Fabricating the Robust g-C3N4 Nanosheets/Carbons/NiS Multiple Heterojunctions for Enhanced Photocatalytic H2 Generation: An Insight into the Trifunctional Roles of Nanocarbons

In this work, robust nanocarbons, including graphite (G), carbon nanotube (CNT), reduced graphene oxide (rGO), carbon black (CB), and acetylene black (AB), have been successfully coupled into the interfaces between g-C3N4 and NiS using a facile precipitation method. The results demonstrated that nanocarbons played trifunctional roles in boosting the photocatalytic H-2 evolution over g-C3N4, which can not only act as effective H-2-evolution co-catalysts but can also serve as conductive electron bridges to collect photogenerated electrons and boost the H-2-evolution kinetics over the NiS co-catalysts. More interestingly, the nanocarbons can also result in the downshift of valence band of g-C3N4, thus facilitating the fast oxidation of triethanolamine and charge-carrier separation. Particularly, in all five ternary multiheterostructured systems, the g-C3N4-0.5%CB-1.0% NiS (weight ratio) and g-C3N4-0.5%AB-1.0%NiS photocatalysts exhibited the highest H-2-evolution rates of 366.4 and 297.7 mu mol g(-1) h(-1), which are 3.17 and 2.57 times higher than that of g-C3N4-1.0%NiS, respectively. Apparently, the significantly enhanced H-2-evolution activity of multiheterostructured g-C3N4/carbon/NiS composite photocatalysts can be mainly ascribed to the trifunctional nanocarbons, which serve as the conductive electron bridges rather than the general co-catalysts. More importantly, it is revealed that the amorphous carbons with higher electrical conductivity and weaker electrocatalytic H-2-evolution activity are more suitable interfacial bridges between g-C3N4 and NiS co-catalysts for maximizing the H-2 generation. This work may give a new mechanistic insight into the development of multiheterostructured g-C3N4-based composite photocatalysts using the combination of trifunctional nanocarbon bridges and earth-abundant co-catalysts/semiconductors for various photocatalytic applications.