Good Electrochromic Properties Research Articles

Electrochemical Fabrication of MoO2/MoO3-Based Photo-Anodes for Water Splitting

After Fujishima and Honda reported their results on the photo-electrolysis of water [1], three materials have received special attention for the fabrication of photo-anodes: (i) titanium dioxide (TiO2), (ii) hematite (α-Fe2O3) and (iii) tungsten trioxide (WO3). These materials present high efficiencies for solar energy capture and conversion to chemical energy, stability under water splitting operating conditions and/or low cost and earth abundance [2]. However, when operating under illumination in the visible range (ca. 45% of the solar spectrum), these semiconductors need of an external potential (bias) to promote the oxygen evolution reaction on its surface, due to the position of their conduction and valence bands [3]. In this context, it has been observed that molybdenum trioxide (MoO3) films have good electrochromic and photochromic properties, while molybdenum dioxide (MoO2) has an unusual metal-like electrical conductivity [4,5]. Therefore, these compounds emerge as an interesting alternative for the fabrication of photo-anodes for water splitting. Photo-anodes are usually fabricated by coating or electrodepositing films of the photo-active material on a substrate (e.g. fluorine-doped tin dioxide(FTO)-coated glass) that acts as current collector and support. The technique chosen has to allow a good control of the structural parameters (particle size, crystallinity and thickness) of the photo-active films produced, since they will affect the photo-electrochemical behavior of the cell [6]. Furthermore, if the photo-anodes are to be produced on a large scale, the production rate and the reproducibility that the fabrication technique offers is crucial. Electrochemical coating processes have been applied successfully in industry for decades. Therefore, this research project aims to produce photo-anodes for water splitting by electrodeposition of MoO2/MoO3 thin films on FTO-coated glass. Fabrication of MoO2/MoO3-based photo-anodes was carried out by applying a potential of -1.377 V vs Ag/AgCl (3 M KCl) during 3 hours on an FTO-coated glass working electrode immersed in a solution containing 0.075 M molybdenum trioxide (MoO3), 0.075 M nickel sulfate (NiSO4·6H2O), 0.5 M tri-sodium citrate (Na3C6H5O7·2H2O) and 0.7 M ammonium hydroxide (NH4OH). The MoO2/MoO3 films produced are ca. 2.3 μm thick, present an irregular microstructure (determined using scanning electron microscopy, SEM) and show a dark brown pigmentation, typical of molybdenum oxides. X-ray photoelectron spectroscopy (XPS) analysis of the samples determined that these films contain molybdenum in at least two different oxidation states (IV and VI), possibly forming MoO2 and MoO3. These results are in good agreement with the energy dispersive X-ray spectroscopy (EDX) analysis of the samples, which showed that the films produced contain approximately 33.7% in weight of molybdenum. Finally, a Tauc plot (Figure 1) was generated using the UV-visible absorption spectra of the samples, which determined that the photo-anodes fabricated present an allowed direct band gap of 2.6 eV. The anodes photo-activity for oxygen evolution was studied by cyclic voltammetry using an electrochemical cell with a 0.1 M Na2SO4 solution as electrolyte. The results obtained (Figure 2) show that in the dark the oxygen evolution reaction on the surface of the MoO2/MoO3-based photo-anodes is triggered at ca. 1.2 V vs Ag/AgCl (3 M KCl), while under UV illumination (wavelength = 364 nm) this phenomenon occurs at ca. 0.6 V vs Ag/AgCl (3M KCl). From the results exposed before, it is clear that the photo-anodes produced present interesting semiconducting and catalytic properties for their application in an electrochemical cell for water splitting. Consequently, future work will be centered in the design and fabrication of a photo-electrochemical cell using these electrodes and improvements will be introduced by heat treatment or doping of the electrodeposited MoO2/MoO3 films. ACKNOWLEDGMENTS The authors thank the Chilean Government for a CONICYT studentship for MGG. REFERENCES Fujishima, A. and Honda, K. (1972). Electrochemical photolysis of water at a semiconductor electrode. Nature, 238, 37-38.Ong, CK. (2013). Design and performance of photo-electrochemical reactors with Fe2O3 photo-anodes for water splitting. PhD thesis, Imperial College London.Xu, Y. and Schoonen, M. (2000). The absolute energy positions of conduction and valence bands of selected semiconducting minerals. American Mineralogist, 85, 543-556.Al-Kandari, H., Mohamed, AM., Al-Kharafi, F., Zaki, MI. and Katrib, A. (2012). Modification of the catalytic properties of MoO2−x(OH)y dispersed on TiO2 by Pt and Cs additives. Applied Catalysis A: General, 417, 298-305.Dukstiene, N., Sinkeviciute, D. and Guobiene, A. (2012). Morphological, structural and optical properties of MoO2 films electrodeposited on SnO2∣glass plate. Central European Journal of Chemistry, 10, 1106-1118.Krol, R. and Grätzel, M. (2012). Photoelectrochemical Hydrogen Production. Springer, Nueva York. Figure 1

