*3.3. Synthesis of g-C3N4*

Bulk-g-C3N4 was prepared by direct pyrolysis of urea according to a reported procedure with modification [68]. In detail, 125 g of urea was put into an alumina crucible with a cover and heated with a heating rate of 10 ◦C/min to 600 ◦C for 4 h. After being cooled to room temperature, the pale yellow of bulk-g-C3N4 (CN) was obtained. The bulk-CN was exfoliated into a nanosheet structure by thermal exfoliation in the presence of acid, and 2.5 g of bulk-CN was stirred in 65 % of HNO3 solution (100 mL) for 12 h. The dispersion was filtrated and washed several times with D.I. water, followed by annealing at 500 ◦C for 4 h to obtain g-C3N4 nanosheet (CNS).
