Focus: Water Structure Down Flat
A new study reveals the structure of water bound to a metal surface, a fundamental yet unanswered question in electrochemistry for the last two decades. Instead of settling into a crinkly layer, as researchers had thought, x-ray experiments show that water molecules form a plane of “flat ice” on a platinum surface. The authors of the study, which appears in the 30 December print issue of PRL, say the finding may help to better understand corrosion and the catalytic reactions exploited by hydrogen fuel cells.
Although water microscopically coats most solid surfaces, especially metals, the precise arrangement of a layer of the Mickey-Mouse-head-shaped molecules has proven elusive. “Water is probably the most important adsorbent layer we have on surfaces, and the bonding [pattern] was still unresolved,” says Anders Nilsson of the Stanford Synchrotron Radiation Laboratory in Menlo Park, California. Water’s hydrogen atoms (the “ears”) determine that pattern by sticking to adjacent water molecules, but show up only as a faint trace in x-ray data. Difficult, low intensity experiments are required to probe them, Nilsson says.
Researchers knew that in a single layer of metal-bound water, alternate rows of oxygen atoms are bound to the metal. Molecules in the non-metal-bonded rows, most researchers assumed, have one hydrogen poking straight up, which would lead to a buckled surface. But recent spectroscopic and theoretical work surprisingly suggested that the layer might really be flat, with those hydrogens pointing down toward the surface.
Eying to settle the question, Nilsson, Hirohito Ogasawara of Stockholm University, and their coworkers, put a layer of “heavy” (deuterium-enriched) water on a platinum crystal at 140 K and examined the structure with x rays. They expected the x-ray absorption of the oxygen-hydrogen bond to reveal whether the relevant hydrogen atom was above the plane of oxygen atoms, in the buckled configuration, or below. “By knowing how the molecular orbitals are oriented, and knowing which orbitals you’re dealing with, you can tell where the hydrogen atoms are,” Nilsson explains. The absorption spectrum indicated that some of the hydrogen atoms are in fact pointing down, and additional experiments revealed that they are bonded to the platinum atoms. According to this structure, every water molecule is bonded to the surface, rather than half of them. “Confirmation of the H-down bonding mode of adsorbed water is a very important accomplishment,” says Ted Madey of Rutgers University in Piscataway, New Jersey.
The result should improve our detailed knowledge of water-metal interactions, says Nilsson, including electrical potentials of metals, corrosion, and catalysis. Platinum is a common catalyst for the formation of water from hydrogen and oxygen, the key reaction in hydrogen fuel cells. While applications may be far removed, says Madey, the work “certainly is going to provide the fundamental understanding that can lead to [technological] improvements.”
JR Minkel is a freelance science writer in New York City.
Related theory paper: A. Michaelides et al., “General Model for Water Monomer Adsorption on Close-Packed Transition and Noble Metal Surfaces,” Phys. Rev. Lett. 90, 216102 (2003)