Multifaceted Gold−Palladium BimetallicNanorods and Their Geometric,Compositional, and Catalytic TunabilitiesLichao Sun,† Qingfeng Zhang,† Guangfang Grace Li, Esteban Villarreal, Xiaoqi Fu,‡ and Hui Wang*

Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States

*S Supporting Information

ABSTRACT: Kinetically controlled, seed-mediated co-reductionprovides a robust and versatile synthetic approach to multimetallicnanoparticles with precisely controlled geometries and compositions.Here, we demonstrate that single-crystalline cylindrical Au nanorodsselectively transform into a series of structurally distinct Au@Au−Pdalloy core−shell bimetallic nanorods with exotic multifacetedgeometries enclosed by specific types of facets upon seed-mediatedAu−Pd co-reduction under diffusion-controlled conditions. Byadjusting several key synthetic parameters, such as the Pd/Auprecursor ratio, the reducing agent concentration, the cappingsurfactant concentration, and foreign metal ion additives, we havebeen able to simultaneously fine-tailor the atomic-level surfacestructures and fine-tune the compositional stoichiometries of themultifaceted Au−Pd bimetallic nanorods. Using the catalytichydrogenation of 4-nitrophenol by ammonia borane as a model reaction obeying the Langmuir−Hinshelwood kinetics,we further show that the relative surface binding affinities of the reactants and the rates of interfacial charge transfers, bothof which play key roles in determining the overall reaction kinetics, strongly depend upon the surface atomic coordinationsand the compositional stoichiometries of the colloidal Au−Pd alloy nanocatalysts. The insights gained from this work notonly shed light on the underlying mechanisms dictating the intriguing geometric evolution of multimetallic nanocrystalsduring seed-mediated co-reduction but also provide an important knowledge framework that guides the rational design ofarchitecturally sophisticated multimetallic nanostructures toward optimization of catalytic molecular transformations.

KEYWORDS: seed-mediated co-reduction, alloy nanoparticles, hetero-nanostructures, high-index facets, low-index facets,nanocrystal growth, nanocatalysis

Metallic nanoparticles exhibit a set of size- and shape-dependent optical and catalytic properties that canbe systematically fine-tuned and rationally optimized

for widespread applications in photonics,1,2 spectroscopies,3,4

biomedicine,5−7 and catalysis.8−12 In comparison to theirmonometallic counterparts, multimetallic nanoparticles exhibitremarkably further enhanced architectural, optical, and catalytictunabilities, allowing anomalous properties to emerge benefit-ing from the synergy between multiple constituents.13−17

Although great success in geometry-controlled synthesis ofmonometallic nanoparticles has been achieved,18−22 the precisearchitectural control of multimetallic nanostructures representsa significantly more challenging task. While some fundamentalprinciples of monometallic nanocrystal growth may also applyto multimetallic systems, the nucleation and growth ofmultimetallic nanoparticles involve substantially more compli-cated structure-transforming processes due to the interplay ofmultiple thermodynamic and kinetic factors that synergistically

guide the deposition, interdiffusion, segregation, and architec-tural arrangements of the constituent elements.11,12,15,23−25

Among a diverse set of multimetallic nanostructures, Au−Pdbimetallic nanoparticles have been of particular interest due totheir superior catalytic performances typically unachievable intheir monometallic counterparts.13,16,26 The performanceoptimization of Au−Pd bimetallic nanocatalysts essentiallyrelies on the precise control over the particle geometries andcompositions,13,26−29 which can be achieved through deliber-ately designed solution-phase colloidal syntheses. Au and Pdboth adopt the face-centered cubic (fcc) crystalline structurewith reasonably small lattice mismatch (∼4.9%) and aremiscible over a wide stoichiometric range, making it possibleto selectively synthesize a whole set of architecturally distinct

Received: January 12, 2017Accepted: February 23, 2017Published: February 23, 2017

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Au−Pd bimetallic nanoparticles ranging from epitaxial core−shell heterostructures to homogeneous alloys by kineticallymaneuvering the nanocrystal nucleation and growth.27−46

Despite their intrinsically different redox potentials andreduction kinetics, Au and Pd precursors can be co-reducedto form Au−Pd alloy nanocrystals through a one-pot synthesisunder diffusion-controlled conditions when the surfacedeposition occurs far more rapidly than the diffusion of thereactants to the nucleus surfaces.35,37,39,42,43,46 The kineticcontrol of the nanocrystal growth, when further coupled withthermodynamic stabilization of specific facets by surface-capping ligands or foreign metal adatoms, also enables one tofine-tailor the crystallographic facets exposed on the alloynanoparticle surfaces.13,37,43,46 The capabilities to fine-tuneboth the geometries and compositions of Au−Pd alloynanoparticles can be further enhanced using preformednanocrystalline seeds with well-defined shapes to mediate theco-reduction of Au and Pd with a mild reducing agent, such asascorbic acid (AA).24 The structural evolution of the nano-crystals during seed-mediated co-reduction is mechanisticallycomplex, entangling multiple kinetically controlled andthermodynamically driven processes that are sensitivelydependent upon a series of interplaying factors, such as theAu/Pd precursor ratio, the pH of the nanocrystal growthsolution, the reaction temperature, the surface-cappingsurfactants, and the structures of the seeds.24,32,35,37,41 Subtlechanges in any of the above-mentioned synthetic parametersmay introduce drastic modifications to the geometries andcompositions of the resulting nanostructures.24,32,35,37,41 Theseinteresting observations motivated us to further decipher thecomplex mechanisms dictating the structural evolution ofnanocrystals during seed-mediated co-reduction with the goalsof pushing the architectural control of multimetallic nano-particles to a higher level of precision and versatility.Here, we investigate the kinetically controlled co-reduction

of Au and Pd seeded by single-crystalline Au nanorods (NRs)as a model system to shed light on the mechanistic complexityassociated with the architectural evolution of anisotropic alloynanostructures in seed-mediated co-reduction processes.Chemically synthesized cylindrical Au NRs are essentiallyenclosed by multifaceted surfaces comprising a variety of localhigh-index and low-index facets, which are coated withstructure-directing capping surfactants, such as cetyltrimethy-lammonium bromide (CTAB), and arguably submonolayers offoreign metal adatoms, such as Ag.47−52 How such surface

structural complexity and geometric anisotropy of the Au NRseeds are translated into the architectural diversity of theovergrown multimetallic nanostructures through seed-mediatedco-reduction remains a fundamentally intriguing open question.As shown in this work, cylindrical Au NRs may selectivelytransform into an entire family of geometrically distinctanisotropic polyhedral nanostructures enclosed by specifictypes of well-defined facets without demolishing theircrystalline integrity upon seed-mediated Au−Pd co-reduction,allowing us to fine-control both the compositional stoichiome-tries and the atomic-level surface structures of Au−Pdbimetallic NRs by judiciously tailoring several key syntheticparameters. The precise control of both the particle facets andcompositions enables us to gain detailed, quantitative insightsinto the structure−composition−property relationships under-pinning the intriguing catalytic behaviors of colloidal Au−Pdalloy nanocatalysts.

