Tungstate nanosheet ink as a photonless and electroless chromic device

Abstract

Cesium tungstate (Cs₄W₁₁O₃₆²⁻) nanosheets coated on an aluminum (Al) substrate turned blue in an aqueous solution, acting as an ink, and rapidly decolorized under air exposure, acting as an eraser. The decoloration rate of Cs₄W₁₁O₃₆²⁻ nanosheets was several orders of magnitude more efficient than that of conventional tungsten trioxide (WO₃) particles, owing to efficient electron transfer in the nanosheet structure.

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1 Introduction

Chromism is a process that causes a reversible color change in materials in response to an external stimulus and is useful for various industrial applications, such as displays, sensors, and data storage devices. Photochromism and electrochromism are transformations that are induced by stimulation with light irradiation and an electric field, respectively.^1–4 Among chromic materials, tungsten trioxide (WO₃) exhibits efficient photochromic and electrochromic property reactivity.^5–8 However, during the coloration of WO₃ under light irradiation, electrons are trapped at deep levels, and the rate of the decoloration process is very slow in air.⁹ As an electrochromic material, WO₃ requires complicated apparatus and an electrode structure, including an electric power supply and a complicated sandwich-type cell with a transparent electroconductive substrate, such as indium-tin-oxide (ITO), and a counter electrode filled with an electrolyte solution.^2,4

Recently, we reported that a WO₃-based chromic device is capable of functioning without light irradiation or an electric power supply.¹⁰ The WO₃-coated aluminum (Al) substrate turns a dark color upon exposure to an aqueous solution, with subsequent exposure to oxygen restoring the original light-yellow color of the material. Although this phenomenon is similar to solvatochromism in organic chromophore molecules,¹¹ the color change in WO₃/Al is due to the galvanic potential between WO₃ and Al, rather than as a result of the polarity of the solvent. This WO₃ based chromic device exhibits high stability against thermal or chemical treatment rather than a solvatochromic device using organic chromophore molecules. Herein, we developed an efficient galvanic chromic device composed of a film of layered cesium tungstate (Cs₄W₁₁O₃₆²⁻) nanosheets, which are transparent in the visible-light range due to the quantum confinement effect. Cs₄W₁₁O₃₆²⁻ nanosheets coated on an aluminum (Al) substrate exhibited a rapid, reversible color change. Cs₄W₁₁O₃₆²⁻/Al turned blue in an aqueous acid solution, and rapidly decolorized under air exposure. In this device, the aqueous solvent acts as an ink, while oxygen exposure acts as an eraser. The color change was due to the galvanic potential between Cs₄W₁₁O₃₆²⁻ and Al, and the decoloration rate of Cs₄W₁₁O₃₆²⁻ is 25-fold higher than that of WO₃ particles. The efficient color change is due to the nanostructure of the Cs₄W₁₁O₃₆²⁻ nanosheets, which promotes effective electron transfer. Thus, the sheet-like structure of Cs₄W₁₁O₃₆²⁻ has suitable properties for constructing a high-performance chromic device.

2 Experimental

2.1 Synthesis of cesium tungstate (Cs₄W₁₁O₃₆²⁻) nanosheets

Layered Cs₄W₁₁O₃₆²⁻ nanosheets were exfoliated from bulk Cs₆W₁₁O₃₆ by a chemical treatment, referring to the previous report.¹² Powder forms of Cs₂CO₃ and WO₃ were mixed at a molar ratio of 3 : 11 and then ground to a fine powder with a mortar and pestle. The mixed powder was calcined at 1173 K for 6 h. After annealing, the resulting dark blue solid (Cs₆W₁₁O₃₆) was ground to powder and 0.500 g of the powder was suspended in 50 mL of a 12 M hydrogen chloride (HCl) solution. After stirring the solution for 24 h at room temperature, the solution was centrifuged at 3000 rpm for 10 min and the acidic supernatant was discarded. Fifty milliliters of a 12 M HCl solution was then added and the suspension was again stirred for 24 h. The solution was centrifuged at 3000 rpm for 10 min and the formed yellow precipitate (protonated cesium tungstate; H₂Cs₄W₁₁O₃₆) was washed six times with pure water. The precipitate was dispersed in 50 mL of an aqueous tetra(n-butyl)ammonium hydroxide (TBAOH; Aldrich Ltd.) solution to achieve neutral conditions. This suspension was shaken for 10 days at room temperature yielding a colloidal suspension with a milky appearance. For comparison, commercial WO₃ colloidal solution (Sekisui-Jushi Ltd.) was used in the present study as a conventional chromic material.

