Unconventional Substitution for BiVO₄ to Enhance Photoelectrocatalytic Performance by Accelerating Polaron Hopping

Bismuth vanadate (BiVO₄) as a fascinating semiconductor for photoelectrocatalytic (PEC) water oxidation with suitable band gap (E_g) has been limited by the issue of poor separation and transportation of charge carriers. Herein, we propose an unconventional substitution of V⁵⁺ sites by Ti⁴⁺ in BiVO₄ (Ti:BiVO₄) for the similar ionic radii and accelerated polaron hopping. Ti:BiVO₄ increased the photocurrent density 1.90 times up to 2.51 mA cm^-2 at 1.23 V vs RHE and increased the charge carrier density 1.81 times to 5.86 × 10¹⁸ cm^-3. Compared with bare BiVO₄, Ti:BiVO₄ improves the bulk separation efficiency to 88.3% at 1.23 V vs RHE. The DFT calculations have illustrated that Ti-doping modification could decrease the polaron hopping energy barrier, narrow the E_g, and decrease the overpotential of the oxygen evolution reaction (OER) concurrently. With further spin-coated FeOOH cocatalyst, the photoanode has a photocurrent density of 3.99 mA cm^-2 at 1.23 V vs RHE. The excellent PEC performance of FeOOH/Ti:BiVO₄ is attributed to the synergistic effect of the FeOOH layer and Ti doping, which could promote charge carrier separation and transfer by expediting polaron migration.