Browsing by Author "Iordanova R."
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Item A study of the effect of citric acid on the crystallinity of ZnO/TiO2 nanopowders(2013-12-17) Shalaby A.; Bachvarova-Nedelcheva A.; Iordanova R.; Dimitriev Y.It is well known that the citric acid is used as a chelating agent which forms poly (basic acid) chelates with the metal cations in a nano-powder synthesis route. The aim of this investigation is to study the influence of the citric acid on the crystallization process in obtaining ZnO/TiO2 nano-composites. Samples have been synthesized by the aqueous sol-gel method applying two different schemes with and without citric acid, respectively. Several compositions containing different TiO2 (10, 90 mol %) and ZnO (10, 90 mol %) amounts were investigated. The as-prepared powders were calcinated from 100 to 500°C. The phase formation and structural transformation at every step of the synthesis route were followed by X-ray phase analysis and IR spectroscopy and the obtained results were compared.Item Combustion gel method for synthesis of nanosized ZnO/TiO2 powders(2013-05-21) Gegova R.; Dimitriev Y.; Bachvarova-Nedelcheva A.; Iordanova R.; Loukanov A.; Iliev T.Nanosized powders in the ZnO-TiO2 system were synthesized by combustion gel method using different precursors: Zn(NO3)2.6H2O, Ti(OC2H5)4 and Ti(OC4H9)4. During the combustion process only one phase, ZnO was obtained. The phase formation and structural transformation at every step of routes for the synthesis were followed by XRD and IR analysis. Our previous investigation in the same system, applying the hydrolytic sol-gel method showed that the type of precursor and the order of adding the components influenced on the microstructure of the final product in the ZnO-TiO2 system, which was proved also in this study. The obtained powders possess photocatalytical activity against organic dye (Malachite green) in the UV and Vis region.Item Effect of B2O3 on the Structure, Properties and Antibacterial Abilities of Sol-Gel-Derived TiO2/TeO2/B2O3 Powders(2023-10-01) Bachvarova-Nedelcheva A.; Iordanova R.; Stoyanova A.; Georgieva N.; Nemska V.; Foteva T.This paper studies the influence of B2O3 on the structure, properties and antibacterial abilities of sol-gel-derived TiO2/TeO2/B2O3 powders. Titanium(IV) butoxide, telluric(VI) acid and boric acid were used as precursors. Differences were observed in the degree of decomposition of Ti butoxide in the presence of H3BO3 and H6TeO6 acids. The phase transformations of the obtained gels in the temperature range of 200–700 °C were investigated by XRD. Composite materials containing an amorphous phase and different crystalline phases (metallic Te, α-TeO2, anatase, rutile and TiTe3O8) were prepared. Heating at 400 °C indicated a crystalline-to-amorphous-phase ratio of approximately 3:1. The scanning electron microscopy (SEM) analysis showed the preparation of plate-like TiO2 nanoparticles. The IR results showed that the short-range order of the amorphous phases that are part of the composite materials consists of TiO6, BO3, BO4 and TeO4 structural units. Free B2O3 was not detected in the investigated compositions which could be related to the better connectivity between the building units as compared to binary TiO2/B2O3 compositions. The UV-Vis spectra of the investigated gels exhibited a red shift of the cut-off due to the presence of boron and tellurium units. The binary sample achieved the maximum photodegradation efficiency (94%) toward Malachite green dye under UV irradiation, whereas the ternary sample photoactivity was very low. The compositions exhibited promising antibacterial activity against E. coli NBIMCC K12 407.Item Eu3+ doped glasses in the Bi2O3-B2O3-WO3-Al2O3 system(2015-01-01) Aleksandrov L.; Iordanova R.; Khanna A.; Georgiev N.; Dimitriev Y.The structure of many Eu3+ doped Bi2O3-B2O3-WO3-Al2O3 glasses of a different composition is studied using infrared spectroscopic technique (IR). It is found that distorted BiO6 (of a band at 480 cm-1), BO3 (of bands at 1390, 1230, 620 cm-1), BO4 (of a band at 1020 cm-1) and WO4 polyhedra (of a band at 880 cm-1) are the main building units forming the amorphous networks. WO6 groups exist in the glass network of a high WO3 content (of bands at 880 cm-1, 620 cm-1, and a shoulder at 670 cm-1). WO3 does not stimulate formation of BO4 structural units. According to the differential thermal analysis (DTA) the glass