Low temperature growth of SWNTs on a nickel catalyst by thermal chemical vapor deposition

Single-walled carbon nanotubes (SWNTs) have been grown on a silica-supported monometallic nickel (Ni) catalyst at temperatures ranging from as low as 450 °C to 800 °C. Different spectroscopic techniques, such as Raman, photoluminescence emission (PLE), and ultra violet-visible-near infrared (UV-vis-NIR) absorption spectroscopy were used to evaluate the diameter and quality of the SWNTs grown over the Ni catalyst at different temperatures. The analysis revealed that high quality SWNTs with a very narrow diameter distribution were obtained at a growth temperature of 500 °C. In the PLE and absorption spectra, differences were observed between the SWNTs grown on Ni and those grown on cobalt (Co). This result expands the potential of growing a specific (n, m) tube species with relatively high abundance by tuning the catalyst composition. Furthermore, the prerequisites for the low temperature growth of SWNTs over a monometallic transition metal catalyst have been elucidated.      

Size engineering of metal nanoparticles to diameter-specified growth of single-walled carbon nanotubes with horizontal alignment on quartz

The electronic, physical and optical properties of single-walled carbon nanotubes (SWNTs) are governed by their diameter and chirality, and thus much research has been focused on controlling the diameter and chirality of SWNTs. To date, control of the catalyst particle size has been thought to be one of the most promising approaches to control the diameter or chirality of SWNTs owing to the correlation between catalyst particle size and tube diameter.In this study, we demonstrate the size engineering of catalytic nanoparticles for the controlled growth of diameter-specified and horizontally aligned SWNTs on quartz substrates. Uniformly sized iron nanoparticles derived from ferritin molecules were used as a catalyst, and their size was intentionally decreased via thermal heat treatment at 900?°C under atmospheric Ar ambient. ST-cut quartz wafers were used as growth substrates in order to elucidate the effect of the size of the nanoparticles on the tube diameter and the effect of catalyst size on the degree of parallel alignment on the quartz substrates. SWNTs grown by chemical vapor deposition using methane as feedstock exhibited a high degree of horizontal alignment when the particle density was low enough to produce individual SWNTs without bundling. Annealing for 60?min at 900?°C produced a reduction of nanoparticle diameter from 2.6 to 1.8?nm and a decrease in the mean tube diameter from 1.2 to 0.8?nm, respectively. Raman spectroscopy results corroborated the observation that prolonged heat treatment of nanoparticles yields thinner tubes with narrower size distributions. The results of this work suggest that straightforward thermal annealing can be a facile way to obtain uniform-sized SWNTs as well as catalytic nanoparticles.     

乙二醇法制备负载型钴基-堇青石催化剂及其催化燃烧甲苯的活性

用乙二醇法和浸渍法制备两种负载在堇青石上的钴基负载催化剂,并用XRD、XPS、TPR和TPD对制备的催化剂进行表征,比较两种催化剂催化氧化(燃烧)甲苯的活性。研究结果表明,与浸渍法制备的Co/cord催化剂相比,乙二醇法制备的催化剂Co/cord-EG对甲苯催化燃烧活性明显提高。应用乙二醇法制备Co/cord-EG负载型催化剂,Co元素主要以Co2+存在于载体表面;而传统浸渍法制备的Co/cord催化剂,Co元素是以Co2+与Co3+的两种化学状态存在于载体表面。应用乙二醇法制备Co/cord-EG负载型催化剂,有助于在载体表面形成比较均匀的吸附活性位。应用乙二醇法制备的Co/cord-EG催化剂,催化活性组分能在载体表面上以更小颗粒而且更高的分散度存在,从而能明显提高其催化活性。     

Controllable preparation and properties of single-/double-walled carbon nanotubes

