Characterization of TiO<Subscript>2</Subscript> powders and thin films prepared by non-aqueous sol–gel techniques

Stable TiO₂ sols were prepared using two non-aqueous sol–gel processes with titanium n-butoxide and titanium isopropoxide, respectively. Crystallization and phase transitions of powders and thin films were studied by ex situ and in situ X-ray diffraction. For both methods, TiO₂ began to crystallize around 320 °C in air. Using helium instead of air during heat treatment slowed down the crystallization and substoichiometric powders were formed. TiO₂ thin films were obtained by spin coating. The morphology of the films was evaluated using scanning electron microscopy. The films were homogeneous and transparent in the visible range. The effect of the heating atmosphere and the type of substrate was investigated.     

Correlation between structural and optical properties of TiO<Subscript>2</Subscript>:ZnO thin films prepared by sol–gel method

We have studied structural and optical properties of thin films of TiO₂, doped with 5% ZnO and deposited on glass substrate (by the sol–gel method). Dip-coated thin films have been examined at different annealing temperatures (350–450 °C) and for various layer thicknesses (89–289 nm). Refractive index, porosity and energy band gap were calculated from the measured transmittance spectrum. The values of the index of refraction are in the range of 1.97–2.44, the porosity is in the range of 0.07–0.46 and the energy band gap is in the range of 3.32–3.43. The coefficient of transmission varies from 50 to 90%. In the case of the powder of TiO₂, doped with 5% ZnO, and aged for 3 months in ambient temperature, we have noticed the formation of the anatase phase (tetragonal structure with 20.23 nm grains). However, the undoped TiO₂ exhibits an amorphous phase. After heat treatments of thin films, titanium oxide starts to crystallize at the annealing temperature 350 °C. The obtained structures are anatase and brookite. The calculated grain size, depending on the annealing temperature and the layer thickness, is in the range of 8.61–29.48 nm.     

Preparation,dielectric and ferroelectric properties of (Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>)<Subscript>0.94</Subscript>Ba<Subscript>0.06</Subscript>TiO<Subscript>3</Subscript> thin films by a sol–gel process

(BiFeO₃)_1−x –(BaTiO₃) _x solid solution thin films are grown on Pt/Ti/SiO₂/Si substrates by chemical solution deposition method. Films with x = 0.00, 0.05 and 0.10 were prepared by annealing at 500°C. X-ray diffraction patterns indicate that the pure BiFeO₃ film adopts random orientation while the solid solution films are highly (100) preferentially oriented. Improved electric property at room temperature was observed in the BaTiO₃ incorporated BiFeO₃ films. The remanent polarization of the film with x = 0.0, 0.05 and 0.10 are 76.6, 51.9 and 19.7 μC/cm² respectively under a measuring electric field of 0.94 MV/cm. The BaTiO₃ incorporated BiFeO₃ films show improved fatigue endurance. By the solid solution with BaTiO₃, the leakage current density is reduced effectively.     

Structure and dielectric properties of HfO<Subscript>2</Subscript> films prepared by a sol–gel route

Mono- and multilayer HfO₂ sol–gel thin films have been deposited on silicon wafers by dip-coating technique using a solution based on hafnium ethoxide as precursor. The densification/crystallization process was achieved by classical annealing between 400 and 600 °C for 0.5 h (after drying at 100 °C). Systematic TEM studies were performed to observe the evolution of the thin film structure depending on the annealing temperature. The overall density of the films was determined from RBS spectrometry correlated with cross section (XTEM) thickness measurements. After annealing at 450 °C the films are amorphous with a nanoporous structure showing also some incipient crystallization. After annealing at 550 °C the films are totally crystallized. The HfO₂ grains grow in colonies having the same crystalline orientation with respect to the film plane, including faceted nanopores. During annealing a nanometric SiO₂ layer is formed at the interface with the silicon substrate; the thickness of this layer increases with the annealing temperature. Capacitive measurements allowed determining the value of the dielectric constant as 25 for four layer films, i.e. very close to the value for the bulk material.     

