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Electron Spectroscopy of Rutile-Type Metal Oxides Sigrun Eriksen B.Sc

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Electron Spectroscopy of Rutile-Type Metal Oxides Sigrun Eriksen B.Sc 1 Electron Spectroscopy of Rutile-type Metal Oxides r Sigrun Eriksen B.Sc. A.R.C.S. A thesis submitted for the degree of Doctor of Philosophy of the University of London and for the Diploma of Membership of Imperial College. Department of Chemistry Imperial College London SW7 2AZ August 1987 2 Abstract This thesis presents a study of defective and defect-free rutile TiO (110) surfaces, using ultraviolet photoelectron spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS) and high resolution electron energy loss spectroscopy (HREELS). The use of argon ion and electron bombardment for creation of surface defects is investigated. An oxygen -20 -2 desorption cross section of 1.5 x 10 cm is found for electron irradiation, and a desorption mechanism suggested. An electronic excitation due to the oxygen vacancies is identified; this modifies the effective surface dielectric constant. The oscillator strength for this excitation is found to be 0.1, indicating an allowed transition. The adsorption of water on the TiO2(110) surface is also studied. From HREELS and He(II) UPS, it is found that water will not adsorb on a defect-free TiO^MIO) surface at 300K, but will adsorb on a surface in which oxygen vacancy defects have been created. It is shown that this adsorption involves dissociation of the water molecule, leaving 0-H species bonded to the surface. Finally, the effects of oxygen vacancies on the rutile SnO^MIO) surface are investigated. It is confirmed that argon ion bombardment gives rise to selective oxygen loss, and the Sn species on the 2 + 4 + resulting defective surface are Sn and Sn ; total oxygen loss is never greater than 502. This process also creates deep band-gap states which do not give rise to conductivity. Electron irradiation of the surface is found to be capable of desorbing at least 702 of surface 2 + 4 + oxygen, and as well as Sn and Sn , clusters of metallic tin are formed on the surface. EELS shows that this situation leads to a broad spread of bandgap states, whose distribution is not readily controllable. Acknowledgements I would like to express my thanks to all those people who have helped and assisted during the course of this work. In particular, I am very grateful to the following people: Russell Egdell, for supervision, advice and encouragement throughout the last three years. Wendy Flavell, for support, advice and valuable discussions, as well as considerable help with the SnO^ work. David Bassett for introducing me to surface science in the first place, for encouragement over the years and for the loan of a very useful power supply. Chris Jones, Bill Young, Mark Appleton, Humphrey Drummond and Steve Bleazard for a great deal of assistance with the computing; specifically Humphrey for help with Padread, Bill for the FFT algorithm, Mark for the Epson Emulator and Chris for introducing me to Pascal and the Z80. Chris Jones, again, for unfailing support, endless patience, and the loan of the Rotring drawing equipment. John Albery and his group, past and present, for providing a lively social life, and invaluable relief from the frustrations of research. Gail Craigie, for typing the figure captions and Zoeta Brown for numbering the pages. 