RESULTS AND DISCUSSIONThe beauty of using Au NRs as the seeds to mediate the Au−Pd co-reduction lies in three key aspects. First, Au NRsrepresent an anisotropically shaped but single-crystalline seedstructure that is ideal for the mechanistic study of seed-mediated overgrowth of nanocrystals into thermodynamicallyunexpected geometries.52−58 Second, the bilayers of CTABsurfactants self-assembled on the Au NR surfaces47,51,52 mayserve as a molecular barrier to fine-regulate the diffusion ratesof the reactants from the bulk solution to the seed surfaces,ensuring that the Au−Pd co-deposition occurs under diffusion-controlled conditions to form alloy structures. Third, thestructural and compositional changes of NRs during seed-mediated co-reduction can be monitored using straightforwardoptical extinction spectroscopy because of the aspect ratio-,morphology-, and composition-dependent plasmonic character-istics of NRs.20,52,59−61 The single-crystalline Au NRs used inthis work were synthesized using a binary surfactant-guided,seed-mediated growth method developed by Murray and co-workers.62 The as-synthesized Au NRs exhibit a uniformcylindrical morphology with average diameters of 26.7 ± 3.30nm and lengths of 102 ± 7.16 nm (Figure S1 in SupportingInformation). As shown in Figure S2A in SupportingInformation, cylindrical Au NRs transformed into Au elongatedtrisoctahedral (ETOH) NRs each of which was enclosed by 24{221} facets at the ends and 4 {110} facets on the lateral sidesupon exposure to an overgrowth solution containing 0.4 mM

Scheme 1. Geometric Derivation of an EHOH and a TCCB NR Starting from a Nanocube

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HAuCl4, 20 mM CTAB, and 10 mM AA, in agreement with ourprevious observations.52 In contrast, cuboidal NRs composed ofAu NR cores coated with epitaxial Pd shells were obtainedupon exposure of the Au NR seeds to 0.4 mM H2PdCl4, 20mM CTAB, and 10 mM AA (Figure S2B in SupportingInformation). Moire ́ patterns were clearly observed in the coreregion of each cuboidal NR in the TEM image, characteristic of

heteroepitaxial Au−Pd core−shell structures with super-imposed lattice mismatch.63 More detailed structural character-izations of the Au@Pd core−shell cuboidal NRs have beenpreviously reported.64 Such strikingly distinct structural trans-formations essentially originate from the intrinsic differences inthe deposition kinetics and the relative surface energies of Auand Pd. Au ETOH NRs enclosed by high-energy {221} and

Figure 1. Structures of Au−Pd alloy EHOH and TCCB NRs. (A) Schematic illustration of the transformations of Au NRs into EHOH andTCCB NRs upon seed-mediated Au−Pd co-deposition. The co-deposition reactions were carried out in the presence of 20 mM CTAB, 10 mMAA, and various [H2PdCl4]/[HAuCl4] molar ratios with the total concentration of H2PdCl4 + HAuCl4 fixed at 0.4 mM. (B) SEM image ofEHOH NRs synthesized at a [H2PdCl4]/[HAuCl4] ratio of 1:9. The inset highlights one individual EHOH NR. TEM images (left), geometricmodels (middle), and SAED patterns (right) of individual EHOH NRs viewed along the (C) [100] and (D) [110] projections. (E,F) TEMimages and geometric models of individual EHOH NRs at other orientations. (G) SEM image of TCCB NRs synthesized at a [H2PdCl4]/[HAuCl4] ratio of 2:1. The inset highlights one individual TCCB NR. TEM images (left), geometric models (middle), and SAED patterns(right) of individual TCCB NRs viewed along the (H) [100] and (I) [110] projections. (J,K) TEM images and geometric models of individualTCCB NRs at other orientations. (L) PXRD patterns of the bimetallic NRs synthesized at various [H2PdCl4]/[HAuCl4] molar ratios aslabeled in the figure. The standard diffraction patterns for bulk Au and Pd are also included. The spectra are offset for clarity. SEM images andEDS elemental maps of (M) EHOH and (N) TCCB NRs. (O) Pd atomic percentages of the Au−Pd bimetallic NRs synthesized at various[H2PdCl4]/[HAuCl4] ratios. The Pd atomic percentages were quantified by PXRD, EDS, and ICP-MS. The error bars show the standarddeviations obtained from three samples synthesized under each experimental condition.

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{110} facets were essentially a kinetically controlled productwhen fast surface deposition and nanocrystal overgrowthoccurred at sufficiently high [AA]/[HAuCl4] ratios ([AA]/[HAuCl4] = 25 in this case), whereas the electroless depositionof Pd on Au under otherwise identical conditions wassignificantly slower than that of Au, resulting in cuboidal NRswhose surfaces were dominated by thermodynamically morestable {100} facets.When the seed-mediated Au−Pd co-reduction occurred in

the presence of both H2PdCl4 and HAuCl4, the Au NRsunderwent significantly more complicated and diverse geo-metric transformations. By judiciously adjusting the Pd/Auprecursor ratios, we have successfully synthesized Au−Pdbimetallic NRs with two distinct exotic multifaceted geometries,an elongated hexoctahedral (EHOH) NR enclosed by 56 facetsand a truncated concave cuboidal (TCCB) NR enclosed by 32facets, both of which can be derived from a simple {100}-faceting cube geometry. As illustrated in Scheme 1, an EHOHshape can be geometrically derived from a cube by first pullingout the center of each {100} facet to form a tetrahexahedron(THH) enclosed by 24 high-index {hk0} facets, then pushingthe center of each square edge toward the body center to createa hexoctahedron (HOH), and finally introducing elongationalong the [001] crystalline axis. A TCCB geometry can bederived by first elongating a cube along the [001] crystallineaxis to form a cuboid, then creating surface concavity to form aconcave cuboid (CCB) enclosed by 24 high-index {hkk} facets,and finally truncating the 8 corners with {111} facets.A typical seed-mediated co-reduction reaction was carried

out at 30 °C under ambient atmosphere in the presence of 20mM CTAB and 10 mM AA at varying [H2PdCl4]/[HAuCl4]ratio with the total concentration of H2PdCl4 + HAuCl4 fixed at0.4 mM. As shown by electron microscopy images in FigureS3A−H in Supporting Information and schematically illustratedin Figure 1A, the cylindrical Au NRs evolved into EHOH NRsat relatively low Pd/Au precursor ratios below 1:2, whereasTCCB NRs formed at a Pd/Au precursor ratio greater than 1:1.We used scanning electron microcopy (SEM), transmissionelectron microscopy (TEM), selected area electron diffraction(SAED), powder X-ray diffraction (PXRD), energy-dispersivespectroscopy (EDS), and inductively coupled plasma massspectrometry (ICP-MS) to fully characterize the crystallinestructures and compositions of the EHOH and TCCB NRs. Asshown in Figure 1B, the multifaceted bimetallic NRssynthesized at a [H2PdCl4]/[HAuCl4] ratio of 1:9 exhibited awell-defined EHOH geometry enclosed by 56 high-index facetswith Miller index of {hkl} (h > k > l > 0). The EHOH NRsdisplayed orientation-dependent projection contours in theTEM images (Figure 1C−F). The shape profiles and measurededge angles of individual EHOH NRs projected along the[100] and [110] zone axes (Figure 1C,D) showed an excellentmatch with the geometric models of a EHOH object enclosedexclusively by 56 {421} facets. The crystalline orientation andepitaxial fcc structure of each EHOH NR were further verifiedby SAED (right panels of Figure 1C,D). Figure 1G shows theSEM images of the TCCB NRs synthesized at a Pd/Auprecursor ratio of 2:1, each of which was identified to beenclosed by 24 high-index {311} side facets and 8 low-index{111} facets at the corner truncations after carefully comparingthe projected contours of individual TCCB NRs in the TEMimages with their geometric models when viewed along the[100], [110], and other projections (Figure 1H−K).

