Anatase TiO2 nanoparticles/carbon nanotubes nanofibers: preparation, characterization and photocatalytic properties

In the present work, we report the preparation and photocatalytic properties of hybrid nanofibers/mats of anatase TiO₂ nanoparticles and multi-walled carbon nanotubes (MWNTs) using combined sol–gel and electrospinning techniques. Poly(vinyl pyrrolidone) is used as a base polymer in the electrospinning suspension to assist the formation of nanofibers and subsequently removed by calcination. The hybrid nanofibers are characterized using XRD, Raman spectra, FT-IR, XPS, SEM, TEM and N₂ adsorption measurements. The results show that MWNTs are encapsulated by in-situ formed anatase TiO₂ nanoparticles, with chemical bonding C–O–Ti between anatase TiO₂ nanoparticles and MWNTs. Hybrid nanofibrous mats with moderate content of MWNTs (mass ratio TiO₂:MWNTs = 100:20) exhibit enhanced adsorption ability and excellent photocatalytic activity. The composition, diameter and morphology of hybrid nanofibers can be tuned by varying sol–gel formulation, electrospinning parameter and post-treatment condition. TiO₂/MWNTs hybrid nanofiber and mats have promising applications in water purification and solar cell areas.     

Synthesis and characterization of pure anatase TiO<Subscript>2</Subscript> nanoparticles

Pure anatase TiO₂ nanoparticles were synthesized by microwave assisted sol–gel method and further characterized by powder X-ray diffraction (XRD), energy dispersive x-ray analysis (EDAX), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV–Visible spectrophotometer, SEM images showed that TiO₂ nanoparticles were porous structure. The XRD patterns indicated that TiO₂ after annealed at 300 °C for 3 h was mainly pure anatase phase. The crystallite size was in the range of 20–25 nm, which is consistent with the results obtained from TEM images. Microwave heating offers several potential advantages over conventional heating for inducing or enhancing chemical reactions.     

The control of porosity at nano scale in resorcinol formaldehyde carbon aerogels

Organic aerogels were synthesized by sol–gel polymerization of resorcinol (R) with formaldehyde (F) catalyzed by sodium carbonate (C) followed by vacuum drying. The influence of the resorcinol/sodium carbonate ratio (R/C) on the porous structure of the resultant aerogels was investigated. The nitrogen adsorption–desorption measurements show that the aerogels possess a well developed porous structure and mesoporosity was found to increase with increasing the R/C ratio. Carbon aerogels were obtained by carbonization of RF aerogels. The carbonization temperature impacts the microstructure of the aerogels by pore transformations during carbonization probably due to the formation of micropores and shrinkage of the gel structure. The results showed that a temperature of 1073 K is more effective in the development of the pore structure of the gel. Activated carbon aerogels were obtained from the CO₂ activation of carbon aerogels. Activation results in an increase in the number of both micropores and mesopores, indicative of pore creation in the structure of the carbon. Activation at higher temperatures results in a higher degree of burn off and increases the pore volume and the surface area remarkably without change of the basic porous structure, pore size, and pore size distribution.     

Preparation of lanthanum-doped TiO<Subscript>2</Subscript> photocatalysts by coprecipitation

