Electrochromic windows based on viologen-modified nanostructured TiO2 films

The construction of an electrochromic window based on a modified transparent nanostructured metal oxide film (TiO₂ anatase, 4.0 μm thick) supported on conducting glass (F-doped tin oxide, 10 Ω/square, 0.5 μm thick) is described. The nanostructured TiO₂film is modified by adsorption of a monolayer of the redox chromophore bis-(2-phosphonoethyl)-4,4′-bipyridinium dichloride, the electrolyte is 0.05 M LiClO₄ and 0.05 M ferrocene in γ-butyrolactone and the counter electrode is conducting glass. The performance of the above device (colouration efficiency of 170 cm² C⁻¹ at 608 nm, switching time of 1 s and stability over 10 000 standard test cycles) suggests an electrochromic technology with commercial potential.     

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.     

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.     

Quasi-solid-state dye-sensitized solar cells employing nanocrystalline TiO2 films made at low temperature

Quasi-solid-state dye-sensitized solar cells with enhanced performance were made by using nanocrystalline TiO₂ films without any template deposited on plastic or glass substrates at low temperature. A simple and benign procedure was developed to synthesize the low-temperature TiO₂ nanostructured films. According to this method, a small quantity of titanium isopropoxide (TTIP) was added in an ethanolic dispersion of TiO₂ powder consisting of nanoparticles at room temperature, which after alkoxide's hydrolysis helps to the connection between TiO₂ particles and to the formation of mechanically stable thick films on plastic or glass substrates. Pure TiO₂ films without any organic residuals consisting of nanoparticles were formed with surface area of 56 m²/g and pore volume of 0.383 cm³/g similar to that obtained for Degussa-P25 powder. The structural properties of the films were characterized by microscopy techniques, X-ray diffractometry, and porosimetry. Overall solar to electric energy conversion efficiencies of 5.3% and 3.2% (under 1sun) were achieved for quasi-solid-state dye-sensitized solar cells employing such TiO₂ films on F:SnO₂ glass and ITO plastic substrates, respectively. Thus, the quasi-solid-state device based on low-temperature TiO₂ attains a conversion efficiency which is very close to that obtained for cells consisting of TiO₂ nanoparticles sintered at high temperature.     

Low-temperature synthesis of TiO2 nanoparticles and preparation of TiO2 thin films by spray deposition

Low-temperature (180–240 °C) synthesis of nanocrystalline titanium dioxide (TiO₂) by surfactant-free solvothermal route is investigated. Titanium iso-propoxide is used as the precursor and toluene as the solvent. Different precursors to solvent weight ratios have been used for the synthesis of TiO₂ nanoparticles. For the weight ratios 15/100, 25/100 and 35/100 the X-ray diffractograms show the formation of nanocrystalline TiO₂. The X-ray diffraction and transmission electron microscopy studies shows that the product has anatase crystal structure (for temperatures <200 °C) with average particle size below 15 nm. The films deposited by spray deposition method using these nanoparticles show the crystalline and porous nature of the films. The present method of deposition also avoids the post-treatment (sintering) of the films. The nanoparticles thus prepared and the films can be used for gas sensing and biological applications and also as photo-electrodes for dye-sensitized solar cells.     

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.     

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.     

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.     

Using room temperature ionic liquid to fabricate PEDOT/TiO2 nanocomposite electrode-based electrochromic devices with enhanced long-term stability

In this work, poly(3,4-ethylenedioxythiophene)(PEDOT) was electrochemically incorporated with nano- and mesoporous TiO₂ films to form PEDOT/TiO₂ nanocomposite electrochromic electrodes. TiO₂ films were introduced to enhance the interfacial adhesion of the polymers to the substrates and thus increase the long-term stability of electrodes of electrochromic devices (ECDs). Room temperature ionic liquid (RTIL)- 1-butyl-3-methyl-imidazolium tetrafluoroborate ([BMIM]BF₄) was employed to serve as electrolyte during the entire fabrication processes. With these efforts, the ECDs were found to retain up to 95% of their optical response and electroactivity after 10,000 deep, and double potential steps, exhibiting enhanced long-term stability.     

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.     

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.     

Synergistic effect of Cu2O/TiO2 heterostructure nanoparticle and its high H2 evolution activity

Cu₂O/TiO₂ nanoparticles were prepared by solvothermal method, which formed the heterostructure of Cu₂O/TiO₂. Due to the heterostructure, the H₂ evolution rate under simulated solar irradiation was increasingly promoted. Meanwhile a certain amount of Cu particles which were confirmed by Transmission Electro Microscopy (TEM) and X-Ray Photoelectron Spectroscopy (XPS), formed on the surface of Cu₂O/TiO₂, and the photoactivity was accordingly further enhanced. The stabilized activity was maintained after many times irradiation. It is interesting that after a few hours irradiation the amount of Cu particles on the surface kept unchanged in the presence of Cu₂O and TiO₂. The Cu particles that formed during hydrogen generation reaction play a key role in the further enhancement of the hydrogen production activity. In this study, it is the first time to study the details on the formation of the stable ternary structure under simulated solar irradiation and their synergistic effect on the photoactivity of the water splitting.     

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.     

