TiO2 Sol-gel Films Research Articles

Photoelectrochromic performance of tungsten oxide based devices with PEG–titanium complex as solvent-free electrolytes

Photoelectrochromic (PEC) devices were assembled with an electrochemically deposited tungsten oxide (WO3) thin film on transparent conductive glass (ITO) substrates, a composite polymeric electrolyte (PE), and a Ruthenium 535 sensitized TiO2 sol–gel film deposited on ITO substrates. The polymeric electrolyte was prepared by the sol–gel method using a viscous polyethylene glycol (PEG, MW 600) and titanium isopropoxide as precursors in an acidic medium. A lithium salt (LiI) was added to the PEG–Ti complex for photoelectrochemical purposes. The color change of WO3 based PEC devices under 100mW/cm2 illumination at short circuit conditions indicates that the introduction of titanium complex inside the polymeric electrolyte accelerates the coloring speed from 26 to 9s. Electrochemical impedance spectroscopy analyses of PEC devices show that the presence of titanium complex in PE reduces the charge transfer resistance and increases the charge storage capacity of the cells. This improvement could come from the oxidation of iodides and reduction of titanium ions inside the composite electrolytes that increase the lithium ion's mobility.

Preventing Sodium Poisoning of Photocatalytic TiO2 Films on Glass by Metal Doping

Titanium dioxide (TiO2) has received much attention as a photocatalyst, specifically in applications that require a mechanically robust thin film. TiO2 is particularly useful because it does not absorb visible light, making it well suited for coatings on glass. Photocatalytic activity of TiO2 films is strongly dependent on the substrate and it has been well established that sodium diffusion from glass has a negative effect on this activity. While the prevention of sodium poisoning is possible through the use of a precoating, this requires an additional coating and/or calcination step. Other remedies, such as acid treatment of the glass surface, are also time consuming. Therefore, it is a more attractive option to negate the effects of sodium diffusion without a separate processing step. In this paper, we examined the effects of silver, cobalt, copper, gallium, molybdenum, and tantalum doping on the prevention of sodium poisoning of sol–gel TiO2 films by comparing the photocatalytic activities on glass and SiO2 precoated glass. While sodium poisoning degraded the photocatalytic activity of undoped TiO2 films by 70%, it was only 10% for Mo‐ and Ta‐doped TiO2 films. Molybdenum was superior to other dopants in terms of photocatalytic activity, both in the presence of sodium and in a sodium‐free environment.

Bandgap modification of TiO2 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, characterization and photocatalytic properties of sol–gel TiO2 films

The application of heterogeneous photocatalysis is described as an advanced oxidation process (AOP) for the degradation of the diazo reactive dye using immobilized TiO2 as a photocatalyst. Starting TiO2 solutions were prepared with and without the addition of polyethylene glycol (PEG) and TiO2 films were directly deposited on a borosilicate glass substrate using the sol–gel dip-coating method. The surface morphology and the nanoscale roughness of TiO2 films were studied by means of atomic force microscopy (AFM). Structural properties of TiO2 were identified by X-ray diffraction (XRD). The decomposition behaviour of organic compounds from the gels was investigated using thermal gravimetry (TG) and differential scanning calorimetry (DSC). Photocatalytic activities of TiO2 films in the process of degradation of the commercial diazo textile dye Congo red (CR), used as a model pollutant, were monitored by means of UV/vis spectrophotometry. The kinetics of the degradation of the CR dye was described with the Langmuir–Hinshelwood (L–H) kinetic model.The addition of PEG to the TiO2 solution resulted in the changes in the film surface morphology, and affected the ratio of anatase–rutile crystal phases and the photocatalytic activity of TiO2. The TiO2 film prepared with PEG is characterized by higher roughness parameters (Ra, Rmax, Rq, Rz and Zmax), a lower amount of the rutile phase of TiO2, a higher amount of the anatase phase of TiO2 and a better photocatalytic activity compared to the TiO2 film without the addition of PEG.

