高温煅烧Mo改性的Fe／MgO催化剂用于制备管径分布均一的单壁碳纳米管

研究了在以甲烷化学气相沉积法制备单壁碳纳米管的过程中高温煅烧预处理(900℃煅烧10h)对Mo改性Fe/MgO催化剂的作用.发现这种预处理有利于Fe在催化剂中的稳定和分散,从而制备出管径均一的单壁碳纳米管.采用能谱元素分析、高分辨透射电镜、X射线衍射、比表面积测量、拉曼光谱和热重分析对样品进行了表征.结果表明,在碳纳米管生长的过程中,铁元素在催化剂表面富集,单壁碳纳米管生长于富集铁的纳米颗粒上,并存在碳管直径与铁颗粒尺寸的依赖关系.Mo存在时可煅烧形成FeMoO4复合氧化物,后者比MgFe2O4相更加稳定.Mo/Fe比例对提高单壁碳纳米管的生长密度、纯度与管径均一性等均有明显影响.上述研究对进一步精确控制制备单壁碳纳米管有重要意义.     

FeCo/MgO催化生长体相单壁碳纳米管的直径调控

单壁碳纳米管的直径可控生长是碳纳米管生长与应用领域的重要问题。直径在0.9–1.2 nm范围内的碳纳米管非常适合应用于近红外荧光生物成像领域和量子器件单光子光源之中。本文使用FeCo/MgO催化剂生长出了直径在这一范围内的体相单壁碳纳米管,并研究了催化剂制备和CVD生长条件对碳纳米管直径的影响。催化剂前驱体的制备是获得小尺寸催化剂颗粒的关键步骤。在浸渍过程中,使用难水解的金属硫酸盐作为前驱体、降低浸渍pH以及加入络合剂分子都会抑制溶液干燥过程中金属盐的水解,从而控制催化剂的尺寸,使其适合于生长出直径可控的单壁碳纳米管。在CVD生长过程中,使用乙醇作为碳源、使用较低的碳氢比例也有利于小直径碳纳米管的生长。     

单壁碳纳米管的手性测量

单壁碳纳米管的制备方法主要包括物理法和化学法两种,不同制备方法的产物中包含很多种不同手性指数的单壁碳纳米管及杂质,而不同手性的单壁碳纳米管其作用完全不同,从而有不同的应用。采用化学方法——钴钼催化剂气相沉积法和物理方法——激光烧蚀气相沉积法制备单壁碳纳米管,用紫外–可见–近红外吸收光谱、荧光光谱及拉曼光谱法分别对其进行手性测量。结果表明,钴钼催化剂气相沉积法制得样品富含半导体型单壁碳纳米管,而激光烧蚀气相沉积法制得样品中主要为金属型单壁碳纳米管。分别得到了两种制备方法样品的手性分布。     

Fe/MgO催化裂解甲烷制备碳纳米带

采用溶胶-凝胶法制备了Fe/MgO催化剂(E)。以甲烷为碳源,通过催化化学气相沉积法在E上生长出了碳纳米带(F),其结构和形貌经SEM和TEM表征。在最佳制备条件[E中Fe负载量为50%,甲烷流速70mL·min-1,于870℃反应1 h]下制备的F宽度约135 nm,厚约89 nm,长度在几十微米量级。     

单壁碳纳米管的控制生长与生长动力学

正结构决定性能,作为典型一维碳材料的单壁碳纳米管具有非常独特的光电性质,因而,单壁碳纳米管的结构控制制备一直是人们关注的热点问题,也成为该领域最具挑战的课题之一~(1,2)。目前,化学气相沉积方法是可控制备碳纳米管的主要方法,在化学气相沉积反应过程中,碳源在催化剂表面裂解成核,进而生长出结构不同、长度各异的单壁碳纳米管。为深入探索碳纳米管的生     

利用Fe3O4/DR膜作为催化剂前体在硅片上生长多壁碳纳米管

Fe3O4纳米粒子与正离子性的重氮树脂在硅基底的表面形成稳定自组装膜,还原后可通过化学气相沉积(CVD)法在硅基底的表面生长多壁碳纳米管.以聚丙烯酸包裹Fe3O4纳米颗粒能够有效地防止纳米粒子的团聚,并提高组装效率,得到均匀的纳米粒子自组装膜,从而获得在硅基底上均匀分布的多壁碳纳米管.     

