Bulk Anatase Research Articles

Facile Sol-Gel Synthesis of C, N Codoped-TiO2 for Efficient Degradation of Palm Oil Mill Effluent.

Wastewater treatment has been regarded as an effective solution in lowering the potential environmental hazards caused by palm oil mill effluent (POME). To ensure the efficient remediation of POME, the implementation of a promising strategy is necessary to overcome the limitations of conventional water treatment methods for the treatment of this pollutant. In this study, the synthesis of carbon, nitrogen codoped titanium dioxide nanoparticles (C, N-TiO2 NPs) was successfully performed by a sol-gel approach for the treatment of POME as a model pollutant under solar light irradiation. The synthesized C, N-TiO2 NPs displayed unique characteristics including an anatase phase with an average crystallite size of 11.35 nm and irregular spherical structures. Additionally, C, N-TiO2 possessed a lower band gap energy of 2.95 eV than 3.2 V of bulk anatase TiO2 and slower electron-hole (e--h+) pair recombination rate as evidenced by photoluminescence (PL) studies. The adsorption isotherm study of POME was most compatible with the Langmuir isotherm model, and the POME degradation kinetics proceeded according to first-order kinetics. Accordingly, the photocatalytic degradation of POME displayed a maximum degradation efficiency of 100% under the optimum condition of pH 7 in the presence of 0.12 g of the C, N-TiO2 photocatalyst within 150 min. The scavenging study showed that the superoxide radical (•O2 -) was the primary active species in the POME photodegradation. Finally, the reusability analysis confirmed that the C, N-TiO2 NPs could be reused for a maximum of five cycles, making them promising photocatalysts for wastewater treatment.

Polarizable force field for TiO2 based on the Drude oscillator model

A polarizable force field has been developed for molecular dynamics simulations of titanium dioxide in an aqueous environment. The force field uses the standard functional form with an additional term accounting for polarizability, i.e., induction interaction, and has been derived exclusively from ab initio calculations by partitioning of electron density. Polarizability is described using the Drude oscillator model where each non-hydrogen atom is represented by two point charges connected by a harmonic potential. It is demonstrated that the force field provides a realistic description of the structure of anatase and rutile bulk materials from x-ray experiments. In addition, it is shown that, when combined with the popular SWM4-NDP polarizable water model, the force field describes the water structure at the titanium dioxide–water interface in agreement with results from ab initio molecular dynamics simulations. Importantly, our new model provides significant improvement of results for water differential adsorption enthalpy measured by calorimetry experiments compared to previous non-polarizable force field. The new force field allows for accurate simulations of titanium dioxide–aqueous interfaces for systems of a size of ∼105 atoms and simulation times up to the microsecond time scale.

Size-dependent anatase to rutile phase transition under acoustic shocked conditions – A case study of TiO2 bulk and nanoparticles

In the present work, we attempt to explore the size-dependent structural stability of bulk (0.28 µm) and nano-sized (25 nm) anatase TiO2 particles under acoustic shocked conditions. Based on the observed X-ray diffractometric, Raman spectroscopic and transmission electron microscopic results with respect to the number of shock pulses, at the 90-shocked condition, the nano-size particles undergo the anatase to the rutile phase transition, whereas the bulk size TiO2 particles remain to be in the anatase phase. From the comprehensive assessment, it is demonstrated that the bulk TiO2 samples have higher structural stability compared to the nano-size samples. The nano-sized anatase-TiO2particles have values of lower thermal conductivity compared to the bulk-sized particles such that the value of lower thermal conductivity is a prominent crucial factor in initiating the dynamic recrystallization which results in the anatase-to-rutile phase transition. Based on the observed present experimental results and previous reports, it has been authenticated that, while increasing the particle size from nano-scale to micro-scale, the critical shock number for the anatase to rutile phase transition is significantly increased.

