Ti 3d States Research Articles

Correlation between electronic and structural orders in 1T−TiSe2

The correlation between electronic and crystal structures of 1T-TiSe2 in the charge density wave (CDW) state is studied by x-ray diffraction. Three families of reflections are used to probe atomic displacements and the orbital asymmetry in Se. Two distinct onset temperatures are found, TCDW and a lower T* indicative for an onset of Se out-of-plane atomic displacements. T* coincides with a DC resistivity maximum and the onset of the proposed gyrotropic electronic structure. However, no indication for chirality is found. The relation between the atomic displacements and the transport properties is discussed in terms of Ti 3d and Se 4p states that only weakly couple to the CDW order.

Electronic structures and properties of TiAl/Ti2AlNb heterogeneous interfaces: A comprehensive first-principles study

The electronic structures and properties of ten TiAl/Ti2AlNb heterogeneous interfaces with three Ti2AlNb phases (B2, D019, O) and different terminations/boundaries were comprehensively studied via first-principles calculations. Their interfacial bonding features, strength and relative stability are quantitatively and qualitatively revealed by the equilibrium energy, Fermi energy, interfacial energy (γint), electron localization function and bonding charge density (Δρ), respectively. Accordingly, it was revealed by the partial and total electron density of states that the decreasing tendency of the bond strength in the order of Ti-Nb > Ti-Al > Nb-Al at the interfaces was attributed to the electrons hybridization of Ti-d states, Nb-d states and Al-p states. Since the γint decreased in the order of TiAl/B2 > TiAl/O > TiAl/D019, the TiAl/D019 interfaces with the lowest energy was regarded as the most stable phase boundaries and was further validated by the electron back-scattered diffraction analysis. It is found that the D019 phase is dominated at the interface boundaries while the B2 serves as a transition phase distributed on its both sides. On the contrary, the metastable O phase can form in the bulk Ti2AlNb but not at the interface due to their largest distortion when precipitating at the phase boundary. This work provides an insight into the atomic and electronic basis for the bonding and strengthening mechanisms of TiAl/Ti2AlNb heterogeneous interfaces.

Growth and photoemission studies of titanium carbide coating by laser carburizing

Carburizing of metals is well known technique to modify the tribological properties like hardness and corrosion resistance of metal surfaces. Besides this the electronic structures of nitrides and carbides of titanium metal have great importance from both fundamental and technological point of view. Resonance photoemission electron spectroscopy is a well-known technique to reveal the electronic structure of these alloys. In the present work titanium carbide (TiC) coating has been synthesized by laser carburizing on commercial available pure titanium sheet in a controlled methane atmosphere. X-ray diffraction studies reveal the formation of TiC phase after laser treatment. The hardness of TiC was performed by nano-indentation and its electronic structure has been characterized by x-ray and valence band photoelectron spectroscopy. The measured hardness is six times higher than the pure Ti substrate. Resonance photoemission spectrum of TiC gives maxima at 46 eV, which is a characteristic feature of Ti 3d states in the valence band of TiC coating.

A first principle study of magnetic and opto-electronic properties of half metallic Heusler alloy, Co2TiSi

We have investigated the half metallic Heusler alloy, Co2TiSi via first principle calculation using the Wien2K code and studied the electronic, magnetic and optical properties. Density of state (DOS) profile represents the half metallic character for Co2TiSi i.e, 100% spin polarization at the Fermi level. Partial (p-) DOS show that major contribution is due to Co-3d and Ti-3d states across the Fermi level. Band structure also shows a band gap of ~ 1.0 eV for spin down states whereas metallic character for the spin up states. Calculated electronic and magnetic behavior along with estimated magnetic moment (1.98 μB) found to be in good agreement with its experimental results.

A new type of noncovalent surface-π stacking interaction occurring on peroxide-modified titania nanosheets driven by vertical π-state polarization.

Noncovalent π stacking of aromatic molecules is a universal form of noncovalent interactions normally occurring on planar structures (such as aromatic molecules and graphene) based on sp2-hybridized atoms. Here we reveal a new type of noncovalent surface–π stacking unusually occurring between aromatic groups and peroxide-modified titania (PMT) nanosheets, which can drive versatile aromatic adsorptions. We experimentally explore the underlying electronic-level origin by probing the perturbed changes of unoccupied Ti 3d states with near-edge X-ray absorption fine structures (NEXAFS), and find that aromatic groups can vertically attract π electrons in the surface peroxo-Ti states and increase their delocalization regions. Our discovery updates the concept of noncovalent π-stacking interactions by extending the substrates from carbon-based structures to a transition metal oxide, and presents an approach to exploit the surface chemistry of nanomaterials based on noncovalent interactions.

