Vertically-oriented Graphene Nanosheets Research Articles

Effect of nitrogen doping on the structure and electrochemical properties of vertical graphene sheets prepared by HWP-CVD

3D nitrogen-doped vertically-oriented graphene nanosheets (3D N/VGs) were controllably prepared by helicon wave plasma chemical vapor deposition (HWP-CVD) employing argon, methane, and nitrogen by varying the N2 flow rate (RN2) without any catalyst and substrate heating during the growth. The effect of plasma gas phase species on the growth structure of N/VGs thin films was studied. The influence of nitrogen doping content on the structure and properties of VGs was analyzed. A growth model for the preparation of N/VGs by HWP-CVD was proposed. The results show that the elevated doping content of nitrogen atoms promotes enhanced CH4 dissociation and access to more H atoms, leading to an elevated nanosheet number density and a reduced growth rate. More excited N atoms could be embedded into the C vacancy to replace the missing C atom and restore the hexagonal network and increasing the nitrogen content. N/VGs with lower defect densities obtained with enhanced doping of nitrogen. The effective average distance between defects increases with nitrogen content, and the tunneling phenomenon is apparent, increasing resistance. Electrochemical performance tests showed that VGs with high nitrogen atomic content have higher specific capacitance and rate performance, up to 1340 μF/cm2 and 91.7%, respectively.

Scalable growth of vertically oriented graphene nanosheets with high rate by a high-flux mesoplasma chemical vapor deposition

The Vertically Oriented Graphene Nanosheets (VGNs) possesses many promising properties and attracts intense interests in various applications due to its unique nanoarchitecture and intrinsic properties of graphene. However, a scalable growth of VGNs film with high rate is still an open challenge that hampers both the scientific research and mass applications. Herein, we report for the first time a large area growth of VGNs film with high rate via a high-density and high-flux mesoplasma chemical vapor deposition by rotating the substrate. A uniform VGNs film on a large area with a high growth rate on the order of ten micrometer per minute has been obtained. A continues growth process of VGNs film with high rate is also predicted by sweeping the substrate, which gives a new route for the scalable growth of VGNs film with high rate for the future mass applications.

All Si3 N4 Nanowires Membrane Based High-Performance Flexible Solid-State Asymmetric Supercapacitor.

Recently, much attention has been drawn in the development of flexible energy storage devices due to the increasing demands for flexible/portable electronic devices with high energy density, low weight, and good flexibility. Herein, vertically oriented graphene nanosheets (VGNs) are in situ fabricated on the surface of free-standing and flexible Si3 N4 nanowires (NWs) membrane by plasma-enhanced chemical vapor deposition (PECVD), which are directly used as flexible nanoscale conductive substrates. NiCo2 O4 hollow nanospheres (HSs) and FeOOH amorphous nanorods (NRs) are finally prepared on Si3 N4NWs @VGNs, which are served as the positive and negative electrodes, respectively. Profiting from the structural merits, the synthesized Si3 N4NWs @VGNs@NiCo2 O4HSs and Si3 N4NWs @VGNs@FeOOHNRs membrane electrodes exhibit remarkable electrochemical performance. Using Si3 N4NWs membrane as the separator, the assembled all Si3 N4NWs membrane-based flexible solid-state asymmetric supercapacitor (ASC) with a wide operating potential window of 1.8 V yields the outstanding energy density of 96.3 Wh kg-1 , excellent cycling performance (91.7% after 6000 cycles), and good mechanical flexibility. More importantly, this work provides a rational design strategy for the preparation of flexible electrode materials and broadens the applications of Si3 N4NWs in the field of energy storage.

Fast preparation of vertical graphene nanosheets by helicon wave plasma chemical vapor deposition and its electrochemical performance

We investigated the growth process of vertically-oriented graphene nanosheets (VGs) on a Si substrate by helicon wave plasma chemical vapor deposition (HWP-CVD). The results show that VGs has good vertical orientation on Si and the growth process does not require the catalyst. Modulation of height, thickness and mesh size of vertical graphene nanosheets can be achieved by changing the growth time. It is found that the height of VGs is linear with time. Extremely high growth rate, approximately 0.26 μm/min, is achieved, in contrast to previous studies. The results of Cyclic voltammetry (CV) measurement indicating that VGs have a very rapid current–voltage response, an excellent electrochemical reversibility and excellent capacitive behavior. The VGs was able to deliver an outstanding specific capacitance value of 345 μF/cm2, and demonstrated an excellent rate capability, which retained 94.41%.

