Generating electricity is not the only way to turn sunlight into energy we can use on demand（即期）. The sun can also drive reactions to create chemical fuels, such as hydrogen, that can in turn power cars, trucks and trains. The trouble with solar fuel production is the cost of producing the sun-capturing semiconductors and the catalysts to generate fuel. The most efficient materials are far too expensive to produce fuel at a price that can compete with gasoline（汽油）.

"In order to make commercially viable devices for solar fuel production, the material and the processing costs should be reduced significantly while achieving a high solar-to-fuel conversion efficiency," says Kyoung-Shin Choi, a chemistry professor at the University of Wisconsin-Madison.

In a study published last week in the journal Science, Choi and postdoctoral researcher Tae Woo Kim combined cheap, oxide-based materials to split water into hydrogen and oxygen gases using solar energy with a solar-to-hydrogen conversion efficiency of 1.7 percent, the highest reported for any oxide-based photoelectrode（光电极） system.

Choi created solar cells from bismuth vanadate using electrodeposition -- the same process employed to make gold-plated jewelry or surface-coat car bodies -- to boost the compound's surface area to a remarkable 32 square meters for each gram.

"Without fancy equipment, high temperature or high pressure, we made a nanoporous semiconductor of very tiny particles that have a high surface area," says Choi, whose work is supported by the National Science Foundation. "More surface area means more contact area with water, and, therefore, more efficient water splitting."

Bismuth vanadate needs a hand in speeding the reaction that produces fuel, and that's where the paired catalysts come in.

While there are many research groups working on the development of photoelectric semiconductors, and many working on the development of water-splitting catalysts, according to Choi, the semiconductor-catalyst junction gets relatively little attention.

"The problem is, in the end you have to put them together," she says. "Even if you have the best semiconductor in the world and the best catalyst in the world, their overall efficiency can be limited by the semiconductor-catalyst interface."