Unprecedented effect of CO2 calcination atmosphere on photocatalytic H2 production activity from water using g-C3N4 synthesized from triazole polymerization

Gaseous atmosphere during graphitic carbon nitride (g-C3N4) preparation can have a significant influence on modifying the morphological texture, polymeric structure, charge carrier behavior, and consequently the photocatalytic performance. Herein, we developed a new one-step method to fabricate g-C3N4 through direct pyrolysis of 3-amino-1,2,4-triazole in CO2 atmosphere (C3N4-T-CO2) with no additive. Surprisingly, the H2 production activity of C3N4-T-CO2 photocatalyst under visible light irradiation was 2.4 and 1.7 times as high as that of g-C3N4 obtained under air and N2 atmosphere with the same heating process, respectively. Detailed characterizations indicated that the CO2 calcination atmosphere induced less nitrogen vacancies with no charge transport ability, more NHx groups, and faster rate of the electron transport between heptazine rings for C3N4-T-CO2 among three samples. It is also suggested that the larger number of NHx in C3N4-T-CO2 could enhance the interlayer electron transport through the hydrogen-bonding interaction between C3N4 layers. Time-resolved photoluminescence, single-particle fluorescence, and femtosecond time-resolved transient absorption measurements were performed to elucidate the efficient charge transfer and trapping processes in C3N4-T-CO2. For the first time, such unprecedented effect of CO2 calcination atmosphere was observed for g-C3N4. This work not only presents a promising strategy in designing highly effective g-C3N4 photocatalyst for solar energy conversion, but also makes an insight into the charge transfer process in g-C3N4 photocatalyst.