@article {1350,
	title = {Enhancing Colloidal Metallic Nanocatalysis: Sharp Edges and Corners for Solid Nanoparticles and Cage Effect for Hollow Ones},
	journal = {Acc Chem Res},
	year = {2013},
	note = {MEDLINE AN 2013361444(Journal; Article; (JOURNAL ARTICLE))},
	month = {//},
	abstract = {There are two main classes of metallic nanoparticles: solid and hollow. Each type can be synthesized in different shapes and structures. Practical use of these nanoparticles depends on the properties they acquire on the nanoscale. Plasmonic nanoparticles of silver and gold are the most studied, with applications in the fields of sensing, medicine, photonics, and catalysis. In this Account, we review our group{\textquoteright}s work to understand the catalytic properties of metallic nanoparticles of different shapes. Our group was the first to synthesize colloidal metallic nanoparticles of different shapes and compare their catalytic activity in solution. We found that the most active among these were metallic nanoparticles having sharp edges, sharp corners, or rough surfaces. Thus, tetrahedral platinum nanoparticles are more active than spheres. We proposed this happens because sharper, rougher particles have more valency-unsatisfied surface atoms (i.e., atoms that do not have the complete number of bonds that they can chemically accommodate) to act as active sites than smoother nanoparticles. We have not yet resolved whether these catalytically active atoms act as catalytic centers on the surface of the nanoparticle (i.e., heterogeneous catalysis) or are dissolved by the solvent and perform the catalysis in solution (i.e., homogenous catalysis). The answer is probably that it depends on the system studied. In the past few years, the galvanic replacement technique has allowed synthesis of hollow metallic nanoparticles, often called nanocages, including some with nested shells. Nanocage catalysts show strong catalytic activity. We describe several catalytic experiments that suggest the reactions occurred within the cage of the hollow nanocatalysts: (1) We synthesized two types of hollow nanocages with double shells, one with platinum around palladium and the other with palladium around platinum, and two single-shelled nanocages, one made of pure platinum and the other made of pure palladium. The kinetic parameters of each double-shelled catalyst were comparable to those of the single-shelled nanocage of the same metal as the inside shell, which suggests the reactions are taking place inside the cavity. (2) In the second set of experiments, we used double-shelled, hollow nanoparticles with a plasmonic outer gold surface and a non-plasmonic inner catalytic layer of platinum as catalysts. As the reaction proceeded and the dielectric function of the interior gold cavity changed, the plasmonic band of the interior gold shell shifted. This strongly suggested that the reaction had taken place in the nanocage. (3) Finally, we placed a catalyst on the inside walls of hollow nanocages and monitored the corresponding reaction over time. The reaction rate depended on the size and number of holes in the walls of the nanoparticles, strongly suggesting the confinement effect of a nanoreactor.[on SciFinder (R)]},
	isbn = {1520-4898},
	author = {Mahmoud, Mahmoud A. and Narayanan, Radha and El-Sayed, Mostafa A.}
}
@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 {1121,
	title = {Some aspects of colloidal nanoparticle stability, catalytic activity, and recycling potential},
	journal = {Topics in Catalysis},
	volume = {47},
	number = {1-2},
	year = {2008},
	note = {Narayanan, Radha El-Sayed, Mostafa A.},
	month = {Mar},
	pages = {15-21},
