Ti-rich Conditions Research Articles

Exploring defect engineering in monolayer TiS[formula omitted] for next-generation electronic devices: Insights from first-principles study

Titanium trisulfide monolayer (TiS3) is a quasi-1D crystal with promising applications in a range of fields, including photoelectrochemical cells and thermoelectrics. Despite the emerging importance of monolayer TiS3, little is known regarding its intrinsic defects. In this work, we systematically investigate the stability, electronic, and magnetic properties of native defects in single layer TiS3, using Density Functional Theory. We consider S and Ti monovacancies, divacancies, extended Ti-nS (n=2−8) vacancy complexes, S and Ti antisites, as well as their complexes. We show that the likelihood of the formation of sulfur vacancies is strongly dependent on the symmetrically inequivalent lattice site at which the S vacancy is located. Under S-rich conditions, single sulfur vacancies are shown to be energetically more favorable than divacancies. In contrast, under Ti-rich conditions sulfur divacancies are more favorable than single S vacancies. We show that STi and TiS1 are most likely to form under S-rich and Ti-rich conditions, respectively, with low formation energies of -10.42 eV and -9.18 eV, and are therefore likely to be prevalent in single layer TiS3. We further consider defect levels in the band-gap, and show that several of these intrinsic defects are potential n-type or p-type dopants, while others will form deep electron/hole traps. Further, we show that while a number of these intrinsic defects induce no spin in the host, most intrinsic defects will induce a net magnetic moment in the host. The commonality of these defects in TiS3, will significantly impact applications such as spintronics, thermoelectrics, and the manufacturing of electronic devices.

First-principles study of the effects of Vo on the magnetic and photocatalytic properties of bilayer anatase TiO2(001): C/N/S

In this study, we investigated the effect of Vo on the magnetic and photocatalytic properties of bilayer anatase TiO2 (001): C/N/S using first principles within the density functional theory framework. Through research, it has been discovered that the stability and electronic structure of oxygen vacancies can vary depending on their positions, ultimately resulting in diverse photocatalytic performances. When the distance between N and Vo is relatively close, bilayer TiO2(001) system exhibited a relatively low formation energy under Ti-rich conditions, a high cohesive energy, and no virtual frequency formed in the phonon spectrum. Thus, it has good stability. The bilayer Ti24O46N (001) system displayed the most evident red shift in the absorption spectrum, the largest electric dipole moment, the smallest work function, relatively high carrier activity, a notable difference in mobility between electrons and holes, reduced recombination of electrons and holes, an extended carrier lifetime, and superior oxidation reaction capability. Consequently, the bilayer Ti24O46N (001) surface system emerged as the most suitable catalyst for the photo-degradation of water to produce oxygen.

In situ synthesis of TiB2 rod crystal reinforced PcBN mechanical properties: First-principles calculations and experimental study

In this work, a combination of first-principles calculations and experiments is used to not only reveal the growth mechanism of hexagonal rod-shaped crystals TiB2 and its theoretical influencing factors, but also to experimentally regulate the growth of TiB2 rod-shaped crystals to obtain polycrystalline cubic boron nitride (PcBN) composites with excellent comprehensive mechanical properties. The calculation results reveal that under Ti-rich or B-rich conditions, both the surface energies of (0001) and (101̄0) are lower than the surface energy of (112̄0). The growth of TiB2 is interpreted at the atomic level as being composed of (0001) and (101̄0) surfaces, and it preferentially grows as rod crystals along the (0001) crystal plane. The calculations further indicate that the hexagonal morphology of TiB2 is determined by the atomic concentration within the melt. It exhibits a higher propensity for nucleation under Ti-rich conditions than under B-rich conditions. And it is predicted that a lower relative concentration of Ti atoms promotes the growth of TiB2 into hexagonal rod crystals. In addition, PcBN composites with TiB2 reinforced phase are synthesized in situ by modulating the Ti-Al molar ratio. The experimental results demonstrate that as the concentration of Ti atoms decreases, the nucleation of TiB2 decreases, while the growth of TiB2 rod crystals increases, which aligns with the predicted calculations. Moreover, the presence of rod crystals greatly enhances the densities and flexural strength of PcBN composites. When the Ti-Al molar ratio is adjusted to 1:2, a substantial quantity of high-quality TiB2 rod crystals are cultivated, leading to PcBN composites exhibiting outstanding mechanical properties.

Roles of Defects and Sb-Doping in the Thermoelectric Properties of Full-Heusler Fe2TiSn.

