Fabricating red-blue-switching dual polymer electrochromic devices using room temperature ionic liquid

In this work, room temperature ionic liquid (RTIL)—1-butyl-3-methyl-imidazolium hexafluorophosphate ([BMIM]PF₆)—was employed to fabricate dual polymer electrochromic devices (DPECDs). [BMIM]PF₆ was used as the electrolyte both in the electrochemical synthesis of conducting polymers (CPs) and in the fabrication of DPECDs. The electrochemically deposited poly(3,4-ethylenedioxythiophene) (PEDOT) and poly(3-methylthiophene) (PMeT) were employed to serve as two complementary coloring electrochromic thin films. Through combining these two electrochromic layers, the assembled DPECDs were found to switch between deep red and deep blue, which are two primary colors for a display. By employing RTIL as electrolyte, the devices retained 65% of their optical contrast and electroactivity after 5×10³ deep double potential steps, showing enhanced stability and durability. The DPECDs also exhibited stable electrochromic performance, with a maximum optical contrast of 26% at 665 nm, and achieved a high coloring efficiency of 460 cm² C^-1.     

Formation of ultrafast-switching viologen-anchored TiO2 electrochromic device by introducing Sb-doped SnO2 nanoparticles

Ultrafast-switching viologen-anchored TiO₂ electrochromic device (ECD) was developed by introducing Sb-doped SnO₂ (Sb_xSn_1−xO₂, ATO) as counter electrode (CE), and the switching behavior of the fabricated ECD was investigated as a function of Sb-doping concentration. About 9-nm-sized Sb_xSn_1−xO₂ (x=0–0.3) nanoparticles were synthesized by a solvothermal reaction of tin (IV) chloride and antimony (III) chloride at 240 °C, and employed to fabricate 2.4-μm-thick transparent CE. Working electrode (WE) was formed from the 7-nm-sized TiO₂ nanoparticle by a doctor blade method, and the thickness of the nanoporous TiO₂ electrode was 4.5 μm. The phosphonated viologen, bis(2-phosphonylethyl)-4,4′-bipyridinium dibromide, was then adsorbed on the prepared films for the construction of the ECD. The response time was strongly dependent on the doping concentration of Sb in ATO, and the fastest switching response was observed at 3 mol%. At this composition, the coloration time was 5.7 ms, and the bleaching time was 14.4 ms, which is regarded as one of the best results so far reported.     

Electrochromic devices based on surface-modified nanocrystalline TiO2 thin-film electrodes

Nanocrystalline TiO₂ thin film electrodes on conductive glass were modified with monolayers of different electrochromic compounds (mono-, di- and trimeric N,N′-dialkyl- or-diphenyl-4,4′-bipyridinium salts) equipped with TiO₂ anchoring groups (An=benzoate, salicylate, phosphonate). The synthesis of these compounds is reported. Different approaches have been studied to increase the surface concentration Γ_CS of electrochemically active coloring centers (CS) on TiO₂. The electrodes were checked coulometrically and spectroelectrochemically under potentiostatic conditions in MeCN/TEAP. Γ_CS of mono- and oligomeric viologens was shown to depend on the ratio (CS/An) of CS to anchoring groups (An). A cone-shaped trimeric arborol-type viologen was prepared with the intention to fill out the space above the convex surface of the nanoparticles particularly well. Preliminary results of a new type of TiO₂ solid-phase supported synthesis of the viologens is reported. Electrochromic devices including filters and displays have been prepared. The filter devices (12–100 cm²) consist generally of OTE/TiO₂-poly-viologen/glutaronitrile-LiN(SO₂CF₃)₂+spacer/Prussian Blue/OTE and exhibit optical density changes up to 2 (transparent to blue or yellowish to green and red-brown (at higher potential)) at switching times in the range of 1–3 s. Even higher optical density changes (at slower switching times) were achieved with systems such as OTE/TiO₂-poly-viologen/glutaronitrile-LiN(SO₂CF₃)₂+spacer/Prussian Blue-TiO₂/OTE. The display devices prepared include reflective displays with two to four separately addressable segments ((OTE/TiO₂ (both structured)-oligo-viologen/microcrystalline rutile (reflective layer)/molten salt+spacer/Zn) or (OTE/TiO₂ (both structured)-oligo-viologen/microcrystalline rutile (reflective layer)/glutaronitrile-LiN(SO₂CF₃)₂+spacer/Prussian Blue/OTE), as well as transparent systems with up to four addressable segments such as: OTE/TiO₂ (both structured)-poly-viologen/glutaronitrile-LiN(SO₂CF₃)₂+spacer/Prussian Blue/OTE.     

