Mixed–Halogen Layer Approach of Band Engineering and Anisotropic Charge Migration in X1X2 Sillén Nanosheets Boost Cocatalyst-free Photocatalytic Hydrogen Evolution

Abstract

Efficient photocatalysts responsive to visible-light is essential for sustainable hydrogen (H2) production via solar water splitting. While bismuth-based layered oxyhalides hold great promise, conventional Sillén compounds (X1, X2, and X1X2) with one type of halogens suffer from limitations such as wide band gap, unsuitable band edge position for water reduction, or self-oxidative photodecomposition due to the dominance of halide p-orbitals at the valence band maximum (VBM). In this work, for the first time, a mixed-halide X1X2 Sillén compound, SrBi3O4Cl2Br, is synthesized via solid-state reaction, and subsequently exfoliated into nanosheets. This rational tuning of mixed-halide composition leads to a favorable band alignment with enhanced reduction potential, enabling cocatalyst-free hydrogen evolution under visible-light. The balanced Br content suppresses the halide p-orbital dominance at the VBM and retains excellent photostability, while the larger size of bromine allows the interlayer expansion in the mixed-halide Sillén and weakens the interlayer interactions. This facilitates relatively easy exfoliation of SrBi3O4Cl2Br Sillén nanoplates into well-defined nanosheets with the simultaneous presence of exposed {101} facet at the edges and the prominently formed {001} facet in the plane of nanosheets. The anisotropic charge migration of photogenerated electrons to {101} and holes to {001} facet facilitates the special charge carrier separation in SrBi3O4Cl2Br nanosheets exhibiting remarkable 7.7-fold photocatalytic hydrogen evolution (PHE) than respective SrBi3O4Cl2Br nanoplates and 3-fold than SrBi3O4Cl3 nanosheets, correlating with its elevated reduction potential. These findings underscore the potential of mixed-halide engineering in layered structured photocatalysts, providing a promising strategy for designing next-generation materials for efficient solar-to-hydrogen conversion.