We used PXRD to characterize the crystalline structures andcompositions of the Au−Pd bimetallic NRs. As the [H2PdCl4]/[HAuCl4] ratio progressively increased, the diffraction peakscorresponding to the fcc Au became asymmetrically broadenedand eventually split into two sets of fcc diffraction peaks, whichcould be assigned to the Au NR cores and Au−Pd alloy shells(Figure 1L). Through deconvolution of the (111) diffractionpeak into a Au and a Au−Pd alloy peak (Figure S4 inSupporting Information), we were able to calculate the latticeparameters of the Au−Pd alloys using the Bragg’s law:

λθ

=d2 sin( )111

111 (1)

where λ = 1.5406 Å for Cu Kα, d111 is the (111) lattice spacingof the alloy, and θ111 is the angle of incidence on the {111}plane. We further calculated the Pd/Au stoichiometric ratios inthe alloy shells using an empirical rule known as the Vegard’slaw,65 which states that the lattice parameters of ahomogeneous binary alloy are linearly related to its composi-tional stoichiometries, as described by the following equation:

= + −−

d xd x d(1 )Au Pd Au Pdx x(1 ) (2)

where x is the atomic fraction of Au in the Au−Pd alloy, anddAu and dPd are the lattice constants of fcc Au and Pd,respectively. More definitive evidence on the alloy structures ofthe overgrown shells was obtained by EDS elemental mapping(Figure 1M,N and Figures S5 and S6 in SupportingInformation). PXRD, EDS, and ICP-MS all consistentlyshowed that the Pd atomic percentage of the bimetallic NRsincreased with the Pd/Au precursor ratio in the overgrowthsolution (Figure 1O). While the EDS and ICP-MS results werein excellent agreement, the atomic percentages of Pd calculatedfrom PXRD were about 5−15% higher than those quantified byEDS and ICP-MS due to the fact that both EDS and ICP-MSmeasured the compositions of the entire particles but PXRDprovided the compositional information on the Au−Pd alloyshells specifically.The structural and compositional changes of NRs upon seed-

mediated co-reduction introduced interesting modifications totheir optical characteristics (Figure S3I in SupportingInformation). As the [H2PdCl4]/[HAuCl4] ratio progressivelyincreased up to 1:2, cylindrical Au NRs transformed intoEHOH NRs with increasing Pd content, causing a systematicred shift and broadening of both the longitudinal and transverseplasmon resonance peaks of the NRs. As the [H2PdCl4]/[HAuCl4] ratio further increased, however, the TCCB NRsbecame increasingly more Pd-rich and their vertices weretruncated to a greater extent, resulting in a blue shift andfurther damping of the plasmon resonances. The interestingextinction spectral evolution originated from both the structuraltransitions of the NRs and the plasmon damping caused byalloying of Au with Pd.As previously shown by Skrabalak and co-workers, the

crystalline habits and surface structures of the seeds are bothkey factors guiding the geometric evolution of the Au−Pdbimetallic nanoparticles during seed-mediated co-reduction.40

In this work, both the EHOH and TCCB NRs were derivedfrom their parental single-crystalline Au NR seeds that weresynthesized following Murray’s protocol in the presence ofoleate/CTAB binary surfactants.62 These Au NRs, regardless oftheir dimensions and aspect ratios, essentially inherited thecrystalline habits of elongated tetrahexahedral (ETHH) NRs,

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each of which was enclosed by 24 high-index {730} facets.52,62

However, when the Au NRs were thinner than ∼30 nm, theircharacteristic facets were underdeveloped with significanttruncations at the edges and vertices of the NRs. Therefore,the as-synthesized Au NRs exhibited a cylindrical morphology,capped with rounded ends and enclosed by a variety of ill-defined local facets. By decreasing the amount of seeds whilekeeping the other synthetic parameters the same, we were ableto synthesize ETHH NRs with significantly increased lateralsizes enclosed by fully developed {730} facets (Figure S7A inSupporting Information). We further used these well-definedETHH NRs as the seeds to mediate the Au−Pd co-reduction,through which we successfully synthesized both EHOH andTCCB NRs under appropriate conditions (Figure S7B,C inSupporting Information). Our results strongly indicated that itwas the intrinsic crystalline habits rather than the exposed facetsof the Au NRs that determined their structural transformationsinto the exotic EHOH and TCCB geometries upon seed-mediated co-reduction.The Au−Pd bimetallic EHOH and TCCB NRs also inherited

the structural anisotropy from their parental Au NR seeds.Skrabalak and co-workers previously reported that Au or Pdseeds with various polyhedral symmetries, such as nanocubes,nanobipyramids, nano-octahedra,24,38,44 selectively evolved intoa series of Au−Pd bimetallic branched nanostructures or low-index-faceting nanopolyhedra upon seed-mediated co-reduc-tion, suggesting a strong correlation between the crystallinesymmetries of seeds and the structures of the overgrown Au−Pd bimetallic nanoparticles. They also used Au NRs as theseeds to mediate the co-reduction of Au and Pd, through whichelongated branched Au−Pd alloy nanoparticles with ill-definedfacets were obtained.44 Here, we showed that by fine-maneuvering the kinetics of Au−Pd co-reduction on Au NRseeds, the synthesis of anisotropic Au−Pd bimetallic NRsenclosed by well-defined high-index facets became possible.The spatially uniform alloy shell structures and the high-

index-faceting EHOH and TCCB geometries are both theconsequences of kinetically fast nanocrystal overgrowth underdiffusion-controlled conditions. The seed-mediated co-reduc-tion essentially involves two key processes, the diffusion ofreactants from the bulk solution to the seed surfaces and thesurface deposition of metals, possibly with additionalcomplications caused by the interfacial diffusion of adatoms44

and the galvanic replacement between the metal precursors andthe Ag underpotential deposition (UPD) layer on the Au NRsurfaces.64 As demonstrated by Xie and co-workers,39 when thesurface deposition occurs much more rapidly than the diffusion,the overall nanocrystal growth kinetics becomes diffusion-controlled and the ratios of the Au and Pd componentsdeposited onto the seed surfaces remain constant during thecrystal growth process, resulting in alloy nanoparticles withuniform spatial distributions of Au and Pd. A linear relationshipwas observed when we plotted the Pd atomic percentages inthe overgrown shells (quantified by PXRD) as a function of themolar percentage of Pd precursor in the reaction mixtures(Figure S8 in Supporting Information), which provided strongevidence on the co-deposition of Au and Pd at fixed relativerates during the nanocrystal overgrowth. The slope of the linewas around 0.5, suggesting that the atomic deposition rate ofAu was approximately twice that of Pd. The extrapolation of theline to 100% Pd precursor yielded an intercept around 0.5,indicating that about 50% of the Pd precursors were co-deposited with Au on the seed surfaces while other possible

nondepositing species, such as unreacted PdCl42−, ultrasmall Pd

atomic clusters, and (C19H42N)2PdBr4 complex,38,66 might also

exist in the supernatant. The nanocrystal growth kinetics wasalso a key factor determining the geometries and surfacestructures of the overgrown nanoparticles. Fast nanocrystalgrowth at sufficiently high AA concentrations (e.g., 10 mMunder the current synthetic conditions) favored the formationof kinetically trapped high-index-faceting nanostructures, asexemplified by the EHOH and TCCB NRs. Further increase ofAA concentration from 10 to 100 mM did not introduce anysignificant modifications to the dimensions and morphologiesof the EHOH and TCCB NRs (Figure S9 in SupportingInformation). In contrast, the formation of nanocrystalsenclosed by low-index facets requires relatively slow growthkinetics that allow the nanocrystals to fully evolve intothermodynamically stable geometries. Under the conditionsfor slow nanocrystal growth, however, the surface deposition ofAu and Pd on the seed surfaces becomes the rate-limiting step,and as a consequence, hetero-nanostructures composed of Au-rich cores and Pd-rich or even monometallic Pd shells start toform due to the intrinsically faster consumption of the Auprecursor compared to that of the Pd precursor.24,30,39

