Photocatalytic activity of oxide coatings on fired clay substrates

The coating of fired clay substrates with various metal oxides, such as anatase, rutile, zinc oxide and tin oxide was achieved using a simple spraying technique followed by a thermal treatment. The photocatalytic activity of the layer was characterized through measurement of the absorption spectrum, in the range 400–800 nm, of methylene blue deposited on top of the coating. Results show that the presence of anatase enhances the degradation of methylene blue when it is exposed to ultraviolet light. Thermal treatment at 1050 °C transforms anatase crystals into thermodynamically stable rutile. This results in a decrease of the photocatalytic activity, which can be explained by increase of the grain size and by a difference in the crystal structure. Measurements of the photocatalytic activity of ZnO and SnO₂ show that these two oxides also exhibit photocatalytic properties. In particular, ZnO is a promising alternative material to anatase.     

Synthesis and photocatalysis of TiO2 hollow spheres by a facile template-implantation route

Process variables such as reaction temperature (55 to 90 °C), calcination temperature (450 to 750 °C), and concentration of TiCl₄ precursor (26 to 105 mM) have been examined in order to tailor the surface area, crystallite size, and the anatase/rutile ratio of the polycrystalline TiO₂ microcapsules prepared by a template-implantation route in heptane solvent. The hollow capsules are all non-aggregating with nanoporous shell structure. Among the process variables examined, the Brunauer–Emmett–Teller (BET) surface area and the anatase/rutile ratio are found critically dependent on the reaction temperature, in which a reduced reaction temperature (from 90 to 55 °C) leads to a higher BET value (from 8.4 to 36.4 m^?2 g^?1), a predominant anatase phase (weight fraction of the anatase phase increases from 0.20 to 0.84), and an improved photodegradation of aqueous methylene blue (MB) dye under UV exposure (the degradation rate increases from 0.5×10^?2 to 5.5×10^?2 min^?1).     

Effects of SiO2 addition on TiO2 crystal structure and photocatalytic activity

A series of TiO₂–SiO₂ mixtures – having the following stoichiometry Ti_1?xSi_xO₂, with x = 0, 0.1, 0.3 and 0.5 atoms per formula unit – were prepared by using precursor oxides and fired at three temperatures (900, 1000 and 1200 °C). The modifications in the structure and, consequently, on the photocatalytic activity, induced by the addition of SiO₂ into the TiO₂ powder, were thoroughly investigated by using various analytical techniques: X-ray powder diffraction, electron microscopy (FE-SEM and TEM), XPS, FT-IR, DRS and BET analysis. The results underlined as essentially no solid solution occurs between the two crystalline end-members. Nevertheless, silica addition caused a retarding effect on anatase-to-rutile phase transformation and on the crystallite growth.The photocatalytic activity of the powders was assessed in gas phase and the results were explained by taking into account the anatase and rutile relative amounts in the samples, their crystallite size, the surface hydroxyl groups adsorbed on the photocatalysts and the surface area of the mixtures.     

Chemical oxidation of residual carbon from ZnAl2O4 powders prepared by combustion synthesis

The removal of carbon residue from ZnAl₂O₄ nanopowders by annealing at 500–800 °C leads to a decrease of specific surface area from 228.1 m²/g to 47.6 m²/g. At the same time, the average crystallite size increased from 5.1 nm to 14.9 nm. In order to overcome these drawbacks, a new solution for removing the carbon residue has been suggested: chemical oxidation using hydrogen peroxide. In terms of carbon removal, a H₂O₂ treatment for 8 h at 107 °C proved to be equivalent to a heat treatment of 1 h at 600 °C. The benefits of chemical oxidation over thermal oxidation were obvious. The specific surface area was much larger (188.1 m²/g) in the case of the powder treated with H₂O₂. The average crystallite size (5.8 nm) of ZnAl₂O₄ powder treated with H₂O₂ was smaller than the crystallite size (8.2 nm) of the ZnAl₂O₄ powder annealed at 600 °C.     

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).     

