Mesoporous TiO2 Thin Films Research Articles

Improved photovoltaic performance of dye-sensitized solar cells with modified self-assembling highly ordered mesoporous TiO2 photoanodes

Strategies for improving the photovoltaic performance of dye-sensitized solar cells (DSSCs) are proposed by modifying highly transparent and highly ordered multilayer mesoporous TiO2 photoanodes through nitrogen-doping and top-coating with a light-scattering layer. The mesoporous TiO2 photoanodes were fabricated by an evaporation-induced self-assembly method. In regard to the modification methods, the light-scattering layer as a top-coating was proved to be superior to nitrogen-doping in enhancing not only the power conversion efficiency but also the fill factor of DSSCs. The optimized bifunctional photoanode consisted of a 30-layer mesoporous TiO2 thin film (4.15 μm) and a Degussa P25 light-scattering top-layer (4 μm), which gives rise to a ∼200% higher cell efficiency than for unmodified cells and a fill factor of 0.72. These advantages are attributed to its higher dye adsorption, better light scattering, and faster photon–electron transport. Such a photoanode configuration provides an efficient way to enhance the energy conversion efficiency of DSSCs.

In Situ versus ex Situ Assembly of Aqueous-Based Thioacid Capped CdSe Nanocrystals within Mesoporous TiO2 Films for Quantum Dot Sensitized Solar Cells

We report a straightforward in situ deposition method to directly assemble aqueous thioglycolic acid capped CdSe colloidal quantum dots within mesoporous TiO2 thin films by a low-temperature hydrothermal route. This approach integrates linker assisted adsorption and colloidal quantum dot synthesis in a single step due to the use of thioglycolic acid as the capping agent for the quantum dots and tethering agent for the TiO2. It permits high loading and uniform distribution of colloidal quantum dots within mesoporous TiO2 electrodes with a greatly improved photovoltaic performance as quantum dot sensitized solar cells, reaching efficiencies as high as 2.2% under one sun illumination conditions after ZnS treatment, compared to the ex situ assembly technique of adsorption of presynthesized semiconductor nanocrystals.

Influence of reduction processes on the colour and photochromism of amorphous mesoporous TiO2 thin films loaded with a silver salt

Silver nanoparticles were created inside mesoporous titania thin films by different reduction processes. We investigated the influence of the reduction method on the colour and photochromism of these amorphous TiO(2) films. The results highlight brown films by optical reduction, gray films by thermal reduction, and red, purple or orange films by chemical reduction. The different size distributions and localizations of the nanoparticles, characterized by UV-visible spectroscopy and electron microscopy, give various photochromic behaviours when exposed to visible laser light. We especially report the bleaching of different film colours under laser exposure.

Charge Recombination to Oxidized Iodide in Dye-Sensitized Solar Cells

The goal of this study was to determine whether electrons injected into TiO2 in dye-sensitized solar cells (DSSCs) react with di-iodide, I2•–, a known intermediate in sensitized iodide oxidation. The approach was to utilize time-resolved absorption spectroscopy to quantify the yield of I2•– disproportionation under conditions where I2•– reduction by electrons photoinjected into TiO2, TiO2(e–)s, could be competitive. The DSSC was based on [Ru(dtb)2(dcb)]2+, where dtb is 4,4′-(C(CH3)3)2-2,2′-bipyridine and dcb is 4,4′-(COOH)2-2,2′-bipyridine, sensitized mesoporous nanocrystalline TiO2 thin films sintered onto an optically transparent fluorine-doped tin oxide (FTO) conductive substrate. A transparent Pt counter-electrode and a 0.5 M LiI/0.05 M I2/acetonitrile electrolyte completed the DSSC. After pulsed 532 nm laser excitation, the first iodide oxidation product observed spectroscopically was I2•–. Under all conditions studied, there was no direct evidence for the reaction between TiO2(e–) and I2•–, and the ...

Study on the Electrical and Thermal Conductivity of Ordered Mesoporous TiO2 Thin Film Incorporated with Pt Nanoparticles

Ordered mesoporous TiO2 films with incorporated Pt nanoparticles were prepared using titanium tetraisopropoxide, hexachloroplatinic acid hexahydrate, and Pluronic P-123 as a titania precursor, a Pt precursor, and a structure-directing agent, respectively. Pt nanoparticles were introduced to enhance the electrical properties of the mesoporous films, which have excellent thermal insulation properties. We confirmed that the synthesized composite films with Pt nanoparticles had an ordered pore structure with anatase phase TiO2 by both small-angle and wide-angle X-ray diffraction analyses. Although the porosity of the composite film decreased from 37.6 to 29.3% when Pt nanoparticles were included, the conductivity ratio (σ/κ) of the film increased greatly, up to approximately 1370 K V-2, due to the greater increase in electrical conductivity than thermal conductivity. Based on our results, we conclude that by incorporating Pt nanoparticles into ordered mesoporous TiO2 film, the thermoelectric properties of the mesoporous films can be improved.

