Graphene Plane Research Articles

Effects of Crumpling Stage and Porosity of Graphene Electrode on the Performance of Electrochemical Supercapacitor.

The performance characteristics of supercapacitors composed of crumpled graphene electrodes and aqueous NaCl electrolytes are investigated through Molecular Dynamics (MD) simulations using a newly developed crumpled graphene-based supercapacitor model. Results suggest that the three-dimensional configuration of crumpled graphene boosts electrolyte-electrode interaction. This improved interaction, which includes a larger ion-accessible zone, increases the specific capacitance of the supercapacitor by roughly 400% (16.4 μF/cm2) compared to planar graphene electrodes. Examining the effect of different stages of crumpling and the inclusion of pores on the electrode surface shows that the stages of crumpling substantially influence the supercapacitor performance. A smaller crumpling radius, meaning fully crumpled stage, improves the performance as increased crumpling leads to better packing efficiency, which aids in more ion separation. Furthermore, adding pores on the surface of crumpled graphene improves the ion accessibility by creating additional adsorption sites. An exceptional capacitance of 19.8 μF/cm2 is obtained for a porosity of 20%. However, the results suggest that the in-plane-porosity of the electrode needs to be optimized as there is a decline in specific capacitance after that point (20% porosity), indicating a suboptimal relationship between the charge distribution, specific surface area (SSA) and the porosity of the electrode.

Self-Driven Graphene Photodetector Arrays Enabled by Plasmon-Induced Asymmetric Electric Field.

Gap surface plasmon (GSP) modes enhance graphene photodetectors (GPDs)' performance by confining the incident light within nanogaps, giving rise to strong light absorption. Here, we propose an asymmetric plasmonic nanostructure array on planar graphene comprising stripe- and triangle-shaped sharp tip arrays. Upon light excitation, the noncentrosymmetric metallic nanostructures show strong light-matter interactions with localized field close to the surface of tips, causing an asymmetric electric field. These features can accelerate the hot electron generation in graphene, forming a directional diffusion current. Accordingly, the artificial GPDs exhibit a wavelength-dependence behavior covering the wavelength range from 0.8 to 1.6 μm, with three photoresponse maxima corresponding to the nanostructures' resonances. Additionally, the polarization-dependent GPDs can realize a responsivity of ∼25 mA/W and a noise equivalent power of ∼0.44 nW/Hz1/2 at zero bias when excited at the resonance of 1.4 μm. Overall, our study offers a new strategy for preparing compact and multifrequency infrared GPDs.

Printed and Laser Induced Graphene Electrochemical Biosensors for Rapid Environmental and Biomedical Monitoring

This presentation will demonstrate how low-cost printed and flexible graphene circuits can overcome the challenges associated with conventional electrical circuits to open the door to a highly sensitive monitoring of biochemical threats and toxins including nerve agents/pesticides. The use of inkjet and aerosol printing; spin coating; and laser writing (i.e., laser induced graphene) can be used to create such circuits with high resolution (line widths as low as 20 µm). Moreover, the use of rapid-pulse laser annealing of the printed graphene and/or salt porogens added into graphene inks before printing can be used to further improve the electrical conductivity (< 10 ohms/sq.) and electroactive surface area (nano/micro structuring) of the graphene circuits as well as be used to tune the surface of the printed graphene from one that is hydrophilic (water contact angle (WCA) = 48°) to one that is superhydrophobic (WCA = 158°). More recently we demonstrate how graphene synthesis, patterning and surface wettability can all be controlled through a laser induced graphene (LIG) process that creates a high surface area graphene surface directly from polyimide. Such LIG circumvents the need to create solution-phase graphene inks for printing. These printing and laser fabrication techniques are also amenable for use on flexible and chemically/thermally sensitive surfaces including polymers and paper.Moreover, the laser etched printed graphene and LIG exhibits an inherent 3D morphology that significantly increases the electrode electroactive surface over traditional 2D planar graphene electrodes. This large increase in surface area significantly improves the loading of enzyme onto the electrode surface and subsequently increases the heterogenous charge transport during catalytic pesticide sensing. Such ion sensors displayed near-Nernstian sensitivities, wide sensing ranges over four orders of magnitude extending from the 10–2 M down to detection limits in the 10–6 M range or for fertilizer ions (nitrate, ammonium, potassium, calcium, and magnesium) reaching down to detection limits less than 1 ppm in some cases in soil and water samples. The pesticide sensing capability of the sensor is able to reach an extremely low picomolar detect limit for organophosphates (paraoxon) and to the nanomolar regime for neonicotinoids in surface waters. We also demonstrate how this flexible LIG electrodes can be coupled with polymeric microfluidics and adhered to the body for continuous monitoring of lactate, glucose, and sodium in the sweat. Finally, we demonstrate how such surface wettability patterning of the printed graphene and LIG itself through laser etching can be used to make hydrophilic LIG tracks surrounded by near superhydrophic sidewalls for open microfluidic fluid transport without the need for polymeric microfluidics. Such open microfluidic tracks enable the splitting and transport of fluid samples to distinct electrochemical sensors for multiplexed biosensing of both fertilizer ion and pesticide monitoring from water samples and assist in harvesting sprayed pesticides in the field. Figure 1

