Shape-Controlled Synthesis of Pd Nanocrystals in Aqueous Solutions

FEATUR

www.afm-journal.de

Shape-Controlled Synthesis of Pd Nanocrystals inAqueous Solutions

EARTIC

By Byungkwon Lim, Majiong Jiang, Jing Tao, Pedro H. C. Camargo,

Yimei Zhu, and Younan Xia*

LE

This article provides an overview of recent developments regarding synthesis

of Pd nanocrystals with well-controlled shapes in aqueous solutions. In a

solution-phase synthesis, the final shape taken by a nanocrystal is determined

by the twin structures of seeds and the growth rates of different

crystallographic facets. Here, the maneuvering of these factors in an aqueous

system to achieve shape control for Pd nanocrystals is discussed. L-ascorbic

acid, citric acid, and poly(vinyl pyrrolidone) are tested for manipulating the

reduction kinetics, with citric acid and Br– ions used as capping agents to

selectively promote the formation of {111} and {100} facets, respectively. The

distribution of single-crystal versus multiple-twinned seeds can be further

manipulated by employing or blocking oxidative etching. The shapes obtained

for the Pd nanocrystals include truncated octahedron, icosahedron,

octahedron, decahedron, hexagonal and triangular plates, rectangular bar,

and cube. The ability to control the shape of Pd nanocrystals provides a great

ctrical, and
opportunity to systematically investigate their catalytic, ele
plasmonic properties.

1. Introduction

Palladium is a key catalyst invaluable to many industrialprocesses; notable examples include hydrogenation/dehydro-genation reactions, low-temperature reduction of automobilepollutants, and petroleum cracking.[1–3] It has also demonstratedremarkable performance in hydrogen storage at room tempera-ture and atmospheric pressure.[4] In organic chemistry, a largenumber of carbon-carbon bond forming reactions such asSuzuki, Heck, and Stille coupling all depend on catalysts basedupon Pd(0) or its compounds.[5–7] It has been shown that theactivity and selectivity

[*] Prof. Y. Xia, Dr. B. Lim, P. H. C. CamargoDepartment of Biomedical Engineering, Washington UniversitySt. Louis, Missouri 63130 (USA)E-mail: [email protected]

M. JiangDepartment of Chemistry, Washington UniversitySt. Louis, Missouri 63130 (USA)

Dr. J. Tao, Dr. Y. ZhuCondensed Matter Physics & Materials Science DepartmentBrookhaven National LaboratoryUpton, New York 11973 (USA)

DOI: 10.1002/adfm.200801439

Adv. Funct. Mater. 2009, 19, 189–200 � 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinh

of a catalyst can be greatly enhanced bythe use of nanocrystals enclosed by specificcrystal facets that are intrinsically moreactive for a particular reaction.[8–11] Sincethe facets exposed on a nanocrystal aredetermined by its shape, an exquisite shapecontrol of Pd nanocrystals is thereforehighly desired for tailoring their catalyticproperties and also a prerequisite for highperformance in various catalytic applica-tions.

Over the last few years, polyol synthesishas been a preferred method of preparingnoble metal nanocrystals with well-definedshapes because of the ability of polyolssuch as ethylene glycol (EG) to dissolvemany metal salts (precursors to the noblemetals), and also due to the temperature-dependent reducing power of such poly-ols.[12–28] The primary step of this processinvolves the reduction of a metal salt by a

polyol at an elevated temperature in the presence of a polymericstabilizer such as poly(vinyl pyrrolidone) (PVP). Despite itssuccess in controlling the shape of many noble metalnanocrystals, however, the major products are often restrictedto cuboctahedrons or truncated cubes due to the fast reductionand growth rates associated with the strong reducing power of apolyol.[13,17,18,22] In addition, polyol synthesis is often troubled bythe irreproducible results associated with the shape of metalnanocrystals due to the presence of trace amounts of impurities(known or unknown) that are usually contained in commercialchemical reagents such as EG. For example, we have shown thatfor the polyol synthesis of Ag nanocrystals based upon EG, eventhe presence of a ppm level of Cl� impurity could drastically alterthe morphology of the final products.[13] Furthermore, themechanism by which metal ions are reduced in a polyol synthesisis still poorly understood. Our recent results indicate that in thetemperature range of 140–160 8C, the primary reducing agent isglycolaldehyde, being produced via thermal oxidation of EGby theoxygen in air, rather than acetaldehyde derived from thedehydration of EG, which has been assumed as the reducingagent in a typical polyol synthesis for several decades.[29] Ofcourse, knowledge of the exact mechanism underlying thereaction pathway is essential to both reproducibility and scale-upproduction of metal nanocrystals with well-controlled shapes.

Compared to polyol synthesis, a water-based system shouldprovide a more environmentally sound route to the production of

eim 189

FEATUREARTIC

LE

www.afm-journal.de

190

noble metal nanocrystals because it does not involve toxic organicsolvents. In a water-based synthesis, the reduction of a metalprecursor can be readily achieved by introducing variousreducing agents that are safe and easy to handle, with typicalexamples including L-ascorbic acid, citric acid, and alcohol.Importantly, by using chemicals with different reducing powers,one can easily manipulate the reduction kinetics and conduct asystematic study on the formation mechanism of differentlyshaped Pd nanocrystals. In addition, high-purity water is morereadily accessible than an organic solvent, so we do not worryabout unexpected results that might be caused by trace amountsof impurities. A water-based system also provide a number ofother merits such as simplicity, convenience, and the potential forlarge-scale production.[30–37] For these reasons, we and othergroups have recently started to pay more attention to the water-based system as a more attractive route to the shape-controlledsynthesis of noble metal nanocrystals. This Feature Articleprovides a brief account of these efforts, with a primary focus onour own work. More specifically, we want to demonstrate thefacile synthesis of Pd nanocrystals with a rich variety of shapesincluding truncated octahedron, icosahedron, octahedron, dec-ahedron, hexagonal and triangular thin plates, rectangular bar,and cube. It is worth noting that some of these shapes (e.g.,octahedron and thin plates) could not be achieved using the polyolmethod.

2. Reducing Agents

Reduction kinetics plays a key role in controlling the nucleationand growth of nanocrystals. In this work, we employed L-ascorbicacid, citric acid, and PVP as reducing agents (Fig. 1). Amongthem, L-ascorbic acid, which is commonly known as vitamin C,can serve as a strong reducing agent for fast reduction of a Pdprecursor, as well as other noble metal salts.[38] Citric acid worksas a reducing agent in a manner similar to the mechanism of aconventional citrate-based synthesis of noble metal nanocrys-

Figure 1. Structural illustration of L-ascorbic acid, citric acid, and OH-

terminated PVP and their oxidized forms due to the redox reactions with

Pd2þ ions.

� 2009 WILEY-VCH Verlag GmbH

tals.[32,35,39,40] For most chemical syntheses of metal nanocrystals,PVP has been widely used as a steric stabilizer to protect theproduct from agglomeration. As we recently demonstrated using13C NMR spectroscopy, however, the ends of commerciallyavailable PVP (if it is synthesized in an aqueous medium) areterminated in the hydroxyl (–OH) group due to the involvementof water and hydrogen peroxide in polymerization.[30] Therefore,it can act like a long-chain alcohol and serve as a class of weakreducing agents. Note that the reducing power of an alcoholdecreases as its alkyl chain becomes longer. In a water-basedsystem, the difference in reducing power for these reagentsenables one to control the reduction kinetics, and thus the shapeof Pd nanocrystals. Table 1 summarizes the reaction conditionsemployed in this work and the shapes of Pd nanocrystals obtainedunder each condition. To appreciate the difference in reductionrate associated with the reducing agents, the concentration of[PdCl4]

2� was monitored by UV-vis spectroscopy during the earlystage of each reaction. As illustrated in Figure 2, the absorptionpeak at 425 nm that corresponds to [PdCl4]

2� completelydisappeared at t¼ 10min in the case of L-ascorbic acid,demonstrating the fast reduction of [PdCl4]

2� by L-ascorbic acidunder this reaction condition. In contrast, the reduction rate wasmuch slower in the case of PVP, indicating the weak reducingpower of PVP. In the case of citric acid, it exhibited a moderatereducing power between PVP and L-ascorbic acid. These resultsalso demonstrate that a wide range of reduction rates could beaccessed by using different reducing agents.