Sol–gel titania films on YHfAl foils and their influence on inhibition of transition aluminas during oxidation at high temperatures

In this experimental work, sol–gel technique was used as surface coating of YHfAl alloys to avoid the formation of transition aluminas (δ and θ). This technique has an advantage of low cost and rapidity. The preparation conditions of TiO2 sol–gel coatings were presented with a description of their evolution with temperature during the first stages of YHfAl oxidation. It is noted that the rutile is the only detected phase at 800°C. The sol–gel thin films were prepared in order to inhibit the formation of transition aluminas. An important gain mass was recorded at a temperature of 875°C and the first grains of aluminas were appeared after 0.5h of oxidation. A sol–gel TiO2 film of nearly 60nm thick has proven its efficiency and has allowed to reduce the presence of transition aluminas.

Ion-irradiation enhanced epitaxial growth of sol–gel TiO2 films

We report the epitaxial growth of sol–gel TiO2 films by using ion-irradiation enhanced synthesis. Our present study shows that the ion-beam process can provide highly crystalline TiO2 even at 350°C. Nuclear energy deposition at amorphous/crystalline interface plays a dominant role in the epitaxial growth of the films at the reduced temperature via a defect-migration mechanism. In addition, the ion irradiation allows for increasing the film density by balancing the crystallization rate and the escape rate of organic components.

Gold Nanoparticle-Doped TiO2Semiconductor Thin Films: Gas Sensing Properties

TiO2 sol–gel films doped with gold nanoparticles are used as both optical and conductometric sensors for the detection of CO and H2. Tests performed at 360 °C show for the first time CO- and H2-induced reversible variations in the optical absorption of thin TiO2/Au sol–gel films. The absorbance changes are strongly dependent on both the testing wavelength and the film microstructure. Together with the sensing dynamics observed through an electric interface, it is possible to obtain a better understanding of the mechanisms involved in the detection of both gases. The presence of H2 elicits almost ideal, step-like dynamic responses using conductometric detection.

Thrombocompatibility of sol-gel titanium dioxide coatings deposited on stainless steel substrate

Titanium dioxide (TiO2) can be used as protective and biocompatible coatings. Properties of these coatings strongly depend on a method and conditions of the deposition process. The aim of this study was to investigate the contact of whole blood with sol-gel TiO2 films deposited on stainless steel (316L) substrates by the dip–coating method and treated at different temperatures. The TiO2 films were annealed at temperatures in the range of 300–800°C. Different structures of TiO2 (amorphous, anatase and rutile) were obtained. Samples were characterised in regard to morphology, thickness, roughness, wettability and phase composition. An in vitro cell adhesion and cytofluorymetry tests were also performed to characterise the quantity and morphology of adhered blood platelets and platelet activation and aggregation in the blood samples after contact with the studied surfaces.The TiO2 sol-gel coating annealed at 800°C (rutile) showed better thrombocompatibility in comparison with other studied coatings.

Formation, densification and properties of sol–gel TiO2 films prepared from triethanolamine-chelated soluble precursor powders

Amorphous, soluble powders were synthesized with triethanolamine (TEA) as chelating agent as precursors for TiO2 sol–gel solutions. Dip coating on glass substrates and subsequent annealing yielded thin films with excellent optical properties. Furthermore as-dried films were scraped of substrates, annealed at different temperatures and characterized in order to investigate the structural changes during processing. The observations were systematically compared with previous studies on precursor powders based on acetylacetone. Results indicate that triethanolamine provides both a sufficient hydrolytical stability of the Ti-precursor during coating and an adequate plasticity of the film material throughout thermal densification. These characteristics significantly improve the practical workability of the respective process. Additionally former assumptions regarding the relationship between film properties and intermediate structural features were verified and refined.

Tribological properties of anatase TiO2 sol–gel films controlled by mutually soluble dopants

Tribological properties of TiO2 sol–gel thin films with mutually soluble dopants were studied on a glass substrate. The results showed that the formation of mutually soluble solid solution played a very important role in the growth of titania grains. The fine-grained TiO2 films controlled by SiO2 dopant were superior to pure TiO2 film in wear resistance and endurance life, although both films greatly improve the surface characteristics of glass substrate, enhancing its tribological characteristics. High resistance to microfracture because of the very small grain size as well as a good adhesion of the film to the substrate is believed to be the determining factors influencing the tribological properties of SiO2 doped TiO2 films. However, excessive SiO2 seriously deteriorates wear resistance of film due to phase separation. The wear mechanisms were also discussed based on the observation of the surface morphologies by scanning electron microscope (SEM).

