XPS Valence Band Spectra Research Articles

Structural and XPS studies of PSi/TiO2 nanocomposites prepared by ALD and Ag-assisted chemical etching

PSi/TiO2 nanocomposites fabricated by atomic layer deposition (ALD) and metal-assisted chemical etching (MACE) were investigated. The morphology and phase structure of PSi/TiO2 nanocomposites were studied by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) with an energy dispersive X-ray spectroscopy (EDX) and Raman spectroscopy. The mean size of TiO2 nanocrystals was determined by TEM and Raman spectroscopy. X-ray photoelectron spectroscopy (XPS) was used to analyze the chemical elemental composition by observing the behavior of the Ti 2p, O 1s and Si 2p lines. TEM, Raman spectroscopy and XPS binding energy analysis confirmed the formation of TiO2 anatase phase inside the PSi matrix. The XPS valence band analysis was performed in order to investigate the modification of PSi/TiO2 nanocomposites electronic structure. Surface defects states of Ti3+ at PSi/TiO2 nanocomposites were identified by analyzing of XPS valence band spectra.

Characterization of nitrogen ion implanted TiO2 photocatalysts by XAFS and XPS

A nitrogen doped TiO2 as a visible-light response photocatalyst was prepared by N+ implantation technique. N+-implanted TiO2 samples promoted the photocatalytic activity for degradation of methylene blue under visible-light irradiation. XANES and XPS analyses indicated two types of chemical state of nitrogen, one photo-catalytically active N substituting the O sites and the other inactive NOx (1⩽x⩽2) species. In the valence band XPS spectrum of the high activity sample, the additional electronic states were observed just above the valence band edge of a TiO2. The electronic state would be originated from the substitutional nitrogen and be responsible for the band gap narrowing, i.e., visible light response of TiO2 photocatalysts.

Probing the chemical nature of surface oxides during coal char oxidation by high-resolution XPS

Coal chars, like most solid carbons, have a pronounced tendency to chemisorb oxygen at low and moderate temperatures. Characterization by XPS of surface oxides on carbon has been accomplished with the aim of providing a better atomistic insight into: (a) reactions involved in molecular oxygen adsorption on coals and (b) the relations between the nanostructure of solid carbons and the chemistry of oxidation. High-resolution C 1s and O 1s core level and valence band XPS spectra effectively reflected the oxidative functionalization of different types of coals and synthetic carbons upon oxidation in air at moderate temperatures (300 and 500°C). More specifically, analysis of C 1s and valence band spectra could be directed to monitor the structural evolution of the carbons in terms of extension of sp2 versus sp3 conjugation, carbon vacancies and oxidized carbon. Comparison of the O 1s spectra, on the other hand, provided a tool to characterize the nature of oxygen bonding on carbon and to determine the relative abundance of carbon–oxygen species. Results underline the important role of epoxy groups in the early stages of oxidation, providing a mechanistic framework for the identification of the stable and metastable intermediates in the heterogeneous oxidation of coal by molecular oxygen.

Surface defects control for ZnO nanorods synthesized by quenching and their anti-recombination in photocatalysis

ZnO nanorods with controllable surface defects was synthesized by high-temperature quenching method, and the recombination of photogenerated electron–hole pairs had been drastically suppressed, thus significantly improving the photocatalytic reactivity. The as-prepared samples were characterized for the surface structure, chemical state, phase structure as well as optical absorption using field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Brunauer–Emmett–Teller (BET) analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), fluorescent spectrophotometer (PL), diffuse reflectance UV–visible spectroscopy (DRS) and zeta potential. With XPS valence band spectra characterization, its light absorption enhancement in UV–vis range was found due to induced additional electronic states above the valence band edge. Specific types of defects related to the quenching process were further investigated. Moreover, the concentration of surface defects and the recombination of carriers were controllable by quenching temperature, also affected by cooling rates. It provides a time-saving and straightforward method to suppressed recombination of photo-induced carriers and increased UV–vis light absorption for highly efficient ZnO-based photocatalyst applied to environmental remediation.

