Undoped Anatase TiO2 Research Articles

Self-assembled titanium-deficient undoped anatase TiO2 nanoflowers for ultralong-life and high-rate Li+/Na+ storage

Anatase TiO2 is a promising safe and high-rate anode for Li- and Na-ion batteries owing to its moderate redox potential and multi-dimensional ion diffusion paths. However, the capacity, rate and cycle life of anatase TiO2 are severely hindered by the low Li+/Na+ diffusion coefficients. Ti vacancies have been predicted to significantly improve Li+ diffusion kinetics by previous theoretical calculations. However, experimental evidence is still lacking because the existing methods to create Ti vacancies commonly rely on aliovalent doping, i.e., the co-existence of Ti vacancies and foreign anions precludes revelation of the true role and contribution of Ti vacancies alone. The current work reports the synthesis of mesoporous flower-like titanium-deficient anatase TiO2 (TDAT). The formation mechanisms of the Ti vacancies and the micro-architectures are tentatively discussed. Its undoped nature allows elucidation of the unambiguous roles of Ti vacancies on Li+/Na+ storage. Electrochemical results show high capacity, high rate and ultra-long cycle stability for both Li+/Na+ storage in TDAT. DFT calculations reveal that the presence of Ti vacancies results in reduced energy barrier for Li+/Na+ intercalation, enhanced diffusion kinetics, additional Li+/Na+ storage sites and diffusion pathways. For Na+ storage, it achieves a high capacity of 219.9 mAh g−1 at 50 mA g−1, and superior stability over ultra-long 15,000 cycle test at 2000 mA g−1. This work complements with the prevailing view of anion vacancy for improved Li+/Na+ storage.

Thickness induced modifications in the valence, conduction bands and optoelectronic properties of undoped and Nb-doped anatase TiO2 thin films

Nb-doped anatase TiO2 (NTO) is a promising wide band gap transparent semiconductor as it is an inexpensive, environment-friendly and chemically stable compound. However, its overall electronic structure and optoelectronic properties are tailored by tuning the microstructure, which are effected by varying process conditions. To this end, here series of thickening NTO and undoped anatase TiO2 (TO) films are deposited on quartz substrate in RF magnetron sputtering, followed by vacuum annealing at 823 K. Following fabrication, detailed electronic structure, defect states, microstructure and optoelectronic characterization of all these films are performed. Here, energy positions of the valence and conduction band edges are correlated with the oxygen and titanium content of these films. Whereas, TO films are found to be highly insulating, NTO films show highly conducting characteristics. The lowest electrical resistivity of 4.80 × 10−3 Ω cm is obtained for ~74 nm thick NTO film. Additionally, relative positions and intensities of the shallow donor level defect states of anatase are analyzed and compared with ZnO to gain fundamental insight into the differences in their electrical behavior. Along with visible region, these films also exhibit high transmittance in NIR range, thereby making these promising transparent conductors in the entire spectral range.

Unveiling the interplay between induced native defects and room temperature magnetic ordering in titanium deficient disordered-TiO2 nanoparticles

Understanding the origin of magnetic ordering in an undoped semiconductor with native defects is an open question, which is being explored actively in research. In this investigation, the interplay between magnetic ordering and excess induced native defects in undoped anatase TiO2 nanoparticles is explained using an experimental and theoretical approach. It is demonstrated that structurally disordered TiO2 nanoparticles with a high concentration of native defects such as titanium interstitials and oxygen vacancies are synthesized using controlled atmospheric rapid cooling (i.e. quenching) process. The structural disorders in the lattice have been examined using various spectroscopic and microscopic analyses revealed the existence of Ti deficiency in both pristine and quenched TiO2 nanoparticles. A possible origin of magnetic ordering in titanium deficient anatase TiO2 system is elucidated based on first-principle calculations. It was found that the overall magnetic moment of Ti deficient TiO2 system is determined by the distance between Ti interstitials and its neighboring vacancies (i.e. either V Ti or V Os). However, quenched TiO2 nanoparticles possess excess Ti interstitials, Ti and O vacancies and therefore the net magnetic moment of the system is reduced due to anti-ferromagnetically coupled neighboring Tilattice ions.

