Stanislav A. Moshkalev

Self-Assembled and One-Step Synthesis of Interconnected 3D Network of Fe3O4/Reduced Graphene Oxide Nanosheets Hybrid for High-Performance Supercapacitor Electrode

In the present work, we have synthesized three-dimensional (3D) reduced graphene oxide nanosheets (rGO NSs) containing iron oxide nanoparticles (Fe3O4 NPs) hybrids (3D Fe3O4/rGO) by one-pot microwave approach. Structural and morphological studies reveal that the as-synthesized Fe3O4/rGO hybrids were composed of faceted Fe3O4 NPs induced into the interconnected network of rGO NSs. The morphologies and structures of the 3D hybrids have been characterized using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectrometer (XPS). The electrochemical studies were analyzed by cyclic voltammetry, galvanostatic charge/discharge measurements, and electrochemical impedance spectroscopy, which demonstrate superior electrochemical performance as supercapacitors electrode application. The specific capacitances of 3D hybrid materials was 455 F g-1 at the scan rate of 8 mV s-1, which is superior to that of bare Fe3O4 NPs. Additionally, the 3D hybrid shows good cycling stability with a retention ratio of 91.4 after starting from ∼190 cycles up to 9600 cycles. These attractive results suggest that this 3D Fe3O4/rGO hybrid shows better performance as an electrode material for high-performance supercapacitors.

Plasma power measurement and hysteresis in the E-H transition of a rf inductively coupled plasma system

An experimental investigation of the argon plasma behavior near the E–H transition in an inductively coupled Gaseous Electronics Conference reference cell is reported. Electron density and temperature, ion density, argon metastable density, and optical emission measurements have been made as function of input power and gas pressure. When plotted versus plasma power, applied power corrected for coil and hardware losses, no hysteresis is observed in the measured plasma parameter dependence at the E–H mode transition. This suggests that hysteresis in the E–H mode transition is due to ignoring inherent power loss, primarily in the matching system.

Microwave-assisted synthesis and deposition of a thin ZnO layer on microwave-exfoliated graphene: optical and electrochemical evaluations

A rapid and facile microwave-assisted method has been developed for the deposition of a zinc oxide layer (ZnOL) in situ on partially microwave exfoliated graphene (MpEG). The formation of the ZnO layer on microwave partially exfoliated graphene (ZnOL@MpEG) hybrid only requires zinc nitrate and the MpEG sheets are reacted under a low level of microwave irradiation (700 W) for 5 min. The deposited thin ZnOL on the MpEG sheets is uniformly well-dispersed and covers the whole MpEG sheets. The as-prepared ZnOL@MpEG hybrids demonstrate enhanced electrochemical properties as supercapacitors and also show quenching phenomena for photoluminescence. The fluorescence quenching observed for the ZnOL@MpEG hybrids compared to ZnO, indicates photoinduced electron transfer from ZnO to the MpEG layers, which shows recombination of hole and electron charge carriers. The electrochemical measurements exhibit that the ZnOL@MpEG hybrids have a large integral area of the cyclic voltammetry loop, indicating that such hybrids are promising for application in supercapacitors. The material displayed a high specific capacitance of 347 F g−1 at a current density of 5.0 A g−1. A mechanism for the formation of the ZnOL@MpEG hybrids via the microwave method has also been proposed.

Mode transitions and hysteresis in inductively coupled plasmas

Optical emission spectroscopy as a noninvasive plasma diagnostic was employed to study mode transitions and hysteresis in an inductively coupled plasma in Ar and Ar∕N2 mixtures. Using selected Ar lines, basic plasma parameters, relevant to the analysis of the mode transitions, were evaluated. Small changes of the electron energy distribution function in the vicinity of the mode transition were detected. The role of metastable Ar atoms in mode transitions and in a hysteresis was clarified. Enhanced production of metastables in the hysteresis region as well as faster transitions in plasmas with higher influence of metastables were observed.Optical emission spectroscopy as a noninvasive plasma diagnostic was employed to study mode transitions and hysteresis in an inductively coupled plasma in Ar and Ar∕N2 mixtures. Using selected Ar lines, basic plasma parameters, relevant to the analysis of the mode transitions, were evaluated. Small changes of the electron energy distribution function in the vicinity of the mode transition were detected. The role of metastable Ar atoms in mode transitions and in a hysteresis was clarified. Enhanced production of metastables in the hysteresis region as well as faster transitions in plasmas with higher influence of metastables were observed.

