Bulk Rutile TiO2 Research Articles

High-temperature solid-state rutile-to-anatase phase transformation in TiO2

TiO2 has many polymorphs, among which rutile is the most thermodynamically stable. Bulk anatase TiO2 is usually a metastable phase at room temperature. Hence, phase transformation of bulk rutile TiO2 to anatase TiO2 by heat treatment is thermodynamically forbidden. However, the stability of TiO2 polymorphs is dependent on particle size and doping level. In this work, we harnessed this characteristic to develop a solid-state high-temperature route of sequential NH3 and O2 treatment that transforms rutile TiO2 to anatase phase via grain fracture and N-doping. As an additional advantage, this technique produces a mixture of anatase and rutile phases, with anatase being a major phase. Biphasic TiO2 is advantageous from a charge separation perspective for photocatalytic applications. Furthermore, the bandgap of anatase TiO2 can be manipulated by controlling the O2 treatment time to tune it for specific applications.

Performance of Dye-Sensitized Solar Cell Using Size- Controlled Synthesis of TiO2 Nanostructure

Titanium dioxide (TiO2) or titania shows a great interest in solar cell application due to its morphology and crystalline structure. Moreover, it is an affordable compound that could make solar cells more cost economical than traditional silicon solar cells. In this study, one-step hydrothermal method is demonstrated to synthesis rutile TiO2 nanorods and nanoflowers morphology in nanoscale dimension on different hydrothermal reaction times for Dye-sensitized solar cells application. Increasing the reaction time could influence in formation of higher crystalline rutile phase titania nanostructure before abruptly decreases as the prolong hydrothermal process carry out. The length of the nanorods produced shows increasing behaviour and the growth of nanoflowers are become denser obviously. Band gap estimation is 2.75 eV slightly lower than bulk rutile TiO2. It shows that the growth mechanism under different reaction times has great influences on the morphologies and alignment of the nanostructure. Further, the DSSCs fabricated using 15 hours reaction time exhibited the best photovoltaic performance with highest efficiency of 3.42% and highest short-circuit photocurrent of 0.7097V.

A symbiotic hetero-nanocomposite that stabilizes unprecedented CaCl2-type TiO2 for enhanced solar-driven hydrogen evolution reaction.

Symbiotic hetero-nanocomposites prevail in many classes of minerals, functional substances and/or devices. However, design and development of a symbiotic hetero-nanocomposite that contains unachievable phases remain a significant challenge owing to the tedious formation conditions and the need for precise control over atomic nucleation in synthetic chemistry. Herein, we report a solution chemistry approach for a symbiotic hetero-nanocomposite that contains an unprecedented CaCl2-type titania phase inter-grown with rutile TiO2. CaCl2 structured TiO2, usually occurring when bulk rutile-TiO2 is compressed at an extreme pressure of several GPa, is identified to be a distorted structure with a tilt of adjacent ribbons of the c-axis of rutile. The structural specificity of the symbiotic CaCl2/rutile TiO2 hetero-nanocomposite was confirmed by Rietveld refinement, HRTEM, EXAFS, and Raman spectra, and the formation region (TiCl4 concentration vs. reaction temperature) was obtained by mapping the phase diagram. Due to the symbiotic relationship, this CaCl2-type TiO2 maintained a high stability via tight connection by edge dislocations with rutile TiO2, thus forming a CaCl2/rutile TiO2 heterojunction with a higher reduction capacity and enhanced charge separation efficiency. These merits endow symbiotic CaCl2/rutile TiO2 with a water splitting activity far superior to that of the commercial benchmark photocatalyst, P25 under simulated sunlight without the assistance of a cocatalyst. Our findings reported here may offer several useful understandings of the mechanical intergrowth process in functional symbiotic hetero-nanocomposites for super interfacial charge separation, where interfacial dislocation appears to be a universal cause.

UV-black rutile TiO2: An antireflective photocatalytic nanostructure

This work presents an experimental study on the specific quantitative contributions of antireflective and effective surface areas on the photocatalytic and antibacterial properties of rutile TiO2 nanospikes. They are studied when continuously distributed over the whole surface and when integrated into well-defined microstructures. The nanospikes were produced following MeV ion beam irradiation of bulk rutile TiO2 single crystals and subsequent chemical etching. The ion beam irradiation generated embedded isolated crystalline nanoparticles inside an etchable amorphous TiO2 layer, and nanospikes fixed to the not etchable TiO2 bulk substrate. The produced nanospikes are shown to resist towards aggressive chemical environments and act as an efficient UV antireflective surface. The photocatalytic activity experiments were performed under the ISO 10678:2010 protocol. The photonic and quantum efficiency are reported for the studied samples. The combined micro- and nanostructured surface triples the photonic efficiency compared to the initial flat surface. Results also revealed that the antireflective effect, due to the nanostructuring, is the dominating factor compared to the increase of surface area, for the observed photocatalytic response. The obtained results may be taken as a general strategy to design and precisely evaluate photoactive nanostructures.

