Vertically-aligned Graphene Research Articles

Self-Assembly for Creating Vertically-Aligned Graphene Micro Helices with Monolayer Graphene as Chiral Metamaterials.

Graphene's emergence enables creating chiral metamaterials in helical shapes for terahertz (THz) applications, overcoming material limitations. However, practical implementation remains theoretical due to fabrication challenges. This paper introduces a dual-component self-assembly technique that enables creating vertically-aligned continuous monolayer graphene helices at microscale with great flexibility and high controllability. This assembly process not only facilitates the creation of 3D microstructures, but also positions the 3D structures from a horizontal to a vertical orientation, achieving an aspect ratio (height/width) of ≈2700. As a result, an array of vertically-aligned graphene helices is formed, reaching up to 4mm in height, which is equivalent to 4 million times the height of monolayer graphene. The benefit of these 3D chiral structures made from graphene is their capability to infinitely extend in height, interacting with light in ways that are not possible with traditional 2D layering methods. Such an impressive height elevates a level of interaction with light that far surpasses what is achievable with traditional 2D layering methods, resulting in a notable enhancement of optical chirality properties. This approach is applicable to various 2D materials, promising advancements in innovative research and diverse applications across fields.

Tin Oxide/Vertically Aligned Graphene Hybrid Electrodes Prepared by Sonication-Assisted Sequential Chemical Bath Deposition for High-Performance Supercapacitors.

Hybrid electrodes comprising metal oxides and vertically aligned graphene (VAG) are promising for high-performance supercapacitor applications because they enhance the synergistic effect owing to the large contact area between the two constituent materials. However, it is difficult to form metal oxides (MOs) up to the inner surface of a VAG electrode with a narrow inlet using conventional synthesis methods. Herein, we report a facile approach to fabricate SnO2 nanoparticle-decorated VAG electrodes (SnO2@VAG) with excellent areal capacitance and cyclic stability using sonication-assisted sequential chemical bath deposition (S-SCBD). The sonication treatment during the MO decoration process induced a cavitation effect at the narrow inlet of the VAG electrode, allowing the precursor solution to reach the inside of the VAG surface. Furthermore, the sonication treatment promoted MO nucleation on the entire VAG surface. Thus, the SnO2 nanoparticles uniformly covered the entire electrode surface after the S-SCBD process. SnO2@VAG exhibited an outstanding areal capacitance (4.40 F cm-2) up to 58% higher than that of VAG electrodes. The symmetric supercapacitor with SnO2@VAG electrodes showed an excellent areal capacitance (2.13 F cm-2) and a cyclic stability of 90% after 2000 cycles. These results suggest a new avenue for sonication-assisted fabrication of hybrid electrodes in the field of energy storage.

Vertically aligned graphene prepared by the electrochemical anodization of graphite foil for supercapacitor electrodes

Vertically aligned graphene (VAG) improves the specific capacitance of electric double layer capacitor (EDLC) electrodes, because VAGs increase the specific surface area of electrode and promote ion diffusion. Herein, we demonstrate a facile approach for preparing the VAGs with outstanding areal specific capacitance by electrochemical anodization of graphite foil (GF). As the anodizing time increases, the graphite layers in the anodized GF (AGF) electrodes expand due to ion intercalation and bubbles. Each graphite layer is vertically aligned, resulting in the formation of VAG sheets on the GF surface. The AGF electrode exhibits an excellent specific areal capacitance of 2.79 Fcm−2 at a current density of 2 mAcm−2, which is higher than that of the GF electrode by a factor of 17.43. The EDLC device using AGF electrodes exhibits an outstanding specific areal capacitance of 1.43 Fcm−2 at a current density of 2 mAcm−2 and a favorable cycling stability retaining 90% of its initial capacitance after 2000 cycles at a current density of 20 mAcm−2. This AGF electrode fabrication method is simple, cost-effective, and eco-friendly. We believe that the fabrication of AGF electrodes may be a significant contribution to the development of high-performance supercapacitors as well as the eco-friendly energy industry.

