1s Binding Energy Research Articles

Proton transfer and hydrogen bonding in the organic solid state: a combined XRD/XPS/ssNMR study of 17 organic acid–base complexes

The properties of nitrogen centres acting either as hydrogen-bond or Brønsted acceptors in solid molecular acid-base complexes have been probed by N 1s X-ray photoelectron spectroscopy (XPS) as well as (15)N solid-state nuclear magnetic resonance (ssNMR) spectroscopy and are interpreted with reference to local crystallographic structure information provided by X-ray diffraction (XRD). We have previously shown that the strong chemical shift of the N 1s binding energy associated with the protonation of nitrogen centres unequivocally distinguishes protonated (salt) from hydrogen-bonded (co-crystal) nitrogen species. This result is further supported by significant ssNMR shifts to low frequency, which occur with proton transfer from the acid to the base component. Generally, only minor chemical shifts occur upon co-crystal formation, unless a strong hydrogen bond is formed. CASTEP density functional theory (DFT) calculations of (15)N ssNMR isotropic chemical shifts correlate well with the experimental data, confirming that computational predictions of H-bond strengths and associated ssNMR chemical shifts allow the identification of salt and co-crystal structures (NMR crystallography). The excellent agreement between the conclusions drawn by XPS and the combined CASTEP/ssNMR investigations opens up a reliable avenue for local structure characterization in molecular systems even in the absence of crystal structure information, for example for non-crystalline or amorphous matter. The range of 17 different systems investigated in this study demonstrates the generic nature of this approach, which will be applicable to many other molecular materials in organic, physical, and materials chemistry.

Notes on the adsorption of octyl hydroxamic acid on pyrochlore and calcite

XPS measurements showed that octyl hydroxamic acid (OHA) adsorbed on pyrochlore by both chemisorption (N 1s binding energy peak at around 399eV) and physisorption (N 1s binding energy peak at above 400eV). The ratio of the integrated areas of the physisorption peak and chemisorption peak increased from nearly zero to about 4, and remained at 4 with increasing OHA concentrations. However, adsorption isotherm showed a continuous increase in the adsorption density of OHA. The discrepancy was found to be the result of how the OHA-treated pyrochlore was separated from the suspensions. Maximum adsorption of OHA on pyrochlore in fact remained at the second plateau (about 50μmol/m2), i.e., N 1s peak ratio of 4, and the rest of the physisorbed OHA could be washed off. On calcite, only a small and symetrical N 1s binding energy peak could be detected, indicating only physisorbed OHA.

The influence of surface structure on H4SiO4 sorption and oligomerization on goethite surfaces: An XPS study using goethites differing in morphology

Two important weathering products are solution phase silicic acid (H4SiO4) and the iron oxides. The chemistry of H4SiO4 on iron oxide surfaces is a complex mix of sorption and polymerization and this affects many iron oxide properties. For example the presence of H4SiO4 directs goethite growth to shorter and fatter needles which implies different H4SiO4 chemistry on different goethite faces and the purpose of this paper is to directly explore this and related phenomena. Isotherms and the Si 2s X-ray photoelectron spectra were measured for H4SiO4 on three goethite samples having different morphologies. Both the shape of the isotherms and the shifts in the Si 2s binding energies (BE) indicated different H4SiO4 chemistry on acicular crystalline goethite compared to previously reported behavior on ≈2nm sized disordered particles of ferrihydrite. The H4SiO4 isotherm on acicular goethites had a plateau for silicic acid concentrations (Si(sol)) between 0.01 and 0.3mM but the isotherm increases steeply for Si(sol)>0.3mM. The Si 2s BE for acicular goethites indicated monomeric sorption occurring at Si(sol)<0.3mM but then the BE increases for Si(sol)>0.3mM indicating the onset of H4SiO4 polymerization as also indicated by the increase in isotherm slope. The data are consistent with a model where monomers sorb on both the (110) and (021) faces (Pbnm space group) but at Si(sol) over ≈0.3mM polymers form only on the (021) face. The arrangement of monomer sorption sites on the acicular goethite (021) face acts as a template for H4SiO4 polymerization. This would explain why the onset of polymerization occurs quite distinctly on the acicular goethite surface compared with that previously observed on ferrihydrite where there is a gradual increase in the significance of polymerization across the H4SiO4 isotherm.

