J. Phys. Chem. C 2007, 111, 17597-17602
17597
D2O Adsorption on an Ultrathin Alumina Film on NiAl(110) Cheol-Woo Yi and Ja´ nos Szanyi* Institute for Interfacial Catalysis, Pacific Northwest National Laboratory, P.O. Box 999, MSIN: K8-80, Richland, Washington 99352 ReceiVed: June 8, 2007; In Final Form: August 23, 2007
The structure of an ordered, ultrathin Al2O3 film grown on a NiAl(110) single-crystal surface and its interaction with D2O were studied by low-energy ion scattering spectroscopy (LEISS), X-ray photoelectron spectroscopy (XPS), temperature programmed desorption (TPD), and infrared reflection absorption spectroscopy (IRAS). LEISS demonstrated that the surface was mainly terminated by an oxygen layer, and TPD data of adsorbed D2O revealed that most of the water desorbed molecularly from Al2O3/NiAl(110). However, a small amount of dissociated water molecules during adsorption and/or TPD measurements was observed, and as a consequence the alumina film thickness increased after water adsorption/desorption. These results suggest that atomic oxygen and/or hydroxyl species, which are formed by dissociation of water, interact with subsurface aluminum atoms through defect sites and cause the increase in the alumina film thickness. In addition, a few monolayers of water can be transformed from the amorphous solid water (ASW) to the crystalline ice (CI) phase, as seen by IRAS.
Introduction The chemistry of Al2O3 has been studied extensively because of its importance in coatings and microelectronics and, even more importantly, its popularity as a substrate in many heterogeneous catalytic processes, due to its high surface area, inertness, chemical stability, and so on.1,2 However, much of the work has focused on high surface area γ-Al2O3 powders, and very few surface chemistry studies have been conducted on well-defined Al2O3 surfaces, due to the difficulty of experimental investigations of the single crystal, originating from its inherently low electron conductivity. Recently, long range ordered Al2O3 thin films have been prepared by the oxidation of a NiAl(110) single crystal and characterized by low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray diffraction (XRD), and high-resolution electron energy loss spectroscopy (HREELS).2-5 However, due to the ambiguity of the assignment of the ultrathin alumina film to any specific phase,6 several surface structures have been proposed. On the basis of STM and HREELS results, it was reported that the film had a γ-Al2O3-like structure.7,8 Stierle et al. reported the formation of ultrathin ordered κ-Al2O3-like layers on NiAl(110) studied by extended surface X-ray diffraction (SXRD) and LEED.6 More recently, Kresse et al. reported that the surface was different from all Al2O3 bulk phases because aluminum atoms are pyramidally and tetrahedrally coordinated and the overall stoichiometry of the film is not Al2O3 but Al10O13.3 The interaction of water with metal-oxide surfaces plays an important role in heterogeneous catalysis, environmental science, and electrochemistry. Water adsorption on metals and metaloxide surfaces has been thoroughly discussed in comprehensive reviews by Thiel et al.9 and more recently by Henderson.10 Water adsorbs molecularly and/or dissociatively on metal and metal-oxide surfaces. Generally, on perfect metal-oxide surfaces, * To whom correspondence should be addressed. E-mail: janos.szanyi@ pnl.gov.
water adsorbs molecularly onto the surface, whereas the dissociative adsorption of water on oxide surfaces is dominated by defect sites, as was clearly demonstrated on MgO(100), Fe3O4(111), R-Al2O3(0001), and TiO2(110).1,9-13 Water dissociation occurring on regular surface sites on certain metal oxides has recently been discussed.11,14 The adsorption of water on FeO(111) and Fe3O4(111) is a good example, demonstrating the molecular/dissociative adsorption with respect to the surface morphology.11 On the FeO(111) surface, exposing only closely packed oxygen atoms, weakly bound, molecularly adsorbed water species were observed.11,14 Alternatively, on the Fe3O4(111) surface, terminated by both Fe and O atoms, water adsorbs dissociatively and forms adsorbed hydroxide on the surface.11 On the other hand, the surfaces of thin alumina films have been believed to be inert toward dissociation of water at low pressure (
