Photocatalytic properties of porous TiO2/Ag thin films

In this study, nanocrystalline TiO₂/Ag composite thin films were prepared by a sol-gel spin-coating technique. By introducing polystyrene (PS) spheres into the precursor solution, porous TiO₂/Ag thin films were prepared after calcination at a temperature of 500 °C for 4 h. Three different sizes (50, 200, and 400 nm) of PS spheres were used to prepare porous TiO₂ films. The as-prepared TiO₂ and TiO₂/Ag thin films were characterized by X-ray diffractometry (XRD) and by scanning electron microscopy to reveal structural and morphological differences. In addition, the photocatalytic properties of these films were investigated by degrading methylene blue under UV irradiation.When PS spheres of different sizes were introduced after calcination, the as-prepared TiO₂ films exhibited different porous structures. XRD results showed that all TiO₂/Ag films exhibited a major anatase phase. The photodegradation of porous TiO₂ thin films prepared with 200 nm PS spheres and doped with 1 mol% Ag exhibited the best photocatalytic efficiency where ∼ 100% methylene blue was decomposed within 8 h under UV exposure.     

Photocatalytic properties of nanocrystalline TiO2 thin film with Ag additions

In the present study, nanocrystalline TiO₂/Ag composite thin films were prepared by a sol–gel spin coating technique. While, by introducing polystyrene (PS) microspheres, porous TiO₂/Ag films were obtained after calcining at a temperature of 500 °C. The as-prepared TiO₂ and TiO₂/Ag thin films were characterized by X-ray diffractometry, and scanning electron microscopy to reveal the structural and morphological differences. In addition, the photocatalytic properties of these films were investigated by degrading methylene blue under UV irradiation.After 500 °C calcination, the microstructure of PS-TiO₂ film without Ag addition exhibited a sponge-like microstructure while significant sintering effect was noticed with Ag additions and the films exhibited a porous microstructure. Meanwhile, coalescence of nanocrystalline anatase-phase TiO₂ can be observed with respect to the sharpening of XRD diffraction peaks. The photodegradation of porous TiO₂ doped with 1 mol% Ag exhibited the best photocatalytic efficiency where 72% methylene blue can be decomposed after UV exposure for 12 h.     

Reflectance modulation of transparent multilayer thin films for energy efficient window applications

Spectrally selective glazing system attracts great attention for energy efficient window applications. In this study, multilayer films consisting of high/low/high (TiO₂/SiO₂/TiO₂) refractive index materials were prepared by sol–gel synthesis and spin coating process. Film thicknesses were examined by spectroscopic ellipsometry (SE) and focused ion beam (FIB) techniques, and refractive indices of TiO₂ and SiO₂ single layer films were also measured by SE. The reflectance spectra experimentally measured from multilayer films were in good agreement with the theoretical calculations by incorporating variable refractive index into the transfer matrix, so it is possible to modulate reflectance of multilayer films by controlling experimental variables for energy efficient transparent window applications.     

Enhanced effect of vacuum-deposited SiO2 overlayer on photo-induced hydrophilicity of TiO2 film

Photo-induced hydrophilicity of SiO₂/TiO₂ multilayer film prepared by using the vacuum deposition method was investigated by means of water contact angle measurement. Using black light irradiation of the films centering at a wavelength of 365 nm, an extreme photo-induced hydrophilicity was achieved when the TiO₂ film was covered by SiO₂ overlayer ranging from 10 to 20 nm in thickness. These multilayer films exhibited much more extreme hydrophilicity than the TiO₂ film without SiO₂ overlayer. The surface analyses revealed that the enhanced photo-induced hydrophilic surface of the multilayer films exhibited an improved photo-catalytic activity towards decomposition of organic substances on their surfaces. It was found that significant growth of the SiOH group occurred in the uppermost surface of the SiO₂ overlayer of the multilayer films through the depth profile measurement of TOF-SIMS. This result suggests that the photo-generated reactive species such as hole created in the TiO₂ film may transmit the SiO₂ layer to reach the surface. The enhanced photo-induced hydrophilicity of the films can be explained by a synergetic effect of the improved photo-catalytic activity of the multilayer film and the stable hydrophilicity of SiO₂ itself.     

