Anatase Samples Research Articles

Tetrakis-2,3-[5,6-di-(2-pyridyl)-pyrazino]porphyrazine, and its Cu(II) complex as sensitizers in the TiO 2-based photo-degradation of 4-nitrophenol

The tetrapyrrolic macrocycle tetrakis-2,3-[5,6-di-(2-pyridyl)-pyrazino]porphyrazine, [Py 8TPyzPzH 2], and its copper(II) derivative, [Py 8TPyzPzCu] were used as sensitizers adsorbed on TiO 2 (anatase) particles for the UV (300–400 nm) photo-degradation of 4-nitrophenol (4-NP) in water suspensions. A 3.5-fold increase of photocatalytic power over bare TiO 2-anatase has been observed in the case of the Cu(II) complex and of more than a factor of two for the free base. Compared with previous data obtained with anatase samples impregnated with some structurally similar porphyrins and phthalocyanines derivatives, these results represent a significant improvement. On the contrary, upon impregnation with [Py 8TPyzPzCu], both TiO 2-rutile and TiO 2-Degussa P25 did not show any improvement of their photocatalytic efficiency. Experimental evidences indicate that formation of singlet oxygen does not contribute to the beneficial effect of the macrocycles. Hydrogen peroxide was found not to participate to the 4-NP depletion. Some photo-bleaching of the adsorbed dye has also been observed.

Photocatalytic Activity of BulkTiO2Anatase and Rutile Single Crystals Using Infrared Absorption Spectroscopy

A systematic study on the photocatalytic activity of well-defined, macroscopic bulk single-crystal TiO(2) anatase and rutile samples has been carried out, which allows us to link photoreactions at surfaces of well-defined oxide semiconductors to an important bulk property with regard to photochemistry, the life time of e-h pairs generated in the bulk of the oxides by photon absorption. The anatase (101) surface shows a substantially higher activity, by an order of magnitude, for CO photo-oxidation to CO(2) than the rutile (110) surface. This surprisingly large difference in activity tracks the bulk e-h pair lifetime difference for the two TiO(2) modifications as determined by contactless transient photoconductance measurements on the corresponding bulk materials.

Carbon-coated TiO 2 composites for the photocatalytic degradation of low concentration benzene

Carbon-coated TiO 2 was synthesized by mixing TiO 2 powders and polyethylene glycol, followed by heat treatment in nitrogen atmosphere. All samples were characterized by powder X-ray diffraction, UV diffuse reflectance spectroscopy, high-resolution transmission electron microscopy, and nitrogen adsorption. The photocatalytic activity of carbon-coated TiO 2 for benzene degradation was investigated with a benzene concentration of ∼1.2×10 −5. Results showed that the residual carbon content was influenced greatly by heat treatment temperature (HTT) and the amount of PEG, which decreased and increased with increasing the temperature and the amount of PEG, respectively. The crystallinity of TiO 2 was improved when the HTT increased. However, the carbon residue had an inhibition effect on the crystal growth of TiO 2. The carbon-coated anatase samples were shown to exhibit higher photocatalytic activities than the pristine TiO 2 because of the adsorption enrichment of benzene by carbon around the anatase particles and of the effective charge separation due to the electronic conduction of carbon. Another important factor affecting photocatalytic activity was the crystallinity of the anatase phase. High photocatalytic activity for benzene requires a balance between the carbon content and the anatase crystalline structure.

Physicochemical study of the ‘anatase/electrolyte solution’ interface in suspension and closely packed particles of the solid

The electric double layer (EDL) developed at the interface of anatase in contact with aqueous electrolyte solutions was investigated at 25 °C. Potentiometric titrations (PT), measurements of the electrophoretic mobility (EM) in suspensions, and streaming potential (SP) measurements were taken. The surface charge over a wide pH range (ca. 3–10) and the point of zero charge (pzc = 6.3 ± 0.1) of anatase was easily determined by means of the suspension titration curve and the blank one, obtained at a single ionic strength value. Streaming potential measurements were conducted in anatase particles appropriately packed to form plugs. Two different plugs were prepared differing in the degree of particles' packing and, consequently, in the respective porosities. It was found that surface conductivity is lower at higher packing (lower porosity), because of the reduction of the total surface area in contact with the electrolyte. Moreover, it was found that the surface conductivity of the anatase samples increased at pH values away from pzc, while the mobility of the counter ions behind the shear plane decreased. This trend was attributed to the increase of the absolute surface charge. This increase caused an increase in the amount of the counter ions and, therefore, in the conductivity due to these ions. On the other hand, stronger electrostatic interactions between the surface of the solid and the counter ions reduced their mobility. The packing density of the anatase particles in the respective plugs, affected the values of ζ-potential calculated from SP measurements when the effect of surface conductivity was neglected. Copyright © 2010 John Wiley & Sons, Ltd.

