Dissociation And Charge Transfer Of H₂O On Cu?110?Probed In Real Time Using Ion Scattering Spectroscopy

With the large scale development of graphene devices based on copper substrates,water-gas shift reaction in the heterogeneous catalysis,and nuclear waste buried underground packed in the copper jar,the investigation of the interaction between water and copper is becoming more and more important.In particular,it is important to study the process of adsorption and dissociation of water on Cu,because it is essential to understand the corrosion process and the H-bond interactions at a fundamental level.Most studies have been focused on the adsorption and dissociation of water on Cu(110)at low temperatures under ultrahigh vacuum conditions.At low temperatures,water is found to adsorb molecularly intact on clean Cu(110).Water desorbs near 290 K via an OH recombination mechanism.Does it continue to occur at high temperatures? Is there a strong chemical interaction between a Cu surface and a hydroxyl group? To the best of our knowledge,the available experimental results do not adequately clarify this issue.The change in surface composition should affect the electronic states near the Fermi level of the surface.Ultraviolet photoelectron spectroscopy can detect valence states,but it may enhance water dissociation on the surface.X-ray photoelectron spectroscopy only detects innershell electrons,but X-ray/electron induced dissociation of water always has to be considered in the related studies.Moreover,it cannot detect hydrogen.Temperature-programmed desorption can detect the desorbed water molecules from the crystal edges and from its backside or the heating sample holder,which somewhat complicates the interpretation of the experimental data.Thus it is necessary to develop a new experimental method.Based on these issues mentioned above,we developed a new method named ion scattering spectroscopy to in situ detect H and O in real time.Moreover,charge transfer has been used to detect the electronic states near the Fermi level of the surface.In this work,we first performed the time-of-flight ion scattering spectroscopy to study the adsorption and desorption kinetics of water on Cu(110).It is found that on Cu(110)there exists a first layer of chemisorbed hydroxyl followed by a large number of physically adsorbed water molecules and that the water adsorption rate decreases with increasing water coverage.Water adsorption meets the first-order Langmuir adsorption model at room temperature.The relative intensities of H and O reduce rapidly around 423 K,which indicates that the weakly bound H2 O layers can be removed by heating the surface.The OH/O phases have coexisted above room temperature,accompanied by specific surface reactions.In particular,traces of atomic oxygen on the surface are observed even at 723 K,which is an obvious evidence for surface water oxidization.In order to probe the change in the surface electronic states near the Fermi level of the surface in real time,we then performed charge transfer experiments with energetic fluorine ions.It is found that negative-ion fractions monotonically decrease with increasing temperature.The reason has been qualitatively discussed in terms of the electronic structure at elevated temperature.A simple calculation has been performed to compare with the experimental data.As expected,the calculated work function increases with the increase in annealing temperature.It is confirmed that charge transfer can be used as a sensitive probe of the electronic interaction of adsorbates and metal surfaces in real time that cannot be obtained by other techniques.