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.     

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).     

Evolution of Co n Al clusters and chemisorption of hydrogen on Co n Al clusters

Multiwall carbon nanotubes (MWCNTs, 1?C3???M in length and 20?C25?nm in diameter) were initially functionalized with a 2,2??:6??2??-terpyridine-chelated ruthenium(II) complex by covalent amidation. The resulting functionalized ruthenium MWCNTs (RuMWCNTs, 1?C2???M in length and 10?C20?nm in diameter) were characterized by thermogravimetric analysis, X-ray photoelectronic spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and scanning electron microscopy (SEM). Thermogravimetric experiments of RuMWCNTs show that the functional group coverage of terpyridine?Crutheniun?Cterpyridine (tpy?CRu?Ctpy) is 0.7036?mmol/1.0?g carbon. The XPS results show N1s and Ru3d^5/5 signals, confirming the presence of tpy?CRu?Ctpy groups on the surface of MWCNTs. The FTIR spectra of the RuMWCNTs display the typical stretching mode of the carboxyl group (amide I) and a combination of amide N?CH and C?CN stretching mode (amide II). The Raman D- and G-line peak intensity ratio of RuMWCNTs (ID/IG 2.21) exceeds that of pristine MWCNTs (ID/IG 1.93), suggesting covalent bonding of tpy?CRu?Ctpy to MWCNTs and supporting the disruption of the graphitic integrity due to the proposed covalent functionalization. High-resolution SEM images confirm that tpy?CRu?Ctpy moieties are interconnected or attached as aggregated structures (100?C200-nm range) on the surfaces of the carbon nanotubes after functionalization. The electrical property of RuMWCNTs depicts higher resistance (10.10?M???) than that of OX-MWCNTs (15.38?k??).     

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.     

On the determination of diameter distribution in multi‐wall carbon nanotubes by Raman spectroscopy: issues related to excitation laser energy

Multi‐wall carbon nanotubes (MWCNTs) produced by the arc discharge between two graphite rods in liquid nitrogen have been investigated with the use of Raman spectroscopy and transmission electron microscopy (TEM). The effects of the applied voltage on the structural properties of the produced MWCNTs have been observed, in particular, as it concerns the size of the innermost and outermost diameters. The apparent discrepancies observed between the Raman and TEM results are explained through the observation of a selective response of the MWCNTs to the excitation laser energy in Raman spectroscopy, similar to the case of single‐wall CNTs. Electronic structure calculations correlate such behavior with optical transitions, in the presence of strong electronic localizations, spatially confined in a few neighboring walls. Copyright © 2011 John Wiley & Sons, Ltd.     

On the merits of Raman spectroscopy and thermogravimetric analysis to asses carbon nanotube structural modifications

Raman spectroscopy, thermogravimetric analysis (TGA), Fourier transform infrared spectroscopy, and transmission electron microscopy are used to assess structural changes generated on the surface of multi-walled (MWCNTs) and single-walled (SWCNTs) carbon nanotubes. Different levels of structural modifications are generated by the use of acidic oxidative treatments. It is found that Raman spectroscopy is a very powerful technique to assess structural modification of SWCNTs with initial low defect concentration. For MWCNTs grown by chemical vapor deposition, which already contain a high density of structural defects in their as-produced state, Raman spectroscopy is not a very sensitive tool to detect the generation of further defects or other structural modifications introduced through acidic treatments. For this later case, TGA is a sensitive technique to assess structural modifications on the nanotubes.     

超长定向含铁多壁碳纳米管阵列的制备

采用无模板化学气相沉积法,以二茂铁为催化剂,二甲苯为碳源,利用单温炉加热装置制备了定向碳纳米管阵列。运用扫描电子显微镜、透射电子显微镜、拉曼光谱和X射线衍射仪等对定向碳纳米管阵列的形貌、成分和物相进行细致的分析和表征。结果表明:制得的碳纳米管阵列具有良好的定向性和多壁管状结构,并且石墨化程度高;碳纳米管中除碳元素外,管中包含有少量以纳米颗粒和纳米线形式存在的铁及其化合物,主要成分是铁和碳化铁。结合碳纳米管的制备和透射电子显微镜分析表征结果,认为超长碳纳米管阵列的生长模式为底部生长方式,即经历催化剂分解、催化、成核、长大、中毒、凝聚成粒和连接成线的循环过程,正是由于碳源和催化剂的连续供应促成了碳纳米管阵列的快速定向生长。     

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.     

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     

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.     

Microwave-induced high surface functionalization of multi-walled carbon nanotubes for long-term dispersion in water

Multi-walled carbon nanotubes (MWCNTs) were functionalized by monoethanolamine (MEA), diethanolamine (DEA) and triethanolamine (TEA) by a rapid microwave-assisted method. Surface functional groups and morphology of MWCNTs were analyzed by ultraviolet-visible spectroscopy, Fourier transform infrared spectroscopy, thermogravimetric analysis, Raman spectroscopy and transmission electron microscopy. The water dispersibility of samples was also measured over time and compared with that of MWCNT functionalized by ethylenediamine (EDA). Accordingly, the dispersibility was in the sequence MEA-MWCNTs > DEA-MWCNTs > TEA-MWCNTs > EDA-MWCNTs > pristine MWCNTs. The significant long-term dispersibility of MEA-MWCNTs was related to its exceptionally high surface functionalization (89.3 %) and lack of chemical bridging between adjacent MWCNTs. This strategy, along with the low-cost functionalizing agents, could pave the way for large-scale dispersion of CNTs in the polar solvents.     

