Influence of calcination temperature on structural and optical properties of TiO2-SiO2 thin films prepared by sol-gel dip coating

We prepared TiO₂-SiO₂ thin films with various TiO₂/SiO₂ ratios by sol-gel dip coating method and explored the dependence of their structural and optical properties on calcination temperature. The absorption peaks relevant to Si—O, Si—O—Ti and Ti—O bonds appeared in the FTIR spectra. With increasing TiO₂ content, the intensity of Si—O bond peaks decreases and that of Ti—O bond peaks increases. The XRD results show that the temperature of transformation from amorphous to anatase phase is lowered as TiO₂ content increases. The crystallite size of anatase phase in composite thin films increases with increasing TiO₂ content and calcination temperature. At 1000°C, the mixed phase of anatase and rutile appears in the pure TiO₂ thin films. The rutile films are denser than the anatase films. The increase in refractive index of composite thin films with calcination temperature is related to the decreased thickness and increased density as a result of evaporation of water and organic matters below 400°C. On the other hand, it is related to the change in the crystal phase and crystallite size of the films over 400°C.     

Effects of oxygen flow ratios and annealing temperatures on Raman and photoluminescence of titanium oxide thin films deposited by reactive magnetron sputtering

Titanium oxide (TiO_x) thin films were deposited on the Si(100) substrates by direct-current reactive magnetron sputtering at 3-15 % oxygen flow ratios (FO₂% = FO₂/(FO₂ + FAr) × 100%), and then annealed by rapid thermal annealing (RTA) at 350-750 °C for 2 min in air. The phase, bonding and luminescence behaviors of the as-deposited and annealed TiO_x thin films were analyzed by X-ray diffraction (XRD), Raman spectroscopy and photoluminescence (PL) spectroscopy, respectively. The as-deposited TiO_x films were amorphous from XRD and showed weak Raman intensity. In contrast, the distinct crystalline peaks of anatase and rutile phases were detected after RTA at 550-750 °C from both XRD and Raman spectra. A mixture of anatase and rutile phases was obtained by RTA at 3 FO₂% and its amount increased with annealing temperature. Only the anatase phase was detected in the 6-15 FO₂% specimens after RTA. The PL spectra of all post-annealed TiO_x films showed a broad peak in visible light region. The PL peak of TiO_x film at 3 FO₂% at 750 °C annealing can be fitted into two Gaussian peaks at ~ 486 nm (2.55 eV) and ~ 588 nm (2.11 eV) which were attributed to deep-level emissions of oxygen vacancies in the rutile and anatase phases, respectively. The peak around 550 nm was observed at 6-15 FO₂% which is attributed to electron-hole pair recombination from oxygen vacancy state in anatase phase to valence band. The variation of intensity of PL peaks is concerned with the formation of the rutile and anatase phases at different FO₂% and annealing temperatures.     

The effect of the H2 flow rate on the structure and optical properties of TiO2 films deposited by inductively coupled plasma assisted chemical vapor deposition

The optical and photocatalytic properties of TiO₂ are closely related to crystalline structures, such as rutile and anatase. In this paper, TiO₂ films were produced by inductively coupled plasma (ICP) assisted chemical vapor deposition (CVD) without extra heating of the substrate, and the effect of H₂ addition on the structure and optical properties of the films was investigated. After increasing the partial pressure of H₂, the structure of the TiO₂ films changed from anatase to rutile, which usually appears at high temperatures (> 600 °C). The light transmittance decreased with increasing the H₂ flow rate due to the increased surface roughness. The photocatalytic activity of the anatase TiO₂ film was better than that of the rutile TiO₂ film.     

