Coordination of Ti4+ Sites in Nanocrystalline TiO2 Films Used for Photoinduced Electron Conduction: Influence of Nanoparticle Synthesis and Thermal Necking

Nanocrystalline titania (TiO₂) anatase films are widely utilized as substrates for electron conduction in photoelectrochemical devices. In this paper, we subjected the lattice disorder of TiO₂ anatase nanoparticles and the resulting nanocrystalline films to analysis with X-ray absorption fine structure spectroscopy. The TiO₂ nanoparticles were synthesized from dehydration of a titanate and from a conventional sol–gel method. Although both specimens had similar first shell Ti⁴⁺ coordination numbers (CNs) of ca. 5.7, the titanate-derivative TiO₂ was shown to be phase-pure anatase and the sol–gel TiO₂ contained a minute amount of brookite impurity. After nanoparticle necking into films, the former TiO₂ exhibited a negligible decrease in the CN, whereas the latter showed a significant decrease to a value of ca. 4.9. As a result, the titanate-derivative film is more efficient than the sol–gel one in transmitting electrons injected from a photoexcited dye. Significant lattice distortion near the grain boundaries of films are believed to occur during necking of the nanoparticles containing impurities. We have demonstrated that the synthesis of phase-pure nanoparticles is essentially important in fabricating films with a minimal degree of lattice disorder.     

Nanostructure of TiO2 Nano-Coated SiO2 Particles

We characterized SiO₂–TiO₂ nano-hybrid particles, prepared using the sol–gel method, using high-resolution transmission microscopy. A few nanometer-ordered TiO₂ anatase crystallites could be observed on the monodispersed SiO₂ nanoparticle surface. The quantum size effect of the TiO₂ anatase crystallites is attributed to the blue shift of the absorption band. The rough surface of the SiO₂–TiO₂ nano-hybrid particles was derived from the developed growth planes of the TiO₂ anatase crystallites, grown from fully hydrolyzed Ti alkoxide that did not react with acetic acid during the crystallization process at 600°C thermal annealing.     

Microstructural Evolution of Titania Nanocrystallites by a Hydrothermal Treatment: A HRTEM study

Nanocrystalline anatase titanium dioxide films were formed by a hydrothermal treatment at temperatures below 100°C. The microstructural evolution of the crystallites was monitored by high-resolution transmission electron microscopy (HRTEM). HRTEM analysis of the as-grown film revealed the presence of nanocrystalline brookite TiO₂ within an amorphous matrix. After 90 min of the hydrothermal treatment, the TiO₂ nanoparticles remained of brookite form and a small increase in grain size was observed. Intergranular amorphous TiO₂ between the nanoparticles was observed in both as-grown and 90-min-treated films. However, after further 90 min of the treatment, TiO₂ nanoparticles were transformed into anatase phase with a rapid growth in grain size.     

Fabrication and Microstructure Characterization of Continuous Porous TiO2/Al2O3 Composite Membrane

Using a multipass extrusion process, continuous porous Al₂O₃ body (∼41% porosity) was produced and used as a substrate to fabricate continuous porous TiO₂/Al₂O₃ composite membrane. The diameter of the continuous pores of the porous Al₂O₃ body was about 150 μm. The TiO₂ nanopowders dip coated on the continuous pore-surface Al₂O₃ body existed as rutile and anatase phases after calcination at 520°C in air. However, after aging of the fabricated continuous porous TiO₂/Al₂O₃ composite membrane in 20% NaOH at 60°C for 24 h, a large number of TiO₂ fibers frequently observed on the pore surface. The diameter of the TiO₂ fibers was about 150 nm having a high specific surface area. However, after 48-h aging period, the diameter of the TiO₂ fibers increased, which was about 3 μm. Most of the TiO₂ fibers had polycrystalline structure having nanosized rutile and anatase crystals of about 20 nm.     

Solid-State Diffusive Amorphization in TiO2/ZrO2 Bilayers

Thin films of crystalline TiO₂ were deposited on self-assembled organic monolayers from aqueous TiCl₄ solutions at 80°C; partially crystalline ZrO₂ films were deposited on top of the TiO₂ layers from Zr(SO₄)₂ solutions at 70°C. In the absence of a ZrO₂ film, the TiO₂ films had the anatase structure and underwent grain coarsening on annealing at temperatures up to 800°C; in the absence of a TiO₂ film, the ZrO₂ films crystallized to the tetragonal polymorph at 500°C. However, the TiO₂ and ZrO₂ bilayers underwent solid-state diffusive amorphization at 500°C, and ZrTiO₄ crystallization could be observed only at temperatures of 550°C or higher. This result implies that metastable amorphous ZrTiO₄ is energetically favorable compared to two-phase mixtures of crystalline TiO₂ and ZrO₂, but that crystallization of ZrTiO₄ involves a high activation barrier.     

