The dehydrogenation of ethylene on Pt(111) was studied by scanning tunneling microscopy (STM) under ultra-high vacuum conditions. Previous experiments have shown that thermal dehydrogenation following saturation exposure of ethylene on Pt(111) resulted in the formation of well-defined carbon clusters. The aggregation to form the carbon clusters leaves open Pt areas that could be available for additional adsorption. It had not been previously determined whether the adsorption and dehydrogenation of additional ethylene would lead to the growth of the initial clusters or the nucleation of additional clusters of the same size. The present study confirms previous reports that the initial carbon clusters are 15 ± 2 Å in diameter, 2.5 ± 0.3 Å in height, and contain an average of 34 ± 9 carbon atoms per cluster. We show that exposing this surface to additional ethylene at room temperature and annealing leads to an increase in the number of particles of the same size, with no growth in size of the initial particles. Dosing and dehydrogenation/annealing cycles were repeated until the dehydrogenation activity of the Pt(111) surface was completely suppressed, which occurred after the fourth such cycle. Continued cycling leads to the beginning of the formation of a graphite adlayer on the platinum, presumably via agglomeration of the clusters at a high cluster density.

Ozone is known to readily oxidize the heavier alkali halides in the form of sea salt aerosols, and this chemistry is thought to be a probable candidate for the formation of reactive gas-phase halogens in the troposphere. Water that becomes adsorbed at the interface following the oxidation process is believed to play a vital role in the mechanisms that release these gas-phase halogens. We have carried out studies that utilize X-ray photoemission spectroscopy and atomic force microscopy to follow the surface chemistry and topography of KIO3 films as they are exposed to water vapor at room temperature. The KIO3 films were grown under dry conditions by the heterogeneous reaction of ozone with a model low defect density KI(100) single crystal. As the water vapor pressure is increased above the surface dissolution point, the KIO3 layer becomes solvated and ionic mobility at the surface increases. This mobility results in a ripening process that creates large crystallites of KIO3. The inhomogeneous KIO3 layer continues to evolve at relative humidities near the bulk deliquescence point of the KI(100) substrate. Under these conditions, surface-adsorbed water molecules dissociate at defect sites generated during the initial heterogeneous reaction process and provide the necessary protons to initiate a Dushman-like surface reaction, where IO3− and I− react to release gas-phase iodide compounds. Vacancies created in lattice sites during the release of gas-phase iodide species are replaced by OH− groups to form a stable adlayer of KOH (which is likely partially hydrated). As a result of the KOH × H2O adlayer, which does not react further with O3, only a portion of the particle’s iodide content is available for reaction.

It is now well established by numerous experimental and computational studies that the adsorption propensities of inorganic anions conform to the Hofmeister series. The adsorption propensities of inorganic cations, such as the alkali metal cations, have received relatively little attention. Here we use a combination of liquid-jet X-ray photoelectron experiments and molecular dynamics simulations to investigate the behavior of K+ and Li+ ions near the interfaces of their aqueous solutions with halide ions. Both the experiments and the simulations show that Li+ adsorbs to the aqueous solution−vapor interface, while K+ does not. Thus, we provide experimental validation of the “surfactant-like” behavior of Li+ predicted by previous simulation studies. Furthermore, we use our simulations to trace the difference in the adsorption of K+ and Li+ ions to a difference in the resilience of their hydration shells.

It is now well established by numerous experimental and computational studies that the adsorption propensities of inorganic anions conform to the Hofmeister series. The adsorption propensities of inorganic cations, such as the alkali metal cations, have received relatively little attention. Here we use a combination of liquid-jet X-ray photoelectron experiments and molecular dynamics simulations to investigate the behavior of K+ and Li+ ions near the interfaces of their aqueous solutions with halide ions. Both the experiments and the simulations show that Li+ adsorbs to the aqueous solution−vapor interface, while K+ does not. Thus, we provide experimental validation of the “surfactant-like” behavior of Li+ predicted by previous simulation studies. Furthermore, we use our simulations to trace the difference in the adsorption of K+ and Li+ ions to a difference in the resilience of their hydration shells.

