Preparation And Hydrogen-production Performance Of G-C₃N₄ Based Photocatalysts

Graphite carbon nitride?g-C₃N₄?has attracted tremendous attention in the field of photocatalysis due to its special band structure,good physical and chemical properties as well as low price and abundant resource.However,pure g-C₃N₄ usually exhibits poor photocatalytic property because its photogenerated electrons and holes are easy to recombine.To achieve outstanding photocatalytic performance of g-C₃N₄,therefore,it is highly desirable to realize the effective separation of its photogenerated carriers.Recent studies have shown that cocatalyst modification is an effective way for g-C3N4 to improve the separation of photogenerated carriers and photocatalytic activity.Based on the studies,this article mainly focuses on the synthesis,structure characterization,photocatalytic hydrogen-production and catalytic mechanism of the g-C₃N₄ with co-catalyst modification.Specifically,?1?facile synthesis of carbon dots?CDs?modified g-C₃N₄ for enhanced photocatalytic H₂-evolution performance;?2?photocatalytic synthesis of metal sulfide cocatalysts to enhance photocatalytic H₂-evolution activity of g-C₃N₄.The main results could be summarized and shown as follows:Firstly,a facile hydrothermal approach was developed to prepare CDs/g-C₃N₄photocatalysts using L-ascorbic acid and g-C3N4 as the precursors.Upon in situ thermal polymerization of L-ascorbic acid on the surface of g-C₃N₄,the carbon dots were homogeneously and solidly modified on the g-C₃N₄ surface.Compared with the reported method for CDs/g-C3N4,our hydrothermal method is more facile and simple.It was found that the CDs/g-C₃N₄?10 wt%?sample showed the highest H₂-production rate(ca.2.2?mol h^-1),which was ca.4.4 times than the pure g-C₃N₄(ca.0.5?mol h^-1).After a Pt co-catalyst was loaded onto the as-prepared sample,the Pt-CDs/g-C₃N₄ photocatalyst formed could even split pure water to produce H₂.The enhanced photocatalytic performance of CDs/g-C3N4 may originate from rapid photogenerated electron transfer,and then inhibit photogenerated carrier recombination.As a result,more photogenerated electrons can be used to rapidly reduce H⁺to H₂.Secondly,NiS_x cocatalysts were in situ synthesized on the surface of g-C₃N₄ by a photocatalytic method to form NiS_x/g-C₃N₄ using sulfur,nickel nitrate and g-C₃N₄as precursors.Firstly,the S was reduced to S_x^2-by the photogenerated electrons of g-C₃N₄.Subsequently,the formed S_x^2-combined with Ni²⁺in the solution to form NiS_x,which was simultaneously deposited on the surface of g-C₃N₄.Compared with the reported approaches for NiS_x/g-C₃N₄ photocatalysts,our approach is obviously green,simple,and mild.Additionally,it can also inhibit the agglomeration of NiS_x cocatalysts and regulate NiS_x deposition sites.The photocatalytic results showed that NiSx/g-C3N4?0.3 wt%?exhibited the highest hydrogen production rate of 12.2?mol h^-1,which was comparable to g-C₃N₄ loaded by 1 wt%Pt.In addition,CoS_x/g-C₃N₄,AgS_x/g-C₃N₄ and CuS_x/g-C₃N₄ were also prepared by the photocatalytic method and exhibited higher photocatalytic H₂-evolution performance than pure g-C₃N₄.Obviously,it can be developed for a general method to synthesize g-C3N4 with modification of metal sulfide cocatalysts.Furthermore,the reason for enhanced photocatalytic performance is that metal sulfide cocatalysts promote the separation of photogenerated carries of g-C₃N₄.