Characterization of MAPLE deposited WO3 thin films for electrochromic applications

Tungsten trioxide (WO3) is a widely studied material for electrochromic applications. The structure, morphology and optical properties of WO3 thin films, grown by matrix assisted pulsed laser evaporation (MAPLE) from monoclinic WO3 nano-sized particles, were investigated for their possible application as electrochromic layers. A KrF* excimer (λ=248 nm, ζFWHM=25 ns) laser source was used in all experiments. The MAPLE deposited WO3 thin films were studied by atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXRD) and Fourier transform infrared spectroscopy (FTIR). Cyclic voltammetry measurements were also performed, and the coloring and bleaching were observed. The morpho-structural investigations disclosed the synthesis of single-phase monoclinic WO3 films consisting of crystalline nano-grains embedded in an amorphous matrix. All thin films showed good electrochromic properties, thus validating application of the MAPLE deposition technique for the further development of electrochromic devices.

Effects of oblique angle deposition on optical and morphological properties of WO3 nanorod films for electrochromic application

In this work, the tungsten trioxide (WO3) films were prepared on ITO glass by reactive dc magnetron sputtering. The effects of oblique angle deposition (OAD) between 0 – 85° were investigated for optical and morphological properties of WO3 films and its electrochromic application. The crystal structure, morphologies and optical properties were characterized by grazing – incidence X-ray diffraction, field emission scanning electron microscopy and spectrophotometer, respectively. All WO3 films exhibited amorphous structure and the WO3 nanorod morphologies were demonstrated when the OAD more than 60°. It is owing to the shadowing effect and limits adatom diffusion. The electrochromic property of WO3 films was investigated for colored and bleached states with their optical transmission. It was found that WO3 nanorod structure prepared by OAD exhibited good electrochromic property due to very large effective surface area and more ion insertion in WO3 nanorod films. In addition, WO3 nanorod film prepared by OAD at 80° exhibited an optimum electrochromic property with optical contrast of 62%.

Preparation of PAA/WO3 composite films with enhanced electrochromism via layer-by-layer method

Abstract The polyacrylic acid/tungsten oxide (PAA/WO3) composite films with good electrochromic properties have been prepared by a layer-by-layer method. The porous PAA layers were used as a template for the deposition of WO3, and finally, the composite film showed a network structure with high porosity. The electrochromic performance of the PAA/WO3 composite film was investigated by means of cyclic voltammogram (CV), chronoamperometry (CA), and transmittance measurements. The PAA/WO3 film also exhibited a remarkable electrochromism ability with the reversible color change from transparent colorless to dark blue and the transmittance change from 83% to 24% at 620 nm. This research provided a cost-effective approach for the preparation of PAA/WO3 composite films with controllable microstructure and good electrochromic property.