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.

Ab initio prediction of the structural, electronic and optical behavior of novel combinations of ternary perovskite oxides ATiO3 (A = Rb, Cs, Fr) using the Hubbard ‘U’ correction for optoelectronic devices

The structural and optoelectronic properties of ATiO3 (A = Rb, Cs, and Fr) perovskite oxides in the cubic phase are investigated using an ab initio method based on density functional theory. The exchange correlation function is determined by the full-potential linearized augmented plane wave approach. RbTiO3 shows an indirect band gap energy of 2.43 eV, whereas direct band gap energies of 1.90 eV and 1.66 eV are calculated for CsTiO3 and FrTiO3 compounds, respectively. The electronic density of states reveals that d states from A-cations (Rb, Cs, and Fr), along with Ti-d states, are located in the conduction band. The charge density demonstrates the ionic nature of the A-cations (Rb, Cs and Fr), while oxygen atoms share isolines with Ti atoms, leading to covalent bonds between them. Optical analyses show that the considered compounds have good optical conductivity and absorptivity when high-energy photons are incident on the surface of the materials, in particular showing minimal reflectivity. In view of these results, we consider that perovskites belong to a semiconductor category of materials suitable for application in photovoltaic and optoelectronic devices.

Eliminating the isolated states from the band structure of doped TiO2: A first principle study

Band structure of anatase [Formula: see text] could be modified by doping foreign atoms. With ab-initio calculations, considering praseodymium, nitrogen and gadolinium as potential dopants, some mono-, co- and tri-doped models are introduced. The structure modification due to each model is compared with a standard [Formula: see text] system. Pr doping reduced the band gap of [Formula: see text] by introducing Pr 4f states below the conduction band, and the Fermi level appears below the conduction band minimum. The addition of nitrogen in Pr-doped [Formula: see text] pushed back the Fermi level to its intrinsic position while reducing the band gap. The tri-doped system comprising of Pr, N and Gd as dopants provided a populated band gap. The impurity states associated with Gd appeared in the middle of the band gap. The density of states analysis reveals that Pr 4f states change the location of N 2p states in the band structure. Instead of appearing as isolated states, the N 2p states are mixed with O 2p states while the Pr 4f states are coupled with the Ti 3d states in Pr, N-[Formula: see text]. The Pr, N, Gd tri-doped [Formula: see text] system provided strong visible light absorption due to the stepwise transition of electrons by Gd f states. While providing reasonable visible light absorption, the Pr, N-[Formula: see text] is expected to provide strong visible light photoactivity due to the clear band gap. Due to the absence of isolated states, the generated photo-excited carriers would be efficiently utilized in the oxidation/reduction processes.

Intrinsic electronic defect states of anatase using density functional theory

In this work an overall electronic structure including the position and formation energies of various intrinsic defects are computed for anatase using Density Functional Theory aided by Hubbard correction (DFT + U). The intrinsic point defects considered here are, oxygen vacancy (VO), oxygen interstitial (Oi), titanium vacancy (VTi) and titanium interstitial (Tii). Out of all the intrinsic defects considered here, VTi and Tii are found to be most stable under equilibrium condition. Whereas, conduction band in anatase is consisted of mainly Ti 3d with a minor component of O 2p states, valence band is found to be mainly composed of O 2p with a minor contribution from Ti 3d states. VO and Tii are found to form localized states in the band gap. Moreover, anisotropy in the effective mass is seen. Finally, an alignment of band diagrams for all the intrinsic defect states is performed using vacuum potential from slab-supercell calculation as reference. This first principle study would help in the understanding of defect-induced insulating to conducting transition in anatase, which would have significant impact in the photocatalytic and optoelectronic area.