It is now well established by numerous experimental and computational studies that the adsorption propensities of inorganic anions conform to the Hofmeister series. The adsorption propensities of inorganic cations, such as the alkali metal cations, have received relatively little attention. Here we use a combination of liquid-jet X-ray photoelectron experiments and molecular dynamics simulations to investigate the behavior of K+ and Li+ ions near the interfaces of their aqueous solutions with halide ions. Both the experiments and the simulations show that Li+ adsorbs to the aqueous solution−vapor interface, while K+ does not. Thus, we provide experimental validation of the “surfactant-like” behavior of Li+ predicted by previous simulation studies. Furthermore, we use our simulations to trace the difference in the adsorption of K+ and Li+ ions to a difference in the resilience of their hydration shells.

It is now well established by numerous experimental and computational studies that the adsorption propensities of inorganic anions conform to the Hofmeister series. The adsorption propensities of inorganic cations, such as the alkali metal cations, have received relatively little attention. Here we use a combination of liquid-jet X-ray photoelectron experiments and molecular dynamics simulations to investigate the behavior of K+ and Li+ ions near the interfaces of their aqueous solutions with halide ions. Both the experiments and the simulations show that Li+ adsorbs to the aqueous solution−vapor interface, while K+ does not. Thus, we provide experimental validation of the “surfactant-like” behavior of Li+ predicted by previous simulation studies. Furthermore, we use our simulations to trace the difference in the adsorption of K+ and Li+ ions to a difference in the resilience of their hydration shells.

We report molecular dynamics simulations of acetonitrile–water binary solutions at concentrations of 0.032–0.59 mole fraction. We find that at low bulk concentration acetonitrile has an enhanced population near the liquid/vapor interface. The surface-bound acetonitrile molecules exhibit anisotropic orientations and lie nearly flat along the solution surface with their terminal methyl groups directed toward the vapor. Upon increasing the bulk concentration, the formation of acetonitrile domains is promoted by interactions between hydrophobic methyl moieties. Dipole–dipole interactions facilitate a pseudonematic, antiparallel pairing of near-neighbor molecules both in the bulk solution and near the liquid/vapor interface. Near the interface the preferred orientation of acetonitrile flattens further to accommodate antiparallel pairing of neighboring molecules such that the methyl group remains above the solution. This study paints a surprisingly complex picture of a binary organic–water solution that manifests behavior similar to liquid crystals through preferred orientations and pseudonematic antiparallel pairing.

Titania has attracted significant interest due to its broad catalytic applications, many of which involve titania nanoparticles in contact with aqueous electrolyte solutions. Understanding the titania nanoparticle/electrolyte interface is critical for the rational development of such systems. Here, we have employed liquid-jet ambient pressure X-ray photoelectron spectroscopy (AP-XPS) to investigate the solid/electrolyte interface of 20 nm diameter TiO2 nanoparticles in 0.1 M aqueous nitric acid solution. The Ti 2p line shape and absolute binding energy reflect a fully oxidized stoichiometric titania lattice. Further, by increasing the X-ray excitation energy, the difference in O 1s binding energies between that of liquid water (O 1sliq) and the titania lattice (O 1slat) oxygen was measured as a function of probe depth into the particles. The titania lattice, O 1slat, binding energy decreases by 250 meV when probing from the particle surface into the bulk. This is interpreted as downward band bending at the interface.

