High-contrast electrochromic multilayer films of molybdenum-doped hexagonal tungsten bronze (Mo_0.05–HTB)

Abstract

The creation of new materials for lightweight and high performance displays is an extremely active research field. Inorganic electrochromic ultra-thin film displays possess a keen advantage over other technologies because of their multiple color, fast switching, improved durability, high contrast, high coloration efficiency and low cost. Here we present the successful fabrication of multiple colored electrochromic multilayer films consisting of molybdenum-doped hexagonal tungsten bronze formulated as Na_0.2Mo_xW_1−xO₃·ZH₂O (x = 0.05) (Mo_0.05–HTB), using a layer-by-layer (LBL) assembly method. The resultant organic–inorganic films exhibit a linear increase in film thickness with assembly progressing. The color and electrochromic contrast of the films are adjustable by changing the mole ratio of Mo to W. Electrochromic contrast, coloration efficiency, switching speed, stability and optical memory for the as-prepared films were carefully investigated. The Mo–HTB/polyethyleneimine films undergo a color transition from colorless to light blue, and finally to dark blue over the potential range from −1.0 to 1.0 V vs Ag/AgCl reference electrode. These results exhibit the validation of an LBL-assembly based intermixing strategy for the design of multiple-hue electrochromic films.

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1. Introduction

In recent years, the subsequent deposition of nanoparticle and charged polyelectrolytes has become a new procedure for fabricating well-defined thin films. The interest in assembling nanoparticle into thin films by so-called electrostatic layer-by-layer self-assembly (ELSA) method rests on the simplicity of layer formation, the excellent thickness control, and its broad applicability.¹ The most importance of this method is that the nanoparticle is immobilized on the substrate while its main functions and properties did not changed.² By this method, the nanocomposite fabrication of organic polymer and inorganic nanoparticles has been studied due to their potential applications.

Polyoxometalates (POMs) attract increasing attention worldwide due to their intriguing structures and diverse properties.^3,4 Polyoxometalates represent a well-known class of structurally well-defined metal oxide clusters with a wealth of topological, chemical and physical properties, and are of wide application in different fields such as catalysis,⁵ medicine,⁶ and materials sciences.⁷ One of the most important properties of these metal oxide clusters is that they can accept electrons to become mixed-valency colored species (“poly blue” or “heteropoly blue”), which make them suitable for photochromic and electrochromic materials.^8,9 Many researches focus on the electrochromic and photochromic films containing POMs.^10,11 Although a lot of electrochromic materials, such as transition metal oxides,^12,13 Prussian blue,¹⁴ viologens,¹⁵ conductive polymers,¹⁶ lanthanide complexes,¹⁷ transition metal complexes and metal phthalocyanines,^18,19 have received considerable attention, these materials have limitations in application because of the lifetime requirements, slow response for achieving full contrast, their cost-intensive production process and high price. Furthermore, the films construction process of these materials is time consuming.²⁰

In this paper, we present a rational way to incorporate molybdenum-doped hexagonal tungsten bronze (Mo_x–HTB) into the ultrathin multilayer films with polyethyleneimine (PEI). Using the Mo_x–HTB as electrochromic materials, the [Mo_x–HTB/PEI]_n film could display color changing from bleach, light blue to dark blue. And their high contrast, suitable response time, improved durability, high coloration efficiency and low operation potential should be promising to meet the requirement for developing flexible displays and electrochromic devices.

2. Experimental

2.1 Materials

The PEI (25000 M_w, Sigma Inc.) was used as received. The polyelectrolytes were dissolved in MilliQ-filtered deionized (DI) water and then pH was adjusted to 7 with diluted HCl solution. The concentration of the PEI solution is 1 × 10⁻³ M. The quartz substrates were purchased from BaiTa Quartz and Glass Apparatus Company, Jiangsu Province. The ITO substrates were obtained from the Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences. All other reagents were of reagent grade. Elemental analyses were undertaken by energy dispersive spectroscopy (EDS) on a JEOL JSM-840 operated at 20 kV. XRD spectra were collected on a Rigaku D/max-IIB X-ray diffractometer in the range of 5–70° (2θ), employing Cu Kα radiation (1.542 Å). UV–vis spectra were collected on a 756CRT UV–visible spectrometer. Cyclic voltammograms, double-potential steps and other electric chemistry measurements were undertaken on a CHI 660 electrochemical workstation, conventional three-electrodes, Ag/AgCl, Pt. All the voltages involved in the article were vs Ag/AgCl reference electrode.

