Formation of single-wall carbon nanohorn aggregates hybridized with carbon nanocapsules by laser vaporization

Single-wall carbon nanohorn (SWNH) aggregates hybridized with carbon nanocapsules (CNCs) were fabricated at a high yield (∼70%). The carbon was laser-vaporized for 2 s into an Ar gas atmosphere with one of the following: Fe, Al, Si, Co, Ni, Cu, Ag, La₂O₃, Y₂O₃, and G₂O₃. By optimizing the Ar gas pressure and metal content, we were able to produce hybridized SWNH structures for Fe, Co, Ni, Cu, and Ag. Possible mechanisms for governing hybrid production, which occurs with smaller CNCs (<100 nm) with only certain metals and carbide, are discussed on the basis of thermal and catalytic graphitization. PACS 61.46.Df; 68.37.Lp; 81.16.Mk     

Fabrication of completely filled carbon nanotubes with copper nanowires in a confined space

Carbon nanotubes (CNTs) filled completely with polycrystalline Cu nanowires were synthesized by laser vaporization of Cu and graphite under high-pressure Ar gas atmosphere. Depending on the Ar gas pressure (0.1–0.9 MPa) and the Cu content (1–40 at.%) in graphite targets for laser vaporization, various products with different morphologies were observed by scanning and transmission electron microscopy. The ratios of the Cu-filled CNTs and carbon nanocapsules particularly increased as Ar gas pressure was increased. The maximum ∼60% fraction of Cu-filled CNTs with outer diameter of 10–50 nm and length of 0.3–3 μm was achieved at 0.9 MPa from graphite containing 20 at.% Cu. Most of the encapsulated Cu-nanowires were surrounded by single, double, or triple graphitic layers. Although the yield of the Cu-filled CNTs was also dependent on the Cu content in the graphite targets, no unfilled CNTs were produced even for low Cu content. The growth of Cu-filled CNTs is explained by the formation of molten Cu–C composite particles with an unusually C-rich composition in a space confined by high-pressure Ar gas, followed by precipitating Cu and C from the particles and subjecting them to phase separation.     

Effect of silicon on the formation of graphitic polyhedra and balloon-like particles

We conducted laser vaporization of graphite and graphite containing 1 at. % silicon in Ar gas atmosphere. Comparison of the products from the laser vaporization indicated that the coexistence of Si promoted graphitization in grown carbon particles of 90–1500 nm. Polyhedral graphite (PG) particles and balloon-like carbon (BC) particles with shells of graphitic layers were grown under control of Ar gas pressures of 0.1–0.7 MPa. We discuss possible roles of Si in graphitic structure growth and the formation mechanisms of the PG and BC particles. PACS 81.16.Mk; 61.46.Df; 68.37.Lp     

Effect of adding W to Fe catalyst on the synthesis of SWCNTs by arc discharge

Combining iron (Fe) and tungsten (W) as a bimetallic catalyst, we synthesized high-yield single-wall carbon nanotubes (SWCNTs) of narrow diameter distribution by a hydrogen–argon arc discharge method. Raman spectra indicate that the diameters of SWCNTs prepared using the Fe–W catalysts are about 0.5 nm smaller than those using Fe catalyst alone. The transmission electron microscopy and X-ray diffraction studies show that the SWCNTs prepared by the bimetallic catalyst coexist with few graphite flakes and other amorphous carbon. At the W content of 2–4 at%, tungsten cannot be found in the SWCNT samples. Thus by using a simple two-step purification process, high-purity SWCNT samples can be obtained. We have demonstrated the growth mechanism for the high melting metal (such as W, Mo)–Fe catalyst synthesis of SWCNTs by the arc discharge method.     

The effect of Fe and Ni catalysts on the growth of multiwalled carbon nanotubes using chemical vapor deposition

The effect of Fe and Ni catalysts on the synthesis of carbon nanotubes (CNTs) using atmospheric pressure chemical vapor deposition (APCVD) was investigated. Field emission scanning electron microscopy (FESEM) analysis suggests that the samples grow through a tip growth mechanism. High-resolution transmission electron microscopy (HRTEM) measurements show multiwalled carbon nanotubes (MWCNTs) with bamboo structure for Ni catalyst while iron filled straight tubes were obtained with the Fe catalyst. The X-ray diffraction (XRD) pattern indicates that nanotubes are graphitic in nature and there is no trace of carbide phases in both the cases. Low frequency Raman analysis of the bamboo-like and filled CNTs confirms the presence of radial breathing modes (RBM). The degree of graphitization of CNTs synthesized from Fe catalyst is higher than that from Ni catalyst as demonstrated by the high frequency Raman analysis. Simple models for the growth of bamboo-like and tubular catalyst filled nanotubes are proposed.     

