Effect of graphitization on fluorination of carbon nanocones and nanodiscs

The reactivity with pure fluorine gas of a mixture of carbon nanodiscs and nanocones was investigated. The starting materials were the as-prepared mixture, which results from cracking of heavy oils, and the sample post-treated at 2700 °C in argon atmosphere in order to increase the graphitization degree. The effect of this graphitization on the resulting fluorinated carbons was highlighted in terms of structural and morphological features using ¹³C and ¹⁹F solid-state nuclear magnetic resonance, electron paramagnetic resonance, Raman spectroscopy, atomic force microscopy and scanning electron microscopy. These characterizations were used to explain the electrochemical properties of these materials when used as an electrode in a primary lithium battery.     

The synthesis of microporous carbon by the fluorination of titanium carbide

Fluorination of titanium carbide with pure fluorine gas leads to the formation of a nanoporous carbon with a monodisperse pore size distribution centered at 0.59 nm. As with chlorination, a selective etching of titanium atoms from the carbide matrix occurs but at lowers temperatures (130–300 °C) than chlorination.Complementary physico-chemical characterizations such as quantitative XRD, Raman spectroscopy, solid state ¹³C nuclear magnetic resonance have been performed to determine accurately the amount and hybridization state of carbon in the studied samples. Textural analysis by scanning electron microscopy and N₂ sorption at 77 K shows that titanium trifluoride acts as a protective layer which limits the transformation of the new carbon formed into carbon fluorides.     

Low-temperature synthesis of multilayer graphene/amorphous carbon hybrid films and their potential application in solar cells

ABSTRACT: The effect of reaction temperature on the synthesis of graphitic thin film on nickel substrate was investigated in the range of 400 [DEGREE SIGN]C to 1,000 [DEGREE SIGN]C. Amorphous carbon (a-C) film was obtained at 400 [DEGREE SIGN]C on nickel foils by chemical vapor deposition; hybrid films of multilayer graphene (MLG) and a-C were synthesized at a temperature of 600 [DEGREE SIGN]C, while MLG was obtained at temperatures in excess of 800 [DEGREE SIGN]C. Schottky-junction solar cell devices prepared using films produced at 400 [DEGREE SIGN]C, 600 [DEGREE SIGN]C, 800 [DEGREE SIGN]C, and 1,000 [DEGREE SIGN]C coupled with n-type Si demonstrate power conversion efficiencies of 0.003%, 0.256%, 0.391%, and 0.586%, respectively. A HNO3 treatment has further improved the efficiencies of the corresponding devices to 0.004%, 1.080%, 0.800%, and 0.820%, respectively. These films are promising materials for application in low-cost and simple carbon-based solar cells.     

Combustion synthesis of graphene materials

Combustion synthesis (CS) of graphene by a novel type of exothermic self-sustain reaction between a refractory ceramic compound (silicon carbide) and polymer (polytetrafluoroethylene, PTFE) under the inert gas (argon) environment is reported. The synthesis of graphene is confirmed by both transmission electron microscopy and Raman spectroscopy. It is important that the produced graphene has low (<1 wt.%) oxygen content. The mechanism for CS of graphene is also discussed. It is experimentally shown that fluorocarbon gases (e.g. tetrafluoroethylene, C₂F₄) released due to PTFE decomposition in the combustion wave, reduces SiC to tetrafluorosilane (SiF₄) gas and meso-porous carbon particles with folded “native” graphene layers on their surfaces. The continuous supply of carbon, in the form of fluorocarbon gases, and the high reaction temperature (∼1400 K) enables further rapid growth of “free-standing” graphene sheets on the surface of those graphene-coated particles. The developed method for synthesis of graphene does not require an external energy source, since it occurs in a self-sustained synergetic manner. This approach is also flexible in terms of tuning the synthesis conditions, and allows easy scale-up.     

