Growth Mechanism Of Nanotubes Research Articles

Mechanism of WS2 Nanotube Formation Revealed by in Situ/ex Situ Imaging.

Multiwall WS2 nanotubes have been synthesized from W18O49 nanowhiskers in substantial amounts for more than a decade. The established growth model is based on the "surface-inward" mechanism, whereby the high-temperature reaction with H2S starts on the nanowhisker surface, and the oxide-to-sulfide conversion progresses inward until hollow-core multiwall WS2 nanotubes are obtained. In the present work, an upgraded in situ SEM μReactor with H2 and H2S sources has been conceived to study the growth mechanism in detail. A hitherto undescribed growth mechanism, named "receding oxide core", which complements the "surface-inward" model, is observed and kinetically evaluated. Initially, the nanowhisker is passivated by several WS2 layers via the surface-inward reaction. At this point, the diffusion of H2S through the already existing outer layers becomes exceedingly sluggish, and the surface-inward reaction is slowed down appreciably. Subsequently, the tungsten suboxide core is anisotropically volatilized within the core close to its tips. The oxide vapors within the core lead to its partial out-diffusion, partially forming a cavity that expands with reaction time. Additionally, the oxide vapors react with the internalized H2S gas, forming fresh WS2 layers in the cavity of the nascent nanotube. The rate of the receding oxide core mode increases with temperatures above 900 °C. The growth of nanotubes in the atmospheric pressure flow reactor is carried out as well, showing that the proposed growth model (receding oxide core) is also relevant under regular reaction parameters. The current study comprehensively explains the WS2 nanotube growth mechanism, combining the known model with contemporary insight.

Experimental and theoretical aspects of the growth of vertically aligned CNTs by CCVD on AZO substrate

An efficient and reproducible growth of vertically aligned carbon nanotubes by CCVD requires accurate and specific setting of the synthesis parameters and the properties of catalyst thin layers. In this work, the growth of vertically aligned carbon nanotubes onto AZO (= aluminum doped zinc oxide) glass substrate covered by Al2O3 and Fe-Co catalyst layer system is presented. Investigation of the effect of catalyst composition and synthesis temperature on CVD growth revealed the optimum condition of the synthesis. The analysis of as-prepared samples by SEM, TEM and Raman spectroscopy was carried out to prove the structure and quality of carbon deposit. Theoretical considerations have supported speculative ideas about the role of the support layer, the transformation of the catalyst layer in the presence of hydrogen gas and the growth mechanism of carbon nanotubes. The mechanism of CNT growth is modelled and the order of magnitude of experimentally observed vertical linear growth rate of CNT (several nm/s) is reproduced.

A substrate constituent Na-catalyzed growth of carbon nanotubes on glass substrate by atmospheric pressure PECVD

A growth mechanism of carbon nanotubes (CNTs) without any externally added catalyst is reported in this study. Instead of any externally added catalyst, the constituents of the used soda lime glass substrates seem to play the pivotal role in the growth of CNTs. XPS analysis of the synthesized CNTs reveal that among the various constituents of soda lime glass, sodium (Na) emerges as the primary catalyst for initiating CNT growth. The adopted PECVD technique involving an especially designed spiral-shaped fused hollow cathode source for generating the required atmospheric pressure plasma makes such synthesis easily feasible. The important key factors influencing CNT growth include substrate temperature and plasma pre-treatment of the substrate. Plasma pre-treatment of the substrate at an elevated temperature leads to the formation of catalyst nanoparticles on the surface of the substrate. This report features a detailed CNT growth mechanism proposed on the basis of optical emission spectroscopy and XPS analyses. To be precise, this study demonstrates a novel Na-catalysed synthetic approach for achieving uniform CNT growth on glass substrates without using any externally added catalyst, facilitating the preparation of clean CNTs.

Review of: "The growth mechanism of nanotubes for the production of nanotransistors and nanochips"

Review of: "The growth mechanism of nanotubes for the production of nanotransistors and nanochips"

Mechanism of carbon nanotube growth in expanded graphite via catalytic pyrolysis reaction using carbores P as a carbon source.

