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.     

Aqueous Titanate Sols from Ti Alkoxide-α-Hydroxycarboxylic Acid System and Preparation of Titania Films from the Sols

Aqueous titanate sols were prepared by reactions of titanium tetraisopropoxide (TIP) with -hydroxycarboxylic acids in water. IR and Raman spectra, and elemental analyses of the precipitates obtained from the sols revealed that the carboxylates were chelated to titanium but the Ti species were polymerized to form a cluster that had a colloidal nature. Spin-coating of titania (TiO₂) thin films from these sols was also examined. Interestingly, it was found that (004) preferentially oriented anatase films with refractive index of 2.54 were obtained from TIP-lactic acid-NH₃ (1:1:1, molar ratio) aqueous sol. This crystallographic orientation was characteristic of the TIP-lactic acid-NH₃ (1:1:1) system, and was not affected by the kinds of substrates used and the heating rate of the film. TEM observation indicated that small anatase grains had already formed at 200°C. Therefore, the crystallographic orientation might depend strongly on the structure of the chemical species of the precursor solutions.     

TiO2 nanotube, nanowire, and rhomboid-shaped particle thin films fixed on a titanium metal plate

Titanium dioxide thin films having various nanostructures could be formed by various treatments on sodium titanate nanotube thin films approximately 5 μm thick fixed on titanium metal plates. Using an aqueous solution with a lower hydrochloric acid concentration (0.01 mol/L) and a higher reaction temperature (90 °C) than those previously employed, we obtained a hydrogen titanate nanotube thin film fixed onto a titanium metal plate by H⁺ ion-exchange treatment of the sodium titanate nanotube thin film. Calcination of hydrogen titanate nanotube thin films yielded porous thin films consisting of anatase nanotubes, anatase nanowires, and anatase nanoparticles grown directly from the titanium metal plate. H⁺ ion-exchange treatment of sodium titanate nanotube thin films at 140 °C resulted in porous thin films consisting of rhomboid-shaped anatase nanoparticles.     

Sol-Gel Preparation and Characterization of 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 0.03 and 0.06 Ag/Ti nominal atomic ratios. After drying at 80°C, the films were fired at 300°C and 500°C for 30 min. The films were analysed by X-ray diffraction (XRD) with glancing angle, and X-ray photoelectron spectroscopy (XPS), with depth profiling of the concentration. 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 very outer layer where it was present as Ag⁺. For the films prepared with a Ag/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 0.019. For the films heated to 500°C, two layers were found, where the Ag/Ti ratios were 0.015 near the surface and 0.026 near the substrate.     

Preparation and Characterization of Bismuth Doped TiO2 Thin Films

A series of Bismuth-doped titanium oxide (Bi-doped TiO₂) thin films on glass substrates have been prepared by sol-gel dip coating process. The prepared catalysts were characterized by XRD and XPS. The photocatlytic activity of the thin film catalysts was evaluated through the photodegradation of aqueous methyl orange under UV illumination. The experiments demonstrated that the Bi-doped TiO₂ prepared was anatase phase. The doped bismuth was in the 3⁺ oxidation state. The presence of Bi significantly enhanced the photocatalytic activity of TiO₂ films. At calcination temperature of 500°C, with doping concentration of 2 wt %, Bi-doped TiO₂ thin film showed the highest photocatalyic activity.     

Preparation of TiO2 Thin Film Photocatalysts by Dip Coating Using a Highly Viscous Solvent

TiO₂ thin film photocatalysts coated onto soda lime glass were prepared by a dip coating process using a highly viscous solvent. The source of the TiO₂ was tetraisopropyl orthotitanate, and -terpineol was used as the solvent. Two types of thin film preparation procedures based on dip coating (a sol-gel system and thermal decomposition system) were used to prepare the samples. TiO₂ thin films were obtained after calcination at 450°C for 1 hour. The film thickness obtained with a single dipping was proportional to the viscosity of the dip coating solutions. The obtained thin films were transparent with a thickness of 1 m. The crystal form of the obtained photocatalyst films was anatase alone. The thin films were formed with aggregated nano-sized TiO₂ single crystals (7–15 nm). The photocatalytic activity of the TiO₂ thin films, as evaluated by the photooxidation of NO (1 ppm) in dry air, was as high as our previous TiO₂ thin films prepared by the sol-gel method.     

Formation of Anatase Nanocrystals in Sol-Gel Derived TiO2-SiO2 Thin Films with Hot Water Treatment

We have successfully prepared transparent and porous anatase nanocrystals-dispersed films by treating the sol-gel derived TiO₂-SiO₂ films containing poly(ethylene glycol), PEG, with hot water. This process was done at temperatures lower than 100°C under atmospheric pressure, and thus anatase nanocrystals-dispersed films can be formed on various kinds of substrates including organic polymers with poor heat resistance. The changes in structure and composition of the TiO₂-SiO₂ gel films with hot water treatment were related to the formation process of anatase nanocrystals in the TiO₂-SiO₂ gel films with hot water treatment. The formation of anatase nanocrystals was found to proceed to hydrolysis of Si–O–Ti bonds and dissolution of SiO₂ component. In addition, porous film structure formed by leaching of PEG with hot water treatment led to homogenous dispersion of anatase nonocrystals in the films.     

