Experimental and theoretical exploration of bismuth oxyhalide (BiOX, X = Cl, Br, I) nanoparticles in thermoelectric, optoelectronic, and photocatalytic applications.

Here, we performed first-principles density functional theory (DFT) modeling coupled with the Boltzmann transport equation to explore hydrothermally synthesized bismuth oxyhalides (BiOX, X = Cl, Br, I) nanoparticles. The correlation between BiOX crystallographic and elastic properties was established from the equation of state and elastic tensor simulations. The phonon calculations inferred the dynamical stability and infrared activity of BiOX. The Raman absorptions were reproduced in Raman tensor simulations. The field emission scanning electron microscopy revealed average particle sizes of 154, 221, and 71 nm for BiOCl, BiOBr, and BiOI, respectively. The elemental identification and chemical state analysis were performed with energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy. The diffuse reflectance-derived indirect electronic band gaps of BiOCl (3.54 eV), BiOBr (2.83 eV), and BiOI (1.85 eV) were modeled from the hybrid Heyd-Scuseria-Ernzerhof screened and tuned approach with long-range van der Waals interaction and relativistic spin-orbit coupling. The effective mass analysis revealed that holes in BiOCl and BiOBr are heavier than electrons, whereas the opposite holds in BiOI. High degree of anisotropy was revealed in lattice thermal transport in BiOX. The thermoelectric figure of merit