One-step, high-yield synthesis of g-C3N4 nanosheets for enhanced visible light photocatalytic activity

Abstract

A facile template-free one-step synthesis method of ultrathin g-C₃N₄ nanosheets was developed through thermal polycondensation of melamine. The higher temperature, prolonged time and tightly sealed crucible reaction system contributed to the formation of ultrathin g-C₃N₄ nanosheets. The as-synthesized g-C₃N₄ nanosheets were applied to the visible light photocatalytic degradation of RhB. The photocatalytic activity was significantly enhanced with increased calcination temperature from 500 °C to 650 °C and prolonged calcination time from 4 h to 10 h. Interestingly, the obtained ultrathin g-C₃N₄ nanosheets simultaneously possess high yield and excellent photocatalytic activity. Moreover, g-C₃N₄ nanosheets can maintain photochemical stability after five consecutive runs. The remarkably enhanced photocatalytic activity can be interpreted as the synergistic effects of the enhanced crystallinity, the large surface area, the reduced layer thickness and size and the reduced number of defects. A new layer exfoliation and splitting mechanism of the formation of the ultrathin nanosheets was proposed. This work provides a new strategy to develop a facile eco-friendly template-free one-step synthesis method for potential large-scale synthesis of ultrathin nanosheets with high yield, high photocatalytic efficiency and stable activity for environmental and energetic applications.

Fig. 1. Pictures of the sealed crucible before reaction (a–c) and g-C₃N₄ sample S_650-10 in crucible (d).

Fig. 2. Photograph of volume comparison of samples with the same mass (20 mg).

Fig. 3. SEM images of S_600-4 (a), S_600-6 (b), S_600-8 (c and d) and S_600-10 (e) and (f) samples.

Fig. 4. SEM images of S_500-10 (a), S_550-10 (b and c), S_600-10 (d and e) and S_650-10 (f and g) samples and TEM image of S_650-10 (h) sample.

Fig. 5. AFM image of S_650-10 and the corresponding height profile of randomly chosen section.

Fig. 6. XRD patterns (a) and FT-IR spectra (b) of S_500-10, S_550-10, S_600-10 and S_650-10 samples.

Fig. 7. XPS spectra of S_650-10 sample. (a) XPS survey. (b) C_1s spectra. (c) N_1s spectra.

Fig. 8. (a) N₂ adsorption–desorption isotherms, the inset is a magnification when the relative pressure is 0.6–1.0. (b) The corresponding PSD curves and (c) the correlation between S_BET, APD and temperature of S_500-10, S_550-10, S_600-10, S_650-10 samples.

Fig. 9. (a) UV-vis DRS spectra and (b) plots of (αhν)^1/2vs. photon energy of S_500-10, S_550-10, S_600-10 and S_650-10 samples.

Fig. 10. Room-temperature PL spectra of S_500-10, S_550-10, S_600-10 and S_650-10 samples under 328 nm excitation.

Fig. 11. TGA curve of g-C₃N₄ sample S_650-10.

Scheme 1. Schematic for the formation (a) and the reaction paths (b) of g-C₃N₄ nanosheet from the melamine precursor.

Fig. 12. The time-dependent curves of RhB concentration change under visible light irradiation (λ > 400 nm) ((a) pre-absorption, (b) without pre-absorption). The temporal absorption spectra of RhB in the presence of S_650-10 (c) and S_600-10 (d) samples. (e) Linear transform −ln (C/C₀) of the kinetic curves of RhB degradation. (f) Photocatalytic reaction cycles of S_650-10 sample. (g) The color change pictures and (h) concentration change curve of RhB solution over S_650-10 sample with time under natural sunlight.