助剂铬对Ni/MgO催化剂CVD法制备碳纳米管的促进作用

采用溶胶-凝胶法制备了助剂Cr改性的Ni/MgO催化剂, 用化学气相沉积(CVD)法在600 ℃下裂解甲烷生长碳纳米管, 研究了助剂Cr的引入对催化剂微结构和制备碳纳米管性能的影响. 催化剂样品用XRD, TPR和CO-TPD进行了分析, 制备的碳纳米管用TEM和XRD进行了表征. 实验结果表明, NiO和MgO之间存在着强相互作用而形成固溶体, Ni/MgO催化剂经氢气处理后其中的镍氧化物只有极少部分被还原成为镍. 助剂铬的引入明显促进了镍的还原, 使得催化剂表面的Ni活性中心数增多, 从而使催化剂的活性和性能得到了明显的改进. 在加入助剂后碳纳米管的产率明显增加, 当Cr质量分数为8%时, 碳纳米管的产量为未加助剂时产量的5倍, 碳纳米管和催化剂的质量比达到1928. 当Cr含量进一步增加时, Ni在催剂表面聚集形成大颗粒, 制备出的产品中含有大量的碳纳米纤维和无定形碳. 以8%Cr-Ni/MgO催化剂合成的碳纳米管具有比较高的产率且质量较好.     

CeO2助Ni/MgO催化剂用于化学气相沉积法制备多壁碳纳米管

以柠檬酸燃烧法制备的Ni/MgO,Ni/CeO2-MgO和Ni/CeO2为催化剂,CH4为碳源,采用化学气相沉积法制备多壁碳纳米管(MWCNTs),通过N2吸附、X射线衍射、H2程序升温还原和X射线光电子能谱对催化剂进行表征,并运用热重和透射电镜表征了碳纳米管的质量和形貌.结果表明,CeO2的加入可有效地降低还原温度和...     

单壁碳纳米管的CVD合成及管径分布

甲烷在以活性氧化铝为载体的Fe、Co、Ni、Ru等催化剂上于850 ℃分解并生成直径为0.8～5 nm的单壁碳纳米管.预先将催化剂在1100 ℃焙烧,能够减少产物中无定形碳的生成.拉曼光谱结果表明,由该法制备的碳纳米管的管径分布主要受温度的影响,较低温度有利于较小直径的单壁碳纳米管的生成和较好的管径选择性.     

在不同激发波长下的单壁碳纳米管的拉曼光谱研究

采用直流电弧法制备单壁碳纳米管样品,用457.5和632.8nm两种不同的激发光分别测得单壁碳纳米管的正常拉曼光谱和共振拉曼光谱.通过理论分析得到了单壁碳纳米管的直径分布,进一步推测了其类型及结构参数;对单壁碳纳米管的正切拉伸模的成分进行了归属.在632.8nm激发波长下得到了IG/ID值随激光功率变化的曲线,认为在2.5mW时,单壁碳纳米管缺陷的结构可能发生了改变.在用457.5nm波长激发的单壁碳纳米管的拉曼光谱中,首次发现了1421cm-1的拉曼谱峰.     

Performance of dye-sensitized solar cells with various carbon nanotube counter electrodes

Double-wall carbon nanotubes (DWCNTs), single-wall carbon nanotubes (SWCNTs), and multi-wall carbon nanotubes (MWCNTs) were investigated as an alternative for platinum in counter-electrodes for dye-sensitized solar cells. The counter-electrodes were prepared on fluorine-doped tin oxide glass substrates by the screen printing technique from pastes of carbon nanotubes and organic binder. The solar cells were assembled from carbon nanotubes counter-electrodes and screen printed anodes made from titanium dioxide. The cells produced with DWCNTs, SWCNTs or MWCNTs have overall conversion efficiencies of 8.0%, 7.6% and 7.1%, respectively. Electrochemical impedance spectroscopy measurements revealed that DWCNTs displayed the highest catalytic activity for the reduction of tri-iodide ions. The large surface area and superior chemical stability of the DWCNTs facilitated the electron-transfer kinetics at the interface between counter-electrode and electrolyte and yielded the lowest transfer resistance, thereby improving the photovoltaic activity. A short-term stability test at moderate conditions confirmed the robustness of solar cells based on the use of DWCNTs, SWCNTs or MWCNTs.

