Study of the influence of phase transformations of the biogenic Hydroxyapatite/Nb2O5 heterostructure on the photodegradation of different dye mixtures under sunlight irradiation

Abstract

The calcination temperature is responsible for decomposing hydroxyapatite (HAp) and altering the crystalline structure of Nb₂O₅. In this work, we report the synthesis of a biogenic HAp/Nb₂O₅ heterostructure via a microwave-assisted hydrothermal method. Marine shell residues were used as a calcium precursor for the synthesis of HAp. Samples without calcination and calcined between 600 and 1200 °C were obtained to observe the influence of phase transformations on the photocatalytic performance. XRD and SEM characterization revealed the formation of calcium niobate crystalline phases at 800 °C and tricalcium phosphate phases at 1000 °C. The presence of these phases promoted textural and spectroscopic changes in the heterostructure. Photocatalytic tests under solar irradiation in different mixtures involving cationic and anionic dyes revealed that the best sample was the one without calcination, as it exhibited fewer deep defects and a larger specific surface area. Moreover, the heterostructure of the mixtures maintained photodegradation stability by at least 78 % after five reuse cycles. Inhibitor tests, EPR analyses and UV light testing indicated that superoxide (•O₂⁻), hole (h⁺) and hydroxyl (^•OH) are the active radicals and that there is a minimal sensitization influence. After calculation of the conduction and valence bands, a proposed mechanism for the obtained heterostructure indicated that it is of the direct Z scheme type.