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.     

Photocatalytic degradation of trichloroethylene over TiO<Subscript>2</Subscript>/SiO2 in an annulus fluidized bed reactor

The effects of superficial gas velocity (Ug), wavelength and intensity of ultraviolet (UV) light, oxygen and H₂O concentration on the photocatalytic degradation of TCE (Trichloroethylene) over TiO₂/SiO₂ catalyst have been determined in an annulus fluidized bed photoreactor. The key factor in determining the performance of the annulus fluidized bed photoreactor is found to be an optimum superficial gas velocity (Ug) that provides the optimum UV lighttransmit through the proper size of bubbles in the photoreactor. The degradation efficiency of TCE increases with light intensity but decreases with wavelength of the UV light and H₂O concentration in the fluidized bed of TiO₂/silica-gel photocatalyst. The optimum concentration of O₂ for TCE degradation is found to be approximately 10%. The annulus fluidized bed photoreactor is an effective tool for high TCE degradation with efficient utilization of photon energy. This paper is dedicated to Professor Dong Sup Doh on the occasion of his retirement from Korea University.     

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.     

Spectral Selective Solar Light Enhanced Photocatalysis: TiO<Subscript>2</Subscript>/TiAlN Bilayer Films

We demonstrate that spectral selective photocatalytic multilayer films can be tailored such that they can harness the full solar spectrum for enhanced photocatalytic gas-phase oxidation of acetaldehyde. Thin films of anatase TiO₂ were deposited on a thin solar absorber TiAlN film to fabricate bilayer TiO₂/TiAlN films by dc magnetron sputtering on aluminium substrates. The structural and optical properties of the films were characterized by X-ray diffraction and Raman spectroscopy. The reaction rate and quantum yield for acetaldehyde removal was measured and an almost tenfold enhancement of the quantum yield was observed for the TiO₂/TiAlN films compared with the single TiO₂ film, on par with enhancements achieved with new heterojunction photocatalysts. The results were interpreted by a temperature-induced change of the reaction kinetics. Absorption of simulated solar light illumination resulted in a temperature increase of the TIAlN film that was estimated to be at most 126 K. We show that a concomitant temperature increase of the top layer TiO₂ by 100 K shifts the water gas-surface equilibrium from multilayer to submonolayer coverage. We propose that this is the main reason for the observed enhancement of the photocatalytic activity, whereby gas phase molecules may come in direct contact with free surface sites instead of having to diffuse through a thin water film. The implications of the results for judicious control of temperature and relative humidity for efficient gas-phase photocatalysis and exploitation of selective solar absorbing films are discussed.     

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.     

Electrochemically assisted photocatalytic degradation of oxalic acid on particulate TiO2 film in a batch mode plate photoreactor

Electrochemically assisted photocatalytic degradation of oxalic acid was studied in a batch mode plate photoreactor composed of particulate TiO₂ film immobilized on Ti metal plate (Ti/TiO₂ electrode) and Pt wires immersed in a flowing film of aqueous solution (Pt counter electrode). The degradation rate of oxalic acid was followed as a function of the potential of the Ti/TiO₂ electrode, the oxygen concentration and the light intensity. The presence of oxalic acid caused an increase in the measured photocurrent by one order of magnitude which is due to its reaction with photogenerated holes. The degradation rate increased with increasing potential up to 0.5 V vs SCE, then the increase was more gradual. Electrochemically assisted photocatalytic degradation of oxalic acid also proceeded in the absence of oxygen. The photogenerated electrons caused hydrogen evolution (low oxygen concentration) or predominantely oxygen reduction (high oxygen concentration) on the Pt counter electrode.     

Photocatalytic degradation of imidacloprid pesticide in aqueous solution by TiO2 nanoparticles immobilized on the glass plate

The purpose of this study is to appraise the photocatalytic degradation of imidacloprid pesticide in an aqueous solution. To this end, imidacloprid was degraded using TiO₂ nanoparticles immobilized on a glass plate under UV light illumination. The effects of operational parameters (initial concentration of imidacloprid, pH, and light intensity) on the activity of TiO₂ nanophotocatalyst and the kinetics of the reaction were investigated. The results indicated that TiO₂ had impressive photocatalytic proficiency in the presence of UV-C light irradiation for the removal of imidacloprid from the aqueous solution. The highest efficiency for the removal of imidacloprid (R%?=?90.24) was obtained in the initial concentration of 20?mg?L^?1 imidacloprid, pH?=?5, and light intensity of 17?W?m^?2 after 180?min. The results of the mineralization studies represented a subtractive trend of total organic carbon (TOC) and an increase in the mineralization products during the reaction time.     

