Spectroscopic ellipsometry characterization of TiO2 thin films prepared by the sol–gel method

TiO₂ thin films were prepared on SiO₂/Si(100) substrates by the sol–gel process. XRD results indicate that the major phase of TiO₂ thin films is anatase. The surface morphology and cross-section are observed by FE-SEM. The surface of thin films is dense, free of cracks and flat. The average grain size is about 60–100 nm in diameter. The thickness of single layer TiO₂ thin films is about 60 nm, which increases with the concentration of solution. Ellipsometric angles ψ, Δ are investigated by spectroscopic ellipsometry. The optical constant and the thickness of TiO₂ thin films are fitted according to Cauchy dispersion model. The results reveal that the refractive index and the extinction coefficient of TiO₂ thin films in wavelength above 800 nm are about 2.09–2.20 and 0.026, respectively. The influences of processing conditions on the optical constants and thicknesses of TiO₂ thin films are also discussed.     

Preparation, characterization and catalytic properties of Co–Nb2O5–SiO2 catalysts

Co–Nb₂O₅–SiO₂ catalysts were prepared using three different sol–gel procedures: (i) the colloidal sol–gel method using NbCl₅ and SiCl₄ as precursors; (ii) the polymeric sol–gel method using niobium ethoxide and tetraethyl-orthosilicate (TEOS); (iii) an intermediate procedure between the colloidal and polymeric sol–gel method in which the precursors were those utilized in the CSG but dissolved in a mixture of anhydrous ethanol and CCl₄. In all procedures, the elimination of the solvent carried out between 80 and 110°C was followed by a reduction in hydrogen flow (30 ml min⁻¹) at 773 K. Following these procedures, samples containing 10 wt.% Co and 15 wt.% niobium oxide (expressed as Nb₂O₅) were obtained. The characterization of the catalysts was performed using various techniques: N₂ adsorption and desorption curves at 77 K, NH₃- and H₂-chemisorption, TPO, XPS, XRD, and solid state ¹H MAS-NMR. Hydrogenolysis of butane was evaluated. The low reaction rates are assigned to the effect of the metal size, whereas the isobutane selectivity as well as the relatively high stability is due to the acidity of the support.     

Photoresponse and AC impedance characterization of TiO2–SiO2 mixed oxide for photocatalytic water decomposition

TiO₂–SiO₂ mixed oxides were prepared by sol–gel processes with one-stage (mix up fully hydrolyzed titania- and silica-sol), two-stage (with pre-hydrolysis) and modified two-stage synthesis routes. The photoresponse and AC impedance characterization of the derived catalysts are studied and correlated for the first time with the photocatalytic activities in water decomposition under UV illumination. Synergistic effects in terms of photocatalytic activity and electronic properties including band-gap energy, flat band potential and doping density were observed on atomically mixing TiO₂ and SiO₂ by the two-stage synthesis route. Meanwhile, the decline of photocurrent density were found on TiO₂–SiO₂ relative to bare TiO₂, which could be attributed to low quality crystalline structure of the former compared to that of the latter. The superior photocatalytic performance of TiO₂–SiO₂ is ascribed to the higher flat band potential, band-gap energy, and doping density than those of bare TiO₂.     

Local structure and photocatalytic activity of B2O3–SiO2/TiO2 ternary mixed oxides prepared by sol–gel method

The local structure and the photoactivity of B₂O₃–SiO₂/TiO₂ ternary mixed oxides (SiO₂ content was fixed as 30 at.% with respect to TiO₂) was investigated by using XRD, FT-IR, BET, UV-vis spectra, and electron paramagnetic resonance (EPR) measurement. In FT-IR analysis, boron was incorporated into the framework of titania matrix with replacing Ti---O---Si with Si---O---B or Ti---O---B bonds. Also, paramagnetic species such as O⁻ and Ti³⁺ defects were formed by the boron incorporation. In SiO₂/TiO₂ mixed oxides, a blue shift in the light absorption band was observed due to the quantization of band structure. All B₂O₃–SiO₂/TiO₂ samples had pure anatase phase and no rutile phase was formed even though the calcination temperature was over 900 °C. Incorporating boron oxides of more than 10% enlarges the grain size of anatase phase and causes a red shift of the light absorption spectrum. The surface area was monotonically decreased with increasing the content of boron content. As a result, the photoactivity of B₂O₃–SiO₂/TiO₂ ternary mixed oxides was greatly influenced by the content of boron oxide. The highest photoactivity (g moles/min l) was obtained when the boron content was 5% and seven times higher than that of silica/titania binary mixed oxide. In addition, the specific photoactivity (g moles/m² l) was maximum still at 5%. It was concluded that the large reduction of surface area, the change of band structure, and more formation of bulk Ti³⁺ sites are responsible for the deterioration in the photoactivity of B₂O₃–SiO₂/TiO₂ ternary mixed oxides when the content of boron is over 10%, although their crystallinity was enhanced by increasing the calcination temperature with keeping anatase phase.     