Self-assembly hydrothermal assisted synthesis of mesoporous anatase in the presence of ethylene glycol

Mesoporous nanocrystalline anatase was prepared hydrothermally employing P123 as structure-directing agent. Ethylene glycol was used as a key synthesis parameter to fine tune the morphology, crystal size and pore size of the resultant mesophases. The incorporation of EG in the synthesis gel resulted in the formation of 1–2 μm sphere-like shapes and led to an increase in the specific surface area from ∼95 to ∼170 m²/g, decrease in the average pore size from ∼11 to ∼4.8 nm, and decrease in the average crystallite size from ∼17 to ∼12 nm. These mesophases were used as photocatalysts for the UV degradation of methylene blue and methyl orange. The mesoporous anatase phases photodegraded MB ∼1.5–3× faster than commercially available P25 and showed limited photocatalytic behavior for methyl orange.     

Suspension high velocity oxy-fuel spraying of a rutile TiO2 feedstock: Microstructure,phase evolution and photocatalytic behaviour

Nano-structured TiO₂ coatings were produced by suspension high velocity oxy fuel (SHVOF) thermal spraying using water-based suspensions containing 30 wt% of submicron rutile powders (~180 nm). By changing the flame heat powers from 40 kW to 101 kW, TiO₂ coatings were obtained with distinctive microstructures, phases and photocatalytic behaviour. Spraying with low power (40 kW) resulted in a more porous microstructure with the presence of un-melted nano-particles and a lower content of the anatase phase; meanwhile, high powers (72/101 kW) resulted in denser coatings and rougher surfaces with distinctive humps but not necessarily with a higher content of anatase. Linear sweep voltammetry (LSV) was used to evaluate the photocatalytic performance. Surprisingly, coatings with the lowest anatase content (~20%) using 40 kW showed the best photocatalytic behaviour with the highest photo-conversion efficiency. It was suggested that this was partially owing to the increased specific surface area of the un-melted nano-particles. More importantly, the structural arrangement of the similarly sized TiO₂ nano-crystallites between rutile and antase phases also created catalytic “hot spots” at the rutile−anatase interface and greatly improved the photo-activity.     

Enhancement of the interfacial polarization resistance of La0.6Sr0.4Co0.2Fe0.8O3-δ cathode by microwave-assisted combustion method

Thermogravimetry, phase formation, microstructural evolution, specific surface area, and electrical properties of La_0.6Sr_0.4Co_0.2Fe_0.8O_3−δ (LSCF) cathode were studied as functions of its preparation technique. The pure perovskite LSCF cathode powder was synthesized through glycine–nitrate process (GNP) using microwave heating technique. Compared with conventional heating technique, microwave heating allows the rapid combustion to occur simultaneously between the nitrates and glycine in a controllable manner. The resulting powder is a single-phase nanocrystallite with a mean particle size of 113 nm and a high specific surface area of 12.2 m²/g, after calcination at 800 °C. Impedance analysis indicates that microwave heating has significantly reduced the polarization resistance of LSCF cathode. The area specific resistance (ASR) value of 0.059 and 0.097 Ω cm² at 800 °C and 750 °C, respectively, were observed. These values were twofold lower than the corresponding ASR of the cathode (0.133 and 0.259 Ω cm² at 800 °C and 750 °C, respectively) prepared through conventional heating. Results suggest that the microwave heating GNP strongly contributes to the enhancement of the LSCF cathode performance for intermediate temperature solid oxide fuel cells.     

Photocatalytic Nb2O5-doped TiO2 nanoparticles for glazed ceramic tiles

TiO₂ nanoparticles are currently used as coating for self-cleaning building products. In order to achieve high self-cleaning efficiency for outdoor applications, it is important that titania is present as anatase phase. Moreover, it is desirable that the particle sizes are in nano-range, so that a large enough surface area is available for enhanced catalytic performance. In this work, TiO₂ nanoparticles doped with 0–5 mol% Nb₂O₅ were synthesized by co-precipitation. Nb₂O₅ postponed the anatase to rutile transformation of TiO₂ by about 200 °C, such that after calcination at 700 °C, no rutile was detected for 5 mol% Nb₂O₅-doped TiO₂, while undoped TiO₂ presented 90 wt% of the rutile phase. A systematic decreasing on crystallite size and increasing on specific surface area of TiO₂ were observed with higher concentration of Nb₂O₅ dopant. Photocatalytic activity of anatase polymorph was measured by the decomposition rate of methylene blue under ultraviolet and daylight illumination and compared to commercial standard catalyst (P25). The results showed enhanced catalysis under UV and visible light for Nb₂O₅-doped TiO₂ as compared to pure TiO₂. In addition, 5 mol% Nb₂O₅-doped TiO₂ presented higher photocatalytic activity than P25 under visible light. The enhanced performance was attributed to surface chemistry change associated with a slight shift in the band gap.     

