Mo 4d States Research Articles

First-principles analysis of the C–N bond scission of methylamine on Mo-based model catalysts

The C-N bond breaking of methylamine on clean, carbon (nitrogen, oxygen)-modified Mo(100) [denoted as Mo(100) and Mo(100)-C(N,O), respectively], Mo(2)C(100), MoN(100), and Pt(100) surfaces has been investigated by the first-principles density functional theory (DFT) calculations. The results show that the reaction barriers of the C-N bond breaking in CH(3)NH(2) on Mo(100)-C(N,O) are higher than that on clean Mo(100). The calculated energy barrier can be correlated linearly with the density of Mo 4d states at the Fermi level after the adsorption of CH(3)NH(2) for those surfaces. Moreover, the DFT results show that the subsurface atom, e.g., carbon, can reduce the reaction barrier. In addition, We noticed that the activation energies for the C-N bond breaking on Mo(2)C(100) and MoN(100) are similar to that on Pt(100), suggesting that the catalytic properties of the transition metal carbides and nitrides for C-N bond scission of CH(3)NH(2) might be very similar to the expensive Pt-group metals.

Exciton transition and electronic structure of PbMoO4 crystals studied by polarized light

Polarized reflectivity spectra of PbMo0 4 crystals have been measured using synchrotron radiation up to 20 eV. The optical constants for the crystallographic axes are derived by using a Kramers-Kronig analysis. It is found that the exciton band at 3.6 eV shows a doublet structure with distinct dichroism. X-ray photoemission spectroscopy (XPS) and the calculation of the electronic structure by a discrete variational Xα method are also carried out. The calculation shows that the valence band and the conduction band are mainly composed of the O 2p and Mo 4d states, respectively, and the Pb state contributes appreciably to the top of the valence band and the bottom of the conduction band. The valence-band XPS spectrum of PbMoO 4 is compared with that of PbW0 4 , which reveals a remarkable difference between them. This difference reflects different magnitudes of hybridization of Mo 4d or W 5d state to the valence band. The exciton transition is explained in terms of the cationic Pb 6s → 6p excitation model taking into account the crystal-field splitting and the spin-orbit interaction of Pb 6p state. From a comparison of the doublet structure of the exciton band of PbMo0 4 and PbW0 4 , it is suggested that the electron-hole exchange interaction plays an important role for the exciton transitions in both materials.

Electronic structure of Fe (0–5 at.%) dopedMoO2 thin films studied by resonant photoemission spectroscopy

The electronic structure of pulsed laser-depositedMo1−xFexO2 (x = 0,0.02 and 0.05) thin films has been investigated using resonant photoemission spectroscopynear the Mo 4p absorption edge. In all the samples a broad Fano-like resonance peak at∼46 eV is observed in the whole area of the valence band, which indicates the contribution ofthe Mo 4d states in the entire valence band region. The doping of Fe in these films leads toa decrease in Mo 4d states contributing to electronic states at lower binding energy region.In addition to this, we also observe a shoulder at 4.9 eV in the valence band spectra ofdoped samples. It is proposed that the origin of this shoulder is due to the Fe hybridizedstates.

Li-Ion Intercalation in Thermal Oxide Thin Films of MoO3 as Studied by XPS, RBS, and NRA

XPS, RBS, and NRA have been combined to study the mechanisms of Li-ion electrochemical intercalation in MoO3 thin films prepared by thermal oxidation of molybdenum metal. A direct anaerobic and anhydrous transfer was used from a glovebox (O-2 and H2O < 1 ppm), where the samples were electrochemically treated at selected potentials between 1.7 and 3.2 V versus Li+/Li, to the XPS analysis chamber. The thermal oxide film grown at T = 450 +/- 10 degrees C and P(O-2) = 100 +/- 10 mbar for t = 5 min consisted of a 20 nm thick MoO3 outer layer overlying a 13 nm thick inner layer of lower oxides (Mo2O5 and MoO2). Combined RBS/NRA analysis allowed the dosing of intercalated lithium and the determination of the composition of the lithiated phases. Li0.50MoO3, Li1.20MoO3, and Li0.21MoO3 were obtained after intercalation at 2.58 and 1.73 V and deintercalation at 3.2 V, respectively, showing that similar to 1.2 mol of Li can be initially intercalated in the potential range 1.7-3.2 V (capacity of 223 mA h/g), and similar to 0.2 mol of Li per mol of MoO3 is trapped in the oxide matrix after the initial stages of intercalation. The XP Mo3d core level spectra evidenced the reduction of Mo6+ ions to Mo5+ ions after intercalation at 2.58 V and further to Mo5+ and Mo4+ ions after intercalation at 1.73 V with resulting Mo6+/Mo5+/Mo4+ ratios of 53:47:00 at % and 37:39:24 at %, respectively. Reoxidation of molybdenum is observed after deintercalation but 40 at % Mo5+ subsist at 3.2 V due to the trapping of lithium strongly bonded to the oxide matrix. The Li1s core level (at E-B = 55.80 eV) is most intense at 1.73 V and does not vanish at 3.2 V. Broadening of the Mo3d core level peaks are assigned to the distortion of the oxide matrix. Changes of the electronic structure after intercalation result from the occupation of the Mo4d states (at E-B = 1.0 eV) originally empty in the pristine oxide. The XP C1s and O1s core level spectra show the irreversible formation of a solid electrolyte interphase (SEI) layer including lithium carbonate and Li-alkoxides.

