Structure and Electron Spin Resonance of Annealed Sol-Gel Glasses Containing Ag

In this work, SiO₂ samples with silver, prepared using the sol-gel method, were analyzed after being thermally treated in air in the range of 100 to 800°C. The sol-gel starting solutions were prepared by mixing tetra-ethyl-orthosilicate (TEOS), water and ethanol. Samples with 4 different H₂O/TEOS molar ratios (3.3, 5, 7.5 and 11.7 respectively) and with different nominal Ag concentrations were prepared (1, 2 and 4%vol. of Ag). It was found that upon annealing, different silver spices were formed, such as Ag₂ ⁺, Ag⁺, Ag°, and metallic silver aggregates. The identification of these spices was carried out by means of X-ray diffraction, Electron Paramagnetic Resonance (EPR), optical emission and optical absorption. It was also found that the specific type of silver spices observed depends on the structure of the SiO₂ matrix and on the annealing temperatures. It was found that samples prepared from precursor solutions with a low H₂O/TEOS ratio have a more open structure, and therefore silver diffuses faster and forms agglomerates at lower temperatures. Samples prepared from solutions with larger H₂O/TEOS ratios have a more dense structure, which allows the formation of atomic or molecular spices in addition to silver particles. A systematic study of this system was carried out using EPR on samples prepared from solutions having different H₂O/TEOS molar ratios, various Ag concentrations and subjected to different thermal treatments.     

Silver as anode in cryolite—alumina-based melts

The anodic behaviour of silver was investigated in cryolite—alumina-based melt. Silver has a lower melting point (ca. 960°C) than the other metals considered as possible inert materials for aluminium electrolysis. The working temperature used in aluminium industry is approximately 960°C, depending on the melt composition. Therefore, the stability of silver during the anodic process was tested at 870°C in an acidic electrolyte consisting of 65.5 mass % Na₃AlF₆ + 22.9 mass % AlF₃ + 5.7 mass % CaF₂ + 3.9 mass % LiF + 2 mass % Al₂O₃ with the melting point ca. 850°C. The electrolyte without alumina was prepared as well, with the melting point ca. 860°C. The resulting cryolite ratio (CR = n(NaF)/n(AlF₃)) for both electrolytes was equal to 1.6. The behaviour of the silver anode was studied by voltammetry measurements. The electrochemical study showed that an oxidation reaction occurred at a potential below the oxygen evolution potential. Silver was not found to be stable under oxygen evolution. The degradation of the silver anode was apparent after electrolysis.     

Organophosphine/phosphite‐stabilized silver(I) complexes bearing N‐acetylbenzamide ligand: synthesis,characterization and potential application in metal organic chemical vapor deposition

New N‐silver(I) acetylbenzamide complexes of type L_n?AgNC₉H₈O₂ (L = PPh₃; n = 1, 2a; n = 2, 2b; n = 3, 2c; L = P(OEt)₃; n = 1, 2d; n = 2, 2e; n = 3, 2f) were prepared. These complexes were obtained in high yields and characterized by elemental analysis, ¹H NMR, ¹³C{H} NMR, ³¹P{H} NMR and IR spectroscopy, respectively. The molecular structure of 2b has been determined by X‐ray single‐crystal analysis in which the silver atom is in a distorted tetrahedral geometry and crystallizes as cis–trans. New N‐silver(I) acetylbenzamide complexes have a four‐membered ring, which could influence their chemical and physical properties and modulate volatility. Metal organic chemical vapor deposition experiments were carried out successfully at 400°C and 450°C using 2e as precursor for the deposition of silver films, respectively. The high‐purity silver film obtained at 400°C is dense and homogeneous. Copyright © 2012 John Wiley & Sons, Ltd.     

