Characterization of carbon nanotube filters and other carbonaceous materials by Raman spectroscopy—II: study on dispersion and disorder parameters

Recently, we have reported on the characterization of various carbonaceous materials including multiwalled carbon nanotube (MWCNT) filters, which have specific molecular filtering capabilities and good mechanical strength and can be produced in bulk as highly aligned arrays of bundles of CNTs. We have extended our studies using Fourier transform‐Raman spectroscopy with 1064 nm excitation wavelength and a rotating sample holder in the region 1000–2800 cm⁻¹, in addition to 532 and 785 nm, which were used for Raman excitation in our previous study. Raman spectra were analyzed for band positions and line shape with special emphasis on the D‐, G‐ and G′‐ bands. For the single‐walled species, Carbotrap and graphite spectra were also recorded with 488 nm excitation. A dispersion study has been made from the Raman data available with the different excitation wavelengths. Slight band shifts and band broadening could be observed under the two sample conditions, one with the stationary sample and the other with sample rotation. The spectral changes are related to the excessive heating caused in a stationary sample by laser irradiation. Based on our findings in this study combined with our earlier study, we can state that only a careful line shape analysis and study of intensity pattern of the D‐ and G‐Raman bands under well‐defined measurement conditions lends itself as a good measure of degree of alignment in the MWCNT bundles. Copyright © 2010 John Wiley & Sons, Ltd.     

Surface‐enhanced Raman scattering and fluorescence emission of gold nanoparticle–multiwalled carbon nanotube hybrids

Multiwalled carbon nanotubes (MWCNTs) are grafted with gold (Au) nanoparticles of different sizes (1–12 and 1–20 nm) to form Au–MWCNT hybrids. The Au nanoparticles pile up at defect sites on the edges of MWCNTs in the form of chains. The micro‐Raman scattering studies of these hybrids were carried using visible to infrared wavelengths (514.5 and 1064 nm). Enhanced Raman scattering and fluorescence is observed at an excitation wavelength of 514.5 nm. It is found that the graphitic (G) mode intensity enhances by 10 times and down shifts by approximately 3 cm⁻¹ for Au–MWCNT hybrids in comparison with pristine carbon nanotubes. This enhancement in G mode due to surface‐enhanced Raman scattering effect is related to the interaction of MWCNTs with Au nanoparticles. The enhancement in Raman scattering and fluorescence for large size nanoparticles for Au–MWCNTs hybrids is corroborated with localized surface plasmon polaritons. The peak position of localized surface plasmons of Au nanoparticles shifts with the change in environment. Further, no enhancement in G mode was observed at an excitation wavelength of 1064 nm. However, the defect mode (D) mode intensity enhances, and peak position is shifted by approximately 40 cm⁻¹ to lower side at the same wavelength. The enhanced intensity of D mode at 1064 nm excitation wavelength is related to the double resonance phenomenon and shift in the particular mode occurs due to more electron phonon interactions near Fermi level. Copyright © 2012 John Wiley & Sons, Ltd.     

Changes in the vibrational modes of carbon nanotubes induced by electron‐beam irradiation: resonance Raman spectroscopy

Influence of electron‐beam (e‐beam) irradiation on multi‐walled (MW) and single‐walled (SW) carbon nanotube films grown by microwave chemical vapor deposition technique is investigated. These films were subjected to an e‐beam energy of 50 keV from a scanning electron microscope for 2.5, 5.5, 8.0, and 15 h, and to 100 and 200 keV from a transmission electron microscope for a few minutes to ∼2 h continuously. Such conditions resemble an increased temperature and pressure regime enabling a degree of structural fluidity. To assess structural modifications, they were analyzed prior to and after irradiation using resonance Raman spectroscopy (RRS) in addition to in situ monitoring by electron microscopy. The experiments showed that with extended exposures, both types of nanotubes displayed various local structural instabilities including pinching, graphitization/amorphization, and formation of an intramolecular junction (IMJ) within the area of electron beam focus possibly through amorphous carbon aggregates. RRS revealed that irradiation generated defects in the lattice as quantified through (1) variation of the intensity of radial breathing mode (RBM), (2) intensity ratio of D to G band (I_D/I_G), and (3) positions of the D and G bands and their harmonics (D* and G*) and combination bands (D + G). The increase in the defect‐induced D band intensity, quenching of RBM intensity, and only a slight increase in G band intensity are some of the implications. The MW nanotubes tend to reach a state of saturation for prolonged exposures, while the SW ones transform from a semiconducting to a quasi‐metallic character. Softening of the q = 0 selection rule is suggested as a possible reason to explain these results. Furthermore, these studies provide a contrasting comparison between MW and SW nanotubes. Copyright © 2006 John Wiley & Sons, Ltd.     

