X-ray Photoelectron Spectroscopy Experiments Research Articles

Semiconducting Ta3N5-Polymer Nanohybrid that Photoelectrolyze Water

The successful preparation of a nanohybrid electrode made from Ta3N5 nanoparticle powders interlinked by Nafion is described. The basic photoelectrochemical properties as anode for water photolysis are investigated and gas evolution as well as corrosion reactions are determined by differential electrochemical mass spectroscopy (DEMS) and X-ray photoelectron spectroscopy (XPS). The sample shows oxygen evolution but also photocorrosion, oxidation of the Ta3N5 nanoparticles via the O 1s line in XPS experiment. The photocorrosion reaction results in a core-shell structure consisting of a tantalum oxide shell and a tantalum nitride core as seen in XPS and x-ray diffraction.

Probing the Low-Temperature Water–Gas Shift Activity of Alkali-Promoted Platinum Catalysts Stabilized on Carbon Supports

We report on the direct promotional effect of sodium on the water-gas shift activity of platinum supported on oxygen-free multiwalled carbon nanotubes. Whereas the Na-free Pt catalysts are shown to be completely inactive, the addition of sodium is found to improve the water-gas shift activity to levels comparable to those obtained with highly active Pt catalysts on metal oxide supports. The structure and morphology of the catalyst surface was followed using aberration-corrected HAADF-STEM, which showed that atomically dispersed platinum species are stabilized by the addition of sodium. In situ atmospheric-pressure X-ray photoelectron spectroscopy (AP-XPS) experiments demonstrated that oxidized platinum Pt-OHx contributions in the Pt 4f signal are higher in the presence of sodium, providing evidence for a previously reported active-site structure of the form Pt-Nax-Oy-(OH)z. Pt remained oxidized in all redox experiments, even when a H2-rich gas mixture was used, but the extent of its oxidation followed the oxidation potential of the gas. These findings offer new insights into the nature of the active platinum-based site for the water-gas shift reaction. A strong inhibitory effect of hydrogen was observed on the reaction kinetics, effectively raising the apparent activation energy from 70 ± 5 kJ/mol (in product-free gas) to 105 ± 7 kJ/mol (in full reformate gas). Increased hydrogen uptake was observed on these materials when both Pt and Na were present on the catalyst, suggesting that hydrogen desorption might limit the water-gas shift reaction rate under such conditions.

Critical Water Effect on the Plasmon Band and Visible Light Activity of Au/ZnO Nanocomposites

Small amounts of water (between and 0.05 and 0.35% V/V) critically determine the morphology and plasmon band of Au/ZnO nanostructures obtained by Au3+ photoreduction on ZnO nanoparticles dispersed in 2-propanol. All the synthesized materials exhibit plasmon induced activity to drive the solvent oxidation; however, the temporal evolution of acetone shows a clear induction time followed by the sudden boost in the rate of the oxidation product, which depends on the photodeposition conditions. X-ray photoelectron spectroscopy (XPS) indicates that visible irradiation produces the transformation of surface Au(0) in Au(+). Besides, an increment in the ZnO surface area ascribed to the photoinduced fragmentation of aggregated networks of Au/ZnO nanocomposites is evidenced by XPS and simple adsorption experiments. The changes in the surface properties correlate with the onset in the catalytic activity. Possible mechanisms are discussed to account for the experimental findings.

The improved piezoelectric properties of ZnO nanorods with oxygen plasma treatment on the single layer graphene coated polymer substrate

The step towards the fabrication of nanodevices with improved performance is of high demand; therefore, in this study, oxygen plasma treated ZnO nanorods based piezoelectric nanogenerator is developed on the single layer graphene coated PET flexible polymer substrate. ZnO nanorods on the single layer graphene are grown by hydrothermal growth method and the structural study is carried out by using X‐ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The piezoelectric properties of ZnO nanorods with and without plasma treatment were investigated by atomic force microscopy (AFM). The oxygen plasma treated sample of ZnO nanorods showed significant increase in the piezoelectric potential which could be due to the decrease in the defects levels in the ZnO and also increase in the mechanical properties of ZnO nanorods. Furthermore X‐ray photoelectron spectroscopy (XPS) confirms that the filling of vacancies by oxygen in the matrix of ZnO using oxygen plasma treatment has gave an enhanced piezoelectric potential compared to the sample of ZnO nanorods not treated with oxygen plasma. In addition to XPS experiment, cathodoluminescence (CL) technique was used for the determination of defect level in the ZnO nanorods after the treatment of oxygen plasma and the obtained information supported the XPS data of oxygen plasma treatment sample by showing the decreased level of defect levels in the prepared sample. From the XPS and CL studies, it is observed that the defect level has significant influence on the piezoelectric potential of the ZnO nanostructures.

