School of Chemistry and Chemical Engineering, Anhui University, Hefei 230601, P. R. China
基金项目：
the National Natural Science Foundation of China(551872003);the National Natural Science Foundation of China(51572003);the Anhui Provincial Natural Science Foundation, China(1908085J21);the Anhui Provincial Natural Science Foundation, China(1908085QB83)
School of Chemistry and Chemical Engineering, Anhui University, Hefei 230601, P. R. China
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-C3N4via 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.