Highly Active Crystalline Mesoporous TiO2 Films Coated onto Polycarbonate Substrates for Self-Cleaning Applications

SiO2 film precoating is found to be very effective for the protection of polycarbonate (PC) as a typical example for heat-sensitive substrate materials, thus preventing its photodegradation under UV light. Subsequently, transparent photoactive mesoporous TiO2 and Sachtleben Hombikat UV100 thin film coatings on SiO2/PC have been developed for self-cleaning applications. The newly prepared photocatalysts films have been compared with either a film prepared from a commercial photocatalyst (Sachtleben Hombikat UV-100), SG-TiO2, or Pilkington Glass ActivTM by the determination of their photonic efficiencies for degradation of methylene blue and the photoinduced surface wettability. The efficiency of the photocatalytic reactions was found to be about three times higher in the case of the mesoporous TiO2 films compared with films prepared from UV-100 or Pilkington Glass ActivTM. This is the first report focusing on the synthesis of mesoporous TiO2 films on polycarbonate substrates, and, to the best of our knowledge, the measured photonic efficiency of ζ = 0.078% for these meso-TiO2 films on PC is among the highest ζ-values reported up to now even when using soda-lime glass substrates.

Efficient Photocatalytic Decomposition of Glucose, Starch, and Cellulose to CO2 Using a Mesoporous Semiconductor Thin Film

UV light-activated highly efficient photocata- lytic decomposition of aqueous glucose and polysaccha- rides (starch and cellulose) to CO2 was successfully achieved by using a mesoporous TiO2 thin film coated on a fluorine-doped transparent conductive glass (FTO). The external quantum efficiency (g) of 0.08 (=8%) was obtained for glucose photodecomposition at neutral pH based on the total incident UV light, and the internal quantum efficiency (g 0 ) was 8 (=800%) based on the pho- ton that was effective for activating the reactant, demon- strating that the major decomposition mechanism is dark auto-oxidation of the activated reactant by O2. Glucose gave g 0 values of 19 at pH 12 and 25 at pH 2 demonstrating that when a glucose molecule was once activated by one photon, the molecule can undergo auto-oxidative decom- position to CO2 at these pH under dark. Water-soluble starch was also photodecomposed completely to CO2 with estimated g 0 value of 8.6. Water-soluble carboxymethyl cellulose (CMC) also underwent decomposition to CO2 with similar efficiency of g 0 = 5. Solid state cellulose powders could be photodecomposed to CO2 by sandwich- ing them between FTO-coated TiO2 thin films.

Oriented Mesoporous TiO2 Thin Film as Photoanode for Cathodic Protection of Stainless Steel

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Schottky junction/ohmic contact behavior of a nanoporous TiO2thin film photoanode in contact with redox electrolyte solutions

The nature and photoelectrochemical reactivity of nanoporous semiconductor electrodes have attracted a great deal of attention. Nanostructured materials have promising capabilities applicable for the construction of various photonic and electronic devices. In this paper, a mesoporous TiO2 thin film photoanode was soaked in an aqueous methanol solution using an O2-reducing Pt-based cathode in contact with atmospheric air on the back side. It was shown from distinct photocurrents in the cyclic voltammogram (CV) that the nanosurface of the mesoporous n-TiO2 film forms a Schottky junction with water containing a strong electron donor such as methanol. Formation of a Schottky junction (liquid junction) was also proved by Mott–Schottky plots at the mesoporous TiO2 thin film photoanode, and the thickness of the space charge layer was estimated to be very thin, i.e., only 3.1 nm at −0.1 V vs Ag/AgCl. On the other hand, the presence of [Fe(CN)6]4− and the absence of methanol brought about ohmic contact behavior on the TiO2 film and exhibited reversible redox waves in the dark due to the [Fe(CN)6]4−/3− couple. Further studies showed that multiple Schottky junctions/ohmic contact behavior inducing simultaneously both photocurrent and overlapped reversible redox waves was found in the CV of a nanoporous TiO2 photoanode soaked in an aqueous redox electrolyte solution containing methanol and [Fe(CN)6]4−. That is, the TiO2 nanosurface responds to [Fe(CN)6]4− to give ohmic redox waves overlapped simultaneously with photocurrents due to the Schottky junction. Additionally, a second step photocurrent generation was observed in the presence of both MeOH and [Fe(CN)6]4− around the redox potential of the iron complex. It was suggested that the iron complex forms a second Schottky junction for which the flat band potential (Efb) lies near the redox potential of the iron complex.