Band bending at free Pb(Zr,Ti)O3 surfaces analyzed by X-ray photoelectron spectroscopy

Abstract This paper analyses in detail the core levels evolution of Pb(Zr,Ti)O 3 , i.e. Pb 4f, Zr 3d, Ti 2p, O 1s in various conditions: absolutely freshly prepared sample, sample stored under air, and the effects of in vacuum annealing. The aim of the study is to quantify separately the chemical reactivity at the surface and the band bending effects due to the ferroelectric polarization. It is found that freshly prepared samples present mostly inwards (↓) polarization. This phenomenon is mostly revealed by the Ti 2p and O 1s spectra, manifested as a distinct component with 1.8 eV lower binding energy in the O 1s binding energy and by 1.1 eV in the Ti 2p binding energy. Sample aging under air suppresses the inwards polarization, and most signal comes from surfaces not presenting ferroelectric permanent polarization perpendicular to the sample surface. This process conducts also to the formation of Pb(CO 3 ) 2 on the surface. Annealing to temperatures up to 400 °C stabilizes a surface composed by a main part of surface without polarization perpendicular to the surface, and with some areas presenting outwards (↑) polarization. These areas have, most probably, different terminations, the polarized area being (Ti,Zr)O 2 terminated.

Evaluating the Structure of Catalysts Using Core-Level Binding Energies Calculated from First Principles

X-ray photoelectron spectroscopy (XPS) is a powerful and popular surface characterization technique, and the measured shifts in the core electron binding energies are sensitive to the chemical structure and local environment of the surface species. C 1s binding energies were calculated with density functional theory (DFT) for 17 structures including eight well-characterized structures on a Co(0001) surface and nine on a Pt(111) surface, while B 1s binding energies were calculated for six well-characterized structures and compared with experimental values. DFT calculations describe the 2.8 eV variation in the C 1s binding energies on Co surfaces, the 4.2 eV variation in the C 1s binding energies on Pt surfaces, and the 5.5 eV variation in the B 1s binding energies in the test sets with average deviations of 85, 73, and 53 meV, respectively. The shift in the C 1s and the B 1s binding energies can be correlated with the calculated charges, though only within homologous series. To illustrate how binding energ...

The influence of potassium doping on hydrogen adsorption on carbon nanocone material studied by thermal desorption and photoemission

Hydrogen adsorption/desorption on potassium doped carbon nanocones was studied by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy. TPD shows that the hydrogen storage was enhanced by up to 40% after potassium doping. Hydrogen adsorption on K-modified carbon nanocone material seems more stable than that on the undoped material. The XPS results indicate that there is charge transfer from potassium to carbon. The C 1s binding energy increases with increased potassium doping and the peak becomes wider. These binding energy shifts may be explained by work function changes related to potassium doping. The K 2p spectra indicate that there are two different local environments for potassium on the carbon cone material.

The Influence of Surface Structure on H4SiO4 Oligomerization on Rutile and Amorphous TiO2 Surfaces: An ATR-IR and Synchrotron XPS Study

