Influence of the gas atmosphere during the synthesis of g-C ₃N ₄ for enhanced photocatalytic H ₂ production from water on Au/g-C ₃N ₄ composites

Abstract

The atmosphere plays a key role in the polycondensation synthesis and the g-C ₃N ₄ structure.

Abstract

The design of Au/g-C ₃N ₄ nanocomposites for enhanced H ₂ production from water under solar and visible light irradiation is presented by varying the g-C ₃N ₄ synthesis atmosphere (air, N ₂, H ₂, Ar and NH ₃). We showed for the first time that the synthesis of g-C ₃N ₄ in a pure NH ₃ atmosphere led to enhanced photocatalytic performances between 3 and 9 times higher than g-C ₃N ₄ prepared in other gas atmospheres. The resulting, novel 0.3 wt% Au/g-C ₃N ₄–NH ₃ photocatalyst produced up to 324 μmol h ⁻¹ g _cat ⁻¹ and 26 μmol h ⁻¹ g _cat ⁻¹ of H ₂ 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-C ₃N ₄, 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-C ₃N ₄–NH ₃ and lower deformation of nanosheets. Additionally, the ratio between tri-s-triazine and s-triazine based C ₃N ₄ 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-C ₃N ₄–NH ₃ allowed more homogeneous coverage with small Au NPs yielding more intimate contact and higher quality of the interface between Au NPs and the g-C ₃N ₄ support.