Photoresponse and donor concentration of plasma-sprayed TiO<Subscript>2</Subscript> and TiO<Subscript>2</Subscript>-ZnO electrodes

The photoelectrochemical characteristics of plasma-sprayed porous TiO₂, TiO₂-5%ZnO, and TiO₂-10%ZnO electrodes in 0.1 N NaOH solution were studied through a three-electrode cell system. The microstructure, morphology, and composition of the electrodes were analyzed using an electron probe surface roughness analyzer (ERA-8800FE), scanning electron microscopy, and x-ray diffraction. The results indicate that the sprayed electrodes have a porous microstructure, which is affected by the plasma spray parameters and composition of the powders. The TiO₂-ZnO electrodes consist of anatase TiO₂, rutile TiO₂, and Zn₂Ti₃O₈ phase. The photoresponse characteristics of the plasma-sprayed electrodes are comparable to those of single-crystal TiO₂, but the breakdown voltage is close to 0.5 V (versus that of a saturated calomel electrode). The short-circuit photocurrent density (J _SC) increases with a decrease of donor concentration, which was calculated according to the Gartner-Butler model. For the lowest donor concentration of a TiO₂-5%ZnO electrode sprayed under an arc current of 600 A, the short-circuit J _SC is approximately 0.4 mA/cm² higher than that of the TiO₂ electrodes under 30 mW/cm² xenon light irradiation. The J _SC increases linearly with light intensity. The original version of this paper was published as part of the DVS Proceedings: “Thermal Spray Solutions: Advances in Technology and Application,” International Thermal Spray Conference, Osaka, Japan, 10–12 May 2004, CD-Rom, DVS-Verlag GmbH, Düsseldorf, Germany.     

Surface characteristics and phase transformation of highly ordered TiO<Subscript>2</Subscript> nanotubes

Highly ordered TiO₂ nanotubes with 85 μm length were fabricated by the electrochemical method for 17 h at 60 V in an ethylene glycol/0.5 % NH₄F/5 % water mixture solution. The nanotube arrangements and surface morphologies of the anodic titania films were clearly dependent on the electrolytes used in the fabrication process. The activation energy for the transformation from an amorphous to anatase structure was evaluated using differential scanning calorimetry (DSC) results and a Kissinger plot. The activation energy for anatase transformation on the anodic titania nanotubular film was estimated to be 208.9 kJ/mol.     

Hetero-Orientation Epitaxial Growth of TiO<Subscript>2</Subscript> Splats on Polycrystalline TiO<Subscript>2</Subscript> Substrate

In the present study, the effect of titania (TiO₂) substrate grain size and orientation on the epitaxial growth of TiO₂ splat was investigated. Interestingly, the splat presented comparable grain size with that of substrate, indicating the hereditary feature of grain size. In addition, hetero- and homo-orientation epitaxial growth was observed at deposition temperatures below 400 °C and above 500 °C, respectively. The preferential growth of high-energy (001) face was also observed at low deposition temperatures (≤?400 °C), which was found to result from dynamic nonequilibrium effect during the thermal spray deposition. Moreover, thermal spray deposition paves the way for a new approach to prepare high-energy (001) facets of TiO₂ crystals.     

Superior performance of high-velocity oxyfuel-sprayed nanostructured TiO<Subscript>2</Subscript> in comparison to air plasma-sprayed conventional Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-13TiO<Subscript>2</Subscript>

Air plasma-sprayed conventional alumina-titania (Al₂O₃-13wt.%TiO₂) coatings have been used for many years in the thermal spray industry for antiwear applications, mainly in the paper, printing, and textile industries. This work proposes an alternative to the traditional air plasma spraying of conventional aluminatitania by high-velocity oxyfuel (HVOF) spraying of nanostructured titania (TiO₂). The microstructure, porosity, hardness (HV 300 g), crack propagation resistance, abrasion behavior (ASTM G65), and wear scar characteristics of these two types of coatings were analyzed and compared. The HVOF-sprayed nanostructured titania coating is nearly pore-free and exhibits higher wear resistance when compared with the air plasma-sprayed conventional alumina-titania coating. The nanozones in the nanostructured coating act as crack arresters, enhancing its toughness. By comparing the wear scar of both coatings (via SEM, stereoscope microscopy, and roughness measurements), it is observed that the wear scar of the HVOF-sprayed nanostructured titania is very smooth, indicating plastic deformation characteristics, whereas the wear scar of the air plasma-sprayed alumina-titania coating is very rough and fractured. This is considered to be an indication of a superior machinability of the nanostructured coating.     

