Materials Research Bulletin 47 (2012) 3637–3643

Controlled synthesis of concave tetrahedral palladium nanocrystals by reducingPd(acac)2 with carbon monoxide

Hai Zhu a, Quan Chi a, Yanxi Zhao a, Chunya Li a, Heqing Tang a, Jinlin Li a, Tao Huang a,*, Hanfan Liu a,b

a Key Laboratory of Catalysis and Materials Science of the State Ethnic Affairs Commission & Ministry of Education, Hubei Province, College of Chemistry and Materials Science, South-

Central University for Nationalities, Wuhan 430074, Chinab Institute of Chemistry, Chinese Academy of Science, Beijing 100080, China

A R T I C L E I N F O

Article history:

Received 15 February 2012

Received in revised form 17 May 2012

Accepted 14 June 2012

Available online 23 June 2012

Keywords:

A. Metals

A. Nanostructures

B. Chemical synthesis

C. Electrochemical measurements

A B S T R A C T

CO reducing strategy to control the morphologies of palladium nanocrystals was investigated. By using

CO as a reducing agent, uniform and well-defined concave tetrahedral Pd nanocrystals with a mean size

of about 55 � 2 nm were readily synthesized with Pd(acac)2 as a precursor and PVP as a stabilizer. The

structures of the as-prepared Pd nanocrystals were characterized by transmission electron microscopy (TEM),

X-ray powder diffraction (XRD), ultraviolet–visible (UV–vis) absorption spectroscopy and electrochemical

measurements. The results demonstrated that CO was the most essential for the formation of the concave

tetrahedral Pd nanostructures. The morphologies and sizes of the final products can be well controlled by

adjusting the flow rate of CO. The most appropriate CO flow rate, temperature and time for the formation of

the ideal concave tetrahedral Pd nanocrystals was 0.033 mL s�1, 100 8C and 3 h, respectively.

� 2012 Elsevier Ltd. All rights reserved.

Contents lists available at SciVerse ScienceDirect

Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Shape-controlled synthesis of noble metal nanostructures hasbeen paid much attention for decade owing to their potentialapplications in many fields such as catalysis [1–4], photonics [5,6],optoelectronics [7,8], plasmonics [9,10], microelectronics [11,12],information storage [13], sensing [14–17], biological labeling [18],and among others. The unique properties of metallic nanoparticlesare strongly dependent on their sizes and shapes, as well as theircompositions, crystallinities and structures. So, special nanos-tructures with uniform sizes and well-defined shapes are requiredto tune their properties with a greater versatility for variousapplications. Up to now, different techniques such as template-directed [19,20], solvent thermal [21], microwave dielectricheating [22,23], chemical and electrochemical methods [24,25]have been developed for the preparation of metallic nanoparticles,and many reducing agents were employed to control particle sizesand morphologies.

In case of Pd, it has attracted much interest due to itsextraordinary properties. It has been widely used as catalystsfor some organic reactions [26–28] and low temperature reductionof pollutants exhausted from automobiles [29,30] because of its

* Corresponding author at: College of Chemistry and Materials Science, South-

Central University for Nationalities, Wuhan 430074, China. Tel.: +86 27 67843521;

fax: +86 27 67842752.

E-mail address: huangt6628@yahoo.com.cn (T. Huang).

0025-5408/$ – see front matter � 2012 Elsevier Ltd. All rights reserved.

http://dx.doi.org/10.1016/j.materresbull.2012.06.048

exceptional sensitivity and remarkable adsorbing capacity towardhydrogen [31]. Pd nanoparticles were also used as electrocatalystsfor direct alcohol oxidation [24,25,32,33]. Since the catalyticefficiency of Pd nanoparticles highly depends on both its size andits shape, a lot of efforts have been devoted to the fabrication ofuniform Pd nanostructures with controllable sizes and well-defined morphologies. So far, various morphological Pd nanopar-ticles have been prepared by using different methods. The mostrepresentative Pd nanostructures are rods [34–38], wires [21],sheets [39], cubes/bars [40,41], tetrahedral [42,43], octahedral[44], and multipod [45] as well as icosahedra [23,46,47] andtetrahexahedra [24,25]. In the preparing process, polyols (such asethylene glycol, tetraethylene glycol) [23,36,47], citric acid [46],ascorbic acid [48,49], vitamin B1 [50], alkylamine [51,52],formaldehyde [53] as well as CO gas [39] were generally usedas reducing agents. Meanwhile, polyvinylpyrrolidone (PVP),alkylammonium ions or other surfactants were the most widelyused mediating materials to serve as surface-regulating agents orstabilizers. However, the forming mechanisms for some nanos-tructures have not yet been revealed.