Aus der Chemie von Uranhexachlorid und Uranpentachloridazid

On the Chemistry of Bauxite Extraction. III. Studies in the System Na₂O? MgO? CaO? Al₂O₃? TiO₂? H₂O The formation of crystalline compounds in the system Na₂O? MgO? CaO? Al₂O₃? TiO₂? H₂O was studied at 100°C and atmospheric pressure. Magnesium titanates or magnesium aluminates were not detected in the investigated range of concentrations. In the presence of Ca(OH)₂ the same compounds are formed as in the system Na₂O? CaO? Al₂O₃? TiO₂? H₂O. In addition, sodium aluminates are formed at high sodium and aluminium concentrations.     

Beiträge zur Chemie des Bauxitaufschlusses. Untersuchungen im System Na2O?CaO?Al2O3?TiO2?H2O im Temperaturbereich 100–275°C

On the Chemistry of Bauxite Extraction. II. Studies in the System Na₂O? CaO? Al₂O₃? TiO₂? H₂O between 100 and 275°C The formation of crystalline compounds in the system Na₂O? CaO? Al₂O₃? TiO₂? H₂O was studied between 100 and 275°C. With caustic alkali concentrations up to 300 g Na₂O/l the calcium aluminate 3 CaO · Al₂O₃ · 6 H₂O is formed. With rising temperatures two different calcium titanates, among them perovskite, CaTiO₃, are identified. Above 200°C perovskite is formed at all concentrations investigated.     

Highly efficient and green oxidation of alkanes and alkylaromatics with hydrogen peroxide catalysed by silver and vanadyl on mesoporous silica‐coated magnetite

A heterogeneous catalyst (FeSi/Ag/VO) based on silver and vanadyl as active sites and mesoporous silica‐coated nanospheres of magnetite (Fe₃O₄@m‐SiO₂) as support was successfully prepared by deposition of Ag nanoparticles and the covalent grafting of vanadyl(IV) acetylacetonate on Fe₃O₄@m‐SiO₂. The catalyst exhibited excellent activity for the oxidation of alkanes, benzene and alkylaromatics using green oxidant H₂O₂ and oxalic acid in acetonitrile at 60 °C.     

Effect of CuO–Bi<Subscript>2</Subscript>O<Subscript>3</Subscript> on low temperature sintered MnZn-ferrite by sol–gel auto-combustion method

A sol–gel auto-combustion method was investigated to incorporate small amounts of additives of Cu and Bi uniformly into soft magnetic MnZn-ferrite nanoparticles, which were prepared by Fe(NO₃)₃·9H₂O, Mn(NO₃)₂ and Zn(NO₃)₂·6H₂O dissolved in water and citric acid. The powder was characterized by the X-ray diffraction analysis and transmission electron microscope method. The effects of nano-particle sized powders in microstructure development and adding CuO–Bi₂O₃ into MnZn-ferrite on phase formation, densification process as well as magnetic properties were studied by scanning electron microscope and vibrating sample magnetometer techniques. The sample without additive can be sintered well at 930 °C, while the samples with a small amount of the additive can be sintered at less than 900 °C. Obviously, the micron-sized powders exhibited high sintering activity. It was also found that CuO–Bi₂O₃ additive promoted the growth of grains and improved magnetic properties. The permeability and the saturation magnetization were improved substantially by adding CuO–Bi₂O₃ into MnZn-ferrite and the sintering temperature was lowered to 875 °C, which may be associated with the redistribution of cations on the tetrahedral (A) sites and octahedral (B) sites within the spinel lattice.     

Non-isothermal decomposition of silver maleate dihydrate and anhydrous silver fumarate

The non-isothermal decompositions of silver maleate dihydrate (C₄H₂O₄Ag₂2H₂O) and anhydrous silver fumarate (C₄H₂O₄Ag₂) were studied up to 500°C, in a dynamic atmosphere of air, by means of TG and DTA measurements. Both compounds showed some sublimation (at 120°C for silver maleate and at 180°C for silver fumarate) prior to the onset of decomposition (at 170°C for silver maleate and at 280°C for silver fumarate). The gaseous decomposition products of both compounds were found, using IR spectroscopy, to be dominated by maleic anhydride and CO₂. Minor proportions of ethylene, ethyl alcohol, acetone, methane and isobutene were also identified. Metallic silver was the final solid product, as identified by X-ray diffractometry. NMR analysis was used to monitor the isomerization of the maleate radical into the more stable fumarate above 230°C. Kinetic parameters (E _a and lnA) were calculated from the effect of heating rate, (2, 5, 10, and 20 deg min^?1) on the DTA measurements. A mechanism is suggested for the decomposition pathways of these compounds, on basis of the results obtained and, also, on similarities with analogous systems.     

