Georgi Avdeev

Effects of mechanical activation on structure and photocatalytic properties of ZnO powders

AbstractZinc oxide (ZnO) was mechanically activated in air using a planetary ball mill using varying milling speeds and time. The obtained samples were analyzed by X-ray diffraction analysis (XRD), scanning electron microscopy (SEM) and UV-Vis diffusion reflectance spectroscopy. The photocatalytic activity of the mechanically treated ZnO powders were investigated in the reaction of Malachite Green (MG) degradation in aqueous solution under UV-light irradiation. A decrease in the crystallite size (from 90 to 10 nm) accompanied by an increase of microstrains and lattice parameters were established applying different milling speeds. The agglomeration of the particles was observed by SEM analysis. The absorption spectra of the initial and mechanically activated ZnO samples show shifting of the band position from 360 to 330 nm, which can be related to decrease in the crystalline size. The ZnO powders activated at lower milling speeds (for shorter time intervals) exhibit highest photocatalytic activity.

Au nanostructure fabrication by pulsed laser deposition in open air: Influence of the deposition geometry

We present a fast and flexible method for the fabrication of Au nanocolumns. Au nanostructures were produced by pulsed laser deposition in air at atmospheric pressure. No impurities or Au compounds were detected in the resulting samples. The nanoparticles and nanoaggregates produced in the ablated plasma at atmospheric pressure led to the formation of chain-like nanostructures on the substrate. The dependence of the surface morphology of the samples on the deposition geometry used in the experimental set up was studied. Nanocolumns of different size and density were produced by varying the angle between the plasma plume and the substrate. The electrical, optical, and hydrophobic properties of the samples were studied and discussed in relation to their morphology. All of the nanostructures were conductive, with conductivity increasing with the accumulation of ablated material on the substrate. The modification of the electrical properties of the nanostructures was demonstrated by irradiation by infrared light. The Au nanostructures fabricated by the proposed technology are difficult to prepare by other methods, which makes the simple implementation and realization in ambient conditions presented in this work more ideal for industrial applications.

Electrodeposition of Ni–Cu alloys at high current densities: details of the elements distribution

We report results from an experimental and modeling study of the far-from-equilibrium electrodeposition of Ni–Cu alloys by changing systematically the current density with focus on the surface structuring and surface content of the elements. General finding is that Cu prevails in the convex regions of the deposit, while Ni prevails in the concave ones. No deposition of a monophase is observed by the XRD analyses—in all samples are registered two phases under the form of solid solutions of Cu and Ni. With the increasing current density, the percentage of the major component increases towards deposition of pure Cu and Ni. Some oscillations of the overpotential with the current density are correlated with the Cu content in the Cu-rich solid solution. An original model based on Cellular Automata (CA) rationalizes the hypothesis that during their simultaneous deposition, Cu preserves the diffusion-limited growth mode, while the Ni discharges in a kinetically controlled one. The model is expected to be valid for lower current densities.

Influence of ThO2 on the photocatalytic activity of TiO2

AbstractMicrocomposites consisting of TiO2 (or Ce-doped TiO2) and ThO2 (0.5–2% of the TiO2 mass) are produced by sol-gel synthesis of TiO2 in presence of ThO2. X-ray diffraction study reveals the effects of ThO2 (compared to the ThO2-free TiO2, obtained by the same method) on the anatase interplanar distances, crystallites size and phase composition. The photocatalytic tests in presence of the composites under UV irradiation reveal an increase of the Malachite Green degradation rate constant. The effect depends on the Th relative content, temperature of annealing of the catalyst and addition of other doping agent. The highest photocatalytic activity is observed for TiO2 obtained at 550°C and containing 1% ThO2. The composite exhibits activity in dark, also. The presence of Ce4+ ions is not an obligatory requirement for the realization of the ThO2 effect. The reported results suggest that the radioactivity of the Th and/or its decay products is one of the main factors responsible for the increased photocatalytic activity of TiO2.

