Effect of thermal annealing on the structure and microstructure of TiO2 thin films

Nanostructured TiO₂ thin films have been prepared through chemical route using sol-gel and spin coating techniques. The deposited films were annealed in the temperature range 400–1000°C for 1 h. The structure and microstructure of the annealed films were characterized by GAXRD, micro-Raman spectroscopy and AFM. The as-deposited TiO₂ thin films are found to be amorphous. Micro-Raman and GAXRD results confirm the presence of the anatase phase and absence of the rutile phase for films annealed up to 700°C. The diffraction pattern of the film annealed at 800 to 1000°C contains peaks of both anatase and rutile reflections. The intensity of all peaks in micro-Raman and GAXRD patterns increased and their width (FWHM) decreased with increasing annealing temperature, demonstrating the improvement in the crystallinity of the annealed films. Phase transformation at higher annealing temperature involves a competition among three events such as: grain growth of anatase phase, conversion of anatase to rutile and grain growth of rutile phase. AFM image of the asdeposited films and annealed films indicated exponential grain growth at higher temperature.      

Study of the structural phase transformation,and optical behavior of the as synthesized ZnO–SnO2–TiO2 nanocomposite

Bulk nanocomposites ZnO–SnO₂–TiO₂ were synthesized by solid-state reaction method. The X-ray diffraction patterns and Raman spectra of bulk nanocomposite as a function of sintering temperature (700 °C–1300 °C) indicate that the structural phases of SnO₂ and TiO₂ depend on the sintering temperature while the ZnO retains its hexagonal wurtzite phase at all sintering temperatures and SnO₂ started to transform into SnO at 900 °C and completely converted into SnO at 1100 °C, whereas the titanium dioxide (TiO₂) exhibits its most stable phase such as rutile at low sintering temperature (≤900°C) and it transforms partially into brookite phase at high sintering temperature (≥ 900 °C). The optical band gap of nanocomposite ZnO–SnO₂–TiO₂ sintered at 700 °C, 900 °C, 1100 °C and 1300 °C for 16 hours is calculated using the transformed diffuse reflectance ultra violet visible near infra red (UV–VisNIR) spectra and has been found to be 3.28, 3.29, 3.31 and 3.32 eV, respectively.     

Enhanced photocatalytic performance of TiO2 particles via effect of anatase–rutile ratio

In the present work anatase–rutile transformation temperature and its effect on physical/chemical properties as well as photocatalytic activity of TiO₂ particles were investigated. The characterisation of the synthesised and annealed TiO₂ particles were determined by X-Ray Powder Diffraction (XRD), scanning electron microscope (SEM), dynamic light scattering (DLS) and Brunauer–Emmett–Teller surface area analysis (BET). The refraction in the ultraviolet–visible (UV–vis) range was assessed using a dual-beam spectrophotometer. The photocatalytic performance of the particles was tested on methylene blue solution. The XRD data indicated that the percentage of rutile increased with the annealing temperature and almost 100% of anatase transformed to rutile at 1000 °C. In addition, the phase transformation was a linear function of annealing temperature so phase composition of TiO₂ can be controlled by changing the annealing temperature. The SEM and BET results presented the increase of agglomerate size and the decrease of specific surface area with the increasing annealing temperature. This proved that anatase has smaller particle size and higher surface area than rutile. The photocatalytic activity of the annealed TiO₂ powders reduced with the increase of annealing temperature. The samples annealed at 900 °C and 925 °C with anatase: rutile ratio of 92:8 and 77:23, respectively, showed the best activity. These results suggested that the photocatalytic activity of TiO₂ particles is a function of phase composition. Thus it can be enhanced by changing its phase composition which can be controlled by annealing temperature.     

Structural study of TiO2 thin films by micro-Raman spectroscopy

The Raman spectroscopy method was used for structural characterization of TiO₂ thin films prepared by atomic layer deposition (ALD) and pulsed laser deposition (PLD) on fused silica and single-crystal silicon and sapphire substrates. Using ALD, anatase thin films were grown on silica and silicon substrates at temperatures 125–425 °C. At higher deposition temperatures, mixed anatase and rutile phases grew on these substrates. Post-growth annealing resulted in anatase-to-rutile phase transitions at 750 °C in the case of pure anatase films. The films that contained chlorine residues and were amorphous in their as-grown stage transformed into anatase phase at 400 °C and retained this phase even after annealing at 900 °C. On single crystal sapphire substrates, phase-pure rutile films were obtained by ALD at 425 °C and higher temperatures without additional annealing. Thin films that predominantly contained brookite phase were grown by PLD on silica substrates using rutile as a starting material.     