Thermal,XRD, and magnetization studies on ZnAl<Subscript>2</Subscript>O<Subscript>4</Subscript> and NiAl<Subscript>2</Subscript>O<Subscript>4</Subscript> spinels,synthesized by citrate precursor method and annealed at 450 and 650 °C

Two aluminate spinel materials (ZnAl₂O₄ and NiAl₂O₄) were synthesized by the citrate precursor method. The citrate precursors consisting of coprecipitated citrates of Zn²⁺ or Ni²⁺ and aluminum were first subjected to thermal analysis (TG-DSC) for determining the optimum temperature for annealing. Two step decomposition was observed incorporating dehydration and formation of the aluminate. The second step gives an endo peak (−2937 J/g) at 356 °C in the DSC curve of the coprecipitated nickel(II) citrate–aluminum citrate gel in O₂ atmosphere. Kinetic/mechanistic analysis of the TG data has also been carried out and values of E _a, ΔS ^#, ΔG ^#, and A were approximated. On the basis of the findings, 450 °C has been chosen for annealing of the gels. Annealing has also been done at 650 °C for 1 h in muffle furnace in an attempt to obtain nanometric particles of aluminates (MAl₂O₄) {M = Ni, Zn} and to find out their magnetic properties which could render them useful for chemical sensing applications, etc. The TG-DSC curves of various powders which were obtained on annealing at the two temperatures did exhibit thermal instability when carried out in N₂ atmosphere. NiAl₂O₄ and ZnAl₂O₄ spinels (particle size 17 and 34 nm, respectively) are obtained in pure crystalline phase at 650 °C. ZnAl₂O₄ prepared this way shows coercivity values of 470 and 58.37 G and NiAl₂O₄, 107 and 23.24 G when annealed at 450 and 650 °C, respectively. ZnAl₂O₄ prepared by a polymer precursor method and annealed at 1000 °C, has earlier been reported to have coercivity value of 469 G. Thus, the citrate precursor method is good for the synthesis of ZnAl₂O₄, producing single phase nanocrystalline powder of high quality and crystallinity. The value of magnetization was found to be small in the present case for the NiAl₂O₄ spinel obtained at 450 °C.     

Sol-Gel Processing of Sm2+-Doped Glass and Its Spectral Hole Burning at Room Temperature

The sol-gel process was applied to the preparation of Sm²⁺ ion-doped silicate glasses, which show persistent spectral hole burning at room temperature. The gels synthesized by the hydrolysis of metal alkoxides and SmCl₃·6H₂O were heated in air at 500°C, were then reacted with H₂ gas to form the Sm²⁺ ion. The Al₂O₃−SiO₂2 glasses are appropriate to reduce the Sm³⁺ ion with H₂ gas and show intense photoluminescence of Sm²⁺ ion. Persistent spectra hole burning was observed in the excitation spectrum for the⁷F₀→⁵D₀ transition of the Sm²⁺ ion by the irradiation of DCM dye laser. The hole width and depth were ∼16 cm⁻¹ and ∼10% of the total intensity, respectively, at 20°C.     

