Electrochemical formation of an ultrathin electroactive film from 1,10-phenanthroline on a glassy carbon electrode in acidic electrolyte.

The electrochemical reduction of 1,10-phenanthroline in aqueous acidic electrolyte at a glassy carbon electrode led to the covalent modification of the electrode. Thereafter, the deposited film can be switched to an electroactive form by electrochemical oxidation. An electroactive film can be also generated by alternate reductive and oxidative voltammetric cycling in a 1,10-phenanthroline/aqueous sulfuric acid solution. First, the electrochemical procedure for the formation of a film is presented. Second, the morphology and chemical structure of 1,10-phenanthroline coatings were investigated by atomic force microscopy, time-of-flight secondary ion mass spectrometry, X-ray photoelectron spectroscopy, and electrochemical techniques. The ultrathin (<15 nm) electrodeposited films consist of oligomeric species. The coatings deposited in alternate and/or continuous reductive and oxidative steps contain oxygen atoms incorporated into the oligomer backbone. The preliminary results point out the formation of a dione derivative that is responsible for the electroactivity of the grafted layer.

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.

Improvement of the platinum nanoparticles-carbon substrate interaction by insertion of a thiophenol molecular bridge.

The effect of thiophenol layer grafted on carbon for platinum catalyst stabilization was studied. The grafted layer was prepared by reduction of 4-thiophenoldiazonium ions in the presence of Vulcan XC72 substrate. The grafted layer was characterized by elemental analysis, thermogravimetric analysis coupled with mass spectrometry, and X-ray photoelectron spectroscopy. Platinum nanoparticles prepared by the "water in oil" microemulsion method were then deposited on modified substrates and bare Vulcan XC72. The platinum stability improvement was characterized by in situ X-ray diffraction and electrochemical aging. These experiments enabled to evidence a lower crystallite growth during heat treatment under hydrogen atmosphere and a lower active surface area loss for platinum particles deposited on modified substrates compared to those deposited on bare Vulcan XC72. This stability improvement can be attributed to a better interaction between platinum particles and carbon substrate due to the thiophenol molecular bridge.