| The two greatest challenges we humans face in 21st century are shortage of energy and environmental pollution,which have seriously restrained the sustainable development of our society.For the sake of ourselves,it is time to seek for a green and reproducible energy to satisfy our thirst for energy,and avoid any pollution to the earth at the same time.Photocatalysis technology is expected to work out the current dilemma we face,not only the solar energy is easy-available and endless to us but also it can be converted to hydrogen,electricity,chemical energy and so on.Recently,a new semiconductor,graphitic carbon nitride(g-C3N4),has drawn great attention from researchers due to its excellent properties,such as proper band gap,nontoxic,physicochemical stability and easily-synthesis process.However,the utility of g-C3N4 in daily life is still some way off because of the limited visible-light absorption and easy-recombination of photogenerated electron-hole pairs.To solve the problems above,our group modified g-C3N4 through elemental doping and constructing heterojunctions.The photocatalytic hydrogen evolution experiment was used to estimate the photocatalytic properties of the obtained samples before and after modification.Meanwhile,we also try to explain the mechanism of the charge carriers' separation,transfer and reaction processes based on the characterization and experiments.The details of this thesis are shown as follow:1.In section 2,we successfully introduced a series amount of phosphorus(P)atoms into the framework of g-C3N4 by directly calcinating the mixture of urea and 4-(diphenylphosphino)benzoic acid(4-DPPBA)in a one-pot method.After modification,the P doped g-C3N4(PCN)showed improved optical and electrical properties.Under the irradiation of visible light,PCN-5(the addition amount of 4-DPPBA is 5 wt%)showed the best photocatalytic hydrogen evolution rate(2610.80 ?mol h-1 g-1),which is nearly 10 times higher than that of pure g-C3N4(265.00 ?mol h-1 g-1).The improvement of photocatalytic activity in PCN samples may be thanks to the fact that electron-rich P atoms(compared to C atoms),acting as an electron donor,effectively trap photogenerated holes and restrain its recombination with electrons.Hence,the rest of electrons can be used in the reactions of reducing water on the surface of photocatalyst.To describe this process in detail,a possible mechanism was proposed.Besides,the PCN samples still maintain a high photocatalytic activity after 20 hours'recycling experiments,suggesting its good stability.Our work provides a way for broad application prospects in solar-to-fuel conversion.2.In section 3,we design and synthesize a novel Z-scheme g-C3N4/Ag/CoTPP ternary composites by a moderate two-step method.Ag nanowires combined with CoTPP·+through redox reactions and then Ag/CoTPP anchored onto the surface of g-C3N4 by mixing them in aqueous solution.The synergistic effect between g-C3N4 and Ag/CoTPP efficiently hinders the rapid recombination of photogenerated electron-hole pairs and enhances the photocatalytic activity.Under irradiating of visible light,both g-C3N4 and CoTPP can be excited and then the electrons on the CB of g-C3N4 combine with the holes on the VB of CoTPP through Ag nanowires,leaving the more reductive electrons on the CB of CoTPP and the more oxidative holes on the VB of g-C3N4,respectively.Compared with the pure g-C3N4,photocatalytic hydrogen evolution rate of the optimal sample is nearly 77 times higher than that of the former.Moreover,the reusability of the optimal sample was tested by recycling experiments.Finally,a possible mechanism of the photocatalytic process was put forward based on the results and analyses. |
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