X-ray Photoelectron Spectroscopy Binding Energy Shifts Research Articles

XPS Binding Energy Shifts in 2D Ti3C2Tz MXene go largely Beyond Intuitive Explanations: Rationalization from DFT Simulations and Experiments.

MXenes are prototypes of surface tunable 2D materials with vast potential for properties tuning. Accurately characterizing their surface functionalization and its role in electronic structure is crucial, X-ray photoelectron spectroscopy (XPS) being among the go-to methods to do so. Despite extensive use, XPS analysis remains however intricate. Focusing on the benchmark MXene Ti3C2Tz, Density Functional Theory (DFT) calculations of core-level binding energy shifts (BE.s.) are combined with experiments in order to provide a quantitative interpretation of XPS spectra. This approach demonstrates that BE.s. are driven by the complex interplay between chemical, structural, and subtle electronic structure effects preventing analysis from intuitive arguments or comparison with reference materials. In particular, it is shown that O terminations induce the largest BE.s. at Ti 2p levels despite lower electronegativity than F. Additionally, F 1s levels show weak sensitivity to the F local environment, explaining the single contribution in the spectrum, whereas O 1s states are significantly affected by the local surface chemistry. Finally, clear indicators of surface group vacancies are given at Ti 2p and O 1s levels. These results demonstrate the combination of calculations with experiments as a method of the highest value for MXenes XPS spectra analysis, providing guidelines for otherwise complex interpretations.

One-Step Synthesis and Operando Electrochemical Impedance Spectroscopic Characterization of Heterostructured MoP-Mo2N Electrocatalysts for Stable Hydrogen Evolution Reaction.

This study presents a novel synthesis of self-standing MoP and Mo2N heterostructured electrocatalysts with enhanced stability and catalytic performance. Facilitated by the controlled phase and interfacial microstructure, the seamless structures of these catalysts minimize internal resistivity and prevent local corrosion, contributing to increased stability. The chemical synthesis proceeds with an etching step to activate the surface, followed by phosphor-nitriding in a chemical vapor deposition chamber to produce MoP-Mo2N@Mo heterostructured electrocatalysts. X-ray diffraction analyses confirmed the presence of MoP, Mo2N, and Mo phases in the electrocatalyst. Morphology studies using scanning electron microscopy characterize the hierarchical growth of structures, indicating successful formation of the heterostructure. X-ray photoelectron spectroscopy (XPS) analyses of the as-synthesized and postcatalytic activity samples reveal the chemical shift in terms of the binding energy (BE) of the Mo 3d XPS peak, especially after catalytic activity. The XPS BE shifts are attributed to changes in the oxidation state, electron transfer, and surface reconstruction during catalysis. Electrochemical evaluation of the catalysts indicates the superior performance of the MoP-Mo2N@Mo heterostructured catalyst in hydrogen evolution reactions (HER), with lower overpotentials and enhanced Tafel slopes. The stability tests reveal changes in double layer capacitance over time, suggesting surface reconstruction and an increased active surface area during catalysis. Operando electrochemical impedance spectroscopy (EIS) further elucidates the dynamic changes in resistance and charge transfer during HER. Overall, a comprehensive understanding of the synthesis, characterization, and electrochemical behavior of the developed MoP-Mo2N@Mo heterostructured electrocatalyst, as presented in this work, highlights its potential utilization in sustainable energy applications.

Transition Metal β-Diketonate Adhesion Promoters in Epoxy-Anhydride Resin.

Epoxy to copper adhesion supports the reliability of numerous structures in electronic packaging. Compared to substrate pre-treatment, processing and cost considerations are in favor of adhesion promoters loaded in epoxy formulations. In this work, first row transition metal β-diketonates present such a compelling case when added in epoxy/anhydride resins: over 30% (before moisture aging) and 50% (after moisture aging) enhancement in lap shear strength are found using Co(II) and Ni(II) hexafluoroacetylacetonate. From extensive X-ray photoelectron spectroscopy (XPS) analyses on the adhesively failed sample surfaces, increased population of oxygen-containing functional groups, especially esters, is linked to the adhesion improvement. Assisted by XPS depth profile on the fractured epoxy side and in situ Fourier-transform infrared spectroscopy (FTIR), the previously discovered latent cure characteristics endowed by the metal chelates interacting with phosphine catalysts are regarded pivotal for pacing the anhydride consumption and allowing interfacial esterification reactions to occur. Further examinations on the XPS binding energy shifts and dielectric properties of the doped epoxy also reveal metal-polymer coordination that contribute to the adhesion and moisture resistance properties. These findings should stimulate future research of functional additives targeting at cure kinetics control and polar group coordination ideas for more robust epoxy-Cu joints.