4 Contents page Abstract 2 Acknowledgements 3 Table of Contents 4 List of Figures 8 List of Tables 11 List of Symbols and Abbreviations 12 Chapter 1 Introduction 15 1.1 The Structure of the Rutile Oxides 15 1.2 Applications of Ti02 21 1.3 Previous Studies of Ti02 - a Summary 22 1.3.1 Defects on Ti02 Surfaces 22 1.3.2 Adsorbates on Ti02 Surfaces 25 1.3.3 A Brief Look at Some Other Titanium Oxides 29 1.4 Applications of Sn02 30 1.5 Previous Studies of Sn02 - a Summary 31 1.6 An Outline of this Thesis 34 Chapter 2 Theory 36 2.1 Principles of Photoelectron Spectroscopy 36 2.1.1 Surface Sensitivity 38 2.1.2 Ultraviolet Photoelectron Spectroscopy 40 2.1.3 X-ray Photoelectron Spectroscopy 41 2.1.4 Fine Structure in PES 47 2.2 Auger Electron Spectroscopy 49 2.3 Low Energy Electron Diffraction 51 2.3.1 LEED for Lattice Characterisation 52 2.4 Electron Energy Loss Spectroscopy 53 2.4.1 The Nature of Dipole Scattering 54 2.4.2 Excitations in Dipole Scattering 57 2.4.3 EELS of Ionic Solids - a Brief Historical Survey 60 2.4.4 Introducing the Theory of Dipole Scattering 61 2.4.5 The Energy Loss Probability 62 2.4.6 The Explicit Form of the Dielectric Constant 71 Chapter 3 Apparatus and Experimental Methods 75 3.1 The Need for Ultrahigh Vacuum 75 3.2 The Ultrahigh Vacuum System 75 3.3 The HREEL Spectrometer 80 3.3.1 The Spectrometer Construction 80 3.3.2 The Effect of the Analyser Acceptance Angle 82 3.3.3 Calibration of the Spectrometer 86 3.3.4 Operation of the Spectrometer 86 3.3.5 Overcoming Difficulties 90 3.4 The General Surface Analysis Facilities 92 3.4.1 The Radiation Sources 93 3.4.2 The Energy Analyser 94 3.5 The LEED Optics 97 3.6 Data Collection and Processing 101 3.6.1 Data Collection 101 3.6.2 Data Processing 102 3.7 Sample Preparation and Mounting 105 3.7.1 Initial Sample Treatment 105 3.7.2 Sample Mounting 105 3.7.3 Cleaning the Samples In Vacuo 111 Chapter 4 The (110) Surface of Rutile TiO^ 112 4.1 Sample Preparation 112 4.1.1 Preparation of a Defect Free TiO2(110) Surface 112 4.1.2 Criteria for Stoichiometry 113 4.1.3 Criteria for Surface Order 116 4.1.4 Criteria for Cleanliness 121 6 4.2 HREELS of the Stoichiometric TiO^dlO) Surface 124 4.2.1 Symmetry and Phonons in the HREELS of TiO^ 124 4.2.2 The Effect of Anisotropy 131 4.2.3 The HREEL Spectrum of TiOgdIO) 131 4.3 The Creation of Oxygen Deficiency in TiO^MIO) 135 4.3.1 The Knotek Fiebelman Mechanism 138 4.3.2 The Practice of Electron Bombardment 138 4.3.3 The Oxygen Desorption Cross Section 140 4.3.4 The Depth of the Oxygen Vacancy Defects 145 4.4 The Effect of Oxygen Deficiency on UPS, ELS and HREELS 147 4.4.1 The UPS of Defective TiOgdIO) 147 4.4.2 The ELS of Defective Ti02(110) 151 4.4.3 The HREELS of Defective TiO (110) 155 4.5 Towards an Explanation 163 4.5.1 The Oscillator Strength of the Excitation 163 4.5.2 The Nature of the Defect Sites 164 4.5.3 The Final Model 165 4.5.4 Notes on Previous Work 166 Chapter 5 The Adsorption of Water on TiO^f110) 168 5.1 A Detailed Look at Some Previous Work 168 5.1.1 Water on TiO^ 168 5.1.2 Water on SrTi03 174 5.1.3 Water on Other Oxides 180 5.2 Some Experimental Aspects 181 5.2.1 Water Dosage 182 5.2.2 Sample Mounting and Attempts to Cool the Surface 182 5.3 UPS of Water on TiOgdIO) 184 5.3.1 The Spectra Obtained 184 5.3.2 A Discussion of the Results 187 5.A HREELS of Water on TiO (110) 190 Chapter B The (110) Surface of Rutile SnO^ 194 6.1 Sample Preparation 194 6.2 Oxygen Loss and the Desorption Cross Section 196 6.2.1 Oxygen Loss by Argon Ion Bombardment 196 6.2.2 Oxygen Loss by Electron Bombardment 198 6.2.3 ELS of