	abstract = {In this review article, we examine many important aspects of the nanocatalysis field such as size and shape dependent nanocatalysis, the stability of nanoparticles during its catalytic function, and their recycling potential. We provide an overview of some of the work in the literature pertinent to these topics and also discuss some of our own work in these important areas. Some examples of how the catalytic activity is affected by the size of the nanoparticles are discussed as well as how the catalytic process affects the nanoparticle size after its catalytic function. The synthesis of platinum nanoparticles of different shapes is surveyed and the dependence of nanoparticle shape on the catalytic activity is discussed. In addition, changes in the nanoparticle shape and resulting changes in the catalytic activity are also discussed. The recycling potential of the metal nanocatalysts is also highlighted. In addition, a simple examination of the mechanism of nanocatalysis is discussed.},
	isbn = {1022-5528},
	doi = {10.1007/s11244-007-9029-0},
	author = {Narayanan, Radha 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 {1167,
	title = {Effect of colloidal nanocatalysis on the metallic nanoparticle shape: The Suzuki reaction},
	journal = {Langmuir},
	volume = {21},
	number = {5},
	year = {2005},
	note = {Narayanan, R El-Sayed, MA},
	month = {Mar},
	pages = {2027-2033},
	abstract = {Dominantly tetrahedral shaped poly(vinylpyrrolidone) -platinum (PVP-Pt) nanoparticles are shown to catalyze the Suzuki reaction between phenylboronic acid and iodobenzene but are not as active as the spherical palladium nanoparticles studied previously. The dominantly tetrahedral PVP-Pt nanoparticles (55 +/- 4\% regular tetrahedral, 22 +/- 2\% distorted tetrahedral, and 23 +/- 2\% spherical nanoparticles) are synthesized by using the hydrogen reduction method. The transmission electron microscopy (TEM) results show that a transformation of shape from tetrahedral to spherical Pt nanoparticles takes place 3 h into the first cycle of the reaction. After the first cycle, the spherical nanoparticles have a similar size distribution to that of the tetrahedral nanoparticles before the reaction and the observed shape distribution is 18 +/- 6\% regular tetrahedral, 28 +/- 5\% distorted tetrahedral, and 54 +/- 5\% spherical nanoparticles. After the second cycle of the Suzuki reaction, the shape distribution is 13 +/- 5\% regular tetrahedral, 24 +/- 5\% distorted tetrahedral, and 63 +/- 7\% spherical nanoparticles. After the second cycle, the transformed spherical nanoparticles continue to grow, and this could be due to the strong capping action of the higher molecular weight PVP (M-w = 360 000), which makes the nanoparticles more resistant to aggregation and precipitation, unlike the Pd nanoparticles capped with the lower molecular weight PVP (M-w = 40 000) used previously. The transformation in shape also occurs when the nanoparticles are refluxed in the presence of the solvent, sodium acetate, and iodobenzene and results in spherical nanoparticles with a similar size distribution to that of the tetrahedral nanoparticles before any perturbations. However, in the presence of phenylboronic acid, the regular tetrahedral nanoparticles remain dominant (51 6\%) and maintain their size. These results support our previous studies in which we proposed that phenylboronic acid binds to the nanoparticle surface and thus acts as a capping agent for the particle and reacts with the iodobenzene. Recycling the nanoparticles results in a drastic reduction of the catalytic activity, and this must be due to the transformation of shape from the dominantly tetrahedral to the larger dominantly spherical nanoparticles. This also supports results in the literature that show that spherical platinum nanoparticles do not catalyze this reaction.},
	isbn = {0743-7463},
	doi = {10.1021/la047600m},
	author = {Narayanan, Radha and El-Sayed, Mostafa A}
}
@article {1166,
	title = {FTIR study of the mode of binding of the reactants on the Pd nanoparticle surface during the catalysis of the Suzuki reaction},
	journal = {Journal of Physical Chemistry B},
	volume = {109},
	number = {10},
	year = {2005},
	note = {Narayanan, R El-Sayed, MA},
	month = {Mar},
	pages = {4357-4360},
