Solvent-free mechanochemical access to phase-pure Cs–Co–Cl halometalates with tuneable electronic properties for energy applications

Abstract

We report a solvent-free mechanochemical route for the selectively synthesis of three different caesium cobalt chlorides: CsCoCl₃, Cs₂CoCl₄, and Cs₃CoCl₅, by simply tuning the CsCl : CoCl₂ precursor ratio. This is the first comprehensive comparative study of these phases synthesized in pure form, enabling a clear correlation between composition, crystal structure, and optoelectronic properties. Each phase exhibits a unique Co²⁺ coordination geometry: octahedral in CsCoCl₃ and tetrahedral in Cs₂CoCl₄ and Cs₃CoCl₅, as revealed by XRD, SEM-EDS, Raman, and XPS, with several features reported here for the first time. All phases display high thermal stability and narrow optical bandgaps (1.65–1.70 eV), supported by ligand field analysis and CIE colorimetry. Valence and conduction band energies determined by VB-XPS and cyclic voltammetry reveal a systematic, composition-driven tuning of energy levels across the series. Importantly, the band edge alignment are suitable for visible-light-driven hydrogen evolution and photovoltaic applications. SCAPS-1D simulations predict power conversion efficiencies up to 17.1%, positioning these halocobaltates as promising absorbers. Altogether, this work introduces a scalable synthesis route and demonstrates the potential of cobalt-based halide frameworks as modular systems for solar energy conversion and photocatalysis.