Tungstate nanosheet ink as a photonless and electroless chromic device †

Cesium tungstate (Cs4W11O36 2 ) 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 Cs4W11O36 2 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.


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.6][7][8] However, during the coloration of WO 3 under light irradiation, electrons are trapped at deep levels, and the rate of the decoloration process is very slow in air. 9As an electrochromic material, WO 3 requires complicated apparatus and an electrode structure, including an electric power supply and a complicated sandwichtype cell with a transparent electroconductive substrate, such as indium-tin-oxide (ITO), and a counter electrode lled with an electrolyte solution. 2,4ecently, we reported that a WO 3 -based chromic device is capable of functioning without light irradiation or an electric power supply. 10The WO 3 -coated aluminum (Al) substrate turns a dark color upon exposure to an aqueous solution, with subsequent exposure to oxygen restoring the original lightyellow color of the material.Although this phenomenon is similar to solvatochromism in organic chromophore molecules, 11 the color change in WO 3 /Al is due to the galvanic potential between WO 3 and Al, rather than as a result of the polarity of the solvent.This WO 3 based chromic device exhibits high stability against thermal or chemical treatment rather than a solvatochromic device using organic chromophore molecules.Powder forms of Cs 2 CO 3 and WO 3 were mixed at a molar ratio of 3 : 11 and then ground to a ne powder with a mortar and pestle.The mixed powder was calcined at 1173 K for 6 h.Aer annealing, the resulting dark blue solid (Cs 6 W 11 O 36 ) was ground to powder and 0.500 g of the powder was suspended in 50 mL of a 12 M hydrogen chloride (HCl) solution.Aer 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.Fiy 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; H 2 Cs 4 W 11 O 36 ) 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 WO 3 colloidal solution (Sekisui-Jushi Ltd.) was used in the present study as a conventional chromic material.5500MT, Perkin Elmer Instruments, Japan) analysis was conducted to determine the chemical states of cesium tungstate using standard Mg Ka X-rays.Cross-sectional images of the thin lms were observed using scanning electron microscopy (SEM; model S-4500, Hitachi Co., Tokyo Japan).Surface morphologies of the mono-layered nanosheet lm 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 Cs 4 W 11 O 36 2À /Al and WO 3 /Al samples were recorded using a spectrophotometer (UV-vis, model V-660, Jasco Instruments, Japan) by a diffuse-reectance method with BaSO 4 as background.Powder X-ray diffraction (XRD) patterns were recorded with a diffractometer (model SmartLab, Rigaku Instruments, Japan) using Cu Ka radiation (l ¼ 0.15405 nm).Furthermore, XRD patterns of the thin lms were recorded using out-of-plane and in-plane methods.Coloration was evaluated aer dipping the lms in an aqueous HCl solution (pH 2.0) for 5 min using a spectrophotometer as described above.

Results and discussion
Layered Cs 4 W 11 O 36 2À nanosheets were exfoliated from bulk Cs 6 W 11 O 36 by a chemical treatment, according to a previous paper. 12In the present study, we optimized the ratio of the Cs 2 CO 3 and WO 3 starting materials to achieve a high production yield of the Cs 6 W 11 O 36 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. 12However, all XRD peaks were assigned to CsW 2 O 6 , 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 ¼ 6/11 to achieve a high production yield of the layered       than that of the electrons in WO 3 lms.We also evaluated the absorption spectra of thin lms in a reectance mode without using an integration sphere unit and could see the obvious color change even in a reectance mode (see ESI, Fig. S1 †).Aer exposure to air, the blue substrates returned to their initial color, because the trapped electrons in W 5+ 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 Cs 4 W 11 O 36 2À nanosheet lm is markedly faster than that of the WO 3 lm.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 rst-order kinetics, a nding that is consistent with the photochromic decoloration process in WO 3 . 15The half-life for the decoloration of Cs 4 W 11 O 36 2À nanosheets was 25 s, which was 25-fold more efficient than that of WO 3 particles (650 s).
The Al substrate can be generally oxidized under anodic polarization by an electric eld more positive than its redox potential (+1.662 V vs. NHE).In contrast, Cs 4 W 11 O 36 2À and WO 3 are reduced under cathodic polarization at a potential more negative than their at-band potentials.When a thin lm of a metal oxide semiconductor is coated on an Al substrate, a galvanic potential is generated between the lm and the substrate.As the galvanic potential is the driving force that initiates the coloration reaction, the thickness of the semiconductor lm should be less than that of the space charge layer between the metal substrate and the semiconductor lm.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 lm became inhomogeneous and blue spots were observed around pinholes and cracks of thin lms (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 10 16 cm À3 for undoped metal oxide.Our Cs 4 W 11 O 36 2À and WO 3 lms were insulative before coloration, thus the thickness of our lm should be less than several micrometers for coloration.Furthermore, the thinner lm is better for the diffusion of electrons.But when the thickness of cesium tungstate is too thin, visible light absorption in the thin lm is not so large and coloration is not obvious.
Therefore, the thickness of cesium tungstate nanosheet lms in the present study has been adjusted to $500 nm.
In the chromic devices synthesized here, Cs 4 W 11 O 36 2À or WO 3 is reduced in an acid solution to form color centers, particularly W 5+ , together with the intercalation of protons in the crystal lattice, 16 whereas Al is oxidized to form Al 3+ ions, which dissolve into the aqueous acid solution.Fig. 6 shows XPS spectra of the W-4f orbital before and aer coloration of the thin lm of the Cs 4 W 11 O 36 2À 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 In the decoloration process, reduced tungsten species are oxidized and return to their initial color upon exposure to oxygen.Specically, 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 2À 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 10 18 to 10 20 cm À3 in tungsten oxide compounds. 17,18Under 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.
In addition to rapid coloration, our Cs

