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The interaction of water with metal oxide surfaces is of key importance to several research fields and applications. Because of its ability to photo-catalyze water splitting, reducible anatase TiO2 (a-TiO2) is of particular interest. Here, we combine experiments and theory to study the dissociation of water on bulk-reduced a-TiO2(101). Following large water exposures at room temperature, point-like protrusions appear on the a-TiO2(101) surface, as shown by scanning tunneling microscopy (STM). These protrusions originate from hydroxyl pairs, consisting of terminal and bridging OH groups, OHt/OHb, as revealed by infrared reflection absorption spectroscopy (IRRAS) and valence band experiments. Utilizing density functional theory (DFT) calculations, we offer a comprehensive model of the water/a-TiO2(101) interaction. This model also explains why the hydroxyl pairs are thermally stable up to ∼480 K. [ABSTRACT FROM AUTHOR]

The growth rate of crystalline ice (CI) in amorphous solid water (ASW) films was investigated using reflection absorption infrared spectroscopy. Two different experiments were set up to measure rates of the crystallization front propagation from the underlying crystalline template upward and from the vacuum interface downward. In one set of experiments, layers of ASW (5% D2O in H2O) were grown on a CI template and capped with a decane layer. In isothermal experiments from 140 to 150 K, crystallization was observed from the onset (no induction time) and the extent of crystallization increased linearly with time. In a second set of experiments, uncapped ASW films without a CI template were studied. The films were created by placing a 100 ML isotopic layer (5% D2O in H2O) at various positions in a 1000 ML ASW (H2O) film. The CI growth rates obtained from the two configurations (capped films with a CI template and uncapped films without a CI template) are in quantitative agreement. The results support the idea that for ASW films in a vacuum, a crystalline layer forms at the surface that then acts as a CI template for a growth front that moves downward into the film. [ABSTRACT FROM AUTHOR]

The crystallization kinetics of transiently heated, nanoscale water films were investigated for 188 K < Tpulse < 230 K, where Tpulse is the maximum temperature obtained during a heat pulse. The water films, which had thicknesses ranging from approximately 15–30 nm, were adsorbed on a Pt(111) single crystal and heated with ∼10 ns laser pulses, which produced heating and cooling rates of ∼109–1010 K/s in the adsorbed water films. Because the ice growth rates have been measured independently, the ice nucleation rates could be determined by modeling the observed crystallization kinetics. The experiments show that the nucleation rate goes through a maximum at T = 216 K ± 4 K, and the rate at the maximum is 1029±1 m−3 s−1. The maximum nucleation rate reported here for flat, thin water films is consistent with recent measurements of the nucleation rate in nanometer-sized water drops at comparable temperatures. However, the nucleation rate drops rapidly at lower temperatures, which is different from the nearly temperature-independent rates observed for the nanometer-sized drops. At T ∼ 189 K, the nucleation rate for the current experiments is a factor of ∼104−5 smaller than the rate at the maximum. The nucleation rate also decreases for Tpulse > 220 K, but the transiently heated water films are not very sensitive to the smaller nucleation rates at higher temperatures. The crystallization kinetics are consistent with a "classical" nucleation and growth mechanism indicating that there is an energetic barrier for deeply supercooled water to convert to ice. [ABSTRACT FROM AUTHOR]

Isotopic exchange reactions in mixed D2O and H2O amorphous solid water (ASW) films were investigated using reflection absorption infrared spectroscopy. Nanoscale films composed of 5% D2O in H2O were deposited on Pt(111) and graphene covered Pt(111) substrates. At 130 K, we find that the reaction is strongly dependent on the substrate with the H/D exchange being significantly more rapid on the Pt(111) surface than on graphene. At 140 K, the films eventually crystallize with the final products on the two substrates being primarily HOD molecule on Pt(111) and a mixture of HOD and unreacted D2O on graphene. We demonstrate by pre-dosing H2 and O2 on Pt(111) that the observed differences in reactivity on the two substrates are likely due to the formation of hydrogen ions at the Pt(111) surface that are not formed on graphene. Once formed the mobile protons move through the ASW overlayer to initiate the H/D exchange reaction. [ABSTRACT FROM AUTHOR]

