Size- and shape-dependent electrochemistry of gold nanoparticles.

Abstract

This dissertation describes 1) size-dependent electrochemical oxidation/stripping of chemically synthesized gold nanoparticles (Au NPs), 2) copper underpotential deposition (Cu-VPD) on different sized chemically-synthesized and electrochemically-deposited Au NPs attached to glass/indium-tin-oxide (ITO) electrodes, and 3) electrochemical synthesis of Au nanoplates directly on glass/ITO electrodes. The motivation of this work was to better understand the characteristics and electrochemical properties of metal nanostructures with different sizes and shapes. We synthesized Au nanoparticles with average diameters ranging from 5 to 45 nm by a chemical seed-mediated growth method and electrostatically attached them to amino-functionalized glass/ITO electrodes. Linear sweep voltammograms (LSVs) obtained on electrodes coated with Au NPs in 0.01 M potassium bromide plus 0.1 M HCI04 showed a positive shift in oxidation potential from 680±1 mV to 773±6 m V with increasing Au NP diameter, consistent with increasing NP stability with increasing size. In copper underpotential deposition (UPD) studies, Au NPs were chemically-synthesized in solution by seed-mediated growth, and Au NPs were electrochemically deposited directly on the glass/ITO electrode from HAuCl4 in H2S04 by chronocoulometry. Potentials used were -0.2 V, 0.4 V, and 0.8 V versus Ag/AgCI and charges employed well 6x10-4 Coulombs (C), 1x10-3 C, and 6.6x10-3 C. Cyclic voltammograms (CV s) obtained on the electrodes in 0.01 M Cu(CIO4)2 plus 0.1 M HCIO4 in the region from 0.1 to 1.6 V vs. Hg/HgO showed that the UPD-peak is highly sensitive to the size of the Au NPs. The amount of Cu deposited onto the Au surface was inversely proportional to the size of the NP. This shows that a more dense Cu UPD layer forms on smaller NPs, likely due to a greater number of defects with decreasing NP size. CVs obtained in the region from 0.8 to -0.7 V revealed a decrease in the reduction potential of Cu2+ from -508 m V to -553 m V with increasing NP size. This directly shows that smaller NPs are better catalysts for metal deposition. We also electrochemically deposited Au NPs directly on the surface of the glass/ITO electrode from HAuCl4 in H2S04 at a potential of 0.8 V versus Ag/AgCl using chronocoulometry for - 3x10-3 C, 6x10-3 C, 9x10-3 C, and 1.2x10-2 C. We compared the yield of nanoplates formed under the different conditions and studied the mechanism of their growth. In addition, we obtained UV-vis spectra of the Au NPs and Au nanoplates. The growth mechanism involves: formation of uniformly distributed flower-like nanostructures, smashing into an irregular-shaped nanoplate and their growing, then etching and aggregation of the nanoplates with the nanoparticles. The maximum yield was observed 40-50%. In this thesis, we describe the experimental setup used in this research and the results. The results of this research are very important for understanding the fundamental electrochemical properties of NPs, which could lead to applications in several different fields. It is…