Anti-soiling effect of porous SiO<Subscript>2</Subscript> coatings prepared by sol–gel processing

Thin films being composed of a nanoporous SiO₂ network and silica nanoparticles were prepared on glass substrates by sol–gel processing. The surfaces combine anti-reflective (AR) and anti-soiling properties as demonstrated in laboratory testing and long term outdoor exposure. Films were characterized by scanning electron microscopy (SEM). It is shown that both, the structure of the nanoporous matrix as well as the particle density, contribute to this effect, the influence of relative humidity (RH) during dust exposure was investigated. Due to their generally improved solar transmittance and dust-repellant properties the coatings are believed to have a vast potential for many photovoltaic and solar thermal applications.     

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.     

Preparation of LiTaO<Subscript>3</Subscript> nanoparticles by a sol–gel route

A sol–gel route was developed to prepare pure ultrafine LiTaO₃ powders using Ta₂O₅, Li₂CO₃, citric acid (CA) as chelating agent, ethylene glycol (EG) as esterification agent and polyethylene glycol (PEG) as dispersant. The effects of pH value and heat treatment temperature of powder precursor on the synthesis of LiTaO₃ powders were investigated. The phase content and morphology of the final product were evaluated by XRD, SEM and TEM. A transparent gel was produced when heating a mixed-solution of CA, EG, Li and Ta ions with a molar ratio of [CA]:[EG]:[Li]:[Ta] = 3.0:6.0:1.0:1.0 and 2‰ PEG additions with a pH value of 7 at water bath temperature of 80 °C. The results showed that single phase LiTaO₃ powders with average particle sizes of nanometers were produced after heat treatment of the powder precursor at 650, 700, 800, and 900 °C respectively for 2 h.     

Bright blue pigment CoAl<Subscript>2</Subscript>O<Subscript>4</Subscript> nanocrystals prepared by modified sol–gel method

Nanocrystalline films of magnetite have been prepared by a novel sol–gel route in which, a solution of iron (III) nitrate dissolved in ethylene glycol was applied on glass substrates by spin coating. Coating solution showed Newtonian behaviour and viscosity was found as 0.0215 Pa.s. Annealing temperature was selected between 291 and 350 °C by DTA analysis in order to obtain magnetite films. In-plane grazing angle XRD and TEM studies showed that magnetite phase was present upon annealing the films at 300 °C. The films had crack free surfaces and their thicknesses varied between ~10 and 200 nm. UV–Vis spectrum results showed that transmittance of the films increases with decreasing annealing temperature and increasing spinning rate. Up to 96% transmittance was observed between the wavelengths of 900–1,100 nm. Vibrating sample magnetometer measurements indicated that magnetite thin films showed ferromagnetic behavior and the saturation magnetization value was found as ~35 emu/cm³.     

Low temperature synthesis of Co<Subscript>2</Subscript>SiO<Subscript>4</Subscript>/SiO<Subscript>2</Subscript> nanocomposite using a modified sol–gel method

The paper presents a study on the preparation of Co₂SiO₄/SiO₂ nanocomposites by a new modified sol–gel method. We have prepared gels starting from tetraethylorthosilicate (Si(OC₂H₅)₄), cobalt nitrate Co(NO₃)₂·6H₂O and some diols: ethylene glycol (C₂H₆O₂), 1,2propanediol (C₃H₈O₂) and 1,3propanediol (C₃H₈O₂), for a final composition: 30% CoO/70% SiO₂. During the heating of the gels at 140 °C, a redox reaction takes place between NO₃ ⁻ ions and diol with formation of some carboxylate anions. These carboxylate anions react with the Co(II) ions to form coordination compounds embedded in silica matrix, as evidenced by FT-IR spectrometry and thermal analysis. These Co(II) coordinative compounds thermally decompose in the range 250–300 °C to the corresponding oxides: CoO and/or Co₃O₄ inside the matrices pores. When CoO results, it reacts with SiO₂ at low temperature leading to Co₂SiO₄, which crystallizes at 700 °C. XRD patterns of the samples annealed at temperatures lower than 700 °C were characteristic to amorphous phases. The samples annealed at temperatures ≥700 °C, contain Co₂SiO₄ (olivine) as unique crystalline phase inside the amorphous silica matrix, according to XRD patterns. As evidenced by TEM images, Co₂SiO₄ nanoparticles are homogenously dispersed inside the silica matrix.     

