Graphitic Carbon Films Research Articles

Novel Graphene-Based Reversible Physisorption Materials for High Hydrogen Uptake

Carbon-based nanomaterials (graphene, carbon nanotube, microporous carbon, graphitic carbon etc.) are excellent H2 storage materials due to high molecular H2 uptake at the micropores and large H2 adsorption at the graphene network via the formation of dipoles in H2 molecules through London dispersion forces. On the other hand, incorporation of transition metal nanoparticles within the nanocarbon network facilitates dissociation of H2 molecules into constituents H-atoms over the metal nanoparticles, followed by diffusion into the carbon nano-matrix via ‘spill-over’ effect for polarization/hybridization of metal-hydrogen to create high H2 binding energy for high hydrogen adsorption. Therefore, a nanocomposite of graphene-microporous carbon-metal nanoparticle network is expected to deliver excellent H2 uptake capacity and acts as a novel reversible physisorption material for hydrogen storage applications, which is very important for sustainable renewable energy integration.A simple, cost effective sputtering method is used to fabricate metal incorporated graphitic microporous carbon film and differential resistance measurement showed high H2 uptake. Average number of graphene layer formation is dependent on sputtering parameters, which manifest bi-to-multilayer graphene. With increase in graphene layers, H2 adsorption also increases due to a fourfold effect: higher molecular hydrogen uptake at the graphene layers, more active sites into the micropores, promotion of molecular dissociation into atomic hydrogen by the metal nanoparticles, and the formation of hydrogenated carbon via destruction of the π-bonds.

Ecofriendly-synthesized pro-microporous C-Co3O4 nanoparticles conjugated with controlled electropolymerization of pyrrole for high-efficient symmetric supercapacitors

High energy density and high power density supercapacitor electrode materials are prepared by a simple pseudo sol-gel method, followed by electrochemical polymerization for electrochemical energy storage. A thin graphitic carbon-film coated Co3O4 (C-Co3O4) is synthesized as an efficient energy storage material with the help of an aqueous phytochemical extract. The green-synthesized C-Co3O4 is pro-microporous with the maximum pore volume centered at 0.7 nm pore width with a distribution of >85 % pore volume with ≤4.5 nm pore width. A thin layer of conducting polypyrrole (PPy) is electropolymerized on a nickel foam/C-Co3O4 electrode (NF/C-Co3O4/PPy). Specific capacitance of NF/C-Co3O4 and NF/C-Co3O4/PPy electrodes are 320 F g−1 and 1176 F g−1, respectively, at 20 mV/s and 221 F g−1 and 695 F g−1, respectively, at 0.5 A g−1. The enhanced capacitance of NF/C-Co3O4/PPy could be ascribed to the synergistic effect between C-Co3O4 and the conducting PPy nanolayer. The energy density of the symmetric supercapacitor (SC) fabricated with NF/C-Co3O4 is 8.1 Wh kg−1 at 800 W kg−1 with a potential window of 0.8 V. The energy density of the symmetric SC fabricated with NF/C-Co3O4/PPy is enhanced ~5 times with the use of the PPy thin film and is 34.4 Wh kg−1 at 1100 W kg−1 with a potential window of 1.1 V. The capacitance retention of NF/C-Co3O4/PPy was 93.9 % over 8000 cycles while that of NF/C-Co3O4 was 79.6 % only. The symmetric SC exhibited a high power density of 35.2 kW kg−1 with an energy density of 4.2 Wh kg−1. The inexpensive green approach coupled with the promising superior capacitance characteristics made C-Co3O4/PPy nanocomposites excellent candidates for high energy density aqueous supercapacitors.

Facile fabrication of conductive graphitic carbon films as transparent electrodes in organic solar cells by ion beam irradiation of polystyrene films and carbonization

Conductive graphitic carbon films were fabricated from insulating polystyrene (PS) by ion beam irradiation and subsequent thermal treatment, and used as a transparent anode for organic solar cells (OSCs). Analyses of optical and chemical properties revealed that the thermally resistant oxidized carbon clusters in the PS precursors (spin-coated on quartz) were efficiently generated by ion beam irradiation at a fluence of 2 x 1016 ions/cm2, and transformed to graphitic carbon thin films through high-temperature treatment. The graphitic carbon films exhibited thickness-dependent conductivity (the highest conductivity of 374 S/cm), while their work function (5.20 eV) and surface roughness (0.560–0.596 nm) were unaffected by the thickness of the films. Furthermore, the introduction of the 10.2 nm-thick graphitic carbon films into poly(3-hexylthiophene) (P3HT): [6,6]-phenyl-C61 butyric acid methyl ester (PCBM)-based OSCs resulted in a power conversion efficiency (PCE) of 1.40%, demonstrating the feasibility of using the graphitic carbon films as transparent electrodes.