Influence of physico-structural properties on the photocatalytic activity of sol-gel derived TiO2 thin films

Sol-gel TiO2 films were deposited from various sols and heat-treated at different temperatures. Fourier transform infrared spectroscopy, ellipsometry, X-ray diffraction, and scanning electron microscopy methods have been used to study physico-structural properties of the films. These properties are discussed with respect to the experimental parameters. It is shown that, depending on sol formulations and annealing temperatures, a large range of crystallite size, crystallization degree, film porosity, and surface morphology can be covered. The photocatalytic properties of the films have been studied in relation to their physico-structural properties through the photodecomposition of malic acid. This study shows that the photocatalytic activity undergoes a complex dependence on the physico-structural properties.

Silver negative-ion implantation to sol–gel TiO2 film for improving photocatalytic property under fluorescent light

We have investigated improvement of photocatalytic properties of spin-coated sol–gel anatase-type TiO2 with a bandgap of 3.2eV by negative ion-implantation. Silver-negative ions were implanted to anatase-type sol–gel TiO2 films (about 500nm in thickness) on a glass at 65 and 30keV with various doses in 1016–1017ions/cm2. Subsequent annealing was performed at up to 600°C for 1h. After annealing at above 300°C, all the samples showed strong optical absorption of surface plasmon resonance of Ag nanoparticles at around the photon energy of 2.2eV. This means that Ag nanoparticles were formed in the TiO2 films. In the photocatalytic properties under fluorescent light, the Ag-implanted sample (30keV, 1×1016ions/cm2) after annealing at 400°C showed an improved photocatalytic efficiency by 1.58 times than a rutile single crystal TiO2 with bandgap of 3.0eV less than that of the anatase. Ag nanoparticles formed at shallower depth are considered to work well for catalysis.

Semiconductor TiO2Gas Sensor for Controlling Nitrocarburizing Processes

TiO2 films can serve as oxygen sensors for controlling the nitrogen potential in the process of nitrocarburizing. In contrast to conventional semiconductor-base oxygen sensors that lose stability under extreme thermal and chemical conditions in nitriding and carburizing atmospheres, TiO2 films obtained by the sol-gel method offer a promising alternative. In the present work TiO2 films with a density of about 80% and a grain size of 30 – 50 nm are deposited by the sol-gel technology. Steel substrates and commercial substrates from Al2O3 aluminum oxide with platinum electrodes are coated and calcinated. In order to preserve the sensitive element of the sensor a special casing is constructed for operation under conditions very close to the atmosphere of nitrocarburizing. The sensitive element consists of an Al2O3 substrate with built-in Pt-electrodes, which is coated by a thin sol-gel TiO2 film. The direct current in the furnace is measured for nitrocarburizing in various mixtures (N2, O2, H2, and NH3) at a temperature of 500 – 600°C. A linear dependence log[R] – log[p O2], where R is the electrical resistivity, is obtained for the films in the studied range of partial pressure of oxygen independently p O2 of the presence of NH3 or H2 in the atmosphere. The dependence log[R] – log[p O2] for a nitriding furnace with a capacity of 90 liters is shown to be stable with 3% scattering per month.

ATR-IR spectroscopic studies of the influence of phosphate buffer on adsorption of immunoglobulin G to TiO2

In situ attenuated total reflectance infrared (ATR-IR) spectroscopy has been applied to the study of the influence of phosphate on the extent of protein adsorption onto TiO2. Immunoglobulin G (Ig.G) was adsorbed onto a TiO2 sol–gel film from solutions containing phosphate or NaCl. Monitoring of the amide II absorbance (v=1545 cm−1) confirmed reduced protein adsorption from the phosphate containing solution. In situ ATR-IR spectroscopy was also used to study phosphate induced desorption of Ig.G. Solutions containing various phosphate concentrations were passed over a TiO2 film with Ig.G adsorbed to it. As the concentration of phosphate increased the amide II absorbance decreased confirming the removal of bound Ig.G from the TiO2 surface. As the amide II absorbance decreased the phosphate absorbance (v=1080 cm−1) increased suggesting accumulation of phosphate at the TiO2 surface. Not all of the bound protein could be displaced from the TiO2 surface by phosphate suggesting the presence of weakly and strongly bound Ig.G.

Spectral and photocatalytic characteristics of TiO2 CVD films on quartz.