Effective nitrogen doping into TiO2 (N-TiO2) for visible light response photocatalysis

The thickness-controlled TiO2 thin films are fabricated by the pulsed laser deposition (PLD) method. These samples function as photocatalysts under UV light irradiation and the reaction rate depends on the TiO2 thickness, i.e., with an increase of thickness, it increases to the maximum, followed by decreasing to be constant. Such variation of the reaction rate is fundamentally explained by the competitive production and annihilation processes of photogenerated electrons and holes in TiO2 films, and the optimum TiO2 thickness is estimated to be ca. 10nm.We also tried to dope nitrogen into the effective depth region (ca. 10nm) of TiO2 by an ion implantation technique. The nitrogen doped TiO2 enhanced photocatalytic activity under visible-light irradiation. XANES and XPS analyses indicated two types of chemical state of nitrogen, one photo-catalytically active N substituting the O sites and the other inactive NOx (1⩽x⩽2) species. In the valence band XPS spectrum of the high active sample, the additional electronic states were observed just above the valence band edge of a TiO2. The electronic state would be originated from the substituting nitrogen and be responsible for the band gap narrowing, i.e., visible light response of TiO2 photocatalysts.

Synthesis of BiOI@(BiO)2CO3 facet coupling heterostructures toward efficient visible-light photocatalytic properties.

In this paper, BiOI@(BiO)2CO3 facet coupling heterostructures were synthesized via exfoliation and ion exchange, and characterized by X-ray diffraction (XRD) patterns, scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), UV-vis diffuse reflectance spectra (DRS), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL) spectra and valence-band XPS spectra. With the reaction time increasing, more BiOI can be transformed to (BiO)2CO3, and BiOI@(BiO)2CO3 facet coupling heterostructures were obtained. The photocatalytic results showed that BiOI@(BiO)2CO3 displays much higher photocatalytic activity than BiOI and (BiO)2CO3 under visible-light. The photocatalytic mechanism study revealed that BiOI@(BiO)2CO3 has strong adsorption for RhB molecules due to the ultrathin nanosheets and higher BET, and displays better separation efficiency of photoinduced charge carriers and higher photocurrent due to the {001}/{001} facet coupling.

Pit assisted oxygen chemisorption on GaN surfaces.

A comprehensive analysis of oxygen chemisorption on epitaxial gallium nitride (GaN) films grown at different substrate temperatures via RF-molecular beam epitaxy was carried out. Photoemission (XPS and UPS) measurements were performed to investigate the nature of the surface oxide and corresponding changes in the electronic structure. It was observed that the growth of GaN films at lower temperatures leads to a lower amount of surface oxide and vice versa was observed for a higher temperature growth. The XPS core level (CL) and valence band maximum (VBM) positions shifted towards higher binding energies (BE) with oxide coverage and revealed a downward band bending. XPS valence band spectra were de-convoluted to understand the nature of the hybridization states. UPS analysis divulged higher values of electronic affinity and ionization energy for GaN films grown at a higher substrate temperature. The surface morphology and pit structure were probed via microscopic measurements (FESEM and AFM). FESEM and AFM analysis revealed that the film surface was covered with hexagonal pits, which played a significant role in oxygen chemisorption. The favourable energetics of the pits offered an ideal site for oxygen adsorption. Pit density and pit depth were observed to be important parameters that governed the surface oxide coverage. The contribution of surface oxide was increased with an increase in average pit density as well as pit depth.

