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.     

Evaluation of residual iron in carbon nanotubes purified by acid treatments

A detailed analysis by X-ray photoelectron spectroscopy was carried out on multi-walled carbon nanotube (MWCNT) surfaces after non-oxidative and oxidative purification treatments in liquid-phase. The MWCNT were produced by pyrolysis of camphor and ferrocene, that provides a high yield but with high iron contamination (∼15% wt). The elimination and/or oxidation of iron nanoparticles were monitored by Fe2p and O1s core level. Oxygen-based functional groups attachment was also investigated by C1s fitting. The effectiveness of each treatment in iron removal was evaluated by thermogravimetric analysis (TGA) and atomic absorption spectroscopy (AAS). The integrity of the MWCNT structure was verified by Raman spectroscopy (RS) and transmission electron microscopy (TEM). A purity degree higher than 98% was achieved only with non-oxidative treatments using sonification process.     

Electron energy loss spectroscopy analysis of the interaction of Cr and V with MWCNTs

The presented scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) results show the strong reaction of Cr and V with the graphitic walls of MWCNTs. For Vanadium, an interfacial VC layer could be observed at the interface between VN and MWCNTs, when the samples were heated in situ to 750 °C. Knowledge about this interfacial VC layer is important for the formation of VN-MWCNT hybrid materials, used in supercapacitor electrodes, often synthesized at high temperatures. Chromium reacts at 500 °C with the MWCNTs to form Cr₃C₂ and in some cases, dissolved the MWCNT completely. Together with the previously published results about the interaction of MWCNTs with Cu (no interaction) and Ni (a slight rehybridisation trend for the outermost MWCNT-wall observed with EELS) (Ilari et al., 2015) the influence of the valence d-orbital occupancy of 3d transition metals on the interaction strength with CNTs is shown experimentally. For a transition metal to form chemical bonds towards CNT-walls, unoccupied states in its valence d-orbitals are needed. While Ni (2 unoccupied states) interacts only slightly, Cr (5 unoccupied states) and V (7 unoccupied states) react much stronger and can dissolve the MWCNTs, at least partially.     

Interfacial interaction in a composite based on multi-walled carbon nanotubes and amorphous tin oxide

The specific features of changes in the electronic structure of multi-walled carbon nanotubes (MWCNTs) due to the interaction with an amorphous tin oxide in the SnO_x/MWCNT composite formed by magnetron sputtering have been investigated using X-ray spectroscopy. It has been shown that the formation of chemical bonds responsible for significant changes in the local and electronic structures of the outer layers of MWCNTs occurs at the boundaries of the “amorphous oxide/MWCNT” contacts. The vacuum annealing of the composite leads to the disturbance of the chemical interaction at interfaces of the composite and to a partial recovery of the local structure of the outer layers of MWCNTs. A decrease in the amount of oxygen in the tin oxide under vacuum annealing conditions causes an increase in the number of unpaired Sn 5s electrons, which, in turn, enhances the charge transfer through the interfaces in the composite and leads to a splitting of the π^*-subsystem of the outer layers of MWCNTs.     

The influence of treatment duration on multi-walled carbon nanotubes functionalized by H2SO4/HNO3 oxidation

Variation in the nature of multi-walled carbon nanotubes (MWCNTs) subjected to different degrees of oxidation was investigated. The microstructure was determined by high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) methods, and the surface chemistry was evaluated in terms of the functional groups determined by X-ray photoelectron spectroscopy (XPS) and thermal analysis-mass spectroscopy (TA-MS). In addition, TGA was used to indicate the thermal stability of the nanotubes. Results demonstrate that the graphitic structure of nanotubes oxidized with a mild mixture of H₂SO₄/HNO₃ was preserved. Decrease in the degree of crystallinity started with widening of the C(0 0 2) XRD diffraction peak, followed by this peak shifting towards lower angles. The oxygen content increased with increasing treatment time. A defect peak incorporated in deconvolution of XPS C1s spectra was helpful for detecting the generation of defect sites. The predominant surface functionalities of the nanotubes have been changed from basic to acidic groups after treatment for one day. The samples oxidized for two days had the most abundant surface -COOH and the highest oxidation resistance. The oxidation mechanism of MWCNTs in mild H₂SO₄/HNO₃ mixture was proposed, which was a successive and iterative process, including the initial attack on active sites, and next the hexagon electrophilic attack generating new defects and introducing more oxygen, and then the tubes becoming thinner and shorter.     

