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 Ba₂Bi₂ O₆ 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 were activated by 20% atomic substitution of Bi by La in Ba2Bi2O6 (i.e. Ba2Bi1.6La0.4O6 ), achieving a photocurrent density of -0.85 mA cm-2 at +0.68 VRHE under simulated sunlight conditions. However, this high photocurrent was not accompanied with hydrogen evolution and is attributed instead to the reduction of bismuth through its oxidation states. Tauc plot analyses of incident photon-to-current efficiencies revealed a bandgap reduction from 2.70 to 2.53 eV with optimal La amount substitution, supporting the observed improvement in light absorption and photocurrent. Mott-Schottky plots showed a clear slope for Ba2Bi1.6La0.4O6, 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 revealed that La substitution doubles the microstrain, and computational analysis confirmed this trend, showing that La substitution increase lattice parameters, induces strain, 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 photoelectrochemical hydrogen evolution.