Therefore, it has remained a significant challenge to synthesizelow-index-faceting Au−Pd bimetallic nanoparticles with spa-tially uniform alloy compositions.To overcome this synthetic challenge, we used Au NRs

whose surfaces were capped with a self-assembled bilayer ofCTAB surfactants as the seeds to guide the electroless co-deposition of Au and Pd. The CTAB bilayers served as amolecular barrier that significantly decelerated the diffusion ofthe reactants from the bulk solution to the seed surfaces,allowing us to slow down the metal deposition by eitherdecreasing the AA concentration or increasing the CTABsurface packing density while still maintaining the diffusion-controlled conditions such that the formation of low-index-faceting bimetallic NRs with homogeneous alloy shell structuresbecame possible. At a fixed [H2PdCl4]/[HAuCl4] ratio of 1:9,the corners and edges of the EHOH NRs got truncated to formirregularly shaped, multifaceted NRs while the Pd/Au atomicratios remained essentially unchanged as the AA concentrationprogressively decreased from 10 to 0.6 mM (Figure S10A−E inSupporting Information). Such geometric evolution causedweakening and a blue shift of the plasmon resonance peaks inthe extinction spectra (Figure S10F in Supporting Informa-tion). When the [H2PdCl4]/[HAuCl4] ratio was fixed at 2:1,TCCB NRs became less concave on their side facets and moretruncated at their corners, gradually evolving into quasi-cuboidal (QCB) NRs and eventually into truncated cuboidal(TCB) NRs as the AA concentration progressively decreased(Figure S11A−D in Supporting Information). Interestingly,both the Pd/Au atomic ratios and the alloy structures werewell-preserved regardless of the variation of AA concentration(Figures S11E and S12 in Supporting Information). Althoughthe plasmon bands appeared very weak and broad due tosignificant plasmon damping effects caused by the relativelyhigh fraction of Pd in the alloy shells (∼35%), a blue shift of theplasmon resonance was still clearly observed as the morphologyof the NRs switched from TCCB to QCB and eventually toTCB NRs (Figure S11F in Supporting Information).The transitions from high-index-faceting to low-index-

faceting geometries were also observed when the CTABconcentration increased. The CTAB bilayers exhibit differentpacking densities and thereby different stabilization effects on

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various types of facets of Au and Pd nanocrystals, allowing us tofurther fine-tailor the surface structures of the overgrown alloyshells while maintaining the co-reduction at diffusion-controlledconditions. When increasing the CTAB concentration in therange from 4 to 150 mM at a fixed [H2PdCl4]/[HAuCl4] ratioof 1:9 and AA concentration of 10 mM, increasingly moresignificant corner truncations and suppression of surfaceconvexity were observed, causing a progressive blue shift ofthe plasmon resonance in the extinction spectra, though thevariation of CTAB concentration did not cause any significantchanges in the Pd/Au stoichiometries (Figure S13 inSupporting Information). These results indicated that thesurface packing density of CTAB, which could be controlled bythe CTAB concentration in the bulk solution, provided anadditional parameter for the control of tip/edge sharpness andsurface convexity of the EHOH NRs. By deliberately varyingboth CTAB concentration and the [H2PdCl4]/[HAuCl4] ratio,we were able to synthesize EHOH NRs with almost identicalaspect ratios, surface convexity, and tip/edge sharpness butdifferent Pd/Au stoichiometric ratios (Figure S14 in SupportingInformation) in a highly controllable and precise manner. Atthe [H2PdCl4]/[HAuCl4] ratio of 2:1, the TCCB NRs becameless concave on their side surfaces accompanied by more

significant corner truncations as the CTAB progressivelyincreased, eventually transforming into TCB NRs at sufficientlyhigh CTAB concentrations (Figure S15 in SupportingInformation). The Pd/Au stoichiometric ratios, however,remained unchanged regardless of the variation of CTABconcentration. At the CTAB concentration of 150 mM, thedensely packed CTAB bilayers on the seed surfaces significantlyslowed down the diffusion and thus the nanocrystal growthkinetics, resulting in the formation of well-defined low-index-faceting TCB NRs even at AA concentrations as high as 100mM (Figure S16 in Supporting Information). It is worthmentioning that in previously reported seed-mediatedsyntheses of multimetallic nanoparticles, the seed particleswere typically coated with various surface-capping surfactants,which may play crucial roles in guiding the structural evolutionof multimetallic nanocrystals. While significant mechanisticinsights have been gained on the effects of seed shapes andstructures,24,38,44 the roles of surface-capping surfactants stillremain poorly understood and need to be further explored. Ourresults clearly show that the packing density of CTAB on theseed surfaces can also be used as a key synthetic knob for thegeometry control of the Au−Pd bimetallic nanoparticles

Figure 2. Structures of Au−Pd alloy TCB NRs. (A) SEM image of Au−Pd bimetallic TCB NRs synthesized at a [H2PdCl4]/[HAuCl4] molarratio of 2:1 in the presence of 150 mM CTAB and 10 mM AA. The inset highlights one individual TCB NR. TEM images (left), geometricmodels (right), and SAED patterns (middle insets) of individual TCB NRs viewed along the (B) [100] and (C) [110] projections. (D) SEMimage and EDS elemental maps of Au−Pd TCB NRs. SEM images of TCB NRs synthesized at [H2PdCl4]/[HAuCl4] ratios of (E) 1:1 and (F)4:1 in the presence of 150 mM CTAB and 10 mM AA. (G) PXRD patterns of the TCB NRs synthesized at various [H2PdCl4]/[HAuCl4] molarratios as labeled in the figure. The standard diffraction patterns for cubic-phase bulk Au and Pd are also included. The spectra are offset forclarity. (H) Pd atomic percentages of the TCB NRs synthesized at various [H2PdCl4]/[HAuCl4] molar ratios. The Pd atomic percentageswere quantified by PXRD, EDS, and ICP-MS. The error bars show the standard deviations obtained from three samples synthesized underidentical experimental conditions.

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through seed-mediated co-reduction under diffusion-controlledconditions.A TCB NR represents a prototypical low-index-faceting

geometry which can be derived from a {100}-facetingnanocuboid by creating 8 {111} facets on the truncatedcorners. Figure 2 shows more detailed structural and composi-tional information on the Au−Pd bimetallic TCB NRssynthesized in a sufficiently high concentration of CTAB at150 mM. The electron microscopy images and SAED patterns(Figure 2A−C) showed that the bimetallic shells overgrown onthe Au NR seeds were single-crystalline in nature, and eachTCB NR was enclosed by 6 low-index {100} facets on the sidefaces and 8 low-index {111} facets at the truncated corners.The EDS elemental mapping results clearly showed that the Auand Pd elements were homogeneously distributed in theovergrown bimetallic shells (Figure 2D and Figure S17 inSupporting Information). By varying the [H2PdCl4]/[HAuCl4]ratio while keeping CTAB concentration at 150 mM, we wereable to synthesize TCB NRs with essentially the same geometrybut different compositional stoichiometries (Figure 2E,F). ThePXRD results further verified that the overgrown bimetallicshells were composed of Au−Pd alloys (Figure 2G), absent ofsegregated monometallic Pd shells. The compositionalstoichiometries of the alloy shells were quantified by PXRDusing the Vegard’s law and further compared with the bulkcompositions quantified by EDS and ICP-MS in Figure 2H.The elemental mapping results with the current spatialresolution (∼2 nm), however, did not allow us to further

quantify the local compositional gradient over even smallerlength scales in the alloy shells. Such a local compositionalgradient has been previously observed in various Au−Pdbimetallic nanostructures synthesized by seed-mediated co-reduction67,68 and may also exist in the Au−Pd bimetallic TCBNRs synthesized using our protocol, which is further implicatedby the broadening of the alloy peaks in the PXRD patterns(Figure 2G).The capability of fine-tuning the surface structures and

compositions of the bimetallic NRs through seed-mediated co-reduction can be further enhanced with the aid of foreign metalions, such as Cu2+ and Ag+. We have previously demonstratedthat the facets of monometallic Au NRs can be fine-tailoredwith atomic-level precision using Cu+ ions and CTAB as a pairof surface-capping competitors to guide the particle geometryevolution during NR overgrowth.52 However, the effects offoreign metal ions on the architectural evolution of bimetallicnanocrystals during seed-mediated Au−Pd co-reduction stillremain poorly understood. As shown in Figure 3A−D, at a fixedPd/Au precursor ratio of 1:9, the geometry of the overgrownbimetallic NRs evolved from EHOH NRs to concave cuboidalNRs and eventually to TCB NRs with significantly decreasedtransverse dimensions as an increasing amount of Cu2+ wasintroduced into the overgrowth solution, a trend analogous tothat we recently observed on monometallic Au NRs.52 There isalready compelling evidence showing that Cu2+ ions arereduced to Cu+ ions by AA under similar experimentalconditions.52,69 The Cu+ ions then serve as a surface-capping

Figure 3. Effects of Cu2+ ions on the structural transformations of NRs upon seed-mediated Au−Pd co-reduction. SEM images of the Au−Pdalloy NRs synthesized in the presence of 20 mM CTAB and 10 mM AA at a [H2PdCl4]/[HAuCl4] ratio of 1:9 and various Cu2+ concentrationsof (A) 2 μM, (B) 5 μM, (C) 10 μM, and (D) 400 μM. (E) Pd/Au atomic ratios (quantified by EDS) and (F) extinction spectra of Au−Pdbimetallic NRs obtained at various Cu2+ concentrations ([H2PdCl4]/[HAuCl4] = 1:9). SEM images of the Au−Pd alloy NRs synthesized in thepresence of 20 mM CTAB and 10 mM AA at a [H2PdCl4]/[HAuCl4] ratio of 2:1 and various Cu2+ concentrations of (G) 0.05 μM, (H) 0.2μM, (I) 20 μM, and (D) 200 μM. (K) Pd/Au atomic ratios (quantified by EDS) and (L) extinction spectra of Au−Pd bimetallic NRs obtainedat various Cu2+ concentrations ([H2PdCl4]/[HAuCl4] = 2:1). The error bars in panels E and K show the standard deviations obtained fromthree samples synthesized under identical experimental conditions. All SEM images share the scale bar in panel A.

