P. J. Estrup

The chemical physics of surfaces

1. Introduction.- 1.1. Surface States and Surface Sites.- 1.1.1. The Chemical Versus Electronic Representation of the Surface.- 1.1.2. The Surface State on the Band Diagram.- 1.1.3. The Fermi Energy in the Surface State Model.- 1.1.4. Need for Both Surface Site and Surface State Models.- 1.2. Bonding of Foreign Species to the Solid Surface.- 1.2.1. Types of Interaction.- 1.2.2. The Chemical Bond.- 1.2.3. Acid and Basic Surface Sites on Solids.- 1.2.4. Adsorbate Bonding on Various Solid Types.- 1.2.5. Movement of Surface Atoms: Relaxation, Reconstruction, and Relocation.- 1.2.6. The Electronic Energy Level (Surface State) of a Sorbate/Solid Complex.- 1.3. Surface Hydration on Ionic Solids.- 1.4. Surface Heterogeneity.- 2. Space Charge Effects.- 2.1. General.- 2.1.1. The Double Layer Involving Two Planar Sheets of Charge.- 2.1.2. The Space Charge due to Immobile Ions: The Depletion Layer.- 2.1.3. The Double Layer in the Band Diagram, Fermi Energy Pinning.- 2.2. Space Charge Effects with Reactive Surface Species.- 2.2.1. The Accumulation Layer.- 2.2.2. The Inversion Layer.- 2.3. Electron and Hole Transfer between the Solid and Its Surface.- 2.3.1. Basic Physical Model of Electron and Hole Capture or Injection.- 2.3.2. Electron and Hole Transfer with Large Changes in the Surface Barrier.- 2.3.3. Charge Transfer to a Surface Species in a Polar Medium: The Fluctuating Energy Level Mechanism.- 3. Experimental Methods.- 3.1. Surface Measurements Based on Electrical and Optical Techniques.- 3.1.1. Work Function.- 3.1.2. Surface Conductivity.- 3.1.3. Electroreflectance.- 3.1.4. Field Effect.- 3.1.5. Surface Photovoltage.- 3.1.6. Capacity of the Double Layer.- 3.1.7. Channel Measurements.- 3.1.8. Powder Conductance.- 3.1.9. Ellipsometry.- 3.1.10. Other Electrical and Optical Measurements.- 3.2. The Surface Spectroscopies.- 3.2.1. Ultraviolet Photoelectron Spectroscopy (UPS).- 3.2.2. Energy Loss Spectroscopy (ELS).- 3.2.3. Soft X-Ray Appearance Potential Spectroscopy (SXAPS).- 3.2.4. Field Emission (FEM).- 3.2.5. Field Ion Microscopy (FIM).- 3.2.6. Ion Neutralization Spectroscopy (INS).- 3.2.7. Low-Energy Electron Diffraction (LEED).- 3.2.8. Methods of Chemical Composition Determination for the Surface.- 3.2.9. Studies of Chemical Reactions due to the Impinging Beam.- 3.3. Chemical Measurements.- 3.3.1. Infrared Absorption.- 3.3.2. Temperature-Programmed Desorption.- 3.3.3. Adsorption of Gaseous Acids and Bases or of Indicators.- 4. The Adsorbate-Free Surface.- 4.1. Introduction.- 4.1.1. The Classification of Solids.- 4.1.2. Preparation of a Clean Surface.- 4.2. Theoretical Models.- 4.2.1. Quantum Models.- 4.2.2. Semiclassical Models: The Madelung Model for Ionic Solids.- 4.2.3. Models for Electron Pair Sharing: Lewis and Bronsted Sites.- 4.2.4. Comparison of the Various Surface States and Sites.- 4.3. Measurements on Adsorbate-Free Ionic Solids.- 4.3.1. Reconstruction on Ionic Solids.- 4.3.2. Physical Measurements on Ionic Solids.- 4.3.3. Chemical Measurements on Ionic Solids.- 4.4. Measurements on Adsorbate-Free Covalent or Metallic Solids.- 4.4.1. Reconstruction on Covalent and Metallic Solids.- 4.4.2. Electrical Measurements of Intrinsic Surface States on Covalent Solids.- 4.4.3. Measurement by the Surface Spectroscopies.- 5. Bonding of Foreign Species at the Solid Surface.- 5.1. Reconstruction and Relocation in Bonding.- 5.2. The Semiclassical Model of Bonding: The Surface Molecule.- 5.2.1. Surface Molecule Versus Rigid Band Model.- 5.2.2. Adsorbate Bonding to Covalent or Metallic Solids.- 5.2.3. Adsorbate Bonding to Ionic Solids.