Science and Technology of Chemiresistor Gas Sensors [1 ed.] 9781617286650, 9781600215148

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Copyright © 2007. Nova Science Publishers, Incorporated. All rights reserved. Science and Technology of Chemiresistor Gas Sensors, Nova Science Publishers, Incorporated, 2007. ProQuest Ebook Central,

Copyright © 2007. Nova Science Publishers, Incorporated. All rights reserved. Science and Technology of Chemiresistor Gas Sensors, Nova Science Publishers, Incorporated, 2007. ProQuest Ebook Central,

Copyright © 2007. Nova Science Publishers, Incorporated. All rights reserved.

SCIENCE AND TECHNOLOGY OF CHEMIRESISTOR GAS SENSORS

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. Science and Technology of Chemiresistor Gas Sensors, Nova Science Publishers, Incorporated, 2007. ProQuest Ebook Central,

Copyright © 2007. Nova Science Publishers, Incorporated. All rights reserved. Science and Technology of Chemiresistor Gas Sensors, Nova Science Publishers, Incorporated, 2007. ProQuest Ebook Central,

SCIENCE AND TECHNOLOGY OF CHEMIRESISTOR GAS SENSORS

DINESH K. ASWAL AND

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SHIV K. GUPTA EDITORS

Nova Science Publishers, Inc. New York

Science and Technology of Chemiresistor Gas Sensors, Nova Science Publishers, Incorporated, 2007. ProQuest Ebook Central,

Copyright © 2007 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.

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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. LIBRARY OF CONGRESS CATALOGING-IN-PUBLICATION DATA Aswal, Dinesh K. Science and technology of chemiresistor gas sensors/Dinesh K. Aswal and Shiv K. Gupta. p. cm. Includes bibliographical references and index. ISBNH%RRN 1. Gas Detectors. 2. Nanotechnology. I. Gupta, Shiv K., 1930- II. Title TP754.A79 2006 681’.2—dc22

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CONTENTS

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Preface

vii

Chapter 1

Overview of Gas Sensor Technology Noboru Yamazoe and Kengo Shimanoe

Chapter 2

Chemiresistor Gas Sensors: Materials, Mechanisms and Fabrication Manmeet Kaur, D. K. Aswal and J. V. Yakhmi

1

33

Chapter 3

One-electrode Semiconductor Gas Sensors G. Korotcenkov

Chapter 4

Nanostructured Metal Oxides and their Hybrids for Gas Sensing Applications K. Kalyanasundaram, P. I. Gouma

147

The Dynamic Measurements of SnO2 Gas Sensors and their Applications Jinhaui Liu, Xingjiu Huang and Fanli Meng

177

Chapter 5

95

Chapter 6

Resistive Oxygen Sensors Avner Rothschild and Harry L. Tuller

215

Chapter 7

Tellurium Thin Films Based Gas Sensor Shashwati Sen, S. K. Gupta and V. C. Sahni

257

Chapter 8

Vibrating Capacitor Method in the Development of Semiconductor Gas Sensors János Mizsei

Chapter 9

Porous Silicon Based Hydrogen Sensors Niranjan S. Ramgir and Shekhar Bhansali

Chapter 10

Bases of Noise Spectroscopy for Enhancing Metallic Oxide Gas Sensors Selectivity Jean-Luc Seguin, Sami Gomri, Jacques Guerin, Khalifa Aguir

Index Science and Technology of Chemiresistor Gas Sensors, Nova Science Publishers, Incorporated, 2007. ProQuest Ebook Central,

297 333

351 373

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PREFACE Gas sensor technology has advanced remarkably during past few decades and has become one of the indispensable technologies for modern society. Varieties of gas sensors are commercially available and, using innovative ideas, efforts are being made to develop next generation gas sensors having very small size with very low power consumption. The ultimate model for this is probably given by the sensory organs of our own body, which are implanted finely and work well with a very modest amount of energy. In order to achieve this goal, it is essential that various aspects of gas sensors are seriously considered. These include understanding of gas sensing mechanisms, development of new materials and methods to synthesize them into selective sensors, innovations in nanostructured materials, measurement methods, microfabrication of sensors, exploring intelligent sensing systems, etc. This book imparts some of these issues pertaining to the chemiresistive gas sensors and, consists of ten chapters. The first chapter is an introduction, and provides an overview of different kinds of gas sensors that have been developed or proposed. The background of the sensor technology, gas sensors and their construction principles, commercially available sensors, and some proposals for the next generation technology have been discussed. The second chapter presents the basics of chemiresistor gas sensors, experimental determination of various sensor parameters and a literature review on the various existing chemiresistor materials, namely, metal-oxides and non-oxides. The various aspects of tinoxide sensors, such as, structure, additives, mechanisms and fabrication methods are discussed. Methods adopted for the fabrication of selective tin oxide gas sensors for H2S, NH3, NO and H2 are discussed. The third Chapter focuses on the fabrication and characteristics of one electrode gas sensors. Design features of one-electrode semiconductor gas sensors, specificity of their response to gases and application are described, and their characteristics have been compared with pellistor and conventional two electrode semiconductor sensors. Present and future requirements of these sensors and, typical characteristics of sensors supplied by some of the manufacturers are discussed. The possibilities of further development in this area are also presented. Chapter four reviews the processing techniques used to fabricate gas sensing nanomaterials, such as, SnO2 nanoribbons and MoO3 nanowires. The electronic properties and gas detection behavior of these nanostructures are discussed. Commonly observed p- to n- type transition phenomena in nanostructured metal oxides is explained on the basis of the

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viii

Dinesh K. Aswal and Shiv K. Gupta

sensing mechanism involved in gas detection and the size-related effects of nanomaterials. Insights are given on the future challenges in resistive gas sensing and how nanotechnology may be helpful in overcoming these burdens. Chapter five describes the measurements and analyses of the dynamic response of a gas sensor. The detailed theory of the dynamic measurement is discussed. The influencing factors, such as, applied potential, heating waveform (rectangular, triangular, saw-tooth, pulse, sinusoidal, and trapezoidal) and frequency of heater voltage on the dynamic responses are discussed. A new approach for fast detection of organophosphorus pesticides in actual vegetables using solid phase microextraction (SPME) coupled with tin dioxide gas sensor is presented. Chapter six focuses on sensors, operating on the chemoresistive principle, designed to monitor and detect oxygen. The figures of merit of oxygen sensors are reviewed and related to the mechanisms controlling defect formation and transport. Temperature cross-sensitivity is identified as a key limitation of such semiconductive oxygen sensors and, as a consequence, attention is focused on the source of temperature sensitivity and means for its minimization. The perovskite oxide, SrTi0.65Fe0.35O3-y (STF35), has been identified as very promising candidate given its near intrinsic zero temperature dependence and relatively high sensitivity to partial pressure of oxygen. Special attention is focused on the special features of solid solution systems, such as STF, in which additives such as Fe, at high concentrations, can no longer be viewed as dopants and, as such, systematically induce major modifications in the defect equilibria as well as the electronic band structure. The long term stability of such sensors is examined in relation to oxygen partial pressure excursions and potential interactions with substrate materials and fuel contaminants such as SO2. Chapter seven deals with various gas sensing aspects of elemental semiconductor tellurium thin films. Nature of bonding, crystal structure and doping in Te has been discussed. Effect of grain size and defects in crystallites on gas response has been investigated in detail and interaction mechanism of the films with different gases has been reported. Chapter eight reviews the capabilities of the surface potential sensitive vibrating capacitor in conjunction with semiconductor gas sensors. This chapter starts from a simplest adsorption induced work-function tests and extends to scanning vibrating capacitor yielded olfactory pictures and other chemical pictures. Olfactory pictures from semiconductor surfaces offer new opportunities to improve the selectivity of gas analysis. Chemical pictures from thin films and other surfaces may reveal the inhomogeneities of the gas sensor layer technology, and thus making scanning vibrating capacitor an excellent tool for solid state surface characterization. Chapter nine describes in detail the method of porous silicon fabrication and its utility in designing hydrogen sensors. Moreover, the limitations and scope of different sensing modalities (for sensing H2) leveraging the PS architecture are discussed. The final chapter ten focuses on investigations aiming to develop theoretical models and experimental methods for applying noise spectroscopy to metallic oxide gas sensors. The possible sources of noise in a metallic oxide sensors and their coupling with the sensing mechanisms are analyzed. A theoretical description of adsorption-desorption noise in metal oxide gas sensors is presented. Application of the noise model for simulating the oxygen chemisorption is described. The validity of the method, its limitations, and directions for its extension to more general cases such as gas mixtures are discussed.

