shape-controlled synthesis of pd nano crystals in aqueous solutions

8/6/2019 Shape-Controlled Synthesis of Pd Nano Crystals in Aqueous Solutions

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Shape-Controlled Synthesis of Pd Nanocrystals inAqueous Solutions

By Byungkwon Lim, Majiong Jiang, Jing Tao, Pedro H. C. Camargo,Yimei Zhu,and Younan Xia*

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. [13] 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. [57] It has been shown that theactivity and selectivity

of a catalyst can be greatly enhanced by the use of nanocrystals enclosed by speciccrystal facets that are intrinsically moreactive for a particular reaction. [811] Sincethe facets exposed on a nanocrystal aredetermined by its shape, an exquisite shapecontrol of Pd nanocrystals is therefore

highly 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-denedshapes 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.[1228] 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 by the 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 that for the polyol synthesis of Ag nanocrystals based upon EG, eventhe presence of a ppm level of Cl impurity could drastically alterthe morphology of the nal 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 140160 8 C, 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] Of course, 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

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

This article provides an overview of recent developments regarding synthesisof Pd nanocrystals with well-controlled shapes in aqueous solutions. In asolution-phase synthesis, the nal shape taken by a nanocrystal is determinedby the twin structures of seeds and the growth rates of different crystallographic facets. Here, the maneuvering of these factors in an aqueoussystem to achieve shape control for Pd nanocrystals is discussed. L-ascorbicacid, citric acid, and poly(vinyl pyrrolidone) are tested for manipulating thereduction kinetics, with citric acid and Br

ions used as capping agents to

selectively promote the formation of {111} and {100} facets, respectively. Thedistribution of single-crystal versus multiple-twinned seeds can be further manipulated by employing or blocking oxidative etching. The shapesobtainedfor 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 opportunity to systematically investigate their catalytic, electrical, andplasmonic properties.

Adv. Funct. Mater. 2009, 19, 189200 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 189


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FEATUREARTCLE noble metal nanocrystals because it does not involve toxic organic

solvents. 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 differently shaped Pd nanocrystals. In addition, high-purity water is morereadily accessible than an organic solvent, so we do not worry about unexpected results that might be caused by trace amountsof impurities. A water-based system also provide a number of other merits such as simplicity, convenience, and the potential forlarge-scale production. [3037] 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 specically, 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 AgentsReduction 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-

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 using13 C NMR spectroscopy, however, the ends of commercially available PVP (if it is synthesized in an aqueous medium) areterminated in the hydroxyl (OH) group due to the involvement

of water and hydrogen peroxide in polymerization.[30]

Therefore,it can act like a long-chain alcohol and serve as a class of weak reducing 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 shapesof 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 early stage of each reaction. As illustrated in Figure 2, the absorptionpeak at 425 nm that corresponds to [PdCl 4]2

completely disappeared at t 10 min in the case of L-ascorbic acid,demonstrating the fast reduction of [PdCl 4]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 OctahedronsIn a solution-phase synthesis of metal nanocrystals, the number

of twin planes in seeds plays the most important role indetermining the shapes taken by the nal products. [25,41] Whenthe reduction is relatively fast, there are sufcient Pd atoms that can 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

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Figure 1. Structural illustration of L-ascorbic acid, citric acid, and OH-terminated PVP and their oxidized forms due to the redox reactions withPd2 ions.

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.