Enhanced electrochromic performance of WO3 nanowire networks grown directly on fluorine-doped tin oxide substrates

A WO3 nanowire film was prepared directly on conducting glass and showed good electrochromic properties.

Synthesis and electrochemical properties of a novel poly(ether sulfone) with oligoaniline pendants

A novel poly(ether sulfone) with oligoaniline pendants(PESOP) was prepared by K2CO3-mediated nucleophilic polycondensation and characterized with Fourier-transform infrared spectrometry(FTIR), 1H nuclear magnetic resonance(1H NMR) and gel permeation chromatography(GPC). The cyclic voltammetry(CV) curve of PESOP reveals the reversible electroactivity. In the electrochromism study, PESOP film exhibited good electrochromic properties with a high contrast value, moderate switching time and acceptable coloration efficiency. After running 1000 cycles, the PESOP/indium-tin oxide(ITO) electrochromic device remained unchanged except for the drifting of base line, which certified its good electro-photo stability of the fabricated electrochromic device. Moreover, the polarization technique and electrochemical impedance spectroscopy(EIS) were used to study the anticorrosion performance of PESOP coatings on the cold rolled steel(CRS) in a 5%(mass fraction) NaCl solution. All the results indicate that the obtained PESOP is a competitive candidate as anticorrosion material.

English

Recent developments in the synthesis of transition metal oxides in the form of thin films have opened up opportunities in the construction of electrochromic devices with enhanced properties. Transition metal oxides like WO3 and MoO3 have good electrochromic properties and these oxides can also change their optical properties when a voltage pulse is applied. These electrochromic materials are used for displays, gas sensors, rear-view mirrors and smart windows for energy saving applications. There are numbers of different techniques used for the deposition of stable thin films of these oxide materials. Tungsten Oxide (WO3) is the best suited material for energy conservation applications. Thin films of WO3 are deposited by various techniques that offers flexibility, allows the fabrication of required topographical, physical, crystallographic, desired geometrical and metallurgical structures. This review paper is aimed to summarize applications of WO3 thin films as an electrochromic material and the effect of various deposition techniques on its electrochromic and optical properties.

The effects of heat treatment on optical, structural, electrochromic and bonding properties of Nb2O5 thin films

Nb2O5 thin films were deposited onto heated glass substrates by RF magnetron sputtering using a Nb2O5 target. The films were annealed in air at temperatures between 400 and 700 °C for 6 h. Effects of the crystalline structure on optical, structural, electrochromic and bonding properties of the Nb2O5 thin films were investigated by X-ray diffraction, atomic force microscopy, X-ray photoelectron spectroscopy, optical spectrophotometry and electrochemical measurements. The film refractive index varied between 2.09 and 2.22 at the wavelength of 550 nm depending on the annealing temperature. The decrease of the optical band gap revealed for the films with increasing annealing temperature is attributed to oxygen-ion vacancies in the film structure. The orthorhombic structure of Nb2O5 films resulted in good electrochromic properties with high colouration efficiencies of 19.56 cm2/C and 53.24 cm2/C at 550 nm and 1000 nm, respectively. The optical, structural and electrochromic properties of the different crystalline polymorphic forms of the Nb2O5 films make them attractive for optical applications.

Synthesis and anticorrosive properties of a novel electroactive polyurea containing oligoaniline pendants

A novel polyurea containing oligoaniline pendants (PU-p-OA) was synthesized by one-step synthetic route. The molecular structure of PU-p-OA was confirmed by Fourier transform infrared spectra (FTIR), nuclear magnetic resonance (NMR), and gel permeation chromatography (GPC), and its thermal stability was probed via thermal gravimetric analysis (TGA). Due to the introduction of the oligoaniline segments, PU-p-OA showed expected spectroscopic properties and reversible electroactivity. The electrochromic behavior of a PU-p-OA/ITO electrode was investigated by spectrochronoamperometry in detail, exhibiting good electrochromic properties with high contrast value. Moreover, the inhibition effect of the electroactive PU-p-OA coatings on the cold rolled steel (CRS) in 5 wt% sodium chloride solution was investigated by Tafel plot analysis and electrochemical impedance spectroscopy. Furthermore, comparative anticorrosive study among PU-p-OA, normal polyurea (NPU), parent aniline tetramer (PAT), and NPU/PAT blend was accomplished, which indicated that the obtained PU-p-OA was a good corrosion inhibitor for CRS in brine medium.

