Water Adsorption and Coadsorption with Potassium on Graphite(0001)

Water adsorption on graphite (0001)

Adsorption of water, ions, and biomolecules constitutes the first events occurring at biomaterial-biosystem interfaces. In this work, the adsorption and coadsorption of water and glycine on TiO2 were studied by thermal desorption spectroscopy (TDS). The first water monolayer desorbs in three peaks around 180K, 300K, and 400K, which are assigned to water molecularly adsorbed at oxygen sites, at Ti4+ sites, and to recombination of dissociated water, respectively. A fourth desorption peak (160K), appearing at coverages > 0.8 monolayer, is attributed to water clusters and multilayers. The water-TiO2 interaction is changed if the surface is annealed in vacuum, which leads to increased hydroxylation. Desorption spectra from glycine overlayers evaporated on TiO2 in situ show that around 40% of the first monolayer desorbs as intact molecules ( approximately 300-450 K) and the remainder as dissociation fragments and surface reaction products around 600 K. At coverages > 0.6 monolayers, intact molecules desorbing from cluster multilayers at 310 K are detected. The glycine desorption spectra are unaffected by coadsorbed water. In contrast, coadsorption of glycine displaces water from more strongly bound states in the monolayer to more weakly bound states and clusters, making the surface more hydrophobic. The study shows that TDS is a powerful method for characterizing biomaterial surfaces with regard to their interaction with biologically relevant molecules.

Coadsorption of water with either ammonia or hydrogen fluoride on an Ag(110) substrate has been studied to examine the relationships between intermolecular hydrogen bonding near the surface and preferential adsorption at the surface. The experiments were conducted in ultrahigh vacuum (UHV) with facilities for thermal desorption spectroscopy (TDS) and high-resolution electron energy loss spectroscopy (HREELS). Adsorption was performed at 110 K for ammonia and water and at 90 K for hydrogen fluoride and water. Ammonia and water are simply coadsorbed at 110 K with no evidence of specific hydrated complexes nor of ionization to NH4+. Coadsorbed water enhances the population of the chemisorbed state of ammonia, which we estimate to have a saturation coverage of 0.12 monolayer (ML) without water, through a mechanism of dielectric screening. At coverages approaching one monolayer and higher, water stabilizes the bulk of ammonia, increasing the average desorption temperature by as much as 21 K from 134 K. Stabilization energies estimated from these temperature shifts are of the order of 5 kJ mol–1, much less than typical hydrogen bond energies, ca. 25 kJ mol–1. In contrast, hydrogen fluoride and water form a well defined monohydrate HF · H2O, as evidenced by the coincident thermal desorption curves of both species. The monohydrate behaves as an adlayer azeotrope and thermal desorption measurements can be used to follow the sublimation temperature as a function of adlayer composition. From these data so-called bubble-point and dew-point curves can be developed by analogy with normal binary solutions.

Interaction of water with potassium on graphite: A HREELS study

The formation and reactivity of hydroxyl species originating from coadsorption of water and oxygen on Ni(110) single-crystal surfaces have been studied by using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). The resulting surface population of hydroxyl intermediates at a given water–oxygen coverage combination was found to be temperature-dependent. This was demonstrated by the differences in hydroxyl coverages determined by TPD and XPS: while the TPD data were determined to mostly reflect the maximum coverages that can be reached for a given set of gas exposures at low temperatures, the XPS results measure the OH coverages formed at the temperature of dosing. Our results indicate that, besides the stoichiometric and reversible H2O(ads) + O(ads) = 2OH(ads) step, a second water-decomposition reaction on the oxygen-precovered surface deposits additional hydroxyl adsorbates. Depletion of surface oxygen can be induced by thermal reaction with coadsorbed ammonia as well, a result that provides direct evidence for the OH(ads) disproportionation reaction.

