Sol–gel preparation of titania multilayer membrane for photocatalytic applications

The main goal of the present study is to prepare a titania membrane with high permeability and photocatalytic activity for environmental applications. In this investigation a mesoporous titania multilayer membrane on alumina substrate is successfully fabricated via the sol–gel processing method. The prepared titania polymeric sol for the membrane top layer has an average particle size of 11.7 nm with a narrow distribution. The resulting TiO₂ multilayer membrane exhibits homogeneity with no cracks or pinholes, small pore size (4 nm), large specific surface area (83 m²/g), and small crystallite size (10.3 nm).The permeability and photocatalytic properties of the titania membrane were measured. The photoactivity of the titania membrane was examined to be 41.9% after 9 h UV irradiation based on methyl orange degradation. This measurement indicates high photocatalytic activity per unit mass of the catalyst. Through multilayer coating procedure, the photocatalytic activity of the membrane improved by 60% without sacrificing the membrane permeation. The prepared TiO₂ photocatalytic membrane has a great potential in developing high efficient water treatment and reuse systems due to its multifunctional capability such as decomposition of organic pollutants and physical separation of contaminants.     

TiO2 nanofibers doped with rare earth elements and their photocatalytic activity

In the present study rare earth doped (Ln³⁺–TiO₂, Ln = La, Ce and Nd) TiO₂ nanofibers were prepared by the sol–gel electrospinning method and characterized by XRD, SEM, EDX, TEM, and UV-DRS. The photocatalytic activity of the samples was evaluated by Rhodamine 6G (R6G) dye degradation under UV light irradiation. XRD analysis showed that all the synthesized pure and doped titania nanofibers contain pure anatase phase at 500 °C but at 700 °C it shows both anatase and rutile phase. XRD result also shows that Ln³⁺-doped titania probably inhibits the phase transformation. The diameter of nanofibers for all samples ranges from 200 to 700 nm. It was also observed that the presence of rare-earth oxides in the host TiO₂ could decrease the band gap and accelerate the separation of photogenerated electron–hole pairs, which eventually led to higher photocatalytic activity. To sum up, our study demonstrates that Ln³⁺-doped TiO₂ samples exhibit higher photocatalytic activity than pure TiO₂ whereas Nd³⁺-doped TiO₂ catalyst showed the highest photocatalytic activity among the rare earth doped samples.     

Preparation and photocatalytic properties of macroporous honeycomb alumina ceramics used for water purification

In this work, porous alumina ceramics with highly ordered capillaries were successfully fabricated by ionotropic gelation process of alginate/alumina suspensions. By varying the initial solid loading (10–30 wt%) of slurries, the porosity of alumina ceramics ranged from 60.4% to 79.5% with controlled pore size (180–315 μm). Due to the well-crosslinked macroporous structure and large specific surface areas, the porous ceramics were utilized as the photocatalyst supports of TiO₂ catalysts whose photocatalytic activity was characterized by degrading methyl blue under UV irradiation. TiO₂ coatings prepared by sol–gel method demonstrated excellent adhesion to the substrates. When the solid loading of supports reached 15 wt%, the TiO₂ coatings showed the highest photocatalytic efficiency of 79.52%. Besides, TiO₂ films possessed nearly the same photocatalytic activity as titania/water suspension. Thus, the honeycomb ceramic prepared by self-organization process holds promise for use as photocatalyst supports in water purification without recycling process of powders.     

TiO2 photocatalysis in cementitious systems: Insights into self-cleaning and depollution chemistry

The present work offers a general overview about application of titanium dioxide (or titania), TiO₂, photocatalysis to concrete technology in relation to enhanced aesthetic durability and depollution properties achieved by implementing TiO₂ into cement. Chemistry of degradation of Rhodamine B (RhB), a red dye used to assess self-cleaning performances of concretes containing TiO₂, as well as oxidation of nitrogen oxides (NOx), gaseous atmospheric pollutants responsible for acid rains and photochemical smog, is investigated using two commercial titania samples in cement and mortar specimens: a microsized, m-TiO₂ (average particle size 153.7 nm ± 48.1 nm) and a nanosized, n-TiO₂ (average particle size 18.4 nm ± 5.0 nm). Experimental data on photocatalytic performances measured for the two samples are discussed in relation to photocatalyst properties and influence of the chemical environment of cement on titania particles. Impacts on applications in construction concrete are also discussed.     

