Synergy of Adsorption and Plasmonic Photocatalysis in the Au–CeO2 Nanosystem: Experimental Validation and Plasmonic Modeling

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).