Journal of Materials Chemistry Amezhao/pdf/184.pdf · nanocrystals with diﬀerent shapes as seeds.30 In this work, for the rst time we report a new, facile and one-step polyol route

Journal ofMaterials Chemistry A

PAPER

Department of Mechanical Engineering, Th

Technology, Clear Water Bay, Kowloon, Ho

ust.hk; Fax: +852 23581543; Tel: +852 2358

† Electronic supplementary informa10.1039/c2ta00725h

Cite this: J. Mater. Chem. A, 2013, 1,906

Received 9th July 2012Accepted 30th October 2012

DOI: 10.1039/c2ta00725h

www.rsc.org/MaterialsA

906 | J. Mater. Chem. A, 2013, 1, 906

One-step polyol synthesis of Rh-on-Pd bimetallicnanodendrites and their electrocatalytic properties forethanol oxidation in alkaline media†

Shuiyun Shen and Tianshou Zhao*

We report a new, facile and one-step polyol route for the synthesis of Rh-on-Pd bimetallic nanodendrites

that are composed of Pd cores with Rh branches. Ethylene glycol is used as a reducing agent while

hexadecyltrimethylammonium bromide (CTAB) is used as a structure-directing agent. The as-synthesized

nanodendrites are characterized by transmission electron microscopy, energy-dispersive X-ray

spectrometry, X-ray diffraction and X-ray photoelectron spectroscopy. It is demonstrated that the

morphology and number of Rh branches can be regulated by varying, respectively, the molar ratio of Pd

to Rh precursors and the CTAB content. An intriguing finding is that CTAB not only directs the growth

of Rh branches but also enables the formation of uniformly-shaped Pd cores. This effective one-step

polyol synthesis can be ascribed to the different reduction kinetics between Pd and Rh ions resulting in

the formation of Pd cores prior to the growth of the Rh branches. The electrocatalytic properties of the

carbon supported Rh-on-Pd bimetallic nanodendrites as the catalyst for ethanol oxidation in alkaline

media are investigated. Cyclic voltammetry results show that the Rh-on-Pd/C catalysts display a much

higher CO2 selectivity than a Pd/C catalyst. In particular, the ratio of the forward to backward peak

current density (jf/jb) of the Rh-on-Pd (3 : 1)/C catalyst is 2.2, which is three times that of the Pd/C catalyst.

1 Introduction

Over the past decade, the rational design and synthesis ofbimetallic nanomaterials has attracted increasing interestbecause of their fascinating optical, electronic and catalyticproperties relative to their monometallic counterparts. It hasbeen recognized that compared with single metal nano-materials, bimetallic nanomaterials can have greatly improvedcatalytic performances including activity, selectivity andstability.1–3 Bimetallic nanomaterials can be in the form ofeither alloy or core–shell structures.4–6 Recently, special atten-tion has been paid to the synthesis of M1-on-M2 bimetallicnanodendrites that are composed of an array of branchesformed from one metal (M1) supported on a core of anothermetal (M2).7,8 With respect to conventional core–shell nano-materials with smooth surfaces, the open and branched M1shell of M1-on-M2 bimetallic nanodendrites not only exhibits ahigh surface area but also facilitates the mass transport ofreactants to active sites, thus providing a great opportunity toimprove the catalytic activity of M1.9–16 Bimetallic nano-dendrites, including Pt-on-Au,9,10 Pt-on-Pd11–15 and Pt-on-Ag,16

e Hong Kong University of Science and

ng Kong SAR, China. E-mail: metzhao@

647

tion (ESI) available. See DOI:

–912

have been obtained and are of particular interest as electro-catalysts in fuel cell applications. It has been shown by Xiaet al.12 that on the basis of an equivalent Pt mass, a Pt-on-Pd/Ccatalyst is two and a half times more active for the oxygenreduction reaction in acidic media than the state-of-the-art Pt/Ccatalyst. In addition to a higher activity, Pt-on-Pd bimetallicnanodendrites also possess a better stability than Pt nano-particles. Yang and Peng13 demonstrated that aer 30 000 cyclesof linear potential sweeps, the Pt-on-Pd/C catalyst lost �12% ofits initial electrochemical surface area while the loss for thecommercial E-TEK Pt/C catalyst was as high as 39%. Theimproved stability could be due to the favored interfacialstructures between Pt and Pd as well as the larger particle size ofPt-on-Pd nanodendrites preventing the dissolution or aggrega-tion of Pt active sites during the oxygen reduction reaction.Despite those successful demonstrations, it is noted that theabove-mentioned M1-on-M2 bimetallic nanodendrites are allprepared through a two-step seed-mediated growth route, inwhich preformed M2 nanocrystals serve as seeds to direct thegrowth of M1 branches. This strategy requires multiple stepsand complex procedures. Therefore, it is desirable to develop aneffective one-step route for the synthesis of M1-on-M2 bime-tallic nanodendrites. Up to now, to the best of our knowledge,there are only a few reports associated with the one-stepsynthesis of M1-on-M2 bimetallic nanodendrites.17–20 In all ofthese cases, a block copolymer-mediated synthesis route wasemployed and the temporal separation of the formation of

