Selective removal of metallic single-walled carbon nanotubes by combined in situ and post-synthesis oxidation

A combined in situ and post-synthesis gas phase oxidation approach for selective removal of metallic single-walled carbon nanotubes (m-SWCNTs) is reported. The in situ oxidation is performed by introducing a small amount of oxygen during the synthesis of SWCNTs by floating catalyst chemical vapor deposition, and the post-synthesis oxidation is conducted by heat-treating the synthesized SWCNTs in air at 400 °C. A combination of characterization techniques shows that m-SWCNTs were selectively removed as a result of their higher reactive activity to oxygen compared to semiconducting SWCNTs, and the diameter distribution of the SWCNTs is narrowed to a range of 1.5-2.0 nm. The mechanism of the combined in situ and post-synthesis oxidation approach is discussed.     

Magnetite nanoparticles stabilized with polymeric bilayer of poly(ethylene glycol) methyl ether-poly(?-caprolactone) copolymers

In this work, we report a synthetic method of water dispersible magnetite nanoparticles having oleic acid and poly(ethylene glycol) methyl ether-poly(?-caprolactone) (mPEG-PCL) amphiphilic block copolymer as polymeric stabilizers. The particles were prepared by coprecipitation of Fe(II) and Fe(III) in NH₄OH and had bilayer surface with hydrophobic inner layer and hydrophilic corona. mPEG-PCL copolymer was synthesized by a ring-opening polymerization of ?-caprolactone using mPEG as a macroinitiator in the presence of stannous octoate catalyst. FTIR and thermogravimetric analysis (TGA) indicated the presence of the copolymer on the particle surface. Roles of reaction parameters, such as stabilizer concentrations and time of ultrasonicating treatment, on percent of magnetite in the complex and its magnetic properties were investigated. Transmission electron microscopy (TEM) showed the average particle size about 9.0 ± 1.1 nm in diameter. Vibrating sample magnetometry (VSM) measurement indicated that the magnetite nanoparticles were superparamagnetic at room temperature. Approximately 6.8 ± 0.5% of indomethacin model drug (68 μg/mg of magnetite) was effectively entrapped on the particles.     

The effect of catalyst calcination temperature on the diameter of carbon nanotubes synthesized by the decomposition of methane

The effect of catalyst calcination temperature on the uniformity of carbon nanotubes (CNTs) diameter synthesized by the decomposition of methane was studied. The catalysts used were CoO-MoO/Al₂O₃ without prior reduction in hydrogen. The results show that the catalyst calcination temperature greatly affects the uniformity of the diameter. The CNTs obtained from CoO-MoO/Al₂O₃ catalysts, calcined at 300 °C, 450 °C, 600 °C, and 700 °C had diameters of 13.4 ± 8.4, 12.6 ± 5.1, 10.7 ± 3.2, and 9.0 ± 1.4 nm, respectively, showing that an increase in catalyst calcination temperature produces a smaller diameter and narrower diameter distribution. The catalyst calcined at 750 °C was inactive in methane decomposition. Transmission electron microscopy (TEM) studies showed that CNTs grown on the catalyst calcined at 700 °C were of uniform diameter and formed a dense interwoven covering. High-resolution TEM shows that these CNTs had walls of highly graphitized parallel graphenes.     

Mechanistic investigations of single-walled carbon nanotube synthesis by ferrocene vapor decomposition in carbon monoxide

The single-walled carbon nanotubes (SWCNTs) were synthesized by the carbon monoxide disproportionation reaction on Fe catalyst particles formed by ferrocene vapor decomposition in a laminar flow aerosol (floating catalyst) reactor. On the basis of in situ sampling of the product collected at different locations in the reactor, kinetics of the SWCNT growth and catalyst particle crystallinity were studied. Catalyst particles captured before SWCNT nucleation as well as inactive particles were determined to have cementite (Fe₃C) phase, while particles with γ- and α-Fe phases were found to be embedded in the SCWNTs. The growth rate in the temperature range from 804 to 915 °C was respectively varied from 0.67 to 2.7 μm/s. The growth rate constant can be described by an Arrhenius dependence with an activation energy of E_a = 1.39 eV, which was attributed to the carbon diffusion in solid iron particles. CNT growth termination was explained by solid-liquid phase transition in the catalyst particles. A high temperature gradient in the reactor was found to not have any effect on the diameter during the SWCNT growth and as a result on the chirality of the growing SWCNTs.     

