Site-specific interaction of H2O with ZnO single-crystal surfaces studied by thermal desorption and UV photoelectron spectroscopy

The adsorption and condensation of H₂O(D₂O) on ZnO(101̄0), (0001)Zn and (0001̄)O surfaces was investigated by means of thermal desorption (TDS) and UV photoelectron spectroscopy (UPS). The clean ZnO single-crystal surfaces were prepared by Ar-ion sputtering and annealing and characterised by Auger electron spectroscopy, LEED, UPS and work-function measurements. On all three surfaces six different adsorption states were found. In the monolayer regime there is a stronger bonding to Zn sites (desorption temperature 340 K) than to O sites (190 K), The bonding to the Zn sites seems to be accompanied by some clustering. Before the chemisorption layer is completed a first ice state is found whose desorption temperature shifts from 162 to 168 K with increasing exposures. At higher exposures the multilayer ice state is found at 152 K. On the (0001̄)O face defect-induced features were identified. The water lone-pair orbital 1b₁, whose energy falls between the O p and the Zn 3d emission of the substrate and which is known to show bonding shifts, was analysed using angle-resolved UPS. In the monolayer, the main chemisorption states are found at E_B^V(1b₁) = ?9.6 eV for the (0001)Zn face and at ? 10.6 eV for the (0001̄)O face and are compared with the multilayer ice emission at 1̄1.1 eV. The difference in binding energies shows the same trend as the TDS data. For the (101̄0) face the 1b₁ emission is very broad, indicating some overlap between different states.     

A photoelectron spectroscopic investigation of the interaction between H2O and oxygen on Ag(110)

The adsorption and reaction of H₂O with adsorbed oxygen atoms on Ag(110) was examined by UPS. In agreement with previous EELS results, H₂O formed multilayers of ice upon adsorption at 140 K. The ice layers could be easily distinguished from monolayer coverages of chemisorbed H₂O (present above 160 K) by UPS. The ice layers produced (1) strong attenuation of the emission from the Ag d-bands, (2) a nearly 2 eV shift of H₂O valence levels to higher binding energy and (3) strong attenuation of emission from the H₂O 3a₁ orbital. H₂O was observed to react stoichiometrically with O(a) above 250 K to produce a pure layer of adsorbed hydroxyl species. The UPS spectra for these species exhibited features at ?5.8 and ?8.7 eV, as well as strong features above the d-bands. These spectra were compared with those for OH(a) on other surfaces, and the difficulties of identifying OH by UPS due to contamination by excess H₂O are discussed.     

H2O interaction with clean and oxygen precovered Ni(110)

The interaction of water vapour with clean as well as with oxygen precovered Ni(110) surfaces was studied at 150 and 273 K, using UPS, ΔΦ, TDS, and ELS. The He(I) (He(II)) excited UPS indicate a molecular adsorption of H₂O on Ni(110) at 150 K, showing three water-induced peaks at 6.5, 9.5 and 12.2 eV below E_F (6.8, 9.4 and 12.7 eV below E_F). The dramatic decrease of the Ni d-band intensity at higher exposures, as well as the course of the work function change, demonstrates the formation of H₂O multilayers (ice). The observed energy shift of all water-induced UPS peaks relative to the Fermi level (ΔE_max = 1.5 eVat 200 L) with increasing coverage is related to extra-atomic relaxation effects. The activation energies of desorption were estimated as 14.9 and 17.3 kcal/mole. From the ELS measurements we conclude a great sensitivity of H₂O for electron beam induced dissociation. At 273 K water adsorbs on Ni(110) only in the presence of oxygen, with two peaks at 5.7 and 9.3 eV below E_F (He(II)), being interpreted as due to hydroxyl species (OH)^δ? on the surface. A kinetic model for the H₂O adsorption on oxygen precovered Ni(110) surfaces is proposed, and verified by a simple Monte Carlo calculation leading to the same dependence of the maximum amount of adsorbed H₂O on the oxygen precoverage as revealed by work function measurements. On heating, some of the (OH)^δ? recombines and desorbs as H₂O at ? 320 K, leaving behind an oxygen covered Ni surface.     