Nanoscale physics and defect state chemistry at amorphous-Si/In0.53Ga0.47As interfaces

Interface and surface passivation of thin layers of III–V compound semiconductors is a key enabler of many technological applications spanning from nano-electronics to nano-photonics. The interaction between thin amorphous Si (a-Si) layers and clean, group-III-rich reconstructed In0.53Ga0.47As interfaces is studied by combining high-resolution synchrotron radiation x-ray photoelectron spectroscopy and time-dependent surface photovoltage (SPV) experiments. From 0.6 to 2.4 monolayers (ML) of a-Si are deposited on non-intentionally doped, p- and n-type In0.53Ga0.47As. For each deposition step, the observed surface and interface chemistry is directly correlated to the measured surface Fermi level position, band bending and SPV.Group-III-reconstructed In0.53Ga0.47As surfaces are observed to be intrinsically unstable against reaction with Si and two different instability regimes have been identified. First, for low deposition temperature, Si reacts strongly and intermixes with the In0.53Ga0.47As surface inducing In and Ga out-diffusion even at a sub-monolayer amount. For 2.4 ML of a-Si, a net positive interface charge of 1.24 × 1012 #/cm2 and a band of defects close to the conduction band are detected. For post-annealing at temperatures lower than 380 °C, the interface rearranges. At temperatures higher than 380 °C, out-diffusion of As in the a-Si is found to be the main instability driver.

Effect of carrier density and valence states on superconductivity of oxygen annealed Fe1.06Te0.6Se0.4 single crystals

The variations of carrier density and valence states in oxygen annealed Fe1.06Te0.6Se0.4 single crystals were studied systematically. It was found that the carrier density nH increases after oxygen annealing by Hall coefficient measurements. The X-ray photoelectron spectroscopy experiments reveal that the oxygen annealing changes Fe0 and Te0 states to Fe2+/3+ and Te4+, respectively, while the valence variation of Se is negligible. Our results indicate that the improvement of superconductivity, such as the zero resistance transition temperature Tczero, shielding and Meissner fraction value 4πχ and upper critical field Hc2, could be closely related to the proper manipulation of nH and the valence states by oxygen annealing in the system.

Preparation of an ordered ultra-thin aluminosilicate framework composed of hexagonal prisms forming a percolated network

A flat ultra-thin (0.5nm thick) aluminosilicate framework was grown using Ru(0001) as a template. The structure and composition were determined by a combination of scanning tunneling microscopy, low energy electron diffraction, infrared reflection absorption spectroscopy and X-ray photoelectron spectroscopy experiments. This film is composed mainly of an ordered arrangement of double six-membered rings d6r (a.k.a. hexagonal prisms) and it covers ∼45% of the surface, forming a two-dimensional percolated network. The remaining “uncovered” area leaves the Ru(0001) surface exposed through irregularly shaped holes of sizes in the mesopore scale range. The film morphology is different from that observed for pure silica, where a monolayer structure bound to the Ru substrate was produced under the same preparation conditions. The results provide further insights into the factors that influence the formation of two-dimensional frameworks intended to be used as model systems for surface science studies of rigid porous materials.

Effects of interface oxidation on the transport behavior of the two-dimensional-electron-gas in AlGaN/GaN heterostructures by plasma-enhanced-atomic-layer-deposited AlN passivation

The effects of interface oxidation on the transport behavior of the 2-D electron gas (2DEG) in AlGaN/GaN heterostructures by plasma-enhanced-atomic-layer-deposited AlN (PEALD-AlN) passivation were investigated using temperature-dependent Hall-effect and X-ray photoelectron spectroscopy (XPS) characterizations. AlGaN/GaN heterostructure with a 4-nm-thick PEALD-AlN passivation exhibits good 2DEG transport behavior and stability at moderately high temperature (e.g., 275 °C). However, serious oxidation of the AlN/GaN (cap layer) interface occurs as the sample is heated up to 400 °C in low-pressure atmosphere, as verified by an increased Ga-O bond in Ga 3d core-level spectra. The oxidation leads to a significant reduction of 2.47 × 1012 cm−2 in the 2DEG density in the channel. A modified AlN passivation structure with Al2O3/AlN (10/4 nm) stack is shown to be able to effectively suppress the oxidation of the AlN/GaN interface, demonstrating an enhanced 2DEG density and high-temperature stability even when the sample is heated up to 500 °C. Based on XPS and 2DEG recovery experiments, it is suggested that acceptor-like deep levels have been generated in the near-surface region of AlGaN/GaN heterostructure because of the oxidation, and trapping of these deep levels results in significant depletion of the 2DEG in the channel. The effects of PEALD-AlN passivation on the strain in the AlGaN barrier of AlGaN/GaN heterostructures are also evaluated with high-resolution X-ray diffraction technique.