2.2 Thin film fabrication of Cs₄W₁₁O₃₆²⁻ nanosheets on an aluminum substrate

An aluminum (Al) substrate (1 cm × 1 cm × 1 mm) was washed by ultrasonication in ethanol for 10 min and surface contaminants were removed by UV–ozone treatment for 30 min. A colloidal suspension of Cs₄W₁₁O₃₆²⁻ nanosheets was coated on the Al substrate using a dipping method. The samples were then dried in air at room temperature. For comparison, we prepared a WO₃ particle-coated Al substrate by spin-coating of commercial WO₃ colloidal solution.¹⁰

2.3 Measurement and analysis

Transmission electron microscopy (TEM; model JEM-2010, JEOL Instruments, Japan) was used to observe the structure of Cs₄W₁₁O₃₆²⁻ nanosheets using an acceleration voltage of 200 kV. X-ray photoelectron spectroscopy (XPS; model ESCA-5500MT, Perkin Elmer Instruments, Japan) analysis was conducted to determine the chemical states of cesium tungstate using standard Mg Kα X-rays. Cross-sectional images of the thin films were observed using scanning electron microscopy (SEM; model S-4500, Hitachi Co., Tokyo Japan). Surface morphologies of the mono-layered nanosheet film on the mica substrate were observed by atomic force microscopy (AFM; model SPM-9700, Shimadzu Instruments, Japan). AFM measurements were performed in the tapping mode under ambient conditions. Absorption spectra of the Cs₄W₁₁O₃₆²⁻/Al and WO₃/Al samples were recorded using a spectrophotometer (UV-vis, model V-660, Jasco Instruments, Japan) by a diffuse-reflectance method with BaSO₄ as background. Powder X-ray diffraction (XRD) patterns were recorded with a diffractometer (model SmartLab, Rigaku Instruments, Japan) using Cu Kα radiation (λ = 0.15405 nm). Furthermore, XRD patterns of the thin films were recorded using out-of-plane and in-plane methods. Coloration was evaluated after dipping the films in an aqueous HCl solution (pH 2.0) for 5 min using a spectrophotometer as described above.

3 Results and discussion

Layered Cs₄W₁₁O₃₆²⁻ nanosheets were exfoliated from bulk Cs₆W₁₁O₃₆ by a chemical treatment, according to a previous paper.¹² In the present study, we optimized the ratio of the Cs₂CO₃ and WO₃ starting materials to achieve a high production yield of the Cs₆W₁₁O₃₆ crystal structure. Fig. 1(a) shows the XRD pattern of the calcined powder when the molar ratio of Cs/W in the starting materials was 8/11, which is the same condition as described in a previous report.¹² However, all XRD peaks were assigned to CsW₂O₆, which was not a layered structure but formed a three-dimensional tunnel structure. In the present study, the ratio of the starting materials was optimized as Cs/W = 6/11 to achieve a high production yield of the layered Cs₆W₁₁O₃₆ structure. The XRD pattern of Cs₆W₁₁O₃₆ synthesized in the present work is shown in Fig. 1(b). Fig. 1(c) shows the XRD pattern of protonated cesium tungstate (H₂Cs₄W₁₁O₃₆).

Fig. 1 XRD patterns of the powder forms of: (a) bulk CsW₂O₆, (b) layered Cs₆W₁₁O₃₆, and (c) protonated layered H₂Cs₄W₁₁O₃₆.

