Control of Nitrogen Vacancy in g-C3N4 by Heat Treatment in an Ammonia Atmosphere for Enhanced Photocatalytic Hydrogen Generation

Graphite phase carbon nitride (g-C3N4) has shown excellent potential when applied to photocatalytic hydrogen (H-2) 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 H-2 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-C(3)N(4)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-C(3)N(4)offers low photocatalytic efficiency owing to the high recombination rate of photogenerated electron-hole pairs. Currently, controlling the concentration of N vacancy in g-C(3)N(4)is an immense challenge. Herein, we report an effective means of achieving controllable N vacancies in g-C(3)N(4)via urea in-situ generated NH3 at an elevated temperature. Specifically, g-C(3)N(4)was first prepared with dicyandiamide as a precursor and subjected to rapid post-thermal treatment at 650 degrees C in a tubular furnace for 10 min, in which a desired amount of urea was mixed with g-C(3)N(4)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-C(3)N(4)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-C(3)N(4)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 H-2 generation. The highest H-2 generation rate from the optimized added amount of urea was 6.5 mu mol.h(-1), which was three times higher than that when using a g-C(3)N(4)sample thermally treated without urea addition. The H-2 generation enhancement was also the result of the increased separation efficiency of photogenerated electron-hole pairs as exemplified by the significantly decreased