This journal is ª The Royal Society of Chemistry 2013

Paper Journal of Materials Chemistry A

different metals was believed to be the key to the one-stepsynthesis. Triblock Pluronic copolymers such as Pluronic F127and Pluronic P123 are employed as structure-directing agentsand ascorbic acid as a reducing agent.17–20

An important advantage of anion-exchangemembrane directethanol fuel cells (AEM DEFCs) is that the kinetics of theethanol oxidation reaction (EOR) in alkaline media are muchfaster than those in acidic media, making it possible to use lessexpensive Pd catalysts.21–26 However, on the Pd catalyst, ethanolis selectively oxidized to acetic acid through a 4-electronpathway and this leads to a loss of 66.7% in the faradic effi-ciency of the fuel cell. Recently, it has been reported that duringthe EOR, Rh has great potential to achieve C–C bond cleavagedue to the preferential formation of an oxametallacyclicconformation on Rh surfaces.27–29 However, as Rh is a far lessactive catalyst for the EOR, it is usually combined with Pt or Pdas the electrocatalyst. In a recent achievement, Rh-on-Pdbimetallic nanodendrites were successfully synthesizedthrough a two-step seed-mediated growth route using Pdnanocrystals with different shapes as seeds.30 In this work, forthe rst time we report a new, facile and one-step polyol routefor the synthesis of Rh-on-Pd bimetallic nanodendrites. In thisroute, ethylene glycol (EG) is used as a reducing agent whilehexadecyltrimethylammonium bromide (CTAB) is used as astructure-directing agent. We show that both the morphologyand number of Rh branches can be regulated by varying,respectively, the molar ratio of Pd to Rh precursors and theCTAB content. We investigate the use of carbon supported Rh-on-Pd bimetallic nanodendrites as the electrocatalyst for theEOR in alkaline media, and make comparisons with mono-metallic Pd/C and Rh/C catalysts.

2 Experimental2.1 Materials

Palladium(II) chloride (PdCl2), rhodium(III) chloride (RhCl3),CTAB and hexadecyltrimethylammonium chloride (CTAC) wereall purchased from Aldrich. EG, ethanol (CH3CH2OH) andpotassium hydroxide (KOH) were fromMerck KGaA. Vulcan XC-72 carbon with an average particle size of 30 nm was purchasedfrom E-TEK, while 5 wt% polytetrauoroethylene (PTFE) emul-sion was received from Dupont.

2.2 Synthesis of Rh-on-Pd bimetallic nanodendrites

In a typical synthesis of Rh-on-Pd bimetallic nanodendrites witha Pd/Rh molar ratio of 1 : 1, 1.0 mL of 56.4 mM PdCl2, 2.95 mLof 19.1 mM RhCl3, 0.824 g of CTAB and 20 mL of EG were mixedwith 20 mL of deionized water. The mixture solution was stirredfor 1 h at room temperature, and then transferred to a Teon-lined stainless-steel autoclave. The sealed vessel was heated at120 �C for 12 h before it was cooled to room temperature. Theas-obtained precipitate was then collected by ltration, washedwith ethanol and deionized (DI) water, and dried at 70 �C in anoven. For comparison, both monometallic Pd and Rh nano-particles were synthesized using the same procedure withoutthe other metal precursor in the starting solution. A series of


comparative experiments were also carried out: (1) by loweringthe CTAB content to 0.206 g; (2) by replacing CTAB with thesame number of moles of CTAC; (3) in the absence of thestructure-directing agent. For the synthesis of Rh-on-Pd bime-tallic nanodendrites with different Pd/Rh molar ratios, thePd/Rh molar ratio in the starting solution was varied from 1.0 to7.0. For simplicity, Rh-on-Pd bimetallic nanodendrites withPd/Rh molar ratios of 1 : 1, 3 : 1, 5 : 1 and 7 : 1 are, respectively,labelled as Rh-on-Pd (1 : 1), Rh-on-Pd (3 : 1), Rh-on-Pd (5 : 1)and Rh-on-Pd (7 : 1).

