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(1)AGH UNIVERSITY OF SCIENCE AND TECHNOLOGY FACULTY OF MATERIALS SCIENCE AND CERAMICS Department of Analytical Chemistry. Ph.D. Thesis Dorota Katarzyna Flak. New metal oxide nanoparticles for gas sensors. Supervisors: Prof. Dr. hab. Mieczysław Rękas Dr. Artur Braun. Kraków 2013.

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(3) Dedicated to my Family and Friends.

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(5) Acknowledgements First of all I would like to thank to my supervisors Prof. Mieczysław Rękas and Dr. Artur Braun for their great scientific guidance and contribution to my development as a scientist. I am very thankful to Prof. Mieczysław Rękas, who was supervising me already since my Master Thesis project and who gave me further on great possibility to develop my research interests as a PhD candidate. I am very grateful for the given freedom concerning my research ideas and also for his great care. I am especially grateful to Dr. Artur Braun, who after some personal changes accepted me as a member of his group and then provided me with a big trust into my ideas and with unique opportunities to work with collaborators from several scientific institutes not only within Europe, but also USA and South Korea. His scientific contribution to my work is invaluable. I would like to acknowledge Prof. Dariusz Kata and Prof. Jolanta Janczak-Rush for the opportunity to conduct my PhD studies within the International PhD School Switzerland-Poland and Prof. Thomas Graule for hosting my stay at Laboratory for High Performance Ceramics, Empa in Switzerland. In particular I would like to thank Dr. Katarzyna Michałów-Mauke, who as a cosupervisor during my Master Thesis project and later on as a friend introduced me with a great passion to the world of research and showed me clear and valuable guidelines. I am indebted to Prof. Simon Mun and his students for hosting me at Hanyang University in South Korea and for his great assistance during the XPS beam-time and inspiring lectures and discussions. I am also grateful to Dr. Farid El Gabaly from Sandia National Laboratories in Livermore for preparing sputtered metal contacts on my specimens for AP-XPS and EIS and for support during the XPS beam-time. I sincerely appreciate Prof. Katarzyna Zakrzewska and Prof. Marta Radecka for being my co-examiners and for their continuous concern about young scientist and always great support as I have experienced during several conferences and scientific meetings. Thanks to my colleagues in Switzerland: Dr. Francesca Bortolani, Dr. Claudia Strehler, Dr. Qianli Chen, Dr. Tzu-Wen Huang, Michal Gorbar, Evelyn Schlenther, Dr. Selma Erat, Dr. Rita Tóth, Dr. Krisztina Gajda-Schrantz, Dr. Debajeet Bora, Dr. Edvardas Kazakevicius, Federico Dalcanale, Florent Boudoire and all HPC members for the great working atmosphere and “helpful hand” during experiments and valuable discussions.. I.

(6) Equal thanks are owned to my colleagues in Poland for all the support and encouragements: Justyna Kupis, Dr. Filip Ciepiela, Dr. Dariusz Burnat, Jan Wyrwa, Małgorzata Dziubaniuk, Barbara Łysoń-Sypień. I would like to thank to all people who introduced and helped me with handling experiments and supported with the discussion on the obtained results. Thanks to Dr. Katarzyna Michałów-Mauke and Dr. Andre Heel for introduction to the FSS and characterization of nanoparticles, to Prof. Dr. Magdalena Parlińska-Wojtan for the TEM analysis, to Jan Wyrwa for the strong technical support during gas sensor and EIS measurements, to Dr. Jong-Bae Park and Dr. Giuseppino Fortunato for XPS measurements, to Dr. Max Döbeli for EBS measurements, and to Dr. Zhi Liu and Dr. Antje Vollmer for hosting me among other scientists at their beamlines. In the end I am thankful to all people I have the pleasure to meet and work with during my PhD studies and which are not mentioned here by name.. II.

(7) Table of contents List of symbols and acronyms ............................................................................................................................... V List of tables....................................................................................................................................................... VIII List of figures ........................................................................................................................................................ IX 1. Introduction ........................................................................................................................................... - 1 -. 2. Semiconductor metal oxide (MOx) based gas sensors ........................................................................ - 4 -. 2.1. Review of research on metal oxide based gas sensors ............................................................................ - 4 -. 2.2. Gas sensing semiconductor metal oxide (MOx) nanoparticles ............................................................... - 7 -. 2.2.1. Gas sensing technology from the perspective of nanotechnology .......................................................... - 7 -. 2.2.2. Metal oxide nanoparticles for gas sensing application ........................................................................... - 8 -. 2.3. Electronic structure and optical properties of semiconductor metal oxides ......................................... - 15 -. 2.4. Non-stoichiometry and surface properties of MOx vs. electrical properties and gas sensing behavior - 20 -. 2.5. Defect chemistry of Fe2O3 .................................................................................................................... - 23 -. 3. General model and basic principles of the interaction of gas molecules with the semiconductor metal oxides (MOx).................................................................................................... - 28 -. 3.1. Electronic band structure of n-type and p-type semiconductor MOx upon oxidizing (acceptor type) and reducing (donor type) gas molecules.............................................................................................. - 30 -. 3.2. Interaction of oxygen gas molecules with n-type semiconductor metal oxides.................................... - 32 -. 3.3. Interaction of reducing gas molecules with n-type semiconductor metal oxides ................................. - 35 -. 3.3.1. Interaction of ammonia gas molecules with n-type semiconductor metal oxides ................................ - 36 -. 3.4. Influence of the crystallite/grain size on the sensing mechanism ......................................................... - 37 -. 3.5. Effect of the water vapor molecules on the conductivity of semiconductor metal oxides.................... - 39 -. 4. Flame spray synthesis of metal oxide nanoparticles ........................................................................ - 41 -. 5. Experimental methods ........................................................................................................................ - 44 -. 5.1. Determination of the crystal structure and morphology (XRD, BET, TEM) ....................................... - 44 -. 5.2. Characterization of the surface chemical composition and stoichiometry (XPS, resonant EBS) ......... - 46 -. 5.3. Investigation of the electronic structure (NEXAFS) ............................................................................. - 47 -. 5.4. In-situ observation of the chemical interaction of the oxygen and hydrogen molecules at the surface and the electronic structure of the metal oxides (AP-XPS, AP-NEXAFS) .............................. - 48 -. 5.5. Determination of the optical band gap (DRS)....................................................................................... - 49 -. 5.6. Investigation of the nature of the electrical transport properties (EIS) ................................................. - 50 -. 5.7. Evaluation of the gas sensing performance towards hydrogen and ammonia detection (DC measurements)....................................................................................................................................... - 52 -. 6. Results and discussion......................................................................................................................... - 54 -. 6.1. Un-modified and Ti-modified Fe2O3 nanoparticles .............................................................................. - 54 -. III.