C2H4在Ru(1010)表面吸附与分解的研究

用X射线电子能谱(XPS)、热脱附谱(TDS)和紫外光电子能谱(UPS)方法研究了乙烯(C_{2H4)在Ru(1010)表面的吸附,在低温下(200K以下)乙稀(C2Ｈ4})可以在Ru(1010)表面上以分子状态稳定吸附,在200K以上乙烯(C₂H ₄)则发生了脱氢分解反应.TDS结果表明乙烯(C₂H₄)分 解后的主要产物为乙炔(C< 关键词： 乙烯 钌(1010)表面 吸附与分解     

XPS,UPS and thermal desorption studies of alcohol adsorption on Cu(110): I. Methanol

The adsorption of methanol on clean and oxygen dosed Cu(110) surfaces has been studied using temperature programmed reaction spectroscopy (TPRS), ultra-violet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). Methanol was adsorbed on the clean surface at 140 K in monolayer quantities and subsequently desorbed over a broad range of temperature from 140 to 400 K. The UPS He (II) spectra showed the 5 highest lying emissions seen in the gas phase spectrum of methanol with a chemisorption bonding shift of the two highest lying orbitais due to bonding to the surface via the oxygen atom with which these orbitals are primarily associated. A species of quite a different nature was produced by heating this layer to 270 K. Most noticeably the UPS spectrum showed only 3 emissions and the maximum coverage of this state was approximately $\text{1}\text{2}$ monolayer. The data are indicative of the formation of a methoxy species, thus showing that methanol is dissociated on the clean Cu(110) surface at 270 K. The same dissociated species was observed on the oxygen dosed surface, the main difference in this ease being the production of large amounts of H₂CO observed in TPRS at 370 K.     

Adsorption behavior of H2O on clean and oxygen precovered Ni(s)(111)

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 (T_m = 155 K). In the angular resolved UPS spectra three H₂O induced emission maxima at 6.2, 8.5 and 12.3 eV below E_F were found for θ ≈ 0.5. Angular and polarization dependent UPS measurements show that the C_2v 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 T_m = 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.     

Adsorption and decomposition of H2O on a K-covered Pt(111) surface

The adsorption of H₂O on clean and K-covered Pt(111) was investigated by utilizing Auger, X-ray and ultra-violet photoemission spectroscopies. The adsorption on Pt(111) at 100–150 K was purely molecular (ice formation) in agreement with previous work. No dissociation of this adsorbed H₂O was noted on heating to higher temperatures. On the other hand, adsorption of H₂O on Pt(111) + K leads to dissociation and to the formation of OH species which were characterized by a work function increase, an O 1s binding energy of 530.9 eV and UPS peaks at 4.7 and 8.7 eV below the Fermi level. The amount of OH formed was proportional to the K coverage for θ_K > 0.06 whereas no OH could be detected for θ? 0.06. Dissociation of H₂O occurred already at T = 100 K, with a sequential appearance of O 1s peaks at 531 and 533 eV representing OH and adsorbed H₂O, respectively. At room temperature and above only the OH species was observed. Annealing of the surface covered with coadsorbed K/OH indicated the high stability of this OH species which could be detected spectroscopically up to 570 K. The adsorption energy of H₂O coadsorbed with K and OH on Pt(111) is increased relative to that of H₂O on Pt. The work function due to this adsorbed H₂O increases whereas it decreases for H₂O on Pt(111). The energy shifts of valence and O1s core levels of H₂O on Pt + K as deduced from a comparison of gas phase and adsorbate spectra are 2.8–4.2 eV compared to ≈ 1.3–2.3 eV for H₂O on Pt (111). This increased relaxation energy shift suggests a charge transfer screening process for H₂O on Pt + K possibly involving the unoccupied 4a₁ orbital of H₂O. The occurrence of this mode of screening would be consistent with the higher adsorption energy of H₂O on Pt + K and with its high propensity to dissociate into OH and H.     

Photoemission studies of H2S,H2 and S adsorbed on Ru(110): Evidence for an adsorbed SH species

H₂S, H₂ and S adsorbed on Ru(110) have been studied by angle-integrated ultraviolet photoemission (UPS) as part of a study of the effect of adsorbed sulfur, a common catalytic poison, on this Ru surface. For low exposures of H₂S at 80 K, the work function rises to a value 0.16 eV above that of clean Ru(110) while the associated UPS spectra (hν = 21.2 eV) exhibit features similar to those of H(ads) and S(ads) and different from those of molecular H₂S. We conclude that H₂S dissociates completely at low coverages on Ru(110) at 80 K. At intermediate exposures the work function drops and the UPS spectra show new features which are attributed to the presence of an adsorbed SH species. This appears to be the first direct observation of this surface complex. At higher exposures the work function saturates at a value 0.36 eV below the clean value; the UPS spectra change markedly and indicate the adsorption of molecular H₂S. Heating adsorbed H₂S leaves a stable layer of S(ads) on Ru(110). The surface with adsorbed sulfur strongly modifies the adsorption at 80 K of a number of molecules relative to the clean Ru(110) surface.     