Optical and vibrational spectra of Bi 1.8 Fe 1.2(1 – x ) Ga 1.2 x SbO 7 solid solutions with pyrochlore-type structure

Optical and vibrational spectra of Bi1.8Fe1.2(1 – x)Ga1.2xSbO7 solid solutions with pyrochlore structure have been studied. It has been shown that the compounds have strongly disordered structure. It has been established that a decrease in iron content causes a marked shift of Bi1.8Fe1.2(1 – x)Ga1.2xSbO7 absorption edge. According to assessment of band gap width, Bi1.8Ga1.2SbO7 is the most wide-band compound (2.90 eV), while Bi1.8Fe1.2SbO7 is the most narrow-band compound (1.88 eV).

Crystallization and growth morphology of barium titanate and fresnoite from a glass with the composition 20.1Na2O·23.1BaO·23TiO2·9.8B2O3·21SiO2·3Al2O3

A glass with the composition 20.1Na2O·23.1BaO·23TiO2·9.8B2O3·21SiO2·3Al2O3 (in mol%) was synthesized using a traditional glass melting technique. Its thermal properties were characterized by differential thermal analysis (DTA), and appropriate time–temperature schedules were applied in order to prepare glass-ceramics containing barium titanate. X-ray diffraction (XRD) analysis showed the formation of cubic barium titanate and fresnoite at an annealing temperature of 650 °C and for times t ≥ 3 h, while annealing for 1 h resulted in the sole crystallization of barium titanate. Supplying longer crystallization times resulted in the additional formation of increasing fresnoite concentrations. The crystallization behavior of the glass is characterized using electron microscopy techniques, i.e. energy dispersive X-ray spectroscopy (EDXS) and electron backscatter diffraction (EBSD), which reveal the simultaneous growth of fine, densely-branched barium titanate and fresnoite crystals.

Fabrication of ZnO nanostructures by PLD

Different types of ZnO nanostructures were fabricated on metal (Au or Ag) coated silicon substrates by applying the pulsed laser deposition (PLD) method. The samples were prepared at substrate temperatures in the range of 300 – 650 °C, oxygen pressure of 5 Pa, and laser fluence ≤ 1 J.cm-2– process parameters usually used for thin-film deposition. The metal layer is essential for the preparation of nanostructures. The nanostructures grown at different substrate temperatures showed obvious morphological differences. The substrate temperature increase led to changes in the morphology of the nanostructures from nanowhiskers to nanowalls when a thin Au layer was used. It was also observed that the type and thickness of the metal layer affect the morphology of the nanostructure.

Preparation of metal nanorods substrates for SERS application

Pulsed laser deposition was used together with a glancing angle deposition technique in order to produce suitable metal substrates for surface enhanced Raman scattering (SERS) applications. The surface morphology, crystallinity, optical properties, and enhancement factor of these substrates were investigated. Rodamine-6G was used as a SERS active molecule pumped at 785 nm. The Au nanorods with average lengths of 50 – 70 nm having diameter in the range of 10 – 20 nm exhibited a SERS enhancement factor of 105. The as-prepared metal substrates were found to meet the criteria of a sensitive, uniform, and easily produced SERS substrate.

Synthesis and some aspects of the formation mechanism of carbon structures under hydrothermal conditions

The formation of carbon structures upon the reactions of organic compounds with strong mineral acids under hydrothermal conditions was studied. The reactions were found to give amorphous carbon, microcrystalline graphite, nanodiamonds, diamond-like carbon, diamonds, C60 and C70 fullerenes, and carbolite. Hydrocarbons of various compositions including aromatics and naphthenes were detected among the reaction products. A possible mechanism of the formation of carbon structures with sp2 bond hybridization under hydrothermal conditions was proposed.

Growth, composition and dielectric properties of Pb3Ni1.5Mn5.5O15 single crystal

Single crystals of Pb3Ni1.5Mn5.5O15 were successfully grown by the high temperature solution growth method. Their composition was determined by energy dispersive X-ray analysis. The phase homogeneity of the grown crystals was examined and confirmed by powder X-ray phase analysis. The structure was characterized by X-ray single crystal diffractometry and was indexed in the trigonal space group P-3c1 (No. 165) with lattice parameters a = 9.9142(2) A and c = 13.4923(18) A. The dielectric properties of Pb3Ni1.4Mn5.6O15 were investigated in the temperature range 150 – 500 K. The AC -measurements were carried out for 3 different frequencies: 10 kHz, 100 kHz and 1 MHz. From the DC measurements thermal activation energy was estimated.