We describe a facile method for the formation of dynamic nanostructured surfaces based on the modification of porous anodic aluminum oxide with poly(N-isopropyl acrylamide) (PNIPAAm) via surface-initiated atom transfer radical polymerization. The dynamic structure of these surfaces was investigated by atomic force microscopy (AFM), which showed dramatic changes in the surface nanostructure above and below the aqueous lower critical solution temperature of PNIPAAm. These changes in surface structure are correlated with changes in the macroscopic wettability of the surfaces, which was probed by water contact angle measurements. Principal component analysis was used to develop a quantitative correlation between AFM image intensity histograms and macroscopic wettability. Such correlations and dynamic nanostructured surfaces may have a variety of uses.
Absolute values of the current density at microscopic electroactive sites on polycrystalline Ti electrodes have
been measured with a scanning electrochemical microscope (SECM). The active sites on the oxide-film-coated Ti electrode have radii ranging from 3.3 to 28.6 μm and are randomly distributed at a surface density
of ∼180 sites/cm2. Localized current densities as large as 0.14 A/cm2 are observed for the oxidation of Br-
in aqueous solution, approximately 4 orders of magnitude larger than the average current density based on
the geometrical area of the electrode. Quantitative analysis of large-area SECM images demonstrates that at
least ∼65% of the total current during Br- oxidation is associated with sites that occupy a minuscule fraction
(0.01−0.1%) of the total exposed electrode area. The SECM data also suggest that the rate of Br- oxidation
approaches the diffusion-limited value at sites having radii less than ∼10 μm; a precipitous decrease in the
electron-transfer rate is observed at electroactive sites of slightly larger dimensions.
Scanning electrochemical microscopy (SECM) has been used to study the oxidation of iodide at Ta electrodes covered by a thin (∼2.5 nm) film of Ta(2)O(5). SECM images of surface activity reveal that the voltammetric response of a macroscopic Ta electrode comprises the individual responses of a large number of microscopic sites, each with its own unique electrochemical behavior. Oxide film growth and metal dissolution occur simultaneously with iodide oxidation, resulting in a complex voltammetric response. The component of the voltammetric current due to iodide oxidation can be separated from the total current by SECM analysis. The growth of nanometer-thick oxide films can also be studied using SECM by monitoring the rate at which iodide is oxidized at the electrode surface.
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