TiO2, TiO2/Ag and TiO2/Au photocatalysts prepared by spray pyrolysis

TiO₂, TiO₂/Ag and TiO₂/Au photocatalysts exhibiting a hollow spherical morphology were prepared by spray pyrolysis of aqueous solutions of titanium citrate complex and titanium oxalate precursors in one-step. Effects of precursor concentration and spray pyrolysis temperature were investigated. By subsequent heat treatment, photocatalysts with phase compositions from 10 to 100% rutile and crystallite sizes from 12 to 120 nm were obtained. A correlation between precursor concentration and size of the hollow spherical agglomerates obtained during spray pyrolysis was established. The anatase to rutile transformation was enhanced with metal incorporations and increased precursor concentration. The photocatalytic activity was evaluated by oxidation of methylene blue under UV-irradiation. As-prepared TiO₂ particles with large amounts of amorphous phase and organic residuals showed similar photocatalytic activity as the commercial Degussa P25. The metal incorporated samples showed comparable photocatalytic activity to the pure TiO₂ photocatalysts.     

Excellent antimicrobial properties of mesoporous anatase TiO2 and Ag/TiO2 composite films

Optically transparent, crack-free, mesoporous anatase TiO₂ thin films were fabricated. The Ag/TiO₂ composite films were prepared by incorporating Ag in the pores of TiO₂ films with an impregnation method via photoreduction. The as-prepared composite films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectronic spectra (XPS) and N₂ adsorption. The release behavior of silver ions in the mesoporous composite film was also studied. Moreover, the antimicrobial behaviors of the mesoporous film were also investigated by confocal laser scanning microscopy. The antibacterial activities of the composite films were studied by a fluorescence label method using Escherichia coli (E. coli) as a model. The as-prepared mesoporous TiO₂ films showed much higher antimicrobial efficiency than that of glass and commercial P25 TiO₂ spinning film. The facts would result from the high surface area, small crystal size and more active sites for the mesoporous catalysis. After the doping of Ag, a significant improvement for the antimicrobial ability was obtained. To elucidate the roles of the membrane photocatalyst and the doped silver in the antimicrobial activity, cells from a silver-resistant E. coli were used. These results indicated that Ag nanoparticles in the mesoporous were not only an antimicrobial but also an intensifier for photocatalysis. The as-prepared mesoporous composite film is promising in application of photocatalysis, antimicrobial and self-clean technologies.     

Structural colors from TiO2/SiO2 multilayer flakes prepared by sol-gel process

Preparation of TiO₂/SiO₂ multilayer flakes and their application to decorative powders were investigated. In contrast to conventional products prepared through the multicoating of core platelets, the coreless TiO₂/SiO₂ multilayer flakes were prepared by detaching multilayer films from their substrates. These flakes exhibited structural colors, when the optical path length of both the TiO₂ and SiO₂ layers are adjusted to be one fourth of the wavelength of visible light. A multicoating of more than five layers resulted in the propagation of cracks, which prevented the preparation of thick flakes. Paint films fabricated using the multilayer flakes and acrylic resins showed reflectance spectra that were comparable with those obtained for multicoatings on substrates.     

Meldola blue immobilized on a new SiO2/TiO2/graphite composite for electrocatalytic oxidation of NADH

Meldola blue immobilized on a new SiO₂/TiO₂/graphite composite was applied in the electrocatalytic oxidation of NADH. The materials were prepared by the sol-gel processing method and characterized by several techniques including scanning electronic microscopy coupled to energy dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electronic microscopy (HRTEM). Si and Ti mapping profiles on the surface showed a homogeneous distribution of the components. Ti2p binding energy peaks indicate that the formation of Si-O-Ti linkage is presumably the responsible for the high rigidity of the matrices. The good electrical conductivity presented by the composites (5 and 11 S cm⁻¹) can be related to a homogeneous distribution of graphite particles observed by TEM. After the materials characterization, a SiO₂/TiO₂/graphite electrode was prepared and some chemical modifications were performed on its surface to promote the adsorption of meldola blue. The resulting system presented electrocatalytic properties toward the oxidation of NADH, decreasing the oxidation potential to −120 mV. The proposed sensor showed a wide linear response range from 0.018 to 7.29 mmol l⁻¹ and limit of detection of 0.008 mmol l⁻¹. SiO₂/TiO₂/graphite has shown to be a promising material to be used as a suitable support in the construction of new electrochemical sensors.     