Electrochemical functionalisation of SWNTs with poly(3,4‐ethylenedioxythiophene) evidenced by anti‐Stokes/Stokes Raman spectroscopy

The capability of anti‐Stokes/Stokes Raman spectroscopy to evaluate chemical interactions at the interface of a conducting polymer/carbon nanotubes is demonstrated. Electrochemical polymerisation of the monomer 3,4‐ethylenedioxythiophene (EDOT) on a Au support covered with a single‐walled carbon nanotube (SWNT) film immersed in a LiClO₄/CH₃CN solution was carried out. At the resonant optical excitation, which occurs when the energy of the exciting light coincides with the energy of an electronic transition, poly(3,4‐ethylenedioxythiophene) (PEDOT) deposited electrochemically as a thin film of nanometric thickness on a rough Au support presents an abnormally intense anti‐Stokes Raman spectrum. The additional increase in Raman intensity in the anti‐Stokes branch observed when PEDOT is deposited on SWNTs is interpreted as resulting from the excitation of plasmons in the metallic nanotubes. A covalent functionalisation of SWNTs with PEDOT both in un‐doped and doped states takes place when the electropolymerisation of EDOT, with stopping at +1.6 V versus Ag/Ag⁺, is performed on a SWNT film deposited on a Au plate. The presence of PEDOT covalently functionalised SWNTs is rationalised by (1) a downshift by a few wavenumbers of the polymer Raman line associated with the symmetric C C stretching mode and (2) an upshift of the radial breathing modes of SWNTs, both variations revealing an interaction between SWNTs and the conjugated polymer. Raman studies performed at different excitation wavelengths indicate that the resonant optical excitation is the key condition to observe the abnormal anti‐Stokes Raman effect. Copyright © 2010 John Wiley & Sons, Ltd.     

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.     

Excitation energy dependence of Raman bands in multiwalled carbon nanotubes

Vertically aligned multiwalled carbon nanotubes (MWCNTs) were grown on 1‐ and 3‐nm cobalt (Co) films, at various growth times by microwave plasma enhanced chemical vapor deposition technique and their microstructural properties were analyzed with the help of Raman spectra that were obtained from different sources of laser excitation energies (E_L: 2.41, 1.96 and 1.58 eV). The variation of D and G band positions in MWCNTs grown on 1‐ and 3‐nm Co films follows a similar behavior, and an anomalous behavior was observed in the E_L dependence of the D′‐band wavenumber. In the second‐order spectra, the G′ band varied strongly according to structure with the laser excitation energy (E_L). The I_D/I_G ratio decreased with the increase of E_L for all MWCNTs; however, for a fixed E_L, the I_D/I_G dispersion is higher at lower E_L. The crystallite sizes were estimated using I_DI_G and E_L. We have shown that, for all MWCNTs, I_D/I_G ratio is inversely proportional to . Copyright © 2010 John Wiley & Sons, Ltd.     

