Sasho Vassilev

Effect of Mg doping and MgO-surface modification on the cycling stability of LiCoO2 electrodes

Two synthetic routes including Mg doping and MgO-surface modification were applied to the preparation of LiCoO2 showing enhanced reversible cycling behaviour as cathode material in lithium ion batteries. Mg-doped LiCoO2 was obtained by the citrate precursor method in the temperature range 750–900°C. The surface of LiCoO2 was modified by coating with Mg(CH3COO)2 and subsequent heating at 600°C. XRD, chemical oxidative analysis and electron paramagnetic resonance (EPR) of Ni3+ spin probes were used to characterize the Mg distribution in LiCoO2. Substitution of Co by Mg in the CoO2-layers was found to have a positive effect on the cycling stability, while Mg dopants in LiO2-layers did not influence the capacity fade. The accumulation of MgO on the surface of LiCoO2 improves the cycling stability without loss of initial capacity.

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.

Structural investigations of the Se-Ag-I system

Glasses from the Se–Ag–I system have been investigated. Structural information is gathered based on results collected by a combination of several types of diffraction measurements. First coordination sphere at r ¼ 2:3–2:5 A termined by radial distribution function calculations can be assumed to be composed by Se–Ag and Ag–I correlations in the network units andSe–Se correlation in the Se cluster units. The interatomic d istances andaverage bondangles decrease with the introduction of additives to selenium. The Raman spectra reveal that the introduction of silver and iodine, in particular, brings about a shift of the Se chain stretching mode towards lower wave numbers (251–236 cm � 1 ). Apparently, the newly formed structure after the introduction of additives into selenium is pretty compact and is becoming more covalent with a stronger interchain interaction. Depending on the concentration of additives the breathing modes of Ag–Se and Ag–I bonds also can be seen. 2003 Elsevier B.V. All rights reserved.

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.

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.

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.

Structural Characterization of Mg Treated LiCoO2 Intercalation Compounds

Searching for electrode materials for lithium-ion batteries with improved cycling properties, two approaches have been developed recently for lithium transition metal oxides with layered crystal structure [11-10]. The first consists in doping with electrochemically inactive metal ions such as Mg, Al, Ti etc [1-6]. The second comprises surface modification with electrochemically inactive metal ions [5,7-10]. The results obtained show that the second strategy is more effective for LiCoO2, especially in the case of coating with Al2O3 [9], while doping with Mg has a positive effect on the cycling stability of LiNiO2 and Co-doped LiNiO2 [1-3,6].

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.

The precision of XRD apparatus can be assessed by accounting for secondary extinction of a single reflection

The purpose of this study is to gain insight into the operative capabilities of XRD apparatus to supply measurement data informative for its precision. One can assess apparatus precision using parameterized criteria bounded inextricably by a measurement procedure that insists particularly on a change of the incident X-ray beam intensity I0. Quantifying kinematical intensity Ikin [=PI0QS/2μ] and secondary extinction (SE) coefficient g [=kPI0S/2μ], the intensity I0 defines the level of interaction between X-radiation and crystal media. Here P is the texture factor (pole density): it represents the relative volume fraction of crystallites whose -poles contribute to reflection [1], k is the empirical extinction coefficient, and other symbols have their usual meaning (see [2] as well). The second equation above read that decrease of the level of interaction of the diffraction process is controlled by g in terms of I0 under otherwise equal conditions. Hence, the intensity measured at a level of interaction is affected by SE as strongly as higher I0 is. This is illustrated by the figure below showing profiles of 200 reflection corresponding to the main <100> component of a textured sample of silver measured at three different levels of interaction under the same time(τ)-generator current(i)factor as defined: τi= τ*i* = τ**i**. The legend inside the figure represents i-values used to cause discrete changes in the I0-intensities that reflect in the areas under profiles and integral breadths B due to respective SE effects. Whereas these measurements show a simple qualifying of the SE effects, the single reflection method [2] is a proper tool for quantification of the same data.