Morphology-Controlled Synthesis of Palladium Nanostructures by SonoelectrochemicalMethod and Their Application in Direct Alcohol Oxidation

Qingming Shen, Qianhao Min, Jianjun Shi, Liping Jiang, Jian-Rong Zhang, Wenhua Hou,*and Jun-Jie Zhu*

School of Chemistry and Chemical Engineering, Key Laboratory of Mesoscopic Chemistry, Key Laboratory ofAnalytical Chemistry for Life Science, Nanjing UniVersity, Nanjing, 210093, People’s Republic of China

ReceiVed: September 04, 2008; ReVised Manuscript ReceiVed: NoVember 12, 2008

A simple sonoelectrochemical method was used to realize the morphology-controlled synthesis of palladiumnanostructures at room temperature. The palladium spherical nanoparticles, multitwinned particles, and sphericalspongelike particles (SSPs) were successfully prepared in the presence of different surfactants or polymers.It was found that the size and shape of the Pd nanostructures could be controlled by varying current densityand pH value of the precursor solution. The Pd nanostructures were characterized by transmission electronmicroscopy, high-resolution transmission electron microscopy, field emission scanning electron microscopy,energy-dispersive X-ray, and X-ray diffraction. The possible formation mechanism was discussed. In addition,the electrocatalytic properties of the Pd nanostructures for direct alcohol oxidation in alkaline media weresystematically investigated. The results showed that SSPs had a higher electrochemical active surface andresult in more stable and better electrocatalytic properties than other Pd nanostructures for the ethanol electro-oxidation.

1. Introduction

Morphology-controlled synthesis of nanostructures has at-tracted much attention because the size and morphology of mostnanostructures have a great effect on their chemical and physicalproperties.1

As one of the most studied materials, palladium has attractedconsiderable interest for its applications in many field.2-4 Forexample, it has been extensively used as catalysts for the low-temperature reduction of pollutant gases and organic reactions,2,3

and it has prominent performance in hydrogen adsorption andsensing.4 Recently, Pd nanomaterials were also used as elec-trocatalysts for the electrooxidation of ethanol, and the resultsrevealed that Pd was a good electrocatalyst for direct alcoholfuel cells (DAFCs) in alkaline media.5 Since the catalyticefficiency highly depends on both the size and the shape ofpalladium nanomaterials, much effort has been made towardsize and morphology-controlled synthesis of Pd nanostructuresand their catalytic activity.1h,5c,6

Up to now, a lot of approaches have been developed toachieve the shape controlled synthesis of Pd nanostructures.Most of them are synthesized in the presence of some materials,such as use of surfactants, polymers, DNA, or RNA to mediatethe reaction, the use of coordinating ligand to direct the growthand use of template to control the morphology as well as thethermolysis of palladium complexes.1h,4e,f,5c-d, 7,8 While somemethods may involve the use of toxic organic solvents orreagents and complicated process, only a few reports were onthe synthesis of uniform palladium nanostructures in an aqueoussolution.1h,8 Therefore, it is urgent to develop a simple andenvironmentally friendly method for the preparation of size andshape-controlled Pd nanostructures to meet the critical needs

of reproducible and controllable synthetic route for the tech-nological applications.

Recently, the sonoelectrochemical method, which combinessonochemistry and electrochemistry, has been proven to be afast, simple, and effective route for shape-controlled synthesisof nanostructured material.1b,9 The method is accomplished byapplying an electric current pulse to nucleate and perform theelectrodeposit, followed by a burst of ultrasonic wave to removethe products from the sonic probe cathode. During the sonicprocess, the surface state of the nanoparticles falling from thecathode might change under the remarkable ultrasonic conditionand the morphology of the obtained nanoparticles will transformuntil the surface state becomes stable.1b,9 Furthermore, the shapeand size of these nanomaterials could be controlled by adjustingvarious parameters such as current density, time of depositionand sonication, temperature, sonic power, etc.1b,9 Up to now,several metal nanoparticles have been prepared by the sono-electrochemical method;1b,9a-f nevertheless palladium nano-structures have rarely been prepared by this technique.9b

In this paper, a sonoelectrochemical method for the morphol-ogy-controlled synthesis of Pd nanostructures was described.This method is fast, simple, and environmentally friendly. Thepalladium spherical nanoparticles (SNPs), multitwinned particles(MTPs), and spherical spongelike particles (SSPs) were obtainedin various solutions by this method. Meanwhile, the currentdensity and the pH value of the precursor solution also have agreat effect on the size and shape of the Pd nanostructures. Aspontaneous assembly mechanism was proposed for the forma-tion of SSPs. These Pd nanostructures possess good catalyticactivity for the direct oxidation of ethanol in alkaline media.Because of the porous nanostructure, the SSPs exhibited a higherelectrocatalytic activity and more stable electroproperties thanother Pd nanostructures for ethanol electro-oxidation. These Pdnanostructures can be desirable for applications in DAFCs andethanol sensors.