	abstract = {In the Suzuki reaction between phenylboronic acid and iodobenzene catalyzed by palladium nanoparticles, our previous studies suggested that the phenylboronic acid adsorbs on the nanoparticle surface and then interacts with the iodobenzene that is present in solution. In the present study, FTIR is used to examine the change in the vibrational frequencies of phenylboronic acid in films with and without the addition of palladium nanoparticles. The large change in the B-O stretching frequency of phenylboronic acid from 1348 to 1376 cm(-1) in the presence of sodium acetate and palladium nanoparticles strongly suggests that the mode of binding of phenylboronic acid to the Pd nanoparticle surface involves a B-O-Pd type of bonding. Shifts in the B-C stretching mode and the out-of-plane phenyl C-C ring deformation bands associated with phenylboronic acid provide additional confirmations of the binding process. It is also shown that the phenylboronic acid needs to be in the deprotonated form in the presence of sodium acetate (phenylboronate anion) to bind to the palladium nanoparticle surface. No changes in the characteristic bands of iodobenzene were observed in films made in the presence of the palladium nanoparticles. The FTIR studies provide proof of the mode of binding that occurs in the nanoparticle Surface for the first time and also confirms the mechanism of the Suzuki reaction that we proposed previously.},
	isbn = {1520-6106},
	doi = {10.1021/jp044659t},
	author = {Narayanan, Radha and El-Sayed, Mostafa A}
}
@article {820,
	title = {Raman Studies on the Interaction of the Reactants with the Platinum Nanoparticle Surface during the Nanocatalyzed Electron Transfer Reaction},
	journal = {The Journal of Physical Chemistry B},
	volume = {109},
	year = {2005},
	note = {doi: 10.1021/jp053526k},
	month = {2005},
	pages = {18460 - 18464},
	publisher = {American Chemical Society},
	abstract = {Raman studies are conducted to understand the specific interactions between the individual reactants and the platinum nanoparticle surface during the nanocatalyzed electron transfer reaction between hexacyanoferrate (III) ions and thiosulfate ions. When Pt nanoparticles are added to the thiosulfate ion solution, a shift in the symmetric SS stretching mode is observed compared to the frequency observed for the free thiosulfate ions in solution, suggesting that binding to the Pt nanoparticle surface occurs via the S- ion. It is also observed that there are no shifts in the symmetric and asymmetric OSO bending or SO stretching frequencies. This suggests that the thiosulfate ions do not bind to the nanoparticle surface via the O- ion. When platinum nanoparticles are added to the hexacyanoferrate(III) ion solution, evidence is found for both adsorbed hexacyanoferrate(III) ions and a platinum cyanide complex. For adsorbed hexacyanoferrate(III) ions, the CN stretching frequency is observed at 2101 cm-1 and the Fe?C stretching frequency is found at 368 cm-1. The observed CN stretching frequencies located at 2147 and 2167 cm-1 provide strong evidence that there is a Pt(CN)42- platinum cyanide complex formed. In addition, the Pt?C?N band is also observed at 2054 cm-1. These observed bands provide spectroscopic evidence that the hexacyanoferrate(III) ions dissolve by forming a complex with the surface platinum atoms of the nanoparticles. Raman spectra of the product mixtures are obtained after the completion of the reaction when carried out with higher reactant concentrations to observe the Raman spectra, but with a similar 10:1 ratio of thiosulfate to hexacyanoferrate(III) ions as used previously, with and without PVP?Pt nanoparticles at a correspondingly higher concentration. It is observed that there are no shifts in the characteristic Raman bands associated with hexacyanoferrate(II) ions and no evidence for the formation of adsorbed hexacyanoferrate(II) species or platinum cyanide complexes in the presence of the platinum nanoparticles. In addition, there is evidence for the shifted symmetric SS stretching mode, suggesting that some of the unreacted thiosulfate (present in large excess) is bound to the Pt nanoparticle surface. Thus, under the actual reaction conditions, the hexacyanoferrate(III) ions preferentially react with adsorbed thiosulfate ions to form the reaction products, and this supports the surface catalytic mechanism we proposed previously.Raman studies are conducted to understand the specific