Preparation of nanocrystalline TiO2 porous films from terpineol-ethanol-PEG system

Nanocrystalline TiO₂ porous films were prepared by a sol-gel dip coating method from a solution using poly(ethylene glycol) as a template, ethanol as a solvent and terpineol as a highly viscous solvent. The thickness of films increases greatly by increasing the amount of terpineol. However, the overmuch incorporation of terpineol hinders the formation of porous structure and accelerates the crack creation. The crack-free film having a three-dimensionally extended porous structure was obtained at the ethanol/terpineol ratio of 8:1, the film thickness with three depositions being 2.5 μm. Pretreatment of the precursor at an appropriate temperature also plays an important role in pore formation. Single-phase anatase TiO₂ porous films were obtained after calcinations at 550°C for 1 h, and the crystal size increases with an increase in the amount of terpineol. The pore formation mechanism is discussed primarily in relation to the phase separation in the system and self-assembly of PEG.     

Preparation of macroporous and mesoporous TiO2 film with various solvents

To investigate the effects of solvents on the microstructure, porous TiO₂ films have been prepared by sol-gel method using hydroxypropyl cellulose as the additive. Ethylene glycol monomethyl ether, propylene glycol monomethyl ether, ethanol, and the mixed solution of half ethylene glycol monomethyl ether and half ethanol were chosen as the solvents. Infrared spectra, scanning electron microscope micrographs, and X-ray diffraction patterns have been studied to characterize the microstructure of the sol and film samples. The results showed that mesoporous TiO₂ films with the pore size around 20 nm, 10 nm and 6 nm were obtained when propylene glycol monomethyl ether, the mixed solution of half ethylene glycol monomethyl ether and half ethanol, and ethanol were used as solvent, respectively. It was found that COC group in the solvent was beneficial to enlarge the pore size, because the oxygen bridge in the COC groups could be pulled out and act with titanoxane polymers. When ethylene glycol monomethyl ether was used as the solvent, macroporous TiO₂ film with pore size around 200 nm was obtained. This can be ascribed to the high enough concentration of effective COC groups of the solvent.     

Fabrication of porous TiO2 film via hydrothermal method and its photocatalytic performances

Porous anatase (TiO₂) films were fabricated onto stainless steel substrates with Ti(OC₄H₉)₄ as a precursor via hydrothermal process. The crystallization and porous structure of TiO₂ film were dependent on the time and temperature of the hydrothermal reaction. A TiO₂ film with orderly porous structure and high crystallization was obtained upon treatment at 150 °C for 2 h. The grain size of TiO₂ is ca. 6 nm, and pore diameter is ca. 10 nm. Diffusion of Fe into the porous TiO₂ film occurred; Fe also diffused onto the surface of the film with the extension of hydrothermal reaction time or increase of the reaction temperature. The diffusion reaction has a large effect on the formation of porous TiO₂ film as well as its interface texture. However, it does not change the crystal phase of the TiO₂. The resultant TiO₂ film showed high photocatalytic activity towards degradation of gaseous formaldehyde.     

Reticulated macroporous ceramic foam supported TiO2 for photocatalytic applications

Ceramic macroporous reticular alumina foams with a pore size of 15, 20 and 25 pores per inch (ppi) were prepared by the Schwartzwalder method and sintered at 1200 °C to preserve a high porosity. TiO₂ thick films were supported on the foam surface by a wash coating process, using Degussa P25 as a TiO₂ nanopowder source. After annealing at 600 °C, films with an adequate adhesion and with a thickness of 5–10 μm were obtained. An increasing pore size of the supported foams improves the specific photocatalytic activity of the TiO₂ coated scaffolds, the flow of solution through the highly active porous foam structure and the better access of light to the active TiO₂ surface.     

Low-temperature fabrication of porous anatase TiO2 film with tiny slots and its photocatalytic activity

The films consisting of 4,4′-dihydroxydiphenyl propane (BPA) and anatase TiO₂ colloidal particles were prepared by a dip-coating method. The BPA in the films was preferentially dissolved in anhydrous ethanol and thus eliminated from the films, so the porous anatase TiO₂ films were obtained. The surface morphology and the properties of the films were studied. The results show that the co-continuous morphology of the blends is favorable to the formation of porous surface structure with homogeneous and continuous tiny slots. The porosity induces the effective adsorption of Rhodamine B (RB) on the catalyst surface, and thus the photocatalytic activity is enhanced.     