* To whom correspondence should be addressed. E-mail: jjzhu@nju.edu.cn(J.-J.Z.); whou@nju.edu.cn (W.H.). Phone/Fax: +86 25 8359 4976.

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10.1021/jp807881s CCC: $40.75 2009 American Chemical SocietyPublished on Web 01/06/2009

2. Experimental Section

2.1. Materials. Palladium(II) chloride (PdCl2), potassiumnitrate (KNO3), and sodium hydroxide (NaOH) were purchasedfrom Chinese Shanghai Regent Co. Polyvinylpyrrolidone (PVP,Mw ) 39000) and hexadecyltrimethyl ammonium bromide(CTAB) were purchased from Sinopharm Chemical ReagentCo., Ltd. Polydiallyldimethylammonium chloride (PDDA, 20wt % in H2O) was purchased from Sigma-Adrich. Different pHvalues were adjusted with sodium hydroxide (NaOH). Potassiumhydroxide (KOH) and ethanol (C2H5OH, anhydrous) were ofanalytical grade and used without further purification. Allsolutions were prepared with Millipore water.

2.2. Sonoelectrochemical Setup. The sonoelectrochemicaldevice employed has been described elsewhere.1b,9 In brief, atitanium horn (ultrasonic liquid processor VC-750, 20 kHz,Sonics & Materials) acts both as the cathode and the ultrasoundemitter. The electroactive part of the sonoelectrode is the planarcircular surface with an area of 1.23 cm2 at the bottom of thehorn. The immersed cylindrical part is covered by an isolatingplastic jacket. This sonoelectrode produces a sonic pulse thatis triggered immediately following a current pulse. A CHI6301Belectrochemical workstation (CH Instruments Co., USA) wasoperated in the pulse current regime (without using a referenceelectrode). A platinum sheet (1.0 cm × 1.0 cm) was used as acounter electrode. The pulse on time of the current was 0.5 s,the pulse off time of the current was 0.5 s, and the duration ofthe ultrasonic pulse was 0.3 s.

2.3. Sonoelectrochemical Synthesis of Pd Nanostructures.2.3.1. The Pd SNPs. Pd SNPs were prepared according toreference with slight modification.9b Briefly, 2 mL of H2PdCl4(56.5 mM) solution and 2 mL of KNO3 (1 M) solution wereadded into 60 mL of CTAB (10 g L-1) solution while stirring.Then, the pH value of the solutions was adjusted to 8 withNaOH solution (0.1 M). The SNPs were produced with a currentdensity of 25 mA cm-2, an ultrasound intensity of approximately20 w, and a reaction time of 1.5 h. After the reaction, theresulting brown-black solution was purified by centrifugation,and the precipitate was washed with Millipore water for severaltimes and then dried in air.

2.3.2. MTPs. The reactions were carried out in the sameconditions described above except that the CTAB was changedto PVP (5 g L-1), and the pH value was adjusted to 6.

2.3.3. SSPs. The reactions were carried out in the presenceof PDDA (7.5 g L-1), and the pH value was adjusted to 6. Theother reaction conditions were the same as mentioned above.

The obtained Pd nanostructures were sufficiently stable inwater, even after storage for several weeks. Various reactionswere carried out with different current densities and pH values.

