The Modification And Photocatalytic Performance Of Graphite-like Carbon Nitride

Facing the ever-increasing energy demands and environmental crisis,the whole society is calling for a clean and cost-efficient method to redefine future.Photocatalysis,which is inspired by natural photosynthetic process,has gained growing attention for its ability to directly utilize solar energy for the production of solar fuels,chemical products and degradation of pollutants.A photocatalytic reaction includes three main steps:i)generation of photo-excited electron-hole pairs;ii)separation and transfer of photoexcited charge carriers;iii)the charge carrier-involved redox/oxidation reactions.However,photocatalysis is still in laboratory to date because of its low efficiency arising from the fast charge recombination and slow charge transfer.In order to elevate the photon energy conversion efficiency,a photocatalyst or photocatalytic system must harmonically guarantee high efficiencies of all three main processes concurrently instead of only one of them.The design and fabrication of efficient photocatalysts becomes critical in photocatalysis.One of the most promising photocatalysts is the tri-s-triazine-based graphitic carbon nitride?CN?.As the most stable form of carbon nitrides,CN exhibits excellent chemical and thermal stability,and can be easily prepared from low-cost precursors,such as urea,melamine and dicyandiamide.Also,its nontoxicity makes CN friendly not only to human but also to environment.Furthermore,CN has suitable energy levels with high enough redox potential for water-splitting,and suitable band-gap to respond to visible light.Since the discovery of the photocatalytic ability of CN in 2009,improving its moderate performance has been becoming one of the major subjects of research on CN.Given that CN has the character of both organic material and semiconductor,the strategies for its activity enhancement are rather diverse,including red-shifting the absorption onset through copolymerization,increasing surface area by texture modification and forming composite with other semiconductors to prevent recombination.Here in this dissertation,three strategies are proposed to elevate the photocatalytic performance of CN.First in Chapter 2,aiming to promote the separation of charge carriers in CN,we propose a strategy of constructing composite photocatalyst.In detail,a new MoS₂ quantum dot-decorated CN heterostructured photo-catalyst?MoS₂-QDs/CN?has been synthesized via a facile impregnation method.The obtained composite photocatalyst exhibits enhanced absorption in visible light range compared with pristine CN.The introduction of MoS₂-QDs into the composite effectively prevents charge recombination and enhances the photocatalytic H₂ evolution activity.The highest H₂ evolution rate of 393.2?mol h^-1 g^-1has been achieved on 1wt%MoS₂-QDs/CN photocatalyst,which is 6.6 times higher than that on pristine CN.The band-structure of MoS₂-QDs/CN composite results in a type-II heterostructure,which is preferable in preventing the recombination of photo-induced carriers.The photocatalytic performance of MoS₂-QDs/CN is superior to the previously reported CN decorated with bulk MoS₂because of the more prominent edge activity of MoS₂-QDs.MoS₂-QDs have great potential to substitute for noble metal cocatalyst for enhanced photocatalytic H₂ evolution performance.This novel MoS₂-QDs/CN composite is a promising candidate for visible light H₂ evolution,and will provide inspiration on the development of other heterostructured photocatalysts for solar fuel production.In Chapter 3,we develop an efficient copolymerization strategy for synthesizing pyrimidine-modified carbon nitride?PMCN?,which provides a hyperchannel for photo-exited electron transfer from CN matrix to the anchored Pt nanoparticles.Owing to the extraordinary electro-withdrawing ability of pyrimidine groups and its strong interaction with the Pt particles,such a hyperchannel has been evidenced to be capable of largely promoting the electron transfer across the catalyst interface,i.e.,from the copolymer matrix to cocatalyst Pt,which is the limiting step during the photocatalytic reaction.Consequently,the Pt-loaded PMCN demonstrates a remarkably enhanced photocatalytic activity of an extra high hydrogen evolution rate of 3279.7?mol h^-1 g^-1 and AQY of 6%at 420 nm,15.3times that of CN.This work not only provides an effective copolymerization strategy to modulate electronic configuration of CN,but also highlights the importance of metal-support interaction in the photocatalyst system,revealing the effectiveness of electron transfer hyperchannel construction in the rational design of photocatalysts for substantially elevated solar-fuel conversion.In Chapter 4,we demonstrate an effective but facile strategy to introduce nickel single-atom sites into CN,and more importantly,to precisely modulate their electron configurations.Modulating the oxidation state of Ni single-atom sites into the intermediate state with a precise Ni²⁺/Ni⁰ratio of 2 will bring the most abundant unpaired d-electrons in the photocatalyst,thus optimizing the electronic structure for remarkably enhanced photocatalytic performance.The resulting photocatalytic H₂ production rate is as high as354.9?mol h^-1 g^-1,which is the highest among the reported CN photocatalysts decorated with metal single atoms,especially cheap transition-metal single atoms.This discovery not only demonstrates the great potential of cheap transition metals as highly potential active sites,but also reveals the unique role of d-electrons of the metal single atoms in CN framework in photocatalytic reactions.This work provides a brand-new electron-level route to explore the working mechanism and enhance the performances of single-atom catalysts.