Aggregation behavior of graphitic C₃N₄ nanosheets in aqueous environment: Kinetics and mechanisms

The aggregation behaviors of graphitic carbon nitride (g-C₃N₄) nanosheets under various electrolytes and pH conditions were systematically investigated. The aggregation of g-C₃N₄ nanosheets was significantly enhanced with increasing electrolyte concentrations. The divalent electrolytes (CaCl₂ and MgCl₂) were more effective than monovalent electrolytes (NaCl and KCl) in promoting the aggregation of g-C₃N₄ nanosheets. At the same valence, cations with higher atomic weight were more effective in enhancing the aggregation of g-C₃N₄ nanosheets. The measured critical coagulation concentrations (CCC) of g-C₃N₄ nanosheets were 4.7 mM KCl, 9.2 mM NaCl, 1.0 mM CaCl₂ and 1.9 mM MgCl₂ at pH 6.0, which were lower than some of other two-dimensional nanoparticles. The CCC values of g-C₃N₄ nanosheets were decreased to 5.5 mM NaCl at pH 2.0, but increased to 29.0 mM NaCl and 2.1 mM CaCl₂ at pH 10.0, indicating that the aggregation degree of g-C₃N₄ nanosheets was decreased with increasing pH. The Fe/Al hydrated complexes generated at the specific pH inhibited the aggregation of g-C₃N₄ nanosheets and enhanced the stability. Overall, findings from this study demonstrated that the electrolytes and pH conditions played important and combined roles on the aggregation of g-C₃N₄ nanosheets. In addition, the aggregation behaviors of g-C₃N₄ nanosheets could be well predicted with the DLVO theory.