Carbon nanotubes (CNTs) had potential applications in energy conversion and storage devices, and it could be prepared by expanded graphite loaded with catalyst at high temperature, however, the mechanism of carbon nanotube growth in expanded graphite need further confirmation. In this work, carbon nanotubes' in situ growth in expanded graphite (EG) were prepared via catalytic pyrolysis reaction using carbores P as a carbon source and Co(NO3)3•6H2O as a catalyst. The results of X-ray diffraction (XRD), scanning electron microscope (SEM) and energy dispersive X-ray spectroscope (EDS) indicated the carbon nanotubes could generate in, EG with the presence of carbores P as a carbon source and cobalt nitrate as a catalyst. More interestingly, the growth mechanism of carbon nanotubes could be concluded by the results of differential thermal analysis-thermogravimetry-mass spectrometry (DTA-TG-MS) and X-ray photoelectron spectroscopy (XPS) analysis. The pyrolysis products of carbores P were mainly hydrocarbon gas such as CH4 gas, which reacts with Co(NO3)3·6H2O catalyst to reduces CoOx to Co particles, then the carbon form pyrolysis was deposited the on the surface catalyst Co particles and, after continuous solid dissolution and precipitation, carbon nanotubes were at last generated in EG at last.

Low temperature multi-catalytic growth and growth mechanism of carbon nanotubes on carbon fiber surfaces

In order to reduce the etching effect of the catalysts to carbon fibers caused by high temperature during the chemical vapor deposition (CVD) process, four multi-element catalysts, Fe–Co, Fe–Ni, Co–Ni and Fe–Co–Ni, were used to realize the low temperature growth of carbon nanotubes (CNTs) on carbon fibers at 350 °C–400 °C. The results show that the growth state of CNTs has a great relationship with the type of catalysts. The catalytic efficiency of Fe–Co catalysts is low, but the graphitization degree of CNTs is relatively high. The Fe–Co–Ni catalysts has high catalytic efficiency but low graphitization degree of CNTs. The tensile strength of carbon fiber/CNTs reinforcements prepared by Fe–Ni catalysts at 400 °C is the highest, reaching 3.99 GPa, which is 11.14% higher than that of desized fiber. The melt drop phenomenon of the catalysts was found by TEM, indicating the formation of the liquid phase catalysts during the growth of CNTs. This phenomenon can change the diffusion mode of carbon atoms in the catalyst and significantly reduce the growth activation energy of CNTs, so that CNTs can grow at lower temperatures. Based on the detailed analysis of the CVD process, a low temperature growth model of CNTs on carbon fibers was proposed.

Catalytic carbon-bed sintering of CNTs/calcium aluminate cement and its effects on thermal mechanical properties of refractory castables

To improve the dispersion of carbon nanotubes in refractory castables and exert the strengthening effect of carbon nanotubes on castables, the core-shell CNTs/CAC (carbon nanotubes/calcium aluminate cement) was synthesized by the catalytic carbon-bed sintering method. The effects of catalyst content on the microstructure of CNTs/CAC powders were evaluated by Raman spectroscopy, SEM, and HRTEM. The results suggested that the optimal addition amount of catalyst was 0.75 wt%. The diameter of the bamboo-like carbon nanotubes was ∼50 nm and the length of the carbon nanotubes was ∼120 μm. Carbon nanotubes and small calcium aluminate particles were interspersed with each other and wrapped in a compact calcium aluminate shell with a thickness of 3∼5 μm. Moreover, the DFT calculation was employed to investigate the growth mechanism of carbon nanotubes. Electrons were transferred from Ni to H, O, and C atoms, increasing bond length and weakening the strength of C–H and CO bonds which was conducive to the growth of carbon nanotubes. The formation of the core-shell structure was in direct connection with the liquid-phase eutectic phase. Calcium carbonate and alumina reacted preferentially to form C12A7, and then C12A7 continued to react with alumina to form CA and CA2. With the increase of sintering temperature, a large amount of liquid phase was formed. Carbon nanotubes were wrapped by the liquid phase of calcium aluminate to form a core-shell structure. The CNTs/CAC cement bonded Al2O3–SiC castables had great mechanical properties and thermal shock resistance. Compared with Al2O3–SiC castables bonded with Secar71, the HMOR and thermal shock residual strength ratio of Al2O3–SiC castables bonded with CNTs/CAC cement were increased by 24.7% and 22.1%, respectively.

Study on potential gradient in Ti anodization

Porous oxides or nanotubes are obtained through the anodization of valve metal. However, the mechanism of nanotube growth remains unclear. Traditional field-assisted dissolution (FAD) theory has many limitations, such as its inability to explain the connotation of the three stages in the current–time curve. By placing a container between two electrodes, the ions move around the container in a ring in the present study. The potential gradient during anodization was innovatively changed. Finally, the current–time curve obtained during anodization using the new device is quite different from that obtained using conventional anodizing device. This phenomenon is explained by the electronic current and ionic current theory in this paper.