Preparation of freestanding and crack-free titania–silica aerogels and their performance for gas phase,photocatalytic oxidation of VOCs

Freestanding and crack-free titania–silica aerogels with high titanium content (i.e., Ti/Si = 1) were successfully prepared by adjusting the hydrolysis of the two alkoxide precursors to a comparable rate during the sol–gel processing. Two titania–silica aerogels were prepared by ethanol and CO₂ supercritical drying methods. Well-dispersed, nanometer-sized anatase crystal domains (ca. 10 nm) were crystallized by high temperature, ethanol supercritical drying. The crystalline domains were solidly anchored to the aerogel network by Ti–O–Si bonds. Titania–silica aerogels prepared by CO₂ supercritical drying method were devoid of TiO₂ crystals. A molecular-level mixing was achieved and anatase TiO₂ was only crystallized with difficulty by high temperature calcination (1073 K). Both aerogels were mesoporous and displayed similar open pore structure that is readily accessible to reactant molecules. However, only the titania–silica aerogel with anatase TiO₂ prepared by ethanol supercritical drying was active for the gas phase, photocatalytic oxidation of volatile organic compounds (i.e., isopropanol and trichloroethylene). Catalysts prepared from Degussa P25 TiO₂ displayed lower activity under similar reaction conditions.     

Polyimide/nano-TiO2 hybrid films having benzoxazole pendent groups: In situ sol–gel preparation and evaluation of properties

Polyimide/titania (PI/TiO₂) nanocomposite films have been successfully fabricated through the in situ formation of TiO₂ within a PI matrix via sol–gel method. Poly(amic acid) (PAA), which is the precursor of PI, was successfully synthesized by mixing pyromellitic dianhydride (PMDA), with equimolar amount of a diamine monomer having a pendent benzoxazole unit and two flexible ether linkages in N,N-dimethylformamide (DMF) solvent. Tetraethyl orthotitanate [Ti(OEt)₄] and acetylacetone were then added to the resulted PAA. After imidization at high temperature, PI/TiO₂ hybrid films were formed. The structure and morphology of the hybrid nanocomposites with different titania contents (0 wt%, 5 wt%, 10 wt%, and 15 wt%) were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, and transmission electron microscopy. The results indicate that the TiO₂ nanoparticles were homogeneously dispersed in the hybrid films. The thermogravimetric analysis of nanocomposites confirms the improvement in the thermal stability with the increase in the percentage of titania nanoparticle. Transmission electron microscopy showed that the nanoparticles with an average diameter of 25–40 nm were dispersed in the polymer matrix.     

Iron-doped TiO2 nanotubes with high photocatalytic activity under visible light synthesized by an ultrasonic-assisted sol-hydrothermal method

A series of iron-doped anatase TiO₂ nanotubes (Fe/TiO₂ NTs) catalysts with iron concentrations ranging from 0.88 to 7.00 wt% were prepared by an ultrasonic-assisted sol-hydrothermal process. The structures and the properties of the fabricated Fe/TiO₂ NTs were characterized in detail and photocatalytic activity was examined using a reactive brilliant red X-3B aqueous solution as pollutant under visible light. The lengths of the NTs were determined to range from 20 nm to 100 nm. The incorporation of the iron ions (Fe³⁺) into the TiO₂ nanotubes shifted the photon absorbing zone from the ultraviolet (UV) to the visible wavelengths, reducing the band gap energy from 3.2 to 2.75 eV. The photocatalytic activity of the Fe/TiO₂ NTs was 2–4 times higher than the values measured for the pure TiO₂ nanotubes.     

