Water-assisted Chemical Vapor Deposition Research Articles

Catalytic mechanism of Co-containing layered double hydroxide (Co-LDH) as a precursor in carbon nanotube (CNT) synthesis

Abstract The synthesis of carbon nanotubes (CNTs) by water-assisted chemical vapor deposition using Co-containing layered double hydroxides (Co-LDH) as catalyst precursors and the catalytic mechanism of Co-LDH catalyst precursors during CNT synthesis were investigated. Co-Al and Co-Mg-Al LDH were prepared by the hydrothermal or homogeneous precipitation method using urea. XPS analysis indicated an increase in Co0 content after a reduction process during CNT synthesis, and the TEM images show that metallic Co nanoparticles were formed while maintaining the LDH platelet-like crystal form. Consequently, these metal particles were found to act as the catalyst for the formation of CNTs. Finally, through control of the flow rate of C2H4 gas during CNT synthesis, the formation of single-walled CNTs with diameters of 4 nm or less was demonstrated.

Water-assisted controllable growth of atomically thin WTe2 nanoflakes by chemical vapor deposition based on precursor design and substrate engineering strategies

WTe2 nanostructures have intrigued much attention due to their unique properties, such as large non-saturating magnetoresistance, quantum spin Hall effect and topological surface state. However, the controllable growth of large-area atomically thin WTe2 nanostructures remains a significant challenge. In the present work, we demonstrate the controllable synthesis of 1T′ atomically thin WTe2 nanoflakes (NFs) by water-assisted ambient pressure chemical vapor deposition method based on precursor design and substrate engineering strategies. The introduction of water during the growth process can generate a new synthesized route by reacting with WO3 to form intermediate volatile metal oxyhydroxide. Using WO3 foil as the growth precursor can drastically enhance the uniformity of as-prepared large-area 1T′ WTe2 NFs compared to WO3 powders. Moreover, highly oriented WTe2 NFs with distinct orientations can be obtained by using a-plane and c-plane sapphire substrates, respectively. Corresponding precursor design and substrate engineering strategies are expected to be applicable to other low dimensional transition metal dichalcogenides, which are crucial for the design of novel electronic and optoelectronic devices.

Water‐Assisted Growth: Optimizing the Optoelectronic Properties of Face‐On Oriented Poly(3,4‐Ethylenedioxythiophene) via Water‐Assisted Oxidative Chemical Vapor Deposition (Adv. Funct. Mater. 14/2021)

In article number 2008712, Meysam Heydari Gharahcheshmeh, Karen K. Gleason, and co-workers present remarkable texture and nanostructural engineering of poly(3,4-ethylenedioxythiophene) (PEDOT) thin films grown by water-assisted oxidative chemical vapor deposition, which yields a record high electrical conductivity of 7520 S/cm. In PEDOT thin films with a highly face-on orientation, the charge transport between chains becomes a rate-limiting factor. Thus, electrical conductivity enhancement is achievable by reducing the π–π stacking distance through tuning the process parameters.

Study on mechanical properties and wear behavior of in-situ synthesized CNTs/ CuCrZrY composites prepared by spark plasma sintering

The Cr particles catalyst uniformly distributed on the surface of CuCrZrY composite powders were prepared by solid solution and aging treatment, the treated CuCrZrY composite powders were used to in-situ synthesis of CNTs/CuCrZrY composite powders by water-assisted chemical vapor deposition (CVD), which improved the wettability and dispersion of CNTs and Cu matrix. The CNTs/CuCrZrY composites were prepared by spark plasma sintering (SPS), and 60% deformation was performed on the hot rolling mill. The results shown that the tensile strength of CNTs/CuCrZrY composites was improved to 423 MPa, which was 53% and 14% higher than Cu and CuCrZrY composites, respectively. The softening resistance temperature of CNTs/CuCrZrY composites reaching 670 °C, and the thermal stability of CNTs/CuCrZrY composites had been significantly improved. The sliding friction coefficient between the CNTs/CuCrZrY composites and GCr15 steel balls at 20 °C and 300 °C were 0.254 and 0.290, which were 55.9% and 56.7% lower than Cu, respectively.