Characterization of inhomogeneity in TiO2 thin films prepared by pulsed dc reactive magnetron sputtering

This article discusses an analytical method for characterizations of TiO₂ thin films and determinations of the degree of their inhomogeneity. The TiO₂ films were prepared by a pulsed dc magnetron sputtering with an operating pressure as a main experimental parameter. The obtained films were primarily characterized for film crystallinity, microstructures and optical properties by spectroscopic ellipsometry. The measured ellipsometric data were analyzed by the single-, the double, and the triple-layer models in order to match with the inhomogeneous film structure proposed in the Thornton structure zone model. The results were then compared with those obtained from grazing-incidence X-ray diffraction, field-emission scanning electron microscopy and high-resolution transmission electron microscopy. The study revealed that the pulsed dc sputtered TiO₂ films could be best described by the inhomogeneous triple-layer physical model. Although the films deposited at lower operating pressure had a dense structure with a mirror-like surface topography, the films deposited at higher operating pressure had the porous structure with the rough surface and the void.     

Fabrication of large-area and high-quality colloidal crystal films on nanocrystalline porous substrates by a room temperature floating self-assembly method

When colloidal crystal films are deposited onto nanocrystalline porous substrates by the commonly used colloidal crystallization method of vertical deposition self-assembly, the colloidal crystal tends to be poorly adhered to the porous film. Herein, we present a fabrication of large-area, three-dimensional (3D) colloidal crystal thin films on nanocrystalline porous substrates by a room temperature floating self-assembly method that has recently been developed for colloidal crystal deposition. Firstly, colloidal suspensions were prepared by dispersing monodisperse colloidal microspheres at high volume fraction in a mixture of ethanol and water. At room temperature, these suspensions were spread onto nanocrystalline porous TiO₂ films. The colloidal particles assembled into 3D ordered structures at the air−liquid interface of the suspensions as a result of rapid evaporation of the solvents. After the solvents (water and ethanol) had evaporated completely, the colloidal crystals were directly deposited on the nanocrystalline porous TiO₂ films. Scanning electron microscopy images and normal-incidence transmission spectra of the samples showed that the colloidal crystal films deposited on the nanocrystalline porous TiO₂ substrates by this method had very high crystalline quality. In addition, the effect of the degree of surface roughness of the nanocrystalline porous substrate on the crystalline quality of the colloidal crystals has been studied.     

Dimensional effects observed for the electrical, dielectrical and optical properties of TiO2 DC magnetron thin films

In this paper, we present the dimensional effects observed for TiO₂ films deposited by DC circular magnetron sputtering. TiO₂ thin films are deposited in an Al–TiO₂–Al structure to investigate their non-linear characteristics, from which the carrier effective mass and the barrier potential at the Al–TiO₂ interface is calculated, and an important relationship between the effective carrier mass and film thickness is observed. The dielectric constant of TiO₂ thin films is also investigated, and is observed to vary with TiO₂ film thickness. Further, TiO₂ band gap is observed to vary with film thickness.     

Structure of nanotubular titanium oxide templates prepared by electrochemical anodization in H2SO4/HF solutions

The electrochemical formation of nanotubular titanium oxide films was investigated in 1 M H₂SO₄ and 0.05-0.4 wt.% HF electrolytes. Depending on anodization condition, i.e. cell voltage, anodization time, HF concentration, TiO₂ porous films having different thickness (from 350 to 500 nm) and pore diameter (from 40 to 150 nm) were obtained. By varying the cell voltage from 10 V to 40 V it was possible to gradually change the crystal structure of titanium oxide from anatase to rutile. The effect of annealing temperature and duration on crystal structure was also considered.     