Characterization of vertically-oriented graphene nanosheets grown on copper substrate

Vertical Graphene Nanosheets (VGNs) can be described as graphite sheet nanostructures with edges that are composed of stacks of planar graphene sheets standing almost vertically on the substrate. The present study reports VGNs synthesized on the copper foil substrate under 5 mTorr gas pressure and CH4/Ar plasma in an Electron Cyclotron Resonance – Chemical Vapor Deposition (ECR-CVD) reactor. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) have been run to study the morphology of the VGNs which show a petal-like structure. The Raman spectrum shows two major fingerprints of graphene structures, G peak at 1570 cm−1 and G' peak at 2672 cm−1. The X-ray Diffraction (XRD) pattern of VGNs has four peaks which the main peak is C(0 0 2) and interlayer spacing found to be 0.34 nm. The Diffuse Reflectance result demonstrates a minimum reflectance at ∼270 nm which is apparently observable in the UV range of all graphene samples and drop of reflectance at ∼650 nm can be induced by the interaction of copper with VGNs structure and also it shows high absorptance value 90.42% which makes it a good candidate for solar selective absorber application.

Vertically oriented graphene nano-sheets grown by plasma enhanced chemical vapor deposition technique at low temperature

Abstract In this study, it was aimed to obtain vertically-oriented graphene nano-sheets (VGNs) on copper foils using CH4 gas as the precursor material by means of plasma enhanced chemical vapor deposition (PECVD) technique. VGNs were obtained at 50 W, 75 W and 100 W Radio Frequency (RF) powers by this technique and the effect of RF power on the morphological structure of graphene layers was investigated. VGNs were also analyzed by Raman spectroscopy, scanning electron microscopy (SEM) and water contact angle (WCA) measurements. The SEM images showed that the VGNs became more tightly packed with the increase of RF power and thus these coating parameters had a significant effect on the morphological structure. WCA analyzes for VGNs were also performed and it was found that VGNs obtained at 100 W RF power had higher contact angle value than the other RF powers. These VGNs, which have both large surface area and optically transparent, are thought to be materials that have the potential to be used in supercapacitors and solar cells.

Efficient and controllable growth of vertically oriented graphene nanosheets by mesoplasma chemical vapor deposition

Owing to the unique nanoarchitecture and graphene in nature, the vertically oriented graphene nanosheets (VGN) possesses many promising properties and holds great potential for various applications. Although the plasma-enhanced chemical vapor deposition (PECVD) techniques have been the effective and widely used approaches, they remain the challenge for the controllable and efficient growth of VGN for the mass and intended applications. Also, each method provides limited structures. Herein, for the first time, we report a synthesis of VGN with various controllable structures and fast growth rates by mesoplasma CVD at Ar-H2-CH4 gas mixture. Five types of structures, including cauliflower-like, petal-like, quasi-maze-like, maze-like, and floc-like structures are synthesized by simply adjusting the growth parameters. The growth dynamics and structure transition of VGN are analyzed from the aspect of the deposition precursor density and the H/C ratio. Even at no H2 addition, VGN is synthesized at mesoplasma condition and an extremely high rate of 18.08 μm/min is obtained, which is at least one order of magnitude faster than any other methods reported. This work provides a new route to increase the growth rate of VGN and also demonstrates the advantages of mesoplasma CVD for the growth of high quality films with high efficiency.