190 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Funct. Mater. 2009, 19, 189200


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FEATUREARTCLE ensemble of ve tetrahedrons with twin-related adjoining faces, a

gap 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 gaps

formed 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 energyof the {111} facets canno longer remedy the excessive strain energy required to sustainthe twinned structures. As a result, MTPs are thermodynamically favored primarily at relatively small sizes. Alternatively, adecahedron can grow along the vefold axis to generate a

pentagonal rod without any signicant 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 generation

and 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 O 2/Cl

pair for both the nuclei and seeds. Compared totwinned seeds, single-crystal seeds are moreresistant to oxidative etching due to the lack of defect 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, Na 2PdCl4 is themost commonly used precursor for Pd becauseof its stability in air and good solubility in a

variety of solvents. When the reaction isconducted in air with Na 2PdCl4 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 agent to 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 heating11 mL of an aqueous solution containing17.4 mM Na2PdCl4, 31 mM L-ascorbic acid,

and 87 m M PVP at 1008

C. Figure 3AC showstypical transmission electron microscopy (TEM) images of Pd samples taken at different stages of the reaction. At t 30min, thesample contained both single-crystal truncatedoctahedrons and MTPs (Fig. 3A). As thereaction proceeded to t 1 h 30 min, however,all the twinned particles disappeared from thesolution (Fig. 3B). During the next 1 h 30 min,there was no signicant change in shape, but the remaining truncated octahedronsincreased in size until they reached an average

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Figure 3. AC) TEM images of Pd samples obtained by heating 11 mL of an aqueous solutioncontaining 17.4 m M Na2PdCl4, 31mM L-ascorbic acid, and 87mM PVP at 100

8 C for A) 30 min,B) 1 h 30 min, and C) 3 h. In (A), twinned particles are indicated bytw. D) HRTEM image of asingle truncated octahedron shown in (C) recorded along the [011] zone axis and the corre-sponding FT pattern (inset). The lattice spacings of 1.94 and 2.24A can be indexed as {200} and{111} of fcc Pd, respectively. In the FT pattern, the spots circled and squared can be indexed asthe {200} and {111} reections, respectively. E) Illustration of the proposed mechanism by whichsingle-crystal truncated octahedrons were obtained in high yields.



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diameter of about 8 nm. Figure 3C shows a typical TEM image of the sample obtained at t 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 2.24Acan be indexed as {200} and {111} of fcc Pd, respectively. BothTEM and HRTEM analyses conrmed the absence of Pdnanocrystals with twinned structures in the nal product.

The higher reactivity of twinned structures towards oxidativeetching can be attributed to the higher density of twin defects ontheir surfaces, which are much higher in energy relative to the single-crystal regions and thus aremore susceptible to an oxidative environ-ment. [13,17] As a result, MTPs are preferentially attacked 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 [PdCl 4]2

with L-ascorbic acidare added to the existing truncated octahedrons, by which 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.

4. Citric Acid for Moderate Reductionand Surface Capping in theFormation of Icosahedrons,Octahedrons, and DecahedronsThe multiple-twinned nanocrystals of Pd such asicosahedrons and decahedrons produced duringthe reaction are often difcult 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 not only 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 specically, we demonstrate that the shape of the nal Pd nanocrystals can be controlled by varying the concentrations of Na 2PdCl4 and citricacid to selectively produce Pd icosahedrons,

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

We synthesized Pd icosahedrons by heating 11 mL of anaqueous solution containing 5.8 m M Na2PdCl4, 28 mM citric acid,and 29mM PVP at 90 8 C for 26h. Both scanning electronmicroscopy (SEM) and TEM studies revealed that Pd icosahe-

drons with edge lengths of approximately 25nm were obtained asthe major product ( $ 80%) in addition to a small amount of octahedrons ( $ 20%), as shown in Figure 4A and B. Interestingly,we found that the shape of Pd nanocrystals was highly sensitive tothe concentration of Na 2PdCl4. For instance, increasing theconcentration of Na 2PdCl4 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

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Figure 4. A) SEM and B) TEM images of Pd icosahedrons synthesized by heating 11 mL of an aqueous solution containing 5.8 m M Na2PdCl4, 28mM citric acid, and 29 mM PVP at 90

8 Cfor 26 h; C) SEM and D) TEM images of Pd octahedrons prepared under the same conditionas in (A) except that the concentrations of Na 2PdCl4 and PVP were increased to 7.4 and37 mM, respectively; E) SEM and F) TEM images of Pd decahedrons prepared under thesame condition as in (A) except that the concentrations of Na 2PdCl4, PVP, and citric acidwere increased to 17.4, 87, and 84 mM, respectively (modied with permission from [35],copyright 2007 Wiley-VCH).



