Solar photocatalytic decolorization of dyes in solution with TiO2 film

Application of TiO₂ film to solar photocatalysis of organic dyes, including Methylene Blue (MB) (λ_max, 660 nm), RR195 (λ_max, 540 nm) and RY145 (λ_max, 420 nm), was investigated. It was found that after 6-h solar irradiation, the extent of color degradation of dyes using solar photocatalytic system without TiO₂ film was quite limited. The color removal percentage for MB, RR195, and RY145 was found to be 23.3, −9.3, and −20.7%, respectively, resulting from competitions between the photosensitizing reaction and formation of colored intermediates during solar irradiation. However, as TiO₂ film was applied, the color degradation capability of solar photocatalytic system was significantly improved, in spite of the fact that only approximately 7% of solar irradiation belongs to the UV region. The color removal percentage for MB, RR195, and RY145 was up to 93.6%, 85.3%, and 71.1%, respectively, after 6-h irradiation. We believed that in such a solar photocatalytic system immobilized with TiO₂ film, both the maximum absorbance wavelength of the dye and the adsorbability of the dye on TiO₂ film played significant roles on the rate and efficiency of color removal of the dye solutions. Moreover, the possible reaction mechanism was proposed. The solar photocatalytic process with immobilized TiO₂ film was found to follow the pseudo-first order reaction kinetics. Color removal rate of MB was almost twice of that of RY145. Accordingly, the photocatalytic degradation process using solar light as an irradiation source, and immobilized TiO₂ as a photocatalyst, showed potential application for the decolorization of wastewater.     

Enhanced photocatalytic activity of highly transparent superhydrophilic doped TiO2 thin films for improving the self-cleaning property of solar panel covers

Undoped and metal doped nanocrystalline TiO₂ transparent thin films were synthesized on glass substrates via sol-gel/dip-coating method. TiO₂ thin film coatings can be applied to the surfaces of solar panels to impart self-cleaning properties to them. The structural and optical properties of few nanometer-thick films were characterized by XRD, SEM, CA, AFM, XPS, and UV–Vis spectrophotometry techniques. The stoichiometric TiO₂ films crystallized in anatase phase, with a particle size of ～100 nm, which were uniformly distributed on the surface. The prepared films with a roughness of ～1–5 nm, increased the hydrophilicity of the glass surface. Reducing the amount of Ti precursor (X) favored the improvement of film quality. To improve the photocatalytic activity of the TiO₂ thin film, it was doped with Ni, Cd, Mo, Bi and Sr metal ions. The effect of metal doping on the photocatalytic activity of the films was investigated using the degradation process of methylene blue (MB) dye as the model contaminant. Among the prepared coatings, the Sr–TiO₂ film showed the highest efficiency for MB degradation. It increased the dye degradation efficiency of the films under both UV and Vis lights. The kinetic investigations also showed that the degradation of MB by TiO₂ and M ? TiO₂ films obeyed the pseudo-first order kinetics.     

AUGMENTATION OF PHOTOCATALYTIC ACTIVITY OF TiO2 THIN FILMS PREPARED BY A SOL-GEL TECHNIQUE

In order to increase the photocatalytic activity on TiO₂ thin film per its external surface area, the structure of flat thin film was modified by adding a small amount of polyethylene glycol (PEG) to TiO₂ sol solution. By firing PEG contained in a TiO₂ gel film, a porous structure was developed. The photocatalytic activities of the thin films prepared thus were evaluated by the degradation of 2-propanol in the aqueous solutions under black light illumination. The photocatalytic activity of TiO₂ thin film prepared with added PEG 400 by 2.6 wt% or PEG 2000 by 9.5 wt% was increased by about 30% as compared to that prepared without added PEG.     

Photoassisted mineralization of remazole red F3B over NiO/TiO2 and CdO/TiO2 nanoparticles under simulated sunlight

In this work, a series of titania-supported NiO and CdO materials were synthesized by a modified sol-gel process. The prepared photocatalysts were characterized by X-ray diffraction (XRD), UV-Vis diffuse reflectance spectroscopy (DRS), and transmission electron microscopy (TEM). The activities of titania-supported NiO and CdO photocatalysts for photocatalytic degradation of Remazole Red F3B (RR) dye, under simulated sunlight, were investigated. The photocatalytic mineralization of an RR dye solution over various NiO-x/TiO₂ and CdO-x/TiO₂ photocatalysts under simulated sunlight was investigated. It was worthy noticing that the photocatalytic activity of titania improved using the prepared catalysts. The prepared TiO₂, NiO-5/TiO₂, and CdO-2/TiO₂ photocatalysts exhibited higher photocatalytic activity under simulated sunlight than did commercial TiO₂. The prepared photocatalysts were stable after photocatalytic degradation of the dye. The observed photocatalytic mineralization of the dye was 51 and 71% over NiO-10/TiO₂ and CdO-2/TiO₂ after 180 min of irradiation, respectively. Juxtaposing a p-NiO-5/TiO₂ semiconductor provided a potential approach for decreasing charge recombination. The prepared photocatalystsNiO-5/TiO₂ and CdO-2/TiO₂ are promising composites for the solar detoxification of textile wastewater.     