In this paper, we discussed recent studies done in our laboratories with a floating catalyst chemical vapor deposition (CVD) method. We can grow single- or double-walled carbon nanotubes (SWNTs/DWNTs) with different kinds of catalysts. Single-walled carbon nanotubes without amorphous carbon coating were prepared by thermally decomposing acetylene (C₂H₂) at the temperature range of 750–1200 °C with ferrocene as catalyst. While with sulfur promoted ferrocene catalyst, double-walled carbon nanotubes were mass-produced by pyrolizing C₂H₂ at the temperature range of 900–1100 °C. Furthermore, tunable growth of DWNTs with different diameter was achieved in our experiment. It is found that DWNTs produced at lower carbon partial pressure have much smaller inner tubes, even DWNTs with the smallest inner diameter of 0.4 nm was found in here. As convenient and effective tool, radial breathing mode (RBM) of Raman scattering technique can be used to distinguish SWNTs from DWNTs. In further studies of Raman scattering with DWNTs, the possible match of the inner tubes and the outer tubes according to the RBM bands was assigned, and different chirality types were discussed according to the diameter and chirality dependence of resonant Raman vibration. We also investigated the temperature-dependent frequency shift of resonant Raman spectra of DWNTs in the range of 78–650 K. We found that different RBM peaks, which are relative to different tube diameters, have different temperature coefficient of frequency shift, and the larger diameter carbon nanotubes have more RBM frequency downshift with increasing temperature. It is ascribed to the RBM frequency variation to the temperature dependence of the stretching force constant of C–C bond. Besides, Polarized Raman spectra were preformed on well-aligned SWNTs structure fabricated through post-growth method and found that the angular dependence of Raman intensity is consistent well with the predictions of the resonance Raman theory.     

A simple method of producing single-walled carbon nanotubes from a natural precursor: Eucalyptus oil

Single-walled carbon nanotubes (SWNTs) have been synthesized by catalytic decomposition of eucalyptus oil, on a high silica-zeolite support impregnated with Fe/Co catalyst at 850 °C by the spray pyrolysis method. Catalyst with 5 wt.% (molar ratio of Co:Fe = 1:1), impregnated in zeolite was suitable for effective formation of carbon nanotubes (CNTs). As-grown CNTs were characterized by SEM, TEM and Raman spectroscopy. Raman spectroscopy reveals that as-grown CNTs are well graphitized. Raman spectroscopy also reveals that the as-prepared SWNTs have a diameter of about 0.79-1.71 nm.     

Direct observation of single-walled carbon nanotube growth at the atomistic scale

The growth dynamics of a single-walled carbon nanotube (SWNT) is observed in real-time using an in situ ultrahigh vacuum transmission electron microscope at 650 degrees C. SWNTs preferentially grow on smaller sized catalyst particles (diameter 相似文献   

Moderate temperature synthesis of single-walled carbon nanotubes on alumina supported nickel oxide catalyst

A simple nickel oxide catalyst has been developed in synthesizing single-walled carbon nanotubes (SWNTs) at moderate temperature. The catalyst used in the experiment was without a preceding reduction in hydrogen flow. The synthesis of SWNTs was performed at a temperature of 700 °C, which represents a moderate reaction temperature. The presence of SWNTs on the catalyst was confirmed by transmission electron microscope (TEM) and Raman spectroscope. The Raman spectrum shows a strong intensity at the radial breathing mode, indicating that the occurrence of SWNTs was dominant. Raman data further reveals that the synthesized SWNTs had the diameters in the range from 0.58 to 2.02 nm.     

Optical Absorption of Single‐Wall Carbon Nanotubes Produced by Arc‐Discharge Method with Different Concentration of Ni/Co Catalyst

Abstract

Material containing single‐wall carbon nanotubes (SWNTs) was obtained by arc‐discharge method. The optical absorption spectra were measured. Several narrow absorption bands were observed in a range from 0.4 to 3 eV caused by optical transitions between van Hove singularities. Comparing the results of “tight‐binding” electronic structure calculation for SWNT with the experimental spectra we found that each feature of spectrum can be attributed to interband transition in SWNTs. We have also found that the total yield of SWNTs and their diameter distribution depend on the catalyst content.     

超临界干燥法制备催化剂合成单壁纳米碳管及表征

采用超临界干燥法制备了活性高、比表面大的Fe/Mo/Al₂O₃催化剂.通过超临界干燥,催化剂的比表面由294m²/g提高到401m²/g.在1000℃下用该催化剂催化裂解甲烷, 合成了大产量、高质量的单壁纳米碳管(SWNTs).利用SEM、TEM、HRTEM、TGA 和Raman等手段对所制备的SWNTs进行了表征.结果表明:超临界法制备催化剂合成的粗产品中SWNTs含量在30%以上,大大高于同一配方催化剂采用常规干燥法的产率(约2%); SWNTs的管径分布在0.8-1.0nm之间,其形态以束状为主.     

