Structural and reactive properties at the metal-halogen interface: The interaction of bromine with chromium (110)

The chemisorption and subsequent reaction of bromine on Cr(110 has been studied by Auger spectroscopy, LEED, Δφ, and thermal desorption measurements. For gas doses of < 7.5 × 10¹⁸ molecules m^?2, very efficient dissociative chemisorption leads to a series of well-ordered, out-of-registry compression structures. Uniquely, however, the overlayer falls back into registry at saturation coverage; at this point the appearance of glide symmetry indicates that the three-fold coordinated adsorption sites are occupied exclusively. Bromine → metal charge transfer occurs during adsorption (in contrast to Cr(100)). On raising the temperature at low coverages, the surface phase decomposes by evaporation as CrBr molecules; at higher coverages the desorption product switches to CrBr₂. Continuous growth of bulk CrBr₂ sets in at high gas exposures, this corrosion reaction proceeding at a rate which is ten times slower than the rate of overlayer formation. The chromium dibromide layer also evaporates as CrBr₂(g). Structural relationships with related metal-halogen systems are discussed.     

Chemisorption and corrosion at the metal-halogen interface: Overlayer growth and compound formation by bromine on Cr(100)

The interaction of bromine with Cr(100) has been studied in the regime 300–1300 K and over a wide range of coverage. The initial reaction results in the formation of an overlayer which exhibits a continuous series of compression structures whose end members are c(2 × 2) and c(2 × 4). The sticking probability of Br₂ remains constant at 0.9 ± 0.1 during this process. Beyond this stage, nucleation and growth of CrBr₂ commences at a rate which is ten times slower than that of the initial chemisorption reaction. The stoichiometry of the bromide phase is kinetically controlled and it grows by the Volmer-Weber mechanism to yield an epitaxial corrosion layer of a pseudohexagonal form of CrBr₂. Intensity changes and kinetic energy shifts in the Cr and Br Auger spectra indicate the occurrence of substantial valence charge transfer from metal to halogen. The structural, thermochemical and electronic properties of the system are discussed with reference to a specific model.     

The optical spectrum of the doubly charged molecular nitrogen ion: Rotational analysis of the (0-0) and (1-1) bands of the D1Σu+-X1Σg+ transition of N22+: Comparison of observations with Ab-initio calculation results

Emission spectra obtained in the 1550–1650 Å region with a 10-m vuv spectrograph are conclusively assigned to the N₂²⁺ ion. The 1589-Å band, previously observed by Carroll, and a new band of the same system, have been rotationally analyzed. Ab-initio calculations have been performed which support the assignment of these two bands to the D¹Σ_u⁺-X¹Σ_g⁺ system. The calculations also explain the observed breaking-off points in the branch structure as well as weakening and broadening of the other expected bands. These phenomena arise from electron configuration changes and perturbation effects in the ground state.     

Surface states at the metal-electrolyte interface

Normal incidence electroreflectance spectra of Ag and Au single crystal electrodes in aqueous electrolytes reveal pronounced structural features, which can be assigned to optical transitions involving empty surface states. The transition energies show a marked dependence on the electrode potential as predicted by the calculation of Ho, Harmon and Liu. It is demonstrated that electroreflectance of metal electrodes in the double layer charging region is a sensitive tool for studying metallic surface states.     

Surface cation ratios of binary oxide solid solutions

The cation ratios in the (100) surface of cubic oxides of bulk composition A_0.99B_0.01O, in which A and B are divalent cations were calculated using a monolayer model. Divalent cations including Ni, Co, Mn, Fe and Mg were studied. The cations were represented by a shell model, and the surface compositions were calculated by minimizing the total Gibbs free energy of the solid. The bulk energy change on surface segregation was evaluated by minimizing the energy change on substituting one bulk cation by another in an infinitely dilute solid solution and the surface energy was evaluated by a statistical method to accommodate the possibility that both cations can exist at non-negligible densities at the surface. The bulk energy change on surface segregation was found to be dominant in most cases. The surface energy change, the entropy change, and the change in crystal field stabilization energy were all secondary. Thus, except when the difference in ionic radii is very small, the larger cation is always more preferred at the surface. This conclusion appears to be in agreement with available experimental data.     