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$ 10% other shapes including triangular plates anddecahedrons (Fig. 4C and D). These results suggest that formation of octahedrons was more favorable than that of icosahedrons at relatively high Na 2PdCl4 precursor concentra-tions. The reaction condition could be further manipulated toobtain Pd decahedrons by increasing both the Na 2PdCl4 and citric

acid concentrations beyond those employed for the formation of icosahedrons and octahedrons. Figure 4E and F shows typicalSEM and TEM images of Pd decahedrons obtained by increasingthe concentrations of Na 2PdCl4 and citric acid to 17.4 and 84 mM,respectively. Both images revealed that Pd decahedrons with sizesof 2540 nm were produced as a major product ( > 80%).

As demonstrated in this work, citricacid favors the formationof Pd nanocrystals enclosed by the {111} facets, such as icosahe-drons, octahedrons, and decahedrons. It has been shown by simulation 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. [4345] When thePd precursorconcentration 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 of time. 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 of decahedrons 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 O 2/Cl

pair. As a result, single-crystaloctahedronsare morelikely to remain in the nal product. To improve the

yield of decahedrons, a higher concentration forthe capping agent (citric acid or citrate ions in thiscase) is required to ensure sufcient capping of the {111} facets, which not only reduces the surfaceenergy and thus compensates for the extra strainenergy caused by twinning but also efciently protects them from oxidative environment. Forthese reasons, the decahedrons can be preservedand thus accumulate throughout the reaction.

5. PVP for Slow Reduction in theFormation of Hexagonal and

Triangular NanoplatesIf 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 nal nanocrystal shape typically deviates 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 weak reducing power, as described in Section 2, thereby enabling akinetic control over both nucleation and growth.

Among various shapes, 2D anisotropic nanostructures of noble metals such as hexagonal and triangular nanoplates have

drawn 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. [4649] 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 a metal precursor in the presence of a specic capping agent or surfactant, [52] and mild annealing of self-organized nanocrystals on carbon substrates. [53] Here wedemonstrate that Pd nanoplates with hexagonal and triangularshapes can be simply produced by reducing Na 2PdCl4 with PVP,without the involvement of additional capping agents or specicsubstrates.

Figure 5A showsa TEM imageof Pd nanoplates synthesized by heating 11 mL of an aqueous solution containing 17.4 m MNa2PdCl4 and 87 m M PVP at 100

8 C for 3 h. Note that this reaction

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Figure 5. A) TEM image of hexagonal and triangular Pd nanoplates synthesized by heating11mL of an aqueous solution containing 17.4m M Na2PdCl4 and 87m M PVP at 100

8 Cfor3h.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) HRTEMimagetaken from theat topface of a singlenanoplate andcorresponding FT pattern (inset).In the FT pattern, the spots circled and squaredcan be indexed to the {220} and forbidden 1/3{422} reections, respectively, in which the latter indicates the presence of planar defectssuch as stacking faults in the {111} planes. C) Proposed mechanism for the formation of Pdnanoplates. Pd atoms nucleate to form nuclei with a metastable rhcp structure with theinclusion of stacking faults. At a slow reduction rate, these nuclei can evolve into plate-likeseeds, which further grow into hexagonal and triangular nanoplates with their top andbottom faces enclosed by the {111} facets.



