Synthesis and characterisation of nanoporous carbide-derived carbon by chlorination of vanadium carbide

Nanoporous carbide-derived carbon (CDC) was synthesised from vanadium carbide (VC) powder via gas phase chlorination in the temperature range from 500 to 1100 °C. The XRD analysis of nanoporous carbon powder samples was carried out to investigate the structural changes (graphitisation) of nanoporous carbons synthesised. The first-order Raman spectra showed the absorption peak at ∼1582 cm⁻¹ and the disorder-induced (D) peak at ∼1345 cm⁻¹. The low-temperature N₂ adsorption experiments were performed and a specific surface area up to 1305 m² g⁻¹ and total pore volume up to 0.66 cm³ g⁻¹ were obtained.     

Spark plasma sintering of double-walled carbon nanotubes

Bulk samples of double-walled carbon nanotubes are prepared for the first time. The best spark plasma sintering conditions are (1100 °C, 100 MPa). Raman spectroscopy and scanning electron microscopy show that the nanotubes are undamaged. The density is equal to 1.29 g cm⁻³ and the pores are all below 6 nm in diameter. The electrical conductivity is equal to 1650 S cm⁻¹. The transverse fracture strength is equal to 47 MPa.     

Origin of the 2450 cm Raman bands in HOPG, single-wall and double-wall carbon nanotubes

The second order Raman signals around the G′-band region of graphite and carbon nanotubes have been investigated at more than 15 excitation laser lines. Two distinct Raman bands have been observed around 2700 cm⁻¹; a prominent one is due to the so-called G′-band and the other is a weak band around 2450 cm⁻¹. Both two bands can be from the double resonance process involving two phonons around the K-point in the phonon dispersion of a two-dimensional graphite. The 2450 cm⁻¹-band has exhibited little power dependence, whereas the intensity of G′-band has shown large photon energy dependence as already reported. The 2450 cm⁻¹-band and the G′-band correspond to non-dispersive q = 0 and fully-dispersive q = 2k, respectively. From the phonon dispersion and the corresponding phonon frequency, the 2450 cm⁻¹-band can be assigned as an overtone mode of LO phonon (i.e. 2LO). This is revealed by calculated Raman spectra of graphite with proper electron-phonon matrix elements. The present study is the first report on the origin and assignment of the 2450 cm⁻¹-band, which is based on the double resonance Raman scattering.     

Comparative study of first- and second-order Raman spectra of MWCNT at visible and infrared laser excitation

Comparative studies of first- and second-order Raman spectra of multi-walled carbon nanotubes (MWCNT) and three other graphitic materials - carbon fiber, powdered graphite and highly ordered pyrolytic graphite - are reported. Three laser excitation wavelengths were used: 514.5, 785 and 1064 nm. In first-order Raman spectra, the positions of the bands D, G and D′ (1100-1700 cm⁻¹) presented very similar behavior, however the intensity (I) ratio I_D/I_G ratio showed differed behaviors for each material which may be correlated to differences in their structural ordering. In the second-order spectra, the G′ band varied strongly according to structure with the infrared laser excitation.     

Raman microspectroscopy of soot and related carbonaceous materials: Spectral analysis and structural information

Experimental conditions and mathematical fitting procedures for the collection and analysis of Raman spectra of soot and related carbonaceous materials have been investigated and optimised with a Raman microscope system operated at three different laser excitation wavelengths (514, 633, and 780 nm). Several band combinations for spectral analysis have been tested, and a combination of four Lorentzian-shaped bands (G, D1, D2, D4) at about 1580, 1350, 1620, and 1200 cm⁻¹, respectively, with a Gaussian-shaped band (D3) at ∼1500 cm⁻¹ was best suited for the first-order spectra. The second-order spectra were best fitted with Lorentzian-shaped bands at about 2450, 2700, 2900, and 3100 cm⁻¹. Spectral parameters (band positions, full widths at half maximum, and intensity ratios) are reported for several types of industrial carbon black (Degussa Printex, Cabot Monarch), diesel soot (particulate matter from modern heavy duty vehicle and passenger car engine exhaust, NIST SRM1650), spark-discharge soot (Palas GfG100), and graphite. Several parameters, in particular the width of the D1 band at ∼1350 cm⁻¹, provide structural information and allow to discriminate the sample materials, but the characterisation and distinction of different types of soot is limited by the experimental reproducibility of the spectra and the statistical uncertainties of curve fitting. The results are discussed and compared with X-ray diffraction measurements and earlier Raman spectroscopic studies of comparable materials, where different measurement and fitting procedures had been applied.     