Syntheses and Applications of Inorganic Electrochromic Materials and Devices

Tungsten oxide (WO3) nanorods were synthesized by a hydrothermal process. Transparent film was fabricated by an aggregation-deposition process of WO3 nanorods on transparent conductive glass. The as-synthesized WO3 nanorods exhibit good electrochromic properties. WO3 nanopaltes were directly grown on transparent conductive glasses by a crystal-assisted hydrothermal process, showing enhanced response and coloration efficiency. These properties of WO3 nanostructures endow their promising application in large-area smart windows. Prussian blue films were fabricated and used in color-changeable betteries. A bi-functional Prussian blue/Aluminium device has been demonstrated for self-powered electrochromic window and self-rechargeable transparent battery. Without applying any external bias, Prussian blue can be bleached to Prussian white by connecting the Prussian blue and Al electrodes. After bleaching, the blue colour of the device can be spontaneously recovered. In this regard, the designed device can be used as an energy-saving electrochromic smart window which requires no external power source. The as-prepared device can also be used as a battery with a high output voltage. Most importantly, after discharging, such a battery can automatically self-recharge. The as-prepared device exhibits promising applications in broad fields, for example, as a self-powered smart window to perform indoor light and heat management, and as a self-rechargeable transparent battery to power some delicate devices.

Synthesis of NiO and NiO/TiO2 films with electrochromic and photocatalytic activities

NiO and NiO/TiO2 films were prepared on fluorine doped tin oxide (FTO) coated glass substrates by electrochemial deposition and sol–gel method. Choline chloride–urea ionic liquid was used as electrodeposition media. The composition, morphology and microstructure of the films were characterized by a scanning electron microscope (SEM) and X-ray diffractometer (XRD). Cyclic voltammetry (CV), cycle life, response time and transmittance of the films were measured and compared with each other. Photocatalytic activity of the NiO/TiO2 film was examined and explained through the degradation of methyl orange. Results show that, after 8000 cycles, both NiO and NiO/TiO2 films still maintain their good electrochromic property and stability. The NiO/TiO2 film not only performs excellent electrochromic function but also displays a certain photocatalytic activity. And interestingly, the NiO/TiO2 film at bleached and colored states demonstrates different photocatalytic activities, which are higher than that at as-prepared state.

Synthesis and electrochemical properties of electroactive hyperbranched poly(aryl ether ketone) bearing oligoaniline segments

A novel electroactive hyperbranched poly(aryl ether ketone) (EHPAEK) with oligoaniline segments has been prepared by K2CO3-mediated nucleophilic aromatic polycondensation. The structure of EHPAEK was confirmed by Fourier-transform infrared spectra (FTIR), nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC). Its thermal stability and spectroscopic properties were also studied using thermogravimetric analysis (TGA) and UV–vis spectroscopy. Due to the oligoaniline segment, EHPAEK was found to be electroactive, which was explored by cyclic voltammetry in 0.5M H2SO4. The electrochromism of EHPAEK thin film was examined using EHPAEK/ITO as the working electrode coupled with UV–vis spectroscopy, which exhibited good electrochromic properties with high contrast value, moderate switching times, acceptable coloration efficiency. Furthermore, the Tafel plots analysis and electrochemical impedance spectroscopy was applied to study the anticorrosion of the EHPAEK coatings on the cold rolled steel (CRS) in 3.5wt% NaCl electrolyte solution. The enhanced corrosion protection ability of the EHPAEK coatings was ascribed to the redox catalytic capabilities of the oligoaniline segment.