Electronic structure of Cu2ZnGeSe4 single crystal: Ab initio FP-LAPW calculations and X-ray spectroscopy measurements

High-quality Cu2ZnGeSe4 single crystal has been successfully grown by a solution-fusion method and the crystal structure of the compound is refined within tetragonal kesterite-type (space group 14¯), with the unit cell parameters a=5.6112(1) Å and c=11.0473(3) Å. X-ray photoelectron core-level and valence-band spectra for pristine and Ar+-ion irradiated surfaces of the Cu2ZnGeSe4 crystal are measured. Our X-ray photoelectron spectroscopy (XPS) data indicate that the Cu2ZnGeSe4 single crystal surface is very rigid with respect to Ar+ ion-irradiation. The electronic structure of the Cu2ZnGeSe4 compound has been theoretically studied using first principles calculation employing the full potential linearized augmented plane wave (FP-LAPW) method. Calculations of the total and partial densities of states of atoms constituting Cu2ZnGeSe4 reveal that the dominant contributors to the valence band of the compound studied are the Cu 3d states, which contribute mainly at the top of the band, while the central portion of the band is composed mainly from the Cu 3d and Se 4p states in almost equal proportion. Additionally, the present FP-LAPW calculations indicate that the bottom of the valence band is dominated by the Zn 3d states, while the bottom of the conduction band is composed mainly from the unoccupied Ge 4s and Se 4p states. The data of the present FP-LAPW calculations are confirmed by comparison on a common energy scale of the X-ray emission bands representing the energy distribution of the valence Cu d, Zn d, Ge p and Se p states and the XPS valence-band spectrum of the Cu2ZnGeSe4 single crystal.

Deduction of the chemical state and the electronic structure of Nd2Fe14B compound from X-ray photoelectron spectroscopy core-level and valence-band spectra

Characterization of chemical state and electronic structure of the technologically important Nd2Fe14B compound is attractive for understanding the physical nature of its excellent magnetic properties. X-ray photoelectron spectroscopy (XPS) study of such rare-earth compound is important and also challenging due to the easy oxidation of surface and small photoelectron cross-sections of rare-earth 4f electrons and B 2p electrons, etc. Here, we reported an investigation based on XPS spectra of Nd2Fe14B compound as a function of Ar ion sputtering time. The chemical state of Fe and that of B in Nd2Fe14B compound can be clearly determined to be 0 and −3, respectively. The Nd in Nd2Fe14B compound is found to have the chemical state of close to +3 instead of +3 as compared with the Nd in Nd2O3. In addition, by comparing the valence-band spectrum of Nd2Fe14B compound to that of the pure Fe, the contributions from Nd, Fe, and B to the valence-band structure of Nd2Fe14B compound is made more clear. The B 2p states and B 2s states are identified to be at ∼11.2 eV and ∼24.6 eV, respectively, which is reported for the first time. The contribution from Nd 4f states can be identified both in XPS core-level spectrum and XPS valence-band spectrum. Although Nd 4f states partially hybridize with Fe 3d states, Nd 4f states are mainly localized in Nd2Fe14B compound.

Surface state modulation through wet chemical treatment as a route to controlling the electrical properties of ZnO nanowire arrays investigated with XPS

ZnO is a wide bandgap semiconductor that has many potential applications including solar cell electrodes, transparent thin film transistors and gas/biological sensors. Since the surfaces of ZnO materials have no amorphous or oxidised layers, they are very environmentally sensitive, making control of their semiconductor properties challenging. In particular, the electronic properties of ZnO nanostructures are dominated by surface effects while surface conduction layers have been observed in thin films and bulk crystals. Therefore, the ability to use the ZnO materials in a controlled way depends on the development of simple techniques to modulate their surface electronic properties. Here, we use monochromatic x-ray photoelectron spectroscopy (XPS) to investigate the use of different wet chemical treatments (EtOH, H2O2) to control the electronic properties of ZnO nanowires by modulating the surface depletion region. The valence band and core level XPS spectra are used to explore the relationship between the surface chemistry of the nanowires and the surface band bending.

Electronic structure of β-RbNd(MoO4)2 by XPS and XES

β-RbNd(MoO4)2 microplates have been prepared by the multistage solid state synthesis method. The phase composition and micromorphology of the final product have been evaluated by XRD and SEM methods. The electronic structure of β-RbNd(MoO4)2 molybdate has been studied employing the X-ray photoelectron spectroscopy (XPS) and X-ray emission spectroscopy (XES). For the molybdate, the XPS core-level and valence-band spectra, as well as XES bands representing energy distribution of the Mo 4d- and O 2p-like states, have been measured. It has been established that the O 2p-like states contribute mainly to the upper portion of the valence band with also significant contributions throughout the whole valence-band region. The Mo 4d-like states contribute mainly to a lower valence band portion.

