Physicochemical properties of the anion-modified surfaces of silver nanostructures
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Nano-sized structures of silver were prepared, characterized, and then chemically-modified with the adsorption of molecules and ions on the metal surface. Nanostructures prepared as aqueous colloids were found to have highly regular polyhedral shapes by transmission electron microscopy. Electron diffraction techniques indicated that isolated Ag structures were composed of a single crystalline phase or were multiply-twinned, both having a lattice constant of 4.05 A;Adsorption of iodide and bromide ions on the silver surface was monitored by surface-enhanced Raman spectroscopy. A characteristic halide-metal stretching vibration was observed at 112 cm-1 for I- and at 156 cm-1 for Br-. Extinction spectra of the halide-modified Ag colloids showed a frequency-shift and damping of the surface plasmon resonance band assigned to particle aggregation. This was confirmed using two dimensional arrays of particles in which the surface-modifier caused only damping with no change in the plasmon frequency;Addition of cytochrome c to the halide-modified colloid resulted in the reduction of the protein. Competitive binding of I- ions between cytochrome c and the metal necessitated the use of a redox active indophenol dye for quantitative measurements of reduction efficiencies. Two distinct processes were identified: reduction at iodide coverages up to one monolayer on the Ag surface and reduction in the presence of excess I- in solution. The latter was characterized by I- reacting with silver in a 1:1 stoichiometry to form molecular AgI. The former, which resulted in only 5% reduction of the electron acceptors, was a consequence of partial charge transfer from I- to the metal, producing a unique iodide-Ag surface complex different than molecular AgI;Raman spectra of the complex excited at 413 nm and at temperatures less than 150 K contained a strong vibrational progression with a fundamental band at 123 cm-1 and up to six overtones. An excitation profile composed of 77 points in the wavelength range 409-433 nm revealed a series of resonance emission bands due to resonance Raman scattering and resonance and relaxed fluorescence. A model was discussed which involved the photoinduced formation of a new I2 species adsorbed on small Ag clusters.