The nature of excess electrons in anatase and rutile from hybrid DFT and RPA.

The behavior of excess electrons in undoped and defect free bulk anatase and rutile TiO2 has been investigated by state-of-the-art electronic structure methods including hybrid density functional theory (DFT) and the random phase approximation (RPA). Consistent with experiment, charge trapping and polaron formation is observed in both anatase and rutile. The difference in the anisotropic shape of the polarons is characterized, confirming for anatase the large polaron picture. For anatase, where polaron formation energies are small, charge trapping is observed also with standard hybrid functionals, provided the simulation cell is sufficiently large (864 atoms) to accommodate the lattice relaxation. Even though hybrid orbitals are required as a starting point for RPA in this system, the obtained polaron formation energies are relatively insensitive to the amount of Hartree-Fock exchange employed. The difference in trapping energy between rutile and anatase can be obtained accurately with both hybrid functionals and RPA. Computed activation energies for polaron hopping and delocalization clearly show that anatase and rutile might have different charge transport mechanisms. In rutile, only hopping is likely, whereas in anatase hopping and delocalization are competing. Delocalization will result in conduction-band-like and thus enhanced transport. Anisotropic conduction, in agreement with experimental data, is observed, and results from the tendency to delocalize in the [001] direction in rutile and the (001) plane in anatase. For future work, our calculations serve as a benchmark and suggest RPA on top on hybrid orbitals (PBE0 with 30% Hartree-Fock exchange), as a suitable method to study the rich chemistry and physics of TiO2.

Cytotoxicity of TiO2 nanoparticles to mussel hemocytes and gill cells in vitro: Influence of synthesis method, crystalline structure, size and additive

Increasing the production and applications of TiO2 nanoparticles (NPs) has led to grow concerns about the consequences for the environment. In this study, we investigated the effects of a set of TiO2 NPs on the viability of mussel hemocytes and gill cells using neutral red and thiazolyl tetrazolium bromide assays. For this, we compared the cytotoxicity of TiO2 NPs (0.1–100 mg Ti/L) produced by different techniques: rutile NPs (60 nm) produced by milling and containing disodium laureth sulfosuccinate (DSLS), rutile NPs (10, 40 and 60 nm) produced by wet chemistry and anatase/rutile NPs (∼100 nm) produced by plasma synthesis. The commercially available P25 anatase/rutile NPs (10–20 nm) were also tested. Exposures were performed in parallel with their respective bulk forms and the cytotoxicity of the additive DSLS was also tested. Z potential values in distilled water indicated different stabilities depending on the NP type and all NPs tested formed agglomerates/aggregates in cell culture media. In general, TiO2 NPs showed a relatively low and dose-dependent toxicity for both cell models with the two assays tested. NPs produced by milling showed the highest effects, probably due to the toxicity of DSLS. Size-dependent toxicity was found for NPs produced by wet chemistry (10 nm > 40 nm and 60 nm). All TiO2 NPs tested were more toxic than bulk forms excepting for plasma produced ones, which were the least toxic TiO2 tested. The mixture bulk anatase/rutile TiO2 was more toxic than bulk rutile TiO2. In conclusion, the toxicity of TiO2 NPs varied with the mode of synthesis, crystalline structure and size of NPs and can also be influenced by the presence of additives in the suspensions.

Ground state of the singly ionized oxygen vacancy in rutile TiO2

Results from electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) experiments are used to establish the model for the ground state of the singly ionized oxygen vacancy in the interior of bulk rutile TiO2 crystals. Hyperfine from 47Ti and 49Ti nuclei show that the unpaired electron in this S = 1/2 defect is localized on one titanium ion adjacent to the oxygen vacancy (i.e., the spin is not shared by two titanium ions). These defects are formed at low temperature (∼35 K) in as-grown oxidized crystals when sub-band-gap 442 nm laser light converts doubly ionized nonparamagnetic oxygen vacancies to the singly ionized paramagnetic charge state. The g matrix is obtained from EPR spectra and the 47Ti and 49Ti hyperfine and nuclear electric quadrupole matrices (A and Q) are obtained from ENDOR spectra. Principal values of the 47Ti and 49Ti hyperfine matrices are 64.54, 11.57, and 33.34 MHz. All the matrices have a principal axis along the [001] direction. In the basal plane, principal axes of the hyperfine and quadrupole matrices also coincide. The principal axes of the g matrix in the basal plane, however, deviate significantly from those of the A and Q matrices, thus indicating mixing of d orbitals due to the low symmetry at the Ti3+ ion site and participation of excited-state orbitals.

