Surface properties and photocatalytic activity of nanocrystalline titania films

In this work the efficiency and physicochemical details of a thin film produced by help of a microwave assisted sol gel technique is compared to different commercial powders (Degussa P25 and Hombikat UV100) deposited on glass substrates. Furthermore, a supercritical produced TiO₂ powder (SC 134) was included in the comparison.The prepared TiO₂ films were characterized using XRD, XPS, AFM, DSC and DLS. The photocatalytic activity was determined using stearic acid as a model compound. Investigation of the prepared films showed that the Degussa P25 film and the sol–gel film were the most photocatalytic active films. The activity of the films was found to be related to the crystallinity of the TiO₂ film and the amount of surface area and surface hydroxyl groups. Based on the XPS investigation of the films before and after UV irradiation it was suggested that the photocatalytic destruction of organic matter on TiO₂ films proceeds partly through formation of hydroxyl radicals which are formed from surface hydroxyl groups created by interactions between adsorbed water and vacancies on the TiO₂ surface. Furthermore a correlation between the amount of OH groups on the surface of the different TiO₂ films and the photocatalytic activity was found.     

Synergistic effect between TiO2 sol–gel and Degussa P25 in dye photodegradation

The TiO₂ powders were synthesized by versatile and low cost sol–gel process using HNO₃ and titanium tetra-isopropoxide (volumetric ratio of 4:1) following by the hydrolysis reaction. The powders show the two polymorphs of TiO₂: 96 % anatase and 4 % brookite, due to acidic condition (pH = 3). Thin films of titanium oxide were obtained by dip-coating, using the sol–gel of titanium oxide mixed with commercial Degussa P25 into a weight ratio 1:1 or 1:1.5, to enhance the synergistic effect of anatase/rutile ratio aiming at increasing the efficiency of the TiO₂ photocatalyst in dyes degradation. The thin film surface (charge and morphology) was controlled by polymer (poly-ethylene glycol) and surfactant (Sodium dodecyl sulphate, Triton X100) addition. The titanium oxide was characterized by particle size analyzer, contact angle measurements, X-ray diffraction, scanning electron microscopy, and atomic force microscopy. The photocatalytic properties of powders and coatings were evaluated based on the degradation efficiency of two reference dyes (methyl orange and methylene blue). The results outline that poly(ethylene glycol) and films morphologies are the most influential factors that affecting the photocatalytic activity.     

Magnetically Recoverable Core–Shell Nanocomposites with Enhanced Photocatalytic Activity

Core–shell structured Fe₃O₄/SiO₂/TiO₂ nanocomposites with enhanced photocatalytic activity that are capable of fast magnetic separation have been successfully synthesized by combining two steps of a sol–gel process with calcination. The as‐obtained core–shell structure is composed of a central magnetite core with a strong response to external fields, an interlayer of SiO₂, and an outer layer of TiO₂ nanocrystals with a tunable average size. The convenient control over the size and crystallinity of the TiO₂ nanocatalysts makes it possible to achieve higher photocatalytic efficiency than that of commercial photocatalyst Degussa P25. The photocatalytic activity increases as the thickness of the TiO₂ nanocrystal shell decreases. The presence of SiO₂ interlayer helps to enhance the photocatalytic efficiency of the TiO₂ nanocrystal shell as well as the chemical and thermal stability of Fe₃O₄ core. In addition, the TiO₂ nanocrystals strongly adhere to the magnetic supports through covalent bonds. We demonstrate that this photocatalyst can be easily recycled by applying an external magnetic field while maintaining their photocatalytic activity during at least eighteen cycles of use.     