Processing and photocatalytic properties of Cu-grafted TiO2 powder from acid treated BaTiO3

The respective influences of calcination, drying methods, and washing conditions on the morphologies, surface properties, and photocatalytic activities of TiO₂ powders prepared from acid treatments of BaTiO₃ were investigated. Rutile powder was obtained using the treatment under strong acid conditions. It possesses a bundle-like shape and comprises rutile nanorods. After calcination, characteristic voids were observed in the particles. Anatase powder was obtained by adjusting pH values of a BaTiO₃ suspension to 2.5-3. Drying at 110 °C engendered the formation of spheroidal anatase, although freeze-dried anatase particles assembled into a flake-like shape. The freeze-dried samples show lower crystallinity. With grafting Cu ions, rutile exhibited better photocatalytic performance for the decomposition of gaseous 2-propanol (IPA) under visible light, although it did not work effectively for anatase.     

Hydrothermal growth of ferromagnetic Fe-doped TiO2 films

Polycrystalline Fe-doped TiO₂ anatase films were deposited on (001) oriented SrTiO₃ single crystal substrates at temperatures less than 200 °C from acidic aqueous solutions of titanyl sulfate and iron (III) nitrate. Epitaxial anatase TiO₂ films were obtained when the films were annealed in air at 900 °C. Room-temperature ferromagnetic Fe-doped TiO₂ thin films were obtained after repeated deposition and annealing steps. The observed saturation magnetization (>0.28μ_B/Fe) could not be attributed to the presence of secondary phase magnetic iron oxides or iron clusters.     

The preparation of the anatase and rutile forms of Ag-TiO2 and hydrogen production from methanol/water decomposition

This study examined hydrogen production over Ag-TiO₂ photocatalysts containing Ag_xO, a conducting component. X-ray photon spectroscopy (XPS) confirmed that the Ag and Ag₂O components were dominant in the Ag-TiO₂ photocatalysts treated at 500 and 800 °C, respectively. The Ti2p bands in Ag-TiO₂ were shifted to lower binding energies, which were assigned to Ti³⁺, compared to pure TiO₂, and the shift was greater in the rutile structure than in the anatase. The measured full widths at half maximum (FWHM) of the Ag3d and Ti2p peaks were larger in the anatase structure than in the rutile structure in both TiO₂ and Ag-TiO₂. The H₂ production from methanol photodecomposition was greater over the rutile structure than over the anatase structure of TiO₂. Moreover, the amount of hydrogen was enhanced over Ag-TiO₂ compared to pure TiO₂; the production reached 17,124 μmol after 24 h over rutile Ag-TiO₂. After methanol photodecomposition, the amount of Ag component in the Ag-TiO₂ photocatalysts increased, while the Ag₂O component decreased.     

A heterostructured SnO2–TiO2 thin film prepared by Langmuir–Blodgett technique

SnO₂–TiO₂ heterostructure films were prepared through Langmuir–Blodgett (LB) route. LB films of octadecyl amine (ODA)–titanyl oxalate multilayer deposited on Si (100) and decomposed at 600 °C showed rutile and anatase phases of ultrathin TiO₂ film. Subsequently, multilayer LB film of ODA–stannate deposited on the pre deposited TiO₂ film after decomposition at 600 °C resulted in thin SnO₂ films on the TiO₂ thin film. The phase analysis of the SnO₂–TiO₂ film showed cassiterite phase of SnO₂ as well as the rutile/anatase mixture of TiO₂ indicating a SnO₂–TiO₂ heterostructured film. Surface morphology of the pure TiO₂ film and SnO₂–TiO₂ film were analyzed by using AFM. Electrical characterization by AC impedance analysis suggested SnO₂–TiO₂ heterostructure formation. DC current voltage measurement showed increase in photocurrent indicating visible light absorption and efficient charge separation under the sunlight type radiation.     