Carbon nanotubes/Ni and chain-like carbon nanospheres/Ni nanocomposites: selective production and their microwave absorption performances

It was well recognized that constructing the dielectric/magnetic nanocomposites was considered as an effective way to develop excellent microwave absorption materials (MAMs). Herein, we proposed a simple water-assisted chemical vapour deposition process to selectively produce carbon nanotubes (CNTs)/Ni and chain-like carbon nanospheres (CCNSs)/Ni nanocomposites in high yield by controlling the decomposition temperature. The ultrahigh yield of CCNSs could be achieved when C2H2 was catalytically decomposed at 515 °C, which was up to ca. 211.0. The results suggested that electromagnetic and microwave absorption properties of as-prepared samples were highly dependent on their microstructures and composition parameters, which could be regulated by the introduction of water vapour and decomposition temperature. It was worth mentioning that the obtained CCNSs/Ni nanocomposites could simultaneously present an optimal reflection loss of − 28.32 dB with a matching thickness of 1.68 mm, and an effective frequency bandwidth of 4.60 GHz with the matching thickness of 1.71 mm. Our results provided an effective and facile strategy to produce CCNSs/Ni in high yield, which provided a new idea for the designing and synthesis of lightweight and excellent MAMs.

Atmospheric pressure atomic layer deposition of iron oxide nanolayer on the Al2O3/SiO2/Si substrate for mm-tall vertically aligned CNTs growth

Atmospheric pressure atomic layer deposition (AP-ALD) of iron oxide, as a catalyst for water-assisted chemical vapor deposition (WA-CVD) growth of vertically aligned carbon nanotubes (VA-CNTs), was studied. Fe2O3 film was deposited on the porous Al2O3/SiO2/Si substrate using ferrocene and O2 as precursors. The self-liming film growth was obtained at the temperature window of about 180–250 °C with the optimum dosing and purging duration of about 20 min. The rather longer dosing time here compared to the conventional ALD is attributed to the high pressure as well as high porosity of the substrate. Considering the lower deposition rate at the beginning of the growth, it is suggested that the substrate-inhibited growth mode takes place in this work. The grown films have been characterized by SEM, AFM, Raman, and XRR. Raman spectroscopy peaks were consistent with the formation of Fe2O3. SEM images showed that the uniform coating of Fe2O3 follows the Volmer–Weber growth mode. The catalyst thickness-dependent growth of VACNTs revealed that the deposition through 12 cycles at 250 °C provides sufficient thickness, which resulted in the growth of mm-tall VA-CNTs within a relatively short time of WA-CVD. The changes in the catalyst, i.e. surface roughness and particle size, as a function of WA-CVD time were investigated. The time-dependent evolutions were mainly due to the subsurface diffusion and Ostwald ripening that ultimately led to the growth termination of CNTs.

In situ preparation of magnetic nickel-containing functionalized carbon nanotubes to support palladium as a catalyst for the Heck reaction

Magnetic and hydroxylated carbon nanotubes (NiCNTs-OH) were synthesized in situ via a green, facile, and efficient water-assisted chemical vapor deposition (H2O-CVD) method. Pd nanoparticles (NPs) were then anchored to the surfaces of the NiCNTs-OH and the resulting Pd/NiCNTs-OH were studied for their catalytic efficiency in the Heck reaction. Transmission electron microscopy images show that the prepared Pd/NiCNTs-OH consist of a CNT-based framework with Pd NPs of 2–4 nm and Ni NPs encapsulated by the tubes. Vibrating sample magnetometer measurements reveal that the catalysts are easily recycled by magnetic separation. Raman spectroscopy, X-ray photoelectron spectroscopy, and density functional theory calculations suggest that defects and oxygen-containing groups are introduced by the H2O-CVD method, providing sufficient anchoring sites for Pd NP loading. Hence, the Pd/NiCNTs-OH exhibit excellent yields in the Heck reaction of various aryl iodides with methyl acrylate. This work thus demonstrates a green and facile method for the preparation of magnetic nano-catalysts that can be employed for various catalytic reactions.