transition temperature (Tg) of the investigated glasses is situated in the range of 400°C - 485°C. The DTA curves of all glasses show two exothermic peaks corresponding to the formation of two crystal phases. Photoluminescence emissions due to the 4f transitions 5D0→7Fj (j=0-4) of Eu3+ ions are observed. The addition of WO3 to the glass compositions decreases the emission intensity of investigated glasses.Item Glass formation and structure of glasses in the ZnO-WO3-La2O3-Al2O3 system(2015-01-01) Ataalla M.; Milanova M.; Aleksandrov L.; Iordanova R.; Staneva A.; Dimitriev Y.The glass formation region in the section 90(xZnO.yWO3.zLa2O3).10Al2O3 of a constant Al2O3 content of 10 mol % and varying amounts of WO3, ZnO and Nd2O3 from ZnO-WO3-La2O3-Al2O3 system is determined by the melt quenching technique (with cooling rates of 10 Ks-1 - 102 Ks-1). The glasses are obtained from compositions of a high WO3 content (60 mol % - 75 mol %). The amorphous state of the samples is verified by X-ray diffraction (XRD). The thermal parameters of the investigated glasses are obtained by differential thermal analysis (DTA). The glass transition temperature (Tx) is in the range 440°C - 510°C, while the crystallization temperature (Tg) varies from 520°C to 650°C. The glasses are characterized by a good thermal stability in respect to crystallization. ΔT varies in the range of 60°C - 135°C (ΔT = Tx-Tg). The glasses structure is studied by IR spectroscopy. It is established that the amorphous network is built up mainly by WO6 octahedra connected through W-O-W bridges.Item INFLUENCE OF Nb2O5 AND B2O3 ON THE PHOTOCATALYTIC AND ANTIBACTERIAL ACTIVITY OF SOL-GEL DERIVED TiO2 NANOPОWDERS(2024-01-01) Bachvarova-Nedelcheva A.; Iordanova R.; Georgieva N.; Nemska V.; Foteva T.; Stoyanova A.TiO2 based powders containing B2O3 and Nb2O5 were obtained through an aqueous sol-gel method. The as-prepared gels were step wisely heated in the temperature range 200°C - 500°C and subsequently characterized by XRD, IR and UV-Vis analysis. The TiO2 (anatase) is the single crystalline phase which has been detected up to 500°C. Photocatalytic tests showed that the investigated samples possess photocatalytic activity toward Malachite green organic dye and TiO2/Nb2O5 exhibited higher photocatalytic activity than TiO2/B2O3 sample. The compositions exhibited good antimicrobial activity against E. coli NBIMCC K12 407 and Bacillus subtilis NBIMCC 3562.Item PHOTOCATALYTIC AND ANTIBACTERIAL ASSESSMENT OF SOL - GEL DERIVED TiO2/TeO2/ZnO POWDERS(2022-01-01) Bachvarova-Nedelcheva A.; Iordanova R.; Georgieva N.; Nemska V.; Stoyanova A.Two ternary gel compositions containing simultaneously TiO2, TeO2 and ZnO were selected for detailed investigations. The phase transformations of the gels upon the heating were investigated by XRD analysis and it was established that the obtained composites contain an amorphous phase and different crystalline phases: TiO2 (anatase), TiO2 (rutile), a-TeO2 and ZnTeO3, depending on composition. The IR results showed that the short range order of the amorphous phases which are part of the composite materials consist of TiO6, ZnO4 and TeO4 structural units. By UV-Vis spectroscopy was measured the absorption edge of the gels which is about 363 nm and the calculated Eg values are about 3.41 eV. The performed photocatalytic tests showed that the samples possess photocatalytic activity toward Malachite green organic dye. Both compositions exhibited good antimicrobial activity against E. coli K12.Item Photocatalytic properties of ZnO/TiO2 powders obtained via combustion gel method(2013-03-01) Bachvarova-Nedelcheva A.; Iordanova R.; Stoyanova A.; Gegova R.; Dimitriev Y.; Loukanov A.The present study is a continuation of our previous investigations on the ZnO-TiO2 system. By applying different sol-gel methods we proved that the type of precursor and the order of adding the components influence the microstructure of the final product. This study focuses on the combustion sol-gel synthesis and photocatalytic properties of nanosized (∼7-20 nm) ZnO-TiO2 powders. The photocatalytic tests were performed toward two model organic dyes, Malachite Green and Reactive Black 5, in the UV and Vis region. For synthesis of ZnO/TiO2 powders, different precursors such as Zn(NO3)2·6H2O, Zn(CH 3COO)2·2H2O, Ti(OC2H 5)4 and Ti(OC4H9)4 were used. During the combustion process various phases (ZnO, TiO2 - anatase and rutile, ZnTiO3) were obtained. The