Enhanced thermal transport and corrosion resistance by coating vertically-aligned graphene on zirconium alloy for nuclear reactor applications

Owing to the requirement of higher efficiency, safety reliability and lower maintenance cost for nuclear reactors, the performance improvement of zirconium (Zr) alloy cladding coating materials is becoming increasingly significant. Graphene, with its inherent properties like ultrahigh thermal conductivity, excellent chemical stability, and super-hydrophobicity, has demonstrated enormous potential in the field of corrosion prevention and thermal management, indicating its promising application in cladding materials. Herein, plasma-enhanced chemical vapor deposition (PECVD) was employed for growing vertically-aligned graphene (VG) array on Zr alloy (ZA) substrates. In the deposition process, C2H4 gas was introduced into the reactor as the carbon source and the reactor was kept at a growth temperature range of 650–800 °C, which is 300–400 °C lower than that for traditional graphene synthesis using thermal chemical vapor deposition (CVD). The effects of other growth experimental parameters such as gas flow ratio, radio-frequency power (RF-power), growth temperature, reaction pressure and deposition time were systematically investigated. The corrosion resistance and thermal transport performance of vertical graphene enhanced Zr alloy (VGZA) were studied by using electrochemical corrosion method and heat dissipation test system. The results showed that the samples loaded with vertical graphene could effectively improve the corrosion resistance and heat transport property.

Dendrite-free lithium deposition enabled by a vertically aligned graphene pillar architecture

Uncontrollable dendrite growth and poor reversibility of lithium (Li) metal anode hinder the practical implementation of Li metal based batteries. Herein, we report a feasible and scalable electrospraying strategy for fabricating a vertically aligned graphene (VAG) pillar architecture with low tortuosity and high specific surface area as both current collector and host material for Li metal anode. Columnar and dense Li with large granular size plates into the VAG electrode by the aid of the pillar structure, which could ensure continuous ion transfer and lower actual current density for uniform nucleation, as well as provide more accommodation with proper space size. The hollow graphene pillars and the surrounding sufficient space enables smooth Li plating under high capacity depth and abstains volume fluctuation of the whole electrode. The VAG pillar modified Li anode delivers low overpotential of 25 mV over 280 h under a high rate of 3 mA cm−2, and high Coulombic efficiencies for 250 cycles at a capacity of 1 mAh cm−2. More importantly, full cells paired with the high capacity LiFePO4 cathode exhibit excellent cycling stability and rate capability, demonstrating the high Li utilization efficiency and improved ion migration kinetic in the low tortuous VAG architecture.

Low-temperature synthesis of vertically aligned graphene through microwave-assisted chemical vapour deposition

The intrinsic properties of vertically aligned graphene (VG), such as a high ratio of exposed active edge sites compared to inert basal sites, non-stacking morphology and large surface-to-volume ratio make it applicable to numerous advanced technologies such as sensors, flexible electronics and fuel cells. Plasma-enhanced chemical vapour deposition has emerged as a promising technique to synthesise graphene at lower temperatures than a conventional chemical vapour deposition (CVD) process, providing a better control over the deposition parameters to tailor graphene properties.This study presents a cost effective, scalable, single step synthesis of a high quality VG at low temperatures. The samples were synthesised in a microwave (MW) assisted CVD reactor, which allowed for the decomposition of CH4 and H2 gas mixtures in Ar as a carrier gas, and VG growth directly onto Si wafer substrates without any additional heat applied to the substrate. Deposition conditions, such as MW power, gas ratio, and substrate-to-plasma separation were optimized to tailor the morphology, growth rate and electrochemical performance of the VG. The physiochemical analysis revealed that the fabricated VG consisted of high quality vertically aligned sp2 graphene, whilst a cyclic voltammetry analysis confirmed that the structures are electrochemically active and demonstrate a typical characteristics of quasi-reversible processes.

Optoelectronic synapses using vertically aligned graphene/diamond heterojunctions

The present work demonstrates that vertically aligned graphene (VG)/diamond heterojunctions can function as optoelectronically-controllable synapses, which are connection parts of neurons and key elements for the memory functions of human brain. These junctions mimic several of the fundamental characteristics of biological synapses, including producing an excitatory postsynaptic current (EPSC), exhibiting a transition from short-term memory (STM) to long-term memory (LTM) states, and paired pulse facilitation (PPF), all of which are optically controllable. These junctions also exhibit photo-sensing properties and photo-controllable synaptic plasticity, meaning that they have similar functions as occur in the human brain and retina. Arrays fabricated from these junctions were found to function as image sensors that could provide an optical memory function and selectively memorize information depending on the relative importance of the data. These results are expected to assist in the realization of neuromorphic optical computers simulating human visual memory systems that are modulated by specific interests.

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 (Csc) 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 Csc, 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.

