Effect of silver incorporation on crystallization and microstructural properties of sol–gel derived titania thin films on glass

Titanium dioxide (TiO₂) thin films, with and without silver (Ag), were prepared on float glass via sol–gel processing. The float glass substrates were pre-coated with a silica-barrier layer prior to the deposition of TiO₂-based thin films. Silver nanoparticle incorporation into the TiO₂ matrix was achieved by thermal reduction of Ag ions dissolved in a titanium-n-butoxide (Ti[O(CH₂)₃CH₃]₄) based sol during calcination in air at 250, 450 and 650 °C. Thin films were characterized using glancing incidence X-ray diffraction, UV–visible spectroscopy, X-ray photoelectron spectroscopy, and Raman spectroscopy. The effects of Ag concentration and calcination temperature on microstructure and on chemical and physical properties of the thin films have been reported. The size and chemical state of Ag particles, as well as the phase characteristics of the titania matrix were strongly influenced by Ag concentration and calcination temperature. Results from this study can be utilized in both processing and structure-functional property optimization of sol–gel based Ag-TiO₂ thin films by aqueous routes.     

Characterization and preparation of nanocrystalline MgCuZn ferrite powders synthesized by sol–gel auto-combustion method

Nanocrystalline Mg–Cu–Zn ferrite powders were successfully synthesized through nitrate–citrate gel auto-combustion method. Characterization of the nitrate–citrate gel, as-burnt powder and calcined powders at different calcination conditions were investigated by using XRD, DTA/TG, IR spectra, EDX, VSM, SEM and TEM techniques. IR spectra and DTA/TGA studies revealed that the combustion process is an oxidation–reduction reaction in which the NO₃ ⁻ ion is oxidant and the carboxyl group is reductant. The results of XRD show that the decomposition of the gel indicated a gradual transition from an amorphous material to a crystalline phase. In addition, increasing the calcination temperature resulted in increasing the crystallite size of Mg–Cu–Zn ferrite powders. VSM measurement also indicated that the maximum saturation magnetization (64.1 emu/g) appears for sample calcined at 800 °C while there is not much further increase in M _s at higher calcination temperature. The value of coercivity field (H _c) presents a maximum value of 182.7 Oe at calcination temperature 700 °C. TEM micrograph of the sample calcined at 800 °C showed spherical nanocrystalline ferrite powders with mean size of 36 nm. The toroidal sample sintered at 900 °C for 4 h presents the initial permeability (μ _i) of 405 at 1 MHz and electrical resistivity (ρ) of 1.02 × 10⁸ Ω cm.     

Preparation and electrocatalytic performance of Fe–N–C for electroreduction of H2O2

Fe–N–C catalysts were prepared through metal-assisted polymerization method. Effects of carbon treatment, Fe loading, nitrogen source, and calcination temperature on the catalytic performance of the Fe–N–C for H₂O₂ electroreduction were measured by voltammetry and chronoamperometry. The Fe–N–C catalyst shows optimal performance when prepared with pretreated active carbon, 0.2 wt.% Fe, paranitroaniline (4-NA) and one-time calcination. The Fe–N–C catalyst displayed good performance and stability for electroreduction of H₂O₂ in alkaline solution. An Al–H₂O₂ semi-fuel cell was set up with Fe–N–C catalyst as cathode and Al as anode. The cell exhibits an open-circuit voltage of 1.3 V and its power density reached 51.4 mW cm⁻² at 65 mA cm⁻².     

Synthesis of LiCoO<Subscript>2</Subscript> by <Emphasis Type="SmallCaps">l</Emphasis>-apple acid assisted sol–gel method and its electrochemical behavior in aqueous lithium-ion battery

The synthesis process of LiCoO₂ prepared by l-apple acid (l-HOOCCH(OH)CH₂COOH) assisted sol–gel method is studied by using Fourier transforms infrared spectroscopy, mass spectroscopy, simultaneous thermogravimetric and differential thermal analysis, X-ray diffraction analysis, and elemental analysis. The results show that lithium and cobalt ions are trapped homogeneously on an atomic scale throughout the precursor. Lithium carbonate and Co₃O₄ are intermediate products during heat treatment of the precursor. Moreover, the kinetics for formation of LiCoO₂ by l-apple acid assisted sol–gel method is faster than the case of the conventional solid-state reaction between lithium carbonate and Co₃O₄. In comparison with the solid-state reaction, the sol–gel method significantly shortens the required reaction time for synthesizing LiCoO₂, and also reduces the particle size. In the electrochemical test, it is found that the specific discharge/charge capacities as well as the coulomb efficiency substantially increase with increasing the calcination temperature. It is considered that LiCoO₂ with a good-layered structure facilitates the insertion and de-insertion of lithium ions in aqueous electrolyte. As a result, the combination of the sol–gel method with proper calcination processes is highly successful in producing LiCoO₂ powders with large specific capacity and good cycle performance in aqueous lithium-ion battery.     

