| Energy shortage and environmental pollution are common problems for all over the world,which are related to the sustainable development of human society.At present,human energy consumption mainly depends on non-renewable fossil fuels such as oil,coal and natural gas,which are the non-renewable nature.The huge amount of CO2 was released by the burning of these fossil fuels,which was caused environmental pollution.Therefore,it is urgent to develop new renewable energy.Due to the advantages of the inexhaustible solar energy and the high combustion value,green and environmental protection of hydrogen energy,it is considered as a kind of hydrogen production technology with great development potential by converting solar energy into hydrogen energy using photocatalyst,which has received extensive research attention.Traditional photocatalysts are mainly inorganic semiconductor materials based on metal oxides or sulfides.Their commercial application was seriously hindered due to the complex preparation procedures,high energy consumption,high toxicity and low activity under visible light.As a kind of new type photocatalyst materials,advantages of organic polymer g-C3N4 including its simple preparation,low cost,good thermal stability and band gap,has been attracted people's attention,but the shortcomings such as the low specific surface area of a single and optical electronic-hole recombination rate higher,were lead to low its photocatalytic activity and visible light response.How to develop a new type of g-C3N4 photocatalyst with high visible light response activity through the selection of structure design and preparation strategy has become a frontier topic in this research field,which has received great attention from many researchers in recent years.In this dissertation,a series of novel g-C3N4 composite photocatalyst materials were synthesized.By introducing organic microporous polymer into g-C3N4 through polymerization,the specific surface area of g-C3N4 was increased,and the photocatalytic activity sites of g-C3N4 were enhanced.By constructing semiconductor heterojunction,the effective separation of photogenerated electrons and holes were improved,and the photocatalytic activity was greatly boosted.The visible light response activity of g-C3N4 composite photocatalyst material was improved by effective regulation of electronic structure and band gap(through impregnation,calcining and in-situ synthesis).The structure-activity relationship between composite photocatalyst and photocatalytic performance of g-C3N4 composite photocatalyst was constructed by deeply studying the structural factors affecting the photocatalytic performance of g-C3N4 composite photocatalyst,which is of great significance for the design and development of composite photocatalyst materials with high performance.Specific research contents include the following four aspects:(1)Preparation of metal oxide/g-C3N4 composite photocatalyst and study on photocatalytic hydrogen evolution:A series of MgO/g-C3N4 and MnO/g-C3N4 composite photocatalyst materials were prepared by loading the precursor of metal oxide onto the surface of g-C3N4 by dipping and calcination,and their photocatalytic hydrogen evolution performance of the prepared composite was studied.The results show that the increase of the specific surface area of the composite materials was helpful to increase photocatalytic active sites.Due to coordination covalent bonds between metal ions of metal oxide nanoparticles and N of g-C3N4 nanoplates,UV-Visible absorption spectrum was redshifted and energy gap bandwidth was reduced,and absorption of visible light was enhanced.The heterojunction between metal oxide and g-C3N4 promotes the effective separation of photogenic e--h+ and improves the photocatalytic activity of hydrogen-producing by photocatalytic splitting water.Under visible light irradiation,the hydrogen evolution rate of MgO/g-C3N4 photocatalyst can reach as high as 30.1 ?mol h-1,and the hydrogen production rate of MnO/g-C3N4 photocatalyst can also reach 55.9 ?mol h-1,which is much higher than the hydrogen evolution rate of 5.79 ?mol h-1 of bare g-C3N4.These results show that the introduction of metal oxides contributes to the separation of photo e--h+and greatly improves the photocatalytic activity of hydrogen evolution from water splitting.Therefore,the preparation of metal oxide/g-C3N4 composite photocatalyst by combining metal oxide with g-C3N4 was an effective way to improve the photocatalytic performance.