Coloured electrochromic windows based on nanostructured TiO2 films modified by adsorbed redox chromophores

A series of viologens has been synthesised, characterised and tested for their suitability as redox chromophores in electrochromic devices. These viologens contain a phosphonic acid moiety and are irreversibly adsorbed at a transparent nanoporous-nanocrystalline TiO₂ electrode. An electrochromic device consisting of a sandwich of a viologen-modified TiO₂ electrode/electrolyte (γ-butyrolactone, 0.05M LiClO₄, 0.05M ferrocene)/conducting glass shows excellent electrochromic properties: fast switching times (1–2 s), large changes in absorbance, high colouration efficiencies (up to 200 cm²/C) and good long-term stability (>10 000 cycles). Further, the colour changes from transparent or a faint yellow to either a deep blue or a deep green, depending on the nature of the viologen.     

Effect of morphological properties of ionic liquid-templated mesoporous anatase TiO2 on performance of PEMFC with Nafion/TiO2 composite membrane at elevated temperature and low relative humidity

Three high-purity TiO₂ (anatase) powders (T_PF6, T_BF4, and T_conventional) were prepared by the sol–gel method with/without ionic liquid as template and calcinations at 450 °C. These powders were, then, characterized to investigate their differences in morphological properties. Electrochemical performances of the H₂/O₂ PEMFCs employing the Nafion composite membranes with these three TiO₂ powders as fillers were studied over 80–120 °C under 50% and 95% relative humidity (RH). The result showed that the order of the fillers effect on the performance at 80 and 90 °C was the same as that of the TiO₂ filler's specific surface area (i.e. T_PF6 > T_conventional > T_BF4 > P25, a commercially available nonporous TiO₂ powder). However, the order between T_conventional and T_BF4 was reversed at 110 and 120 °C under 50% RH. This indicates that the size and the amount of mesopores, which better confined the water molecules, were significant contributing factors to the performances at the higher temperatures. The best power density obtained under 50% RH at 120 °C and a voltage of 0.4 V was from the PEMFC with the T_PF6-containing Nafion composite membrane. It was about 5.7 times higher than the value obtained from that with the recast Nafion membrane.     

Elevated temperature cycling stability and electrochemical impedance of LiMn2O4 cathodes with nanoporous ZrO2 and TiO2 coatings

In this study, nanoporous zirconia (ZrO₂) and titania (TiO₂) coatings are shown to stabilize the cycling performance of lithium-ion batteries with LiMn₂O₄ spinel cathodes. The effect of firing temperature on the coating pore size is discussed and the resulting performance of the coated cathodes is evaluated. Stabilization mechanisms, such as neutralization of acidic electrolytes by ZrO₂ and TiO₂ coatings, are examined. It is proposed that the establishment of a complex nanoporous network for lithium-ion transport results in a more uniform current distribution at the particle surface, thereby suppressing capacity fade that may be associated with surface instabilities of the spinel electrode.     

