XPS Binding Energy Research Articles

Nanoisland formation of small Ag -clusters on HOPG as determined by inner-shell photoionisation spectroscopy

Abstract XPS (3d) and Auger spectra (MNN) of deposited Ag n -clusters on a non-sputtered HOPG surface have been measured. Most of the XPS and Auger spectra appear very similar to the corresponding bulk spectrum, caused by a high mobility and agglomeration of the clusters on the inert carbon surface, independent of the initial cluster size. The metallic fingerprint of the cluster-agglomerated islands is sustained under UHV conditions for several days. Oxidised Ag-islands reveal a positive binding energy shift in distinct contrast to the negative XPS shift usually observed for silver oxide bulk compounds. The XPS binding energy shift has been used for estimation of the diameter of the cluster-assembled nanoislands (6–9 nm) using an electrostatic model.

Theoretical and experimental local reactivity parameters of 3-substituted coumarin derivatives.

Local reactivity descriptors, such as atomic charges, atomic electrostatic potential and atomic Fukui indices were computed for a series of 3-substituted coumarin (2-oxo-2H-1-benzopyran) derivatives, using density functional theory (DFT) and Möller-Plesset methods (MP2). The variation of those properties as a function of the substituents was compared with the variation of the measured XPS binding energies. The atomic electrostatic potentials and XPS binding energies serves as indicators of the electrophilicity of a given center within a molecule, while the atomic Fukui indices describe its degree of electronic localization, known as atomic softness. The correlation between those theoretical and experimental properties allowed us to follow the effect of electron withdrawing substituents on the electrophilicity of a given atomic center. The Fukui indices provided additional information about the softening/hardening of the center of interest due to presence of different substituents to the coumarin system. On the basis of these analysis, the 1,2-addition would be favored for 3-acetyl, 3-phosphono, and 7-diethylamino substituents, while 3-carboxyl, 3-ethoxycarbonyl, and 3-nitro substituent would favor 1,4-addition. The substituted coumarins would preferably react with soft nucleophiles at position 2 and with hard nucleophiles at position 4.

Highly active mixed-metal nanosheet water oxidation catalysts made by pulsed-laser ablation in liquids.

Surfactant-free mixed-metal hydroxide water oxidation nanocatalysts were synthesized by pulsed-laser ablation in liquids. In a series of [Ni-Fe]-layered double hydroxides with intercalated nitrate and water, [Ni1-xFex(OH)2](NO3)y(OH)x-y·nH2O, higher activity was observed as the amount of Fe decreased to 22%. Addition of Ti(4+) and La(3+) ions further enhanced electrocatalysis, with a lowest overpotential of 260 mV at 10 mA cm(-2). Electrocatalytic water oxidation activity increased with the relative proportion of a 405.1 eV N 1s (XPS binding energy) species in the nanosheets.

Self-doping for visible light photocatalytic purposes: construction of SiO2/SnO2/SnO2:Sn2+nanostructures with tunable optical and photocatalytic performance

This work explores a hydrothermal route to synthesis of self-doped SiO2/SnO2/SnO2:Sn2+ nanostructures with controlled hierarchical structures and lattice parameters for tunable optical and photocatalytic properties. The effects of Sn2+ self-doping in the SnO2 lattice on the local structure, electronic structure, and photodegradation of methyl orange were systematically investigated. It is found that two layers of SnO2 and SnO2:Sn2+ nanocrystals with thicknesses of 13 and 8 nm were coated on SiO2 spheres showing monodispersed features. By Sn2+ doping, a blue shift of the Sn 3d XPS binding energy was observed, which is thought to be mainly related to the difference in electronegativity of Sn4+, Sn2+ and variation of lattice parameters. In contradiction to the quantum size effect, the self-doped SiO2/SnO2/SnO2:Sn2+ nanostructures showed an abnormal red shift of band gap energy, which is related to doping effects and variations of lattice parameters. With well-defined lattice structure and electronic structure, the photocatalytic performance of SiO2/SnO2/SnO2:Sn2+ nanostructures can be well regulated and optimized.

