Experimental and theoretical investigation of electrochemically synthesized AuPt dendrimer-encapsulated nanoparticles (DENs)

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2020-06-22

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Lapp, Aliya Siegel

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Herein, experiment and theory are combined to study the efficacy of synthesizing core@shell Au@Pt dendrimer-encapsulated nanoparticles (DENs) through electrochemical means. DENs are small (~1-2 nm), catalytically active nanoparticles (NPs) with precise sizes and compositions. These features assist pairing experiment with theory and catalytic interpretation. The small sizes of DENs can impart interesting physical and chemical properties that differ from bulk phase materials. Core@Pt shell NPs with monolayer (ML)-thick shells minimize the use of Pt, which is a key catalyst for various reactions but is expensive and scarce. Owing to the fact that the available techniques for Pt ML deposition were originally developed for bulk Au, their applicability to ~1.6 nm Au DEN cores is not trivial. In this dissertation, we explore several electrochemical strategies for synthesizing Au@Pt DENS: hydride-terminated (HT) Pt electrodeposition and underpotential deposition, followed by galvanic exchange with Pt (UPD/Pt GE) using two different UPD metals (Cu and Pb). Each of these techniques deposits a single Pt ML onto bulk Au surfaces. However, when they are applied to ~1.6 nm Au DENs, the AuPt NP structures that form are both dissimilar to one another and to the corresponding structures for bulk Au. The HT synthesis is found to lead to an alloy structure, whereas AuPt NP structure formed upon UPD/Pt GE depends on the choice of the UPD metal (Cu or Pb). More specifically, Cu UPD/Pt GE produces a core@shell structure, whereas an alloy structure is afforded by Pb UPD/Pt GE. These conclusions are supported by extensive experimental characterization and density functional theory (DFT) calculations. For the HT method, a core@shell structure can ultimately be obtained, but requires 3-5 total HT pulses. Due to the fact that catalysis tends to be highly structure sensitive, the results of these studies are important for rational design of catalysts. Indeed, we show that varying the number of HT pulses (from 1-10) can be used to tune electrocatalysis for formic acid oxidation (FAO).

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