Performance Enhancement of Moisture-driven Power Generators by Photofragmentation of Inorganic Salt Particles.

We developed a novel method based on the photofragmentation of inorganic salt particles for improving the moisture-electric energy transformation performance of a moisture-driven power generator (MPG). Infrared laser irradiation on cellulose nanofiber films (CNFs) prepared by a TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation of bleached pulp induced a photothermal conversion of CNFs to porous graphitic carbon films (GCFs) with the catalyst-derived Na2O2 particles. Since the laser beam was focused on the top surface of CNF, the gradients of the photothermal conversion of CNFs and Na2O2 concentration were created along the thickness direction. Subsequent irradiation with ultraviolet (UV) light induced the photofragmentation of the micrometer-sized Na2O2 particles into smaller ones, which increased the surface area of the salt particles in contact with the GCFs and consequently increased the number of effective dissociable charge carriers. When the GCF was exposed to moisture, the dissociated sodium ions migrated along the preformed concentration gradient, producing continuous outputs of current and voltage. At 90% relative humidity, the maximum voltage and current density outputs of the MPG increased from 0.91 V and 18.7 μA/cm2 before UV irradiation to 1.10 V and 56.2 μA/cm2 after UV irradiation, respectively. Additionally, we demonstrated that a green light-emitting diode could be turned on without capacitors or rectifiers during normal breathing while wearing a face mask with three GCF arrays attached (each 3 mm × 3 mm × 0.1 mm in size).

Investigation of the mechanical properties of graphitic carbon nitride pellet and film by simulation and nanoindentation experiments

Investigation of the mechanical properties of graphitic carbon nitride pellet and film by simulation and nanoindentation experiments

Direct fabrication of high performance moisture-driven power generators using laser induced graphitization of sodium chloride-impregnated cellulose nanofiber films

High performance moisture-driven power generators were developed by laser induced graphitization (LIG) of sodium chloride-impregnated cellulose nanofiber films (CNFs). CNFs impregnated with different amounts of NaCl were obtained by immersing CNFs in NaCl solutions of various concentrations. A CO2 laser engraver was employed to convert the CNFs to porous graphitic carbon films (GCFs) under ambient conditions. By focusing the laser beam on the top surface of the CNF, the laser intensity was the highest on the top surface and gradually decreased toward the bottom surface. Since the focal temperature of the laser beam was higher than the boiling point of NaCl, the NaCl particles near the top surface evaporated more rapidly, creating a NaCl concentration gradient along the thickness direction. When the GCF was exposed to moisture, the dissociated ions migrated between the top and bottom surfaces, producing an electrical current. The maximum voltage and current outputs were 0.65 V and 550 μA/cm2, respectively, at 90% relative humidity (RH). Due to abundant dissociated ions, the current output was produced continuously rather than pulsed, and was the highest current reported so far. We demonstrated that six vertically stacked GCFs (each 3 mm × 3 mm × 240 µm in size) at 75% RH were sufficient to turn on green light emitting diodes (LEDs) operating at an onset potential of 2 V for 48 h without any auxiliary devices, such as rectifier circuits and capacitors.