Formation mechanisms of nanocomposite layers based on multiwalled carbon nanotubes and non-stoichiometric tin oxide

Nanocomposite layers based on multiwalled carbon nanotubes (MWCNTs) and non-stoichiometric tin oxide (SnO _x ) have been grown by magnetron deposition and CVD methods. In the case of the CVD method, the study of the structure and phase composition of obtained nanocomposite layers has shown that a tin oxide “superlattice” is formed in the MWCNT layer volume, fixed by SnO _x islands on the MWCNT surface. During magnetron deposition, the MWCNT surface is uniformly coated with tin oxide islands, which causes a change in properties of individual nanotubes. Electrical measurements have revealed the sensitivity of nanocomposite layers to (NO₂)⁻ molecule adsorption, which is qualitatively explained by a change in the conductivity of the semiconductor fraction of p-type MWCNTs.     

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.     

Sulfur impregnation of multi‐walled carbon nanotubes via SF6/NH3 plasma exposure

Plasma treatments are established methods to functionalise carbon nanotubes (CNTs) and modify their surface structure. This paper presents a mild glow‐discharge plasma treatment of aligned arrays of multi‐walled carbon nanotubes employing sulfur hexafluoride (SF₆), ammonia (NH₃), and their mixtures as process gases. For the latter, sulfur was detected at the tip and sidewalls of the nanotubes via energy‐dispersive X‐ray spectroscopy, while electron microscopy served as method to verify the structural integrity of the CNTs after the plasma treatment. This approach provides the basis for an easy and quick alternative to existing sulfur functionalisation methods of MWCNTs. Furthermore, the proposed method can conveniently be applied to carbon nanotube arrays on substrate while preserving their structure and alignment.

SEM‐EDX map of SF₆/NH₃ plasma‐treated multi‐walled carbon nanotubes on substrate. Green, yellow and red correspond to silicon, carbon and sulfur signals, respectively.     

Surface‐enhanced Raman scattering studies of carbon nanotubes using Ag‐core Au‐shell nanoparticles

Surface‐enhanced Raman scattering from carbon nanotube bundles adsorbed with plasmon‐tunable Ag‐core Au‐shell nanoparticles (Ag@Au nps) was carried out for the first time. By utilizing nanoparticles whose plasmon resonance peak (541, 642 nm) closely matches the commonly used Raman excitation sources (532, 632.81 nm), we can observe a large enhancement in the Raman signatures of carbon nanotubes. We obtain greater enhancement in the Raman signal for the above case when compared to nanotubes adsorbed with conventional Ag, Au or other ‘off resonant’ Ag@Au nps. The power‐dependent SERS experiment on single‐walled nanotubes (SWNTs) with resonant Ag@Au nps reveals a linear behavior between the G‐band intensity and the photon flux density, which is in agreement with the vibrational pumping model of SERS. The observed enhancement by resonance matching is pronounced for carbon nanotubes and may lead to insights into understanding nanotube–nanoparticle interaction. Copyright © 2009 John Wiley & Sons, Ltd.     

Raman spectroscopic characterization of novel carbon‐bonded filter compositions for steel melt filtration

In this work, Raman scattering results on novel carbon‐bonded filter compositions are presented. Such filters are already used for steel melt filtration; however, the potential of this carbon‐bonded Al₂O₃–C system regarding material characteristics and filtration efficiency has not been fully understood yet. In order to investigate thermally induced structural changes of the filter compositions, micro‐Raman spectroscopy was applied. Analyzing the position, intensity, and full width at half maximum of G and D peaks in the Raman spectra, it could be determined that the carbon appears in graphitic form and the graphitic cluster size was estimated. We found an increase of the lateral cluster size L_a with increasing coking temperature. Copyright © 2013 John Wiley & Sons, Ltd.     

New features in the anti‐Stokes and Stokes Raman spectra of single‐walled carbon nanotubes that are highly separated into their semiconducting and metallic nanotube components

Surface‐enhanced Raman scattering studies were performed using nonresonant (514.5 nm) and resonant (676.4 nm) optical excitations on single‐walled carbon nanotubes thoroughly separated into semiconducting (pure 99%) and metallic (pure 98%) components. Regardless of the support (Au or Ag), the metallic nanotubes do not present an anomalous anti‐Stokes Raman emission. Regardless of whether an on‐resonant or off‐resonant optical excitation is used, only the semiconducting nanotubes produce an abnormal anti‐Stokes Raman emission that grows when increasing the excitation light intensity or temperature. The Raman studies under light polarized relative to the main nanotube axis demonstrate that only semiconducting nanotubes are sensitive toward changes in the polarization of the excitation light. Copyright © 2014 John Wiley & Sons, Ltd.     

