Preparation of Unique TiO2 Nano-particle Photocatalysts by a Multi-gelation Method for Control of the Physicochemical Parameters and Reactivity

A novel multi-gelation method to prepare TiO₂ nano-particle photocatalysts showed good performance in controlling the important parameters determining the photocatalytic reactivity, i.e., the particle size, surface area, crystallinity, pore-volume, pore-diameter as well as the anatase and rutile phase composition of the catalysts. In particular, this method at higher pH swing times could prevent the phase transition from anatase to rutile, leading to higher photocatalytic activity. By adopting variations in the pH swing, the TiO₂ nano-particle photocatalysts showed significantly higher photocatalytic reactivity for the complete oxidation of 2-propanol diluted with water into CO₂ and H₂O. It can be considered a viable alternative method for the preparation of high performance TiO₂ nano-particle photocatalysts for widespread commercial applications.     

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.     

Hydrothermal synthesis of S-doped TiO2 nanoparticles and their photocatalytic ability for degradation of methyl orange

A simple synthesis route to nanocrystalline S-doped TiO₂ photocatalysts by a hydrothermal method at 180 °C was developed and the photocatalytic activity of the obtained powders for the degradation of methyl orange was studied. The products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The phase composition (anatase/rutile ratio) and the photocatalytic activity of the final materials were found to be markedly influenced by the amount of the incorporated sulphur. On increasing the S-dopant amount, the anatase/rutile ratio and the photocatalytic activity of the as-prepared powders increased.     

Preparation, crystal structure, and photocatalytic activity of TiO2 films by chemical vapor deposition

Photocatalytic activities of TiO₂ films were experimentally studied. TiO₂ films with different crystal structures (amorphous, anatase, rutile) were prepared by a Low Pressure Metal Organic Chemical Vapor Deposition (LPMOCVD) at different reaction temperatures and also by a Sol-Gel method using TTIP (Titanium Tetra Iso-Pro-poxyde). The Effect of CVD preparation method, CVD reaction conditions, crystal structure and wave-length of UV light on the photocatalytic decomposition rate of methylene blue in aqueous solution were studied. First, the characteristics of CVD preparation of TiO₂ films, such as the CVD film growth rate, crystal structure and morphology of the grown TiO₂ films, were experimentally studied as a function of CVD reaction temperature. Secondly, photocatalytic activities of TiO₂ films were evaluated by using two types of photo-reactors. The results indicated that TiO₂ films prepared by CVD exhibit higher photocatalytic activity than a catalyst prepared by the Sol-Gel method. Among the CVD grown TiO₂ films, anatase and rutile showed high photocatalytic activities. However, amorphous TiO₂ films showed lower activities. The activity of the photocatalysts of anatase films was excellent under all types of UV-lamps. The activity of CVD-prepared anatase films was four to seven times higher than that of photocatalyst films prepared by the Sol-Gel method.     

Bandgap Modulation of TiO2 and its Effect on the Activity in Photocatalytic Oxidation of 2-isopropyl-6-methyl-4-pyrimidinol

Nano-size TiO₂ particles were obtained using various mesoporous materials and the particle sizes were determined by the pore sizes of mesoporous materials. The bandgap increments due to quantum size effect were observed in both anatase TiO₂ and rutile TiO₂. The bandgap increment in anatase TiO₂ did not affect the efficiency of photocatalytic oxidation of 2-isopropyl-6-methyl-4-pyrimidinol. However, the bandgap increment in rutile TiO₂ enhanced the photocatalytic activity by virtue of the increase in redox potential of TiO₂.     

