Atomic Force Microscopy Study of TiO2 Films Obtained by the Sol-Gel Method

Atomic Force Microscopy (AFM) was used to study the influence of thermal treatments on the structural and textural properties of the sol-gel TiO2 films obtained from Ti(OC3H7i)4. X-ray diffraction (XRD), ellipsometric and porosity measurements have also been made.The TiO2 sol-gel films were homogeneous, transparent and amorphous. Heat treatments in the 400–600°C range indicate that the films have a strong tendency to crystallization. The high initial homogeneity of the TiO2 films was preserved during the crystallization process. AFM shows that the thermally treated films exhibit uniform, monodispersed crystals.     

Preparation and Characterization of Sol-Gel Derived Y2O3 Thin Films

Sol-gel derived Y2O3 thin films have been prepared on platinum coated silicon wafers and fired to temperatures ranging from 400°C to 750°C. Multiple coats were used to obtain films up to 0.5 m thick with an intermediate firing of 400°C between coatings. Top Pt electrodes were sputtered to form monolithic capacitors. These films exhibited a dielectric constant of 18 and a leakage current of 10–11–10–7 A/cm2, making them attractive candidates for high dielectric constant dielectric films in high density DRAMs.     

Structural study of VOx doped aluminium fluoride and aluminium oxide catalysts

The structural properties of vanadium doped aluminium oxyfluorides and aluminium oxides, prepared by a modified sol-gel synthesis route, were thoroughly investigated. The influence of the preparation technique and the calcination temperature on the coordination of vanadium, aluminium and fluorine was analysed by different spectroscopic methods such as Raman, MAS NMR and ESR spectroscopy. In all samples calcined at low temperatures (350 °C), vanadium coexists in two oxidation states V^IV and V^V, with V^IV as dominating species in the vanadium doped aluminium oxyfluorides. In the fluoride containing solids aluminium as well as vanadium are coordinated by fluorine and oxygen. Thermal annealing of 800 °C leads to an extensive reorganisation of the original matrices and to the oxidation of V^IV to V^V in both systems.     

Sol-Gel Derived Calcium Phosphate Coatings for Biomedical Applications

Hydroxyapatite (HA) coatings have received considerable attention because they exhibit bone bonding capabilities. Unfortunately the common forms of coating production result in cracking and degradation of HA due to the thickness of the coatings and the elevated temperatures employed. This study demonstrates the production of sub-micron, crack-free calcium phosphate coatings on quartz glass substrates using a sol-gel dip-coating technique and firing temperatures below 1000°C.Coatings fired at 1000°C comprised a mixture of hydroxyapatite (HA) and tricalcium phosphate (TCP). XPS analysis of the coating surface showed that the Ca/P ratio lay in the range 1.5–1.67, and that there was a contribution from carbon in the form of carbonate.It is proposed that the sol-gel coatings comprising a resorbable (TCP) and an insoluble (HA) phase have potential benefits in certain implant applications.     

A Generic Technique for Coating Doped Sol-Gel Films onto the Inside of Tubes for Use as Colorimetric Sensors

Sol-gel thin films have been doped with bromophenol blue and eriochrome cyanine RC for measuring pH and Cu⁺⁺ in solution respectively. The films were coated inside glass tubes by a novel method. Films doped with bromophenol blue responded to pH changes from pH 3 to 8. They were stable to variations in temperature from 20°C to 40°C. The sensing films have been subjected to leaching studies in different pH buffer solutions. Copper ions in solution have been measured to a minimum concentration of 0.6 ppm by eriochrome immobilised in sol-gel films. The interferences of other metal ions were also studied. Doped sol-gel films coated onto the inside of test tubes offer a simple, none-invasive, reusable and fast optical chemical sensing technique for the measurement of colour by spectroscopy or colorimetry.     