Controllable synthesis of N-doped aligned carbon nanotubes from melamine-based carbon by water-assisted chemical vapor deposition

N-doped well-aligned thin multiwall carbon nanotubes (CNTs) have been prepared by a facile water-assisted chemical vapor deposition (CVD) process using melamine as both the carbon precursor and N-dopant and ferrocene as both the catalyst and carbon precursor. It is shown that the synthetic temperature has strong effects on the growth of carbonaceous structure. Carbon films started to grow at reaction temperature of 700 °C, and the optimal temperature for the synthesis of aligned CNTs, based on the experiment results, was found to be 900 °C. The lengths of the CNTs can be tuned by changing the annealing time of the pyrolysis process, it could reach as long as 40.2 μm. The atomic percentage of nitrogen doping in aligned CNTs could be achieved to 1.71%. It was found that there were many joints and defects in the as-prepared CNTs which may offer more nucleation sites on the chemically inert CNT surface for various specialized applications such as supercapacitor.

Controlled synthesis of N-doped carbon microspheres from melamine-based carbon by chemical vapor deposition

N-doped carbon hollow microspheres (NCMs) have been prepared by a facile water-assisted chemical vapor deposition (CVD) process using melamine as both the carbon precursor and N-dopant and ferrocene as both the catalyst and carbon precursor. It is shown that the size of the NCMs can be tuned just by changing the annealing temperature of the CVD furnace. The atomic percentage of nitrogen doping in NCMs could be achieved to 7.85% without sacrificing the hollow spherical structure. Probable growth mechanism of NCMs was proposed in this paper. The work reported here may pave a way for designing novel various atom-doped carbon microspheres.

Water-assisted rapid growth of monolayer graphene films on SiO2/Si substrates

Metal-free growth of high-quality monolayer graphene films by chemical vapor deposition (CVD) on dielectric substrates is of great significance for the development of high-performance graphene-based electronic and optoelectronic devices. However, the existing CVD processes suffer from poor structural uniformity (or growth quality) and/or a slow growth rate due to the negligible catalytic activity of dielectric substrates. Here, we report a water-assisted CVD process for rapid growth of monolayer graphene film on SiO2/Si substrate without using metal catalysts or ultra-high temperature. Even trace amount of water enables the preferential formation of monolayer graphene films with significantly reduced structural defects. Particularly, this strategy enables the rapid growth of monolayer graphene by effectively lowering the growth kinetic barrier. We attribute the benefits of water on simultaneously improving the structural uniformity and growth rate to its mild oxidative effect and the role in accelerating the release of oxygen from the SiO2/Si substrate, respectively. Our work provides helpful information for efficient and controllable CVD growth of high-quality graphene on dielectric substrates.

Preparation and Antibacterial Activity Studies of TiO2 Nanostructured Materials

Different morphologies of nanostructured TiO2 materials can be tailor-made as a function of growth time varied from 1 to 7 h with a step of 2 h. TiO2 nanostructures were prepared on titanium foil by a facile water-assisted chemical vapor deposition method at 850°C. Scanning electron microscopy examination of the samples reveled that, under the treatment time of 7 h, the morphology of nanostructures transformed from cubic nanostructure-like to agglomerated nanorod-like and nanoplate shape. The X-ray diffraction through synthesized TiO2 nanostructures exhibited the crystalline structure of rutile TiO2 formed during the synthesis process. X-ray diffraction analysis showed that at deposition time of 7 h the intensity peak of [110] plane strongly decreased and seems that the [210] plane peak is the preferred orientation at this condition. Present paper deals with studies of the antibacterial activity of different synthesized samples after 24 h in the presence of Escherichia coli and Staphylococcus aureus cells. It is found that the growth time has an essential role in the morphology and antibacterial properties of TiO2 nanostructures.

One-step synthesis of hierarchical metal oxide nanosheet/carbon nanotube composites by chemical vapor deposition

We report here the one-step water-assisted CVD growth of metal oxide nanosheets/carbon nanotubes (CNTs) composites. The cross-linked composites were characterized by scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. The results showed that helical CNTs were obtained when the CVD growth process was prolonged to 1 h, and the typical fish-bone-type CNTs can be observed by HRTEM. Moreover, a related growth mechanism is proposed to explain the growth of such novel nanocomposites.

