Synthesis And Modification Of Graphitic Carbon Nitride For Visible Light Photocatalytic Hydrogen Evolution

Energy crisis and environmental pollution are not scaremongering.It is imperative to find clean energy from renewable sources to prevent them.Hydrogen,which only produces water during combustion,has been regarded as an environmentally friendly renewable energy source that can replace traditional fossil energy.Semiconductor-based photocatalytic water splitting for H₂ is a very worthwhile way to solve energy and environmental problems.The core of this technology is to find a semiconductor photocatalyst with high photocatalytic activity.Upon discovery of H₂ evolution from water splitting using graphitic carbon nitride?g-C₃N₄?,photocatalysts based on this non-metal semiconductor have drawn growing attention owing to its uniquely electric,optical,and physiochemical properties as well as its cost-effective advantage.However,g-C₃N₄ suffers from the following drawbacks:small specific surface area,high recombination rate of charge carriers and narrow visible light absorption??<450 nm?,thus resulting in low photocatalytic activity under sunlight.In order to overcome the above shortcomings of g-C₃N₄,we used isonicotinic acid and ammonium citrate tribasic as precursors to react with urea or g-C₃N₄ to modify g-C₃N₄,and successfully prepared nitrogen-rich carbon nitride,nitrogen-deficient carbon nitride,and carbon nitride modified by carbon quantum dots.The structures and photocatalytic activity of the prepared materials were carefully studied.The details of the work are summarized as follows:?1?Isonicotinic acid?IA?was demonstrated the capability to tune the nitrogen content in carbon nitride?CN?to prepare both nitrogen-rich CN?CN-NR100?and nitrogen-deficient CN?CN-ND500?.The reaction of IA with urea introduced more nitrogen into the tri-s-triazine units of CN.Elemental doping can effectively narrow the band gap of CN,so as to effectively broaden its absorption of visible light.On the other hand,IA reacting with CN can take away part of the amino groups of CN.By introducing nitrogen atoms and forming nitrogen defects to modulate the band structure and electronic structure of CN,it can significantly enhance the absorption of visible light,increase the specific surface area and improve the charge separation of CN.Therefore,CN-NR100 and CN-ND500 manifest excellent photocatalytic hydrogen evolution rates of 73?mol h^-1?about 4 times of that of CN?and 120?mol h^-1?6.7times of that of CN?,respectively.?2?The ammonium citrate tribasic?ACT?was used as a precursor for the preparation of carbon quantum dots?CQDs?.The CQDs-modified carbon nitride?CN-ACT-1?was prepared by putting ACT and urea in a crucible of different specifications.During the heating process,the CN was etched by ammonia gas which came from the decomposition of ACT,resulting in a big specific surface area of CN.On one hand,as an electron acceptor,CQDs can rapidly capture the photogenerated electrons,so as to separate photogenerated electrons and holes.On the other hand,large specific surface area can provide more active sites for photocatalytic reaction.These changes were beneficial to the photocatalytic hydrogen evolution of the samples.The hydrogen evolution of CN-ACT-1 reached 120 mol h^-1,which was 5 times of that of pure CN.Compared with other reports on CQDs modification of CN,this method avoids the complex process of preparing CQDs,and makes use of a simple and environmentally friendly precursor to prepare CQDs-modified carbon nitride in a pot.