Synthesis of multilayer Nano-ZrO2 coated polystyrene spheres on fabrication of three-dimensional ordered macroporous structures

Monodispersed ZrO₂ precursor nanoparticles with a diameter of 25 nm were successfully synthesized by using diglycol as complexing reagent. The kinetic of particle growth as a function of concentration ratio of ZrOCl₂: diglycol was investigated. The as-synthesized ZrO₂ precursor nanoparticles were homogenously coated on the surface of polystyrene particles. Multilayer coating process was implemented by using poly (acrylic acid) (PAA) to modify the surface charges of the coated spheres, which was characterized by zeta-potential, particles size distribution and microstructural observation. The multilayer-coated polystyrene (PS) spheres have been used as templates to produce macroporous materials. Ordered macroporous ZrO₂ materials were obtained after the ZrO₂ precursor nanoparticles coated PS spheres were formed by centrifugation and calcined at 550°C for 3 h. The porous wall thickness could be well controlled by using the multilayer nano-ZrO₂ coated PS spheres with different coating thickness.     

Synthesis of TiO2/Pt/TiO2 multilayer films via radio frequency magnetron sputtering and their enhanced photocatalytic activity

TiO₂/Pt/TiO₂ (TPT) multilayered films with different thicknesses of Pt layers from about 0.75 to 12 nm were prepared by radio frequency magnetron sputtering method. The as-prepared films were characterized by X-ray diffraction, scanning electron microscopy, atomic force microscopy, UV-visible diffuse reflectance spectroscopy, and photoluminescence spectroscopy. The photocatalytic activity of the samples was evaluated by the photocatalytic decolorization of methyl blue aqueous solution under ultraviolet light irradiation. The photocatalytic activities of all TPT multilayer films were higher than that of the pure TiO₂ films. When Pt thickness was increased to 3 nm, the measured photocatalytic activity of the TPT film was highest, and exceeded that of the pure TiO₂ films by a factor of more than three times. Such enhancement was ascribed to the presence of Pt layer, which inhibits the recombination of the photogenerated charge and carriers, as well as modifies the crystallinity of the TiO₂ top layer.     

Fabrication and characterization of novel composite membranes composed of photonic crystals and TiO2 nanotube array films

Novel composite membranes composed of photonic crystals (PCs) and TiO₂ nanotube array (TNA) films have been fabricated by combining the room temperature floating self-assembly (RTFSA) method, recently developed by our research group, and the liquid-phase deposition technique. By applying this combined procedure, polystyrene (PS) opal PC/TNA and TiO₂ inverse opal PC/TNA composite membranes were prepared. Scanning electron microscopy and ultraviolet/visible spectroscopy analyses showed that the membrane samples possessed very high crystalline quality. Notably, the ordered packing of the PS microspheres from the top to the bottom of the opal PC film was not affected by the surface roughness of the porous TNA substrate. This is attributed to the self-assembly mechanism of the colloidal particles, which produces a three-dimensional ordered structure in the RTFSA method. Herein, the crystallization of the colloidal particles occurred at the surface of the colloidal suspension, and the crystal growth proceeded downward from the surface of the suspension to the substrate.     

SiO2/TiO2 multilayer films grown on cotton fibers surface at low temperature by a novel two-step process

A novel two-step process was developed to synthesize and deposit SiO₂/TiO₂ multilayer films onto the cotton fibers. In the first step, SiO₂ particles on cotton fiber surface were synthesized via tetraethoxysilane hydrolysis in the presence of cotton fibers, in order to protect the fibers against photo-catalytic decomposition by TiO₂ nanoparticles. In the second step, the growth of TiO₂ nanoparticles into the modified cotton fiber surface was carried out via a sol-gel method at the temperature as low as 100 °C. The as-obtained SiO₂/TiO₂ multilayer films coated on cotton fibers were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, atomic force microscopy and X-ray diffraction, respectively.     

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.     

