Sprayed CeO2 thin films for electrochromic applications

Cerium dioxide (CeO₂) thin films were prepared by spray pyrolysis using hydrated cerium chloride (CeCl₃·7H₂O) as source compound. The films prepared at substrate temperatures below 300°C were amorphous, while those prepared at optimal conditions (T_s=500°C,s=5 ml/min) were polycrystalline, cubic in structure, preferentially oriented along the (2 0 0) direction and exhibited a transmittance value greater than 80% in the visible range. The cyclic voltammetry study showed that films of CeO₂ deposited on ITO pre-coated glass substrates were capable of charge insertion/extraction when immersed in an electrolyte of propylene carbonate with 1 M LiClO₄.These films also remained fully transparent after Li+ intercalation/deintercalation.     

Effect of citric acid on properties of CeO2 films for electrochromic windows

An alcohol based sol–gel process involving cerium chloride heptahydrate and citric acid in different mole ratios has been employed for the deposition of CeO₂ films. The structural, electrochemical, and optical properties of the films have been investigated using a wide range of techniques. Differential thermal analysis has shown the onset of crystallization of CeO₂ at 389 °C. The addition of an additive (citric acid) to the precursor sol has led to homogeneity and also a reduced ion storage capacity in the films. This observation emphasizes on the use of optimum content of the citric acid such that the films are suitable in terms of transparency as well as uniformity characteristics and also exhibit good electrochemical response. As is evidenced by the SEM study, the degree of polycrystalline grain formation in the citric acid derived films is observed to be less. The XPS results have confirmed the presence of Ce⁴⁺ state in the films. The optically passive behavior of the films is affirmed by their negligible transmission modulation upon Li ion insertion and extraction. A higher proportion of citric acid has also resulted in a reduced porosity and diminished crystallite size of the cerianite phase. The effect of the CeO₂ films on the switching kinetics of the tungsten oxide (WO₃) films has revealed an increase in the coloration time of the latter with the diminished crystallite size of CeO₂ nanograins in the former.     

Al2O3-coated nanoporous TiO2 electrode for solid-state dye-sensitized solar cell

This paper reports the preparation of a core-shell nanoporous electrode consisting of an inner TiO₂ porous matrix and a thin overlayer of Al₂O₃, and its application for solid-state dye-sensitized solar cell using p-CuI as hole conductor. Al₂O₃ overlayer was coated onto TiO₂ porous film by the surface sol–gel process. The role of Al₂O₃ layer thickness on the cell performance was investigated. The solar cells fabricated from Al₂O₃-coated electrodes showed superior performance to the bare TiO₂ electrode. Under illumination of AM 1.5 simulated sunlight (89 mW/cm²), a ca. 0.19 nm Al₂O₃ overlayer increased the photo-to-electric conversion efficiency from 1.94% to 2.59%.     

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.     

Cu(OH)2-modified TiO2 nanotube arrays for efficient photocatalytic hydrogen production

Cu(OH)₂/TNAs photocatalyst was prepared by loading Cu(OH)₂ nanoparticles on TiO₂ nanotube arrays (TNAs) using a chemical bath deposition method. The amount of Cu(OH)₂ loaded on the arrays was controlled by the repeated deposition times. The prepared catalyst was used to generate hydrogen under simulated solar light irradiation, and the results demonstrated that the hydrogen yield of Cu(OH)₂/TNAs was 20.3 times that of the pure TNAs. Furthermore, the photocatalytic efficiency for hydrogen production decreased only 5.8% after five cycles, indicating that Cu(OH)₂/TNAs photocatalyst showed excellent stability and reusability. This work presents an applicable and facile method to fabricate a highly active and stable photocatalyst for hydrogen production.     