Ir promotion of TiO2 supported Au catalysts for selective hydrogenation of cinnamaldehyde

We synthesized Ir-on-Au (Au–Ir) nanoparticles (NPs) using 8–9nm Au NPs, which had a partial coverage of Ir. Both the studied systems allowed the obtainment of cinnamyl alcohol with high selectivities (>83%). However, Au–Ir/TiO2 delivered a hydrogenation rate 5 times higher than that of Au/TiO2. The deposition of Ir onto the surface of Au and the presence of surface Au–Ir alloy were confirmed by UV–vis and HRTEM respectively. Moreover. Moreover, the strongly shifted XPS binding energy implyed the electron transfer from Ir to Au, which was believed to be responsible for the enhanced H2 activation capacity of Au.

Positive XPS binding energy shift of supported CuN-clusters governed by initial state effects

An initial state effect is established as origin for the positive 2p core electron binding energy shift found for CuN-clusters supported by a thin silica layer of a p-doped Si(100) wafer. Using the concept of the Auger parameter and taking into account the usually neglected Coulomb correlation shift in the Auger final state (M4,5M4,5) it is shown that the initial state shift is comparable to the measured XPS shift while the final state relaxation shift contributes only marginally to the binding energy shift. The cluster results differ from the negative surface core-level shift of crystalline copper which has been explained in terms of a final state relaxation effect.

Effect of Oxygen Stoichiometry on Microstructural and Magnetic Properties of FePt/TaO$_{\rm x}$ Bilayer Fabricated by Ion-Beam-Bombardment Deposition

The effect of TaOx capping layer on microstructures and magnetic properties of FePt thin films via annealing was studied. The structural ordering of FePt from face-centered cubic to face-centered tetragonal phases depends strongly on the oxygen contents in the capping TaOx layer. The role of the TaOx layer is used to separate the FePt grains, as revealed by TEM. The annealed FePt/TaOx (15%O2/Ar) exhibited the largest out-of-plane coercivity (~2.4 kOe) amongst all samples, compared to that (Hc ~ 4.2 kOe) in the reference FePt layer. At low oxygen content in FePt/TaOx bilayers, the Ta atoms may act as defects to obstruct the FePt ordering whereas at high oxygen contents, the excess oxygen atoms diffuse into the FePt layer and react with Fe to form iron oxides which give rise to the low coercivities, as characterized by the XPS depth profile and binding energy analysis.

Density functional theory calculations of XPS binding energy shift for nitrogen-containing graphene-like structures

Our results validate the use of independent DFT predicted BE shifts for defect identification and constraining ambient pressure XPS observations for Me-Nx moieties in pyrolyzed carbon based ORR electrocatalysts. This supports the understanding of such catalysts as vacancy-and-substitution defects in a graphene-like matrix.

Stability of Platinum Nanoparticles Supported on SiO2/Si(111): A High-Pressure X-ray Photoelectron Spectroscopy Study

The stability of Pt nanoparticles (NPs) supported on ultrathin SiO(2) films on Si(111) was investigated in situ under H(2) and O(2) (0.5 Torr) by high-pressure X-ray photoelectron spectroscopy (HP-XPS) and ex situ by atomic force microscopy (AFM). No indication of sintering was observed up to 600 °C in both reducing and oxidizing environments for size-selected Pt NPs synthesized by inverse micelle encapsulation. However, HP-XPS revealed a competing effect of volatile PtO(x) desorption from the Pt NPs (~2 and ~4 nm NP sizes) at temperatures above 450 °C in the presence of 0.5 Torr of O(2). Under oxidizing conditions, the entire NPs were oxidized, although with no indication of a PtO(2) phase, with XPS binding energies better matching PtO. The stability of catalytic NPs in hydrogenation and oxidation reactions is of great importance due to the strong structure sensitivity observed in a number of catalytic processes of industrial relevance. An optimum must be found between the maximization of the surface active sites and metal loading (i.e., minimization of the NP size), combined with the maximization of their stability, which, as it will be shown here, is strongly dependent on the reaction environment.