Effects of acid treatment duration and sulfuric acid molarity on purification of multi-walled carbon nanotubes

Multi-walled carbon nanotubes were synthesized using a Fe-Ni bimetallic catalyst supported by MgO using thermal chemical vapor deposition. Purification processes to remove unwanted carbon structures and other metallic impurities were carried out by boiling in sulfuric acid solution. Various analytical techniques such as TGA/DSC, Raman spectroscopy, SEM, HRTEM and EDAX were employed to investigate the morphology, graphitization and quality of the carbon nanotubes. The obtained results reveal the molarity of sulfuric acid and immersed time of the carbon nanotubes in the acid solution is very effective at purifying multi-walled carbon nanotubes. It was also found that 5 M concentration of boiling sulfuric acid for a 3 h treatment duration led to the highest removal of the impurities with the least destructive effect. Moreover, it was observed that acid treatment results in decreasing of CNTs’ diameter.     

Synthesis of worm-shaped carbon nanofibers over a sodium chloride support

Worm-shaped carbon nanofibers (WCNFs) were synthesized in bulk by chemical vapour deposition at 680 °C using iron carboxylate as catalyst precursors and sodium chloride as catalyst support. The products were characterized by scanning electron microscopy, transmission electron microscopy, Raman spectroscopy and X-ray diffraction method. The purity of the purified products was determined by thermal analysis. The WCNF yield was 6700% relative to catalyst. The simplicity, environmental friendliness and use of easily available low-cost precursors are the advantage of this synthesis technique.     

Formation of single-wall carbon nanotubes in Ar and nitrogen gas atmosphere by using laser furnace technique

The formation of single-wall carbon nanotubes (SWNTs) by using laser vaporization technique in different ambient gas atmosphere was investigated. SWNTs were prepared with Rh/Pd (1.2/1.2 atom%)-carbon composite rod in Ar and nitrogen gas atmosphere, respectively. Raman spectra of raw carbon materials including SWNTs and photoluminescence mapping of dispersed SWNTs in a surfactant solution demonstrate that the diameter distribution of SWNTs prepared in Ar atmosphere is narrower than those obtained by using CVD technique (e.g. HiPco nanotube), even when the ambient temperature is as high as 1150 ^○C. It was also found that nitrogen atmosphere gives wider diameter distribution of SWNTs than that obtained with Ar atmosphere. Furthermore, the relative yield of fullerenes (obtained as byproducts) is investigated by using HPLC (high-performance liquid chromatography) technique. It was found that the relative yield of higher fullerenes becomes lower, when nitrogen is used as an ambient gas atmosphere. Based on these experimental findings, a plausible formation mechanism of SWNTs is discussed.     

Synthesis of the vertically aligned carbon hexagonal nanoprism arrays and their application for field emission

A simple approach is demonstrated for effectively growing large-area vertically aligned carbon hexagonal nanoprism arrays on molybdenum substrates by the catalyst-assisted pulsed laser deposition techniques. The carbon hexagonal nanoprisms have uniform shape and length, almost aligned vertically on the substrate, and the average diameters are about 30 nm. The internal angles of the nanoprisms present 60°. The vertically aligned nanorods have also been obtained for a comparison in the presence of catalyst Fe. The sample with vertically aligned carbon hexagonal nanoprism arrays exhibits better field emission behaviors than that with aligned carbon nanorod arrays.     

Fabrication of carbon nanoflowers by plasma-enhanced chemical vapor deposition

Two and three-dimensional (2D and 3D) carbon nanoflowers have been prepared on silicon (1 1 1) substrates by plasma-enhanced chemical vapor deposition, using CH₄, H₂ and Ar as reactive gases in the presence of Fe catalyst. The flower patterns are controlled by the flux ratio of the carrier gas, the reaction pressure and the growth temperature. Through observation by scanning electron microscopy, we find that the 2D carbon nanoflowers are formed by various nanoleaves while the 3D flowers are composed of hundreds of nanofibers. The former is related closely to the flux ratio of gas and the reaction pressure, while the latter depended mainly on the growth temperature. The nucleation and growth process of the nanoflowers seem to be a vapor/liquid/solid mechanism.     

