Noncatalytic synthesis of carbon nanotubes, graphene and graphite on SiC

Graphene and carbon nanotubes (CNT) can be produced by vacuum decomposition of SiC, but discrepancies and conflicting data in the literature limit the use of this method for CNT synthesis. A systematic study of the effects of SiC surface morphology and carbon transport through the gas phase leads to reproducible and controlled growth of arrays of small-diameter (1–4 walls) nanotubes, which show pronounced radial breathing modes in Raman spectra, on either carbon or silicon (0 0 0 1) face of 6H SiC wafers at 1400–1900 °C. These nanotube arrays have a very high density and are catalyst-free with no internal closures. They show a higher oxidation resistance compared to CNTs produced by catalytic chemical vapor deposition (CVD). Their integration with graphite/graphene or silica layers on SiC wafers is possible in a simple 2-step process and opens new horizons in nanoscale device fabrication.     

Rapid synthesis of few-layer graphene over Cu foil

We report a unique process for rapid synthesis of few-layer graphene films on Cu foil by microwave plasma chemical vapor deposition (MPCVD). We show that the plasma/metal interaction can be advantageous for a rapid synthesis of such thin films. The process can produce films of controllable quality from amorphous to highly crystalline by adjusting plasma conditions during growth processes of ～100 s duration and with no supplemental substrate heating. Films have been characterized using Raman spectroscopy, scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. The results help to identify the stages involved in the MPCVD deposition of thin carbon films on Cu foil, and the findings open new pathways for a rapid growth of few-layer graphene films.     

On the oxidation degree of few-layer graphene oxide sheets obtained from chemically oxidized multiwall carbon nanotubes

Multiwall carbon nanotubes (MWCNT), generated by catalytic decomposition of methane using a Fe–Mo/MgO catalyst, were subjected to oxidation and exfoliation obtaining aqueous suspensions of few-layer graphene oxides (FLGO). FLGO with different oxidation ratios were obtained using a modified Hummers method by varying the amount of oxidizing agent (KMnO₄), followed by ultrasonic exfoliation. The exhaustive characterization of the resulting materials clearly showed surface functional groups evolution during oxidation and structural and morphological changes. Oxidation caused expansion and separation of the graphene layers by oxygenated groups formation and water intercalation. The full opening of the MWCNT resulted in FLGO as carbon nanoribbons and sheets from a KMnO₄/MWCNT ratio of 6.     

Effects of humic acid on copper adsorption onto few-layer reduced graphene oxide and few-layer graphene oxide

The effects of humic acid (HA) on copper (Cu(II)) adsorption onto few-layer reduced graphene oxide (FRGO) and few-layer graphene oxide (FGO) were investigated using a batch equilibration method, micro-Fourier transform infrared spectroscopy, and extended X-ray absorption fine structure spectroscopy (EXAFS). The results showed that HA was adsorbed on FRGO through π–π interaction. The adsorbed HA introduced O-containing functional groups and negative charges to FRGO surfaces, increasing Cu(II) adsorption through chemical complexation and electrostatic attraction. In contrast, HA was adsorbed onto FGO mainly through polar interactions, due to its rich O-containing functional groups. The adsorbed HA had little effect on Cu(II) adsorption onto FGO because the shielding effect of HA on Cu(II) adsorption was offset by newly introduced adsorption sites of HA on FGO. EXAFS results suggested that Cu(II) was adsorbed on FRGO and FGO mainly through the coordination with their O-containing functional groups. When HA was added at pH 4.0 and 6.0, more Cu(II) was adsorbed on HA-coated FRGO. At pH 8.0, a portion of Cu(II) in solution precipitated on FRGO surface, while the presence of HA led to the formation of FRGO-HA-Cu ternary surface complexes instead of Cu(II) precipitation.     

Flame synthesis of carbon nanotubes using a double-faced wall stagnation flow burner

The potential of using a double-faced wall stagnation flow (DWSF) burner in mass production of multi-walled carbon nanotubes (MWCNTs) has been evaluated. Due to minimized heat losses across a catalyst plate and redistributed temperature profiles on the plate surface, the DWSF burner can produce abundant and uniform MWCNTs, the quality and yield of synthesized MWCNTs being controlled by varying flame stretch rates.     

