Preparation And Photocatalytic Hydrogen Evolution Of Modified Graphitic Carbon Nitride

Converting solar energy to hydrogen energy through semiconductor photocatalysis is considered to be a significant method to alleviate energy crisis and environmental pollution.Graphitic carbon nitride photocatalysts as one typical kind of conjugated polymer has been widely studied in water splitting by virtue of its two-dimensionalπ-conjugated structure,suitable energy band,visible light absorption and easy preparation.However,graphitic carbon nitride obtained by calcination of N-rich precursors suffered from the limited visible light absorption,the fast recombination rate of photo-generated charges,low electronic conductivity and low specific surface area.Specially,the rapid photo-generated charges recombination resulted in sluggish reaction rate and inferior photocatalytic quantum efficiency.Therefore,element doping,construction of micro-nano structure and heterojunction were used to improve the photocatalytic H₂ evolution of graphitic carbon nitride in this thesis.To deep insight into the separation and transfer of photocharges,the intrinsic structure,microstructure and energy band gap were investigated in details.The oxygen and phenyl co-doped graphitic carbon nitride（DA-C₃N₄-OP）nanoshteets were facilely synthesized via one-step copolymerization.The incorporated phenyl as terminal groups and oxygen as a substitution for nitrogen in heptazine were verified by the correosponding characterizations.And the thickness of DA-C₃N₄-OP nanoshteets was estimated to be around 4.0 nm by AFM images.As a result,DA-C₃N₄-OP nanoshteets exhibited the highest visible-light-driven H₂generation rate of 7.394 mmol g^-1 h^-1,which was 7.67 times higher than that of pristine C₃N₄.The enhancement of photocatalytic activity for DA-C₃N₄-OP nanoshteets resulted from the below two reason:（1）The construction of nanosheets structure shortened the photoexcited charges transport length to the surface and inhibited the recombination of photocharges;（2）The donor-acceptor（D-A）system was introduced into DA-C₃N₄-OP nanosheets by oxygen and phenyl co-doping strategy to build the molecular dipole-induced internal electric field and facilitate the transport of photoinduced charge carriers.This work not only provides a simple strategy to construct efficient C₃N₄ nanosheet photocatalyst with D-A system,but also promote the deep insight into the effect of molecular dipole originated from D-A system on the transport of photoinduced charges and photocatalytic activity for graphitic carbon nitride material.The phenyl-bridged graphitic carbon nitride（Ph-CN-MCA）with a hollow sphere structure composed of porous nanosheets was successfully synthesized by calcination of a trimesic acid-doped melamine-cyanuric acid（MCA）supramolecular.According to characterizations,phenyl was confirmed to be incorporated into the structure of Ph-CN-MCA by substituting the bridged N atoms.The porous and hollow sphere structure was revealed by scanning electron microscopy（SEM）and transmission electron microscopy（TEM）images,which augmented the light absorption intensity and promote the mass transfer and diffusion.Additionally,the increased degree of polymerization in Ph-CN-MCA promoted the reduction capacity for photocatalytic H₂ evolution.As confirmed by UV-vis absorption spectra（DRS）,the extended visible light absorption of Ph-CN-MCA was due to the the incorporated phenyl.For Ph-CN-MCA,the enhanced electronic conductivity,improved separation and transfer of photoexcited charges were attributed to the comprehensive influences of the structural and electronic properties as demonstrated by steady-state PL spectra,time-resolved PL spectra and photo-electrochemical measurements.Further,the separated HOMO and LUMO of Ph-CN-MCA were verified by theoretical calculations,which facilitated the inherent dissociation of photogenerated charges.Hence,compared with the photoactivity of reference CN-MA catalyst prepared from melamine,the photocatalytic H₂ evolution of Ph-CN-MCA displayed approximately 48.42 times higher.Covalent bonding oxygen-doped carbon nitride/graphitic carbon nitride（O-CN/CN）Z-scheme heterojunctions have been synthesized via calcination and solvothermal treatment.Based on XRD,FT-IR,XPS,solid-state ¹³C NMR spectra and organic element analysis,O-CN was grown in-situ on the surface of CN by forming new C–N bond at O-CN/CN heterojunction interface.The DRS spectra revealed that the introduction of O-CN to O-CN/CN hybrids extended the visible light absorption.TR-PL spectra and photo-electrochemical measurements indicated that covalent bonding heterojunction reduced the transmission resistance of photogenerated charges,promoted the photocharges transfer and the lifetime of active charge carriers.According to photocatalytic mechanism,the O-CN/CN heterojunction was classified as a typical direct Z-scheme heterojunction.In this case,the separation and transfer of photogenerated charges were obviously improved,and the strong driving force for H₂ evolution was retained.As a result,the most efficient O-CN/CN-3 photocatalyst displayed the 12.4-fold promotion of photocatalytic hydrogen performance with respect to the pristine counterpart CN.The present results developed new insights into covalent bonding direct Z-scheme heterojunction strategies for CN to enhance photocatalytic H₂ evolution.