Structural and thermal properties of monolithic silica–phosphate (SiO<Subscript>2</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript>) gel glasses prepared by sol–gel technique

A sol–gel process for producing monolithic silica–phosphate (SiO₂–P₂O₅) system different concentrations of P₂O₅, starting with tetra-ethoxysilane TEOS, and triethyl-phosphate as sources of SiO₂ and P₂O₅ was performed. The gels were heat-treated at temperatures ranging from 100 up to 900 °C. The structural and chemical analyses of the samples were determined by using X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). It was found from the XRD that the existence of phosphorus enhances the crystallization of silica gel, while the FTIR indicated the main functional groups of silica–phosphate. It is important to study the effect of hydroxyl in silica–phosphate glass. The results obtained are promising to use the prepared samples in a variety of applications, ranging from traditional application such as lighting products) to the modern application (such as optical fibers. Optical studies were measured by using the spectrophotometer in wavelength range 0.2–2.5 μm. The refractive index (n) was calculated for the prepared samples, it was found to be strongly affected by structural rearrangement resulting from the elimination of the solvent and the Si–OH, Si–O–Si and Si–O–OH bonding by phosphate and aluminum and it increases by increasing phosphate concentrations. The weight losses have investigated for prepared samples.     

Novel sol–gel method for preparation of high concentration ε-Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/SiO<Subscript>2</Subscript> nanocomposite

ε-Fe₂O₃/SiO₂ nanocomposite was prepared by novel solgel method using single precursor for both nanoparticles and matrix. This method allows to prepare the samples free of α-Fe₂O₃ with 40% of Fe₂O₃ in SiO₂. Nanoparticles of 12 nm diameter were obtained by annealing at 1,000 °C. The samples were characterized by powder X-ray diffraction and transmission electron microscopy. Mössbauer spectroscopy identified ε-Fe₂O₃ as the only magnetically ordered phase at room temperature. Magnetic measurements revealed progressive necking of hysteresis loops measured at 300 and 2 K. In both cases the intrinsic coercivity reaches only 0.25 T. Measurements up to 14 T shows monotonous decreasing trend of saturated magnetization with increasing temperature.     

Catalytic properties of Fe/SiO<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> systems,obtained via sol–gel method

The high efficacy of iron-containing catalysts based on SiO₂–Al₂O₃ systems obtained via sol–gel method in the oxidative destruction of carmoisine azo dye in aqueous solutions is demonstrated. It is found that the stability of the catalysts with respect to the leaching of iron ions into a solution during catalysis grows along with the aluminum content in the composition of aluminosilicate supports. It is concluded that the synthesized catalysts are promising materials for purifying wastewaters contaminated with organic dyes.     

Luminescent properties of Sr<Subscript>3</Subscript>Al<Subscript>2</Subscript>O<Subscript>6</Subscript>: Eu,Pr prepared by sol–gel method

A new red-emitting long afterglow Sr₃Al₂O₆: Eu²⁺, Pr³⁺ phosphor was synthesized by sol–gel methods using Sr(NO₃)₂, Al(NO₃)₃·9H₂O, Eu(NO₃)₃ and Pr(NO₃)₃ as raw materials. The crystalline structure of the phosphor powders were characterized by X-ray diffraction. Luminescent properties of the phosphor powders were analyzed by the fluorescence spectrophotometer. Sr₃Al₂O₆: Eu²⁺, Pr³⁺ phosphor powders with single Sr₃Al₂O₆ phase were prepared at 1200 °C for 2 h in the reducing atmosphere. Pr³⁺ doped made the light intensity and the light-lasting time of Sr₃Al₂O₆: Eu²⁺, Pr³⁺ phosphors improved. The emission peaks of the Sr₃Al₂O₆: Eu²⁺, Pr³⁺ phosphor powders lay at 612 nm with the excitation of 472 nm and the longest afterglow time could last for about 15 min at Pr³⁺ content of 0.06.     

