Isoelectric point and adsorption activity of porous g-C₃N₄

The isoelectric point (IEP) is an important physicochemical parameter of many compounds, such as oxides, hydroxides, and nitrides, and can contribute to estimation of the surface charges of compound particles at various pH conditions. In this work, three types of graphitic carbon nitrides (g-C₃N₄) were synthesized by directly heating melamine, thiourea, and urea. The prepared samples showed different microstructures and IEPs that influenced their adsorption activity. Differences in microstructure resulted from the various precursors used during synthesis. The IEPs of the obtained g-C₃N₄ were measured to be approximately 4–5, which is due to the equilibrium of chemical reactions between hydrogen ions, hydroxyl ions, and amine groups on the g-C₃N₄ surface. The IEP of g-C₃N₄ prepared from thiourea was lower than those of the corresponding samples prepared from melamine and urea. The adsorption activity of methylene blue on g-C₃N₄ prepared from urea and thiourea was excellent, which indicates that g-C₃N₄ is a promising adsorbent. This work provides a useful reference for choosing precursors with which to prepare g-C₃N₄ and combining g-C₃N₄ with other compounds in solution. Copyright © 2015 Elsevier B.V.