A simple one-step fabrication of micrometer-scale hierarchical TiO 2 hollow spheres

Hierarchical TiO 2 hollow spheres had been prepared based on bubble templates by a simple one-step hydrothermal method. The diameter of hollow spheres was about 700 nm and the shell thickness of them was 69 nm. They were composed of similar spindle- or needle-like building units. Furthermore, hydrothermal time had an important influence on the morphology and crystallinity of hollow spheres. Moreover, the UV–Vis diffuse reflectance spectra of TiO 2 hollow spheres heated at 150 °C for 10 h showed the strongest absorption in the UV–Vis region and the Raman spectrum demonstrated the anatase sample. Additionally, a possible formation mechanism of TiO 2 hollow spheres was proposed. So this novel and simple method would provide a development direction to fabricate all kinds of inorganic hollow spheres by one-step procedure.

Sulfur doped anatase TiO 2 single crystals with a high percentage of {0 0 1} facets

Extending the response range of wide-bandgap (3.2 eV) anatase TiO 2 photocatalysts into the visible light range can play an important role in promoting the practical applications of photocatalysts. Here, we report a route to prepare sulfur doped anatase TiO 2 single crystal sheets with a high percentage of {0 0 1} facets. The resultant TiO 2 sheets were investigated by a combination of experimental characterizations and electronic structure calculations. The synthesized sulfur doped anatase samples show an additional visible light absorption band from 400 nm to ca. 550 nm and some visible-light photocatalytic activity in OH radical generation and photodecomposition of organic dyes. The Ti–S bond structure causes not only visible light absorption but also changes to an extent the surface structures of doped anatase TiO 2 sheets. Theoretically, localized 3p states of S formed in the bandgap are implicated for the visible light absorption of the sulfur doped anatase TiO 2.

Vibrational dynamics of anataseTiO2: Polarized Raman spectroscopy andab initiocalculations

Vibrational properties of anatase titanium dioxide single crystal are thoroughly investigated by means both of experimental and theoretical approaches. Polarized Raman-scattering experiments were carried out at room temperature in backscattering geometry on a microcrystalline anatase sample under different crystal orientations set by a micromanipulator. All the independent components of the polarizability tensor were separately obtained. In particular, it was possible to evidence, in turn, the different components of the ${\text{A}}_{1g}$ mode, thus unambiguously determining its peak wave number, and then, definitively resolve and identify the ${\text{A}}_{1g}$ and ${\text{B}}_{1g}$ modes of anatase. Moreover, ab initio calculations based on density-functional perturbation theory within the generalized gradient approximation have been performed to evaluate infrared (IR) and Raman modes, dielectric tensors, Born effective charges and TO-LO splitting of the IR modes. The agreement between computational and experimental results is much better than the one achieved in previous computational works.

Electrochemical lithium storage of TiO 2 hollow microspheres assembled by nanotubes

TiO 2 hollow microspheres with the shell consisting of nanotubes have been successfully synthesized via a template-free hydrothermal process and subsequent treatments. The electrochemical properties of the anatase sample have been investigated by cyclic voltammetry and galvanostatic method. The initial Li insertion/extraction capacity at a current density of 0.2 C reach 290 and 232 mAh g −1 respectively. Moreover, as-prepared TiO 2 delivers a reversible capacity of ca. 150 mAh g −1 after 500 cycles at 1 C, and it also shows superior high rate performance (e.g., 90 mAh g −1 at 8 C) without any modification.

