Ti 3d Band Research Articles

HYDROGEN DEFECTS ON THE SURFACE OF LEAD-FREE FERROELECTRIC Na\(_{0.5}\)Bi\(_{0.5}\)TiO\(_3\) MATERIALS

Hydrogen atomic chemisorption and adsorption at all possible vacancy sites on the surface of perovskite Na0.5Bi0.5TiO3 (110) were explored using calculations from density functional theory. Our calculations reveal that a pristine Na0.5Bi0.5TiO3 (110) surface can exhibit direct and indirect transitions characterized by optical bandgaps of 2.68 eV and 2.75 eV, respectively. The density of states of the Na0.5Bi0.5TiO3 (110) surface indicates predominant electron transitions from O-2p valence bands to Ti-3d and Bi-6p conduction bands in the low-energy range. Hydrogen substitution at vacancies can induce magnetism, whereas hydrogen adsorption enhances both the magnetism and conductivity of the surface system. In particular, the charge density reveals electron transfer between cations and hydrogen atoms, providing insight into the variations in hydrogen bonding properties in these surface systems. These observations expand our understanding of hydrogen chemisorption and adsorption on defect sites, providing a novel avenue for investigating and potentially developing environmentally friendly multiferroic materials for advanced electronic devices.

Rashba splitting in polar-nonpolar sandwich heterostructure: a DFT study

In this study, we employ density functional theory based first-principles calculations to investigate the spin-orbit effects in the electronic structure of a polar-nonpolar sandwich heterostructure namely LaAlO3 / SrTiO3 / LaAlO3 . Our focus on the Ti-3d bands reveals an inverted ordering of the SrTiO3 - t2g orbital near the n-type interface, which is consistent with earlier experimental work. In contrast, toward the p-type interface, the orbital ordering aligns with the natural ordering of SrTiO3 orbitals, influenced by crystal field splitting. In the presence of SOC, a notable inter-orbital coupling between t2g and eg orbitals is observed within the tetragonal slab, a phenomenon not reported before in the SrTiO3 -based 2D systems. Additionally, our observations highlight that the cubic Rashba splitting in this system surpasses the linear Rashba splitting, contrary to experimental findings. This comprehensive analysis contributes to a refined understanding of the role of orbital mixing in Rashba splitting in the sandwich oxide heterostructures.

Structural and Photoluminescence Properties of Ca2+-Substituted Self-Activated Photoluminescence Material of Na2TiSiO5

The effects of Ca2+ substitution on the structural and optical properties of Na2TiSiO5 were studied. It was expected that the Ca2+ ions would replace Na+ ions and change the coordination of Ti4+-O2- as a luminescence center. Na2(1‑x)CaxTiSiO5 (x = 0.00, 0.50, 1.00) samples were synthesized using the solid-state method, and their structural properties, the local Ti4+-O2- coordination, the absorption spectra, and the photoluminescence properties were studied. The electronic structure of Na2(1-x)CaxTiSiO5 with x = 0.00 and 1.00 was also calculated using the Full Potential Linear Augmented Plane Wave (FP-LAPW) method to explain some of the observed properties. The Ca2+ substitution resulted in a phase transformation from an orthorhombic to a monoclinic structure. The number of TiO6 octahedra increased with the increase of Ca2+ and correlated with the decrease of PL emission intensity of the Na2(1‑x)CaxTiSiO5 samples. The Ti-3d bands in the CaTiSiO5 were more dispersive than in the Na2TiSiO5 and were responsible for the PL emission intensity reduction.