2.2 Nanoparticle synthesis

The hexagonal tungsten bronze nanoparticles were prepared according to the methodology of Griffith and Luca.²¹ Typically, a Na₂WO₄ solution (10 ml, 1.0 M) was acidified with HCl (1.0 M) to the pH range of 1.65–1.75. Hydrothermal treatments (155 °C, 48 h) of the resultant solution were undertaken in Teflon-lined Parr acid digestion bombs to afford a pale green solid which was washed with deionized water until the pH of the eluant was neutral, and dried to constant weight at 75 °C in air. The preparation of molybdenum-doped phases HTB was undertaken by adding Na₂MoO₄ solution (1.0 M) to the Na₂WO₄ solution prior to acidification and then hydrothermal treatment of the resulting solution for 24 h. Powder X-ray diffraction patterns of the Mo–HTB phases in the range 5–70° (2θ) is shown in Fig. 1, which is in good agreement with the XRD patterns for Na_0.2Mo_xW_1−iO₃·ZH₂O (x < 0.15) presented in ref. 21. The EDS spectroscopy shows that the mole ratio of Mo to W is 0.05 : 0.95, and the structure formula of the molybdenum-doped hexagonal tungsten bronze should be presented as Mo_0.05–HTB.

Fig. 1 Powder X-ray diffraction patterns of the Mo_0.05–HTB phases in the range 2–70°.

2.3 Preparation of the film

The quartz and ITO substrates were rinsed in methanol, acetone and DI water for 15 min each. The quartz substrates and ITO-coated glass were cleaned by immersing into a Piranha solution (H₂SO₄ : H₂O₂ = 7 : 3 v/v) for a few minutes and then in DI water for 30 min. The Mo_0.05–HTB nanoparticle was assembled at pH 2.7 with PEI by LBL. The fabrication of the multilayer films were carried out as follows: the cleaned substrates were immersed in PEI solution for 20 min, rinsed with DI water, and dried under a nitrogen stream. Then the PEI-coated substrates were exposed to Mo_0.05–HTB composite and PEI solutions for 20 min, respectively. Water rinsing and nitrogen drying steps were performed after each adsorption step.

UV–vis spectroscopy was used after each bilayer-deposition to monitor the layer-by-layer assembling process of the films. The LBL growth of [Mo_0.05–HTB/PEI] films under these conditions is linear, as shown in Fig. 2. Each layer grows rapidly and could reach the maximum absorbance only within 20 min, and the film could be observed by naked eye just after one pair of layers of assembly.

Fig. 2 UV–vis spectra of Mo_0.05–HTB in solution and Mo_0.05–HTB/PEI films. In the insert the absorbance at 600 nm was plotted vs the number of bilayers.

3. Results and discussion

3.1 Electrochemistry: cyclic voltammetry

Electrochemical measurements were conducted in a standard buffer solution (pH 6.8) containing 0.1 M CH₃COONa and CH₃COOH. The working electrode, the counter-electrode, and the reference electrode were Mo_0.05–HTB/PEI coated ITO glass, platinum wire, and Ag/AgCl electrode, respectively. Fig. 3 shows the cyclic voltammogram of the films modified on ITO electrode. The cyclic voltammogram displays two pairs of bad shaped redox peaks. One is at 0.285 V (A₁) and −0.321 V(C₁) and, another is at −0.318 V (A₂) and −0.714 V (C₂). The peaks of A₁ and C₁ could be designated to the reduction of Mo(VI) → Mo(V) and the peaks of A₂ and C₂ could be designate to the reduction of W(VI) → W(V).

Fig. 3 Cyclic voltammograms of the [Mo_0.05–HTB/PEI]₄ films on ITO electrode.

