J. Phys. Chem. C 2009, 113, 1001–1005
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Porous Palladium Nanoflowers that Have Enhanced Methanol Electro-Oxidation Activity Zhen Yin,† Huajun Zheng,‡ Ding Ma,*,† and Xinhe Bao*,† State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, The Chinese Academy of Sciences, Dalian 116023, People’s Republic of China, and State Key Laboratory Breeding Base of Green Chemistry Synthesis Technology, Zhejiang UniVersity of Technology, Hangzhou 310014, People’s Republic of China ReceiVed: August 20, 2008; ReVised Manuscript ReceiVed: NoVember 4, 2008
A liquid phase approach was used for the synthesis of novel porous Pd nanoflowers by the self-organization of primary nanoparticle building blocks. Palladium acetylacetonate was used as the metal precursor with 1,2-hexadecanediol as the reduction agent. The porous Pd nanostructure obtained has an average size of 50 nm and was composed of three-dimensionally connected Pd nanoparticles with an average diameter of 5.5 nm. The new Pd nanostructure has higher electrocatalytic activity and better stability for the electro-oxidation of methanol in an alkaline media than Pd nanoparticles. Introduction The electro-oxidation of methanol has attracted enormous attention in the past several decades due to the application in the direct methanol fuel cells (DMFCs).1-5 Much work on the electro-oxidation of methanol has been done on Pt-based6-9 and bimetallic catalysts, especially PtRu,10-12 because these are recognized as good anode catalysts. However, the high cost of Pt and its low activity for alcohol electro-oxidation in an alkaline medium are major obstacles to its use in DMFCs. Hence, the development of Pt-free electrocatalysts of methanol oxidation has become the primary work for many different groups.4,13,14 As an alternative to the relatively more expensive Pt, Pd has been used as the effective catalysts for methanol oxidation in recent years.14,15 Significantly, Pd nanostructures, which have larger surface area-to-volume ratio and more active centers, have been widely used in various electrochemical reactions.14,16 However, the control over the dimension and the morphology of these Pd nanostructures has been difficult, especially when using the conventional method of the chemical reduction of Pd salts to prepare Pd nanostructures. Therefore, there have been few reports on the application of Pd nanostructures to the electro-oxidation of methanol in an alkaline media.4 Porous or flower-shaped three-dimensional nanomaterials of metals and metal oxides, which have been successfully fabricated made by several groups recently,17-20 have attracted widespread attention for their advantages of larger surface area and more active centers. For example, Yang et al. reported that Pt or Pt-based nanostructures can be prepared by a nonhardtemplate approach. The porous PtRu nanostructures obtained have shown extraordinary electrochemical properties that were better than those of commercial bimetallic PtRu catalysts.11,21,22 However, little attention has been paid to three-dimensional Pd nanostructures.23 Most research efforts have been limited to the fabrication and application of Pd nanoparticles, and it remains a challenge to directly synthesize three-dimensional Pd nanoflowers via a chemical route and to compare these with Pd nanoparticle catalysts. * To whom correspondence should be addressed. E-mail:
[email protected],
[email protected]. Fax: +86-0411-84694447. Phone: +86-0411-84379256. † The Chinese Academy of Sciences. ‡ Zhejiang University of Technology.
