EFFECT OF STOICHIOMETRY ON THE STRUCTURE OF TIO2(110)

Scanning tunneling microscopy has been used to investigate the structure of TiO2(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 1×1 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 1×2 phase always accompanied the 1×1 termination, nucleating at step edges. With increasing O-vacancy density, the 1×2 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-lateral displacement of fivefold-coordinated Ti atoms along [11̄0], and a relaxation of Ti3+ ions towards the bulk. Coexisting with the 1×2 phase is a structure with 2×2 local symmetry that is periodic along [11̄0]. 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 1×1 phase, suggesting that the 2×2 phase acts to relieve the strain imposed by formation of the 1×2 morphology. Further reduction of the substrate results in faceting to (100), (011), and (111) planes. © 1995 The American Physical Society.