Local and electronic structure of Sr1-xGdxTiO3 probed by X-ray absorption spectroscopy

Gadolinium-doped strontium titanate (Sr1-xGdxTiO3) is a typical perovskite structure material which has been studied due their thermomechanical, termoelectrical and electrochemical properties. In this study, local and electronic structure of Sr1-xGdxTiO3 samples were analyzed through X-ray absorption spectroscopy measurements. The results obtained with the adjustment of extended X-ray absorption fine structure (EXAFS) spectra at Sr K-edge show that crystallographic model of Pm-3m space group is consistent with local structure around Sr atoms, as expected. This same analysis also reveals an increasing of the Debye-Waller as a function of the Gd content in some shells, which is associated with disorder induced by Sr vacancies due to the heterovalent Gd incorporation. EXAFS spectra at Gd LIII-edge for Sr1-xGdxTiO3 samples indicates regular GdO12 dodecahedra without displacement of Gd atoms from centrosymmetric position. A disorder was also identified in the shells beyond the first 12 O neighbors in which neither the crystallographic cubic structure of the SrTiO3 nor the orthorhombic structure of the GdTiO3 fits well. X-ray absorption near edge spectroscopy (XANES) spectrum at Ti LIII,II-edges shows an asymmetric peak because of the splitting between the eg orbitals of 3d band for SrTiO3 sample. The addition of Gd atoms to SrTiO3 structure cause an enlargement of this peak and this split is associated with a small displacement of Ti atoms from their centrosymmetric position. Several features of the XANES spectra at O K-edge for Sr1-xGdxTiO3 samples are affected by the increase of Gd concentration. According to our calculated projected density of states, these transitions are related to a reduction in the number of unoccupied O 2p - Ti 3d states caused by the split of Ti 3d band. Moreover, these XANES spectra also show a dependence of the increasing of the hybridization between O 2p and Gd 5d4f6s states.

Separation of Surface- and Bulk-specific Ti L-edge XANES Spectra of Rutile (110) Surface

The electronic structure of the rutile TiO2(110) surface passivated with pyrrolidine dithiocarbamate (PDTC) was investigated by depth-resolved Ti L-edge X-ray absorption spectroscopy. We measured a series of Ti LMM Auger electron angular distributions as a function of photon energy. X-ray absorption spectra measured at the grazing and normal emission directions were deconvoluted into surface- and bulk-region-specific spectra. The spectra at an emission polar angle of approximately 47.5° were identical to the emission-angle integrated spectrum. This suggests that the TiO2 surface electronic structures have two main components: those of the surface layers, with a thickness of approximately 0.47 nm, and those of the bulk regions. The separated surface spectrum suggested that the Ti 3d band is partially occupied, but no additional peak related to the defect state was found. These results showed that the surface treatment was effective in removing the electron recombination defect sites at the photovoltaic cel...

Splitting of the Ti-3d bands of TiSe2 in the charge-density wave phase

Very high resolution angular resolved photoemission (ARPES) spectra on TiSe2 in two distinct polarization geometries (vertical and horizontal) at temperatures between 300K and 22K enabled the observation of details of bands near the Fermi level not reported so far. Calculations of the electronic band structure based on density functional theory (DFT) using B3LYP hybrid functional and MBJ potential (with and without spin-orbit coupling) were performed to obtain the orbital symmetry and dispersion. Two degenerate conduction bands (CB’s) were observed at the Γ-point, a weak CB- emission at the A-point, and two non degenerate CB’s (i.e. splitting of CB) at the M/L-point of the Brillouin Zone (BZ). The splitting was detected at L for both polarizations, while at M remarkably only for horizontal polarization. These results cannot be fully accounted for by current theories for the charge density wave (CDW) and point to a reduced symmetry of the electronic states, possibly due to the chiral CDW.

Photochemistry of Carboxylate on TiO2(110) Studied with Synchrotron Radiation Photoelectron Spectroscopy.