FT-EPR Spectroscopic Study of Silicophosphate Glasses Derived from Gels

(100-x) mole% SiO₂-x mole% P₂O₅-glasses withx=1–9 have been prepared by the sol-gel process using tetraethylorthosilicate and triethylphosphate as precursors. The gels were fired at various temperatures up to 950°C and then exposed to γ-ray irradiation to induce paramagnetic centers. CW-EPR and FT-EPR Spectroscopies were employed at temperatures between 4 and 300 K in order to determine the resulting structures. The dried gels exhibited four types of O ₂ ⁻ -ions trapped in pores of different sizes. The gels fired atT=670°C exhibited theE’ ₁-center and non-bridging oxygen as a results of the fracture of the Si-O-Si bonds. At higher temperatures, the spectra of the POHC, POHC ^b ,E’, and CH ₃ ⁻ -centers have been detected that are a function ofx andT.     

纳米银三角片的光诱导法可控制备及其抗菌活性

采用一种快速高效的光诱导法合成了纳米银三角片,系统地考察了光照时间、柠檬酸钠用量、OH-用量和合成方法对银纳米三角片的合成产率、尖端形貌及其稳定性的影响。研究结果表明,采用光诱导法合成纳米银三角片的最优条件为光照时间3.5 h,反应物物质的量之比n_AgNO3：n_Na3C6H5O7：n_NaBH4=1：10：0.8,OH-浓度0.125 mmol·L^-1,合成出粒径70~80 nm的纳米银三角片,透射电镜和紫外可见分光光度的表征结果表明：光诱导法相对于直接化学还原法制得的纳米银三角片具有较好的微观形态、产率及稳定性。抗菌测试结果表明三角片形态的纳米银比球型颗粒形态的纳米银具有更优异的抗菌性能。     

纳米银三角片的光诱导法可控制备及其抗菌活性

采用一种快速高效的光诱导法合成了纳米银三角片,系统地考察了光照时间、柠檬酸钠用量、OH-用量和合成方法对银纳米三角片的合成产率、尖端形貌及其稳定性的影响。研究结果表明,采用光诱导法合成纳米银三角片的最优条件为光照时间3.5 h,反应物物质的量之比n_(AgNO_3)∶n_(Na_3C_6H_5O_7)∶n_(NaBH_4)=1∶10∶0.8,OH-浓度0.125 mmol·L~(-1),合成出粒径70~80 nm的纳米银三角片,透射电镜和紫外可见分光光度的表征结果表明:光诱导法相对于直接化学还原法制得的纳米银三角片具有较好的微观形态、产率及稳定性。抗菌测试结果表明三角片形态的纳米银比球型颗粒形态的纳米银具有更优异的抗菌性能。     

Synthesis and characterization of nanocrystalline zinc aluminate spinel by sol–gel technique using modified alkoxide precursor

Nanosized zinc aluminate spinel (gahnite, ZnAl₂O₄) powders were prepared by sol−gel technique at low sintering temperatures. Aluminium-sec-butoxide [Al(O^sBu)₃] and zinc nitrate hexahydrate Zn(NO₃)₂ . 6H₂O were used as starting materials. Gels with and without chelating agent were prepared. Ethyl-acetoacetate (C₆H₁₀O₃) was used as a chelating agent in order to control the rate of hydrolysis of Al(O^sBu)₃. The dried gels and thermally treated samples were characterized by means of Differential Thermal Analysis and Thermo-Gravimetric Analysis (DTA, TGA), X-ray Diffraction (XRD), Fourier Transform Infrared spectroscopy (FTIR) and Transmission Electron Microscopy (TEM). The surface area was measured by Brunauer-Emmet-Teller (BET) adsorption–desorption isotherms. It has been established that chelation enables to obtain a transparent gel. The thermal evolution of gels was characterized by two crystallization processes in the range 200–400 °C and 600–700 °C. Both processes yielded pure ZnAl₂O₄ as evidenced by XRD, i.e. zinc aluminate spinel powders were produced by gel heat-treatment at temperatures as low as 300 °C. The average gahnite crystallite size for the samples sintered in the temperature range of 400–1000 °C has been calculated from the broadening of XRD lines revealing that nanocrystalline powders were prepared. The surface areas measured for the samples fired at 700 °C for 2 h were 43.1 and 62.6 m² g⁻¹, for sample without and with the chelating agent, respectively. TEM micrographs confirmed the nano-scale size of particles.     

