Arrays of nanometer-scale electrodes were prepared by cyanide-etching of hexadecanethiol monolayers confined to Cu-underpotential deposition (UPD)-modified Au(111). This process results in fabrication of arrays having average electrode radii ranging from 6 to 80 nm. The arrays were independently characterized by cyclic voltammetry and scanning tunneling microscopy (STM). A recessed disk model was used to calculate an expected limiting current based on the STM-derived geometrical data, and this was correlated with the electrochemically determined value. General agreement was found between these values, but nonidealities in the distribution of electrodes within the arrays led to substantial scatter in the data. We conclude that improved array characterization will result in a reliable means to test for predicted deviation from standard microelectrode theory at nanoscale electrodes. Electrochemical responses were measured for the arrays in the presence of both Ru(NH3)6 3+ and benzoquinone, redox probes having distinctly different heterogeneous electron-transfer rates. Highly irreversible electrochemical kinetics were observed for the kinetically slow redox probe which is a consequence of the small size of these electrodes.

Gold nanoparticles supported on hydrous tin-oxide ͑Au-SnO x ͒ are active for the four-electron oxygen reduction reaction in an acid electrolyte. The unique electrocatalytic of the Au-SnO is confirmed by the low amount of peroxide detected with rotating ring-disk electrode voltammetry and Koutecký-Levich analysis. In comparison, 10 wt % Au supported on Vulcan carbon and SnO x catalysts both produce significant peroxide in the acid electrolyte, indicating only a two-electron reduction reaction. Characterization of the Au-SnO x catalyst reveals a high-surface area, amorphous support with 1.7 nm gold metal particles. The high catalytic activity of the Au-SnO is attributed to metal support interactions. The results demonstrate a possible path to non-Pt catalysts for proton exchange membrane fuel cell cathodes.

No abstract

We report the preparation and characterization of viologen-functionalized generation 2, 4, and 6 poly-(amidoamine) (PAMAM) dendrimers. An amidation reaction between the succinimide ester of 1-ethyl-1′-(3-propionic acid)-4,4′-bipyridylium dibromide and the dendrimer primary amines resulted in 13-34% end group functionalization. The water-soluble dendrimers were examined by 1 H NMR, MALDI-TOF MS, and UV-vis spectroscopy to determine the extent of functionalization. UV-vis spectra, obtained following chemical reduction of the viologenated dendrimers, indicated that the viologen radical cations were largely dimerized, which demonstrates the close proximity of the terminal viologens. Dynamic light scattering (DLS) measurements indicate that the oxidized viologenated-dendrimers are predominately unaggregated in aqueous electrolyte solutions. Diffusion coefficients determined by chronoamperometry for the three generations of viologenated PAMAM dendrimer are within 15% of values calculated using the Stokes-Einstein relationship for individual unaggregated dendrimers. Reversible voltammetry was obtained for all three generations of dendrimers for the first viologen reduction wave. The magnitude of the peak or steady-state currents indicated incomplete electrolysis of the dendrimer viologen moieties, whereas the presence of a single wave showed that all electrochemically addressable groups were equivalent. Cycling through the second reduction wave resulted in electrode passivation due to irreversible electroprecipitation. Adsorption of a stable film of oxidized viologenated-dendrimer on a Au electrode is also demonstrated by cyclic voltammetry.

No abstract