structure and morphology of the resulting particles were characterized by XRD and SEM analysis. All samples exhibited a good photocatalytic activity in both UV and Vis regions. © 2012 Versita Warsaw and Springer-Verlag Wien.Item Photocatalytic properties of ZnO/TiO2 powders obtained via combustion gel method(2013-03-01) Bachvarova-Nedelcheva A.; Iordanova R.; Stoyanova A.; Gegova R.; Dimitriev Y.; Loukanov A.The present study is a continuation of our previous investigations on the ZnO-TiO2 system. By applying different sol-gel methods we proved that the type of precursor and the order of adding the components influence the microstructure of the final product. This study focuses on the combustion sol-gel synthesis and photocatalytic properties of nanosized (∼7-20 nm) ZnO-TiO2 powders. The photocatalytic tests were performed toward two model organic dyes, Malachite Green and Reactive Black 5, in the UV and Vis region. For synthesis of ZnO/TiO2 powders, different precursors such as Zn(NO3)2·6H2O, Zn(CH 3COO)2·2H2O, Ti(OC2H 5)4 and Ti(OC4H9)4 were used. During the combustion process various phases (ZnO, TiO2 - anatase and rutile, ZnTiO3) were obtained. The structure and morphology of the resulting particles were characterized by XRD and SEM analysis. All samples exhibited a good photocatalytic activity in both UV and Vis regions. © 2012 Versita Warsaw and Springer-Verlag Wien.Item Sol-Gel Obtaining of TiO2/TeO2 Nanopowders with Biocidal and Environmental Applications(2023-02-01) Bachvarova-Nedelcheva A.; Iordanova R.; Naydenov A.; Stoyanova A.; Georgieva N.; Nemska V.; Foteva T.TiO2/TeO2 powders were obtained by an aqueous sol-gel method. Telluric acid (H6TeO6) and titanium butoxide were used as precursors. The as-prepared gel was step-wisely heated in the temperature range 200–700 °C and subsequently characterized by XRD, IR, and UV-Vis analysis and SEM. Mixtures containing TiO2 (anatase), α-TeO2 (paratellurite), and TiTe3O8 were established by XRD as final products, depending on heating temperature. The thermal stability of the obtained gels in the temperature range 100–400 °C was investigated. It was found by IR spectroscopy that the samples heated up to 300–400 °C consist mainly of an organic–inorganic amorphous phase which is transformed into an inorganic one above these temperatures. The microstructure of the gels was verified by scanning electron microscopy (SEM). The photocatalytic degradation of the synthesized nanopowders toward Malachite green organic dye (MG) was examined in order to evaluate the potential applications for environmental remediation. The prepared TiO2/TeO2 samples showed up to 60% decoloration efficiency after 120 min exposure to UV-light. The composition exhibited good antimicrobial activity against E. coli K12. The properties of the obtained material were investigated by the reactions of complete catalytic oxidation of different alkanes and toluene, and it could be suggested that TiO2/TeO2 powders are promising material for use as an active phase in environmental catalysts.Item Synthesis and characterization of glass doped reduced graphene oxide(2015-01-01) Shalaby A.; Aleksandrov L.; Iordanova R.; Staneva A.; Dimitriev Y.Graphene is very attractive material which has high conductivity and good electrical properties. By using glass as a matrix to incorporate the graphene, it is possible to improve the conductivity and the electrical properties of the glass. In our previous study we synthesized composite material by adding of reduced graphene oxide (RGO) to silica. In this work we continue our experiments to synthesize a composite material on a glass basis in the PbO-ZnO-B2O3 system. The differential thermal analysis of the present glass showed that the glass transition temperature (Tg) is situated at ca 415°C. The synthesized glass was doped with 20 % of reduced graphene oxide (RGO). A mixture of glass with RGO was heated at different temperature values for different time taking into consideration the glass transition temperature. The characterization is performed by X-ray phase analysis, DTA and SEM analysis. It was found according to the XRD results that graphene was kept inside the glass matrix till 600°C.Item Synthesis of amorphous and crystalline LaBWO6 using mechanochemical activation(2015-01-01) Gancheva M.; Aleksandrov L.; Iordanova R.; Dimitriev Y.This article presents data referring to the mechanochemical activated synthesis of amorphous and