Preparation And Photocatalytic Properties Of Carbon Nitride-based Photocatalysts

Semiconductor photo catalytic technology,as one of the most attractive technologies,has been used in the direct collection,conversion and storage of renewable solar energy to produce sustainable green solar fuels,environmental restoration,and medical antibacterial applications.Under the condition that a photon is injected into a semiconductor photocatalyst,the electrons obtained in the valence band are excited to transition to the conduction band,and the holes generated in the valence band participate in the photocatalytic redox reaction.The properties of highly efficient photocatalysts play a crucial role in determining the overall quantum efficiency of the photocatalytic reaction system.Graphene-like carbon nitride(g-C3N4)is a new type of non-metal photocatalyst containing only carbon and nitrogen.It has an excellent layered porous structure,cheap and readily available raw materials,simple preparation methods,and unique physical,chemical,optical,and electrical properties,which have attracted many researchers to develop and design a variety of g-C3N4-based photocatalysts.However,the thermal polymerization method,which is one of the most important ways to prepare g-C3N4,will inevitably cause certain crystal and intra-layer defects and disordered structures in the synthesis process of g-C3N4.The quantum efficiency is low and the recombination rate of photo-generated carriers is high.At the same time,as a photocatalyst for visible light catalysis,the band gap energy of g-C3N4 is still high(2.7 eV).These factors have severely restricted its application in visible light catalysis.Therefore,in order to address the defects of the g-C3N4 material proposed above,a single or a combination of optimization strategies such as size control,carbon doping and ion intercalation are used to study the photo catalyst's ability to degrade dyes and decompose water to produce hydrogen under visible light.Meanwhile,a diversity of characterization methods are used to explore the possible mechanism of photocatalytic performance changes before and after modification.The main research contents and research results are as follows:Nano-structured carbon nitride improves the visible light catalytic degradation of g-C3N4.Morphological control is one of the direct methods to optimize the photo catalytic activity of g-C3N4.The ultra-thin carbon nitride sheet layer prepared by thermally peeling off the bulk g-C3N4 significantly reduces the accumulation of visible light,and greatly enhances the absorption of visible light.The band gap can be reduced from the original 2.75 eV to 2.63 eV.Fluorescence test results indicate that the electron-hole recombination rate is reduced,the lifetime of photo-generated carriers is prolonged,and the degradation rate of Rhodamine B can reach to 91.2%in 90 min.This study provides a nano structured carbon nitride matrix for subsequent modification.Biochar modification provides a good carbon/g-C3N4 contact interface for g-C3N4-carbon-based composites.Biochar(BC)is prepared from rice straw that is abundant in rural areas.BC can not only provide an electronic carrier for g-C3N4-based photocatalytic materials,but the modification of BC can also provide new ideas for realizing the sustainable resource utilization of agricultural waste.Significantly,the introduction of an appropriate amount of BC can maximize the photocatalytic activity of the BC-g-C3N4 composite photocatalyst.The optimal ratio of biochar and melamine is 1:15.The g-C3N4-carbon-based composite prepared at this ratio retains the g-C3N4 polymer backbone,and BC is bound to g-C3N4 through ?-? stacking interaction and C-O-C bond bridges,and the ? provided by biochar conjugate electron systems can improve the charge separation efficiency.The enhanced photocatalytic performance of the composite is attributed to the introduction of carbon to increase the material's absorption of visible light and increased specific surface area(54.09 m2 g-1).At the same time,the excellent contact interface and synergy between BC and g-C3N4 extend the lifetime of the excited electrons of the semiconductor and reduce the light-induced electron-hole recombination rate during photocatalysis.Ion intercalation optimizes the performance of photoinduced hydrogen evolution for water production by g-C3N4.Creating photo-generated carrier channels between g-C3N4 layers is an effective strategy to improve the photocatalytic performance of g-C3N4.The carbon nitride intercalation compound(DCN)synthesized through a simple molten salt route is an efficient polymer photocatalyst with high quantum yield.It indicates that the alkali metal is coordinated into the C-N plane nitrogen tank of carbon nitride,and the chloride ion bridges the g-C3N4 layer,which promotes the effective separation of photo-generated carriers and reduces the rate of electron-hole recombination and formed a new photo-generated carrier channel.As a result,after 1 h of light reaction,the hydrogen production efficiency of DCN was 0.5 mmol h-1 g-1,which reached 10 times the original hydrogen production rate of g-C3N4.This study will provides new opportunities for using cheap semiconducting polymers as energy transducers.