Plasma Heavily Nitrogen-Doped Vertically Oriented Graphene Nanosheets (N-VOGNs) for High Volumetric Performance On-Chip Supercapacitors in Ionic Liquid

Background: The research of innovative nanostructured materials will be helpful in designing high performance supercapacitor devices in the near future. In this direction, graphenebased structures, such as Vertically Oriented Graphene Nanosheets (VOGNs), have emerged as promising architectures due to their interesting chemical, structural and electrochemical properties. One of the main strategies to enhance the performance of supercapacitive materials consists of the functionalization or modification of their surfaces by means of various approaches focused on doping or the deposition of electroactive material coatings/films among others. Particularly, in this study, the doping effect, by nitrogen plasma, on VOGNs was investigated as innovative electrodes for on-chip supercapacitors. Methods: Nitrogen-doped VOGNs (N-VOGNs) were characterised at morphological (SEM and TEM techniques), structural (XPS and Raman techniques) and electrochemical levels, respectively. Subsequently, the potential of such doped graphene nanostructure was evaluated in a symmetric supercapacitor device using a coin cell configuration. The properties of the supercapacitor were examined in terms of capacitance, energy and power density and lifetime. Results: A maximal N-content of 17% was achieved for VOGNs with a vertical length around 370 nm, through exposure to a N2 microwave-plasma in an electron cyclotron resonance (ECR)-CVD reactor. The deconvolution of N1s spectrum of N-VOGNs reflected the presence of three main peaks corresponding to pyrrolic (400.2 eV), pyridinic (399.0 eV) and graphitic (401.2 eV) nitrogen forms, demonstrating the incorporation of nitrogen into graphene structure. The doping effect reflected an important impact on the morphological (surface defects and reactions, porosity) and structural (conductivity) properties, which allowed to enhance greatly the capacitive properties compared to undoped VOGNs. Conclusion: N-VOGN based supercapacitors have demonstrated excellent capacitive properties such as high volumetric energy (28 mWh cm-3) and power (360 W cm-3) densities as well as an outstanding cycling stability (retention of 80% after 300 000 galvanostatic charge-discharge cycles). These results are very promising compared to the state-of-the art dealing with carbonaceous structurebased supercapacitors. Consequently, this study paves the way to explore the in-depth potential of such nanostructure in the development of innovative high performance supercapacitors. Keywords: Doping, graphene nanosheets, ionic liquid, supercapacitors, vertically oriented graphene, VOGN.

Unveiling the ionic exchange mechanisms in vertically-oriented graphene nanosheet supercapacitor electrodes with electrochemical quartz crystal microbalance and ac-electrogravimetry

This work presents the first electrochemical quartz crystal microbalance (EQCM) results for vertically-oriented graphene nanosheets (VOGNs) as supercapacitor electrodes. Conventional EQCM technique delivered primary insights on the ionic exchange mechanisms between VOGNs and organic electrolytes, showing a major contribution of anions. A more advanced electrogravimetric methodology, ac-electrogravimetry, was then used to access specific dynamic attributes for each species exchanged at the VOGN electrode surface. Accordingly, under the conditions of this study, anions were confirmed to be the major energy storage vector with high kinetic values and low transfer resistance while cations and free solvent molecules are given non-negligible supporting roles.

6-inch uniform vertically-oriented graphene on soda-lime glass for photothermal applications

Vertically-oriented graphene (VG) has many advantages over flat lying graphene, including a large surface area, exposed sharp edges, and non-stacking three-dimensional geometry. Recently, VG nanosheets assembled on specific substrates have been used for applications in supersensitive gas sensors and high-performance energy storage devices. However, to realize these intriguing applications, the direct growth of high-quality VG on a functional substrate is highly desired. Herein, we report the direct synthesis of VG nanosheets on traditional soda-lime glass due to its low-cost, good transparency, and compatibility with many applications encountered in daily life. This synthesis was achieved by a direct-current plasma enhanced chemical vapor deposition (dc-PECVD) route at 580 °C, which is right below the softening point of the glass, and featured a scale-up size ∼6 inches. Particularly, the fabricated VG nanosheets/glass hybrid materials at a transmittance range of 97%–34% exhibited excellent solarthermal performances, reflected by a 70%–130% increase in the surface temperature under simulated sunlight irradiation. We believe that this graphene glass hybrid material has great potential for use in future transparent “green-warmth” construction materials.

Process-specific mechanisms of vertically oriented graphene growth in plasmas.