Moreover, most of the as-obtained nanocrystals were generallyflat or convex shapes. Recently, though concave polyhedral Pdnanocrystals [53] were synthesized in the presence of formalde-hyde, it is only a limited success for them to date. The reportindicated that the formation of the concave tetrahedral/trigonalbipyramidal Pd nanocrystals was dependent on the aldehydegroup [53]. However, the formation mechanism has been stillunclear. In fact, decomposition of formaldehyde may produce CO

H. Zhu et al. / Materials Research Bulletin 47 (2012) 3637–36433638

and H2. Therefore, it can be assumed that the formation of theconcave Pd nanostructures would be related to CO molecules. If so,concave polyhedral Pd nanocrystals should be obtained in thepresence of CO. Indeed, our recent experiment confirmed this idea.Herein, the synthesis of concave tetrahedral Pd nanocrystals wasdemonstrated by using CO as a reducing agent, Pd(acac)2 as aprecursor and PVP as a stabilizer. The formation mechanism wasalso explored preliminarily.

2. Experimental

2.1. Materials

Pd(acac)2 (99%) was purchased from Acros Chemicals. PVP(average molecular weight, Mw = 30,000) and N,N-Dimethylfor-mamide (DMF) were purchased from Sinopharm Chemical ReagentCo., Ltd. (Shanghai, China). All reagents were of analytical gradeand used as received without further purification. High purity CO(99.999%) and H2 (99.999%) were used.

2.2. Methods

In a typical synthesis, 160 mg PVP and 25 mg Pd(acac)2 weredissolved in 10 mL DMF. After thorough mixing, the resultinghomogeneous yellow solution was transferred to a glass three-necked flask. The final concentration of Pd(acac)2 was 8.2 mM andthe molar ratio of PVP/Pd(acac)2 was 17/1. Under vigorouslystirring, CO gas was bubbled continuously into the solution at aflow rate of 0.033 mL s�1. The flow rate of CO was controlled byusing mass flow controller. Following the exclusion of air, the flaskwas heated at 100 8C for 3 h under atmospheric pressure. Afterbeing cooled to room temperature, the resulting black homoge-neous Pd colloids were precipitated by acetone, separated bycentrifugation and further purified by ethanol. Under the sameconditions, the reaction was also conducted by using pure H2 or themixed CO–H2 gas instead of CO.

Transmission electron microscopy (TEM) images were taken ona FEI Tecnai G2 20 transmission electron microscopy operated at200 kV. The sample for TEM observation was prepared by placing adrop of the colloidal dispersion onto a copper grid coated with aperforated carbon film, followed by evaporating the solvent atambient temperature. The average particle size and the distribu-tion were determined from the enlarged micrographs on the basisof the measurement of about 300 particles. X-ray powderdiffraction (XRD) measurement was recorded on a Bruker D8

Fig. 1. Typical TEM image (a) and high magnification TEM (b) of the concave tetrahedr

Advance X-ray Diffractometer employing Cu Ka radiation with40 kV and 50 mA. Ultraviolet–visible (UV–vis) absorption spectrawere measured on a Lambda BIO35 spectrophotometer.

2.3. Electrochemical measurements

Pd-modified working electrodes were fabricated by depositingethanolic dispersion of purified concave tetrahedral Pd nanocrys-tals onto a glassy carbon electrode followed by natural drying. Asaturated calomel electrode (SCE) and a platinum foil were used asthe reference and counter electrode, respectively. Firstly, toinvestigate the CO adsorption on the freshly-prepared concavetetrahedral Pd nanocrystals, the CO stripping voltammetry wasrecorded in 0.1 M H2SO4 at a sweep rate of 2 mV/s withoutintroducing any additional CO. Then a second potential scanningwas followed at the same sweep rate. After that, CO gas (99.999%)was bubbled for 15 min through the 0.1 M H2SO4 solution in whichthe modified electrode was immersed before measurements. Themodified electrode was quickly transferred into a fresh 0.1 MH2SO4 solution and the CO stripping voltammetry was recordedonce again.

