Nitrogen Based Materials:Preparation,Characterization And Their Energy Conversion And Storage Properties

Energy shortages and environmental pollution are among the most important issues that constrain the development of human society in the future.The exploration and development of new clean renewable energy is the key to solving future energy and environmental problems.As an inexhaustible new energy source,solar energy is one of the most promising new clean energy sources to solve future energy shortages and environmental pollution problems.Photocatalytic technology can convert solar energy into chemical energy to realize solar energy conversion and storage.It is one of the most potential technical means to effectively use solar energy to solve energy and environmental problems.At present,the efficiency of photocatalysis is still low,which has become the most important problem that restricts its practical application.Therefore,exploring new efficient photocatalytic materials and further improving the energy conversion efficiency is of great significance for promoting the progress of photocatalytic technology and solving future energy and environmental problems.Among many photocatalytic materials,nitrogen-based semiconductor materials are a kind of photocatalytic materials with great development potential because of their good visible light absorption,suitable band structure,high stability and good semiconductor performance.However,due to the limitation of the preparation technology,the composition,defects,and morphology of nitrogen-based semiconductor materials are not well regulated,and these parameters are important for light absorption and carrier separation of nitrogen-based semiconductor materials.Therefore,this paper selects several potential nitrogen-based materials as research objects,such as: TaON,Ta₃N₅,Nb₄N₅,g-C₃N₄,etc..By studying the growth process and preparation technology of nitrogen-based materials,the precise regulation of the composition,morphology and defects of nitrogen-based materials can be realized,thereby further expanding the light absorption and promoting the separation of photo-generated carriers.Our work provides new methods and theoretical basis for the designand preparation of new high-efficiency nitrogen-based photocatalytic materials.Combining with the characteristics of the material itself and photoelectrochemistry,supercapacitors,etc.,some research works on photoelectrochemical water splitting and nitrogen-based supercapacitors were carried out.In Chapter 1,the basic methods and principles of solar energy conversion and storage are firstly introduced.Then,the research status of common material systems,as well as their basic advantages and disadvantages are summarized,and the nitrides are discussed in detail.Finally,the basic methods for solving the disadvantages of current materials were briefly introduced,and then we put up the signi ficance and main research contents of this thesis.In Chapter 2,a new CCl₄ assisted nitridation method was developed for solving the difficulty of preparing oriented nitride materials through preparation method.[100]-oriented TaON films were fabricated by using LiTaO₃ single crystal as the substrates and [153]-orientd Ta₃N₅ films were also prepared by using KTaO₃ as the substrates.This method can prepare oriented free-standing films by endotaxial growth with the aid of CCl₄,which solve the problem that the conventional high temperature nitridation method cannot prepare the highly oriented tantalum-base nitrides.The film prepared by this method has a unique rod-like structure perpendicular to the surface of the film.Based on the crystal structure analysis between the single crystal substrate and TaON product and the corresponding experimental results,the basic growth mechanism of theoriented TaON free-standing film is proposed.In detail,firstly,the precursor LiTaO₃ and the product TaON have similar atomic arrangement in a specific orientation,which greatly reduces the stress during the conversion process.In the preparation process,by adding CCl₄,the impurity element Li in the LiTaO₃ precursor can be evaporated in the formation of LiCl gas under high temperature condition,and some tantalum atoms are removed in the form of chlorides,which leaves enough space for subsequent nitridation.When the reaction is completed,the solidification of LiCl causes the corresponding stress released at the interface,resulting in the complete peeling of TaON film.Bystudying the growth mechanism of oriented TaON thin films prepared by CCl₄ assisted nitridation method,it provides a basic theoretical basis for the further preparation of other materials by this method.Based on the results,it is expected to use this method to synthesize and prepare other materials with unique structures and performance.In Chapter 3,the performance of the oriented TaON and its modification were studied.Firstly,we studied the photoelectrochemical water splitting properties of the high oriented TaON.The results show that the high oriented crystal structure of TaON significantly improves its photoelctrochemical water splitting performance,which is about twice that of non-oriented TaON.In addition,we found that the presence of nitrogen vacancy defects inhibits the photoelectrochemical performance of TaON.By annealing under NH₃ atmosphere,the nitrogen vacancies were successfully removed without destroying the oriented structure.Then the photoelectrochemical performance of the TaON film was greatly enhanced,its photocurrent can be increased by 3.7 times to 2.84 mA cm^-2.In order to solve the problems that TaON is easily oxidized to form corresponding oxides?TaO_x?during the photoelectrochemical test,we carried out researches on the modification of the TaON with cocatalyst.Firstly,we studied the role of Co-Pi in the oriented TaON photoelectrode.The results show that although Co-Pi cocatalyst can effectively improve the photocurrent density of TaON photoelectrode,its uneven distribution makes it still cannot solve the stability problem of TaON electrode.Aiming at this problem,we constructed a new cobalt-based cocatalyst Co₄N by molten salt method.The uniform load of Co4 N cocatalyst around TaON particles can be achieved by this method.The photoelectrochemical performance test shows that the uniformly covered Co₄N cocatalyst not only can effectively increase the performance of the TaON photoelectrode but also can effectively suppress the oxidative corrosion of the TaON electrode.This work firstly proved the excellent photoelectrochemical properties of oriented TaON,and then verified the effective application of the new cocatalyst system Co₄N in TaON material