2.4. Characterization. The transmission electron microscopy(TEM) images were recorded on a FEI Tecnai-12 transmissionelectron microscope with an accelerating voltage of 120 kV anda JEOL 200CX electron microscope using an acceleratingvoltage of 160 kV. High-resolution TEM (HRTEM) imageswere taken using a JEOL 2010 electron microscope at anaccelerating voltage of 200 kV. Samples for TEM and HRTEMwere prepared by placing two drops of the dispersed Pdnanoparticles solution on a carbon film coated copper grid (400mesh) and then drying under air. Field emission scanningelectron microscopy (FESEM) images were obtained using aFEI Sirion 200 field emission scanning electron microscope.Sample for FESEM was prepared by placing two drops of thedispersed Pd nanoparticles solution on an indium tin oxide (ITO)conductive glass and then drying under air. Wide-angle powderX-ray diffraction (XRD) patterns were obtained with a Philips

X’pert Pro X-ray diffractometer (Cu KR radiation, λ ) 0.15418nm). The UV-vis spectra of PdCl2 solution were recorded bya Shimadzu UV-3600 spectrophotometer at a certain pH value.

Electrochemical experiments were performed on an AutolabPGSTAT-30 potentiostat/galvanostat (Eco Chemie BV, TheNetherlands) using a standard three-electrode cell. A platinumwire was used as the counter electrode and a saturated calomelelectrode (SCE) as the reference. The working electrode wasglassy carbon electrode (GCE, diameter: 3.0 mm) that waspolished with Al2O3 powders (Aldrich, 0.05 µm) and rinsed withMillipore water prior to the test. A 10-µL solution of Pddispersed solution, which was prepared by sonicating 10 mgPd nanoparticles in 5 mL Millipore water (2.0 mg mL-1), wasdrop-cast onto a GCE. The Pd loading was 0.07 mg cm-2. Cyclicvoltammograms (CVs) were obtained in 1.0 M KOH and 1.0M KOH + 1.0 M C2H5OH solutions at room temperature at ascan rate of 50 mV s-1 in the potential range of -0.8 to 0.2 V(vs SCE). Chronoamperometric experiments were carried outat -0.3 V in 1.0 M KOH solution containing 1.0 M C2H5OHat room temperature for 1800 s.

3. Results and Discussion

3.1. Characterization of Different Pd Nanostructures.Figure 1 shows the TEM images of the Pd nanostructuresprepared in different surfactants or polymer solutions. As shownin Figure 1a, spherical nanoparticles were obtained in CTABsolution.9b The high-magnification TEM image in Figure 1bdemonstrates the diameter of the SNPs is about 7 nm. The insetin Figure 1b is the SAED pattern of SNPs. In the SAED pattern,mainly four kinds of crystalline orientations are observed, whichare indexed to be {111}, {200}, {220}, and {311} reflectionsof face-centered cubic (fcc) Pd. To further investigate themorphology of SPNs, HRTEM was used, shown in Figure 1c,the lattice plane spacing calculated from the image is 0.23 nm,which corresponds to that of the {111} lattice planes of fcc Pd.

Figure 1d shows the low-magnification TEM image ofuniform MTPs synthesized in PVP solution. From the high-magnification micrograph (Figure 1e), the MTPs with a diameterof ∼40 nm can clearly be observed. Recently, the similar PdMTPs were also synthesized in aqueous PVP solution. The PVPserved as a stabilizer and capping agent, which can be stronglybound to the {100} facets of Pd and affect the relative growthrates of different facets and thus favor the formation ofMTPs.1h,8d,e The corresponding SAED pattern is shown in theinset of Figure 1e. The presence of distinct points, correspondingto the planes of fcc Pd, indicates the formation of the singlecrystalline Pd. From the HRTEM image (Figure 1f), a 6-foldtwinned nanoparticle can be clearly seen, and the twin bound-aries are indicated by white lines. The inset in Figure 1f is theenlarge image of the square zone area. The clearly markedinterplanar d spacing is 0.23 nm, which corresponds to that ofthe {111} lattice planes of fcc Pd.

The TEM image in Figure 1g indicates that the obtained Pdnanomaterials in PDDA solution are all SSPs with a diameterabout 100 nm. The spongelike structure is evident in high-magnification micrograph, which is built up by tens of primarynanoparticles with an average dimension of 5 nm (Figure 1h).It appears that these small primary nanoparticles are intercon-nected with one another to form larger secondary sphericalspongelike architectures with recognizable boundaries or voidsbetween the component subunits. The SAED pattern of SSPs(inset in Figure 1h) also shows mainly four kinds of crystallineorientations, but the diffraction spots in all orientations areslightly elongated. The elongated diffraction spots in the SAED

