Ti 3d States Research Articles

Electronic properties of anatase TiO2 doped by lanthanides: A DFT+U study

The effects of mono-doping of 4f lanthanides with and without oxygen vacancy defect on the electronic structures of anatase TiO2 have been studied by first-principles calculations with DFT+U (DFT with Hubbard U correction) to treat the strong correlation of Ti 3d electrons and lanthanides 4f electrons. Our results revealed that dopant Ce is easy to incorporate into the TiO2 host by substituting Ti due to its lower substitutional energy (∼−2.0eV), but the band gap of the system almost keeps intact after doping. The Ce 4f states are located at the bottom of conduction band, which mainly originates from Ti 3d states. The magnetic moment of doped Ce disappears due to electron transfer from Ce to the nearest O atoms. For Pr and Gd doping, their substitutional energies are similar and close to zero, indicating that both of them may also incorporate into the TiO2 host. For Pr doping, some 4f spin-down states are located next to the bottom of the conduction band and narrow the band gap of the doping system. However, for Gd doping, the 4f states are located in deep valence band and there is no intermediate band in the band gap. The magnetic moment of dopant Gd is close to the value of isolated Gd atom (∼7μB), indicating no overlapping between Gd 4f with other orbitals. For Eu, it is hard to incorporate into the TiO2 host due to its very higher substitutional energy. The results also indicated that oxygen vacancy defect may enhance the adsorption of the visible light in Ln-doped TiO2 system.

Resonant photoemission at the L3 absorption edge of Mn and Ti and the electronic structure of 1T-Mn0.2TiSe2

Resonant valence band x-ray photoelectron spectra (ResPES) excited near the 2p3/2 core level energies, 2p x-ray photoelectron spectra (XPS) and L3,2 x-ray absorption spectra (XAS) of Ti and Mn in single crystals of 1T-Mn0.2TiSe2 were studied for the first time. The ionic–covalent character of the bonds formed by the Mn atoms with the neighboring Se atoms in the octahedral coordination is established. From the XPS and XAS measurements compared with the results of atomic multiplet calculations of Ti and Mn L3,2 XAS, it is found that the Ti atoms are in the ionic state of 4 + and the Mn atoms are in the state of 2 + . In ResPES of Mn0.2TiSe2 excited near the Ti 2p3/2 and Mn 2p3/2 absorption edges the Ti 3d and Mn 3d bands at binding energies just below the Fermi level are observed. According to theoretical calculations of E(k) the Ti 3d states are localized in the vicinity of the Γ point and the Mn 3d states are localized along the direction K–Γ–M in the Brillouin zone of the crystal.

First-principles study of ferromagnetism in Ti-doped ZnO with oxygen vacancy

Using first-principles density functional calculations, we have investigated the electronic structures of Ti-doped ZnO (Ti is in 4+ oxidation state) with and without oxygen vacancy. The ferromagnetic property is identified in the presence of oxygen vacancy despite Ti being nonmagnetic in its natural phase. The ferromagnetism originates from the charge transferring from donor derived-defect band to unoccupied Ti-3d states and the hybridization between Ti-3d and O-2p band near the Fermi level. On increasing the oxygen vacancy concentration, a transition from a long-ranged magnetic order to a short-ranged interaction is found and the oxygen vacancies prefer to distribute non-uniformly in Ti-doped ZnO.

Electronic structure of Ti4Fe2O as determined from ab initio calculations and X-ray spectroscopy studies

The TiLα, FeLα and OKα ultrasoft X-ray emission bands obtained in experiment reflect, respectively, the energy distribution of mainly the Ti3d, Fe3d and O2p electronic states in Ti4Fe2O compound, which is an efficient hydrogen absorber for energy cells. Full and partial densities of electronic states for all atoms constituting the indicated oxide were calculated by a modified method of associated plane waves (APW) using the WIEN2k software package. The APW calculation data for Ti4Fe2O compound as well as superposition of TiLα, FeLα and OKα ultrasoft X-ray emission bands on a single energy scale indicate that O2p states in the oxide are localized mainly near the bottom of the valence band, the major contribution near the ceiling of the valence band belonging to Fe3d and Ti3d states. According to the APW calculation, the major contribution to the bottom of Ti4Fe2O conduction band is made by Fe3d* and Ti3d* states. The APW data for Ti4Fe2O are supported by the cluster calculation performed for this compound using a FEFF82 software package.

