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(1010)surfaces.When dep...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(1010)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(1010)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 Cr^(3+)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 Cr^(3+)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.展开更多
文摘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(1010)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(1010)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 Cr^(3+)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 Cr^(3+)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.
