Bismuth oxide (Bi₂O₃) represents a promising photocatalytic material owing to its favorable optical properties and exceptional chemical stability under operational conditions. Notably, Bi₂O₃ exhibits rich polymorphism with four distinct crystal phases, making it uniquely suited for materials design strategies. Selective control over Bi₂O₃ polymorphism provides a powerful materials design approach for tuning optical and electronic properties in photocatalytic applications. While the thermodynamically stable monoclinic α-phase (Eg = 2.8 eV) dominates under conventional conductive/convective heating (furnace annealing), the metastable tetragonal β-phase exhibits superior photocatalytic properties with its reduced bandgap of 2.4 eV, enabling enhanced visible light harvesting. However, accessing β-Bi₂O₃ through conventional thermal pathways (α→δ at ~730°C, δ→β upon cooling at 645-650°C) faces significant challenges for practical applications. These high-temperature requirements are incompatible with temperature-sensitive substrates, such as fluorine-doped tin oxide (FTO), which degrades above ~550°C, and may also be problematic for nanostructured materials and thin films, where the thermal behavior can differ significantly from that of bulk systems.

We demonstrate Flash Photonic Heating (FPH) as a transformative radiative heating technique that overcomes substrate thermal limitations while enabling the selective control of Bi₂O₃ polymorphs. FPH employs millisecond white light radiative pulses with ultra-rapid heating rates (10⁶-10⁷ °C/s) to induce the α-to-β phase transformation in Bi₂O₃ films deposited on FTO substrates, circumventing conventional thermal constraints. X-ray diffraction analysis confirmed successful β-phase formation, while optical absorption and surface photovoltage (SPV) spectroscopy revealed distinct characteristics consistent with the α-to-β transformation and the narrower bandgap of β-Bi₂O₃. Notably, the substrate integrity was maintained throughout the process, demonstrating the compatibility of this approach with device-relevant architectures.

This work establishes FPH as a rapid, substrate-compatible, and scalable method for accessing metastable semiconductor phases through radiative heating. The ability to selectively stabilize β-Bi₂O₃ demonstrates a broader paradigm for structural fine-tuning at the atomic level, where precise control over crystal phases can dramatically alter optoelectronic properties. This approach opens new avenues for developing high-performance photocatalytic materials with designer properties tailored for advanced catalytic materials design in light-driven chemical transformations.