Authors:Oana Ştefănescu, Corneliu Davidescu, Mircea Ştefănescu, and Marcela Stoia
In this paper we present a study regarding the obtaining of iron oxides embedded in silica matrix, using a modified sol-gel
method. This method consists in the formation, inside the silica matrix, of some Fe(III)-carboxylate compounds, resulted in
the redox reaction between Fe(NO3)3 and diol.
We have synthesized four gels, starting from tetra-ethyl orthosilicate, Fe(NO3)3·9H2O and different diols: ethylene glycol, 1,2-propanediol, 1,3-propanediol and 1,4-butanediol, for a final composition 50% Fe2O3/50% SiO2. The obtained gels have been thermally treated at 130°C, when the redox reaction Fe(NO3)3-diol took place with formation of the precursors in the xerogels pores. The thermal decomposition of all four precursors
took place up to 300°C.
The samples obtained by annealing at 300, 500, 700°C of the four xerogels lead to crystalline phases inside the amorphous
silica matrix. γ-Fe2O3/SiO2 may be obtained as unique phase depending on the diols nature. The formation of the precursors inside the silica matrix and
the evolution of the crystalline phases were analyzed by thermal analysis, FTIR spectrometry and XRD.
Authors:Oana Ştefănescu, Marcela Stoia, Mircea Ştefănescu, and Titus Vlase
Hybrid organic–inorganic materials, silica–diol, were synthesized by the sol–gel process from mixtures of tetraethylorthosilicate
(TEOS) and diols: ethylene glycol (HO–CH2–CH2–OH) and 1,3 propane diol (HO–CH2–CH2–CH2–OH), in acid catalysis. The gels have been synthesized for a molar ratio H2O:TEOS = 4:1 and different molar ratios diol/TEOS: 0.25; 0.5; 0.75; 1.0; 1.25 and 1.5. The resulting gels were studied by
thermal analysis and FT-IR spectroscopy, in order to evidence the interaction of diols with silica matrix. Thermal analysis
indicated that the condensation degree increases with the molar ratio diol/TEOS until a certain value. The thermal decomposition
of the organic chains bonded within the silica network in the temperature range 250–320 °C, leaded to a silica matrix with
modified morphology. The adsorption–desorption isotherms type is different for the samples with and without diol. Thus, the
specific surface areas have values <11 m2/g for the samples without diol and >200 m2/g for the samples with diols, depending on the annealing temperature.
Authors:Mircea Stefanescu, Marcela Stoia, Oana Stefanescu, and Paul Barvinschi
This article presents a study on obtaining Ni, Zn ferrite starting from Fe(III), Ni (II), Zn (II) nitrates and some polyols: 1,2-propane diol, 1,3-propane diol and glycerol. While heating, a redox reaction takes place between nitrate anion and polyol, with formation of carboxylate type precursors. The obtained precursors have been investigated by thermal analysis, FT-IR spectrometry and atomic absorption spectroscopy. The thermal decomposition of the synthesized precursors up to 350 °C leads to the formation of Ni, Zn ferrite as unique phase, evidenced by XRD. The average diameter of the ferrite crystallites, estimated from XRD data, takes values within the range 20–50 nm, depending on the annealing temperature. Transmission Electron Microscopy has evidenced the obtaining of spherical, agglomerated nanoparticles. The magnetic properties of the synthesized samples, measured in cvasistatic magnetic field (50 Hz) are characteristic for the Ni, Zn ferrite nanoparticles, with narrow hysteresis cycle and values of the saturation magnetization <70 emu/g.
