Solution-processed La-substituted Ba2Bi2O6 photocathodes with enhanced photoelectrochemical activity: a combined experimental and computational study

Abstract

In the search for solution-processed p-type semiconductors for photocathodic hydrogen evolution, we have developed a spin-coating method for fabricating the double perovskite Ba₂Bi₂O₆ using polyvinylpyrrolidone as a polymer template and a sol–gel mixture of barium and bismuth acetates. This method produces films with high porosity and well-defined grain boundaries, enlarging semiconductor/electrolyte interfacial area. Photocurrent responses are activated by 20% atomic substitution of Bi by La in Ba₂Bi₂O₆ (i.e. Ba₂Bi_1.6La_0.4O₆), achieving a photocurrent density of −0.85 mA cm⁻² at +0.68 V_RHE under simulated sunlight conditions. However, this high photocurrent is not accompanied by hydrogen evolution and is attributed instead to the reduction of bismuth through its oxidation states. Tauc plot analyses of incident photon-to-current efficiencies reveal a bandgap reduction from 2.70 to 2.53 eV with an optimal amount of La substitution, supporting the observed improvement in light absorption and photocurrent. Mott–Schottky plots show a clear slope for Ba₂Bi_1.6La_0.4O₆, while Ba₂Bi₂O₆ exhibited a flat response, indicating poor conductivity in Ba₂Bi₂O₆ that was only activated by La substitution. Williamson–Hall analysis of X-ray diffraction reveals that La substitution doubles the microstrain, while scanning transmission electron microscopy confirms the uniform elemental composition and excellent crystallinity of the La-substituted films. Computational analysis supports the experimental results, showing that La substitution increases the lattice parameters, induces microstrain, and reduces the bandgap. These findings demonstrate a solution-based approach for preparing Ba₂Bi₂O₆ double perovskite photocathodes and show how La substitution enhances photocurrent performance. They also highlight the remaining challenge of directing these photocurrents toward improved photoelectrochemical hydrogen evolution.