The bonding of water molecules to platinum surfaces

Using high resolution electron energy loss spectroscopy in ultra-high vacuum we have studied the vibrational spectrum of submonolayer and multilayer quantities of water adsorbed on platinum (100) surfaces. For adsorbed multilayers the spectrum resembles the spectrum of ice I. For submonolayer quantities of H₂O we find three different OH stretching vibrations, 2850, 3380, and 3670 cm^?1. The highest frequency is attributed to free OH groups. The vibration around 3380 cm^?1 indicates H bonding between oxygen atoms. It is therefore concluded that the water molecules cluster even at low coverage. The lowest OH stretching frequency is attributed to H bonding to platinum. We find also evidence for additional oxygen lone pair orbital bonding to the surface which disappears however when the first monolayer is completed. The relation to currently considered models in electrochemistry of aqueous solutions is discussed.     

UPS/XPS study of reactive and non-reactive SrTiO3 (100) surfaces: Adsorption of H2O

SrTiO₃ surfaces, prepared by ion bombardment alone and in conjunction with annealing, exhibit very different properties upon exposure to H₂O. In the case of ion bombardment only, H₂O adsorption occurs, signaled by the appearance of a peak in the HeII spectrum at 10.8 eV binding energy which saturates for dosages lower than 5 L. Our data indicate that this adsorption is non-dissociative. No adsorption was detected for bombarded and annealed samples. The difference in behavior is believed to be related to the oxidation state of Ti. The significance of these results for previously published data is discussed.     

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.     

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.     

Water adsorption on the Be(0001) surface: from monomer to trimer adsorption

In this paper, the density functional theory has been used to perform a comparative theoretical study of water monomer, dimer, trimer, and bilayer adsorptions on the Be(0001) surface. In our calculations, the adsorbed water molecules are energetically favoured adsorbed on the atop sites, and the dimer adsorption is found to be the most stable with a peak adsorption energy of ～437 meV. Further analyses have revealed that the essential bonding interaction between the water monomer and the metal substrate is the hybridization of the water 3a₁-like molecular orbital with the (s, p_z) orbitals of the surface beryllium atoms. While in the case of the water dimer adsorption, the 1b₁-like orbital of the H₂O molecule plays a dominant role.     

The electronic structure of the valence bands of solid NH3 and H2O studied by ultraviolet photoelectron spectroscopy

The results of a photoelectron study using ultraviolet 40.81 eV photons (UPS) of the outermost bands of the molecular solids NH₃ and H₂O are reported. The binding energies, the energy separation, the band widths and the branching ratio of the two outermost bands of solid NH₃ are found not to be significantly different from the 3a_l and 1e molecular orbital states of the gaseous NH₃ UPS spectrum. This implies that hydrogen bonding has not produced any significant change in the electronic structure of the valence bands of solid NH₃. Because of a much smaller intermolecular hydrogen bond length in solid H₂O compared to that in solid NH₃, the hydrogen bond does, however, produce a significant change in the valence bands of H₂O on solidification, and because of the orbital geometry it predominantly affects the 3a_l molecular orbital state.     

UPS measurements of molecular energy level of condensed gases

There have been numerous attempts to use ultraviolet photoemission spcctroscopy (UPS) to monitor the chemical states of adsorbed gas molecules on metal surfaces. To interpret the data correctly, one has to determine the effect of photoemission on the measured energy levels of the molecule. We have measured the UPS spectra of seven gases (C₆H₆, C₅H₅N, CH₃OH, C₂H₅OH, H₂CO, H₂O, NH₃) condensed on a LN₂ cooled MoS₂ substrate at hv = 21.2 eV. The inertness of the MoS₂ substrate assures that no strong chemical bonding exists between the substrate and adsorbed molecules. For each gas, the spectrum of the condensed phase is similar to the corresponding spectrum of the gas phase except all the energy levels are shifted up by the same amount. This shift ranges from 1 to 1.65 eV for the gases studied. The energy shift is attributed to the dielectric screening of the hole produced during the optical excitation.     

SIMS,EID and flash-filament investigation of O2, H2, (O2 + H2) and H2O interaction with vanadium

Comparative investigations of secondary ion emission, electron induced ion emission and flash filament signals from polycrystalline vanadium surfaces exposed to well-defined O₂, H₂, H₂O and (O₂ + H₂) doses (<500 L) have been carried out. The vanadium target could be heated and bombarded by either electrons (300 eV) or ions (3 keV) under ultra high vacuum conditions (<10^?10 Torr). The investigations were carried out with a computer controlled ultra high vacuum mass spectrometer. The experimental results establish exact reproducible spectra of well defined surface layers. They give detailed insight into the reactions between H₂, O₂ H₂O and vanadium, and some interactions between these species. They further indicate the importance of bulk and surface diffusion as well as the influence of the probing ion and electron bombardment. A clear distinction between bulk oxygen, surface oxides, and adsorbed oxygen for the vanadium-oxygen interaction at room temperature could be established. For the interaction of hydrogen with clean and oxygen covered vanadium surfaces the formation of adsorbed hydrogen, bulk solution of hydrogen, and the formation of OH groups and H₂O could be demonstrated. A detection limit below 10^?5 of one single monolayer for metal bonded hydrogen could be established.     