1268 J. Phys. Chem. C, Vol. 113, No. 4, 2009 Shen et al.

pattern suggest the presence of multiple nanodomains with asmall misorientation deviating from perfect single crystallinebehavior.1f,9e HRTEM of SSPs clearly shows a lower contrastbetween the crystallites, which reveals that nanopores separatemany primary nanoparticles (Figure 1i). It has been previouslyreported that larger crystalline domain can be formed by thesmall primary nanoparticles via assembly and recrystallization.The clearly marked interplanar d spacing is 0.23 nm, whichcorresponds to that of the {111} lattice planes of fcc Pd. Thesedata suggested that the every attached building unit was nearlysingle crystalline and had different crystallographic orientationsin the SSPs. In addition, the Ostwald ripening process of theprimary nanoparticles is believed to occur by the coalescencebetween the individual building units, leading to the formationof the quasisingle crystal.9e

The FESEM images in parts a and b of Figure 2a confirmthe large-scale production and narrow-size distribution of MTPsand SSPs. Figure 2a shows that MTPs have the polyhedronmorphology, and the average size is about 40 nm. While Figure2b clearly shows that SSPs are truly spherical and built up bytens of primary nanoparticles, and the average size is about 100nm, which is consistent with the TEM results.

The chemical composition of the obtained Pd nanostructureswas determined by EDX analysis. In the EDX spectrum (Figure3), except for the copper and carbon signals from the TEM grid,only peaks of Pd are observed. This result indicates that theobtained nanostructures are exclusively composed of palladium.

The XRD patterns (Figure 4) reveal that all the diffractionpeaks of the obtained Pd nanostructures could be indexed tothe pure fcc phase of palladium (JCPDS card No. 05-0681).The peak broadening is consistent with the nanoscale structuralfeatures of the Pd nanocrystals; the mean size of the nanocrystalscan be calculated by the Scherrer formula from the peak widthat half-maximum.10 The calculated values obtained from thewidth of the {111} peak are 6.3, 34.9, and 4.5 nm, respectively.The mean size of SNPs and MTPs match well with the valuesmeasured from the TEM images, while the mean size of SSPsis much smaller than that shown in TEM image. It is due to thefact that the size calculated from XRD reflects the size ofcrystalline grains, while that shown in TEM image reflects the

Figure 1. TEM and HRTEM images of the Pd nanostructures prepared in different solutions. (a-c) SNPs prepared in CTAB solution. (d-f) MTPsprepared in PVP solution. (g-i) SSPs prepared in PDDA solution. The insets in parts b, e, and h are SAED patterns of the corresponding Pdnanostructures. The inset in part f is the enlarge image of the square zone area. The white lines in part f indicate the twin boundaries separating thedifferent adjacent facts.

Figure 2. FESEM images of (a) MTPs and (b) SSPs.

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overall size of nanoparticles. The Pd SSPs are polycrystallineassembled by tens of primary particles with a size of 5 nm andthe mean size of these primary particles is very similar withthe result calculated from XRD.

3.2. Influence of Current Density. The current density wasfound to have an important effect on the size and morphologyof the obtained Pd nanostructures. Parts a and b of Figure 5 areTEM images of SSPs prepared at different current densities inCTAB solution. It can be seen that the lower current densityresulted in larger particles (Figure 5a), and the higher currentdensity resulted in smaller particles (Figure 5b).9b,e When thecurrent density was increased from 13 mA cm-2 to 42 A cm-2,the diameter of Pd nanoparticles was decreased from 12 to 4nm. Similar results were also obtained in the case of MTPssynthesized in PVP solution. When the current density was 13mA cm-2, the obtained Pd MTPs was a little aggregated andhave a size in range of 40-80 nm (Figure 5c). When the currentdensity was increased to 42 mA cm-2, the uniform MTPs witha diameter of 40 nm were prepared (Figure 5d). In the case ofSSPs, as the current density was increased from 13 to 42 mAcm-2, the size and morphology of Pd nanoparticles changedobviously. When the current density was 13 mA cm-2, theaggregated Pd nanoparticles in a range of 100-200 nm wereprepared (Figure 5e). When the current density was increasedto 42 mA cm-2, the SSPs with a diameter of 60 nm wereobtained (Figure 5f). The above results are consistent with theprevious reports.9b-e The simple explanation for this is that atlow current density (13 mA cm-2) the reduction rate of Pd isslower and the number of nuclei is small, and the rate of growthis faster than that of nucleation, which results in largernanoparticles. As the current density is increased (42 A cm-2),the reduction rate of Pd2+ increases, and the rate of nucleationis faster than that of growth, and the enhanced reduction rateleads to the generation of more nuclei and the formation ofsmaller Pd nanoparticles.9b-e

