Visible light-induced electronic structure modulation of Nb- and Ta-doped α-Fe₂O₃ nanorods for effective photoelectrochemical water splitting

The photoelectrochemical (PEC) water splitting activity of Nb and Ta-doped hematite (α-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 α-Fe₂O₃ nanorods depends strongly on the species and concentrations of dopants. The doping of α-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 α-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 α-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 α-Fe₂O₃. The correlation between PEC activity and the electronic structure of Nb/Ta-doped α-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.