EMSL: Greg Kimmel's Publications

Akin MC, NG Petrik, and GA Kimmel. 2009. "Electron-Stimulated Reactions and O-2 Production in Methanol-Covered Amorphous Solid Water Films." Journal of Chemical Physics 130(10):Art. No. 104710. Abstract 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 x 1015 cm-2) at 50 K using 100 eV incident electrons. The major ESD products from a monolayer 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 non-thermal 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 ~ 0.6 x 1015 cm-2 and 1.6 x 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 x 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 semi-quantitatively accounts for the observations is presented.

Kimmel GA, J Matthiesen, M Baer, CJ Mundy, NG Petrik, RS Smith, Z Dohnalek, and BD Kay. 2009. "No Confinement Needed: Observation of a Metastable Hydrophobic Wetting Two-Layer Ice on Graphene." Journal of the American Chemical Society 131(35):12838-12844. Abstract The structure of water at interfaces is crucial for processes ranging from photocatalysis to protein folding. Here, we investigate the structure and lattice dynamics of two-layer crystalline ice films grown on a hydrophobic substrate - graphene on Pt(111) - with low energy electron diffraction, reflection-absorption infrared spectroscopy, rare-gas adsorption/desorption, and ab-initio molecular dynamics. Unlike hexagonal ice, which consists of stacks of puckered hexagonal "bilayers", this new ice polymorph consists of two flat hexagonal sheets of water molecules in which the hexagons in each sheet are stacked directly on top of each other. Such two-layer ices have been predicted for water confined between hydrophobic slits, but not previously observed. Our results show that the two-layer ice forms even at zero pressure at a single hydrophobic interface by maximizing the number of hydrogen bonds at the expense of adopting a non-tetrahedral geometry with weakened bonds.

Petrik NG, Z Zhang, Y Du, Z Dohnalek, I Lyubinetsky, and GA Kimmel. 2009. "Chemical Reactivity of Reduced TiO2(110): The dominant role of surface defects in oxygen chemisorption." Journal of Physical Chemistry C 113(28):12407-12411. doi:10.1021/jp901989x Abstract O2 chemisorption on reduced, rutile TiO2(110) with various concentrations of oxygen vacancies (Ov) and bridging hydroxyls (OHb) is investigated with scanning tunneling microscopy, temperature programmed desorption and electron-stimulated desorption. On the annealed surface, 2 oxygen molecules can be chemisorbed per Ov. The same amount of O2 chemisorbs on surfaces where each Ov is converted to two OHb’s by exposure to water (i.e. 1 O2 per OHb). Surfaces with few or no Ov’s or OHb’s can be created by exposing the hydroxylated surface to O2 at room temperature, and the amount of O2 that chemisorbs on these surfaces at low temperatures is only ~20% of the amount on the annealed (reduced) surface. In contrast, the amount of chemisorbed O2 increases by more than a factor of two when the OHb concentration is enhanced – without changing the concentration of sub-surface Ti interstitials. The results indicate that the reactivity of TiO2(110) is primarily controlled by the amount of electron-donating surface species such as Ov’s and/or OHb’s, and not Ti3+ interstitials.

Petrik NG, and GA Kimmel. 2009. "Nonthermal Water Splitting on Rutile TiO2: Electron-Stimulated Production of H-2 and O-2 in Amorphous Solid Water Films on TiO2(110)." Journal of Physical Chemistry C 113(11):4451-4460. Abstract Electron-stimulated desorption (ESD) of H2, O2 and H2O from 0 - 60 ML films of amorphous solid water (ASW) adsorbed on TiO2(110) are investigated as function of film thickness and isotopic composition. For 100 eV incident electrons, both the H2 and O2 ESD yields have maxima when the ASW coverage is ~ 20 monolayer (ML), while the H2O ESD yield increases monotonically with water coverage. All the products reach a coverage-independent yield above 40 - 50 ML. Experiments using isotopically layered films of H2O and D2O demonstrate that the molecular hydrogen is produced in reactions that occur preferentially at or near both the ASW/TiO2 interface and the ASW/vacuum interface. However, electronic excitations or ionic defects created within the interior of the ASW films by the energetic electrons can subsequently migrate to the interfaces where they initiate reactions. Electron irradiation of ASW films results in the formation of bridge-bonded hydroxyls on the TiO2(110). These hydroxyls do not contribute to the H2 produced near the ASW/TiO2 interface. Instead, the results suggest that this H2 is produced from a stable precursor, trapped near the substrate. The proposed mechanism for the H2 production near the ASW/TiO2(110) interface is supported by a kinetic model that semi-quantitatively reproduces the main features of the non-thermal reactions.

Zhang Z, Y Du, NG Petrik, GA Kimmel, I Lyubinetsky, and Z Dohnalek. 2009. "Water as a Catalyst: Imaging Reactions of O-2 with Partially and Fully Hydroxylated TiO2(110) Surfaces." Journal of Physical Chemistry C 113(5):1908-1916. Abstract The reactions of molecular oxygen with bridging hydroxyl groups, OHb, formed by H2O dissociation on bridging oxygen vacancies of TiO2 (110) are studied at low and high OHb coverages as a function of the O2 exposure, using scanning tunneling microscopy (STM), temperature programmed desorption (TPD), and electron simulated desorption (ESD) techniques. On partially hydroxylated surfaces, the sudden simultaneous disappearance of oxygen vacancies and oxygen adatoms formed by O2 dissociation is observed at high O2 exposures. On fully hydroxylated TiO2 surfaces, which enable us to compare results of STM, TPD and ESD studies, most of OHb’s are removed via reacting with O2. Hence, fully hydroxylated TiO2 surfaces can be converted to nearly stoichiometric surfaces, albeit with some amount of adsorbed molecular water. Formation of mobile H2O molecules and water-assisted diffusion of the reactants plays an important role in the kinetics of the processes on both partially and fully hydroxylated surfaces.

