Crystallographic shear of polymorphic TiO<Subscript>2</Subscript> nanocondensates with enhanced Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> dissolution via pulsed laser ablation

TiO₂ nanoparticles with enhanced solid solution of Cr up to 16 wt% in polymorphs of rutile, anatase, brookite, α-PbO₂-type, and occasionally baddeleyite-type were synthesized via pulse laser ablation on ceramic TiO₂ target dissolved with Cr₂O₃ or clamped Cr/Ti plates in air. Analytical electron microscopic observations indicated these nanocondensates have prevalent crystallographic shear (CS) along specific planes to form superstructures. The rutile type typically shows (100) and (010) CS besides the conventional ones rotating about the [111] zone axis as reported for ambient samples. The CS planes are parallel to (001) for anatase, (001) and ([`1] \overline{1} 10) for brookite, whereas (001) and {1[`3] \overline{3} 1} for the α-PbO₂-type TiO₂ with varied extent of Cr dissolution. Surface modification, as a result of Cr dissolution and/or internal stress, was observed for all the polymorphs.     

Synthesis and nanostructural investigation of TiO<Subscript>2</Subscript> nanorods doped by SiO<Subscript>2</Subscript>

TiO₂ Nano rods can be used as dye-sensitized solar cells, various sensors and photocatalysts. These nanorods are synthesized by a hydrothermal corrosion process in NaOH solution at 200°C using TiO₂ powder as the source material. In the present work, the synthesis of TiO₂ nanorods in anatase, rutile and Ti₇O₁₃ phases and synthesis of TiO₂ nanorods by incorporating SiO₂ dopant, using the sol–gel method and alkaline corrosion are reported. The morphologies and crystal structures of the TiO₂ nanorods are characterized using field emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) study. The obtained results show not only an aggregation structure at high calcination temperatures with spherical particles but also Ti–O–Si bonds having four-fold coordination with oxygen in SiO^4 −.     

Preparation of TiO<Subscript>2</Subscript>-coated LiCo<Subscript>1/3</Subscript>Ni<Subscript>1/3</Subscript>Mn<Subscript>1/3</Subscript>O<Subscript>2</Subscript> by electrostatic self-assembly method and its properties

A precursor of TiO₂–LiCo_1/3Ni_1/3Mn_1/3O₂ was prepared by electrostatic self-assembly method. The final product was obtained by heating the precursor at 400–450 °C for 4–6 h in air. X-ray diffraction (XRD), scanning electron microscope (SEM), X-ray photoelectron spectroscopy (XPS), and electrochemical tests were used to examine the structural, morphology, elementary valence, and electrochemical characteristics. XRD indicated that the TiO₂-coated material can be indexed by α-NaFeO₂ layered structure, which belongs to hexagonal-type space group R3m. XPS results confirmed the existence of TiO₂ compound on the surface of the coated sample. The SEM image showed that the material had spherically porous morphology with the uniform size about 6 μm. The initial charge–discharge capacity of the TiO₂-coated LiCo_1/3Ni_1/3Mn_1/3O₂ material was 168.8/160.0 mAh/g. After 60 cycles, the discharge capacity of the TiO₂-coated LiCo_1/3Ni_1/3Mn_1/3O₂ sample was 147.0 mAh/g, and the coulombic efficiency was 94.0%. Compared with the uncoated sample, the electrochemical performance of TiO₂-coated LiCo_1/3Ni_1/3Mn_1/3O₂ was improved.     

The enhanced catalytic degradation of SiO<Subscript>2</Subscript>/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript>/C@TiO<Subscript>2</Subscript> photo-Fenton system on p-nitrophenol

Heterogeneous photo-Fenton SiO₂/Fe₃O₄/C@TiO₂ (SFCT) catalyst with a core-multishell structure and a diameter of about 550 nm was successfully prepared and was characterized by scanning electron microscopy (SEM), TEM, XRD, Raman, and Fourier transform infrared (FT-IR). The results illustrated that anatase TiO₂ coexisted with rutile TiO₂, in which the anatase phase was the main crystal phase. In addition, the catalytic activity of SFCT catalyst had been evaluated in the catalytic degradation on p-nitrophenol (PNP). The influence factors on the PNP degradation, including SFCT component ratio (m _SFC/ m _TiO2), H₂O₂ dosage, solution pH, and PNP concentration, had been investigated. And the contrast experiments about the photo-Fenton catalytic mechanism revealed that the SFCT-2 catalyst possessed a superior activity in the neutral environment due to the optimal activity matching between Fe₃O₄ and TiO₂, and it exhibited the stable catalytic performance after five successive recycles. Therefore, the SFCT-2 catalyst had a promising application for the photo-Fenton degradation of organic contaminant.     