3.2 Electrochromism investigation

For the multilayer films [Mo_0.05–HTB/PEI]_n (n = 4), the Mo_0.05–HTB is reduced to display light blue at ca. −0.3 V, and finally dark blue at ca. −1.0 V. This blue color is bleached when more positive potentials are applied, as shown in Fig. 4. With decreasing potential, the reduction degree of Mo_0.05–HTB film is deepened, and the colors of the films turn to dark blue quickly. The visible spectra also show exponential increases of the absorbance from 400 nm to 800 nm at applied potential of −1000 mV for the [Mo_0.05–HTB/PEI]₄ film, while the plot of absorbance vs. wavelength is almost flat at 1000 mV, as shown in Fig. 5. Another interesting feature of the [Mo_0.05–HTB/PEI] film is that its colors and electrochromic contrast can be adjusted by changing the mole ratio of Mo to W. Increasing the mole ratio of Mo could enhance the electrochromic contrast of the films. The colors of the HTB films can also be tuned by incorporating with other transition metal atoms, such as Co, Cu, Fe, Mn, Zn, Cd, etc, in to HTB. Cu_x–HTB, Fe_x–HTB and Co_x–HTB present colors of light green, orange and pink, respectively. The color range of these transition metal–HTB is larger than that of Mo_x–HTB. More detailed research on these films is underway and will be presented in the near future. The results confirm the feasibility to successfully fabricate multicolor films based on nanoparticles of HTB species.

Fig. 4 Electrochromic photos of the [Mo_0.05–HTB/PEI]₄ film taken after the film had been immersed for 30 s in an electrochemical cell at the noted potential; the electrolyte meniscus is visible on the upper portion of the file. Cell conditions were identical to CV conditions.

Fig. 5 Visible absorbance spectra of the [Mo_0.05–HTB/PEI]₄ film, applied potentials of 1000 mV and −1000 mV respectively.

There are some important parameters in identifying and characterizing the electrochromic materials. These parameters generally are electrochromic contrast, coloration efficiency, switching speed, stability and optical memory.²²

The electrochromic contrast was reported as a percentage of transmittance change (Δ%T) at a specified wavelength where the electrochromic material has the highest optical contrast. From Fig. 5 we can see that the electrochromic contrast is almost 72.3% at 800 nm. Gao et al. reported electrochromic films based of POMs, which were ionized in solution prior to the assembly of LBL films.²⁰ The electrochromic contrasts of the {P₂W₁₇/[Cu^II(phen)₂]}₃₀ and {P₂W₁₇/[Fe^II(phen)₃]}₃₀ films at 650 nm are 37.1% and 36.8%, respectively. In our case, the electrochromic contrast of the LBL films are higher since each layer of the films is composed of nanoprticles in which more molecules could exert electrochromic property than those in ordinary cation/anion LBL films. DeLongchamp et al. reported electrochromic films composed of Prussian blue (PB) nanoparticles with average size of 4–5 nm. They fabricated 60 bilayers of [LPEI/PB] and 30 bilayers of [PANI/PB] and the electrochromic contrasts of the [LPEI/PB]₆₀ and [PANI/PB]₃₀ were about 83.5% (at 700 nm) and 63.7% (at 750 nm), respectively.^23,24 While in our work, the electrochromic films are composed of Mo_0.05–HTB nanoparticles with average size of 50–100 nm and only three or four bilayers of [Mo_0.05–HTB/PEI] can reach the electrochromic contrast of 72.3%. When the particles used are too big, say 500 nm, we could not get well defined films. It is also reasonable to deduce that electrochromic films with smaller particles could hardly reach suitable electrochromic contrast unless they are composed of many bilayers. A significant merit in using nanoparticles with suitable size to fabricate electrochromic films with excellent electrochromic contrast is to simplify fabrication process.

The coloration efficiency is a practical parameter to measure the power requirements of electrochromic materials. In essence, it determines the amount of optical density change (ΔOD) induced as a function of the injected/ejected electronic charge (Q_d), i.e., the amount of electronic charge necessary to produce the optical change. The coloration efficiency is given by the equation

η = log[T_b/T_c]/Q_d

in which η is the coloration efficiency at a given λ. T_b and T_c are the bleached and colored transmittance values, respectively. From Fig. 6 we can see that the absorbance at about 800 nm increases 0.6 unit from bleaching to coloration for [Mo–HTB/PEI]₄ films. The transmittance could be given by the equation

T = 10^−A

where T is the transmittance and A is the absorbance at a given λ. From Fig. 6 we can know the values of T_b, T_C (at 800 nm) and Q_d, which are 51.0%, 12.6% and 5.1 mC, respectively. The value of η at 800 nm is therefore 119.1 for the [Mo_0.05–HTB/PEI]₄ films.