Energetic stability of graphene nanoflakes and nanocones

We investigate the relative energetic stability of a variety of nanographene structures such as graphene nanoflakes, nanoribbons, nanodisks, and nanocones. We calculate the cohesive energies with respect to hydrogen passivation, edge nature (zigzag versus armchair) and shape (triangular, rectangular, hexagonal). The cohesive energy is confirmed to increase with size for all these structures. We pay particular attention to optimally-compact circular flakes and compare our theoretical results with round disks produced in a plasma torch atmosphere. We find in the calculations that round shape does not have preferred relative stability. This suggests that the observed disks are grown under conditions where carbon atoms are highly mobile. For graphene nanocones we obtain a similar result. Experimentally, the open base of a 19-degree-cone is observed perpendicular to the cone axis, but this does not correspond to the most stable configuration as obtained by the calculations. Instead, we find that both, disks and cones, prefer minimal length of the edge termination rather than a maximum in the cohesive energy. With respect to our results we discuss for polycyclic aromatic hydrocarbons (PAH) and atomic clusters, as models for graphene flakes, the significance of the cohesive energy for the observed abundances.     

Hierarchical graphene nanocones over 3D platform of carbon fabrics: a route towards fully foldable graphene based electron source

A three dimensional field emitter comprising hierarchical nanostructures of graphene over flexible fabric substrate is presented. The nanostructuring is realized through plasma treatment of graphene, coaxially deposited over individual carbon fiber by means of simple aqueous phase electrophoretic deposition technique. Hierarchical graphene nanocone, acting as a cold electron emitter, exhibits outstanding electron emission performance with a turn-on field as low as 0.41 V μm(-1) and a threshold field down to 0.81 V μm(-1). Electric field modification around the special woven like geometry of the underlying base fabric substrate serves as the booster to the nanostructured graphene related field amplification at the electron emission site. Superb robustness in the emission stability can be attributed to suppressed joule heating on behalf of higher inborn accessible surface area of graphene nanocone as well as excellent electrical and thermal conductivity of both the graphene and carbon fabrics. Superior flexibility of this high-performance graphene based emitter ensures their potential use in completely foldable and wearable field emission devices.     

One-pot synthesis of MnO2/graphene/carbon nanotube hybrid by chemical method

A branched hybrid of MnO₂/graphene/carbon nanotube (CNT) is generated in a one-pot reaction process by chemical method. Some ultrathin MnO₂/graphene nanosheets, around 5 nm in thickness, are randomly distributed on the CNT surface. Morphology, phase structure, microstructure and vibrational properties of the hybrid were characterized by field emission scanning electron microscope, X-ray diffractometer, high resolution transmission electron microscope and Raman spectrometer. Elemental distribution of the hybrid was determined by energy dispersive X-ray mapping performed in scanning transmission electron microscope mode. The key factor of the formation mechanism is associated with both redox and oxidation–intercalation reactions. Graphene flakes are partly exfoliated from the surface layers of the CNTs, and the redox reaction between KMnO₄ and hydroxyl groups occurs on both sides of these flakes, resulting in the formation of a MnO₂/graphene/CNT hybrid. Brunauer–Emmett–Teller surface area measurements indicate that the hybrid has over four times the specific surface area of the pristine CNTs.     

Synthesis of three-dimensional carbon nanotube/graphene hybrid materials by a two-step chemical vapor deposition process

A three-dimensional carbon nanotube (CNT)/graphene hybrid material was synthesized by a two-step chemical vapor deposition (CVD) process. Due to the separated CVD processes for graphene and CNTs, the structures of the hybrid materials could be easily controlled. It is revealed that graphene film was tightly connected with one end of the CNT arrays, forming “jellyfish” structures. Moreover, our results indicate that the presence of graphene influenced the precipitation and growth rate of CNTs. The precipitation of CNTs was postponed due to the existence of graphene. However, the average growth rate of CNTs in the graphene region for the whole process was faster than that in the region without graphene.     

Facile approach to prepare multi-walled carbon nanotubes/graphene nanoplatelets hybrid materials

A facile approach was developed to prepare multi-walled carbon nanotubes/graphene nanoplatelets hybrid materials through covalent bond formation. First, poly(acryloyl chloride) was grafted onto oxidized multi-walled carbon nanotubes through the reaction between the acyl chloride groups of poly and the hydroxyl groups of oxidized multi-walled carbon nanotubes. Second, the remaining acyl chloride groups of poly were allowed to react with the hydroxyl groups of hydroxylated graphene nanoplatelets. Scanning electron microscopy and transmission electron microscopy data showed that the multi-walled carbon nanotubes and graphene nanoplatelets were effectively connected with each other. And Fourier transform infrared spectroscopy data indicated the formation of covalent bonds between carbon nanotubes and graphene nanoplatelets. Conformational changes were monitored by Raman spectroscopy. This novel kind of carbon hybrid materials may have the potential application in a wide field, especially in increasing the toughness and strength of the matrix resin.     