- 5.2.4. Multilayer Adsorption: The Development of a New Phase.- 5.3. Quantum Models of the Adsorbate/Solid Bond.- 5.3.1. Solid State Theories: The Semi-infinite Crystal.- 5.3.2. Cluster Models.- 5.3.3. The Interacting Surface Molecule (the Model Hamiltonian Analysis).- 5.3.4. Other Quantum Models.- 5.3.5. Remarks.- 5.4. Measurement of Adsorbate Surface States on Covalent or Metallic Solids.- 5.4.1. Screening Shifts and Other Inaccuracies in Measurement.- 5.4.2. Bond Angles.- 5.4.3. Surface State Energy Levels of Sorbate/Sorbent Bonds.- 5.5. The Chemistry of Surface States.- 5.5.1. Change of Surface State Energy Associated with Bonding.- 5.5.2. The Influence of a Polar Medium or Coadsorbate on the Surface State Energy.- 5.5.3. Surface States due to Multiequivalent Foreign Adsorbates.- 5.6. The Formation of Surface State Bands.- 6. Nonvolatile Foreign Additives on the Solid Surface.- 6.1. General.- 6.2. Dispersion of Additives.- 6.2.1. Techniques for Dispersing Additives.- 6.2.2. Measurement of Dispersion.- 6.2.3. Sintering of Dispersed Particles: Surface Diffusion of Adsorbates.- 6.3. The Cluster, the Transition between a Molecule and a Solid.- 6.4. The Control of Surface Properties with Additives.- 6.4.1. Theoretical Discussion.- 6.4.2. Observations of Additive Effects.- 6.5. The Real Surface.- 7. Adsorption.- 7.1. Adsorption Isotherms and Isobars.- 7.1.1. Physical Adsorption.- 7.1.2. Heat and Activation Energy of Adsorption, Irreversible Chemisorption.- 7.1.3. The Adsorbate Superstructure.- 7.2. Ionosorption on Semiconductors.- 7.2.1. The Surface State Representation of Adsorbed Species.- 7.2.2. Observations of Ionosorption.- 7.3. Adsorption with Local Bonding.- 7.3.1. Adsorption on Ionic Solids.- 7.3.2. Adsorption on Platinum.- 8. The Solid/Liquid Interface.- 8.1. Introduction.- 8.2. Theory.- 8.2.1. Double Layers and Potentials in Electrochemical Measurements.- 8.2.2. Charge Transfer between the Solid and Ions in Solution.- 8.2.3. Energy Levels of Surface Species Relative to Band Edges.- 8.3. Observations with Semiconductor Electrodes.- 8.3.1. Measurement Methods.- 8.3.2. Radical Generation (Current Doubling).- 8.3.3. Measurements of Energy Levels and Band Edges.- 8.3.4. Other Charge Transfer Measurements, Capture Cross Section.- 8.4. Comparison of the Solid/Liquid with the Solid/Gas Interface.- 9. Photoeffects at Semiconductor Surfaces.- 9.1. General.- 9.2. Simple Hole/Electron Recombination.- 9.2.1. Theory.- 9.2.2. Experimental Results.- 9.3. Photoadsorption and Photodesorption.- 9.3.1. Theory.- 9.3.2. Experimental Observations of Photoadsorption and Photodesorption.- 9.4. Photocatalysis.- 9.4.1. Photodecomposition of Adsorbed Species.- 9.4.2. Photostimulated Catalytic Reactions.- 9.5. Direct Excitation of Surface States by Photons.- 10. Surface Sites in Heterogeneous Catalysis.- 10.1. General Concepts.- 10.1.1. The Role of the Catalyst.- 10.1.2. Some Correlations in Heterogeneous Catalysis.- 10.2. Surface Sites Associated with Steps and Other Geometrical Factors.- 10.3. The Role of Acid and Basic Sites in Catalytic Reactions.- 10.4. Covalent Bonding to Coordinatively Unsaturated Metal and Cationic Sites.- 10.5. Sites in Oxidation Catalysis.- 10.5.1. Introduction.- 10.5.2. Oxygen Exchange Sites in Oxidation Catalysis.- 10.5.3. Dangling Bonds as Active Sites for Adsorption and Electron Exchange.- 10.5.4. Wide Bands as Electron Sources and Sinks: n-Type and p-Type Semiconductors.- 10.6. Examples of Oxidation Catalysis.- 10.6.1. Platinum.- 10.6.2. Partial Oxidation Catalysts: Bismuth and Iron Molybdate.- References.- Author Index.