Preparation of Highly Active TiO2 Nano-particle Photocatalysts by a Flame Aerosol Method for the Complete Oxidation of 2-Propanol

A flame aerosol method has been employed to prepare spherical TiO₂ nano-particle photocatalysts with controlled anatase/rutile phase ratios without calcination at higher temperatures. This method was found to have important advantages since the main factors in achieving high photocatalytic activity such as the particle size, crystallinity and the anatase/rutile phase ratios could be easily controlled. In particular, the incorporation of small amounts of bimetals, such as Fe and Zn, were found to initiate the formation of well-crystalline, small and uniform spherical nano-size particles with a well-defined anatase/rutile phase ratio of around 60/40, similar to P-25 TiO₂. This suppressed the recombination of the photoformed charge carriers leading to a significant increase in the photocatalytic reactivity of the TiO₂ nano-particles. The incorporation of very small amounts of mono-metals, such as Fe, Cr and Zn (around 1 at.%), within the TiO₂ nano-particles led to a slight increase in the photocatalytic activity of the TiO₂ nano-particle photocatalysts for the complete oxidation of 2-propanol dissolved in water into CO₂ and H₂O as compared with the unincorporated pure TiO₂. The incorporation of bimetals of Fe and Zn within TiO₂ (Fe/Zn–TiO₂) nano-particles, on the other hand, led to a remarkable enhancement in the photocatalytic activity as compared with the unincorporated and mono-metal incorporated TiO₂.     

Shock-consolidated TiO2 bulk with pure anatase phases fabricated by explosive compaction using underwater shockwave

Titanium dioxide (TiO₂) bulk with pure anatase phases was fabricated by an explosive compaction technique using an underwater shockwave. Dynamic shock pressure of 6 GPa was used to consolidate anatase TiO₂ powders. Its microstructural, crystalline structural and photocatalytic characteristics were observed and measured by various techniques, such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and photocatalytic activity measurement system. It was confirmed that the relative density of anatase TiO₂ compact is about 96% (3.73 g/cm³) of the theoretical density (3.89 g/cm³) and a strong surface bonding between particles is formed by a shock energy. In X-ray diffraction analysis, high purity anatase phases, broadened peaks due to lattice defects and decreased crystallite size were found. For the photocatalytic activities, the anatase TiO₂ compact was quite excellent compared to the commercial sintered TiO₂ bulk.     

TiO2 nanotubes and CNT–TiO2 hybrid materials for the photocatalytic oxidation of propene at low concentration

In this work, we investigated titanium dioxide (TiO₂) nanotubes and CNT–TiO₂ hybrid materials for the photocatalytic oxidation (PCO) of propene at low concentration (100 ppmv) in gaseous phase. The materials were prepared via sol–gel method using sacrificial multi-walled carbon nanotubes (CNT) as templates and subsequent heat treatments to obtain the desired crystalline phase (anatase, rutile or a mixture of both) and eventually to remove the carbon template. We also studied rutile nanotubes for the first time and demonstrate that the activity strongly depends on the crystalline composition, following rutile < anatase < anatase/rutile mixture. The enhanced activity of the anatase–rutile mixture is attributed to the decrease in the electron–hole pair recombination due to the multiphasic nature of the particles. The key result of this work is the exceptional performance of the CNT–TiO₂ hybrid, which yielded the highest observed photocatalytic activity. The improved performance is attributed to synergistic effects due to the hybrid nature of the material, resulting in small anatase crystalline sizes (CNT act as heat sinks) and a reduced electron–hole pair recombination rate (CNTs act as electron traps). These results demonstrate the great potential of hybrid materials and stimulate further research on CNT-inorganic hybrid materials in photocatalysis and related areas.     

