Bismuth (Bi) has a fairly low melting point of 544 K making it practical as a liquid metal medium in a number of applications. Under ambient atmospheric conditions the surface of solid Bi oxidizes. While the solid Bi interface has been fairly well characterized with surface science studies upon exposure to oxygen, to date little is known about the molecular level reactivity of the liquid interface. Using ambient pressure X-ray photoelectron spectroscopy the liquid-gas interfacial chemistry of Bi was examined upon exposure to oxygen gas and water vapor at 550 K up to a maximum pressure of 0.5 Torr. Water vapor remained unreactive towards the liquid interface over the entire pressure range. Oxygen remined unreactive up to 10(-4) Torr, whereas above this pressure oxidation was observed forming Bi2O3. The oxidation exposure onset was > 5 x 10(5) Langmuirs, significantly higher than what is required for solid Bi interfaces.

Ionic liquids (ILs) are considered to be one of the steppingstones to fabricate next generation electrochemical devices given their unique physical and chemical properties. The addition of water to ILs significantly impact electrochemical related properties including viscosity, density, conductivity, and electrochemical window. Herein we utilize ambient pressure X-ray photoelectron spectroscopy (APXPS) to examine the impact of water on values of the electrochemical shift (S), which is determined by measuring changes in binding energy shifts as a function of an external bias. APXPS spectra of C 1s, O 1s and N 1s regions are examined for the IL 1-butyl-3-methylimidazolium acetate, [C4 mim][OAc], at the IL/gas interface as a function of both water vapor pressure and external bias. Results reveal that in the absence of water vapor there is an IL ohmic drop between the working electrode and quasi reference electrode, giving rise to chemical specific S values of less than one. Upon introducing water vapor, S values approach one as a function of increasing water vapor pressure, indicating a decrease in the IL ohmic drop as the IL/water mixture becomes more conductive and the potential drop is driven by the electric double layer at the electrode/IL interface.

The ZnO/Cu2O heterojunction promises high efficiency in photocurrent conversion and other light-driven processes, but the lattice mismatch between ZnO and Cu2O leads to slow electron transfer and low conversion efficiency. In addition, the stability of Cu2O is still the main challenging and limiting factor for device applications in real environments. CuxO is a mixed semiconductor of CuO and Cu2O, which is a promising alternative to Cu2O in device fabrication due to its better stability and photocatalytic efficiency. In this work, CuxO nanorods were attached to vertically aligned gold-decorated ZnO nanorods, creating a hierarchical ZnO/Au/CuxO nanoforest. In addition, the hierarchical surface shows superhydrophobicity, which can prevent Cu2O degradation by water and oxygen. Femtosecond time-resolved transient absorption spectroscopy was employed to investigate the electron transfer dynamics in the ZnO/Au/CuxO heterojunction. The nanoforest demonstrates enhanced electron mobility, increased lattice match, and higher photocurrent conversion efficiency compared with bare ZnO, CuxO, or ZnO/CuxO.

Molecular level information about thermodynamic variations (enthalpy, entropy, and free energy) of a gas molecule as it crosses a gas-liquid interface is strongly lacking from an experimental perspective under equilibrium conditions. Herein, we perform in situ measurements of water interacting with the ionic liquid (IL) 1-butyl-3-methylimidazolium acetate, [C(4)mim] [Ace], using ambient pressure X-ray photoelectron spectroscopy in order to assess the interfacial uptake of water quantitatively as a function of temperature, pressure, and water mole fraction (x(w)). The surface spectroscopy results are compared to existing bulk water absorption experiments, showing that the amount of water in the interfacial region is consistently greater than that in the bulk. The enthalpy and entropy of water sorption vary significantly between the gas-liquid interface and the bulk as a function of x(w), with a crossover that occurs near x(w) = 0.6 where the water-IL mixture converts from being homogeneous (x(w) < 0.6) to nanostructured (x(w) > 0.6). Free energy results reveal that water at the gas-IL interface is thermodynamically more favorable than that in the bulk, consistent with the enhanced water concentration in the interfacial region. The results herein show that the efficacy for an ionic liquid to absorb a gas phase molecule is not merely a function of bulk solvation parameters but also is significantly influenced by the thermodynamics occurring across the gas-IL interface during the mass transfer process.

