Bifunctional Zr-Doped and Zr/Sr Dual-Doped CeO2 Nanomaterials for Photocatalytic and Hydrogen Evolution Reactions with Cytotoxic Evaluation

Zr-doped and Zr/Sr dual-doped cerium oxide nanomaterials were synthesized via a green solution combustion method using an aqueous seed extract of Lobularia maritima . This plant-mediated approach offers a sustainable and cost-effective route for the controlled synthesis of doped CeO 2 . Comprehensive characterization using p-XRD, FT-IR, FE-SEM, EDX, Raman, and UV–visible diffuse reflectance spectroscopy confirmed the successful incorporation of dopants and the structural and optical properties of the nanomaterials . Notably, UV–visible analysis revealed a substantial reduction in band gap energy from 2.95 eV for Zr-doped CeO 2 to 2.02 eV for Zr/Sr co-doped CeO 2 , indicating enhanced visible-light harvesting capability. Both doped and co-doped samples exhibited an average crystallite size below 10 nm, which is favorable for surface-driven catalytic reactions. The photocatalytic activity of the synthesized nanomaterials was evaluated using methylene blue as a model organic pollutant under visible light irradiation. Among the investigated samples, the Zr/Sr dual-doped CeO 2 nanoparticles demonstrated outstanding photocatalytic performance, achieving complete (100%) degradation of MB within 120 min. This superior activity is attributed to the Sr and Zr co-doping, which induces a high density of oxygen vacancies, suppresses electron-hole recombination, and promotes efficient charge carrier separation, thereby enhancing reactive oxygen species generation. Beyond photocatalytic activity, the multifunctional potential of the synthesized nanomaterials was further evaluated through cytotoxicity studies using the MTT cell viability assay, offering insight into their prospective biomedical applications. The IC 50 values of Zr 0.075 -Ce 0.925 O 2 and Sr 0.05 -Zr 0.05 -Ce 0.90 O 2 nanoparticles against A549 cells were 266.78 and 338.80 µg/mL, respectively, with cell viabilities of 28.39% and 39.18% observed at a concentration of 500 µg/mL. In contrast, for MCF-7 cells, the IC₅₀ values were 350 and 198.52 µg/mL, with corresponding viabilities of 42.52% and 24.22% at 500 µg/mL. Moreover, electrochemical studies revealed that the Sr/Zr dual-doped CeO 2 nanocomposite exhibits significantly enhanced electrocatalytic activity toward the hydrogen evolution reaction, as evidenced by a markedly lower Tafel slope (129 mV dec -1 ) compared to Zr-CeO 2 (673 mV dec -1 ) and pure CeO 2 (840 mV dec -1 ).