'Hot' oxygen atoms on titanium dioxide motivated by more than just temperature

Like two ballroom dancers waltzing together, the two atoms of an oxygen molecule severed by a metal catalyst usually behave identically. But new research reveals that on a particular catalyst, split oxygen atoms act like a couple dancing the tango: one oxygen atom plants itself while the other shimmies away, probably with energy partially stolen from the stationary one.

Scientists from the Department of Energy’s Pacific Northwest National Laboratory found the unanticipated behavior while studying how oxygen interacts with reduced titanium oxide surfaces. The chemists are trying to understand how molecular oxygen -- the stuff we breathe -- interacts with metals and metal oxides, which are used as catalysts in a variety of environmental and energy applications. Researchers worldwide are exploring the use of titanium dioxide, especially in hydrogen production for solar fuel cells.

The team was a bit surprised by the unequal sharing of resources among the oxygen atoms.

"It is unique that one atom stays in place and the other one is mobile and probably gets most of the energy," says lead scientist Igor Lyubinetsky, who performed the work at the DOE's Environmental Molecular Sciences Laboratory, a national scientific user facility located on the PNNL campus, with funding by DOE's Office of Basic Energy Sciences. Their work will be published as the cover article in the Journal of Physical Chemistry C on February 21, 2008, and previously appeared online January 5, 2008.

Researchers have yet to determine if this short-lived extra mobility plays a role in chemical reactions, but understanding the basic chemistry might be important in processes that break down pollutants or split water to generate hydrogen.

Previous research has revealed much about how oxygen molecules interact with metals. For example, when molecular oxygen (O2) hits a platinum surface, the platinum helps split the molecule apart and each oxygen atom zips over the surface in opposite directions, eventually sticking to the metal. Chemists call the pumped up atoms "hot" because the extra energy released by the breaking and reforming bonds gives the atoms their boost.

Titanium dioxide is not only a popular catalyst, but it also serves as a great model oxide to study basic chemistry. PNNL scientists, led by Lyubinetsky, wanted to know if molecular oxygen behaved on titanium dioxide the way it behaves on metals such as platinum. Oxides have different properties than metals: Rust, for example, is iron oxide, which flakes off from iron metal.

To find out, the team started with a slice of titanium oxide crystal, oriented so that titanium and oxygen atoms line up on the surface in alternating strips, forming grooves of titanium troughs between oxygen rows. By heating the sample, the team created imperfections on the surface, or spots where an oxygen atom vacated its row. Using scanning tunneling microscopy, the researchers found that molecular oxygen only broke apart when it encountered a vacancy, indicating that oxygen molecules bounce along flawless titanium oxide surfaces and don't react, as expected from previous results.

The team also expected one of the atoms to make the vacancy its home, and the second to situate itself right next to its former partner. Instead, the chemists found that the second oxygen behaved like a "hot" atom and was free to move one or two crystal lattice spaces away. Out of 110 molecules the team counted, more than three quarters of the hot atoms hopped one or two spaces away before becoming mired on the surface.

"This is one of the first time chemists have looked at oxygen on metal oxides at the atomic level, and this finding was unexpected," says Lyubinetsky.

But a skittering atom requires some sort of energy to propel it, so the researchers explored how a splitting oxygen molecule divvied up its energetic resources. The team found that a free oxygen atom at room temperature (about 20 C or 68 F) is virtually immobile on a titanium oxide surface. However, previous calculations have suggested that the energy is released from the rearrangement of the bonds -- from within the oxygen molecule and between the oxygen atom and titanium surface -- and the team has concluded this might be the source of the hot atom's burst after its partner anchored itself in the vacancy: the calculated energy was about two to three times that required to get an immobilized oxygen unstuck. Lyubinetsky postulates that the hot oxygen atom uses this energy to move around on the titanium oxide surface.

The scientists are trying to better understand the mechanism because it might be significant in basic catalytic chemistry.

"This finding may be important in surface reactivity. We don't know yet," Lyubinetsky says. The chemical event could, for example, be affected by the extra energy the oxygen atom possesses. The effect might also play into whether surface oxygen atoms interfere with the chemistry between the catalyst and other reagents.

In any event, the result will keep chemists tango-ing with new questions for a long time.

Reference: Y. Du, Z. Dohnalek, and I. Lyubinetsky, Transient Mobility of Oxygen Adatoms upon O2 Dissociation on Reduced TiO2(110), J. Phys. Chem. C, 2008, 10.1021/jp077677u. Published online January 5, 2008; print February 21, 2008

Source: Pacific Northwest National Laboratory

Related stories:

Solar cell directly splits water for hydrogen
Plants trees and algae do it. Even some bacteria and moss do it, but scientists have had a difficult time developing methods to turn sunlight into useful fuel. Now, Penn State researchers have a proof-of-concept device that can split water and produce recoverable hydrogen.
Uniform tungsten trimers stand and deliver
Like tiny nano-soldiers on parade, the cyclic tungsten trioxide clusters line up molecule-by-molecule on the titanium dioxide platform. One tungsten atom from each cluster is raised slightly, holding forth the potential to execute catalytic reactions.
The sweet smell of nano-success
Materials scientists at Lehigh University and catalyst chemists at Cardiff University have uncovered secrets of the "nanoworld" that promise to lead to cleaner methods of producing, among other things, spices and perfumes.
New Nanotechnology Discovery Controls Electronic Properties of High-K Oxides
Time is fast running out for the semiconductor industry as transistors become ever smaller and their insulating layers of silicon dioxide, already only atoms in thickness, reach maximum shrinkage. In addition, the thinner the silicon layer becomes, the greater the amount of chemical dopants that must be used to maintain electrical contact. And the limit here also is close to being reached.

But a Cornell University researcher has caused an information industry buzz with the discovery that it is possible to precisely control the electronic properties of a complex oxide material -- a possible replacement for silicon insulators -- at the atomic level. And this can be done without chemicals. Instead, the dopant is precisely nothing.

In a paper in a recent issue of Nature (Aug. 5, 2004), David Muller, associate professor of applied and engineering physics at Cornell, and his collaborator, Harold Hwang of the University of Tokyo, report that by removing oxygen atoms from layers in thin films of the oxide strontium titanate, they can precisely control the conducting ability of the material by creating empty spaces, or vacancies, that act as electron-donating dopants. And they have used a scanning transmission electron microscope (STEM) to tell exactly where the missing atoms are in the material.
Researchers Observe Hydrogen-Bond Exchange
Hydrogen bonds are quite small, on the level of a few angstroms. They can also be passed between two different molecules very quickly, at speeds of tens of times per second. But in spite of these properties, researchers have recently observed hydrogen-bond exchange taking place in real-time.
A 'squeeze' in cuprates may explain superconducting temperatures
New experiments at Cornell have verified a theory that variations in the distance between atoms in cuprate superconductors account for differences in the temperature at which the material begins to superconduct. A better understanding of the process could lead to superconductors that work at higher temperatures.
Researchers synthesize compound to flush HIV out of hiding
Any hunter will tell you that when your quarry goes into hiding, you have to flush it out to get a good shot at it. Such is the case with HIV, the virus that causes AIDS.
Ceramic, heal thyself
A new computer simulation has revealed a self-healing behavior in a common ceramic that may lead to development of radiation-resistant materials for nuclear power plants and waste storage.

News discussion:

General Science news