Structural and optical properties of carbon nitride polymorphs

Graphitic carbon nitride (g-C3N4) is considered as one of the most promising photocatalysts for environmental improvement and energy storage. It presents a graphene – like 2D structure, composed of heptazine units connected by amino groups. Here we report multi-technique experimental data (XRD patterns, Raman, steady-time and time resolved Luminescence, Photoluminescence Excitation spectra, Reflectivity spectra) to analyze the relationship between structural and optical properties. We found that the low temperature phase of carbon nitride, namely Melem, presents the main optical features shifted towards higher energies with respect to the higher temperature phase, i.e. Liebig's melon structure or Polymeric Carbon Nitride (PCN). Time resolved luminescence in the ns-domain was therefore exploited to further analyze the kinetics of the excitons generated during optical excitation. Two decay trends were observed in both the structures, with decay time constants depending on sample structure and cell parameters: the faster component, with sub-nanosecond decay time, is related to the recombination from the σ* band whilst the slower decay time is assigned to recombinations within the single triamino-tri-s-triazine unit. The correlation between structural parameters and optical properties is discussed and a model on the optical levels is proposed.