Direct Z-scheme anatase/rutile bi-phase nanocomposite TiO2 nanofiber photocatalyst with enhanced photocatalytic H2-production activity

Design and preparation of direct Z-scheme anatase/rutile TiO₂ nanofiber photocatalyst to enhance photocatalytic H₂-production activity via water splitting is of great importance from both theoretical and practical viewpoints. Herein, we develop a facile method for preparing anatase and rutile bi-phase TiO₂ nanofibers with changing rutile content via a slow and rapid cooling of calcined electrospun TiO₂ nanofibers. The phase structure and composition, surface morphology, specific surface area, surface chemical composition and element chemical states of TiO₂ nanofibers were analyzed by X-ray powder diffraction (XRD), field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM), nitrogen adsorption and X-ray photoelectron spectroscopy (XPS). By a rapid cooling of 500 °C-calcined electrospun TiO₂ precursor, anatase/rutile bi-phase TiO₂ nanofibers with a roughly equal weight ratio of 55 wt.% anatase and 45 wt.% rutile were prepared. The enhanced H₂ production performance was observed in the above obtained anatase/rutile composite TiO₂ nanofibers. A Z-scheme photocatalytic mechanism is first proposed to explain the enhanced photocatalytic H₂-production activity of anatase/rutile bi-phase TiO₂ nanofibers, which is different from the traditional heterojunction electron–hole separation mechanism. This report highlights the importance of phase structure and composition on optimizing photocatalytic activity of TiO₂-based material.     

Low-temperature preparation of AgIn5S8/TiO2 heterojunction nanocomposite with efficient visible-light-driven hydrogen production

AgIn₅S₈ and AgIn₅S₈/TiO₂ heterojunction nanocomposite with efficient photoactivity for H₂ production were prepared by a low-temperature water bath deposition process. The resultant AgIn₅S₈ shows an absorption edge at ∼720 nm, corresponding to a bandgap of ∼1.72 eV, and its visible-light-driven photoactivity (100.1 μmol h⁻¹) for H₂ evolution is 9 times higher than that (11 μmol h⁻¹) of the product derived from a hydrothermal process, while the obtained AgIn₅S₈/TiO₂ heterojunction nanocomposites prepared by using commercially available TiO₂ nanoparticles (P25) as TiO₂ source exhibit remarkably improved photoactivity as compared to the pristine AgIn₅S₈, and the AgIn₅S₈/TiO₂ nanocomposite with molar ratio of 1:10 shows a maximum photocatalytic H₂ evolution rate (371.1 μmol h⁻¹), which is 4.3 times higher than that (85 μmol h⁻¹) of the corresponding AgIn₅S₈/TiO₂ nanocomposite derived from a hydrothermal method. This significant enhancement in the photocatativity of the present AgIn₅S₈/TiO₂ nanocomposite can be ascribed to the better dispersion of the AgIn₅S₈ formed on TiO₂ nanoparticle surfaces and the more intimate AgIn₅S₈/TiO₂ heterojunction structure during the water bath deposition process under continuously stirring as compared to the corresponding nanocomposite derived from a hydrothermal method. This configuration of nanocomposite results in fast diffusion of the photogenerated carriers in AgIn₅S₈ towards TiO₂, which is beneficial for separating spatially the photogenerated carriers and improving the photoactivity. The present findings shed light on the tuning strategy of spectral responsive region and photoactivity of photocatalysts for efficient light-to-energy conversion.     

Photocatalytic hydrogen production from aqueous methanol solution with CuO/Al2O3/TiO2 nanocomposite

The photocatalytic hydrogen production from aqueous methanol solution was investigated with ZnO/TiO₂, SnO/TiO₂, CuO/TiO₂, Al₂O₃/TiO₂ and CuO/Al₂O₃/TiO₂ nanocomposites. A mechanical mixing method, followed by the solid-state reaction at elevated temperature, was used for the preparation of nanocomposite photocatalyst. Among these nanocomposite photocatalysts, the maximal photocatalytic hydrogen production was observed with CuO/Al₂O₃/TiO₂ nanocomposites. A variety of components of CuO/Al₂O₃/TiO₂ photocatalysts were tested for the enhancement of H₂ formation. The optimal component was 0.2 wt% CuO/0.3 wt% Al₂O₃/TiO₂. The activity exhibited approximately tenfold enhancement at the optimum loading, compared with that with pure P-25 TiO₂. Nano-sized TiO₂ photocatalytic hydrogen technology has great potential for low-cost, environmentally friendly solar-hydrogen production to support the future hydrogen economy.     