Growth and characterization of (K<Subscript>0.5</Subscript> Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> thin films by a sol–gel method

(K_0.5 Na_0.5)NbO₃ (KNN) perovskite materials have been developed as a promising lead-free piezoelectric material for environmentally benign piezoelectric devices. KNN films with about 320 nm thickness were fabricated on Pt(111)/SiO₂/Si(100) substrates by a sol–gel method from stoichiometric and A-site ion excess precursor solutions. Two different annealing methods were also used to investigate the crystallographic evolution of the films. A layer-by-layer annealing process results in highly (001) oriented KNN from the annealing temperature of 550 °C, while the final annealing method leads to weaker crystalline peaks with a random orientation. The KNN films from the K and Na excess precursor solutions show similar crystallization behavior. However, the ferroelectric hysteresis loops of the films were greatly improved by compensating for an A-site vacancy. In particular, the KNN films from K-excess precursor solutions show better ferroelectric properties compared to the films prepared from Na excess solutions.     

TiO<Subscript>2</Subscript> thin films on soda-lime and borosilicate glass prepared by sol–gel processing: influence of the substrates

TiO₂ films were deposited on soda-lime and borosilicate glass substrates, their optical and microstructural properties were investigated. X-ray diffraction showed significant differences between the sample series. Films deposited on the upper surface of soda-lime glass substrates showed higher indices of refraction than those prepared on the lower surface that had been in contact with the tin bath during float glass production. Results indicate that these differences not only result from different optical properties of the TiO₂ backbone material due to alkali contamination but that also different film porosities can measured by ellipsometric porosimetry.     

Structure and multiferroic properties of Bi<Subscript>5</Subscript>FeTi<Subscript>3</Subscript>O<Subscript>15</Subscript> thin films prepared by the sol–gel method

The Bi₅FeTi₃O₁₅ (BFTO) films of layered structure have been fabricated on Pt/Ti/SiO₂/Si substrates by the sol–gel method. The thermal decomposition behaviors of precursor powder were examined using thermo-gravimetric and differential scanning calorimeters analysis. The optimal heat treatment process for BFTO films were determined to be low-temperature drying at 200 °C for 4 min and high-temperature drying at 350 °C for 5 min followed by annealing at 740 °C for 60 min, which led to the formation of compact films with uniform grains of ~300 nm. The structural, surface topography, ferroelectric and magnetic properties of the films were investigated. The remnant polarization (2P _r) of BFTO thin films under an applied electric field of ~550 kV/cm are determined to be 67.5 μC/cm² . Meanwhile, the weak ferromagnetic properties of the BFTO films were observed at room temperature.     

Structural and optical characterization of Zn doped TiO<Subscript>2</Subscript> nanoparticles prepared by sol–gel method

Undoped and zinc-doped TiO₂ nanoparticles (Ti_1−xZn_xO₂ where x = 0.00–0.10) were synthesized by a sol–gel method. The synthesized products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and UV–VIS spectrometer. XRD pattern confirmed the tetragonal structure of synthesized samples. Average grain size was determined from X-ray line broadening using the Debye–Scherrer relation. The crystallite size was varied from 10 to 40 nm as the calcination temperature was increased from 350 to 800 °C. The incorporation of 3–5 mol% Zn²⁺ in place of the Ti⁴⁺ provoked a slight decrease in the size of nanocrystals as compared to undoped TiO₂. The SEM and TEM micrographs revealed the agglomerated spherical-like morphology with a diameter of about 10–30 nm and length of several nanometers, which is in agreement with XRD results. Optical absorption measurements indicated a blue shift in the absorption band edge upon 3–5 mol% zinc doping. Direct allowed band gap of undoped and Zn-doped TiO₂ nanoparticles measured by UV–VIS spectrometer were 2.95 and 3.00 eV at 550 °C, respectively.     