Annealing effects on microstructural and optical properties of Nanostructured-TiO2 thin films prepared by sol-gel technique

In this paper we report on the effect of annealing on the microsctructural and optoelectronic properties of titanium dioxide (TiO₂) thin films prepared using sol-gel method onto silicon (Si) (100) and quartz substrates. The annealing temperatures range from 200 to 1000 °C. The Microstructural properties of annealed thin films were investigated by Thermal gravimetric analyses (TGA), X-ray diffraction (XRD) and Raman Spectroscopy. The surface morphology of the film was examined using Atomic Force Microscopy (AFM) method. The optical properties of TiO₂ thin films were characterized using UV-VIS and Spectroscopic ellipsometry. The results have shown that the TiO₂ thin films persist in the anatase phase even after annealing at 800 °C. The phase transformation from anatase to rutile occurred only when the films were annealed at 1000 °C. AFM studies revealed nanocrystalline structure where their shape and density depend strongly on the annealing temperatures. The elaborated nanostructured-TiO₂ thin films present a high transparency in the visible range. Spectroscopic ellipsometry (SE) study was used to determine the effect of annealing temperature on the thickness and on the optical constant of TiO₂ thin films. Spectroscopic ellipsometry and UV-VIS shows that the band gap of TiO₂ thin films was found to decrease when the annealing temperature increases. The Anatase phase was find to show higher photocatalytic activity than the rutile one.     

Phase transformation of nanostructured titanium dioxide thin films grown by sol–gel method

Nanostructured TiO₂ thin films were deposited on quartz glass at room temperature by sol–gel dip coating method. The effects of annealing temperature between 200^∘C to 1100^∘C were investigated on the structural, morphological, and optical properties of these films. The X-ray diffraction results showed that nanostructured TiO₂ thin film annealed at between 200^∘C to 600^∘C was amorphous transformed into the anatase phase at 700^∘C, and further into rutile phase at 1000^∘C. The crystallite size of TiO₂ thin films was increased with increasing annealing temperature. From atomic force microscopy images it was confirmed that the microstructure of annealed thin films changed from column to nubbly. Besides, surface roughness of the thin films increases from 1.82 to 5.20 nm, and at the same time, average grain size as well grows up from about 39 to 313 nm with increase of the annealing temperature. The transmittance of the thin films annealed at 1000 and 1100^∘C was reduced significantly in the wavelength range of about 300–700 nm due to the change of crystallite phase. Refractive index and optical high dielectric constant of the n-TiO₂ thin films were increased with increasing annealing temperature, and the film thickness and the optical band gap of nanostructured TiO₂ thin films were decreased.     

Correlation between microstructure and optical properties of nano-crystalline TiO2 thin films prepared by sol-gel dip coating

Titanium dioxide thin films have been prepared from tetrabutyl-orthotitanate solution and methanol as a solvent by sol-gel dip coating technique. TiO₂ thin films prepared using a sol-gel process have been analyzed for different annealing temperatures. Structural properties in terms of crystal structure were investigated by Raman spectroscopy. The surface morphology and composition of the films were investigated by atomic force microscopy (AFM). The optical transmittance and reflectance spectra of TiO₂ thin films deposited on silicon substrate were also determined. Spectroscopic ellipsometry study was used to determine the annealing temperature effect on the optical properties and the optical gap of the TiO₂ thin films. The results show that the TiO₂ thin films crystallize in anatase phase between 400 and 800 °C, and into the anatase-rutile phase at 1000 °C, and further into the rutile phase at 1200 °C. We have found that the films consist of titanium dioxide nano-crystals. The AFM surface morphology results indicate that the particle size increases from 5 to 41 nm by increasing the annealing temperature. The TiO₂ thin films have high transparency in the visible range. For annealing temperatures between 1000 and 1400 °C, the transmittance of the films was reduced significantly in the wavelength range of 300-800 nm due to the change of crystallite phase and composition in the films. We have demonstrated as well the decrease of the optical band gap with the increase of the annealing temperature.     

