Effect of hydrodynamics during sol–gel synthesis of TiO2 nanoparticles: From morphology to photocatalytic properties

In this study, the role of mixing hydrodynamics during the sol–gel synthesis of titania nanoparticles and the consequences on their photocatalytic properties were investigated. For the first time three different T-mixer geometries were tested. Alcoholic solutions of titanium tetra-isopropoxide and water were mixed in three different T-mixers with turbulence promoters and thus different mixing characteristics. The changes of nanoparticle sizes during the induction time of the sol–gel process were followed by dynamic light scattering and velocity and turbulence fields were simulated by Computational Fluid Dynamics (CFD) for the three T-mixer geometries. The results indicated that macro-mixing is crucial during the first step as it determines the nucleation rate and then the primary particle size. The micro-mixing has an influence on particle properties, especially on particle stability. Titanium dioxide nanoparticles synthesized by the sol–gel process were deposited on alumina supports. A homogeneous film of about 200 nm was deposited in all cases. Degradation of Acid Orange 7 (AO7) was used to evaluate the photocatalytic activity of TiO₂ coatings. No difference was observed between the photoactivity of synthesized TiO₂. Total mineralization of the dye occurred after 24 h irradiation.     

Catalytic oxidation of gas-phase mercury over Co/TiO2 catalysts prepared by sol–gel method

Co/TiO₂ catalysts prepared by sol–gel method were studied for gas-phase mercury oxidation in this paper. Experimental results revealed that the optimal loading of Co was 7.5%, which yielded more than 90% oxidation efficiency within the temperature range of 120–330 °C. The high activity was mainly attributed to the enrichment of well dispersed Co₃O₄. We also found that oxygen performed a key role in mercury oxidation process, while HCl could corrode the oxidized mercury and release it to gas phase. Thereafter, a modified Mars–Maessen mechanism was proposed and the possible simultaneous oxidization of NO and mercury was studied.     

Microwave assisted synthesis of Dy2Ti2O7 ultrafine powders by sol–gel method

Dy₂Ti₂O₇ ultrafine powders ranging from 100 to 300 nm were successfully synthesized by sol–gel method. Particularly, the dried gel precursor was treated at different temperatures (700–1000 °C) via microwave-heating, which contributed to decreasing the grain size and reaction time. The phase composition and structural evolution of the final products were examined by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). Furthermore, the resultant powders were selected to fabricate ceramics and rubber based absobers. Their sinterability, mechanical properties and neutron absorption ability were also studied. Results showed that the highest flexural strength of 99.0 MPa were obtained for Dy₂Ti₂O₇ samples when sintered at 1600 °C for 2 h in air atmosphere. Meanwhile, the neutron absorption rate of Dy₂Ti₂O₇ ceramics and rubber based absorbers could reach 97.39% and 80.00% respectively when the thickness of samples was set as 5.0 mm.     

Influence of the synthesis process on the features of Y2O3-stabilized ZrO2 powders obtained by the sol–gel method

Y₂O₃-stabilized ZrO₂ (YSZ) powders have been prepared by the sol–gel method using different synthesis parameters. Specifically, zirconium n-propoxide was dissolved in propanol at pH 0.5 or 5 (provided by HNO₃), with or without acetic acid in the hydrolysis medium. Subsequently, the YSZ powders obtained by gelation and drying of these solutions was characterized using scanning and transmission electron microscopies, X-ray diffractometry, and N₂-adsorption. Compacts made from these YSZ powders which were then sintered were also analyzed. It was found that the pH of the hydrolysis medium has a notable influence on the microstructure, morphology, color, crystallinity, and sintering behavior process of these YSZ sol–gel powders. It was also found that the use of acetic acid also affects the YSZ powder features, and results in compacts with higher residual porosity after sintering. Finally, the compacts prepared from the YSZ powders obtained at pH 5 and without acetic acid exhibit the greatest sinterability.     

Synthesis and optical characterization of Er-doped bismuth titanate nanoparticles grown by sol–gel hydrothermal method

The Er³⁺-doped bismuth titanate (Bi₄Ti₃O₁₂, BIT) nanoparticles were synthesized by a combined sol–gel and hydrothermal method under a partial oxygen pressure of 30 bar. The composition and morphology were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Raman scattering. They showed pure and homogeneous spherical BIT nanoparticles with a size below the 30 nm. The incorporation of Er ions showed a strong decrease in the lattice parameters, as well as averaged particle size. The photoluminescence up-conversion (excitation wavelength =1480 nm) showed an enhancement of the infrared emission (980 nm) as Er concentration increased, achieving a maximum for 6% mol, while photoluminescence spectra (excitation wavelength =473 nm) showed a strong green emission (529 and 553 nm) with a maximum at 4% mol.     

