Nataliya Murafa

Transformation of brookite-type TiO2 nanocrystals to rutile: correlation between microstructure and photoactivity

Nanometric particles of pure brookite TiO2 were synthesized by modified thermolysis of reactant solutions containing titania powder, HCl, urea and PEG 10000. Unique flower-like brookite agglomerates with an average diameter of ∼400–450 nm composed of single brookite nanocrystals of ∼4–5 nm were obtained at 105 °C. The brookite → rutile transformation has been studied and TiO2 mixtures with variable amount of anatase, brookite and rutile polymorphs at different temperatures (from 200 to 800 °C) were obtained. High resolution transmission electron microscopy (HRTEM), electron diffraction pattern and BET/BJH analyses were used to characterize the phase assemblages, crystallite size and pore volume of the pure-phase brookite and TiO2 mixtures. In order to understand the metastable–stable TiO2 phase transformation X-ray powder diffraction (XRD) was performed. The photoactivity of pure brookite and TiO2 powders with different compositions of the brookite–anatase–rutile and anatase–rutile polymorphs obtained during the transitions was examined by photocatalyzed degradation of 4-chlorophenols in aqueous solution. The titania sample having the highest catalytic activity was obtained at 500 °C, contained 3.2% brookite, 42.9% anatase and 53.9% rutile and is referred to as TiO[B])/500 .

Photocatalytic Activity of Boron-Modified Titania under UV and Visible-Light Illumination

Nanosized boron(III) oxide-doped titania was prepared by homogeneous hydrolysis of titanium oxo-sulfate with urea in aqueous solutions in the presence of amorphous boron. The prepared samples were annealing at 700 degrees C. The structure of as-prepared samples was characterized by X-ray powder diffraction (XRD) and selected area electron diffraction (SAED) and surface area (BET) and porosity determination (BJH). The morphology and microstructure characteristics were obtained by scanning electron microscopy (SEM) and high-resolution electron microscopy (HRTEM). The method of UV/vis diffuse reflectance spectroscopy was employed to estimate band gap energies of the boron-doped titania. The photoactivity of the prepared samples was assessed by the photocatalytic decomposition of Orange II dye in an aqueous slurry during irradiation at 365 and 400 nm wavelength. The prepared titania samples doped with boron(III) oxide showed better photocatalytic activity in comparison with the reference TiO(2) sample. These photocatalysts showed better photocatalytic performance under visible-light irradiation.

Direct mechanosynthesis of pure BiFeO3 perovskite nanoparticles: reaction mechanism

In this work, a mechanochemical procedure is proposed as a simple and fast method to synthesize the pure BiFeO3 perovskite phase as a nanostructured material without the need for purification treatments, while the mechanochemical reaction mechanism has been investigated and correlated with that of the conventional solid-state reaction. Thus, different milling conditions have been used as a tool for tailoring the crystallite size of the resulting BiFeO3 nanoparticles. The materials prepared by the mechanochemical reaction could be annealed or sintered without the formation of undesirable phases. Both the ferroelectric and ferromagnetic transitions were observed by DSC. Finally, the dielectric constants of the prepared material at different frequencies as a function of the temperature have been measured, showing that the material is clearly an isolator below 200 °C, characteristic of a high quality BiFeO3 material.

Photocatalytic degradation of acetone and butane on mesoporous titania layers

Mesoporous titania was prepared by homogeneous hydrolysis of titanium oxo-sulfate with urea in aqueous solutions in the presence of the cationic and anionic surfactants, cetyl trimethyl ammonium bromide (CTAB) and sodium dodecyl benzene sulfonate (SDBS), respectively. Following annealing at 600 °C the structure of prepared samples was determined with X-ray powder diffraction (XRD) and selected area electron diffraction (SAED). The morphology and microstructure characteristics were also obtained by scanning electron microscopy (SEM) and high resolution electron microscopy (HRTEM). Surface area (BET) and porosity were also determined. From the mesoporous titania samples, a 300 μm thin layer on glass desk 10 × 15 cm was created. The photocatalytic activity of the prepared layers was assessed from the kinetics of the photocatalytic degradation of butane and acetone in the gas phase.

