Origin of hydroxyl pair formation on reduced anatase TiO2(101).

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.

Direct visualization of radiation-induced transformations at alkali halide-air interfaces.

Radiation driven reactions at mineral/air interfaces are important to the chemistry of the atmosphere, but experimental constraints (e.g. simultaneous irradiation, in situ observation, and environmental control) leave process understanding incomplete. Using a custom atomic force microscope equipped with an integrated X-ray source, transformation of potassium bromide surfaces to potassium nitrate by air radiolysis species was followed directly in situ at the nanoscale. Radiolysis initiates dynamic step edge dissolution, surface composition evolution, and ultimately nucleation and heteroepitaxial growth of potassium nitrate crystallites mediated by surface diffusion at rates controlled by adsorbed water. In contrast to in situ electron microscopy and synchrotron-based imaging techniques where high radiation doses are intrinsic, our approach illustrates the value of decoupling irradiation and the basis of observation.

Observation of Molecular Hydrogen Produced from Bridging Hydroxyls on Anatase TiO2(101).

Anatase TiO2 is used extensively in a wide range of catalytic and photocatalytic processes and is a promising catalyst for hydrogen production. Here, we show that molecular hydrogen was produced from bridging hydroxyls (HOb) on the (101) surface of single-crystal anatase (TiO2(101)). This stands in contrast to rutile TiO2(110), where HOb pairs react to form H2O. Electron bombardment at 30 K produced bridging oxygen vacancies in the surface. Deuterated bridging hydroxyls (DOb) were subsequently formed via dissociation of adsorbed D2O and confirmed by infrared reflection-absorption spectroscopy. During temperature-programmed desorption (TPD) spectroscopy, D2 desorption was observed at 520 K. Density functional theory calculations show that both H2 and H2O production from HOb are endothermic at 0 K on TiO2(101), but H2 (H2O) desorption is entropically driven above 230 K (800 K). The calculated activation barrier for H2 desorption is 1.40 eV, which is similar to the desorption energy obtained from analysis of the D2 TPD spectra. The H2 desorption likely proceeds in two steps: H atom diffusion on the surface and then recombination.

Crystallization growth rates and front propagation in amorphous solid water films.

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.

Homogeneous ice nucleation rates and crystallization kinetics in transiently-heated, supercooled water films from 188 K to 230 K.

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.

Communication: Proton exchange in low temperature co-mixed amorphous H2O and D2O films: The effect of the underlying Pt(111) and graphene substrates.

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.

Electron-stimulated reactions in nanoscale water films adsorbed on α-Al2O3(0001).

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.

Homogeneous Nucleation of Ice in Transiently-Heated, Supercooled Liquid Water Films.

We have investigated the nucleation and growth of crystalline ice in 0.24 µm thick, supercooled water films adsorbed on Pt(111). The films were transiently heated with ∼10 ns infrared laser pulses, which produced typical heating and cooling rates of ∼109-1010 K/s. The crystallization of these water films was monitored with infrared spectroscopy. The experimental conditions were chosen to suppress ice nucleation at both the water/metal and water/vacuum interfaces. Furthermore, internal pressure increases due to curvature effects are precluded in these flat films. Therefore, the experiments were sensitive to the homogeneous ice nucleation rate from ∼210 to 225 K. The experiments show that Jmax, the maximum for the homogeneous ice nucleation rate, J(T), needs to be ≥1026 m-3 s-1 and is likely to be ∼1029±2 m-3 s-1. We argue that such large nucleation rates are consistent with experiments on hyperquenched glassy water, which typically have crystalline fractions of ∼1% or more.

Adsorption and Photodesorption of CO from Charged Point Defects on TiO2(110).

The adsorption and photochemistry of CO on rutile TiO2(110) are studied with scanning tunneling microscopy (STM), temperature-programmed desorption, and angle-resolved photon-stimulated desorption (PSD) at low temperatures. Site occupancies, when weighted by the concentration of each kind of adsorption site on the reduced surface, show that the adsorption probability is the highest for the bridging oxygen vacancies (VO). The probability distribution for the different adsorption sites corresponds to very small differences in CO adsorption energies (<0.02 eV). UV irradiation stimulates diffusion and desorption of CO at low temperature. CO photodesorbs primarily from the vacancies with a bimodal angular distribution, indicating some scattering from the surface, which also leads to photostimulated diffusion. Hydroxylation of VO's does not significantly change the CO PSD yield or the angular distribution, which suggests that photodesorption can be initiated by recombination of photogenerated holes with excess electrons localized near the charged point defect (either VO or bridging hydroxyl).