We report photoelectron spectroscopy measurements from binary acetonitrile–water solutions, for a wide range of acetonitrile mole fractions (xCH3CN = 0.011–0.90) using a liquid microjet. By detecting the nitrogen and carbon 1s photoelectron signal of CH3CN from aqueous surface and bulk solution, we quantify CH3CN’s larger propensity for the solution surface as compared to bulk solution. Quantification of the strong surface adsorption is through determination of the surface mole fraction as a function of bulk solution, xCH3CN, from which we estimate the adsorption free energy using the Langmuir adsorption isotherm model. We also discuss alternative approaches to determine the CH3CN surface concentration, based on analysis of the relative amount of gas- versus liquid-phase CH3CN, obtained from the respective photoelectron signal intensities. Another approach is based on the core-level binding energy shifts between liquid- and gas-phase CH3CN, which is sensitive to the change in solution surface potential and thus sensitive to the surface concentration of CH3CN. Gibbs free energy of adsorption values are compared with previous literature estimates, and we consider the possibility of CH3CN bilayer formation. In addition, we use the observed changes in N 1s and C 1s peak positions to estimate the net molecular surface dipole associated with a complete CH3CN surface monolayer, and discuss the implications for orientation of CH3CN molecules relative to the solution surface.

In the work described here, the electronic structure of sulfuric acid in water is explored by liquid-jet photoelectron spectroscopy. From the S2p photoelectron spectra of H2SO4 (aq), measured over a large concentration range and aided by previously reported HSO4–/SO42- and HSO4–/H2SO4 concentration ratios in the bulk solution, we obtain detailed electronic structure information of each species. Comparing our results with previous studies on the dissociation of nitric acid, we argue that the solvation structure of H2SO4 (aq) changes around 5–7 M concentration, at which point a dramatic change in both the HSO4– photoelectron peak width and binding energy occurs.

Ambient pressure X-ray photoelectron spectroscopy (AP-XPS) was used to explore ion behavior at liquid/vapor interfaces of aqueous NaCl, RbCl, and RbBr solutions. Interfacial depth profiles of ions were obtained from XPS spectra at a series of photoelectron kinetic energies. Depth profiles of the ratio of anion to cation show little difference among the solutions. Previously, these depth profiles were determined from the ratio of anion to cation signal-peak areas. However, using molecular dynamics simulations (MD), the individual anion and cation depth profiles are both observed to differ as a function of solution, but the differences are masked when only the anion-to-cation ratios are considered. Using the Cl–/Owater ratio determined from the XPS measurements, surface-enhanced concentrations of Cl– are observed in the NaCl solution, but not in the RbCl solution, in agreement with predictions from MD simulations. We also report studies of aqueous solutions of RbBr. In contrast to an aqueous RbCl solution, our combination of AP-XPS experiments and MD simulations suggests that anion/cation ratios are enhanced at the surface for this system due to the separation of bromide and rubidium in the double layer near the surface, while the interfacial concentration of bromide does not differ considerably from the bulk.

Ordered linear arrays of titanium dioxide nanoparticles were fabricated on highly oriented pyrolytic graphite utilizing a step edge decoration method. Ag- or Pt-based nanoparticles were then photodeposited onto the titanium dioxide nanoparticles (∼18 nm) to simultaneously verify photocatalytic activity and to demonstrate a viable route to load the titanium dioxide nanoparticles with metals. Scanning electron microscopy and atomic force microscopy determined the morphology, size, and distribution of the particles. X-ray photoelectron spectroscopy confirmed the identity of the titanium dioxide nanoparticles, and transmission electron microscopy showed that some of the particles were rutile single crystals. Energy-dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy determined the chemical composition of the metal-based nanoparticles selectively loaded on the linear arrays of titanium dioxide nanoparticles.Keywords: Ag; HOPG; photodeposition; Pt; step decoration; TiO2

Nitric acid is a prevalent component of atmospheric aerosols, and the extent of nitric acid dissociation at aqueous interfaces is relevant to its role in heterogeneous atmospheric chemistry. Several experimental and theoretical studies have suggested that the extent of dissociation of nitric acid near aqueous interfaces is less than that in bulk solution. Here dissociation of HNO3 at the surface of aqueous solution is quantified using X-ray photoelectron spectroscopy of the nitrogen local electronic structure. The relative amounts of undissociated HNO3(aq) and dissociated NO3–(aq) are identified by the distinguishable N1s core-level photoelectron spectra of the two species, and we determine the degree of dissociation, αint, in the interface (approximately the first three layers of solution) as a function of HNO3 concentration. Our measurements show that dissociation is decreased by ∼20% near the solution interface compared with bulk solution and furthermore that dissociation occurs in the topmost solution layer. The experimental results are supported by first-principles MD simulations, which show that hydrogen bonds between HNO3 and water molecules at the solution surface stabilize the molecular form even at low concentration by analogy to the stabilization of molecular HNO3 that occurs in bulk solution at high concentration.