Kinetics of Ag/TiO2-photocatalyzed iodide ion oxidation

Ag-doped TiO2 (anatase) samples (mass fraction w Ag = 0.01 and w Ag = 0.02) of 15.9 and 14.5 nm mean particle size and 11.46 and 10.14 m2 g−1 BET surface area were prepared by photodeposition. Doping results in surface plasmon resonance of the metallic silver nanoclusters at around 500 nm, but the absorption edge remains unaltered at 365 nm. Ag-doping remarkably enhances the photooxidation of iodide ion under UV light; iodine formation with Ag/TiO2 with w Ag = 0.01 is 16 times greater than with bare TiO2. The reaction conforms to Langmuir–Hinshelwood kinetics with regard to both I− and O2. Increase of pH slows down iodine formation and sacrificial electron donors arrest the reaction. Pre-sonication of the catalyst slurry hinders the photocatalysis. Generation of iodine is much greater in acetonitrile than in water. Under the experimental conditions, Ag/TiO2 with w Ag = 0.01 is more efficient than Ag/TiO2 with w Ag = 0.02, and the enhanced photocatalysis is likely to be because of suppression of electron–hole pair recombination. Kinetic analysis reveals that increasing the Ag mass fraction from 0.01 to 0.02 enhances the surface pseudo-first-order rate constant but inhibits the adsorption of iodide ion and the oxygen molecule on the illuminated oxide surface.

Photocatalytic degradation of organic molecules in water: Photoactivity and reaction paths in relation to TiO2 particles features

Abstract Anatase–rutile and anatase–brookite–rutile composite nanocrystals were synthesized by a controlled sol–gel reaction followed by hydrothermal growth and by mechanically mixing pure crystalline phase precursors, respectively. Their physicochemical and photocatalytic properties were investigated in comparison with those of several home-made and commercial pure anatase, rutile and mixed phase TiO 2 samples. In particular, the phase composition and the crystallite size were determined by X-ray diffraction measurements. The photocatalytic degradation of the azo dye Acid Red 1 (AR1) and of formic acid (FA) in aqueous suspension were employed as test reactions. Hydrogen peroxide evolution, the main reductive process occurring in parallel, was also monitored during the runs. The investigated photocatalysts exhibit different photoactivity scales towards the two organic substrates, depending on the prevailing photodegradation path. High surface area anatase samples were particularly active in FA degradation, principally occurring through direct interaction with photoproduced valence band holes, whereas rutile samples, even possessing high surface areas, but exhibiting poor wettability, were scarcely active. A good linear correlation was found between H 2 O 2 evolution and the rate of AR1 photodegradation on the investigated series of photocatalysts, whereas no H 2 O 2 was detected during FA mineralization, apart in the case of a low surface area anatase sample, whose peculiar photocatalytic properties were ascribed to the presence of Ti 3+ ions in its structure.

Hard template synthesis of crystalline mesoporous anatase TiO2 for photocatalytic hydrogen evolution

Mesoporous crystalline anatase TiO2 has been synthesized by using the nanocasting method with mesoporous silica KIT-6 as the hard template and titanium alkoxide as the precursor. Three key synthetic parameters play important roles in the formation of crystalline porous materials. These are precursor/template ratios, calcination temperatures and immersion time (in 2 M NaOH to remove template). The synthesized materials are characterized with powder X-ray diffraction (XRD), N2 adsorption–desorption isotherm analysis and energy-dispersive X-ray spectroscopy (EDX) elemental analysis. Owing to the template confinement effect, mesoporous anatase TiO2 instead of rutile phase was obtained at the calcination temperature of 750 °C. Materials with a surface area as high as 207 m2 g−1 have been obtained. The photocatalytic activities of the as-prepared mesoporous TiO2 were studied with a loading of 0.5% Pt. The materials with higher surface areas showed higher H2 evolution activities. The H2 evolution activity of the mesoporous anatase sample prepared under optimal conditions was 5.5 times higher than that of bulk anatase TiO2. The work represents a step forward in the development of highly efficient photocatalysts based on crystalline high surface area porous materials and our results indicate that the hard template method may be an effective method to achieve this goal.