Diaqua oxalato strontium(II) complex as a precursor for facile fabrication of Ag-NPs@SrCO3, characterization,optical properties,morphological studies and adsorption efficiency

Diaqua oxalato strontium(II) complex [Sr(C₂O₄)(H₂O)₂] was prepared via a precipitation reaction. Thermal treatment of the as-synthesized precursor at 550?°C resulted in formation of strontium carbonate (SrCO₃) nanocrystals. A new composite of silver nanoparticles decorated with strontium carbonate (Ag-NPs@SrCO₃) was fabricated by heating a mixture of silver oxalate and strontium carbonate in air at 150?°C for 2?h. The spectral, morphological and thermal properties of the materials have been studied using different physicochemical techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX), high-resolution transmission electron microscopy (HR-TEM), Fourier infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), diffrential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA). From the Debye–Scherrer equation the calculated particle size of Sr(C₂O₄)(H₂O)₂], SrCO₃ and Ag-NPs@SrCO₃ are 62.1, 58.7, and 58.5?nm, respectively. The SEM and TEM images indicate tetragonal structure of [Sr(C₂O₄)(H₂O)₂] while SrCO₃ and Ag-NPs@SrCO₃ appeared as cubic structures. The calculated energy band gap of SrCO₃ and Ag-NPs@SrCO₃ using the Tauc equation are estimated at 5.9 and 4.7?eV, respectively. The adsorption capacity of the materials is tested for the adsorption of Congo red anionic dye and exhibited promising results. The adsorption capacity followed the order Ag-NPs@SrCO₃>SrCO₃>?[Sr(C₂O₄)(H₂O)₂] with efficiencies of 73.90, 67.55, and 60.50%, respectively.     

Thermal Decomposition of Strontium Iron Citrate

The citrate precursor method has been used to synthesise ultrafine SrFe₁₂O¹⁹. The thermal decomposition of citrate precursor SrFe₁₂O₆ (C₆H₆O₇)₁₃ was investigated by TG, DTG and DTA techniques, gas and chemical analyses. The citrate precursor on decomposition in static air atmosphere yields pure and stoichiometric SrFe₁₂O₁₉. The decomposition consists of three major steps, the formation of acetone dicarboxylate complex occurs around 165°C. The citrate groups are completely destroyed in the temperature range 195–315°C resulting in the formation of complex carbonate with the evolution of acetone and CO₂ gas. The decomposition of carbonate results in the formation of ultrafine SrFe₁₂O₁₉ below 550°C with the evolution of CO₂ gas. The ultrafine particles are observed as platelet clusters having crystallite size 13 nm and surface area 76.4 m² g^?1. The citrate precursor and the decomposed products were characterised by IR, NMR, XRD, SEM and surface area measurements.     

CuAl2O4 powder synthesis by sol-gel method and its photodegradation property under visible light irradiation

Nanocrystalline spinel CuAl₂O₄ powders were prepared by sol-gel method from nitrate Cu(NO₃)₂·3H₂O, Al(NO₃)₃·9H₂O and complex C₆H₈O₇·H₂O. Sintering was carried out at 400, 500, 600, 700, 800°C respectively for 2 h in air. The XRD patterns started to appear CuAl₂O₄ peaks after sintering of 500°C and consist of only CuAl₂O₄ peaks as spinel crystal after sintering of 700°C. The powders were analyzed by TEM and UV-vis diffuse reflectance spectrum to be round, about 10–30 nm in size and Eg=1.77 eV. Photodegradation property of nanocrystalline CuAl₂O₄ powders was investigated by using methyl orange as model pollutant and mercury lamp (λ>400 nm) as energy source. The results indicated that CuAl₂O₄ powders sintered at 700°C had the excellent visible photocatalytic property. Under the irradiation of visible light, methyl orange could be degraded 97% in 120 min.     