Size-controlled synthesis of mesoporous silica nanoparticles using rice husk by microwave-assisted sol–gel method

Mesoporous silica nanoparticles with distinct characteristics like particle size, tunable pores, and high surface area have received much interest for environmental remediation, energy conversion, and biological applications. In this work, we synthesized spherical silica nanoparticles with tunable particle size and mesoporous properties using a low-cost silica source (rice husk) and polyethylene glycol (PEG) via microwave-assisted sol–gel synthesis. The formation of an amorphous silica structure was found using XRD and FTIR analysis. FESEM analysis showed that altering the PEG concentration from .01 to .005 M produced spherical silica nanoparticles with 100–500 nm in size. Nitrogen adsorption–desorption demonstrated that silica nanoparticles obtained with .005, .007, and .01 M of PEG had unique pore sizes and distributions, with specific surface areas of 51.475, 62.367, and 84.251 m²/g, respectively. These results might be due to PEG molecules’ capping effect, which acts as a soft template to regulate particle size, pore size, and dispersion by interacting with sodium silicate precursor. Hence, this approach can be a facile and cost-effective method to prepare mesoporous silica nanoparticles with controllable nanoscale characteristics for suitable applications.     

Elaboration of pure and doped TiO2 nanoparticles in sol–gel reactor with turbulent micromixing: Application to nanocoatings and photocatalysis

Pure and doped nanocoatings were prepared of from chemically active TiO₂ colloids. The colloids are generated in sol–gel reactor with turbulent mixing of two reactive fluids, containing titanium tetra-isopropoxide and water, in T-mixer. The particle size is measured by the light-scattering technique using optical-fibre probe. The TiO₂ nanoparticles doped with nitrogen are obtained by the injection of a doping agent into the reaction zone at the mixing stage. The mixing at the nucleation stage has a strong impact on the nanoparticles polydispersity and mean size. The prepared nanocoatings show photocatalytic activity under UV and visible light illumination.     

Influence of the synthesis route on sol–gel SiO2–TiO2 (1:1) xerogels and powders

Five different sol–gel routes are used in order to synthesize mixed SiO₂–TiO₂ materials. Simple mixing of the Ti and Si precursors, pre-hydrolyzing of TEOS, modification of the Ti alkoxide with acetic acid, isoamyl alcohol and acetylacetone lead to translucent gels with different time of gelation. Different techniques such as TGA, DTA, XRD and IR spectroscopy are used to characterize each material. IR spectroscopy revealed the presence of Si–O–Ti and Si–O–Si bonds for all the xerogels letting suppose a composite microstructure of the gels. Pre-hydrolyzing of TEOS and modification of Ti alkoxide with isoamyl alcohol are the most appropriate routes to retain the anatase phase up to 1100 °C.     

Characterisation of urushiol formaldehyde polymer/multihydroxyl polyacrylate/SiO2 nanocomposites prepared by the sol–gel method

Novel anti-ultraviolet UFP/MPA/SiO₂ nanocomposite coatings were prepared from urushiol formaldehyde polymer (UFP) and multihydroxyl polyacrylate resin (MPA) via the sol–gel process. FT-IR spectroscopy was employed to reveal the nanocomposite structure between UFP/MPA and nano-SiO₂. TEM was used to observe the size scale and the distribution of nanoparticles throughout the polymer matrix. Simultaneously, the dynamic mechanical properties were characterised through dynamic mechanical thermal analysis (DMTA). The influence of the SiO₂ content on the physical mechanical and anticorrosive properties of UFP/MPA/SiO₂ nanocomposites was investigated. Moreover, the 1000 h anti-ultraviolet tests showed that the ultraviolet resistance of UFP/MPA/SiO₂ coatings improved. The best anti-ultraviolet and anticorrosive properties were achieved when the SiO₂ content was 5 wt.%.     

Synthesis of silica nanoparticles from Vietnamese rice husk by sol–gel method

Silica powder at nanoscale was obtained by heat treatment of Vietnamese rice husk following the sol–gel method. The rice husk ash (RHA) is synthesized using rice husk which was thermally treated at optimal condition at 600°C for 4 h. The silica from RHA was extracted using sodium hydroxide solution to produce a sodium silicate solution and then precipitated by adding H₂SO₄ at pH = 4 in the mixture of water/butanol with cationic presence. In order to identify the optimal condition for producing the homogenous silica nanoparticles, the effects of surfactant surface coverage, aging temperature, and aging time were investigated. By analysis of X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, the silica product obtained was amorphous and the uniformity of the nanosized sample was observed at an average size of 3 nm, and the BET result showed that the highest specific surface of the sample was about 340 m²/g. The results obtained in the mentioned method prove that the rice husk from agricultural wastes can be used for the production of silica nanoparticles.     