Photoemission Study of N-Doped TiO2(110) with NH3

High-resolution photoemission spectroscopy using synchrotron radiation was employed to understand the bonding state of N incorporated into a reduced TiO2(110)-1×1 surface. N-doping into TiO2(110) was accomplished both by dosing NH3 at elevated temperatures (800 K) and by using a NHx+ ion implantation method onto TiO2(110) at room temperature. Our results indicate that the N-doped TiO2(110) obtained after final annealing to 900 K contains N species incorporated into the subsurface region of TiO2substitutionally replacing the lattice O2–. Interestingly, we find that some of the N species are found to have a direct N–H bond (NH species). The valence band indicates that the band-gap state related to the Ti 3d state is enhanced with increasing subsurface N species, which explains an attractive interaction between N species and defects with a charge in the Ti 3d state, such as reduced Ti3+ species and oxygen vacancies.

Photoemission Study on the Adsorption of Ethanol on Chemically Modified TiO2(001) Surfaces

Ethanol is a prototype molecule used in probing catalytic reactivity of oxide catalysts such as . In the present study, we adsorbed ethanol on (001) at room temperature (RT) and the corresponding bonding state of ethanol was systematically studied by x-ray photoemission spectroscopy (XPS) using synchrotron radiation. Especially, we compared (001) surfaces prepared in ultra-high vacuum (UHV) with different surface treatments such as -sputtering and oxidation with molecular , respectively. We find that the saturation coverage of ethanol at RT varies depending on the amount of reduced surface defects (e.g., ) which are introduced by -sputtering. We also find that the oxidized (001) surface has other type of surface defects (not related to Ti 3d state) which can dissociate ethanol for further reaction above 600 K. Our C 1s core level spectra indicate clearly resolved features for the two chemically distinct carbon atoms from ethanol adsorbed on (001), showing the adsorption of ethanol proceeds without C-C bond dissociation. No other C 1s feature for a possible oxidized intermediate was observed up to the substrate temperature of 650 K.

Concentration-dependent electronic structure and optical absorption properties of B-doped anatase TiO 2

The concentration-dependent electronic structures and optical properties of B-doped anatase TiO 2 have been calculated using the density functional theory. The calculated results indicate that the electronic structures of B-doped TiO 2 have changed compared with those of pure TiO 2, which is mainly due to the new midgap states induced by B doping. As to the optical properties, we calculate the imaginary part of dielectric function ε 2( ω) and optical absorption spectra of pure and B-doped TiO 2. Two transitions E 1 and E 2 emerged after B doping. The intensity of absorption is enhanced by B doping both in the UV and visible regions. According to the results of imaginary part of dielectric function ε 2( ω) and DOS, it can be concluded that the two optical transitions correspond to the transitions from the O 2p states in the top of valence band to the midgap states and from the midgap states to the Ti 3d states in the bottom of conduction band, respectively. These results have important implications for the further development of photocatalytic materials.

Electronic exchanges between adsorbed Ni atoms and TiO 2(1 1 0) surface evidenced by resonant photoemission

Nickel was deposited on stoichiometric TiO 2(1 1 0) surface in the 0.02–2.1 equivalent monolayer (eqML) range and analyzed by means of photoemission and resonant photoemission. In the case of very low coverage (lower than 0.1 eqML), deposited nickel reacts with the surface through an electronic transfer from nickel atoms towards titanium ions. This exchange caused the filling of unoccupied Ti3d states leading to the increase of a peak in the TiO 2 band gap. These states can be better characterized through resonant photoemission experiments at the Ti 3p → 3d absorption edge: for very low coverage, these states in the TiO 2 band gap have resonant behavior of Ti3d electrons rather than Ni3d ones, confirming the filling of Ti3d states and thus electron transfer between nickel and titanium. For coverage higher than 0.14 eqML, nickel peaks (both Ni3p core level and valence band) should be related to the presence of metallic nickel in small clusters.