Sol–gel combustion synthesis of chromium doped yttrium aluminum perovskites

A water-based sol–gel combustion synthesis was optimized in order to achieve pure yttrium aluminium perovskite (YAlO₃, YAP). The method involved three main steps: xerogel formation, combustion treatment and calcination of the combusted precursors. TG DTA, FTIR and XRPD, used to investigate the precursor phase evolution, confirmed the strong influence of gel synthesis conditions, combustion temperature and calcination rate on YAP purity. Both stoichiometry and kinetics control during all the preparation steps are crucial in order to avoid the formation of undesired Y₃Al₅O₁₂ (YAG) and Y₄Al₂O₉ (YAM) stable phases. This method allowed to synthesize, for the first time from an aqueous sol–gel process, single phase samples of YAP. A series of samples with composition: YAl_(1−x)Cr_(x)O₃ where x is 0, 0.035, 0.135, 0.250, 0.500, 0.750 was obtained. A consistent decrease of calcination temperature, with respect to conventional solid state synthesis, was achieved by sol–gel combustion, avoiding polyphase materials even without the use of mineralizers. This is an important result in several applications like the synthesis of ceramic pigments.     

Preparation of porous manganese oxide nanomaterials by one-pot synthetic sol–gel method

For the first time, the simple epoxide addition, sol–gel method has been employed to successfully prepare porous, high surface area manganese (II) aerogel nanomaterials. These uniform materials can then undergo calcination at relatively low temperature to selectively yield the mixed-valent Mn₃O₄ complex illustrating both an ease of preparation and synthesis versatility.     

Facile preparation and electrochemical properties of large-scale Li<Subscript>1+<Emphasis Type="Italic">x</Emphasis></Subscript>V<Subscript>3</Subscript>O<Subscript>8</Subscript> nanobelts

Large-scale Li_1+x V₃O₈ nanobelts were successfully fabricated using filter paper as deposition substrate through a simple surface sol–gel method. The nanobelts were as long as tens of micrometers with widths of 0.4–1.0 μm and thickness of 50–100 nm. The nanobelts were characterized by X-ray diffration (XRD), Fourier infrared (FT-IR) spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM). The formation mechanism of the nanobelts was investigated, showing that the morphology of the nanobelts is mainly determined by the calcination temperature. Electrochemical properties of the Li_1+x V₃O₈ nanobelts were characterized by charge–discharge experiments, and the results demonstrate that the Li_1+x V₃O₈ nanobelts exhibit a high discharge capacity (278 mAh g⁻¹) and excellent cycling stability.     

Synthesis of ZrO2–SiO2 mixed oxide by alcohol-aqueous heating method

The preparation of ZrO₂–SiO₂ mixed oxide has been carried out simply by heating an alcohol-aqueous mixture. Preparation conditions such as volume ratio of alcohol to water and prehydrolysis time of tetraethoxysilane had more important effect on the homogeneity of mixed oxide. A self-catalytic effect on the resultant Zr–O–Si formation is observed. By employing FT-IR, XRD, NH₃-TPD and pyridine adsorption FT-IR techniques, the mixing homogeneity of the mixed oxides in terms of the amount of Zr–O–Si hetero-linkages has been investigated in detail. The results indicate that the amount of Zr–O–Si hetero-linkages increases with the increase of ZrO₂ content in mixed oxides. Moreover, the phase segregation and acidity generation of mixed oxides are studied and factors responsible for good mixing are also discussed. Using this approach, a series of highly homogeneous mixed oxides with ZrO₂ content of 20–70 mol% are obtained at the optimized preparation condition.     