The polyol process is a facile way to synthesize different metal nanoparticles.24,25 Xia et al. discovered that the addition of a trace amount of iron species (FeII or FeIII) to the polyol process and the control of the reaction atmosphere can significantly alter the growth kinetics of Pt nanostructures to give Pt with different morphologies such as branched multipods.26 Herein, we demonstrate, for the first time, that three-dimensional porous Pd nanostructures can be synthesized in large quantity using a modified polyol process with Pd(acac)2 as the palladium precursor. It is believed that the surfactant, oleylamine, has acted as the capping agent in addition to being the directing agent for the oriented assembly of primary Pd nanoparticles and thus to get the three-dimensionally connected porous Pd nanostructures. Cyclic voltammetry and chronoamperometry measurements were employed to investigate the electrochemical activity and stability of the obtained nanostructure for the methanol oxidation under alkaline conditions. The results indicated that the novel Pd nanostructures had much higher activity and better stability for methanol electro-oxidation in an alkaline media than the Pd nanoparticles. Experimental Section Synthesis. We performed the synthesis using a minor modification of the polyol process in an organic solvent and at high temperature.27 In a typical run, a solution of 0.5 mmol of Pd(acac)2 (Alfa Aesar, 99.4%) and 1 mL of oleylamine (Acros, 97%) in 6 mL of o-dichlorobenzene (DCB, Acros, 99%) was injected into a boiling mixture (at a temperature of ∼180 °C) of 0.3 g of 1,2-hexadecanediol (HDD, Aldrich, 90%) and 10 mL of DCB under vigorous magnetic stirring. The color of solution began to change quickly from the original orange to dark brown after approx 10 min reaction, indicating the formation of Pd nanoparticles. To monitor the process of nanostructure formation, a set of samples were withdrawn from the reaction mixture at different time intervals using glass pipettes. To minimize temperature fluctuation during sampling, the glass pipettes were held just above the solution surface and preheated for 30 s before immersion. To get samples for XRD and other studies, the reaction mixture was maintained at the reflux temperature for a specific time and then cooled to room temperature. The samples were washed with ethanol to remove most of the DCB and oley-
10.1021/jp807456j CCC: $40.75 2009 American Chemical Society Published on Web 12/30/2008
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Figure 1. TEM images of Pd nanoflowers at different magnification (A) and (B); (C) HR-TEM image of Pd nanoflowers; (D) ADF-STEM image of Pd nanoflowers.
lamine. Then, the mixture was centrifuged at 10000 rpm for 10 min, and the precipitate was separated from the supernatant and redispersed in ethanol for additional washing. The process was repeated three or four times to get the Pd nanoflowers. At the same time, Pd nanoparticles with a narrow size distribution to use as an activity reference for the Pd nanoflowers were also prepared by a facile method different from the reported synthesis route.28-31 Details of the synthesis procedure, transmission electron microscopy (TEM) images, and histogram of the size distribution (see Figure S1 of Supporting information) are given. Characterization. Electron microscope specimens were prepared by dispersing the suspension of nanoflowers in hexane and drop-casting it onto TEM copper grids. The TEM images were recorded on a FEI Tecnai G2 Spirit microscope equipped with a Gatan CCD camera at an accelerating voltage of 120 kV. High-resolution TEM (HR-TEM) and scanning transmission electron microscope (STEM) images were recorded on a FEI Tecnai G2 microscope at an accelerating voltage of 300 kV equipped with an energy dispersive X-ray (EDX) system from EDAX Inc. with a point resolution of 0.20 nm. Powder X-ray diffraction (PXRD) patterns were taken using a Rigaku D/max2500 diffractometer with a Cu KR X-ray source (λ ) 1.5405 Å) operated at 40 kV with a scan rate of 0.083° 2θ/s. The particle size distribution was obtained by analyzing about 100 particles in the TEM images using the Image software from Gatan. Electrocatalytic Study. The electrochemical catalytic activity of the Pd nanoflowers was characterized using a three-electrode system on a Solartron 1255B potentialstat. The Pd nanoflowers used in this study were washed and concentrated and then homogeneously mixed with polytetrafluoroethylene (PTFE) and conductive carbon black in a mass ratio of 5:5:90 in absolute ethanol and spread onto a 10 mm × 10 mm sized nickel foam as the working electrode (WE). The mass of nanoparticles on each WE was 0.5 mg. Cyclic voltammograms (CVs) were carried out at room temperature in 0.25 M methanol + 0.5 M KOH solution, which was degassed with N2 prior to the
