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

Recent Progress on Graphene-Based Derivatives for Enhanced Energy Storage Devices

2D materials have been widely utilized in efficient energy storage applications in recent years. In efforts to mitigate global warming concerns, the novel trends of using nonfossil fuel cells have drawn significant attention of scientific communities towards various 2D materials owing to their excellent physiochemical properties, preferably suitable for developing energy storage systems. Amongst the 2D materials, graphene and its derivatives, due to their tunable surface properties, have emerged as prominent 2D materials for energy storage. Graphene, leveraged with its highly reactive surface sites, can be tailored with myriads of functional groups to enhance its applicability as an energy storage material. This review focuses on the recent advancements in utilizing various dimensions of graphene, including 0D GQDs, 1D GNRs, 2D GO/rGO, and 3D architectures, along with vertical graphene and graphene paper for efficient energy storage devices. The review addresses the limitations of pristine graphene and highlights the benefits of functionalization and synergistic material combinations. This also discusses the challenges and future perspectives related to the commercialization and large-scale production of graphene-based energy storage devices.

Advancing Graphene Synthesis: Low-Temperature Growth and Hydrogenation Mechanisms Using Plasma-Enhanced Chemical Vapor Deposition

This study explores the low-temperature synthesis of graphene using plasma-enhanced chemical vapor deposition (PECVD), emphasizing the optimization of process parameters to achieve controlled growth of pristine and hydrogenated graphene. Graphene films were synthesized at temperatures ranging from 700 °C to as low as 400 °C by varying methane (25-100 sccm) and hydrogen (25-100 sccm) gas flow rates under 10-20 mBar pressures. Raman spectroscopy revealed structural transitions: pristine graphene grown at 700 °C exhibited strong 2D peaks with an I(2D)/I(G) ratio > 2, while hydrogenated graphene synthesized at 500 °C showed increased defect density with an I(D)/I(G) ratio of ~1.5 and reduced I(2D)/I(G) (~0.8). At 400 °C, the material transitioned to a highly hydrogenated amorphous carbon film, confirmed by photoluminescence (PL) in the Raman spectra. Atomic force microscopy (AFM) showed pristine graphene with a root mean square roughness (Rq) of 0.37 nm. By carefully adjusting PECVD synthesis parameters, it was possible to tune the surface roughness of hydrogenated graphene to levels close to that of pristine graphene or to achieve even smoother surfaces. Conductive AFM measurements revealed that hydrogenation could enhance graphene's contact current under specific conditions. The findings highlight the role of PECVD parameters in tailoring graphene's structural, morphological, and electronic properties for diverse applications. This work demonstrates a scalable, low-temperature approach to graphene synthesis, offering the potential for energy storage, sensing, and electronic devices requiring customized material properties.

Direct Growth of Vertical Graphene on Fiber Electrodes and Its Application in Alternating Current Line-Filtering Capacitors

Fiber-shaped electrochemical capacitors (FSECs) have garnered substantial attention to emerging portable, flexible, and wearable electronic devices. However, achieving high electronic and ionic conductivity in fiber electrodes while maintaining a large specific surface area is still a challenge for enhancing the capacitance and rapid response of FSECs. Here, we present an electric-field-assisted cold-wall plasma-enhanced chemical vapor (EFCW-PECVD) method for direct growth of vertical graphene (VG) on fiber electrodes, which is incorporated in the FSECs. The customized reactor mainly consists of two radio frequency coils: one for plasma generation and the other for substrate heating. Precise temperature control can be achieved by adjusting the conductive plates and the applied power. With induction heating, only the substrate is heated to above 500 °C within just 5 min, maintaining a low temperature in the gas phase for the growth of VG with a high quality. Using this method, VG was easily grown on metallic fibers. The VG-coated titanium fibers for FSECs exhibit an ultrahigh rate performance and quick ion transport, enabling the conversion of an alternating current signal to a direct current signal and demonstrating outstanding filtering capabilities.

Continuous batch synthesis with atmospheric-pressure microwave plasmas

Plasmas under atmospheric pressure offer a high-temperature environment for material synthesis, but electrode ablation compromises purity. Here, we introduce an atmospheric-pressure microwave plasma (AMP) operated without electrodes to overcome the existing limitations in pure material synthesis. The distribution of the electrostatic field intensity inside a waveguide during AMP excitation was examined via electrostatic field simulations. The lateral and radial gas temperature distributions were also studied using optical emission spectroscopy. The AMP exhibited a uniform ultrahigh temperature (9,000 K), a large volume (102-104 cm3), and a response time on the millisecond level. AMP efficiently synthesized silicon nanoparticles, graphene, and graphene@Si-Fe core-shell nanoparticles within tens of milliseconds, ensuring purity and size control. We propose the "heat impulse" metric for evaluating the plasma characteristics (n a, T g, and t) in material synthesis, extended to other high-temperature plasmas. AMP is compact, cost-effective, and easy to assemble, promising for eco-friendly mass production of pure materials.

Art etching of graphene

The growth of graphene on a metal substrate using chemical vapor deposition (CVD), assisted by hydrocarbons such as CH4, C3H8, C2H6, etc. leads to the formation of carbon clusters, amorphous carbon, or any other structure. These carbon species are considered as unwanted impurities; thus a conventional etching step is used simultaneously with CVD graphene growth to remove them using an etching agent. Meanwhile, art etching is a specific method of producing controlled non-Euclidean and Euclidean geometries by employing intricate and precise etching parameters or integrated growth/etching modes. Agents such as H2, O2, CH4, Ar, and others are applied as art etching agents to support the art etching technology. This technique can generate nanopores and customize the properties of graphene, facilitating specific applications such as nanodevices, nanosensors, nanofilters, etc. This comprehensive review investigates how precursor gases concurrently induce graphene growth and art etching during a chemical vapor deposition process, resulting in beautifully etched patterns. Furthermore, it discusses the techniques leading to the creation of these patterns. Finally, the challenges, uses, and perspectives of these non-Euclidean and Euclidean-shaped art etched graphene geometries are discussed.

Infrared Lightwave Memory-Resident Manipulation and Absorption Based on Spatial Electromagnetic Wavefield Excitation and Resonant Accumulation by GdFe-Based Nanocavity-Shaped Metasurfaces

An arrayed nanocavity-shaped architecture consisting of the key GdFe film and SiO2 dielectric layer is constructed, leading to an efficient infrared (IR) absorption metasurface. By carefully designing and optimizing the film system configuration and the surface layout with needed geometry, a desirable IR radiation absorption according to the spatial magnetic plasmon modes is realized experimentally. The simulations and measurements demonstrate that GdFe-based nanocavity-shaped metasurfaces can be used to achieve an average IR absorption of ~81% in a wide wavelength range of 3-14 μm. A type of the patterned GdFe-based nanocavity-shaped metasurface is further proposed for exciting relatively strong spatial electromagnetic wavefields confined by a patterned nanocavity array based on the joint action of the surface oscillated net charges over the charged metallic films and the surface conductive currents including equivalent eddy currents surrounding the layered GdFe and SiO2 materials. Intensive IR absorption can be attributed to a spatial electromagnetic wavefield excitation and resonant accumulation or memory residence according to the GdFe-based nanocavity-shaped array formed. Our research provides a potential clue for efficiently responding and manipulating and storing incident IR radiation mainly based on the excitation and resonant accumulation of spatial magnetic plasmons.

