Toward Next-Generation Optoelectronic Devices: A Study of the Lead-Free Double Perovskite Cs₂CuSbCl₆

<title>Abstract</title> <p> Halide double perovskite (HDPV) Cs₂CuSbCl₆ nanoparticles were successfully synthesized via a facile precipitation route under acidic conditions. Comprehensive structural characterization of the as-prepared sample by X-ray diffraction (XRD) and infrared (IR) spectroscopy confirmed the formation of a cubic Cs₂CuSbCl₆ phase with Fm\(\:\stackrel{-}{3}\)m symmetry and an average crystallite size of 29.34 nm. High-resolution transmission electron microscopy (HRTEM) further verified the nanoscale morphology of Cs₂CuSbCl₆, revealing an average particle size of 31.34 nm. Thermogravimetric analysis (TGA) was conducted to evaluate the thermal stability of the HDPV and to derive the associated kinetic and thermodynamic parameters. Diffuse reflectance spectroscopy (DRS) revealed a narrow direct band gap of 1.2 eV coupled with a notably low Urbach energy of 0.22 eV. The normal dispersion of the refractive index was well described by the Cauchy model. Furthermore, the Wemple–DiDomenico and Drude models were applied to determine the dispersion and oscillator energies, static dielectric constant, static refractive index, and free-carrier concentration, revealing strong consistency among the derived optical parameters. Based on the Mulliken electronegativity approach, the conduction band (E _CB ) and valence band (E _VB ) potentials of Cs₂CuSbCl₆ were determined to be + 1.25 V and + 2.45 V in relation to the Normal Hydrogen Electrode (NHE), respectively. The E _VB value exceeds the redox potentials of both H₂O/O₂ (+ 1.23 V) and OH⁻/•OH (+ 1.99 V), indicating strong oxidative capability sufficient to drive both direct water oxidation and hydroxyl radical generation. In contrast, the E _CB potential of + 1.25 V is too positive to support proton reduction to H₂, since hydrogen evolution requires an E _CB more negative than 0 V vs. NHE. These band positions suggest that Cs₂CuSbCl₆ is better suited for photocatalytic oxidation processes, such as pollutant degradation, than for photocatalytic hydrogen production. </p>