Influence of the gas atmosphere during the synthesis of g-C3N4 for enhanced photocatalytic H2 production from water on Au/g-C3N4 composites

Abstract

The design of Au/g-C₃N₄ nanocomposites for enhanced H₂ production from water under solar and visible light irradiation is presented by varying the g-C₃N₄ synthesis atmosphere (air, N₂, H₂, Ar and NH₃). We showed for the first time that the synthesis of g-C₃N₄ in a pure NH₃ atmosphere led to enhanced photocatalytic performances between 3 and 9 times higher than g-C₃N₄ prepared in other gas atmospheres. The resulting, novel 0.3 wt% Au/g-C₃N₄–NH₃ photocatalyst produced up to 324 μmol h⁻¹ g_cat ⁻¹ and 26 μmol h⁻¹ g_cat ⁻¹ of H₂ corresponding to internal quantum yields of 1.85 and 0.60% under solar and visible light irradiation respectively, with an unusually low amount of triethanolamine used as the sacrificial agent (1 vol%). This enhanced activity was correlated to the structural, optical, porosity, and surface properties of g-C₃N₄, and to the quality of the interface with Au NPs. From an in-depth structure–activity correlation study, we highlighted the combined effects of a higher surface area with larger contribution of mesoporous volume, higher crystallization degree of g-C₃N₄–NH₃ and lower deformation of nanosheets. Additionally, the ratio between tri-s-triazine and s-triazine based C₃N₄ was determined and used for the first time to point out the effect of different continuous gas flow atmospheres during synthesis. Furthermore, the suitable surface chemistry of g-C₃N₄–NH₃ allowed more homogeneous coverage with small Au NPs yielding more intimate contact and higher quality of the interface between Au NPs and the g-C₃N₄ support.