Authors:Mirela Barbu, Mircea Stefanescu, Marcela Stoia, Gabriela Vlase, and Paul Barvinschi
In this article, we present a new method for the obtaining of ZnCr2O4 and MgCr2O4 embedded in silica matrix. This method consists in the formation of Cr(III), Zn(II) and Cr(III), Mg(II) hydroxycarboxylate/carboxylate compounds, during the redox reaction between the nitrate ion and diol (1,3-propanediol), uniformly dispersed in the pores of hybrid gels. The thermal decomposition of these precursors leads to a mixture of corresponding metal oxides. The gels were synthesized starting from mixtures of Cr(NO3)3·9H2O, Zn(NO3)2·6H2O and Cr(NO3)3·9H2O, Mg(NO3)2·6H2O with tetraethyl orthosilicate and 1,3-propanediol for final compositions 50% ZnCr2O4/50% SiO2 and 50% MgCr2O4/50% SiO2. The obtained gels have been thermally treated at 140 °C, when the redox reaction nitrates-diol took place with formation of the precursors within the xerogels pores. The thermal decomposition of all precursors took place up to 300 °C, with formation of oxides mixtures (Cr2O3 + x and ZnO) and (Cr2O3 + x and MgO), respectively. At 400 °C, Cr2O3 + x turn to Cr2O3 which reacts with ZnO forming ZnCr2O4/SiO2. Starting with 400 °C, Cr2O3 reacts with MgO to an intermediary phase MgCrO4, which decomposes with the formation of MgCr2O4/SiO2. The formation of the precursors inside the silica matrix and the evolution of the crystalline phases were studied by thermal analysis, FT-IR spectrometry, XRD, and TEM.
Authors:Mircea Stefanescu, Marcela Stoia, Costica Caizer, Thomas Dippong, and Paul Barvinschi
This paper presents a study for the preparation of CoxFe3−xO4 (x = 0.02, 0.2, 0.5, 0.8, 1.0, 1.1, 1.5) nanoparticles, starting from metal nitrates: Co(NO3)2·6H2O, Fe(NO3)3·9H2O and ethylene glycol (C2H6O2). By heating the solutions metal nitrates-ethylene glycol, the redox reaction took place between the anion NO3− and OH–(CH2)2–OH with formation of carboxylate anions. The resulted carboxylate anions reacted with Co(II) and Fe(III) cations to form
coordinative compounds which are precursors for cobalt ferrite. XRD and magnetic measurements have evidenced the formation
of cobalt ferrite for all studied molar ratios. The average diameter of the cobalt ferrite crystallites was estimated from
XRD data and showed values in the range 10–20 nm. The crystallites size depends on the annealing temperature. The magnetization
of the synthesized samples depends on the molar ratio Co/Fe and on the annealing temperature.
Authors:Marcela Stoia, Paul Barvinschi, Lucian Barbu Tudoran, Mirela Barbu, and Mircea Stefanescu
Nickel ferrite powders were synthesized by thermal decomposition of the precursors obtained in the redox reaction between the mixture of Ni(NO3)2·6H2O and Fe(NO3)3·9H2O with polyalcohol: 1,4-butanediol, polyvinyl alcohol and also with their mixture. During this reaction the primary C–OH groups were oxidized at –COOH, while secondary C–OH groups at C=O groups. The carboxylic groups formed coordinate to the present Ni(II) and Fe(III) cations leading to carboxylate type compounds, further used as precursors for NiFe2O4. These precursors were characterized by thermal analysis and FT-IR spectrometry. All precursors thermally decomposed up to 350 °C leading to nickel ferrite weakly crystallized. By annealing at higher temperatures, nanocrystalline nickel ferrite powders were obtained, as resulted from XRD. SEM images have evidenced the formation of nanoparticulate powders; these powders present magnetic properties characteristic to the oxidic system formed by magnetic nanoparticles.