Temperature-mediated growth of single-walled carbon-nanotube intramolecular junctions

Single-walled carbon nanotubes (SWNTs) possess superior electronic and physical properties that make them ideal candidates for making next-generation electronic circuits that break the size limitation of current silicon-based technology. The first critical step in making a full SWNT electronic circuit is to make SWNT intramolecular junctions in a controlled manner. Although SWNT intramolecular junctions have been grown by several methods, they only grew inadvertently in most cases. Here, we report well-controlled temperature-mediated growth of intramolecular junctions in SWNTs. Specifically, by changing the temperature during growth, we found that SWNTs systematically form intramolecular junctions. This was achieved by a consistent variation in the SWNT diameter and chirality with changing growth temperature even though the catalyst particles remained the same. These findings provide a potential approach for growing SWNT intramolecular junctions at desired locations, sizes and orientations, which are important for making SWNT electronic circuits.     

Synthesis-condition dependence of carbon nanotube growth by alcohol catalytic chemical vapor deposition method

We report the dependence of growth yield of single-walled carbon nanotubes (SWNTs) on heat-treatment time and catalyst film thickness by the alcohol catalytic chemical vapor deposition method. Three types of heat-treatment, synthesis of 30 min, synthesis of 30 min after annealing of 30 min, and synthesis of 60 min, were investigated. Thickness of Co catalyst film was varied from 1 to 10 nm. In the case of thinner Co film less than 3 nm, long synthesis time of 60 min is favorable for the effective SWNT growth, because of the small amount of Co catalyst. In the case of thicker Co film more than 3 nm, an amount of grown SWNTs by 30 min synthesis after 30 min annealing and by 60 min synthesis was much higher than that by 30 min synthesis without annealing, showing that total heat-treatment time of 60 min is important for the SWNT growth. Results suggest that the conversion from the thicker film of Co to nano-particle which acts as catalyst takes place during the first 30 min.     

Diameter and Density Control of Single‐Walled Carbon Nanotube Forests by Modulating Ostwald Ripening through Decoupling the Catalyst Formation and Growth Processes

A continuous and wide range control of the diameter (1.9?3.2 nm) and density (0.03?0.11 g cm^?3) of single‐walled carbon nanotube (SWNT) forests is demonstrated by decoupling the catalyst formation and SWNT growth processes. Specifically, by managing the catalyst formation temperature and H₂ exposure, the redistribution of the Fe catalyst thin film into nanoparticles is controlled while a fixed growth condition preserved the growth yield. The diameter and density are inversely correlated, where low/high density forests would consist of large/small diameter SWNTs, which is proposed as a general rule for the structural control of SWNT forests. The catalyst formation process is modeled by considering the competing processes, Ostwald ripening, and subsurface diffusion, where the dominant mechanism is found to be Ostwald ripening. Specifically, H₂ exposure increases catalyst surface energy and decreases diameter, while increased temperature leads to increased diffusion on the surface and an increase in diameter.     

Diameter‐ and Metallicity‐Selective Enrichment of Single‐Walled Carbon Nanotubes Using Polymethacrylates with Pendant Aromatic Functional Groups