(8) 6.1.1. Crystal structure and morphology ......................................................................................................... - 55 -. 6.1.2. Band gap - DRS optical measurements ................................................................................................. - 60 -. 6.1.3. Chemical composition and electronic structure - XPS, resonant EBS .................................................. - 65 -. 6.1.3.1 Effect of oxidizing and reducing processing conditions on the chemical composition and electronic structure of Fe2O3 nanoparticles ............................................................................................................ - 66 6.1.3.2 Effect of Ti-modification on the chemical composition and electronic structure of Fe2O3 nanoparticles .......................................................................................................................................... - 70 6.1.4. Electronic structure under UHV conditions - NEXAFS ....................................................................... - 73 -. 6.1.5. In-situ observations of the surface chemistry and electronic structure of Fe2O3 nanoparticles AP-XPS ................................................................................................................................................. - 76 -. 6.1.6. In-situ observations of the surface chemistry and electronic structure of Fe2O3 nanoparticles AP-NEXAFS ......................................................................................................................................... - 84 -. 6.1.7. Electrical properties - EIS ..................................................................................................................... - 87 -. 6.1.8. Gas sensing performance....................................................................................................................... - 95 -. 6.2. TiO2, ZnO and SnO2 nanoparticles ...................................................................................................... - 101 -. 6.2.1. Crystal structure and morphology ....................................................................................................... - 101 -. 6.2.2. Band gap - DRS optical measurements ............................................................................................... - 104 -. 6.2.3. Chemical composition and electronic structure - XPS, resonant EBS ................................................ - 106 -. 6.2.3.1 Effect of oxidizing and reducing processing conditions on the chemical composition and electronic structure of MOx nanoparticles............................................................................................................ - 106 6.2.4. Electrical properties - EIS ................................................................................................................... - 111 -. 6.2.5. Gas sensing performance..................................................................................................................... - 115 -. 6.3. Tungsten trioxide WO3 ........................................................................................................................ - 118 -. 6.3.1. Crystal structure and morphology ....................................................................................................... - 118 -. 6.3.2. Band gap - DRS optical measurements ............................................................................................... - 120 -. 6.3.3. Chemical composition and electronic structure - XPS, resonant EBS ................................................ - 121 -. 6.3.3.1 Effect of oxidizing and reducing processing conditions on the chemical composition and electronic structure of WO3 nanoparticles ........................................................................................................... - 121 6.3.4. Electronic structure under UHV conditions - NEXAFS ..................................................................... - 124 -. 6.3.5. Electrical properties - EIS ................................................................................................................... - 125 -. 6.3.5.1 Impedance behavior under H2 and NH3 and effect of temperature ..................................................... - 125 6.3.6. Gas sensing performance..................................................................................................................... - 129 -. 7. Summary and conclusions ................................................................................................................ - 131 -. 8. Bibliography....................................................................................................................................... - 139 -. IV.

(9) List of symbols and acronyms AC. alternating current. AMT. ammonium metatungstate. AP-NEXAFS. ambient pressure near edge X-ray absorption fine structure (p ≤ 130 mTorr). AP-XPS. ambient pressure X-ray photoelectron spectroscopy. BET. Brunauer-Emmett-Teller adsorption isotherm. BZ. benzene. C(H2), C(NH3). concentration of hydrogen and ammonia, respectively. CB. conduction band. CGB. grain boundary capacitance. CPE. constant phase element. CPS. counts per second. CVD. chemical vapor deposition. dBET, dTEM. particle size calculated based on BET and TEM particle analysis. DC. direct current. DRIFT. diffuse reflectance infrared fourier transform. DRS. diffuse reflectance spectroscopy. dXRD. crystallite size. e. electron charge. e’, h⦁. electron, hole. [e’], [h⦁]. concentration of electrons and holes, respectively. Ea. activation energy. EB. binding energy. EBS. elastic backscattering. EC. conduction band energy. EC. equivalent circuits. ED, EA. donor and acceptor energy levels, respectively. EDD, EDA. deep donor and acceptor energy levels, respectively. EF. Fermi energy level. Eg. band gap energy. Ei. impurity energy level. EIS. Electrochemical Impedance Spectroscopy. EPR. electron paramagnetic resonance. ERC. mid gap energy level. ESD, ESA. shallow donor and acceptor energy levels, respectively. ESR. electron spin resonance. EtOH. ethyl alcohol. EV. valence band energy. EXAFS. extended X-ray absorption fine structure. FC. ferrocene. FSS. flame spray synthesis. V.

(10) G, G0. overall conductance, zero conductance (in reference atmosphere), respectively. gD , g A. degeneracy factors for donor and acceptor levels. GL. Gaussian-Lorentzian fit function. h. Planck constant. hν. photon energy. ICSD. inorganic crystal structure database. ICSD. inorganic crystal structure database. K. absorption coefficient. kB. Boltzmann constant. ks. rate constant. K S , K e, K R. equilibrium constant of Schottky defect formation, thermal electron-hole generation and reduction reaction, respectively. L. inductive element; sample thickness. LDA. local density approximation light emitting diodes. LED *. me , mh. *. effective mass of charges: electrons and holes, respectively metal atom in metal lattice position metal interstitial. MOx. semiconductor metal oxide. my. power coefficient in Equation 2-4. mσ. power coefficient in Equation 2-7. nc. carrier concentration. NC, NV. effective density of states in conduction band and valence band, respectively. NEXAFS. near edge X-ray absorption fine structure. Nζ. computable constant defined by Equation 2-12 oxygen in lattice position or. pH. ,. 2. adsorbed forms of oxygen hydrogen partial pressure. pH O 2. water vapor partial pressure. pO. 2. oxygen partial pressure. ppm. particles per milion. PSD. particle size distribution. qVS or eVS. surface potential energy barrier. r. crystallite/grain radius. R, RGB, RB, RE. overall electrical resistance, grain boundary resistance, bulk resistance, electrode resistance, respectively; resistor; reflectance. RDiff. diffuse reflectance. ROx. reducing agent. RRef. reference reflectance. RT. room temperature. RTGO. rheotaxial growth and oxidation. S. scattering coefficient; sensor response; surface adsorption site. SAED. selected area electron diffraction pattern VI.

(11) SSA. specific surface area. T. absolute temperature. TDiff. diffused transmittance. TEM. transmission electron microscopy. TEY. total electron yield. ⦁. titanium substituional dopant in Fe2O3. TMT. tin tetramethyl. TPD. temperature programmed desorption. TTIP. titanium tetraisopropoxide. UHV. ultra high vacuum. VB. valence band. ⦁⦁. ,. ⦁. iron vacancy doubly and singly ionizes oxygen vacancy. XANES. X-ray absorption near edge structure. XAS. X-ray absorption spectroscopy. XPS. X-ray photoelectron (photoemission) spectroscopy. XRD. X-ray diffraction. y. deviation from the stoichiometry. Y’, Y”. real and imaginary part of admittance. Z’, Z”. real and imaginary part of impedance, respectively. Zn(acac)2. zinc acethylacetonate. α. absorption coefficient. α0. material constant. γ. ΔG. power coefficient (Tauc plot). ε0, εr. vacuum permittivity and dielectric constant, respectively. Θ. Bragg diffraction angle. λ. wavelength. λD. Debye screening length. λFSS. the oxygen to fuel ratio. µ: µe, µh. charge mobility: electron and hole. σ. electrical conductivity. σg. geometric standard deviation calculated from the PSD curves. τ0.95. response time. change of the free dissociation energy. VII.

(12) List of tables Table 3-1 Adsorbed species on the SnO2 surface upon gas and temperature exposure and its impact on the conductivity (after Yamazoe et al. [291]). ................................................................................................. - 29 Table 4-1 Flame spray synthesis parameters of metal oxide nanoparticles. ............................................. - 43 Table 6-1 Structural parameters of un-modified and Ti-modified Fe2O3 nanoparticles ........................... - 57 Table 6-2 Summary of the band gap energy and additional transition energies found for un-modified and Ti-modified Fe2O3 nanoparticles................................................................................................................ - 64 Table 6-3 Structural parameters of FSS-made nanoparticles. (SSA - specific surface area; dXRD - crystallite size calculated using Scherrer’s formula; dBET - calculated particle size on the base of XRD and BET results; dTEM - calculated particle size on the base of statistical TEM image analysis). .......................... - 101 Table 6-4 Summary of the band gap energy and additional transition energies found with the first derivative analysis for TiO2, ZnO and SnO2 nanoparticles....................................................................................... - 104 Table 6-5 Structural parameters of FSS-made WO3 nanoparticles. (SSA - specific surface area; dXRD crystallite size calculated using Scherrer’s formula; dBET - calculated particle size on the base of XRD and BET results; dTEM - calculated particle size on the base of statistical TEM image analysis). ................. - 119 -. VIII.