离子束辅助沉积对类金刚石膜结构影响的计算机模拟

采用分子动力学(MD)模拟研究了离子束辅助沉积(1BAD)生长类金刚石(DLC)膜的物理过程.分 别选C₂分子和Ar离子作为沉积源和辅助沉积粒子.改变Ar的入射能量和到达比(A r／C)，研 究了它对DLC膜结构的影响.重点讨论了Ar辅助沉积引起表面原子的瞬间活性变化对薄膜结构 产生的影响.分析表明，由于Ar离子的轰击引起的能量和动量的传递，大大地增强了C原子在 表面的反冲动能及迁移概率，增加了合成薄膜的SP³键含量.研究结果和实验 观察一致，并从合成机理上给出了一些定量解释. 关键词： 类金刚石膜 离子束辅助沉积 分子动力学模拟     

Growth of single-walled carbon nanotubes from size-selected catalytic metal particles

Single-walled carbon nanotubes (SWNTs) were synthesized using size-controlled catalyst nanoparticles created by the pulsed laser ablation method. Specifically, the alloy particles (Co/Mo or Co/Pt) were prepared by ablation of the target alloy materials in an inert gas atmosphere. Size selection was performed using a differential mobility analyzer (DMA). The obtained nanoparticles were deposited on a quartz substrate from which SWNTs were grown by the alcohol catalytic CVD (ACCVD) technique that was developed by the authors group. AFM and Raman scattering analysis revealed that SWNTs were successfully synthesized. It seems the Co/Mo alloy catalyst was more effective for the synthesis of SWNTs than the Co/Pt catalyst, though this is a preliminary result to be further investigated. PACS 36.40.-c; 61.46.+w; 65.80.+n; 78.30.Na; 81.07.de     

CVD synthesis and purification of single-walled carbon nanotubes on aerogel-supported catalyst

Single-walled carbon nanotubes (SWNTs) were synthesized by disproportionation of carbon monoxide on an aerogel-supported Fe/Mo catalyst. A simple acidic treatment followed by an oxidation process produced a high purity (>99%) of SWNTs. The nanotubes obtained are bundled SWNTs and free of amorphous-carbon coating. Several factors that affect the yield and the quality of the SWNTs were also studied. This method shows great promise for large-scale production of SWNTs. Received: 30 August 2001 / Accepted: 3 December 2001 / Published online: 4 March 2002     

Effect of metal oxide and oxygen on the growth of single-walled carbon nanotubes by electric arc discharge

The effect of oxygen on the growth of single-walled carbon nanotubes was studied with Ni–Co alloy powder as catalyst under helium atmosphere of 500 Torr by electric arc discharge. The oxygen included in nickel or (and) cobalt oxides was added in catalyst. The content of oxygen in atmosphere was controlled by changing vacuum degree inside furnace before inputting buffer gas. The examinations of TEM and Raman scattering showed that oxygen in metal oxide as catalyst promotes the nucleation of SWCNT by taking effect on the metal catalyst particles. However, O₂ in atmosphere has the role of oxidizing amorphous particles along with nanotubes. When its molar proportion is higher than 0.22 ppm (Parts per million), the carbon nanotubes produced are oxidized and their purity decreases. The diameter of single-walled carbon nanotube obtained under different condition has a narrow distribution around 1.28 nm.     

The synthesis of multi-walled carbon nanotubes (MWNTs) by catalytic pyrolysis of the phenol-formaldehyde resins

A series of carbon nanomaterials, particularly multi-walled carbon nanotubes (MWNT), are obtained as products from catalytic pyrolysis of the cross-linked phenol-formaldehyde resins with different ferrocene under inert atmosphere. The morphology and structure of the samples were evaluated by TEM and XRD techniques. CNTs morphology is dependent on the iron nanoparticles and their forms (Fe, Fe₃C) resulted from ferrocene decomposition. The amount of nanotubes increases with iron content released from ferrocene catalyst during the pyrolysis process. Fe₃C nanoparticles drive the nucleation and the growth of carbon nanotubes during the pyrolysis process. Long (up to microns) well-defined MWNTs with small defects, ropes and disordered carbon are representatives in the pyrolyzed resins composition.     

Vertically aligned carbon nanotubes from natural precursors by spray pyrolysis method and their field electron emission properties

Vertically aligned carbon nanotubes have been synthesized from botanical hydrocarbons: Turpentine oil and Eucalyptus oil on Si(100) substrate using Fe catalyst by simple spray pyrolysis method at 700°C and at atmospheric pressure. The as-grown carbon nanotubes were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), thermogravimetric analysis (TGA), differential thermal analysis (DTA), and Raman spectroscopy. It was observed that nanotubes grown from turpentine oil have better degree of graphitization and field emission performance than eucalyptus oil grown carbon nanotubes. The turpentine oil and eucalyptus oil grown carbon nanotubes indicated that the turn-on field of about 1.7 and 1.93 V/μm, respectively, at 10 μA/cm². The threshold field was observed to be about 2.13 and 2.9 V/μm at 1 mA/cm² of nanotubes grown from turpentine oil and eucalyptus oil respectively. Moreover, turpentine oil grown carbon nanotubes show higher current density in relative to eucalyptus oil grown carbon nanotubes. The maximum current density of 15.3 mA/cm² was obtained for ∼3 V/μm corresponding to the nanotubes grown from turpentine oil. The improved field emission performance was attributed to the enhanced crystallinity, fewer defects, and greater length of turpentine oil grown carbon nanotubes.     