Polymer material-supported titania nanofibers with different polyvinylpyrrolidone to TiO2 ratios for degradation of vaporous trichloroethylene

Polymer-supported TiO₂ nanofibers with different polyvinylpyrrolidone (PVP)-to-TiO₂ (PT) ratios were synthesized and their photocatalytic efficiencies were examined for the decomposition of trichloroethylene (TCE). The spectral results of the TiO₂ nanofibers confirmed the presence of TiO₂ crystal phases in the nanofibers. The TiO₂ nanofiber with the highest PT ratio of 1:1.3 showed the highest TCE decomposition (94%), followed by TiO₂ nanofibers with PT ratios of 1:0.7 (91%), 1:0.35 (88%), and 1:0.1 (84%). The conditions of low input concentration and humidity were suggested for the optimal decomposition of TCE. Consequently, the TiO₂ nanofiber webs could be used effectively to decompose TCE.     

Microwave photocatalysis II: novel continuous‐flow microwave photocatalytic experimental set‐up with titania‐coated mercury electrodeless discharge lamps

BACKGROUND: A continuous‐flow microwave photocatalytic reactor was set up consisting of a glass tube equipped with microwave powered mercury electrodeless discharge lamps (EDLs) coated with thin films of nanoporous titanium(IV) oxide. The effect of operational parameters on photocatalytic degradation of aqueous mono‐chloroacetic acid (MCAA) by a TiO₂/UV/MW process was investigated. RESULTS: Studies were carried out at a relatively high concentration of MCAA (0.1 mol L^?1), and revealed that reaction temperature and light intensity of the EDLs depend inversely on the flow rate, but that the 366 nm line intensity of EDL is directly proportional to the reaction temperature. The photodegradation of MCAA was enhanced by heating and significantly enhanced by air bubbling of the reaction mixture in the glass reservoir at laboratory temperature. The photocatalytic efficiency increased with the number of titania‐coated EDLs inserted in the glass tube reactor. CONCLUSIONS: It was found that the operational parameters (i.e. flow rate, reaction temperature, number of titania‐coated EDLs, and air bubbling) had important effects on degradation efficiency. The photocatalytic degradation of MCAA on thin films of titanium(IV) oxide in the continuous‐flow microwave photoreactor can be enhanced in the TiO₂/UV/MW system. Copyright © 2009 Society of Chemical Industry     

Photocatalytic activity of metal ion (Fe or W) doped titania

Iron or tungsten-doped nano TiO₂ were successfully synthesized from TiCl₄. All of the samples showed anatase phase of TiO₂. For the iron-doped TiO₂, Iron ion was well dispersed in the TiO₂ lattice. However, tungsten-doped TiO₂ formed 12-tungstate with anatase TiO₂. As the concentration of tungsten increased, 12-tungstate disappeared. The photocatalytic oxidation of acetaldehyde was evaluated to examine the photocatalytic characteristics of metal-doped TiO₂. Because of the surface containing metal oxide or metal precursors at high concentration metal ion, increasing the concentration of W or Fe ion decreased the reactivity. The reaction rate was drastically increased after 300 °C heat treatment. Furthermore, the photocatalytic activity of iron- or tungsten-doped TiO₂ was higher than that of synthesized pure TiO₂ and commercial TiO₂.     

Operando FTIR study of the photocatalytic oxidation of acetone in air over TiO2–ZrO2 thin films

Operando FTIR spectroscopy has been used to study the photocatalytic oxidation of acetone vapors over semiconductors films containing TiO₂ and ZrO₂. Preparation of these coatings was carried out by dipping a silicon wafer in stable sols containing particles of TiO₂, Ti_1−xZr_xO₂, or a mixture of ZrO₂ and TiO₂. These differences in chemical composition and phase homogeneity were selected in order to determine their effect on the photocatalytic performance. A transmission cell specifically designed for in situ studies of photocatalytic coatings was utilized for the FTIR experiments under reaction conditions. In contrast with investigations with powdered photocatalysts, the use of thin films guarantees that the whole semiconductor is irradiated, and for that reason purely photochemical reactions are monitored. Acetone adsorption takes place molecularly and is higher on the Ti_1−xZr_xO₂ coating. This fact is very likely related to the higher specific surface of the samples containing Zr. However, the maximum photocatalytic rate for acetone degradation corresponds to the films composed by a binary mixture of TiO₂ and ZrO₂. On the other hand, remarkable differences on the type and concentration of intermediates appearing as a result of the photocatalytic oxidation of acetone are found for the coatings studied. A simple kinetic model was applied to analyze the evolution of both gas phase and surface species. The parameters obtained indicate that each specific surface process is affected in a different way by the variation in the composition of the photoactive films.     