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ionic species to compete with the CTAB surfactants, therebymodulating the relative growth rates of various types of facets.Apparently, the passivation of the seed surfaces with Cu+ ionsfavors the formation of thermodynamically more stable low-index facets during the seed-mediated Au−Pd co-reduction. Analternative mechanism regarding the effects of Cu2+ involves theselective passivation of certain facets by a submonolayer of Cuadatoms generated from UPD.46 While neither PXRD nor EDSshowed any evidence of the presence of metallic Cu in theovergrown NRs, the existence of transient, localized Cu UPDlayers on the seed surfaces during the seed-mediated co-reduction cannot be completely ruled out at this point.The Cu2+ ions greatly influenced not only the geometries but

the compositions of the overgrowth bimetallic NRs, as well. Asthe Cu2+ concentration gradually increased, the Pd/Au atomicratios (quantified by EDS) continued to decrease until the Pdsignals became almost undetectable at sufficiently high Cu2+

concentrations above 100 μM (Figure 3E). The structural andcompositional transformations of NRs were well-reflected bythe evolution of the key plasmonic features in the opticalextinction spectra (Figure 3F). Surface concavity and cornertruncation caused a red shift and blue shift of the plasmonresonances, respectively, while the loss of Pd content from thealloy shells resulted in narrowing and strengthening of theplasmon bands. Similar effects of Cu2+ ions were also observedat a fixed Pd/Au precursor ratio of 2:1 (Figure 3G−L). As theCu2+ concentration increased, the high-index-faceting TCCBNRs gradually transformed into low-index-faceting quasi-

cuboidal NRs, accompanied by the decrease of the Pd/Auatomic ratios.Our observations strongly suggested that the capping of the

seed surfaces with Cu+ ions selectively suppressed theelectroless deposition of Pd with respect to that of Au.Considering the Au NR seeds as the catalysts for the electrolessco-deposition, it was reasonable to hypothesize that the seedsurfaces became partially poisoned toward the catalyticdeposition of Pd upon surface adsorption of Cu+, resulting inself-nucleation of Pd atoms in solution and the formation ofdiscrete Pd islands on the seed surfaces. Consequently, amixture of self-nucleated Pd nanocrystal aggregates, Au NRsdecorated with Pd nanocrystallites, and irregularly shapedbyproducts (mostly l ikely a CTAB−Pd complex,(C19H42N)2PdBr4, according to the EDS results), wereobtained in the presence of 400 μM Cu2+ (Figure S18 inSupporting Information). Interestingly, no such surface poison-ing effect of Cu2+ ions on Pd deposition was observed duringthe growth of Pd concave nanocubes on Pd seeds69 andseedless one-pot growth of Au−Pd bimetallic nanocrystals,37,46

strongly indicating that such surface poisoning by Cu+ ions washighly likely to be intimately tied to the surface structures of theAu NR seeds. The exact mechanisms involved in the Cu2+-guided Au−Pd co-deposition on Au NR seeds, however, stillremain ambiguous at this stage and require further scrutiny.In contrast to Cu+ ions which exist as surface-adsorbed ionic

species, Ag+ ions have been used to guide the geometricevolution of Au nanocrystals by forming a Ag UPD layer that

Figure 4. Effects of Ag+ ions on the structural transformations of NRs upon seed-mediated Au−Pd co-reduction. SEM images of theovergrown NRs synthesized in the presence of 20 mM CTAB and 10 mM AA at a [H2PdCl4]/[HAuCl4] ratio of 1:9 and various Ag+

concentrations of (A) 1.6 μM, (B) 20 μM, (C) 40 μM, and (D) 160 μM. (E) Pd/Au and Ag/Au atomic ratios (quantified by EDS) and (F)extinction spectra of the overgrown NRs obtained at various Ag+ concentrations ([H2PdCl4]/[HAuCl4] = 1:9). SEM images of the overgrownNRs synthesized in the presence of 20 mM CTAB and 10 mM AA at a [H2PdCl4]/[HAuCl4] ratio of 2:1 and various Ag+ concentrations of(G) 0.5 μM, (H) 4.0 μM, (I) 40 μM, and (D) 160 μM. (K) Pd/Au and Ag/Au atomic ratios (quantified by EDS) and (L) extinction spectra ofthe overgrown NRs obtained at various Ag+ concentrations ([H2PdCl4]/[HAuCl4] = 2:1). The error bars in panels E and K show the standarddeviations obtained from three samples synthesized under identical experimental conditions. All SEM images share the scale bar in panel A.

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selectively passivates certain facets exposed on the Aunanocrystal surfaces.37,70−72 We recently demonstrated thatthe Ag+-guided Au NR overgrowth involved two underlyingpathways, Ag UPD and Au−Ag electroless co-deposition, andthe manipulation of the pathway switch enabled the cylindricalAu NRs to selectively transform into a library of anisotropicnanostructures with interesting geometric, compositional, andoptical characteristics.58 Here, we systematically investigatedthe effects of Ag+ on the structural and compositional evolutionof NRs during seed-mediated co-reduction of Au and Pd byvarying the concentration of Ag+ ions in the overgrowthsolution while keeping the Pd/Au precursor ratios at 1:9. As theAg+ concentration gradually increased in the range from 0.5 to10 μM, the EHOH NRs gradually evolved into quasi-cuboidalNRs as the degree of surface convexity decreased (Figure 4Aand Figure S19 in Supporting Information). Further increase ofAg+ concentration from 10 to 160 μM witnessed the transitionfrom quasi-cuboidal NRs to irregularly shaped NRs andeventually to elongated octahedral (EOH) NRs (Figure 4B−D). Each EOH NR was enclosed by 8 well-defined {111} facetsat the two ends while the 4 side faces exhibited nanoscaleroughness. As shown in Figure 4E, the Pd/Au atomic ratioswere independent of the Ag+ concentration, which was instriking contrast to what we observed in the presence of Cu2+.More interestingly, while the Ag signals remained almostundetectable by EDS at the Ag+ concentrations below 10 μM,the Ag/Au atomic ratios significantly increased as the Ag+

concentration further increased (Figure 4E), indicating thepathway switch from Ag UPD-guided Au−Pd co-deposition toAu−Ag−Pd trimetallic co-deposition. While the seed-mediatedAu−Pd co-reduction guided by Ag UPD resulted in Au@Au−Pd alloy core−shell NRs whose surfaces were covered by asubmonlayer of Ag UPD adatoms, the Au−Ag−Pd trimetallicco-deposition led to the formation of Au−Ag−Pd ternary alloyshells73 on the Au NR seeds as shown by the EDS elementalmapping results (Figure S20 in Supporting Information). Thesestructural and compositional changes introduced interestingmodifications to the plasmonic features of the Au−Ag−Pdtrimetallic NRs as shown in the optical extinction spectra(Figure 4F).The pathway switch was also observed when varying the Ag+