The potential of Fe2TiSn full-Heusler compounds for thermoelectric applications has been suggested theoretically, but not yet proven experimentally, due to the difficulty in obtaining reproducible, homogeneous, phase-pure and defect-free samples. In this work, we studied Fe2TiSn1-xSbx polycrystals (x from 0 to 0.6), fabricated by high-frequency melting and long-time high-temperature annealing. We obtained fairly good phase purity, a homogeneous microstructure, and good matrix stoichiometry. Although the intrinsic p-type transport behavior is dominant, n-type charge compensation by Sb-doping is demonstrated. Calculations of the formation energy of defects and electronic properties carried out using the density functional theory formalism reveal that charged iron vacancies VFe2- are the dominant defects responsible for the intrinsic p-type doping of Fe2TiSn under all types of (except Fe-rich) growing conditions. In addition, Sb substitutions at the Sn site give rise either to SbSn, SbSn1+, which are responsible for n-type doping and magnetism (SbSn) or to magnetic SbSn1-, which act as additional p-type dopants. Our experimental data highlight good thermoelectric properties close to room temperature, with Seebeck coefficients up to 56 μV/K in the x = 0.2 sample and power factors up to 4.8 × 10-4 W m-1 K-2 in the x = 0.1 sample. Our calculations indicate the appearance of a pseudogap under Ti-rich conditions and a large Sb-doping level, possibly improving further the thermoelectric properties.

First principles of Fe/Co/Ni doping and the coexistence of O vacancy in the magnetic and optical properties of rutile TiO2(110) surface

The research on the photocatalytic properties of TiO2 by doping transition metals (Fe/Co/Ni) has achieved certain progress. However, the effects of Fe/Co/Ni doping and the coexistence of O vacancies on the photocatalytic properties of TiO2 have been rarely reported. Oxygen vacancies are the most common point defects in TiO2 materials, and the precise control of O vacancies in experiments presents difficulty. To solve this problem, this study used first principles under the framework of density functional theory to investigate the effects of Fe/Co/Ni doping and the coexistence of O vacancies on the magnetic and optical properties of rutile TiO2(110) surface. The findings indicated that the formation energy of the (110) surface of rutile TiO2 doped with transition metals Fe, Co, and Ni and coexisting with O vacancies was smaller under Ti-rich conditions than under O-rich conditions. Under Ti-rich conditions, the doping system easily formed and exhibited a high stability. The findings on magnetic properties indicated that the Fe/VO-TiO2, Co/VO-TiO2, and Ni/VO-TiO2 systems all exhibited magnetism on the (110) surface. The Fe/VO-TiO2(110) surface had the largest magnetic moment and the best magnetic properties. The study of optical properties showed that the Fe/VO-TiO2(110) surface had the longest carrier lifetime, strongest surface activity, and most evident red shift in the absorption spectrum. The oxidation capability of the Fe/VO-TiO2(110) surface was relatively the best. Fe doping and the coexistence of O vacancies are the most beneficial to the photocatalysts for the production of O2 by dissociation of water on the surface of rutile TiO2(110).

Applying Hydrogenation to Stabilize N-TiO2 and Enhance Its Visible Light Photocatalytic Activity

Up to now, the explanation for the origin of enhanced photocatalytic activity of N doped TiO2 (N-TiO2) with H incorporation, which is observed in experiment, is still lacking. In our work, the effects of hydrogenation on the stability and electronic properties of N-TiO2 have been systematically investigated by first-principles calculations. Our results of the study on stability demonstrate that, both full and part hydrogenation could stabilize N-TiO2 by largely reducing the formation energy of N doping under Ti-rich conditions. Moreover, the calculated results on the electronic structure show that, for the completely hydrogenated N-TiO2, band gap becomes slightly larger, which is caused by the full passivation for unpaired electron from N atom. However, for the partially hydrogenated N-TiO2, due to the interaction between hydrogenated and unhydrogenated N atoms, its valence band maximum shifts to higher energy by 0.32 eV and the valence band states mix with the wide band-gap states, which results in a higher light absorption capacity and carrier separation. Our results not only explain the enhancement of visible light photocatalytic activity experimentally found in N-TiO2 specimen with H incorporation, but also indicate that, tuning the hydrogenation degree is a hopeful routine to improve the photocatalytic performance of N-TiO2.

