Hierarchical Structures of Carbon Nanotubes and Arrays of Chromium‐Capped Silicon Nanopillars: Formation and Electrical Properties

The formation of stochastically oriented carbon‐nanotube networks on top of an array of free‐standing chromium‐capped silicon nanopillars is reported. The combination of nanosphere lithography and chemical vapor deposition enables the construction of nanostructures that exhibit a hierarchical sequence of structural sizes. Metallic chromium serves as an etching mask for Si‐pillar formation and as a nucleation site for the formation of carbon nanotubes through the chemical vapor deposition of ethene, ethanol, and methane, respectively, thereby bridging individual pillars from top to top. Iron and cobalt were applied onto the chromium caps as catalysts for CNT growth and the influence of different carbon sources and different gas‐flow rates were investigated. The carbon nanotubes were structurally characterized and their DC electrical properties were studied by in situ local‐ and ex situ macroscopic measurements, both of which reveal their semiconductor properties. This process demonstrates how carbon nanotubes can be integrated into Si‐based semiconductors and, thus, this process may be used to form high‐surface‐area sensors or new porous catalyst supports with enhanced gas‐permeation properties.     

微波等离子体化学气相沉积法低温合成纳米碳管

纳米碳管的低温合成是纳米碳管合成的一个重要研究方向。在众多的合成方法 中,化学气相沉积法,特别是等离子体化学气相沉积法在纳米碳管的低温合成方面 意义重大。本研究利用溶胶-凝胶法结合等离子体还原,获得了负载在SiO_2上的纳 米金属钻颗粒。以甲烷为碳源、氢气为载气,在纳米金属钴颗粒的催化作用下,利 用微波等离子体化学气相沉积法在低于500 ℃ 的温度条件下合成了纯度较高的纳 米碳管。     

Zigzag Assembly of Carbon Nanotubes inside Au Microtrenches

We report the self-assembly of zigzag patterns consisting of aligned carbon nanotubes inside Au microtrenches by chemical vapor deposition using ferrocene/xylene solution as the precursor. The zigzag nanotubes have uniform size and constant interpattern distance, which can be controlled by simply changing the width of the Au trenches. We demonstrated the tunable length and orientation of nanotubes during self-assembly, leading to a predictable motion of zigzag patterns. A growth model was proposed for the zigzag assembly of nanotubes, including the formation and subsequent splitting of an amorphous carbon layer on the pattern top. Rows of nanotube micropatterns regularly distributed along the Au trench are potential candidates as integrated arrays of thermal or mechanical detectors and actuators.     

Cone-type multi-shell in the hollow core of multi-wall carbon nanotube

Cone-stacked nanotubes with thick layers of deposited carbon obtained by a catalytic chemical vapor deposition method are annealed at high temperature. Phase separation between cone-stacked nanotubes and amorphous carbon layers occurs during annealing due to different structural transformation behaviors, resulting in the formation of multi-wall carbon nanotubes containing a cone-type multi-shell arrangement. The apex angles of the nanocones are about 40°. This type of carbon nanotube shows different physical and electrical properties.     

The template synthesis of double coaxial carbon nanotubes with nitrogen-doped and boron-doped multiwalls

We present the first synthesis of double coaxial carbon nanotubes with nitrogen-doped and boron-doped multiwalls by the template technique with two-step chemical vapor deposition. X-ray photoelectron spectra confirm the coaxial formation of different-doped structures. The electrical conductance and oxygen chemisorption measurement indicate dual electrical properties and chemical activity at their outer and inner layers.     

氧化铝模板上定向纳米碳管的快速生长及超声切短

在阳极氧化铝（AAO）模板上电沉积催化剂,快速生长了定向纳米碳管,纳米碳管以顶部生长模式生长.采用了超声的方法来切短露头于AAO模板的纳米碳管,增加纳米碳管膜的定向性.结果显示随着超声时间的增加,纳米碳管的定向性增加.位于纳米碳管膜顶部的催化剂在碳管切短的同时被去除,得到了顶部开口的纳米碳管.解释了纳米碳管被超声切短的机理.     

Direct growth of aligned multiwalled carbon nanotubes on treated stainless steel substrates

Growth of aligned carbon nanotubes (CNTs) on electrically conductive substrates is promising for many applications; however, the lack of complete understanding of the substrate effects on CNT growth poses a lot of technical challenges. Here, we report the direct growth of aligned multiwalled nanotubes (MWNTs) on chemically treated stainless steel (Type 304) using a chemical vapor deposition (CVD) process. A detailed X-ray photoelectron spectroscopy (XPS) analysis has been carried out for the various treated samples in order to better understand the correlation between the surface properties of the substrates and the MWNT growth. The XPS studies revealed that the CNTs prefer to grow on the enriched surface of iron oxides obtained by the chemical treatment rather than on the passive chromium oxide films present on the surface of the as-received stainless steel substrates. The density and alignment of the MWNTs could therefore be controlled by tuning the ratio of the iron oxides to chromium oxides through the chemical treatment on the stainless steel surfaces. On the basis of this method, selective growth of CNT patterns on stainless steel has also been demonstrated.     

