TiO2 is a material that has various applications, such as photocatalyst, electron transport layer for perovskite solar cell, and so on. Representative crystal phases of TiO2 are rutile, anatase, and brookite among its crystal phases. Anatase and brookite are metastable states, thus it is not simple to obtain the epitaxial thin film of these. Gas phase methods, like pulse laser deposition [1], molecular beam epitaxy [2], and so on, are typically used for fabrication of epitaxial thin films of anatase TiO2. It has been reported that the epitaxial orientation is (001)[100]TiO2||(001)[100]substrate for using LaAlO3 (001) and SrTiO3 (001) as a substrate. On the other hand, for the fabrication of (001)-faceted anatase TiO2 crystal by hydrothermal method, the effectiveness of fluoride ions has been reported [3]. Recently, we reported the epitaxial growth of anatase TiO2 by solvothermal method on LaAlO3 (001) with a precursor containing fluoride ions and the local structure analysis around dopant atoms of the fabricated thin film [4].
To further the local structure analysis for understanding of the relation between photocatalytic activity and dopant, it was found that the change of the substrate is necessary. In this presentation, the successful of epitaxial growth of anatase TiO2 thin film on SrTiO3 by the invented solvothermal method and the growth dependence on precursor compositions will be presented.
Schematic drawings of the experimental procedures are shown in Figure 1 (a). The results of crystallinity and orientation evaluation by X-ray diffraction are shown in Figure 1 (b) and (c). It shows that the fabricated thin film was anatase TiO2 and was grown epitaxially. In addition, the dependence of the crystallinity on the precursor compositions was found. Fluoride ions added as a surfactant remained on the surface of the fabricated thin film, as shown by X-ray photoelectron spectroscopy.
Continuing the growth investigation, structural analysis will be performed to reveal the local structure around the dopant and the adsorption structure of fluorine atoms on the surface in the future.
References
[1] H. Sakama, G. Osada, M. Tsukamoto, A. Tanokura, and N. Ichikawa, Thin Solid Films 515, 535 (2006).
[2] M. Murakami, Y. Matsumoto, K. Nakajima, T. Makino, Y. Segawa, T. Chikyow, P. Ahmet, M. Kawasaki, and H. Koinuma, Appl. Phys. Lett. 78, 2664 (2001).
[3] H. G. Yang, C. H. Sun, S. Z. Qiao, J. Zou, G. Liu, S. C. Smith, H. M. Cheng, and G. Q. Lu, Nature 453, 638 (2008).
[4] K. Ono, K. Kimura, T. Kato, K. Hayashi, R. M. G. Rajapakse, and M. Shimomura, Chem. Eng. J. 451, 138893 (2023).
