Investigating the vibrational structures of lead bismuth oxyhalides PbBiO2X (X = Cl, Br, I) using temperature-dependent low-wavenumber Raman spectroscopy and the emission properties of the trapped states

Photocatalysts PbBiO₂X exhibit promising catalytic behavior in the visible range. Here we study the vibrational structures and emission properties of the trapped states in these catalysts using temperature-dependent Raman spectroscopy and emission spectroscopy to understand the catalytic activity. The Raman spectra of PbBiO₂X in the range of 77–623 K were obtained under both vacuum and atmospheric pressure conditions. Five bands in the range of 50–500 cm⁻¹ for all three materials were observed and assigned. From density functional theory calculations, the structural configurations of PbBiO₂X are assigned to the I4/mmm structure. Phonon redshifts with temperature are mainly attributed to thermal expansion of the crystal volume. PbBiO₂X exhibited more photoluminescence (PL) in near infrared after heated in a vacuum environment. The emission intensity decreased first with temperature, then increased to reach a maximum, and decreased at high temperatures. The decrease at low temperatures is for thermal quenching with activation energies of 41.4/45.5/32.6 meV for X = Cl/Br/I. At medium temperatures, the increased PL with activation energies of 321/530/590 meV, is explained to thermal activation to high-energy radiative trapped states. At > 400 K, the decreased PL is attributed to electron-hole dissociation, with activation energies of 570/644 meV for Cl/Br.