Towards the production of carbon xerogel monoliths by optimizing convective drying conditions

Resorcinol-formaldehyde hydrogels prepared at various resorcinol/sodium carbonate ratios, R/C, were convectively air dried. The influence of the drying operating conditions, i.e. air temperature and velocity, on the pore texture, shrinkage and cracking of the dried gels were investigated. Shrinkage was found to be isotropic. The shrinkage behaviour and the textural properties of the gels are independent of the drying operating conditions, but are completely determined by the value of the synthesis variables. The analysis of the drying kinetics shows two main drying periods. During the first phase, shrinkage occurs and the external surface of the material remains completely wet: heat and mass transfers are limited by external resistances located in a boundary layer. When shrinkage stops, the second period begins: the evaporation front recedes inside the solid and internal transfer limitations prevail. The drying time can be reduced by increasing the air temperature and/or velocity, but the temperature increase is limited when monolithicity is required, especially when the pores are small. For example, at a temperature of 160 °C and a velocity of 2 m/s, about 1 h is needed to dry a 2.8 cm in diameter and 1 cm in height cylinder containing macropores (pore width > 50 nm after drying). The same cylinder presenting small mesopores (pore width = 10-15 nm after drying) requires 20 h at 30 °C and 2 m/s to reach complete dryness without the development of cracks.     

Carbon aerogels, cryogels and xerogels: Influence of the drying method on the textural properties of porous carbon materials

Carbon materials with tailored texture can be obtained from drying and pyrolysis of resorcinol-formaldehyde gels. The pore texture of both dried and pyrolyzed material depends on the drying process. Several more or less expensive methods (supercritical drying, freeze-drying, evaporative drying) were tested in order to determine which process is the most suitable for the synthesis of a porous carbon with a definite texture. Supercritical drying leads to the highest pore volume and the widest texture range, but residual surface tensions and shrinkage are not avoided when the pore size is small or when the material density is low; this hampers to fix both the pore volume and the pore size easily. Monoliths are very difficult to obtain by freeze-drying, and the appearance of huge channels due to ice crystal growth at high dilution ratio hinders the fabrication of low density materials. Moreover, gels with small pores do not remain frozen throughout drying, which leads to surface tensions and shrinkage. Although generally replaced by more complicated techniques, evaporative drying is suitable when dense carbons are needed or when the only selection criterion is the pore size: all pore sizes are reachable, but this parameter is in this case strongly correlated to the pore volume.     

Porous carbon xerogels with texture tailored by pH control during sol-gel process

Despite commonly accepted ideas, evaporative drying does not always completely destroy the pore texture of phenolic gel. This work shows that very porous carbon materials can be synthesized by evaporative drying and pyrolysis of aqueous resorcinol-formaldehyde gels provided that the operating variables are correctly chosen. Moreover, in this manner monoliths can be easily produced. The pore texture of the materials was studied before and after pyrolysis in order to determine which synthesis and/or pyrolysis variables have an influence on the final texture of the carbon. Results show that it is possible to tailor the morphology of these materials by varying the initial pH of the precursors solution in a narrow range. Micro-macroporous, micro-mesoporous, microporous or totally non-porous carbon materials were obtained. The specific surface area is independent from the initial pH whereas the total void volume varies from 0.4 to 1.4 cm³/g when the initial pH decreases from 6.25 to 5.45. These materials can be used as catalysts supports or for electrochemical applications, the texture control being an interesting advantage.     

Synthesis of transition metal-doped carbon xerogels by solubilization of metal salts in resorcinol–formaldehyde aqueous solution

The pore texture of carbon materials obtained from evaporative drying and pyrolysis of resorcinol–formaldehyde aqueous gels is controlled by the initial pH of the precursors solution. In order to produce transition metal-containing carbons with tailored texture, various metallic salts were dissolved in the precursors solution. When necessary, a complexing agent (HEDTA or DTPA) was added to render the metal ions soluble. Ni, Fe and Pd loaded carbon xerogels were synthesized and their pore texture was studied after evaporative drying and after pyrolysis. The carbon texture was also studied with regard to the nature of the metal and the amount of complexing agent. The solubilization of transition metal salts in the resorcinol–formaldehyde aqueous solution does not prevent the texture regulation, even though this texture control is influenced: the limits of the pH interval leading to micro–mesoporous carbon materials can slightly differ when a metal salt and/or a complexing agent are added. The pH range shift depends mainly on the amount and nature of the complexing agent, but also slightly on the nature of the metal ion. Nevertheless, the metal particles obtained are rather big (diameter > 15 nm). For catalytic applications, the metal dispersion must be enhanced, especially in the case of expensive metals.     

A practical route to the production of carbon nanocages

Carbon-coated iron nanoparticles with diameters ranging from 5 to 50 nm and a few layers of graphitic shells have been synthesized in a mass scale by laser-induction complex heating evaporation. Through the studies on the dissolution behavior of the inner iron cores under the treatments of HCl and HNO₃ acids at different conditions, a practical route with the combined treatments of HNO₃ and HCl acids has been optimized to produce carbon nanocages. The nanocages thus obtained have some channels and are full of defects in the shells, as characterized by high-resolution transmission electron microscopy and Raman spectroscopy. Similar treatments should also be applicable to some other carbon-coated metal nanoparticles, e.g., Ni-C and Co-C, for the same purpose.     