Structure and electrorheological properties of nanoporous BaTiO<Subscript>3</Subscript> crystalline powders prepared by sol–gel method

In this paper, a novel nanoporous barium titanate (BaTiO₃) crystalline powder was synthesized by using triblock poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) based systems (P-123) as the soft template via a sol–gel method and their structure-dependent electro rheological property was studied. The pore diameter and specific surface area of BaTiO₃ were precisely controlled by varing the calcined temperature. The chemical composition, structure and surface morphology of BaTiO₃ were characterized by X-ray diffraction (XRD), thermo gravimetric analysis (TGA), and nitrogen adsorption–desorption method, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The result revealed that the pore volume and specific surface area of BaTiO₃ decreased with the increment of calcined temperature. The electro rheological fluids (ERFs) were obtained by dispersing BaTiO₃ crystallites in silicon oil and three kinds ERFs were fabricated by using three kinds of BaTiO₃ which were prepared under different calcined temperature (550, 600 and 900 °C) as the precursors. The behaviors of the ERFs were evaluated via a rotational rheometer fixed with electric field generator. The results showed that electro rheological effect was related to the pore volume and specific surface area of BaTiO₃. Due to the distinct advantage of sol–gel method for preparing nanoporous BaTiO₃ without contamination of the materials, the markedly low current destiny of the ERFs was obtained. The yield stress of ERFs with large specific surface area of BaTiO₃ reached the maximum of 3 kPa, which is higher than that of ERFs using traditional pure BaTiO₃ crystallites (lower than 1 kPa).     

TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> hard coating on polycarbonate substrate by microwave assisted sol–gel technique

To enhance the poor scratch resistance of polycarbonate, a silica (SiO₂) and titania (TiO₂) transparent inorganic coatings was designed and synthesized using a microwave assisted sol–gel heating. Due to the transparency of PC to microwave, the idea was to obtain a localized heating only on the coating film. The obtained films were fully characterized to mainly evaluate the effect of titania content, added both as nanoparticles and from tetraethyl orthotitanate, TEOT, on scratch resistance and surface morphology. Particular attention was paid to preserve the transparency of the final product. The results allowed to define that TEOT addition enhances the adhesion between coating and polycarbonate, even if the optimized quantity have to be defined in order to avoid a decrease of coating mechanical resistance. In this work optimized TEOT level results to be the associated to 5 wt% of TiO₂, which enable the better balancing between adhesion and mechanical resistance performances.     

Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-based pigments synthesized by a new proteic sol–gel method

Alumina-based pigments were synthesized by the proteic sol–gel method. In this method, coconut water is employed as polymeric agent instead of the conventional alkoxide precursors. To this study, three common chromophore metallic ions (Mn³⁺, Co³⁺, and Cr³⁺) were chosen in order to verify the method efficiency. Differential thermal analysis (DTA), thermogravimetry (TG), and XRD techniques were used to characterize the synthesis process. The colorimetric characterization of the produced pigments was done according to the CIE-L*a*b* 1976 norm which is recommended by the CIE (International Commission on Illumination). The synthesized pigments presented intense and uniform colors in accordance to the literature results for each chromophore ion. The produced pigments also presented agglomerated with an average grain size of 180 nm when calcined at 800 °C.     

