Adatoms

Chemists working for the department of energy have discovered a new water catalyzed transformation on titanium dioxide surfaces.

Their work begins with a clean surface of TiO2 (rutile) which appears as rows of Ti atoms alternating with rows of oxygen atoms. They then treat the surface with oxygen, which induces some of the titanium atoms to bond an additional oxygen, which sticks up from the relatively flat surface. These new oxygens (adatoms) are generally stationary, or they only move up and down rows, as the energy barrier to move across rows is too high at ordinary temperatures. But, when the surface is treated with water, the adatoms are suddenly mobile across the entire surface.

The explanation is fairly simple. A water molecule attaches to a titanium near the adatom, then transfers one of its hydrogens to the adatom. The original adatom can pull the remaining hydrogen off of the bonded water molecule and leave, resulting in a net migration of the adatom. This process can occur down rows, but this was already possible. More importantly, it allows transfer of the adatoms across rows.

The authors' justification of the research is a little bit lacking:
"Oxygen and water are involved in many, many reactions," said physicist Igor Lyubinetsky at the Department of Energy's Pacific Northwest National Laboratory, who reported the team's results in March 6 issue of the Physical Review Letters. "This mobility might interfere with some reactions and help others.""

It is an interesting reaction, however. The most interesting part, for me, was that the mechanism of this process was discovered by actually watching the process through a microscope. At low temperatures the process is slow enough that with "scanning tunneling microscopy" the process can be observed in real time.



http://www.sciencedaily.com/releases/2009/03/090316133442.htm
http://scitation.aip.org/getpdf/servlet/GetPDFServlet?filetype=pdf&id=PRLTAO000102000009096102000001&idtype=cvips&prog=normal