TiO<Subscript>2</Subscript> nanofibers embedding single crystalline TiO<Subscript>2</Subscript> nanowires

Epitaxially grown titanium dioxide (TiO₂) nanofibers embedding single crystalline TiO₂ nanowires (NWs) were successfully fabricated by electropinning poly(vinyl pyrrolidone)/ethanol solutions mixed with hydrothermally synthesized TiO₂ NWs and titanium isopropoxide precursors and subsequently calcinating the electrospun nanofibers. Utilizing scanning electron microscopy (SEM) and transmission electron microscopy (TEM), the morphologies of TiO₂ NWs and nanofibers were investigated. High resolution TEM (HR-TEM) and selected area electron diffraction (SAED) allowed us to indentify the fact that, during the calcination process under the optimized condition, titanium isopropoxide precursors were epitaxially crystallized on the surface of single crystalline TiO₂ NWs. Based on the X-ray diffraction (XRD) experiments, it was also realized that the crystalline structure of hydrothermally synthesized TiO₂ NWs and epitaxially crystallized TiO₂ nanofibers is anatase and that TiO₂ composite nanofibers embedding TiO₂ NWs exhibited a higher crystallinity than the pristine TiO₂ nanofibers. Additionally, ultraviolet visible (UV–Vis) spectra of nanofibers indicated that optical properties of TiO₂ nanofibers can be tuned by introducing the single crystalline TiO₂ NWs.     

Photocatalytic degradation of organic compounds in aqueous systems by Fe and Ho codoped TiO<Subscript>2</Subscript>

Undoped, single-doped, and codoped TiO₂ nanoparticles were prepared by the sol-gel method and characterized with X-ray diffraction (XRD), the Brunauer-Emmett-Teller (BET)-specific surface area (S_BET), UV-Vis absorption spectra (UV-Vis), and X-ray photoelectron spectroscopy (XPS). Their photocatalytic activity was evaluated by methyl orange (MO) degradation in an aqueous suspension under UV or simulated solar light illumination. XRD showed that all samples calcined at 600°C preserved the anatase structure, and doping inhibited the increase of crystallite size. The BET result revealed that doping improved the surface area of TiO₂. UV-Vis indicated that Fe³⁺-doping broadened the absorption profile of TiO₂. XPS demonstrated that doping was advantageous to absorb more surface hydroxyl groups or chemisorbed water molecules. Photocatalytic degradation showed that the photocatalytic activity of TiO₂ codoped with Fe³⁺ and Ho³⁺ ions was markedly improved. This was ascribed to the fact that there was a cooperative action in the two doped elements. Fe³⁺-doping broadens the absorption profile, improves photo utilization of TiO₂, and then generates more electronhole pairs. Ho³⁺-doping restrains the increase in grain size and retards the recombination of photo-generated electrons and holes.     

Microstructure of SiO<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> hybrid electrospun nanofibers and their application in dye degradation

SiO₂/TiO₂ hybrid nanofibers were prepared by electrospinning and applied for photocatalytic degradation of methylene blue (MB). The phase structure, specific surface area, and surface morphologies of the SiO₂/TiO₂ hybrid nanofibers were characterized through thermogravimetry (TG), X-ray diffraction (XRD) analysis, Brunauer–Emmett–Teller (BET) analysis, scanning electron microscopy (SEM), etc. XRD measurements indicated that doping of silica into TiO₂ nanofibers can delay the phase transition from anatase to rutile and decrease the grain size. SEM and BET characterization proved that silica doping can remarkably enhance the porosity of the SiO₂/TiO₂ hybrid nanofibers. The MB adsorption capacity and photocatalytic activity of the SiO₂/TiO₂ hybrid nanofibers were distinguished experimentally. It was found that, although increased silica doping content could enhance the MB adsorption capacity, the intrinsic photocatalytic activity gradually dropped. The SiO₂ (10 %)/TiO₂ composite nanofibers exhibited the highest MB degradation rate, being superior to SiO₂ (20 %)/TiO₂ or pure TiO₂.     

