5 research outputs found

    Covalent Modification of Glassy Carbon Surfaces by Electrochemical Grafting of Aryl Iodides

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    The reduction of an aryl iodide is generally believed to involve a clean-cut two-electron reduction to produce an aryl anion and iodide. This is in contradiction to what is observed if a highly efficient grafting agent, such as an aryldiazonium salt, is employed. The difference in behavior is explained by the much more extreme potentials required for reducing an aryl iodide, which facilitates the further reduction of the aryl radical formed as an intermediate. However, in this study we disclose that electrografting of aryl iodides is indeed possible upon extended voltammetric cycling. This implies that even if the number of aryl radicals left unreduced at the electrode surface is exceedingly small, a functionalization of the surface may still be promoted. In fact, the grafting efficiency is found to increase during the grafting process, which may be explained by the inhibiting effect the growing film exerts on the competing reduction of the aryl radical. The slow buildup of the organic film results in a well-ordered structure as shown by the well-defined electrochemical response from a grafted film containing ferrocenyl­methyl groups. Hence, the reduction of aryl iodides allows a precisely controlled, albeit slow, growth of thin organic films

    On Electrogenerated Acid-Facilitated Electrografting of Aryltriazenes to Create Well-Defined Aryl-Tethered Films

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    The mechanism of electrogenerated acid-facilitated electrografting (EGAFE) of the aryltriazene, 4-(3,3-dimethyltriaz-1-enyl)¬≠benzyl-1-ferrocene carboxylate, was studied in detail using electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry. The measurements support the previously suggested mechanism that electrochemical oxidation of the EGA agent (i.e., <i>N,N‚Ä≤</i>‚Äďdiphenylhydrazine) occurs on the forward oxidative sweep to generate protons, which in turn protonate the aryltriazene to form the corresponding aryldiazonium salt close to the electrode surface. On the reverse sweep, the electrochemical reduction of the aryldiazonium salt takes place, resulting in the electrografting of aryl groups. The EGAFE-generated film consists of a densely packed layer of ferrocenyl groups with nearly ideal electrochemical properties. The uncharged grafted film contains no solvent and electrolyte, but counterions and solvent can easily enter and be accommodated in the film upon charging. It is shown that all ferrocene moieties present in the multilayered film are electrochemically active, suggesting that the carbon skeleton possesses a sufficiently high flexibility to allow the occurrence of fast electron transfers between the randomly located redox stations. In comparison, EQCM measurements on aryldiazonium-grafted films reveal that they have a substantially smaller electrolyte uptake during charging and that they contain only 50% electroactive ferrocenyl groups relative to weight. Hence, half of these films consist of entrapped supporting electrolyte/solvent and/or simply electrochemically inactive material due to solvent inaccessibility

    Surface-Attached Poly(glycidyl methacrylate) as a Versatile Platform for Creating Dual-Functional Polymer Brushes

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    Novel types of dual-functional surface-attached polymer brushes were developed by post-polymerization modification of poly¬≠(glycidyl methacrylate) brushes on glassy carbon substrates. Azide and alcohol groups were initially introduced by epoxide ring-openings of the side chains. These polymer brushes represent an attractive chemical platform to deliberately introduce other molecular units at specific sites. In this work, ferrocene and nitrobenzene redox units were immobilized through the two groups to create redox polymers. In-depth analysis by infrared reflection‚Äďabsorption spectroscopy and X-ray photoelectron spectroscopy revealed an almost quantitative conversion of the modification reactions. The electrochemical activity of the ferrocenyl part of this diode-like system was fully expressed with an electron transfer rate constant = 1.2 s<sup>‚Äď1</sup> and surface density = 0.19 nmol cm<sup>‚Äď2</sup> per nm section of the film, independent of its thickness. In contrast, for the nitrobenzene moieties diffusion of counterions (i.e., tetraalkylammonium) easily becomes the rate-controlling step, thereby leaving a substantial fraction of them electrochemically inactive

    Functionalizing Arrays of Transferred Monolayer Graphene on Insulating Surfaces by Bipolar Electrochemistry

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    Development of versatile methods for graphene functionalization is necessary before use in applications such as composites or as catalyst support. In this study, bipolar electrochemistry is used as a wireless functionalization method to graft 4-bromobenzenediazonium on large (10 √ó 10 mm<sup>2</sup>) monolayer graphene sheets supported on SiO<sub>2</sub>. Using this technique, transferred graphene can be electrochemically functionalized without the need of a metal support or the deposition of physical contacts. X-ray photoelectron spectroscopy and Raman spectroscopy are used to map the chemical changes and modifications of graphene across the individual sheets. Interestingly, the defect density is similar between samples, independent of driving potential, whereas the grafting density is increased upon increasing the driving potential. It is observed that the 2D nature of the electrode influences the electrochemistry and stability of the electrode compared to conventional electrografting using a three-electrode setup. On one side, the graphene will be blocked by the attached organic film, but the conductivity is also altered upon functionalization, which makes the graphene electrode different from a normal metal electrode. Furthermore, it is shown that it is possible to simultaneously modify an array of many small graphene electrodes (1 √ó 1 mm<sup>2</sup>) on SiO<sub>2</sub>

    Controlled Electrochemical Carboxylation of Graphene To Create a Versatile Chemical Platform for Further Functionalization

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    An electrochemical approach is introduced for the versatile carboxylation of multi-layered graphene in 0.1 M Bu<sub>4</sub>NBF<sub>4</sub>/MeCN. First, the graphene substrate (i.e., graphene chemically vapor-deposited on Ni) is negatively charged at ‚ąí1.9 V versus Ag/AgI in a degassed solution to allow for intercalation of Bu<sub>4</sub>N<sup>+</sup> and, thereby, separation of the individual graphene sheets. In the next step, the strongly activated and nucleophilic graphene is allowed to react with added carbon dioxide in an addition reaction, introducing carboxylate groups stabilized by Bu<sub>4</sub>N<sup>+</sup> already present. This procedure may be carried out repetitively to further enhance the carboxylation degree under controlled conditions. Encouragingly, the same degree of control is even attainable, if the intercalation and carboxylation is carried out simultaneously in a one-step procedure, consisting of simply electrolyzing in a CO<sub>2</sub>-saturated solution at the graphene electrode for a given time. The same functionalization degree is obtained for all multi-layered regions, independent of the number of graphene sheets, which is due to the fact that the entire graphene structure is opened in response to the intercalation of Bu<sub>4</sub>N<sup>+</sup>. Hence, this electrochemical method offers a versatile procedure to make all graphene sheets in a multi-layered but expanded structure accessible for functionalization. On a more general level, this approach will provide a versatile way of forming new hybrid materials based on intimate bond coupling to graphene via carboxylate groups
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