A pulsed laser heating system has been developed that enables investigations of the dynamics and kinetics of nanoscale liquid films and liquid/solid interfaces on the nanosecond time scale in ultrahigh vacuum (UHV). Details of the design, implementation, and characterization of a nanosecond pulsed laser system for transiently heating nanoscale films are described. Nanosecond pulses from a Nd:YAG laser are used to rapidly heat thin films of adsorbed water or other volatile materials on a clean, well-characterized Pt(111) crystal in UHV. Heating rates of ~1010 K/s for temperature increases of ~100-200 K are obtained. Subsequent rapid cooling (~5 × 109 K/s) quenches the film, permitting insitu, post-heating analysis using a variety of surface science techniques. Lateral variations in the laser pulse energy are ~±2.7% leading to a temperature uncertainty of ~±4.4 K for a temperature jump of 200 K. Initial experiments with the apparatus demonstrate that crystalline ice films initially held at 90 K can be rapidly transformed into liquid water films with T > 273 K. No discernable recrystallization occurs during the rapid cooling back to cryogenic temperatures. In contrast, amorphous solid water films heated below the melting point rapidly crystallize. The nanosecond pulsed laser heating system can prepare nanoscale liquid and supercooled liquid films that persist for nanoseconds per heat pulse in an UHV environment, enabling experimental studies of a wide range of phenomena in liquids and at liquid/solid interfaces. [ABSTRACT FROM AUTHOR]

We have examined the adsorption of the weakly bound species N2, O2, CO, and Kr on the (√37 × √37)R25.3° water monolayer on Pt(111) using a combination of molecular beam dosing, infrared reflection absorption spectroscopy, and temperature programmed desorption. In contrast to multilayer crystalline ice, the adsorbate-free water monolayer is characterized by a lack of dangling OH bonds protruding into the vacuum (H-up). Instead, the non-hydrogen-bonded OH groups are oriented downward (H-down) to maximize their interaction with the underlying Pt(111) substrate. Adsorption of Kr and O2 have little effect on the structure and vibrational spectrum of the "√37" water monolayer while adsorption of both N2, and CO are effective in "flipping" H-down water molecules into an H-up configuration. This "flipping" occurs readily upon adsorption at temperatures as low as 20 K and the water monolayer transforms back to the H-down, √37 structure upon adsorbate desorption above 35 K, indicating small energy differences and barriers between the H-down and H-up configurations. The results suggest that converting water in the first layer from H-down to Hup is mediated by the electrostatic interactions between the water and the adsorbates. [ABSTRACT FROM AUTHOR]

Low-energy (100 eV) electron-stimulated reactions in layered H2O/CO/H2O ices are investigated. For CO layers buried in amorphous solid water (ASW) films at depths of 50 monolayers (ML) or less from the vacuum interface, both oxidation and reduction reactions are observed. However, for CO buried more deeply in ASW films, only the reduction of CO to methanol is observed. Experiments with layered films of H2O and D2O show that the hydrogen atoms participating in the reduction of the buried CO originate in the region that is 10-50 ML below the surface of the ASW films and subsequently diffuse through the film. For deeply buried CO layers, the CO reduction reactions quickly increase with temperature above ∼60 K. We present a simple chemical kinetic model that treats the diffusion of hydrogen atoms in the ASW and sequential hydrogenation of the CO to methanol to account for the observations. [ABSTRACT FROM AUTHOR]

Six H-bonds in the periodic di-interstitial structure that accounts for scanning tunneling microscope images of '[formula]' and '[formula]' wetting layers on Pt(111) are some 0.2 Å shorter than H-bonds are in ice Ih. According to a broadly obeyed correlation, this density functional theory result implies a stringent test of the di-interstitial motif, namely the presence of an OH-stretch band red-shifted from that of ice Ih by more than 1000 cm-1. Infrared absorption spectra satisfy the test, in showing a feature centered at about 1965 cm-1, which grows in as deposited water orders. [ABSTRACT FROM AUTHOR]