A series of novel CVD films of titanium(IV) oxide of different thicknesses, spanning the range 10-91 nm, are prepared on quartz, via the reaction of titanium(IV) chloride and ethyl acetate, using a CVD technique. The films are clear, mechanically robust and comprise thin layer of nanocrystalline anatase titania of different thicknesses that absorb UV light. The UV-Visible spectral profiles of all the CVD TiO2 films of different thickness are the same and obey Lambert's law (absorbance is porportional to film thickness). A plot of the reciprocal length for the TiO2 coating versus wavelength is reported. The photocatalytic activity of each film to mediate the destruction of a thin layer of stearic acid is investigated. The rate depends directly upon the fraction of light absorbed and the apparent quantum yield for the overall process is 0.00035, which appears low compared with that for sol-gel TiO2 films.

Preparation and isolation of gold nanoparticles coated with a stabilizer and sol-gel compatible agent

In this work an attractive technique is presented that brings together the advantage of the micelle reverse technique to control the particle growth and the efficiency of the amine silanes as sol-gel-compatible surface modifiers. The diamino silane is far from being a passive agent in the formation process of the gold particle; it strongly modifies the growth of the gold particle in the reverse micelle. The diamino silane allows the gold particles to keep their individual properties unaltered throughout the process, which ends with their incorporation into a SiO2–TiO2 sol-gel thin film.

Effect of humidity on the photocatalytic degradation of trichloroethylene in gas Phase over TiO2 thin films treated by different conditions

The effect of humidity on the photocatalytic degradation reaction of trichloroethylene (TCE) in gas phase was investigated by using pretreated TiO2 sol-gel films. It was observed that the photocatalytic activity of the TiO2 films depended more greatly on the pretreatment conditions, for example, UV pre-illumination, than on the moisture content. When the relative humidity was over 50%, the reaction rates decreased regardless of treatment conditions of the photocatalyst. The photocatalytic decomposition rate of TCE increased with the increase of light intensity. However, the influence of humidity on the reaction rate was less significant under the increased light intensity. The intermediates and byproducts of the reaction were not changed in different humidity conditions.

Low temperature preparation of high refractive index and mechanically resistant sol-gel TiO2 films for multilayer antireflective coating applications

TiO2 sol-gel thin films have attracted a large attention for applications which require high refractive index transparent layers. In this work, sol-gel TiO2 layers were prepared by Aerosol-gel deposition followed by a thermal treatment procedure in air. Depending on the experimental conditions, abrasion resistant and high refractive index layers could be obtained after post-treatment at only 110°C. In this paper, the experimental parameters which allow the preparation of functional TiO2 sol-gel layers at such low temperature are discussed. It is concluded that the preparation of high refractive index and mechanically resistant TiO2 layers can be interpreted in terms of competition between polycondensation and densification mechanisms. This result allows to envisage the sol-gel processing at low temperature of multilayer antireflective coatings.

Ultrathin SiOx Film Coating Effect on the Wettability Change of TiO2 Surfaces in the Presence and Absence of UV Light Illumination

Wettability of sol–gel TiO2 film surfaces has been studied by following the H2O contact angle (θ) as functions of illumination time and subsequent dark storage time. Upon illumination of the TiO2 surface (λex>300 nm), the value of θ rapidly decreased to reach approximately zero (photoprocess). When the resultant superhydrophilic sample was stored in the dark, the θ value increased slowly with increasing storage time (dark process). Ultrathin SiOx films were formed on the surface of TiO2 by repeating chemisorption of 1,3,5,7-tetramethylcyclotetrasiloxane and its photooxidation. With increasing thickness of the SiOx monolayer coating, the rate for the dark process significantly decreased, while that for the photoprocess also decreased. Both X-ray photoelectron spectroscopic studies and product analyses during the photoprocess clarified that photocatalytic oxidation of organic adsorbates in air causes the superhydrophilicity of the TiO2 surfaces.

Raman spectroscopy of sol–gel ultrathin films enhanced by surface plasmon polaritons

We present experimental results of a Kretschmann-prism coupler method used in Raman scattering experiments with ultrathin (few nanometers) TiO2 sol–gel films deposited on a metallic silver layer characterized by surface plasmon resonance (SPR) experiments. In this article, we describe a new optical device as an application of SPR properties and coupled with a micro-Raman scattering setup. The enhancement factor (150) of the Raman intensity in comparison to the classical way allows us to determine the critical thin films structure. Optical results are then discussed and compared to transmission electron microscopy.