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Left, an electron micrograph (TEM)
of a metal, in this case platinum, deposited on cellulose. Inset,
crystalline cellulose without metal. Right, TEM showing the
pattern of platinum clustering along hydroxyl sites on the
cellulose surface.
Image: Pacific Northwest National
Laboratory |
Exarhos called Shin’s experimental work "the
first report of the efficacy of nanocrystalline cellulose templates in
driving the formation of ordered metal and metal oxide nanoparticles
at surfaces." Exarhos has dubbed these cellulose nanocrystals "molecular
factories."
Using acid-treated cellulose fibers from cotton as
a natural template, the PNNL team has been able to grow gold, silver,
palladium, platinum, copper, nickel and other metal and metal-oxide
nanocrystals quickly and of uniform size, Shin said. The metals
display catalytic, electrical and optical that would not be present in
larger or odd-sized crystals.
The acid converts the cellulose to a large, stable
crystallized molecule rich in oxygen-hydrogen, or hydroxyl, groups,
predictably spaced along the long chemical chains, or polymers, that
comprise the cellulose molecule’s backbone. When most metal salts
dissolved in solution are added in a pressurized oven and heated 70 to
200 degrees centigrade or warmer for four to 16 hours, uniform metal
crystals form at the hydroxyl sites.
The researchers called this method a "green process,"
requiring only heat, the crystalline cellulose and the metal salts.
Other attempts to get uniform nanometals have resulted in crystals of
widely variable sizes that require strong, caustic chemicals as
reducing and stabilizing agents.
"We have some preliminary catalytic results," Shin
said, involving "coupling reactions of organic molecules for palladium
and UV-irradiated degradation of organic dyes in water with selenium
metals. "Smaller particles - 15 to 20 nanometers - showed faster and
higher catalytic conversion ratio compared to commercial catalysts." |
