As-prepared Multi-walled Carbon Nanotubes Research Articles

Novel Cu and Pd-Cu Catalysts Supported on Multi-Walled Carbon Nanotubes for Steam Reforming and Decomposition of Methanol

In this study, multi-walled carbon nanotubes (MWCNTs) were prepared by chemical vapour deposition (CVD) using acetylene as a carbon source over an iron catalyst. As-prepared MWCNTs were used to support modern mono-copper, palladium, and bimetallic palladium-copper catalysts, and their feasibility for hydrogen production was tested during steam reforming of methanol (SRM) and methanol decomposition (DM). The structural characteristics of the MWCNTs were evaluated using the SEM and XRD methods. The physicochemical properties of the monometallic and bimetallic catalysts were analysed using the TPR and XRD methods. The promotion effect of palladium on methanol conversion rate and H2 productivity in the case of the copper catalysts was demonstrated. The enhanced activity of the Cu/MWCNTs after palladium promotion was due to the formation of Pd-Cu alloy compound.

Temperature-dependent synthesis of multi-walled carbon nanotubes and hydrogen from plastic waste over A-site-deficient perovskite La0.8Ni1-xCoxO3-δ

Thermochemical conversion of plastic wastes into carbon nanotubes (CNTs) and hydrogen is a promising management option to eliminate their hazardous effect. The yields and morphologies of CNTs strongly depend on the catalyst design and reaction conditions. To boost the efficiency, tuning of bimetallic nanoparticles as catalyst is an effective approach. For that reason, A-site-deficient perovskite La0·8Ni1-xCoxO3-δ (LN1-xCx, x = 0.15, 0.5, 0.85) was developed and used as a catalyst precursor to achieve in situ formation of bimetallic Ni–Co nanoparticles. At an optimized Ni-to-Co ratio, the LN0.5C0.5 exhibited the highest yields of multi-walled CNTs, namely 840 and 853 mg/gcatalyst from high density polyethylene and polypropylene, respectively. This could be attributed to the higher catalytic capability of LN0.5C0.5 catalyst for the decomposition of hydrocarbons into hydrogen and carbon. In both cases, multi-walled CNTs had regular shapes when the reaction temperature was 700 °C. At higher reaction temperatures, the morphological changes of carbon products were observed from multi-walled CNTs to carbon nano-onions. The Raman spectra showed that compared with the commercial multi-walled CNTs, the as-prepared multi-walled CNTs had a lower degree of defects.

Evaluation of carbon-based nanostructures suitable for the development of black pigments and glazes

Five different carbon-based samples were chosen for the fabrication of black pigments and glazes: historical ‘unknown’ wood charcoal, graphite, tailored graphitized soot, as-prepared multi-walled carbon nanotubes (MWCNTs) and purified MWCNTs. Ceramic glazes with carbon pigments were prepared with Pb3O4 and SiO2 in molar ratio of 2.85:1.9 and using 5 wt. % of pigment at 900 °C. The black pigments and glazes were characterized by scanning electron microscopy (SEM), optical microscopy, diffuse reflectance spectroscopy and contact angle measurements. International Commission on Illumination (CIE) parameters L*, a* and b* (CIELab-parameters) of the samples were also determined. It was demonstrated that nanostructured carbon pigments showed better black colour and higher quality of black ceramic glaze. Our investigation let us to conclude that these results possibly were affected by initial microstructure of applied carbon derivatives. The obtained nanostructured carbon black pigments could be successfully used for artefact restoration.

Efficient gas-phase purification using chloroform for metal-free multi-walled carbon nanotubes

Gas-phase treatments using chloroform (CHCl3) and water vapor were used to remove metal impurities (MIs) from as-prepared multi-walled carbon nanotubes (AP-MWCNTs), which is a non-destructive way to produce highly pure CNTs with extremely low MI content and with retention of CNT properties. After purification using CHCl3, the MIs in the AP-MWCNTs had significantly decreased to 12 ppm, representing a purification efficiency of 99.8%. This purification method, applicable to various AP-MWCNTs with high contents and different compositions of MIs, is more effective than liquid-phase purification using acids and does not damage the CNT structure and morphology. This moderate purification is due to a combination of factors. Chlorine radicals and hydrochloric acid from the thermal decomposition of CHCl3 react with MIs encased by graphitic carbon layers to form metal chlorides that sublime at high temperature. Hydrogen, oxygen, and water vapor generated during the formation of the metal chlorides etch graphitic carbon layers and lead to the formation of new pores and cracks, which provide fast reaction and diffusion paths. During metal purification, the MWCNT surface becomes lightly coated with chlorine or chlorinated carbon, which are eliminated from the surface post-treatment using water vapor.

Enhancement of photocurrent in Cu2ZnSnS4 quantum dot-anchored multi-walled carbon nanotube for solar cell application

Photoconductivity of kesterite CZTS (Cu2ZnSnS4) quantum dots (QDs) anchored on multi-walled carbon nanotubes (MWCNTs) hybrid nanostructures has been investigated here, which take the advantages of both CZTS QDs and MWCNTs. The objective material CZTS QDs-anchored MWCNTs hybrid nanostructure have been prepared by a simple solution casting approach without doing any surface modifications. As-prepared MWCNTs, CZTS QDs and CZTS QDs–MWCNTs hybrid nanostructure have been characterized by XRD, Raman, FTIR, XPS, FESEM and TEM, and UV–visible spectroscopy. The improved optical gain of CZTS QDs–MWCNTs has been observed by UV–visible spectroscopy analysis. Other characterizations and TEM micrographs confirmed the adherence of CZTS QDs on MWCNTs. The photoconductivity of the above prepared hybrid nanostructure has been measured both in dark and under illumination (1.5G solar simulator). The hybrid nanostructure demonstrated improved photocurrent compared to bare CZTS QDs and MWCNTs. The obtained improved photocurrent is due to high optical gain by CZTS QDs and fast charge carrier transport by MWCNTs throughout in the hybrid nanostructure. The enhanced photocurrent and the stability of the CZTS–MWCNTs hybrid nanostructure ensure a possible application in future solar cell.