Ab initio investigation of interfacial layer formation in the Mo∕Si boundary for extreme ultraviolet lithography

The energetics and the electronic structure of the Si∕Mo(110) surface were investigated using the density functional theory calculations in an attempt to clarify the initial stage of silicide layer formation. The energy barriers for the migration of Si adatom to an adjacent hollow site passing the bridge site were calculated to be 0.64eV [Mo (110)] and 0.74eV [Si-substituted Mo (110)]. In order to analyze the bonding nature between Si adatom and Mo (110) surface, layer density of states and total valence electron density were obtained. It can be reasonably inferred that Mo 4d states mainly contributed to the bonding between Mo and Si and, consequently, four Mo–Si covalent bonds were formed around the Si atom at the hollow site.

Electronic structure of face-centred cubic MoO 2: A comparative study by the full potential linearized augmented plane wave method, X-ray emission spectroscopy and X-ray photoelectron spectroscopy

X-ray emission spectroscopy (XES) and X-ray photoelectron spectroscopy (XPS) methods were employed in the present paper to investigate the electronic structure of face-centred cubic (fcc) molybdenum dioxide, fcc-MoO 2. For the mentioned compound, the XES O Kα and Mo Lβ 2,15 bands reflecting the valence O p- and Mo s,d-like states, respectively, were derived and compared on a common energy scale with the XPS valence-band spectrum. For comparison, the similar experimental studies of the electronic structure were made for a usual orthorhombic form of molybdenum trioxide, MoO 3. Band-structure calculations of fcc-MoO 2 were made using the full potential linearized augmented plane wave (FP-LAPW) method. A rather good agreement of the experimental XES and XPS results and the theoretical FP-LAPW data for the electronic properties of fcc-MoO 2 has been achieved in the present paper. A new near-Fermi sub-band was detected on both the XES Mo Lβ 2,15 band and the XPS valence-band spectrum when going from orthorhombic MoO 3 to fcc-MoO 2. The FP-LAPW calculation reveals that the main contributors into the aforementioned sub-band of fcc-MoO 2 are the Mo 4d(e g) states. Further, the FP-LAPW data indicate that the O 2p and Mo 4d(t 2g) states contribute into both the central part and the bottom of the valence band of fcc-MoO 2, while the Mo 4d(e g) states contribute almost exclusively into the bottom of the valence band of the oxide. A significant portion of density of states (mainly Mo 4d(e g) states) detected by the FP-LAPW calculation at the Fermi energy of fcc-MoO 2 indicates that the oxide is rather unstable.

Comparative study of optical spectra and electronic structures of CaMoO4 and CaWO4 crystals

AbstractPolarized reflectivity spectra of CaMoO4 and CaWO4 single crystals have been measured up to 30 eV by using synchrotron radiation as a light source. The optical constants for a ‐ and c ‐axes are derived from the reflectivity spectra by a Kramers–Kronig analysis. A clear correspondence is found in the main structures of the optical spectra between molybdate and tungstate. It is revealed that the lowest bands peaking at 5.0 eV in CaMoO4 and at 6.0 eV in CaWO4 consisit of some fine structures with distinct dichroism. They are attributed to the joint density‐of‐states; in other words, no structures indicative of exciton transition are found at the fundamental absorption edge. From the comparison of the spectral profiles in CaMoO4 and CaWO4, the structures below 10 eV are ascribed to the transitions from the O 2p valence band to the conduction band mainly made of Mo 4d (W 5d) states, while those above 10 eV are assigned to the transitions to the Ca conduction band. The polarization dependence of the structures below 10 eV is explained well in terms of the intramolecular transition in an oxyanion complex in the scheelite crystal. The energy band structure of CaWO4 is discussed taking into account of the previous results such as the thermostimulated luminescence and the two‐photon excitation. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Electronic structure of the mixed-valent system V2−xMoxO5

We have investigated the electronic structure of the mixed-valent system V2−xMoxO5 with varying doping concentration by means of X-ray photoelectron and X-ray emission spectroscopy. A series of V2−xMoxO5 compounds was obtained by the hydrochemical method. By comparing the XPS valence band with the V Lα, O Kα and Mo Lβ2.15 XES spectra we could localize the V3d, Mo4d and O2p states in the valence band. It has been found that in the course of the Mo doping, the density of states just below the Fermi level increases. The enhancement is due to both Mo4d and V3d states and seems to be relatively unaffected by the hybridization with O2p states situated at higher binding energies. The trends in the measured spectra are discussed in comparison to band structure calculations performed for defect-containing supercells.