(2)Preparation of ZnO/Fe2O3/g-C3N4 composite photocatalyst and study on photocatalytic hydrogen evolution:A series of ZnO/Fe2O3/g-C3N4 photocatalytic materials were prepared by loading precursors of Fe2O3 and ZnO onto the surface of g-C3N4 by calcination method,and their photocatalytic hydrogen production performance was studied.The results show that the increase of specific surface area of ZnO/Fe2O3/g-C3N4 photocatalysis contributes to the increase of active sites of photocatalysis.Due to the coordination between Fe and Zn of the metal oxides and N atoms of g-C3N4,UV-Visible absorption spectrum was redshifted and energy gap bandwidth was reduced,and absorption of visible light was enhanced.Due to the combination of p-type semiconductor Fe2O3 and n-type semiconductor ZnO,a p-n heterojunction was effectively constructed to improve the separation efficiency of e-h+pairs,and the composition of photogenic e-h+ was fabricated.Under visible light irradiation,the hydrogen evolution rate of ZnO/Fe2O3/g-C3N4 composite photocatalyst can reach up to 25.5 ?mol h-1,which is much higher than 5.79 ?mol h-1 of bare g-C3N4.These results indicate that the introduction of p-n heterojunction of metal oxides into the structure was conducive to the separation of photogenic e-h+,which greatly improved the photocatalytic activity of hydrogen evolution from water splitting.Therefore,the preparation of metal oxide/g-C3N4 composite photocatalyst by forming p-n heterojunction and g-C3N4 was make an effective way to improve the photocatalytic performance.(3)Preparation of organic microporous conjugated polymer/g-C3N4 composite and study on photocatalytic hydrogen evolution:A series of photocatalyst including PPB/g-C3N4,PPPy/g-C3N4 and PPyPP/g-C3N4 were formed by loading organic conjugated microporous polymer onto the surface of g-C3N4 through in-situ recombination using 1,4-phenylenebiboronic acid,1,3,5-tribromobenzene and 1,3,6,8-tetrabromopyrene.The photocatalytic hydrogen evolution and degradation of organic pollutants were evaluated as photocatalysts.The study shows that the introduction of organic conjugated microporous polymer greatly was improved the specific surface of composite photocatalytic materials,the reduction of energy gap was enhanced the absorption of visible light,and the polymer heterojunction formed between organic polymer and g-C3N4 was also promoted the effective separation of photogenic e-h+,thus significantly improving its photocatalytic performance.Under UV-Visible irradiation,the photocatalytic activity of organic conjugated microporous polymer/g-C3N4 photocatalyst was much higher than that of bare g-C3N4.At the same time,this kind of photocatalyst material has remarkable effects on photocatalytic degradation of organic pollutants methylene blue and removal of Cr(VI).These results show that the introduction of organic microporous polymer into g-C3N4 to prepare organic photocatalyst materials was make an effective way to improve the photocatalytic performance.(4)Preparation of composite photocatalyst for soluble linear polymer PFSO/g-C3N4 and study on photocatalytic hydrogen evolution:The soluble linear polymer PFSO(copolymer of fluorene and benzothiophenone)was loaded onto g-C3N4 through impregnation to form organic composite photocatalyst PFSO/g-C3N4,and its photocatalytic hydrogen evolution performance was evaluated.The results show that the polymer bulk heterojunction which constructed with soluble sulfoxide polymer loaded on g-C3N4 nanometer sheet was promoted the efficient separation of photogenerated electrons and holes.The introduction of sulfoxide(O=S=O)not only increased the photocatalytic activity sites,but also improved the hydrophilicity of the composite photocatalyst.In addition,the polymer containing sulfone group has good co-planar structure,which is beneficial to the migration and transport of photogenic electrons.Therefore,the PFSO/g-C3N4 photocatalyst showed a high photocatalytic activity of hydrogen evolution from water splitting,and its hydrogen evolution activity was up to 149.0 ?mol h-1 under visible light irradiation.This work provides a new idea for the preparation of efficient composite photocatalyst with visible light response. |
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