3. L-Ascorbic Acid for Fast Reduction andFormation of Truncated Octahedrons

In a solution-phase synthesis of metal nanocrystals, the numberof twin planes in seeds plays the most important role indetermining the shapes taken by the final products.[25,41] Whenthe reduction is relatively fast, there are sufficient Pd atoms thatcan be added to the surface of seeds for continuous growth,leading to a rapid size increase. In this case, the seeds tend to takea single-crystal or multiple-twinned structure in an attempt tominimize the total surface energy of the system under a given

Younan Xia was born inJiangsu, China, in 1965. Hereceived a B.S. degree inchemical physics from theUniversity of Science andTechnology of China (USTC)in 1987, an M.S. degree ininorganic chemistry from theUniversity of Pennsylvania in1993 and a Ph.D. in physicalchemistry from HarvardUniversity in 1996. He iscurrently at WashingtonUniversity in the Department

of Biomedical Engineering, where his research centers on thedesign and synthesis of nanostructured materials withcontrolled properties.

& Co. KGaA, Weinheim Adv. Funct. Mater. 2009, 19, 189–200

FEATUREARTIC

LE

www.afm-journal.de

Figure 2. UV-vis spectra from tests on 17.4mM Na2PdCl4 solutions con-

taining 31mM L-ascorbic acid, 84mM citric acid, and 87mM PVP as a

reducing agent after heating for 10min; heating was performed at 100 8Cfor the solutions containing L-ascorbic acid and PVP and at 90 8C for the

solution containing citric acid. These conditions correspond to those for

the syntheses of Pd truncated octahedrons, decahedrons, and hexagonal

and triangular plates, respectively. The absorption peak at 425 nm is directly

proportional to the concentration of the [PdCl4]2� species.

Table 1. Summary of shapes that have been obtained under different experimental conditions.

Precursor (mM) Reducing

agent (mM)

Capping

agent (mM)

Stabilizer

(mM)

Solution

temp. (8C)Reaction

time (h)

Shape Schematic drawing[b]

Na2PdCl4 (17.4) L-ascorbic

acid (31)

– PVP

(87)[a]100 3 truncated

octahedron

Na2PdCl4 (5.8) citric acid

(28)

citric acid

(28)

PVP

(29)[a]90 26 icosahedron


(28)

citric acid

(28)

PVP

(37)[a]90 26 octahedron


(84)

citric acid

(84)

PVP

(87)[a]90 26 decahedron

Na2PdCl4 (17.4) PVP

(87)[a]– PVP

(87)[a]100 3 hexagonal/

triangular

plate


acid (31)

KBr

(230)

PVP

(87)[a]100 3 bar


acid (31)

KBr

(460)

PVP

(87)[a]100 3 cube/

pentagonal

rod


acid (31)

KBr

(230)

PVP

(87)[a]80 3 cube

Na2PdCl4 (17.4) PVP (87)[a] KBr

(460)

PVP

(87)[a]100 3 cube

[a]The concentration of PVP was calculated in terms of the repeating unit. All of these syntheses were carried out with a fixedmolar ratio of PVP to Na2PdCl4 at 5. [b]The blue and

gray colors represent the {100} and {111} facets, respectively. Twin planes are delineated with red lines.

Adv. Funct. Mater. 2009, 19, 189–200 � 2009 WILEY-VCH Verl

volume. For a face-centered cubic (fcc) structure, the surfaceenergies of the low-index crystallographic facets that typicallyencase a nanocrystal are in the order of g{111}< g{100}< g{110}. This sequence implies that a single-crystal seed shouldtake an octahedral or tetrahedral shape in order to maximize theexpression of {111} facets and minimize the total surface energy.Both shapes, however, have larger surface areas than a cube of thesame volume. As a result, it is not unexpected for a single-crystalseed to evolve into a truncated octahedron (or the so-called Wulffpolyhedron) enclosed by eight {111} and six {100} faces. Thisshape has a nearly spherical profile and thus the smallest surfacearea to minimize the total interfacial free energy.

Multiple-twinned seeds with a fivefold symmetry (e.g.,icosahedral and decahedral seeds) are another class of favorableseeds typically observed in a solution-phase synthesis. In additionto single-crystal seeds, these fivefold twinned seeds can beproduced under the same reaction condition and grow into the so-called multiple-twinned particles (MTPs) including icosahedronsand decahedrons. These twinned species achieve the lowest totalfree energy by maximizing the surface coverage with the {111}facets. As MTPs grow rapidly into large sizes, however, the totalfree energy of the system will drastically go up because of theincrease in strain energy as caused by the twin defects. Forexample, in an ideal decahedron, which can be described as an

ag GmbH & Co. KGaA, Weinheim 191

FEATUREARTIC

LE

www.afm-journal.de

192

ensemble of five tetrahedrons with twin-related adjoining faces, agap of 7.35 8 is generated as the theoretical angle between two{111} planes of a tetrahedron is 70.53 8. As a result, the spacemust be compensated for by increasing the separation betweenadjacent atoms, giving rise to internal lattice strain. Similar to adecahedron, internal strains are also involved in closing the gapsformed in an icosahedron, which consists of twenty tetrahedrons.If the MTPs expand in a lateral dimension, the lattice strain willkeep increasing and the low surface energy of the {111} facets canno longer remedy the excessive strain energy required to sustainthe twinned structures. As a result, MTPs are thermodynamicallyfavored primarily at relatively small sizes. Alternatively, adecahedron can grow along the fivefold axis to generate a

Figure 3. A–C) TEM images of Pd samples obtained by heating 11mL of an

containing 17.4mM Na2PdCl4, 31mM L-ascorbic acid, and 87mM PVP at 100

B) 1 h 30min, and C) 3 h. In (A), twinned particles are indicated by tw. D) H

single truncated octahedron shown in (C) recorded along the [011] zone a

sponding FT pattern (inset). The lattice spacings of 1.94 and 2.24 A can be ind

{111} of fcc Pd, respectively. In the FT pattern, the spots circled and squared

the {200} and {111} reflections, respectively. E) Illustration of the proposedme

single-crystal truncated octahedrons were obtained in high yields.


pentagonal rod without any significant increase in strain energy,if the {100} facets on the side surface can be stabilized.[16,42]

In essence, the population of seeds with different twinstructures is mainly determined by the statistical thermody-namics related to the free energies of different species incombination with the reduction kinetics regarding the generationand addition of metal atoms to the nuclei. In practice, thedistribution of single-crystal versus twinned seeds can be furthermanipulated via the introduction of other processes such asoxidative etching, in which zero-valent metal atoms are oxidizedback to ions. When the synthesis is conducted in air, acombination of a ligand from the metal ion such as Cl� andO2 from air can result in a powerful etchant such as the O2/Cl