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Preface

ix

There is certain amount of overlap in the contents of some chapters, and this is expected as the individual authors had freedom to write their chapters without consulting others. We thank all the authors who have contributed to this book. We are greatly indebted to Dr V.C. Sahni and Dr J.V. Yakhmi for their support and guidance during the course of this work. At Nova Science Publishers, we wish to thank Ms. Maya Columbus who encouraged us to take this project. Finally, we are grateful to our wives Neelam Aswal and Vandana Gupta for their continuous espouse.

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Dinesh K. Aswal Shiv K. Gupta

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In: Science and Technology of Chemiresistor Gas Sensors ISBN: 978-1-60021-514-8 Editors: D. K. Aswal, S. K. Gupta, pp. 1-32 © 2007 Nova Science Publishers, Inc.

Chapter 1

OVERVIEW OF GAS SENSOR TECHNOLOGY Noboru Yamazoe∗ and Kengo Shimanoe Faculty of Engineering Sciences, Kyushu University Kasuga-shi, Fukuoka 816, Japan

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ABSTRACT Various kinds of gas sensors have been developed or proposed and, some of them have been recognized as key devices to monitor, handle or control of gases for various purposes. Overview of gas sensor technology is tried through brief description of the background of the technology, introduction of representative gas sensors and their construction principles, introduction of a variety of gas sensor markets, mature through challenging, introduction of fundamental aspects of semiconductor gas sensors as a case study, and presentation of some proposals for next generation technology.

1. BACKGROUND OF GAS SENSOR TECHNOLOGY The atmospheric air we live in contains numerous kinds of gaseous species, some of which are vital or useful to our life while many others are hazardous more or less. Apart from the gases of natural origins, there are many kinds of gases of artificial origins such as combustion processes. Vital gases like O2 and humidity should be kept at adequate levels, while emission of hazardous gases should be controlled at harmless levels. Such control or handling of gases can be made possible only when one can know the concentration of each gas in real time. Gas sensors have been developed as those devices which respond to selected gases in situ and generate electric (or optical) signals dependent on their concentrations in real time. The concentration ranges concerned are largely different depending on the kinds of gases and purposes of gas sensing. ∗

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Figure 1 illustrates critical concentrations of interest for some selected gases as well as the respective concentration ranges covered by gas sensors currently. Solid lines indicate the ranges of concentration safely covered by commercial gas sensors, while broken lines indicate those reportedly covered in laboratory test. Lower hydrocarbons like CH4, propane and butane or H2, used popularly in domestic homes as fuels, are not poisonous but can cause a miserable disaster to people once they leak out into air and explode. The major role of gas sensors in this case is to protect people from the disaster by detecting the gas leakage as early as possible when necessary. The gas sensors are usually set to give an alarm when the concentrations of the leakage gases have exceeded 1/10 of lower explosion limit (LEL, usually a few to several %). For toxic gases like CO and H2S, on the other hand, alarming should be carried out well before their concentrations reach fatal levels. Safety from a sudden accident is a main purpose of using sensors in these cases. For offensive odors (NH3, H2S, etc), volatile organic compounds (VOCs, HCHO, CH3CHO, aromatics, etc) and air pollutants (NO2, SO2, CO2, O3, etc), critical concentrations of interest (standards) have been determined from a viewpoint of securing amenity, health or environment during rather long time exposure. The standards are fairly low as indicated by several kinds of marks in Figure 1. Especially the standard for some VOCs like benzene has been set as low as 1 ppb, a concentration level far out of reach by any of current gas sensors.

Figure 1. Various target gases and their concentration ranges covered by commercialized gas sensors (solid line) and in laboratory test (broken line). Star marks indicate the concentrations registered by respective laws in Japan. Science and Technology of Chemiresistor Gas Sensors, Nova Science Publishers, Incorporated, 2007. ProQuest Ebook Central,

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Overview of Gas Sensor Technology Table 1. Classification of representative gas sensors Type Semiconductor

Device structure

Resistor (block, film)

Responding property Resistance

Sensing materials SnO2, WO3, ・ ・ ・ metal porphyrines

Target gases Inflammable gases, CO, H2S, NH3, NO2, O3, NO, ・ ・ ・

TiO2 O2 (A/F control) (high temperature operation)

Gas sensitive gate-MISFET

Pd gate WO3 (semiconductor) gate Threshold voltage LiNO2 (ionic conductor) gate Cellulose (dielectric) gate

Solid electrolyte

Gas concentration cell Auxiliary phaseattached gas cell (TYPE III)

EMF

Mixed gas cell (short circuit) + gas diffusion hole

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Dielectric material

Insulator

Piezoelectric crystal

Resistor

Resistor

Adsorbentcoated quartz oscillator Adsorbentcoated SAW device Gas sensitive wave guide

Optical fiber Optical fiber + gas reaction layer

Catalytic combustion

Catalyst bead + Pt coil (imbedded)

H2O

Stabilized zirconia K2CO3 NASICON

O2 (A/F) CO2 Na vapor

LiNO2-attached NASICON Li2CO3-attached 〃

NO2 CO2

Electrolytic cell Electrolytic current Stabilized zirconia + gas diffusion layer (hole) Mixed gas cell (open circuit)

H2, NH3 NO2 NO2

O2, H2O, NO, NO2

nH2O Sb2O5・ (H+ conductor)

H2, CO

Short circuit current

Ion exchange membrane (H+ or OH- conductor)

CO

AC resistance (due to capacitance change)

Some organic polymers

Mixed potential

AC resistance (due to ionic conductance change) Resonant frequency

WO3-CuO composite MgCr2O4-TiO2 composite

H2O

Organic polymers

H2O

Various adsorbents

H2O, odorants, organic compounds ・ ・ ・

Delay time

Light absorption

WO3 wave guide

Light absorption Fluorescent light (Emission, extinction)

Reaction layer: Dye metal complex, protein, ・ ・ ・

Resistance of Pt coil (change due to heat of combustion)

Pt-supported alumina bead

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H2 Alcohol, O2, NO, ・ ・ ・ (in liquid phase)

Inflammable gases

4

Noboru Yamazoe and Kengo Shimanoe

It is remarked that there are many gases missing in Figure 1. Probably most important ones are O2 and humidity, sensors for which have been used extensively for car emission control and air conditioning, respectively, as described later. The gases generated or consumed by biological or physiological activities (respiration, photosynthesis, disease, etc.) of our bodies, animal or plants would be important for diagnosing health conditions. Similarly, the gases emitted from foods and food materials during ageing, rotting or cooking would give important information about the freshness and quality of those origins. As suggested from these examples, various gas sensors are needed for various purposes covering safety, health, amenity, environmental preservation, energy saving, health care and diagnosis, senior people support, foods, cooking and so on. In reality, however, only a limited number of gas sensors have been developed up to the level of practical use, many others still remaining under investigation. Gas sensor technology has thus grown to be indispensable for supporting several aspects of our civil life already, but as a whole it has plenty of room for further growth in the future. It is desired to exert efforts toward innovation of gas sensors as well as extension of their applications in practice.

