Chemisorption on a quantum dot

The quantum kinetic equations for a “quantum dot-adatom” system are derived. It is demonstrated that the inclusion of the interaction between a quantum dot and an adatom leads to an increase in the quantum dot radius. The perturbations of the electron density of the quantum dot and the adatom upon chemisorption are calculated.     

Chemisorption on a model insulator

The adsorption both of a single atom and a monolayer of atoms on the (001) surface of a model two-band crystal with the CsCI structure is investigated using the Green's function formalism and the phase shift technique. The electronic structure of the surface is described within the Linear Combination of Atomic Orbitals (LCAO) scheme and the Tight Binding (TB) approximation. Each adatom is represented by a single non-degenerate energy level. The adatoms are placed on the surface in both the on-site and the centered fourfold-site configuration. The change in the density of electronic states upon chemisorption is found, and comparison is made with similar results on a metal surface. It is shown that many, but not all, of the qualitative features in chemisorption on metallic surfaces can be transferred to the case of an insulating surface. In addition, it is shown that there are systematic variations in the density of states with adatom coverage which depend upon the absorption site.     

Chemisorption theory

The aim of chemisorption theory is to achieve for any adsorbateladsorbent system complete quantum predictions of the equilibrium positions of all the nuclei, the ground-state (electronically adiabatic) potential energy surface, the electronic excitation spectrum, and the response of the system to external probes. We are still a very long way from this goal, but some simplified models can now be explored in depth. Serious quantitative work begins with Toya,^1,2 and my review ends with the work of Appelbaum and Hamann³ These works provide examples of two of the three approaches to chemisorption theory. Toya's is a configuration interaction (CI) approach, using the states of the noninteracting adsorbate/solid system as basis states; Appelbaum and Hamann try to integrate Schrodinger's equation directly. The third approach is the linear combination of atomic orbitals molecular orbital (LCAO-MO) theory, and I begin with it because it is generally familiar in its simpler forms through its widespread use in molecular quantum mechanics. However, I remark at this stage that the direct integration of Schrodinger's equation may be more economical in computer time than the traditional quantum chemistry approach; it is certainly more economical if the nonlocal Hartree-Fock exchange potential is replaced by a local approximation to it.     

Chemisorption on a quantum-well wire

The specific features of chemisorption on a quantum-well wire are studied. It is shown that the energy of an electron of an adatom chemisorbed on the quantum-well wire changes jumpwise with variations in the wire radius.     

Chemisorption of water on nickel surfaces

Magnetic and volumetric measurements associated with neutron inelastic spectroscopy demonstrate that water is dissociatively adsorbed on nickel. In the case of Raney nickel, for low coverages, the oxygen is fixed on surface aluminium atoms and hydrogen occupies two nickel sites. At high coverage, the water molecule is fixed via oxygen bonding to the hydrogen covered nickel. On nickel prepared from its hydroxide, adsorption is reversible, the water molecule occupying six metal sites.     

Chemisorption of oxygen on aluminum surfaces

A critical review of the oxygen—aluminum system is presented. The primary emphasis concerns the electronic properties of aluminum surfaces exposed to oxygen. The chemisorption and oxidation aspects are considered. Cluster and slab model calculations are discussed fully and the results are related to relevant experimental data. Some of the unresolved issues are listed. A comprehensive guide to the oxygen—aluminum literature is provided.     

Hydrogen Chemisorption on Disordered Binary Alloys

The one-dimensional tight-binding model and the Green's function method are used to study the chemisorption energy of an atomic H on disordered binary alloys within the average T-matrix approximation. The one-electron chemisorption theory is used to describe the chemisorption process. The chemisorption energy is calculated as a function of the concentration percentages of the two components of the disordered binary alloys. The effect of the surface segregation of the CuNi system is also investigated.     

Chemisorption of atomic oxygen on silicon surface

We investigated the electronic structure of oxygen adsorbed on silicon surface in a head-on position, and showed that only the corresponding bonding structure is able to explain the features in UPS and ELS spectra that were not interpreted satisfactorily before. We also found that charge is transferred from backbonds to SiO bond and induces empty states localized between the first and second layers of the silicon surface.     

Chemisorption of carbon monoxide on copper-nickel alloy surfaces

The adsorption of CO on Cu-Ni alloy surfaces has been studied at 300 and 120 K using LEED, AES, TDS, and work function measurement. The alloys have been prepared as thin (111) epitaxial films evaporated on mica, and as poly crystalline foils. At 300 K the alloy surfaces show an adsorption behavior similar to that of pure Ni: the work function increases to a saturation value which is higher for Ni-rich surfaces than for Cu-rich. The isosteric heat of adsorption (106 kJ/mole) is nearly as high as with pure Ni. At 120 K the alloys exhibit a more Cu-like adsorption behavior: the work function passes through a minimum which becomes deeper at higher Cu surface concentration. The isosteric heat of adsorption at low temperatures (50 kJ/mole) is nearly as low as for pure Cu. From TDS it can be shown, that the binding energy of the highest (Ni-like) adsorption states increases with increasing Ni surface concentration. At the (111) alloy surfaces no LEED superstructures due to CO adsorption could be observed.     

Chemisorption of NO on Re studied by XPS

The adsorption of nitric oxide on clean rhenium has been studied by X-ray photoelectron spectroscopy. At room temperature only dissociative adsorption occurs; at 100 K, NO adsorbs molecularly at low coverages. Higher NO exposures lead to the formation of NO₂ which dissociates upon warming above 180 K.     