Enhanced photocatalytic activity of Au-buffered TiO2 thin films prepared by radio frequency magnetron sputtering

Au-buffered TiO₂ thin films have been prepared by radio frequency magnetron sputtering method. The structural and morphological properties of the thin films were characterized by X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The photocatalytic activity of the samples was evaluated by the photodecomposition of methylene blue. The Au-buffer thin layer placed between the TiO₂ thin films significantly enhanced photocatalytic activity by 50%. Annealing the Au-buffered TiO₂ thin film at 600 °C decreased the film roughness, but it increased the surface area and anatase crystalline size, enhancing the photocatalytic activity.     

Effects of V and Zn codoping on the microstructures and photocatalytic activities of nanocrystalline TiO2 films

To enhance the photocatalytic activity of TiO₂, V and Zn co-doped TiO₂ films were synthesized by the sol–gel method. The experimental results indicated that the films were composed of round-like nano-particles or aggregates. V and Zn codoping could not only obviously increase the specific surface area of TiO₂ but also result in the narrowed band gap of TiO₂ sample. The photocatalytic activities of the TiO₂ films were evaluated by the photocatalytic decomposition of organic dyes in aqueous solution. Compared with un-doped TiO₂ film or single doped TiO₂ film, V and Zn co-doped TiO₂ film exhibited excellent photocatalytic activities under both UV light and visible light. The improvement mechanism by V and Zn codoping was also discussed.     

Photocatalytic functional coatings of TiO2 thin films on polymer substrate by plasma enhanced atomic layer deposition

We prepared photocatalytic TiO₂ thin films which exhibited relatively high growth rate and low impurity on polymer substrate by plasma enhanced atomic layer deposition (PE-ALD) from Ti(NMe₂)₄ [tetrakis (dimethylamido) Ti, TDMAT] and O₂ plasma to show the self-cleaning effect. The TiO₂ thin films with anatase phase and bandgap energy about 3.3 eV were deposited at growth temperature of 250 °C and the photocatalytic effects were compared with commercial Activ glass. From contact angles measurement of water droplet and photo-induced degradation test of organic liquid, TiO₂ thin films with anatase phases showed superhydrophilic phenomena and decomposed organic liquid after UV irradiation. The anatase TiO₂ thin film on polymer substrate showed highest photocatalytic efficiency after 5 h UV irradiation. We attribute the highest photocatalytic efficiency of TiO₂ thin film with anatase structure to the formation of suitable crystalline phase and large surface area.     

Removal of persistent tartrazine dye by photodegradation on TiO2 nanoparticles enhanced by immobilized calcinated sewage sludge under visible light

TiO₂ nanoparticles on calcined sewage sludge (TiO₂/sludge) were prepared by the sol-gel method and were fully characterized. The photocatalytic efficiency of TiO₂/sludge was evaluated by tartrazine dye degradation by halide lamp. TiO₂/sludge exhibited a high photocatalytic oxidation efficiency (more than 90%) of tartrazine compared with naked TiO₂ (less than 20%) due to the synergy effect of sewage sludge. The optimization of experimental conditions was 0.5 g/l TiO₂/Sludge, pH 8 with 50 mg/l tartrazine dye. The addition of sewage sludge to TiO₂ improves the efficiency of dye mineralization. The prepared catalyst showed easier separation and effective reuse.     

Photoelectrocatalytic degradation of bisphenol A in aqueous solution using a Au–TiO2/ITO film

A series of Au–TiO₂/ITO films with nanocrystaline structure was prepared by a procedure of photo-deposition and subsequent dip-coating. The Au–TiO₂/ITO films were characterized by X-ray diffraction, scanning electronic microscopy, electron diffraction, X-ray photoelectron spectroscopy, and UV–VIS diffuse reflectance spectroscopy to examine the surface structure, chemical composition, the chemical state of metal, and the light absorption properties. The photocatalytic activity of the Au–TiO₂/ITO films was evaluated in the photocatalytic (PC) and photoelectrocatalytic (PEC) degradation of bisphenol A (BPA) in aqueous solution. Compared with a TiO₂/ITO film, the degree of BPA degradation using the Au–TiO₂/ITO films was significantly higher in both the PC and PEC processes. The enhancement is attributed to the action of Au deposits on the TiO₂ surface, which play a key role by attracting conduction band photoelectrons. In the PEC process, the anodic bias externally applied on the illuminated Au–TiO₂/ITO film can further drive away the accumulated photoelectrons from the metal deposits and promote a process of interfacial charge transfer.     