Study of the resistance behavior of anatase and rutile thick films towards carbon monoxide and oxygen at high temperatures and possibilities for sensing applications

The resistance changes of thick films of titania were examined in the presence of CO at elevated temperatures ranging from 500 to 1000 °C. Three materials were examined, commercial samples of anatase and rutile and a Cr 3+-doped rutile prepared in the laboratory. The anatase film exhibited a n-type response to both oxygen and CO in the tested O 2 range of 2–21% and CO from 10 to 2000 ppm at 600 °C, with the response disappearing beyond 700 °C. On the other hand, the rutile films showed an entirely different electrical behavior. Until 1000 °C, p-type response was observed to oxygen concentrations from 2 to 21%. With CO concentrations up to 2000 ppm, p-type behavior was observed in 2–21% oxygen concentrations between 600 and 700 °C. However, when the temperature was above 750 °C, an n-type behavior to CO with concentrations ranging from 200 to 2000 ppm was observed, and response was observed until 850 °C. A coating of colloidal platinum on the rutile film induced a transition of the response of the thick film from p to n-type to both oxygen and CO at 600 °C. With the Cr-rutile sample, p-type behavior was observed for both CO and O 2. The possible mechanisms for the observed p–n transition of the two rutile films are discussed. Strategies for developing CO sensors for high temperature applications based on the above observations are formulated.

Photoactivity of mechanochemically prepared nanoparticulate titanium dioxide investigated by EPR spectroscopy

Titanium dioxide nanopowders were prepared by mechanochemical synthesis in a high-energy ball mill using TiOSO4·xH2O and Na2CO3 followed by annealing in the temperature range 200–800°C. The UVA photonic efficiency of radical processes on synthesized TiO2 powders was determined by in situ EPR spectroscopy, using, as indicators, the N-oxide spin trapping agents (5,5-dimethyl-1-pyrroline N-oxide and 5-(diisopropoxy-phosphoryl)-5-methyl-1-pyrroline N-oxide) or the radical cation of 2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonate, ABTS+). The results obtained by monitoring the photoinduced generation of hydroxyl radical spin adducts correlated with those found by the investigation of the photoreduction of ABTS+. The presence of iron and chromium ions, which were evidenced in samples milled in steel, decreased the photonic efficiency of radical processes. The presence of a sulfate salt matrix during the annealing process distinctly inhibits the transformation of anatase to rutile. The highest photocatalytic activity was shown by anatase samples which were prepared by milling in corundum and annealed at 700°C. They were composed of crystallites with a mean size of 25–30nm and well developed crystal faces.

The origin of oxygen vacancy induced ferromagnetism in undopedTiO2

Using the full-potential linearized augmented plane wave method, we have investigatedthe oxygen vacancy defect induced ferromagnetism in both rutile and anataseTiO2. It has been found that the oxygen vacancy induces lattice distortion in rutileTiO2,whereas there is no such meaningful change in the anatase structure. Interestingly, the lattice distortedrutile TiO2 shows an oxygen vacancy induced ferromagnetic state with a magnetic moment of0.22 µB in the Ti atom neighboring the vacancy site, while only0.06 µB is observed in theTi atom in anatase TiO2. We attribute the sizable magnetic moment due to the oxygen vacancy in rutileTiO2 to the charge redistribution owing to lattice distortion. Experimentallymeasured magnetic hysteresis curves for undoped rutile and anataseTiO2 films clearly display ferromagnetic behavior at room temperature. The observed magneticstrength of the rutile sample turns out to be larger than that of the anatase sample, inaccordance with the theoretical calculations.

The solubility of Co in TiO 2 anatase and rutile and its effect on the magnetic properties

Co-doped anatase and rutile bulk-samples prepared by the sol–gel technique are found to be paramagnetic at room-temperature. Only further annealing in Ar/H 2 gas results in a ferromagnetic behavior. X-ray diffraction and electron-microscope studies reveal for low doping levels <4% the formation of Co-doped rutile samples and the formation of CoTiO 3 as a new phase. Co 3O 4 can be detected in anatase samples with Co doping levels ⩾4%. The observed Co oxides are reduced by Ar/H 2 to Co metal. The room-temperature ferromagnetism can therefore be traced back to a segregation of metallic Co.

Study of Nitrogen-Modified Titanium Dioxide as an Adsorbent for Azo Dyes

The removal of two azo dyes from aqueous solution was investigated using a highly active nitrogen-modified titanium dioxide (TiO2/N). This photocatalyst was prepared by pressure modification of a commercial sample of anatase, TiO2 (supplied by Zaklady Chemiczne Police SA, Poland). The adsorption of two azo dyes (Reactive Red 198 — a monoazo dye and Direct Green 99 — a polyazo dye) onto the unmodified and nitrogen-modified TiO2 surface was determined. Adsorption onto unmodified TiO2 led to an isotherm which could be described by the Freundlich model, whereas that onto the nitrogen-modified TiO2 surface was best described by the Langmuir model. The adsorption isotherms of Direct Green 99 onto TiO2/N from aqueous solutions at various pH values were compared with those onto unmodified TiO2 during dye removal under UV irradiation. Such a comparison is important since it provides information as to how solutes interact with adsorbents. Zeta potential measurements of TiO2 provided an explanation for the observed behaviour in terms of the surface charge. The IR spectrum of the photocatalyst obtained exhibited new absorption bands at 1430–1440 cm−1 which were attributed to the bending vibrations of NH+4 and at 1535 cm−1 associated with NH2 groups or NO2 and NO groups.

