Radio-frequency Catalytic Chemical Vapor Deposition Research Articles

The study of adenine and guanine electrochemical oxidation using electrodes modified with graphene-platinum nanoparticles composites

Composite materials based on graphene and platinum nanoparticles (Gr-Pt-2 and Gr-Pt-3) were synthesized over two catalysts (Ptx/MgO, where x=2 or 3wt.%) using radio-frequency catalytic chemical vapor deposition (RF-CCVD), with methane as carbon source. After morphological (TEM/HRTEM) and structural characterization (XRD, XPS, FTIR and UV-Vis) they were used to modify two gold electrodes and subsequently employed for the investigation of adenine and guanine electrochemical oxidation. For the Au/Gr-Pt-2 electrode, the oxidation peak potential of adenine was observed at +1.19V vs. Ag/AgCl, while, for Au/Gr-Pt-3 electrode, this was negatively shifted to +1.09V vs. Ag/AgCl. In addition, the oxidation current densities were approximately 2.7×10−4 and 6.9×10−4 A·cm−2 (for Au/Gr-Pt-2 and Au/Gr-Pt-3, respectively) demonstrating that the Gr-Pt-3 composite had a better electro-catalytic activity towards the oxidation of adenine. A similar behavior was observed for guanine oxidation. The excellent electro-catalytic properties of the Gr-Pt-3 sample were correlated with the fact that the composite material had a higher amount of platinum nanoparticles which were not fully covered by graphene layers (about 50%). In addition, it has a larger surface area (335 m2·g−1) compared with that of the Gr-Pt-2 sample (271 m2·g−1), which also greatly improved the electron kinetics.

Electrochemical oxidation of adenine using platinum electrodes modified with carbon nanotubes

Single- (SWNTs), double- (DWNTs), and multi-walled (MWNTs) carbon nanotubes were synthesized by controlled Radio Frequency-catalytic Chemical Vapor Deposition (RF-cCVD). Their morphological and structural characteristics were identified using Transmission Electron Microscopy (TEM/HRTEM) and X-ray powder diffraction (XRD). Next, three platinum electrodes with identical amounts of each nanotube material (denoted Pt-SW, Pt-DW, and Pt-MW) were modified in order to test the electro-catalytic characteristics of the carbon nanotubes and further used for the electrochemical oxidation of adenine. The signal recorded with the Pt-SW electrode was very poor, due to the predominantly semiconducting properties of these nanotubes. In contrast, the signal recorded with Pt-DW or Pt-MW was well-defined, with the peak potentials at 1.07 and 1.01V vs Ag/AgCl, respectively. In both cases, the detection limit (DL) for adenine was found to be 3×10−6M.

Catalytic one-step synthesis of Pt-decorated few-layer graphenes

This study presents the influence of the various reaction parameters (catalyst concentration, reaction temperature, hydrocarbon flow rate, and reaction time) on the one-step growth of few-layer graphenes decorated with metallic platinum (Pt) nanoparticles. The latter were synthesized over a Pt/MgO catalyst using radio-frequency catalytic chemical vapor deposition (CCVD) from methane. A mixture of nanotubes and graphenes in various ratios was produced depending on the synthesis conditions. The nanotubes have a few walls (generally 2 or 3, but the maximum number observed was 7) with diameters in the range of a few to several nanometers (Dmaxext ≈ 7 nm). The graphene structures have a few square-shaped layers with dimensions ranging between 80 and 100 nm, which are decorated with Pt nanoparticles (with 70% having diameters between 4 and 8 nm) both on the flat surfaces as well as on the edges. Our experimental findings demonstrate that it is relatively easy to obtain predominantly graphenes using the Pt/MgO (2 wt% Pt) catalyst system at 1000 °C, with a 100 mL min−1 hydrocarbon flow rate and a reaction time of 30 minutes.