Evidence for the benefit of adding a carbon interphase in an all-carbon composite

T-800 polyacrylonitrile-based carbon fibres were coated with pyrolytic carbon and carbon/carbon/carbon (C/C/C) composites were prepared from them using coal tar pitch as the matrix precursor. Composites were characterised regarding pyrolysis behaviour, mechanical properties, microtexture, nanotexture, and fracture behaviour. All of the composite components, including interfacial areas, were characterised by high resolution transmission electron microscopy. It was checked that the presence of the carbon interphase did not affect the development of the matrix texture, nor deeply modified the fibre surface energetics. However, adding a pyrolytic carbon interphase resulted in improved mechanical properties for the C/C/C composites with respect to the similarly prepared but interphase-free composites (C/C), such as the increase in the flexural strength by a factor of five, and that of the flexural modulus a factor of two. It is shown that such a benefit brought by adding a carbon interphase is possible merely through the appropriate texture of the latter. Typically, the interphase has to exhibit features from both the matrix (anisotropic) and the fibres (isotropic) so that to reduce the discontinuity effect at the fibre/matrix contact, and should also exhibit features (elongated porosity) that promote multiple micro-cracking from the primary cracks, so that to somewhat absorb part of the fracture energy.     

The effects of nitrogen and boron doping on the optical emission and diameters of single-walled carbon nanotubes

Using TEM, absorption and photoluminescence-excitation spectroscopy we have shown that nitrogen and nitrogen/boron doping of single-walled carbon nanotubes produce significant changes in both the optical properties and the diameter distribution of nanotubes produced by the arc-discharge method. Smaller diameter tubes are preferentially formed in the presence of boron. In addition the presence of nitrogen is found to significantly affect the emission properties of the nanotube ensemble, causing a shift in the dominant emission to lower energies, possibly due to changes in the bundling structure of the nanotubes in solution, but only very small changes are observed in the emission energies for individual nanotubes.     

Synthesis of carbon nanocapsules and carbon nanotubes by an acetylene flame method

The fabrication of carbon nanocapsules and carbon nanotubes (CNTs) using an acetylene flame method was investigated. Carbon nanocapsules, a graphitic structure of nanoparticles with a hollow core, were synthesized using catalyst-free acetylene flames while CNTs were formed with the presence of cobalt-based catalysts in addition to acetylene flames. When the synthesis of these materials was carried out, the results showed that a massive amount of high-purity carbon nanocapsules with a particle size in the range of 15-30 nm can be produced with the acetylene flame method. The CNTs produced were multi-walled carbon nanotubes measuring a few micrometers in length and 20-30 nm in diameter. The acetylene flame method holds great potential for the cost-effective production of CNTs as well as carbon nanocapsules.     

TEM image simulation study of small carbon nanotubes and carbon nanowire

Recent findings of extremely small diameter carbon nanotube and nanowire in the core of a multi-walled carbon nanotube (MWCNT) have attracted interests from broad range of researchers. Direct observation of carbon nanotube is usually done using a transmission electron microscope (TEM). When nanotubes become smaller, it becomes harder to correctly understand the TEM images, not only because of the weak scattering, but also due to the artifact that starts to appear because of the interference effect and the inappropriate defocus condition.In this study, we have shown that the artifact such as ghost fringes due to inappropriate defocus conditions of the TEM appear in the core of an MWCNT, and can be misinterpreted as either carbon nanowire or small carbon nanotube. It is also shown that, in the TEM image, it is hard to distinguish a single-walled nanotube bundle from a double-walled carbon nanotube bundle. Finally, we propose that the cross-sectional observation is necessary for the correct characterization of single- and double-walled carbon nanotube bundles.     

Synthesis of well-aligned carbon nanotubes with open tips

Large amounts of well-aligned carbon nanotubes (CNTs) with open tips have been produced by pyrolysis of iron(II) phthalocyanine. The aligned CNTs have an average length about 10 μm and diameters ranging from 92 to 229 nm. Some of produced CNTs showed Y-junction structure due to the self-joint growth of two neighboring CNTs. The well-aligned CNTs indicated a bamboo-shaped multiwalled structure and fairly good crystallinity. The aligned CNTs follow tip growth mechanism.     

Removal of crystal violet from wastewater using resorcinol-formaldehyde carbon xerogels

Resorcinol-formaldehyde activated carbon xerogels (RF-ACXs) were prepared at different conditions. RF-ACXs were characterized by Fourier transform infrared spectroscopy, surface area and porosity analyzer, X-rays diffraction, energy dispersive X-ray diffraction, and scanning electron microscopy. Batch adsorption was used to evaluate the ability of RF-ACXs to remove crystal violet (CV) from wastewater. CV adsorption capacities of RF-ACXs ranged from 66.47 to 121.53 mg·g^?1 for CV initial concentration ranged from 0 to 120 mg/L at 60°C. Kinetic and equilibrium adsorption data were analyzed using collection of models. The best results were achieved with the pseudo-second order kinetic model and Langmuir’s equilibrium model. Regenerability and reusability of RF-ACXs were investigated.     

Growth of carbon nanotubes and nanofibres in porous anodic alumina film

By acetylene pyrolysis at 650 and 550°C, carbon nanotubes were synthesized successfully in porous alumina templates anodized in sulfuric and/or oxalic acid solution. For templates anodized in oxalic acid followed by boiling in distilled water, thermal decomposition of acetylene at 650°C in the pores results in the formation of carbon nanofibres. For templates anodized in sulfuric acid, only carbon nanotubes were formed, even if boiling in water was adopted to process it. This indicates that the modifications of the catalytic effects in acetylene pyrolysis by boiling in water are different for these two types of templates. All the carbon nanotubes and nanofibres have similar lattice structures under HRTEM examination. No carbon nanotubes or nanofibres can be formed when the chemical vapour deposition temperature decreases to 500°C.