Synchrotron x-ray spectroscopy was employed to determine the effects of nanostructuring on electronic band structure in V2O5, a promising cathode material and widely used catalyst. V2O5 nanoparticle and bulk powders were characterized via P-XRD, electron microscopy, and diffuse reflectance ultraviolet/visible/near-infrared spectroscopy to confirm the optical bandgap. X-ray emission spectroscopy revealed the nanoparticle valence band O 2p states to be upshifted relative to the bulk, while x-ray absorption spectroscopy and resonant inelastic x-ray scattering showed the lowest V 3d conduction band states to be static. Together, these changes (in conjunction with an increased density of unoccupied lower conduction band states) produce a shrunken bandgap in the V2O5 nanoparticles that defies the Burstein-Moss effect. Changes in nanoparticle band structure are generally attributed to oxygen vacancy defects. While nanostructure bandgap reduction is in line with much previous computational work, it is unexpected from most previous experimental results. To our knowledge, this is the first synchrotron x-ray spectroscopy study of a shrunken bandgap achieved in pure V2O5 nanoparticles.

We present a combined Langmuir-Pockels trough and ambient pressure X-ray photoelectron spectroscopy (APXPS) study of the compression of stearic acid surfactant layers on neat water. Changes in the packing density of the molecules are directly determined from C 1s and O 1s APXPS data. The experimental data are fit with a 2D model for the stearic acid coverage. Based on the results of these proof-of-principle experiments, we discuss the remaining challenges that need to be overcome for future investigations of the role of surfactants in heterogeneous chemical reactions at liquid-vapor interfaces in combined Langmuir-Pockels trough and APXPS measurements.

Ambient pressure X-ray photoelectron spectroscopy (APXPS) has been used to study the initial stages of methanol adsorption on vanadium oxide surfaces. V 2p, O 1s, C 1s, and K 2p XPS spectra were collected as a function of relative methanol pressure in a series of isotherm and isobar experiments on two VO2/TiO2 (100) films with different surface vanadium oxidation states. The binding energies and O 1s/C 1s peak area ratios for adsorbates were consistent with a mixture of molecular methanol, methoxide, hydroxide, and water, indicating that both molecular and dissociative methanol adsorption occur. In contrast to water adsorption experiments on similar films, an adsorption onset was observed at a consistent temperature, rather than a consistent relative pressure, indicating that a more complex reaction mechanism is at play. Vanadium oxidation state, C 1s peak position, and the area of carbon and oxygen adsorbate peaks were correlated, suggesting that reduced surface sites play a critical role in enhancing both the dissociative and molecular adsorption of methanol. The two fairly similar VO2/TiO2 (100) films showed quite different behavior, with the more reduced surface showing greater reactivity toward methanol. The difference in reactivity could be linked to different levels of potassium in the two films, which appears to play an important role in determining the vanadium oxidation state and has important practical consequences for the design of catalytic systems.

We report that oxygen vacancies have a profound impact on phase separation and thermoelectric properties of ITO films grown at room temperature. Oxygen vacancies in non-stoichiometric In1.8Sn0.2O2.5 films aided the formation of In-rich metallic clusters. It yields a high electrical conductivity σ = 1540 Scm−1 and Seebeck coefficient |α| = 27.2 μVK−1, which resulted in the highest power factor (α2σ = 113.8 μW m−1K−2) but low optical transmission (Top ∼ 25%). An increase in oxygen partial pressure resulted in stochiometric In1.8Sn0.2O3 films which improved the optical transparency by 300% (Top ∼75.4%), but power factor was reduced by ∼85% due to a decrease in α and σ. A decrease in α was due to the lack of energy filtering of charge carriers in the stoichiometric ITO film which did not have In-rich metallic clusters. XPS results showed that the valence band energy shifts with a change in oxygen partial pressure due to a decrease in carrier density, which implied a change in Fermi energy due to the reverse Moss-Burstein effect. Our results showed that phase separation can be obtained in nanocomposite ITO films by tuning their stoichiometry simply by varying the oxygen partial pressure during deposition of thermoelectric materials at low temperatures.

In this work, ambient pressure x-ray photoelectron spectroscopy (APXPS) is used to study the initial stages of water adsorption on vanadium oxide surfaces. V 2p, O 1s, C 1s, and valence band XPS spectra were collected as a function of relative humidity in a series of isotherm and isobar experiments. Experiments were carried out on two VO2 thin films on TiO2 (100) substrates, prepared with different surface cleaning procedures. Hydroxyl and molecular water surface species were identified, with up to 0.5 ML hydroxide present at the minimum relative humidity, and a consistent molecular water adsorption onset occurring around 0.01% relative humidity. The work function was found to increase with increasing relative humidity, suggesting that surface water and hydroxyl species are oriented with the hydrogen atoms directed away from the surface. Changes in the valence band were also observed as a function of relative humidity. The results were similar to those observed in APXPS experiments on other transition metal oxide surfaces, suggesting that H2O-OH and H2O-H2O surface complex formation plays an important role in the oxide wetting process and water dissociation. Compared to polycrystalline vanadium metal, these vanadium oxide films generate less hydroxide and appear to be more favorable for molecular water adsorption.

Reverse osmosis using aromatic polyamide membranes is currently the most important technology for seawater desalination. The performance of reverse osmosis membranes is highly dependent on the interplay of their surface chemical groups with water and water contaminants. In order to better understand the underlying mechanisms of these membranes, we study ultrathin polyamide films that chemically resemble reverse osmosis membranes, using ambient pressure X-ray photoelectron spectroscopy. This technique can identify the functional groups at the membrane-water interface and allows monitoring of small shifts in the electron binding energy that indicate interaction with water. We observe deprotonation of free acid groups and formation of a 'water complex' with nitrogen groups in the polymer upon exposure of the membrane to water vapour. The chemical changes are reversed when water is removed from the membrane. While the correlation between functional groups and water uptake is an established one, this experiment serves to understand the nature of their chemical interaction, and opens up possibilities for tailoring future materials to specific requirements.

The unique electronic band structure of indium nitride InN, part of the industrially significant III-N class of semiconductors, offers charge transport properties with great application potential due to its robust n-type conductivity. Here, we explore the water sensing mechanism of InN thin films. Using angle-resolved photoemission spectroscopy, core level spectroscopy, and theory, we derive the charge carrier density and electrical potential of a two-dimensional electron gas, 2DEG, at the InN surface and monitor its electronic properties upon in situ modulation of adsorbed water. An electric dipole layer formed by water molecules raises the surface potential and accumulates charge in the 2DEG, enhancing surface conductivity. Our intuitive model provides a novel route toward understanding the water sensing mechanism in InN and, more generally, for understanding sensing material systems beyond InN.