Low-temperature hydrothermal synthesis of highly photoactive mesoporous spherical TiO2 nanocrystalline

TiO₂ microspheres with mesoporous textural microstructures and high photocatalytic activity were prepared by hydrothermal treatment of mixed solution of titanium sulfate and urea with designed time. The prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and N₂ adsorption-desorption measurements. The photocatalytic activity was evaluated via the photocatalytic oxidation of acetone in air at room temperature. The results show that the hydrothermal time significantly influences on the morphology, microstructure and photocatalytic activity of the as-prepared samples. With increasing hydrothermal time, specific surface areas and pore volumes decrease, contrarily, the crystallite size and relative anatase crystallinity increase. The photocatalytic efficiency of the as-prepared samples is obviously higher than that of commercial Degussa P25 (P25) powders. Especially, the as-prepared TiO₂ powders by hydrothermal treatment for 7 h shows the highest photocatalytic activity, which exceeds that of P25 by a factor of more than 2 times.     

Preparation, characterization and photocatalytic activity of polycrystalline Bi2O3/SrTiO3 composite powders

Bi₂O₃/SrTiO₃ composite powders have been prepared and their photocatalytic activities were investigated by photooxidation of methanol. These powders were characterized by UV-Visible diffuse reflectance spectra, SEM and X-ray diffraction (XRD). The results revealed that all the Bi₂O₃/SrTiO₃ composite powders exhibited higher photocatalytic activity than pure SrTiO₃, Bi₂O₃ and TiO₂ (P25) under visible light irradiation (λ>440 nm). The effects of the Bi₂O₃ contents on the photocatalytic activities of the composite powders were examined, the photocatalytic activities increased with the content of Bi₂O₃ increasing to a maximum of 83% and then decreased under visible light irradiation. The effects of the calcination temperatures on the photocatalytic activities of the composite powders were also investigated.     

Preparation of Fe2O3/SrTiO3 composite powders and their photocatalytic properties

Fe₂O₃/SrTiO₃ composite powders have been prepared and their photocatalytic activities were investigated by photooxidizing methanol. These powders were characterized by ultraviolet (UV)-visible diffuse reflectance spectra, scanning electron microscope (SEM) and X-ray diffraction (XRD). The results showed that the Fe₂O₃/SrTiO₃ composite powders with optimum proportion exhibited higher photocatalytic activity than pure SrTiO₃, Fe₂O₃ and TiO₂ (P25) under visible light (λ>440 nm) irradiation. The SEM image of the composite powders showed that SrTiO₃ and Fe₂O₃ particles contacted well. Further research revealed that the calcination temperature is an important factor in the preparation of the composite powder with relatively high photocatalytic ability.     

Photocatalytic activity of Nb2O5/SrNb2O6 heterojunction on the degradation of methyl orange

The pure SrNb₂O₆ powders were prepared at 1400 °C by a conventional solid-state method and characterized by X-ray powder diffraction and UV-vis diffuse reflection spectrum. The powders of Nb₂O₅ and SrNb₂O₆ were ball-milled together and annealed to form the Nb₂O₅/SrNb₂O₆ composite. Photocatalytic activities of the composites were investigated on the degradation of methyl orange. The results show that the proportion of Nb₂O₅ to SrNb₂O₆ and the annealing temperature greatly influence the photocatalytic activities of the composites. The best photocatalytic activity occurs when the weight proportion of Nb₂O₅ to SrNb₂O₆ is 30% and the annealing temperature is 600 °C. The tremendously enhanced photocatalytic activity of the Nb₂O₅/SrNb₂O₆ composite compared to Nb₂O₅ or SrNb₂O₆ is ascribed to the heterojunction effect taking place at the interface between particles of Nb₂O₅ and SrNb₂O₆. The powders also show a higher photocatalytic activity than commercial anatase TiO₂.     