Synthesis,calcination and characterization of Nanosized ceria powders by self-propagating room temperature method

Nanometric ceria powders with fluorite-type structure were obtained by applying self-propagating room temperature method. The obtained powders were subsequently thermally treated (calcined) at different temperatures for different times. Powder properties such as specific surface area, crystallite size, particle size and lattice parameter have been studied. Roentgen diffraction analysis (XRD), BET and Raman scattering measurements were used to characterize the as-obtained (uncalcined) powder as well as powders calcined at different temperatures.It was found that the average diameter of the as-obtained crystallites is in the range of 3–5 nm whereas the specific surface area is about 70 m²/g. The subsequent, 15 min long, calcination of as-obtained powder at different temperatures gradually increased crystallite size up to ～60 nm and reduced specific surface down to 6 m²/g. Raman spectra of synthesized CeO_2?y depicts a strong red shift of active triply degenerate F_2 g mode as well as additional peak at 600 cm^?1. The frequency of F_2 g mode increased while its line width decreased with an increase in calcination temperature. Such a behavior is considered to be the result of particle size increase and agglomeration during the calcination. After the heat treatment at 800 °C crystallite size reached value larger than 50 nm. Second order Raman mode, which originates from intrinsic oxygen vacancies, disappeared after calcination.     

Probing the oxidation behavior of Ti2AlC MAX phase powders between 200 and 1000 °C

Oxidation of commercial Ti₂AlC MAX phase powders at 200–1000 °C has been investigated by XRD, XPS, SEM, STA and TGA coupled with FTIR. These powders are a mixture of Ti₂AlC, Ti₃AlC₂, TiC and Ti_1.2Al_0.8. Oxidation at 400 °C led to disappearance of carbide phases from Ti 2p, Al 2p and C 1s XPS spectra. At 600 °C, powders changed from dark grey to light grey with a significant volume increase due to crack formation. Powders were severely oxidized by detecting rutile with minor anatase TiO₂. At 800 °C, α-Al₂O₃ was detected while anatase transformed into rutile TiO₂. The cracks were healed and disappeared. At 1000 °C, the Ti₂AlC powders were fully oxidized into rutile TiO₂ and α-Al₂O₃ with a change of powder color from light grey to yellow. FTIR detected the release of C as CO₂ from 200 °C onwards but with additional CO above 800 °C.     

Enhanced photocatalytic activity of electrospun TiO2/ZnO nanofibers with optimal anatase/rutile ratio

The photocatalytic characteristics of the TiO₂/ZnO nanofibers synthesized by electrospinning followed by calcinating at different temperatures to alter the anatase-to-rutile ratio are investigated. The results demonstrate that the photocatalytic activity of TiO₂/ZnO nanofibers is enhanced by optimizing the anatase/rutile ratio among the trade-off effects of the band-gap energy, the electron/hole recombination rate, and the surface area. When calcined at 650 °C, the TiO₂/ZnO nanofibers with optimal anatase/rutile ratio (48:52) balancing these trade-off effects have the highest photocatalytic efficiency both in the degradation of RhB in liquid and conversion of NO gas.     