First-principles investigations of oxygen adsorption at TiNi surface and the TiO2/TiO–TiNi interface

Ab-intio study of the interaction of atomic and molecular oxygen with the TiNi(110) surface was performed using the projector augmented wave method with generalized gradient approximation for the exchange-correlation functional. The oxygen adsorption energies were calculated and the preferential adsorption sites of oxygen atom on the surface were determined. Our results confirmed the formation of a Ni-rich layer at the alloy–oxide interface. Atomic and electronic structure of both TiO/TiNi(110) and TiO2/TiNi(110) interfaces were analyzed. The formation energies (Ef) of point defects at the interfacial layers as well as in bulk TiNi, monoclinic TiO, and rutile TiO2 were estimated. It was shown that Ef of Ti–Ni swap defect has a lower energy than that for the Ni antisites at the TiO2(100)/TiNi(110) interface. In the case of TiO(100)/TiNi(110) interface, the formation energies of Ti–Ni swap and Ni-antisites defects are close to each other. Our results demonstrated that Ef of Ni-defect in TiO is twice less than that in TiO2. The increase in the formation energies of defects in TiO2 reveal the increase in diffusion barriers of Ni atoms in comparison with those in oxides with a lower oxygen content, which hampers Ni segregation.

Defect Chemistry of Rutile TiO2 from First Principles Calculations

In the current contribution, we study the defect chemistry of bulk rutile TiO2 through first principles defect calculations in combination with thermodynamic modeling. We apply three levels of theory, PBE, PBE+U, and the HSE hybrid functional. The defect calculations reveal that the included isolated defects, vTiq, vOq, Tiiq, OHOq, and HOq, display shallow thermodynamic charge transition levels, whereas oxygen interstitials, Oiq,display a deep (0/-2) transition level. The formation energies of the complexes between vTi4/, vO••, and OHO• are as low or even lower than those of the isolated defects, and all included complexes display exothermic binding enthalpies, indicating their possible presence under thermodynamic equilibrium. Through thermodynamic modeling, we show that the finite temperature defect structure of rutile TiO2 largely is dominated by vTi4/, OHO•, and their mutual complexes under wet oxidizing conditions and lower temperatures and by vTi4/, vO••, and their complexes at higher temperatures. Hence, we indicate that TiO2 cooled from high temperatures or grown at low temperatures will display defect structures very different from previously known, which together with previous results for defects in surfaces gives a new basis for understanding for instance photocatalytic properties of TiO2.

Study of Nickel Segregation at the TiNi-Titanium Oxide Interface

Ab-intio investigations of atomic and molecular oxygen on TiNi(110) surface are performed by using the projector augmented wave method with generalized gradient approximation for the exchange-correlation functional. Our results confirm the formation of a Ni-rich interface TiO2(100)/TiNi(110), for which the formation energies (Hf) of point defects at the interfacial layers were estimated. It is shown that Hf of swap Ti-Ni defect has a lower energy than that for the Ni antisites at the interfacial layers. The formation energies of point defects in bulk TiNi, monoclinic TiO, and rutile TiO2 are also calculated. Our results demonstrate that Hf of Ni-antisites in TiO is twice less than that in TiO2. The formation of small Ni clusters is also discussed.

Facile synthesis of rutile TiO2 nanorod microspheres for enhancing light-harvesting of dye-sensitized solar cells

Sub-micrometer sized rutile TiO2 nanorod microspheres with diameters of 500–700 nm and with a specific surface area of 63.7 m2 g−1 were synthesized via a salt-assisted hydrothermal method. Morphological evolution as a function of reaction time was carried out to gain insight into the formation mechanism of the nanorod microspheres. Unlike traditional bulk rutile TiO2 particles, the nanorod microspheres could provide dual-functions of adsorbing dye molecules and strong light-harvesting efficiency when they were fabricated as a scattering overlayer in dye-sensitized solar cells (DSSCs). Furthermore, the inherent nanorods would provide excellent electron percolation pathways for charge transfer as confirmed by electrochemical impedance spectroscopy. Consequentially, DSSC with the scattering overlayer exhibited a 39% increment of cell efficiency (7.32%) compared with the DSSC without one (5.28%), and the efficiency was also a little higher than the DSSC with the same thickness composed of only nanocrystallites (7.14%).

