**2. Materials and Methods**

### *2.1. C3N4 Synthesis and Characterization*

Graphitic carbon nitride nanosheets (C3N4) were synthesized with urea (20 g) in a vacuum tube furnace. The ramping rate was 5 ◦C/min, and the temperature was kept at 550 ◦C under nitrogen flow for4h[47]. The synthesized C3N4 was cooled down to ambient temperature in the vacuum tube furnace and then washed with deionized water three times. The C3N4 yield in each batch was approximately 2% (*w*/*w*). The procedures were repeated several times to prepare sufficient C3N4 for experimentation. All batches of C3N4 were mixed thoroughly and then freeze-dried in a lyophilizer (<1.5 mbar, −50 ◦C, FreeZone® Benchtop Freeze Dryers, Model 70020, Labconco Corporation, Kansas City, MO, USA)

For imaging characterization, the synthesized C3N4 was dispersed in deionized water and a small volume diluted with methanol (1:1 *v*/*v*). One drop (2 μL) was deposited onto carbon-coated Cu grids and left to dry at room temperature. Images were taken in a Hitachi model HT7800 transmission electron microscope equipped with a lanthanum hexaboride (LaB6) filament in high contrast mode at an accelerating voltage (HV) of 80 kV. (Hitachi Incorporation, Tokyo, Japan)

For analysis by Fourier-transform infrared spectroscopy (FTIR, Spectrum One, PerkinElmer Inc, Waltham, MA, USA) equipped with the universal ATR sampling accessory, approximately 50 mg C3N4 was pressed into a thin layer and loaded onto the instrument's crystal; the samples were scanned, and spectra of C3N4 in the range of 450–4000 cm−<sup>1</sup> were collected [48].

In addition, 50 and 250 mg/L C3N4 suspensions were separately prepared in deionized water and half-strength Hoagland's solution. Hydrodynamic diameter and zeta potential were measured by dynamic light scattering (90 Plus Particle Size Analyzer, Brookhaven, Upton, NY, USA) [48].