Conclusions
We have developed a novel, reversible chromic device consisting of a Cs 4 W 11 O 36 2À nanosheet lm 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.
Herein, we developed an efficient galvanic chromic device composed of a lm of layered cesium tungstate (Cs 4 W 11 O 36 2À ) nanosheets, which are transparent in the visible-light range due to the quantum connement effect.Cs 4 W 11 O 36 2À nanosheets coated on an aluminum (Al) substrate exhibited a rapid, reversible color change.Cs 4 W 11 O 36 2À /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 Cs 4 W 11 O 36 2À and Al, and the decoloration rate of Cs 4 W 11 O 36 2À is 25-fold higher than that of WO 3 particles.The efficient color change is due to the nanostructure of the Cs 4 W 11 O 36 2À nanosheets, which promotes effective electron transfer.Thus, the sheet-like structure of Cs 4 W 11 O 36 2À has suitable properties for constructing a highperformance chromic device.

Cs 6 W
11 O 36 structure.The XRD pattern of Cs 6 W 11 O 36 synthesized in the present work is shown in Fig. 1(b).Fig.

Fig. 3
Fig. 3 XRD patterns for a thin film of Cs 4 W 11 O 36 2À nanosheets.Outof-plane and in-plane XRD measurements are shown.

Fig. 4
Fig. 4 TEM image of commercial WO 3 particles (a), cross-sectional SEM image of the commercial WO 3 -coated thin film (b), and out-ofplane XRD pattern of the thin film of commercial WO 3 (c).

Fig. 5
Fig.5UV-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.
Fig. 6 XPS spectra of the W-4f orbital of the Cs 4 W 11 O 36 2À nanosheet coated on an Al substrate: (a) before acid treatment and (b) after acid treatment.

Fig. 7 .Fig. 8
Fig. 7 The plausible scheme for the reversible color change of a thin film of Cs 4 W 11 O 36 2À .

Fig. 9
Fig. 9 Photos of Cs 4 W 11 O 36 2À nanosheet films after dipping in acid with pH ¼ 3 (a), in alkaline solution with pH ¼ 11 (b), and before coloration (c). 12 6 W 11 O 36 and protonated H 2 Cs 4 W 11 O 36 resuspended in water, and Cs 4 W 11 O 36 1(c) shows the XRD pattern of protonated cesium tungstate (H 2 Cs 4 W 11 O 36 ).Fig. 2(a) shows photographs of aggregates of bulk Cs 2À nanosheets in water (pH 7).As Cs 4 W 11 O 36 2À nanosheets have an anionic surface they are highly dispersed in water even under neutral conditions.TEM analysis revealed that bulk Cs 4 W 11 O 36 2À was efficiently exfoliated and formed a sheetlike structure in water.The line prole obtained from an atomic force microscopy (AFM) image indicated that the thickness of the Cs 4 W 11 O 36 2À nanosheet was about 2.3 nm (Fig. 2(c)), which is consistent with the reported thickness of a monolayer of a Cs 4 W 11 O 36 2À nanosheet. 12A colloidal suspension of Cs 4 W 11 O 36 2À nanosheets was coated on an Al substrate This coloration effect was induced by the injection of electrons from the Al substrate to the Cs 4 W 11 O 36 2À nanosheets or WO 3 particles, leading to the reduction of W 6+ to W 5+ .Simultaneously with the reduction reaction, protons are injected into the Cs 4 W 11 O 36 2À and WO 3 crystals. 13,14Under these conditions, a broad absorption peak around 650 nm was observed for the WO 3 lm, whereas the Cs 4 W 11 O 36 2À nanosheet lm absorbed more 4 W 11 O 36 2À nanosheet is superior to conventional WO 3 particles because of its colorlessness in the visible light range before dipping in aqueous solution.Furthermore, our Cs 4 W 11 O 36 2À nanosheet is very stable under alkaline conditions, whereas conventional WO 3 particles are dissolved in alkaline solution.Fig. 9 shows the photos before and aer dipping in aqueous solution.Our Cs 4 W 11 O 36 2À nanosheet exhibited reversible coloration even under alkaline conditions.And our Cs 4 W 11 O 36 2À nanosheet is very stable under alkaline conditions, while conventional WO 3 particles are dissolved in alkaline solution.