The Monolayer Thickness Dependence of Quantized Double-Layer Capacitances of Monolayer-Protected Gold Clusters

Abstract

This report describes how the electrochemical double-layer capacitances of nanometer-sized alkanethiolate monolayer-protected Au clusters (MPCs) dissolved in electrolyte solution depend on the alkanethiolate chain length (C4 to C16). The double-layer capacitances of individual MPCs (C(CLU)) are sufficiently small (sub-attoFarad, aF) that their metal core potentials change by >0.1 V increments for single electron transfers at the electrode/solution interface. Thus, the current peaks observed are termed "quantized double layer charging peaks", and their spacing on the potential axis varies with C(CLU). Differential pulse voltammetric measurements of C(CLU) in solutions of core-size-fractionated (i.e., monodisperse) MPCs are compared to a simple theoretical model, which considers the capacitance as governed by the thickness of a dielectric material (the monolayer, whose chain length is varied) between concentric spheres of conductors (the Au core and the electrolyte solution). The experimental results fit the simple model remarkably well. The prominent differential pulse voltammetric charging peaks additionally establish this method, along with high-resolution transmission electron microscopy and laser ionization-desorption mass spectrometry, as a tool for evaluating the degree of monodispersity of MPC preparations. We additionally report on a new tactic for the preparation of monodisperse MPCs with hexanethiolate monolayers.

Absolute Standard Redox Potential of Monolayer-Protected Gold Nanoclusters

“…The extraction treatment removed not only the byproducts and unreacted residues but also some fractions of smaller core size MPCs. 14 The obtained MPCs were further annealed in dichloromethane by adding a 500-fold excess of C6-SH versus MPC dissolved. After stirring for 4 days, dichloromethane was evaporated.…”

“…[3][4][5][6] At ambient temperature, both freely diffusing and electrode-attached MPCs demonstrate the voltammetric responses featuring a series of evenly spaced current peaks. 4,[7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] This ensemble behavior is equivalent to a series of classical Coulomb staircase charging events or sequential electrochemical redox reactions, which occur as MPCs encompass an intermediate dimension between small molecules and bulk materials. Previously, the sequential electrontransferhasbeenobservedforfullerene(andderivatives) [22][23][24][25][26] and Pt-carbonyl nanoclusters [Pt n (CO) m ], [27][28][29] which led Weaver et al 30 to develop an electrostatic model to relate the electron transfer energetics of molecular capacitances in gas-and solution-phase systems.…”

Solvent Effect on Redox Properties of Hexanethiolate Monolayer-Protected Gold Nanoclusters

“…The electrochemistry of MPCs has been extensively studied by several groups. [3][4][5]7 A series of factors, including the solvent, 8 the temperature, 9 as well as the solvation and/or penetration of solvent and supporting electrolyte, 8,10,11 have been investigated. So far, the model of quantized capacitance charging developed by Murray et al 6 has been extensively employed to explain the multivalent redox behavior of MPCs.…”

Solid‐State Electrochemical Method for Determining Core and Shell Size in Pd@PdO Nanoparticles

“…On the basis of the reported values for the length of extended tetrabutylammonium (TBA þ ) units (0.94 -1.0 nm) [9,23], an average length of 1.8 nm will be taken for TOA þ units for following calculations. It should be noted, however, that the extended chain assumption is probably most valid for short chain lengths and small nanoparticles, since the radial dilution of chain density is least at shorter chain lengths and small nanoparticle sizes while for larger chain lengths and/or large particle sizes there is place for capping disorganization [24].…”