Aerosol-assisted synthesis of mesoporous titania nanoparticles with high surface area and controllable phase composition

Mesoporous titania nanoparticles (denoted as MTN) with high surface area (e.g., 252 m² g⁻¹) were prepared using tetrapropyl orthotitanate (TPOT) as a titania precursor and 10–20 nm or 20–30 nm silica colloids as templates. Co-assembly of TPOT and silica colloids in an aerosol-assisted process and immediate calcination at 450 °C resulted in anatase/silica composite nanoparticles. Subsequent removal of the silica colloids from the composite by NaOH solution created mesopores in the TiO₂ nanoparticles with pore size corresponding to that of silica colloids. Effects of silica colloids’ contents on MTN porosity and crystallites’ growth at a higher calcination temperature (e.g., 1000 °C) were investigated. Silica colloids suppressed the growth of TiO₂ crystallites during calcination at a higher calcination temperature and controllable contents of the silica colloids in precursor solution resulted in various atomic ratios of anatase to rutile in the calcinated materials. The mesostructure and crystalline structure of these titania materials were characterized by transmission electron microscope (TEM), scanning electron microscope (SEM), X-ray diffraction (XRD), differential thermal analysis (DTA)-thermo-gravimetric analysis (TGA), and N₂ sorption.     

Structure and magnetic properties of CoFe<Subscript>2</Subscript>O<Subscript>4</Subscript> ferrites synthesized by sol–gel and microwave calcination

CoFe₂O₄ ferrites were synthesized sol–gel with cobalt chloride, ferric chloride and citric acid as the main raw material. X-ray diffraction, vibrating sample magnetometer and simultaneous thermal analysis were applied to character the structure and magnetic properties of traditional and microwave calcined samples. The samples with pH 5 and molar ratio of citric acid to metal nitrate 1–1.2 showed the optimal structure and magnetic properties. Microwave calcination reduces the synthesis time from 2 h for conventional calcination to 15–30 min. The saturation magnetization (σ _s ) for sample microwave-calcined at 550 °C for 30 min reaches to 75.89 emu/g, much higher than that of conventional-calcined samples.     

Synthesis of electrospun BaSrTiO<Subscript>3</Subscript>/PVP nanofibers

Ba_1−x Sr _x TiO₃(x = 0–0.5, BST) nanofibers with diameters of 150–210 nm were prepared by using electrospun BST/polyvinylpyrrolidone (PVP) composite fibers by calcination for 2 h at temperatures in the range of 650–800 °C in air. The morphology and crystal structure of calcined BST/PVP nanofibers were characterized as functions of calcination temperature and Sr content with an aid of XRD, FT-IR, and TEM. Although several unknown XRD peaks were detected when the fibers were calcined at temperatures less than 750 °C, they disappeared with increasing the temperature (above 750 °C) due to its thermal decomposition and complete reaction in the formation of BST. In addition, the FT-IR studies of BST/PVP fibers revealed that the intensities of the O–H stretching vibration bands (at 3430 and 1425 cm⁻¹) became weaker with increasing the calcination temperature and a broad band at 540 cm⁻¹, Ti–O vibration, appeared sharper and narrower after calcination above 750 °C due to the formation of metal oxide bonds. However, no effect of Sr content on the crystal structure of the composites was detected.     

Electrochemical performance of LiVPO<Subscript>4</Subscript>F/C composite cathode prepared through amorphous vanadium phosphorus oxide intermediate

LiVPO₄F/C composites with better electrochemical performance were prepared by calcination of LiF and amorphous vanadium phosphorus oxide (VPO) intermediate synthesized by a sol–gel method using H₃PO₄, V₂O₅ and citric acid as raw materials. The properties of LiVPO₄F/C composites were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and electrochemical tests. The analysis of XRD patterns and Fourier transform infrared spectra (FTIR) reveal that VPO intermediate prepared by sol–gel method is amorphous and VPO₄ may exist in VPO intermediate. The compositions of LiVPO₄F/C composites are related to the calcination temperature for preparation of amorphous VPO/C intermediate and LiVPO₄F/C composite prepared by VPO/C synthesized at 700°C consists of a single crystal phase of LiVPO₄F. The electrochemical tests show that LiVPO₄F/C composite prepared by VPO/C synthesized at 700°C exhibits higher discharge capacity and excellent cycle performance. This LiVPO₄F/C composite displays discharge capacity of 133 mAh g⁻¹ at 0.5 C (78 mA g⁻¹) and remains capacity retention of 96.8% after 30 cycles, even at a high rate of 5 C, the composite exhibits high discharge capacity of 115 mAh g⁻¹ and capacity retention of 97% after 100 cycles.     