3. Results and discussion

Fig. 1 shows TEM images of the typical Pd nanocrystalsprepared using CO as a reducing agent with a flow rate of0.033 mL s�1 at 100 8C for 3 h. As can be seen, uniform and well-defined concave tetrahedral Pd nanocrystals were the preferentialnanostructure, though followed by a few trigonal bipyramidalshapes. The average side length of the concave tetrahedral Pdnanocrystals was 55 � 2 nm. The as-obtained morphological fea-tures were the same as those reported by Zheng and co-workers [53].It was clearly observed that each face of the tetrahedron wasexcavated with a trigonal pyramid at the center. According to Zheng’sstudies on this morphology [53], each cut-out pyramid at the center ofthe tetrahedral Pd holds a {1 1 1} facet and three {1 1 0} facetsexposed. And the concave tetrahedral are single crystalline, while theconcave trigonal bipyramids are single twinned with a stacking faultalong the {1 1 1} planes. The concave tetrahedral Pd nanocrystalsobtained by Zheng was related to the aldehyde group [53]. Combinedwith our results herein, the formation of the concave tetrahedral Pdwas essentially dependent upon CO molecules.

Fig. 2 shows the XRD pattern of the concave tetrahedral Pdnanocrystals obtained in the typical experiment above. Fivecharacteristic peaks of Pd at 2u = 40.58, 46.88, 68.48, 82.48, 86.98,

al Pd nanocrystals prepared with a CO flow rate of 0.033 mL s�1 at 100 8C for 3 h.

90807060504030

{222}

{311}{220}

{200}

{111}

Intensity(a.u)

2θ(degree)

Fig. 2. XRD patterns of the typical concave tetrahedral Pd nanocrystals.

H. Zhu et al. / Materials Research Bulletin 47 (2012) 3637–3643 3639

corresponding to the {1 1 1}, {2 0 0}, {2 2 0}, {3 1 1} and {2 2 2}lattice planes, were observed. All the diffraction peaks can be well-indexed to face-centered cubic (fcc) Pd according to the JCPDS cardNo.00-001-1201, indicating that the as-prepared concave tetrahe-dral Pd had a high purity and high crystallinity.

To verify the critical effect of CO on concave tetrahedralstructure, the mixed CO–H2 gas (1:1 by volume) was used as areducing agent instead of pure CO and bubbled into the reactionsystem at a flow rate of 0.033 mL s�1 under the same otherconditions. The result was shown in Fig. 3a. It can be seen that thedominant products were tetrahedral nanocrystals with an averagesize of 35 � 2 nm as well as trigonal bipyramids. The reductionpotential involving CO is �0.12 V, so the CO reduction was dominantfor the mixed CO–H2 gas. Though the existence of H2 interfered withthe reduction by CO and its adsorption on Pd surfaces and resulted inthe unobvious concavity of the as-obtained nanocrystals, it stillconfirmed that the formation of tetrahedral as well as trigonalbipyramidal nanostructures with vague concavities was reallydependent on the use of CO. Similar experiment was conducted byusing pure H2 instead. As shown in Fig. 3b, by contrast, pompon-likeself-assemblies [54] of Pd nanoparticles were obtained, and nopolyhedral nanocrystals were observed. These results showed thatconcave tetrahedral or other polyhedral nanostructures could notgenerate in the absence of CO. Accordingly, this also proved ourassumption that concave polyhedral Pd nanocrystals should beobtained by using CO as a reducing agent instead of formaldehyde.

