Microwave-assisted Chemical Vapor Deposition Research Articles

Effect of hydrogen flow rate on the synthesis of carbon nanofiber using microwave-assisted chemical vapour deposition with ferrocene as a catalyst

Carbon nanostructure materials are becoming of considerable commercial importance, with interest growing rapidly over the decade since the discovery of carbon nanofibers. In this study, a new novel method is introduced to synthesize the carbon nanofibers by gas-phase, where a single-stage microwave-assisted chemical vapour deposition approach is used with ferrocene as a catalyst and acetylene and hydrogen as precursor gases. Hydrogen flow rate plays a significant role in the formation of carbon nanofibers, as being the carrier and reactant gas in the floating catalyst method. The effect of process parameters such as microwave power, radiation time and gas ratio of C2H2/H2 was investigated statistically. The carbon nanofibers were characterized using scanning and transmission electron microscopy and thermogravimetric analysis. The analysis revealed that the optimized conditions for carbon nanofibers production were microwave power (1000 W), radiation time (35 min) and acetylene/hydrogen ratio (0.8). The field emission scanning electron microscope and transmission electron microscope analyses revealed that the vertical alignment of carbon nanofibers has tens of microns long with a uniform diameter ranging from 115 to 131 nm. High purity of 93% and a high yield of 12 g of CNFs were obtained. These outcomes indicate that identifying the optimal values for process parameters is important for synthesizing high quality and high CNF yield.

A Comparative Analysis of Energy and Water Consumption of Mined versus Synthetic Diamonds

In our research, we analyzed the energy and water consumption in diamond mining and laboratory synthesis operations. We used publicly available reports issued by two market leaders, DeBeers and ALROSA, to estimate water and energy use per carat of a rough diamond. The efficiency of the two most popular synthesis technologies for artificial diamonds—High-Pressure-High-Temperature (HPHT) and Microwave-assisted Chemical Vapor Deposition (M-CVD)—was examined. We found that the modern HPHT presses, with open cooling circuits, consume about 36 kWh/ct when producing gem-quality and average-sized (near-) colorless diamonds. ALROSA and DeBeers use about 96 kWh/ct and 150 kWh/ct, respectively, including all energy required to mine. Energy consumption of M-CVD processes can be different and depends on technological conditions. Our M-CVD machine is the least energy-efficient, requiring about 215 kWh/ct in the single-crystal regime, using 2.45-GHz magnetron for the support synthesis. The M-CVD methods of individual synthetic companies IIa Technology and Ekati Mine are different from our results and equal 77 and 143 kWh/ct, respectively. Water consumption for the HPHT and M-CVD methods was insignificant: approximately zero and 0.002 m3/ct, respectively, and below 0.077 m3/ct for ALROSA-mined diamonds. This study touches upon the impact of the diamond production methods used on the carbon footprint.

Low-temperature synthesis of vertically aligned graphene through microwave-assisted chemical vapour deposition

The intrinsic properties of vertically aligned graphene (VG), such as a high ratio of exposed active edge sites compared to inert basal sites, non-stacking morphology and large surface-to-volume ratio make it applicable to numerous advanced technologies such as sensors, flexible electronics and fuel cells. Plasma-enhanced chemical vapour deposition has emerged as a promising technique to synthesise graphene at lower temperatures than a conventional chemical vapour deposition (CVD) process, providing a better control over the deposition parameters to tailor graphene properties.This study presents a cost effective, scalable, single step synthesis of a high quality VG at low temperatures. The samples were synthesised in a microwave (MW) assisted CVD reactor, which allowed for the decomposition of CH4 and H2 gas mixtures in Ar as a carrier gas, and VG growth directly onto Si wafer substrates without any additional heat applied to the substrate. Deposition conditions, such as MW power, gas ratio, and substrate-to-plasma separation were optimized to tailor the morphology, growth rate and electrochemical performance of the VG. The physiochemical analysis revealed that the fabricated VG consisted of high quality vertically aligned sp2 graphene, whilst a cyclic voltammetry analysis confirmed that the structures are electrochemically active and demonstrate a typical characteristics of quasi-reversible processes.