(13) List of figures Figure 1-1 Overview of a complex investigation into the physico-chemical properties of metal oxide nanoparticles prepared by FSS..................................................................................................................... - 3 Figure 2-1 Unit cell of corundum-like hematite (left) and cubic magnetite (right). The red spheres indicate iron atoms and silver spheres indicate oxygen atoms. ............................................................................... - 10 Figure 2-2 Unit cell of anatase (left) and rutile (right) TiO2. The yellow spheres indicate Ti atoms (or Sn in case of SnO2) and silver spheres indicate oxygen atoms. .......................................................................... - 13 Figure 2-3 Crystal structure of monoclinic WO3 with corner-sharing arrangement of octahedral and with highlighted unit cell (left), ReO3-type crystal structure with outlined unit cell (right). ............................ - 14 Figure 2-4 Unit cell of wurtzite. The blue spheres indicate Zn atoms and silver spheres indicate oxygen atoms. ......................................................................................................................................................... - 15 Figure 2-5 Calculated electronic band gap structure of un-modified α-Fe2O3 (a) and Ti-modified α-Fe2O3 (after [248]). ............................................................................................................................................... - 18 Figure 2-6 Calculated electronic band gap structure of anatase phase of TiO2 (after [251]). ................... - 18 Figure 2-7 Optical transitions in semiconductors with a direct and indirect band gap. ............................ - 20 Figure 2-8 (Left) Energy levels of shallow and deep donors (SD and DD) and acceptors (SA and DA) and mid gap level (RC) in semiconductor band gap; (Right) Energy levels corresponding to the different point defects in semiconductor metal oxide. ....................................................................................................... - 24 Figure 2-9 Phase diagram of the Fe2O3-TiO2 system after [287].............................................................. - 27 Figure 3-1 Sensing mechanism of the n-type semiconductor metal oxide with oxygen deficiency (CB conduction band, eVs - potential energy barrier, T - temperature, R - resistance). ................................... - 29 Figure 3-2 Schematic representation of conduction processes in the sensing layer and the corresponding energy bands representation for n-type (left) and p-type (right) semiconductor metal oxide MOx. (Case of porous sensing layer composed of loosely sintered nanocrystalline grains of. >. ) [294]. ................. - 30 -. Figure 3-3 (Upper panel) Scheme of energy bands for n-type semiconductor metal oxides MOx with depletion layer upon exposure to oxidizing (acceptor type) gas molecules and with accumulation layer upon exposure to reducing (donor type) gas molecules. (Lower panel) Simplified conduction as a function of the concentration of adsorbed gas molecules. ................................................................................................. - 31 Figure 3-4 (Upper panel) Scheme of energy bands for p-type semiconductor metal oxides MOx with accumulation layer upon exposure to oxidizing (acceptor type) gas molecules and with depletion layer upon exposure to reducing (donor type) gas molecules. (Lower panel) Simplified conduction as a function of the concentration of adsorbed gas molecules. ................................................................................................. - 32 Figure 3-5 Schematic representation of the conduction mechanism in two types of materials: small crystallite/grains ( ≤. ) (left), large crystallite/grains (right) ( >. ) (VB - valence band, CB -. conduction band, EF - Fermi level, - Debye screening length, r - crystallite/grain radius). For simplicity of consideration the crystallite/grains have an idealistic spherical shape. ..................................................... - 38 Figure 4-1 (Left) Scheme of the set-up of the flame spray syntheis plant; (right) image of the FSS plant at Empa Dübendorf, Laboratory for High Performance Ceramics. ............................................................... - 43 Figure 5-1 (Left) schematic cross-section of the Scienta 4000R-HiPP ambient pressure photoemission spectrometer (APPES) (figure reproduced after Aksoy et al. [315]); (right) hematite sample with sputtered Au electrodes mounted on the sample holder. ........................................................................................... - 49 -. IX.

(14) Figure 5-2 Scheme of the custom-made electrochemical cell (Probostat-like) used for impedance measurements. ............................................................................................................................................ - 52 Figure 5-3 Scheme of the experimental set-up for gas sensing measurements. ........................................ - 53 Figure 6-1 Images of (a) maghemite (γ-Fe2O3) and (b) hematite (α-Fe2O3) nanoparticles. ...................... - 54 Figure 6-2 (a) Powder diffraction patterns of α-Fe2O3 and γ-Fe2O3 and (b) TiO2-modified Fe2O3. ......... - 58 Figure 6-3 Transmission electron micrographs (TEM) of (a, b) maghemite (γ-Fe2O3), (d) hematite (αFe2O3) nanoparticles; selected area electron diffraction patterns (SAED) of (c) maghemite (e) hematite nanoparticles. ............................................................................................................................................. - 58 Figure 6-4 Transmission electron micrographs (TEM) and selected area electron diffraction patterns (SAED) of Ti-modified Fe2O3 nanoparticles: (a) 5 mole % TiO2; (b) 10 mole % TiO2; (c) 20 mole % TiO2. .................................................................................................................................................................... - 59 Figure 6-5 Particle size distribution (PSD) of as-prepared γ-Fe2O3 and Ti-modified Fe2O3 nanoparticles with 5, 10 and 20 mole % TiO2. ................................................................................................................. - 60 Figure 6-6 Diffuse reflectance spectra of un-modified Fe2O3, TiO2 (left panel) and Ti-modified Fe2O3 (right panel) nanoparticles compared with samples after mild oxidation. Note: right hand side abscissa refers to TiO2. ........................................................................................................................................................... - 61 Figure 6-7 The first derivative analysis of diffuse reflectance spectra of Ti-modified Fe2O3 (5, 10, 20 mole %) (lower panel) compared with those of TiO2, γ-Fe2O3 and α-Fe2O3 (upper panel). The inset numbers are the calculated values of the transition energies. ......................................................................................... - 62 Figure 6-8 Tauc’ plots as a variation of (αhν )1/ 2 and (αhν )1/ 2 vs. photon energy for Ti-modified Fe2O3 (5, 10, 20 mole %) compared with those of TiO2, γ-Fe2O3 and α-Fe2O3: (top) direct allowed and (bottom) indirect allowed transition. ......................................................................................................................... - 65 Figure 6-9 XPS O 1s core level spectra of oxidized and reduced γ-Fe2O3 and α-Fe2O3 nanoparticles. Color lines correspond to the Gaussian-Lorentzian fit......................................................................................... - 67 Figure 6-10 XPS Fe 2p core level spectra of oxidized and reduced γ-Fe2O3 and α-Fe2O3 nanoparticles. Color lines correspond to the Gaussian-Lorentzian fit. ............................................................................. - 68 Figure 6-11 XPS valence band spectra of oxidized (blue line) and reduced (green line) γ-Fe2O3 (left) and αFe2O3 (right) nanoparticles. ........................................................................................................................ - 69 Figure 6-12 Resonant 5 MeV elastic backscattering spectra (EBS) of γ-Fe2O3 (left) and α-Fe2O3 (right) nanoparticles. ............................................................................................................................................. - 70 Figure 6-13 XPS O 1s core level spectra of oxidized Ti-modified Fe2O3 nanoparticles. Color lines correspond to the Gaussian-Lorentzian fit. ................................................................................................ - 71 Figure 6-14 XPS Fe 2p core level spectra of oxidized Ti-modified Fe2O3 nanoparticles. Color lines correspond to the Gaussian-Lorentzian fit. ................................................................................................ - 71 Figure 6-15 XPS Ti 2p core level spectra of oxidized Ti-modified Fe2O3 nanoparticles. Color lines correspond to the Gaussian-Lorentzian fit. ................................................................................................ - 72 Figure 6-16 Calculated contribution of the components of: a) O 1s core level, b) Fe 2p core level and c) Ti 2p core level for Ti-modified Fe2O3 nanoparticles and given to the oxidizing processing conditions. ..... - 72 Figure 6-17 XPS valence band region of Ti-modified Fe2O3 nanoparticles given to the oxidizing processing conditions. .................................................................................................................................................. - 73 Figure 6-18 Normalized O K-edge (top left), Fe L-edge (top right) and Ti L-edge (bottom) NEXAFS spectra of Ti-modified Fe2O3 (5, 10, 20 mole %) compared with those of γ-Fe2O3, α-Fe2O3 and TiO2.... - 76 X.