First-principles calculation of X-ray photoelectron spectroscopy binding energy shift for nitrogen and phosphorus defects in 3C-silicon carbide

We systematically investigated the formation energies and the core-level X-ray photoelectron spectroscopy binding energy (XPSBE) shifts of nitrogen (N) 1s and phosphorus (P) 2p for defects including N and P in 3C-SiC by a first-principles calculation using the generalized gradient approximation, whose reliability for n-type defects was confirmed by some tests using the HSE06 hybrid functional. XPSBEs were separated into the local potential average around the impurity and the relaxation energy of the wave function to analyze the relationship between the XPSBE shift and the defect structures. It is difficult to understand the relaxation energy intuitively. The electrons localized around the impurity atom, which have energy levels in energy gaps, make a large contribution to the relaxation energies. Considering the formation energies, we predicted some XPS peaks expected to be found.

First-principles X-ray photoelectron spectroscopy binding energy shift calculation for boron and aluminum defects in 3C-silicon carbide

We systematically investigated the core-level X-ray photoelectron spectroscopy (XPS) binding energy shifts of B 1s and Al 2p and formation energies for defects including boron and aluminum in 3C-silicon carbide (SiC) by first-principles calculation. We analyzed the relation between the XPS binding energy shift and defect states and found that the defects with localized electrons in the band gap or energy gap in the valence band have larger XPS relaxation energies (XPSREs) than those without localized electrons. In contrast, spread electrons and electrons localized away from the core hole hardly affect the XPSREs. Further, we analyzed the dependence on crystal matrices, that is, elemental and compound semiconductors, on XPS spectra by comparing the XPS spectra of Si and 3C-SiC. Although the variation of the local potential in 3C-SiC is larger than that in Si, the variations of their relaxation energies are comparable.

Initial stages in functionalization of polystyrene upon treatment with oxygen plasma late flowing afterglow

A remote microwave discharge was applied as a source of oxygen (O) atoms for treatment of polystyrene in the preparation chamber of an x-ray photoelectron spectroscopy (XPS) instrument. The density of O atoms as measured with a calibrated catalytic probe was 2 × 1018 m−3. Such a small O atom density allowed for investigating the initial stages of polymer functionalization. The samples were subjected to various fluences of O atoms from approximately 2 × 1021 to 3 × 1023 m−2, and the functional groups were determined by XPS without breaking vacuum conditions. The results showed rapid destruction of the aromatic rings even at low O atom fluences. The initial functionalization caused the formation of hydroxyl groups followed by carbonyl groups, but the measurable amount of the carboxyl or ester groups, let alone carbonate, appeared only after the fluence of approximately 1 × 1022 m−2. The concentration of functional groups of the high XPS binding energy shift increased at elevated fluences until it became comparable with those of the low energy shift at the largest fluence used in this investigation, i.e. 3 × 1023 m−2. The O concentration on the polymer surface as determined from XPS survey spectra was first rapidly increasing up to the fluence of a few 1022 m−2 and then continue increasing slowly with the increasing fluence even beyond 1 × 1023 m−2. This investigation, which was performed in situ, gives us new knowledge for understanding the initial steps of polymer functionalization.

Investigation of various phases of Fe–Si structures formed in Si by low energy Fe ion implantation

The compositional phases of ion beam synthesized Fe–Si structures at two high fluences (0.50×1017atoms/cm2 and 2.16×1017atoms/cm2) were analyzed using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). The distribution of Fe implanted in Si was simulated using a dynamic simulation code (TRIDYN) incorporating target sputtering effects. The Fe depth profiles in the Si matrix were confirmed with Rutherford backscattering spectrometry (RBS) and XPS depth profiling using Ar-ion etching. Based on XPS binding energy shift and spectral asymmetry, the distribution of stable Fe–Si phases in the substrate was analyzed as a function of depth. Results indicate Fe implantation with a fluence of 0.50×1017atoms/cm2 and subsequent thermal annealing produce mainly the β-FeSi2 phase in the whole thickness of the implanted region. But for the samples with a higher fluence Fe implantation, multiple phases are formed. Significant amount of Fe3Si phase are found at depth intervals of 14nm and 28nm from the surface. Initially, as-implanted samples show amorphous Fe3Si formation and further thermal annealing at 500°C for 60min formed crystalline Fe3Si structures at the same depth intervals. In addition, thermal annealing at 800°C for 60min restructures the Fe3Si clusters to form FeSi2 and FeSi phases.