The low-energy, electron-stimulated desorption (ESD) of molecular products from amorphous solid water (ASW) films capped with methanol is investigated versus methanol coverage (0–4×1015 cm-2) at 50 K using 100 eV incident electrons. The major ESD products from a monolayer (ML) of methanol on ASW are quite similar to the ESD products from bulk methanol film: H2, CH4, H2O, C2H6, CO, CH2O, and CH3OH. For 40 ML ASW films, the molecular oxygen, hydrogen, and water ESD yields from the ASW are suppressed with increasing methanol coverage, while the CH3OH ESD yield increases proportionally to the methanol coverage. The suppression of the water ESD products by methanol is consistent with the nonthermal reactions occurring preferentially at or near the ASW/vacuum interface and not in the interior of the film. The water and molecular hydrogen ESD yields from the water layer decrease exponentially with the methanol cap coverage with 1/e constants of ∼6×1014 and 1.6×1015 cm-2, respectively. In contrast, the O2 ESD from the water layer is very efficiently quenched by small amounts of methanol (1/e∼6.5×1013 cm-2). The rapid suppression of O2 production by small amounts of methanol is due to reactions between CH3OH and the precursors for the O2—mainly OH radicals. A kinetic model for the O2 ESD, which semiquantitatively accounts for the observations, is presented. [ABSTRACT FROM AUTHOR]

Electron-stimulated reactions in thin [<3 ML (monolayer)] water films adsorbed on TiO2(110) are investigated. Irradiation with 100 eV electrons results in electron-stimulated dissociation and electron-stimulated desorption (ESD) of adsorbed water molecules. The molecular water ESD yield increases linearly with water coverage θ for 0<=θ<=1 ML and 1<θ<=2 ML. However, for θ>1 ML, the water ESD yield per additional water molecule adsorbed (i.e., the slope of the ESD yield versus coverage) is 3.5 times larger than for θ<1 ML. In contrast, the number of water molecules dissociated per incident electron increases linearly for θ<=2 ML without changing slope at θ=1 ML. The total electron-stimulated sputtering rate, as measured by postirradiation temperature programed desorption of the remaining water, is larger for θ>1 ML due to the increased water ESD for those coverages. The water ESD yields versus electron energy (for 5–50 eV) are qualitatively similar for 1, 2, and 40 ML water films. In each case, the observed ESD threshold is at ∼10 eV and the yield increases monotonically with increasing electron energy. The results indicate that excitations in the adsorbed water layer are primarily responsible for the ESD in thin water films on TiO2(110). Experiments on “isotopically layered” films with D2O adsorbed on the Ti4+ sites (D2OTi) and H2O adsorbed on the bridging oxygen atoms (H2OBBO) demonstrate that increasing the water coverage above 1 ML rapidly suppresses the electron-stimulated desorption of D2OTi and D atoms, despite the fact that the total water ESD and atomic hydrogen ESD yields increase with increasing coverage. The coverage dependence of the electron-stimulated reactions is probably related to the different bonding geometries for H2OTi and H2OBBO and its influence on the desorption probability of the reaction products. [ABSTRACT FROM AUTHOR]

The growth of crystalline ice films on Pt(111) and Pd(111) is investigated using temperature programed desorption of the water films and of rare gases adsorbed on the water films. The water monolayer wets both Pt(111) and Pd(111) at all temperatures investigated [e.g., 20–155 K for Pt(111)]. However, crystalline ice films grown at higher temperatures (e.g., T>135 K) do not wet the monolayer. Similar results are obtained for crystalline ice films of D2O and H2O. Amorphous water films, which initially wet the surface, crystallize and dewet, exposing the water monolayer when they are annealed at higher temperatures. Thinner films crystallize and dewet at lower temperatures than thicker films. For samples sputtered with energetic Xe atoms to prepare ice crystallites surrounded by bare Pt(111), subsequent annealing of the films causes water molecules to diffuse off the ice crystallites to reform the water monolayer. A simple model suggests that, for crystalline films grown at high temperatures, the ice crystallites are initially widely separated with typical distances between crystallites of ∼14 nm or more. The experimental results are consistent with recent theory and experiments suggesting that the molecules in the water monolayer form a surface with no dangling OH bonds or lone pair electrons, giving rise to a hydrophobic water monolayer on both Pt(111) and Pd(111). [ABSTRACT FROM AUTHOR]

