Natal’ya S. Komarova

Enhancement of the Carbon Nanowall Film Capacitance. Electron Transfer Kinetics on Functionalized Surfaces

The effects of electrochemical oxidation and surfactant adsorption on behavior of vertically oriented carbon-nanowall (CNW)-based electrodes are studied. Electrochemical oxidation is carried out by the electrode polarization in aqueous solutions at high anodic potentials corresponding to water electrolysis, whereas the modification of surface by surfactants is accomplished by the adsorption of molecules characterized by the cage-like structure. Using the methods of cyclic voltammetry and impedancemetry, it is shown that a substantial increase in the capacitance of CNW-based electrodes is observed in both cases (30-50-fold and 3-5-fold, respectively). The as-grown and modified electrodes are characterized by scanning electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. A substantial increase in a number of oxygen-containing functional groups is observed on the CNW surface after the electrode polarization at high anodic potentials. The kinetics of redox reactions on the CNW film surface is studied by comparing the behavior of systems [Ru(NH3)6](2+/3+), [Fe(CN)6](4-/3-), Fe(2+/3+), and VO3(-)/VO(2+). It is demonstrated that oxidation of nanowalls makes the electron transfer in the redox reaction VO3(-)/VO(2+) and the redox system Fe(2+/3+) considerably easier due to coordination of discharging ions of these systems with the functional groups; however, no such effect is observed for the redox-systems [Fe(CN)6](3-/4-) and [Ru(NH3)6](2+/3+).

Electrochemical modification of electrodes based on highly oriented carbon nanowalls

The original and modified vertically oriented carbon nanowalls (CNWs) were applied onto conducting substrates by the plasma-chemical method. Their electrochemical behavior was studied by the methods of cyclic voltammetry and impedance measurements. The modified and original electrodes were characterized by using the methods of scanning and transmitting electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. The nanowalls were modified with the functional groups (FG) via the electrolysis of aqueous solutions at the anodic potentials. Their adsorption properties were studied in the solutions of organic surfactants with the skeleton structure. It is shown that, in the first case, the number of oxygen-containing FG on the CNW surface significantly increases and, in both cases, the electrode capacitance considerably increases (by 30–50 and 3–5 times, respectively). A correlation between the rate constants k0 of [Ru(NH3)6]2+/3+, [Fe(CN)6]4–/3–, and Fe2+/3+ redox reactions and a degree of nanowall surface functionalization is revealed. The values of k0 were estimated in the automatic mode using a specially developed program by comparing the potential differences between the peaks of cyclic voltammograms ΔE, which were measured in a wide range of potential scan rate v, and the calculated ΔE (k0, v) dependences, which were obtained by solving the corresponding diffusion equations. It is shown that the functionalization of CNWs leads to a substantial (by ~103 times) increase in k0 for the Fe2+/3+ redox system and has almost no effect on the electron transfer in the [Fe(CN)6]3–/4– and [Ru(NH3)6]2+/3+ systems.

Adsorption characteristics of electrodes containing nanostructured carbon of different morphology

The effect of camphor adsorption on the differential capacitance of electrodes of nanostructured carbon of different morphology (single-walled carbon nanotubes, filiform carbon, and columnar structures) in aqueous electrolyte solutions and also on the electrochemical reactions in these systems is studied. It is shown that irrespective of the ac frequency, the differential capacitance of the nanopaper and columnar electrodes increases 3–5-fold throughout the studied potential range. This experimental fact is explained by the substantial increase in the electrode surface accessible for electrolyte, which is a manifestation of the Rehbinder effect in electrochemistry. The revealed different kinds of effects of camphor adsorption layers formed at the nanostructured carbon/electrolyte interface on the electron transfer processes are as follows: partial inhibition of both the electron injection and the K3[Fe(CN)6] reduction; complete suppression of the reduction of sodium nitrate and nitrite; the absence of effects on the OH radical reduction and solvated electron oxidation.

Electrochemical Behavior of Electrodes Containing Nanostructured Carbon of Various Morphology in the Cathodic Region of Potentials

Voltammograms for electrodes containing nanostructured carbon of various morphology (single-walled carbon nanotubes, filament, columnar structures) are obtained in neutral aqueous electrolytic solutions. Experimental proofs for the existence of injection of solvated electrons into electrolytic solutions at moderate cathodic potentials are presented for all the electrodes. It is established that this effect is connected with the presence of atomically sharp areas on the electrode surfaces. It is assumed that the reason for the appearance of solvated electrons is the autoelectron emission at the interface between the conducting surface of the carbon material and the electrolytic solution. By studying the nitrate anion reduction it is shown that the reduction over-voltage of stable compounds may be lowered by substituting a fast homogeneous reaction of solvated electrons with the initial substance for the hindered heterogeneous stage of the first electron transfer.

Adsorption of surface-active compounds with the skeleton molecular structure from dimethylsulfoxide solutions on carbon nanotubes

The adsorption of adamantane, adamantanol, thiocamphor, and sodium cryptate on electrodes of single-walled carbon nanotubes (SWNT) from dimethylsulfoxide (DMSO) solutions is studied by measuring the differential capacitance (C) vs. potential (E) dependences and cyclic voltammograms. In the tested systems, the high surface activity of these surfactants is observed to result in a noticeable increase in the C of such electrodes in the range of 0.2 ≤ E ≤ −(0.9−1.1) V (SCE). As in the case of aqueous solutions, this experimental fact is explained by the appearance of the so-called Rehbinder effect (the adsorption-induced decrease in the strength), which, in this particular case, consists in a decrease in the surface energy of a solid with the formation of adsorption layers on the side surfaces of SWNTs combined into bundles to afford the partial splitting of these bundles and, as a consequence, the increase in the nanotube surface accessible to the electrolyte. At the same time, the obtained results suggest that for the adsorption of surfactants from nonaqueous solvents (in contrast to aqueous), the interaction between solvent and adsorbate molecules may become important.

Electron injection from nanostructured carbon electrodes at moderate cathodic potentials

Voltammograms of electrodes based on nanostructured carbon of different morphology (single-wall carbon nanotubes, carbon nanofilaments, columnar structures) are taken in hexamethylphosphortriamide solutions. For all listed electrodes, direct experimental proof of the electron injection to the electrolyte solution at moderate cathodic potentials is obtained. It is found that this phenomenon is associated with the existence of atomically sharp spots at the electrode surface, which operate as local cathodes for the electron emission. It is shown that the current-voltage characteristics for the electron injection to the electrolyte solution differ from those for the field electron emission current at the electrode/vacuum interface.

Electrochemical defluorination of carbon nanotubes

An electrochemical procedure for the removal of fluorine from the walls of fluorinated monolayer carbon nanotubes was suggested. According to element analysis data, fluorine was completely removed. According to UV-Vis-Nir and Raman spectroscopic studies, the initial structure of fluorinated carbon nanotubes was almost completely restored.