Electro-catalytic properties of graphene composites containing gold or silver nanoparticles

Composite nanostructures based on few-layers graphene with encased gold or silver nanoparticles (denoted as Gr-Au and Gr-Ag, respectively) were separately prepared in a single-step synthesis by radio frequency catalytic chemical vapor deposition (RF-cCVD) over Aux/MgO and Agx/MgO catalytic system (where x=3wt.%), respectively. Their morphological properties were investigated by electron microscopy techniques (TEM/HRTEM), which demonstrated that the number of graphitic layers within the sheet varied between 2 and 7. Thorough TEM analysis also indicated that gold nanoparticles had a mean size of 22nm, while silver nanoparticles were found to be larger with a mean size of 35nm. X-ray powder diffraction proved that the crystallinity of the Gr-Au or Gr-Ag samples is less influenced by the type of metallic nanoparticles (silver or gold) encased between the graphitic layers. The mean value of the crystalline domain perpendicular to graphene (002) crystallographic plane was determined to be approximately 2.25nm (for Gr-Au sample) and 2.14nm (for Gr-Ag sample), both corresponding to 6 graphitic layers. Gr-Ag and Gr-Au nanostructures were used to modify platinum substrates and subsequently employed for the electrochemical analysis of carbamazepine. A significant decrease in the electrochemical oxidation potential of carbamazepine (150mV) was obtained with both modified electrodes. The detection limit (DL) was found to be 2.75×10−5M and 2.92×10−5M for the Pt/Gr-Ag and Pt/Gr-Au electrode, respectively.

Chirality-enriched semiconducting carbon nanotubes synthesized on high surface area MgO-supported catalyst

Single-walled carbon nanotubes (SWCNTs) with a narrow diameter distribution were synthesized by radio frequency-Catalytic Chemical Vapor Deposition (RF-CCVD) through the pyrolysis of CH4. Fe–Co bimetallic catalytic nanoclusters were supported on high-surface area MgO nanopowders and used in the nanotube synthesis process. Nanolog absorption fluorescence analysis was used to characterize the chiralities of the as-produced SWCNTs over this nanostructural catalyst. In the final SWCNT sample, the (7,5) semiconducting carbon nanotube species were found to be dominant, with a low chirality variation.

Carbon-covered magnetic nanomaterials and their application for the thermolysis of cancer cells

Three types of graphitic shelled-magnetic core (Fe, Fe/Co, and Co) nanoparticles (named as C-Fe, C-Fe/Co, and C-Co NPs) were synthesized by radio frequency-catalytic chemical vapor deposition (RF-cCVD). X-ray diffraction and X-ray photoelectron spectroscopy analysis revealed that the cores inside the carbon shells of these NPs were preserved in their metallic states. Fluorescence microscopy images indicated effective penetrations of the NPs through the cellular membranes of cultured cancer HeLa cells, both inside the cytoplasm and the nucleus. Low RF radiation of 350 kHz induced localized heating of the magnetic NPs, which triggered cell death. Apoptosis inducement was found to be dependent on the RF irradiation time and NP concentration. It was showed that the Fe-C NPs had a much higher ability of killing the cancer cells (over 99%) compared with the other types of NPs (C-Co or C-Fe/Co), even at a very low concentration of 0.83 microg/mL. The localized heating of NPs inside the cancer cells comes from the hysteresis heating and resistive heating through eddy currents generated under the RF radiation. The RF thermal ablation properties of the magnetic NPs were correlated with the analysis provided by a superconducting quantum interference device (SQUID).

Novel synthesis process for ceramic carbon nanotube nanocomposites with nanojunctions

AbstractA single‐step process of radio frequency‐catalytic chemical vapor deposition (RF‐cCVD) was used for the synthesis of pelletized ceramic carbon nanotube (CNT) composites. The surface resistance of the insulating magnesium oxide (MgO) was found to drop dramatically as the result of the graphitic nanostructures formed within the ceramic matrix after the catalytic process. It was found that the electrical properties of the ceramic CNT composites were highly dependent on the concentration of the loading metal that was responsible for the formation of different carbon species on the MgO pellet. Hall effect measurements of ceramic single‐walled CNT composites indicated an n‐type electrical behavior with high carrier densities of around 2.5 × 1020/cm3 and electrical mobilities of 1.1 cm2/V s. The process is shown to be useful for generating ceramic CNT devices that may be used for a variety of applications.

High-aspect ratio and horizontally oriented carbon nanotubes synthesized by RF-cCVD

Horizontally aligned long carbon nanotubes were efficiently synthesized on Si substrates by using a radio-frequency catalytic chemical vapor deposition method. The morphological as well as the growth properties of these nanotubes were systematically investigated with various analytical techniques including microscopy and Raman spectroscopy. Different reaction parameters such as temperature, type of hydrocarbon gas and catalyst amount were varied and their effects on the nanotube size, quality and alignment are reported. High-aspect ratio and horizontally oriented nanotubes were found to grow following the “tip growth” mechanism. The fast and localized heating rate produced by the RF generator helps nanotubes to separate and lift the nano-particles away from the support and hence contributes to the growth of CNTs with a very high-aspect ratio. Carbon nanotubes synthesized with methane show a better horizontal alignment compared to those synthesized with acetylene, which might be due to the flow rate of the hydrocarbon gas.