crystalline LaBWO6. A stoichiometric mixture of La2O3, B2O3 and WO3 of a 1:1:2 molar ratio was subjected to intense mechanical treatment in air using a planetary ball mill (Fritsch - Premium line - Pulversette No. 7). The obtained samples were analyzed by XRD, DTA, IR and UV-Vis spectroscopy. The process of mechanochemical amorphization was accomplished within 20 h at a milling speed of 500 rpm at room temperature. An additional heat-treatment of the mechanochemically obtained amorphous phase was carried out in order to obtain single crystal LaBWO6 phase. The short range order of the amorphous network was defined by BO3, BO4 and WO4 structural units. Glassy LaBWO6 was obtained by us with the application of the melt quenching method prior to these experiments. The infrared and optical spectra showed that WO4 units were more distorted in the structure of the glassy LaBWO6 when compared to those of both amorphous phases. The crystallization of LaBWO6 was achieved after thermal treatment at 690°C of the mechanochemically obtained amorphous and glass composition.Item Synthesis Of Znteo3 by crystallization of glasses in TeO2-Bi2O3-ZnO-Nb2O5 system(2018-01-01) Ganev S.; Bachvarova-Nedelcheva A.; Iordanova R.; Ranguelov B.; Parvanov S.The aim of the present work is to verify the synthesis of zinc tellurite by crystallization of glasses in TeO2 - Bi2O3 - ZnO - Nb2O5 system. The investigation refers to TeO2 - based glasses containing up to 10 mol % of Nb2O5 and Bi2O3 and ZnO content raging from 5 mol % to 10 mol %. Several glasses are selected and heat treated for 6h at 500°C. The XRD analysis detects formation of several ZnTeO3, TeO2 and ZnO crystalline phases. Additional information for the formation of ZnTeO3 crystalline phase is obtained by infrared spectroscopy (IR). There is a difference in the IR spectra of the crystalline sample of 50TeO2.50ZnO (ZnTeO3) and those of the multicomponent glasses. The spectra analysis shows that the glass network consists mainly of TeO4 (tbp) units, while TeO3 (tp) are the main structural units in the crystalline ZnTeO3 phase. The microstructure of the selected crystallized samples is examined by SEM analysis.Item Synthesis, structure and optical properties of TiO2/TeO2/MnOm (M = Zn, B) gels: A comparison(2015-01-01) Gegova R.; Iordanova R.; Bachvarova-Nedelcheva A.; Dimitriev Y.We report in this study the preparation of gels of a nominal composition 50TiO2-25TeO2-25MnOm (M = Zn, B) and our attempt to verify the oxide components behaviour using Ti butoxide, Te (VI) acid, Zn acetate and boric acid as precursors. The phase and structural transformations of the prepared gels upon heat treatment in the temperature range of 200 °C - 700°C are thoroughly studied. The powdered samples still contain organics at 200°C as verified by XRD. Composites containing an amorphous phase along with different crystalline phases (elementary Te, α-TeO2, TiO2 (anatase), ZnTeO3 and TiTe3O8) are formed with temperature increase (above 300°C). The IR results obtained show that the amorphous phases (300 °C - 400°C) of the composite materials consist of TiO6, TeO4, ZnO4, BO3 and BO4 units in different ratio depending on the composition. The UV-Vis spectra of the gels prepared exhibit a red shift of the cut-off compared to pure Ti butoxide gel behavior.Item The influence of nd and sm on the structure and properties of sol-gel-derived tio2 powders(2021-07-01) Bachvarova-Nedelcheva A.; Yordanov S.; Iordanova R.; Stambolova I.; Stoyanova A.; Georgieva N.; Nemska V.TiO2 nanopowders modified by Nd and Sm were prepared using the sol-gel technique. It was found by XRD analysis that the samples containing Sm are amorphous up to 300◦C, while those with Nd preserve a mixed organic-inorganic amorphous structure at higher temperatures (400◦C). The TiO2 (rutile) was not detected up to 700◦C in the presence of both modified oxides. TiO2 (anatase) crystals found at about 400◦C in the Sm-modified sample exhibited an average crystallite size of about 25–30 nm, while doping with Nd resulted in particles of a lower size—5–10 nm. It was established by DTA that organic decomposition is accompanied by significant weight loss occurring in the temperature range 240–350◦C. Photocatalytic tests showed that the samples heated at 500◦C possess photocatalytic activity under UV irradiation toward Malachite green organic dye. Selected compositions exhibited good antimicrobial activity against E. coli K12 and B. subtilis.