Preparation and characterization of TiO2 generated from synthetic wastewater using TiCl4 based coagulation/flocculation aided with Ca(OH)2

This study focused on the preparation of undoped and Ca-doped titania from flocculation generated sludge. Initially, TiCl4 was utilised to perform coagulation and flocculation in synthetic wastewater and an optimised dose of coagulant was determined by evaluating the turbidity, dissolved organic carbon (DOC) and zeta potential of the treated water. Later, using Ca(OH)2 as a coagulant aid, the effects on effluent pH, turbidity and DOC removal were investigated. Both Ca-doped and undoped anatase TiO2 were prepared from the flocculated sludge for morphological and photocatalytic evaluation. During the standalone use of TiCl4, maximum turbidity and DOC removal were found at 11.63 and 14.54 mg Ti/L, respectively. At the corresponding coagulant dose, rapid deprotonation of water caused the pH of the effluent to reach below 3.77 mg Ti/L. Whereas, when using Ca(OH)2 as a coagulant aid, a neutral pH (7.26) was attained at a simultaneous dosing of 32.40 mg Ca/L and 14.54 mg Ti/L. When aided with Ca(OH)2, the turbidity removal was further increased by 54.28% and the DOC removal was somewhat similar to the standalone use of TiCl4. TiO2 was prepared by incinerating the collected sludge at 600 °C for 2 h. Both XRD and SEM analysis were conducted to observe the morphology of the prepared titania. The XRD pattern of the TiO2 showed only an anatase phase along with the presence of a high atomic proportion of Ca (4.14%). Consequently, a high amount of Ca atoms inhibited the level of TiO2 phase and no obvious presence of CaO was observed. The prepared Ca-doped TiO2 at the optimised dose of Ca(OH)2 was found to be inferior to the undoped TiO2 during the photodegradation of acetaldehyde. However, a reduced dose of Ca(OH)2 (<15 mg Ca/L) exhibited a substantial increase in photoactivity under UV irradiance.

Evidence of oxygen and Ti vacancy induced ferromagnetism in post-annealed undoped anatase TiO2 nanocrystals: A spectroscopic analysis

The evidence of oxygen and titanium (Ti) vacancy induced room-temperature ferromagnetism (RTFM) is observed in anatase TiO2 nanocrystals investigated by spectroscopic techniques. RTFM significantly depends on the annealing temperature due to modification of intrinsic defects. Magnetic moment (MS) and Curie temperature (TC) are found to increase initially with increase of annealing temperature and then decreases on further annealing. The samples annealed at lower temperature, is found to possess significant amount of singly ionized oxygen vacancy (VO+) defects. In addition, coincident doppler broadening and positron annihilation spectroscopic analysis provides an indication that such oxygen vacancy may merge with one or two Ti vacancy (VTi) and thereby forming larger-sized defect-combinations like divacancy VTi+O and trivacancy VTi+O+Ti which act as dominant positron trapping centre within the nanocrystalline TiO2. Hence, the combination of VO and VTi defects play the critical role in inducing RTFM in TiO2 nanocrystals which can yield promising spintronic applications.

Effect of Co2+ doping on Raman spectra and the phase transformation of TiO2:Co2+ nanowires

Co2+-doped anatase TiO2 nanowires were successfully synthesized by hydrothermal method. Inhibitory effect of Co2+-doping on the anatase to rutile phase transformation was investigated by X-ray diffraction, Raman scattering. The replacement of Ti4+ ions with Co2+ ions led to a slight expansion of the unit cell volume and a shift and broadening of all the characteristic Raman scattering modes of the anatase TiO2:Co2+ nanowires. The absorption spectra exhibited the band-band transitions of TiO2 host and the transitions within Co2+ ions located in octahedral field of the anatase TiO2 nanowires. Co2+-doping reduced both the indirect and the direct band gaps. Photoluminescence spectra of the undoped anatase TiO2:Co2+ nanowires showed the transitions between the bands and the transitions related to surface defect states, oxygen vacancies. The Co2+-doping caused only the quenching phenomenon of emission of the TiO2 nanowires.

Improving the photocatalytic properties of anatase TiO2 (101) surface by co-doping with Cu and N: Ab initio study

Substitutionally and interstitially Cu/N co-doped anatase TiO2 (101) surface is investigated by using density functional theory (DFT) calculations. The results suggest improved visible light photocatalytic activity over undoped anatase TiO2. Sizable lattice relaxation around the dopants is observed, followed by a formation of N–O bond. Depending on the local arrangement of atoms, localized states above the valence band maximum, deep into the band gap, and below the conduction band minimum are found. In addition, our calculation also predict band gap narrowing. The hybridization of the Cu 3d and N 2p states within the band gap and the other electronic and optical properties suggest a synergistic effect of the dopants in the enhancement of the visible light absorption on the (101) anatase surface.