2. TYPES AND PRINCIPLES OF GAS SENSORS

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2.1. Types of Gas Sensors A gas sensor is designed to transform chemical information (concentration) of a particular gas present in the designated space into an electric (or optical) signal. As critical requirements, it must be compact enough to be installed in a commodity appliance and cheap enough to be available for citizens. From these requirements, intrinsic molecular properties such as molecular weight and optical absorption can not be used conveniently for gas sensors unlike the case of usual analytical instruments. Instead, gas sensors usually adopt indirect methods utilizing functional properties of materials and/or devices. Semiconductors, ionic conductors (solid electrolytes), piezoelectric crystals, catalytic combustion catalysts, optical fibers, and other functional materials have been introduced into gas sensors. Table 1 summarizes various types of gas sensors using these functional materials. Semiconductor gas sensors are divided into two main groups using metal oxides and Si, for which sensor devices are constructed in form of resistor and MISFET (Metal/Insulator/Semiconductor/Field Effect Transistor), respectively. An example of resistor type device is shown in Figure 2, while that of another type will be shown later. In the latter group, various kinds of materials can be introduced to gas sensitive gate, allowing detection of various gases. Although, the former group is dominating currently, importance of the latter group will increase in the future. Solid electrolytes (ionic conductors) have conventionally been used to construct a gas cell or electrolytic cell. Actually, however, other modifications have been developed recent years. Among them, auxiliary phase-attached type (Type III) can be considered as combining a gas-sensitive half cell with a gas insensitive half cell [1, 2]. Importance of this type arises from the capabilities of sensing oxidic gases like CO2, NO2, and SO2. Mixed gas cell has been developed as a commercial CO sensor operative at temperature as mentioned later. Insulative ceramics or polymers have been utilized as relative humidity sensing materials successfully. Physical adsorption (condensation) of humidity into

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Overview of Gas Sensor Technology

pores of ceramics or network of polymers enhances ionic conduction through pores or network, which is transformed into a change in AC resistance. Catalytic combustion sensor utilized heat of combustions for detecting inflammable gases. Typically it is composed of a combustion catalyst (Pt-loaded alumina) bead and an imbedded Pt coil (See Figure 2). Electrical resistance of the Pt coil increases upon exposure to an inflammable gas, which is measured on a wheat stone bridge circuit by referring to a gas-insensitive (compensation) bead.

Sensing material

Support Catalyst

Pt wire

Au electrode

(a) Resistor type

(b) Catalytic combustion type

Figure 2. Schematic drawing of gas sensors for (a) resistor type and (b) catalytic combustion type.

The other functional materials have been introduced into gas sensors based on interesting working principles. Most of them are still under way of development with regard to feasibility in practice or commercialization.

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2.2. Receptors and Transducers – Construction Principles In principle, gas sensors are constructed by combining two key-functions, i.e., a function to recognize gas molecules in problem (gas recognition) and another to transduce the recognition into a signal output (signal transduction), as schematically shown in Figure 3. Gas recognition is usually carried out through the interaction (adsorption, reaction, or electrochemical reaction) of gas molecules with adequately chosen materials or electrodes (receptors). The interaction exerts physical or chemical effects on or around the receptors, such as formation of adsorbates or reaction products, generation of heat of reaction, changes of the receptors in mass, dimension, surface properties or bulk properties, and generation of electrode potential or occurrence of electrode reactions. Any one of these effects can be chosen to be converted into electrical or optical signals through adequately chosen material properties or device properties (transducers). The presence of receptors and transducers are obvious in same gas sensors. In a potentiometric oxygen sensor using stabilized zirconia, for instance, the zirconia/electrode interface is an electrochemical receptor to oxygen, while the electrochemical cell formed between sensing and reference electrodes is a transducer which generates electromotive force as a sensor signal. In a quartz oscillator gas sensor, an adsorbent covering the electrode at one side acts as a receptor, while the resulting mass increase is transduced into a decrease in resonant frequency by the oscillator (transducer). In some sensors, however, discrimination between receptor and transducer is not always so obvious, as will be illustrated for the case of resistive semiconductor gas sensors later.

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Noboru Yamazoe and Kengo Shimanoe Gas sensor Gas molecules

Receptor (chemical)

Signal Transducer (physical)

Gas sensor

Receptor (function)

Transducer (function)

Signal

Semiconductor

Oxide surface (Change in work function)

Grain boundaries (Work functiondependent resistance)

Resistance change

Solid electrolyte (gas concentration cell type)

Electrode (Change in electrode potential)

Chemical cell (Comparison between sensing and reference electrodes)

Piezoelectric Crystal (Quartz oscillator)

Adsorbate attached (Increase in mass of electrode)

Quartz oscillator (Mass-dependent resonant frequency)

EMF

Resonant frequency change

Figure 3. Construction of gas sensors with receptor and transducer.

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It should be noted that the gas recognition is based mainly on chemical processes while the signal transduction is on physical processes. This means that the former is more susceptible to the conditions applied, especially temperature. Sensitivity, selectivity, and rate of response are usually dependent strongly not only on the kind of receptor used but also on the temperature applied. In many gas sensors, a heating facility is installed to optimize the temperature of receptors used. In addition, polarization also becomes an influential factor in the electrochemical sensors of limiting current type.

2.3. Modes of Gas Sensing Gas sensors have so far been developed mostly by using those receptors which interact with gas molecules reversibly. In this context, there have been three modes in the way of gas sensing, i.e., equilibrium, steady state and complete reaction. Recently, however, yet a new mode, so-called accumulation mode, which utilizes irreversibly interacting receptors, has been investigated. Each mode is described briefly below.

Equilibrium Mode Gas adsorption on solid surface is usually governed by adsorption equilibrium. If one selects a receptor with a moderate adsorption constant to the gas in problem, the amount of adsorption can be well correlated with its concentration in the gas phase so that a gas sensor is available by combining it with a transducer. This mode has been adopted in various adsorption-based sensors such as humidity sensors, piezo-crystal gas sensors and semiconductor NO2 sensors. Similarly, electrode potential is governed by the equilibrium of electrochemical reactions so that it can be used as a receptor. This mode is operative in solid

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Overview of Gas Sensor Technology

7

electrolyte-based potentiometric sensors and FET sensors for oxygen, oxygenic gases (e.g., CO2 and NO2), halogens, etc.

Steady State Mode This mode is well illustrated by semiconductor sensors for an inflammable gas. The gas reacts with the surface (adsorbed) oxygen of oxide semiconductor (receptor) while the consumed oxygen is supplied back from the gas phase. The surface thus reaches a steady state. The resulting change of oxide surface property (work function) is transduced into a change in electric resistance. A similar situation appears on the sensing electrodes of potentiometric oxygen sensors when a reducing or oxidizing gas is coexistent with oxygen. In this case, electrochemical reactions of the gas and oxygen take place simultaneously, resulting in the generation of mixed potential at the electrode. This principle has been used in mixed potential type sensors for inflammable gases and nitrogen oxides.