The lanthanum-doped TiO₂ (La³⁺-TiO₂) photocatalysts were prepared by coprecipitation and sol–gel methods. Rhodamine B was used as a model chemical in this work to evaluate the photocatalytic activity of the catalyst samples. The optimum catalyst samples were characterized by XRD, N₂ adsorption–desorption measurement, SEM and electron probe microanalyses to find their differences in physical and chemical properties. The experimental results showed that the La³⁺-TiO₂ catalysts prepared by coprecipitation exhibited obviously higher photocatalytic activities as compared with that prepared by the conventional sol–gel process. The optimum photocatalysts prepared by the coprecipitation and sol–gel process have similar adsorption equilibrium constants in Rhodamine B solution and particle size distribution in water medium although there are larger differences in their surface area, morphology and pore size distribution. The pores in the sol-gel prepared catalysts are in the range of mesopores (2–50 nm), whereas the pores in the coprecipitation prepared catalysts consist of bigger mesopores and macropores (>50 nm). The morphology of the primary particles and agglomerates of the La³⁺-TiO₂ catalyst powders was affected by doping processes. The inhibition effect of lanthanum doping on the phase transformation is greater in the coprecipitation process than in the sol–gel process, which could be related with the different amount of Ti–O–La bonds in the precursors. This finding could be used for preparing the anatase La³⁺-TiO₂ catalysts with more regular crystal structure through a higher heat treatment temperature. The optimum amount of lanthanum doping is ca. 1.0 wt.% and the surface atomic ratio of [O]/[Ti] is ca. 2.49 for 1.0 wt.% La³⁺-TiO₂ catalysts prepared by the two processes. The obviously higher photocatalytic activity of the La³⁺-TiO₂ samples prepared by the coprecipitation could be mainly attributed to their more regular anatase structure and more proper surface chemical state of Ti³⁺ species. The optimum preparation conditions are 1.0 wt.% doping amount of lanthanum ions, calcination temperature 800 °C and calcination time 2 h using the coprecipitation process. As compared with the sol-gel process, the coprecipitation process used relatively cheap inorganic raw materials and a simple process without organic solvents. Therefore, the coprecipitation method provides a potential alternative in realizing large scale production.     

Hot-water treatment of sol–gel derived SiO2–TiO2 microparticles and application to electrophoretic deposition for thick films

SiO₂–TiO₂ spherical microparticles of about 0.7 μm in diameter were prepared by the sol–gel method. Anatase nanocrystals were formed in the microparticles and their specific surface area was increased after a hot-water treatment at 90 °C. From the changes in the concentration of I₂ photocatalytically generated from KI aqueous solution, the activity of the SiO₂–TiO₂ microparticles was found to increase with increasing the hot-water treatment time. Particulate, thick films were electrophoretically deposited on indium tin oxide (ITO)-coated glass substrates using the anatase nanocrystal-precipitated SiO₂–TiO₂ microparticles. The thickness of the electrophoretically deposited particulate film increased to be approximately 10 μm with an increase in applied voltage. The resultant thick film showed a high photocatalytic activity.     

Innovative one-step immobilization of TiO2 on polymer material by the sol–gel method under IL/MW conditions

An innovative one-step immobilization of titanium dioxide (TiO₂) nano-particles on organic polymer (PMMA) substrate at ambient condition is reported in this article. This immobilization can be achieved by the sol–gel method under ionic liquid/microwave heating conditions. In this method, a sol–gel reaction is conducted at specific sites of the polymer surface. These sites are the tiny cavities of the rough surface resulting from the softening and swelling effect of an alcohol, such as isopropyl alcohol, on the polymer surface under microwave irradiation. The roughness of the polymer surface is an important factor for the effective immobilization. In addition, ionic liquid can induce low temperature surface anatase crystallization of immobilized titanium dioxide in a short time. From the field emission scanning electron microscopy and energy dispersive spectroscopy observation, the TiO₂ particles could be effectively immobilized on the PMMA substrate. Raman spectra analysis data showed that the immobilized TiO₂ was anatase phase. The experimental data also shows that the immobilized TiO₂ prepared by this novel method has good immobilization stability and photocatalytic water treatment performance.     

Ti K-edge XANES study of the local environment of titanium in bioresorbable TiO2–CaO–Na2O–P2O5 glasses

Ti K-edge XANES (X-ray absorption near edge structure) spectroscopy has been used to study the local coordination of titanium in biocompatible and bioresorbable TiO₂–CaO–Na₂O–P₂O₅ glasses. Both conventional melt-quenched glasses of composition (TiO₂) _x (CaO)_0.30(Na₂O)_(0.20−x)(P₂O₅)_0.50, where x = 0.01, 0.03 and 0.05, and sol–gel derived (TiO₂)_0.25(CaO)_0.25(P₂O₅)_0.50 glass have been studied. The results show that in all the materials studied, titanium is surrounded by an octahedron of oxygen atoms. Further analysis reveals that the TiO₆ site in the amorphous samples is not heavily distorted relative to that in rutile, anatase or CaSiTiO₅. The spectra from the (TiO₂)_0.25(CaO)_0.25(P₂O₅)_0.50 sol–gel samples reveal greater distortion in the TiO₆ site in the dried gel compared to the heat-treated sol–gel glass. The XANES spectra from melt-quenched glass samples soaked in distilled water for various times do not shown any evidence of degradation of the titanium site over periods of up to 14 days.     

