oxygen to Na (Si-O-Na). From figure 12 it was determined that the NBO/total oxygen content was found to be 58.3%. The NBO content could be further deconvoluted to reveal the presence of Si-O-Na, Si-O-Cu(I), Si-O-Cu(II), where the Si-O-Si content is gradually replaced by Si-O-Cu(I), Si-O-Cu(II) at binding energies of 530.1 eV and 530.2 eV respectively, and Si-O-Na which was present at 529.6 eV. This highlights the ability of XPS to separate all possible contributions to the NBO signal. Overall these results suggest that Cu enters the glass structure as a network modifying ion [45].

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Figure 13. (a.) O 1s photoelectron spectra and (b.) chemical shifts of O 1s binding energy for (R)Na2OB2O3-(K)SiO2 where K=0.5

Studies have also been conducted on the role of boron (B) in a glass as it acts as an intermediate oxide in a glass. Studies undertaken by Miura et al looked at the change in coordinations state of B in sodium boro-silicate glasses. This was achieved by taking a (R)Na2O-B2O3-(K)SiO2 glass and by adjusting the R=Na2O/B2O3 (0.5-6.0) and the K=SiO2/B2O3 (0.2-5.0) ratios. This study was undertaken as sodium borosilicate glass should have various chemical bondings such as Si-O-Si, Si-O-Bn, Bn-O-Bn, where n=3 or 4, the coordination number of oxides around boron atom for the BO component and two types, Si-ONa+ and B-O-Na+ for the NBO component. For this review the values for K=0.5 and R=0.21.2 will be discussed [46]. From figure 13 it can be seen that the O 1s signal shifts towards lower energy side with increasing R at a constant K, and the signals are expressed by one symmetric function in the small R region at a small constant K, K=0.5 which says the signal is composed of only BO component[46]. In figure 13a the NBO component appears at R=0.6 and increases in intensity with increasing R. From figure 13b it can be seen that the binding energy of O 1s shifts toward the lower energy side with increasing R. It was determined from this study that the O 1s binding energy at the small R is due to the formation of BO4 groups and the lower O 1s drop at larger R is described by the conversion of BO4 to BO3 groups with NBOs. It was however concluded from this study that sodium oxides are shared between silicate and borate groups networks even in small R glasses and that borate and silicate networks are mixed in the borosilicate glasses than the existence of particular building units such as reedmergnerite (NaBSi3O8) and diborate groups [46]. Another area of research where XPS has been applied is in the properties and structure of glasses in relation to processing method. Glasses have traditionally been prepared by the melt-quench route, however interest in sol-gel (sintered gel) based glasses has led to claims that sol-gel processing may have a number of distinct advantages over melt-quench

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counterparts. Sol-gel processing has the ability to prepare highly pure and homogenous glasses at temperatures far less than required by the melting of batch powders [47]. Other cited advantages include controlled stoichiometry, flexibility of processing to form monolith, thin films or high surface area powders [48]. XPS is useful for investigating sol-gel based glass as changes occur at the molecular level as the liquid is transformed to a gel and then to a glassy state. Studies by Roy et al have determined differences existing between melt-quench and sol-gel glass considering 15Na2O-85SiO2 glasses. Figure 14 shows the O 1s photoelectron spectrum for 15Na2O-85SiO2 a.) melt-quench glass, b.) gel heated to 400oC and c.) gel heated to 500oC. From figure 14 it can be seen that there is a higher concentration of NBOs in the melt-quench glass as compared to both sol-gel glasses, however it is evident that the fraction of NBOs increases with the temperature of heat treatment, approaching the value of the melt-quench glass of the same composition. This is related to factors related to the gel converting to a glass. It is unlikely that NBOs are present around the Si atom at the initial stages of gel formation involving hydrolysis and polycondensation. Polycondensation reactions in the glass form Si-O-Si, and subsequent high temperature treatment of the gel promotes depolymerisation reactions resulting in Si-O--Na+. This breakup leads to primarily ionic bonding between the alkali and NBO[47, 48]. Table 1 summarises the O 1s spectra components for 15Na2O-85SiO2 considering binding energy, FWHM and area under the curve. It can be noted from table 1 that the fraction of NBO for the 15Na2O-85SiO2 is 7% when heated to 400oC and increases to 12.2% upon heating to 500oC, which is close to the meltquench (13%) glass of the same composition. The binding energy of the BO was also found to be closer to the melt-quench glass as the temperature of the gel-glass increased [47]. This suggests that the gel structure evolves towards Si-O-Si glassy network upon heating to high temperatures[48]. However the structure of the silicate gels approaches that of the meltquench glass but may not reach it even upon heating it to temperatures higher than the glass transition temperature (480oC for the 15Na2O-85SiO2) [47]. In relation to these glasses XPS was also used to determine if any phase separation was present in 15Na2O-85SiO2 and 15Li2O-85SiO2 glasses. The addition of modifier oxides to glass former SiO2 often leads to liquid-liquid immiscibility and because glasses in the 15Na2O-85SiO2 range tend to phase separate, it is reasonable to assume that the two components of the Na 1s spectra for heat-treated gels represent two distinct alkali environments in the two phases. Figure 15 shows the Na 1s spectrum for 15Na2O-85SiO2 considering a.) the melt quench glass and b.) gel heated to 500oC. Table 1. Summary of O 1s components for 15Na2O-85SiO2 Specimen

Binding Energy eV)

FWHM (eV)

Area under curve (% )

Melt-Quench

532.9

1.56

87.0

530.6

1.35

13.0

533.2

1.92

93.0

531.4

2.21

7.0

532.8

1.89

87.8

530.9

1.67

12.2

o

Gel - 400 C o

Gel - 500 C

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Figure 14. O 1s photoelectron spectrum for 15Na2O-85SiO2 a.) melt-quench glass, b.) gel heated to 400oC and c.) gel heated to 500oC

Figure 15. Na 1s spectrum for 15Na2O-85SiO2 a.) melt-quench glass, b.) gel heated to 500oC

Figure 15a describes the Na 1s from the melt-quench glass and it can be well described with one peak at a binding energy of 1072.4 eV. On the otherhand, by heating the gels to 400500oC, at least two peaks are needed to describe the total Na 1s peak [47]. The first is present at 1072.4 eV, which matches the Na 1s peak of the melt-quench glass, while the second appears at a lower binding energy of 1071.6 eV. From this study it can be assumed that glasses prepared by the sol-gel route are more easily phase separated than the melt-quench samples and that melt-quench glasses that exhibit a single Na 1s peak which signifies

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homogenous single-phase chemnical environment of the alkai environment. The phase separation in the sol-gel glasses may be attributed to the high water content required when performing the sol-gel process and high mobility of the atomic species. This study concludes that the sol-gel route provides faster kinetics for phase separation in glasses [47, 48].

6. EVOLUTION OF BIOACTIVE GLASSES Recently work has focuses on the development of bioactive glasses which can be used to replace hard tissue in the body. Glasses in the system Na2O-CaO-P2O5-K2O-MgO-B2O3-SiO2 are known to form chemical bonds with natural bone tissue, and will subsequently promote bone growth [49, 50]. Bioglass is a commercially used bioactive glass and is in the system Na2O-CaO-SiO2-P2O5. This glass has been used in the form of particulates and has been successfully utilized in over a million cases periodontal bone repair [51]. These glasses have a highly reactive surface when immersed in plasma (or analogous solution). Surface dissolution of the glass leads to the formation of a silica-rich gel layer and, subsequently, the precipitation of a calcium phosphate layer [50]. The bioactivity is therefore related to the composition and structure of the glass. These bioactive silica based materials present an open network which enables the inclusion of a wide range of alkali and alkaline-earth cations. These cations are termed network modifiers (Na+, K+, Ca2+) and function in disrupting the continuity of the glass network due to the breaking of some of the Si-O-Si bonds leading to the formation of non-bridging oxygen groups (Si-O-NBO) [49, 50]. The terms bridging and non-bridging oxygen (BO and NBO, respectively) are used to describe the environment around a central silicon atom in silicate glasses. A BO refers to the oxygen that bonds two Si atoms together (i.e. Si–O–Si). A NBO bonds one Si atom to a metal cation (e.g. Si-O-Pb) [52]. This is a key factor in the bioactive process, because the concentration of Si-O-NBO groups controls the dissolution rate of the silica through the formation of Silanol (SiOH) groups at the glass surface [49, 50, 53]. The study of the bonding configuration in glasses by XPS has proved adventitious as it has enabled researchers to improve the design and properties of bioactive glasses. XPS has the ability to determine the intrinsic binding energy of the atomic orbitals, which shifts chemically according to the surrounding environment of the atom [49]. Serra et al recently carried out studies FT-IR and XPS studies on glass compositions ranging from vitreous silica (V. silica), window glass (W. glass) and two novel bioactive glass formulations, BG50 and BG42. Table 2 shows the glass formulations considered for this study. Table 2. Glass formulations Na2 O

K2 O

CaO

P2 O5

MgO

B2 O3

SiO2

V. silica

0.3

0.1

0.1

-

0.1

-

99.1

W. glass

13.1

0.4

8.8

0.1

4.3

-

71.9

BG50

15.0

15.0

15.0

-

2.0

3.0

50.0

BG42

20.0

10.0

20.0

3.0

5.0

-

42.0

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Figure 16. (a) Si2p photoelectron spectra for: (i) V. silica, (ii) W. glass, (iii) BG50 & (iv) BG42. (b) Chemical shifts of O1s binding energy as a function of the glass composition

XPS was initially utilized to show the Si 2p photoelectron spectra (figure 16a) for the V. Silica, W. glass and the bioactive glasses. The Si 2p signal is expressed by a symmetric function and it shifts towards lower energies (ΔE=1.21 eV) when the SiO2 content decreases (increasing alkali content). A similar observation can also be made for the O 1s peak (figure 16b) where by increasing the alkali content the O 1s binding energy decreases from 533.0 eV to 532.2 eV. This suggests increasing network disruption within the glass as a function of alkali content [49].

Figure 17. O 1s photoelectron spectrum for bioactive glass BG42

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Figure 18. XPS wide scan spectra of (a.) A - sample BVA, B - in PB solution, C - in HAb/PB (b.) after treatment in HAb/PB with respect to exposure time

The O 1s photopeaks provide very useful information, particularly relating to work on oxide glasses. This is because the oxide ions in the silicate glass have different types of chemical bonding. It is possible to analyze the bonding states of the oxides by deconvoluting the O 1s spectrum. Figure 17 is the deconvoluted curve of a O 1s spectrum for the bioactive glass BG42 [49]. Resolving the peak at around 523 eV is associated with Si-O-Si vibration (BO groups), while there is also a shoulder peak at a lower binding energy, and this is associated with Si-ONa or Ca, K (NBO groups). Vitreous silica is formed by a continuous random network of SiO4 tetrahedral connected to the adjacent tetrahedral by BOs to form a three dimensional connected glass network. When modifiers are included into this network, the electron density of the bonding states of the Si and O atoms are modified. The charge difference between the BOs and NBO atoms is reflected in an XPS chemical shift of the O 1s peak. The breakage of the Si-O-Si bonds and the formation of Si-O-NBO groups play a key role on the biological response regarding surface interactions in biomaterials when used in close contact with body fluids [49, 54]. It is widely accepted that the success of an implanted biomaterial is related to how the material’s surface influences a biological response as a result of contact between the material and the biological system. This response is largely a result of interactions between the thin surface layers on the biomaterial and the surrounding environment. XPS is a useful tool in such instances, providing chemical information from the first 50-100Å of the sample surface as well as lending insight into the chemistry and physics of surfaces and at interfaces [55]. Polzonetti et al looked at the surface chemical composition of a plasma coated biologically active glass (BVA - CaO(20%)-SiO2(47%)-Na2O(19%)-K2O(5%)-MgO(1%)-Al2O3(1%)P2O5(7%)) compared to a soda lime glass (BVH - CaO(9%)-SiO2(71%)-Na2O(14%)-

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K2O(1%)-MgO(3%)-Al2O3(2%)). Both glasses were plasma coated onto Ti-6Al-4V disks (812mm) to form glass coatings of around 80 µm thickness. Both BVA and BVH were treated in phosphate buffer (PB) or in buffered human albumin solution (HAb/PB) as a function of exposure time. Further testing was conducted to determine the in vitro biocompatibility of the biomaterial surface with murine fibroblasts and polymorphonuclear granulocytes (PMNs) [55]. XPS analysis was used initially to determine the elemental surface composition of the two plasma-sprayed glass samples and any changes occurring when immersed in PB or HAb/PB. For this review focus will be directed on the BVA glass. Figure 18a shows compositional changes of BVA when immersed in PB and HAb/PB for 24 hours. From figure 18a it can be seen that there was a marked increase in the Ca and P content and a net decrease in the Si on the sample surface for BVA. This is due to the formation of a calcium phosphate (CaP) surface layer forming, caused by the adsorption of P ions from the PB solution and the migration of Ca ions from the sample bulk to the surface [55]. The formation of CaP rich films on the surface of bioactive glasses upon interaction with simulated body fluids has been well documented and is related to the migration of Ca2+ and PO3-4 ions through the SiO2 layer. HAb/PB solution was used for testing as plasma protein adsorption is one of the first events that takes place between blood and an implanted biomaterial. Albumin is the predominant plasma protein and is known to passivate biomaterial surface, diminishing bacterial and platelet adhesion which results in reducing inflammation and thrombogenic responses. It was determined that HAb adsorbed on the surfaces of both materials, BVA and BVH [55]. XPS wide scan spectra revealed the presence of N 1s signal caused by surface nitrogen atoms contained in HAb. From figure 18b it can be seen that most of the changes in surface composition was complete after 24 hours. At this stage; (1) a CaP surface layer had formed on the surface of the material and (2) there was HAb adsorption as indicated by the appearance of the N 1s signal. The CaP-albumin layer completely covered the glass surface and the Si 2p signal almost disappeared after 24 hours. Further exposure to 48 and 96 hours did not result in further appreciable modification of the surface composition. The BVH sample did not form any CaP layer upon exposure to HAb/PB. Further biocompatibility testing revealed that both materials did not inhibit fibroblast growth or increase PMN activation. However the cellular response was higher towards the BVA glass confirming the increased bioactivity of this glass compared to BVH [55]. Further testing on the bioactivity is reported by Sharma et al where a 34SiO2-(45-x)CaO16P2O5-4.5MgO-0.5CaF2-xFe2O3 (where x=5, 10, 15, 20) glass was produced. This glass was of interest as the inclusion of Fe2O3 shows important applications in cancer treatment by elimination of cancerous cells in bones; by means of hyperthermia. The material is placed in the region of the tumour and is subjected to an alternating magnetic field. The tumour is effectively heated and the temperature locally rises to 42-45oC. As a result the cancerous cells perish while the healthy one survives [56]. However this particular study was undertaken to determine knowledge on the structure and oxidation state of iron (Fe), and also the bioactivity of these glasses by using Simulated Body Fluid (SBF). SBF was developed by Kokubo et al [51, 57] and has an ionic composition similar to that of human blood plasma. It is characterized by the precipitation of Ca2+ and PO3-4 ions from the supersaturated SBF solution which forms a CaP surface layer. This method is used extensively in determining the bioactivity of biomaterials. In this case XPS was used to determine any surface compositional changes of the material in relation to immersion time in SBF. From this study it was determined that

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exposure in SBF resulted in an increase in P content and a net decrease in Si on the sample surface. This was due to the formation of a CaP surface layer caused by the adsorption of P ions from the solution [56]. However it was found that additions of Fe2O3 resulted in reduced CaP surface deposition. This was attributed to the substitution of Fe for Ca and the resultant decrease of Ca2+ migration to the surface through SiO2 rich layer. The rate of CaP deposition was found to decrease with increasing Fe content in the glass as it reduced the dissolution rate of the glass, therefore taking more time to precipitate CaO-P2O5 rich surface layers from the supersaturated solution. Overall it was determined by XPS that by increasing the Fe content in the glass its bioactivity was reduced [56].

7. GLASS BASED BIOMATERIALS: GLASS IONOMER CEMENTS In some cases the glass phase is one part of a system that incorporates a functional biomaterial. For example, glass ionomer cements (GICs) are materials that are used extensively in dentistry for luting, lining and tooth restoration. They consist of a glass phase, which is typically a Ca-Al-Si or CaF-Al-Si glass mixed with a polyacrylic acid and water [53]. Upon mixing the H+ from the acid chains disassociate and degrade the surface of the glass thus releasing metal cations (Al3+, Ca2+, Sr2+, Zn2+) into the polysalt cement mix. These cations then crosslink the carboxylate groups on the acid chains resulting in a hard set material [58-60]. The composition and concentration of modifiers in the glass phase will have a significant impact on the resulting properties. Studies on the processing route (i.e. the solgel and melt-quench route) have also been investigated by XPS on the glass phase of these materials and the resulting properties determined. Towler et al used XPS to investigate 0.12CaO-0.04SrO-0.36ZnO-0.48SiO2 (BT 101) glasses for use as the glass phase of a novel GIC. GICs formulated from both the solgel and melt-quench route were then investigated for rheology and mechanical properties. Initially quantitative XPS was used to determine the relative concentrations of ions present in the glass [61]. Table 3 shows the quantitative XPS analysis of the melt derived and solgel glass. From table 3 it can be seen that there is a reduced concentration of Zn and Sr in the solgel glass and that no Ca was detected. Conversely a much higher Si concentration was found in the solgel glass as compared to the melt-quench glass [61]. Figure 19 shows the O 1s spectrum of the a.) melt-quench glass and b.) solgel glass. Table 3. Quantitative analysis of melt-derived and solgel BT 101 glass

Zn 2p Sr 3p Ca 2p C 1s O 1s Si 2p

Melt-quench (% At Conc)

Solgel (% At Conc)

7.0 1.5 2.5 13.3 46.2 29.5

1.7 1.3 0 6.2 53.4 37.4

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A.W W. Wren, F.R R. Laffir, N.P. Mellot et al.

Fiigure 19. Shows the O 1s specttrum of the a.) melt-quench m glaass and b.) solggel glass

Fiigure 20. a.) rheeology and b.) mechanical m propperties of GICs produced from m melt-quench and a solgel gllass

Deconvolu ution of the O 1s spectrum inn figure 19b gives g a well reesolved peak at a 532.9 eV asssociated with h Si–O–Si unnits (BO). Thee accompanyiing shoulder at a lower bindding energy

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(531 eV) can be attributed to NBO where one of the Si atoms is replaced by a modifying cation. The O 1s spectrum of melt derived glass (figure 19a) was described with two peaks. By constraining the peak width (FWHM) of the higher binding energy (BO) to that of figure 19b, assuming that both arise from a chemically similar environment, the peak can be fitted as evident in figure 19a. From figure 19 it is evident that there is a marked difference in the ratio of BO:NBO, with the melt-derived glass containing a significantly higher concentration of NBOs. It was determined from the cation:Si ratio that the melt-quench glass (1:2.7) contained approximately five time the concentration of modifier than the solgel (1:12.5) derived glass of similar composition[61]. Increased concentrations of modifier cations facilitates the replacement of BO with NBO within the silica glass network, and these groups will have a direct relationship on properties such as the working time (Tw), setting time (Ts), compressive strength ( c) and biaxial flexural strength ( f). Figure 20 shows the properties a.) rheology and b.) mechanical properties of GICs produced from melt derived and solgel glass. From figure 20a it is evident that cements formulated from the solgel glass had a much greater Tw and Ts than the melt-quench glass. The Ts in particular for the solgel (1644 s) is far too long to be any way clinically applicable. Explanations regarding the extended Tw and Ts in the solgel-derived (78s, 1644s) glass compared to the melt-quench (19s, 25s) is related to the ratio BO:NBO in the glass. The solgel glass contains a significantly lower concentration of NBOs as compared to the melt-derived glass. This reduced concentration of NBOs lowers the rate of silica dissolution and hence the glasses ability to degrade and form a crosslinked network during setting [61]. The high concentration of NBOs in the melt-quench system results in a glass that is more susceptible to acid attack which facilitates a more rapid setting reaction. This is also the likely explanation for the higher c, 63 MPa and f, 26 MPa (figure 20b) attributed to the melt-quench glass, where a more acid degradable glass will facilitate greater ion release and crosslinking during the cements setting reaction. This process is directly linked to the concentration of NBOs in the glass network [61]. Some of the more beneficial properties of GICs rely largely on the release of beneficial ion from the glass phase over prolonged periods of time. GICs are regarded as ion reservoirs where they are able to uptake ions over time and re-release them into their local environment [62]. In commercial GICs, such as Fuji IX and Ketac Molar, the release of ions such as Fluoride (F-) has generated considerable interest as it has been largely attributed to preventing secondary carie formation as F- is cited as being a potent antibacterial ion [58, 63-66]. Studies by Hadley et al looked at changes in F- content in Ca-Al-Si glass based GIC (LG30) when immersed KF rich solution. LG30 glass was used as it contains neither F nor any other monovalent cation, and for this reason it has been previously used for ion-uptake studies. This study predominantly used Secondary Ion Mass Spectroscopy (SIMS), however XPS was used to confirm the SIMS results [62]. Figure 21 shows the XPS trace of LG30 after immersion in KF solution. From this study it was determined that F ions from the KF solution were incorporated into the cement matrix (figure 21). By using the ion beams of the SIMS to sputter away cement surface layers it was possible to measure up to 10µm into the LG30 cement. It was determined by depth profiling that the F content reduces with increasing depth. The F content was found to drop from 6.2mmol/g at 2µm from the surface to 0.2mmol/g at 10µm, which suggests that in GICs ion uptake and release predominantly occur at the immediate surface layer [62]. These results were confirmed by Jones et al where XPS and SIMS was used to investigate surface reactions occurring in GICs. They found that when immersed in KF

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A.W. Wren, F.R. Laffir, N.P. Mellot et al.

solution, LG30 was found to take up F possibly by the formation of a CaF2 rich surface layer. This study also used XPS to identify some of the organic compounds that form from the breakdown of PAA during the setting of a GIC. The C 1s spectrum was used to measure the binding energies of carbon atoms bound to different functional groups such as C-OH, C=O, COO- as these organic polymers have significantly different binding energies and the resulting C lineshape reflects the relative contribution of each organic component [67]. Further research on the bonding mechanism of PAA to mineralized tissues has been performed by Fukuda et al. This was achieved through quantification of the degree of ionic bonding between the carboxyl groups of the PAA with calcium present in the hydroxyapatite [68]. To investigate this, a thin film of 50w/w% PAA solution was pressed to a uniform film between polyethylene and glass plates. One of the thin films was removed and transferred to the surface of a synthetic hydroxyaptite (HAp) plate as well as enamel and dentin substrates. The plates were ultrasonically rinsed in pure water to remove any polymer that did not react with the surface. This allowed investigation of the chemical bonding efficacy of a polyalkenoic acid consisting solely of acrylic acid units (PAA) to HAp [68]. Figure 22 shows XPS wide-scan spectra of untreated HAp and HAp treated with PAA.

Figure 21. Shows the XPS trace of LG30 after immersion in KF solution

Table 4. Comparison of the organic and mineral ratios for the different constituents Dentin

Glass Powder

DI

GI

Al/Si

~

0.81

0.94

0.62

Sr/Si

~

0.46

0.31

0.24

Na/Si

~

0.11

0.14

0.08

F/Si

~

1.07

0.76

0.59

Ca/Si

~

~

0.46

0.08

Ca/N

1.00

~

0.62

0.43

Ca/P

1.59

~

0.90

0.08

C/N

4.25

~

19.00

~

Ca/C

0.20

~

0.03

0.01

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23

Figure 22. XPS wide-scan spectra of untreated HAp and HAp treated with PAA

Figure 23. XPS narrow-scan spectra of the C 1s region of a.) PAA, HAp and HAp treated with PAA and b.) Identification of -COOH and –COO Ca2+ groups on HAp treated with PAA

The wide scan spectra of untreated HAp and HAp treated with PAA only exhibit a slight difference, a twofold peak at a binding energy of approximately 285 eV (C 1s) that appeared when HAp was exposed to PAA. This peak is likely attributed to C-C and C-H at 284.6 eV and C-O at 286.0 eV. However this wide scan spectra does not reveal the carboxyl groups of PAA which should appear at a binding energy of 288.6 eV[68]. The narrow scan of pure PAA (figure 23a) reveals this peak at 288.6 eV, representing the carboxyl groups of the PAA and also identifies other organic groups attributed to PAA at a binding energy of 284.6 eV (C-C, C-H bindings). Figure 23a also shows the presence of carboxyl groups present on the surface of HAp when treated with PAA. From figure 23b it can be seen that when HAp was treated with PAA, a peak representing carboxyl groups from PAA appeared and shifted to a lower binding energy. Deconvolution of the shifted peak of carboxyl groups at 288.6 eV representing unreacted -COOH and a peak at

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A.W. Wren, F.R. Laffir, N.P. Mellot et al.

