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.     

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.     

Effect of preparation conditions of CuO–CeO2–ZrO2 catalyst on CO removal from hydrogen-rich gas

The activities of CuO–CeO₂–ZrO₂ catalysts synthesized by four methods, e.g. sol–gel, co-precipitation, one-step impregnation, and two-step impregnation, were compared for CO removal from hydrogen-rich gas. The influence of the precipitant and calcination temperature on the catalytic activity was investigated, and a series of analytical methods, such as XRD, H₂-TPR, TG-DSC, and SEM, were used to characterize the catalysts. It was indicated that CuO–CeO₂–ZrO₂ catalyst prepared by co-precipitation method exhibits the widest operation temperature range with the 99% conversion of CO and relatively high selectivity. The optimized preparation conditions were confirmed using Na₂CO₃ as a precipitant, and calcining at 500 °C. It was proposed that the high activity and selectivity result from the high dispersion of copper and strong interaction among CuO, CeO₂, and ZrO₂. The effects of precipitants on the grain size and morphology of the catalyst is larger than that of calcination temperature.     

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.     

Comparative studies of “all sol–gel” electrochromic devices with optically passive counter-electrode films, ormolyte Li ion-conductor and WO3 or Nb2O5 electrochromic films

Laminated electrochromic (EC) devices are becoming increasingly important for making “smart” windows and switchable displays. Mostly, polymeric Li⁺ ionic conductors in combination with vacuum deposited active electrochromic and counter-electrode films are used. In this paper we report on the development of all sol–gel EC devices, that is, those where all three internal layers are prepared via the sol–gel route, including the ionically conductive inorganic–organic hybrid (ormolyte). The electrochemical and optical properties of EC devices are presented and the cycling stability and reversibility of their optical modulation assessed. The results show that WO₃/ormolyte/SnO₂ : Mo, WO₃/ormolyte/SnO₂ : Sb, WO₃/ormolyte/SnO₂ : Sb : Mo, Nb₂O₅/ormolyte/SnO₂ : Sb : Mo and WO₃/ormolyte/LiCo-oxide exhibit a transmission modulation dependent on the thickness of the active electrochromic and counter-electrode films and the thickness of the ormolyte layer. Electrochemical and optical properties of individual films are described and correlated with the stability of the all sol–gel EC devices.     

Electrochromic properties of heat-treated thin films of CeO2–TiO2–ZrO2 prepared by sol–gel route

CeO₂–TiO₂–ZrO₂ thin films were prepared using the sol–gel process and deposited on glass and ITO-coated glass substrates via dip-coating technique. The samples were heat treated between 100 and 500 °C. The heat treatment effects on the electrochromic performances of the films were determined by means of cyclic voltammetry measurements. The structural behavior of the film was characterized by atomic force microscopy and X-ray diffraction. Refractive index, extinction coefficient, and thickness of the films were determined in the 350–1000 nm wavelength, using nkd spectrophotometry analysis.Heat treatment temperature affects the electrochromic, optical, and structural properties of the film. The charge density of the samples increased from 8.8 to 14.8 mC/cm², with increasing heat-treatment temperatures from 100 to 500 °C. It was determined that the highest ratio between anodic and cathodic charge takes place with increase of temperature up to 500 °C.     

Tri-layer antireflection coatings (SiO2/SiO2–TiO2/TiO2) for silicon solar cells using a sol–gel technique

Antireflection coatings (ARCs) have become one of the key issues for mass production of Si solar cells. They are generally performed by vacuum processes such as thermal evaporation, reactive sputtering, and plasma-enhanced chemical vapor deposition. In this work, a sol–gel method has been demonstrated to prepare the ARCs for the non-textured monocrystalline Si solar cells. The spin-coated TiO₂ single-layer, SiO₂/TiO₂ double-layer and SiO₂/SiO₂–TiO₂/TiO₂ triple-layer ARCs were deposited on the Si solar cells and they showed good uniformity in thickness. The measured average optical reflectance (400–1000 nm) was about 9.3, 6.2 and 3.2% for the single-layer, double-layer and triple-layer ARCs, respectively. Good correlation between theoretical and experimental data was obtained. Under a triple-layer ARC condition, a 39% improvement in the efficiency of the monocrystalline Si solar cell was achieved. These indicate that the sol–gel ARC process has high potential for low-cost solar cell fabrication.     