�

aqueous solution

8C for A) 30min,

RTEM image of a

xis and the corre-

exed as {200} and

can be indexed as

chanism by which

& Co. KGaA, Weinheim

pair for both the nuclei and seeds. Compared totwinned seeds, single-crystal seeds are moreresistant to oxidative etching due to the lack ofdefect zones on the surface. By takingadvantage of this selectivity, the populationof differently structured seeds in a solution-phase synthesis can be manipulated in con-trollable fashion. For example, as we havedemonstrated for Ag system, all twinned seedscan be removed from the solution by adding atrace amount of Cl� to the reaction, leavingbehind only single-crystal truncated octahe-drons or cubes in the products.[13] Here wedemonstrate that truncated octahedrons of Pdcan be produced in high yields by coupling thefast reduction of a Pd precursor with oxidativeetching for the selective removal of twinnedstructures. Experimentally, Na2PdCl4 is themost commonly used precursor for Pd becauseof its stability in air and good solubility in avariety of solvents. When the reaction isconducted in air with Na2PdCl4 as a precursor,no additional Cl� is needed to initiate oxidativeetching as this ligand will be released fromNa2PdCl4 during the reaction. In this synth-esis, L-ascorbic acid is used as a reducing agentto ensure the fast reduction of a Pd precursor,which is critical to the formation of thermo-dynamically favorable species such as trun-cated octahedrons and MTPs.

In a typical protocol, we synthesizedtruncated octahedrons of Pd by heating11mL of an aqueous solution containing17.4mM Na2PdCl4, 31mM L-ascorbic acid,and 87mM PVP at 100 8C. Figure 3A–C showstypical transmission electron microscopy(TEM) images of Pd samples taken at differentstages of the reaction. At t¼ 30min, thesample contained both single-crystal truncatedoctahedrons and MTPs (Fig. 3A). As thereaction proceeded to t¼ 1 h 30min, however,all the twinned particles disappeared from thesolution (Fig. 3B). During the next 1 h 30min,there was no significant change in shape, butthe remaining truncated octahedronsincreased in size until they reached an average

Adv. Funct. Mater. 2009, 19, 189–200

FEATUREARTIC

LE

www.afm-journal.de

diameter of about 8 nm. Figure 3C shows a typical TEM image ofthe sample obtained at t¼ 3 h, which revealed that truncatedoctahedrons of Pd had become the major product (>95%). Thestructure of these truncated octahedrons was supported by high-resolution TEM (HRTEM) analysis. Figure 3D shows a HRTEMimage of a single truncated octahedron recorded along the [011]zone axis and the corresponding Fourier transform (FT) pattern(inset); both data indicate that the Pd truncated octahedron is apiece of single crystal with the exposed facets being {111} and{100} planes. The fringes with lattice spacings of 1.94 and 2.24 Acan be indexed as {200} and {111} of fcc Pd, respectively. BothTEM and HRTEM analyses confirmed the absence of Pdnanocrystals with twinned structures in the final product.

The higher reactivity of twinned structures towards oxidativeetching can be attributed to the higher density of twin defects on
their surfaces, which are much higher in energyrelative to the single-crystal regions and thus aremore susceptible to an oxidative environ-ment.[13,17] As a result, MTPs are preferentiallyattacked by the enchant, oxidized, and dissolvedinto the solution in the early stage of a synthesis(Fig. 3E). During the initial dissolution stage,etching of MTPs could increase the concentrationof [PdCl4]
2�. As the reaction continues, Pd atomsformed by reducing [PdCl4]

2� with L-ascorbic acidare added to the existing truncated octahedrons, bywhich they grow into larger sizes. Our resultsdemonstrate that the oxidative etching can alsoserve as a powerful means for purifying and thuscontrolling the shape of Pd nanocrystals for awater-based system.

Figure 4. A) SEM and B) TEM images of Pd icosahedrons synthesized by heating 11mL of

an aqueous solution containing 5.8mM Na2PdCl4, 28mM citric acid, and 29mM PVP at 90 8Cfor 26 h; C) SEM and D) TEM images of Pd octahedrons prepared under the same condition

as in (A) except that the concentrations of Na2PdCl4 and PVP were increased to 7.4 and

37mM, respectively; E) SEM and F) TEM images of Pd decahedrons prepared under the

same condition as in (A) except that the concentrations of Na2PdCl4, PVP, and citric acid

were increased to 17.4, 87, and 84mM, respectively (modified with permission from [35],

copyright 2007 Wiley-VCH).

4. Citric Acid for Moderate Reductionand Surface Capping in theFormation of Icosahedrons,Octahedrons, and Decahedrons

The multiple-twinned nanocrystals of Pd such asicosahedrons and decahedrons produced duringthe reaction are often difficult to retain in a typicalsolution-phase synthesis conducted in air due tothe intrinsically corrosive environment, as dis-cussed in Section 3. To remedy this issue, citricacid can be introduced into the reaction. Interest-ingly, citric acid (or citrate ions) can serve as notonly a reducing agent but also a capping agent tostabilize these structures thanks to its strongbinding to the {111} facets of Pd.[32,35] In addition,these species can effectively block oxidativeetching by competing with oxygen adsorptiononto the Pd surface or exhausting the adsorbedoxygen atoms.[32] In this way, it becomes possibleto produce Pd icosahedrons and decahedrons.More specifically, we demonstrate that the shape ofthe final Pd nanocrystals can be controlled byvarying the concentrations of Na2PdCl4 and citricacid to selectively produce Pd icosahedrons,


octahedrons, and decahedrons whose surface is covered by the{111} facets.

We synthesized Pd icosahedrons by heating 11mL of anaqueous solution containing 5.8mM Na2PdCl4, 28mM citric acid,and 29mM PVP at 90 8C for 26 h. Both scanning electronmicroscopy (SEM) and TEM studies revealed that Pd icosahe-drons with edge lengths of approximately 25 nm were obtained asthe major product (�80%) in addition to a small amount ofoctahedrons (�20%), as shown in Figure 4A and B. Interestingly,we found that the shape of Pd nanocrystals was highly sensitive tothe concentration of Na2PdCl4. For instance, increasing theconcentration of Na2PdCl4 by �25% while carefully keepingthe concentration of citric acid and the molar ratio of PVP toNa2PdCl4 the same as in the synthesis of icosahedrons led tothe formation of octahedrons as the major species (�90%) in the


FEATUREARTIC

LE

www.afm-journal.de

194

product, with �10% other shapes including triangular plates anddecahedrons (Fig. 4C and D). These results suggest thatformation of octahedrons was more favorable than that oficosahedrons at relatively high Na2PdCl4 precursor concentra-tions. The reaction condition could be further manipulated toobtain Pd decahedrons by increasing both the Na2PdCl4 and citricacid concentrations beyond those employed for the formation oficosahedrons and octahedrons. Figure 4E and F shows typicalSEM and TEM images of Pd decahedrons obtained by increasingthe concentrations of Na2PdCl4 and citric acid to 17.4 and 84mM,respectively. Both images revealed that Pd decahedrons with sizesof 25–40 nm were produced as a major product (> 80%).

