Subject Number :S-16-2011, 2024
Support Type : Common use (including technical support necessary for the training),
Proposal Title (English) : Improved photoelectrochemical performance of Au@TiO2-coated Fe2O3 nanorods studied by scanning transmission x-ray microscopy
Username (English) : Y. R. Lu1,2, Y. F. Wang1, Y. C. Huang1,2, J. W. Chiou2, C. L. Dong1, W. F. Pong1, T. Ohigashi3 and N. Kosugi3
Affiliation (English): 1Department of Physics, Tamkang University, Tamsui 251, Taiwan2Department of Electrophysics, National Chiao Tung University, 3Department of Applied Physics, National University of Kaohsiung, 4, UVSOR Facility, Institute for Molecular Science, Okazaki 444-8585, Japan

Since Fujishima and Honda had found the photoelectrochemical (PEC) water splitting could be made directly by TiO2 under UV irradiation thirty years ago, this direct solar fuel conversion is regarded as one of the most promising clean and renewable energy resource. The main drawback of TiO2 is the bandgap is too large so it can only absorb the UV light. Hematite (α-Fe2O3) is a promising material for solar water splitting owing to its smaller band gap energy about 2.1-2.3 eV, which can capture 40% of incident sunlight. However, it is only suitable for oxygen evolution half reaction and electron-hole recombines very easily due to its intrinsic short hole diffusion length and poor electron conductivity. Doping is an effective way to adjust the band edge and overcome the charge transfer ability in hematite.[1-2] In addtion, by taking advantage of plasmonic effect caused by metal nanocrystals, the PEC performance could be further improved. In this work, the hematite nanorods coated with a thin layer of TiO2 with/without Au nanoparticles embbeded in between have been studied by STXM to elucidate the correlation between the PEC properties and interfacial electonic structures.
Figures 1 present the O K-edge (left panel) and Fe L-edge (right panel) STXM stack mappings of selected single nanorod of bare-, TiO2 coated and Au embedded TiO2-coated Fe2O3. The stack mappings display yellow, red and green areas, corresponding to the different regions that are associated with different regions and chemical properties of the nanorods. Figures 1 also present the XANES spectra, which correspond to their stack mappings. While there is no significant difference of XANES of Ti L-edge (not shown here) could be revealed, the variation of both O K-edge and Fe L-edge are observed. Spectroscopic results indicate the surface region contains more defects compared with core region, which enhance the density of states both in O and Fe sites. However, the TiO2 layer and embedded Au nanoparticles may inhibit the charge recombination.
The enhanced PEC performance of TiO2-coated Fe2O3 with Au nanoparticles embedded is likely to be not only the plasmonic effect caused by Au nanoparticles, but also the intrinsic improvement of charge transfer ability that is attributable to embedded Au nanoparticles.

Figs. 1 O K-edge (left panel) and Fe L-edge (right panel) scanning transmission X-ray microscopy image and its corresponding stack mappings of selected single (top) bare-, (middle) TiO2-coated, and (bottom) TiO2/Au- coated Fe2O3 nanorod.

References
[1] Y. Fu et al., ChemNanoMat, (2016). 2, 704
[2] Y. Fu et al., Phys. Chem. Chem. Phys. (2016). 18, 5203.
[3] Y. R. Lu et al., to be submitted.