A layer-by-layer deposition mechanism for producing a pyrolytic carbon coating on carbon nanotubes

Pyrolytic carbon (PyC) was deposited on carbon nanotubes (CNTs) in order to modify them by introducing defects to their surface. The deposition of PyC was carried out at temperature between 800 and 1000 °C using propane as carbon source with or without a hydrogen carrier gas at low pressure of 20 kPa. The structure of PyC coatings was examined using transmission electron microscopy. The PyC coating could be distinguished from the original CNT walls due to the difference of the structure, with the coating showing a less orderly layer structure. When H2 was introduced during deposition, PyC coating started to form at 900 °C, and the deposition rate increased rapidly with increasing temperature. Without H2, PyC coating with a thickness of a few layers could be formed at temperatures between 800 and 900 °C in 10 min. The outmost layer of the PyC coating showed a structure of rough and curved carbon fragment. A layer-by-layer mechanism is proposed for the deposition consisting of alternating fragment formation (nucleation) and lateral growth to layer.