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All-Oxide α-Fe₂O₃/H:TiO₂ Heterojunction Photoanode: A Platform for Stable and Enhanced Photoelectrochemical Performance through Favorable Band Edge Alignment

Kodan, Nisha, Agarwal, Khushboo, Mehta, B. R.
Journal of physical chemistry 2019 v.123 no.6 pp. 3326-3335
X-ray photoelectron spectroscopy, absorption, annealing, electric current, electrodes, energy, hematite, hydrogen, lighting, microscopy, titanium dioxide
To improve the photoelectrochemical (PEC) performance of hematite thin-film photoanode, we report a novel heterostructure based on Fe₂O₃/TiO₂ and hydrogenated Fe₂O₃/TiO₂ (Fe₂O₃/H:TiO₂) for faster charge transfer owing to passivation of surface states in Fe₂O₃ via TiO₂ overlayer and favorable band alignment using hydrogen annealing of TiO₂ overlayer. The valence band offset, band gap and work function values have been measured using X-ray photoelectron spectroscopy, optical absorption, and Kelvin probe force microscopy to construct the energy band diagram of the heterostructure photoanodes. The results confirm the upshift in the valence band edge of TiO₂ over Fe₂O₃ after hydrogen treatment of TiO₂ overlayer, which leads to the formation of type II band edge alignment in Fe₂O₃/H:TiO₂ heterojunction and improved PEC performance compared to Fe₂O₃/TiO₂, pristine Fe₂O₃ and TiO₂ thin-film photoelectrodes. The well-straddled and improved band alignment in Fe₂O₃/H:TiO₂ heterostructure gives rise to substantial enhancement in photocurrent density, up to 3.36 mA/cm² at 1.23 V (vs reversible hydrogen electrode (RHE)) with a low onset potential of 0.1 V (vs RHE), under AM1.5 illumination condition. The observed photocurrent density in Fe₂O₃/H:TiO₂ heterostructure is 15-fold higher than bare Fe₂O₃ (0.22 mA/cm²) photoanode. This work shows how a simple bilayer junction and its hydrogen treatment can be used to enhance the PEC response of heterojunctions and offers valuable insights into the further development of all-oxide heterojunctions.