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.     

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 H2O on clean and oxygen-dosed silver single crystal surfaces

The adsorption of H₂O on the surface of a single-crystal sphere of silver with exposed (111), (100) and (112) facets has been examined using ESDIAD (electron stimulated desorption ion angular distribution), LEED (low energy electron diffraction) and TDS (thermal desorption spectroscopy). The purpose of the study was (a) to examine the influence of substrate geometry for adsorption of H₂O on a metal surface for which the adsorbate-substrate interaction is weak, and (b) to study the influence of a surface impurity, oxygen, on the surface chemistry and local bonding structure of H₂O on Ag. We have found no evidence for either long-range or short-range local bonding order for adsorbed H₂O at 80 K on any of the surfaces studied. This appears to be a consequence, in part, of the lattice mismatch between the Ag crystal structure and the two-dimensional H₂O ice crystal structure. Adsorbed H₂O reacts with preadsorbed oxygen to form OH species which are bonded with the molecular axis perpendicular to Ag(111) and (100) but “inclined” on (112) surfaces, as identified using ESDIAD. The “inclined” OH species are associated with atomic steps on the (112) surface.     

甲醇分子在金红石TiO2(110)和锐钛矿TiO2(001)表面吸附和反应的活性位点

通过高分辨的扫描隧道显微术研究并比较了金红石型TiO₂(110)-(1×1)和锐钛矿型TiO₂(001)-(1×4)两种表面的活性位点． 在金红石型TiO₂(110)-(1×1)表面, 观察到氧空位缺陷是O₂和CO₂分子的活性吸附位点,而五配位的Ti原子是水分子和甲醇分子的光催化反应活性位点．在锐钛矿型TiO₂(001)-(1×4)表面,观察到完全氧化的表面,Ti原子更可能是六配位的,H₂O和O₂分子均不易在这些Ti原子上吸附．经还原后表面出现富Ti的缺陷位点, 这些缺陷位点对H₂O和O₂分子表现出明显的活性． 锐钛矿型TiO₂(001)-(1×4)表面的吸附和反应活性并不具有很高的活性,某种程度上其表现出的活性似乎低于金红石型TiO₂(110)-(1×1)表面．     

Adsorption of H2O on Al(111)

The adsorption of H₂O on Al(111) has been studied by ESDIAD (electron stimulated desorption ion angular distributions), LEED (low energy electron diffraction), AES (Auger electron spectroscopy) and thermal desorption in the temperature range 80–700 K. At 80 K, H₂O is adsorbed predominantly in molecular form, and the ESDIAD patterns indicate that bonding occurs through the O atom, with the molecular axis tilted away from the surface normal. Some of the H₂O adsorbed at 80 K on clean Al(111) can be desorbed in molecular form, but a considerable fraction dissociates upon heating into OH_ads and hydrogen, which leaves the surface as H₂. Following adsorption of H₂O onto oxygen-precovered Al(111), additional OH_ads is formed upon heating (perhaps via a hydrogen abstraction reaction), and H₂ desorbs at temperatures considerably higher than that seen for H₂O on clean Al(111). The general behavior of H₂O adsorption on clean and oxygen-precovered Al(111) (θ_O ? monolayer) is rather similar at low temperature, but much higher reactivity for dissociative adsorption of H₂O to form OH _adsis noted on the oxygen-dosed surface around room temperature.     

SrTiO3薄膜生长初期Sr，Ti，O原子分子反应机理的理论研究

在激光分子束外延(LMBE)生长SrTiO₃(STO)薄膜过程中,激光闪蒸出的Sr,Ti,O原子的微观反应过程及粒子形态是STO薄膜生长初期形成的关键.采用密度泛函理论中的广义梯度近似(DFT/GGA)方法,在PW91/DNP 水平上研究了Sr,Ti,O原子在真空中的优先反应过程和形态,计算研究了SrO,TiO₂和STO分子形成的反应机理,获得了相应的中间体和过渡态及反应活化能,并运用前线轨道理论分析了STO分子的形成机理.对比计算了STO分子可能的几何构型,得 关键词： 3薄膜')" href="#">SrTiO₃薄膜 反应机理 活化能     

Methyl orange adsorption by microporous and mesoporous TiO2-SiO2 and TiO2-SiO2-Al2O3 composite xerogels