Bulk synthesis of linear carbon chains confined inside single-wall carbon nanotubes by vacuum discharge

Carbyne, an infinite carbon chain, has attracted much interest and induced significant controversy for many decades. Recently, the presence of linear carbon chains (LCCs), which were confined stably inside double-wall carbon nanotubes (DWCNTs) and multiwall carbon nanotubes (MWCNTs), has been reported. In this study, we present a novel method to produce LCCs in a film of carbon nanotubes (CNTs). Our transmission electron microscopy and Raman spectroscopy revealed the formation of a bulk amount of LCCs after electric discharge of CNT films, which were used as field emission cathodes. The LCCs were confined inside single-wall CNTs as well as DWCNTs. Furthermore, two or three LCCs in parallel with each other are encapsulated when the inner diameter of CNT is larger than approximately 1.1 nm.     

Electrochemistry of polystyrene intercalated vertically aligned single- and double-walled carbon nanotubes on gold electrodes

Electrochemical electrodes incorporating double- and single-walled carbon nanotubes (CNTs) were fabricated on cysteamine modified flat gold substrates. Through covalent coupling of the amine end groups with carboxyl functionalized CNTs, a dense forest of vertically aligned CNTs was produced. To these a 30 nm thick insulating polystyrene layer was spin coated, resulting in exposure of the uppermost carbon nanotube ends. The electrochemical performance of each electrode was then determined using the redox probe ruthenium hexaamine. Once surrounded by polymer, the double-walled CNTs (DWCNTs) showed an improved electron transfer rate, compared to the single-walled electrode. This improvement was attributed to the protection of the electronic properties of the inner wall of the DWCNT during the chemical modification and suggests that DWCNTs may offer a useful alternative to SWCNTs in future electrochemical sensors and biosensors.     

The use of organometallic transition metal complexes in the synthesis of shaped carbon nanomaterials

A review of the use of organometallic complexes in the synthesis of shaped carbon nanomaterials (SCNMs), in particular carbon nanotubes (CNTs) has been undertaken. This review reveals that a limited number of organometallic complexes have been used as catalysts (typically ferrocene, Fe(CO)₅) to make carbon materials that have distinctive shapes. Depending on the reaction conditions employed, ferrocene can be used to synthesize single walled (SWCNTs), double walled (DWCNTs) and multiwalled nanotubes (MWCNTs) as well as fibres and other SCNMs. The type of reactor used as well as the reaction conditions (temperature, pressure, gas flow rates, etc.) and the presence of hydrogen and heteroatoms (N, O, S, P, etc.) also play a role in determining the final carbon types (and their sizes) that have been synthesized. The influence of the various chemical and physical factors on the carbons produced are discussed. The current mechanism used to explain the formation of CNTs is described.     

Critical Evaluation of Adsorption–Desorption Hysteresis of Heavy Metal Ions from Carbon Nanotubes: Influence of Wall Number and Surface Functionalization

Single‐, double‐, and multi‐walled carbon nanotubes (SWCNTs, DWCNTs, and MWCNTs), and two oxidized MWCNTs with different oxygen contents (2.51 wt % and 3.5 wt %) were used to study the effect of the wall number and surface functionalization of CNTs on their adsorption capacity and adsorption–desorption hysteresis for heavy metal ions (Ni^II, Cd^II, and Pb^II). Metal ions adsorbed on CNTs could be desorbed by lowering the solution pH. Adsoprtion of heavy metal ions was not completely reversible when the supernatant was replaced with metal ion‐free electrolyte solution. With increasing wall number and amount of surface functional groups, CNTs had more surface defects and exhibited higher adsorption capacity and higher adsorption–desorption hysteresis index (HI) values. The coverage of heavy metal ions on the surface of CNTs, solution pH, and temperature affect the metal ion adsorption–desorption hysteresis. A possible shift in the adsorption mechanism from mainly irreversible to largely reversible processes may take place, as the amount of metal ions adsorbed on CNTs increases. Heavy metal ions may be irreversibly adsorbed on defect sites.     

