Novel 2D carbon-carbon composites made of carbon nanofibers supported on a carbon preform were functionalized by non thermal plasma treatment, before being used as supports for metallic cobalt nanoparticles. It was shown that the degree of functionalization of the carbon nanofibers depends on the plasma power input, the treatment time and the CNF loading. The size of the cobalt nanoparticles generated after subsequent reduction of the Co-containing plasma treated CNF/C composites under hydrogen flow seems to be independent of the amount of supported cobalt. Changes in surface characteristics were analyzed using thermogravimetric analyses coupled to a mass spectrometer, X-ray photoelectron spectroscopy analyses and Raman spectroscopy. Transmission electron microscopy was used to complementary characterize the final size, dispersion and location of the so generated Co nanoparticles.
Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are of great interest due to their exceptional mechanical, thermal and electronic properties in many applications including nanocomposites, gas storage materials, electronics, chemical sensors and heterogeneous catalysis. However, chemical applications are challenged by the inertness of the carbon materials due to the lack of functional sites after purification. Various covalent and non-covalent surface derivatization routes have been developed to insert functional groups on the surface of CNTs and CNFs. One of the approaches consists in a strong oxidizing treatment with KMnO4 or H2O2 or mineral acids such as HNO3, H2SO4, or HCL, but with damaging effects of the outer electronic surface of the carbon materials. An alternative way to the surface oxidation treatment, in milder conditions, is the functionalization of CNFs by non thermal plasma. Different types of plasma were tested for the functionalization of the CNT or CNF materials. It included microwave, dielectric barrier discharge, radiofrequency and corona discharge plasmas using various type of gases such as O2, Ar/O2, NH3, air, air/H2O, and N2. The plasma discharge generates highly energetic electrons and activated species such as radicals and ions which can interact with the surface of the CNTs and CNFs, in particular with the aromatic C-C bonds. These interactions are able to create active sites leading to the grafting of functional groups that can be further used for the growth of metallic or oxidic nanoparticles for catalytic applications, as well as for the reinforcement of adhesive properties between the CNF and a polymeric matrix. Compared with the chemical treatments, the non thermal plasma method presents many advantages since it is a fast, non destructive method with low energy consumtion and it can be generated in air, at atmospheric pressure, thus avoiding the use of an expensive vacuum system or special gases.
A large number of studies have been published concerning the growth of nanoparticles on carbon nanotubes or nanofibers following different chemical treatments. Very few, however, describe a non thermal plasma functionalization of CNFs prior to their use as spports for metallic nanoparticles.
In the present work, the plasma functionalization of novel 2D carbon-carbon composites constituted of carbon nanofibers supported on a carbon preform is presented. The pristine and air plasma-treated CNF/C composites were further decorated with Co nanoparticles. The chemical functional groups generated by the non-thermal plasma treatment on the surface of the composites should favor strong interactions with a cobalt precursor, thereby leading after reduction to a homogeneous distribution of supported Co nanoparticles. Such surface functional groups are indeed expected to act as anchoring sites for the cobalt nanoparticles, thus allowing their size, dispersion and stability to be controlled. The effect of treatment time and plasma power input on the functionalization degree of the CNF/C composites and hence on the cobalt particles formation was characterized through TGA-MA, XPS and TEM analyses.