Fabrication and characterization of few-layer graphene

Few-layer graphene were produced on transition-metal templates by using super-short-pulse laser produced plasma deposition techniques. Transparent few-layer graphene with the average thickness less than 8 nm, width up to 350 nm, and length more than 3 μm were successfully obtained. An unusual edge reconstruction was observed: a double layer folded back on itself, representing four layers. Wrinkles were found within the few-layer graphene, which is likely due to the presence of nanoparticles underneath the few-layer graphene causing distortion. This method of growing few-layer graphene can provide valuable information for understanding the growth mechanism of graphene, which may facilitate its controllable synthesis and applications.     

Flame synthesis of graphene films in open environments

Few-layer graphene is grown on copper and nickel substrates at high rates using a novel flame synthesis method in open-atmosphere environments. Transmittance and resistance properties of the transferred films are similar to those grown by other methods, but the concentration of oxygen, as assessed by X-ray photoelectron spectroscopy, is actually less than that for graphene grown by chemical vapor deposition under near vacuum conditions. The method involves utilizing a multi-element inverse-diffusion-flame burner, where post-flame species and temperatures are radially-uniform upon deposition at a substrate. Advantages of the specific flame synthesis method are scalability for large-area surface coverage, increased growth rates, high purity and yield, continuous processing, and reduced costs due to efficient use of fuel as both heat source and reagent. Additionally, by adjusting local growth conditions, other carbon nanostructures (i.e. nanotubes) are readily synthesized.     

Adatom complexes and self-healing mechanisms on graphene and single-wall carbon nanotubes

Point defects play a role in the functionalization, chemical activation, carrier transport, and nano-engineering of graphitic systems. Here, we use first-principles calculations to describe several processes that alter the properties of graphene and single-wall carbon nanotubes (SWCNTs) in the presence of self-interstitials (SI’s). We find that, while two or four SI’s are stabilized in hillock-like structures that stay idle unless the system is heated to very high temperatures, clustering of three C adatoms leads to the formation of mobile protrusions on graphene and large enough SWCNTs. For different SI concentrations and SWCNT size, the interplay between mobile and immobile species may favor one of the two competing processes, self-healing or formation of adatom superstructures.     

Chemical-free graphene by unzipping carbon nanotubes using cryo-milling

Current paper reports synthesis of chemical free graphene by unzipping of the carbon nanotubes (CNTs) using high strain rate deformation at 150 K. A specially designed cryomill operating at 150 K was used for the experiments. The mechanism of unzipping was further explored using molecular dynamics (MD) simulations. Both experimental and simulation results reveal two modes of unzipping through radial and shear loading.     

Thickness-dependent azobenzene doping in mono- and few-layer graphene

Doping effects on exfoliated graphenes induced by methyl orange (MO) have been studied by spatially resolved Raman spectroscopy. When the MO molecules were adsorbed on the top of graphenes, the charge transfer between them caused two outcomes simultaneously. One is the strong chemical doping in graphenes and the other is the enhanced Raman signals of MO molecules. Our finding provides a potential approach for manipulating the electronic properties of graphenes and investigating the vibrational properties of molecules. Moreover, the thickness-dependent doping effects in graphenes are unambiguous distinguished by Raman imaging. The possible origin was discussed and designated to the different band structures of graphenes and the screening effect.     

Hybrid carbon nanotubes/graphene modified acrylic coats

Water-borne one-component acrylic coating compositions and coats modified with carbon nanotubes (CNT), graphene (GN), as well as hybrid carbon nanofillers (CNT/GN mixtures) have been prepared and evaluated. These coating materials were successfully formulated on a basis of commercial components, i.e. acrylic resin aqueous dispersion, CNT and/or GN and auxiliary agents. Influence of a kind and ratio of incorporated carbon nanofillers on the following coats properties was investigated: electrical surface resistivity, transparency, gloss, mechanical (hardness and cross-cut adhesion) and thermal properties (glass transition temperature, storage modulus and thermal stability). The presence of carbon nanostructure significantly improved electrical conductivity, hardness, storage modulus and thermal stability of the coats. Moreover, the nanofillers did not negatively influence adhesion and glass transition temperature.     

