Effect of stoichiometry on the structure ofTiO2(110)

Abstract

Scanning tunneling microscopy has been used to investigate the structure of ${\mathrm{TiO}}_{2}$(110) as a function of oxygen-vacancy density. The results are consistent with imaging Ti atoms, as expected given the positive sample bias employed. Our results for the stoichiometric 1\ifmmode\times\else\texttimes\fi{}1 surface are in line with the structure expected on the basis of bulk termination. On the surfaces prepared here, a small density of the reduced 1\ifmmode\times\else\texttimes\fi{}2 phase always accompanied the 1\ifmmode\times\else\texttimes\fi{}1 termination, nucleating at step edges. With increasing O-vacancy density, the 1\ifmmode\times\else\texttimes\fi{}2 phase increased in area, eventually dominating the surface. Atomically resolved images indicate a structure that involves the loss of every other bridging O row, a 0.5-\AA{} lateral displacement of fivefold-coordinated Ti atoms along [11\ifmmode\bar\else\textasciimacron\fi{}0], and a relaxation of ${\mathrm{Ti}}^{3+}$ ions towards the bulk. Coexisting with the 1\ifmmode\times\else\texttimes\fi{}2 phase is a structure with 2\ifmmode\times\else\texttimes\fi{}2 local symmetry that is periodic along [11\ifmmode\bar\else\textasciimacron\fi{}0]. This appears to arise from a transfer of O from the remaining bridging O rows to reoccupy O-vacancy sites and to sit atop fivefold-coordinated Ti atoms. Accompanying this rearrangement of O atoms, fivefold Ti atoms relax back to their positions in the 1\ifmmode\times\else\texttimes\fi{}1 phase, suggesting that the 2\ifmmode\times\else\texttimes\fi{}2 phase acts to relieve the strain imposed by formation of the 1\ifmmode\times\else\texttimes\fi{}2 morphology. Further reduction of the substrate results in faceting to (100), (011), and (111) planes.