Post-synthetic modification of graphitic carbon nitride with PCl3 and POCl3 for enhanced photocatalytic degradation of organic compounds

Graphitic carbon nitride (g-C3N4) was synthesised from melamine at 550 °C in the air for a period of 4 h. As such prepared g-C3N4 was dispersed in PCl3 and POCl3 with and without pyridine at an ambient temperature for us to dope g-C3N4 with phosphorus. The bulk structural properties of g-C3N4 examined by X-ray diffraction (XRD) and Fourier transformed infrared (FTIR) spectroscopy were not changed. On the contrary, a surface modification in terms of pore size distribution studied using physisorption of nitrogen and electron microscopy was observed. Using PCl3 (in the presence of pyridine), nitrogen vacancies were filled with phosphorus and phosphoramidate groups were formed (with and without pyridine). When POCl3 was used nitrogen vacancies were removed and the surface structure was rearranged, but no phosphorus was doped in g-C3N4. The band gap energies varied from 2.69 to 2.73 eV and specific surface areas varied from 8 to 11 m2 g−1. The g-C3N4 surface structure rearrangement was associated with altered electronic properties which led to higher photocatalytic activity observed by the degradation of Ofloxacin, Amoxicillin and Rhodamine B (RhB) under LED irradiation of 420 nm. A degradation efficiency decreased in the order: Ofloxacin > RhB > Amoxicillin. Superoxide radicals were found to be able to react with all the organic compounds, but holes could react only with Ofloxacin and RhB. All the modified materials were more active than the pristine g-C3N4 and the best photocatalyst was prepared through the reaction with PCl3 in the presence of pyridine.