3.3. Influence of pH Value. In addition, we studied theinfluence of pH values on the size and morphology of Pdnanostructures. Parts a and b of Figure 6 show the TEM imagesof Pd nanostructures prepared under different pH values inCTAB solution. When the pH is 6, the agglomerated Pdnanoparticles were obtained (Figure 6a). In pH 10 solution,uniform Pd nanoparticles with a size of 6 nm were produced(Figure 6b). In MTPs synthesized in PVP solution, aggregatedMTPs were obtained at pH 4 (Figure 6c), and the MTPsprepared at pH 8 had a diameter of 30-60 nm (Figure 6d).Similar result was also found in the preparation of SSPs. Whenthe pH is equal to 4, the Pd nanoparticles were aggregatedtogether and formed dendritic nanostructures, as shown in Figure6e. While in pH of 8 solution, the SSPs with a size of 70 nmwere prepared (Figure 6f). It can be concluded that lower pHvalue results in larger particles (parts a, c, and e of Figure 6);higher pH value leads to smaller particles (parts b, d, and f ofFigure 6). In addition, the pH value also has an effect on thedispersion of the particles.

As mentioned above, the pH value of the precursor solutiondoes have a great effect on the size and shape of the Pdnanostructures. As the pH value of PdCl2 aqueous solution wasincreased by adding NaOH, the color of PdCl2 solution turnedfrom brown-yellow to brown. It is known that, with the increaseof pH, the Cl- complex anion can be displaced by water, andthen neutral hydrated ion undergo hydrolysis and loss of proton,

Figure 3. Representative EDX spectrum of Pd nanostructures.

Figure 4. XRD patterns of palladium (a) SNPs, (b) MTPs, and (c)SSPs.

Figure 5. TEM images of Pd nanostructures at different currentdensities. (a, c, e) Current density of 13 mA cm-2. (b, d, f) Currentdensity of 42 mA cm-2. (a and b) SNPs; (c and d) MTPs; (e and f)SSPs.

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similar to the case of AuCl4-. These processes can berepresented by the following equation11

PdCl42-+H2OT [PdCl3(H2O)]

-+Cl- (1)

[PdCl3(H2O)]-T [PdCl3(OH)]

2-+H+ (2)

[PdCl3(OH)]2-+H2OT [PdCl2(OH)(H2O)]

-+Cl- (3)

[PdCl2(OH)(H2O)]-T [PdCl2(OH)2]

2-+H+ (4)

[PdCl2(OH)2]2-+H2OT [PdCl(OH)2(H2O)]

-+Cl- (5)

[PdCl(OH)2(H2O)]-T [PdCl(OH)3]

2-+H+ (6)

[PdCl(OH)3]2-+H2OT [Pd(OH)3(H2O)]

-+Cl- (7)

[Pd(OH)3(H2O)]-T [Pd(OH)4]

2-+H+ (8)

The color change of PdCl2 solution with the increase of pHwas investigated by UV-vis absorption spectroscopy. As shownin Figure 7a, without the addtion of NaOH aqueous solution(pH ) 2), two absorption peaks at 206 and 235 nm wereobserved, which could be assigned to the ligand-to-metal chargetransfer (LMCT) of [PdCl3(H2O)]- complex (curve A).12 Withthe addition of NaOH aqueous solution (pH ) 4), these twopeaks decreased with the appearance of a new absorption bandat 290 nm (curve B). This absorption band could be attributedto the LMCT of [PdCl2(OH)2]2- and [PdCl3(OH)]2-.12 With the

further addition of NaOH, this absorption band increased (curveC) and reached its maximum at pH 8 (curve D). The similarphenomena were also found in the UV-vis absorption spectraof PdCl2 in CATB, PVP, and PDDA solution at different pHvalues, as shown in Figure 7b. With the increase of the pH value,the absorption spectra of the Pd complex also changed signifi-cantly. It indicates that OH- could partially replace Cl-, H2O,CTAB, PVP, or PDDA to coordinate with Pd2+ in the solutionof CTAB, PVP, or PDDA, respectively. The different pH valuesof Pd solution result in different existing forms of Pd complexand, therefore, lead to the different reduction rates ofpalladium.11d,13 The higher pH value results in the slowerreduction rate and, therefore, yields the smaller Pd nanoparticles.