The low-energy, electron-stimulated production of molecular oxygen from thin amorphous solid water (ASW) films adsorbed on Pt(111) is investigated. For ASW coverages less than ∼60 ML, the O2 electron-stimulated desorption (ESD) yield depends on coverage in a manner that is very similar to the H2 ESD yield. In particular, both the O2 and H2 ESD yields have a pronounced maximum at ∼20 ML due to reactions at the Pt/water interface. The O2 yield is dose dependent and several precursors (OH, H2O2, and HO2) are involved in the O2 production. Layered films of H216O and H218O are used to profile the spatial distribution of the electron-stimulated reactions leading to oxygen within the water films. Independent of the ASW film thickness, the final reactions leading to O2 occur at or near the ASW/vacuum interface. However, for ASW coverages less than ∼40 ML, the results indicate that dissociation of water molecules at the ASW/Pt interface contributes to the O2 production at the ASW/vacuum interface presumably via the generation of OH radicals near the Pt substrate. The OH (or possibly OH-) segregates to the vacuum interface where it contributes to the reactions at that interface. The electron-stimulated migration of precursors to the vacuum interface occurs via transport through the hydrogen bond network of the ASW without motion of the oxygen atoms. A simple kinetic model of the nonthermal reactions leading to O2, which was previously used to account for reactions in thick ASW films, is modified to account for the electron-stimulated migration of precursors. [ABSTRACT FROM AUTHOR]

The growth of amorphous solid water (ASW) films on Pt(111) is investigated using rare gas (e.g., Kr) physisorption. Temperature programmed desorption of Kr is sensitive to the structure of thin water films and can be used to assess the growth modes of these films. At all temperatures that are experimentally accessible (20–155 K), the first layer of water wets Pt(111). Over a wide temperature range (20–120 K), ASW films wet the substrate and grow approximately layer by layer for at least the first three layers. In contrast to the ASW films, crystalline ice films do not wet the water monolayer on Pt(111). Virtually identical results were obtained for ASW films on epitaxial Pd(111) films grown on Pt(111). The desorption rates of thin ASW and crystalline ice films suggest that the relative free energies of the films are responsible for the different growth modes. However, at low temperatures, surface relaxation or “transient mobility” is primarily responsible for the relative smoothness of the films. A simple model of the surface relaxation semiquantitatively accounts for the observations. [ABSTRACT FROM AUTHOR]

The electron-stimulated sputtering of thin amorphous solid water films deposited on Pt(111) is investigated. The sputtering appears to be dominated by two processes: (1) electron-stimulated desorption of water molecules and (2) electron-stimulated reactions leading to the production of molecular hydrogen and molecular oxygen. The electron-stimulated desorption of water increases monotonically with increasing film thickness. In contrast, the total sputtering—which includes all electron-stimulated reaction channels—is maximized for films of intermediate thickness. The sputtering yield versus thickness indicates that erosion of the film occurs due to reactions at both the water/vacuum interface and the Pt/water interface. Experiments with layered films of D2O and H2O demonstrate significant loss of hydrogen due to reactions at the Pt/water interface. The electron-stimulated sputtering is independent of temperature below ∼80 K and increases rapidly at higher temperatures. [ABSTRACT FROM AUTHOR]

