@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 {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 {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 {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}
}