3.4. Mechanism of the Formation of SSPs. Recently,similar spherical spongelike particles of Rh, Pd, Ni, Ru, Pt, andPtRu were also prepared.14 In the meantime, a spontaneousnanoparticle assembly mechanism has already been suggestedfor the sonoelectrochemical synthesis of the 3D dendritic Ptnanostructures,9e and this mechanism may be extended to explainthe formation of the Pd SSPs nanostructures in this paper.

Figure 8 shows the schematic illustration of the formation ofSSPs. First, palladium ions were reduced by electricity andformed palladium primary nanoparticles on the electrode. Then,the palladium primary nanoparticles were separated in thesolution by the ultrasonic. The primary nanoparticles couldspontaneously assemble together and formed small SSPs. Withthe increase of time and the continuous formation of primarynanoparticles, the small SSPs grew up and finally formedspherical SSPs. In addition, under the ultrasonic condition, theOstwald ripening process is increased, which leads to the smallprimary nanoparticles in favor of the crystallite reorganization.9e

The crystal lattice in the HRTEM image (Figure 1i) confirmsthat this structure is assembled by primary nanoparticles.

According to these results and previous reports,14,9e it can beconcluded that the appropriate stabilizing agent (such as PDDA)and the combination of electrochemical and sonochemicalprocesses are essential for the formation of the uniform SSPs.

3.5. Ethanol Electrooxidation. It is well-known that Pd isa good electrocatalyst for ethanol oxidation in alkaline media.5

The catalytic properties of SNPs, MTPs, and SSPs Pd nano-structures for the direct oxidation of ethanol were studied usingCVs. Figure 9a shows the CVs of the SNPs (curve A), MTPs(curve B), and SSPs (curve C) Pd-modified electrodes in 1.0M KOH solution without containing ethanol. The anodic peaksbetween -0.75 and -0.55 V (vs SCE) originate from thedesorption of atomic hydrogen on the Pd electrocatalysts (Figure9a). The electrochemically active specific surface areas of Pd-modified GCEs can been evaluated through the hydrogendesorption region of CVs measured in 1.0 M KOH solution inthe absence of ethanol (Figure 9a).5 The area of hydrogendesorption on the CV curves represents the charge passed forthe hydrogen desorption, QH, which is proportional to theelectrochemically active area of Pd-modified GCEs. The QHvalues of SNPs, MTPs, and SSPs Pd-modified GCE werecalculated to be 4.7, 2.5, and 6.3 mC cm-2, respectively. Thehigh electrochemically active specific surface area of Pd SSPsmodified GCE can be attributed to the nanopores within theSSPs.

Figure 9b shows the CV curves of ethanol oxidation on thePd-modified GCE in a 1.0 M KOH solution containing 1.0 Methanol. The onset potentials for the ethanol oxidation on SNPs,MTPs, and SSPs were at -0.70, -0.64, and -0.76 V,respectively, which are 250, 190, and 310 mV more negativethan the -0.45 V observed on the Pd film electrode.5b The

Figure 6. TEM images of Pd nanostructures at different pH values.(a) Pd nanostructures synthesized at pH ) 6, (b) SNPs synthesized atpH ) 10, (c) MTPs synthesized at pH ) 4, (d) MTPs synthesized atpH ) 8, (e) Pd nanostructures synthesized at pH ) 4, (f) SSPssynthesized at pH ) 8.

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negative shift of the onset anodic potential shows that the ethanolis easier to oxidize.5b The observed ethanol oxidation peaks wereat -0.2 V in the forward sweep and at -0.3 V in the backwardsweep, the magnitude of the forward anodic peak current isproportional to the reaction activity for the ethanol oxidation atthe Pd-modified GCE.5 The peak current density of the SNPs,MTPs, and SSPs Pd-modified GCE were 51, 13, and 77 mAcm-2, respectively. It is clear that the catalytic activity of SSPsis six times that of the MTPs and about 1.5 times that of theSNPs. This fact demonstrates that Pd SSPs exhibit higherelectrocatalytic activity for the oxidation of ethanol than SNPsand MTPs. The peak current density of SSPs is even slightlylarger than that of Pd nanowire arrays (74 mA cm-2) despite alower Pd loading (0.07 mg cm-2 for Pd SSPs modified GCE,0.24 mg cm-2 for Pd nanowire arrays modified GCE).5b Theremarkable high oxidation current for the Pd SSPs modifiedGCE is directly relate to the morphology of the SSPs. Theporous structure might play a crucial role in determining thecatalytic performance.