New insights into the role of nitrogen doping in microporous carbon on the capacitive charge storage mechanism: From ab initio to machine learning accelerated molecular dynamics

Fundamental understandings of the relationship between ion-electrode interaction and structural feature in porous carbon electrodes at a molecular level provides guidelines for the design of high-performance electric double layer supercapacitors. It is certified by experiments that porous carbon structures doped with nitrogen show enhanced capacitive performance. However, in the theoretical simulations, the fundamental charge storage mechanism is still elusive. In particular, the recent experimental result shows that the generally ignored nitrolic nitrogen (N5) in porous carbon exhibits a positive effect on capacitance, while graphitic nitrogen (N3) does the opposite, which is against with the simulation results based the 2D-modeled porous graphene structure. Here, we perform ab initio molecular dynamics simulations on the N3 and N5-doped carbon/electrolyte interfaces, including both 2D planar and 3D microporous carbon electrodes. Our calculation indicates that N3 in the 3D pore hinders the electrolyte transport, while N5-doped micropore still serves as an electrolyte transport channel through the formation of H-bond. The charge storage mechanism is further elucidated by the analysis of the well equilibrated interfaces obtained from the machine learning force field accelerated molecular dynamics. Our work provides a new insight into the effect of nitrogen doping in 3D porous carbon, which is exactly opposite to the 2D planar graphene. Therefore, we emphasize that differences in the electrochemical conditions of 2D planar and 3D microporous carbon electrodes should be fully considered when analyzing the effects of surface chemistry on charge storage mechanisms.

Role of Vacancy Defects and Nitrogen Dopants for the Reduction of Oxygen on Graphene.

Disentangling the roles of nitrogen dopants and vacancy defects (VG) in metal-free carbon catalysts for the oxygen reduction reaction (ORR) ideally requires studying both the dopants and defects separately. Here, we systematically introduced nitrogen dopants and VGs via plasma treatment into the basal plane of monolayer graphene as a model carbon catalyst to investigate their specific roles in ORR catalysis. An increased defect density including dopants is positively associated with boosted ORR activity. Nitrogen dopants are responsible for an improved current via a 2e- pathway generating hydroperoxide, while VGs result in enhanced kinetics and water production. We therefore infer that VGs in graphene are responsible for the improved ORR kinetics, while nitrogen dopants majorly influence the selectivity of ORR reaction products. The nitrogen dopants without VGs lead to a higher overpotential compared with the pristine graphene. Instead of the attribution of the ORR active site to only nitrogen species in carbon materials, the improved ORR activity in nitrogen-doped carbon materials should be attributed to the active sites constituted of VGs, oxygen dopants, and nitrogen dopants. Through this work, we provide important insights into the intertwined roles of nitrogen and VGs as well as oxygen dopants in nitrogen-doped metal-free catalysts for a more efficient ORR.