Optical and photocatalytic properties of nanocrystalline TiO2 synthesised by solid-state chemical reaction

Nanoparticulate TiO₂ is of interest for a variety of technological applications, including optically transparent UV-filters and photocatalysts for the destruction of chemical waste. The successful use of nanoparticulate TiO₂ in such applications requires an understanding of how the synthesis conditions effect the optical and photocatalytic properties. In this study, we have investigated the effect of heat treatment temperature on the properties of nanoparticulate TiO₂ powders that were synthesised by solid-state chemical reaction of anhydrous TiOSO₄ with Na₂CO₃. It was found that the photocatalytic activity increased with the heat treatment temperature up to a maximum at 600 °C and thereafter declined. In contrast, the optical transparency decreased monotonically with the heat treatment temperature. These results indicate that solid-state chemical reaction can be used to prepare powders of nanoparticulate TiO₂ with properties that are optimised for use as either optically transparent UV-filters or photocatalysts.     

Highly active V-TiO2 for photocatalytic degradation of methyl orange

Two kinds of vanadium-doped TiO₂ powders photocatalysts were prepared by sol-gel method in even doping and uneven doping modes, and were characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), transmission electron microscopy (TEM) and UV-vis diffuse reflectance spectroscopy (DRS). The photocatalytic activity of TiO₂ photocatalysts doped by vanadium evenly with lower dopant level up to 0.002 mol.% is better than that of undoped TiO₂, while with higher dopant level the activity is worse. TiO₂ photocatalysts doped by vanadium unevenly with a p-n junction semiconductor structure, was shown to have a much higher photocatalytic destruction rate than that of TiO₂ photocatalysts doped by vanadium evenly and undoped TiO₂, which is ascribed mainly to the electrostatic-field-driven electron-hole separation in TiO₂ particles doped by vanadium unevenly.     

The effects of activated carbon supports on the structure and properties of TiO2 nanoparticles prepared by a sol-gel method

TiO₂-coated activated carbon (TiO₂/AC) composites and pure TiO₂ powders were prepared by a sol-gel method using tetrabutylorthotitanate as a precursor. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential thermal analysis (DTA), X-ray photoelectron spectrum (XPS) and nitrogen absorption. The photoactivity of samples was evaluated by methylene blue (MB) degradation. The analysis results show that compared with pure TiO₂ powders, the spherical-shaped TiO₂ particles are well-dispersed in the AC matrix and the size of the resulting TiO₂ crystallites decreases to below 40 nm with increasing phase transformation temperature. The AC matrix creates anti-calcination effects and shows interfacial energy effects that control the growth of the TiO₂ particles, baffle the anatase to rutile phase transition, and cumber the TiO₂ particles to agglomerate. Compared with the surface areas of TiO₂ powders, the combination of TiO₂ and AC forms composites with high surface areas which are slightly affected by calcination temperature. By AC support, the photoactivity of TiO₂ is increased in MB photocatalytic course, possible because active carbon increases photocatalytic activity of TiO₂ particles by producing high concentration of organic compound near TiO₂, and small-size TiO₂ particles are well-dispersed on the surface of AC.     

Preparation of nanosized crystalline TiO2 particles at low temperature for photocatalysis

Nanocrystalline titanium dioxide (TiO₂) particles were prepared by a modified alkoxide method under acidic conditions at temperatures ranging from 60°C to 90°C. The reaction temperature was used to control the crystalline phase of the TiO₂ particles. At 60°C and 75°C rutile was formed whilst at 90°C anatase and brookite were formed.The photocatalytic activity of the prepared particles was tested for the degradation of sucrose. The photocatalytic activities of the prepared nanosized TiO₂ were compared to those obtained from Degussa P-25 TiO₂ as well as TiO₂ crystalline samples prepared using the conventional sol–gel/heat treatment method. At low organic concentrations, Degussa P-25 exhibited higher photocatalytic behaviour than all the prepared particles while, at high organic concentrations, the nanosized TiO₂ particles prepared at low temperature displayed an activity comparable to Degussa P-25 but much higher than the heat treated sample. The formation of excess intermediates during the degradation of higher sucrose loadings is believed to hinder the photoactivity of Degussa P-25, while the prepared TiO₂ particles are able to maintain their activity for the degradation of the intermediates of sucrose.     