Immobilization and Characterization of Lactate Dehydrogenase on TEOS Derived Sol-Gel Films

Physical adsorption and physical entrapment techniques have been utilized for the immobilization of lactate dehydrogenase (LDH) on tetraethylorthosilicate (TEOS) derived sol-gel films. The enzyme (LDH) activity has been assayed as a function of time, temperature, pH and pyruvate concentration. The results of photometric measurements used for monitoring the reaction yield a response time of about 1 min, linearity over a concentration range of 0–1.5 × 10^-3 M and detection limit of 5 × 10^-5 M. The TEOS sol-gel films containing LDH have been found to be stable for about 30 days at temperatures 4 to 10°C.     

Optical Switching in VO2 Thin Films

Vanadium dioxide thin films have been deposited from vanadium alkoxides VO(OR)3. An amorphous film is formed that transforms into crystalline VO2 upon heating at 500°C under a reducing atmosphere. Optically transparent VO2 thin films are then obtained that exhibit both electrical and optical switching around 70°C. The switching temperature together with the shape of the hysteresis loop can be modified by doping VO2 films with foreign cations. Doped MxVO2 (M = W6+, Nb5+, Ti4+, Cr3+ or Al3+) thin films have been prepared under the same conditions by mixing the vanadium alkoxide and a metal salt in an alcoholic solution. The switching temperature decreases when the film is doped with high-valent cations (W6+) and increases with low-valent cations (Al3+, Cr3+). The transition temperature first decreases and then increases when TiIV is added to the VO2 film while the width of the hysteresis loop is significantly reduced.     

Orientation and Surface Properties of Sol-Gel Derived Y2O3 Films

The orientation, surface and optical properties of sol-gel derived Y₂O₃ films have been investigated. Transparent Y₂O₃ films were prepared on quartz glass substrates by sol-gel processes using YCl₃·6H₂O as a starting material. The water droplet contact angles of the films reached constant values between 79° and 90° after the films were left for 8 to 10 days in air at ambient temperature, indicating that the film surface exhibited hydrophobicity. When 2-(2-methoxyethoxy)ethanol (MEE) was added to the sol, yttria in the films crystallized to a strongly oriented cubic phase at firing temperatures between 400°C and 500°C. The intensity of the XRD peaks increased as the firing temperature was increased to 900°C. However, yttria crystallized to a non-oriented cubic phase when MEE was not used. The refractive index and packing density of the Y₂O₃ films increased from 1.55 to 1.68 and from 0.67 to 0.79, respectively, as the firing temperature was raised from 400°C to 900°C, indicating that sol-gel derived Y₂O₃ films are lower in density than evaporated ones.     

Effects of organic-inorganic hybrid coatings on durability of cross-linked polyethylene

Cross-linked polyethylene (XLPE) films have been coated with nanostructured hybrid organic-inorganic coatings in order to improve their durability. For this purpose, bi- and mono-layer coatings containing different amount of silica and different organic polymers have been prepared through sol-gel reactions and applied to XLPE commercial films. The thermo-oxidative stability, electrical strength and conductivity of XLPE coated films have been investigated after ageing in air at temperatures above the on-service conditions, i.e. at 105 and 120 °C for 1900 and 600 h, respectively. The performed investigations (FT-IR, DSC, TGA and electrical properties) showed that all the coatings tested were able to strongly protect XLPE against oxidation, and that the coating with a PVOH/SiO₂ layer gave the best protection. The increase of thermal resistance induced by the coatings reflects on the electrical strength after ageing, which is higher for coated samples than for uncoated ones. Moreover, while ageing has only a slight effect on electrical conductivity in different coated samples, a strong increase of conductivity was observed after ageing for highly oxidated uncoated samples.     