One step room temperature photodeposition of Cu/TiO2 composite films and its conversion to CuO/TiO2

Cu/TiO₂ composite films were prepared at low temperature on glass substrates by a photodeposition method. Films were deposited by irradiating the substrate while in contact with an aqueous TiO₂ suspension containing copper(II) nitrate and ethanol. Cu/TiO₂ composite films of 500 nm in thickness were deposited at room temperature after a short irradiation time (15 min) with a 125 W mercury vapour lamp. According to scanning electron microscopy observations, the obtained films were homogeneous and porous. Energy dispersive X-ray spectroscopy analysis revealed a 3:1 Cu:Ti atomic ratio. Grazing angle X-ray diffraction analysis showed that the films contained Cu and TiO₂ as major components and Cu₂O as a minor component. Heat treatment at 400 °C in air for a period of 3 h transformed the initial material into a CuO/TiO₂ composite, improved the adhesion to the substrate and favoured a more regular distribution of copper oxide according to backscattering micrographs.     

Synthesis of porous titania thin films using carbonatation reaction and its hydrophilic property

Super-hydrophilic porous titania thin films containing mesoscale channels were successfully synthesized by the carbonatation reaction. An amorphous Li-Ti-O film with a smooth surface was prepared on a glass substrate by radio frequency magnetron sputtering using a Li₂TiO₃ target in an Ar atmosphere. The as-deposited films were carbonated at 400 °C for 10 h in CO₂, resulting in the formation of Li₂CO₃ and the development of characteristic through-channels. After dissolution of the Li₂CO₃ in distilled water, porous TiO₂ thin films with mesoscale through-channels were fabricated. The obtained porous films had a large surface area and show a rapid decrease in the water contact angle or excellent super-hydrophilicity.     

One-step electrodeposition of TiO2/dye hybrid films

Crystalline TiO₂ films can be formed by anodic electrodeposition from TiCl₃ solution. In this study we investigated the co-deposition of TiO₂ with different organic dye molecules as structure-directing agents. The best results were obtained with the pH-indicator bromothymol blue, which leads to the formation of thick, crystalline and porous films. The films were tested in dye-sensitized solar cells and gave a five times higher efficiency compared to the film deposited with sodium dodecyl sulfate in our earlier study.     

Effect of porosity on the Seebeck coefficient of mesoporous TiO2 thin films

Mesoporous TiO₂ films were prepared by using titanium tetraisopropoxide as the titania precursor and triblock copolymer as the structure directing agent. The synthesized mesoporous TiO₂ film was confirmed to have the ordered pore structure with rutile phase by small angle and wide angle X-ray diffraction analyses. The mesoporous TiO₂ film has the porosity range from 21.6 to 35.6%, and its Seebeck coefficient was changed according to its porosity, up to −88.6 μV/K. From the obtained Seebeck coefficient, the ordered mesoporous TiO₂ film was found to be a good candidate of thermal sensing layer of thin film thermal sensor.     

Organic solvent based TiO<Subscript>2</Subscript> dispersion paste for dye-sensitized solar cells prepared by industrial production level procedure

In order to prepare the TiO₂ liquid dispersions for the electrodes of dye-sensitized solar cells with industrial mass production level at a reasonable cost, the present study investigates the preparation of TiO₂ liquid dispersions by a general industrial dispersion technique using readily available P25. To determine the TiO₂ dispersion offering the best light–electricity energy conversion efficiency, the suitability of various types of solvents and resins for use in TiO₂ dispersion are tested. In general, organic solvent based TiO₂ dispersions are found to allow the formation of more uniform thin films in comparison with water-based dispersions. A preparation using ethyl cellulose as the resin and the terpineol as the solvent is found to exhibit the best conversion efficiency. We have also found that using two kinds of resins of different molecular weights gave rise to better efficiency. Among 26 metal compounds tested in this study, the best metal dopant was Ag. XRD and XPS measurements confirm that the Ag exists as metal Ag and silver oxide.     

Cu2ZnSnS4 films by paste coating and their optoelectronic properties

Cu₂ZnSnS₄ (CZTS) thin films were prepared by a paste coating method as the absorb layer of solar cells. This method is more eco-friendly using ethanol as solvent and more convenient than traditional sol–gel method. The effects of sulfurization temperature on properties of thin film were studied. The results of X-ray diffraction and Raman spectroscopy showed the formation of kesterite structure of CZTS films. The scanning electron microscopy images revealed that CZTS thin film obtained at 550 °C were compact and uniform. The optical band gap of the CZTS film was about 1.5 eV, and the CZTS film had an obvious optoelectronic response. Moreover, CZTS solar cell was prepared with a conversion efficiency of 0.47 %.     

