Comparison of effectiveness of bulk vs. exfoliated graphitic carbon nitride towards the degradation of Rhodamine B

Graphitic carbon nitride (GCN) is a promising metal-free, non-toxic and stable visible light photocatalyst [1]. The effectiveness of GCN towards the degradation of pollutants varies with the strategies and conditions employed for its synthesis. Exfoliation, doping, intercalation, formation of heterojunctions, inducing defects, protonation and making it mesoporous are some of the strategies adopted to improve its efficiency [2]. Melamine, urea, thiourea and dicynamide are some of the commonly used nitrogen rich organic precursors for the synthesis of GCN [3]. Among them, the bulk GCN derived from urea has been reported to have a relatively higher surface area, better adsorption capacity and degradation efficiency than those derived from other precursors [4]. Achieving better degradation by manipulating the characteristic properties of the catalyst such as surface area, high rate of recombination of charged particles, formation of electron holes has been the prime focus. Preliminary studies conducted using bulk GCN derived from melamine reveals poor efficiency when compared to those derived from urea, which warrants modification. The present study aims to compare the effectiveness of bulk GCN derived from urea and exfoliated GCN derived from melamine towards the degradation of Rhodamine B as a model pollutant. Bulk GCN was derived from urea at 550 °C for 4 h. The exfoliated GCN was derived from melamine using ammonium chloride as an activator (1:4 ratio) at 550 °C for 4 h. The bulk and exfoliated GCN were characterized using scanning electron microscopy, X-ray diffraction measurement, Fourier transform infrared spectroscopy and B.E.T surface area analyzer for determining their morphological features, structural characteristics, nature of functional groups and, surface area and pore volume. The efficiency of the bulk and exfoliated GCN towards the degradation of Rhodamine B was evaluated under simulated solar irradiation conditions (63 W; 6650 lumens; 52921 lux) using a visible light photo reactor as a function of pH, catalyst loading and irradiation time. Before subjecting to light irradiation, the solution mixture containing 10 ppm of Rhodamine B and 100 mg of GCN was magnetically stirred for 30 min to attain adsorption-desorption equilibrium. The extent of degradation of Rhodamine B as a function of irradiation time is measured at 546 nm using a UV-visible spectrophotometer. There is not much difference in the structural characteristics as well as the nature of functional groups, as evidenced by the X-ray diffraction pattern and FT-IR spectra. Nevertheless, when compared to the bulk GCN derived from urea, the efficiency of degradation of Rhodamine B is much better for exfoliated GCN (fig. 1). The extent of degradation of Rhodamine B at its natural pH of 6.0 using bulk GCN derived from urea is  ~ 74% after 60 min whereas for GCN derived from melamine using ammonium chloride as an activator it is ~90%. The difference in efficacy is mainly due to the difference in surface area of the catalyst, which is relatively higher for exfoliated GCN derived from melamine than the bulk GCN derived from urea. The study concludes that the conditions employed for the synthesis of GCN can be effectively manipulated to impart the desired characteristics for it so as to achieve better efficiency of degradation of Rhodamine B.