Thermal Oxidation of Carbonaceous Nanomaterials Revisited: Evidence of Mechanism Changes

Kinetic data, for example, activation energy and reaction order, are crucial for the understanding of chemical reactions and processes. Here, we describe a novel method for obtaining kinetic data based on thermogravimetric measurements (TGA) that exploits in each measurement multiple successive isothermal steps (SIS). We applied this method to the notoriously challenging carbon combustion process for vastly different carbons for oxygen molar fractions between 1.4 % and 90 %. Our obtained apparent EA values are within the wide range of results in the literature and vary in a systematic way with the oxygen partial pressure. The improved accuracy and large amount of obtainable data allowed us to show that the majority of experimentally obtained apparent data for apparent EA are neither in a kinetic regime nor in a diffusion‐controlled one but rather in a transition regime

Conductive inks of graphitic nanoparticles from a sustainable carbon feedstock

Microwave plasma splitting of biogas to solid carbon forms is a promising technique to produce large quantities of sustainable carbon based nano materials. Well defined graphitic nano carbons have been produced exhibiting graphene multilayers in turbostratic packing. After heat treatment, the purified material has been used to formulate stable, aqueous dispersions. These dispersions are used directly as inks, allowing the preparation of conductive membranes with remarkable resistivity. Nano carbons derived by plasma processes constitute a promising alternative to carbon black because they can be prepared from renewable sources of methane or natural gas, are calibrated in size, exhibit high conductivity, and have promising perspectives for chemical and material science purposes

Covalent Functionalization of Carbon Nanotubes Through Organometallic Reduction and Electrophilic Attack

Organometallic reagents such as butyllithium are known to covalently functionalize the sidewalls of carbon nanotubes. The function grafted corresponds to the organic part of the alkali compound, while one negative charge is transferred to the nanotube for each function. Carbon nanotubes reduced by organolithium compounds were used here as nucleophilic reactive species through these transferred and delocalized charges. Various halogenated electrophiles in excess were reacted with them in anhydrous conditions. The grafting of the corresponding chemical function onto the carbon nanotubes through a Lewis metathetic exchange reaction was demonstrated by chemical, thermal, and spectroscopic analyses. This synthetic route applied successfully to both single-walled and multi-walled nanotubes and to a series of electrophiles. The extent of functionalization was found to depend on stoechiometries used, although a direct correlation could not be obtained

Electrospun lignin-based twisted carbon nanofibers for potential microelectrodes applications

International audienceLignin nanofibers (NFs) have been fabricated by electrospinning of a new formulation system: lignin, polyvinyl alcohol (PVA), dimethyl sulfoxide (DMSO) mixture. The ternary phase diagram of this system has been realized in order to define bead free nanofibers electrospinnability domains. Resulting NFs mat were carbonized in a temperature range 550–1200 °C and then characterized by X-Ray diffraction, Raman spectroscopy, and electrochemical measurements. It revealed the influence of the carbonization temperatures on the structural properties in the resulting CNFs. Lignin NFs yarns have been for the first time manufactured by twisting the initial electrospun lignin NFs precursor mats. The structure and properties of the obtained lignin-based twisted carbon nanofibers (TCNFs) have been characterized as function of the degree of twist. Electrochemical capacitance and electrical conductivity of the TCNFs have revealed the influence of an increase in the twisting level (from 330 mF.g⁻¹ to a few mF.g⁻¹ and from 11 to 22 S.cm⁻¹). It has been established that the compaction and the densification of highly TCNFs yarns are the reasons of these electrochemical and electrical behaviors. Manufactured bio-resourced conductive TCNFs yarns can be promising candidates as microelectrodes in biosensor or biofuel cells applications

