Reverse Monte Carlo studies of nanoporous carbon from TiC

The structures of nanoporous carbon prepared by chlorination of TiC at five different temperatures (700-1100 deg. C) have been studied by means of reverse Monte Carlo modelling of neutron diffraction data S(q), 0.3<q<10.5 A-1, using an atomic configuration (8000 atoms) with a density corresponding to 0.62 of graphite. Four different starting models were tested: (i) random atom configuration (ii) separated graphite sheets and (iii) two defect models created by removing atoms in a graphite structure to obtain the wanted density. To increase the feasibility of the resulting atom configurations, a number of hard and soft constraints were introduced into the software. The hard constraints were (i) a minimum C-C distance of 1.0 A (ii) a co-ordination constraint for nearest-neighbour distances of up to 1.6 A to avoid zero- or single- co-ordinated atoms and (iii) no atoms between 1.7 and 2.1 A to avoid small unphysical peaks in the radial distribution function. A soft constraint was centred C-C-C angles around 120 deg. with a variance of 6 deg. The best fit between observed and calculated S(q) was obtained for the defect models. An evaluation of the porosity and surface area corresponding to the atomic configuration showed a significant difference between the 700 and 1000 deg. C samples and the one prepared at 1100 deg. C in agreement with HREM and sorption studies