Vibrational Structures of Iodine-Vacancy Bismuth Oxyiodides Using Temperature-Dependent Low-Wavenumber Raman Spectroscopy
Hsueh Han Yang, Kang-Yu Hsiao, Fuyu Liu, Chiing‐Chang Chen, I‐Chia Chen
Abstract
The reaction of BiOI to form iodine-poor derivatives under high temperatures was investigated by using temperature-dependent low-wavenumber Raman spectroscopy. The Raman spectra of BiOI and of the iodine-poor derivatives Bi 4 O 5 I 2, Bi 7 O 9 I 3, and Bi 5 O 7 I were recorded. The phonon mode displacement vectors were calculated in the software CRYSTAL, and the PBE0/pob-TZVP method was used to analyze these Raman bands. The appearance of a Raman band at 95 cm –1 of a heated sample of BiOI in the atmosphere was assigned to the production of Bi 4 O 5 I 2, and up to 623 K, no other iodine-poor derivatives were generated. After 632.8 nm laser irradiation under vacuum for a few minutes before exposure to the atmosphere, Bi 7 O 9 I 3 and β-Bi 2 O 3 were produced at various laser powers. Red shifts of low-wavenumber bands with temperature were obtained, and the slopes of the linear dependence χ for three phonon modes of BiOI were obtained. The obtained χ values were more negative in the atmosphere than in a vacuum. This can be explained by the reaction of BiOI to form Bi 4 O 5 I 2, during which the lattice volume may have been further expanded. The near-IR emission intensity of photoexcited BiOI was found to decrease with temperature. This turnoff process with a low activation energy is explained by thermal quenching of charge carriers.