Novel gold monolayer protected clusters: synthesis, characterization, separation, and functionalization Public Deposited

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  • March 21, 2019
  • Wolfe, Rebecca L.
    • Affiliation: College of Arts and Sciences, Department of Chemistry
  • Chapter One is an introduction to monolayer protected clusters (MPCs) and their unique size-dependent properties. This chapter also serves as an introduction to the methods of synthesis, characterization, functionalization, and separation of MPCs that have been explored in the literature. Chapter Two demonstrates how the Brust synthesis of thiolate-protected gold clusters has been modified to produce identifiable proportions of a hexanethiolate-protected Au225 core nanoparticle that display quantized double layer charging voltammetry consistent with a Au225 core dimension. Transmission electron microscopy (TEM) and thermogravimetric results indicate an average nanoparticle formula of Au225[(S(CH2)5CH3)]75. A simulated pulse voltammogram that accounts for the TEM nanoparticle dispersity matches reasonably well with that of the polydisperse synthetic sample containing the Au225 component. In confirmation of the size determination, an HPLC analysis using ratiometric absorbance and electrochemical detectors gives a core radius of 1.0 nm for the Au225 nanoparticle. Chapter Three describes the synthesis and compositional analysis of four different gold clusters with protecting monolayers comprised solely of ferrocene hexanethiolate ligands. The gold nanoparticles have average core diameters of 1.4, 1.6, 2.0, and 2.2 nm with estimated average atom counts of 55, 140, 225, and 314 Au atoms and average monolayer coverages of 37, 39, 43, and 58 ferrocenated ligands, respectively. The data show unequivocally that the number of ferrocene hexanethiolate ligands bound to each core size is constrained by the steric requirements of the ferrocene head group; the ligand numbers are significantly smaller than those for hexanethiolate ligands bonded to analogoussized Au cores. Voltammetry of dilute solutions of these nanoparticles shows a large ferrocene oxidation wave and, at more negative potentials, smaller one-electron waves for the quantized double-layer charging of the Au cores. Together, the ferrocenes and core of the ferrocenated Au314 nanoparticle deliver 60 electrons at the ferrocene oxidation potential, which amounts to a very large volume charge capacity, 7 x 109 C/m3, for an undiluted nanoparticle sample. Chapter Four describes how Au nanoparticles fully coated with w-ferrocenyl hexanethiolate ligands, with average composition Au225(w-ferrocenyl hexanethiolate)43, exhibit a unique combination of adsorption properties on Pt electrodes. The adsorbed layer is so robust that electrodes bearing sub-monolayer, monolayer, and multilayer quantities of these nanoparticles can be transferred to fresh electrolyte solutions and then exhibit stable ferrocene voltammetry over long periods of time. The adsorption kinetics is quite slow and monolayer and sub-monolayer deposition can be described by a rate law that is first order in bulk concentration of the nanoparticles and in available surface of the platinum electrode. The adsorption mechanism is proposed to involve ion-pair bridges between oxidized (ferrocenium) sites and certain specifically adsorbed electrolyte anions on the electrode. Adsorption is promoted by potential scanning through the ferrocene redox wave and by high concentrations of Bu4N+X- electrolyte (X- = ClO4 -, PF6 -) in the CH2Cl2 solvent; there is no adsorption if X- = p-toluenesulfonate or if the electrode is coated with an alkanethiolate monolayer. The electrode double layer capacity is unchanged in the presence of the ferrocenated layers, and the adsorbed nanoparticles can be gradually desorbed by scanning to potentials more negative than the electrode’s potential of zero charge. The full-width-half-maxima of the symmetrical voltammetric peaks of an adsorbed monolayer of ferrocenated nanoparticles are considerably narrower (typical 35 mV) than expected (90.6 mV, at 298 K) for a one electron transfer or for reactions of multiple, independent redox centers with identical formal potentials. The peak narrowing is explicable by a surface activity effect involving large, attractive lateral interactions between nanoparticles and by a proposed series of reactions of ferrocene sites whose formal potential values become successively altered as ion-pair bridges are formed. Chapter Five presents the use of anion-induced adsorbed Au225(w-ferrocenyl hexanethiolate)43 on carbon-paper-supported carbon aerogel (nanofoam) electrodes as novel materials for supercapacitors. The specific capacitance (in F/g) of the carbon nanofoam electrode increases by more than 8000% upon adsorption of the ferrocenyl functionalized gold MPCs. This remarkable increase in capacitance can be attributed to the pseudocapacitance derived from the redox charging of the monolayer as well as the doublelayer capacitance arising from the charging of the gold core. The carbon nanofoam electrode is also ground into a fine powder to increase the surface area, and similar studies with adsorbed ferrocenated MPCs are performed. While the capacitance of the powder does increase upon adsorption of the MPC, it does not surpass that of the intact MPCmodified nanofoam electrode. Scanning electron microscopy shows that the powder is quite polydisperse in size and shape, and the dropcast method to analyze the powder electrochemically leads to nonuniform distrubition of the powder onto surfaces. Chapter Six investigates the catalytic properties of Pd, Au, Ag, and bimetallic AgAu MPCs. A polar protecting monolayer shell consisting of N-(2-mercaptopropionyl)glycine ligands (also known as tiopronin) is used to allow for solubility in water. The reaction that is catalyzed by all four MPCs is the reduction of 4-nitrophenol in the presence of sodium borohydride, which on its own does not reduce the substrate. The four MPCs all successfully catalyze reduction, and first-order rate constants are derived and found to be comparable with other literature values. Pd MPCs are not surprisingly significantly faster than the other three MPCs, but, more interestingly, the bimetal AgAu MPC catalyzes the reduction faster than that of the monometallic Ag and Au MPCs.
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  • Murray, Royce W.
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  • University of North Carolina at Chapel Hill
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