Gas Sensors Based on Conducting Metal Oxides

Description

Gas Sensors Based on Conducting Metal Oxides: Basic Understanding, Technology and Applications focuses on two distinct types of gas sensors based on conducting metal oxides. Ion conduction, applied in so-called solid-state electrolytic sensors for one, and electronic conduction used in semiconductivity gas sensors for the other. The well-known λ–probe, a key component to optimize combustion in car engines, is an example of the former type, and the in-cabin car air-quality control SnO2 and WO2 sensor array stands for the semiconductivity type. Chapters cover basic aspects of functioning principles and describe the technologies and challenges of present and future sensors.

Provides reader background and context on sensors, principles, fabrication and applications
Includes chapters on specific technological applications, such as exhaust sensors, environmental sensors, explosive gases alarms and more
Presents a structured presentation that allows for quick reference of vital information

Front Cover 2
Gas Sensors Based on Conducting Metal Oxides 5
Copyright Page 6
Contents 7
List of contributors 11
Biography 13
Preface to the series 15
Preface 19
- References 20
Book outline 21
- Chapter 2: Gas sensors based on oxygen ion conducting metal oxides (Ulrich Guth and Hans-Dieter Wiemhöfer) 21
- Chapter 1: Introduction (Nicolae Barsan and Klaus Schierbaum) 21
- Chapter 3: Basics of semiconducting metal oxides–based gas sensors (Alexandru Oprea, David Degler, Nicolae Barsan, Anne Hem... 22
- Chapter 4: Technological realization of semiconducting metal oxide–based gas sensors (Jong-Heun Lee) 23
- Chapter 5: Application of SMOX-based sensors (Kuniyuki Izawa, Heiko Ulmer, Anna Staerz, Udo Weimar, and Nicolae Barsan) 23

Front Cover 2
Gas Sensors Based on Conducting Metal Oxides 5
Copyright Page 6
Contents 7
List of contributors 11
Biography 13
Preface to the series 15
Preface 19
- References 20
Book outline 21
- Chapter 2: Gas sensors based on oxygen ion conducting metal oxides (Ulrich Guth and Hans-Dieter Wiemhöfer) 21
- Chapter 1: Introduction (Nicolae Barsan and Klaus Schierbaum) 21
- Chapter 3: Basics of semiconducting metal oxides–based gas sensors (Alexandru Oprea, David Degler, Nicolae Barsan, Anne Hem... 22
- Chapter 4: Technological realization of semiconducting metal oxide–based gas sensors (Jong-Heun Lee) 23
- Chapter 5: Application of SMOX-based sensors (Kuniyuki Izawa, Heiko Ulmer, Anna Staerz, Udo Weimar, and Nicolae Barsan) 23
- Chapter 6: Conclusion and outlook (Nicolae Barsan and Klaus Schierbaum) 23
1 Introduction 25
- References 33
- Further reading 35
2 Gas sensors based on oxygen ion conducting metal oxides 37
- 2.1 Solid-state electrolyte sensors for oxygen-containing gases 37
  - 2.1.1 Fundamentals of oxygen ion conductivity in oxides 37
  - 2.1.2 Basic operation principles of solid-state electrolyte sensors with ion conductors 39
- 2.2 Potentiometric sensors 42
  - 2.2.1 Fundamentals 42
  - 2.2.2 Application of potentiometric sensors 46
- 2.3 Amperometric sensors 50
- 2.4 Coulometric sensors 52
- 2.5 Mixed-potential sensors 54
  - 2.5.1 Principle 54
  - 2.5.2 Catalytic behavior of electrode materials 54
  - 2.5.3 Electrode materials for mixed potential sensors 58
    - 2.5.3.1 Binary oxides 62
    - 2.5.3.2 Electrode materials for hydrogen 63
    - 2.5.3.3 Electrode materials for hydrocarbons 66
    - 2.5.3.4 Mixed oxides 70
    - 2.5.3.5 Electrode materials for nitrogen oxides NOx (Figs. 2.54–2.55) 72
  - 2.6 Solid-state electrolyte sensors in dynamic mode 75
  - 2.7 Conclusion 81
  - References 82
  - Further reading 84
- 3 Basics of semiconducting metal oxide–based gas sensors 85
  - 3.1 Origin of conduction in SMOX 86
    - 3.1.1 Preliminary considerations on the origin of conduction in SMOXs 86
    - 3.1.3 Types of defects involved in electronic conduction 88
      - 3.1.3.1 Intrinsic defects 88
      - 3.1.3.2 Extrinsic defects 90
        