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Complete Reaction Mode Under highly reactive conditions where all the gas molecules in problem are consumed completely over a receptor, the rate of consumption (reaction) is determined by the rate of gas supply. When the concentration is sufficiently small, the rate of gas supply is proportional to its concentration in the surrounding atmosphere. The rate itself or a secondary quantity induced can be used a sensor signal transduction. Catalytic combustion type sensors for inflammable gases are a typical example. This mode can be used conveniently for an electrolytic cell put under polarized conditions, as shown in Figure 4 [3]. The cell is covered with a sheet with a gas diffusion hole or a gas-permeating membrane to secure gas supplylimiting conditions. When the rate of gas supply and polarization are well balanced, limiting current is made proportional to gas concentration outside exemplified for the limiting type oxygen sensor.

Figure 4. Limiting current type oxygen sensor. (a) Principle of oxygen sensing by using electrochemical oxygen pump cell through pinhole. (b) Current-voltage characteristic of a limiting current-type oxygen sensor at 700 oC in various O2 concentrations. Science and Technology of Chemiresistor Gas Sensors, Nova Science Publishers, Incorporated, 2007. ProQuest Ebook Central,

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Noboru Yamazoe and Kengo Shimanoe

Accumulation Mode In case the affinity between gas molecules and receptor is extremely strong, the gas molecules are adsorbed irreversibly and the resulting adsorbates are accumulated on the receptor progressively with time up to its full capacity. If an adequate transducer is available to monitor the accumulated adsorbates, one can deduce the time-dependent gas concentration as a first derivative of the accumulation curve. The receptor after use is regenerated or disposed. This mode seems to be suited for detecting very dilute gases and the resulting sensor can also be used as a dosimeter. Problems of this mode are associated with the selection of receptors and transducers. Combinations of a complexes-forming receptor and an optical transducer have been proposed.

3. BIRTH AND GROWTH OF GAS SENSOR TECHNOLOGY

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3.1. Brief History Some gas sensors, such as a galvanic oxygen meter using electrolytic solution and a methane detector of catalytic combustion type, had in fact been developed well before 1960. However, those were aimed at being used by professional experts working in quite limited fields. Most of gas sensors aiming at non-professional use were inaugurated in 1960s or later. As the first one of such devices, a resistive gas sensor using oxide semiconductor for detecting inflammable gases was reported in 1962 by Seiyama et al [4]. Through an independent exploration, Taguchi invented a gas leakage alarm using the same type device to protect people from disasters caused by mal-functions of gas appliances in domestic homes, and applied for a patent in the same year [5]. The semiconductor gas sensors were subjected further to several revisions including the addition of sensitizers (noble metals like Pd) before being commercialized in about 1970. In the market the sensors were proven to be quite effective in decreasing disastrous gas-accidents dramatically and thus established their position as a safety device. This success triggered extensive investigations on such sensors targeting various inflammable or toxic gases. As is known well, the oxygen sensor using stabilized zirconia electrolyte has been most important for car emission control. Originally the electrochemical cell using the same solid electrolyte was developed to measure standard free energy of formation of metal oxides or activity of oxygen in molten metals [6]. Probably the first attempt to apply this type cell to gas phase was made in 1963 by Goto et al [7]. They showed that an electrochemical cell devised by attaching a Pt sensing electrode and a solid reference electrode (metal-metal oxide mixture) to a CaO-stabilized zirconia tube generated EMF very stably depending on the oxygen partial pressure in various gas ambient at high temperature. It is important that a simple Pt electrode placed on the zirconia surface could work well for oxygen sensing, because it enables one to construct an oxygen concentration cell by simply attaching such electrodes at both of reference and sensing sides. In fact, such a cell was constructed and tested for oxygen sensing performances in various atmospheres including combustion products in the article of Lawrence et at published in 1969 [8]. The significance of this cell was soon recognized when Muskie act, requiring drastic decreases of air pollutants from car

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Overview of Gas Sensor Technology

emissions, was legislated in USA in 1970. Oxygen sensing principle and actual device structure of oxygen sensor for car are shown in Figure 5, as a result of electrochemical reaction of O2 at sensing at reference electrodes, EMF is generated in proportion to logarithm of the ratio of O2 partial pressures at respective electrodes. Reference electrode is exposed to open air, while sensing one is to emissions between engine and catalyst. EMF lowers almost stepwise in the vicinity of stoichiometric A/F (mass ratio) whereas three kinds of air pollutants are eliminated by catalytic reactions in a narrow range of A/F called “window”, as shown in Figure 6. Therefore A/F ratio is controlled with the windows by a feed-back control system incorporating the oxygen sensor. In this regard, the oxygen sensor is now one of the key devices for car industry. Oxygen concentration cell

E O2-

Air

O2

Zirconia 1/2O2 + 2eE=

RT 4F

ln

O2-

PO2 (s) PO2 (r)

AIR-FLOW METER

EXHAUST GAS ENGINE

AIR

3-WAY CATALYST OXYGEN SENSOR

INJECTOR EFI CONTROLLER

CONVERSION EFFICIENCY (%)

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Figure 5. Zirconia oxygen sensor. (a) Principle of EMF generation by oxygen concentration cell. (b) Schematic drawing of oxygen sensor used for A/F control of gasoline engines. WINDOW

100

HC

STOICHIOMETRIC A/F

0

12

O2 sensor response

16 14 AIR-FUEL RATIO A/F 0.9

3way-catalyst emission control system

NOx

CO

1.1 1.0 EXCESS AIR λ

Exhaust emissions with 3-way catalyst

Figure 6. Engine emission control system using oxygen sensors and 3-way catalyst (a) and catalytic conversion efficiency of three pollutants as a function of A/F ratio.

Humidity sensors have also been used popularly for various aspects of civil life ranging from air-conditioning to protection of electronic instruments from dew formation. Unlike classic hygrometers, their history is rather short. In 1976, Matsusita Electric Industrial Co. commercialized electronic ovens attached with a ceramic humidity sensor for automated

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cooking [9]. The sensor made of a porous composite of metal oxides was shown to change its AC impedance with a change in humidity at elevated temperature. This event stimulated broad interest to humidity sensors in Japan. Various humidity sensors including dew point sensors have been developed by using ceramics as well as polymers for application at room temperature or elevated temperature. The above three groups of sensors have comprised substantial part of gas sensor technology so far, although many other groups have been developed or proposed. It may be possible to draw a few points from this brief overview. First, no doubt gas sensor technology has advanced in the areas where seeds and needs are matched well as every technology is so. It is interesting to note that the gas components targeted in the technology, i.e., inflammable gases, oxygen and water vapor, are those which are intimately encountered in our daily life. It suggests that new needs in the future would also be deeply associated with our daily life. In this sense, next candidates will be sought in the fields related to health, energy, environments, foods, agriculture, etc. Second, seeds and needs do not always emerge simultaneously. Often a good seed cuts open a new need or encounters an unexpected need. In this regard, it is most important for researchers to create a sound gas sensor which works well under specified, sometime harsh, conditions. Third, gas sensor technology has so far been contributed much by scientists and engineers in Japan. Many gas sensors have been invented or subjected to innovations by Japanese researchers. In addition, International Meeting on Chemical Sensors, now rotating world-wide as a serial conference held every two years, was initiated in Japan in 1983 under the leadership of the late Professor Seiyama. Probably this reflects strong passion Japanese had toward technologies after the defeat in the Second World War. With almost everything lost, there were no ways to recover other than having high spirits to raise new technologies in Japan and sensors appear to have been selected as one of such new technological targets.