Preparation of MnOx/TiO2 ultrafine nanocomposite with large surface area and its enhanced toluene oxidation at low temperature

The TiO₂ support materials were synthesized by a chemical vapor condensation (CVC) method and the subsequent MnO_x/TiO₂ catalysts were prepared by an impregnation method. Catalytic oxidation of toluene on the MnO_x/TiO₂ catalysts was examined with ozone. These catalysts had a smaller particle size (9.1 nm) and a higher surface area (299.5 m² g⁻¹) compared to MnO_x/P25-TiO₂ catalysts. The catalysts show high catalytic activity with the ozone oxidation of toluene even at low temperature. As a result, the synthesized support material by the CVC method gave more active catalyst.     

Electroreduction of oxygen on Pt nanoparticle/carbon nanotube nanocomposites in acid and alkaline solutions

The kinetics of O₂ reduction on novel electrocatalyst materials deposited on carbon substrates were studied in 0.5 M H₂SO₄ and in 0.1 M NaOH solutions using the rotating disk electrode (RDE) technique. Pt nanoparticles (PtNP) supported on single-walled (PtNP/SWCNT) and multi-walled carbon nanotubes (PtNP/MWCNT) were prepared using two different synthetic routes. Before use, the CNTs were cleaned to minimize the presence of metal impurities coming from the catalyst used in the synthesis of this material, which can interfere in the electrochemical response of the supported Pt nanoparticles. The composite catalyst samples were characterised by transmission electron microscopy (TEM) showing a good dispersion of the particles at the surface of the carbon support and an average Pt particle size of 2.4 ± 0.7 nm in the case of Pt/CNTs prepared in the presence of citrate and of 3.8 ± 1.1 nm for Pt/CNTs prepared in microemulsion. The values of specific activity (SA) and other kinetic parameters were determined from the Tafel plots taking into account the real electroactive area of each electrode. The electrodes exhibited a relatively high electrocatalytic activity for the four-electron oxygen reduction reaction to water.     

Production of single-walled carbon nanotubes with narrow diameter distribution using iron nanoparticles derived from DNA-binding proteins from starved cells

Iron nanoparticles derived from DNA-binding proteins from starved cells (Dps) were used to grow single-walled carbon nanotubes (SWCNTs) with narrow diameter distribution. An atomic force microscopy, Raman spectroscopy, and photoluminescence were used for evaluation of diameter or chirality distribution of the SWCNTs. We found that thin SWCNTs (1.1 nm diameter) were grown from the large Dps-derived nanoparticles (2.4 nm diameter) on and above the substrates. From the size comparison with ferritins and Co-filled apoferritins, we also found that SWCNTs become thinner as the catalyst becomes smaller. The synthesis of smaller catalysts (ca. 1 nm diameter) and their use for growth becomes crucial for the control of SWCNT diameter.     

MWCNT activation and its influence on the catalytic performance of Pt/MWCNT catalysts for selective hydrogenation

Multi-walled carbon nanotubes were submitted to three activation procedures: nitric acid oxidation, ball-milling and air oxidation. The influence of these treatments on nanotubes surface chemistry and morphology was evaluated by XPS, Raman and infrared spectroscopy, TGA, TPD, nitrogen adsorption and TEM. The three activated materials were used to prepare Pt supported catalysts from the organometallic precursor [Pt(CH₃)₂(COD)]. The influence of the activation treatments, together with that of a post-reduction thermal treatment, on the performances of the catalytic systems in the selective hydrogenation of cinnamaldehyde was investigated. It was shown that the best compromise between catalyst activity and selectivity requires a low amount of oxygenated groups on the support surface of the final catalyst, typically less than 700 μmol/g CO + CO₂ evolved during TPD experiments, together with an optimized platinum particle size ranging between 10 and 20 nm.     

