Guido Ketteler

Electron Spectroscopy of Aqueous Solution Interfaces Reveals Surface Enhancement of Halides

It has been suggested that enhanced anion concentrations at the liquid/vapor interface of airborne saline droplets are important to aerosol reactions in the atmosphere. We report ionic concentrations in the surface of such solutions. Using x-ray photoelectron spectroscopy operating at near ambient pressure, we have measured the composition of the liquid/vapor interface for deliquesced samples of potassium bromide and potassium iodide. In both cases, the surface composition of the saturated solution is enhanced in the halide anion compared with the bulk of the solution. The enhancement of anion concentration is more dramatic for the larger, more polarizable iodide anion. By varying photoelectron kinetic energies, we have obtained depth profiles of the liquid/vapor interface. Our results are in good qualitative agreement with classical molecular dynamics simulations. Quantitative comparison between the experiments and the simulations indicates that the experimental results exhibit more interface enhancement than predicted theoretically.

In situ x-ray photoelectron spectroscopy studies of water on metals and oxides at ambient conditions

In-situ X-ray photoelectron spectroscopy studies of water metals and oxides at ambient conditions Ev Vi si ua t w tio ww n Ed .a ct itio iv n eP of DF ac .c tiv om eP DF fo rm So or ftw e d e ar ta e. ils on S Yamamoto 1 , H Bluhm 2 , K Andersson 1,3,6 , G Ketteler 4,7 , H Ogasawara 1 , M Salmeron 4,5 and A Nilsson 1,3 Stanford Synchrotron Radiation Laboratory, P.O.B. 20450, Stanford, CA 94309, USA. Lawrence Berkeley National Laboratory, Chemical Sciences Division, Berkeley, CA 94720, USA. FYSIKUM, Stockholm University, AlbaNova University Center, SE-106 91 Stockholm, Sweden. Lawrence Berkeley National Laboratory, Materials Sciences Division, Berkeley, CA 94720, USA. Department of Materials Science and Engineering, University of California, Berkeley, CA 94720, USA. E-mail: nilsson@slac.stanford.edu Running head: In-Situ XPS studies of water on metals and oxides at ambient conditions Present address: Center for Individual Nanoparticle Functionality (CINF), Department of Physics, Technical University of Denmark, Fysikvej 312, DK-2800 Kgs. Lyngby, Denmark. Present address: Department of Applied Physics, Chalmers University of Technology, SE-412 96 Goteborg, Sweden. Abstract . X-ray photoelectron spectroscopy (XPS) is a powerful tool for surface and interface analysis, providing the elemental composition of surfaces and the local chemical environment of adsorbed species. Conventional XPS experiments have been limited to ultrahigh vacuum (UHV) conditions due to a short mean free path of electrons in a gas phase. The recent advances in instrumentation coupled with third-generation synchrotron radiation sources enables in-situ XPS measurements at pressures above 5 Torr. In this review, we describe the basic design of the ambient pressure XPS setup that combines differential pumping with an electrostatic focusing. We present examples of the application of in-situ XPS to studies of water adsorption on the surface of metals and oxides including Cu(110), Cu(111), TiO 2 (110) under environmental conditions of water vapor pressure. On all these surfaces we observe a al general trend where hydroxyl groups form first, followed by molecular water adsorption. The importance of surface OH groups and their hydrogen bonding to water molecules in water adsorption on surfaces is discussed in detail.

Autocatalytic water dissociation on Cu(110) at near ambient conditions

Autocatalytic dissociation of water on the Cu(110) metal surface is demonstrated on the basis of X-ray photoelectron spectroscopy studies carried out in situ under near ambient conditions of water vapor pressure (1 Torr) and temperature (275-520 K). The autocatalytic reaction is explained as the result of the strong hydrogen-bond in the H2O-OH complex of the dissociated final state, which lowers the water dissociation barrier according to the Brønsted-Evans-Polanyi relations. A simple chemical bonding picture is presented which predicts autocatalytic water dissociation to be a general phenomenon on metal surfaces.

Water growth on metals and oxides: binding, dissociation and role of hydroxyl groups

We discuss the role of the presence of dangling H-bonds from water or from surface hydroxyl species on the wetting behavior of surfaces. Using scanning tunneling and atomic force microscopies and photoelectron spectroscopy, we have examined a variety of surfaces, including mica, oxides and pure metals. We find that in all cases, the availability of free, dangling H-bonds at the surface is crucial for the subsequent growth of wetting water films. In the case of mica, electrostatic forces and H-bonding to surface O atoms determine the water orientation in the first layer and also in subsequent layers with a strong influence in its wetting characteristics. In the case of oxides like TiO2, Cu2O, SiO2 and Al2O3, surface hydroxyls form readily on defects upon exposure to water vapor and help nucleate the subsequent growth of molecular water films. On pure metals, such as Pt, Pd and Ru, the structure of the first water layer and whether or not it exhibits dangling H-bonds is again crucial. Dangling H-bonds are provided by molecules with their plane oriented vertically, or by OH groups formed by the partial dissociation of water. By tying the two H atoms of the water molecules into strong H-bonds with pre-adsorbed O on Ru can also quench the wettability of the surface.