concentration at a fixed Pd/Au precursor ratio of 2:1.Transition of TCCB NRs into cuboidal NRs occurred whenthe Ag+ concentration increased from 0 to 10 μM as aconsequence of Ag UPD-guided Au−Pd co-deposition. Whenthe Au−Pd−Ag trimetallic co-deposition started to dominatethe nanocrystal overgrowth at Ag+ concentrations greater than10 μM, the corners of the cuboidal NRs were significantlytruncated accompanied by nanoscale surface roughening of theNRs, eventually leading to the formation of dumbbell-shapedNRs with highly roughened surfaces at a Ag+ concentration of160 μM (Figure 4G−J and Figure S21 in SupportingInformation). While the Pd/Au atomic ratios remainedessentially unchanged as the Ag+ concertation varied, the Ag/Au atomic ratios displayed a sudden increase upon the pathwayswitch from Ag UPD-guided Au−Pd co-deposition to Au−Ag−Pd trimetallic co-deposition (Figure 4K). The EDS elementalmapping results showed that each Au−Pd−Ag trimetallicdumbbell-shaped NR was composed of a Au NR core coatedwith a Au−Pd−Ag ternary alloy shell (Figure S22 in SupportingInformation). The surface roughness of the Au−Pd−Agdumbbell-shaped NRs originated most likely from the interplaybetween co-deposition and galvanic replacement during the

nanocrystal overgrowth. At the initiate stage, Ag was co-deposited with Au and Pd to form a ternary alloy layer on thesurfaces of the Au NR seeds. The deposited Ag in the alloyshells was oxidized by the AuCl4

− and PdCl42− ions in the

solution through galvanic replacement reactions and was thenredeposited due to an abundant supply of reducing agent, AA.As a consequence, Au−Pd−Ag ternary alloy shells withroughened surfaces formed gradually due to the continuousgeneration of surface atomic vacancies and the nanoscalemigration of surface atoms during the sustained galvanicreplacement concurrent with the trimetallic co-deposition. Thenanoscale surface roughening caused by Ag+-guided electrolessPd deposition on Au nanocrystal cores has also been previouslyobserved by us64 and other groups.74,75 The evolution ofextinction spectral features of the overgrown Au−Pd−Agtrimetallic NRs reflecting the above-mentioned structuraltransformations and compositional changes is shown in Figure4L.The precise control over the particle geometries and

compositions realized through the kinetically controlled seed-mediated co-reduction enabled us to quantitatively correlate thesurface structures and compositions with the catalytic activitiesof colloidal Au−Pd alloy nanocatalysts. As schematicallyillustrated in Figure 5A, we used catalytic reduction of 4-nitrophenol (NP) into 4-aminophenol (AP) by ammoniaborane (AB) at room temperature as a model reaction, whichhas been widely used as a benchmark hydrogenation reactionfor assessing the catalytic activities of metallic nanocatalysts.76,77

In a basic environment, NP exists in the form of 4-nitrophenolate, which exhibited a strong absorption peak at∼400 nm whose intensity gradually decreased as the hydro-genation reaction proceeded. Therefore, the reaction kineticscould be monitored in real time using UV−visible absorptionspectroscopy (Figure 5B). Without any metallic nanocatalysts,the hydrogenation reaction was extremely slow with noobservable progression over extended time periods (Figure5C). In the presence of Au−Pd bimetallic EHOH NRs,however, the hydrogenation reaction started immediately aftermixing NP with excessive AB in aqueous K2CO3 solution at pH10.4 (Figure 5C). This catalytic reaction is essentially driven byactive hydrogen species generated upon metal-catalyzedhydride formation, which can be harnessed to hydrogenatesurface-adsorbed NP. Therefore, it involves the adsorption ofboth NP and AB onto the catalyst surfaces, followed by abimolecular reaction between the surface-adsorbed species.Both NP and AB compete for the same active sites on thecatalyst surfaces, and the reaction kinetics can be well-describedby the Langmuir−Hinshelwood model using the followingsecond-order rate law:77−79

θ θ− =t

kd[NP]

d NP AB (3)

where k is the intrinsic rate constant and θNP and θAB are thesurface coverage of NP and AB, respectively. Considering thesurface adsorption/desorption equilibria for both NP and AB,the rate law should take the following expression:

− =+ +t

k S K KK K

d[NP]d

( ) [AB] [NP](1 [AB] [NP])

02

AB NP

AB NP2

(4)

where S0 is the total active surface area of the catalysts and KABand KNP are the equilibrium constants for adsorption/desorption of AB and NP, respectively. When KAB[AB] ≫

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KNP[NP], the rate law can be further simplified to the followingexpression:

− =+t

k S K KK

d[NP]d

( ) [AB] [NP](1 [AB])0

2AB NP

AB2

(5)

When AB was in great excess with respect to the NP, [AB]remained constant at its initial value, [AB]0, throughout theentire process. Therefore, this catalytic reaction follows pseudo-first-order kinetics as described by the following rate equations:

− =t

kd[NP]

d[NP]obs (6)

− = − =⎛⎝⎜

⎞⎠⎟

II

k tln[NP][NP]

ln0 0

obs(7)

and

=+

kk S K K

K( ) [AB](1 [AB] )obs

02

AB 0 NP

AB 02

(8)

Equations 6 and 7 are the derivative and integral forms of therate law for the pseudo-first-order reaction, and kobs is theapparent pseudo-first-order rate constant, which can beobtained through least-squares fitting of the experimentalresults using eq 7. I0 and I are the absorption at 400 nm beforethe reaction starts and at a certain reaction time, respectively.To quantitatively assess the facet- and composition-depend-

ent catalytic activities of the Au−Pd alloy nanocatalysts, weperformed kinetic measurements in the presence of excessiveAB ([NP]0 = 56.0 μM and [AB]0 = 4.48 mM) at nominally the

same catalyst concentration (1.0 × 108 particles mL−1). Weestimated the particle concentrations of the colloidal Au−Pdbimetallic nanocatalysts based on the concentration of the AuNRs used for the seed-mediated Au−Pd co-reduction. Theconcentration of Au NRs was estimated based on theconcentration of the initial Au seeds (∼2 nm in diameter)used for the NR growth as described in greater detail in ourprevious publication.80 Using the seed-mediated co-reductionmethod, we were able to precisely tailor the geometricparameters of the EHOH, TCCB, and TCB NR samplessuch that the aspect ratios, corner and edge sharpness, andmore importantly the facets of each geometry can be keptalmost identical for rigorous comparison despite the differencein compositional stoichiometries. The synthetic conditions forthe samples used in the catalytic measurements are listed indetail in Table S1 in Supporting Information. In Figure 5C−E,we showed the kinetics of the hydrogenation reactionscatalyzed by EHOH, TCCB, and TCB NRs, respectively. Alinear relationship was observed when plotting −ln(I/I0) as afunction of reaction time, t, verifying that the catalytic reactionsobeyed pseudo-first-order kinetics.In Figure 5F, we compared the kobs values on EHOH, TCCB,

and TCB NRs with varying compositional stoichiometries. Asthe Pd/Au stoichiometric ratio increased, the kobs progressivelyincreased for both TCCB and TCB NRs but decreased in thecase of EHOH NRs, which was counterintuitive because Pd hasbeen previously demonstrated to be a much better catalyst thanAu for the hydrogenation of NP.81 We hypothesized that suchcounterintuitive composition dependence might be a character-istic of the {hkl} facets exposed on the EHOH NR surfaces.

Figure 5. Catalytic tunability of Au−Pd alloy NRs. (A) Schematic illustration of the catalytic hydrogenation of NP by AB on the surface ofAu−Pd alloy NRs. (B) Two-dimensional color-coded intensity maps of time-resolved UV−vis spectra collected after mixing AB with NP in thepresence of Au−Pd alloy EHOH NRs (Pd/Au atomic ratio of 0.30). Plots of −ln(I/I0) at λ = 400 nm as a function of reaction time, t, for (C)EHOH NRs, (D) TCCB NRs, and (E) TCB NRs with various Pd/Au atomic ratios as labeled in the figures. The solid lines show the least-squares fitting results to the reaction kinetic curves. The initial concentrations of NP and AB were 56.0 μM and 4.48 mM, respectively. Theerror bars in panels C, D, and E show the standard deviations obtained from three experimental runs. (F) Comparison of the apparentpseudo-first-order rate constants, kobs, on EHOH, TCCB, and TCB NRs with various Pd/Au atomic ratios.