Chemisorption and catalytic oxidation of CO on palladium

The adsorption of CO on clean, low index crystal faces of palladium is analyzed in terms of the fixed site and mobile models. Isotherms give evidence of adsorbed molecule interaction or heterogeneity, although in the case of Pd(111) the Clausius-Clapeyron heats do not, and partial mobility is indicated. The oxidation mechanism in the mixed-Langmuir formulation is analyzed in terms of the absolute rate theory, and in terms of the isolated domain or island model for mixed adsorption. The data, which are qualitatively similar on all crystal faces can be adequately accounted for on the basis of a non-equilibrium mixed-Langmuir adsorption on a surface with long-range heterogeneity.     

Chemisorption and decomposition of nitric oxide on ruthenium

Nitric oxide, strongly chemisorbed on ruthenium, is desorbed almost completely as oxygen and nitrogen. Oxygen, nitrogen, and nitric oxide were observed singly on ruthenium with field emission microscopy. Thermal desorption spectroscopy from Ru(101̄0) showed that molecular nitrogen is only physisorbed but nitrogen from NO decomposition is strongly chemisorbed. Nitrogen from NO shows three binding states, the most strongly bound being present to only a small extent. NO shows three and two binding states when adsorbed at 120 K and 295 K respectively. Work function measurements gave Δφ = 1.3 eV for a monolayer of NO. NO is dissociatively adsorbed above 250 K but a lower temperature limit was not established. The decomposition of NO on $\text{Ru}\text{(10}\text{1}\text{0)}$ under high vacuum conditions is catalytic in that no oxides of ruthenium were observed to form in the process.     

Chemisorption,photodesorption and conductivity measurements on ZnO surfaces

Experimental results of a mass-spectro metric analysis of photodesorption from ZnO single crystals at different temperatures are reported. They provide direct evidence that CO₂ is the only photodesorbed species from both the single crystal and powder samples studied. The CO₂ photodesorption occurs only when the incident photon energy exceeds the ZnO band gap energy. Excellent agreement between the illumination time dependence of the CO₂ photodesorption and surface conductivity data in both single crystals and powder samples strongly suggests a substrate dependent mechanism in which photodesorption occurs by the neutralization of chemisorbed CO₂^? “ion-molecules” by photogenerated holes. In addition, measurements of the chemisorption kinetics of oxygen on ZnO single crystal and powder surfaces are reported. The results are compared with CO₂ and CO chemisorptiun experiments to show that, of these gases, only oxygen chemisorbs from the gas phase. Auger analysis of oxygen saturated and photodesorbed surfaces of ZnO show a significant relation between the carbon content and the photo-desorptive and conductive activities of those surfaces. These observations indicate that impurity carbon atoms on ZnO surfaces can be oxidized by electron capture to produce chemisorbed CO₂^? “ion-molecules’ which will then readily photodesorb by bandgap radiation. This proposed process is discussed together with further supporting evidence.     

Chemisorption and dissociation of carbon monoxide on rhodium surfaces

The adsorption, desorption and decomposition of CO on Rh surfaces have been investigated using field emission microscopy and thermal desorption spectroscopy. Thermal dissociation of CO cannot be detected on clean Rh surfaces at pressures up to 10^?1 Torr and temperatures below 1000 K. This holds also for atomically rough surfaces like (210). CO dissociation can be promoted under the influence of an electron beam directed to the surface, a high electric field in the presence of CO in the gas phase and by means of discharge techniques. The growth of crystallites formed by CO dissociation and the diffusion of carbon into the bulk has been followed as a function of temperature and surface structure. The tip regions around (110) are very active in these processes. Carbon crystallites on these surfaces disappear around 1000 K by diffusion into the lattice whereas crystallites present around (311) surfaces persist up to 1150 K. The results are discussed in relation to the activity of Rh in CO/H₂ reactions.     

Chemisorption and surface barrier structure

The effects of chemisorption on the potential barrier at a metal surface are discussed. Simple physical considerations predict an outward shift in the barrier origin, an increased saturation of the barrier, and changes in the inner potential. However, an analysis of low-energy electron diffraction (LEED) data for p(2 × 1) O/W(110) and p(2 × 1) H/W(110) indicates that these changes are small, and that the observed modifications are due primarily to changes in the surface scattering properties accompanying chemisorption. LEED fine structure analysis should then be a useful technique for identifying adsorption sites on metal surfaces.     

Chemisorption of periodic overlayers of atomic nitrogen on graphite

Regular surface phases which could be formed by chemisorption of atomic nitrogen on graphite have been investigated by the CO-NDO method. The maximum adsorption energy is shown by nitrogen atoms directly chemisorbed over each carbon atom in the substrate. The corresponding phase is a zero-gap semiconductor, with electronic features sharply differing from those of graphite.     

Chemisorption of periodic overlayers of atomic oxygen on graphite

The crystalline orbital-NDO method has been applied to the regular surface phases allowed by the graphite symmetry for oxygen atoms chemisorbed at equivalent positions in the oxygen/carbon atomic ratios $\text{1}\text{2}$ and $\text{1}\text{1}$. The maximum adsorption energy is shown by chemisorption of oxygen atoms over the C-C bonds, with formation of an epossidic bond.