Understanding the effect of host structure of nitrogen doped ultrananocrystalline diamond electrode on electrochemical carbon dioxide reduction

Despite recent literature reporting the remarkable electrochemical CO2 reduction reaction (CO2RR) performance of nitrogen-doped graphitic carbon materials (sp2-carbon) and nitrogen-doped diamond materials (sp3-carbon), no systematic studies have been conducted on the catalytic activities of hybrid carbon nanomaterials between diamond and graphitic extremes. In this study, nitrogen-doped ultra-nanocrystalline diamond thin films were prepared by a microwave-assisted chemical vapor deposition technique. The ratio of sp2-carbon phase to sp3-carbon phase was controlled by varying growth conditions. Our results confirm that nitrogen-doped sp2-carbon (graphitic) rich electrodes have better selectivity for the CO2RR products over the nitrogen-doped sp3-carbon rich electrodes, indicating that the host structure of nitrogen dopants is crucial for the catalytic activity. Nitrogen-doped sp2-carbon electrodes present Faradaic efficiency for CO production up to 82% with excellent activity and selectivity. The vital role of the host structure and the potential catalytic sites were detailed by density functional theory (DFT) calculations.

CVD growth of ultrapure diamond, generation of NV centers by ion implantation, and their spectroscopic characterization for quantum technological applications

Abstract Applications of nitrogen-vacancy (NV) centers in diamond in quantum technology have attracted considerable attention in recent years. Deterministic generation of ensembles of NV centers can advance the research on quantum sensing, many-body quantum systems, multipartite entanglement and so on. Here we report the complete process of controlled generation of NV centers in diamond as well as their characterisation: growing diamond films through chemical vapor deposition (CVD), ion implantation and spectroscopic characterization of the defect centers using a confocal microscope. A microwave-assisted CVD set-up is presented which we constructed for the preparation of single-crystalline homoepitaxial diamond films. The films were prepared with minimized nitrogen concentration, which is confirmed through photoluminescence measurements. We demonstrate an in situ ultra high vacuum (UHV) implantation and heating process for creation of NV centers using a novel experimental set-up. For the first time hot implantation has been shown which prevents surface charging effects. We do not observe graphitization due to UHV heating. By optimizing the implantation parameters it has been possible to implant NV centers in a precise way. We present large area mapping of the samples to determine the distribution of the centers and describe the characterization of the centers by spectroscopic techniques. Reducing the decoherence caused by environmental noise is of primary importance for many applications in quantum technology. We demonstrate improvement on coherence time T_{2} of the NV spins by suppression of their interaction with the surrounding spin-bath using robust dynamical decoupling sequences.

Production of carbon nanotubes on bio-char at low temperature via microwave-assisted CVD using Ni catalyst

Microwave-assisted chemical vapor deposition (CVD) was employed for the synthesis of carbon nanotubes (CNTs) on pine nut shell (PNS) chars. The impact of reaction temperature, catalyst, carbon source, and the renewable carbon substrate on the synthesis of the CNTs were investigated. Microwave irradiation, the type of carbon source, and the presence of nickel catalyst were found to be the governing factors during the formation and growth of CNTs at low temperatures. 600 °C was found to be the optimum temperature for the formation of the CNTs. The high-resolution transmission electron microscope (HRTEM) results showed that the CNTs synthesized at 600 °C had a multiwall structure and a d-spacing of 0.34 nm. The diameter and length of CNTs were ~50 nm and 2600–3200 nm, respectively. Raman spectroscopy analysis revealed that a high degree of carbon order was achieved at 600 °C due to the formation of CNTs on the surface of bio-char. The formation and growth mechanisms of CNTs under microwave irradiation were proposed and discussed.