Recent Progress of Vertical Graphene: Preparation, Structure Engineering, and Emerging Energy Applications

Vertical graphene (VG) nanosheets have garnered significant attention in the field of electrochemical energy applications, such as supercapacitors, electro-catalysis, and metal-ion batteries. The distinctive structures of VG, including vertically oriented morphology, exposed, and extended edges, and separated few-layer graphene nanosheets, have endowed VG with superior electrode reaction kinetics and mass/electron transportation compared to other graphene-based nanostructures. Therefore, gaining insight into the structure-activity relationship of VG and VG-based materials is crucial for enhancing device performance and expanding their applications in the energy field. In this review, the authors first summarize the fabrication methods of VG structures, including solution-based, and vacuum-based techniques. The study then focuses on structural modulations through plasma-enhanced chemical vapor deposition (PECVD) to tailor defects and morphology, aiming to obtain desirable architectures. Additionally, a comprehensive overview of the applications of VG and VG-based hybrids d in the energy field is provided, considering the arrangement and optimization of their structures. Finally, the challenges and future prospects of VG-based energy-related applications are discussed.

Nanoparticles synthesis in microwave plasmas: peculiarities and comprehensive insight

Low-pressure plasma processes are routinely used to grow, functionalize or etch materials, and thanks to some of its unique attributes, plasma has become a major player for some applications such as microelectronics. Plasma processes are however still at a research level when it comes to the synthesis and functionalization of nanoparticles. Yet plasma processes can offer a particularly suitable solution to produce nanoparticles having very peculiar features since they enable to: (i) reach particle with a variety of chemical compositions, (ii) tune the size and density of the particle cloud by acting on the transport dynamics of neutral or charged particles through a convenient setting of the thermal gradients or the electric field topology in the reactor chamber and (iii) manipulate nanoparticles and deposit them directly onto a substrate, or codeposit them along with a continuous film to produce nanocomposites or (iv) use them as a template to produce 1D materials. In this article, we present an experimental investigation of nanoparticles synthesis and dynamics in low-pressure microwave plasmas by combining time-resolved and in-situ laser extinction and scattering diagnostics, QCL absorption spectroscopy, mass spectrometry, optical emission spectroscopy and SEM along with a particle transport model. We showed for the first time the thermophoresis-driven dynamic of particle cloud in electrodless microwave plasmas. We showed that this effect is linked to particular fluctuations in the plasma composition and results in the formation of a void region in the bulk of the plasma surrounded by a particle cloud in the peripherical post-discharge. We also reveals and analyze the kinetics of precursor dissociation and molecular growth that result in the observed nanoparticle nucleation.

Graphene nanowalls formation investigated by Electron Energy Loss Spectroscopy

The properties of layered materials are significantly dependent on their lattice orientations. Thus, the growth of graphene nanowalls (GNWs) on Cu through PECVD has been increasingly studied, yet the underlying mechanisms remain unclear. In this study, we examined the GNWs/Cu interface and investigated the evolution of their microstructure using advanced Scanning transmission electron microscopy and Electron Energy Loss Spectroscopy (STEM-EELS). GNWs interface and initial root layers of comprise graphitic carbon with horizontal basal graphene (BG) planes that conform well to the catalyst surface. In the vertical section, the walls show a mix of graphitic and turbostratic carbon, while the latter becomes more noticeable close to the top edges of the GMWs film. Importantly, we identified growth process began with catalysis at Cu interface forming BG, followed by defect induction and bending at 'coalescence points' of neighboring BG, which act as nucleation sites for vertical growth. We reported that although classical thermal CVD mechanism initially dominates, growth of graphene later deviates a few nanometers from the interface to form GNWs. Nascent walls are no longer subjected to the catalytic action of Cu, and their development is dominated by the stitching of charged carbon species originating in the plasma with basal plane edges.

Optimizing Optical Dielectrophoretic (ODEP) Performance: Position- and Size-Dependent Droplet Manipulation in an Open-Chamber Oil Medium

An optimization study is presented to enhance optical dielectrophoretic (ODEP) performance for effective manipulation of an oil-immersed droplet in the floating electrode optoelectronic tweezers (FEOET) device. This study focuses on understanding how the droplet's position and size, relative to light illumination, affect the maximum ODEP force. Numerical simulations identified the characteristic length (Lc) of the electric field as a pivotal factor, representing the location of peak field strength. Utilizing 3D finite element simulations, the ODEP force is calculated through the Maxwell stress tensor by integrating the electric field strength over the droplet's surface and then analyzed as a function of the droplet's position and size normalized to Lc. Our findings reveal that the optimal position is xopt= Lc+ r, (with r being the droplet radius), while the optimal droplet size is ropt = 5Lc, maximizing light-induced field perturbation around the droplet. Experimental validations involving the tracking of droplet dynamics corroborated these findings. Especially, a droplet sized at r = 5Lc demonstrated the greatest optical actuation by performing the longest travel distance of 13.5 mm with its highest moving speed of 6.15 mm/s, when it was initially positioned at x0= Lc+ r = 6Lc from the light's center. These results align well with our simulations, confirming the criticality of both the position (xopt) and size (ropt) for maximizing ODEP force. This study not only provides a deeper understanding of the position- and size-dependent parameters for effective droplet manipulation in FEOET systems, but also advances the development of low-cost, disposable, lab-on-a-chip (LOC) devices for multiplexed biological and biochemical analyses.

Impact of probe sonication and sulfuric acid pretreatment on graphene exfoliation in water

Graphene is a 2D material with promising commercial applications due to its physicochemical properties. Producing high-quality graphene economically and at large scales is currently of great interest and demand. Here, the potential of producing high-quality graphene at a large scale via water-phase exfoliation methods is investigated. By altering exfoliation parameters, the production yield of graphene and flake size are evaluated. Pretreatment of the precursor graphite powder using acidic solutions of H2SO4 at different concentrations is found to increase further the yield and structural quality of the exfoliated graphene flakes. These findings are confirmed through various spectroscopy and surface characterization techniques. Controlling flake size, thickness, and yield are demonstrated via optimization of the sonication process, centrifuge time, and H2SO4 pretreatment.