Electrochromic properties of MoO<Subscript>3</Subscript> thin films derived by a sol–gel process

Molybdenum trioxide (MoO₃) films were deposited on ITO/Glass substrates by the sol–gel method using a spin-coating technique and heat treated at various temperatures under different ambient atmosphere. Effects of the process parameters on the electrochromic properties of MoO₃ films were studied using cyclic voltammetry (CV) in a propylene carbonate (PC) non-aqueous solution containing 1 M lithium perchlorate (LiClO₄). Electrochromic MoO₃ film on lithium intercalation was investigated by in-situ transmittance measurement during the CV process. The MoO₃ films showed reversible recharge ability on Li⁺/e⁻ intercalation/deintercalation. Experimental results revealed that the heat-treatment temperature, the ambient atmosphere, and the thickness will have the string influence on the electrochromic properties of MoO₃ thin films. X-ray diffraction (XRD) results show that the amorphous MoO₃ films can be obtained with the heat-treatment temperature below 300 °C in O₂ ambient atmosphere. The optimum electrochromic MoO₃ film, with a thickness of 130 nm, exhibits a maximum transmittance variation (ΔT%) of 30.9%, an optical density change (ΔOD) of 0.213, an intercalation charge (Q) of 8.47 mC/cm², an insertion coefficient x in Li _x MoO₃ was 0.21 and a coloration efficiency (η) of 25.1 cm²/C between the colored and bleached states at a wavelength (λ) of 550 nm.     

Preparation of Ga<Subscript>2</Subscript>Se<Subscript>3</Subscript> thin films by sol–gel technique

In this study we describe the preparation of Ga₂Se₃ semiconductor compound thin films by sol–gel method for different crystal formation temperatures. The films were characterized by X-ray diffraction analyses (XRD), UV–visible spectrometer, and scanning electron microscope (SEM). The XRD spectrum showed that the formation of Ga₂Se₃ crystals were between 743 and 823 K. The thin film crystals that were formed at 773 K corresponded to the β phase and the preferred crystal structure was monoclinic. The value of band gap from optical absorption spectra for the Ga₂Se₃ thin films was estimated to be about E _g ~ 2.56 eV. The thickness of the one-coat Ga₂Se₃ thin films, which was measured by a Spectroscopic Ellipsometer, was about ~200 nm. The average grain sizes of scattered particles were within the limits between 200 and 500 nm.     

NiTiO<Subscript>3</Subscript> nanoparticles encapsulated with SiO<Subscript>2</Subscript> prepared by sol–gel method

NiTiO₃ (NTO) nanoparticles encapsulated with SiO₂ were prepared by the sol–gel method resulting on core-shell structure. Changes on isoelectric point as a function of silica were evaluated by means of zeta potential. The NTO nanoparticles heat treated at 600°C were characterized by X-ray diffraction, transmission electron microscopy (TEM) and energy dispersive X-ray analysis. TEM observations showed that the mean size of NTO is in the range of 2.5–42.5 nm while the thickness of SiO₂ shell attained 1.5–3.5 nm approximately.     

Photocatalytic degradation of trichloroethylene using N-doped TiO<Subscript>2</Subscript> prepared by a simple sol–gel process

Visible-light-driven N-doped TiO₂ was prepared by a simple sol–gel process using nitric acid not only as the acid catalyst of the sol–gel reaction but also as the source of nitrogen. The photocatalytic performance of the N-doped TiO₂ was investigated by using FTIR spectroscopy to monitor the degradation of trichloroethylene (TCE) during UV and visible irradiation. The photocatalytic degradation of TCE was well-reproduced several times. The activity of Ti–O–N species was supported experimentally. The N-doped TiO₂ was found to be responsive to visible light and was stable during repeated runs and maintained the nitrogen species and its activity for at least four months.     

Bandgap modification of TiO<Subscript>2</Subscript> sol–gel films by Fe and Ni doping

Nickle and iron doped TiO₂ thin films were prepared on glass substrates by sol–gel dip coating process. Indirect and direct optical energy gaps were calculated with the incorporation of different concentrations of both the ions. Indirect bandgap was found to be a strong function of the dopant concentration, whereas direct energy gap has negligible dependence on the nature of dopant and its concentration. Direct energy gap has always been found to retain a value higher than the indirect energy gap. Variation of observed energy gap properties shows a trend similar to that reported on the basis of numerical calculations or the samples obtained by other techniques. Increase in refractive index and corresponding density of the film sample does not support the change in turn over frequency. The influence of crystalline phase change is also ruled out by XRD investigations. It is concluded that red shift of band edge absorption takes place by incorporation of dopant and sol–gel dip coating technique offers an effective low cost route to the production of these coatings.     