Chemoselective hydrogenation of cinnamaldehyde over MOFs-derived M2Si@C (M = Fe, Co, Ni) silicides catalysts

Chemoselective hydrogenation of CC and CO bonds in the cinnamaldehyde (CMA), was successfully performed with highly dispersed transition metal silicides-embedded in the porous carbon matrix (M2Si@C (M = Fe, Co, Ni)) nanocatalysts with a particle size of 8–12 nm, which were synthesized by a facile approach via microwave-assisted chemical vapor deposition (MWCVD) using organosilane and metal-organic-framework (MOFs-74)-templated porous carbon matrix-encapsulated transition metal (M@C) precursors. Compared with M@C, the activity of M2Si@C (M = Fe, Co, Ni) nanocatalysts obviously increases, which can be attributed that the overlayered carbon on metals has been destroyed during MWCVD processing, leading to the exposure of more active sites of catalysts. Due to the inherent electronic properties of metallic active sites in the metal silicides, the catalytic activity of Ni2Si@C was much higher than that of Co2Si@C and Fe2Si@C in the chemoselective hydrogenation of CMA. At total conversion of CMA, the Co2Si@C nanocatalyst was chemoselective for the hydrogenation of polar CO bonds (selectivity to cinnamyl alcohol of ∼60%), whereas Ni2Si@C nanocatalyst was highly chemoselective for the hydrogenation of non-polar CC bonds (selectivity to hydrocinnamaldehyde of ∼90%). The doping of silicon atoms with more electropositive into metals lattices in the M-Si intermetallic compounds (IMCs) influenced the adsorption of substrate molecule on the catalyst surface, slightly leading to the different reaction routes. In addition, the Co2Si@C and Ni2Si@C respectively maintain the robust stability in the hydrogenation of CMA. This result provided guidance on the design of catalyst via tuning the polarization properties of the inter-atoms in the IMCs according to the target product.

Highly selective hydrogenation of phthalic anhydride to phthalide over CoSix/CNTs catalyst prepared by multi-step microwave-assisted chemical vapor deposition

CoSix/CNTs catalysts with different CoSix phases (Co2Si, CoSi) have been rapidly synthesized via a multi-step microwave-assisted chemical vapor deposition method. The catalysts were evaluated for the liquid phase hydrogenation of phthalic anhydride to produce phthalide, which is an important industrial intermediate for pharmaceuticals, fine chemicals and organic synthesis. The prepared CoSix/CNTs catalysts exhibited not only similar selectivity (81.0% selectivity to phthalide at 220 °C) compared with the Co/CNTs catalyst, but also exhibited much better activity (94.1% conversion of phthalic anhydride at 220 °C) than Co/CNTs catalyst (80.0% conversion of phthalic anhydride at 220 °C) under mild reaction conditions (190–230 °C and 4.0 MPa H2).

Homoepitaxial growth of single crystalline CVD-diamond

The influence of temperature, pressure and holder geometry on the homoepitaxial single crystalline diamond growth in a microwave-assisted chemical vapor deposition reactor will be introduced and discussed in detail. Optimized diamond growth conditions were determined for homoepitaxy on (100)-oriented high-temperature high-pressure (HPHT) seed crystals using Raman scattering and confocal-micro-photoluminescence spectroscopy measurements.

Mass Production of Carbon Nanofibers Using Microwave Technology.

Carbon nanotubes (CNFs) were produced by gas phase single stage microwave assisted chemical vapour deposition (MA-CVD) using ferrocene as a catalyst and acetylene (C2H2) and hydrogen (H2) as precursor gases. The effect of the process parameters such as microwave power, radiation time, and gas ratio of C2H2/H2 was investigated. The CNFs were characterized using scanning and transmission electron microscopy (TEM), and by thermogravimetric analysis (TGA). Results reveal that the optimized conditions for CNF production were 1000 W reaction power, 35 min radiation time, and 0.8 gas ratio of C2H2/H2. TEM analyses revealed that the uniformly dispersed CNFs diameters ranging from 115-131 nm. The TGA analysis showed that the purity of CNF produced was 93%.

