Smart glass is gaining popularity as an energy-efficiency product for buildings, cars and airplanes. Photo: Steven Marquez/Colorado State University.
Smart glass is gaining popularity as an energy-efficiency product for buildings, cars and airplanes. Photo: Steven Marquez/Colorado State University.

‘Smart glass’, an energy-efficiency product found in newer windows of cars, buildings and airplanes, slowly changes between transparent and tinted at the flick of a switch. ‘Slowly’ is the operative word, though; typical smart glass takes several minutes to reach its darkened state, and cycling repeatedly between light and dark also tends to degrade the tinting quality over time.

Now, chemists at Colorado State University have devised a potentially major improvement to both the speed and durability of smart glass by providing a better understanding of how the glass works at the nanoscale. As they report in a paper in the Proceedings of the National Academy of Sciences, their research offers an alternative nanoscale design for smart glass.

The project started as a grant-writing exercise for graduate student and first author Colby Evans, whose idea – and passion for the chemistry of color-changing materials – turned into an experiment involving two types of microscopy and enlisting several collaborators. Evans is advised by Justin Sambur, assistant professor in the Department of Chemistry, who is the paper's senior author.

The smart glass that Evans and his colleagues studied is ‘electrochromic’; it works by using a voltage to drive lithium ions into and out of thin, clear films of a material called tungsten oxide. "You can think of it as a battery you can see through," Evans said. Typical tungsten oxide smart glass panels take from seven to 12 minutes to transition between clear and tinted.

The researchers focused on electrochromic tungsten oxide nanoparticles, which are 100 times smaller than the width of a human hair. Their experiments revealed that single nanoparticles, by themselves, tint four times faster than films of the same nanoparticles. That's because, in the films, interfaces between the nanoparticles trap lithium ions, slowing down the tinting behavior. Over time, these ion traps also degrade the material's performance.

To support their claims, the researchers used bright field transmission microscopy to observe how tungsten oxide nanoparticles absorb and scatter light. Making samples of ‘smart glass’, they varied how much nanoparticle material they placed in the samples and watched how the tinting behaviors changed as more and more nanoparticles came into contact with each other. They then used scanning electron microscopy to obtain higher-resolution images of the length, width and spacing of the nanoparticles, so they could tell, for example, how many particles were clustered together, and how many were spread apart.

Based on their experimental findings, the authors proposed that the performance of smart glass could be improved by making a nanoparticle-based material with optimally spaced particles, to avoid ion-trapping interfaces.

Their imaging technique offers a new method for correlating nanoparticle structure and electrochromic properties, which means improving smart window performance is just one potential application. Their approach could also guide applied research in batteries, fuel cells, capacitors and sensors.

"Thanks to Colby's work, we have developed a new way to study chemical reactions in nanoparticles, and I expect that we will leverage this new tool to study underlying processes in a wide range of important energy technologies," Sambur said.

This story is adapted from material from Colorado State University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.