Browsing by Author "Lee, Wan-Yi"

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Nb-Doped Hematite Nanorods for Efficient Solar Water Splitting: Electronic Structure Evolution versus Morphology Alteration
(2016-07)
Fu, Yanming
;
Dong, Chung-Li
;
Lee, Wan-Yi
;
Chen, Jie
;
Guo, Penghui
In this study, Nb-doped alpha-Fe2O3 nanorod photoanodes were prepared via a facile solution-based process by directly adding NbCl5 in the aqueous solution. The morphology and electronic structure of Nb-doped alpha-Fe2O3 films depended strongly on the dopant concentrations. Compared with the undoped sample, the optimal Nb-doped alpha-Fe2O3 film showed an approximately 4-fold photocurrent increase under solar light at 1.0 V versus Ag/AgCl, and the incident photon-to-current conversion efficiency was increased by 2.5 times, reaching 13.7% at 350 nm and 1.23 V versus RHE. The enhancement in PEC activity, as induced by moderate Nb doping, was attributed to the increased charge carrier density for promoted charge transfer ability as well as the smaller nanorod diameter for shortened charge transfer distance. However, superfluous Nb dopants would destroy the nanorod structure and greatly reduce the thickness of alpha-Fe2O3 films, leading to poor optical absorption and thus decreased photoelectrochemical performance.
Visible light-induced electronic structure modulation of Nb- and Ta-doped alpha-Fe2O3 nanorods for effective photoelectrochemical water splitting
(2018-02-09)
Chang, Han-Wei
;
Fu, Yanming
;
Lee, Wan-Yi
;
Lu, Ying-Rui
;
Huang, Yu-Cheng
The photoelectrochemical (PEC) water splitting activity of Nb and Ta-doped hematite (alpha-Fe2O3) nanorods was investigated with reference to electronic structures by in situ synchrotron x-ray absorption spectroscopy (XAS). Current density-potential measurements demonstrate that the PEC activity of alpha-Fe2O3 nanorods depends strongly on the species and concentrations of dopants. The doping of alpha-Fe2O3 nanorods with a low level of Nb or Ta can improve their electrical conductivity and thereby facilitate charge transport and reduced electron-hole recombination therein. The photoconversion effects of Nb and Ta-doped alpha-Fe2O3 by in situ XAS in the dark and under illumination revealed opposite evolutions of the spectral intensities of the Fe L-edge and Nb/Ta L-edge, indicating that charge transfer and a conduction pathway are involved in the photoconversion. Analytic in situ XAS results reveal that the alpha-Fe2O3 that is doped with a low level of Nb has a greater photoconversion efficiency than that doped with Ta because Nb sites are more active than Ta sites in alpha-Fe2O3. The correlation between PEC activity and the electronic structure of Nb/Ta-doped alpha-Fe2O3 is examined in detail using in situ XAS and helps to elucidate the mechanism of PEC water splitting in terms of the electronic structure.