Date on Master's Thesis/Doctoral Dissertation

8-2010

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemistry

Committee Chair

Zamborini, Francis Patrick

Author's Keywords

Electrochemical synthesis; Chemical synthesis; Galvanic exchange; Silver nanowires; Silver nanostructures

Subject

Nanoparticles; Nanowires; Electrochemical metallizing; Silver-plating

Abstract

This dissertation describes galvanic exchange of silver (Ag) nanostructures with PdCl42- in 1) various PdCl42- concentrations, 2) the presence of 0.1 M cetyitrimethylammonium bromide (CTAB), and 3) the presence of 0.1 M potassium bromide (KBr). It also describes the electrochemical seed-mediated growth of Ag nanorods directly on indium tin oxide (ITO) coated glass electrodes, and the synthesis of Ag nanostructures directly on thin sheets of graphene and the Raman enhancement of the graphene. Ag nanorods (NRs) and nanowires (NWs) synthesized directly on surfaces from surface-attached Au nanoparticles (NPs) by seed-mediated growth react spontaneously with PdC142- solutions by galvanic exchange. The morphology of the resulting AgPd alloy nanostructures depends on the galvanic exchange rate when performed in aqueous solutions with no other additives. The rate increases with increasing PdCl42- concentration over a concentration range of 1.0x 10-5 to 1.0x 10-4 M. A concentrations of 7.5 x 10-5 M or higher lead to rapid galvanic exchange resulting in Pd deposition over the entire Ag nanostructure at the early stages of exchange. When the concentration of PdCl42- is in the range of 1.0 x 10-5 to 5.0 X 10-5 M, Pd deposition occurs preferentially at high energy twin plane defects in the form of well-spaced nanoparticles during the early stages of exchange. Composition analysis by linear sweep voltammetry shows that the UV-Vis data do not reflect the composition well. The rate at all concentrations is slower that the diffusion-limited rate and the reaction does not reach completion at any concentration studied (Max = 70% Pd) The galvanic exchange in the presence of 0.1 M CTAB and concentrations of PdCl42- from 1.0x10-5 M to 1.0x10-4 M show several differences compared to exchange in water. First, the rate of exchange does not depend strongly on PdCl42- concentration. Second, the rate of exchange is slower overall and the morphology of the PdAg NWs is different. The galvanic exchange also occurs preferentially on twin -plane defect sites, but the Pd deposition is more continuous and the etching of the terraces more pronounced. Third, the extent of exchange is different. The Ag is fully exchanged in the presence of CTAB after 1 h in 5.0x10-5 M PdCl42-, while 25-30% Ag remains on the surface for exchange in water only. The galvanic exchange rate is slower than the diffusion-limited rate at all PdCl42 concentrations. In presence of 0.1 M KBr, the rate of exchange is fastest out of all environments studied (water, CTAB, KBr). The resulting morphology is very different compared to morphologies obtained during exchange in water or CTAB. Exchange in 7.5x10-5 M PdCl42- in 0.1 M KBr leads to the destruction of the nanostructures and an evenly distributed rough morphology after exchange in 5.0x10-5 M and 2.5x10-5 M PdCl42- M. The electrodeposition of Ag in pH 10.6 buffer occurs preferentially on Au nanoparticle seeds attached to glass/ITO electrodes over a potential range of -0.1 to -0.2 V vs. Ag/AgCl. At -150 mV (vs. Ag/AgCl), the deposited nanostructures contain a small percentage ofNRs and NWs, whereas other potentials produces mostly spherical or flower-like structures. This shows that the formation of NRs/NWs depends strongly on the reduction potential, which is an important finding for fundamental and applied studies. Ag and Au nanostructures, including NRs/NWs, can be immobilized on thin sheets of graphene by attaching hydrophobic hexanethiolate Au seeds and growing them by seed-mediated growth. The presence of the Au and Ag nanostructures leads to the Raman enhancement of graphene by 50 and 150 fold, respectively. This shows that metal nanostructures are in intimate contact with graphene. These heterostructures could find use in catalysis, sensing, nanoelectronics, and optoelectronics applications.

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