Surface structures of In-Pd intermetallic compounds. I. Experimental study of In thin films on Pd(111) and alloy formation.

A combination of experimental methods was used to study the structure of In thin films deposited on the Pd(111) surface and the alloying behavior. X-ray photoelectron spectroscopy (XPS), low-energy electron diffraction (LEED), and scanning tunneling microscopy results indicate that surface alloying takes place at room temperature. Below 2 monolayer equivalents (MLEs), the LEED patterns show the formation of three rotational domains of InPd(110) of poor structural quality on top of the Pd(111) substrate. Both core-levels and valence band XPS spectra show that the surface alloy does not yet exhibit the electronic structure characteristic of the 1:1 intermetallic compound under these conditions. Annealing the 1 MLE thin film up to 690 K yields to a transition from a multilayer InPd near-surface intermetallic phase to a monolayer-like surface alloy exhibiting a well ordered (√3×√3) R30(∘) superstructure and an estimated composition close to In2Pd3. Annealing above 690 K leads to further In depletion and a (1 × 1) pattern is recovered. The (√3×√3) R30(∘) superstructure is not observed for thicker films. Successive annealing of the 2 MLE thin film leads the progressive disappearance of the InPd diffraction spots till a sharp (1 × 1) pattern is recovered above 690 K. In the high coverage regime (from 4 to 35 MLE), the formation of three rotational domains of a bcc-In7Pd3 compound with (110) orientation is observed. This In-rich phase probably grows on top of interfacial InPd(110) domains and is metastable. It transforms into a pure InPd(110) near-surface intermetallic phase in a temperature range between 500 and 600 K depending on the initial coverage. At this stage, the surface alloy exhibits core-level chemical shifts and valence band (VB) spectra identical to those of the 1:1 InPd intermetallic compound and resembling Cu-like density of states. Annealing at higher temperatures yields to a decrease of the In concentration in the near-surface region to about 20 at.% and a (1 × 1) LEED pattern is recovered.

Electronic structure of cage-like compound UB12 – Theory and XPS experiment

The electronic structure of cubic UB12 has been investigated by X-ray photoelectron spectroscopy (XPS) and first principles calculations employing the fully relativistic full-potential local-orbital (FPLO-9) method. The calculated densities of states (DOS) revealed unique fairly sharp five-peaks of the U 5f states with the lowest energy peak crossing the Fermi level. The calculated and experimental valence band XPS spectra are in very good accord, indicating a substantial delocalization of the U 5f electrons. On the other hand, our experimental U 4f core-level XPS spectrum exhibits both the 1-eV and 7-eV satellites, pointing to some localization effects of the U 5f electrons, which may exhibit a mixed-valence behavior. Our Fermi surfaces of UB12 calculated for ambient and such high pressure as 55GPa show only negligible differences, which can explain the earlier reported stability of the cubic crystal structure of the studied system under high pressure.

X-ray spectroscopy study of the electronic structure of non-centrosymmetric Ag2CdSnS4 single crystal

We report on studies of the electronic structure of Ag2CdSnS4 single crystal grown by the horizontal gradient freeze technique. For the crystal under consideration, X-ray photoelectron core-level and valence-band spectra for pristine and Ar+-ion irradiated surfaces have been measured. The present X-ray photoelectron spectroscopy (XPS) results indicate that Ag2CdSnS4 single crystal surface is very rigid with respect to Ar+ ion-irradiation. In particular, Ar+ bombardment of the single crystal surface does not induce any significant changes of values of the binding energies of core-level electrons as well as the energy distribution of electronic states within the valence-band region. The Ar+ bombardment does not change the elemental stoichiometry of the Ag2CdSnS4 surface. For the Ag2CdSnS4 compound, the X-ray emission bands representing the energy distribution of the valence Ag d, Cd d and S p states were recorded and compared on a common energy scale with the XPS valence-band spectrum. Results of such a comparison indicate that the S 3p states contribute predominantly in the upper and central portions of the valence band of the Ag2CdSnS4 single crystal. In addition, our data reveal that the Ag 4d and Cd 4d states contribute mainly in the central portion and at the bottom of the valence band, respectively.

