Microstructure of tobacco chars and the origin of associated graphite as determined by high-resolution transmission electron microscopy (HRTEM)

摘要

Microstructures of tobacco chars were studied by high-resolution transmission electron microscopy (HRTEM). The chars were prepared by pyrolyzing tobacco for 10 min under helium at temperatures from 350 to 1200℃. The char samples were then embedded and sectioned by microtome for analysis by HRTEM. The observed microstructures could then be related to specific cell components. The samples were also examined to locate and determine the origin of the graphitic structures previously observed by the authors in powdered tobacco chars applied to lacey carbon grids. Although graphite is a minor phase in these chars, their presence in such low temperature chars implies either highly specialized and localized conditions of formation or contamination from the lacey carbon grids. When microtomed sections were examined instead, graphitic structures were indeed found in the sections, proving that they were not the result of contamination. However, they were always associated with transition metals and were probably formed by catalysis. When whole tobacco powder was examined, the results from electron energy loss spectroscopy (EELS), energy dispersive spectroscopy (EDS), and microstructure analyses showed considerable scatter. When only the cell wall material from the microtomed sections was considered, the results showed identifiable trends. Considerable oxygen depletion occurred by 550℃. Concentration of aromatic ring structures, as documented by EELS and graphene sheet development by HRTEM, increased above 550℃. Clays were also common and some may have formed in situ in the tobacco.