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Electrochemical Optical‐Modulation Device with Reversible Transformation Between Transparent, Mirror, and Black

2012; Volume: 24; Issue: 23 Linguagem: Inglês

10.1002/adma.201200060

1521-4095

Shingo Araki, Kazuki Nakamura, Kanae Kobayashi, Ayako Tsuboi, Norihisa Kobayashi,

Gas Sensing Nanomaterials and Sensors

A novel electrochromic device with three optical states, transparent, specular mirror, and black, is demonstrated. The cell is constructed by sandwiching gel electrolyte containing silver nitrate between one flat and one particle-modified indium thin oxide electrode. The optical states can be switched by altering the potential across the two electrodes. All changes are reversible and show good stability over 2500 cycles of testing. Chromogenic materials can change their optical properties in response to external stimuli, such as light irradiation1-3 (photochromic materials), temperature change4, 5 (thermochromic materials), exposure to gas6, 7 (gaschromic materials), and the application of an electrical voltage8-14 (electrochromic materials). Great interest has been drawn to the electrochromic effect exhibited by inorganic materials, because of their possible applications in information displays,15, 16 light shutters,17, 18 and variable reflectance mirrors.19, 20 Among these, fenestration technologies for the modulation of light and solar energy transmittance–"smart windows"–are receiving much attention because they are able to achieve energy efficiency (by reducing the need for air conditioning) and indoor comfort21, 22 (due to less glare and thermal discomfort). These electrochromic devices can be developed using three groups of materials. The first involves the insertion of ions (H+, Li+) into hosts such as WO3, NiO, MoO3, V2O5, and other transition metal oxides; it is characterized by reversible and persistent changes in optical properties when the ions are interspersed.10, 22-24 The second group exploits hydrogen-induced phase transitions in rare earths, and mixtures of rare earth or transition metals with magnesium, to achieve variable reflectance devices.25-27 In the third group, reversible electrodeposition uses the deposition and dissolution of a metal (Cu, Ni, Ag, Bi, Pb, and others) onto a transparent conducting substrate in order to manipulate its optical state.28-31 Devices for reversible electrodeposition systems are basically composed of a pair of transparent electrodes with an electrochromic material dissolved in an electrolyte between them. The passage of electric current causes metal deposition on the substrate to construct a thin metal film that alters the optical properties of the surface. In such electrochromic devices, the ease with which the coatings are deposited and dissolved determines the response rate and the reversibility of the changes. Bismuth-copper deposition devices are the most widely studied in this category.29, 32, 33 Copper promotes reversibility and makes the co-deposition pattern flatter than that produced by bismuth alone. Because it switches rapidly and reversibly from black to transparent, the bismuth-copper deposition device can be used in information displays. In the reversible silver deposition systems that have been investigated previously, only a black colour has been obtained by Ag deposition.34, 35 A mirror-like deposition has been achieved using a silver dicyano ion complex or other additives, but it had difficulties for obtaining of fully dissolved transparent states.36-38 When inorganic electrochromic materials are used for "smart window" systems, light absorbing devices and light reflecting devices both have advantages and disadvantages. In the reduction of inside heating in summer, light absorbing devices are only partially effective, because the window itself is heated by the absorption of infrared light. Therefore, light reflecting devices are more effective than light absorbing ones for reducing inside heating. In contrast, for the purpose of glare reduction, light absorbing devices are more effective than light reflecting devices. The visibility of outside through the light absorbing device might be preferable because the outside view is not interrupted with strong reflection of the inside object. Here, we present a novel, multi-functional electrochromic device that incorporates both colour changing and variable reflectance. Such multifunctional electrochromic devices have significant advantages over mono-functional devices for radiant energy control, since they offer the advantages of both absorption and reflection in a single unit. We have developed a novel metal deposition-based electrochromic device that can achieve three optical states—transparent, silver-mirror and black—in a single material. Its underlying mechanism is based on the electrodeposition of silver particles on two facing transparent electrodes, a flat indium