Effects of yttrium, ytterbium with tungsten co-doping on the light absorption and charge transport properties of bismuth vanadate photoanodes to achieve superior photoelectrochemical water splitting

Effective water splitting by a photoelectrochemical cell using a BiVO₄ photoanode is limited by the light absorption and charge transport properties. Dual doping is one of the promising strategies to overcome this problem. In the present work, the role of individual element W, Y, and Yb doping and (Y,W) and (Yb,W) co-doping in BiVO₄ is evaluated towards photoelectrochemical water splitting. Doping Y³⁺ or Yb³⁺ at the Bi³⁺ site leads to an extended absorption region (λ ≤ 550) with a reduced band gap but decreases charge transport properties in comparison to W⁶⁺ doping at the V⁵⁺ site. Dual doping (Y,W) or (Yb,W) in BiVO₄ also shows slightly lower PEC performance than that with the W:BiVO₄ photoanode. Charge separation/transfer efficiency measurement reveals that the defects generated especially at the surface with Y³⁺ or Yb³⁺ substitutional doping act as a recombination center, leading to lower PEC performance. However, by forming a heterojunction with WO₃ along with a surface catalyst (Fe:NiO/Co-Pi) layer for efficient charge transfer, the (Y,W):BiVO₄ photoanode shows enhanced performance. In particular, the WO₃/(Y,W):BiVO₄/Fe:NiO/Co-Pi photoanode shows near complete suppression of recombination of electrons and holes with an improved absorption efficiency (∼85%) and efficient charge transfer (73 ± 4%) as well as shows a remarkable photoelectrochemical performance of 5.8 ± 0.3 mA cm^-2 at 1.23 V under 1 sun illumination in the K₂HPO₄ electrolyte. The photoanode also showed high incident and absorbed photon to current conversion efficiency (∼98%), demonstrating a simultaneous positive effect on the light absorption and charge transport properties.