Thin-film water is ubiquitous in nature, occurring on virtually all surfaces exposed to the ambient environment. In particular, alkali halide salts below their deliquescence point are expected to be coated with water films from one molecular layer to a few nanometers thick. While salt ion mobility in thin-film water has been characterized in the literature, little is known about the chemistry occurring within these films. Here we investigate the surface chemistry change of a mixed bromine salt (KBr/KBrO3) using X-ray photoelectron spectroscopy, secondary electron microscopy, and energy-dispersive X-ray spectroscopy. At 68% relative humidity, the Br− surface concentration was observed to deplete with increasing water vapor exposure time. Known bulk solution kinetics for the reaction of Br− + BrO3− has a second-order dependence on H+ concentrations. However, in the present experiments there was no addition of an external acid. These results suggest that the pH and chemical reactions within thin-film water are uniquely differently from bulk solution. Because bromine chemistry in the atmosphere is strongly influenced by pH, these results have implications for the cycling of bromine where thin-film water is present.

We have used a combination of temperature-programmed desorption (TPD) experiments and molecular dynamics (MD) simulations to characterize interactions between water and carboxylic acid-terminated surfaces. In the TPD experiments, D2O water interacts with alkylthiol self-assembled monolayers (SAMs) comprised of 3-mercaptopropionic acid (C3 acid), 15-mercaptopentadecaonic acid (C15 acid), 16-mercaptohexadenoic acid (C16 acid), or two-component monolayers of these constituents. Water TPD spectra exhibit broad, first-order desorption profiles with maximum desorption temperatures ranging from 168 K during desorption from the C16 acid to a maximum desorption temperature of 200 K when water desorbs from the C3 acid surface. For water desorption from the C15 acid and for the two-component surfaces, desorption traces adopt intermediate profiles between these two extremes. Desorption activation energies range from 42 and 50 kJ mol−1. In the MD studies, submonolayer concentrations of adsorbed water interact with slabs that simulate the one- and two-component SAMs employed in the TPD experiments as well as 4-mercaptobutanoic acid (C4 acid). The MD simulations are characterized by distributions of the water−carboxylic acid interaction energies and the orientation of the carbonyl groups. The water−carboxylic acid interaction energy distributions show excellent qualitative agreement with the desorption activation energy values determined from the TPD experiments. Analysis of the carbonyl group orientation from the MD simulations shows strong effects of SAM chain length and whether the SAM contains an odd or even number of carbon atoms. These “odd−even” and chain length effects support many of the results from the TPD experiments and the MD simulations. We discuss the effectiveness of employing MD simulations in concert with TPD studies as well as the atmospheric implications for the interaction of water with highly oxidized, multicomponent surfaces.

Depth-resolved ion spatial distributions of nitrate and nitrite anions in aqueous solution have been quantitatively measured using X-ray photoemission spectroscopy on a 15 μm aqueous liquid jet containing 3 M NaNO3, 3 M NaNO2, or an equimolar mixture of the two. The surface region, which extends to photoelectron kinetic energies of 400−500 eV, is partially depleted in anions relative to the bulk 3 M concentration. The nitrate and nitrite solutions exhibit similar depth-dependent anion profiles. The results presented here are compared with recent molecular dynamics simulations of a NaNO3 solution and are found to agree at high photoelectron kinetic energies. At shallower probe depths, the experiment measured a surface anion concentration less than that predicted by theory. Possible origins of the discrepancy are discussed in terms of the confined size of the simulation box and uncertainties that remain in regard to the inelastic mean free path of photoelectrons in aqueous media. The importance of our findings is discussed in relation to the observed increase in photochemical activity of nitrate-containing aerosols in the atmosphere.