Nonlocal XANES pre-edge feature of FeTiO3 ilmenite-type at Ti and Fe K-edge

The pre-K-edge feature of Ti and Fe XANES of ilmenite FeTiO3 were investigated. The electronic structure of FeTiO3 was examined based on the partial density of electronic state distribution on both local and nonlocal atoms. A strong contribution of nonlocal metal-metal intervalence electronic states of Ti and Fe pre-K-edge was observed in X-ray absorption near-edge structure (XANES) measurement. This observation agrees well with the calculated local state density. The nonlocal excitation transitions in the pre-edge feature contribute to local pre-K-edge at both Ti and Fe. Ti pre-K-edge consists of highly hybridized Ti-3d(t2g) with a small part of dipole-allowed 4p and a small 4s density transition state feature which is predominated by nonlocal of Fe-3d mixed with 4p state delocalized via O-2p as the main component peak. The peak feature at Ti-3d(eg) energy state is assigned to hybridization of Ti-3d(eg) and Ti-4p state contributed with nonlocal of the hybridized Fe-sp and the Ti-pd state. Fe pre-K-edge shows two main prominent transition peaks of hybridized Fe-pd state and the delocalized local Fe-sp with nonlocal Ti-3d(t2g) and 4p mixing states centered at 7112.38 and 7116.48 eV, respectively. The local and nonlocal excited transition energy states and density of both Ti pre-K-edge and Fe pre-K-edge features are corresponded with each other through the O-2p state transition pathway.

Preparation and properties of black Ti-doped zirconia ceramics

Black zirconia ceramics has many problems, such as uneven color and poor phase stability. The reason for this phenomenon is that the colorant uses a colorant with a complex black spinel structure, and also the application of a dyeing method. To improve the color uniformity and phase stability of black ceramics, TiO2 was introduced to replace the traditional complex black spinel, a high-temperature reduction method (HTR) was employed to prepare a black Ti-doped zirconia ceramic, stead of the conventional dyeing method. After the HTR at 1400 °C, the concentration of Ti3+ and oxygen vacancies increased, and the lightness of the ceramic samples decreased from 93.48 to 44.17. The black ceramic is uniform in color and stable in phase, avoiding the disordered phase of the spinel doped multiphase zirconia ceramic. The results indicate that the overall color of the samples depends on the development mechanism of the color center. The formation of color centers (F+ centers) in this work was related to the electronic transition of Ti 3d states and oxygen vacancies generated during the HTR. F+ centers cause a strong light absorbance over the entire visible light range (400–750 nm), so that the color of the ceramics is black. Based on their good color attributes, black ceramics have great potential in industry, for example as the back plate of mobile phones.

Study on the electronic structure and photocatalytic properties of a novel monolayer TiO<sub>2</sub>

By means of state-of-the-art density functional theory (DFT) computations, We designed a new two-dimensional material TiO<sub>2</sub>. We further investigated the stability, electronic structure, carrier mobility, and optical properties of monolayer TiO<sub>2</sub>. Our results show that monolayer TiO<sub>2</sub> has good kinetic, thermodynamic and mechanical stability and can exist stably at room temperature. The results were demonstrated using the binding energy, phonon spectrum, molecular dynamics simulation, and elastic constant calculation. The band structure indicates that the monolayer TiO<sub>2</sub> is an indirect bandgap semiconductor with energy gaps of 1.19 eV (GGA+PBE) and 2.76 eV (HSE06), respectively. The results of state density show that the Ti-3d state electrons constitute the top of the valence band and Ti-4s state electrons constitute the bottom of the conduction band. The electron states of O atoms contribute very little near the Fermi energy level and are mainly distributed in the deep energy level. In addition, the carrier mobility of monolayer TiO<sub>2</sub> is smaller than monolayer MoS<sub>2</sub>, and the electron and hole mobility can reach 31.09 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup> and 36.29 cm<sup>2</sup>·V<sup>–1</sup>·s<sup>–1</sup>, respectively. Due to the anisotropy of hole mobility and electron mobility, the composite rate of electrons and holes is relatively low. This ensures longer service life and better photocatalytic activity of monolayer TiO<sub>2</sub>. Furthermore, under the condition of uniaxial strain and biaxial strain, the energy gap of monolayer TiO<sub>2</sub> has a clear response. The energy gap is more sensitive to biaxial strain than uniaxial strain, indicating that monolayer TiO<sub>2</sub> can be applied to various semiconductor devices. The band-edge potential and optical properties of semiconductors indicate that two-dimensional TiO<sub>2</sub> is capable of photo-splitting water production, H<sub>2</sub> at –5~2% single/biaxial strain, and O<sub>2</sub>, H<sub>2</sub>O<sub>2</sub>, O<sub>3</sub>, etc. at –5~5% single/biaxial strain. Moreover, the monolayer TiO<sub>2</sub> has a high absorption coefficient for visible and ultraviolet light. In conclusion, the monolayer TiO<sub>2</sub> has a potential application prospect in the field of optoelectronic devices and photocatalytic materials in the future.