Interaction of H 2O with the RuO 2(1 1 0) surface studied by HREELS and TDS

Ziel dieser Arbeit war es, die Reaktivität von V2O3(0001) zu untersuchen. In dieser Arbeit wird sich zunächst mit dem epitaktischen Wachstum von V2O3-Filmen auf Au(111)und W(110) befaßt. Stöchiometrie und Geometrie der dünnen Filme wurden mit Röntgenphotoelektronenspektroscopie (XPS), Röntgenabsorptionsspektroskopie (NEXAFS) und Beugung niederenergetischer Elektronen (LEED) charakterisiert. Wir haben gezeigt, dass die Oberfläche zwei Terminierungen aufweist, die sich durch die An- bzw. Abwesenheit von zusätzlichen Sauerstoffatomen auf der Oberfläche unterscheiden. Diese Sauerstoffatome bilden Vanadylgruppen mit den Oberflächenvanadiumatomen, deren Streckschwingung mit Infrarotabsorptionsspektroskopie (IRAS) detektiert werden kann. Die elektronische Struktur des V2O3(0001) dünnen Filmes wurde mittels UV-Photoelektronenspektroskopie (UPS), XPS und NEXAFS untersucht. Wir haben bewiesen, dass die Bildung von Vanadylgruppen an der Oberfläche einen Metall-Isolator Übergang hervorruft. Für jede Oberflächenterminierung wurde ein elektronenenergieverlustspektrum (HREELS) gezeigt und mit einem Spektrum des isomorphischen Cr2O3 verglichen. Wasseradsorptionsexperimente zeigen, dass Wasser sowohl molekular als auch dissoziativ auf beiden Oberflächen adsorbiert. Die Dissoziationswahrscheinlichkeit hängt von der Terminierung und von der Bedeckung ab. Sie ist am höchsten bei großer Bedeckung auf der -V=O Oberfläche. CO2 Adsorption wurde mit UPS, XPS, HREELS und IRAS untersucht. CO2 physisorbiert auf der -V=O Oberfläche. Den IRA Spektren entnehmen wir, dass CO2 auf der -V Oberfläche als gewinkelte Spezies adsorbiert. Heizen dieser Spezies auf 200 K führt zu Karbonatbildung. Die Adsorption von CO verhält sich ähnlich wie die von CO2: nur kleine Menge adsorbieren auf der -V=O Oberfläche, während die -V Oberfläche viel reaktiver zu sein scheint. Winkelaufgelöste UPS Messungen deuten auf eine flache CO Adsorptionsgeometrie auf der -V=O Oberfläche hin. NEXAF- und IRA-Spektren zeigen dagegen, dass bereits bei 90 K sich CO2 auf der -V Oberfläche bildet.

The interaction of water with the (001) surface of α-Cr 2O 3 was examined with temperature programmed desorption (TPD), high resolution electron energy-loss spectroscopy (HREELS) and X-ray photoelectron spectroscopy (XPS). Two α-Cr 2O 3(001) surfaces were examined, both of which were grown on α-Al 2O 3(001) substrates using oxygen plasma-assisted molecular beam epitaxy (MBE). The two surfaces differed in that one was grown with α-Fe 2O 3 interlayers whereas the other was grown directly on α-Al 2O 3(001). The in-plane lattice spacing of the α-Cr 2O 3(001) surface on α-Fe 2O 3/α-Al 2O 3(001) was 2% expansively strained relative to the unstrained α-Cr 2O 3(001) surface grown directly on α-Al 2O 3(001). Both the strained and unstrained surfaces exhibited similar water TPD behavior, with the possible exception that the desorption states of water on the strained surface were shifted slightly to lower temperatures relative to those on the unstrained surface. Water adsorbs on α-Cr 2O 3(001) in both molecular and dissociative states, with the former desorbing in TPD at 295 K and the latter at 345 K. TPD uptake measurements and XPS data suggest that each surface Cr 3+ atom has the capacity to bind two water molecules, one in a molecular state and one in a dissociative state. Water in the dissociative state is comprised of two distinct OH groups based on HREELS, one of which is a terminal group with a ν(OH) mode at 3600 cm −1 and the other of which is presumably a bridging group with a ν(OH) mode at 2885 cm −1. These losses shift to 2645 and 2120 cm −1 with D 2O adsorption. The low loss energy for the bridging OH/OD group indicates its involvement in a very strong hydrogen-bonded interaction with another species, presumably the oxygen atom of the terminal OH group. This pairing behavior is likely responsible for the first-order desorption kinetics observed for the recombinative desorption state at 345 K. The hydrogen-bonding interaction is unusually strong, as exemplified by the very low ν(OH) frequency for the bridging OH group. Studies on the oxygen pre-exposed surface indicate that oxygen atoms, formed by O 2 dissociation, block the H 2O dissociative channel but do not block the molecular adsorption channel.