3.2. Mature Markets Gas sensor technology expanded largely since 1980s based on the three groups of gas sensors just mentioned. Although no precise statistics are available, current yearly production of gas sensors in the world is estimated to be around 20 million pieces of semiconductor sensors, about 30 million sets of zirconia oxygen sensors for car emission control, and far more of humidity sensors. These figures are remarkable, though not so big as those of physical sensors (the whole sale of chemical sensors still remains less than one tenth of that of total sensors). These figures seem to assure that gas sensor technology has been fully recognized to be very important in our society. At the same time, conventional markets for these sensors have become almost matured. As a matter of course, those markets have witnessed heavy competitions not only in sensor production cost but also among the kinds of sensors. Once a new competitive sensor appears, it can replace the old one. It is true that gas sensor technology has advanced through such competitions. Two examples are shown below to illustrate that improvements and innovations are always needed even in mature markets. The market for gas leakage alarms were originally opened and filled with semiconductor sensors. In European countries, however, catalytic combustion type sensors have been popular traditionally. They have revised the classic sensors sufficiently to replace the semiconductor sensors by the classic ones.

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Another example can be picked up for CO sensors, which are installed indoor or in garages as a safety device to gas appliances or car emissions, respectively. This market was also opened by semiconductor sensors. A competitor in this market has turned out to be a limiting current type sensor using proton conducting membrane, which is schematically shown in Figure 7. Apparently this sensor is an extended version of an electrochemical device we proposed about twenty yeas ago [10]. In an effort to realize gas sensors operative at room temperature, we introduced a proton conducting membrane into the device. As we found, upon exposure to H2 or CO, the device generates mixed potential at the sensing electrode under open circuit conditions, while current flows through the membrane under short circuit conditions. The CO sensor above has been derived from this device by attaching gas diffusion holes and water reservoir (to keep sufficient humidity inside). Being capable of operating at room temperature, this sensor has no heater attached and so can be driven by batteries (cordless sensor). This advantage appears to have made the sensor favored by many users, especially in USA. Currently this type and semiconductor type occupy almost equal shares in the CO sensor market.

CO A

H+ conducting membrane Water

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Sensing Electrode（ SE）

Counter Electrode（CE）

Electrode reactions under open circuit conditions SE:CO + H2O → CO2 + 2H+ + 2e1/2O2 + 2H+ + 2e- → H2O (Mixed potential)

CE: 1/2O2 + 2H+ + 2e-

H2O

Figure 7. Limiting current type CO sensor operative at room temperature based on mixed gas cell and diffusion hole.

3.3. Emerging Markets In addition to mature markets just mentioned, new markets are emerging for gas sensors. Most of them are concerned with inflammable or odorous gases at rather low concentration levels so that semiconductor sensors are frequently applied. Here some major examples are briefly introduced.

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Air Quality Sensor In a closed space like living rooms and car rooms, the quality of air is lowered gradually during use with increases in the concentrations of CO2, body- or food-related gases, cigarette smoke, dust and so on. As a way to keep clean air without carrying out ventilation, air cleaners have been commercialized and are being sold up to a rather large quantity especially in Japan. When necessary, the cleaner pumps air inside automatically to eliminate those pollutants except CO2 by filtration and catalytic oxidation. A semiconductor sensor responding to the organic pollutants is installed inside as air quality sensor. Auto-damper sensor Car rooms are designed to ventilate outside air constantly through air-inlet. However, this induces inconveniences when cars run through tunnels or dirty air regions. To solve this problem, an auto-damper system (ADS) has been developed. It adopts two types of semiconductor gas sensors which respond to inflammable gases (typically hydrocarbons) and NOx (typically NO2), respectively. As shown in Figure 8, it opens or closes the air inlet depending on whether the outside air is clean or polluted. Production of ADS has been increasing year after year, exceeding three millions in quantity in 2001 [11, 12].

600

500 35

400

300

30

200 25

ADS

100

Production of ADS/ x 106

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NOx, HC etc.

Production of car/ x 106

40

Clean air 20

93

94

95

96

97

98

99

00

01

0

Year

(b) Figure 8. Working principle of auto damper system (a) and statistics of its supply for cars per year (b).

Gas Sensor -combined Fire Alarm Fire alarms have traditionally based on a smoke detector or a heat detector. In the event of fire, however, various inflammable gases are produced and, in particular, H2 among them diffuses far more rapidly than smoke or heat does. It has been claimed that detection of the gases can be more useful than the conventional methods for earlier detection of fires in many case. Seemingly in correspondence with this claim, semiconductor gas sensors have begun to be incorporated into the conventional fire alarms in Japan, as shown in Figure 9 [11, 13]. The market of the combined type fire alarms is expanding rapidly in recent years.

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Figure 9. Statistics of yearly sales for conventional and combination type fire alarms.

Quality-discerning Odor Analyzer Generally speaking, response of semiconductor gas sensor depends on the kind and concentration of the target gas. Therefore one can not obtain information on both simultaneously from the response of a single sensor. More precisely, however, the response also depends on two molecular properties of the gas, reactivity and diffusivity, resulting in a volcano-shaped correlation against temperature, as described later. For the same sensor, the temperature at the top of volcano as well as the shape of volcano can be assumed as characterizing the target gas based on the above molecular properties. Likewise, when plural sensors largely different in surface catalytic property or microstructure are available, their response ratios at a properly selected temperature can also serve as a guide to judge the molecular properties. Based on this concept, an odor analyzer which discerns the quality and strength of odors has been commercialized [14]. It combines two types of gas sensors (A and B), of which A is more sensitive to light odorants (NH3, H2S, etc) than to heavy odorants (aromatic compounds, large unsaturated hydrocarbons, etc) and B shows just opposite sensing properties. As shown in Figure 10, the ratio of the responses of two sensors represents the quality of the odorant while the sum of their vectors gives a measure of the strength of the odor (or concentration of the odorant). This analyzer has been claimed to be effective in measuring the strength of unpleasant odors in the field as well as finding out the origins of the odors. It is noteworthy that this kind of information can be acquired by using just a couple of sensors.

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1200

clove

perfume

Output of A-sensor /mV

1000 Light gas (standard)

800 600

cardamon peppermint oil 400 leek breath mint cinnamon 200 onion 0 0

200

400

600

methyl mercaptane

Heavy gas (standard) 800 1000 1200

Output of B-sensor /mV

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Figure 10. Signal outputs of dual sensors of quality-discerning odor analyzer in two dimensional space (Futaba electronics).

