Dynamic windows that switch from being transparent to opaque and back again could transform homes, offices, cars and aeroplanes, reducing heating and cooling costs without the need for blinds or curtains. Smart windows are already being used on airlines and these are based on materials such as methyl violegen, which changes colour when electrically charged. However, such devices are expensive, have a bluish tint and can take over 20 minutes to dim. What is more, they can become less opaque over time.

A team led by Michael McGehee has now designed a completely different type of window based on a plate of conductive glass containing an electrolyte solution with Cu2+/Pb2+ or Cu2+/Ag+ ions in it. It also comprises a transparent ITO working electrode modified with Pt nanoparticles anchored via a self-assembled monolayer of 3-mercaptopropionic acid and a metal counter-electrode frame. This transparent electrode is similar to the ones employed in flat-panel displays.

Transparent to opaque in less than three minutes

"When we want to attenuate light passing through the structure, we apply a voltage to the electrode and electroplate metal (Cu-Pb or Cu-Ag) onto it," explains McGehee. "When we want it to be transparent again, we apply the opposite voltage and remove the metal. This process is known as reversible metal electrodeposition.

"In our experiments, we have shown that the window can go from being transparent (transmitting around 80% of light) to opaque (less than 5% transmission) in less than three minutes and that we can go through the cycle more than 5000 times without degradation in uniformity or optical contrast."

Scaling to larger devices

The technology could be used for windows in all kinds of buildings, he tells nanotechweb.org. "It could also be used in the sunroof of a car or its rear windows. Dynamically tinting windows are already used in Boeing’s 787 jets and we think that there could be advantages to switching to the kinds of windows we have developed. We might even imagine making sunglasses with such adjustable tuning."

The researchers, reporting their work in Joule https://doi.org/10.1016/j.joule.2017.06.001, say that they are now busy scaling up the windows to make them much larger than the 25 cm2 area that they have demonstrated. "While it is easy to simply make the windows bigger, it is a challenge to maintain the switching speed for larger devices because we need to have a reasonably constant voltage across the electrode," explains McGehee. "When the windows are large and we switch them fast, there is a voltage drop in the electrode.

"We also need to demonstrate that the windows will be stable over many years and in real-life operating conditions by performing accelerated tests under conditions that are even harsher than the windows would normally encounter. The cost of the devices also needs to be cut so that it is at least half that of dynamic windows already on the market."