Synthesis and Characterization of Nanocellulose/Polyacrylonitrile‐based Carbon Nanofibers: An Approach toward Low Cost and Sustainable Agro Waste based Carbon Nanomaterials

AbstractThe cost of polymer precursors and high energy need in carbonization for the preparation of carbon materials are two limiting factors in the commercialization of carbon‐based materials. In this study, carbon nanofiber films were prepared by adding a very small concentration of polymer polyacrylonitrile in nanocellulose (extracted from agro waste i.e., rice straws) with an overall aim of producing carbon nanomaterials with cost effective, renewable and sustainable carbon source. Polyacrylonitrile (PAN) added nanocellulose solution was drop‐casted to prepare Nanocellulose/Polyacrylonitrile‐based nanofibers film (NC/PAN). It was further stabilized in air atmosphere at 280 °C followed by carbonized in nitrogen atmosphere at 700 °C. It was found that addition of NC/PAN have 18 % lower activation energies than PAN. Results of tunneling electron microscopy showed that polyacrylonitrile/nanocellulose films carbonized at 700 °C temperature exhibited amorphous carbon nanofibers structure similar to bare polyacrylonitrile based carbon film carbonized at higher temperature of 1000 °C‐1200 °C. Furthermore, polyacrylonitrile/nanocellulose based carbon nanofibers have showed high degree of carbonization ((ID)/(IG)=0.87) as compared to bare polyacrylonitrile based carbon nanofibers film ((ID)/(IG)=1.02). This may be attributed to chiral nature of nanocellulose which served as a template to orientation of graphene planes and showed efficient carbonization.

Theoretical study on the energy storage performance of Electrical double layer supercapacitors with choline amino acid ionic liquids electrolyte

Choline amino acid ionic liquids have attracted much attention in recent years due to their simple extraction process, rich sources, and easy degradation. Due to the high viscosity of such ionic liquids, the addition of organic solvents can adjust the transport performance of ionic liquids. Here, we explore the effects of different solvents (methanol, ethanol) and different temperatures on the diffusion properties and conductivity of ionic liquids through molecular dynamics simulations. The results showed that with the increase of temperature, the conductivity linearly increased. The addition of organic solvents can improve the conductivity of ionic liquids, and the addition of methanol solvent has a higher compatibility with choline amino acid ionic liquids. The conductivity after mixing methanol and choline glycine in a certain proportion is 5 times higher than before. In addition, this article investigated the performance of choline glycine ionic liquid electrolytes in Electrical Double Layers (EDLs) near planar graphene electrodes. The results indicate that similar double layer structures and double layer capacitors can be observed regardless of whether solvents are added.

Facile design and superior electrochemical performance of fluorine doped graphene slice for supercapacitor electrodes

A facile method has been developed to enhance the electrochemical characteristics of graphene by synthesizing fluorine-doped graphene (FG) slice via a hydrothermal reaction involving graphene oxide and hydrofluoric acid. The resultant FG slice demonstrates enhanced volumetric capacitance and robust cycling durability. The advancements in electrochemical behavior are owing to the integration of covalent C-F and semi-ionic C-F bonds aligned perpendicularly to the graphene planes. This FG slice is suitable for a range of practical applications, including use in sensors, as catalysts, and within the realm of nanomaterial-based electronics.