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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 5080 nm. Most of the triangular nanoplates have truncated corners.Figure 5B shows a HRTEM image taken from the at top face of a

single 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 FT pattern 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} reections, respectively. Observation of the forbidden spots associated with 1/3{422} diffractionssuggests that planar defects such as stacking faults are present in 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 nucleation

theory, 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 by stacking faults is low and the rhcp structure is slightly more stablethan fcc at this stage. [5557] When the reduction is relatively fast,these nuclei can evolve into polyhedral seeds such as singlecrystal, truncated octahedrons and vefold twinned decahedronsin an effort to lower the total surface energy, as discussed inSection 3. If the reduction is considerably slowed, however, these

nuclei 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 of Pd 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 most important factor in achieving the kinetically controlled synthesisof Pd nanoplates.

6. Bromide as a Capping Agent in Promoting the{100} FacetsThe 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 xed interms of twin structure, the nal shape taken by the nanocrystalwill be determined by the relative rates at which different crystallographic facets grow, in which the facets with a slowergrowthrate willbe exposed more on the nanocrystals surface. Forexample, if the fast growing facets correspond to the {111} of atruncated octahedron, the nal crystal shape will be a cubeenclosed by the slow growing {100} facets. In contrast, if the fast growing facets correspond to the {100} faces, the nal crystalshape will be an octahedron enclosed by slow growing {111}facets. Such a dynamic evolution can selectively enlarge one set of facets 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 can

change the order of free energies of different facets through theirchemical interaction with a metal surface. This alternation may signicantly affect the relative growth rates of different facets andthus lead to different morphologies for the nal 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 primarily onto 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 areof 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 eld-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

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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 11 mL

of an aqueous solution containing 17.4m MNa2PdCl4, 31mM L-ascorbic acid, 87 mM PVP,and 230 m M KBr at 1008 C for 3 h. These reactionconditions are the same as in the synthesis of truncated octahedrons shown in Figure 3C except for the addition of KBr. Figure 6A and B showsTEM images of the resulting product at different magnications. 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 68 nm and24 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 1.94A, which can be indexed as{200} of fcc Pd. The corresponding FT pattern iscomposed of spots with a square symmetry. Theseresults conrm that the Pd nanobar is a piece of single 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 totheir preferential chemisorption on the {100} facets of truncated

octahedrons. At a relatively low concentration of Br

ions, theformation of nanobars can be attributed to anisotropic growth of cubic 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 relatively thin layer of Br ions. In this case,oxidative etching could removesome of the Br ions from the Pd surface. As in the case of corrosion of Pd nanocubes by pitting process or galvanicreplacement between Ag nanocubes and HAuCl 4, 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 selective

activation 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 Ag 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 prepared

under the same condition as in Figure 6A except that theconcentration of KBr was increased to 460m M. It can be seen that Pd nanocubes with an average size of 10 nm were formed as themajor product ( $ 95%) in addition to a small amount of vefoldtwinned pentagonal nanorods ( $ 5%) with a diameter of about 8 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 symmetry indicates that the side facesof 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 signicant. In this case,all the faces of a Pd nanocube could be stabilized by a monolayerof Br ions, which could effectively block the localized oxidativeetchingand thus prohibit its anisotropic growth into a nanobar. At the same time, some of the decahedral seeds may evolve intopentagonal nanorods under these reaction conditions. It has been

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Figure 6. A and B) TEM images of Pd nanobars prepared under the same condition as inFigure 3C except that the reaction was conducted in the presence of 230m M KBr. C) HRTEMimage of a single nanobar and the corresponding FT pattern (inset). The lattice spacing of 1.94 Acan be indexed as {200} of Pd. In the FT pattern, the spots circled and squared can beindexed to the {200} and {220} reections, respectively. D) Schematic illustration of themechanism responsible for the formation of nanobars.



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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 vefold 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 sufcient 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

concentration, 1D anisotropic growth of nano-cubes into nanobars could be greatly suppressedby lowering the reaction temperature. When thereaction was performed at 80 8 C while keepingother reaction parameters the same as inFigure 6A, nanobars with an aspect ratio of 24

were rarely found in the product and most of thePd nanocrystals were of a cubic shape with sizes of 1012nm, although some of them were slightly elongated 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 signicantly eliminatedand, as a result, the initially formed nanocubescould maintain their cubic morphology andaccumulate throughout the reaction without growing into nanobars via selective activationprocesses.