Nanostructured Mesoporous Titanium Dioxide Thin Film Prepared by Sol-Gel Method for Dye-Sensitized Solar Cell

Titanium dioxide (TiO2) paste was prepared by sol-gel and hydrothermal method with various precursors. Nanostructured mesoporous TiO2thin-film back electrode was fabricated from the nanoparticle colloidal paste, and its performance was compared with that made of commercial P25 TiO2. The best performance was demonstrated by the DSSC having a 16 μm-thick TTIP-TiO2back electrode, which gave a solar energy conversion efficiency of 6.03%. The ability of stong adhesion on ITO conducting glass substrate and the high surface area are considered important characteristics of TiO2thin film. The results show that a thin film with good adhesion can be made from the prepared colloidal paste as a result of alleviating the possibility of electron transfer loss. One can control the colloidal particle size from sol-gel method. Therefore, by optimizing the preparation conditions, TiO2paste with nanoparticle and narrow diameter distribution was obtained.

Morphology and photocatalysis of mesoporous titania thin films annealed in different atmosphere for degradation of methyl orange

The effect of different annealing atmosphere on the morphology and photocatalytic activity of mesoporous TiO2 thin films by dip-coating technique is investigated. The annealing temperature and atmosphere causes significant change of the morphology, thermal stability, photoluminescence, and photocatalytic properties of mesoporous TiO2 films. As-prepared mesoporous thin films have an amorphous structure that is transformed to crystalline TiO2 with well-maintained mesoporous structure at 450°C in various annealing atmosphere except that the mesoporous structure is fully destroyed for films annealed in air. Films annealed in N2 show the enhanced UV photodegradation of methyl orange (MO) resulting from the enhanced crystallinity in these films evidenced by XRD and Raman spectra. These findings can be used to tailor the structure and morphology of mesoporous TiO2 films and thus improve their photocatalytic activity for efficient removal of dye effluents in wastewater.

Self-accumulated Ag nanoparticles on mesoporous TiO2 thin film with high bactericidal activities

Antibacterial activity of sol–gel synthesized Ag–TiO2 nanocomposite layer (30 nm) deposited on rough anatase (a) TiO2 thin film (∼ 200 nm in thickness) was investigated against Escherichia coli bacteria, in dark and also in exposure to UV light. The nanocomposite thin films were transparent with a surface plasmon resonance absorption band at a wavelength of 410 nm. The metallic silver nanoparticles with an average diameter of 30 nm and fcc crystalline structure were self-accumulated on surface of a mesoporous and aqueous TiO2 layer with a capillary pore structure having a pore radius of 3.0 nm. By adding the silver nanoparticles in the TiO2 layer, recombination of the photoexcited electron–hole pairs in the (a)TiO2 thin film was delayed, while the pore structure was unchanged. Decrease in the recombination rate and accumulation of the silver nanoparticles on the film surface let the mesoporous Ag–TiO2/(a)TiO2 nanocomposite thin films have excellent antibacterial activity against E. coli bacteria. It was found that the relative rate of reduction of the viable bacteria in dark (in exposure to the UV light) for the Ag–TiO2/(a)TiO2 nanocomposite thin film was 3.2 × 10–2 min–1 (26 × 10–2 min–1) which was 4.6 (2.0) times greater than the corresponding value for the (a)TiO2 thin film. The behavior of silver ion release showed that the dominant mechanism of the release process in long time was based on water diffusion through the capillary mesoporous of the TiO2 layer, unlike the usual diffusion of water on the surface of silver-based bulk materials. Therefore, the synthesized Ag–TiO2/(a)TiO2 nanocomposite thin film can be utilized as a promising and effective bactericidal material in the future.

Efficient Photocatalytic Decomposition of Aqueous NH3 Using a Simple Mesoporous Semiconductor Thin Film with External Quantum Efficiency of 35 (= 3.5 × 103%) Based on Total Incident UV Light

UV light-activated highly efficient photocatalytic decomposition of ammonia to N2 in water was successfully achieved by using only a simple mesoporous TiO2 thin film coated on a transparent substrate with external quantum efficiency of 35 (= 3.5 × 103%) based on the total incident UV light. The internal quantum efficiency exceeded 250 (= 2.5 × 104%) in a large scale 22.2 L submodule meaning that ammonia was once activated at TiO2 by UV light, the activated ammonia underwent more than 250 times oxidative decomposition by O2 probably via a kind of chain reaction mechanism.

Positron annihilation study of structural effect on photocatalytic activity of mesoporous TiO2 thin films

Abstract Mesoporous TiO 2 films were synthesized by using triblock copolymers via a sol–gel process in aqueous solution. It was found that a film calcined at 600 °C has the highest photocatalytic activity. By application of positron annihilation Doppler broadening spectroscopy combined with XRD, SEM, and N 2 adsorption desorption techniques, the film structural properties were examined systematically. It is revealed that an excellent photocatalytic activity could be achieved only if a film maintains a suitable TiO 2 grain size, i.e. ∼7.5 nm in the present study, a median specific surface area and a high crystallinity in anatase state.