interactions between the individual reactants and the platinum nanoparticle surface during the nanocatalyzed electron transfer reaction between hexacyanoferrate (III) ions and thiosulfate ions. When Pt nanoparticles are added to the thiosulfate ion solution, a shift in the symmetric SS stretching mode is observed compared to the frequency observed for the free thiosulfate ions in solution, suggesting that binding to the Pt nanoparticle surface occurs via the S- ion. It is also observed that there are no shifts in the symmetric and asymmetric OSO bending or SO stretching frequencies. This suggests that the thiosulfate ions do not bind to the nanoparticle surface via the O- ion. When platinum nanoparticles are added to the hexacyanoferrate(III) ion solution, evidence is found for both adsorbed hexacyanoferrate(III) ions and a platinum cyanide complex. For adsorbed hexacyanoferrate(III) ions, the CN stretching frequency is observed at 2101 cm-1 and the Fe?C stretching frequency is found at 368 cm-1. The observed CN stretching frequencies located at 2147 and 2167 cm-1 provide strong evidence that there is a Pt(CN)42- platinum cyanide complex formed. In addition, the Pt?C?N band is also observed at 2054 cm-1. These observed bands provide spectroscopic evidence that the hexacyanoferrate(III) ions dissolve by forming a complex with the surface platinum atoms of the nanoparticles. Raman spectra of the product mixtures are obtained after the completion of the reaction when carried out with higher reactant concentrations to observe the Raman spectra, but with a similar 10:1 ratio of thiosulfate to hexacyanoferrate(III) ions as used previously, with and without PVP?Pt nanoparticles at a correspondingly higher concentration. It is observed that there are no shifts in the characteristic Raman bands associated with hexacyanoferrate(II) ions and no evidence for the formation of adsorbed hexacyanoferrate(II) species or platinum cyanide complexes in the presence of the platinum nanoparticles. In addition, there is evidence for the shifted symmetric SS stretching mode, suggesting that some of the unreacted thiosulfate (present in large excess) is bound to the Pt nanoparticle surface. Thus, under the actual reaction conditions, the hexacyanoferrate(III) ions preferentially react with adsorbed thiosulfate ions to form the reaction products, and this supports the surface catalytic mechanism we proposed previously.},
	isbn = {1520-6106},
	doi = {10.1021/jp053526k},
	url = {http://dx.doi.org/10.1021/jp053526k},
	author = {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}
}
@article {819,
	title = {Effect of Colloidal Catalysis on the Nanoparticle Size Distribution:  Dendrimer-Pd vs PVP-Pd Nanoparticles Catalyzing the Suzuki Coupling Reaction},
	journal = {The Journal of Physical Chemistry B},
	volume = {108},
	year = {2004},
	note = {doi: 10.1021/jp037169u},
	month = {2004},
	pages = {8572 - 8580},
	publisher = {American Chemical Society},
	abstract = {A comparison of the stability and catalytic activity of PAMAM?OH generation 4 dendrimer?Pd nanoparticles (1.3 {\textpm} 0.1 nm) with the previously studied PVP?Pd nanoparticles (2.1 {\textpm} 0.1 nm) in the Suzuki coupling reaction between phenylboronic acid and iodobenzene is conducted. After the first cycle, the average size of the PVP?Pd nanoparticles increases by 38\% and the dendrimer?Pd nanoparticles increases by 54\%. After the second cycle, the PVP?Pd nanoparticles decrease in size by 24\% whereas the dendrimer?Pd nanoparticles continue to increase in size by 35\%. The strong encapsulating action of the PAMAM?OH generation 4 dendrimer?Pd nanoparticles could make the rate of conversion to the full nanoparticle size slow, resulting in a large excess Pd metal atom concentration in solution, resulting in the continuous growth of the nanoparticles during the catalytic reaction. The effect of the individual reactants on the stability of the dendrimer?Pd nanoparticles has also been investigated and found to be similar to that observed for the PVP?Pd nanoparticles previously. It was found that the nanoparticle size growth occurs while refluxing