Using precipitated Cr on the surface of Cu-Cr alloy powders as catalyst synthesizing CNTs/Cu composite powders by water-assisted CVD

Given that the conventional catalyst is easily soluble in the matrix to result in the poor performance of the CNTs/Cu composite materials, the Cr nano-particles precipitated on the surface of Cu-Cr particles are first used as catalysts to prepare the CNTs/Cu composite powders by means of water-assisted chemical vapor deposition in situ synthesis. The results show that the morphological difference of the precipitated Cr nano-particle is obvious with the change of solution and aging treatment, and the morphology, length and diameter of the synthetic CNTs are also different. The catalyst of Cr nano-particle has the best morphology and the synthesized CNTs had a good wettability with Cu particles when the Cu-Cr composite powders was solution-treated at 1023 K for 60 min and then was aged at 723 K for 120 min. The length, diameter, yield and purity of the synthesized CNTs can be also affected by the moisture content in the reaction gas. It is the most suitable for the growth of CNTs when the moisture content is 0.4%, and the high purity and defect-free CNTs with the smooth pipe wall, a diameter of 20 ∼ 30 nm and a length of up to 1800 nm can be obtained. The yield of CNTs with the moisture content of 0.4% reached to 138%, which was increased by 119% to compare with that without moisture. In this paper, a feasible technology was offered for the preparation of high performance CNTs/Cu composites.

High-performance field emission device utilizing vertically aligned carbon nanotubes-based pillar architectures

The vertical aligned carbon nanotubes (CNTs)-based pillar architectures were created on laminated silicon oxide/silicon (SiO2/Si) wafer substrate at 775 °C by using water-assisted chemical vapor deposition under low pressure process condition. The lamination was carried out by aluminum (Al, 10.0 nm thickness) as a barrier layer and iron (Fe, 1.5 nm thickness) as a catalyst precursor layer sequentially on a silicon wafer substrate. Scanning electron microscope (SEM) images show that synthesized CNTs are vertically aligned and uniformly distributed with a high density. The CNTs have approximately 2–30 walls with an inner diameter of 3–8 nm. Raman spectrum analysis shows G-band at 1580 cm−1 and D-band at 1340 cm−1. The G-band is higher than D-band, which indicates that CNTs are highly graphitized. The field emission analysis of the CNTs revealed high field emission current density (4mA/cm2 at 1.2V/μm), low turn-on field (0.6 V/μm) and field enhancement factor (6917) with better stability and longer lifetime. Emitter morphology resulting in improved promising field emission performances, which is a crucial factor for the fabrication of pillared shaped vertical aligned CNTs bundles as practical electron sources.

A systematic study of the controlled generation of crystalline iron oxide nanoparticles on graphene using a chemical etching process.

Chemical vapor deposition (CVD) of carbon precursors employing a metal catalyst is a well-established method for synthesizing high-quality single-layer graphene. Yet the main challenge of the CVD process is the required transfer of a graphene layer from the substrate surface onto a chosen target substrate. This process is delicate and can severely degrade the quality of the transferred graphene. The protective polymer coatings typically used generate residues and contamination on the ultrathin graphene layer. In this work, we have developed a graphene transfer process which works without a coating and allows the transfer of graphene onto arbitrary substrates without the need for any additional post-processing. During the course of our transfer studies, we found that the etching process that is usually employed can lead to contamination of the graphene layer with the Faradaic etchant component FeCl3, resulting in the deposition of iron oxide FexOy nanoparticles on the graphene surface. We systematically analyzed the removal of the copper substrate layer and verified that crystalline iron oxide nanoparticles could be generated in controllable density on the graphene surface when this process is optimized. It was further confirmed that the FexOy particles on graphene are active in the catalytic growth of carbon nanotubes when employing a water-assisted CVD process.

Water-assisted and controllable synthesis of core/shell/shell structured carbon-based nanohybrids, and their magnetic and microwave absorption properties