Manipulating solar absorption and electron transport properties of rutile TiO2 photocatalysts via highly n-type F-doping

In this work, we report a facile and nontoxic one-pot hydrothermal method for synthesizing F-doped rutile single crystalline TiO2 with tuneable solar absorption. The optical band gap of the catalyst can be easily manipulated from 3.05 to 2.58 eV via altering the initial F : Ti molar ratio of reaction precursors. The photoanodes made of rutile TiO2 single crystals with appropriate F-doping concentration show excellent photoelectrocatalytic activity towards water oxidation under ultraviolet and visible light illumination. The best photoelectrocatalytic performance under UV irradiation can be obtained by F-doped TiO2 with an initial F : Ti molar ratio of 0.1, which is almost 15 times higher than that of un-doped TiO2. Further, the F-doped TiO2 photoanodes also exhibit superior photoelectrocatalytic activity under visible irradiation, and the best performance can be achieved by F-doped TiO2 photoanode with an initial F : Ti molar ratio of 0.05. The superior photoelectrocatalytic activity could be attributed to the highly n-type dopant introduced by fluorine, which significantly tunes the electrical conductivities and band structures of the resulting TiO2 photoanodes, and thus the photoelectrocatalytic activities under both UV and visible irradiation. Different techniques have been employed to characterize the electrical conductivity, charge carrier density and band structures of the F-doped rutile TiO2 films, such as photoelectrochemical method, electrical impedance spectroscopy (EIS) measurements, Mott–Schottky plots and XPS valence band spectra.

X-ray photoelectron study and first principle calculations of the electronic structure of PbFe1/2Nb1/2O3 single crystal in the ferroelectric and paraelectric phases

The present study is the first in which the theoretical prediction of the changes in the density of states of single crystal of PbFe1/2Nb1/2O3 (PFN) multiferroic depending on a temperature at the ferroelectric – paraelectric transition is experimentally confirmed by the X-ray photoelectron spectroscopy. The valence band XPS spectra of PFN single-crystal at three temperatures T1 = 297 K (monoclinic phase), T2 = 368 K (tetragonal) and T3 = 403 K (cubic) were obtained by X-ray photoelectron study using ESCALAB 250. It is found out that the experimental valence band XPS spectra of PFN single-crystals in ferroelectric and paraelectric phases consist of two main peaks with a difference in relative intensity and energy position of the second peak. The density of states of PFN was calculated for tetragonal ferroelectric (368 K) and cubic paraelectric (403 K) phases. The first main peak corresponds to strongly hybridized Fe3d, Nb3d and O2p states while the second peak is mainly composed of Pb6s states. According to the experimental data, the position of the second main peak of the total density of states of PFN in the ferroelectric monoclinic phase is shifted toward the first peak relatively to its position in the paraelectric phase by 0.28 eV and for the tetragonal phase by 0.15 eV, the latter value coincides well with the calculated value equal to 0.17 eV.

Valence band offset at the CdS/Cu2ZnSnS4 interface probed by x-ray photoelectron spectroscopy

The valence band offset (VBO) at the interface CdS/Cu2ZnSnS4 was investigated by x-ray photoelectron spectroscopy (XPS). The VBO was measured by two different procedures: an indirect method involving the measurements of the core levels together with the XPS bulk valence band (VB) spectra and a direct method involving the analysis of XPS VB spectra at the interface. The indirect method resulted in a VBO value of (−1.20 ± 0.14) eV while the direct method returned a similar value of (−1.24 ± 0.06) eV but affected by a lower uncertainty. The conduction band offset (CBO) was calculated from the measured VBO values. These two measured values of the VBO allowed us to calculate the CBO, giving (−0.30 ± 0.14) eV and (−0.34 ± 0.06) eV, respectively. These values show that the CBO has a cliff-like behaviour which could be one of the reasons for the Voc limitation in the CdS/CZTS solar cells.