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The detailed mechanisms underpinning the synergy betweenAu and Pd on the {hkl} facets still remain elusive at this pointand need further experimental and computational investiga-tions. Regardless of the Pd/Au atomic ratios, the EHOH andTCCB NRs were catalytically more active than the TCB NRsbecause high-index facets typically exhibit catalytic activitiesmuch higher than those of low-index facets due to the presenceof highly abundant, catalytically active undercoordinated surfaceatoms, which has been well-demonstrated on monometallicnanocatalysts. The TCCB NRs exhibited activities significantlyhigher than those of the EHOH NRs with similar Pd−Auatomic ratios, indicating that {311} facets were catalyticallymore active than {421} facets mostly likely due to higherabundance of undercoordinated surface atoms on {311} facetsthan on {421} facets. Among all the nanostructures undercurrent investigations, the monometallic Au TCB NRssynthesized following our previously published protocol(Figure S23 in Supporting Information) exhibited the lowestcatalytic activity due to their monometallic nature and lowerabundance of active sites on their low-index facets. It is worthmentioning that the undercoordinated atoms located at theedges and vertices where multiple facets merge only account fora negligibly small fraction of the surface atoms because eachfacet exposed on the NR surfaces is typically larger than 5nm.8,82 Therefore, the relative reaction rates well-reflected thecorrelation between the characteristic surface atomic coordina-tions and the intrinsic catalytic activities of various types of

facets. When two types of facets coexisted on the NRs, forexample, {311} and {111} facets on TCCB NRs and {100} and{111} facets on TCB NRs, the reaction kinetics reflected theoverall catalytic activities with predominant contributions fromthe more active facets.To gain further mechanistic insights into the catalytic

tunability of Au−Pd alloy NRs, we varied the initialconcentration of AB, [AB]0, while keeping the initialconcentration of NP, [NP]0, fixed at 56.0 μM. At sufficientlyhigh [AB]0/[NP]0 ratios, the hydrogenation reactions catalyzedby the EHOH, TCCB, and TCB NRs could all be well-described as pseudo-first-order reactions, whereas they startedto show significant deviation from pseudo-first-order kineticswhen the [AB]0/[NP]0 ratio became lower than 20:1 (Figure6A−C). However, the reactions still followed first-orderkinetics at their initial stages, regardless of the [AB]0/[NP]0ratios. By fitting the linear part of the kinetic trajectories at theinitial stage of the reactions, we obtained the initial rateconstants, kinitial, which are related to several key kinetic andthermodynamic parameters as described by the followingequation:

=+ +

kk S K KK K( ) [AB]

(1 [AB] [NP] )initial0

2AB 0 NP

AB 0 NP 02

(9)

Because the catalytic reactions occurring at [AB]0/[NP]0ratio greater than 20:1 obeyed the pseudo-first-order kineticsthroughout the entire process, kinitial became equivalent to kobs.

Figure 6. Langmuir−Hinshelwood kinetics of catalytic hydrogenation of NP by AB on Au−Pd alloy NRs. Plots of −ln(I/I0) at λ = 400 nm as afunction of reaction time, t, for (A) EHOH NRs (Pd/Au atomic ratio of 0.083), (B) TCCB NRs (Pd/Au atomic ratio of 0.53), and (C) TCBNRs (Pd/Au atomic ratio of 0.57) at various [AB]0/[NP]0 ratios as labeled in the figure. The solid lines show the least-squares fitting resultsto the linear part of the curves at the early stage of the reactions. The initial concentration of NP was fixed at 56.0 μM. The error bars inpanels A, B, and C show the standard deviations obtained from three experimental runs. (D) Plots of initial rate constants, kinitial, on EHOH,TCCB, and TCB NRs as a function of [AB]0. The results of the least-squares fitting using the Langmuir−Hinshelwood kinetic equation areshown as solid curves. (E) Comparison of KNP, KAB, and k(S0)

2 on EHOH, TCCB, and TCB NRs.

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In Figure 6D, we plotted kinitial as a function of [AB]0 for thereactions catalyzed by EHOH NRs (Pd/Au atomic ratio of0.083), TCCB NRs (Pd/Au atomic ratio of 0.53), and TCBNRs (Pd/Au atomic ratio of 0.57). In all cases, the kinitial firstincreased and then decreased as [AB]0 progressively decreased,achieving its maximal values around [AB]0 of 0.56 mM ([AB]0/[NP]0 = 10). The volcano-type relationship between kinitial and[AB]0 could be well-interpreted in the context of Langmuir−Hinshelwood kinetics model. We further performed least-squares curve fitting on the experimental results using eq 9,which allowed us to get the equilibrium constants for thesurface adsorption/desorption of AB and NP, KAB and KNP,respectively, and k(S0)

2, which is the intrinsic rate constant, k,multiplied by the square of the total catalyst surface area, S0. Asshown in Figure 6E, KNP was significantly smaller than KAB forboth the high-index-faceting EHOH and TCCB, whereas thelow-index-faceting TCB NRs exhibited a higher affinity to NPthan to AB. Moreover, KAB exhibited much higher values onEHOH and TCCB NRs than on TCB NRs, whereas KNP

displayed the opposite trend, strongly indicating that theundercoordinated surface atoms on high-index facets may serveas the high affinity sites for AB adsorption while thecoordinatively saturated surface atoms on the low-index facetsfavored NP adsorption. The k(S0)

2 values followed the trend ofTCCB > EHOH > TCB NRs, which was in line with therelative abundance of undercoordinated surface atoms ({311} >{421} > {100}/{111}). Considering that the EHOH, TCCB,and TCB NRs under current investigations had comparable S0values (see more detailed description of how the particlesurface areas are estimated in Supporting Information), thedependence of k on the surface atomic coordinations and thePd/Au stoichiometries was clearly demonstrated by the kineticresults reported here.To more comprehensively understand the factors determin-

ing k, we investigated the pH-dependent catalytic tunability ofthe Au−Pd bimetallic NRs toward the hydrogenation reactions.As previously reported, the hydrogenation of NP involves thetransfer of surface hydrogen species supplied by AB andinterfacial transfer of electrons mediated by the metallicnanocatalysts.83 The modification of the plasmonic features ofthe metallic nanocatalysts introduced by these interfacial chargetransfers has been precisely monitored by single-particle dark-field spectroscopy.83 k is essentially related to the rates of thesetransfer processes, which are determined not only by thesurface structures and compositions of the Au−Pd alloynanocatalysts but also by the pH of the reaction medium. Ithas been previously reported that hydrogenation reactionsproceeded more rapidly at higher pH values.84 As shown inFigure S24 in Supporting Information, the catalytic reactionsfollowed the pseudo-first-order kinetics throughout the entirepH range from 10.4 to 13.9, and the catalytic activities of all themultifaceted NRs significantly increased with the pH valuesregardless of their geometries and compositions. At each pH,however, the kobs values always followed the same trend ofTCCB > EHOH > TCB NRs. Our results clearly show that thecatalytic tunability observed on the multifaceted NRs essentiallystems from the interplay between the competitive surfaceadsorption of the reactants and the interfacial charge transfersinvolved in the reactions, both of which are dependent uponthe surface atomic coordinations and the compositionalstoichiometries of the Au−Pd alloy nanocatalysts.