Physicochemical properties and photocatalytic hydrogen evolution of TiO2 films prepared by sol–gel processes

Anatase TiO₂ films were obtained on glass substrates using a sol–gel method using titanium isopropoxide as a precursor. The thickness of the film was about 140 nm for one coating, and the thickness is controlled by the number of coating cycles. The spectra of UV-VIS absorption indicated that the absorption edge of the TiO₂ films is ca. 385 nm, corresponding to the band gap energy of 3.20 eV. We obtained TiO₂ films having a high activity for the hydrogen evolution from photocatalytic water cleavage. By loading with 0.3 wt% Pt rate of hydrogen production increases. No influence of film thickness and calcination temperature on the photocatalytic property is observed.     

Comparative studies of “all sol–gel” electrochromic windows employing various counter-electrodes

Electrochromic (EC) “smart” windows for buildings represent an effective way to modulate the intensity of incoming solar radiation. While it is accepted that WO₃ films represent the best option for the working electrode, the choice of the best counter-electrode is still debatable. Optical properties of counter-electrodes such as Ce, Fe, V and Sn oxides are presented. Electrochromic windows were made with a sol–gel WO₃ active colouring film (150°C), Ce, Fe, V oxide counter-electrodes and a sol–gel organic–inorganic hybrid (Li⁺ormolyte) ion conductor. The electrochromic responses of these devices predicted from the charge capacities, photopic transmittances and coloration efficiencies of individual films are compared with measured values.     

Effect of stabilizer on structural, optical and electrochemical properties of sol–gel derived spin coated TiO2 films

Thin films of TiO₂ were prepared using two different sol–gel routes. The two routes employed diethanolamine (DEA) and acetylacetone as stabilizing agents with titanium isopropoxide (Ti(OPrⁱ)₄) in ethanol as the deposition solution. The densification at 500 °C achieved the nanophase TiO₂ films, which were investigated by performing structural, optical and electrochemical studies. Ion storage capacity and transmission measurements showed superior response of the films derived from DEA. Between the films obtained from the two routes, the appearance of the rutile phase at lower temperature for the film synthesized using DEA was predicted on the basis of the thermal analysis of the corresponding xerogel. The nanocrystalline nature of the films was evident from the X-ray diffraction, atomic force microscopy, and scanning electron microscopy. The films deposited from both the stabilizers exhibited electrochromism in 1 M LiClO₄-propylene carbonate electrolyte on cathodic polarization.     

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.     

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.     

Optical and electrochemical properties of CeO2 thin film prepared by an alkoxide route

Transparent CeO₂ thin solid films, used as ion storage layer in electrochromic devices were prepared by the sol–gel method using an alkoxide route combined with the dip-coating technique. The precursor sol was prepared from a mixture of cerium (IV) methoxyethoxide in anhydrous 2-butanol. Electrochemical Li⁺ intercalation/deintercalation was performed by cyclic voltammetry and the results indicate that the CeO₂/LiClO₄ system is electrochemically reversible. The total inserted/extracted charge of the CeO₂ film was determined by chronoamperometric measurements, which showed an ion storage capacity of 14 mC/cm². The solid-state diffusion of lithium ion into the CeO₂ thin films was investigated by electrochemical impedance spectroscopy.     