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.     

CF<Subscript>4</Subscript> plasma-induced grafting of fluoropolymer onto multi-walled carbon nanotube powder

Grafting of fluoropolymer onto multi-walled carbon nanotube (MWCNTs) powder by CF₄ plasma treatment was investigated in this study. In order to achieve a uniform treatment of powder, a rotating barrel was designed and fixed between the two discharge electrodes. The influence of various plasma parameters, such as power and treatment time, on the fluorination of MWCNT surface was systematically analyzed by X-ray photoelectron spectroscopy (XPS), Fourier transformed infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The results revealed that a successful fluorination of MWCNT powder with a maximum of fluorine content of 12% could be achieved by using our plasma equipment. Our work provides a new way for the homogeneous fluorination of MWCNT powder and is valuable for industrial production. PACS 52.50.Dg; 52.40.Mj; 52.59. Ye; 52.77.-j     

SnO2-decorated multiwalled carbon nanotubes and Vulcan carbon through a sonochemical approach for supercapacitor applications

Multiwalled carbon nanotubes (MWCNTs) and Vulcan carbon (VC) decorated with SnO₂ nanoparticles were synthesized using a facile and versatile sonochemical procedure. The as-prepared nanocomposites were characterized by means of transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, and Fourier transform infra red spectroscopy. It was evidenced that SnO₂ nanoparticles were uniformly distributed on both carbon surfaces, tightly decorating the MWCNTs and VC. The electrochemical performance of the nanocomposites was evaluated by cyclic voltammetry and galvanostatic charge/discharge cycling. The as-synthesized SnO₂/MWCNTs nanocomposites show a higher capacity than the SnO₂/VC nanocomposites. Concretely, the SnO₂/MWCNTs electrodes exhibit a specific capacitance of 133.33 F g⁻¹, whereas SnO₂/VC electrodes exhibit a specific capacitance of 112.14 F g⁻¹ measured at 0.5 mA cm⁻² in 1 M Na₂SO₄.     

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.     

X-Ray Spectral Studies of the Interface Interaction in CuO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/MWCNTs Nanocomposite

Results of a comprehensive study of the interface interaction of a nanostructured CuO_x and multiwalled carbon nanotubes (MWCNTs) in CuO_x/MWCNT nanocomposite by X-ray absorption spectroscopy (XANES, NEXAFS) and X-ray photoelectron spectroscopy (XPS) methods using a synchrotron radiation are presented. It is established that a nanostructured CuO_x in CuO_x/MWCNT nanocomposite is predominantly formed by CuO and has the form of flakelike particles 200–500 nm in size uniformly dispersed over an array of nanotubes. A chemical interaction of CuO_x and nanotubes with formation of covalent carbon–oxygen bonds, which does not lead to a significant destruction of the outer layers of carbon nanotubes, is observed at the interfaces of the nanocomposite.     

Remarkable influence of carbon nanotubes on microwave absorption characteristics of strontium ferrite/CNT nanocomposites

In this research work, magnetic multi-walled carbon nanotube (MWCNT) nanocomposites have been created by the assembly of Zn-Sn substituted strontium ferrite film onto the surface of MWCNTs. X-ray diffraction and transmission electron microscopy were used to demonstrate the successful attachment of ferrite films to MWCNTs. Mössbauer spectroscopy indicates that the Zn-Sn ions preferentially occupy the 2b and 4f₂ sites. Vibrating sample magnetometry confirms the relatively strong dependence of saturation magnetization with the volume percentage of MWCNTs. Microwave absorption of the MWCNTs/doped strontium ferrite nanocomposites is evidently enhanced compared to that of pure MWCNTs and ferrite. The maximum reflection loss increased significantly with an increase in volume percentage of MWCNTs in nanocomposites. Reflection loss evaluations indicated that the nanocomposites display a great potential application as wide-band electromagnetic wave absorbers.     