Coadsorption of water and sodium on the Ru(001) surface

Coadsorption of water and selected aromatic molecules to model the adhesion of epoxy resins on hydrated surfaces of zinc oxide and iron oxide

Coadsorption of H2O and Cl was studied on a Ag(110) surface under conditions of ultrahigh vacuum with thermal desorption spectroscopy, low energy electron diffraction, and electron stimulated desorption ion angular distribution. The experiments were conducted over the temperature range of 100–650 K for water coverages ranging from zero to several multilayers and chlorine coverages θCl of 0–0.75 monolayers (ML). Water adsorption is stabilized by chlorine; the thermal desorption peak for water interacting with chlorine, called the α2 state, shifts to higher temperature by 25–40 K from the α1 state for desorption from the clean surface. A c(2×2) bilayer for H2O forms for coadsorption with less than 0.25 ML of Cl. The surface solvation number (SSN), defined as the number of stabilized water molecules per chlorine atom, varies from 13 to about 4 as θCl increases from 0 to 0.25. The unusually large SSN and the c(2×2) structure is evidence that Cl(a) promotes water adsorption to the metal surface itself in an effect called adsorbate-induced hydrophilicity. Coadsorption with higher chlorine coverages produces a p(4×3) structure for 0.25<θCl<0.4 and a c(4×4) structure for 0.4<θCl<0.5. Chlorine interacts directly with water in these structures in the form of surface solvation seen in previous studies of water coadsorption. The p(4×3) and c(4×4) patterns are evidence that coadsorbed water alters the distribution of chlorine on the surface. These results are interpreted in terms of the balance of forces among the two adsorbed species and the surface.

The adsorption of methanol and water on H-ZSM-5

An HREELS and TPD study of water on TiO 2(110): the extent of molecular versus dissociative adsorption

The adsorption, desorption, and dissociation of water on the GaAs(001)-(4×2) surface have been studied using Auger electron spectroscopy (AES), temperature-programmed desorption, and high-resolution electron energy loss spectroscopy. We have found that water first adsorbs molecularly at 100 K and dissociates readily upon annealing by virtue of overlapping desorption and dissociation temperatures between 150 and 200 K. The dissociation probability of water on the GaAs(001)-(4×2) surface is approximately 0.8 at low coverages (exposures below 0.5 L). However, the decomposition products of water exhibit a high recombination probability, making the oxidation of GaAs difficult. A large fraction of surface hydroxyls are rehydrogenated to produce desorbing water at temperatures between 300 and 700 K. Hence, we have applied a cycling treatment (repeated adsorption of water at 100 K followed by annealing to 750 K) in order to effectively oxidize the GaAs surface. During cycling, we have monitored GaAs–oxide growth using AES. In addition, thermal desorption spectra recorded after exposure of the cycling-treated GaAs surface to water at 100 K point to molecular adsorption and intact desorption of water with little evidence of dissociation, which suggests that the surface has been significantly oxidized by the cycling treatment of water.

We present an experimental and theoretical investigation of K atom desorption from the basal plane of graphite at 83 K induced by low energy photons (3–6 eV). The 2D potassium overlayer is characterized by low energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), thermal desorption spectroscopy (TDS), and work function measurements. At monolayer coverage (5.2×1014 atoms cm−2), the dependence of the cross section on photon energy has a threshold at ℏω≈3.0 eV and rises up to a maximum of 1.8±0.4×10−20 cm2 at 4.8 eV. The coverage dependence of the photoyield reflects the existence of two phases of adsorbed K, dilute ionized photo-active and close-packed photo-neutral, respectively. The observed photodesorption is a single-photon, nonthermal event, consistent with a substrate-mediated mechanism. The desorption results from attachment of optically excited hot electrons to the empty 4s state of ionized potassium. The theory predicts in this case a Gaussian line shape of the photoyield vs photon energy. Fitting the model parameters to the experimental data, we determine (i) the energy and slope of the excited state potential energy curve, and (ii) the position and width of the potassium-induced 4s resonance. The present findings combined with other available data for potassium on graphite are used to construct 1D potential energy curves along the surface normal for K+ and K0. The calculated cross sections for s- and p-polarized light are in qualitative agreement with the measurements.