| Photocatalysis is a green sustainable development technology that uses renewable solar energy to convert light energy to chemical energy.In recent years,as the core of photocatalytic technology,nano-photocatalysts have developed rapidly in the fields of water splitting,carbon dioxide reduction,nitrogen fixation,volatile organic compound removal,and water treatment,and have become an effective method to solve energy and environmental crises.The use of renewable solar energy to replace traditional non-renewable fossil fuels has opened up a new way of thinking.However,the current efficiency of nano-photocatalysts is low,and it is still far from the expected value of 10%for the photocatalytic efficiency of industrial production.Therefore,the rational construction of high-efficiency nano-photocatalysts is of great significance to promote the development of photocatalysis.The photocatalytic efficiency(?)is jointly determined by the light absorption efficiency(?abs),the separation efficiency of photo-generated charges(?cs),and the surface reaction rate(?rea),that is,?=?abs×?cs×?rea.Therefore,broadening the light absorption range of nano-photocatalysts,improving the efficiency of photo-generated charge transfer and separation,and increasing the number of active centers on the surface are effective strategies to improve photocatalytic efficiency.This dissertation takes the rational construction of high-efficiency photocatalysts as the research goal,selects graphite phase carbon nitride(g-C3N4)with a relatively wide light absorption range as the basic research material,uses photodeposition method to control the charge transfer path purposefully,and introduces Ga N4 site to improve the performance of the photocatalyst,and is committed to solving the two scientific problems of the low charge separation efficiency of the photocatalyst and the small number of surface active centers,and promotes the development of high-efficiency nano-photocatalysts.In addition,three innovative results have been achieved:(1)It was discovered that the direction of the photoinduced electron flow was controlled by the photodeposition method,and it is revealed that the photodeposition method is an effective method to realize the purpose construction of type II heterojunction and direct Z-scheme(DZS)photocatalyst;(2)It is proposed to selectively utilize the photo-oxidation and photo-reduction pathways in the photodeposition method to adjust the direction of charge transfer,and proved that it is a new strategy for the formation of DZS-type photocatalysts;(3)For the first time,Ga-doped polymeric carbon nitride(GCN)photocatalyst was prepared by a one-step method.The photocatalytic hydrogen production activity of GCN is as high as 9904?mol h-1 g-1,which is 162 times the photocatalytic hydrogen production activity of polymeric carbon nitride without co-catalyst,and 3.3times the activity of photocatalytic hydrogen production of polymeric carbon nitride with 1.0 wt%Pt.According to theoretical and experimental results,the reason for its high efficiency is that the introduction of Ga N4 active sites promotes charge separation and increases the number of surface reaction activity centers.This work opens a new chapter for the research of high-efficiency photocatalysts that integrate active sites and surface reactions.The specific work content of this dissertation mainly revolves around the following aspects:(1)Photodeposition method realizes the purpose construction of type II heterojunction and direct Z-scheme(DZS)photocatalyst.The content of this chapter is to achieve high-efficiency charge separation as the research purpose.The research plan is to construct a composite photocatalytic material with Cd S and g-C3N4 as components by photoreduction deposition and chemical hydrothermal deposition.The studies have found that the composite materials synthesized by the two methods exhibit different properties in photocatalytic activity and stability.It is inferred that the reaction mechanisms of the two materials are type II heterojunction and direct Z-scheme,respectively.In order to further verify the correctness of the above inferences,the charge flow direction and PL spectrum test were performed.The results showed that,and the electrons,which generated from the composite photocatalytic material synthesized by photoreduction deposition is excited by light,tend to flow from g-C3N4 to Cd S,and the holes tend to remain in g-C3N4,so the reaction mechanism is the traditional type II heterojunction.The difference from the above results is that the photogenerated electrons generated by the hydrothermal synthesis composite photocatalytic material after being excited tend to remain in g-C3N4,and the holes remain in Cd S,so the reaction mechanism is direct Z-scheme heterojunction.Furthermore,by comparing the SPS spectrum,EIS spectrum and AQE,it is found that the charge separation efficiency of type II heterojunction synthesized by photoreduction deposition method is significantly higher than that of direct Z-scheme heterojunction synthesized by hydrothermal method.The reason is that the photoreduction deposition method can control the flow of photogenerated electrons generated after excitation of g-C3N4 to the reduction sites on its surface,so that Cd S formed after S is reduced can be selectively deposited on the surface of g-C3N4.Therefore,the photogenerated electrons of the composite material are more likely to flow from g-C3N4 to Cd S.However,due to the randomness of the deposition site of the sample synthesized by the hydrothermal method,the direction of the charge flow is also high randomness.In summary,the photodeposition method achieves the purposeful construction of type II heterojunctions by regulating the direction of charge flow,and the separation efficiency of photogenerated charges is further improved compared to the purposeless synthesis method.Furthermore,combining the similarity in composition between type II heterojunction and direct Z-scheme,the photodeposition method is an effective way to achieve type II heterojunction and direct Z-scheme purpose construction with high charge separation efficiency.