Two-dimensional Raman spectroscopic study on the structural changes of a poly(3-chlorothiophene) film during the heating process

Two-dimensional Raman spectroscopy has been applied to provide the information on charge carriers and thermal stability of a doped poly(3-chlorothiophene) (PCTh) film. The strong spectral intensity at 1420 cm⁻¹ shows that positive polarons are the major charge carriers in doped PCTh. On the other hand, peaks in the 2D contour maps separate the overlapped bands around 1386 cm⁻¹, confirming the existence of positive bipolarons in PCTh. The positive asynchronous cross peak located at 1420/1386 cm⁻¹ further indicates that bipolarons have a higher thermal stability compared with polarons in the doped PCTh. The increase of the spectral intensity at 1454 cm⁻¹ and the decrease of the spectral intensity at 1420 cm⁻¹ indicate that during the heating process, a structural change occurs in the PCTh film.     

Nitrogen doping effects on the structure behavior and the field emission performance of double-walled carbon nanotubes

The nitrogen (N) doping effect and field emission properties of double-walled carbon nanotubes (DWCNTs) were investigated. Diameter transformation and defect generation in the N-doped DWCNTs mainly depend on the amount of nitrogen employed. By applying N-doping into DWCNTs (1.5 N at.%), the average diameters of the DWCNTs were increased from 1.7 to 2.4 nm, and the crystallinity (I_G/I_D) was decreased from 13.5 to 5. Field emission properties were enhanced by the N doping into DWCNTs. The turn-on field, corresponding to a current density of 0.1 μA/cm², was about 0.9 V/μm for the N-doped DWCNTs (1.5 N at.%). The field enhancement factor of the N-doped DWCNTs was higher than that of the undoped DWCNTs. It was found that the field emission properties were controlled by pyridine-like N in the graphite due to N-doping.     

One-step fabrication of high quality double-walled carbon nanotube thin films by a chemical vapor deposition process

High-quality double-walled carbon nanotubes (DWCNTs) thin-films have been fabricated in one-step by the catalytic chemical vapor deposition gas-flow reaction process with acetone as a carbon source in an argon flow. The DWCNTs film is formed through the self-assembly of the DWCNTs in the gas flow, which is achieved by controlling the gas rates in the synthesis reaction. The DWCNT film is self-supported and consists of preferentially aligned high-quality DWCNT bundles. Raman spectral analysis shows a low intensity ratio of the D band and the G band with I_D/I_G being 0.025 indicating a high-quality of DWCNTs at a macroscopic scale. Property measurements show that the DWCNT film is mechanically robust and highly electrically conductive. The formation of high-quality DWCNTs can be attributed to the reaction in the argon environment that is inert and does not attack the DWCNTs at the high synthesis temperature (1170 °C). This one-step fabrication process is feasible for large-scale productions of high-quality DWCNTs films with promising structural and functional applications.     

