Molecular dynamics simulation of evolution of nanostructures and functional groups in glassy carbon under pyrolysis

With the continued and sustained growth of the field of carbon MEMS (C-MEMS) in several application areas, there is a corresponding increased and renewed interest in understanding the exact molecular structure of the ensuing glassy carbon (GC) material and, therefore, potential to engineer some of its key characteristics. Associated with that interest, the effects of key process parameters like temperature ramping, holding and cooling times on carbonization, the final nanostructures and – importantly – on the nature and distribution of functional groups remain to be of significant research importance to the GC community. While recent in-situ imaging approaches during the synthesis process have opened up opportunities for better understanding of these nanostructures, the 2D nature of the approaches, however, had introduced significant barriers. In this paper, therefore, we present a simulation-based approach using reactive molecular dynamics (MD) where we model the synthesis process and investigate in detail (i) the intermediate and final stages of carbonization with particular focus on GC nanostructures and (ii) formation of functional groups and the processes and parameters that drive their evolution. Our simulation captured the pyrolysis-driven synthesis process where the polymer structure degraded followed by increased carbonization and formation of flakes of 3-, 4-, membered carbon rings that further grew into graphene-type 6-member carbon rings as well as 5- and 7-member rings. These eventually evolved into a complex multi-layer 3D nanostructure consisting of connected but distinct zones of (a) dominant cage-like tubular components and (b) long chains of flat graphene-like parts rich with functional groups. Taken together, the MD simulations demonstrate that (i) holding time at the maximum temperature has the most significant effect in the formation of complex cage-like 3D nanostructures with stacked layers, (ii) the closed cage-like zones have crystallite diameter (La) in the range of 3.2 Å - 9.5 Å with clear and well-defined curved and stacked graphene-like sheets of stacking thickness (Lc) = 3.0 Å - 3.3 Å, (iii) carbonyl, ether, epoxide, hydroxyl, and alkyne groups form predominately on the edges of flat components of GC where abundant non-6 membered rings, dangling bonds, long sp carbon chains, and intermediate structures exist, and to a lesser extent on edges of vacancy defects inside cage-like zones.