Hydrothermal synthesis of efficient TiO<Subscript>2</Subscript>-based photocatalysts

We carried out a thorough study of the micromorphology, physicochemical properties, and photocatalytic activity of nanocrystalline anatase powders formed during the hydrothermal treatment of titanyl sulfate solutions of various compositions. A new procedure is proposed for the hydrothermal synthesis of nanocrystalline anatase having specific surface areas of ∼50 m²/g and photocatalytic activities exceeding the activity of the commercially available Degussa P25 formulation.     

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.     

Characterization of nanocrystalline anatase TiO<Subscript>2</Subscript> thin films

Nanoporous thin films were deposited onto glass substrates by painting with a solution of nanocrystalline anatase TiO(2) particles (with a size of either 6 nm or 16 nm) suspended in an organic solvent. Upon drying in air for about 1 day, the films were tempered at 450 degrees C in air for 1 h. This procedure results in stoichiometric TiO(2) films with a thickness of several micro m and a milky whitish appearance. Scanning force microscopy of the surface revealed that the nanoparticles of the films agglomerated into structures with lateral dimensions of some 100 nm. Transmission electron microscopy was utilized to investigate the structural arrangement of the crystallites in the films. High-resolution electron diffraction and X-ray diffraction analyses demonstrated, furthermore, that the material consists exclusively of a single TiO(2) phase, namely anatase, and that the films do not exhibit any preferential texture. The elemental stoichiometry and the possible presence of impurities were monitored throughout the films by means of secondary-ion mass spectrometry depth profiling. Electrical measurements have been carried out as a function of both the sample temperature T and the ambient oxygen partial pressure p(O(2)). From these data the electrical conductivity sigma of the porous films was determined in dependence of those parameters.     

Charge separation by tetrahexahedron-SrTiO<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> heterojunction as an efficient photocatalyst

Charge separation plays a key role in the conversion of solar energy into chemical energy for use in the redox reaction and as well as in the photocatalytic activity. In this study, SrTiO₃ particles with different morphologies including irregular, tetrahexahedron, and cube were synthesized by an in situ solvothermal method. The photocatalytic activity of the synthesized nanoparticles was investigated in the photocatalytic decomposition of methylene blue under UV light irradiation. Tetrahexahedron SrTiO₃ particles exhibited high decomposition activity (70 %), which is about two times higher than those of the irregular and cubic SrTiO₃ particles. The high decomposition activity of tetrahexahedron SrTiO₃ particles could be attributed to the improvement of charge separation achieved on different crystal facets. To reach a good charge separation, tetrahexahedron SrTiO₃/TiO₂ coupled nanoparticles were fabricated by impregnation method. Results showed that coupling tetrahexahedron SrTiO₃ with TiO₂ could produce efficient charge separation between tetrahexahedron SrTiO₃ and TiO₂ due to their matched band edges. In order to achieve better charge separation, the tetrahexahedron SrTiO₃/90 %TiO₂ sample was calcined at different temperatures in the 450–750 °C range. Tetrahexahedron SrTiO₃/90 %TiO₂ coupled nanoparticles calcined at 650 °C show high photocatalytic activity compared with other samples. The prepared samples were characterized by using various techniques such as X-ray diffraction, scanning electron microscopy, photoluminescence emission spectra, and UV–Vis diffuse reflectance spectroscopy.     

Enhancement of visible-light photocatalytic activity of Cu-doped TiO<Subscript>2</Subscript> nanoparticles

Bare TiO₂ and Cu-doped TiO₂ nanoparticles with different nominal doping amounts of Cu ranging from of 0.5 to 5.0 mol% were synthesized using the modified sol–gel method. The samples were physically characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, energy-dispersive X-ray spectroscopy, Brunauer–Emmett–Teller-specific surface area, UV–Vis diffuse reflectance spectroscopy, zeta potential, X-ray photoelectron spectroscopy, inductively coupled plasma, and photoluminescence techniques. The Cu-doped TiO₂ exhibited good photocatalytic activity in mineralization of oxalic acid and formic acid under visible light irradiation. Photomineralization of oxalic and formic acids under visible light irradiation revealed greatly enhanced photoactivity exhibited by the 2.0 mol% Cu-doped TiO₂ photocatalyst compared to bare TiO₂ . The enhanced photocatalytic performance arises from copper ion doping in the TiO₂ structure, leading to an extended photoresponsive range, enhanced photogenerated charge separation, and transportation efficiency.     