Preparation and characterization of TiO2 bulk porous nanosolids

A novel TiO₂ bulk porous nanosolid (also called a “TiO₂ nanosponge”) was prepared by a solvothermal hot-press (SHP) technique, using TiO₂ nanoparticles (average particle size of 15 nm) and various solvents as the starting materials. The pore diameters of the nanosolids were rather uniform, and the maximum value of pore volume and specific surface area were 0.249 cm³/g and 59.455 m²/g, respectively. The pore volume and diameter of TiO₂ bulk porous nanosolid could be controlled by changing either the type or the amount of the solvent used in the experiment. No residual solvents could be detected by FTIR analysis, which means that the solvents had entirely escaped from the samples during the process of preparing TiO₂ bulk porous nanosolids.     

Preparation of TiO2 hollow microparticles by spraying water droplets into an organic solution of titanium tetraisopropoxide

Hollow, spherical TiO₂ microparticles several tens of micrometers in diameter were prepared by spraying water into an organic phase containing titanium tetraisopropoxide (TTIP) as a titanium source. The rapid hydrolysis of TTIP at the water-oil interface resulted in the formation of a TiO₂ shell covering the water droplet. Hexane and cyclohexane were better solvents than isopropanol for fabricating hollow spherical microparticles, suggesting the importance of immiscibility of the solvent with water in this synthesis method. The average particle size increased as the distance from the nozzle to the surface of the TTIP solution was increased. The shell thickness was reduced by the addition of ethanol to the sprayed water droplet. These results demonstrate the controllability of the structure of TiO₂ hollow microparticles, including the diameter and the shell thickness.     

Gas sensors with porous three-dimensional framework using TiO2/polymer double-shell hollow microsphere

The correlations between the structures and gas-sensing properties of porous thin-film gas sensors made of packed hollow spheres are investigated. For this purpose, hollow polymeric spheres were used as templates. Double-shell hollow spheres were prepared by encapsulating the polymeric hollow spheres with TiO₂ shells. Solid polymeric spheres were used as templates for comparison. Porous thin-film gas sensor with interconnected three-dimensional pores was prepared by using the TiO₂ encapsulated hollow spheres. The double-shell hollow spheres and porous titania films were characterized by XRD, BET, TEM and SEM. The gas-sensing properties of the sensors toward NO₂ depend on the type of template and the three-dimensional porous structure of the films. Using the hollow sphere template and adding precursors during the film formation procedure help to prevent the collapse of hollow sphere and form the mesopores in films after removing the template. These films show enhanced gas sensitivity when compared to TiO₂ polycrystalline films. Such improvement in sensitivity results from the porous architecture of the hollow microsphere films which not only increase the active surface area but also promotes the gas diffusion.     

Preparation and properties of porous polyimide films with TiO2/polymer double shell hollow spheres

Without the template removal process, double shell hollow spheres were prepared by encapsulating the polymeric hollow spheres with TiO₂ shells. TiO₂ encapsulated hollow spheres were incorporated with the polyamic acid solutions and imidized to prepare the porous polyimide films. The final porous texture of the film depends on the as-doped arrangement and the properties of the hollow spheres. The TiO₂ shell exhibits good thermal stability and chemical resistance to prevent the collapse of hollow spheres during the polyamic acid solution/hollow particle mixing and the imidization procedures. The effects of preparation conditions on the morphology of the encapsulated hollow spheres were investigated. The dielectric constant of the porous polyimide films (pore diameter around 0.6 µm) decreased to 2.8. The influences of hollow particles on the porous textures, the dielectric and dynamic mechanical properties of the porous films were studied.     

High thermal stability thick wall mesoporous titania thin films

Mesoporous TiO₂ thin films were prepared by using tetrabutyl titanate as the inorganic precursor and triblock copolymer (Pluronic F127) as the structure directing agent. The obtained mesostructured TiO₂ thin film exhibits a high thermal stability, which can sustain 600 °C thermal treatment. The small angle XRD and wide angle XRD patterns indicate that the samples have mesoporous channel and are composed of anatase. The corresponding TEM images show that the homogeneous mesostructure and very thick pore walls (about 9–13 nm) are formed in the obtained thin films, which could be responsible for the high thermal stability of the framework. In addition, the samples have narrow pore diameter distribution and a mean pore size of 7.4 nm.