High Binding Energy Component Research Articles

Formation of cesium carbonate in ion-implanted graphite, examined with dual-source x-ray photoelectron spectroscopy, density functional theory calculations and thermodynamic modelling

A sample of highly orientated pyrolytic graphite (HOPG) was implanted with Cs+ ions in order to study the effect on the graphite matrix and the chemical form of Cs once embedded. The mean implantation depth was calculated to be ∼22 nm, allowing for investigation with X-ray Photoelectron Spectroscopy, (XPS), where both a traditional Al Kα X-ray source and a higher energy Ag Lα source was used for an increased sampling depth deeper into the surface. Analysis found that there was a reduction of sp2 C–C bonding with increasing implantation dose, resulting in a total loss of sp2 character at ∼6 atomic % Cs+. A striking increase in higher binding energy components (285.8–289.4 eV) in the C 1s spectra cannot be attributed solely to the presence of C–O species, instead indicating a dramatic re-ordering of the graphitic lattice to accommodate and neutralize the embedded Cs. XPS analysis indicates the formation of cesium carbonate within the graphitic material; this is reinforced by analysis of a Cs2CO3 reference sample. Additionally, thermodynamics equilibrium simulations, supported by simplified density functional theory (DFT) calculations, are performed, leading to strong agreement with the XPS findings; that is, the most stable form of Cs within the graphite matrix is expected to be cesium carbonate (Cs2CO3), which is in thermodynamic equilibrium with degraphitised HOPG and oxygenated degraphitised HOPG.

Jahn–Teller distorted Cu1−xNixCr2O4 (x = 0, 0.5, 1) nanoparticles

In this report, x-ray photoelectron spectroscopy data for three types of Cu-doped nickel chromite nanoparticles are presented. CuCr2O4, Cu0.5Ni0.5Cr2O4, and NiCr2O4 were prepared using the citric acid-assisted sol-gel method. All samples were amorphous after the initial gel preparations were dried, but after calcination at temperatures higher than 700 °C, the samples crystallized in a spinel structure. X-ray diffraction patterns for the samples were well fitted with the I41/amd space group. Survey scans were conducted as well as detailed scans of Cu 2p, Ni 2p, Cr 2p, and O 1s levels. The O 1s core level was well-fitted with three components, including lattice oxygen (low binding energy) and two higher binding energy components, which corresponded to surface-absorbed hydroxyl groups originating from water and alcohol used in the synthesis of the particles. The line shape of the Cu 2p core level was compatible with mixed 2+ and 1+/0+ oxidation states for Cu ions in these nanoparticles. The strong intensity of the high binding energy satellite indicated that the majority of the Cu 2p ions were in 2+ oxidation state. Finally, Ni 2p showed a mixed 3+/2+ oxidation state for Ni ions in these nanoparticles.

Multiferroic nanoparticles of Ni doped CoCr2O4: An XPS study

Three spinel compounds, namely CoCr2O4, (Co0.75Ni0.25)Cr2O4, and (Co0.5Ni0.5)Cr2O4 were investigated using x-ray photoelectron spectroscopy techniques. These compounds were synthesized using sol-gel techniques. The gel was dried and subsequently calcined at 600 °C to achieve crystallinity and the correct cubic phase. The survey scans of the analyzed samples with detailed spectra of the Co 2p, Ni 2p, Cr 2p, O 1s, and the valence band were collected. The core levels are found to be asymmetric and have multiple components. Ni and Co have 2+/3+ oxidation states, whereas Cr prefers 3+ oxidation state. The O 1s core level demonstrates three components. The low binding energy component refers to the lattice oxygen atoms, whereas the two high binding energy components correspond to the surface adsorbed hydroxyl groups resulting from water and the alcohol used for the synthesis of these nanoparticles. The data emanating from this study are reported.

Initial oxidation kinetics of Si(113)-(3 × 2) investigated using supersonic seeded molecular beams

The initial oxidation of silicon surfaces with (113) orientation has been investigated using high-resolution photoelectron spectroscopy with synchrotron radiation. In the present study, we investigated both the Si 2p state and the O 1s state to evaluate the oxide thickness and composition, and to assess the strain at the SiO2/Si interface. The oxidized components in the Si 2p state (Si1+, Si2+, Si3+, Si4+) were analyzed. In the O 1s state, a low-binding-energy component (LBC) and a high-binding-energy component (HBC) were analyzed. To investigate the non-thermal oxidation process, we utilized supersonic seeded molecular beam (SSMB) to obtain oxygen molecules with high translational kinetic energies (Et). We demonstrate that the oxide quality and oxidation kinetics are largely altered by changing the value of Et.