(Invited) Understanding the Switching Mechanism in RRAM Devices and the Dielectric Breakdown of Ultrathin High-k Gate Stacks from First Principles Calculations

RRAM devices received increased interest lately as advanced non-volatile memory technologies in terms of low operating power, high density, better non-volatility, fast switching speed, and compatibility with conventional CMOS process. However, up to date the fundamental physical principles controlling the switching are not well understood. We have employed first-principles simulations based on density functional theory (DFT) to elucidate the effect of oxygen vacancy defects on the electronic structure of rutile TiO2 and NiO using the local density approximation with correction of on-site Coulomb interactions (LDA+U). The vacancy filament induces several defect states within the band gap, which can lead to the defect-assisted electron transport and account for on-state low resistance conduction in bulk rutile TiO2 and NiO. For CMOS devices on the other hand the reliability of the gate stack is becoming a significant challenge with the continuous scaling of transistors, due to the ultrathin oxides and defects in the gate stack. The degradation of the gate oxides has been observed under electrical stress, due to traps generated by defects, e.g. oxygen vacancies present in these materials. First principles methods based on density functional theory are used to determine the location of the defect states in the band gap when these defects are at the various interfaces of the gate stack and how they contribute to the oxide breakdown in ultrathin gate stacks.

DFT + U study of defects in bulk rutile TiO2

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

Bulk and Surface Properties of Rutile TiO2 from Self-Consistent-Charge Density Functional Tight Binding.

Bulk rutile TiO2 and its (110) surface have been investigated with a computationally efficient semiempirical tight binding method: self-consistent-charge density functional tight binding (SCC-DFTB). Comparisons of energetic, mechanical, and electronic properties are made to density functional theory (DFT) and to experiment to characterize the accuracy of SCC-DFTB for bulk rutile TiO2 and TiO2(110). Despite the fact that the SCC-DFTB parameters for Ti, Ti-Ti, and Ti-O were developed in the context of small biologically relevant Ti containing compounds, SCC-DFTB predicts many properties of bulk TiO2 and the TiO2(110) surface with accuracy similar to local and gradient-corrected DFT. In particular, SCC-DFTB predicts a direct band gap of TiO2 of 2.46 eV, which is in better agreement with experiment, 3.06 eV, than DFT utilizing the local density approximation (LDA), 2.0 eV. SCC-DFTB also performs similar in terms of accuracy as LDA-DFT for the phonon frequencies of the bulk lattice and for the relaxed geometry of the TiO2(110) surface. SCC-DFTB does, however, overestimate the surface energy of TiO2(110) compared to LDA-DFT. Nevertheless, the overall accuracy of SCC-DFTB, which is substantially more computationally efficient than DFT, is encouraging for bulk rutile TiO2 and TiO2(110).

Ab initio studies of hydrogen and acceptor defects in rutile TiO2

The behaviour of hydrogen in both defect free and acceptor doped bulk rutile TiO2 is investigated through defect calculations performed within the density functional theory formalism. Both interstitial and substitutional hydrogen defects are shown to behave as shallow donors in the material, thereby existing as effectively positive hydroxide defects, OH*(O), and substitutional hydrogen defects, H*O. Of the investigated isolated hydrogen defects, the OH*(O) defect is shown to have the lowest formation energy over the whole Fermi level range under both oxidising and reducing conditions. However, the formation energy of H*O is only 0.35 eV higher under conditions where TiO2 is in equilibrium with Ti2O3, indicating that it exists as a minority defect in rutile TiO2 under highly reducing conditions such as under growth of TiO2 on a Ti metal surface. The enthalpy of hydration of oxygen vacancies is calculated to be -1.64 and -1.60 eV for the 2 x 2 x 3 and 3 x 3 x 3 supercells, indicating that OH*(O) defects will prevail in wet atmospheres, even at high temperatures. Hydrogen incorporated in the material can furthermore be expected to associate significantly with various acceptor centres (e.g., substitutional N and Al acceptors and Ti vacancies). The binding energies of the substitutional NH(x)O (imide) defect and of the association complexes between H and fully ionised substitutional Al, (Al'Ti x OH*(O)), and Ti vacancies, (V(Ti)x OH*(O)and (V(Ti) x 4OH*(O)) (i.e., Ruetschi type defects), are calculated to be -0.36, -0.12, -0.47 and -0.82 eV, respectively.