CO2 Sensor Carbon dioxide is harmless at low concentration levels, but at higher levels it exerts undesirable effects on human bodies. In Japan, it is legislated in the Ordinance on Health Standard in the Office that its concentration should be kept below 1000 ppm in the work space. On the other hand, it is an indispensable source of carbon in the photosynthesis of carbohydrates for plants. Artificial addition of CO2 into green-house is frequently carried out to increase sweetness of fruits. In this regard, CO2 sensors have had potential needs. Recently optical CO2 sensors have been commercialized, though those seem to be rather too costly to proliferate in civil life. More recently, far less costly CO2 sensors (Figure 11), for which a conventional solid electrolyte cell is attached with an auxiliary phase of metal carbonate by the sensing electrode side (so called Type III solid electrolyte sensor [1, 2]), have been developed and brought into market in practice. The latter sensors exhibit good CO2 sensing capability as shown in Figure 12, a performance almost as good as the former ones do. Those have been applied to air conditioners for auto-ventilation. Market is not so big right now, but it is anticipated to expand as energy problems deepen. Apart from this, a big need is about to appear for CO2 sensors. Car industries are going to replace present car air-conditioners by new type ones using CO2 as a heat mediator in very near future. For safety purpose, CO2 sensors are demanded for the new type in order to detect leakage of CO2 if it happens. The two types of sensors mentioned above are being put under rigorous field test in car industries.

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Au mesh

Carbonate

NASICON Inorganic adhesive

Pt black

Quartz tube

Pt-mesh Pt wire

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Figure 11. Structure of Type III CO2 sensor [15].

Figure 12. Sensing capabilities of Type III solid electrolyte sensors to CO2, NO2 or SO2 at respective operating temperatures indicated [16].

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NOx Sensors NOx, usually an abbreviation for NO and NO2, is one of the most typical air pollutants, mainly produced artificially from the combustion of fossil fuels. Although NOx emissions from fixed combustion facilities (e.g. steam power plants) and gasoline-cars have been reduced drastically in advanced countries thanks to the advances of NOx removal techniques, the present control levels are considered to be still far from ideal level especially for gasolinecars. In addition, NOx removal techniques are still under investigation for diesel car emissions. Onboard NOx sensors are thus badly needed not only to be sure about the emission levels but also to avoid over-specifications of NOx removal processes. These sensors have to work under harsh conditions of high temperature and various disturbing gases. As such sensors, two types of electrochemical devices (A and B) fabricated by laminating zirconia slabs have been commercialized or proposed so far. In device A (Figure 13), the emission having flown in the first cavity is deprived of free oxygen by electrochemical pumping while NOx left is reduced electrochemically in the next cavity. The total NOx concentration is measured from the reduction current. In device B (Figure 14), the emission is subjected sequentially to removal of reducing components by oxidation over Pt catalyst in the first cavity, electrochemical oxidation to convert NOx into NO2 in the next cavity, and exposure to sensing electrode under fixed partial pressure of oxygen in the final cavity. Mixed potential is generated at the sensing electrode as a function of NO2 concentration and so the total NOx can be measured from the device EMF. Both devices aim particularly at application to diesel car emission control. Outer electrode Second diffusion path First diffusion path Second internal cavity First internal cavity Inner electrode Inner pumping electrode

ZrO2-1

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O2-

Part I

Part II

Part III

ZrO2-2

O2-

ZrO2-3 ZrO2-4 ZrO2-5 ZrO2-6

Measuring electrode Reference electrode

V

A 450 mV

Heater

Air duct

Outer pumping electrode

150 mV

Figure 13. Limiting current type NOx sensor using a stack of stabilized zirconia slabs (after Kato et, al. [17]).

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Overview of Gas Sensor Technology Oxidational-catalyst electrode Counter electrode

Pt heater

NOx conversion electrode NOx sensing electrode (Cr2O3) Air duct (O2 pumping)

Exhaust gases diffusion path

Air duct (O2 pumping) Inner cavity Pt heater O2 sensing electrode

V O2

VNOx

Reference electrode Vout

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Figure 14. Mixed potential type NO2 sensor using a stack of stabilized zirconia slabs (after Ono et. al. [18]).

Figure 15. Structure and sensing capabilities of an NO2 sensor for which an NO2-sensitive half cell is combined with a field effect transistor (FET) chip.

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Sensing of NOx or NO2 in various aerial spaces is also of great importance for health and environment. In urbane areas, NOx concentrations can go up to alarming levels to health around the sites of heavy traffic. In car inspection factories, workers may be exposed to high levels of NOx. The relevant sensors do not need to work at high temperature but need to have high sensitivity capable of measuring NOx at low concentration levels around its environmental standard (40-60 ppb in Japan). We have shown that such high sensitivity to NO2 can be achieved by devising an electrochemical cell of the same type (Type III) as used for CO2 sensor [19]. As shown in Figure 12, NO2 can be detected at low concentrations down to about 10 ppb. As an extension, this device has been modified into one using a field effect transistor (FET) chip (see Figure 15), which shows excellent sensing performances as well [20]. It is possible to measure total NOx with these devices if a catalyst is incorporated to convert NO into NO2 beforehand.

3.4. Challenging Markets

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Once a new gas sensor (seed) is developed to match strong demand in our society (need), a prosperous new market can be expected. There are many such demands indeed, waiting the development of new sensors. Some examples are listed below.

Onboard Car-Emission Sensors Car emission control is going to be made more stringent as time goes. To meet it, various onboard gas sensors are demanded. The target gases are NOx, hydrocarbons and CO for gasoline car emissions. For diesel car emissions, NH3 will also be included in the targets because a NOx removal system using urea water is under investigation. Urea water generates NH3 which reduces NOx selectively. Overdoses of urea water, however, causes emissions of NH3, which is another air-pollutant. Such overdoses can be avoided if NH3 in emissions is monitored. Apart from gaseous components, particulate material (PM) is problematic in diesel car emissions and its removal through filtration and/or combustion is being explored extensively. Sensory detection of fine PM particles after such removal processes is highly demanded. Environmental Monitoring Most of typical air pollutants, such as NOx, SO2, CO and hydrocarbons in urbane area are currently monitored through analytical methods. Because of the high cost of equipments and maintenance, monitoring sites are limited to very few per town. If reliable gas sensors are available for this purpose, monitoring sites would be increased dramatically. In addition, monitoring can be made continuous instead of the intermittent one currently adopted, e.g., once or twice an hour. These advantages can be crucial in environmental monitoring, since distributions of air pollutants are not steady but always fluctuating with time, being affected by many factors such as distance from origins, geography, weather, wind and so on. The data collected by sensor network would provide important information to citizens and administrations. Volatile organic compounds (VOCs) are also wanted to be brought among the targets of sensory detection. Some of them are emitted from building materials and painting materials, being known to cause sick house syndrome. Those gases are rather thick in

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concentration and not out of reach by some of current sensors. Some others like aromatics, however, have been allotted very low values of environmental standard (1 ppb for benzene in Japan) so that drastic innovations of sensors would be needed for environmental monitoring of them.