Control of proton transport and hydrogenation in double-gated graphene

The basal plane of graphene can function as a selective barrier that is permeable to protons1,2 but impermeable to all ions3,4 and gases5,6, stimulating its use in applications such as membranes1,2,7,8, catalysis9,10 and isotope separation11,12. Protons can chemically adsorb on graphene and hydrogenate it13,14, inducing a conductor–insulator transition that has been explored intensively in graphene electronic devices13–17. However, both processes face energy barriers1,12,18 and various strategies have been proposed to accelerate proton transport, for example by introducing vacancies4,7,8, incorporating catalytic metals1,19 or chemically functionalizing the lattice18,20. But these techniques can compromise other properties, such as ion selectivity21,22 or mechanical stability23. Here we show that independent control of the electric field, E, at around 1 V nm−1, and charge-carrier density, n, at around 1 × 1014 cm−2, in double-gated graphene allows the decoupling of proton transport from lattice hydrogenation and can thereby accelerate proton transport such that it approaches the limiting electrolyte current for our devices. Proton transport and hydrogenation can be driven selectively with precision and robustness, enabling proton-based logic and memory graphene devices that have on–off ratios spanning orders of magnitude. Our results show that field effects can accelerate and decouple electrochemical processes in double-gated 2D crystals and demonstrate the possibility of mapping such processes as a function of E and n, which is a new technique for the study of 2D electrode–electrolyte interfaces.

Experimental demonstration of cyclotron emissions in micro-scale graphene structures

A solid-state implementation of a cyclotron radiation source consisting of arrays of semicircular geometries was designed, fabricated, and characterized on commercially available graphene on hBN substrates. Using a 10 µm design radius and device width, respectively, such devices were expected to emit a continuous band of radiation spanning from 3 to 6 GHz with a power 3.96 nW. A peak emission was detected at 4.15 GHz with an effective array gain of 22 dB. This is the first known experimental measurement of cyclotron radiation from a curved planar graphene geometry. With scaling, it may be possible achieve frequencies in the THz range with such a device.

Analytical modeling of nucleation and growth of graphene layers on CNT array and its application in field emission of electrons

Carbon Nanotube (CNT) arrays and graphene have undergone several investigations to achieve efficient field emission (FE) owing to CNT’s remarkable large aspect ratio and graphene’s exceptional FE stability. However, when dense CNT arrays and planar graphene layers were used as field emitters, their field enhancement factor reduced dramatically. Therefore, in this paper, we numerically analyze the growth of a dense CNT array with planar graphene layers (PGLs) on top, resulting in a CNT-PGL hybrid and the associated field enhancement factor. The growth of the CNT array is investigated using Plasma Enhanced Chemical Vapor Deposition (PECVD) chamber in C2H2/NH3 environment with variable C2H2 flow, Ni catalyst film thickness, and substrate temperature followed by PGL precipitation on its top at an optimized cooling rate and Ni film thickness. The analytical model developed accounts for the number density of ions and neutrals, various surface elementary processes on catalyst film, CNT array growth, and PGLs precipitation. According to our investigation, the average growth rate of CNTs increases and then decreases with increasing C2H2 flow rate and catalyst film thickness. CNTs grow at a faster rate when the substrate temperature increases. Furthermore, as the chamber temperature is lowered from 750 °C to 250 °C in N2 environment and Ni film thickness grows, the number of the graphene layers increases. The field enhancement factors for the CNT array and hybrid are then calculated based on the optimal parameter values. The average height of the nanotubes, their spacing from one another, and the penetration of the electric field due to graphene coverage are considered while computing the field enhancement factor. It has been found that adding planar graphene layers to densely packed CNTs can raise its field enhancement factor. The results obtained match the current experimental observations quite well.

Optical matter based on graphene surface plasmons.

In this work, we have proposed a graphene planar structure as an optical binding device of dielectric nanoparticles. Surface plasmons (SPs) on a graphene sheet, generated thanks to the near field scattering of the incident plane wave by the nanoparticles placed close to the graphene sheet, act as a powerful intermediary for enhancing the optical force between nanoparticles to organize the particle structure at length scales comparable with the plasmon wavelength, i.e., at the light sub-wavelength scale. In particular, we have paid attention to the formation of one-dimensional arrays of nanoparticles. Our results show that both the equilibrium separation between particles and the energy potential binding depend on the number of particles forming the array and that the former tends to the plasmon wavelength (the array constant) for a number of particles large enough. We have obtained simple analytical expressions that explain the main results obtained by using the rigorous theory. Our contribution can be valuable for the knowledge in the low-frequency optical binding framework, from terahertz to far-infrared spectrum.