2.3 Characterization

Transmission electron microscopy (TEM) and high-resolutionTEM (HRTEM) images were obtained using a high-resolutionJEOL 2010F TEM system that was operated with a LaB6 lamentat 200 kV and equipped with an energy-dispersive X-ray spec-trometer (EDS). X-ray diffraction (XRD) analysis was carried outwith a Philips powder diffraction system (model PW 1830) usinga Cu Ka source operating at 40 keV at a scan rate of 0.025� s�1.X-ray photoelectron spectroscopy (XPS) characterization wasperformed by a Physical Electronics PHI 5600 multi-techniquesystem using Al monochromatic X-rays at a power of 350 W. Thesurvey and regional spectra were obtained using passing ener-gies of 187.85 and 23.5 eV, respectively. Fourier transforminfrared (FT-IR) spectroscopy was conducted with an infraredspectrometer (Bio-Rad, FTS 6000).

2.4 Electrochemical measurements

For electrochemical studies, all the samples were loaded onVulcan XC-72 carbon and a 20 wt% total metal (Pd and Rh)loading was guaranteed. Cyclic voltammetry (CV) tests wereconducted using a potentiostat (EG&G Princeton, model 273A)in a conventional three-electrode cell. A glass carbon electrode(GCE) with an area of 0.1256 cm2 was used as the underlyingsupport of the working electrode, platinum foil as the counterelectrode, and an Hg/HgO/KOH (1.0 mol L�1) (MMO (mixedmetal oxide), 0.098 V vs. SHE) electrode as the reference elec-trode. The GCE was modied by depositing a catalyst layer ontoit and served as the working electrode. The catalyst ink was rstprepared through ultrasonically dispersing 10 mg of 20 wt%Pd/C, Rh/C or Rh-on-Pd/C catalyst in 1.9mL of ethanol, to which0.1 mL of 5 wt% PTFE emulsion was added. 12 mL of the ink wasthen pipetted onto the GCE and dried in air to yield a metalloading of 96 mg cm�2. The CV tests were performed at roomtemperature and in 1.0 M KOH solution containing 1.0 Methanol, which was deaerated by bubbling nitrogen (99.9%) for30 min in advance. The CV tests were performed in the potentialrange from �0.926 to 0.274 V at a scan rate of 50 mV s�1. All thepotentials in this paper refer to the MMO electrode.

3 Results and discussion

Fig. 1a shows a representative TEM image of the Rh-on-Pd (1 : 1)bimetallic nanodendrites synthesized by the one-step polyolroute in the presence of CTAB. It can be seen that for all thenanodendrites, a darker center is surrounded by a lighter

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Fig. 1 TEM (a) and HRTEM (b) images of Rh-on-Pd (1 : 1) bimetallicnanodendrites.

Fig. 2 TEM images of the monometallic Pd (a) and Rh (b) nanoparticles. Theinsets show the corresponding HRTEM images.

Journal of Materials Chemistry A Paper

branched shell, and this intense contrast demonstrates thatwell-dened Rh-on-Pd bimetallic nanodendrites are obtained.The absence of isolated Pd or Rh nanoparticles indicates a highyield (�100%) of Rh-on-Pd bimetallic nanodendrites. Thediameter of Rh-on-Pd (1 : 1) bimetallic nanodendrites rangesfrom 15 to 30 nm. EDS analysis over the entire region of Fig. 1a(Fig. S1, ESI†) conrms the existence of both Pd and Rh, andthe Pd/Rh molar ratio is 1.11, which is in good agreement withthe nominal ratio of 1.0 in the starting solution. To examine thespecic structures of Rh-on-Pd bimetallic nanodendrites, theHRTEM image of a single Rh-on-Pd (1 : 1) bimetallic nano-dendrite is shown in Fig. 1b. It can be clearly observed that anumber of Rh branches are uniformly distributed on the Pdcore and have an average diameter of �3 nm. The HRTEMimage in Fig. 1b also reveals that the nanodendrites possessgood crystallinity with well-dened fringes. No obvious grainboundaries between the Pd core and the Rh branches are seen,indicating that the Rh branches are grown epitaxially on thesurface of the Pd core. This can be attributed to the fact thatthere is only a small lattice mismatch of 2.3% between Pd andRh.30 EDS analysis of the whole single Rh-on-Pd (1 : 1) bime-tallic nanodendrite in Fig. 1b (Fig. S2, ESI†) reveals that the