A WO3 Nanoporous-Nanorod Film Formed by Hydrothermal Growth of Nanorods on Anodized Nanoporous Substrate

A tungsten oxide (WO3) nanoporous-nanorod film was produced for the first time by hydrothermally growing WO3 nanorods on nanoporous WO3 substrate. The nanoporous substrates were prepared using the anodization method; hydrothermal reaction was performed on as-anodized and annealed (200–500°C) nanoporous substrates for nanorod growth. The as-anodized substrate and the substrates annealed at ≤300°C dissolved after hydrothermal reaction because of the amorphous and low crystallinity behaviors of the WO3 substrates. However, the substrates annealed at ≥400°C did not dissolve during the hydrothermal reaction; instead, nanorods were grown on the substrates, forming a nanoporous-nanorod structure. The WO3 nanoporous-nanorod film (nanorods grown on substrate annealed at 400°C) showed good electrochromic properties with high current density (−13.22 and +7.30 mA cm−2), good cycling stability, and short coloring (∼6 s) and bleaching (∼3.4 s) times. These good properties are attributed to the combination of two nanostructures (nanoporous and nanorods) that contributed to a large WO3 surface area.

Electroactive polyurea bearing oligoaniline pendants: Electrochromic and anticorrosive properties

In this study, a novel polyurea with oligoaniline pendants was prepared in a single step from readily accessible monomer. Fourier-transform infrared spectra (FTIR), nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) were employed to characterize the molecular structure of the obtained electroactive polyurea (EPU). Its spectroscopic properties and thermal stability were also detected and evaluated. The electrochemical activity of EPU was explored by cyclic voltammetry in 0.5 M H2SO4 confirming a surface controlled process. The spectrochronoamperometry was applied to investigate the electrochromic performance of EPU/ITO electrode, which exhibited good electrochromic properties with high contrast value, moderate switching times, acceptable coloration efficiency and excellent stability. Furthermore, corrosive protection of the EPU coatings on the cold rolled steel (CRS) in 5 wt% NaCl solution were studied by tafel plots analysis and electrochemical impedance spectroscopy. The results indicated that the synthesized EPU is a good candidate for inhibiting the corrosion of CRS in brine medium.

Synthesis and electrochromic properties of polybismaleimides containing triphenylamine units

The bismaleimide (BMI) monomer containing triphenylamine was prepared firstly. Then BMI was reacted with different diamines by Michael addition to obtain prepolymers, respectively. Finally, polyimides (PIs) were obtained through melting procedure. The effects of the structure of BMI moiety and thermal curing condition on thermal stability of PIs were studied by thermogravimetric analysis. PIs have excellent processing properties for film casting. Cyclic voltammetry of PI films was carried out on an indium-doped tin oxide-coated glass substrate. Results which exhibit two reversible oxidations at 0.71–0.78 and 1.06–1.16 V vs Ag/AgCl. The electrochromic performance was investigated by spectroelectrochemical methods. These anodically coloring polymer films not only showed good electrochromic properties but also exhibited high optical contrast ratio of transmittance with a color changing from yellow to green. After 100 cyclic switches, the polymer films still retained excellent redox and electrochromic activity.