(15) Figure 6-19 Observed and Gaussian-Lorentzian fit of the O 1s AP-XPS spectra of α-Fe2O3 (left) and γFe2O3 (right) under applied gas atmosphere and thermal conditions (see description insets in the figure). .................................................................................................................................................................... - 81 Figure 6-20 Observed and Gaussian-Lorentzian fit of the Fe 2p AP-XPS spectra of α-Fe2O3 (left) and γFe2O3 (right) under applied gas atmosphere and thermal conditions (see description insets in the figure). .................................................................................................................................................................... - 82 Figure 6-21 XPS valence band spectra VB of α-Fe2O3 (left) and γ-Fe2O3 (right) under applied gas atmosphere and thermal conditions (see description insets in the figure). ................................................ - 84 Figure 6-22 Normalized O K-edge NEXAFS of α-Fe2O3 (left) and γ-Fe2O3 (right) under applied gas atmosphere and thermal conditions (see description insets in the figure). ................................................ - 86 Figure 6-23 Normalized Fe L-edge NEXAFS of α-Fe2O3 (left) and γ-Fe2O3 (right) under applied gas atmosphere and thermal conditions (see description insets in the figure). ................................................ - 86 Figure 6-24 Complex impedance plots (Z” vs. Z’) of α-Fe2O3 nanoparticles under air atmosphere and different temperatures as an example of a typical complex impedance response of nanocrystalline metal oxides. ........................................................................................................................................................ - 88 Figure 6-25 Complex impedance plots (Z” vs. Z’) and admittance plots (Y” vs. Y’) of α-Fe2O3 nanoparticles under hydrogen atmosphere and different temperatures as an example of a typical complex impedance and admittance response of nanocrystalline metal oxides. ...................................................... - 88 Figure 6-26 Complex impedance plots (Z” vs. Z’) of Fe2O3 - 10 mole % TiO2 nanoparticles under air atmosphere and different temperatures as an example of a typical complex impedance and admittance response of nanocrystalline metal oxides. ................................................................................................. - 88 Figure 6-27 Complex impedance plots (Z” vs. Z’) and admittance plots (Y” vs. Y’) of Fe2O3 - 10 mole % TiO2 nanoparticles under hydrogen atmosphere and different temperatures as an example of a typical complex impedance and admittance response of nanocrystalline metal oxides. ....................................... - 89 Figure 6-28 Impedance plots (Z” vs. Z’) (a, b) and admittance plots (Y” vs. Y’) (d) of Ti-modified Fe2O3 (5, 10, 20 mole %) under air and hydrogen atmosphere at 573 K and corresponding equivalent circuits (c). .................................................................................................................................................................... - 90 Figure 6-29 Conductivity changes of un-modified and Ti-modified Fe2O3 (5, 10, 20 mole %) upon exposure to synthetic air and hydrogen atmosphere as a function of temperature. ................................... - 91 Figure 6-30 Trend of the activation energy Ea for the un-modified and Ti-modified Fe2O3 (5, 10, 20 mole %) nanoparticles upon exposure to synthetic air and hydrogen atmosphere. ............................................ - 93 Figure 6-31 Dynamic changes in the electrical resistance R of FSS-made Fe2O3 based sensors: γ-Fe2O3 (left) and α-Fe2O3 (right) at 573 K as a function of time upon exposure to hydrogen atmosphere (1000 ppm - 25 000 ppm). Insets show the response time of the sensor for chosen concentrations. .......................... - 96 Figure 6-32 Sensor characteristics of hydrogen (left) and ammonia detection (right) of un-modified Fe2O3 nanoparticles (α-Fe2O3 and γ-Fe2O3) under different operating temperatures. Note: minus and plus signs of sensor response correspond to the n-type and p-type conductivity, respectively. ..................................... - 96 Figure 6-33 Sensor characteristics of hydrogen detection of Ti-modified Fe2O3 (5, 10, 20 mole %) at different operating temperatures (523 K, 573 K and 673 K). Note: minus and plus signs of sensor response correspond to the n-type and p-type conductivity, respectively. ............................................................... - 97 Figure 6-34 Sensor characteristics of ammonia detection of Ti-modified Fe2O3 (5, 10, 20 mole %) at different operating temperatures (523 K, 573 K, 632 K and 673 K). Note: minus and plus signs of sensor response correspond to the n-type and p-type conductivity, respectively. ................................................ - 99 -. XI.

(16) Figure 6-35 Comparison of the sensor response towards 1000 ppm of both H2 (upper right) and NH3 (lower right) and 750 ppm of NH3 (lower left) of un-modified and Ti-modified Fe2O3 nanoparticles as a function of composition and operating temperature. Note: minus and plus signs of sensor response correspond to the ntype and the p-type conductivity, respectively. ........................................................................................ - 100 Figure 6-36 XRD patterns of: (left) FSS-made TiO2, (middle) ZnO and (bottom) SnO2 nanoparticles with assignment of Bragg reflections. .............................................................................................................. - 102 Figure 6-37 Transmission electron micrographs: (a) TEM image of TiO2, (b) SAED of TiO2, (c) TEM image of ZnO, (d) HRTEM of ZnO, (e) SAED of ZnO, (f) TEM of SnO2, (g) HRTEM of SnO2, (h) SAED of SnO2. Please note different scale markers. .......................................................................................... - 103 Figure 6-38 The first derivative analysis of diffuse reflectance spectra and calculated values of the transition energies for oxidized (bottom panel) and reduced (upper panel) TiO2, ZnO and SnO2. ......... - 106 Figure 6-39 O 1s core level spectra of oxidized (bottom panel) and reduced (upper panel) TiO2, ZnO and SnO2 nanoparticles fitted with the Gaussian-Lorentzian function. .......................................................... - 107 Figure 6-40 Metal ion core level spectra of oxidized (black line) and reduced (grey line) TiO2, ZnO and SnO2 nanoparticles: (left) Ti 2p, (middle) Zn 2p; (right) Sn 3d............................................................... - 108 Figure 6-41 Valence band spectra of oxidized (black line) and reduced (grey line) TiO2, ZnO and SnO2 nanoparticles. ........................................................................................................................................... - 109 Figure 6-42 Resonant 5 MeV elastic backscattering spectra (EBS) of TiO2 (left), ZnO (middle) and SnO2 (right) nanoparticles. ................................................................................................................................ - 111 Figure 6-43 Nyquist plots for TiO2 under air atmosphere (left panel) and hydrogen atmosphere (right panel) for chosen temperatures: RT - room temperature, 473 K and 673 K. ...................................................... - 113 Figure 6-44 Nyquist plots for ZnO under air atmosphere (left panel) and hydrogen atmosphere (right panel) for chosen temperatures: RT - room temperature, 473 K and 673 K. ...................................................... - 114 Figure 6-45 Nyquist plots for SnO2 under air atmosphere (left panel) and hydrogen atmosphere (right panel) for chosen temperatures: RT - room temperature, 473 K and 673 K. ........................................... - 114 Figure 6-46 Conductivity changes of metal oxide samples upon exposure to synthetic air and hydrogen atmosphere as a function of temperature. ................................................................................................ - 115 Figure 6-47 Change of the relative resistance of SnO2 as a function of temperature and (a) hydrogen and (b) ammonia concentration. ........................................................................................................................... - 117 Figure 6-48 Change of the relative resistance of (a, b) TiO2, (c, d) ZnO and (e, f) SnO2 as a function of (left panel) hydrogen and (right panel) ammonia concentration. .................................................................... - 118 Figure 6-49 Diffraction patterns: (a) XRD and (b) SAED; (c) TEM and (d) HRTEM micrographs of WO3 nanoparticles. ........................................................................................................................................... - 119 Figure 6-50 The first derivative analysis of diffuse reflectance spectra and calculated values of the transition energies for reduced (left) and oxidized (right) WO3 nanoparticles. ....................................... - 121 Figure 6-51 XPS O 1s core level of (a) reduced and (b) oxidized WO3 nanoparticles; (c) XPS W 4f core level and (d) valence band VB of (green line) reduced and (blue line) oxidized WO3 nanoparticles. .... - 123 Figure 6-52 Resonant 5 MeV elastic backscattering spectra (EBS) of WO3 nanoparticles. ................... - 123 Figure 6-53 Normalized O K-edge NEAXFS spectrum of WO3 nanoparticles under UHV conditions.- 124 Figure 6-54 Impedance plots (Z” vs. Z’) of WO3 nanoparticles under synthetic air atmosphere and different temperatures (RT - 673 K). ...................................................................................................................... - 127 -. XII.