Solar photocatalytic degradation of 2-sec-butyl-4,6-dinitrophenol (DNBP) using TiO2/SiO2 aerogel composite photocatalysts

Silica aerogels and TiO₂/silica aerogel composite photocatalysts were synthesized by sol–gel technique at ambient pressure using orthosilioate and tetra-n-butyl titanate as precursors, respectively. The prepared composite photocatalysts were characterized by XRD, TEM, BET surface area, FT-IR and UV–vis absorption spectra. The results showed that the TiO₂/silica aerogel composite photocatalysts possess high surface area. The addition of silica aerogels inhibited the grain growth and phase transformation of anatase to rutile during calcination. The TiO₂/silica aerogel composite sample calcined at 500 °C with an optimal silica aerogel content of 7 wt.% afforded the highest photocatalytic activity. The photocatalytic degradation of 2-sec-butyl-4,6-dinitrophenol (DNBP) was investigated by using this novel TiO₂/silica aerogel composite photocatalyst under solar light irradiation. The effects of irradiation time, pH, catalyst concentration, temperature and initial DNBP concentration were examined as operational parameters. The optimal operational parameters were found as follows: pH as solution pH 4.82, 8 g L⁻¹ catalyst concentration, 20 °C, and 240 min irradiation time. The kinetics of DNBP degradation by TiO₂/silica aerogel composite fit well a pseudo-first-order kinetic model. The repeatability of photocatalytic activity was also tested. This study showed the feasible and potential use of TiO₂/silica aerogel composite photocatalysts in degradation of toxic organic contaminants.     

Preparation and photocatalytic activity of TiO2 powders from titanium citrate complex using two-step hydrothermal treatments

White, almost carbon-free TiO₂ powders were prepared from a titanium citrate complex ((NH₄)₄[Ti₂(C₆H₄O₇)₂(O₂)₂]·4H₂O) using a two-step hydrothermal treatment. The product yield, carbon contamination, and crystalline phase of TiO₂ depended on both the temperature and pH value for each treatment. Titanium was precipitated as a solid phase (H₂Ti₂O₅·H₂O) using the first hydrothermal treatment in the basic condition (pH = 12) at temperatures less than 150 °C. Then white rutile or anatase powder was crystallized using the second hydrothermal treatment at 200 °C. By changing the pH condition of the second hydrothermal treatment, rutile and anatase were synthesized selectively. The photocatalytic decomposition activity of obtained rutile powder for gaseous 2-propanol under visible light was increased by Cu-grafting.     

TEM microstructure characterization of nano TiO2 coated on nano ZrO2 powders and their photocatalytic activity

Nano TiO₂ coated commercial nano ZrO₂ powders (20 and 80 wt.%) were synthesized by a sol–gel process. Their microstructure and crystal structure depending on the calcination temperatures were investigated using XRD and HRTEM techniques. In the as-received powders, the nano TiO₂ particles attached to the ZrO₂ particles existed in an amorphous phase. After calcination at 450 and 600 °C, most of the TiO₂ powders were crystallized to an anatase type, whereas at 750 °C, they were changed to a rutile phase.From the comparison of photocatalytic activity, the nano TiO₂ coated ZrO₂ powders calcined at 450 and 600 °C showed excellent efficiency for the removal of methyl orange (MO). However, in the powders calcined at 750 °C, the photocatalytic activity was decreased due to the appearance of a rutile phase.     

Structural and electrical characterization of TiO2 films grown by spray pyrolysis

The sol-gel spray pyrolysis method was used to grow TiO₂ thin films onto silicon wafers at substrate temperatures between 315 and 500 °C using pulsed spray solution feed followed by annealing in the temperature interval from 500 to 800 °C in air. According to FTIR, XRD, and Raman, the anatase/rutile phase transformation temperature was found to depend on the film deposition temperature. Film thickness and refractive index were determined by Ellipsometry, giving the refractive indexes of 2.1-2.3 and 2.2-2.6 for anatase and rutile, respectively. According to AFM, film roughness increases with annealing temperature from 700 to 800 °C from 0.60 to 1.10 nm and from 0.35 to 0.70 nm for the films deposited at 375 and 435 °C, respectively. The effective dielectric constant values were in the range of 36 to 46 for anatase and 53 to 70 for rutile at 10 kHz. The conductivity activation energy for TiO₂ films with anatase and rutile structure was found to be 100 and 60 meV, respectively.     

