Size effect in the characterization of microporous activated nanostructured carbon

Two kinds of microporous activated nanostructured carbons were studied by X-ray diffraction, Raman spectroscopy, electron paramagnetic resonance (EPR) spectroscopy and electrical conductivity measurement. They have different specific surface areas: 1500 m2/g (AC I) and 2500 m2/g (AC II). From X-ray diffraction analysis the average values of the nanodomain sizes of the AC I and AC II carbon particles are 1.7 nm and 1.5 nm, respectively. The Raman spectra and conductivity measurements have consistently confirmed the gradation of the crystallite sizes. It was observed also that the EPR spectra shapes and their temperature evolution between 300 K and 4 K were depending on the types of carbon sample. Room temperature spectra show dissimilar line-widths ΔB = 0.5 mT and 1.6 mT for AC I and AC II, respectively. The AC I carbon EPR spectra show: (a) a broadening at the beginning of the cooling process, similar to the EPR behavior of graphite and, subsequently, (b) a quick narrowing below 130 K. For the super-activated carbon (AC II), a monotonic narrowing process is observed on decreasing the temperature. These different behaviors were attributed to the size effect of graphenes in both activated carbons.