Nano-Impact Single-Entity Electrochemistry Enables Plasmon-Enhanced Electrocatalysis
Authors: Ganguli Sagar , Zhao Ziwen , Parlak Onur , Hattori Yocefu , Sa Jacinto , Sekretareva Alina 
Published: February 2024

Plasmon-enhanced electrocatalysis (PEEC), based on a combination of localised surface plasmon resonance excitation and electrochemical bias applied to plasmonic material, can result in improved electrical-to-chemical energy conversion compared to conventional electrocatalysis.1 Here, we demonstrate the advantages of nano-impact single-entity electrochemistry (SEE) over conventional ensemble electrochemical measurements for investigating the intrinsic activity of plasmonic catalysts towards PEEC at the single-particle level using glucose electrooxidation on gold nanoparticles (AuNPs) as a model reaction.

First, we performed conventional ensemble measurements using AuNPs coated highly oriented pyrolytic graphite (HOPG) electrodes and AuNPs coated ITO electrodes. Identical photocurrents (Figures A-B,F) were observed at wavelengths where the AuNPs plasmons are active (532 nm) and inactive (650 nm) in an electrolyte consisting of 100 mM NaOH and 30 mM glucose. Controlled experiments demonstrated that in ensemble measurements with working electrodes, where the plasmonic AuNPs are in direct contact with the conductive support, the underlying support material, such as HOPG, is the primary photo-absorber and responsible for the photocurrent. Near-identical photo-absorption of HOPG across the visible region causes consistent temperature increase at different wavelengths. This increased temperature at the electrode-electrolyte interface enhances the charge transfer kinetics and this gets reflected in the identical photocurrent at different wavelengths.2 In ensemble measurements, plasmonic effects have minimal impact on photocurrents. We suggest that this is due to continuous equilibration of the Fermi level (EF) of the AuNPs with the EF
of the working electrode, leading to fast neutralisation of hot carriers by the measuring circuit (Figure G). Therefore, the hot carriers generated in plasmonic materials can not participate in catalysing chemical reactions.

In SEE, a catalytically inactive carbon ultramicroelectrode (C-UME) is used as the working electrode and AuNPs diffused (~5 pM) in the electrolyte act as the catalyst. When the AuNPs collide with biased C-UME or are at close distances that enable charge tunnelling, electrocatalysis occurs on the AuNPs. This leads to formation of current spikes in the chronoamperograms. Contrary to ensemble measurements, different rate enhancements were detected at different wavelengths, with the charge/spike being higher under 532 nm illumination (1.230±0.058 fC) compared to dark (0.870±0.034 fC) and 650 nm illumination (0.920±0.036 fC) in an electrolyte consisting 10 mM NaOH and 30 mM glucose (Figures C-E,F). This wavelength dependence of photocurrent is in accordance with the absorption properties of AuNPs and therefore confirms that plasmonic effects are contributing to the photocurrents. We suggest that this is due to decoupling of the EF of the dispersed AuNPs and the C-UME. Within the charge transfer distance from the electrode or even at collision, EF level equilibration does not necessarily occur in SEE when the colliding particle does not stick to the electrode.3 The first possibility for current enhancement arises from the injection of hot carriers. When the AuNPs are not in direct contact with the electrode to enable rapid EF equilibration but at a distance sufficient for charge transfer, the hot electrons will be transferred to the C-UME due to the large difference in the energies of the electrode and hot electrons (Figure G). At the same time, the hot holes inside the AuNPs can not be neutralised by electron transfer from the electrode at the same rate as hot electrons are removed to the external circuit as the energies of the holes are either higher than that of the electrons at the electrode or only slightly lower depending on the exact value of EF of the AuNPs and the energy distribution of the hot holes.4 These remaining hot holes participate in charge transfer with glucose molecules. As the hot carrier generation in AuNPs is a wavelength-dependent phenomenon, the participation of hot charge carriers in the catalytic reaction is possible only under 532 nm but not under 650 nm irradiation.5 The second possibility is the facilitation of the catalytic reaction by thermal contribution due to the recombination of charge carriers. As the diffused AuNPs are exposed to laser irradiation, the hot carriers will recombine leading to lattice heating. When a 532 nm laser-irradiated AuNP comes into contact with the working electrode, thereby becoming active for the catalytic reaction, its temperature is already higher compared to the AuNPs in the dark or 650 nm illumination. The interfacial charge transfer will become faster on the heated AuNP leading to enhanced charge/nano-impact.

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Figure 1