Iodine incorporated direct band gap materials for enhanced electrochemical hydrogen evolution: role of structural...

All inorganic two-dimensional (2D) (trigonal Cs₃Sb₂X₉ phase) halide perovskite like materials generate enormous research interest due to their excellent optoelectronic properties. Understanding the relationships among phase stability, bonding interactions, and electronic properties is crucial for the design and optimization of the optoelectronic characteristics of these materials for advanced technological applications. In this article, we have thoroughly studied the phase stability of different polymorphs of Cs₃Sb₂X₉ (X = Cl, I) and their potential electrocatalytic H₂ evolution reaction. The synthesis of Cs₃Sb₂I₉via the solution method at low temperature resulted in a 0D hexagonal phase. Furthermore, heating the hexagonal phase at 600 °C in a sealed silica tube led to structural phase transformation into the thermodynamically stable 2D (layered) trigonal phase. Mixed halides, Cs₃Sb₂Cl_9-xI_x, were synthesized to systematically modulate the optical and electronic properties. Optical study shows that pristine trigonal Cs₃Sb₂Cl₉ and hexagonal Cs₃Sb₂I₉ exhibit indirect band gaps of 2.87 eV and 1.95 eV, respectively, whereas the trigonal polymorph of Cs₃Sb₂I₉ has a direct band gap of 1.89 eV. Progressive substitution of I^- for Cl^- in the trigonal layered Cs₃Sb₂Cl_9-xI_x phase results in an indirect to direct band gap transition. Theoretical study confirms the band type transition from an indirect to direct band gap in Cs₃Sb₂Cl_9-xI_x upon substitution of one mole of iodine at the chlorine site. It is observed that with I^- substitution, the valence band maximum remains unaltered, whereas the conduction band minimum changes from the A point to the Γ point. From a detailed analysis of the density of states (DOS) and the structural variation, it is evident that electronic structure modification induced by structural distortion plays an important role in driving the band type transition in these compounds. Electrocatalytic studies show that the direct band gap layered Cs₃Sb₂I₉ compound has superior hydrogen evolution reaction (HER) performance (20 460 μmol g^-1 h^-1) compared with the indirect band gap 0D Cs₃Sb₂I₉ phase and other direct band gap mixed compositions. The direct band gap nature and relatively weak Sb-I bond are primarily responsible for the observed superior electrocatalytic hydrogen evolution reaction in trigonal Cs₃Sb₂I₉.