Growth rate of crystalline ice and the diffusivity of supercooled water from 126 to 262 K.

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.

A nanosecond pulsed laser heating system for studying liquid and supercooled liquid films in ultrahigh vacuum.

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 ∼10(10) K/s for temperature increases of ∼100-200 K are obtained. Subsequent rapid cooling (∼5 × 10(9) K/s) quenches the film, permitting in-situ, 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.

Complete Wetting of Pt(111) by Nanoscale Liquid Water Films.

The melting and wetting of nanoscale crystalline ice films on Pt(111) that are transiently heated above the melting point in ultrahigh vacuum (UHV) using nanosecond laser pulses are studied with infrared reflection absorption spectroscopy and Kr temperature-programmed desorption. The as-grown crystalline ice films consist of nanoscale ice crystallites embedded in a hydrophobic water monolayer. Upon heating, these crystallites melt to form nanoscale droplets of liquid water. Rapid cooling after each pulse quenches the films, allowing them to be interrogated with UHV surface science techniques. With each successive heat pulse, these liquid drops spread across the surface until it is entirely covered with a multilayer water film. These results, which show that nanoscale water films completely wet Pt(111), are in contrast to molecular dynamics simulations predicting partial wetting of water drops on a hydrophobic water monolayer. The results provide valuable insights into the wetting characteristics of nanoscale water films on a clean, well-characterized, single-crystal surface.

Anticorrelation between surface and subsurface point defects and the impact on the redox chemistry of TiO2(110).

By using a combination of scanning tunneling microscopy (STM), density functional theory (DFT), and secondary-ion mass spectroscopy (SIMS), we explored the interplay and relative impact of surface versus subsurface defects on the surface chemistry of rutile TiO2 . STM results show that surface O vacancies (VO ) are virtually absent in the vicinity of positively charged subsurface point defects. This observation is consistent with DFT calculations of the impact of subsurface defect proximity on VO formation energy. To monitor the influence of such lateral anticorrelation on surface redox chemistry, a test reaction of the dissociative adsorption of O2 was employed and was observed to be suppressed around them. DFT results attribute this to a perceived absence of intrinsic (Ti), and likely extrinsic interstitials in the nearest subsurface layer beneath inhibited areas. We also postulate that the entire nearest subsurface region could be devoid of any charged point defects, whereas prevalent surface defects (VO ) are largely responsible for mediation of the redox chemistry at the reduced TiO2 (110).

Turning things downside up: adsorbate induced water flipping on Pt(111).

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 H-up is mediated by the electrostatic interactions between the water and the adsorbates.

Electron-stimulated reactions in layered CO/H2O films: hydrogen atom diffusion and the sequential hydrogenation of CO to methanol.

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.

Probing the photochemistry of chemisorbed oxygen on TiO2(110) with Kr and other co-adsorbates.

Weakly-bound atoms and molecules (Ar, Kr, Xe, CO, CH4, CO2, CH3OH, N2O, and N2) are used to probe the photochemical interactions of chemisorbed oxygen on rutile TiO2(110). Ultraviolet irradiation of chemisorbed oxygen co-adsorbed with the probe species leads to photon-stimulated desorption (PSD) of some of the probe species (e.g. Kr and CH4), but not others (e.g. CO2 and N2O). Without chemisorbed oxygen, the PSD yields of all the probe species are very low or not observed. Surprisingly, both chemisorbed O2 and oxygen adatoms, Oa, are photo-active for desorption of Kr and other weakly-bound species. To our knowledge, this is the first evidence for photo-activity of Oa on TiO2(110). The Kr PSD yield increases with increasing coverage of Kr and of chemisorbed oxygen. For Kr, the angular distribution of the photodesorbed atoms is approximately cosine. The Kr distribution is quite different from the angular distribution for the O2 PSD, which is sharply peaked along the surface normal. We propose that various forms of chemisorbed oxygen are excited by reactions with electrons and/or holes created in the TiO2 substrate by UV photon irradiation. The photo-excited oxygen collides with, and transfers energy to, neighboring co-adsorbed atoms or molecules. For co-adsorbates with a small enough binding energy to the substrate, desorption may result. The observed phenomenon provides a new tool for studying photochemical processes.

Multiple Nonthermal Reaction Steps for the Photooxidation of CO to CO2 on Reduced TiO2(110).