Silicic acid (H(4)SiO(4)) is ubiquitous in natural aquatic systems. Applications of TiO(2) in these systems will be influenced by H(4)SiO(4) sorption and oligomerization reactions on the TiO(2) surface, and this can affect many aspects of TiO(2) reactivity. The spatial arrangement of sorption sites on a metal oxide surface can promote specific lateral interactions, such as oligomerization, between sorbed species. In this work we explore the relationship between surface structure and interfacial H(4)SiO(4) oligomerization by quantifying the extent of H(4)SiO(4) sorption and oligomerization on three TiO(2) phases; a rutile phase having well-developed (110) faces (R180), a rutile phase with poorly developed (110) faces (R60), and an amorphous TiO(2) (TiO(2(am))). The in situ ATR-IR spectra measured over time as 0.2 mM H(4)SiO(4) reacted with TiO(2) were quite different on the three TiO(2) phases. The percentage of the surface H(4)SiO(4) that was present as oligomers increased over time on all phases, but after 20 h almost all H(4)SiO(4) on the R180 surface was oligomeric, while the H(4)SiO(4) on TiO(2(am)) was predominantly monomeric. The extent of H(4)SiO(4) oligomerization on R60 was intermediate. When the TiO(2) phases reacted with 1.5 mM H(4)SiO(4) the ATR-IR spectra showed oligomeric silicates dominating the surface of all three TiO(2) phases; however, after 20 h the percentage of the surface H(4)SiO(4) present as three-dimensional polymers was ∼30, 10, and 0% on R180, R60, and TiO(2(am)) respectively. The Si 2s photoelectron peak binding energy (BE) and the H(4)SiO(4) surface coverage (Γ(Si)) were measured by XPS over a range of Γ(Si). For any given Γ(Si) the Si 2s BE's were in the order R180 > R60 > TiO(2(am)). A higher Si 2s BE indicates a greater degree of silicate polymerization. The ATR-IR and XPS results support the existing model for interfacial H(4)SiO(4) oligomerization where linear trimeric silicates are formed by insertion of a solution H(4)SiO(4) between suitably orientated adjacent bidentate sorbed monomers. The TiO(2(am)) has previously been shown to consist of ∼2 nm diameter particles with a highly disordered surface. When compared to the TiO(2(am)) surface, the regular arrangement of TiO(6) octahedra on the rutile (110) face means that sorbed H(4)SiO(4) monomers on adjacent rows of singly coordinated oxygen atoms are oriented so as to favor linear trimer formation. Higher silicate polymers can form between adjacent trimers, and this is favored on the rutile (110) surfaces compared to the TiO(2(am)). This is also expected on the basis of the arrangement of surface sites on the rutile (110) surface and because the high surface curvature inherent in a ∼2 nm spherical TiO(2(am)) particle would increase the spatial separation of adjacent trimers.

Influence of deposition pressure on hydrogenated amorphous carbon films prepared by d.c.‐pulse plasma chemical vapor deposition

Hydrogenated amorphous carbon films (a‐C : H) were prepared by d.c.‐pulse plasma chemical vapor deposition using CH4 and H2 gases. The microstructure and hardness of the resulting films were investigated at different deposition pressures (6, 8, 11, 15, and 20 Pa). The growth rate increased sharply from 3.2 to 10.3 nm/min with increasing the pressure from 6 to 20 Pa. According to Raman spectra, XPS, and Fourier transform infrared analysis, the films deposited at the pressure of 6 and 8 Pa have high sp3 content and show typical diamond‐like character. However, the microstructures and bond configuration of the films deposited at 11, 15, and 20 Pa have high sp2 content and favored fullerene‐like nanostructure. The hardness and sp2 content were shown to reach their minimum values simultaneously at a deposition pressure of 8 Pa and then increased continuously. The film with fullerene‐like nanostructure obtained at 20 Pa displays a high Raman ID/IG ratio (~1.6), and low XPS C 1s binding energy (284.4 eV). The microstructural analysis indicates that the films are composed of a hard and locally dense fullerene‐like network, i.e. a predominantly sp2‐bonded material. The rigidity of the films is basically provided by a matrix of dispersed cross‐linked sp2 sites. Copyright © 2012 John Wiley &amp; Sons, Ltd.

Adsorption of O2 and CO2 on the Si(111)-7 × 7 surfaces

The interaction of O2 and CO2 with the Si(111)-7×7 surface has been studied with X-ray photoelectron spectroscopy (XPS). It was found that both O2 and CO2 molecules can readily oxidize the Si(111)-7×7 surface to form thin oxide films. Two oxygen species were identified in the oxide film: oxygen atoms binding to on-top sites of adatom/rest atoms with an O 1s binding energy of ~533eV as well as to bridge sites of adatom/rest atom backbonds at ~532eV. These two oxygen species can be interconverted thermally during the annealing process. Due to the low oxidation capability, the silicon oxide film formed by CO2 has a lower O/Si ratio than that of O2.

Organic Reactions in Ionic Liquids Studied by in Situ XPS

We demonstrate the application of in situ X-ray photoelectron spectroscopy (XPS) to monitor organic, liquid-phase reactions. By covalently attaching ionic head groups to the reacting organic molecules, their volatility can be reduced such that they withstand ultra high vacuum conditions. The applied method, which is new for the investigation of organic reactions, allows for following the fate of all elements present in the reaction mixture--except for hydrogen--in a quantitative and oxidation-state-sensitive manner in one experiment. This concept is demonstrated for the alkylation of a tertiary amine attached to an imidazolium or phosphonium moiety by the anion 4-chlorobutylsulfonate ([ClC(4)H(8)SO(3)](-)). In the course of the reaction, the covalently bound chlorine is converted to chloride and the amine to ammonium as reflected by the distinct shifts in the N 1s and Cl 2p binding energies.

