Structural, optical and electrochemical insights of Ca²⁺ and Al³⁺-doped ZnS nanostructures for pseudocapacitive supercapacitor electrodes

Rising global energy demand and the drive toward sustainable electrical grids have intensified the need for electrode materials that combine low cost, environmental safety and high charge-storage capacity; ZnS is an attractive but under-utilised candidate whose poor intrinsic conductivity has restricted its supercapacitor performance. In this work, Ca (2%)-doped and Al co-doped ZnS nanoparticles were prepared using the hydrothermal method with the variation of concentration of Al (0%, 2% and 4%). These results illustrate the structural, optical and defect characterization systematically to study the impact of dual doping on the nanoparticles. The incorporation of Al leads to the formation of cubic zinc blende structure with smaller crystallites as seen by the x-ray powder diffraction analysis. Scanning electron microscopy showed that the nanoparticle microstructure and particle size varied with increasing dopant concentration, with greater agglomeration and smaller particle size observed at higher doping levels. XRD analysis was used to check the phase purity and energy dispersive X-ray spectroscopy (EDAX) was used to confirm that calcium and aluminium were successfully incorporated into the ZnS lattice without any formation of secondary phase. FTIR and RAMAN studies indicate the presence of Zn–S bonding, lattice distortion from the dopants and an increase in phonon interactions. The UV–Vis analysis results showed that the optical band gap increases from ~ 3.73 eV (Ca 2%-doped ZnS) to ~ 3.82 eV (4% Al co-doped ZnS), which is due to the quantum confinement and the Burstein–Moss effects. Photoluminescence studies show that the defect-related blue emission intensity is highest for Ca 2 and decreases progressively with Al doping, suggesting the growth of non-radiative recombination centers. The combined effect of Ca and Al co-doping suggests that the defect density, particle size and electronic structure of the material are important parameters to be explored to further improve the electrochemical performance for supercapacitor applications. It is expected that the higher amount of defects and surface area will help to improve the storage capacity of charges via faradaic and surface-controlled processes. In this regard, promising strategy is suggested in the present study to tailor nanomaterials such as ZnS for next-generation energy storage devices. Electrochemically, Al co-doping progressively lowers the charge-transfer resistance (from ~ 340 Ω for Ca-doped ZnS to ~ 165 Ω for 4% Al co-doped ZnS) and raises the specific capacitance, with the 4% Al co-doped electrode delivering the best performance (~ 412 F g⁻¹ by cyclic voltammetry and ~ 428 F g⁻¹ by galvanostatic charge-discharge), an energy density of ~ 14.8 Wh kg⁻¹ and a power density of ~ 725 W kg⁻¹.