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A layer of TiO2 nanoparticles with size of 100 nm was deposited onto a fluorine-doped tin oxide (FTO) conducting electrode via dip-coating technique, followed by sintering treatment. Multi-state electrodeposition-based electrochromic device was subsequently fabricated by sandwiching gel electrolyte between a modified FTO electrode and a flat FTO electrode. When applied with a suitable voltage, the fabricated device changed its optical state from transparent to mirror or black depending on whether the silver nanoparticles were deposited on the unmodified or the modified FTO electrode. The silver was reversely dissolved into gel electrolyte once the applied voltage was removed. The fabricated device exhibited transmittance below 1% in the black state and reflectance over 70% in the mirror state. Moreover, the transmittance increased to 11% after 2000 cycles, indicative of good device stability. Introduction Electrochromic devices based on reversible electrodeposition have simple sandwich-type structure, with electrolyte and electrodeposition components interspersed between two transparent conducting electrodes. Their optical properties can be manipulated by depositing metal (copper, bismuth, plumbum, nickel, silver, etc.) onto transparent conducting electrodes in the presence of an applied voltage and, reversely, dissolving metal into electrolyte upon removal of the voltage [1-9]. Ag-based electrodeposition systems [9-12] have been widely investigated, but most of them only show black states through Ag deposition. Even though a mirror-like deposition via a silver dicyano ion complex or other additives has been achieved, the complexity and difficulty of achieving completely transparent state hinders its development and further application. Hence, it is necessary to develop Ag-based electrochromic devices with reversible multi-state using a simple and controllable method [13-15]. Araki et al. [13] deposited Ag onto an indium tin oxide (ITO) nanoparticle-modified ITO electrode via spin-coating and obtained reversible black/mirror state. Araki and his co-workers [14] achieved multi-color states for electrochromic device by controlling the growth of Ag grains under different voltages. In a previous study, our group fabricated an electrodeposition-based Ag/Cu electrochromic device with a reversible three-state optical transformation by using conducting FTO electrode spin-coated with TiO2 nanoparticles [15]. Another technique to deposit thin film on the surface is dip-coating. Deepa, etc. [16] reported that compared with films fabricated via spin-coating, the WO3 thin film fabricated with dip-coating method showed improved optical performance. However, this deposition method has not yet been applied in the fabrication of electrodeposition-based Ag/Cu electrochromic devices. In this study, we fabricated an electrodeposition-based electrochromic device with reversible three-state optical transformation (among transparent, silver-mirror, and black) by depositing TiO2 nanoparticles onto FTO electrode using dip-coating technique. When applied with a suitable voltage, silver was deposited onto the flat FTO electrode or the TiO2 nanoparticles modified FTO electrode depending on the polarity of the applied voltage, resulting in change of optical states from transparent to mirror or black state. Reversely, Ag was dissolved into the gel electrolyte between two facing electrodes upon removal of the voltage. 4th International Conference on Advanced Materials and Information Technology Processing (AMITP 2016) © 2016. The authors Published by Atlantis Press 503 Experimental To prepare the TiO2 nanoparticle dispersion, TiO2 nanoparticles (2.5 g) with lauric acid (0.25 g) and ethyl cellulose (0.75 g) were placed into a ball-mill jar firstly, mixed with terpineol (16 mL) and ethyl alcohol (10 mL) immediately before milling. TiO2 nanoparticle slurry was obtained after 50 min milling, followed by diluting the slurry with ethyl alcohol. To prepare the gel electrolyte, TBABr (806 mg), silver nitrate (85 mg), and copper chloride (13 mg) were dissolved in 10 mL of DMSO, followed by addition of PVB (1.32 g, 10 wt%). 10 ml ethyl alcohol was added to 5 mL TiO2 nanoparticles dispersion and ultrasonically mixed for 30 min. FTO electrode was fixed on the dip-coater, immersed into the above mentioned dispersion, and lifted with a speed of 1000 um/s. Subsequently, the dip-coated FTO electrode was sintered for 30 min at 500°C. In order to assemble an electrodeposition-based electrochromic device, Teflon sheets were used as spacers and were cut into 25×25 mm with a 20×20 mm hole in the middle. The gel electrolyte was contained in the hole of the Teflon sheet, a space formed when the Teflon sheet was sandwiched between a flat FTO electrode and a TiO2 nanoparticle-modified FTO electrode. Field-emission scanning electron microscope (FESEM, S-3400, Hitachi) was used to observe the morphology of the TiO2 thin film. Electrochemical test was conducted by using an electrochemical workstation (CHI660D, CHI) and the optical properties of the device were evaluated by measuring the transmittance and reflectance spectra using a UV-Vis spectrophotometer (Cary 5000, Agilent). Results and discussion As shown in Figure1, the fabricated electrodeposition-based electrochromic device exhibits three reversible optical states, including mirror, transparent, and black depending on the applied voltage. Without applied voltage, the device is transparent as shown in Figure 1a, in which the printed letters on the background can be clearly seen through the device. When a positive voltage (+2.5 V) is applied on the modified FTO working electrode, the device turns into mirror state as shown in Figure 1b, in which the reflection of the red pen can be clearly visible. The black state can be obtained by applying a negative voltage on the modified FTO electrode (Figure 1c). Figure 1. Photographs of the electrochromic device with transparent state (a), mirror state (b), and black state (c) As illustrated in Figure 1, the deposition of an Ag layer on the flat or modified FTO electrode leads to absorption and multiple scattering of incidence light, resulting in mirror or black state. Therefore, the morphological feature of dip-coated TiO2 thin film is critical to the formation of the black state. Photograph and in-plane SEM image of the dip-coated TiO2 thin film are presented in Figure 2. FTO electrode turns from transparency to opaque after the deposition of TiO2 thin film, as illustrated in Figure 2a. Uniform pores and grains on the surface of the as-deposited TiO2 thin film can be observed (Figure 2b). Slight aggregation and agglomeration of TiO2 nanoparticles, formed during the dispersion of TiO2 nanoparticles in ethyl alcohol, the dip-coating of the TiO2 thin film, and the sintering treatment of as-prepared TiO2 thin film-can be observed. Figure 2. Photograph(a) and SEM image(b) of TiO2 nanoparticle-modified FTO electrodes via dip-coating

Electrochromic Device with Three-state Optical Transformation Achieved by Modifying Electrode via Dip-coating Technique