Stoichiometric and oxygen deficient MoO3(010) surfaces

The (010) surface of single crystal MoO₃ has been prepared and examined using LEED, XPS, UPS, and ELS. Three methods yield the stoichiometric surface: scraping in UHV and annealing, ion etching followed by reoxidation (770 K, 10² Pa O₂), or oxygen treatment to remove carbon contamination. LEED shows the surface periodicity is the same as that of the bulk (010). The MoO₃ valence band is 7 eV wide with density of states maxima at 1.5, 3.6, and 5.6 eV below the top of the valence band. Heating MoO₃ in vacuum reduces the surface region. XPS indicates the O/Mo atomic ratio decreases to 2.85 ± 0.12 on heating to 600 K. Ar ion bombardment disorders the surface and reduces the surface O/Mo atomic ratio to 1.6. Annealing of reduced surfaces at > 770 K incompletely reoxidizes them by diffusion of oxygen from the bulk. UPS of reduced and annealed MoO₃ exhibits two new emission features in the bandgap at 0.9 and 2.0 eV above the top of the valence band. These features originate from Mo derived states of a defect involving two or more Mo atoms, such as crystallographic shear planes. Because of the insulating nature of MoO₃, surface charging and electron beam induced damage were substantial hindrances to electron spectroscopic examination.     

Photoelectron spectroscopy of nanocrystalline anatase TiO2 films

Nanocrystalline TiO₂ (anatase) films were prepared using either colloidal suspensions or a sol-gel route. The electronic structure of these films was analyzed using X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS). Apart from pristine films, films containing defects introduced by annealing under ultra-high vacuum conditions or by ion bombardment were investigated. Generally, annealing in the temperature range up to 720 K results in no significant changes in the XPS and UPS spectra as compared to the pristine state, indicating that the amount of defect formation is too low to be observable by these techniques. On the other hand, ion irradiation causes the appearance of distinct defect states; these could be identified in agreement with previous data from photoemission studies on rutile and anatase single crystals. From UPS, a valence-band width of ∼4.6 eV was determined for the nanocrystalline anatase films.     

UPS and XPS studies of the chemisorption of O2, H2 AND H2O on reduced and stoichiometric SrTiO3(111) surfaces; The effects of illumination

About one monolayer of Ti³⁺ species is detectable at the surface of reduced SrTiO₃(111) single crystals by XPS and UPS. O₂, H₂ and H₂O have been adsorbed in the dark and the decrease on the concentration of the Ti³⁺ species has been monitored as a function of the gas exposures. Subsequent band gap illumination partially restores the Ti³⁺ initial concentration in the cases of O₂ and H₂ exposures but not in the case of H₂O. The Ti³⁺ photogeneration on the oxygen covered surface is associated with oxygen photodesorption as indicated by XPS and UPS. UPS measurements give evidence for surface hydroxylation resulting from water and hydrogen adsorption. The activity of the stoichiometric SrTiO₃(111) crystal face for O₂ and H₂ adsorption is very low when compared with the reduced SrTiO₃ samples.     

Core and valence level photoemission studies of iron oxide surfaces and the oxidation of iron

The core and valence level XPS spectra of Fe_xO (x ~ 0.90–0.95); Fe₂O₃ (α and γ); Fe₃O₄; and FeOOH have been studied under a variety of sample surface conditions. The oxides may be characterized by a combination of valence level differences and core-level effects (chemical shifts, multiplet splittings, and shake-up structure). Fe^II and Fe^III states are distinguishable, but octahedral and tetrahedral sites are not. The O 1 s BE cannot be used to distinghuish between the oxides since it has a nearly constant value. Fe 3d valence level structure spreads some 10 eV below E_F, much broader than suggested by previous UPS and photoelectron-spin-polarization (ESP) measurements for Fe_xO and Fe₃O₄. Fe surfaces (films, foils, (100) face) yield predominantly Fe^III species when exposed to high exposures of oxygen or air, though there is evidence for some Fe^II also. At low exposures the Fe^II/Fe^III ratio increases.     

The adsorption of water on clean and oxygen covered Cu(110)

The water adsorption on clean and oxygen precovered Cu(110) surfaces is studied by means of UPS, LEED, work function measurements and ELS. At 90 K on the clean surface molecular water adsorption is indicated by UPS. The H₂O molecules are bonded at the oxygen end and the H-O-H angle is increased as compared with the free molecule. In the temperature range between 90 and 300 K distorted H₂O molecules and adsorbed hydroxyl species (OH) are detected, which are desorbed at room temperature. On an oxygen covered surface hydroxyl groups are formed by dissociation of adsorbed water molecules at a lower temperature than on the clean surface. Multilayers of condensed water are found below 140 K in both cases.     

