Mar 29 2009
Titanium dioxide (TiO2) catalysts are widely used for producing hydrogen gas for fuel cells. TiO2 is a photoactive material that converts sunlight into electronic charge, and then uses these charges to electrochemically spilt water into hydrogen and oxygen gas. Now, Jingbo Li and colleagues at the Chinese Academy of Science in Beijing and collaborators in the USA have designed a way to enhance TiO2 catalytic efficiency by doping the crystal with pairs of atoms.
Electrons in semiconductors such as TiO2, can only exist within certain allowed energy ranges, known as bands. To generate charge, a photon of light must excite an electron from the valence band-which is full of electrons-to the mostly empty and higher energy, conduction band. In TiO2, most wavelengths of light do not have sufficient energy to excite electrons across the gap between bands, thereby making the catalytic splitting of water inefficient.
Experiments have shown that substituting atoms, such as nitrogen for oxygen, or a new transition metal for titanium, into the TiO2 crystal structure reduces the optical bandgap. However, efficiencies remain low because these single chemical dopants localize the excited charges and prevent the electrochemical reaction from proceeding.
Using detailed theoretical analysis, Li's team discovered that specific pairs of chemical dopants, such as molybdenum and carbon (Mo+C), created new electronic bands in TiO2 that improved the water-splitting efficiency.
"In our calculations, one Ti atom and one O atom were substituted simultaneously by (Mo+C) pairs in a 48-atom supercell representing the entire crystal," says Li. While the researchers examined many types of dopants, only the (Mo+C) pair decreased the bandgap by more than a third while retaining high catalytic activity.
The researchers found that pairing dopants stopped any localization effects, allowing charge to be transferred to the water-splitting reaction. "We expect the carrier lifetime in our passivated codoped system should be longer than in unpassivated systems with single dopants," says Li.
The design principle outlined by Li is applicable to many other semiconductors and catalyts. "Due to developments in first-principles theory and computational power, computer-aided, knowledge-based design of materials is now possible. This has become a vital tool for accelerating scientific discovery," says Li. (From Nature)