The influence of titanium dioxide phase composition on dyes photocatalysis

A comparative study of TiO₂ powders prepared by sol–gel methods is presented. Titanium tetraisopropoxide was used as the precursor for the sol–gel processes. The effects of the annealing treatment on phase, crystallite size, porosity and photodegradation of dyes (methyl orange and methylene blue) were studied. The phase structure, microstructure and surface properties of the films were characterized by using X-ray diffraction (XRD) and Atomic Force Microscopy (AFM). The X-ray diffraction was used for crystal phase identification, for the accurate estimation of the anatase–rutile ratio and for the crystallite size evaluation of each polymorph in the samples. It was found that the only TiO₂ anatase phase of the synthesized TiO₂ develops below 500 °C, between 600 and 800 °C the anatase coexist with rutile and above 800 °C only the rutile phase was found in the samples. Attention has been paid not only to crystal structures, but also to the porosity, the particle size and the photocatalytic properties. However, the annealing temperature was found to have significant influence on the photocatalytic properties. Different TiO₂ doctor blade thin films were obtained mixing the sol gel powder (100% anatase) and TiO₂ Aldrich with TiO₂ Degussa P25. The surfactant (Triton X100 or sodium dodecyl sulfate) affects the packing density of the particles during deposition and the photocatalytic degradation efficiency of the dyes. The photocatalytic degradation kinetics of methyl orange and methylene blue using TiO₂ thin film were investigated.     

Photocatalytic reduction of nitrate over TiO2 and Ag-modified TiO2

This research work presents the efficiency of the TiO₂ and Ag–TiO₂ thin films prepared by the sol–gel method and coated onto the surface of 304 stainless steel sheets used in the photocatalytic nitrate reduction processes. The Ag–TiO₂ thin films had the weight by weight (w/w) ratio of Ag⁺/TiO₂ of 0.1% as Ag atom. The XRD results showed that the crystalline phase structure of TiO₂ on the Ag–TiO₂ thin films was anatase. The optical band gaps of the TiO₂ and 0.1% Ag–TiO₂ thin films were respectively 3.27 and 2.70 eV, while the surface of the prepared catalysts was hydrophobic with the respective average water contact angles of 94.8° and 118.5° for the TiO₂ and 0.1% Ag–TiO₂ thin films. The net efficiencies of photocatalytic nitrate reduction of TiO₂ and 0.1% Ag–TiO₂ were 41.4% and 70.0%, respectively. The loading of Ag only influenced the nitrate removal efficiency without affecting the stoichiometric ratio of formate to nitrate. The net stoichiometric ratio of formate to nitrate of all experiments was 2.8:1.0, which is close to the stoichiometric ratio of 2.5:1.0 of the nitrate reduction to nitrite and then to nitrogen gas.     

Mesoporous and mesostructured TiO2 coatings for photocatalytic applications

The correlation between the textural properties and the photocatalytic activity of nanocrystalline Titanium dioxide (TiO₂)-anatase films obtained by sol–gel has been investigated. Mesoporous and mesostructured TiO₂-anatase films were prepared using different titanium precursors and Pluronic (F127) and polyethylene glycol hexadecyl ether P5884 (Brij58) surfactants via acid catalysis. Ca(NO₃)₂ and WCl₆ were incorporated to TiO₂ sols to investigate the effect of the doping on the photocatalytic behaviour. The microstructure and textural properties were characterised by X-ray diffraction, spectral ellipsometry and transmission electronic microscopy. The photocatalytic properties were evaluated in aqueous solution (methyl orange) and in gas phase (trichloroethylene, sulphide acid and methyl-ethyl-ketone) using multilayer films deposited on glass-slides. TiO₂-B-Brij-58 films exhibited the most efficient photocatalytic activity either in aqueous or gas medium. The Ca doping strongly enhances the photocatalytic activity associated with the reduced recombination of electrons and holes in the catalyst.     

Mesoporous interstitial and substitutional nitrogen-containing titanium dioxide photocatalysts prepared by acid-assisted sol–gel procedure