TiO2 photocatalysts and diamond electrodes

Photocatalysis and electroanalysis are two seemingly disparate research areas, but they are linked by the fact that both involve the use of well-known materials, TiO₂ and diamond, respectively, in new ways in the service of both environmental and medical sciences. In the present article, recent developments in the area of TiO₂ photocatalysis and diamond electrochemistry are summarized, with emphasis on our findings at the University of Tokyo. In the photocatalysis section, we present the fundamental aspects of TiO₂ photocatalysis and its practical applications, including air purification, self-cleaning surfaces and transparent superhydrophilic coatings. The diamond electrochemistry section deals with the electrochemical characterization and applications of diamond electrodes, which exhibit high sensitivity and excellent stability for electroanalysis, in contrast to conventional electrode materials. A particularly interesting environmental application of diamond electrodes has been developed; this involves the trace analysis of lead without the use of mercury.     

Structural and photocatalytic degradation characteristics of hydrothermally treated mesoporous TiO2

Mesoporous TiO₂ photocatalysts have been synthesized using polyethylene glycol (PEG) as a template direction agent in diluted acetic acid aqueous solution. This medium slows down the hydrolysis reaction of titanium sources due to the hydrolytic retardant and the strong chelating effects of acetic acid. A hydrothermal treatment process was introduced to better control the resultant mesoporous structures. The effects of PEG molecular weight and thermal treatment temperature on the resultant structure and photoactivity were investigated. Morphological, structural and phase compositional properties of the resultant photocatalysts were systematically characterized using transmission electron microscopy, X-ray diffraction and nitrogen adsorption/desorption analysis. The mesoporous structure with diameters between 13.3 and 17.0 nm and mean porous sizes that ranged from 9.6 to 13.3 nm were obtained when the molecular weight of PEG were varied from 200 to 20,000. The mesoporous diameters were changed significantly from 9.8 to 18.4 nm with mean porous sizes slightly increasing from 8.0 to 10.0 nm when the calcination temperature was varied from 350 to 550 °C. The activities of the resultant TiO₂ photocatalysts were evaluated using 2,4,6-tribrominated phenol as a testing compound that represents a class of toxic brominated flame retardants. The experimental results revealed that the photocatalytic activity depends on the phase and on the structural characteristics of the resultant photocatalysts.     

Facile synthesis of mesoporous Ag-loaded TiO2 thin film and its photocatalytic properties

Ag nanoparticles highly dispersed into TiO₂ thin films are synthesized via a remarkably simple one-pot route in the presence of a P123 triblock copolymer as template directing and reducing agents, where the reduction of Ag⁺ to Ag⁰ by in situ heat-induced reduction through the oxidation of template at 400 °C and the controlled polymerization of TiO₂ take place simultaneously. The obtained mesoporous Ag/TiO₂ films deposited on soda-lime glass were optically transparent and crack-free. SEM and Kr adsorption clearly prove that Ag/TiO₂ films at different Ag contents are mesoporous with large surface area and regularly ordered mesopores and the thickness of the obtained films is ∼280 ± 20 nm. The pristine TiO₂ film exhibits a specific surface area of 63 cm²/cm² and specific pore volume of 0.013 mm³/cm² that it is decreased to 42 cm²/cm² and 0.010 mm³/cm² respectively as a result of Ag-loaded mesoporous TiO₂. The newly prepared photocatalysts Ag/TiO₂ films were evaluated for their photocatalytic degradation of 2-chlorophenol as a model reaction. It was found that the meso-ordered Ag/TiO₂ films are more photoactive 8 times than nonporous commercial photocatalysts Pilkington Glass Activ™. The recycling tests indicated that Ag/TiO₂ films was quite stable during that liquid-solid heterogeneous photocatalysis since no significant decrease in activity was observed even after being used repetitively for 10 times, showing a good potential in practical application. In general, the cubic mesoporous Ag/TiO₂ nanocomposites are stable and can be recycled without loss of their photochemical activity.     