Evaporation‐Induced Alignment of Cylindrical Mesopores in TiO2 Thin Films

Hexagonally packed cylindrical mesopores that are vertically aligned at the surface of mesoporous TiO2 thin film are formed by self‐assembly of the hydrolyzed titanium alkoxides in the presence of poly(styrene‐b‐ethylene oxide) building blocks. The mesoporous thin film exhibits a high degree of lateral ordering and thermal stability. Long‐range lateral ordering and orientation of mesopores at the TiO2 film surface arise from a strong unfavorable interaction between the polystyrene and poly(ethylene oxide) building blocks, together with the cooperative assembling between the building blocks and inorganic species in association with the directionality of solvent evaporation.

Photocatalytic activity of (sulfur, nitrogen)-codoped mesoporous TiO2 thin films

Nitrogen and sulfur co-doped mesoporous TiO2 thin films were fabricated using thiourea as a doping resource by the combination of the sol–gel and evaporation-induced self-assembly (EISA) processes. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), N2 adsorption–desorption, and UV–vis spectra were performed to characterize the as-synthesized mesoporous TiO2 materials. The XPS result shows that O–Ti–N and O–Ti–S bonds in the (S, N)-codoped mesoporous TiO2 were formed. The resultant mesoporous (S, N)-codoped TiO2 exhibited anatase framework with a high porosity and a narrow pore distribution. After being illuminated for 3 h, methyl orange (MO) could be degraded completely by the co-doped sample under the ultraviolet irradiation, whereas mesoporous TiO2 film without doping could only degrade 60% MO. After being illuminated by visible light, the water contact angles of the mesoporous co-doped TiO2 samples decreased slightly, but the pure TiO2 mesoporous film exhibited no change in the hydrophilicity.

Enhanced Photocatalysis by Doping Cerium into Mesoporous Titania Thin Films

Mesoporous TiO2 thin films doped with varying amounts of cerium have been synthesized via a supramolecular-templated sol−gel route. The cerium dopant strongly affects the mesostructure, photocatalytic, and optical properties of the mesoporous TiO2 films. At an appropriately low level of cerium doping (i.e., Ce/Ti < 10 mol %), the film showed an increase in the degree of mesoscopic orderliness with an obvious degradation in the nanocrystallinity, which is related to the fact that cerium adversely affects the growth of titania nanocrystallites during thermal annealing. The photocatalytic ability of the film with Ce/Ti = 0.3 mol % was highest for degradation of methylene blue under UV irradiation, which is 3.5-fold of that of the undoped mesoporous TiO2 film. This remarkable enhancement in photoreactivity is attributed to the facilitated transferring of electrons to oxygen in association with cerium cations. The cerium-doped mesoporous TiO2 thin films also demonstrate photoluminescence at room temperature, w...

Preparation and characterization of mesoporous TiO2 thin film

Ordered hexagonal mesoporous TiO2 thin film was prepared by the evaporation-induced self-assembly (EISA) method using triblock copolymer (Pluronic P123) and tetrabutyl orthotitanate (Ti(OBun)4, TBOT) in 1-methoxy-2-propanol (C4H10O2, PGME) solvent. The arrangement of mesopores was identified by small-angle X-ray diffraction and transmission electron microscopy (TEM). The well-ordered hexagonal mesoporous TiO2 had a high specific surface area of 239 m2/g and an average pore size of 6.3 nm. The structure of mesoporous TiO2 thin film was anatase with a 5.1 nm crystallite. The absorption band shift of the mesoporous TiO2 toward longer wavelengths as calcined at 350 °C due to the residual carbon.

Highly ordered transparent mesoporous TiO2 thin films: an attractive matrix for efficient immobilization and spectroelectrochemical characterization of cytochrome c

We demonstrate remarkably fast incorporation and high loading of cytochrome c within thin films of periodically ordered nanocrystalline TiO(2) deposited on transparent electrodes. The immobilized cytochrome c is not denaturated and it can be reversibly reduced without mediator over the time scale of a few seconds as evidenced by spectroelectrochemistry.

Highly dispersed Au nanoparticles incorporated mesoporous TiO2 thin films with ultrahigh Au content

Highly dispersed gold nanoparticles (NPs) incorporated mesoporous titania thin films with ultrahigh Au content (53.3 wt%) were synthesized by a deposition-precipitation method using urea as precipitator, which showed high off-resonant third-order optical nonlinear susceptibility (χ(3) = 2.69 × 10−8 esu) measured by the Z-scan technique at 1064 nm. The enhanced χ(3) value of the materials was attributed to the homogeneous dispersion and the ultrahigh concentration of the gold NPs, and the high linear refractive index of the titania matrix.