Interstitial M⁺ (M⁺ = Li⁺ or Sn⁴⁺) Doping at Interfacial BiVO₄/WO₃ to Promote Photoelectrochemical Hydrogen Production

Doping metals together with heterostructure assemblages is critical to address the challenges encountered while using bismuth vanadate (BVO) to yield improved light-harvesting, charge transfer, and solar-to-hydrogen conversion efficiency. To date, most approaches have focused on substitutional doping using hexavalent metal ions (Mo6+ and W6+) at vanadium or bismuth sites to improve the photoelectrochemical (PEC) performance. Unlike conventional substitution, which produces V-substituted sites that function as hole traps and reduce the activity, herein, we used a simple hydrothermal and metal–organic decomposition approach to introduce interstitial [Li+ or Sn4+]n-doping in BVO (n = 0.25, 0.5, 1.0, 1.5, and 2.0 mM) interfaced with tungsten oxide (WO). The resulting Sn-doped BVO/WO (0.5 mM) shows a reproducible photocurrent density of 1.65 ± 0.07 mA cm–2 and 4.28 ± 0.15 mA cm–2 at 1.23 VRHE for water oxidation and sulfite oxidation, respectively, with a superior quantum efficiency (60% at 470 nm) and long-term durability (>10000 s) under standard AM 1.5 G light irradiation (1 sun). The results show that the Sn-doped BVO/WO exhibited an enhanced PEC performance approximately three times better than that of pristine BVO/WO, thus enabling continuous H2 production (∼800 μmol·cm–2) and highlighting the beneficial role of strategically controlled interstitial dopant concentration. Mott–Schottky analysis revealed an increase in the donor concentration for Li–BVO/WO (∼2.3-fold) and Sn–BVO/WO (3.5-fold), related to the reference BVO/WO photoelectrode. This work highlights the use of low-cost dopants and heterojunction photocatalysts to carry out hydrogen evolution reactions at a significantly improved rate.