Electron Paramagnetic Resonance Quantifies Hot-Electron Transfer from Plasmonic Ag@SiO<sub>2</sub> to Cr<sup>6+</sup>/Cr<sup>5+</sup>/Cr<sup>3+</sup>
Constantinos Moularas, Christos Dimitriou, Yiannis Georgiou, G.A. Evangelakis, Nikos Boukos, Yiannis Deligiannakis
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@SiO 2 nanoaggregates, to drive sequential Cr 6+ reduction toward Cr 5+ /Cr 3+ . Employing flame spray pyrolysis (FSP), core–shell Ag@SiO 2 nanoaggregates were engineered with varying SiO 2 -shell thickness, in the range of 1–5 nm. Using EPR spectroscopy, the spin Hamiltonian parameters for the S = 1/2 {oxalate-Cr 5+ } and S = 3/2 {oxalate-Cr 3+ } systems at the Ag@SiO 2 /Cr interface are analyzed and used to quantitatively monitor the sequential electron-transfer steps during Cr 6+ reduction. In the absence of the SiO 2 shell, the oxidative path via the dark reduction of Cr 6+ due to the oxidation of bare Ag was deducted accordingly. Importantly, we show that the SiO 2 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.