Degradation and chemical stability of graphitic carbon nitride during ultraviolet light irradiation

Abstract

Graphitic carbon nitride (g-C3N4) was synthesized by pyrolysis of urea at 450 °C for 2 h. X-ray diffraction and transmission electron microscopy confirmed the formation of the stacked hexagonal layers and platelet-like structure of g-C3N4. The sample was irradiated by ultraviolet (UV, 370 nm) light for about 62 h and X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), time-of-flight secondary ion mass spectroscopy (ToF-SIMS), Nitrogen adsorption–desorption isotherm, and photoluminescence spectroscopy (PL) were used to investigate the degradation. XPS spectra of C 1s electrons revealed four components associated with adventitious carbon, the –C≡N group, heptazine/triazine and carboxyl/carbonate species. Quantitative XPS analysis indicated an increase in the peak areas associated with the –C≡N group and carboxyl/carbonate species, accompanied by a decrease in the areas of the components associated with the heptazine/triazine and adventitious carbon. XPS spectra of N 1s were fitted using four components which were assigned to pyridinic nitrogen (C–N=C), nitrile group (N≡C), amine groups (NHx) and tertiary nitrogen (N-(C)3). The relative areas of peaks associated with the amine and nitrile groups have slightly increased after UV irradiation whereas the peak associated with pyridinic nitrogen decreased. FTIR spectra suggested a reduction in the C–N and C=N heterocycle bonds and slight variation in the C≡N bonds after UV irradiation. BET analysis revealed that the surface area and pore volume have decreased after irradiation. ToF-SIMS analysis indicate the formation of NOx, CNO, NHx, and C4H3NO2 species after irradiation. PL spectra of the g-C3N4 showed a broad blue-green emission which was fitted using three bands and ascribed to the δ*-LP, π-LP, and π-π* transitions within the bandgap. During UV irradiation, the g-C3N4 powder lost about 22% of its blue-green emission intensity after nearly 45 h and then remained stable for further irradiation time. This study confirmed the degradation of g-C3N4 under UV irradiation, which should be further studied for better integration of this material in photoexcited applications.