The interaction of water molecules with solid surfaces has been a subject of considerable interests, due to its importance in the fields from atmospheric and environmental phenomena to biology, catalysis and electrochemistry [1-2]. Among various kinds of surfaces, a lot of theoretical and experimental studies have been performed regarding water on MgO(100), however, to date, there has been no direct observation of water molecules on MgO by scanning tunneling microscope (STM) as compared with those on metal surface. Here, we will present the direct observation and manipulation of single water molecules on ultrathin MgO(100) films using low-temperature scanning tunneling microscope (LT-STM). Our results rationalize the previous theoretical predictions of isolated water molecules on MgO including the optimum adsorption sites and non-dissociative adsorption of water. Moreover, we were able to dissociate a water molecule by exciting the vibrational mode of water, which is unattainable on metal surfaces. The enhanced residual time of tunneling electrons in molecules on the insulating film is responsible for this unique pathway toward dissociation of water.

[1] P. A. Thiel and T. E. Madey Surf. Sci. Rep. 7, 211 (1987).

Under ambient relative humidity (RH) conditions thin film water coats metals, insulators, semiconductors and ice surfaces [1]. The water-oxide interface plays an important role in chemical, biological and industrial systems [2]. Water on MgO(100) is one of the most widely studied systems, both theoretically and experimentally, due to the simple rock salt cubic structure of MgO. Carbon dioxide is an important greenhouse gas, and the carbonation of MgO in mineral deposits has been suggested as a potential medium for carbon dioxide sequestration [3]. Here we present results from the investigation of the interaction of water and carbon dioxide with MgO(100)/Ag(100) films using the ambient pressure photoemission spectroscopy (APPES) setup at beamline 11.0.2 of the Advanced Light Source at Lawrence Berkeley National Laboratory [4]. With APPES we can quantitatively probe on a molecular level the chemical changes of the MgO(100) surface using XPS and NEXAFS while in equilibrium with water and/or carbon dioxide gases. Using 0.5 Torr isobars, we have characterized the uptake of water on MgO surfaces up to 20% RH. At roughly 0.1% RH, the MgO surface is fully hydroxylated with a ML of OH and sub-ML molecular water. At 20% RH there is roughly 1 ML of molecular water interacting with a fully hydroxylated MgO surface. Preliminary results for carbon dioxide reactivity with MgO to form carbonate showed dependency on whether the MgO surface was dry (oxide terminated) or wet (hydroxyl/water terminated). Thus, the presence of thin film water influences carbon sequestration. Ongoing studies are addressing the influences of changing RH on MgO carbonation.

References

[1] G.E Ewing 2006 Chem Rev. 106 1511.

[2] G.E. Brown et al. 1999 Chem. Rev. 99 77.

[3] T. Koljonen et al. 2004 Energy 29 1521.

[4] H. Bluhm et al. 2007 MRS Bulletin 34 1022.

Adsorbed water plays an important role in a wide range of surface phenomena. In this talk, the focus is on water interactions with oxide and carbonate surfaces in dynamic equilibrium with water vapor under ambient conditions of temperature and relative humidity. The coverage and nature of the adsorbed water layer as a function of relative humidity are investigated. Both experiment and theory show that the adsorbed water layer is inhomogeneous and the surface is not uniformly covered even at high relative humidity. The impact of the nature of the adsorbed water layer on the surface chemistry of carbonate and oxide surfaces under ambient conditions will be discussed.

Water dissociation on the surface is the first step in many reactions and wetting processes. A fundamental understanding of this process is necessary and applies to various fields, such as corrosion, catalysis, electrochemistry and hydrogen generation. Thermally dissociated water molecules on an oxygen precovered Cu(110) surface are investigated using scanning tunneling microscopy (STM) at low temperature. Chain structures of elongated molecules are obtained by dosing water on an oxygen precovered Cu(110) at 77K followed by annealing to ~180K. Density functional theory calculations reveal that the elongated molecular shapes are made of two hydrogen bonded OH molecules which form a dimer by tilting one OH to the other¹. The weak interaction between dimers gives rise to the changes in shape of the cluster. We propose that this structural variation is the result of molecular fluctuations and from the interaction with atomic hydrogen produced by water dissociation. Switching between the different dimer shapes can be accomplished by exciting the OH-stretch mode with tunneling electrons.

¹T. Kumagai et. al Phys. Rev. B 79, 035423 (2009)

Although extensive research has been aimed at the structure of ice films [1], important details of the morphology evolution, especially for films in the nm-thickness range, have remained elusive. Our capability to image up to 30 molecular layers of ice with STM [2], enables us to track the film evolution during growth and annealing. In particular, we investigate the role of new-layer nucleation and surface self-diffusion.

As reported before by others, we observe that water deposited onto Pt(111) below 120K forms amorphous films, whereas metastable cubic ice appears between 120K and ~150K. At 140K and a mean film thickness of ~1nm the film consists of 2-3 nm high crystallites, embedded in a one bilayer high wetting layer. Analyzing the annealing behavior of these crystallites we find [3] that the rate at which new layers nucleate, and not surface diffusion, determines how fast individual crystallites equilibrate. The resistance to nucleate new layers even during deposition has a surprising effect on thicker films: The deposited water attaches preferentially to growth spirals around screw dislocations, promoting the formation of metastable cubic ice [2].

Finally, we report the first measurements of surface self-diffusion on ice. Using uniform 5-nm-thick ice films (grown at 145K) as templates, we deposited a fraction of a monolayer of water at 115K to create metastable arrays of 2D-islands. By quantifying the Ostwald ripening of these arrays upon annealing between 115 and 135 K, we extract temperature-dependent diffusion rates and determine the activation energy for surface self-diffusion to be (0.4+-0.1) eV [4].

[1] A. Verdaguer, G. M. Sacha, H. Bluhm, and M. Salmeron, Chem. Rev. 106, 1478 (2006).

[2] K. Thürmer and N. C. Bartelt, Phys. Rev. B 77, 195425 (2008).

[3] K. Thürmer and N. C. Bartelt, Phys. Rev. Lett. 100, 186101 (2008).

[4] S. Nie, N. C. Bartelt, and K. Thürmer, Phys. Rev. Lett. 102, 136101 (2009).