Investigating The Photoelectric And Photocatalytic Properties Of Graphitic Carbon Nitride

With the increasing consumption of fossil energy and enhancement of environmental awareness,the development of new clean energy and its technology has been the inevitable trend.Photocatalytic hydrogen production technology uses the light absorption properties of semiconductors to produce electron-hole pairs with strong oxidation activity,and decomposes water into hydrogen and oxygen.This technology actualizes the conversion from solar to hydrogen energy,which is in line with the concept of green chemistry.It has received extensive attention in solving the problems of environmental pollution and energy shortage.The core issue of photocatalytic hydrogen production is to obtain catalysts with high stability,low toxicity and high energy efficiency.The frequently studied oxides semiconductor photocatalysts,such as Ti O₂ either have poor response to visible light,or have high biological toxicity or low chemical stability,limiting the utilization and transformation of solar energy.Nevertheless among many new semiconductor photocatalysts,graphite phase nitrogen carbide(g-C₃N₄)is a kind of non-metallic semiconductor photocatalyst,which has suitable band gap,unique electronic structure and good chemical thermal stability.Therefore it has been widely researched in the field of photocatalytic hydrogen production,photocatalytic degradation,and photocatalytic CO₂ reduction.However,high carrier recombination efficiency and low specific surface area reduce the photocatalytic activity of g-C₃N₄.Among these structural modification and modification methods of g-C₃N₄,defect regulation and the construction of heterojunctions are considered to be one of the ways with high potential to tackle the problem of g-C₃N₄ high photogenerated carrier recombination.As a matter of fact,on the one hand,the intervention of defects is often accompanied by incomplete polymerization of precursors,resulting in the coexistence of a variety of functional groups.The role of defects,especially intrinsic defects,is often covered up.Thus,it is significantly important to understand the generation of defects and their effect on the photocatalytic hydrogen production activity of g-C₃N₄ from the perspective of polymerization thermodynamics.On the other hand,metal semiconductors are commonly utilized in the construction of g-C₃N₄heterojunctions.Although this way can promotes the separation of electrons and holes,the interfacial ion bonding may lead to the reduction of active sites.Given all the above,in this paper,by introducing cyan and hydroxyl defects into g-C₃N₄ or using organic conjugated polymer PIDTT-DTBO and g-C₃N₄ to construct heterojunction photocatalyst.The surface active sites of g-C₃N₄ are adjusted,the visible light absorption range is expanded,the recombination probability of photocarriers is reduced,and the photocatalytic activity of g-C₃N₄ is improved.Furthermore,the formation of defects and heterojunctions was revealed by structural characterization of XRD,TEM,SEM,XPS and FTIR.The relationship between the specific defects or heterostructure and the photocatalytic hydrogen production performance of the material was revealed by the photoelectrochemical test and DFT theoretical calculation,and the internal mechanism of improving the photocatalytic hydrogen production performance by the related structural regulation was proposed.The research contents specifically present as follows:(1)The performance of photocatalytic hydrogen production of g-C₃N₄ modified by single defects(nitrogen defects,cyan defects,hydroxyl defects)and synergistic defects(cyan defects and hydroxyl defects)was studied in this paper.It was found that the catalytic activity of cyan and hydroxyl co-modified g-C₃N₄ was significantly improved,and the hydrogen production rate reached 2253.6?mol h^-l g^-1,which was about 3 times higher than that of g-C₃N₄ modified by cyan or hydroxyl single defect.Through the experimental feature of transient photocurrent,electrochemical impedance spectroscopy and transient PL,it is revealed that the synergistic effect of cyan and hydroxyl defects effectively improved the detached efficiency of g-C₃N₄ photogenerated carriers.Moreover,it also can reduce the band gap and expand the photoabsorption range,thus improving the photocatalytic hydrogen production activity of g-C₃N₄.The density functional theory calculations demonstrate that the modification of nitrogen defects and cyan defects will introduce gap states near the valence band top(VBM)and conduction band bottom(CBM)of g-C₃N₄ respectively.The cyan-modified g-C₃N₄ will then form a deep energy trap state and become the recombination center of carriers,which hinders the charge separation and the process of photocatalytic hydrogen production.Such synergistic modification of cyan group and hydroxyl group will effectively reduce the band gap of g-C₃N₄,improve the electronic structure,increase the migration rate of electron hole pair,reduce the recombination probability,and improve the photocatalytic hydrogen production performance.(2)Study on photocatalytic hydrogen production performance of heterojunction of donor-acceptor(D-A)polymer and g-C₃N₄.In this paper's research,two kinds of heterojunction photocatalysts Po/Pt/CN and Pt/Po/CN with different structures were constructed by using D-A conjugated polymer PIDTT-DTBO(Po),platinum nanoparticles(Pt)and g-C₃N₄ nano-sheet(CN).As shown by the hydrogen productive performance of the materials,it was demonstrated that the photocatalytic hydrogen productive efficiency of 3%Po/Pt/CN was significantly increased than that of CN,which was about 8408.9?mol h^-1 g^-1.No significant change was observed in the photocatalytic hydrogen productive performance of Pt/3%Po/CN.Besides,by using UV-vis DRS to characterize the optical absorbing properties of the materials,it is exhibited that the optical absorbing properties of the two composites have been remarkably improved.Further photocurrent response and impedance spectra tests show that 3%Po/Pt/CN possessed higher current density and lower transfer resistance.In addition,the steady-state and transient fluorescence spectra show that the mismatched energy level structure of Pt/3%Po/CN will lead to the interface recombination of the carriers,and this will make against to improving the photocatalytic performance of the materials.In conclusion,the performance of photocatalytic hydrogen production has been significantly improved based on the heterostructure constructed by defect-regulated g-C₃N₄.