(17) Figure 6-55 Impedance plots (Z” vs. Z’) of WO3 nanoparticles under H2 atmosphere (750 - 3000 ppm) and different temperatures (RT - 673 K). ....................................................................................................... - 128 Figure 6-56 Impedance plots (Z” vs. Z’) of WO3 nanoparticles under NH3 atmosphere (750 - 3000 ppm) and different temperatures (RT - 673 K). ................................................................................................ - 128 Figure 6-57 Conductivity changes of WO3 nanoparticles upon exposure to synthetic air, H2 atmosphere (1000 - 25 000 ppm) (left) and NH3 atmosphere (750 - 3000 ppm) (right) as a function of temperature . .................................................................................................................................................................. - 129 Figure 6-58 Change of the relative resistance of WO3 as a function of (a) hydrogen, (b) ammonia concentration and temperature. ................................................................................................................ - 130 -. XIII.

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(19) Introduction. 1. Introduction Since the early 1920s, numerous investigations have been undertaken to demonstrate. the influence of the gas atmosphere on conductivity, free carrier mobility, work function and surface potential of semiconductor materials. Upon these results a new phenomenon of the reversible changes of the semiconductor conductivity in response to changes in gas concentration has been introduced i.a. by the theory of surface traps of Bardeen and Brattain [1]. This discovery brought the solution for monitoring escalating air pollution resultant of the intense development of societies and growing industrialization. In view of these facts monitoring and control of the anthropogeneous, industrial and automotive exhausts, the highly sensitive, selective, stable, reliable and long-life sensors capable of detecting and measuring gases in the ambient and at the generating source are the utmost necessity. Though instrumental techniques such as gas chromatography or infra-red absorption widely used for the accurate analysis of the gas samples even at the trace levels are available, they fail due to a high initial costs, complicated maintenance, size and weight, high power consumption, the need of the qualified personnel and low-time resolution. In such systems in-situ continuous monitoring of the gas environments becomes time-consuming and un-realistic. It has been already four decades of the rapid growth in the sensor technology since the Taguchi Gas Sensor introduction to the market. However, most of the research encompasses so called users and developers approach, i.e. testing of the phenomenological parameters of available sensors and empirical optimization of the sensor technologies, devoted to the developing of new gas sensing materials as well as their preparation methods, testing structures, conditioning and measuring procedures. The use of doped or otherwise modified metal oxides and their composites was a major trend in recent years for the conventional semiconductor gas sensors. However, the interrelation of synthesis, processing, operation, performance and reliability of such sensors is not yet completely understood. Of particular scientific and technological interest is the interaction of gases with metal oxides, which is also relevant e.g. for heterogeneous catalysis or fuel cells next to gas sensors. The understanding of the principles of surface and/or bulk reaction responsible for gas sensing is expected to lead to the development of gas sensors with increased sensitivity, selectivity and stability. Recent development of the spectroscopic tools, devoted to the assessment of the physicochemical processes taking place in a sensing material in a real time and under operating conditions, has introduced a -1-.

(20) Introduction. new trend in the research on gas sensors, undertaken by basic research scientists. In-situ and operando methods are important for all device-related studies, including gas sensors. The historical “pressure gap” dilemma is now overcome with ambient pressure or close to ambient pressure XPS and NEXAFS [2]. By applying spectroscopies along with the phenomenological techniques of sensor testing they contribute to the deep understanding of sensing mechanisms on the molecular scale. The present work can be identified with the approach undertaken by basic science researchers, however aims towards closing the gap between the basic and applied research by applying different spectroscopic techniques. In order to obtain a promising material for gas sensing application, the high temperature flame spray synthesis route was chosen for the synthesis of the semiconductor metal oxide nanoparticles, as it allows reliably control of crystallinity and phase composition, so that the required electronic structure is also met. Further on complex spectroscopic techniques were applied so as to elaborate scientific understanding of the role of the electronic structure and the resulting electric behavior of non-stoichiometric semiconducting metal oxides under applied atmospheric and thermal conditions. For the first time the AP-XPS and AP-NEAXFS studies at high temperature under hydrogen and oxygen ambient were performed on Fe2O3 nanoparticles, as a candidate for gas sensors. The chemistry of the gas/solid interface and nature of the conduction mechanism taking place at this interface are discussed and correlated with the reducing gas sensing performance over the wide range of gas concentration, as in the context of a future projected hydrogen economy, efficient and reliably operating hydrogen gas sensors not only in low ppm region but also over the wide range of gas concentration, will be a necessary commodity. The thesis consists of the theoretical and experimental parts organized in the following chapters: Chapter 1: includes in its contents an introduction to the topic of the metal oxide nanoparticles for resistive gas sensors and motivation of this work Chapter 2: covers the discussion and the literature review on the semiconductor metal oxide based gas sensors, semiconductor metal oxide nanoparticles for gas sensing applications in respect of their electronic structure, optical properties, non-stoichiometry and surface properties as well as phase modification vs. gas sensing application Chapter 3: describes the general model and basic principles of the interaction of gas molecules with the semiconductor metal oxides -2-.

(21) Introduction. Chapter 4: encloses the description of the applied synthesis route (flame spray synthesis) and its advantages as well as details of the synthesis of each metal oxide Chapter 5: gives an overview on the analytical assessment chosen for this work and its description Chapter 6: describes results of the undertaken analytical assessment with the strong focus on the applied spectroscopic techniques (DRS, standard XPS and NEXAFS, ambient pressure XPS and NEXAFS, EIS); encloses the discussion of the results in the view of the great potential of the investigated metal oxide nanoparticles for reducing gas sensing application, as verified with the gas sensor performance assessment Chapter 7: summarizes results of this work, by highlighting the main achievements as proposed correlations found between structural, optical, electronic properties vs. electrical transport and gas sensing performance of FSS-prepared semiconductor metal oxide nanoparticles.. Figure 1-1 Overview of a complex investigation into the physico-chemical properties of metal oxide nanoparticles prepared by FSS.. -3-.

(22) Semiconductor metal oxide (MOx) based gas sensors. 2. Semiconductor metal oxide (MOx) based gas sensors. 2.1. Review of research on metal oxide based gas sensors The origin of the semiconductor metal oxide (MOx) based resistive gas sensors. research topic is associated with names of Bardeen and Brattain [1], who in 1952 for the first time reported on direct evidence for the existence of a space charge layer at the free surface of a semiconductor (in that report of germanium) and its manifestation and applicability for the gas sensing materials. Later on in 1962 this aspect was further investigated by Seiyama et al. [3], who proposed a new gas detector based on semiconductor metal oxides (with the example of ZnO). This idea was them adopted and developed by Taguchi and in 1962 he delivered the first patent on SnO2 based resistive type of gas sensor [4] and implemented this idea to the commercial production (at present Figaro Engineering Inc. with the most popular product Taguchi Gas Sensors TGS). Since then there has been permanent search for new semiconducting materials and on the development of the gas sensor technology in order to satisfy numerous performance requirements. The phenomenological parameters of available sensors have been extensively studied. Along with the research and development devoted to the optimization of sensor material and performance testing, also a deep insight into the basic properties and working principles of these gas sensing materials is an ongoing quest of basic research scientists. Numerous literature in the field of resistive gas sensors delivers a broad overview on the development in this field and gives an image of future aspect [514]. The performance features of gas sensors can be tailored by the use of new materials with different crystal structure, particle size and morphology, such as multi-component metal oxide systems (composites, solid solutions, doped materials either with the noble and/or transition metal oxides or with inert impurities), what can be usually achieved by applying different synthesis routes. Another opportunity is an unusual architecture of such materials e.g. heterostructures such as (nano-)composites or more complex core-shell cluster structures. Plasmonic systems are considered to be also an interesting possibility. Such modifications besides the formation of favored material microstructure (grain size and morphology, surface architecture, porosity etc.) can strongly affect its physicochemical properties e.g. formation of preferred active phases, modification of stoichiometry, improvement of catalytic activity of base oxide, stabilization of a particular valence state, chemical and thermal stabilization, increase concentration of the -4-.

