| In order to solve the growing problems of environmental pollution and energy poverty,photo-and electro-catalytic reactions have attracted much attention in recent years.Researchers can effectively use photo-and electro-catalytic reactions to utilize clean energy and then solve environmental problems.However,the slow kinetics limits its practical application.As promising catalytic material,carbon-nitrogen nanosheets possess the advantages of adjustable electronic structure and abundant active sites,which can reduce the energy barrier of catalytic reduction reaction.However,the present synthetic method of carbon-nitrogen nanosheets is complicated and the catalytic performance is difficult to meet the demand.Therefore,it is one of the challenging problems to construct carbon-nitrogen nanosheets for high activity.Moreover,catalytic reaction is an atomic-level reaction,and atomically defect regulation can change the atomic structure,improve the activity,and explore the mechanism of catalytic reaction,becoming a key issue in the field of catalysis.Focusing on this key scientific problem,this paper can take carbon-nitrogen nanosheets as the main research object,use atomically defect engineering to introduce defects such as nonmetallic heteroatoms and metal single atoms and construct heterogeneous structures for improving the band structure and electronic structure,optimizing the carrier transport rate and electron transfer efficiency,enhancing the material surface mass transfer and catalytic reactions,and then exploring the mechanism of improved catalytic reduction performance by microstructure effects.The main contents are as follows:（1）Preparation of oxygen atom-modified porous carbon-nitrogen nanosheets for photocatalytic hydrogen evolutionGraphite-phase carbon-nitrogen（g-C3N4）material shows great potential in photocatalytic hydrogen evolution.However,the traditional g-C3N4 material has many shortcomings such as poor carrier diffusion,small specific surface area and low charge separation efficiency.In this work,porous oxygen-modified g-C3N4 nanosheets（OCN-n,n represents the number of heat treatments）were obtained by multiple thermal oxidation treatments.During the thermal treatment process,oxygen atoms can bond with the surface of the material structure by chemical adsorption,resulting in a large number of structural defects,which is beneficial for the formation of holes and monolayer structures.In addition,oxygen atoms can be chemically doped into the structure to significantly reduce the band gap and regulate the band structure.OCN-3 formed by three times of heat treatment has the best performance under visible light with a hydrogen production rate of 3519.6μmol g-1 h-1.（2）Configuration design of defective carbon-nitrogen nanosheet/titanium dioxide heterogeneous core-shell structure for photocatalytic hydrogen evolutionIn order to reduce the generating carrier recombination rate of g-C3N4 material and improve the photocatalytic hydrogen production activity,this work used hard template sacrifice and self-assembly order regulation method to obtain carbon-nitrogen nanosheets/titanium dioxide heterogeneous core-shell structure with different configurations.The titanium dioxide nanocrystals are connected with the outer side of carbon-nitrogen nanosheets to form TiO2/g-C3N4 material,while the titanium dioxide nanocrystals are connected with the inner side of carbon-nitrogen nanosheets to form g-C3N4/TiO2 material.The configuration regulation of the material can affect optical property.At the heterogeneous interface of g-C3N4/TiO2,the formed Ti–C bond can improve the structural stability and carrier transport rate of material.The electrons transfer from O atoms to Ti atoms in TiO2 lattice to increase the electron cloud density of Ti atoms,and the electron cloud distribution in the interfacial layer is regulated by Ti–C bond to avoid the photoelectron-hole recombination of TiO2 and g-C3N4 materials,thus promoting the photocatalytic hydrogen evolution reaction.g-C3N4/TiO2 with the optimal configuration has the highest hydrogen production rate(5816.5μmol g-1 h-1)under visible light.（3）Preparation of metal single atoms-carbon-nitrogen nanosheets for electrocatalytic oxygen reduction performanceCarbon-nitrogen nanosheets not only have photocatalytic performance,but also have electrocatalytic performance.Due to the low intrinsic electrocatalytic activities of carbon-nitrogen nanosheets,metal single atoms can improve the oxygen reduction performance.However,it is difficult to ensure the activity and stability of metal single atom on two-dimensional carriers by traditional preparation methods.This work used lamellar micelles self-assembled by surfactants as two-dimensional soft templates,and let carboxyl groups of the soft templates to coordinate with metal ions to enhance metal-carrier interaction and anchor metal atoms after thermal treatment process.Based on the interlayer confinement strategy,a series of transition metal single atoms-defective carbon-nitrogen nanosheets（Fe-NSC,Co-NSC,Ni-NSC）were obtained.By changing the type and amount of metal sources,materials with different metal atoms,loads and atomic structures can be obtained.Compared with other active sites of single-atoms materials,Fe sites in Fe-NSC can more effectively change the adsorption/desorption ability of intermediates and reduce the reaction energy barrier for the formation of O–OH*rate-determination decision step.Thus,Fe-NSC has the best half-wave potential（0.83 V）and limit current density(-5.03 mA cm-2).After 40000 s cycle reaction,the current density of Fe-NSC electrode decreased by 19.1%,which is better than that of commercial Pt/C（32.4%）.The Zn-air fuel battery assembled with Fe-NSC as the positive electrode showed a high open circuit voltage（1.45 V）and specific capacity of 612.1 mAh g Zn-1at 20 mA cm-2.（4）Preparation of dual metal atoms-defective carbon-nitrogen nanosheets for electrocatalytic oxygen reduction performanceIn order to further improve the electrocatalytic activity and stability of metal atoms on carbon-nitrogen nanosheets,Fe-Co dual metal atoms/carbon-nitrogen nanosheet material（FeCo-NSC）was prepared.Fe and Co dual metal atoms were stabilized on the carbon nanosheet by coordinating with N and S heteroatoms to form the bridge bimetallic atom configuration with FeN4S1/Co N4S1.Due to the synergistic effect of Fe-Co dual metal atoms,FeCo-NSC material has a better half-wave potential（0.86 V）and limit current density(-5.26 mA cm-2).The coordination between heteroatoms and dual metal atoms can not only accelerate the electron transfer to Co atoms,thus to increase the local electron cloud density of metal atoms,but also improve the structural stability during the reaction process.After 40000 s cycle reaction,the current density of FeCo-NSC electrode decreased by only 11.2%.The Zn-air fuel battery assembled by FeCo-NSC can reach an open circuit potential of 1.51 V and a specific capacity of 782.1 mAh g Zn-1 at 20 mA cm-2.（5）Preparation of Co single atom-titanium dioxide/Co single atom-carbon-nitrogen nanosheets for electrocatalytic oxygen reduction performanceIn order to realize the transformation of oxygen reduction path,this work introduced cobalt single atom-titanium dioxide nanosheets(Co-Ti1–xO2)into Co-NSC materials and constructed Co-Ti1–xO2/Co-NSC heterostructure for realizing the material conversion from four electrons to two electrons electrocatalytic oxygen reduction and applying in electrocatalytic hydrogen peroxide production field.Due to the change of binding energy between the material and intermediate OOH*,Co-Ti1–xO2/Co-NSC can have 2e-ORR performance with a high initial-potential（0.76 V）and H2O2 selectivity（～93.2%）.The H-type reaction cell assembled by Co-Ti1–xO2/Co-NSC material can produce 0.225 mM concentration of H2O2 with yield of～2220m M g-1 h-1 in 60 min and degrade various dyes within 20 min. |
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