in the presence of only the solvent, sodium acetate, or iodobenzene. However, the presence of phenylboronic acid is found to inhibit the particle growth, suggesting that it acts as a capping agent. Thus, the reaction mechanism must involve the adsorption of phenylboronic acid to the nanoparticle surface, which subsequently reacts with the iodobenzene in solution. This is similar to the mechanism found previously on PVP?Pd nanoparticles, suggesting that the mechanism is insensitive to the capping material used. The ratio of the yield of biphenyl formed in the second cycle to that in the first cycle is higher for the dendrimer?Pd nanoparticles catalyzed reaction than for the PVP?Pd nanoparticles. This could be due to the greater stability of the dendrimer?Pd nanoparticles and the increase in its size during the reaction. The larger PVP?Pd nanoparticles studied previously is believed to aggregate and precipitate out of solution during the second cycle. The presence of excess dendrimer is found to severely diminish the catalytic activity of the dendrimer?Pd nanoparticles and also diminishes the change in the Pd nanoparticle size during the catalysis.A comparison of the stability and catalytic activity of PAMAM?OH generation 4 dendrimer?Pd nanoparticles (1.3 {\textpm} 0.1 nm) with the previously studied PVP?Pd nanoparticles (2.1 {\textpm} 0.1 nm) in the Suzuki coupling reaction between phenylboronic acid and iodobenzene is conducted. After the first cycle, the average size of the PVP?Pd nanoparticles increases by 38\% and the dendrimer?Pd nanoparticles increases by 54\%. After the second cycle, the PVP?Pd nanoparticles decrease in size by 24\% whereas the dendrimer?Pd nanoparticles continue to increase in size by 35\%. The strong encapsulating action of the PAMAM?OH generation 4 dendrimer?Pd nanoparticles could make the rate of conversion to the full nanoparticle size slow, resulting in a large excess Pd metal atom concentration in solution, resulting in the continuous growth of the nanoparticles during the catalytic reaction. The effect of the individual reactants on the stability of the dendrimer?Pd nanoparticles has also been investigated and found to be similar to that observed for the PVP?Pd nanoparticles previously. It was found that the nanoparticle size growth occurs while refluxing in the presence of only the solvent, sodium acetate, or iodobenzene. However, the presence of phenylboronic acid is found to inhibit the particle growth, suggesting that it acts as a capping agent. Thus, the reaction mechanism must involve the adsorption of phenylboronic acid to the nanoparticle surface, which subsequently reacts with the iodobenzene in solution. This is similar to the mechanism found previously on PVP?Pd nanoparticles, suggesting that the mechanism is insensitive to the capping material used. The ratio of the yield of biphenyl formed in the second cycle to that in the first cycle is higher for the dendrimer?Pd nanoparticles catalyzed reaction than for the PVP?Pd nanoparticles. This could be due to the greater stability of the dendrimer?Pd nanoparticles and the increase in its size during the reaction. The larger PVP?Pd nanoparticles studied previously is believed to aggregate and precipitate out of solution during the second cycle. The presence of excess dendrimer is found to severely diminish the catalytic activity of the dendrimer?Pd nanoparticles and also diminishes the change in the Pd nanoparticle size during the catalysis.},
	isbn = {1520-6106},
	doi = {10.1021/jp037169u},
	url = {http://dx.doi.org/10.1021/jp037169u},
	author = {Narayanan, Radha and El-Sayed, Mostafa A}
}
@article {1177,
	title = {Effect of nanocatalysis in colloidal solution on the tetrahedral and cubic nanoparticle SHAPE: Electron-transfer reaction catalyzed by platinum nanoparticles},
	journal = {Journal of Physical Chemistry B},
	volume = {108},
	number = {18},
	year = {2004},
	note = {Narayanan, R El-Sayed, MA},
	month = {May},
	pages = {5726-5733},