(23) Semiconductor metal oxide (MOx) based gas sensors. free charge carriers, and their increase of the electron exchange rate, decrease of the surface potential and inter-crystallite energy barriers [6, 13, 15-18]. The selection of the second component in solid solution/composites and doped-materials is based on its ability to form metal oxides with different valence states, chemical inertness towards base material, catalytic activity, type of conductivity, mutual solubility (to some extent) [1930]. One has to be also aware that modifications of the contact geometries and operating conditions have as well a great impact on the gas sensor performance [6, 15]. Concerning the design and technology of sensors a large fraction of the research and development is devoted to development of the suitable synthesis route of metal oxide nanoparticles and deposition methods as well as to the miniaturization of the final device [31-35]. List of the developed synthesis routes is impressive and includes amongst the following techniques: sol-gel [15, 36-42], precipitation from solutions [43-51], gas phase condensation [52-55], decomposition of metaloorganic precursors [56], oxidation of metals [48, 57], hydrothermal treatment of colloidal solutions [41, 58-61], non-aqueous synthesis [62], laser ablation [63, 64], mechano-chemical processing [21], template synthesis [65, 66], thermal evaporation [67, 68], plasma enhanced atomic layer deposition [69], molten-salt method [70], electrospinning [71, 72], solid-state reaction [27, 28, 45, 48, 73], and flame spray synthesis [74-80]. Nowadays, most of the state-of-the-art semiconductor metal oxide based gas sensors is based on the nanocrystalline materials prepared with above mentioned methods and deposited as thick films. An extensive activity is as well observed in the aspect of the deposition techniques with the aim of elaboration of compatible techniques with the gas sensing device fabrication technology. The following categorized deposition techniques are available and under continuous development: sputtering and evaporation techniques (magnetron sputtering, e-beam evaporation, rheotaxial growth and oxidation - RTGO), aerosol techniques (chemical vapor deposition - CVD, flame spray synthesis deposition), sol-gel based techniques (spin-coating, drop-coating, spray-coating, immersion, dip-pen nanolithography) and thick film deposition techniques (screen-printing, drop deposition). These deposition techniques are usually associated with the synthesis methods described and referenced above. A comprehensive overview on the recent developments in the deposition techniques is given by Graf et al. [8]. The choice of a highly sensitive and selective gas sensor devices working in real operating conditions is still limited on the market, hence the intensive research, however not only on the materials and sensor design, but also on. -5-.

(24) Semiconductor metal oxide (MOx) based gas sensors. the deep insight and understanding of basic operating principles. Macroscopic performance parameters of the gas sensor should be strongly related to these basic principles, what in turn will bring the opportunity for the eventual gas sensor optimization. This ultimate need of the basic understanding of the gas sensing mechanism was strongly underlined and several times successfully approached by the group of scientist from the Professor’s Weimar group from the Institute of the Physical and Theoretical Chemistry, at Tübingen University in Germany, as well as by the scientists from the Siemens AG, Corporate Technology in München. They delivered several reports in this respect [12, 81-87] as well presented and discussed their ideas and results at several conferences devoted to the gas sensor field. In order to reach the set goal, they suggest in addition to the phenomenological sensor testing (such as conductivity measurements) to apply spectroscopies, perform quantum mechanical calculations, determine simple gas sensing operation models, assess the thermodynamic and kinetic aspects of sensing mechanism on the molecular scale. It was several times suggested, that surface spectroscopy in this respects is an essential tool. Spectroscopic techniques can be then applied either under idealistic atmospheric conditions (UHV - ultra high vacuum), under ex-situ conditions (after e.g. thermal and atmospheric treatment) or under in-situ or operando real operation conditions. The firstly performed in-situ measurements included Fourier. Transform Infrared. (FTIR),. Electron. Paramagnetic Resonance. (EPR). spectroscopy as well as complimentary Raman spectroscopy [88, 89]. Later on this idea was adopted by Pohle et al. [86, 87], who applied the infra red emission spectroscopy (IRES) to study the adsorption of water, hydrogen-containing gases and oxygen on semiconductor metal oxides. Gurlo et al. [82] gave a detailed introduction to the in-situ and operando measurements on the gas sensors among others, giving a raise to the intensive activity in this respect. Important contribution was also given by Koziej et al. [85], who reported on the influence of annealing temperature on the CO sensing mechanism of SnO2-based gas sensors. They applied operando simultaneous electrical and infrared studies with Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). Similar approach was presented in the report given by Grossmann et al. [81], where the hydrogen, water and oxygen molecules interplay and its influence on the gas sensing mechanism was presented. In turn Sänze et al. [90] delivered interesting operando Raman spectroscopic study on the ethanol gas sensing mechanism. Turn from the IR and Raman spectroscopic techniques towards X-ray absorption (XAS) techniques. -6-.

(25) Semiconductor metal oxide (MOx) based gas sensors. was then taken again by Koziej et al. [84] and Hübner et al. [83]. They applied in-situ XAS in fluorescence mode, both Extended X-ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES also called NEXAFS - Near Edge X-ray Absorption Fine Structure) spectroscopies on Pd-SnO2. Summarizing, today’s physics, chemistry and technology of gas sensors demands a better understanding of the sensing processes. It is expected that derived knowledge and experience from the above briefly reviewed reports on the spectroscopic assessment of the sensing mechanism of semiconductors metal oxide based gas sensors under real operating conditions, will further affect an increased interest among scientist. This in turn can result in tremendous advancement of the gas sensor technology. This spectroscopic approach should be also adapted to the new sensing materials and not only, as discussed above, to the most common sensing material SnO2. 2.2. Gas sensing semiconductor metal oxide (MOx) nanoparticles. 2.2.1. Gas sensing technology from the perspective of nanotechnology. Coarse grained, micron scaled polycrystalline metal oxides were the first materials commonly used in the construction of gas sensors. However, their sensing signal and resulting sensitivity is very poor. After the Feynmann’s lecture “There’s plenty room at the bottom” in 1959 at Caltech, scientist of the gas sensor community have been also inspired and noticed the new opportunities in materials scaled down to nanometers range. Nanostructuring of metal oxides can enhance the performance of functional materials as it gives them unique properties such as: increased surface-to-volume ratio, which provides more surface area for both chemical and physical interactions; significantly altered surface energies that allow tuning and engineering of the material’s properties, as atomic species near the surface have different bond structures than those embedded in the bulk; quantum confinement effects, due to the inherently small size of nanostructured materials, that significantly influences charge transport, electronic band structure and optical properties. Since then, the particular focus is put on nanosenors and their peculiar behavior, mostly due to quantum nature of observed effects. Nanosensors of non-porous, uniform nanoparticles potentially perform better. As it is known sensing signal results from the interaction of gas molecules with the surface layer of the semiconductor metal oxides. Thickness of this layer λ is usually less than 100 nm. Using materials composed of coarse grains of a radius r greater than λ. ≫. , the value of sensing signal is diminished by -7-.