Doping effects on catechol functionalized anatase TiO2(101) surface for dye-sensitized solar cells

Doping effects of Al, Mg and Cr on the structural and photoelectric properties of catechol functionalized anatase TiO2(101) surface (CFAS) have been studied using density-functional theory. The results indicate that the adsorption processes of CFAS and catechol functionalized doped anatase TiO2(101) surfaces (CFDAS) are all exothermic and these adsorption systems are quite stable. The relative lower formation energy of Al-doped TiO2 means that it is energetically favorable structure under Ti-rich conditions. For band structure of catechol-Cr-doped-TiO2, the electron transition energy will be reduced, and visible light absorption will be extended on account of the decreased band gap and widespread impurity states. The positive Fermi energy shift of Cr-doped TiO2 suggests that it is beneficial to increase the open circuit voltage compared with pure TiO2 under the same conditions. When catechol as a model organic sensitizer functionalizing the pure and Al, Mg and Cr doped TiO2 (101) surfaces, a positive shift of the Fermi energies is observed in comparison with those materials without catechol functionalization. Compared with the optical properties of CFDAS, Cr doping has a greater effect on the optical properties of anatase TiO2 (101) surface than that of Al or Mg doping. The results show that Cr doped anatase TiO2 (101) surface is a better photoanode material and can be applied in Dye-Sensitized Solar Cells.

First-principles study on the stability and work function of low-index surfaces of TiB2

Abstract The structures, electronic properties, surface energies and work functions of low-index surfaces of TiB2 compound are studied using the first-principles calculations based on density functional theories. The structural relaxations and electronic properties show that the Ti-terminated (0 0 0 1) and (1 0 1 - 0) surfaces are more stable than the corresponding B-terminated surfaces, and the (1 1 2 - 0) surface is the most stable surface of all surfaces. Through the comparison of surface energies, it can be obtained that the Ti-terminated (0 0 0 1) and (1 0 1 - 0) surfaces are thermodynamically more favorable under Ti-rich condition, and B-terminated (0 0 0 1) surface is thermodynamically more favorable under B-rich condition. Therefore, the morphologies of TiB2 particles consisting of (0 0 0 1) and (1 0 1 - 0) surfaces are related to the atomic concentration in the melt. In addition, the B-terminated (0 0 0 1) surface has the largest work function among all surfaces.

A density functional study of the effect of hydrogen on electronic properties and band discontinuity at anatase TiO2/diamond interface

Tailoring the electronic states of the dielectric oxide/diamond interface is critical to the development of next generation semiconductor devices like high-power high-frequency field-effect transistors. In this work, we investigate the electronic states of the TiO2/diamond 2 × 1–(100) interface by using first principles total energy calculations. Based on the calculation of the chemical potentials for the TiO2/diamond interface, it is observed that the hetero-interfaces with the C-OTi configuration or with two O vacancies are the most energetically favorable structures under the O-rich condition and under Ti-rich condition, respectively. The band structure and density of states of both TiO2/diamond and TiO2/H-diamond hetero-structures are calculated. It is revealed that there are considerable interface states at the interface of the anatase TiO2/diamond hetero-structure. By introducing H on the diamond surface, the interface states are significantly suppressed. A type-II alignment band structure is disclosed at the interface of the TiO2/diamond hetero-structure. The valence band offset increases from 0.6 to 1.7 eV when H is introduced at the TiO2/diamond interface.

Temperature-dependent growth window of CaTiO3 films grown by hybrid molecular beam epitaxy

The authors report the effects of growth temperature on the self-regulated growth window of CaTiO3 thin films grown by hybrid molecular beam epitaxy (hMBE). Films were grown on (001) (La0.3Sr0.7)(Al0.65Ta0.35)O3 at temperatures between 700 and 950 °C. Calcium was supplied by a standard thermal effusion cell, while the metalorganic precursor titanium tetra-isopropoxide (TTIP) was used as the titanium source. The stoichiometric growth conditions were mapped using a combination of x-ray diffraction, reflection high energy electron diffraction, and atomic force microscopy. It is found that the growth window widened and shifted to higher TTIP fluxes with increasing temperature. Further, the shift of the growth window edge to Ti-rich conditions is three times larger than the growth window edge to Ca-rich conditions, which is discussed in general terms of the kinetic processes involved in hMBE.

Density functional study of carbon vacancies in titanium carbide

It is well established that TiC contains carbon vacancies not only in carbon-deficient environments but also in carbon-rich environments. We have performed density functional calculations of the vacancy formation energy in TiC for C- as well as Ti-rich conditions using several different approximations to the exchange-correlation functional, and also carefully considering the nature and thermodynamics of the carbon reference state, as well as the effect of varying growth conditions. We find that the formation of carbon vacancies is clearly favorable under Ti-rich conditions, whereas it is slightly energetically unfavorable under C-rich conditions. Furthermore, we find that the relaxations of the atoms close to the vacancy site are rather long-ranged, and that these relaxations contribute significantly to the stabilization of the vacancy. Since carbon vacancies in TiC are also experimentally observed in carbon-rich environments, we conclude that kinetics may play an important role. This conclusion is consistent with the experimentally observed high activation energies and sluggish diffusion of vacancies in TiC, effectively causing a freezing in of the vacancies.