MnO x /ZrO2 gel-derived materials for hydrogen peroxide decomposition

Manganese-yttrium-zirconium mixed oxide nanocomposites with three different Mn loadings (5, 15 and 30 wt%) were prepared by sol–gel synthesis. Amorphous xerogels were obtained for each composition. Their structural evolution with the temperature and textural properties were examined by thermogravimetry/differential thermal analysis, X-ray diffraction, diffuse reflectance UV–vis spectroscopy and N₂ adsorption isotherms. Mesoporous materials with high surface area values (70–100 m² g⁻¹) were obtained by annealing in air at 550 °C. They are amorphous or contain nanocrystals of the tetragonal ZrO₂ phase (T-ZrO₂) depending on the Mn amount and exhibit Mn species with oxidation state higher than 2 as confirmed by temperature programmed reduction experiments. T-ZrO₂ is the only crystallizing phase at 700 °C while the monoclinic polymorph and Mn₃O₄ start to appear only after a prolonged annealing at 1,000 °C. The samples annealed at 550 °C were studied as catalysts for H₂O₂ decomposition in liquid phase. Their catalytic activity was higher than that of previously studied Mn/Zr oxide systems prepared by impregnation. Catalytic data were described by a rate equation of Langmuir type. The decrease of catalytic activity with time was related to dissolution of a limited fraction (up to 15%) of Mn into the H₂O₂/H₂O solution.     

Preparation, structure, morphology, and dc and ac conductivity of the 88SiO2-6Li2O-6Nb2O5 (% mole) sol-gel derived glass-ceramics

The glass composition 88SiO₂-6Li₂O-6Nb₂O₅ (mole %) was successfully prepared by the sol-gel technique. The dried and translucent gel was heat-treated at temperatures between 500°C and 800°C. Lithium niobate crystallites, an important ferroelectric material, were detected in the gel derived glass-ceramics treated above 650°C. In the samples treated at 700 and 800°C the Li₂Si₂O₅ crystalline phase is present. The 800°C treated sample also presents the Li₃NbO₄ phase. The structure and morphology of the samples were studied by X-ray powder diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM). The SEM revealed that all the samples, heat-treated above 650°C, present crystallites embedded in the glass matrix. The particles detected in the 600°C treated sample are essentially amorphous, or with an incipient structure. The temperature dependence of the dc electrical conductivity (σ _dc ) shows two regions with different activation energies. The conductivity behaviour of the sample is mainly due to the mobile ion number. The ac conductivity (σ _ac ), measured at 1 kHz decreases with the rise of the treatment temperature due to the increase of the LiNbO₃ crystallites amount. The electrical behavior of the glass and glass-ceramics reflects the important role carried out by the treatment temperature in the gel-glass structure.     

Thermo-Raman Studies on Dehydration of Na4P2O7⋅10H2O and Phase Transformations of Na4P2O7

In this work, dehydration of sodium diphosphate decahydrate Na₄P₂O₇⋅10H₂O and phase transformations of Na₄P₂O₇ in open air have been studied in detail by thermo-Raman spectroscopy. The spectra were measured continuously in a temperature range from room temperature up to 600°C for the bands of P₂O₇ ^4- and H₂O. The spectral variation showed one step of dehydration and four-phase transformations. The thermo-Raman intensity(TRI) and differential thermo-Raman intensity (DTRI) curves calculated from the characteristic bands of H₂O also showed one step of dehydration with the loss of all hydrated water in the temperature interval from 45 to 69°C. Thermogravimetric measurements supported this result. The thermo-Raman investigation indicated the transformation of Na₄P₂O₇ from low temperature phase to high temperature phase proceed through pre-transitional region from 75 to 410°C before the major orientational disorder at 418°C and minor structural modifications at 511,540 and 560°C. The results from differential scanning calorimetry and differential thermal analysis on Na₄P₂O₇ showed endotherms at 407,517, 523, 548, 557°C and 426, 528, 534, 555, 565°C, respectively. This revised version was published online in July 2006 with corrections to the Cover Date.