Causes of different catalytic activities of metals in formation of single-wall carbon nanotubes

When single-wall carbon nanotubes (SWNTs) were formed by pulsed Nd:YAG laser ablation or arc discharge, the yield depended on the metal catalyst: NiCo> Ni∼NiFe≫Co∼Fe>Pd∼Pt. It appears that an effective catalyst for SWNT growth must satisfy three conditions: it must be a good graphitization catalyst, have low solubility in carbon, and have a stable crystallographic orientation on graphite. NiCo, Ni, and NiFe satisfy these three conditions. The poor catalytic activities of Co, Fe, Pd, and Pt for SWNT formation would be explained by the ineffectiveness of Pt and Pd as graphitization catalysts, crystallographic orientation instability of Co crystals on graphite, and high solubility of Fe in graphite. Received: 29 October 2001 / Accepted: 7 November 2001 / Published online: 23 January 2002     

金镍复合膜上碳纳米管的定向生长及生长过程中金的作用

以金镍复合膜作催化剂，在96%的高氢气浓度下实现了碳纳米管的定向生长，并对其生长过 程进行了深入探讨.结果表明，高氢气浓度下碳纳米管生长的实现与本实验所选用的催化剂 ——金镍复合膜有密切关系.催化剂中金的参与，促进了碳在催化剂中的扩散，提高了碳在 催化剂中的活度.与催化剂中没有金的情况相比较，金的参与有利于镍吸收气氛中的碳，从 而使镍更容易达到碳饱和，有利于在高的氢气浓度下实现碳纳米管的定向生长. 关键词： 金镍复合膜 高氢气浓度 原子氢 碳活度     

Controlling the size and the activity of Fe particles for synthesis of carbon nanotubes

The properties of carbon nanotubes (CNTs) are controlled by their structure and morphology. Therefore, their selective synthesis, using catalytic chemical vapor deposition, requires precise control of a number of parameters including the size and activity of the catalyst nanoparticles. Previously, an environmental scanning transmission electron microscope (ESTEM) has been used to demonstrate that electron beam-induced decomposition (EBID) of Fe containing precursor molecules can be used to selectively deposit Fe catalyst nanoparticles that are active for CNT growth. We have extended these in situ ESTEM observations to further our understanding of the EBID parameters, such as deposition time, and substrate temperature, that control the size and placement of Fe catalyst particles for two precursors, namely diiron nonacarbonyl (Fe(2)(CO)(9)) and ferrocene (Fe(C(5)H(5))(2)). We found that the diameter of deposited particles increased with increasing deposition time. Electron energy-loss spectra, collected during deposition, show the incorporation of C in the Fe particles. The C content decreased as the substrate temperature was increased and was negligible at 100°C for Fe(2)(CO)(9). However, C and Fe were co-deposited at all temperatures (up to 450°C) when Fe(C(5)H(5))(2) was used as an iron source. After deposition, the substrate was heated to the CNT growth temperature in flowing hydrogen to remove the co-deposited C, which was an important step to activate the deposited Fe catalyst for the growth using acetylene. Our measurements revealed that the Fe nanoparticles fabricated from Fe(2)(CO)(9) had higher activity for CNT growth compared to the ones fabricated using Fe(C(5)H(5))(2). We also found that the co-deposited carbon could not be removed by heating in hydrogen in the case of Fe(C(5)H(5))(2). The particles deposited from Fe(C(5)H(5))(2) at 300°C to 450°C formed a core-shell structure with Fe surrounded by graphitic carbon. We speculate that the reduced activity for Fe(C(5)H(5))(2) is due to the C content in the deposit.     

A shadowed off-axis production of Ge nanoparticles in Ar gas atmosphere by pulsed laser deposition

In this work, we report on the production of Ge nanoparticles (NPs) in an inert Ar gas atmosphere by pulsed laser deposition (PLD) at room temperature (RT). The direct deposition of energetic particles/droplets resulting from the ablation process of the target material has been avoided by using an original and customized off-axis shadow mask (shadowed off-axis) deposition set-up where the NPs deposition on the substrate takes place by means of scattering between the NPs formed in the vapor phase and the background Ar atoms. It is found that the Ar gas pressure parameter has a relevant role in the crystallization process, with better crystallinity obtained as the background Ar pressure is raised for the given experimental conditions.