Preparation of Ag/HOPG model catalysts with a variable particle size and an in situ xps study of their catalytic properties in ethylene oxidation

The preparation of model silver catalysts supported on highly oriented pyrolytic graphite is described, and the effect of the Ag particle size on the catalytic ethylene oxidation into ethylene oxide, studied by in situ XPS and mass spectrometry, is considered. For a mean particle diameter of 8 nm, the adsorbed oxygen species characterized by an O 1s binding energy of 530.8 ± 0.2 eV (electrophilic oxygen) forms on the silver surface exposed to the ethylene-oxygen reaction mixture. Larger silver particles with a mean diameter of 40 nm additionally contain the adsorbed oxygen species characterized by an O 1s binding energy of 529.2 ± 0.2 eV (nucleophilic oxygen). The presence of both oxygen species on the surface of the larger particles ensures the formation of ethylene oxide, while the sample with the smaller silver particles is inactive in the epoxidation reaction. The O 1s signal at 530.8 eV is partly due to oxygen dissolved in the subsurface layers of silver.

X-ray photoelectron and absorption spectroscopy of mixed lanthanum copper oxychalcogenides LaCuOSe 1− xTe x (0 ≤ x ≤ 1)

The complete oxychalcogenide solid solution LaCuOSe 1− x Te x (0 ≤ x ≤ 1) was examined by X-ray photoelectron and X-ray absorption near-edge spectroscopy. Whereas the La 3d, Cu 2p and most of the chalcogen ( Ch) core-line peaks are invariant, the O 1s binding energies (BE) show shifts (up to 0.35 eV) with increasing substitution of Se by Te that reflect the competing effects of the charges of neighbouring La atoms and more distant Ch atoms, if the charge potential model is applied. This observation supports the occurrence of interlayer La– Ch interactions between the [LaO] and [Cu Ch] layers in LaCuO Ch. The Se 3d BEs show smaller shifts (up to 0.15 eV) that indicate the development of more negative charge on the Se atoms with increasing x. The Cu K-edge and Se K-edge spectra reveal subtle absorption edge energy and intensity changes that can be explained by the evolution of Cu-to- Ch charge transfer within the [Cu Ch] layers.

On the interaction of encapsulated pH indicator species within a silica matrix produced by three sol–gel routes

Solid acid–base sensors were prepared by encapsulating two pH indicators (brilliant yellow or acridine) within a silica matrix by the sol–gel method using three different routes: (1) non-hydrolytic, (2) acid catalyzed and (3) base catalyzed. The interactions of the silica-indicator with the resulting materials were then investigated by cyclic and differential pulse voltammetry. Complementary, ultraviolet–visible, photoacoustic spectroscopy was employed for the characterization of the interactions by monitoring the band shifts (bathochromic or hypsochromic, depending on the sol–gel route) between the neat pH indicators and those encapsulated within the silica network. Furthermore, X-ray photoelectron spectroscopy showed that the N 1s binding energy in brilliant yellow was shifted for the material resulting from the acid route. The electrochemical behavior and the pH indicator interactions with the silica network were dependent on the nature of the employed sol–gel route. For the sensors prepared with acridine, the interactions with the silica network took place through the nitrogen group from the pyridinic ring. For the brilliant yellow indicator, different behaviors were observed depending on the route, suggesting different processes during preparation or analysis. For the basic catalyzed and non-hydrolytic routes, it was not possible to assign a specific interaction. Nevertheless, it seemed that interactions might have taken place through the hydroxyl and/or sulphonic groups. Furthermore, for the brilliant yellow sensor prepared through the acid route, it was possible to show that the interaction probably or partially occurred through the azo groups.

