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Direct Electrochemical Observation of Single Platinum Cluster Electrocatalysis on Ultramicroelectrodes Matthew Glasscott, and Jeffrey E. Dick Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.8b02219 • Publication Date (Web): 12 Jun 2018 Downloaded from http://pubs.acs.org on June 12, 2018
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Analytical Chemistry
Direct Electrochemical Observation of Single Platinum Cluster Electrocatalysis on Ultramicroelectrodes Matthew W. Glasscott and Jeffrey E. Dick* Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, NC 27599
ABSTRACT: We demonstrate a method to electrodeposit and observe the electrocatalysis of small platinum clusters and nanoparticles (NPs) in real time as they form on an ultramicroelectrode (UME). Water droplets (rdrop~700 nm), stabilized by sodium dodecylsulfate (SDS), were suspended in a solution of dichloromethane (DCM) and tetrabutylammonium perchlorate ([TBA][ClO4]), which was used to mitigate charge balance during droplet electrolysis. When droplets collided with an UME biased sufficiently negative to drive water reduction, large blips of current were observed. Droplets were synthesized with varying concentrations of H2PtCl6 (from 24.4 mM to 32 nM), which can be reduced to Pt0 at 0.8 V more positive than water reduction on a Au or C UME. The observation of current blips synthesized with mM amounts of H2PtCl6 indicated water droplets deliver H2PtCl6 to the electrode surface, where a cathodic potential caused Pt NPs to form. The formation of clusters was observed by biasing the electrode potential more negative than water reduction on Pt but more positive than water reduction on Au, giving current blips for droplets containing µM to nM amounts of H2PtCl6. These blips corresponded to the electrocatalysis of thousands to tens of atoms (clusters). Droplet electrolysis allows for a large amplification such that the electrocatalysis of clusters can be observed in real-time. Single, isolated clusters were further characterized voltammetrically on carbon fiber UMEs. The isolation step used the amperometric method, which unambiguously depicted cluster formation in real time. Carbon was chosen due to its large potential window and slow kinetics toward many inner-sphere reactions, such as proton reduction, used in this study. Voltammetric characterization of proton reduction in HClO4 and NaClO4 allowed for cluster size analysis using the limiting current. The reduction of proton on the clusters (E1/2 ~ -0.6 V vs. Ag/AgCl) occurred at ca. 400 mV more negative than bulk, polycrystalline platinum. Keywords: Clusters, Electrocatalysis, Electrodeposition, Nanoparticles, Ultramicroelectrode
While nanoparticles and clusters are attractive as catalysts due to unique electronic properties and high surface area to volume ratios, synthetic routes generally leave a distribution of shapes and sizes. These variations make mechanistic investigations on ensembles of nanoparticles and clusters difficult to interpret since ensemble measurements report values on the average size and morphology of the nanostructure. By studying single catalytic centers, particle differences can be accounted for and variations in size and morphology rigorously evaluated.1-4 Currently, however, many techniques lack the spatiotemporal resolution to make measurements on such small catalyst nanostructures5 on a catalyst-by-catalyst basis.
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Figure 1. Schematic representation of the experiment. A.) A water droplet collides with a gold UME, and the water is reduced. B.) A water droplet collides with a platinum UME, and water is reduced at more positive potentials than on gold. C.) A water droplet containing H2PtCl6 collides with a gold UME, and the H2PtCl6 is converted to a Pt nanocluster at 0.8 V more positive than water reduction on gold. D.) nM to µM amounts of H2PtCl6 are dissolved in the water droplet. At sufficiently negative potentials, a Pt cluster will form on the Au surface and reduce water, generating a large electrocatalytic signal.
Since Lemay’s landmark report in 2004 describing the time-resolved detection of single insulating microspheres at ultramicroelectrodes (UMEs), several techniques have been developed for studying single insulating and conductive nanoparticles (NPs) colliding with UME surfaces.6-25 One such technique used to detect catalytic NPs, termed electrocatalytic amplification (EA)26, has been used to observe catalytic NPs colliding on a relatively inert surface one at a time.27, 28 The key in these experiments is that the kinetics of the reaction being driven on the NP must be much more facile than on the UME.15 For instance, proton reduction occurs at more positive potentials on a platinum UME compared to a gold or carbon UME. Thus, if a gold or carbon UME is biased at a potential in an acidic solution such that proton reduction is not favored on the UME surface, and a Pt NP is incident on that surface, the electrocatalysis of single Pt NPs can be observed in amperometry,28-30 potentiometry, 15, 31, 32 and voltammetry.33 Using this conceptually elegant method, groups have demonstrated catalysis measurements on a range of small NPs15, 31, 34. Unfortunately, these methods suffer from NP aggregation and ligand-stabilizer effects3537 , which complicate the evaluation of size effects on NP electrocatalysis. Recently, Bard and co-workers developed a method of depositing isolated platinum atoms and clusters up to 9 atoms on carbon and bismuth surfaces from femtomolar solutions of H2PtCl6.38, 39
Here, we show direct electrochemical observations of cluster electrocatalysis unambiguously and in real time by nucleating a small cluster of Pt in a water droplet upon collision with an UME. In principle, this method can be used to size-selectively grow single clusters of platinum and study the electrocatalytic behavior of clusters one at a time.40 The water droplets, which included ~15 mM PBS, were stabilized by sodium dodecylsulfate (SDS) and suspended in a dichloromethane (DCM) and tetrabutylammonium perchlorate [TBA][ClO4] solution. Previously, Kim and co-workers reported the adsorption of similar droplets to UME surfaces by driving a heterogeneous reaction (ferrocene oxidation) and observing step-like decreases in the steady-state current with time.41 Oil-in-water droplets have been shown to act as attoliter to zeptoliter electrochemical reactors, delivering contents to an electrode surface.42, 43 The use of water droplet reactors offers important advantages over other deposition techniques: 1.) In principle, one can control the size of the cluster by controlling the initial amount of H2PtCl6 dissolved in the water phase44 as well as the polydispersity of the droplet sizes.12 2.) Whereas previous reports on single cluster catalysis rely on building a cluster from femtomolar solutions of H2PtCl6, the working concentration of H2PtCl6 in these experiments is between 10 nM and 10 µM (the dissolution of nM to µM H2PtCl6 in a droplet with radius ~500 nm corresponds to 10s to 1,000s of molecules of H2PtCl6). 3.) The contact radius between a droplet and the electrode has been reported to be