We investigate the adsorption behavior of methyl orange (MO) on composite xerogels. Mesoporous TiO₂-SiO₂ and TiO₂-SiO₂-Al₂O₃ xerogels are prepared by hydrolysis of tetraethyl orthosilicate (TEOS), tetrabutyl orthotitanate (TBOT), and AlCl₃ ·6H₂O using HCl as a catalyst and cetyltrimethylammonium bromide (CTAB) as a templating agent. The corresponding microporous xerogels are also prepared without addition of CTAB. These xerogels are then characterized by XRD, FTIR, and BET.We find that MO is adsorbed by the mesoporous xerogels in acidic solutions, showing Langmuir type adsorption isotherms. We also find that the adsorption decreases with increasing temperature. It is suggested that adsorption exhibits a sieve effect and that MO is adsorbed through electrostatic attraction and hydrogen bonding between MO and the xerogels.     

The location of excess electrons on H2O/TiO2(110) surface and its role in the surface reactions

Excess electrons play a key role in many of the properties of Titanium dioxide (TiO₂). Understanding their behaviour is important for improving the performance of TiO₂ in energy-related applications. Here, we describe a DFT + U study of the locations of the unpaired electron (UPE) on rutile TiO₂(110) (R-TiO₂(110)) surface and H₂O/R-TiO₂(110) surface. Our results show that the subsurface are preferred with R-TiO₂(110) surface. In contrast to previous studies, we find that the UPE tends to migrate to the surface H₂O-Ti_5c (the five-coordinated titanium (Ti_5c) at surface with H₂O adsorption) with the increasing of H₂O coverage and UPE concentration. In addition, we have shown that the UPE plays an important role in the O-H bond dissociation and other important elementary reactions in photo-catalytic H₂O dissociation on R-TiO₂(110) such as H, OH and H₂ desorption. Specifically, it enhances the O-H bond dissociation, as well as H and H₂ desorption from bridging hydroxyl and Ti_5c-OH (the Ti_5c with OH adsorption), but hinders the OH and H desorption from Ti_5c. We believe our results afford a further understanding of the adsorbent dependent UPE migration, and the role of UPE in the surface reactions.     

Ab initio study of water adsorption on TiO2-terminated (100) surface of SrTiO3 with and without Cr doping

Water molecule adsorption on TiO₂-terminated (100) surface of SrTiO₃ with and without Cr doping is investigated by first principle calculation based on density functional theory. The band gap is shrunk compared with that of bulk due to the existence of defect states on the surface and 3d states of dopants. As a result the absorption energy edge is reduced and locates in the visible region. When adsorbed on the surface, energy levels of water molecules as a whole are lowered with respect to the Fermi energy, but the higher levels are split and electrons are transferred from low levels to high levels due to the decrease of the density of states in low energy region. Weak bonding is formed between water hydrogen atoms and surface oxygen atoms. This bonding causes the electron transferring from substrate to molecule and the occupation of the corresponding states.     

Au adsorption and Au-mediated charge transfer on the SrO-termination of SrTiO3 (0 0 1) surface

Atomic Au adsorption on the SrO-termination of SrTiO₃ (0 0 1) surface has been studied by means of the first-principles calculations based on the density functional theory (DFT). It indicates that charge polarization dominantly contributes to the bonding between Au and SrO-termination. Interfacial charge transfer induces dipole moment and changes work function. The mediating role Au played in charge transfer from electron-doped SrTiO₃:Nb to NO has been simulated. Charge transfer from SrTiO₃:Nb to Au is ascertained indicating that Au plays as an electron trapping center. SrO-termination has weak activity to NO while the molecule can be strongly adsorbed on negatively charged Au atom. It has been represented that Au mediates the charge transfer from SrTiO₃:Nb to NO. Antibonding orbital (π_2p*) of NO accommodates the charge and thus molecular bond is weakened (activated). Fukui functions demonstrate the role Au played in transiting the charge transfer from electronically excited SrTiO₃ to target species. Evidence that metal deposited on photocatalyst surface effectively separates the electron-hole pairs and improves the photocatalytic activity is presented in the current work.     

TiO2分子在GaN(0001)表面吸附的理论研究

采用基于密度泛函理论的平面波超软赝势法,计算了TiO₂分子在GaN(0001)表面的吸附成键过程、吸附能量和吸附位置. 计算结果表明不同初始位置的TiO₂分子吸附后,Ti在fcc或hcp位置,两个O原子分别与表面两个Ga原子成键,Ga-O化学键表现出共价键特征,化学结合能达到7.932-7.943eV,O-O连线与GaN[1120]方向平行,与实验观测(100)[001] TiO₂//(0001)[1120]GaN一致. 通过动力学过程计算分析,TiO₂分子吸附过程经历了物理吸附、化学吸附与稳定态形成的过程,稳定吸附结构和优化结果一致. 关键词： GaN(0001)表面 2分子')" href="#">TiO₂分子 密度泛函理论 吸附