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Process Gas Monitoring Various reagents used for chemical processing in factories can be hazardous to worker if exposed. To protect workers, working environment measurement law and Ministry of labors and welfare law have been legislated in Japan. Under these laws, leakage of hazardous gases into working environments is monitored by using analytical equipments so far. Replacement of those equipments by adequate sensors can be a great benefit to factories. In silicon machining factories, for example, NH3, HNO3, HCl, HF, PH3, etc are included in the waiting list for such sensory detection, in order to protect workers and precious instruments. Applications of gas sensors are awaited in metallurgical and chemical factories as well. In hospitals, N2O is frequently used as an anesthetic for an operation. It has been wasted into the atmosphere after use. However, reservations of its use have been increased; N2O has a rather high earth-worming coefficient in addition to a suspicion that it may cause a disturbance to a pregnant nurse working in the operation room. In some advanced countries, the waste N2O is decomposed to N2 and O2 catalytically while the N2O concentration in the operation room is monitored by means of a spectroscopic analyzer. An N2O gas sensor is thus needed not only for the working space monitoring but also for confirming the level of N2O decomposition. Ethylene oxide (C2H4O) is a gaseous sterilizer popularly used for hospital goods. It is strongly hazardous to human beings so that a sensitive sensor to detect it is highly requested especially in field hospitals. The sensor reported so far [21-23] remains yet to be improved. Wearable Sensors and Ubiquitous Sensors Sensors compatible with new sensing styles are being explored extensively. Two examples are raised here. Wearable sensors mean the sensors people wear or those fixed to goods people wear or bring about, such as clothes, belts, wrist watches or mobile phones. Unlike conventional sensors fixed at a specific site, wearable sensors moves together with individual people. Gas sensors, if made wearable, would provide information about hazardous gases if present at every site or about the gases emitted from personal bodies like breath odor. For this purpose, however, two crucial points should be cleared. That is, sensors should be made compact enough in size as well as small enough in power consumption to be driven by batteries. Wearable O2 sensor has been put in market (see Figure 16 [24]). Ubiquitous sensors are the sensors incorporated into ubiquitous IT network. Signals of individual sensors are sent, through wireless communication, to a key station where a host computer is installed for signal processing, analysis and display [25]. A group of sensors arranged in a wide space can be controlled collectively with a host computer, making it possible to obtain more elaborate information about the space. More importantly, however, ubiquitous sensors seem to pave a way to making sensors more compatible with IT.

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Figure 16. Wrist watch type O2 sensor [24].

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4. FUNDAMENTAL ASPECTS OF SEMICONDUCTOR GAS SENSORS Resistive gas sensors using semiconducting oxides have been proven to be well suited for detecting inflammable or oxidative gases from their advantageous features in sensitivity, stability, robustness and so on. To meet newly emerging targets like odors and VOCs, however, the sensors should be improved rather drastically, especially in sensitivity. Such improvements will be almost impossible unless fundamental aspects of the sensors are clarified. Although the working principle of sensors in this group, a change in resistance, is simple seemingly, the gas sensing mechanism involved is never so simple. The sensing body (resistor) used in this group is a porous assembly of fine crystalline grains of metal oxide, as schematically illustrated in Figure 17. Probably the complex nature of the sensing body structure has hindered basic understanding of the sensing mechanism for a long time. The inflammable gas molecules in problem diffuse in the sensing body through pores while at the same time they are consumed gradually by a reaction with the surface of oxide grains exposed. The resistance, on the other hand, is a collective property of the whole body. We have pursued this problem through carrying out several experiments. Our conclusion is that the gas sensing phenomenon can basically be reduced into a combination of three basic factors, i.e., receptor function (surface properties), transducer function (inter-grain properties) and utility (kinetic factor determined by diffusion and surface reaction), as mentioned below [26-40].

4.1. Three Basic Factors The three basic factors of semiconductor gas sensors are schematically illustrated in Figure 18. Receptor function concerns the ability of the oxide surface to interact with the target gas. If the sensor is made of a neat oxide, the surface oxygen, especially adsorbed oxygen, of the oxide acts as a receptor. In air, oxygen is adsorbed on the oxide grains as negatively charged ions, inducing a surface space charge layer depletive of electrons or increasing the work function of grains. Upon exposure to the target gas, the adsorbed oxygen

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is consumed and decreased down to a steady state level, resulting in a corresponding decrease in work function. When the surface is loaded with a foreign receptor like PdO, it acts as a receptor stronger than the adsorbed oxygen, as shown in Figure 19, eventually giving rise to a far larger decrease in work function upon exposure to the gas.

Figure 17. Roles grain surface (a), grain boundaries (b) and gas sensing body (c) in semiconductor gas sensor. L: thickness of surface space charge layer, D: grain size (diameter).

Receptor function

Foreign receptor

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Space charge layer

Redox property Acid-base property Gas-specific promoter

Transducer function Carrier mobility Grain size Doping Schottky barrier height

Utility

Gas diffusion and reaction

・Pore size ・Diffusion depth Secondary particle size Film thickness

Figure 18. Three basic factors in semiconductor gas sensor.

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in air

Under exposure to gas（H2,CO）

-

O

-

O

-

O

-

-

O

SnO2

O

SnO2

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Pd

PdO Pd-SnO2

SnO2

-

O

SnO2

SnO2

PdO：Strong acceptor of electrons and strong foreign receptor Pd (reduced state)：No interaction with SnO2

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Figure 19. Receptor function of neat and Pd-loaded SnO2 grains; mechanism of electronic sensitization by Pd is schematically shown.

Transducer function concerns the ability to convert the change in the work function of grains into a change in electrical resistance. This function has been explained by assuming a formation of double Schottky barriers for transport of electrons through grain boundaries. The resistance changes with a change in the barrier height and so with a change in the work function. This model is well consistent with the grain size effects shown in Figure 20 [31-33]. The resistance and gas sensitivity of the device hardly depend on the grain size (diameter, D) when D is larger than a critical value (Dc, 6 nm for SnO2), which corresponds to twice the thickness (Ls) of surface space charge layer of the oxide, whereas the two quantities increase sharply with decreasing D when D is smaller than Dc. The model is also consistent with the sensitizing effects of certain foreign receptors as well. It is mentioned, however, that the double-Schottky barrier model is not necessarily a single model consistent with the above two effects. Recent experiments have suggested a possibility of the participation of the tunneling effect for electron transfer through grain boundaries [41]. Conduction mechanism should be investigated thoroughly as a very important basis of the sensors of this group. In the above discussion, no attention has been paid on the location of the oxide grains in the sensing body. The target gas molecules diffuse into the sensing body while reacting with the oxide surface. If reaction rate is too large compared with diffusion, gas molecules are mostly consumed in the shallow region of the sensing body and can not reach the grains located at inner sites, leaving them unutilized for gas sensing and thus resulting in a loss in sensor response. The third factor, utility, is necessary to take such a kinetic effect into account. For quantitative analysis of the phenomenon, one needs a sensing body well defined in microstructure. An analysis was carried out recently for a thin film device derived from SnO2 sols [38-40]. As illustrated in Figure 21, depth profiles of target gas concentration inside the film depend markedly on the magnitude of m. Here m is a non-dimensional

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Overview of Gas Sensor Technology

300 oC 150

L: Debye length 10 8

100

10 7 10 6

50

in air 0

10 5 10 4

in 800 ppm H2 2L

0

5 10 15 20 25 30 Crystallite size (diameter) / nm

Resistance / Ω

Sensor response (Ra/Rg)

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quantity defined by m = L(k/Dk)1/2, where L is film thickness, k rate constant of the surface reaction and Dk Knudsen diffusion coefficient. For m < 1, significant part of target gas can reach the bottom of the film, while the gas can not reach the bottom at m=3 and is accessible to only the surface region when m is large. As a result, utility factor, defined as the ratio of the sensor response (resistance ratio) at a given m to the ideal one at m =0, decreases fairly sharply with increasing m when m is larger than 3. It is important that utility factor can go down even to zero as L or k/Dk is large. Since Dk= (4r/3)(2RT/πM)1/2, where r is pore radius, M is molecular mass of target gas and RT has its usual meaning, Dk increases proportionally with r. Thus r and L are micro-structural parameters which affect utility factor. It is also possible to estimate roughly the effect of operating temperature (T) on sensor response. As a rate constant of chemical reaction, k increases exponentially with increasing T, whereas Dk is proportional to square root of T. Thus the ratio k/Dk and then m also increase almost exponentially with increasing T. This means that, under the conditions of fixed microsstructural parameter, utility factor eventually goes down to zero as T is raised. It can be shown that this behavior of utility factor is responsible for the occurrence of a well known volcano-shaped correlation between sensor response and T as simulated under assumption for different film thicknesses in Figure 22 [38, 39]. In the lower temperature side where utility factor remain close to unity, sensor response increases with increasing T or k, while the sensor response goes down with increasing T in the higher temperature side where utility factor decreases sharply. It is also seen from the figure that such attenuation of sensor response appears at lower temperature region as film thickness increases.