Multichromic polymers containing alternating bithiophenes derivatives and 4-cyanotriphenylamine unit and their application for electrochromic devices

Two structural properties related monomers 4-di(4-(2-thienyl)phenyl)aminobenzonitrile (M1) and 4-di(4-(2-(3,4-ethylenedioxythiophene))phenyl)aminobenzonitrile (M2) were successfully synthesized. Polymer P1 and P2 were characterized successfully, which were performed by cyclic voltammetry (CV), UV–vis spectroscopy and scanning electron microscopy (SEM). Spectroelectrochemical analysis revealed that the polymers film had good electrochromic properties and exhibited multi-electrochromic behavior. The polymer films showed reversible electrochemical oxidation accompanied by strong color changes with a high contrast ratio, rapid switching time and high coloration efficiency. Besides, it is interesting to find that the P1/PEDOT device and P2/PEDOT device displayed two colors (yellow, dark-blue and green, blue) belonging to primary colors, respectively. The corresponding devices based on P1 or P2 and PEDOT showed reasonable optical contrast (ΔT%), fast response time and excellent cyclic voltammetry stability; hence they are promising candidates for various applications.

Tuning the Fluorescent Response of a Novel Electroactive Polymer with Multiple Stimuli

A novel polymer featuring oligoaniline pendants that exhibits reversible electroactivity and good electrochromic properties with high contrast value, acceptable switching times, and excellent coloration efficiency is presented. This polymer can undergo reversible changes in fluorescence in response to reductive and oxidative chemical stimulus, pH, and electrical potential. The fluorescence switching operation shows reasonable reversibility and reproducibility when subjected to multiple stimuli. In this elegant fluorescence switching system, the oligoaniline pendants are used as fluorophore and regulatory units simultaneously.

Synthesis and properties of multifunctional poly(amic acid) with oligoaniline and fluorene groups

A novel multifunctional poly(amic acid) bearing oligoaniline, fluorene groups (PAAOF) has been prepared through the one-step synthetic route. The structure of PAAOF was confirmed via nuclear magnetic resonance (NMR), Fourier-transform infrared spectra (FTIR), and gel permeation chromatography (GPC). Moreover, the electrochemical measurement results revealed that PAAOF material have an expected electrochemical activity, and good electrochromic properties with high contrast value and satisfactory coloration efficiency. The photophysical properties of the as-synthesized PAAOF at various oxidation states were studied. The results indicated that the fluorescence of PAAOF could be tuned by modulating the oxidation states of oligoaniline segments. In the fluorescence tuning, the fluorene groups are fluorophore, and the oligoaniline segments are used as regulatory unit.

Electrochromic Properties of WO3-MoO3 Nanocomposite Films Prepared by Electrodeposition Method

WO3-MoO3 nanocomposite films with good electrochromic properties were prepared by the electrodeposition method. The surface morphologies and composition of the composite films were characterized by scanning electron microscope (SEM) and energydispersive X-ray analysis (EDS). Spectroelectrochemistry and electrochromic properties of the WO3-MoO3 nanocomposite films were characterized using cyclic voltammetry (CV) and spectral optical absorbance. The effect the content of MoO3 in the composite films on the electrochromic properties has been studied. It has been observed that the electrochromic properties of the composite films could be adjustable by changing the MoO3 quantity in the composite films.

Multifunctional hyperbranched polyamide: Synthesis and properties

A novel multifunctional hyperbranched polyamide bearing oligoaniline, azobenzene and triphenylbenzene (PAOAF) has been prepared through a pyridine/LiCl-mediated acylation reaction. The structure of PAOAF was confirmed via nuclear magnetic resonance (NMR), Fourier-transform infrared spectra (FTIR), X-ray diffraction (XRD) and gel permeation chromatography (GPC). The thermal stability was probed via thermogravimetric analysis (TGA). The electrochemical activity of PAOAF was explored by cyclic voltammetry in 1.0 M H2SO4 confirming a surface controlled process. Tunable dielectric property of PAOAF has been accomplished by controlling the isomerization of azobenzene groups using UV/Vis irradiation. The electrochromic performance of a PAOAF/ITO electrode was studied by spectrochronoamperometry in detail, exhibiting good electrochromic properties with high contrast value and satisfactory coloration efficiency. Moreover, the fluorescence of PAOAF was modulated by controlling the oxidation degree of oligoaniline segments because of the energy migration occurring between oligoaniline and triphenylbenzene groups.