A Front‐Illuminated Nanostructured Transparent BiVO4 Photoanode for >2% Efficient Water Splitting

DOI: 10.1002/aenm.201501645 Although the nanoporous BiVO 4 photoanode does not fulfi l all criteria from the perspective of practical tandem-electrode applications, it manifested the effectiveness of nanostructuring and represented a more facile approach, compared with gradient doping and heterojunction formation, for electron– hole separation. Nanostructuring is essential for performance enhancement for many energy conversion semiconducting materials with relatively poor carrier dynamics, such as hematite [ 22–24 ] and tantalum nitride. [ 25 ] To seek for the champion nanostructure, however, is a very demanding task. Such strong linkage between nanostructure and performance should also exist for BiVO 4 . We argue that those obstacles to front illumination performance could also be overcome by nanostructure perfection. Here, we demonstrate that largely enhanced front-illumination performance along with improved charge separation effi ciency and light transmittance can be realized simultaneously via appropriate nanostructuring and sophisticated morphology control for nondoped nanostructured BiVO 4 electrodes. We also explored the use of a bimetallic NiFe-(oxy)hydroxide/borate (NiFeO x -B i ) oxygen evolution catalyst (OEC) as a cocatalyst for BiVO 4 electrodes to make the best of the outstanding charge separation capability for water splitting. As a result, the solar energy conversion effi ciency exceeded 2% for the fi rst time for BiVO 4 based photoanodes. Moreover, such an effi ciency was achieved under front illumination, together with a transmittance larger than 50% above 600 nm wavelength, making our electrode a perfect candidate for tandem applications. Our BiVO 4 electrode possesses a characteristic worm-like network morphology, with an optimized diameter of the nanostructure unit ≈120 nm ( Figure 2 f). Its monoclinic scheelite crystal structure was confi rmed by X-ray diffraction (XRD) (Figure S4, Supporting Information). To distinguish from the reported nanoporous BiVO 4 electrode, it will be referred as nanoworm BiVO 4 electrode hereafter. A two-step approach was used for its preparation, similar to that used for the synthesis of nanoporous BiVO 4 electrode. A BiOI fl ake precursor was fi rst prepared by electrodeposition on an indium tin oxide (ITO) substrate, followed with addition of vanadyl acetyl acetonate dissolved in dimethyl sulfoxide (DMSO) and annealing in air. The thermally stable ITO glass from GEOMATEC has a much smoother surface and is more conductive (5 Ω sq −1 ) compared with most fl uorine-doped tin oxide (FTO) glasses, so that the voltage loss can be minimized. Our new process for BiOI deposition involved the use of a more diluted Bi 3+ solution and was more acidic, resulting in a much denser packing of BiOI 2D fl akes on ITO substrate (Figure 2 d). Owing to its increased packing density and also the surface smoothness of ITO, the deposited BiOI layer tended to peel off during deposition, With growing concern over the depletion of fossil fuels, increasing research interests are being attracted to sustainable energy developments. [ 1–6 ] Photoelectrochemical (PEC) water splitting is a quintessential example of solar energy harnessing for direct chemical energy conversion and storage. BiVO 4 , fi rst studied as water oxidation photocatalyst in a powder suspension system, [ 7,8 ] was soon found to be a promising photoanode material for PEC water splitting due to its appropriate band structure, potential stability, and wide availability. [ 9–11 ] In the past several years, exciting progress has been achieved for BiVO 4 photoanode development in terms of both PEC performance and durability. [ 12–15 ]