Applications of plasma-produced vertically oriented graphene nanosheets (VGNs) rely on their unique structure and morphology, which can be tuned by the process parameters to understand the growth mechanism. Here, we report on the effect of the key process parameters such as deposition temperature, discharge power and distance from plasma source to substrate on the catalyst-free growth of VGNs in microwave plasmas. A direct evidence for the initiation of vertical growth through nanoscale graphitic islands is obtained from the temperature-dependent growth rates where the activation energy is found to be as low as 0.57 eV. It is shown that the growth rate and the structural quality of the films could be enhanced by (a) increasing the substrate temperature, (b) decreasing the distance between the microwave plasma source and the substrate, and (c) increasing the discharge power. The correlation between the wetting characteristics, morphology and structural quality is established. It is also demonstrated that morphology, crystallinity, wettability and sheet resistance of the VGNs can be varied while maintaining the same sp3 content in the film. The effects of the substrate temperature and the electric field in vertical alignment of the graphene sheets are reported. These findings help to develop and optimize the process conditions to produce VGNs tailored for applications including sensing, field emission, catalysis and energy storage.

Effect of Precursor on Antifouling Efficacy of Vertically-Oriented Graphene Nanosheets.

Antifouling efficacy of graphene nanowalls, i.e., substrate-bound vertically-oriented graphene nanosheets, has been demonstrated against biofilm-forming Gram-positive and Gram-negative bacteria. Where graphene nanowalls are typically prepared using costly high-temperature synthesis from high-purity carbon precursors, large-scale applications demand efficient, low-cost processes. The advancement of plasma enabled synthesis techniques in the production of nanomaterials has opened a novel and effective method for converting low-cost natural waste resources to produce nanomaterials with a wide range of applications. Through this work, we report the rapid reforming of sugarcane bagasse, a low-value by-product from sugarcane industry, into high-quality vertically-oriented graphene nanosheets at a relatively low temperature of 400 °C. Electron microscopy showed that graphene nanowalls fabricated from methane were significantly more effective at preventing surface attachment of Gram-negative rod-shaped Escherichia coli compared to bagasse-derived graphene, with both surfaces showing antifouling efficacy comparable to copper. Attachment of Gram-positive coccal Staphylococcus aureus was lower on the surfaces of both types of graphene compared to that on copper, with bagasse-derived graphene being particularly effective. Toxicity to planktonic bacteria estimated as a reduction in colony-forming units as a result of sample exposure showed that both graphenes effectively retarded cell replication.

Direct Electrodeposition to Fabricate Vertically-Oriented Graphene Nanosheets Modified Electrode and its Application for Determination of Levodopa in the Presence of Uric Acid and Ascorbic Acid

The electrodeposition of reduced graphene oxide (rGO) film with preferred vertical orientation was fabricated on a glassy carbon electrode by using pulse potential method in a graphene oxide solution. The rGO film was applied in developing a high-sensitive electrochemical sensor for detection of levodopa (L-dopa). The new electrochemical sensor displayed a significant response to L-dopa because of its excellent electric conductivity and large specific surface area. Moreover, the determination of L-dopa on this modified electrode was not interfered by the coexistence of ascorbic acid (AA) and uric acid (UA). Under the selective conditions, the modified electrode showed a linear voltammetric response to L-dopa within a certain concentration ranging from [Formula: see text][Formula: see text] mol L[Formula: see text], with the detection limit of [Formula: see text] mol L[Formula: see text]. Besides, the modified electrode also exhibited an excellent selectivity, stability, reproducibility and repeatability.

Direct anodic exfoliation of graphite onto high-density aligned graphene for large capacity supercapacitors