Photocatalytic activity of titanium dioxide coatings: Influence of the firing temperature of the chemical gel

Heterogeneous photocatalysis can be exploited for the decomposition of micro-organisms which have developed on the surfaces of building materials. In this work, the efficiency of titanium dioxide coatings on fired clay products is examined. The sol–gel method is used to synthesize a fine TiO₂ powder with a specific surface area of 180 m² g^?1. Thermal treatment of the chemical gel at 340 °C leads to crystallisation in the anatase phase and with further temperature increase, crystallite growth. For thermal treatments in the range 580–800 °C, there is a progressive transition from anatase to rutile. However, despite a decrease in specific surface area of the powder attributed to aggregation/agglomeration, the coherent domain size deduced from X-ray diffraction measurements remains almost constant at 23 nm. Once the transition is completed, increase of thermal treatment temperature above 800 °C leads to further crystallite growth in the rutile phase. The thermally treated titania powders were then sprayed onto fired clay substrates and the photocatalytic activity was assessed by the aptitude of the coating to degrade methylene blue when exposed to ultraviolet light. These tests revealed that the crystallite size is the important controlling factor for photocatalytic activity rather than the powder specific surface area or the anatase/rutile polymorph ratio.     

Dispersion of mixtures of submicrometer and nanometre sized titanias to obtain porous bodies

The stability and rheological behaviour of bimodal titania suspensions was studied. Bimodal mixtures were prepared by mixing nanosized TiO₂ powders with an average primary size of ～20–40 nm and surface area of ～50 m² g^?1 and/or a colloidal titania suspension of the same nanopowders dispersed in water with a submicrometer sized titania. The dispersing conditions were studied as a function of pH, type and content of dispersant, and sonication time for a constant solids content of 30 vol% (62 wt%). The mixtures were slip cast and presintered at low temperatures (800–1000 °C) in order to obtain porous materials with anatase as the major phase. The pore size distribution, microstructure and phase composition were characterised using MIP, SEM and XRD techniques, respectively.     

A comparative study of the effect of Pd-doping on the structural,optical, and photocatalytic properties of sol–gel derived anatase TiO2 nanoparticles

Pd-doped anatase TiO₂ nanoparticles were synthesized by a modified sol–gel deposition technique. The synthetic strategy is applicable to other transition and post-transition metals to obtain phase-pure anatase titania nanoparticles. This is important in the sense that anatase titania forms the most hydroxyl radicals (compared to other polymorphs like rutile, brookite, etc.) for better photocatalytic performance. XRD and Raman data confirm the phase-pure anatase formation. Doping of Pd²⁺ into Ti⁴⁺ sites (for substitutional doping) or interstitial sites (for interstitial doping) creates strain within the nanoparticles and is reflected in the XRD peak broadening and Raman peak shifts. This is because of the ionic radii difference between Ti⁴⁺(∼68 pm) and Pd²⁺(∼86 pm). XPS data confirm the formation of high surface titanol groups at the nanoparticle surface and a large number of loosely bound Ti³⁺–O bonds, both of which considerably enhance the photocatalytic activity of the doped nanoparticles. A comparative study with other metal doping (Ga) shows that TiO₂: Pd nanoparticles have more Ti³⁺–O bonds, which enhance the charge transfer rate and hence improve the photocatalytic activity compared to other transition and post-transition metal-doped titania nanostructures.     

Effect of TiO2 doping on the characteristics of macroporous Al2O3/TiO2 membrane supports

A cost-effective tubular macroporous ceramic support consisting of alumina and titania was prepared by extrusion and subsequent heat treatment. An Al₂O₃/TiO₂ composite support with high porosity (41.4%), an average pore size of 6.8 μm and sufficient mechanical strength (32.7 MPa) was obtained after sintering at 1400 °C. The formation mechanism of this support as investigated with X-ray micromapping, SEM and XRD indicated that the appearance of Al₂TiO₅ plays a key role in the fabrication of high performance composite membrane supports at relatively low temperature. The amount of Al₂TiO₅ present in the composite has a strong impact on the properties of supports, especially with regard to the mechanical strength. A composite of 85 wt.% Al₂O₃/15 wt.% TiO₂ sintered at 1400 °C for 2 h exhibited both high permeability (pure water flux of 45 m³ m^?2 h^?1 bar^?1), together with an excellent corrosive resistance towards hot NaOH and HNO₃ solutions.     

Synthesis of hierarchical mesoporous titania with interwoven networks by eggshell membrane directed sol–gel technique

Hierarchical mesoporous titania with interwoven networks was successfully prepared through a surface sol–gel process followed by a calcination treatment and using eggshell membrane (ESM) as the biotemplate. The biotemplating synthesis was systematically investigated by controlling calcination temperature (550–800 °C), heating rate (1–35 °C/min), impregnant pH value (1–3), and so on. Different from traditional immersion techniques, the nucleation, the growth, and the assembly of mesoporous TiO₂ in our work depended more on some reactions involving ESM biomacromolecules. As-prepared ESM-morphic TiO₂ was composed of intersectant fibers assembled by 6 nm nanocrystallites at 3D with hierarchical pores from 2 nm up to 8 μm.     