Chronoamperometry tests were performed to investigate thelong-term performance of these Pd-modified electrodes duringalcohol oxidation in a 1.0 M KOH solution containing 1.0 Methanol. Figure 10 shows current-time curves for alcoholoxidation measured at a fixed potential of -0.30 V for allmodified electrodes. The polarization current for the ethanolelectrooxidation reaction shows a rapid decay during the initialperiod because of the poisoning of the intermediate species.5

The Pd SSPs modified electrode exhibits a higher initial currentand much slower current decay as compared with those of the

SNPs and MTPs modified electrodes because the former has alarge number of active sites initially available for ethanolactivation. At the end of the 1800-s test, the oxidation currenton the Pd SSPs is still considerably higher than those of theothers. These results are consistent with the CVs measurementsmentioned above, further confirming the relatively better toler-ance to the intermediate species and higher catalytic activity ofPd SSPs electrodes toward ethanol electrooxidation.

The capping agents have a big influence on the electrocatalyticproperties of the Pd nanostructures. First of all, the interactionsbetween the different capping agent and Pd metal surfacedetermine the different Pd nanostructures. Second, the interac-tion between the capping agents and Pd surface also has a big

Figure 7. UV-vis absorption spectra of (a) PdCl2 aqueous solution and (b) PdCl2 in CTAB, PVP, and PDDA solutions at different pH values.

Figure 8. Schematic illustration of the formation of SSPs.

Figure 9. CVs of the SNPs (curve A), MTPs (curve B), and SSPs (curve C) Pd-modified electrode (a) in 1.0 M KOH solution without ethanol and(b) in1.0 M KOH solution containing 1.0 M ethanol.

Figure 10. Chronoamperometric curves for ethanol electrooxidationat -0.3 V (vs SCE) on SNPs (curve A), MTPs (curve B), and SSPs(curve C) Pd-modified GCEs in a 1.0 M KOH containing 1.0 M ethanolsolution.

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effect on the electrocatalytic properties. As we know, PVPinteracts strongly with the metal surface and thus block asignificant number of active sites.15 However, the interactionof CTAB with Pd surface is considerably much weaker thanthat of PVP, and more active sites are reserved.16 In the case ofthe PDDA, the catalytic active sites are not blocked, and thecharge and mass transport are also feasible in the SSPs Pdnanostructures.17

According to the detailed analyses presented above, it is clearthat the unique structure of the Pd SSPs results in its higherelectrocatalytic activity and better electrochemical stability forthe oxidation of ethanol. The interconnected structure of Pd SSPsnot only leads to a high surface area but also provides enoughabsorption sites for involved molecules in a limited space.18 Inaddition, PDDA acts as a stabilizing agent absorbing on theSSPs and provides more catalytic active sites, better conduc-tance, and more stable electroproperties to the SSPs. Finally,the good tolerance of the Pd SSPs catalysts to the intermediatespecies also has an advantage in enhancing the catalyticactivities.

4. Conclusions

In conclusion, we developed a simple method for themorphology-controlled synthesis of Pd nanostructures. The PdSNPs, MTPs, and SSPs were successfully prepared in thepresence of different surfactants or polymers. In addition, thecurrent density and the pH value of the precursor solution alsohave a great effect on the size and shape of the Pd nanostruc-tures. The Pd SSPs nanostructures possess excellent electro-catalytic properties and may be of great potential in DAFCs.This simple approach can be a useful method for making Pdnanostructures which can be used in the fields of nanoelectron-ics, catalysis, hydrogen storage, fuel cells, and other relatedfields.

Acknowledgment. We are grateful for the financial supportof the European Community Sixth Framework Program througha STREP grant to the SELECTNANO Consortium, ContractNo. 516922, the National Natural Science Foundation of China(20635020, 20521503, 20773065), and the 973 Program(2007CB936302).

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