Electrochemical determination of rosuvastatin calcium in pharmaceutical and human body fluid samples using a composite of vertically aligned carbon nanotubes and graphene oxide as the electrode material

A novel electrochemical method for the determination of rosuvastatin calcium (RC) using a composite of vertically aligned carbon nanotubes and graphene oxide (VACNT-GO) as the electrode material is proposed. The VACNT-GO electrode was synthesized using a microwave-assisted chemical vapor deposition method followed by oxygen plasma treatment. The VACNT-GO electrode presented superhydrophilic behavior and fast charge transfer, which are very attractive for electrochemical applications. The RC molecule presented an oxidation process at +1.26V vs. Ag/AgCl (3.0molL−1 KCl) in phosphate buffer solution (pH 2), and this redox behavior was explored for its voltammetric determination. Using square-wave adsorptive anodic stripping voltammetry (SWAdASV) under optimized conditions, the analytical curve was linear in RC concentration range from 0.48 to 46.72μmolL−1 with a limit of detection of 0.06μmolL−1. The developed SWAdASV method was applied for RC quantification in commercial pharmaceutical samples, and the results are consistent with those obtained by the comparative spectrophotometric method at a confidence level of 95%. Moreover, RC was determined in human body fluid samples of urine and human serum.

Intrinsic and extrinsic absorption of chemical vapor deposition single-crystal diamond from the middle ultraviolet to the far infrared

Optical absorption is reported in high-quality single-crystal Type IIa diamonds grown by microwave-assisted chemical vapor deposition. Laser calorimetry and spectrophotometry were used at wavelengths from the ultraviolet to the infrared. There is minimal evidence of extrinsic absorption on or near the short wavelength side of the middle-ultraviolet (MUV) region (200–300 nm) or in the infrared absorption bands. At the long wavelength side of the MUV and extending into the visible, extrinsic absorption dominates, which is related to the amount of nitrogen and nitrogen-related lattice defects in diamond. In the samples studied nitrogen concentrations were between 10 and 40 ppb. At 10.6 μm, the measured absorption coefficient of a representative sample is comparable with, or less than, those of the highest quality samples previously reported. Evidence is presented showing that our measurements serve as upper bounds on the absorption coefficients and call into question previous calorimetric reports of intrinsic absorption in the visible, near infrared, and at 10.6 μm.

Fast carbon nanofiber growth on the surface of activated carbon by microwave irradiation: A modified nano-adsorbent for deep desulfurization of liquid fuels

Carbon nanofibers were fabricated on the surface of regular activated carbon (AC) by microwave-assisted chemical vapor deposition (CVD) method. Methane and ethane were comparatively used as the source gases to figure out the textural and particulate properties and efficiency of the produced nano-adsorbents in sulfur adsorption from a fuel model. AC granules were firstly impregnated with nickel and exposed to a flow of hydrocarbon–nitrogen mixture through a stationary bed. A 900W household microwave apparatus was used as the energy source to decompose the hydrocarbon and grow up nanofibers on the surface. Effect of catalyst fraction, radiation time and inlet gas flow rate were studied as the influential synthesis parameters. The 0.5wt% Ni-impregnated AC was exposed to equal mole fraction of N2–hydrocarbon mixture with WHSV of 6.7ml/gmin and the result revealed the highest yield of nanofibers growth on the surface. The diameter of nanofibers made from ethane was almost twofold over the diameter of nanofibers made from methane. Thiophene and dibenzothiophene removal were comparatively investigated by a series of batch adsorption experiments with a model fuel comprising n-octane. The sulfur removal efficiency was improved up to 87% compared with the original AC. During two hours contact time, the sulfur content of solution was decreased from 290 to 45ppm by the modified AC surface.

