Role of thermal decomposition process in the photocatalytic or photoluminescence properties of BiPO₄ polymorphs

Thermal decomposition process was used to obtain modified photocatalytic and/or photoluminescence properties of bismuth phosphate polymorphs. The precursor BiPO₄ , 0.7H₂ O was synthesized by a coprecipitation route. The observed polymorphs were the hexagonal P3₁ 21 hydrate phase BiPO₄ , 0.7H₂ O at 25°C, a mix system of hexagonal and monoclinic P2₁ /n phases at 200°C and 400°C, a mix system of monoclinic phases P2₁ /n and P2₁ /m at 600°C, and a unique monoclinic phase P2₁ /m at 900°C. The X-ray diffraction analyses allowed evidencing lattice deformations due to structural defects. The photocatalytic activities in the presence of rhodamine B in aqueous solution were determined using UV light irradiation. The best photocatalytic efficiencies were observed with the mix systems resulting from thermal decomposition at 400 and 600°C. Photoluminescence experiments performed under UV-laser light irradiation revealed unexpected emissions in the green-orange range, with optimal intensities for the mix systems observed at 400°C. The role of structural defects resulting from decomposition process is discussed. PRACTITIONER POINTS: Thermal decomposition is used to introduce structural defects in BiPO₄ polymorphs The BiPO₄ intermediate systems are used to photodegrade rhodamine B Active species trapping experiments are performed Photoluminescence experiments highlight green-orange emissions Structural defects are at the origin of this photoluminescence.