Water Diffusion and Clustering on Pd(111)
T. Mitsui,1
M. K. Rose,2
E. Fomin,2
D. F. Ogletree,1
M. Salmeron1*
The adsorption, diffusion, and clustering of water molecules
on a Pd(111) surface were studied by scanning tunneling microscopy. At
40 kelvin, low-coverage water adsorbs in the form of isolated molecules, which diffuse by hopping to nearest neighbor sites. Upon
collision, they form first dimers, then trimers, tetramers, and so on.
The mobility of these species increased by several orders of magnitude
when dimers, trimers, and tetramers formed, and decreased again
when the cluster contained five or more molecules. Cyclic hexamers were
found to be particularly stable. They grow with further exposure to
form a commensurate hexagonal honeycomb structure relative to the
Pd(111) substrate. These observations illustrate the change in relative
strength between intermolecular hydrogen bonds and molecule-substrate
bonds as a function of water cluster size, the key property that
determines the wetting properties of materials.
1 Lawrence Berkeley National Laboratory,
University of California, Berkeley, CA, 94720 USA.
2 Department of Physics, University of California,
Berkeley, CA, 94720 USA.
*
To whom correspondence should be addressed. E-mail:
salmeron{at}stm.lbl.gov
