Publication date: 31st March 2013
The direct splitting of water into hydrogen and oxygen gases, with sunlight as the only input of energy, could provide a vital source of fuel in the context of a low-carbon economy. Hematite is a promising photo-anode material for the oxidation of water to oxygen (OER) – the more difficult half of the overall reaction converting water to hydrogen and oxygen gases. However, despite its suitable valence band position, visible light absorption, and good chemical stability, hematite is limited by rather weak absorption and poor charge transport. This leads to a trade-off between light absorption and carrier collection in flat devices. Mesoporous hematite is an interesting morphology for water splitting, as the small feature sizes favour good hole-collection at the semiconductor-electrolyte interface. While the efficiency of hematite layers is rather low, significant improvements are observed upon doping with various elements, such as Ti, Si or Sn.
We have developed efficient photo-active hematite layers by a chemical doping with a Sn-precursor during sol-gel synthesis. In this way, we incorporate around 3 % Sn atoms into the hematite lattice. This leads to a significant increase in photocurrent, which can be attributed to a higher efficiency of hole-transfer from the semiconductor to the OER, ηtrans. This could, in turn, be explained by faster transfer of holes to the OER, or reduced surface electron-hole recombination, and is the subject of on-going research. In order to further improve our photoelectrochemical performance, we have developed additional surface treatments with Co3O4 nanoparticles which reduce losses through electron-hole recombination.
a) Dependence of ηtrans on the percentage Sn-precursor added to the hematite synthesis. b) Impact of Co3O4 treatment on 250 nm thick electrodes synthesized with 20% Sn-precursor in the synthesis. J-V curves measured at pH 13 under simulated solar irradiation AM 1.5. c) SEM cross section of a 350 nm thick meso-porous layer.
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