The relationship between the structural properties of bimetallic Pd–Sn/SiO 2 catalysts and their performance for selective citral hydrogenation

The effect of Sn addition to Pd on the selective liquid-phase hydrogenation of citral to α,β-unsaturated alcohols (UA: nerol and geraniol) was examined. Pd–Sn/SiO 2 bimetallic catalysts were prepared by successive impregnation method and were characterized by transmission electronic microscopy (TEM) coupled with energy dispersive X-ray spectroscopy (EDX), temperature-programmed reduction (TPR), Fourier transform infrared (FTIR) spectroscopy of adsorbed CO, and X-ray photoelectron spectroscopy (XPS). Sn addition to Pd/SiO 2 catalysts significantly modifies their properties for citral hydrogenation performed at 130 °C, under 7 MPa and in isopropanol solvent, inducing a promoting effect on the UA selectivity. This promoting effect is related to the existence of a Pd–Sn interaction highlighted by EDX analysis, TPR under hydrogen and FTIR of adsorbed CO. The latter technique suggested the presence of a geometric effect on catalytic activity. Maximum UA selectivities (>75% at 30% citral conversion) were obtained when an alloy of the Pd 3Sn type is formed in the bimetallic particles, as confirmed by TPR and XPS. Moreover, FTIR measurements of the adsorbed CO singleton frequency as well as XPS binding energy shifts strongly imply an electron transfer from Sn to Pd, which is proposed to be responsible for enhanced adsorption of citral C O bond on the surface of Pd–Sn/SiO 2 bimetallic catalysts. To our knowledge, it is the first time that modified Pd catalysts lead to such important UA selectivity values during α,β-unsaturated aldehyde hydrogenation.

Investigation of HfO2 high-k dielectrics electronic structure on SiO2/Si substrate by x-ray photoelectron spectroscopy

In this Letter, x-ray photoelectron spectroscopy (XPS) was used to investigate electronic structures of HfO2 with various thicknesses on SiO2/Si substrate prepared by atomic layer deposition with post deposition annealing of 600 °C, which is compatible with a gate last process in the fabrication of complementary metal oxide semiconductor. The hafnium silicate formed between HfO2 and SiO2 interface plays a key role in generating an internal electric field. This can be attributed to the presence of oxygen vacancies (Vo) at the interface. The XPS binding energy shifts of O 1s, Si 2p, and Hf 4f spectra with increasing HfO2 thicknesses can be explained by the presence of the internal field and differential charging effects. Electrical measurements of capacitor structures support the XPS result.

Framework Zinc-Substituted Zeolites: Synthesis, and Core-Level and Valence-Band XPS

A proof of framework substitution of T atoms of faujasite by divalent zinc is obtained by high-resolution X-ray photoelectron spectroscopy (XPS). Distinct and systematic XPS binding energy shifts between cationic and framework zinc are reported for both the core-levels and the valence-band Zn3d emission. A strong preference of T-atom substitution with zinc for silicon rather than aluminum is observed. Synthesis of a zinc-containing faujasite gives rise to a clean embedding of zinc into the framework. A subsequent postsynthesis modification by aqueous solution of zinc oxide in sodium hydroxide further enhances the framework zinc concentration. In the present investigation, a total of 15% Si atoms substituted by zinc yielded over a 2-fold enhancement of the monovalent cation-exchange capacity of faujasite.

Structural characterisation of silica supported CoMo catalysts by UV Raman spectroscopy, XPS and X-ray diffraction techniques

The structure of Co-Mo catalysts supported on commercial silica, doped with various amounts of sodium ions, was investigated by means of X-ray diffraction (XRD), UV-Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Two series of samples were considered. One series was prepared by classic incipient wet impregnation (WI) and the other series by co-impregnation in the presence of nitrilotriacetic acid (NTA). The effect of sodium on the catalyst structure depended on the preparation procedure: in the case of the WI catalysts, sodium promoted the transformation of the polymolybdate species into monomolybdate Na 2MoO 4; in the case of the NTA prepared samples, sodium induced transformation from heptamolybdates, Mo 2O 7 2− and mixed CoMo oxides into MoO 4 2− units with distorted tetrahedral symmetry without forming Na 2MoO 4 compound. In addition to purely structural modifications, sodium induced an electronic effect, evidenced by Mo 3d and Co 2p XPS binding energy shifts. The catalytic behaviour of the samples, previously tested in the HDS of thiophene, was explained in terms of structural and electronic changes.