As demonstrated in this work, citric acid favors the formation ofPd nanocrystals enclosed by the {111} facets, such as icosahe-drons, octahedrons, and decahedrons. It has been shown bysimulation that icosahedral, decahedral, and truncated octahedralclusters are favored for Pd at small (with the number of atomsN< 100), medium (100<N< 6500), and large sizes (N> 6500),respectively.[43–45] When the Pd precursor concentration is low, theseeds are most likely to adopt an icosahedral structure due to theslow addition of Pd atoms and remain small for a long period oftime. As a result, Pd icosahedrons can be produced in high yields.When the concentration of Pd precursor is increased, thegeneration and addition of Pd atoms becomes faster and thereforemost seeds will take a decahedral or truncated octahedral shapebecause of their rapidly increased size. At a relatively low
concentration of citric acid, however, formation ofdecahedrons is less favorable than that of octahe-drons because of lattice strain caused by twindefects and their preferential dissolution viaoxidative etching in the presence of an O2/Cl
�

pair. As a result, single-crystal octahedrons aremorelikely to remain in the final product. To improve theyield of decahedrons, a higher concentration forthe capping agent (citric acid or citrate ions in thiscase) is required to ensure sufficient capping ofthe {111} facets, which not only reduces the surfaceenergy and thus compensates for the extra strainenergy caused by twinning but also efficientlyprotects them from oxidative environment. Forthese reasons, the decahedrons can be preservedand thus accumulate throughout the reaction.

Figure 5. A) TEM image of hexagonal and triangular Pd nanoplates synthesized by heating

11mL of an aqueous solution containing 17.4mMNa2PdCl4 and 87mM PVP at 100 8C for 3 h.

Note that this reaction condition is the same as in Figure 3C except for the exclusion of L-

ascorbic acid from the reaction. In this case, PVP serves as a reducing agent. B) HRTEM

image taken from the flat top face of a single nanoplate and corresponding FT pattern (inset).

In the FT pattern, the spots circled and squared can be indexed to the {220} and forbidden 1/

3{422} reflections, respectively, in which the latter indicates the presence of planar defects

such as stacking faults in the {111} planes. C) Proposed mechanism for the formation of Pd

nanoplates. Pd atoms nucleate to form nuclei with a metastable rhcp structure with the

inclusion of stacking faults. At a slow reduction rate, these nuclei can evolve into plate-like

seeds, which further grow into hexagonal and triangular nanoplates with their top and

bottom faces enclosed by the {111} facets.

5. PVP for Slow Reduction in theFormation of Hexagonal andTriangular Nanoplates

If the reduction becomes considerably slow, bothnucleation and growth may deviate from athermodynamically controlled pathway. This typeof synthesis is known as a kinetically controlledprocess, and the final nanocrystal shape typicallydeviates from those favored by thermodynamics(i.e., structures with higher free energies). In onecase, thin plates with hexagonal and triangularshapes can be formed, with both top and bottomfaces covered by the {111} facets. In practice,kinetically controlled synthesis can be achieved by


substantially slowing down the reduction rate. For this purpose,PVP can be used as an ideal reducing agent thanks to its weakreducing power, as described in Section 2, thereby enabling akinetic control over both nucleation and growth.

Among various shapes, 2D anisotropic nanostructures ofnoble metals such as hexagonal and triangular nanoplates havedrawn increasing attention because of their unique opticalproperties and potential use in chemical and biological sensing.The nanoplates exhibit unique localized surface plasmonresonance (LSPR) features, such as quadrupole resonance peaksthat are absent in small nanospheres, and are supposed to beparticularly active for surface-enhanced Raman scattering (SERS)thanks to their sharp corners and edges.[46–49] To date, a numberof different synthetic routes have been developed to generatenanometer- or micrometer-sized thin plates of various noblemetals, including light-induced conversion of nanospheres tonanoplates,[50,51] reduction of ametal precursor in the presence ofa specific capping agent or surfactant,[52] and mild annealing ofself-organized nanocrystals on carbon substrates.[53] Here wedemonstrate that Pd nanoplates with hexagonal and triangularshapes can be simply produced by reducing Na2PdCl4 with PVP,without the involvement of additional capping agents or specificsubstrates.

Figure 5A shows a TEM image of Pd nanoplates synthesized byheating 11mL of an aqueous solution containing 17.4mM

Na2PdCl4 and 87mM PVP at 100 8C for 3 h. Note that this reaction


FEATUREARTIC

LE

www.afm-journal.de

condition is the same as in Figure 3C except for the exclusion of L-ascorbic acid from the reaction. As shown in Figure 5A, theproduct mainly consists of nanoplates (> 90%) with bothhexagonal and triangular shapes and sizes in the range of 50–80 nm. Most of the triangular nanoplates have truncated corners.Figure 5B shows a HRTEM image taken from the flat top face of asingle nanoplate and the corresponding FTpattern (inset). In theHRTEM image, the fringes with a lattice spacing of 1.4 A can beseen and indexed as {220} of fcc Pd. The FTpattern is composedof spots with a six-fold rotational symmetry, indicating that the topand bottom faces of Pd nanoplates are enclosed by the {111}planes. The spots circled and squared can be indexed to the {220}and forbidden 1/3{422} reflections, respectively. Observation ofthe forbidden spots associated with 1/3{422} diffractionssuggests that planar defects such as stacking faults are presentin the {111} plane perpendicular to the electron beam.[54]

The prevalence of a plate-like morphology in this synthesis canbe attributed to the slow reduction rate, which is associated withthe weak reducing power of PVP. As an fcc metal, the crystalstructure of Pd provides no intrinsic driving force to grow into 2Dnanostructures such as thin plates. Compared to a polyhedralnanocrystal of the same volume, a nanoplate with top and bottomfaces covered by the {111} facets exhibits a large surface area, andthus its total free energy is relatively high regardless of the surfacecoverage by the {111} facets. As a result, formation of plate-likemorphology is not favored in terms of thermodynamics. To obtainthis class of highly anisotropic shapes, one needs to break thecubic symmetry of the lattice. One way to accomplish this is toincorporate planar defects such as stacking faults into thenanocrystals. For an fcc lattice, the stacking sequence of {111}layers should be ABCABCABC. When stacking faults areintroduced, however, they disrupt the stacking sequence forone or two layers (e.g., ABCABABC). In the classical nucleationtheory, it is assumed that the nucleus takes a spherical shape dueto surface tension and the same crystal structure as the bulk solid.However, it has recently been shown by simulation that for fcccrystals, nuclei formed in the early stages of nucleation tend totake a random hexagonal close-packed (rhcp) structure – arandom mixture of both hexagonal close packing (hcp) andcubic close packing – with the inclusion of stacking faults ratherthan a pure fcc phase because the strain energy caused bystacking faults is low and the rhcp structure is slightly more stablethan fcc at this stage.[55–57] When the reduction is relatively fast,these nuclei can evolve into polyhedral seeds such as singlecrystal, truncated octahedrons and fivefold twinned decahedronsin an effort to lower the total surface energy, as discussed inSection 3. If the reduction is considerably slowed, however, thesenuclei with a metastable rhcp structure could remain small for along period of time, due to the slow addition of atoms, andgradually evolve into plate-like seeds while retaining theirstructure characterized by the presence of similar stacking faultsin a vertical direction (Fig. 5C). These seeds can further grow intohexagonal and triangular nanoplates via preferential addition ofPd atoms onto their edges because they are bound by a mix of{110} and {100} facets with a higher surface energy than the topand bottom {111} faces. Therefore, the slow reduction ratederived from the weak reducing power of PVP seems to be mostimportant factor in achieving the kinetically controlled synthesisof Pd nanoplates.