Effects of Thermal Annealing on Formation of Micro Porous Titanium Oxide by the Sol–Gel Method

We investigated the structural and optical properties of microporous titanium oxide (TiO₂) fabricated by the sol–gel method using templates of colloidal crystals with polystyrene spheres when the annealing temperature was changed between 600° and 1000°C. From X-ray diffraction patterns and SEM images, the rutile TiO₂ annealed at a high temperature did not form periodic porous bodies, while the anatase TiO₂ annealed at lower than 800°C formed periodic porous bodies. The porous TiO₂ obtained acts as an air-sphere/TiO₂ photonic crystal with an FCC structure. It is suggested that TiO₂ sol annealed at a lower temperature do not lead to phase transition from the anatase phase to the rutile phase to obtain the air-sphere/TiO₂ photonic crystal by the sol–gel method using templates of colloidal crystals.     

Dopants in Flame Synthesis of Titania

The effect of dopants on the characteristics of titania particles made by oxidation of TiCl₄ in a laminar diffusion flame reactor is presented. Introduction of dopant SiCl₄ inhibits the transformation of anatase to rutile, due to the formation of interstitian solid solution of SiO₂ and TiO₂. Silica decreases the sintering rate of titania and decreases the primary particle size, and, as a result, the specific surface area increases. Intruduction of SnCl₄ enhances the transformation of anatase to rutile, due to the similar crystalline structure of SnO₂ and rutile titania. However, the presence of SnO₂ and rutile titania. However, the presence of SnO₂ does not affect the primary particle size or the specific surface area of titania particles. Introduction of AICI₃ enhances the transformation of anatase to rutile, due to the formation of excess oxygen vacancies as Al₂O₃ and TiO₂ form a substitutional solid solution.     

Preparation of TiO2-Coated Al2O3 Particles by Chemical Vapor Deposition in a Rotary Reactor

A process of coating Al₂O₃ particles with TiO₂ by hydrolysis of Ti(OC₄H₉)₄ using chemical vapor deposition in a rotary reactor has been developed. The process resulted in (1) a coating film of TiO₂ which was compact and uniform with the fraction of TiO₂ being 0.1%–10.0% and (2) an amorphous TiO₂ coating at a low reaction temperature converted to anatase at a reaction temperature higher than 673 K.     

Rapid Formation of Active Mesoporous TiO2 Photocatalysts via Micelle in a Microwave Hydrothermal Process

Rapid formation of active, mesoporous, and crystalline TiO₂ photocatalysts via a novel microwave hydrothermal process is presented. Crystalline anatase mesoporous nanopowders 100–300 nm in size with worm hole-like pore sizes of 3–5 nm were prepared by a modified sol–gel of titanium tetra-isopropoxide, accelerated by a microwave hydrothermal process. The organic surfactant, tetradecylamine, which is used as a self-assembly micelle in the sol–gel and microwave hydrothermal process, enables to harvest crystallized mesoporous anatase nanoparticles with a high-surface area. Mesoporous worm hole-like and crystalline powders with surface areas of 243–622 m²/g are obtained. X-ray diffraction, N₂-adsorption isotherms (Barrett–Joyner–Halenda and Brunauer–Emmet–Teller method), scanning electron microscope, and transmission electron microscope are used to identify the characteristics and morphologies of the powders. It is shown that crystallization by calcination at 400°C/3 h inevitably reduced the surface area, while the microwave hydrothermal process demonstrated a rapid formation of crystalline mesoporous TiO₂ nanopowders with a high-surface area and excellent photocatalytic effects.     

Formation of Titania Submicrometer Patterns by the Combination of Synthesis from an Aqueous Solution and Transcription of a Resist Pattern

A CrN–coated silicon wafer printed with a resist pattern was used as a substrate, and a novel method for forming titania (TiO₂) patterns was investigated. A TiO₂ thin film formed over the whole surface of the substrate by soaking in a mixed solution of (NH₄)₂TiF₆ and B₂O₃ at room temperature. The resist material then was dissolved off, using acetone, together with the TiO₂ formed on it. A minute pattern of the TiO₂ thin film, transcribing the resist pattern, formed. The minimum line width of the TiO₂ thin film was 300 nm, which was also the minimum line width of the resist pattern.     