Chemical state analysis, optical band gap, and photocatalytic decolorization of cobalt-doped ZnO nanospherical thin films by DC/RF sputtering technique

We have used direct current radio frequency (DC/RF) sputtering to deposit cobalt- incorporated zinc oxide (Co:ZnO) thin films on glass substrate. The x-ray diffraction (XRD) was applied for structural observation while topography was examined by atomic force microscopy (AFM). The optical studies were carried out by UV–vis absorption spectroscopy. The elemental composition was performed via x-ray photoelectron spectroscopy (XPS). The chemical state studies were also investigated. The shifting of XRD pattern towards higher diffraction angle confirmed successful incorporation. The optical observation showed “d–d” electron transition during the thin film deposition. The photocatalytic response was carried through the degradation of methylene blue (MB). The improved photocatalytic behavior of Co:ZnO thin films were attributed to the high binding energy component (HBEC) of oxygen vacancies. We have also proposed the tentative mechanism of cobalt incorporation into ZnO through XPS finding. We also checked the stability of deposited thin films. Co:ZnO thin films are new attractive candidates for degradation of waste dyes.

Investigation of the interrelation between the chemical state and the electric properties in Al-doped ZnO films

Transparent conducting Al-doped ZnO (AZO) thin films were prepared on glass substrates by radio frequency magnetron sputtering in pure Ar. The influence of the annealing atmosphere on the microstructure, chemical state, electric and optical properties of the AZO films was investigated with X-ray diffraction, field-emission scanning electron microscopy, X-ray photoelectron spectroscopy and Hall measurements. The AZO thin films annealed under vacuum had the highest carrier concentration of 2.488 × 1020 cm−3 and a Hall mobility of 16.35 cm2 V−1 s−1, while the AZO thin films annealed in air had the lowest carrier concentration of 4.182 × 1017 cm−3 and a Hall mobility of 2.375 cm2 V−1 s−1. The fitted narrow-scan O1s spectra revealed that O1s was composed of three components. The AZO thin films annealed under vacuum appeared to have a higher proportion of medium binding energy which correspond to O2− ions in the oxygen-deficient regions within the ZnO matrix, and have a lower proportion of high binding energy component which correspond to loosely bound chemisorbed oxygen. It believed that the oxygen vacancies and chemisorbed oxygen of the films played an important role in the electrical conductance. The carrier concentration increased with the formation of oxygen vacancies. The Hall mobility increased with desorption of the loosely bound oxygen.

Adsorption of CO<sub>2</sub> on Polyethyleneimine 10k—<i>Mesoporous silica</i> Sorbent: XPS and TGA Studies

A CO2 solid sorbent based on polyethyleneimine 10k (PEI-10k) impregnated into mesoporous silica (MPS) foam was synthesized and utilized to capture CO2 at temperatures ranged from 65°C to 95°C. The calculated nitrogen and carbon contents in the bulk of the PEI-10k/MPS sorbent were similar to the XPS results measured on the surface of the foam, suggesting that the PEI was homo-geneously distributed throughout the MPS support. After CO2 adsorptionthe N 1s peak was broadened and could be resolved into two components: a high binding energy component (~401 eV) and a lower binding energy one (396 eV), respectively. The former nitrogen states are consistent with a protonated amine, presumably, due to carbamate formation. The lower binding energy component (~396 eV) could possibly be due to strongly chemisorbed CO2. The maximum sorption capacity was about 4 mmole CO2/g sorbent at 85°C and 1 bar. This capacity was doubled by raising the CO2 pressure to 24.95 bars. The adsorption results can be described by a Langmuir adsorption isotherm.

Evidence of room temperature ferromagnetism in argon/oxygen annealed TiO2 thin films deposited by electron beam evaporation technique

TiO2 thin films deposited by electron beam evaporation technique annealed in either O2 or Ar atmosphere showed ferromagnetism at room temperature. The pristine amorphous film demonstrates anatase phase after annealing under Ar/O2 atmosphere. While the pristine film shows a super-paramagnetic behavior, both O2 and Ar annealed films display hysteresis at 300K. X-ray photo emission spectroscopy (XPS), Raman spectroscopy, Rutherford’s backscattering spectroscopy (RBS), cross-sectional transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS) were used to refute the possible role of impurities/contaminants in magnetic properties of the films. The saturation magnetization of the O2 annealed film is found to be higher than the Ar annealed one. It is revealed from shifting of O 1s and Ti 2p core level spectra as well as from the enhancement of high binding energy component of O 1s spectra that the higher magnetic moment is associated with higher oxygen vacancies. In addition, O2 annealed film demonstrates better crystallinity, uniform deposition and smoother surface than that of the Ar annealed one from glancing angle X-ray diffraction (GAXRD) and atomic force microscopy (AFM). We conclude that although ferromagnetism is due to oxygen vacancies, the higher magnetization in O2 annealed film could be due to crystallinity, which has been observed earlier in Co doped TiO2 film deposited by pulsed laser deposition (Mohanty et al., 2012 [10]).