(2)The DZS-type photocatalyst was constructed rationally by selectively using the two paths of photooxidation and photoreduction in the photodeposition method.Compared with type II heterojunction,direct Z-scheme photocatalyst is an ideal material for high-efficiency photocatalysis,because it achieves high-efficiency charge separation efficiency while still maintaining high redox capability.Based on the previous work content and conclusions,this chapter takes the reasonable construction of direct Z-scheme with high efficiency of charge separation as the research goal,and proposes the theoretical strategy by photooxidation and photoreduction deposition to consciously construct direct Z-scheme.The specific content is as follows:when starting from SC-I,SC-II needs to be deposited to the hole-rich site through photooxidation process,and if SC-II is the starting material,SC-II needs that SC-I is deposited in the photogenerated electron-rich area by means of photoreduction.In order to verify the feasibility of the above strategy,using g-C3N4 and Ti O2 as starting materials,these two routes were demonstrated by using photo-oxidation and photo-reduction deposition methods,respectively.When g-C3N4 is used as SC-I,Fe2O3 can be selectively deposited on g-C3N4 as SC-II through the photooxidation pathway,and a g-C3N4/Fe2O3 composite material is formed.Experiments on charge tracing and active radical detection have been carried out to prove that photogenerated electrons remain on g-C3N4,while holes remain on Fe2O3.Therefore,Fe2O3/g-C3N4synthesized by photooxidation deposition method is direct Z-scheme photocatalyst.Similarly,using Ti O2 as the raw material of SC-II,Cd S can be deposited on the surface of Ti O2 as SC-I through photoreduction.Similarly,the formed Cd S/Ti O2composite material is also direct Z-scheme photocatalyst.In summary,through the careful selection of SC-I and SC-II materials,and the reasonable control of the flow of photo-generated charges by photo-deposition,the reasonable construction of direct Z-scheme photocatalysts can be realized,and this is proved that the photo-deposition method is an effective strategy to achieve direct Z-scheme construction that is conducive to high-efficiency charge separation efficiency.(3)Preparation of a high-efficiency GCN photocatalyst that integrates active sites and surface reactions.The content of this chapter is to solve the scientific problems of single-component semiconductor charge separation efficiency and the small number of surface active sites as the overall research goal,so as to avoid charge separation caused by the loss of interfacial charge in the reaction process of heterojunction photocatalytic materials.The problem of difficulty in achieving effective improvement in efficiency is the research direction.Taking polymeric carbon nitride(CN)as the research object,it is modified by element doping to realize the construction of a high-efficiency photocatalyst that integrates single-component active sites and surface reactions.The study found that in the Ga-doped carbon nitride(GCN)synthesized by one-step pyrolysis of urea,the Ga element is dispersed on the CN surface at the atomic level,and it is in a cationic state while coordinating with four nitrogen atoms to form Ga N4 sites.The photocatalytic hydrogen production activity of GCN is as high as 9904?mol h-1 g-1,which is 162 times the photocatalytic hydrogen production activity of polymeric carbon nitride without co-catalyst,and 3.3 times the activity of photocatalytic hydrogen production of polymeric carbon nitride with 1.0 wt%Pt.The results of TRPL,TPV and theoretical calculations show that the electrons generated by photoinduced can be injected into the Ga N4 sites on the GCN surface,thus realizing the effective spatial separation of photo-generated charges.In addition,because the empty p orbitals on Ga atoms can temporarily store photogenerated electrons at Ga N4 sites,this provides favorable conditions for the adsorption of protons and promotes the movement of photogenerated electrons from Ga N4 sites to protons.On this basis,the photogenerated electrons combine with protons to generate hydrogen atoms and fall back to the ground state,which simplifies the desorption of positively adsorbed hydrogen on the Ga N4 site.LSV and theoretical calculations further confirmed that the hydrogen-producing active center of GCN is Ga N4 site,and the atomic-level dispersion maximizes the number of surface active sites at extremely low Ga doping levels.This opens a new chapter in the research of high-efficiency photocatalysts that integrate active sites and surface reactions. |
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