908 | J. Mater. Chem. A, 2013, 1, 906–912

Pd/Rh molar ratio is 1.06, which is also consistent with thenominal ratio of 1.0 for the precursors. The EDS analyses cor-responding to the regions included in circles A and B in Fig. 1b(Fig. S3 and S4, ESI†) show that both Pd and Rh can be detectedin the central region, while only Rh is found in the peripheralregion, demonstrating the formation of Rh-on-Pd bimetallicnanodendrites. For comparison, the TEM images of mono-metallic Pd and Rh nanoparticles synthesized with the sameprocedure are shown in Fig. 2a and b, respectively. The insetsshow the corresponding HRTEM images. As can be seen inFig. 2a, the monometallic Pd nanoparticles are highly dispersedand have denite geometric shapes with an average diameter of20 nm. Fig. 2b shows that interconnected small Rh nano-particles with an average diameter of 3 nm are obtained.

The as-synthesized Rh-on-Pd (1 : 1) bimetallic nanodendriteswere characterized by XRD and XPS and compared with themonometallic Pd and Rh nanoparticles. Fig. 3 shows the XRDpatterns of the monometallic Pd and Rh nanoparticles and theRh-on-Pd (1 : 1) bimetallic nanodendrites. There exist vediffraction peaks for themonometallic Pd nanoparticles that arecharacteristic of the face-centered cubic (fcc) crystalline


Fig. 4 Pd 3d and Rh 3d XPS spectra of monometallic Pd and Rh nanoparticlesand Rh-on-Pd (1 : 1) bimetallic nanodendrites.


structure and assigned to the (111), (200), (220), (311) and (222)planes. For the monometallic Rh nanoparticles and the Rh-on-Pd (1 : 1) bimetallic nanodendrites, four peaks are clearlyobserved, but thepeak for the (222) plane is not obvious. It canbeobserved that the Rh-on-Pd (1 : 1) bimetallic nanodendritesshow strongdiffractionpeaks corresponding to the Pd coreswithshoulders corresponding to the Rh branches, suggesting that Pddoes not alloy with Rh during the simultaneous reductionprocess. The XPS spectra of the Rh-on-Pd (1 : 1) bimetallicnanodendrites in the Pd 3d andRh 3d regions are shown in Fig. 4and compared with those of the monometallic Pd and Rhnanoparticles. As seen from Fig. 4, for the Rh-on-Pd (1 : 1)bimetallic nanodendrites, the Pd 3d spectrum shows a doubletthat consists of a high energy band (Pd 3d3/2) at 340.9 eV and alow energy band (Pd 3d5/2) at 335.5 eV, and the Rh 3d spectrumshows a doublet that consists of a high energy band (Rh 3d3/2) at312.2 eV and a low energy band (Rh 3d5/2) at 307.4 eV, indicatingthe existence of metallic Pd and Rh. The Pd 3d spectrum of themonometallic Pd nanoparticles shows a doublet that consists ofa high energy band (Pd 3d3/2) at 340.7 eV and a low energy band(Pd 3d5/2) at 335.3 eV, and the Rh 3d spectrum of the mono-metallic Rhnanoparticles shows adoublet that consists of a highenergy band (Rh 3d3/2) at 312.2 eV and a low energy band(Rh 3d5/2) at 307.4 eV. The very small differences in both the Pdand Rh binding energies between the Rh-on-Pd (1 : 1) nano-dendrites and the monometallic Pd and Rh nanoparticlessuggest that no electronic effect between Pd andRh exists, whichfurther conrms the non-alloy structure.