Intrinsic Hole Migration Rates in TiO2from Density Functional Theory

Formation and migration of hole polarons in bulk rutile and anatase TiO2 were modeled using density functional theory (DFT) combined with the Marcus/Holstein theory of electron/polaron transfer. We previously applied a similar methodology to model electron polarons and extended the approach to hole polarons. Holes were formed by removal of an O(2p) valence electron, and the quantum mechanical characterization of hole hopping (reorganization energy and electronic coupling) was carried out with the DFT+U method, a method that corrects for self-interaction errors and facilitates charge localization, combined with Hartree−Fock cluster calculations. Several elementary hole transfer processes along various directions were investigated for both rutile and anatase. Hopping along most directions in rutile and along one direction in anatase was found to be adiabatic in character, i.e., thermal processes coupled to phonons. The activation energies for these processes were found to be about twice as large as the acti...

First-Principles Study on Rutile TiO2 Quantum Dots

The electronic structure of rutile TiO2 quantum dots (QDs) are investigated via the first-principles band structure method. We first propose a model to passivate the rutile TiO2 surfaces for the local density approximation calculations. In this model pseudohydrogen atoms are used to passivate the surface dangling bonds, which remove the localized in-gap surface states in the TiO2 QDs. As the size of the QD decreases, the band gap evolves as Eg(dot) = Eg(bulk) + 73.70/d1.93, where Eg(dot) and d are the band gap and diameter of the QD, and Eg(bulk) is the band gap of the bulk rutile TiO2. The valence band maximum and the conduction band minimum states of the QDs are distributed mostly in the interior of the QDs, and they well inherit the atomic characteristics of those states of the bulk rutile TiO2.

Spectral, tensor, and ab initio theoretical analysis of optical second harmonic generation from therutile TiO2(110) and (001) faces

We give an overview of our recent experimental study on the optical second harmonic (SH) response of therutile TiO2(110) and (001) faces, and the analysis of these results by phenomenologicalelectromagnetic theory using nonlinear susceptibility tensors and by abinitio theory using the self-consistent full potential linearized augmentedplane-wave (FLAPW) method within the local-density approximation. Since bulk rutileTiO2 has a uniaxial crystal structure of symmetryD4h14, the nonlinear optical response of its surface and bulk showed remarkable anisotropy. TheTiO2(110) face exhibited stronger reflected SH response when the incident electricfield was directed parallel than perpendicular to the [001] axis, while theTiO2(001) face exhibited relatively isotropic SH response. The anisotropy of the SH intensity patternsdepended remarkably on the incident photon energy and the polarization combination. Byusing a phenomenological electromagnetic theory, we performed a simultaneous analysis ofthe SH intensity patterns from the (110) and (001) faces as a function of thesample rotation angle around its surface normal. As a result we could separatethe contributions from the surface second-order and bulk higher-order nonlinearsusceptibilities. We also found that the SH intensity spectra as a function of the SHphoton energy depended strongly on the sample rotation angle and the polarizationcombination of the fundamental and SH light. The onset of the SH resonance of theTiO2(110) face was located at eV when the induced nonlinear polarization was perpendicular to the surface. It waslocated at eV when the induced nonlinear polarization is parallel to the [001] direction in the surfaceplane. These onset energies were higher than the onset energy of the bulk linear absorptionat 3.0 eV. On the other hand, the onset energy of the SH resonance of the (001) face wasfound at eV. A discussion is given on the physical meaning of the observed SH intensity spectra.Furthermore, an ab initio calculation of the nonlinear optical response from theTiO2(110) surface using the FLAPW method was performed. The calculated results agreed very wellwith the experimental SH intensity patterns and spectra. We found both from thephenomenological and ab initio calculation that the main SH response from theTiO2(110) surface originated from the Ti–O–Ti–O– zigzag chains on theTiO2(110) surface.

Preparation, characterization and photoreactivity of titanium(IV) oxide encapsulated in zeolites

The preparation, characterization and photoreactivity of titanium(IV) oxide encapsulated inside either sodium- or ammonium-exchanged zeolite Y (NaY and NH4Y), mordenite (NaM and NH4M) or potassium zeolite L (KL) are presented. The material was prepared by an ion-exchange method with ammonium titanyl oxalate monohydrate aqueous solution, (NH4)2TiO(C2O4)2·H2O. The results of X-ray photoelectron spectroscopy, chemical analysis and photolysis studies support the contention that the exchanged titanium species locate dominantly inside the zeolite cavities, and not on the external surfaces. The TiO2+-exchanged zeolite exhibits characteristics of TiO2, such as photoreduction of methyl viologen to methyl viologen radical cation. A significant blue shift of the UV–VIS reflectance spectrum (onset ca. 330 nm), compared with those of bulk anatase and rutile TiO2, and a lack of an X-ray diffraction pattern of TiO2 in the TiO2–zeolite samples indicate that the particle size of the TiO2 encapsulated inside the zeolite is on the nanometre scale.