Multi-walled carbon nanotubes formed by condensed-phase conversions of Fe-C-based nanopowder in vacuum

We describe the formation of multi-walled carbon nanotubes (MWCNTs) which have grown during annealing at 800 °C of Fe-C-based nanopowder in vacuum. The Fe-C-based nanopowder was produced by a laser pyrolysis of gas-phase reactants. The as-synthesized and annealed samples were characterised by TEM, XRD, Mössbauer spectroscopy, Raman spectroscopy, and magnetic measurements. Under the TEM, MWCNTs were observed in the annealed sample. In addition, larger particles with the graphitic shells and various graphitic structures were found. XRD and Mössbauer analysis confirmed that only two iron phases were present in the annealed sample: α-Fe and Fe₃C. Phase transformations taking place during the thermal treatment of the sample are discussed.     

Effect of Thermal Treatment on the Structure of Multi-walled Carbon Nanotubes

The effects of vacuum annealing and oxidation in air on the structure of multi-walled carbon nanotubes (MWCNTs) produced by a large-scale catalytic chemical vapor deposition (CCVD) process are studied using Raman spectroscopy and transmission electron microscopy (TEM). A detailed Raman spectroscopic study of as-produced nanotubes has also been conducted. While oxidation in air up to 400°C removes disordered carbon, defects in tube walls are produced at higher temperatures. TEM reveals that MWCNTs annealed at 1,800°C and above become more ordered than as-received tubes, while the tubes annealed at 2,000°C exhibit polygonalization, mass transfer and over growth. The change in structure is observable by the separation of the Raman G and D′ peaks, a lower R-value (I _D/I _G ratio), and an increase in the intensity of the second order peaks. Using wavelengths from the deep ultraviolet (UV) range (5.08 eV) extending into the visible near infrared (IR) (1.59 eV), the Raman spectra of MWCNTs reveal a dependence of the D-band position proportional to the excitation energy of the incident laser energies.     

Mild hydrothermal treatment to prepare highly dispersed multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) with improved dispersion property have been prepared by a mild and fast hydrothermal treatment. The hydrothermal process avoids using harsh oxidants and organic solvents, which is environmental friendly and greatly decreases the damage to intrinsic structure of MWCNTs. The modified MWCNTs were highly soluble in polar solvents such as water, ethanol and dimethylformamide. Morphological observation by TEM indicated that the diameter and inherent structure were well reserved in modified MWCNTs. X-ray photoelectron spectroscopy and Raman spectroscopy were used to quantify functional groups created on the MWCNT surface, and to determine rational parameters of hydrothermal process.     

Origin of the low-frequency band in Raman spectra of multi-walled carbon nanotubes synthesized by the CVD method

The origin of the low-frequency band (250–300 cm^?1) in the Raman spectra of multi-walled carbon nanotubes (MWCNTs) produced by the CVD method has been studied. The studies performed by Raman spectroscopy, transmission electron microscopy, Auger spectroscopy, and X-ray photoelectron spectroscopy after chemical and thermal treatments allow the assumption that this band belongs to radial vibrations of carbon atoms in internal walls of MWCNTs.     

Functionalization of multiwall carbon nanotubes with nitrogen containing polyelectrolyte by a simple method

Commercially available multiwall carbon nanotubes (MWCNTs) have been functionalized with poly(diallyl dimethylammonium) chloride (PDDA), a nitrogen containing polyelectrolyte by a simple on-off ultrasonication method. The results obtained from Raman and X-ray photoelectron spectroscopy (XPS) studies confirm the functionalization of MWCNTs with PDDA. An up- shift in the positions of C1s XPS peak and a down-shift in the positions of the N1s XPS peak, has been observed along with an up-shift in the G-peak position in the Raman spectra, which suggest the occurrence of inter-molecular charge transfer from carbon atoms in MWCNTs to N⁺ centres in PDDA. The preliminary linear sweep voltammetry (LSV) results show good electrocatalytic activity of MWCNTs functionalized with nitrogen containing polyelectrolyte, which is comparable to the results with platinum based electrodes. Thus, MWCNTs non-covalently functionalized with a nitrogen containing polyelectrolyte (PDDA) by a simple on-off ultrasonication method could be advantageous for developing efficient metal-free electrocatalysts for the oxygen reduction reaction (ORR).     