Synthesis and stabilization of anatase form of biomimetic TiO2 nanoparticles for enhancing anti-tumor potential

Abstract The present study emphasizes the stabilization of the biologically active anatase form of titanium dioxide (TiO2) nanoparticles (NP). TiO2 NP require stringent conditions for chemical synthesis and are usually a mixture of biologically inactive bulk rutile and the active bulk anatase forms. We utilized the culture pellet of the Exiguobacterium aestuarii SBG4 MH185868 to synthesize and stabilize the anatase form of TiO2 NP. The NP showed λ max at ∼350 nm and scanning electron microscope micrographs indicated their oval and spherical shape. Steric stabilized anatase TiO2 NP exhibited substantial cytotoxicity of up to 80% reduction in cell viability at 100 µg against cervical cancer derived HeLa and SiHa cell lines, whereas the rutile form showed least cytotoxicity. Clonogenic inhibition assay of HeLa cells showed dose-dependent decline with a 75% reduction in colony formation at 100 µg TiO2 NP and cell migration assay revealed significant inhibition in recovery of the wound/scratch in presence of anatase form of TiO2 NP (10–33% at 24 h and 42–79% at 48 h). Co-incubation of HeLa cells with anatase TiO2 NP in chorioallantoic membrane of embryonated chick eggs prevented the formation of new capillaries (20 ± 5% compared with control groups), indicating appreciable anti-angiogenic activity of the NP. Further, TiO2 NP tagged with doxorubicin and paclitaxel exhibited enhanced cytotoxicity against cancer cells at very low concentrations of 9 and 120 nM itself, indicating their anti-tumor potential. In conclusion, biomimetic anatase TiO2 NP have significant anti-proliferative and anti-angiogenic activity and can have potential application in tagging with generic anti-cancer drugs for enhanced cytotoxicity against cancer cells.

Experimental and Theoretical Study on Electronic and Optical Properties of TiO2 (anatase) Bulk and Al Doped Thin Film Phases

Experimental and Theoretical Study on Electronic and Optical Properties of TiO2 (anatase) Bulk and Al Doped Thin Film Phases

Vacancy-induced magnetic states in TiO2 surfaces

We present a combined experimental and theoretical study of surface-related magnetic states in TiO2. Our experiments on nano-sized thin films of pure TiO2 have suggested that the observed room-temperature magnetism originates from defects, in particular, from the surface of thin films as well as from point defects, such as oxygen vacancies located mainly at the surface. Clarifying this phenomenon is very important for harnessing magnetic properties of pristine TiO2 films in future spintronic applications but a detailed experimental investigation is very demanding. Therefore, quantum-mechanical density functional theory calculations were performed for (i) bulk anatase TiO2, (ii) bulk-like TiO2-terminated vacancy-free (001) surfaces, (iii) vacancy-containing TiO-terminated (001) surfaces, (iv) TiO0.75-terminated (001) surfaces with additional 25% surface oxygen vacancies, as well as (v) oxygen-terminated (001)-surfaces. Our fixed-spin-moment calculations identified both the bulk and the bulk-like terminated vacancy-free TiO2-terminated (001) surfaces as non-magnetic. In contrast, oxygen vacancies in the case of TiO-terminated and TiO0.75-terminated (001) surfaces lead to ferromagnetic and rather complex ferrimagnetic states, respectively. The spin-polarized atoms are the Ti atoms (due to the d-states) located in the surface and sub-surface atomic planes. Last, the O-terminated surfaces are also magnetic due to the surface and sub-surface oxygen atoms and sub-surface Ti atoms (but their surface energy is high).

Multi‐Phase Sputtered TiO2‐Induced Current–Voltage Distortion in Sb2Se3 Solar Cells

