Paper TF-WeE4
Band-engineering of (TiO₂)_1−x(TaON)_x Thin Films for Photochemical Applications

Wednesday, December 5, 2018, 6:40 pm, Room Naupaka Salons 4

Titanium dioxide (TiO₂) has been extensively studied for photocatalytic applications. Recently, we synthesized epitaxial thin films of anatase tantalum oxynitride (TaON), which has a smaller bandgap and larger refractive index than TiO₂ [1,2]. Alloying of anatase TiO₂ and TaON would enable band structure engineering of TiO₂ in a controller manner. In this study, we have grown thin films of an anatase (TiO₂)_1−x(TaON)_x (TTON) solid solution and investigated their optical properties and band alignment. Epitaxial thin films of TTON (0.1 ≤ x ≤ 0.9) were deposited on (LaAlO₃)_0.3(SrAl_0.5Ta_0.5O₃)_0.7 substrates by nitrogen plasma-assisted pulsed laser deposition technique. X-ray diffraction confirmed epitaxial growth of phase-pure anatase TTON, of which lattice constants changed with x in agreement with Vegard’s law. Optical properties and band alignment were examined spectroscopic ellipsometry and X-ray photoelectron spectroscopy, respectively. The bandgap of TTON systematically decreased with increasing x, mainly due to upward shift in the valence band maximum through evolution of shallow N 2p band. Meanwhile, the position of the conduction band minimum was insensitive to x. The band alignment of anatase TTON was found to be suitable for photocatalytic water splitting with visible light. The refractive index of anatase TTON monotonically increased with x, possibly originating from the higher covalency of metal-N bonds than that of metal-O bonds.

References

[1] A. Suzuki, Y. Hirose, D. Oka, S. Nakao, T. Fukumura, S. Ishii, K. Saso, H. Matsuzaki, and T. Hasegawa, Chem. Mater.26, 976 (2014)