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

Abstract

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 BiVO 4 (BV) from solution processed twodimensional 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 SnO 2 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/cm 2 for sulfite oxidation and 2.55 mA/cm 2 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 BiVO 4 -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.

Controlled Band Offsets in Ultrathin Hematite for Enhancing the Photoelectrochemical Water Splitting Performance of Heterostructured Photoanodes

“…In order to overcome the limits of traditional photoelectrochemical (PEC) materials, numerous semiconductors have been studied as an alternative that has a small band gap, good charge separation efficiency, and a long carrier lifetime. − Fe 2 O 3 , WO 3 , and BiVO 4 (BVO) can represent photoanodes with the high theoretical STH conversion efficiencies of 15.5, 5.8, and 9.2%, respectively . Although WO 3 is earth-abundant, a good light absorber, and stable in an aqueous solution, it has a relatively low STH conversion efficiency. , BVO has good STH conversion efficiency and low production cost, but it has poor surface charge-transfer ability. − Fe 2 O 3 is stable in water and has good STH conversion efficiency. , However, it requires a high overpotential for water oxidation, poor transport ability of majority carriers, short diffusion length of minority carriers, and low absorption coefficient . To achieve the theoretical maximum conversion efficiency, a design strategy that goes beyond the limits of these PEC materials is important.…”

Sustained Water Oxidation with Surface- and Interface-Engineered WO3/BiVO4 Heterojunction Photoanodes

“…Furthermore, the relative ECSAs of the WBV films with different particle sizes (Figure a–e) were calculated from the electrochemical double layer capacitance ( C dl ) by measuring CV under dark conditions (Figure S8). It is noted that C dl is linearly proportional to the ECSA . The linear slope values of WBV photoanodes with different BiVO 4 particle sizes were compared to observe the surface characteristics (Figure f).…”

“…It was found that the WBV photoanode with the smallest BiVO 4 particle size showed a larger slope value than the other WBV photoanodes, suggesting the maximum ECSA (Figure f). Thus, a WBV photoanode with a smaller BiVO 4 particle size possessed larger interfacial areas and porosity that allowed access to more electrolyte at the electrode surface, resulting in enhanced PEC properties . To achieve the heterojunction effect of the WBV film, it is necessary to load an optimum amount of BiVO 4 on WO 3 nanosheets (Figure S4d).…”

Constructing a full-space internal electric field in a hematite photoanode to facilitate photogenerated-carrier separation and transfer

“…Currently, oxide semiconductors have received extensive attention due to their intrinsic characteristics of adjustable chemical composition, controllable band gaps, stable performance, etc. [6][7][8][9][10] The studied materials in the PEC-WS eld have expanded from wide bandgap semiconductors (e.g., TiO 2 (ref. 11) and ZnO 12 ) to narrow bandgap semiconductors (e.g., BiVO 4 (ref.…”