Fabrication and characterization of CdS-sensitized TiO<Subscript>2</Subscript> nanotube photoelectrode

CdS quantum dot (Qd)-sensitized TiO₂ nanotube array photoelectrode is synthesised via a two-step method on tin-doped In₂O₃-coated (ITO) glass substrate. TiO₂ nanotube arrays are prepared in the ethylene glycol electrolyte solution by anodizing titanium films which are deposited on ITO glass substrate by radio frequency sputtering. Then, the CdS Qds are deposited on the nanotubes by successive ionic layer adsorption and reaction technique. The resulting nanotube arrays are characterized by scanning electron microscopy, X-ray diffraction (XRD) and UV–visible absorption spectroscopy. The length of the obtained nanotubes reaches 1.60 μm and their inner diameter and wall thickness are around 90 and 20 nm, respectively. The XRD results show that the as-prepared TiO₂ nanotubes array is amorphous, which are converted to anatase TiO₂ after annealed at 450 °C for 2 h. The CdS Qds deposited on the TiO₂ nanotubes shift the absorption edge of TiO₂ from 388 to 494 nm. The results show that the CdS-sensitized TiO₂ nanotubes array film can be used as the photoelectrode for solar cells.     

Preparation and investigation of composite SiO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> films

SiO₂-TiO₂ films [Si:Ti = 1:(0.06–2.3)] are obtained by the sol-gel method. The structural and photoluminescent properties of the films and powders heat-treated at different temperatures are studied. It is shown that after 700°C the composite consists of TiO₂ crystallites that are structurally similar to anatase and distributed in an amorphous SiO₂ matrix. The photoluminescence spectra have maxima at 450–500 nm. The photoluminescence intensity depends on the treatment temperature and TiO₂ content. __________ Translated from Zhurnal Prikladnoi Spektroskopii Vol. 74, No. 3, pp. 357–361, May–June, 2007.     

Remarkable catalytic activity of Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> nanocomposites prepared by hydrothermal method for the degradation of methyl orange

Visible light Bi₂O₃/TiO₂ nanocomposites are successfully prepared with different dosages of Bi₂O₃ by hydrothermal process. All the as-prepared samples are characterized by X-ray diffraction (XRD), scanning and transmission electron microscopes (SEM and TEM), Brunauer-Emmett-Teller analysis (BET), N₂ adsorption-desorption measurement, and UV-Vis diffuse reflectance spectra (DRS). XRD and Raman spectra reveal the anatase phase of both TiO₂ and Bi₂O₃/TiO₂ nanocomposites. X-ray diffraction patterns demonstrate that the bismuth ions did not enter into the lattice of TiO₂, and Bi₂O₃ is extremely dispersive on the surface of TiO₂ nanoparticles. The incorporation of Bi₂O₃ in TiO₂ leads to the spectral response of TiO₂ in the visible light region and efficient separation of charge carriers. The enhanced visible light activity is tested by the photocatalytic degradation of methyl orange under light illumination, and the performance of Bi₂O₃/TiO₂ nanocomposites are superior than that of pure TiO₂ which is ascribed to the efficient charge separation and transfer across the Bi₂O₃/TiO₂ junction. Bi₂O₃/TiO₂ nanocomposite (20 mg) loaded with 0.25 of Bi₂O₃ dispersed in 50 ml of 5 ppm methyl orange solution exhibited the highest photocatalytic activity of 98.86% within 240 min of irradiation, which is attributed to the low band gap, high surface area, and the strong interaction between Bi₂O₃ and TiO₂.     

Gas sensor based on nanosize In<Subscript>2</Subscript>O<Subscript>3</Subscript>:Ga<Subscript>2</Subscript>O<Subscript>3</Subscript> film

Nanosize films of In₂O₃:Ga₂O₃ (96:4 weight %) have been deposited on a glassceramic substrate by the method of rf magnetron sputtering. The surfaces of fabricated films were studied with use of a scanning electron microscope; sizes of grains were determined and the thicknesses of films were measured. In order to prepare a gas-sensitive structure, a thin catalytic palladium layer and ohmic comb contacts were deposited on the In₂O₃:Ga₂O₃ film surface by the method of ion-plasma sputtering. The sensitivity of sensors based on the glassceramic/In₂O₃:Ga₂O₃ (96:4 weight %)/Pd structure to different concentrations of propane and butane gas mixture, as well as to methane was investigated at temperatures of working substance from 250 to 300°C.     