thin oxide (ITO) electrode and an ITO particle-modified electrode, that sandwich a gel electrolyte in which electrochromic material is dissolved. The gel electrolyte contains silver nitrate, copper chloride and tetrabutylammonium bromide (TBABr) in a dimethyl sulfoxide (DMSO)-based gel. Because there is an excess of bromide ions, the silver and bromide ions form complex ions in the organic electrolyte. The cell's default state is transparent, but applying a voltage to one or the other electrode causes the electrodeposition of Ag on its surface. When Ag is deposited on the flat ITO electrode a variable reflectance surface is created (switchable mirror); when Ag is deposited on the rough ITO particle-modified electrode, on the other hand, the cell turns black and functions as a light absorbing device. Both these changes are reversed when the electrical charge ceases. The three optical states of transparent, silver-mirror and black can thus be controlled by changing the voltages applied. Figure 1 shows 3-electrode cyclic voltammograms (CVs) and transmittance changes at 700 nm of the electrochromic solution when the flat (a) or the particle-modified (b) ITO electrodes are activated. Supplementary Figure 1S shows the CVs of the solutions with the flat ITO electrode, for each of the components of the solution. Cyclic voltammograms (bottom) and changes in transmittance at 700 nm (top) of dimethyl sulfoxide (DMSO) based electrolyte solution, including 5 mM AgNO3, 100 mM TBABr, 5 mM CuCl2, for (a) the flat ITO electrode and (b) the ITO particle-modified electrode. For the flat ITO electrode, as the potential moves from zero in a negative direction, cathodic current is observed from –0.7 V, reaching a peak at –1.2 V. This cathodic current can be attributed to the electrochemical reductions in Ag+ and Cu+ that are clearly identifiable in the CVs of either AgNO3 or CuCl2 dissolved solution (Figure 1S, b & c). Beyond -0.7 V, the transmittance of the cell decreases as a result of the electrodeposition of Ag and Cu particles on the flat ITO electrode under a negative potential, eventually producing a mirror-like surface. As the potential sweeps from –2.0 V in a positive direction, anodic current appears at –0.3 V, reaching a peak at +0.2 V. This anodic peak reflects the oxidation of the electrodeposited Ag and Cu particles, leading to an increase in the cell's transmittance as they dissolve. However, the transmittance of the cell was not restored to its initial value at +0.3 V, probably because of the lower electroactivity of Ag deposition in the electrolyte solution. An additional oxidation current appears at around +0.8 V and is attributable to oxidation of Cu+ to Cu2+ (Figure 1S c). Since the oxidation potential of Cu+ to Cu2+ is higher than that of Ag metal to Ag+, Cu2+ mediates the oxidation of the Ag deposition. Through this mediation, Ag deposition is fully oxidized to Ag+, resulting in an increase of the transmittance to its initial value. This is supported by the observation that the Ag deposition could not fully dissolve at +1.0 V during the CV cycle without CuCl2, while the transmittance in the transparent state decreased with an increase in the number of cycles (Figure 2S). In the case of the rough ITO particle-modified electrode, the CV curve is similar to that for the flat ITO electrode (Figure 1b). Since similar electrochemical Ag+ ion redox reactions and changes in the cell transmittance are observed, the electrochemical reactivity of the electrochromic solution is maintained on the rough ITO particle-modified electrode. In contrast to the flat ITO, Ag deposition on the rough ITO particle-modified electrode causes the colour to change to a non-reflective black colour when the negative potential is applied. In order to achieve both silver mirror and black colour states in a single cell, a prototype 2-electrode cell was constructed that had two facing electrodes: a flat ITO electrode and an ITO particle-modified electrode. The bias was defined to be positive if the flat ITO electrode was connected to the anode. From the CV measurements of the 2-electrode cell, the current flow due to electrodeposition of Ag was observed to start at –2.0 V, resulting in a specular mirror state (Figure 4S a). During a voltage sweep from negative voltage to positive voltage, the Ag electrodeposits began dissolving at –0.5 V and disappeared from the electrode at +1.0 V. Under the electrochemical deposition or dissolution reaction of Ag, an auxiliary redox process of Cu2+/Cu+ or Br3−/Br− would occur on the opposite electrode. On the other hand, the application of a voltage above 2.0 V (as voltage polarity is defined above) to the ITO particle-modified electrode caused dissolved Ag+ ion to be deposited on the ITO particle-modified electrode, resulting in a black colour state (Figure 4S b). Hence, the bias voltages for the electrodeposition of Ag were determined to