1s Signal Research Articles

Reversible oxidation of ethylene on ferroelectric BaTiO3(001): An X-ray photoelectron spectroscopy study

Adsorption and desorption of ethylene on BaO-terminated (001) barium titanate are investigated by X-ray photoelectron spectroscopy. Carbon is found in an oxidized state, at a binding energy similar to that resulting from CO adsorption on BaTiO3(001). The amount of carbon adsorbed on the surface is also similar to the case of CO/BaTiO3(001). Upon heating the substrate up to the loss of its ferroelectric polarization, the C 1s signal from the oxidized spectral region vanishes. At the same time, there was no noticeable oxygen depletion of the surface after repeated C2H4 adsorption and desorption. The substrate remains stable after repeated oxidative adsorption and desorption of ethylene. Desorption occurs at different temperatures, depending on the adsorption temperature, which suggests different adsorption geometries: non-dissociated adsorption at high temperature with ethylene bond on two surface oxygen atoms, and locally dissociated adsorption at lower temperatures, in “formaldehyde-like” local configurations.

Surfaces of Atmospheric Droplet Models Probed with Synchrotron XPS on a Liquid Microjet.

ConspectusThe atmosphere is a key part of the earth system comprising myriad chemical species in all basic forms of matter. Ubiquitous nano- and microscopic aerosol particles and cloud droplets suspended in the air play crucial roles in earth's climate and the formation of air pollution. Surfaces are a prominent part of aerosols and droplets, due to the high surface area to bulk volume ratios, but very little is known about their specific properties. Many atmospheric compounds are surface-active, leading to enhanced surface concentrations in aqueous solutions. Their distribution between the surface and bulk may determine heterogeneous chemistry and many other properties of aerosol and cloud droplets, but has not been directly observed.We used X-ray photoelectron spectroscopy (XPS) to obtain direct molecular-level information on the surface composition and structure of aqueous solutions of surface-active organics as model systems for atmospheric aerosol and cloud droplets. XPS is a vacuum-based technique enabled for volatile aqueous organic samples by the application of a high-speed liquid microjet. In combination with brilliant synchrotron X-rays, the chemical specificity of XPS allows distinction between elements in different chemical states and positions within molecular structures. We used core-level C 1s and N 1s signals to identify the alkyl and hydrophilic groups of atmospheric carboxylic acids, alkyl-amines, and their conjugate acids and bases. From this, we infer changes in the orientation of surface-adsorbed species and quantify their relative abundances in the surface. XPS-derived surface enrichments of the organics follow trends expected from their surface activities and we observed a preferential orientation at the surface with the hydrophobic alkyl chains pointing increasingly outward from the solution at higher concentrations. This provides a first direct experimental observation of well-established concepts of surface adsorption and confirms the soundness of the method.We mapped relative abundances of conjugate acid-base pairs in the aqueous solution surfaces from the respective intensities of distinctive XPS signals. For each pair, the protonation equilibrium was significantly shifted toward the neutral form in the surface, compared to the bulk solution, across the full pH range. This represents an apparent shift of the pKa in the surface, which may be toward either higher or lower pH, depending on whether the acid or base form of the pair is the neutral species. The surface shifts are broadly consistent with the relative differences in surface enrichment of the individual acid and base conjugates in binary aqueous solutions, with additional contributions from nonideal interactions in the surface. In aqueous mixtures of surface-active carboxylate anions with ammonium salts at near-neutral pH, we found that the conjugate carboxylic acids were further strongly enhanced. This occurs as the coadsorption of weakly basic carboxylate anions and weakly acidic ammonium cations forms ion-pair surface layers with strongly enhanced local abundances, increasing the probability of net proton transfer according to Le Chatelier's principle. The effect is stronger when the evaporation of ammonia from the surface further contributes to irreversibly perturb the protonation equilibrium, leaving a surplus of carboxylic acid. These surface-specific effects may profoundly influence atmospheric chemistry mediated by aqueous aerosols and cloud droplets but are currently not taken into account in atmospheric models.