Effect of coating and plasma treatments on the induced coupled plasma-reactive ionic etching of boron-doped diamond for microelectromechanical systems (MEMS) applications

Induced coupled plasma etching was performed on poly-crystalline boron-doped diamond (BDD) films synthesized by microwave assisted chemical vapor deposition (MWCVD). Effects of the input power and of the gas composition and pressure on etching performances have been investigated. A basic aluminium layer and a new hybrid material have been studied as protecting film for the patterning. Best conditions to get high etch rates were a gas ratio Ar/O2 of 27% (vol.) under 10 mTorr at 200 W (input power). Aluminium (Al) masking induced un-intentional whiskers formation which was removable using CHF3 as a reactive gas with the drawback to lower the process selectivity. In order to get both a high etching rate and a high selectivity, a hybrid material based on an organic/mineral resin was then used as the masking material. This performing photoresist film allowed etching a 3.6-μm-thick BDD film at 200 nm min−1.

Proposed model for biomineralization of novel nanohydroxyapatite/vertically aligned multiwalled carbon nanotube scaffolds

For the first time, the growth mechanism of biominerals formed on plate-like nanohydroxyapatite (nHAp) electrodeposited on superhydrophilic vertically aligned multi-walled carbon nanotubes (VAMWCNT-O2) is presented and a model for the specific growth preference is discussed. VAMWCNT-O2 films were obtained by microwave-assisted chemical vapor deposition method and funcionalized by oxygen plasma. nHAp/VAMWCNT-O2 nanocomposites were fabricated with a direct electrodeposition of the thin nHAp films onto the VAMWCNT-O2 films. The biomineralized "scaffolds" were obtained by soaking nHAp/VAMWCNT-O2 in simulated body fluid for 7, 14 and 21 days. Results show that the carboxyl functional groups directly attached onto VAMWCNT tips after oxygen plasma treatment were essential for the acceleration of the OH- formation and the deposition of plate-like nHAp crystals.

Superior Field Emissions from Boron-Doped Nanocrystalline Diamond Compared to Boron-Doped Microcrystalline Diamond

Boron-doped microcrystalline diamond (BMD) and nanocrystalline diamond (BND) thin films were grown on Si substrates by microwave-assisted chemical vapor deposition, and their field emission properties were evaluated. BND exhibited a lower turn-on field and higher field enhancement factor than BMD. Furthermore, in a long-term emission stability test, BND showed only a 4% increase in the current density after 12 h of emission, whereas the current density of BMD decreased by - 59%. These results indicate that BND is a more stable and viable current emitter than BMD.

Formation of nano-pores in nano-crystalline diamond films

Various nano-pores in nano-crystalline diamond (NCD) thin films have been fabricated and characterized. Therefore in this work two aspects of NCD thin films synthesized by microwave assisted chemical-vapour-deposition (MWCVD) have been investigated. Firstly, the influence of CVD-growth conditions on the film morphology and chemical grain boundary composition and their impact on the mechanical properties. Second, the formation of nano-pores by selective etching of the non-diamond phase. Freestanding NCD membranes were fabricated and bulged to calculate the Young’s modulus which can reach surprisingly high values (1100GPa) close to single crystal diamond. The presence of nano-pores was verified by electrochemical experiments where ions have been used to detect the porosity.

Electrochemical properties of N-doped hydrogenated amorphous carbon films fabricated by plasma-enhanced chemical vapor deposition methods

