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Multicolor Electrochromism Showing CMY Three Primary Color States Based on Size- and Shape-Controlled Silver Nanoparticles Ayako Tsuboi, Kazuki Nakamura, and Norihisa Kobayashi Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5039039 • Publication Date (Web): 24 Oct 2014 Downloaded from http://pubs.acs.org on October 25, 2014
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Chemistry of Materials
Multicolor Electrochromism Showing CMY Three Primary Color States Based on Size- and Shape-Controlled Silver Nanoparticles Ayako Tsuboi,†‡ Kazuki Nakamura†, and Norihisa Kobayashi*† †
Department of Image and Materials Science, Graduate School of Advanced Integration Science, Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan ‡
Research Fellow of the Japan Society for the Promotion of Science (JSPS)
ABSTRACT: Inorganic electrochromic (EC) materials have various advantages for use in display devices. We demonstrate here a new multicolor EC device based on an electrochemical silver deposition mechanism. Ag nanoparticles electrochemically deposited on an electrode exhibit a wide variety of optical states based on their localized surface plasmon resonance (LSPR). As LSPR bands and the resultant colors change depending on the size and shape of the nanoparticles, morphological control of the Ag nanoparticles enables multiple color representation. In order to exploit this color variation in inorganic EC devices, we investigated the electrochemical deposition of size- and shape-controlled Ag nanoparticles by varying the surface morphology of the electrode and by applying a step voltage, which consisted of the application of two consecutive different voltages. Using only silver deposition, we have successfully achieved the first LSPR-based multicolor EC device that enables reversible color changes, including three primary colors—cyan, magenta, and yellow—as well as transparent, black, and mirror (CMYK + mirror) in a single cell.
shutters,15 smart windows,2,16 and variable reflectance mirrors17).
INTRODUCTION Electrochromism (EC) is the phenomenon by which reversible optical changes are induced by electronic energy and the resulting electrochemical redox reactions of materials. The obtained color changes are based on a change in the electronic state of a material caused by electron transfer between the EC material and an electrode. Recently, many inorganic EC materials have emerged, and these materials can be classified into three different types. The first and most typical type of inorganic EC materials are transition metal oxides,1-4 such as WO3, NiO, MoO3, and V2O5. In metal oxide systems, following application of the potential a cation with a small ionic radius (H+, Li+, etc.) can enter the crystal lattice of the metal oxide owing to charge compensation; the resulting electrochromic color change occurs through intervalence charge transfer. The second type of EC material is based on a reversible mirror switching phenomenon that occurs in metals (i.e., yttrium and lanthanum) and alloys of rare earth-magnesium or nickelmagnesium. The reflection states of these metals are controlled by hydrogenation/dehydrogenation reactions.5-7 The third type of EC material involves the reversible electrodeposition of metals, such as Ag, Bi, Cu, Ni, Pb, etc. In this system, the reduction of metal cations dissolved in solution results in deposition of metal particles or films on a transparent electrode to control its color or reflection state.8-11 These EC materials offer many advantages, including low operation voltages, memory effects, and high visibilities under sunlight. Therefore, inorganic EC materials are expected to achieve applications in information displays (i.e., electronic paper and digital signage12-14) or in light modulating devices (i.e., light
We have previously reported an EC device based on silver deposition that achieved three reversible optical changes— transparent, silver-mirror, and black—in a single cell.18 The driving principle of this EC device is the exploitation of Ag nanoparticle deposition on two different transparent electrodes: a flat indium tin oxide (ITO) electrode and a rough ITO particle-modified electrode. The EC material, consisting of a gel electrolyte in which Ag+ is dissolved, is sandwiched between the two electrodes. The default state of this device is transparent, whereas by applying a negative voltage to either one of the electrodes Ag is electrodeposited on the electrode surface. Following Ag deposition on the flat ITO electrode, the device becomes mirrored. Conversely, when Ag deposition occurs on the ITO particle-modified electrode, which has a rough surface, the device turns black. Although the development of multifunctional EC materials that allow control of various colors and their densities is necessary for the practical realization of full-color EC displays,1924 studies of inorganic EC materials that achieve multichromatic states are sparse. To address this issue, the localized surface plasmon resonance (LSPR) band of the electrodeposited Ag nanoparticles can be used as a tool to control multichromatic states, as indicated by recent work on optical filters or printing based on the surface plasmon resonance of metal arrays with periodic structures.25-27 In the course of our research on Ag deposition, we recently found a reversible multicolor changing phenomenon, from transparent to magenta and