(26) Semiconductor metal oxide (MOx) based gas sensors. the factor of ⁄ . Therefore, nanomaterials with decreased particle size and increased. specific surface area (high surface-to-volume ratio) are highly recommended for gas sensing applications. In such case, the surface phenomenon overtakes the chemistry and physics of the bulk. This effect is also important for the miniaturization trend of the gas sensing devices. Consequently by use of nanosensors the following improvements of sensor performance parameters can be introduced: increased sensitivity, lower and broader detection limits, fast response and recovery time, possibility of direct detection [91] etc. Currently several nanostructures is used in the development of nanosensors, mostly nanoparticles, nanorods, nanotubes as well as embedded nanostructures and selfassembled materials [11]. Nanoparticles (NPs) may behave as a zero-dimensional quantum dots with discrete energy levels, that can be controlled by their size [92], hence their outstanding optical and electrical properties. Nanoparticles are usually basic units of the bulk nanostructured sensors. The high ratio of surface atoms with free valences to the total amount of atoms as well as nanosized microstructure of nanosenors leads to the high current flow, when exposed to the detected gas molecules. Stark and Pratsinis [93], one of the developers of highly efficient nanoparticle synthesis route - flame spray synthesis, have also recognized the importance of scaling the size of new functional materials down to the nanometers range: “Nano is big. Nano will do it. You need nano in your product line, in your project titles. As a matter of fact, the small Greek word has become highly fashionable in science: While in the 1980’s, the word nano was limited largely to the unit system and the Snowwhite story, in the early 1990’s, it became a word on its own”. 2.2.2. Metal oxide nanoparticles for gas sensing application. Polycrystalline metal oxides, such as Fe2O3, SnO2, TiO2, WO3 and ZnO are very well known for their gas-sensing properties, as well as for their ease of production by conventional methods, yielding bulk material either with crystalline or non-crystalline structure [45, 60, 94-97]. SnO2 still remains the most common material for resistive type of gas sensors due to its high sensitivity and good morphological and chemical stability. However, the ZnO was the pioneering material in gas sensing [3]. Nowadays, the spectrum of metal oxide nanoparticles for gas sensing applications gradually broadens. It should be also noted here that these semiconductor metal oxides due to their unique physical properties are intriguing materials, not only gas sensors, but also for a diverse -8-.

(27) Semiconductor metal oxide (MOx) based gas sensors. research topics and technological/industrial applications such as catalysis and photocatalysis, photovoltaics, LED, fuel cells, batteries, data storage. Iron oxide polymorphs - Fe2O3 (α and γ) Iron (III) oxides in their most thermodynamically stable polymorphic phase of hematite α-Fe2O3 and metastable maghemite γ-Fe2O3 (transforms into α-Fe2O3 at temperature of around 823 K) are intriguing materials for gas sensing applications. The one of the strong advantages is their abundance as they are constituents of rocks, soil, sediments, and products of the chemical weathering, bacterial processes and corrosion of the steel. From the physical properties point of view they are narrow band gap semiconductors within the range of 1.8 - 2.2 eV (depending on their modification), chemically stable, exhibiting interesting magnetic and optical behavior. α-Fe2O3 has the corundum structure (with hexagonal unit cell) constructed of iron atoms surrounded by six oxygen atoms are not bonded at the corners of a regular octahedron (Figure 2-1). γFe2O3 in turn is iso-structural with magnetite (cubic crystal system) (Figure 2-1), but with cation deficient sites. Eight cations occupy tetrahedral sites and remaining are randomly distributed over the octahedral sites. Vacancies are confined to the octahedral sites. Fe2O3 is a magnetic insulator with [Ar]3d5 electronic configuration. The valency of Fe plays an important role in electrical conduction. Nearly stoichiometric Fe2O3 is quite inert for the surface gas adsorption, probably due to the stability of the half-occupied d-band configuration. Defects-rich surface or defect induction by the incorporation of impurities (e.g. foreign metal cations Ti4+, Sni4+, Zr4+, Si4+) strongly modifies physico-chemical properties by providing more active sites for the interaction with gas molecules [23, 25, 27-29, 55, 98, 99]. Thus, the broad variety of their application such as magnetic separation systems [100], biomedical materials (e.g. drug delivery systems) [101], magnetic storage devices [102], heterogenous catalysts (e.g. Fisher-Tropsch reaction) [103], visible light active photocatalysts [104-106], high efficient solar cells (photoanode materials for water splitting and hydrogen generation) [107, 108], lithium-ion batteries [109], next to the leading in this article gas sensor application [55, 61, 110, 111]. As resistive gas sensors pure or modified Fe2O3 nanoparticles have been so far employed for the detection of ethanol (EtOH) [27, 28, 55, 61, 109, 111-114], liquefied petroleum gas (LPG) [55, 112, 113, 115], hydrogen (H2) [77, 78, 109, 113, 116], ammonia (NH3) [78, 112], acetone ((CH3)2CO) [55, 65, 112, 117], hydrogen sulfide (H2S) [51, 118], water. -9-.

(28) Semiconductor metal oxide (MOx) based gas sensors. vapor (H2O↑) [119, 120], propanol (C3H7OH) [111], aliphatic hydrocarbons (e.g. CH4, C4H10) [110], formaldehyde (HCHO) [121] and acetic acid (CH3COOH) [61]. Recently, also switching of conductivity type of iron (III) oxides, which depends on the material doping, measurement conditions and chosen gas agent, has also attracted considerable attention [111]. Lee et al. [122] reported about the p-type conductivity of αFe2O3 nanowires with induced oxygen vacancies, which switched the conductivity to ntype after annealing in reducing atmosphere at low temperature. Gurlo et al. [123] showed for α-Fe2O3, that by changing the gas composition and operating temperature it was possible to control the crossover of conductivity from p-type to n-type. Vasiliev et al. [116] showed that doping of α-Fe2O3 with low concentrations of ZnO, caused changes of the conductivity from p-type to n-type, under exposure to reducing gases. This effect can have a significant impact on the gas sensing properties of iron oxide, in particular on their selectivity towards either reducing or oxidizing gases. From the electrical and electronic behavior point of view n-type conductivity (usually expected and reported for hematite) is associated with the presence of the free electrons established by the oxygen vacancies and excess of iron sites, while p-type conductivity due to presence of the iron vacancies and associated electron holes.. Figure 2-1 Unit cell of corundum-like hematite (left) and cubic magnetite (right). The red spheres indicate iron atoms and silver spheres indicate oxygen atoms.. Tin dioxide - SnO2 Tin (IV) dioxide (also called stannic oxide) is the most extensively studied gas sensing material and it is the dominant choice for solid-state gas sensors in domestic, commercial and industrial settings. SnO2 benefits gas sensors with its morphological and chemical stability, low operating temperatures, high sensitivities, mechanical simplicity of sensor - 10 -.

(29) Semiconductor metal oxide (MOx) based gas sensors. design, possibility of miniaturization and low manufacturing costs. However, SnO2 has also some disadvantages such as poor selectivity, unsatisfactory response and recovery times, and as well low thermodynamic stability at elevated temperatures. The most thermodynamically stable crystal structure of tin dioxide is rutile structure (see Figure 2-2). SnO2 is a n-type wide band gap semiconductor with Eg of around 3.6 eV, chemically stable, exhibiting strong non-stoichiometry. Its electronic configuration is [Kr]4d10. The non-stoichiometric character results from its interesting surface architecture. The bridging oxygen ions lying above the main surface plane can be easily detached either by the thermal treatment or by the ion particles bombardment. The two electrons set free can occupy then 5s and 5p orbital mixture on the surface of reduced Sn ions. Due to its outstanding optical, electrical and mechanical properties, SnO2 is a versatile material and has found wide range of applications. It is an attractive material not only for the gas sensors but also as a catalyst of the organic compounds oxidation [124, 125], a key component in Li batteries [126, 127], master element in opto-electronic devices [68, 128], antistatic coatings [129], electrodes in glass melting furnaces [130], special coatings for energy-conserving windows [131], dye-sensitized solar cells [132], photocatalysts [133135]. Available literature on the gas sensing properties of pure or modified SnO2 nanoparticles proves variety of gases possible for detection: carbon monoxide (CO) [15, 21, 43, 49, 52, 63, 69, 99, 136-143], nitrogen dioxide (NO2) [21, 99, 139, 144], hydrogen [15, 45, 57, 69, 140], ammonia [69, 71], hydrogen sulfide [25, 99], ethanol [46, 47, 52, 69-71, 144, 145], methane [37, 44, 49, 59, 140, 143, 146], other volatile organic compounds (VOC – e.g. acetone, toluene, isopropanol, gasoline, toluene, cyclohexane, benzene, triethylamine, methanol, propanol, buthanol etc.) [30, 37, 70-72, 144, 145, 147] and oxygen (O2) [137, 147], chlorine (Cl2) [144], nitrogen (N2) [148]. The sensing performance of the SnO2 can be strongly affected by the modifications with the additives of the foreign metals [21, 98, 149-153] as well as noble metals [47, 49, 63, 79, 137, 143, 150, 154-156] or metal oxides [23, 25, 48, 99, 150, 157-160]. Such complex systems benefit from the combination of the best performance parameters of their individual components. Some of the modifications can lead to distinct changes of the uppermost property of SnO2, namely its conductivity type. Galatsis et al. [21] reported that modification of SnO2 with Fe can lead to its conductivity change from the n-type to ptype and this change is strongly related to the given thermal conditions.. - 11 -.

