Visible-light-driven photocatalytic water splitting in double perovskite halides Cs ₂ TlInX ₆ (X = Cl, Br, I): a first-principles study

Abstract This study investigates the structural, electronic, optical, mechanical, and photocatalytic properties of double perovskites Cs 2 TlInX 6 (X = Cl, Br, I) using first-principles DFT with GGA and SCAN functionals. All compounds exhibit structural and mechanical stability, supported by negative formation energies, tolerance factors within 0.7 ≤t ≤1, and compliance with Born’s criteria. SCAN-based electronic calculations show that only the Cl- and Br-based compounds possess band gaps within the optimal 1.8–3.0 eV range for visible-light photocatalysis. Optical spectra confirm strong absorption in the Vis-UV region with α ≈10 5 cm −1 , indicating good light-harvesting ability. Band-edge alignment using Mulliken electronegativity reveals that Cs 2 TlInCl 6 and Cs 2 TlInBr 6 have VBM positions above the OER potential and CBM below the HER potential, satisfying the thermodynamic requirements for overall water splitting. These findings highlight Cs 2 TlInCl 6 and Cs 2 TlInBr 6 as promising lead-free photocatalysts for solar-driven water splitting.