Fe2WO6/TiO2, an efficient visible-light photocatalyst driven by hole-transport mechanism

Fe₂WO₆ nanocrystal with an average size of ~ 300 nm was prepared by solid-state reaction between Fe₂O₃ and WO₃ at 950 °C. The coupled structure of Fe₂WO₆/TiO₂ was then fabricated by covering the Fe₂WO₆ surface with polycrystalline TiO₂ by sol–gel process. Under visible-light irradiation, the Fe₂WO₆/TiO₂ exhibits remarkable photocatalytic activity in decomposing gaseous 2-propanol and aqueous salicylic acid which is comparable to that of typical nitrogen-doped TiO₂. It is deduced that its high catalytic activity originates from the hole-transfer from Fe₂WO₆ to TiO₂. Evidences for the hole-transport were provided by monitoring the hole-scavenging reactions, employing iodide (I⁻) and 1,4-terephthalic acid (TA), respectively.     

Mesoporous TiO2–SiO2 aerogels with hierarchal pore structures

Freestanding blocks of binary oxides, TiO₂–SiO₂ aerogel containing highly ordered mesophase structures were successfully prepared by a new synthesis method involving partial solvent evaporation followed by supercritical extraction and drying. The new method allows the routine preparation of large, crack-free aerogels of high titanium content (i.e., Ti/Si ? 0.75 or up to 50 wt.% Ti), ordered mesopores (i.e., 2–20 nm), large surface area (i.e., 400–900 m² g^?1) and pore volume (i.e., 0.7–2.6 cm³ g^?1). Aerogels with well-ordered mesopores were obtained for Ti/Si atom ratios of 0.04–0.08. The size of ordered mesopore domains decreases with increasing titanium loading, and TS75 aerogels with Ti/Si = 0.75 display discontinuous microdomains of ordered mesoporosity within disordered phases interspersed with crystalline anatase TiO₂. The greater permeability of the TS75 pore network resulted in fifteen times better activity for photocatalytic oxidation of airborne trichloroethylene compared to commercial Degussa P25 TiO₂ and more than twice that TiO₂–SiO₂ aerogel (TS100) of similar titanium loading but with disordered and tortuous pore network.     

Synthesis of carbon-doped photocatalytic TiO2 nano-powders by AFD process

Carbon-doped titanium dioxide (TiO₂) nano-powders were synthesized by the aerosol flame deposition (AFD) process using 2-butanol liquid sol containing 20 wt% of titanium isopropoxide (TTIP). They were mesoporous nano-powders with particle size ranging from 20 to 40 nm, the specific surface area of 36 m²/g, and the pore size of 19 nm. They had the anatase structure and showed high photocatalytic activity not only under UV-A light but also under fluorescent light. They reduced the concentration of methylene blue (MB) from 5.0 to 1.5 ppm within 2 h under UV-A light and from 5.0 to 2.0 ppm within 4 h under fluorescent light. And, they killed 99% of Escherichia coli (E. coli) cells within 2 h and bacterial growth of the E. coli was not observed for 12 h under both UV-A and fluorescent lights.     

Low temperature hydrothermal synthesis of monodispersed flower-like titanate nanosheets

Monodispersed flower-like titanate superstructure was successfully prepared by simple hydrothermal process without any surfactant or template. N₂-sorption analysis, scanning electron microscopy (SEM), and X-ray diffraction (XRD) observation of as-synthesized product revealed the formation of flower-like titanate with diameter of about 250–450 nm and BET surface area (S_BET) of 350.7 m² g^?1. Upon thermal treatment at 500 °C, the titanate nanosheets were converted into anatase TiO₂ with moderate deformation of their structures. The as-prepared flower-like titanate showed high photocatalytic activity for H₂ evolution from water splitting reaction. Moreover, the sample heat treated at 500 °C exhibited higher photocatalytic activity than that of commercial TiO₂ anatase powder (ST-01).     