Growth and characterization of chloronitroaniline crystals for optical parametric oscillators: I. XPS study of Mo-based compounds

The surface electronic structures and compositions of Mo-based compounds (oxides and nitrides) have been elucidated using X-ray photoelectron spectroscopy (XPS). The Mo nitride samples were synthesized using the temperature-programmed nitridation of a low surface area MoO 3 with NH 3. The Mo oxides were also used to compare XPS results. No significant change was observed in the surface electronic structure of core levels of Mo 3d, Mo 3p, N 1s, and O 1s for Mo nitrides after the hydrogen reduction. A species with a Mo 3d 5/2 binding energy of 228.4±0.2 eV from Mo nitrides was newly observed and was calculated to be Mo 1.3+. This value indicates that the absolute magnitude of charge transfer from Mo 4d states to N 2p states is 1.3 electrons per molybdenum in Mo nitrides. The binding energies of Mo 3d 5/2 were shifted to lower ones (232.5→230.0→228.4 eV) as the oxides were transformed to nitride during nitridation (Mo 6+→Mo 4+→Mo δ+ ). There are two different regions (nitride and oxide regions) present on the Mo nitride surface as shown from the deconvoluted Mo 3d peaks. For the Mo nitrides prepared in this study the nitride fraction was observed to be a majority on the surface.

57Fe Mössbauer study of Prm(Fe, Mo)n compounds with m:n=2:17 and 1:12

The effects of Mo substitution in Pr2Fe17−xMox and PrFe12−xMox compounds were investigated by X-ray diffraction, magnetic measurements and Mossbauer spectroscopy. Different fitting procedures, using magnetic sextets at low temperature, magnetic sextets and quadrupole doublets at room temperature and paramagnetic doublets well above the Curie points, had to be considered in order to analyze the Mossbauer spectra. The Mossbauer data obtained on the Mo-containing compounds stand for distributions of the hyperfine fields due to the random distributions of Mo over the 6c sites in the 2:17 and 8i sites in the 1:12 compounds. The variations in the isomer shifts and hyperfine fields with composition are discussed in relationship to the local environment details at the different iron sites. Site-specific information on the electronic effects due to Fe(3d)–Mo(4d) states hybridization are derived.

Photoemission and inverse-photoemission spectra of quasi-two-dimensional Mo 4O 11

Electronic structures of Magnéli-phase η- and γ-Mo 4O 11 have been studied by means of ultraviolet photoemission and inverse-photoemission spectroscopies. Resonant photoemission spectra have been also measured in the photon energy range from 32 eV to 90 eV The energy positions of Mo 4d states can be qualitatively explained in terms of a simple band structure model.

Photoemission and inverse-photoemission spectra of quasi-two-dimensional Mo4O11

Electronic structures of Magnéli-phase η- and γ-Mo4O11 have been studied by means of ultraviolet photoemission and inverse-photoemission spectroscopies. Resonant photoemission spectra have been also measured in the photon energy range from 32 eV to 90 eV The energy positions of Mo 4d states can be qualitatively explained in terms of a simple band structure model.

Polarized X-ray absorption spectra and electronic structure of molybdenite (2H-MoS2)

Polarized S K- and L-edge, Mo L3- and L2-edge x-ray absorption near-edge structure (XANES) of natural molybdenite (2H-MoS2) have been measured with synchrotron radiation. These results are qualitatively interpreted using the energy band model of molybdenite and provide important information on the unoccupied states of molybdenite. The valence band (VB) maximum of molybdenite is characterized by fully occupied Mo 4dz 2, and the conduction band (CB) minimum of molybdenite is characterized by unoccupied Mo 4d states. The unoccupied Mo 4d band is split into two sub-bands, designated as t 2g − /t 2g + and e g − /e g + sets. Although the relative energy of these two sets are difficult to be evaluated, probably the former has the lower energy than the latter, both two sets have the combination wave functions of the other unoccupied Mo 4d components, rather than the simple 4dx 2 — y2 and 4dxy states. The unoccupied Mo 4d sub-bands contain significant DOS of both S 3 p- and 3 s-like states, indicating strong hybridization with S 3s and 3 p states. In the lower energy sub-band, the DOS of the S pz- and px,y-like states are very similar. However, in the higher energy sub-band, the DOS of the S 3 px,y-like state is lower than that of the S 3pz state. Polarized S K-edge XANES also reveal the features of antibonding S pz- and px,y-like states in molybdenite. The feature assigned to the S 3 pz-like states is stronger and sharper, and shifts to lower energy by about 2 eV relative to that for the S 3 px,y-like states.