Novel La-doped Bi<Subscript>2</Subscript>WO<Subscript>6</Subscript> photocatalysts with enhanced visible-light photocatalytic activity

Novel La-doped Bi₂WO₆ composites were successfully prepared via a facile solvothermal method and well characterized by X-ray diffraction, Brunner?Emmet?Teller measurements, scanning electron microscopy, transmission electron microscopy/high-resolution, energy dispersive spectrometry, X-ray photoelectron spectroscopy, ultraviolet–visible spectroscopy, and Fourier transform infrared spectroscopy. The photocatalytic activity of modified catalysts was evaluated by degrading tetracycline hydrochloride under visible light (450?W Xe lamp irradiation). It was found 5%La-Bi₂WO₆ had the highest light-absorption ability, great morphology, and microstructures. The La dopant enlarged surface area and increased crystal defects, which may enhance the optical absorption activity and inhibit the recombination of the photo-generated charge carrier, respectively. After 150?min illumination, the photocatalysts that 5%La-Bi₂WO₆ and pure Bi₂WO₆ exhibited the best and worst photocatalytic performance, respectively (96.25% vs. 88.92%).

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Preparation of highly active photocatalyst anatase TiO<Subscript>2</Subscript> by mixed template method

TiO₂ nanocrystals with diameters 8–10 nm have been prepared through sol–gel method using a mixed template of polyethylene glycol (PEG) and cetytrimethylammonium bromide (CTAB) at low temperature. The samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), high-resolution (HR) TEM and fourier transform infrared spectroscopy (FT-IR) etc. XRD analysis showed the TiO₂ photocatalysts prepared with mixed template are pure anatase. FTIR spectrum revealed that the cationic surfactant provides CTA⁺ molecules and bonds to Ti–O to prevent the condensation reaction. PEG plays a dispersant role in controlling the structure of nano-TiO₂ particles. CTAB and PEG incorporated with each other to restrain the growth of crystal nucleus and control the size of grain. The self-assembling process has been confirmed by HRTEM. PEG played different role in mixed template from the single template. The photocatalytic activity of samples was determined by using as a model reaction. The results showed that TiO₂ photocatalysts with mixed template have higher photocatalytic activity than P25.     

Light expanded clay aggregate (LECA) as a support for TiO<Subscript>2</Subscript>, Fe/TiO<Subscript>2</Subscript>, and Cu/TiO<Subscript>2</Subscript> nanocrystalline photocatalysts: a comparative study on the structure,morphology, and activity

In this work, TiO₂ and doped TiO₂ photocatalysts (Fe/TiO₂ and Cu/TiO₂) were synthesized by the sol–gel method. The main objective of this study was to investigate the influence of dopants on the structure, morphology, and activity of the catalysts in powder and immobilized states. XRF, XRD, and SEM methods were used to characterize the catalysts. The structure and phase distribution of the nanocrystalline powders were identified by XRD. Nanoparticles crystallite size and the degree of crystallinity were affected by doping. The anatase contents of catalysts were achieved as follows: TiO₂ (5.89 %) < Fe/TiO₂ (42.17 %) < Cu/TiO₂ (70.28 %). It was indicated that the activity of the catalysts strongly depends on the anatase content. Under the same circumstances, copper-modified TiO₂ exhibited a twofold higher photocatalytic activity compared with TiO₂. The nanostructured catalysts were immobilized on light expanded clay aggregate (LECA) granules in order to investigate the effect of a novel support on the activity of the catalysts. Morphological changes are recognizable in the SEM images. Activity tests indicated that the best catalytic performance was assigned to Cu/TiO₂/LECA. After 120 min of irradiation, 61 % degradation of phenol in synthetic wastewater was achieved. The high photocatalytic activity of Cu/TiO₂/LECA confirms that LECA is as an excellent support.     

Preparation of Nb<Subscript>2</Subscript>O<Subscript>5</Subscript> and N co-doped TiO<Subscript>2</Subscript> photocatalysts and their enhanced photocatalytic activities under visible light

Niobium pentaoxide (Nb₂O₅) and nitrogen co-doped TiO₂ photocatalysts were prepared by mechanical alloying with Nb₂O₅, TiO₂ and urea as raw materials. The pure TiO₂ powders of Degussa P25 were chosen as precursor. Chemical and physical properties of the Nb₂O₅ and N co-doped TiO₂ photocatalysts were investigated, including XRD patterns, XPS spectra, DRS spectra, FT-IR spectra and N₂ adsorption-desorption isotherms. Experiments on photodegradation of methylene blue (MB) and sulfosalicylic acid (SSA) under visible light were carried out to evaluate the photoactivities of the prepared samples, and the chemical oxygen demand (COD) analysis was also conducted as a comparison.     

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.     

