posted on 2014-02-11, 00:00authored byLucas R. Parent, David
B. Robinson, Patrick
J. Cappillino, Ryan J. Hartnett, Patricia Abellan, James E. Evans, Nigel D. Browning, Ilke Arslan
The
prevalent approach to developing new nanomaterials is a trial-and-error
process of iteratively altering synthesis procedures and then characterizing
the resulting nanostructures. This is fundamentally limited in that
the growth processes that occur during synthesis can be inferred only
from the final synthetic structure. Directly observing real-time nanomaterial
growth provides unprecedented insight into the relationship between
synthesis conditions and product evolution and facilitates a mechanistic
approach to nanomaterial development. Here, we use in situ liquid-stage
scanning transmission electron microscopy to observe the growth of
mesoporous palladium in a solvated block copolymer (BCP) template
under various synthesis conditions, and we ultimately determined a
refined synthesis procedure that yields extended structures with ordered
pores. We found that after sufficient drying time of the casting solvent
(tetrahydrofuran, THF), the BCP assembles into a rigid, cylindrical
micelle array with a high degree of short-range order but poor long-range
order. Upon slowing the THF evaporation rate using a solvent-vapor
anneal step, the long-range order was greatly improved. The electron
beam induces nucleation of small particles in the aqueous phase around
the micelles. The small particles then flocculate and grow into denser
structures that surround, but do not overgrow, the micelles, forming
an ordered mesoporous structure. The microscope observations revealed
that pore disorder can be addressed prior to metal reduction and is
not invariably induced by the Pd growth process itself, allowing for
more rapid optimization of the synthetic method.