Vertically Aligned Graphene Prepared by Photonic Annealing for Ultrasensitive Biosensors.

Graphene exhibits excellent physical, electronic, and chemical properties that are highly desirable for biosensing applications. However, most graphene biosensors are based on graphene lying flat on a substrate and therefore do not utilize its maximum specific surface area for ultrasensitive detection. Herein, we report the novel use of photonic annealing on a flexographically printed graphene-ethyl cellulose composite to produce vertically aligned graphene (VAG) biosensors for ultrasensitive detection of algal toxins in drinking water. These VAG structures, which maximized the specific surface area of graphene, were formed by partial removal of the polymeric binder upon applying intense pulsed light on the printed graphene. A label-free and low-cost VAG biosensor based on a non-faradaic electrochemical impedance spectroscopy technique was fabricated. The biosensor exhibited a limit of detection of 1.2 ng/L for microcystin-LR in local tap water. Such an ultrasensitive VAG biosensor is suitable for low-cost mass production using an integrated roll-to-roll flexographic printing with rapid photonic annealing technique.

Engineering high-defect densities across vertically-aligned graphene nanosheets to induce photocatalytic reactivity

The fabrication of graphene nanostructures, with a variety of morphologies and densities of defective sites, can be a promising tool to tune their characteristics towards photocatalytic applications, without the need for external dopants. In this study, the impact of morphological properties in terms of the orientation and defect concentrations of graphene nanostructures is demonstrated to support the development of active photocatalytic sites across graphitic structures. Vertically-aligned graphene nanosheets were grown across carbon fibres via electron cyclotron resonance microwave plasma chemical vapour deposition, to yield a range of different wall densities and edge functionalities. The variation of growth conditions was correlated to the photocatalytic activity for the degradation of methylene blue dye under ultra-violet and visible light. The chemical state of oxygen content hybridized with nanosheets was studied by X-ray photoelectron spectroscopy and correlated to the growth conditions and photocatalytic performance. The fastest degradation rate of dye was found on the graphene samples which were grown at 800 °C for 240 min, with a kinetic constant of 46.6 × 10−4 min−1. Such performance has not been observed to date for any graphitic materials and is shown to be on the same order of performance as the conventional photocatalytic materials.

Electrochemical properties of vertically aligned graphenes: tailoring heterogeneous electron transfer through manipulation of the carbon microstructure.

The electrochemical response of different morphologies (microstructures) of vertically aligned graphene (VG) configurations is reported. Electrochemical properties are analysed using the outer-sphere redox probes Ru(NH3)62+/3+ (RuHex) and N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD), with performances de-convoluted via accompanying physicochemical characterisation (Raman, TEM, SEM, AFM and XPS). The VG electrodes are fabricated using an electron cyclotron resonance chemical vapour deposition (ECR-CVD) methodology, creating vertical graphene with a range of differing heights, spacing and edge plane like-sites/defects (supported upon underlying SiO2/Si). We correlate the electrochemical reactivity/response of these novel VG configurations with the level of edge plane sites (%-edge) comprising their structure and calculate corresponding heterogeneous electron transfer (HET) rates, k0. Taller VG structures with more condensed layer stacking (hence a larger global coverage of exposed edge plane sites) are shown to exhibit improved HET kinetics, supporting the claims that edge plane sites are the predominant source of electron transfer in carbon materials. A measured k0eff of ca. 4.00 × 10−3 cm s−1 (corresponding to an exposed surface coverage of active edge plane like-sites/defects (% θedge) of 1.00%) was evident for the tallest and most closely stacked VG sample, with the inverse case true, where a VG electrode possessing large inter-aligned-graphene spacing and small flake heights exhibited only 0.08% of % θedge and a k0eff value one order of magnitude slower at ca. 3.05 × 10−4 cm s−1. Control experiments are provided with conventional CVD (horizontal) grown graphene and the edge plane of highly ordered pyrolytic graphite (EPPG of HOPG), demonstrating that the novel VG electrodes exhibit ca. 3× faster k0 than horizontal CVD graphene. EPPG exhibited the fastest HET kinetics, exhibiting ca. 2× larger k0 than the best VG. These results are of significance to those working in the field of 2D-carbon electrochemistry and materials scientists, providing evidence that the macroscale electrochemical response of carbon-based electrodes is dependent on the edge plane content and showing that a range of structural configurations can be employed for tailored properties and applications.

