Electrochemical characterization of surface-immobilized metal nanostructures: stability, atomic level doping, catalysis, and sensing applications.

Abstract

This dissertation has two main themes. The first theme involves voltammetric analysis of the stability of Au nanoparticles (NPs) under electrochemical and thermal treatment as a function of size, ligand stabilizer, and atomic composition. The second theme involves the use of Au NPs, electrophoretic deposition (EPD), and anodic stripping voltammetry (ASV) for electrochemical detection of analytes. The electrochemical size stability of 4.1, 15.1, and 50.3 nm average diameter Au NPs upon treatment with multiple electrochemical oxidation-reduction cycling in acidic electrolyte is monitored by observing changes in the peak oxidation potential (Ep) in ASV and the electrochemically measured surface area-to-volume ratios (SA/V) of the Au NPs. The Au NPs exhibit size-dependent size stability. The ripening rate depends on cycling scan rate, NP coverage on the electrode, and anodic polarization. Also, the NPs take part in a combination of anodic dissolution and electrochemical Ostwald ripening as the mechanism for ripening. The rate and extent of thermal ripening also depends on the Au NP diameter. Au NPs of 0.9 nm and 1.6 nm diameter begin to sinter at 70-80 oC while 4.1 nm diameter Au NPs remain stable when heated up to 400 oC for 60 minutes. The Ep in ASV tracks the Au NP size changes, where the ratio of peak currents for sintered size and original size as a function of temperature provides the sintering transition temperature, which is 109, 132, and 509 oC for 0.9 nm, 1.6 nm, and 4.1 nm diameter Au NPs, respectively. The stability of citrate-coated Au NPs against electrooxidation increases with increasing size of the NPs. This trend becomes opposite when the NPs are coated with alkanethiolate self-assembled monolayers (SAMs). The Au oxidation current measured by CV and chronocoulometry (CC) follows the order of 50.3 nm > 15.1 nm > 4.1 nm Au NPs, and this result is supported by UV-Vis combined with CC experiments. The NP composition plays an important role in the catalytic activity and stability towards the oxygen reduction reaction (ORR) in alkaline solution. Atomic level doping of 1.6 nm diameter Au NPs with 1-2% Ag by anti-galvanic replacement (AGR) dramatically improves the ORR of the Au NPs to a level similar to Ag NPs, but also dramatically improves the stability. In terms of synthesis, the work on AGR shows successful atomic level doping of 1.6 nm Au NPs with 1-2% Ag and Cu, where the cluster size remains stable during doping and