Abstract:

Graphite phase carbon nitride (g-C₃N₄) has shown excellent potential when applied to photocatalytic hydrogen (H₂) generation upon exposure to visible light. However, the photocatalytic activity during hydrogen generation remains very low because of the high recombination rate of photogenerated electron-hole pairs and poor conductivity. Of the various strategies to improve H₂ generation efficiency, N vacancies have proven to be effective at increasing the photocatalytic performance of g-C₃N₄. However, creating a N vacancy is primarily dependent on the post-heating of g-C₃N₄ in air at an elevated temperature, which generates a high concentration of N vacancies and consequent decreased crystallinity of g-C₃N₄. Thus, as-produced g-C₃N₄ offers low photocatalytic efficiency owing to the high recombination rate of photogenerated electron-hole pairs. Currently, controlling the concentration of N vacancy in g-C₃N₄ is an immense challenge. Herein, we report an effective means of achieving controllable N vacancies in g-C₃N₄ via urea in-situ generated NH₃ at an elevated temperature. Specifically, g-C₃N₄ was first prepared with dicyandiamide as a precursor and subjected to rapid post-thermal treatment at 650 ℃ in a tubular furnace for 10 min, in which a desired amount of urea was mixed with g-C₃N₄ as the source material for NH₃. X-ray diffraction analysis showed increased crystallinity and an unchanged crystal structure as compared to pristine g-C₃N₄. X-ray photoelectron spectroscopy and elemental analysis verified the reduced levels of N-vacancy concentration with urea added as the NH₃ source when compared to the g-C₃N₄ post-heated in air without the addition of urea. In addition, UV-Vis spectra displayed an increased visible light absorption due to the generated N vacancies. Moreover, the specific surface area of g-C₃N₄ was progressively enlarged with an increase in the amount of urea added. The high crystallinity, low N-vacancy concentration, increased light absorption, and enlarged surface area translated into markedly increased photocatalytic H₂ generation. The highest H₂ generation rate from the optimized added amount of urea was 6.5 μmol·h^-1, which was three times higher than that when using a g-C₃N₄ sample thermally treated without urea addition. The H₂ generation enhancement was also the result of the increased separation efficiency of photogenerated electron-hole pairs as exemplified by the significantly decreased photoluminescence spectra and large transient photocurrent. The results of this study demonstrate the simultaneous production of highly crystalline g-C₃N₄ and controllable creation of N vacancy by in-situ generated NH₃ through thermal decomposition of urea. This study reveals the immense potential of NH₃ at controlling the N-vacancy concentration of g-C₃N₄ for increased photocatalytic H₂ generation.

Key words: Semiconductor, Carbon nitride, Photocatalytic water splitting, NH₃, Nitrogen vacancy