So far, however, it was reported that only well-defined ultrathinhexagonal nanosheets were produced by using CO as a reducing

Fig. 3. TEM images of Pd nanoparticles prepared using mixed CO-H2 gas (a) and pure H2 (

agent [39]. To explain the concave tetrahedral feature obtainedhere, CO flow rate was considered especially. Fig. 4 shows TEMimages of Pd nanocrystals prepared at different flow rates of COgas. As can be seen, with the increase of the CO bubbling rate from0.033 to 0.05, 0.1, 0.3 mL s�1, the amount of concave tetrahedradecreased, while other various morphologies increased. When theCO flow rate increased to 0.05 mL s�1, particles with uncertainmorphologies and nonuniform sizes were obtained, thoughconcave tetrahedral particles were still in the majority (Fig. 4b).It is noteworthy that a few nanosheets, accompanying withtetrahedral and other morphological particles, were generated at aCO flow rate of 0.1 mL s�1 (Fig. 4c) and apparent hollow nanosheetswere produced at 0.3 mL s�1 (Fig. 4d). Moreover, the particle sizesshrank gradually and the degree of concavity decreased. Theaverage side length of the concave tetrahedral Pd nanocrystals was55 � 2, 51 � 5, 31 � 2 nm from (a), (b) to (c), respectively. Theseresults indicated that the flow rate of CO had a great effect onmorphologies and sizes of the final products. A relatively slower rateof CO gas inlet, which may lead to a slower reducing rate and a loweradsorption of CO on the Pd nucleus crystallite, was favorable for theformation of uniform and well-defined concave tetrahedral Pdnanocrystals (Fig. 4a). While CO gas was inlet with a faster rate, onone hand, the reducing rate was increased, on the other hand, theadsorption of CO on the nucleus crystallite was enhanced and theadsorbing selectivity decreased. As a result, varied morphologies of Pdnanocrystals were produced and the average particle sizes decreased.In addition, this effect of CO flow rate indicated that concavetetrahedral Pd nanocrystals required growth under kinetic control. Itis advantageous to the formation of concave tetrahedral Pdnanocrystals at a slow flow rate of CO due to kinetic control. Withthe increase of CO flow rate, irregular smaller nanoparticles appeareddue to a faster rate of atomic addition, and even apparent hollownanosheets were generated because of the growth confinement effectof CO with a faster flow rate.

Furthermore, the effects of reaction temperature and time onthe formation of concave tetrahedral Pd nanocrystals were alsoinvestigated. Fig. 5 shows TEM images of Pd products prepared atdifferent temperature under the same other conditions as thetypical synthesis. Compared with those obtained at 100 8C, nearlyno concave tetrahedral Pd nanoparticles were obtained at 80 8C, asshown in Fig. 5a, while many irregular apparent hollow hexagonalnanosheets accompanied with a few smaller concave tetrahedraland other morphological particles were produced at 120 8C, asshown in Fig. 5c. This is because at a lower temperature both theformation and growth of nucleus crystallite slowed down due to

b) as a reducing agent, respectively, instead of CO under the same other conditions.

Fig. 4. TEM images of Pd nanocrystals prepared at different flow rates of CO. (a) 0.033 mL s�1; (b) 0.05 mL s�1; (c) 0.1 mL s�1; (d) 0.3 mL s�1.

Fig. 5. The effects of reaction temperature on the morphologies of Pd nanocrystals. (a) 80 8C; (b) 100 8C; (c) 120 8C. The inset of (c) shows a locally magnified TEM image of an

apparent hollow nanosheet.

H. Zhu et al. / Materials Research Bulletin 47 (2012) 3637–36433640

slower rate of atomic addition with slow reducing rate of Pd(acac)2,whereas the adsorbing rate as well as the nonselective adsorptionof CO on Pd facets enhanced. As a result, shaped-nanoparticleswere hard to form beyond kinetic control. At a higher temperature,however, the adsorbing rate of CO decreased while the selectiveadsorption of CO on Pd {1 1 1} planes enhanced, and consequentlya few nanosheets which are thermodynamically favorable wereproduced due to a faster growing rate. The apparent hollowhexagonal nanosheets were actually sheet-like porous structures

with thicker edges, as shown in the inset magnified TEM of Fig. 5c.So, an appropriate temperature in favor of kinetic control wasrequired for the formation of concave tetrahedral nanostructures.

TEM image of Pd nanocrystals produced in a 6 h reaction isshown in Fig. 6. Obviously, not only did the number of concavetetrahedral Pd decrease significantly, but also the degree ofconcavity faded out. On the other hand, many different concavepolyhedral nanocrystals twinned with a stacking fault along the{1 1 1} were obtained. In addition, the side length of the Pd

Fig. 6. TEM images of the as-prepared Pd nanocrystals in a 6 h reaction.

