Electron Paramagnetic Resonance Quantifies Hot-Electron Transfer from Plasmonic Ag@SiO2 to Cr6+/Cr5+/Cr3+

Abstract

Understanding the plasmon-mediated electron-transfer mechanisms from plasmonic nanostructures to redox-active metals is a technically challenging and still developing procedure. Electron paramagnetic resonance (EPR) spectroscopy is well established as a state-of-the-art tool to selectively detect the redox evolution of paramagnetic metals; however, its use in plasmon-driven charge-transfer processes has not been explored so far. Herein, we present a quantitative study on the mechanism of hot-electron transfer, from plasmonic Ag@SiO2 nanoaggregates, to drive sequential Cr6+ reduction toward Cr5+/Cr3+. Employing flame spray pyrolysis (FSP), core–shell Ag@SiO2 nanoaggregates were engineered with varying SiO2-shell thickness, in the range of 1–5 nm. Using EPR spectroscopy, the spin Hamiltonian parameters for the S = 1/2 {oxalate-Cr5+} and S = 3/2 {oxalate-Cr3+} systems at the Ag@SiO2/Cr interface are analyzed and used to quantitatively monitor the sequential electron-transfer steps during Cr6+ reduction. In the absence of the SiO2 shell, the oxidative path via the dark reduction of Cr6+ due to the oxidation of bare Ag was deducted accordingly. Importantly, we show that the SiO2 shell plays a key role in hot-electron transfer, as the 1 nm shell allows a predominant hot-electron transfer via a light-induced decrease of the activation barrier, suppressing the oxidative path and excluding photothermal effects.