Vertically-oriented Graphene Research Articles

Perspective of Using Vertically Oriented Graphene as an Electrochemical Biosensing Platform

Electrochemical sensors based on vertically oriented graphene (VG) have gained attention in recent years due to the unique properties of VG, such as its large surface area, biocompatibility, and high electrical conductivity. In this paper, we studied an electrochemical sensor with interdigitated electrodes modified with VG as an essential interface for the identification of two types of human colon adenocarcinoma cells: SW403 (high invasiveness) and HT29 (low invasiveness). Both cell lines have epithelial morphology, and we tested the electrochemical sensor on different concentrations of SW403 and HT29 cells. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used for morphological characterization of VG deposited on the working interdigitated electrodes, Raman spectroscopy was used to evaluate the graphitic nature of the VG growth on electrodes, and atomic force microscopy (AFM) was used to study the rugosity of the VG. Fourier-transform infrared spectrometry (FTIR) was used to study the configurations of the chemical bonds in the VG used for the working electrode of the electrochemical sensor. Vertically oriented graphene improves the sensor’s response on the cell lines, as evaluated by electrochemical impedance spectroscopy (EIS).

Microfluidic Printing of Vertically-Oriented Nanosheets/MOFs Hetero-Interface for Intensive Pseudocapacitive Storage.

Efficient manufacture of electroactive vertically-oriented nanosheets with enhanced electrolyte mass diffusion and strong interfacial redox dynamics is critical for realizing high energy density of miniature supercapacitor (SC), but still challenging. Herein, microfluidic droplet printing is developed to controllably construct vertically-oriented graphene/ZIF-67 hetero-microsphere (VAGS/ZIF-67), where the ZIF-67 is coordinately grown on vertically-oriented graphene framework via Co─O─C bonds. The VAGS/ZIF-67 shows ordered porous channel, high electroactivity and strong interfacial interaction, providing rapid electrolyte diffusion dynamics and high faradaic capacitance in KOH solution (1674 F g-1 , 1004 C g-1 ), which are verified by computational fluid dynamics (CFD) and density functional theory (DFT). Moreover, the VAGS/ZIF-67 based SC exhibits large energy density (100Wh kg-1 ), excellent durability (10000 cycles and high/low temperature), and robust power-supply applications in portable electronics.

Efficient flexible electrodes for lithium-ion batteries utilizing well-dispersed hybrid Mo2C nanoparticles on vertically-oriented graphene nanowalls

Flexible lithium-ion batteries (LIBs) have gained significant interest as potential power source solutions for wearable and flexible electronic devices. However, the fabrication of flexible LIBs with optimal flexibility, mechanical stability, and high energy density remains a formidable challenge for researchers. Transition metal carbides are being investigated as capable anode materials for advanced lithium-ion batteries. In this study, we explore the growth of molybdenum carbides (Mo2C) and vertically-oriented graphene nanowalls (VGNWs) on flexible graphite paper (Papyex®) and their potential application as anode material for Lithium-Ion batteries (LIBs). Our approach involves a bottom-up synthesis of binder-free hybrid electrodes through the deposition of Mo carbide nanostructures on VGNWs using a combination of chemical vapour deposition, magnetron sputtering, and thermal annealing processes. The hybrid structure of Mo2C/VGNWs exhibits distinct and specialized attributes, including remarkable structural durability, small particle size, and a porous configuration. These features effectively promote the accessibility of electrons and ions to the interface between the electrode and electrolyte. Our electrochemical tests demonstrate that the Mo2C/VGNWs hybrids show superior lithium storage behavior when compared to VGNWs/Papyex® electrodes. The synergistic effects of the Mo2C nanoparticles and the highly conductive VGNW are mainly responsible for the enhanced electrochemical characteristics.

Position-Induced Controllable Growth of Vertically Oriented Graphene Using Plasma-Enhanced Chemical Vapor Deposition.

Because the morphology of vertically oriented graphene (VG) synthesized by the plasma-enhanced chemical vapor deposition process determines the application performance of VG, morphology control is always an important part of the research. A concise correspondence between plasma and the morphology of VG is the key to investigating the morphology control of VG, which is still under research. In this study, a simple but effective parameter, position, is used to grow VG, by which the continuous morphology evolution of VG is realized. As a result, the morphology of VGs varies from a porous structure to a "wall-like" structure, thus leading to a continuous change in its hydrophobicity and thermal emissivity. An ultrahigh emissivity of 0.999 with superhydrophobicity is obtained among these VGs, showing great potential in the area of the black body and infrared thermometer. Finally, the states of active particles in plasma depending on the positions are diagnosed to investigate their relations with the morphology of VGs.