Two different types of nitrogen-containing TiO₂ were synthesized via an acid-modified sol–gel method. Yellow-colored interstitial and white substitutional nitrogen-containing TiO₂ powders were characterized by means of X-ray diffraction, X-ray photoelectron spectroscopy, Brunauer–Emmett–Teller specific surface area and Barrett–Joyner–Halenda pore size distribution analyses, scanning electron microscopy, transmission electron microscopy and UV–vis absorption spectroscopy. The band gaps of interstitial and substitutional nitrogen-containing TiO₂ ceramics were estimated from UV–vis spectroscopy data to be 2.6 and 2.8 eV, respectively. Prepared substitutional nitrogen-containing TiO₂ featured steep light absorption edge with an approximately parallel characteristic to that in pure TiO₂. This fact is due to band-to-band visible light absorption ability of substitutional nitrogen-containing TiO₂. The photocatalytic properties of the produced nitrogen-containing TiO₂ samples were compared for the degradation of Direct Red 28 azo dye. Further studies were also devised to compare the catalytic efficiency of the nitrogen-containing TiO₂ powders with the pure TiO₂ synthesized via the similar sol–gel method. The produced nitrogen-containing TiO₂ samples revealed superior photocatalytic properties in comparison with pure TiO₂ due to their remarkable band gap narrowing, surface oxygen vacancies and much more surface defects. The results also revealed that the substitutional nitrogen-containing TiO₂ is the most effective photocatalyst under ultraviolet and visible light irradiation.     

The influence of precursor’s composition and concentration on cadmium doped TiO<Subscript>2</Subscript> film

The purpose of this article is to emphasize that new nanomaterials offer a number of attractive alternatives for solar energy use in wastewater photocatalysis. The wastewater from the textile industry contains dyes and heavy metals. Thin films of cadmium doped TiO₂ (Cd-TiO₂) were coated by a doctor blade using TiO₂ Degussa P25 and cadmium precursor (cadmium nitrate). The photocatalytic efficiency of cadmium doped TiO₂ is strongly influenced by crystal structure, particle size, particle morphology, porosity and doping. The pore size distribution and the roughness analysis have been studied by the atomic force microscopy (AFM) of the thin films. The photocatalytic activity of the samples was tested in Methyl orange and Methylene blue photodegradation. Cadmium doped TiO₂ catalyst does not significantly decrease the efficiency of photodegradation processes, and, in some situations, improves dye photodegradation.     

Enhanced photocatalytic activity of Ni doped TiO2 nanowire–nanoparticle hetero–structured films prepared by hydrothermal and sol–gel methods

TiO₂ nanowire-nanoparticle hetero-structured films were prepared via a sol–gel method and coated on glass substrates by dipping method for photocatalytic activity. In this study 0, 1, 3, and 5 mol% of Ni doped were studied. One-dimensional TiO₂ nanowires (NWs) were prepared by hydrothermal treatment with TiO₂ nanoparticles (NPs) which are commercially available. XRD, FESEM, DRS, and XPS were used to characterize the prepared nanowire-nanoparticle hetero-structures films. 3%Ni doped TiO₂ hetero-structured film (TNi3) had the highest photocatalytic activity on the degradation of methylene blue (MB). TNi3 films provided about 4.3 times of degradation rate compared to undoped TiO₂ (T). It revealed that TNi3 film resulted in shifting the absorption wavelength towards narrowing the energy band gap and small crystallite size. Therefore, the TNi3 film exhibited a photocatalytic activity on the degradation of MB under visible light irradiation greater than undoped film.     

Synthesis and photocatalytic activity of nanocrystalline TiO2 co-doped with nitrogen and cobalt(II)

Nitrogen-modified cobalt-doped TiO₂ materials were successfully prepared via a modified sol–gel method. The structure and properties of the catalysts were characterized via X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution TEM, ultraviolet–visible light diffuse reflectance spectra (UV–Vis DRS), N₂ adsorption–desorption isotherms, and energy-dispersive X-ray spectroscopy. The XRD patterns of the pure and co-doped TiO₂ samples indicate that the predominant phase was anatase. The average grain size obtained from TEM was approximately 10 nm. The Brunauer–Emmett–Teller analysis results indicate that the specific surface area was 77.7 m² g^?1. The UV–Vis DRS results for the co-doped sample reveal an absorption edge that had been red-shifted to 500 nm. The photocatalytic activities of the samples were evaluated through photodegradation of papermaking wastewater under UV and visible light irradiation. Compared with the cobalt-doped TiO₂ sample and Degussa P25, the 3 mol% N-doped mesoporous N/Co-TiO₂ photocatalyst exhibited the highest photocatalytic activity, which can be ascribed to the synergistic effect of the N and Co co-doping.     

