Investigation Of The Electronic And Hydrogen Storage Properties For Metal Decorated SiC Nanotubes

SiC nanotubes were firstly synthesized by shape memory method in 2001 and attracted a lot of attention subsequently.Studies have indicated that the Si-C bond of nanotubes is polar bond,which can enhance the bonding ability of nanotubes with metal atoms or gas molecules.In this paper,using first-principles within the density-functional theory,we have systematically investigated the structure,electronic properties and hydrogen storage performances of the systems of alkali metal(AMs:Li,Na,K),alkali earth metal(AEMs:Be,Mg,Ca)and transition metal atoms(TMs:Be,Mg,Ca)decorated SiC nanotubes.The main conclusions as follows:(1)We systematically studied the structure and electronic properties of the systems of alkali metal and alkaline earth metal atoms decorated SiC nanotubes(M@SiCNT).The results show that all metal atoms can be stably adsorbed on the SiC nanotubes surface except Mg atoms.When Li,Na and K atoms decorated the center of hexagonal ring(H),the Be atom decorated the top of zigzag Si-C bond(Z),and the Ca decorated the top of silicon atom(TSi),those decorated systems are the most stable.And,the binding energies of Li,Na,K and Ca on the surface of nanotube are all larger than the cohesive energies of corresponding bulk metals.It is found that the energy barrier of Li,Na,K and Ca diffusing from a stable site to the adjacent stable site is large enough.Therefore,the Li,Na,K and Ca atoms are not easy to form clusters on the surface of SiC nanotubes.The electron results show that metal atoms bind to SiC nanotubes through orbital hybridization mechanism,and the charge transfer of the metal atoms to the nanotube substrates results in a local electric field between the metal atoms and the SiC nanotubes,which is conducive to the adsorption of H2 molecules;the alkali metal atoms decorated systems have gold property,the Be atom decorated system is direct semiconductor,and the Ca decorated system is indirect semiconductor.(2)We systematically studied the structure and electronic properties of the systems of transition metal atoms decorated Si vacancy defects(VSi and C vacancy defects(VC)of SiC nanotubes.The results show that the existence of defects improves the binding energy of metal atoms with nanotubes,and the binding energies of the systems of transition metal atoms decorated Si vacancy defects(X@VSi are larger than the cohesive energy of the corresponding metal atom bluk systems,which prohibits the formation of metal clusters.The electron results show that the transition metal atoms bind to substrates through orbital hybridization mechanism;Due to charge transfer,a local electric field is formed between the metal atoms and the defect SiC nanotubes,which is conducive to gas adsorption;the adsorption of metal atoms enhances the conductivity of Vsi and Vc defective structures,but the decorated systems are still semiconductors;The magnetic moment of all decorated systems are in the range of 1?3/?B,and the magnetic moment is mainly derived from the d electron of the metal atom.(3)We systematically studied the hydrogen storage properties of the systems of metal atoms decorated pristine SiC nanotubes(M@SiCNT,M=Li,Na,K,Ca)and metal atoms decorated Si vacancy defects SiC nanotubes(X@VSi,X=Sc,Ti,V).It is shown that the hydrogen storage capacity of SiC nanotubes is improved by the modification of metal atoms.The hydrogen storage capacity of high metal atomic coverage system Li4@SiCNT,Na4@SiCNT and Ca4@SiCNT is 4.56 wt%,6.19 wt%and 4.79 wt%,respectively,and the adsorption energies of H2 molecules are all within the ideal adsorption energy range.Further studies show that hydrogen can be stored in Li,Na and Ca decorated SiCNT substrates under normal temperature and mild pressure.Therefore,the Li,Na and Ca decorated SiCNT systems are expected to be used as hydrogen storage materials.The adsorption energies of H2 molecules on the K decorated SiCNT substrates are less than 0.20 eV,thus the adsorbed H2 molecules will be spontaneously dissociate at room temperature.The adsorption energies of H2 molecule on X@VSi are in the range of 0.202?0.534 eV,but each metal atom can only adsorb 3 H2 molecules,so the hydrogen storage capacity of X@VSi systems is low.Based on the difference charge density curve and the projected state density(PDOS),it is found that the adsorption mechanism of H2 molecules on M@SiCNT substrate is polarization mechanism and weak orbital hybridization mechanism,while the adsorption mechanism of H2 molecules on X@VSi substrate is mainly through strong orbital hybridization mechanism.