Growth of Ag and Au Nanoparticles on Reduced and Oxidized Rutile TiO2(110) Surfaces

The nucleation and growth of Au and Ag nanoparticles on rutile TiO2(110)–(1 × 1) surfaces in different oxidation states is studied by means of photoelectron spectroscopy (PES) and scanning tunneling microscopy (STM). Au and Ag nanoparticles were found to bind much more strongly to oxidized TiO2(110) model supports than to reduced TiO2(110) surfaces, as directly revealed by STM. Detailed PES studies addressing small Au and Ag particles complete this picture and show that the PES core level spectra acquired on Au/TiO2(110) and Ag/TiO2(110) can be best described by fitting with two binding energy (BE) components. Particularly for coverages in the sub-monolayer regime and for depositions at low temperatures (100 K) the PES core level spectra must be fitted with at least two BE components. The higher BE component is attributed to atoms at the interface between the metal clusters and the TiO2(110) support. For Au/TiO2(110), the two BE components were evident in the core level spectra for higher coverage than for Ag/TiO2(110), consistent with different growth modes for Au (initially 2D) and Ag (3D) on TiO2(110). Finally, strong evidence for charge transfer from Ag nanoparticles to the TiO2(110) support is presented, whereas the charge transfer between Au nanoparticles and the TiO2(110) support is very small.

The effects of X-ray irradiation and temperature on the formation and stability of chemical species on enargite surfaces during XPS

An investigation of the species present on the surface of natural enargite and the effects of X-ray radiation on the surface after electrochemical oxidation at pH 10 has been carried out using X-ray photoelectron spectroscopy. Surface species present on fractured and polished unoxidized enargite have been compared with those present on the polished surface after oxidation at pH 10. The effect of X-ray irradiation time as a function of temperature on the forms and intensities of the photoelectron signals from the enargite surface was studied. High binding energy (BE) components and shake-ups of the Cu 2p signals, characteristic of a copper (II) hydroxide surface layer, reduced in intensity with increasing X-ray exposure time. Associated changes in the relative concentrations of surface oxygen species were also observed. Comparing a surface exposed to X-rays at 35°C with that maintained at −135°C, temperature was shown to significantly influence the rate of change. The observed changes were evaluated in terms of a proposed two-stage mechanism involving the dehydration of Cu(OH) to CuO, followed by photoreduction of CuO to Cu2O. The photoreduction effects of X-ray irradiation should be significantly reduced by sample cooling for consistent analysis of the enargite surface using XPS.

Time-evolution of thermal oxidation on high-index silicon surfaces: Real-time photoemission spectroscopic study with synchrotron radiation

The initial oxidation on high-index silicon surfaces with (113) and (120) orientations at 820K has been investigated by real-time X-ray photoemission spectroscopy (Si 2p and O 1s) using 687eV photons. The time evolutions of the Sin+ (n=1–4) components in the Si 2p spectrum indicate that the Si2+ state is suppressed on high-index surfaces compared with Si(001). The O 1s state consists of two components, a low-binding-energy component (LBC) and a high-binding-energy component (HBC). It is suggested that the O atom in strained SiOSi contributes to the LBC component. The reaction rates are slower on high-index surfaces compared with that on Si(001).

Examination of the proposition that Cu(II) can be required for charge neutrality in a sulfide lattice — Cu in tetrahedrites and sphalerite