Lithographically Cut Multiwalled Carbon Nanotubes: Opening Caps, Controlling Length Distribution, and Functionalization

This work presents a microwave digestion heating method using a 40 ml mixture of potassium permanganate and sulfuric acid (3/1 mol) to cut, end-open, and functionalize as-prepared multiwalled carbon nanotubes (A-MWCNTs). Also, the ability of the microwave digestion method was studied at different reaction times (5, 10, 20, 30, and 40 minutes). To harness the extent of treatment, shortened multiwalled carbon nanotubes (S-MWCNTs) were fully characterized by UV-visible, Fourier transform infrared spectroscopy, and Raman spectroscopies, as well as, transmission electron microscopy and optical microscopy. The results showed that, with this mild treatment, the A-MWCNTs (>80 µm) were efficiently and significantly shortened to below 1 µm after 40 minutes. Also, heating the closed reaction vessel under microwave irradiation at 90°C for 20 minutes was very effective in the functionalization of MWNTs. At the same time, the S-MWCNTs formed the best state of dispersion in water without the help of surfactants that provided possibility for further functionalizations. For the first 10 minutes, crystallinity increased then decreased until 20 minutes, and after that improved slightly. Finally, the mechanism of this process is explained schematically. This method could be used as high-efficiency and damage-free process to the cut, end-open, and functionalization of CNTs that led to dispersion of CNTs and ultimately for such application.

Optical Studies on Multiwalled Carbon Nanotubes via Modified <i>Wolff-Kishner</i> Reduction Process

Multiwalled carbon nanotubes (MWCNTs) have been successfully synthesized by using a relatively simple method, known as modified Wolff-Kishner reduction process. Transmission electron microscopy (TEM) has shown the as-prepared MWCNTs possess straight morphologies with average inner and outer diameters, between 2 to 7 nm and 5 to 15 nm, respectively. Ultraviolet-visible (UV-Vis) absorption measurement has been conducted for the first time. The correlation between energy of the π plasmon absorbance (Eπ) and nanotube diameter (dCNT) for the as-prepared MWCNTs shows contradiction to the empirical relationship as Eπ = 4.8 + 0.7 / (dCNT)2. Other optical characteristics like transmittance, reflectance and refraction index also have been studied. This work explores the optical property of the as-prepared MWCNTs and thus provides better understanding about the feasibility of this synthesis technique.

Length Characterization of Multi-Walled CNT by Capillary Hydrodynamic Fractionation (CHDF)

Multi-walled carbon nanotubes (MWCNT) should be treated via cutting process for engineering applications because almost as-prepared MWCNT are highly entangled and aggregated. Especially, the size distribution of MWCNT after cutting process is very important factor to determine the final properties of polymer-MWCNT composite. In this study, we take a novel method for CNT characterization, a capillary hydrodynamic fractionation (CHDF), which is a traditional chromatographic method for colloid particles. The length distributions of pre-treated MWCNT by two different methods were investigated by CHDF. One is a high-power ultrasonication directly and the other is an acid treatment followed by a high-power ultrasonication. For MWCNT, retention volume apparently depends on a combination of the length, diameter and rigidity of CNTs. We found that a high-power ultrasonication renders MWCNT short into segments of a few micrometers and the additional treatment by acid solution made those much shorter and more flexible.

Study of Methane Storage and Adsorption Equilibria in Multi-Walled Carbon Nanotubes

Adsorbed natural gas has various advantages and is relatively more economical than liquefaction and compression. Carbon nanotubes can be introduced as a new candidate for natural gas storage. In this study, adsorption of methane was firstly studied on the as-prepared multi-walled carbon nanotubes, and then chemical and physical treatment of MWCNTs was performed to enhance the methane adsorption. Treatment by acid washing and annealing with air improved purity, surface area and methane adsorption. The adsorption and equilibrium isotherm data of T-MWCNTSs, were measured by a static volumetric technique at different temperatures of 298, 291, 273 and 263 K and pressures up to 7 MPa. The maximum value of methane storage at normal temperature of 298 K was achieved to 2.81 mmole/g in our experiments. This amount of methane adsorption is equal to 108 v/v, meanwhile the target value of the adsorbed natural gas is 120 v/v to become as the accepted material for ANG process. The isosteric heat of adsorption of T-MWCTs was determined in the studied range of pressures and temperatures. The results revealed an energetically heterogeneous surface in methane adsorption. Furthermore, different isotherm models were fitted on the experimental adsorption data and the model parameters were correlated. Within the different studied isotherms, Sips equation provided best fitting to the experimental data.

Synthesis and evaluation on performance of hydrogen storage of multi-walled carbon nanotubes decorated with platinum

By means of chemical reduction, nanoparticles of platinum were deposited on the surface of multi-walled carbon nanotubes (MWCNTs). The performance of hydrogen storage of as-prepared MWCNTs decorated with platinum was investigated. The results indicate that: (1) Hydrogen uptake is more quick and intense for decorated MWCNTs than that for not decorated ones at 10.931 MPa and room temperature. The saturation of hydrogen uptake of the former only lasts about 30 min, while the latter needs about 150 min; (2) The amount of hydrogen uptake of decorated MWCNTs is about 1.13wt%, which is larger than that of not decorated ones (about 0.54wt%); (3) However, more than 37% hydrogen absorbed by decorated MWCNTs is chemisorbed.