Size-dependent XPS spectra of small supported Au-clusters

High resolution XPS spectra (4f) of mass-selected AuN-clusters supported by a thin natural silica layer are presented in the size range N=1–35 atoms per cluster. The XPS binding energy as well as peak width is found to increase systematically with decreasing cluster size. Furthermore a size-dependent asymmetry is observed on the high energy tail of the photoemission peaks. Analysing the XPS shift and line shape using the dynamic electrostatic model helps to identify initial and final state effects such as negative surface core level shift, inhomogeneous broadening, dynamic final-state screening and chemisorption-like interaction between cluster and support.

Particle Size and Structural Control of ZnWO4 Nanocrystals via Sn2+ Doping for Tunable Optical and Visible Photocatalytic Properties

In this work, we report on the microwave-assisted hydrothermal synthesis of Sn2+-doped ZnWO4 nanocrystals with controlled particle sizes and lattice structures for tunable optical and photocatalytic properties. The samples were carefully characterized by X-ray diffraction, transmission electron microscopy, inductive coupled plasma optical emission spectroscopy, UV–vis diffuse reflectance spectroscopy, and Barrett–Emmett–Teller technique. The effects of Sn2+ doping in ZnWO4 lattice on the crystal structure, electronic structure, and photodegradation of methylene orange dye solution were investigated both experimentally and theoretically. It is found that part of the Sn2+ ions were homogeneously incorporated in the ZnWO4 host lattice, leading to a monotonous lattice expansion, and part of Sn2+ ions were expelled at surface sites for decreased crystallinity and particle size reduction. By Sn2+ doping, ZnWO4 nanocrystals showed a significant XPS binding energy shift of Zn 2p, W 4f, and O 1s, which is attributed to the combination of electronegativity between Sn2+ and Zn2+, lattice variation, and particle size reduction. Meanwhile, the BET surface areas were also greatly enlarged from 40.1 to ∼110 m2·g–1. Contrary to the theoretical predictions of the quantum size effect, Sn2+-doped ZnWO4 nanocrystals showed an abnormal band gap narrowing, which can be well-defined as a consequence of bulk and surface doping effects as well as lattice variations. With well-controlled particle size, crystallinity, and electronic structure via Sn2+ doping, the photocatalytic performance of Sn2+-doped ZnWO4 nanocrystals was optimized at Sn2+ doping level of 0.451.

Characterization of SiC in DLC/a-Si films prepared by pulsed filtered cathodic arc using Raman spectroscopy and XPS

DLC/a-Si films were deposited on germanium substrates. a-Si film was initially deposited as a seed layer on the substrate using DC magnetron sputtering. DLC film was then deposited on the a-Si layer via a pulsed filtered cathodic arc (PFCA) system. In situ ellipsometry was used to monitor the thicknesses of the growth films, allowing a precise control over the a-Si and DLC thicknesses of 6 and 9nm, respectively. It was found that carbon atoms implanting on a-Si layer act not only as a carbon source for DLC formation, but also as a source for SiC formation. The Raman peak positions at 796cm−1 and 972cm−1 corresponded to the LO and TO phonon modes of SiC, respectively, were observed. The results were also confirmed using TEM, XPS binding energy and XPS depth profile analysis.

Adsorption Geometry Determines Catalytic Selectivity in Highly Chemoselective Hydrogenation of Crotonaldehyde on Ag(111)

