The Theoretical Study On The Electronic Structures And Interaction Of Keggin-type Polyoxometalate/Two-dimensional Materials Nanocomposites

Polyoxometalates(POMs)comprise a large class of nanoscale and molecular well-defined transition-metal oxide clusters and have applications in catalysis,photochemistry,electrochemistry,single-molecule magnets,microelectronics,and energy storage.The synthetic chemistry of POMs has experienced rapid development,leading to the explosion of new POMs.Given that many functional POMs are available,their assembly chemistry requires more urgent development because it determines the progress of POM-based materials and devices directly.In general,the structures of POMs are are sensitive to pH,which limit their applications.A effective solution strategy is used to modify and assemble POMs with suitable matrices such as organic molecules,polymers,silica,and carbon materials to achieve their enhanced stability and the synergistic functionalities of POMs and matrices.Recently,carbon materials have been proved as suitable matrices for the assembly of POMs owing to their excellent chemical stability and strong affinity for POMs.Graphene,as a new carbon nanomaterial composed of a monolayer of two-dimensional carbon atoms,has attracted considerable attention in recent years owing to its various extraordinary properties such as high mechanical stiffness,large specific surface area,high thermal conductivity,and excellent carrier mobility.Moreover,graphene is also a stable matrix material for the assembly of additional components such as small molecules,polymers,and inorganic nanoparticles for the construction of functional nanocomposites.Some of the important properties of POMs,such as catalysis,capacitance,and lithium-storage ability,can be enhanced by the large specific area and good conductivity of graphene.Up to now,several works regarding POM/graphene nanocomposites have been reported.Although the studies on the properties of POMs/graphene(carbon nanotubes)nanocomposites are relatively mature,the theoretical researches on this aspect are still relatively weak.Based on the theoretical simulations,it is possible to effectively predict and analyze the related properties of the material,which plays an important role in understanding the experimental phenomena and results,and can be guidence for the synthesis of new materials in experiments.In the present thesis,we carried out systematic theoretical studies on the interaction between Keggin-type POM and several nonmetallic two-dimensional(2D)materials for exploring their potential application.The main purpose of this thesis is:(1)design new nanocomposites based on POM and 2D materials;(2)analyze the effect of the applied electric field on the electronic structure of the POM/2D composite,and predict the application in molecular electronics;(3)explore the influencing factors of the interaction between POM and2 D materials,especially the effects of different central heteroatoms.The first part of this paper mainly introduces the preparation and application ofPOM/carbon material nanocomposites,and summarizes the research on POM/carbon material nanocomposites in recent years.The second part gives a brief introduction to the first principle method and the density functional theory(DFT)method.The rest is the main part of the article.1.The redox and acidic properties of ?-Keggin anions [PW11O39ME]4-(M = Nb,Ta;E = O,S,Se)have been investigated by using density functional theory(DFT)method.The results confirm that the substitution of one O atom by S and Se atom in POMs enhances the redox properties and acidity of POMs.2.The electronic structures,magnetization,and transport properties of pristine zigzag graphene nanoribbons(ZGNRs)and ZGNR with different conductive metal adatoms(Al,Cu,Ag and Au)are investigated using DFT+NEGF calculations.The results show that the metal adatoms have crucial effects on the devices' transport properties and exhibit different effects on 6ZGNR-based and 7ZGNR-based devices.The presence of any metal adatoms lead to an increase in the current to different extent along the bias window compared to the pristine6 ZGNR.and a decrease in the case of 7ZGNR-based devices.Furthermore,6ZGNR-Al conducts best among these 6ZGNR-metal systems,which is agreement with that of atomic wires of Ag,Al,Au,and Cu.3.The study on the geometric and electronic properties for the adsorption of POM on three 2D materials-graphene,silicon carbide(Si C)and hexagonal boron nitride(h-BN)were peroformed by using DFT calculations.The results show that POM prefers to adsorb on SiC with chemisorption,while the adsorption on graphene and BN are between weak chemisorption and strong physisorption.The adsorption of POM changes the electronic properties of complexes.After the PMo12 adsorption,the band gaps of SiC-PMo12 and BN-PMo12 are significantly reduced compared to SiC and BN.On the other hand,the band structures of 2D–PMo12 complexes can be easily altered by external electric field.The band gaps of 2D–PMo12 decrease under the negative electric fields and increase under the positive electric fields with the increasing of with the external electric field.4.The interaction between Keggin-type POM with different central heteroatoms Nan[XMo12O40]n-(X = B,Al,Ga,Si and P)and borophene is theoretically investigated by the first-principles principle.The effects of different heteroatoms on the interaction and electronic structures are analyzed.By analyzing the adsorption energy,the electronic structure and the charge transfer properties,it is found that the interaction between POM and borophene is enhanced when the central heteroatom varies from top to bottom within the same group(B-Al-Ga),while the interaction gradually decreases when heteroatom changes from left to right along the same period(Al-Si-P).