In another demonstration, we found that introduction of Br ions into a kinetically controlled process (with PVP as a reducing agent)could enable the production of Pd nanocubes at high yields. Figure 8C and D shows TEM imagesof Pd nanocubes with an average size of $ 10nmprepared under the same condition as in thesynthesis of the hexagonal and triangular nano-plates shown in Figure 5A except for the presenceof 460 mM KBr in the reaction solution. It is clearthat the Br ions induced a cubic morphology forthe resulting Pd nanocrystals. The formation of nanocubes in this synthesis implies that single-crystal seeds are involved in the nucleation step, in

which Br

ions could break the rhcp structure of initially 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. It should 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-dened shapes can serve as primary sites, i.e., seeds, forheterogeneous nucleation of added metal 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 nal 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

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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 460 mM. C and D) HRTEMimages and the corresponding FT patterns (insets) of a single nanocube and pentagonalnanorod, 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 beindexed to the {200}, {220} and {112} reections, respectively. E) Schematic illustration of the mechanism responsible for the formation of pentagonal nanorods.



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FEATUREARTCLE

extremely powerful in Au, Ag, and Pt systems, [6670] littleattention has been paid to the seeded growth of Pd nanocrystals.Here we demonstrate that Pd nanocrystals can be completely converted from cubes to truncated and regular octahedrons via

seeded growth. The Pd nanocubes with an average size of $ 10 nm shown in Figure 8C were rst prepared as described inSection 6.2 and then used as the seeds toinitiate further nanocrystal growth whenadditionalPd precursor was added and thenslowly reduced by PVP. As more Pd atomswere added to the Pd nanocubes, they grew into truncated or regular octahedronsdepending on the amount of Na 2PdCl4added to the reaction (Fig. 9A).

In a typical synthesis, we obtained Pdtruncated octahedrons by adding 1 mL of the as-prepared Pd nanocube suspension to9 mL of an aqueous solution containing10.5mM Na2PdCl4 and 52.5m M PVP andheating at 90 8 C for 3h. As shown inFigure 9B, the resulting product mainly contained Pd nanocrystals with a truncatedoctahedral shape ( > 95%) and edge lengthsof 1215 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

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-metersbeing keptthe same, Pd nanocrystals with aregular octahedral shape were obtained in a high

yield (>

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 of nanocubes. [69,71] The shape conversion of Pdnanocrystals observed in the present work might also 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 speciccapping agents, the continued addition of Pdatoms prefers to the {100} faces of the Pdnanocrystal as the relative surface energies of the 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 of Na2PdCl4, this process yields truncated octahe-drons bound by both the {111} and {100} facets.When the concentration of Na 2PdCl4 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 shape

and size of Pd nanocrystals by taking advantage of a rich variety of shapes that have been already achieved.

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Figure 8. A and B) TEM images of Pd nanocubes prepared under the same condition as inFigure 6A except that the reaction temperature was lowered to 80 8 C. C and D) TEM imagesof Pd nanocubes prepared under the same condition as in Figure 5A except that the reactionwas conducted in the presence of 460m M KBr.

Figure 9. A) Schematic illustration of seeded growth of Pd octahedrons with and without truncationat corners from cubic Pd seeds. B) TEM and C) HRTEM images of Pd truncated octahedronsobtained by adding 1mL of the as-prepared Pd nanocube solution (shown in Fig. 8C) to 9 mL of anaqueous solution containing 10.5m M Na2PdCl4 and 52.5mM PVP and heating at 90

8 C for 3h.D) TEM and E) HRTEM images of Pd octahedrons prepared under the same condition as in (B)except that the concentrations of Na 2PdCl4 and PVP were increased to 21 and 105mM, respectively.



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shape-controlled synthesis of pd nano crystals in aqueous solutions

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