The chemoselective hydrogenation of crotonaldehyde to crotyl alcohol was studied by temperature programmed desorption/reaction, high resolution XPS and NEXAFS. The organic molecule adsorbed without decomposition, all three possible hydrogenation products were formed and desorbed, and the clean overall reaction led to no carbon deposition. Selectivities up to 95% were found under TPR conditions. The observed behavior corresponded well with selectivity trends previously reported for Ag/SiO2 catalysts and the present findings permit a rationalization of the catalytic performance in terms of pronounced coverage-dependent changes in adsorption geometries of the reactant and the products. Thus at low coverages the C=O bond in crotonaldehyde lay almost parallel to the metal surface whereas the C=C was appreciably tilted, favoring hydrogenation of the former and disfavoring hydrogenation of the latter. With increasing coverage of reactants, the C=C bond was forced almost parallel to the surface, rendering it vulnerable to hydrogenation, thus markedly decreasing selectivity towards formation of crotyl alcohol. Butanol formation was the result of an overall two-step process: crotonaldehyde → crotyl alcohol → butanol, further hydrogenation of the desired product crotyl alcohol being promoted at high hydrogen coverage due to the C=C bond in the unsaturated alcohol being driven from a tilted to a flat-lying geometry. Finally, an explanation is offered for the strikingly different behavior of Ag(111) and Cu(111) in the chemoselective hydrogenation of crotonaldehyde in terms of the different degrees of charge transfer from metal to C=O π bond, as suggested by C 1s XPS binding energies.

Electronic Structure of the Water Oxidation Catalyst cis,cis-[(bpy)2(H2O)RuIIIORuIII(OH2)(bpy)2]4+, The Blue Dimer

The first designed molecular catalyst for water oxidation is the "blue dimer", cis,cis-[(bpy)(2)(H(2)O)Ru(III)ORu(III)(OH(2))(bpy)(2)](4+). Although there is experimental evidence for extensive electronic coupling across the μ-oxo bridge, results of earlier DFT and CASSCF calculations provide a model with magnetic interactions of weak to moderately coupled Ru(III) ions across the μ-oxo bridge. We present the results of a comprehensive experimental investigation, combined with DFT calculations. The experiments demonstrate both that there is strong electronic coupling in the blue dimer and that its effects are profound. Experimental evidence has been obtained from molecular structures and key bond distances by XRD, electrochemically measured comproportionation constants for mixed-valence equilibria, temperature-dependent magnetism, chemical properties (solvent exchange, redox potentials, and pK(a) values), XPS binding energies, analysis of excitation-dependent resonance Raman profiles, and DFT analysis of electronic absorption spectra. The spectrum can be assigned based on a singlet ground state with specific hydrogen-bonding interactions with solvent molecules included. The results are in good agreement with available experimental data. The DFT analysis provides assignments for characteristic absorption bands in the near-IR and visible regions. Bridge-based dπ → dπ* and interconfiguration transitions at Ru(III) appear in the near-IR and MLCT and LMCT transitions in the visible. Reasonable values are also provided by DFT analysis for experimentally observed bond distances and redox potentials. The observed temperature-dependent magnetism of the blue dimer is consistent with a delocalized, diamagnetic singlet state (dπ(1)*)(2) with a low-lying, paramagnetic triplet state (dπ(1)*)(1)(dπ(2)*)(1). Systematic structural-magnetic-IR correlations are observed between ν(sym)(RuORu) and ν(asym)(RuORu) vibrational energies and magnetic properties in a series of ruthenium-based, μ-oxo-bridged complexes. Consistent with the DFT electronic structure model, bending along the Ru-O-Ru axis arises from a Jahn-Teller distortion with ∠Ru-O-Ru dictated by the distortion and electron-electron repulsion.

The role of the Auger parameter in XPS studies of nickel metal, halides and oxides

The critical role of the Auger parameter in providing insight into both initial state and final state factors affecting measured XPS binding energies is illustrated by analysis of Ni 2p(3/2) and L(3)M(45)M(45) peaks as well as the Auger parameters of nickel alloys, halides, oxide, hydroxide and oxy-hydroxide. Analyses of the metal and alloys are consistent with other works, showing that final state relaxation shifts, ΔR, are determined predominantly by changes in the d electron population and are insensitive to inter-atomic charge transfer. The nickel halide Auger parameters are dominated by initial state effects, Δε, with increasing positive charge on the core nickel ion induced by increasing electronegativity of the ligands. This effect is much greater than the final state shifts; however, the degree of covalency is reflected in the Wagner plot where the more polarizable iodide and bromide have greater ΔR. The initial state shift for NiO is much smaller than those of Ni(OH)(2) or NiOOH and the effective oxidation state is much less than that inferred from the average electronegativity of the ligand(s). Auger parameter analysis indicates that the bonding in NiO appears to have stronger contributions from initial state charge transfer from the oxygen ligands than that in the hydroxide and oxyhydroxide consistent with the considerable differences in the Ni-O bond lengths in these compounds with some relaxation of this state occurring during final state phenomena. The Auger parameter of NiOOH is, however, shifted positively, like the iodide, indicating greater polarizability of the ligands and covalency in this bonding. There is support for more direct use of relative bond lengths in interpreting differences between related compounds rather than more general electronegativity or similar parameters.