Effects of annealing temperature on the photocatalytic activity of N-doped TiO2 thin films

The effects of annealing temperature on the photocatalytic activity of nitrogen-doped (N-doped) titanium oxide (TiO₂) thin films deposited on soda-lime-silica slide glass by radio frequency (RF) magnetron sputtering have been studied. Glancing incident X-ray diffraction (GIAXRD), Raman spectrum, scanning electron microscopy (SEM), atomic force microscopy (AFM) and UV-vis spectra were utilized to characterize the N-doped TiO₂ thin films with and without annealing treatment. GIAXRD and Raman results show as-deposited N-doped TiO₂ thin films to be nearly amorphous and that the rutile and anatase phases coexisted when the N-doped TiO₂ thin films were annealed at 623 and 823 K for 1 h, respectively. SEM microstructure shows uniformly close packed and nearly round particles with a size of about 10 nm which are on the slide glass surface for TiO₂ thin films annealed at 623 K for 1 h. AFM image shows the lowest surface roughness for the N-doped TiO₂ thin films annealed at 623 K for 1 h. The N-doped TiO₂ thin films annealed at 623 K for 1 h exhibit the best photocatalytic activity, with a rate constant (k_a) of about 0.0034 h⁻¹.     

Fabrication of high-efficiency anatase TiO2 photocatalysts using electrospinning with ultra-violet treatment

In metal oxide nanofiber fabrication using the electrospinning method, heat treatment is performed at temperatures of 500°C or higher for crystallization and polymer desorption. Therefore, it is difficult to fabricate low-temperature phase metal oxides that crystallize at low temperatures. TiO₂, a representative metal oxide often used as photocatalysts, is known to have higher photocatalytic activity in the low-temperature phase (anatase structure) than in the high-temperature phase (rutile structure). Studies on the fabrication of TiO₂ anatase nanofibers using conventional electrospinning have reported disadvantages such as the partial expression of rutile structures and low crystallinity. This study developed an anatase TiO₂ nanofiber as a high-efficiency catalyst based on the electrospinning method and a residual organic matter cleaning method that employs ultra-violet (UV) light. We fabricated nanofibers using the electrospinning method and implemented TiO₂ nanofibers with the anatase structure through heat treatment at 260°C. Residual organics remaining after heat treatment of the fabricated crystalized TiO₂ nanofibers were removed by exposing them to UV light, thereby improving photocatalytic efficiency. The photocatalytic efficiency of the fabricated TiO₂ nanofibers was confirmed through a methylene blue (MB) decomposition experiment under visible light irradiation. The photocatalytic efficiency (time taken for the concentration of the MB solution to reach 50%) of the UV-treated TiO₂ nanofibers was approximately six times higher than of P25 and the heat-treated nanofibers.     

Photoelectro-Fenton combined with photocatalytic process for degradation of an azo dye using supported TiO2 nanoparticles and carbon nanotube cathode: Neural network modeling

In this work, treatment of an azo dye solution containing C.I. Basic Red 46 (BR46) by photoelectro-Fenton (PEF) combined with photocatalytic process was studied. Carbon nanotube-polytetrafluoroethylene (CNT-PTFE) electrode was used as cathode. The investigated photocatalyst was TiO₂ nanoparticles (Degussa P25) having 80% anatase and 20% rutile, specific surface area (BET) 50 m²/g, and particle size 21 nm immobilized on glass plates. A comparison of electro-Fenton (EF), UV/TiO₂, PEF and PEF/TiO₂ processes for decolorization of BR46 solution was performed. Results showed that color removal follows the decreasing order: PEF/TiO₂ > PEF > EF > UV/TiO₂. The influence of the basic operational parameters such as initial pH of the solution, initial dye concentration, the size of anode, applied current, kind of ultraviolet (UV) light and initial Fe³⁺ concentration on the degradation efficiency of BR46 was studied. The mineralization of the dye was investigated by total organic carbon (TOC) measurements that showed 98.8% mineralization of 20 mg/l dye at 6 h using PEF/TiO₂ process. An artificial neural network (ANN) model was developed to predict the decolorization of BR46 solution. The findings indicated that artificial neural network provided reasonable predictive performance (R² = 0.986).     