MoS x Thin Films by Thermolysis of a Single-Source Precursor

Thin films of MoS _x have been prepared on silicon substrates by spin coating and thermolysis of 0.5 M solutions of alkyldiammonium tetrathiomolybdates in 1,2-ethanediamine (EDA) and 1,2-propanediamine (12PDA). The films have been heat treated in air at temperatures between 80 and 250°C and under N₂ atmosphere at temperatures between 300 and 800°C. X-ray diffraction shows a restricted crystallisation and amorphous residues in both kind of films. EDA-based films exhibit a high tendency to crystallise whereas 12PDA-based films form associated structures with the solvent preventing precursor crystallisation. An insight into the processes occurring in film formation is gained by infrared spectroscopy which indicates a beginning of the decomposition of the 12PDA-based film at temperatures as low as 80°C with incorporation of the diamine solvent. In contrast, the EDA-based films show first signs of a decomposition at 150°C. The decomposition of the intermediate MoS₃ in both cases starts between 250 and 300°C. By means of SNMS depth profiles carbon contents up to 21 and 32 atom-% were found in EDA- and 12PDA-based films, respectively. The films show a significant deficit of sulphur which is compensated by the carbon. Near the surface of the coatings a loss of carbon is observed.     

Comparison of electrode structures and photovoltaic properties of porphyrin-sensitized solar cells with TiO2 and Nb, Ge, Zr-added TiO2 composite electrodes

Electrode structures and photovoltaic properties of porphyrin-sensitized solar cells with TiO2 and Nb-, Ge-, and Zr-added TiO2 composite electrodes were examined to disclose the effects of partial substitution of Ti atom by the other metals in the composite electrodes. The TiO2 and Nb-, Ge-, and Zr-added TiO2 composite electrodes were prepared by sol-gel process using laurylamine hydrochloride as a template for the formation of micellar precursors yielding well-defined mesoporous nanocrystalline structures, as in the cases of the formation of silica and titania tubules and nanoparticles by the templating mechanism. The TiO2 and Nb-, Ge-, and Zr-added TiO2 composite electrodes were characterized by transmission electron microscopy, BET surface area analysis, X-ray diffraction analysis, Raman spectroscopy, and impedance measurements. The TiO2 anatase nanocrystalline structure is retained after doping a small amount (5 mol %) of Nb, Ge, or Zr into the TiO2 structure, suggesting the homogeneous distribution of the doped metals with replacing Ti atom by the doped metal. The power conversion efficiency of the porphyrin-sensitized solar cells increases in the order Zr-added TiO2 (0.8%) < Nb-added TiO2 (1.2%) < TiO2 (2.0%) < Ge-added TiO2 cells (2.4%) under the same conditions. The improvement of cell performance of the Ge-added TiO2 cell results from the negative shift of the conduction band of the Ge-added TiO2 electrode. The Ge-added TiO2 cell exhibited a maximum power conversion efficiency of 3.5% when the porphyrin was adsorbed onto the surface of the Ge-added TiO2 electrode with a thickness of 4 microm in MeOH for 1 h.     

Physical Properties of Sol-Gel Coatings

One of the most important applications of sol-gel technology is the fabrication of coatings. This is because of the possibility of applying oxide coatings with practically all types of chemical compositions at low ambient temperatures on many substrates of various shapes through the use of liquid solutions. Both oxides and different types of organic-inorganic hybrid coatings have been reported. Both oxides and hybrid coatings are usually amorphous at ambient temperatures but some oxides can be converted to the crystalline phase with heating. Regardless of the intended applications of the coatings their physical properties are always of importance. For instance, an anti-reflective coating for an automobile mirror is of little practical value unless it is fairly scratch-resistant. In this review which covers published information in the past fifteen years, some of the more important results of physical properties of sol-gel derived coatings are discussed firstly for oxides and then for organic-inorganic hybrids. It appears that properties such as the hardness of oxide coatings are inadequate unless the heat-treatment temperatures are in excess of about 400°C. The hybrid coatings, especially when they contain a dispersed phase of a hard solid like colloidal silica, can be processed at temperatures below about 150°C and can improve the performance of organic plastics such as the polycarbonates. There is insufficient scientific understanding of the relationship between physical properties and other interdependent variables such as processing conditions, chemistry and coating thickness. More research in this area will undoubtedly contribute to the availability of better and new coatings via the sol-gel approach.     