Vertically oriented graphene nanosheets (VOGNs) fabricated on conductive substrates with a large amount of edge planes and open channels are ideal for electrochemical double-layer (EDL) capacitor electrodes. However, preparation of such a structure with high-density of graphene nanosheets is challenging. Herein, a facile, environment-friendly, and economical technique to prepare high-quality VOGNs directly on conductive graphite plates with a high mass loading is described. The VOGNs are obtained by electrochemical anodization of graphite and a large amount of aligned reduced graphene oxide (rGO) is produced and adheres strongly to the graphite substrate (G@rGO). The symmetrical supercapacitors composed of the G@rGO electrodes exhibit a high volumetric capacitance of 3.9Fcm−3 and energy density of 0.66WhL−1 (based on the volume of the whole electrode) at a current density of 7.5mAcm−3 in 6M KOH. The rate performance and long-term cycling stability are very good. The outstanding capacitive performance can be attributed to the unique structure of the G@rGO electrode which facilitates transportation of ions between the electrolyte and graphene surface, minimizes the distributive nature of charge storage, expedites the formation of EDL, and enhances the electrochemical utilization of graphene and stability by avoiding restacking and aggregation of graphene nanosheets.

DC and Microwave Plasmas for Synthesis of Vertically Oriented Graphene

Plasma-enhanced chemical vapor deposition of vertically oriented graphene (VG) employing direct current glow discharge and microwave plasmas are presented and compared. The VG nanosheets with various morphologies can be obtained for diverse applications.

High-Voltage, High-Frequency Electrochemical Capacitor

Electric double layer capacitors (EDLCs) with electrical performance like aluminum electrolytic capacitors have been developed.(1) They can be smaller in size and offer higher reliability. Operation of such devices relies on charge stored on vertically oriented graphene nanosheet (VOGN) carbon. Vertically oriented graphene nanosheets are prepared using RF plasma enhanced chemical deposition of a hydrocarbon feed gas.(2) Under proper growth conditions, VOGN are ohmically connected to the metal current collector on which they grow, offer high electronic conductivity, and have an “open” structure totally free of porous electrode behavior. Each of these characteristics is necessary for efficient operation at frequencies of 1 kHz and higher.EDLC cells, like battery cells, operate at low voltage. Thus cells must be series connected to operate at high-voltage. Figure 1 shows an approach devised for interconnecting the EDLC cells to create a high-voltage, high-frequency EDLC. It involves interconnecting planar EDLC cells using the substrate metallization. As shown, interdigitated gaps are created in the VOGN that has been grown on a thin metal layer covering an insulating substrate, for instance, nickel on alumina. The gap is created using a laser to cut through the VOGN and metal down to the insulating substrate. This then creates multiple electrically-isolated regions (six are shown in the figure). Two adjacent regions become a single EDLC after electrolyte is applied over the gap and the surface of that region. To make a series connection of cells, each gap must be individually covered with an electrolyte, making sure that the electrolyte band does not touch the neighboring bands. This is a “bipolar” design in two dimensions, the substrate metallization serving as the bipolar plate. Connecting M cells in series each rated at voltage Vo with capacitance Co and series resistance Ro yields a capacitor with a voltage rating M·Vo, capacitance Co/M, and series resistance M·Ro. The frequency response of this high-voltage capacitor is nearly identical to that of one of its cells because the interconnects add minimal resistance. Voltage-balance resistors can be added using, for instance, thick-film printing processes. Details of the fabrication process are described and electrical performance data on these high-voltage, high-frequency electric double layer capacitors is presented. References 1J.R. Miller el al, “Graphene Double-Layer Capacitor with ac Line-Filtering Performance”, Science 329 , 1637 (2010). 2Cai M, Outlaw RA, Butler SM, and Miller JR, “A High Density of Vertically-Oriented Graphenes for Use in Electric Double Layer Capacitors”, Carbon 50, 5481–5488 (2012)..

Ni(OH)2 nanoflakes electrodeposited on Ni foam-supported vertically oriented graphene nanosheets for application in asymmetric supercapacitors

Binderless Ni(OH)2 nanoflakes grown on Ni foam (NF)-supported vertically oriented graphene nanosheets (V-GNs) has been fabricated as a positive electrode material for asymmetric supercapacitor (ASC), coupled with activated carbon (AC) as a counter electrode material. The introduction of V-GNs leads to dense growth of nanocrystalline β-Ni(OH)2 that is confirmed by X-ray diffraction, transmission electron microscopic and scanning electron microscopic analyses. The electrochemical performances of the Ni(OH)2/GNs/NF electrode are characterized by cyclic voltammetry and charge–discharge tests, which exhibit high specific capacitance of 2215Fg−1 at a scan current density of 2.3Ag−1, enhanced cycling stability and high rate capability. The Ni(OH)2/GNs/NF-AC-based ASC can achieve a cell voltage of 1.4V and a specific energy density of 11.11Whkg−1 at 0.5mAcm−2 with a nearly 100% coulombic efficiency at room temperature.

