John T. Newberg

The nature of water on surfaces of laboratory systems and implications for heterogeneous chemistry in the troposphere

A number of heterogeneous reactions of atmospheric importance occur in thin water films on surfaces in the earths boundary layer. It is therefore important to understand the interaction of water with various materials, both those used to study heterogeneous chemistry in laboratory systems, as well as those found in the atmosphere. We report here studies at 22 °C to characterize the interaction of water with such materials as a function of relative humidity from 0–100%. The surfaces studied include borosilicate glass, both untreated and after cleaning by three different methods (water, hydrogen peroxide and an argon plasma discharge), quartz, FEP Teflon film, a self assembled monolayer of n-octyltrichlorosilane (C8 SAM) on glass, halocarbon wax coatings prepared by two different methods, and several different types of Teflon coatings on solid substrates. Four types of measurements covering the range from the macroscopic level to the molecular scale were made: (1) contact angle measurements of water droplets on these surfaces to obtain macroscopic scale data on the water-surface interaction, (2) atomic force microscopy measurements to provide micron to sub-micron level data on the surface topography, (3) transmission FTIR of the surfaces in the presence of increasing water vapor concentrations to probe the interaction with the surface at a molecular level, and (4) X-ray photoelectron spectroscopy measurements of the elemental surface composition of the glass and quartz samples. Both borosilicate glass and the halocarbon wax coatings adsorbed significantly more water than the FEP Teflon film, which can be explained by a combination of the chemical nature of the surfaces and their physical topography. The C8 SAM, which is both hydrophobic and has a low surface roughness, takes up little water. The implications for the formation of thin water films on various surfaces in contact with the atmosphere, including building materials, soil, and vegetation, are discussed.

Experimental and theoretical investigation of the electronic structure of Cu2O and CuO thin films on Cu(110) using x-ray photoelectron and absorption spectroscopy

The electronic structure of Cu(2)O and CuO thin films grown on Cu(110) was characterized by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). The various oxidation states, Cu(0), Cu(+), and Cu(2+), were unambiguously identified and characterized from their XPS and XAS spectra. We show that a clean and stoichiometric surface of CuO requires special environmental conditions to prevent loss of oxygen and contamination by background water. First-principles density functional theory XAS simulations of the oxygen K edge provide understanding of the core to valence transitions in Cu(+) and Cu(2+). A novel method to reference x-ray absorption energies based on the energies of isolated atoms is presented.

Highly Compressed Two-Dimensional Form of Water at Ambient Conditions

The structure of thin-film water on a BaF2(111) surface under ambient conditions was studied using x-ray absorption spectroscopy from ambient to supercooled temperatures at relative humidity up to 95%. No hexagonal ice-like structure was observed in spite of the expected templating effect of the lattice-matched (111) surface. The oxygen K-edge x-ray absorption spectrum of liquid thin-film water on BaF2 exhibits, at all temperatures, a strong resemblance to that of high-density phases for which the observed spectroscopic features correlate linearly with the density. Surprisingly, the highly compressed, high-density thin-film liquid water is found to be stable from ambient (300 K) to supercooled (259 K) temperatures, although a lower-density liquid would be expected at supercooled conditions. Molecular dynamics simulations indicate that the first layer water on BaF2(111) is indeed in a unique local structure that resembles high-density water, with a strongly collapsed second coordination shell.

Reaction of bromide with bromate in thin-film water.

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/KBrO(3)) 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(-) + BrO(3)(-) 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.

In Situ XPS for Evaluating Ceria Oxidation States in SOFC Anodes

Ceria is being considered as an electrocatalyst component for fuelflexible SOFC anodes. With Ce 3+ and Ce 4+ oxidation states, ceria can function as a mixed ionic-electronic conductor, transporting both O 2- ions and electrons. It remains unclear what role these oxidation states play during SOFC operation. This study addresses these issues using ambient-pressure XPS in a single-chamber cell to characterize thin-film CeO2-x electrodes on a YSZ electrolyte during electrochemical oxidation of H2 and reduction of H2O at 620 °C. Voltage-current curves for the ceria film electrodes reveal sharp changes in resistance as a function of applied voltage from positive to negative bias. XPS measurements during electrochemical characterization show the extent of near-surface Ce 4+ reduction to Ce 3+ with increasing cell voltage. Increased oxidation to Ce 4+ at large negative voltages correlates with a resistance drop due to H2 oxidation activity. These findings provide insight into how CeO2-x influences electrochemical fuel oxidation or H2O reduction.