Surface Modification of Sericite Using TiO2 Powders Prepared by Alkoxide Hydrolysis: Whiteness and SPF Indices of TiO2-Adsorbed Sericite

Titania (TiO₂) powders have been prepared from the 0.025-M titanium isopropoxide/ethanol solution and the 0.5-M distilled water/ethanol solution. The prepared TiO₂ powders showed an anatase phase and a rutile phase after heat treatment at 500°C for 2 h and 1000°C for 2 h, respectively. The heterocoagulation adsorption between TiO₂ powder and sericite surface in water was achieved in the range of pH 3.63.7 (where this pH range shows a maximum Zeta-potential difference for two powders). On the other hand, an anomalous transformation behavior appeared in the TiO₂-adsorbed sericite after heat treatment at 1000°C. The surface modification of sericite through the TiO₂-adsorption improved the whiteness as well as the SPF (Sun Protection Factor) indices.     

Nonlinear optical response of titanium oxide nanostructured thin films

Titanium (IV) oxide thin films prepared by low temperature (95 °C) hydrothermal growth were observed to undergo important structural modifications upon variation of the deposition period, modifications strongly affecting the nonlinear optical (NLO) response of the films. Depending on the growth time, the films were observed to contain anatase or rutile TiO₂. It was found that only anatase TiO₂ exhibits significant nonlinear optical response.     

A low-temperature process to synthesize rutile phase TiO2 and mixed phase TiO2 composites

This works employed K₂Ti₄O₉, a novel Ti source, to prepare TiO₂ powders. By a “low-temperature dissolution-reprecipitation process” (LTDRP), rutile phase TiO₂ was successfully synthesized after reacting at 50 °C for 48 h. The obtained sample showed a specific surface area about 45 m²/g, and excellent activity in photo-destruction of NO_x gas. The coupling of rutile phase TiO₂ with commercial anatase TiO₂ showed significant effect in further enhancing the photocatalytic activity.     

Photocatalytic degradation of dyes and organic contaminants in water using nanocrystalline anatase and rutile TiO2

Nanocrystalline TiO₂ was synthesized by controlled hydrolysis of titanium tetraisopropoxide. The anatase phase was converted to rutile phase by thermal treatment at 1023 K for 11 h. The catalysts were characterized by X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), Fourier-transform infrared absorption spectrophotometry (FT-IR) and N₂ adsorption (BET) at 77 K. This study compare the photocatalytic activity of the anatase and rutile phases of nanocrystalline TiO₂ for the degradation of acetophenone, nitrobenzene, methylene blue and malachite green present in aqueous solutions. The initial rate of degradation was calculated to compare the photocatalytic activity of anatase and rutile nanocrystalline TiO₂ for the degradation of different substances under ultraviolet light irradiation. The higher photocatalytic activity was obtained in anatase phase TiO₂ for the degradation of all substances as compared with rutile phase. It is concluded that the higher photocatalytic activity in anatase TiO₂ is due to parameters like band-gap, number of hydroxyl groups, surface area and porosity of the catalyst.     

Preparation and characterization of Co-doped TiO2 materials for solar light induced current and photocatalytic applications

Different amounts of Co-doped TiO₂ powders and thin films were prepared by following a conventional co-precipitation and sol–gel dip coating technique, respectively. The synthesized powders and thin films were subjected to thermal treatments from 400 to 800 °C and were thoroughly investigated by means of X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive analysis with X-rays, FT-infrared, FT-Raman, diffuse reflectance spectroscopy, ultraviolet–visible spectroscopy, BET surface area, zeta potential, flat band potential measurements, band-gap energy, etc. The photocatalytic ability of the powders was evaluated by methylene blue (MB) degradation studies. The thin films were characterized by photocurrent and ultraviolet–visible (UV–Vis) spectroscopy techniques. The characterization results suggest that the Co-doped TiO₂ powders synthesized in this study consist mainly anatase phase, and possess reasonably high specific surface area, low band gap energy and flat band potentials amenable to water oxidation in photoelectrochemical (PEC) cells. The photocatalytic degradation of MB over Co-doped TiO₂ powders followed the Langmuir–Hinshelwood first order reaction rate relationship. The 0.1 wt.% Co-doped TiO₂ composition provided the higher photocurrent, n-type semi-conducting behavior and higher photocatalytic activity among various Co-doped TiO₂ compositions and pure TiO₂ investigated.     