Comparison of optical and structural properties of nanostructure TiO2 thin film doped by Sn and Nb

The thin films of TiO₂ doped by Sn or Nb were prepared by sol–gel method under process control. The effects of Sn and Nb doping on the structural, optical and photo-catalytic properties of applied thin films have been studied by X-ray diffraction (XRD) high resolution transmission electron microscopy and UV–Vis absorption spectroscopy. Surface chemical state of thin films was examined by atomic X-ray photoelectron spectroscopy. XRD results suggest that adding impurities has a great effect on the crystallinity and particle size of TiO₂. Titania rutile phase formation in thin film was promoted by Sn⁴⁺ addition but was inhibited by Nb⁵⁺ doping. The activity of the photocatalyst was evaluated by photocatalytic degradation kinetics of aqueous methylene blue under UV and Visible radiation. The results show that the photocatalytic activity of the Sn-doped TiO₂ thin film have a larger degradation efficiency than Nb-doped TiO₂ under visible light, but under UV light photocatalytic activity of the Nb-doped TiO₂ thin film is better.     

An ambient temperature aqueous sol–gel processing of efficient nanocrystalline doped TiO2-based photocatalysts for the degradation of organic pollutants

An environmentally-friendly aqueous sol–gel process for producing undoped and Cu²⁺, Ni²⁺, Zn²⁺ or Pb²⁺-doped TiO₂ photocatalysts exhibiting a remarkably high photocatalytic activity without requiring any calcination step has been developed. The physicochemical properties of the catalysts were characterized by ICP-AES, XRD, UV–Vis spectroscopy and nitrogen adsorption–desorption. It has been found that the catalysts are composed of nanocrystallites of anatase with a size of 6–7 nm and a specific surface area varying from 184 to 275 m² g^?1. A screening of the photocatalytic activity of the undoped and doped photocatalysts has been performed by evaluating the degradation of 4-nitrophenol under artificial light (330 nm < λ < 800 nm) after 7 h of illumination using a custom-designed multisample photoreactor. The activity measured for the TiO₂-Undoped catalyst was found to be five times higher than the activity measured for uncalcined TiO₂ catalysts produced by other sol–gel methods. We propose that this interesting result is due to the particular morphology of the xerogels obtained. It has also been demonstrated that the presence of the dopant leads to an enhancement of the photocatalytic activity in all cases. The role of particular dopants in modulating the photocatalytic activity will be discussed. Finally, the possibility of producing undoped and Zn²⁺-doped films presenting a higher activity than the commercial photocatalytic coating (Saint Gobain Glass Bioclean^®) without requiring any calcination step has been demonstrated. These preliminary results constitute an important step forward in the development of photocatalytic films using a sol–gel process compatible with the constraints associated with large-scale industrial processing.     

Comparison of photocatalysis degradation of 4-nitrophenol using N,S co-doped TiO2 nanoparticles synthesized by two different routes

The photocatalytic degradation of 4-nitrophenol as pollutant in aqueous solutions was investigated under visible light irradiation over two different N?CS-codoped anatase TiO₂ catalysts prepared by sol?Cgel methods using titanium isopropoxide and titanium tetrachloride as two different precursors. The catalysts were characterized by XRD, SEM, DRS, EDAX and FT-IR. The effects of various operating parameters including the initial concentration of 4-nitrophenol (2?C14?ppm), solution pH (5?C8) and kinetic reactions were studied. The optimum solution pH was at around 6. For comparison purpose, the photodegradation activity of the commercial Degussa P-25 TiO₂ catalyst has also been studied. The results indicated that photocatalytic activity of N?CS-codoped TiO₂ with titanium isopropoxide as precursor was higher than N?CS-codoped TiO₂ with titanium tetrachloride as precursor and Degussa P-25.     