Investigations of BaFe0.5Nb0.5O3 nano powders prepared by a low temperature aqueous synthesis and resulting ceramics

A facile method to prepare nanoscaled BaFe_0.5Nb_0.5O₃ via synthesis in boiling NaOH solution is described herein. The nano-crystalline powder has a high specific surface area of 55 m² g⁻¹ and a crystallite size of 15 nm. The as-prepared powder does not show any significant crystallite growth up to 700 °C. The activation energy of the crystallite growth process was calculated as 590 kJ mol⁻¹. Dense ceramics can be obtained either after sintering at 1200 °C for 1 h or after two-step sintering at 1000 °C for 10 h. The average grain sizes of ceramic bodies can be tuned between 0.23 μm and 12 μm. The thermal expansion coefficient was determined as 11.4(3)·10⁻⁶ K⁻¹. The optical band gap varies between 2.90(5) and 2.63(3) eV. Magnetic measurements gave a Néel temperature of 20 K. Depending on the sintering regime, the ceramic samples reach permittivity values between 2800 and 137,000 at RT and 1 kHz.     

Gravimetric and spectroscopic studies of the chemical combination of moisture by as-fired and reheated terracotta

The application of microbalance measurements to investigate the chemical recombination of moisture with fired clay ceramics is demonstrated. The kinetics of mass gain at constant temperature and relative humidity (RH) are studied for terracotta fired at temperatures between 800 °C and 1200 °C. The experimental results show that mass gain proceeds in two stages. The second stage mass gain is taken as a measure of the capacity for long-term chemical combination with moisture based on the (time)^1/4 law. A maximum rate of mass gain is obtained for terracotta fired at 1000 °C. It is shown that reheating a sample of fired terracotta at any temperature between 500 °C and the original firing temperature will return the material to its as-fired state. This is supported by Raman spectroscopy. Exceeding the original firing temperature during reheating alters the subsequent rate of reaction to moisture, suggesting a method for determining the original firing temperature of ceramic artefacts.     

Facile synthesis of mesoporous titanium dioxide nanocomposites with controllable phase compositions by microwave-assisted esterification

Mesoporous titanium dioxide nanocomposites with controllable phase compositions and high surface areas were synthesized through convenient, fast, and one-step microwave-assisted esterification method. The introduction of microwave in the synthetic reactions not only accelerates the esterification reaction but also promotes the fast crystallization. By changing reaction temperature, microwave irradiation time, the amount of staring materials and the composition of solvents, pure anatase, pure rutile or mixed phase titanium dioxide nanocomposites were obtained in minutes. Meanwhile, the size of crystallite can be controlled by the reaction temperature. The as-synthesized materials display significative photocatalytic activities without any further disposal, among which the material with 3.6% rutile shows the best effect in degradation of methylene blue under UV-light irradiation. Moreover, the nitrogen adsorption–desorption results illustrate the obtained samples are mesophases. It is also interesting that the pure rutile has specific surface area as high as 210 m² g^?1 calculated by BET equation. The method can control the hydrolysis of TiCl₄ well and dramatically shorten the preparation time of titanium dioxide.     

Synthesis and photocatalytic performance of hollow sphere particles of SiO2-TiO2 composite of mesocellular foam walls

Hollow Microspheres of SiO₂-TiO₂ photocatalysts whose walls are made up of mesoporous cellular foams were synthesized with the aid of hexane as a swelling agent and P123 as a pore template by an emulsion templating method. Pore structure of materials and crystal phase of titanium oxide was tailored by hydrothermal and calcination temperature during synthesis of samples. The samples were characterized with field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), N₂ adsorption–desorption experiments, X-ray photoelectron spectroscopy (XPS) and X ray diffraction (XRD) techniques. The effect of pore structure and titania phase on photoactivity were evaluated by methylene blue (MB) degradation test under UV light as well. Results showed that hydrothermal temperature during synthesis process has a significant effect on pore and window sizes of mesostructured cellular foam. Interestingly, for the sample hydrothermally treated at higher temperature (130 °C), anatase to rutile transformation was avoided after calcination treatment as high as 800 °C. The highest photocatalytic activity was detected from the sample hydrothermally treated at 130 °C and calcined at 800 °C for which the highest degree of crystallinity and anatase phase as well as enhanced pore connectivity was obtained.     

