The effect of Pd addition to Fe as catalysts on growth of carbon nanotubes by TCVD method

The effects of Pd addition to Fe (Pd/Fe = 0, 2.5/7.5, 5/5, 7.5/2.5 and 1) and growth temperatures (920 and 970 °C) on density, diameter and growth mode of carbon nanotubes (CNTs) have been studied. SEM observations and TG analyses confirmed that the CNT yields depend on Pd/Fe ratios as (7.5/2.5) > (5/5) > Pd > (2.5/7.5) > Fe at both growth temperatures. TEM data showed that addition of Pd results in tip growth mode. From Raman spectroscopy data, the order of samples’ structural quality (I_G/I_D ratio) are Fe > Pd/Fe (2.5/7.5) > (5/5) > (7.5/2.5) > Pd and the I_G/I_D ratios increase by decreasing the growth temperature. Films with higher concentration of Fe (Pd/Fe = 0, 2.5/7.5) contain some single-walled carbon nanotubes.     

Selective growth, characterization, and field emission performance of single-walled and few-walled carbon nanotubes by plasma enhanced chemical vapor deposition

Single-walled carbon nanotubes (SWCNTs) and few-walled carbon nanotubes (FWCNTs) have been selectively synthesized by plasma enhanced chemical vapor deposition at a relative low temperature (550 °C) by tuning the thickness of iron catalyst. The parametric study and the optimization of the nanotube growth were undertaken by varying inductive power, temperature, catalyst thickness, and plasma to substrate distance. When an iron film of 3-5 nm represented the catalyst thickness for growing FWCNT arrays, SWCNTs were synthesized by decreasing the catalyst thickness to 1 nm. The nanotubes were characterized by field emission scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. Electron field emission properties of the nanotubes indicate that the SWCNTs exhibit lower turn-on field compared to the FWCNTs, implying better field emission performance.     

Growth of carbon nanotubes by Fe-catalyzed chemical vapor processes on silicon-based substrates

In this paper, a site-selective catalytic chemical vapor deposition synthesis of carbon nanotubes on silicon-based substrates has been developed in order to get horizontally oriented nanotubes for field effect transistors and other electronic devices. Properly micro-fabricated silicon oxide and polysilicon structures have been used as substrates. Iron nanoparticles have been obtained both from a thin Fe film evaporated by e-gun and from iron nitrate solutions accurately dispersed on the substrates. Single-walled nanotubes with diameters as small as 1 nm, bridging polysilicon and silicon dioxide “pillars”, have been grown. The morphology and structure of CNTs have been characterized by SEM, AFM and Raman spectroscopy.     

Synthesis of multi-walled carbon nanotubes using CoMnMgO catalysts through catalytic chemical vapor deposition

The Co Mg O and Co Mn Mg O catalysts are prepared by a co-precipitation method and used as the catalysts for the synthesis of carbon nanotubes（CNTs） through the catalytic chemical vapor deposition（CCVD）. The effects of Mn addition on the carbon yield and structure are investigated. The catalysts are characterized by temperature programmed reduction（TPR） and X-ray diffraction（XRD） techniques, and the synthesized carbon materials are characterized by transmission electron microscopy（TEM） and thermo gravimetric analysis（TG）. TEM measurement indicates that the catalyst Co Mg O enclosed completely in the produced graphite layer results in the deactivation of the catalyst. TG results suggest that the Co Mn Mg O catalyst has a higher selectivity for CNTs than Co Mg O. Meanwhile, different diameters of CNTs are synthesized by Co Mn Mg O catalysts with various amounts of Co content, and the results show that the addition of Mn avoids forming the enclosed catalyst, prevents the formation of amorphous carbon, subsequently promotes the growth of CNTs, and the catalyst with decreased Co content is favorable for the synthesis of CNTs with a narrow diameter distribution.The Co Mn Mg O catalyst with 40% Co content has superior catalytic activity for the growth of carbon nanotubes.     