Photocatalytic degradation of dichloroacetyl chloride adsorbed on TiO<Subscript>2</Subscript>

Dichloroacetyl chloride (DCAC) attracted our attention as an intermediate product of the photocatalytic degradation of trichloroethylene (TCE). The adsorption and photocatalytic reaction of DCAC on TiO₂ have been investigated by FTIR spectroscopy. The influence of the surface structure of several TiO₂s on the reaction mechanism was discussed in order to understand the complete degradation mechanism of TCE as well as DCAC. DCAC was transformed into dichloroacetic acid (DCAA) on the relatively hydrophobic TiO₂ surface by the small amount of the water molecules weakly adsorbed on the surface. This DCAA was degraded to phosgene, CO₂, and CO during UV irradiation. For the hydrophilic TiO₂, DCAC was mainly transformed into the dichloroacetate anion. UV irradiation allowed this species to produce chloroform in addition to phosgene, CO₂, and CO. It is suggested that DCAC easily reacts with the Ti–OH group on the hydrophilic TiO₂ and forms the bidentate titanium chelate of dichloroacetate, which efficiently degrades into chloroform.     

Visible-light-driven photocatalytic degradation of microcystin-LR by Bi-doped TiO<Subscript>2</Subscript>

Bi-doped nano-crystalline TiO₂ (Bi–TiO₂) has been synthesized by sonocrystallization at low temperature. The Bi–TiO₂ materials have narrower bandgaps than pristine TiO₂, which endow them with significant visible light absorption. Accordingly, these materials had enhanced photocatalytic activity in the degradation of organic dye pollutants and the cyanotoxin microcystin-LR (MC-LR) under visible irradiation. It was found that degradation of MC-LR is rather efficient. After irradiation with visible light for 12 h the original MC-LR was removed completely, and 78% of the organic carbon was mineralized into CO₂ after irradiation for 20 h. The hydroxyl radical (·OH) is the major active species responsible for the degradation reaction. Identified intermediates primarily originate from attack of ·OH radicals on the double bonds between C4 and C5 (C6 and C7) of Adda and the ethylenic bond of Mdha in MC-LR. Some peptide bonds are also broken with longer irradiation time.     

Photooxidation of Ethylbenzene with TiO<Subscript>2</Subscript> and Metal Coated TiO<Subscript>2</Subscript> and its Kinetics

Summary. Photooxidation of ethylbenzene with oxygen to give ethylbenzene hydroperoxide has been achieved in a stirred photochemical reactor that was cooled by a water system by irradiation with a 400W high-pressure mercury lamp and using TiO₂ powder and metal coated TiO₂. The effects of the amount of copper or silver coated on TiO₂ and of the temperature on the rate of oxidation have been investigated. It is suggested that thermal cleavage of the O–O bond and photochemically generated singlet oxygen should be considered as the initiating step in a radical chain mechanism. An optimum loading of 6% Ag or 4–5% Cu was observed for photooxidation of ethylbenzene.     

Photocatalytic activity of Au/TiO<Subscript>2</Subscript> nanocomposite for azo-dyes degradation

The structural properties of Au/TiO₂ catalyst were studied by X-ray diffraction, UV-visible diffuse reflectance, photoluminescene, scanning transmission and electron microscope, and temperature programmed reduction. The photocatalytic activity of the catalysts was evaluated for the degradation of various azo-dyes such as methylene blue, methyl orange, reactive blue-4, and eosin-B under solar irradiation. It was found that TiO₂ catalyst modified with gold exhibits higher percentage of degradation compared to starting TiO₂. For example, TiO₂ showed 35% of methyl orange degradation whereas gold modified TiO₂ possessed 82%. Effect of different parameters such as pH and dye concentration has been evaluated and the photocatalytic activity was correlated with physico-chemical properties. The dye degradation rate followed first order kinetics.     

Compatibility of TiO<Subscript>2</Subscript> with a CaO-CaCl<Subscript>2</Subscript> melt

Interaction of TiO₂ with a CaO-CaCl₂ melt was studied to determine whether electrolytic production of titanium from oxide raw materials is possible.     

Solvothermal preparation of copper(II) porphyrin-sensitized mesoporous TiO<Subscript>2</Subscript> composites: enhanced photocatalytic activity and stability in degradation of 4-nitrophenol

