Improved photocatalytic water-splitting efficiency and superior optoelectronic aspects in a lead free CsRbGe(Cl,Br)6: Strain engineering

The realization of a sustainable fuel economy rests squarely on the development of earth-abundant, robust photocatalysts. Despite the remarkable optoelectronic performance of lead-halide perovskites, their commercial viability remains fundamentally obstructed by two persistent liabilities-structural decomposition under operational conditions and the ecological hazards of lead leaching. Here, we address this challenge through a comprehensive first-principles investigation of the strain-engineered germanium-based vacancy-ordered double perovskite CsRbGe(Cl,Br) 6 . Using density functional theory calculations via hybrid functional (HSE06) coupled with ab-initio molecular dynamics, it is demonstrated that both compounds exhibit exceptional energetic, mechanical, and dynamical stability. Critically, we uncover a direct bandgap that decreases monotonically from 3.22 to 1.93 eV for the chloride system and from 2.09 to 1.24 eV for the bromide system under −5% compressive strain to +5% tensile strain, enabling precise optical tuning across the visible spectrum. Optical absorption coefficients exceeding 10 5 cm −1 and low reflectivity ( < 28%) further establish these materials as superior light harvesters. Out of all the strain conditions, CsRbGeCl 6 under −5% strain is the most desirable hydrogen evolution reaction photocatalyst. Its conduction band edge is the most negative at −0.76 V vs normal hydrogen electrode, thus providing the most powerful thermodynamic drive for proton reduction. For the oxygen evolution reaction, all valence band positions are lower than the water oxidation potential. However, CsRbGeCl 6 at −5% strain has the best valence band edge (2.36 V vs normal hydrogen electrode), making it the most stable for overall water splitting out of all systems. Thus, this work not only identifies CsRbGe(Cl,Br) 6 as a lead-free, strain-tunable platform for photocatalysis but also establishes a generalizable paradigm for designing high-performance renewable energy materials through controlled lattice deformation.