By controlling the pyrolysis temperature, core/shell/shell structured Fe/Fe5C2/carbon nanotube bundles (Fe/Fe5C2/CNTBs), Fe/Fe3C/helical carbon nanotubes (Fe/Fe3C/HCNTs) and Fe/Fe3C/chain-like carbon nanospheres (Fe/Fe3C/CCNSs) with high encapsulation efficiency could be selectively synthesized in large-scale by water-assisted chemical vapor deposition method. Water vapor was proved to play an important role in the growth process. Because of α-Fe nanoparticles tightly wrapped by two layers, the obtained core/shell/shell structured nanohybrids showed high stabilities and good magnetic properties. The minimum reflection loss values of the as-prepared nanohybrids reached approximately −15.0, −46.3 and −37.1 dB, respectively. The excellent microwave absorption properties of the as-prepared core/shell/shell structured nanohybrids were considered to the quarter-wavelength matching model. Moreover, the possible enhanced microwave absorption mechanism of the as-prepared Fe/Fe3C/HCNTs and Fe/Fe3C/CCNSs were discussed in details. Therefore, we proposed a simple, inexpensive and environment-benign strategy for the synthesis of core/shell/shell structured carbon-based nanohybrids, exhibiting a promising prospect as high performance microwave absorbing materials.

Chemical Vapor-Deposited Carbon Nanotubes as Electrode Material for Supercapacitor Applications

ABSTRACTWe report the capacitance of entangled carbon nanotubes (CNTs) synthesized on flexible carbon fabric via water-assisted chemical vapor deposition. The CNTs were grown at atmospheric pressure with iron (Fe) as the catalyst, ethylene (C2H4) and 5%/95% H2/Ar as precursor gasses, and aluminum oxide as a buffer/barrier layer. The effect of the catalyst thickness (5 and 10 nm) on the specific capacitance was studied. The capacitance behavior of CNTs was evaluated by cyclic voltammetry measurements via a three-electrode system. The highest specific capacitance, approximately 56 F/g, was obtained for electrodes with 5nm Fe thickness. A nearly rectangular shaped cyclic voltammogram was exhibited for the CNTs grown on the carbon fabric. A specific power density of 0.012 KW/Kg and specific energy density 0.15 Wh/Kg were calculated from the galvanic charge/discharge (CD) curves. In addition, electrochemical impedance spectroscopy (EIS) revealed a characteristic supercapacitive behavior with a low equivalent series resistance of 7 Ωcm2.

Water-assisted growth of large-sized single crystal hexagonal boron nitride grains

A water-assisted chemical vapor deposition method is developed for the fast growth of large-sized h-BN single crystal domains.

Highly pure, millimeter-tall, sub-2-nanometer diameter single-walled carbon nanotube forests

We report the highly efficient synthesis of sub-2-nm diameter single-walled carbon nanotube (SWCNT) forests with homogenous and controllable diameter (1.5–2.8 nm), millimeter-scale height, and extremely high purity. This control was achieved by combining a previously reported sandwich catalyst (Al/Fe/Al) with an atmospheric synthesis process and the water-assisted thermal chemical vapor deposition method. In this way, a dense array of small and stable catalyst nanoparticles, suitable to support vertical growth, while avoiding gas diffusion limitations, could be prepared from which SWCNT forests could be synthesized efficiently to achieve both high purity and mm-scale height. The mm-tall sub-2-nm diameter SWCNT forests showed good quality (Raman G/D ratio of ∼40) and exceptionally high as-grown purity (outer specific surface area of 1215 m2/g; ideal: 1315 m2/g), which can be directly used without additional post-growth purification process to avoid any potential damage to the CNTs. This result represents a significant advance in synthesizing highly pure, mm-scale length SWCNT forests with sub-2-nm diameters.

Fabrication of copper/single-walled carbon nanotube composite film with homogeneously dispersed nanotubes by electroless deposition

Copper/single-walled carbon nanotube (SWCNT) composite films were fabricated by electroless deposition. The SWCNTs were formed using the water-assisted chemical vapor deposition method. To prepare copper/SWCNT composite plating baths with homogeneously dispersed SWCNTs, magnetic stirring, ultrasonic homogenization, and collision-type atomization were examined as mechanical methods for the disintegration of SWCNT bundles. Appropriate dispersants were added to the plating baths to avoid re-aggregation of the disintegrated SWCNTs. The degree of SWCNT dispersion in the copper/SWCNT composite plating bath was evaluated using a particle size analyzer, and the microstructures of the copper/SWCNT composite films were analyzed using scanning electron microscopy and X-ray diffraction. Collision-type atomization most significantly improved the degree of SWCNT dispersion in the composite plating bath. Using this treatment method, copper/SWCNT composite films with homogeneous dispersions of SWCNTs were successfully fabricated by electroless deposition.