High electrochromic performance of co-sputtered vanadium–titanium oxide as a counter electrode

This study examined the material and electrochromic properties of vanadium–titanium oxides (V–Ti oxides) as a counter electrode material in electrochromic devices. These oxides were deposited on an ITO substrate using a co-sputtering method at different levels of RF power. Electrochemical experiments of these oxides were carried out using half-cell and semi full-cell tests which are good methods for measuring the potential applied to each electrode. The change in electrochromic properties after 1000 cycles of a semi full-cell test was examined. Reversibility and durability of an electrochromic device were improved by increasing the titanium content in V–Ti oxides.     

Single- and double-layered mesoporous TiO2/P25 TiO2 electrode for dye-sensitized solar cell

Nanocrystalline mesoporous titania (MP-TiO₂) was synthesized by surfactant-assisted templating method using tetraisopropyl orthotitanate modified with acetylacetone and laurylamine hydrochloride as template. The short-circuit photocurrent density (J_sc) of the cell made from MP-TiO₂ was much higher than that of the cell made from commercial P25 titania. The incident photon to current conversion efficiency (IPCE) spectra of thin MP-TiO₂ cell were higher than that of thick P25 cell in the region between 400 and 475 nm but lower than that of thick P25 cell in the red region, because the thickness of thin transparent MP-TiO₂ film was not enough to scatter the light leading to low absorbed spectra in red region. IPCE spectra of MP-TiO₂ can be improved by using the cell made from blended MP-TiO₂ with P25. The cell performance was improved with increasing sintering temperature. Double-layered titania cells were also fabricated to further improve the cell performance by increasing light scattering and amount of adsorbed dye. The solar energy conversion efficiency (η) up to 8.1% was obtained by using the double-layered titania cell sintered at 450 °C for 2 h.     

Preparation of TiO2 nanocrystalline electrode for dye-sensitized solar cells by 28 GHz microwave irradiation

Microwave preparation of TiO₂ nanocrystalline electrode for use in dye-sensitized solar cells is examined. A multi-mode microwave heating system operating at a frequency of 28 GHz is used to produce rapid processing. Well-sintered TiO₂ nanocrystalline thin film is successfully fabricated on transparent conductive FTO glass electrode. Photoelectron energy conversion efficiency of 5.51% is achieved in an electrode prepared by 28 GHz microwave irradiation at 0.7 kW for 5 min.     

A note on fast protonic solid state electrochromic device: NiOx/Ta2O5/WO3−x

In the present investigation, the electrochromic properties of a fast protonic solid state device: NiO_x/Ta₂O₅/WO_3−x prepared at room temperature (300 K) is reported. The non-stoichiometric tungsten oxide thin film is prepared by reactive DC magnetron sputtering technique on ITO coated glass; the oxides of tantalum (300 nm) and nickel (100 nm) are prepared by electron beam evaporation. This proton device has a coloration efficiency of 82.4 cm²/C and coloration and bleaching time of 6 and 5 s, respectively, and a transmittance variation of 60%. The work function of WO_3−x thin films by Kelvin probe in uncolored and colored states are 4.73 and 4.30 eV, respectively.     

Pt modified TiO2 nanotubes electrode: Preparation and electrocatalytic application for methanol oxidation

Pt nanoparticles decorated TiO₂ nanotubes (Pt/TiO₂NTs) modified electrode has been successfully synthesized by depositing Pt in TiO₂NTs, which were prepared by anodization of the Ti foil. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electrochemical methods were adopted to characterize their structures and properties. The Pt/TiO₂NTs electrode shows excellent electrocatalytic activity toward methanol oxidation reaction (MOR) in alkaline electrolyte without UV irradiation.     

