Is precarbonization necessary for effective laser graphitization?

Laser-graphitization offers a new approach to obtaining high-quality 3D graphene films simply and efficiently. Over the years this method was developed to include different carbonaceous precursors and irradiation parameters. Precarbonization is a step in the synthesis where the precursors are annealed at high temperatures under an inert atmosphere to increase the carbon content in the samples prior to the laser reaction. Some studies report this step while others do not, thus its necessity and beneficially have to be evaluated. In this study, we compare the laser products of as-prepared and precarbonized carbon nanodots (CNDs). The study investigates the samples from the initial synthesis step of microwave-assisted formation of CNDs, through separation of the CNDs by particle size, the precarbonization step conducted at different temperatures, to the final 3D graphene films products. By analyzing the chemical structure and graphitization level of the intermediates, we could realize the chemical characteristics responsible for successful laser-graphitization. We have found that precarbonization indeed leads to improved graphitic products, only to a certain extent. The best graphitization level is achieved when the CNDs were precarbonized to 300 °C obtaining I_D/I_G ratio = 0.44 as compared to 1.08 when as-prepared CNDs are used. On the other hand, when annealed to 400 °C the laser produces more defected graphene films (I_D/I_G ratio = 0.83). Moreover, both the surface chemistry and the core structure of the CNDs affect the final results. A careful examination of the surface chemistry reveals the coverage of oxygen- and nitrogen-containing functional groups required for the optimal thermochemical reaction.