Modification of carbon substrates by aryl and alkynyl iodonium salt reduction

Different carbon materials were modified using iodonium ion reduction creating radicals, which after reaction with carbon surfaces formed grafted layers of molecules. Several molecules (4-bromophenyl, 4-fluorophenyl, 6-chlorohexyne, and 4-bromobutyne) were grafted on glassy carbon and Vulcan XC72 carbon substrates. Carbon substrates were shown to be free of halogen atoms; therefore, the quantification of the grafted groups containing halogen atoms was facilitated. The grafting of the different molecules was first electrochemically studied on glassy carbon electrodes using cyclic voltammetry, in order to determine the reduction potential of the corresponding iodonium ions. Voltammetric study using Fe(CN)(6)(4-) and Fe(CN)(6)(3-) probe molecules and XPS characterization were also used to evidence the effectiveness of grafting from iodonium ion reduction reaction. Reduction potentials were found in the range from -0.9 V vs SCE to -1.0 V vs SCE, lower than those for corresponding diazonium ion reduction reaction on glassy carbon (close to -0.3 V vs SCE). Therefore, grafted layers from iodonium ions were carried out on carbon Vulcan XC72 powder using NaBH(4) as reducing agent. Functionalized carbon powders were characterized by elemental analysis, thermogravimetric analysis, and X-ray photoelectron spectroscopy to evidence the presence of grafted molecules on the materials. However, low grafting yields were obtained. Then, several synthesis parameters were studied to optimize the grafting reactions, such as the control of the addition of reactants and their concentrations, leading to increase the surface concentration by a factor 2. At last, according to XPS measurements the grafting of alkinyliodonium ions led to very low surface concentrations (0.5 wt % for 6-chlorohexyne), whereas elemental analysis and TGA indicate ca. 2.4 wt % and ca. 5 wt %, respectively.