Temperature programmed desorption study of acetylene on a clean,H-covered,and O-covered Pt(111) surface

The reactions of acetylene on a clean, a H-covered and an O-covered Pt(111) surface were studied by temperature programmed desorption for various coverages of acetylene, and acetylene to H or O ratios. The desorption products were quantitatively determined. On a clean surface, acetylene decomposes to hydrogen and surface carbon. A small amount of self-hydrogenation to ethylene also occurs during decomposition. On a H-covered surface, hydrogenation to CH₄, C₂H₆, and ethylene, and decomposition to hydrogen and surface carbon occur simultaneously. The reactions on these two surfaces can be explained by the presence of two sites. One site is a bare surface Pt atom on which decomposition is the primary reaction pathway. The other site is a Pt atom with adsorbed H on which hydrogenation is the primary reaction pathway. On the O-covered surface, the decomposition reaction takes place together with an oxidation reaction which yields CO, CO₂, and water. The oxidation reaction probably proceeds via an intermediate that has a stoichiometry of CH. Results on the O-covered surface are consistent with the model that oxygen absorbs in islands, and the oxidation reaction takes place at the perimeter of the islands. These results are compared with those of ethylene reaction on the same Pt surfaces.     

Ion-enhanced gas-surface chemistry: The influence of the mass of the incident ion

There are many examples of situations in which a gas-surface reaction rate is increased when the surface is simultaneously subjected to energetic particle bombardment. There are several possible mechanisms which could be involved in this radiation-enhanced gas-surface chemistry. In this study, the reaction rate of silicon, as determined from the etch yield, is measured during irradiation of the Si surface with 1 keV He⁺, Ne⁺, and Ar⁺ ions while the surface is simultaneously subjected to fluxes of XeF₂ or Cl₂ molecules. Etch yields as high as 25 Si atoms/ion are observed for XeF₂ and Ar⁺ on Si. A discussion is presented of the extent to which these results clarify the mechanisms responsible for ion-enhanced gas-surface chemistry.     

Basic studies of the oxygen surface chemistry of silver: Chemisorbed atomic and molecular species on pure Ag(111)

The interaction of oxygen with Ag(111) has been studied over the pressure range 10^?2?1.0 Torr. Thermal desorption measurements using isotopically labelled molecules unambiguously establish the presence of a stable chemisorbed dioxygen species which co-exists with adsorbed atomic oxygen. Dissolved oxygen undergoes exchange with the latter species but not with the former. The maximum dioxygen population is found to be markedly sensitive to gas dosing pressure; a model is proposed which accounts for these observations and for related observations on alkali-doped Ag. XP and UP spectral features can be correlated with the two types of oxygen species; angle-resolved XP and Auger spectra indicate that O₂ (a) resides on the metal surface whereas O(a) is located within the surface. The XP spectra also suggest that in the case of O₂(a) the molecular axis may lie perpendicular to the surface.     

The interaction of hydrogen with stepped surfaces of copper: A pairwise-additive model

A pairwise-additive model, with Morse functions as the two-body potentials, has been employed to study stepped surfaces of copper. In general, the hydrogen-atom-copper bond energy is increased when one or more copper atoms are added to the surface at sites different from that which the hydrogen atom is approaching. Approaches of the hydrogen atom to a step face produced hydrogen-copper binding energies larger than those found in the absence of the step. Variations in these general observations are noted for alterations of the approach site environment.     