The facile and high-quality formation of the Rh-on-Pdbimetallic nanodendrites can be ascribed to the formation of Pdcores prior to the growth of Rh branches, which is due to thedifferent reduction kinetics of the Pd and Rh ions.19 Because thestandard reduction potential of [PdCl4]

2�/Pd (+0.591 V vs. SHE)is higher than that of [RhCl6]

3�/Rh (+0.431 V vs. SHE), Pd(II)species will be preferentially reduced over Rh(III) species duringthe reduction process. The preformed Pd nanocrystals can beused as in situ seeds, allowing for the subsequent growth of Rhbranches. In this work, EG was used as a reducing agent whileCTAB was used as a structure-directing agent. Owing to the

Fig. 3 XRD patterns of monometallic Pd and Rh nanoparticles and Rh-on-Pd(1 : 1) bimetallic nanodendrites.


interactions between Br� ions and Pd, CTAB can adsorb onthe surface of the preformed Pd nanocrystals, and thus facilitatethe formation of the Rh branches.31–33 In the absence of thestructure-directing agent, only aggregated nanoparticles aresynthesized (Fig. S5-a, ESI†). Upon replacing CTAB with thesame number of moles of CTAC, well-dened Rh-on-Pd (1 : 1)bimetallic nanodendrites are also obtained (Fig. S5-b, ESI†). Asshown in Fig. 2a and 5a, highly dispersed Pd nanoparticles withdenite geometric shapes are obtained in the presence of CTABor CTAC. In contrast, as shown in Fig. 5b, only large Pd aggre-gates are obtained in the absence of CTAB and CTAC. Therefore,it is concluded that CTAB and CTAC not only direct the growthof the Rh branches but also enable the formation of uniformly-shaped Pd cores.

By simply changing the CTAB content in the starting solution,the number of Rh branches can be easily controlled. As can beseen inFig. 6a,when theCTAB content is decreased from0.824 to0.206 g, Rh-on-Pd (1 : 1) bimetallic nanodendrites with more Rhbranches are synthesized as compared with those shown inFig. 1a. The HRTEM image of a single Rh-on-Pd (1 : 1) bimetallicnanodendrite is shown in Fig. 6b. It is also observed that the Rh-on-Pd (1 : 1) bimetallic nanodendrites in Fig. 6 possess larger Pdcores than those in Fig. 1, which is attributed to the fact thatduring the formation of the Pd cores, CTAB serves as a stabilizingagent, andduring the reductionprocess, theuse of lessCTABwilllead to larger but fewer Pd cores. Fig. 7 shows TEM and HRTEMimages of Rh-on-Pd bimetallic nanodendrites with different Pd/Rhmolar ratios. As shown in Fig. 7a, when the Pd/Rhmolar ratiois increased from 1 : 1 to 3 : 1, there are only a few Rh brancheson each Rh-on-Pd (3 : 1) bimetallic nanodendrite. The HRTEMimage of a single Rh-on-Pd (3 : 1) bimetallic nanodendrite in theinset of Fig. 7a reveals that when the Pd/Rh molar ratio is 3 : 1,only a few Rh protrusions are formed on the Pd surface. Uponfurther increasing the Pd/Rhmolar ratio to 5 : 1 and 7 : 1, the Rhbranches become less visible in the TEM andHRTEM images, asshown in Fig. 7b and c.

As can be judged from the FT-IR spectrum (Fig. S6, ESI†),aer being washed/ltrated with ethanol and water severaltimes, all the samples had clean surfaces and could be used as

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Fig. 5 TEM images of monometallic Pd nanoparticles in the presence of CTAC (a)and with no structure-directing agent (b). The inset shows the correspondingHRTEM image.

Fig. 6 TEM (a) and HRTEM (b) images of Rh-on-Pd (1 : 1) bimetallicnanodendrites synthesized using a lower CTAB content.


electrocatalysts. Fig. 8a compares stabilized CV curves for theEOR on the monometallic Pd/C, Rh/C and bimetallic Rh-on-Pd/C catalysts in 1.0 M KOH solution containing 1.0 M ethanol.For clear observation, scans in the positive direction are shownin Fig. 8b, and magnied curves between�0.926 and�0.3 V areshown in Fig. 8c. In Fig. 8b, it can be seen that the Pd/C catalystis more active than Rh/C for the EOR in alkaline media. Thepeak current density of the EOR on Pd/C is 102.8 mAmg�1 whileit is only 20.6 mA mg�1 on Rh/C. However, as shown in Fig. 8c,the Rh/C catalyst has better ethanol oxidation kinetics than thePd/C at lower potentials. The onset potential of the EOR on theRh/C catalyst is �0.70 V, and this is 150 mV more negative thanthat on the Pd/C. For the EOR, the dissociative adsorption ofethanol to adsorbed COads and CHx species usually occurs atlower potentials, and this leads to the complete oxidation ofethanol to CO2 when in the presence of the oxygen-containingspecies.34–36 According to Fig. 8c, all the Rh-on-Pd/C catalystshave better ethanol oxidation kinetics at the potentials lowerthan �0.35 V as compared with the Pd/C. The onset potential ofthe EOR is �0.66 V on Rh-on-Pd (1 : 1)/C, �0.63 V on Rh-on-Pd(3 : 1)/C,�0.61 V on Rh-on-Pd (5 : 1)/C and�0.60 V on Rh-on-Pd