Synthesis and characterization of sol–gel derived bioactive CaO–SiO<Subscript>2</Subscript>–P<Subscript>2</Subscript>O<Subscript>5</Subscript> glasses containing magnetic nanoparticles

Magnetic bioglasses in the system CaO–SiO₂–P₂O₅ were prepared by interaction of acetic acid vapors with iron nitrate dispersed on the surface of sol–gel derived porous silicate network. Upon pyrolysis, the created iron acetate species transform into magnetic iron oxide nanoparticles. X-ray diffraction (XRD), FT-infrared (FT-IR) spectroscopy and surface area measurements (BET) were employed to monitor the evolution of glass structural features during the synthetic pathway as well as the structure and the texture of the resultant glasses. XRD, Raman spectroscopy and vibration magnetic measurements (VSM) revealed the features of magnetic phases, developed in the form of γ-Fe₂O₃ and magnetite. The obtained glasses exhibit in vitro bioactivity, expressed by spontaneous formation of hydroxyapatite on their surface after immersion in SBF at 37 °C, confirmed with μ-Raman and FT-IR spectroscopies.     

Structure and multiferroic properties of Bi<Subscript>5</Subscript>FeTi<Subscript>3</Subscript>O<Subscript>15</Subscript> thin films prepared by the sol–gel method

The Bi₅FeTi₃O₁₅ (BFTO) films of layered structure have been fabricated on Pt/Ti/SiO₂/Si substrates by the sol–gel method. The thermal decomposition behaviors of precursor powder were examined using thermo-gravimetric and differential scanning calorimeters analysis. The optimal heat treatment process for BFTO films were determined to be low-temperature drying at 200 °C for 4 min and high-temperature drying at 350 °C for 5 min followed by annealing at 740 °C for 60 min, which led to the formation of compact films with uniform grains of ~300 nm. The structural, surface topography, ferroelectric and magnetic properties of the films were investigated. The remnant polarization (2P _r) of BFTO thin films under an applied electric field of ~550 kV/cm are determined to be 67.5 μC/cm² . Meanwhile, the weak ferromagnetic properties of the BFTO films were observed at room temperature.     

Colour tuneability and white light generation in Yb<Superscript>3+</Superscript>–Ho<Superscript>3+</Superscript>–Tm<Superscript>3+</Superscript> co-doped SiO<Subscript>2</Subscript>–LaF<Subscript>3</Subscript> nano-glass-ceramics prepared by sol–gel method

Nanostructured transparent glass-ceramics with composition of 95SiO₂–5LaF₃ co-doped with 0.3Yb³⁺, 0.1Ho³⁺ and 0.1Tm³⁺ (mol%) were synthesized by thermal treatment of precursor sol–gel derived glasses. X-ray diffraction and transmission electron microscopy analysis point out the precipitation of hexagonal LaF₃ nanocrystals with diameter ranging from 11 to 20 nm in these nano-glass-ceramics. White light generation by means of efficient blue, green and red up-conversion luminescence under infrared excitation at 980 nm was observed and involved mechanisms were analyzed. Colour tuneability is achieved by varying the up-conversion emission ratios as a function of pump power.     

Erbium and ytterbium co-doped SiO<Subscript>2</Subscript>:GeO<Subscript>2</Subscript> planar waveguide prepared by the sol–gel route using an alternative precursor

The sol–gel method combined with a spin-coating technique has been successfully applied for the preparation of rare-earth doped silica:germania films used for the fabrication of erbium-doped waveguide amplifiers (EDWA), presenting several advantages over other methods for the preparation of thin films. As with other methods, the sol–gel route also shows some drawbacks, such as cracks related to the thickness of silica films and high hydrolysis rate of certain precursors such as germanium alkoxides. This article describes the preparation and optical characterization of erbium and ytterbium co-doped SiO₂:GeO₂ crack-free thick films prepared by the sol–gel route combined with a spin-coating technique using a chemically stable non-aqueous germanium oxide solution as an alternative precursor. The non-crystalline films obtained are planar waveguides exhibiting a single mode at 1,550 nm with an average thickness of 3.9 μm presenting low percentages of porosity evaluated by the Lorentz–Lorenz Effective Medium Approximation, and low stress, according to the refractive index values measured in both transversal electric and magnetic polarizations. Weakly confining core layers (0.3% < Δn < 0.75%) were obtained according to the refractive index difference between the core and buffer layers, suggesting that low-loss coupling EDWA may be obtained. The life time of the erbium ⁴I_13/2 metastable state was measured as a function of erbium concentration in different systems and based on these values it is possible to infer that the hydroxyl group was reduced and the formation of rare-earth clusters was avoided.     