Investigation of picosecond laser-induced graphene for dopamine sensing: Influence of laser wavelength on structural and electrochemical performance

This research focuses on a straightforward synthesis method for laser-induced graphene using green (532 nm) and UV (355 nm) picosecond laser irradiation of graphene oxide dispersion. Structural analysis revealed that the laser operating at 355 nm is more favorable to producing a product with a higher content of the restored sp2 network, a higher graphene in-plane crystallite size, a higher concentration of pyrrolic N, and higher values of the optical bandgap. Conversely, the 532 nm laser yielded a sample with higher concentrations of epoxy groups, sp3 defects, and amorphous carbon. Electrochemical analysis for dopamine determination showed potential for both samples as electrode materials. However, the analytical parameters of the sample treated with the 355 nm laser surpass those of the 532 nm laser. It was assumed that the enhanced electrochemical activity results from the presence of pyrrolic N and vacancies in the structure of the sample treated with the 355 nm laser.

Universal Murray’s law for optimised fluid transport in synthetic structures

Materials following Murray’s law are of significant interest due to their unique porous structure and optimal mass transfer ability. However, it is challenging to construct such biomimetic hierarchical channels with perfectly cylindrical pores in synthetic systems following the existing theory. Achieving superior mass transport capacity revealed by Murray’s law in nanostructured materials has thus far remained out of reach. We propose a Universal Murray’s law applicable to a wide range of hierarchical structures, shapes and generalised transfer processes. We experimentally demonstrate optimal flow of various fluids in hierarchically planar and tubular graphene aerogel structures to validate the proposed law. By adjusting the macroscopic pores in such aerogel-based gas sensors, we also show a significantly improved sensor response dynamics. In this work, we provide a solid framework for designing synthetic Murray materials with arbitrarily shaped channels for superior mass transfer capabilities, with future implications in catalysis, sensing and energy applications.

Theoretical study of electrochemical hydrogen evolution reaction of Pt–Co diatomic sites catalyst

Hydrolytic Hydrogen is a clean hydrogen production method. To mitigate the substantial costs associated with water electrolysis, electrocatalysts are crucial in decreasing the cathodic reaction involved in hydrogen hydrolysis, specifically, the overpotential for hydrogen evolution reaction (HER). Compared to traditional catalysts and single-atom catalysts, Pt–Co diatomic sites catalyst exhibit excellent hydrogen evolution reaction activity and stability. We have established eight different structures of dual-center metal catalysts based on graphene planes. A systematic study was conducted on the catalytic performance and mechanism of these configurations, among which the structure with the best hydrogen evolution reaction activity was CoN4/PtC2N2, with a Gibbs free energy (ΔG) of 0.004 eV. Electronic structure analysis confirms that H follows the Sabatier mechanism on diatomic catalysts, and the catalyst exhibits excellent HER performance for moderate adsorption strength of H atoms. The d-band centers of Co and Pt atoms in the electrocatalyst CoN4–PtC2N2 are lower than those of a single system, indicating that the electronic effect between the CoN4 and PtC2N2 sites can enhance the inherent HER activity of the catalyst. This study expands the application scope of dual-site catalysts and provides new insights for the design and synthesis of dual-site hydrogen evolution catalysts.

Lithiation bridged molecular conducting magnets

Combining ferromagnetism and metallic conductivity has been extensively studied in contemporary materials science. However, building such multifunctionalities in molecular chemistry is particularly challenging, on which two collec-tive properties are long considered as mutually exclusive. Here, we report molecular conducting magnet consisting of two-dimensional graphene and molecular planar magnetic networks bridged by electrochemical lithiation, exhibiting the co-existence of distinct magnetism and conductivity and magneto-electric coupling effects. The room temperature magnetoresistance and magnetoelectric coupling coefficient of such complex solid reach 10.7 % and 1.77 V/Oe cm, respectively, which can be further tailored by modulating the interaction between graphene and molecular magnetic network. In addition, the graphene passivation simultaneously introduces environmental stability of molecular mag-nets. The electrochemical lithiation offers a pathway towards coupling magnetism and conductivity in molecular complexes.

