Theoretical Study On The Electronic Structures Of Water-confinement Systems

Water is one of the most important materials in all fields of scientific research,livehood and production, which is a main resource for all known lives surviving onEarth. It plays an important role in the evolution of life. The structural properties ofwater clusters and the change properties in the confinement environment are closelyrelated to some basic life process, which has long been a focus in physics, chemistry,life sciences, materials science and other fields. In this paper, by synthetically usingstructural ptimization and molecular dynamics simulations based on densityfunctional tight-binding method, the properties of water layer on low-dimensionalgraphene surface and between interlayer graphene were theoretically studied onatomic level. The work mainly includes the following five spoecific parts:Firstly, the inevitable defects of graphene could alter its properties and applications.However, in this section, based on calculations using the third generation of thequantum approximate density functional tight-binding methodology, we clearly showthat common Stone-Wales defects and divacancy defects do not affect hydrophilic andhydrophobic properties of graphene, meanwhile, an adsorbed water single layer doesnot noticeably affect the electronic properties of the defect-containing graphene.Specifically, the type of the defect, the ratio of defects sites to total area and theinduced deformation of graphene arise negligible alteration in the interactionstrength (less than0.1kcal/mol) between the water single layer and graphene.Thepresence of a water single layer causes also negligible changes in the energy gap andinduce a small charge transfer to the aqueous layer (less than0.1e per unit cell). Theresults indicate graphene has a good relative stability which make its electronicproperties mainly determined by its own structural characteristics and not considerably affected by the adsorbed water layer. Such conclusions are significantfor studies and applications of graphene in life sciences and material science wherehydrophilic and hydrophobic properties and electronic properties are important.Secondly, based on the third generation of the quantum approximate densityfunctional tight-binding methodology, we provide a detailed geometric and electronicstructure characterization of a nano-confined water film within two parallel graphenesheets. We find that, when the distance between the graphene layers is reduced to4.5, the O-H bonds of the water molecules become almost parallel to graphene.However, further reducing the distance to4.0induces an abnormal phenomenoncharacterized by several O-H bonds pointing to the graphene surface. Electronicstructure analyses revealed that the charge transfers of these nano-confined watermolecules are opposite in these two situations. In the former scenario, the electronloss of each water molecule in the confined aqueous monolayer is approximately0.008e, with electrons migrating to graphene from the p orbitals of water oxygenatoms; In the latter case, the electron transfer is reversed, with the water monolayergaining electrons from graphene in excess of0.017e per water molecule. Our currentstudy highlights the importance of the nano-confinement on the electronic structuresof interfacial water, which can be very sensitive to small changes in physicalconfinement such as a small reduction in the graphene interlayer distance, and mayhave implications in de novo design of graphene nano-channels with unique watertransport properties for nanofluidic applications.Thirdly, in order to further explore the reverse impact of interlayer water moleculeson the confinement, based on the dynamics simulation and optimization of the thirdgeneration of the quantum approximate density functional tight-binding methodology(DFTB3), we studied the influence of water layer on the conformations and electronicproperties of bilayer graphene in graphene-water-graphene system in this section. Theresults show that, under the strong confinement with an interlayer distance about4.0, the existence of water layer changes the stable structure of bilayer graphene fromAB stacking towards AA stacking. This reverse structural adjustment allows thereduction of repulsive energy in water layer and weakens electron cloud overlap in thesystem through interplay between orientations of water molecules and the confiningspace provided by the honeycomb graphene layers. Meanwhile, for bilayer graphenein different stacking orders, the interlayer water modulates their frontier orbitals andgaps differently and the systems show distinguishable electron distribution and charge transfer from water to graphene. These results are of significance in understanding theinterlayer water system and the design of relevant confinement devices.Fourthly, for further exploring the impact of confined water molecules on theinterlayer distance of graphene bilayer and the electrical properties of the systemunder non-neutral environment, we study the interaction of bilayer graphene with theinterlayer water in neutral and ionic environment at this section. The results show that,when the sandwich system carries-1e charge, the interlayer distance betweengraphene layers increases. The interaction between water and graphene weakens, andthe (Highest occupied molecular orbital) HOMO is from one of graphene bilayer, theorbital coupling which almost show no coupling between water molecules andgraphene and between graphene bilayer. While the sandwich system carries+1echarge, the interlayer distance of bilayer graphene layer decreases, the interactionbetween graphene bilayer as well as graphene and water molecules are enhanced, andboth two layers of graphene contributes to the HOMO orbital of system, there is evena certain degree of orbit coupling between graphene bilayer. And obviously differentto the interlayer water system, the interlayer distance of pure bilayer graphene in ionicenvironment is less than that in neutral environment, regardless of the positive ornegative electrons. This research is important for the understanding sandwich watersystems in ion environment, and hoping contribute to the design ofnano-electronic-control switching.Finally, to discuss the influence of confinement effects on the interlayer interactionsbetween graphene and some small molecules as well as possible applications, basedon the third generation of the quantum approximate density functional tight-bindingmethodology, we study the interaction of small molecule such H2O, CH3or NH4andgraphene with different size of periodic cell. The results show that, due to the differentmolecular structure and the van der Waals radius of H2O, NH3, CH4, which bringdifferent influences on the interlayer distance and deformation of graphene, as well asthe electrical properties of the system. Interestingly, the electronic balance of systemsobvious flips when inserting H2O or CH4molecule. the former H2O loses electronsabout order of0.001e, this is related to the parallel conformations of water moleculesand graphene bilayer, because the atomic radius of O is significantly larger than thatof H, the overlap of electron clouds between O and graphene, leads to the electronicmigration from the p orbitals of oxygen atoms to graphene. CH4molecule gains someelectrons, even up magnitude of0.01e, this is due to4-H atoms of CH4pointing toward the graphene bilayer in this case, the hydrogen atoms infiltrates the surface ofgraphene sheets, leading to1s orbital of H atom being disturbed by the delocalized πbonds of p electrons of C atoms in graphene, resulting in the direction of chargetransfer reverses from the above described situation. Our studies contribute to thebetter understanding of the sandwich confinement system, and may have implicationsfor the identification of small molecule and the design of gas sensor.