Operando Study of Photocatalytically Induced Semiconductor to Metallic Phase Transition in BWO using Bragg Coherent Diffraction Imaging

Photocatalytic materials often face challenges such as charge recombination and long-term stability, intricately linked to their local structure, including strain and defects. Imaging strain and defects in such nanomaterials with nanoscale resolution in working environments remains elusive. We report three-dimensional imaging of defects, crystal morphology, and strain dynamics in individual photocatalytic Bi₂WO₆ (BWO) nanoflakes under operando temperature, gas, and photo-driven conditions using Bragg Coherent Diffractive Imaging (BCDI). Due to the layered nature of BWO, we observe that defects are static at room temperature and act as active sites or traps where electrons and holes recombine, causing the strain energy that could drive photocatalytic reactions to be lost as heat. Maintaining a constant above room temperature (40°C) enables the mobility of these defects, mediated by the thermal activation of charge carriers, potentially enhancing their mobility and reducing recombination rates. Further flow of Argon (Ar) gas stabilizes the reaction environment, while a mixed Hydrogen-Nitrogen (H₂ + N₂) flow promotes a hydrogen-induced Semiconducting to Metallic (SM) electronic phase transition and a structural phase transformation, supported by Density Functional Theory (DFT) calculations. DFT and BCDI confirm that during the SM phase transition, a new structural phase nucleates near the defect and spreads inhomogeneously. These experimental conditions and operando imaging open a powerful avenue for facilitating improvement and rational design of nanostructured photocatalytic materials.