Irina Stambolova

Preparation of ceria films by spray pyrolysis method

Abstract The deposition of CeO2 films on fused-silica substrates by spray pyrolysis of a water–ethanol solution of a cerium nitrate precursor has been studied. Polycrystalline films have been obtained at a substrate temperature of 300–450°C after annealing of the deposit in air at temperatures in the range 350–500°C. It has been established that the best uniform ceria films with nanometric scale grains are prepared at a substrate temperature of 400°C with 0.5 h annealing of the deposit at 500°C. At lower spraying temperatures large CeO2 crystallites have been observed on the film surface along with the fine grains. When the substrate temperature exceeds 400°C, numerous cracks caused by thermal stresses appear in the films.

Visible light photocatalytic activity of TiO2 deposited on activated carbon

Four photocatalyst samples, prepared from beech sawdust, were synthesized by an original method, combining pyrolysis and impregnation — two of them: TiO2 + activated carbon and other two — only activated carbon. The pyrolysis process has been carried out at two different temperatures — 680°C and 830°C. The prepared samples were characterized by a series of methods — XRD, BET, SEM and DTA/TG. The most important result was achieving visible light photocatalytic activity with an azo dye pollutant for both materials. The TiO2/AC-680°C sample demonstrated higher activity under visible light illumination than the TiO2/AC-830°C sample. The visible light activity was attributed to the active carbon component in the composite materials, which was evidenced by the photocatalytic tests with bare carbon (without any TiO2) manifesting visible light activity. The AC-680°C carbon was superior to the AC-830°C under visible illumination probably due to its higher specific surface area and porous texture. UV-light testing of the photocatalytic activity revealed that the TiO2/AC-680°C sample was higher than that of the TiO2/AC-830°C under polychromatic UV-A illumination (320–400 nm with a maximum at l = 365 nm). The TiO2/AC-680°C sample was also more efficient with the monochromatic UV-C illumination (l = 254 nm).Graphical abstract

Sprayed nanostructured TiO2 films for efficient photocatalytic degradation of textile azo dye

Spray pyrolysis procedure for preparation of nanostructured TiO(2) films with higher photocatalytic effectiveness and longer exploitation life is presented in this study. Thin films of active nanocrystalline TiO(2) were obtained from titanium isopropoxide, stabilized with acetyl acetone and characterized by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and X-ray diffraction (XRD). The activity of sprayed nanostructured TiO(2) is tested for photocatalytic degradation of Reactive Black 5 dye with concentrations up to 80 ppm. Interesting result of the work is the reduction of toxicity after photocatalytic treatment of RB5 with TiO(2), which was confirmed by the lower percentage of mortality of Artemia salina. It was proved that the film thickness, conditions of post deposition treatment and the type of the substrate affected significantly the photocatalytic reaction. Taking into account that the parameters are interdependent, it is necessary to optimize the preparation conditions in order to synthesize photocatalytic active films.

Microwave-assisted and conventional sol-gel preparation of photocatalytically active ZnO/TiO2/glass multilayers

AbstractFor the first time a combination of microwaves and/or the conventional treatment method was used to dry and heat multilayered sol-gel ZnO/TiO2/glass structures. Compact or porous TiO2 films were deposited as a bottom layer, covered with a ZnO film.The structures were characterized by X-ray Diffraction (XRD), Energy Dispersive X-ray Spectroscopy (EDX) and Scanning Electron Microscopy (SEM). Only peaks of wurtzite ZnO crystalline phase were registered on the X-Ray diffractograms.The microwave irradiation leads to a formation of poorly crystallized multilayers with very small crystallites and enhanced surface roughness. This results in a better photocatalytic activity of these structures than the structures of the samples treated conventionally.It was established that the morphology of the bottom titania layer affects the reaction of photocatalytic degradation of Malachite Green dye (MG). The structures with the compact bottom TiO2 films showed higher activities than those on porous TiO2 films.This study offers an energy saving method of producing ZnO/TiO2/glass multilayered structures of various morphologies and pronounced photocatalytic properties. The method does not involve any calcination step, normally applied to achieve a good degree of crystallization. This makes the method suitable for protecting substrates of low thermal stability.

