Optical and Structural Property Tuning in Physical Vapor Deposited Bismuth Halides Cs₃Bi₂(I_1-Br)₉ (0 ≤ x ≤ 1)

Crystalline films of lead-free all-inorganic Cs₃Bi₂X₉ (X = Br, I) perovskites have been deposited by radio frequency (RF)-magnetron sputtering providing high-quality, single-phase films as confirmed by structural, morphological, and optical property characterization. Progressive tuning of crystal structure characteristics and optical absorbance has been achieved in mixed Br/I phases Cs₃Bi₂(I_1-Br)₉ (0 ≤ x ≤ 1), highlighting a shift of the band gap from about 2.0 eV for Cs₃Bi₂I₉ to 2.64 eV for Cs₃Bi₂Br₉. X-ray diffraction and Raman scattering allowed defining the range of alloyed compositions where single-phase compositions are found. Finally, preliminary photocatalytic activity tests on the degradation of methylene blue provided solid data indicating the future possible exploitation of Bi-based perovskite derivative materials as active photocatalysts.