@article {1120,
	title = {Can the observed changes in the size or shape of a colloidal nanocatalyst reveal the nanocatalysis mechanism type: Homogeneous or heterogeneous?},
	journal = {Topics in Catalysis},
	volume = {48},
	number = {1-4},
	year = {2008},
	note = {Narayanan, Radha Tabor, Christopher El-Sayed, Mostafa A.},
	month = {May},
	pages = {60-74},
	abstract = {The surface energy of metallic nanocrystals is relatively high compared to bulk materials due to the metal-metal bond deficiency of the surface atoms. This results in an insufficient chemical valency. In addition, smaller nanoparticles possess a higher degree of curvature, weakening, the bonding of their surface atoms. This is especially true for non-spherical shapes, which are comprised of a large number of sharp corner and edge sites. These atomic sites possess higher surface energies due to the lower number of shared bonds with the nanoparticle, resulting in instability of the surface atoms and rendering them physically unstable and chemically active. In many instances, the constant "bombardment" of these surface atoms by the solvent molecules as well as by the reactant molecules when these nanocrystals are in colloidal solution could lead to surface atom dissolution, both physically and/or chemically. This phenomenon could alter the functionality of the metallic colloidal nanoparticle from supplying catalytically active sites (in heterogeneous catalysis) to serving as a reservoir of catalytically active species to the solution (in homogeneous catalysis). In the latter type, if the atoms of the nanocatalyst appear in the products, the nanoparticle is no longer a catalyst but a reactant. In this review we attempt to answer the question raised in the title by examining our Previous work on the changes in size, shape, and other physical and chemical properties of colloidal transition metal nanoparticles during the nanocatalysis of two fundamentally different and important reactions: (1) the gentle electron-transfer reaction at room temperature involving the reduction of hexacyanoferrate (III) ions with thiosulfate ions and (2) the more harsh Suzuki cross-coupling reaction between phenylboronic acid and iodobenzene that takes place at 100 degrees C for 12 h. Changes in the nanoparticle dimensions were followed with TEM and HRTEM. Raman and FTIR spectroscopies were used to follow the chemical changes. For each change, we will use the above definition to see if the observed change can help us determine whether the catalysis is homogeneous or heterogeneous.},
	isbn = {1022-5528},
	doi = {10.1007/s11244-008-9057-4},
	author = {Narayanan, Radha and Tabor, C. E. and El-Sayed, Mostafa A}
}
@article {1164,
	title = {Carbon-supported spherical palladium nanoparticles as potential recyclable catalysts for the Suzuki reaction},
	journal = {Journal of Catalysis},
	volume = {234},
	number = {2},
	year = {2005},
	note = {Narayanan, R El-Sayed, MA},
	month = {Sep},
	pages = {348-355},
	abstract = {Carbon-supported PVP-Pd nanoparticles prepared by adsorption of colloidal PVP-Pd nanoparticles onto activated carbon are used as catalysts for the Suzuki reaction between phenylboronic acid and iodobenzene to form biphenyl. These carbon-supported nanoparticles result in a lower biphenyl yield during the first cycle than the colloidal Pd nanoparticles that we studied previously. The carbon-supported Pd nanoparticles retain 69\% of its activity upon recycling (second cycle), which is almost double the recycling potential observed in colloidal Pd nanoparticles (37\% retention of activity). In addition, the carbon-supported Pd nanoparticles retain 73 +/- 3\% of their catalytic activity during the second through fifth cycles of the Suzuki reaction, while the catalytic activity of the colloidal Pd nanoparticles greatly decreases during that time frame. The carbon support that the palladium nanoparticles are adsorbed onto helps to preserve its catalytic activity for longer time periods. The effect of catalysis and recycling on the nanoparticle size is also investigated. The average size of the carbon-supported palladium nanoparticles is 1.9 +/- 0.1 nm initially, 2.6 +/- 0.1 nm after the first cycle, and 3.1 +/- 0.1 nm after the second cycle. The continued growth of the supported nanoparticles suggests that the carbon support protects the palladium nanoparticles during the harsh Suzuki reaction and prevents aggregation and precipitation unlike the colloidal palladium nanoparticles. In addition, a narrow size distribution during the growth process (Ostwald ripening) is observed for the carbon-supported nanoparticles. This could be due to the adsorption method for preparing carbon-supported Pd nanoparticles because excess unaggregated palladium atoms will not be adsorbed onto the carbon support. (c) 2005 Elsevier Inc. All rights reserved.},
	isbn = {0021-9517},
	doi = {10.1016/j.jcat.2005.06.024},
	author = {Narayanan, Radha and El-Sayed, Mostafa A}
}
@article {1165,
	title = {Catalysis with transition metal nanoparticles in colloidal solution: Nanoparticle shape dependence and stability},
	journal = {Journal of Physical Chemistry B},
	volume = {109},
	number = {26},
	year = {2005},
	note = {Narayanan, R El-Sayed, MA},
	month = {Jul},
	pages = {12663-12676},
	abstract = {While the nanocatalysis field has undergone an explosive growth during the past decade, there have been very few studies in the area of shape-dependent catalysis and the effect of the catalytic process on the shape and size of transition metal nanoparticles as well as their recycling potential. Metal nanoparticles of different shapes have different crystallographic facets and have different fraction of surface atoms on their corners and edges, which makes it interesting to study the effect of metal nanoparticle shape on the catalytic activity of various organic and inorganic reactions. Transition metal nanoparticles are attractive to use as catalysts due to their high surface-to-volume ratio compared to bulk catalytic materials, but their surface atoms could be so active that changes in the size and shape of the nanoparticles could occur during the course of their catalytic function, which could also affect their recycling potential. In this Feature Article, we review our work on the effect of the shape of the colloidal nanocatalyst on the catalytic activity as well as the effect of the catalytic process on the shape and size of the colloidal transition metal nanocatalysts and their recycling potential. These studies provide important clues on the mechanism of the reactions we studied and also can be very useful in the process of designing better catalysts in the future.},
	isbn = {1520-6106},
	doi = {10.1021/jp051066p},
	author = {Narayanan, Radha and El-Sayed, Mostafa A}
}
@article {836,
	title = {Chemistry and properties of nanocrystals of different shapes},
	journal = {Chemical Reviews},
	volume = {105},
	year = {2005},
	pages = {1025-1102},
	publisher = {ACS Publications},
	isbn = {0009-2665},
	doi = {10.1021/cr030063a},
	url = {http://dx.doi.org/10.1021/cr030063a},
	author = {Burda, Clemens and Chen, X. and Narayanan, Radha and El-Sayed, Mostafa A}
}
@article {1176,
	title = {Changing catalytic activity during colloidal platinum nanocatalysis due to shape changes: Electron-transfer reaction},
	journal = {Journal of the American Chemical Society},
	volume = {126},
	number = {23},
	year = {2004},
	note = {Narayanan, R El-Sayed, MA},
	month = {Jun},
	pages = {7194-7195},
	isbn = {0002-7863},
	doi = {10.1021/ja0486061},
	author = {Narayanan, Radha and El-Sayed, Mostafa A}
}