Current methods for the synthesis of single‐walled nanotubes (SWNTs) produce mixtures of semiconducting (sem‐) and metallic (met‐) nanotubes. Most approaches to the chemical separation of sem‐/met‐SWNTs are based on small neutral molecules or conjugated aromatic polymers, which characteristically have low separation/dispersion efficiencies or present difficulties in the postseparation removal of the polymer so that the resulting field‐effect transistors (FETs) have poor performance. In this Full Paper, the use of three polymethacrylates with different pendant aromatic functional groups to separate cobalt–molybdenum catalyst (CoMoCAT) SWNTs according to their metallicity and diameters is reported. UV/Vis/NIR spectroscopy indicates that poly(methyl‐methacrylate‐co‐fluorescein‐o‐acrylate) (PMMAFA) and poly(9‐anthracenylmethyl‐methacrylate) (PAMMA) preferentially disperse semiconducting SWNTs while poly(2‐naphthylmethacrylate) (PNMA) preferentially disperses metallic SWNTs, all in dimethylforamide (DMF). Photoluminescence excitation (PLE) spectroscopy indicates that all three polymers preferentially disperse smaller‐diameter SWNTs, particularly those of (6,5) chirality, in DMF. When chloroform is used instead of DMF, the larger‐diameter SWNTs (8,4) and (7,6) are instead selected by PNMA. The solvent effects suggest that diameter selectivity and change of polymer conformation is probably responsible. Change of the polymer fluorescence upon interaction with SWNTs indicates that metallicity selectivity presumably results from the photon‐induced dipole–dipole interaction between polymeric chromophore and SWNTs. Thin‐film FET devices using semiconductor‐enriched solution with PMMAFA have been successfully fabricated and the device performance confirms the sem‐SWNTs enrichment with a highly reproducible on/off ratio of about 10³.     

Effect of Co and Ni nanoparticles formation on carbon nanotubes growth via PECVD

The effect of cobalt (Co) and nickel (Ni) nanoparticle catalysts on the growth of carbon nanotubes (CNTs) were studied, where the CNTs were vertically grown by plasma enhanced chemical vapour deposition (PECVD) method. The growth conditions were fixed at a temperature of 700 °C with a pressure of 1000 mTorr for 40 minutes with various thicknesses of sputtered metal catalysts. Only multi-walled carbon nanotubes are present from the growth as large average diameter of outer tube (～10–30 nm) were measured for both of the catalysts used. Experimental results show that high density of CNTs was observed especially towards thicker catalysts layers where larger and thicker nanotubes were formed. The nucleation of the catalyst with various thicknesses was also studied as the absorption of the carbon feedstock is dependent on the initial size of the catalyst island. The average diameter of particle size increases from 4 to 10 nm for Co and Ni catalysts. A linear relationship is shown between the nanoparticle size and the diameter of tubes with catalyst thicknesses for both catalysts. The average growth rate of Co catalyst is about 1.5 times higher than Ni catalyst, which indicates that Co catalyst has a better role in growing CNTs with thinner catalyst layer. It is found that Co yields higher growth rate, bigger diameter of nanotube and thicker wall as compared to Ni catalyst. However, variation in Co and Ni catalysts thicknesses did not influence the quality of CNTs grown, as only minor variation in I_G/I_D ratio from Raman spectra analysis. The study reveals that the catalysts thickness strongly affects not only nanotube diameter and growth rate but also morphology of the nanoparticles formed during the process without influencing the quality of CNTs.     

Low temperature synthesis of single-walled carbon nanotubes in an inductively coupled plasma chemical vapor deposition system

In this work, we present a parametric study on the low temperature synthesis of single-walled carbon nanotubes (SWNTs) in an inductively coupled plasma (ICP) CVD system using dry bi-layered catalytic thin-films (Fe/Al and Ni/Al, deposited by electron-beam evaporation method) as the catalysts. With a low substrate temperature of 550 degrees C and above, SWNTs were successfully synthesized on both catalysts, as revealed from the characteristic peaks of SWNTs in the micro-Raman spectra. By the reduction of plasma power and the shortening of the process times, the lowest synthesis temperature of SWNTs achieved in our system was approached to 500 degrees C on Ni/Al catalysts; on the other hands, the lowest temperature for Fe/Al catalysts was 550 degrees C. Our results suggest that as compared with Fe/Al, Ni/Al is more favorable for plasma-enhanced CVD (PECVD) synthesis of SWNTs at low temperatures. This work can be used for further improvements and better understanding on the production processes of SWNTs by PECVD methods.     

Y/Ni催化剂中FeS的添加对SWNTs制备的影响

Y/Ni是电弧放电法制备单壁碳纳米管(SWNTs)时常用的催化剂.本文研究了FeS在Y/Ni催化剂中的添加对SWNTs制备的影响.结果发现FeS的添加不仅能增加网状产物的百分比,还能使网状产物中SWNTs的含量明显增多.这表明Y/Ni中FeS的添加能使更多的石墨转化为SWNTs.导致这一结果的原因可能有两个:(1)硫可能会扩大反应腔体中适合SWNTs生长的空间;(2)硫作为交联剂能促进碳元素的石墨化.     

