CHAMPAIGN, Ill. - For more than a decade, scientists have known about the ability of monolayers of organic molecules to self-assemble, but the self-assembly of inorganic molecules had remained an elusive goal. Recently, two scientists at the University of Illinois devised a way of modifying metal surfaces with self-assembled monolayers of an inorganic compound.
"We have shown that certain kinds of inorganic molecules will spontaneously form adherent, ordered arrays on solid surfaces," said Andrew Gewirth, a U. of I. professor of chemistry. "Our chemistry provides a facile and versatile route to functionalized surfaces with potentially superior stability and mechanical properties."
The inorganic compound that Gewirth and colleague Walter Klemperer, also a professor of chemistry at the U. of I., have "grown" on silver surfaces is called a silicotungstate and is part of a class of molecules called polyoxometallates. These molecules are known to function as superacids, corrosion inhibitors, catalysts and photochemical oxidants.
"The silicotungstate anion contains a central silicon core surrounded by a series of tungsten-oxygen clusters," Klemperer said. "The molecule presents oxygen atoms to the silver surface, and it is the affinity of silver for these atoms that drives the self-assembly process."
Recent studies by the researchers have shown that the silicotungstate monolayer also will
self-assemble on other oxophillic surfaces (such as copper and aluminum) once the native oxide layer is removed. Because the monolayer can accommodate a wide range of organic, organometallic and inorganic functional groups, the films can hold catalysts for various kinds of electrochemical processes. The films also can be used as effective corrosion inhibitors.
"One of the problems with typical monolayer films is that they are easily oxidized, a property that makes them unsuitable as corrosion inhibitors," Gewirth said. "But, because the bond we are making between the metal surface and the silicotungstate anion is with oxygen, the molecule already is highly oxidized. Therefore, the film is very stable and prevents corrosion from occurring."
Because the silicotungstate anions will orient themselves to maximize the bonding that occurs between the oxygen atoms and the metal surface, the structure and functionality of the monolayer can be changed by changing the shape of the molecule.
"This imparts a kind of 'tuneability' to the film," Klemperer said. "Our concept is to predesign these molecules to incorporate different functional groups in them and then bind them to the surface. It's almost like having a tool box where you have different building blocks that you can put down to form a variety of shapes and patterns, and then continue to build on top of them."
The researchers' latest findings appear in the June 19 issue of the Journal of the American Chemical Society.