288.2 eV originates from carboxyl groups bonded to calcium contained within HAp. It is also interesting to note that the area under the unreacted –COOH (288.6 eV) peak is quite similar to the area under the –COO…Ca2+ peak (288.2 eV). This suggests that approximately 50 % of the carboxyl groups reacted to the calcium content of HAp [68]. Similar results were attained by applying PAA to treated enamel and dentin that was gathered from freshly extracted, prophylatically cleaned human teeth. Wide-scan XPS of untreated enamel/dentin and enamel/dentin treated with PAA resulted in a significant increase in intensity of the C 1s peak as a result of the application of PAA. Overall this study revealed that the molecular structure of PAA significantly influences the chemical bonding efficacy to HAp as well as enamel and dentin [68]. Further work was conducted by Sennou et al on characterizing the interactions occurring between a GIC (GC Fuji Type II) and a dentin substrate [69]. XPS was used to perform quantitative analysis on the surface and interface between the GIC and GIC/dentin substrate. For this study the GIC was deposited onto a dentin surface in the order for 1.5mm, placed in a steam saturated bath (at 34oC) for 7 minutes until setting was complete. This was done in order to approximate clinical conditions. The samples were then desiccated and the interfaces split apart to obtain a dentin side interface (DI) and a GIC side interface (GI). XPS wide scan spectrum was the used to identify the composition of the various elements present at the interfaces. Initial wide scan XPS analysis was used to identify the major constituents of both original materials, dentin and the GIC [69]. The dentin was composed predominantly of elements from the mineralized (hydroxyapatite) and the organic (type I collagen) components, namely Ca, P, C, N and O. The glass phase of the GIC was found to contain Al, Si, F, Na and in this glass it was found that Ca was completely replaced by Sr. A set GIC was also investigated where the wide scan spectrum identified the elements already identified, however a large concentration of carbon was also identified (derived from PAA). The resulting C 1s spectrum supported earlier work by Jones et al where C peaks were decomposed into peaks at 285 eV and 288.6 eV which corresponds to aliphatic carbon and also carboxylic carbon COO[69]. Table 4 compares the organic and mineral ratios for the different constituents. From table 4 it can be noted that the Ca/N and Ca/P ratios in the DI are less than observed in dentin, and that Ca is present in the GI interface even though this GIC does not contain Ca. This suggests that Ca migrates from the dentin to the GIC through diffusion. Considering elements of the GIC (Al, Si, Sr, F, Na) it can be seen that the ratio of diffusible elements Al, F, Sr, Na to Si increase from the GI towards the DI. Al, Sr, F and Na depletion occurs in the GI, suggesting that these elements are migrating preferentially towards the dentin. In comparing the elements of dentin and the GIC, it can be seen that there is a decrease in the Ca/C and an increase in the C/N ratio in DI, which reflects the migration of PAA towards the dentin [69]. It can also be seen with the Ca/Si and Ca/C ratios that the DI is formed of elements from both dentin and the GIC, while the GIC contains its own elements and a small quantity if Ca from the dentin. This study was performed to investigate compositional changes occurring at the immediate adhesion layer as XPS has the advantage of examining to depths of tens of Å. Previous work on the bonding of a GIC have been conducted using SEM equipped with EDS. However this technique analyzes to a depth in the order of a micron and as such, any information on adhesion, which relates only to the first atomic layers, is diluted [69]. XPS has also been used to detect the release of organic molecules from the surface of GICs which are added in order to improve the materials properties. Studies by Palmer et al

X-Ray Photoelectron Spectroscopy: Studies from Industrial & Bioactive Glass…

25

investigated the release of chlorhexidine acetate (CHA) from an experimental GIC formulated from a fluoro-alumino silicate glass with additions of CHA ranging from 0%-11.28% by weight. Figure 24 shows a.) a wide scan XPS measurement of an undoped GIC and CHA powder and b.) N 1s and Cl 1s narrow region XPS spectra where XPS was performed on undoped GIC pellets (i), CHA powder (ii), GIC pellets doped with CHA by inclusion during mixing (iii) and pellets made up in the same way (11.28%) but immersed in water for 490 days (iv), and also GIC pellets immersed in 2% CHA solution for 48 hours (v) [70]. From figure 24a it can be seen that the major peaks from the undoped GIC sample are C and O, however smaller peaks attributed to Al, Si, Ca, P and F were also detected. The spectrum from the CHA powder also shows significant C and O peaks, then main difference being the presence of strong N 1s and Cl 2p peaks due to the N and Cl present in the CHA molecule. Figure 24b shows a weak N1s signal for the undoped GIC (i) which suggest some slight contamination occurred; there was however no Cl observed. The CHA powder (ii) shows strong N 1s and Cl 2p signals. The Cl 2p signal is a spin-orbit doublet with peaks at 200.8 eV and 202.4 eV. There is also a small feature present at 198.0 eV which is representative of Cl-. These peak shapes and positions can be used to identify the presence of CHA within the GIC samples. The peak intensities can be used to estimate the elemental composition of the powder. The GIC doped with CHA (iii) shows that that the peak shapes are similar to that of the CHA powder, although the N 1s and Cl 2p peaks are relatively small. It was found that after 420 days leeching in water (iv) the 11.28% CHA showed intense peaks which are quite similar to those recorded for the GIC immersed in CHA solution (2%, 48hrs) [70]. XPS measurement in this study determined that since the N 1s and Cl 2p peaks for the leached specimen were more intense than for the ‘doped’ specimen, it suggests that the release into solution is not just a surface wash-off effect. Explanations for the increase in CHA signals would be the degradation of any CHA-depleted surface layer or that CHA in solution could be re-adsorbed onto the surface from the leaching solution. This seems to be the more plausible explanation since the spectra from the immersed in CHA solution are similar to those from the leached sample [70].

Figure 24. Shows a.) wide scan XPS measurement of an undoped GIC and CHA powder and b.) A- N 1s and B-Cl 1s narrow region XPS spectra

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A.W. Wren, F.R. Laffir, N.P. Mellot et al.

X-ray Photoelectron Spectroscopy has been utilized for all stages in the development of, in this case, glasses used in industrial and biomedical applications, from glass surface corrosion and design to the development of bioactive glasses. Bioactive glasses, are generating a considerable amount of interest as they have to ability to stimulate wound healing in damaged tissues in the human body. These glasses also have the ability to form functional biomaterials which are widely commercially used. XPS as an analytical tool has enabled research and development of many of these materials and continues to do so in order to further knowledge and understanding of novel glasses and glass based-materials.

REFRENCES [1]

[2] [3] [4] [5] [6] [7] [8] [9] [10] [11]

[12] [13] [14]

[15]

Siegbahn, K.C., Nordling, A., Fahlman, K., Hamrin, J., Hedman, Ndberg. R.C., Johansson, T., Bergmark, S. E. Karlsson, Lindgren, I. & Lindberg, B. (1967). Atomic, molecular and solid-state structure studied by means of electron spectroscopy. Nova Acta Regiae Societatis Scientiarum Upsaliensis, 20, 282. Hertz, H. (1887). Uber einen Einfluss der ultravioletten Lichtes auf die elektrische Entladung. Annalen der Physik, 31, 983-1000. Einstein, A. (1905). On a heuristic viewpoint concerning the production and transformation of light. Annalen der Physik, 17, 132-48. UK Surface Analysis Forum, Chapter 6, http://www.uksaf.org/tutorials. Watts, J. F. & Wolstenholme, J. (2003). An introduction to surface analysis by XPS and AES. (2 Ed.) Chichester, Wiley. Briggs, D. & Seah, M. P. (1990). Practical surface analysis. (2 Ed.) Chichester, Wiley & Sons. Attard, A. & Barnes, C. (1998). Surfaces. Oxford, Oxford Unversity Press. Beamson, G. & Briggs, D. (2000). The XPS of Polymers Database, UK: Surface Spectra Ltd. Shelby, J. E. (1997). Introduction to glass science and technology. Cambridge, RSC Publishing. Pantano, C. (1993). X-Ray Photoelectron Spectroscopy of glass; experimental techniques of glass science. Westerville Ohio, American Ceramic Society. Henderson, S., Castle J. & Zhadan P. (1997). The SPM and XPS Characterization of the corrosion resistance of glasses. (57 Ed), Advances in the characterization of ceramics. 55-66. Holland, D., Gee, I., Mekki A. & McConville, C. (2001) Role of surface science in the determination of glass structure. Physics and Chemistry of Glasses, 42, 247-54. Mellott, N. P., Brantley, S. L., Hamilton J. P. & Pantano, C. G. (2001) Evaluation of surface preparation methods for glass. Surface and Interface Analysis, 31, 362-68. Mellott, N. P. & Pantano, C. G. (2009). Multicomponent aluminosilicate glass: mechanisms of acid corrosion and surface layer formation. Journal of Non-Crystalline Solids, Submitted Manuscript. Pantano, C. G. & Hamilton, J. P. (2000). Characterization and structure of leached surface layers on glass. Rivista della Stazione Sperimentale del Vetro, 6, 81-86.

X-Ray Photoelectron Spectroscopy: Studies from Industrial & Bioactive Glass…

27

[16] Trens, P., Denoyel R. & Guilloteau, E. (1996) Evolution of surface composition, porosity, and surface area of glass fibers in a moist environment. Langmuir, 12, 124550. [17] Jallot, E., Benhayoune, H., Kilian, L., Irigaray, J. L., Barbotteau, Y., Balossier G. & Bonhomme, P. (2001). Dissolution kinetics, selective leaching, and interfacial reactions, of a bioglass coating enriched in alumina. Journal of Colloid and Interface Science, 233, 83-90. [18] Ruud, G., Garafolini, S., Hensley D. & Mate, M. (1993). Atomic Force Microscopy and X-Ray Photoelectron Spectroscopy investigation of the onset of reactions on alkali silicate glasses. Journal of the American Ceramic Society, 76(10), 2555-60. [19] Matousek, J., Maryska M. & Helebrant, A. (1996). Cation concentration profiles in float glass surfaces during corrosion in aqueous solutions. Glastechniche Beriche, 69 (1), 7-11. [20] Hamilton, J. & Pantano, C. (1997). Effects of glass structure on the corrosion behavior of sodium-aluminosilicate glasses. Journal of Non-Crystalline Solids, 222, 167-74. [21] Schultz-Munzenburg, C., Meisel W. & Gutlich, P. (1998) Changes in lead silicate glasses induced by leaching. Journal of Non Crystalline Solids, 238, 83-90. [22] Hamilton, J., Pantano C. & Brantley, S. (2000) Dissolution of albite glass and crystal. Geochimica et Cosmochimica Acta, 64(15), 2603-15. [23] Bois, L., Guittet, M. J., Barre, N., Trocellier, P., Guillope, S., Gautier, M., Verdier, P. & Laurent, Y. (2000). Aqueous alteration of lanthanum aluminosilicate glasses. Journal of Non Crystalline Solids, 276, 181-94. [24] Bange, K., Anderson, A., Rauch, F., Lehuede, P., Radlein, E., Tadokoro, N., Mazzoldi, P., Rigato, V., Matsumoto, K. & Farnworth, M. (2001). Multi-method characterization of soda lime glass corrosion; Part 1. Analysis techniques and corrosion in liquid water. Glasetechniche Beriche, 74(5), 124-41. [25] Smith, N. & Pantano, C. (2008). Leached layer formation on float glass surfaces in the presence of acid interleave coatings. Journal of the American Ceramic Society, 91(3), 736-44. [26] Petit, J., Della Mea, G., Dran, J., Magonthier, M., Mando P. & Paccagnela, A. (1990). Hydrated-layer formation during dissolution of complex silicate glasses and minerals. Geochimica et Cosmochimica Acta, 54(7), 1941-55. [27] Sprenger, D., Bach, H., Meisel W. & Gutlich P. (1990). XPS study of leached glass surfaces. Journal of Non-Crystalline Solids, 126(1-2), 111-29. [28] Dal Bianco, B. & Bertoncello, R. (2008). The development of growth rings on ancient glass surfaces: description and simulation of weathering. Journal of Non-Crystalline Solids, 354, 773-79. [29] Hench, L. L. & Clark, D. E. (1978). Physical chemistry of glass surfaces. Journal of Non-Crystalline Solids, 28, 83-105. [30] Hench, L. L. (1975). Characterization of glass corrosion and durability. Journal of Non-Crystalline Solids, 19, 27-39. [31] Clark, D. E., Dilmore, M. F., Ethridge E. C. & Hench, L. L. (1976). Aqueous corrosion of soda-silicate and soda-lime-silicate glass. Journal of the American Ceramic Society, 59(1-2), 62-65.

28

A.W. Wren, F.R. Laffir, N.P. Mellot et al.

[32] Doremus, R. H., Mehrotra, Y., Lanford W. A. & Burman, C. (1983). Reaction of water with glass: Influence of a transformed surface layer. Journal of Materials Science, 18, 612-22. [33] Bunker, B. C., Arnold, G. W., Day, D. E. & Bray, P. J. (1986). The effect of molecular structure on borosilicate glass leaching. Journal of Non-Crystalline Solids, 87, 226-53. [34] White, W. B. (1988). Glass structure and glass durability. Materials Research Society Symposium Proceedings, 125, 109. [35] Bunker, B. C. (1994). Molecular mechanisms for corrosion of silica and silicate glasses. Journal of Non-Crystalline Solids, 179, 300-08. [36] Koenderink, G. H. Brzesowsky R. H. & Balkenende, A. R. (2000). Effect of the initial stages of leaching on the surface of alkaline earth sodium silicate glasses. Journal of Non-Crystalline Solids, 262, 80-96. [37] Sanders, D. M. & Hench, L. L. (1973). Mechanisms of glass corrosion. Journal of the American Ceramic Society, 56(7), 373-77. [38] Doremus, R. H. (1979). Chemical durability of glass. Treatise on Materials Science and Technology, 17, 41-69. [39] Smets, B. M. J. & Tholen, M. G. W. (1985). The pH dependence of aqueous corrosion of glass. Physics and Chemistry of Glasses, 26(3), 60-63. [40] Barkatt, B., Gibson, C., Macedo, P. B., Montrose, C. J., Sousanpour, W., Barkatt, A., Boroomand, M. A., Rogers V. & Penafiel, M. (1986). Mechanisms of defense waste glass dissolution. Nuclear Technology, 73, 140-63. [41] Clark, D. E., Pantano C. G. & Hench, L. L. (1979). Corrosion of glass. New York, Books for industry and the glass industry, 1-39. [42] Fadley, C. (2009). X-Ray Photoelectron Spectroscopy: from origins to future directions. Nuclear Instruments and Methods in Physics Research A, 601, 8-31. [43] Bluhm, H. (2009). Photoelectron Spectroscopy of surfaces under humid conditions. Journal of Electron Spectroscopy and Related Phenomena, In Press. [44] Mekki, A. & Salim, M. (1999). XPS study of transition metal doped silicate glasses. Journal of Electron Spectroscopy and Related Phenomena, 103, 227-32. [45] Mekki, A., Holland D. & McConville, C. F. (1997). X-ray photoelectron spectroscopy study of copper sodium silicate glass surfaces. Journal of Non-Crystalline Solids, 215(2-3), 271-82. [46] Miura, Y., Kusano, H., Nanba T. & Matsumoto, S. (2001). X-ray photoelectron spectroscopy of sodium borosilicate glasses. Journal of Non-Crystalline Solids, 290(1), 1-14. [47] Roy, B., Jain, H., Saha S. K. & Chakravorty, D. (1995). Comparison of Structure of alkali-silicate glasses prepared by sol-gel and melt-quench methods. Journal of NonCrystalline Solids, 183, 268-76. [48] Roy, B. & Jain, H. (1998). Phase separation and structural differences between alkiali silicate glasses prepared by the Sol-gel and melt-quench methods. Journal of American Ceramic Society, 81(9), 2360-70. [49] Serra, J., González, P., Liste, S., Serra, C., Chiussi, S., León, B., Pérez-Amor, M., Ylänen H. O. & Hupa, M. (2003). FTIR and XPS studies of bioactive silica based glasses. Journal of Non-Crystalline Solids, 332(1-3), 20-27.

X-Ray Photoelectron Spectroscopy: Studies from Industrial & Bioactive Glass…

29

[50] Serra, J., Gonzalez, P., Liste, S., Chiussi, S., Leon, B., Perez-amor, M., Ylanen, H. O. & Hupa, M. (2002). Influence of the non-bridging oxygen groups on the bioactivity of silicate glasses. Journal of Material Science:Materials in Medicine, 13, 1221-25. [51] Kokubo, T. Kim, H. M. & Kawashita, M. (2003). Novel bioactive materials with different mechanical properties. Biomaterials, 24(13), 2161-75. [52] Dalby, K. N. Nesbitt, H. W. Zakaznova-Herzog V. P. & King, P. L. (2007). Resolution of bridging oxygen signals from O 1s spectra of silicate glasses using XPS: Implications for O and Si speciation. Geochimica et Cosmochimica Acta, 71, 4297-313. [53] Nicholson, J. W. & Wilson, A. D. (1993). Acid-Base cements - Their biomedical and industrial applications. Chemistry of solid state materials. Vol. 3. Cambridge, Cambridge University Press. [54] Simon, V., Todea, M., Takacs, A. F., Neumann M. & Simon, S. (2007). XPS study on silica-bismuthate glasses and glass ceramics. Soid State Communications, 141, 42-47. [55] Polzonetti, G., Iucci, G., Frontini, A., Infante, G., Furlani, C., Avigliano, L., Del Principe, D., Palumbo G. & Rosato, N. (2000). Surface reactions of a plasma-sprayed CaO-P2O5-SiO2-based glass with albumin, fibroblasts and granulocytes studied by XPS, fluorescence and chemiluminescence. Biomaterials, 21(15), 1531-39. [56] Sharma, K., Dixit, A., Singh, A., Jagannath, S., Bhattacharya, S., Prajapat, C. L., Sharma, P. K., Yusuf, S. M., Tyagi, A. K. & Kothiyal, G. P. (2009). Preparation and studies on surface modifications of calcium-silico-phosphate ferrimagnetic glassceramics in simulated body fluid. Materials Science and Engineering, 29(7), 2226-33. [57] Kokubo, T. & Takadama, H. (2006). How useful is SBF in predicting in vivo bone bioactivity. Biomaterials, 27, 2907-15. [58] Billington, R. W. Williams J. A. & Pearson, G. J. (2006). Ion processes in glass ionomer cements. Journal of Dentistry, 34(8), 544-55. [59] Nicholson, J. W. (1998). Chemistry of Glass Ionomer Cements. Biomaterials, 19, 48594. [60] Crisp, S. & Wilson, A. D. (1973). Reactions in Glass ionomer cements. Journal of Dental Research, 53(6), 14O8-13. [61] Wren, A. W., Clarkin, O. M., Laffir, F. R., Ohtsuki, C., Kim I. Y. & Towler, M. R. (2009). The effect of glass synthesis route on mechanical and physical properties of resultant glass ionomer cements. Journal of Material Science: Materials in Medicine, 10, 1991-99. [62] Hadley, P. C., Milella, E., Gerardi, C., Hill, R. G. & Billington, R. W. (2001). Distribution of fluoride in glass ionomer cement determined using SIMS. Biomaterials, 22, 1563-69. [63] Seppa, L., Forss H. & Ogaard, B. (1993). The effect of fluoride application on fluoride release and the antibacterial action of glass ionomers. Journal of Dental Research, 72(9), 1310-14. [64] Marczuk-Kolada, G., Jakoniuk, P., Mystkowska, J., Luczaj-Cepowicz, E., Waszkiel, D., Dabrowski, J. R. & Leszczynska, K. (2006). Fluoride release and antibacterial activity of selected dental materials. Postepy Higieny Medycyny Doswiadczalnej, 60, 416-20. [65] DeBruyne, M. A. A. & DeMoor, R. J. G. (2004). The use of glass ionomer cements in both conventional and surgical endodontics. International Endodontics Journal, 37, 91104.

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A.W. Wren, F.R. Laffir, N.P. Mellot et al.

[66] Mount, G. J. (2001). An atlas of glass-ionomer cements: A clinician's guide. (3Ed), Informa Healthcare. [67] Jones, F. H., Hutton, B. M., Hadley, P. C., Eccles, A. J., Steele, T. A., Billington, R. W. & Pearson, G. J. (2003). Fluoride uptake by glass ionomer cements: a surface analysis approach. Biomaterials, 24(1), 107-19. [68] Fukuda, R., Yoshida, Y., Nakayama, Y., Okazaki, M., Inoue, S., Sano, H., Suzuki, K., Shintani H. & Meerbeek, B. V. (2003). Bonding efficacy of polyalkenoic acids to hydroxyapatite, enamel and dentin. Biomaterials, 24(11), 1861-67. [69] Sennou, H. E., Lebugle, A. A. & Grégoire, G. L. (1999). X-ray photoelectron spectroscopy study of the dentin-glass ionomer cement interface. Dental Materials, 15(4), 229-37. [70] Palmer, G., Jones, F. H., Billington, R. W. & Pearson, G. J. (2004) Chlorhexidine release from an experimental glass ionomer cement. Biomaterials, 25(23), 5423-31.

In: X-Ray Photoelectron Spectroscopy Editor: Johanna M. Wagner

ISBN: 978-1-61668-915-5 © 2011 Nova Science Publishers, Inc.

Chapter 2

XPS FOR INVESTIGATING CULTURAL HERITAGE MATERIALS Claudia Altavilla1 and Enrico Ciliberto2* 1

Dipartimento di Ingegneria Chimica e Alimentare -Università degli Studi di Salerno, Via ponte don Melillo 1, 84084 Fisciano, (SA) Italy 2 Dipartimento di Scienze Chimiche -Università degli Studi di Catania Viale A.Doria 6, 95125, (CT), Italy

ABSTRACT The continuous demand for new analytical methods in studies relating to art and archaeology has prompted researchers to investigate the possible applications of a variety of non-conventional and innovative analytical techniques. In this chapter, the authors describe several interesting applications of X-ray photoelectron spectroscopy in order to undertand in greater detail the chemical phenomena which are present on the surface of archeological and artistic objects that they have been studied over the last ten years. After a preliminary discussion on the potentialities and limitations of XPS in this fascinating research field, a series of case studies on different kinds of art crafts is reported. The use of XPS, in monitoring the efficiency and the damage of restoring techniques on the surfaces of several masterpieces such as Michelangelo’s David and the S. Marco Basilica (Venice) mosaics, is also critically discussed. Finally, some innovative techniques, such as Self Assembled Monolayers, used for the protection of material surfaces are tested by the use of XPS.

1. INTRODUCTION In this chapter, how XPS may be applied to materials constituting artistic and/or archaeological artefacts is discussed in order to focus on: (i) Paint layers, (ii) Glass, (iii) *

Corresponding author: E-mail:[email protected]; [email protected].

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Stone, (iv) Metal alloys and (v) Surface modifications induced by cleaning and protective treatments. Over the last forty years numerous surface sensitive and surface specific techniques have been developed for the study of properties such as surface structure, chemical composition, the oxidation states and chemical environment of elements. The most commonly used techniques involve the scattering, absorption or emission of photons, electrons, atoms and ions. XPS is a surface sensitive technique that gives a kinetic energy spectra of the electrons emitted as a result of the material bombardment with X-ray photons. Whereas the excited photons deeply penetrate into the solid sample, the photoelectrons can escape from only a very short distance beneath the surface. The mean escape depth is usually quite low, commonly less than 3 nanometers in the range of electron kinetic energies that is of practical concern in the solid state XPS[1,2,3]. This can either be considered positive or negative, depending on the preferences. If trying to apply XPS in order to gain electronic structural information about the bulk material, then there would be problems. Thus, a fracture of the material under ultra high vacuum conditions could be required as in the case of ceramics, glass, alloys etc. On the other hand, the surface sensitivity of XPS can be exploited vantageously when interested in the surface of the material rather than its bulk. Many phenomena such as the interaction between materials and external environments can be beneficially studied by XPS with a sensitivity order of greater magnitude than surface specific techniques[4,5,]. Therefore, this feature does not represent a limit of the application of this powerful kind of spectroscopy when investigating real materials where patina, crusts and pollution layers generally affect the surface of ancient artefacts. On the contrary, different ideas represent, according to us, a good example of ‘superficial science’ due to our extensive experience in this field having demonstrated that XPS can provide very important information which is extremely useful in solving archaeometric and conservation problems in the many different types of material used in the so called ‘Cultural Heritage’ handcrafts. What the term ‘Cultural Heritage’ indicates is the whole of Man’s cultural patrimony, i.e. everything which refers to the history of civilization. This includes all that is considered to be of interest from a variety of points of view – archaeological, historical and artistic. It also embraces all the works and documents of value contained in archives or preserved in book collections. In this field of interest, it is well known that, in principle, scientific investigations can be used in two areas: archaeometric and conservative studies. The archaeometric topic corresponds to answering the following questions: • • • •

How old is the object? What are the raw materials and where did they come from? What manufacturing technique was used? What was the use of the object?