        The unintentional extrinsic defects most encountered in SMOXs 90
        
        Intentional extrinsic defects 90
      - 3.1.4 Experimental methods for the characterization of localized states 90
        
        3.1.4.1 Experimental techniques operating under thermodynamic equilibrium of the defect configuration 91
        
        3.1.4.2 Experimental techniques operating away from the thermodynamic equilibrium of the defect configuration 92
        
        3.1.4.3 Experimental techniques not using electronic state transition 93
      - 3.1.5 Localized levels in the metal oxides utilized for gas sensors—an overview 93
        
        3.1.5.1 Intrinsic doping2/native doping of SMOXs 93
        
        Tin dioxide, SnO2, ([Kr]4d10 ⊗ 2·[He]2s22p6) 96
        
        Zinc oxide, ZnO, ([Ar]3d10 ⊗ [He]2s22p6) 97
        
        Titanium dioxide, TiO2, ([Ar]3s23p6 ⊗ 2[He]2s22p6) 98
        
        Gallium oxide, Ga2O3, (2·[Ar]4d10 ⊗ 3·[He]2s22p6) 98
        
        Tungsten trioxide, WO3, ([Xe]4f14 ⊗ 3·[He]2s2p6) 98
        
        Indium oxide, In2O3, (2·[Kr]4d10 ⊗ 3·[He]2s22p6) 98
        
        Ternary and quaternary oxides 99
        
        p-type SMOXs 99
        
        3.1.5.2 Extrinsic doping of SMOX 101
        
        Unintentional/accidental extrinsic doping 101
        
        Intentional extrinsic doping 102
        
        3.1.5.3 Alternative causes of the native conduction of SMOXs 103
        
        The surface/interface accumulation layers 103
        
        3.1.6 Discrepancies and open questions emerging from the experimental investigation of defects in SMOXs 104
        
        3.1.6.1 Inadvertences in the experimental data 104
        
        3.1.6.2 Principle scientific queries 104
        
        3.1.6.3 The origin of the discrepancies among experimental parameters relevant for the conduction of SMOXs 105
        
        The thermodynamic equilibrium/nonequilibrium conditions under which the experimental method operates 105
        
        The erroneous identification of the evaluated defect in indirect experimental methods 105
        
        The accidental coincidence of the native energy level with the energy of an extrinsic center 106
        
        3.1.7 Theoretical approaches to the nature and level position of localized states in SMOXs 106
        
        3.1.7.1 General features of the theoretical approaches employed in the simulation of the SMOXs’ defects 106
        
        3.1.7.2 Theoretical approaches to intrinsic defect levels in SMOXs 108
        
        n-type, large gap SMOXs 108
        
        p-Type, middle and small gap SMOXs 109
        
        3.1.7.3 The persistent photoconductivity as source of native conduction in SMOXs 110
        
        3.1.7.4 Theoretical approaches for extrinsic doping in SMOXs: unintentional doping 111
        
        3.1.7.5 Theoretical approaches for extrinsic doping in SMOXs: intentional doping 112
        
        3.1.8 The contradictions of different theoretical approaches among themselves and in the context of experimental results; r... 114
        
        3.1.8.1 Disagreements originating in the limited performance of the theoretical approaches 114
        
        DFT errors: the underestimation of the band gap 114
        
        DFT errors: supercell and computational issues 115
        
        3.1.8.2 Disagreements originating in the mismatch between the theoretical and the experimental data 116
        
        Comparing the experimental data from equilibrium measurements with the theoretical simulations 116
        
        Comparing the experimental data from nonequilibrium measurements with the theoretical simulations 117
        
