A comparison study of alkali metal-doped g-C3N4 for visible-light photocatalytic hydrogen evolution

Photocatalytic hydrogen production based on semiconductor photocatalysts has been considered as one of the most promising strategies to resolve the global energy shortage. Graphitic carbon nitride (g-C3N4) has been a star visible-light photocatalyst in this field due to its various advantages. However, pristine g-C3N4 usually exhibits limited activity. Herein, to enhance the performance of g-C3N4, alkali metal ion (Li+, Na+, or K+)-doped g-C3N4 are prepared via facile high-temperature treatment. The prepared samples are characterized and analyzed using the technique of XRD, ICP-AES, SEM, UV-vis DRS, BET, XPS, PL, TRPL, photoelectrochemical measurements, photocatalytic tests, etc. The resultant doped photocatalysts show enhanced visible-light photocatalytic activities for hydrogen production, benefiting from the increased specific surface areas (which provide more active sites), decreased band gaps for extended visible-light absorption,land improved electronic structures for efficient charge transfer. In particular, because of the optimal tuning of both microstructure and electronic structure, the Na-doped g-C3N4 shows the most effective utilization of photogenerated electrons during the water reduction process. As a result, the highest photocatalytic performance is achieved over the Na-doped g-C3N4 photocatalyst (18.7 μmol/h), 3.7 times that of pristine g-C3N4 (5.0 μmol/h). This work gives a systematic study for the understanding of doping effect of alkali metals in semiconductor photocatalysis.