AbstractDespite the recent success of CdS/Sb2Se3 heterojunction devices, cadmium toxicity, parasitic absorption from the relatively narrow CdS band gap (2.4 eV) and multiple reports of inter‐diffusion at the interface forming Cd(S,Se) and Sb2(S,Se)3 phases, present significant limitations to this device architecture. Among the options for alternative partner layers in antimony chalcogenide solar cells, the wide band gap, non‐toxic titanium dioxide (TiO2) has demonstrated the most promise. It is generally accepted that the anatase phase of the polymorphic TiO2 is preferred, although there is currently an absence of analysis with regard to phase influence on device performance. This work reports approaches to distinguish between TiO2 phases using both surface and bulk characterization methods. A device fabricated with a radio frequency (RF) magnetron sputtered rutile‐TiO2 window layer (FTO/TiO2/Sb2Se3/P3HT/Au) achieved an efficiency of 6.88% and near‐record short–circuit current density (Jsc) of 32.44 mA cm−2, which is comparable to established solution based TiO2 fabrication methods that produced a highly anatase‐TiO2 partner layer and a 6.91% efficiency device. The sputtered method introduces reproducibility challenges via the enhancement of interfacial charge barriers in multi‐phase TiO2 films with a rutile surface and anatase bulk. This is shown to introduce severe S‐shaped current–voltage (J–V) distortion and a drastic fill–factor (FF) reduction in these devices.

Periodic hybrid-DFT study on the N-doped TiO2 (001) nanotubes as solar water splitting catalysts: A comparison with the rutile and anatase bulk phases

TiO2 is a routinely used catalyst due to its photocatalytic activity. The main problem of TiO2 is its large band gap. Doping metal and non-metal atoms in TiO2 crystals reduce the band gap. In this work, first-principle calculations by the full-potential numeric atom-center orbital theory were used to investigate detailed electronic properties of N-doped one monolayer (1 ML) and two monolayers (2 ML) (4,4), (8,8) and (16,16) TiO2 (001) nanotubes. The proposed model first validated by comparing its data with the experimental available data for rutile and anatase bulk phases, and then was used to predict the electronic properties of proposed nanotubes. PBE0 hybrid method was used to align the valence and conduction bands and to predict their standard reduction potential. In addition, different routine generalized gradient approximation (GGA) and meta-GGA exchange-correlation functionals were used to show how much their errors are in predicting the band gaps of nanotubes. Hybrid data showed that a 2 ML (8,8) nanotube is a proper candidate for photocatalyst not only for its band gap but also from its formation energy point of view. Contrary to the N-doped rutile and anatase phases, there is no electron-hole recombination center for the N-doped 2 ML (8,8) TiO2 nanotube.

Modeling stoichiometric and oxygen defective TiO2 anatase bulk and (101) surface: structural and electronic properties from hybrid DFT.

We present a periodic hybrid DFT investigation of the structural and electronic properties of both stoichiometric and oxygen-defective TiO2 anatase bulk and (101) surface, in singlet and triplet spin states. In all cases, an excellent agreement with available photoelectron spectroscopy data has been obtained, reproducing the offsets of the deep defect levels positions from the conduction band minimum of TiO2 created upon oxygen vacancy (VO) formation. For the bulk, different local structural polaronic distortions around the VO site have been evidenced depending on the spin state considered. Although a similar conclusion has been drawn for the defective surface for the nine different vacancy positions which have been considered, large migration of the twofold coordinated surface O atom has also been evidenced, up to the initial vacancy site in some cases. The very good agreement obtained with available experimental data regarding the offsets from the conduction band minimum of the deep defect levels positions both for the bulk and the (101) surface of TiO2 anatase is encouraging for the application of the proposed hybrid-based computational strategy to TiO2 surface-related processes such as TiO2-based photocatalysis in which oxygen vacancies are known to play a key role. All calculations have been performed with Crystal17, considering different hybrid functionals with both effective core pseudopotentials and all-electron atom-centered basis sets, as well as additional empirical dispersion effects with the D2 and D3 models.

Ti–N coordination bonds boost Z-scheme interfacial charge transfer in TiO2/C-deficient g-C3N4 heterojunctions for enhanced photocatalytic phenolic pollutant degradation