Flame aerosol synthesis and characterization of pure and carbon coated titania nano powder

Carbon coated titanium dioxide (TiO₂) nano powder has been synthesized by combustion of TiCl₄ precursor in a laminar diffusion flame using inexpensive liquid petroleum gas (LPG) as a fuel, air as an oxidant, nitrogen gas as a carrier, and characterized with regard to phase(s), surface area, carbon content, morphology and optical absorption. The product is shown to contain both the anatase and rutile phases and exhibits (i) decrease in rutile content, (ii) increase in BET specific surface area, and (iii) increase in the amount of carbon (soot) present with increase in fuel flow rate. Further, the maximum attainable temperature depends on carbon content and determines the phase content and morphology of nano powder, e.g. spherical particles result and display reduced agglomeration when carbon content is more. The rutile phase essentially emerges by transformation of the anatase phase, formed initially with lattice parameters somewhat smaller than the bulk due to oxygen deficiency. On the other hand, use of oxygen (instead of air) leads to formation of spherical particles (average diameter ～104 nm) of a pure anatase phase (as transformation to rutile phase is totally suppressed) with lattice parameters, a=3.776(5) Å, c=9.507(5) Å, close to bulk.     

Catalytic activity of La-modified TiO2 for soot oxidation by O2

TiO₂ and La-doped TiO₂ samples with La loading between 0.2% and 2% have been prepared, characterised, and tested in the catalytic oxidation of soot by O₂. La doping prevents anatase to rutile phase transformation and crystallite size growth when heated at 800 °C, as deduced from XRD, Raman spectroscopy and BET surface area measurements. The optimum La loading was found to be 0.2%. The catalytic activity of TiO₂ and La-doped TiO₂ samples tested mainly depends on crystallite size, the lower the size the better the activity. TiO₂ phase composition is not a key factor in the catalytic oxidation of soot.     

One-step preparation of size-defined aggregates of TiO2 nanocrystals with tuning of their phase and composition

One-step route based on the thermal decomposition of the double salt (NH₄)₂TiO(SO₄)₂ (ammonium titanyl sulfate, ATS) is presented to prepare size-defined aggregates of Ti-based nanoparticles with structural hierarchy. The component of Ti-based networks is tunable from anatase/rutile TiO₂, nitrogen-doped TiO₂, TiN_xO_1−x, to TiN depending on the atmospheres and reaction temperatures. The as-prepared Ti-based powders were characterized by X-ray diffraction analysis (XRD), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectra (DRS), and BET surface area techniques. It is found that TiO₂ in the predominant rutile phase could be achieved by the thermal decomposition of ATS in flowing Ar gas. Furthermore, the nitrogen-doped TiO₂, TiN_xO_1−x solid solution and TiN were prepared by the thermal decomposition of ATS in flowing NH₃ gas by varying the temperatures. The network of anatase TiO₂ with a specific surface area up to 64 m² g⁻¹ contains large mesopores with a mean diameter of ca. 15 nm, and the large pore size allows more accessible surface and interface available for the photocatalytic degradation of large-molecule dyes. The photocatalytic activity of the prepared TiO₂ and nitrogen-doped TiO₂ under UV–vis light irradiation is compared to Degussa P-25 using the photocatalytic degradation of methylene blue (MB) as a model reaction. The anatase TiO₂ nanoparticles derived from one-step route show the highly efficient photocatalytic activity for the degradation of MB in comparison with Degussa P-25. The presence of large-sized rutile in the TiO₂ powder decreases the specific surface area and thus the powder exhibits a lower photocatalytic activity.     

Porcelain containing anatase and rutile nanocrystals

The aim of this work is to study the dielectric of two porcelains containing TiO₂ in the form of anatase and rutile. TiO₂ was added in compositions by means of raw kaolin with a relative high quantity of anatase, or the addition of anatase powder (10 wt%) in the initial mixture. An alternative porcelain containing kaolin–anatase mixture was obtained by a preliminary firing at 1300 °C. Beside kaolin, compositions also contain quartz and alkaline feldspar.The microstructural observations show various crystalline phases and micropores, which also have an effective role in affecting the properties. The dielectric characterization of fired porcelain, in the frequency range of 10⁵–10⁹ Hz, shows that permittivity value can be increased from 7.19 to 8.41, depending in TiO₂ crystal type, morphology and content. Permittivity depends also on mullite, quartz and cristobalite, quantities, but the role of TiO₂ phase is predominant. The macroscopic permittivity of porcelains can be calculated using a mixing rule, which fit accurately experimental results.