Raman spectroscopic evidence for interfacial interactions in poly(bithiophene)/single-walled carbon nanotube composites

Electrochemical polymerization of 2,2′-bithiophene (BTh) on single-walled carbon nanotube (SWCNT) films has been studied by Raman scattering and infrared absorption spectroscopy. Covalent functionalization of SWCNTs with poly(bithiophene) (PBTh) in its un-doped and doped states is demonstrated. The occurrence of a charge transfer process at the interface of PBTh and SWCNTs, is shown by: (i) an up-shift of the Raman lines associated with the radial breathing modes of SWCNTs that reveals both a doping process and an additional twisting together as a rope with the conducting polymer as binding agent; (ii) a new Raman band in the range 1430-1450 cm⁻¹ indicating the functionalization of SWCNTs with PBTh in doped and un-doped states; (iii) strong absorption bands situated in the interval 600-800 cm⁻¹ resulting from steric hindrance produced by the nanotube binding to the polymeric chain. Treatment of the PBTh/SWCNT composite with aqueous NH₄OH solution forms un-doped PBTh covalently functionalized SWCNTs. At the resonant excitation of the metallic tubes, an additionally enhanced Raman process is generated by plasmon excitation in the metallic nanotubes. It is evidenced by a particular behavior in the Stokes and anti-Stokes branch of the PBTh Raman line at 1450 cm⁻¹.     

SERS studies on single-walled carbon nanotubes submitted to chemical transformation with sulfuric acid

Surface-enhanced Raman scattering (SERS) at 676.44 nm and 1064 nm excitation wavelengths was used to investigate chemical transformation of single-walled carbon nanotubes (SWNTs) deposited on a gold support. Sulfuric acid was used as the chemical reagent. Special attention was paid to the changes in the Raman bands associated to radial and tangential vibration modes. Partial restoration of the Raman spectra by a subsequent alkaline treatment indicates a transformation with a certain degree of reversibility. The recovery reaction achieved with a 0.5 M KOH solution showed that the variations of tangential and radial band groups are not correlated. The intensity changes of the radial bands is a principal indicator for the chemical transformation of the SWNTs. Particular attention was paid to radial bands at 164 and 176 cm⁻¹, observed with 1064 nm and 676.44 nm excitation wavelength, respectively, and their 14 cm⁻¹ up-shifted replicas i.e. the bands at 178 and 190 cm⁻¹. A different behavior of these bands in the anti-Stokes side was observed.     

Characterisation of activated nanoporous carbon for supercapacitor electrode materials

Commercial nanoporous carbon RP-20 was activated with water vapor in the temperature range from 950 °C to 1150 °C. The XRD analysis was carried out on nanoporous carbon powder samples to investigate the structural changes (graphitisation) in modified carbon that occurred at activation temperatures T ? 1150 °C. The first-order Raman spectra showed the absorption peak at 1582 cm⁻¹ and the disorder (D) peak at 1350 cm⁻¹. The low-temperature N₂ adsorption experiments were performed at −196 °C and a specific surface area up to 2240 m²g⁻¹ for carbon activated at T = 1050 °C was measured. The cell capacitance for two electrode activated nanoporous carbon system advanced up to 60 F g⁻¹ giving the specific capacitance ∼240 F g⁻¹ to one electrode nanoporous carbon ∣1.2 M (C₂H₅)₃CH₃NBF₄ + acetonitrile solution interface. A very wide region of ideal polarisability for two electrode system (∼3.2 V) was achieved. The low frequency limiting specific capacitance very weakly increases with the rise of specific area explained by the mass transfer limitations in the nanoporous carbon electrodes. The electrochemical characteristics obtained show that some of these materials under discussion can be used for compilation of high energy density and power density non-aqueous electrolyte supercapacitors with higher power densities than aqueous supercapacitors.     