Versatile Catalytic System for the Large-Scale and Controlled Synthesis of Single-Wall, Double-Wall, Multi-Wall, and Graphene Carbon Nanostructures

This work reports a low-cost production method of carbon nanotubes (CNTs) and few-layer graphene sheets in large quantities through radio-frequency catalytic chemical vapor deposition (RF-cCVD). As the reaction temperature was varied over a range of 700−1000 °C, different types of high-quality carbon nanostructures were synthesized on a single Fe−Co/MgO catalytic system, while utilizing acetylene as the hydrocarbon source. The reaction efficiencies were found to be dependent on the morphological properties of the corresponding nanostructures and varied significantly with the synthesis temperature. Highly crystalline multi-wall, double-wall, and single-wall CNTs were obtained when the temperature was in the range of 700−950 °C, whereas large quantities of few-layer graphene sheets (1−5 layers) were synthesized at 1000 °C. The carbon nanostructures were characterized via several analytical techniques, such as electronic microscopy, Raman spectroscopy, X-ray diffraction, etc. It was determined that the synthesis temperature had a strong influence, both on the catalyst system and on the type of the nanostructures synthesized. By utilizing the same catalytic system and varying only one reaction parameter, it may be possible to reduce the production cost of CNTs and graphene nanosheets significantly, so that they may be utilized in various advanced applications.

Synthesis of narrow diameter distribution carbon nanotubes on ZnO supported catalysts

Multiwall carbon nanotubes, as indicated by electron microscopy and spectroscopy, were grown over a novel Co(5 wt.%)/ZnO catalyst by using a cold wall radio-frequency catalytic chemical vapor deposition method. The nanotubes were found to have a rather narrow diameter distribution, which can be explained by the formation over the ZnO support particles of uniform Co clusters, as shown by high resolution transmission electron microscopy studies. The growth of carbon nanotubes over such a support could represent a fast and reliable method to develop in one step 3D ZnO–CNTs heterogeneous composites of any size or shape, for use in advanced applications.

Effect of hydrogen on the growth and morphology of single wall carbon nanotubes synthesized on a Fe [sbnd]Mo/MgO catalytic system

Single wall carbon nanotubes were synthesized from thermal pyrolysis of methane on a Fe Mo/MgO catalyst by radio frequency catalytic chemical vapor deposition (RF-CVD) using argon as a carrier gas. Controlled amounts of hydrogen ( H 2 / CH 4 = 0 – 1 v / v ) were introduced in separate experiments along with the carbon source. The properties and morphology of the synthesized single wall carbon nanotubes were monitored by transmission electron microscopy, Raman scattering, and thermogravimetric analysis. The nanotubes with the highest crystallinity were obtained with H 2 / CH 4 = 0.6 . By monitoring the Radial Breathing Modes present in the Raman spectra of the single-wall carbon nanotube samples, the variation of the structural and morphological properties of the carbon nanotubes with the flow level of hydrogen, reflect changes of the catalyst systems induced by the presence of hydrogen.

Surface area and thermal stability effect of the MgO supported catalysts for the synthesis of carbon nanotubes

Surface area (SA) of the oxide support was found to play a major role on the morphology, quality of the single wall carbon nanotubes (SWNTs) growth. Fe–Co bimetal was supported onto MgO powders with different surface areas, and used for the SWNTs synthesis from the pyrolysis of CH4 through radio frequency catalytic chemical vapor deposition (RF-CCVD). The high surface area MgO showed specificity towards the growth of SWNTs even at the very high metal loadings of 17.4%. As the MgO surface areas decreased, the agglomeration of the active metal species on the support, nanotubes with wider diameter distributions and more graphitic walls were obtained. This surface area effect suggested that the high surface area networks were responsible for a more effective confinement of the active metal nanoparticles that impeded their migration during the SWNTs thermal growth process. Therefore, by adjusting the surface area or chemical morphology of the catalyst systems, SWNTs with narrower diameter distribution and specific chiralities can be grown.