Controllable Fabrication of Vertical Graphene with Tunable Growth Nature by Remote Plasma-Enhanced Chemical Vapor Deposition

As an important member of the graphene family, vertical graphene (VG) has broad applications like field emission, energy storage, and sensors owing to its fascinating physical and chemical properties. Among various fabrication methods for VG, plasma enhanced chemical vapor deposition (PECVD) is most employed because of the fast growth rate at relatively low temperature for the high-quality VG. However, to date, relations between growth manner of VG and growth parameters such as growth temperature, dosage of gaseous carbon source, and electric power to generate plasma are still less known, which in turn hinder the massive production of VG for further applications. In this study, the growth behavior of VG was studied as functions of temperature, plasma power, and gas composition (or chamber pressure). It was found that the growth behavior of VG is sensitive to the growth conditions mentioned above. Although conditions with high growth temperature, large flow rate of mixed gas of methane and carrier gases, and high plasma power may be helpful for the fast growth of VG, brunching of VG is simultaneously enhanced, which in turn decreases the vertical growth nature of VG. High-quality VG can be achieved by optimizing the growth parameters. It was revealed that the vertical growth nature of VG is governed by the electric field at the interfacial layer between VG and the substrate, for which its strength is influenced by the density of plasma. These findings are important for the general understanding of the VG growth and provided a feasible way for the controllable fabrication of VG using the remote PECVD method which is usually believed to be unsuitable for the fabrication of VG.

Bottom-up Synthesis of Nanosheets at Various Interfaces

Nanostructured materials with high aspect ratios have been widely studied for their unique properties. In particular, nanosheets have safety, dispersibility, and nanosized effects, and nanosheets with exceptionally small thicknesses exhibit unique properties. For non-exfoliable materials, the bottom-up nanosheet growth using various interfaces as templates have been investigated. This review article presents the synthesis of nanosheets at the interfaces and layered structure; it explains the features of each interface type, its advantages, and its uniqueness. The interfaces work as templates for nanosheet synthesis. We can easily use the liquid-liquid and gas-liquid interfaces as the templates; however, the thickness of nanosheets usually becomes thick because it allows materials to grow in thickness. The solid-gas and solid-liquid interfaces can prevent nanosheets from growing in thickness. However, the removal of template solids is required after the synthesis. The layered structures of various materials provide two-dimensional reaction fields between the layers. These methods have high versatility, and the nanosheets synthesized by these methods are thin. Finally, this review examines the key challenges and opportunities associated with scalable nanosheet synthesis methods for industrial production.

Advancements in Plasma-Enhanced Chemical Vapor Deposition for Producing Vertical Graphene Nanowalls

In recent years, vertical graphene nanowalls (VGNWs) have gained significant attention due to their exceptional properties, including their high specific surface area, excellent electrical conductivity, scalability, and compatibility with transition metal compounds. These attributes position VGNWs as a compelling choice for various applications, such as energy storage, catalysis, and sensing, driving interest in their integration into next-generation commercial graphene-based devices. Among the diverse graphene synthesis methods, plasma-enhanced chemical vapor deposition (PECVD) stands out for its ability to create large-scale graphene films and VGNWs on diverse substrates. However, despite progress in optimizing the growth conditions to achieve micrometer-sized graphene nanowalls, a comprehensive understanding of the underlying physicochemical mechanisms that govern nanostructure formation remains elusive. Specifically, a deeper exploration of nanometric-level phenomena like nucleation, carbon precursor adsorption, and adatom surface diffusion is crucial for gaining precise control over the growth process. Hydrogen's dual role as a co-catalyst and etchant in VGNW growth requires further investigation. This review aims to fill the knowledge gaps by investigating VGNW nucleation and growth using PECVD, with a focus on the impact of the temperature on the growth ratio and nucleation density across a broad temperature range. By providing insights into the PECVD process, this review aims to optimize the growth conditions for tailoring VGNW properties, facilitating applications in the fields of energy storage, catalysis, and sensing.

Hybrid Nanostructured Compounds of Mo2C on Vertical Graphene Nanoflakes for a Highly Efficient Hydrogen Evolution Reaction

Organizing a post-fossil fuel economy requires the development of sustainable energy carriers. Hydrogen is expected to play a significant role as an alternative fuel as it is among the most efficient energy carriers. Therefore, nowadays, the demand for hydrogen production is increasing. Green hydrogen produced by water splitting produces zero carbon emissions but requires the use of expensive catalysts. Therefore, the demand for efficient and economical catalysts is constantly growing. Transition-metal carbides, and especially Mo2C, have attracted great attention from the scientific community since they are abundantly available and hold great promises for efficient performance toward the hydrogen evolution reaction (HER). This study presents a bottom-up approach for depositing Mo carbide nanostructures on vertical graphene nanowall templates via chemical vapor deposition, magnetron sputtering, and thermal annealing processes. Electrochemical results highlight the importance of adequate loading of graphene templates with the optimum amount of Mo carbides, controlled by both deposition and annealing time, to enrich the available active sites. The resulting compounds exhibit exceptional activities toward the HER in acidic media, requiring overpotentials of 82 mV at -10 mA/cm2 and demonstrating a Tafel slope of 56 mV/dec. The high double-layer capacitance and low charge transfer resistance of these Mo2C on GNW hybrid compounds are the main causes of the enhanced HER activity. This study is expected to pave the way for the design of hybrid nanostructures based on nanocatalyst deposition on three-dimensional graphene templates.

Direct Growth of Patterned Vertical Graphene Using Thermal Stress Mismatch between Barrier Layer and Substrate

Vertical graphene (VG) combines the excellent properties of conventional graphene with a unique vertical nanosheet structure, and has shown tremendous promise in the field of electronics and composites. However, its complex surface morphology brings great difficulties to micro-nano fabrication, especially regarding photolithography induced nanosheet collapse and remaining chemical residues. Here, we demonstrate an innovative method for directly growing patterned VG on a SiO₂/Si substrate. A patterned Cr film was deposited on the substrate as a barrier layer. The VG was synthesized by PECVD on both the patterned Cr film and the exposed SiO₂/Si substrate. During the cooling process, the patterned Cr film covered by VG naturally peeled off from the substrate due to the thermal stress mismatch, while the VG directly grown on the SiO₂/Si substrate was remained. The temperature-dependent thermal stress distribution in each layer was analyzed using finite element simulations, and the separation mechanism of the Cr film from the substrate was explained. This method avoids the contamination and damage caused by the VG photolithography process. Our work is expected to provide a convenient and reliable solution for the manufacture of VG-based electronic devices.

A Review on CNTs-Based Electrochemical Sensors and Biosensors: Unique Properties and Potential Applications

Carbon nanotubes (CNTs), are safe, biocompatible, bioactive, and biodegradable materials, and have sparked a lot of attention due to their unique characteristics in a variety of applications, including medical and dye industries, paper manufacturing and water purification. CNTs also have a strong film-forming potential, permitting them to be widely employed in constructing sensors and biosensors. This review concentrates on the application of CNT-based nanocomposites in the production of electrochemical sensors and biosensors. It emphasizes the synthesis and optimization of CNT-based sensors for a range of applications and outlines the benefits of using CNTs for biomolecule immobilization. In addition, the use of molecularly imprinted polymer (MIP)-CNTs in the production of electrochemical sensors is also discussed. The challenges faced by the current CNTs-based sensors, along with some the future perspectives and their future opportunities, are also briefly explained in this paper.

