Line Koefoed

One-step preparation of bifunctionalized surfaces by bipolar electrografting

Bipolar electrochemistry (BPE) is widely used to trigger electrochemical reactions on conducting objects without direct electrical wiring. In this study a novel methodology is reported, which for the first time allows simultaneous deposition of two different organic films at each end of a glassy carbon substrate (1 × 1 cm2). The approach is based on the use of an organic bifunctional molecule, which may be oxidatively and reductively electrografted at the same time. The reduction process goes through the diazonium group, while the oxidation proceeds via the primary amine. The double functionalized plates are investigated by ellipsometry, cyclic voltammetry, condensation imaging, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. Post-modification of one of the anchoring layers illustrates the versatility of the system, pointing to its potential use in fields going from molecular electronics to targeted drug delivery.

Facile electrochemical transfer of large-area single crystal epitaxial graphene from Ir(1 1 1)

High-quality growth of graphene and subsequent reliable transfer to insulating substrates are needed for various technological applications, such as flexible screens and high speed electronics. In this paper, we present a new electrochemical method for the transfer of large-area, high-quality single crystalline graphene from Ir(1 1 1) to Si/SiO2 under ambient conditions. The method is based on intercalation of tetraoctylammonium ions between the graphene layer and the Ir surface. This simple technique allows transfer of graphene single crystals having the same size as the substrate they are grown on (diameter ≈7 mm). In addition, the substrate can be reused for further growth cycles. A detailed Raman map analysis of the transferred graphene reveals straight lines, in which the Raman peaks characteristic for graphene are shifted. These lines originate from scratches in the Ir(1 1 1) crystal introduced by the polishing procedure. Furthermore, areas with numerous wrinkles exist inbetween these lines, forming a network across the entire graphene crystal. Hence, the initial characteristics and imprints left on the sheet of graphene in terms of strain and wrinkles from the growth process remain after transfer.

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

An electrochemical approach is introduced for the versatile carboxylation of multi-layered graphene in 0.1 M Bu4NBF4/MeCN. First, the graphene substrate (i.e., graphene chemically vapor-deposited on Ni) is negatively charged at -1.9 V versus Ag/AgI in a degassed solution to allow for intercalation of Bu4N(+) 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 Bu4N(+) 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 CO2-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 Bu4N(+). 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.

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

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.

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

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 × 10 mm(2)) monolayer graphene sheets supported on SiO2. 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 × 1 mm(2)) on SiO2.