Effect of zinc doping on the bandgap and photoluminescence of Zn2+-doped TiO2 nanowires

This study was focused on the effect of Zn2+ dopant concentration on the absorption edge and photoluminescence of anatase TiO2 nanowires synthesized by hydrothermal technique. For the undoped anatase TiO2 nanowires, the indirect band gap of 3.26eV and the direct band gap of 3.58eV are assigned to the indirect Γ3→X1b and direct X2b→X1b transitions, respectively. The Zn2+-doping makes the absorption edge of TiO2:Zn2+ nanowires shift towards the lower energy side (red shift). On the other hand, the replacing Ti4+ ions with Zn2+ ions creates oxygen vacancies (VO) and shallow defects associated with VO. Just these defects are responsible for the enhanced luminescence of Zn2+-doped TiO2 nanowires.

Ferromagnetism of undoped anatase TiO2 based on the first-principles calculations

Compared with conventional semiconductors, the diluted magnetic semiconductors, in which the cations are substituted by transition metal ions, have attracted a great deal of attention due to their promising applications in spintronics. Recently, the unexpected room temperature ferromagnetism has been found in many undoped oxides. These findings challenge our understanding of magnetism in these systems, because neither cations nor anions have unpaired d or f electrons. Generally, the candidate defects responsible for the unexpected ferromagnetism must fulfill two conditions at the same time: (i) the defects should prefer a spin-polarized ground state with a nonzero local magnetic moments; (ii) the exchange interactions between local magnetic moments induced by defects should be ferromagnetic energetically. Among these oxides, TiO2 has recently attracted much attention because of its unique properties and potential applications in spintronics, laser diodes and biomaterials. In order to explore the origin of ferromagnetism in such an undoped TiO2 system, the electronic structures and magnetic properties of oxygen vacancy (VO) and Ti vacancy (VTi) in anatase TiO2 have been studied systematically by the first-principles calculation based on the density functional theory with the LDA+U method (UTi-3d = 5.8 eV). It is found that two electrons introduced by VO are captured by two neighbor Ti4+ ions, and thereby the Ti4+ ions are restored to Ti3+ ions with opposite spin orientation. Therefore, the single VO cannot induce local magnetic moment. The defect energy level locates near the Fermi level for VTi. Six oxygen atoms neighboring VTi constitute an octahedron, and the defect energy level is split into a single state A, a double state E and a triple state T in the octahedral crystal field. The occupation of four unpaired electrons introduced by six oxygen atoms is a+1t+3t-0e0 (subscripts + and - mean up-spin and down-spin, respectively), and the VTi can induce 4 B local moments. Furthermore, the magnetic coupling interaction between local magnetic moments induced by two VTi is ferromagnetic, and the magnetic coupling constant (JO) is 88.7 meV. It means the ferromagnetism can continue up to room-temperature. The VO cannot induce local magnetic moment, but it can enhance the coupling strength between two VTi, which can explain the origin of ferromagnetism observed experimentally in undoped anatase TiO2, i.e., the VTi induces local magnetic moment, while VO enhances the long range ferromagnetic coupling interaction between VTi. Especially, for the ferromagnetic coupling between local magnetic moments, we have proposed the second type direct exchange interaction model, which has been recommended in detail.

Strong out-of-plane magnetic anisotropy in ion irradiated anatase TiO2 thin films

The temperature and field dependence of the magnetization of epitaxial, undoped anatase TiO2 thin films on SrTiO3 substrates was investigated. Low-energy ion irradiation was used to modify the surface of the films within a few nanometers, yet with high enough energy to produce oxygen and titanium vacancies. The as-prepared thin film shows ferromagnetism which increases after irradiation with low-energy ions. An optimal and clear magnetic anisotropy was observed after the first irradiation, opposite to the expected form anisotropy. Taking into account the experimental parameters, titanium vacancies as di-Frenkel pairs appear to be responsible for the enhanced ferromagnetism and the strong anisotropy observed in our films. The magnetic impurities concentrations was measured by particle-induced X-ray emission with ppm resolution. They are ruled out as a source of the observed ferromagnetism before and after irradiation.