One-pot, efficient functionalization of multi-walled carbon nanotubes with diamines by microwave method

Diamines are known to act as a medium to bind miscellaneous compounds to carbon nanotubes (CNT). However, they are commonly applied in a tedious manner. Here, multi-walled carbon nanotubes (MWCNTs) were functionalized by a series of diamine molecules (ethylenediamine, 1,6-hexamethylenediamine and 1,4-diaminobenzen) in a one-pot, rapid microwave-assisted method. Surface functionality groups and morphology of MWCNTs were analyzed by infrared spectroscopy, thermogravimetric analysis, Raman spectroscopy, scanning electron microscopy, and transmission electron microscopy. The results consistently confirmed the formation of diamines functionalities on MWCNTs, while the structure of MWCNT has remained relatively intact. This simple and efficient process may play an important role for realizing miscellaneous functionalization of CNTs.     

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.     

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.     

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.     

170keV质子辐照对多壁碳纳米管薄膜微观结构与导电性能的影响

碳纳米管具有优异的导电性, 是未来电子元器件的理想候选材料, 应用前景广阔. 针对碳纳米管在空间电子元器件的应用需求, 本文研究了170 keV质子辐照对多壁碳纳米管薄膜微观结构与导电性能的影响. 采用扫描电子显微镜(SEM)、拉曼光谱仪(Raman)、X射线光电子能谱仪(XPS)及电子顺磁共振谱仪(EPR)对辐照前后碳纳米管试样的表面形貌和微观结构进行分析; 利用四探针测试仪对碳纳米管薄膜进行导电性能分析. SEM分析表明, 170 keV质子辐照条件下, 当辐照注量高于5×10¹⁵ p/cm² (protons/cm²)时, 碳纳米管薄膜表面变得粗糙疏松, 纳米管发生明显弯曲、收缩及相互缠结现象. 目前, 质子辐照纳米管发生的收缩现象被首次发现. 基于Raman和XPS分析表明, 170 keV质子辐照后碳纳米管的有序结构得到改善, 且随辐照注量增加, 碳纳米管的有序结构改善明显. 结构的改善主要是由于170 keV质子辐照碳纳米管所产生的位移效应导致缺陷重组. EPR分析表明, 随着辐照注量的增加, 碳纳米管薄膜内的非局域化电子减少. 利用四探针测试分析表明, 碳纳米管薄膜的导电性能变差, 这是由于170 keV质子辐照导致碳纳米管薄膜中的电子特性及形态发生改变. 本文研究结果有助于利用质子辐照对碳纳米管膜结构和性能进行调整, 从而制备出抗辐射的纳米电子器件.     

Comparison of different types of polypyrimidine/CNTs/Pt hybrids in fuel cell catalysis

In this paper, we mainly studied the preparation of platinum-containing composite materials with carbon nanotubes wrapped by polypyrimidine-conjugated polymers and the performance of the composites. The polymer-based carbon nanotubes/Pt catalysts were prepared successfully and confirmed by infrared spectroscopy, XPS, XRD, and TEM images. The performance of polypyrimidine/multi-walled carbon nanotubes (MWCNTs)/Pt and polypyrimidine/double-walled carbon nanotubes (DWCNTs)/Pt was compared with the polypyrimidine/single-walled carbon nanotubes (SWCNTs)/Pt. The amount of the loaded Pt on the polypyrimidine/MWCNTs and polypyrimidine/DWCNTs was calculated to be 50.5 wt% and 52.7 wt% respectively. The effective specific surface area of the polypyrimidine/MWCNTs/Pt (45.7 m²/g) and polypyrimidine/DWCNTs/Pt (42.47 m²/g) was observed by electrochemical cyclic voltammetry. These studies strongly imply that the MWCNTs were better candidates than DWCNTs and SWCNTs in the application of polypyrimidine/CNT materials as catalyst for fuel cells.     

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.     

Effect of different microwave-based treatments on multi-walled carbon nanotubes

Three new methods for the functionalization of multi-walled carbon nanotubes (MWCNTs) are reported using microwave (MW) energy and water as a mild chemical agent. In the first method we reported the effect of MW irradiation on a dispersion of MWCNTs in deionized water, in the second method we studied the exposing of MWCNT to microwave irradiation in the presence of steam, and in the third method we used microwave oven-generated plasma for the functionalization of MWCNTs. We also performed thermal oxidation and acid treatment as two conventional methods for oxidative functionalization of carbon nanotubes, to compare their effect with our results. Transmission electron microscopy (TEM) and Raman spectroscopy results showed that although these microwave methods introduced some defects to the carbon nanotubes, the damage was less severe than conventional treatments. X-ray photoelectron spectroscopy (XPS) measurements confirmed that the functionalization of carbon nanotubes by these methods favored hydroxyl groups, which are useful when further functionalization is required.