Preparation and characterization of doped TiO2 nanodandelion

Doped dandelion-like TiO₂ microspheres assembled nanorods were synthesized from rutile powders using either urea or thiourea leading to N- or S-doped TiO₂. The rutile particles reacted in concentrated NaOH and urea (or thiourea) solution under hydrothermal conditions (200 °C for 24 h), yielding N- and S-doped TiO₂ nanodandelions with diameters ranging from 0.7 to 1.3 μm. SEM, HRTEM, X-ray diffraction (XRD) and IR spectra were used to characterize the synthesis of powders. The results show that concentrated urea (or thiourea) and NaOH are used as additives that help in the construction of the dandelion-like structures. The fabricated nanostructures exhibit high photocatalytic activity in the photodegradation of aqueous Methylene Blue solution.     

Field emission from the structure of well-aligned TiO2/Ti nanotube arrays

Well-aligned TiO₂/Ti nanotube arrays were synthesized by anodic oxidation of titanium foil in 0.5 wt.% HF in various anodization voltages. The images of filed emission scanning electron microscopy indicate that the nanotubes structure parameters, such as diameter, wall thickness and density, can be controlled by adjusting the anodization voltage. The peaks at 25.3° and 48.0° of X-ray diffraction pattern illuminate that the TiO₂ nanotube arrays annealed at 500 °C are mainly in anatase phase. The filed emission (FE) properties of the samples were investigated. A turn-on electric field 7.8 V/µm, a field enhancement factors approximately 870 and a highest FE current density 3.4 mA/cm² were obtained. The emission current (2.3 mA/cm² at 18.8 V/µm) was quite stable within 480 min. The results show that the FE properties of TiO₂/Ti have much relation to the structure parameters.     

Structural investigation of thin TiO2 films prepared by evaporation and post‐heating

Thin films of TiO₂ have been prepared by reactive evaporation of Ti₂O₃ at substrate temperatures from 150 °C to 350 °C and by post‐heating at 150 °C to 850 °C. The mass density of the films increases with increasing substrate and annealing temperature. The crystalline structure of the film prepared at 350 °C is anatase and becomes rutile upon annealing at 850 °C. All other films are amorphous as‐prepared and become anatase upon annealing above 250 °C. The crystallinity is higher for films prepared at lower temperature and does not increase with annealing temperature. Coatings with reproducible optical properties are obtained when deposited and post‐annealed at 250 °C.     

Fabrication of asymmetric alumina membranes: I. Effect of SrO addition on thermal stabilization of transition aluminas

The effect of SrO addition on the thermal stabilization of transition aluminas with the aim of producing membrane layers (supported and unsupported) has been investigated. Al₂O₃–x wt.% SrO composite powders (x = 1, 3, 5, 8) were synthesized by co-precipitation of the hydroxides from solutions of AlCl₃ and Sr(NO₃)₂ salts using NH₄OH as a precipitating agent. Optimum SrO dopant concentration regarding the transition aluminas stabilization effect was determined to be 5 wt.% based on XRD analysis. STA analysis showed a 30 °C shift versus higher temperatures in the transformation of final transitional alumina (θ-Al₂O₃) to stable alpha phase due to addition of 5 wt.% SrO. The mechanism of transition aluminas thermal stabilization as a result of SrO addition is thoroughly discussed. Unsupported alumina membranes were prepared by drying boehmite sols at 600, 800, 1000 and 1100 °C. The effect of calcination temperature on surface area, pore size distribution of unsupported membranes containing 5 wt.% SrO has been investigated. The microstructure of unsupported and supported membranes revealed quite different. Smaller grains in the supported layers were attributed to the interaction between support and membrane.