Preparation and characterization of nitrogen-doped TiO 2 nanoparticles by the laser pyrolysis of N 2O-containing gas mixtures

Nitrogen-doped TiO 2 nanoparticles have been prepared by the IR laser pyrolysis technique. A sensitized mixture of TiCl 4 (vapors) and N 2O was used as titanium and nitrogen precursors, respectively. The structural properties of the resultant N-doped nanoparticles such as the phase formation and the average particle size and distributions were investigated by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy. The phase composition varied from almost pure anatase to mixtures of rutile and anatase. A decrease of the mean particle diameters from about 18 nm in case of the almost pure anatase sample to about 13 nm in case of the anatase–rutile mixture is observed. XPS analysis suggests and interstitial character of the doping process.

Tailoring nanostructured TiO 2 for high power Li-ion batteries

It is shown that the rate performance of anatase TiO 2 can be significantly improved by addition of a small amount (few percent) of carefully selected oxides such as silica or RuO 2. Specifically, silica serves primarily as a suppressant of particle growth during heating of anatase precursor—in our case titania nanotubes. The addition of RuO 2 is supposed to enhance the electronic conductivity. The beneficial impact of the combined use of silica and RuO 2 in the preparation of anatase-based electrodes is also demonstrated on a commercially available sample of anatase.

Hydrothermal Synthesis of Nanostructured TiO2 Particles and Characterization of Their Photocatalytic Antimicrobial Activity

Nanostructured titania particles were synthesized by using hydrothermal processing and the photocatalytic antimicrobial activities were characterized. Both sol-gel synthesized and commercial TiO2 (anatase) samples were processed with two step hydrothermal treatments, under alkaline and neutral conditions. Scanning Electron Microscope (SEM) images showed that alkaline treatment yields nanofibers and lamellar structured particles from the commercial anatase and sol-gel synthesized samples respectively. Further treatment of nanofibers and nanostructured lamellar particles with distilled water results with crystal growth and the formation of nano structured bipyramidal crystalline particles. The photocatalytic antimicrobial activities of the samples were determined against Escherichia coil under irradiation. It was observed that the samples treated under alkaline conditions have improved activity than the original anatase samples. Limited activity and resulting time lag in bacterial inactivation were observed for hydrothermally treated samples with distilled water. However, a post treatment comprising the UV irradiation in aqueous conditions enhanced the photocatalytic activity.

Structural properties and electrochemical behavior of CNT‐TiO2 nanocrystal heterostructures

AbstractDifferent methods of preparation of CNT‐TiO2 (anatase) heterostructures have been tested in order to obtain information on the relationship between the preparation method, resulting structure of the material and its electrochemical behavior during Li+ insertion. The most complete TiO2 coverage of individual non‐functionalized MWCNT was achieved in block co‐polymer (Pluronic P123) templated TiO2 thin films. This heterostructure has the highest specific capacity of Li+ insertion into TiO2 lattice to form Lix TiO2 (688 C/g, x = 0.57), however, this was accompanied by the worst reversibility (76%). On the contrary, electrodeposited TiO2 on SWCNT covered only bundles of nanotubes and its Li+ insertion capacity was 507 C/g (x = 0.42). However, the reversibility for all electrodeposited samples was almost complete and varied around 98%. Furthermore, Li+ insertion behavior studied by the cyclic voltammetry technique showed unambiguously the presence of minute amounts of monoclinic TiO2(B) in addition to anatase in samples from both synthesis methods. Thirdly, non‐functionalized MWCNT have been mixed by phase pure anatase nanocrystals of average size 30 nm made by hydrolysis of Ti‐butoxide in ethanol dispersion of CNT. Electrochemical Li+ insertion showed x = 0.54 (charge capacity of 652 C/g) with 85% reversibility. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)