Photoelectrochemical behaviour of ruthenium disulphide electrodes in contact with aqueous electrolytes

RuS₂, which is a semiconductor, has been prepared as a sintered material. As an electrode, it has a high catalytic activity for oxygen evolution from aqueous solution in the dark. On illumination, an oxidation reaction occurs at potentials more negatively by 1 V. This photoreaction with water was studied as a function of the photon energy, the pH of the electrolyte, and the electrochemical history of the electrode. The interpretation of the behaviour is complicated by the apparent dependence of the double layer potential on the total electrode potential and the pH which leads to a considerable shift of the bands of the semiconductor with respect to the redox levels of the electrolyte. Kinetic evidence also indicates the involvement of surface states in a reduction of the oxidation products of water. The remarkable photoelectrochemical stability of RuS₂ is discussed in terms of a high concentration of Ru d-states in the valence band and the presence of S₂^2? ions in the crystal lattice. The prospects for RuS₂ as a photocatalytic material for oxygen evolution seem promising, but a full evaluation is at present hampered by the occurrence of unfavourable side reactions, probably caused by the polycrystalline and porous nature of the available material. In this first investigation we study the microcrystalline sintered product which would be more likely to be a practical electrode. Later we will present a more fundamental study using single crystals of RuS₂.     

Interaction of CO,CO2 and O2 with nonpolar,stepped and polar Zn surfaces of ZnO

The nonpolar (10$\text{1}$0), stepped (40$\text{4}$1) and (50$\text{5}$1), and the polar (0001) surfaces of ZnO were prepared. Stable unreconstructed nonpolar and stepped surfaces were obtained. LEED analyses showed that the step height and the step width of the stepped surfaces were similar to the theoretical values. The polar surface showed a 1 × 1 LEED pattern of six-fold symmetry after annealing at 500°C, and evidence of a more complicated pattern at 300–400°C. Temperature programmed desorption of CO resulted in the desorption of CO from the stepped and the polar surfaces. However, desorption of CO₂ was observed from the stoichiometric nonpolar surface, and no desorption from the reduced nonpolar surface. CO₂ was also observed by interacting CO with all surfaces at elevated temperatures. A total of four temperature programmed desorption peaks of CO₂, α, β, γ, and δ were observed. The α and β peaks were observed on the nonpolar and the stepped surfaces, and the γ peak was observed on the polar surface. The α peak was assigned to adsorption on a surface ZnO pair, and the β peak was assigned to adsorption on an anion vacancy or a step. While adsorbed water enhanced the β, preadsorbed methanol reduced it. O₂ adsorption was similar on the nonpolar and the stepped surfaces, but was weak on the polar surface.     

The crystallographic shear planes of the non-stoichiometric molybdenum oxides as revealed by RHEED: Investigation from the Mo18O52(100) surface

Large Mo₁₈O₅₂ crystals are obtained by an appropriate crystallization method. The examination of their well developed surfaces by the use of reflection high energy electron diffraction (RHEED) proves that these are (100) surfaces stepped along [010] directions. It is concluded that this oxide surface conformation can be connected to the particular Mo₁₈O₅₂ structure which is built up of MoO₃ slabs of finite width mutually joined by crystallographic shear planes (CS planes). Transmission electron microscopy (TEM) analysis from Mo₁₈O₅₂ crystal flakes confirms that these are single crystals without disorder in the periodicity of the CS planes.     

Nitrogen-induced reconstruction of a stepped iron surface

The dissociative adsorption of nitrogen on Fe(12, 1, 0) is found to induce an increase in the density of steps and finally cause faceting of the surface with large nitrogen exposures. The epitaxial relationship between α-Fe and Fe₄N in certain crystallographic directions can account for the observed structural changes.     