EIS analysis on low temperature fabrication of TiO2 porous films for dye-sensitized solar cells

A low temperature (<150 °C) fabrication method for preparation of TiO₂ porous films with high efficiency in dye-sensitized solar cells (DSSCs) has been developed. The Ti(IV) tetraisopropoxide (TTIP) was added to the paste of TiO₂ nanoparticles to interconnect the TiO₂ particles. The electrochemical impedance spectroscopy (EIS) technique was employed to quantify the charge transport resistance at the TiO₂/dye/electrolyte interface (R_ct2) and electron lifetime in the TiO₂ film (τ_e) under different molar ratios of TTIP/TiO₂ and also at various TiO₂ thicknesses. It was found that the R_ct2 decreased as the molar ratio increased from 0.02 to 0.08, however, it increased at a molar ratio of 0.2 due to the reduction in surface area for dye adsorption. In addition, the characteristic frequency peak shifted to lower frequency at a molar ratio of 0.08, indicating the longer electron lifetime. As for the thickness effect, TiO₂ film with a thickness around 17 μm achieved the best cell efficiency. EIS study also confirmed that, under illumination, the smallest R_ct2 was associated with a TiO₂ thickness of 17 μm, with the R_ct2 increased as the thickness of TiO₂ film increased. In the Bode plots, the characteristic frequency peaks shifted to higher frequency when the thickness of TiO₂ increased from 17.2 to 48.2 μm, indicating the electron recombination increases as the thickness of the TiO₂ electrode increases.Finally, to make better use of longer wavelength light, 30 wt% of larger TiO₂ particle (300 nm) was mixed with P25 TiO₂ as light scattering particles. It effectively increased the short-circuit current density and cell conversion efficiency from 7.44 to 8.80 mA cm⁻² and 3.75 to 4.20%, respectively.     

TiO2-coated nanostructures for dye photo-degradation in water

The photocatalytic efficiency of a thin-film TiO₂-coated nanostructured template is studied by dye degradation in water. The nanostructured template was synthesized by metal-assisted wet etching of Si and used as substrate for the deposition of a thin film of TiO₂ (10 nm thick) by atomic layer deposition. A complete structural characterization was made by scanning and transmission electron microscopies. The significant photocatalytic performance was evaluated by the degradation of two dyes in water: methylene blue and methyl orange. The relevance of the reported results is discussed, opening the route toward the application of the synthesized nanostructured TiO₂ for water purification.     

Thickness effect in the photocatalytic activity of TiO2 thin films derived from sol-gel process

TiO₂ films in various thicknesses were prepared by sol-gel method, and their photocatalytic activities in the decomposition of gaseous 2-propanol were evaluated. It was found that the photocatalytic activities of transparent TiO₂ films increase with the increase of film thickness: The photocatalytic activity of TiO₂ films in 670 nm-thickness was 3.7 times that of films in 70 nm-thickness. We proposed that the higher photocatalytic activities for the thicker TiO₂ films originate from the greater amount of photogenerated electron and hole pairs, which are transferred from the inside to the surface of TiO₂ films. We also provided some experimental evidences supporting this mechanism.     

Photocatalytic decomposition of NO on titanium oxide thin film photocatalysts prepared by an ionized cluster beam technique

Transparent TiO₂ thin film photocatalysts were prepared on transparent porous Vycor glass (PVG) by an ionized cluster beam (ICB) method. The UV‐VIS absorption spectra of these films show specific interference fringes, indicating that uniform and transparent TiO₂ thin films are formed. The results of XRD measurements indicate that these TiO₂ thin films consist of both anatase and rutile structures. UV light (λ > 270 nm) irradiation of these TiO₂ thin films in the presence of NO led to the photocatalytic decomposition of NO into N₂, O₂ and N₂O. The reactivity of these TiO₂ thin films for the photocatalytic decomposition of NO is strongly dependent on the film thickness, i.e., the thinner the TiO₂ thin films, the higher the reactivity. This revised version was published online in July 2006 with corrections to the Cover Date.     

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.     