Synchrotron-Radiation Photoemission Study of Growth and Stability of Au Clusters on Rutile TiO2(110)-1×1

利用同步辐射高分辨光电子能谱研究了金团簇在部分还原TiO₂-(1×1)表面的生长和稳定性．价带谱实验结果观察到非常少量金团簇的沉积导致了Ti3+的3d峰完全消失,表明金团簇成核在TiO₂-(1×1)表面的氧缺陷位．Au4f芯电子光电子能谱实验结果证明了TiO₂-(1×1)表面氧缺陷位向金团簇转移电荷．还对比研究了化学剂量比和部分还原的TiO₂-(1×1)表面上金团簇的热稳定性．当金团簇尺寸相近时部分还原的TiO₂-(1×1)表面上金团簇要比化学剂量比的TiO₂-(1×1)面上金团簇稳定;在相同的表面上尺寸大的金团簇要比尺寸小的金团簇稳定．     

A photoemission study of the interaction of Ni(100), (110) and (111) surfaces with oxygen

Photoelectron spectroscopic studies of the oxidation of Ni(111), Ni(100) and Ni(110) surfaces show that the oxidation process proceeds at 295 and 485 K in two distinct steps: a fast dissociative chemisorption of oxygen followed by oxide nucleation and lateral oxide growth to a limiting coverage of 3 NiO layers. The oxygen concentration in the 295 K saturated oxygen layer on Ni(111) was confirmed by ¹⁶O(d,p) ¹⁷O nuclear microanalysis. At 295 and 485 K the oxide growth rates are in the order Ni(110) > Ni(111) > Ni(100). At 77 K the oxygen uptake proceeds at the same rate on all three surfaces and shows a continually decreasing sticking coefficient to saturation at ~2.1 layers (based upon NiO). An O 1sb.e. = 529.7 eV is associated with NiO, and O ls b.e.'s of ~531.5 and 531.3 eV can be associated, respectively, with defect oxide (Ni₂O₃) or (in the presence of H₂O) with an NiO(H) species. The binding energies (Ni 2p, O 1s) of this NiO(H) species are similar to those for Ni(OH)₂. Defect oxides are produced by oxidation at 485 K, or by oxidation of damaged films (e.g. from Ar⁺ sputtering) and evaporated films. Wet oxidation (or exposure to air) of clean nickel surfaces and oxides, and exposure of thick oxide to hydrogen at high temperature results in an O 1s b.e. ~531.3 eV species. Nuclear microanalysis ²H(³He,p) ⁴He indicates the presence of protonated species in the latter samples. Oxidation at 77 K yields O 1s b.e.'s of 529.7 and ~531 eV; the nature of the high b.e. species is not known. Both clean and oxidised nickel surfaces show a low reactivity towards H₂O; clean nickel surfaces are ~10³ times less reactive to H₂O than to oxygen.     

An X-ray photoelectron spectroscopic study of the adsorption of N2, NH3, NO,and N2O on dysprosium

The adsorption of N₂, NH₃, NO, and N₂O onto clean polycrystalline dysprosium at 300 and 115 K and the changes undergone by the adsorbed species upon heating from 115 K have been investigated using X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES). At 115 K, N₂ adsorbs dissociatively, vielding two peaks in the N 1s region at 396.2 and 398.2 eV corresponding respectively to a nitride and to chemisorbed nitrogen N(a). No peaks corresponding to molecularly adsorbed N₂ (BE 400.2 eV [10]) were observed. Upon heating the sample the N(a) is converted into the nitride species, as evidenced by a decrease in the 398.2 eV peak and a corresponding increase in the 396.2 eV peak. At a warm-up temperature of 300 K, the N(a) species accounts for only ～10% of the total nitrogen on the surface. Ammonia adsorbed at 115 K shows three distinct peaks, at 401.7, 399.3 and 396.2 eV, corresponding to molecular, partly dissociated, and completely dissociated (nitride) ammonia. Upon heating multilayer ammonia to 175 K, it desorbs to leave predominantly the peak corresponding to the partly dissociated species. Upon further heating the molecular and partly dissociated ammonia is converted into the nitride species. At 400 K only nitride-type nitrogen remains on the surface. The adsorption of NO and N₂O at 115 K is predominantly dissociative. NO has N 1s peaks at 403.1 and 396.3 eV corresponding possibly to molecularly adsorbed NO, and to nitride species. After N₂O adsorption there is very little nitrogen on the surface. Adsorption of N₂ and NO at 300 K yields only the peak at 396.2 eV, whereas NH₃ yields, in addition to this peak, a small intensity (～20%) of the peak at 398.2 eV (partly dissociated ammonia).     