Analysis of the magnetic losses in iron-based soft magnetic composites with MgO insulation produced by sol-gel method

This work investigated the magnetic losses of heat treated iron-based soft magnetic composites with a thin MgO insulating layer produced by sol-gel method. The samples were characterized by energy dispersive X-ray spectroscopy, X-ray analysis and Fourier transform infrared spectroscopy. The results show that the surface of the powders contains a thin layer of MgO insulation. The loss results indicate that the hysteresis part for both the core loss and total loss factor was approximately the same for the MgO-insulated compacts and conventional SOMALOY^TM samples with phosphate insulation after annealing at 600 °C. But the MgO-insulated compacts exhibited significantly lower eddy current contribution of both core loss and total loss factor with respect to SOMALOY^TM samples after annealing. Also the contribution of eddy current in the iron particles for MgO insulated compacts (k_p=0.91) was noticeably higher than this contribution for SOMALOY^TM samples (k_p=0.18) after annealing due to the higher electrical resistivity of the MgO-insulated compacts.     

二维相关吸收光谱法研究多壁碳纳米管与腐植酸吸光组分间非均相吸附行为

溶解性有机质（DOM）是水环境中一类复杂的溶解性有机混合物,不仅可影响污染物归趋及生物有效性,且DOM自身属于消毒副产物（DBPs）前驱体。为此,如何高效去除水中DOM已成为当前环境污染控制与治理技术研究的热点问题之一。以商品化腐植酸（HA）为DOM典型代表物质,利用紫外可见吸收光谱结合二维相关光谱法（2D-COS）从动力学、吸附等温线及热力学等角度研究了原始多壁碳纳米管（MWCNT）、羟基化MWCNT及羧基化MWCNT与HA吸光组分间吸附行为。2D-COS提高了HA一维吸收光谱分辨率,经二维相关吸收光谱图分析显示3种MWCNTs对HA吸光组分的吸附变化顺序均为275 nm→400 nm,表明MWCNTs与HA吸光组分间为非均相吸附行为。动力学结果显示MWCNTs与275 nm处HA吸光组分间吸附速率高于MWCNTs与400 nm处HA吸光组分间吸附速率,表明275 nm处HA吸光组分可优先吸附至三种MWCNTs上。在25和35 ℃条件下,MWNCTs与HA吸光组分间吸附等温线表现为非线性,且Freundlich模型拟合决定系数R2大于Langmuir模型拟合决定系数,表明Freundlich吸附等温模型比Langmuir吸附等温模型更有利于描述MWCNTs与HA吸光组分间吸附等温线。275 nm处HA吸光组分的饱和吸附容量（q_max）及相同给定平衡浓度下单点吸附系数K_d高于400 nm处HA吸光组分,进一步表明MWCNTs与HA吸光组分间为非均相吸附。此外,当给定平衡浓度（c_e=0.5 cm-1和c_e=1.5 cm-1）越低,MWCNTs与HA吸光组分间结合能力越大,即HA吸光组分浓度较低时更易与MWCNTs上高能吸附位点相结合。相同平衡浓度下MWCNTs与特定HA吸光组分间吸附能力顺序表现为MWCNT8>MWCNT8-OH>MWCNT8-COOH,表明功能化基团可影响MWCNTs与HA吸光组分间吸附特性。另外,相同条件下单点吸附系数K_d与MWCNTs比表面积间无显著相关性（p>0.05）,表明比表面积不是造成MWCNTs与特定HA吸光组分间结合能力差异的主要因素;K_d与MWCNTs中孔孔隙度间呈显著正相关（p<0.05）而与微孔孔隙度间无显著相关性（p>0.05）,这主要是由于HA吸光组分可进入MWCNTs中孔而难以通过MWCNTs微孔。最后热力学分析结果显示Gibbs自由能变（ΔG0）<0、焓变（ΔH0）>0和熵变（ΔS0）>0,表明MWCNTs吸附HA吸光组分的过程为吸热反应,可自发进行,升温可促进MWCNTs对HA吸光组分的吸附,且吸附过程中固液界面的无序性增加。相同温度及吸光组分下,MWCNT8的ΔG0值小于MWCNT8-OH及MWCNT8-COOH,进一步表明MWCNT8与特定HA吸光组分结合能力强于MWCNT8-OH及MWCNT8-COOH。证明了2D-COS可识别HA中具有不同吸附行为的吸光组分,二维相关吸收光谱技术可作为研究MWCNTs与DOM间非均相吸附行为的有效工具。同时,有助于更好地理解MWCNTs与DOM间相互作用特性及机理,不仅可为水环境中DOM去除研究提供基础数据及新的研究思路,且有利于更好地预测自然环境中MWCNTs及DOM迁移传输及环境归趋。     