Structural and electronic modifications on TiO2 anatase by Li, K or Nb doping below and above the solubility limit

Undoped and Nb-, K- or Li-doped TiO2 nanopowders were synthesized using a bottom-up approach by sol–gel method. The changes of crystal structure in TiO2 upon Nb, K or Li doping were followed with X-ray diffraction (XRD) and Raman spectroscopy techniques. XRD results have shown that anatase-to-rutile phase transition as a function of temperature was suppressed by potassium and niobium incorporation, whereas it was facilitated by lithium incorporation. X-ray detectable secondary phases were not observed in any of the cases. Additional Raman lines were observed at 362, 323 and 242 cm−1 for K- or Nb-doped anatase, which were missing in both Li-doped and undoped anatase TiO2. Those additional lines were attributed to brookite phase or brookite-like distortions introduced to Ti–O octahedra in anatase. Solubility limits of the aforementioned cations were determined with depth profiling using a surface etching technique. ICP-OES results of etched powders have shown that most of the K or Li was on the surface of the particles and only 0.2 cation% K and 0.7 cation% Li have been incorporated into bulk anatase. A wedge-shaped specimen was utilized for chemical analysis of Nb-doped anatase. EDXS measurements in a TEM were carried out above and below solubility limit. Strong surface accommodation took place in the sample doped above the solubility limit.

Titanium-defected undoped anatase TiO2 with p-type conductivity, room-temperature ferromagnetism, and remarkable photocatalytic performance.

Defects are critically important for metal oxides in chemical and physical applications. Compared with the often studied oxygen vacancies, engineering metal vacancies in n-type undoped metal oxides is still a great challenge, and the effect of metal vacancies on the physiochemical properties is seldom reported. Here, using anatase TiO2, the most important and widely studied semiconductor, we demonstrate that metal vacancies (VTi) can be introduced in undoped oxides easily, and the presence of VTi results in many novel physiochemical properties. Anatase Ti0.905O2 was synthesized using solvothermal treatment of tetrabutyl titanate in an ethanol-glycerol mixture and then thermal calcination. Experimental measurements and DFT calculations on cell lattice parameters show the unstoichiometry is caused by the presence of VTi rather than oxygen interstitials. The presence of VTi changes the charge density and valence band edge of TiO2, and an unreported strong EPR signal at g = 1.998 presents under room temperature. Contrary to normal n-type and nonferromagnetic TiO2, Ti-defected TiO2 shows inherent p-type conductivity with high charge mobility, and room-temperature ferromagnetism stronger than Co-doped TiO2 nanocrystalline. Moreover, Ti-defected TiO2 shows much better photocatalytic performance than normal TiO2 in H2 generation (4.4-fold) and organics degradation (7.0-fold for phenol), owing to the more efficient charge separation and transfer in bulk and at semiconductor/electrolyte interface. Metal-defected undoped oxides represent a unique material; this work demonstrates the possibility to fabricate such material in easy and reliable way and thus provides new opportunities for multifunctional materials in chemical and physical devices.

Structural, Electronic and Optical Properties of S-Doped Anatase TiO&lt;sub&gt;2&lt;/sub&gt;&lt;sub&gt;&lt;/sub&gt;

Structural, electronic and optical properties of S-doped anatase TiO2 have been investigated using the first-principles density-functional theory. The band structure and density of states show that the band gap of anatase TiO2 narrows due to the presence of the S-3p energy level in the top of the valence bands. Compared with the optical absorption edge of the undoped anatase TiO2, the optical absorption edge of S-doped anatase TiO2 shifts to a lower energy, which indicates that S-doped anatase TiO2 can be used for visible-light absorption applications.

Exceptional thermal stability of undoped anatase TiO2 photocatalysts prepared by a solvent-exchange method

A new solvent-exchange method for preparation of undoped anatase TiO2 nanoparticles with unprecedented thermal stability has been presented.

Photocarrier dynamics in anatase TiO2 investigated by pump-probe absorption spectroscopy

The dynamics of photogenerated electrons and holes in undoped anatase TiO2 were studied by femtosecond absorption spectroscopy from the visible to mid-infrared region (0.1–2.0 eV). The transient absorption spectra exhibited clear metallic responses, which were well reproduced by a simple Drude model. No mid-gap absorptions originating from photocarrier localization were observed. The reduced optical mass of the photocarriers obtained from the Drude-model analysis is comparable to theoretically expected one. These results demonstrate that both photogenerated holes and electrons act as mobile carriers in anatase TiO2. We also discuss scattering and recombination dynamics of photogenerated electrons and holes on the basis of the time dependence of absorption changes.