Photocatalytic TiO2 films and membranes for the development of efficient wastewater treatment and reuse systems

In order to develop efficient photocatalytic TiO₂ films and membranes for application in water and wastewater treatment and reuse systems, there is a great need to tailor-design the structural properties of TiO₂ material and enhance its photocatalytic activity. Through a simple sol–gel route, employing self-assembled surfactant molecules as pore directing agents along with acetic acid-based sol–gel route, we have fabricated nanostructured crystalline TiO₂ thin films and TiO₂/Al₂O₃ composite membranes with simultaneous photocatalytic, disinfection, separation, and anti-biofouling properties. The highly porous TiO₂ material exhibited high specific surface area and porosity, narrow pore size distribution, homogeneity without cracks and pinholes, active anatase crystal phase, and small crystallite size. These TiO₂ materials were highly efficient in the decomposition of methylene blue dye and creatinine, destruction of biological toxins (microcystin-LR), and inactivation of pathogenic microorganisms (Escherichia coli). Moreover, the photocatalytic TiO₂ membranes exhibited not only high water permeability and sharp polyethylene glycol retention but also less adsorption fouling tendency. Here, we report results on the synthesis, characterization, and environmental application and implication of photocatalytic TiO₂ films and membranes.     

Lithium-ion battery anode properties of TiO2 nanotubes prepared by the hydrothermal synthesis of mixed (anatase and rutile) particles

From mixed (anatase and rutile) bulk particles, anatase TiO₂ nanotubes are synthesized in this study by an alkaline hydrothermal reaction and a consequent annealing at 300-400 °C. The physical and electrochemical properties of the TiO₂ nanotube are investigated for use as an anode active material for lithium-ion batteries. Upon the first discharge-charge sweep and simultaneous impedance measurements at local potentials, this study shows that interfacial resistance decreases significantly when passing lithium ions through a solid electrolyte interface layer at the lithium insertion/deinsertion plateaus of 1.75/2.0 V, corresponding to the redox potentials of anatase TiO₂ nanotubes. For an anatase TiO₂ nanotube containing minor TiO₂(B) phase obtained after annealing at 300 °C, the high-rate capability can be strongly enhanced by an isotropic dispersion of TiO₂ nanotubes to yield a discharge capacity higher than 150 mAh g⁻¹, even upon 100 cycles of 10 C-rate discharge-charge operations. This is suitable for use as a high-power anode material for lithium-ion batteries.     

Formation of the Ti4O7 phase through interaction between coated carbon and TiO2

Fine particles ofphotocatalytic anatase TiO₂ prepared through hydrolysis of titanium tetraisopropoxide were coated by carbon. A reduced phase, Ti₄O₇, was formed through interaction between TiO₂ and the coating carbon. EXAFS analysis on this Ti₄O₇ phase showed an intermediate Ti-Ti distance between those in anatase and rutile, which agreed with the structure composed of two-dimensional slabs of Ti-O octahedra separated by a shear plane. This carbon-coated Ti₄O₇ was confirmed to have photocatalytic activity, even though a little lower than anatase, examining the decomposition of methylene blue in water under LTV irradiation.     

The effects of firing conditions on the properties of electrophoretically deposited titanium dioxide films on graphite substrates

Thick anatase films were fabricated on graphite substrates using a method of anodic aqueous electrophoretic-deposition using oxalic acid as a dispersant. Thick films were subsequently fired in air and in nitrogen at a range of temperatures. The morphology and phase composition were assessed and the photocatalytic performance was examined by the inactivation of Escherichia coli in water. It was found that the transformation of anatase to rutile is enhanced by the presence of a graphite substrate through reduction effects. The use of a nitrogen atmosphere allows higher firing temperatures, results in less cracking of the films and yields superior bactericidal performance in comparison with firing in air. The beneficial effects of a nitrogen firing atmosphere on the photocatalytic performance of the material are likely to be a result of the diffusion of nitrogen and carbon into the TiO₂ lattice and the consequent creation of new valence band states.     