Fabrication of interdigitated micro-supercapacitor devices by direct laser writing onto ultra-thin, flexible and free-standing graphite oxide films

In this work we present graphene-based in-plane flexible interdigitated micro-supercapacitor devices fabricated through direct laser writing onto ultra-thin graphite oxide (GO) films. The fabrication route is simple, fast, additive-free, mask-free and cost effective. This involves direct micro-writing of reduced graphene oxide (rGO) by a pulsed UV laser on a very small area (1.14 cm2). The fabricated micro-supercapacitor contains nineteen pairs of rGO electrodes separated by the unreduced portion of the GO film. The single laser patterned rGO electrode presents low resistivity, while the unpatterned portion is non-conducting. Under the optimized laser parameters the 2.2 μm ultra-thin GO films were completely and uniformly reduced. The electrochemical measurements showed that the micro-supercapacitor, packed in a glass cavity, and in the presence of a liquid electrolyte have a capacitance nearly 288% higher (288.7 mF cm−3) compared to the as-fabricated device (0.36 mF cm−3). The as-fabricated micro-supercapacitor without electrolyte also shows some capacitance due to the presence of free ions in the unreduced portion of GO which plays a crucial role. Furthermore, the cycling stability of the as-fabricated micro-supercapacitor is robust, with not much performance degradation for more that 5000 cycles.

Low contact resistivity and strain in suspended multilayer graphene

Method to prepare suspended multilayer graphene (MLG) flakes and to form highly conductive (contact resistivity of ∼0.1 kΩ μm2) and tight mechanical connection between MLG and metal electrodes is described. MLG flakes prepared from natural graphite were precisely deposited over tungsten electrodes using dielectrophoresis, followed by high-temperature thermal annealing in high-vacuum. Considerable strain induced in the suspended part of flakes was revealed by Raman imaging.

Nucleation and growth of carbon nanotubes in catalytic chemical vapor deposition

The process of nucleation of multiwall carbon nanotubes in chemical vapor deposition process with nickel as catalyst and methane as a carbon precursor is analyzed. The nucleation is considered as a specific instability developed on the surface of a metal catalyst particle supersaturated with carbon. The energy released in graphitization of carbon from the metal-carbon solution is shown to be crucial for the nanotube nucleation. The energy released may be high enough for substantial metal heating resulting in partial liquefaction of the catalyst particle. The proposed mechanism can be called vapor-solid-liquid-solid (VSLS) as the catalyst particle may be in a mixed solid-liquid (or liquidlike) state during nucleation and unstable phases of nanotube growth.

Highly sensitive and selective electrochemical dopamine sensing properties of multilayer graphene nanobelts

In the present study, we report the electrochemical sensing property of multi-layer graphene nanobelts (GNBs) towards dopamine (DA). GNBs are synthesized from natural graphite and characterized by using techniques like field-emission scanning electron microscopy, atomic force microscopy and Raman spectroscopy. An electrochemical sensor based on GNBs is developed for the detection of DA. From the cyclic voltammetry and amperometry studies, it is found that GNBs possess excellent electrocatalytic activity towards DA molecules. The developed DA sensor showed a sensitivity value of 0.95 μA μM(-1) cm(-2) with a linear range of 2 μM to 0.2 mM. The interference data exhibited that GNB is highly selective to DA even in the presence of common interfering species like ascorbic acid, uric acid, glucose and lactic acid.

Supercapacitors based on patronite–reduced graphene oxide hybrids: experimental and theoretical insights

Here we report the hydrothermal synthesis and detailed study on supercapacitor applications of a patronite hybrid, VS4/reduced graphene oxide, which showed an enhanced specific capacitance of ∼877 F g−1 at a current density of 0.5 A g−1. In comparison to bare vanadium sulfide and reduced graphene oxide, the hybrid showed ∼6 times and ∼5 times higher value of specific capacitance, respectively. The obtained energy density (117 W h kg−1) and power density (20.65 kW kg−1) are comparable to those of other reported transition metal sulfides and their graphene hybrids. Theoretical calculations using density functional theory confirm an enhanced quantum capacitance of VS4/graphene composite systems, owing primarily to the shifting of the graphene Dirac cone relative to the band gap of VS4. The results infer that the hybrid has the potential to be used as a high performance supercapacitor electrode.

Nonlocal laser annealing to improve thermal contacts between multi-layer graphene and metals

The accuracy of thermal conductivity measurements by the micro-Raman technique for suspended multi-layer graphene flakes has been shown to depend critically on the quality of the thermal contacts between the flakes and the metal electrodes used as the heat sink. The quality of the contacts can be improved by nonlocal laser annealing at increased power. The improvement of the thermal contacts to initially rough metal electrodes is attributed to local melting of the metal surface under laser heating, and increased area of real metal-graphene contact. Improvement of the thermal contacts between multi-layer graphene and a silicon oxide surface was also observed, with more efficient heat transfer from graphene as compared with the graphene-metal case.