Browsing by Author "Okada, Kiyoshi"

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Fully Depleted Ti-Nb-Ta-Zr-O Nanotubes: Interfacial Charge Dynamics and Solar Hydrogen Production
(2018-07-11)
Chiu, Yi-Hsuan
;
Lai, Ting-Hsuan
;
Chen, Chun-Yi
;
Hsieh, Ping-Yen
;
Ozasa, Kazunari
Poor kinetics of hole transportation at the electrode/electrolyte interface is regarded as a primary cause for the mediocre performance of n-type TiO2 photoelectrodes. By adopting nanotubes as the electrode backbone, light absorption and carrier collection can be spatially decoupled, allowing n-type TiO2, with its short hole diffusion length, to maximize the use of the available photoexcited charge carriers during operation in photoelectrochemical (PEC) water splitting. Here, we presented a delicate electrochemical anodization process for the preparation of quaternary Ti-Nb-Ta-Zr-O mixed-oxide (denoted as TNTZO) nanotube arrays and demonstrated their utility in PEC water splitting. The charge transfer dynamics for the electrodes was investigated using time-resolved photoluminescence, electrochemical impedance spectroscopy, and the decay of open-circuit voltage analysis. Data reveal that the superior photoactivity of TNTZO over pristine TiO2 originated from the introduction of Nd, Ta, and Zr elements, which enhanced the amount of accessible charge carriers, modified the electronic structure, and improved the hole injection kinetics for expediting water splitting. By modulating the water content of the electrolyte employed in the anodization process, the wall thickness of the grown TNTZO nanotubes can be reduced to a size smaller than that of the depletion layer thickness, realizing a fully depleted state for charge carriers to further advance the PEC performance. Hydrogen evolution tests demonstrate the practical efficacy of TNTZO for realizing solar hydrogen production. Furthermore, with the composition complexity and fully depleted band structure, the present TNTZO nanotube arrays may offer a feasible and universal platform for the loading of other semiconductors to construct a sophisticated heterostructure photoelectrode paradigm, in which the photoexcited charge carriers can be entirely utilized for efficient solar-to-fuel conversion.
ZnO-graphene composites as practical photocatalysts for gaseous acetaldehyde degradation and electrolytic water oxidation
(2015-01-25)
Chen, Yu-Chih
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Katsumata, Ken-ichi
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Chiu, Yi-Hsuan
;
Okada, Kiyoshi
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Matsushita, Nobuhiro
A facile, green one-pot hydrothermal method has been developed to prepare ZnO-reduced graphene oxide (rGO) composites with controllable rGO content. Time-resolved photoluminescence spectra revealed that an increased electron-transfer rate constant was observed for ZnO-rGO with increasing rGO contents, suggesting that an increased number of photoexcited charge carriers were separated and available for photocatalysis utilization. The photocatalytic properties of the ZnO-rGO composites were investigated by using gaseous acetaldehyde (CH3CHO), a typical volatile organic compound (VOC), as the test pollutant. This is the first time that ZnO-rGO composites have been used as photocatalysts for gaseous CH3CHO degradation. Compared to pure ZnO, ZnO-rGO composites with suitable rGO contents (1.0 and 3.0 wt%) displayed significantly enhanced photocatalytic activity in both CH3CHO degradation and CO2 generation. This enhancement was ascribed to the rGO support that can promote the carrier utilization efficiency of ZnO by readily accepting the photoexcited electrons from ZnO. As a result, abundant photogenerated holes remained on the ZnO and were available for participation in the CH3CHO photodegradation. Furthermore, the photoactivity of ZnO-rGO toward electrolytic water oxidation was evaluated. The results showed that ZnO-rGO composites achieved 50% increase in water oxidation current over pure ZnO under white light illumination. The demonstration from this work may facilitate the use of ZnO-rGO composites in photodegradation of VOCs as well as for photoelectrochemical applications, which is imperative from the environmental and energetic points of view. (C) 2014 Elsevier B.V. All rights reserved.