Origin of the visible light absorption of Co2+ and NH4+ co‐doped hydrogen titanate nanotube thin films

AbstractHydrogen titanate nanotube (HTNT) thin films were synthesized by hydrothermal method and then Co2+ and NH4+ co‐doped hydrogen titanate nanotube (Co, N‐HTNT) thin films were prepared by ion‐exchange method. The Co, N‐HTNT thin films exhibit strong absorption in the visible light range compared with the HTNT and NH4+ doped hydrogen titanate nanotube (N‐HTNT) thin films. The first‐principles calculations reveal that NH4+ doping has no effect on the visible light absorption of HTNTs. The red shift of Co, N‐HTNTs is only due to the mixture of the Co 3d and O 2p states in the top of the valence band, which results in the band gap narrowing. Relative to HTNTs and N‐HTNTs, both the valence band maximum (VBM) and conduction band minimum (CBM) of Co, N‐HTNTs shift to lower potential based on the valence band XPS spectra. Furthermore, the up‐shift of the VBM is much larger than that of the conduction band, which can result in band gap reduction explaining the origin of the visible light absorption of Co, N‐HTNTs.

Effects of CeO2 on the XPS valence band spectra of coal under the combustion initialization stage at 400°C

In order to get the catalytic mechanism of CeO2 on graphite and coal at 400°C, the morphologies of coal, graphite, and CeO2 before and after combustion were analyzed through X-ray photoelectron spectroscopy (XPS). It is found that the particle size of coal is mostly between 11.727 and 64.79 μm, while the particle size of CeO2 is between 1.937 and 11.79 μm. The agglomeration of coal and CeO2 can be seen by scanning electron microscopy (SEM) after reaction. XPS results show that with the addition of CeO2, the intensity of binding energy gets stronger, but there is no energy peak transition. Comparing the character of coal with and without the addition of CeO2, it can be seen that the C-C bond fractures first at 400°C, while the C-H energy-band takes electrons at the same time to be far away from the Fermi level, and the O 2s, O 2p, and C sp hybrid orbitals are all excited. Adding CeO2 can enhance the activity of the whole coal. In addition, through XPS analysis, combined with the oxygen transfer theory and the electron transfer theory, the catalytic mechanism of CeO2 for pulverized coal combustion could be obtained.

Spectroscopic Studies of In<SUB>2</SUB>O<SUB>3</SUB> Nanostructures; Photovoltaic Demonstration of In<SUB>2</SUB>O<SUB>3</SUB>/p-Si Heterojunction

The thermal oxidation process of the indium nitride (InN) nanorods (NRs) was studied. The SEM studies reveal that the cracked and burst mechanism for the formation of indium oxide (In2O3) nanostructures by oxidizing the InN NRs at higher temperatures. XRD results confirm the bcc crystal structure of the as prepared In2O3 nanostructures. Strong and broad photoluminescence spectrum located at the green to red region with maximum intensity at 566 nm along with a weak ultraviolet emission at 338 nm were observed due to oxygen vacancy levels and free excitonic transitions, respectively. The valence band onset energy of 2.1 eV was observed from the XPS valence band spectrum, clearly justifies the alignment of Fermi level to the donor level created due to the presence of oxygen vacancies which were observed in the PL spectrum. The elemental ratio In:O in as prepared In2O3 was found to be 42:58 which is in close agreement with the stoichiometric value of 40:60. A downward shift was observed in the Raman peak positions due to a possible phonon confinement effect in the nanoparticles formed in bursting mechanism. Such single junction devices exhibit promising photovoltaic performance with fill factor and conversion efficiency of 21% and 0.2%, respectively, under concentrated AM1.5 illumination.