6. Bromide as a Capping Agent in Promoting the{100} Facets

The preparation of single-crystal Pd nanocrystals encased withonly one type of facet is desirable, particularly for systematicstudies of, and applications in, catalysis. Once the seed is fixed interms of twin structure, the final shape taken by the nanocrystalwill be determined by the relative rates at which differentcrystallographic facets grow, in which the facets with a slowergrowth rate will be exposedmore on the nanocrystal’s surface. Forexample, if the fast growing facets correspond to the {111} of atruncated octahedron, the final crystal shape will be a cubeenclosed by the slow growing {100} facets. In contrast, if the fastgrowing facets correspond to the {100} faces, the final crystalshape will be an octahedron enclosed by slow growing {111}facets. Such a dynamic evolution can selectively enlarge one set offacets at the expense of others on a nanocrystal. In a solution-phase synthesis, the seeds can grow into nanocrystals withdrastically different shapes by controlling the relative growth ratesof different facets. In particular, impurities or capping agents canchange the order of free energies of different facets through theirchemical interaction with a metal surface. This alternation maysignificantly affect the relative growth rates of different facets andthus lead to different morphologies for the final pro-ducts.[22,24,26,58,59] For example, as we have demonstrated for aAg system, PVP can serve as a capping agent whose oxygen atomsbind most strongly to the {100} facets of Ag. For single-crystal Agseeds terminated with only {111} and {100} facets, thispreferential capping drives the addition of Ag atoms primarilyonto the poorly passivated {111} facets, resulting in the formationof nanocubes enclosed by the {100} facets.[12,60,61] However, PVPis too big to have a capping effect on small Pd nanocrystals. As aresult, single-crystalline Pd nanocrystals (typically< 10 nm) tendto take the truncated octahedral shape when prepared in thepresence of PVP as shown in Figure 3C.[17] Interestingly, Br� ionscan provide this function as a small ionic capping agent, capableof preferentially chemisorbing onto the {100} facets of Pdnanocrystals.[26] In this way, Br� ions stabilize the {100} facetsand thus favor the formation of Pd nanocrystals enclosed by the{100} facets, as for nanobars and nanocubes.

6.1. Synthesis of Nanobars

1D nanostructures of Pd are of particular interest because they arepromising building blocks for fabricating nanoscale electronicdevices. For instance, it has been shown that Pd can be used forresistance-based detection of hydrogen gas due to its exceptionalsensitivity towards hydrogen.[62] In addition, Pd can form reliableand reproducible ohmic contacts with carbon nanotubes (CNTs)as it has a relatively high work function and can easily wet thecarbon surface, which makes it useful for CNT-based devicessuch as field-effect transistors.[63,64] Here we demonstrate that Pdnanobars with 1D anisotropic structure can be produced in highyields under the fast reduction of a Pd precursor by L-ascorbic acidin the presence of Br� ions. During the course of this work, it wasfound that the selective activation process could initiateanisotropic growth of Pd nanocrystals if the concentration of


FEATUREARTIC

LE

www.afm-journal.de

Figure 6. A and B) TEM images of Pd nanobars prepared under the same condition as in

Figure 3C except that the reaction was conducted in the presence of 230mM KBr. C) HRTEM

image of a single nanobar and the corresponding FT pattern (inset). The lattice spacing of

1.94 A can be indexed as {200} of Pd. In the FT pattern, the spots circled and squared can be

indexed to the {200} and {220} reflections, respectively. D) Schematic illustration of the

mechanism responsible for the formation of nanobars.

196

Br� ions was relatively low, resulting in theformation of nanobars. Unlike nanorods com-monly observed in Ag and Au systems,[16,42,65] Pdnanobars are single crystals with a rectangularcross section and bound by the {100} facets.

We synthesized Pd nanobars by heating 11mLof an aqueous solution containing 17.4mM

Na2PdCl4, 31mM L-ascorbic acid, 87mM PVP,and 230mM KBr at 100 8C for 3 h. These reactionconditions are the same as in the synthesis oftruncated octahedrons shown in Figure 3C exceptfor the addition of KBr. Figure 6A and B showsTEM images of the resulting product at differentmagnifications. It can be clearly seen that theproduct mainly consists of nanobars (>95%) witha larger dimension along one direction than thosealong the other two directions. The width andaspect ratio of the nanobars were 6–8 nm and2–4 nm, respectively. The structure of nanobarswas characterized by HRTEM study, an imagefrom which is Figure 6C. The HRTEM image of asingle nanobar recorded along the [001] zone axisdisplayed well-resolved, continuous fringes with alattice spacing of 1.94 A, which can be indexed as{200} of fcc Pd. The corresponding FT pattern iscomposed of spots with a square symmetry. Theseresults confirm that the Pd nanobar is a piece ofsingle crystal enclosed by the {100} facets.

As discussed in Section 3, fast reductioncoupled with oxidative etching yields truncatedoctahedrons, which are expected to further evolveinto nanocubes in the presence of Br� ions due to
their preferential chemisorption on the {100} facets of truncatedoctahedrons. At a relatively low concentration of Br� ions, theformation of nanobars can be attributed to anisotropic growth ofcubic Pd nanocrystals driven by selective activation of one of theirsix {100} faces (Fig. 6D).[26] When the concentration of Br� ions islow, the surface of a nanocube is most likely coated by a relativelythin layer of Br� ions. In this case, oxidative etching could removesome of the Br� ions from the Pd surface. As in the case ofcorrosion of Pd nanocubes by pitting process or galvanicreplacement between Ag nanocubes and HAuCl4, the oxidativeetching can selectively take place on only one of the six {100}faces, making this particular face more active than others. As aresult, this localized oxidative etching creates a favorable site forthe subsequent addition of Pd atoms (i.e., growth) and thusfacilitates the preferential growth on this face. This selectiveactivation process could break the symmetry of a nanocube andeventually lead to its anisotropic growth into a nanobar.
6.2. Synthesis of Nanocubes

Noble metal nanocrystals with a cubic shape have recently beensynthesized by a polyol method in the presence of a polymeric orionic capping agent such as PVP or Agþ ions.[12,21,22] Here weshow that Pd nanocubes can be produced simply by modifyingthe reaction conditions used for the synthesis of Pd nanobars.


Figure 7A and B shows TEM images of the Pd sample preparedunder the same condition as in Figure 6A except that theconcentration of KBr was increased to 460mM. It can be seen thatPd nanocubes with an average size of 10 nm were formed as themajor product (�95%) in addition to a small amount of fivefoldtwinned pentagonal nanorods (�5%) with a diameter of about8 nm and lengths up to 200 nm. The HRTEM image of a singlenanocube (Fig. 7C) clearly shows continuous fringes with aperiod of 1.94 A, which is consistent with the {200} lattice spacingof fcc Pd. The corresponding FT pattern (Fig. 7C, inset) reveals asquare symmetry for the spots. These results indicate that the Pdnanocube is a piece of single crystal bound by the {100} facets.Figure 7D represents a HRTEM image taken from the end of asingle pentagonal nanorod, which reveals that the {111} twinboundary is straight and continuous along the entire longitudinalaxis of the nanorod. The corresponding FTpattern (Fig. 7D, inset)can be interpreted as the overlapping of the [100] and [112] zoneaxes of fcc Pd. The presence of spots with a square symmetryindicates that the side faces of the nanorod are bound by the {100}facets.