Photoactive TiO2/MoS2 electrode with prolonged stability

TiO₂/MoS₂ composites are synthesized by means of a novel and environmentally friendly electrodeposition technique. MoS₂ is prepared via the hydrothermal process from Na₂MoO₄ and CH₄N₂S with the use of different surfactants such as Pluronic, SDBS, CTAB, and PEG-1000. TiO₂ in the form of nanotubes grows on Ti foil via anodization. The physicochemical properties of the obtained materials are studied by means of XRD, Raman spectroscopy, and SEM. MoS₂ forms a layered structure, while TiO₂ features well-defined and highly crystallized nanotubes. The deposition of MoS₂ on the surface of TiO₂ via the electrochemical method is conducted in a three-electrode cell with the use of a water-based suspension of MoS₂ at a pH close to neutral. The morphology and photoelectrochemical properties of the MoS₂-modified TiO₂ photoanodes are studied. The stability of TiO₂/MoS₂ composites is successfully investigated by repeating photocurrent-time characteristics in the period of 8 months. A shift in flat-band potential accompanied by increased photocurrent and hydrogen production indicate a type II heterostructure.     

TiO2(B) as a promising high potential negative electrode for large-size lithium-ion batteries

Needle-like TiO₂(B) powder was obtained from K₂Ti₄O₉ precursor by ion exchange to protons, followed by dehydration. The charge and discharge characteristics of the TiO₂(B) powder were investigated as a high potential negative electrode in lithium-ion batteries. It had a high discharge capacity of 200–250 mAh g⁻¹ at around 1.6 V vs. Li/Li⁺, which was comparable with that of TiO₂(B) nanowires and nanotubes prepared via a hydrothermal reaction in alkaline solution. It showed very good cycleability, and gave a discharge capacity of 170 mAh g⁻¹ even in the 650th cycle. It also had a high rate capability, and gave a discharge capacity of 106 mAh g⁻¹ even at 10 °C.     

Co-deposition of TiO2/CdS films electrode for photo-electrochemical cells

A densely packed TiO₂ thin film onto an indium doped–tin oxide (ITO) substrate was synthesized at room temperature by chemical deposition and a CdS thin film was deposited onto the pre-deposited TiO₂ film by a doctor blade route (powder of CdS was obtained from chemical deposition). TiO₂/CdS film was annealed at 300 °C for 1 h in air for crystallinity improvement. The first grown TiO₂ film was nanocrystalline, whereas the CdS film was polycrystalline as evidenced by X-ray diffraction (XRD) and selected area electron diffraction (SAED). Scanning electron microscopy (SEM) images show formation of mono-dispersed CdS spherical grains onto compact, densely packed spherical nanocrystalline grains of TiO₂. The TiO₂/CdS bilayer film was used in a photo-electrochemical cell as a working electrode, and a platinum electrode as a counter electrode (0.1 M lithium iodide electrolyte) under 80 mW/cm² light illumination intensity.     

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.     

Enhanced hydrogen generation using ZrO2-modified coupled ZnO/TiO2 nanocomposites in the absence of noble metal co-catalyst

Mesoporous ZrO₂-modified coupled ZnO/TiO₂ nanocomposites were prepared by a surfactant assisted sol–gel method. The photocatalytic performance of these materials was investigated for H₂ evolution without noble metal co-catalyst using aqueous methanol media under AM1.5 simulated light. The H₂ evolution was compared with coupled ZnO/TiO₂, TiO₂, ZnO and Degussa P25. The ZrO₂-modified nanocomposites exhibited higher H₂ generation, specifically 0.5 wt.% ZrO₂ loading produced 30.78 mmol H₂ g⁻¹ compared to 3.55 mmol H₂ g⁻¹ obtained with coupled ZnO/TiO₂. A multiple absorbance thresholds at 435 nm and 417 nm were observed with 0.5 wt.% ZrO₂ loading, corresponding to 2.85 eV and 2.97 eV band gap energies. The high surface area, large pore volume, uniform crystallite sizes and enhanced light harvesting observed in ZrO₂-modified nanocomposites were contributing factors for effective charge separation and higher H₂ production. The possible mechanism of H₂ generation from aqueous methanol solution over ZrO₂-modified nanocomposite is presented.     

