| Graphitic carbon nitride(g-C3N4)is a two-dimensional layered semiconductor material.In recent years,it has attracted much more attention as an eximious photocatalyst in decomposing water to prepare hydrogen under visible-light irradiation for its simple synthesis method,appropriate electronic band structure,excellent chemical stability and abundant sources.However,low specific surface area,easy carrier recombination,low solar absorption range and other problems seriously limit the photocatalytic activity of bulk g-C3N4.Therefore,the improvement of the photocatalytic hydrogen production performance by the structure regulation of g-C3N4 has been widely concerned.In this thesis,three new precursor reforming strategies were proposed to modify g-C3N4.We have successfully constructed three kinds of g-C3N4 nanostructures with high photocatalytic hydrogen production performances;,including:non-metallic element oxygen(O)-doped g-C3N4 nanosheets,metal element potassium(K)-doped g-C3N4 nanosheets and the g-C3N4 bulk/nanosheet homojunctions.As the result shown:the above-mentioned three modification methods all enhanced the reduction potential(conductive band edge)of g-C3N4 and prolonged the photogenerated carriers lifetime,which improved the performances of photocatalytic hydrogen production drastically.The main results of this thesis are as follows:(1)The O-doped g-C3N4 nanosheets(denoted as CNO)were synthesized by a direct thermal polymerization of hydrogen peroxide-assisted hydrothermal-reformed melamine.When the addictive amount of H2O and H2O2 were 35 mL and 15 mL respectively(the mass of melamine keeps at 7.2 g),the specific surface area of CNO increased to 31.7 m2· g-1 and the photocatalytic hydrogen production rate of CNO was highly up to 1050.3 ?mol·h-1·g-1 under visible light irradiation(?>420 nm),which was about 10.7 times than that of the bulk g-C3N4.Furthermore,the appearance quantum efficiency(AQE)reached to 13.04%(?=420 nm).The effective combination of O-doping and ultrathin nanosheets structure raised the conductive band edge of designed CNO compared with bulk g-C3N4,increased the active sites in photocatalytic reduction reaction,and effectively inhibited the rapid recombination of photogenerated charge carriers.Thus,CNO demonstrated an excellent photocatalytic performance in hydrogen production.(2)The K-doped g-C3N4 nanosheets were prepared by a direct thermal polymerization of potassium hydroxide-assisted hydrothermal-reformed melamine.This strategy can realize the synchronization of K atom interlayer doping and block g-C3N4 stripping.When the molar ratio of melamine to KOH was 2:1(denoted as CNK21,and the mass of melamine keeps at 7.2 g),the hydrogen production achieved maximum(919.5 ?mol·h-1g-1)under visible-light irradiation(?=420 nm),which was about 13.1 times than that of the bulk g-C3N4.Furthermore,the AQE reached to 6.98%at the wavelength of 420 nm.The significantly enhanced hydrogen production of CNK21 sample could be attributed to the nanosheet structure with high specific surface area and the increased reduction potential in K-doped atom layer,which could provide abundant of photocatalytic active sites and contribute to increase the transfer as well as suppress the recombination of photo-induced carriers,respectively.(3)A bulk/nanosheet g-C3N4 nanocomposite(named as BCN/CNNS homojunction)was prepared by a direct thermal polymerization of glucose-assisted hydrothermal-reformed melamine.Its hydrogen production activity achieved maximum when the additive amount of glucose was 50 mg(the mass of melamine keeps at 7.2 g).The photocatalytic hydrogen production rate of BCN/CNNS was enhanced up to 1361.9 ?mol·h-1g-1 under visible-light irradiation(?>420 nm),which was about 16.4 times than that of the bulk g-C3N4.Furthermore,the AQE reached to 6.8%(?= 420 nm)and achieved to 1.82%(?= 550 nm).This type II homojunction could not only broaden the visible light absorption range of g-C3N4,but also contribute to the photo-induced carriers transfer,result in the improved photocatalytic hydrogen production.Moreover,we have also prepared new g-C3N4 homojunctions by hydrothermal treatment of other melamine precursors by substitution of glucose with fructose,sucrose,lactose,maltose and maltitol,which exhibited sililar morphologies and structures with the above BCN/CNNS homojunction as well as excellent photocatalytic activities for hydrogen production. |
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