Photoelectron spectroscopy of ceria: Reduction, quantification and the myth of the vacancy peak in XPS analysis

Primarily due to its inherent redox chemistry, ceria (CeO2) is of use in many diverse areas of research. However, there is a wealth of misinterpretation of the oxygen spectra when discussing the result of damage or reduction to the CeO2 lattice, especially with regard to a signal in this region attributed to oxygen vacancies. In this paper, it is shown that this peak cannot be due to vacancies and that a better understanding of the changes in stoichiometry of CeO2 is best viewed from that of the Ce(III) component when considered in tandem with the O 1s signal.

Use of X-Ray Photoelectron Spectroscopy as an analytical tool for the study of contamination by Teflon in the synthesis of Mo(VI)Cl2O2Bipy/TiO2 catalysts

The preparation of catalysts can involve various sources of contamination, which can seriously affect the quality of the prepared materials. In the present work, a case of fluorine contamination in a set of catalyst samples was studied, in which using the X-Ray Photoelectron Spectroscopy (XPS) technique, it was evidenced by the F 1s signal that this element was present in the form of Teflon, since its binding energy corresponded mainly to the CF2 species. Furthermore, using the C 1s signal, it was also possible to corroborate the presence of the CF2 group, which is associated with the main component of the Teflon carbon chains. The use of this information made it possible to identify that the solvent dehydration procedure (previous step to obtaining the catalysts) could lead to contamination with Teflon since it involved various accessories with Teflon, organic solvents and high temperature; the Teflon tape and the magnetic stirrer being the possible sources of contamination.

Efficient Hot Electron Transfer and Extended Separation of Charge Carriers at the 1P Hot State in Sb2Se3/CdSe p-n Heterojunction.

Utilization of hot carriers is very crucial in improving the efficiency of solar energy devices. In this work, we have fabricated an Sb2Se3/CdSe p-n heterojunction via a cation exchange method and investigated the possibility of hot electron transfer and relaxation pathways through ultrafast spectroscopy. The enhanced intensity of the CdSe hot excitonic (1P) bleach in the heterostructure system confirmed the hot electron transfer from Sb2Se3 to CdSe. Both the 1S and 1P signals are dynamically very slow in the heterosystem, validating this charge migration phenomenon. Interestingly, recovery of the 1P signal is much slower than that of 1S. This is very unusual as 1S is the lowest-energy state. This observation indicates the strength of hot electron transfer in this unique heterojunction, which helps in increasing the carrier lifetime in the hot state. Extended separation of charge carriers and enhanced hot carrier lifetime would be extremely helpful in extracting carriers and boost the performance of optoelectronic devices.

Selective alteration mechanisms of sodium tripolyphosphate towards serpentine: Implications for flotation of pyrite from serpentine

Since the preferential dissolution of hydroxyl ion, serpentine is positively charged under weak alkaline conditions, and easily covers on sulfide minerals through electrostatic attraction in flotation process. To address this problem, sodium tripolyphosphate (STPP) was adopted as a novel depressant for serpentine in this work. Micro-flotation results demonstrated that compared with SHMP and CA, STPP possessed a superior effect for removing the adverse impact of serpentine on pyrite flotation with a maximum increase about 75% in pyrite recovery at pH 9. Results of ICP-OES tests showed that STPP could selectively adsorb on serpentine and facilitate the dissolution of magnesium cations from bulk serpentine at pH 9, while had a little influence on pyrite. The synergetic actions overcompensated the positive surface charge on serpentine and its zeta potential was rendered negative. Hence, the electrostatic repulsion between the negatively charged pyrite and serpentine destroyed their hetero-coagulation, which was verified by the DLVO calculations. DFT calculations and XPS analyses indicated that the oxygen anions in STPP could chelate with the magnesium cations exposed on the magnesium surface of serpentine (001) plane with the generation of P-O-Mg bonds, which led to the adsorption of STPP on serpentine. Besides, XPS results also confirmed the stepped-up dissolution of magnesium cations from serpentine with a decrease in the Mg 1s signal of serpentine after adding STPP. In general, STPP may be applied as an efficient dispersant in the flotation separation of sulfide ores from serpentine.

