1. (a) XRD patterns of 2D g-C₃N₄, and 10% CoN/2D g-C₃N₄ samples, and CoN, (b) FT-IR spectra of 2D g-C₃N₄, and CoN/2D g-C₃N₄ samples, (c) UV-Vis diffuse reflectance spectra of 2D g-C₃N₄, and CoN/2D g-C₃N₄ samples, and CoN, (d) N₂ adsorption-desorption isomers of 2D g-C₃N₄ and 10% CoN/2D g-C₃N₄

2. SEM images of (a) 2D g-C₃N₄, (b) CoN and (c) 10% CoN/2D g-C₃N₄, and (d, e) TEM images and (f) HR-TEM image of 10% CoN/2D g-C₃N₄

3. (a) Photocatalytic hydrogen evolution with photocatalysts under visible light irradiation, and (b) hydrogen evolution stability test of 10% CoN/2D g-C₃N₄ under visible light irradiation (10% TEOA as sacrificial agent, 10 mg catalyst usage, xenon lamp as light source, λ>400 nm)

4. (a) Steady-state PL spectra excited at 384 nm, (b) photocurrent-time dependence, (c) electrochemical impedance spectra (EIS) of 2D g-C₃N₄ and 10% CoN/2D g-C₃N₄, and (d) Motschottky (MS) curves of 2D g-C₃N₄ and 10% CoN/2D g-C₃N₄

5. ESR spectra of (a, c) DMPO-^·O₂^- and (b, d) ·OH O₂^- and (b, d) ·OH (a, b) under visible-light irradiation and (c, d) without light irradiation of the 2D g-C3N4 and 10% CoN/2D g-C3N4

S1. (a) SEM image of 10% CoN/2D g-C₃N₄, (b-d) EDS elemental mappings from C, N and Co corresponding to 10% CoN/2D g-C₃N₄

S2. (a) XPS survey spectra of 10% CoN/2D g-C₃N₄ and 2D g-C₃N₄, (b) Co2p XPS spectra of CoN and 10% CoN/2D g-C₃N₄, (c) C1s and (d) N1s XPS spectra of 2D g-C₃N₄ and 10% CoN/2D g-C₃N₄