Effect of reaction parameters on the growth of MWCNTs using mesoporous Sb/MCM-41 by chemical vapour deposition

Mesoporous Si-MCM-41 molecular sieve was synthesized hydrothermally and different wt.% of Sb (1.0, 2.0, 3.0, 5.0, 10.0, 15.0 and 20.0) was loaded on it by wet impregnation method. The Sb/MCM-41 materials were characterized by various physico-chemical techniques such as XRD, TGA and TEM. The TEM image showed a honeycomb structure of the host material. They were used as catalytic templates for the growth of MWCNTs by CVD method with different temperatures at 700, 800, 900 and 1000 °C using acetylene as a carbon precursor. The reaction temperature was optimized for the better formation of MWCNTs and they were purified and then characterized by XRD, SEM, HR-TEM and Raman spectroscopy techniques. The formation of MWCNTs with diameter in the range of 4−6 nm was observed from HR-TEM. The good thermal stability and high productivity of catalyst observed in this study revealed that the 2 wt.% Sb loaded MCM-41 could be a promising support for the catalytic synthesis of MWCNTs at 800 °C by CVD method.     

Effects of different carbon precursors on synthesis of multiwall carbon nanotubes: Purification and Functionalization

Cyclohexanol and xylene were used as carbon precursors, for synthesis of multiwall carbon nanotubes (MWCNTs) arrays in a CVD system at temperature of 750 °C, using nitrogen as carrier gas and ferrocene as catalyst. Different characterization methods were employed to compare the MWCNTs structure synthesized by these two precursors. All scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermal gravimetric analysis (TGA) and Raman spectroscopy results illustrated that using cyclohexanol could significantly reduce formation of amorphous carbon and catalyst particles in the as-grown CNTs. The less amorphous carbon can be attributed to in situ oxidation in presence of oxygen atom of cyclohexanol. Characterizations showed that MWCNTs with high purity could be obtained using cyclohexanol as carbon precursor. The as-grown MWCNTs were purified by oxidation and acid treatment. Characterization of the purified MWCNTs using HNO₃/H₂SO₄ (1/3 or 1/1), 8 M HCl or 8 M HNO₃ was carried out. The results showed that 8 M HNO₃ could be considered as the best chemical to obtain more pure MWCNTs, less amorphous and metal particles and less damaged MWCNTs. The Raman spectroscopy results demonstrated that HNO₃/H₂SO₄ (1/3) treatment could more disorder the MWCNTs structure and this was attributed to the bigger destroying effect of this acid treatment. Furthermore, the TEM analysis of MWCNTs before and after acid treatment revealed that acid treatment could remove encapsulated catalyst particles. The FTIR analysis illustrated that purification of the MWCNTs with nitric acid could connect the functional groups onto the outer surface of MWCNTs and this resulted in more dispersion of the MWCNTs in water.     

Evaluation of crystalline perfection degree of multi‐walled carbon nanotubes: correlations between thermal kinetic analysis and micro‐Raman spectroscopy

Commercially available and laboratory‐prepared multi‐walled carbon nanotubes (MWCNTs) are systematically investigated by the use of micro‐Raman spectroscopy (MRS), thermogravimetric analysis (TGA) and complementary techniques (scanning electron microscopy (SEM) and transmission electron microscopy (TEM)) with the aim of establishing a standardised post‐growth diagnostic protocol for the assessment of their overall crystalline quality. By studying a set of ‘reference’ samples, clear correlations are evidenced between the Raman graphitisation indexes (D/G, G′/G and G′/D intensity ratios) commonly adopted to describe the crystalline arrangement of nanotubes, and their reactivity towards oxygen, as measured by the apparent activation energy needed for their oxidation, inferred from the kinetic analysis in quasi‐isothermal conditions. The higher the crystalline perfection degree, the higher the energy needed for oxidising them. The efficacy of the found correlations in indirectly assessing the reactivity of nanotubes prepared under different conditions is successfully demonstrated by the use of a second set of samples. The physical meaning and range of validity of the shown correlations are further discussed. Copyright © 2010 John Wiley & Sons, Ltd.     