Enhanced dielectric tunability properties of Ba(ZrxTi1−x)O3 thin films using seed layers on Pt/Ti/SiO2/Si substrates

The compositionally graded and homogeneous Ba(Zr_xTi_1−x)O₃ (BZT) thin films were fabricated on LaNiO₃ (LNO) buffered Pt/Ti/SiO₂/Si and Pt/Ti/SiO₂/Si substrates by a sol–gel deposition method, respectively. These films crystallized into a single perovskite phase. The BZT thin films deposited on LaNiO₃/Pt/Ti/SiO₂/Si substrates had a highly (1 0 0) preferred orientation and exhibited a preferred (1 1 0) orientation when the thin films were deposited on Pt/Ti/SiO₂/Si substrates. The LNO and Ba(Zr_0.30Ti_0.70) served as seed layer on Pt/Ti/SiO₂/Si substrates and analyze the relationship of seed layer, microstructure and dielectric behavior of the thin films. The compositionally graded thin films from BaTiO₃ to BaZr_0.35Ti_0.65O₃ were fabricated on LNO/Pt/Ti/SiO₂/Si substrates. The tunability behavior of compositionally graded films was analyzed in order to produce optimum effective dielectric properties. The dielectric constant of BaZr_xTi_1−xO₃ compositionally graded thin films showed weak temperature dependence. This kind of thin films has a potential in a fabrication of a temperature stable tunable device.     

Photocatalytic properties of TiO2 sol-gel modified nanocomposite films

TiO₂ nanocomposite films with different concentrations of TiO₂ MT-150A nanoparticles were immobilized on glass substrates using a dip coating process. The crystalline structure and surface chemical state of nanocomposite film properties were examined by X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS), respectively. The specific surface area and morphology of TiO₂ MT-150A nanoparticles were evaluated by the BET method and Field Emission Scanning Electron Microscopy (FE-SEM). The photocatalytic activities of films were evaluated by the methyl orange decoloring rate. XPS measurements showed that the oxygen amount (%) was related to the film composition. The composite film with 10 g/L MT-150A loading yielded the highest amount of surface oxygen (26.82%) and TiO₂ rutile showed the lowest amount of surface oxygen (13.67%) in the form of surface hydroxyl groups. The remaining oxygen was identified as lattice oxygen. In addition, the nanocomposite film with 10 g/L MT-150A loading yielded the highest photocatalytic activity.     

The effect of crystallographic orientation and solution aging on the electrical properties of sol–gel derived Pb(Zr0.45Ti0.55)O3 thin films

Lead zirconate titanate—Pb(Zr_0.45Ti_0.55)O₃ thin films are grown on Pt_1 1 1/Ti/SiO₂/Si_1 0 0 substrates by a sol–gel method with 1 0 0/0 0 1 and 1 1 1 preferred orientations. Film orientation was controlled mainly by the annealing process and temperature. Films with 1 0 0/0 0 1 orientation consist of a uniform microstructure with micron size grains, whereas films with 1 1 1 orientation contain sub-micron grains. The electrical properties were influenced markedly by the microstructure and orientation of the films. The 1 1 1 oriented films exhibit a square-like hysteresis loop with remnant polarization (P_r) reaching 46 μC/cm² under 550 kV/cm, whereas 1 0 0/0 0 1 oriented films have a P_r of 20 μC/cm² with more slim hysteresis curves. Aging of the precursor solutions resulted in films growing with 1 0 0/0 0 1 texture and displaying inferior electrical properties.     

TiO2 optical coating layers for self-cleaning applications

TiO₂ films deposited by various coating techniques were investigated for self-cleaning applications. The optical coating layers of TiO₂ films prepared from a sol–gel precursor were deposited on glass substrates using spin coating, dip coating and screen printing techniques. Effects of film deposition techniques on crystal structure, microstructure, thickness, photocatalytic activity, hydrophilicity and optical properties of the films were investigated using XRD, AFM, SEM, surface profilometer, UV–vis spectrophotometer and contact angle measurement. Dip coating the TiO₂ optical film two and three times resulted in superhydrophilic surfaces. Increasing number of dipping times was found to increase the photocatalytic activity.     

