Modulated phase and morphology of bismuth oxyiodide supported on natural halloysite by alcoholic solvents

The study investigated the impact of three alcoholic solvents—ethanol (Et), ethylene glycol (EG), and glycerol (GLY)—on the solvothermal synthesis of the supported photocatalyst BiOI/natural halloysite. Characterization using FTIR, X-ray diffraction, SEM, TEM, DR-UV-Vis, and fluorescence spectroscopy provided insights into the structure, phase, morphology, and optical properties. Natural halloysite (HAL) was sourced from Gamalama volcanic soil. Consequently, varied BiOI and Bi₅O₇I phases, sizes, and morphologies were observed with different solvents. Ethylene glycol and ethanol produced spherical BiOI particles (1–5 μm in diameter), while glycerol yielded tube-shaped Bi₅O₇I particles (1 μm). The incorporation of halloysite hindered BiOI agglomeration, leading to an increase in the bandgap energy. The bandgap energy for BiOI (Et) was 1.98 eV, whereas for BiOI/HAL (Gly), it was 2.79 eV. Natural halloysite effectively reduced electron-hole recombination, as confirmed by fluorescence spectroscopy. This study elucidates how the selection of solvent and the addition of halloysite modulate the properties of the resulting photocatalyst. This study is the first to report the use of natural halloysite from Gamalama as a supporting material for Bismuth Oxyiodide (BiOI). Our findings reveal that natural halloysite can prevent BiOI agglomeration, increase bandgap energy, and reduce electron-hole recombination.