Synthesis, Characterization, and Materials Chemistry of Group 4 Silylimides

This paper focuses on the development of potential single source precursors for M-N-Si (M = Ti, Zr or Hf) thin films. The titanium, zirconium, and hafnium silylimides (Me(2)N)(2)MNSiR(1)R(2)R(3) [R(1) = R(2) = R(3) = Ph, M = Ti(1), Zr (2), Hf (3); R(1) = R(2) = R(3) = Et, M = Ti (4), Zr (5), Hf (6); R(1) = R(2) = Me, R(3) = (t)Bu, M = Ti (7), Zr (8), Hf (9); R(1) = R(2) = R(3) = NMe(2), M = Ti (10), Zr (11), Hf (12)] have been synthesized by the reaction of M(NMe(2))(4) and R(3)R(2)R(1)SiNH(2). All compounds are notably sensitive to air and moisture. Compounds 1, 2, 4, and 7-10 have been structurally characterized, and all are dimeric, with the general formula [M(NMe(2))(2)(μ-NSiR(3))](2), in which the μ(2)-NSiR(3) groups bridges two four-coordinate metal centers. The hafnium compound 3 possesses the same basic dimeric structure but shows additional incorporation of liberated HNMe(2) bonded to one metal. Compounds 11 and 12 are also both dimeric but also incorporate additional μ(2)-NMe(2) groups, which bridge Si and either Zr or Hf metal centers in the solid state. The Zr and Hf metal centers are both five-coordinated in these species. Aerosol-assisted CVD (AA-CVD) using 4-7 and 9-12 as precursors generates amorphous films containing M, N, Si, C, and O; the films are dominated by MO(2) with smaller contributions from MN, MC and MSiON based on XPS binding energies.

Nano-Roughened Teflon-Like Film Coated Polyethylene Terephthalate Substrate with Trifluoromethane Plasma Enhanced Hydrophobicity and Transparency

Nano-roughened Teflon-like films with manipulated topography and chemical bonds are demonstrated to concurrently enhance hydrophobicity and transparency. By fluorocarbon plasma deposition with tunable F/C precursor ratio on polyethylene terephthalate substrate, the nano-roughened Teflon-like film prepared with an F/C precursor ratio of 3 reveals a repellent water-contact angle with long-term bonding stability due to its extremely low surface energy density of 28 mN/m caused by enhancing the highly cross-linking -CF2 functional groups. Increasing the F/C precursor ratio up to 4 oppositely reduces the F/C composition ratio and down-shifts the XPS binding energy peak of the C-F bond in -CFx groups with increasing bond length, which inevitably shrinks the water contact angle after thermal stability test for 24 h. Enlarging the reaction time with trifluoromethane plasma enriches the fluorine atom content to improve the cross-linking capability of -CFx and -C-CFx bonds, simultaneously promoting the high hydrophobicity and transparent surface with water contact angle up to 133° and minimum absorption coefficient of 5623 cm−1 at 450 nm, respectively.

CuO-loaded nano-porous SnO 2 films fabricated by anodic oxidation and RIE process and their gas sensing properties

The fabrication of nano-porous SnO 2 films with 1-D nano-channels by anodic oxidation has been a challenge due to a collapse of pore structure during annealing process and an existence of non-porous upper barrier layer. A new approach is developed to synthesize the nano-porous SnO 2 films by employing Sn thin film instead of Sn foil and a reactive ion etching (RIE) process. The integrity of nano-porous structure was maintained by anodizing the Sn thin film deposited on SiO 2/Si substrate, and 1-D nano-channels were opened up by removing the non-porous layer by RIE. As a potential application to gas sensor, CuO was impregnated to the nano-porous SnO 2 films and their gas sensing properties were investigated. The nano-porous CuO–SnO 2 film sensors exhibited the enhanced sensitivity and selectivity toward H 2S gas, and their H 2S sensing mechanism was discussed in terms of the formation/disruption of resistive p-CuO/n-SnO 2 junction based on the variation of XPS binding energy and electrical resistance during gas exposure.