Pressureless sintering and mechanical properties of SiO2–Al2O3–MgO–K2O–TiO2–F (CaO–Na2O) machinable glass–ceramics

To develop a high strength machinable glass–ceramic through pressureless sintering, the glassy compositions were obtained by mixing a mica-based frit and a frit in the SiO₂–CaO–Na₂O system. According to XRD results, the glass compositions mainly crystallized into phlogopite and diopside after sintering. The optimum sintered glass–ceramic with desirable mechanical properties, machinability and sinterability was achieved by addition of 30 wt.% SiO₂–CaO–Na₂O glass powder to 70 wt.% mica glass composition. SEM results confirmed presence of needle-like diopside crystals which played a reinforcement role to the platelet phlogopite and glassy matrix combination. The measurements showed bending strength and fracture toughness enhanced up to 144.6 ± 17.6 MPa and 1.7 ± 0.2 MPa m^1/2, respectively.     

Hydrophilic/hydrophobic conversion of Ni-doped TiO2 thin films on glass substrates

Nickel-doped titanium oxide thin films were prepared on soda–lime–silica glass substrates by using a metal naphthenate. Films prefired at 500 °C for 10 min were finally annealed at 600 °C for 30 min in air. Contact angle measurement was used for analyzing hydrophilic/hydrophobic conversion. NiTiO₃, rutile and anatase peaks were obtained for the film after nickel doping. The film containing nickel showed a shift towards the visible in the absorption threshold.     

Low temperature synthesis of MgxAl2(1−x)Ti(1+x)O5 films by sol–gel processing

Aluminium titanate films thicker than 0.5 μm have been synthesized by sol–gel methods. The films have been deposited via repetitive dip-coating on silicon wafers and their thermal stability has been tested as a function of the annealing time and temperature. The sol–gel approach has allowed the formation of the aluminium titanate phase at temperatures (700 °C) much lower than those necessary for solid-state reactions (1450 °C). Magnesium oxide has been used to improve the thermal stability of the films at high temperatures. The behavior of samples prepared with two different Mg content, i.e. Mg_0.2Al_1.6Ti_0.8O₅ and Mg_0.6Al_0.8Ti_1.6O₅, has been studied. The films have proven to be stable at 1150 °C, for up to 90 h. X-ray photoelectron spectroscopy has shown that after firing at 500 °C the surface chemical composition of the films is in accordance with the nominal one, whilst at higher annealing temperatures some differences, attributed to diffusion effects, have been observed.     

Effect of ammonium polyacrylate on rheology of anatase TiO2 nanoparticles dispersed in silicon alkoxide sols

Ammonium polyacrylate (NH₄PA) was introduced into powdered mixtures consisting of anatase-structured TiO₂ nanoparticles and silicon alkoxide precursors at the sol level, and the rheological behavior of the mixtures was examined under various solid loadings (φ=0.05–0.13 in volumetric ratios), shear rates (  s⁻¹) and NH₄PA concentrations. The alkoxide precursors were mixtures of tetraethyl orthosilicate (TEOS, Si(OC₂H₅)₄), ethyl alcohol (C₂H₅OH), H₂O and HCl in a constant [H₂O]/[TEOS] ratio of 11. The nanoparticle–sol mixtures generally exhibited a pseudoplastic flow behavior over the shear-rate regime examined. The NH₄PA appeared to serve as an effective surfactant which facilitates the suspension flow by reducing the flow resistance at low NH₄PA concentrations. At φ=0.10, a viscosity reduction ca. 85% was found at  s⁻¹ when the NH₄PA concentration was held at 2.5 wt.% of the solids. As the NH₄PA exceeded a critical level, e.g., [NH₄PA]≥3.0 wt.%, the NH₄PA acted as a catalyst which quickly turned the TiO₂–silica sol mixtures (φ=0.10) into a gelled structure, resulted in a pronounced increase of mixture viscosity. The maximum solids concentration (φ_m) of the mixtures was experimentally determined from a derivative of relative viscosity, i.e., (1−η_r^−1/2)–φ dependence. The estimated φ_m increased from 0.127 to 0.165 when NH₄PA of 0.5 wt.% was introduced into the TiO₂–silica sol mixtures.     

