Photo-catalytic removal of rhodamine B by nickel doped graphitic carbon nitride: anomalous dependence of removal efficiency on carrier recombination

This work reports the synthesis of pure and nickel-doped graphitic carbon nitride (GCN) by simple thermal decomposition of urea at a moderate temperature (550 °C). The concentration of nickel was increased systematically to see the effect of dopant concentration. Both the pure and doped samples were characterized by techniques like X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (FESEM), UV–Vis, Fourier transformed infrared (FTIR), and Raman spectroscopy. The thermal stability of the sample was analyzed with the help of a thermal gravimetric and differential thermal analysis (TG–DTA) study. XPS confirms the successful doping, whereas both XRD and FESEM show that when pure GCN gets doped with the transition metal, the crystal structure, as well as the morphology of the sample, changes. The Kubelka–Munk plot shows the monotonic decrease of the optical band gap of the material. Photoluminescence (PL) spectra of the GCN showed quenching of the intensity when nickel was introduced. The efficacy of the as synthesized material in removing textile dyes like Rhodamine B (Rh-B) through the process of photo-catalysis (PC) which is catalytic reaction under the influence of photons of suitable wavelength has been tested. It was seen that the observed PL quenching, in contradiction to the expected result, does not support PC when attempting to remove Rh–B under UV light. The pure GCN shows the best removal efficiency, achieving approximately 100% within a time duration of 70 min. The reaction mechanism has been explained in view of the pseudo-1st-order reaction. The anomaly of the PC results was qualitatively explained in terms of the abundance of oxygen and changes in the surface area. This is the 1st report where GCN shows a simultaneous enhancement of PL and PC properties.