Propargyl carbamate-functionalized Cu(II)-metal organic framework after reaction with chloroauric acid: An x-ray photoelectron spectroscopy data record

A copper-containing metal organic framework was prepared using the new organic linker 5-(2-{[(prop-2-yn-1-yloxy)carbonyl]-amino}ethoxy)isophthalic acid [1,3-H2YBDC (where Y = alkYne and BDC = Benzene DiCarboxylate)] and functionalized with gold particles by reaction with HAuCl4 under thermal treatment in methanol. The resulting system was investigated by complementary techniques to obtain information on its structure and morphology. In the present work, x-ray photoelectron spectroscopy (XPS) was employed to analyze the chemical composition of a representative specimen. Besides wide scan spectra, data obtained by the analysis of the C 1s, O 1s, N 1s, Cu 2p, and Au 4f signals are presented and critically discussed. The results highlight the reduction of Au(III) to mostly Au(I) species. Overall, the data presented herein may act as useful guidelines for the eventual tailoring of material properties and their possible implementation toward functional applications in heterogeneous catalysis.

Ab initio-guided X-ray photoelectron spectroscopy quantification of Ti vacancies in [formula omitted] thin films

Ab initio calculations were employed to investigate the effect of oxygen concentration dependent Ti vacancies formation on the core electron binding energy shifts in cubic titanium oxynitride (Ti1−δOxN1−x). It was shown, that the presence of a Ti vacancy reduces the 1s core electron binding energy of the first N neighbors by ∼0.6 eV and that this effect is additive with respect to the number of vacancies. Hence it is predicted that the Ti vacancy concentration can be revealed from the intensity of the shifted components in the N 1s core spectra region. This notion was critically appraised by fitting the N 1s region obtained via X-ray photoelectron spectroscopy (XPS) measurements of Ti1−δOxN1−x thin films deposited by high power pulsed magnetron sputtering. A model to quantify the Ti vacancy concentration based on the intensity ratio between the N 1s signal components, corresponding to N atoms with locally different Ti vacancy concentration, was developed. Herein a random vacancy distribution was assumed and the influence of surface oxidation from atmospheric exposure after deposition was considered. The so estimated vacancy concentrations are consistent with a model calculating the vacancy concentration based on the O concentrations determined by elastic recoil detection analysis and text book oxidation states and hence electroneutrality. Thus, we have unequivocally established that the formation and population of Ti vacancies in cubic Ti1−δOxN1−x thin films can be quantified by XPS measurements from N 1s core electron binding energy shifts.

In situ XPS studies of MoS2-based CO2 hydrogenation catalysts

Various formulations of Co, Ni and K modified MoS2-based catalysts were synthesized hydrothermally and compared in their catalytic performance in CO2 hydrogenation at 21 bar and 220 °C–330 °C. The products were CO, CH4 and methanol. The addition of K reduced the selectivity to CH4 and moved the maximum of the methanol formation rate to a higher temperature. The materials were characterized by N2 physisorption, temperature programmed oxidation, temperature programmed reduction, and x-ray diffraction spectra (XRD). Under reducing conditions the catalysts were stable until 700 °C and under oxidizing conditions until 300 °C. XRD had shown mainly a MoS2 phase, as well as metal sulfide in the cobalt and nickel promoted catalysts. Different species of Mo and O were observed under reaction conditions by in situ x-ray photoelectron spectroscopy (XPS). When switching from H2 to CO2 + H2 an increase in the amount of oxygen, both lattice oxygen and surface adsorbates, was observed as a consequence of CO2 addition and H2O formation in the reaction. The amount of lattice oxygen correlates with the minor amount of Mo(VI) detected. Increasing reaction temperature and thus conversion led to an increase of the O 1s signals at 533 eV assigned to surface OH, formate and adsorbed H2O. XPS measurements in CO2 + H2 feed showed an effect of K addition on the adsorbate-related O 1s peak, which appeared at lower binding energy (532 eV) and was assigned to carbonates. This may indicate a different reaction mechanism in the presence of the promoter.