Formation of Superhydrophobic Alumina Coating Films with High Transparency on Polymer Substrates by the Sol-Gel Method

Transparent, superhydrophobic coating films have been prepared on polymer substrates at low temperatures through the sol-gel method. Al₂O₃ gel films were prepared on poly(ethylene terephthalate) substrates from Al(O-sec-C₄H₉)₃ chemically modified with ethyl acetoacetate. A small roughness of about 20–50 nm was found to form on the surface of the Al₂O₃ gel films dried at room temperature and then immersed in hot water at 60°C. The electron diffraction measurements have shown that this roughened surface consists of pseudoboehmite nanocrystals. The coating of hydrolyzed fluoroalkyltrimethoxysilane on the Al₂O₃ gel films with the small roughness produced transparent, superhydrophobic films with contact angle for water larger than 150°.     

Structural and Optical Properties of Sol-Gel Derived Aluminosilicate Planar Waveguides Doped with Er3+ Ions

Er3+ doped-aluminosilicate thin films were prepared on silica and silica/Si substrates by the sol-gel process and dip-coating. The sol-gel aluminosilicate planar waveguides were prepared from silicon and aluminium alkoxides. Their structural characterization has been carried out by Raman spectroscopy, Atomic Force and Scanning Electron Microscopies. The results indicated that these films present an amorphous structure until an annealing temperature of 900°C, while at temperatures higher than 1000°C, crystallization occurs. An estimate of microcrystallite sizes using Raman spectroscopy is given, which agrees with data from scanning electron microscopy. The optical properties have been investigated by Fluorescence spectroscopy in the visible region.     

Structural and IR Spectroscopic Analysis of Sol-Gel Processed CuFeMnO4 Spinel and CuFeMnO4/Silica Films for Solar Absorbers

Black colored CuFeMnO₄ spinel powders and films were prepared using sol-gel process from Mn-acetate and Fe- and Cu-chloride precursors. Films were deposited by dip-coating technique and heat-treated at 500°C. For CuFeMnO₄/silica films 3-aminopropyl-triethoxysilane (3-APTES) or tetraethoxysilane (TEOS) were used in molar proportion (Mn : Cu : Fe) : silica = 1 : 1. Films and powders were prepared by heating at 500°C. IR spectroscopic measurements were employed to follow the hydrolysis-condensation reactions in (Mn : Cu : Fe)/3-APTES sols hydrolysed with water, and (Mn : Cu : Fe)/TEOS sols hydrolysed with (NH₃)_aq (Stöber processing). The resulting coatings were examined with transmission electron microscopy (TEM) combined with electron dif-fraction analyses, Rutherford back scattering (RBS) and proton induced X-ray emission (PIXE) techniques. Results revealed that (Mn : Cu : Fe)/3-APTES films had a composite structure consisting of the upper Cu_1.4Mn_1.6O₄ spinel and the lower amorphous SiO₂ layer. RBS measurements confirmed the composite structure, showing also that the composition of the film was Mn : Cu : Fe = 1 : 0.96 : 0.29, i.e. close to the precursors ratio Mn : Cu : Fe = 3 : 3 : 1. (Mn : Cu : Fe)/TEOS films prepared from sols which were catalysed with (NH₃)_aq consisted of amorphous monodispersed spherical SiO₂ particles with a size of about 400–420 nm. Solar absorbance (a _s) and thermal emittance (e _T) values of CuFeMnO₄ (500°C) and (Mn : Cu : Fe)/TEOS films (500°C) showed that CuFeMnO₄ films could be used as potential selective coatings for solar absorbers in solar collector systems.     