There are different questions relating to the conservation field: • • •

Is the object degrading ? If yes, Why? (Are there any intrinsic and/or environmental effects?) Is it possible to stop degradation?

XPS for Investigating Cultural Heritage Materials •

33

If the object needs consolidation and/or a protective treatment, are there any compatibly problems between the materials and chemical compounds of the intervention? (The question is not arbitrary because if the restoration is not accurate the artefact can be severely damaged).

Figure 1. XPS survey spectrum obtained from a daguerreotype surface

In Figure 1, a wide XPS spectrum obtained from the surface of a daguerreotype picture is shown. Daguerreotypes are the first recognized photographs made on sensitized silver-coated metal [6]. During the XIX century, they were popular for about 20 years, until the improved collodion print process came into use. They are microscopically interesting, partly because they exhibit a peculiar change in the image with a changing incidence of light ( Figure 2).

Figure 2. An example of a daguerreotype: the daguerreotype is a negative image, but the mirrored surface of the metal plate reflects the image and makes it appear positive when the silvered surface has a dark ground reflected into it.

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Figure 3. A 2D plot of silver Auger parameters: the arrow indicates the position corresponding to peaks measured on the daguerreotype surface

The reason for this change is explained by the unusual surface structure. The series of signals indicates the presence of many elements such as Ag (3p), Ag (3d), Ag (MNN); Cu (2p), Cu (3p), Cu (LMM); Cl (2s), Cl (2p); S (2s), S (2p). A similar result gives two different deductions: i) the technique of making the photo layer consisted in applying a silver thin film on a copper substrate; ii) the decomposition of the external layer is due to sulphur (sulphide) and chlorine (chloride) species. Only by running a simple wide spectrum is it then possible to have both archaeometric and conservative information. Evidently, a more detailed study of the spectrum, performed by recording expanded regions of the bands of interest, gives more detailed information on the material. For instance, by taking into account the silver signals, in particular the silver 3d5/2 and the Auger signal M4N4.5N4.5, it is possible to calculate the Auger parameter and plot the values in an Auger KE vs 3d BE graph. The result, shown in figure 3, clearly indicates that silver is oxidized forming Ag2S on the surface of the daguerreotype. Every chemical and/or physical procedure performed to restore the picture by cleaning its surface must then take into account the presence of this degradation phase that gives a blackish colour to the picture. However, most real specimens are not electrically conductive. In this case, when they are under X-ray bombardment, the positive holes produced by the photoionization process can be neutralized only by capturing the stray electrons from the vacuum system, due to the lack of conduction from the sample backing [7,8]. This generally causes a change in the observed position of the XPS peaks. This consequently leads to considering the shift in the spectrum interpretation after an accurate B.E. measurement of a reference peak. Standard X-ray sources, owing to their metal windows, generally produce a lot of low energy electrons and the mechanism of capturing vacuum electrons from the analysed surface is quite efficient. On the contrary, the monochromatic X-ray sources very often produce a greater shift (more than 10 eV) that must be removed with a slow electron flood gun [9]. Moreover, real samples coming from archaeological or artistic handicrafts are generally composed of many chemical phases. In this case, often a series of charging domains can be observed because the particles

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of different materials can charge to different extents and the interpretation of the spectrum is complicated accordingly. However, by carefully regulating the intensity as well as the energy of the electron flux, it is possible to recognise and distinguish the different domains (9) as well as obtain a greater simplification of the spectrum. Finally, it is worth noting that painting specimens and porous lithic materials can contain a lot of volatile species that must be removed before running a XPS spectra. Water is the most common volatile compound contained and its removal can be intially carried out by an accurate drying in a drier system and, secondly, by long term pumping in a separate vacuum system. This operation is useful in order to also eliminate organic volatile molecules such as solvents, etc. In the following sections, several XPS studies on different materials are discussed.

2. PIGMENTS IDENTIFICATION AND ALTERATION IN PAINT LAYERS There are many reasons behind the studies of a particular pigment in a work of art or a paint layer. For example, the identification of pigments in a paint can be useful in understanding the technique used by the artist as well as reconstructing his palette of colors. Moreover, the dating and authentication of a work of art can be supported by the chemical analyses of the pigments, answering questions such us whether a certain pigment was even in existence or not at the time the painting was created. A painting is a stratified system in which it is possible to recognize several important elements: the support, the preparation and the paint layer. There are many kinds of support: canvas, wooden panels, plaster on a wall, glass. The choice of the material depends on the painting technique. The preparation, called “ground and/or imprimitura”, is an important element in a painting. It is the interlayer between the support and paint layer itself. Generally, the ground is made by a fine mixture of inert and bonding agents to assure the adhesion and prevent the eventual detachment of the paint. For example, in the Marriage of the Virgin, a painting on a wood panel by Pietro Perugino (1450-1523), the preparation was made of six layers of gypsum, containing small amounts of animal glue and other organic materials. A thin final layer, called “imprimitura”, about 10 micrometers covering the ground was very rich in white lead as well as small amounts of thin yellow lead [10]. Today, thanks to scientific analysis (supporting the traditional and historical sources), it is possible to verify and identify the ingredients of the secret receipes that an artist used to prepare the own “imprimitura”[11]. In a “fresco” painting, the ground is the so called “arriccio”, a layer of fairly coarse plaster applied to the wall before the “intonaco” (the smoother top layer of plaster that forms the painting surface). The paint layer is a mixture of pigments finely ground and dispersed in an oleic (in oil painting) or protein (in tempera painting) fluid medium with a thickness of about 10-30 microns. After the drying process, the binder protects the grains of pigment and guarantees the adhesion of the paint layer to the imprimitura. In the case of a fresco, the binder is a thick layer of lime, spread onto the wall immediately before to being painted on with the aqueous suspension of pigments. The Ca(OH)2 reacts with CO2 forming a transparent film of calcium carbonate that encloses the pigment grains protecting them for a prolonged time.

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Pigment grain

Paint Layer Imprimitura Preparation Ground

Support

Figure 4. schematic cross section of a paint layer

A schematic representation of a paint cross section is reported in figure 4. Another important aspect in the study of pigments and paint layers is the restoration and conservation of the painting. The conservation of a work of art is essentially influenced by two factors: the materials that it is made of and the environment in which it is or was in the past. Organic materials are generally more sensitive to deterioration with respect to inorganic ones, but nothing is totally preserved by ageing. Sudden changes of temperature or relative humidity, as well as the presence of pollutants can have a synergic effect and accelerate the deterioration processes of many kinds of works of art [12]. In particular, in the case of paintings, aggression by pollutants (SO2, NOx) triggers chemical reactions that involve both the pigments and binders. The result is often the alteration of the chemical nature and colors of the paint layer which damages the readability as well as durability of the work. A large variety of material characterization techniques are adopted in the study of paintings. Spectroscopic techniques are frequently used to identify the composition of the pigments and binders for restoration and conservation purposes. Recent reviews on Spectroscopic techniques in cultural heritage conservation are given by Bitossi et al.[13], Dawson [14] and Aucouturieret al.[15]. In this context, X-ray photoelectron spectroscopy (XPS) is a surface technique with great potential [16,17]. The alteration of a pigment by the environment may not be homogeneous. It logically starts from the surface. The products formed at the early stage of degradation are in the form of very thin layers (only a few nanometers thick) and the analytical techniques usually adopted for the characterization of solids, such as X-ray diffraction (XRD) or electron-probe microanalysis (EPMA), cannot be used. The use of surface analysis methods may be of great help in understanding the alteration mechanism and prepare conservation programs. It is also possible to study the paint coating in a non-destructive manner. Following XPS analysis, samples may be used in further tests or simply be replaced in the original work of art. In this paragraph, the results of a systematic XPS study of several pictorial models (pigment + binder) of historical interest are discussed. The effects of temperature, humidity,

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reactive gases (SO2, NO2), exposition to UV radiation were evaluated analyzing pure pigments, as well as both artificially and naturally aged paint layers. The artificial ageing was carried out in a climatic chamber for 4 weeks. The scheme with the ageing conditions of a week cycle is reported in table 1. The pigments and the paint layers used as pictorial models were: copper reinate + linseed oil and smalt + (leather glue). XPS analysis was performed before and after the ageing treatment. Each sample was analyzed on the external surface (paint layer-air), exposed to a controlled polluted environment containing SO2 and NO2, as well as the internal surface (paint layer –glass substrate) of the same sample.

2.1. The Case of Copper Resinate Copper resinate is the name commonly given to the transparent green glazes that are coloured by copper salts of resin acids. This pigment was most commonly used in Europe in the fifteenth and sixteenth centuries, but it does not seem to be a usual feature of the palette anywhere after the end of the sixteenth century, because its tendency to discolour was already widely known by artists of the time [18]. Recently, studies on the darkening phenomenon of copper resinate observed in a XV century easel painting were carried out by X-ray absorption spectroscopy (XAS) at the GILDA beamline of the European synchrotron radiation facility (ESRF, Grenoble, France). It is reasonable to assume that discoloration takes place as a result of strong copper–organic matrix interactions (copper-ion extraction and/or metal-catalysed oxidation of the organic matrix) followed by the modification of the Cu coordination structure[19]. X-ray Photoelectron Spectroscopy (XPS) was adopted to investigate, understand and describe the modifications of copper resinate in oil paintings, caused by the external environment through analysis of pictorial models [20]. The samples were prepared according to early recipes[21]. The pigment was obtained by mixing Venice turpentine, basic verdigris and turpentine essence and grinding the green glaze to powder. The paint layer models were obtained by traditional methods, using linseed oil as a binder. XPS techniques make it possible to discriminate between the different oxidation states and chemical environments of the elements. In this case, copper was in the cupric (Cu II) state, and experimental precautions must be taken into account as a known risk of reduction to the Cu(I) state by reduction under the incident X-ray beam[22,23,24,25]. Table 1. Week cycle of artificial ageing treatment of paint layers Day 1 2 3 4 5 6/7

Ageing agent NO2(10ppm)+SO2(10ppm) UV(365nm) NO2(10ppm)+SO2(10ppm) UV(365nm) NO2(10ppm)+SO2(10ppm) UV(365nm)

Temperature 18 °C 30 °C 18 °C 30 °C 18 °C 30 °C

R.H. 75 % 75 % 75 % 75 % 75 % 75 %

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Cu2p

Cu(I)

35'

shake-up

Cu(II)

0'

(1.5 eV)

970

960

950

940

930

920

B.E. (eV)

Figure 5. Cu 2p spectra from a sample of pure copper resinate acquired at different X-ray irradiation times 4800 00

external surface Internal surface

S2p

4700

external surface Internal surface

N1s

4600 4500

00

4400 00

4300 4200

00

4100 4000

00

3900 3800

00

160

3700

162

164

166

168

170

B.E.(eV)

172

174

176

3600

410

408

406

404

402

400

398

B.E. (eV)

Figure 6. S 2p and N 1s expanded regions acquired on the external (black ) and internal (red ) sides of copper resinate in linseed oil film artificially aged

After 35min of X-ray bombardment Cu(II) appears to be completely reduced to Cu(I) as suggested by the shape of the L3M45M45 Auger [20] (not reported here) and Cu2p signals that are shown in the figure 5. In order to avoid the alteration of the oxidation of the chromophore element, the XPS acquisition time was kept under 10min. This phenomenon is important to evaluate the possible damage produced on real paintings, very often analyzed by techniques that used X-Ray[26]. The effect of artificial ageing on the paint layer was investigated with the XPS analysis of both the external surface (paint layer –air) as well as the internal surface (paint layer glass substrate). The results show that there is a strong difference between the two. The spectrum of the external interface presents pollutant signals (S 2s, S 2p, N 1s) that were absent in the analogous spectrum recorded on the internal interface of the same sample as well as the freshly prepared samples. The XPS spectra collected on both sides of the

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artificially aged paint layer are reported in figure 6. The corresponding B.E. values clearly indicate that sulphur and nitrogen possess oxidation numbers of +6 and +2 respectively. This evidence strongly suggests that a red–ox reaction between SO2 and NO2 occurred on the layer surface[20]. Only the external surface of the layer is affected by deterioration processes because the oil medium sufficiently protects the enclosed pigment grains.

Figure 7. OMFC images(500x) of Copper resinate grains (on the left) and Copper resinate in linseed oil (on right)

Table 2. Surface Atomic concentration of some chromospheres of different pigments in oil paint layers Linseed oil Atomic conc. (%)

White lead 2,8

White zinc 1,2

Copper resinate 46,3

Another interesting aspect that emerged during the XPS analysis was the high atomic concentration of copper ions on the surfaces (46%) with respect to analogous systems obtained using the same binder and other pigments (see table 2). This result strongly suggests that the green pigment is partially soluble in linseed oil. The positive interaction between the cromophore and the binder was also confirmed by the evaluation of the Auger Parameter of copper in the pure pigment (1851.1 eV) with respect to the oil paint layer (1849.3 eV). This property could explain the greater reactivity of this pigment with the external environment [19, 20].

2.2. The Case of Smalt Smalt is a pigment obtained by the grinding down of blue-colored potassium glass. The blue color is due to the small but variable amounts of cobalt, added as cobalt oxide during the manufacture process. The principal sources of cobalt used in the preparation of smalt in Europe since the Middle Ages appear to have been the mineral members of the smaltite family ([Co, Ni]As3−2), a group of the diarsenide system of Co and Ni with variable

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concentrations of Fe(II). Cobalt ores are roasted to obtain the oxide (CoO). This is melted together with quartz and potash or added to molten glass. The mixture is poured into cold water, in which the thermal shock disintegrates the blue melt into particles. Finally the product is ground in water mills and elutriated [27,28]. The origin of this fascinating blue pigment can be traced back to an invention of a Bohemian glass-maker, Christoph Schurer, around 1550 [28]. However, earlier cases of smalt used as a pigment have been found, for example in the frescoes by Domenico Ghirlandaio (1449-94) in the church of S. Maria Novella in Florence as well as the lunette of the main portal of the Church of S. Maria la Vetere in Militello Val di Catania (Sicily), the portal being dated 1506 [28,29]. Using the same protocol of artificial ageing reported previously for copper resinate, pure smalt and smalt in “leather-glue1”, were analysed by XPS in order to study the effect of pollutant species (SO2 and NO2) and UV radiations [30] (see table 1). The wide scan X-ray photoelectron spectrum (see figure 8) of the pure smalt sample clearly revealed the core level lines Si 2s, Si 2p, Co 3p, Co 2p, K 2s, K 2p, As 3p, As 3d, O 1s as well as the Auger lines Co LMM, As LMM and OKVV. The arsenic traces derive from ores used during the preparation of cobalt oxide. The silicon and potassium signals are typical of potassium glass. All of these elements are, thus, an important factor in the discrimination of this pigment from other blue pigments.

Figure 8. XPS wide spectrum of smalt. The signals of all the elements, useful in identifying the blue pigment, are clearly visible. In the insert, the binding energy region attributed to multiplet splitting of the Co2p photoelectron signal

Cobalt can exist in different coordination geometries and different spin states for the same oxidation state. In the insert of figure 8, the XPS spectrum of Co2p photoelectron signal of pure smalt is shown. The Co 2p1/2-Co 2p3/2 separation (16.0 eV) and the strong satellite peaks about 5.6 eV above the main Co2p lines confirm that cobalt, in smalt, is Co2+ in high1

binder traditionally obtained by extracting collagen proteins from rabbit furs

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spin state (S=3/2) with tetrahedral coordination[30]. These results confirm the studies carried out by Bacci and Piccollo on similar pigments using fibre optics reflectance spectroscopy in the visible and near infrared region [31]. Table 3. Atomic concentration ratios between silicon and cobalt in samples aged under different conditions. Smalt in leather glue Si/Co

not aged

naturally aged (2 yr)

3.5

4.9

75%R.H. +10 ppm NO2 + 10 ppm SO2 7.8

Further interesting information was obtained from the analysis of the O1s photoelectron core level spectrum, with the main peak being at 532.4 eV and a shoulder at BE of 530.4 eV. According to the nature of a glass network, the incorporation of K2O during the making process of the glass produces the breaking of tetrahedral Si-O bonds in the glass network (see the paragraph on glass) and generates the shoulder at lower binding energy (530,4 eV ) [30]. As it is widely known, potassium glass is more vulnerable to leaching than sodium glass, and high humidity conditions as well as the presence of pollutants could cause leaching of metal salts from the silicate network. For this reason, smalt is not a stable pigment. Discoloration of smalt on sixteenth-century paintings is not at all unusual. Earlier studies have implicated a number of factors in the degradation [32]. Concurrent loss of cobalt into the medium has been proposed. However, due to the percentage of cobalt in smalt being very low – typically less than 10% – this has proved difficult to confirm through conventional analyses [33]. A process which, as well as causing deterioration of the glass itself, could cause saponification or condensation reactions in the oil medium, and would account for the excessive yellowing often observed in paint samples containing deteriorated smalt [34]. XPS was useful in evaluating and demonstrating that the leaching of smalt pigment had occurred and also involved the cobalt ions [30] producing the widely know discolouration. The trend of the atomic concentration ratio between Si/Co, at different environmental and times conditions, is reported in table 3. The XPS analysis of the internal and external surfaces of artificially aged paint layers reveals the presence of pollutant species on both sides. The S2p photoelectron core level spectrum has two components at 169.5 eV due to sulphur of sulphate group[35,36]and at 167.8eV that can be attributed to sulphite species[37,38]. This indication seems to be in accordance with the experimental evidence that tempera paintings are extremely sensitive to polluted environments because paint film is permeable to reactive vapours.

3. STONE An important part of the world’s cultural heritage is represented by stone monuments. This enormous artistic legacy includes historical and modern buildings as well as decorative stone structures, individual monuments and an array of monuments constructed with dimension stones or carved from bedrock, such as sculptures and/or artefacts of different size and shape. Many stone types have been used for constructing monuments. Due to their

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geological history, the stone types show significant differences in their physical or mechanical properties as well as their chemical composition. Despite stone often being considered to be "eternal", all stone monuments are affected by ageing. Variations in colour, lack of cohesion between the various components of the rock, detached portions of the material, formation efflorescence of soluble salts, patinas and crusts, appearance of fractures and the growth of various vegetal organisms are the most relevant examples of problems that need both attention and to be solved during a restoration of a stone monument. These alteration/deterioration phenomena are the result of an interaction between the material and external environmental agents, such as rain, wind, temperature and pollutants. Many interesting studies have been published on the effects of a polluted atmosphere on stone with emphasis on the more chemical aspects [39,40,41]. The physical-chemical analytical techniques employed in the study of building materials provide very accurate qualitative and quantitative results on the alterations related to the patina or crust as well as the bulk chemistry of the exposed stone. Scanning electron microscopy (SEM), Electron probe X-ray microanalysis (EPXMA), Fourier-transform infrared analysis (FTIR), X-Ray diffraction (XRD), energy dispersive X-Ray fluorescence , Ion Chromatography, are the most used techniques for the studies of sulphate black crusts as well as to evaluate the effect of exposition time of the sample stone to weathering[42,43]. However, weathering processes, which might be expected to require many years before visible changes become apparent, can be monitored in order to estimate the initial stages of the chemical alterations at the stone-atmosphere interface by XPS analysis. This surface technique was successfully used by Maravelaki-Kalaitzaki et al. to study the decay phenomena of Istria stone, a material with low porosity and low roughness, where decay products are limited to the outermost layers[44]. Mossotti et al.[45] applying XPS to fresh Salem limestone, detected a sulphate layer of 10nm thickness on the calcite grains after an exposure period to pollutants. XPS was also used to evaluated the surface composition of limestone after treatment with a protective material of a fluorinated phosphoric ester, by XPS [46]. Torrisi used XPS to study commercial fluorinated compounds as a protection of calcarenite surfaces, before and after ageing in a climatic chamber simulating exposure to solar radiation, [47]. Ciliberto and al. used XPS to study the penetration depth of the products introduced by Sanpaolesi’s treatment in the marble matrix of the Donatello’ pulpit. The masterpieces of the famous Italian sculptor were in fact restored with a mixture of ZnSiF6 and MgSiF6 in 1941. The surface analysis showed that both the fluoride- and silicon-containing compounds enrich the most external layers of the marble. Two small fragments, about 1.0-0.5cm wide and 50 mol%) was less than the experimental value; that of the large structures, greater. The dependence of the relative content of modifier-lead did not agree with the experimental data (Figure 11). The calculated content of modifier-lead for the low-lead glasses was 15% less than the experimental value; for high-lead glasses, much greater than the experimental value. In the second approximation it was assumed that two substructures formed by different network-formers, Pb and Si, were present in the lead-silicate glasses. The glasses were represented as two solutions in order to model this structure. The first solution consisted of the starting oxides; the second solid solution, the final products (silicates). Like in the first version, the glass-gas equilibrium states were calculated and the resulting structural components of the glasses were estimated. The calculated content of Si-O structural components agreed with the X-ray photoelectron analysis. However, the relative content of modifier-lead did not agree with the experimental value (Figure 11), just like in the first version. The calculated amount of modifier for the high-lead glasses was greater; for low-lead glasses, less. The substantial differences in the content of modifier-lead between the experimental and calculated results (in the first and second approximations) were most likely due to formation in the high-lead glasses of polymeric Pb-O structural units that were decomposed on going from the metastable glassy state to the crystalline one.

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Fig.11 for chapter of Kanunnikova O. “X-ray photoelectron spectroscopy applied to atomic structure analysis of silicate glasses thin layers”

A priori information obtained from X-ray photoelectron analysis (fraction of networkformer-lead atoms forming polymeric Pb-O structures) was used to estimate the content of metastable components in the high-lead glasses. We hypothesized that Pb-O structures containing less than seven Pb atoms could form in the high-lead glasses because the bonding degree of the Pb-O structure in high-lead glasses was similar to that of the Si-O structure in low-lead glasses (Table 5). We assumed the formation in the glasses (with 50.0 and 66.7 PbO) of structural groups similar in composition to metastable lead silicates Pb3SiO8, Pb3Si2O10, Pb5Si2O14, Pb5SiO12, Pb7SiO16, Pb7Si2O18. The glasses were represented as solid solutions consisting of lead oxides and stable and metastable lead silicates. We used concentrations of stable silicates obtained from the second approximation. Thermodynamic data for the metastable lead silicates were obtained by the literature method [48]. The estimated distribution of structural components indicated that PbO dominated structures including three and five Pb atoms in glasses with 50.0 and 66.7% PbO.

THE SECONDARY-ELECTRON EMISSION COEFFICIENTS OF LEAD SILICATE GLASSES The composition and chemical state of surface layers of glasses determine their emission capacity, i.e., the secondary electron emission coefficient. The mechanism of secondary electron emission is studied in [51]. The authors developed theory of a plasmon mechanism of secondary emission of electrons by dielectrics, in which the main role is attributed to the process of generation and disintegration of plasmons arising as a consequence of inelastic interaction of primary electrons with the electron structure of solids. The probability of plasmon excitation depends on the concentration of valence electrons and the minimum

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energy required for excitation of valence electrons. The latter is approximately equal to the forbidden-band width. A.L. Shakhmin [52] suggested for the first time the formula for calculating of the secondary-electron emission coefficients taking into account the electron contributions from the glasses surface layer chemical elements obtained from XPS spectra. The calculations were based on the additivity of the emission characteristics of glasses on the assumption that the main contribution to the secondary electrons generation belongs to the mechanism of the plasmon generation and decay.