An efficient method to prepare high-performance dye-sensitized photoelectrodes using ordered TiO<Subscript>2</Subscript> nanotube arrays and TiO<Subscript>2</Subscript> quantum dot blocking layers

High-performance dye-sensitized photoelectrodes using ordered TiO₂ nanotube arrays (TNTs) and TiO₂ quantum dot blocking layers are fabricated. The free-standing TNT membranes with perfect ordered morphology are prepared by three times of anodic oxidation on Ti foils. These TNT membranes can be easily transported to conductive glasses to fabricate front-side illuminated photoelectrodes. By changing anodic oxidation duration, the thickness of TNT membranes can be controlled, which shows significant influence on the UV-Vis reflectance and absorption abilities of TNT-based photoelectrodes and further influence photovoltaic performance of dye-sensitized solar cells (DSSCs). The highest power conversion efficiency (PCE) of DSSCs about 6.21 % can be obtained by using TNT membranes prepared with anodic oxidation of 3 h. For further improving photovoltaic performance of DSSCs, TiO₂ quantum dot (QDs) blocking layers are inserted between conductive glasses and TNT membranes in the photoelectrodes, which show remarkable effects. The highest PCE of DSSCs with this kind of blocking layers can increase to 8.43 %, producing 35.75 % enhancement compared with that of the counterparts without TiO₂ QD blocking layers.     

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.     

Growth and properties of MnIn<Subscript>2</Subscript>S<Subscript>4</Subscript> single crystals

Homogeneous MnIn₂S₄ single crystals ∼14 mm in diameter and ∼40 mm long were grown by directional solidification of melt. For these MnIn₂S₄ single crystals, the composition was determined by electron probe microanalysis, structure by X-ray diffraction, and melting temperature by differential thermal analysis. Transmission spectra were studied in these single crystals, in the region of the intrinsic absorption edge within 10–300 K. The transmission spectra were used to determine the bandgap width, and it was plotted as a function of temperature. The thermal expansion of MnIn₂S₄ single crystals was studied dilatometrically in the range 80–700 K, and the thermal expansion coefficient was determined.     

Bactericidal activity and UV-filtering property of TiO<Subscript>2</Subscript>-based photocatalysts coated on curtain fabrics

Synthesized and commercial titanium dioxide (TiO₂) were coated onto household curtain fabrics for anti-microbial and ultraviolet (UV) shielding functions. The coating was performed by inducing a deposition of the TiO₂ layer from the Ti precursor onto the fabric surface. A silane adhesive agent was employed to improve the adhesion between the coating and the fabric. Ag nanoparticles were also incorporated into some samples to further improve the anti-bacterial activity, which was evaluated by a standard qualitative test (AATCC 147). Efficiency for UV shielding was evaluated by measuring a UV–visible reflection of the coated fabrics both before and after subjecting it to several washing cycles. The results showed that the TiO₂-coated fabrics had potential as both anti-bacterial and UV shielding for the curtain industry.     

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.     

Controlled growth of ZnO nanorod templates and TiO<Subscript>2</Subscript> nanotube arrays by using porous TiO<Subscript>2</Subscript> film as mask

Silver nanoparticles well dispersed in a spherical Poly(vinylpyrollidone)(PVP) matrix were simply prepared by spray pyrolysis of aqueous solutions of AgNO₃ and PVP without any reducing agent. Highly monodisperse silver particles were obtained above the initial mass ratio of PVP/AgNO₃ ∼ 1 and in a certain narrow temperature range. Below the critical mass ratio the silver particles grew to larger ones polydispersely. As the ratio increased above it, they became smaller maintaining their monodispersity. The use of PVP considerably decreased the reduction temperature of the silver nitrate from 450 °C to 250 °C under the same pyrolysis conditions, due to its reducing nature. As the pyrolysis temperature increased above the decomposition temperature of PVP, the silver particles in the matrix grew to merge to a single particle while their crystallite size did not increase as much. The spherical assemblies of the silver nanoparticles were hardly disengaged even after severe washing off the matrix materials. The mechanism of the nanoparticle growth was also discussed.     

Synthesis and characterization of Co(OH)<Subscript>2</Subscript>/TiO<Subscript>2</Subscript> nanotube composites as supercapacitor materials

A novel composite of Co(OH)₂ and TiO₂ nanotubes was synthesized by a chemical precipitation method. Co(OH)₂/TiO₂ nanotube composites and its microstructure were characterized by transmission electron microscopy (TEM), X-ray diffraction pattern (XRD). The electrochemical capacitance performance of this composite was investigated by cyclic voltammetry and charge–discharge tests with a three-electrode system in 6 M KOH solution. We synthesized different weight ratios of Co(OH)₂/TiO₂ nanotubes, a maximum specific capacitance of 229 F/g was obtained for the composite. Based on these tests, we propose that TiO₂ nanotubes provide the three-dimensional nanotube network structure for the composite and make the Co(OH)₂ dispersed. For these reasons, the TiO₂ nanotubes used as a framework for Co(OH)₂ improve the utilization of Co(OH)₂ greatly.