You know it as a white smudge across the lifeguard’s nose, or a soothing cream on your baby’s bottom. But someday, the white ointment that protects our skin could generate electricity on your roof or in your car.
Many Americans know zinc oxide as the key ingredient in sunscreen and an effective balm for diaper rash. But it’s also a semiconductor that can be used in new types of solar cells, optical gas sensors and ultraviolet lasers, and in the production of hydrogen gas that could replace petroleum as a source of fuel for cars and trucks.
“The main advantage of zinc oxide over other semiconductor materials is that it is abundant, inexpensive and it can be used to make very good quality devices with inexpensive processes,” says Amir Hassanpour, a physicist at Concordia University in Montreal. “This could lead to cheap devices with very good performance that do not depend on rare materials that are becoming increasingly scarce.”
He and his colleagues, including physicist Pablo Bianucci and chemists Nicoleta Bogdan and John Capobianco, devised a new method for processing zinc oxide to be used as a semiconductor. Their breakthrough approach could be deployed to make more efficient solar panels and hydrogen fuel cells, among other technologies. They published their study in the journal Materials and Design.
At the microscopic level, zinc oxide is a forest of tiny “trees” called nanorods — one-dimensional structures that provide a path for transporting electricity. To work well in such devices as gas sensors, the nanorods must be arranged in specific patterns, which, until now were expensive to produce and required sophisticated equipment.
“It’s easy to grow zinc oxide as a forest of randomly positioned nanorods, where each one has a diameter between 100 and 1,000 times smaller than a human hair, but it’s not easy to tell the nanorods where they are supposed to grow,” Bianucci says.
Scientists covered a smooth surface with zinc oxide and heated it to 400 C. They then covered the first layer with a polymer and used an electron beam to punch holes in it. Later, they put the zinc oxide and the patterned mask inside a bottle of zinc salt and other chemicals dissolved in pure water. They then heated the solution. The zinc and the water reacted only inside the holes, so the nanorods formed only in those holes. After removing the polymer, the nanorod patterns remained.
By growing the nanorods in specific patterns, scientists can create photonic crystals—special structures that trap light, Bianucci explained. Photonic crystals could be used to make more efficient ultraviolet lasers or optical gas sensors that would change color when a certain gas is present.
Such a sensor could, for example, detect dangerous amounts of carbon monoxide. “Or it could also be used for the detection of trace gases that you do not want to be there, for instance, ethylene in fruit storage facilities, since the presence of ethylene can cause fruit to be spoiled,” Hassanpour says.
Zinc oxide nanorods would be valuable asset for gas sensors because they change in measurable ways when exposed to different gases, according to the scientists. While it is possible to make sensors with nanorods that are not in specific patterns, making more sensitive devices “requires a precise control over the nanorod positioning,” Hassanpour says.
Zinc oxide nanorods can also be used to make more efficient solar panels and cheaper hydrogen fuel cells. Since zinc oxide absorbs energy from sunlight, putting nanorods in water causes the absorbed energy from the sun to break the bond between the oxygen and hydrogen atoms in water molecules, generating hydrogen gas.
“The hydrogen can be collected, and then used as a fuel, ideally replacing gasoline or other fossil fuels,” Hassanpour says. “By conserving the hydrogen, it can be easily transferred as fuel to a consumer such as a hydrogen car.”