Different Effects of Fluoride Surface Modification on the Photocatalytic Oxidation of Phenol in Anatase and Rutile TiO2 Suspensions

The effect of calcination temperature on the photocatalytic degradation of phenol with aqueous suspensions of synthetic anatase and rutile TiO₂ under UV light irradiation (λ > 320 nm), was studied in the absence and presence of NaF. The presence of fluoride accelerates the degradation of phenol in anatase TiO₂, with this positive effect increasing at first, before declining with increasing calcined temperature. A negative effect of fluoride was observed for all the rutile TiO₂ suspensions. The selectivity of catechol increased in the presence of fluoride in both anatase and rutile TiO₂ suspensions.     

Use of coaxial photocatalytic reactor (CAPHORE) in the TiO2 photo-assisted treatment of mixed E. coli and Bacillus sp. and bacterial community present in wastewater

This paper reports the photocatalytic disinfection of water contaminated by a mixture of Escherichia coli and Bacillus sp. as well as that of wastewater containing a larger microbial community. The photocatalytic reactions were carried out in a coaxial photocatalytic reactor called CAPHORE, using TiO₂ P-25 of Degussa. E. coli is more sensitive than Bacillus sp. to photocatalytic treatment. Bacterial inactivation was dependent on organic matter and dissolved oxygen (DO) concentration.

Of the bacterial community present in partially treated wastewater, E. coli appears to be more sensitive to the treatment than Enterococcus sp., coliforms (other than E. coli), and Gram-negative (other than coliforms). After photocatalytic treatment, no bacterial recovery of previous groups was observed for 24 h in the dark. However a very low bacterial inactivation rate was observed for the whole bacterial population present in wastewater and detected by non-selective media. The effective disinfection time (EDT), the time necessary for total inactivation of bacteria without re-growth in a subsequent dark period referenced at 24 h (or 48 h), was reached only for Enterococcus sp., and coliform groups. EDT₂₄ was not reached for the whole population.     

Photocatalytic treatments on dental mirror surfaces using hydrolysis of titanium alkoxide

Anatase titanium dioxide (TiO₂) photocatalytic thin films were directly formed on glass slide and commercial dental mirror substrate surfaces by a hydrolysis of titanium alkoxide, and the hydrophilicity, the degree of oxidizing power and the transparency of the anatase TiO₂-coated substrate surfaces. The contact angles of water and the decomposition rates of methylene blue on the anatase TiO₂ photocatalytic thin films improved with the increasing duration of a tetraethyl orthotitanate (TEOT) hydrolysis, but they hardly changed for the longer duration. The reflectance of anatase TiO₂ photocatalytic thin films coated on glass slide substrate surfaces was higher as the duration of a TEOT hydrolysis increased. Similar tendencies concerning hydrophilicity and transparency were recognized in cases of commercial dental mirror substrate surfaces. A hydrolysis of titanium alkoxide obtained superhydrophilic and antibacterial treatments with excellent transparency on commercial dental mirror substrate surfaces.     

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.     

Sonosynthesis of nanostructured TiO2 doped with transition metals having variable bandgap

Here is described a sonosynthesis method to produce nanostructured TiO₂ pure and doped (Al, C, Co, Fe and Rh). The synthesized TiO₂ is amorphous and is transformed to anatase, brookite or rutile by heat treatments at temperatures between 100 and 300 °C. Pure TiO₂ can be partially transformed to brookite between 100 and 300 °C. The band gap in all heat treated samples from 100–600 °C is relatively constant, 3.2 eV, except for those doped with Fe. This effect on the band gap is the results of a bi/tri-crystal (anatase:brookite:rutile) framework. Rhodium is the most effective dopant to narrow the band gap, the opposite effect is observed with C. In single phase frameworks the bandgap can be modified ranges from 2.38 to 4.10 eV depending on the dopant. TiO₂ lattices are rigid enough to promote an outwards diffusion of the dopants to the surface of the particles forming nanostructured precipitates. The precipitates develop a network of quantum-dots with sizes between 5 and 10 nm.