⎧ N ( x)[ρ ( x) μ ( x)]⎫ hw 2 ( x) = hw 2 (0)⎨ ⎬, [ ] ( 0 ) ( 0 ) ( 0 ) N ρ μ ⎩ ⎭

(3)

where ρ(0) – quartz density; ρ(x) – glass density; μ(0) – quartz molecular weight; μ(х) – glass molecular weight; hw(0) – glassy quartz plasmon energy. Provided the change of the secondary electrons yield depth is small (estimated as 5%) it is obtained:

σ ( x) hw( x) ⋅ ln (E p hw( x) ) , = σ (0) hw(0) ⋅ ln (E p hw(0) )

(4)

where (x) is the coefficient of secondary electron emission of glass with a PbO content equal to x; σ(0) is the coefficient of secondary electron emission of glassy SiO2, σ(0) = 3.6; hw(0) = 22.5; Ep is the energy of primary electrons, at which a maximum σ(x) is registered: ρ(0) and ρ(x) are the densities of quartz (2.22 g/cm3) and glass: (0) and (x) are the molar masses of quartz and glass. Below given is a formula to calculate the electron contributions of different atoms in the glass structure: N(x) = N(Si) + N(OSi-O-Si) + N(OPb-O-Pb) + N(OPb-

) + N(Pb)

(5)

N(Si) = 4(1-х) is silicon in silicon-oxygen chains; N(OSi-O-Si ) = 4α(2-х) is bridged oxygen in Si-O-Si; N(OPb-O-Pb) = 4γ(2-х) is oxygen bonded to lead-network-former; N(OPb- ) = 4β(2-х) is oxygen bonded to modifier-lead; N(Pb) = 4τ х is lead-network-former; x is lead oxide content in glass. The coefficients α, γ, τ, β were calculated from the deconvolution of O1s and Pb4f spectra. The modifier-lead was assumed not to contribute into value of secondary electron emission coefficient. Using these formulas, A.L. Shakhmin determined the coefficients of secondary electron emission for lead silicate glasses. A good qualitative correlation was obtained between experimental and estimated variation regularities of the secondary electron emission coefficient on varying the lead oxide content in glasses (Table 7). This confirms the validity

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of the theory proposed in [52] and the possibility of predicting values of the secondary electron emission coefficient in glasses depending on the glass composition and structure. However, there are quantitative differences that grow from 10% for low-lead glasses to 25% for high-lead glasses. The reason for this consists of the choice of nuclei for analysis of the O1s and Pb 4f spectra. It was a priori assumed in [46] that oxygen atoms in glassy quartz and lead atoms in glass with a 30% content of PbO exist in a unique chemical state; hence the O 1s spectrum of glassy quartz and Pb 4f spectrum of glass 30%PbO+ 70%SiO2, were selected as nuclei for decomposition. There is no reason to assume that lead atoms in this glass exist in a unique chemical state. Table 7 lists the results of estimation of secondary electron emission coefficients for binary lead silicate glasses implemented by decomposition of Pb 4f and O 1s spectra. The relative content of modifying lead is substantially overestimated, i.e. the coefficient in the calculation formula for Nx is overestimated, which is the main reason for the elevated values of est. The values of secondary electron emission coefficients calculated by us agree well with experimental data. Table 7. The results of estimation of secondary electron emission coefficients for binary lead silicate glasses [PbO], mol% 30.0 40.0 50.0 55.0 66.7

σexp [ ] 2.77 2.67 2.64 2.57 2.30

σestim [ ] 3.044 3.04 2.99 2.98 2.92

Relative content of modifier-lead, % [ ] 100 95.0 86.7 88.3 65.8

σestim [this work] 2.82 2.70 2.63 2.62 2.35

Relative content of modifierlead, % [this work ] 80.0 75.0 48.0 35.0 10.0

Table 8. Effect of Ar+ ion bombardment on the atomic structure of the lead-silicate glasses surface layers [PbO], mol % 30 40 55

Ar+ ion dose, ion/cm2 5 1016 3 1017 No changes of content and structure No changes of content and structure No changes of structure; Pbnet→Pbmod Pb content decreases by 10%. (increases by 3 times) [SiO4]3,4→ [SiO4]5,6 (increases by 2 times)

7 1017 – No changes Pb content decreases by 10%. Pbnet→Pbmod→Pb0 (increases by 3 times) (~6%) [SiO4]3,4→[SiO4]5,6 (increases by 2.5 times)

66.7

Pb content decreases by 10%. Pbnet→Pbmod→Pb0 (increases by 2 times) (~6%)

_

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The effect of Ar ion bombardment (1016 - 1018 ion/cm2) upon the atomic structure of the lead-silicate glasses was studied (Table 8) [53]: The composition and structure of low-lead glasses (content of [PbO] 30 and 40 mol.%) remains practically constant right up to the dose of 2.8.1017 ion/cm2. In the range of average and high concentrations of lead oxide ([PbO] content ~ 50 mol.% and 66.7 mol.%, respectively) primary sputtering of lead is observed – its content decreases by ~10%. In average-lead and high-lead glasses structural changes occur. Part of networkformer-lead transforms into a modifier state. Besides, in glasses with [PbO] content ~ 50 mol.% at doses 3 .1017 ion/cm2, and in glasses with [PbO] content 66.7 mol.% at lower doses – 5 .1016 ion/ 2 lead deoxidizes to Pb0, resulting in the destruction of multi-unit siliconoxygen structures and the increase of the fraction of few-unit structures.

CONCLUSION The progress of science has been aided by a continuing advance in the scientific tools for studying the atomic structure of materials. Two of the standard tools for this are "diffraction" and "spectroscopy". Diffraction provides a way of directly measuring the positions of atoms inside a material, and is very widely used. But its applying is difficult for the case of glasses. Spectroscopic tools such as EXAFS and NMR have provide successful for investigating the cation surroundings and network structure. However, these methods cannot give a quantitative description beyond the first coordination shell. X-ray photoelectron spectroscopy detect changes in the states of atoms within a material and traditionally used to analyze content and chemical state of elements in thin surface layer. An attempt has been made to infer information about the positions of atoms from the changes in the states of atoms. The technique, based on the photoelectron spectra analysis, which allows one to infer information about content and statistics of the intermediate-range order structural units and cross-linking degree of glass structure was developed.

REFERENCES [1] [2]

Zaiman, JM. Models of disorder. Cambridge University Press, 1979. Shelby, JE. Introduction to Glass Science and technology, 2-nd Edition. The Royal So iety of Chemistry, Cambridg, England, 2005. [3] Liebau, F. Structural chemistry of silicates. Springer-Verlag, 1985. [4] Sitarz, M; Mozgawa, W; Molec, MJ. Struct., 1999, v.511-512, 281-285. [5] Pasquarello, A; Car, R. Phys. Rev. Lett., v.80, №23, 5145-5147. [6] Elliot, RJ. Non-Cryst. Solids, 1995, v.182, №1-2, 1-8. [7] Kerner, RJ. Non-Cryst. Solids, 1995, v.182, №1-2, 9-21. [8] Ito, Y; Winkler, D; Jain, H; Williams, DS. J. Non-Cryst. Solids, 1997, v.222, 83-93. [9] Volmair, K; Kob, W; Biner, K. Phys. Rev. B., 1996, v.54, №22, 1588-15827. [10] Zyubin, AS; Dembovskii, SA. Materialovedenie, 1999, №1, 2-19.

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[11] Briggs, D; Seach, MP. Practical surface analysis by Auger and X-ray photoelectron spectroscopy. Division of Materials Applications, National Physical Laboratory, Teddington, Middlesex, UK, John Wiley&Sons, 1983. [12] Povstugar, VI; Shakov, AA; Mikhailova, SS. et al. J.Anal. Chem., 1998, v.53, N8, 697701. [13] Nefedov, VI. X-ray photoelectron spectroscopy of inorganic materials [in Russia]. Moscow: Chimia, 1984. [14] Vasil’ev, LS. Bull. Rus. Acad. Sci.: Physics, 2008, v.72, N4, 471-475. [15] Nefedov, VI; Sergushin, NP; Selyn, Ya; et al. J. Microsc. Spectrosc. Electron., 1976, v.1, N4, 551-570. [16] Nefedov, VI; Sergushin, NP; Selyn, Ya. J. Electron Spectrosc.Rel. Phen., 1976, v. 8, N1, 81-84. [17] Grunhaner, FJ; Grunhaner, PJ; Vasques, PR; et al. Phys. Rev. Lett., 1979, v.43, №22, 1683-1686. [18] Newton, MD. in Structure and bonding in crystal, v.1, New York: Acad. Press, 1988, 175-193. [19] Kanunnikova, OM. Technique of thin silicate films analysis [in Russia]. Issledovano v Rossii. http://zhurnal.ape.relarn.ru/articles/2006/225.pdf. [20] Kanunnikova, OMJ. Adv. Mater., 2006, № 6, 88-92. [21] Kiselev,VF; Krylov, OV. Adsorption processes on the surface of semiconductors and dielectrics [in Russia]. Nauka, Moscow, 1978. [22] Ramkumar, K; Saxena, AN. J. Electrochem. Soc., 1992, v.139, N5, 1437-1442. [23] Susman, S; Volin, KJ; Price, DI. et al. Phys. Rev.B, 1991, v.43, (1B), 1194-1197. [24] Brekhovskikh, SM; Viktorova, Yu. N; Landa, LM. Radiation effects in glasses [in Russia]. Energoizdat, Moscow, 1982. [25] Leamy, HJ; Gilmer, GH; Dirks, AG. Curr. Top. Mater. Sci., 1980, v.6, 309- 315. [26] Yudin, VV. Stochastic Magnetic Structure of Films with a Micropore System [in Russia], Nauka, Moscow, 1987. [27] Kanunnikova, OM; Lomaeva, SF; Shakov, AA; Gilmutdinov, FZ. Glass and ceramics, 2003, 60, N 1-2, 51-56. [28] Appen, AA. Chemistry of glasses [in Russia]. Leningrad: Chimia, 1970. [29] Toropov, NA; Barzakovskii, VP; Lapin, VV. Diagrammy sostoyanii silicatnih system [in Russia]. Leningrad: Nauka, 1969. [30] Brosset, C. Phys. Chem. Glasses, 1963, v.4, 99-106. [31] Mydlar, MF; Kreidl, HJ; Hendren, JK; Clayton, GT. Phys. Chem. Glasses, 1970, v. II, №6, 196-204. [32] Gotz, J; Hoebbel, D; Wieker, WJ. Non-Cryst. Solids, 1976, v.22, 391-398. [33] Morikawa, H; Takagi, Y; Ohno, H. J.Non-Сryst.Solids, 1982, v.53, 173-182. [34] Imaoka, M; Hasagawa, H; Yasui, I. J. Non-Cryst. Solids, 1986, v.85, 393-412 [35] Furukawa, T; Brawer, SA; White, WB. J. Mater. Sci., 1978, v.13, 268-282. [36] Fayon, F; Bessada, C; Massiot, D. J. Non-Cryst. Solids, 1998, v. 232-234, 403-408. [37] Schrikhande, VK; Sudarsan, V; Kothiyal, GP; Kulshreshtha, SK. J. Non-Cryst. Solids, 2001, v.283, 18-26. [38] Tiago Takaishi, Masahide Takahashi, Jisun Jin et al. J. Am. Ceram. Soc., 2005, v.88, №6, 1591-1596. [39] Fayon, F; Landron, C; Sacurai, K. et al. J. Non-Cryst. Solids, 1999, v. 243, №1, 39-44.

X-Ray Photoelectron Spectroscopy Applied to Atomic Structure… [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]

[50] [51] [52] [53]

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Rybica, J; Rybica, A. Comp. Meth. Sci. Techn, 1990, v.5, 67-74. Rybica, J; Rybica, A; Witkowska, A. et al. J. Non-Cryst. Solids, 2000, v.276, 19-26. Nemilov, SV. Inorg. Mater., 1968, v.4, 952-955. Smets, BMJ; Lommen, TP. J. Non-Cryst. Solids, 1982, v. 48, 423-430. Wang Paul, W; Zhang Lipeng. J. Non-Cryst. Solids, 1996, v. 194, № 1-2, 129-134. Shakhmin, AA; Tuytikov AM. Phys.Chem. Glasses, 1990, v. 16, №6, 833-839. Ezikov, VI; Pasishnik, SV. Fiz. Khim. Stekla, 1989, v.15, N6, 900-911. Anfilogov, VN; Bykov, VN; Osipov, AA. Silicate alloys [in Russia]. Moscow: Nauka, 2005, 145-162. Schneider, J; Mastelaro, VR; Zanotto, ED; et al. J. Non-Cryst.Solids, 2003, v.325, 164178. Goncharov, O. Yu. Estimation of thermodynamic properties of gafnium bromide and gafnium iodide [in Russia]. Issledovano v Rosii. http:// zhurnal. ape. relarn. ru/articles /2006/273.pdf. Goncharov, O.Yu; Kanunnikova, OM. Rus. J. Phys. Chem., 2009, v.83, N12, 20072012. Gusarov, AI; Murashov, SV. Surf. Sci., 1994, v.320, 361-368. Shakhmin, AL; Murashov, SV. Technical Physics Letters, 2000, v. 26, N5, 62-66. Kanunnikova, OM. Fiz. Khim. Obrab. Mater., 2007, N3, 8-12.

In: X-Ray Photoelectron Spectroscopy Editor: Johanna M. Wagner

ISBN: 978-1-61668-915-5 © 2011 Nova Science Publishers, Inc.

Chapter 4

INFLUENCE OF NO2 ON ATMOSPHERIC CORROSION OF ZINC EXPOSED IN A CLIMATE CHAMBER G. Juan Castaño Corrosion and Protection Group, University of Antioquia, P. O. Box 1226, Medellin, Colombia

ABSTRACT Nitrogen oxides are among the main pollutants of the atmosphere. In many places, the content of nitrogen dioxide exceeds that of sulfur dioxide, which is the main aggressive agent in atmospheric corrosion of zinc and galvanized steel. However, studies on the effect of NO2 on zinc corrosion are scarce and their results are variable and at times seemingly contradictory. The dry deposition on zinc surfaces of nitrogen dioxide (NO2), alone and in combination with sulfur dioxide (SO2), at temperatures of 35 and 25°C and relative humidity (RH) of 90%, was simulated in a climatic chamber. Pollutant concentrations belonged to highly polluted industrial atmosphere levels. From the gravimetric results obtained and from the characterization of the corrosion products by Xray Photoelectron Spectroscopy (XPS) and Ion Exchange Chromatography (IC), we have verified an accelerating effect when NO2 acts in conjunction with SO2 at 25°C and 90% RH. At 35°C and 90% RH, the accelerating effect is smaller, which suggests it may be favoured by the presence of moisture. In those cases where an accelerating effect has been observed, high-resolution XPS spectra showed a greater proportion of sulfates and the lack of nitrogen compounds in the corrosion products, indicating that NO2 participates indirectly as a catalyst of the oxidation of SO2 to sulfate. However, the corrosive action of NO2 alone is negligible compared with SO2.

INTRODUCTION Zinc-coated steels are widely used in automotive and construction industries, and in manufacturing electronic equipment. Therefore, these materials are exposed to a wide variety of outer and inner atmospheres. The general picture of the atmospheric corrosion of zinc has

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been summarised in several review works [1-5], and extensive research has been conducted on the atmospheric corrosion resistance of different types of zinc coatings [5-9]. In galvanized steels, a zinc coating exhibits better atmospheric corrosion resistance than steel substrate. Steel is protected by zinc for three reasons: 1) the barrier effect—zinc coating prevents the direct contact with aggressive species of the medium; 2) galvanic protection— when there are discontinuities, pores or cracks in the coating, zinc exerts a galvanic or cathodic protection in these sites, forming a dense and adherent film of corrosion products that fills discontinuities. The degree of protection depends on the dimensions of the discontinuities and the thickness of the coating; 3) the pH of their corrosion products—the corrosion products of zinc are alkaline and tend to provide additional protection because they neutralize the acidity of the moisture condensed on the surface [10, 11]. The crucial SO2 effect in zinc atmospheric corrosion has been corroborated through a large number of studies [4, 5, 11-13]. When the critical relative humidity is reached, SO2 adsorbed in the humidity layer that forms on zinc is transformed into sulfite and, subsequently, into sulfate. The humidity layer is acidified by the oxidation of SO2 to sulfate, which greatly accelerates the rate of corrosion. Haynie and Upham [14] proposed a simple linear function to correlate SO2 concentration with the corrosion rate of zinc. In subsequent investigations these equations have been adjusted and the contribution of other variables has been analyzed [15-18]. Svensson and Johansson [19] proposed the following reactions for the process of SO2 adsorption and the formation of sulfite and sulfate ions: SO2 (g) SO2 (ads) + H2O HSO32- (ads) SO32- (ads) + ½O2 HSO32- (ads) + ½O2

SO2 (ads) HSO32- (ads) + H+ (ads) SO32- (ads) + H+ (ads) SO42- (ads) SO42- (ads) + H+ (ads)

(1) (2) (3) (4) (5)

The acidity of the aqueous film promotes the dissolution of the previously formed corrosion products: ZnO (s) + 2H+ (ads) Zn(OH)2 (s) + 2H+(ads)

Zn2+(ads) + H2O Zn2+(ads) + H2O

(6) (7)

NaCl is less corrosive to zinc that SO2. This salt has high hygroscopicity, which allows the corrosion to begin at a lower relative humidity. Furthermore, the chloride ion participates actively in the corrosion reactions. To produce the attack, it is necessary to overcome the critical relative humidity (RHc), which only requires that a microscopic amount of liquid is formed around the salt particle. For this reason, in marine atmospheres, RHc values as low as 40–50% have found. The corrosion rate is higher near the coast and diminishes with distance from the shore, because the salinity of the air significantly decreases inland [20, 21].

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Because of its high hygroscopicity, chlorides may enhance the SO2 effect, which increases corrosion rates [16, 22]. However, in the case of high concentrations of chloride and SO2 acting simultaneously, there is a significant decrease of corrosion rate, apparently because the metal surface originates such pH conditions that allow the formation of relatively protective corrosion products [14]. In the past decade, Odnevall and Leygraf [23-25] made a 90-day, systematic study of atmospheric corrosion of zinc exposed to different atmospheres. They proposed a sequence of formation of corrosion products for the various atmospheres. In most cases, all main corrosion products can be detected after one month of atmospheric exposure. In aggressive environments (marine and industrial) the formation of sulfur and chlorine compounds occurs very fast (often in a single day). While the process advances, these compounds generally increase in amount, although some can disappear as a result of their transformation into other components, depending on the atmosphere. In rural atmospheres, zinc hydroxycarbonate (3Zn(OH)2.2ZnCO3) is formed initially. This compound can be found even after long exposure time outdoors, although gradually transforms into hydroxysulfate (Zn5SO4(OH)6.nH2O) under a sheltered condition [23]. Due to structural resemblance between the two phases, the sulfate groups can enter through the layers of zinc hydroxide to gradually form hydroxysulfates. The latter is the final corrosion product if environmental conditions do not change radically [25]. In urban and industrial environments, zinc hydroxycarbonate formed at first is converted into hydroxysulfate, although eventually zinc chlorosulfate (Zn4Cl2(OH)4SO4.5H2O) is formed. In these atmospheres, hydroxysulfates formation is faster than in rural atmosphere, because of the higher deposition of sulfur compounds. The last step, in some cases, can be the formation of hydroxychloride (Zn5(OH)8Cl2.H2O). In marine environments, hydroxycarbonate is gradually transformed into sodium zinc chlorohydroxysulfate (NaZn4Cl(OH)6SO4.6H2O), which would be the final corrosion product [23]. Odnevall and Leygraf found that there is a structural resemblance between hydroxycarbonate, hydroxychloride, hydroxysulfate and sodium zinc chlorohydroxysulfate [24], since these compounds have layered structures with sheets of Zn2+ in octahedral and tetrahedral coordination, with the main difference being the chemical content and bonding between the sheets. This structural resemblance may facilitate the transformation from one phase into another under appropriate environmental conditions. The basic salts (hydroxychloride, hydroxysulfate and sodium zinc chlorohydroxysulfate) are formed discontinuously on the surface, looking like islands. These islands grow gradually to form a continuous layer that completely covers the metal and protects it from further attacks [24, 26]. However, if the surface moisture reaches a low pH, either permanently or occasionally (for example, due to higher pollution with SO2, acid rain or HCl-polluted atmospheres), zinc hydroxide and basic salts does not form [3, 15]. Then, the formation of water soluble sulfates and chlorides is facilitated, which can be washed by rain and will not protect the metal. Thus, the corrosion rate could increase. Very little information has been published on the atmospheric corrosion of zinc over long time periods (10–20 years) [27, 28]. In a long-term study in Spain, the characteristics of corrosion products formed on zinc panels (after 13–16 years) in various types of atmospheres in Spain (rural, urban, industrial, mild marine and severe marine) were studied [28], and the authors found a linearly increasing amount of zinc corrosion with time at the different test sites, except the marine atmospheres, where the zinc deviated from the usual behavior

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(constant corrosion rate). The serrated pattern of the zinc corrosion rate versus time graph found in severe marine atmosphere suggests the occurrence of corrosion product layer formation-compaction-detachment processes. Hydroxychloride and sodium zinc chlorohydroxysulfate were found as main constituents in marine atmospheres, while hydroxysulfate was detected in industrial atmospheres and zincite in rural and urban atmospheres. In marine atmospheres, the cross-sections of the corrosion product layers formed show the existence of two clearly differentiated subzones: an inner sublayer, crossed by cracks, and an outer sublayer, completely microcracked and thus exhibiting many extremely fine microscopic channels. The spongy structure of the outer sublayer facilitates the entry of oxygen, water and pollutants, for which reason this sublayer is in continuous growth. The inner sublayer is compact and stable, impeding the passage of water and pollutants, making it that which controls the kinetics of the corrosion process. The growing demand for environmental protective actions has led to lower atmospheric sulfur levels, at least in developed countries, as a result of the use of cleaner coals and sophisticated gas emission controls [29-31]. This has raised the relative significance of other atmospheric compounds and the interest in studying their effects on zinc and galvanized steel, including CO2 [32, 33], ozone [19, 34], NaNO3 [35] and NO2 [19, 34, 36-38]. There is an increasing interest in nitrogen oxides (NO2 and NO), because their levels in the atmosphere remained constant or even increased [33, 39] somewhat as a result of the higher burning temperatures used [29] and the lack of specific control measures, and urban NOx levels are now higher than SO2 levels in most cities [40]. NO2 levels can rise from 0.15 ppm in normal conditions to 1900 ppm in situations of temperature inversion [41]. Hence, it is important to be aware of the NO2 effect in the atmospheric corrosion of zinc, since its effect in the corrosive phenomenon, although slight, can cause the premature failure of zinc coatings. This can be especially important in situations where zinc is used in electronic equipment components. Studies on the effect of NOx are scarce and their results are variable and at times seemingly contradictory. In field tests, no correlation has been found between NO2 and zinc corrosion [42]. In contrast, in laboratory tests the researchers studying the possible accelerating effect of NO2 in combination with SO2 can be split into two groups: on the one hand those who find a synergistic effect [19, 36, 37, 43]; and on the other hand those who consider such an effect to be non-existent or insignificant compared to what SO2 can do by itself [34, 38, 44, 45]. With regard to the detractors, Henriksen and Rode [38], Strandberg et al. [45] and Oesch and Faller [34] found no effect of NO2 on zinc corrosion at 22–25°C and 90–95% RH. Among the proponents of the accelerating effect of NO2, especially at high RH, Svensson and Johansson [19, 36] suggest that this effect is due to the fact that NO2 catalyses the oxidation of SO2 to sulfate on the metal surface and discourages the formation of zinc sulfite. These authors observed that in the presence of NO2 the relationship between the corrosion product mass and the corroded zinc mass increased, indicating that the corrosion products are sulfate richer than those formed in tests with only SO2. They also found that SO2 deposition was greater in atmospheres with both pollutants, which implies an increase in the S4+ to S6+ oxidation rate (reactions (1)-(5)), and that gaseous NO was not produced, which means that the increase in the oxidation rate was not due to a direct relationship between NO2 and SO2 to form NO and H2SO4. The indirect intervention of NO2 was also verified by the absence of nitrites and nitrates on the zinc surface. There seem to be certain characteristics of

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the effect of NO2 on metals that could explain the lack of agreement on how it affects atmospheric corrosion, e.g. the absence or non-permanence of nitrogen compounds in the corrosion products; the existence of a critical pollutant concentration to trigger a synergistic NO2-SO2 effect; and the possible occurrence of photolytic reactions in field tests which may involve ozone or organic compounds and which obviously do not occur in laboratory tests [46]. In field tests, the large number of variables and overlapping effects makes it extremely difficult to determine the role played by NO2. In contrast, in laboratory tests in carefully controlled conditions close to those existing in real atmospheres it is possible to study the separate effect of this compound and to investigate the effect of its combination with SO2 [19, 26]. In this work, the role of NO2 in the atmospheric corrosion of zinc was analyzed from a detailed characterization of corrosion products. Laboratory tests with exposure parameters close to the conditions observed in real atmospheres were performed, with the aim of simulating close-to-reality corrosion mechanisms. Low-pollutant concentrations and shortterm exposures were carried out. For these reasons, XPS were used for the analysis of very thin corrosion layers formed.

EXPERIMENTAL PROCEDURE Exposure [47, 48] was conducted in a HERAEUS HC2033 climatic chamber with gaseous pollutant dosage system, laminar air flow, plus a rate of pollutants (SO2 and/or NO2) mixed with N2 from a gas pipe, and the desired levels of RH and temperature strictly controlled. The air flow was 1 m3/h, permitting the atmosphere inside the cabinet be renewed eight times every hour, in completely laminar regime conditions Commercial grade zinc plates ((Pb95%). Smaller contributions from components at lower binding energies in spectra of the 150 nm films are arguably attributed to the presence of Ti2O3, TiO. In contrast, the only other contributing species to the Ti 2p spectra of the disks is from elemental Ti. It was previously reported that

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when time sputtering under vacuum conditions is increased, titanium dioxide is reduced to lower oxidation states (Ti2O3 and TiO) by the preferential removal of oxygen [30].