Metallocene-Filled Single-Walled Carbon Nanotube Hybrids.

In this paper, the growth mechanism, structure, growth processes, growth kinetics, and optical, vibronic and electronic properties of metallocene-filled single-walled carbon nanotubes (SWCNTs) are considered. A description of the procedures used to fill the nanotubes is provided. An investigation of doping effects on metallicity-mixed SWCNTs filled with metallocenes by Raman spectroscopy, near edge X-ray absorption fine structure spectroscopy, photoemission spectroscopy, and optical absorption spectroscopy is described. The studies of doping effects on metallicity-sorted SWCNTs filled with metallocenes are discussed. Doping effects in metallicity-mixed and sorted SWCNTs upon the chemical transformation of encapsulated molecules are analyzed. A discussion of the modification of the electronic properties of filled SWCNTs is presented. Applications of metallocene-filled SWCNTs in electrochemistry, thermoelectric power generation, chemical sensors, and magnetic recording are discussed.

Growth of Boron Nitride Nanotube Over Al‐Based Active Catalyst and its Application in Thermal Management

The effective identification of the active catalytic phase is essential to elucidate the growth mechanism of boron nitride nanotubes (BNNTs) and realize their controllable and scalable synthesis. However, owing to the complexity of chemical reactions during BNNT growth via chemical vapor deposition (CVD) and the lack of techniques for in situ characterization at high temperatures (1100–1300 °C), identifying the true catalyst during BNNT growth is challenging. Herein, an aluminum (Al)‐based active catalyst for BNNT growth via CVD is investigated. The initial Al2O3 nanoparticle catalyst precursor is transformed into an Al‐B phase prior to BNNT growth. Based on our density functional theory‐based molecular dynamic simulations of BNNT nucleation, AlB x (x = 1.5 to 2) shows catalytic activity for the formation of BN chains and BN six‐membered rings. Confirmatory experiments demonstrate that AlB2 is the active Al‐based catalyst during BNNT growth. A nanocomposite is prepared from cellulose nanocrystal, and purified BNNTs exhibited a high in‐plane thermal conductivity of 13.33 W m−1 K−1 at 20 wt% BNNTs. A further application for light‐emitting diode chip cooling demonstrates excellent heat‐dissipation performance of the nanocomposite film. Thus, this study can guide the controllable synthesis of high‐quality BNNTs and facilitate their use in thermal interface materials.

Growth mechanism of carbon nanotubes from Co-W-C alloy catalyst revealed by atmospheric environmental transmission electron microscopy.

High-melting point alloy catalysts have been reported to be effective for the structure-controlled growth of single-wall carbon nanotubes (SWCNTs). However, some fundamental issues remain unclear because of the complex catalytic growth environment. Here, we directly investigated the active catalytic phase of Co-W-C alloy catalyst, the growth kinetics of CNTs, and their interfacial dynamics using closed-cell environmental transmission electron microscopy at atmospheric pressure. The alloy catalyst was precisely identified as a cubic η-carbide phase that remained unchanged during the whole CNT growth process. Rotations of the catalyst nanoparticles during CNT growth were observed, implying a weak interfacial interaction and undefined orientation dependence for the solid catalyst. Theoretical calculations suggested that the growth kinetics are determined by the diffusion of carbon atoms on the surface of the η-carbide catalyst and through the interface of the catalyst-CNT wall.

Platinum supported on hydroxyl-grafted poly (3,4-ethylenedioxythiophene)-modified RuCo, N-tridoped bamboo-like carbon nanotubes for direct methanol fuel cell