Gallium-indium eutectic (eGaIn) is a liquid metal being explored in a number of applications because of its high thermal/electrical conductivity and favorable deformability. A key function regulating the mechanical properties of eGaIn is the ability to rapidly form a passivating oxide layer, which is known to occur upon exposure to air under ambient conditions. Nevertheless, little is known about the molecular level surface reactivity of eGaIn toward oxygen and water vapor under in situ conditions. Herein we present ambient pressure X-ray photoelectron spectroscopy results examining the liquid-gas interface of eGaIn in the presence of oxygen and water vapor. By examining each gas independently, results reveal that both oxygen and water vapor react with Ga in eGaIn to form the same oxidized products: Ga3+ oxide (Ga2O3) as an outer layer and Ga1+ oxide (Ga2O) as an interlayer. Despite similar product formation, stark differences are observed in pressure-dependence and adsorbate-induced binding energy shifts. The results herein suggest both oxygen and water vapor uniquely contribute to the oxidative passivation of eGaIn under ambient conditions.

The interaction of water vapor with monolayer NiO/Ag(100) was examined using both experimental and computational techniques. Initial film growth was characterized by scanning tunneling microscopy and low energy electron diffraction showing the formation of NiO(1 x 1). X-ray photoelectron spectroscopy (XPS) reveals that the initial film was mainly composed of NiO oxide with a small amount of hydroxyl groups (OH) attributed to the dissociation of background water vapor at highly reactive edge sites. Density functional theory (DFT) reveals that the adsorption of a water monomer on NiO/Ag(100) terrace sites prefers to be in the molecular rather than the dissociate state. XPS results indicate that upon exposing the oxide film to high water vapor pressures (maximum 333.3 Pa), extensive hydroxylation occurs which is attributed to water dissociation at terrace sites. DFT reveals that upon aggregation of water monomers to dimers at the oxide interface the dissociated dimer is energetically stable. The results herein are consistent with previous MgO/Ag(100) studies, further revealing that for certain metal oxides the formation of water dimers at the metal oxide-vapor interface is a key mechanism leading to extensive terrace site hydroxylation.

Upon freezing aqueous solutions, dissolved solutes are forced into small liquid regions within the frozen ice. This phenomenon affects solute concentrations, solution pH and can accelerate reaction kinetics. The ability to predict this behavior quantitatively has significant implications in environmental and biological studies. For a system with rapid reversible reactions like acids and buffers, a liquid volume decrease thermodynamically necessitates multiple equilibrium shifts which heretofore have been largely unaccounted for in freeze chemistry studies. Herein it is shown that multiple equilibrium shifts explain observed concentration changes of ions and pH shifts both qualitatively and quantitatively within the liquid region of ice upon freezing. Apparent charge imbalances upon freezing are often interpreted as being due to ions becoming trapped within the ice matrix. However, the results herein show that multiple equilibrium shifts allow charge balance to be maintained within the liquid regions of ice while solute concentrations and pH vary upon freezing. It is also shown that for species involved in reversible reactions, concentrations can dramatically increase and/or decrease upon freezing, questioning the common “freeze concentration” assertion that freezing only gives rise to increases in solute concentrations in the absence of solute trapping in the solid ice matrix. The combination of in situ spectroscopy and the multiple equilibrium shift model developed herein can be extended to other systems to help elucidate the effects of freezing on solution pH, solute concentrations and reaction kinetics important in environmental chemistry and cryopreservation.

The ionic liquid-gas interface of 1-hexyl-3-methyl-imidazolium chloride, [HMIM][Cl], was examined in the presence of water vapor using lab-based ambient pressure x-ray photoelectron spectroscopy (APXPS) at room temperature. The interfacial water uptake was measured quantitatively in the pressure range of high vacuum up to a maximum of 5 Torr (27% RH) and back to high vacuum in a systematic manner. Water mole fractions in the interface determined from APXPS were compared to previously published tandem differential mobility analysis results on [HMIM][Cl] nanodroplets. Our findings show that water constitutes a significantly larger mole fraction at the interface when compared to the bulk. Additionally, the reverse isotherms showed that the uptake of water at the interface of [HMIM][Cl] is a reversible process.

The liquid vacuum interface was investigated for a ionic liquid (IL) mixture containing 1-ethyl-3-methylimidazolium acetate, [C2MIM] [OAc], and 1ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [C2MIM] [TFSI]. Herein, we detail a quantitative connection between molecular simulations and angle-resolved X-ray photoemission spectroscopy for an IL vacuum interface. Results show that for a mixture with a low concentration of [TFSI](-), the anion [OAc](-) is slightly depleted from the interface, whereas the [TFSI](-) anion is significantly enhanced relative to the bulk. Both experiments and simulations reveal that the mole fraction of [TFSI](-) increases significantly from the bulk value in the top 17 angstrom. Furthermore, simulations show that [TFSI](-) has a preferred orientation at the liquid-vacuum interface.

