Controlled Electrochemical Synthesis of the Partially Unzipped Multi-Wall Carbon Nanotubes
Authors: Danilov Michail Olegovich , Rusetskii Igor A , Dovbeshko Galina I , Kolbasov Gennadii Ya , Tang Jianguo 
Published: February 2020

Graphene is a two-dimensional monolayer of sp2-hybridized carbon atoms with a cellular structure. By the chemical methods, graphene is obtained in the form of a mixture of mono-, bi- and multilayer (3-10 monolayers) of irregular structured flakes or flat sheets. Multi-walled carbon nanotubes (MWCNT) are several graphene layers rolled into a tube. Using any oxidative effects, you can unzip such MWCNT. Currently, there are the following methods for obtaining graphene from MWCNT [1]: redox reaction, modification via incorporation of nitrogen and alkaline earth atoms; plasma sputtering; unzipping by microwave radiation, ultrasonic and electrical treatment, treatment with metallic catalytic particles, tip hydrogenation in high-temperature conditions, as well as unrolling by electrochemical methods. One of the promising technological methods is the electrochemical method of unzipping nanotubes. Changing the voltage, current and time, it is possible to control the process of unzipping.
The aim of our work was to check the possibility of controlled unzipping of MWCNT under the influence of electrochemical anodic oxidation. As a result we obtain etched carbon nanotubes, partially unzipped, fully unzipped and nanocomposite fragments of graphene nano-plates. Due to its properties, graphene materials are used as catalysts carriers for electrodes of chemical power sources. The study of the electrocatalytic properties of such materials as an oxygen electrode can indirectly confirm the degree of unzipping of MWCNT, since the catalytic properties depend on the structure and specific surface of the electrode material.
MWCNT were subjected to electrochemical anodic oxidation. The outer diameter of the MWCNT was ~ 10–30 nm, the specific surface area was 230 m2 / g. The bulk density is 25–30 g / dm3, the number of walls is from 8 to 15.
Samples of unzipped nanotubes were examined using electron microscope, X-ray phase analysis, Raman spectra. From the obtained materials, by pressing, two-layer oxygen electrodes were made. Studies were carried out in the mock up of the fuel cell. The zinc electrode was used as anode. The dependence of the electrochemical characteristics of oxygen electrodes on the time of the oxidation of MWCNTs was investigated. Anodic oxidation of MWCNTs was carried out in concentrated sulfuric acid at a potential of +3 V. Oxidation was carried out for 0.5, 1.5, 4, and 5 hours. It was found that with an anodic oxidation time increasing to 4 hours, the electrochemical characteristics of oxygen electrodes (current density at one and the same potential) increase. With further increase in oxidation time (≥ 5 hours), the characteristics do not change much. This is due to the production of etched carbon nanotubes (Figure 1a), with a further increase in the oxidation time (≤ 1.5 hours), partially unzipped nanotubes are obtained (Figure 1b). Next, we obtain fully unzipped nanotubes (Fig. 1c) and fragments of graphene nanoplates (Fig. 1d). We carried out the oxidation of MWCNTs for 4 h at various potentials (from 1.8 V to 4.5 V). In this case, a similar dependency was also found. With increasing potential up to 3 V, an increase in the electrochemical characteristics of the electrodes was observed. As the potential increased to 4.5 V, the electrode characteristics deteriorated. This dependence confirms the fact that the gradual unzipping of the nanotubes, depended on the applied oxidative effects. By controlling the treatment process, we obtained unzipped nanotubes with a controlled level of unzipping, a modified surface, the number of defects and functional groups on the surface.
That is, Raman spectra, confirm that the anodic oxidation of MWCNTs results in partially unzipped nanotubes, fully unzipped nanotubes and fragments of graphene nanoplates. These materials are obtained by changing the oxidation time or the applied potential. The material contains all marker bands characterizing graphite structures, namely, D-, G- and 2D-bands. The position, intensity, and half-width of the bands are a reflection of the changes that occur in the samples as a result of the applied effects. Oxygen electrodes, based on electrochemically obtained samples, were stable in galvanostatic mode at current densities of 200 mA / cm2 for the six months. Thus, by oxidizing multi-walled carbon nanotubes at different anodic potentials, as well as by changing the processing time, we obtain controlled unzipping of MWCNT. The resulting materials are the good catalyst carrier for electrodes of chemical power sources. By applying various catalysts to them, it is possible to obtain hybrid nanocomposites with predetermined characteristics.

O. Danilov, I.A. Slobodyanyuk, I.A. Rusetskii, et al.Graphene Science Handbook. Fabrication Methods, (Eds. M. Aliofkhazraei, et al.), CRC Press, 2016, pp. 205.

Figure 1