First-Principles Study of Photocatalytic Activity of Amorphous TiO2 Surface

Photocatalysts are materials that exhibit many functions on their surface when they exposed to light, such as antifouling, air purification, deodorization, antibacterial, and water purification. Titanium oxide (TiO2) is known as a representative photocatalytic material because it is inexpensive and safe. It is expected to be a solution to recent energy and environmental pollution problems. Therefore, studies have been actively conducted to improve the catalytic activity. Our group experimentally found that the catalytic activity of an amorphous TiO2 surface is approximately 13 times higher than that of a normal anatase crystalline surface. Therefore, it is considered that we can further improve the catalytic activity by controlling the surface structure. TEM-EELS has been widely used in recent years to observe nanoscale atomic arrangements and electronic structures with high spatial resolution. ELNES (electron energy-loss near-edge structures) spectrum obtained by EELS can provide sensitive information on chemical bonding, valence states and coordination. We observed the amorphous surface of TiO2 samples with high catalytic activity using the atomic resolution electron microscope (JEM-ARM200F @Nitech) and found that spectrum for the surface region were significantly different from those for bulk region. In this study, we aim to clarify the factors that increase the catalytic activity to propose materials with higher catalytic activity using first-principles calculations. Amorphous surface models were generated by performing high-temperature melting, rapid cooling, and structural relaxation using first-principles MD (molecular dynamics) simulations with SIESTA code [1]. During the MD processes, the coordination number of Ti with oxygen in the amorphous surface region decreased, from 6 to 4 coordination. First-principles Ti L-edge ELNES calculations for the bulk crystalline region and the surface amorphous region were performed with QMAS code [2]. They reproduced the difference between experimental ELNES spectrum for bulk region and surface region well. Therefore, we indicate that the amorphous region of real materials is composed of 4-coordinated Ti. Furthermore, we investigate the reactivity of water molecules which is essential for photocatalytic reactions on the TiO2 surface and the influence of oxygen defects inside TiO2. We indicate unique characteristics of the amorphous surface to clarify the origin of highly active catalytic reactions. References: [1] J. Solar et al., J. Phys. Cond. Matt. 14, 2745 (2000). [2] T. Tamura et al., Phys. Rev. B 85, 205210 (2012). Figure 1