Platinum nanoparticle decorated vertically aligned graphene screen-printed electrodes: electrochemical characterisation and exploration towards the hydrogen evolution reaction.

We present the fabrication of platinum (Pt0) nanoparticle (ca. 3 nm average diameter) decorated vertically aligned graphene (VG) screen-printed electrodes (Pt/VG-SPE) and explore their physicochemical characteristics and electrocatalytic activity towards the hydrogen evolution reaction (HER) in acidic media (0.5 M H2SO4). The Pt/VG-SPEs exhibit remarkable HER activity with an overpotential (recorded at -10 mA cm-2) and Tafel value of 47 mV (vs. RHE) and 27 mV dec-1. These values demonstrate the Pt/VG-SPEs as significantly more electrocatalytic than a bare/unmodified VG-SPE (789 mV (vs. RHE) and 97 mV dec-1). The uniform coverage of Pt0 nanoparticles (ca. 3 nm) upon the VG-SPE support results in a low loading of Pt0 nanoparticles (ca. 4 μg cm-2), yet yields comparable HER activity to optimal Pt based catalysts reported in the literature, with the advantages of being comparatively cheap, highly reproducible and tailorable platforms for HER catalysis. In order to test any potential dissolution of Pt0 from the Pt/VG-SPE surface, which is a key consideration for any HER catalyst, we additively manufactured (AM) a bespoke electrochemical flow cell that allowed for the electrolyte to be collected at regular intervals and analysed via inductively coupled plasma optical emission spectroscopy (ICP-OES). The AM electrochemical cell can be rapidly tailored to a plethora of geometries making it compatible with any size/shape of electrochemical platform. This work presents a novel and highly competitive HER platform and a novel AM technique for exploring the extent of Pt0 nanoparticle dissolution upon the electrode surface, making it an essential study for those seeking to test the stability/catalyst discharge of their given electrochemical platforms.

Vertically-aligned graphene nanowalls grown via plasma-enhanced chemical vapor deposition as a binder-free cathode in Li–O2 batteries

In the present report, vertically-aligned graphene nanowalls are grown on Ni foam (VA-G/NF) using plasma-enhanced chemical vapor deposition method at room temperature. Optimization of the growth conditions provides graphene sheets with controlled defect sites. The unique architecture of the vertically-aligned graphene sheets allows sufficient space for the ionic movement within the sheets and hence enhancing the catalytic activity. Further modification with ruthenium nanoparticles (Ru NPs) drop-casted on VA-G/NF improves the charge overpotential for lithium–oxygen (Li–O2) battery cycles. Such reduction we believe is due to the easier passage of ions between the perpendicularly standing graphene sheets thereby providing ionic channels.

Supercapacitors: High‐Stacking‐Density, Superior‐Roughness LDH Bridged with Vertically Aligned Graphene for High‐Performance Asymmetric Supercapacitors (Small 37/2017)

In article number 1701288, by Chang Yu, Jieshan Qiu, and co-workers, high-stacking-density, superior-roughness layered double hydroxide (LDH) bridged with vertically aligned graphene (VG) is rationally designed and successfully configured. The VG nanosheets are capable of promoting mass and electron transfer, and high-stacking-density LDH induces a superior hydrophilic behavior, thus leading to a greatly improved performance for asymmetric supercapacitors.

High-Stacking-Density, Superior-Roughness LDH Bridged with Vertically Aligned Graphene for High-Performance Asymmetric Supercapacitors.

The high-performance electrode materials with tuned surface and interface structure and functionalities are highly demanded for advanced supercapacitors. A novel strategy is presented to conFigure high-stacking-density, superior-roughness nickel manganese layered double hydroxide (LDH) bridged by vertically aligned graphene (VG) with nickel foam (NF) as the conductive collector, yielding the LDH-NF@VG hybrids for asymmetric supercapacitors. The VG nanosheets provide numerous electron transfer channels for quick redox reactions, and well-developed open structure for fast mass transport. Moreover, the high-stacking-density LDH grown and assembled on VG nanosheets result in a superior hydrophilicity derived from the tuned nano/microstructures, especially microroughness. Such a high stacking density with abundant active sites and superior wettability can be easily accessed by aqueous electrolytes. Benefitting from the above features, the LDH-NF@VG can deliver a high capacitance of 2920 F g-1 at a current density of 2 A g-1 , and the asymmetric supercapacitor with the LDH-NF@VG as positive electrode and activated carbon as negative electrode can deliver a high energy density of 56.8 Wh kg-1 at a power density of 260 W kg-1 , with a high specific capacitance retention rate of 87% even after 10 000 cycles.