Thermal oxidative cutting of multi-walled carbon nanotubes

Commercially available, multi-walled carbon nanotubes grown by CVD are usually inherently entangled, but can be separated by cutting. However, most cutting methods both cause damage to the nanotubes and involve a lengthy work-up procedure. The use of abrupt, repeated exposure to oxidising conditions in air proved to be an efficient (68% yield) means of producing material with open ends, moderate functionalisation, and enhanced solvent dispersibility; the average lengths were reduced from over 5 μm to approximately 650 nm. Additionally, the character of the surface oxides can be tuned to have either an acidic or basic character by using a simple thermal treatment. These approaches could be deliberately integrated into conventional CVD processes, but also have implications for the products of standard nanotube syntheses. Raman spectroscopy and electron microscopy were used to study the impact of cutting on the intrinsic graphitic structure and the length distribution. X-ray photoelectron spectroscopy was used to determine the extent of functionalisation. The cut carbon nanotubes were dispersed in dimethylformamide (DMF), a Lewis basic solvent, and chloroform, a Lewis acidic solvent, using mild sonication. Through the use of an experimentally determined extinction coefficient (ε = 35.10 ml mg⁻¹ cm⁻¹), the relative dispersibility of the cut and functionalised carbon nanotubes in DMF and chloroform was determined.     

Single-source precursor route to carbon nanotubes at mild temperature

Carbon nanotubes were synthesized via a single-source precursor route at 500 °C, using iron carbonyl both as carbon source and catalyst. The X-ray power diffraction pattern indicates that the products are hexagonal graphite. Transmission electron microscope (TEM) images of the sample reveal carbon nanotubes with an average inner (outer) diameter of 30 nm (60 nm). High-resolution TEM indicates that fabrication of the carbon nanotube walls was composed of ca. 40 graphene layers. The Raman spectrum shows two strong peaks at 1587 and 1346 cm⁻¹, corresponding to the typical Raman peaks of graphitized carbon nanotubes. This method avoids the separation of raw material from solvent and simplifies the operation process. At the same time, the research provides a new route to large-scale synthesis of carbon nanotubes.     

Fabrication of nickel oxide-embedded titania nanotube array for redox capacitance application

Titania nanotube array with an enlarged tube diameter of 110 nm and length of 700 nm was grown on titanium metal by a potentiostatic anodization in hydrofluoric acid-phosphoric acid-ethenyl glycol electrolyte. Nickel hydroxide was introduced into this titania nanotubes by either an electrodeposition-oxidation or hydrothermal process. Nickel oxide-titania composite was finally formed by heating treatment at 300 °C. Such a well-defined nanocomposite supported on titanium substrate was designed as a functional nanotube array electrode for the redox capacitance application. The morphology, microstructure and electrochemical properties of the nanocomposites were investigated by field emission scanning electron microscope, X-ray diffraction, energy dispersive X-ray diffraction and cyclic voltammetry measurements. It was found that nickel oxide could be embedded in titania nanotubes and extend from inner wall to top layer with an open pore mouth. The entire tube lengths were approx. 770 nm and 710 nm, meanwhile nickel-to-titanium atom ratios were determined as 9.6 at% and 36.4 at% for nickel oxide-embedded titania by an electrochemical and hydrothermal synthesis, respectively. The corresponding specific redox capacitance was also increased from 26 mF cm⁻² to 85 mF cm⁻² with highly reversible charge-discharge stability. Such an improvement was mostly ascribed to more accessible reaction interface of electroactive nickel oxide through its higher loading and a uniform dispersion on titania nanotubes. The capacitance was further increased up to 128 mF cm⁻² for 36.4 at% nickel-containing nickel oxide-titania/titanium electrode when a porous graphite carbon instead of a platinum sheet was used as a cathode.     

Preparation of double-walled carbon nanotubes from fullerene waste soot by arc-discharge

Fullerene waste soot (FWS) was used as raw material to fabricate double-walled carbon nanotubes (DWCNTs) by arc-discharge in a mixture of Ar and H₂ (2:1, v/v) at 300 Torr. The results of transmission electron microscope and Raman spectroscopy indicate that the high quality FWS-derived DWCNTs can be synthesized by arc-discharge method.     