be –2.5 V to obtain the mirror state and 2.5 V to obtain the black state. Figure 2a shows the transmission spectra of the 2-electrode electrochromic cell in the transparent state, negatively polarized to the flat ITO electrode (–2.5 V/10 s) and negatively polarized to the rough ITO particle-modified electrode (2.5 V/10 s). In both the latter cases, the cell transmittance decreased to below 20%. The maximum transmittance change at 700 nm was 86.7% for –2.5 V and 80.5% for 2.5 V. The reflection spectra of the cell were also measured under the bias voltages of –2.5 V and 2.5 V (Figure 2b). The reflectance of the cell in the transparent state was approximately 20%, whereas under the bias voltage of –2.5 V it was increased over the entire range of visible wavelengths, reaching 100% for values over 500 nm. This result clearly indicates that the cell changed to a mirror state after electrochemical deposition of Ag metal on the flat ITO electrode. The lower reflectance of the cell around 400–500 nm could be attributed to absorption by the surface plasmon band of the deposited Ag particles. In contrast, when the bias voltage of 2.5 V was applied both transmittance and reflectance decreased to approximately 10%, showing that the cell changed its colour to black following electrodeposition of Ag on the rough ITO particle-modified electrode. Transmittance (a) and reflectance spectra (b) of a 2-electrode electrochromic cell with two facing electrodes, a flat ITO electrode and an ITO particle-modified electrode. 50 mM AgNO3, 250 mM TBABr, 10 mM CuCl2 and 10 wt% PVB were dissolved into DMSO based electrolyte solution. Dotted line: transparent state; solid line: Ag deposited on the flat ITO electrode (–2.5 V application); dashed line: Ag deposited on the ITO particle-modified electrode (+2.5 V application). Figure 3 shows photographs of the electrochromic cell in the transparent, black, and mirror states. In the transparent state, the blue origami bird on far side of the cell can be clearly seen through the cell (Figure 3c). When a negative voltage is applied to the flat ITO electrode (–2.5 V), the cell changes to the mirror state and the reflected image of the red origami bird can be seen (Figure 3b). When the ITO particle-modified electrode is activated, the cell changes to black (Figure 3d). These changes can be clearly seen in the supplementary movie file. At the start the cell is transparent state; then a negative voltage is applied to the flat ITO electrode (–2.5 V), leading to the specular mirror state. Subsequently, the voltage of 0.5 V is applied to dissolve the Ag deposition. Next, the negative voltage is applied to the ITO particle-modified electrode to trigger the black state (2.5 V); then the "bleaching" voltage of -0.5 V is applied. Photographs of the 2-electrode electrochromic cell. (a) Side view, (b) mirror state (–2.5 V application), (c) transparent state (before voltage application), and (d) black state (+2.5 V application). The red origami bird is on the near side of the cell and the blue origami bird is on the far side. The surface morphologies of the flat ITO and ITO particle-modified electrodes before and after Ag deposition were examined using a scanning electron microscopy (SEM) and atomic force microscopy (AFM). Figure 5S shows SEM images, AFM images and surface profiles of the flat ITO and ITO particle-modified electrodes before Ag deposition. The flat ITO electrode has a smooth surface, with an RMS value of 3.5 nm. In contrast, ITO particles ∼200 nm in size are observed on the ITO particle-modified electrode, creating a highly rough surface (RMS 92.0 nm). SEM images of the Ag-deposited electrodes are shown in Figure 4. On both of the electrodes, Ag and Cu elements were detected by energy dispersive X-ray spectroscopy (EDX) measurement (Ag: ∼10%, Cu: ∼2% as atomic percent), suggesting that the Ag and Cu were co-deposited on the electrode surface, as indicated by the electrochemical measurements. The diameter of the Ag particles on the flat ITO electrode was approximately 90 nm and was almost uniform, so that the Ag particles were connected to each other, forming a planar deposition surface. Such uniform nucleation of the Ag particles on the flat ITO electrode would be greatly facilitated by the complex AgBrn(1-n) anion, leading to the mirror-like state of metal reflection. On the other hand, Ag particles deposited on the ITO particle-modified electrode were found to be aggregated in large particles around the ITO particles. The diameter of the aggregated particles is approximately 300–500 nm and the aggregated particles are not connected each other, resulting in a rough, non-reflective surface. The RMS factor of the Ag-deposited ITO particle-modified electrode was 100.6 nm, substantially larger than that of the Ag-deposited flat ITO electrode (8.3 nm). AFM images and surface profiles are shown in Figure 6S. Furthermore, deposited Ag