N_2H_4在NiFe(111)合金表面吸附稳定性和电子结构的第一性原理研究

First-principles study of stability and electronic structure of N_2H_4 adsorption on NiFe(111) alloy surface

采用基于色散校正的密度泛函理论进行了第一性原理研究,详细分析了肼(N_2H_4)在Ni_8Fe_8/Ni(111)合金表面稳定吸附构型的吸附稳定性和电子结构及成键性质.通过比较发现,肼分子以桥接方式吸附在表面的两个Fe原子上是最稳定的吸附构型,其吸附能为-1.578 eV/N_2H_4.同时发现,肼分子在这一表面上吸附稳定性的趋势为:桥位比顶位吸附更有利,且在Fe原子上比在Ni原子上的吸附作用更强.进一步分析了不同吸附位点上稳定吸附构型的电子结构、电荷密度转移以及电子局域化情况.结果发现:相同吸附位点的电子态密度图基本一致,并且N原子的p轨道和与之相互作用的表面原子的d轨道之间存在态密度上的重叠;吸附后电荷密度则主要从肼分子转移到表面原子之上;在电子局域化函数切面图中也发现吸附后电子被局域到肼分子的N原子和相邻的表面原子之间.这些电子结构的表征都充分说明肼分子与表面原子之间通过电荷转移形成了强烈的配位共价作用.

We use the density functional theory（DFT） with dispersion correction to investigate the stability and electronic structure of hydrazine（N_2H_4） adsorpted on Ni_8Fe_8/Ni（111） alloy surface.The geometries and adsorption characteristics of the structure on the Ni_8Fe_8 alloy surface are presented.Results show that N_2H_4 bridging between two iron atoms gives the strongest adsorption with an adsorption energy of —1.578 eV/N_2H_4.Top modes turn out to be the local minima with adsorption energies of —1.346 eV/N_2H_4（for the top site on a Fe atom） and —1.061 eV/N_2H_4（for the top site on a Ni atom）.It is demonstrated that the bridging mode is more favorable than the top mode on the NiFe alloy surface with a coverage of 1/16 ML,and Fe atom can provide stronger adsorption site than Ni atom.The van der Waals contribution is significant with a value of about 0.4 eV/N_2H_4.Meanwhile,the van der Waals contribution is larger for adsorption on Fe atom than on Ni atom,and for adsorption of the bridging mode than of the top mode.We also find that the structure of N_2H_4 in the anti molecule,rather than the gauche molecule,is bound on the top site of Fe atom on the NiFe alloy surface with a coverage of 1/16 ML,which demonstrates that the repulsive adsorbate-adsorbate interaction is weak on the surface with low coverage.The strong interaction between the surface atom and the adsorbate contributes to the result that the lone pair electrons of N_2H_4 in gauche conformer are attracted by the Fe atom.In addition,for the five adsorption structures of N_2H_4 on Ni8Fes/Ni（lll） alloy surface,we analyze the projected electronic density of states（DOS）,induced charge density and electron localisation function（ELF） slices through the Fe-N or Ni-N bonds of the adsorbed molecule on the alloy surface.It shows that the electronic DOS presents the mixture between HOMO of N_2H_4and the d orbital of the surface atom,which corresponds to charge transfer between the substrate and the adsorbate.The charges are transferred mainly from N_2H_4 to the surface atoms,and the extents of charge transfer are different for the bridging mode and the top one which is present in the induced charge density.Furthermore,the region of localisation in the ELF slices can be found for the adsorptions between the N atom of N_2H_4 and the Fe or Ni atom of surface,which gives a clear view of the coordination bonds for the interactions of N—Fe or N—Ni.