We have measured the electron-stimulated desorption (ESD) of D2, O2, and D2O, the electron-stimulated dissociation of D2O at the D2O/Pt interface, and the total electron-stimulated sputtering in thin D2O films adsorbed on Pt(111) as a function of the D2O coverage (i.e., film thickness). Qualitatively different behavior is observed above and below a threshold coverage of ∼2 monolayers (ML). For coverages less than ∼2 ML electron irradiation results in D2O ESD and some D2 ESD, but no detectible reactions at the water/Pt interface and no O2 ESD. For larger coverages, electron-stimulated reactions at the water/Pt interface occur, O2 is produced and the total electron-stimulated sputtering of the film increases. An important step in the electron-stimulated reactions is the reaction between water ions (generated by the incident electrons) and electrons trapped in the water films to form dissociative neutral molecules. However, the electron trapping depends sensitively on the water coverage: For coverages less than ∼2 ML, the electron trapping probability is low and the electrons trap preferentially at the water/vacuum interface. For larger coverages, the electron trapping increases and the electrons are trapped in the bulk of the film. We propose that the coverage dependence of the trapped electrons is responsible for the observed coverage dependence of the electron-stimulated reactions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The electron-stimulated production of molecular hydrogen (D2, HD, and H2) from amorphous solid water (ASW) deposited on Pt(111) is investigated. Experiments with isotopically layered films of H2O and D2O are used to profile the spatial distribution of the electron-stimulated reactions leading to hydrogen within the water films. The molecular hydrogen yield has two components that have distinct reaction kinetics due to reactions that occur at the ASW/Pt interface and the ASW/vacuum interface, but not in the bulk. However, the molecular hydrogen yield as a function of the ASW film thickness in both pure and isotopically layered films indicates that the energy for the reactions is absorbed in the bulk of the films and electronic excitations migrate to the interfaces where they drive the reactions. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The structure and lattice dynamics of two-layer crystalline ice films grown on a hydrophobic substrate, graphene on Pt(111), are studied with low energy electron diffraction, reflection-absorption infrared spectroscopy, rare-gas adsorption/desorption and ab initio molecular dynamics. The results have shown that the two-layer ice has formed even at zero pressure at a single hydrophobic interface by maximizing the number of hydrogen bonds at the expense of adopting a nontetrahedral geometry with weakened hydrogen bonds.

The radiation-induced decomposition and desorption of nanoscale amorphous solid water (D2O) films adsorbed on an α-Al2O3(0001) surface was studied at low temperature in ultrahigh vacuum using temperature programmed desorption (TPD) and electron stimulated desorption (ESD) with a mono-energetic, low energy electron source. ESD yields of molecular products (D2, O2 and D2O) and the total sputtering yield increased with increasing D2O coverage up to ∼15 water monolayers (i.e.∼15 × 1015 cm−2) to a coverage-independent level for thicker water films. Experiments with isotopically-layered water films (D2O and H2O) demonstrated that the highest water decomposition yields occurred at the interfaces of the nanoscale water films with the alumina substrate and vacuum. However, the increased reactivity of the water/alumina interface is relatively small compared to the enhancements in the non-thermal reactions previously observed at the water/Pt(111) and water/TiO2(110) interfaces. We propose that the relatively low activity of Al2O3(0001) for the radiation-induced production of molecular hydrogen is associated with lower reactivity of this surface with hydrogen atoms, which are likely precursors for the formation of molecular hydrogen. [ABSTRACT FROM AUTHOR]

Understanding deeply supercooled water is key to unraveling many of water's anomalous properties. However, developing this understanding has proven difficult due to rapid and uncontrolled crystallization. Using a pulsed-laser-heating technique, we measure the growth rate of crystalline ice, G(T), for 180 K < T < 262 K, that is, deep within water's "no man's land" in ultrahigh-vacuum conditions. Isothermal measurements of G(T) are also made for 126 K ≤ T ≤ 151 K. The self-diffusion of supercooled liquid water, D(T), is obtained from G(T) using the Wilson-Frenkel model of crystal growth. For T > 237 K and P ~ 10-8 Pa, G(T) and D(T) have super-Arrhenius ("fragile") temperature dependences, but both cross over to Arrhenius ("strong") behavior with a large activation energy in no man's land. The fact that G(T) and D(T) are smoothly varying rules out the hypothesis that liquid water's properties have a singularity at or near 228 K at ambient pressures. However, the results are consistent with a previous prediction for D(T) that assumed no thermodynamic transitions occur in no man's land. [ABSTRACT FROM AUTHOR]