system.In Chapter 4,in view of the problem that the low light utilization and the highcarrier recombination efficiency of carbon nitride,we first carried out the element doping research,using supramolecular self-assembly method with chitosan and melamine as the precursor,we constructed a supramolecular structure,and the carbonself-doped carbon nitride material was successfully constructed with subsequent calcination process.Theoretical and experimental studies show that carbon elementssuccessfully enter the crystal lattice of carbon nitride by replacing bridging nitrogen atoms,which promotes the formation of big ? bonds,so that photogenerated electrons can efficiently transfer between adjacent heptazines.Then the separation efficiency of photogenerated carriers and light utilization efficiency are improved.This work proves that the element doping of carbon nitride can be achieved by supramolecular self-assembly method,which provides a new idea and way for the preparation of high-performance,low-cost carbon nitride with element doping.In addition,we systematically studied the nitrogen vacancy defects of carbon nitride.The carbon nitride materials with different nitrogen vacancies?ring nitrogen vacancies and pendent nitrogen vacancies?were prepared under different atmospheres?H₂ and CCl₄ atmospheres?.Based on experimental and theoretical calculation studies,both nitrogen vacancies can introduce defect levels into the band gaps,and the single occupied defect level can serve as electron trapping site to capture photogenerated electron and then as the hydrogen generation sites.In addition,the nitrogen vacancies can provide additional transfer channels for electrons,thereby increasing the separation efficiency of photogenerated carriers.Photocatalytic performance studies show that carbon nitrides with pendent nitrogen vacancies show higher photocatalytic activity dueto the higher electron localization compared to ring nitrogen vacancies.This work provides detailed theoretical and experimental evidence for understanding the photocatalytic mechanism of carbon nitride with nitrogen vacancy defects,which is helpful for the synthesis of highly efficient carbon nitride materials.Moreover,we constructed carbon nitride material?GCN-THP?with a unique 1D hollow prism structure under CCl₄ atmosphere,and then studied the effect ofmicrostructure on carbon nitride performance.The growth mechanism of carbon nitridewith hollow prism was constructed by studying the related morphologies and surfacestates during different preparation processes.The method realizes a simple,low-cost and safe preparation process for fabricating 1D hollow carbon nitride structure by using low-polymerization melamine as the template.Studies show that the unique 1D hollow nanostructure can effectively promote the light scattering and absorption as well as thecarrier separation efficiency,thus effectively improve the photocatalytic water splitting performance of carbon nitride materials.This work provides a new method forconstructing carbon nitride materials with unique nanostructure,which is expected to be extended to other material systems.In Chapter 5,the problems that conventional carbon nitride is usually in powder form and has poor recyclability are considered,thus we developed a method to construct g-C₃N₄ continuous films on a variety of substrates by using CCl₄.Studies on the thin films prepared on different substrates show that the basic principle of the method is that under the action of CCl₄,the metal ions in the substrates can be extracted in situ to the surface of the substrates,then with the reaction between electron-rich nitrogen atoms in g-C₃N₄ and the metal cations,metal cations can be embedded into the molecular structure of g-C₃N₄.Finally,these metal ions act as anchor points to continuously grow g-C₃N₄ molecules into thin films.The film prepared by this method is a continuous film?non-particle interaction film?,and thus can exist independently from the substrate.Moreover,the film's physical and chemical properties can be easily adjusted by controlling the preparation conditions.Performance studies show that the film prepared by this method not only has good photocatalytic performance,but also has good cycle stability and recyclability.The method has the advantages of simple operation,no special process,low cost and expandability,and provides a possibility for the reuse of g-C₃N₄ and the application in the field of thin films,and the method is expected to be extended to the construction of other thin film systems.In Chapter 6,we studied nitride supercapacitor materials for electrical storageproblems after solar energy is converted into electrical energy.The Nb₄N₅ material was successfully prepared by the CCl₄ assisted nitridation method using LiNbO₃ singlecrystal as the precursor.It is found by XPS that the niobium atoms have different valence states at the interior and the out surface of the sample,where the interior niobium atoms have lower valence states while the niobium atoms on the out surface have higher surface valence states.In addition,crystal structure studies find that Nb₄N₅ has alternating Nb atomic layer and the N atomic layer along the [211] direction.The unique structural arrangement and chemical composition result in the electrons inside the electrode rapidly transporting to the electrode surface when Nb₄N₅ is used as an electrode for energy storage,thereby promoting the adsorption of protons on the surface.Therefore,although the prepared sample has a small specific surface area,itssupercapacitor test shows that it still has excellent supercapacitor performance,where its power and energy density can reach 12.5 mW cm^-2 and 24.6 ?Wh cm^-2,respectively,and the cyclic test shows that there was no significant performance degradation after2000 cycles.This work proves that the nitride capacitor electrode material with excellent performance can be constructed by CCl₄ assisted nitridation method.Based on the crystal structure,the influence of crystal structure and composition on the material properties is studied.It is expected to provide new ideas and methods for developing excellent non-oxide capacitor electrode material in the future.In short,the structure and properties of materials have close relationship.In this study,different structures are prepared by different preparation methods and the relations between them are systematically studied.The studies on the relationships between material structure and properties and the growth mechanism of the construction method is of great value and significance for the future construction of efficient solar energy conversion and storage materials.