2.4. Carbon Contamination The XPS survey spectra demonstrated some carbon contamination on the titanium surface, which is typical; adventitious, unavoidable hydrocarbon contaminants are adsorbed spontaneously from ambient air onto the surface [13, 22, 31]. Differences in surface architecture and subsequently surface area may account for the differing levels of carbon contamination, despite all surface essentially consisting of TiO2. Spectra of the C 1s regions, shown in Figure 1, were deconvoluted into contributions from aliphatic hydrocarbon (C– H/C–C), alcohol and ether (C–O), and acid (O–C=O) functional groups. The C 1s binding energies measured for each are 285.0, 286.6, and 288.5 eV, respectively. Table 1. Atomic fractions of elements detected on surfaces of the titanium samples by XPS Sample 3 nm Ti on glass 12 nm Ti on glass 150 nm Ti on glass 3 nm Ti on Si wafer 12 nm Ti on Si wafer 150 nm Ti on Si wafer Mechanically polished titanium Mechano-chemically polished titanium *

Zn 0* 0 0 0 0 0 0.6

O 53.9 51.3 51.2 46.3 46.0 48.8 53.8

Ti 13.6 14.1 19.3 17.7 18.7 21.7 18.6

Ca 0.6 1.2 0 0 0 2.3 1.3

N 0.5 0.7 0.8 1.7 2.1 2.3 0.5

C 26.9 32.0 28.7 25.6 33.0 27.1 14.4

Si 4.0 0 0 8.5 0 0 0

2.4

37.7

46.5

0.5

0.4

11.6

0

less than the detection limit (ca. 0.1%)

Table 2. Binding energies and relative contributions to the total peak area of components fitted to Ti 2p3/2 spectra of the titanium samples

Binding energies (eV) 3 nm Ti on glass 12 nm Ti on glass 150 nm Ti on glass 3 nm Ti on Si wafer 12 nm Ti on Si wafer 150 nm Ti on Si wafer Mechanically polished titanium Mechano-chemically polished titanium

TiO2 458.4 100 100 88.2 100 100 82.1 85.4 95.1

Ti2O3 456.9 0 0 1.6 0 0 4.8 3.3 0

TiO 456.0 0 0 5.6 0 0 7.1 6.3 0

Ti 454.1 0 0 4.5 0 0 6.0 5.0 4.9

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Figure 1. Titanium 2p and carbon 1s spectra of the 3, 12, and 150 nm titanium films and the bulk titanium.

In a previous study, CO2 adsorption on TiO2(110) was investigated [32]. There are two mechanisms of adsorption: one is linearly bound at five-coordinate Ti4+ sites and the second is adsorption at oxygen vacancy sites. Water binds strongly in monolayer TPD states at 265, 332 and 490 K, assigned to molecular adsorption at five-coordinate Ti4+ sites, and dissociative adsorption at minority step and vacancy sites, respectively. Pre-adsorbed H2O blocks CO2 adsorption, and pre-adsorbed CO2 is displaced by H2O. A new surface species characterized by a CO2 TPD state at 217 K is formed when CO2 and H2O are simultaneously dosed on the vacuum annealed TiO2(110) surface. Although not definitive, SSIMS results suggest that a bicarbonate species is the product of this interaction of CO2 and H2O [32].

2.5. Conclusions The effectiveness with which medical implants are integrated into the body strongly depends on the efficiency with which the implant surfaces come into contact with the

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biological environment [33]. Flat titanium films have been shown to provide an effective substrate to study the effects of topography on protein adsorption on a nanometer scale [34]. In this study, the effect of sputtering conditions and polishing technique on the surface oxidation states of titanium thin films and bulk titanium respectively was investigated. Titanium dioxide was the main component detected on all surfaces, which plays an important role in protecting the titanium surface and in biocompatibility. Two additional titanium oxidation states were detected on 150 nm films, Ti2O3 and TiO, which may be a result of the increased sputtering time. Carbon contamination adsorbed on titanium surfaces was detected, and is difficult to eliminate as it can occur spontaneously from the atmosphere. These studies would be the good reference point for preparation of titanium implant surfaces with maximum biological compatibility.

3. CASE STUDY 2 – BACTERIAL MODIFICATION OF POLY(ETHYLENE TEREPHTHALATE) POLYMER SURFACES Most commonly used plastics are cheap to manufacture and highly available [35], and are both physically and chemically stable [36]. The durability of these materials makes them ideal for a wide variety of applications, but is also one of the primary factors leading to their accumulation in the environment [37]. Currently, there are three plastic disposal methods in widespread use, each with its own inherent drawbacks. Disposal of plastic in landfill is the simplest of these methods, however there is potential for secondary pollutants to be released into groundwater [38]. Similarly, incineration of plastic waste leads to the formation and subsequent release into the atmosphere of dangerous secondary pollutants[38]. Recycling is the most environmentally friendly technique, however it a more expensive process, and often additives and impurities make it extremely difficult for some plastics to be recycled [38]. In this case study, surfaces of a commonly consumed plastic polymer, poly(ethylene terephthalate) (PET), were exposed to bacterial communities present in samples of coastal seawater. The PET was the sole carbon and energy source provided for the bacteria, to maximise the desirability for bacteria to interact with the surface. Changes in the elemental and chemical composition of the PET surfaces were detected by XPS.

3.1. Experimental Design Two reaction vessels were prepared: one containing 100 mL of seawater and approximately 5 g of 1 cm2 sterilised pieces of PET water bottles, the second with twice the volume of seawater and number of PET tokens. Further details are published elsewhere [39]. Individual PET pieces were sampled from the flasks at the end of nine months. Samples were cleaned by sonication in 70% ethanol for one hour. XPS analysis was then performed, both on the modified samples taken from reaction vessels and on reference PET samples not exposed to bacterial interactions, to detect modifications to the polymer surface made by the bacteria. XPS analysis was performed with the same parameters as described in section 2.1

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3.2. Elemental Analysis Survey scans of reference and modified PET surfaces reveal changes in the elemental composition that occurred over the incubation period (Figure 2, Table 3). Firstly, magnesium and silicon were not detected on the modified surface. These may have moved into solution and simply been washed off the surface. Four elements were detected on the modified surfaces that were not detected on the reference surface. Three of these elements, sodium, potassium, and chlorine are almost certainly present as a result of adsorption, as seawater contains a high concentration of salts containing these elements. The fourth element, sulfur, may also originate from the seawater, but could also arise as a result of bacterial action. It is known that some marine bacteria are able to take up various sulfur-containing compounds from their environment [40], and sulfur is an important element in the amino acid cysteine. Throughout the incubation period there was also an increase in oxygen and nitrogen on the surface, compared to the carbon composition.

3.3. High-Resolution Spectra High-resolution spectra for the C 1s, O 1s and N 1s peaks of both surfaces are presented in Figure 3. The C 1s and O 1s spectra of the reference PET surface are, for the most part, in good agreement with those reported in literature [41-43]. A similar carbon components ratio of 4.8:1.4:1 (PET1:PET2:PET3, see Table 4) was obtained, and like previous studies there was a deviation from the ratio 3:1:1 according to PET (commercial-grade) stoichiometry. This deviation is most likely due to the over-representation of the hydrocarbon and C–O components, as a result of contaminants present in commercial-grade PET. The ratio of oxygen components obtained, 1.1:1 (PETa:PETb, see table 4), also agreed well with the literature [41-43], and the theoretical value of 1:1. In all three cases, the inclusion of one or two additional components not present in spectra of the reference sample is required to fit the peaks in the corresponding spectra of the modified sample. In the case of the C 1s spectrum, components attributable to aliphatic carbon atoms and to carbon atoms in carbonyl groups were added. Additional contributions to the O 1s spectrum appear to be associated with an oxide (discussed below) and an alcohol or ether (C–O) species. The N 1s spectrum appears to comprise contributions from positively charged quaternary amine or ammonium ions, as well as from the uncharged C–N species detected on the control sample. The relative contributions of components in each spectrum are listed in Table 4. Table 3. Atomic fractions of elements detected on the surfaces of reference and modified PET surfaces

Reference PET Modified PET *

Atomic fraction (%) of elements on each surface C O N K Si Na Cl S * 77.8 20.5 1.0 0 0.6 0 0 0 72.9 21.5 2.6 1.4 0 0.9 0.4 0.3

Element not detected on sample surface

Mg 0.1 0

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Figure 2. Survey spectra of reference (top), and modified (bottom) PET surfaces

3.4. Bacterial Modifications There are three notable differences between the C 1s spectra of the reference and modified surfaces, two of which may be explained by the surface being modified by the same mechanism. An increase in the PET2 (C–O) component relative to PET1 (CAr), and the appearance of an aliphatic component might be the result of ring opening by bacterial oxygenases. Oxygenases and dioxygenases are enzymes commonly involved in the metabolism of aromatic substrates [44-46]. The third difference in the spectra of the two

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surfaces is the appearance of a carbonyl (C=O) component, distinct from that of the PET3 (O=C–O) ester group. This can be attributed to the formation of amide bonds, and will be discussed further below. Two new components were also detected in the O 1s spectrum of the modified sample. One of the components appears to be associated with an inorganic oxide. Coupled with the S 2p binding energy of 167.5 eV, it is thought that the oxide component may be present as a sulfite salt. The second new component can be attributed to an alcohol or ether group (C–O). This, together with the appearance of a carbonyl component in the C 1s spectrum, as described above, and the significant increase in nitrogen on the surface, may occur as result of cleavage of the polymer chain, according to the mechanism shown in Figure 4. This chain cleavage would presumably be mediated by bacterial enzymes. This hypothesis is supported by the increased intensity of the N 1s spectrum of the modified surface, which though partly attributable to the appearance of quaternary amine or ammonium ions (perhaps the counterions to the sulfite mentioned above), is also the result of an increased contribution from the C–N component.

Figure 3. High resolution spectra of the carbon, oxygen and nitrogen 1s peaks of reference and modified PET surfaces

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Hayden Webb, Vi Khanh Truong, T. Robert Jones Table 4. Relative contributions and binding energies of components detected in high-resolution spectra of C 1s, O 1s, and N 1s peaks

Reference PET Relative Binding contribution energy (eV) (%) 67.3 284.7 – – 18.8 286.4 – – 13.9 288.8 – – 52.7 531.8 – – 47.3 100 –

533.4 399.8 –

Component C 1s – PET1i C 1s – Aliphatic C (C – H) C 1s – PET2ii C 1s – Carbonyl (C = O) C 1s – PET3iii O 1s – Oxide O 1s –PETaa O 1s – Alcohol/Ether (COR) O 1s – PETbb N 1s – Organic N (C – N) N 1s – Ammonium (N+)

Modified PET Relative Binding contribution energy (eV) (%) 53.1 284.6 13.3 285.0 21.1 286.2 3.0 287.4 9.4 288.7 18.0 530.8 45.3 531.8 7.9 532.7 28.7 82.6 17.4

533.4 399.4 401.1

i, ii, iii, a, b

PET1, PET2, PET3, PETa and PETb refer to atoms in the configurations indicated on the above molecular structure

Figure 4. Proposed PET polymer chain scission mechanism by amide bond formation

3.5. Carbon Contamination Carbon contamination is also likely to occur on polymer surfaces, as it does on titanium surfaces. The deviation of the carbon components ratio from the theoretical expected ratio is evidence to that fact [41-43]. However, resolving carbon contamination from polymer surfaces is more difficult, due to the high percentage of carbon in polymer chains. Contaminant components with similar binding energies to those of the material surface may

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go unnoticed, as they simply contribute to the same peak [47]. For this reason, careful controls need to be prepared in order to properly detect changes on polymer surfaces.

3.6. Conclusions When incubated over a period of nine months in an environmental sample of marine bacteria in a seawater medium, PET plastic surfaces undergo a number of chemical changes. Some elemental changes are minor and result from the adsorption of compounds in the seawater itself; however bacteria do appear to act upon the surface. Evidence for two mechanisms that may be the beginnings of plastic biodegradation processes have been found: ring cleavage by bacterial oxygenases, and chain scission by amide bond formation.

4. SUMMARY The high sensitivity and accuracy of XPS, coupled with low analysis depth and the detailed chemical information it provides, make it an ideal analytical tool for studying surfaces important in biological contexts. XPS analysis has been used to detect the oxidation states of titanium in thin films as an indication of biocompatibility, with thinner films having a higher proportion of titanium dioxide. Modifications to PET polymer surfaces by marine bacteria were also detected by XPS, which may be the initial stages of plastic biodegradation.

5. REFERENCES [1] [2]

[3] [4]

[5]

[6]

[7]

Gentle, I; Barnes, G. Interfacial Science, 2005, Oxford University Press: New York. 148-153. Ivanova, EP; Truong, VK; Wang, JY; Berndt, CC; Jones, RT; Yusuf, II; Peake, I; Schmidt, HW; Fluke, C; Barnes, D; Crawford, RJ. Impact of Nanoscale Roughness of Titanium thin films surfaces on Bacterial Retention. Langmuir, 2009, in press. Kasemo, B. Biological Surface Science. Surf. Sci., 2002, 500, 656-677. Landa, AS; van der Mei, HC; Busscher, HJ. Detachment of linking film bacteria from enamel surface by oral rinses and penetration of sodium lauryl sulphate through an artificial oral biofilm. Advances in Dental Research, 1997, 11, 528-538. Teng, F; Guan, YT; Zhu, WP. Effect of biofilm on cast iron pipe corrosion in drinking water distribution system: Corrosion scales characterization and microbial community structure investigation. Corrosion Science, 2008, 50, 2816-2823. Pavlekovic, M; Schmid, MC; Schmider-Poignee, N; Spring, S; Pilhofer, M; Gaul, T; Fiandaca, M; Löffler, FE; Jetten, M; Schleifer, KH; Lee, NM. Optimization of three FISH procedures for in situ detection of anaerobic ammonium oxidizing bacteria in biological wastewater treatment. Journal of Microbiological Methods, 2009, 78, 119126. Singh, R; Paul, D; Jain, RK. Biofilms: implications in bioremediation. Trends in Microbiology, 2006, 14, 389-397.

122 [8]

[9]

[10]

[11] [12]

[13]

[14]

[15] [16]

[17]

[18] [19]

[20] [21]

[22]

Hayden Webb, Vi Khanh Truong, T. Robert Jones Kamalesh, S; Tan, P; Wang, J; Lee, T; Kang, ET; Wang, CH. Biocompatibility of electroactive polymers in tissues. Journal of Biomedical Materials Research, 2000, 52, 467-478. Chu, CL; Wang, RM; Hu, T; Yin, LH; Pu, YP; Lin, PH; Dong, YS; Guo, C; Chung, CY; Yeung, KWK; Chu, PK. XPS and biocompatibility studies of titania film on anodized NiTi shape memory alloy. Journal of Materials Science: Materials in Medicine, 2009, 20, 223-228. Pommet, M; Juntaro, J; Heng, JYY; Mantalaris, A; Lee, AF; Wilson, K; Kalinka, G; Shaffer, MSP; Bismarck, A. Surface Modification of Natural Fibers Using Bacteria: Depositing Bacterial Cellulose onto Natural Fibers To Create Hierarchical Fiber Reinforced Nanocomposites. Biomacromolecules, 2008, 9, 1643-1651. Long, M; Rack, HJ. Titanium alloys in total joint replacement - a materials science perspective. Biomaterials, 1998, 19, 1621-1639. Jang, HK; Whangbo, SW; Kim, HB; Im, KY; Lee, YS; Lyo, IW; Whang, CN; Kim, G; Lee, HS; Lee, JM. Titanium oxide films on Si(100) deposited by electron-beam evaporation at 250°C. Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2000, 18, 917-921. Cai, K; Müller, M; Bossert, J; Rechtenbach, A; Jandt, KD. Surface structure and composition of flat titanium thin films as a function of film thickness and evaporation rate. Applied Surface Science, 2005, 250, 252-267. Jeyachandran, YL; Karunagaran, B; Narayandass, SK; Mangalaraj, D; Jenkins, TE; Martin, PJ. Properties of titanium thin films deposited by dc magnetron sputtering. Materials Science and Engineering: A, 2006, 431, 277-284. Rack, HJ; Qazi, JI. Titanium alloys for biomedical applications. Materials Science and Engineering: C, 2006, 26, 1269-1277. Lamolle, S; F; Monjo, M; Lyngstadaas, SP; Ellingson, JE; Haugan, HJ. Titanium implant surface modification by cathodic reduction in hydrofluoric acid: surface characterization and in vivo performance. Journal of Biomedical Materials Research Part A, 2008, 88, 581-588. Solar, RJ; Pollack, SR; Korostoff, E. In vitro corrosion testing of titanium surgical implant alloys: an approach to understanding titanium release from implants. Journal of Biomedical Materials Research, 1979, 13, 217-250. Tang, L; Tsai, C; Gerberich, WW; Kruckeberg, L; Kania, DR. Biocompatibility of chemical-vapour-deposited diamond. Biomaterials, 1995, 16, 483-488. Milošev, I; Metikoš-Huković, M; Strehblow, HH. Passive film on orthopaedic TiAlV alloy formed in physiological solution investigated by X-ray photoelectron spectroscopy. Biomaterials, 2000, 21, 2103-2113. LeClair, P; Berera, GP; Moodera, JS. Titanium nitride thin films obtained by a modified physical vapor deposition process. Thin Solid Films, 2000, 376, 9-15. Lim, YJ; Oshida, Y. Initial contact angle measurements on variously treated dental/medical titanium materials. Bio-Medical Materials and Engineering, 2001, 11, 325-341. Liu, X; Chu, PK; Ding, C. Surface modification of titanium, titanium alloys, and related materials for biomedical applications. Materials Science and Engineering R: Reports, 2004, 47, 49-121.

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[23] Kim, TN; Balakrishnan, A; Lee, BC; Kim, WS; Smetana, K; Park, JK; Panigrahi, BB. In vitro biocompatibility of equal channel angular pressed (ECAP) titanium. Biomedical Materials, 2007, 2, S117-S120. [24] Le Guéhennec, L; Soueidan, A; Layrolle, P; Amouriq, Y. Surface treatments of titanium dental implants for rapid osseointegration. Dental Materials, 2007, 23, 844854. [25] Kim, T; Balakrishnan, A; Lee, B; Kim, W; Dvorankova, B; Smetana, K; Park, J; Panigrahi, B. In vitro fibroblast response to ultra fine grained titanium produced by a severe plastic deformation process. Journal of Materials Science: Materials in Medicine, 2008, 19, 553-557. [26] Le Guehennec, L; Lopez-Heredia, MA; Enkel, B; Weiss, P; Amouriq, Y; Layrolle, P. Osteoblastic cell behaviour on different titanium implant surfaces. Acta Biomaterialia, 2008, 4, 535-543. [27] Truong, VK; Rundell, S; Lapovok, R; Estrin, Y; Wang, JY; Berndt, CC; Barnes, DG; Fluke, CJ; Crawford, RJ; Ivanova, EP. Effect of ultrafine-grained titanium surfaces on adhesion of bacteria. Applied Microbiology and Biotechnology, 2009, 83, 925-937. [28] National Institute of Standards and Technology, NIST X-ray Photoelectron Spectroscopy Database, version, 3.5. 2003, Gaithersburg, USA. [29] Harju, M; Areva, S; Rosenholm, JB; Mäntylä, T. Characterization of water exposed plasma sprayed oxide coating materials using XPS. Applied Surface Science, 2008, 254, 5981-5989. [30] McCafferty, E; Wightman, JP. An X-ray photoelectron spectroscopy sputter profile study of the native air-formed oxide film on titanium. Applied Surface Science, 1999, 143, 92-100. [31] Bertóti, I; Mohai, M; Sullivan, JL; Saied, SO. Surface characterisation of plasmanitrided titanium: an XPS study. Applied Surface Science, 1995, 84, 357-371. [32] Henderson, MA. Evidence for bicarbonate formation on vacuum annealed TiO2(110) resulting from a precursor-mediated interaction between CO2 and H2O. Surface Science, 1998, 400, 203-219. [33] MacDonald, DE; Markovic, B; Allen, M; Somasundaran, P; Boskey, AL. Surface analysis of human plasma fibronectin adsorbed to commercially pure titanium materials. Journal of Biomedical Materials Research, 1998, 41, 120-130. [34] Giorgetti, L; Bongiorno, G; Podestà , A; Berlanda, G; Scopelliti, PE; Carbone, R; Milani, P. Adsorption and stability of streptavidin on cluster-assembled nanostructured TiOx films. Langmuir, 2008, 24, 11637-11644. [35] Yamada-Onodera, K; Mukumoto, H; Katsuyaya, Y; Saiganji, A; Tani, Y. Degradation of polyethylene by a fungus, Penicillium simplicissimum YK. Polymer Degradation and Stability, 2001, 72, 323-327. [36] Ivar do Sul, JA; Costa, MF. Marine debris review for Latin America and the wider Caribbean region: from the 1970s until now, and where do we go from here? Marine Pollution Bulletin, 2007, 54, 1087-1104. [37] Derraik, JGB. The pollution of the marine environment by plastic debris: a review. Marine Pollution Bulletin, 2002, 44, 842-852. [38] Zhang, J; Wang, X; Gong, J; Gu, Z. A study on the biodegradability of polyethylene terephthalate fiber and diethylene glycol terephthalate. Journal of Applied Polymer Science, 2004, 93, 1089-1096.

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[39] Webb, HK; Crawford, RJ; Sawabe, T; Ivanova, EP. Poly(ethylene terephthalate) polymer surfaces as a substrate for bacterial attachment and biofilm formation. Microbes and Environments, 2009, 24, 39-42. [40] Gonzalez, JM; Kiene, RP; Moran, MA. Transformation of Sulfur Compounds by an Abundant Lineage of Marine Bacteria in the alpha -Subclass of the Class Proteobacteria. Applied and Environmental Microbiology, 1999, 65, 3810-3819. [41] Uchida, E; Uyama, Y; Iwata, H; Ikada, Y. XPS analysis of the poly(ethylene terephthalate) film grafted with acrylamide. Journal of Polymer Science Part A: Polymer Chemistry, 1990, 28, 2837-2844. [42] Cui, NY; Upadhyay, DJ; Anderson, CA; Meenan, BJ; Brown, NMD. Surface oxidation of a Melinex 800 PET polymer material modified by an atmospheric dielectric barrier discharge studied using X-ray photoelectron spectroscopy and contact angle measurement. Applied Surface Science, 2007, 253, 3865-3871. [43] Amor, SB; Jacquet, M; Fioux, P; Nardin, M. XPS characterisation of plasma treated and zinc oxide coated PET. Applied Surface Science, 2009, 255, 5052-5061. [44] Hara, H; Masai, E; Miyauchi, K; Katayama, Y; Fukuda, M. Characterization of the 4carboxy-4-hydroxy-2-oxoadipate aldolase gene and operon structure of the protocatechuate 4,5-cleavage pathway genes in Sphingomonas paucimobilis SYK-6. Journal of Bacteriology, 2003, 185, 41-50. [45] Fukuhara, Y; Kasai, D; Katayama, Y; Fukuda, M; Masai, E. Enzymatic properties of terephthalate 1,2-dioxygenase of Comamonas sp. strain E6. Bioscience, Biotechnology, and Biochemistry, 2008, 72, 2335-2341. [46] Jiang, XW; Liu, H; Xu, Y; Wang, SJ; Leak, DJ; Zhou, NY. Genetic and biochemical analyses of chlorobenzene degradation gene clusters in Pandoraea sp. strain MCB032. Archives of Microbiology, 2009, 191, 485-492. [47] Johansson, LS; Campbell, JM. Reproducible XPS on biopolymers: cellulose studies. Surface and Interface Analysis, 2004, 36, 1018-1022.

In: X-Ray Photoelectron Spectroscopy Editor: Johanna M. Wagner

ISBN: 978-1-61668-915-5 © 2011 Nova Science Publishers, Inc.