Preparation of Titanium Oxide Layer on Silica Glass Substrate with Titanium Naphthenate Precursor

Nanocrystalline TiO₂ thin films on silica glass substrates were prepared by using a naphthenic acid precursor. As-deposited thin films were heat treated at 500, 600, 700 and 800^∘C for 30 min in air. The TiO₂ thin films were analyzed by High Resolution X-ray diffraction, ultra violet—visible—near infrared spectrophotometer, field emission—scanning electron microscope and scanning probe microscope. After annealing at 600 and 700^∘C, the XRD patterns consist of only anatase peaks of TiO₂ film. Rutile(110) peak begins to appear at an annealing temperature of 800^∘C. Relative high transmittance at visible range was obtained for all films except the film annealed at 800^∘C. Optical band gap, E_g, is in the range between 3.53 and 3.78 eV except the TiO₂ film annealed at 500^∘C. The best hydrophilicity was achieved with a high-temperature annealing.     

Fabrication of Ordered TiO2 Porous Thin Films by Sol-Dipping PS Template Method

Monolayer polystyrene spheres (∼400 nm) array templates were assembled orderly on clean glass substrates by dip-drawing method from emulsion of PS and porous TiO₂ thin films were prepared by using sol-dipping template method to fill TiO₂ sol into the interstices among the close-packed PS templates and then annealing to remove the PS templates. The effects of TiO₂ precursor sol concentration and dipping time in sol on the porous structure of the thin films were studied. The results showed pore size of the ordered TiO₂ porous thin film depended mainly on PS size and partly on TiO₂ sol concentration. The shrinkage of pore diameter was about 10% for 0.2 M and 20% for 0.4 M TiO₂ sol concentrations. X-ray diffraction (XRD) spectra indicated the porous thin film was anatase structure. The transmittance spectrum showed that optical transmittance of the porous thin film kept above 70% beyond the wavelength of 430 nm. Optical band-gap of the porous TiO₂ thin film (fired at 550^∘;C) was 3.12 eV.     

Syntheses of TiO2(B) nanowires and TiO2 anatase nanowires by hydrothermal and post-heat treatments

TiO₂(B) nanowires and TiO₂ anatase nanowires were synthesized by the hydrothermal processing in 10 M NaOH aq. at 150 °C followed by the post-heat treatment at 300-800 °C. As-synthesized Na-free titanate nanowires (prepared by the hydrothermal treatment and repeated ion exchanging by HCl (aq.) were transformed into TiO₂(B) structure with maintaining 1-D morphology at 300-500 °C, and further transformed into anatase structure at 600-800 °C with keeping 1-D shape. At 900 °C, they transformed into rod-shaped rutile grains. Microstructure of these 1-D TiO₂ nanomaterials is reported.     

Low-temperature, solution processing of TiO2 thin films and fabrication of multilayer dielectric optical elements

High-quality TiO₂ thin films have been deposited from aqueous titanium-peroxo solutions via spin coating. The effects of precursor solution pH on the crystallization behavior, morphology, density, and refractive index of the films are reported. From X-ray diffraction measurements, the amorphous as-deposited films are found to crystallize in the anatase phase at 250 °C. Surface and cross-section SEM images reveal that films deposited from an acidic precursor are more uniform and denser than those deposited from a basic precursor. X-ray reflectivity measurements show that films with smooth surfaces and high densities (up to 87% of single-crystal anatase) can be produced at temperatures as low as 300 °C. Measured densities are consistent with high refractive indices at 633 nm of 2.24 and 2.11 for films derived from acidic and basic precursors, respectively. The uniformity and dense nature of the films have allowed fabrication of multilayer dielectric optical elements with thermal processing at only 300 °C. The distributed Bragg reflector with four bilayers exhibits a reflectance of 92% and a stop band width of 150 nm. The optical microcavity has a quality factor of 20. The optical properties of all elements agree well with theoretical models, indicating good optical quality. Use of the precursor chemistry for direct photopatterning of TiO₂ films without a polymer resist is also demonstrated.     