Rational designing of chemical bond-bridged TiO2-based Z-scheme heterojunctions and deep understanding mechanism of significantly enhanced visible-light photocatalytic performance of the heterojunctions is a hotspot in photocatalysis. Herein, a defect-engineering strategy is developed to fabricate Ti–N coordination bond-bridged TiO2/C-deficient g-C3N4 heterojunctions (TiO2/VC-CN), and the preparation process includes formaldehyde-assisted preorganization of molten urea in the presence of titanium isopropoxide followed by thermal condensation. The detailed characterization results evidence the formation of a unique atomic-level heterojunction contact interface via Ti–N coordination bonds in TiO2/VC-CN, in which N atoms around the C vacancies of g-C3N4 donate electrons to unoccupied Ti 3d orbitals of TiO2. At the optimal TiO2 doping level, the TiO2/VC-CN heterojunctions exhibit remarkably high visible-light photocatalytic oxidation performance in the degradation of recalcitrant phenolic pollutants, methylparaben and acetaminophen, which outperforms commercially available P25 TiO2, as-prepared anatase TiO2 and bulk g-C3N4. The experimental results combined with theoretic calculations reveal that the outstanding interfacial Ti–N coordination bonding remarkably boosts the direct Z-scheme charge carrier transfer in the TiO2/VC-CN heterojunctions, which results in the generation of plentiful reactive oxidation species including O2−, OH and 1O2 for efficient removal of the target pollutants.

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.

Titanium Phosphate Grafted on Mesoporous SBA-15 Silica as a Solid Acid Catalyst for the Synthesis of 5-Hydroxymethylfurfural from Glucose

The grafting of titania on SBA-15 followed by its phosphation was presented to prepare a mesoporous Lewis–Brønsted bifunctional solid acid catalyst for the tandem conversion of glucose via fructose to 5-hydroxymethylfurfural (HMF). Titania was dispersed on SBA-15 as an amorphous surface layer containing abundant coordinatively unsaturated tetrahedral Ti ions, which was reactive and readily transformed upon phosphation into a new titanium phosphate phase with the chemical formula identified as Ti2O3(H2PO4)2·2H2O. The ordered mesoporous structure was well maintained after three modification cycles, affording a desirable surface area of over 300 m2/g. The SBA-15-supported titanium phosphate layer affords higher overall acidity and Brønsted to Lewis acid ratio, compared with the conventional post-phosphated bulk anatase titania. The tetrahedral Ti ions and the adjacent protonated phosphate groups on the titanium phosphate layer could form Lewis–Brønsted acid pairs at molecular level proximity, which largely enhanced the selective tandem catalysis for glucose conversion via fructose to HMF. An optimized HMF yield of 71% was achieved at 160 °C in a water–methyltetrahydrofuran biphasic system over the SBA-15-supported titanium phosphate catalyst. The catalyst exhibited good hydrothermal stability with a rather limited silicon and phosphate leaching, and no distinct pore collapse or performance loss over three sequential reaction runs.

Nanotube formation of Ti-6Al-4V alloy and its corrosion behavior

In this study, the nanotube formation and corrosion behavior of Ti-6Al-4V alloy with α- and β-phases were researched with various experimental instruments. For this study, nanotube formation on the sample was performed using the anodic oxidation method by applying a voltage of 30 V using a direct current power supply for 1 h using a 1.0 M H3PO4 + 0.8 wt.% NaF solution at 25 °C. The corrosion properties of specimens were examined by potentiodynamic testing (potential range of -1000–1500 mV) in a 0.9% NaCl solution using a potentiostat. The nanotube morphology of the alloys was observed by field-emission scanning electron microscopy, X-ray diffractometry, and energy dispersive X-ray spectroscopy. The nanotubes in the α-phase were uniformly formed on the surface of the Ti-6Al-4V alloy, but nanotubes in the β-phase were irregularly formed. The nanotube-formed alloy had a wider passivation range and higher corrosion current density than the bulk alloy. The morphologies of the corrosion surfaces show that the edge trace around the β-phase was reduced, and the anatase phase increased on the corroded surface. In cross-sectioned nanotube layer after corrosion test, a deep pit was observed in the part where the nanotubes were removed. XRD analysis showed that the anatase and rutile phases were not formed in bulk alloys, but anatase and rutile phases were detected in the nanotube formed alloy, and the peak of the anatase increased on the corroded surface. From the result of potentiostatic test, the variation of current density from the beginning is almost the same, but the current density of the nanotube formed surface was slightly higher than that of non-nanotube formed surface with time.

