Synergy of Adsorption and Plasmonic Photocatalysis in the Au–CeO<sub>2</sub> Nanosystem: Experimental Validation and Plasmonic Modeling
Manash P. Nath, Sritam Biswas, Pabitra Nath, Biswajit Choudhury
Abstract
Adsorption-mediated water treatment leaves adsorbents as secondary pollutants in the environment. However, photocatalysis aids in decomposing the contaminant into its nontoxic forms. In this context, we demonstrate an adsorption–photocatalysis pairing in Au–CeO2 nanocomposites for a total methylene blue (MB) removal from water. We synthesized Au–CeO2 through the citrate (cit) reduction method at different Au loading and studied its adsorption capacity with kinetics and thermodynamic models. We observe that the high adsorption capacity of Au–CeO2 is primarily because of the presence of Ce3+ states in CeO2 and citrate ligands on Au NPs. The Ce3+ states interact and transfer their electrons to supported Au NPs, rendering a negative charge over Au. The negatively charged Au surface and the carboxyl (−COO–) group of citrate ligands mediate an electrostatic interaction/adsorption of cationic MB. The total removal of MB is expedited under white light and lasers. A control experiment with Au NPs shows less adsorption–photocatalysis. The size of Au NPs and Au–CeO2 interfacial interaction is responsible for the surface plasmon resonance spectral position at 550–600 nm. Linear sweep voltammetry (LSV) and plasmonic field simulation show surface plasmon-driven photocatalysis in Au–CeO2. LSV shows a 3-fold higher photocurrent density in Au–CeO2 than colloidal Au NPs under white light. The simulated electric field intensity in Au–CeO2 is maximum at SPR excitation and the closest interfacial separation (d = 0 nm). The plasmon-driven photocatalysis in colloidal Au NPs is mainly due to the interaction of hot electrons with the adsorbed MB molecule. Notably, near-field light concentration, hot electrons, and interfacial charge separation are responsible for excellent MB removal in the Au–CeO2 nanosystem. The total MB removal through adsorption–photocatalysis pairing is 99.3% (Au–CeO2), 30.7% (Au NPs), and 13% (CeO2).