Aqueous Solution-Based Switchable Silver Mirror Devices
Authors: Nakamura Saho , Aoki Atsushi 
Published: December 2024

Dimming glass using electrochromic materials that can control the amount of transmitted light by switching between the colored and transparent states. At the colored state, light is absorbed and is changed to heat resulting in increasing temperature. Meanwhile, a switchable metal mirror between mirror and transparent states, is expected not to absorb light but to reflect light. In the previous study, the switchable silver mirror based on reversible electroplating and electrodissolving on the ITO electrode has been realized. The ionic liquid has been used as a solvent for the silver plating bath. It’s unique solvent, which has the melting salt in room temperature, nonvolatility and ionic electrolyte whereas it’s expensive. Furthermore, aqueous solution is usually employed in the metal plating bath whereas toxic cyanide ion is containing in the bath for stabilizing metal ion. Therefore, in this study, we investigated switchable mirror characteristics using an aqueous solution-based Ag plating bath instead of ionic liquids and wihtout containing cyanide ions.

First, an Ag plating bath was prepared by dissolving silver nitrate, sodium sulfate as an electrolyte, sodium citrate as an additive, and ammonia as ligand for silver ion in pure water. Using the Ag plating bath, Ag plating was performed on the ITO modified by a Pd catalyst by applying a constant current of -4 mA/cm2 for 20 sec and the formation of a mirror surface was confirmed. However, the average transmittance in the visible light region is high at about 10%, and the Ag surface looks whitish. This is thought to be due to light scattering caused by the presence of unevenness on the electroplated Ag surface. The surface irregularities would be caused by the lack of dense modification of Pd catalyst, which is the core of silver for smooth plating of silver, so that the conditions to make more highly dispersed Pd catalyst were investigated. Pd strike plating was performed by changing the current density at the same charge. When the current density increased from -5 to -40 mA/cm2, the grain size of Pd catalyst gradually decreased from 130 nm to 61.6 nm. The dispersibility of Pd catalyst also increased with decrease in particle size. When the Pd-modified ITO at the current density of -40 mA/cm2 was plated with Ag by applying a constant current of -4 mA/cm2, a mirror surface with a low transmittance of less than 1% was successfully formed.

After Ag plating, Ag dissolution was also investigated by applying a constant current of +4.0 mA/cm2 under the potential of +0.25 V vs. Ag/Ag+ and then a constant potential of +0.25 V vs. Ag/Ag+ . However, black residue was remained on the ITO after dissolving the plated Ag on the Pd-modified ITO. Therefore, the catalyst was changed from Pd to platinum in order to complete Ag dissolution from the ITO. Pt has the advantage of catalytic property and electrochemical stability compared to Pd. Complete dissolution was achieved by plating and dissolution using Pt-modified ITO under the same conditions as those for Pd-modified ITO.

The repeating cycle of Ag plating and dissolution was carried out on the Pt-modified ITO until three cycles. At the three cycles, the un-plated region appeared on the upper part of the mirror surface. In order to resolve this problem, we changed the configuration of the equipment to prevent the influence of the Ag plating bath interface, and succeeded in disappearing the un-plated region. Therefore, it is considered that the Ag plating is affected by the oxygen at the plating bath interface during Ag plating.

Switchable silver mirror device will be fabricated using an aqueous Ag plating bath. The device shape would prevent the influence of atmospheric oxygen because of the closed system. The device will be repeatedly switched between the transparent and mirror states to evaluate the cycling characteristics of the aqueous solution-based switchable silver mirror device.