Synthesis and microwave dielectric properties of Ca<Subscript>0.6</Subscript>La<Subscript>0.267</Subscript>TiO<Subscript>3</Subscript> nanocrystalline powders by sol–gel method

Ca_0.6La_0.267TiO₃ nanocrystalline powders were successfully synthesized by the sol–gel method using PEG1000 as a dispersant in this study. The sinterability of the powders and the microwave dielectric properties of the ceramics were also investigated. The XRD diffraction result showed that pure Ca_0.6La_0.267TiO₃ powder with orthorhombic perovskite structure could be synthesized at 600 °C for 2 h without any detectable intermediate phase. The average grain size of the as-synthesized powder was as low as 35 nm. Compared with Ca_0.6La_0.267TiO₃ ceramics fabricated by conventional solid-state process, the bulk materials prepared by sintering as-prepared nanopowders performed better in densification and microwave dielectric properties. The ceramics sintered at 1,300 °C exhibited a higher relative density of 98.3% combined with a dielectric constant (ε _r ) of 120.3, a quality factor (Q × f) of 23,550 GHz and a temperature coefficient of resonant frequency (τ _f ) of +220.7 ppm/°C, respectively.     

Preparation and characterization of TiO<Subscript>2</Subscript> sol–gel modified nanocomposite films

In this study, the role of TiO₂ MT-150A loading in the polymeric sol was investigated for the synthesis of immobilized TiO₂ nanocomposite films on glass substrate using the MT-150A nanoparticles-modified sol–gel method. The nanocomposite film properties were examined using different material characterization techniques including X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, UV–Vis spectrophotometer, Scotch tape test and pencil hardness test. The hydrophilicity of films during UV/Vis irradiation and storage in a dark place were evaluated by a contact angle analyzer. The MT-150A loading had a significant effect on the amount of crystallite phases in the films. However, increasing the MT-150A loading in the sol resulted in an increase in rutile phase content. In addition, increasing MT-150A loading in the sol yielded films with higher hydrophilicity but a concentration of 10–30 g/L MT-150A in the sol was found as the maximum for obtaining films with good adherence on the glass substrate.     

Oxalic acid sensors based on sol–gel nanostructured TiO<Subscript>2</Subscript> films

Titanium (IV) oxide semiconducting layers were prepared by means of the templated sol–gel method and deposited on conductive ITO substrates. The films were described by a series of techniques involving X-ray diffraction (XRD), Raman spectroscopy, X-ray reflectivity (XRR), atomic force microscopy (AFM), scanning electron microscopy (SEM) and ultraviolet–visible spectroscopy (UV–Vis). The photo-excitation properties of the films were characterized by electrochemical tests and evaluated from the obtained polarization curves. The generated photocurrents were measured in the presence of the hole-scavengers—oxalic acid and formic acid in the electrolyte. It was shown that especially in the case of oxalic acid the developed system can be used as an efficient and simpler concentration sensor. The relationship between values of the generated photocurrent and the layers’ thicknesses was also investigated.     

Structural and textural features of TiO<Subscript>2</Subscript>/SAPO-34 nanocomposite prepared by the sol–gel method

This paper focuses on the synthesis of nanocomposite materials, TiO₂/SAPO-34, using the sol–gel method, which involves preparing a mixture between as-synthesized or calcined SAPO-34 zeolite and TiO₂ gel under hydrothermal crystallization and then calcining it at 400 °C for the formation of the TiO₂ anatase phase. The structural and textural features of the obtained materials were determined by various physico-chemical techniques such as thermogravimetric analysis, X-ray diffraction, scanning electronic microscopy, nitrogen sorption at 77 K, energy dispersive X-ray analysis and ultraviolet–visible spectrometry. The DRX results showed that calcination at 400 °C of the mixture between the calcined SAPO-34 and TiO₂ gel led to the collapse of the original framework of zeolite, but formed the anatase TiO₂ in a nano-spherical morphology; however, the use of as-synthesized SAPO-34 supports provides a mixture phase between SAPO-34 and TiO₂ anatase after calcination. The photocatalytic properties of the SAPO-34/TiO₂ and TiO₂-type materials were tested for the removal of methylene blue (MB) dye. The MB degradation proved to increase as a function of contact time, catalyst mass and the initial concentration of MB.     

