25 August 1998

It's Curtains For The Curtains

Scientists at the Department of Energy's Lawrence Berkeley National Laboratory and the University of California, Berkeley have discovered serendipitously an inexpensive material that changes color on exposure to light.

The material, which they were studying in an effort to improve the performance of an advanced rechargeable battery, may be used to develop windows that switch from transparent to opaque spontaneously by the application of a small voltage.

Robert Kostecki and Frank McLarnon of Berkeley Lab's Environmental Energy Technologies Division were studying an electrode consisting of thin transparent films of nickel hydroxide [Ni(OH)2] and titanium dioxide [TiO2]. This layered sandwich was formed on glass.

"We were looking for an additive to improve the performance of rechargeable alkaline batteries which use nickel hydroxide electrodes," says chemist Kostecki. "So we added the titanium dioxide film to the nickel hydroxide film in an attempt to inhibit unwanted oxygen gas formation. I wanted to see what would happen when I exposed it to ultraviolet light. When we did this, we saw that the electrode, which had been nearly transparent, darkened. This result indicated that the combination has potential use as either a photochromic device or an electrochromic device, or both."

A photochromic material is one that changes from transparent to a color when it is exposed to light, and reverts to transparency when the light is dimmed or blocked. An electrochromic material changes color when a small electric charge is passed through it. Both photochromic and electrochromic materials have potential applications in many types of devices - privacy in the back of your car, at the flick of a switch?