Superhydrophobic and Electrochemical Performance of CF2-Modified g-C3N4/Graphene Composite Film Deposited by PECVD

The physicochemical properties of functional graphene are regulated by compositing with other nano-carbon materials or modifying functional groups on the surface through plasma processes. The functional graphene films with g-C₃N₄ and F-doped groups were produced by controlling the deposition steps and plasma gases via radio frequency plasma-enhanced chemical vapor deposition (RF-PECVD). The first principles calculation and electrochemistry characteristic of the functional graphene films were performed on Materials Studio software and an electrochemical workstation, respectively. It is found that the nanostructures of functional graphene films with g-C₃N₄ and F-doped groups were significantly transformed. The introduction of fluorine atoms led to severe deformation of the g-C₃N₄ nanostructure, which created gaps in the electrostatic potential of the graphene surface and provided channels for electron transport. The surface of the roving fabric substrate covered by pure graphene is hydrophilic with a static contact angle of 79.4°, but the surface is transformed to a hydrophobic state for the g-C₃N₄/graphene film with an increased static contact angle of 131.3° which is further improved to 156.2° for CF₂-modified g-C₃N₄/graphene film exhibiting the stable superhydrophobic property. The resistance of the electron movement of CF₂-modified g-C₃N₄/graphene film was reduced by 2% and 76.7%, respectively, compared with graphene and g-C₃N₄/graphene.

Graphene Synthesis Techniques and Environmental Applications

Graphene is fundamentally a two-dimensional material with extraordinary optical, thermal, mechanical, and electrical characteristics. It has a versatile surface chemistry and large surface area. It is a carbon nanomaterial, which comprises sp² hybridized carbon atoms placed in a hexagonal lattice with one-atom thickness, giving it a two-dimensional structure. A large number of synthesis techniques including epitaxial growth, liquid phase exfoliation, electrochemical exfoliation, mechanical exfoliation, and chemical vapor deposition are used for the synthesis of graphene. Graphene prepared using different techniques can have a number of benefits and deficiencies depending on its application. This study provides a summary of graphene preparation techniques and critically assesses the use of graphene, its derivates, and composites in environmental applications. These applications include the use of graphene as membrane material for the detoxication and purification of water, active material for gas sensing, heavy metal ions detection, and CO₂ conversion. Furthermore, a trend analysis of both synthesis techniques and environmental applications of graphene has been performed by extracting and analyzing Scopus data from the past ten years. Finally, conclusions and outlook are provided to address the residual challenges related to the synthesis of the material and its use for environmental applications.

Carbon-Nanowall Microporous Layers for Proton Exchange Membrane Fuel Cell

The cathode microporous layer (MPL), as one of the key components of the proton exchange membrane fuel cell (PEM-FC), requires specialized carbon materials to ensure the two-phase flow and interfacial effects. In this respect, we designed a novel MPL based on highly hydrophobic carbon nanowalls (CNW). Employing plasma-assisted chemical vapor deposition techniques directly on carbon paper, we produced high-quality microporous layers at a competitive yield-to-cost ratio with distinctive MPL properties: high porosity, good stability, considerable durability, high hydrophobicity, and substantial conductivity. The specific morphological and structural properties were determined by scanning electron microscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. Thermo-gravimetric analysis was employed to study the nanostructures' thermal stability and contact angle measurements were performed on the CNW substrate to study the hydrophobic character. Platinum ink, serving as a fuel cell catalyst, was sprayed directly onto the MPLs and incorporated in the FC assembly by hot-pressing against a polymeric membrane to form the membrane-electrode assembly and gas diffusion layers. Single-fuel-cell testing, at moderate temperature and humidity, revealed improved power performance comparable to industrial quality membrane assemblies (500 mW cm-2 mg-1 of cathodic Pt load at 80 °C and 80% RH), with elevated working potential (0.99 V) and impeccable fuel crossover for a low-cost system.

Effect of Na-ion intercalation on the thermal conductivity of carbon honeycomb nanostructure

This work studies the thermal conductivity of Na-ion intercalated carbon honeycomb (CHC) via the combination of first-principles calculation and molecular dynamics simulation. The effects of ion concentration, ion charge, temperature, and strain are explored. The simulation results show that the thermal conductivity of CHC presents a nonmonotonic dependence on the ion concentration. The enhanced phonon scattering and increased phonon group velocities of CHC induced by its interaction with the Na ions are responsible for the nonmonotonic dependence. Both the increases in the ion charge and temperature reduce the thermal conductivity. In contrast, a compressive strain of around -3% can increase the thermal conductivity by eliminating the phonon softening effect caused by the volume expansion of CHC during the ion intercalation. However, further increasing the strain negatively or positively from -3% leads to a decrease in the thermal conductivity. The simulation results presented in this work are beneficial in understanding the thermal properties of CHC when it is used as an electrode in ion batteries and supercapacitors.

Direct fabrication of high-quality vertical graphene nanowalls on arbitrary substrates without catalysts for tidal power generation

The non-catalytic preparation of high-quality vertical graphene nanowalls (VGN) and graphene-based high output power hydrovoltaic effect power generation devices has always been difficult to achieve. In this work, we successfully prepared VGN with defect density, few layers and submicron domain size on a variety of substrates without catalysts through reasonable adjustment of growth conditions by the hot-wire chemical vapor deposition (HWCVD) method. The Raman test of the VGN prepared under optimal conditions showed that its I_D/I_G value was less than 1, and I_2D/I_G was more than 2.8. The deposition pressure was a key factor affecting the crystallization quality of the VGN. A suitable deposition pressure of 500 Pa could screen the active carbon clusters involved in the growth of nanowalls. The VGN prepared had excellent electrical properties and output of dropping-ion-droplet nano-power generation devices. Because of the larger crystal domain area and smaller contact angle of the VGN, the maximum output power exhibited at 100 Pa was 15.7 μW, which exceeded the value produced by other reported hydrovoltaic energy harvesting devices. All of them confirmed that VGN with improved quality had high application prospects in nano-energy devices.