Spin-doping effect on the electronic structure of Sr 1−( x+ y) La x+ yTi 1− xCr xO 3

We have investigated spin-doping effects on the valence-band electronic structure of Sr 1−( x+ y) La x+ y Ti 1− x Cr x O 3 at y = 0.2 using high-resolution photoemission spectroscopy and first-principles band structure calculations. The Cr 3d photoemission spectral weight located between 2.0 and 3.5 eV increased with x, indicating that x works as a rough measure of the spin density. However, the increase in the spectral weight was actually more rapid than the band theory prediction. The near-Fermi level ( E F) weight due to the Ti 3d states showed significant reduction with x. Considering that the band theory predicted nearly constant density of states near E F, this reduction of the near- E F weight should be interpreted as a manifestation of strong Ti–Cr site disorder effects.

Ab-initio calculations of electronic structure and optical properties of TiAl alloy

The electronic structures and optical properties of TiAl intermetallic alloy system are studied by the first-principle orthogonalized linear combination of atomic orbitals method. Results on the band structure, total and partial density of states, localization index, effective atomic charges, and optical conductivity are presented and discussed in detail. Total density of states spectra reveal that (near the Fermi level) the majority of the contribution is from Ti-3d states. The effective charge calculations show an average charge transfer of 0.52 electrons from Ti to Al in primitive cell calculations of TiAl alloy. On the other hand, calculations using supercell approach reveal an average charge transfer of 0.48 electrons from Ti to Al. The localization index calculations, of primitive cell as well as of supercell, show the presence of relatively localized states even above the Fermi level for this alloy. The calculated optical conductivity spectra of TiAl alloy are rich in structures, showing the highest peak at 5.73 eV for supercell calculations. Calculations of the imaginary part of the linear dielectric function show a prominent peak at 5.71 eV and a plateau in the range 1.1–3.5 eV.

Electron energy loss spectroscopy investigation of Pb and Ti hybridization with O at the PbTiO3/SrTiO3 interface

Studies of oxide interfaces, and how they affect physical properties, have recently gained large attention. For ferroelectric materials, the evolution of the order parameter close to an interface is important to understand regarding the stability of the ferroelectric phase, and how to optimize devices taking advantage of the polarization at the interface. Here we employ electron energy loss spectroscopy (EELS) in scanning transmission electron microscopy to compare the electronic and structural properties in both bulk and interface regions of epitaxial PbTiO3 thin films grown on SrTiO3 substrates. At the interface, changes in EELS spectra of the Ti-L3,2 and O-K edges, as compared to the bulk of the thin film, reveal a reduction in the hybridization of Ti 3d and Pb 6sp states with O 2p, and thus tetragonal distortion of the TiO6 octahedron. Real-space multiple-scattering calculations of the O-K edge support the experimental results. Moreover, the analysis of the Ti-valence reveals that the change is gradual over ∼2–3 nm. The data implies a decreasing ferroelectric order parameter over ∼2–3 nm close to the PbTiO3/SrTiO3 interface with a nonzero value at the interface with an additional screening of the polarization over ∼1–2 nm into the SrTiO3 substrate from the Ti atoms.

Characterizing TiO2(110) surface states by their work function

The unreconstructed TiO(2)(110) surface is prepared in well-defined states having different characteristic stoichiometries, namely reduced (r-TiO(2), 6 to 9% surface vacancies), hydroxylated (h-TiO(2), vacancies filled with OH), oxygen covered (ox-TiO(2), oxygen adatoms on a stoichiometric surface) and quasi-stoichiometric (qs-TiO(2), a stoichiometric surface with very few defects). The electronic structure and work function of these surfaces and transition states between them are investigated by ultraviolet photoelectron spectroscopy (UPS) and metastable impact electron spectroscopy (MIES). The character of the surface is associated with a specific value of the work function that varies from 4.9 eV for h-TiO(2), 5.2 eV for r-TiO(2), 5.35 eV for ox-TiO(2) to 5.5 eV for qs-TiO(2). We establish the method for an unambiguous characterization of TiO(2)(110) surface states solely based on the secondary electron emission characteristics. This is facilitated by analysing a weak electron emission below the nominal work function energy. The emission in the low energy cut-off region appears correlated with band gap emission found in UPS spectra and is attributed to localised electron emission through Ti(3+)(3d) states.