Magneto-optical selection rules of curved graphene nanoribbons and carbon nanotubes

We derive the generalized magneto-absorption spectra for curved graphene nanoribbons and carbon nanotubes by using the Peierls tight-binding model. The main spectral characteristics and the optical selection rules result from the cooperative or competitive relationships between the geometric structure and a magnetic field. In curved ribbons, the dominant selection rule remains unchanged during the variation of the curvature. When the arc angle increases, the prominent peaks are split, with some even vanishing as the angle exceeds a critical value. In carbon nanotubes, the angular-momentum coupling induces extra selection rules, of which more are revealed due to the increase of either (both) of the factors: tube diameter and field strength. Particularly once the two factors exceed certain critical values, the optical spectra could reflect the quasi-Landau-level structures. The identifying features of the spectra provide insight into optical excitations for curved systems with either open or closed boundary condition.     

Chirality dependent surface adhesion of single-walled carbon nanotubes on graphene surfaces

The adhesion of single-walled carbon nanotubes to the surface of graphene has been studied, mixing shortened tubes of different chiralities and few-graphene crystal membranes. The spontaneous atomic match of the two lattices was directly imaged using high-resolution transmission electron microscopy and carefully analyzed by means of electron diffraction, and we found evidences that surfaces of few-graphene crystal act as tangential nano-sieves, preferentially retaining zig-zag tubes to their surface.     

Laser-induced unzipping of carbon nanotubes to yield graphene nanoribbons

Irradiation of undoped and doped multi-walled carbon nanotubes by an excimer laser (energy ～200-350 mJ) yields graphene nanoribbons (GNRs). The GNRs so obtained have been characterized by transmission electron microscopy, atomic force microscopy and Raman spectroscopy.     

Electron-beam engineering of single-walled carbon nanotubes from bilayer graphene

Bilayer graphene nanoribbons (BGNRs) with a predefined width have been produced directly from bilayer graphene using a transmission electron microscope (TEM) in scanning mode operated at 300 kV. The BGNRs have been subsequently imaged in high-resolution TEM mode at 80 kV. During imaging, the interaction of the electrons with the sample induces structural transformations in the BGNR, such as closure of the edges and thinning, leading to the formation of a single-walled carbon nanotube (SWCNT). We demonstrate using molecular dynamics simulations that the produced SWCNT is, in fact, a flattened SWCNT with elliptical circumference. Density functional theory calculations show that the band gap of the flattened semiconducting SWCNTs is significantly smaller than that of the undeformed semiconducting SWCNTs, and this effect is particularly profound in narrow SWCNTs.     

Easy synthesis of carbon nanotubes with polypyrrole nanotubes as the carbon precursor

An easy synthesis route for carbon nanotubes with polypyrrole nanotubes as a carbon precursor has been developed. Polypyrrole nanotubes were fabricated via a reactive self-degraded template method. Carbon nanotubes were further obtained by pyrolysis of the polypyrrole nanotube at 900 °C under a nitrogen atmosphere. The resultant carbon nanotube structure was found to be amorphous carbon on the basis of XRD, Raman spectra and high-resolution transmission electron microscopy (HRTEM) studies.     

Single step synthesis of graphene nanoribbons by catalyst particle size dependent cutting of multiwalled carbon nanotubes

Graphene nanoribbons are emerging as an interesting material for the study of low dimensional physics and for the applications in future electronics due to its finite energy band gap. However, its applicability for large scale nanoelectronics may not be effectively realized unless graphene nanoribbons could be produced using a simple, viable, cost-effective and scalable technique. Here, we report the one step facile synthesis of few layered graphene nanoribbons (GNRs) by catalytically unzipping multi-walled carbon nanotubes (MWCNTs) based on the solubility of carbon atoms in transition metals. The process is free from aggressive oxidants (such as KMnO(4), KClO(4), H(2)SO(4), HNO(3), etc.) and utilizes the in situ grown nickel nanoparticles for nanotube unzipping. This is an additional advantage over previously used techniques to synthesize GNRs. To observe the effect of catalyst particle size and reaction temperature on cutting length of the nanotubes, a simulation study has been done based on solubility of carbon atoms in metal nanoparticles.     

Sol-gel synthesis and investigation of metal-oxide catalysts for the preparation of carbon nanomaterials

In this work, the investigation of metal-oxide catalysts of various chemical compositions used in the synthesis of carbon nanotubes was performed. The physical-mechanical (dispersity) and structural (specific surface area, pore volume) characteristics of the catalysts were measured and the influence of the regularities of these parameters on the catalytic activity, as well as on the quality and yield of the final product, carbon nanotubes, were revealed.