Unraveling the Site-Selective Doping Mechanism in Single-Crystalline BiVO4 Thin Films for Photoelectrochemical Water Splitting

Doping with extrinsic elements in bismuth vanadate (BiVO4, BVO) has been widely exploited for boosting photoelectrochemical (PEC) water splitting. However, changes in the carrier transport behavior of doping, in particular for the Bi-site doping in BVO, are still not well understood. Here, we explore the impact of site-selective doping on structural variation and electron transport in BVO as well as its implication for designing photoanodes with improved photoelectrochemical (PEC) performance. Two types of single-crystalline doped BVO films, with the V site substituted with Mo (Mo-V) ions or the Bi site substituted with Gd ions (Gd-Bi), are prepared by pulsed-laser deposition. Compared to pristine BVO, both types of doped photoanodes are found to require thinner films for delivering larger photocurrents, but the underlying doping mechanism appears to be different. Combined with temperature-dependent Raman characterization, X-ray photoelectron microscopy, and solid-state electron transport measurements, it is suggested that Gd-Bi doping facilitates carrier transport by introducing structural distortion and a gentle increase in the oxygen vacancy concentration. This is in contrast to MoV doping, which substantially enhances the donor density and facilitates carrier hopping. This work provides a perspective on understanding the impact of site-selective doping on BVO photoanodes for efficient PEC water splitting.