Sol-Gel Preparation and Photoelectrochemical Properties of Fe2TiO5 Thin Films

Polycrystalline Fe₂TiO₅ films were prepared on nesa silica glass substrates by the sol-gel method, and their photoanodic properties were measured in a three-electrode wet cell with an aqueous buffer solution of pH = 7. Gel films were crystallized into Fe₂TiO₅ when fired at 500°C. The photoanodic current significantly increased when the films were fired at 700°C, and then decreased with increasing firing temperature. Thicker films obtained by repeating the gel film deposition and firing showed smaller photocurrent, and the 50 nm thick film prepared via non-repetitive deposition exhibited the maximum photocurrent. Although the photoresponse was extended to wavelengths near 500 nm, the maximum quantum yield was as low as 0.12 at a wavelength of 340 nm.     

Effects of Heat Treatment on Photocatalytic Property of Sol-Gel Derived Polycrystalline TiO2

Polycrystalline TiO₂ in the bulk form was prepared by hydrothermal treatment of amorphous TiO₂ gel, and its photocatalytic activity for degradation of ethanol and physical properties were investigated after drying and calcination (300–650°C). The maximum photocatalytic activity reached when the polycrystalline TiO₂ was calcinated at temperatures from 300 to 500°C, which was almost the same as for a commercial TiO₂ powder. It was markedly reduced above 600°C. Photocatalytic activity of TiO₂ was affected by crystal growth of anatase, progress of sintering, anatase-rutile transformation and morphology of the polycrystalline TiO₂. It was a characteristic of the polycrystalline TiO₂ prepared in this study that the photocatalytic activity did not vary so much with the calcination temperature in the range of 300–500°C.     

Synthesis,Ferroelectric and Optical Properties of (Pb,Ca)TiO3 Thin Films by Soft Solution Processing

Calcium modified lead titanate sol was synthesized using a soft solution processing, the so-called polymeric precursor method. In soft chemistry method, soluble precursors such as lead acetate trihydrate, calcium carbonate and titanium isopropoxide, as starting materials, were mixed in aqueous solution. Pb_0.7Ca_0.3TiO₃ thin films were deposited on platinum-coated silicon and quartz substrates by means of the spinning technique. The surface morphology and crystal structure, dielectric and optical properties of the thin films were investigated. The electrical measurements were conducted on metal-ferroelectric-metal (MFM) capacitors. The typical measured small signal dielectric constant and dissipation factor at a frequency of 100 kHz were 299 and 0.065, respectively, for a thin film with 230 nm thickness annealed at 600°C for 2 h. The remanent polarization (2P_r) and coercive field (E _c) were 32 C/cm² and 100 kV/cm, respectively. Transmission spectra were recorded and from them, refractive index, extinction coefficient, and band gap energy were calculated. Thin films exhibited good optical transmissivity, and had optical direct transitions. The present study confirms the validity of the DiDomenico model for the interband transition, with a single electronic oscillator at 6.858 eV. The optical dispersion behavior of PCT thin film was found to fit well the Sellmeir dispersion equation. The band gap energy of the thin film, annealed at 600°C, was 3.56 eV. The results confirmed that soft solution processing provides an inexpensive and environmentally friendly route for the preparation of PCT thin films.     

Ba Titanate and Barium/Strontium Titanate Thin Films from Hydroxide Precursors: Preparation and Ferroelectric Behavior

Randomly oriented ferroelectric BaTiO₃ and (Ba_0.6Sr_0.4) TiO₃ thin films on platinum coated Si (100) were prepared by a sol-gel method. The precursor solutions were derived from barium hydroxide or a mixture of barium/strontium hydroxides dissolved in acetic acid and titanium butoxide. Polarization versus applied voltage hysteresis studies indicated a remanent polarization of 3 µC/cm² and a coercive field of 43.4 kV/cm for BaTiO₃ films annealed at 800°C for 1 h. Corresponding parameters for (Ba_0.6Sr_0.4)TiO₃ films annealed at 800°C were found to be 7.2 µC/cm² and 102.7 kV/cm, respectively. Microstructural study of the surface morphology of these films indicated grains of less than 0.1 µm in size. The leakage current for (Ba_0.6Sr_0.4)TiO₃ films was found to be two orders of magnitude lower than that for BaTiO₃ films.     

Preparation and characterization of SiO2/TiO2 composite microspheres with microporous SiO2 core/mesoporous TiO2 shell

SiO₂/TiO₂ composite microspheres with microporous SiO₂ core/mesoporous TiO₂ shell structures were prepared by hydrolysis of titanium tetrabutylorthotitanate (TTBT) in the presence of microporous silica microspheres using hydroxypropyl cellulose (HPC) as a surface esterification agent and porous template, and then dried and calcined at different temperatures. The as-prepared products were characterized with differential thermal analysis and thermogravimetric (DTA/TG), scanning electron microscopy (SEM), X-ray diffraction (XRD), nitrogen adsorption. The results showed that composite particles were about 1.8 μm in diameter, and had a spherical morphology and a narrow size distribution. Uniform mesoporous titania coatings on the surfaces of microporous silica microspheres could be obtained by adjusting the HPC concentration to an optimal concentration of about 3.2 mmol L⁻¹. The anatase and rutile phase in the SiO₂/TiO₂ composite microspheres began to form at 700 and 900 °C, respectively. At 700 °C, the specific surface area and pore volume of the SiO₂/TiO₂ composite microspheres were 552 and 0.652 mL g⁻¹, respectively. However, at 900 °C, the specific surface area and pore volume significantly decreased due to the phase transformation from anatase to rutile.     

