Producing hydrogen via solar water splitting using a photoelectrochemical cell (PEC) persists as one of the most exciting research topics in the field of solar fuels. The construction of efficient PECs requires the integration of multiple components including a photoanode, a photocathode, an oxygen evolution catalyst, and a hydrogen evolution catalyst. Therefore, the compatibility and stability of all of these elements in a given operating condition are crucial. When the stability of a semiconductor electrode used as the photoanode or photocathode is limited in an acidic or basic condition which is optimum for the operation of the other components, a thin protective layer has been deposited on the semiconductor surface to prevent its chemical dissolution. Surface coating of a thin and conformal TiO₂ layer has been proven to be successful for protecting photoelectrodes since TiO₂ is chemically and electrochemically stable in a wide range of pH conditions under both anodic and cathodic conditions.

In order to prevent the semiconductor surface from coming into direct contact with the corrosive electrolyte, complete coverage of the photoelectrode with TiO₂ is required. At the same time, the TiO₂ layer should be thin enough not to interfere with the charge transport properties of the photoelectrode. As a result, atomic layer deposition (ALD) has been the only successful tool used to date to produce an effective protective layer. However, the slow processing time and economic viability of ALD methods motivated us to develop an inexpensive and facile solution-based synthesis method for the deposition of high quality TiO₂ coating layers.

In this presentation, we report a new electrochemical method to deposit a thin and conformal TiO₂ layer on nanoporous BiVO₄ that has an intricate, high surface area morphology. BiVO₄ is a promising n-type photoanode material with a relatively low bandgap (2.4~2.5 eV). However, its usage has been limited to neutral and mildly basic conditions (pH 5~9) because it is chemically unstable in strongly acidic and basic conditions. Our method allows for the deposition of a 5~6 nm thick TiO₂ layer on BiVO₄ within 1 min and the resulting BiVO₄/TiO₂ electrodes exhibit chemical stability in basic solutions (pH 12~13). Sulfite oxidation measurements of BiVO₄ and BiVO₄/TiO₂ electrodes show that the thin TiO₂ protective layer does not significantly reduce the hole transfer to the electrolyte. Finally, we demonstrate the photoelectrochemical stability of the BiVO₄/TiO₂ electrode for photoelectrochemical water oxidation in basic solutions by coupling the BiVO₄/TiO₂ electrode with appropriate oxygen evolution catalysts.