M.W. Roberts

Adsorption of nitrogen and ammonia by polycrystalline iron surfaces in the temperature range 80–290 K studied by electron spectroscopy

Abstract X-ray and uv induced photoelectron spectroscopy have provided information on the various molecular states of nitrogen formed on polycrystalline iron surfaces from dinitrogen and ammonia. At 85 K two distinct states are observed with N 2 (g) which have N(1s) binding energy values of 405.3 and 400.2 eV. These are in equilibrium with N 2 (g), are weakly held, and are desorbed on warming to 290 K leaving a nitrogen free surface. The two states are assigned to a molecularly adsorbed and linear species the former characterised by an N(1s) value of 400.2 eV and the latter by 405.3 eV. At 290 K nitrogen is adsorbed with a very low sticking probability (⩽10 −6 ) giving rise to an N(1s) value of 397.2 eV. This is undoubtedly the dissociatively chemisorbed species. At a nitrogen pressure of l Torr adsorption is “instantaneous” and the N(1s) value is 397 eV. No evidence for the unstable bridged and linear forms of nitrogen is obtained at 290 K although they may well be precursors to the formation of the strongly chemisorbed nitrogen species. Shifts in the N(1s) binding energy induced by subsequent oxygen adsorption are discussed briefly. At 85 K ammonia adsorbs largely in the molecular form with a broad N(1s) peak centred at about 400 eV but on warming to 290 K this splits to give two peaks one at 397 eV and the other at 400 eV. Interaction at 290 K leads to a dominant peak at 397.2 eV and a subidiary one at 400 eV. Helium (1) spectra support the assignment of the 397.2 eV peak to dissociated species (N, NH) and the 400 eV peak to molecular adsorption. The conclusions with N 2 and NH 3 are substantiated further by comparing the data with results for nitric oxide. The concentration of nitrogen adatom species formed from NO at 290 K and 10 −6 Torr is some ten times that formed from N 2 at 1 Torr and three times that from NH 3 at 10 −6 Torr and the same temperature.

Models for an adsorbed layer and their evaluation by comparison of LEED and optical diffraction patterns: The system W(112)O2

The LEED patterns produced by the system W(112)-(2 × 1)-O at θ < 0.5 are known to consist of sharp integral and streaked half order features and have been previously interpreted in terms of separate islands of adsorbed oxygen out of phase with each other in the [111] direction. The diffraction properties of various structures of this type are examined theoretically and by optical diffraction methods using laser light. The results show that if the island model was correct integral order spots would be streaked, in addition to those of half order. An alternative model is therefore developed which correctly simulates the LEED patterns. On the basis of a number of assumptions regarding the formation of a mobile precursor layer, diffusion mechanism and diffusion distances within this layer, and attractive forces between adsorbed and diffusing species, a simple statistical method is used to follow the progress of individual molecules between gas phase and final adsorption site. The resulting adsorbate layer consists of out of phase regions, many of which are in contact even at low coverage; vacant sites are permitted at random positions within each region. Changes which have been observed in details of the LEED patterns at high (2 × 1) coverage are shown to be consistent with an adsorbate rearrangement leading to elimination of the antiphase boundaries.

Chemisorption of oxygen by aluminium

Abstract The variation in the work function of aluminium films during oxygen interaction has enabled information to be obtained relating to the stability of chemisorbed oxygen and its subsequent incorporation into the sub-surface region. At both −195 °C and −183 °C chemisorbed oxygen is incorporated; the activation energy of the thermally induced process occurring from the completed oxygen monolayer is ⩽3 kcal mole −1 . At 23 °C two distinct kinds of behaviour were observed with oxygen; either very little change in work function or a substantial increase (0.6 eV). The present data are discussed with particular relation to the recent results and conclusions of Huber and Kirk.

XPS studies of donor and acceptor chemisorption of NO and CO on nickel oxide surfaces

Systematic variation on the defect (excess) oxygen concentration in the surface of nickel oxide preheated to 700, 1100 and 1450°C, has been revealed in O(1s) and Ni(2p32) X-ray photoelectron spectra. The magnitude of the surface charge, after evacuation at 500°C, is also directly related to the defect properties. The effect of chemisorption of NO and CO on the free hole concentration of the p-type semiconductor surface has been studied by monitoring the surface charge. NO adsorption, predominantly as negatively charged (electron acceptor) molecules removes much of the surface charge whereas CO adsorption, as COδ+ species (donor chemisorption), increases the surface charge on all samples.