Surface chemistry and optical property of TiO2 thin films treated by low-pressure plasma

The low temperature RF plasma treatment was used to control the surface chemistry and optical property of TiO₂ thin films deposited by RF magnetron sputtering with a very good uniformity at 300 °C substrate heating temperature. The XRD pattern indicates the crystalline structure of the film could be associated to amorphous structure of TiO₂ in thin film. The plasma treatment of TiO₂ film can increase the proportion of Ti³⁺ in Ti2p and decrease in carbon atoms as alcohol/ether group in C1s at the surface. The optical transmittance of the film was enhanced by 50% after the plasma treatment. The surface structure and morphology remain the same for untreated and low-pressure plasma-treated films. Therefore, increase in the optical transmission could be due to change in surface chemistry and surface cleaning by plasma treatment.     

Sol-Gel Nanosized Semiconducting Titania-Based Powders for Thick-Film Gas Sensors

Pure, 5 at%, and 10 at% Ta- or Nb-doped TiO₂ nanosized powders were prepared by the sol-gel method. The powders heated to 400°C have the crystalline anatase structure. While the pure TiO₂ powder heated to 850°C has the rutile structure, the addition of Ta and Nb inhibited the anatase-to-rutile phase transformation at this temperature. Ta was soluble in the titania lattice up to the concentration of 10 at%, while the solubility of Nb was 5 at%. Thick films were fabricated with these powders by screen printing technology and then fired at 650°C and 850°C for 1 h. SEM observations showed that the anatase-to-rutile phase transformation induces a grain growth of about one order of magnitude for pure TiO₂. The addition of Ta and Nb is effective to keep the TiO₂ grain size at the nanometric level even at 850°C. Conductance measurements showed that a good gas response is observed only for the nanostructured titania-based films. The CO response of these materials is only slightly affected by humidity.     

Study of the decomposition of aqueous citratoperoxo-Ti(IV)-gel precursors for titania by means of TGA-MS and FTIR

An aqueous solution-gel route is developed for the preparation of TiO₂. In this report, we study an aqueous citratoperoxo-Ti(IV)-precursor at pH 2.0, which is compatible with polyvinyl alcohol (PVA) and therefore can be applied for the preparation of a thick mesoporous TiO₂ film.With regard to deposition of films, it is important to gain insight in the behaviour of the precursor during thermal treatment. Therefore, the thermal decomposition mechanism of a citratoperoxo-Ti(IV)-gel and a PVA modified citratoperoxo-Ti(IV)-gel is studied. Weight losses and evolved gasses are characterized by TGA-MS (5 °C min⁻¹), while gel structure and changes in the solid upon heating are studied by means of FTIR. For both gels, decomposition in dry air can be divided into five regions. After drying of the samples in the first region (∼100 °C), decomposition of the organic matter not coordinated to the metal ions occurs (∼200 °C). The third region (∼310 °C) involves the decomposition of citrato ligands. Finally, the residual organic matter is combusted in the last two regions. Only in dry air it is possible to fully remove the organic matrix in both gels at temperatures below 600 °C.It is also proven that the citratoperoxo-Ti(IV)-complexes, seen at pH 7.0, exist in the precursor gel already at pH 2.0.     

Pyrolysis study of sol-gel derived TiO2 powders: Part IV. TiO2-anatase prepared by hydrolysing titanium(IV) isopropoxide without chelating agents

An amorphous TiO₂ gel was obtained by hydrolysing titanium(IV) isopropoxide with a stoichiometric amount of water using SnCl₂ as catalyst. In these operative conditions, a TiO₂ gel matrix containing a lower fraction of organic residual was obtained with respect to samples prepared by previously modifying the titanium alkoxide precursor with chelating ligands. Dried gel powders were characterized by N₂ adsorption analyses, FT-IR and XRD measurements. Thermogravimetric (TG) and differential thermal analysis (DTA) coupled with mass spectrometric (MS) and gas chromatographic (GC) measurements were performed in order to identify the organic products released from TiO₂ gel pyrolysis. The Tg-MS semiquantitative analysis of the main evolved species allowed to describe both the chemical composition of the initial TiO₂ gel and the chemical rearrangements occurring in the matrix during heating up to its crystallisation to anatase form at 420°C.This revised version was published online in November 2005 with corrections to the Cover Date.