Müller Markus, Stankic Slavica, Diwald Oliver, Knözinger Erich, Sushko Peter V, Trevisanutto Paolo E, Shluger Alexander L
Institute of Materials Chemistry, Vienna University of Technology, Veterinärplatz 1/GA, A-1210 Vienna, Austria.
J Am Chem Soc. 2007 Oct 17;129(41):12491-6. doi: 10.1021/ja0736055. Epub 2007 Sep 25.
Site-specific functionalization of oxide nanostructures gives rise to novel optical and chemical surface properties. In addition, it can provide deeper insights into the electronic surface structure of the associated materials. We applied chemisorption of molecular hydrogen, induced by ultraviolet (UV) light, followed by vacuum annealing to MgO nanocubes to selectively decorate three-coordinated oxygen ions (oxygen corner sites, for simplicity) with protons. Fully dehydroxylated nanocubes exhibit 3.2 +/- 0.1 eV photoluminescence induced by 4.6 eV light, where both emission and absorption are associated with three-coordinated oxygen sites. We find that partially hydroxylated nanocubes show an additional photoluminescence feature at 2.9 +/- 0.1 eV. Interestingly, the excitation spectra of the 2.9 and 3.2 eV emission bands, associated with protonated and nonprotonated oxygen corner sites, respectively, nearly coincide and show well-pronounced maxima at 4.6 eV in spite of a significant difference in their local atomic and electronic structures. These observations are explained with the help of ab initio calculations, which reveal that (i) the absorption band at 4.6 eV involves four-coordinated O and Mg ions in the immediate vicinity of the corner sites and (ii) protonation of the three-coordinated oxygen ions eliminates the optical transitions associated with them and strongly red-shifts other optical transitions associated with neighboring atoms. These results demonstrate that the optical absorption bands assigned to topological surface defects are not simply determined by the ions of lowest coordination number but involve contributions due to the neighboring atoms of higher coordination. Thus, we suggest that the absorption band at 4.6 eV should not be regarded as merely a signature of the three-coordinated O2- ions but ought to be assigned to corners as multiatomic topological features. Our results also suggest that optical absorption signatures of protonated and nonprotonated sites of oxide surfaces can be remarkably similar.
氧化物纳米结构的位点特异性功能化产生了新颖的光学和化学表面性质。此外，它还能更深入地了解相关材料的电子表面结构。我们对MgO纳米立方体应用了紫外（UV）光诱导的分子氢化学吸附，随后进行真空退火，以用质子选择性地修饰三配位氧离子（为简单起见，即氧角位点）。完全脱羟基的纳米立方体在4.6 eV光激发下呈现出3.2±0.1 eV的光致发光，其发射和吸收均与三配位氧位点相关。我们发现部分羟基化的纳米立方体在2.9±0.1 eV处呈现出额外的光致发光特征。有趣的是，分别与质子化和非质子化氧角位点相关的2.9 eV和3.2 eV发射带的激发光谱几乎重合，并且尽管它们的局部原子和电子结构存在显著差异，但在4.6 eV处都显示出明显的最大值。借助从头算计算对这些观察结果进行了解释，计算结果表明：（i）4.6 eV处的吸收带涉及角位点紧邻处的四配位O和Mg离子；（ii）三配位氧离子的质子化消除了与之相关的光学跃迁，并使与相邻原子相关的其他光学跃迁发生强烈红移。这些结果表明，归因于拓扑表面缺陷的光吸收带并非简单地由最低配位数的离子决定，而是涉及高配位数相邻原子的贡献。因此，我们认为4.6 eV处的吸收带不应仅仅被视为三配位O2-离子的特征，而应归因于作为多原子拓扑特征的角位点。我们的结果还表明，氧化物表面质子化和非质子化位点的光吸收特征可能非常相似。