Direct Formation of Zirconia-Doped Titania with Stable Anatase-Type Structure by Thermal Hydrolysis

Nanoparticles of anatase-type titania (TiO₂) doped with zirconia (ZrO₂) were directly synthesized from acidic precursor solutions of TiOSO₄ and Zr(SO₄)₂ by simultaneous hydrolysis, under mild hydrothermal conditions, at 200° and 240°C. Doping ZrO₂ into TiO₂ suppressed the crystal growth of anatase and shifted the phase transformation from anatase-type to rutile-type structure to a high temperature. The presence of an anatase-type structure with high crystallinity and high phase stability, even after annealing at 1000°C for 1 h, was fully achieved by both the doping of ZrO₂ into TiO₂ through direct precipitation and the simultaneous hydrolysis of the sulfate solutions.     

Antibacterial Activity of Photocatalytic Titanium Dioxide Thin Films with Photodeposited Silver on the Surface of Sanitary Ware

The antibacterial activity of photocatalytic titanium dioxide (TiO₂) thin films with photodeposited silver on the surface of sanitary ware was studied. Samples were prepared by coating a TiO₂ sol that was calcined at 880°–980°C and photodeposited with silver ions onto the glazed layer of the sanitary ware. The relationships between the antibacterial activity and the fabrication conditions were investigated by X-ray diffractometry, scanning electron microscopy, and colorimetry. The phase of TiO₂ identified in the thin films was a mixture of anatase and rutile. The amount of rutile phase increased as the calcination temperature increased, and grain growth of the TiO₂ particles was observed. The activity was dependent on the TiO₂ thickness, the calcination temperature, and the amount of silver. These results suggest that the antibacterial activity was strongly affected by the amount of anatase in the thin films.     

Conversion of SiO2 Diatom Frustules to BaTiO3 and SrTiO3

Diatom frustules were used as bio-templates to synthesize functional ceramics via solid–gas displacement reactions. Silica-based frustules were exposed to TiF₄ at 330°C to form TiOF₂, which was later converted to TiO₂ (anatase) by heat treatment in air at 600°C. The TiO₂ frustules were then exposed to molten Ba(OH)₂ or Sr(OH)₂ to form BaTiO₃ or SrTiO₃, respectively. In both cases, near-complete conversion was achieved while retaining the morphology of the original silica frustules. BaTiO₃ and SrTiO₃ frustules exhibit nearly phase pure, nanocrystalline perovskite structure.     

Influence of Dipolar Fields on the Photochemical Reactivity of Thin Titania Films on BaTiO3 Substrates

The photochemical properties of TiO₂ films supported on BaTiO₃ were investigated to test the hypothesis that dipolar fields from a ferroelectric substrate would affect the reactivity of the supported film. Photochemical reaction products were formed on the TiO₂ surface in patterns that correspond to the underlying domain structure of BaTiO₃. As the film thickness increases from 10 to 100 nm, the titania more effectively screens the ferroelectric field, and the pattern of reaction products is obscured. It is concluded that dipolar fields from the ferroelectric substrate influence charge carrier transport in the film and spatially localize the reaction products.     

Formation of Monoclinic Hafnium Titanate Thin Films Via the Sol–Gel Method

Hafnium titanate films are generating increasing interest because of their potential application as high- k dielectrics materials for the semiconductor industry. We have investigated sol–gel processing as an alternative route to obtain hafnium titanate thin films. Hafnia-titania films of different compositions have been synthesized using HfCl₄ and TiCl₄ as precursors. The HfO₂–TiO₂ system composition with 50 mol% of TiO₂ and 50 mol% of HfO₂ has allowed the formation of a hafnium titanate film after annealing at 1000°C. The films exhibited a homogeneous nanocrystalline structure and a monoclinic hafnium titanate phase that has never been obtained before in thin films. The films resulted in the formation of homogeneously distributed nanocrystals with an average size of 50 nm. Different compositions, with higher or lower hafnia contents, produced anatase crystalline films after annealing at 1000°C.     