Fig. 2(a) shows photographs of aggregates of bulk Cs₆W₁₁O₃₆ and protonated H₂Cs₄W₁₁O₃₆ resuspended in water, and Cs₄W₁₁O₃₆²⁻ nanosheets in water (pH 7). As Cs₄W₁₁O₃₆²⁻ nanosheets have an anionic surface they are highly dispersed in water even under neutral conditions. TEM analysis revealed that bulk Cs₄W₁₁O₃₆²⁻ was efficiently exfoliated and formed a sheet-like structure in water. The line profile obtained from an atomic force microscopy (AFM) image indicated that the thickness of the Cs₄W₁₁O₃₆²⁻ nanosheet was about 2.3 nm (Fig. 2(c)), which is consistent with the reported thickness of a monolayer of a Cs₄W₁₁O₃₆²⁻ nanosheet.¹² A colloidal suspension of Cs₄W₁₁O₃₆²⁻ nanosheets was coated on an Al substrate using a dipping method. A cross-sectional scanning electron microscopy (SEM) image of the coated Al substrate is shown in Fig. 2(d). The thickness of the Cs₄W₁₁O₃₆²⁻ nanosheet layer could be controlled by adjusting the concentration of the colloidal suspension and the dipping duration. The thickness of Cs₄W₁₁O₃₆²⁻ nanosheet films was adjusted to ∼500 nm by considering the depth of the space charge layer as discussed below.

Fig. 2 Photographs of colloidal solutions of Cs₆W₁₁O₃₆ and H₂Cs₄W₁₁O₃₆ powders, and Cs₄W₁₁O₃₆²⁻ nanosheets (a). TEM (b) and AFM (c) images of Cs₄W₁₁O₃₆²⁻ nanosheets. The AFM line profile of the Cs₄W₁₁O₃₆²⁻ nanosheet is shown below the AFM image. The cross-sectional SEM image of a thin film of Cs₄W₁₁O₃₆²⁻ nanosheets (d).

In-plane and out-of-plane X-ray diffraction (XRD) patterns of the Cs₄W₁₁O₃₆²⁻ nanosheet films are shown in Fig. 3. Peaks observed in the out-of-plane pattern were assigned to the c-axis of the exfoliated Cs₄W₁₁O₃₆²⁻ plane, while peaks detected in the in-plane pattern were perpendicular to the c-axis. These results indicate that the exfoliated c-plane of Cs₄W₁₁O₃₆²⁻ nanosheets is attached to the substrate and that the thin film is highly oriented in the direction of the c-axis, which is perpendicular to the Al substrate.

Fig. 3 XRD patterns for a thin film of Cs₄W₁₁O₃₆²⁻ nanosheets. Out-of-plane and in-plane XRD measurements are shown.

Structural analysis of the commercial WO₃ particles as a comparison sample was conducted and the results are shown in Fig. 4. The thickness of the commercial WO₃ film was similar to that of the Cs₄W₁₁O₃₆²⁻ nanosheet film (about 500 nm). The XRD pattern of the commercial WO₃ revealed that its crystal phase displayed a monoclinic WO₃ structure, and that of the WO₃ film was polycrystalline without any crystal orientation.

Fig. 4 TEM image of commercial WO₃ particles (a), cross-sectional SEM image of the commercial WO₃-coated thin film (b), and out-of-plane XRD pattern of the thin film of commercial WO₃ (c).