Preparation and photocatalytic activity of nitrogen-doped TiO2 hollow nanospheres

TiO₂ hollow nanospheres were prepared using silicon oxide as a template. N-doped titanium oxide hollow spheres, TiO_2−xN_x were synthesized by reacting TiO₂ hollow spheres with thiourea at 500 °C. XRD and XPS data showed that oxygen was successfully substituted by nitrogen through the nitrogen-doping reaction, and finally N-doped TiO₂ hollow spheres were formed. The N-doped TiO₂ hollow spheres showed new absorption shoulder in visible light region so that they were expected to exhibit photocatalytic activity in the visible light. The photocatalytic activity of N-doped TiO₂ hollow spheres under visible light was similar to that of normal spherical TiO_2−xN_x in spite of the structural difference.     

Ag/TiO2 sol prepared by a sol–gel method and its photocatalytic activity

Ag/TiO₂ sol with narrow particle size distribution was synthesized using TiCl₄ as the starting material. TiCl₄ was converted to Ti(OH)₄ gel. The Ag/TiO₂ sol was prepared by a process where H₂O₂ was added and then heated at 90–97 °C. After condensation reaction and crystallization, a transparent sol with suspended Ag/TiO₂ was formed. Ag/TiO₂ was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, contact angle analysis, and X-ray photoelectron spectroscopy. The photocatalytic properties of Ag/TiO₂ film were evaluated by degradation of methylene blue in aqueous solution under UV light irradiation. The suspended Ag/TiO₂ particles were rhombus primary particles with the major axis ca. 40 nm and the minor axis ca. 10 nm. Ag nanoparticles were well dispersed on TiO₂ and the particle size was only 1–2 nm. Ag could restrain the recombination of photo-generated electrons and holes effectively. Transparent thin films could be obtained through dip-coating glass substrate in the sol. The thin film had strong hydrophilicity after being illuminated by UV light. Ag/TiO₂ film showed a significant increase in photocatalytic activity compared to the TiO₂ film. The high amount of surface hydroxyls on Ag/TiO₂ film also played an important role in its photocatalytic activity.     

Immobilized TiO<Subscript>2</Subscript> nanoparticles produced by flame spray for photocatalytic water remediation

Anatase/rutile mixed-phase titanium dioxide (TiO₂) photocatalysts in the form of nanostructured powders with different primary particle size, specific surface area, and rutile content were produced from the gas-phase by flame spray pyrolysis (FSP) starting from an organic solution containing titanium (IV) isopropoxide as Ti precursor. Flame spray-produced TiO₂ powders were characterized by means of X-ray diffraction, Raman spectroscopy, and BET measurements. As-prepared powders were mainly composed of anatase crystallites with size ranging from 7 to 15 nm according to the synthesis conditions. TiO₂ powders were embedded in a multilayered fluoropolymeric matrix to immobilize the nanoparticles into freestanding photocatalytic membranes. The photocatalytic activity of the TiO₂-embedded membranes toward the abatement of hydrosoluble organic pollutants was evaluated employing the photodegradation of rhodamine B in aqueous solution as test reaction. The photoabatement rate of best performing membranes significantly overcomes that of membranes produced by the same method and incorporating commercial P25-TiO₂.     

Studies on nanocrystalline (Sr,Pb)TiO3 solid solutions prepared via a facile self-propagating combustion method

Macroporous nanocrystalline (Sr,Pb)TiO₃ solid solutions were prepared by a facile self-propagating combustion method. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected area electron diffraction (SAED), energy dispersive X-ray spectrum (EDS) and X-ray photoelectron spectroscopy (XPS). (Sr,Pb)TiO₃ solid solutions showed enhanced photocatalytic activity for the degradation of methyl orange (MO) than pure SrTiO₃ and an optimum performance was observed for Sr_29/32Pb_3/32TiO₃. The possible mechanism for the enhanced photocatalytic activity on (Sr,Pb)TiO₃ solid solutions was proposed.     

