Cation Exchange
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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
Cation Exchange
- 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 ???
Editor's Notes
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- - Modification the properties of crystalline materials by the cation exchange of atoms . Cation exchange has been investigated in a wide range of nanocrystals of varying composition, size, and shape. crystal structure and morphology of the reaction products are strongly dependent on the size and shape of the nanocrystal. They chose to work with CdSe nanocrystal because of the high degree of control over size and shape that has been achieved.
- - Ag+ ion solution in a slightly larger amount than necessary to replace all the Cd2+ ions in the nanocrystals. - Methanol more strongly binds to any free binary cations in solution and thus favors the forward reaction.
- - The XRD line widths of the initial and recovered case are nearly identical. Moreover, the absorption and fluorescence peak positions, which show strong size dependence due to the quantum confinement effect (2), are also nearly identical for the initial and recovered CdSe nanocrystals.
- - TEM images of also indicate that size and shape are preserved for both the initial and recovered CdSe the. Which demonstrates a fundamental feature of cation exchange reactions in nanocrystals
- - It is readily apparent that thinner nanorods (Fig. 2A) reorganize to the equilibrium spherical shape during the forward reaction, which indicates that the anion sublattice is completely disrupted during the reaction (Fig. 2B). Thicker nanorods maintain their nonequilibrium shapes (Fig. 2, E, F, I, and J). - Thus, there exists a certain size limit below which the structural rigidity of the anion sublattice is not maintained during the cation exchange reaction. - The width and length dependence of the morphology changes we observed (e.g., compare Fig. 2, E and F, with Fig. 2, G and H) also suggest that nanorod thickness is a more relevant variable than nanorod length in determining the shape change.
- - CdS hollow spheres maintain overall morphology during the cation exchange, although a smoothing of the rough surface and a small increase in volume are observed. In the case of CdTe tetrapods, slight expansion (=5%) of the width of each branch is observed after the transformation to Ag2Te.
- - The observed changes in size can be accounted for by changes in the crystal unit cell symmetry and lattice parameters during the transformation. Where small increases in the width observed in the transformation of thicker CdSe rods to tetragonal Ag2Se rods (Fig. 2, F and J) reflect the changes in dimension upon change of the crystal structure as shown in Fig. 4A.
- Attempts to induce anion exchange have not been successful under similar experimental conditions, possibly because of the much larger size of the anions relative to the cations, which makes diffusion more difficult. It can be a versatile route for expanding the range of nanoscale materials with divers compositions, structures, and shapes without having to develop new synthetic methods to produce each individual nanostructure