Electrical and photovoltaic properties of devices based on PbPc–TiO2 thin films

The electrical, optical and photovoltaic properties of organic–inorganic hybrid devices consisting of Al/TiO₂/PbPc/ITO and Al/PbPc/TiO₂/ITO structures have been investigated through analyzing the current–voltage characteristics, optical absorption and photocurrent action spectra of the devices. The combined presence of oxygen, light and an electric field in the photocurrent decay of Al/TiO₂/PbPc/ITO device have been studied. It is observed that under illumination, the oxygen radical anions and excitons are formed, which subsequently drift towards the interface with TiO₂, where an internal electric field is present. The excitons that reach to the interface are subsequently dissociated into free charge carriers due to the electric field present at the interface. The exciton diffusion length for PbPc calculated from the dependence of luminescence with the PbPc film is about 13 nm. We have also studied the effect of PbPc thickness and hole mobility on the device performance of organic photovoltaic device consisting of PbPc as an optically active layer, TiO₂ as the electron–transporting layer and ITO and Al used as electrodes. We have shown that the power conversion efficiency in the device is primarily limited by the short-exciton diffusion length combined with the low-hole mobility in PbPc layer. The model of charge transport in Al/TiO₂/PbPc/ITO device explained the experimental results where the total current density is a function of injected carriers at electrode–organic semiconductor surface, the leakage current through the organic layer and collected photogenerated current that results from the effective dissociation of excitons.     

Selective removal of CO from hydrogen-rich stream over CuO/CexSn1−xO2–Al2O3 catalysts

The solid solutions of Ce_xSn_1−xO₂ incorporated with alumina to form Ce_xSn_1−xO₂–Al₂O₃ mixed oxides, by the suspension/co-precipitation method, were used to prepare CuO/Ce_xSn_1−xO₂–Al₂O₃ catalysts for the selective oxidation of CO in excess hydrogen. Incorporating Al₂O₃ increased the dispersion of Ce_xSn_1−xO₂, but did not change their main structures and did not weaken their redox properties. Doping Sn⁴⁺ into CeO₂ increased the mobility of lattice oxygen and enhanced the activity of the 7%CuO/Ce_xSn_1−xO₂–Al₂O₃ catalyst in the selective oxidation of CO. The selective oxidation of CO was weakened as the doped fraction of Sn⁴⁺ exceeded 0.5. Incorporating appropriate amounts of Sn⁴⁺ and Al₂O₃ could obtain good candidates 7%CuO/Ce_xSn_1−xO₂–Al₂O₃(20%), 1–x=0.1–0.5, for a preferential oxidation (PROX) unit in a polymer electrolyte membrane fuel cell system for removing CO. Its activity was comparable with, and its selectivity was much larger than, that of the noble catalyst 5%Pt/Al₂O₃.     

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.     

Optical properties of sol–gel dip-coated Ta2O5 films for electrochromic applications

Amorphous Ta₂O₅ films were prepared by sol–gel dip process on different substrates. The dip-coating technique was used to prepare amorphous Ta₂O₅ films by hydrolysis and condensation of tantalum ethoxide, Ta(OC₂H₅)₅, precursor. Stable coating solutions were prepared using acetic acid as a chelating ligand and catalyzer. Single layer and multi-layered Ta₂O₅ films were fabricated at a dipping rate of 107 mm/min. The microstructure, stoichiometry and optical properties of these films were investigated as a function of the film thickness. Room temperature CV measurements clearly revealed a protonic conductor behavior for Ta₂O₅ films. Optical properties such as refractive index, extinction coefficient and optical band gap value of the Ta₂O₅ films were calculated from optical transmittance measurements. It was found that the refractive index and extinction coefficient values were affected by the thickness of the coatings. The refractive index at a wavelength of 550 nm increased from 1.70 to 1.72 with increasing film thickness. The optical band gap value (3.75±0.12 eV) of the coating was unaffected by the film thickness. These results indicate that sol–gel-deposited Ta₂O₅ films have a promising application as proton conductors in electrochromic devices.     

Effect of TiO2 mixtures on the optical, structural and electrochromic properties of Nb2O5 thin films

Metal oxide films are important for various optical devices and especially for solar energy materials. TiO₂-mixed Nb₂O₅ thin films have been produced by sol–gel dip-coating method. Several parameters such as heat treatment, thickness, and mixture percentages are studied for the effect of the optical, structural and electrochromic properties of the materials. Optical parameters of the films were calculated through transmission and reflection measurement by a refractive index, extinction coefficient and thickness analyzer. Structural, electrochromic and surface analyses of the films were done by X-ray diffractometer, potentiostat/galvanostat and atomic force microscope systems.     

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.     

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.