Evidence from photoelectron spectroscopy for dissociative adsorption of oxygen on nickel oxide

XPS and UPS spectra from oxygen adsorption at high temperature on polycrystalline nickel oxide surfaces pre-heated at 700°C and 1450°C are presented. Adsorption results in complete loss of surface charge on both surfaces. There is an increase in intensity of the 0(1s) 529.7 eV peak, attributed to O2− ions, and the Ni(2p32) spectra show a shift of intensity to the 854.6 eV peak due to Ni2+ ions. The results are compared with previous data from kinetic, conductivity and electrochemical measurements. Agreement with a model of O2− and nickel vacancy production is demonstrated. UPS spectra indicate considerable reorganisation of electronic charge in the surface of the 1450°C pre-heated oxide, after oxygen adsorption, giving an almost stoichiometric surface similar to that of a cleaved single crystal.

The adsorption of oxygen on Cu(210)

Abstract The system Cu(210)-O 2 has been examined using LEED and AES, combined with optical simulation of diffraction patterns to investigate the detailed structure of the adsorbed layer. Exposure at 300 K and 5 × 10 −9 Torr resulted in LEED patterns showing pronounced streaks. The corresponding structures are believed to require an adsorption mechanism in which O 2 dissociation can occur only at a limited number of surface sites and in which O atoms after dissociation diffuse over quite large distances (≳10 nm) before becoming chemisorbed. Heating these structures to 500–600 K produced a sharp (2 × 1) pattern; this step is thought to involve equilibration of the adsorbed layer. Further combinations of exposure (⩾1 × 10 −6 Torr) and heating (up to 500 K) resulted in a series of (2 × 1) and (3 × 1) patterns, while heating to 800 K at any stage of the oxygen interaction regenerated the clean surface.

The interaction of hydrogen sulphide with Cu(001)

The interaction of H2S with a clean Cu(001) surface has been studied by LEED and Auger Electron Spectroscopy. Extra diffraction spots appear at (h + 12, k) and (h, k + 12) and these are interpreted in terms of two domains of (2 × 1) rotated at 90° to each other. This corresponds to a maximum surface coverage (S/Cu) of 0.5 at 295 K and a H2S exposure of ≈ 10L. There is prominent streaking in the half-order directions. Less than 5% of a monolayer of S is detectable by Auger spectroscopy.

Chemisorption of nitric oxide by nickel

Abstract The adsorption of nitric oxide on clean and pre-oxidized nickel has been investigated by X-ray photoelectron spectroscopy. Three distinct states of chemisorption have been recognised at room temperature; one is dissociative while two involve molecularly adsorbed NO. Pre-exposing the nickel surface to oxygen enabled the activity of the surface to be controlled such that adsorption was confined to only one of the molecular states. The two molecular states are suggested to arise from “bent” and “linear” forms of NO.

Nitrogen chemisorption by iron

Abstract The conversion of molecularly adsorbed nitrogen present on ion surfaces to the dissociatively chemisorbed state has been observed by X-ray photoelectron spectroscopy at low temperature. The molecularly adsorbed nitrogen reversibly adsorbed at 80 K is also immediately displaced when exposed to molecular hydrogen at the same temperature. A possible explanation is given for the low sticking probability (∼10−7) of nitrogen chemisorption observed on iron above room temperature.

Xps studies of surface charge on nickel oxide

Abstract Surface charge present on nickel oxide has been studied with samples prepared by heating the oxide at temperatures between 700 and 1450°C The effect of the charge on O(1s).C (1s) and Ni(2p) spectra has been examined in detail. The magnitude of the charge at 20°C is shown to be related to the defect concentrations and hence the bulk conductivity of the oxides The effect of increasing the sample temperature (up to 120°C) is to facilitate removal of the charge due to increased surface conductivity