Plasmon-Induced Tuning of Cerium Oxidation States in Au@CeO${}_x$ Core@Shell Nanoparticles

Abstract

CeO${}_x$-based nanoforms are widely used in catalysis, or biomedical applications due to their redox activity and oxygen storage capacity. The key parameters determining their surface chemistry are the Ce${}^{3+}$/Ce${}^{4+}$ ratio and the ability to transition between Ce${}^{4+}$ and Ce${}^{3+}$ states. We synthesized Au@CeO${}_x$ core@shell nanoparticles with different thicknesses of CeO${}_x$ shells and different Ce${}^{3+}$/Ce${}^{4+}$ ratios through a photothermal reaction driven by localized surface plasmon resonances (LSPRs) at the Au nanoparticle surface induced by visible light. We introduce a way to further enhance the Ce${}^{3+}$/Ce${}^{4+}$ ratio in the shell by exposing the Au@CeO${}_x$ nanoparticles to visible light using a green laser (532 nm, 50 mW). Our findings based on photoelectron spectroscopy indicate that the Ce${}^{4+}$-to-Ce${}^{3+}$ transition results from LSPR-induced superheating of the Au@CeO${}_x$ interface, leading to the formation of oxygen vacancies and reduction of Ce${}^{4+}$ ions. This process is reversible upon air exposure suggesting that the ability to transition between the Ce${}^{4+}$ and Ce${}^{3+}$ states is retained in the Au@CeO${}_x$ nanoparticles. Our study presents the CeO${}_x$-based nanoforms with a tunable cerium valence state ratio, highlighting the potential of plasmonic control in optimizing their photocatalytic and enzyme-mimetic properties.

18 pages including the Main text and the Supplementary Material, 8 Figures, accepted in Applied Physics Letters