Atomic structures and electronic properties of Cr-doped ZnO() surfaces

Abstract

An integrated approach combining scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) is used to investigate the atomic structures and electronic properties of Cr-doped ZnO(▪) surfaces. When deposited at 300 K, Cr at low surface coverage (< 0.1 ML) appeared either as isolated atoms on the surface terrace of ZnO(▪) or substituting Zn atoms in the ZnO lattice. Their structural models could be identified from atomic-resolution STM images and their oxidation states were found as Cr3+ based on XPS measurements. Rectangular islands nucleated at step edges along the [0001] direction could also be observed during the initial growth of Cr at 300 K and were assigned as Cr islands. The density of Cr islands as well as their average size increased with the increasing of Cr surface loading. Thermal treatments at above 600 K could facilitate the decomposition of Cr islands and the re-dispersion of Cr atoms into the ZnO lattice, indicating a strong interaction between Cr and ZnO. The adsorption of CO at 78 K showed no preferential adsorption at Cr3+ sites embedded in the surface lattice of ZnO. However, the re-dispersion of Cr atoms into the ZnO bulk at above 600 K could induce a significant upward band bending, causing a negative shift of core level XPS peaks of Zn 2p and O 1s by ~0.5–0.7 eV. Our study has thus constructed a model catalyst for Cr-doped ZnO and provided atomic insight for understanding ZnO-based catalysts.