To prepare well-defined models of ZnO-based catalysts, in particular of Cu/ZnO used for methanol synthesis, we studied the structure of Zn and ZnO thin films grown on a Cu(111) single crystal surface using metal vapor deposition. Structural characterization was performed by scanning tunneling microscopy, Auger electron spectroscopy, and low-energy electron diffraction. In agreement with previous studies, Zn wets the Cu surface forming mixed surface layer depending on Zn coverage. Surface oxidation of the Zn film into ZnO, as monitored by STM, showed that the reaction starts at step edges and propagates inside the terrace at increasing temperature. However, the process is affected by Zn migration into the Cu bulk and hence the film formation critically depends on the heating rate. In another approach using Zn deposition in oxygen ambient and subsequent annealing in vacuum, the resulted films were well-ordered and showed a long-range coincidence structure, assigned to the formation of a single ZnO(0001) layer on top of Cu(111). Independent of preparations conditions, the ZnO overlayer did not cover the entire surface, leaving considerable areas exposing Cu(111) or Cu₂O/Cu(111) surface. Reactivity measurements for CO oxidation and reverse water gas shift reactions at nearly atmospheric pressures showed no promotional effects of the ZnO overlayer under conditions studied. Moreover, Zn irreversibly migrates into the Cu crystal bulk in an O₂ rich ambient, and the surface chemistry is governed, in essence, by a poorly defined Cu-oxide film. However, the ZnO/Cu model catalysts are fairly stable in a mixture of CO₂ and H₂

Thin (0001)-oriented films of ZnO on metals may exhibit interlayer relaxations, resulting in the hexagonal boron nitride-like crystal structure. The driving force for such reconstruction is the polar instability of either Zn- or O- terminated surfaces of ZnO(0001). Here, we examined surface hydroxylation as another possible stabilization mechanism. Zinc oxide films grown on Pt(111) were studied by infrared reflection–absorption spectroscopy (IRAS) as a function of film thickness and morphology as imaged by scanning tunneling microscopy. Despite prepared in pure oxygen ambient, the “as grown” films on Pt(111) expose hydroxyl groups. In contrast, the bilayer films on Ag(111) do not exhibit OH species, not even upon dosing of hydrogen or water. The results show that hydrogen may efficiently be provided by a Pt support, even for the multilayer films, via hydrogen dissociation and subsequent diffusion of H atoms through the film. Thermal stability of the OH-terminated surfaces depends on the film thickness, with a monolayer film being the least stable. Removal of OH species from a monolayer film proceeds through water desorption and may be accompanied by hydrogen spillover onto more stable multilayer structures. Stabilization of the polar ZnO surface in the metal-supported films seems to be a delicate balance between interlayer relaxation and hydroxylation and depends on the metal support

This investigation concerns the initial chemical reactions that affect the ionization of matrixes in matrix-assisted laser desorption/ionization (MALDI). The study focuses on the relaxations of photon energy that occur on a comparable time scale to that of ionization, in which the available laser energy is shared and the ionization condition is changed. The relaxations include fluorescence, fragmentation, and nonradiative relaxation from the excited state to the ground state. With high absorption cross section and long excited-state lifetime, photoionization of matrix plays an important role if sufficient laser energy is used. Under other conditions, thermal ionization of the molecule in the ground state is predicted to be one of the important reactions. Evidence of change in the branching ratio of initial reactions with the matrix and the excitation wavelength was obtained with α-cyano-4-hydroxycinnamic acid, sinapinic acid, 2,5-dihydroxybenzoic acid, and 2,4,6-trihydroxyacetophenone. These matrixes are studied by obtaining their mixed crystal absorption spectra, fluorescence properties, laser-induced infrared emission, and product ions. The exact ionization pathway depends on the chemical properties of matrixes and the excitation conditions. This concept may explain the diversity of experimental results observed in MALDI experiments, which provides an insight into the ensemble of chemical reactions that govern the generation of ions.

The initial ionization reaction in matrix-assisted laser desorption/ionization (MALDI) was examined on the basis of the appearance of photoelectrons. The threshold laser fluence for the ejection of photoelectrons from 2,5-dihydroxybenzoic acid (DHB), sinapinic acid (SA), and trihydroxyacetopheone (THAP) on stainless steel (SS) substrates was 0.05, 0.41, and 8.39 mJ/cm(2), respectively. These values are considerably lower than those for MALDI ions, indicating that the electron detachment likely precedes other ionization reactions. The SS substrate played an insignificant role in the production of photoelectrons because suspended DHB produced a photoelectron signal similar to DHB on the SS surface, and decreasing the DHB thickness on the SS reduced the photoelectron intensity. For crystalline DHB and SA, the photoelectron intensity increased with the laser (337 nm) fluence in a relationship of less than second order, suggesting considerable reductions of ionization potentials in comparison with free molecules. According to ab initio calculations, the ionization potential of DHB clusters reduces as the cluster size increases from monomer to octamer. The impact of these abundant electrons on the ion production in MALDI is discussed.