Chapter 6

XPS AS A POWERFUL TOOL TO INVESTIGATE THE SURFACE PROPERTIES OF SIMPLE, DOPED AND MIXED METAL OXIDES Silvia Gross*, Lidia Armelao ISTM-CNR, Dipartimento di Scienze Chimiche, Università degli Studi di Padova, and INSTM UdR Padova, Via Marzolo 1, I-35131 Padova, Italy corresponding author: Silvia Gross e-mail: [email protected]

ABSTRACT XPS is a well established surface sensitive method for the analysis of solid materials, allowing the determination of surface composition and the assessment of oxidation state and chemical environment of the different species. In fact, core level XPS is highly sensitive to the chemical environment, i.e. the binding energy may be strongly dependent on the oxidation state of the investigated ion, as well as on the neighbouring atoms. Oxide-based materials (simple, multicomponent and composites), are a fascinating and technologically appealing class of materials, exhibiting a huge variety of chemical, structural, optical, magnetic, electric and electronic properties. Since in oxides the functional properties are mainly related to the complex interplay among composition, microstructure, shape, characteristic size, charge density, chemical environment, presence of defects etc., the understanding of their actual chemical nature and electronic structure is a primary concern to fully exploit their functionalities in different fields. In this chapter, after a general overview on metal oxides and on their electronic structure and peculiarities, the application of XPS to their investigation will be introduced and discussed and some selected application fields will be considered. The effect of ion sputtering in oxides will be moreover shortly examined. Furthermore, in this contribution, the issues related to the determination of the actual chemical state of metal species in mixed oxides will be also extensively addressed and discussed. The possibility of XPS to evidence the presence of M-O-M’ mixed bonds will be highlighted.

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Silvia Gross, Lidia Armelao In the second part of the chapter, several different selected case studies will be analysed in detail, which deal with different oxide systems: simple oxides, also in mixedvalence state, (ZnO, WO3, CuxOy, VxOy), mixed oxides (e.g. SiO2-HfO2, SiO2-ZrO2, SiO2-TiO2, ZrO2-TiO2), doped oxides (FexOy:TiO2, M(Pt, Ag):TiO2, M (Au, Pt, Pd, Rh):SiO2). In particular, the use of this method for the investigation of the chemical composition of different typologies of surfaces (thin films, powders) will be discussed. A further aspect to be dealt with is the perspective combination of XPS, also at variable pressure, with other site-specific spectroscopic methods (XAS, XEOL) and theoretical tools (e.g. DFT), which represents a valuable tool to unravel and predict issues related to the reactivity and to other functional properties of these materials.

ABBREVIATIONS AND ACRONYMS AP-XPS AR-XPS BE DFT ESCA FWHM KE SIMS UHV XAS XANES XEOL XPS XRD

Atmospheric Pressure XPS Anglular Resolved XPS Binding Energy Density Functional Theory Electron Spectroscopy for Chemical Analysis Full Width Half Maximum Kinetic Energy Secondary Mass Ion Spectrometry Ultra High Vacuum X-ray Absorption X-ray Absorption Near-Edge Spectroscopy X-ray Excited Optical Luminescence X-ray Photoelectron Spectroscopy X-Ray Diffraction

GENERAL REMARKS The focus of the present chapter is i) on the application of X-ray Photoelectron spectroscopy (XPS) to the investigation of metal oxides surfaces (both in form of powder as well as of thin films) and ii) on the wide spectrum of different single, multicomponent and composite oxide systems which can be thoroughly studied from the point of view of composition and chemical nature of the species (oxidation state, chemical environment, electronic structure). The chapter begins with a general overview on metal oxides, with particular focus on their surface and on the effect of the electronic structure on their functional properties. In the following, the application of XPS to the characterisation of oxides is introduced, with particular emphasis on the determination of the oxidation state of the metal atom. The third and last part of the chapter is instead devoted to the description of several case studies (taken from our own past and present experience) dealing with the XPS investigation of different single, doped or mixed oxide systems.

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For each specific case study, a general introduction on the dealt system is firstly provided (with hints on its technological relevance and synthesis), which is then followed by the description of its compositional and chemical features as gained by XPS analysis. Finally, an outlook on potential applications of advanced XPS-related methods such as atmospheric pressure XPS and combination of XPS with other advanced spectroscopic methods (XAS, XEOL) will be shortly sketched out.

1. Metal oxides: a general overview 1.1. General remarks on metal oxides Metal oxides are a very diversified and fascinating class of materials whose chemicophysical, structural and functional properties span an extremely wide range of different features; for instance, if electric behaviour is considered, their properties range from superconductors (e.g. high-Tc copper oxides), to good metallic conductors (e.g. V2O3, ReO3) through semiconductors (e.g. NiO, Ti2O3) to excellent insulators (e.g. MgO) [1,2]. Their electrical and magnetic properties present an exciting wide variability: we can observe ferroelectric and thermoelectric oxides (e.g. BaTiO3) [3], antiferroelectric (e.g. WO3), whereas the magnetic behavior ranges from ferromagnetism (CrO2) to antiferromagnetism (FeO, NiO), to paramagnetism (RuO2). Thermodynamic correlations between band gaps, electronegativity and chemical behavior of oxides have been discussed in a paper by Portier et al. [4]. Also from a structural and chemical point of view, their properties span the full range, and their chemical character can range from acidic through amphoteric to basic. The acid-base classification relies on the thermodynamic properties of the hydroxides in solution [2], and the periodic chemical properties of oxides have been extensively reviewed in dedicated textbooks [5]. Oxides are known for all the elements of the periodic table, except the lighter noble gases. As far as metallic oxides are concerned, they can be precisely stoichiometric (e.g. CaO) or rather show stoichiometry variability over a narrow or wide range of composition, depending on the availability of different stable oxidation states for the metal (e.g. MnxOy). Furthermore, transition elements, [1] in which a variable oxidation state is energetically feasible and occurs very frequently, show in their oxides, sulphides and other related binary compounds, a wide nonstoichiometric behaviour, related to their oxygen defectivity (for example CeO4-x) [6]. These nonstoichiometric oxide phases are of great importance in semiconductor devices, heterogeneous catalysis, and in the basic understanding of functional properties such as thermoelectric, photoelectric, magnetic and conduction properties in solids. A further noticeable contribution to the wide variation of electric and optical properties is provided by the defectivity of oxides (point, line, plane defects) [7]. The technological interest of metal oxides is highly relevant in several fields, ranging from catalysis and photocatalysis [8], to gas sensing [9], to superconductors [10] and optics [11]. 1.2 Importance of surface in metal oxides The surface of metal oxides plays a key role in determining their functional properties and in a wide range of chemico-physical phenomena, such as passivation against corrosion,

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catalytic activity, oxidative behaviour, bonding in sintered ceramics, interaction with gases in solid-state oxide-based gas sensors for monitoring of pollution, stability of electrode/electrolytes in fuel cells, surface conductivity in dielectric materials, optical properties on oxide nanocomposite glasses etc. The technological importance of metal oxide surfaces in different application fields has been extensively reviewed in the seminal book on this topic by Cox [1]. In all these and many other cases, the functional properties and performances of the oxides are ruled by the interfaces between metal oxides and other materials or by the interaction, occurring at the surface, between the oxide and the surrounding environment. The investigation of the chemico-physical nature of oxides surface, and the understanding of their correlations with the oxide behaviour and properties represents consequently a key issue not only from the scientific, but also from the technological point of view. Actually, in the last decays, an increasing number of papers has appeared, in which the study of metal oxide surface, both theoretically [13] and experimentally, is addressed. It should be underlined that this surface science task is complicated by inherent features of metal oxides such as defectivity, complex surface crystallography, variation of possible oxidation states, and surface orientation. Surfaces with apparently similar composition, but prepared slightly differently, can display completely different surface properties, which in turn results in different chemical behaviour and interaction with the environment (e.g. physior chemisorbed molecules). It becomes consequently a primary issue the availability of analytical tools providing detailed information of the microstructure and composition of metal oxide surface, as outlined in an exhaustive review by Fernandez-Garcia et al. [7]. The assessment of composition, intended as chemical nature and atomic percentages of the present species, oxidation state and chemical environment, can be afforded by XPS.

1.3 The electronic structure of oxides and its effect on functional properties When dealing with electronic structure, it is important to distinguish between transition metal and non-transition metal oxides, since the former class presents a wider variation of electronic and functional behaviours, also including important aspects such as variation of the oxidation state, the formation of defects and the chemisorptions of molecules on the oxide surface. The electronic structure of the oxide strongly affect some outstanding functional properties such as the optical, the electric, the magnetic and the catalytic ones and the different models to approach their theoretical investigation have been exhaustively described in references [2, 13, 14]. The electronic structure of metal oxides is much more complex than that of semiconductors and metals, and their appropriate modelling presents many difficulties related also to their structural complexity. Furthermore, the experimental task of surface scientists involved in the characterisation of these surfaces is complicated by the insulator nature of many technologically important oxides such as MgO and Al2O3. Since many of the most powerful surface science methods involve the use of charged particles (either electrons or ions), the surface charging in samples with negligible bulk conductivity prevents their application in investigation of these systems, thus limiting the range of applicable analytical tools. The electronic features of oxides surfaces, which can be studied by using different theoretical approaches as well as spectroscopic methods such as XANES and optical absorption [14], can be suitably addressed also by using valence and core-level photoemission, as extensively outlined in the following paragraphs.

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1.4 Simple, doped and multicomponent oxides and composites In this chapter, we address and discuss the application of XPS to the chemical investigation of simple (MOx), doped (M: M’Ox) and multicomponent oxides (MOx:M’Oy). Although the presented systems are very different, from the chemical and functional point of view, the common features they present is they were all prepared by soft-chemistry methods starting from molecular precursors. The preparation route is in many cases the sol-gel method, whereas in some of the presented cases studies, they were prepared by other wetchemistry methods, as extensively described in the given references.

2. Application of XPS to the characterisation of oxides: general remarks

2.1 Background and general introduction on XPS X-ray Photoelectron Spectroscopy (XPS) is a well established, versatile and generally applicable spectroscopic technique for the study of surfaces. [15]. Although at the beginning intended as a method for investigation of bulk materials or gases, it was already acknowledged in 1966 that XPS probing depth is rather small, and it turned out that the technique could be fruitfully applied to surface investigation [16,17]. Its primary importance derives (i) from the extent of chemical bonding and compositional details obtainable from peak position analysis, (ii) from the possibility to analyze the in-depth distribution and (iii) from the low level of radiation damage introduced into the sample by soft X-ray excitation. Applications of XPS to material science and technology range from basic materials research to analytical support and process monitoring. Examples of XPS experiments in materials research include detailed analysis of the chemical and electronic structure of buried interfaces, the determination of surface stoichiometry and chemistry, and of the Fermi level position in semiconductors, insulators and metals. The basic principle underlying this method is apparently simple. X-ray Photoelectron Spectroscopy (XPS) is one of a wide family of surface characterization methods based on the detailed energy analysis of electrons emitted from a sample surface into ultrahigh vacuum [16, 18]. Basically, photoelectron spectroscopy is based on photo-ionization and energydispersive analysis of the emitted photoelectrons to study the composition and the electronic state of the surface region of a sample. Traditionally, since the technique has been used for surface studies, it has been divided, depending on the energy of the exciting radiation into: - X-ray Photoelectron Spectroscopy using soft (ca. 200-2000 eV) X-rays to examine core-levels. - Ultraviolet Photoelectron Spectroscopy (UPS) using UV (10-45 eV) photons to investigate valence levels. As mentioned, XPS is based on the photoelectric effect, consisting in the emission of electrons (photoelectrons) from a material when irradiated with electromagnetic radiation bearing sufficiently high energy (Figure 2.1). This process is currently known as photoionization, the electrons ejected being the photoelectrons. The energy of a photon is given, in the monoelectronic approximation, by the Einstein relation [19]: E=hν

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where h is the Plank's constant (6.62 ·10-34 J s) and ν is the frequency (Hz) of the radiation.

e(-)

hν

sample

Figure 2.1 Photoionization process

In XPS photons are absorbed by atoms in molecules or solids, leading to ionization and to the emission of core (inner shell) electrons. At variance, in UPS the photon interacts with outer levels of the molecule or solid, leading to emission of valence electrons (Figure 2.2). e-

e-

vacuum UV photon

φs EF

EF

core levels

X photon

b) UPS

a) XPS

Figure 2.2 XPS (a) and UPS (b) photoemission processes

The kinetic energy distribution of the emitted photoelectrons can be measured using any appropriate electron energy analyser and a photoelectron spectrum can thus be recorded. The process of photoionization can be considered in several ways, one of them is to look at the overall process as follows: A + hν › A+ + ewhere A is a generic atom. Conservation of energy then requires that: E(A) + hν = E(A+ ) + E(e-)

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Since the electron energy is present only as kinetic energy (KE) this can be rearranged to give the following expression for the KE of the photoelectron: KE = hν - ( E(A+ ) - E(A)) The final term in brackets, representing the difference in energy between the ionized and the neutral atoms, is generally called the binding energy (BE) of the emitted electron; this then leads to the following commonly quoted equation: KE = hν - BE It is worth noting that the binding energies (BE) of energy levels in solids are conventionally measured with respect to the Fermi level of the solid, rather than to the vacuum level. This involves a correction to the equation given above in order to account for the work function (Φs) of the solid: KE = hν - BE - Φs For each element, there is a characteristic binding energy associated with each core atomic orbital, i.e. each element gives rise to a characteristic set of peaks in the photoelectron spectrum at kinetic energies determined by the incident photon energy and by the respective binding energies. The presence of peaks at particular energies therefore indicates the presence of a specific element in the investigated sample. Furthermore, the intensity of the peaks is related to the concentration of the element within the sampled region. XPS is sensitive to as low as 0.5 atomic percent and detects almost all elements, except H and He. Thus, the technique yields an elemental semi-quantitative analysis with an indetermination of ± 5-10% and it is sometimes known by the alternative acronym, ESCA (Electron Spectroscopy for Chemical Analysis). The most commonly employed X-ray sources are Mg Kα radiation (hν = 1253.6 eV) and Al Kα radiation (hν = 1486.6 eV). The emitted photoelectrons have therefore kinetic energies in the range of ca. 0 - 1250 eV or 0 - 1480 eV. Since such electrons have very short mean free paths in solids (tens of angstroms), the technique is necessarily surface sensitive. Whatever the energy of the incident radiation, the energy levels involved in the photoemission process is different. The process is consequently polyenergetic, even if it is induced by a monoenergetic radiation. Since the occupied energy levels are quantized, the photoemission distribution n(E) is characterized by a discontinuous pattern with different peaks. A thorough investigation of this energy distribution represents the basis of the analytical method. It may be further noted that there are significant differences in the natural widths of the various photoemission peaks and the peak intensities are not simply related to the electron occupancy of the orbitals. Emission from some levels does not give rise to a single photoemission peak, but to a closely spaced doublet. Spin-orbit coupling is generally treated using one of two models which correspond to the two limiting ways in which the coupling can occur, namely the LS (or Russell-Saunders) coupling approximation and the j-j coupling approximation. The Russell-Saunders coupling approximation is best applied only to light atoms and this splitting can alternatively be described using individual electron l-s coupling.

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This spin-orbit splitting is not present with s-levels (l = 0), but it occurs with p, d e f corelevels whichshow characteristic spin-orbit doublets. The binding energy of an electron depends not only upon the level from which photoemission is occurring, but also upon (i) the formal oxidation state of the atom and (ii) the local chemical and physical environment. Changes in either (i) or (ii) give rise to small shifts (few tenths of eV) in the peak positions in the spectrum, the so-called chemical shifts. This ability to discriminate between different oxidation states and chemical environments is one of the major advantages of the XPS technique. In practice, the possibility to resolve between atoms exhibiting slightly different chemical shifts is limited by the peak widths which are governed by a combination of factors. For traditional X-ray sources resolution can be improved by using X-ray monochromators. As already pointed out, XPS uses an X-ray beam to excite the sample and measures the energies of characteristic emitted photoelectrons. Information on the elements and their chemical bonding allows the identification of functional groups and molecular types. For most metals, XPS can distinguish between oxidized and reduced species. Although XPS is a surface sensitive technique, greater depths can be investigated through ion etching. Ion-assisted depth profiling can be applied to all solid materials, including insulators such as polymers and glasses. It can be widely used to analyze the chemical nature of the materials, to evaluate contamination and to determine oxidation states. As previously mentioned, even if the penetration depth of X-rays is in the micron range [20], the electron matter interaction and the related phenomena, (such as formation of hole-pairs electrons or collective oscillations of electrons) limit the depth escape of the photoelectrons. The sampled thickness is thus related to the mean free path of the outcoming electrons (Λe) and the electrons undergoing inelastic scattering originate an increase of the background in the higher BE part of the spectrum (Figure 2.3).

Figure2.3 Electron mean free path as a function of energy

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The number of emerging photoelectrons is given by the relation:

I = I o exp(−

z ) Λ e (Ek )senϑ

where Io is the initial number of electron at a depth z and ϑ is the angle between the surface and the direction along which the photoelectrons are detected. A plot of Λe as a function of KE (Figure 2.3) shows that, in the energy range typical of XPS experiments, i.e. 0-1500 eV, Λe is of the order of some nm [21]. It is this characteristic which endows the

method with a higher surface sensitivity. The quantity Λe(KE)senϑ is the escape depth and it corresponds to the thickness of the analyzed surface layer (Figure 2.4). As it can be seen in the following figure, the narrower is the angle ϑ, the lower is this sampled thickness.

Figure 2.4 Sampled thickness and incidence angle

The surface sensitivity of the technique can therefore be enhanced by lowering the angle ϑ by positioning the sample so that the photoelectronic emission is grazing to the surface. These analyses are known as AR-XPS (Angular Resolved XPS). During photoionization process, holes are formed in the core levels. They decay through a recombination with an electron coming from higher energy states according to two distinct and competitive processes (Figure 2.5): 1. X-ray fluorescence, in which the excess energy is emitted as photons (radiative decay); 2. Auger emission, in which the excess energy is released to an electron (Auger electron), which is emitted (non radiative decay). The peaks observed in a XPS spectrum can be grouped into two basic types: peaks due to photoemission from core levels and valence levels (levels occupied by electrons of low binding energy (0-20 eV), which are involved in de-localized bonding orbitals), and peaks due to X-ray-excited Auger emission. Photoelectron spectra of conducting or semiconductor materials generally display one or several weaker satellite peaks on the low-energy side of photoelectron peaks. They are due to plasmon excitation and to the conduction band.

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vacuum

φ s

φ s

EF M, etc. L 2,3 L1 K core hole X fluorescence

Auger emission

Figure 2.5 Decay processes for a hole created in a core level

Additional effects due to multiple excitations may generate further peaks [22] or modify line profiles; these effects include the "shake-up" (second electron raised from the valence band to the conduction band), and the "shake-off" effect (second electron ejected from the valence band into the continuum) [23]. There is an important problem concerning the measurement of binding energies shifts which should be mentioned. Insulating materials usually acquire a surface positive charge under X-ray bombardment, due to electron extraction. The positive charge left on the sample surface causes the electrons emitted from the sample to lose kinetic energy and appear to have a higher binding energy. Since carbon is the most commonly detected element in contamination layers and photoelectrons from C1s core level have a known binding energy of 284.6-285.0 eV, it is often used for referencing purposes [24]. The difference between the measured position of C 1s photoelectrons in the energy spectrum and the above value gives the charging value. Various methods have been developed for quantifying the XPS measurements [16]. For the analyses presented in this chapter, the method that utilizes peak area sensitivity factors was chosen and should be briefly discussed. Assuming the homogeneity of the examined sample, the number of photoelectrons per second in a specific spectra peak is given by: I=n·f·σ·θ·y·λ·A·T where n is the number of atoms of the element per cm3 of the sample, f is the X-ray flux in photons/(cm2sec), σ is the photoelectric cross-section for the atomic orbital of interest in cm2, θ is an angular efficiency factor for the instrumental arrangement based on the angle between the photon path and the detected electron, y is the efficiency in the photoelectric process for formation of photoelectrons of the normal photoelectron energy, λ is the mean free path of the photoelectrons in the sample, A is the area of the sample from which photoelectrons are detected, and T is the detection efficiency for electrons emitted from the sample. For a certain X-ray source with a fixed angle relative to the analyzer, the equation can be written in the form:

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I=n·S where S is the atomic sensitivity factor. Considering a strong line from each of two elements, the ratio n1:n2 is given by the ratio (I1/S1):(I2/S2). Even if it is true that such

quantities as σ and λ vary somewhat from material to material, their ratios σ1/σ2 and λ1/λ2 remain nearly constant. Thus, for any spectrometer, it is possible to develop a set of relative values of S for all elements. The XPS investigations here described for the various oxide systems have been carried out using a Perkin Elmer Φ 5600 Multi Technique System located at the Department of Chemistry of the University of Padova (Italy), equipped with a dual Mg - Al anode as the Xray source. Magnesium and aluminium are the two materials used universally as anodes in soft X-ray sources (Mg Kα = 1253.6 eV, FWHM = 0.70 eV; Al Kα = 1486.6 eV, FWHM = 0.85 eV). The instrument is shown schematically in Figure 2.6. It consists of the X-ray source of magnesium and aluminium, a fast-entry lock system for sample introduction (pre-chamber), an UHV analysis chamber, a detection system, an ion source for sputtering, a low energy electron gun for surface charge compensation, and a pumping system. XPS analysis requires Ultra High Vacuum (UHV) conditions for two reasons. Firstly, the electrons travelling from the sample surface toward the energy analyzer should encounter as few gas molecules as possible, otherwise they will be scattered and lost from the analysis. For this purpose it is sufficient a vacuum of 10-3-10-4 Pa. The requirement for a much better vacuum level arises from the nature of the technique itself that is very sensitive to surface contamination. This condition is fulfilled at 10-8-10-9 Pa. pump system

ion source

electron source

analysis chamber

X-ray source

analyser

computer

detector

Figure 2.6 Perkin Elmer Ø 5600 Multi Technique System

prechamber

UV source

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2.2 XPS for the investigation of simple, mixed and multicomponent oxides From the general introduction on the theoretical background, features and potentialities of the XPS method, it becomes clear that the information delivered by this kind of analysis concerns i) the chemical nature of the species present on the solid surface, ii) their oxidation state and chemical environment, iii) their relative atomic percentages. As already anticipated in a previous paragraph, the electronic structure and characteristics of oxides surface, can be suitably addressed also by using valence (see Section 2.5) and corelevel photoemission [14]. Furthermore, since the experimental binding energies, in this case of oxygen and metal main peaks, are strongly affected by the neighbourhood atoms, an insight also on the presence of mixed bonds (e.g. Ti-O-Si), witnessing the formation of multicomponent oxide homogeneous at atomic level, can be afforded by the method. Last, but not least, in the case of metal doped-oxides or mixed oxides, the possibility to perform, by mild sputtering, depth profiling of the samples affords to get an insight i) on the actual distribution of the dopant species in the former case and ii) on the degree of species intermixing in the latter one.

2.3 Determination of the oxidation state and chemical environment Several major factors have been highlighted, which can contribute to the shift in the BE observed for atom in solids, among them: valence electron density, crystal field potential, work function and relaxation energies [25]. Also the differences between both initial state potential energy and final state extra-atomic relaxation energy have been established to strongly affect the BE shifts. To unambiguously determine the oxidation state of a species in a sample, instead of corelevel BE values, it can be advantageous to use the Auger parameter. The Auger parameter concept was firstly developed by Wagner [26]. The Auger parameter is currently defined as the i.e. the sum of the KE of the Auger transition and of the BE of a main XPS transition, not corrected for charging effects. The Auger parameter of a particular element in different samples depends on the extraatomic relaxation energy of the element in this sample. This latter value is in turn related to the chemical, electronic and structural nature of the corresponding material. In this way, the Auger parameter can be used to obtain physico-chemical information about the bonding characteristics of the element in the investigated compound [27a]. The basic assumption with respect to charging problems is that peak shifts caused by charging should be the same for different electron lines of the same element. Thus, by using a photoelectron line and an Auger line of the element of interest, the difference between their kinetic energies (i.e. the Auger parameter) can provide a parameter that is independent on the overall charging of the sample. Although recent studies evidence that some experimental precautions should be taken when using the Auger parameter [27a], its main recognised advantage is that, in first approximation, it can be considered independent on energy referencing problems, whilst still being measurable to a high precision. Hence small shifts in its value can be measured and interpreted. This in turn enables a reliable determination of the actual chemical state

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(oxidation state and chemical environment) of the species under investigation. Also, by using a simple electrostatic model, Moretti and Porta [27b] were able to relate the Auger parameter shift with nuclearity and geometry of small metallic (Pd or Cu) cluster entrapped in zeolites.