Electron energy loss and Auger study of epitaxially grown Cu on Ni(100)

Auger and electron energy loss spectra have been measured on films of Cu epitaxially grown on Ni(100). The films were prepared under UHV conditions using a quartz crystal for monitoring the deposition rate. LEED measurements were taken to determine the orientation of the films. The presence of a monolayer of Cu on Ni(100) is enough to suppress the 3p-3d transition on the surface of the sample. The electron energy loss spectra were studied as a function of the primary electron energy (50 to 300 eV). The experimental results were qualitatively analyzed using recent theoretical calculations of Cooper and co-workers. The effect of a small Cu coverage on Ni(100) on the chemisorption of CO and O₂ was also studied. A strong suppression of CO chemisorption at room temperature was observed. In the case of O₂, large exposures are necessary in order to observe a significant amount of oxygen on the surface. The absence of any appreciable chemisorption on the surface of the metal is attributed to the lack of empty d-surface states.     

Ion-induced auger electron emission of Mg,Al and Si as a function of ion energy

Experiments have been performed in which ion-induced Auger electron yields of Mg, Al and Si were measured as a function of ion energy. A computer simulation based on a binary collision cascade model yielded results that were in close agreement with the experimental findings. From this a model is proposed, where the creation of inner shell vacancies occurs during symmetrical collisions in the collision cascade generated by the ion. From the minimum energy required for Auger emission a distance of closest approach between the particles can be derived, using the Molière approximation of the screened Coulomb potential to describe the collisions. This distance can be correlated with the radii of the interacting orbitals derived from Hartree-Fock-Slater calculations.     

Modelling of kink nucleation and propagation along steps of finite length

A discrete model for kink nucleation and propagation along steps of finite length is developed. The step velocities were calculated as a function of the step length, the first order rate constants, k_s and k_K, for kink nucleation and propagation respectively, and a kink interaction parameter. The two limiting kinetic regimes of short and long steps as well as the intermediate transitional regime were considered. For very short steps, S² ? $\text{k}{}_{\mathrm{<mtext>K</mtext>}}\text{k}{}_{\mathrm{<mtext>s</mtext>}}$, the velocity varied directly with the number of sites S. Steps which met the criterion S² ? $\text{k}{}_{\mathrm{<mtext>K</mtext>}}\text{k}{}_{\mathrm{<mtext>s</mtext>}}$, were shown to have velocities on the order (2k_Kk_s)^{$\text{1}\text{2}$}, independent of step length. Step velocities were constant over large ranges of the kink interaction parameter. The steps were shown to be “sharp” for zero or attractive kink interaction, but with sufficiently large kink repulsion very diffuse steps could be formed. The density of kinks and numbers of kinks nucleated along an atom row were also calculated.     

Chemical properties of anion vacancies on zinc oxide

A ZnO(404&#x0304;1) surface, which is a stepped [4(101&#x0304;0) × (0001)] surface containing a high density of anion vacancies was prepared. When compared with the nonpolar (101&#x0304;0) surface, the (404&#x0304;1) surface adsorbed O₂ and methanol more strongly, but CO₂ more weakly. The decomposition products of methanol were different on these two surfaces.     

The autocatalytic decomposition of acetic acid on Ni(110)

The flash decomposition of CH₃COOH was studied on a clean nickel (110) surface following adsorption at 30° C in order to access the applicability of chemical reaction rate theory to a homologous series of reactants on a well-defined surface. As was observed for formic acid, acetic acid adsorbed at 30° C to yield gaseous H₂O and to form islands of adsorbed anhydride intermediates; the decomposition proceeded by a two-dimensional auto-catalytic mechanism to form H₂, CO₂, Co and surface carbon. The decomposition of the anhydride was rate determining for the formation of CO₂ and H₂. The rate of decomposition was well described by the equation governing the formic acid decomposition on the same surface. The activation energy for this first order decomposition was determined to be 28.2 $\text{kcal}\text{gmol}$ and the pre-exponential factor, v, was found to be 6.4 × 10¹⁴ s^?1 with a fraction of initiation sites of 0.004. These values were nearly the same as those observed for the decomposition of HCOOH, suggesting identical intramolecular mechanisms for the unimolecular decomposition of the adsorbed intermediates. The relative values of v for the decomposition of HCOOH, DCOOH and CH₃COOH indicated that the motion of the H, D or CH₃ group was involved in the rate-limiting step.