Protic Salts of High Nitrogen Content as Versatile Precursors for Graphitic Carbon Nitride: Anion Effect on the Structure, Properties, and Photocatalytic Activity

Graphitic carbon nitride (g-C3N4) is traditionally obtained by thermal polycondensation of nitrogen-rich organic nonionic precursors. Herein, we provide a new and simple strategy for the synthesis of g-C3N4 through direct thermal condensation of single protic guanidine salts with various anions. The thermal condensation process, and the structure and photophysical properties (optical properties, thermal stabilities, morphology and porosity, and photocatalytic activity) of the obtained g-C3N4 materials are investigated intensively through thermogravimetric analysis, powder X-ray diffraction, X-ray photoelectron spectroscopy, UV/Vis diffuse reflectance spectroscopy, nitrogen adsorption isotherms, and photodegradation of Rhodamine B. The results reveal that during thermal condensation, the nitrogen-rich basic core in the guanidine salts serves as a carbon and nitrogen source to form the skeleton of g-C3N4, and the counter anions mediate the thermal condensation in situ and modify slightly the structure and properties of g-C3N4, even though they are not incorporated into the final structure. Among all g-C3N4 materials, including the pristine g-C3N4 from melamine, the g-C3N4 derived from guanidine thiocyanate exhibits a high activity toward the photocatalytic degradation of Rhodamine B dye because of its relatively high surface area, enhanced absorption, small bandgap, and low photoluminescence intensity.