High-fidelity modeling of breakdown in helium: initiation processes and secondary electron emission

Understanding the role of physical processes contributing to breakdown is critical for many applications in which breakdown is undesirable, such as capacitors, and applications in which controlled breakdown is intended, such as plasma medicine, lightning protection, and materials processing. The electron emission from the cathode is a critical source of electrons which then undergo impact ionization to produce electrical breakdown. In this study, the role of secondary electron yields due to photons (γ ph) and ions (γ i) in direct current breakdown is investigated using a particle-in-cell direct simulation Monte Carlo model. The plasma studied is a one-dimensional discharge in 50 Torr of pure helium with a platinum cathode, gap size of 1.15 cm, and voltages of 1.2–1.8 kV. The current traces are compared with experimental measurements. Larger values of γ ph generally result in a faster breakdown, while larger values of γ i result in a larger maximum current. The 58.4 nm photons emitted from He(21P) are the primary source of electrons at the cathode before the cathode fall is developed. Of the values of γ ph and γ i investigated, those which provide the best agreement with the experimental current measurements are γ ph = 0.005 and γ i = 0.01. These values are significantly lower than those in the literature for pristine platinum or for a graphitic carbon film which we speculate may cover the platinum. This difference is in part due to the limitations of a one-dimensional model but may also indicate surface conditions and exposure to a plasma can have a significant effect on the secondary electron yields. The effects of applied voltage and the current produced by a UV diode which was used to initiate the discharge, are also discussed.

High hydrogen uptake by a metal-graphene-microporous carbon network

High-surface area carbon nanomaterials are promising candidate as reversible physisorption materials for hydrogen storage in personal transportation vehicles at moderate temperatures and pressures. Metal-incorporated graphitic microporous carbon is considered to be an excellent H2 storage material due to high molecular hydrogen uptake at the micropores coupled with considerable H2 adsorption at the graphitic network. A simple, cost-effective sputtering technique is adopted to fabricate metal-incorporated graphitic microporous carbon film and differential resistance measurements are carried out in H2 ambient to observe the high hydrogen uptake within the samples. Ultrathin amorphous carbon film is irradiated with metal nanoparticles which converts it into graphitic carbon, whereas sputtering plasma acts as dry-etchant to activate it into microporous carbon. Average number of graphene layer formation is observed to be dependent on sputtering parameters (current/voltage/time), which manifest bi-layer to multi-layer graphene sheets. With increase in the graphene layers, hydrogen adsorption also increases due to a four-fold effect - higher molecular hydrogen uptake at the graphene layers, more active sites into the micropores, promotion of molecular dissociation into atomic hydrogen by the metal nanoparticles, followed by adsorption at the active surface sites and the formation of hydrogenated carbon via destruction of the π-bonds.

Coupling between structural properties and charge transport in nano-crystalline and amorphous graphitic carbon films, deposited by electron-beam evaporation

Nano-crystalline and amorphous films of graphitized carbon were deposited by electron-beam evaporation of bulk graphite. Structural properties and the size of graphite nanoclusters (L ≈ 1.2–5 nm) in the films were determined from the analysis of their Raman spectra. Electrical properties of the bulk nano-crystalline graphite reference sample and the deposited graphitic carbon films were measured by means of the Hall effect technique within the temperature range from 290 to 420 K. The electrical conductivity σ and Hall mobility μH of all samples exhibited exponential temperature dependences, indicating on the non-metallic behavior. Electrical properties of the amorphous graphitic carbon thin films, deposited at low substrate temperatures (620 and 750 K) were analyzed in the scope of the hopping charge transport mechanism via localized states. We have shown that the charge transport in the bulk and thin film (920 K) nano-crystalline graphite samples is carried out via the tunneling and thermionic emission over potential barriers at the grain boundaries.This paper contributes towards better understanding of coupling between structural and electrical properties of graphitic carbon thin films.

Quasi-aligned SiC@C nanowire arrays as free-standing electrodes for high-performance micro-supercapacitors

Currently, it still remains a grand challenge to enhance energy density of micro-supercapacitors (MSCs) without sacrificing power density, cycle life and other key performance. Herein, we reported the exploration of free-standing electrodes for MSCs based on quasi-aligned SiC nanowire arrays wrapped by ultrathin graphitic carbon film (SiC@C). Such rationally designed SiC@C architecture bring an optimal specific capacitance of 78.98 mF cm−2 at a current density of 0.2 mA cm−2, which was enhanced by more than 700% in comparison to that of pure SiC counterpart (9.56 mF cm−2). The aqueous-based MSCs exhibit a high specific capacitance, fast charge-discharge rate, excellent rate capability, and long-term cycling stability (94.1% capacity retention after 10000 charge-discharge cycles). The ionic-liquid-based MSCs had an extended operating voltage up to 2.6 V with an energy density up to 2.84 μW h cm−2 (at a power density of 65.1 μW cm−2), which was a new record of SiC-based SCs, and superior to most of those of nanowire-based MSCs in other material systems ever reported.