Facile synthesis of reduced graphene oxide/MWNTs nanocomposite supercapacitor materials tested as electrophoretically deposited films on glassy carbon electrodes

This paper reports on a facile synthesis method for reduced graphene oxide (rGO)/multi-walled carbon nanotubes (MWNTs) nanocomposites. The initial step involves the use of graphene oxide to disperse the MWNTs, with subsequent reduction of the resultant graphene oxide/MWNTs composites using l-ascorbic acid (LAA) as a mild reductant. Reduction by LAA preserves the interaction between the rGO sheets and MWNTs. The dispersion-containing rGO/MWNTs composites was characterized and electrophoretically deposited anodically onto glassy carbon electrodes to form high surface area films for capacitance testing. Pseudo capacitance peaks were observed in the rGO/MWNTs composite electrodes, resulting in superior performance with capacitance values up to 134.3 F g^?1 recorded. This capacitance value is higher than those observed for LAA-reduced GO (LAA-rGO) (63.5 F g^?1), electrochemically reduced GO (EC-rGO) (27.6 F g^?1), or electrochemically reduced GO/MWNTs (EC-rGO/MWNTs) (98.4 F g^?1)-based electrodes.     

Free transverse vibration of single-walled carbon nanocones

Free transverse vibrations of single-walled carbon nanocones (SWCNCs) are investigated through beam model and molecular dynamics (MD) simulations. The fundamental frequencies of transverse vibrations of cantilevered SWCNCs with different apex angles, top radii and lengths are obtained from the MD simulations. The Timoshenko beam model provides a better prediction of the frequencies than the Euler–Bernoulli beam model. MD results indicate that apex angles have significant effects on the frequencies of SWCNCs with same lengths. It is found from the MD that the fundamental frequency of SWCNC is higher than an equivalent SWCNT. The effect of different top radii of SWCNC on the frequency is also examined.     

All-carbon field emission device by direct synthesis of graphene and carbon nanotube

In this paper, an all carbon-based field emission device (FED) fabricated by graphene and carbon nanotubes (CNTs) is presented. Through the combination of highly conductive graphene and photolithographically patterned CNT, the resistivity of the interface is lowered and the FED performance is enhanced. FE measurements indicated that the fabricated all carbon-based FED demonstrated stable electron emission properties with uniform luminance.     

Fabrication of graphene/single wall carbon nanotubes/polyaniline composite gels as binder-free electrode materials

A three-dimensional (3D) graphene-based hydrogels system containing one-dimensional (1D) carbon material-single wall carbon nanotubes (SWCNTs) and pseudocapacitor material-polyaniline (PANI) was prepared by combination of cross-linking, reduced and in situ polymerization. The polyaniline nanoparticles were combined with the reduced graphene sheet by π-π conjugation. The as-perpared composite gels could be directly used as electrode materials without binders. Due to the synergistic effect between SWCNTs, graphene sheet and PANI, the graphene/single wall carbon nanotubes/polyaniline (GH/SWCNTs/PANI) composite gel shows the enhanced electrochemical performances. The resultant GH/SWCNTs/PANI gel electroactive material shows a gravimetric specific capacitance of 145.4 F/g at 0.5 A/g and has improved 45% compared with initial graphene hydrogel (GH) at the same current density. And it keeps high retention of 98.8% of the initial capacity after 10,00 charge/discharge cycles at high current density of 10 A/g. The great cycle stability achieved is fundamentally attributed to the support of graphene sheet and single wall carbon nanotubes, which favors stress distribution and charge transfer during the longtime charge/discharge process. The graphene-based hydrogels could be a potential applicant for high rate charge/discharge applications. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 46948.     

