Blume Raoul, Niehus Horst, Conrad Horst, Böttcher Artur, Aballe Lucia, Gregoratti Luca, Barinov Alexei, Kiskinova Maya
Institut für Physik der Humboldt-Universität, Newtonstrasse 15, 12489 Berlin, Germany.
J Phys Chem B. 2005 Jul 28;109(29):14052-8. doi: 10.1021/jp044175x.
The oxidation states formed during low-temperature oxidation (T < 500 K) of a Ru(0001) surface are identified with photoelectron spectromicroscopy and thermal desorption (TD) spectroscopy. Adsorption and consecutive incorporation of oxygen are studied following the distinct chemical shifts of the Ru 3d(5/2) core levels of the two topmost Ru layers. The evolution of the Ru 3d(5/2) spectra with oxygen exposure at 475 K and the corresponding O2 desorption spectra reveal that about 2 ML of oxygen incorporate into the subsurface region, residing between the first and second Ru layer. Our results suggest that the subsurface oxygen binds to the first and second layer Ru atoms, yielding a metastable surface "oxide", which represents the oxidation state of an atomically well ordered Ru(0001) surface under low-temperature oxidation conditions. Accumulation of more than 3 ML of oxygen is possible via defect-promoted penetration below the second layer when the initial Ru(0001) surface is disordered. Despite its higher capacity for oxygen accumulation, also the disordered Ru surface does not show features characteristic for the crystalline RuO2 islands. Development of lateral heterogeneity in the oxygen concentration is evidenced by the Ru 3d(5/2) images and microspot spectra after the onset of oxygen incorporation, which becomes very pronounced when the oxidation is carried out at T > 550 K. This is attributed to facilitated O incorporation and oxide nucleation in microregions with a high density of defects.
利用光电子能谱显微镜和热脱附（TD）光谱法确定了Ru(0001)表面在低温氧化（T < 500 K）过程中形成的氧化态。通过研究最顶层两个Ru层的Ru 3d(5/2)芯能级的明显化学位移，研究了氧的吸附和连续掺入情况。Ru 3d(5/2)光谱随475 K下氧暴露量的变化以及相应的O2脱附光谱表明，约2 ML的氧掺入到次表面区域，位于第一层和第二层Ru层之间。我们的结果表明，次表面氧与第一层和第二层Ru原子结合，产生一种亚稳表面“氧化物”，它代表了在低温氧化条件下原子有序的Ru(0001)表面的氧化态。当初始Ru(0001)表面无序时，通过缺陷促进氧穿透到第二层以下，有可能积累超过3 ML的氧。尽管无序Ru表面具有更高的氧积累能力，但它也没有显示出结晶RuO2岛的特征。氧掺入开始后，Ru 3d(5/2)图像和微区光谱证明了氧浓度横向不均匀性的发展，当在T > 550 K下进行氧化时，这种不均匀性变得非常明显。这归因于在具有高密度缺陷的微区中氧掺入和氧化物成核更容易。