Fig. 6 Voltage, current, Coulomb and absorbance at 800 nm of [Mo_0.05–HTB /PEI]₄ modified ITO during subsequent double-potential steps (−1000 to +1000 mV, periods 10 s).

The switching speed could be characterized by the response time of the film, which was investigated by the double-potential experiments as well as the visible absorbance spectroscopy. The input electronic signal is a double-potential squared wave signal, which changes from +1000 mV to −1000 mV, and the period is 10 s, see Fig. 6. If we define that 90% of maximum absorbance is the criterion of the color change accomplishment, the coloration time is almost 4.5 s and the bleaching time is less than 1.5 s. To our knowledge, the switching speed and the bleaching time of the [Mo_0.05–HTB/PEI]₄ film seem to be faster than those of most reported electrochromic films containing inorganic anion and cationalized polymers. For example, in Gao's work,²⁰ the coloration time and the bleaching time of the [P₂W₁₇/[Cu^II(phen)₂]]₃₀ films are 25 s and 15 s, respectively; in DeLongchamp's work,²⁴ the coloration and bleaching time of the [LPEI/PB]₆₀ films are 7 s and 4 s, respectively; in Yagi's work,²⁵ those of WO₃/[Ru(bpy)₃]²⁺/PSS film are 5.5 s and 2.3 s, respectively. In the [Mo_0.05–HTB/PEI]_n film, the quantum size effect of Mo_0.05–HTB nanoparticle increases the speed of electron transfer, and decreases the resistance.²⁶

The stability of the films is usually associated with electrochemical stability since the degradation of active redox couple results in the loss of electrochromic contrast and hence the performance of the EC materials. So we measured the stability of the [Mo_0.05–HTB/PEI]₄ films under two different conditions. First, we measured the absorbance of the films at 800 nm after 100, 200, 1000 and 5000 cycles in the electrochemical cell. From Fig. 7, we could see that after 5000 cycles, there was no significant decrease (less than 10%) in absorbance. In succession, we try to survey the film stability under more rigorous conditions, such as in H₂SO₄ (98%). Fig. 8 shows the curve of the absorbance at 800 nm and 260 nm vs treatment time in H₂SO₄ (98%) of 80 °C. After 60 min treatment, there was no significant decrease in absorbance. We consider that the Mo_0.05–HTB layer acts as a shield layer over PEI, makes the films stable when exposed to corrosive environments.

Fig. 7 Absorbance of visible spectra at 800 nm for [Mo_0.05–HTB /PEI]₄ modified ITO which are recorded after 100, 200, 1000, 5000 cycles of subsequent double-potential steps (−1000 to +1000 mV, periods 10 s).

Fig. 8 Absorbance at 260nm and 800 nm of [Mo_0.05–HTB /PEI]₄ modified ITO (bleached) which were treated in H₂SO₄ (98%) of 80 °C solution in 60 min.

The optical memory is defined as the time the material retains its absorption degree after the electric field is removed. In our work, the self-erasing time of the [Mo_0.05–HTB/PEI]₄ film is no more than 180 s in the CH₃COONa–CH₃COOH (NaAc–HAc) solution. And the self-erasing time is no more than 70 s in the air, as shown in Fig. 9. The self-erasing process was mostly because of the oxidization of the Mo_0.05–HTB by oxygen. We also found that self-erasing process was accelerated when the films were suffered to the radiation of UV rays, in both the NaAc–HAc solution and the air. It could be easily concluded that the UV ray can excite the electrons in the Mo and W cluster, make it easier for them to lose electrons when exposed to oxygen, which was the main motivity of the self-erasing process.

Fig. 9 Self-erasing process at 800 nm of the [Mo_0.05–HTB /PEI]₄ modified ITO in the HAc–NaAc solution and in the air.

Conclusions

A multiple-hue electrochromic film has been developed on the basis of the LBL assembly of two materials: PEI, a colorless conjugated polymer, and an inorganic nanoparticle dispersion of Mo_0.05–HTB whose colors based on the oxidative alteration of charge-transfer. The film materials consisting of Mo_0.05–HTB show good electrochromism with suitable response time, low operation potential, high electrochromic contrast, high coloration efficiency, good color reversibility and stability, suggesting that these materials may become promising candidates for the application of electrochromic materials.

Acknowledgements

This work was financially supported by the Natural Science Foundation of China (No.20271001).

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