Authors:Marcela Stoia, Mircea Stefanescu, Mirela Barbu, Paul Barvinschi, and Lucian Barbu-Tudoran
The present study deals with preparation and characterization of spinel mixed oxide systems NiM2IIIO4, where MIII = FeIII, CrIII. In order to obtain 50% NiFe2O4/50% SiO2 and 50% NiCr2O4/50% SiO2 nanocomposite, we have used a versatile route based on the thermal decomposition inside the SiO2 matrix, of some particular precursors, coordination compounds of the involved MII and MIII cations with dicarboxylate ligands. The ligands form in the redox reaction between metal nitrates mixture and 1,3-propanediol at the heating around 140 °C of the gels (tetraethylorthosilicate–metal nitrates–1,3-propanediol–water). The as-obtained precursors, embedded in silica gels, have been characterized by FT-IR spectrometry and thermal analysis. Both precursors thermally decompose up to 350 °C leading to the formation of the corresponding metal oxides inside the silica matrix. X-ray diffraction of the annealed powders have evidenced the formation of NiFe2O4 starting with 600 °C, and NiCr2O4 starting with 400 °C. This behavior can be explained by the fact that, by thermal decomposition of the Fe(III) carboxylate at 300 °C, the spinelic phase γ-Fe2O3 is formed, which interacts with the NiO, forming the ferrite nuclei. By thermal decomposition of chromium carboxylate, a nonstoichiometric chromium oxide (Cr2O3+x) is formed. In the range 380–400 °C, Cr2O3+x turns into Cr2O3 which immediately interacts with NiO leading to the formation of nickel chromites nuclei inside the pores of silica matrix. Both spinels have been obtained as nanocrystalites homogenously dispersed as resulted from XRD and TEM data.
Authors:Marcela Stoia, Mirela Barbu, Mircea Ştefănescu, Paul Barvinschi, and Lucian Barbu-Tudoran
In this article, we present a detailed study regarding the preparation of nanosized zinc and magnesium chromites starting from a 4% poly(vinyl)alcohol (PVA) aqueous solution and metal nitrates. The controlled thermal treatment of these solutions has permitted the isolation of an intermediary solid product, used as precursor of the preferred mixed oxides: zinc and magnesium chromites. The as-obtained precursors were characterized by FT-IR spectrometry and thermal analysis. FT-IR spectrometry has evidenced the disappearance of the NO3− anions at 140 °C, due to the redox interaction with PVA. The thermal decompositions of the synthesized precursors were different, as resulted from both thermal analysis and FT-IR spectrometry. Thus, while ZnCrPVA precursor decomposes up to 400 °C with formation of zinc chromite, the precursor MgCrPVA decomposes up to 500 °C, with formation of MgCrO4 as intermediary amorphous phase. By thermal decomposition of MgCrO4 at 500 °C, weakly crystallized MgCr2O4 powder is obtained. The obtained chromite powders consist of fine nanoparticles with diameters ranging from 10 to 30 nm at 500 °C; on raising the annealing temperature to 1000 °C, chromite particles become octahedral, with diameter up to 500 nm, but with no sign of sintering.
Authors:Eszter Laczkó-Zöld, Andrea Komlósi, Timea Ülkei, Erzsébet Fogarasi, Mircea Croitoru, Ibolya Fülöp, Erzsébet Domokos, Ruxandra Ştefănescu, and Erzsébet Varga
In this study, we analyzed extracts of Ribes (black currant, red currant and gooseberry) fruits obtained with methanol, methanol 50% and water. For each extract total polyphenol content, total flavonoid content and total anthocyanin content was assessed. The antioxidant activity of extracts was evaluated by 1,1-Diphenyl-2-picrylhydrazyl (DPPH) and 2,2’-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS) radical scavenging capacity and by the photo-chemiluminescence (PCL) method. Identification and quantification of individual phenolic compounds was performed by means of high performance liquid chromatograph coupled with diode array detector (HPLC-DAD) analyses. From each fruit, best extraction of polyphenols was obtained with methanol 50%. In case of red currants and gooseberry there was no significant difference in flavonoids and anthocyanins extraction rate by the different extraction solvents. For black currants the methanol and methanol 50% extract presented the highest antioxidant activity. For red currants extracts with methanol 50% showed stronger antioxidant activity (IC50 = 5.71 mg/ml for DPPH, IC50 = 1.17 mg/ml for ABTS) than those with methanol or water. In case of gooseberry by the DPPH test the water extract proved to be the most active (IC50 = 5.9 mg/ml). In the PCL test black currants methanol 50% extract was over 6 times more powerful as the ones from red currants. In case of gooseberries, water extract presented the highest antioxidant activity (41.84 μmol AAE/g). In black currant cyanidin-3-glucoside was the major compound. Quercetin 3-O-glucoside was identified in each sample. From cinnamic acid derivatives neochlorogenic acid was present in black currants in the highest amount (356.33 μg/g).