Electromechanical properties of nanotube–PVA composite actuator bimorphs

Oxidized multiwalled carbon nanotube (oxidized-MWNT)/polyvinyl alcohol (PVA) composite sheets have been prepared for electromechanical actuator applications. MWNT have been oxidized by nitric acid treatments. They were then dispersed in water and mixed with various amounts of PVA of high molecular weight (198 000 g mol−1). The composite sheets were then obtained through a membrane filtration process. The composition of the systems has been optimized to combine suitable mechanical and electrical properties. Thermogravimetric analysis, mechanical tensile tests and conductivity measurements show that the best compromise of mechanical and electrical properties was obtained for a PVA weight fraction of about 30 wt%. In addition, one face of the sheets was coated with gold to increase the conductivity of the sheets and promote uniform actuation. Pseudo-bimorph devices have been realized by subsequently coating the composite sheets with an inert layer of PVA. The devices have been tested electromechanically in a liquid electrolyte (tetrabutylammonium/tetrafluoroborate (TBA/TFB) in acetonitrile) at constant frequency and different applied voltages, from 2 to 10 V. Measurements of the bimorph deflections were used to determine the stress generated by the nanotube–PVA sheets. The results show that the stress generated increases with increasing amplitude of the applied voltage and can reach 1.8 MPa. This value compares well with and even exceeds the stress generated by recently obtained bimorphs made of gold nanoparticles

Burn Them Right! Determining the Optimal Temperature for the Purification of Carbon Materials by Combustion

International audienceA new purification procedure for carbon nanoforms is proposed. It was tested on multiwall carbon nanotubes (MWCNTs) prepared by arc discharge, which is among the most challenging of cases due to the chemical and structural similarity between the MWCNTs and most of the impurities to be removed. Indeed, the various methods for synthesizing carbon nanoforms lead to a distribution of carbonaceous products, such as carbon shells, carbon spheres, fullerenes, and a variety of other species. Thus, many strategies to purify the desired products have been developed. Among the most successful ones, thermal oxidation (combustion) seems particularly efficient. To be successful while preserving a reasonable amount of MWCNTs, the combustion temperature has to be carefully selected. Moreover, the ideal combustion temperature does not only depend on the material to be treated but also on the overall system used to perform the reaction, including the reactor type and the parameters of the gaseous reactant. Typically, the optimization of the purification relies on multiple experiments and analysis of the products. However, to the best of our knowledge, a strategy to determine a priori the most suitable temperature has not been reported yet. We demonstrate here that a thermogravimetric method, namely the constant decomposition rate thermal analysis (CRTA), is particularly well adapted to answer this question. An isothermal treatment based on the results obtained from a CRTA program allowed arc-MWCNTs to be successfully purified from graphenic shells while optimizing the yield of the MWCNTs. This strategy is believed to be valuable not only for purifying MWCNTs but also for the purification of other carbonaceous forms, including new carbon nanoforms

Optimized carbon nanotube fiber microelectrodes as potential analytical tools

The preparation and interesting electrochemical properties of carbon nanotube (CNT) fiber microelectrodes are reported. By combining the advantages of CNT with those of fiber electrodes, this type of microelectrode differs from CNT-modified or CNT-containing composite electrodes, because they are made solely of CNT without other components, for example additives or binders. The performance of these electrodes has been characterized with regard to, among others, the electrocatalytic oxidation of analytes via dehydrogenase-mediated reactions. In this context the reversible regeneration of the coenzyme NAD+ using a mediator is a key step in the development of new amperometric sensor devices and we have successfully immobilized mediator molecules that are very efficient for this purpose on the surface of the CNT fiber electrode. The microelectrodes thus obtained have been compared with classic carbon microelectrodes and have promising behavior in biosensing applications, especially after specific pretreatments such as CNT alignment inside the fiber or expansion of the specific surface by chemically induced swelling

Carbon Nanotube Fiber Microelectrodes: Design, Characterization, and Optimization

We report on the preparation and interesting electrochemical behavior of carbon nanotube fiber microelectrodes (CNTFM). By combining the advantages of carbon nanotubes (CNT) with those of fiber electrodes, this type of microelectrode differs from CNT modified or CNT containing composite electrodes, because it's made of only CNT without any other components like additives or binders. The active CNT surface is easily regenerated. The performance of CNTFMs has been characterized, among others, by surface modification with phosphomolybdic acid. It is shown that adsorption behavior of these catalyst molecules is highly improved with a controlled orientation of CNT. A better CNT alignment inside the fiber can be achieved by a hot stretching procedure

Structuration of lignin-graphene oxide based carbon materials through liquid crystallinity

International audienceThis paper presents the approach of combining lignin with graphene oxide (CO) flakes in order to introduce a higher structuration inside amorphous carbon materials....