	abstract = {The stability of tetrahedral and cubic platinum nanoparticles during the catalysis of the electron-transfer reaction between hexacyanoferrate (III) and thiosulfate ions in colloidal solution at room temperature was studied by using TEM and HRTEM. Before the reaction, the dominantly tetrahedral nanoparticles have a shape distribution of 55 +/- 4\% regular tetrahedral, 22 +/- 2\% distorted tetrahedral, and 23 +/- 2\% spherical nanoparticles, and the dominantly cubic nanoparticles have an initial shape distribution of 56 4\% regular cubes, 13 +/- 1\% distorted cubes, and 31 +/- 3\% truncated octahedral nanoparticles. The amount of tetrahedral nanoparticles decreases by 60 +/- 5\% after the first cycle and by 62 +/- 4\% after the second cycle of the reaction. In the case of cubic nanoparticles, the amount of cubic nanoparticles decreases by 39 +/- 5\% after the first cycle and by 66 +/- 5\% after the second cycle compared to before the reaction. After the first and second cycles of the reaction, there are a greater percentage of distorted tetrahedral and distorted cubic nanoparticles present. The rate of the dissolution of the surface Pt atoms is faster for the tetrahedral nanoparticles than for the cubic nanoparticles. This suggests that tetrahedral nanoparticles, with their sharp corners and edges, are more sensitive and more liable to shape changes during nanocatalysis. The presence of just hexacyanoferrate ions in the solution with the nanoparticles is found to increase the amount of distorted tetrahedral and distorted cubes present much more than during the reaction. The presence of only the thiosulfate ions does not seem to affect the size or shape distribution which might result from the capping ability of this anion and thus protects the nanoparticles.},
	isbn = {1520-6106},
	doi = {10.1021/jp0493780},
	author = {Narayanan, Radha and El-Sayed, Mostafa A}
}
@article {1175,
	title = {Shape-dependent catalytic activity of platinum nanoparticles in colloidal solution},
	journal = {Nano Letters},
	volume = {4},
	number = {7},
	year = {2004},
	note = {Narayanan, R El-Sayed, MA},
	month = {Jul},
	pages = {1343-1348},
	abstract = {The activation energies and the average rate constants are determined in the 298 K-318 K temperature range for the early stages of the nanocatalytic reaction between hexacyanoferrate (111) and thiosulfate ions using 4.8 +/- 0.1 nm tetrahedral, 7.1 +/- 0.2 nm cubic, and 4.9 +/- 0.1 nm "near spherical" nanocrystals. These kinetic parameters are found to correlate with the calculated fraction of surface atoms located on the corners and edges in each size and shape.},
	isbn = {1530-6984},
	doi = {10.1021/nl0495256},
	author = {Narayanan, Radha and El-Sayed, Mostafa A}
}
@article {1186,
	title = {Effect of catalysis on the stability of metallic nanoparticles: Suzuki reaction catalyzed by PVP-palladium nanoparticles},
	journal = {Journal of the American Chemical Society},
	volume = {125},
	number = {27},
	year = {2003},
	note = {Narayanan, R El-Sayed, MA},
	month = {Jul},
	pages = {8340-8347},
	abstract = {The small size of nanoparticles makes them attractive in catalysis due to their large surface-to-volume ratio. However, being small raises questions about their stability in the harsh chemical environment in which these nanoparticles find themselves during their catalytic function. In the present work, we studied the Suzuki reaction between phenylboronic acid and iodobenzene catalyzed by PVP-Pd nanoparticles to investigate the effect of catalysis, recycling, and the different individual chemicals on the stability and catalytic activity of the nanoparticles during this harsh reaction. The stability of the nanoparticles to the different perturbations is assessed using TEM, and the changes in the catalytic activity are assessed using HPLC analysis of the product yield. It was found that the process of refluxing the nanoparticles for 12 h during the Suzuki catalytic reaction increases the average size and the width of the distribution of the nanoparticles. This was attributed to Ostwald ripening in which the small nanoparticles dissolve to form larger nanoparticles. The kinetics of the change in the nanoparticle size during the 12 h period show that the nanoparticles increase in size during the