Sol–gel derived nanocrystalline CeO2–TiO2 coatings for electrochromic windows

Mixed CeO₂–TiO₂ coatings synthesized by sol–gel spin coating process using mixed organic–inorganic Ti(OC₃H₇)₄ and CeCl₃·7H₂O precursors with different Ce/Ti mole ratios were investigated by a wide range of characterization techniques. The attempts were directed towards achieving coatings with high transparency in the visible region and good electrochemical properties. Elucidation of the structural and optical features of the films yielded information on the aspects relevant to their usage in transmissive electrochromic devices. The films have been found to exhibit properties for counter electrode in electrochromic smart windows in which they are able to retain their transparency under charge insertion, high enough for practical uses. The high optical modulation and fastest switching for WO₃ film in the device configuration with the Ce/Ti (1:1) film is interpreted in terms of conducive microstructural changes induced by addition of TiO₂ in an amount equivalent to CeO₂.     

Cycle test and degradation analysis of WO3/PC+LiClO4/CeO2·TiO2 electrochromic device

We fabricated an electrochromic full cell device adopting WO₃ as a working electrode, and 1 M LiClO₄ in PC with 3% water addition as an electrolyte and CeO₂·TiO₂ with various thicknesses as an ion storage layer. CeO₂·TiO₂ with less than 100 nm shows large charge density but the long-term cyclability is not good due to lithium ion diffusion into ITO thin film. Therefore, the thickness of CeO₂·TiO₂ ion storage layer should be coated at more than 200 mm/min. Long-term cycle test results show that CeO₂·TiO₂ ion storage layer with more than 150 nm thickness and two time coating enhance the long-term stability. SIMS analysis results show that the degradation is due to the remaining lithium ion in the working electrode, WO₃.     

Electrodeposition of TiO2/SiO2 nanocomposite for dye-sensitized solar cell

For the working electrode of dye-sensitized solar cell (DSC), TiO₂/SiO₂ nanocomposite materials were electrodeposited on transparent fluorine doped tin oxide-coated glass by cathodic electrodeposition at room temperature. The electrode and DSC fabricated with TiO₂/SiO₂ nanocomposite were characterized with photocurrent density, X-ray diffraction (XRD), field emission-scanning electron microscopy (FE-SEM) and a photovoltaic performance test. On the electrodeposition, the addition of an appropriate amount of SiO₂ in the bath containing TiO₂ slurry was essential to achieve the superior crystallinity, photocurrent density and photovoltaic performance of the resulting TiO₂/SiO₂ electrode, which was significantly superior to a bare TiO₂ electrode. This enhanced performance of optimized TiO₂/SiO₂ electrode was ascribed to the role of SiO₂ as an energy barrier, increasing the physical separation of injected electrons and oxidized dyes/redox couple, and thereby retarding the recombination reactions in the resulting DSC.     