Silver Deposition onto Modified Silicon Substrates

Trimethylphosphine(hexafluoroacetylacetonato)silver(I) was used as a precursor to deposit silver onto silicon surfaces. The deposition was performed on silicon-based substrates including silica, H-terminated Si(100), and OH-terminated (oxidized) Si(100). The deposition processes at room temperature and elevated temperature (350 °C) were compared. The successful deposition resulted in nanostructures or nanostructured films as confirmed by atomic force microscopy (AFM) and scanning electron microscopy (SEM) with metallic silver being the majority deposited species as confirmed by X-ray photoelectron spectroscopy (XPS). The reactivity of the precursor depends drastically not only on the temperature of the process but also on the type of substrate. Density functional theory (DFT) was used to explain these differences and to propose the mechanisms for the initial deposition steps.

Water Dissociation on MnO(1x1)/Ag(100)

In this work we utilize experimental and simulation techniques to examine the molecular level interaction of water with a MnO(1 × 1) thin film deposited onto Ag(100). The formation of MnO(1 × 1)/Ag(100) was characterized by low energy electron diffraction and scanning tunneling microscopy. Density functional theory (DFT) shows MnO(1 × 1) is thermodynamically more stable than MnO(2 × 1) by ∼0.4 eV per MnO. Upon exposure to 2.5 Torr water vapor at room temperature, X-ray photoemission spectroscopy results show extensive surface hydroxylation attributed to reactivity at MnO(1 × 1) terrace sites. DFT calculations of a water monomer on MnO(1 × 1)/Ag(100) show the dissociated form is energetically more favorable than molecular adsorption, with a hydroxylation activation barrier 0.4 eV per H2O. These results are discussed and contrasted with previous studies of MgO/Ag(100) which show a stark difference in behavior for water dissociation.

Investigation of the influence of oxygen plasma on supported silver nanoparticles

Silver deposition precursor molecule trimethylphosphine(hexafluoroacetylacetonato)silver(I) [(hfac)AgP(CH3)3] was used to deposit silver onto water-modified (hydroxyl-terminated) solid substrates. A silicon wafer was used as a model flat surface, and water-predosed ZnO nanopowder was investigated to expand the findings to a common substrate material for possible practical applications. Following the deposition, oxygen plasma was used to remove the remaining organic ligands on a surface and to investigate its effect on the morphology of chemically deposited silver nanoparticles and films. A combination of microscopic and spectroscopic techniques including electron microscopy and x-ray photoelectron spectroscopy was used to confirm the change in the morphology of the deposited material consistent with Ostwald ripening as a result of plasma treatment. Particle agglomeration was observed on the surfaces, and the deposited metallic silver was oxidized to Ag2O following plasma treatment. The fluorine-containing ligands were completely removed. This result suggests that chemical vapor deposition can be used to deposit silver in a very controlled manner onto a variety of substrates using different topography methods and that the post-treatment with oxygen plasma is effective in preparing materials deposited for potential practical applications.

Synthesis and Characterization of ZnO/CuO Vertically Aligned Hierarchical Tree-like Nanostructure

Vertically aligned ZnO nanowire-based tree-like structures with CuO branches were synthesized on the basis of a multistep seed-mediated hydrothermal approach. The nanotrees form a p-n junction at the branch/stem interface that facilitates charge separation upon illumination. Photoelectrochemical measurements in different solvents show that ZnO/CuO hierarchical nanostructures have enhanced photocatalytic activity compared to that of the nonhierarchical structure of ZnO/CuO, pure ZnO, and pure CuO nanoparticles. The combination of ZnO and CuO in tree-like nanostructures provides opportunities for the design of photoelectrochemical sensors, photocatalytic synthesis, and solar energy conversion.

ZnO(101̅0) Surface Hydroxylation under Ambient Water Vapor

The interaction of water vapor with a single crystal ZnO(101̅0) surface was investigated using synchrotron-based ambient pressure X-ray photoelectron spectroscopy (APXPS). Two isobaric experiments were performed at 0.3 and 0.07 Torr water vapor pressure at sample temperatures ranging from 750 to 295 K up to a maximum of 2% relative humidity (RH). Below 10-4 % RH the ZnO(101̅0) interface is covered with ∼0.25 monolayers of OH groups attributed to dissociation at nonstoichiometric defect sites. At ∼10-4 % RH there is a sharp onset in increased surface hydroxylation attributed to reaction at stoichiometric terrace sites. The surface saturates with an OH monolayer ∼0.26 nm thick and occurs in the absence of any observable molecularly bound water, suggesting the formation of a 1 × 1 dissociated monolayer structure. This is in stark contrast to ultrahigh vacuum experiments and molecular simulations that show the optimum structure is a 2 × 1 partially dissociated H2O/OH monolayer. The sharp onset to terrace site hydroxylation at ∼10-4 % RH for ZnO(101̅0) contrasts with APXPS observations for MgO(100) which show a sharp onset at 10-2 % RH. A surface thermodynamic analysis reveals that this shift to lower RH for ZnO(101̅0) compared to MgO(100) is due to a more favorable Gibbs free energy for terrace site hydroxylation.