Oxidation of CH4 over Pd supported on TiO2-doped SiO2: Effect of Ti(IV) loading and influence of SO2

Titania-modified silicas with different weight% of TiO₂ were prepared by sol–gel method and used as supports for Pd (1 wt%) catalysts. The obtained materials were tested in the oxidation of methane under lean conditions in absence and in presence of SO₂. Test reactions were consecutively performed in order to evaluate the thermal stability and poisoning reversibility. Increasing amounts of TiO₂ improved the catalytic activity, with an optimum of the performance for 10 wt% TiO₂ loading. Moreover, the titania-containing catalysts exhibited a superior tolerance towards SO₂ by either adding it to the reactants or feeding it as a pure pretreatment atmosphere at 350 °C. Catalysts were characterized by XPS, XRD, FT-IR and BET measurements. According to the structural and surface analyses, the mixed oxides contained Si–O–Ti linkages which were interpreted as being responsible for the enhanced intrinsic activity of supported PdO with respect to PdO on either pure SiO₂ or pure TiO₂. Moreover, the preferential interaction of the sulfur molecule with TiO₂ and the easy SO_x desorption from high surface area silica were the determining factors for the superior SO₂ tolerance of the TiO₂-doped catalysts.     

Liquid phase deposition of mesoporous TiO2/DNA hybrid film: Characterization and photoelectrochemical investigation

A photoelectroactive TiO₂/DNA hybrid film was synthesized via the liquid phase deposition (LPD) process. Scanning electron microscopic (SEM) characterization showed that the compact TiO₂ film was changed to a mesoporous structure when DNA was present in the deposition solution, which might be the result of TiO₂ particles growing along the backbones of the double-helical structure of DNA molecules. Although UV absorption spectra and cyclic voltammograms indicated that the deposited TiO₂ on the substrate surface was decreased in the presence of DNA, an enhanced photocurrent response was observed. The electrochemical impedance and cyclic voltammetric measurements using K₃[Fe(CN)₆] as a redox probe suggested that the mesoporous film provided a relatively more efficient electron transfer interface, which could improve the photoelectron transfer rate from the semiconducting film to the electrode and reduce the recombination of photoelectrons and holes. This results in an enhanced photocurrent. Even after long-term and continuous UV irradiation, the mesoporous film exhibited a promoted photoelectrochemical response. The promoted photoelectrocatalytic degradation of methylene blue was obtained on the TiO₂/DNA composite film, which is consistent with the enhanced photocurrent, and this demonstrates that DNA behaved as a useful biomaterial for the synthesis of a photoelectroactive hybrid film with improved performance.     

Effect of Ag addition on the properties of Pd–Ag/TiO2 catalysts containing different TiO2 crystalline phases

Anatase and rutile TiO₂ were used for preparation of the TiO₂ supported Pd and Pd–Ag catalysts for selective hydrogenation of acetylene. It was found that Pd/TiO₂-anatase exhibited higher acetylene conversion and ethylene selectivity than rutile TiO₂ supported ones. However, addition of Ag to Pd/TiO₂-anatase catalyst resulted in lower ethylene selectivity while that of Pd/TiO₂-rutile increased. It is suggested that Ag addition suppressed the beneficial effect of the Ti³⁺ sites presented on the anatase TiO₂ during selective acetylene hydrogenation whereas without Ti³⁺, Ag promoted ethylene selectivity by blocking sites for over-hydrogenation of ethylene to ethane.     