Dielectric enhancement of BaTiO3/BaSrTiO3/SrTiO3 multilayer thin films deposited on Pt/Ti/SiO2/Si substrates by sol–gel method

Polycrystalline BaTiO₃/Ba_0.6Sr_0.4TiO₃/SrTiO₃ (BT/BST/ST) multilayer thin films with different periodicities have been deposited on Pt/Ti/SiO₂/Si substrates by using a sol–gel method. The multilayer thin films were crack free, compact and crystallized in a perovskite structure. The dielectric constant of the multilayer thin films was significantly increased and the dielectric loss was almost the same as those of uniform BT, ST and BST thin films. The dielectric constant increased with increasing stacking periodicity as the thickness of each individual layer decreased. The multilayer thin films showed excellent dielectric properties and can be promisingly used for the dielectric layer of silicon-based embedded capacitors in package substrate.     

Effect of plasma treatment on chemical bonding states of porous TiO2 thin films prepared from polymer-blended solution

Dense TiO₂ (D-TiO₂) thin films and porous TiO₂ (P-TiO₂) thin films were prepared by using a polymer-blended solution. The film porosity decreased gradually or disappeared with an increase in the polyethylene glycol (PEG) or TiO₂ content of the solution. To modify their surface properties, the thin films were treated with atmospheric pressure (AP) plasma by using a reactive gas. The surface morphologies of the O₂-plasma-treated TiO₂ (O-TiO₂) thin films were smooth and did not change significantly. The decolorization efficiency of the P-TiO₂ thin films was found to be enhanced when compared to that of the D-TiO₂ thin films. The enhancement was due to an increase in the specific surface area and the number of hydroxyl groups, and a decrease of Ti₂O₃ states.     

A heterostructured SnO2–TiO2 thin film prepared by Langmuir–Blodgett technique

SnO₂–TiO₂ heterostructure films were prepared through Langmuir–Blodgett (LB) route. LB films of octadecyl amine (ODA)–titanyl oxalate multilayer deposited on Si (100) and decomposed at 600 °C showed rutile and anatase phases of ultrathin TiO₂ film. Subsequently, multilayer LB film of ODA–stannate deposited on the pre deposited TiO₂ film after decomposition at 600 °C resulted in thin SnO₂ films on the TiO₂ thin film. The phase analysis of the SnO₂–TiO₂ film showed cassiterite phase of SnO₂ as well as the rutile/anatase mixture of TiO₂ indicating a SnO₂–TiO₂ heterostructured film. Surface morphology of the pure TiO₂ film and SnO₂–TiO₂ film were analyzed by using AFM. Electrical characterization by AC impedance analysis suggested SnO₂–TiO₂ heterostructure formation. DC current voltage measurement showed increase in photocurrent indicating visible light absorption and efficient charge separation under the sunlight type radiation.     

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.     

Production of free standing composite membranes or of patterned films after sol–gel reactions in an exponential layer-by-layer architecture

This paper aims at describing that polyelectrolyte multilayer (PEM) films, made of biocompatible molecules, Hyaluronic acid (HA) and Poly(6-lysine) (PLL), can entrap negatively charged precursors of titania or silica. These precursors undergo hydrolysis and polycondensation in contact with the amino groups of PLL leading to the formation of oxides (TiO₂ or SiO₂) within the films. Moreover, this work shows that these composite thin films, a few μm thick, can be easily detached as free standing membranes containing photoactive TiO₂. To this aim the PEM film containing TiO₂ was thermally cross-linked to form amide bonds. Immersion of the film in diluted hydrofluoric acid (2% (v/v)) allowed then the formation of a free standing membrane. We extend the concept of sol–gel reaction in PEM films to films made from poly(L-glutamic acid) (PGA) and Poly(allylamine) (PAH) and show that the composition of the multilayer film influences the distribution of the inorganic material in the films: the TiO₂ distribution seems homogeneous in (PLL–HA)_n films whereas the (PAH–PGA)_n films are self patterned with inorganic aggregates tens of micrometers in diameter. The free standing membranes or self patterned composite films obtained in this investigation may present interesting bio-applications.     