Effects of defects generated in ALD TiO<Subscript>2</Subscript> films on electrical properties and interfacial reaction in TiO<Subscript>2</Subscript>/SiO<Subscript>2</Subscript>/Si system upon annealing in vacuum

Thin TiO₂ layers grown at 130°C on SiO₂-coated Si substrates by atomic layer deposition (ALD) using TTIP and H₂O as precursors were annealed, and the effects of the annealing temperature on the resulting electrical properties of TiO₂ and the interface properties between a Pt electrode and TiO₂ were examined using transmission line model (TLM) structures. The as-deposited TiO₂ thin film had an amorphous structure with OH groups and a high resistivity of 6×10³Ω-cm. Vacuum annealing at 700 °C transformed the amorphous film into an anatase structure and reduced its resistivity to 0.04Ω-cm. In addition, the vacuum-annealing of the TiO₂/SiO₂ structure at 700°C produced free silicon at the TiO₂-SiO₂ interface as a result of the reaction between the Ti interstitials and SiO₂. The SiO₂ formed on the TiO₂ surface caused a Schottky contact, which was characterized by the TLM method. The use of the TLM method enabled the accurate measurement of the resistivity of the vacuum-annealed TiO₂ films and the characterization of the Schottky contacts of the metal electrode to the TiO₂.     

Preparation and properties of a nano TiO<Subscript>2</Subscript>/Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> composite superparamagnetic photocatalyst

Nano TiO₂/Fe₃O₄ composite particles with different molar ratios of TiO₂ to Fe₃O₄ were prepared via sol-gel method. X-ray diffraction, transmission electron microscopy, and vibration sample magnetometry were used to characterize the TiO₂/Fe₃O₄ particles. The photocatalytic activity of the particles was tested by degrading methyl blue solution under UV illumination (254 nm). The results indicate that with the content of TiO₂ increasing, the photocatalytic activity of the composite particles enhances, while the magnetism of the particles decreases. When the molar ratio of TiO₂ to Fe₃O₄ is about 8, both the photocatalytic activity and magnetism of the TiO₂/Fe₃O₄ particles are relatively high, and their photocatalytic activity remains well after repeated use.     

Dielectric properties of Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> thin films deposited onto Ti and TiO<Subscript>2</Subscript> layer

The present study describes the dielectric properties of RF sputtered Ta₂O₅ thin films as a function of the buffer layer and annealing condition. The buffer layers were Ti or TiO₂. And the thin film was annealed in various conditions. The X-ray pattern results showed that the phase of the RF sputtered Ta₂O₅ thin films was amorphous and this state was kept stable to RTA (rapid thermal annealing) even at 700°C. Measurements of the electrical and dielectric properties of the reactive sputtered Ta₂O₅ fabricated in two simple metal insulator semiconductor (MIS) structures, (Cu/Ta₂O₅/Ti/Si/Cu and Cu/Ta₂O₅/TiO₂/Si/Cu) indicated that the amorphous Ta₂O₅ grown on Ti possesses a high dielectric constant (30–70) and high leakage current (10⁻¹–10⁻⁴ A/cm²), whereas a relatively low dielectric constant (−10) and low leakage current (−10⁻¹⁰ A/cm²) were observed in the amorphous Ta₂O₅ deposited on the TiO₂ buffer layer. In addition, the leakage current mechanisms of the two amorphous Ta₂O₅ thin films were investigated by plotting the relation of current density (J) vs. applied electric field (E). The Ta₂O₅/Ti film exhibited three dominant conduction mechanism regimes contributed by the Ohmic emission at low electrical field, by the Schottky emission at intermediate field and by the Poole-Frenkel emission at high field. In the case of Ta₂O₅/TiO₂ film, the two conduction mechanisms, the Ohmic and Schottky emissions, governed the leakage current density behavior. The conduction mechanisms at various electric fields applied were related to the diffusion of Ta, Ti and O, followed by the creation of vacancies, in the rapid thermal treated capacitors.     