Preparation of carbon nanosheets deposited on carbon nanotubes by microwave plasma-enhanced chemical vapor deposition method

Carbon nanosheets were synthesized by microwave plasma-enhanced chemical vapor deposition method on carbon nanotubes substrate which was treated by hydrogen plasma. The results showed that the diameters of carbon nanotubes first got thick and then “petal-like” carbon nanosheets were grown on the outer wall of carbon nanotubes. The diameters of carbon nanotubes without and with carbon nanosheets were 100-150 and 300-500 nm, respectively. Raman spectrum indicated the graphite structure of carbon nanotubes/carbon nanosheets. The hydrogen plasma treatment and reaction time greatly affected the growth and density of carbon nanosheets. Based on above results, carbon nanosheets/carbon nanotubes probably have important applications as cold cathode materials and electrode materials.     

Synthesis of multi-wall carbon nanotubes by the pyrolysis of ethanol on Fe/MCM-41 mesoporous molecular sieves

Ordered hexagonal arrangement MCM-41 mesoporous molecular sieves were synthesized by the traditional hydrothermal method, and Fe-loaded MCM-41 mesoporous molecular sieves (Fe/MCM-41) were prepared by the wet impregnation method. Their mesoporous structures were testified by X-ray diffraction (XRD) and the N₂ physical adsorption technique. Carbon nanotubes (CNTs) were synthesized by the chemical vapor deposition (CVD) method via the pyrolysis of ethanol at atmospheric pressure using Fe/MCM-41 as a catalytic template. The effect of different reaction temperatures ranging from 600 to 800 ^°C on the formation of CNTs was investigated. The resulting carbon materials were characterized by various physicochemical techniques such as transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), and Raman spectroscopy. The results show that multi-wall carbon nanotubes (MWCNTs) with an internal diameter of ca. 7.7 nm and an external diameter of ca. 16.9 nm were successfully obtained by the pyrolysis of ethanol at 800 ^°C utilizing Fe/MCM-41 as a catalytic template.     

Effect of alcohol sources on synthesis of single-walled carbon nanotubes

The effect of oxidant species forming an alcohol molecule for hot-filament chemical vapor deposition (HFCVD) on single-walled carbon nanotubes (SWCNTs) growth has been investigated. To use a graphite rod as a filament for HFCVD to decompose alcohol sources, contamination-free sample surface can be obtained and SWCNTs are successively and densely grown at a lower temperature than those by conventional thermal CVD. It is found that the higher the molecule number of alcohol among CH₃OH, C₂H₅OH, and 2-C₃H₇OH is, the lower the initial growth rate of SWCNTs is. As for CH₃OH, diameter distribution of SWCNTs is dynamically changed with the growth time passed, and a negative growth rate is observed at the later stage of growth. The growth kinetics depending on the alcohol sources is discussed on the basis of a capability of the oxidant species to burn away SWCNTs and deactivation of Co catalysts used for the growth.     

Short time synthesis of high quality carbon nanotubes with high rates by CVD of methane on continuously emerged iron nanoparticles

We report the variation of yield and quality of carbon nanotubes (CNTs) grown by chemical vapor deposition (CVD) of methane on iron oxide-MgO at 900-1000 °C for 1-60 min. The catalyst was prepared by impregnation of MgO powder with iron nitrate, dried, and calcined at 300 °C. As calcined and unreduced catalyst in quartz reactor was brought to the synthesis temperature in helium flow in a few minutes, and then the flow was switched to methane. The iron oxide was reduced to iron nanoparticles in methane, while the CNTs were growing.TEM micrographs, in accordance with Raman RBM peaks, indicate the formation of mostly single wall carbon nanotubes of about 1.0 nm size. High quality CNTs with I_G/I_D Raman peak ratio of 14.5 are formed in the first minute of CNTs synthesis with the highest rate. Both the rate and quality of CNTs degrades with increasing CNTs synthesis time. Also CNTs quality sharply declines with temperature in the range of 900-1000 °C, while the CNTs yield passes through a maximum at 950 °C. About the same CNTs lengths are formed for the whole range of the synthesis times. A model of continuous emergence of iron nanoparticle seeds for CNTs synthesis may explain the data. The data can also provide information for continuous production of CNTs in a fluidized bed reactor.     