Atmospheric pressure chemical vapour deposition of vanadium nitride and oxynitride films on glass from reaction of VCl4 with NH3

Atmospheric pressure chemical vapour deposition of vanadium nitride and oxynitride coatings was achieved on glass substrates from the reaction of VCl4 and NH3 at 350–650°C. The coatings show excellent uniformity, surface coverage, adherence and a range of colours (yellow, blue–silver) dependent on deposition temperature and film thickness. Growth rates were of the order of 1 µm min−1. All films deposited at greater than 450°C were crystalline, single phase with an fcc NaCl diffraction pattern, a = 4.065–4.116(2) A. Scanning electron microscopy (SEM) revealed different film thicknesses and surface morphologies consistent with an island growth mechanism. X-Ray photoelectron spectroscopy (XPS) revealed that individual samples were homogeneous. Compositional variations between films were related to reaction conditions; VN0.8O0.18–VN0.56O0.8. XPS binding energy shifts were seen for V 2p at 513.3 eV for O 1s at 530.8 eV and for N 1s at 397.1 eV and were largely invariant between samples. Energy dispersive X-ray analysis (EDXA) and electron probe studies gave elemental ratios that were in agreement with the XPS measurements, indicating no chlorine incorporation. Raman spectra showed a broad band at 600 cm−1. Sheet resistance measurements indicated that all films were metallic. Optically, the films were reflective in the range 800–2600 nm and showed reasonable transmission in the range 300–800 nm. The films show promise as solar-control coatings.

Self-Assembly of Tetraphenylporphyrin Monolayers on Gold Substrates

Two tetraphenylporphyrins, TPP-P-disulfide and TPP-P-thiol, were synthesized and used to form self-assembled monolayers (SAMs) on gold surfaces. The monolayers were characterized using X-ray photoelectron spectroscopy (XPS), ultraviolet/visible absorption spectroscopy (UV/vis), and scanning tunneling microscopy (STM). XPS binding energy shifts revealed that the porphyrins were chemisorbed to the surface through a sulfur−gold bond. A red shift without a significant blue-shifted component of the Soret band in the absorption spectra demonstrated that the porphyrin molecules are aligned on the gold surface in a side-by-side orientation. Round STM features, approximately 2 nm in diameter, correspond closely to the diameter of tetraphenylporphyrin (1.8 nm). Taken together, these data indicated the formation of monolayers of uniformly spaced TPP-P-disulfide and TPP-P-thiol molecules, with the porphyrin ring oriented parallel to the gold surface. Furthermore, porphyrin monolayers were stable for at least a week a...

An XPS Study of the Interaction of Ultrathin Cu Films with Pd(111)

The interaction of ultrathin Cu films with Pd(111) was studied by X-ray photoelectron spectroscopy (XPS). The effects of Cu coverage and annealing temperature were investigated. The XPS data show that at room temperature Cu grows on Pd(111) layer-by-layer without alloying. Furthermore, Cu 2p3/2 core-level shifts as a function of film thickness indicate that the Cu−Pd interactions perturb the electronic properties of two to three layers of Cu atoms. The Cu 2p3/2 binding energy of a Cu monolayer at room temperature was shifted by −0.47 eV relative to a Cu(100) surface. XPS core-level shifts demonstrate that by annealing to temperatures higher than 450 K the Cu overlayer alloys with the Pd substrate. After annealing to 900 K, the Cu 2p3/2 binding energy for 1.0 ML Cu coverage was observed to shift −0.49 eV relative to that of 1 ML Cu/Pd(111). The XPS binding energy shifts are discussed in terms of both initial and final state effects.

Adsorption of N,N‘-Disalicylidene-1,2-propanediamine on 304 Stainless Steel

The behavior of the jet fuel metal deactivator additive (MDA), N,N′-disalicylidene-1,2propanediamine in some high-temperature tests, has raised questions concerning whether it would decompose on contact with hot metal surfaces in aircraft engines and fuel systems. The adsorption of this additive was studied on oxidized and oxide-free 304SS surfaces using X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD). XPS binding energy (BE) shifts indicated thermal decomposition on the surfaces. TPD revealed the growth of both physisorbed and chemisorbed states in accordance with the Stranski-Krastanov mechanism. MDA was only weakly chemisorbed onto the 304SS surface, with desorption energies of 47 kJ/mol on an oxidized surface and 33 kJ/mol on the oxide-free surface. These findings suggest that stainless steel would not likely contain a contiguous chemisorbed MDA multilayer that the adsorbed MDA could be removed from internal surfaces of aircraft fuel systems in regions of high-temperature and turbulent flow.