Production of TiO<Subscript>2</Subscript> from CaTiO<Subscript>3</Subscript> by alkaline roasting method

CaTiO3 was decomposed by alkaline roasting method for the production of TiO2.The process included alkaline roasting, water leaching and acid leaching steps.In the alkaline roasting step, the factors such as roasting temperature and NaOH/CaTiO3 molar ratio were investigated and 99.5% TiO2 could be extracted from CaTiO3.In addition, it is believed that only ion-exchange between Ca2+ and Na+ takes place, while the structure of TiO 32-in CaTiO3 was not destroyed during the roasting process.In the acid leaching ...     

Microstructure and microwave dielectric properties of ZnO-B2O3 added （Ca0.254Li0.19Sm0.14）TiO3 ceramics

The effects of ZnO-B2O3 (ZB2) on the sintering behavior and microwave dielectric properties of (Ca0.254Li0.19Sm0.14)TiO3 ceramics were investigated.The densities of the specimens reached the maximum value by adding 3 wt.% ZB2 and then decreased.The sintering temperature of the specimens was lowered from 1300 to 1100°C without degradation of the microwave dielectric properties.The (Ca0.254Li0.19Sm0.14)TiO3 + 3 wt.% ZB2 sintered at 1100°C for 3 h showed good microwave dielectric properties,εr = 108.2,Qf = 6545 GHz,and τf = 6.5 ppm/°C,respectively,indicating that ZB2 was an effective sintering aid to improve the densification and microwave dielectric properties of (Ca0.254Li0.19Sm0.14)TiO3 ceramics.     

Mechano-thermal treatment of TiO<Subscript>2</Subscript>-Al powder mixture to prepare TiAl/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> composite

In this research, a composite comprising an intermetallic matrix and dispersed Al₂O₃ particles was processed. A mixture of TiO₂ and Al was mechanically activated in the presence of a process control agent and/or without it, in a high-energy planetary ball mill. As a subsequent process, the sample was sintered at various temperatures. The phase composition and morphology of the samples were evaluated by XRD and SEM techniques, respectively. The thermal behavior of the samples milled for 8 h with PVA and/or without it, were also assessed by the DTA technique and compared with one another. The DTA results revealed that addition of PVA shifted the aluminothermic reduction of TiO₂ to higher temperatures; therefore, final composite phases were developed at higher temperatures. The results also showed that addition of PVA during milling caused the final microstructure to coarsen. The XRD pattern of the sample sintered at 700 °C exhibits the existence of TiAl, Ti₃Al, and Al₂O₃ phases. In the sample sintered at 850 °C, the remaining Ti₃Al peak was attenuated and completely disappeared at 1000 °C.     

Synthesis and photocatalytic properties of lanthanum doped anatase TiO<Subscript>2</Subscript> coated Fe<Subscript>3</Subscript>O<Subscript>4</Subscript> composites

A composite photocatalyst (La/TiO<,2>/Fe<,3>O<,4>) with a lanthanum doped TiO<,2> (La/TiO<,2>) shell and a magnetite core was prepared by coating photoactive LafTiO<,2> onto a magnetic Fe<,3>O<,4> core. The morphological, structural, and optical properties of as-prepared samples were charac- terized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and UV-vis absorption spectroscopy. The effect of lanthanum content on the photocatalytie properties was studied, and the result revealed that 0.15 mol% La/TiO<,2>/Fe<,3>O<,4>exhibited the highest photoactiv- ity. The photocatalytic properties of the prepared photocatalyst under UV and visible light were investigated in aqueous solution using methyl orange (MO) as a target pollutant. The results showed that the prepared photocatalyst was activated by visible light and used as an ef- fective catalyst in photooxidation reactions. In addition, the possibility of cyclic usage of the prepared photocatalyst was also confirmed. Moreover, La/TiO<,2> was tightly bound to Fe<,3>O<,4> and could be easily recovered from the medium by a simple magnetic process.     