Facile growth of vertically-aligned graphene nanosheets via thermal CVD: The experimental and theoretical investigations

Owing to the distinctively morphological and structural features, vertically-aligned graphene nanosheets (VGs) possess many unique properties and hold great promise for applications in various fields. For controllable preparation and wide application of VGs, the establishing reliable growth method and profound understanding of the growth mechanism are of vital significance. Up to date, VGs are normally produced by plasma-enhanced chemical vapor deposition (PECVD) and it's considered that plasma is an indispensible factor for the vertical alignment of graphene sheets. Herein, for the first time, we report the facile and controllable VGs growth via a thermal CVD by precisely tuning growth parameters. Experimental observations in combination with detailed energy calculations reveal that the flow rate of carbon precursor determines the growth dynamics of graphene in CVD. This work offers a novel and reliable technique for VGs preparation and provides new insights into the intrinsic mechanism of vertical graphene growth. Furthermore, benefiting from the ultra-high density of edge sites, thin thickness, and outstanding electrical conductivity of VGs, the as-prepared VGs exhibit excellent field-emission performance such as ultra-low turn-on electric field and threshold field down to 1.07 and 1.65 V μm−1, respectively.

Graphene-based electronic biosensors

Abstract

High Packing Density Unidirectional Arrays of Vertically Aligned Graphene with Enhanced Areal Capacitance for High-Power Micro-Supercapacitors.

Interfacial integration of a shape-engineered electrode with a strongly bonded current collector is the key for minimizing both ionic and electronic resistance and then developing high-power supercapacitors. Herein, we demonstrated the construction of high-power micro-supercapacitors (VG-MSCs) based on high-density unidirectional arrays of vertically aligned graphene (VG) nanosheets, derived from a thermally decomposed SiC substrate. The as-grown VG arrays showed a standing basal plane orientation grown on a (0001̅) SiC substrate, tailored thickness (3.5-28 μm), high-density structurally ordering alignment of graphene consisting of 1-5 layers, vertically oriented edges, open intersheet channels, high electrical conductivity (192 S cm-1), and strong bonding of the VG edges to the SiC substrate. As a result, the demonstrated VG-MSCs displayed a high areal capacitance of ∼7.3 mF cm-2 and a fast frequency response with a short time constant of 9 ms. Furthermore, VG-MSCs in both an aqueous polymer gel electrolyte and nonaqueous ionic liquid of 1-ethyl-3-methylimidazolium tetrafluoroborate operated well at high scan rates of up to 200 V s-1. More importantly, VG-MSCs offered a high power density of ∼15 W cm-3 in gel electrolyte and ∼61 W cm-3 in ionic liquid. Therefore, this strategy of producing high-density unidirectional VG nanosheets directly bonded on a SiC current collector demonstrated the feasibility of manufacturing high-power compact supercapacitors.

Optical properties of vertically aligned graphene sheets

ABSTRACT We have developed deposition of vertically aligned graphene sheets on Cu foils by surface wave microwave plasma CVD and the transfer from Cu foil to quartz substrate to evaluate optical reflectances and transmittances of the inherent vertical aligned graphene sheets. Both reflectance and transmittance spectra are almost independent of incident angles in the range between 300 and 800nm. The reflectance is lower than 0.067%, which is lower than those of the commercial black alumite plate. The transmittances are less than the detection limit of the system. It is considered that the obtained low reflectance is attributed to the unique structure of the vertically aligned graphene sheets.

Morphological Characterization of Graphene Flake Networks Using Minkowski Functionals

Two Minkowski functionals were tested in the capacity of morphological descriptors to quantitatively compare the arrays of vertically-aligned graphene flakes grown on smooth and nanoporous alumina and silica surfaces. Specifically, the Euler-Poincare characteristic and fractal dimension graphs were used to characterize the degree of connectivity and order in the systems, i.e. in the graphene flake patterns of petal-like and tree-like morphologies on solid substrates, and meshlike patterns (networks) grown on nanoporous alumina treated in low-temperature inductivelycoupled plasma. It was found that the Minkowski functionals return higher connectivity and fractal dimension numbers for the graphene flakepatterns with more complex morphologies, and indeed can be used as morphological descriptors to differentiate among various configurations of vertically-aligned graphene flakes grown on surfaces.