Preparation and characterization of nanocrystalline porous TiO2/WO3 composite thin films

TiO₂ materials possessing not only photocatalytic but also electrochromic properties have attracted many research and development interests. Though WO₃ exhibits excellent electrochromic properties, the much higher cost and water-sensitivity of WO₃ as compared with the TiO₂ may restrict the practical application of WO₃ materials. In the present study, the feasibility of preparing nanocrystalline porous TiO₂/WO₃ composite thin films was investigated.Precursors of sols TiO₂ and/or WO₃ and polystyrene microspheres were used to prepare nanocrystalline pure TiO₂, WO₃, and composite TiO₂/WO₃ thin films by spin coating. The spin-coated thin films were amorphous and, after heat treating at a temperature of 500 °C, nanocrystalline TiO₂, TiO₂/WO₃, and WO₃ thin films with or without pores were prepared successfully. The heat-treated thin films were colorless and coloration-bleaching phenomena can be observed during cyclic voltammetry tests. The heat-treated thin films exhibited good reversible electrochromic behavior while the porous TiO₂/WO₃ composite film exhibited improved electrochromic properties.     

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.     

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.     

Effect of titanium oxide-polystyrene nanocomposite dielectrics on morphology and thin film transistor performance for organic and polymeric semiconductors

Previous studies have shown that organic thin film transistors with pentacene deposited on gate dielectrics composed of a blend of high K titanium oxide-polystyrene core-shell nanocomposite (TiO₂-PS) with polystyrene (PS) perform with an order of magnitude increase in saturation mobility for TiO₂-PS (K = 8) as compared to PS devices (K = 2.5). The current study finds that this performance enhancement can be translated to alternative small single crystal organics such as α-sexithiophene (α-6T) (enhancement factor for field effect mobility ranging from 30-100× higher on TiO₂-PS/PS blended dielectrics as compared to homogenous PS dielectrics). Interestingly however, in the case of semicrystalline polymers such as (poly-3-hexylthiophene) P3HT, this dramatic enhancement is not observed, possibly due to the difference in processing conditions used to fabricate these devices (film transfer as opposed to thermal evaporation). The morphology for α-sexithiophene (α-6T) grown by thermal evaporation on TiO₂-PS/PS blended dielectrics parallels that observed in pentacene devices. Smaller grain size is observed for films grown on dielectrics with higher TiO₂-PS content. In the case of poly(3-hexylthiophene) (P3HT) devices, constructed via film transfer, morphological differences exist for the P3HT on different substrates, as discerned by atomic force microscopy studies. However, these devices only exhibit a modest (2×) increase in mobility with increasing TiO₂-PS content in the films. After annealing of the transferred P3HT thin film transistor (TFT) devices, no appreciable enhancement in mobility is observed across the different blended dielectrics. Overall the results support the hypothesis that nucleation rate is responsible for changes in film morphology and device performance in thermally evaporated small molecule crystalline organic semiconductor TFTs. The increased nucleation rate produces organic polycrystalline films with small grain size which are better connected and exhibit lower barriers for charge transport and as such higher field effect mobilities are measured in these devices.     

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.     

TiOx/Ag/TiOx multilayer for application as a transparent conductive electrode and heat mirror

Amorphous TiO_x films and Ag layer were deposited by electron-beam evaporation on soda-lime glass at room temperature. The details regarding the structure, surface morphology, and optical properties of the as-prepared TiO_x films were examined by X-ray diffraction, scanning electron microscopy, and ultra-violet (UV) -visible-near-infrared (NIR) spectrometry. The TiO_x films exhibit amorphous phase with an optical band gap of 3.35 eV. The polygrains oriented along the (111) and (200) directions in the Ag films were adopted to supply carriers into the TiO_x film and lower the sheet resistance of the stacked layer. The multilayer exhibited a sufficiently large Ag thickness (>15 nm), low resistance, high UV transmittance, visible transmittance, and high NIR reflection. Dependence of Ag thickness, TiO_x bottom-layer, and TiO_x overlayer on the optical and electrical properties of TiO_x/Ag/TiO_x were explored. A figure of merit (FOM) was used to find an optimal structure for a multilayer with superior conductivity and visible transparency. An FOM of 9.8 × 10^?2 (Ω^?1) at the visible wavelength of 550 nm for a TiO_x/Ag/TiO_x stacked layer with an 18-nm-thick Ag and a 20-nm-thick TiO_x was achieved. The TiO_x/Ag/TiO_x sample annealed at 500 °C 10 min also shows a good thermal stability.