In the formation of a cubic morphology, the presence of Br�

ions at a relatively high concentration is significant. In this case,all the faces of a Pd nanocube could be stabilized by a monolayerof Br� ions, which could effectively block the localized oxidativeetching and thus prohibit its anisotropic growth into a nanobar. Atthe same time, some of the decahedral seeds may evolve intopentagonal nanorods under these reaction conditions. It has been


FEATUREARTIC

LE

www.afm-journal.de

Figure 7. A and B) TEM images of a Pd sample prepared under the same condition as in

Figure 6A except that the concentration of KBr was increased to 460mM. C and D) HRTEM

images and the corresponding FT patterns (insets) of a single nanocube and pentagonal

nanorod, respectively. The lattice spacings of 1.94 and 1.4 A can be indexed as {200} and

{220} of Pd, respectively. In the FT patterns, the spots circled, squared, and triangled can be

indexed to the {200}, {220} and {112} reflections, respectively. E) Schematic illustration of

the mechanism responsible for the formation of pentagonal nanorods.

suggested that the twin defects in a decahedron, which bringabout internal strain in the lattice, are largely responsible for theanisotropic growth of pentagonal nanorods.[16,42] In principle,when atoms in a decahedron are located far from the central axis,the strain in the lattice will be extremely high. Thus, the strain willbe greatly increased if a decahedron grows in lateral dimensions.In contrast, elongation of a decahedron in a direction parallel tothe twin planes does not increase the lattice strain. Consequently,the decahedrons can preferentially grow along the fivefold axisinto pentagonal nanorods to retain a low strain energy. This kindof anisotropic growth requires the presence of a capping agent forthe stabilization of newly formed {100} side faces. Accordingly,when a sufficient amount of Br� ions are introduced into thereaction at the fast reduction rate, some of the decahedral seedscan quickly evolve into pentagonal nanorods with their side {100}faces being stabilized by chemisorbed Br� ions.

By tuning the experiment condition, we could further improvethe yield of Pd nanocubes. It was found that at a low Br� ion

Adv. Funct. Mater. 2009, 19, 189–200 � 2009 WILEY-VCH Verlag GmbH & Co. KG

concentration, 1D anisotropic growth of nano-cubes into nanobars could be greatly suppressedby lowering the reaction temperature. When thereaction was performed at 80 8C while keepingother reaction parameters the same as inFigure 6A, nanobars with an aspect ratio of 2–4were rarely found in the product and most of thePd nanocrystals were of a cubic shape with sizes of10–12 nm, although some of them were slightlyelongated along one direction (Fig. 8A and B). Asdemonstrated in our previous studies, the etchingpower of the O2/Cl

� pair is reduced at low reactiontemperatures.[33] In this case, the localizedoxidative etching could be significantly eliminatedand, as a result, the initially formed nanocubescould maintain their cubic morphology andaccumulate throughout the reaction withoutgrowing into nanobars via selective activationprocesses.

In another demonstration, we found thatintroduction of Br� ions into a kineticallycontrolled process (with PVP as a reducing agent)could enable the production of Pd nanocubes athigh yields. Figure 8C and D shows TEM imagesof Pd nanocubes with an average size of �10 nmprepared under the same condition as in thesynthesis of the hexagonal and triangular nano-plates shown in Figure 5A except for the presenceof 460mM KBr in the reaction solution. It is clearthat the Br� ions induced a cubic morphology forthe resulting Pd nanocrystals. The formation ofnanocubes in this synthesis implies that single-crystal seeds are involved in the nucleation step, inwhich Br� ions could break the rhcp structure ofinitially formed nuclei through their chemicalinteraction with Pd surfaces and lead to itstransformation into an fcc structure. During thegrowth step, these single-crystal seeds couldfurther evolve into nanocubes via preferentialchemisorption of Br� ions on the {100} facets. Itshould be pointed out that pentagonal nanorods

are absent in the product because the multiple-twinned seeds arenot favored in this kinetically controlled synthesis.

7. Seeded Growth

With regards to crystal growth, pre-formed nanocrystals withwell-defined shapes can serve as primary sites, i.e., seeds, forheterogeneous nucleation of addedmetal atoms.When the addedatoms have the same crystal structure and lattice constant as theseed, the crystal structure of the seed is transferred to the entirenanocrystal via epitaxial growth, although the final shape of ananocrystal may deviate from that of the initial seed due to thecrystal habit governed by the growth rates of different crystal-lographic facets. This so-called seeded growth approach offers analternative way for synthesizing uniform nanocrystals by takingadvantage of the absence of homogenous nucleation during thegrowth step. Although seeded growth has been proven to be

aA, Weinheim 197

FEATUREARTIC

LE

www.afm-journal.de

Figure 8. A and B) TEM images of Pd nanocubes prepared under the same condition as in

Figure 6A except that the reaction temperature was lowered to 80 8C. C and D) TEM images

of Pd nanocubes prepared under the same condition as in Figure 5A except that the reaction

was conducted in the presence of 460mM KBr.

198

extremely powerful in Au, Ag, and Pt systems,[66–70] littleattention has been paid to the seeded growth of Pd nanocrystals.Here we demonstrate that Pd nanocrystals can be completelyconverted from cubes to truncated and regular octahedrons viaseeded growth. The Pd nanocubes with an average size of�10 nm shown in Figure 8C were first prepared as described in

Figure 9. A) Schematic illustration of seeded growth of Pd octahedrons with and without truncation

at corners from cubic Pd seeds. B) TEM and C) HRTEM images of Pd truncated octahedrons

obtained by adding 1mL of the as-prepared Pd nanocube solution (shown in Fig. 8C) to 9mL of an

aqueous solution containing 10.5mM Na2PdCl4 and 52.5mM PVP and heating at 90 8C for 3 h.

D) TEM and E) HRTEM images of Pd octahedrons prepared under the same condition as in (B)

except that the concentrations of Na2PdCl4 and PVP were increased to 21 and 105mM, respectively.

Section 6.2 and then used as the seeds toinitiate further nanocrystal growth whenadditional Pd precursor was added and thenslowly reduced by PVP. As more Pd atomswere added to the Pd nanocubes, they grewinto truncated or regular octahedronsdepending on the amount of Na2PdCl4added to the reaction (Fig. 9A).

In a typical synthesis, we obtained Pdtruncated octahedrons by adding 1mL ofthe as-prepared Pd nanocube suspension to9mL of an aqueous solution containing10.5mM Na2PdCl4 and 52.5mM PVP andheating at 90 8C for 3 h. As shown inFigure 9B, the resulting product mainlycontained Pd nanocrystals with a truncatedoctahedral shape (>95%) and edge lengthsof 12–15 nm. HRTEM imaging of a singletruncated octahedron recorded along the[011] zone axis and the corresponding FTpattern indicated that the Pd truncatedoctahedron was a piece of single crystal withthe exposed facets of both {111} and {100}(Fig. 9C). The fringes with lattice spacings

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinh

of 1.94 and 2.24 A can be indexed as {200} and{111} of Pd, respectively. When the concentrationsof Na2PdCl4 and PVP were increased to 21 and105mM, respectively, with other reaction para-meters being kept the same, Pd nanocrystals with aregular octahedral shape were obtained in a highyield (>95%), as shown in Figure 9D and E.