Synthesis of Graphene-Based Nanocomposites for Environmental Remediation Applications: A Review

The term graphene was coined using the prefix "graph" taken from graphite and the suffix "-ene" for the C=C bond, by Boehm et al. in 1986. The synthesis of graphene can be done using various methods. The synthesized graphene was further oxidized to graphene oxide (GO) using different methods, to enhance its multitude of applications. Graphene oxide (GO) is the oxidized analogy of graphene, familiar as the only intermediate or precursor for obtaining the latter at a large scale. Graphene oxide has recently obtained enormous popularity in the energy, environment, sensor, and biomedical fields and has been handsomely exploited for water purification membranes. GO is a unique class of mechanically robust, ultrathin, high flux, high-selectivity, and fouling-resistant separation membranes that provide opportunities to advance water desalination technologies. The facile synthesis of GO membranes opens the doors for ideal next-generation membranes as cost-effective and sustainable alternative to long existing thin-film composite membranes for water purification applications. Many types of GO-metal oxide nanocomposites have been used to eradicate the problem of metal ions, halomethanes, other organic pollutants, and different colors from water bodies, making water fit for further use. Furthermore, to enhance the applications of GO/metal oxide nanocomposites, they were deposited on polymeric membranes for water purification due to their relatively low-cost, clear pore-forming mechanism and higher flexibility compared to inorganic membranes. Along with other applications, using these nanocomposites in the preparation of membranes not only resulted in excellent fouling resistance but also could be a possible solution to overcome the trade-off between water permeability and solute selectivity. Hence, a GO/metal oxide nanocomposite could improve overall performance, including antibacterial properties, strength, roughness, pore size, and the surface hydrophilicity of the membrane. In this review, we highlight the structure and synthesis of graphene, as well as graphene oxide, and its decoration with a polymeric membrane for further applications.

In Situ Grown Vertically Oriented Graphene Coating on Copper by Plasma-Enhanced CVD to Form Superhydrophobic Surface and Effectively Protect Corrosion

Graphene exhibits great potential for the corrosion protection of metals, because of its low permeability and high chemical stability. To enhance the anticorrosion ability of Cu, we use plasma-enhanced chemical vapor deposition (PECVD) to prepare a vertically oriented few-layer graphene (VFG) coating on the surface of Cu. The Cu coated with VFG shows superhydrophobic surface with a contact angle of ~150°. The VFG coating is used to significantly increase the anticorrosion ability, enhanced by the chemical stability and the unique geometric structure of vertically oriented graphene. The corrosion rate of VFG-Cu was about two orders of magnitude lower than that of bare Cu. This work highlights the special synthesized way of PECVD and superhydrophobic surface of vertical structures of graphene as coatings for various applications.

In Situ Monitored (N, O)-Doping of Flexible Vertical Graphene Films with High-Flux Plasma Enhanced Chemical Vapor Deposition for Remarkable Metal-Free Redox Catalysis Essential to Alkaline Zinc-Air Batteries

Rechargeable zinc-air batteries (ZABs) have attracted great interests for emerging energy applications. Nevertheless, one of the major bottlenecks lies in the fabrication of bifunctional catalysts with high electrochemical activity, high stability, low cost, and free of precious and rare metals. Herein, a high-performance metal-free bifunctional catalyst is synthesized in a single step by regulating radicals within the recently invented high-flux plasma enhanced chemical vapor deposition (HPECVD) system equipped with in situ plasma diagnostics. Thus-derived (N, O)-doped vertical few-layer graphene film (VGNO) is of high areal population with perfect vertical orientation, tunable catalytic states, and configurations, thus enabling significantly enhanced electrochemical kinetic processes of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with reference to milestone achievements to date. Application of such VGNO to aqueous ZABs (A-ZABs) and flexible solid-state ZABs (S-ZABs) exhibited high discharge power density and excellent cycling stability, which remarkably outperformed ZABs using benchmarked precious-metal based catalysts. The current work provides a solid basis toward developing low-cost, resource-sustainable, and eco-friendly ZABs without using any metals for outstanding OER and ORR catalysis.

Chloroform-Assisted Rapid Growth of Vertical Graphene Array and Its Application in Thermal Interface Materials

With the continuous progress in electronic devices, thermal interface materials (TIMs) are urgently needed for the fabrication of integrated circuits with high reliability and performance. Graphene as a wonderful additive is often added into polymer to build composite TIMs. However, owing to the lack of a specific design of the graphene skeleton, thermal conductivity of graphene-based composite TIMs is not significantly improved. Here a chloroform-assisted method for rapid growth of vertical graphene (VG) arrays in electric field-assisted plasma enhanced chemical vapor deposition (PECVD) system is reported. Under the optimum intensity and direction of electric field and by introducing the highly electronegative chlorine into the reactor, the record growth rate of 11.5 µm h-1 is achieved and VG with a height of 100 µm is successfully synthesized. The theoretical study for the first time reveals that the introduction of chlorine accelerates the decomposition of methanol and thus promotes the VG growth in PECVD. Finally, as an excellent filler framework in polymer matrix, VG arrays are used to construct a free-standing composite TIM, which yields a high vertical thermal conductivity of 34.2 W m-1  K-1 at the graphene loading of 8.6 wt% and shows excellent cooling effect in interfacial thermal dissipation of light emitting diode.

Enriching WPCs and NFPCs with Carbon Nanotubes and Graphene

Carbon nanotubes (CNTs) and graphene, with their unique mechanical, electrical, thermal, optical, and wettability properties, are very effective fillers for many types of composites. Recently, a number of studies have shown that CNTs and graphene may be integrated into wood–plastic composites (WPCs) and natural-fibre-reinforced polymer composites (NFPCs) to improve the existing performance of the WPCs/NFPCs as well as enabling their use in completely new areas of engineering. The following review analyses the results of the studies presented to date, from which it can be seen that that inclusion of CNTs/graphene may indeed improve the mechanical properties of the WPCs/NFPCs, while increasing their thermal conductivity, making them electroconductive, more photostable, less sensitive to water absorption, less flammable, and more thermally stable. This study indicates that the composition and methods of manufacturing of hybrid WPCs/NFPCs vary significantly between the samples, with a consequent impact on the level of improvement of specific properties. This review also shows that the incorporation of CNTs/graphene may enable new applications of WPCs/NFPCs, such as solar thermal energy storage devices, electromagnetic shielding, antistatic packaging, sensors, and heaters. Finally, this paper recognises key challenges in the study area, and proposes future work.

Insights into the Mechanism for Vertical Graphene Growth by Plasma-Enhanced Chemical Vapor Deposition

Vertically oriented graphene (VG) has attracted attention for years, but the growth mechanism is still not fully revealed. The electric field may play a role, but the direct evidence and exactly what role it plays remains unclear. Here, we conduct a systematic study and find that in plasma-enhanced chemical vapor deposition, the VG growth preferably occurs at spots where the local field is stronger, for example, at GaN nanowire tips. On almost round-shaped nanoparticles, instead of being perpendicular to the substrate, the VG grows along the field direction, that is, perpendicular to the particles' local surfaces. Even more convincingly, the sheath field is screened to different degrees, and a direct correlation between the field strength and the VG growth is observed. Numerical calculation suggests that during the growth, the field helps accumulate charges on graphene, which eventually changes the cohesive graphene layers into separate three-dimensional VG flakes. Furthermore, the field helps attract charged precursors to places sticking out from the substrate and makes them even sharper and turn into VG. Finally, we demonstrate that the VG-covered nanoparticles are benign to human blood leukocytes and could be considered for drug delivery. Our research may serve as a starting point for further vertical two-dimensional material growth mechanism studies.