Fig. 5(a) shows UV-vis absorption spectra recorded by a diffuse reflectance method for the WO₃ and Cs₄W₁₁O₃₆²⁻ thin films. Before being immersed in aqueous solution, the WO₃ thin film exhibited optical absorption below 460 nm under atmospheric conditions, a property that is consistent with the band-gap of WO₃. In contrast, the absorption edge of the Cs₄W₁₁O₃₆²⁻ film was observed around 350 nm. These results demonstrate that Cs₄W₁₁O₃₆²⁻ nanosheets have a wider band gap than that of WO₃ particles, owing to the quantum confinement effect of the nanosheet structure. For this reason, the Cs₄W₁₁O₃₆²⁻ nanosheet film appeared transparent, whereas the film of WO₃ particles was yellow. However, after dipping the Cs₄W₁₁O₃₆²⁻ nanosheet and WO₃ films in an aqueous acid solution with a pH value of 2.0 for 2 min, both films turned blue. This coloration effect was induced by the injection of electrons from the Al substrate to the Cs₄W₁₁O₃₆²⁻ nanosheets or WO₃ particles, leading to the reduction of W⁶⁺ to W⁵⁺. Simultaneously with the reduction reaction, protons are injected into the Cs₄W₁₁O₃₆²⁻ and WO₃ crystals.^13,14 Under these conditions, a broad absorption peak around 650 nm was observed for the WO₃ film, whereas the Cs₄W₁₁O₃₆²⁻ nanosheet film absorbed more photons near the infrared region around 800 nm rather than the WO₃ film (Fig. 5(a)). These results indicate that the potential energy of trapped electrons in Cs₄W₁₁O₃₆²⁻ nanosheets is lower than that of the electrons in WO₃ films. We also evaluated the absorption spectra of thin films in a reflectance mode without using an integration sphere unit and could see the obvious color change even in a reflectance mode (see ESI, Fig. S1†).

Fig. 5 UV-vis spectra for thin films before and after acid treatment (a). Decay in color (at 600 nm) for thin films in air (b). The inset of (b) shows the relationship between natural logarithm and time.

After exposure to air, the blue substrates returned to their initial color, because the trapped electrons in W⁵⁺ species reduce oxygen molecules in air. Fig. 5(b) shows the changes in the absorbance value at 600 nm for the decoloration process under air exposure. Notably, the rate of decoloration of the Cs₄W₁₁O₃₆²⁻ nanosheet film is markedly faster than that of the WO₃ film. A plot of the relationship between the natural logarithm of absorbance and time shows that the decay follows a linear trajectory (Fig. 5(b), inset). These results indicate that the decoloration process follows first-order kinetics, a finding that is consistent with the photochromic decoloration process in WO₃.¹⁵ The half-life for the decoloration of Cs₄W₁₁O₃₆²⁻ nanosheets was 25 s, which was 25-fold more efficient than that of WO₃ particles (650 s).

The Al substrate can be generally oxidized under anodic polarization by an electric field more positive than its redox potential (+1.662 V vs. NHE). In contrast, Cs₄W₁₁O₃₆²⁻ and WO₃ are reduced under cathodic polarization at a potential more negative than their flat-band potentials. When a thin film of a metal oxide semiconductor is coated on an Al substrate, a galvanic potential is generated between the film and the substrate. As the galvanic potential is the driving force that initiates the coloration reaction, the thickness of the semiconductor film should be less than that of the space charge layer between the metal substrate and the semiconductor film. We investigated the thickness dependence of the coloring phenomena. When the thickness of the tungstate nanosheet was more than two micrometers, the color of this film became inhomogeneous and blue spots were observed around pinholes and cracks of thin films (see ESI, Fig. S2†). In contrast, the color was homogeneous, when the thickness was 500 nm. These results indicate that the thickness for homogeneous coloration should be less than several hundred nanometers. The distance of the space charge layer has been estimated by using Poisson's equation, and the estimated distance of the space charge layer in the present study is about several micrometers by using a reasonable carrier density of 10¹⁶ cm⁻³ for undoped metal oxide. Our Cs₄W₁₁O₃₆²⁻ and WO₃ films were insulative before coloration, thus the thickness of our film should be less than several micrometers for coloration. Furthermore, the thinner film is better for the diffusion of electrons. But when the thickness of cesium tungstate is too thin, visible light absorption in the thin film is not so large and coloration is not obvious. Therefore, the thickness of cesium tungstate nanosheet films in the present study has been adjusted to ∼500 nm.