Improvement of Fe/MgO catalysts by calcination for the growth of single- and double-walled carbon nanotubes

Calcination at 900-1000 degrees C for 8-12 h of an Fe/MgO catalyst prepared by impregnation was found to result in a uniform MgFe2O4/MgO solid solution that showed a successful settling of well-dispersed iron species into the MgO lattice. During methane reduction, many iron-containing particles with a diameter of about 4 nm were formed on the catalyst surface to provide numerous active sites for the growth of single- and double-walled carbon nanotubes. There was a significant improvement of the Fe/MgO catalyst that resulted in a high yield of impurity-free nanotubes. Using C2H4 cracking at 600 degrees C and transmission electron microscope observations, the Fe species distribution in the catalysts and microscope images of nanotube growth were described in detail. H2 reduction of the calcined Fe/MgO catalyst was found to cause the formation of iron layers on the catalyst surface, which resulted in the growth of only carbon layers. The results are useful for understanding changes in the metal species distribution in the catalysts and the nanotube growth mechanism, and they provide a simple method to improve Fe/MgO catalysts.     

The study of carbon nanotubes as conductive additives of cathode in lithium ion batteries

Carbon nanotubes (CNTs), including multi-walled CNTs (MWCNTs) and single-walled CNTs (SWCNTs), are employed as conductive additives in lithium ion batteries. The effects of MWCNTs’ carbon precursors, diameter, and weight fraction on the electrochemical behavior of MWCNTs/LiCoO₂ composite cathode are investigated. Meanwhile, a comparison is made between SWCNTs /LiCoO₂ and MWCNTs/LiCoO₂. Among the three kinds of carbon precursors: CH₄, natural gas, and C₂H₂, MWCNTs prepared from CH₄ are very fit for acting as conductive additives due to their better crystallinity and lower electrical resistance. MWCNTs with smaller diameter favor improving the electrochemical behavior of MWCNTs/LiCoO₂ composite cathode at higher charge/discharge rate owing to their advantage in primary particle number in unit mass. To make full use of LiCoO₂ at higher rate, it is necessary to add at least 5 wt.% of MWCNTs with a diameter 10~30 nm. However, SWCNTs are not expected to be added into LiCoO₂ composite cathode since they tend to form bundles.     

多壁碳纳米管储氢的物理吸附特性

The physisorption of hydrogen stored in armchair multi-walled carbon nanotubes (MWCNTs) is simulated by the grand canonical Monte Carlo (GCMC) method on the condition of 10 MPa at normalt emperature. Hydrogen-hydrogen and hydrogen-carbon interactions are both modeled with Lennard-Jones potential. The hydrogen storage in double-walled carbon-nanotubes (DWCNTs) has been investigated on the condition that the internal or external radius is changed while the other radius remains constant. The results show that hydrogen molecules are mostly absorbed near the tube walls, and the hydrogens to rage capacityisim proved effectively when the difference between the internal radius and the external radius increases from 0.34 to 0.61 or 0.88 nm. Its simple theoretic explanation also is given. Further more, the capacity of hydrogen physisorbed in there-walled carbon nanotubes (TWCNTs) is calculated when the wall-wall distance is 0.34, 0.61 and 0.88 nm respectively. Then its hydrogen storage capacity is compared with that of single-walled carbon nanotubes (SWCNTs) and DWCNTs, and it is discovered that the capacity of hydrogen physisorbed in MWCNTs decreases as the number of wall increases.     

Growth of Aligned Carbon Nanotubes on Large Scale by Methane Decomposition with Deactivation Inhibitor

The effects of additives containing iron or nickel during chemical vapor deposition(CVD)on the growth of carbon nanotubes(CNTs)by methane decomposition on Mo/MgO catalyst were investigated. Ferrocene and nickel nitrate were introduced as deactivation inhibitors by in-situ evaporation during CVD. The precisely controlled in-situ introduction of these inhibitors increased the surface renewal of catalyst, and therefore prevented the catalyst from deactivation.Using this method,aligned multi-walled CNTs with parallel mesopores can be produced on a large scale.