        3.1.8.3 Improving the experimental and theoretical accuracy 118
        
        3.1.9 Conclusion and perspectives 119
        
        3.2 Experimental investigations 120
        
        3.2.1 Spectroscopic techniques 121
        
        3.2.1.2 Infrared spectroscopy 122
        
        3.2.1.1 Radio and microwave spectroscopies 122
        
        3.2.1.3 Raman spectroscopy 127
        
        3.2.1.4 Ultraviolet, visible and near infrared spectroscopy 128
        
        3.2.1.5 X-ray absorption and emission spectroscopies 129
        
        3.2.1.6 Mössbauer spectroscopy 133
        
        3.2.2 Electrical techniques 134
        
        3.2.2.1 DC-resistance measurements 135
        
        3.2.2.2 Impedance spectroscopy 136
        
        3.2.2.3 Operando work function measurements 138
        
        3.2.2.4 Hall-effect measurements 141
        
        3.2.3 Residual gas analysis 142
        
        3.2.4 Examples for operando studies of doped SnO2 sensing materials 143
        
        3.2.5 Summary and outlook 147
        
        3.3 Computational atomic-scale calculations based on first-principles density functional theory (DFT) 148
        
        3.3.1 Multilevel-modeling of metal oxide gas sensors 149
        
        3.3.2 Atomic scale investigations using DFT calculations 150
        
        3.3.3 Reactivity 151
        
        3.3.4 Getting into kinetics 154
        
        3.3.5 Doping 155
        
        3.3.6 Charge transfer in DFT calculations 156
        
        3.4 Modeling gas sensing with SMOXs 157
        
        3.4.1 History 158
        
        3.4.2 The conduction mechanism 160
        
        3.4.2.1 n-type SMOXs 160
        
        3.4.2.2 p-type SMOXs 162
        
        3.4.3 Review of the modeling assumptions in the case of SnO2 163
        
        References 167
      - 4 Technological realization of semiconducting metal oxide–based gas sensors 191
        
        4.1 Transducer technology evolution from pellets to microhotplates 192
        
        4.1.1 Thick films on Al2O3 tubes and substrates 192
        
        4.1.2 MEMS-based sensors 193
        
        4.1.3 Sensors on plastic foils 197
        
        4.2 Synthesis of sensitive materials: synthetic strategies and gas-sensing characteristics 197
        
        4.2.1 Historic perspective 197
        
        4.2.2 Hydrothermal/solvothermal self-assembly reaction 198
        
        4.2.3 Template-based synthesis: tubular nanofibers, hemispheres, hollow spheres, and inverse opals 199
        
        4.2.4 Spray pyrolysis: dense, hollow, yolk-shell, and hierarchically porous spheres 203
        
        4.2.5 Core-shell nanostructures (nanowires, nanofibers, spheres): modulation of the electron depletion layer 205
        
        4.2.6 Oxide nanowires or spheres decorated with different oxides 207
        
        4.2.7 Catalyst-loaded microreactor 208
        
        4.3 Design of sensing films for highly sensitive and selective gas detection 209
        
        4.3.1 Slurry deposition, spin coating, Langmuir–Blodgett method, and dielectrophoretic positioning 209
        
        4.3.2 Screen printing 211
        
        4.3.3 Inkjet printing 212
        
        4.3.4 Near-field electrospinning 213
        
        4.3.5 Flame spray pyrolysis 214
        
        4.3.6 Vapor deposition: sputtering, e-beam evaporation, GAD, ALD, and PLD 214
        
        4.3.6.1 Sputtering and e-beam evaporation 214
        
        4.3.6.2 Glancing angle deposition (oblique angle deposition) 215
        
        4.3.6.3 Atomic layer deposition 217
        
        4.3.6.4 Pulsed layer deposition 219
        
        4.3.7 Direct growth of nanowires 219
        
        4.3.8 Sensing films on MEMS hotplates 220
        
        4.3.9 Sensing films on flexible substrates 222
        
        4.4 Conclusions 224
        
        References 224
      - 5 Application of SMOX-based sensors 241
        
        5.1 Application of SMOX-based sensors 241
        
        5.1.1 The history of gas and the ensuing need for detection 241
        
        5.1.1.1 The beginning of SMOX-based sensors used for domestic safety 244
        
        5.1.1.2 In-cabin air quality monitoring 244
        
        5.1.2 Applications in development and future applications 246
        
        5.1.2.1 Advancement in in-cabin monitoring and inclusion on airplanes and space stations 246
        
        5.1.2.2 Indoor air quality monitoring 248
        
        5.1.2.3 Industrial safety 250
        
        5.1.2.4 Antiterrorism detection 251
        
        5.1.2.5 Food freshness 253
        
        5.1.2.6 Breath analysis 254
        
        5.1.3 The requirements of commercial SMOX-based gas sensors – example domestic safety 256
        
        5.1.3.1 Mechanical strength 260
        
        5.1.3.2 Setting of alarm point 260
        
        5.1.3.3 Sensor signal 261
        
        5.1.3.4 Selectivity 262
        
        5.1.3.5 Gas response speed 263
        
        5.1.3.7 Long-term stability 264
        
        5.1.3.6 Warm-up behavior 264
        
        5.1.3.8 Thermal effects 265
        
        5.1.3.9 Moisture effect 266
        
        5.1.3.10 Poisoning by siloxanes 267
        
        5.1.3.11 Battery operation and combination of methane and carbon monoxide detections 269
        
        5.2 Conclusion 271
        
        References 275
        
        6 Conclusion and outlook 283
        
        References 286
        
        Index 287
        
        Back Cover 294
    - 3.1.2 The role of defects in the electronic conduction of SMOXs 88

Basic Understanding, Technology and Applications

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