In the chromic devices synthesized here, Cs₄W₁₁O₃₆²⁻ or WO₃ is reduced in an acid solution to form color centers, particularly W⁵⁺, together with the intercalation of protons in the crystal lattice,¹⁶ whereas Al is oxidized to form Al³⁺ ions, which dissolve into the aqueous acid solution. Fig. 6 shows XPS spectra of the W-4f orbital before and after coloration of the thin film of the Cs₄W₁₁O₃₆²⁻ nanosheet. The shape of the XPS curve before acid treatment was deconvoluted into two doublets, where the main doublet at 36.0 eV could be assigned to a W-4f_7/2 orbital and the other doublet at 38.1 eV was due to W-4f_5/2, corresponding to the W⁶⁺ oxidation state. As shown in Fig. 6(b), a second doublet appeared after acid treatment at lower binding energies of 34.2 and 36.3 eV, which were assigned to W-4f_7/2 and 4f_5/2, respectively, and corresponded to the W⁵⁺ oxidation state. These results revealed the formation of W⁵⁺ species after dipping the Cs₄W₁₁O₃₆²⁻ nanosheet into aqueous solution. The percentage of W⁵⁺ after acid treatment was 21.4%.

Fig. 6 XPS spectra of the W-4f orbital of the Cs₄W₁₁O₃₆²⁻ nanosheet coated on an Al substrate: (a) before acid treatment and (b) after acid treatment.

In the decoloration process, reduced tungsten species are oxidized and return to their initial color upon exposure to oxygen. Specifically, electrons trapped in tungsten ions in the crystal lattice diffuse to the nanosheet surface where they react with oxygen molecules in air, reconverting the color of the tungsten species to their initial states. As shown in Fig. 5(a), the potential energy of trapped electrons in Cs₄W₁₁O₃₆²⁻ is lower than that of trapped electrons in WO₃, indicating that electrons in Cs₄W₁₁O₃₆²⁻ have higher reduction potential. Furthermore, the diffusion length of trapped electrons in Cs₄W₁₁O₃₆²⁻ is much shorter than that of WO₃ particles, as the thickness of Cs₄W₁₁O₃₆²⁻ nanosheets (2.3 nm) is markedly smaller than the diameter of WO₃ particles (several tens of nm). Thus, the high coloration efficiency observed for Cs₄W₁₁O₃₆²⁻ nanosheets is due to efficient electron transfer within the sheet-like nanostructure of this material. The plausible scheme for the reversible color change is shown in Fig. 7.

Fig. 7 The plausible scheme for the reversible color change of a thin film of Cs₄W₁₁O₃₆²⁻.

The repeatability of coloration and decoloration processes is important for practical applications. Fig. 8 shows the repeatability of the reversible color change in the Cs₄W₁₁O₃₆²/Al film upon alternating exposure to acid and ambient air. We confirmed that coloration and decoloration could be repeated more than 50 times. During the coloration process, the Al substrate was gradually dissolved, however, this dissolution of Al is expected to cease after the full capacity of the Cs₄W₁₁O₃₆²⁻ nanosheet for electrons is reached. In the present study, the Cs₄W₁₁O₃₆²⁻ nanosheet layer had a thickness of only 2.3 nm, making its capacity for electrons very small. Here, we speculate on the reasonable life-time of our device based on the electrochemical point of view. We assume a reasonable carrier density in a bulk as small polarons from 10¹⁸ to 10²⁰ cm⁻³ in tungsten oxide compounds.^17,18 Under these considerations, one coloration/decoloration cycle consumes the Al substrate, which corresponds to the Al thickness from 0.0027 nm to 0.27 nm. Therefore, the long-term durability can be expected in our chromic device, and our chromic device is very robust under repeated coloration and decoloration processes.

Fig. 8 Repeatability of the reversible color change in Cs₄W₁₁O₃₆²⁻ nanosheets/Al by acid treatment and exposure to air at room temperature.

In addition to rapid coloration, our Cs₄W₁₁O₃₆²⁻ nanosheet is superior to conventional WO₃ particles because of its colorlessness in the visible light range before dipping in aqueous solution. Furthermore, our Cs₄W₁₁O₃₆²⁻ nanosheet is very stable under alkaline conditions, whereas conventional WO₃ particles are dissolved in alkaline solution. Fig. 9 shows the photos before and after dipping in aqueous solution. Our Cs₄W₁₁O₃₆²⁻ nanosheet exhibited reversible coloration even under alkaline conditions. And our Cs₄W₁₁O₃₆²⁻ nanosheet is very stable under alkaline conditions, while conventional WO₃ particles are dissolved in alkaline solution.