Direct synthesis of mesoporous silicalite-1 supported TiO2–ZrO2 for the dehydrogenation of EB to styrene with CO2

Direct synthesis route was developed to support TiO₂–ZrO₂ binary metal oxide onto the carbon templated mesoporous silicalite-1 (CS-1). Metal hydroxide modified carbon particles could play a role as hard template and simultaneously support metal components on the mesopores during the crystallization of zeolites. Such supported TiO₂–ZrO₂ binary metal oxides (TZ/CS-1) showed better resistance to deactivation in the oxidative dehydrogenation of ethylbenzene (ODHEB) in the presence of CO₂. These catalysts were found to be active, selective and catalytically stable (10 h of time-on-stream) at 600 °C for the dehydrogenation of ethylbenzene (EB) to styrene (Sty).     

Preparation and characterization of CeO2/TiO2 nanoparticles by flame spray pyrolysis

Nanocrystalline TiO₂, CeO₂ and CeO₂-doped TiO₂ have been successfully prepared by one-step flame spray pyrolysis (FSP). Resulting powders were characterized with X-ray diffraction (XRD), N₂-physisorption, Transmission Electron Microscopy (TEM) and UV-Vis spectrophotometry. The TiO₂ and CeO₂-doped TiO₂ nanopowders were composed of single-crystalline spherical particles with as-prepared primary particle size of 10-13 nm for Ce doping concentrations of 5-50 at%, while square-shape particles with average size around 9 nm were only observed from flame-made CeO₂. The adsorption edge of resulting powder was shifted from 388 to 467 nm as the Ce content increased from 0 to 30 at% and there was an optimal Ce content in association with the maximum absorbance. This effect is due to the insertion of Ce^3+/4+ in the TiO₂ matrix, which generated an n-type impurity band.     

Effect of molar ratio of TiO2/SiO2 on the properties of particles synthesized by flame spray pyrolysis

Titania (TiO₂)–silica (SiO₂) nanoparticles were synthesized from sprayed droplets of a mixture of TEOS and TTIP by flame spray pyrolysis (FSP). The effect of molar ratio between TEOS and TTIP in the mixture on the particle properties such as particle morphology, average particle diameter, specific surface area, crystal structure, etc., were determined using TEM, XRD, BET, and FT-IR. A UV-spectrometer was also used to measure the absorption spectrum and the band gap energy of the product particles. As the molar ratio of TEOS/TTIP increased by increasing TEOS concentration at the fixed TTIP concentration, the average particle diameter of the mixed oxide nanoparticles increased with maintaining uniform dispersion between TiO₂ and SiO₂, and crystal structure was transformed from anatase to amorphous. The band gap energy of the TiO₂–SiO₂ nanoparticles increased with respect to the increase of the molar ratio due to the decrease of width of UV-absorption spectrum. Photocatalytic activity of TiO₂–SiO₂ composite particles decreased with the concentration of TEOS.     

Anatase TiO2 spheres with high surface area and mesoporous structure via a hydrothermal process for dye-sensitized solar cells

Spherical pure anatase TiO₂ spheres with a mesoporous structure and high surface area of up to 116.5 m² g⁻¹ were prepared by a simple urea-assisted hydrothermal process and investigated as dye-sensitized solar-cell electrodes. Although the particle diameters of the prepared TiO₂ spheres ranged from 0.4 to 1.3 μm, due to the high specific surface area, mesoporous TiO₂ sphere electrode was obtained with enhanced light harvesting and a larger amount of dye loading. An overall light conversion efficiency of 7.54% under illumination of simulated AM 1.5G solar light (100 mW cm⁻²) was achieved using the mesoporous TiO₂ spheres electrode, which was significantly higher than a commercial Degussa P25 TiO₂ nanocrystals electrode (5.69%).     