Sol–gel preparation of pure and doped TiO2 films for the photocatalytic oxidation of ethanol in air

Stable sols of TiO₂ were synthesized by a non-aqueous sol–gel process using titanium (IV) isopropoxide as precursor. The microstructure, optical and morphological properties of the films obtained by spin-coating from the sol, and annealed at different temperatures, were investigated using scanning electron microscopy, transmission electron microscopy, diffuse reflectance spectroscopy and ellipsometry. The crystalline structure of the films was characterized by X-ray diffraction and their photocatalytic activity was evaluated for the oxidation of ethanol in air. The influence of the calcination temperature, pre-heat treatment and the number of layers was studied. Simultaneous thermo-gravimetric and differential thermal analysis measurements were carried out to ascertain the thermal decomposition behavior of the precursors. In order to obtain a higher photoresponse in the visible region, a series of vanadium-, niobium- and tantalum-doped TiO₂ catalysts was synthesized by the same sol–gel method. For V doping two different precursors, a vanadium alkoxide and V₂O₅, were used. The effect on the crystallization and photocatalytic activity of the doped TiO₂ films was investigated. Furthermore, to identify the effective composition of the samples, they were characterized by X-ray photoelectron spectroscopy and the surface area of the powders was measured by N₂ adsorption. The 10 wt.% doped catalysts exhibit high photocatalytic activity under visible light and among them the best performance was obtained for the sample containing Ta as dopant. The crystallite sizes are closely related to the photocatalytic activity.     

Study of the Efficiency of UV and Visible‐Light Photocatalytic Oxidation of Methanol on Mesoporous RuO2–TiO2 Nanocomposites

Mesoporous RuO₂–TiO₂ nanocomposites at different RuO₂ concentrations (0–10 wt %) are prepared through a simple one‐step sol–gel reaction of tetrabutyl orthotitanate with ruthenium(III) acetylacetonate in the presence of an F127 triblock copolymer as structure‐directing agent. The thus‐formed RuO₂–TiO₂ network gels are calcined at 450 °C for 4 h leading to mesoporous RuO₂–TiO₂ nanocomposites. The photocatalytic CH₃OH oxidation to HCHO is chosen as the test reaction to examine the photocatalytic activity of the mesoporous RuO₂–TiO₂ nanocomposites under UV and visible light. The photooxidation of CH₃OH is substantially affected by the loading amount and the degree of dispersion of RuO₂ particles onto the TiO₂, which indicates the exclusive effect of the RuO₂ nanoparticles on this photocatalytic reaction under visible light. The measured photonic efficiency ξ=0.53 % of 0.5 wt % RuO₂–TiO₂ nanocomposite for CH₃OH oxidation is maximal and the further increase of RuO₂ loading up to 10 wt % gradually decreases this value. The cause of the visible‐light photocatalytic behavior is the incorporation of small amounts of Ru⁴⁺ into the anatase lattice. On the other hand, under UV light, undoped TiO₂ shows a very good photonic efficiency, which is more than three times that for commercial photocatalyst, P‐25 (Evonik–Degussa); however, addition of RuO₂ suppresses the photonic efficiency of TiO₂. The proposed reaction mechanism based on the observed behavior of RuO₂–TiO₂ photocatalysts under UV and visible light is explored.     

Photocatalytic Degradation of Glyphosate in Water by N‐Doped SnO2/TiO2 Thin‐Film‐Coated Glass Fibers

Photocatalytic degradation of glyphosate contaminated in water was investigated. The N‐doped SnO₂/TiO₂ films were prepared via sol–gel method, and coated on glass fibers by dipping method. The effects of nitrogen doping on coating morphology, physical properties and glyphosate degradation rates were experimentally determined. Main variable was the concentration of nitrogen doping in range 0–40 mol%. Nitrogen doping results in shifting the absorption wavelengths and narrowing the band gap energy those lead to enhancement of photocatalytic performance. The near optimal 20N/SnO₂/TiO₂ composite thin film exhibited about two‐ and four‐folds of glyphosate degradation rates compared to the undoped SnO₂/TiO₂ and TiO₂ films when photocatalytic treatment were performed under UV and solar irradiations, respectively, due to its narrowest band gap energy (optical absorption wavelength shifting to visible light region) and smallest crystallite size influenced by N‐doping.     

