K. V. Mironovich

Carbon nanowalls: the next step for physical manifestation of the black body coating

The optical properties of carbon nanowall (CNW) films in the visible range have been studied and reported for the first time. Depending on the film structure, ultra-low total reflectance up to 0.13% can be reached, which makes the CNW films a promising candidate for the black body-like coating, and thus for a wide range of applications as a light absorber. We have estimated important trends in the optical property variation from sample to sample, and identified the presence of edge states and domain boundaries in carbon nanowalls as well as the film mass density variation as the key factors. Also we demonstrated that at much lower film thickness and density than for a carbon nanotube forest the CNWs yield one order higher specific light absorption.

Oxygen Reduction by Lithiated Graphene and Graphene-Based Materials

Oxygen reduction reaction (ORR) plays a key role in lithium-air batteries (LABs) that attract great attention thanks to their high theoretical specific energy several times exceeding that of lithium-ion batteries. Because of their high surface area, high electric conductivity, and low specific weight, various carbons are often materials of choice for applications as the LAB cathode. Unfortunately, the possibility of practical application of such batteries is still under question as the sustainable operation of LABs with carbon cathodes is not demonstrated yet and the cyclability is quite poor, which is usually associated with oxygen reduced species side reactions. However, the mechanisms of carbon reactivity toward these species are still unclear. Here, we report a direct in situ X-ray photoelectron spectroscopy study of oxygen reduction by lithiated graphene and graphene-based materials. Although lithium peroxide (Li2O2) and lithium oxide (Li2O) reactions with carbon are thermodynamically favorable, neither of them was found to react even at elevated temperatures. As lithium superoxide is not stable at room temperature, potassium superoxide (KO2) prepared in situ was used instead to test the reactivity of graphene with superoxide species. In contrast to Li2O2 and Li2O, KO2 was demonstrated to be strongly reactive.

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.

Ag on carbon nanowalls mesostructures for SERS

We have elaborated substrates for surface enhanced Raman scattering (SERS) based on carbon nanowalls (CNW) deposited with Ag nanoparticles and thin Ag films. For carbon nanowalls deposited with silver nanoparticles, the achieved analytical enhancement factor SERS was from 50 to 2500. Much higher analytical enhancement factor of SERS, up to 5×104, was obtained for carbon nanowalls deposited with thin Ag film. In this case the SERS signal is determined by fractal structure of carbon nanowalls covered by Ag films. Such fractal structure provides a strong inhomogeneous localization of light, formation of a large number of hot spots and leads as a result to significant enhancement of SERS signal.

Simulation and optical spectroscopy of a DC discharge in a CH4/H2/N2 mixture during deposition of nanostructured carbon films

Two-dimensional numerical simulations of a dc discharge in a CH4/H2/N2 mixture in the regime of deposition of nanostructured carbon films are carried out with account of the cathode electron beam effects. The distributions of the gas temperature and species number densities are calculated, and the main plasmachemical kinetic processes governing the distribution of methyl radicals above the substrate are analyzed. It is shown that the number density of methyl radicals above the substrate is several orders of magnitude higher than the number densities of other hydrocarbon radicals, which indicates that the former play a dominant role in the growth of nanostructured carbon films. The model is verified by comparing the measured optical emission profiles of the H(n ≡ 3), C2*, CH*, and CN* species and the calculated number densities of excited species, as well as the measured and calculated values of the discharge voltage and heat fluxes onto the electrodes and reactor walls. The key role of ion–electron recombination and dissociative excitation of H2, C2H2, CH4, and HCN molecules in the generation of emitting species (first of all, in the cold regions adjacent to the electrodes) is revealed.

Etching of carbon nanowalls during synthesis in the plasma of direct current discharge

Anisotropic etching of carbon nanowalls by hydrogen during synthesis in plasma discharge of direct current is considered. This effect brings about generation of defects in the bottom part of the side surface of the nanowalls during their vertical growth. Based on the theoretical model of the discharge, it is shown that a decrease in the intensity of such etching is accompanied by an increase in the concentrations of such hydrocarbon radicals as C, CH, CH2, C2H, C3, and C3H, which indicates their possible role in the so-called process of healing of vacancies in the structure of nanowalls. In addition, it has been shown that an increase in synthesis temperature also can contribute to a decrease in the etching intensity.

Gaining cycling stability of Si- and Ge-based negative Li-ion high areal capacity electrodes by using carbon nanowall scaffolds

We report an approach to stabilize the electrochemical performance of silicon- and germanium-based thin film anodes by using carbon nanowall matrices. Silicon and germanium layers were deposited onto vertically oriented carbon nanowall scaffolds and this procedure has been repeated multiple times producing multilayered structures with increased silicon and germanium areal mass loading. It was demonstrated that the areal specific capacity of multilayered anodes achieves up to 2 mA h cm−2 without sacrificing cycling stability. Based on post-mortem SEM analysis of the electrodes we speculate that the reason for the improved cycling stability of multilayered highly loaded silicon/graphene composites is the ability to relax the mechanical stresses in the films.

Synthesis of nanostructured zinc oxide films in propane flow

A new method is proposed for the synthesis of nanostructured zinc oxide (ZnO) films in propane that is used as a source of reactive carbon and carrier gas. The growth of films consisting of columnar ZnO structures under these conditions takes place at a relatively low temperature (about 630°C) of the evaporator.