A DFT study of the electronic structures and optical properties of (Cr, C) co-doped rutile TiO2

To get an effective doping model of rutile TiO2, we systematically study geometrical parameters, density of states, electron densities, dielectric functions, optical absorption spectra for the pure, C mono-doping, Cr mono-doping and (Cr,C) co-doping rutile TiO2, using density functional calculations. We find that a C doped system presents higher stability under Ti-rich condition, while Cr doped and (Cr,C) co-doped systems are more stable under O-rich condition. For (Cr,C) co-doping situation, the imaginary part of the dielectric function reflects the higher energy absorption efficiency for incident photons. Moreover, co-doping system exhibits much bigger red-shift of optical absorption edge compared with Cr/C single doping systems, because of the great reduction of the direct band gap. The calculated optical absorption spectra show that the (Cr,C) co-doping rutile TiO2 has higher photocatalytic activity in the visible light region.

Electronic and optical properties of Zr, N mono-doped and co-doped anatase TiO2: first-principles calculations

The growth conditions, crystal structural, electronic, optical and photocatalytic properties of undoped, N-, Zr mono-doped TiO2 and four kinds of Zr/N co-doped TiO2 (Zr–N1, Zr–N2, Zr–N3 and Zr–N4) with different Zr and N co-doped positions were investigated by first-principles calculations. It is found that N substituted preferentially for O under Ti-rich conditions. While Zr substituted preferentially for Ti under O-rich conditions, and has the lowest formation energies (−4.56 eV) due to Zr having the same valence electron distribution as Ti. And, co-doped samples prepared in the experiment may contain all co-doped modes; owing to the formation energies of them being very close under Ti-rich or O-rich conditions. On the other hand, the optical absorption threshold and the optical absorption intensity of Zr–N1, Zr–N2 and Zr–N4 co-doped TiO2 are more enlarged than the N mono-doped and Zr–N3 co-doped TiO2, due to the smaller carrier effective mass and the curved and wide impurity level (IL) appearing in the band gap, which not only increase the carrier mobility and reduce the electron–hole recombination, but also reduces the electronic transition energies. Thus, Zr–N1, Zr–N2 and Zr–N4 co-doped configurations can explain the mechanism of visible light absorption and photocatalytic activity enhancement for co-doped TiO2 and give a good explanation of the previous experimental observation.

The synergetic effect of V and Fe-co-doping in TiO2 studied from the DFT + U first-principle calculation

Based on the density functional theory (DFT+U), a detailed study on the energetic, electronic, and optical properties of Fe-, V-, and Fe & V-co-doping anatase and rutile TiO2 was performed The synergetic effect of Fe & V bimetal co-doping on the optical absorption was discussed on electronic level. Two kinds of co-dopants were considered, which included edge-shared and corner-shared co-doping. It was shown that Fe and V atoms prefer to replace Ti atom in the O-rich contions than in the Ti-rich conditions. Co-doping in anatase reduces the formation energies in both cases, while the formation energies for rutile cannot be decreased. The Bader charge analysis indicates the +3 of Fe atom and +4 of V atom, and the obvious electron exchange between Fe and V atom in co-doping cases can be identified, which indicates the presence of synergetic effect induced by co-doping. The cooperation of Fe & V co-dopants was also supported by the result of projected density of states and spin charge density differences, as the hybridization of Fe3d with V3d orbitals was seen within the TiO2 forbidden band. Different from single-dopant systems, the V3d-Fe3d co-interaction leads to the formation of some spin mid-gap states, which have an obvious effect on the optical absorptions.

Rutile TiO2(011)-2 × 1 Reconstructed Surfaces with Optical Absorption over the Visible Light Spectrum.

The stable structures of the reconstructed rutile TiO2(011) surface are explored based on an evolutionary method. In addition to the well-known "brookite(001)-like" 2 × 1 reconstruction model, three 2 × 1 reconstruction structures are revealed for the first time, all being more stable in the high Ti-rich condition. Importantly, the predicted Ti4O4-2 × 1 surface model not only is in excellent agreement with the reconstructed metastable surface detected by Tao et al. [Nat. Chem. 3, 296 (2011)] from their STM experiment but also gives a consistent formation mechanism and electronic structures with the measured surface. The computed imaginary part of the dielectric function suggests that the newly predicted reconstructed surfaces are capable of optical absorption over the entire visible light spectrum, thereby offering high potential for photocatalytic applications.