The heterogeneous surface reaction of OH with dry KI(100) results in iodide vacancies in the surface lattice sites that are filled with OH to generate a stable layer of KOH. Under high-vacuum conditions, in which surface ions are not mobile, the reaction is self-passivating and generates two molecular layers of potassium hydroxide, releasing 1.6 × 1016 iodide ions per cm2 of surface area. Reaction rates are identical with those of NaI(100). A similar surface reaction occurs with alkali bromides (KBr(100)), albeit at a much slower rate to generate approximately one-tenth of a monolayer of KOH, whereas no observable reaction occurs with KCl(100) under the conditions of this experiment. The heterogeneous reaction of OH with alkali halides is found to be dependent solely on the identity of the halide anion and independent of the alkali metal cation with the relative reaction rates following the anion ordering, I− > Br− > Cl−. The release of halide-containing species is expected to impact the chemistry of the marine boundary layer.

X-Ray photoemission spectroscopy operating under ambient pressure conditions is used to probe ion distributions throughout the interfacial region of a free-flowing aqueous liquid micro-jet of 6 M potassium fluoride. Varying the energy of the ejected photoelectrons by carrying out experiments as a function of X-ray wavelength measures the composition of the aqueous–vapor interfacial region at various depths. The F− to K+ atomic ratio is equal to unity throughout the interfacial region to a depth of 2 nm. The experimental ion profiles are compared with the results of a classical molecular dynamics simulation of a 6 M aqueous KF solution employing polarizable potentials. The experimental results are in qualitative agreement with the simulations when integrated over an exponentially decaying probe depth characteristic of an APPES experiment. First principles molecular dynamics simulations have been used to calculate the potential of mean force for moving a fluoride anion across the air–water interface. The results show that the fluoride anion is repelled from the interface, consistent with the depletion of F− at the interface revealed by the APPES experiment and polarizable force field-based molecular dynamics simulation. Together, the APPES and MD simulation data provide a detailed description of the aqueous–vapor interface of alkali fluoride systems. This work offers the first direct observation of the ion distribution at an aqueous potassium fluoride solution interface. The current experimental results are compared to those previously obtained for saturated solutions of KBr and KI to underscore the strong difference in surface propensity between soft/large and hard/small halide ions in aqueous solution.

Arrays of linear, one-dimensional (1D) silver nanoparticle rows have been synthesized that demonstrate strong surface enhanced Raman scattering (SERS) that is dependent on the polarization of the incident electromagnetic radiation. Ordered arrays of 1D rows of spherical silver nanoparticles were fabricated on highly oriented pyrolytic graphite (HOPG) by physical vapor deposition (PVD) at 400 °C. Scanning electron microscopy confirmed the formation of arrays of highly parallel rows of nanoparticles. The rows are typically hundreds of microns long with particle gaps less than 10 nm and 10−1000 nm spacing between adjacent 1D rows. The polarization dependence of the SERS was characterized using thiophenol as a Raman probe molecule that was adsorbed as a monolayer on the silver nanoparticle surfaces. When incident light is polarized along the axis of the nanoparticle rows, the intensity of the Raman-scattered light was ≈20 times stronger than Raman scattered light when the incident radiation was polarized perpendicular to the axis of the nanoparticle rows. This polarization selectivity is in good agreement with our calculations that explore the electromagnetic response of the interacting nanoparticles with an incident light field.

The crystal structure of KIO3 grown by heterogeneous surface oxidation of KI (001) with ozone is reported. Under ambient reaction conditions (RH ∼35%, room temperature) a thick layer of KIO3 grows at the gas−solid interface. Two doublets are present in the I(4d) X-ray photoelectron spectroscopy structure measurements, characteristic of unreacted KI (I−) from the substrate and the oxidized KIO3 (I5+) reaction product. X-ray diffraction measurements confirm the presence at the interface of randomly oriented polycrystalline-triclinic KIO3 with an average particle diameter of 15 nm. KIO3 particle diameters determined from the X-ray diffraction peak widths are consistent with the results of atomic force microscopy. There is no X-ray powder diffraction evidence to suggest that the underlying KI substrate is altered in any manner during this heterogeneous interfacial reaction.

