Synthesis and characterization of carbon nitride with tunable properties for photocatalytic degradation of phenol

Abstract

Carbon nitride (CN) has been regarded as a potential visible light photocatalyst due to its light absorption up to ca. 450 nm and possesses band gap energy (Eg) of ca. 2.7 eV. CN can be prepared by thermal polymerization method using carbon and nitrogen-rich compound as the precursor. However, most of the reported CNs were associated with a defect-rich and less-ordered structure as well as low surface area that could affect their performance. In this study, CNs of high surface area, improved structural order, low Eg and low electron charge transfer resistance (Rct) that are practicable for photocatalytic degradation of phenol under a wide range of sunlight irradiation have been successfully prepared. At the early stage, various salt melts of KCl-LiCl, KCl-NaCl, and KCl-ZnCl2 were used in order to induce the crystallinity of CN. Despite all the salt melts helped to improve the optical properties as revealed by diffuse reflectance ultraviolet-visible (DR UV-Vis) spectroscopy, only salt melts of KCl-LiCl could form crystalline CN as shown by X-ray diffraction (XRD) patterns with the formation of crystalline poly(triazine imide). The fluorescence and electrochemical impedance (EIS) spectroscopy confirmed that the higher crystallinity has suppressed the electron hole recombination and decreased the values of Rct. Improved photocatalytic degradation of phenol (24%), of ca. 2.5 times better than that of amorphous CN (10%), was achieved on crystalline sample of CN-KCl-LiCl. Besides, optimizations of synthesis parameters including amount of precursor, synthesis temperature, synthesis time and amount of salt melt were conducted. Current study revealed that increasing the amount of precursor from 1 g to 3 g led to the decrease in photocatalytic activity from 12% to 5% of phenol degradation. Increasing reaction temperature from 500?C to 550°C increased the photocatalytic activity from 7% to 24%. However, the photocatalytic activity decreased to 20% when the reaction temperature was increased to 600°C. In addition, short synthesis time (2 h) and long synthesis time (6 h) have led to the low photocatalytic activity with 17% and 20% of phenol degradation, respectively. Meanwhile, low (2.5 g) and high (7.5 g) amounts of salt melts showed low photocatalytic activities of 14% and 11%, respectively. The optimized conditions for the synthesis of CN with high crystalline phase were 2 g of precursor, reaction temperature of 550?C, reaction time of 4 h and 5 g of salt melts. By employing the optimized synthesis parameters, both amorphous and crystalline CN were prepared using melamine (Mel) as the precursor. The photocatalytic testing of the crystalline CN-Mel showed an improved activity of ca. 1.5 times higher (30%) compared to amorphous CN-Mel (19%). Further modification to increase the surface area was carried out by creating porous structure using Pluronic P123 (P123) surfactant. Increasing the mass ratio of P123 to precursor from 0.02 to 0.05 improved the photocatalytic activity from 20% to 46%, but decreased to 37% at the high mass ratio (0.1). The high photocatalytic activity was due to its high surface area (160 m2 g-1) and low Rct values (11.49 kO). In order to improve the light absorption, modification of porous crystalline CN with 2,4,6-triaminopyrimidine (TAP) was conducted. Low addition of TAP (0.02 mass ratio) has significantly improved the photocatalytic activity up to 60%. The high activity was mainly due to the combination of high surface area (137 m2 g-1), low Eg (2.62 eV) and low Rct (14.52 kO) value. However, increasing the mass ratio of TAP from 0.03 to 0.1 decreased the photocataytic activity from 53% to 19%. Overall, this study has demonstrated that CN with tunable properties improved photocatalytic degradation of phenol that was nearly three times higher than unmodified CN under visible light region.