Structural transformation study of TiO2 nanoparticles annealing at different temperatures and the photodegradation process of eosin-Y

Hydrothermal method was used to prepare TiO₂ nanoparticles with annealing temperature at 500 °C–700 °C. The mixture of anatase-rutile phase was investigated by powerful tool of X-ray diffraction (XRD). The structural parameters of anatase and rutile mixture phaseTiO₂ nanoparticles were calculated from the Rietveld refinement. The transformation rate of rutile was increased linearly with an annealing temperature of 500 °C–700 °C. The spherical morphology of the anatase and rutile mixed phase were obtained by scanning electron microscope and transmission electron microscope. The spherical particle of the anatase and rutile TiO₂ shows with great aggregation with different size and within the range of few tens nm. The EDAX study revealed the presence of titanium and oxygen. The best photocatalytic activity was identified as the 87.04% of anatase and 12.96% of rutile mixer phase of TiO₂. Various factors could be involved for a better photocatalytic activity.     

Fabrication of coaxial nanowire heterostructures: SiO x nanowires with conformal TiO2 coatings

Silica nanowires, grown via the active oxidation of a silicon substrate, have been coated with TiO₂ using two coating methods: solution-based deposition of Ti-alkoxides and atomic layer deposition. Analysis of as-deposited and annealed films shows that it is possible to produce stable conformal coatings of either the anatase or rutile phases of TiO₂ on nanowires with diameters greater than 100 nm when annealed between 500–600°C and 800–900°C, respectively, with annealing at higher temperatures (1050°C) producing coatings with a highly facetted rutile morphology. The efficacy of the process is shown to depend on nanowire diameter, with nanowires having diameters less than about 100 nm fusing together during solution-based coating and decomposing during TiO₂ atomic layer deposition. The use of a suitable buffer layer is shown to be an effective means of minimizing nanowire decomposition. Finally, annealing coated nanowires under active oxidation conditions (1100°C) is shown to be an effective technique for depositing additional conformal SiO _x coatings, thereby providing a means of fabricating multi-layered coaxial nanostructures.     

Evolution of structural and optical properties of rutile TiO2 thin films synthesized at room temperature by chemical bath deposition method

Nanocrystalline thin films of TiO₂ were prepared on glass substrates from an aqueous solution of TiCl₃ and NH₄OH at room temperature using the simple and cost-effective chemical bath deposition (CBD) method. The influence of deposition time on structural, morphological and optical properties was systematically investigated. TiO₂ transition from a mixed anatase–rutile phase to a pure rutile phase was revealed by low-angle XRD and Raman spectroscopy. Rutile phase formation was confirmed by FTIR spectroscopy. Scanning electron micrographs revealed that the multigrain structure of as-deposited TiO₂ thin films was completely converted into semi-spherical nanoparticles. Optical studies showed that rutile thin films had a high absorption coefficient and a direct bandgap. The optical bandgap decreased slightly (3.29–3.07 eV) with increasing deposition time. The ease of deposition of rutile thin films at low temperature is useful for the fabrication of extremely thin absorber (ETA) solar cells, dye-sensitized solar cells, and gas sensors.     

A Spectroscopic Ellipsometry Study of TiO2 Thin Films Prepared by dc Reactive Magnetron Sputtering： Annealing Temperature Effect

TiO2 thin films are obtained by dc reactive magnetron sputtering. A target of titanium （99.995%） and a mixture of argon and oxygen gases are used to deposit TiO2 films on to silicon wafers （100）. The crystalline structure of deposited and annealed film are deduced by variable-angle spectroscopic ellipsometry （VASE） and supported by x-ray diffractometry. The optical properties of the films are examined by VASE. Measurements of ellipsometry are performed in the spectral range O. 72-3.55 e V at incident angle 75^o. Several SE models, categorized by physical and optical models, are proposed based on the ＇simpler better＇ rule and curve-fits, which are generated and compared to the experimental data using the regression analysis. It has been found that the triple-layer physical model together with the Cody-Lorentz dispersion model offer the most convincing result. The as-deposited films are found to be inhomogeneous and amorphous, whereas the annealed films present the phase transition to anatase and rutile structures. The refractive index of TiO2 thin films increases with annealing temperature. A more detailed analysis further reveals that thickness of the top sub-layer increases, whereas the region of the bottom amorphous sub-layer shrinks when the films are annealed at 300℃.     

