Brookite TiO2 is a promising material for active photocatalysts. However, the principal mechanism that determines the distinctive photocatalytic activity between anatase, rutile, and brookite TiO2 has not yet been fully elucidated. Therefore, in this work, we studied the behavior of photogenerated electrons and holes in these TiO2 powders by using femtosecond to millisecond time-resolved visible to mid-IR absorption spectroscopy. We found that most of the photogenerated electrons in brookite TiO2 are trapped at powder defects within a few ps. This electron trapping decreases the number of surviving free electrons, but it extends the lifetime of holes as well as the trapped electrons because the probability of electrons to encounter holes is decreased by this electron-trapping. As a result, the number of surviving holes increases, which is beneficial for photocatalytic oxidation. In contrast, the reactivity of electrons is decreased to some extent by trapping, but they still remain active for photocatalytic reductions. Electron trapping also takes place on anatase and rutile TiO2 powders, but the trap-depth in anatase is too shallow to extend the lifetime of holes and that of rutile is too deep than the thermal energy (kT) at room temperature for the electron-consuming reactions. As a result of the moderate depth of the electron trap in brookite, both electrons and holes are reactive for photocatalytic reductions and oxidations. These results have clearly demonstrated that the presence of an appropriate depth of the electron trap can effectively contribute to enhance the overall photocatalytic activity.

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Introduction Recently, global warming has become a serious problem all over the world. Being a greenhouse gas, the high levels of CO2 are a major contributor toward global warming. For address this problem, the photoelectrochemical CO2 reduction by using semiconductor photoelectrodes has attracted much attention as a potential means of converting solar energy into chemical energy in the form of usable organic fuels (CO, HCOOH, CH3OH, CH4, etc.). Titanium oxide (TiO2) has been a sustainable photocatalyst for environmental remediation because of its nontoxicity, availability, high oxidative potential and chemical stability. It is well-known that the TiO2 has three polymorphs: rutile, anatase, and brookite. In previous work, we have successfully synthesized the rutile and brookite TiO2 nanorods, which showed high levels of activity for degradation of 2-propanol and acetaldehyde under UV irradiation compared to the activity levels of anatase fine particles (ST-01). Therefore, in this study, we fabricated the TiO2 photoelectrodes by using the rutile and brookite nanorods, and investigated their photoelectrochemical properties under UV light irradiation. Moreover, we performed photoanode-driven photoelectrochemical CO2 reduction by using the TiO2 photoanodes and different metal counter electrodes (Pt, Ti, Cu, Sn, Zn) under UV light irradiation. Experimental Two polymorphs of TiO2 nanorods (rutile and brookite) were prepared by hydrothermal synthesis method, which described in our previously reports [1],[2]. Their photoelectrodes were fabricated by electrophoresis deposition method on the FTO conducting glasses. To improve photocurrent response, the necking treatment of the TiO2 photoelectrode was demonstrated: 0.01M titanium(IV) isopropoxide solution was dropped on the TiO2 electrode and then annealed in air at 400 ℃. The photoelectrochemical properties were investigated a three−electrode configuration using a silver−silver chloride (Ag/AgCl) reference electrode and different metal counter electrodes (Pt, Ti, Cu, Sn, Zn). The electrolyte was 0.1M KHCO3 aqueous solution. The electrolyte was stirred and purged with CO2 gas for 30 min before measurement. The photoelectrochemical CO2 reduction was carried out by using Xe lamp irradiation (100 mW/cm2, λ<800 nm). After photo-irradiation for 1h, the CO2 reduction products were evaluated by using gas-chromatography and ion chromatography. Results and discussion We investigated the photoelectrochemical CO2 reduction by using TiO2 photoanode and various metal counter electrodes (Pt, Ti, Cu, Sn, Zn). The CO2 reduction products were strongly dependent on the metal counter electrodes. In the case of Pt counter electrode, H2 was main product, and no CO2 reduction products such as CO, HCOOH, and CH3OH, were detected. This is because Pt has low overpotential for H2 generation compared to the other four metals (Ti, Cu, Sn, Zn). On the other hand, CO and/or HCOOH generation was generated when other four metals (Ti, Cu, Sn, Zn) was used as counter electrode. This is due to a relatively high overpotential for H2 generation. Among them (Ti, Cu, Sn, Zn), the Sn counter electrode showed the highest CO and HCOOH generation, which mechanism will be discussed in poster section. As described above, the photoelectrochemical CO2 reduction was demonstrated by using TiO2 photoanode combined with various metal counter electrodes under UV light irradiation. However, one fault of TiO2 is that it is inactive in the absence of ultraviolet (UV) light because of its large band gap (~3.0 eV for rutile and 3.2 eV for brookite). As nearly 50% of the incident solar energy on the earth’s surface falls within the visible light energy range, visible light-responsive photoanodes are strongly demanded. Therefore, we also introduced the photoelectrochemical properties of black TiO2 which could be synthesized by thermal annealing in reducing atmosphere. The detail information will be discussed in poster section. References [1] A.Tanibata, N.Murakami, T.Ohno,visible light response of shape controlled rutile TiO2 nanorod photocatalyst by LSPR Absorption(2012) [2] M. Kobayashi, K. Tomita, V. Petrykin, M. Yoshimura, M. Kakihana, J. Mater. Sci., 43 (2008) 2158–62