Controllable Preparation And Properties Of Carbon Nitride And Its Composites

In recent years,carbon nitride has drawn increasing interesting as a photocatalyst for CO₂ reduction,water splitting,environmental remediation,etc.due to its high visible-light-response,remarkable chemical,physical and thermal stability.However,it still suffers some drawbacks,such as rapid recombination of photogenerated carriers,low specific surface area,low recovery properties and inefficient utilization of light energy.Therefore,this work adopts the plasma technology with fast synthesis speed,low energy consumption,green environmental protection and controllability to overcome the problems of carbon nitride.The green controllable synthesis and surface modification of carbon nitride are realized by bombarding,heating and etching carbon nitride precursor and its products with high-energy particles.Through the construction of heterojunction and theoretical calculation,the preparation and modification mechanism of carbon nitride by plasma technology are systematically discussed,and a controllable plasma preparation and modification process is established to obtain high-performance carbon nitride,which paves the way for its application in the field of photocatalysis.Based on this,the main contents and results of this study are as follows:1.The nitrogen vacancy carbon nitride sphere（NVCNS）was synthesized by using H₂ plasma treatment to tune nitrogen vacancy（NV）.The as-synthesized NVCNS exhibits an efficient and stability photocatalytic H₂O₂ yield of 4413.1μmol g_cat^-1h^-1,which is 2.5 and 4.6 times higher than that of carbon nitride sphere(CNS,1766.4μmol g_cat^-1h^-1)and bulk CN(BCN,956.6μmol g_cat^-1h^-1)under Xe lamp with an intensity of100 m Wcm^-2,respectively.Particularly,the charges recombination rate is remarkably reduced by introducing N defect state and thus promotes the electron accumulation and O₂ adsorption through theoretical calculation and experiments.Furthermore,the NV creates abundant unsaturated sites and induces strong interlayer interactions,leading to effective electronic excitation and promotion of charge transport.2.Carbon nitride with nitrogen vacancy is a robust photocatalyst with proven enhancing H₂O₂ production ability.However,nitrogen vacancy control is extremely challenging with the majority of reports representing it as a few vacancies.Herein,the amorphous carbon nitride sphere（ACN）with two N_2C-site vacancies in one heptazine ring unit is prepared by a one-step H₂ plasma approach.First-principles calculations and experimental results provide consistent evidence that two N_2C vacancies are located in one heptazine ring unit structure after amorphous transformation.Plasma-induced ACN is stable with a hierarchical continuous nanosheet network structure and exhibits an ultrahigh specific surface area of～405.76 m²g^-1,which is 83 times higher than that of pristine carbon nitride(4.89 m²g^-1)and significantly enhanced photocatalytic H₂O₂production,yielding 1874μmolg^-1h^-1.Besides,the existence potential drop of 2.61 e V for the electrostatic potential in ACN is key to charge carrier separation.Moreover,the amorphous transformation leads to a new strong band tail,which remarkably enhances the absorbance edge of ACN up to 593 nm,resulting in a wider range of visible-light absorption to enhance H₂O₂ production.The results have provided an effective approach for promoting the practical application of ACN in photocatalytic H₂O₂production.3.We directly transformed graphitic carbon nitride to heavily K-doped nanocrystalline carbon nitride via an alkali molten salt post-treatment.The etched and intercalated carbon nitride structure functionalized by selected alkali metal atoms can effectively counteract the“self-purification to obtain heavily K-doped nanocrystalline carbon nitride with occupancy rates of up to 80%where the three K ions in a triangular cave strongly rivet interaction between layers and stabilize its nanostructures.The as-synthesized heavily K-doped nanocrystalline carbon nitride exhibits extraordinary long-lived photoelectrons up toμs,a superior electron mobility and an extended light absorption with nearly 100%photon utilization for oxygen reduction reaction（ORR）to H₂O₂ at 420 nm.This work has paved a way for developing heavily doped polymer nanocrystalline by using a facile strategy which can possibly extended to a wider range of photoreduction applications.4.We synthesized a graphite carbon nitride/Mn₂O₃（CN/Mn₂O₃）p-n heterojunction grown by a plasma-assisted liquid-based method using a special precursor complex of melamine-cyanuric acid supramolecular on fluorine-doped tin oxide（FTO）substrate.The as-grown CN/Mn₂O₃/FTO p-n heterojunction exhibits a significantly enhanced photoelectrochemical performance with a photocurrent density of 25μA/cm² compared with its powder form（0.9μA/cm²）formed by common drop coating method,and a tunable valence band（VB）with excellent mechanical strength which is benefit for photocatalytic oxidation reaction and recovery.The formation of CN/Mn₂O₃/FTO p-n heterojunction can promote the transfer of photoelectrons due to an internal electric field at the interface of p-n junction,leading to the accumulation of electrons and holes in the VB of CN and conduction band（CB）of Mn₂O₃.The results have shed a light on promoting the practical application of carbon nitride,and the developed plasma-assisted liquid-based growth method can be extended to the preparation of other type of functional heterojunctions.