Reactivity of sub 1 nm supported clusters: (TiO2)n clusters supported on rutile TiO2 (110)

Metal oxide clusters of sub-nm dimensions dispersed on a metal oxide support are an important class of catalytic materials for a number of key chemical reactions, showing enhanced reactivity over the corresponding bulk oxide. In this paper we present the results of a density functional theory study of small sub-nm TiO(2) clusters, Ti(2)O(4), Ti(3)O(6) and Ti(4)O(8) supported on the rutile (110) surface. We find that all three clusters adsorb strongly with adsorption energies ranging from -3 eV to -4.5 eV. The more stable adsorption structures show a larger number of new Ti-O bonds formed between the cluster and the surface. These new bonds increase the coordination of cluster Ti and O as well as surface oxygen, so that each has more neighbours. The electronic structure shows that the top of the valence band is made up of cluster derived states, while the conduction band is made up of Ti 3d states from the surface, resulting in a reduction of the effective band gap and spatial separation of electrons and holes after photon absorption, which shows their potential utility in photocatalysis. To examine reactivity, we study the formation of oxygen vacancies in the cluster-support system. The most stable oxygen vacancy sites on the cluster show formation energies that are significantly lower than in bulk TiO(2), demonstrating the usefulness of this composite system for redox catalysis.

Interface effects in the electronic structure of TiO 2 deposited on MgO, Al 2O 3 and SiO 2 substrates

We report the Ti 2p X-ray absorption (XAS) and resonant photoemission (RPES) spectra of one equivalent TiO 2 monolayer grown on MgO, Al 2O 3 and SiO 2 substrates. The Ti 2p XAS spectra of these systems were compared to atomic multiplet calculations projected in different octahedral crystal fields. The comparison indicates that the crystal field splitting and the Ti–O hybridization decrease along the MgO, Al 2O 3 and SiO 2 series. The analysis of the RPES spectra provides the Ti 3d contributions to the valence band in these systems. These were compared to configuration interaction calculations of a TiO 6 cluster for different Ti 3d–O 2p hybridizations. The Ti 3d states in the valence band shift to lower binding energies along the MgO, Al 2O 3 and SiO 2 series. These effects are attributed to changes in the electronic structure at the interface, which, in turn, are related to the formation of cross-linking Ti–O–M (M = Mg, Al, and Si) bonds.

Electronic structure of ZrTiO 4 and HfTiO 4: Self-consistent cluster calculations and X-ray spectroscopy studies

Total and partial densities of states of the constituent atoms of ZrTiO 4 and HfTiO 4 titanates have been calculated using a self-consistent cluster method as incorporated in the FEFF8 code. The calculations reveal the similarity of the electronic structure of both titanates and indicate that the valence band of the compounds under consideration is dominated by contributions of O 2p states. These states contribute throughout the whole valence-band region; however their maximum contributions occur in the upper portion of the band. Other significant contributors in the valence-band region are Ti 3d and Zr 4d states in ZrTiO 4 and Ti 3d and Hf 5d states in HfTiO 4. All the above d-like states contribute throughout the whole valence-band region of the titanates; however maximum contributions of the Ti 3d states occur in the upper portion, whilst those of the Zr 4d (Hf 5d) states are in the central portions of the valence band. The FEFF8 calculations render that the bottom of the conduction band of ZrTiO 4 and HfTiO 4 is dominated by contributions of Ti 3d ⁎ states, with also smaller contributions of Zr 4d ⁎/Hf 5d ⁎ and O 2p ⁎ states. To verify the above FEFF8 data, the X-ray emission bands, representing the energy distributions of mainly O 2p, Ti 3d and Zr 4d states, were measured and compared on a common energy scale. These experimental data are found to be in agreement with the theoretical FEFF8 results for the electronic structure of ZrTiO 4 and HfTiO 4 titanates. Additionally, X-ray photoelectron valence-band and core-level spectra were recorded for the constituent atoms of the titanates under study.