Photocatalytic activity of undoped and sulfur-doped TiO2 films grown by MOCVD for water treatment under visible light

Titanium dioxide ceramic coatings have been used as catalysts in green technologies for water treatment. However, without the presence of a dopant, its photocatalytic activity is limited to the ultraviolet radiation region. The photocatalytic activity and the structural characteristics of undoped and sulfur-doped TiO₂ films grown at 400 °C by metallorganic chemical vapor deposition (MOCVD) were studied. The photocatalytic behavior of the films was evaluated by methyl orange dye degradation under visible light. The results suggested the substitution of Ti⁴⁺ cations by S⁶⁺ ions into TiO₂ structure of the doped samples. SO₄^2? groups were observed on the surface. S-TiO₂ film exhibited good photocatalytic activity under visible light irradiation, and the luminous intensity strongly influences the photocatalytic behavior of the S-TiO₂ films. The results supported the idea that the sulfur-doped TiO₂ films grown by MOCVD may be promising catalysts for water treatment under sunlight or visible light bulbs.     

Photocatalytic properties of TiO2 sol-gel modified nanocomposite films

TiO₂ nanocomposite films with different concentrations of TiO₂ MT-150A nanoparticles were immobilized on glass substrates using a dip coating process. The crystalline structure and surface chemical state of nanocomposite film properties were examined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The specific surface area and morphology of TiO₂ MT-150A nanoparticles were evaluated by the BET method and Field Emission Scanning Electron Microscopy (FE-SEM). The photocatalytic activities of films were evaluated by the methyl orange decoloring rate. XPS measurements showed that the oxygen amount (%) was related to the film composition. The composite film with 10 g/L MT-150A loading yielded the highest amount of surface oxygen (26.82%) and TiO₂ rutile showed the lowest amount of surface oxygen (13.67%) in the form of surface hydroxyl groups. The remaining oxygen was identified as lattice oxygen. In addition, the nanocomposite film with 10 g/L MT-150A loading yielded the highest photocatalytic activity.     

Aging Studies on Food Packaging Films Containing β-Cyclodextrin-Grafted TiO2 Nanoparticles

Polymeric materials, such as polyvinyl alcohol (PVA) and ethylene–PVA copolymers (EVOH) are widely used in the food sector as packaging materials because of their excellent properties. TiO₂ nanoparticles (NPs) show photocatalytic activity; when added to the aforementioned polymers, on the one hand, they are expected to provide bactericidal capacity, whereas on the other hand, they could favor nanocomposite degradation. These types of nanoparticles can be derivatized with cyclodextrin macromolecules (CDs), which can act as food preservative carriers, increasing the packaging food protective properties. In this work, films containing β-Cyclodextrin (βCD)-grafted TiO₂ nanoparticles and PVA or EVOH were prepared. Regarding the photocatalytic activity of the nanoparticles and the possible environmental protection, accelerated aging tests for PVA, EVOH, and their composites with cyclodextrin-grafted TiO₂ nanoparticle (NP) films were performed by two methods, namely, stability chamber experiments at different conditions of temperature and relative humidity and UV light irradiation at different intensities. After analyzing the systems color changes (CIELAB) and Fourier transform infrared spectroscopy (FTIR) spectra, it was observed that the film degradation became more evident when increasing the temperature (25–80 °C) and relative humidity percentage (28–80%). There was no significant influence of the presence of CDs during the degradation process. When irradiating the films with UV light, the largest color variation was observed in the nanocomposite films, as expected. Moreover, the color change was more relevant with increasing NP percentages (1–5%) due to the high photocatalytic activity of TiO₂. In addition, films were characterized by FTIR spectroscopy and variation in the signal intensities was observed, suggesting the increase of the material degradation in the presence of TiO₂ NPs.     

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.     

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).     

Enhancing Effects of Ultrasound Treatment on the Preparation of TiO2 Photocatalysts

TiO₂ nanoparticles were synthesized by the hydrolysis and condensation of TiCl₄ in a mixed solvent of iso-propyl alcohol and water with or without ultrasound treatment. As-prepared powders were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), energy filtering transmission electron microscopy (EF-TEM), particle size analysis and BET surface area analysis. The specific surface area, thermal stability and crystallization of the as-prepared samples treated with ultrasound were higher than those of samples treated without ultrasound. To examine the photocatalytic activity of the as-prepared TiO₂, the photodegradation of MB which is a typical dye resistant to biodegradation has been investigated on TiO₂ powders in aqueous heterogeneous suspensions. The photocatalytic degradation of a aqueous solution of methylene blue shows a remarkable increase when it is carried out with ultrasound in all cases.