2.4 Depth profiles and effects of ion sputtering on the chemical state of oxide Depth profiling is a common way to determine the distribution of species within a sample. This technique involves bombarding the sample with a primary ion beam and monitoring the concentration of species as a function of depth either with a secondary probe beam (XPS) or by using the ions generated from the primary beam (SIMS). During XPS depth profile measurements, the surface of the sample is eroded by cycles of sputtering, caused by Ar+ ions. Between two cycles, the surface is analyzed by using an X-ray beam as described above. The obtained results are usually presented in diagrams of intensity versus etching time, which can in turn related (upon measurement of the eroded depth by a profilometer) to the depth of the eroded zone. The sputtering-induced changes of surface composition and topography have been thoroughly discussed in a dedicated review by Hofmann [28]. The main drawback of ionbeam bombardment of solid surfaces is the occurrence of preferential sputtering, i.e. the preferential removal of one species with respect to the others, due to different sputtering yields, and leading to marked deviation of the obtained surface composition with respect to the actual one. A further detrimental effect induced by the sputtering, and being itself a consequence of the preferential sputtering of a light and frequently occurring element as oxygen, is the chemical reduction of the metal atoms in oxides, as already reported for many metal oxides of Fe, Co, Ti and Nb [29]. This preferential removal of oxygen has been related to both mass difference and surface BE effects, and has been interpreted as thermal spike mechanism rather than collisional cascade sputtering. This means that preferential sputtering of some oxide species can be related to the relative stability and decomposition rate of the given oxides. Improved methods and models as well as standardization protocols [28] have been implemented to minimize the error sources in quantitative determination upon sputtering, and achieve a high-resolution and accurate depth profiling. A valuable alternative to sputtering, if the interesting depth is less than 3 nm (for example in catalysis, photocatalysis and chemisorptions processes, corrosion and surface degradation phenomena) is Angle-Resolved XPS (AR-XPS), which is non destructive and it is particularly advantageous if chemical state information is important, since this is, as previously evidenced, very rapidly lost upon depth-profiling involving ion-sputtering. [30]. In fact, as already outlined in the general description of the method, the surface sensitivity of the technique can be enhanced by lowering the incident angle ϑ by positioning the sample so that the photoelectronic emission is grazing to the surface. Although the method requires a careful optimisation of the experimental parameters, the analysis of the results is not straightforward, and the obtained data, especially those related to the quantitative analysis, should be handled with extreme care. The method can be profitably used to non-destructively probe the surface of solids, up to a limited depth.

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2.5 Valence band XPS for the investigation of oxides Although Ultraviolet Photoemission Spectroscopy has been assessed as a very powerful tool for the investigation of the electronic structure of the surface states [31], also valence band photoemission, i.e. this characterized by energy lower than 20 eV, can be conveniently used to study the band structure of oxides. Since in the valence region electrons occupy delocalized or bonding orbitals, the spectrum of this region consists of closely spaced levels originating a band structure. For large oxide particles, valence bands similar to those of the bulk materials are expected, whereas as the size of the particles decreases, unique and diagnostic features can appear [32]. By using this part of the spectrum, band gap can be determined, [33] and the presence of oxygen vacancies in the oxide nanoparticles can be detected. Typically, electronic states associated with oxygen vacancies usually appear above the valence band of the oxide in the band gap [34].

3.Case studies

3.1 Simple oxides General remarks Hereafter we present four different case studies concerning different functional pure oxides. In two of them (Cu and V oxides) the metal can occur in difference oxidation state, and the aim of the XPS investigation was in these cases also to assess the actual valence of the metal in the investigated oxides.

3.1.1 ZnO General introduction and motivations ZnO nanostructures with different shapes and morphologies have attracted, in the last 10 years, an amazing interest [11,12,35], which can be traced back to the outstanding functional properties, particularly electrical, electronic and optical, of ZnO [36]. ZnO colloidal nanosized particles are particularly appealing for their functional properties and their can be approached by classical methods of colloid chemistry [37], non-aqueous routes [38], as well as by solvo- and hydrothermal methods [39]. In our recent work, [40] we addressed the synthesis of ZnO by using a facile and reproducible wet-synthesis route based on the controlled hydrolysis and condensation of zinc acetylacetonate as precursor in 4 different solvents (1,2 propanediol, water, ethanol and glycerol) as dispersing media. Irrespective of the nature of the solvent, X-ray Diffraction (XRD) analysis shows the formation of crystalline hexagonal ZnO. Indeed, different particles sizes and very different morphologies were obtained. The composition of the obtained ZnO was determined by elemental analysis, X-ray Photoelectron Spectroscopy (XPS), FT-IR and Thermogravimetric Analysis (TGA) analysis, whereas time resolved UV-Vis and XAFS

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studies were performed to get a deeper insight on the formation mechanisms of these nanostructures [40].

XPS analysis XPS measurements were performed on ZnO obtained from all four solvents to get information about the composition and the chemical environment of zinc and oxygen atoms, and also to assess the complete decomposition of acetlylacetonate to give the desired ZnO and the possible co-presence of zinc hydroxide. To this aim, O1s peaks were de-convoluted using commercial software. The BE values for Zn 2p3/2 and O1s region, along with the O/Zn atomic ratios are summarized in Table 3.1. For ZnO, tabulated values for Zn2p3/2 BE are comprised between 1021.1 and 1022.4 eV, whereas the expected value for Zn(OH)2 would be 1022.7 eV [13,16,41-43]. The experimental values (see table 3.1) for the four samples are in the range 1021.1-1021.6 eV, values similar to zinc oxide, and in agreement with XRD data, evidencing formation of nanocrystalline zinc oxide, whereas they are quite different from those reported for Zn(OH)2. On the basis of this data, presence of the hydroxide could be ruled out. However, the deconvolution of O1s in three components reveals the presence of a component ascribable to OH species at the surface, thus highlighting the importance of a complete and thorough analysis of all the available XPS regions. In particular, peak components at 530.8 eV, 531.5 eV, 530.9 eV and 531.7 eV for samples obtained from ethanol, 1,2 propanediol, water and glycerol respectively, were ascribed to the hydroxide. The presence of OH groups was also confirmed by FT-IR analysis. Table 3.1 BE (eV) values for Zn2p3/2 and O1s and O/Zn atomic ratios relative to the four ZnO samples synthesised from different media. Values of BE were corrected considering charging effects.

BE (eV)

EtOH

1,2

H2O

propanediol Zn 2p3/2

Glycer ol

1021.3

1021.1

1021.1

1021.6

529.8

530.0

529.4

530.4

530.8

531.5

530.9

531.7

532.4

532.9

532.2

532.9

1.1

2.0

1.1

1.3

(eV) O1s O/Zn (atomic ratio)

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3.1.2 WO3 General introduction and motivations Amorphous or polycrystalline tungstic oxide (WO3) films are particularly attractive because they show remarkable catalytic activity both in oxidation and reduction reactions [1]. Moreover, owing to their electrical and optical properties, WO3 films have been extensively studied as prototypes of electrochromic devices [44]. Such properties, which are related to the transformation of WV into WVI, are strongly dependent on the microstructure and purity of the deposited film. Therefore, the synthesis strategy should include an accurate control over both the morphology and composition of the thin films and the sol-gel technique is particularly suitable, in principle, for controlling both these factors. Several studies have appeared over the time on the use of the sol-gel technique for the production of tungsten oxide thin films starting from different precursors [45]. We have studied the synthesis of WO3 layers on silica starting from the freshly prepared W2(OR)6 (R = tert-butyl) alkoxide compound [46]. In this case, the absence of inorganic anions in the molecular precursor (to be compared with previous sol-gel deposition from WOCl4), gives in principle the possibility of obtaining high purity oxide films.

XPS analysis The chemical characterization by XPS evidenced the absence of spurious peaks. The BE value of the W 4f7/2 peak is typical of WO3 (35.8 eV) [43] for coatings deposited and annealed under different conditions (300 < T < 700°C, 1h < t < 6.5h). The O1s region shows a more complex pattern, which can be fitted with two components: the one with lower BE at 530.2 eV is typical of WO3, whereas a second component with higher BE (532.1 eV) is likely associated with the presence of W-OH groups due to incomplete polycondensation of the alkoxide precursor. Indeed, a decrease of this component on passing from the sample annealed at 300°C to the one heated at 500°C can be easily appreciated, as a consequence of the more extensive condensation promoted at increasing temperatures [47]. An important peculiarity is the amount and distribution of residual carbon within the films, originating from incomplete removal of the degradation products of the organic residues. This was evaluated by analysing the C1s peak during an etching process (3 keV, Ar+ ions). Whereas the sample treated at 300°C for 6.5 h presents a decreasing amount of carbon when going deeper into the film, the one treated at 500°C for 1 h shows a slight increase at the interface with the silica substrate. The complexity of the phenomenon is evident when considering that heating has the effect of pyrolysing the organic residues, but the concomitant gel densification works against the displacement of the pyrolysis products, which remain confined in the film. Consequently, XPS evidenced as higher temperature treatments do not necessarily imply lower carbon content since the densification of the materials competes with the elimination of the pyrolysis products.

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3.1.3 VxOY General introduction and motivations Due to the good performance of vanadium pentoxides in catalysis and lithium insertion batteries, vanadium pentoxide nanocomposites are widely studied materials [48]. Hybrids of vanadium pentoxide with conducting polymers such as polypyrrole, polythiophene, [49] polyaniline, [50]. poly(3-alkylpyrroles), [51] poly(N-propane sulfonic acid aniline) are particularly interesting. As far as the preparation of V2O5 is concerned, non-aqueous sol-gel chemistry so far failed to produce V2O5 nanostructures, leading to V2O3 at best instead, due to reduction of the precursors. In a recent study hereafter discussed, Smarsly et al. [52] demonstrate a new methodology for the generation of V2O5 nanostructures based on ionic liquids (ILs) serving as both reaction medium and structure-providing template at the same time. Ionic liquids are organic salts with a low melting point, usually below 100 °C. [53]. Due to their special solvent structure that derives from the ion-ion interaction and extended Hbridge motifs they are also interesting templates in the sol-gel synthesis of nanoparticles. Zhou et al. described for the first time the synthesis of anatase nanoparticles in imidazoliumbased Ils [54]. This approach was used also for other systems, such as the synthesis of titania in ionic liquids. Nevertheless, the promising application of ILs acting as templates and solvent at the same time in the synthesis of inorganic nanostructures in general is still quite unexplored. The advantage of using VOCl3 as precursor together with an imidazolium-based IL as solvent and structure-directing agent relies on the fact that VOCl3 forms addition compounds with nitrogen donors [55]. Based on this fact, Niederberger et al. synthesized vanadium oxide nanotubes using long alkyl chain amines as template [56]. In this work, a facile synthetic strategy for different IL-vanadium pentoxide nanocomposites, which is performed at relatively moderate temperatures, starting from a homogeneous solution of VOCl3 in ILs, is presented [52]. This work demonstrated that such facile synthetic approach is indeed peculiar for ILs, because for instance it takes advantage of the liquid state and the good solubilization behaviour, in combination with the variation of the ions.

XPS analysis In this case, XPS was mainly devoted not only to get information on composition, and chemical environment of the species, but particularly to assess the actual oxidation state(s) of vanadium in the prepared materials. It is well known [2] that vanadium can occur in V(III), V(IV), V(V) valences, and it is particularly important, especially for functional properties, to be able to relate the oxidation state with the observed functional properties. To this aim, we also analysed, for referencing purposes, the corresponding pure vanadium oxides, V2O3, V2O4 and V2O5, respectively. As far as the composition of the V2O5-IL nanocomposite is concerned, the presence of all involved species (O, V, C, N, B, F) could be evidenced by the survey spectrum on the surface

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of the samples. The atomic percentages of the different species are reported in Table 3.2. As it can be seen, the experimental O/V atomic ratio is 2.8, which would be in agreement with the stoichiometric values expected for V2O5, i.e. 2.5. Table 3.2. Atomic composition of the V2O5-C2mimBF4 nanocomposite. Sample

%N

%O

%V

%B

%F

%C

V2O5

11.3

26.5

9.4

2.6

17.2

33.0

To get a better insight on the chemical state of the different species, selected spectra of the regions of interest were acquired. The deconvolution of the V2p3/2 region reveals the presence of two components (Figure 3.1).

Figure 3.1 Deconvolution of the XPS V2p3/2 region of V2O5-C2mimBF4 (in this figure values of BE are not corrected for charging effects)

The BE value of the more intense component, (see table 3.3) corrected for charging effects, is 516.1 eV, a value considerably lower than that reported for V2O5 (about 517.0517.4 eV) and closer to the lower values reported for VO2 (516.3 eV), V2O3 (515.7-516.4 eV). [16,42,43]. The less intense component resulted to be peaked at 515.1 eV, a value which suggests the presence of vanadium in V(III) oxidation state. Also the value of the BE of oxygen, i.e. 529.0 eV, is lower than that expected V2O5 (529.8-530.4). Instead, these values are more similar to V(III) and V(IV) oxidic species [57]. To better determine the actual oxidation state of vanadium, the Auger parameter of vanadium was also evaluated by the sum of V2p(BE) (main component) + VLMM(KE), resulting to be 984.3 eV. This value is also slightly higher than that reported in literature, 984.0 eV [58] and also than the value of the Auger parameter experimentally determined by a reference V2O5 sample, i.e. 983.7 eV. On account of these findings, an unambiguous determination of the oxidation state of vanadium could not be carried out, and the copresence of different oxidation states and chemical environment around the metal centre can be instead assumed.

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In conclusion, XPS quantitative measurements and the comparison of the experimental binding energies with literature values and reference samples suggest the presence of the vanadium pentoxide, but the copresence of V in lower oxidation states (likely V(III)) was suggested both by the presence of a second component in the V2p region and also by the detected BE values. Table 3.3 BE values of the different vanadium species. Sample

V2O5C2mimBF4 V2O5 reference V2O3 reference V2O4 reference

BE V2p (eV)

BE O1s (eV)

KE VLMM (eV)

Auger Parameter (eV)

516.1

529.0

466.6

984.3

517.4

530.2

466.6

983.6

514.8

529.0

467.5

983.7

517.3

530.2

467.0

983.9

3.1.4 CuxOy General introduction and motivations Copper oxide-based materials have been widely investigated due to their potential applications in many technological fields. Cupric oxide (CuO, tenorite) is a monoclinic p-type semiconductor with a band gap of 1.2–1.5 eV [59] whereas cuprous oxide (Cu2O, cuprite) is a cubic p-type semiconductor with a band gap of 2.0 eV [60]. Copper oxides and metallic copper have been employed as heterogeneous catalysts for several environmental processes, e.g. NO selective reduction, CO oxidation and NO2 decomposition [61]. Furthermore, copper (II) oxide thin films could be employed in the production of gas sensing devices, owing to the conductivity changes induced by the reaction of gases with surface adsorbed oxygen [62]. Conversely, when combined with small noble metal particles, copper (II) oxide displays high activity as optical gas sensor [63]. Regarding Cu2O, it is a very promising material for the development of photovoltaic devices like solar cells [64] and for the photocatalytic decomposition of water [65]. On this basis, we focused on the sol–gel synthesis and characterization of nanocrystalline copper oxide thin films with the aim to achieve a good control over film chemical composition [66]. In the preparation of such systems, stoichiometry and phase composition represent a major concern, and they can be suitably tailored by a proper choice of the molecular precursors and of the annealing conditions. The chemical composition and copper oxidation state were investigated by X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XE-AES). Copper oxide thin films (thickness ≈ 100nm) were produced from copper acetate and were subsequently heated between 100 and 900 °C in different flowing gas atmospheres, i.e. air, nitrogen and forming gas (4% H2 in N2).

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Particular attention was focused on the chemical evolution of the systems as a function of the annealing treatment, e.g. temperature and atmosphere.

XPS analysis

a.u.)) Intensity (a.u.

XPS detailed scans spectra were recorded for C1s, O1s, Cu2p and CuLMM regions, and the different copper oxidation states were evaluated using the Auger α parameter [67], defined as the sum of the BE value of the Cu2p3/2 XPS band and the kinetic energy (KE) of the CuLMM Auger peak. In the XPS spectra all samples revealed signals from Cu, O and C. The latter was ascribed to atmospheric contamination (ca. 20 at.% on the surface), as indicated by its disappearance after mild Ar+ sputtering. Detailed Cu2p region scans for the samples containing only CuO (600 °C, 1 h, air) or Cu2O (900 °C, 5 h, N2) crystalline phases are displayed in Figure3.2.

Binding Energy (eV (eV))

Figure3.2. XPS Cu 2p line for copper oxide samples annealed in different conditions under air or nitrogen stream.

The discrimination between Cu(II) and Cu(I) oxides is usually accomplished by the analysis of the Cu2p region, not only for the different peak position (Cu2p3/2(CuO) = 933.7 eV, Cu2p3/2(Cu2O) = 932.5 eV) [16,43] but also for the bandshape profile. The Cu2p signals for Cu(II)-containing compounds are characterized by the presence of intense shake-up satellites located at BE values ca. 9 eV higher than the main spin-orbit split components[68]. Conversely, in the case of cuprous derivatives shake-up satellites are almost undetectable. The Cu2p3/2 peak position for the sample containing only the Cu2O crystalline phase showed an intermediate value between those reported for Cu(I) and Cu(II) oxides [69]. This finding may indicate the formation of CuO in the outer layers of the film due to partial oxidation of Cu(I). In fact, the Cu:O atomic ratio was always close to 1 on the sample surface. Further indication of Cu2O and CuO co-presence might be traced back to the intensity ratio between each spin-orbit split component and the corresponding shake-up satellite. The analysis of the Cu2p bandshape profile for the two specimens indicated a lower intensity ratio for the sample annealed in N2, suggesting thus the co-existence of Cu(I) and Cu(II) species on the sample surface. Moreover, the Auger α parameters for the films annealed in air and nitrogen resulted in 1851.6 eV and 1849.7 eV, respectively. A comparison with literature values for CuO

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(1851.8 eV) and Cu2O (1848.7–1849.4 eV) [16,69] confirmed unambiguously the presence of both copper oxides in the near surface region of the film annealed under inert atmosphere. Finally, under reducing conditions, the formation of CuO, Cu2O and Cu was progressively observed also basing on X-Ray Diffraction (XRD) studies, leading to a mixture of Cu(II) and Cu(I) oxides and metallic copper after treatment at 900 °C for 5 h. In this regard, recent studies [70] have demonstrated that the copresence of copper oxides results in a synergic effect in oxidation catalytic processes of ethanol. These XPS results evidenced that a proper choice of processing conditions allows the tailoring of the chemical composition in copper oxide based films. The different oxidation states of copper can be determined in detail by combing the features of the photoelectron lines with the evaluation of the Auger parameter.

3.2 METAL-DOPED OXIDES General remarks In the last decade the research activity on composite materials based on metal-doped systems, both as ionic species or metallic particles dispersed in/on suitable oxide matrices has enormously increased due to the possibility of tailoring their functional properties basing on their chemical composition and surface structure, particle size, shape and distribution [71, 72]. Unique characteristics are also derived from interfacial interactions among guest clusters and oxide matrices due to the presence of defects and compositional gradients that might play a pivotal role on the functional performances of the resulting systems [73]. A full exploitation of the nanosystem potential relies on the deep understanding of their properties, with particular regard to their chemical composition. At present, these systems stimulate considerable interest in various applicative fields such as (photo)catalysis, sensing, magnetism, photonics and energetics. Among the various supporting/embedding hosts, metal oxides have received a significant attention [1], owing to their different structures and chemico-physical characteristics. In particular, oxide matrices such as SiO2 and TiO2 have been thoroughly investigated thanks to features such as optical transparency in the visible range, structural and thermal stability and weak interactions with guest particles [74]. We report here some case studies concerning the preparation of M’/MO2 nanocomposites systems (M’= Au, Rh, Pd, Pt; M = Si, Ti) from molecular precursors, focusing on their composition and its dependence on synthesis parameters. The XPS investigation of M’/MO2 nanocomposites has already been addressed also by other authors such as for Ag [75, Au [76], and bimetallic systems [77], whereas XPS investigations of free and supported metal clusters (e.g. Au, Ag, Pt, etc.), relating their corelevel BEs with their chemical state, cohesive energy, size and strength of the interaction with support has been the topic of contribution by Wertheim [78], Calliari [79], Minati [80]. In the case of M’/MOx nanocomposite, Speranza et al. [75] have outlined as size-dependent electronic effect in metallic nanoclusters embedded in oxide matrices can be conveniently addressed by XPS. In particular, in the mentioned study, the authors were able to correlate changes in the photoelectron core-level BE, in Auger parameter and valence band line-shapes with the size of the particles and the extent of quantum confinement.

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3.2.1 Rh-SiO2 General introduction and motivations SiO2 can be conveniently exploited as a matrix material to embed or support transition metals as ionic species or metallic particles. The versatility of the sol-gel method [81], suitable for producing thin films [82], powders with definite porosity [83], low-density materials [84], and samples with proper geometry [85], justifies this interest, which is generally agreed to be connected with the appearance of functionalities proper to transition metal species. As for VIIIB and IB elements, their inclusion as metal nanoparticles in sol-gel SiO2 matrix immediately led to applications in heterogeneous catalysis. [86]. More intriguing research topics concern the use of sol-gel SiO2-metal composites for sensors [87], optical waveguides [88], electrochromic materials [89] and, in general, for the multi-faceted interests of the electronic industry. Thus, nanometric particles embedded in sol-gel SiO2 films are studied for electrochromic film preparation [90] and for the peculiar magnetic properties [91] arising from extreme metal dispersion; Ag, Pt and Au dispersed in SiO2 are of interest in nonlinear optics technology; Pd, Ag and Au are studied for special optical effects [92], including absorption-reflection optics and photo-sensitization. Among the noble metals, Rh has also been studied [93]: Rh presents some fundamental features which are of interest in the appearance of specific optoelectronic functions for metals included in an active sol-gel SiO2 matrix. The easy change of oxidation numbers is an important property for electrochromic functions, the possible stabilization of Rh(I) species may favor the formation of the nanometric and monomodal Rh distribution required in non-linear optics, and the availability of mixed Rh(I)-Rh(III) oxides may give rise to particular magnetic features. To correlate the features of Rh-SiO2 nanocomposites with optoelectronic and magnetic functions, the study was primarily oriented to identifying the basic features of the rhodium-doped silica system, particularly how the SiO2 matrix affects the chemical evolution of different Rh precursors during sol-gel processing. The properties of metallic species in an inorganic oxide network are traced back to mutual interactions between metal and matrix, which may stabilize definite oxidation and coordination states, favour particular metallic dispersions, and avoid the collapse of small particles on heating. On the other hand, valuable concentrations of a transition metal ion in the solution of gelling SiO2 precursors (Si(OEt)4, TEOS) may change the kinetics of the sol-gel process, and this phenomenon may in turn be affected by the chemical nature of the metallic species originally used as precursors. RhCl3×nH2O and [RhCl(C2H4)2]2 were used as typical Rh(III) and Rh(I) compounds, respectively, the former being the most common Rh compound, the latter representing a classic Rh(I) organometallic species. Rh1%-SiO2 samples (Rh(III)-SiO2 and Rh(I)-SiO2 from RhCl3×nH2O and [RhCl(C2H4)2]2, respectively) were synthesized, dried under vacuum for one week and subsequently annealed in air at increasing temperatures [94].

XPS analysis XPS analysis resulted fundamental to evaluate rhodium chemical state in the various samples [43]. First, XPS was used to investigate the evolution of the single Rh precursor compounds. In the 380°C treated RhCl3×nH2O sample, the two 3d5/2 and 3d3/2 components for

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the Rh signal, typical of the spin-orbit coupling, were present as sharp peaks at 310.0 and 314.8 eV. The absence of low energy components for the Rh 3d signal indicated the presence of Rh(III) species without significant presence of lower oxidation states. The spectrum of the 980°C sample revealed a strong signal of metallic Rh at 307.5 eV with an appreciable shoulder at high energy, tentatively ascribed to the lower contribution of Rh(I). At variance, the spectra for the Rh 3d region of [RhCl(C2H4)4]2 treated at 160, 450, and 1000°C indicated that at low temperature a mixture of Rh(I) (Rh3d5/2 = 309.4 eV) and metallic rhodium (Rh3d5/2 = 307.5 eV) is present (Figure 3.3).

Figure 3.3. Rh 3d XPS spectra of [RhCl(C2H4)2]2 precursor heated at 160, 450 and 1000°C under inert atmosphere.