We present a dedicated synchrotron radiation photoelectron spectroscopy (SR-PES) study of a photochemical reaction on the surface of rutile TiO2(110). The photoreaction kinetics of carboxylate species (trimethyl acetate, TMA) upon irradiation by UV and soft X-rays were monitored, and we show that it is possible to control the reaction rates from UV light and soft X-rays independently. We directly observe Ti4+ → Ti3+ conversion upon irradiation, attributed to electron trapping at Ti sites close to surface OH groups formed by deprotonation of the parent molecule, trimethylacetic acid (TMAA). TMA photolysis on two surface preparations with different oxygen vacancy densities shows that the vacancy-related charge quenches the amount of charge that can be trapped at hydroxyls upon irradiation. During the initial stages of reaction the correlation between the amount of photodepleted TMA and the amount of charge trapped in the Ti 3d band gap state is nearly 1:1. A first-order kinetics analysis reveals that the reaction rate decreases with decreasing TMA coverage. There is also a coverage-dependent difference in the electronic structure of TMA moieties, primarily involving the carboxyl anchor group. These changes are consistent with a decreased hole affinity of the adsorbed TMA and hence a decreased reaction rate. This discovery adds to the previously presented picture of a reactivity that is inversely proportional to the number of surface hydroxyls, suggesting that the balance between the amounts of TMA, OH, and trapped charge needs to be considered.

PEFC Cathode Catalysts Using Doped and Non-Doped Titanium Oxide Support

Stability of catalyst support materials is one of important issue for improving the durability of PEFC cathode. Due to vulnerability of carbon black to oxidation at high potentials during start-up/shut-down cycles, electro-conductive oxides, nitrides, and carbides are recently attracting much attentions as promising alternative support materials [1]. Among them, sub-stoichiometric (oxygen-deficient) titanium oxide, which is known as Magnéli phase (TinO2n-1), is attractive candidate because of its superior corrosion-resistant and electrically conductive properties. As reported previously, TiOx-supported Pt catalyst shows improved ORR activity with much higher stability against high potentials under RDE and practical MEA evaluation conditions [2]. In addition to introducing oxygen-deficiency to the TiO2 lattice, n-type doping with higher valent cation such as Nb5+ is commonly used technique to obtain conductive titanium oxide. Doped Nb5+ cations introduce carrier electron into Ti-3d band, which leads to the increase of conductivity of the oxide. In this work, titanium oxide supports with different dopant concentration and reduced level were prepared, and its activity for ORR and stability against potential cycles were examined.Nb-doped TiO2 (Nb: 1-10 at.%) nanoparticles were prepared by flame spray method using diluted organometallic compounds as metal sources. The obtained Nb-TiO2 powder is a mixture of rutile and anatase structures and consists of 20-50 nm particles with BET area of 60-80 m2/g. Reducing treatments of the oxide was carried out by irradiating with Nd:YAG laser operated at λ = 355 nm in acetonitrile [3]. Pt nano-particles were deposited on the oxide support (10-20 wt.%Pt) by impregnation-reduction method. The electrochemical measurements was conducted by rotating disk electrode (in 0.1M HClO4) and MEA (at 80°C, 100%RH). Durability of the catalyst was examined according to the FCCJ-2011 protocols (1.0 V-1.5 V vs RHE for catalyst support durability, 0.6 V-1.0 V vs RHE for Pt NPs durability) [4]. Figure 1 shows specific ORR activity (at 0.9 V) of Pt/TiOx and Pt/Nb-TiOx along with that of a conventional Pt/C in O2-saturated solution at 25°C. As niobium content of the oxides increases, the ORR activities decrease. From XRD patterns of the catalysts, formation of ordered Pt-Ti alloy phase was confirmed for Pt/1%Nb-TiOx, whereas no ordered alloy phase was observed for Pt/10%Nb-TiOx. This fact indicates that higher Nb-content suppress the Pt-Ti alloy formation, which leads to lower ORR activity at Pt/Nb-TiOx catalysts. In addition, without reducing treatment with laser irradiation, Pt/Nb-TiOx shows further lower ORR activity. Alloy formation seems to be almost the same with the reduced catalysts so that this activity loss would be partly due to resistivity difference of the Nb-TiOx catalyst supports. To examine the catalyst stability at high potentials, potential cycling test between 1.0 V and 1.5 V was conducted in 0.1 M HClO4 at 60°C. CVs of Pt/1%Nb-TiOx with reducing treatment are fairly stable even after 5,000 cycles, which was similar stability with non-doped Pt/Ti4O7 catalyst [2]. On the other hands, the catalysts with higher Nb content showed much inferior CV stability. Additional data including MEA performance/stability and further discussions will be presented at the meeting.