Effect of annealing on TiO2 nanoparticles

TiO₂ nanoparticles have been prepared by simple chemical precipitation method and annealed at different temperatures. The as-prepared TiO₂ are amorphous, and they transform into anatase phase on annealing at 450 °C, and rutile phase on annealing at 900 °C. The X-ray diffraction results showed that TiO₂ nanoparticles with grain size in the range of 21–24 nm for anatase phase and 69–74 nm for rutile phase have been obtained. FESEM images show the formation of TiO₂ nanoparticles with small size in structure. The FTIR and Raman spectra exhibited peaks corresponding to the anatase and rutile structure phases of TiO₂. Optical absorption studies reveal that the absorption edge shifts towards longer wavelength (red shift) with increase of annealing temperature.     

An investigation of phase transformation of titania slag using microwave heating

The influences of microwave heating on the phase transformation of titania slag were systematically investigated. The thermal stability, surface chemical functional groups and microstructure of the titania slag before and after microwave heating, at a temperature of 950?°C for 60 min, were also analyzed using thermogravimetry and differential thermal analysis (TG-DSC), Fourier transform infrared spectroscopy (FT-IR) spectrum and scanning electron microscope (SEM), respectively. The TG-DSC analysis revealed that the phase transformation of the titania slag from anatase TiO₂ to rutile TiO₂ occurred between 750 and 1000 °C. The FT-IR rustles demonstrate that the banding form of Ti⁴⁺, Ti³⁺ and Ti²⁺ ions and the methyl groups on the surface of the titania slag has changed and a new chemical bond Ti–OH was formed. The results of SEM showed that a large number of regulation rutile TiO₂ crystals were found on the surface of the microwave-treated samples and the synthetic rutile has been synthesized successfully using microwave heating.     

Effect of Nb doping on morphology,crystal structure,optical band gap energy of TiO2 thin films

Nb–TiO₂ nanofibers and thin films were prepared using a sol–gel derived electrospinning and spin coating, respectively, by varying the Nb/Ti molar ratios from 0 to 0.59 to investigate the effect of Nb doping on morphology, crystal structure, and optical band gap energy of Nb–TiO₂. XRD results indicated that Nb–TiO₂ is composed of anatase and rutile phases as a function of Nb/Ti molar ratio. As the Nb/Ti molar ratio rose, the anatase to rutile phase transformation and the reduction in crystallite size occurred. The band gap energy of Nb–TiO₂ was changed from 3.25 eV to 2.87 eV when the anatase phase was transformed to rutile phase with increasing the Nb doping. Experimental results indicated that the Nb doping was mainly attributed to the morphology, the crystal structure, the optical band gap energy of Nb–TiO₂, and the photocatalytic degradation of methylene blue.     

Annealing temperature effect on the properties of mercury-doped TiO2 films prepared by sol-gel dip-coating technique

This work presents the annealing temperature effect on the properties of mercury (Hg)-doped titanium dioxide (TiO₂). Thin films and polycrystalline powders have been prepared by sol-gel process. The structure, surface morphology and optical properties, as a function of the annealing temperature, have been studied by atomic force microscopy (AFM), Raman, reflectance and ellipsometric spectroscopies. In order to determine the transformation points, we have analyzed the xerogel-obtained powder by differential scanning calorimetry (DSC). Raman spectroscopy shows the crystalline anatase and rutile phases for the films annealed at 400 °C and 1000 °C respectively. The AFM surface morphology results indicate that the particle size increases from 14 to 57 nm by increasing the annealing temperature. The complex index and the optical band gap (E_g) of the films were determined by the spectroscopic ellipsometry analysis. We have found that the optical band gap decreases by increasing the annealing temperature.     

