Hierarchical Nanoporous BiVO₄ Photoanodes with High Charge Separation and Transport Efficiency for Water Oxidation

To fabricate high efficiency photoanodes for water oxidation, it is highly required to engineer their nanoporous architecture and interface to improve the charge separation and transport efficiency. By focusing on this aspect, we developed hierarchical nanoporous BiVO₄ (BV) from solution processed two-dimensional BiOI (BI) crystals. The orientation of the BI crystals was controlled by changing the solvent volume ratios of ethylene glycol (EG) to ethanol (ET), which resulted in different hierarchical and planar BV morphologies through a chemical treatment followed by thermal heating. The morphology with optimal particle dimension, connectivity, and porosity can offer a highly enhanced electrochemically active surface area (ECSA). The hierarchical BV owning a maximum ECSA showed the best photoelectrochemical (PEC) performance in terms of the highest photocurrent density and charge separation efficiency. However, to further improve the performance of the electrode, conformal and ultrathin SnO₂ underlayers were deposited by a powerful atomic layer deposition technique at the interface to effectively block the defect density, which significantly improved the photocurrents as high as 3.25 mA/cm² for sulfite oxidation and 2.55 mA/cm² for water oxidation at 0.6 V versus the reversible hydrogen electrode (RHE). The electrode possessed record charge separation efficiency of 97.1% and charge transfer efficiency of 90.1% at 1.23 V_RHE among to-date reported BiVO₄-based photoanodes for water oxidation. Furthermore, a maximum applied bias photon-to-current efficiency (ABPE) of 1.61% was found at a potential as low as 0.6 V_RHE, which is highly promising to make a tandem cell. These results indicate that the construction of the hierarchical nanoporous photoanode with an enhanced ECSA and its proper interface engineering can significantly improve the PEC performance.