Surface studies by electron diffraction

Abstract The problems in the analysis of surface structures by means of electron diffraction, particularly at low energy, are reviewed. A brief introduction is given to the basic scattering and diffraction phenomena occurring at a solid surface after which the nature of the experimental diffraction data is described. The theoretical interpretation of the diffracted intensity by various kinematical and dynamical methods is outlined and the present difficulties in obtaining a complete surface structure determination are examined. The different types of ordered and disordered layers which may be formed on a surface—usually by adsorption of foreign species—are discussed and the interpretation of the corresponding diffraction patterns is illustrated by some examples.

Surface structures and phase diagram for the H/W(001) chemisorption system

The chemisorption of hydrogen on tungsten (001) induces displacive rearrangements (’’reconstruction’’) of the substrate surface, depending on the temperature T and the adsorbate coverage ϑ. By means of LEED and other techniques, the surface phase diagram in the T–ϑ plane (ϑ<0.7, T<500 K) has been studied. The predominant structure is (√2×√2) which is stabilized by hydrogen in the region ϑ<0.3, but is transformed to an incommensurate structure at higher coverages. Adsorption below ?200 K produces no ordered phase. The LEED patterns from a terraced surface exhibit reduced symmetry and reveal the direction of the lattice distortion; the atomic displacements are found to be along 〈10〉 for the H‐induced structures, in contrast to the 〈11〉 displacements on clean W(001). The substrate rearrangement is believed to be a major cause of the observed coverage dependence of the desorption energy and other adsorbate properties.

Desorption of hydrogen from tungsten (100)

Abstract The desorption energy, Ed, and the preexponential factor, ν, for the desorption of hydrogen from W(100) have been determined as a function of coverage, θ, from a series of isobars. The values of Ed and ν depend strongly on coverage and weakly on temperature; they show a compensation effect, both quantities decreasing at about 1 4 of the hydrogen saturation coverage: Ed drops from about 40 kcal per mole of H2 to about 20 kcal per mole of H2 and ν from 103 to 10−5 s−1 cm2. These values differ from those extracted from earlier thermal desorption studies, but they are compatible with the measurements reported. Correlation of Ed(θ) and ν(θ) with structural data for the H/W(100) surface shows that the higher Ed and ν values are associated with desorption from a reconstructed substrate. Effects of the reconstruction also appear in the coverage dependence of the partial molar entropy of the H/W surface layer,

Two-dimensional phases in chemisorption systems

ss, which is zero or less at low coverages and rises to about 10 cal/K per mole of H at 1 4 of saturation. Analysis of

Phase transformations of the H/W(110) and H/Mo(110) surfaces

s s (θ) indicates that the hydrogen-induced reconstruction reduces the entropy of the tungsten surface by 1k per W atom.

Adsorption of carbon monoxide on the molybdenum (100) surface

Abstract A discussion is given of two-dimensional phase transitions observed in chemisorption systems. The general problems encountered in experimental studies of these phenomena are reviewed. This is followed by a survey of the available data, after which some specific chemisorption systems are discussed: H/Fe(110) as an example of a lattice gas system; H/W(100) as an example of adsorbate-induced reconstruction; O/Ni(111) and H/W(110) as examples of critical-exponents studies.

The geometry of surface layers

We present results of combined theoretical and experimental studies of the surface phases produced by hydrogen adsorption on tungsten (110) and molybdenum (110) planes. We have formulated a theoretical model which has the following ingredients: (1) a possible adsorbate‐induced lateral shift reconstruction of the substrate surface; (2) two‐body and three‐body interactions among hydrogen atoms; and (3) a coupling of the overlayer ordering to the substrate reconstruction. This model gives good agreement with the observed phase diagram for H/W(110) which shows a (2×1) phase at low coverages and a (2×2) phase, as well as substrate reconstruction, at high coverages. In the case of H/Mo(110), for which no reconstruction is observed, the lack of a (2×1) phase suggests the presence of a strong three‐body interaction among the adatoms.

The dissociation of CO on modified Mo(110) surfaces

Abstract The interaction of CO with Mo(100) has been studied by means of thermal desorption spectrometry, work function measurements and electron stimulated desorption, in conjunction with LEED and AES. Results have been obtained for adsorption at room temperature and at temperatures down to 200 K. The study confirms previous results, showing that the β-states formed at room temperature are atomic. The thermal desorption data for the β-states are analyzed to give directly the desorption activation energy as a function of coverage. This energy is found to vary smoothly from an initial value of 3.7 to a final value of 2.9 eV molecule, indicating an average repulsive interaction between a pair of adjacent adatoms of 0.2 eV. The data at low temperature indicate that a molecular state, virgin-CO, is produced in competition with β-CO and probably one other state, from a common precursor. The step leading to virgin-CO has both a low activation energy and a low pre-exponential factor, suggesting that a reorientation of the molecule is required.

The kinetics of CO dissociation on Mo(001)

If we wish to understand the properties of a solid surface, we must begin with some picture of the geometrical arrangement of atoms in the outermost layers of the solid. Such a structural model represents an essential ingredient in any microscopic description of a material.