XPS AND DFT STUDY ON THE MIGRATION OF LITHIUM IN MONTMORILLONITE

The distribution of lithium in montmorillonite was studied by using X-ray photoelectron spectroscopy (XPS) and compared with calculations based on the density functional theory (DFT). Interlayer Li ion in montmorillonite migrated to octahedral sites by heat treatments at a temperature range from 250 to 350°C for up to 72 hours. Li (Is) binding energy of hectorite used for reference and Li exchanged montmorillonite were 55.0 and 57.0 eV, which were attributed to the octahedral and the exchangeable lithium, respectively. After the heat treatment at 250°C for 18 hours, a new component peak was appeared at 55.4 eV which was attributed to the octahedral lithium ion; and the ratio of the peak area to the total Li (1s) peak area was increased with prolonging the heat treatment. However, even after the heat treatment at 350°C for 72 hours, the lithium ions were distributed to the both sites. Two distinct Li locations in smectites were studied using DFT calculations. One was in the ditrigonal cavity, and the other was in the octahedral sheet of smectite. The optimized structure of the cluster models suggested that the Li ion favorably interacts with a hydroxyl oxygen atom and/or basal oxygen atoms at the bottom of the ditrigonal cavity in the both cases. Calculated 1s binding energy of the Li ion in the octahedral site was smaller than that of the Li ion in the ditrigonal cavity site, which shows good coincidence with the XPS chemical shift of the Li (1s) peak.

Role of Pt-pyridinic nitrogen sites in methanol oxidation on Pt/polypyrrole-carbon black Catalyst

In the present study, polypyrrole-carbon black (Ppy-C) composite is synthesized via in situ chemical polymerization method and then Pt nanoparticles are deposited on the surface of Ppy-C, where tartaric acid is employed as both a reductant and a stabilizer for Pt nanoparticles. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) indicate that Pt nanoparticles have a higher dispersion and smaller particle size (∼2.3nm) on Ppy-C composite than that on carbon black. Electrochemical measurements show that Pt/Ppy-C catalyst Exhibits 60% higher electrocatalytic activity towards methanol oxidation than Pt/C. X-ray photoelectron spectroscopy (XPS) characterization indicates Pt4f binding energy in Pt/Ppy-C is shifted by+0.2eV as compared with that in Pt/C, while N 1s binding energy in Pt/Ppy-C shifted by −0.5eV as compared with that in Ppy-C composite. The Pt-pyridinic nitrogen sites suppress the posioning CO adsorption on Pt nanoparticles and facilitate the methanol electrocatalytic oxidation. This work is important for the understanding of the role of Ppy in the fuel cell electrocatalyst, and the development of advanced catalysts for methanol oxidation.

Electronic structure of the SrTiO3/LaAlO3 interface revealed by resonant soft x-ray scattering

Interfaces in heterostructures composed of two different transition-metal oxides are now a strongly growing research topic in condensed-matter physics due to the anomalous electronic properties very different from those of the component materials. Here we present resonant soft x-ray scattering studies of SrTiO3/LaAlO3 superlattices. From the existence of the forbidden Bragg peak (003), we found evidence for differences between the bulk and the interface electronic structures. The photon-energy dependence near the O 1s edge and polarization dependence showed that there are two structures split by ∼ 1 eV, which were not observed in the O 1s x-ray absorption spectrum. The origin of this splitting is still unclear, but we consider that this is not only due to the splitting of t2g states at the interface but also due to the difference between the O 1s binding energies for bulk and interface oxygen.

Effects of alkyl side chains on properties of aliphatic amino acids probed using quantum chemical calculations

Effects of alkyl side chains (R-) on the electronic structural properties of aliphatic amino acids are investigated using quantum mechanical approaches. The carbon (C 1s) binding energy spectra of the aliphatic amino acids are derived from the C 1s spectrum of glycine (the parent spectrum) by the addition of spectral peaks, depending on the alkyl side chains, appearing in the lower energy region IP < 290 eV (where IP is the ionization potential). The two glycyl parent spectral peaks of the amide 291.0 eV [C((2))] and carboxylic 293.5 eV [C((1))] C atoms are shifted in the aliphatic amino acids owing to perturbations depending on the size and structure of the alkyl chains. The pattern of the N 1s and O 1s spectra in glycine is retained in the spectra of the other amino acids with small shifts to lower energy, again depending on the alkyl side chain. The Hirshfeld charge analyses confirm the observations. The alkyl effects on the valence binding energy spectra of the amino acids are concentrated in the middle valence energy region of 12-16 eV, and hence this energy region of 12-16 eV is considered as the `fingerprint' of the alkyl side chains. Selected valence orbitals, either inside or outside of the alkyl fingerprint region, are presented using both density distributions and orbital momentum distributions, in order to understand the chemical bonding of the amino acids. It is also observed that the HOMO-LUMO energy gaps of the aliphatic amino acids are reduced with the growth of the alkyl side chain.

