Quantized Double-Layer Charging of Highly Monodisperse Metal Nanoparticles Jocelyn F. Hicks, † Deon T. Miles, ‡ and Royce W. Murray* Contribution from the Kenan Laboratories of Chemistry, UniVersity of North Carolina CB#3290, Chapel Hill, North Carolina 27599-3290 Received July 16, 2002 Abstract: We describe unprecedented resolution of electrochemically observed quantized double layer (QDL) charging, attained with use of reduced solution temperatures and with an annealing procedure that produces hexanethiolate monolayer protected gold clusters (C6 MPCs) with a high level of monodispersity in charging capacitance, CCLU. The spacing ΔV ) e/CCLU on the electrochemical potential axis between one electron changes in the electronic charge of nanoscopic metal particles is determined by their effective capacitance CCLU. The high monodispersity of the C6 MPCs with Au140 cores facilitates (a) detailed rotated disk and cyclic voltammetric measurements, (b) simulation of QDL waveshapes based on assumed reversible, multivalent redox-like behavior, (c) determination of nanoparticle diffusion rates, and (d) observation of as many as 13 changes in the MPC charge state, from MPC 6- to MPC 7+ . The single electron QDL charging peaks are quite evenly spaced (ΔV constant) at potentials near the MPC potential of zero charge, but are irregularly spaced at more positive and negative potentials. The irregular spacing is difficult to rationalize with classical double layer capacitance ideas and is proposed to arise from a correspondingly structured (e.g., not smooth) density of electronic states of the nanoparticle core, resulting from its small HOMO/LUMO gap and incipiently molecule-like behavior. Introduction Monolayer-protected gold clusters (MPCs) are nanoparticles coated with dense, protecting monolayers of organothiolate, 1 organophosphine, 2 or organoamine ligands. 3 The thiolate mono- layer inhibits aggregation of the MPC core, even in the absence of solvent. MPC stability facilitates design and manipulation 1 of its monolayer functionality and detailed analytical charac- terization. For MPCs having a 1 to 2 nm core dimension, small variations in the number of core metal atoms may potentially evoke significant variations in nanoparticle properties. MPC samples having a mixture of core sizes can accordingly exhibit a mixture of properties (i.e., dispersity). Dispersity in properties may additionally arise from the variability of the ligand shells; there is inadequate analytical information available on mono- layer variability. Because thiolate-coated MPCs prepared using the Brust 1c reaction or its modifications 4 are somewhat poly- disperse, there have been several studies aimed at reducing their polydispersity, and analyzing it using solubility fractionation, 5 etching, 6 extraction, 7 chromatography, 8 capillary electrophore- sis, 9 and mass spectrometry. 10 Of further value are procedures amenable to producing quantities of monodisperse MPCs sufficient for subsequent synthetic, as well as physical, inves- tigations. The quantized double layer (QDL) charging 1a,7,11,12 of MPC cores occurs because the effective capacitances (C CLU ) of * To whom correspondence should be addressed. E-mail: rwm@ email.unc.edu. † Current address: Dupont Experimental Station, Wilmington, DE 09880- 0262. ‡ Current address: University Of The South, 735 University Ave., Sewanee, TN 37383. (1) (a) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (b) Whetten, R. L.; Shafigullin, M. N.; Khoury, J. 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