Effect of calcination temperature, pH and catalyst loading on photodegradation efficiency of urea derived graphitic carbon nitride towards methylene blue dye solution

Abstract

In this study, the photodegradation of methylene blue (MB) dye was performed using urea based graphitic carbon nitride (g-C₃N₄). Interestingly, it has been observed that the calcination temperature for the synthesis of g-C₃N₄ along with factors (pH and catalyst loading) influencing the photodegradation process, can make an impactful improvement in its photodegradation activity towards MB dye solution. The concept behind the comparatively improved photoactivity of g-C₃N₄ prepared at 550 °C was explored using various characterisation techniques like XRD, FTIR, SEM, BET and DRS. The FTIR and XRD patterns demonstrated that synthesis of g-C₃N₄ took place properly only when the calcination temperature was above 450 °C. The evolution of morphological and optical properties based on calcination temperature led to dramatically increased BET surface area and a decreased optical band gap value of g-C₃N₄ prepared at 550 °C. The effects of pH conditions and catalyst concentration on the MB dye degradation rate using optimally synthesised g-C₃N₄ are discussed. The value of the apparent rate constant was found to be 12 times more in the case of photodegradation of the MB dye using g-C₃N₄ prepared at 550 °C at optimum pH and catalyst loading conditions when compared with g-C₃N₄ prepared at 450 °C showing the lowest photoactivity potential. Further, high stability of the photocatalyst was observed for four cyclic runs of the photocatalytic reaction. Hence, g-C₃N₄ can be considered as a potential candidate for methylene blue photodegradation.

Fig. 1. Schematic for synthesis of g-C₃N₄ showing the plausible intermediates at different temperature range.

Fig. 2. XRD pattern of g-C₃N₄ prepared at different calcination temperature.

Fig. 3. FTIR pattern of g-C₃N₄ prepared at different calcination temperature.

Fig. 4. SEM images of g-C₃N₄ prepared at (a) 450 °C (b) 500 °C (c) 550 °C (d) 600 °C (e) 650 °C (f) TEM image of g-C₃N₄ prepared at 550 °C.

Fig. 5. (a) Nitrogen adsorption–desorption isotherm; (b) BET adsorption isotherm (c) BJH pore size distribution curve of g-C₃N₄ prepared at different calcination temperature.

Fig. 6. (a) UV-vis diffused absorbance spectra (b) estimated band gaps of g-C₃N₄ prepared at different calcination temperature.

Fig. 7. (a) Comparison of photocatalytic activity of g-C₃N₄ prepared at different temperature (b) ln(C₀/C(t)) for MB degradation with g-C₃N₄ prepared at different temperature as a function of simulated solar irradiation time.

Fig. 8. (a) Comparison of photocatalytic activity of g-C₃N₄ at different pH condition (b) ln(C₀/C(t)) for MB degradation with g-C₃N₄ at different pH condition as a function of simulated solar irradiation time.

Fig. 9. (a) Comparison of photocatalytic activity of g-C₃N₄ at different catalyst concentration (b) ln(C₀/C(t)) for MB degradation with g-C₃N₄ at different catalyst concentration as a function of simulated solar irradiation time.

Fig. 10. Recyclability experiment of the photocatalytic degradation of dyes using g-C₃N₄ prepared at 550 °C.

Fig. 11. Photocatalytic mechanism of g-C₃N₄.