experiments. Pt black sheet and a standard calomel electrode (SCE) served as the counter and reference electrodes, respectively. All the electrode potentials in this paper are quoted versus the reversible hydrogen electrode (RHE). Results and Discussion Figure 1 shows the TEM image of Pd nanoflowers sampled after the reaction for 60 min reaction at the reflux temperature. It can be seen that the products were relatively uniform flowershaped nanostructures with diameter of around 50 nm. The magnified TEM image shown in Figure 1B revealed that each of those nanoflowers was actually a three-dimensionally interconnected porous network with Pd nanoparticles as primary building blocks. The primary Pd nanoparticles were quite uniform in size (with an average size of 5.5 nm, see Figure S2 of Supporting Information) and were face-centered cubic structured as characterized by the corresponding XRD profiles (see Figure S3 of Supporting Information). The full-width at half-maximum of the (111) diffraction was used to estimate the average size of the Pd nanoparticles by the Scherrer equation. The calculated sizes of the nanoparticles agreed well with the TEM observation. The HRTEM images showed that the primary Pd nanoparticles were single crystalline (Figure 1 and Figure S4 of Supporting Information), with lattice spacing of 2.25 Å, which could be assigned to the (111) planes of Pd. STEM analyses of the Pd nanoflowers were also carried out. The annular dark-field (ADF) images in Figure 1D and energydispersive X-ray (EDX) maps in Figure S5 of Supporting Information confirmed that these nanostructures were threedimensionally connected Pd nanocrystallites. To understand how the nanostructures formed, samples taken from the synthesis mixture at different time intervals were observed by TEM. Figure 2 shows the morphology evolution of the Pd nanostructures. At t ) 11 min (Figure 2A and Figure S6 of Supporting Information), the sample mainly contained randomly dispersed Pd nanoparticles with an average size of 4.4 nm. As the reaction proceeded to t ) 20 min (Figure 2B and Figure S6 of Supporting Information), all the nanoparticles
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Figure 2. TEM images of the products at different stages of formation: (A) t ) 11 min; (B) t ) 20 min; (C) t ) 30 min; (D) t ) 45 min.
Figure 3. Effect of reaction time on the average size of primary Pd nanoparticles. The size distribution was shown in Figure S6 of Supporting Information.
Figure 4. Cyclic voltammetric curves comparing the Pd nanoflowers with Pd nanoparticles in 0.25 M MeOH + 0.5 M KOH solution taken at a scan rate of 1 mV/s.
continued to grow, and the average particle size was 5.0 nm (Figure S6 of Supporting Information). Meanwhile, as indicated in Figure 2B, some nanoparticles began to aggregate under the directing role of the organic reagent-oleylamine. In the following 30 to 45 min, there was no significant change in both the size and shape of the primary Pd nanoparticles. Parts C and D of Figure 2 and Figure S6 of Supporting Information show typical TEM images and size distributions
of the samples obtained at t ) 30 min and t ) 45 min. It can be seen from these pictures that with longer reaction time the oleylamine-directed aggregation of the primary building blocks proceeded while the increase of the size of the primary nanoparticles began to slow down. After 60 min reaction, the size of the primary nanoparticles became constant at 5.5 nm. Interestingly, compared with that of the sample after 20 min reaction, the size distribution of the primary Pd nanoparticles became broader as the reaction proceeded. Meanwhile, after 20 min reaction, the flowerlike morphology gradually developed, and there was a well-defined flowerlike morphology after 45 min reaction. The formation of the Pd nanoflowers can be described as follows: (1) In the first 10 min, the color of the synthesis mixture remained unchanged. At this stage, palladium ions were slowly reduced by 1,2hexadecanediol to give free Pd clusters in the solution. (2) With one min further reaction, the color of the mixture changed from orange to dark brown, indicating the concentration of Pd clusters has reached the critical point and a homogeneous nucleation occurred under the influence of very fast autocatalytic Pd ion reduction to give the primary Pd nanoparticles. (3) With the depletion of Pd ions, the reaction was dominated by thermodynamic growth, namely, the Ostwald ripening process, which led to the dissolution of small Pd nanoparticles in favor of the growth of large ones. The size dependence of the primary Pd nanoparticles on reaction time in Figure 3 was the evidence for this. The formation of the porous Pd nanoparticle network is believed to be controlled by the very fast autocatalytic reduction of Pd ions, which resulted in a very high concentration of Pd ions within a relatively small domain. With the directing role of oleylamine, three-dimensionally connected Pd nanoflowers were obtained with a longer reaction time. The electrochemical catalytic activity of Pd nanoflowers for the oxidation of methanol was studied using cyclic voltammetry
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Figure 5. (a) CV curves of Pd nanoflowers for the oxidation of methanol at different scan rates in 0.25 M MeOH + 0.5 M KOH solution. (b) Linear relationship of oxidation peak current versus peak potential at different scan rates.
Figure 6. Chronoamperometric curves for the oxidation of methanol catalyzed by the porous Pd nanoflowers and Pd nanoparticles at 0.2V in 0.25 M MeOH + 0.5 M KOH solution.
