In most of the cases, the interaction occurs with a charge transfer to the surface, resulting in a negative WF change (see and references there in). Water-surface interaction through hydrogen bonds is rare. Generally speaking, it is widely accepted, that when water molecules are adsorbed on surfaces, they interact mostly through their O atoms. Differently, it seems that their role in the water-surface interaction strongly depends on the conditions of the adsorption process and the type of the substrate. From the above discussion, it is obvious that there is no a common behavior of alkalis. In addition, Cs on the MgO(100) surface dissociates the adsorbed water too. In contrast, Shi and Jacobi reported that Cs submonolayers of 0.25 and 0.33 ML on the Ru(001) surface, cause dissociation of water, while for 0.08 ML predeposited cesium, H 2O adsorbs molecularly on surface. Also Cs submonolayer quantity on the Cu(110) surface, does not dissociate but instead increase the adsorption energy of water. For example, although Li, Na and K enhance the dissociation on nickel surfaces under certain conditions, Li and Cs do not allow dissociation on the Ag(110) surface at any instance. In general, alkalis promote the dissociation of water depending on the coverage, temperature of adsorption, the kind of alkali atom and the type of substrate. Concerning the electropositive additives, alkali metals have attracted much of interest mainly because most of them are constituents of many electrochemical cells. In contrast, the electronegative adsorbed oxygen, in most of the cases promotes the dissociation of water on surfaces, although there are cases where preadsorbed O prevents the dissociation process. On the other hand, additives such as CO, H and Br on metal surfaces, do not favor the dissociative adsoprtion of water (see Sect. 6.2.1 in and references therein). Moreover, the surface imperfections like steps, edges and defects can also promote water dissociation. In addition, water-surface bonding can be influenced by the presence of additives and their coverage on surface, thus enhancing the dissociative adsorption. Furthermore, in some cases the structure as well as the temperature, the composition and the morphology of the substrate (steps or terraces), seems also to play a decisive role in determining whether or not dissociation of water will take place on surface. Although the enthalpy change seems to be a rough thermodynamic criterion of what adsorption pathway will be followed, kinetic effects cannot be ignored. Basically there are two possibilities for the adsorbed water molecules on a surface: a) the associative or molecular adsorption, and b) the dissociative adsorption. The so far provided knowledge is well presented in excellent reviews, by giving information about how the water behaves on different type of surfaces, such as metals, oxides and semiconductors. Over the past decades, the interaction of water with solid surfaces has intensively attracted the research interest in several scientific fields, such as corrosion chemistry, heterogeneous catalysis, electrochemistry, solar energy conversation, fuel cells, photocatalysis etc. Finally, we propose atomistic adsorption models for both processes of cesium with water adsorption. Based on that, we suggest a catalytic reaction of water dissociation according to the Langmuir–Hinshelwood mechanism. It appears that the co-adsorbed cesium with water modifies the surface potential providing an effective template for cesium oxide, Cs 2O development. In contrast to the sequential adsorption, during the co-adsorption process the oxidation of cesium takes place above a critical coverage of cesium (≥ 0.45 ML). For a full cesium layer covered surface, the adsorbed water retracts the metallicity of cesium due to electrostatic interactions. Instead, water dissociation appears to merely occur on defective sites of the substrate in accordance with previous studies. This seems to be due, first to the strong interaction between the alkaline adatoms and the substrate, and secondly to the limited maximum pre-deposited amount of cesium (≤ 0.45 ML). Based on the results and by adopting the Lewis acid–base model, we conclude that during the sequential adsorption the water molecules are mostly adsorbs non-dissociatively on surface, without oxidizing the alkaline overlayer. The catalytic role of cesium with respect to the dissociation of water on surface was investigated, by applying two different adsorption processes at room temperature (RT): (1) The adsorption of water on the cesium covered surface (sequential adsorption), and (2) the co-adsorption process (simultaneous adsorption) on surface. The interaction of water with cesium on the strontium titanate surface SrTiO 3(100), was studied, mainly by means of work function measurements and thermal desorption spectroscopy.
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