Rich photoluminescence emission of SnO2–SiO2 composite aerogels prepared with a co-fed precursor sol–gel process

Tin oxide–silica composite aerogels were successfully prepared with a co-fed precursor sol–gel process. The crystallinity of the tin oxide nanoparticles, embedded in the mesoporous SiO₂ network, was improved with increasing the post-reaction thermal treatment temperature. The composite aerogels exhibited a rich photoluminescence (PL) emission contributed by both SnO₂ and SiO₂. The PL peak of 346 nm was from the near band edge emission of the tin oxide nanoparticles, and the ones located at 310 and 476 nm were attributable to the oxygen deficiencies of the silica network. Three more emission peaks, 387, 432, and 522 nm, were observed, with the 387 nm peak contributed by the oxygen vacancies V_O⁺⁺, the 432 nm peak by the Sn interstitials, and the 522 nm peak by the oxygen vacancies V_O⁺, respectively, of the tin oxide nanoparticles. The intensities of these three defect level emissions were found decreased, as compared to that of the near band edge emission, with increasing the post-reaction thermal treatment temperature as the tin oxide crystallinity improved.     

Preparation and characterization of SiO2/polydopamine/Ag nanocomposites with long-term antibacterial activity

Silver nanoparticles (Ag NPs) with diameter of approximately 10 nm were prepared by the reduction of silver nitrate using green synthesis, an eco-friendly approach. The synthesized Ag NPs were homogeneously deposited on silicon dioxide (SiO₂) particles modified with dopamine, leading to the formation of SiO₂/polydopamine (PD)/Ag nanocomposites (NCs) with a core–shell–satellite structure investigated by transmission electron microscopy. The Ag content of SiO₂/PD/Ag NCs determined by inductively coupled plasma optical emission spectrometry was approximately 5.92 wt%. The antibacterial properties of both Ag NPs and SiO₂/PD/Ag NCs against Vibrio natriegens (V. natriegens) and Erythrobacter pelagi sp. nov. (E. pelagi) were investigated by bacterial growth curves and inhibition zone. Compared to Ag NPs, the SiO₂/PD/Ag NCs exhibited superior long-term antibacterial activity, attributed to its controlled release of Ag⁺ ions.     

Dependence of the properties of compositionally graded Pb(Zr,Ti)O3 ferroelectric films of the bottom electrode

Compositionally graded Pb(Zr_,Ti)O₃ thin films were prepared on the Pt(1 1 1)/Ti/SiO₂/Si, LNO/Si(1 0 0) and LNO/Pt(1 1 1)/Ti/SiO₂/Si substrates by a modified sol–gel method and rapid heat-treatment. The composition depth profile of a typical up-graded film was determined using a combination of auger electron spectroscopy and Ar-ion etching. The crystallographic orientation and the microstructure of the resulting graded PZT thin films on the different substrates were characterized by XRD. The dielectric and ferroelectric properties of the graded PZT films were discussed. The graded PZT films on LNO/Pt/Ti/SiO₂/Si and LNO/Si(1 0 0) substrates have larger dielectric constant and remnant polarizations compared to that grown on Pt/Ti/SiO₂/Si substrates.     

Novel electro-optic properties of epitaxially grown (Pb, La)(Zr, Ti)O3 (PLZT) thin films derived by advanced sol–gel methods

Advanced sol–gel methods using a secondary solvent addition into (Pb, La)(Zr, Ti)O₃ (PLZT) sol–gel solution and a methanol pre-treatment of sapphire substrates are demonstrated. For the secondary solvent addition, the additive affected the crystallinity and electro-optic (EO) property of PLZT films and only methanol addition can improve them. In addition, the methanol pre-treatment is also appeared to be effective to improve film characteristics.

Through these optimizations, epitaxially grown PLZT thin films on r-cut sapphire are obtained and a high Pockels coefficient which is comparable to those of bulk PLZTs is achieved. It is believed that these PLZT thin films are applicable for integrated EO devices and open the door for the future data communication systems.     

Low dielectric loss and enhanced tunability of Ba(Zr0.3Ti0.7)O3-based thin film by sol–gel method

In this work, dopants and buffer layers were employed to simultaneously lower the dielectric loss and enhance the dielectric tunability of Ba(Zr_0.3Ti_0.7)O₃ (BZT) thin films. The BZT, 1 mol% La doping BZT (BZTL) with and without La_0.5Sr_0.5CoO₃ (LSCO) buffer layers were prepared by sol–gel technique. The dielectric properties of the thin films were investigated as a function of frequency and current bias field. As a result, the BZTL thin film with LSCO buffer layer showed lower dielectric loss and higher tunability simultaneously, which can be a promising candidate for tunable microwave device applications.     