The role of phosphoric acid in the anodic electrocatalytic layer in high temperature PEM fuel cells

The poisoning effect and the role of H3PO4 (PA) at the anodic electrocatalytic layer of a high temperature polymer electrolyte membrane (HT PEM based on ADVENT TPS®) fuel cell are discussed under the light of cyclic voltammetry, CO stripping, and X-ray photoelectron spectroscopy (XPS) experiments. The catalytic layer was based on both the pyridine-modified multi-wall carbon nanotubes, 30 wt% Pt/(ox.MWCNT)–Py, and on commercial 30 wt% Pt/C, with varying PA loadings on the electrode. At low PA loadings ( 80 % in the long term. This is believed to be a consequence of the more uniform distribution of PA, thus eliminating the PA displacement from the Pt interphase. It is hypothesized that the minimization of the PA poisoning effect at PA > 3 gPA/gPt, may also be a result of more efficient hydration of the catalytic layer that is being achieved through the hydration of the PA in the membrane and in the catalyst layer by the cathodically produced water vapors.

Effect of the valence states of titanium on the lattice structure and ionic conductivity of Li0.33La0.55TiO3 solid electrolyte

Li0.33La0.55TiO3 solid electrolytes with a pure phase were synthesized by the citric acid-supported sol-gel method and then sintered under controlled redox atmospheres of air, argon and hydrogen. Although of similar morphology and relative density, Li0.33La0.55TiO3 samples sintered under reduction atmosphere such as argon and hydrogen exhibited a tetragonal structure with lattice distortion. The distortion and volume expansion of the crystal lattice was identified as originating from the transformation of the Ti valence state, and this was more clearly observed under sintering of the hydrogen atmosphere. The qualitative analysis of the Ti valence state using an X-ray Photoelectron Spectroscopy experiment was performed and indicated that the largest amount of Ti3+(18.6%) was formed in the Li0.33La0.55TiO3 samples sintered under hydrogen atmosphere. The relationship between the lattice distortion and the lithium ion conductivity of the Li0.33La0.55TiO3 sintered under different atmospheres is also discussed on the basis of the lattice distortion.

Gold intercalation of boron-doped graphene on Ni(111): XPS and DFT study

The intercalation of a graphene layer adsorbed on a metal surface by gold or other metals is a standard procedure. While it was previously shown that pristine, i.e., undoped, and nitrogen-doped graphene sheets can be decoupled from a nickel substrate by intercalation with gold atoms in order to produce quasi-free-standing graphene, we find the gold intercalation behavior for boron-doped graphene on a Ni(111) surface to be more complex: for low boron contents (2–5%) in the graphene lattice only partial gold intercalation occurs and for higher boron contents (up to 20%) no intercalation is observed. In order to understand this different behavior, a density functional theory investigation is carried out, comparing undoped as well as substitutional nitrogen- and boron-doped graphene on Ni(111). We identify the stronger binding of the boron atoms to the nickel substrate as the factor responsible for the different intercalation behavior in the case of boron doping. However, the calculations predict that this energetic effect prevents the intercalation process only for large boron concentrations and that it can be overcome for smaller boron coverages, in line with our x-ray photoelectron spectroscopy experiments.

Preferential Au precipitation at deformation-induced defects in Fe–Au and Fe–Au–B–N alloys

Abstract The influence of deformation-induced defects on the isothermal precipitation of Au was studied in high-purity Fe–Au and Fe–Au–B–N alloys. Preferential Au precipitation upon annealing at 550 °C is observed at local plastic indentations. In fractured Fe–Au–B–N, solute Au atoms were found to heterogeneously precipitate at grain boundaries and local micro-cracks. This is supported by in-situ creep tests that showed a strong tendency for Au precipitation at cracks and cavities also formed during creep loading at 550 °C. Complementary X-ray photoelectron spectroscopy experiments indicate a strong tendency of Au, B and N segregation onto free surface during aging. The observed site-specific precipitation of Au holds interesting opportunities for defect healing in steels subjected to creep deformation.