Role of chemical pressure on optical and electronic structure of Ho2GexTi2−xO7

Chemical pressure plays a crucial role in determining the electronic properties of the quantum materials. Investigation of electronic structure of Ho2GexTi2−xO7 (x = 2, 1.9, 1.75, 1.5 1, 0.5, 0.25, 0.1 and 0) series has been performed. Pyrochlore and Pyrogermanate, Re2B2O7 (Re = Ho3+, B = Ti4+ and Ge4+; rare earth titanates and germanates), substituted with increasing amount of Ge4+ at the Ti4+ site and vice versa develops structural distortions. Distinct shrinkage effect has been established in the Ho2Ti2O7 matrix upon Ge+4 substitutions at B site, resulting in the modification of band gap value. Band gap of 5.24 eV drastically drops to 3.92 eV with immediate Ti4+ substitution in Ho2Ge2O7. Electronic states of Ho3+ (4f forbidden transitions) had also been identified. We observe favored sub level transition (Specific Stark component) corresponding to5F5 to 5I8 electronic transition for Ho3+ at λexc. = 450 nm. The upper valence band consisted of O 2p state hybridized with Ho 5p and Ti and Ge 4p states and conduction band primarily formed by Ho 5d state hybridized with Ti 3d and Ge 4d states as obtained from density of states (DOS) calculations. Strong hybridization between Ho 5p1/2 and Ti 3p orbital upon Ti4+ inclusion in Ho2Ge2O7 has been observed through both theoretical studies using LDA—1/2 and UV–Vis, photoluminescence, ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy. The evolution of total DOS of all studied composition shows that valence band edge is more sensitive than conduction band to composition. These results provide chemical pressure as an excellent tool to tailor the band gap and fine tune the intermediate electronic states in Ho2GexTi2−xO7.

Band gap modulation effect on electronic and optical properties in PbTiO3 under stress: a DFT study

First principles calculation was implemented to explore the effect of band gap modulation under stress in cubic PbTiO3 by using generalized gradient approximation (GGA) and ultrasoft pseudo-potential (USP). The band gaps at different external pressures show indirect band nature with significant reduction in band gapes. The density of states indicated that the upper valence states were influenced by O-2p and in the meantime Ti-d states have main contribution in conduction band. The inclusion of external stress not only changes electronic structure but also significantly changes the optical properties such as complex dielectric function, absorption, energy loss function, refractive index and reflectivity. The shifting of absorption edges toward higher energies confirm the presence of blue shift and make this material an appealing candidate for optoelectronic devices.

Step-wise transitions of electrons between valence and conduction bands: a tri-doped TiO2 approach

The unwanted structural changes and isolated unoccupied states in the band structure of TiO2 due to doping could be minimized by proper selection of dopants. A good combination of dopants would improve the optical response in visible regime and remove the isolated states from the band gap while keeping minimum structural distortion. Enhanced visible light absorption and effective utilization of photo-generated carriers would improve the photoactivity of TiO2 under solar irradiations. Mono-doped, co-doped and tri-doped models of molybdenum (Mo), carbon (c), and nitrogen (N) are developed in the structure of anatase TiO2 and the corresponding change in the geometrical structure, band structure, and optical response is noted. The Mo and C co-doping produced least structure modification when compared to a reference TiO2 system. The Mo 3d states are mixed with Ti 3d states reducing the band gap of Mo-doped TiO2. While shifting the Fermi level to the middle of the band gap, the C 2p states are introduced around the middle of the gap of C doped TiO2. The impurity states are occupied and maximum reduction in the band gap is found in Mo, C codoped TiO2. The addition of N in the Mo, C codoped TiO2 modified the band structure by creating isolated states in the band gap. The photo-response of Mo, C, N tri-doped TiO2 is prominent among the simulated models attributed to the step wise transition of electrons by the C 2p and N 2p states. The improved optical response allows more visible photons for absorptions but the isolated states may annihilate it. The Mo, C codoped system provided minimum structure changes with clear band structure and reasonable visible light absorption.

Screening Doping Strategies To Mitigate Electron Trapping at Anatase TiO2 Surfaces.