The photooxidation of CO on reduced, rutile TiO2(110) is studied on a millisecond time scale. For CO coadsorbed with a saturation coverage of chemisorbed O2 (θsat), the CO2 photon-stimulated desorption (PSD) signal is initially zero, increases to a maximum after several tens of milliseconds, and then decreases at longer times. The initial CO2 PSD signal increases ∼5 times more quickly for an oxygen coverage of 0.5θsat. The initial rate of increase of the CO2 PSD signal is proportional to the flux of UV photons. The results show that two or more nonthermal reaction steps are required to photooxidize CO adsorbed on TiO2(110). The intermediate species involved in the reactions is stable for at least 100 s at 30 K. Previous models had suggested that CO photooxidation required only one nonthermal reaction. The likely initial and final charge states of the system suggest that an electron-mediated reaction and a hole-mediated reaction are needed for the complete photooxidation reaction.

Thermal and nonthermal physiochemical processes in nanoscale films of amorphous solid water.

Amorphous solid water (ASW) is a disordered version of ice created by vapor deposition onto a cold substrate (typically less than 130 K). It has a higher free energy than the crystalline phase of ice, and when heated above its glass transition temperature, it transforms into a metastable supercooled liquid. This unusual form of water exists on earth only in laboratories, after preparation with highly specialized equipment. It is thus fair to ask why there is any interest in studying such an esoteric material. Much of the scientific interest results from the ability to use ASW as a model system for exploring the physical and reactive properties of liquid water and aqueous solutions. ASW is also thought to be the predominant form of water in the extremely cold temperatures of many interstellar and planetary environments. In addition, ASW is a convenient model system for studying the stability of amorphous and glassy materials as well as the properties of highly porous materials. A fundamental understanding of such properties is invaluable in a diverse range of applications, including cryobiology, food science, pharmaceuticals, astrophysics, and nuclear waste storage, among others. Over the past 15 years, we have used molecular beams and surface science techniques to probe the thermal and nonthermal properties of nanoscale films of ASW. In this Account, we present a survey of our research on the properties of ASW using this approach. We use molecular beams to precisely control the deposition conditions (flux, incident energy, and incident angle) and create compositionally tailored, nanoscale films of ASW at low temperatures. To study the transport properties (viscosity and diffusivity), we heat the amorphous films above their glass transition temperature, T(g), at which they transform into deeply supercooled liquids prior to crystallization. The advantage of this approach is that at temperatures near T(g), the viscosity is approximately 15 orders of magnitude larger than that of a normal liquid. As a result, the crystallization kinetics are dramatically slowed, increasing the time available for experiments. For example, near T(g), a water molecule moves less than the distance of a single molecule on a typical laboratory time scale (∼1000 s). For this reason, nanoscale films help to probe the behavior and reactions of supercooled liquids at these low temperatures. ASW films can also be used for investigating the nonthermal reactions relevant to radiolysis.

Polarization- and Azimuth-Resolved Infrared Spectroscopy of Water on TiO2(110): Anisotropy and the Hydrogen-Bonding Network.

We have investigated the structure and dynamics of thin water films adsorbed on TiO2(110) using infrared reflection-absorption spectroscopy (IRAS) and ab initio molecular dynamics. Infrared spectra were obtained for s- and p-polarized light with the plane of incidence parallel to the [001] and [11̅0] azimuths for water coverages ≤ 4 monolayers. The spectra indicate strong anisotropy in the water films. The vibrational densities of states predicted by the ab initio simulations for 1 and 2 monolayer coverages agree well with the observations. The results provide new insight into the structure of water on TiO2(110) and resolve a long-standing puzzle regarding the hydrogen bonding between molecules in the first and second monolayers on this surface. The results also demonstrate the capabilities of polarization- and azimuth-resolved IRAS for investigating the structure and dynamics of adsorbates on dielectric substrates.

Adsorption Geometry of CO versus Coverage on TiO2(110) from s- and p-Polarized Infrared Spectroscopy.

The adsorption of CO on reduced, rutile TiO2(110) is investigated using IR reflection-absorption spectroscopy and temperature-programmed desorption. Experiments using s- and p-polarized IR light incident along the [001] and [11̅0] azimuths give detailed information on the adsorption geometry of the CO as a function of the CO coverage, θCO. The results indicate that for θCO ≤ 1 ML, CO adsorbs oriented perpendicular to the surface at Ti5c sites. For 1 < θCO ≤ 1.5 ML, the bonding geometry of the CO adsorbed at Ti5c sites is unchanged, whereas the additional CO molecules adsorb at Ob sites parallel to the surface and parallel to the [11̅0] azimuth. The results do not support previous suggestions that CO at Ti5c sites tilt ∼20° from normal at high coverages. The results demonstrate the utility of polarization-resolved infrared reflection-absorption spectroscopy for elucidating adsorption geometries on dielectric substrates.