Preparation And Visible Light H₂ Evolution Performance Of Carbon Nitride-carbon Nanofiber Composites

Polymeric graphitic phase carbon nitride（g-C₃N₄）as a non-metallic semiconductor material has attracted much attention because of its non-toxicity,stability and suitable electronic energy band structure.However,the disadvantages of bulk g-C₃N₄,such as severe aggregation,weak visible light absorption,low crystallinity and poor separation efficiency of photogenerated electron-hole（e^--h⁺）pairs,restrict its photocatalytic H₂ evolution rate（HER）.To address the above disadvantages of g-C₃N₄ photocatalysts,firstly,multi-channel hollow carbon nanofibers（MHCNF）with excellent electrical conductivity and large specific surface area were successfully prepared.Subsequently,1D/2D MHCNF/g-C₃N₄ was prepared by in situ condensation using urea and MHCNF as the precursors,and the effect of MHCNF loading in the 1D/2D MHCNF/g-C₃N₄composites on the HER of g-C₃N₄was investigated.On this basis,crystallized carbon nitride（CCN）materials were further prepared by molten salt method（Li Cl,KCl）.CCN reduced the numbers of internal defects in g-C₃N₄ and promoted the separation and transfer of photogenerated e^--h⁺pairs.Finally,1D/1D CNF/CCN composite photocatalyst was prepared through electrostatic self-assembly combined with microwave hydrothermal method by CCN and carbon nanofibers（CNF）as precursors.The detailed work is as follows.1.Multi-channel hollow carbon nanofibers（MHCNF）were prepared by electrostatic spinning,pre-oxidation,carbonization and acidification with polyacrylonitrile（PAN）as the main raw material and polystyrene（PS）spheres as the guest raw material,respectively.MHCNF possess 1D carbon nanostructure making them have unique 1D electron transport paths and high conductivity properties,which accelerate the separation efficiency of photogenerated e^--h⁺of g-C₃N₄.In this work,the MHCNF/g-C₃N₄ composite photocatalysts were subsequently prepared by in situ thermal polycondensation methed by using the mixure of urea and MHCNF as precursors.The detailed characterization revealed that the introduction of MHCNF significantly improved the absorption intensity of visible light and the photogenerated charge carriers transfer and separation rate of g-C₃N₄.This is attributed to the fact that MHCNF is a light absorber with excellent conductivity.In addition,g-C₃N₄ has a 2D porous structure,which can shorten the distance of electron transfer from the bulk phase to the surface reaction sites.The HER of MHCNF/g-C₃N₄ sample is 2495.0μmol g^-1 h^-1 at the optimal MHCNF loading under 420 nm LED light.The mechanism of enhanced photocatalytic activity of MHCNF/g-C₃N₄ was proposed.2.On the basis of the above work,the effect of crystallinity of g-C₃N₄ on photocatalytic activity of g-C₃N₄ was further investigated.High crystallinity of g-C₃N₄ reduces the number of recombination centers of its charge carriers.In this work,CCN with 1D nanorod structure was prepared by molten salt method,and the HER of CCN was 6.1 times higher than that of g-C₃N₄.That was ascribed to the fact that the molten salt modified g-C₃N₄（CCN）had internal orderly arrangement structure,resulting in the decreased number of photogenerated carrier recombination center.However,a single photocatalyst has its own limitations.Therefore,the CNF/CCN composite photocatalysts were prepared by electrostatic self-assembly induced microwave hydrothermal method using CCN and CNF as raw materials.The experiment results showed that the introduction of CNF effectively increased reaction sites,the visible light absorption intensity,and the e^--h⁺transfer and separation rates of CCN.The HER of CNF/CCN samples with optimal CNF loading under 420 nm LED irradiation is 6398μmol g^-1 h^-1,which is 2.6 and 16.0 times higher than that of CCN and g-C₃N₄,respectively.