Ab initio calculations for XPS chemical shifts of poly(vinyl-trifluoroacetate) using trimer models

X-ray photoelectron spectra (XPS) of the polymer poly(vinyl-trifluoroacetate) show C(1s) binding energy shifts which are unusual because they are influenced by atoms which are several bonds away from the probed atom. In this work, the influence of the trifluoroacetate substituent on the 1s ionization potential of the carbon atoms of the polyethylene chain is investigated theoretically using mono-substituted, diad and triad models of trimers representing the polymer. Carbon 1s ionization energies are calculated by the Hartree–Fock theory employing Koopmans' theorem. The influence of the configuration and conformation of the functional groups as well as the degree of substitution are found to be important determinants of XPS spectra. It is further found that the 1s binding energy correlates in a linear fashion, with the total electrostatic potential at the position of the probe atom, and depends not only on nearest neighbor effects. This may have implications for the interpretation of high-resolution XP spectra.

Interfacial electronic structures at Fe/pentacene/Fe interfaces

We investigate the interfacial electronic structures of the heterojunction Fe/pentacene/Fe on Cu(100) substrate, using synchrotron-based photoemission spectroscopy. No chemical reaction is observed at either Fe/pentacene or pentacene/Fe interface. The hole injection barrier was estimated to be about 0.95 eV between pentacene and under the Fe thin film. C K-edge NEXAFS revealed that the long axis of pentacene molecule was almost perpendicular to the surface plane. With increasing Fe thickness deposited on pentacene film, the pentacene's HOMO shifts to higher binding energy whereas the C 1s binding energy showed an interesting unusual behaviour due to the initial band bending gradually suppressed by the increasing core-hole screening effect.

Assessing the role of silicate polymerization on metal oxyhydroxide surfaces using X-ray photoelectron spectroscopy

The iron oxide phases that precipitate in weathered environments can contain large amounts of adsorbed silicic acid (H 4SiO 4). This adsorbed silicate can be present as monomeric or oligomeric silicate species and affects many properties of iron oxides such as morphology, phase stability and surface charge. Therefore understanding the chemistry of H 4SiO 4 at iron oxide interfaces is important to describe the geochemistry of iron oxides and the elements that are associated with iron oxides. In this study we have measured the Si 2s and Si 2p binding energies (BE) in the XPS spectra of silicate adsorbed onto preformed ferrihydrite under conditions where the silicate surface chemistry is well defined. The positions of both the Si 2s and Si 2p peaks are indicative of the degree of polymerization of adsorbed silicate, but the Si 2p peaks overlap with an Fe 3p multiplet splitting peak. For ferrihydrite-adsorbed H 4SiO 4 the Si 2s peak BE increased from 153.1 to 153.9 eV as the H 4SiO 4 surface concentrations (Γ Si in mol Si (mol Fe) − 1 ) increased from 0.03 to 0.2. The low Γ Si Si 2s BE value is slightly lower than the Si 2s BE of the orthosilicate almandine (153.3 eV), while the high Γ Si value lies midway between almandine and the tectosilicate quartz (154.6 eV). The positions of the Si 2s peaks correlate with the Si–O stretching bands (ν(Si–O)) in the FTIR spectra and are consistent with a model in which monomeric silicate species are present at low Γ Si and one dimensional oligomers are formed at higher Γ Si. The Si 2s BE and ν(Si–O) for synthetic and natural ferrihydrites co-precipitated with silicates are substantially lower than those for silicate adsorbed onto preformed ferrihydrite, clearly indicating a lower degree of silicate polymerization for silicate adsorbed onto the internal surfaces between ferrihydrite domains. The Si 2s and ν(Si–O) for co-precipitated ferrihydrites appear to be indicative of the age of the ferrihydrite and the rate of ferric precipitation.