N,S和Cl改性纳米TiO_2薄膜的光电性能

采用溶胶-凝胶技术,以钛酸四正丁酯为原料,分别添加浓硝酸、浓硫酸或浓盐酸,制备氮、硫或氯改性纳米TiO2薄膜.紫外-可见漫反射光谱、光电流谱、交流阻抗谱和电位～时间变化测试表明,氮改性纳米TiO2薄膜使表面结构优化,从而可提高纳米TiO2电极的光电转换效率.     

The research on the high quality TiO2, MoO3-doped WO3 electrochromic film

The high quality TiO2, MoO3-doped WO3 electrochromic film was prepared by the sol–gel method for the first time. The sol, which has hydrogen peroxide (H2O2) and oxalic acid (H2C2O4), was very stable at room temperature and quite suitable for the deposition of films. The WO3 electrochromic film prepared from this doped sol had excellent performance, such as short response time, no cracks, good adhesion to the substrate, high coloring efficiency and longevity of service.     

Photoelectrochemical Properties of Sol-Gel Processed Ag-TiO2 Nanocomposite Thin Films

Ag-TiO₂ thin films were prepared with a sol-gel route, using titanium isopropoxide and silver nitrate as precursors, at 0, 0.03 and 0.06 Ag/Ti nominal atomic ratios. After drying at 80°C, the films were fired at 300°C, 500°C, and 600°C for 30 min and 5 h. Glancing angle X-ray diffraction (XRD) analysis and X-ray photoelectron spectroscopy (XPS), with depth profiling of the concentration, were used to study the films. XPS analysis showed the presence of C and N as impurities in the nanocomposite films. Their concentration decreased with increasing the firing temperature. Chemical state analysis showed that Ag was present in metallic state, except for the outer layer where it was present as Ag⁺. For the films prepared with a Agt/Ti concentration of 0.06, depth profiling measurements of the film fired at 300°C showed a strong Ag enrichment at the outer surface, while composition remained almost constant within the rest of the film, at Ag/Ti atomic ratio of 0.02. Two layers were found for the films heated to 500°C, where the Ag/Ti ratios were 0.015 near the surface and 0.03 near the substrate. The photoelectrochemical properties of Ag-TiO₂ were studied for thin films deposited on ITO substrates. Photocurrents of Ag-TiO₂ nanocomposite electrodes fired at 300°C were observed even at visible light, for wavelengths longer than 400 nm.     

Fabrication and Characterization of Sol-Gel Derived Potassium Sodium Strontium Barium Niobate

Potassium sodium strontium barium niobate (K_0.2Na_0.2Sr_0.48Ba_0.32Nb₂O₆, KNSBN) powder and thin films dip coated onto Si(100) substrates have been prepared by sol-gel route. The thermal evolution of these sol-gel derived KNSBN was studied by differential thermal analysis (DTA) and thermogravimetry (TG). Their structural changes at different annealing temperatures were examined by X-ray diffraction (XRD) and Raman spectroscopy. Our results suggest that the KNSBN tetragonal tungsten bronze (TTB) phase is formed via an intermediate orthorhombic phase. Pure TTB phase KNSBN was obtained at annealing temperatures of 1200°C and 600°C for powder and films respectively.     

Preparation and optical properties of Bi2S3 microcrystallite doped glass and thin film by the sol-gel process

The Bi₂S₃ microcrystallite doped thin films and glass lumps have been successfully prepared by the sol-gel process from the hydrolysis of a complex solution of Si(OC₂H₅)₄ Bi(NO₃)₃ · 5H₂O and SC(NH₂)₂, and the size of the microcrystallites in glass heated for different times at 400°C was decided by the method of HRTEM. The optical transmission valley shifted towards longer wavelengths with longer heat-treatment time at 500°C in the Bi₂S₃ doped thin films, showing the experimental evidence of quantum size effects. The red-shift of emission peaks in luminenscence spectrum excited with longer wavelength is attributed to the broad distribution of particle size in Bi₂S₃ doped glass.