the Defective Surface 205 Appendix A Classical Theory of Dipole Electron Scattering 208 Appendix B Program Collect 216 Appendix C Program Padread 221 Appendix D Program Stripper 229 Appendix E Program Fitter 235 Appendix F Program Eelsim 240 References 250 8 List of Figures page Figure 1: (a) structure of the rutile unit cell and (b) orientation of orbitals in the rutile unit cell 16 Figure 2: DOS diagram for TiO^ 18 Figure 3: DOS digram for SnO^ 19 Figure 4: Atomic arrangement of the (110) face of rutile 20 Figure 5: Illustration of the photoelectric effect 37 Figure 6: Variation of inelastic mean free path with photoelectron energy 39 Figure 7: UPS He(I) of defect free TiO (110) 42 Figure 8: UPS He(II) of defect free TiO (110) 43 Figure 9: Wide scan XPS of defect free TiO (110) 45 Figure 10:: Narrow scan XPS of defect free TiO^t110) 46 Figure 1 1 :: The Auger process 50 Figure 12:: Polar plot of the dipole scattering lobe in HREELS 55 Figure 13:: Illustration of the dipole selection rule 56 Figure 14 :: Phonon dispersion curve 59 Figure 15:: EELS dipole scattering mechanisms 63 Figure 16:: Plot of variation of {Q/[Q^+1]^> with Q 65 Figure 17:: Geometry of the EELS two-layer model 69 Figure 18:: Plot of variation of e(u>) with u) 73 Figure 19:: Photograph of the Leybold Heraeus vacuum system 77 Figure 20:: Schematic diagram of the vacuum system 79 Figure 21 :: Diagram of the construction of the HREELS optics 81 Figure 22:: The geometry of scattering in EELS 84 Figure 23:: Photograph of the HREELS instrumentation panel 88 Figure 24:: Montage of XPS MgKa Ag spectra at different Eq 98 Figure 25:: Plot of variation in FWHM with E in the spectra of figure 24 99 9 page Figure 26: Plot of variation in peak height with Eq in the spectra of figure 24 100 Figure 27: XPS of TiOgtHO) before and after stripping by the computer program "Stripper" 103 Figure 28: XPS of TiO^ with "fitted" curves as created by the computer program "Fitter" 104 Figure 29: Photograph of two of the samples used 106 Figure 30: Drawing of the room-temperature sample mount 109 Figure 31 : Drawing of the "low temperature" sample mount 110 Figure 32: XPS of TiO (110), defective and defect-free, showing Ti4+ and Ti3+ core peaks 115 Figure 33: ELS of TiO^lHO), defective and defect-free 117 Figure 34: UPS He(I) of TiO (110), defective and defect-free 118 Figure 35: UPS He(II) of TiOgtllO), defective and defect-free 119 Figure 36: LEED photograph of the ordered TiO^MIO) surface 120 Figure 37: Plot of the variation in HREELS TiO (110) elastic peak intensities with deviation of the collection angle from the specular 122 Figure 38: As figure 37, but for a well-ordered GaAs surface 123 Figure 39: XPS TiO_(110) showing the carbon peak for clean and contaminated surfaces 125 Figure 40: Diagram of the translations in the rutile unit cell involved in phonon excitations 129 Figure 41 : HREELS of defect-free TiO^tHO) 132 Figure 42: Calculated HREELS of defect-free TiO^MIO) 134 Figure 43: XPS of a typical argon-etched TiOgMIO) surface 137 Figure 44 : Diagram of the Knotek Feibelman Mechanism 139 Figure 45: Diagram of the electron bombardment assembly 141 Figure 46: XPS of TiO_(110) showing changes as electron bombardment progresses 142 Figure 47: Log plot of variation in surface oxygen concentration with electron bombardment for the TiOgUIO) surface 144 Figure 48: UPS He(I) of defect free and argon ion bombarded TiO (110) 148 Figure 49: UPS He(I); detail of fig.
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