Sedona Francesco, Rizzi Gian Andrea, Agnoli Stefano, Llabrés i Xamena Francesc X, Papageorgiou Anthoula, Ostermann Dieter, Sambi Mauro, Finetti Paola, Schierbaum Klaus, Granozzi Gaetano
Dipartimento di Scienze Chimiche and Unità di Ricerca INFM-CNR, Università di Padova, Via Marzolo, I-35131 Padova, Italy.
J Phys Chem B. 2005 Dec 29;109(51):24411-26. doi: 10.1021/jp0543173.
Ultrathin ordered titanium oxide films on Pt(111) surface are prepared by reactive evaporation of Ti in oxygen. By varying the Ti dose and the annealing conditions (i.e., temperature and oxygen pressure), six different long-range ordered phases are obtained. They are characterized by means of low-energy electron diffraction (LEED), X-ray photoemission spectroscopy (XPS), and scanning tunneling microscopy (STM). By careful optimization of the preparative parameters, we find conditions where predominantly single phases of TiO(x), revealing distinct LEED pattern and STM images, are produced. XPS binding energy and photoelectron diffraction (XPD) data indicate that all the phases, except one (the stoichiometric rect-TiO2), are one monolayer thick and composed of a Ti-O bilayer with interfacial Ti. Atomically resolved STM images confirm that these TiO(x) phases wet the Pt surface, in contrast to rect-TiO2. This indicates their interface stabilization. At a low Ti dose (0.4 monolayer equivalents, MLE), an incommensurate kagomé-like low-density phase (k-TiO(x) phase) is observed where hexagons are sharing their vertexes. At a higher Ti dose (0.8 MLE), two denser phases are found, both characterized by a zigzag motif (z- and z'-TiO(x) phases), but with distinct rectangular unit cells. Among them, z'-TiO(x), which is obtained by annealing in ultrahigh vacuum (UHV), shows a larger unit cell. When the postannealing of the 0.8 MLE deposit is carried out at high temperatures and high oxygen partial pressures, the incommensurate nonwetting, fully oxidized rect-TiO2 is found The symmetry and lattice dimensions are almost identical with rect-VO2, observed in the system VO(x)/Pd(111). At a higher coverage (1.2 MLE), two commensurate hexagonal phases are formed, namely the w- [(square root(43) x square root(43)) R 7.6 degrees] and w'-TiO(x) phase [(7 x 7) R 21.8 degrees]. They show wagon-wheel-like structures and have slightly different lattice dimensions. Larger Ti deposits produce TiO2 nanoclusters on top of the different monolayer films, as supported both by XPS and STM data. Besides the formation of TiO(x) surfaces phases, wormlike features are found on the bare parts of the substrate by STM. We suggest that these structures, probably multilayer disordered TiO2, represent growth precursors of the ordered phases. Our results on the different nanostructures are compared with literature data on similar systems, e.g., VO(x)/Pd(111), VO(x)/Rh(111), TiO(x)/Pd(111), TiO(x)/Pt(111), and TiO(x)/Ru(0001). Similar and distinct features are observed in the TiO(x)/Pt(111) case, which may be related to the different chemical natures of the overlayer and of the substrate.
通过在氧气中对钛进行反应蒸发，在Pt(111)表面制备了超薄有序二氧化钛薄膜。通过改变钛剂量和退火条件（即温度和氧气压力），获得了六种不同的长程有序相。利用低能电子衍射（LEED）、X射线光电子能谱（XPS）和扫描隧道显微镜（STM）对它们进行了表征。通过仔细优化制备参数，我们找到了主要生成具有独特LEED图案和STM图像的TiO(x)单相的条件。XPS结合能和光电子衍射（XPD）数据表明，除了一种（化学计量比的rect-TiO2）之外，所有相都是单层厚的，由具有界面钛的Ti-O双层组成。与rect-TiO2相比，原子分辨的STM图像证实这些TiO(x)相浸润Pt表面。这表明了它们的界面稳定性。在低钛剂量（0.4单层当量，MLE）下，观察到一种非公度的类似 kagomé 的低密度相（k-TiO(x)相），其中六边形共享它们的顶点。在较高的钛剂量（0.8 MLE）下，发现了两种密度更高的相，两者都以锯齿形图案为特征（z-和z'-TiO(x)相），但具有不同的矩形晶胞。其中，通过在超高真空（UHV）中退火获得的z'-TiO(x)显示出更大的晶胞。当对0.8 MLE沉积物进行高温和高氧分压的后退火时，发现了非公度的不浸润、完全氧化的rect-TiO2。其对称性和晶格尺寸与在VO(x)/Pd(111)系统中观察到的rect-VO2几乎相同。在较高的覆盖率（1.2 MLE）下，形成了两种共格的六方相，即w-[(√43×√43)R 7.6°]和w'-TiO(x)相[(7×7)R 21.8°]。它们显示出类似车轮的结构，并且具有略有不同的晶格尺寸。XPS和STM数据均表明，更大的钛沉积物在不同的单层膜顶部产生TiO2纳米团簇。除了形成TiO(x)表面相之外，通过STM在衬底的裸露部分发现了蠕虫状特征。我们认为这些结构可能是多层无序的TiO2，代表有序相的生长前体。我们将关于不同纳米结构的结果与类似系统的文献数据进行了比较，例如VO(x)/Pd(111)、VO(x)/Rh(111)、TiO(x)/Pd(111)、TiO(x)/Pt(111)和TiO(x)/Ru(0001)。在TiO(x)/Pt(111)的情况下观察到了相似和不同的特征，这可能与覆盖层和衬底的不同化学性质有关。