The development of highly active and efficient heterogeneous catalytic oxidation system has become an attractive research field. In this paper, a catalyst (RuCo/N-CNT@PEDOT-OH/Pt) from platinum nanoparticles (Pt NPs) supported on hydroxyl-grafted poly(3,4-ethylenedioxythiophene) (PEDOT–OH)-modified RuCo, N-tridoped bamboo-like carbon nanotubes (RuCo/N-CNT) are used for direct methanol fuel cell (DMFC). The electrocatalytic activity of RuCo/N-CNT@PEDOT-OH/Pt is systematically compared with RuCo/N-CNT/Pt (Pt NPs supported on RuCo/N-CNT without PEDOT-OH) in the methanol oxidation reaction (MOR). The growth mechanism of carbon nanotubes and the role of heteroatom doping in the electrocatalytic process is explored. The catalysts show excellent electrocatalytic performance with high stability for MOR. It is found that the mass activity (MA) of the RuCo/N-CNT@PEDOT-OH/Pt (1961.3 mA mg−1Pt) for MOR was higher than that of RuCo/N-CNT/Pt (1470.1 mA mg−1Pt) and the commercial Pt/C catalysts (281.0 mA mg−1Pt), indicating the positive effect of the PEDOT-OH in the electrocatalytic MOR. In addition, density functional theory (DFT) calculations verify the possible mechanism pathways of the obtained RuCo/N-CNT@PEDOT-OH/Pt catalyst. This presented catalyst offers new inspiration for designing efficient electrocatalysts for methanol oxidation.

Growth modes of single-walled carbon nanotubes on catalysts.

Understanding the growth mechanism of single-walled carbon nanotubes (SWCNTs) and achieving selective growth requires insights into the catalyst structure-function relationship. Using an in situ aberration-corrected environmental transmission electron microscope, we reveal the effects of the state and structure of catalysts on the growth modes of SWCNTs. SWCNTs grown from molten catalysts via a vapor-liquid-solid process generally present similar diameters to those of the catalysts, indicating a size correlation between nanotubes and catalysts. However, SWCNTs grown from solid catalysts via a vapor-solid-solid process always have smaller diameters than the catalysts, namely, an independent relationship between their sizes. The diameter distribution of SWCNTs grown from crystalline Co7W6, which has a unique atomic arrangement, is discrete. In contrast, nanotubes obtained from crystalline Co are randomly dispersed. The different growth modes are linked to the distinct chiral selectivity of SWCNTs grown on intermetallic and monometallic catalysts. These findings will enable rational design of catalysts for chirality-controlled SWCNTs growth.

Nucleation of Single-Wall Carbon Nanotubes from Faceted Pt Catalyst Particles Revealed by in Situ Transmission Electron Microscopy.

Revealing the nucleation and growth mechanism of single-wall carbon nanotubes (SWCNTs) from faceted solid catalysts is crucial to the control of their structure and properties. However, due to the small size and complex growth environment, the early stages and dynamic process of SWCNT nucleation have rarely been directly revealed, especially under atmospheric conditions. Here, we report the atomic-resolved nucleation of SWCNTs from the faces of truncated octahedral Pt catalysts under atmospheric pressure using a transmission electron microscope equipped with a gas-cell. It was found that the graphene layers were initially formed preferentially on (111) surfaces, which then joined together to form an annular belt and a hemispherical cap, followed by the elongation of the SWCNT. Based on the observations, an annular belt assembly nucleation model and a possible chirality control mechanism are proposed for SWCNTs grown from well-faceted Pt catalysts, which provides useful guidance for the controlled synthesis of SWCNTs by catalyst design.

Aluminum oxide nanotubes fabricated via laser ablation process: Application as superhydrophobic surfaces

Laser-induced surface structuring is a promising technique for the novel formation of nanostructures that can effectively be used to modify the surface and synthesis of nanostructures. Metallic and semiconductor oxide nanotubes presently have been attracted in an extensive domain of materials that range from optoelectronic, gas sensor, catalyst, and superhydrophobic surface applications. In this work, aluminum oxide nanotubes were successfully fabricated by laser ablation processes of a 40-nm gold nanolayer coated on an aluminum surface plate inside the ethanol. A continuous-wave fiber laser with a power of 40 W was used to create Al2O3 nanotubes. The electron diffraction pattern analysis of these nanotubes demonstrated that the Al2O3 nanotubes had a single crystal and hexagonal crystal structure with the lattice parameter of a = 0.432 nm as well as zone axis of z=12¯13¯. These nanotubes had a highly-crystalline structure with about 50–200 nm diameters, 10–50 nm wall thicknesses, and 5–20 µm length as well as open-ended. In addition, aluminum oxide nanotubes structures indicated a near superhydrophobic behavior with about 155° ± 2° as a contact angle using experimental and simulation approaches. Meanwhile, molecular dynamics simulation was applied to investigate the wetting property of the aluminum surface and the aluminum surface containing Al2O3 nanoparticles and Al2O3 nanotubes. The effect of the Al2O3 nanotubes on wetting properties was considerable compared to the Al2O3 nanoparticles. The growth mechanism of Al2O3 nanotubes was proposed and illustrated in detail. Also, the obtained findings of this work can be applied to superhydrophobic surfaces in industrial production.