Adsorption, Desorption, and Reaction of Methyl Radicals on Surface Terminations of α-Fe2O3

The adsorption, desorption, and reaction of gas phase methyl radicals were studied on the (0001)-oriented α-Fe2O3 surface in ultrahigh vacuum. Two different surface terminations were compared: An Fe3O4 (111) layer and the so-called “biphase” surface thought to be a mixture of FeO and Fe2O3 terminations. Gas phase methyl radicals were prepared by pyrolysis of azomethane. On Fe3O4 (111) methyl radical adsorption forms surface methoxide species as determined by the C(1s) XPS binding energy. Temperature programmed reaction spectroscopy produced direct desorption of methyl radicals at all coverages and the formation of ethane at high coverages in two desorption peaks at 331 and 439 K. The activation energies for desorption were 84 and 133 kJ/mol in the two regimes. The two surface terminations exhibit saturation coverages that differ by ca., 30×: 1.5 × 1014 and 5.2 × 1012 per cm2 for the Fe3O4(111) and “biphase” terminations, respectively. These results are interpreted in terms of bonding models and difference...

Homoleptic Platinum(II) and Palladium(II) Organothiolates and Phenylselenolates: Solvothermal Synthesis, Structural Determination, Optical Properties, and Single‐Source Precursors for PdSe and PdS Nanocrystals

Homoleptic d(8)-metal organothiolates and phenylselenolates [M(EC(6)H(5))(2)](infinity) (E=S, M=Pt 1, M=Pd 2, M=Ni 5; E=Se, M=Pt 3, M=Pd 4) were prepared as crystalline solids under solvothermal conditions. Their structures were solved using powder X-ray diffraction data. In each case, the EC(6)H(5) (E=S, Se) ligand binds to two metal ions (M=Pt, Pd, and Ni) to form chain-like structures with planar (in 1) or zig-zag (in 2-5) conformations. The [M(SR)(2)](infinity) complexes (M=Pt, R=4-tert-butylphenyl 6; R=2-naphthyl 8; R=4-nitrophenyl 10 and M=Pd, R=4-tert-butylphenyl 7; R=2-naphthyl 9; R=4-nitrophenyl 11) were prepared under similar solvothermal conditions. Based on the XPS binding energies and elemental analyses, complexes 6-11 have the same [M(SR)(2)](infinity) formulation as 1 and 2. The cyclic complex [Pd(6)(SCH(3))(12)] 12 was prepared as a crystalline solid by solvothermal annealing treatment of the amorphous precipitate. A chain-like polymer structure is proposed for both [Pd(SC(12)H(25))(2)](infinity) 13 and [Pd(SC(16)H(33))(2)](infinity) 14; these polymeric chains self-assemble to give layer-like structures. Solid-state diffuse reflectance spectra reveal that the optical band gap E(g) (eV) of complexes 1, 6, 8, 10 and of 2, 7, 9, 11 are in the range of 2.10-3.00 eV and 2.10-2.63 eV, respectively, and 5 has the lowest E(g) value (1.72 eV). Heating solid samples of 4 and 13 under solvothermal conditions afforded phase-pure Pd(17)Se(15) and PdS nanocrystals, respectively. Field-effect transistors fabricated with a drop-cast thin film made from Pd(17)Se(15) nanocrystals prior treated with an ethanolic solution of 1-hexadecanethiol displayed ambipolar charge transporting properties with hole and electron mobility being 7x10(-2) cm(2) V(-1) s(-1) and 6x10(-2) cm(2) V(-1) s(-1), respectively.