Micromechanical properties of poly(butylene terephthalate) nanocomposites with single- and multi-walled carbon nanotubes

Polymer nanocomposites with carbon nanotubes (CNT) are becoming important structural materials because of their superior mechanical properties and easy processability. The objective of the work is to investigate the influence of small amounts of single walled carbon nanotubes (SWCNT), as well as multi-walled carbon nanotubes (MWCNT), on the microhardness of a thermoplastic polymer such as poly(butylene terephthalate) (PBT). The nanocomposites were obtained by introducing the CNT into the reaction mixture during the synthesis of PBT. The polymers without carbon nanotubes (reference material) and with carbon nanotubes were synthesized using an in-situ polycondensation reaction process. Weight percentages ranging from 0.01 to 0.2 wt% of the single walled and from 0.01 to 0.35 wt% of the multi-walled nanotubes were dispersed in 1,4-butanediol (BD) by ultrasonication and by ultra high speed stirring. The nanocomposites were extruded followed by injection molding. The samples were characterized by electron microscopy and microindentation hardness techniques. The variations of the micromechanical properties (indentation hardness) of the nanocomposites with nanotube content and with temperature are discussed in the light of the stress transfer between the polymer matrix and nanotubes, the degree of dispersion, the nature of the tubes and other structural parameters.     

Surface‐enhanced Raman spectra of individual multiwalled carbon nanotubes with small innermost diameters

The Raman spectra of individual multiwalled carbon nanotubes (MWCNTs) with the innermost diameters of 0.6–0.9 nm are studied by surface‐enhanced Raman scattering. The influences of small innermost diameters to Raman features are investigated. A clear and relatively sharp Raman peak appears at 1510 cm⁻¹ when the innermost diameter is close to 0.6 nm. Lorentzian fits of G band indicate that its splitting is affected by the small innermost diameter of MWCNT. Moreover, the splitting of 2iTO mode is also observed at 2800–3000 cm⁻¹. Copyright © 2012 John Wiley & Sons, Ltd.     

Effects of gas composition on the growth of multi-walled carbon nanotube

This paper studies the effects of different gas compositions on the growth of multi-walled carbon nanotube (MWCNT) films by using an electron cyclotron resonance chemical vapor deposition (ECR-CVD) method. The Raman spectrum was employed to explore the composition of the MWCNT films grown under different mixtures of C₃H₈ and H₂. The results showed that the optimum relative intensity ratio of the D band to G band (i.e., I_D/I_G) is 2 for the cases considered in this study. In addition, the morphology and microstructure of the MWCNTs were examined by field emission scanning electron microscopy (FE-SEM) and field emission gun transmission electron microscopy (FEG-TEM). Furthermore, atomic force microscopy (AFM) and scanning thermal microscopy (SThM) were used to study the surface topography and thermal properties of the MWCNTs.     

Controllable purification, cutting and unzipping of multi-walled carbon nanotubes with a microwave method

A rapid microwave-assisted method was developed for the purification, cutting and unzipping of arrays of multi-walled carbon nanotubes (MWCNTs) using a mixture of KMnO₄ and H₂SO₄. To harness the extent of treatment, MWCNT products were fully characterized at different reaction times by UV-visible and Raman spectroscopies as well as scanning and transmission electron microscopies. The results show that the carbon nanoparticles and the amorphous carbon which coated the MWCNTs were removed after about 10 minutes. The excessive oxidation of MWCNTs then leads to cutting and unzipping of graphitic walls. Moreover, while the catalyst residues outside the MWCNTs were rapidly extracted up to 10 minutes, the removal of catalyst residues inside the MWCNTs did not begin before 20 minutes. This method can be considered as an efficient route for the purification, cutting and unzipping of MWCNTs due to its fast and controllable procedure.     

