Green synthesis of Ca/Sr co-doped TiO2 nanoparticles: impact on morphology, band gap reduction, supercapacitor, and visible-light photocatalytic efficiency

Doping with different metals or non-metals significantly influences the morphology, structure, band gap, electrochemical properties, and photocatalytic activity of metal oxide nanoparticles. In this study, the effect of Ca and Sr co-doping on TiO2 nanoparticles (NPs) was investigated to enhance their electrochemical and photocatalytic properties. Ca and Sr co-doped TiO2 NPs were synthesized via a green synthesis method and applied for supercapacitor studies and pollutant dye degradation. Structural and morphological characterizations were performed using XRD, FTIR, SEM, EDX, UV–Vis, DLS, and BET analysis. XRD confirmed a predominant anatase phase in all synthesized materials. Optical analysis revealed a band gap reduction from 3.21 eV (undoped TiO2) to 2.31 eV for 9 wt% Ca and Sr co-doped TiO2, enabling enhanced visible light absorption. SEM and DLS analyses confirmed nanoscale particle sizes ranging from 27.74 to 9.02 nm. BET analysis showed an increased specific surface area of 299.9 m2/g for the co-doped sample, compared to 137.3 m2/g for undoped TiO2. Electrochemical studies demonstrated a higher specific capacitance of 256.25 F/g for Ca and Sr co-doped TiO2 at scan rates of 100 mVs−1, surpassing singly doped and undoped samples. Photocatalytic studies revealed that 9 wt% Ca and Sr co-doped TiO2 achieved 96.62% degradation of methylene blue (MB) (100 mg/L) in 50 min under visible light, attributed to effective charge separation of electron–hole pairs. These findings confirm that Ca and Sr co-doping enhances the electrochemical and photocatalytic performance of TiO2 NPs, making them promising materials for energy storage and environmental remediation applications.