Highly-branched vertically-oriented graphene nanosheets with dense open graphitic edge planes as Pt support for methanol oxidation

Vertically-oriented graphene (VG) with predominantly edge plane structure is considered as a promising catalyst support candidate for the methanol oxidation reaction (MOR) in direct methanol fuel cells (DMFCs). Inspired by the fact that catalyst nanoparticles are spontaneously attached to the exposed graphene edges, a novel highly-branched vertically-oriented graphene (HBVG) based catalyst support is proposed in the current work. With employing atmospheric normal glow discharge (ANGD) and microwave plasma-enhanced chemical vapor deposition (PECVD) methods, HBVG and maze-like VG (MVG) are synthesized on the surface of carbon paper (CP), respectively. Pt nanoparticles are deposited on pristine CP, HBVG-CP, and MVG-CP with the same route of repeated double-potential pulse electrodeposition (RDPE). Experimental results show that HBVG-CP with dense open graphitic edge planes can desirably provide a considerable number of nuclei sites for the fast nucleation and well dispersion of Pt nanoparticles. Meanwhile, a certain amount of oxygen-containing functional groups on the HBVG surface will benefit the rapid removal of CO and accumulated carbonaceous species. Based on the electrochemical measurements on Pt utilization efficiency, catalytic activity, CO tolerance, and long-term stability, Pt/HBVG-CP is demonstrated to be able to present enhanced catalytic performance over the Pt/CP and Pt/MVG-CP counterparts, holding a great potential for fuel cell applications.

Multi‐pin dc glow discharge PECVD for uniform growth of vertically oriented graphene at atmospheric pressure

Atmospheric normal glow discharge plasma‐enhanced chemical vapor deposition (ANGD‐PECVD) holds great potential for the industrial in‐line continuous growth of vertically oriented graphene (VG) nanosheets. However, conventional single‐pin‐to‐plate electrode arrangement owns the drawback of producing a non‐uniform electric field, subsequently leading to a fairly poor uniformity of the as‐grown VG nanosheets. This work offers a proof of concept on a novel multi‐pin ANGD‐PECVD method for the uniform growth of VG nanosheets. Based on the numerical calculation, the dual‐pin ANGD can provide a more uniform electric field compared with the single‐pin counterpart, due to the compensation from adjacent pins and a lower integrated curvature of the discharge electrode. Experimental results demonstrate that ANGD with a four‐pin discharge electrode is able to significantly improve the uniformity of the localized current density at the growth substrate, and further realize the growth of VG nanosheets with excellent homogeneity in terms of morphology and structure. Successful application of the as‐grown VG nanosheets working as the active materials of a supercapacitor is illustrated at the end of this work.

Plasma-enhanced chemical vapor deposition synthesis of vertically oriented graphene nanosheets

Vertically oriented graphene (VG) nanosheets have attracted growing interest for a wide range of applications, from energy storage, catalysis and field emission to gas sensing, due to their unique orientation, exposed sharp edges, non-stacking morphology, and huge surface-to-volume ratio. Plasma-enhanced chemical vapor deposition (PECVD) has emerged as a key method for VG synthesis; however, controllable growth of VG with desirable characteristics for specific applications remains a challenge. This paper attempts to summarize the state-of-the-art research on PECVD growth of VG nanosheets to provide guidelines on the design of plasma sources and operation parameters, and to offer a perspective on outstanding challenges that need to be overcome to enable commercial applications of VG. The review starts with an overview of various types of existing PECVD processes for VG growth, and then moves on to research on the influences of feedstock gas, temperature, and pressure on VG growth, substrate pretreatment, the growth of VG patterns on planar substrates, and VG growth on cylindrical and carbon nanotube (CNT) substrates. The review ends with a discussion on challenges and future directions for PECVD growth of VG.