Snejana Bakardjieva

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 .

TiO2-graphene oxide nanocomposite as advanced photocatalytic materials

BackgroundGraphene oxide composites with photocatalysts may exhibit better properties than pure photocatalysts via improvement of their textural and electronic properties.ResultsTiO2-Graphene Oxide (TiO2 - GO) nanocomposite was prepared by thermal hydrolysis of suspension with graphene oxide (GO) nanosheets and titania peroxo-complex. The characterization of graphene oxide nanosheets was provided by using an atomic force microscope and Raman spectroscopy. The prepared nanocomposites samples were characterized by Brunauer–Emmett–Teller surface area and Barrett–Joiner–Halenda porosity, X-ray Diffraction, Infrared Spectroscopy, Raman Spectroscopy and Transmission Electron Microscopy. UV/VIS diffuse reflectance spectroscopy was employed to estimate band-gap energies. From the TiO2 - GO samples, a 300 μm thin layer on a piece of glass 10×15 cm was created. The photocatalytic activity of the prepared layers was assessed from the kinetics of the photocatalytic degradation of butane in the gas phase.ConclusionsThe best photocatalytic activity under UV was observed for sample denoted TiGO_100 (k = 0.03012 h-1), while sample labeled TiGO_075 (k = 0.00774 h-1) demonstrated the best activity under visible light.

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.

Synthesis and visible light photocatalytic activity of nanocrystalline PrFeO 3 perovskite for hydrogen generation in ethanol–water system

AbstractNanocrystalline PrFeO3 perovskite type orthoferrite was synthesized at 700°C by using three different synthesis methods, namely sol–gel, template and combustion method. The synthesized materials were characterized by XRD, BET-SA, SEM, HRTEM, XPS, FTIR and UV-DRS techniques to understand their physico-chemical properties. Characterization data reveal the formation of nanocrystalline PrFeO3 perovskite composition with improved physical properties, possibly due to lower synthesis temperature used. PrFeO3 synthesized by sol–gel method consists of crystallite size of about 20 nm with absorption maxima at 595 nm wavelength in visible light range. This photocatalyst shows hydrogen generation of about 2847 μmol.g−1.h−1, under visible light irradiation in ethanol–water system. The photocatalyst was further investigated for various operational parameters such as photocatalyst dose variation, illumination intensity, time, etc. in a view to optimize the hydrogen generation as well as to understand mechanistic aspects. This material appears to follow a semiconductor type mechanism for ethanol-assisted visible light photocatalyic water-splitting and can also be an interesting candidate to develop hetero-junction type photocatalysts. ᅟPrFeO3–type perovskite was synthesized by sol–gel, template and combustion methods. PrFeO3 synthesized by sol–gel method consists of crystallite size of about 20 nm with absorption maxima at 595 nm wavelength in visible light range. This photocatalyst shows hydrogen generation of about 2847 µmol.g-1.h-1, under visible light irradiation in ethanol-water system, and follows semiconductor type mechanism with alcohol acting as sacrificial donor.

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.

Aerogel nanoscale magnesium oxides as a destructive sorbent for toxic chemical agents

An autoclave hypercritical drying procedure has been used to prepare precursors of MgO from Mg(OCH3)2. This material was prepared with a specific surface area of 1200 m2g1. The dehydrated materials consisted of much smaller crystallites than conventionally prepared MgO and were free of OCH3 groups. The precursors and samples of magnesium oxide were taken for experimental evaluation of their reactivity with mustard. The largest percentage of the conversion mustard into non-toxic products after the elapse of the reaction was 77%.

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.

Polymer-stabilized nano-sized tellurium films by laser-induced chemical vapour co-deposition process

UV laser-irradiation of a gaseous mixture of dimethyl tellurium and 1,3-disilacyclobutane induces concurrent photolysis of both compounds. Chemical changes taking place are due to expulsion of elemental tellurium from dimethyl tellurium and formation of short-lived and fast polymerizing silene from 1,3-disilacyclobutane. The co-photolysis process results in chemical vapour co-deposition of a nano-sized tellurium–polycarbosilane composite that contains amorphous nano-structures of tellurium stabilized against oxidation by organosilicon polymer.

Material Effect in the Nuclear Fuel–Coolant Interaction: Analyses of Prototypic Melt Fragmentation and Solidification in the KROTOS Facility

Abstract During a severe accident sequence in a pressurized light water reactor, the hot (~3000 K) molten materials (corium) coming from the degraded reactor core may generate a violent interaction if they come in contact with water. This melt-water interaction, called fuel-coolant interaction (FCI), may damage the structures and threaten the reactor integrity if there is a steam explosion. FCI occurs generally in two phases: a premixing phase, during which the molten corium jet is fragmented into large droplets and mixed with the water, and the explosion phase, during which the vapor film that has developed around the fuel droplets is destabilized and the droplets are finely fragmented. The presented work covers a set of experimental studies describing the morphology and nature of the solidified materials after interaction with water. Debris from experiments performed in the KROTOS (Commissariat à l’Énergie Atomique, Cadarache, France); PREMIX, ECO (Karlsruhe Institute of Technology, Karlsruhe, Germany); and MISTEE (Royal Institute of Technology, Stockholm, Sweden) facilities have been characterized by metallographic, analytical, and microscopic techniques. These post-test analyses are able to provide important information on the solidification path and other main phenomena involved during FCI. It was found that the behavior of metallic and oxide melts differs significantly from the standpoint of debris morphology. Oxide melts that underwent simple coarse fragmentation showed spherical or angular rocklike shape, unlike metallic melts. A statistical analysis was performed on the debris from the KROTOS tests; a data set of particles was described by the circularity, solidity, and porosity. The mechanism of water ingression (Kim and Corradini) inside the melt droplet was observed to be the key mechanism of fine (secondary) fragmentation. The particles participating in fine “thermal” fragmentation have significantly higher porosity, up to ~30% for prototypic corium in the KROTOS facility. It was calculated that only a part of the premixed melt participates in fine fragmentation, i.e., about one-third of the melt mass for the KROTOS tests using UO2-ZrO2 mixture.