Chemical interactions which occur at a heterogeneous interface between a gas and substrate are critical in many technological and natural processes. Ambient pressure X-ray photoelectron spectroscopy ( APXPS) is a powerful spectroscopy tool that is inherently surface sensitive, elemental and chemical specific, with the ability to probe sample surfaces in the presence of a gas phase. In this review, we discuss the evolution of lab-based AP-XPS instruments, from the first development by Siegbahn and coworkers up through modern day systems. A comprehensive overview is given of heterogeneous experiments investigated to date via lab-based AP-XPS along with the different instrumental metrics that affect the quality of sample probing. We conclude with a discussion of future directions for lab-based AP-XPS, highlighting the efficacy for this in-demand instrument to continue to expand in its ability to significantly advance our understanding of surface chemical processes under in situ conditions in a technologically multidisciplinary setting. (C) 2018 Elsevier B.V. All rights reserved.

Ambient pressure X-ray photoelectron spectroscopy (APXPS) was used to quantitatively assess the chemical changes of the top few nanometers of the ionic liquid (IL) gas interface of 1-butyl-3-methylimidazolium acetate, [BMW] [OAc], in the presence of water vapor at room temperature. Above 10(-3) Torr the uptake of water into the interfacial region was observed and increases up to a maximum water mole fraction (x(w)) of 0.85 at S Torr. Comparing APXPS to gravimetric analysis measurements, the kinetics of interfacial uptake are rapid compared to bulk water absorption. There is growing evidence from experiments and molecular dynamic simulations that water/IL mixtures undergo a phase transition from being homogeneously mixed to a system composed of nanometer sized, segregated polar and nonpolar regions near x(w) = 0.7 in the bulk. For x(w) > 0.6, APXPS C is spectra show a sudden change in shape. If is suggested that this observed spectral change in C is is due to a similar nanostructuring occurring neat the IL gas interface. Increasing interfacial water gives rise to relative binding energy shifts in O is, C is, and N is regions which increase with x(w) thus "suggesting that water significantly influences the electronic environment of both the anion and cation.

Water is known to affect bulk properties of ionic liquids (ILs) including density, viscosity, conductivity and gas absorption. It is also becoming increasingly recognized that water gives rise to significant changes at IL interfaces. In this chapter we review the surface sensitive analytical techniques and molecular dynamic (MD) simulations utilized to probe the IL-vacuum, IL-gas and IL-solid interfaces in the presence of water. An overview is first given from the perspective of surface science experiments, followed by a focus on results from MD simulations. Experimental studies in most cases examined the IL in the melted state, while a few studies examined the IL-vacuum interface while the IL transitioned between frozen and melted states. Over the past two years there has been a significant increase in atomic force microscopy (AFM) studies probing the effects of water on the IL-solid interface of both neutral surfaces and electrified surfaces. Experimental and MD simulation studies that vary the amount of water often reveal water mole fraction (x(w)) dependent structural changes and IL layering at IL-gas and IL-solid interfaces. Under low xw conditions the concentration of water at the interface can be significantly different than in the bulk. While water is often viewed as a ubiquitous contaminant, it is also possible to envisage utilizing water as a chemical knob to influence xw dependent interfacial IL chemistry and structure with significant implications in gas absorption, electrochemical and surface catalytic studies.

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.

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.

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.

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.

The interface of ionic liquid (IL) 1-ethyl-3-methylimidazolium-bis-(trifluoromethylsulfonyl)-imide, [EMIM][Tf2N], was examined in the presence of water vapor using lab-based ambient pressure X-ray photoelectron spectroscopy (APXPS) with a monochromatic Al Kα X-ray source. Room temperature water vapor exposures to [EMIM][Tf2N] ranged from 10−6 to 2.0Torr with no evidence of water uptake at the IL-vapor interface. The hydrophobic nature of [EMIM][Tf2N] allowed for measuring the attenuation of F 1s, O 1s, N 1s, C 1s and S 2p APXPS peaks as a function of pressure, allowing for the determination of the electron scattering cross-section (σe) of gas phase water. The elemental diversity of [EMIM][Tf2N] allowed for the extraction of σe over a 520eV kinetic energy range in the absence of a variable energy X-ray source. Herein we show that lab-based APXPS in combination with an elementally diverse, unreactive substrate is an effective tool for the determination of electron scattering cross sections for gas phase species.