Anataselike Grain Boundary Structure in Rutile Titanium Dioxide.

The formation of nanoscale phases at grain boundaries in polycrystalline materials has attracted much attention, since it offers a route toward targeted and controlled design of interface properties. However, understanding structure–property relationships at these complex interfacial defects is hampered by the great challenge of accurately determining their atomic structure. Here, we combine advanced electron microscopy together with ab initio random structure searching to determine the atomic structure of an experimentally fabricated Σ13 (221) [11̅0] grain boundary in rutile TiO2. Through careful analysis of the atomic structure and complementary electron energy-loss spectroscopy analysis we identify the existence of a unique nanoscale phase at the grain boundary with striking similarities to the bulk anatase crystal structure. Our results show a path to embed nanoscale anatase into rutile TiO2 and showcase how the atomic structure of even complex internal interfaces can be accurately determined using a combined theoretical and experimental approach.

Theoretical study of the modification of the oxide-reducing capacity of titania from selected dopant elements

The positions of the upper edge of the valence band and the lower edge of the conduction band of semiconducting materials such as TiO2 are closely related to the values of the respective oxidation or reduction potentials. The modifications suffered by a material of this type impact on the displacements of the aforementioned bands and thus on the redox potentials. In the present work, anatase and rutile bulk systems are analyzed for how different metallic and nonmetallic dopants located substitutionally where the “@” symbol indicates that the preceding element replaces the element after it or interstitially this position in the network is indicated with the letter “i” affect these potentials. From the analysis of the modifications, consequence of the doping, it can be noted that substitutional doping of nitrogen (N) at the oxygen site (N@O) improves the reduction potential of anatase and doped with interstitial (Ni) that of rutile. On the other hand, for both polymorphs, interstitial iron (Fei) doping has the best oxidant properties, followed by doping with Fe in titanium (Ti) site (Fe@Ti). The latter also has interesting magnetic properties to facilitate the extraction of the titania from the reaction media. The other doping elements studied, vanadium, platinum, silver, carbon and fluorine only improve the oxidation potential but all to a lesser extent than iron.

Enhanced active oxidative species generation over Fe-doped defective TiO2 nanosheets for boosted photodegradation.

Semiconductor photocatalysis is widely proposed for decomposing multiple pollutants via photo-generated oxidative species. However, the photocatalytic degradation performance in practical settings still remains unsatisfactory due to the limited production of active oxidative species (AOS). In this work, a defect engineering strategy was developed to explore the superiority of oxygen vacancies (Vo) and their structural regulation to enhance AOS production for boosting photodegradation. Taking anatase TiO2 as a model photocatalyst, ultrathin TiO2 nanosheets containing abundant Vo and appropriate Fe doping exhibited an unprecedented 134 times higher activity in the degradation of Rhodamine B (RhB) (rate as high as 0.3073 min−1) than bulk anatase and were superior to most reported photocatalysts. The defect-rich ultrathin TiO2 nanosheets could be further applied in high-efficiency degradation of tetracycline hydrochloride (TC-HCl) with the degradation rate of 0.0423 min−1. The in situ electron paramagnetic resonance, advanced spectroscopic characterization and electrochemical measurement revealed the key role of Vo and Fe doping in facilitating the production of photo-generated holes and superoxide radicals (˙O2−) that were identified to be effective to decompose both RhB and TC-HCl. This research provides insight into defect engineering promoting AOS generation and gives inspiration for the design of efficient photocatalysts for photooxidation applications.

An environmentally friendly FeTiSOx catalyst with a broad operation‐temperature window for the NH3‐SCR of NOx

AbstractA novel FeTiSOx catalyst prepared by a simple hydrolysis coprecipitation method was used for the selective catalytic reduction (SCR) of NOx with NH3, which exhibited high catalytic activity (NOx conversion of >97% and N2 selectivity of >95%) and tolerance for both H2O and SO2 at a broad temperature window of ~325–475°C. The characterization results showed that the formation of Fe–O–Ti and Fe–O–S species could significantly enhance the acidic sites of the catalyst, which play an important role in NH3 absorption and their catalytic activity. The Fe3+ ions in the bulk anatase TiO2 could significantly enhance the redox properties for the SCR reaction and suppress the side reaction of NH3 oxidation to NO or N2O. In addition, the reaction mechanism was discussed based on in situ diffuse reflectance infrared Fourier transform spectroscopy measurements and kinetic investigation, indicating that the reaction was dominated by the Eley–Rideal mechanism over the FeTiSOx catalyst.

