Chemists happy when vanadium blue

By Mitchell Zimmer

Thursday, April 9, 2009

Imagine a switch that could automatically shut off the sun’s heat coming through your window when it gets too

hot – but that would still let in the light.

A grant from the Western Innovation Fund is helping chemist Rob Lipson, above, and a team of scientists coat

window glass with vanadium dioxide, a development that could someday shield a building's interior from the

sun's heat.aptiontexthere

Now give that switch a name – vanadium dioxide.

“Vanadium oxide is a remarkable material,” says Western chemist Rob Lipson.

One form, vanadium dioxide (VO2), is attracting the most attention because of one fascinating property.

“At 68 degrees Celsius it turns from being a semiconductor to a metal,” he adds. In its metallic state VO2 will

then reflect infrared light or heat away.

When this material is coated as a thin film onto glass it has the potential of decreasing the greenhouse effect

within cars and homes. Even though it’s hard to heat the coating to the transition temperature under most

natural conditions, Lipson says there is a way around that in that “it can be made active by passing some

electrical current through the material, to make it metallic.”

In addition, the material has possible applications for thermal sensors, transparent electrical conductors and

laser-intensity limiters. The last application is useful where a strong laser beam is used to do a process, but if the

power becomes too high, VO2 will turn into a metal within a subtrillionth of a second and reflect the light back.

“From a commercial perspective, it is really an incredibly important material,” says Lipson.

Until recently, there was a drawback in producing large-scale applications of VO2 thin films on materials.

As Lipson explains, the synthesis begins with dissolving vanadium and oxygen containing components in a

simple alcohol. When this solution is spin-coated onto a material like glass, and heated, the solvents evaporate

resulting in the thin film of interest.

When the precursor molecule is dissolved in the solvent it “exhibits a beautiful blue colour which turns orange

as you watch it …What it means is essentially that the electron on the vanadium atom in the precursor is being

removed (so-called oxidation).”

From a commercial standpoint, that’s not good. At this point the solution fails to make good VO2 thin films. So

what to do?

Lipson and his laboratory tried a number of standard solvents that didn’t work well.

“We were forced to make the films immediately because if we waited even just a couple of hours we were out

of luck due to the oxidation effect.”

Lipson’s graduate student at the time, now Dr. Cheng Lu, tried different alcohols to overcome this problem. He

discovered one certain type of solvent called allylic alcohols that turned off the oxidation.

“I have no idea why he chose those alcohols,” says Lipson “but when he did we obtained a blue solution that

remained blue for months… and that means the electron on the vanadium is stable to oxidation.”

“Using allylic alcohol removes that chemical imperative for a company to make up their solutions and make the

films very quickly.”

This advance allowed Lipson to pursue and receive a grant from the Western Innovation Fund to find ways of

scaling up the production of these films to an industrial scale.

Preparing these films in the lab involves taking a drop of the precursor solution, putting it on a small substrate

and spinning the substrate so that the drop makes a thin even coat. That procedure would not work on a large

scale.

“What you really want to do is try to spray it. To this end I have enlisted the help of (professor) Jesse Zhu in

Engineering, who has the spraying capabilities, allowing us to try that out.”

At this early point in the research and development process Lipson and co-workers do not have to use substrates

the size of a window pane, they just need to show proof the principle works on pieces of glass that are a few

inches square in dimension.

The other partner in the project is Leo Lau of Chemistry and Director of Surface Science Western, who will

measure how well the film adheres to the substrate.