Chemical observation of anodic TiO<Subscript>2</Subscript> for biomaterial application

Anodic TiO₂ film on Ti a substrate was fabricated at 170 V in sulfuric acid solutions containing phosphoric acid and hydrogen peroxide. The pores in the oxide layer were not uniform in size, shape, or direction of growth, particularly near the interface between the substrate and the oxide layer, compared with those of the surface layer. The formation of an irregular type of pores seemed to be caused by the spark discharge phenomena, which occurred extensively when the anodic voltage was increased. The pore diameter and the cell size increased and the number of cells per unit area decreased over time. From the analysis of chemical states in the XPS spectrum, the incorporated phosphate species were found mostly in the forms of HPO₄, PO₄, and PO₃. In addition, Ti, O, and P were well dispersed in TiO₂ oxide film, as shown in the results of element mapping analyses by an energy dispersive X-ray spectrometer. An SEM observation of osteoblast-likes cells grown on treated TiO₂ oxide film samples for 24h showed clear differences in cell density and spreading pattern. MC3T3 osteoblast-like cells were flat with a polygonal configuration and were attached to the substrate by cellular extensions.     

Controlling the size of nanograins in TiO<Subscript>2</Subscript> nanofibers

The feasibility of controlling the size of nanograins in TiO₂ nanofibers by varying the duration of calcination is demonstrated in this study. This result opens up a new dimension in understanding the physical properties of nanomaterials and their applications. Similar to the sintering behavior commonly observed in bulk ceramics, the nanograins in nanofibers grow in size under a longer calcination time.     

Preparation of TiO<Subscript>2</Subscript> Nanopowders by Plasma Spray and Characterizations

TiO₂ powders with the range of 10-60 nm were prepared successfully by plasma spray in the self-developed plasma spray equipment. The prepared nanopowders were characterized by transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. The results showed that the prepared TiO₂ nanopowders were the mixture of anatase phase and rutile phase, the main phase was anatase. There were O, Ti, and C elements in powders; Ti element still existed in tetravalent. The photocatalytic degradation of methyl orange indicated that all methyl orange (20 mg/L) can be degraded fully when the addition of prepared TiO₂ nanopowders and illumination time were 1 g/L and 150 min, respectively.     

TiO<Subscript>2</Subscript> nanotube structures for enhanced cell and biological functionality

Nanostructures have pronounced effects on biological processes such as growth of cells and their functionality. Advances in biomaterial surface structure and design have resulted in improved tissue engineering. Nanotechnology can be utilized for optimization of titanium implants with a formation of vertically aligned TiO₂ nanotube arrays on the implant surface. The anodic oxidation of the titanium implant surface to form a TiO₂ nanotube array involves electrochemical processes and self assembly. In this paper, the mechanism of nanotube formation, nanotube bio-characteristics, and their emerging role in soft and hard tissue engineering as well as in regenerative medicine will be reviewed, and the beneficial effects of surface nanotubes on cell adhesion, proliferation, and functionality will be discussed in relation to potential orthopedics applications.     

Influence of the structure of TiO<Subscript>2</Subscript>, CeO<Subscript>2</Subscript>, and CeO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> supports on the activity of Ru catalysts in the catalytic wet air oxidation of acetic acid

Ru catalysts, supported on TiO₂, CeO₂, and CeO₂-TiO₂, were prepared by the impregnation method. The effect of the structure of the supports on the activity of Ru catalysts was investigated in the catalytic wet air oxidation (CWAO) of acetic acid under 230°C and 5 MPa in a batch reactor. Physical properties including the surface area, crystalline phase, and surface components of the Ru catalysts were characterized by N₂ adsorption, X-ray powder diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The CeO₂-based Ru catalysts had good activity, and the prepared RuO₂/CeO₂ catalyst showed markedly higher activity than the RuO₂/CeO₂-TiO₂ catalyst. The surface structure, the high amount of chemisorbed oxygen on the catalyst surface, and the suitable pH_pzc value of the supports played an important role in the activity of the Ru catalysts in CWAO of acetic acid.     

