The tunable mirror

25 October 2014 - Gadgets

A tunable mirror.PNGWouldn't that be nice. A mirror that you can switch between a reflective mode and a transparent mode with a push of a button. A nice gadget that would make. For some reason police interrogation rooms come to mind. Devices exist based on silver electrodeposition backed by an electrolyte solution (called 'reversible electrochemical mirror' or REM). However, because the silver slowly dissolves, the device requires a continuous voltage. Cracking of the silver mirror is also an issue. Ideally the reflective mode would not have to require a voltage and one way to accomplish this as detailed recently by a group of Korean researchers (Park et al. DOI) is making dissolution of the silver mirror less straightforward.

Here is how they did it. One piece of an ITO glass electrode was treated with an oxygen plasma leaving a surface covered with hydroxyl groups. This surface was further treated with (3-mercaptopropyl) trimethoxysilane (MPTMS) forming a self-assembled monolayer. The second ITO electrode remained bare. The electrolyte solution in between consisted of silver nitrate, copper chloride, tetrabutylammonium bromide, polyvinyl butyral and DMSO. By applying a voltage across both electrodes a bimetallic Ag/Cu nanoparticle film formed predominantly on the surface-treated side. Reversing the voltage quickly dissolved the film, forming a soluble AgBr salt.

Thanks to the surface treatment it now takes 60 seconds for the metallic film to dissolve after switching off the power (as opposed to immediate for untreated film). But 60 seconds is still disappointing and the fast dissolution is due to the presence of halides in the electrolyte. In current thinking silver is oxidised (the article does not explain how) and then carried of by halide ions to form species such as AgBrn(1-n). In this way high halide concentrations speed up silver dissolution.

The researchers then found a way to slow silver dissolution to two hours by replacing some of the DMSO by an ionic liquid. The idea behind it is that the ionic liquid assists in forming an electrical double layer with the positive ions of the ionic liquid occupying the surface where the halide ions have no longer have any businesses.