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

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

Synthesis of Graphene and Related Materials by Microwave-Excited Surface Wave Plasma CVD Methods

Several kinds of chemical vapor deposition (CVD) methods have been extensively used in the semiconductor industries for bulk crystal growth, thin film deposition, and nanomaterials synthesis. In this article, we focus on the microwave-excited surface wave plasma CVD (MW-SWP CVD) method for growth of graphene and related materials. The MW-SWP CVD system consisting of waveguide, slot antenna, and dielectric windows is significant for generating high density plasma with low electron temperature, enabling low temperature growth of materials without damaging the surface of base substrates. The synthesis of graphene and hexagonal boron nitride (hBN) films has been achieved on metals, semiconductors, insulators, and dielectric substrates for application in photovoltaics, sensors, batteries, supercapacitors, fuel cells, and various other electronic devices. The details of the synthesis process for graphene films, vertically-oriented graphene, doped-graphene, and hBN films by the MW-SWP CVD method are summarized to understand the growth mechanism, which will enable further development of the plasma CVD process for material synthesis at a low temperature for industrial applications.

Direct Plasma‐Enhanced‐Chemical‐Vapor‐Deposition Syntheses of Vertically Oriented Graphene Films on Functional Insulating Substrates for Wide‐Range Applications

AbstractGraphene has inspired wide‐range research interests and practical applications due to its intriguing materials properties and versatile application prospects. Specifically, vertically oriented graphene (VG) constructed by vertically aligned nanosheets emerges as a unique material type, usually achieved through a plasma‐enhanced chemical vapor deposition (PECVD) route. The VG films possess abundant exposed active edges, large surface areas, relatively high electrical conductivity, and excellent thermal property in both in‐plane and out‐of‐plane directions. Direct synthesizing of such VG films on various functional substrates enables their transfer‐free integrations and direct applications in thermal management, sensing, energy storage and conversion, etc. In this review article, it is planned to showcase the recent advances in direct PECVD growth of VG films on some novel functional insulating substrates (e.g., glass, functional ceramics), mainly in terms of improving the electrical conductivity, enhancing the interfacial adhesion, and graphene growth orientation modulation. Meanwhile, the applications of the PECVD‐derived products in various new fields will also be discussed, such as photothermal conversion, switchable windows, light attenuation in photography, transparent heating, and thermal management for high‐power devices. Finally, current challenges and future opportunities will also be presented regarding controllable syntheses of high‐quality VG films and practical application explorations in different fields.

High-Performance 3D Vertically Oriented Graphene Photodetector Using a Floating Indium Tin Oxide Channel.

Vertically oriented graphene (VG), owing to its sharp edges, non-stacking morphology, and high surface-to-volume ratio structure, is promising as a consummate material for the application of photoelectric detection. However, owing to high defect and fast photocarrier recombination, VG-absorption-based detectors inherently suffer from poor responsivity, severely limiting their viability for light detection. Herein, we report a high-performance photodetector based on a VG/indium tin oxide (ITO) composite structure, where the VG layer serves as the light absorption layer while ITO works as the carrier conduction channel, thus achieving the broadband and high response nature of a photodetector. Under the illumination of infrared light, photoinduced carriers generated in VG could transfer to the floating ITO layer, which makes them separate and diffuse to electrodes quickly, finally realizing large photocurrent detectivity. This kind of composite structure photodetector possesses a room temperature photoresponsivity as high as ~0.7 A/W at a wavelength of 980 nm, and it still maintains an acceptable performance at temperatures as low as 87 K. In addition, a response time of 5.8 s is observed, ~10 s faster than VG photodetectors. Owing to the unique three-dimensional morphology structure of the as-prepared VG, the photoresponsivity of the VG/ITO composite photodetector also presented selectivity of incidence angles. These findings demonstrate that our novel composite structure VG device is attractive and promising in highly sensitive, fast, and broadband photodetection technology.

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.

Vertically-oriented graphene electrodeposited with MnO2 on native SiO2/Si for high-performance supercapacitor electrodes

Supercapacitor electrodes, composed of vertically-oriented graphene (VG) and MnO2, are synthesized on native SiO2-passivated silicon wafers. The VG layers with a thickness of ~3 μm are deposited directly on the substrates via plasma-enhanced chemical vapor deposition (PECVD). The distribution levels of MnO2 formed with the electrodepositing method are investigated by using different types of solutions. The deeper infiltration of MnO2 depositions produced with ethanol/water solution is observed. The capacitance retention of 75.22% from a scan rate of 20 mV/s to 100 mV/s with ethanol/water solution, compared with that of 60.72% using water solution, suggests the better rate capability. The specific capacitance as high as 470.80 mF/cm2 at 20 mV/s is achieved with the increasing loadings of MnO2. The original retention of 99.29% after 5000 cycles demonstrates the excellent cycling stability, which is ascribed to the strong bonds between VG/SiO2/Si and MnO2/VG. The promising supercapacitor electrodes in this study provide a potential for forming high-performance supercapacitor devices based on silicon wafer substrates.