Electron doping and phonon scattering in Ti1+xS2 thermoelectric compounds

Bulk polycrystalline samples in the series Ti1+xS2 (x=0 to x=0.05) were prepared using high-temperature synthesis from the elements and spark plasma sintering. X-ray structure analysis shows that the lattice constant c expands as titanium intercalates between TiS2 slabs. A Seebeck coefficient close to −300μVK−1 is observed for the first time in TiS2 compounds. The decrease in electrical resistivity and Seebeck coefficient that occurs upon Ti intercalation (Ti off stoichiometry) supports the view that charge carrier transfer to the Ti 3d band takes place and the carrier concentration increases. At the same time, the thermal conductivity is reduced by phonon scattering due to structural disorder induced by Ti intercalation. Optimum ZT values of 0.14 and 0.48 at 300 and 700K, respectively, are obtained for x=0.025.

Suppression of the critical thickness threshold for conductivity at the LaAlO3/SrTiO3 interface.

Perovskite materials engineered in epitaxial heterostructures have been intensely investigated during the last decade. The interface formed by an LaAlO3 thin film grown on top of a TiO2-terminated SrTiO3 substrate hosts a two-dimensional electronic system and has become the prototypical example of this field. Although controversy exists regarding some of its physical properties and their precise origin, it is universally found that conductivity only appears beyond an LaAlO3 thickness threshold of four unit cells. Here, we experimentally demonstrate that this critical thickness can be reduced to just one unit cell when a metallic film of cobalt is deposited on top of LaAlO3. First-principles calculations indicate that Co modifies the electrostatic boundary conditions and induces a charge transfer towards the Ti 3d bands, supporting the electrostatic origin of the electronic system at the LaAlO3/SrTiO3 interface. Our results expand the interest of this low-dimensional oxide system from in-plane to perpendicular transport and to the exploration of elastic and inelastic tunnel-type transport of (spin-polarized) carriers.

A DFT study of the stoichiometric and reduced anatase (0 0 1) surfaces

Here we report a systematic study on the structural and energetic properties of the stoichiometric and reduced anatase (001) surfaces by using periodic density functional calculations. Structural and energetic properties for different surface models are investigated, as well as their stability as a function of the slab thickness. The calculations indicated that a 3×3×6 model is adequate to describe the pure and reduced anatase TiO2 (001) surfaces. Our calculations on the reduced system have demonstrated that the outermost oxygen vacancy, in the first atomic layer, is the most stable defect. A comparison between (001) and (101) surfaces reveals that defect creation is easier on the former. Thus, a higher vacancy concentration on that surface is expected. On the other hand, calculations at higher vacancy concentrations show repulsive interactions among defects what might inhibit further vacancy creation. The electronic structure has been examined by means of DFT+U methodology. The Hubbard U parameter influences considerably the location of defect states in the Ti 3d band and is essential to correctly describe the electronic structure. We conclude that the value U=4.0eV is a correct choice, which produces mid states in the band gap, a charge density localization and increases geometry deformation because of repulsive interactions, but effects on energetics are limited.

The effect of Ta doping in polycrystalline TiOx and the associated thin film transistor properties

Tantalum (Ta) is suggested to act as an electron donor and crystal phase stabilizer in titanium oxide (TiOx). A transition occurs from an amorphous state to a crystalline phase at an annealing temperature above 300 °C in a vacuum ambient. As the annealing temperature increases from 300 °C to 450 °C, the mobility increases drastically from 0.07 cm2/Vs to 0.61 cm2/Vs. The remarkable enhancement of thin film transistor performance is suggested to be due to the splitting of Ti 3d band orbitals as well as the increase in Ta5+ ions that can act as electron donors.

