Kinetic Control of Crystal Polymorphism via Flash Photonic Heating: Reversible α ↔ β-Bi₂O₃ Phase Cycling in Thin-Film Photoelectrodes

Metastable semiconductor polymorphs often demonstrate superior optoelectronic properties compared to their thermodynamically stable counterparts, yet remain underexploited due to the inability of conventional thermal processing to access and stabilize these kinetically trapped phases. We introduce flash photonic heating (FPH, heating rates of 10⁶–10⁷ °C/s) for the kinetic control of crystal polymorphism in photocatalytic metal oxides, enabling access to metastable phases with distinct functional properties. Using Bi₂O₃ thin-film photoelectrodes on FTO-coated glass as a model system in both doctor-blade and pulsed-laser-deposited thin films, we demonstrate that pulse duration and peak power are critical FPH parameters, with optimal processing windows that depend on microstructure-determined thermal properties. Ultrashort pulses (∼0.1 ms) create far-from-equilibrium conditions that kinetically trap metastable β-Bi₂O₃, while longer pulses (>1 ms) follow near-equilibrium pathways yielding stable α-Bi₂O₃. The β-phase exhibits intrinsic optoelectronic properties, including a narrower bandgap (2.35 ± 0.05 eV vs. 3.15 ± 0.05 eV for the α-phase) and a 10- to 50-fold higher photocurrent density (at 1.23 V vs. RHE, depending on film microstructure) attributed to reduced carrier recombination. This kinetic control strategy has the potential for broad applicability across polymorphic metal–oxide systems, providing a transformative platform for accessing kinetically trapped metastable phases on practical device substrates.