Shining Light on Carbon Nitrides: Leveraging Temperature To Understand Optical Gap Variations

Polymeric carbon nitride (PCN)-based materials have opened new research avenues in the photocatalytic field. The understanding of parameters underlying the optical properties of PCNs is critical to improve their performance. Herein, the optical properties of PCNs are investigated with a combination of time-dependent density functional theory (TD-DFT) calculations and laboratory characterizations, including variable temperature (VT)-XRD, VT-UV–vis, and VT-IR techniques, on samples prepared using a broad range of synthesis temperatures. Upon increasing the synthesis temperature from 390 to 600 °C, the optical gap Egopt (eV) narrows from 2.92 to 2.67, and on further increasing the synthesis temperature to 700 °C, it widens to 3.03 and another absorption at ∼2.2 eV is observed. The TD-DFT calculations show that Egopt of PCNs converges rapidly with heptazine oligomer size, indicating localized photophysics behavior and justifying a molecular approach. Calculations exploring geometrical variations upon increasing the temperature at which the optical properties were determined revealed that the net effect of these variations was limited, suggesting a vibrational origin of the observed decrease in the gap. The absorption at ∼2.2 eV is tentatively attributed to fully polymerized graphitic PCN oligomers as suggested by the excitation wavelength-dependent photoluminescence emission behavior. The results provide novel insights into the optical properties of PCNs, providing directions for the development of the next generation of PCN-based photocatalysts with optimal light-harvesting abilities.