Significantly improved dehydrogenation of LiAlH4 catalysed with TiO2 nanopowder

The effects of TiO₂ nanopowder addition on the dehydrogenation behaviour of LiAlH₄ have been studied. The 5 wt.% TiO₂-added LiAlH₄ sample showed a significant improvement in dehydrogenation rate compared to that of undoped LiAlH₄, with the dehydrogenation temperature reduced from 150 °C to 60 °C. Kinetic desorption results show that the added LiAlH₄ released about 5.2 wt% hydrogen within 30 min at 100 °C, while the as-received LiAlH₄ just released below 0.2 wt.% hydrogen within same time at 120 °C. From the Arrhenius plot of the hydrogen desorption kinetics, the apparent activation energy is 114 kJ/mol for pure LiAlH₄ and 49 kJ/mol for the 5 wt.% TiO₂ added LiAlH₄, indicating that TiO₂ nanopowder adding significantly decreased the activation energy for hydrogen desorption of LiAlH₄. X-ray diffraction and Fourier transform infrared analysis show that there is no phase change in the cell volume or on the Al-H bonds of the LiAlH₄ due to admixture of TiO₂ after milling. X-ray photoelectron spectroscopy results show no changes in the Ti 2p spectra for TiO₂ after milling and after dehydrogenation. The improved dehydrogenation behaviour of LiAlH₄ in the presence of TiO₂ is believed to be due to the high defect density introduced at the surfaces of the TiO₂ particles during the milling process.     

TiO2 films obtained by microwave-activated chemical-bath deposition used to improve TiO2-conducting glass contact

In traditional solar cells, metal-semiconductor contacts used to extract photogenerated carriers are very important. In dye-sensitized solar cells (DSSC) not much attention has been given to contact between the TiO₂ and the transparent conducting glass (TCO), which is used instead of a metal contact to extract electrons. TiO₂ layers obtained by microwave-activated chemical-bath deposition (MW-CBD) are proposed to improve TiO₂ contact to conducting glass. Spectra of incident photon to current conversion efficiency (IPCE) are obtained for two-photoelectrode TiO₂ photoelectrochemical cells. IPCE spectra show higher values when TiO₂ double layer photoelectrodes are used. In these, the first layer or contacting layer is made by MW-CBD. Best results are obtained for double layer photoelectrodes on FTO (SnO₂:F) as conducting oxide substrate. Modeling of IPCE spectra reveals the importance of electrical contact and electron extraction rate at the TiO₂/TCO interface.     

Highly-efficient electrochromic performance of nanostructured TiO2 films made by doctor blade technique

Electrochromic TiO₂ anatase thin films on F-doped tin oxide (FTO) substrates were prepared by doctor blade method using a colloidal solution of titanium oxide with particles of 15 nm in size. The films were transparent in the visible range and well colored in a solution of 1 M LiClO₄ in propylene carbonate. The transmittances of the colored films were found to be strongly dependent on the Li⁺ inserted charges. The response time of the electrochromic device coloration was found to be as small as 2 s for a 1 cm² sample and the coloration efficiency at a wavelength of 550 nm reached a value as high as 33.7 cm² C⁻¹ for a 600 nm thick nanocrystalline-TiO₂ on a FTO-coated glass substrate. Combining the experimental data obtained from in situ transmittance spectra and in situ X-ray diffraction analysis with the data from chronoamperometric measurements, it was clearly demonstrated that Li⁺ insertion (extraction) into (out of) the TiO₂ anatase films resulted in the formation (disappearance) of the Li_0.5TiO₂ compound. Potential application of nanocrystalline porous TiO₂ films in large-area electrochromic windows may be considered.     

Green synthesis of fibrous hierarchical meso-macroporous N doped TiO2 nanophotocatalyst with enhanced photocatalytic H2 production

A new approach to prepare hierarchical and fibrous meso-macroporous N-doped TiO₂ is attempted at room temperature without using templates by the addition of titanium isopropoxide droplets to the ammonia solution. The catalysts are thoroughly characterized by physico-chemical and spectroscopic method to explore the structural, electronic and optical properties. The photocatalytic activities of the catalyst were evaluated with hydrogen generation. NTP catalyst calcined at 400 °C (NTP-400) exhibited 602.7 μmol/3 h H₂ generation from 10 vol.% methanol under visible light. The excellent photocatalytic activity for NTP-400 is attributed to the porous networks existing in our system with uniform N dispersion throughout the catalyst. The hierarchical and fibrous structures allow easy channelization of electron as in the case of nanotubes for effective surface charge transfer. Along with macroporosity, nitrogen incorporation and mesoporosity play some important roles for enhanced photoactivities.     