The adsorption and reaction of H2O on clean and oxygen covered Ag(110)

The adsorption and reaction of water on clean and oxygen covered Ag(110) surfaces has been studied with high resolution electron energy loss (EELS), temperature programmed desorption (TPD), and X-ray photoelectron (XPS) spectroscopy. Non-dissociative adsorption of water was observed on both surfaces at 100 K. The vibrational spectra of these adsorbates at 100 K compared favorably to infrared absorption spectra of ice Ih. Both surfaces exhibited a desorption state at 170 K representative of multilayer H₂O desorption. Desorption states due to hydrogen-bonded and non-hydrogen-bonded water molecules at 200 and 240 K, respectively, were observed from the surface predosed with oxygen. EEL spectra of the 240 K state showed features at 550 and 840 cm^?1 which were assigned to restricted rotations of the adsorbed molecule. The reaction of adsorbed H₂O with pre-adsorbed oxygen to produce adsorbed hydroxyl groups was observed by EELS in the temperature range 205 to 255 K. The adsorbed hydroxyl groups recombined at 320 K to yield both a TPD water peak at 320 K and adsorbed atomic oxygen. XPS results indicated that water reacted completely with adsorbed oxygen to form OH with no residual atomic oxygen. Solvation between hydrogen-bonded H₂O molecules and hydroxyl groups is proposed to account for the results of this work and earlier work showing complete isotopic exchange between H₂¹⁶O_(a) and ¹⁸O_(a).     

Adsorption of water and methanol on GaAs(110) surfaces studied by ultraviolet photoemission

UV photoemission spectroscopy (UPS) with He 1 radiation (hν = 21.2 eV) has been used to study the interaction of H₂O and CH₃OH with GaAs(110) surfaces prepared by cleavage in ultrahigh vacuum (UHV). For H₂O two molecularly adsorbed phases can be distinguished at 300 K: at low coverage H₂O is chemisorbed by its oxygen lone-pair orbital to the surface whereas for higher exposures a less perturbed species which resembles more a “physisorbed” or condensed H₂O layer is found. At 180 K only the less perturbed species can be identified. Also CH₃OH is chemisorbed molecularly at lower coverage with its oxygen end to the GaAs surface. For higher exposures two additional emission bands are observed which are interpreted as due to the methoxy radical CH₃O resulting from a partial decomposition of CH₃OH.     

The adsorption of SO2 on iron surfaces studied by x-ray photoelectron spectroscopy

The interaction of SO₂ with evaporated iron surfaces in the temperature range 80–450 K was investigated by using X-ray photoelectron spectroscopy. At 300 K, SO₂ decomposed at the initial stage of the interaction and gave adsorbed S with the S2p peak at 161.9 eV and adsorbed O with the O1s at 530.0 eV. Further exposure of SO₂ gave adsorbed SO₄ with S2p at 166.8 eV O1s at 531.3 eV, being different in binding energies from ionic SO₄^2?. This indicates the two stage reaction Of SO₂ with iron surface; SO₂(gas) → S(ads) + 20(ads), SO₂(gas) + 2O(ads) → SO₄(ads). The first reaction did not occur at low temperature or in the presence of adsorbed O. The adsorbed SO₄ formed at 80 K showed a quantitative decomposition reaction into S(ads) and O(ads) in the temperature range 200–350 K.     

Adsorption of H2O on clean and on boron-contaminated Rh surfaces

The adsorption and dissociation of H₂O on Rh(111) and Rh foil surfaces have been studied in UHV using Auger electron, electron energy loss (in the electronic range) and thermal desorption spectroscopy. H₂O adsorbs weakly on clean Rh samples at 110 K. The adsorption is accompanied by the appearance of a broad loss feature at 14–14.5 eV. At higher exposures new losses appeared at 8.6 and 10.5 eV. The desorption of H₂O took place in two stages, with T_p = 183 K (β, chemisorption) and 158 K (α, multilayer formation). There was no indication of dissociation of H₂O on a clean Rh(111) surface. Similar results were obtained for a clean Rh foil. However, when small amounts of boron segregated on the surface of Rh, they exerted a dramatic influence on the adsorptive properties of this surface and caused the dissociation of H₂O. This was exhibited by the formation of H₂, by the buildup of surface oxygen, by the appearance of an intense new loss at 9.4 eV, identified as B-O surface species, and by the development of “boron-oxide”-like Auger fine structure.