The Effect Of Doping On The Adsorption Properties Of MOFs And The Electronic Structures Of Graphene:A First-principles Study

Metal-organic frameworks?MOFs?are polyporous crystalline materials with a periodically topological structure,which consists of metal cations or clusters connecting with organic linkers by self-assembly,showing ordered three-dimensional networks with permanent and size-controlled nano-porosity.Due to its extremely high surface area and controllable pore size,MOFs have a wide application in gas adsorption and separation,hydrogen storage,catalysis,sensors,as well as drug delivery,and so on.Recently,much effort has been devoted to manipulate the adsorption properties of MOFs by doping or functionization at the metallic or ligand sites,which can improve the capability for gas adsorption due to doping induced charge redistribution of MOFs.Besides,it is an important and an alternative approach to introduce new functional groups into MOFs by doping or functionization.Graphene,a new two-dimensional carbon-based nanomaterial,becomes a rapidly rising star on the horizon of condensed-matter physics and material science due to its unique physical and chemical properties.In particular,in the electrochemical energy storage field,graphene has showed a much more remarkably electrochemical properties than other materials,therefore,it is highly imperative to systematically study the electronic properties of electrochemically doped graphene.This can not only help to understand electrochemical energy storage mechanism of graphene,but also can help to further optimize and improve its electrochemical performance for energy storage.Therefore,we have performed a systematically first-principle calculation based on the density functional theory?DFT?in this paper,in order to explore the influence of doping on the adsorption capability toward CO₂ of MOF-74 and the effect of electrochemical doping on the atomic structure,lattice vibration,and electronic structure of monolayer graphene.Firstly,by introducing the alkaline-earth metal?Mg?dopant at the metal sites of Co-MOF-74,the influence of Mg dopant on its electronic structure,adsorption capability toward gaseous CO₂ molecule has been extensively investiaged,and possible activation mechanism for adsorbed CO₂ molecule of Mg-Co-MOF-74 was also discussed.In addition,we have also studied the change of C-C bond length,electronic structure,charge transfer,vibration frequency and bandgap of monolayer graphene upon electrochemical doping with different doping types?electron or hole doping,in another word n-or p-doping?and levels.Specifically,the main results obtained in this work are briefly summarized as below:?1?The adsorption capability toward CO₂ molecule of Co-MOF-74 with one to six Mg dopants?Co-Mg-MOF-74?were studied by using the first-principles calculation.By comparing adsorption energy with the weak interaction,we have found that Co-Mg-MOF-74 shows a strong adsorption for CO₂ molecule,indicating that doping or functionalization at the metal sites of MOF-74 can effectively improve its adsorption ability for gasous molecules.Besides,the vdW force is found to be the dominant interaction force.Among all considered configurations,Mg-Co-MOF-74 with one or two Mg atoms possess an extremely large interaction with CO₂.The adsorption capability toward CO₂ of other configurations showsa tendency as: 6Mg > 5Mg > 4Mg > 3Mg > 6Co,which is in good agreement with previous experimental results reported by Yang Jiao in 2015.This implies that our simulation method and results are quite convincing.Bader charge analysis?BCA?and electron density difference?EDD?calculations are further performed,and we have found that the strong adsorption capability toward CO₂ of Mg-Co-MOF-74 mainly comes from the strong interaction between Mg atom and O1 atom near Mg doping-site,due to the positive and the negative potential of Mg cation and O1 atom,respectively.We have also systematically investigated the catalytic behavior of Co-Mg-MOF-74 toward CO₂ activation.After fully relaxation,the molecular structure CO₂ shows an obviously bent configuration with an O-C-O bond angle less than 180°?ranging within 177.53° 177.79°?and the asymmetric C=O bond?longer bond length for the C=O1 bond near the doping site?,differing from the linear and symmetric configuration of an isolated CO₂ molecule due to the strong interaction between Mg dopant and O1 atom.Based on these calculations,we have affirmatively found that alkalini-earth doping or modification can effectively improve the adsorption strength of MOFs toward gaseous molecules.These results may further providea solid theoretical foundation to improve the gas adsorption capability of MOFs by doping or modification in the future.?2?The atomic and electronic structures of electrochemical doped graphene are investigated by DFT calculation.Two doping types?n-doping and p-doping?with different doping levels are considered in this work by changing the total number of electronin the primitive cell of graphene.For example,n/p-doped graphene with ten different doping levels is simulated by increasing/decreasing the total number of electron.The effect of n/p-doping on the C-C bond-length,Fermi energy shift,and lattice vibration frequency of graphene was explored.In the case of n-doped graphene,we have found that the C-C bond-length increases monotonically,the Fermi energy shifts upward to high energy,while the vibration frequency decreases with increasing the doping concentration.As for p-doping,the situation is little bit more complex.For example,both the C-C bond-length and the vibration frequency decrease firstly and then increases with increasing the doping concentration.The reason for this can be ascribed to the re-balance between the Coulomb repulsive and the attractive interactions by changing the number of electrons.While the Fermi energy shifts continuously downward to low energy by increasing the doping concentration.Considering that n-doped and p-doped graphenes are normally achieved in experiment by alkali?Li and/or K?,alkali-earth atoms?Ca?and FeCl₃,respectively.Therefore,we further investigate the electronic structure of Li-,K-,Ca-,and FeCl₃-doped graphene.The monolayer graphene containing six and eight carbon atoms was constructed to simulate the Li-,Ca-doped?LiC₆,CaC₆?and K-doped?KC₈?graphene,respectively.Similar to previous reports,the most stable site for the alkali and alkali-earth atoms on the graphene surface is found to be the hollow site of six-membered ring of graphene after fully relaxation.Charge population analysis indicates the trend of charge transfer of Li-,K-,Ca-doped graphene is C₈ K < C₆ Ca < C₆ Li with electrons transferred from dopants to graphene.Moreover,it has been found that,due to the breaking of the sub-lattice symmetry,the systems of C₈ K,C₆Ca,and C₆ Li open a small band gap at the Dirac point and change the metallic graphene into a semiconductor.In the case of FeCl₃-doping,in order to minimize the lattice mismatch,a supercell with the 5×5 periods of graphene primitive cell and 2×2 periods of the layered FeCl₃ primitive cell was constructed to simulate the complex system of FeCl₃-doped graphene,leading a small lattice mismatch about 0.7%.Electronic structure calculation shows that the FeCl₃-doped grapheneis metallic without opening an energy gap at the Dirac point due to the graphene symmetry remaining unchanged in this case.This is totally different from the case of the FeCl₃ intercalated bilayer graphene.