Controllable synthesis of metastable tetragonal zirconia nanocrystals using citric acid assisted sol–gel method

Metastable tetragonal ZrO₂ nanoparticles and nanosheets were synthesized with citric acid assisted sol–gel method. In this approach, zirconium acetylacetonato, Zr(acac)₂, citric acid (CA) and ethylene glycol (EG) were used as the source of Zr⁴⁺, the chelating, and solvent agent, respectively. The effects of heat treatment on zirconia phase evolution were investigated. We demonstrate that pure tetragonal nanocrystalline zirconia can be obtained with CA: EG mole ratio=5:1 and calcination temperature 490 °C. The microstructure of the products was characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED) and Raman scattering. Finally, Photoluminescence (PL) of nanosheets and nanoparticles were also investigated.     

Preparation and characterization of TiO2, ZnO,and TiO2/ZnO nanofilms via sol–gel process

The preparation of the TiO₂, ZnO, and TiO₂/ZnO nanofilms was conducted on glass via sol–gel process. The prepared film was detailedly characterized by means of OM, SEM, XRD, and EDS. The results showed that the obtained pure TiO₂ was composed of nanoparticles. For pure ZnO it consisted of nanoparticles and large agglomerates. Both the microstructural morphology and the crystallization of the prepared TiO₂/ZnO composite film were strongly related to the Ti/Zn ratio in the film. With a Ti/Zn ratio less than 1/1, the composite film was absence of cracks. Poor crystallization was definitely observed for the composite film with Ti/Zn ratio of 3/1 and 1/1. The EDS analysis revealed homogeneous distribution of Ti and Zn elements in the film.     

Aqueous sol–gel synthesis of zirconium titanate (ZrTiO4) nanoparticles using chloride precursors

Zirconium titanate powders were synthesized by a straightforward sol–gel method using zirconium and titanium chlorides as metal precursors, deionized water as solvent and oxygen donor, and a NaOH solution for adjusting pH to 7. According to transmission electron microscopy, amorphous particles of nearly 5 nm in size with a relatively spherical morphology were prepared. Thermogravimetry and differential scanning calorimetery analyses on the xerogel at a heating rate of 10 °C/min indicated a crystallization temperature of 690 °C, which is comparable with previous reports. Furthermore, via differential scanning calorimetery studies using the Kissinger's equation, the activation energy for ZrTiO₄ crystallization was determined to be 850 kJ/mol. Structural evaluations in the isothermal regime, using X-ray diffraction experiments, implied the onset of ZrTiO₄ crystallization at 550 °C.     

Optimization of process parameters using D-optimal design for synthesis of ZnO nanoparticles via sol–gel technique

The experimental conditions for the synthesis of ZnO nanoparticles to produce minimal size were optimized using the D-optimal design. The influence of process parameters involves molar ratio of the starting materials, pH and the calcination temperature on the particle size were evaluated using the polynomial regression. The optimum conditions revealed by the model for obtaining a minimum particle size of ZnO were predicted to have a molar ratio of 1.76, pH of 1.50 and calcination at 402.2 °C. The obtainable particle size upon applying the model is 22.9 nm in compare to experimental result of 18 ± 2 nm was obtained.     

Chemical control on the size and properties of nano NiFe2O4 synthesized by sol–gel autocombustion method

Nickel ferrite nanoparticles have been synthesized by sol–gel auto combustion route. The significant role played by nitric acid added to the precursor solution in controlling the reaction rate phase purity, crystallinity, crystallite size, thermal and magnetic properties of nanoparticles was explored and reported. Also, the influence of annealing on the properties were studied. Samples of average crystallite size ranging from 10 nm to 40 nm have been obtained by controlling the HNO₃ concentration and by increasing the annealing temperature. The size-dependent structural, thermal and magnetic properties were investigated and reported. The Hopkinson peak was observed for all the crystalline samples near the Curie temperature. The highest value 47.3 emu/g of saturation magnetization was obtained for the sample prepared with higher concentration (6 mol/L) of HNO₃.     

Design of SiO2/ZrO2 core–shell particles using the sol–gel process

Core–shell particles of SiO₂/ZrO₂ were developed using a sol–gel process. Spherical core particles of SiO₂, 320 nm in diameter, were initially prepared using tetraethylorthosilicate (TEOS), and then uniformly shelled with ZrO₂ nano-particles synthesized with zirconium(IV) butoxide (TBOZ). The deposition of ZrO₂ nano-particles on the SiO₂ core particles was generally promoted when increasing the H₂O and TBOZ concentrations and temperature of the sol–gel process. However, micron-sized homo-aggregates of ZrO₂ were formed above certain concentrations of H₂O and TBOZ due to self-aggregation of the nano-ZrO₂ particles. It was very interesting to discover that a chemical bonding between zirconium and silicon bridged by oxygen (Si–O–Zr bond) was developed during the formation of the ZrO₂ shell around the core particles, as the silane groups on the core particles were condensed with zirconium hydroxyl groups during the deposition of ZrO₂. XPS and FT-IR confirmed the chemical bonding of Si–O–Zr in the core–shell particles.     