Synchrotron XPS and Cu L2,3-edge NEXAFS spectroscopic data for a natural tetrahedrite surface prepared by fracture under UHV were in accord with the composition of the mineral and its expected semiconductivity. The 2p binding energy for the 6-coordinate S atoms was found to be not detectably greater than that for the 4-coordinate S atoms, and surface species were not clearly discernible in either surface-optimized S 2p or Cu 2p spectra. The Cu 2p and Cu L2,3-edge spectra indicated that all Cu in the mineral was indisputably Cu(I). The Cu L2,3-edge spectra of relatively pure natural sphalerite treated with mildly acidic aqueous cupric solution revealed the presence of Cu(II) in the outermost layer of the fracture surfaces, but it was concluded that most of the Cu near the surface of the mineral was in formal oxidation state Cu(I), albeit with higher than normal d9character. The Cu(I) absorption peak was at an energy much lower than for the tetrahedrite absorption edge, but still consistent with Cu(I) in 4-fold coordination by S. The Cu(II) was consistent with Cu bonded both to S atoms in the outermost layer of the sphalerite and to O atoms in chemisorbed water. S 2p spectra determined at different photon energies revealed high binding energy components arising from oligosulfide-like environments in the outermost layers, but not necessarily in a completely restructured lattice and not in a Cu oligosulfide only. The data indicated some loss of Zn in addition to the Zn that had been replaced by Cu in the outermost layers of the sulfide lattice. The presence of these oligosulfide-like environments precluded the detection of S with formal oxidation state greater than (-II) that might have arisen only from Cu(I) in the S lattice. No evidence was obtained for the presence of Cu(II) in a sulfide lattice, but it was not possible to exclude the possibility of a very low concentration because of the presence of the Cu(II) bonded to both S and O at the surface of the treated sphalerite.Key words: tetrahedrite, sphalerite, copper uptake, XPS, NEXAFS.

Experimental studies on vacancy induced ferromagnetism in undoped TiO 2

Room temperature ferromagnetism is observed in undoped TiO 2 films deposited on Si substrates using pulsed laser deposition (PLD). The ferromagnetic properties of the samples depend on the oxygen partial pressure during the PLD synthesis. The appearance of higher binding energy component (HBEC) in the oxygen 1s core peak from X-ray photoelectron spectroscopy (XPS) suggests the presence of oxygen vacancies in these samples. The amount of oxygen during the synthesis determines the vacancy concentration in the samples which is directly related to the magnetic behavior of the samples. The magnetic moment decreases with oxygen vacancy concentration in the samples. Valence band measurements were performed to study the electronic structure of both stoichiometric and reduced TiO 2. The analyses show the presence of Ti 3d band near the Fermi level in reduced TiO 2 samples. These bands are otherwise empty in stoichiometric TiO 2 and reside in the conduction band which makes them unobservable by XPS. The existence of this Ti 3d band near the Fermi level can possibly lead to Stoner splitting of the band.

Core level spectroscopy of MoS 2

X-ray photoelectron spectroscopy has been used to study mineral molybdenite, MoS 2. The fitted core level spectra of sulphur 2p and molybdenum 3d states reveal several photon energy sensitive components. The high binding energy component in both spectra is proposed to originate from the uppermost sulphur or molybdenum atoms of an S–Mo–S sandwich layer of the hexagonal structure, respectively. The other features are suggested to be caused by the edge structures formed during the sample cleavage. The edge facets have much stronger chemical properties than the basal planes and they are known as the active sites of MoS 2 when it is used as a catalyst. The spectral features and the effect of the structure of UHV cleaved MoS 2 on them are discussed.

Evidence of different oxygen states during thermal coloration of tungsten oxide

In this study, we report on the thermal reduction of tungsten oxide studied with in situ photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectrometry (ToF-SIMS). XPS measurements of the O1s core level of the annealed oxide film in vacuum show the appearance of a second state displaced in 1.1 eV towards higher binding energy. We propose that the shifted component is due to the difference in the charge state of oxygen anions and results from oxygen that is not bonded to metal. Simultaneous increase of the O 16 ion yield measured with ToF-SIMS suggests that this higher binding energy component is related to atomic oxygen that is released from the oxide but remains in film.

Study of the oxidation for Si nanostructures using synchrotron radiation photoemission spectroscopy

We report the oxidation characteristics for Si nanostructures. Synchrotron radiation Si 1s X-ray photoemission spectroscopy (XPS) is used to explain the oxidation process and the modification of the electronic structures of Si. Si films of different thicknesses were grown on a graphite surface. Before oxidation, several photoemission components are observed in thin Si films, such as 2 or 4 Å films. The components observed at the higher binding energy regions are identified as Si nanostructures. After oxidation, photoemission results show that the oxidation process is thickness dependent. Quantitatively, in 2 and 4 Å films, the photoemission signals from oxide phase are 18 and 30%, respectively of total intensity. The present result suggests that the higher binding energy components, i.e., Si nanostructures play some significant role during the oxidation, where the reactivity of nanostructures towards oxygen is relatively low or negligible, and nanostructures impose some restrictions on the oxidation of the film.