The latter signal (307.5 eV) became the major component for the 450°C sample and almost the only one at 1000°C. As for the doped-silica specimens, XPS results for the Rh(III)SiO2 sample evidenced the presence of metallic Rh (BE = 307.5 eV); in addition, the Rh3d line showed an evident tail on the high binding energy side, due to the appreciable presence of Rh(I). These features were maintained at 225°C with a less important Rh(I) contribution, whereas at 800 and 1000°C, the peak shape suggested the presence of only metallic rhodium. The XPS spectral evolution of [RhCl(C2H4)2]2-SiO2 derived samples is very similar to that of Rh(III)-SiO2 samples although in the sample prepared at room temperature the more pronounced occurrence of Rh metal may be foreseen from the shape of the signal at 307.5 eV. Therefore, the thermal evolution of the sol-gel SiO2 matrix is not substantially affected by the presence of Rh species, whereas the silica gel deeply influences the thermal reduction of both RhCl3×nH2O and [Rh(C2H4)2Cl]2 precursors with respect to their original behavior as pure compounds. The most evident effect of the inclusion of Rh precursors in the SiO2 gelling solution is the occurrence of reduced Rh in the material obtained at room temperature. This fact may be attributed to reduction by ethanol of RhCl3 and [RhCl(C2H4)2]2 to metallic Rh nanoclusters. XRD and TEM data do not reveal the presence of a segregated Rh phase, as instead detected by XPS signals in the binding energy region typical of metallic rhodium. Through this XPS study we have shown that high dispersions of Rh particles into a sol-gel SiO2 matrix may be easily obtained and that the Rh-precursors influence the dispersion of the

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Rh phase and its thermal stability. XPS evaluation of the rhodium chemical state was fundamental to identifying the basic features of Rh-SiO2 nanocomposite systems prepared via sol-gel starting from different Rh precursors under different processing conditions.

3.2.2 Au-SiO2 General introduction and motivations The formation of metal nanoparticles is an expanding area of research [95], in connection with the use of well-defined molecular precursors, the understanding of the aggregation phenomena and the application to several areas of surface science. Due to the great fundament and applicative potential, it appears of general interest to study the chemical implantation of noble metals on silica matrices starting from molecular complexes, by exploiting specific reactions on silanol (Si-OH) sites. In this framework, gold nanoparticles have attracted considerable attention both for their intrinsic properties [96] and for their implications in catalysis and as CO-sensing systems [63a, 97]. Gold-grafting on silica has been investigated under mild conditions starting with the mononuclear molecular precursors Au(O2CNR2)(PPh3) [98] and AuCl(CO) [99]. Formation of gold nanoparticles can be predicted with the chlorocarbonyl derivative, due to the reducing properties of the carbonyl ligand, whereas by using another molecular precursor such as the N,N-dialkylcarbamato complex grafting of gold(I) onto the silica surface is expected. In the latter case, the primary process consists of the silanolysis of the carbamato ligand with CO2 evolution. [100]

XPS analysis Starting from Au(O2CNR2)(PPh3), the formation of a matrix-grafted gold-silicate displaying the Si–O–Au–P sequence was confirmed by XPS data [101]. Indeed, the XPS spectrum of the Au 4f7/2 line showed a single-component spin–orbit split peak centred around 84.9 eV, [16, 42, 43, 102] which was attributed to a Au(I) species bearing a tertiary phosphine ligand. On the other hand, the choice of AuCl(CO) to implant metallic gold on silica, stemmed from the observation that halo-carbonyl complexes of late transition elements undergo reduction in the presence of water, coordinated carbon monoxide being converted to carbon dioxide [103]. Thus, we expect physi-/chemi-sorption of gold(I) to be the primary process, followed by reduction in the presence of water, as such or arising from condensation of silanol groups. In this case XPS clearly evidenced the presence of metallic gold on the silica support [43]. The more intense Au4f7/2 spin–orbit split component of the Au4f peak was found to be centred around 84.2 eV, which was deconvoluted to two peaks located around 84.0 and 85.1 eV, associated with Au(0) and Au(I) species, respectively, gold clusters being presumably anchored to silica via oxygen-bridging. The relative intensity of the two peaks (Au(0)/Au(I) ratio was found to be approximately 4 and it was unaffected by the experimental conditions (fast or slow addition of the AuCl-(CO) solution to the silica, ageing of the samples at room temperature, storage at 40°C in a water-vapour saturated atmosphere). The Si2p (BE = 103.6 eV) and O1s (BE = 532.9 eV) peaks are characteristic of SiO2, whereas the C1s line was mainly due to adventitious contamination.

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Thus, for both Au(O2CNR2)(PPh3) and AuCl(CO) complexes the anionic ligand is eliminated during the grafting silanolysis reaction (as CO2 and NHR2 for the former, and as HCl for the latter), whereas the neutral ancillary ligand influences the chemical complexity, nuclearity and morphology of the implanted fragment. In the former case, isolated grafted Au(PPh3) moieties are obtained, that do not show any tendency to undergo reduction or clustering. Conversely, deposition of metal gold nanoclusters, through the joint action of adsorbed water and CO is obtained with AuCl(CO).

3.2.3 Pd-SiO2 and Pt-SiO2 General introduction and motivations In analogy with the previously described study, concerning the implantation of gold on silica surfaces starting from well-defined molecular precursors, we have also studied the preparation of supported palladium and platinum nanoparticles from molecular precursors [104]. Their use in hydrogenation catalysis is a topic of current interest. [105, 106]. When the precursor contains a metal characterized by a positive standard reducing potential, its prompt reduction (by heat or by exposure to H2, or to CO/H2O) is to be expected, with formation of metal nanoparticles, whose dimensions depend, among other factors, on the temperature. In earlier reports from Calderazzo, Belli Dell’Amico et al. palladium, platinum and gold have been supported on silica by using the corresponding N,N-dialkylcarbamato complexes as precursors [100,101], as well as the mononuclear chlorocarbonyls AuCl(CO) and cisPtCl2(CO)2 [101,103]. More recently, the dinuclear chlorocarbonyls Pt2(μ-Cl)2Cl2(CO)2 and Pd2(μ-Cl)2Cl2(CO)2 complexes [107, 108] have been used for the same purpose [104]. As the precursors are similar from a molecular viewpoint (both are chloride-bridging species), [108b] the comparison between Pd and Pt behaviour in the grafting process on a silica surface was particularly interesting to be investigated. Indeed, these dinuclear derivatives were expected to bind to the oxygen atoms on the silica surface by splitting of the chloride bridges, with subsequent attack by water thus reducing the platinum(II) and palladium(II) centres, CO being converted to CO2. The presence in the dinuclear precursors of only one CO group per metal atom and, consequently, of two reducing equivalents, was considered to be ideal for converting the central metal atom to the zerovalent state, i.e. to M(s). A further advantage in the use of the chloro-carbonyl derivatives of palladium and platinum is that their reaction with moisture occurs at room temperature or even lower, which is important to limit aggregation of the resulting metal particles. Pt-SiO2 and Pd-SiO2 powders were prepared by reacting silica powder with the corresponding metal precursor in toluene solution. Samples as prepared and treated in water vapour at 40 °C were investigated.

XPS analysis Information on chemical composition of both samples was obtained by XPS analysis. Concerning the former sample, Figure 3.4 shows the XPS high resolution spectra of the Pt4f regions, with the 4f7/2 and 4f5/2 spin–orbit splitting components, for as-prepared powders (A),) and after exposure to water flux (B).

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Figure3.4: XPS spectra of the Pt4f regions for as-prepared powders (A), and after exposure to water flux (B).

Both spectra show a significant line broadening suggesting the presence of platinum centres characterized by different oxidation states and/or chemical environments. Peak deconvolution resulted in two doublets centred at (BE Pt4f 7/2) ≈71.5 eV and ≈73.0 eV, assigned to Pt(0) and Pt(II) species, respectively [42,43]. The most remarkable difference between the samples concerns the amount of metallic platinum, that can be evaluated by comparing the intensity of the Pt(0) and Pt(II) components. While the Pt(0)/Pt(II) ratio is ca. 1 in the as-prepared Pt-SiO2 sample (A), it increases to ca. 2.2 after hydrolysis. Therefore, the impregnation of the silica matrix with the toluene solution of Pt2( -Cl)2Cl2(CO)2, produces a reduction of about half of the Pt(II) ions, whereas further Pt(II)→Pt(0) reduction is achieved through water vapour treatment, up to at least 70% of the total platinum content. For Pd-SiO2 specimens, XPS investigation evidenced that metallic palladium is already present in the as-prepared sample. Figure 3.5 shows the XPS high resolution spectra of the Pd3d regions, with the d5/2 and d3/2 spin–orbit splitting components, for the as-prepared PdSiO2 (A) and after hydrolysis (B),). A moderately broad band-shape profile can be observed especially for the as prepared sample, suggesting the presence of palladium centres with different oxidation states and/or chemical surroundings. The changes between sample A and sample B in the Pd3d spectra became more apparent by fitting two doublet components to the data. In both cases, the positions of the components at (BE Pd3d5/2) ≈335.2 eV and ≈337.0 eV, correspond to Pd(0) and Pd(II) species, respectively [42,43].

Figure3.5: XPS spectra of the Pd3d regions for as-prepared powders (A), and after exposure to water flux (B).

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By comparing the intensities of the Pd(0) and Pd(II) components, metallic palladium represents ca. 65% of the total metal content in the as-prepared Pt–SiO2 sample (A) increasing up to 85% after hydrolysis. Therefore, contacting the toluene solution of the Pd2( -Cl)2Cl2(CO)2 compound with the silica matrix is effective in promoting the Pd(II) to Pt(0) conversion, which is almost complete after controlled exposure to moisture under mild conditions. XPS analysis was proven to be very effective to understand the formation of Pt and Pd metal particles through a substantially identical process at room temperature, starting from molecular precursors of the same type but characterized by a decreasing metal–metal bond strength.

Metal nanoparticles-doped TiO2 Titanium dioxide is at present one of the most intensively investigated system in materials science because of its unique electronic and optical properties [109] which disclose applications in photocatalysis [110,113], chemical sensing, photovoltaics [111] and energy storage [112]. The photocatalytic properties of TiO2 are being considered, among others, for the remediation of polluted air/ water by organic compounds and for H2 production [113]. Due to the wide band-gap energy of TiO2, there is a restriction to the use of low wavelengths of the solar spectrum for its activation. To overcome this problem, various transition metals (e.g. Ag, Pt, Au, Cu, Fe etc) are used as dopants for the improvement of photocatalytic activity of TiO2 thin film [114]. Doping of TiO2 with noble metal nanoparticles (NPs) [115] is a currently intensively explored development in the field of titania-based materials for photocatalytic applications. The surface plasmon of coinage metal nanoparticles allows electron exchange to and from TiO2 nanocrystallites depending on the illumination conditions (visible light or UV, respectively) [116]. TiO2 and metal-doped TiO2 are prone to a variety of applications such as catalytic gasphase conversion of molecules or photo-oxidation of organic molecules in aqueous solutions [117]. For the first aspect, titanium dioxide exhibits a very rich surface organic chemistry, due to the ability of titanium cations to exist in different oxidation states and coordination environments [118]. On the other hand, the interfaces between highly dispersed metal phases on the TiO2 substrate may deeply influence the macroscopic behaviour of these materials, in view of the modifications of the electronic structure, morphology and thermal stability of the metal [119]. The photocatalytic efficiency depends on the preparation method of the catalyst [120] that can influence significantly the composition and the size of the crystals and the surface distribution of hydroxyl groups [121,122]. Different synthetic approaches to the preparation of metal-doped titania have been reviewed by Sallard et al. [76, 123,124].

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3.2.4 Pt-TiO2 General introduction and motivations As already discussed,, [125] the presence of VIII group metals on the surface of the catalyst particles improves the rate of electron transfer to O2 and consequently has a beneficial effect on the photo-oxidation rate of organic species [126] It has been reported that the presence of Pt has a beneficial influence on the photocatalytic activity of TiO2 (rutile) [127], scarcely photoactive in aqueous ambient, and also on that of WO3 [128]. It has been hypothesised that the effect depends on the presence of both Pt(0) and Pt oxidised species and is related to their amounts and more or less homogeneous distribution on the catalyst surface. [129,130]. As the influence of the preparation method on the characteristics of catalyst is concerned, the sol-gel technique appears to be very interesting [82b, 131] because it allows one to obtain very small and highly defective TiO2 microcrystals which are suitable to be easily supported and show good electronic properties. It is worth noting, moreover, that TiO2 doped with various metals can be easily obtained with a single reactive step by sol-gel method [132]. Here, the sol-gel preparation of polycrystalline TiO2 loaded with Pt (1%) is reported. [133] The highly dispersed metal phase was obtained by dissolving, in the titanium alkoxide (titanium(IV) tetraisopropoxide) propan-2-ol solutions, a suitable Pt organometallic precursor (the Zeise’s salt) able to resist to the gelling process but sufficiently unstable to easily decompose to Pt nanoparticles during the first stage of drying treatment. Samples were treated under water vapour stream (Pt-TiO2-v), and subsequent annealed (at 450°C) under oxidizing (O2, Pt-TiO2-t.O2) or reducing (H2, Pt-TiO2-t.H2) conditions, or photocatalytically cleaned (Pt-TiO2-p).

XPS analysis To determine the sample chemical composition for Pt-TiO2 samples by X-Ray Photoelectron Spectroscopy, the BE values for the Ti2p, Pt4f and O1s lines as well as the elemental atomic percentages were measured as a function of the different treatment conditions. All the analysed samples show a single sharp component (FWHM, ca. 2.0 eV) centred at 458.8 eV for the main Ti2p3/2 component of the Ti2p peak. This position is typical for oxygen hexacoordinated Ti(IV) ions. No variations in the Ti2p3/2 BE value were observed after the thermal treatments, even under H2 atmosphere, indicating that titanium atoms in the TiO2 matrix do not undergo reducing processes. As regards O1s and Pt4f lines a significant broadening is observed for all the samples, thus suggesting the co-existence of different oxidation states and/or chemical environments. After peak deconvolution the spectra reveal splitting into two bands for the O1s and Pt4f signals. The O1s line exhibits the major component at 529.9 eV, typical of TiO2 network, whereas the second minor band at higher BE (531.2–532 eV) is mainly due to the presence of Ti OX (X=H, R) species [43]. Concerning platinum, the more intense Pt4f7/2 spin-orbit split component shows two bands which are associated to Pt(0) and Pt(II) species. Variations in the intensity of the oxygen- and platinum-related (see Figure 3.6) components are observable depending on processing conditions.

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Figure 3.6: XPS Pt4f regions of the Pt-TiO2 samples for the different annealing conditions: (a) under H2 flux; (b) under photocatalytic conditions; (c) under O2 flux.

As far as platinum chemical state is concerned, the broad profile of Pt4f signals and the high FWHM values indicate the co-existence of several platinum species in different oxidation states. After peak deconvolution, the spectra reveal splitting into two bands assigned to Pt(0) atoms (BE Pt4f7/2 ≈ 71.1 eV) and to Pt(II) (BE Pt4f7/2 ≈ 73.6 eV) species. Due to the small differences in the BE values, the attribution of Pt(II) to not well identified chlorinated species, arising from partial decomposition of the used Zeise’s salt, or to PtO and to Pt(OH)2, is not straightforward. However, Pt(II) organometallic derivatives are reasonably present in the samples treated at low temperatures, whereas PtO is formed when the powders undergo annealing at higher temperatures. The ratio Pt(0)/Pt(II) is almost constant and close to 1 for all the samples, only the Pt-TiO2-t.O2 and Pt-TiO2-p samples show a strong increase of the oxidised component (see Figure 3.6). Taking into account that platinum is present in the form of nanoparticles, as evidenced by XRD and TEM investigations, platinum atoms lying in the outer layers of the particles undergo preferential oxidation with respect to inside atoms, especially when exposed to strong oxidising conditions. Consequently, each Pt nanocluster can be described as a core of metallic atoms covered by an oxidised surface layer whose thickness varies depending on the different sample treatments. This work shows as the sol-gel method easily allows the preparation of nanocomposite materials consisting of inorganic matrices where a metal phase could be highly dispersed. This result was obtained in one-pot synthesis by an appropriate choice of suitable titanium alkoxide and platinum organometallic precursors. It is worth noticing that XPS measurements were very useful to investigate the surface and compositional features of the Pt-TiO2 photocatalysts, and to understand the chemical transformations occurring during their preparation starting from the precursors.

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3.2.5 Ag- and Au-TiO2

Ag-TiO2 General introduction and motivations In our work [134] we addressed the sol–gel preparation of mesoporous Ag-doped titania films by using commercially available AgNO3 and a non-ionic surfactant (Pluronic F-127) to increase its photocatalytic activity for degradation of organic pollutant. In situ degradation of the azo dye methyl orange in aqueous solution was performed in the presence of various pure and Ag metal ion-doped titania thin films, prepared with and without surfactants. The main aim of the study was to analyze the effect of non ionic surfactant (Pluronic F-127), Ag metal ion doping and their combined effect on the photoinduced hydrophilicity and photocatalytic degradation of ambient air contaminants (gas-solid interface), methyl stearate (solid-solid interface) and azo dye methyl orange (liquid-solid interface). The optical, morphological and hydrophilic properties of the thin films were characterized by UV–Vis spectrophotometer, atomic force microscope (AFM) and water contact angle technique, respectively; whereas the crystalline form, the specific surface area and the chemical state/composition were determined by X-ray diffraction, BET nitrogen sorption analysis and X-ray photoelectron spectroscopy (XPS), respectively.

XPS analysis The surface and the in-depth composition of two different samples produced with low and high temperature sols, (LTS-1, HTS-1) were analysed X-Ray Photoelectron Spectroscopy (XPS) in order to get information on the chemical state of the different species and on their in-depth distribution. In particular, the goal of the investigation was to assess whether the Ag dopant was homogeneously distributed in the titania host matrix and in which chemical state (Ag0 or Ag+) it occurred. The films are characterized by a relatively higher amount of Ag on the surface of the film (12% at. for LTS-1 (A) and 3.4% at. for HTS-1) whereas this amount remarkably decreases upon sputtering, reaching the values lower than 0.5% in the depth of the film. This hints at a segregation of Ag in the outer layers, as already observed in previous publications and ascribed to migration phenomena and/or the thermal annealing [135]. Concerning the chemical state of the species, the BE values for the Ti2p region range, for all the analyzed samples, in the interval 458.2-458.7 eV, which are values typical of titania. [16, 43, 136]. It should however be underlined that, upon sputtering, the Ti2p peak broadens, due to a partial reduction of Ti(IV) to lower oxidation state species. This can be in turn ascribed to a preferential sputtering of oxygen, leading to an oxygen depletion and consequently to a reduction of the metallic species. Concerning oxygen, the O1s region, peaked at 529.5-530.0 eV, values typical of O1s in titania [16, 43, 136], shows a shoulder at higher BE (about 531.0-531.7 eV), which can be ascribed to OH surface groups. The silver Ag3d region, displays the typical doublet with the main Ag5/2 component and the Ag3/2

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component shifted of 6 eV at higher BE. As far as the BE values are concerned, it is worth to distinguish the values on the surface of the samples and after the sputtering. In the latter case, the BE are likely to be also affected by the oxygen depletion induced by the preferential sputtering, and not to reflect a real chemical species of the species. By considering the BE of silver on the surface, where the amount of Ag has its highest values, regardless the sample and the thermal treatment, these have a value ranging between 367.0 and 367.3 eV, which witnesses the presence of silver in its Ag+ oxidation state, according to literature reference data, whereas the presence of metallic silver, characterised by higher BE values (368.1-368.3 eV) could be ruled out.

Au-TiO2 In this work, [76a] the single-pot preparation of mesoporous and highly crystalline titania films with high Au doping was described. As templating agent we used an amphiphilic block polymer poly(- ethylene-co-butylene)-block-poly(ethylene oxide) ‘‘KLE’’. As gold sources, we compared conventional AuCl3 with pre-synthesized, monodisperse Au113+ nanoclusters. The obtained results show that highly crystalline and well ordered structures can be attained even for doping levels as high as 4% in Au/Ti atomic ratio, which so far could be attained only by the more complex post-doping procedures. In addition, and surprisingly, we noticed that doping with Au11 3+ yielded materials with superior activities, likely because of the formation of TiO2 (B)–anatase mixed phases. Optimization of our procedure could therefore lead to simplified production of photocatalytic matrices with improved performances.

XPS analysis XPS was used to analyse the surface and in-depth atomic composition and the chemical state of the different species in the different TiO2 Au gold doped samples. The analysed samples were characterized by different amounts of gold (varying from 1 to 10 atom% as Au : Ti atomic ratio) and preparation routes (AuCl3 or Au113+ as gold precursor). In particular, our interest was focused on the distribution of gold NPs in the titania host matrix and in the investigation of the chemical state and environment of gold. An Au(111) single crystal was analyzed as a reference, the BE of the Au4f peak being at 83.8 eV. In all samples the Au 4f region (Figure 6B) was characterised by a very symmetric and narrow peak without any broadening, implying the presence of only one BE component. Although differences are seen among the samples, the BE values of the Au 4f7/2 component are in the expected ranges (83.1–83.5 eV), in agreement with the value of metallic gold of the Au(111) reference measured and also with the observed plasmon resonance typical of metallic gold. The presence of both Au(III) and Au(I) could be ruled out, since these two species are expected to be observed at higher binding energy (BE), i.e. 86.8–87.5 and 84.6–86.6 eV, respectively, thus suggesting that all of the gold atoms are reduced to the metallic state. In order to investigate whether gold was uniformly distributed in the host titania matrix, a depth profile was carried out. It is well known that, as already discussed, ion sputtering can

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induce noticeable changes not only in the topography, but also in the chemical composition of a multicomponent sample [16,28] mainly due to the well known phenomenon of preferential sputtering, i.e. the preferential depletion, upon sputtering, of one element with respect to the other ones (see Section 2.4). It could be argued that in this case this sputtering technique is not suitable to address the determination of the in-depth distribution of the Au. To avoid dramatic changes in the chemical composition of the analysed sample, we performed the depth profiling by accelerating the Ar+ ions by a low voltage (2 kV), i.e. by bombarding the surface of the films with moderate energy, and by using the same conditions for all the samples. Furthermore, we tried to minimize errors by evaluating not absolute values, but rather the trends of the atomic ratio between Au and Ti, which is less sensitive to compositional changes than the absolute value. On the basis of these considerations, we can consider the presented trends reliable. Although noticeable differences are evident among the samples, which can be traced back to the different compositions of the solutions used for their preparations, in all the measured samples annealed for longer time a higher amount of gold is detected on the surface and in the outer layers of the film, whereas upon sputtering a lower Au: Ti atomic ratio was found in the inner layers. As in the previous case, these differences can be either ascribed to phase segregation or to thermal annealing. The observed differences imply that the metal distribution in a metal oxide matrix also significantly depends not only on the nature of the precursor and synthesis route, but also on the thermal treatment.

3.3 MIXED OXIDES Introduction and synthetic approaches to mixed oxides The development of molecularly homogeneous multicomponent oxide-based glasses is a key concern in many fields of materials chemistry. In particular, a great interest is devoted to the microstructural features of such systems since phase separation may play a critical effect in determining their properties and applications. These systems can be conveniently approached starting from gels, whose molecular homogeneity can be transferred in the final materials and may greatly enhance their performances. The temperature of crystallization of a two component metal oxide is in fact strictly dependent on the homogeneity of the starting gel. For instance, in the case of Al2O3-SiO2 system, the only compound with a stoichiometric composition is mullite, 3Al2O3-2SiO2. In presence of a molecular level homogeneity, crystallization of tetragonal mullite occurs at 980°C, whereas whenever the gel is homogeneous only on the nanometer scale, a spinel phase crystallizes at the same temperature (980°C) whereas mullite is obtained only at 1200°C. In this context, novel multicomponent materials with enhanced mechanical, optical, thermal properties can be obtained by a proper combination of versatile synthetic route and of suitable precursor compounds. In the field of sol-gel derived materials, multicomponent gel evolves, after densification process and thermal annealing, to mixed oxide. However, in the synthesis of multicomponent

XPS as a powerful tool to investigate the surface properties of simple, doped….

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gel, problems often arises with respect to the composition and homogeneity of gels because of the differences in the rates of hydrolysis and the condensation of the various alkoxides used. [82b] With respect to multicomponent systems, different reaction rates can lead to the inherent inhomogeneity of system, leading to severe differences in reactivities, densifying, sintering, crystallizing, phase separation. Moreover, phase separation in binary systems such as, for instance, SiO2-TiO2, depends upon the relative tendencies for homocondensation (SiO-Si and Ti-O-Ti) and heterocondensation (Si-O-Ti), an optimization of the experimental conditions and process parameters in terms of either composition and aging times of the solution has to be pursued. To control the relative reactivities of the precursors and to match their hydrolysis rate, different strategies can be adopted [137, 138]: 1- formation of complexes in the starting solution; 2- controlled addition of water; 3- controlled prehydrolysis of the slowly reacting components; 4- choice of the proper solvent. By a careful variation of the different process parameters, the behaviour of the metal precursor during hydrolysis and condensation can be tuned and the evolution of the multioxide system can be tailored. The molecular structure, the chemical reactivity, the functionality of these precursors and of the derived gel can be controlled by varying both the ligand/metal molar ratio x and the hydrolysis ratio h = H2O/M. As a general rule, more condensed species are obtained as x decreases and h increases. For very low hydrolysis ratio, h