ARTICLE pubs.acs.org/JPCC
Growth of Ordered Ultrathin Tungsten Oxide Films on Pt(111) Zhenjun Li,† Zhenrong Zhang,§,† Yu Kwon Kim,||,† R. Scott Smith,† Falko Netzer,‡ Bruce D. Kay,*,† Roger Rousseau,†,* and Zdenek Dohnalek†,* †
Chemical and Materials Sciences Division, Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, PO Box 999, Mail Stop K8-88, Richland, Washington 99352, United States ‡ Surface and Interface Physics, Institute of Physics, Karl-Franzens University A-8010 GRAZ, Austria
bS Supporting Information ABSTRACT: Ordered ultrathin tungsten oxide films were prepared on a Pt(111) substrate at 700 K via direct sublimation of monodispersed cyclic (WO3)3 trimers. The surface composition, structure, and morphology were determined using a combination of atomically resolved imaging, ensemble-averaged surface-sensitive spectroscopies, and DFT. We find that half of the (WO3)3 tungsten atoms in the first layer get partially reduced to the (5þ) oxidation state. The opening of the (WO3)3 ring leads to the formation of a tungsten oxide layer with a zigzag chain structure with a c(4 2) periodicity. In the second layer, the (WO3)3 clusters remain intact and form an ordered (3 3) array of molecularly bound (WO3)3. DFT calculations provide a detailed understanding of the structure, oxidation states, and the vibrational frequencies for both the c(4 2) and (3 3) overlayers.
1. INTRODUCTION Oxides have attracted great attention in recent years owing to their widespread application in heterogeneous catalysis and photocatalysis, electronic devices, gas sensors, and corrosion protection. For these reasons, considerable effort has been put into the preparation of their well-characterized surfaces and interfaces. Besides the preparation of traditional thin films that possess surfaces of the bulk materials, ultrathin films often exhibit surfaces with novel structural motifs that cannot be prepared otherwise.1-6 Such structures may exhibit new physical and chemical properties radically different from their bulk counterparts. Early transition-metal oxides, in particular, represent a class of catalytically important materials that have been very challenging to prepare as thin films despite their widespread importance in reactions such as the isomerization of alkanes and alkenes,7-10 partial oxidation of alcohols,11 selective reduction of nitric oxide,12-15 and metathesis of alkenes.16-18 There is a substantial amount of work on tungsten oxide clusters supported on various substrates such as SiO2,11,18,19 Al2O3,16 ZrO2,20 and TiO2.10,21-23 It has been suggested that the structure and reactivity of the tungsten oxide species are significantly influenced by the oxide support and the size of the WO3 clusters.24,25 Thin films of many early transition-metal oxides including WO3 have been successfully prepared on various substrates (MoO3,26,27 V2O3,28-31 V2O5,32,33 WO3,34-36 and TiO21,2,6). These thin oxide films can serve both as active model catalysts and model supports for metal nanoparticles. Previously, we have explored the reactivity of monodispersed (WO3)3 clusters supported on TiO2(110) with aliphatic r 2011 American Chemical Society
alcohols22 and formaldehyde.37 In this work, we focus on the growth of ordered tungsten trioxide, WO3, thin films on Pt(111) from monodispersed gas-phase (WO3)3 clusters prepared by sublimation of WO3 powders.23,38 The structure of the ordered WOx films deposited at 700 K onto Pt(111) was investigated using a combination of atomically resolved imaging, ensemble averaged surface sensitive techniques, and DFT calculations. We find that (WO3)3 trimers get partially reduced to the (5þ) oxidation state upon contact with Pt(111). The clusters react, forming an ordered c(4 2) overlayer that exhibits zigzag chainlike structures in the scanning tunneling microscopy (STM) images. In contrast, the (WO3)3 trimers in the second layer are found to form an ordered array of molecularly bound (WO3)3 with (3 3) periodicity.
2. METHODS 2.1. Experimental Details. The experiments were performed in two ultrahigh-vacuum (UHV) systems: a molecular beam scattering apparatus, devoted to ensemble averaged studies, and a scanning probe microscopy apparatus, used for atomically resolved studies. The experimental setup and procedures used in both systems are described below. The molecular-beam scattering apparatus (base pressure