Vertically Oriented Graphene for the Fluorescence Quenching Raman Spectra of Aromatic Dyes

Vertically oriented graphene (VG) is deposited on a wide range of substrates by plasma-enhanced chemical vapor deposition. Moreover, the growth mode of VG is proven to be bottom-up deposition using isotope labeling. The Raman characteristic features of rhodamine 6G (R6G), an aromatic dye, are acquired using VG as a universal fluorescence quencher. The fluorescence quenching mechanism is likely related to the transfer of energy and photon-excited electrons between dye molecules and VG because not only the fluorescence emission spectrum of R6G greatly overlaps with the UV–vis absorption spectrum of VG, but also the resistance of dye-coated VG decreases under light irradiation. In addition, the Raman features of other aromatic dyes, such as eosin Y, rhodamine B, methylene blue, and gallocyanine, are obtained by the same method.

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.

Deciphering the Influence of Electrolytes on the Energy Storage Mechanism of Vertically-Oriented Graphene Nanosheet Electrodes by Using Advanced Electrogravimetric Methods.

Electrolyte composition is a crucial factor determining the capacitive properties of a supercapacitor device. However, its complex influence on the energy storage mechanisms has not yet been fully elucidated. For this purpose, in this study, the role of three different types of electrolytes based on a propylene carbonate (PC) solution containing tetrabutylammonium perchlorate (TBAClO4), lithium perchlorate (LiClO4) and butyltrimethylammonium bis(trifluoromethylsulfonyl)imide (N1114TFSI) ionic liquid on vertically-oriented graphene nanosheet electrodes has been investigated. Herein, in situ electrochemical quartz crystal microbalance (EQCM) and its coupling with electrochemical impedance spectroscopy (EIS), known as ac-electrogravimetry, have allowed the dynamic aspects of the (co)electroadsorption processes at the electrode-electrolyte interface to be examined. A major contribution of ClO4− anions (TBAClO4) was evidenced, whereas in the PC/N1114TFSI mixture (50:50 wt%) both anions (TFSI−) and cations (N1114+) were symmetrically exchanged during cycling. In the particular case of LiClO4, solvation of Li+ cations in PC was involved, affecting the kinetics of electroadsorption. These results demonstrate the suitability of dynamic electrogravimetric methods to unveil the interfacial exchange properties of mobile species for the conception of new high performance energy storage devices.

Enabling directional ion transport over graphene electrode surfaces for kilohertz electrochemical capacitors

The filtering capacitor is an electrochemical energy storage device, which can realize signal stabilization from alternating current to direct current in the modern electronic circuits. Traditional filtering capacitor, an aluminum electrolytic capacitor is usually limited by the bulky and rigid configuration. The graphene-based electrochemical capacitor with a fast frequency response has the potential to be the next-generation filtering capacitor. Kilohertz electrochemical capacitor with alternating current line-filtering performance requires the microstructural engineering of graphene electrodes to facilitate effective electrolyte ion permeation. Vertically-oriented graphene electrode-based electrochemical capacitor by microstructural engineering provides a significant solution for improving the filtering performance, but the underlying mechanism by which the vertical orientation endows graphene electrodes with the fast frequency response is still not clear. Therefore, a fundamental understanding of this mechanism must be developed and improved to ensure the design of superior kilohertz electrochemical capacitors for effective filtering. Here, a vertically-oriented graphene macroporous membrane is prepared to enable directional ion transport over the electrode surface at high frequencies, which is demonstrated by all of the electrochemical analysis, line scanning, schematic illustration and finite element analysis simulation results. Compared with those based on conventional horizontally-oriented and unordered graphene macroporous membrane electrodes, the vertically-oriented electrode-based electrochemical capacitor exhibits excellent filtering performance. Furthermore, it can convert arbitrary alternating current waveforms to straight signals. This work advances the understanding of the mechanism underlying the kilohertz frequency response and enables directional ion transport over the surface of vertically-oriented graphene electrodes, providing new insight into the search for future kilohertz filtering capacitors.

Highly Conductive Nitrogen-Doped Vertically Oriented Graphene toward Versatile Electrode-Related Applications.