Symmetry and models of single-walled TiO<Subscript>2</Subscript> nanotubes with rectangular morphology

The formalism of line symmetry groups for one-periodic (1D) nanostructures with rotohelical symmetry has been applied for symmetry analysis of single-walled titania nanotubes (SW TiO₂ NTs) formed by rolling up the stoichiometric two-periodic (2D) slabs of anatase structure. Either six- or twelve-layer (101) slabs have been cut from TiO₂ crystal in a stable anatase phase. After structural optimization, the latter keeps the centered rectangular symmetry of initial slab slightly compressed along a direction coincided with large sides of elemental rectangles. We have considered two sets of SW TiO₂ NTs with optimized six- and twelve-layer structures, which possess chiralities (−n, n) and (n, n) of anatase nanotubes. To analyze the structural and electronic properties of titania slabs and nanotubes, we have performed their ab initio LCAO calculations, using the hybrid Hartree-Fock/Kohn-Sham exchange-correlation functional PBE0. The band gaps (Δɛ _gap ) and strain energies (E _strain ) of six-layer nanotubes have been computed and analyzed as functions of NT diameter (D _NT). As to models of 12-layer SW TiO₂ NTs of both chiralities, their optimization results in structural exfoliation, i.e., the multi-walled structure should be rather formed in nanotubes with such a number of atomic layers.     

Thermostable photocatalytically active TiO<Subscript>2</Subscript> anatase nanoparticles

Anatase is the low-temperature (300–550 °C) crystalline polymorph of TiO₂ and it transforms to rutile upon heating. For applications utilizing the photocatalytic properties of nanoscale anatase at elevated temperatures (over 600 °C) the issue of phase stabilisation is of major interest. In this study, binary TiO₂/SiO₂ particles were synthesized by a flame aerosol process with TiCl₄ and SiCl₄ as precursors. The theoretical Si/Ti ratio was varied in the range of 0.7–1.3 mol/mol. The synthesized TiO₂/SiO₂ samples were heat treated at 900 and 1,000 °C for 3 h to determine the thermostability of anatase. Pyrogenic TiO₂ P25 (from Evonik/Degussa, Germany) widely applied as photocatalyst was used as non-thermostabilized reference material for comparison of photocatalytic activity of powders. Both the non-calcinated and calcinated powders were characterized by means of XRD, TEM and BET. Photocatalytic activity was examined with dichloroacetic acid (DCA) chosen as a model compound. It was found that SiO₂ stabilized the material retarding the collapse of catalyst surface area during calcination. The weighted anatase content of 85% remains completely unchanged even after calcination at 1,000 °C. The presence of SiO₂ layer/bridge as spacer between TiO₂ particles freezes the grain growth: the average crystallite size increased negligibly from 17 to 18 nm even during the calcination at 1,000 °C. Due to the stabilizing effect of SiO₂ the titania nanoparticles calcinated at 900 and 1,000 °C show significant photocatalytic activity. Furthermore, the increase in photocatalytic activity with calcination temperature indicates that the titania surface becomes more accessible either due to intensified cracking of the SiO₂ layer or due to enhanced transport of SiO₂ into the necks thus releasing additional titania surface.     

Preparation of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles for superthermites by the detonation of an explosive nanocomposite material

This article reports on the preparation of chromium(III) oxide nanoparticles by detonation. For this purpose, a high explosive—hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX)—has been solidified from a solution infiltrated into the macro- and mesoporosity of Cr₂O₃ powder obtained by the combustion of ammonium dichromate. The resulting Cr₂O₃/RDX nanocomposite material was embedded in a cylindrical charge of pure explosive and detonated in order to fragment the metallic oxide into nanoparticles. The resulting soot contains Cr₂O₃ nanoparticles, nanodiamonds, amorphous carbon species and inorganic particles resulting from the erosion by the blast of the detonation tank wall. The purification process consists in (i) removing the carbonaceous species by an oxidative treatment at 500 °C and (ii) dissolving the mineral particles by a chemical treatment with hydrofluoric acid. Contrary to what could be expected, the Cr₂O₃ particles formed during the detonation are twice larger than those of initial Cr₂O₃. The detonation causes the fragmentation of the porous oxide and the melting of resulting particles. Nanometric droplets of molten Cr₂O₃ are ejected and quenched by the water in which the charge is fired. Despite their larger size, the Cr₂O₃ nanoparticles prepared by detonation were found to be less aggregated than those of the initial oxide used as precursor. Finally, the Cr₂O₃ synthesized by detonation was used to prepare a superthermite with aluminium nanoparticles. This material possesses a lower sensitivity and a more regular combustion compared to the one made of initial Cr₂O₃.     