Aging effect of diethanolamine stabilized sol on different properties of TiO2 films: Electrochromic applications

Diethanolamine derived clear precursor sol has been utilized for the deposition of TiO₂ films annealed at 470 °C for 5 min. Effect of the precursor sol's aging on different properties of the films has been examined in the present study. Films obtained from aged sol have exhibited superior electrochemical (diffusion coefficient—2.46×10⁻¹⁰ cm² s⁻¹) and electrochromic characteristics due to enhanced Li ion insertion upon application of electric field. The aged sol derived films have exhibited a higher optical modulation (40% at 550 nm) between the colored and bleached states. The ion storage capacities of the films derived from freshly prepared and aged sols are 4.1 and 8.1 mC cm⁻², respectively, upon applied voltage of ±1.5 V. X-ray diffraction studies have affirmed an increase in the TiO₂ crystallite size upon the use of aged sol for the deposition of films. FTIR investigations have confirmed the conversion of Ti–O–Ti to Ti–O network in the aged sol derived films. SEM studies have evidenced porosity changes in films obtained from the sol aged for different durations. The index of refraction as measured by the ellipsometry method corroborates the SEM results and shows reduced porosity (pore size—38 nm) in films derived from the sol just reaching the state of gelation. Thickness of the aged sol derived film is measured to be the highest i.e. 350 nm. Energy bandgaps of the films for both direct and indirect transitions tend to decrease as a function of sol's aging.     

Bi-based electrochromic cell with mediator for white/black imaging

We studied electrochemical properties of bismuth deposit, which showed reversible color change from colorless clear to black by electrochemical reaction, toward paper-like electronic imaging device. Bismuth salt in an electrolyte solution is colorless clear, but turned to black by the electrodeposition on an electrode. Namely, bismuth ion (Bi³⁺) dissolved in the electrolyte solution (colorless clear) is electrochemically reduced on the electrode to deposit the Bi metal showing black color. The Bi deposit on the electrode is electrochemically oxidized to dissolve the deposit into the solution. These processes enable the color change between colorless clear and black. However, the stability of the switching between colorless and black state is not sufficient due to less electroactivity of bismuth deposit. To improve the switching stability of the present electrochromic cell, various mediators were employed to add into the electrolyte solution. Reversible white/black display cell has been successfully demonstrated.     

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.     

Brown coloring electrochromic devices based on NiO–TiO2 layers

Brown coloring electrochromic 5×10 cm² windows with the configuration K-glass/NiO–TiO₂/electrolyte/CeO₂–TiO₂/K-glass have been prepared and characterized by optoelectrochemical techniques (cyclic voltammetry, chronoamperometry and galvanostatic measurements). The electrochromic layers have been prepared by the sol–gel technique. As electrolyte either a 1 M aqueous KOH solution or a newly developed starch-based gel impregnated with KOH have been used. The CeO₂–TiO₂ sol–gel layers sintered at 550 °C have been previously characterized in 1 M aqueous KOH electrolyte as a function of the thickness up to 2000 cycles and showed a highly reversible behavior without any corrosion effect. The NiO–TiO₂ sol–gel layers sintered at 300 °C have been extensively characterized in the same electrolyte up to about 7000 cycles. All windows present a deep brown color characteristic of the presence of Ni³⁺ (NiOOH) species, that is fully reversible for several thousands of cycles with a rather-fast kinetics (<30 s). The transmittance of the bleached state however slowly decreases with cycling (permanent coloration). The full-bleached condition can be nevertheless recovered by applying a negative potential for a long duration. Deeper coloration is usually obtained by cycling the windows galvanostatically with a current density of 20 μA/cm². The lifetime of the windows is however limited because of the degradation of the NiO-based layers due to the not fully reversible exchange of OH⁻ that turns the layers mechanically fragile and leads eventually to their complete removal from the substrate. Windows working satisfactorily up to 7000 and 17 000 cycles have been obtained using aqueous KOH electrolyte and starch KOH gel electrolyte, respectively. Memory tests showed that the devices bleach at the open circuit potential from T=39% (colored state) to about T=50% in 60 min.