Effective visible light-active boron and carbon modified TiO2 photocatalyst for degradation of organic pollutant

A visible light-active TiO2 photocatalyst modified by boron and carbon was synthesized by sol–gel followed solvothermal process. The resulting photocatalyst was characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV–vis absorption spectroscopy, and electron paramagnetic resonance (EPR). It was found that the boron and carbon modified TiO2 showed obvious absorption in the range 400–500nm. XPS results suggested boron species entered into interstitial site of TiO2 matrix and formed the B–O–Ti bond, while carbon species were in the form of carbonates species. EPR results showed the existence of oxygen vacancy in carbon and boron modified TiO2. This may result in the sensitivity of the as-synthesized photocatalyst to visible light. The resulting boron and carbon modified TiO2 exhibited significantly higher photocatalytic activity than carbon modified TiO2 and undoped anatase TiO2 on the degradation of Acid Orange 7 (AO7) in aqueous solution under visible light irradiation. The presence of carbon originating from organic precursor has great influence on the surface properties of B-doped TiO2.

Presence of Room Temperature Ferromagnetism in Co2+ Doped TiO2 Nanoparticles Synthesized through Shape Transformation

New approach for synthesis of Co2+ doped TiO2 nanoparticles showing room temperature ferromagnetic behavior, through shape transformation of hydrothermally treated scrolled titania nanotubes in the presence of Co2+ ions is described. The XRD and ICP measurements demonstrated successful incorporation of 0.46 at. % Co2+ ions in preserved anatase crystal structure of TiO2 nanoparticles, without presence of Co-oxide clustering, metallic Co, or various Co-Ti oxide species. HRTEM measurements revealed that majority of the nanoparticles have polygonal shapes with average dimension of ∼ 6−10 nm. Obtained value of 3.06 eV for band gap energy of Co2+ doped TiO2 nanoparticles explained altered optical properties of TiO2 matrix and indicates narrowing of the electronic properties in respect to the undoped anatase TiO2 nanomaterials. The Co2+ doped TiO2 nanocrystals enabled synthesis of optically transparent film that shows room temperature ferromagnetic ordering with a saturation magnetic moment of 0.25 μB per Co atom. The proposed explanation for room temperature ferromagnetic behavior is based on the presence of critical amount of oxygen vacancies that mediate interaction between Co2+ spins trapped in the lattice structure of titania nanoparticles with undercoordinated surface defect sites.

Possible origin of ferromagnetism in undoped anataseTiO2

Using first-principles electronic structure calculations we find that the titanium vacancy and divacancy may be responsible for the unexpected ferromagnetism in undoped anatase TiO2. An isolated titanium vacancy produces a magnetic moment of 3.5 mu(B), and an isolated titanium divacancy produces a magnetic moment of 2.0 mu(B). The origin of the collective magnetic moments is the holes introduced by the titanium vacancy or divacancy in the narrow nonbonding oxygen 2p(pi) band. At the center of the divacancy, an O-2 dimer forms during the relaxation, which lowers the total energy of the system and leads to the decrease in the total magnetic moment due to a hole compensation mechanism. For both the two native defects, the ferromagnetic state is more stable than the antiferromagnetic state.

Fe-doped TiO2 prepared by mechanical alloying

The effect of different milling conditions on the formation of Fe-doped TiO2 powders by mechanical alloying was investigated by Mossbauer spectrometry. The milling conditions investigated were ball to powder weight ratio, milling time, rotation velocity of supporting disc, and the type of starting reactive iron and its concentration. X-ray diffraction shows that high energy mechanical milling of undoped anatase TiO2 induce the anatase to rutile phase transformation via high pressure srilankite. Mossbauer spectra for the majority of the doped samples were decomposed into one sextet and one or two doublets. The sextets was attributed to the presence of α-Fe or hematite impurities. The doublets were assigned to Fe3 + incorporated in the TiO2 structure, and to the Fe2 + located either at the surface or the interstitial sites of TiO2. A greater incorporation of Fe in the TiO2 structure was observed when samples were prepared from hematite instead of α-Fe.