Modulating TiO2 photocatalyst by Al doping: Density functional theory approach

In this work, systematic study of the thermal stability, crystal structure and electronic properties of Al doped TiO2 were studied by the first principles calculations. The results showed that Al atoms preferentially occupying the interstitial site under Ti-rich condition, but substituting the Ti atom under O-rich condition. In contrast to pure TiO2, the values of VBM and CBM are reduced for Al substituting Ti doped mode, but increased for Al interstitial atom doped mode. Thus, we can modulate the preparation condition and dosage concentration for preparing the optimal photocatalyst.

Tailoring the electronic and optical properties of anatase TiO2 by (S, Nb) co-doping from a DFT plus U calculation

Abstract The geometrical structure, defect formation, electronic and optical properties of S-doped, Nb-doped, and (S, Nb)-codoped anatase TiO 2 were successfully calculated by the first-principles plane-wave ultrasoft pseudopotential method based on the density functional theory with plus U method. Firstly, the geometrical structure demonstrates that the (S, Nb) co-doping can effectively induce lattice distortion and reduce the recombination of electron–hole pairs. Note that the (S, Nb)-codoped system further reduces the band gap compared with pure and mono-doped TiO 2 due to the mixture of S 2p and Nb 4d states appears in the gap, which results in an obvious red-shift in the optical absorption spectra and improves the photocatalytic activity. Moreover, the (S, Nb) co-doping should be grown under Ti-rich conditions while S or Nb mono-doping is expected to be easier under O-rich conditions. The above results would be beneficial to further developing for titanium dioxide photocatalyst.

Study the high photocatalytic activity of vanadium and phosphorus co-doped TiO2 from experiment and DFT calculations

The origin of exceptionally high photocatalytic activity of phosphorus and vanadium (P/V) co-doped TiO2 under solar-light irradiation have been investigated by using X-ray photoelectron spectroscopy (XPS) and UV–vis diffuse reflectance spectra combined with density functional theory (DFT) calculations. XPS experimental results demonstrated that a portion of the dopant P(P−3) ions were implanted into the crystalline lattice of P/V co-doped TiO2 and closed proximity V dopants at substitution farthermost Ti atom, forming [Psub.−O−V] structural units. On other hand, according to a new “noncompensated” codoping concept base on the DFT calculations, we found P substituted O (No. 1 position) and V substituted farthermost Ti (No. 7 position) were favorable in Ti-rich conditions, synchronously, the defects formation energy of P/V co-doped TiO2 was minimum only −1.383eV, the formation energies were higher at substitutional-O and Ti site under O-rich than Ti-rich conditions. Calculated dopant energy levels of P/V co-doped TiO2 were 0 and 1.875eV at the top of valence band and the bottom of conduction band respectively, the [Psub.−O−V] structural units were responsible for trapping the photoinduced electrons, which prolongs the life of the photoinduced charge carriers and eventually leads to a remarkable enhancement in the photocatalytic activity, the calculation results were consistent with the experiment results. These findings are expected to be crucial for understanding the roles of P and V dopants and their synergistic effect in numerous titaniamediated photocatalytic reactions.

First-principles calculation of rutile and anatase TiO2 intrinsic defect

In the paper, we study how the geometry structure and the growing ambience and the Fermi level of several intrinsic point defects effect the defect formation energy of the rutile and anatase TiO2, which type of point defect will be formed, and how to predict the experimental condition of point defect from theory. The key problem is how to calculat the defect formation energy with charge, and correcte the calculation results. The results show the defect type and the defect concentration are related to the nonequilibrium growth condition. In general, under the O-rich condition, VTi will form spontaneously, and under the Ti-rich condition, Ti4+ and VO easily appear in Schottky defects.

Unexpected relationship between interlayer distances and surface/cleavage energies inγ-TiAl: density functional study

Density functional calculations were performed to study theγ-TiAl (001), (100), (110) and (111) surfaces. The (100) surface is the most stableunder Ti-rich conditions, while the Al-termination (110) surface becomes themost stable with the increase of Al chemical potential. We calculate that inγ-TiAl intermetallic compound the larger the interlayer distance, the larger the surface energyand cleavage energy. This is different from the situation in a pure metal. Thisphenomenon can be explained by the analysis of the bonding characteristics inγ-TiAl. In particular there are both metallic and covalent bonds inγ-TiAl, and the strongest covalent bonds mainly focus on the center of three Ti–Al–Ti atoms. It isthe covalent bonds that affect greatly the cleavage energy, the surface energy and thesurface stability.