PH dependence of uranyl retention in a quartz/solution system: an XPS study

We have investigated the pH dependence of U(VI) retention in quartz/10 −4 M uranyl solution systems, under conditions favoring formation of polynuclear aqueous species and of colloids of amorphous schoepite as U(VI) solubility-limiting phases. X-ray photoelectron spectroscopy was used to gain insights into the coordination environments of sorbed/precipitated uranyl ions in the centrifuged quartz samples. The U4 f XPS spectra made it possible to identify unambiguously the presence of two uranyl components. A high binding energy component, whose relative proportion increases with pH, exhibits the U4 f lines characteristic of a reference synthetic metaschoepite. Such a high binding energy component is interpreted as a component having a U(VI) oxide hydrate character, either as polynuclear surface oligomers and/or as amorphous schoepite-like (surface) precipitates. Its pH dependence suggests that a binding of polynuclear species at quartz surfaces and/or a formation of amorphous schoepite-like (surface) precipitates is favored when the proportion of aqueous polynuclear species increases. A second surface component exhibits binding energies for the U4 f core levels at values significantly lower (Δ E b =1.2 eV) than for metaschoepite, evidencing uranyl ions in a distinct coordination environment. Such a low binding energy component may be attributed to monomeric uranyl surface complexes on the basis of published EXAFS data. Such a hypothesis is supported by a major contribution of the low binding energy component to the U4 f XPS spectra of reference samples for uranyl sorbed on quartz from very acidic 10 −3 M uranyl solutions where UO 2 2+ ions predominate.

A study of the interface formation between gold and a thin conjugated oligomer film

The interface formation between Au and a five-ring n-octyloxy substituted phenylenevinylene (Ooct-OPV5) oligomer film was studied by X-ray and ultraviolet photoelectron spectroscopies (XPS, UPS), in ultrahigh vacuum. When the oligomer was stepwise deposited on a clean Au polycrystalline foil, an abrupt interface was formed. A high binding energy component present in the oligomer related XPS C1s peak up to the completion of the first monolayer, revealed the existence of an interfacial state. The C1s peak exhibited a total upward BE shift of ∼0.5 eV, related to the band bending in the oligomer film. When Au was stepwise deposited on a ∼5 nm thick oligomer layer, UPS and angle resolved XPS revealed extended diffusion of the Au atoms in the oligomer. The C1s and Au4f peaks exhibited ∼0.5 and ∼0.65 eV downward BE shifts respectively. These shifts are a combination of initial and final state effects in gold and charge transfer between the two materials in contact.

Oxygen structures on Fe(1 1 0)

The adsorption of oxygen on a Fe(1 1 0) single crystal has been studied by means of high resolution photoelectron spectroscopy (HRPES) and scanning tunneling microscopy (STM). Core level spectra were analyzed in detail on both clean and adsorbate covered surfaces. A shoulder on the high binding energy side of the Fe 2p core level indicates a structure comprising multiple components interpreted as an exchange split of the final state due to interaction between the 2p and 3d electrons. After adsorption of oxygen, (2×5), (2×2) and (3×1) reconstructions were observed with atomically resolved STM. The iron surface was further exposed to gradually higher doses of oxygen. Deconvolution of the O 1s HRPES spectra revealed two components separated approximately by 0.4 eV. The component at lower binding energy dominates at low coverage, while the high binding energy component increases in intensity with increasing O coverage. The formation of oxides was observed in the Fe 2p spectrum in the region between 708 and 710 eV. Further, well ordered iron oxides were grown by exposure to oxygen at 250 °C. The O 1s core level contained a single component with a binding energy similar to that of the high energy component in the just discussed O 1s spectrum. Low energy electron diffraction and STM images of this structure showed a large moiré pattern with a 22.1 A ̊ ×30.9 Å unit cell.

Si 2p and O 1s photoemission from oxidized Si(0 0 1) surfaces depending on translational kinetic energy of incident O 2 molecules

The influence of translational kinetic energy of incident O 2 molecules for the passive oxidation of the partially oxidized Si(0 0 1) surface has been studied by photoemission spectroscopy. The incident energy of O 2 molecules was controlled up to 3 eV by a supersonic molecular beam technique. Two incident energy thresholds (1.0 and 2.6 eV) were found out in accordance with the first-principle calculations. Si 2p and O 1s photoemission spectra measured at representative incident energies showed the incident energy induced oxidation at the backbonds of the dimer and the second layer (subsurface) Si atoms. Moreover, the difference of oxygen chemical bonds was found out to be as the low and the high binding energy components in the O 1s photoemission spectra. They were assigned to bridge sites oxygen and dangling bond sites oxygen, respectively.