Efficient preparation of TiO2 nanoparticle models using interatomic potentials.

Computational modeling has proven to be extremely useful for understanding how the morphology, size, and structure of TiO2 nanoparticles (NPs) affect their electronic properties and their usage in targeted applications (e.g., photocatalysis). Density functional theory (DFT) based calculations of NPs (on the order of hundreds to thousands of atoms) are, however, computationally highly demanding. Herein, we show that interatomic potentials (IPs) can provide a highly computationally efficient means to prepare NP structures which are sufficiently accurate to significantly reduce the computational cost of subsequent DFT calculations. We first compare the direct DFT optimization of faceted NPs directly cut from the anatase bulk crystal with the same calculation where the NP is preoptimized using four different IPs. We then establish the subsequent computational time saving for the respective complete DFT optimizations. We show that IP-based preoptimizing can greatly speed up DFT convergence, with speed-ups of 3×-10× for single point DFT energy evaluations. Moreover, as IP preoptimized NP structures can be closer to those of DFT energy minima, further speed-ups of 2× for DFT structure optimizations can be achieved. Finally, taking NPs derived from anatase spherical cuts, we show that IP-based molecular dynamics annealing gives rise to significant structural reconstruction with an associated high energetic stabilization, as confirmed by DFT calculations. Although similar results can be achieved using DFT tight binding methods, IP-based methods are 3-4 orders of magnitude faster and thus provide a particularly highly computationally efficient route to the preparation and design of large and diverse NP sets.

Quantum size effect and visible light activity of anatase nanosheet quantum dots

Anatase (001) nanosheets have recently attracted great attention as very active catalysts and photocatalysts. These graphene analogs have very high surface area and unique surface properties. In the present paper, very thin two-layer anatase nanosheets are investigated computationally in the form of quantum dots of various size. Quantum size effect (QSE) was clearly observed for nanosheets with fully hydroxylated edges and size up to 14 nm and the ultimate band gap is around 3.4 eV. Dehydroxylation of nanosheets obscured QSE, decreased band gap and induced visible light absorption. Therefore, contradictory trends reported in experimental studies for anatase QSE can be ascribed to different degree of hydroxylation of the TiO2 samples surface. All anatase nanosheet quantum dots retained their flat graphene-like shape. These findings demonstrate that dehydroxylated anatase nanosheet quantum dots are prospective visible-light active photocatalysts even if their inherent band gap is considerably larger than for bulk anatase.

Morpho-structural and opto-electrical properties of chemically tuned nanostructured TiO2

Titanium dioxide (TiO2) nanoparticles were synthesized by co-precipitation and sol-gel techniques. Effects of synthesis techniques on crystallographic structure, surface morphology, optical and electrical properties were extensively investigated. Rietveld refinement analysis of X-ray diffraction (XRD) spectra was performed by considering pure anatase phase having I41/amd space group of the tetragonal structure of TiO2 and various refinement/ crystallographic parameters were calculated. This pure anatase phase for samples synthesized by both of the techniques was further confirmed by Raman spectra analysis. Metal oxide functional group and residual species were identified by Fourier transform infrared (FTIR) spectra for both samples. The effect of synthesis techniques on the development of microstructure and morphology of the TiO2 nanoparticles was observed in scanning electron microscopy (SEM). Optical properties revealed the redshift of 0.18(5) eV and 0.10(3) eV with respect to bulk anatase TiO2 for the co-precipitation and sol-gel derived samples respectively. A lower dielectric constant and larger conductivity of the sample synthesized by co-precipitation technique attributed to the lower crystallite size and band gap energy.