Effect of surface fluorination on the photocatalytic and photo-induced hydrophilic properties of porous TiO2 films

Porous surface-fluorinated TiO₂ (F-TiO₂) films were prepared through PEG modified sol-gel method and surface fluorination. The as-prepared films were characterized with XRD, FTIR, AFM, XPS and UV-vis DRS. The effects of surface fluorination on the photocatalytic activity and hydrophilicity of porous TiO₂ film were studied by photocatalytic degradation of rhodamine B (RhB) as well as water contact angle (CA) on porous TiO₂ film. The results showed that the surface fluorination increased the adsorption of RhB on the porous TiO₂ film and enhanced the photocatalytic degradation of RhB. The concentration and pH of NaF solutions affected much the photocatalytic activity of porous TiO₂ film. Porous F-TiO₂ film prepared in 40 mM NaF solution at pH 4.0 showed the highest photocatalytic activity. Because of its porous structure, the porous F-TiO₂ film had original water CA of 22.7°, which is much smaller than that of normal F-TiO₂ film. Under UV light irradiation, the water CA of porous F-TiO₂ film decreased to 5.1° in 90 min.     

TiO2 nanoparticles with high ability for selective adsorption and photodegradation of textile dyes under visible light by feasible preparation

Particular TiO₂ nanoparticles with high selective photocatalytic oxidation of anionic dyes are prepared by a feasible hydrothermal method. Moreover, its photocatalytic selectivity can be easily switched to cationic dyes by a simple post-treatment in ammonia solution, which makes the prepared TiO₂ have bi-directional selectivity in dye photodegradation. Based on the photocatalytic performances and the structure and surface characteristics of the catalyst, the bi-directional selectivity of the catalysts is found to be closely related to the adsorption selectivity. The adsorption selectivity originates from surface charge groups, which are introduced during the preparation and post-treatment progresses. This study provides a facile and economical approach towards selective degradation of dyes with high efficiency by the special TiO₂ nanoparticles synthesized through a simple hydrothermal method, which may be used practically in the future.     

The roles of Li and F ions in Li-F-codoped TiO2 system

An overall comparative study was carried out on Li-doped, F-doped, and Li-F-codoped TiO₂ powders in order to elucidate the roles of Li⁺ and F⁻ ions in photocatalyst. The characteristic data were based on the analysis of XRD, XPS, and PL spectra. The effects of atomic ratio of Li/Ti and F/Ti on the photocatalytic activity were also investigated. As the results, Li doping accelerated the phase formation of rutile in lower temperature while F doping prevented the phase transition from anatase to rutile. Li doping inducted a large amount of O_OH on the surface of TiO₂, while F doping consumed much of O_OH. Li⁺ ions acted as the roles of recombination center of electron-hole pairs while F doping could restrain the recombination of electron-hole pairs on the center of Li⁺ ions. The roles of Li⁺ and F⁻ ions were also confirmed in the experimental section, where the photocatalytic activity of TiO₂ was improved greatly by synergistic reaction of Li⁺ and F⁻ ions.     

Low temperature deposition of TiO2 thin films on polyvinyl alcohol fibers with photocatalytical and antibacterial activities

Fine particles of photoactive anatase-type TiO₂ coated polyvinyl alcohol (PVA) fibers (TiO₂/PVA) were prepared successfully via a simple dip-coating method. Tetrabutyl orthotitanate (TBOT) was hydrolyzed in presence of hydrolysis control agent tetrabutylammonium hydroxide (TBA)OH and the TiO₂ fine particles were crystallized under microwave (MW) irradiation. The X-ray diffraction (XRD) and selected-area electron diffraction (SAED) analyses indicated that the fine particles obtained with MW irradiation have much higher crystallinity with a single phase anatase compared with the non-MW-treated solution. The continuous layers of titania were found on PVA fibers by the scanning electron microscopy (SEM) analysis. The fibers with anatase coatings showed high photocatalytic property on the photodegradation of methylene blue (MB) and high antibacterial activity.     