Synthesis and characterization of zinc titanate fibers by sol-electrospinning method

A facile and economic electrospinning approach has been developed for the synthesis of zinc titanate-rutile composite fibers as a nanofibrous mat at the first time. The composite fibers with different morphologies were obtained by calcination of the PVP/Ti(OC₄H₉)₄–Zn(CH₃COO)₂ fibers. The reaction mechanism was characterized by thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffractometer (XRD), field emission scanning electron microscopy (FE-SEM) and Fourier transform infraction spectroscopy (FT-IR) spectra techniques. According to the thermal analysis, the phase of ZnTiO₃ occurred at 450 °C and it decomposed at 885 °C. FE-SEM micrographs indicated that the as-spun fibers were round and had a rather uniform and smooth surface with the diameters in the range of 300–800 nm over its length. Its morphology is greatly affected by the calcination temperatures.     

Luminescent characteristics and microstructures of Sr<Subscript>2</Subscript>CeO<Subscript>4</Subscript> phosphors prepared via sol–gel and solid-state reaction routes

Blue-light-emitting Sr₂CeO₄ phosphors were synthesized via a sol–gel process and the conventional solid-state method in this study. The developed sol–gel process lowered the synthesis temperature of monophasic Sr₂CeO₄ to as low as 900 °C. In comparison with the solid-state derived powders, the sol–gel derived powders had more uniform morphology and smaller particle sizes. In addition, sol–gel derived Sr₂CeO₄ displayed higher luminescent intensity than that prepared via the solid-state route under the same heating conditions. This is attributed to the improved compositional homogeneity and crystallinity in the sol–gel process. During the heating processes, Sr₂CeO₄ tended to thermally decompose at elevated temperatures. This decomposition reaction resulted in the formation of an impurity phase- SrCeO₃ and thereby a decrease in the luminescent intensity. For obtaining Sr₂CeO₄ phosphors with high luminescent intensity, the heating conditions in both processes need to be well modulated.     

Unpromoted and K2O-Promoted Cobalt Molybdate as Catalysts for the Decomposition of Acetic Acid

Summary.  Unpromoted cobalt molybdate was prepared from Co(NO₃)₂·6H₂O and (NH₄)₆Mo₇O₂₄·4H₂O, then calcined between 350 and 600°C for 5 h. K₂O (10 w%), as a promoter, was added to the calcined sample at 350°C from two different sources (i.e. KOH and KNO₃) and was subjected to further calcination at 350°C for 5 h. The catalytic activity of unpromoted catalysts towards the vapour phase decomposition of CH₃COOH was greatly influenced by the increase in the calcination temperature. This is attributed to the diminution of both S _BET and their dual acidic–basic characters. The promoted sample from the KOH source was found to be the most active of the catalysts studied. This is due to its high population of both acidic–basic surface sites and the formation of two new phases. XRD and FTIR analyses of the used catalysts, after the decomposition reaction of acetic acid, showed a remarkable change in its structure compared with the parent samples. E-mail: shalawy99@yahoo.com Received May 8, 2002; accepted (revised) July 9, 2002     

Structural and optical characterization of Zn doped TiO<Subscript>2</Subscript> nanoparticles prepared by sol–gel method

Undoped and zinc-doped TiO₂ nanoparticles (Ti_1−xZn_xO₂ where x = 0.00–0.10) were synthesized by a sol–gel method. The synthesized products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and UV–VIS spectrometer. XRD pattern confirmed the tetragonal structure of synthesized samples. Average grain size was determined from X-ray line broadening using the Debye–Scherrer relation. The crystallite size was varied from 10 to 40 nm as the calcination temperature was increased from 350 to 800 °C. The incorporation of 3–5 mol% Zn²⁺ in place of the Ti⁴⁺ provoked a slight decrease in the size of nanocrystals as compared to undoped TiO₂. The SEM and TEM micrographs revealed the agglomerated spherical-like morphology with a diameter of about 10–30 nm and length of several nanometers, which is in agreement with XRD results. Optical absorption measurements indicated a blue shift in the absorption band edge upon 3–5 mol% zinc doping. Direct allowed band gap of undoped and Zn-doped TiO₂ nanoparticles measured by UV–VIS spectrometer were 2.95 and 3.00 eV at 550 °C, respectively.     