Ultrasonic-assisted chemical oxidative cutting of multiwalled carbon nanotubes with ammonium persulfate in neutral media

A new, facile, and mild approach was developed to cut the conventional long and entangled multiwalled carbon nanotubes (MWCNTs) to short and dispersed ones with length of less than 1 μm by ultrasonic-assisted chemical oxidation with ammonium persulfate (APS) in neutral aqueous solution at room temperature. The resulting products were characterized with Fourier transform infrared (FTIR), Raman, X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and transmission electron microscope (TEM) techniques. The shortened MWCNTs formed stable dispersion state in water without the help of surfactants that provided possibility for further functionalizations and applications.     

Exploring the possible interlinked structures in single‐wall carbon nanotubes under pressure by Raman spectroscopy

High‐pressure Raman measurements on single‐wall carbon nanotubes (SWNTs) have been carried out in a diamond anvil cell by using two wavelength lasers: 830 and 514.5 nm. Irrespective of using a pressure transmitting medium (PTM) or not, we found that nanotubes undergo similar transformations under pressure. The pressure‐induced changes in Raman signals at around 2 and 5 GPa are attributed to the nanotube cross‐section transitions from circle to ellipse and then to a flattened shape, respectively. Especially with pressure increasing up to 15–17 GPa, we observed that the third transition takes place in both the Raman wavenumber and the linewidth of G‐band. We propose explanations that the interlinked configuration with sp³ bonds forms in the bundles of SWNTs under pressure, which was the cause for the occurrence of those Raman anomalies, similar to the structural‐phase transition of graphite above 14 GPa. Our TEM observations and Raman measurements on the decompressed samples support this transition picture. Copyright © 2012 John Wiley & Sons, Ltd.     

Deposition of silver nanoparticles on carbon nanotube by chemical reduction method: Evaluation of surface,thermal and optical properties

Silver was stabilized on multi-walled carbon nanotubes (MWCNTs) by chemical-reduction technique using N,N-dimethylformamide (DMF) as a reducing agent. The influence of silver on the performance of carbon nanotubes (CNTs) was investigated by employing Fourier-transform infrared spectra (FTIR), Raman spectroscopy (RAS), thermal gravimetric analysis (TGA), zeta potential measurement, scanning electron microscope (SEM), electron dispersive X-ray spectrometer (EDX), transmission electron microscopy (TEM), and reflectance spectroscopy (RS). FTIR as well as RS methods evidenced the synthesis procedure using chemical reduction method was successful. Performing TGA of the samples under oxygen atmosphere demonstrated that the silver nanoparticles (Ag NPs) generated on MWCNTs surface can decrease the thermal stability of the particles by the catalytic oxidation of CNTs. In contrary, the thermal stability of the MWCNTs has improved under nitrogen atmosphere. EDX results showed the presence of Ag, Au and Co on the surface of deposited sample. The synthesised silver multi-walled carbon nanotubes (Ag–MWCNTs) were found to have higher UV reflection activity compared with untreated particles. The Ag–CNTs can be used in producing anti-UV composites.     

The Raman spectroscopic studies of aligned MWCNTs treated under high pressure and high temperature

The study of the aligned multiwalled carbon nanotubes (MWCNTs) for interlinking bonding under high pressures and temperatures have been conducted in the diamond anvil cell. The MWCNT samples were analyzed using the Raman spectroscopy, when treated under the combinations of pressure and temperature ranges of 2-20 GPa and 25-500 °C. The analyses show the formation of interlinking bonding at a pressure above 2.5 GPa when treated under the temperature 500 °C, based on the significant change of the relative intensity between D- and G-bands in the Raman spectra. Comparisons of the data obtained before and after the high pressure and high temperature treatments are reported. The result indicates that the aligned MWCNTs may be easier to form the interlinking bonding compared to randomly oriented MWCNTs.     

