Study On Effect Of Polymerization Degree On Photocatalytic Activity Of Graphite Carbon Nitride And Its Mechanism

Graphitic carbon nitride(g-CN)has attracted great attention due to its low cost,excellent performance,and unique band structures.However,the photogenerated electron-hole pairs recombination of g-CN is very serious for practical application.Therefore,it is very important to further improve its photocatalytic performance.Recently,researchers have made efforts to improve the photocatalytic performance of g-CN by increasing the specific surface area,dopped with metals or metal oxides,and other methods.Whereas the mechanism of g-CN photocatalysis still lacks a clear understanding.In this thesis,we first study the photocatalytic activity of g-CN relatedoligomers,and then systematically analyse the main factors affecting the its photocatalytic performance.We chose melamine as the raw material,and prepared g-CN by thermal condensation process under relatively lower heating temperatures,the obtained samples are mainly composed of melem and its oligomers.The structure,morphology,chemical bonding characteristics and charge transfer capability of the samples were systematically studied,and their photocatalytic activities and reaction mechanism were also been evaluated and studied.The results showed that synthesized samples with a realtively low degree of polymerization still possess stable photocatalytic activity,in which the incompletely condensed amino groups at the terminals of the heptazine act important catalytic active sites,making the material photocatalyticaly active under the full-spectrum irradiation.However,the relative lower polymerization degree resulted in a larger band gap,and thus the obtained samples showed poor photocatalytic activity under visible-light irradiation.We further modulated the degree of polymerization of g-CN by changing the thermal condensation time with the indentical heating temperature of 550 ℃.The structure and specific surface area of the obtained samples are found to be no significant change.However,as heating time was prolonged,the content of uncondensed amino groups at the terminals of the heptazine units gradually decreased,and their degree of polymerization showed a gradual increase.The band gaps of the obtained samples gradually decreased from 2.77 e V(heated for 1 h)to 2.68 e V(heated for 12 h),leading to a continuous red shift upon visible-light absorption edges as well as the improved utilization of longer wavelength lights.The fluorescence intensity also decreased,indicating that the separation efficiency of the photogenerated charge carriers in the samples have also been improved.On the othter hand,by prolonging the heating time,the photocatalytic activity of the obtained g-CN samples firstly showed an improvement and thengradually decreased,which indicates that the photocatalytic activity of g-CN is affected by both the catalytically active sites and the visible-light harvesting,and these two factors influence and restrict each other.