Electrochemical performance of mesoporous TiO2 anatase

Mesoporous TiO₂ was prepared via a sol–gel method from an ethylene glycol-based titanium-precursor in the presence of a non-ionic surfactant at pH 2. Only the anatase structure was detected after annealing, while the BET specific surface area was measured as being 90 m² g⁻¹ with a rather monomodal pore diameter close to 5 nm. Electrochemical performances were investigated by cyclic voltammetry and galvanostatic techniques. Mesoporous TiO₂ exhibits excellent rate capability (184 mAh g⁻¹ at C/5, 158 mAh g⁻¹ at 2C, 127 mAh g⁻¹ at 6C, and 95 mAh g⁻¹ at 30C) and good cycling stability.     

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.     

Iron doped nanostructured TiO2 for photoelectrochemical generation of hydrogen

This paper describes the photoelectrochemical studies on nanostructured iron doped titanium dioxide (TiO₂) thin films prepared by sol-gel spin coating method. Thin films were characterized by X-ray diffraction, Raman spectroscopy, spectral absorbance, atomic force microscopy and photoelectrochemical (PEC) measurements. XRD study shows that the films were polycrystalline with the photoactive anatase phase of TiO₂. Doping of Fe in TiO₂ resulted in a shift of absorption edge towards the visible region of solar spectrum. The observed bandgap energy decreased from 3.3 to 2.89 eV on increasing the doping concentration upto 0.2 at.% Fe. 0.2 at.% Fe doped TiO₂ exhibited the highest photocurrent density, ∼0.92 mA/cm² at zero external bias. Flatband potential and donor density determined from the Mott–Schottky plots were found to vary with doping concentration from −0.54 to −0.92 V/SCE and 1.7 × 10¹⁹ to 4.3 × 10¹⁹ cm⁻³, respectively.     

Nickel sulfide as co-catalyst on nanostructured TiO2 for photocatalytic hydrogen evolution

In this study, TiO₂ photocatalysts with nickel sulfide cocatalyst are prepared by loading nickel sulfide on TiO₂ with solvothermal synthesis approach. The materials were prepared by glycol solvothermal method using anatase, nickel nitrate, thiourea as precursor. The prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS), and X-ray photoelectron spectroscopy (XPS). This is the first time to report that NiS is used as a cocatalyst with TiO₂ for the photocatalytic production of H₂. The results revealed that the structure and the amount of the cocatalyst loaded on TiO₂ play important roles in the photocatalytic activity of NiS/TiO₂ composite. The maximum evolution of H₂ was obtained when NiS had hexagonal structure with content in the composite of 7 at% in relation to TiO₂. The rate of H₂ evolution was increased up to about 30 times than that of TiO₂ alone.     

Enhanced desorption properties of LiBH4 incorporated into mesoporous TiO2

LiBH₄ nano-particles are incorporated into mesoporous TiO₂ scaffolds via a chemical impregnation method. And the enhanced desorption properties of the composite have been investigated. The LiBH₄/TiO₂ sample starts to release hydrogen at 220 °C and the maximal desorption peak occurs at about 330 °C, much lower compared to the bulk LiBH₄. Furthermore, the composite exhibits excellent dehydrogenation kinetics, with 11 wt% of hydrogen liberated from LiBH₄ at 300 °C within 3 h. X-ray diffraction and Fourier transform infrared spectroscopy are used to confirm the nanostructure of LiBH₄ in the TiO₂ scaffold. This work demonstrates that confinement within active porous scaffold host is a promising approach for enhancing hydrogen decomposition properties of light-metal complex hydrides.     

Optical and structural properties of TiO2 thin films prepared by sol-gel spin coating

Hydrogen is a renewable and non-polluting fuel. Its production from water using renewable energy is an attractive challenge. In this work we report some results on the preparation of titanium oxide TiO₂ thin films for environmental applications such as water photosplitting. TiO₂ thin films have been prepared by spin coating technique of sol precursor onto glass substrates. The deposited films were annealed at different temperatures in air. The X-ray diffraction (XRD) experiments show that the two well-known anatase and rutile phases were observed depending both on the conditions of deposition and on the temperature of annealing. The best conditions of crystallization were found to be around 400 °C in air. The influence of the number of deposited layer on the crystalline quality of the films was investigated. The surface morphology of the deposited film was characterized by atomic force microscopy (AFM) and scanning electronic microscopy (SEM). The UV-Vis-NIR spectroscopy shows that the film exhibits a high transmission around 90%. The best layers were obtained when concentrated (HCl) was added to the sol solutions. The direct band gap of the films was found to be around 3.7 eV, and their refractive index was found to vary from 2 to 2.4.     

High quantum efficiency Pt/TiO2 catalyst for sacrificial water reduction

Platinum photodeposition on TiO₂ from methanolic solution of chloroplatinic acid (CPA) is investigated to determine the conditions that give optimum photocatalytic activity towards dehydrogenation of methanol. Conditions favoring nucleation of Pt islets rather than their autocatalytic growth enhance the catalytic activity. Photoplatinization from idoplatinic acid, adsorbing more strongly on TiO₂ than CPA produced more active Pt/TiO₂ catalysts. The best catalyst prepared from CPA yielded H₂ from 12.5% methanol solution at a quantum efficiency of 23.9% whereas for idoplatinic acid based catalysts, the quantum efficiency increased to 42.5%.