We report the synthesis and photoelectrochemical assessment of phase pure tetragonal matlockite structured BiOX (where X = Cl, Br, I) films. The materials were deposited using aerosol-assisted chemical vapour deposition. The measured optical bandgaps of the oxyhalides, supported by density functional theory calculations, showed a red shift with the increasing size of halide following the binding energy of the anion p-orbitals that form the valence band. Stability and photoelectrochemical studies carried out without a sacrificial electron donor showed the n-type BiOBr film to have the highest photocurrent reported for BiOBr in the literature to date (0.3 mA cm-2 at 1.23 V vs. RHE), indicating it is an excellent candidate for solar fuel production with a very low onset potential of 0.2 V vs. RHE. The high performance was attributed to the preferred growth of the film in the  direction, as shown by X-ray diffraction, leading to internal electric fields that minimize charge carrier recombination.
Bibliographical noteFunding Information:
I. P. P. and C. J. C. acknowledge EPSRC for funding the work under Grant EP/L017709. D. S. B. acknowledges Mr Kevin Reeves for assistance with SEM. Thank you to Pilkington NSG for the glass substrates. This work made use of the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk), via our membership of the UK's HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202), the Iridis cluster, provided by the EPSRC funded Centre for Innovation (Grant codes EP/K000144/1 and EP/K000136/1) and the UCL Legion HPC Facility (Legion@UCL). Thanks to Dr Ainara Garcia Gallastegui for useful discussions. S. S. acknowledges the Deanship of Scientific Research (DSR), King Abdulaziz University, Jeddah for funding the work under grant D-1-434. D. O. S. acknowledges EPSRC for funding the work under grant EP/N01572X/1. The work at Bath was supported by the ERC (Grant no. 277757) and the EPSRC (Grant no. EP/K016288/1, EP/L017792/1 and EP/M009580/1). S. M. and J. T. acknowledge funding from EU FP7 4G-PHOTOCAT Grant no. 309636.
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