Symmetrical Negative Differential Resistance Behavior of a Resistive Switching Device

With a thin insulator sandwiched between two electrodes, the negative differential resistance (NDR) behavior has been frequently reported for its potential device applications. Here we report the experimental observation of a symmetric NDR characteristic in a resistive switching device based on TiO(2). We propose a charge storage mechanism for the NDR effect, with oxygen molecular ions working as the active source, in a thin insulating layer. Current-voltage measurements demonstrated a highly reproducible state at about 0.65 eV, and the photoelectron spectroscopy measurements showed that it complies well with the Ti3d band gap state. Our first-principle calculations confirm that charge storage and release arise from trapping and detrapping of oxygen molecular ions at the defect sites. The results and mechanism demonstrated here in a thin layer could be extended to other systems approaching molecular dimensions for device applications.

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.

Oxidation of ultra-thin Ti films on Mo(100): Soft X-ray photoelectron spectroscopy study

Titanium oxide films grown on Mo(100) have been investigated by low-energy electron diffraction (LEED) and soft X-ray photoelectron spectroscopy (PES). The film was grown by Ti deposition on Mo(100) and subsequent oxidation of the film by 12L of O2 exposure at room temperature. As the film was annealed at 700–1000°C, the film in which the Ti atoms were in a Ti3+ oxidation state was formed. As the film was annealed at 1100–1500°C, the oxidation state of Ti in the film was converted to Ti2+. The valence electronic structure of the film was measured under the condition that the emission from the Mo substrate was minimized due to a Cooper minimum of the Mo 4d photoionization cross sections (hν=100eV). It was found that the Ti 3d band in normal-emission spectra was increased in intensity when the film was annealed at 1100–1500°C. As the film was annealed at 1300°C for 10s and 20s, the film-covered Mo(100) gave (2×2) and (4×1) LEED patterns, respectively. The two-dimensional band structure of the (2×2) system was investigated by angle-resolved PES, and it was found that the film with a (1×1) periodicity with respect to the Mo(100) substrate existed in the (2×2) system.

Electron Enrichment in 3d Transition Metal Oxide Hetero-Nanostructures

Direct experimental observation of spontaneous electron enrichment of metal d orbitals in a new transition metal oxide heterostructure with nanoscale dimensionality is reported. Aqueous chemical synthesis and vapor phase deposition are combined to fabricate oriented arrays of high-interfacial-area hetero-nanostructures comprised of titanium oxide and iron oxide nanomaterials. Synchrotron-based soft X-ray spectroscopy techniques with high spectral resolution are utilized to directly probe the titanium and oxygen orbital character of the interfacial region's occupied and unoccupied densities of states. These data demonstrate the interface to possess electrons in Ti 3d bands and an emergent degree of orbital hybridization that is absent in parent oxide reference crystals. The carrier dynamics of the hetero-nanostructures are studied by ultrafast transient absorption spectroscopy, which reveals the presence of a dense manifold of states, the relaxations from which exhibit multiple exponential decays whose magnitudes depend on their energetic positions within the electronic structure.

The ground electronic state of TiS2: experimental and theoretical studies

AbstractAn investigation of TiS2 electronic structure was conducted using a first‐principles calculation based on density functional theory (DFT). The results showed that there is indirect S3p ‐Ti3d band overlaps in ground electronic structure of TiS2. The measurements of resistivity and Hall effect were used to characterize the electrical transport behavior of TiS2 under ground state. Both theoretical and experimental results supported that TiS2 within ground state is a semimetal (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Band gap engineering of bulk ZrO2 by Ti doping