A study of the abatement of VOC over V2O5–WO3–TiO2 and alternative SCR catalysts

The conversion of C3 organic compounds (propane, propene, 1- and 2-propanol, allyl alcohol, propanal, acrolein, acetone and 1- and 2-chloropropane) in the presence of excess oxygen has been investigated over two V–W–TiO₂ commercial SCR catalysts differing in the V content and over Mn–TiO₂ alternative SCR catalysts. V–W–Ti catalysts show poor activity in the oxidation of hydrocarbons and oxygenates and give significant amounts of partial oxidation products. Moreover they give rise to CO in excess of CO₂. The sample higher in V is more active. Mn–TiO₂ is definitely more active in oxidation of hydrocarbons and oxygenates, and produces, at total conversion, CO₂ as the only detectable product.

V–W–Ti catalysts are very active in dehydrochlorination of the two 2-chloropropane isomers and retain the same oxidation activity also in the presence of HCl. On the contrary, Mn-based catalysts in the presence of chlorocarbons convert into dehydrochlorination catalysts but lose their catalytic activity in oxidation. V–W–Ti catalysts can be used in Cl-containing atmospheres while Mn–TiO₂ can be proposed for DeNO_x and VOC abatement in Cl-free atmospheres such as for diesel engine exhaust gas purification.     

Effect of solvo-thermal treatment temperature on the properties of sol–gel ZrO2–TiO2 mixed oxides as HDS catalyst supports

ZrO₂–TiO₂ mixed oxide (30–70 mol/mol) was prepared by low-temperature sol–gel followed by solvo-thermal treatment (1 day) at various temperatures (40, 80, 120, 160 and 200 °C). Selected samples of the corresponding single oxides were also prepared. Materials characterization was carried out by N₂ physisorption, XRD, thermal analysis (TG-DTA) and UV–vis DRS, infra-red and Laser-Raman spectroscopies. Binary solids of enhanced pore volume and pore size diameter were obtained by increasing the post-treatment severity. Anatase TiO₂ micro-segregation was evidenced by Raman spectroscopy for the mixed oxide solvo-treated at the highest temperature. This solid also showed the highest crystallization temperature to ZrTiO₄ (702 °C). Mo impregnated (2.8 atom nm⁻²) on various mixed oxides was sulfided under H₂S/H₂ (400 °C, 1 h), the catalysts being tested in the dibenzothiophene hydrodesulfurization (HDS, T = 320 °C, P = 5.59 MPa). By increasing the severity of the solvo-treatment improved supports for MoS₂ phase were obtained. The HDS activity of the catalyst with carrier post-treated at 200 °C was 40% higher (in per total mass basis) than that of sulfided Mo supported on the binary oxide solvo-treated at 80 °C. The ZrO₂–TiO₂-supported catalysts showed higher selectivity to products from the hydrogenation route than their counterparts supported on either single oxide.     

Grain growth in TiO2-added ZnO–Bi2O3–CoO–MnO ceramics prepared by chemical processing

The effect of TiO₂ on the grain growth of the ZnO–Bi₂O₃–CoO–MnO ceramic system prepared by chemical coprecipitation, was studied between 1150 and 1300 °C in air. Bi₂O₃ melts during firing, and then TiO₂ dissolves into Bi₂O₃-rich liquid. TiO₂ initially reacts with Bi₂O₃ to form Bi₄Ti₃O₁₂. Above ≈1050 °C, Bi₄Ti₃O₁₂ reacts with ZnO to form Zn₂TiO₄ spinel phase. The kinetic study of grain growth carried out using the expression Gⁿ–G_oⁿ=K_o·t·exp(−Q/RT) gave grain exponent (n) value as 6 and the apparent activation energy (Q) as 226.46 kJ/mol. 1.00 mol% TiO₂ addition increased the grain growth exponent value from 6 to 7 and apparent activation energy with 1.00 mol% TiO₂ addition was found to be 197.10 kJ/mol. The ZnO grain size gradually increases with increasing TiO₂ content. Addition of TiO₂ may increase the reactivity of the Bi₂O₃-rich liquid towards the ZnO grain, thus affecting the ZnO grain growth.