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Abstract: We report here scanning tunneling microscopy (STM) observations of bridge-bonded oxygen vacancies (OVs) on the TiO2(110) surface diffusing under the influence of 266 nm ultraviolet (UV) laser irradiation. OV pairs, and even OV trimers, were formed as a result of UV light-induced OV diffusion. There are two stable STM representations of the OV-pair defects, which are interchangeable during scanning. An extended irradiation time (68 min) can lead to the formation of a TiO2(110) surface with predominant OV-pair point defects. Our results enrich the understanding of OV behavior upon UV irradiation, and future photocatalytic studies on reduced rutile TiO2(110) surfaces involving 266 nm UV light can benefit from the knowledge of the observed diffusion of OVs and the formation of OV oligomers. We also provide a plausible way to prepare an OV-pair abundant TiO2(110) surface, a requisite for further investigations of the otherwise unapproachable defects.
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Figure 1. (a) A 6.25 nm × 6.25 nm area of the reduced TiO2(110) surface. (b) The area in (a) after 1 min of 266 nm irradiation. The bright irregularities on the upper right corners of (a) and (b) are the same impurity used for locating. (c) The 3D topograph obtained by subtracting (b) from (a), the migration of OVs is visualized. The yellow protrusions represent the initial locations of migrating OVs, and the blue depressions indicate the final locations of OVs. The black arrows represent the moving directions of the OVs. (d) STM image of the area in (a) obtained after 100 s, at 80 K, without any irradiation. (e) STM image of a clean TiO2(110) surface, 6.25 nm × 6.25 nm. (f) The same area in (e) after 15 min of 355 nm UV laser irradiation, no OV diffusion is observed.
Figure 2. (a, b) Direct observation of the formation of three OV pairs. The OVs before merging are marked by blue rectangles, and the newly formed OV pairs are marked by green arrows. (c) A TiO2(110) surface with OV pairs as the primary point defects, prepared via extended UV irradiation time (68 min). The size of the scanning area is 7.1 nm × 9.5 nm.
Figure 3. Transitions between the two STM representations of OV pairs. (a) The initial TiO2(110) surface. (b, c) Transitions between two STM representations of OV pairs during scanning. (d, e) Two stable representations of OV pairs. (d) The bright elliptical protrusion which represents an OV pair. (e) The newly observed three-piece dim spots which also represent the OV pair. The green grid overlaid in (e) represents the TiO2 crystal lattice. The area is 4.0 nm × 4.0 nm in size.
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