Effects of extraction temperature, extraction pressure and nozzle diameter on micronization of cholesterol by RESS process

Micronized cholesterol particles were produced via the Rapid Expansion of Supercritical CO₂ Solutions (RESS) process. Taguchi design was used for designing the experimental plan to investigate the effects of three parameters including extraction temperature (40-60 °C), extraction pressure (100-160 bar) and nozzle diameter (0.15-0.24 mm) on the size and morphology of the cholesterol particles produced by the RESS process. The characterization of the particles was carried out using scanning electron microscopy (SEM) and X-ray diffraction (XRD) measurements to evaluate the performance of RESS process. The average particle size of the original material was 55 μm ± (2.84) while the average particle size of cholesterol after size reduction via the RESS process was between the minimum of 0.62 μm ± (0.03) and the maximum of 4.83 μm ± (0.18) depending upon the experimental conditions used. It was observed that both increasing the temperature from 40 to 60 °C and increasing the nozzle diameter from 0.15 to 0.24 mm result a reducing effect on the average particle size, whereas extraction pressure (100-160 bar) change has slight effect on the average particle size.     

Purification of single-walled carbon nanotubes using a fixed bed reactor packed with zirconia beads

Single-walled carbon nanotube (SWCNT) soot produced by arc discharge was purified through gas and liquid phase oxidations. In the gas-phase oxidation, zirconia beads with different diameters of 1, 5, and 10 mm were packed together with raw SWCNT soot inside a vertical quartz tube to enhance air flow uniformity and an exposed surface area of the raw soot during thermal oxidation in air. A decrease of the bead sizes led to such a stronger oxidation of carbonaceous impurities that ∼10 wt.% higher weight loss was then achieved with the 1 mm beads than without them. A subsequent HNO₃ treatment and the second thermal oxidation were engaged to improve further the purity of SWCNTs. Thermogravimetric (TG) analysis, scanning electron microscopy, high resolution transmission electron microscopy, and Raman spectroscopy were used to characterize the samples. The derivative TG (DTG) curves were deconvoluted to quantitatively determine the SWCNT purity of the samples. Our final purified samples showed a yield of ∼26%, a metal impurity of ∼7% and a SWCNT purity of ∼83% as calculated from the deconvoluted DTG curves.     

Solar synthesis of single-walled carbon nanotubes at medium scale

Single-walled carbon nanotubes were synthesized in gram quantities with a 50 kW solar reactor. Vaporization rate was in the range 6-15 g/h and two samples were collected for each run. The structure of the products was studied by SEM, TEM and Raman spectroscopy as a function of the buffer gas (helium or argon), the target length and the collected location. Good quality samples containing 1.2-1.6 nm diameter SWNTs were produced with helium at 450 hPa, 8 Nm³/h and a 15 cm target length using 2 at.% Ni and 2 at.% Co as catalyst. The material quality increased with the target length in helium. On the contrary, poor quality product was obtained whatever the target length in argon. TGA analysis showed that the best product purity was at least 35 wt%.     

Nondestructive purification of single-walled carbon nanotube rope through a battery-induced ignition and chemical solution approach

A two-step method for the purification of single-walled carbon nanotube (SWCNT) rope containing substantial catalyst particles embedded in carbonaceous shells was developed. The first step was the triggering of rope ignition using a 9-V battery, which resulted in pre-oxidization of the carbon shells on the Fe₃C catalyst and oxidation of the exposed Fe₃C to form Fe₂O₃. In addition, SWCNTs with open-end structures due to ignition-induced cutting remained. In the second step, both oxalic acid (H₂C₂O₄) and hydrochloric acid (HCl) were used as the reactants to remove the Fe₂O₃ particles. No damage on the SWCNT walls after H₂C₂O₄ or HCl purification was found. In addition, adsorption of H₂C₂O₄ was also found on the H₂C₂O₄ purified SWCNT rope and it can be effectively removed by heating the rope at 200 °C in vacuum for 40 min. Samples were characterized by SEM, TEM, Raman spectroscopy, TGA, FTIR, XPS, and UV-Vis-NIR.     