New insights into anatase crystallization behavior in ionothermal synthesis of nanostructured TiO<Subscript>2</Subscript>

The anatase crystallization behaviors in ionothermal synthesis (sol–gel method containing ionic liquid) of nanostructured TiO₂ were studied in this paper. It was found that the specific physical chemical characteristics of the water/ionic liquid mixture caused the formation path and crystallinity of anatase TiO₂ to depend on the H₂O/titanium dioxide precursor (titanium tetraisopropoxide, TIP) molar ratio. Hydroxylated titanium compound was a key intermediate for forming anatase TiO₂. It could be directly formed from hydrolysis of titanium dioxide precursor or ionic liquid-induced water dissolution of the condensation product. X-ray photoelectron spectroscopy (XPS) and thermogravimetric analysis (TGA) data indicated that a higher hydroxyl group ratio content of hydroxylated titanium compound was obtained at medium H₂O/TIP molar ratio and from the system containing hydrophilic ionic liquid, such as 1-butyl-3-methylimidazolium tetrafluoroborate ([BuMIm]⁺[BF₄]⁻). The self organization ability of ionic liquid drove anatase crystallization through dehydration of the Ti–OH group of hydroxylated titanium compound in the thermal annealing process. As for the particle size of TiO₂, TEM results indicated smaller particle size of TiO₂ was obtained at medium H₂O/TIP molar ratio case.     

A sol–gel-derived α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> crystal interlayer modified 316L porous stainless steel to support TiO<Subscript>2</Subscript>, SiO<Subscript>2</Subscript>, and TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> hybrid membranes

A homogeneous α-Al₂O₃ crystal membrane was fabricated by the sol–gel technique on 316L porous stainless steel (PSS) substrate with an average pore size of 1.0 μm. The preparation process was optimized by carefully choosing the binder, the concentrations of the casting solutions and the sintering temperatures of the membranes. Compared to methylcellulose and polyethylene glycol 20000, polyvinyl alcohol 1750 was found to be the most effective binder to fabricate a homogeneously structured Al₂O₃ membrane without defects. The concentration to prepare an uniform coverage membrane with a thickness of ~10 μm was 0.032 mol/L. When sintered at 1000 °C, γ-Al₂O₃ membrane with ~3 μm grains was obtained. When sintered at 1200 °C, γ-Al₂O₃ completely transformed into α-Al₂O₃ and the grains grew to ~5 μm. Accordingly, the process was applied to a bigger pore-sized PSS with an average pore size of 1.5 μm to fabricate an α-Al₂O₃ intermediate layer to initially modify its surface. A single α-Al₂O₃ crystal layer with a thickness of ~5 μm and an average pore size of 0.7 μm was achieved. Subsequently, TiO₂, SiO₂, and TiO₂–SiO₂ hybrid membranes were tried on the modified PSS. Defect-free microfiltration membranes with average pore sizes of ~0.3 μm were readily fabricated. The results indicate that the sol–gel method is promising to initially modify the PSS substrates and the sol–gel-derived α-Al₂O₃ crystal layer is an appropriate intermediate layer to modify the PSS and to support smaller grain-sized top membranes.     