A mesoporous SiO2 intermediate layer for improving light propagation in a bundled tube photoreactor

A photoreactor comprising a bundle of TiO₂-coated quartz tubes was studied by varying its tube wall thickness and tube configuration as well as introducing a mesoporous SiO₂ intermediate layer between the TiO₂ coating and quartz tube. The bundled tube photoreactor (BTP) performance was assessed based on tube light propagation and photocatalytic degradation rate of ethylene. Increasing the tube wall thickness improved the tube light propagation and the degradation rate of ethylene. An array of eight 6-mm tubes was found to be the best BTP configuration in this work. The findings from varying the tube configuration suggested an effectively illuminated surface area as a second important parameter to consider when comparing different BTP performances. Introducing a mesoporous SiO₂ intermediate layer with a thickness between 210 and 400 nm between the TiO₂ coating and quartz tube improved not only the tube light propagation but also the ethylene photocatalytic degradation rate by up to 70%. This improvement was attributed to controlled light refraction from the quartz tube, which can be achieved under the conditions of frustrated total internal reflection.     

Fabrication and characterization of nanoporous SiO2 ceramics via pyrolysis of silicone resin filled with nanometer SiO2 powders

Mesoporous SiO₂ ceramics are fabricated by pyrolysis of silicone resin filled with nanometer SiO₂ powders in air at 1273 K. With the increase of shaping pressure, open porosity and average pore size decrease, while bulk density and fracture strength are improved. The fracture surface of porous SiO₂ ceramics was observed.     

Investigation of surface modification of rutile TiO2 nanoparticles with SiO2/Al2O3 on the properties of polyacrylic composite coating

Surface modification and characterization of TiO₂ nanoparticles as an additive in a polyacrylic clear coating were investigated. For the improvement of nanoparticles dispersion and the decreasing of photocatalytic activity, the surface of nanoparticles was modified with binary SiO₂/Al₂O₃. The surface treatment of TiO₂ nanoparticles was characterized with FTIR. Microstructural analysis was done by AFM. The size, particle size distribution and zeta potential of TiO₂ nanoparticles in water dispersion was measured by DLS method. For the evaluation of particle size and the stability of nanoparticles in water dispersions with higher solid content the electroacoustic spectroscopy was made. To determine the applicability and evaluate the transmittance of the nano-TiO₂ composite coatings UV–VIS spectroscopy in the wavelength range of 200–800 nm was employed. The results showed that surface treatment of TiO₂ nanoparticles with SiO₂/Al₂O₃ improves nanoparticles dispersion and UV protection of the clear polyacrylic composite coating.     

Solid acidities of SiO2–TiO2/montmorillonite composites synthesized under different pH conditions

SiO₂–TiO₂/montmorillonite composites were prepared under acidic, neutral and basic conditions and the solid acidity of the resulting composites were determined. All the SiO₂/TiO₂ ratio of the colloidal particles was set at 10 but the resulting SiO₂/TiO₂ ratios were significantly richer in TiO₂. The XRD patterns of the acidic composite showed expanding and broadening of the (001) reflection by intercalation of colloidal SiO₂–TiO₂ particles, but the neutral and basic composites showed only broadening of the reflections and no intercalation. The specific surface areas of the acidic, neutral and basic composites (375, 237 and 247 m²/g, respectively) were much larger than of montmorillonite (6 m²/g). The average pore sizes were about 4, 15 and 50 nm, and the amounts of solid acidic sites measured by the NH₃-TPD were 178, 95 and 86 µmol/g for the acidic, neutral and basic composites, respectively. The solid acid amount of the acidic composite was twice that of a commercial catalyst, K-10, (85 µmol/g) and much higher than the guest phase SiO₂–TiO₂ gel (16 µmol/g) or the host phase montmorillonite (6 µmol/g). The TPD peak temperatures reflect the acid strength, and were similar in all the samples, ranging from 175° to 200 °C.