Effect of particle dimension on biocompatibility of carbon nanomaterials

With various emerging applications ranging from medicine to materials and electronics, the risk of exposure to nanomaterials is rapidly increasing. Several routes of exposure to nanomaterials exist; the most important being dermal contact and inhalation. In this dermal toxicity study, the cellular effects of carbon-based materials with diameters ranging from micro- to nano-dimension were investigated using mouse keratinocytes (HEL-30). The carbon materials tested included carbon fibers (CF; 10 μm diameter), carbon nanofibers (CNF; 100 nm diameter), multi-walled carbon nanotubes (MWCNT; 10 nm diameter), and single-walled carbon nanotubes (SWCNT; 1 nm diameter). CF and CNF did not significantly affect cell viability; however, MWCNT and SWCNT reduced cell viability in a time-dependent manner up to 48 h, with full recovery of mitochondrial function by the 72 h time point. After a 24 h exposure, cells exposed to MWCNT produced up to 3-fold higher increase in reactive oxygen species than those exposed to SWCNT. The results of this study suggest that high-aspect ratio carbon material toxicity is dependent on dimension and composition.     

Continuous production of single-walled carbon nanotubes using a supported floating catalyst

Using catalytic decomposition, a technique for the production of singe-walled carbon nanotubes (SWCNTs) is reported with a production rate up to 6 g h⁻¹ after purification, and scaling capability up to 220 g h⁻¹. This is achieved by injection of pre-prepared alumina supported catalyst powder into a modified vertical floating reactor. The product is collected in several cyclones connected in series. Wide range Raman studies (laser excitations from λ = 1064 to 488 nm) and temperature programmed oxidation measurements of the samples collected from the different cyclones show that SWCNTs were separated in situ by tube diameter. This is attributed to the different residual times of the catalyst in the reaction zone depending on particle diameter. A series of computational fluid dynamics calculations of the flow and heat transfer in the reactor, as well as modeling of catalyst particle transport reveals the parametrical dependence of the process.     

Effect of potential cycling on structure and activity of Pt nanoparticles dispersed on different carbon supports

Platinum particles synthesized via the Bönnemann methods were dispersed on two different Vulcan XC72 carbon supports. One was used after thermal treatment at 400 °C under nitrogen atmosphere, the other after oxidation of its surface by acid leaching using diluted HNO₃ in water (1/3). Characterization of the carbon support indicated that HNO₃ treatment leads to the decrease of the BET surface and to the increase of the surface acidity of the carbon support. After dispersion of the platinum catalyst, TEM results indicated that the mean particle size was a little higher on the non-oxidized support (Pt/XC72) than that on the functionalized one (Pt/XC72_HNO3), being 2.5 and 2.0 nm, respectively. However, potential cycles from 0.05 to 1.25 V vs. RHE led to a higher increase of the particle size when catalyst is dispersed on the functionalized support, reaching after 400 potential cycles 5.5 nm against 4.0 nm with the non-functionalized one. The effect of the upper limit (1.0 and 1.25 V vs. RHE) of the potential cycles on the active surface area and on the activity towards the oxygen reduction reaction (orr) was determined for both catalysts. Results indicated that the particle growth was not the main degradation process over the whole duration of the electrochemical experiments, but that dissolution/redeposition (Otswald ripening) was also involved. The predominant role of each degradation process depends on the number of cycles, on the upper potential limit and on the carbon surface state, and could be temporally separated. However, the lower activity towards orr was recorded for the (Pt/XC72_HNO3) cycled up to 1.0 V vs. RHE.     

Controlling the diameter of aligned single-walled carbon nanotubes on quartz via catalyst reduction time

One of the main objectives of chemical vapor deposition (CVD) growth of single-walled carbon nanotubes (SWCNT) is control over their diameter and type (metallic or semiconducting). Here, we investigate the evolution of iron catalyst particles on quartz substrates depending on the duration of the reduction step with hydrogen and its effect on the growth of horizontally aligned SWCNT. We find a strong dependence of catalyst particle size and size distribution on the initial iron film thickness and the reduction time at 630 °C. Initial decrease of the particle size is followed by an unexpected increase. Statistical analysis of the Raman radial breathing modes of the SWCNT over large areas gave reliable and reproducible diameter distributions that correlated directly with the catalyst particle size distributions. By changing the reduction time it was possible to reproducibly shift the average SWCNT diameter from 1.5 (±0.3) nm to 1.2 (±0.2) nm while maintaining a nanotube density of 5–6 SWCNT/μm. In order to describe the evolution of the particle size during the reduction process at this particular temperature, we propose a model that includes the diffusion of iron into the quartz and its re-emergence.     

