(38, 39) Hill and Pan recently demonstrated the use of single-particle dark-field spectroscopy in tracking the synthesis of single silver nanoparticles via electrodeposition.
A related technique utilizing the surface plasmon resonance of thin films has also been used to study electrocatalytic behavior of adsorbed single nanoparticles and the metal film itself. Single-particle surface plasmon sensing of catalytic and electrochemical charging was first demonstrated by the Mulvaney group (15, 16, 36) and later reported by the Klar group (37) with a focus on the ability to tune the plasmon resonance of single nanoparticles with static potential control. (18, 34, 35) If scientists and engineers could resolve heterogeneous activity within populations of nanoparticles, the study of highly active subpopulations could then be used to inform design principles and further optimize catalytic properties of entire populations, increasing total activity and yield.
(34) Several groups have recently taken advantage of theoretically predicted shape-dependent activity in their quest to make better catalysts.
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(32, 33) The synthesis of nanocatalysts has historically relied heavily on iterative trial and error optimization schemes. (3, 7, 8, 30, 31) Additionally, surface defect sites have been shown to increase nanocatalyst activity. (18-29) Chemically prepared nanoparticles are inherently heterogeneous in size and shape even under identical growth conditions, leading to heterogeneity in catalytic and electrochemical activity within nanoparticle populations. Nanoparticle size and morphology have proven to be critical in the design and engineering of efficient nanocatalysts. (14-17) The use of these spectral characteristics to infer electrochemical processes is the subject of nanoparticle plasmon spectroelectrochemistry. (12) Changes in surface charge density of gold nanoparticles can also be detected through changes in the surface plasmon resonance energy. Electrochemical methods afford unrivaled control of surface chemistry at metal electrodes but are classically built upon bulk electrochemical current, potential, and charge relationships. (9) Because the catalytic properties of nanoparticles are derived from their ability to store and transfer charge, (10-13) nanoelectrodes prepared by attaching metal nanoparticles at submonolayer coverage on conductive substrates can be used to experimentally test the catalytic properties of nanoparticles. With the growing focus on charge transfer and storage applications using nanostructures, (1-8) a fundamental understanding of the physical and chemical processes governing charge transfer at the nanoscale is of paramount importance. The broad range of responses on even a simple sample highlights the rich experimental and theoretical playgrounds that hyperspectral single-particle electrochemistry opens.
Inconsistencies between experimental results and predictions of this common physical model were identified and highlighted.
The expected changes in nanoparticle free-electron density were modeled using a charge density-modified Drude dielectric function and Mie theory, a commonly used model in colloidal spectroelectrochemistry. Some nanoparticles that showed charge density tuning in the cathodic range also showed signs of an additional chemical tuning mechanism in the anodic range. Additional heterogeneity was observed when single nanoparticles demonstrating reversible charge density tuning in the cathodic regime were measured dynamically in anodic potential ranges. The irreversible reactions in particular would be difficult to discern in alternate methodologies. At cathodic potentials, we identified three distinct behaviors from different nanoparticles within the same sample: irreversible chemical reactions, reversible chemical reactions, and reversible charge density tuning. A hyperspectral imaging method was developed that allowed the identification of heterogeneous plasmon response from 50 nm diameter gold colloidal particles on a conducting substrate in a transparent three-electrode spectroelectrochemical cell under non-Faradaic conditions.
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