Characterisation of ultra-thin films of oxidised bacterial cellulose for enhanced anchoring and build-up of polyelectrolyte multilayers

The process of catalytic oxidation of bacterial cellulose (BC) ultra-thin films with 2,2,6,6-tetramethyl-1-piperidinyloxy was investigated along with their capability to adsorb oppositely charged polyelectrolytes of chitosan and alginate. The time-dependent oxidation of BC films was analysed by X-ray photoelectron spectroscopy and quartz crystal microbalance (QCM) experiments. A negatively charged surface was achieved by inserting carboxylic groups, which was augmented by prolonged media exposure (17.9 %), compared with a fast oxidation process (9.1 %). Polyelectrolyte deposition was followed by QCM, which indicated that BC oxidation increased the first layer uptake of chitosan 17-fold (−105.0 ± 1.5 Hz) in comparison with unoxidised BC (−6.0 ± 0.2 Hz), confirming the capability of oxidised BC surfaces to exhibit strong electrostatic interactions and to support the build-up of a thicker multilayer system. These findings indicate that oxidised BC surfaces are capable of immobilising and detecting several charged species.

The selective oxidation of ethylene glycol and 1,2-propanediol on Au, Pd, and Au–Pd bimetallic catalysts

The oxidation of ethylene glycol (HOCH2CH2OH) and 1,2-propanediol (HOCH(CH3)CH2OH) was investigated over Pd/C, Au/C, and a series of bimetallic catalysts prepared by electroless deposition of Au onto Pd/C. In order to explain the enhanced activity of the bimetallic catalysts, the oxidation kinetics of selectively deuterated reagents were investigated. On Au/C and 0.61Au–Pd/C, a primary kinetic isotope effect was observed for d4-ethylene glycol (HOCD2CD2OH), indicating that C–H bond scission is the rate-limiting step. Density functional theory and X-ray photoelectron spectroscopy experiments show a correlation between an increased electron density in Au core orbitals and more favorable thermodynamics for C–H scission as Au is added to Pd. Computational studies suggest that the rate enhancement on the bimetallic surfaces compared to Pd is likely due to a decrease in coverage of strongly bound adsorbates, while the enhancement over Au is likely due to a decrease in the barrier for C–H scission.

Unusual Inherent Electrochemistry of Graphene Oxides Prepared Using Permanganate Oxidants

Graphene and graphene oxides are materials of significant interest in electrochemical devices such as supercapacitors, batteries, fuel cells, and sensors. Graphene oxides and reduced graphenes are typically prepared by oxidizing graphite in strong mineral acid mixtures with chlorate (Staudenmaier, Hofmann) or permanganate (Hummers, Tour) oxidants. Herein, we reveal that graphene oxides pose inherent electrochemistry, that is, they can be oxidized or reduced at relatively mild potentials (within the range ±1 V) that are lower than typical battery potentials. This inherent electrochemistry of graphene differs dramatically from that of the used oxidants. Graphene oxides prepared using chlorate exhibit chemically irreversible reductions, whereas graphene oxides prepared through permanganate-based methods exhibit very unusual inherent chemically reversible electrochemistry of oxygen-containing groups. Insight into the electrochemical behaviour was obtained through cyclic voltammetry, chronoamperometry, and X-ray photoelectron spectroscopy experiments. Our findings are of extreme importance for the electrochemistry community as they reveal that electrode materials undergo cyclic changes in charge/discharge cycles, which has strong implications for energy-storage and sensing devices.

Photothermal decomposition of HNS at 532 nm

The photodecomposition of solid 2,2′,4,4′,6,6′-hexanitrostillbene (HNS) at 532nm, induced by a 10-nanosecond (ns) laser, is investigated by an elaborately designed in situ X-ray photoelectron spectroscopy (I-XPS) experiment and steady-state Fourier transform IR (FTIR) spectroscopies. Numerical integration of the two-dimensional heat conduction equation indicates that the irradiated sample volume is effectively heated and photothermal decomposition takes place. Agreement of results between the I-XPS and FTIR suggests that the nitro-nitrite (NO2-ONO) isomerization is a main decomposition pathway for HNS.