Grieves GA, NG Petrik, J Herring-Captain, B Olanrewaju, A Aleksandrov, RG Tonkyn, SE Barlow, GA Kimmel, and TM Orlando. 2008. "Photoionization of Sodium Salt Solutions in a Liquid Jet." Journal of Physical Chemistry C 112(22):8359-8364. doi:10.1021/jp7102534 Abstract A liquid microjet was employed to examine the gas/liquid interface of aqueous sodium halide (Na+X-, X=Cl, Br, I) salt solutions. Laser excitation at 193 nm produced and removed cations of the form H+(H2O)n and Na+(H2O)m from liquid jet surfaces containing either NaCl, NaBr or NaI. The protonated water cluster yield varied inversely with increasing salt concentration, while the solvated sodium ion cluster yield varied by anion type. The distribution of H+(H2O)n at low salt concentration is identical to that observed from low-energy electron irradiated amorphous ice and the production of these clusters can be accounted for using a localized ionization/Coulomb expulsion model. Production of Na+(H2O)m is not accounted for by this model but requires ionization of solvation shell waters and a contact ion/Coulomb expulsion mechanism. The reduced yields of Na+(H2O)m from high concentration (10-2 and 10-1 M) NaBr and NaI solutions indicate a propensity for Br- and I- at the solution surfaces and interfaces. This is supported by the observation of multiphoton induced production and desorption of Br+ and I+ from the 10-2 and 10-1 M solution surfaces.

Kimmel GA, and NG Petrik. 2008. "Tetraoxygen on Reduced Ti02(110): Oxygen Adsorption and Reactions with Oxygen Vacancies." Physical Review Letters 100(19):Art. No. 196102. doi:10.1103/PhysRevLett.100.196102 Abstract Oxygen adsorption on reduced TiO2(110) is investigated using temperature programmed desorption and electron-stimulated desorption. At low temperatures, two O2 molecules can be chemisorbed in each oxygen vacancy, Ov. These molecules do not desorb upon annealing to 700 K. Instead for 200 K < T < 400 K, the two O2 convert to another species which has four oxygen atoms, i.e. tetraoxygen, that decomposes at higher temperatures. In contrast when only 1 O2 is adsorbed in an oxygen vacancy, the molecule dissociates upon annealing above ~150 K to heal the vacancy in agreement with previous results.

Kimmel GA, NG Petrik, Z Dohnalek, and BD Kay. 2007. "Crystalline Ice Growth on Pt(111) and Pd(111): Nonwetting Growth on a Hydrophobic Water Monolayer." Journal of Chemical Physics 126(11):Art. No. 114702. Abstract The growth of crystalline water films on Pt(111) and Pd(111) is investigated using temperature programmed 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 monlayer 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).

Lane CD, NG Petrik, TM Orlando, and GA Kimmel. 2007. "Electron-Stimulated Oxidation of Thin Water Films Adsorbed on TiO2(110)." Journal of Physical Chemistry C 111(44):16319-16329. doi:10.1021/jp072479o Abstract Electron-stimulated reactions in thin (< 3 monolayer, ML) water films adsorbed on TiO2(110) are investigated. For electron fluences less than ~1×1016 e-/cm2, irradiation with 100 eV electrons results in electron-stimulated desorption (ESD) of atomic and molecular hydrogen, but no measurable O2. The ESD leaves adsorbed hydroxyls which oxidize the TiO2(110) surface and change the post-irradiation TPD spectra of the remaining water in characteristic ways. The species remaining on the TiO2(110) after irradiation of adsorbed water films are apparently similar to those produced without irradiation by co-dosing water and O2. Annealing above ~600 K reduces the oxidized surfaces, and water TPD spectra characteristic of ion sputtered and annealed TiO2(110) are recovered. The rate of electron-stimulated “oxidation” of the water films is proportional to the coverage of water in the first layer for coverages less than 1 ML. However, higher coverages suppress this reaction. When thin water films are irradiated, the rate of electron-stimulated oxidation is independent of the initial oxygen vacancy concentration, as is the final oxidized state achieved at high electron fluences. To explain the results, we propose that electron excitation of water molecules adsorbed on Ti4+ sites leads to desorption of hydrogen atoms and leaves an OH adsorbed at the site. If hydroxyls are present in the bridging oxygen rows, these react with the OH’s on the Ti4+ sites to reform water and heal the oxygen vacancy associated with the bridging OH. Once the bridge bonded hydroxyls have been eliminated, further irradiation increases the concentration of OH’s in the Ti4+ rows leading to the creation of species which block sites in the Ti4+ rows, perhaps H2O2 and/or HO2.

Lane CD, NG Petrik, TM Orlando, and GA Kimmel. 2007. "Site-Dependent Electron-Stimulated Reactions in Water Films on TiO2(110)." Journal of Chemical Physics 127(22):Art. No. 224706. Abstract Electron-stimulated reactions in thin (<3 monolayer, ML) 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 ESD yield for water molecules adsorbed on the bridging oxygen rows, H2OBBO, is 4-5 times greater than the ESD yield for water adsorbed on Ti4+ sites, H2OTi. In contrast, the probability for electron-stimulated dissociation of adsorbed water is comparable for both H2OTi and H2OBBO. The total electron-stimulated sputtering rate is larger for coverages greater than 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 D2OTi and 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.