The direct growth of planar and vertical graphene on Si(100) via microwave plasma chemical vapor deposition: synthesis conditions effects

In the present research, graphene was synthesized directly on a Si(100) substrate via combining direct microwave plasma-enhanced chemical vapor deposition and protective enclosure. The graphene flake orientation was controlled using suitable synthesis conditions. We revealed that high processing temperatures and plasma powers promote vertical graphene growth. The main related physical mechanisms were raised temperature gradients, thermal stress, ion bombardment, and elevated electric field effects. Lowering the synthesis temperature and plasma power resulted in planar graphene growth. An elevated synthesis temperature and long deposition time decreased the graphene layer number as the carbon desorption rate increased with temperature. Dominating defect types and their relationships to the graphene growth conditions were revealed. Planar graphene n-type self-doping was found due to substrate-based charge transfer. In the case of vertical graphene, the increased contact area between graphene and air resulted in the adsorption of more molecules, resulting in no doping or p-type doping.

In the present research, graphene was synthesized directly on a Si(100) substrate via combining direct microwave plasma-enhanced chemical vapor deposition and protective enclosure.

Molecular Understanding of the Effect of Hydrogen on Graphene Growth by Plasma-Enhanced Chemical Vapor Deposition

Plasma-enhanced chemical vapor deposition (PECVD) provides a low-temperature, highly-efficient, and catalyst-free route to fabricate graphene materials by virtue of the unique properties of plasma. In this paper, we conduct reactive...

Direct fabrication and characterization of vertically stacked Graphene/h-BN/Graphene tunnel junctions

We have used a lithography free technique for the direct fabrication of vertically stacked two-dimensional (2D) material-based tunnel junctions and characterized by Raman, AFM, XPS. We fabricated Graphene/h-BN/Graphene devices by direct deposition of graphene (bottom layer), h-BN (insulating barrier) and graphene (top layer) sequentially using a plasma enhanced chemical vapor deposition on Si/SiO2 substrates. The thickness of the h-BN insulating layer was varied by tuning the plasma power and the deposition time. Samples were characterized by Raman, AFM, and XPS. The I-V data follows the barrier thickness dependent quantum tunneling behavior for equally doped graphene layers. The resonant tunneling behavior was observed at room temperature for oppositely doped graphene layers where hydrazine and ammonia were used for n-doping of one of the graphene layers. The resonance with negative differential conductance occurs when the band structures of the two electrodes are aligned. The doping effect of the resonant peak is observed for varying doping levels. The results are explained according to the Bardeen tunneling model.

Vertical Graphene Arrays as Electrodes for Ultra-High Energy Density AC Line-Filtering Capacitors

High-frequency responsive electrochemical capacitor (EC), as an ideal lightweight filtering capacitor, can directly convert alternating current (AC) to direct current (DC). However, current electrodes are stuck in limited electrode area and tortuous ion transport. Herein, strictly vertical graphene arrays (SVGAs) prepared by electric-field-assisted plasma enhanced chemical vapour deposition have been successfully designed as the main electrode to ensure ions rapidly adsorb/desorb in richly available graphene surface. SVGAs exhibit an outstanding specific areal capacitance of 1.72 mF cm^-2 at Φ₁₂₀ =80.6° even after 500 000 cycles, which is far better than that of most carbon-related materials. Impressively, the output voltage could also be improved to 2.5 V when using organic electrolyte. An ultra-high energy density of 0.33 μWh cm^-2 can also be handily achieved. Moreover, ECs-SVGAs can well smooth arbitrary AC waveforms into DC signals, exhibiting excellent filtering performance.

Physical properties of carbon nanowalls synthesized by the ICP-PECVD method vs. the growth time

Investigation of the physical properties of carbon nanowall (CNW) films is carried out in correlation with the growth time. The structural, electronic, optical and electrical properties of CNW films are investigated using electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, UV-Vis spectroscopy, Hall Effect measurement system, Four Point Probing system, and thermoelectric measurements. Shorter growth time results in thinner CNW films with a densely spaced labyrinth structure, while a longer growth time results in thicker CNW films with a petal structure. These changes in morphology further lead to changes in the structural, optical, and electrical properties of the CNW.

Green Synthesis of Nanomaterials

Nanotechnology is considered one of the paramount forefronts in science over the last decade. Its versatile implementations and fast-growing demand have paved the way for innovative measures for the synthesis of higher quality nanomaterials. In the early stages, traditional synthesis methods were utilized, and they relied on both carcinogenic chemicals and high energy input for production of nano-sized material. The pollution produced as a result of traditional synthesis methods induces a need for environmentally safer synthesis methods. As the downfalls of climate change become more abundant, the scientific community is persistently seeking solutions to combat the devastation caused by toxic production methods. Green methods for nanomaterial synthesis apply natural biological systems to nanomaterial production. The present review highlights the history of nanoparticle synthesis, starting with traditional methods and progressing towards green methods. Green synthesis is a method just as effective, if not more so, than traditional synthesis; it provides a sustainable approach to nanomaterial manufacturing by using naturally sourced starting materials and relying on low energy processes. The recent use of active molecules in natural biological systems such as bacteria, yeast, algae and fungi report successful results in the synthesis of various nanoparticle systems. Thus, the integration of green synthesis in scientific research and mass production provides a potential solution to the limitations of traditional synthesis methods.

Adhesion-Enhanced Vertically Oriented Graphene on Titanium-Covered Quartz Glass toward High-Stability Light-Dimming-Related Applications

Improving the adhesion property of graphene directly grown on an insulating substrate is essential for promoting the reliability and durability of the related applications. However, effective approaches have rarely been reported, especially for vertically oriented graphene (VG) films grown on insulating templates. To tackle this, we have developed a facile synthetic strategy by introducing an ultrathin (10 nm-thick) titanium (Ti) film on a quartz glass substrate as the adhesion layer, for plasma-enhanced chemical vapor deposition (PECVD) growth of VG films. This synthetic process induces the formation of Ti, oxygen (O), carbon (C)-containing adhesion layer (Ti (O, C)), offering improved interfacial adhesion due to the formation of chemical bonds among Ti and C atoms. Dramatically improved surface and interface stabilities have been achieved, with regard to its counterpart without a Ti adhesion layer. Moreover, we have also realized precise controls of the transparent/conductive property, surface roughness, and hydrophobicity, etc., by varying the VG film growth time. We have also demonstrated the very intriguing application potentials of the hybrids in light-dimming related fields, that is, electro-heating defogging lenses and neutral density filters toward medical endoscope defogging and camera photography.