Four new copper(II) porphyrins CuPp(1, 2, 3, 4) with a different number of peripheral ester groups were synthesized and used to sensitize the mesoporous TiO₂ under solvothermal condition, and accordingly, four mesoporous CuPp(1, 2, 3, 4)/TiO₂ composites were obtained. These composites were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, BET nitrogen adsorption–desorption isotherms (BET), UV–vis diffuse reflectance spectroscopy (UV–vis-DRS), and Fourier-transform infrared spectroscopy (FT-IR). The results showed the crystal structure and morphology of mesoporous TiO₂ were not affected by the porphyrin existence on its surface. The photocatalysis properties of mesoporous TiO₂ and CuPp(1, 2, 3, 4)/TiO₂ have been evaluated by conducting the photocatalytic degradation of 4-nitrophenol (4-NP) under visible-light irradiation, and the result showed their higher photocatalytic activities and the order is: CuPp(4)/TiO₂ > CuPp(3)/TiO₂ > CuPp(2)/TiO₂ > CuPp(1)/TiO₂ ? TiO₂. The probable reasons are their large surface area and different number of peripheral groups in CuPp, which separate electron–hole pairs efficiently. The repetition test of CuPp(1, 2, 3, 4)/TiO₂ composites demonstrated that they still maintained superior photocatalytic activity over six recycles.     

Synthesis and photocatalytic activity of TiO<Subscript>2</Subscript> nanoparticles fluorine-modified with TiF<Subscript>4</Subscript>

Fluorine-modified TiO₂ nan oparticles were synthesized by introducing TiF₄ as a fluorine source either before or after the sufficient hydrolysis and condensation of Ti(OEt)₄. The photocatalytic activity of the fluorine-modified catalysts was found to be greatly affected by the fluorine position in TiO₂ nanoparticles. When TiF₄ and Ti(OEt)₄ hydrolyzed with synchronization, the fluorine tended to be doped in the lattice. The formation of Ti³⁺ defects could result in charge recombination in bulk and bring down the photocatalytic activity. In contrast, if TiF₄ was introduced after the sufficient hydrolysis and condensation of Ti(OEt)₄. Ti−F bonds could exist mainly on the TiO₂ particles surface, which not only prevented the growth of anatase crystals but also facilitated the transfer of organic compounds from solution to catalyst surface by reducing the hydrophilic properties.     

Photochemical degradation of an anionic surfactant by TiO<Subscript>2</Subscript> nanoparticle doped with C,N in aqueous solution

Novel C,N-doped TiO₂ nanoparticles were prepared by a solid phase reaction. The catalyst was characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX). The results showed that crystallite size of synthesized C,N-doped TiO₂ particles were in nanoscale. UV light photocatalytic studies were carried out using sodium naphthalenesulfonate formaldehyde condensate (SNF) as a model pollutant. The effects of initial concentration of surfactant, catalyst amount, pH, addition of oxidant on the reaction rate were ascertained and optimum conditions for maximum degradation was determined. The results indicated that for a solution of 20 mg/L of SNF, almost 98.7% of the substance were removed at pH ~ 4.0 and 0.44 g/L photocatalyst load, with addition of 1 mM K₂S₂O₈ and irradiation time of 90 min. The kinetics of the process was studied, and the photodegradation rate of SNF was found to obey pseudo-first-order kinetics equation represented by the Langmuir–Hinshelwood model.     

Nanosized TiO<Subscript>2</Subscript> particles decorated on SiO<Subscript>2</Subscript> spheres (TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript>): synthesis and photocatalytic activities

Nanosized TiO₂ and nano-anatase TiO₂ decorated on SiO₂ spherical core shells were synthesized by using a sol–gel method. The synthesized pure TiO₂ nano particle and TiO₂ grafted on SiO₂ sphere with various ratios have been characterized for their structure and morphologies by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectrophotometry (FTIR) and transmission electron microscopy (TEM). Their surface areas were measured using the BET method. The photocatalytic activity of all nanocomposites was investigated using methylene blue as a model pollutant. The synthesized TiO₂/SiO₂ particles appeared to be more efficient in the degradation of methylene blue pollutant, as compared to pure TiO₂ particles.     

Stable hydrosols for TiO<Subscript>2</Subscript> coatings

The optimum processing parameters required to synthesize, by hydrolysis of titanium isopropoxide (TIP), highly stable hydrosols composed of nanoparticles of the smallest possible size, are deduced both from data available in literature and from our own experiments. The colloids prepared in these conditions are composed of aggregates of anatase (~90%) and brookite crystallites (5–6 nm). They are suitable for coatings and have long-term stability (more than one year) in terms of polymorphic composition, crystallite and agglomerate size. Stable sols composed solely of anatase crystallites (4 nm) can be prepared by partially complexing the TIP by acetylacetone before hydrolysis. It is not possible to produce porous films with these colloids because they are stabilized by electrostatic repulsion which causes the particles to organize themselves, during the drying step, to form materials with a close packed structure. However, coatings with controlled porosity can be prepared from these stable sols through the post addition of polymers, like PEG or block copolymers.     

