The 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 are 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 halogen suffer from limitations such as a wide band gap, an 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 a solid-state reaction, and subsequently exfoliated into nanosheets. This rational tuning of the 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 mixed-halide Sillén and weakens the interlayer interactions. This facilitates relatively easy exfoliation of SrBi₃O₄Cl₂Br Sillén nanoplates into well-defined nanosheets with the simultaneous formation of an exposed {101} facet at the edges and {001} facet in the plane of nanosheets. The anisotropic charge migration of photogenerated electrons to {101} and holes to the {001} facet facilitates the special charge carrier separation in SrBi₃O₄Cl₂Br nanosheets. The mixed-halide Sillén nanosheets exhibit remarkable 7.7-fold photocatalytic hydrogen evolution (PHE) compared to that of the respective nanoplates and 3-fold that of SrBi₃O₄Cl₃ nanosheets, which correlates with their elevated reduction potential. The finding underscores the potential of mixed-halide engineering in layered structured photocatalysts and provides a promising strategy to develop next-generation materials for efficient solar-to-hydrogen conversion.