A study of the effect of JB particles on Saccharomyces cerevisiae (yeast) cells by Raman spectroscopy

The mechanism of interaction of particulate matter with living system is not completely understood. Evaluation of the effect of particulate Indian traditional medicine JB(JB) on Saccharomyces cerevisiae (yeast) cells is the major focus of the present study. In India, JB is considered as a rejuvenating medicine and used for the treatment of diseases such as diabetes and age‐related eye diseases, as well as a health promoting tonic by the traditional practitioners. In presence of JB, higher growth has been observed at the late stationary growth phase of yeast. Ultra‐structure analysis using transmission electron microscopy (TEM) has shown that JB‐treated yeast cells have better morphology over control in the late stationary growth phase. In this investigation, cellular response from yeast cells after interaction with JB particles was measured using Raman spectroscopy. Raman spectroscopy—a noninvasive tool to distinguish between particle‐treated and untreated cells—revealed that treatment with JB is able to slow the degradation of cellular components (e.g. DNA, proteins and lipids) with the aging of yeast cells. Copyright © 2008 John Wiley & Sons, Ltd.     

Metallic nanoparticles on plasma treated carbon nanotubes: Nanohybrids

Multi-wall carbon nanotubes (MWCNTs) were decorated with metal clusters by thermal evaporation. Transmission electron microscopy (TEM) shows that the nature and extent of metal coverage can be varied by plasma treating the MWCNT surface. The metal clusters on oxygen plasma treated arc-discharge MWCNTs have a more dense distribution than the clusters evaporated on as-synthesized arc-discharge MWCNTs. In contrast, the plasma treatment did not affect the cluster distribution on CVD MWCNTs. Analyses of the valence band and the core levels by X-ray photoelectron spectroscopy suggest poor charge transfer between gold clusters and MWCNTs; on the contrary suggest good charge transfer between Ni clusters and MWCNTs.     

Aligned synthesis of multi-walled carbon nanotubes with high purity by aerosol assisted chemical vapor deposition: Effect of water vapor

Aligned multi-walled carbon nanotubes (MWCNTs) with high purity and bulk yield were achieved on a silicon substrate by an aerosol-assisted chemical vapor deposition. The introduction of specific amounts of water vapor played a key role in in situ controlling the purity and surface defects of the nanotubes. The morphology, surface quality and structure of MWCNTs were characterized by secondary and backscattered electron imaging in a field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM). Crystallinity and defects of the MWCNTs’ were investigated by high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. In this work, water vapor was found to provide a weak oxidative environment, which enhanced and purified the MWCNTs’ growth. However, excessive water vapor would inhibit the MWCNTs growth with a poor surface quality. In addition, it has been found that the surface morphology of the CNTs can be modified intentionally through producing some surface defects by tuning the amount of the water vapor, which may offer more nucleation sites on the chemically inert CNT surface for various applications such as catalyst support.     

Analysis of self‐repair mechanisms of Phaseolus vulgaris var. saxa using near‐infrared surface‐enhanced Raman spectroscopy

We used surface‐enhanced Raman spectroscopy (SERS) to investigate ultrastructural changes in cell‐wall composition during the self‐repair of lacerated hypocotyls of Phaseolus vulgaris var. saxa. A detailed study of self‐repair mechanisms requires localized information about cell‐wall structure and morphology in addition to the chemical cell‐wall composition. Characteristic Raman and SER spectra yielded two‐dimensional maps of cross sections of P. vulgaris var. saxa visualizing chemical compositions in the walls of different cell types and during various repair phases. SERS substrate particles were produced by the reduction of gold chloride on the plant tissue surface and characterized with absorption spectroscopy, scanning electron microscopy and energy‐dispersive X‐ray spectroscopy. The SERS results were compared with stained cross sections of the same plant using dark‐field microscopy with focus on lignin and suberin contents in repairing cells. In addition, SERS measurements revealed Au cyanide compounds on the cell surface, indicating the formation of hydrogen cyanide during the self‐repair phase. Copyright © 2009 John Wiley & Sons, Ltd.     