Fig. 9 Photos of Cs₄W₁₁O₃₆²⁻ nanosheet films after dipping in acid with pH = 3 (a), in alkaline solution with pH = 11 (b), and before coloration (c).

4 Conclusions

We have developed a novel, reversible chromic device consisting of a Cs₄W₁₁O₃₆²⁻ nanosheet film on an Al substrate, for which an aqueous acid solution and oxygen exposure act as the ink and eraser, respectively. The efficient coloration of this material is due to its sheet-like nanostructure. Our chromic device is comprised of non-toxic inorganic materials that are robust under heat treatment and does not require light or electric power sources, complicated sandwich cell structures, or a transparent electroconductive substrate, such as ITO or FTO glass. Hence, the chromic device described here is potentially suitable for various industrial and commercial applications, including displays, data storage devices, and sensors.

Acknowledgements

We thank Mr K. Hori at the Center for Advanced Materials Analysis, Tokyo Institute of Technology for help with TEM observations. This research was supported by the Japan Science and Technology (JST) Agency and a Precursory Research for Embryonic Science and Technology (PRESTO) Grant. We also express gratitude to Mr G. Newton for a careful reading of the manuscript.

Notes and references

1. S. K. Deb and J. L. Forrestal, in Photochromism, ed. G. H. Brown, Wiley-VCH, New York, 1971, p. 633.
2. C. G. Granqvist, Handbook of Inorganic Electrochromic Materials, Elsevier, Amsterdam, 1995.
3. R. J. Colton, A. M. Guzman and J. W. Rabalais, Acc. Chem. Res., 1978, 11, 170.
4. B. W. Faughnan and R. S. Crandall, Top. Appl. Phys., 1980, 40, 181.
5. C. S. Blackman and I. P. Parkin, Chem. Mater., 2005, 17, 1583.
6. C. Bechinger, E. Wirth and P. Ledierer, Appl. Phys. Lett., 1996, 68, 2834.
7. C. M. Lampert, Sol. Energy Mater., 1984, 11, 1.
8. P. Delichere, P. Falaras, M. Froment, H. L. Goff and B. Agius, Thin Solid Films, 1988, 161, 35.
9. Z. G. Zhao, Z. F. Liu and M. Miyauchi, Adv. Funct. Mater., 2010, 20, 4162.
10. M. Miyauchi, Y. Li, S. Yanai and K. Yotsugi, Chem. Commun., 2011, 47, 8596.
11. M. S. Paley, E. J. Meehan, C. D. Smith, F. E. Rosenberger, S. C. Howard and J. M. Harris, J. Org. Chem., 1989, 54, 3432.
12. K. Fukuda, K. Akatsuka, Y. Ebina, R. Ma, K. Takada, I. Nakai and T. Sasaki, ACS Nano, 2008, 2, 1689.
13. Z. Liu, Z. Zhao and M. Miyauchi, J. Phys. Chem. C, 2009, 113, 17132.
14. Q. Zhong and K. Colbow, Thin Solid Films, 1991, 205, 85.
15. E. Kikuchi, N. Hirota, A. Fujishima, K. Itoh and M. Murabayashi, J. Electroanal. Chem., 1995, 381, 15.
16. J. Hou, G. Zuo, G. Shen, H. Guo, H. Liu, P. Chen, J. Zhang and S. Guo, Nanoscale Res. Lett., 2009, 4, 1241.
17. E. Salje, J. Appl. Crystallogr., 1974, 7, 615.
18. J. Ederth, A. Hogel, G. A. Niklasson and C. G. Granqvist, J. Appl. Phys., 2004, 96, 5722.

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Footnote

† Electronic supplementary information (ESI) available: UV-vis spectra recorded in reflection mode, photos of films for various thicknesses, and decay in color under alkaline conditions. See DOI: 10.1039/c3tc32513j

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