Bismuth vanadate (BVO) has been widely studied as one of the most promising photoanode materials for photoelectrochemical (PEC) water splitting, owing to a bandgap of 2.4 eV and favorable band positions for water oxidation. However, while BVO has the potential for high anodic photocurrents, it is often limited by electron-hole separation, charge transport, and water oxidation kinetics, requiring development of nanostructured electrode architectures to optimize performance.¹

Here we demonstrate a new method for fabricating tunable BVO photoanodes deposited by atomic layer deposition (ALD). ALD is a gas-phase thin film deposition technique with sub-nanometer thickness control, programmable film stoichiometry, and conformal coverage of 3-D nanostructured templates, enabling rational design of electrode architectures. We present the first deposition of ALD BVO using Bi(OCMe₂iPr)₃ as the bismuth source, vanadium(V)oxytriisopropoxide as the vanadium source, and water as the oxidant. The choice of this recently developed Bi precursor provides full control of Bi:V stoichiometry in contrast to the use of earlier Bi precursors such as triphenylbismuth³, while also providing a per cycle growth rate that is more than three times higher than previous reports.

The BVO films were deposited as a nanolaminate of binary bismuth and vanadium oxides. The films were post-annealed to achieve the photoactive monoclinic BiVO₄ phase. Film composition and photocurrent were investigated as a function of deposition pulse ratio and film thickness. The photoactivity of planar ALD BVO was measured in a three-electrode cell under simulated AM1.5G illumination. Sulfite oxidation was used to optimize the BVO deposition conditions independent of co-catalyst performance. We achieved the highest reported photocurrent to date for ALD photoanodes. Using a planar electrode with a 40nm thick BVO film, a photocurrent of > 2.6 mA/cm² at 1.23 V vs. RHE was demonstrated for sulfite oxidation and a photocurrent of > 1.1 mA/cm² at 1.23 V vs. RHE was demonstrated for water oxidation using an un-optimized cobalt oxide co-catalyst.

ALD provides conformal coverage of high aspect ratio structures. The development of an ALD process for BVO enables core-shell architectures that help address the charge transport and carrier separation challenges by decoupling carrier diffusion and light absorption lengths. To demonstrate this benefit, BVO was deposited on mesoporous tin oxide (ITO) substrates to form 3-D electrode architectures with tunable absorption and charge extraction properties. The photoresponse was enhanced for both sulfite and water oxidation under illumination, demonstrating the power of ALD to improve light absorption and charge extraction in 3-D nanostructured electrode architectures.

(1) Sivula, K.; van de Krol, R. Semiconducting Materials for Photoelectrochemical Energy Conversion. Nat. Rev. Mater. 2016, 1 (2), 15010.

(2) Liu, C.; Dasgupta, N. P.; Yang, P. Semiconductor Nanowires for Artificial Photosynthesis. Chem. Mater. 2014, 26 (1), 415–422.

(3) Stefik, M. Atomic Layer Deposition of Bismuth Vanadates for Solar Energy Materials. ChemSusChem 2016, 9 (13), 1727–1735.