Sol–gel synthesis of Eu<Superscript>3+</Superscript> incorporated CaMoO<Subscript>4</Subscript>: the enhanced luminescence performance

In order to develop high efficiency red emitting phosphors, Ca_0.76MoO₄: Eu _0.24 ³⁺ samples are investigated via sol–gel and solid state reaction. The XRD analysis reveals that both methods can synthesize phosphors with tetragonal CaMoO₄ phase successfully. Smaller crystal size of samples prepared by sol–gel can be observed, and the grain size of these samples are around 100–200 nm, which is also smaller than the one prepared by solid state reaction. All phosphors present a red emission at 616 nm wavelength and one sol–gel synthesized sample exhibits the strongest emission intensity because of the higher quantum efficiency. It is attributed to the removal of lattice defects by sol–gel, in which way luminescent efficiency is enhanced. This can be reflected by the longer fluorescent lifetime of phosphors.     

Li‐doped nanosized TiO2 powder with enhanced photocalatylic acivity under sunlight irradiation

This work reports on the synthesis of Li‐doped TiO₂ nanoparticles using the sol–gel process and solid‐state sintering, and investigates their potential use as a photocatalyst for degradation under sunlight excitation of different organic model compounds in aqueous solution. The structure of the nanocrystals was examined by X‐ray diffraction, UV‐vis ground state diffuse reflectance absorption spectra and X‐ray photoelectron emission spectroscopy. Results showed that samples prepared by sol–gel process and calcined at 400 °C are composed of a mixture of anatase and rutile phases, in contrast to the one prepared by solid‐state sintering, which exhibits an anatase phase with Li being involved in a spinel phase. The photocatalytic degradation of aqueous solutions of different aromatic compounds was successfully achieved under sunlight excitation in presence of Li‐doped TiO₂ prepared via sol–gel process. It was shown that the calcination temperature and the preparation mode greatly affect the photocatalytic efficiency. Copyright © 2010 John Wiley & Sons, Ltd.     

锐钛矿型多孔TiO2薄膜的溶解法制备及性能表征

在具有锐钛矿晶粒的TiO₂溶胶中加入苯丙乳液粒子，使用该混合液浸渍提拉涂膜，然后利用甲苯将薄膜中的苯丙乳液粒子溶解去除，并通过重复涂膜，在室温下获得了具有良好多孔性的锐钛矿型TiO₂薄膜。考察了多孔薄膜的表面形貌、光学性能、吸附性能和光催化性能。结果表明：随薄膜涂膜次数的增加，TiO₂多孔薄膜的吸光度增大，透光率减小，光吸收边波长向长波方向移动。罗丹明B在TiO₂多孔薄膜上的吸附量随涂膜次数的增加先升高，后降低；多次涂膜会在薄膜中产生半封闭的孔洞，经过长时间的毛细渗透等作用能进一步增加薄膜对罗丹明B的吸附。TiO₂多孔薄膜通过吸附+光催化氧化的模式快速分解罗丹明B，其活性主要受到薄膜在光催化反应初期的吸附能力的影响。此外，TiO₂的负载量、光的利用效率、以及光生电荷迁移及其分离等也是影响薄膜光催化活性的因素。     

Synthesis of nanostructured titania/zirconia membrane and investigation of its physical separation and photocatalytic properties in treatment of textile industries wastewater

Nanostructured TiO₂/ZrO₂ composite membranes with varying compositions were obtained by sol–gel technique. The influence of 0–30 mol% zirconia doping on microstructure, water permeability, photocatalytic and physical separation properties, removal of methyl violet of textile industries wastewater and thermal and mechanical stability of titania/zirconia composite membranes was described. Firstly, alumina supports were coated with TiO₂ intermediate layers using the colloidal sol–gel route. The TiO₂/ZrO₂ composite sols were prepared via a polymeric sol–gel method and dip-coated on TiO₂ intermediate layer. The samples were characterized by DLS, TG-DTA, XRD, FTIR, BET-BJH, UV–visible, SEM, TEM and AFM. It was shown that zirconia retards the phase transformation of anatase to rutile until at least 700 °C. The minimum pore size and maximum surface area obtained were 1.2 nm and 153 m²/g, respectively, attributed to the sample with 20 mol% zirconia. The mechanical strength of titania membranes was significantly improved by addition of zirconia. The most methyl violet removal efficiency obtained, with and without UV-irradiation, is 80.8 and 72.6%, respectively, attributed to the sample with 20 mol% zirconia.