Oral Cesium growth and characterization on the SrTiO3(100) surface: Water adsorption

Abstract

In this work, we investigate the growth of Cs overlayer on the SrTiO3(100) surface (STO) at room temperature by meansof Auger electron spectroscopy, low energy electron diffraction, electron energy loss spectroscopy, thermal desorptionspectroscopy and work function measurements [1]. According to the results, cesium grows in a single amorphous layer, showingdifferent morphology from that on other insulating substrates. The Cs overlayer approximates a two-dimensional metallicphase. No indications for the reduction of the substrate and a Cs–O compound are found. Thermal annealing desorbs partof the metallic Cs, inducing at the same time the surface diffusion of the Cs adatoms to higher binding energy states. Thegrowth and adsorption kinetics of Cs on the STO, shows substantial differences to that of other alkalis such as K [2] and Li[3]. The reasons for that are discussed.Having characterized the cesium overlayer on the STO(100) surface, our next step wasto study the adsorption of water on that surface system. The interest on water-surface interaction originates not only fromfundamental scientific reasons, namely to understand better wetting and corrosion phenomena, but also from environmentalconcerns underlining the increasing importance of hydrogen as a fuel for green energy production. Despite the scientific effort,our knowledge of water adsorption with additives on surfaces remains incomplete. Thus here, we investigate experimentallythe role of cesium as a promoter for water adsorption on the STO surface. In general, strontium titanate is a perovskite withoutstanding catalytic properties in photoelectrolysis of water [4]. The motivation of this work is the well known enhancementof the catalytic properties of metal oxides by the addition of alkali species. The results show that water neither dissociatesnor interacts strongly with the predeposited cesium on the surface. In addition, no any Cs-H2O compound, was detected,concluding that water adsorbs non-dissociatively on the cesiated STO surface. In contrast, much earlier experiments showedpartial dissociation of water and oxidation of the clean STO substrate [5], while rapid dissociation of water has also been observedon cesium covered MgO(100) surface [6]. The situation was different when we tried simultaneous adsorption of Cs and H2O onthe STO(100) surface. Those experiments resulted in the detection of Cs2O, only if the corresponding Cs coverage was equivalentor larger than 1 monolayer. In that sense, a prerequisite minimum amount of Cs is necessary in order to react with H2O and formcesium oxide. Thus, we conclude that under certain experimental conditions, coadsorption of Cs with water can dissociate themolecule of H2O, resulting in the oxidation of cesium.