910 | J. Mater. Chem. A, 2013, 1, 906–912

(7 : 1)/C. Usually, the peak in the backward scan represents theremoval of incompletely oxidized species formed in the forwardscan, and a high ratio of the forward peak current density to thebackward one (jf/jb) can be an indication of excellent oxidationof ethanol to CO2 and less accumulation of carbonaceous resi-dues on the catalyst.28,37,38 As shown in Fig. 8a, the jf/jb ratios onthe Rh-on-Pd/C catalysts are much larger than those on thePd/C. The jf/jb ratio on the Pd/C catalyst is 0.7, while it is 2.7 onRh-on-Pd (1 : 1)/C, 2.2 on Rh-on-Pd (3 : 1)/C, 1.4 on Rh-on-Pd(5 : 1)/C and 1.2 on Rh-on-Pd (7 : 1)/C. For clear comparison, theonset potentials, peak current densities and jf/jb ratios for thePd/C and the Rh-on-Pd/C catalysts with different Pd/Rh ratiosare summarized in Table S1, ESI.† Among the Rh-on-Pd/Ccatalysts, the Rh-on-Pd (3 : 1)/C catalyst is regarded as the bestin view of its simultaneous higher peak current density, morenegative onset potential and higher jf/jb ratio. The durabilitiesof the Pd/C, Rh/C and Rh-on-Pd (1 : 1)/C catalysts for the EOR inalkaline media were evaluated and compared through applyingsuccessive CV sweeps (Fig. S7, ESI†). Aer 200 cycles, there wasno decrease in the peak current density of the EOR on the Pd/Cwhile the decrease in the peak current density of the EOR on the


Fig. 7 TEM images of Rh-on-Pd bimetallic nanodendrites with different Pd/Rhmolar ratios: 3 : 1 (a); 5 : 1 (b) and 7 : 1 (c). The insets show the correspondingHRTEM images.

Fig. 8 CV curves (a), scans in the positive direction (b) and magnified curvesbetween �0.926 and �0.3 V (c) of the EOR on monometallic Pd/C, Rh/C andbimetallic Rh-on-Pd/C catalysts. (1.0MKOH+ 1.0M ethanol; scan rate: 50mV s�1).


Rh/C was as high as 18%; there is a small decrease of 3.4% inthe peak current density of the EOR on the Rh-on-Pd (1 : 1)/C,and this improved durability can be attributed to both favoredinterfacial structures between Rh and Pd and the larger particlesize of the Rh-on-Pd bimetallic nanodendrites.13


4 Conclusions

In this work, an effective one-step polyol route has beenproposed for the synthesis of Rh-on-Pd bimetallic nano-dendrites that are composed of Pd cores with Rh branches. EGis used as a reducing agent while CTAB is used as a structure-directing agent. The number of Rh branches can be readilytuned by varying the CTAB content, while the morphology canbe regulated by varying the molar ratio of Pd to Rh precursors.The underlying mechanism for the one-step polyol synthesisinvolves the different reduction kinetics between Pd and Rhions resulting in the formation of Pd cores prior to the growth ofRh branches. The CV results demonstrate that for the EOR inalkalinemedia, the Rh-on-Pd/C catalysts result in better kineticsat lower potentials and much higher jf/jb ratios than the Pd/Ccatalyst, indicating that the Rh-on-Pd bimetallic nanodendrites

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have much higher CO2 selectivities during the EOR in alkalinemedia. Most attractively, it is found that CTAB not only directsthe growth of the Rh branches but also enables the formation ofuniformly-shaped Pd cores. Since both CTAB and CTAC areproven to be good capping agents for the controlled synthesis ofnoble metals in different shapes,39–41 we believe that theproposed polyol route offers a powerful means for the one-stepsynthesis of Rh-on-Pd bimetallic nanodendrites with shape-controllable Pd cores. An effective one-step polyol route for thesynthesis of Rh-on-Pd bimetallic nanodendrites with shape-controllable Pd cores will be pursued in future work.

Acknowledgements

The work described in this paper was fully supported by a grantfrom the Research Grants Council of the Hong Kong SpecialAdministrative Region, China (Project no. HKUST9/CRF/11G).

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