X-ray photoelectron and Raman spectroscopy of nanostructured ceria in soot oxidation under operando conditions

Ceria nanoshapes exhibiting different amounts of {100}, {110} and {111} facets (cubes, rods, octahedra and polyhedra) were prepared, characterized, tested in the carbon soot oxidation reaction, and studied by operando near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and operando Raman spectroscopy up to 550 °C. The specific soot oxidation reaction rate (mgC·m−2·min−1) clearly indicated that the {110} and {100} crystallographic planes of ceria were more active than the {111} ones for the oxidation of carbon soot. As deduced from the Ce 3d and O 1s signals recorded using different photon energies, all ceria nanoshapes experienced progressive reduction upon increasing the temperature under Ar, which was accompanied by the formation of oxygen vacancies. Raman studies revealed that, at this stage, isolated graphene layers in carbon soot reacted with ceria lattice oxygen atoms following a Mars-Van Krevelen mechanism. After exposure to O2 at 550 °C, a broad signal in the O 1s region at 531.2–532.5 eV was ascribed to surface active oxygen species, such as peroxide (O22−) and superoxide (O2−) species, generated from the interaction of molecular O2 with oxygen vacancies, which proved to be highly reactive to oxidize the graphitic structures of carbon soot.

Effect of magnesium on the XPS and Raman spectra of (Ba0.5Sr0.5)(Al0.2-xMgxFe0.8)O3-ξ (x ≤ 0.2)

ABSTRACT The perovskite-type cubic oxides have found useful applications in catalysis, solid oxide fuel cells, gas sensors, and membrane technology. XPS and Raman studies of a recently developed (Ba0.5Sr0.5)(Al0.2-xMgxFe0.8)O3-ξ (x = 0–0.2) oxygen-permeable system are reported here. The effect of magnesium is shown to alter the relative amounts of Fe3+, Fe4+, and oxygen species via Fe (2p3/2 and 2p1/2) and O 1s signals. Besides, it causes a surge in oxygen vacancies, an increase in Fe3+ ions, a decline in B – O bond strength (B = Al/Mg/Fe), and a rise in anion flow. The O1s signals at ~ 528.5 and ~531.0 eV correspond to surface oxide and adsorbed (O2 2−, O2−, or O−) species, respectively. Raman spectra offer evidence for symmetric A1g stretching, oxygen vacancies, distorted BO6 octahedra, A-O stretching vibration modes (A = Ba, Sr), and bending B-O linkages. A good correlation is advanced between the XPS and Raman results, in conformity with lattice expansion, Mossbauer data, improved oxygen permeability, enhanced electrical conductivity, and high structural stability realized by means of partial replacement of Al3+ with Mg2+ in (Ba0.5Sr0.5)(Al0.2Fe0.8)O3-ξ earlier.

Distinguishing Electron and Hole Dynamics in Functionalized CdSe/CdS Core/Shell Quantum Dots Using Complementary Ultrafast Spectroscopies and Kinetic Modeling

The evolution of excitation energy and photogenerated charges in semiconductor quantum dots (QDs) functionalized with molecular acceptors can be probed on ultrafast time scales using techniques such as transient absorption (TA) spectroscopy. However, historical interpretations that the 1S(e)-1S3/2(h) transition in Cd-chalcogenide QDs is fully attributable to electrons may be misleading, and multiexponential models used to fit TA kinetics do not correspond directly to specific photophysical processes. Here, we present visible-wavelength and mid-IR TA and time-resolved photoluminescence measurements to inform a comprehensive kinetic model of the photoexcited CdSe/CdS core/shell QDs functionalized with passivating oleic acid (OA), hole-accepting ferrocene, or electron-accepting naphthalene bisimide (NBI). We show that ∼30% of the 1S signal and 72% of the IR signal can originate from holes in well-passivated core/shell QDs. We also demonstrate evidence of electron trapping in OA-capped core/shell QDs, with additional electron transfer and hole trapping in the QDs functionalized with NBI. Electron (hole) trapping and detrapping occur in 450 ± 100 ps (430 ± 70 ps) and 340 ± 100 ps (1.1 ± 0.4 ns) respectively, while the time constant for electron transfer to NBI is ∼1.8 ns. The comprehensive picture of photophysical processes provided by the complementary ultrafast techniques and kinetic modeling can accelerate both the fundamental science and application development of nanostructured and molecular systems.