Ni(OH)2 derived Ni-MOF supported on carbon nanowalls for supercapacitors

Metal organic frameworks (MOFs) are expected to be promising pseudocapacitve materials because of their potential redox sites and porous structures. Nevertheless, the conductivity inferiority of MOF strongly decreases their structural advantages, therefore resulting in unsatisfying electrochemical performance. Herein, we propose an efficient strategy to enhance conductivity and thus electrochemical properties, in Ni(OH)₂ is electrochemically deposited on carbon nanowalls as the precursor for oriented MOF. The synthesized vertically oriented MOF sheets show an almost triple high capacitance of 677 F g^-1 than MOF powder of 239 F g^-1 at the current density of 2 A g^-1. Correspondingly, an asymmetric supercapacitor is fabricated, which can deliver a maximum energy density of 20.7 Wh kg^-1 and a maximum power density of 23 200 W kg^-1. These promising results indicate that modulating the conductivity of MOF is the key step to pursuit upgrading electrochemical performance.

Advanced Carbon-Nickel Sulfide Hybrid Nanostructures: Extending the Limits of Battery-Type Electrodes for Redox-Based Supercapacitor Applications

Transition-metal sulfides combined with conductive carbon nanostructures are considered promising electrode materials for redox-based supercapacitors due to their high specific capacity. However, the low rate capability of these electrodes, still considered "battery-type" electrodes, presents an obstacle for general use. In this work, we demonstrate a successful and fast fabrication process of metal sulfide-carbon nanostructures ideal for charge-storage electrodes with ultra-high capacity and outstanding rate capability. The novel hybrid binder-free electrode material consists of a vertically aligned carbon nanotube (VCN), terminated by a nanosized single-crystal metallic Ni grain; Ni is covered by a nickel nitride (Ni₃N) interlayer and topped by trinickel disulfide (Ni₃S₂, heazlewoodite). Thus, the electrode is formed by a Ni₃S₂/Ni₃N/Ni@NVCN architecture with a unique broccoli-like morphology. Electrochemical measurements show that these hybrid binder-free electrodes exhibit one of the best electrochemical performances compared to the other reported Ni₃S₂-based electrodes, evidencing an ultra-high specific capacity (856.3 C g^-1 at 3 A g^-1), outstanding rate capability (77.2% retention at 13 A g^-1), and excellent cycling stability (83% retention after 4000 cycles at 13 A g^-1). The remarkable electrochemical performance of the binder-free Ni₃S₂/Ni₃N/Ni@NVCN electrodes is a significant step forward, improving rate capability and capacity for redox-based supercapacitor applications.

Boost of Charge Storage Performance of Graphene Nanowall Electrodes by Laser-Induced Crystallization of Metal Oxide Nanostructures

Major research efforts are being carried out for the technological advancement to an energetically sustainable society. However, for the full commercial integration of electrochemical energy storage devices, not only materials with higher performance should be designed and manufactured but also more competitive production techniques need to be developed. The laser processing technology is well extended at the industrial sector for the versatile and high throughput modification of a wide range of materials. In this work, a method based on laser processing is presented for the fabrication of hybrid electrodes composed of graphene nanowalls (GNWs) coated with different transition-metal oxide nanostructures for electrochemical capacitor (EC) applications. GNW/stainless steel electrodes grown by plasma enhanced chemical vapor deposition were decorated with metal oxide nanostructures by means of their laser surface processing while immersed in aqueous organometallic solutions. The pseudocapacitive nature of the laser-induced crystallized oxide materials prompted an increase of the GNW electrodes' capacitance by 3 orders of magnitude, up to ca. 28 F/cm³ at 10 mV/s, at both the positive and negative voltages. Finally, asymmetric aqueous and solid-state ECs revealed excellent stability upon tens of thousands of charge-discharge cycles.

Applications of Plasma-Assisted Systems for Advanced Electrode Material Synthesis and Modification

Research on advanced electrode materials (AEMs) has been explosive for the past decades and constantly promotes the development of batteries, supercapacitors, electrocatalysis, and photovoltaic applications. However, traditional preparation and modification methods can no longer meet the increasing requirements of some AEMs because some of the special reactions are thermodynamically and/or kinetically unfavorable and thus need harsh conditions. Among various recently developed advanced materials synthesis and modification routes, the plasma-assisted (PA) method has received increasing attention because of its unique and different "species reactivity" nature, as well as its wider and adjustable operating conditions. In this Spotlight on Applications, we highlight some recent developments and describe our recent progress by applying PA systems in the synthesis and modification of AEMs, including direct processing, PA deposition, and plasma milling (P-milling). The mechanisms of how plasma works for specific reactions are reviewed and discussed. It is shown that the PA technique has become a powerful and efficient tool in the following areas, including but not limited to materials synthesis, doping, surface modification, and functionalization. Finally, the prospect and challenges are also proposed for AEM preparation and modification using PA systems. This article aims to provide up-to-date information about the progress of PA technology in the fields of chemistry and materials science.

Oxygen-Assisted Trimming Growth of Ultrahigh Vertical Graphene Films in a PECVD Process for Superior Energy Storage

Combining the advantages of a three-dimensional structure with intrinsic properties of graphene, vertical graphene (VG) synthesized by the plasma-enhanced chemical vapor deposition (PECVD) process has shown great promise to be applied to energy-storage electrodes. However, the practical application of the VG electrodes suffers from the limited height, which is mostly in a scale of few hundreds of nanometers, as shown in the previous studies. The reason for the unacceptable thin VG film deposition is believed to be the height saturation, stemming from the inevitable confluence of the VG flakes along with the deposition time. In this study, we developed an oxygen-assisted "trimming" process to eliminate the overfrondent graphene nanosheets thereby surmounting the saturation of the VG thickness during growth. In this approach, the height of the VGs reaches as high as 80 μm. Tested as supercapacitor electrodes, a desirable capacitance of 241.35 mF cm^-2 is obtained by the VG films, indicating the superior electrochemical properties and the potential for applications in energy storage. It is worth noting, this thickness is by no means the maximum that can be achieved with our synthesis technique and higher capacitance can be achieved by conducting the circulating deposition-correction process in our work.

Significant Capacitance Enhancement via In Situ Exfoliation of Quasi-One-Dimensional Graphene Nanostripes in Supercapacitor Electrodes

Graphene has received much attention as a supercapacitor electrode material due to its chemical inertness in preventing reaction with electrolytes and the large surface area due to its two-dimensional nature. However, when graphene sheets are processed into electrodes, they tend to stack together and form a turbostratic graphite material with a much reduced surface area relative to the total surface area of individual graphene sheets. Separately, electrochemical exfoliation of graphite is one method of producing single-layer graphene, which is often used to produce graphene for supercapacitor electrodes, although such exfoliated graphene still leads to reduced surface areas due to stacking during electrode fabrication. To utilize the large surface area of graphene, graphene must be exfoliated in situ within a supercapacitor device after the device fabrication. However, graphitic electrodes are typically destroyed upon exfoliation, which is largely due to the loss of electrical connectivity among small exfoliated graphene flakes. Here, we report successful in situ exfoliation of graphene nanostripes, a type of quasi-one-dimensional graphene nanomaterial with large length-to-width aspect ratios, as the anode material in supercapacitors. We find that the in situ exfoliation leads to over 400% enhancement in capacitance as the result of retaining the electrical connectivity among exfoliated quasi-one-dimensional graphene nanostripes in addition to increasing the total surface area, paving ways to fully realizing the benefit of graphene electrodes in supercapacitor applications.