10 3 10 2

Figure 20. Grain size effects on resistance and sensor response of SnO2 sensor to 800 ppm H2 (300 oC).

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1.0

=0.3

m=L(k/DK)1/2 0.8

=1

/ C/C Cs s

0.6

0.4

m=3 0.2

=30 0

0

=10

0.2

0.4

0.6

0.8

1.0

x/L Figure 21. Gas concentration profiles inside a porous sensing film at varying m. x: Depth from the surface of film, C and Cs: Concentration of gas at depth x and outside (x=0).

200 nm

Sensor response (a.u.)

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100 nm

300 nm 10000 nm

500 nm 1000 nm

0

0

100

200

300

400

500

600

700

Temperature / oC Figure 22. Sensor response vs. temperature correlation simulated for thick film devices with various film thicknesses.

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The above discussion about a thin film device should be modified somewhat for a device using a sensing body of a thick film (screen printed, thickness of about 10μm or above) or block ( prepared by usual ceramic processes , 1 mm or above in size). The microstructure of the sensing body in the latter devices is more complex. However, it is reasonable to assume that the sensing body consists of a stack of secondary particles of one to a fewμm in diameter, each of which is a cluster of primary particles (crystallites) of oxide with a microstructure similar to that in the thin film. The pores among the secondary particles are usually large (macro-pores) and gas diffusion there is fairly quick. That is, we can assume that the concentration of target gas in the macro-pores is kept equal to that outside of the sensing body. This means that the concentration gradient can be assumed to appear inside of the secondary particles only. Roughly speaking, almost the same conclusions as those for the thin film device would be obtained for the thick film or block type device if an average radius of the secondary particles (Rsec) is substituted for the thickness (L) of the thin film.

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4.2. Higher Order Structure Favorable for High Sensitivity Through discussion in the previous section, four kinds of extrinsic parameters characterizing higher order structure of a sensing body, grain size (D), pore size (r), film thickness (L) for a thin film and secondary particles size (Rsec) for a thick film or block, have been shown to influence sensor response. The first parameter is associated with transducer function; Sensitivity increases with decreasing D in the region below Dc. The others affect utility factor; larger r and smaller L or Rsec are more favorable. It is remarked that D and r are not always independent from each other. It is often experienced that an average pore diameter (2r) is almost comparable to the value of D for a usually prepared specimen. Therefore transducer function and utility factor can not necessarily be optimized simultaneously as far as D is concerned. For usual oxides, however, it is difficult to keep D below Dc which is fairly small usually (6 nm for SnO2) so that optimization of transducer function is not realistic in usual cases. It is of interest to consider how to design higher order structure favorable for high sensitivity. For thin film devices, we can conceive two ways for it, as schematically shown in Figure 23. Increasing grain size (a) would give rise to higher sensitivity through increasing utility factor. Even when grain size is kept the same, higher sensitivity would be obtained if the grains are brought into clusters of a certain size in the film (b). The effect of design (a) has been confirmed in a dramatic form with thin film devices using SnO2 for detection of H2S as shown in Figure 24. SnO2 grains were subjected to grain growth as isolated crystallites in sol suspension under hydrothermal condition (see Figure 25). Response to H2S is seen to increase sharply with increasing D in a range of 5 ~ 16 nm. It is noted that, although the response to H2S remained rather modest with conventionally devices unless loaded with CuO [42], quite large response was achieved with neat SnO2 devices according to the present design. It is also noted that utility factor becomes more susceptible to a change in r (or D) and L as the target gas has larger reactivity or smaller diffusivity (larger molecular weight). The conspicuous effect of grain size just observed reflects that H2S is a utility factor- susceptible gas. Design (b), yet to be tested further, appears to be also effective [43], though not so dramatic as seen above. This design can be effective for thick film or block type devices.

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15000

Sensor response (Ra/Rg)

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Figure 23. Ways to promote utility factor for film devices. (a) By increasing pore size through an increase in grain size. (b) By introducing macropores through forming small clusters (secondary particles) of grains.

Grain size

16nm

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10000

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o

Operating temperature ( C ) Figure 24. Sensor response to 5 ppm H2S as correlated with operating temperature for SnO2 thin film devices different in crystallite size (thickness 200 nm).

Science and Technology of Chemiresistor Gas Sensors, Nova Science Publishers, Incorporated, 2007. ProQuest Ebook Central,

27

Overview of Gas Sensor Technology

SnO2 sols

5 nm

Particle size by LPA (nm)

16 14 20 nm

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Crystallite size by XRD (nm)

(a)

(b)

Copyright © 2007. Nova Science Publishers, Incorporated. All rights reserved.

Figure 25. Growth of SnO2 grains under hydrothermal conditions (SnO2 sol). (a) Coincidence between crystallite size (by XRD) and particle size (by LPA), assuring growth as isolated grains. (b) TEM pictures of smaller grains (5 nm in average diameter) and larger grains (16 nm).

The importance of higher order structure control mentioned above indicates the importance of the methods of preparation and processing of the sensing materials as well. High energy methods like r.f. sputtering may be convenient to obtain thin films but it would be difficult to control the higher order structure. In addition, the oxide grains prepared can be subject to grain growth or sintering during sensor operation at elevated temperature, causing transducer function and utility factor to change. Preparation by conventional ceramics methods hardly seem to allow to control higher order structure well either. We believe that wet preparation methods including colloidal processing are worthy of being exploited thoroughly for this purpose. The wet methods may be better suited for other purposes as well. It is challenging to load each colloidal particle of SnO2 with foreign metal like Pd for surface modification. Doping each particle with other oxides for valence control is also worth challenging.

4.3. Sensor Design to Promote Selectivity Selectivity is defined as the ratio of response to gas A to that to gas B. General guidelines for promoting selectivity can be deduced from considering how the response is correlated with molecular properties. Two molecular properties, reactivity (rate constant k) and molecular weight (M), have emerged as those properties influencing the response through receptor function and utility factor, respectively. Of these, k is of primary importance, deciding roughly the effective temperature range of sensor operation. It is known that k can be modified extensively by means of foreign receptors in some cases. It is convenient to

Science and Technology of Chemiresistor Gas Sensors, Nova Science Publishers, Incorporated, 2007. ProQuest Ebook Central,

28

Noboru Yamazoe and Kengo Shimanoe

classify the situation according to the relative magnitudes of reactivity (kA and kB) and molecular weight (MA and MB) where affixes indicate the respective gases, as follows. Case 1: Case 2:

Case 3:

kA >> kB. Selectivity to A will be favored always as far as utility factor to A is kept close to unity. kA comparable to kB. If MA < MB, the same situation as above will result. If MA >MB, on the other hand, selectivity to A will be promoted by raising operating temperature which decreases utility factor to B more selectively. By doing this, however, sensor response to A may also be reduced. kA