Morphology Control of a Layered Oxyhalide Bi ₄ NbO ₈ Cl Photocatalyst via Flux Synthesis

Particle morphology often governs the performance of photocatalysts. Despite the growing interest in mixed‐anion compounds, robust control of particle morphology and a clear morphology–activity relationship remain to be fully established. Here, we use a flux method to tune the morphology of the layered oxyhalide Bi 4 NbO 8 Cl and define how the morphology governs photocatalysis. In situ synchrotron X‐ray diffraction analysis during the heating of the precursor/flux mixtures revealed that Bi 4 NbO 8 Cl nucleated and grew within the molten flux. The crystallinity, particle size, and surface area were systematically varied by adjusting the calcination temperature and holding time. Under visible light, Bi 4 NbO 8 Cl prepared at relatively low temperature (650°C) with a short holding time (5–10 hr) exhibits the highest O 2 evolution rates, correlating with smaller particle size and larger surface area. Time‐resolved measurements indicate that once the bulk carrier properties are improved by flux synthesis, the morphology, rather than residual differences in bulk kinetics, sets the activity trend within the flux samples. Loading IrO x further enhanced the O 2 evolution, and mediator‐free interparticle Z‐scheme water splitting was demonstrated using the obtained flux‐derived Bi 4 NbO 8 Cl as an O 2 ‐evolving photocatalyst. These results provide practical guidance for controlling the morphology of oxyhalide photocatalysts for efficient solar water splitting.