Bundles of identical double-walled carbon nanotubes

In a sample produced by catalytic chemical vapor deposition (CCVD), the structure of the carbon nanotubes (diameter and helicity) which governs their electronic properties, is determined by electron diffraction. We found that most of the smallest bundles are constituted of identical double-walled carbon nanotubes.     

过渡金属氧化物和金属负载型沸石催化剂上合成 纳米碳管及其表征

采用气相催化沉积法催化合成纳米碳管,比较了不同金属氧化物和金属负载型沸石催化剂以及不同分子筛载体对合成纳米碳管的影响,并用TEM,XRD表征其形貌和结晶度,用DTA-TG考察了纳米碳管的热和稳定性。实验结果表明纳米碳管的形成除了与金属种类有关外,还直接与催化剂的颗粒大小和分散状态有关。粒径在20nm左右的不规则形状的纳米粒子是形成纳米碳管的活性组分,非负载和负载型的催化剂均表明活性组分的粒径与纳米碳管的管径有一定的对应关系。化学提纯后能得到高纯度的纳米碳管;其管壁具有较好的石墨化结构,在空气中的热稳定性大于400℃,而在氮气中能维持到1200℃以上。     

Synthesis and electrochemical properties of carbon nanotubes obtained by pyrolysis of acetylene using AB5 alloy

The paper discusses the efficiency of catalytic synthesis and structure of multi-wall carbon nanotubes obtained by acetylene decomposition over Mm (mischmetal)-based multi-component alloy of AB₅ type. Different parameters of catalytic chemical vapor deposition process have an influence on the efficiency. Some of them were changed to obtain the highest amount of carbon material. The samples were purified by acid and were characterized by BET surface area measurements, scanning electron microscopy, and transmission electron microscopy analysis. However, both catalyst and parameters of process (such as the flow rate of acetylene) need further examination to make it cost effective. The capacitance properties of carbon nanotubes as electrode materials for electrochemical capacitors are discussed. It has been shown that carbon nanotubes show moderate values of capacitance. In the form of a network, the material provides good charge propagation and can be used as a support and additive for different composite electrode materials.     

载气对注入CVD法制备碳纳米管和碳纳米纤维的影响

采用注入化学蒸气沉积法,在石英管壁上制备得到了定向的多壁纳米碳管和纳米碳纤维.在相同的反应体系中,采用不同的载气,得到的碳材料的形态有较大不同.经SEM, TEM及Raman等证实,采用氩气和氮气作为载气时,得到了内部填充铁的碳纳米纤维,而且具有较好的类石墨晶体结构;用氢气作为载气时得到了中空的碳纳米管.通过控制氢气和惰性气体的比例,改变了碳纳米管的直径分布.     

A magnetism-assisted chemical vapor deposition method to produce branched or iron-encapsulated carbon nanotubes

A magnetism-assisted chemical vapor deposition method was developed to synthesize branched or iron-encapsulated carbon nanotubes. In the process, the external magnetic field can promote the coalescence or division of the catalyst particles, causing the formation of branched or encapsulated nanostructures. This finding will extend the understanding of the chemical vapor deposition method in a magnetic field and promote the applications of branched or encapsulated nanostructures.     

Selective growth of SWNTs on partially reduced monometallic cobalt catalyst

SiO(2) supported cobalt (Co) catalyst could be partially reduced and anchored by unreduced Co ions during a carbon monoxide (CO) chemical vapor deposition (CVD) process. This resulted in the formation of sub-nanometre metallic Co clusters catalyzing the growth of single-walled carbon nanotubes (SWNTs) with a narrow diameter distribution.     

Multi-walled BN nanotubes synthesized by carbon-free method

Pure multi-walled BN nanotubes were synthesized via a carbon-free chemical vapor deposition process using boron and gallium oxide mixture as reaction precursor. Transmission electron microscopy was used to investigate their structure, morphology and defects. The wall deformation, dependent on tube diameter, was observed and explained in terms of strain relaxation of bond rotation. Opposed to carbon nanotubes, bending of BN nanotubes typically results in fracture at their concave side. Ring defect-related mechanism was proposed to interpret the fracture. The ring defects also result in the formation of a nanocone with 300° disclination. The nanocones end up with BN nanotubes exhibiting the small innermost shell ∼0.4 nm in diameter.     