Multiwalled carbon nanotubes embedded in sol–gel derived TiO<Subscript>2</Subscript> matrices and their use as room temperature gas sensors

Sol–gel TiO₂/Multiwall Carbon Nanotubes composites obtained by the Ti-isopropoxide route are reported as room temperature sensitive coatings. The effect of functionalizing the carbon nanotubes in strong oxidizing acids prior to the sol–gel synthesis is studied in the detection of acetone (CH₃COCH₃) and ammonia (NH₃). Although, mixed oxide phases were found in the sol–gel oxide and composites, sensing was highly reproducible in the composites, with fast adsorption/desorption cycles at room temperature. Composites based on as-received multiwalled carbon nanotubes show an increase in films resistance during acetone and ammonia adsorption, while composites based on functionalized carbon nanotubes show longer acetone desorption times and a decrease in resistance during ammonia sensing. Acetone sensing in both composites contrast with the slight decrement in resistance observed in either pure titanium oxide or carbon nanotubes, in agreement with preliminary theoretical results. In contrast, the strong dependence of NH₃ to the presence of oxygen and hydroxyl atoms on the carbon nanotube surface is not well understood.     

Chemical and sol–gel processing of tellurite glasses for optoelectronics

Recent developments in the application of sol–gel processing technology for tellurite glass systems are reviewed and reported. The processing of telurite glasses via sol–gel entails some difficulties, mainly due to the anomalously high reactivity of Te(IV) alkoxides toward hydrolysis. Although conventional approaches to steric stabilisation of the alkoxides is not successful for these compounds, various successful approaches have been developed which allow the fabrication of transparent films from these precursors. In particular, diol complexation, chemical liberation of water from esterification processes and peptisation methods have been demonstrated. Other successful approaches involve the use of Te(VI) alkoxides and acids as precursors, with TeO₂ based glasses being formed via TeO₃ intermediates which liberate oxygen during heat treatment. One drawback with all these methods is the inherent thermal instability of the sol–gel derived material, which leads to both the liberation of free tellurium and devitrification of the glass on heat treatment. However this problem is less significant when Te(VI) precursors are used. The fabrication of multicomponent tellurite glasses by sol–gel approaches is very successful. Systems such as TeO₂–TiO₂ and TeO₂–PbO–TiO₂ have been successfully fabricated, and exhibit much greater resistance to devitrification allowing fully dense, transparent glasses to be produced.     

Microwave-assisted sol–gel process for production of spherical mesoporous silica materials

Spherical mesoporous SiO₂ and SiO₂–TiO₂ particles were synthesized by sol–gel method using W/O emulsion under microwave irradiation. In SiO₂ system, W/O emulsion was prepared by mixing partially hydrolyzed Si(OC₂H₅)₄ aqueous solution including C₁₈TAC as template with n-hexane solution including polyglycerol polyricinalate as emulsifier. In SiO₂–TiO₂ system, Ti(OC₂H₅)₄ capped by acetylacetone was added to the aqueous phase. In both cases, spherical products were synthesized by heating of W/O emulsion for 30 min under microwave irradiation. The specific surface area and pore size of spherical products were 800 m²/g and 1.6 nm, respectively, which indicates that the spherical products are mesoporous. These results suggest that sol–gel reaction in water phase proceeds rapidly because microwave quickly and selectively heats up the aqueous solution.     

Synthesis and photocatalytic performance of TiO<Subscript>2</Subscript> nanospheres–graphene nanocomposite under visible and UV light irradiation

In this article, we present a fast and simple method to produce TiO₂ nanospheres–graphene nanocomposite with high photocatalytic activity under visible and UV light irradiation. TiO₂ nanospheres were adsorbed on graphene in sol–gel process. First, titanium (IV) butoxide underwent hydrolysis in graphene oxide (GO) ethanol solution resulting in TiO₂ nanospheres deposition on GO. Next, the material was calcinated to generate the phase transition of TiO₂ into anatase and reduce GO to graphene. The detailed characterization of the material was performed via transmission electron microscopy, energy dispersive X-rays spectrometer, Fourier-transformed infrared spectroscopy, X-ray diffraction, and Raman spectroscopy. Interestingly, the band-gap energy of the prepared photocatalyst was drastically decreased in comparison with the commercial photocatalyst P25 from 3.05 to 2.36 eV. This influenced in the activation of the material under visible light and resulted in high photocatalytic activity in the process of phenol decomposition in visible and UV irradiation.     