Soluble Nanographene C222: Synthesis and Applications for Synergistic Photodynamic/Photothermal Therapy.

Nanographene C222, which consists of a planar graphenic plane containing 222 carbon atoms, holds the record as the largest planar nanographene synthesized to date. However, its complete insolubility makes the processing of C222 difficult. Here we addressed this issue by introducing peripheral substituents perpendicular to the graphene plane, effectively disrupting the interlayer stacking and endowing C222 with good solubility. We also found that the electron-withdrawing substituents played a crucial role in the cyclodehydrogenation process, converting the dendritic polyphenylene precursor to C222. After disrupting the interlayer stacking, the introduction of only a few peripheral carboxylic groups allowed C222 to dissolve in phosphate buffer saline, reaching a concentration of up to 0.5 mg/mL. Taking advantage of the good photosensitizing and photothermal properties of the inner C222 core, the resulting water-soluble C222 emerged as a single-component agent for both photothermal and photodynamic tumor therapy, exhibiting an impressive tumor inhibition rate of 96%.

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.

Engineered nanocomposites through embedding of smaller “organic inorganic” nanoparticles in thermoplastic Poly(2-Vinylpyridine) polymer matrix

Polymer nanocomposites (PNCs) are significant for modern and future applications owing to their multifunctionality promoted by morphology and tailored interfaces between the constituents. However, ‘forward’ engineered polymer (host) composites with smaller size nanoparticles (guest) providing desired properties remains challenging as they depend upon nanoparticles aggregation, size, shape, and loading (volume or weight) fraction. This study strategically designs and develops PNCs comprising thermoplastic poly (2-vinylpyridine) (P2VP) polymer matrix impregnated with spherical polyhedral oligomeric silsesquioxane (N-POSS) nanoparticles (diameter ∼2–5 nm) and anisotropic planar nitrogenated graphene nanoribbons (GNR, strip width ∼5–10 nm) commensurate with polymer chain radius of gyration, Rg, (or segment length ∼1.5 nm) and comparable energy scales of electrostatic interaction and attractive hydrogen bonding. We investigated static and dynamic structure and thermophysical properties to correlate with interfacial regions and the results are compared with larger graphene oxide (GO, lateral dimension ∼100–200 nm) nanosheets and silica (SiO2, ∼25–50 nm) particles. While electron microscopy revealed nanoparticle distribution, the lattice bonding, conjugation length, and mechanical properties are determined from micro-Raman spectroscopy and atomic force microscopy, respectively. The differential scanning calorimetry provided a measure of glass transition temperature, Tg, with positive shift of ∼10–18 °C with nanoparticles loading indicating strength of structural relaxation/chain rigidity behavior and thermogravimetric analysis displayed increased thermal stability and conductivity (decreased interfacial resistance). We also measured temperature dependent dc electrical conductivity and dielectric relaxation spectroscopy gaining insights into percolation and dynamic interfacial layer. This study signified understanding of interactions and interfacial regions, key element to demystify the microscopic structure-property relationships.

Monitoring Aging Effects in Graphite Bisulfates by Means of Raman Spectroscopy

Graphite bisulfate (GBS) compounds consist of graphite layers intercalated by HSO4− ions and H2SO4 molecules. Owing to electrostatic interactions with the graphene plane, HSO4− ions cause point defects in the graphite’s crystalline structure, while H2SO4 molecules are free to move via diffusion in the spaces between the adjacent graphite sheets and segregate to form linear defects. In the present work, we report the results of our investigation using Raman spectroscopy on the temporal evolution of such defects on selected GBS samples over 84 months. Two characteristic lengths correlated with the average distance between defects have been estimated and their evolution with aging was investigated. The results show a decrease in the density of point-like defects after aging, regardless of the pristine structural configuration of the GBS samples, revealing a structural instability. This study can provide significant information for the technological development of industrial processes aimed to produce expanded graphite based on GBS precursors, where the aging of GBS is known to influence the efficiency and quality.