Pdx–MgO1−x/rGO enzyme-mimetic nanozymes for photocatalytic hydrogen evolution and dye degradation

Abstract

The development of efficient, sustainable nanozymes that act as photocatalysts for solar hydrogen (H2) evolution remains a critical research priority in clean energy technologies. In this work, multifunctional Pdx–MgO1−x/rGO (x = 0, 0.3, 0.5, 0.7; where x represents the weight fraction of Pd within a total metal loading of 1 wt%) materials acting as nanozymes were phyto-mechanochemically synthesized, demonstrating high activity for photocatalytic H2 evolution, dye degradation, and enzyme-mimetic activities. The structural and morphological parameters of Pdx–MgO1−x/rGO nanozymes were studied by PXRD, UV-visible DRS, SEM, EDS, XPS, N2 adsorption–desorption isotherm, BJH pore size analysis, Zeta potential, photoluminescence (PL), electron impedance study (EIS), electron spin resonance (ESR), ICP-OES, and photothermal conversion ability. The Pd0.7–MgO0.3/rGO nanozyme with 0.7 wt% loading of Pd in an overall metal–metal oxide loading of 1 wt% showed a very high H2 evolution rate of 2.82 mmol g−1 h−1 which is 1.76 times greater than pristine MgO. The high activity of the Pd0.7–MgO0.3/rGO nanozyme was attributed to the inherent property of Pd metal to quench photogenerated electrons by the Schottky barrier formation mechanism. It is also shown that the optimized dose of Pd0.7–MgO0.3/rGO exhibits 80% degradation efficiency of methylene blue (MB) dye in visible light irradiation, which is much higher than pristine MgO, rGO, MgO/rGO, Pd0.5–MgO0.5/rGO, and Pd0.3–MgO0.7/rGO. Besides this, Pd0.7–MgO0.3/rGO exhibits high reactive oxygen species (ROS), superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx) activity as compared to MgO, rGO, MgO/rGO, Pd0.3–MgO0.7/rGO, and the Pd0.5–MgO0.5/rGO nanozyme. This enhanced performance is mechanistically linked to its synergistic enzyme-mimetic activities, mimicking SOD, catalase, and GPx, which collectively orchestrate a ROS-scavenging cascade to suppress charge recombination and promote redox reactions.