聚(2,5-二丁氧基)对苯乙炔/多壁碳纳米管复合材料的制备和性能研究

通过原位聚合法制备了聚（2,5-二丁氧基）对苯乙炔/多壁碳纳米管（PDBOPV/MWCNTs）复合材料.红外光谱和拉曼光谱证实了在MWCNTs表面的包覆层为PDBOPV.高分辨透射电子显微镜观察发现,PDBOPV/MWCNTs复合材料直径为35—45 nm,其中PDBOPV包覆层厚度约为15 nm.紫外—可见吸收光谱表明随着MWCNTs含量的增加,PDBOPV/MWCNTs的吸收发生红移且强度提高.荧光光谱研究表明随着MWCNTs含量的增加,PDBOPV/MWCNTs的最大发射波长发生蓝移且强度减小,MWCNTs与PDBOPV之间形成了光致电子转移体系,使得π电子离域程度增加,并且导致荧光量子效率降低.根据E_g与入射光子能量hν的关系,拟合了PDBOPV/MWCNTs薄膜的光学禁带宽度,发现随着MWCNTs的增加,E_g逐步减小.采用简并四波混频方法测试它们的三阶非线性极化率χ^（3）,结果发现随着MWCNTs含量的增加,PDBOPV/MWCNTs复合体的非线性光学响应逐渐增强,这说明PDBOPV与MWCNTs之间形成了分子间光致电子转移体系,产生了复杂的分子间π-π电子非线性运动. 关键词： 聚（2 5-二丁氧基）对苯乙炔 多壁碳纳米管 复合材料 光致发光     

Nanotitania-coated multi-walled carbon nanotube composite by facile colloidal processing route for photocatalytic applications

Multi-walled carbon nanotubes (MWCNTs) and titanium dioxide nanocomposites (MWCNTs/TiO₂) were fabricated by a simple novel colloidal processing route and tested as a photocatalyst for degradation of methylene blue under UV irradiation. The novel idea behind this work is to make MWCNTs and TiO₂ nanoparticle suspensions separately highly oppositely charged and utilize the electrostatic force of attraction between two entities to deposit nanotitania onto MWCNTs surface. Particle charge detector, scanning electron microscopy, transmission electron microscope, energy dispersive X-rays, X-rays diffraction (XRD), and Raman spectroscopy were used to characterize the composite. XRD and Raman spectroscopic analysis showed the crystalline structure of deposited TiO₂ over MWCNTs surface structure as anatase phase. It was found that MWCNTs/TiO₂ composite structure have much higher photocatalytic activity compared to TiO₂ nanoparticles. The composite material developed may find potential applications in the degradation of organic pollutants in aqueous medium under UV irradiation.     

Synthesis,characterization, and electrochemical properties of imidazole derivatives functionalized single‐walled carbon nanotubes

The imidazole derivatives functionalized single‐walled carbon nanotubes (SWNTs) were synthesized by a diazonium‐based reaction. We have designed and synthesized two imidazole derivatives to modify SWNTs. The resulting products were characterized by Fourier transform infrared (FT‐IR) spectroscopy, Raman spectroscopy, ultraviolet visible (UV/Vis) spectroscopy, thermo gravimetric analysis (TGA), energy dispersive X‐ray spectroscopy (EDX), transmission electron microscopy (TEM), and atomic force microscopy (AFM). Electrochemical measurements via a cyclic voltammetry method revealed that the weak intramolecular electronic interactions presented between the attached imidazole derivatives groups and the nanotubes. Copyright © 2008 John Wiley & Sons, Ltd.