Effect of surface treatment on the gamma-WO3(001) surface: A comprehensivestudy of oxidation and reduction by scanning tunneling microscopy and low-energy electron diffraction

The structure of the (001) surface of monoclinic gamma -WO3 was characteriz ed using scanning tunneling microscopy (STM), Auger electron spectroscopy ( AES), and low-energy electron diffraction (LEED). The surface structure dep ended strongly on preparation conditions. Oxidizing by heating in O-2 remov ed carbonaceous impurities and resulted in large atomically rough terraces. Subsequent heating in lower O-2 pressures led to metastable superstructure s and c(2X2) terraces. Continued heating progressively reduced the surface, resulting in a series of reconstructions. Each could be explained in terms of ordered oxygen vacancies. First, regions of a c(2X2) reconstruction gre w. This structure corresponds to a WO2 plane in which half the W6+ sites ar e covered by terminal O. Since all the W atoms are in a 6+ oxidation state, this surface is considered "fully oxidized." Certain STM images of the c(2 x2) terraces revealed a pseudo-primitive-(2x2) periodicity that can be rela ted to the monoclinic distortion from cubic symmetry. It is shown that the distortion is always present but the ability to detect it depends on the ti p. The distortion also caused twinning that lead to spot splitting in LEED patterns. The splitting disappeared on heating to 785 K when the bulk conve rted to orthorhombic symmetry. Continued reduction produced areas with (2X2 ), (6X2), and c(4x2) periodicities. These reconstructions are formed when f urther terminal O is removed, and half the total W sites are reduced to W5. Sputtering and ultrahigh vacuum annealing also reduced the surface, ultim ately leading to "fully reduced" (I X 1) terraces with no terminal oxygen a nd only W5+ surface cations. These structural transformations were reversib le: at any stage during reduction. heating in greater than or equal to 10(- 4) Torr Of O-2 reoxidized the surface, regenerating the original rough morp hology. (C) 2001 American Vacuum Society.