SYNTHESIS AND CHARACTERISTICS OF POLY(PARA-PHENYLENEVINYLENE)

Poly(para-phenylenevinylene) [PPV] films and nanotubes were prepared via chemical vapor deposition polymerization (CVDP) by vapor phase pyrolysis of α, α'-dichloro p-xylene on the quartz plate, silicon wafers and Al2O3 membrane substrate with the pore size f = 100 nm. We prepared graphitic carbon films and carbon nanotubes by carbonizing the poly(para-phenylenevinylene) [PPV] films and nano parterns under thermal treatment range from 500°C to 900°C. When the PPV films on quartz plate were treated at 900°C highly oriented graphitic carbon films were obtained. The characterization of the PPV nanotube and nanotube carbon has been investigated by means of IR, SEM, AFM and Raman scattering study.

Ultra‐thin graphitic carbon film for high‐power electronics applications

A novel explosive-emission cathode capable to produce high-current nanosecond electron bunches and enable tuning in terms of size is proposed and demonstrated to performance fit the particular problem. The record parameters, average diode current 6.5 kA and voltage 410 kV, are achieved by covering a conventional, planar copper cathode with a carbon film being composed of graphitic pyrolytic carbon (GrPyC). The demonstrated plasma expansion velocity and current density are comparable with those obtained with conventional graphitic cathodes. However, in contrast to purely graphite cathodes, the shape, size and surface profile of the proposed hybrid GrPyC/Cu cathodes can be readily adjusted allowing adjustments to fit requirements of a variety of high-current electronic applications.

Direct laser writing of nanorough cell microbarriers on anatase/Si and graphite/Si

The formation of hierarchical structures consisting of microstripe barriers decorated with nanorough ablated materials prepared by direct laser writing is described. Linear features of circa 25μm width and 12μm height are achieved on amorphous and crystalline titania and graphitic carbon films deposited on silicon. Ablated protrusions build up barriers decorated by nanoscale Si-film reconstructions, as indicated by EDX maps and micro-Raman spectroscopy. Wettability tests show a dramatic change in water contact angle, which leads to almost full wetting after irradiation, irrespective of the original film composition. Fluorescence microscopy images of human mesenchymal stem cells cultured on 1D and 2D structures demonstrate the short term biocompatibility of the ablated surfaces. It is shown that cells adhere, extend and polarize on feature edges, independently of the type of surface, thus suggesting that the created nanoroughness is at the origin of the antifouling behavior. In particular, irradiated anatase and graphite surfaces demonstrate an increased performance of crystalline films for the creation of cell guiding and trapping devices. The results suggest that such laser processing of films may serve as a time-and-cost-efficient method for the design of few-cells analytical surfaces.

Direct growth of nanocrystalline graphitic carbon films on BaF2 by alcohol CVD

Multilayered nanocrystalline graphitic carbon films were directly formed on BaF2 substrates by the alcohol chemical vapor deposition method using ethanol. Domain size was typically 46 nm as estimated from the peak intensity ratios of G- and D-band of the Raman spectra, which were higher than currently reported values for various dielectric substrates. Sheet resistance measured by the four-probe method was 900 Ω/sq. A possible formation mechanism of the graphitic carbon layer was discussed referring to the results of X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray spectroscopy (EDX) for a composition analysis. It was found that the graphitic carbon layers were formed on the BaO surface, which was produced by the oxidization of the BaF2 substrate by the ethanol source.

Nematic liquid crystalline alignment on graphitic carbon film surfaces and its electrooptical characteristics

A graphitic carbon (g-C) film directly grown on a synthetic quartz glass substrate was applied to a liquid crystal (LC) device as an alignment layer combined with a transparent electrode for a demonstration of high performance. The as-grown g-C films showed a nanometer-size domain with 91.6% transmittance at 550 nm and with a sheet resistance of 5.9 kΩ/sq. The nanodomain of the g-C film surface was associated with a random orientation of the twisted nematic LC (4-pentyl-4′-n-cyanobiphenyl, 5CB) molecules in an in-plane randomly parallel alignment that was analyzed by polarized optical microscopy (POM). We also demonstrated an LC display (LCD) in an in-plane random hybrid twisted nematic (IPR-HTN) configuration using the g-C films compared with a previously proposed configuration using a hydroxypropyl cellulose (HPC) sublayer and a TN configuration using a polyimide film with a rubbing treatment. It was found that the combined g-C alignment layer/electrode provides a low turn-on voltage, a fast response, and a wide viewing angle as an orientation sublayer and an electrode.