Hydrolysis corrosion of alumina thin films produced by pulse DC reactive magnetron sputtering at various operating pressures

Alumina thin films were prepared by pulsed DC reactive magnetron sputtering using operating pressures that were varied from 3 to 20 mTorr. The films were immersed in DI water at temperatures of 55 °C and 65 °C for 30 min to study their hydrolysis corrosion behaviors. Unlike bulk crystalline Al2O3 materials, sputtered alumina films fabricated at operating pressures of 7 mTorr and higher were found to react with DI water within minutes, even under mild conditions. X-ray diffraction (XRD) and spectroscopic ellipsometry (SE) showed that the as-sputtered films had amorphous structures with various degrees of porosity within the films. The calculated porosity was found to increase from 17% to 25% as the operating pressure increased from 3 to 20 mTorr, respectively. Field-emission scanning electron microscopy (SEM) was employed to characterize the morphologies of the corroded films. Attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy showed the presence of hydroxide-containing functional groups on surfaces of alumina films, suggesting that the corrosion was due to a hydrolysis reaction. X-ray photoelectron spectroscopy (XPS) revealed distinct features in the non-corroded and corroded sample groups. For the corroded group (7–20 mTorr), the Al 2p peak showed two transitions, at 74.2 and 75.5 eV, attributed to Al–O and Al–OH, respectively. The O 1s peak intensities associated with the hydroxide content of samples in this group were found to be stronger than those associated with the lattice oxygen. The O 1s signal from adsorbed water at 533.7 eV became much stronger in corroded samples. The results also show that films fabricated at higher operating pressures yielded higher levels of pre-adsorbed hydroxide. Corrosion may progress through collective processes, including the formation of soluble aluminum hydroxide complex species and Al–O bond breaking during the proton transfer reactions between adsorbed water and hydroxide.

Layer-by-layer self-assembled polyurea layers onto MAO surface for enhancing corrosion protection to aluminum alloy 6063

Structural defects such as micro-pores and cracks generated during the micro-arc oxidation process can greatly reduce the corrosion resistance of the coating. This work proposed a method to improve the anti-corrosion property of the micro-arc oxidation (MAO) coating by layer-by-layer (LBL) assembling a polyurea layer on the surface of it. Organic silane (KH) and isophorone diisocyanate (IPDI) were self-assembled alternately on the MAO coating surface and formed a polyurea layer. The assembled layers were investigated using XPS, FTIR, EDS-SEM, EIS and polarization curves measurements. Three organic layers obtained couldn't be seen from SEM morphology. Whereas the XPS results showed the existence of polyurea layers with the disappearance of Al 2p signals and appearance of N 1s signals. The MAO coating surface was hydrophilic, and the contact angles of samples with odd-numbered layer (KH550 surface) were up to 95°, while the samples with even-numbered layer (IPDI surface) were about 87°, meaning that the silane and IPDI were successfully assembled on the MAO surface, improving the hydrophobicity of coating. The KH550/IPDI coating exhibited remarkably corrosion protection, and the corrosion resistance of the coating was maintained above 109 Ω·cm2 after exposure in NaCl solution for 288 h. In addition, the self-corrosion current density was under 1.46 × 10−9 mA·cm−2 after 288 h immersion, indicating that the KH550/IPDI coating would keep the anti-corrosion ability for longer time.

1-Bromonaphthalene, by near-ambient pressure XPS

Near ambient pressure-x-ray photoelectron spectroscopy (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i.e., at greater than 5000 Pa. With NAP-XPS, liquids, biological samples, porous materials, and/or polymeric materials that outgas significantly can be studied. In this submission, we show survey, C 1s, Br 3p, and Br 3d NAP-XPS spectra of 1-bromonaphthalene. Small O 1s and N 1s signals from background gas (N2 and air) are also observed.

MnO2 nanomaterials functionalized with Ag and SnO2: An XPS study

The present study is devoted to the characterization of MnO2-Ag and MnO2-SnO2 nanocomposites of technological interest as gas sensors for food quality control and security applications. In particular, MnO2 nanomaterials were deposited on polycrystalline alumina substrates by plasma enhanced-chemical vapor deposition and functionalized with either Ag or SnO2 nanoparticles by means of radio frequency-sputtering. The target materials were investigated by a multitechnique approach, evidencing the successful formation of nanocomposites with tailored morphology, featuring an intimate contact of their components. Herein, the attention is dedicated to the characterization of system surface composition by x-ray photoelectron spectroscopy. Besides wide scan spectra, data obtained by the analysis of the C 1s, O 1s, Mn 2p, Mn 3s, Ag 3d, Sn 3d, and silver Auger signal spectral features are presented and critically discussed. The results highlight the occurrence of phase-pure MnO2 and SnO2 free from other Mn and Sn oxidation states and of appreciable silver surface oxidation in the target nanomaterials.