(30) Semiconductor metal oxide (MOx) based gas sensors. Titanium dioxide - TiO2 Titanium (IV) oxide (also called titania) is the most common and thermodynamically stable titanium oxides and most thoroughly studied. Naturally occurs in three polymorphic forms of anatase (Figure 2-2 left), rutile (Figure 2-2 right) and brookite (not relevant in this work). It can be also found in other minerals such as ilmenite or titanit. Anatase and brookite are metastable phases transforming into the most stable phase of rutile in the temperature range of 973 - 1173 K.. The comparison of the basic. crystallographic and physical properties of TiO2 can be found in a broad literature [161, 162]. The main physico-chemical properties are the wide band gap energy Eg of around 3.2 eV for anatase and 3.03 for rutile, n-type conductivity (anatase), high refractive index, non-toxicity and chemical stability, high resistance, high light absorption in the UV range and weak over the visible range. Its electronic configuration is as follow [Ar]3d0. Due to its unique properties TiO2 is considered as the most important semiconductor metal oxide for wide spectrum of applications. After Fujishima and Honda discovery in 1972 [163] TiO2 has been extensively studied as a photocatalyst, either as an anodes in light harvesting and water splitting [164, 165] or as a catalyst for photocatalytical decomposition of organic compounds [166-170]. It is commonly used in pigments (varnishes, paints, plastics, paper, porcelain etc.) [171], optical devices (coatings, waveguides, electrochromic layers etc.) [172-174], cosmetics, pharmaceuticals, food additives, biomaterials engineering, catalysts supporters and promoters [175, 176], ceramic membranes [177], self-cleaning and anti-fogging layers [178], electronic materials (resistors, varistors, capacitors) [179] and what is the topic of thesis sensors (gas sensors, lambda sensor, humidity sensor). TiO2 is an interesting material for gas sensing applications due to its abundance, low cost, chemical and thermodynamic stability. However it operates rather at high temperatures. The stoichiometric TiO2 at room temperature is gas insensitive. Therefore surface defects are necessary for chemisorptions of gas molecules. Their presence can be greatly observed as dramatic changes in the electronic structure of TiO2. Defects can be created either by ion or electron bombardment or by thermal treatment. TiO2 in both rutile and anatase form has been widely studied as a gas sensitive material toward detection of such gas molecules as: carbon monoxide [20, 22, 40, 96, 180-182], ethanol [38, 40, 182-184], hydrogen [74, 75, 185-187], methane (CH4) [181], hydrogen sulfide and carbon disulfide (CS2) [188, 189], LPG [190, 191], ammonia [192], trimethylamine [193], other organic volatile compounds (e.g. benzene,. - 12 -.

(31) Semiconductor metal oxide (MOx) based gas sensors. xylene, acetone, formaldehyde, methanol, propanol) [38, 45, 183, 189, 194], nitrogen dioxide [22, 180], oxygen [22, 186, 187] and water vapor [195, 196]. Further improvement in the TiO2-based resistive gas sensors is expected via modifications with catalytic noble metals [60], mixed oxides systems [22, 189, 197-199] or by addition of the pentavalent metal dopants [20, 38, 96, 180, 200].. Figure 2-2 Unit cell of anatase (left) and rutile (right) TiO2. The yellow spheres indicate Ti atoms (or Sn in case of SnO2) and silver spheres indicate oxygen atoms.. Tungsten oxide - WO3 Tungsten (VI) oxide and its sub-stoichiometric forms are transition metal oxides, whose importance is growing equally not only in the field of chromism, photocatalysis but also gas sensing. WO3 is a wide band gap semiconductor (Eg of around 2.6 eV for monoclinic WO3) well known for its non-stoichiometric properties due to considerable amount of oxygen deficiency. WO3 crystals crystallize in the ReO3 structure, showing similarities to perovskite crystal structure based on corner-sharing WO6 octahedra (Figure 2-3). It occurs in several crystal phases that undergo transformation during annealing and cooling in the following sequence: monoclinic II (ε-WO3, < -43°C) → triclinic (δ-WO3, 43°C to 17°C) → monoclinic I (γ-WO3, 17°C to 330°C) → orthorhombic (β-WO3, 330°C to 740°C) → tetragonal (α-WO3, > 740°C). Monoclinic phase is the most stable at room temperature. WO3 is a chromogenic material (electro-, chromo- and thermochromic) as undergoes a reversible color change under particular conditions and shows high catalytic behavior both in oxidation and reduction reactions on its surface. Its electronic configuration is as follow [Xe]4f145d0. Versatile and unique properties of WO3 have drawn the attention of scientist working on the functional materials covering research. - 13 -.

(32) Semiconductor metal oxide (MOx) based gas sensors. fields. ranging. from. condensed. matter. physics. to. solid-state. chemistry:. photoelectrochemical anodes for water splitting [201-204], co-catalysts in acid-catalysed and photocatalysed reactions [205, 206], chromic devices [201, 202, 207-209] as well as gas sensors. It was demonstrated by many groups that WO3 has outstanding sensing performance towards nitrogen oxides (NOx) [42, 53, 210-218], ammonia [214, 219-222], hydrogen sulfide [54, 216, 217], hydrogen [217, 223, 224], ethanol [214, 225], other organic compounds (e.g. methane, ethylene, methyl alcohol, benzene, toluene, styrene, xylene, petrol) [97, 226, 227] as well as carbon monoxide [214, 216, 217] and oxygen [199]. Among metal oxides WO3 is considered to be the most promising candidate for the ammonia detection. As it will be presented in 6.3.6, WO3 is indeed a good ammonia sensing material. WO3-based gas sensors exhibit high sensitivity, fast response and recovery as well low working temperatures. Many attempts have been taken to improve its performance. Activation by noble metals [211, 226] and incorporation in mixed oxide systems [199, 211, 215, 226] was found to increase its sensitivity and selectivity and decreased operating temperature.. Figure 2-3 Crystal structure of monoclinic WO3 with corner-sharing arrangement of octahedral and with highlighted unit cell (left), ReO3-type crystal structure with outlined unit cell (right).. Zinc oxide - ZnO Zinc oxide is a wide band gap semiconductors (Eg around 3.2 eV), which has been investigated as an electronic material for many decades. It is a n-type semiconductor as the dominant defects are identified with the oxygen vacancies. It belongs to the transparent conducting oxides. Its electronic configuration is [Ar]3d104s0. ZnO naturally occurs as mineral zincite and crystallize in the hexagonal wurtzite structure (Figure 2-4), which is the most stable in ambient conditions. Historically it is one of the first materials investigated as gas sensor, mostly due to the high mobility of its conduction electrons, good chemical and thermal stability under operating conditions. It is also attractive due to. - 14 -.