The case of the vanishing seeds: How curiosity-driven research is future-proofing “Smart Windows”
Have you ever walked into a room on a glorious Colorado summer day and felt the heat radiating through the glass?
We usually solve this by cranking up the air conditioning or closing the blinds, losing our mountain view in the process. But what if the window itself could think? A team led by Mike McGehee, a Fellow at RASEI, describes research that improves the robustness of such a device.
For years researchers have been working on “smart windows”, devices that could “sense” the conditions outside and “react” to them. This investigation centers around a promising technology called Reversible Metal Electrodeposition (RME). The technical details of this process are complex, but you can understand the concept by thinking of it as a reversible coat of paint. At the flip of a switch, a thin layer of metal, in this case silver, spreads across the glass to form a layer that tints it, blocking out the heat and the glare. Flip the switch again and the silver dissolves back into a clear liquid, making the window transparent.
Buildings are responsible for consuming around 40% of all generated energy globally, much of which is expended in regulating the temperature, heating and cooling the building interior. Installing smart windows that can react to the environmental conditions could provide a very effective mechanism to reduce energy use and slash energy bills by automatically managing how much heat enters a room. It has been estimated that just by controlling the amount of sunlight that is let into a building through a window, we could cut energy bills by up to as much as 20%.
However, there have been a number of challenges to overcome in order to take this initial discovery from the lab to a product that can be deployed for use in buildings. One challenge is that early versions of these windows started out fast but grew “lazy” over time. After a few thousand uses the tinting / de-tinting process slowed, taking almost four times longer than it did on day one.
This is where the researchers undertook some detailed investigations to identify what was going on, and what could be done to fix it. A collaboration between the McGehee group (at the ý) and the (at the University of Nevada) set out to find out exactly what was happening. The team decided to look closer, using a combination of high-powered x-rays and electrochemical tests. The windows were using tiny “seeds” of platinum to help the silver grow on the glass. Platinum is recognized for being tough and non-reactive, and so should be perfect as a nucleation point for the silver. Using these advanced techniques the team explored exactly what was happening to the platinum seeds during the clearing phase, when the silver “paint” is stripped away.
To their surprise, the platinum was not as tough as they initially thought. In the special liquid environment needed for the windows, the platinum seeds were actually dissolving and washing away when the window was switched to clear. As the number of seeds dropped, the silver had fewer locations to grow from, which was the cause behind the window tinting slowing.
This led the team to ask the question “What can we do to make the seeds more resilient?”, which led them to use gold in place of platinum. While gold and platinum are both precious metals, in water, which is the solvent used inside the window panels, gold is more stable and less susceptible to decomposition and dissolving. When they swapped the platinum seeds for gold ones, the results were immediate. Even after 7,500 cycles, the equivalent of years of daily use, the windows transitioned just as fast as the first time they were used.
These gold-based windows provide an exciting range of opportunities. Not only because of their improved stability over many thousands of cycles, but also because they can express multiple colors by varying the voltage, a feature of the size of the gold particles. This presents opportunities for their use in displays and communications devices. This technology offers a better, smarter window that could passively save significant amounts of energy if deployed in commercial and residential buildings. This work shows how the impact of making fundamental chemical changes can unlock the potential of new technologies.