It has been experimentally observed that Ti doping of bulk ZrO(2) induces a large red-shift of the optical absorption edge of the material from 5.3 to 4.0 eV [Livraghi et al., J. Phys. Chem. C, 2010, 114, 18553-18558]. In this work, density functional calculations based on the hybrid functional B3LYP show that Ti dopants in the substitutional position to Zr in the tetragonal lattice cause the formation of an empty Ti 3d band about 0.5 eV below the bottom of the conduction band. The optical transition level ε(opt)(0/-1) from the topmost valence state to the lowest empty Ti impurity state is found at 4.9 eV in a direct band gap of 5.7 eV. The calculated shift is consistent with the experimental observation. The presence of Ti(3+) species in Ti-doped ZrO(2), probed by means of electron paramagnetic resonance (EPR), is rationalized as the result of electron transfers from intrinsic defect states, such as oxygen vacancies, to substitutional Ti(4+) centers.

The influence on Fermi energy of Li-site change in LizTi1−yNiyS2 on crossing z = 1

The Fermi energy EF of the TiS2 layers of metallic LixTiS2 increases by 1.3 eV as x increases through x = 1 to a two-phase state containing a Li-rich phase with Li shifted from octahedral to tetrahedral sites of the space between TiS2 layers. For x 1 phase, EF lies in the 3d band of the Ti(III)/Ti(II) mixed valence. In order to determine how much of the shift of EF is due to the on-site electron–electron electrostatic energy U of the narrow band Ti-3d electrons and how much is due to the shift of the Li in the interlayer space from octahedral to tetrahedral sites, we have monitored the change in EF within the Ti(IV)/Ti(III) mixed-valence state of Lix(Ti0.9Ni0.1)S2 as x is increased through x = 1. Substitution of Ni for Ti changes the sulfur stacking from hexagonal to cubic, but the stacking changes back to hexagonal on insertion of Li to x ≈ 1. A voltage step at 1.45 V with cubic stacking of the sulfide planes and at 1.15 V with hexagonal stacking reveals a raising of EF by 0.3 eV on going from cubic to hexagonal stacking. Shifting of the Li+ ions from octahedral sites to a Li-rich phase in which they occupy tetrahedral sites on increasing x through x ≈ 1 raises EF by Δm ≈ 0.5 eV, which indicates a U ≈ 1.3 eV − Δm ≈ 0.8 eV in the narrow Ti-3d band of LiTiS2 having a width W > U.

Real-Time TD-DFT Simulations in Dye Sensitized Solar Cells: The Electronic Absorption Spectrum of Alizarin Supported on TiO2 Nanoclusters.

The structural and electronic properties of the alizarin dye supported on TiO2 nanoclusters have been examined by means of time-dependent density-functional (TD-DFT) calculations performed in the time-domain framework. The calculated electronic absorption spectrum of free alizarin shows a first band centered at 2.67 eV that upon adsorption features a red shift by 0.31 eV, in agreement with both experimental and previous theoretical work. This red shift arises from a relative stabilization of the dye LUMO when adsorbed. To analyze the dependence of the electronic properties of the dye-support couple on the size of metal-oxide nanoparticles, different models of (TiO2)n nanoclusters have been used (with n = 1, 2, 3, 6, 9, 15, and 38). As a conclusion, the minimal model is good enough to theoretically reproduce the main feature in the spectrum (i.e., the energy shift of the main band upon binding to TiO2). However, it fails in creating intermediate states which could play a significant role under real experimental conditions (dynamics of the electronic transfer). Indeed, as the size of the nanocluster grows, the dye LUMO moves from the edge to well inside the conduction band (Ti 3d band). On the other hand, to assess the consistency of the time-domain approach in the case of such systems, conventional (frequency-domain) TD-DFT calculations have been carried out. It is found that, as far as the functional and basis set are equivalent, both approaches lead to similar results. While for small systems the standard TD-DFT is better suited, for medium to large sized systems, the real-time TD-DFT becomes competitive and more efficient.