particles in the 2-electrode cell without the Br− ion changed to a deep black colour, also formed larger aggregations (∼1 μm), and produced a rougher surface (RMS = 94.7 nm, Figure 3S b). The rough surface of the Ag deposition would result in a black colour state as result of multiple scattering and/or absorption of the light by the aggregated Ag particles. The aggregated Ag particles would absorb wide wavelength range of the light by the plasmon resonance because of large size distributions of the Ag particles. Furthermore, the scattered long-wavelength light would be absorbed by nearby grains of the aggregated Ag particles, resulting in the black colour state. SEM images of Ag deposited electrodes: (a) flat ITO electrode, and (b) ITO particle-modified electrode. Finally, we carried out a repetition stability test of the cell, switching between transparent, mirror and black states by the sequential application of the following biases: -2.5 V (10 s), 0.5 V (20 s), 2.5 V (10 s), –0.5 V (30 s). The transmittance changes at 700 nm during the repetition stability test are shown in Figure 5. In the first cycle, the change in transmittance was ∼72%, indicating good optical modulation. This change was almost maintained after the 2500th cycle, confirming the high stability of the electrochemical deposition and dissolution. The auxiliary redox process of Cu2+/Cu+ or Br3−/Br− on the counter electrode would prevent degradation of the light modulating properties, such as an irreversible side reaction. Further study of the optimization of the electrochemical reaction on both electrodes could achieve a switching stability adequate for practical use. Transmittance change at 700 nm during a repetition stability test of the 2-electrode electrochromic cell. White square shows dissolved (transparent) state and black square shows deposited (black or mirror) state. Bias voltages were applied in the following sequence: –2.5 V (10 s), 0.5 V (20 s), 2.5 V (10 s), –0.5 V (30 s). In conclusion, we successfully fabricated a novel Ag deposition-based electrochromic cell that can switch between transparent, silver-mirror, and black states in response to a change in the applied voltage. The multi functionality of this novel cell and its high switching stability could make it suitable for use in light modulating devices, such as smart windows and information displays. Materials: Silver nitrate (AgNO3, Kanto Chemical Co. Inc.) and copper chloride (CuCl2, Kanto Chemical Co. Inc.) were used as received. Dimethyl sulfoxide (DMSO, Sigma Aldrich Japan) was used as solvent as received. Tetra-n-butylammonium bromide (TBABr, Kanto Chemical Co. Inc.) was used as supporting electrolyte without further purification. Poly (vinyl butyral) (PVB, Sekisui Chemical Co. Ltd.) was used as a host polymer for electrolyte gelation. The flat ITO electrode (Matema, 10 Ω/□) was used after adequate washing. ITO particle-dispersed solution (Mitsubishi Material, average diameter of primary particle: 30 nm) was used for preparation of the ITO particle-modified electrode. Sample preparation: The gel electrolyte for the electrochromic cell was prepared as follows: 0.5 mmol of AgNO3 (85 mg) as electrochromic material, 2.5 mmol of TBABr (806 mg) as supporting electrolyte, and 0.1 mmol of CuCl2 (13 mg) as electrochemical mediator were dissolved in 10 mL of DMSO. Subsequently, 10 wt% of PVB as host polymer was mixed into the DMSO-based electrolyte solution. The ITO particle-modified electrode was prepared by spin coating with ITO-particle dispersion solution on a flat ITO electrode (500 rpm 5 s, 1500 rpm 15 s). Subsequently, the electrode was baked at 250 °C for 1 h. Fabrication of electrochromic cell: The electrochromic cell was constructed by sandwiching PVB-based gel electrolyte between the flat ITO electrode (as working electrode) and the ITO particle-modified electrode (as counter electrode), maintaining the inter-electrode distance of 500 μm with a Teflon spacer. Cell area was 1 cm × 1 cm. Apparatus: Chronoamperometric measurement was carried out using an ALS model 660A potentiostat/galvanostat equipped with a computer. Transmission spectra and reflection spectra at 700 nm were recorded on an Ocean Optics USB2000 diode array detection system. The surface morphologies and elemental analyses of Ag deposition on the electrodes were carried out using a scanning electron microscope system (JEOL, JSM-6510) equipped with EDX. The surface profiles and root mean square surface roughness were measured by atomic force microscope (AFM, SII, SPA400). The analyzed surface areas of the samples were approximately 100 μm2. The individual variability of the RMS values was within plus or minus 5%. Supporting Information is available from the Wiley Online Library or from the author. Detailed facts of importance to specialist readers are published as "Supporting Information". Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.