Vacuum-level-referenced band tuning induced by ion exchange and exfoliation in layered perovskite KCa₂Nb₃O₁₀

Ion exchange and exfoliation are widely used chemical processes for layered perovskites; however, their impact on vacuum-level-referenced electronic band alignment remains unclear. In this study, we investigate the crystal and electronic band structures of the Dion–Jacobson-type layered perovskite KCa₂Nb₃O₁₀, its protonated form HCa₂Nb₃O₁₀, and exfoliated Ca₂Nb₃O₁₀ nanosheets. Synchrotron x-ray powder diffraction reveals that KCa₂Nb₃O₁₀ crystallizes in the monoclinic space group P2₁/m, whereas HCa₂Nb₃O₁₀ adopts a tetragonal structure with space group P4₂2₁2, indicating enhanced structural symmetry upon protonation. Energy diagrams referenced to the vacuum level are experimentally determined for all three compounds, enabling direct comparison of valence-band positions and in-gap states induced by ion exchange and exfoliation. In HCa₂Nb₃O₁₀, a hydrogen-bonded network is found to play a key role in controlling the formation and energy level of in-gap states. These results demonstrate that chemical processing provides an effective route for vacuum-level-referenced band tuning in layered perovskites, offering a practical guideline for band engineering in photocatalytic and related functional oxide materials.