Studies on the solid-state transformation and chemical modification reactions of carbon black nanoparticles

Abstract

Carbon black (CB) is nanoscale particles with quasi-spherical nature, produced from incomplete combustion of hydrocarbons. It is a well-known type of amorphous carbon which exists in the form of aggregated spheres. CBs are most versatile materials, as the surface and structure can be easily tuned and/or modified, to target specific applications. The modification is generally straightforward in carbon materials both in nano and microscopic forms and it can either be done at the surface via chemical functionalization and/ or as structural transformation, where one form of carbon converts to another form either via catalytic or via non-catalytic processes. CB belongs to the group of non-graphitizable carbon. The studies on the solid-phase transformation process of CB particles are carried out in the quest of obtaining further insight into the carbon nano structures. In the present research work mono- and bi- metallic organic compounds were used as catalyst precursor and alumina boat as substrate to carry out solid- phase transformation studies. In the work, a straight forward, economical and single-step process for the transformation of CB nanoparticles into multi-walled carbon nanotubes (MWCNT), metal-encapsulated multi-walled carbon nanobeads (MWCNB) and nano-onion like carbon nanostructures is carried out. Two CBs from different vendors (Timcal Graphite & Carbon and Cabot Corporation) having different surface areas and particle sizes, are used in these studies. The dependency of various effects (temperature, catalysts, catalyst to carbon ratio, substrate etc.,) on the degree of transformation and morphology of the resulting nanostructures are considered and studied widely in this project. The heating rate and catalyst to carbon weight ratio plays a crucial role in the determination of the success rate of the transformation process. Both these experimental parameters have been optimized to achieve a successful transformation of particles to tubes. Comparison of the effect of platinum and iron as catalyst in this transformation process is also studied and discussed briefly on the basis of electron vacancies in dorbitals of metals. To our best knowledge, low temperature transformation study at 400 C is done and found to be successful in the formation of graphitized nanostructures. Further, this works reported the transformation of CB using microwave energy with and without use of any metal catalysts and achieved in obtaining nano-balls, nanoonions and nano-sticks. Time of exposure to microwave irradiation is found to be crucial in the determination of morphology of the carbon products. Electron microscopes (Scanning, transmission and high-resolution transmission), Raman spectroscopy, Thermal gravimetric analysis (TGA), X-ray diffraction (XRD) methods have been used for the characterization of the samples. The second project deals with the chemical modification of CB surface with the intention to provide simple and effective reaction technique. Hence, di- and tricarboxylic acids have been used to functionalize the carbon surface. In addition, it does not require prolonged heating, filtration and washing as it is required when using mineral acids. As both the maleic and citric acid varies with the concentration of hydroxyl and carboxyl molecules, surface reactions on the carbon is expected to be different. The effect of solvents (water/ ethylene glycol) and the effect of pre-heat treatment of carbon have been studied. Both Timcal 350G and XC72R CB samples have been used in the chemical modification experiments. This work further studied the processes and shown the results varies with the solvent used and pre-heat treatments on CB. The modified samples were characterized by thermo-gravimetric analysis (TGA), surface area analyzer, X-ray diffraction (XRD), particle size and Zeta potential measurements, and Fourier transform infra-red spectroscopy (FTIR). It is found that several material properties, among these the thermal stability and surface area of the modified carbon black, are significantly altered relative to the parental carbon samples.