Controllable Fabrication Of Morphological Graphitic Carbon Nitride And Its Visible-Light Photocatalytic Performance

Graphitic carbon nitride(g-C3N4)has been widely used in the fields of visible light photocatalytic water splitting for hydrogen production,carbon dioxide photoreduction and pollutant elimination due to its excellent stability and suitable bandgap structure.Bulk g-C3N4 suffers from the disadvantages of low surface area,fast charge recombination rate and limited photoabsorption,which greatly restrict its visible photocatalytic activity.Electronic and structural modification are effective methods to optimize the visible photocatalytic performance of g-C3N4.A new solvent free and universal method for the preparation of morphology controlled g-C3N4 is constructed in this paper.Great photocatalytic performance proforms in photocatalytic water splitting for hydrogen production,carbon dioxide photoreduction and RhB photodegradation.The main contents are shown as follows:Morphology control is an effective way to improve the specific surface area of g-C3N4 and promote mass transfer and photoinduced carrier separation.In view of the problems of the traditional hard templating method in the preparation of morphological g-C3N4(complex operation,long period,expensive precursor,etc.),the gas solid phase transformation process of melamine was proposed,and a solvent-free and universal method was constructed.The diversity of g-C3N4 morphology can be regulated precisely through this novel method,specifically,by calcinating the mixtures of melamine and the porous silica templates.During melamine polymerization,silica can not only play the role of templates,producing three dimensional mesoporous system,but also promote the polymerization of melamine in the channel,inhibiting the self polymerization process into the bulk morphology,increasing the specific surface area,restricting the photogenerated carrier recombination and enhancing the photoabsorption,which leads to the highest activity of the visible photocatalytic RhB degradation reaction by nearly 24 times,and the first-order kinetic constant of 0.119 min-1.Through this new method,ordered mesoporous,urchin-like,hollow and tubular g-C3N4 were prepared.Based on the shape selective effect of the MCM-41 templates,a selective deposition-transition strategy was constructed,and the single and double shelled hollow g-C3N4 was designed and synthesized.The light absorption enhancement in the photocatalytic reaction was realized by the multi reflection effect inside the cavity.In the case of the traditional hard templating method for hollow or multi-shelled semiconductor materials,the morphology control depends on the channel filling.On the contrast,the shape selective deposition transition strategy depends on the deposition and polymerization of melamine on the silica surface.Under the presence of MCM-41,melamine initially migrates into the channel of MCM-41.With the temperature increasing,melamine gradually polymerizes into oligomers when the temperature is higher than 450 ?.The shape selective effect of the product of MCM-41 inhibits the polymerization of carbon and nitrogen species inside the channel.The carbon and nitrogen species migrate outside of the "narrow" channel,and deposite and transite on the relative opened-up outer surface,leading to the formation of the shell structure.As the increased specific surface area,the suppressed photoinduced carrier recombination and enhanced photoabsorption are performed,and the photoactivity in the RhB photodegradation reaction of single and double shelled g-C3N4 increase by 2.6 times and 15.6 times,respectively,with the first-order reaction kinetics constant reaching 0.013 min-1 and 0.078 min-1.More shell layers are beneficial to the light absorption.g-C3N4 nanosheets are prepared by the NH4NO3-assisted one-step thermal polymerization of melamine,where the use of environmental pollutants such as HF or NH4HF2 for desilication are avoided.g-C3N4 nanosheets can be prepared on a large scale by polymerizing the mixtures of ammonium nitrate and melamine.The decomposed small molecule gases play the role of template agent and increase the specific surface area of g-C3N4.Therefore,the activity of photocatalytic RhB degradation increases by 4.5 times,and the first order reaction kinetics constant reaches 0.167 min-1.Compared with the traditional exfoliation and templating methods,this new method is suitable for the preparation of g-C3N4 nanosheets,which has no pollution and simple operation.At the same time,a new universal method for the preparation of g-C3N4 nanosheets without additives is constructed.By optimizing the concentration of the precursor in the semi-closed polymerization system,the nucleation-growth process is controlled.With the amount of the precursor reducing,the morphology of g-C3N4 evaluted from bulk or stacking structure to the nanosheet structure,and the specific surface area is significantly increased.Common precursors are suitable for this new method,including melamine,dicyandiamide,urea,thiourea and trithiocyanuric acid.With trithiocyanuric acid as the precursor,the morphology and electronic structure can be simultaneously modified,and the nitrogen vacancy-rich g-C3N4 nanosheets were obtained.Excellent catalytic activity in the visible light catalytic CO2 reduction reaction is performed,and the formation rate of CO reaches 258.7 ?molg-1h-1,which is 21.7 times higher than the bulk one.The enhanced visible light catalytic performance is not only attributed to the increased specific surface area and the inhibited recombination of the photogenerated carrier,but also arise from tail band caused by the nitrogen vacancy.The existence of the tail band promotes the photoabsorption and the activation of CO2.