n/p-Type changeable semiconductor TiO<Subscript>2</Subscript> prepared from NTA

A novel kind of nano-sized TiO₂ (anatase) was obtained by high-temperature (400–700°C) dehydration of nanotube titanic acid (H₂Ti₂O₄(OH)₂, NTA). The high-temperature (400–700°C) dehydrated nanotube titanic acids (HD-NTAs) with a unique defect structure exhibited a p-type semiconductor behavior under visible-light irradiation ( nm, E _photon=2.95 eV), whereas exhibited an n-type semiconductor behavior irradiated with UV light ( nm, E _photon=3.40 eV).     

Temperature dependence of the lowest frequency <Emphasis Type="BoldItalic">E</Emphasis><Subscript><Emphasis Type="Bold">g</Emphasis></Subscript> Raman mode in laser-synthesized anatase TiO<Subscript>2</Subscript> nanopowder

Nanosized titanium dioxide (TiO₂) powder was prepared by a laser-induced pyrolysis. Specific surface area of the as-grown powder measured by BET method was 109 m²/g. The grain size (14.5 nm) estimated from these data coincides well with the crystallite size (12.3 nm) determined by XRD measurements. The average grain size (∼35 nm) obtained from the subsequent SEM measurements refers to considerable agglomeration of nanoparticles. Raman spectroscopy has been used to investigate the structural properties of TiO₂ nanopowder and its anatase structure is confirmed. The blueshift and broadening of the lowest frequency E_g Raman mode at temperature range ∼25–550 K have been analyzed using a phonon-confinement model. Dominant influence of the strong anharmonic effect at higher temperatures was demonstrated. PACS 81.07.Wx; 78.30.-j; 63.22.+m     

Photoactivity passivation of TiO<Subscript>2</Subscript> nanoparticles using molecular layer deposited (MLD) polymer films

Pigment-grade anatase TiO₂ particles (160 nm) were passivated using ultra-thin insulating films deposited by molecular layer deposition (MLD). Trimethylaluminum (TMA) and ethylene glycol (E.G) were used as aluminum alkoxide (alucone) precursors in the temperature range of 100–160 °C. The growth rate varied from 0.5 nm/cycle at 100 °C to 0.35 nm/cycle at 160 °C. Methylene blue oxidation tests indicated that the photoactivity of pigment-grade TiO₂ particles was quenched after 20 cycles of alucone MLD film, which was comparable to 70 cycles of Al₂O₃ film deposited by atomic layer deposition (ALD). Alucone films would decompose in the presence of water at room temperature and would form a more stable composite containing aluminum, which decreased the passivation effect on the photoactivity of TiO₂ particles.     

An investigation into ultra-thin pseudobinary oxide (TiO<Subscript>2</Subscript>)<Subscript>x</Subscript>(Al<Subscript>2</Subscript>O<Subscript>3</Subscript>)<Subscript>1-x</Subscript> films as high-<Emphasis Type="Italic">k</Emphasis> gate dielectrics

As potential gate dielectric materials, pseudobinary oxide (TiO₂)_x(Al₂O₃)_1-x (0.1≤x≤0.6) films (TAO) were deposited on Si (100) substrates by pulsed-laser deposition method and studied systematically via various measurements. By a special deposition process, including two separate steps, the TAO films were deposited in the form of two layers. The first layer was deposited at room temperature and the second layer was completed at the substrate temperature of 400 °C. Detailed data show that the properties of the TAO films are closely related to the ratio between TiO₂ and Al₂O₃. The existence of the first layer deposited at room temperature can effectively restrain the formation of the interfacial layer. And according to the results of X-ray photoelectron spectroscopy and high-resolution transmission electron microscopy performed on the films, no other information belonging to the silicon oxide could be observed. For the (TiO₂)_0.4(Al₂O₃)_0.6 film, the best result has been achieved among all samples and its dielectric constant is evaluated to be about 38. It is valuable for the amorphous TAO film as one of the promising dielectric materials for high-k gate dielectric applications. PACS 77.55.+f; 73.40.Qv; 81.15.Fg     

The influence of Fe,Cu, SiO<Subscript>2</Subscript>, TiO<Subscript>2</Subscript>, and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles in aqueous solution on proton relaxation times