A selective drug-release system consisting of surface-modified electrospun carbon fibers by oxy/fluorination

A selective drug-release system was prepared for a controlled drug release between hydrophobic and hydrophilic drugs by using a prepared membrane. A membrane was constructed by electrospun carbon fibers. A porous structure was created in the carbon fibers by chemical activation to reduce the initial drug-burst phenomenon by drug storage in the pores. The surface of the activated carbon fibers was modified by the addition of hydrophobic/hydrophilic functional groups by oxyfluorination or fluorination treatments to allow the selective release of mixed hydrophobic and hydrophilic drugs. The in vitro drug permeation was studied under various applied electric voltages. The initial drug-release phenomenon was reduced due to the storage effect by the improved pore structure, and the drug-release rate was controlled by the intensity of the applied electric voltage. In addition, selective drug release was observed with the presence of the hydrophobic or hydrophilic functional groups introduced through oxyfluorination or fluorination treatments.     

Templated synthesis of carbon materials mediated by porous clay heterostructures

Mesoporous carbon materials were prepared through template method approach using porous clay heterostructures (PCHs) as matrix and furfuryl alcohol as carbon precursor. Three PCHs prepared using amines with 8, 10 and 12 carbon atoms were used. The effect of several impregnation-polymerization cycles of the carbon precursor, the carbonization temperature and the need of a previous surface alumination were evaluated. The presence of two porosity domains was identified in all the carbon materials. These two domains comprise pores resulting from the carbonization of the polymer film formed in the inner structure of the PCH (domain I) and larger pores created by the clay particles aggregation (domain II). The predominance of the porosity associated to domain I or II can be achieved by choosing a specific amine to prepare the PCH matrix. Carbonization at 700 °C led to the highest development of pores of domain I. In general, the second impregnation-polymerization cycle of furfuryl alcohol resulted in a small decrease of both types of porosity domains. Furthermore the previous acidification of the surface to create acidic sites proved to be unnecessary. The results showed the potential of PCHs as matrices to tailor the textural properties of carbons prepared by template mediated synthesis.     

Seed/catalyst-free growth of zinc oxide nanostructures on multilayer graphene by thermal evaporation

We report the seed/catalyst-free growth of ZnO on multilayer graphene by thermal evaporation of Zn in the presence of O₂ gas. The effects of substrate temperatures were studied. The changes of morphologies were very significant where the grown ZnO structures show three different structures, i.e., nanoclusters, nanorods, and thin films at 600°C, 800°C, and 1,000°C, respectively. High-density vertically aligned ZnO nanorods comparable to other methods were obtained. A growth mechanism was proposed based on the obtained results. The ZnO/graphene hybrid structure provides several potential applications in electronics and optoelectronics.     

Synthesis of fluorinated graphene with tunable degree of fluorination

An easy, low-cost and effective synthesis of fluorinated graphene with tunable C/F atomic ratio (R_C/F) has been realized by the reaction between dispersed graphene oxide and hydrofluoric acid. The results show that fluorine is grafted onto the basal plane of graphene, and the R_C/F can be easily adjusted by controlling the reaction conditions. The as-synthesized fluorinated graphene exhibits a sheet-like morphology with 1–2 layered thickness and tunable bandgap energy from 1.82 to 2.99 eV, which has potential applications in optoelectronic and photonic devices.     

Toughening of silicon nitride ceramics by addition of multilayer graphene

Silicon nitride (Si₃N₄) ceramics have superior mechanical properties allowing their broad application in many technical fields. In this work, Si₃N₄-based composites with 1–5?wt% multilayer graphene (MLG) content were fabricated by spark plasma sintering at different temperatures and holding time in order to improve the fracture resistance of the Si₃N₄ ceramic. Our investigation focused on understanding the relationships between the microstructure and mechanical properties with special attention to the intergranular phases between Si₃N₄ matrix and MLG reinforcement.We have found that nanopores developed at the Si₃N₄-MLG interface due to a reaction between carbon and the oxygen available in the topmost layer of the Si₃N₄ particles. Interface porosity has an optimum for the toughening effect. In 1?wt% MLG/Si₃N₄ composites nanopores are local, but separated at the Si₃N₄-MLG interface, which promote the MLG pull-out mechanism imparting a significant toughening effect on the composite. Beyond the optimal 1?wt% MLG content, MLG platelets agglomerate and excessive porosity are developed at the Si₃N₄-MLG interfaces, leading to weaker matrix- graphene adhesion and thus lower fracture toughness.