The chemistry of Br species associated with sea salt ice and aerosols has been implicated in the episodes of ozone depletion reported at Arctic sunrise. However, Br− is only a minor component in sea salt, which has a Br−/Cl− molar ratio of ∼0.0015. Sea salt is a complex mixture of many different species, with NaCl as the primary component. In recent years experimental and theoretical studies have reported enhancement of the large, more polarizable halide ion at the liquid/vapor interface of corresponding aqueous alkali halide solutions. The proposed enhancement is likely to influence the availability of sea salt Br− for heterogeneous reactions such as those involved in the ozone depletion episodes. We report here ambient pressure X-ray photoelectron spectroscopy studies and molecular dynamics simulations showing direct evidence of Br− enhancement at the interface of an aqueous NaCl solution doped with bromide. The experiments were carried out on samples with Br−/Cl− ratios in the range 0.1% to 10%, the latter being also the ratio for which simulations were carried out. This is the first direct measurement of interfacial enhancement of Br− in a multicomponent solution with particular relevance to sea salt chemistry.

The reaction of ozone with the (100) plane of solid potassium iodide (KI) was investigated using atomic force microscopy (AFM). The reaction forming potassium iodate (KIO3) initiates at step edges prior to reacting on the flat terraces. Small domains of KIO3, initially 3.8 Å in height, are formed on the top of step edges. Following reaction at the step edge, domains of KIO3 are formed across the terraces. With prolonged exposure to ozone, KIO3 domains nucleate further growth until the surface is evenly covered with KIO3 particles that are 4−6 nm in height, at which point the surface is passivated and the reaction terminates.

Abstract

It has been suggested that enhanced anion concentrations at the liquid/vapor interface of airborne saline droplets are important to aerosol reactions in the atmosphere. We report ionic concentrations in the surface of such solutions. Using x-ray photoelectron spectroscopy operating at near ambient pressure, we have measured the composition of the liquid/vapor interface for deliquesced samples of potassium bromide and potassium iodide. In both cases, the surface composition of the saturated solution is enhanced in the halide anion compared with the bulk of the solution. The enhancement of anion concentration is more dramatic for the larger, more polarizable iodide anion. By varying photoelectron kinetic energies, we have obtained depth profiles of the liquid/vapor interface. Our results are in good qualitative agreement with classical molecular dynamics simulations. Quantitative comparison between the experiments and the simulations indicates that the experimental results exhibit more interface enhancement than predicted theoretically.

X-Ray photoemission spectroscopy operating under ambient pressure conditions is used to probe ion distributions throughout the interfacial region of a free-flowing aqueous liquid micro-jet of 6 M potassium fluoride. Varying the energy of the ejected photoelectrons by carrying out experiments as a function of X-ray wavelength measures the composition of the aqueous–vapor interfacial region at various depths. The F− to K+ atomic ratio is equal to unity throughout the interfacial region to a depth of 2 nm. The experimental ion profiles are compared with the results of a classical molecular dynamics simulation of a 6 M aqueous KF solution employing polarizable potentials. The experimental results are in qualitative agreement with the simulations when integrated over an exponentially decaying probe depth characteristic of an APPES experiment. First principles molecular dynamics simulations have been used to calculate the potential of mean force for moving a fluoride anion across the air–water interface. The results show that the fluoride anion is repelled from the interface, consistent with the depletion of F− at the interface revealed by the APPES experiment and polarizable force field-based molecular dynamics simulation. Together, the APPES and MD simulation data provide a detailed description of the aqueous–vapor interface of alkali fluoride systems. This work offers the first direct observation of the ion distribution at an aqueous potassium fluoride solution interface. The current experimental results are compared to those previously obtained for saturated solutions of KBr and KI to underscore the strong difference in surface propensity between soft/large and hard/small halide ions in aqueous solution.