Jensen Thomas N, Rasmussen Morten K, Knudsen Jan, Vlad Alina, Volkov Sergey, Lundgren Edvin, Stierle Andreas, Lauritsen Jeppe V
Interdisciplinary Nanoscience Center (iNANO), Aarhus University, Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark.
Phys Chem Chem Phys. 2015 Feb 28;17(8):5795-804. doi: 10.1039/c4cp05878j.
The correlation between surface structure, stoichiometry and atomic occupancy of the polar MgAl2O4(100) surface has been studied with an interplay of noncontact atomic force microscopy, X-ray photoelectron spectroscopy and surface X-ray diffraction under ultrahigh vacuum conditions. The Al/Mg ratio is found to significantly increase as the surface is sputtered and annealed in oxygen at intermediate temperatures ranging from 1073-1273 K. The Al excess is explained by the observed surface structure, where the formation of nanometer-sized pits and elongated patches with Al terminated step edges contribute to stabilizing the structure by compensating surface polarity. Surface X-ray diffraction reveals a reduced occupancy in the top two surface layers for both Mg, Al, and O and, moreover, vacancies are preferably located in octahedral sites, indicating that Al and Mg ions interchange sites. The excess of Al and high concentration of octahedral vacancies, very interestingly, indicates that the top few surface layers of the MgAl2O4(100) adopts a surface structure similar to that of a spinel-like transition Al2O3 film. However, after annealing at a high temperature of 1473 K, the Al/Mg ratio restores to its initial value, the occupancy of all elements increases, and the surface transforms into a well-defined structure with large flat terraces and straight step edges, indicating a restoration of the surface stoichiometry. It is proposed that the tetrahedral vacancies at these high temperatures are filled by Mg from the bulk, due to the increased mobility at high annealing temperatures.
在超高真空条件下，通过非接触原子力显微镜、X射线光电子能谱和表面X射线衍射的相互作用，研究了极性MgAl2O4(100)表面的表面结构、化学计量和原子占有率之间的相关性。发现在1073 - 1273K的中间温度下对表面进行溅射并在氧气中退火时，Al/Mg比会显著增加。观察到的表面结构解释了Al的过量现象，其中纳米尺寸的凹坑和具有Al终止台阶边缘的细长斑块的形成通过补偿表面极性有助于稳定结构。表面X射线衍射显示Mg、Al和O在最上面两层的占有率降低，而且空位优选位于八面体位置，这表明Al和Mg离子互换了位置。非常有趣的是，Al的过量和八面体空位的高浓度表明MgAl2O4(100)的最上面几层采用了类似于尖晶石状过渡Al2O3薄膜的表面结构。然而，在1473K的高温下退火后，Al/Mg比恢复到其初始值，所有元素的占有率增加，并且表面转变为具有大的平坦平台和直的台阶边缘的明确结构，表明表面化学计量恢复。有人提出，在这些高温下，由于高退火温度下迁移率的增加，四面体空位由体相中的Mg填充。