Nitrogen-doped carbon nanotubes with tunable structure and high yield produced by ultrasonic spray pyrolysis

Nitrogen-doped carbon nanotubes (CN_x) were prepared by ultrasonic spray pyrolysis from mixtures of imidazole and acetonitrile. Imidazole, as an additive, was used to control the structure and nitrogen doping in CN_x by adjusting its concentration in the mixtures. Scanning electron microscopy observation showed that the addition of imidazole increased the nanotube growth rate and yield, while decreased the nanotube diameter. Transmission electron microscopy study indicated that the addition of imidazole promoted the formation of a dense bamboo-like structure in CN_x. X-ray photoelectron spectroscopy analysis demonstrated that the nitrogen content varied from 3.2 to 5.2 at.% in CN_x obtained with different imidazole concentrations. Raman spectra study showed that the intensity ratio of D to G bands gradually increased, while that of 2D to G bands decreased, due to increasing imidazole concentration. The yield of CN_x made from mixtures of imidazole and acetonitrile can reach 192 mg in 24 min, which is 15 times that of CN_x prepared from only acetonitrile. The aligned CN_x, with controlled nitrogen doping, tunable structure and high yield, may find applications in developing non-noble catalysts and novel catalyst supports for fuel cells.     

化学气相沉积法制备定向碳纳米管阵列

以二茂铁和二甲苯分别作为催化剂和碳源，采用一种无模板的化学气相沉积法，使用单温炉设备，成功地制备了高度定向的碳纳米管阵列.分别用扫描电子显微镜、透射电子显微镜和电子能量散射谱、拉曼光谱对碳纳米管阵列进行形貌观察和表征, 并研究了不同工艺参数对碳纳米管阵列形貌的影响.结果表明：在生长温度为800℃，催化剂浓度为0.02g/mL，抛光硅片上容易获得高质量的定向碳纳米管阵列，在此优化条件下生长的定向碳纳米管的平均生长速率可达25μm/min.     

Synthesis of multi-walled carbon nanotubes for NH3 gas detection

It has been recently demonstrated that carbon nanotubes (CNTs) represent a new type of chemical sensor capable of detecting a small concentration of molecules such as CO, NO₂, NH₃.In this work, CNTs were synthesized by chemical vapor deposition (CVD) on the SiO₂/Si substrate by decomposition of acetylene (C₂H₂) on sputtered Ni catalyst nanoparticles. Their structural properties are studied by atomic force microscopy, high-resolution scanning electron microscopy (HRSEM) and Raman spectroscopy. The CNTs grown at 700 °C exhibit a low dispersion in size, are about 1 μm long and their average diameter varies in the range 25–60 nm as a function of the deposition time. We have shown that their diameter can be reduced either by annealing in oxygen environment or by growing at lower temperature (less than 600 °C).We developed a test device with interdigital Pt electrodes on an Al₂O₃ substrate in order to evaluate the CNTs-based gas sensor capabilities. We performed room temperature current–voltage measurements for various gas concentrations. The CNT films are found to exhibit a fast response and a high sensitivity to NH₃ gas.     

Temperature and substrate dependence of structure and growth mechanism of carbon nanofiber

The carbon nanofibers were grown on Ni/Si and Ni/Ti/Si substrates in 1 atm CH₄ atmosphere at 640 °C and 700 °C by thermal chemical vapor deposition method. The carbon nanofibers were characterized by field emission scanning electron microscopy, transmission electron microscopy, and Raman spectrometry for morphology, microstructure, and crystallinity. The electron emission property of carbon nanofibers was also investigated by current-voltage (I-V) measurement. The results showed that the solid amorphous carbon nanofibers could be grown on Ni/Si substrate at 640 °C through tip growth mechanism, the carbon nanotubes could be grown on Ni/Si substrate at 700 °C through tip growth mechanism, and the carbon nanotubes could be grown on Ni/Ti/Si substrate at 700 °C through root growth mechanism.     

