Mechanistic
Insight into the Formation of Cationic
Naked Nanocrystals Generated under Equilibrium Control
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Abstract
Cationic naked nanocrystals (NCs)
are useful building units for
assembling hierarchical mesostructured materials. Until now, their
preparation required strongly electrophilic reagents that irreversibly
sever bonds between native organic ligands and the NC surface. Colloidal
instabilities can occur during ligand stripping if exposed metal cations
desorb from the surface. We hypothesized that cation desorption could
be avoided were we able to stabilize the surface during ligand stripping
via ion pairing. We were successful in this regard by carrying out
ligand stripping under equilibrium control with Lewis acid–base
adducts of BF<sub>3</sub>. To better understand the microscopic processes
involved, we studied the reaction pathway in detail using in situ
NMR experiments and electrospray ionization mass spectrometry. As
predicted, we found that cationic NC surfaces are transiently stabilized
post-stripping by physisorbed anionic species that arise from the
reaction of BF<sub>3</sub> with native ligands. This stabilization
allows polar dispersants to reach the NC surface before cation desorption
can occur. The mechanistic insights gained in this work provide a
much-needed framework for understanding the interplay between NC surface
chemistry and colloidal stability. These insights enabled the preparation
of stable naked NC inks of desorption-susceptible NC compositions
such as PbSe, which were easily assembled into new mesostructured
films and polymer-nanocrystal composites with wide-ranging technological
applications