Recently, it was reported that Ag and Aunanocrystals with a cubic morphology could evolveinto truncated and then regular octahedrons asmore atoms were added to the {100} faces ofnanocubes.[69,71] The shape conversion of Pdnanocrystals observed in the present work mightalso occur through a similar process. In this case,the initial addition of Pd atoms on a nanocuberesults in the generation of the {111} faces at eachcorner of the nanocube. In the absence of specificcapping agents, the continued addition of Pdatoms prefers to the {100} faces of the Pdnanocrystal as the relative surface energies ofthe low-index crystallographic facets for an fccmetal are in the order of g{111}< g{100}< g{110}. As the crystal growth continues, thesurface fraction of the slow growing {111} facesincreases at the expense of the faster growing{100} faces. At a relatively low concentration ofNa2PdCl4, this process yields truncated octahe-drons bound by both the {111} and {100} facets.When the concentration of Na2PdCl4 is further

increased, the {100} facets completely disappear via continuedaddition of Pd atoms, leaving behind regular octahedronsenclosed by the {111} facets. We expect that this approach basedon seeded growth could be utilized to further control the shapeand size of Pd nanocrystals by taking advantage of a rich variety ofshapes that have been already achieved.

eim Adv. Funct. Mater. 2009, 19, 189–200

FEATUREARTIC

LE

www.afm-journal.de

8. Concluding Remarks and Future Direction

We have demonstrated the capability and feasibility of using awater-based system for the high-yield production of Pdnanocrystals with a rich variety of shapes, including truncatedoctahedrons, icosahedrons, octahedrons, decahedrons, hexagonaland triangular thin plates, rectangular bars, and cubes. Thesuccess of this approach depends on a number of parameterssuch as the reduction kinetics, oxidative etching, and surfacecapping. These parameters could be combined to provide aneffective route to maneuver both the twin structures of seeds andthe surface facets, which are two key factors in determining thefinal shape of a nanocrystal. Specifically, the twin structures ofseeds could be controlled by introducing different reducingagents and thus manipulating the reduction kinetics. When thereduction rate was relatively fast, the reaction was underthermodynamic control and, as a result, single-crystal andmultiple-twinned seeds were dominant in the nucleation stage.The distribution of these seeds could be further altered byemploying or blocking oxidative etching. In one case, themultiple-twinned seeds could be selectively removed from thesolution by the O2/Cl

� etchant, leaving behind single-crystaltruncated octahedrons as a major product. Meanwhile, citric acid(or citrate ions) facilitated the formation of icosahedrons anddecahedrons by blocking the oxidative etching through its strongbinding to the {111} surfaces. When the reduction rate wasconsiderably slowed, the reaction became kinetically controlled.In this case, plate-like seeds with planar defects such as stackingfaults formed at the initial nucleation stage and then grew intohexagonal and triangular nanoplates that deviated from thethermodynamically favored shapes. In most cases, the bindingselectivity of a capping agent had a profound impact on the crystalhabit of Pd nanocrystals. They could interact more strongly withspecific facets and thus changed the order of free energies fordifferent crystallographic facets. This kind of chemisorption orsurface capping provided a means for controlling the relativegrowth rates of different facets and thus the final shape of Pdnanocrystals. As demonstrated in this work, Br� ions bind moststrongly to the {100} facets of Pd, resulting in the formation ofnanobars and nanocubes.

Controlling the shape of noble metal nanocrystals provides apowerful tool for tailoring their electronic, plasmonic, andcatalytic properties. For instance, we have recently demonstratedthat Pd nanoplates of 45 nm in edge length exhibit LSPR peaksredshifted to the visible region (up to 610 nm)[31] compared tosmall Pd nanocrystals (typically< 10 nm in size) whose LSPRpeaks are located in the UV region. Since the position of the LSPRpeak determines the wavelength of excitation for the maximumelectromagnetic field enhancement, these Pd nanoplates withredshifted LSPR peaks are expected to interact more intenselywith the laser of a commercial Raman spectrometer (514 and785 nm) and, as a result, exhibit stronger SERS activity. Anotherimportant property of Pd nanocrystals is their catalytic activity andselectivity towards chemical or electrochemical reactions. Forexample, it was recently found that in the electrocatalyticoxidation of formic acid with Pd nanocrystals, the peak currentmeasured for Pd cubes was five times higher than that for Pdoctahedrons.[9] This difference in activity was attributed to thedifferent oxidation rate of formic acid on the {100} and {111}


facets of Pd and illustrates the dependence of the electrochemicalactivity of Pd nanocrystals on the shape. However, the shape effectof Pd nanocrystals on the activity and selectivity towards variouschemical reactions such as hydrogenation and cross-couplingreactions still remains largely unexplored. We expect that themethodology for controlling the shape of Pd nanocrystalsdemonstrated in this work could provide a great opportunity tosystematically evaluate their shape-dependent catalytic properties,as well as to fully explore their applications in fields of sensing,storage of hydrogen gas, SERS, and fabrication of nanoelectronicdevices.

Acknowledgements

This work was supported in part by NSF (both DMR-0451788 and DMR-0804088), ACS (PRF, 44353-AC10), and a 2006 Director’s Pioneer Awardfrom NIH (5DP1D000798). B. L. was also partially supported by the KoreaResearch Foundation Grant funded by the Korean Government (KRF-2006-352-D00067). J. T. and Y. Z. were supported by the U.S. DOE/BES (DE-AC02-98CH10886).

Received: September 26, 2008

Published online: December 12, 2008

[1] M. Fernandez-Garcıa, A. Martınez-Arias, L. N. Salamanca, J. M. Coronado,

J. A. Anderson, J. C. Conesa, J. Soria, J. Catal. 1999, 187, 474.

[2] Y. Nishihata, J. Mizuki, T. Akao, H. Tanaka, M. Uenishi, M. Kimura, T.

Okamoto, N. Hamada, Nature 2002, 418, 164.

[3] J. M. Thomas, B. F. G. Johnson, R. Raja, G. Sankar, P. A. Midgley, Acc. Chem.

Res. 2003, 36, 20.

[4] L. Schlapbach, A. Zuttel, Nature 2001, 414, 353.

[5] M. T. Reetz, E. Westermann, Angew. Chem, Int. Ed. 2000, 39, 165.

[6] Y. Li, X. M. Hong, D. M. Collard, M. A. El-Sayed, Org. Lett. 2000, 2, 2385.

[7] S.-W. Kim, M. Kim, W. Y. Lee, T. Hyeon, J. Am. Chem. Soc. 2002, 124, 7642.

[8] R. Narayanan, M. A. El-Sayed, Nano Lett. 2004, 4, 1343.

[9] S. E. Habas, H. Lee, V. Radmilovic, G. A. Somorjai, P. Yang, Nat. Mater.

2007, 6, 692.

[10] K. M. Bratlie, H. Lee, K. Komvopoulos, P. Yang, G. A. Somorjai, Nano Lett.

2007, 7, 3097.

[11] C. Wang, H. Daimon, T. Onodera, T. Koda, S. Sun, Angew. Chem, Int. Ed.

2008, 47, 3588.

[12] Y. Sun, Y. Xia, Science 2002, 298, 2176.

[13] B. Wiley, T. Herricks, Y. Sun, Y. Xia, Nano Lett. 2004, 4, 1733.

[14] T. Herricks, J. Chen, Y. Xia, Nano Lett. 2004, 4, 2367.

[15] F. Kim, S. Connor, H. Song, T. Kuykendall, P. Yang, Angew. Chem, Int. Ed.

2004, 43, 3673.

[16] B. Wiley, Y. Sun, Y. Xia, Langmuir 2005, 21, 8077.

[17] Y. Xiong, J. Chen, B. Wiley, Y. Xia, S. Aloni, Y. Yin, J. Am. Chem. Soc. 2005,

127, 7332.

[18] Y. Xiong, J. Chen, B. Wiley, Y. Xia, Y. Yin, Z.-Y. Li, Nano Lett. 2005, 5, 1237.

[19] Y. Xiong, J. M. McLellan, J. Chen, Y. Yin, Z.-Y. Li, Y. Xia, J. Am. Chem. Soc.

2005, 127, 17118.

[20] J. Chen, T. Herricks, Y. Xia, Angew. Chem, Int. Ed. 2005, 44, 2589.

[21] H. Song, F. Kim, S. Connor, G. A. Somorjai, P. Yang, J. Phys. Chem. B 2005,

109, 188.