Hydrothermal synthesis of well-dispersed ultrafine N-doped TiO2 nanoparticles with enhanced photocatalytic activity under visible light

Ultrafine nitrogen-doped TiO₂ nanoparticles with narrow particle size distribution, good dispersion, and high surface area were synthesized in the presence of urea and PEG-4000 via a hydrothermal procedure. TEM observation, N₂ adsorption, XRD, UV-vis spectroscopy, the Raman spectroscopy and XPS analysis were conducted to characterize the synthesized TiO₂ particles. The synthesized TiO₂ particles were a mixture of 49.5% anatase and 50.5% rutile with a size of around 5 nm. The photocatalytic activities were tested in the degradation of an aqueous solution of a reactive Brilliant Blue KN-R under both UV and visible light. The synthesized TiO₂ particles showed much higher photocatalytic activity than a commercial P25 TiO₂ powder under both UV and visible light irradiations. The high performance is associated to N doping, the reduced particle size, good dispersion, high surface area, and a quantum size effect.     

First-principles study on anatase TiO2 codoped with nitrogen and praseodymium

The crystal structures, electronic structures and optical properties of nitrogen or/and praseodymium doped anatase TiO₂ were calculated by first principles with the plane-wave ultrasoft pseudopotential method based on density functional theory. Highly efficient visible-light-induced nitrogen or/and praseodymium doped anatase TiO₂ nanocrystal photocatalyst were synthesized by a microwave chemical method. The calculated results show that the photocatalytic activity of TiO₂ can be enhanced by N doping or Pr doping, and can be further enhanced by N+Pr codoping. The band gap change of the codoping TiO₂ is more obvious than that of the single ion doping, which results in the red shift of the optical absorption edges. The results are of great significance for the understanding and further development of visible-light response high activity modified TiO₂ photocatalyst. The photocatalytic activity of the samples for methyl blue degradation was investigated under the irradiation of fluorescent light. The experimental results show that the codoping TiO₂ photocatalytic activity is obviously higher than that of the single ion doping. The experimental results accord with the calculated results.     

Curcumin-sensitized TiO2 for enhanced photodegradation of dyes under visible light

Curcumin was coated on P25 TiO₂ by using impregnation method from freshly prepared curcumin solution. The resulting products (Cur–TiO₂–P25) was studied by several techniques such as X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier-transformed infrared spectroscopy, specific surface area by the Brunauer–Emmett–Teller method, and UV–Vis diffused reflectance spectroscopy. Experimental results revealed that impregnation of curcumin at 0.5, 3, 5, and 7 wt% did not affect the native phase of anatase and rutile in P25 significantly, however, it caused red shift of absorption onset in all curcumin-coated samples. The Cur–TiO₂–P25 showed enhanced adsorption efficiency and increased photocatalytic activity under visible light with optimal result at 5 wt% curcumin content. Commercial anatase and rutile coated with curcumin (Cur–TiO₂–an and Cur–TiO₂–ru) were also prepared by the same method for the use in comparative studies of photodegradation of dyes. Cur–TiO₂–an and Cur–TiO₂–ru were also characterized with some selected equipment above but not as extensively as the Cur–TiO₂–P25. Curcumin coating helped improve photocatalytic efficiencies of P25 and anatase but not for rutile. The mechanism of photocatalytic reaction was proposed that under visible light irradiation, curcumin molecule could act as dye sensitizing agent that injected electron into the conduction band of TiO₂ leading to photodegradation of dyes.     

Influence of the OH groups on the photocatalytic activity and photoinduced hydrophilicity of microwave assisted sol-gel TiO2 film

In the present work the influence of the OH groups on the photocatalytic activity and the photoinduced hydrophilicity of microwave assisted sol-gel TiO₂ films was investigated. The prepared TiO₂ films were characterized using XRD and AFM. Furthermore, the surface of the TiO₂ films was examined by help of XPS in order to determine the amount of OH groups before and after UV irradiation at different humidities. The activity of the TiO₂ films was determined using stearic acid as a model compound and the photoinduced superhydrophilicity was investigated through contact angle measurements.The results of this investigation showed that the microwave assisted sol-gel technique produces highly homogeneous and efficient TiO₂ films without the need for heat treatment for crystallization. Based on the conducted experiments it is suggested that the amount of OH groups on the TiO₂ surface highly influence the photocatalytic activity and the photoinduced superhydrophilicity and that the two mechanisms may be closely related. It is suggested that the superhydrophilicity is obtained through a combination of photocatalytic degradation of organic contaminants and surface structural changes in form of an increased amount of OH-groups.