Photocatalytic degradation of gaseous trichloroethene using immobilized ZnO/SnO2 coupled oxide in a flow‐through photocatalytic reactor

The photocatalytic degradation of gaseous trichloroethene (TCE) was investigated on immobilized ZnO/SnO₂ coupled oxide in a flow‐through photocatalytic reactor. It was found that gaseous photocatalysis is an efficient method for volatile organic compounds' abatement and air purification. Degradation of ～100% was found for TCE at the concentrations examined, up to 400 ppmv, in a flow‐through dry synthetic gas stream. In our tested conditions, the flow rate had little influence on the photocatalytic degradation efficiencies of TCE, while the relative humidity had a significant influence on the photocatalytic degradation of TCE. The photocatalytic degradation efficiencies of TCE increased slowly below 20% relative humidity and then decreased as the relative humidity increased further. The deactivation of used immobilized photocatalyst was not observed within the 200 h testing period in the present experiment, although the surface of the photocatalyst changed greatly during the use of the photocatalyst. Copyright © 2004 Society of Chemical Industry     

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.     

Hybrid photocatalysts for the degradation of trichloroethylene in air

Hybrid photocatalysts based on an adsorbent SiMgOx and a photocatalyst TiO₂ were developed in a plate shape. The ceramic surface was coated with TiO₂ by the slip-casting technique. The effect of the support in the photocatalytic degradation of trichloroethylene (TCE) was analyzed by modifying TiO₂ loading and the layer thickness. Photocatalysts were characterised by N₂ adsorption–desorption, mercury intrusion porosimetry, SEM, UV–vis spectroscopy and XRD. A direct relationship between the TiO₂ content and the photocatalytic activity was observed up to three layers of TiO₂ (0.66 wt.%). Our results indicate that intermediate species generated on the TiO₂ layer can migrate through relatively long distances to react with the OH⁻ surface groups of the support. By increasing the TiO₂ loading of the photocatalyst two effects were observed: trichloroethylene conversion is enhanced, while the efficiency of the oxidation process is decreased at expenses of increasing the selectivity to COCl₂ and dichloroacetylchloride (DCAC). The results are discussed in terms of the layer thickness, TiO₂ amount, TCE conversion and CO₂, and COCl₂ selectivity.     

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.     

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.     

Kinetic study of photocatalytic decolorization of C.I. Basic Blue 3 solution on immobilized titanium dioxide nanoparticles

The photocatalytic decolorization of C.I. Basic Blue 3 (BB3) solution on immobilized TiO₂ nanoparticles was carried out in a rectangular flat-plate photoreactor. The investigated TiO₂ was Millennium PC-500 (anatase, average crystallite size 8 nm, surface area 320.76 m² g⁻¹) immobilized on non-woven paper. A new kinetic model was proposed on the basis of intrinsic element reactions. The proposed kinetic model fairly resembled the classic Langmuir–Hinshelwood (L–H) equation from its expression. It was found that both k_obs (the reaction rate constant) and K_R (the adsorption rate constant) were linearly proportional to the light intensity in a rather large intensity range.     

Photocatalytic activity of TiO2 nanotube layers coated with Ag and Pt

Abstract

Abstract

Thin films of anatase TiO₂ nanotube arrays (TiO₂ NTs) were prepared in this study. Pt and Ag were coated on the TiO₂ NTs films, which intend to increase the photocatalytic activity under ultraviolet-visible (UV-vis) irradiation. The phase and structure of the films were investigated by X-ray diffraction and scanning electron microscopy. Photocatalytic activity was tested by UV-vis absorption spectroscopy and showed that UV-vis light absorption of the films was remarkably improved by coated Ag and Pt by 72% and 183% respectively. The photocatalytic activities of the films towards degraded methyl orange and HCHO were compared and were all found to follow the sequence Pt/TiO₂ NTs>Ag/TiO₂ NTs>TiO₂ NTs. It was also found that the kinetics of HCHO photocatalytic degradation by the films fits the first order reaction model better and has higher efficiency than that of the methyl orange photocatalytic degradation by the same films.     

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.     

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.