Poly(methyl methacrylate)-TiO<Subscript>2</Subscript> nanocomposite obtained by non-hydrolytic sol–gel synthesis

Poly(methyl methacrylate) (PMMA)/titanium dioxide (TiO₂) nanocomposites were prepared by means of in situ generation of TiO₂ through a non-hydrolytic sol–gel process (NHSG), starting from titanium chloride, as titania precursor, benzyl alcohol, as oxygen donor, and commercial PMMA. TiO₂ nanoparticles (average size of 30 nm) were obtained in the anatase and amorphous forms. The in situ generation led to a very homogeneous distribution of particulate fillers within the polymeric matrix avoiding the problems related to distributive and dispersive mixing of conventional compounding methods (top down approach). A slight increase of glass transition temperature was observed for all prepared composites with respect to the pristine PMMA. The NHSG process did not affect the molecular weight of the polymers indicating the absence of any degradation reaction for PMMA. The presence of titania in the anatase phase increases the photodegradation of the PMMA matrix due to UV irradiation.     

Effect of the La<Subscript>2</Subscript>O<Subscript>3</Subscript> sol–gel coating on the alumina scale adherence on a model Fe–20Cr–5Al alloy at 1100 °C

The effect of lanthanum sol–gel coatings was studied in order to improve the alumina scale adherence during the model Fe–20Cr–5Al alloy oxidation, at 1100 °C, in air. Various sol–gel coating procedures were applied. Argon annealing of the lanthanum sol–gel coating was tested at temperatures ranging between 600 and 1000 °C. The coating crystallographic nature was characterized by X-ray diffraction (XRD) depending on the annealing temperature. The oxidation process has been examined at 1100 °C by in situ XRD on blank Fe–20Cr–5Al, sol–gel coated and argon-annealed specimens. This study shows that the coating argon annealing at 1000 °C leads to the preferential formation of LaAlO₃ instead of La₂O₃. This coating procedure leads to an alumina scale formation showing the best adherence under thermal cycling conditions at 1100 °C.     

A review on nano-TiO<Subscript>2</Subscript> sol–gel type syntheses and its applications

Nanomaterials, defined as particles ranging from 1 to 100 nm diameter, have become widely utilized because of their unique physicochemical properties. Among those nanoparticles, titanium dioxide (TiO₂) is frequently used in the production of paints, paper, plastics, welding rod-coating material, cosmetics, etc. TiO₂ is the most commonly used semiconductor photocatalyst. Among the different nanomaterials, it is the most studied. Activated by UV-A irradiation, its photocatalytic properties have been utilized in various applications. A wealth of information on TiO₂ photocatalytic in activation of bacteria has been acquired over the last 20 years. Hence, in this review article we have described synthesis methods mainly sol–gel type method like sol–gel method, ultrasonic-assisted sol–gel method, microemulsion method, colloidal synthesis, and also other method are discussed like solvo-thermal method, thermal plasma process, supersonically expanded plasma jet method, induction plasma torch, reactive plasma processing, plasma electrolytic oxidation, hydrolysis method, thermohydrolysis method, coprecipitation method, citrate–nitrate autocombustion method, etc. Also applications of TiO₂ like medical applications, environmental application, sensor application, photocatalytic applications, and also its health impact for long-term exposure are discussed.     

Two-step sol–gel method for preparing MgO–MnO2–B2O3–Li2O co-added core-shell structural Ba0.55Sr0.40Ca0.05TiO3 powders and the dielectric properties of the composite ceramics

