The material, which they were studying in an effort to improve the performance of an advanced rechargeable battery, may be useful in developing a next generation of energy-efficient windows that switch from transparent to opaque spontaneously upon exposure to increasing levels of sunlight, or 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.

Electrochemical reactions driven by light in the ultraviolet spectrum produce the photochromic behavior. When light strikes the titanium-nickel sandwich, electrons from the Ni(OH)2 layer flow to the TiO2 film. The NiII(OH)2 oxidizes into a form of higher nickel (NiIII and NiIV) oxides. As it does, what was a transparent film gradually darkens into shades of gray and black. When the light is blocked, the reaction reverses itself. Full coloration of the prototype device from transparency to its darkest state requires about 10 minutes.

"They are promising for 'smart' energy-efficient windows and information display panels," says McLarnon. "They can control visible light and solar radiation levels passing through them, so they are able to regulate illumination levels, as well as glare, heat gain and heat loss."

"Smart" windows based on these technologies could remain transparent while the sun is low in the sky, and gradually darken as it rises and begins to heat a building's interior spaces. By keeping the heat out, the building uses less energy for air conditioning, thereby saving money and reducing air pollution associated with burning fossil fuels. Then as the sun sets and exterior light levels decrease, the window will gradually return to transparency. An intriguing advantage of the new material is the ability to "override" its natural response when used as a conventional electrochromic device.

Other possible applications of the material include large-scale photoelectrochromic display panels for computers and other electronic equipment, "smart" windows and rear-view mirrors for cars and trucks, photochromic lenses for sunglasses, and new types of light detectors, optical switches and light intensity meters. Another application is as a low-cost memory device for optical computers. It is the material's ability to store information in a binary form -- transparent or dark, representing zeros and ones -- or to encode data as levels of gray, that makes it a candidate for the display-panel and memory-device applications.

"Several problems have prevented the large-scale fabrication of photochromic and electrochromic devices," says McLarnon. "They include the lack of adequate reversibility (switching back and forth from transparency to a colored state), instability of the material over the long term, and high cost."

Although more research and development is needed, the new material addresses certain problems.

"One advantage is that it turns gray on exposure to light," says Kostecki. "Also, you can deposit it on any type of substrate -- glass, plastic or ceramic -- whether it is conductive or not. Current photochromic materials are expensive, whereas electrochromic materials require a conductive substrate. Finally, titanium dioxide and nickel hydroxide are easy to produce and very inexpensive, and are widely used in ceramics, pigments, catalysts and other products."

The research team still must solve some problems and do additional work. "Now, the material darkens mainly in response to the ultraviolet light. We need to modify the film so it will respond efficiently to the solar spectrum," Kostecki explains. "Also, we need to develop technology to produce TiO2 and Ni(OH)2 films which are as uniform and transparent as possible."

Kostecki and McLarnon were assisted by Thomas Richardson (Chemical Sciences Division) in recent stages of this work. A paper by Kostecki, Richardson and McLarnon titled "Photochemical and Photoelectrochemical Behavior of a Novel TiO2/Ni(OH)2 Electrode" has been published in the July issue of the Journal of the Electrochemical Society (Vol. 145, no.7, pp. 2380-2385), and a patent application is pending.

Berkeley Lab is a U.S. Department of Energy National Laboratory located in Berkeley, California. It conducts unclassified research and is managed by the University of California.

Additional information:

• Department of Energy's Electrochromics Initiative