The direct growth of vertically oriented graphene (VG) on low-priced, easily accessible soda-lime glass can propel its applications in transparent electrodes and energy-relevant areas. However, graphene deposited at low temperature (∼600 °C) on the catalysis-free insulating substrates usually presents high defect density, poor crystalline quality, and unsatisfactory electrical conductivity. To tackle this issue, we select high borosilicate glass as the growth substrate (softening point ∼850 °C), which can resist higher growth temperature and thus afford higher graphene crystalline quality, by using a radio-frequency plasma-enhanced chemical vapor deposition (rf-PECVD) route. A nitrogen doping strategy is also combined to tailor the carrier concentration through a methane/acetonitrile-precursor-based synthetic strategy. The sheet resistance of as-grown nitrogen-doped (N-doped) VG films on high borosilicate glass can thus be lowered down to ∼2.3 kΩ·sq-1 at a transmittance of 88%, less than half of the methane-precursor-based PECVD product. Significantly, this synthetic route allows the achievement of 30-inch-scale uniform N-doped graphene glass, thus promoting its applications as excellent electrodes in high-performance switchable windows. Additionally, such N-doped VG films were also employed as efficient electrocatalysts for electrocatalytic hydrogen evolution reaction.

Nanoconfined fusion of g-C3N4 within edge-rich vertically oriented graphene hierarchical networks for high-performance photocatalytic hydrogen evolution utilizing superhydrophillic and superaerophobic responses in seawater

Two-dimensional photocatalysts often suffer severe aggregation due to the inevitable van der Waals forces between nanosheets, which limits their photocatalytic water-splitting efficiency. Herein, a rational design of confined synthesis of g-C3N4 nanomeshes (GCN) on N-doped vertically-oriented graphene (NVG) arrays for enhanced hydrogen evolution is reported. The aggregation of 2D g-C3N4 nanosheets is effectively avoided via physical separation by electrically conductive NVG networks. Well-defined hierarchical architecture of the GCN/NVG photocatalyst endows with superaerophobicity and simultaneously enhanced light absorption. Experimental and ab initio simulation results suggest that the protruding graphene edges induce charge redistribution, thus enhancing interfacial charge separation. The GCN/NVG samples demonstrate a high areal hydrogen evolution rate of 41.7 μmol h−1 cm−2 (225 L m−2 in 24 h, STP) in water and 45.8 μmol h−1 cm−2 (246.2 L m−2 in 24 h, STP) in simulated seawater. This work creates further opportunities for the development of earth-abundant photocatalysts.

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

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

Enhancing the Heat-Dissipation Efficiency in Ultrasonic Transducers via Embedding Vertically Oriented Graphene-Based Porcelain Radiators.

Ultrasonic transducers with large output power have attracted extensive attentions due to their widespread applications in sonar, acoustic levitation, ultrasonic focusing, and so forth. However, the traditional transducer has almost no heat-dissipation capability itself, strictly relying on the assistant coolant system. Introducing high-performance heat-dissipation component is thus highly necessary. Herein, an embedded porcelain radiator component was designed by combining the excellent thermal conductivity of vertically oriented graphene (VG) with the outstanding heat-dissipation characteristics of thermosensitive ceramics, and a new-type transducer with an embedded VG/ceramic-hybrid radiator was constructed to show high heat-dissipation efficiency (up to ∼5 °C/min). Remarkably, prominent heat-dissipation effectiveness (temperature decline of ∼12 °C), enhanced amplitude and vibration uniformity were also achieved for the new-type transducer along with stabilized operating states. This research should pave ways for extending the applications of VG/ceramic hybrids to heat-dissipation scenarios and provide newfangled thoughts for the performance upgrade of multitudinous high-power devices.

Characterization of vertically-oriented graphene nanosheets grown on copper substrate

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

Ethanol-Precursor-Mediated Growth and Thermochromic Applications of Highly Conductive Vertically Oriented Graphene on Soda-Lime Glass.

Direct growth of vertically oriented graphene (VG) nanowalls on soda-lime glass has practical significance in extending the application of graphene to daily-life-related areas, such as gas sensors and conductive electrodes, via combining their complementary properties and applications. However, VG films derived by low-temperature deposition (e.g., on glass) usually present relatively low conductivity and optical transparency. To tackle this issue, an ethanol-precursor-based, radio-frequency plasma-enhanced chemical vapor deposition (rf-PECVD) route for the synthesis of VG nanowalls is developed in this research, at around the softening temperature of soda-lime glass (∼600 °C) templates. The average sheet resistance, i.e., ∼2.4 kΩ·sq-1 (at transmittance ∼81.6%), is only one-half of that achieved by a traditional methane-precursor-based PECVD route. Based on the highly conductive and optically transparent VG/glass, as well as its scalable size up to 25 in. scale, high-performance reversible thermochromic devices were successfully constructed using VG/glass as transparent heaters. Hereby, this work should propel the scalable synthesis and applications of highly conductive VG films on glass in next-generation transparent electronics and switchable windows.