Angular and local spectroscopic analysis to probe the vertical alignment of N-doped well-separated carbon nanotubes

Vertically aligned well-separated N-doped multiwalled carbon nanotubes (CNTs) were grown on a silicon substrate by plasma enhanced chemical vapor deposition (PECVD). Angular near-edge X-ray absorption fine structure (NEXAFS) was used to investigate the vertical alignment of as-grown CNTs. In addition, both individual tubes and tube bundles were characterized by high-resolution electron energy loss spectroscopy (HREELS). Simultaneous analysis of both spectroscopic techniques provides information on chemical environment, orbital orientation between carbon and heteroatoms, and local curvature effects. We demonstrate the utility of NEXAFS as an in situ probe of CNTs.     

碳纳米管透明导电薄膜的可控制备

碳纳米管具有优秀的导电性能、 透光性能和十分突出的柔性, 在柔性透明导电薄膜中有着良好的应用前景. 如何制备同时拥有良好导电性能和透光性能的碳纳米管薄膜是这一领域研究的核心问题. 本综述介绍了碳纳米管薄膜的制备方法, 并重点讨论了基于漂浮催化剂化学气相沉积法的碳纳米管薄膜的可控制备. 在生长过程中限制碳纳米管的团聚、 增加碳纳米管的长度、 降低杂质的含量是提高碳纳米管薄膜性能的主要策略.     

Formation of Multiwall Carbon Nanotubes on Cylindrical Substrates in Synthesis by Metal-Organic Chemical Vapor Deposition

Processes of deposition of multiwall carbon nanotubes in the synthesis by metal-organic chemical vapor deposition on hollow cylindrical substrates and the effect of the substrate area on the yield of the target product were studied. Making larger the tubular deposition reactor and the area of the cylindrical substrate enabled a substantial increase in the yield of carbon nanotubes. The technological synthesis parameters of the carbon material were optimized. Methods were developed for purification of carbon nanotubes by annealing in air, and materials were obtained with purity of no less than 98%. Physicochemical analysis methods were used to examine the structure and properties of the materials.     

Study on Diameter Controlled Growth of Carbon Nanotubes by LaAl1-xFexO3 Catalysts

A series of LaAl1-xFexO3 catalysts prepared with lanthanum nitrate, aluminium nitrate and iron nitrate was investigated in catalytical syntheses of carbon nanotubes with high yields and purity. The properties of carbon nanotubes prepared by the method of CVD(chemical vapor deposition) with n-hexane as the carbon resource were studied and it was shown that the diameter of carbon nanotubes can be controlled by the molar ratio of iron to aluminum in the catalysts and that the diameter of carbon nanotubes changes a little with the decrease of the iron content in the catalysts. From the TEM pictures of carbon nanotubes, it can be found that the LaAl1-xFexO3 catalysts have a significant influence on the wall thickness of the carbon nanotubes, whereas they have little influence on the inner diameter of the carbon nanotubes.     

Formation and growth mechanism of dissimilar coiled carbon nanotubes by reduced-pressure catalytic chemical vapor deposition

Dissimilar coiled carbon nanotubes were prepared by catalytic chemical vapor deposition (CCVD) on finely divided Co nanoparticles supported on silica gel under reduced pressure and at lower gas flow rates. The morphology of the regular coiled carbon nanotubes were examined by TEM, while the polygonization characteristics of the helix were examined by SAED. Observations were made on other forms of irregular coils with various shapes by TEM. On the basis of the heptagon-pentagon construction theory, we proposed a helix formation mechanism, which involves a carbon core formation centering on a catalytic particle followed by carbon helices growth controlled by kinetics.     

Bulk synthesis of large diameter semiconducting single-walled carbon nanotubes by oxygen-assisted floating catalyst chemical vapor deposition

Semiconducting single-walled carbon nanotubes (s-SWCNTs) with a mean diameter of 1.6 nm were synthesized on a large scale by using oxygen-assisted floating catalyst chemical vapor deposition. The oxygen introduced can selectively etch metallic SWCNTs in situ, while the sulfur growth promoter functions in promoting the growth of SWCNTs with a large diameter. The electronic properties of the SWCNTs were characterized by laser Raman spectroscopy, absorption spectroscopy, and field effect transistor measurements. It was found that the content of s-SWCNTs in the samples was highly sensitive to the amount of oxygen introduced. Under optimum synthesis conditions, enriched s-SWCNTs can be obtained in milligram quantities per batch.