Green gelatine-assisted sol–gel synthesis of ultrasmall nickel oxide nanoparticles

Nickel oxide nanoparticles (NiO NPs) were synthesised using a sol–gel method in a gelatinous medium. Gelatine was used as a size-limiting polymerisation agent for the growth of NiO NPs. X-ray diffraction (XRD) analysis revealed that increasing the calcination temperature increased the crystallite size and decreased the size of the lattice constant. The size-strain plot method (SSP) was used to measure the individual contribution of grain sizes and micro strain on the peak broadening of NiO NPs. Transmission electron microscopy (TEM) showed the ultrasmall size of the NiO NPs with a narrow size distribution(10±0.2 nm). The band gap value of NiO NPs was calculated using ultraviolet–visible (UV–Vis) spectroscopy and decreased with increased calcination temperature.     

Size-controlled synthesis of nano α-alumina particles through the sol–gel method

Nano α-alumina particles were synthesized by a sol–gel method using aqueous solutions of aluminum isopropoxide and 0.5 M aluminum nitrate. 1/3-benzened disoulfonic acid disodium salt (SDBS) and sodium bis-2-ethylhexyl sulfosuccinate (Na(AOT)) were used as surfactant stabilizing agents. Solution was stirred for different periods (24, 36, 48 and 60 h) at 60 °C. The samples were then analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Introduction of surfactant stabilizing agents and different stirring times will affect the size and shape of particle formed and also the degree of aggregation. SDBS, however, produced better dispersion, finer particles and spherical shape nanoparticles, compared to Na(AOT). The finest particle size (20–30 nm) was obtained at 48 h stirring time with SDBS surfactant.     

Temperature dependent magnetic properties of CoFe2O4/CTAB nanocomposite synthesized by sol–gel auto-combustion technique

A CoFe₂O₄/cetyl trimethylammonium bromide (CTAB) nanocomposite has been fabricated by a sol–gel auto-combustion method. Characterization of the material revealed the composition of the crystalline phase as CoFe₂O₄ while FT-IR confirmed the presence of CTAB on the nanoparticles. From X-ray line profile fitting, average crystallite size was estimated to be 22±6 nm. SEM analysis showed a porous sheet-like morphology with internal nanosize grains of about 30 nm. The room temperature coercive field (H_c) of the CoFe₂O₄/CTAB nanocomposite was found to be 1045 Oe which is close to the previously reported room temperature values for bulk CoFe₂O₄. The H_c was observed to decrease almost linearly with the square root of the temperature (√T) according to Kneller's law. From the linear fit of H_c versus √T, the zero-temperature coercivity (H_c0) and superparamagnetic blocking temperature (T_B) of the CoFe₂O₄/CTAB nanocomposite were found to be ∼9.1 kOe and ∼425 K, respectively. The remanence magnetization (M_r), the reduced remanent magnetization (M_r/M_s), and the effective magnetic anisotropy (K_eff) decrease with increasing temperature. The M_r/M_s value of 0.6 at 10 K higher than the theoretical value of 0.5 for non-interacting single domain particles with the easy axis randomly oriented suggests the CoFe₂O₄/CTAB nanocomposite to have cubic magnetocrystalline anisotropy according to the Stoner–Wohlfarth model.     

Preparation of nanocrystalline MoSi2 with enhanced lithium storage by sol–gel and carbonthermal reduction method

Herein nanocrystalline MoSi₂ with enhanced lithium storage was successfully synthesized via a sol-gel and carbonthermal reduction method. Reduction of the gel mixture of Mo precursor and Si precursor by carbon at a desired temperature resulted in the formation of MoSi₂ nanoparticles. The gel mixture was obtained through the hydrolysis of TEOS and ammonium molybdate and the polymerization of hydrolysis products of TEOS. The reducing agent carbon was produced via decarburition of sucrose's hydrolysis products, which have been wrapped in the gel during its formation process. Addition of HCl to the mixed solution controlled the hydrolysis and polymerization rate, and enabled the formation of a gel mixture with homogeneously distributed hydrolysis products of ammonium molybdate, TEOS and sucrose. This achievement likely generates a novel route to synthesize non-oxide compounds such as silicide, carbide through the sol–gel and carbonthermal reduction process. In addition, the as-received MoSi₂ nanoparticles showed considerable activities in the reversible lithiation and delithiation process. When using as an anode for Li-ion batteries, MoSi₂ nanoparticles delivered a specific capacity of 325 mAh g^?1 at C/12 and showed an increasing capacity with cycling.