Spray pyrolysis deposition of polycrystalline magnesia films and their use as buffer layers in Bi(Pb)-Sr-Ca-Cu-O/MgO/Al2O3 (or glass ceramics) structures

The deposition of MgO films on Al2O3 and glass ceramic substrates by spray pyrolysis of a water-ethanol magnesium nitrate precursor solution has been studied. Dense polycrystalline films have been obtained by repeated pyrolysis at 300–350 °C followed by annealing of the deposit in air at 970 °C. It has been established that the finest grains obtained under these experimental conditions correspond to the films deposited on glass ceramic substrates. It has been shown that the MgO films can be useful buffer layers preventing the interaction between the above substrates and the BiSr-Ca-Cu-O films deposited on them by spray pyrolysis of nitrates.

Synthesis and Catalytic Activity of Silver-Coated Perlite in the Reaction of Ozone Decomposition

Samples of Bulgarian perlite mineral deposit (natural alumino-silicate glass) in the Rhodope mountain were coated with Ag particles via a spray pyrolysis method. This method allows uniform distribution of the silver particles on the outer shell surfaces. SEM, XPS, FT-IR, and XRD methods were applied for characterization of the samples. The XRD investigations of the pristine samples revealed the formation of metallic silver and a small quantity of Ag2O3 phase. The presence of silver was also confirmed by XPS analysis. It was established that Ag/perlite composite prepared by spray pyrolysis is a promising catalyst for ozone decomposition. The high catalytic activity is preserved during the complete course of the catalytic reaction. After the catalytic activity test some structural and phase changes in the samples were observed. The XRD patterns of the ozonated samples proved the presence of metallic silver, AgO and Ag2O3 phases. The FT-IR analyses revealed that some bands, assigned to Si-O-Si stretching modes, were shifted to lower frequencies after the ozone decomposition test. In general, the spray pyrolysis method turned out to be a very suitable technique for preparation of highly active silver-coated perlite catalyst for efficient ozone removal.

The effects of substitutions and type of substrates on the morphology of La0.8A0.2Cu0.5Mn0.5O3 (A = Ca, Sr, Ba) thin films

Abstract Thin La0.8A0.2Cu0.5Mn0.5O3 (A=Ca, Sr, Ba) films have been deposited using the sol-gel technique on Al2O3, glass ceramic and single crystal 〈101〉 quartz substrates. It was found that the morphology of pure and Ca containing films is affected by the type of substrate used. The films deposited on Al2O3 have good adhesion and uniform structure due to the rough surface of the substrate and its temperature expansion coefficient which has a value close to the corresponding one of the film. The morphology of Sr and Ba containing films is affected by both the type of substrate and substituting elements. The bigger ionic radius of these elements limits their incorporation in the crystal lattice and leads to the appearance of secondary phases and inhomogeneous films.

Effect of post-synthesis acid activation of TiO2 nanofilms on the photocatalytic efficiency under visible light

Nanosized TiO2 films were deposited by spray pyrolysis and thermally treated at 400oC. Then the films were dipped in 1M aqueous solution of HCl. The activated samples were divided into two parts - one part was dried (A) and another was annealed (AT) in air. The photocatalytic degradation of Reactive Black (RB5) textile dye under visible light was tested. The following instrumental methods: X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) were applied for the phase and surface characterization of obtained samples. According to Raman and XRD analyses all films are anatase. The XRD showed that activated films are better crystallized than non activated TiO2 film. The presence of chlorine at 200.3 eV was registered for acid activated samples by X-Ray photoelectron spectroscopy. The acidic activated films exhibited higher rate of dye photodegradation than that of the reference TiO2 sample. The photocatalytic efficiency decreases in the order A > AT> non activated TiO2 films. The degradation rate constant for acid activated films is two times higher than those of the reference film. The hydroxyl content in TiO2 acidic activated films is greater than that of the non- activated films, which results in significant increase in the photocatalytic activity. In addition, the presence of chlorine may also lead to enhancement in efficiency.

MANGANESE-DOPED ZINC OXIDE NANOPOWDERS FOR PHOTOCATALYTIC DECOLORATION OF REACTIVE BLACK 5 TEXTILE DYE

Zinc oxide photocatalysts doped with manganese (0.04‐1 at.%) were synthesized via precipitation technique. Their characteristics were studied by Xray diraction (XRD) and BET analysis. The calculated mean size of the crystallites was in the range of 31‐45 nm. Manganese doping of ZnO samples results in slight decrease in crystalline size. Increasing the Mn content in ZnO powders results in a lower photocatalytic activity. The pure ZnO nanopowders decolorate Reactive Black 5 dye under ultraviolet light to almost 92% for 2 h.