Investigation of phosphate adsorption onto ferrihydrite by X-ray Photoelectron Spectroscopy

The objective of this study was to characterize phosphate adsorption onto synthetic 2-lines ferrihydrite using surface analysis by X-ray Photoelectron Spectroscopy and batch experiments. Surface analysis of ferrihydrite samples before phosphate sorption gives very reproducible Fe:O surface ratios of (1:3±0.1). Phosphate sorption onto ferrihydrite was investigated by means of pH, initial phosphate concentration, and ionic strength effects. Additionally, potential background electrolyte influence on phosphate adsorption was also determined. Phosphate uptake by ferrihydrite significantly increases with decreasing pH, with a maximum uptake of 104.8mgPO4g−1 obtained at pH=4. Phosphate removal increases with the enhancement of ionic strength in agreement with the formation of inner-sphere complexes. The presence of chloride, nitrate, and sulfate showed no competing effect on phosphate removal efficiency. Sorption kinetics follow a pseudo-second order model (R2>0.99) and the Freundlich isotherm model adequately describes sorption (R2=0.995). The careful examination of high resolution Fe 2p, O 1s, and P 2p spectra before and after phosphate sorption allows the characterization of the modifications occurring onto the ferrihydrite surface. The binding energy of the P 2p peak agrees well with that observed in Fe-PO4 compounds. Additionally, binding energy shifts in the Fe 2p spectra combined to variations in the relative intensity of the components in the high resolution O 1s spectra illustrate well the formation of chemical bonding between iron and phosphate anions at the ferrihydrite surface.

Increasing the Wear Resistance by Interstitial Alloying with Boron via Chemical Vapor Deposition

The wear resistance of a Rh(111) surface can be strongly increased by interstitial alloying with boron atoms via chemical vapor deposition of trimethylborate [B(OCH3)3] at moderate temperatures of about 800 K. The fragmentation of the precursor results in single boron atoms that are incorporated in the fcc lattice of the substrate, as displayed by X-ray photoelectron diffraction. The penetration depth of the boron atoms is in the range of at least 100 nm with the boron distribution displaying a nearly homogeneous depth profile, as examined by combined X-ray photoelectron spectroscopy and Ar ion etching experiments. Compared to the bare Rh(111) surface, the wear resistance of the boron-doped Rh surface is increased to about 400%, as probed by the scratching experiments with atomic force microscopy. The presented synthesis route provides an easy method for case hardening of micro- or nanoelectromechanical devices (MEMS and NEMS, respectively) at moderate temperatures.

Molecular Surface Structural Changes of Plasticized PVC Materials after Plasma Treatment

In this research, a variety of analytical techniques including sum frequency generation vibrational spectroscopy (SFG), coherent anti-Stokes Raman spectroscopy (CARS), and X-ray photoelectron spectroscopy (XPS) have been employed to investigate the surface and bulk structures of phthalate plasticized poly(vinyl chloride) (PVC) at the molecular level. Two types of phthalate molecules with different chain lengths, diethyl phthalate (DEP) and dibutyl phthalate (DBP), mixed with PVC in various weight ratios were examined to verify their different surface and bulk behaviors. The effects of oxygen and argon plasma treatment on PVC/DBP and PVC/DEP hybrid films were investigated on both the surface and bulk of films using SFG and CARS to evaluate the different plasticizer migration processes. Without plasma treatment, SFG results indicated that more plasticizers segregate to the surface at higher plasticizer bulk concentrations. SFG studies also demonstrated the presence of phthalates on the surface even at very low bulk concentration (5 wt %). Additionally, the results gathered from SFG, CARS, and XPS experiments suggested that the PVC/DEP system was unstable, and DEP molecules could leach out from the PVC under low vacuum after several minutes. In contrast, the PVC/DBP system was more stable; the migration process of DBP out of PVC could be effectively suppressed after oxygen plasma treatment. XPS results indicated the increase of C═O/C-O groups and decrease of C-Cl functionalities on the polymer surface after oxygen plasma treatment. The XPS results also suggested that exposure to argon plasma induced chemical bond breaking and formation of cross-linking or unsaturated groups with chain scission on the surface. Finally, our results indicate the potential risk of using DEP molecules in PVC since DEP can easily leach out from the polymeric bulk.

Chemisorbed Monolayers of Corannulene Penta-Thioethers on Gold

Penta(tert-butylthio)corannulene and penta(4-dimethylaminophenylthio)corannulene form highly stable monolayers on gold surfaces, as indicated by X-ray photoelectron spectroscopy (XPS). Formation of these homogeneous monolayers involves multivalent coordination of the five sulfur atoms to gold with the peripheral alkyl or aryl substituents pointing away from the surface. No dissociation of C-S bonds upon binding could be observed at room temperature. Yet, the XPS experiments reveal strong chemical bonding between the thioether groups and gold. Temperature-dependent XPS study shows that the thermal stability of the monolayers is higher than the typical stability of self-assembled monolayers (SAMs) of thiolates on gold.