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Chemistry of Materials
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cyan, based on controlling the size of Ag nanoparticles during electrochemical deposition.28,29 Ag nanoparticles show various colors based on their LSPR, and the absorption wavelength of the LSPR band is known to depend on the size and shape of the nanoparticle.30-37 Therefore, dramatic changes in color can be achieved in deposition-based EC devices by manipulating the LSPR bands. In order to obtain reversible changes among multiple colors by controlling the LSPR band, the “voltage-step method” 38,39 was utilized in a Ag deposition-based EC device. This method is based on the application of two consecutive different voltages (Fig. 1). The first voltage V1, which must be a more negative voltage than the critical voltage for nucleation, is applied for a very short time t1 to initiate Ag nucleation. The second voltage V2, subsequently applied for a time t2, is used to control growth of the Ag nuclei. To inhibit further nucleation, V2 must be more positive than the critical voltage for nucleation. Therefore, growth of the Ag nanoparticles, and hence, the color of the device, is finely controlled by the V2 application time t2. In our previous study, magenta and cyan color states were achieved in a silver deposition-based EC device using this voltage-step method.28,29
Fig. 1 Schematic diagram of the voltage-step method. The first voltage V1, which initiates nucleation, is applied for a brief time t1. Application of V1 is immediately followed by application of the second voltage V2 for a time t2. V2 is more positive than the critical voltage for nucleation and promotes particle growth without further nucleation. For optical modulation devices and display devices, such as OLEDs and electronic paper, full color representation is desired for practical applications, and thus achieving three primary colors (i.e., RGB for emissive devices and CMY for reflective devices) is significant. However, a yellow color state was not achieved in our previously described Ag deposition-based EC device. Spherical Ag nanoparticles homogenously dispersed in solution are reported to exhibit a yellow color if the particle size is very fine (ca. 5–10 nm).37 Therefore, in this present study, to obtain fine and uniform Ag nanoparticles in an EC device, we introduced the voltage-step method to an ITO particle-modified electrode with a rough surface and large surface area, instead of the conventional flat ITO electrode. We measured the electrochemical and optical properties of the EC cell with a flat ITO electrode or ITO particle-modified electrode during the step voltage application. As a result, we obtained a new Ag deposition-based multicolor EC device that could achieve a yellow color in addition to the previously reported cyan and magenta states. Additionally, the relationship between the morphology of the electrodepos-
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ited Ag nanoparticles and the device color was investigated in more detail than in our previous study by analyzing the morphology of the deposited silver. By examining the silver deposited on the electrodes using field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM), we clarified that the morphology of the deposited silver, i.e., not only the particle size but also the shape and the connections between particles, influenced the optical states. Establishing a method to control these various morphologies of deposited silver enabled more precise control and realization of various colors in inorganic EC cells. Finally, the flat ITO electrode and the ITO particle-modified electrode were combined in a single cell in which two voltage application methods (voltage-step and constant-voltage) could be applied to each electrode. As a result, a multicolor EC device with six states (transparent, silver-mirror, black, cyan, magenta, and yellow (mirror + CMYK)) was successfully achieved.
EXPERIMENTAL SECTION Reagents. Silver nitrate (AgNO3), copper chloride (CuCl2), and lithium bromide (LiBr) were obtained from Kanto Chemical Co. Inc. Dimethyl sulfoxide (DMSO) was obtained from Sigma Aldrich Japan. Poly(vinyl butyral) (PVB) was obtained from Sekisui Chemical Co. Ltd. These reagents were used as received. An ITO electrode (Wuhu Token Sciences Co. Ltd.,
