This document summarizes a study on cation exchange reactions in ionic nanocrystals. The study found that complete and fully reversible cation exchange can occur rapidly between CdSe nanocrystals and Ag+ ions. Specifically, mixing CdSe nanocrystals with AgNO3 solution resulted in rapid color change and fluorescence disappearance as the crystals transformed to Ag2Se. Subsequently, mixing the Ag2Se crystals with excess Cd(NO3)2 led to recovery of the original CdSe crystals over 1 minute. X-ray diffraction and other characterization confirmed the transformations while also showing the crystals preserved their original size and shape through the reactions. The ability to perform such reversible cation exchanges at room temperature provides opportunities to modify nanocry

1. Cation Exchange Reactions in
Ionic Nanocrystals
Dong Hee Son, Steven M. Hughes,Yadong Yin, A. Paul Alivisatos
Science,306, 1009 (2004).
Sameh Hamzawy
MESC9

2. Outline
 Introduction
 Experiment
 Results and discussion
 Conclusion

3.  Complete, fully reversible, and relatively faster cation exchange
occurs.
 Modification the properties of crystalline materials by the cation
exchange of atoms .
 Structure and morphology vs. size and shape of the nanocrystal.
 Introduction
 They chose to work with CdSe nanocrystal.
X. Peng et al., Nature 404, 59 (2000).

4.  Result:
 A rapid (<< 1 sec) change of solution color.
 Complete disappearance of fluorescence is observed upon mixing
the solutions.
 Experiment and Results
 Forward Reaction:
 Mixing a solution of CdSe NCs in toluene with a small amount of
methanolic solution of AgNO3 under ambient conditions.
 Ag+ ion solution in a slightly larger amount than necessary.
 Methanol favors the forward reaction.

5.  Result:
 A slower color change back to that of CdSe nanocrystals and the
reappearance of fluorescence are observed over a period of 1
min.
 Experiment and Results
 Reverse Reaction:
 mixing Ag2Se nanocrystals with an excess amount of Cd(NO3)2 in
a mixture of toluene and methanol.

6.  For the Forward Reaction :
 The XRD patterns and optical absorption spectra confirm that the
reaction product is Ag2Se.
 Results Analysis
 For the Reverse(Recovered) Reaction :
 XRD patterns, optical absorption, and fluorescence spectra all
indicate that CdSe is recovered from the reverse cation exchange.
CdSe
Ag2Se
Rec.
CdSe

7.  TEM images of also indicate that size and shape are preserved for
both the initial and recovered CdSe.
 Results Analysis
 The speed and reversibility of the reaction at room temperature
in the nanocrystals is very high,relatively.

8.  As the nanorods become thicker
from (A) to (I), the shape change
during the cation exchange reaction
is suppressed.
 Results Analysis
 Cation exchange reactions on
nanocrystals with highly
anisotropic nonequilibrium shapes,
such as rods, tetrapods, and hollow
spheres.
TEM images of CdSe nanorods of different sizes
and their trans formed Ag2Se crystals

9.  Results Analysis
 CdS hollow spheres almost maintain overall morphology during
the cation exchange.
 In the case of CdTe tetrapods, slight expansion of the width is
observed.

10.  Results Analysis
 changes in size can be accounted for changes in the crystal unit
cell symmetry and lattice parameters during the transformation.

11.  The Ag cation exchange reaction in this study, can easily be
extended to exchange with other cations under ambient
conditions.
 Attempts to induce anion exchange have not been successful
under similar experimental conditions,
 It can be a versatile route for expanding the range of nanoscale
materials with divers compositions, structures, and shapes.
 Conclusion

12. Thank you for your
attention
Questions
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