First-principles study of the polar O-terminated ZnO surface in thermodynamic equilibrium with oxygen and hydrogen

Using density-functional theory in combination with a thermodynamic formalism we calculate the relative stability of various structural models of the polar O-terminated (000-1)-O surface of ZnO. Model surfaces with different concentrations of oxygen vacancies and hydrogen adatoms are considered. Assuming that the surfaces are in thermodynamic equilibrium with an O2 and H2 gas phase we determine a phase diagram of the lowest-energy surface structures. For a wide range of temperatures and pressures we find that hydrogen will be adsorbed at the surface, preferentially with a coverage of 1/2 monolayer. At high temperatures and low pressures the hydrogen can be removed and a structure with 1/4 of the surface oxygen atoms missing becomes the most stable one. The clean, defect-free surface can only exist in an oxygen-rich environment with a very low hydrogen partial pressure. However, since we find that the dissociative adsorption of molecular hydrogen and water (if also the Zn-terminated surface is present) is energetically very preferable, it is very unlikely that a clean, defect-free (000-1)-O surface can be observed in experiment.Comment: 10 pages, 4 postscript figures. Uses REVTEX and epsf macro

Influence of band width on the scattered ion yield spectra of a He + Ion by resonant or quasi-resonant charge exchange neutralization

The influence of the band structure, especially the bandwidth, on the scattered ion yield spectra of a He+ ion by the resonant or quasi-resonant neutralization was theoretically examined using quantum rate equations. When calculating the scattered ion yield spectra of He+ to simulate the experimental data, we observed that the band structure, especially the bandwidth, had a strong influence on the spectra at relatively low incident He+ ion energies of less than several hundred eV. Through many simulations, it was determined that theoretical calculations that include bandwidth calculation can simulate or reproduce the experimentally observed spectra of He+-In, He+-Ga, and He+-Sn systems. In contrast, simulations not including bandwidth simulation could neither reproduce nor account for such spectra. Furthermore, the calculated ion survival probability (ISP) at low incident ion energies tended to decrease with increasing bandwidth. This decrease in ISP probably corresponds to the relatively small scattered ion yield usually observed at low incident ion energies. Theoretically, such a decrease indicates that a He+ ion with a low incident energy can be easily neutralized on the surface when the bandwidth is large

Spectroscopic Investigation of Surface-Dependent Acid–Base Property of Ceria Nanoshapes

In addition to their well-known redox character, the acid–base property is another interesting aspect of ceria-based catalysts. Herein, the effect of surface structure on the acid–base property of ceria was studied in detail by utilizing ceria nanocrystals with different morphologies (cubes, octahedra, and rods) that exhibit crystallographically well-defined surface facets. The nature, type, strength, and amount of acid and base sites on these ceria nanoshapes were investigated via in situ IR spectroscopy combined with various probe molecules. Pyridine adsorption shows the presence of Lewis acid sites (Ce cations) on the ceria nanoshapes. These Lewis acid sites are relatively weak and similar in strength among the three nanoshapes according to the probing by both pyridine and acetonitrile. Two types of basic sites, hydroxyl groups and surface lattice oxygen are present on the ceria nanoshapes, as probed by CO₂ adsorption. CO₂ and chloroform adsorption indicate that the strength and amount of the Lewis base sites are shape dependent: rods > cubes > octahedra. The weak and strong surface dependence of the acid and base sites, respectively, are a result of interplay between the surface structure dependent coordination unsaturation status of the Ce cations and O anions and the amount of defect sites on the three ceria nanoshapes. Furthermore, it was found that the nature of the acid–base sites of ceria can be impacted by impurities, such as Na and P residues that result from their use as structure-directing reagent in the hydrothermal synthesis of the ceria nanocrystals. This observation calls for precaution in interpreting the catalytic behavior of nanoshaped ceria where trace impurities may be present

Adsorption and Reaction of Acetaldehyde on Shape-Controlled CeO₂ Nanocrystals: Elucidation of Structure–Function Relationships

CeO₂ cubes with {100} facets, octahedra with {111} facets, and wires with highly defective structures were utilized to probe the structure-dependent reactivity of acetaldehyde. Using temperature-programmed desorption (TPD), temperature-programmed surface reactions (TPSR), and <i>in situ</i> infrared spectroscopy, it was determined that acetaldehyde desorbs unreacted or undergoes reduction, coupling, or C–C bond scission reactions, depending on the surface structure of CeO₂. Room-temperature FTIR indicates that acetaldehyde binds primarily as η¹-acetaldehyde on the octahedra, in a variety of conformations on the cubes, including coupling products and acetate and enolate species, and primarily as coupling products on the wires. The percent consumption of acetaldehyde ranks in the following order: wires > cubes > octahedra. All the nanoshapes produce the coupling product crotonaldehyde; however, the selectivity to produce ethanol ranks in the following order: wires ≈ cubes ≫ octahedra. The selectivity and other differences can be attributed to the variation in the basicity of the surfaces, defects densities, coordination numbers of surface atoms, and the reducibility of the nanoshapes