Structural and Physicochemical Properties of Carbon Nitride Nanoparticles via Precursor Thermal Treatment: Effect on Methyl Orange Photocatalytic Discoloration

In this work, we synthesized and characterized carbonnitride (CN)nanoparticles obtained by the thermal treatment (550 degrees C) of urea,melamine, dicyandiamine, and dicyandiamine-barbituric acid in an openreactor and evaluated the effect of different precursors on the photocatalyticperformance. CNs obtained from melamine, dicyandiamine, and dicyandiamine-barbituricacid were 3D melon-type structures. On the other hand, CN obtainedfrom urea was a 2D microporous, amorphous structure whose melon orgraphitic arrangement could not be determined. The presence of structuraldefective states (mainly C radicals) was corroborated by EPR studiesof the solids. The photocatalytic activity of CN powders for methylorange (MO) discoloration was investigated using 350 nm and simulatedsolar light (SSL) irradiation. MO removal efficiencies were correlatedwith the particle's energy gap, specific surface area, degreeof crystallinity, and C radical defects produced upon irradiation.Moreover, the obtained conduction and valence band potentials in therange of -0.60 and + 2.14 V vs NHE (pH = 7), respectively,evidence CN capacity to oxidize water to hydroxyl radicals (HO center dot) and reduce O-2 to superoxide radical anions(O-2 (center dot-)). The formation of bothradicals upon SSL irradiation of CN suspensions was confirmed by EPRexperiments. The largest specific surface area, the highest chargecarrier density, and the amount of C radical defects observed forCN obtained from urea (CNu) account for their highest photocatalyticperformance. However, the estimated CNu photonic efficiency of ca.8% still indicates a deficient separation/migration efficiency ofphotoinduced charge carriers. Despite CNu performance being higherthan those of other CNs obtained from the thermal treatment of simpleprecursors, further environmentally friendly strategies are stillneeded to overcome the intrinsic disadvantages of CN before it maybe employed in technological applications. A discussion on probableCN mechanisms forming reactive species and leading to MO decolorizationis given.