Enhanced Photoelectrochemical Water Splitting with Er- and W-Codoped Bismuth Vanadate with WO₃ Heterojunction-Based Two-Dimensional Photoelectrode

A novel two-dimensional (2D) heterojunction photoelectrode composed of WO₃ and (Er,W):BiVO₄ is proposed for water oxidation with efficient photoinduced charge carrier separation and transfer. Er stoichiometric along with W nonstoichiometric codoping was introduced to simultaneously manage vacancy creation during substitutional doping, enhance light absorption, and reduce overall impedance. It was found that Er³⁺ is substituted at the Bi³⁺ sites in the BiVO₄ lattice to provide expanded light absorption from 400 to 680 nm. The fabricated WO₃/(Er,W):BiVO₄ electrode shows photocurrent densities of 4.1 and 7.2 mA cm^-2 at 1.23 and 2.3 V (vs reversible hydrogen electrode, RHE), respectively, under a 1 sun illumination in K₂HPO₄ electrolyte. This electrode has shown remarkably high charge separation efficiency of 93% at 1.23 V (vs RHE). With the addition of a standard surface catalyst (i.e., Co-Pi), the WO₃/(Er,W):BiVO₄/Co-Pi electrode exhibits the highest photocurrent of 5.6 ± 0.3 mA cm^-2 at 1.23 V (vs RHE), nearing the theoretical limit (i.e., 7.5 mA cm^-2) while retaining 98% of the photoelectrochemical cell performance after 3 h. By concomitantly doping the Bi³⁺ and V⁵⁺ sites to enhance absorption, this study demonstrates for the first time a planar WO₃/BiVO₄ heterojunction that reaches 88% of the record-high performance of its nanostructured counterpart. Through a detailed characterization of the electrodes, it is concluded that the stoichiometric Er and nonstoichiometric W codoping extend light absorption region and improve charge separation efficiency by reducing bulk resistance. The photoactive materials with 2D morphology were synthesized using a facile ultrasonic spray-coating technique without any complex process steps and thus it can be scaled for commercial development.