Electron transfers between small nanoparticles

Carducci, Tessa

Electron transfers between small nanoparticles

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March 19, 2019

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Carducci, Tessa
- Affiliation: College of Arts and Sciences, Department of Chemistry

Abstract

Chapter one is an introduction to electrochemistry of monolayer-protected clusters and iridium oxide nanoparticles. Chapter two examines the temperature dependence of electron transfer (ET) kinetics in solid-state films of mixed-valent states of monodisperse, small (< 2 nm) Au monolayer protected clusters (MPCs). The mixed valent MPC films, coated on interdigitated array electrodes (IDAs), are Au25(SR)180/1-, Au25(SR)181+/0, and Au144(SR)601+/0, where SR = hexanethiolate for Au144 and phenylethanethiolate for Au25. Near room temperature and for ca. 1:1 mol:mol mixed valencies, the bimolecular ET rate constants (assuming a cubic lattice model) are ~2 x106 M-1 s-1 for Au25(SR)180/-1, ~3x105 M-1 s-1 for Au25(SR)18+1/0, and ~1x108 M-1 s-1 for Au144(SR)60+1/0. Their activation energy ET barriers are, respectively, 0.38 eV, 0.34 eV, and 0.17 eV. At lowered temperatures (down to ca. 77 K), the thermally activated (Arrhenius) ET process dissipates, revealing a tunneling mechanism in which the ET rates are independent of temperature, but among the different MPCs fall in the same order of ET rate: Au144+1/0 > Au250/-1 > Au25+1/0. Electron transfers (ET) in mixed valent ferrocene/ferrocenium materials are ordinarily facile. In contrast, chapter three shows the presence of ca. 1:1 mixed valent ferrocenated thiolates in the organothiolate ligand shells of < 2 nm dia. Au225, Au144, and Au25 monolayer protected clusters (MPCs) exerts a retarding effect on ET between them at room and at lowered temperatures. At lowered temperatures (down to ca. 77 K), the thermally activated (Arrhenius) process dissipates and ET rates become temperature independent. Among the Au225, Au144, and Au25 MPCs, the temperature independent ET rates fall in the same order of ET rate as at ambient temperatures: Au225 > Au144 > Au25. The MPC ET activation energy barriers are little changed by the presence of ferrocenated ligands, and are primarily determined by the Au nanoparticle core size. Chapter four introduces iridium oxide nanoparticles (IrOx NPs). The electronic conductivity of films of IrOx composed of ca. 2 nm NPs is strongly dependent on the film oxidation state. The IrIVOx NPs can be electrochemically converted to several oxidation states, ranging from IrIII to IrV oxides. The NP films exhibit a very high apparent conductivity, e.g., 10-2 S cm-1, when the NPs are in the oxidized +4/+5 state. When the film is fully reduced to its IrIII state, the apparent conductivity falls to 10-6 S cm-1. Chapter five reports second order rate constants of associative ligand exchanges of Au25L18 MPCs (L = PhenylC2) of various charge states, measured by proton NMR at room temperature and below. Differences in second order rate constants (M-1 s-1) of ligand exchange (positive clusters ~1.9 x 10-5 vs. negative ones ~1.2 x 10-4) show that electron depletion retards ligand exchange. The ordering of rate constants between the ligands benzeneselenol > 4-bromobenzenethiol > benzenethiol reveals that exchange is accelerated by higher acidity and/or electron donation capability of the incoming ligand. Together, these observations indicate that partial charge transfer occurs between the nanoparticle and ligand during the exchange and that this is a rate-determining effect in the process.

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