Influences of synthesis route and preparation process on the electrochemical properties of Fe-doped strontium cobaltite

SrCo_0.8Fe_0.2O_3-δ (SCF), as a promising cathode material for intermediate temperature solid oxide fuel cells, possesses a high catalytic activity for the reduction of O₂ to 2O²⁻. The SCF powder was successfully synthesized by the solid state reaction method and Pechini method and characterized using XRD, particle analysis, and electrochemical performance measurements. Smaller-particle-size SCF materials (SCF-P) with single phase are obtained at lower synthesis temperature by the Pechini method and possess better electrochemical performance as compared with those prepared by the solid state reaction method. The reason is that the Pechini method involves the mixing of elements at atomic level, so pure SCF phase formation can be accelerated and showed high electrocatalytic activity. The preparation procedure of SCF cathode was firstly investigated using electrochemical impedance spectroscopy. Results show that the total polarization resistance and the low-frequency resistance decrease gradually with the reduction of the calcination temperature for the SCF cathodes. The SCF-P cathode sintered at 1,000 °C possesses the highest porosity and the best electrochemical performance. It is the result of a comprehensive function of three-phase boundary length, porosity of cathode, and the adhesion between cathode and electrolyte. The charge-transfer process, together with the adsorption, dissociation, and diffusion of oxygen, has a strong influence on the whole reaction process of the cathode. The influence of binder amounts on the performance of the SCF-P cathodes was also studied.     

Thermo-programmed reduction study of Pt/WO<Subscript>x</Subscript>–ZrO<Subscript>2</Subscript> materials by thermogravimetry

The thermo-programmed reduction study of Pt/WO_x–ZrO₂ materials prepared with different tungsten loading were performed by thermogravimetry. The samples were synthesized by impregnation method and calcined at 600, 700 and 800°C. The characterizations of both un-calcined and calcined materials were carried out using different techniques: thermal analysis (TG and DTA), X-ray diffraction (XRD) and thermo-programmed reduction (TPR). TG and DTA analysis of un-calcined were used to determination of calcination temperatures of the samples. XRD diffractograms were useful to help us in the determination of phase presents. TPR profiles showed between three and four events at different temperatures attributed to platinum reduction and the different stages of tungsten specie reduction.     

Effect of PEG with different <Emphasis Type="Italic">M</Emphasis><Subscript>W</Subscript> as template direction reagent on preparation of porous TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript> with assistance of supercritical CO<Subscript>2</Subscript>

Titania–silica composite have been prepared using polyethylene glycol (PEG) with different molecular weights (M _w), PEG20000, PEG10000, and PEG2000, as template in supercritical carbon dioxide (SC CO₂). The composite precursors were dissolved in SC CO₂ and impregnated into PEG templates using SC CO₂ as swelling agent and carrier. After removing the template by calcination at suitable temperature, the titania–silica composite were obtained. The composite were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, and nitrogen sorption–desorption experiment. Photocatalytic activity of the samples has been investigated by photodegradation of methyl orange. Results indicate that there are many Si–O–Ti linkages in the TiO₂/SiO₂ composite; the PEG template has a significant influence on the structure of TiO₂/SiO₂. In addition, the TiO₂/SiO₂ prepared with PEG10000 exhibited high photocatalytic efficiency. So this work supplies a clue to control and obtain the TiO₂/SiO₂ composite with different photocatalytic reactivity with the aid of suitable PEG template in supercritical CO₂.     

A morphological study of mullite long fiber prepared using polyvinyl butyral as spinning aids

Morphologies of aluminosilicate gel fiber and mullite long fiber prepared from spinning sol of Al(NO₃)₃·9H₂O, tetraethylorthosilicate and ethanol using polyvinyl butyral as a spinning aids have been studied experimentally. The weight loss of fibers as function of drying time and calcination temperature was summarized and the fiber morphologies at different temperature were also discussed. The results indicated that fiber sample lost the main weight at the first several minutes at 40°C. A main axial crack was observed due to the heat stress if gel fiber was not pre-dried before calcination. Accompanied by weight loss during heat treatment, “black” fibers were shown at the temperature range of 300–500°C and some attachment was observed among fiber surface attributed by the decomposition of organic materials. The actual elimination temperature of organic materials was different from our former heat analysis measurement result, and it could be explained by the pre-treating procedure and high heating rate of heat analysis. The morphological study of mullite fiber calcined at different temperature could help to understand and obtain the mullite fiber with smooth surface.