Reactivity of Ultra-Thin ZnO Films Supported by Ag(111) and Cu(111): A Comparison to ZnO/Pt(111)

Abstract

Abstract.We studied structure and reactivity of ZnO(0001) ultrathin films grown on Ag (111) and Cu(111) single crystal surfaces. Structural characterization was carried out by scanning tunneling microscopy, Auger electron spectroscopy, low-energy electron diffraction, and temperature programmed desorption. The CO oxidation behavior of the films was studied at low temperature (450 K) at near atmospheric pressures using gas chromatography. For ZnO/Cu(111), it is shown that under reaction conditions ZnO readily migrates into the Cu crystal bulk, and the reactivity is governed by a CuO x oxide film formed in the reaction ambient. In contrast, the planar structure of ZnO films on Ag(111) is maintained, similarly to the previously studied ZnO films on Pt(111). At sub-monolayer coverages, the "inverse" model catalysts are represented by two-monolayer-thick ZnO(0001) islands on Pt(111) and Ag(111) supports. While the CO oxidation rate is considerably increased on ZnO/Pt(111), which is attributed to active sites at the metal/oxide boundary, sub-monolayer ZnO films on Ag(111) did not show such an effect, and the reactivity was inhibited with increasing film coverage. The results are explained by much stronger adsorption of CO on Pt(111) as compared with Ag(111) in proximity to O species at the oxide/metal boundary. In addition, the water-gas shift and reverse water-gas shift reactions were examined on ZnO/Ag(111), which revealed no promotional effect of ZnO on the reactivity of Ag under the conditions studied. The latter finding suggests that wetting phenomena of ZnO on metals does not play a crucial role in the catalytic performance of ZnObased real catalysts in those reactions.

Graphene-Like ZnO: A Mini Review

“…Different metals with their (111) orientations have been exploited as the growth substrates. Examples include Pd(111) [17], Pt(111) [18,19], Au(111) [20], Ag(111) [4,21,22], Cu(111) [22] and Brass(111) [23]. Prior to deposition, the surfaces of the metal substrates were cleaned by mechanical polishing [4] and by cycles of Ar + ion sputtering followed by annealing in UHV, at 1000 K for Pd [17], 750 K for Ag [4], 670 K [21] or 700 K for Au [20], and 1200 K for Pt [19].…”

“…Prior to deposition, the surfaces of the metal substrates were cleaned by mechanical polishing [4] and by cycles of Ar + ion sputtering followed by annealing in UHV, at 1000 K for Pd [17], 750 K for Ag [4], 670 K [21] or 700 K for Au [20], and 1200 K for Pt [19]. In some cases [17][18][19]22], an annealing step was carried out in the presence of O 2 at high temperature (700-800 K) to remove residual carbon. To desorb the residual oxygen, the sample was then flashed to 1000 K in UHV.…”

“…g-ZnO layers can be obtained by first depositing pure Zn at room temperature on to a clean metal substrate by heating a Zn rod using a Knudsen cell [17,21]; in other cases, pulsed laser deposition [4], an electron beam-assisted evaporator [20] or applied current through a thoriated tungsten wire wrapped around the Zn rod can be used [18,19,22]. When oxygen is injected, it is done so at relatively high pressures ranging from~10 −8 and~10 −5 mbar either during or after Zn deposition at room temperature, followed by an annealing step in UHV (the annealing temperature depends on the metal substrate and typically varies between 550 K and 680 K).…”

Local Characterization of Ultrathin ZnO Layers on Ag(111) by Scanning Tunneling Microscopy and Atomic Force Microscopy

“…On the other hand, the CO oxidation is inhibited when ZnO films are grown on Ag(111). 13 Therefore, the local structural and electronic properties are of fundamental importance for a comprehensive understanding of the catalytic effect in ultrathin oxide materials. In addition, such a local structure of ultrathin oxide films is closely related with the microscopic growth mechanism.…”

CO adsorption on graphite-like ZnO bilayers supported on Cu(111), Ag(111) and Au(111) surfaces

“…20 The CO oxidation behavior of ZnO films grown on Ag(111) and Cu(111) has also been investigated and it has been found that on Ag(111) the planar structure of the films is maintained in the course of the reaction. 21 Graphitic-like ZnO films and nanostructures on Ag(111) have then been characterized theoretically in various studies. 22,23 Ultrathin ZnO nanostructures have also been grown on Au(111) using reactive deposition at room temperature and characterized by X-ray photoelectron spectroscopy, STM, and DFT calculations.…”