Phase Transformation of Nanocrystalline Anatase Powders Induced by Mechanical Activation

The microstructural evolution of nanocrystalline anatase TiO₂ during high-energy planetary ball milling was studied. The results show that mechanical activation induces the transformations of nanocrystalline TiO₂ from anatase to srilankite and rutile at room temperature and under ambient pressure. As the milling time increases, more anatase powder transforms to the srilankite and rutile phase, and the particle size of the powder decreases. There is no indication of the formation of amorphous phase during ball milling.     

Formation and Transformation of TiO2 (Anatase) Solid Solution in the System TiO2—Al2O3

In the system TiO₂—Al₂O₃, TiO₂ (anatase, tetragonal) solid solutions crystallize at low temperatures (with up to ∼ 22 mol% Al₂O₃) from amorphous materials prepared by the simultaneous hydrolysis of titanium and aluminum alkoxides. The lattice parameter a is relatively constant regardless of composition, whereas parameter c decreases linearly with increasing Al₂O₃. At higher temperatures, anatase solid solutions transform into TiO₂ (rutile) with the formation of α-Al₂O₃. Powder characterization is studied. Pure anatase crystallizes at 220° to 360°C, and the anatase-to-rutile phase transformation occurs at 770° to 850°C.     

Electrochemical Properties of Hydroxyapatite Crystal Surfaces on Anatase Photocatalysts

Hydroxyapatite (HAp) was crystallized on anatase titanium dioxide (TiO₂) photocatalytic crystals or their thin films using of a pseudo-body solution method, and electrochemical properties of the HAp-adhered anatase TiO₂ photocatalytic surfaces were discussed. Decomposition rates of methylene blue were faster for the HAp-adhered anatase TiO₂ photocatalysts, although specific surface areas were smaller than those for the commercial anatase TiO₂ ones. Surface potential dispersions on the HAp-adhered anatase TiO₂ thin films before and after an ultraviolet light irradiation were measured by an atomic force microscopy. Changes in the color of leucocrystalviolet mixed in the HAp-adhered anatase TiO₂ photocatalysts or the commercial anatase TiO₂ ones with experimental durations were compared. Movements of electrons from the anatase TiO₂ photocatalytic surfaces to the HAp, crystals and oxidizing reactions on the HAp-adhered anatase TiO₂ photocatalytic surfaces in the dark were discussed on the basis of these experimental results.     

Crystallization and Microstructure Development of Sol–Gel-Derived Titanium Dioxide Thin Films with Single and Multiple Layers

Sol–gel TiO₂ thin films were fabricated at 650°C using sol that was derived from the Ti(O ⁱ Pr)₄-diethanolamine-H₂O- ⁱ PrOH system. Repeated deposition and rapid heating resulted in columnar grains, whereas slow heating—at a rate of 10°–20°C/min—caused equiaxed grains to form. Slow heating caused the film to crystallize as anatase. The anatase-to-rutile ratio of the rapidly heated film increased with increased deposition thickness and repeated depositions. The tendency of the thicker film to crystallize as rutile during rapid heating was attributed to the rapid grain growth of fine anatase grains.     

Structural, Optical, and Photoelectrochemical Properties of Sprayed TiO2 Thin Films: Effect of Precursor Concentration

Thin films of titanium dioxide (TiO₂) have been deposited from methanolic titanyl acetylacetonate precursor solution onto the preheated amorphous and fluorine-doped tin oxide (F:SnO₂)-coated glass substrates at optimized substrate temperature of 470°C by a spray pyrolysis technique. The objective of the study is to investigate the effect of film thickness on the structural, optical and photoelectrochemical (PEC) properties of TiO₂ thin films by varying the precursor concentration from 0.05 M , at the interval of 0.025 M , to 0.125 M . X-ray diffraction shows that the films are polycrystalline with anatase phase having tetragonal crystal structure. Scanning electron microscopy reveals uniform and compact distribution of spherical clusters comprising nanoplatelets of size ∼200–400 nm. The films are nearly stoichiometric as confirmed by energy-dispersive X-ray analysis. Atomic force microscopy study also reveals uniform distribution of grains of size ∼180–370 nm. The films are transparent (with % T >70) and film thickness varies over 162–758 nm range with increase in precursor concentration. The band gap energy varies from 3.31 to 3.36 eV. Thickness of the TiO₂ films is shown to have an influence on the PEC performance and plays an important role in determining the efficiency of the PEC cell. The precursor concentration of 0.1 M is optimized using PEC technique. Incident photon to current conversion efficiency of 71.24% is obtained for 600 nm thick TiO₂ film.