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Research
Research in the Schaak group is driven by synthesis developing new
synthetic methodologies that fill critical gaps in the current
toolbox of techniques available in the solid-state chemistry and
nanoscience communities, and applying these new synthetic tools to
important and often applied problems that could benefit from our unique
capabilities. In all of our endeavors, we integrate ideas and
techniques from solid-state chemistry, solution (molecular) chemistry,
and nanoscience, and this allows us to tackle important and often
longstanding scientific problems that lie at the interface between
chemistry, physics, and materials science. For example, by approaching
problems in traditional high-temperature solid-state chemistry with
ideas and techniques from solution chemistry and nanoscience, we have
been able to avoid solid-solid diffusion as the rate-limiting step in
bulk-scale solid-state reactions and nucleate intermetallic compounds
and other solids at low temperatures (often with structures not
accessible using traditional high-temperature routes). Likewise,
inspired by the dramatic and often unexpected changes in physical
properties that can occur when solid-state materials are dimensionally
confined as nanocrystals, we have been able to synthesize nanocrystals
that are exceptionally complex in terms of composition and structure.
These approaches are helping to establish a toolbox of reactions for
generating well-controlled nanomaterials of complex solids, often of
phases that are inaccessible using traditional synthetic strategies.
Three of our current projects are described below.
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Nanoparticle Toolkit for the Low-Temperature Synthesis of Solid-State Materials
. The rate-limiting step in traditional solid-state reactions is
solid-solid diffusion, which generally necessitates high reaction
temperatures and usually leads to thermodynamically stable structures.
We have been actively developing alternative low-temperature strategies
that side-step the diffusion problem. In particular, we have focused on
using nanoparticles as highly-reactive synthons for accessing binary
and ternary intermetallic compounds of the late transition metals and
post transition metals (Metallurgy in a Beaker). In addition to
establishing the generality of these nanoparticle-directed synthetic
methods, we have accessed new compounds, e.g. AuCuSn 2, which are not
observed using traditional methods. A key point of this work is that
these low-temperature solution approaches provide new variables for
influencing the structure of the phases that nucleate, providing a new
medium and temperature regime for solid-state synthesis. As such, one
of our primary goals is to use these low-temperature techniques to
generate new structures and new materials that cannot be made by other
methods. Also, this work is generating solid-state chemical reactions
that convert metal and intermetallic nanocrystals into derivative
intermetallics, phosphides, oxides, and sulfides in a predictable
manner, and also is helping to establish mechanistic guidelines for
accessing new solids and complex multi-metal nanocrystals.
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Synthesis and Self-Assembly of Complex Nanostructures
. We are interested in developing and generalizing strategies for
accessing size- and shape-controlled nanocrystals of compositionally
and structurally complex solids for applications in catalysis,
plasmonics, and magnetism, as well as their assembly into hierarchical
nanostructures. We have been quite successful using both template and
non-template assembly methods to generate nanoparticle superlattices.
For example, bi-disperse FePt nanocrystals can be made to self-assemble
into AB 2 (AlB 2), AB 5 (CaCu 5), and AB 13
(NaZn 13) superlattice structures. Other recent efforts have focused on
rigorous shape control of alloy and intermetallic nanocrystals, which
remains largely unexplored yet is important for the
previously-described applications. While significant worldwide effort
has been focusing on controlling the shape and size of single-metal
nanocrystals, it is not clear whether these same strategies will
translate to multi-metal systems because of differences in
electronegativity, redox potentials, reduction kinetics, and reactivity
among different elements. We discovered that if size- or
shape-controlled single-metal nanocrystals (for which synthetic methods
are well established) are used as templates, diffusion- or
redox-mediated conversion reactions can be used to form derivative
intermetallics with retention of shape and size dispersity. This
project merges our low-temperature solution techniques with methods
that are appropriate for controlling the morphology of metal
nanocrystals to produce high-quality intermetallic nanocrystals with
significantly greater diversity of compositions, structures, and
properties than have been achievable in the past.
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Nanoscale Intermetallic Catalysts
. The ordered atomic structures of intermetallic compounds
(particularly Pt-based systems) are promising targets for highly active
and selective catalysis, yet relatively little work has been done in
this area because of the lack of appropriate and general methods for
accessing intermetallic nanocrystals, which our group has helped to
pioneer. We have learned how to synthesize and carefully fine-tune the
compositions and structures of intermetallic nanoparticles, including
as SiO 2- and Al 2O 3-supported nanoparticles in the
catalytically-relevant 3-10 nm size regime. These nanoparticles are
tunable, supported, and non-stabilized (e.g. they contain no
deliberately-added surface stabilizers), making them ideal catalytic
systems. We are exploring these systems as CO oxidation catalysts,
which are useful for hydrogen reforming applications, as well as
electrocatalysts for fuel conversion reactions that are central to
emerging fuel cell technologies. |