Defect engineering versus amorphization: divergent photocatalytic pathways in laser-synthesized niobium-based oxides

Abstract

The non-equilibrium conditions inherent in femtosecond laser ablation in liquids (LAL) offer a versatile platform for synthesizing metastable nanomaterials, yet predicting the structural evolution of complex oxides under rapid quenching remains a challenge. Here, we elucidate the divergent structural and functional outcomes of LAL applied to two related wide-bandgap niobium-based oxides: LiNbO₃ and Nb₂O₅. We find that the intrinsic crystallization kinetics of the materials dictate their response to laser-induced fragmentation and condensation. Nb₂O₅, a strong glass-former with complex polymorphism, is trapped in an amorphous state. In contrast, LiNbO₃ exhibits robust thermodynamic stability, favoring rapid nucleation and growth to form polycrystalline, albeit defect-rich, nanoparticles. These structural differences profoundly impact their electronic landscapes. Amorphization in Nb₂O₅ introduces a broad continuum of localized states that facilitate rapid charge recombination. Conversely, defect engineering in crystalline LiNbO₃ yields discrete mid-gap states that enhance visible-light absorption and prolong carrier lifetimes. Consequently, LiNbO₃ nanoparticles demonstrate sustained hydroxyl radical generation under visible irradiation, achieving a photocatalytic dye degradation rate threefold higher than their amorphous Nb₂O₅ counterparts and enabling 90% dye removal after 150 minutes at low catalyst loading. This investigation underscores the critical role of intrinsic crystallization kinetics in LAL synthesis and establishes defect-mediated crystallinity as a superior strategy over amorphization for activating wide-bandgap materials for solar-driven photocatalysis.