High-Performance Coral Reef-like Carbon Nitrides: Synthesis and Application in Photocatalysis and Heavy Metal Ion Adsorption

Synthesis of carbon nitrides (CNx) by refluxing under nitrogen exhibited mixed growth mechanisms of oriented attachment and Ostwald ripening, leading to the formation of coral reef-like microstructures from spherical agglomerates. Some phase transformation from β-phase to α-phase CNx occurred upon refluxing for 1.5 h, producing a biphasic CNx. The N content relative to C was determined from CHN elemental analysis, and the presence of C=N and terminal groups (i.e., COOH and NH2) was consistent with the Fourier transform infrared, nuclear magnetic resonance, and X-ray photoelectron spectroscopic results. The sample refluxed for 2.0 h (CNx/2.0 h) had the highest surface area of 24.5 m2·g-1 and displayed enhanced adsorption capacities for methylene blue (MB) molecules and heavy metal ions Pb2+ (720 mg·g-1), Cd2+ (480 mg·g-1), and As(V) (220 mg·g-1), which was attributed to the presence of COOH functional groups. CNx samples had a negative surface charge that electrostatically attracted the cationic heavy metal ions as well as MB molecules for subsequent photodecomposition under visible-light illumination. The photocatalytic activity of CNx/2.0 h toward phenol, a common pollutant in aqueous waste, was also demonstrated and a possible photocatalytic route was proposed.