Surface chemical bonding states and ferroelectricity of Ce-doped BiFeO<Subscript>3</Subscript> thin films prepared by sol–gel process

Bi_1−x Ce _x FeO₃ (x = 0, 0.05, 0.1, 0.15 and 0.20) (BCFO) thin films were deposited on Pt/TiN/Si₃N₄/Si substrates by sol–gel technique. Crystal structures, surface chemical compositions and bonding states of BCFO films were investigated by X-ray diffraction and X-ray photoelectron spectroscopy (XPS), respectively. Compared to BiFeO₃ (BFO) counterparts, the fitted XPS narrow-scan spectra of Bi 4f_7/2, Bi 4f_5/2, Fe 2p_3/2, Fe 2p_1/2 and O 1s peaks for Bi_0.8Ce_0.2FeO₃ film shift towards higher binding energy regions by amounts of 0.33, 0.29, 0.43, 0.58 and 0.49 eV, respectively. Dielectric constants and loss tangents of the BCFO (x = 0, 0.1 and 0.2) film capacitors are 159, 131, 116, 0.048, 0.041 and 0.035 at 1 MHz, respectively. Bi_0.8Ce_0.2FeO₃ film has a higher remnant polarization (P _r = 2.04 μC/cm²) than that of the BFO (P _r = 1.08 μC/cm²) at 388 kV/cm. Leakage current density of the Bi_0.8Ce_0.2FeO₃ capacitor is 1.47 × 10⁻⁴ A/cm² at 388 kV/cm, which is about two orders of magnitude lower than that of the BFO counterpart. Furthermore, Ce cations are feasibly substituted for Bi³⁺ in the Bi_0.8Ce_0.2FeO₃ matrix, possibly resulting in the enhanced ferroelectric properties for the decreased grain sizes and the reduced oxygen vacancies.     

Bright blue pigment CoAl<Subscript>2</Subscript>O<Subscript>4</Subscript> nanocrystals prepared by modified sol–gel method

Nanocrystalline films of magnetite have been prepared by a novel sol–gel route in which, a solution of iron (III) nitrate dissolved in ethylene glycol was applied on glass substrates by spin coating. Coating solution showed Newtonian behaviour and viscosity was found as 0.0215 Pa.s. Annealing temperature was selected between 291 and 350 °C by DTA analysis in order to obtain magnetite films. In-plane grazing angle XRD and TEM studies showed that magnetite phase was present upon annealing the films at 300 °C. The films had crack free surfaces and their thicknesses varied between ~10 and 200 nm. UV–Vis spectrum results showed that transmittance of the films increases with decreasing annealing temperature and increasing spinning rate. Up to 96% transmittance was observed between the wavelengths of 900–1,100 nm. Vibrating sample magnetometer measurements indicated that magnetite thin films showed ferromagnetic behavior and the saturation magnetization value was found as ~35 emu/cm³.     

Cu-particle-dispersed (K<Subscript>0.5</Subscript>Na<Subscript>0.5</Subscript>)NbO<Subscript>3</Subscript> composite thin films derived from sol–gel processing

Sol–gel processing of Cu-particle-dispersed (K_0.5Na_0.5)NbO₃ (Cu/KNN) thin films was studied in an attempt to develop a method producing piezoelectric composite films with good mechanical performance. The Cu/KNN films were prepared via crystallization annealing at 650–750 °C for 1 min in air, followed by reduction annealing at 400–500 °C for 1–2 h in a 5% H₂ and 95% Ar gas mixture. The resultant composite films consisted of perovskite KNN, metallic Cu, and Cu₄O₃. This suggests that the decomposition of Cu sources takes two different ways in this study. The Cu/KNN composite films containing Cu₄O₃ phases were produced by the crystallization annealing at 700 °C for 1 min followed by the reduction annealing at 500 °C for 1 h. Surface morphology observations reveal that these films have dense KNN matrix with a grain size of ~200 nm and uniformly dispersed Cu or Cu₄O₃ particles with a size of <500 nm.