[22] D. Seo, J. C. Park, H. Song, J. Am. Chem. Soc. 2006, 128, 14863.

[23] B. J. Wiley, Y. Xiong, Z.-Y. Li, Y. Yin, Y. Xia, Nano Lett. 2006, 6, 765.

[24] B. J. Wiley, Y. Chen, J. M. McLellan, Y. Xiong, Z.-Y. Li, D. Ginger, Y. Xia,

Nano Lett. 2007, 7, 1032.


FEATUREARTIC

LE

www.afm-journal.de

200

[25] B. Wiley, Y. Sun, Y. Xia, Acc. Chem. Res. 2007, 40, 1067.

[26] Y. Xiong, H. Cai, B. J. Wiley, J. Wang, M. J. Kim, Y. Xia, J. Am. Chem. Soc.

2007, 129, 3665.

[27] Y. Xiong, Y. Xia, Adv. Mater. 2007, 19, 3385.

[28] C. Li, K. L. Shuford, Q.-H. Park, W. Cai, Y. Li, E. J. Lee, S. O. Cho, Angew.

Chem, Int. Ed. 2007, 46, 3264.

[29] S. E. Skrabalak, B. J. Wiley, M. Kim, E. V. Formo, Y. Xia, Nano Lett. 2008, 8,

2077.

[30] I. Washio, Y. Xiong, Y. Yin, Y. Xia, Adv. Mater. 2006, 18, 1745.

[31] Y. Xiong, I. Washio, J. Chen, H. Cai, Z.-Y. Li, Y. Xia, Langmuir 2006, 22,

8563.

[32] Y. Xiong, J. M. McLellan, Y. Yin, Y. Xia, Angew. Chem, Int. Ed. 2007, 46, 790.

[33] Y. Xiong, H. Cai, Y. Yin, Y. Xia, Chem. Phys. Lett. 2007, 440, 273.

[34] Y. Xiong, I. Washio, J. Chen, M. Sadilek, Y. Xia, Angew. Chem, Int. Ed. 2007,

46, 4917.

[35] B. Lim, Y. Xiong, Y. Xia, Angew. Chem, Int. Ed. 2007, 46, 9279.

[36] B. Lim, J. Wang, P. H. C. Camargo, M. Jiang, M. J. Kim, Y. Xia, Nano Lett.

2008, 8, 2535.

[37] B. Lim, P. H. C. Camargo, Y. Xia, Langmuir 2008, 24, 10437.

[38] H. Lee, S. E. Habas, S. Kweskin, D. Butcher, G. A. Somorjai, P. Yang, Angew.

Chem, Int. Ed. 2006, 45, 7824.

[39] A. Henglein, M. Giersig, J. Phys. Chem. B 2000, 104, 6767.

[40] M. Maillard, P. Huang, L. Brus, Nano Lett. 2003, 3, 1611.

[41] B. Wiley, Y. Sun, B. Mayers, Y. Xia, Chem. –Eur. J. 2005, 11, 454.

[42] Y. Sun, B. Mayers, T. Herricks, Y. Xia, Nano Lett. 2003, 3, 955.

[43] F. Baletto, R. Ferrando, Phys. Rev. B 2001, 63, 155408.

[44] F. Baletto, R. Ferrando, A. Fortunelli, F. Montalenti, C. Mottet, J. Chem.

Phys. 2002, 116, 3856.

[45] F. Baletto, R. Ferrando, Rev. Mod. Phys. 2005, 77, 371.

[46] J. E. Millstone, S. Park, K. L. Shuford, L. Qin, G. C. Schatz, C. A. Mirkin, J.

Am. Chem. Soc. 2005, 127, 5312.

[47] S. S. Shankar, A. Rai, A. Ahmad, M. Sastry, Chem. Mater. 2005, 17, 566.

[48] C. S. Ah, Y. J. Yun, H. J. Park, W.-J. Kim, D. H. Ha, W. S. Yun, Chem. Mater.

2005, 17, 5558.

[49] J. E. Millstone, G. S. Metraux, C. A. Mirkin, Adv. Funct. Mater. 2006, 16,

1209.


[50] R. Jin, Y. Cao, C. A. Mirkin, K. L. Kelly, G. C. Schatz, J. G. Zheng, Science

2001, 294, 1901.

[51] R. Jin, Y. C. Cao, E. Hao, G. S. Metraux, G. C. Schatz, C. A. Mirkin, Nature

2003, 425, 487.

[52] Y. Sun, Y. Xia, Adv. Mater. 2003, 15, 695.

[53] A. Courty, A.-I. Henry, N. Goubet, M.-P. Pileni, Nat. Mater. 2007, 6, 900.

[54] C. Lofton, W. Sigmund, Adv. Funct. Mater. 2005, 15, 1197.

[55] J. Zhu, M. Li, R. Rogers, W. Meyer, R. H. Ottewill, STS-73 Space Shuttle

Crew. W. B. Russel, P. M. Chaikin, Nature 1997, 387, 883.

[56] S. Auer, D. Frenkel, Nature 2001, 409, 1020.

[57] U. Gasser, E. R. Weeks, A. Schofield, P. N. Pusey, D. A. Weitz, Science 2001,

292, 258.

[58] S. Maksimuk, X. Teng, H. Yang, Phys. Chem. Chem. Phys. 2006, 8, 4660.

[59] S. Maksimuk, X. Teng, H. Yang, J. Phys. Chem. C 2007, 111, 14312.

[60] S. H. Im, Y. T. Lee, B. Wiley, Y. Xia, Angew. Chem, Int. Ed. 2005, 44, 2154.

[61] A. R. Siekkinen, J. M. McLellan, J. Chen, Y. Xia, Chem. Phys. Lett. 2006, 432,

491.

[62] F. Favier, E. C. Walter, M. P. Zach, T. Benter, R. M. Penner, Science 2001, 293,

2227.

[63] A. Javey, J. Guo, Q. Wang, M. Lundstrom, H. Dai, Nature 2003, 424, 654.

[64] D. Mann, A. Javey, J. Kong, Q. Wang, H. Dai, Nano Lett. 2003, 3, 1541.

[65] C. J. Murphy, T. K. Sau, A. M. Gole, C. J. Orendorff, J. Gao, L. Gou, S. E.

Hunyadi, T. Li, J. Phys. Chem. B 2005, 109, 13857.

[66] A. Sanchez-Iglesias, I. Pastoriza-Santos, J. Perez-Juste, B. Rodrıguez-

Gonzalez, F. J. G. De Abajo, L. M. Liz-Marzan, Adv. Mater. 2006, 18,

2529.

[67] E. Carbo-Argibay, B. Rodrıguez-Gonzalez, J. Pacifico, I. Pastoriza-Santos, J.

Perez-Juste, L. M. Liz-Marzan, Angew. Chem, Int. Ed. 2007, 46, 8983.

[68] C. Xue, J. E. Millstone, S. Li, C. A. Mirkin, Angew. Chem, Int. Ed. 2007, 46,

8436.

[69] D. Seo, C. I. Yoo, J. C. Park, S. M. Park, S. Ryu, H. Song, Angew. Chem, Int.

Ed. 2008, 47, 763.

[70] M. A. Mahmoud, C. E. Tabor, Y. Ding, Z. L. Wang, M. A. El-Sayed, J. Am.

Chem. Soc. 2008, 130, 4590.

[71] A. Tao, P. Sinsermsuksakul, P. Yang, Angew. Chem, Int. Ed. 2006, 45,

4597.


Documents

Shape-Controlled Synthesis of Pd Nanocrystals in Aqueous Solutions