DFT + U study of defects in bulk rutile TiO2

We present a systematic study of electronic gap states in defected titania using our implementation of the Hubbard-U approximation in the grid-based projector-augmented wave density functional theory code, GPAW. The defects considered are Ti interstitials, O vacancies, and H dopants in the rutile phase of bulk titanium dioxide. We find that by applying a sufficiently large value for the Hubbard-U parameter of the Ti 3d states, the excess electrons localize spatially at the Ti sites and appear as states in the band gap. At U=2.5 eV, the position in energy of these gap states are in fair agreement with the experimental observations. In calculations with several excess electrons and U=2.5 eV, all of these end up in gap states that are spatially localized around specific Ti atoms, thus effectively creating one Ti(3+) ion per excess electron. An important result of this investigation is that regardless of which structural defect is the origin of the gap states, at U=2.5 eV, these states are found to have their mean energies within a few hundredths of an eV from 0.94 eV below the conduction band minimum.

Theoretical study on the electronic and optical properties of Ce 3+-doped TiO 2 photocatalysts

A first-principles study based on the density functional theory has been carried out to evaluate the electronic and optical properties of anatase Ti 1− x Ce x O 2 with various cerium concentrations. It was found that the Ce incorporation leads to a decrease of Ti 3d states and enhancement of 4f states in the bottom of conduction band. At the same time, with the increase of Ce concentration, the band gap decreases gradually, and the optical absorption shifts to long wavelength range. In addition, the Ce incorporation results in a decrease of optical absorption in the range of 180–600 nm.

Density functional theory studies of doping in titania

The structural and electronic properties of rutile and anatase, and the influence of both mono- and co-doping, have been studied using density functional theory. Ge-doped anatase and rutile exhibit different band-gap narrowing mechanisms; in particular, host Ti 3d states move to lower energy regions in anatase and Ge 4s impurity states locate below the conduction band of rutile. For S doping, S 3p states locate above the top of the valence band and mix with O 2p states, leading to band-gap narrowing. For Bi doping, the energy levels of the Bi 6s states lie below the bottom of the conduction band while the Fermi level E F lies above the gap states, indicating that the gap states are fully occupied. For Bi/S co-doping, both S 3p acceptor states and partially occupied Bi 6s donor states hybridised with S 3p appear simultaneously. For N and W monodoping, isolated N 2p states above the top of the valence band and W 5d states below the conduction band lead to band-gap narrowing. N/W co-doping yields significant band-gap narrowing. Both studies for Bi/S and N/W co-doping rationalise recent experimental data which show that these doped anatase systems exhibit higher visible-light photocatalytic efficiency than respective mono-doping.

Recent development and application of Li 4Ti 5O 12 as anode material of lithium ion battery

Lithium-ion batteries with both high power and high energy density are one of the promising power sources for electric devices, especially for electric vehicles (EV) and other portable electric devices. One of the challenges is to improve the safety and electrochemical performance of lithium ion batteries anode materials. Li 4Ti 5O 12 has been accepted as a novel anode material of power lithium ion battery instead of carbon because it can release lithium ions repeatedly for recharging and quickly for high current. However, Li 4Ti 5O 12 has an insulating character due to the electronic structure characterized by empty Ti 3d-states, and this might result in the insufficient applications of LTO at high current discharge rate before any materials modifications. This review focuses first on the present status of Li 4Ti 5O 12 including the synthesized method, doping, surface modification, application and theoretical calculation, then on its near future development.

Electronic structure of Ti-doped Sr4Sc2Fe2As2O6 as a possible parent phase for the new FeAs superconductors

Abstract First principle FLAPW-GGA calculations have been performed with the purpose to understand the effect of Ti-doping on the electronic properties for the newly discovered tetragonal iron arsenide-oxide Sr4Sc2Fe2As2O6 (abbreviated as FeAs42226) as the possible parent phase for the new FeAs superconductors. Our results show that the insertion of Ti into Sc sublattice of this five-component iron arsenide-oxide phase leads to the resolute change of electronic structure of FeAs42226. Namely, the insulating oxygen-containing [Sr4Sc2O6] blocks in Ti-doped FeAs42226 became conducting, and this differs essentially from the known picture for all others FeAs superconductors where the conducting [Fe2As2] blocks are alternated with insulating blocks. Moreover in sharp contrast with FeAs-based superconductors with Fe 3d bands near the Fermi level, for Ti-doped FeAs42226 in this region the Ti 3d states are dominated, whereas the Fe 3d states are suppressed.