CONCLUSIONS

This work exemplifies the mechanistic complexity associatedwith the structural evolution of anisotropic alloy nanoparticlesduring seed-mediated co-reduction. Several key syntheticparameters, such as the Au/Pd precursor ratio, the concen-tration of reducing agent, the concentration of cappingsurfactants, and the foreign metal ion additives, all haveprofound influence on the surface co-deposition of metals andthe overgrowth of nanocrystals, thereby providing fine-adjustable knobs for the precise tuning of both the surfaceatomic coordinations and the compositional stoichiometries ofthe resulting bimetallic nanoparticles. By kinetically maneuver-ing the electroless deposition of Au−Pd alloy shells on Au NRseeds, we have been able to selectively synthesize a family ofmultifaceted Au−Pd bimetallic NRs with exotic anisotropicgeometries, including EHOH NRs each of which is enclosed by56 {hkl} high-index facets, TCCB NRs whose surfaces aredominated by 24 {hkk} high-index side facets with 8 truncatedtips terminated by {111} facets, and low-index-faceting TCBNRs enclosed by thermodynamically stable {100} and {111}facets. The seed-mediated co-reduction under diffusion-controlled conditions also enables the fine-tuning of the Au−Pd atomic ratios in the alloy shells deposited on the Au NRseeds over a broad stoichiometric range without formingheterostructures comprising segregated monometallic domainswhile still well-preserving the surface structural characteristicsof each multifaceted geometry.The great success in precise facet and composition control

further allows us to investigate the detailed correlation betweenthe surface structures, the compositional stoichiometries, andthe catalytic activities of colloidal Au−Pd alloy nanocatalysts.Using the catalytic hydrogenation of NP by AB as a modelreaction obeying the Langmuir−Hinshelwood kinetics, we haveshown that the relative binding affinities of reactants and theinterfacial charge transfer rates, both of which serve as the keyfactors that determine the overall kinetics of the catalyticreactions, exhibit strong dependence on the surface atomiccoordinations and the compositional stoichiometries of theAu−Pd alloy nanoparticles. While this work primarily focuseson the structure−composition−property relationships in thecontext of heterogeneous catalysis, the insights gained from thiswork provide general design principles that guide thearchitectural optimization of multimetallic nanoparticles forwidespread applications far beyond heterogeneous catalysis.The incorporation of catalytically active Pd into plasmonicallytunable Au nanostructures enables detailed studies of catalyticmolecular transformations using surface-enhanced Ramanscattering as a time-resolved spectroscopic tool with molecularfinger printing capability.41,85,86 In addition, Au−Pd bimetallicnanoparticles may provide a materials system that allows us toefficiently harness the plasmonic hot electrons to drive orenhance Pd-catalyzed reactions along unconventional pathwaysdistinct from those involved in conventional catalytic thermalreactions or semiconductor-based photocatalysis.87,88 Further-more, Au−Pd bimetallic nanoparticles may serve as stand-aloneoptical sensors for monitoring hydrogen uptake, storage, andrelease due to Pd’s capabilities to reversibly accommodate largequantities of molecular hydrogen and the Pd-enhancedrefractometric sensitivity of Au plasmons.81,89,90 The optimiza-tion of all the above-mentioned applications is intimately tied toour capabilities to precisely tailor both the geometries andcompositions of the bimetallic nanoparticles, which can be

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achieved, as demonstrated in this work, through deliberatelydesigned colloidal syntheses under seed-mediated and kineti-cally controlled conditions.

METHODSChemicals and Materials. The chemicals and materials used in

this work are listed in detail in Supporting Information. All reagentswere used as received without further purification. Ultrapure water(18.2 MΩ resistivity, Barnstead EasyPure II 7138) was used for allexperiments.Synthesis of Multifaceted Au−Pd Bimetallic NRs. The

multifaceted Au−Pd bimetallic NRs were synthesized through seed-mediated electroless plating of Au−Pd alloy shells on single-crystallineAu NRs. Single-crystalline cylindrical Au NRs were synthesizedfollowing a previously published protocol62 with minor modifications(see details in Supporting Information). The electroless plating of Auand Pd on Au NRs was conducted in the presence of HAuCl4,H2PdCl4, CTAB, and AA at 30 °C under ambient air. The Au−Pdbimetallic NR growth solution was prepared by sequentially addingH2O, HAuCl4, H2PdCl4, and AA into a CTAB solution. After thesolution was gently mixed for 30 s, the Au−Pd co-deposition wasinitiated by the introduction of 100 μL of the preformed Au NRs(dispersed in 100 mM CTAB). The reaction solution was gently mixedfor 30 s immediately after the addition of Au NRs and then leftundisturbed at 30 °C for 1 h. The obtained nanoparticles were thenwashed with H2O twice through centrifugation/redispersion cyclesand finally redispersed in 200 μL of 20 mM CTAB. To investigate theeffects of [H2PdCl4]/[HAuCl4] molar ratios, CTAB, AA, and foreignmetal ions, such as Ag+ and Cu2+, on the structural transformationsduring the seed-mediated Au−Pd co-reduction, the overall concen-trations of HAuCl4, H2PdCl4, CTAB, AA, Ag+, and Cu2+ in thereaction solutions were systematically varied, while the total volume ofthe NR overgrowth solution was always fixed at 5.0 mL.Nanostructure Characterizations. The structures and composi-

tions of the Au−Pd bimetallic NRs were characterized by TEM, SEM,EDS, ICP-MS, and PXRD. The optical extinction spectra of thenanoparticles were measured using a Beckman Coulter Du 640spectrophotometer. Details of nanostructure characterizations arepresented in Supporting Information.Catalytic Reaction Kinetics. We used the hydrogenation of NP

by AB at room temperature as a model reaction to evaluate thecatalytic activities of Au−Pd bimetallic EHOH, TCCB, and TCB NRs.In a typical procedure, 0.1 mL of 1.0 mM NP, 0.1 mL of 10 mMK2CO3, and 0.08 mL of 0.1 M AB (freshly prepared) were sequentiallyadded to 1.5 mL of ultrapure water in a cuvette and mixed thoroughly.Then, 20 μL of Au−Pd alloy NR solution was injected into the system.After being thoroughly mixed for 5 s, UV−vis extinction spectra werecollected in real time to monitor the reaction kinetics. We comparedthe catalytic activities of colloidal EHOH, TCCB, and TCB NRs withvarious Pd/Au ratios at nominally the same particle concentration (1.0× 108 particles mL−1). The effects of AB/NP molar ratios on thereaction kinetics were evaluated by varying the initial concentration ofAB, [AB]0, while fixing the initial NP concentration, [NP]0, at 56.0 μMand the total volume of the solution at 1.8 mL. The effects of pHvalues on the reaction kinetics were evaluated by adding appropriateamounts of KOH instead of K2CO3 while keeping the total volume ofthe reaction mixtures at 1.8 mL. The pH values were measured usingan Oakton waterproof pH 450 portable pH meter (Fisher Scientific).

ASSOCIATED CONTENT

*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsnano.7b00264.

Additional experimental details, tables, and figuresincluding SEM images, TEM images, optical extinctionspectra, EDS, PXRD, and catalytic results as noted in thetext (PDF)

AUTHOR INFORMATIONCorresponding Author*E-mail: wang344@mailbox.sc.edu. Phone: 1-803-777-2203.Fax: 1-803-777-9521.ORCIDHui Wang: 0000-0002-1874-5137Present Address‡School of Chemistry and Chemical Engineering, JiangsuUniversity, Zhenjiang, Jiangsu 212013, China.Author Contributions†L.S. and Q.Z. contributed equally.NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENTSH.W. acknowledges the support by a National ScienceFoundation (NSF) CAREER Award (DMR-1253231) and anASPIRE-I Track-I Award from the University of South Carolina(USC) Office of Vice President for Research. Q.Z. was partiallysupported by a Dissertation Fellowship from USC NanoCenter.Both Q.Z. and G.G.L. were partially supported by SPARCGraduate Research Awards from the USC Office of the VicePresident for Research. E.V. was partially supported by aGAANN Fellowship provided by the Department of Educationthrough GAANN Award P200A120075. As a visiting scholar,X.F. was supported by Jiangsu University Study-Abroad Funds(20162673). This work made use of the X-ray diffractionfacilities at the South Carolina SAXS Collaborative supportedby the NSF MRI program (DMR-1428620) and the electronmicroscopy facilities at the USC Electron Microscopy Center.

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