Catalytic Properties of TiO<Subscript>2</Subscript>/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> Nanoparticles in Plasma Chemical Treatment

We proposed here a new process coupling dielectric barrier discharge (DBD) plasma with magnetic photocatalytic material nanoparticles for improving yield in DBD degradation of methyl orange (MO). TiO₂ doped Fe₃O₄ (TiO₂/Fe₃O₄) was prepared by the sol-gel method and used as a new type of magnetic photocatalyst in DBD system. It was found that the introduction of TiO₂/Fe₃O₄ in DBD system could effectively make use of the energy generated in DBD process and improve hydroxyl radical contributed by the main surface Fenton reaction, photocatalytic reaction and catalytic decomposition of dissolved ozone. Most part of MO (88%) was degraded during 30 min at peak voltage of 13 kV and TiO₂/Fe₃O₄ load of 100 mg/L, with a rate constant of 0.0731 min^?1 and a degradation yield of 7.23 g/(kW h). The coupled system showed higher degradation efficiency for MO removal.     

Optimized photocatalytic degradation of pefloxacin by TiO<Subscript>2</Subscript>/UV process

The photocatalytic degradation of pefloxacin was studied using modified TiO₂ as a photocatalyst. The effect of various parameters such as the amount of the photocatalyst, the initial concentration of pefloxacin, initial pH value on the process were investigated, and the optimal conditions were determined. The optimal amount of the photocatalyst is 0.3 g/L. The photodegradation rate of pefloxacin decreases with the increase of initial concentration. Alkaline medium is favorable for the photocatalytic degradation process. The primary photo-degradation products were analyzed by HPLC-ESI-MS/MS and thus the process mechanism was discussed.     

Thermal behavior of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> mesoporous gels

Summary Titania-based photocatalytic materials were prepared by sol-gel method using Fe³⁺ and polyethyleneglycol (PEG₆₀₀) as additives. Thermogravimetry (TG), differential thermal analysis (DTA) and evolved gas analysis (EGA) with MS detection were used to elucidate processes that take place during heating of Fe³⁺ containing titania gels. The microstructure development of the Fe₂O₃/TiO₂ gel samples with and without PEG₆₀₀ admixtures was characterized by emanation thermal analysis (ETA) under in situ heating in air. A mathematical model was used for the evaluation of ETA results. Surface area and porosity measurements of the samples dried at 120&deg;C and the samples preheated for 1 h to 300 and 500&deg;C were compared. From the XRD measurements it was confirmed that the crystallization of anatase took place after thermal heating up to 600&deg;C.     

Electrotransport properties of ions in aqueous solutions of H<Subscript>2</Subscript>SeO<Subscript>4</Subscript> and Na<Subscript>2</Subscript>SeO<Subscript>4</Subscript>

Coefficients of self-diffusion, absolute speeds of movement of ions and the activation energy of electrical conductivity are found from the conductance measurements of aqueous solutions of selenic acid and sodium selenate at different concentrations in a temperature range of 288–318 K. Both the Stokes and effective radii of ions and their hydrate numbers at 298 K are calculated. The obtained results are interpreted in the frames of Samoilov theory on positive and negative hydration of ions.     

Application of a dioxo-molybdenum(VI) complex anchored on TiO<Subscript>2</Subscript> for the photochemical oxidative decomposition of 1-chloro-4-ethylbenzene under O<Subscript>2</Subscript>

The catalytic activity of dioxo-molybdenum(VI)-dichloro[4,4′-dicarboxylato-2,2′-bipyridine] covalently anchored through the carboxylate function to the surface of TiO₂ has been tested for the oxidative degradation of 1-chloro-4-ethylbenzene in MeCN solution under argon and UV irradiation (λ = 254 nm). After 4–5 h of photochemical reaction, the Mo complex was reoxidized in the presence of O₂ in the dark, and then the reaction was continued under argon. The reaction proceeds by the intermediate formation of 4′-chloroacetophenone that undergoes further decomposition to chlorobenzene, plus small amounts of oxygen-containing organochlorine compounds, CO₂ and H₂O. Similar results were obtained for the decomposition of 4′-chloroacetophenone under the same conditions, which also gave chlorobenzene as one of the main products. The ratio of [final product]/[Mo complex] increases during the decomposition of 1-chloro-4-ethylbenzene (up to 350–400% for 30–35 h of reaction), which provides evidence of a catalytic process. The probable photochemical reactions are discussed.