Insights into the Photocatalytic Memory Effect of Magneto-Plasmonic Ag–Fe3O4@TiO2 Ternary Nanocomposites for Dye Degradation and H2 Production under Light and Dark Conditions

This study explores the preparation of Ag plasmons-sensitized magnetic-Fe3O4 integrated TiO2 (Ag–Fe3O4@TiO2) ternary nanocomposites and their defect-induced electron storage properties to exploit their photocatalytic memory effect toward dye degradation and H2 generation under light and dark conditions. The crystalline phase formation and elemental states of the individual materials and elements in the nanocomposite are analyzed using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The existence of Ti and Fe ions with dual oxidation states of Ti4+/3+ and Fe3+/2+, respectively, is observed by XPS, which revealed the presence of defects in the system. The observed red shift along with a distinct plasmonic band (corresponding to metallic Ag nanoparticles) in the UV–visible absorption spectrum and the observed modified radiative recombination emission in the PL spectrum confirmed the plasmon-driven visible light activity along with the improved carrier separation and transfer characteristics in the synthesized ternary composite. Accordingly, the Ag–Fe3O4@TiO2 photocatalyst degraded almost 100% of MB (methylene blue) and RhB (rhodamine B) dyes under simulated solar light in 90 min, while it is found to be around 42% and 36% in 60 min under dark conditions, respectively (which is preirradiated for 60 min). Further, it produced H2 at the rate of 911 μmol g–1 h–1 under light conditions and is decreased to ∼96 μmol g–1 h–1 under dark conditions which is preirradiated for 1 h. However, when the composite is preirradiated for 3 h, it showed a maximum H2 evolution of 144 μmol g–1 h–1 under dark. Further, the photocurrent and electrochemical impedance under light and dark conditions suggested the mechanism of photocatalytic charge storage and transfer process in the composite. Although the photocatalytic memory effect of the composite is meager toward H2 production due to the insufficient potential of the stored–released electrons to reduce the protons (2H+) to H2 under dark conditions, their degradation efficiency is considerably good.