beginning of the reaction and level off toward the end of the first cycle. When the nanoparticles are recycled for the second cycle, the average size decreases. This could be due to the larger nanoparticles aggregating and precipitating out of solution. This process could also explain the observed loss of the catalytic efficiency of the nanoparticles during the second cycle. It is also found that the addition of biphenyl to the reaction mixture results in it poisoning the active sites and giving rise to a low product yield. The addition of excess PVP stabilizer to the reaction mixture seems to lead to the stability of the nanoparticle surface and size, perhaps due to the inhibition of the Ostwald ripening process. This also decreases the catalytic efficiency of the nanoparticles due to capping of the nanoparticle surface. The addition of phenylboronic acid is found to lead to the stability of the size distribution as it binds to the particle surface through the O- of the OH group and acts as a stabilizer. Iodobenzene is found to have no effect and thus probably does not bind strongly to the surface during the catalytic process. These two results might have an implication on the catalytic mechanism of this reaction.},
	isbn = {0002-7863},
	doi = {10.1021/ja035044x},
	author = {Narayanan, Radha and El-Sayed, Mostafa A}
}
@article {1185,
	title = {Effect of catalytic activity on the metallic nanoparticle size distribution: Electron-transfer reaction between Fe(CN)(6) and thiosulfate ions catalyzed by PVP-platinum nanoparticles},
	journal = {Journal of Physical Chemistry B},
	volume = {107},
	number = {45},
	year = {2003},
	note = {Narayanan, R El-Sayed, MA},
	month = {Nov},
	pages = {12416-12424},
	abstract = {The electron-transfer reaction between hexacyanoferrate(III) ions and thiosulfate ions is known to be catalyzed by platinum nanoparticles. In the present study, the stability and catalytic activity of the PVP-Pt nanoparticle during its catalytic function for this electron-transfer reaction is studied. The stability of the nanoparticles after various perturbations was assessed using TEM, and the kinetics of the reaction was followed using absorption spectroscopy. The studies were conducted on four different concentrations of PVP-Pt nanoparticles. It was found that the average size and width of the PVP-Pt nanoparticles decrease slightly after the first and second cycles of the electron-transfer reaction. The size and size distribution width do not change in the presence of only the thiosulfate reactant, whereas the presence of only the hexacyanoferrate reactant results in a reduction of the nanoparticle size. The reduction in the nanoparticle size in the presence of hexacyanoferrate(HI) ions is proposed to result from the dissolution of surface Pt atoms through complexation with the strong cyanide ligand. Thiosulfate ions bind to the nanoparticle surface and act as a capping material, resulting in the stability of the nanoparticles. Judging from these observations, it is possible that the mechanism of this catalytic reaction involves the thiosulfate ions binding to the free sites on the surface of the nanoparticles, followed by reaction with hexacyanoferrate ions approaching the nanoparticle surface from the solution. Conducting the reaction with the nanoparticles preexposed to thiosulfate results in very little change in the centers and widths of the size distributions of the nanoparticles, thus suggesting that thiosulfate ions bind to the nanoparticle surface and inhibit desorption of Pt atoms by hexacyanoferrate(III) ions. The kinetics of the electron-transfer reaction during the first and second cycles is similar. The activation energy of the nanoparticle catalytic reaction is found to decrease linearly with increasing nanoparticle concentration during both the first and second cycles. If increasing the nanoparticle concentration leads to more aggregation, then these results suggest that the aggregated Pt has greater catalytic activity than the individual nanoparticles.},
	isbn = {1520-6106},
	doi = {10.1021/jp035647v},
	author = {Narayanan, Radha and El-Sayed, Mostafa A}
}
