Non-contact atomic force microscopy study of line defect on rutile TiO₂(110)-(1×2) reconstructed surface

抄録

TiO₂ is one of the famous photocatalysts applied for various fields such as water splitting catalysts, coating materials, and solar cells. For obtaining the design guidelines of high reactive photocatalysts, it is important to understand the surface structure and the elementary process of photocatalytic reaction as the reaction field.

Rutile TiO₂(110) surface have been used as a model surface for the study of a photocatalyst. The rutile TiO₂(110)-(1×1) surface transforms to the (1×2) surface by iterations of Ar⁺ sputtering and high-temperature annealing over 900 °C. The (1×2) surface exhibits an asymmetric periodic row structure [1] accompanied by various local structures as shown in Fig. 1: the single-link structure, the double-link structure, and the line defect. Among these local structures, the line defect has been reported with no reactivity of water adsorption [2]. The origin of no reactivity of water adsorption is still controversial. In this study, we investigated the line defect on rutile TiO₂(110)-(1×2) surface using scanning tunneling microscopy (STM) and non-contact atomic force microscopy (NC-AFM).

Nb-doped (0.05 wt%) rutile TiO₂(110) substrates (Shinkosha) were used for the sample. The (1×2) surface was prepared by iterations of Ar⁺ sputtering (Ar partial pressure: 2.5 × 10^-4 Pa, Energy: 1.5 keV) and annealing at 1000 °C under the ultra-high vacuum (< 5.0 × 10^-8 Pa).

In the case of NC-AFM imaging of metal oxide surfaces, the contrast of the NC-AFM image depends on the tip apex polarity [3]. We obtained NC-AFM images of the line defect with opposite contrast to each other. This result suggests that the line defect has charge polarity. To confirm this assumption, the contact potential difference (CPD) on line defects and (1×2) periodic rows were evaluated using NC-AFM. The comparison of the CPD between line defects and (1×2) periodic rows reveals that the line defects were charged relatively negative from (1×2) periodic rows. Furthermore, the atomic-resolution NC-AFM imaging unveiled the structure of the line defect.

Reference

[1] D. Katsube, et al., Beilstein J. Nanotechnol. 9, 686-692 (2018).

[2] P. Maksymovych, et al., Chem. Phys. Lett. 382, 270-276 (2003).

[3] G. H. Enevoldsen, et al., Phys. Rev. B 76, 205415 (2007).