Construction Of Carbon Nitride Nanostructures And Their Photocatalytic Properties

Photocatalysis,which can use renewable solar energy to catalyze multiple redox reactions,is an environmental-friendly and sustainable method to resolve or mitigate the growing environment and energy problems.Among numerous photocatalysts,graphitic carbon nitride（g-C₃N₄）has received substantial attention due to its moderate bandgap,suitable band potential,high physical and chemical stability,and ease of production.However,bulk g-C₃N₄suffers from low specific surface area and high density of defects in the lattice structure,which would result in limited catalytically active sites and fast recombination of photo induced charge carriers.Thus,it requires further research to realize the practical application of g-C₃N_4.In this thesis,photocatalysts with high specific surface area and low defects density,and hydrogen evolution co-catalysts with fine dispersity,small size were obtained through modification and construction of proper co-catalyst based on g-C₃N₄ polymeric nanomaterials.The photocatalytic activity of g-C₃N₄ was modified via three pointcuts.The main achievement in the project is as follows:A porous g-C₃N₄nanosheet was prepared by a two-step pre-polymerization process.XRD and XPS analysis proved that the pre-polymerization treatment towards the precursor would not change the lattice structure and chemical state of g-C₃N₄.The specific surface area of porous g-C₃N₄ increased from 5.8 m²/g to 71.72 m²/g,which provided more active sites for photocatalytic reactions.The porous characteristic and thickness of nanosheets could be tuned easily by varying the pre-treatment temperature.Photocatalytic hydrogen evolution and degradation tests suggested that g-C₃N₄ with precursor pre-heated at 350°C displayed the largest specific surface area and optimum photocatalytic activity.An oxygen doped g-C₃N₄ with high crystallinity was synthesized by a hydrothermally assisted method.The highly ordered architecture of g-C₃N₄ was testified by XRD,FTIR,EPR and SAED.TG analysis demonstrated that this high crystallinity was originated from the structural water in the precursor,which was introduced in the pre-hydrothermal treatment.Due to the suppressed defects density,the recombination of photo generated electrons and holes was largely quenched and the lifetime of charge carriers was also greatly prolonged.Therefore,the photocatalytic hydrogen production capability and photon efficiency were dramatically improved.The superior photocatalytic activity was contributed by its high crystallinity,large specific surface area and oxygen doping effect.This work also provided new thoughts for production of other polymer based photocatalysts with high activity.A two-step refluxing-calcination method was used to deposit Pt O on the surface of g-C₃N₄ as hydrogen evolution co-catalyst to replace metallic Pt,which is usually adopted in such reaction.XPS characterization indicated that the valence state of Pt species on g-C₃N₄ was+2.Pt O nanodots with reduced size（～2 nm）and high dispersity was observed by TEM.Most importantly,when using metallic Pt as hydrogen evolution co-catalyst,it could also catalyze the hydrogen oxidation reaction,the back reaction of hydrogen evolution,which means the produced di-hydrogen would be transformed back to protons.Whereas,the hydrogen oxidation reaction could be totally restrained when Pt O was employed to replace metallic Pt.Hence,on account of its reduced size,enhanced dispersity,Pt O could guarantee a satisfactory performance for photocatalytic water splitting even under a low loading amount.In addition to its ability to suppress the hydrogen back reaction,Pt O nanodots could be a promising candidate as a hydrogen evolution co-catalyst.