Synthesis of carbon nanotubes using mesoporous Fe-MCM-41 catalysts

MWNTs (multi-walled carbon nanotubes) were made by catalytic CVD process using iron-containing mesoporous silica, Fe-MCM-41, with 4 mol% Fe loading prepared by direct synthesis route. Uniform 5 nm size Fe2O3 nano-particles impregnated onto a mesoporous silica support, SBA-15 were also prepared for CNTs synthesis. The catalysts were characterized using XRD, SEM/TEM, N2 physisorption, UV-vis diffuse reflectance and FT-IR spectroscopies. Acetylene gas was introduced as a carbon source, and the gas mixture of Ar:H2:C2H2 = 14:5:1 pyrolyzed at 750 degrees C for 60 min was found to be the optimum synthesis condition. Fe-MCM-41 due to higher dispersion of nano-sized Fe-species was efficient as catalyst for MWNTs with more uniform size distribution. Cobalt-impregnated Fe-MCM-41 (Co/Fe = 1) produced a small fraction of SWNTs of ca. 2 nm diameter mixed with MWNTs.     

Synthesis-condition dependence of carbon nanotube growth by alcohol catalytic chemical vapor deposition method

We report the dependence of growth yield of single-walled carbon nanotubes (SWNTs) on heat-treatment time and catalyst film thickness by the alcohol catalytic chemical vapor deposition method. Three types of heat-treatment, synthesis of 30 min, synthesis of 30 min after annealing of 30 min, and synthesis of 60 min, were investigated. Thickness of Co catalyst film was varied from 1 to 10 nm. In the case of thinner Co film less than 3 nm, long synthesis time of 60 min is favorable for the effective SWNT growth, because of the small amount of Co catalyst. In the case of thicker Co film more than 3 nm, an amount of grown SWNTs by 30 min synthesis after 30 min annealing and by 60 min synthesis was much higher than that by 30 min synthesis without annealing, showing that total heat-treatment time of 60 min is important for the SWNT growth. Results suggest that the conversion from the thicker film of Co to nano-particle which acts as catalyst takes place during the first 30 min.     

Raman-active modes of single-walled carbon nanotubes derived from the gas-phase decomposition of CO (HiPco process)

Here we report Raman scattering studies of ropes of Single-walled carbon nanotubes (SWNTs) grown by a high CO pressure process. Five samples from five different batches were studied as a function of excitation wavelength. Three of these samples exhibited Raman spectra similar to that found for SWNTs made by pulsed laser vaporization of arc-discharge methods. The other two samples were found by Raman scattering to contain a significant fraction of tubes with diameter < 1.0 nm. These samples exhibited unusual spectra that, however, can be well understood within the existing models for the electronic and phononic states in SWNTs. Spectra recorded with 1064 nm for the sample having a significant fraction of smaller diameter tubes shows strong modes present between 500 and 1200 cm-1. We suggest these modes arise due to the enhancement of Raman cross-section for small diameter tubes.     

En Route to High-Density Chiral Single-walled Carbon Nanotube Arrays using Solid Trojan Catalysts

Solid catalyst is widely recognized as an effective strategy to control the chirality of single-walled carbon nanotubes (SWNTs). However, it is still not compatible with high density in horizontal arrays. “Trojan” catalysts strategy is one of the most effective methods to realize SWNTs with high density and has great potential in chirality control. Here, the co-realization of high density and chirality controlling for SWNTs in a low-temperature growth process is reported based on the developed solid “Trojan” catalyst. High temperature “Trojan” catalyst formation process provides sufficient catalyst number to acquire high density. These liquid “Trojan” catalysts are cooled to solid state by adopting low growth temperature (540 °C), which can be good template to realize the chirality controlling of SWNTs with exposing six-fold symmetry face, (111). Finally, (9, 6) and (13, 1) SWNTs enriched horizontal array with the purity of ≈90% and density of 4 tubes µm⁻¹ is realized. The comparison between the distribution of initial catalysts and the density of as-grown tubes indicates no sacrificing on catalysts number to improve chirality selectivity. This work opens a new avenue on the catalyst's design and chirality controlling in SWNTs growth.