Search

The formation of partial perfluoroalkyl or alkyl radicals from partial perfluoroalkyl or alkyl iodides (ICH2CH2C6F13 and IC6H13) and their reaction with surfaces takes place at low driving force (∼-0.5 V/SCE) when the electrochemical reaction is performed in acetonitrile in the presence of diazonium salts (ArN2(+)), at a potential where the latter is reduced. By comparison to the direct grafting of ICH2CH2C6F13, this corresponds to a gain of ∼2.1 V in the case of 4-nitrobenzenediazonium. Such electrochemical reaction permits the modification of gold surfaces (and also carbon, iron, and copper) with mixed aryl-alkyl groups (Ar = 3-CH3-C6H4, 4-NO2-C6H4, and 4-Br-C6H4, R = C6H13 or (CH2)2-C6F13). These strongly bonded mixed layers are characterized by IRRAS, XPS, ToF-SIMS, ellipsometry, water contact angles, and cyclic voltammetry. The relative proportions of grafted aryl and alkyl groups can be varied along with the relative concentrations of diazonium and iodide components in the grafting solution. The formation of the films is assigned to the reaction of aryl and alkyl radicals on the surface and on the first grafted layer. The former is obtained from the electrochemical reduction of the diazonium salt; the latter results from the abstraction of an iodine atom by the aryl radical. The mechanism involved in the growth of the film provides an example of complex surface radical chemistry.

The scanning electrochemical microscope (SECM) in the lithographic mode is used to assess quantitatively, from both theoretical and experimental points of view, the kinetics of irreversible transformation of electroactive molecular moieties immobilized on a surface as self-assembled monolayers (SAMs). The SECM tip allows the generation of an etchant that transforms the surface locally and irreversibly. The resulting surface patterning is detectable by different surface analyses. The quantification of the surface transformation kinetics is deduced from the evolution of the pattern dimensions with the etching time. The special case of slow etching kinetics is presented; it is predicted that the pattern evolution follows the expansion of the etchant at the substrate surface. The case of a chemically unstable etchant is considered. The model is then tested by inspecting the slow reductive patterning of a perfluorinated SAM. Good agreement is found with different independent SECM interrogation modes, depending on the insulating or conducting nature of the covered substrate. The surface transformation measurements are also compared to the reduction of solutions of perfluoroalkanes. The three-orders-of-magnitude-slower electron transfer observed at the immobilized molecules likely describes the large reorganization associated with the generation of a perfluoroalkyl-centered radical anion.