Measurements of proton nuclear spin-spin and spin-lattice relaxation times are applied for determining the concentration of solid-phase nanoparticles in nanofluids. This approach is tested for metal oxides SiO₂, TiO₂, Al₂O₃ and metal-carbon nanoparticles of 3d-metals Fe and Cu. It is shown that the sensitivity of the method for determining concentrations of 3d-metals is much higher than for oxides (by 2–4 orders of magnitude). It is revealed that measurement of the proton spin-spin relaxation time allows one to determine the concentration of Cu nanoparticles to 0.0001 mg/ml and that of Fe nanoparticles to 0.00001 mg/ml.     

Spectroscopic Properties and Energy Transfer Analysis of Tm<Superscript>3+</Superscript>-Doped BaF<Subscript>2</Subscript>-Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>-GeO<Subscript>2</Subscript>-La<Subscript>2</Subscript>O<Subscript>3</Subscript> Glass

This paper reports on the spectroscopic properties and energy transfer analysis of Tm³⁺-doped BaF₂-Ga₂O₃-GeO₂-La₂O₃ glasses with different Tm₂O₃ doping concentrations (0.2, 0.5, 2.0, 2.5, 3.0, 3.5, 3.5, 4.0 wt%). Mid-IR fluorescence intensities in the range of 1,300 nm−2,200 nm have been measured when excited under an 808 nm LD for all the samples with the same pump power. Energy level structure and Judd-Ofelt parameters have been calculated based on the absorption spectra of Tm³⁺, cross-relaxation rates and multi-phonon relaxation rates have been estimated with different Tm₂O₃ doping concentrations. The maximum fluorescence intensity at around 1.8 μm has been obtained in Tm₂O₃-3 wt% sample and the maximum value of calculated stimulated emission cross-section of Tm³⁺ in this sample is about 0.48 × 10⁻²⁰ cm² at 1,793 nm, and there is not any crystallization peak in the DSC curve of this sample, which indicate the potential utility of Tm³⁺-doped BaF₂-Ga₂O₃-GeO₂- La₂O₃ glass for 2.0-μm optical fiber laser.     

Photoconductivity studies on amorphous and crystalline TiO<Subscript>2</Subscript> films doped with gold nanoparticles

In this work, amorphous and crystalline TiO₂ films were synthesized by the sol–gel process at room temperature. The TiO₂ films were doped with gold nanoparticles. The films were spin-coated on glass wafers. The crystalline samples were annealed at 100°C for 30 minutes and sintered at 520°C for 2 h. All films were characterized using X-ray diffraction, transmission electronic microscopy and UV-Vis absorption spectroscopy. Two crystalline phases, anatase and rutile, were formed in the matrix TiO₂ and TiO₂/Au. An absorption peak was located at 570 nm (amorphous) and 645 nm (anatase). Photoconductivity studies were performed on these films. The experimental data were fitted with straight lines at darkness and under illumination at 515 nm and 645 nm. This indicates an ohmic behavior. Crystalline TiO₂/Au films are more photoconductive than the amorphous ones.     

Photoaccumulating film systems based on TiO<Subscript>2</Subscript>/MoO<Subscript>3</Subscript> and TiO<Subscript>2</Subscript>/MoO<Subscript>3</Subscript>:V<Subscript>2</Subscript>O<Subscript>5</Subscript> nanoheterostructures

The thin-film photocatalysts TiO₂/MoO₃ and TiO₂/MoO₃:V₂O₅ obtained by a combination of sol–gel and sintering techniques were studied using the photooxidation of probing dyes, EPR spectroscopy, X-ray diffraction analysis, and electron microscopy. It was shown that due to charge accumulation caused by UV irradiation, these photocatalysts retain their oxidative activity and ability for self-sterilization in the dark for a long time after irradiation was terminated (up to 5 h for TiO₂/MoO₃:V₂O₅).     

Phase composition and structure of nanocrystalline powders based on ZrO<Subscript>2</Subscript>(Y) and Al<Subscript>2</Subscript>O<Subscript>3</Subscript> obtained by plasma spray pyrolysis

It is shown that a nonequilibrium solid solution ZrO₂(3Y, Al) with tetragonal structure is formed in systems based on ZrO₂(3Y) with Al₂O₃ as a second component. A delay in the γ → α Al₂O₃ transformation and a reduction in the size of the coherently scattering domain of modifications are observed in systems based on Al₂O₃ with ZrO₂(Y) as a second component.