Plasma Assisted Reduction of Graphene Oxide Films

The past decade has seen enormous efforts in the investigation and development of reduced graphene oxide (GO) and its applications. Reduced graphene oxide (rGO) derived from GO is known to have relatively inferior electronic characteristics when compared to pristine graphene. Yet, it has its significance attributed to high-yield production from inexpensive graphite, ease of fabrication with solution processing, and thus a high potential for large-scale applications and commercialization. Amongst several available approaches for GO reduction, the mature use of plasma technologies is noteworthy. Plasma technologies credited with unique merits are well established in the field of nanotechnology and find applications across several fields. The use of plasma techniques for GO development could speed up the pathway to commercialization. In this report, we review the state-of-the-art status of plasma techniques used for the reduction of GO-films. The strength of various techniques is highlighted with a summary of the main findings in the literature. An analysis is included through the prism of chemistry and plasma physics.

Controllable Preparation of 3D Graphene with Different Morphologies for High-Performance Electrode Materials

Synthesis of three-dimensional (3D) graphene with controlled morphologies has been achieved by changing the freeze-drying process of graphene in this paper. The obtained vertically aligned graphene (VAGN) is stand-up and has a uniform, dense, and porous network, while the obtained graphene foam (GF) just has a cross-linked 3D porous framework. In addition, the possible growth mechanisms of these nanostructures have been studied based on the experimental results. Furthermore, the effects of morphologies on their electrochemical performances have been investigated. The result shows that the VAGN-based supercapacitor has a higher specific capacitance (C _sc) of 182 F g^-1 at 0.5 A g^-1 than that of GF (160 F g^-1 at 0.5 A g^-1). The supercapacitors prepared by VAGN and GF retain about 95.7 and 92.7% of C _sc, respectively, after 2000 charge-discharge processes. When the power density of the supercapacitor prepared by VAGN is about 50.3 W kg^-1, its maximum energy density can reach 7.05 Wh kg^-1, which is higher than that of GF. This shows that VAGN has better electrochemical performance than GF. It might be because the aligned structure of VAGN plays an important role in reducing the internal resistance of the electrodes and accelerating ion and electron transport. Three-dimensional graphene with different morphologies might have potential applications as electrode materials for supercapacitors.

Selective formation of pyridinic-type nitrogen-doped graphene and its application in lithium-ion battery anodes

We report a high-yield single-step method for synthesizing nitrogen-doped graphene nanostripes (N-GNSPs) with an unprecedentedly high percentage of pyridinic-type doping (>86% of the nitrogen sites), and investigate the performance of the resulting N-GNSPs as a lithium-ion battery (LIB) anode material. The as-grown N-GNSPs are compared with undoped GNSPs using scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), helium ion-beam microscopy (HIM), and electrochemical methods. As an anode material we find that pyridinic-type N-GNSPs perform similarly to undoped GNSPs, suggesting that pyridinic sites alone are not responsible for the enhanced performance of nitrogen-doped graphene observed in previous studies, which contradicts common conjectures. In addition, post-mortem XPS measurements of nitrogen-doped graphene cycled as a lithium-ion battery anode are conducted for the first time, which reveal direct evidence for irreversible chemical changes at the nitrogen sites during cycling. These findings therefore provide new insights into the mechanistic models of doped graphene as LIB anodes, which are important in improving the anode designs for better LIB performance.

Selectively pyridinic type nitrogen-doped graphene is fabricated and applied as a lithium ion battery anode including post mortem characterization.

Ultrathin Nanosheet-Assembled Co-Fe Hydroxide Nanotubes: Sacrificial Template Synthesis, Topotactic Transformation, and Their Application as Electrocatalysts for Efficient Oxygen Evolution Reaction

Hydrogen as a reliable, sustainable, and efficient energy carrier can effectively alleviate global environmental issues and energy crisis. However, the electrochemical splitting of water for large-scale hydrogen generation is still impeded by the sluggish kinetics of the oxygen evolution reaction (OER) at the anode. Considering the synergistic effect of Co and Fe on the improvement of OER catalytic activity, we prepared Co-Fe hydroxide nanotubes through a facile sacrificial template route. The resultant Co_0.8Fe_0.2 hydroxide nanotubes exhibited remarkable electrocatalytic performance for OER in 1.0 M KOH, with a small overpotential of about 246 mV at 10 mA cm^-2 and a Tafel slope of 53 mV dec^-1. The Co_0.8Fe_0.2P nanotubes were further prepared by a phosphidation treatment, exhibiting excellent OER catalytic performance with an overpotential as low as 240 mV at 10 mA cm^-2. Besides, the Co_0.8Fe_0.2P nanotubes supported on a Ni foam (Co_0.8Fe_0.2P/NF) used as both positive and negative poles in a two-electrode system achieved a cell voltage of about 1.67 V at 10 mA cm^-2 and exhibited outstanding stability. A water splitting system was constructed by Co_0.8Fe_0.2P/NF electrodes connected with a crystalline silicon solar cell, demonstrating the application as an electrocatalyst.

Low-temperature low-power PECVD synthesis of vertically aligned graphene

The need for 2D vertical graphene nanosheets (VGNs) is driven by its great potential in diverse energy, electronics, and sensor applications, wherein many cases a low-temperature synthesis is preferred due to requirements of the manufacturing process. Unfortunately, most of today's known methods, including plasma, require either relatively high temperatures or high plasma powers. Herein, we report on a controllable synthesis of VGNs at a pushed down low-temperature boundary for synthesis, the low temperatures (450 °C) and low plasma powers (30 W) using capacitively coupled plasma (CCP) driven by radio-frequency power at 13.56 MHz. The strategies implemented also include unrevealing the role of Nickel (Ni) catalyst thin film on the substrates (Si/Al). It was found that the Ni catalyst on Si/Al initiates the nucleation/growth of VGNs at 450 °C in comparison to the substrates without Ni catalyst. With increasing temperature, the graphene nanosheets become bigger in size, well-structured and well separated. The role of Ni catalysts is hence to boost the growth rate, density, and quality of the growing VGNs. Furthermore, this CCP method can be used to synthesize VGNs at the lowest temperatures possible so far on a variety of substrates and provide new opportunities in the practical application of VGNs.