One pot ultrasonic plant mediated green synthesis of carbon dots and their application invisible light induced dye photocatalytic studies: a kinetic approach

In this study, leaves of Polyalthia longifolia were utilised as carbon source for the synthesis of carbon dots by ultrasonication. UV-visible spectrum of prepared carbon dots exhibited two absorption peaks at 225 nm and 260 nm, respectively. Fourier transform spectrum of carbon dots exhibited the different functional groups at absorption peak values of 1650 cm−1, 2989 cm−1, 1471 cm−1, 1762 cm−1 and 1126 cm−1, respectively. X-ray diffraction analysis of carbon dots showed the amorphous nature of carbon dots with peak values at 31.38º and 40.93º, respectively. Raman spectrum of synthesised carbon dots illustrated two peaks at 1362 cm−1 and 1510 cm−1. These prepared carbon dots were then employed for photocatalytic removal of congo red dye and methylene blue dye under visible light source. The maximum removal of methylene blue dye was achieved at pH 8, at a dye concentration of 5 mg/L in a time interval of 60 min. In the case of congo red dye, maximum degradation was observed in acidic medium at a pH of 6 with 5 mg/L dye concentration in a time interval of 90 min. Further kinetic models of zero order, first order, second order, parabolic diffusion and modified Freundlich were applied to dye photocatalytic studies. It can be observed that methylene blue dye degradation by carbon dots followed parabolic diffusion model while congo red dye depicted closed correlation with both parabolic diffusion and modified Freundlich kinetic models.