Scanning tunnelling microscopy was used to investigate the structure of (110) as a function of the O-vacancy density. The results were consistent with the imaging of Ti atoms; as expected in view of the positive sample bias which was used. The results for the stoichiometric 1 x 1 surface were consistent with the structure that was expected on the basis of bulk termination. On the present surfaces, a small fraction of the reduced 1 x 2 phase always accompanied the 1 x 1 termination and nucleated at step edges. With increasing O-vacancy density, the 1 x 2 phase increased in area and eventually dominated the surface. Atomically resolved images indicated a structure that involved the loss of alternate bridging O rows, an 0.05nm lateral displacement of 5-fold coordinated Ti atoms along [110], and a relaxation of Ti3+ ions towards the bulk. Coexistent with the 1 x 2 phase was a structure with 2 x 2 local symmetry that was periodic along [110]. This appeared to arise from a transfer of O, from the remaining bridging O rows, to re-occupy O-vacancy sites and to sit on top of 5-fold coordinated Ti atoms. In conjunction with this rearrangement of O atoms, 5-fold Ti atoms relaxed back to their positions in the 1 x 1 phase; thus suggesting that the 2 x 2 phase acted so as to relieve the strain which was imposed by the formation of the 1 x 2 morphology. Further reduction of the substrate resulted in faceting to give (100), (011), and (111) planes.

Effect of Stoichiometry on the Structure of TiO2(110). Murray, P.W., Condon, N.G., Thornton, G.: Physical Review B, 1995, 51[16], 10989-97