H. Zhu et al. / Materials Research Bulletin 47 (2012) 3637–3643 3641

nanocrystals was 44 � 3 nm, smaller than that obtained in a 3 hreaction. These results suggested that the optimum reaction time forproducing concave tetrahedral Pd was 3 h. Thus it can be seen that alonger time was unfavorable for the formation of uniform and well-defined concave tetrahedral morphologies beyond kinetic control.

Interestingly, twinned nanocrystals produced with increasingthe reaction time implied that a ripening process was unfavorableto the growth of concave tetrahedral Pd. This abnormal behavingwas confirmed by UV–vis absorption measurement. Fig. 7 showsthe time-dependent UV–vis absorption spectra for the reactionprocess. As can be seen, at the beginning the reaction solutionshowed two strong characteristic absorption peaks at 268 and326 nm, corresponding to that of Pd(II) ions. With the reactionproceeding, the peak at 326 nm reduced gradually. When thereaction proceeded for 3 h, which corresponded to the time infavor of concave tetrahedral Pd nanostructures, the peak at326 nm still remained obviously though it was weakenedsignificantly, indicating the existence of Pd(II) ions. Another peakwith an enhancement was observed at 269 nm. This meant thatPd(II) ions was not reduced completely under the optimumconditions for the ideal concave tetrahedral nanostructure.According to the peak intensities at 326 nm, the yield of concavetetrahedral Pd related to the amount of Pd precursors at 3 h wasabout 81%. Furthermore, increasing the reaction time continu-ously from 4 h to 8 h, the peak at 326 nm disappeared completely,while the peak at 268 nm shifted to 273 nm. The red shift wasindependent on the time in 4–6 h reaction, but the absorbanceincreased gradually due to the enhancement of surface plasmon

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Fig. 7. UV–vis absorption spectra of the reaction system at different stages.

scattering, implying the changes of morphologies. This wasconsistent with the observation on the 6 h reaction, as shown inFig. 6. With the reaction time was prolonged from 3 h, the concavetetrahedral nanocrystals were transformed into twinned nanos-tructures due to ripening process after the depletion of metalprecursor. Considering the concave tetrahedral nanocrystals arenot thermodynamically favorable, the twinned nanocrystalswould be ripened with a stacking along the {1 1 1} planes,accompanying with the disappearance of concavity. These resultsdemonstrated that the formation of the solid polyhedra at 6 h maybe evolved from the concave ones by filling the cavities as well asripening due to the incomplete reaction at 3 h.

In addition, PVP also plays an important role in controlling themorphologies of final nanoparticles. Though PVP can also serve as areducing agent in the syntheses of metal nanomaterials [21], noproduct was observed in the presence of PVP without using CO andan obvious agglomeration was observed without the addition ofPVP when the other conditions were kept the same as those in theabove typical experiment. So, we believe that PVP serves only asstabilizer to eliminate random agglomeration during the synthesisthough it is found that the formation of concave tetrahedral Pd isdependent on the PVP/Pd(acac)2 ratio.

From the above experimental evidences, CO was essential forthe formation of concave tetrahedral Pd nanocrystals. CO playedan important dual role as a both reducing agent and surfaceadsorbent. CO was adsorbed on Pd surfaces at the same time itreduced Pd(II) ion, while the reaction could not occur withoutCO. The concave faces were developed at the very beginning ofthe formation of Pd nucleus crystallite. Due to the adsorption ofCO, either atomic addition or adatom diffusion was restricted, sothat the morphologies would be confined. CO molecules wereadsorbed on some special facets as soon as Pd nucleus crystalliteswith different crystallographic facets were generated. To confirmthe preferential adsorption of CO on special facets, CO strippingvoltammetry measurements were performed. As shown inFig. 8a, only one CO electro-oxidation (COox) peak at 0.927 V(versus RHE) was observed for the freshly-prepared concavetetrahedral Pd nanocrystals in 0.1 M H2SO4 without introducingany additional CO, which can be assigned to the CO stripping onPd (1 1 0) facets [55]. Then CO electro-oxidation peak disap-peared in the followed second potential scanning, as shown inFig. 8b. These results confirmed not only the presence of {1 1 0}facet but also the preferential adsorption of CO on {1 1 0} faceteven in the products. Subsequently, two peaks were observed inCO stripping curve for the CO-stripped Pd-modified electrodeafter CO dosing, as shown in Fig. 8c. One peak for CO electro-oxidation appeared at 0.925 V (versus RHE) again is attributed toCO stripping on {1 1 0} facets, while another peak at 1.026 V(versus RHE) can be well assigned to the CO stripping on {1 1 1}facets [55]. This shows that CO can be adsorbed onto both {1 1 0}and {1 1 1} facets of concave Pd nanocrystals after CO stripping.This verified that CO was adsorbed only on {1 1 0} facets forfreshly-prepared concave Pd nanocrystals. After CO stripping, COcan be just adsorbed on {1 1 1} besides {1 1 0} facets if dosedwith CO.