Recyclable label-free SERS-based immunoassay of PSA in human serum mediated by enhanced photocatalysis arising from Ag nanoparticles and external magnetic field

In the field of SERS-based immunoassay, the recyclable utilization of noble metal materials is essential for their practical application in clinical medicine. Herein, a kind of novel multifunctional Fe3O4@TiO2@Ag microparticles (MPs) was synthesized through a poly-step reaction, and then their feasibility for recyclable label-free SERS-based immunoassay of prostate specific antigen (PSA) was demonstrated. The photocatalytic efficiency of the Fe3O4@TiO2@Ag MPs was fully adjusted by tuning the thickness of the middle TiO2 shell and the areal density of the outside Ag NPs. Particularly, the Fe3O4 core played a critical role in further improving the photocatalytic activity under external magnetic field. Thereafter, the successfully photocatalytic degradation and recyclable detection of PSA in human serum was confirmed by the change of O 1s, N 1s, and C 1s signals in the XPS characterization and a limit of detection as low as 16.25 pg/mL was realized. Furthermore, the quantitative recyclable detection of PSA in clinical serum samples with prostate cancer was conducted, and the results are basically consistent with those of the traditional chemiluminescent immunoassay, showing small relative deviation less than or equal to 5.7%.

Roman coin, by near-ambient pressure XPS

Near ambient pressure-x-ray photoelectron spectroscopy (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i.e., at greater than 2500 Pa. With NAP-XPS, XPS can probe moderately volatile liquids, biological samples, porous materials, and/or polymeric materials that outgas significantly. In this submission, we show survey, O 1s/Ag 3p, Ca 3p/Ag 3d, and extended valence band (0–130 eV) NAP-XPS spectra of an ancient Roman coin at three different positions. A small N 1s signal from N2 background gas is also observed. On the obverse side, the coin bears the bust of Licinius I. On the reverse side, it bears the image of Jupiter. The Ag 3d region indicates different amounts of silver at different oxidation states in different positions.

Clinoptilolite, a type of zeolite, by near ambient pressure-XPS

Near ambient pressure-x-ray photoelectron spectroscopy (NAP-XPS) is a less traditional form of XPS that allows samples to be analyzed at relatively high pressures, i.e., at greater than 2500 Pa. With NAP-XPS, XPS can probe moderately volatile liquids, biological samples, porous materials, and/or polymeric materials that outgas significantly. In this paper, we show the NAP-XPS survey; O 1s, Ca 2p, C 1s, K 2p, Al 2s, Al 2p, Si 2p, and Si 2s narrow scans; and the extended valence band spectrum of clinoptilolite, a natural zeolite that would be difficult to analyze by conventional XPS. A small N 1s signal from N2(g) is also observed in the survey spectrum. Signals in the narrow scans are fit to Gaussian–Lorentzian sum and Gaussian–Lorentzian product functions.

Kinetics of the Thermal Oxidation of Ir(100) toward IrO2 Studied by Ambient-Pressure X-ray Photoelectron Spectroscopy.

Using time-lapsed ambient-pressure X-ray photoelectron spectroscopy, we investigate the thermal oxidation of single-crystalline Ir(100) films toward rutile IrO2(110) in situ. We initially observe the formation of a carbon-free surface covered with a complete monolayer of oxygen, based on the binding energies of the Ir 4f and O 1s core level peaks. During a rather long induction period with nearly constant oxygen coverage, the work function of the surface changes continuously as sensed by the gas phase O 1s signal. The sudden and rapid formation of the IrO2 rutile phase with a thickness above 3 nm, manifested by distinct binding energy changes and substantiated by quantitative XPS analysis, provides direct evidence that the oxide film is formed via an autocatalytic growth mechanism that was previously proposed for PbO and RuO2.