Nanocrystalline anatase titanium dioxide is an efficient electron transport material for solar cells and photocatalysts. However, low-coordinated Ti cations at surfaces introduce low-lying Ti 3d states that can trap electrons, reducing charge mobility. Here, a number of dopants (V, Sb, Sn, Zr, and Hf) are examined to replace these low-coordinated Ti cations and reduce electron trapping in anatase crystals. V, Sb, and Sn dopants act as electron traps, while Zr and Hf dopants are found to prevent electron trapping. We also show that alkali metal dopants can be used to fill surface traps by donating electrons into the 3d states of low-coordinated Ti ions. These results provide practical guidance on the optimization of charge mobility in nanocrystalline TiO2 by doping.

Understanding the correlation between the electronic structure and catalytic behavior of TiC(001) and TiN(001) surfaces: DFT study

Spin-unrestricted density functional theory (DFT) calculations are performed to explore the electronic structure and water-gas shift (WGS) behavior for the non-polar (001) surface of both titanium carbide (TiC) and titanium nitride (TiN). Previous researches have suggested that the unique electronic structures of these materials can effectively explain their surface adsorption and reactivity. Herein, our DFT calculations demonstrate that TiC(001) surface with occupied nonmetal C-2p states strongly binds CO molecule to the carbon site, and subsequently mediates the WGS along an associative mechanism. In contrast, TiN(001) surface dominated by metal Ti-3d states prefers to adsorb CO molecule at the titanium site, and then mediates WGS reaction through a redox mechanism. This indicates that the surface behavior of TiC(001) and TiN(001) is strongly dependent on their own electronic structure as expected. Therefore, the present studies provide an interesting perspective to understand the correlation between the electronic structure and catalytic behavior of such materials.

Electronic structure of designed [(SnSe)1+δ]m[TiSe2]2 heterostructure thin films with tunable layering sequence

Abstract

Role of Lone‐Pairs in Driving Ferroelectricity of Perovskite Oxides: An Orbital Selective External Potential Study

AbstractThe orbital selective external potential (OSEP) method, recently developed by the authors, allows the energy level of a specific atomic orbital to be shifted, thus allowing for the identification of the role of this orbital in the chemical and physical properties of the system. Using OSEP, the origins of ferroelectricity in two classic ferroelectric perovskites, BaTiO3 and PbTiO3, are systematically revisited. The fact that the hybridization between the Ti 3d state and the O 2p state is essential for the formation of ferroelectricity in both BaTiO3 and PbTiO3 is reproduced, which validates this method. Particularly, for PbTiO3, the Pb 6s lone‐pair electron states can also be tuned using the OSEP method, and its influence on the ferroelectricity is unveiled in detail. Surprisingly, it is found that the electric dipoles formed by lone‐pair lobes contribute only slightly to the overall ferroelectric polarization, whereas the concomitant ionic displacements and lattice distortions are prominently favorable for ferroelectricity. Indeed, it is found that the Pb–O hybridization plays an important role in the ferroelectricity of PbTiO3, which makes the polarization of PbTiO3 significantly larger than that of BaTiO3. This study provides a simple yet straightforward way to demonstrate the mechanisms of ferroelectricity in perovskite oxides, which can be applied to the study of ferroelectric mechanism in other relevant materials.

Improving hydrogen evolution activity of perovskite BaTiO3 with Mo doping: Experiments and first-principles analysis

Hydrogen production through photocatalytic water splitting attracts great attention in fields of energy conversion. To improve the hydrogen evolution efficiency, narrowing the bandgap of photocatalysts by introducing dopant atoms is widely investigated for increasing light absorption. Herein, Mo-doped BaTiO3 samples are synthesized by a traditional solid-state reaction method and all the samples are modified with Pt by a photo-reduction method. Compared with pure BaTiO3, Mo doping into BaTiO3 samples realizes the band-to-band visible-light absorption and shows remarkable improvement in hydrogen production efficiency. Under simulated sunlight irradiation and with 0.4 wt% Pt deposition, BaTiO3 doped with 2 at% Mo exhibits a hydrogen evolution rate of 63 μmol g−1 h−1, about 2 times improvement in comparison to pure BaTiO3 (35 μmol g−1 h−1). Further first-principles calculations based on density-function theory demonstrates an apparent downward movement of the conduction band minimum due to the coupling between the Ti 3d and Mo 3d states, leading to the significant bandgap narrowing and enhancement of the visible-light photocatalytic activity.