Theoretical Investigations on Stability and Reactivity of Zinc Oxide Polar Surfaces

Abstract

To date, materials at nanoscale are used increasingly in many fields of science and technology. ZnO is an interesting metal oxide, which has been successfully synthesized in a wide variety of shapes and sizes. However, recent experiments indicate that ZnO NPs may be very toxic for biological systems. The toxicity of ZnO NPs can, however, be re-duced by a small amount of Fe dopants. This is due to the fact that Fe dopants can sup-press the dissolution of ZnO NPs and the number of released toxic Zn2 cations. Though, the stabilization mechanism of ZnO NPs by Fe is still an open question. In addition, ZnO is also employed as a support to improve the reactivity of catalysts. Even if the Cu/ZnO/Al2O3 catalysts have been used in the industrial processes for many years, the active sites and synergy effects on these catalysts are still unknown. Neither Cu nor ZnO alone can achieve a comparable efficiency of methanol synthesis with respect to the mixed Cu/ZnO system. As the industrial synthesis is carried out under high tempera-ture ( > 500 K) and pressure ( > 50 bar), numerous experimental studies performed under vacuum condition are to be questioned. For the Fe-ZnO case, we first performed calculations of core-level spectroscopy (XANES) to compare with experimental spectra (ISEELS). It has been found that the Fe dopants are present as Fe2 instead of Fe3 in the ZnO NPs. Furthermore, using Möβbauer spectroscopy, we have found surprisingly that the local charge of Fe2 in ZnO NPs is close to that of Fe3 in Fe2O3, instead of Fe2 in FeO. This finding can explain well the observed stabilization of ZnO NPs by Fe doing. In addition, we have studied the stability of ZnO polar and nonpolar surfaces in the presence of Fe dopants. Fe dopants stabilize only the ZnO polar surfaces, which are the least stable part of the ZnO NPs. This confirms that the reduced ZnO dissolution is due to the ZnO polar surfaces stabilization by Fe doing. For the Cu/ZnO case, we have found that the catalytic properties are also correlated with the ZnO(000-1) polar surface. Because Cu dopants destabilize the ZnO(000-1) surface, oxygen vacancies are formed on this surface, accompanied by complete surface reduction. In other words, metallic monolayers can be formed and supported on the ZnO(000-1) sur-face in the atmosphere of CO2, CO, and H2 gases. Furthermore, the reactivity of the sup-ported metallic monolayers was optimized for CO2 adsorption and the subsequent CO2 reduction. This synergy effect can explain why the mixed Cu/ZnO system can achieve an optimal efficiency for methanol synthesis. Moreover, we performed DFT calculations to propose an optimized catalyst, AgO/ZnO, for CO2 reduction in the atmosphere of H2 gas. It has been found that the Ag(111) monolayer supported on the ZnO(000-1) surface has an optimized reactivity for CO2 reduction. In practice, AgO/ZnO was proposed as the precursor to generate the sup-ported Ag(111) monolayers in the atmosphere of H2 gas.