The Ba_0.55Sr_0.4Ca_0.05TiO₃–MgO (BSCT–MgO) composite powders possessing core-shell structure, small particle size and high specific surface area were prepared by the novel two-step sol–gel method which was based on the citric acid (CA)–ethylene glycol (EG) system. In this experiment, the Ba_0.55Sr_0.40Ca_0.05TiO₃ (BSCT) powders were firstly synthesized by chemical co-precipitation method. MgO [(MgO/BSCT)_mass = 1] and MnO₂ [(MnO₂/BSCT)_mole = 0.01] were added into the BSCT powders in the first sol–gel step. 1.5 wt% B₂O₃–Li₂O as sintering aids was added into the composite powders during the second sol–gel step. The BSCT–MgO composite powders were detected to have a perfect core-shell structure which was detected by the transmission electron microscope. Perovskite BSCT and periclase MgO were confirmed to exist in the BSCT–MgO composite powders according to the X-ray diffraction patterns. The Ba_0.55Sr_0.4Ca_0.05TiO₃–MgO (BSCT–MgO) composite ceramics that synthesized with the core-shell powders were sintered at 1,000 °C for 2 h. According to the scanning electron microscope images, the grain sizes of BSCT and MgO in the BSCT–MgO composite ceramics were in the range of 0.5–2.0 μm and 1.0–2.5 μm, respectively. The dielectric constant of the BSCT–MgO composite ceramics was 210 and the dielectric loss was 0.0012 when tested at 1 MHz, room temperature. The BSCT–MgO composite ceramics were expected to be a promising candidate for applying as phase shifters or tunable components, etc, in the microwave field.     

Sol–gel preparation and characterization of perovskite lanthanum lithium titanate

Lanthanum lithium titanate, belonging to the solid solution La_0.66Li_0.33TiO₃, was prepared by sol–gel method from LiNO₃, La(NO₃)₃·6H₂O and Ti(OC₄H₉)₄ in ideal cation stoichiometry for La_0.66Li_0.33TiO₃. The synthesized product was characterized by X-ray powder diffractometry (XRD) and thermal analyses (DTA-TG). Electron microscopy investigations (SEM and TEM) was used to evaluate the morphology of synthesized La_0.66Li_0.33TiO₃. The bulk quantities of nano-sized particles of layered La_0.66Li_0.33TiO₃ have been obtained at 1,000°C using the modified sol–gel method. The mean diameter of La_0.66Li_0.33TiO₃ nano-crystals was about 123 nm.     

Preparation and characterization of novel yellow pigments: hollow TiO<Subscript>2</Subscript> spheres doped with cerium

Ce-doped TiO₂ hollow yellow pigment particles were synthesized by coupling template-directed method with Pechini sol–gel process. The effects of water content, ethanol/acetonitrile volume ratio and tetrabutyl orthotitanate concentration, on the fabrication of PS@TiO₂ composite particles (the key intermediate product) were investigated and the final pigments were characterized in detail by X-ray diffraction, transmission electron microscopy, scanning electron microscope, X-ray photoelectron spectroscopy, and UV–vis diffuse reflection. The results show that the optimal water content and ethanol/acetonitrile volume ratio are 0.09 mol dm⁻³ and 3:1, respectively, for the construction of neat PS@TiO₂ core–shell structure without secondary titania particles, and that the damage of hollow spheres can be avoided by increasing the shell thickness, and that the prepared hollow spheres were well-crystallised with anatase phase TiO₂ and cubic CeO₂. Owing to the intrinsic yellow color and lower density, the as-prepared hollow pigments can be expected to be used for color electronic paper display.     

Electro-conductive porous ceramics prepared by chemical vapor infiltration of TiN

Using pressure-pulsed chemical vapor infiltration (PCVI) method, TiN was partially infiltrated at 850 °C from gas system of TiCl₄ (1%)–N₂ (10%)–H₂ into the highly porous carbon substrates prepared by the carbonization of cotton-wool, filter paper, and wood at 1,000 °C in Ar for 4 h. After 10,000 pulses of PCVI, electro-conductive porous ceramics having the three-dimensionally continuous current paths were obtained, which had the porosity of 80% and more, the resistivity of 10⁻⁵–10⁻⁶ Ω m, and the average pore sizes of 10–40 μm. The geometric surface area per unit volume of the sample was higher than that of the conventional foil-type current collector for lithium-ion battery. The surface area showed the highest value for the sample obtained from carbonized wood substrate.