Web Release Date: September 21,
Millisecond Kinetics of Nanocrystal Cation Exchange Using Microfluidic X-ray Absorption
Spectroscopy
and
Department of Chemistry, University of California, Berkeley, California, Materials Science Division, Lawrence Berkeley National Laboratory, and Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California 94720
Received: May 7, 2007
In Final Form: July 29, 2007
Abstract:
We describe the use of a flow-focusing microfluidic reactor to measure the kinetics of the CdSe-to-Ag2Se
nanocrystal cation exchange reaction using micro-X-ray absorption spectroscopy (
XAS). The small
microreactor dimensions facilitate the millisecond mixing of CdSe nanocrystals and Ag+ reactant solutions,
and the transposition of the reaction time onto spatial coordinates enables the in situ observation of the
millisecond reaction using XAS. Selenium K-edge absorption spectra show the progression of CdSe
nanocrystals to Ag2Se over the course of 100 ms without the presence of long-lived intermediates. These
results, along with supporting stopped-flow absorption experiments, suggest that this nanocrystal cation
exchange reaction is highly efficient and provide insight into how the reaction progresses in individual particles.
This experiment illustrates the value and potential of in situ microfluidic X-ray synchrotron techniques for
detailed studies of the millisecond structural transformations of nanoparticles and other solution-phase reactions
in which diffusive mixing initiates changes in local bond structures or oxidation states.
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