Effect of hydrogen on the growth and morphology of single wall carbon nanotubes synthesized on a FeMo/MgO catalytic system

Single wall carbon nanotubes were synthesized from thermal pyrolysis of methane on a FeMo/MgO catalyst by radio frequency catalytic chemical vapor deposition (RF-CVD) using argon as a carrier gas. Controlled amounts of hydrogen (H₂/CH₄=0-1 v/v) were introduced in separate experiments along with the carbon source. The properties and morphology of the synthesized single wall carbon nanotubes were monitored by transmission electron microscopy, Raman scattering, and thermogravimetric analysis. The nanotubes with the highest crystallinity were obtained with H₂/CH₄=0.6. By monitoring the Radial Breathing Modes present in the Raman spectra of the single-wall carbon nanotube samples, the variation of the structural and morphological properties of the carbon nanotubes with the flow level of hydrogen, reflect changes of the catalyst systems induced by the presence of hydrogen.     

Synthesis of carbon nanotubes directly over TEM grids aiming the study of nucleation and growth mechanisms

A novel approach to produce electron-transparent multi-layer membranes over TEM grids for transmission electron microscopy analysis is presented. The membranes have been used to grow and analyze carbon nanostructured materials in a thermal-chemical vapor deposition process using Ni and Cu as catalysts and silicon oxide thin films as support layers, at temperatures as high as 900 °C. It is important that conditions of the synthesis using the electron-transparent multi-layer membrane system are similar to those for a conventional chemical vapor deposition process, where oxidized silicon wafers are employed. In particular, the thickness of the silicon oxide and the catalyst layers are the same, and similar carbon tubular structures were grown using both substrates. The use of membranes was crucial especially for the study of the nucleation mechanism for carbon nanotubes. These electron-transparent multi-layer membranes are relatively easy to obtain and they can be used for transmission electron microscopy studies of high-temperature synthesis of different nanostructured materials.     

A chemical kinetic model for chemical vapor deposition of carbon nanotubes

Carbon nanotubes (CNTs) are classified among the most promising novel materials due to their exceptional physical properties. Still, optimal fabrication of carbon nanotubes involves a number of challenges. Whatever be the fabrication method, a process optimization can be evolved only on the basis of a good theoretical model to predict the parametric influences on the final product. The work reported here investigates the dependence of the deposition parameters on the controllable parameters for carbon nanotube growth during Chemical vapor deposition (CVD), through a chemical kinetic model. The theoretical model consisted of the design equations and the energy balance equations, based on the reaction kinetics, for the plug flow and the batch reactor, which simulate the CVD system. The numerical simulation code was developed in-house in a g++ environment. The results predicted the growth conditions for CNT: the deposition temperature, pressure and number of atoms, which were found to be influenced substantially by the initial controllable parameters namely the temperature, volumetric flow rate of the carbon precursor, and the reaction time. An experimental study was also conducted on a CVD system developed in the laboratory, to benchmark the computational results. The experimental results were found to agree well with the theoretical predictions obtained from the model.     

Synthesis and characterization of carbon nanotubes on carbon microfibers by floating catalyst method

In this paper, carbon nanotubes were synthesized on carbon microfibers by floating catalyst method with the pretreatment of carbon microfibers at the temperature of 1023 K, using C₂H₂ as carbon source and N₂ as carrier gas. The morphology and microstructure of carbon nanotubes were characterized by field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The composition of carbon nanotubes was determined by energy dispersive X-ray spectroscopy (EDX). The results showed that the surface of treated carbon microfibers was thickly covered by carbon nanotubes with diameters of about 50 nm. EDX image indicated that the composition of carbon nanotubes was carbon. In comparison with the sample grown on untreated carbon microfibers surface, it was found that after carbon microfibers were boiled in the solution of sulfur acid and nitric acid (VH₂SO₄:VHNO₃ = 1:3) and immersed in the solution of iron nitrate and xylene, carbon nanotubes with uniform density can be grown on carbon microfibers surface. Based on the results, we concluded that the pretreatment of carbon microfibers had great effect on the growth of carbon nanotubes by floating catalyst method.     