Sol-gel SiO2/TiO2 bilayer films with self-cleaning and antireflection properties

We report here that a facile sol-gel dip-coating technique can be used to fabricate a SiO₂/TiO₂ bilayer film with self-cleaning and antireflection properties. The bottom SiO₂ layer acts as an antireflection coating due to its lower refractive index; the top TiO₂ layer acts as a self-cleaning coating generated from its photocatalysis and photo-induced superhydrophilicity. The maximal transmittance of SiO₂/TiO₂ bilayer film at normally incident light can be reached 96.7%, independent of the high refractive index and coverage of TiO₂ nanoparticles. However, the photocatalytic activity of the bilayer film shows a close dependence on coverage of TiO₂ nanoparticles. After illuminated by ultraviolet light, the SiO₂/TiO₂ bilayer films are superhydrophilic with water contact angle less than 2°, which favors greatly the self-cleaning function of the films.     

TiO2 and TiO2–SiO2 thin films and powders by one-step soft-solution method: Synthesis and characterizations

Simple soft-solution method has been developed to synthesize films and powders of TiO₂ and mixed TiO₂–SiO₂ at relatively low temperatures. This method is simple and inexpensive. Furthermore, reactor can be designed for large-scale applications as well as to produce large quantities of composite powders in a single step. For the preparation of TiO₂, we used aqueous acidic medium containing TiOSO₄ and H₂O₂, which results in a peroxo-titanium precursor while colloidal SiO₂ has been added to the precursor for the formation of TiO₂–SiO₂. Post annealing at 500 °C is necessary to have anatase structure. Resulting films and powders were characterized by different techniques. TiO₂ (anatase) phase with (1 0 1) preferred orientation has been obtained. Also in TiO₂–SiO₂ mixed films and powders, TiO₂ (anatase) phase was found. Fourier transform infrared spectroscopy (FTIR) results for TiO₂ and mixed TiO₂–SiO₂ films have been presented and discussed. The method developed in this paper allowed obtaining compact and homogeneous TiO₂ films. These compact films are highly photoactive when TiO₂ is used as photo anode in an photoelectrochemical cell. Nanoporous morphology is obtained when SiO₂ colloids are added into the solution.     

Electron microscopy studies of TiO2 micelles

The microstructural properties of nanosized TiO₂ micelles, generated in situ in a water-in-oil (w/o) microemulsion composed of water, dioctyl sulfosuccinate sodiun salt (AOT) and cyclohexane, by controlled hydrolysis of TiCl₄, were investigated. The samples were characterized by analytical electron microscopy combining electron diffraction. TiO₂ film consisted of isotropic grains of anatase. The grain sizes were 5 nm, which is in agreement with the average grain size R, previously obtained by grazing-incidence wide-angle X-ray diffraction (GIWAXD), of 5.0±1.3 nm. Grains were gathered in a number of clusters, differently oriented with respect to the electron beam.     

Oxidative photoelectrochemical technology with Ti/TiO2 anodes

Rutile and anatase TiO₂ films have been grown on Ti plates by thermal (500–800°C) and anodic oxidation followed by thermal annealing (400–500°C), respectively. The photoelectrochemical efficiency of these photoanodes, evaluated by current density measurements in the photooxidation of 4-methoxybenzyl alcohol in deaerated CH₃CN, has been determined. The photocurrent efficiency increases with the thickness of the TiO₂ rutile film up to 1 μm (the most efficient thickness). At the wavelengths furnished by the irradiation apparatus similar thicknesses of anatase and rutile films show nearly the same efficiencies. Anodic bias produces similar relative increases of current intensity in both crystalline forms.