*3.1. C3N4 Characterization*

Figure 1A,B shows the morphology of the synthesized C3N4. The FTIR spectra demonstrate characteristic peaks at 810 cm−<sup>1</sup> and 1600–1200 cm−<sup>1</sup> (Figure 1C), corresponding to the breathing mode of triazine units and the stretching mode of CN heterocycles, respectively [55]. All of this indicates successful C3N4 synthesis. The zeta potential value indicates that C3N4 was negatively charged (−18 to −24 mV in both deionized water and nutrient solutions); half-strength Hoagland's solution further decreased the zeta potential as compared with deionized water. The hydrodynamic diameters of C3N4 in half-strength Hoagland's solution and deionized water were similar at 50 mg/L. However, at 250 mg/L, the hydrodynamic diameters of C3N4 in half-strength Hoagland's solution and deionized water were decreased to approximately 600 and 250 nm, respectively (Figure 1D). The possible explanation could be that a high concentration of C3N4 simply formed large aggregates, which could settle faster in the solution.

**Figure 1.** Characterization of C3N4 nanosheets. (**A**,**B**) represent TEM images of C3N4; (**C**) shows FTIR scheme 3, N4; (**D**) shows the hydrodynamic diameter and zeta potential of C3N4 in deionized water (DI) water and half-strength Hoagland's solution. Values of zeta potential followed by different uppercase letters are significantly different at *p* < 0.05; values of hydrodynamic diameter followed by different lowercase letters are significantly different at *p* < 0.05.