Selective Synthesis of Graphitic Carbon and Polyacetylene by Electrochemical Reduction of Halogenated Carbons in Ionic Liquid at Room Temperature

An electrochemical deposition technique was developed to realize high yield and selective synthesis of target carbon nanomaterials. The electrochemical reduction of halogenated carbons, such as carbon tetrachloride, in an ionic liquid was found to result in the formation of a thick graphitic carbon film over a Ni substrate, whereas polyacetylene formation was confirmed in the presence of a small amount of H2O. Such a novel high yield, selective carbon synthesis is expected to be a promising technique for the design of various carbon nanomaterials.

Catalyst-Free and Large-Area Deposition of Graphitic Carbon Films on Glass Substrates by Pyrolysis of Camphor

The feasibility of obtaining graphitic carbon films on targeted substrates without a catalyst and transfer step was explored through the pyrolysis of the botanical derivative camphor. In a horizontal quartz tube, camphor was subjected to a sequential process of evaporation and thermal decomposition; then, the decomposed product was deposited on a glass substrate. Analysis of the Raman spectra suggest that the deposited film is related to unintentionally doped graphitic carbon containing some linear carbon chains. The films were transparent in the visible range and electrically conductive, with a sheet resistance comparable to that of graphene. It was also demonstrated that graphitic films with similar properties can be reproduciblyobtained, while property control was readily achieved by varying the process temperature.

Transferring-free and large-area graphitic carbon film growth by using molecular beam epitaxy at low growth temperature

Graphitic carbon films prepared by using molecular beam epitaxy (MBE) on metal templates with different thicknesses deposited on SiO2/Si substrates are investigated in this paper. With thick Cu templates, only graphitic carbon flakes are obtained near the Cu grain boundaries at low growth temperatures on metal/SiO2 interfaces. By replacing the Cu templates with thin Ni templates, complete graphitic carbon films with superior crystalline quality is obtained at 600°C on SiO2/Si substrates after removing the Ni templates. The enhanced attachment of the graphitic carbon film to the SiO2/Si substrates with reduced Ni thickness makes the approach a promising approach for transferring-free graphene preparation at low temperature by using MBE.

Graphitic carbon/n-CdTe Schottky-type heterojunction solar cells prepared by electron-beam evaporation

We report on the analysis of the electrical and photoelectrical properties of graphitic carbon/n-CdTe Schottky-type heterojunction solar cells, which have been prepared by the deposition of transparent graphitic carbon films of nanometer thickness onto freshly cleaved n-CdTe substrates by the electron-beam evaporation technique. The presence of the electrically-active interface states at the heterojunction interface was revealed from the analysis of the dominating current transport at forward and reverse bias and from the capacitance–voltage characteristics. The unoptimized graphitic carbon/n-CdTe Schottky-type heterojunctions possess a rectification ratio of RR=1390 and produce a maximum short circuit current density of Jsc=8.47mAcm−2, an open-circuit voltage of Voc=0.435V and fill factor of FF=0.37 under standard illumination conditions (100mWcm−2 AM 1.5).

Growth of uniform carbon thin film containing nanocrystalline graphene clusters from evaporated palm oil by thermal chemical vapor deposition

We report the growth of a relatively uniform graphitic carbon film on a nickel (Ni) substrate from the evaporated refined palm oil by thermal chemical vapor deposition employing the so-called flash cooling technique. Raman spectroscopy measurement confirms that the grown thin film contains nanocrystalline graphene clusters. The graphitic carbon thin film with an average thickness of 2 nm shows an optical transmission of 78% at a wavelength of 550 nm, and a sheet resistance of 13 kΩ/sq. The mechanism of the graphene growth on Ni utilizing the flash cooling technique is discussed by focusing on the possible causes of the highly defective characteristics of the grown film.