Preparation and characterisation of NdAlO<Subscript>3</Subscript> nanocrystals by modified sol–gel method

Nanocrystalline single-phase neodymium monoaluminate (NdAlO₃) has been prepared from neodymium oxide and aluminium nitrate by modified Pechini’s method. Malic acid has been used for the first time as a new complexing agent in the sol–gel process. It has facilitated, without adding 1,2-ethanediol, a low-temperature synthesis at 1,050 °C compared to the temperature of 1,630 °C needed for the solid-state preparation. The characterisation of the nanoparticles has been carried out by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and infrared spectroscopy in the range 75–4,000 cm⁻¹. The smallest particles have size of 30 nm and are anisometric; agglomerates of particles have been also observed. The material has pyknometric density of 3.956 g/cm³ at T = 293.15 K and specific surface area 5.2 m²/g. The binding energies of O 1s, Al 2p, Nd 3d, and Nd 4d electrons have been found chemically shifted in NdAlO₃ compared to the values for the respective elements.     

Direct formation of SiO<Subscript>2</Subscript>/SnO<Subscript>2</Subscript> composite nanoparticles with high surface area and high thermal stability by sol–gel-hydrothermal process

SnO₂/SiO₂ composite nanoparticles were prepared by sol–gel-hydrothermal process and their physico-chemical structure and photocatalytic property were investigated. The results of XRD, TEM and FT-IR indicated that SnO₂ crystallites with the tetragonal rutile structure were well-developed directly during hydrothermal process. The SnO₂/SiO₂ composite nanoparticles owned narrow size distribution, large specific surface area, and good thermal stability. As the presence of 25.0 wt% SiO₂, the SnO₂ nanoparticles were about 4.0 nm in diameter and the specific surface area was 259.0 m²/g. After calcination at 800 °C, the crystalline grain size maintained 16.2 nm and the surface area still remained 132.6 m²/g. The SnO₂/SiO₂ composite nanoparticles showed better photocatalytic activity than pure SnO₂ nanoparticles.     

Synthesis of CaSnO<Subscript>3</Subscript> nanofibers by electrospinning combined with sol–gel

Calcium stannate (CaSnO₃) nanofibers were synthesized by electrospinning technique combined with a sol–gel process. The structure and morphology of the as-prepared CaSnO₃ nanofibers were characterized by X-ray diffraction and scanning electron microscopy, respectively. The samples had a band gap of about 3.87 eV, which was estimated by UV–Vis diffuse reflectance spectroscopy. On the basis of the experiment results, the composite fibers containing polymer and inorganic salt can be changed to pure CaSnO₃ nanofibers only when they were sintered at an appropriate temperature. At the same time, a possible mechanism of the nanofibers forming process was also proposed.     

Preparation of Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin films by sol–gel technique

In this study we describe the preparation of Ga₂Se₃ semiconductor compound thin films by sol–gel method for different crystal formation temperatures. The films were characterized by X-ray diffraction analyses (XRD), UV–visible spectrometer, and scanning electron microscope (SEM). The XRD spectrum showed that the formation of Ga₂Se₃ crystals were between 743 and 823 K. The thin film crystals that were formed at 773 K corresponded to the β phase and the preferred crystal structure was monoclinic. The value of band gap from optical absorption spectra for the Ga₂Se₃ thin films was estimated to be about E _g ~ 2.56 eV. The thickness of the one-coat Ga₂Se₃ thin films, which was measured by a Spectroscopic Ellipsometer, was about ~200 nm. The average grain sizes of scattered particles were within the limits between 200 and 500 nm.