Surface analytical investigations of the electrochemical double layer on silver electrodes in alkaline media

The electrochemical double layer on Ag in aqueous solutions was examined ex situ with X-ray photoelectron spectroscopy (XPS) and UV-photoelectron spectroscopy (UPS). The specimens were removed from the electrolyte with hydrophobic surfaces and under potential control. The potential dependent surface concentrations of the adsorbed anions, cations and the surface excess charge, as well as the amount of adsorbed water were determined. The results obtained for the double layer on Ag in 1 M NaClO 4+0.1 M NaOH were compared with an acidic perchlorate solution and pure 1 M NaOH. We found evidence for a specific adsorption of hydroxide ions in the basic electrolyte; i.e. the OH −-surface concentration has to be considered for a proper determination of the cationic excess charge and the potential of zero charge. The observed systematic XPS binding energy shifts of the adsorbate species with respect to the metal core levels and the measured work function changes suggest that the electrochemical adsorbates are not in electronic equilibrium with the metal.

Pd-Ti bimetallics: A study of the electronic structure using x-ray photoelectron spectroscopy and x-ray-absorption near-edge structure.

We studied the core-level binding-energy shifts and the valence-band behavior of ${\mathrm{Pd}}_{3}$Ti and ${\mathrm{PdTi}}_{2}$ alloys using x-ray photoelectron spectroscopy (XPS) and x-ray-absorption near-edge structure (XANES). The observed Pd core-level shifts in the alloys, relative to the pure metal, were not consistent with the electronegativity predictions. It is proposed that upon alloying there is an increase in the number of sp-like conduction electrons and a decrease in the number of d electrons at the Pd site. This electron redistribution at the noble-metal site in the alloy results in a small net charge flow between Pd and Ti, in the direction in accord with electronegativity predictions. Pd ${\mathit{L}}_{3}$-edge and Ti K-edge XANES provide further evidence of electron redistribution upon alloying. Net charge transfer at the Pd site of ${\mathrm{PdTi}}_{2}$ has been estimated to be 0.29\ifmmode\pm\else\textpm\fi{}0.02 electron counts on the basis of a charge compensation model, results from XPS binding-energy shift, and XANES white-line areas. The observed Pd valence-band narrowing and shifts to a higher binding energy are also explained in terms of dilution and charge redistribution.

Correlation between Lewis donor/acceptor properties determined by XPS and Brönsted acid/base properties determined by rest-potential measurements, for aluminium and silicon oxides

In order to qualify the reactivity of various aluminium and silicon oxide substrates for elastomer adhesion applications, X-ray photoelectron spectroscopy (XPS) and rest-electrochemical potential measurements have been performed on those surfaces. The interpretation of XPS binding energy shift measurements in terms of Fermi level variation from one surface to another, as proposed by Mullins and Averbach in 1988, is discussed in view of results on an aluminosilicate compound surface. The correlation with electrochemical restpotential measurements on an anodized aluminium surface and a silane coupling agent is described and discussed. The possibility of surface reactivity assessment through correlated XPS and electrochemical measurements for metal oxides is demonstrated.

Influence of Cu, Ag, Ir and Pt on the work function and near‐surface electron density of Re mass spectrometer filaments

AbstractThe influence of metal overlayers on the properties of polycrystalline Re mass spectrometer filaments has been examined using contact potential difference (CPD) measurements of the work function, x‐ray photoelectron spectroscopy (XPS) measurements of overlayer binding energy (BE) and ultraviolet photoelectron spectroscopy (UPS) measurements of electron energy distribution curves (EDCs). Work function data for CuRe, AgRe and IrRe are compared to earlier results for AuRe, PdRe and PtRe as part of the effort to understand the influence of surface additions to the work functions of ionization sources. The changes in work function owing to the addition of a metal overlayer do not always follow predictions of bulk electron transfer, such as the Miedema electronegativity. However, the observed changes in work function are consistent with a simple initial‐state understanding of the XPS core‐level data involving charge flow toward and away from the surface. The XPS binding energy shifts are consistent with shifts observed in the valence bands for Cu and Ag on Re.