New insight of the photocatalytic behaviors of graphitic carbon nitrides for hydrogen evolution and their associations with grain size, porosity, and photophysical properties

•Grain size of basic units in g-C3N4 was controlled by synthetic conditions.•C3N4 with small grain showed better photocatalytic activities for hydrogen evolution.•Thermal treatment of urea in N2 produced g-C3N4 with the highest catalytic activity.

The development of efficient catalysts for hydrogen evolution reaction (HER) presents a huge technical challenge. Graphitic carbon nitride (g-C3N4) is a promising metal-free, low cost, environment-friendly photocatalyst for HER that is driven by visible light. In this work, the authors provide new insight into the photocatalytic natures of g-C3N4 materials and their dependences on grain size, porosity, chemical structure, and photophysical properties. Three different precursors (urea, melamine, and dicyandiamide) and two gas atmospheres (air or N2) are used to produce various g-C3N4 materials. The use of urea and air leads to the formation of small grain C3N4 networks and porous structures with large surface areas. HER catalytic activity is promoted by large surface areas and the presence of terminal amine groups, and generation of small-sized Pt nanoparticle co-catalysts with narrow size distribution on the surface of g-C3N4. For samples with similar surface areas, band gaps and lifetimes of photogenerated charge carriers critically determine photocatalytic activities. By examining combinations of the above-mentioned factors, urea driven g-C3N4 produced in a N2 atmosphere is found to exhibit the best photocatalytic activity (up to 130 μmol h−1 g−1).

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