High pressure-high temperature synthesis of diamond from single-wall pristine and iodine doped carbon nanotube bundles

High pressure and high temperature experiments were performed on single-wall carbon nanotube bundles up to 14.5 GPa and 1800 K. Depending on the thermodynamic conditions, we have observed three different behaviors: at ambient temperature and for pressure lower than 24 GPa, minor structural changes are observed. Depending on the loss of hydrostatic conditions or on the combined application of pressure and temperature, partial or total graphitization is observed. For pressures of 14.5 GPa and temperatures of 1800 K the nanotubes are irreversibly transformed into cubic diamond, showing that it is possible to synthesize under high pressure and high temperature pure sp³ carbon structures from single-wall carbon nanotubes. In the case of iodine intercalated nanotubes, the same conditions of 14.5 GPa and 1800 K lead also to the transformation into diamond. No evidence of incorporation of iodine in the sp³ carbon structure was found. On the basis of our results, we discuss possibilities for new carbon-carbon composite engineering from single-wall carbon nanotube bundles.     

Estimation of crystallinity of isotactic polypropylene using Raman spectroscopy

A method to estimate the degree of crystallinity in isotactic polypropylene has been developed. The method is based on integrated intensities of the Raman bands at 808 and 841 cm⁻¹. From the observation of correlation splitting, Raman bands related to different conformational states were identified. This analysis indicates the existence of three different conformational states. The 808 cm⁻¹ band was assigned to helical chains within crystals. The 840 cm⁻¹ band was shown to be composed of a band at 840 cm⁻¹, assigned to shorter chains in helical conformation, and a broader band at 830 cm⁻¹ assigned to chains in non-helical conformation. In order to establish a quantitative relation between Raman scattering intensity and crystallinity samples subjected to different cooling rates and crystallisation temperatures were analysed. These results correlate well with those of differential scanning calorimetry.     

Electrostatic heterocoagulation of carbon nanotubes and LiCoO2 particles for a high-performance Li-ion cell

Nanotubes were coated on the surface of active LiCoO₂ particles using electrostatic heterocoagulation to enhance the electrochemical properties of a Li-ion battery. Only 0.5 wt% of multiwalled carbon nanotubes (MWCNTs) was added as a conducting agent into the LiCoO₂ cathode, which had a density of 4.0 g cm⁻³. We found that our electrode that was prepared using heterocoagulation with 0.5 wt% of thin MWCNTs maintained a volumetric capacitance of 403 mAh cm⁻¹ after 40 cycles from the initial 624 mAh cm⁻¹, compared with previous result of 310 mAh cm⁻¹ obtained from simple mixing with 3 wt% MWCNTs. The high volumetric capacity with smaller swelling using less amount of MWCNTs was attributed to the self-assembled nanotube network formed between active particles during coagulation, which was maintained with volume expansion during cycle testing.     

Quantifying ion-induced defects and Raman relaxation length in graphene

Raman scattering is used to study disorder in graphene subjected to low energy (90 eV) Ar⁺ ion bombardment. The evolution of the intensity ratio between the G band (1585 cm⁻¹) and the disorder-induced D band (1345 cm⁻¹) with ion dose is determined, providing a spectroscopy-based method to quantify the density of defects in graphene. This evolution can be fitted by a phenomenological model, which is in conceptual agreement with a well-established amorphization trajectory for graphitic materials. Our results show that the broadly used Tuinstra-Koenig relation should be limited to the measure of crystallite sizes, and allows extraction of the Raman relaxation length for the disorder-induced Raman scattering process.     

Protonation and sonication effects on aggregation sensitive Raman features of single wall carbon nanotubes

The G-band and the G′-band (∼2600-2700 cm⁻¹) Raman spectra of single walled carbon nanotubes (SWCNTs) in a sodium dodecyl sulphate aqueous solution were investigated as a function of sonication energy density (ρ). The relative intensity of the G-band feature around ∼1520-1560 cm⁻¹ (G⁻) and the linewidth (Γ) of the G′-band decrease monotonically with ρ where the former dependence is probably due to charge transfer (promoted by sonication induced pH decrease/solubilisation) and consequently not an unambiguous indication of solubilisation. In contrast Γ is shown to be independent of pH, which suggests that the linewidth of the G′-band may be an important indicator of the aggregation state of SWCNTs.