Capacitance performance enhancement of TiO<Subscript>2</Subscript> doped with Ni and graphite

Nano-amorphous TiO₂ was prepared by a sol-gel method. The results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) show that the composite electrode material (TiO₂-NiO-C) is made of powder with a grain size of 36.2 nm. Doping of nickel and graphite can increase the electrical conductivity and the specific surface area of nano-amorphous TiO₂. The electrochemical properties of TiO₂-NiO-C, such as self-discharge, leakage current, and cycle life, were studied using cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), and charge-discharge test. With a charge-discharge current density of 500 mA/g, the specific capacity of the TiO₂-NiO-C composite material reaches 12.88 mAh/g. Also, the expense of capacity is only 3.88% after 500 cycles. The electrochemical capacitor with the electrode material of TiO₂-NiO-C shows excellent capacity and cycling performance.     

Phase Relations in the System TiO<Subscript>2</Subscript>-V<Subscript>2</Subscript>O<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> under Oxidizing and Reducing Conditions

The target of this work was to investigate the phase development in the catalyst system consisting of TiO₂ (Anatase) and V₂O₅ (Shcherbinaite) under several gas atmospheres. Thus a set of V₂O₅/TiO₂ specimens was prepared by ball milling and exposed to subsequent annealing in air and feed gas in the temperature range from 400 to 700 °C. The XRD-results showed that the initial phases Anatase and Shcherbinaite remain stable for all atmospheres containing oxygen. In the temperature range above 525 °C the formation of a Rutile solid solution (Rutile-ss) containing VO _x species takes place. However, under reducing conditions (lower oxygen partial pressure) the reduction of V₂O₅ to V₂O₃ was found by X-ray diffraction measurements. There is no miscibility up to 1300 °C followed by the formation of V₂TiO₅ (Berdesinskiite). SEM images underline the reduction by monitoring the change in morphology with respect to the V-containing phases. TiO₂ remains without much alteration. The two phases V₂Ti₇O₁₇ and V₂Ti₃O₉ (Schreyerite) as described in mineralogy have not been observed in these experiments. The knowledge of phase relations helps to find the appropriate processing conditions and to understand the aging phenomena of catalysts.     

Electrochemical behavior of zirconium in molten NaCl-KCl-K<Subscript>2</Subscript>ZrF<Subscript>6</Subscript> system

The electrochemical reduction of Zr4+(complex) ions in NaCl-KCl-K2ZrF6 molten salt on Pt electrode was investigated using cyclic voltammetry and square wave voltammetry at 1023 K.Two cathodic reduction peaks related to Zr4+/Zr2+ and Zr2+/Zr steps were observed in the cyclic voltammograms.The result was also confirmed by square wave voltammetry.The diffusion coefficient of Zr4+(complex) ions at 1023 K in NaCl-KCl-K2ZrF6 melt,measured by cyclic voltammetry,is about 4.22×10-6 cm2/s.The characterization of the deposits obtained by potentiostatic electrolysis at different potentials was investigated by XRD,and the results were well consistent with the electrochemical reduction mechanism of Zr4+(complex) ions.     

Control of microwave dielectric properties in 0.42ZnNb<Subscript>2</Subscript>O<Subscript>6</Subscript>−0.58TiO<Subscript>2</Subscript> ceramics by the quantitative analysis of phase transition

Phase constitutions of ZnNb₂O₆−TiO₂ mixture ceramics were significantly changed according to the sintering temperature. Phase transition procedures and their effect on the microwave dielectric properties of 0.42ZnNb₂O₆−0.58TiO₂ were investigated using X-ray powder diffraction and a network analyzer. The fractions of the phases composing the mixture were calculated by measuring integral intensities of each reflection. The structural transitions in 0.42ZnNb₂O₆−0.58TiO₂ were interpreted as the association of two distinct steps: the columbite and rutile to ixiolite transition present at lower temperatures (900–950°C) and the ixiolite to rutile transition at higher temperatures (1150–1300°C). These transitions caused considerable variation of microwave dielectric properties. Importantly, τ_f was modified to around 0 ppm/°C in two sintering conditions (at 925°C for 2 hr and at 1300°C for 2 hr), by the control of phase constitution.