Synthesis and gas sensing characteristic based on metal oxide modification multi wall carbon nanotube composites

A new type of gas sensing material based on metal oxide modification multi wall carbon nanotube (MO/MWCNT) composites is presented since the interface between the composites enhance the carrier density so as to improve the gas sensitivity. Three kinds of MO/MWCNT composite materials, such as ZnO/MWCNT, SnO₂/MWCNT and TiO₂/MWCNT, have been acquired in situ growth using catalytic pyrolysis method. The MO nano particles have decorated on side of MWCNTs, whereas the introduction of SnO₂ nano particles makes part of MWCNT showing two-dimensional form of carbon nano-wall structure. Among four kinds of cathode of ZnO/MWCNTs, SnO₂/MWCNTs, TiO₂/MWCNTs and pure MWCNT composite film, TiO₂/MWCNTs composite has the lowest threshold electric field required to draw current of 12 μA has been found to be ∼1.2 V/μm, and also TiO₂/MWCNTs composite has the highest sensitivity of 16% to ethanol. The TiO₂/MWCNTs composite is superior to the others both in vacuum electron transportation and gas sensitivity.     

New nanocomposites containing metal nanoparticles, carbon nanotube and polymer

Metal-carbon nanotube-graft-polymer (MCNT-g-P) nanocomposites were synthesized and characterized successfully. In this work, multiwall carbon nanotubes (MWCNT) were opened using HNO₃/H₂SO₄ mixture and filled by metal nanoparticles such as silver nanoparticles through wet chemistry method. Then MWCNT containing metal nanoparticles were used as macroinitiator for ring opening polymerization of ε-caprolactone and MCNT-g-P nanocomposites were obtained. Length of grafted polymer arms onto the MWCNT was controlled using MWCNT/ε-caprolactone ratio. Structure and properties of nanocomposites were evaluated by TEM, DSC, TGA, and spectroscopy methods.     

Observation of the diameter-dependent Raman dispersion effect in chemically oxidized multiwalled carbon nanotubes

Chemical oxidation of multiwalled carbon nanotubes (MWCNTs) using H₂SO₄/HNO₃ solution has been monitored by micro-Raman spectroscopy and X-ray absorption spectroscopy. The diameter distribution variation in MWCNTs due to chemical oxidation has been measured by scanning electron microscopy and transmission electron microscopy. The Raman dispersion behaviors of the intensity ratio and the band positions of the D, G, and G′ bands were found to be correlated with the MWCNT diameter distribution. It was also found that, during the nanotube unzipping process, defect formation complicates the observation of the diameter-dependent Raman dispersion effect. The curvature effect plays an important role in the intensity ratio trend. On the other hand, defect formation dominates the band position trend.     

Surface‐enhanced Raman scattering of pyridine‐functionalized multi‐walled carbon nanotubes

Carbon nanotubes (CNTs) have attracted great attention for their potential use in many applications because of their intrinsic properties. The importance other than the impact of the application of CNT‐embedded membranes in the area of water technology development is immense. In this context, the identification and quantification of CNTs in aqueous resources during relevant water purification processes can be proven of high significance. Surface‐enhanced Raman scattering (SERS) potentially has the sensitivity required for trace analysis and has been previously used for CNT identification on solid substrates. A thorough study for the identification and quantification of small concentrations of multi‐walled CNTs (MWCNTs) in water suspensions via SERS has been performed. The functionalization of MWCNTs with pyridine groups seems to favor the surface enhancement of relevant Raman signal. This study constitutes the first step of a work in progress for the characterization of CNTs at quite low concentration range by SERS in any water suspension. It is based on an ex ante functionalization of the CNTs by pyridine, demonstrating the potential of the method. Our long‐term aim is its general application built, however, in an ex post relevant functionalization of the CNTs in any aqueous solution. Copyright © 2014 John Wiley & Sons, Ltd.