Adsorption behavior of H2O on clean and oxygen precovered Ni(s)(111) |
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| Affiliation: | 1. Department of Physics, Central University of Punjab, Bathinda 151401, India;2. Human Resources Development Section, Raja Ramanna Centre for Advanced Technology, Indore 452013, India;3. Homi Bhabha National Institute, Training School Complex, Anushaktinagar, Mumbai 400094, India;4. CNR-ICCOM, Consiglio Nazionale delle Ricerche, ThC2-Lab, Pisa 56124, Italy;1. National Synchrotron Radiation Laboratory, Collaborative Innovation Center of Chemistry for Energy Materials, University of Science & Technology of China, Hefei, Anhui 230029,China;2. CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science & Technology of China, Hefei, Anhui 230026, China;3. Department of Chemical Physics, University of Science & Technology of China, Hefei, Anhui 230026,China;4. Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China;1. State Key Laboratory of Chemical Engineering, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China;2. Department of Chemical Engineering, Norwegian University of Science and Technology, N-7491 Trondheim, Norway |
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| Abstract: | Photoelectron spectroscopy (UPS), thermal desorption spectroscopy (TDS), isotope exchange experiments, work function change (δφ) and LEED were used to study the adsorption and dissociation behavior of H2O on a clean and oxygen precovered stepped Ni(s)[12(111) × (111)] surface. On the clean Ni(111) terraces fractional monolayers of H2O are adsorbed weakly in a single adsorption state with a desorption peak temperature of 180 K, just above that of the ice multilayer desorption peak (Tm = 155 K). In the angular resolved UPS spectra three H2O induced emission maxima at 6.2, 8.5 and 12.3 eV below EF were found for θ ≈ 0.5. Angular and polarization dependent UPS measurements show that the C2v symmetry of the H2O gas-phase molecule is not conserved for H2O(ad) on Ni(s)(111). Although the Δφ suggest a bonding of H2O to Ni via the negative end of the H2O dipole, the O atom, no hints for a preferred orientation of the H2O molecular axes were found in the UPS, neither for the existence of water dimers nor for a long range ordered H2O bilayer. These results give evidence that the molecular H2O axis is more or less inclined with respect to the surface normal with an azimuthally random distribution. H2O adsorption at step sites of the Ni(s)(111) surface leads in TDS to a desorption maximum at Tm = 225 K; the binding energy of H2O to Ni is enhanced by about 30% compared to H2O adsorbed on the terraces. Oxygen precoverage causes a significant increase of the H2O desorption energy from the Ni(111) terraces by about 50%, suggesting a strong interaction between H2O and O(ad). Work function measurements for H2O+O demonstrate an increase of the effective H2O dipole moment which suggests a reorientation of the H2O dipole in the presence of O(ad), from inclined to a more perpendicular position. Although TDS and Δφ suggest a significant lateral interaction between H2O+O(ad), no changes in the molecular binding energies in UPS and no “isotope exchange” between 18O(ad) and H216O(ad) could be observed. Also, dissociation of H2O could neither be detected on the oxygen precovered Ni(s)(111) nor on the clean terraces. |
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