(CV). The CV curves were measured in an aqueous solution of KOH and methanol between -0.4 and 0.8 V. Methanol oxidation peaks were clearly observed for the Pd nanoflowers catalysts in the CV curves, which were at 0.19 V (vs RHE) in the forward sweep and at -0.061 V in the backward sweep (see Figure 4). The peak mass current density of Pd nanoflowers was 26 mA mg-1 at the peak position of 0.19 V, which indicated that the Pd nanoflowers had excellent electrocatalytic activity for methanol oxidation. During the potential scan in the negative direction, there was an oxidation current, which may be caused by the oxidation of intermediates of methanol dissociative adsorption, at -0.061 V in the backward sweep. This is typical behavior of methanol oxidation on Pt or Pt group catalysts, which showed a double oxidation peak both in the forward sweep and backward sweep.6,11 To provide an activity reference for the porous Pd nanoflowers, we synthesized monodisperse Pd nanoparticles. Figure 4 also shows the difference in the electro-oxidation of methanol between Pd nanoflowers and Pd nanoparticles. The porous nanoflowers were about 60% more active than Pd nanoparticles based on per unit mass of Pd, although the nanoparticles showed a more negative oxidation peak potential at 0.073 V (vs RHE). Also, there were two oxidation peaks (0.32 and 0.38V) in the CV curves for Pd nanoparticles. This implied that the electrochemical oxidation of methanol by Pd nanoparticles went through multiple intermediate steps, which included the adsorption of CO, etc. In other words, the process with the Pd nanoparticles would be difficult to control in direct methanol fuel cells in contrast to the case with the porous Pd nanoflowers. Moreover, for the porous Pd nanoflowers, the oxidation peak current at different scan rates (Figure 5a) indicated that the peak current had a linear relationship with the oxidation peak potential, which showed that the electrocatalytic process was only controlled by the concentration polarization for the methanol oxidation (Figure 5b). Hence, the flowerlike threedimensional Pd nanostructures can be very useful in direct methanol fuel cells.
It is probable that the three-dimensional nanostructure with voids between the nanoparticles was critical for the observed outstanding catalytic performance. Moreover, the shape and defects of the nanoparticles is likely dependent on the relative growth rates along different crystal planes. These growth rates could be quite different for colloidal nanoparticles in the presence of different surfactants. Hence, the growth processes of the primary particles may be different for the two different Pd nanostructures under the different surfactants: the oleylamine for the Pd nanoflowers and the oleylamine and oleic acid for the Pd nanoparticles. Therefore, Pd nanoflowers with threedimensionally connected structure may have more defects which accounts for the better activity observed. The long-term stabilities of Pd nanoflowers and Pd nanoparticles for methanol oxidation were investigated with chronoamperometric curves in 0.25 M methanol + 0.5 M KOH. The results are shown in Figure 6. The current decayed rapidly with the Pd nanoparticles, but the current decayed slowly with the Pd nanoflowers. Chronoamperometric characterization further verified that the mass current density with the porous Pd nanoflowers were at least 60% higher than that with the Pd nanoparticles even though the oxidation reaction reached the steady state. These results showed that Pd nanoflowers had better steady state electrolysis activity than Pd nanoparticles for methanol oxidation in an alkaline media. Conclusion A novel three-dimensionally connected Pd flowerlike nanostructure was synthesized by a modified polyol process without using a template. A very fast autocatalytic reduction of Pd ions in the second step of the reaction played an important role in the formation of the nanostructure. By the directing effect of oleylamine, three-dimensionally connected Pd nanoflowers were obtained. CV studies and chronoamperometric characterization showed that the porous Pd nanoflowers were more active and more stable than Pd nanoparticles for the electro-oxidation of methanol under alkaline condition. The porous Pd nanoflowers should be a good electrocatalyst for a Pt-free DMFC. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20773121). D.M. thanks Chinese Academy of Science for financial support through Bairen project. Supporting Information Available: Detailed description on how to synthesize the Pd nanoparticles and more TEM and size distribution data of the Pd nanoparticles, XRD characterization, HRTEM image, and EDX map of the Pd nanoflowers and size distribution at different reaction time. This material is available free of charge via the Internet at http://pubs.acs.org.
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