Morphological comparison and growth mechanism of TiO2 nanotubes in HBF4 and NH4F electrolytes

At present, the growth mechanism of TiO2 nanotubes in different electrolytes is still controversial. Field assisted dissolution (FAD) theory believes that fluoride ion is the main factor affecting the growth of TiO2 nanotubes. In this paper, we conducted a comparative experiment in HBF4 and NH4F electrolytes which containing the same fluoride ion molarity. By comparing the growth morphology of TiO2 nanotubes in two electrolytes, orderly nanotube arrays were obtained in NH4F electrolyte. However, only dense films and nanotube embryos were formed in HBF4 electrolyte. Obviously, FAD theory cannot explain these phenomena. Ionic current and electronic current theories and oxygen bubble model are reasonable for these phenomena. In the NH4F electrolyte, there are high electronic current and intense oxygen evolution, so the nanotube embryos cannot be seen. The low electronic current in the HBF4 electrolyte and the slow evolution of oxygen gas lead to the discovery of the nanotube embryos. This phenomenon also confirms that the formation process of TiO2 nanotubes is that anodic oxides grow upward around the oxygen bubble to form the nanotube walls.

Unraveling the six stages of the current-time curve and the bilayer nanotubes obtained by one-step anodization of Zr.

The application and growth mechanism of anodic TiO2 nanotubes have been a hot topic in the last ten years, but the formation mechanism of anodic ZrO2 nanotubes has rarely been studied. In one-step constant voltage anodization of Al and Ti, the typical current–time curve has three stages. Moreover, the current–time curves of the three stages can last for 10 min or even 10 hours, resulting in a single layer of nanotubes with the same diameter due to the constant voltage in one-step anodization. However, in this paper, it was found for the first time that the three stages of the current–time curve appeared twice in succession during one-step constant voltage anodization of Zr for only 900 seconds, and bilayer nanotubes with increased diameter were obtained. This six-stage current–time curve cannot be explained by classical field-assisted dissolution and field-assisted flow or stress-driven mechanisms. Here, the formation mechanism and growth kinetics of bilayer ZrO2 nanotubes have been clarified rationally by the theories of ionic current, electronic current and oxygen bubble mold. The interesting results presented in this paper are of great significance for revealing the anodizing process of various metals and the formation mechanism of porous structures.

The effect of atmospheric pressure on the growth rate of TiO2 nanotubes: Evidence against the field-assisted dissolution theory

The growth mechanism of anodic TiO2 nanotubes has been a hot topic in recent years. The classical field-assisted dissolution (FAD) theory holds that the atmospheric pressure does not affect the growth of nanotubes, while the oxygen bubble model holds that the decrease of pressure is beneficial to the nanotube growth. In order to verify the oxygen bubble model, anodizing processes at three different pressures (0.1 MPa, 0.05 MPa and 0.005 MPa), three different currents and three different NH4F concentrations were studied. To the best our knowledge, some interesting results are obtained which cannot be obtained at standard atmospheric pressure (1 atm = 0.1 MPa). (1) In the same NH4F electrolyte, the same current density and the same temperature, the reduction of anodizing atmospheric pressure can improve the growth rate of TiO2 nanotubes. (2) Under the condition of the same temperature and current density, with the increase of NH4F concentration (0.3 wt%, 0.4 wt% and 0.5 wt%), the porosity of the nanotubes increases, and the growth rate of the nanotubes decreases obviously. These findings can serve as counterevidence to the FAD theory.

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Spontaneous chiral self-assembly of CdSe@CdS nanorods

Hydrothermal synthesis of rutile TiO2 nanotubes film on Ti foil for photocatalytic degradation

One-dimensional TiO2 nanotubes film was synthesized on the Ti foil oxidized at 800℃ by hydrothermal method in a highly alkaline solution. The crystal structure, morphology and chemical state of the samples were characterized by X-ray diffraction, scanning electron microscope, transmission electron microscope and X-ray photoelectron spectroscopy. The result indicates that the TiO2 nanotubes film still remains rutile phase structure after hydrothermal treatment. The growth mechanism of TiO2 nanotubes is also discussed that includes dissolution, nucleation and growing process. The photocatalytic activity of TiO2 film is remarkably improved after hydrothermal reaction. Due to the one-dimensional tubular structure, TiO2 nanotubes film hydrothermally synthesized for 6 hours displays the highest photocatalytic degradation rate of methylene blue