Palygorskite and SnO2–TiO2 for the photodegradation of phenol

The photocatalytic removal of phenol was studied using palygorskite-SnO₂–TiO₂ composites (abbreviated as Paly-SnO₂–TiO₂) under ultraviolet radiation. The photocatalysts were prepared by attachment of SnO₂–TiO₂ oxides onto the surface of the palygorskite by in situ sol–gel technique. The products were characterized by XRD, TEM and BET measurements. SnO₂–TiO₂ nanoparticles, with an average diameter of about 10 nm, covered the surface of the palygorskite fibers without obvious aggregation. Compared with palygorskite-titania (Paly-TiO₂), palygorskite-tin dioxide (Paly-SnO₂), and Degussa P25, Paly-SnO₂–TiO₂ and SnO₂–TiO₂ exhibited much higher photocatalytic activity. The photodecomposition of phenol was as high as 99.8% within 1.5 h. The apparent rate constants (k_app) for Paly-SnO₂–TiO_2, TiO₂, and P25 were measured. Paly-SnO₂–TiO₂ showed the highest rate constant (0.03435 min^?1). The chemical oxygen demand (COD) of the phenol solution was reduced from 220.2 mg/L to 0.21 mg/L, indicating the almost complete decomposition of phenol. Reusability of the photocatalyst was proved.     

Visible-light-induced photocatalytic activity of TiO2−xNy prepared by solvothermal process in urea–alcohol system

Nitrogen-doped anatase, rutile and brookite titania photocatalyst TiO_2−xN_y which can be excited by visible light were prepared by mixing aqueous TiCl₃ solutions with urea ((NH₂)₂CO) and various type of alcohols followed by solvothermal treatment at 190 °C. The phase composition, crystallinity, microstructure and specific surface area of titania powders greatly changed depending on the pH and type of solvents. Violet, yellowish and grayish TiO_2−xN_y with excellent visible light absorption and photocatalytic activity were prepared. The TiO_2−xN_y powders prepared in urea–methanol solution showed excellent photocatalytic ability for the oxidative destruction of nitrogen monoxide under irradiation of visible light λ > 510 nm.     

Microwave dielectric properties of nanocrystalline TiO2 prepared using spark plasma sintering

TiO₂ bulk ceramics were fabricated by using both spark plasma sintering (SPS) and the conventional sintering method (CSM). Starting materials were ultra fine rutile powders (<50 nm) prepared via the sol–gel process. CSM achieved the relative sintering density of 99.2% at 1300 °C. The grain size of 1300 °C sintered specimen was 6.5 μm. However, the sintering temperature of SPS for the density of 99.1% was as low as 760 °C, where the grain size was only 300 nm. In order to re-oxidize the Ti³⁺ ions due to the reducing atmosphere of the SPS process and the high temperature of the CSM process, the prepared TiO₂ specimens were annealed in an oxygen atmosphere. The dielectric constant (ɛ_r) and quality factor (Q × f) of SPS-TiO₂ re-oxidized specimens in a microwave regime were 112.6 and 26,000, respectively. These properties were comparable to those of 1300 °C sintered CSM specimens (ɛ_r ∼ 101.3, Q × f ∼ 41,600). These microwave dielectric properties of nanocrystalline TiO₂ specimens prepared using SPS were discussed in terms of grain size variation and Ti⁴⁺ reduction.     

Transparent cubic-ZrO2 ceramics for application as optical lenses

Optical transparent polycrystalline ZrO₂ ceramics were fabricated by solid-state sintering process using first vacuum sintering followed by hot isostatic pressing. In the visible wavelength range (400–800 nm), the in-line transmittance of 5.6-mm thick samples reaches 68% at exemplary wavelength 600 nm (corresponding to an in-line absorbance based on 10 of A₁₀ = 0.08 cm^?1), which is approximately 90% of theoretical limit. The refractive indices of the ZrO₂ optoceramics at 630 nm (n_d) are varying between 2.10 and 2.20, depending on TiO₂ contents, the latter being used as sintering aid. The appearance of birefringence is strongly correlated to the addition of TiO₂ as sintering additive in the ceramic samples, whereas addition of TiO₂ and simultaneous increase in Y₂O₃ content resulted in a decrease of birefringence.