Niobium and tantalum doped titania particles

Niobium and tantalum doped anatase were prepared by thermal hydrolysis of peroxotitanium complex aqueous solutions containing of niobium or tantalum peroxo-complexes at 100 °C for 3 days. Niobium-doping increased the unit cell constants of anatase and changed the morphology of TiO 2 from spindle-like to rectangular or square cross section. Nb and Ta doping in the TiO 2 nanostructure increases the anatase to rutile transformation temperature to >1000 °C. In the visible region, the photocatalytic activity is directly proportional to the concentration and increases with increasing of Nb concentration. The niobium addition enhances the photocatalytic activity of titania in the visible light region.

Photocatalytic properties of Ru-doped titania prepared by homogeneous hydrolysis

Nanocrystalline titania particles doped with ruthenium oxide have been prepared by the homogenous hydrolysis of TiOSO4 in aqueous solutions in the presence of urea. The synthesized particles were characterized by X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), High Resolution Transmission Electron Microscopy (HRTEM), Selected Area Electron Diffraction (SAED) and Nitrogen adsorption-desorption was used for surface area (BET) and porosity determination (BJH). The photocatalytic activity of the Ru-doped titania samples were determined by photocatalytic decomposition of Orange II dye in an aqueous slurry during irradiation at 365 nm and 400 nm wavelengths.

Mechanochemical Synthesis of Visible Light Sensitive Titanium Dioxide Photocatalyst

Phase transition of anatase nanoparticles into the phases TiO2-II and rutile under grinding was studied. The addition of ammonium carbamate to the reaction mixture inhibits the phase conversion and the cold welding of particles. The UV-visible absorption spectrum showed narrowing the band gap width after grinding with an ammonium carbamate additive resulting in shift of the light absorption of the ground sample towards the visible region. By EPR, intensive formation of OH• radical at irradiation of the sample with both UV (λ > 300 nm) and visible (λ > 435 nm) light was observed. High photocatalytic activity of the ground sample in visible light region was demonstrated also by measurement of kinetics of the photocatalytic decomposition of 4-chlorophenol.

Constant rate thermal analysis of a dehydrogenation reaction

The Constant Rate Thermal Analysis (CRTA) procedure has been employed for the first time to study the kinetics of MgH2 dehydrogenation by thermogravimetry under high vacuum. CRTA implies controlling the temperature in such a way that the decomposition rate is maintained constant all over the process, employing the mass change as the experimental signal proportional to the reaction rate. The CRTA curves present a higher resolution power to discriminate the kinetic model obeyed by the reaction in comparison with conventional heating rate curves. The combined kinetic analysis has been applied to obtain the kinetic parameters, which show that MgH2 decomposition under high vacuum obeys first-order kinetics (F1). It has been proposed that the dehydrogenation of MgH2 under high vacuum takes place by instantaneous nucleation in the border line of the bidimensional crystallites followed by the growth of the nuclei.

ArF Laser-induced Deposition of Carbon Encapsulated CoFe Nanoparticles

Iron-cobalt based intermetallic alloys exhibit interesting magnetic properties with high Curie temperatures, the highest saturation magnetisations, high permeability and low losses. Such alloys are useful for technological applications where small volume and high magnetic performances are important. Effects of alloy composition on properties are known and were studied in both bulk and nanoparticles. We present here preparation of carbon encapsulated Co/Fe alloy nanoparticles by laser-induced decomposition of iron/cobalt/carbon precursors in the gaseous phase. Three volatile compounds were used as precursors: iron pentacarbonyl Fe(CO)5, cobalt nitrosyl tricarbonyl Co(NO)(CO)3 and acetylene C2H2. The single pulse of ArF laser (193 nm) explosively decomposes the reaction mixture and results in efficient formation of carbon encapsulated alloy nanoparticles along with gaseous products. Metal precursors are depleted almost completely and CO as the main gaseous product of the decomposition was observed. Minor products (benzene, vinylacetylene, diacetylene) are formed as a result of iron catalyzed dehydropolymerization of acetylene.