Structure of (Ba0.7Sr0.3)TiO3 films prepared by chemical solution deposition on sapphire substrates

Barium strontium titanate (BST) films on single-crystal sapphire substrates are prepared by chemical solution deposition upon annealing at temperatures T = 700, 850, and 1000°C. The structure of the BST films is investigated using transmission electron microscopy, high-resolution electron microscopy, and x-ray diffraction. It is established that, upon annealing at T = 700 and 850°C, the film crystallizes in the tetragonal phase of the (Ba_0.7Sr_0.3)TiO₃ perovskite without texture and transition layers. The mean grain sizes are 17 and 37 nm, respectively. However, an increase in the annealing temperature to 1000°C brings about a decrease in the mean grain size to 25 nm and the appearance of additional phases due to the interaction at the film-substrate interface.     

Effect of annealing temperature on microstructures and optical properties of Ba<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>TiO<Subscript>3</Subscript> films

Evolution of microstructure and optical property with annealing temperature has been examined for Ba_0.9Sr_0.1TiO₃ films derived from one single precursor solution containing polyethylene glycol polymer. The films sintered below 750°C exhibit a uniform phase structure across the cross-sections and an ordinary optical thin film feature, while the Ba_0.9Sr_0.1TiO₃ films crystallized at 750°C or higher temperature render a lamellar texture consisting of dense and porous Ba_0.9Sr_0.1TiO₃ layers and a good performance as a one-dimensional photonic crystal. The discrepancy in cross-sectional morphology and reflectance property observed in these Ba_0.9Sr_0.1TiO₃ films has been preliminarily explained.     

Effect of annealing treatment on the uniformity of CeO2/TiO2 bilayer resistive switching memory devices

Bilayer CeO₂/TiO₂ films with high-k dielectric property were prepared by rf magnetron sputtering technique at room temperature. Effect of annealing treatment on resistive switching (RS) properties of bilayer CeO₂/TiO₂ films in O₂ ambient at different temperature in the range of 350–550 °C was investigated. Our results revealed that the bilayer films had good interfacial property at 500 °C and this annealing temperature is optimum for different RS characteristics. Results showed that bilayer CeO₂/TiO₂ film perform better uniformity and reliability in resistive switching at intermediate temperature (i.e. 450 °C and 500 °C) instead of low and high annealing temperature (i.e. 350 °C and 550 °C) at which it exhibits poor crystalline structure with more amorphous background. Less Gibbs free energy of TiO₂ as compared to CeO₂ results in an easier re-oxidation of the filament through the oxygen exchange with TaN electrode. However, the excellent endurance property (>2500 cycles), data retentions (10⁵ s) and good cycle-to-cycle uniformity is observed only in 500 °C annealed devices. The plots of cumulative probability, essential memory parameter, show a good distribution of Set/Reset voltage.     

Phase transformations and crystallization kinetics in electrospun TiO2 nanofibers in air and argon atmospheres

The effects of atmospheric air and argon environments on thermal-induced phase transformations in electrospun TiO₂ nanofibers have been investigated in situ using synchrotron radiation diffraction. Diffraction results showed that the as-synthesized TiO₂ nanofibers were initially amorphous, but crystallized to form anatase and rutile after thermal annealing in air or argon at elevated temperatures. The crystallization temperature of anatase was delayed by 100 °C in argon relative to in air, and the transformation of anatase into rutile occurs faster in argon atmosphere than in air due to the formation of oxygen vacancies. Non-linear strains formed in both polymorphs and the substantial elevation of rutile thermal expansion pointed to strain anisotropy in the rutile phase and the concomitant fibre breakage.     

Surface morphology of ZnO buffer layer and its effects on the growth of GaN films on Si substrates by magnetron sputtering

ZnO thin films were first prepared on Si(111) substrates using a radio frequency magnetron sputtering system. Then the as-grown ZnO films were annealed in oxygen ambient at temperatures of 700, 800, 900, and 1000°C , respectively. The morphologies of ZnO films were studied by an atom force microscope (AFM). Subsequently, GaN epilayers about 500 nm thick were deposited on the ZnO buffer layers. The GaN/ZnO films were annealed in NH₃ ambient at 900°C. The microstructure, morphology and optical properties of GaN films were studied by x-ray diffraction (XRD), AFM, scanning electron microscopy (SEM) and photoluminescence (PL). The results are shown, their properties having been investigated particularly as a function of the ZnO layers. For better growth of the GaN films, the optimal annealing temperature of the ZnO buffer layers was 900°C.