Nitrogen-doped hydrogenated amorphous carbon thin films (a-C:N:H, N-doped DLC) were synthesized with microwave-assisted plasma-enhanced chemical vapor deposition widely used for DLC coating such as the inner surface of PET bottles. The electrochemical properties of N-doped DLC surfaces that can be useful in the application as an electrochemical sensor were investigated. N-doped DLC was easily fabricated using the vapor of nitrogen contained hydrocarbon as carbon and nitrogen source. A N/C ratio of resulting N-doped DLC films was 0.08 and atomic ratio of sp 3/ sp 2-bonded carbons was 25/75. The electrical resistivity and optical gap were 0.695 Ω cm and 0.38 eV, respectively. N-doped DLC thin film was found to be an ideal polarizable electrode material with physical stability and chemical inertness. The film has a wide working potential range over 3 V, low double-layer capacitance, and high resistance to electrochemically induced corrosion in strong acid media, which were the same level as those for boron-doped diamond (BDD). The charge transfer rates for the inorganic redox species, Fe 2+/3+ and Fe(CN) 6 4−/3− at N-doped DLC were sufficiently high. The redox reaction of Ce 2+/3+ with standard potential higher than H 2O/O 2 were observed due to the wider potential window. At N-doped DLC, the change of the kinetics of Fe(CN) 6 3−/4− by surface oxidation is different from that at BDD. The rate of Fe(CN) 6 3−/4− was not varied before and after oxidative treatment on N-doped DLC includes sp 2 carbons, which indicates high durability of the electrochemical activity against surface oxidation.

The effect of the CH 4 level on the morphology, microstructure, phase purity and electrochemical properties of carbon films deposited by microwave-assisted CVD from Ar-rich source gas mixtures

Carbon thin films were deposited on Si substrates by microwave-assisted chemical vapor deposition (CVD) using variable CH 4 levels in an Ar/H 2 (Ar-rich) source gas mixture. The relationship between the CH 4 concentration (0.5 to 3 vol.%) in the source gas and the resulting film morphology, microstructure, phase purity and electrochemical behavior was investigated. The H 2 level was maintained constant at 5% while the Ar level ranged from 92 to 94.5%. The films used in the electrochemical measurements were boron-doped with 2 ppm B 2H 6 while those used in the structural studies were undoped. Boron doping at this level had no detectable effect on the film morphology or microstructure. Relatively smooth ultrananocrystalline diamond (UNCD) thin films, with a nominal grain size of ca. 15 nm, were only formed at a CH 4 concentration of 1%. At the lower CH 4 concentration (0.5%), faceted microcrystalline diamond was the predominant phase formed with a grain size of ca. 0.5 µm. At the higher CH 4 concentration (2%), a diamond-like carbon film was produced with mixture of sp 2-bonded carbon and UNCD. Finally, the film grown with 3% CH 4 was essentially nanocrystalline graphite. The characteristic voltammetric features of high quality diamond (low and featureless voltammetric background current, wide potential window, and weak molecular adsorption) were observed for the film grown with 1% CH 4, not the films' grown with higher CH 4 levels. The C 2 dimer level in the source gas was monitored using the Swan band optical emission intensity at 516 nm. The emission intensity and the film growth rate both increased with the CH 4 concentration in the source gas, consistent with the dimer being involved in the film growth. Importantly, C 2 appears to be involved in the growth of the different carbon microstructures including microcrystalline and ultrananocrystalline diamond, amorphous or diamond-like carbon, and nanocrystalline graphite. In summary, the morphology, microstructure, phase purity and electrochemical properties of the carbon films formed varied significantly over a narrow range of CH 4 concentrations in the Ar-rich source gas. The results have important implications for the formation of UNCD from Ar-rich source gas mixtures, and its application in electrochemistry. Characterization data by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), visible-Raman spectroscopy and electrochemical methods are presented.

Tuneable optical lenses from diamond thin films

Nanocrystalline diamond (NCD) membranes of 150 nm thickness and diameters in the millimeter range grown by microwave-assisted chemical-vapor deposition were bulged to investigate their mechanical properties and their use as tuneable optical lenses. The NCD films were grown at different CH4/H2 gas mixtures to vary the sp2/sp3 ratio and thereby to tune their mechanical, optical, and surface morphology properties. By applying gas over pressure the membrane forms a lens shaped geometry. From deflection data we calculated Young’s moduli which decrease with increasing CH4/H2 ratio from 1160 GPa at 0.5% to 900 GPa at 7%. Optical lens applications show a variation in the focal point from infinity to 3.5 mm. The data indicate that NCD is a promising material for tuneable optical lenses applications.