Multiwalled carbon nanotube supported Pt–Sn–M (M = Ru,Ni, and Ir) catalysts for ethanol electrooxidation

In the present study, Pt–Sn–M (M = Ru, Ni, and Ir) nanocatalysts were supported on multiwalled carbon nanotube and their electrocatalytic activity for ethanol oxidation in membraneless fuel cells was investigated. The combination of monometallic Pt/MWCNTs, bi-metallic Pt–Sn/MWCNTs, and tri-metallic Pt–Sn–Ru/MWCNT, Pt–Sn–Ni/MWCNT, and Pt–Sn–Ir/MWCNT nanocatalysts were prepared by the ultrasonic assisted chemical reduction method. Transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD) were used for the catalyst characterization. The electrocatalytic activities of the catalysts were investigated in half-cell experiments using cyclic voltammetry (CV), CO stripping voltammetry, and chronoamperometry (CA). During the experiments performed on a single membraneless ethanol fuel cell (MLEFC), the Pt–Sn–Ir/MWCNTs exhibited a better catalytic activity from among all the catalysts prepared, with a power density of 39.25 mW cm^?2.     

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.     

Electronic state of nanodiamond/graphite interfaces

The electronic state of nanodiamond/graphite interfaces in samples prepared by annealing of nanodiamonds (ND) at 1150–1600 K has been probed using X-ray fluorescence spectroscopy and field-emission measurements. Comparison between [_C]K spectra of ND before and after annealing revealed an enhancement of density of high-energy occupied states in the products. A quantum-chemical calculation using a carbon model showed that the observed states could originate from the electrons of dangling bonds produced by peeling of a graphitic shell from the (111) surface of a diamond particle. The developed graphitic layers screen the weakly bonding electrons, which results in a lowering of the efficiency of field-electron emission from the samples with an increase of annealing temperature. PACS  61.46.+w; 78.70.En; 79.70.+q     

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。     

Synthesis and characterization of Pd-on-Pt and Au-on-Pt bimetallic nanosheaths on multiwalled carbon nanotubes

The authors have successfully synthesized Pd-on-Pt (thickness: 12 nm) and Au-on-Pt bimetallic nanosheaths on multiwalled carbon nanotubes (MWCNTs) via a seed-mediated growth approach. Pt nanoparticles as seeds were pre-deposited on MWCNTs with uniform distribution followed by the successive seed-mediated growth of metal atoms reduced by a weak reducing agent, ascorbic acid. The essential role of pre-deposited nanoseed particles on MWCNTs was demonstrated. The as-prepared materials were characterization by transition electron microscopy, energy-dispersive X-ray spectroscopy, and element mapping tools. The current strategy extends the classical seed-mediated growth method to prepare bimetallic nanosheath on MWCNT support.     

Synthesis of nano onion-like fullerenes by chemical vapor deposition using an iron catalyst supported on sodium chloride

Nano-carbon materials were synthesized by the catalytic decomposition of acetylene at 420 °C using iron supported on sodium chloride as catalyst. The catalysts contain about 0.3, 1.6, 3.3, and 5.2 wt% iron. The samples were examined by scanning and transmission electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The results show that nano onion-like fullerenes (NOLFs) surrounding an Fe₃C core were obtained using the catalyst containing 0.3 wt% iron. These had a structure of stacked graphitic fragments, with diameters in the range 15–50 nm. When the product was further heat treated under vacuum at 1,100 °C, NOLFs with a clear concentric graphitic layer structure were obtained. The growth mechanism of NOLFs encapsulating metallic cores is suggested to be in accordance with a vapor–solid growth model.     

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.