Single-walled carbon nanotube synthesis using ferrocene and iron pentacarbonyl in a laminar flow reactor

A gas-phase process of single-walled carbon nanotube (SWCNT) formation, based on thermal decomposition of iron pentacarbonyl or ferrocene in the presence of carbon monoxide (CO), was investigated in ambient pressure laminar flow reactors in the temperature range of 600-1300 °C. Ferrocene was found to be a better catalyst precursor in the studied conditions since iron pentacarbonyl decomposes at lower temperatures resulting in the excessive growth of catalyst particles. In situ sampling carried out at 1000 °C showed that the SWCNT growth occurred from individual metal particles in the heating section of the furnace in the temperature range of 891-928 °C, in which the growth rate was estimated to exceed . FT-IR measurements of gaseous products revealed that the majority of the CO disproportionation took place on the reactor walls. Raman measurements confirmed the results of TEM observations, namely, the formation of very high purity SWCNT product. On-line aerosol number size distributions were measured to detect the conditions of SWCNT formation and the product morphology changes. Mechanism of SWCNT formation in the gas phase from ferrocene and CO is discussed.     

Fabrication of CNTs with controlled diameters and their applications as electrocatalyst supports for DMFC

A facile synthesis procedure based on chemical vapor deposition (CVD) process has been developed to fabricate carbon nanotubes (CNTs) with controlled diameters and high yields utilizing Fe-containing ordered hexagonal mesoporous silicas (HMSs) such as MCM-41 and SBA-15 having varied pore sizes as the catalysts as well as the templates. It is found that unlike Fe/HMS catalysts prepared by co-precipitation method, samples prepared by the impregnation method gave rise to multi-wall CNTs with uniform diameters, which were largely dictated by the pore size of the Fe/HMS catalysts. Among these uniform MWCNTs, sample with a larger diameter (≥ 8 nm) was found to be more favorable as support for Pt catalyst, leading to a homogeneous dispersion of metal nanoparticles. Consequently, the Pt/CNT electrocatalysts so prepared gave rise to superior methanol oxidation activities as well as tolerances for CO poisoning compared to Pt supported on commercial single-wall CNT (Pt/SWCNT) and XC-72 activated carbon (Pt/XC-72) having a similar metal loading.     

SWCNT掺杂改性对纳米TiO_2光催化的影响

采用气相氧化法制备高纯度单壁纳米碳管(SWCNT)。用溶胶-凝胶法制备TiO2/SWCNT复合光催化剂。利用热重分析和X射线测试手段对样品的热处理过程、晶体结构进行了测试。结果表明,550℃时SWCNT有较好的稳定性,纯度较高;SWCNT的加入抑制了颗粒TiO2的变大;SWCNT的掺杂量(质量比)在0.9%时,光催化活性最佳。     

Mass production of high-quality multi-walled carbon nanotube bundles on a Ni/Mo/MgO catalyst

A new catalyst (Ni/Mo/MgO) is reported, with which one can synthesize multi-walled carbon nanotube (MWNT) bundles with a yield of more than 45 times the amount of the pristine catalyst, using a methane-hydrogen mixture as precursor. Powder X-ray diffraction, Raman spectroscopy and thermal gravimetric analysis show that the purity of the as-prepared MWNTs is over 97%. The diameter of the carbon nanotubes is 9-20 nm, measured by high-resolution electron microscopy on 421 individual MWNTs. The high purity of the as-prepared MWNTs allows us to omit the usual complex purification process required for carbon nanotubes synthesized by chemical vapor deposition. Because of its durable high activity, the Ni/Mo/MgO catalyst in its pristine state is ideal for mass production of high-quality MWNTs. The synergism of nickel and molybdenum is considered the main reason for the high yield of carbon nanotubes.