Changing Color With Smart Windows

Nanoparticle based windows could switch colors on demand.

Nanoparticle-based windows could switch colors on demand: One day, you might not need special bulbs to give your room's lighting a different hue -- you'd just tell the windows themselves to change. Rice University researchers have discovered that you can change the colors transmitted through glass by sending a voltage through pairs of gold and silver nanoparticles, which you frequently find in stained glass windows. Jolt a window one way and you'd get a bright red; reverse the voltage and you'd get blue. All you're really doing is forming or removing chemical bridges between the particles.

The technology is still far from production, so don't expect to renovate your home just yet. However, you only need tiny amounts of the precious metals to achieve dramatic effects. As such, you might well find yourself upgrading to color shifting smart windows around the whole home, not just in one or two prime locations.

Medieval artisans unwittingly used nanotechnology when they mixed gold chloride into molten glass to create richly hued stained glass windows. Soon we could have full-color displays or stained-glass windows that change color at the flick of an electrical switch, thanks to the same kinds of light-scattering nanoparticles.

We’re one step closer to such wondrous things with a new method for connecting metal nanoparticles via teensy “drawbridges” of thin layers of silver, developed by researchers at Rice University. This lets them link pairs of nanoparticles that scatter different colors of light together to form simple color displays. The scientists described their work in a new paper in Science Advances.

Nanoparticles are special because they straddle the boundary between the macroscopic and quantum realms, where classical physics and quantum mechanics hold sway, respectively. It’s their size that matters: a nanometer is equivalent to one 25-millionth of an inch, and nanoparticles range in size from a few nanometers to several hundred nanometers. That gives them unusual properties not found in the same elements at the macro scale.

This includes optical properties, notably how metal nanoparticles in particular scatter light. Scientists have analyzed medieval stained glass windows, and found gold and silver nanoparticles are the key to the deep reds and yellows found therein.

The gold nanoparticles absorb blue and yellow light; red light, with its longer wavelength, reflects off them and passes through the glass. Something similar happens with silver nanoparticles; only bright yellow light scatters off and passes through the glass. Make gold spheres a bit larger, and you can get green or orange. Make the silver nanoparticles smaller, and you get blue.

They have interesting chemically reactive properties, too. In 2008, for instance, scientists at Queensland University of Technology found that some stained glass windows actually helped purify the air when the sun shone through them. The secret ingredient was gold nanoparticles. Sunlight activated the nanoparticles so they could destroy volatile organic chemicals (VOCs) — the source of that new car smell, for instance, but toxic if inhaled in larger quantities.

The properties of medieval stained glass are so unique, Welsh scientists recently used their own versions of the material to build a special 3D panoramic camera for the European Space Agency’s 2019 Mars rover mission. In that case, the useful property was the way the glass resists fading — even after hundreds of years of exposure to the sun’s UV radiation. By blocking UV radiation, the nanoparticles in the stained glass chips will capture the true colors of the red planet.

Soon We Could Have Displays and Windows That Change Color with the Flick of a Switch. But it’s proven challenging to induce metal nano particles to switch colors, a critical ability if you want to build a color display with them. Past work has managed some slight shifts in hue by linking nanoparticles with nanowire bridges. The Rice scientists improved on those techniques to make the color shifts stronger via a kind of chemical bridge.

First, the Rice team fixed pairs of gold nanoparticles to a glass surface coated with conductive indium tin oxide, or ITO (it’s in your smartphone screen, for instance). They used the ITO to coat the surface of the gold particles with a silver electroplate. Then they immersed them in saltwater electrolyte with a silver electrode to form a circuit. Zap the nanoparticles with a negative voltage, and a conductive silver “drawbridge” forms. Reverse the voltage, and the bridge withdraws.

“The great thing about these chemical bridges is that we can create and eliminate them simply by applying or reversing a voltage,” group leader Christy Landes said in a press release. “This is the first method yet demonstrated to produce dramatic, reversible color changes for devices built from light-activated nano particles.”