Accordingly, the formation of the concave Pd nanocrystals wasgreatly dependent upon a selective adsorption of CO on {1 1 0}facets in the synthesis process. The CO adsorption relates to COflow rate and temperature. With a low CO flow rate, CO wasadsorbed selectively on {1 1 0} facets by an on-top pattern. As aresult, the nucleus crystallite preferred to grow along the exposed{1 1 1} facets and the corners, and a pyramid with three exposed{1 1 0} facets at the center of each {1 1 1} facet of the tetrahedral Pdwas generated. With a fast CO flow rate, the {1 1 0} and {1 1 1}facets were confined and the growth preferred along the edges dueto the adsorption of CO on both {1 1 0} and {1 1 1} facets, and this

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Fig. 8. CO stripping voltammetry of the concave Pd nanocrystals in 0.1 M H2SO4

solution. (a) The freshly-prepared concave tetrahedral Pd nanocrystals without

introducing any additional CO; (b) the second potential scanning; (c) after dosing

CO for 15 min for clean concave tetrahedral Pd nanocrystals.

H. Zhu et al. / Materials Research Bulletin 47 (2012) 3637–36433642

resulted in the formation of nanosheets and apparent hollownanosheets. On the other hand, it was found that the formingprocess of desired concave tetrahedral Pd nanocrystals dependedon a balance between the growth rate of Pd nucleus crystallite andthe adsorbing rate of CO. The adsorbing rate of CO was dependenton not only its flow rate but also the temperature. At a faster flowrate of CO or a higher temperature, the adsorbing rate was fasterthan growth rate, so that nanosheets as well as apparent hollownanosheets were produced. At a slower CO flow rate or lowertemperature, the CO adsorbing rate was slower than nanocrystalgrowth rate, and then tetrahedral Pd particles with small sizes andvague concavities were generated. The most appropriate flow rateof CO was 0.033 mL s�1 for the formation of ideal concavetetrahedral nanocrystals with obvious concave features.

4. Summary

In summary, well-defined concave tetrahedral Pd nanocrystalswith uniform sizes were successfully synthesized with Pd(acac)2 asa precursor, PVP as a stabilizer and CO as a reducing agent underatmospheric pressure. The effects of CO flow rate, temperature andtime on the as-prepared Pd nanostructures were investigated. Theresults showed that the most appropriate flow rate of CO,temperature and time for the formation of the ideal concavetetrahedral Pd nanocrystals was 0.033 mL s�1, 100 8C and 3 h,respectively. Especially, it was found that CO was crucial for theformation of the concave tetrahedral Pd nanostructures. In theabsence of CO, the reaction did not occur. A kinetic control wasrequired for the formation of ideal concave tetrahedral nanos-tructures. The results suggested that the formation mechanism forthe special morphologies was based on equilibrium between thegrowth rate of the nucleus crystallite and the adsorbing rate of CO.Changes in the flow rate of CO, reaction temperature or time wouldlead to a morphological evolution of Pd nanocrystals due to anequilibrium shift. The morphologies of the final products can bewell controlled by adjusting the flow rate of CO under the optimumconditions.

Acknowledgements

This research was supported by theNational Nature ScienceFoundation of China (Grant No. 20673146) and the ScienceFoundation of South-Central University for Nationalities (GrantNo. YZZ05002).

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