Temperature-dependent quantized double layer charging of monolayer-protected gold clusters.

Abstract

This paper describes low-temperature voltammetry of purified hexanethiolate-coated monolayer-protected Au140 clusters (C6 MPCs). Lowered temperatures enhance the resolution of quantized double layer (QDL) charging peaks in differential pulse voltammetry (DPV) observations. As many as 13 resolved peaks are seen in illustrative voltammetry at 263 K in CH2Cl2 solvent, and the concept of voltammetric peak capacity is introduced. For the one-electron MPC charge steps surrounding the E(PZC) of the MPC (small numbers of electrons added or removed from the core), the capacitance C(CLU) of the MPCs (measured from the voltage spacing between charging peaks) increases by approximately 15% as the solvent temperature is lowered from 273 to 210 K. The experimental C(CLU) temperature dependency (d[ln(C(CLU))]/dT approximately -0.0025, in 0.1 M electrolyte) is discussed in light of temperature dependencies of the compact and diffuse double layer capacitances. It is concluded that the observed temperature dependence is probably a mixed diffuse, compact dependence. The regular voltage spacing of MPC charging peaks near the potential of zero charge is generally consistent with electrical double layer properties, but the irregular pattern of charging of the nanoparticles seen at higher charge states suggests intervention of the incipient molecular behavior of Au140 cores in the spacing of energies at which further electrons are added or removed.