Electronic structure of surface defects in K0.3MoO3

The nature of defects on the cleavage surface of the quasi-one-dimensional oxide conductor K0.3MoO3 has been studied by angle-resolved photoemission and Auger electron spectroscopy. Defects were created on the (201̄) surface by Ne+ ion bombardment and by photon-stimulated desorption (PSD). The crystal cleaves to expose K atoms on the surface. Initial sputtering of the cleaved surface leads to the disappearance of emission from the Mo 4d states at the Fermi level (EF) and the growth of a well-defined emission feature 2 eV below EF. Continued sputtering leads to the reappearance of emission at EF. We explain this behavior by the sequential removal of K and O, respectively, from the surface, and verify this model by using PSD to selectively create only O vacancies. This model is further verified by measuring work function changes during surface modifications and by the use of Auger spectroscopy.

Resonant Photoemission Study of Molybdenum Bronzes: K0.3MoO3, K0.33MoO3and K0.9Mo6O17

The electronic structures of molybdenum bronzes K 0.3 MoO 3 , K 0.33 MoO 3 and K 0.9 MoO 17 have been systematically studied by means of the ultraviolet photoemission spectroscopy (UPS). Resonant photoemission spectra have been measured in the photon energy range from 32 eV to 100 eV. The measurements of total yield spectra have also been made on K 0.3 MoO 3 and K 0.9 Mo 6 O 17 . Resonant enhancements of the photoemission have been observed in the valence and conduction bands with the contribution from the Mo 4d states for the excitation around h v =49 or 50 eV. It is found that the resonant effect is very weak for the structures at a binding energy E B =4.6 eV for K 0.3 MoO 3 , and K 0.33 MoO 3 . The magnitude of these resonances corresponds to the local electronic densities of Mo 4d states at the corresponding E B and can be qualitatively explained by the simple band structure model.

Photo-emission study of the electronic structure of the purple molybdenum bronze K0.9Mo6O17

The electronic structure of K0.9Mo6O17, which is a quasi-two-dimensional charge-density-wave (CDW) system, has been studied by ultraviolet photo-emission spectroscopy (UPS). Angle-integrated spectra were measured in the photon energy range from 32 to 100 eV at room temperature. A resonant enhancement of the Mo 4d states was observed around h nu =50 eV. This energy is much greater than the binding energy of the Mo 4p core level measured by photo-emission. The resonant effect for the structure at the binding energy EB=7.0 eV is appreciably stronger than that at EB=4.6 eV, which demonstrates that the Mo 4d orbitals are strongly hybridised with the O 2p orbitals at the deep binding energies in the valence bands. The angle-resolved UPS spectra were also measured at room temperature.

Photoemission Spectroscopy of Electronic Structures of Cu1.8Mo6S7.6with Synchrotron Radiation

Photoemission spectra have been measured on clean surfaces of Cu 1.8 Mo 6 S 7.6 single crystals with use of synchrotron radiation. Three structures are resolved at E B (binding energy from the Fermi level)=0.3∼0.4, 1.1 and 2.9 eV. The structures at E B =1.1 and 0.3∼0.4 eV have shown remarkable resonance effect above the Mo 4p core excitation threshold and are assigned to the Mo 4d states. On the other hand, the structure at E B =2.9 eV has revealed an appreciable resonance effect around the Cu 3p core excitation region, which suggests that it is due to Cu 3d states. The absence of this type of resonance for the structures at E B =1.1 and 0.4 eV shows that the hybridization of Cu 3d states is negligible in small E B region. Besides, the states around E B =5 eV are dominated by the S 3p states with a noticeable hybridization with the Mo 4d and Cu 3d states.

“Silicide-like electron states in the Si-Mo interface grown at liquid nitrogen temperature”

Ultraviolet photoelectron spectra of the Si(111)-Mo interface grown at ∼ 85° K are presented and compared with spectra of the Mo 4d states at increasing coverages on a substrate of Al. In the sharp transition region between Si and the metal the interface electron states can be understood in terms of the Si(p)-Mo(d) hybridzation; in spite of the sharpness of the interface the electron states in the transition region show the same qualitative features as refractory metal silicides.