Synthesis of nitrogen-doped single-walled carbon nanotubes and monitoring of doping by Raman spectroscopy

Nitrogen-doped single-walled carbon nanotubes （CNx-SWNTs） with tunable dopant concentrations were synthesized by chemical vapor deposition （CVD）, and their structure and elemental composition were characterized by using transmission electron microscopy （TEM） in combination with electron energy loss spectroscopy （EELS）. By comparing the Raman spectra of pristine and doped nanotubes, we observed the doping-induced Raman G band phonon stiffening and 2D band phonon softening, both of which reflect doping-induced renormalization of the electron and phonon energies in the nan- otubes and behave as expected in accord with the n-type doping effect. On the basis of first principles calculations of the distribution of delocalized carrier density in both the pristine and doped nanotubes, we show how the n-type doping occurs when nitrogen heteroatoms are substitutionally incorporated into the honeycomb tube-shell carbon lattice.     

A novel continuous process for synthesis of carbon nanotubes using iron floating catalyst and MgO particles for CVD of methane in a fluidized bed reactor

A novel continuous process is used for production of carbon nanotubes (CNTs) by catalytic chemical vapor deposition (CVD) of methane on iron floating catalyst in situ deposited on MgO in a fluidized bed reactor. In the hot zone of the reactor, sublimed ferrocene vapors were contacted with MgO powder fluidized by methane feed to produce Fe/MgO catalyst in situ. An annular tube was used to enhance the ferrocene and MgO contacting efficiency. Multi-wall as well as single-wall CNTs was grown on the Fe/MgO catalyst while falling down the reactor. The CNTs were continuously collected at the bottom of the reactor, only when MgO powder was used. The annular tube enhanced the contacting efficiency and improved both the quality and quantity of CNTs.The SEM and TEM micrographs of the products reveal that the CNTs are mostly entangled bundles with diameters of about 10-20 nm. Raman spectra show that the CNTs have low amount of amorphous/defected carbon with I_G/I_D ratios as high as 10.2 for synthesis at 900 °C. The RBM Raman peaks indicate formation of single-walled carbon nanotubes (SWNTs) of 1.0-1.2 nm diameter.     

碳纳米片-碳纳米管复合材料的一步合成及其场 发射性质研究

利用等离子体化学气相沉积技术, 在引入Ti过渡层后的Co膜表面一步制备出碳纳米片-碳纳米管复合材料, 研究了Co膜厚度对复合材料形貌及场发射性质的影响. 当Co薄膜厚度为11 nm时, 得到了垂直基片定向生长的碳纳米管和碳纳米片复合物, 此时, 碳纳米片分布在碳纳米管的管壁上和管的顶端, 样品的场发射性能最佳.     

In situ polyphenyl derivatisation and the effect of thermal decomposition of adsorbed and chemisorbed polyphenyls on the structure of multi-wall carbon nanotubes

This study presents the exploitation of an alternative reaction route of deamination of arylamines to perform in situ derivatisation of multi-walled carbon nanotubes (MWCNTs) with polyphenyl (PPh) species of various masses. As a result of consecutive derivatisation, high conversion of PPh grafting of the MWCNTs was realised with the collateral outgrowth of physical modification with adsorbed additional PPhs. Applied derivatisation process exceeds the monolayer coverage related superficial saturation limitations in the overall grafting yield of the nanotubes. Thus, a linear relationship was recognized between the overall quantities of chemisorbed PPhs composed of D₅-phenylene oligomers and the applied excess of diazonium activated reagents, corresponding to ²H MAS NMR spectroscopy results. According to mass spectrometry (MS) investigations, uniform thermal decomposition of the chemisorbed PPhs modified MWCNTs was found besides the more intense and altered decomposition characteristic-featured adsorbate-chemisorbate PPhs contained MWCNTs during sequential pyrolysis under inert atmosphere. This is attributed to the pyrolysis provoked isomerisation, decomposition and the formation of adsorbed and chemisorbed PPh moieties. As a result, a mediated and an even more pronounced degradation in the order of graphitic lattice of the MWCNTs were evidenced in the adsorbate-chemisorbate and the chemisorbate PPhs contained samples by FT-Raman spectroscopy and transmission electron microscopy (TEM), respectively. ²H MAS NMR supplied results of relevant amount of deuterium in the chemisorbate PPh contained sample without traces of aromatic related MS detected volatile products, these allow us to conclude about a thermally stable derivatisation that is interpreted as an endohedral modification of the nanotubes.