水在金属镁表面吸附的第一原理研究

First-principles Study on Water Adsorps on Mg Surface

采用第一性原理赝势平面波方法,通过Materials Studio软件中的castep模块,研究了水分子在Mg(0001)表面吸附行为,揭示了镁合金钝化机理。研究了水分子在Mg(0001)表面顶位T1、桥位B2、穴位H3、穴位F4吸附情况,重点研究水分子置于T1位置的3种情况,即:水分子平行镁表面T1p,水分子垂直表面T1v,水分子所在平面与镁表面有一定倾斜T1i。计算给出了吸附能、电荷密度、差分密度、水的键长键角,分析了结构的稳定性和原子间的成键情况。结果表明:水分子在Mg(0001)表面吸附时候,氧原子与表面成键,且在顶位吸附且吸附构型是倾斜时最稳定。吸附能为-0.429 3 eV(=-41.21 kJ/mol),并且吸附能大于40 kJ/mol,吸附为化学吸附。吸附过程中表面镁原子的电子向水分子转移,使镁表面电位向正方向移动,会使镁产生钝化。吸附使水分子键长增长,键角增大,即水中各原子间作用减弱,可与Mg形成水合分子或MgO、或Mg(OH)2,从而发生钝化。

Using first-principles pseudopotential plane-wave method and through CASTEP Module in the Materials Studio software,authors study adsorption behavior of water molecules on Mg（0001） surface,and reveal the passivation mechanism of magnesium alloy.We study the water molecules adsorbs on Mg（0001） surface on the top site T1,bridge site B2,the hcp hollow site H3,and face-center site F4,focusing on three conditions of the location of water molecules adsorbs on T1,namely： water molecules parallel to the surface of magnesium T1p,water molecules perpendicular to the surface of T1v,and the water molecules have an angle to the surface of magnesium.On our work,we calculated adsorption energy,charge density,differential charge density,bond length and bond angle of water,and analyzed the structural stability and bonding between atoms.The results showed that： the water molecules are most stable when they are adsorbed on Mg（0001） surface,oxygen atom is bonding with the surface,and adsorpted on the top site and with inclined molecular plane.Adsorption energy is-0.429 3 eV（=-41.21 kJ/mol）, and it is greater than 40 kJ/mol,the adsorption is chemical adsorption.During the adsorption,the electronic of Mg atoms on surface transfers to the water molecules and makes the Mg surface positively charged and increase the surface potential and lead Mg passivation.The bond length and bond angle increases during adsorption,that is,the interaction between the atoms of the water weakened,which promotes the passivation of Mg surface and the formation of MgO or Mg（OH）2.