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Preparation and characterization of nanostructured Pd with high electrocatalytic activity
Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 75–81
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Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Preparation and characterization of nanostructured Pd with high electrocatalytic activity Peizhi Guo a,b,∗ , Zhongbin Wei a,b , Wanneng Ye a , Wei Qin a , Qinchao Wang a,b , Xianfen Guo a , Chaojing Lu a,∗∗ , X.S. Zhao a,b,c a
Laboratory of New Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, PR China School of Chemistry, Chemical Engineering and Environment, Qingdao University, Qingdao 266071, PR China c School of Chemical Engineering, The University of Queensland, St Lucia, QLD 4072, Australia b
a r t i c l e
i n f o
Article history: Received 19 October 2011 Received in revised form 6 December 2011 Accepted 7 December 2011 Available online 16 December 2011 Keywords: Pd Hydrothermal synthesis Formic acid Ethanol Electrocatalysis
a b s t r a c t Two kinds of Pd nanoparticle aggregates with different electrocatalytic activity are developed in a facile hydrothermal synthesis method. The average size of Pd nanoparticles synthesized from the mixed solvent containing water and acetone (Pd–M) is ∼9 nm smaller than that of Pd samples synthesized from aqueous systems (Pd–A) with the size of ∼14 nm. Nanoparticles of Pd–M and Pd–A show the crystalline nature based on the XRD and HRTEM results. It is found that Pd–M display a remarkably high electrooxidation activity per unit mass toward formic acid and ethanol, which is 8–12 times as high as that of Pd–A. The electrooxidation activity per unit area of Pd–M is also higher than that of Pd–A. The formation mechanisms of Pd–A and Pd–M as well as the relationship between their microstructures and electrocatalytic activity have been discussed based on the experimental results. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fuel cells are considered as green and efficient power generation assembly due to their high efficiency and little or no pollution [1–4]. Among various types of fuel cells, the direct fuel cells which take small organic molecules as fuels have attracted much attention as power sources for portable electronic devices and fuel-cell vehicles owing to its high energy density and low operating temperature [5]. Challenging issues such as low catalytic activity of electrodes both for the oxygen reduction reaction and for small organic molecules oxidation reaction, high costs of the Pt-based electrocatalysis and susceptibility of the catalysts to be poisoned by the CO-like intermediates formed in the oxidation reaction are the main obstacles to the commercialization of fuel cells [6]. Fundamental research efforts are currently directed towards the development of anode electrocatalysis with high catalytic activity [7].
∗ Corresponding author at: School of Chemistry, Chemical Engineering and Environment, Qingdao University, Qingdao 266071, PR China. Tel.: +86 532 837 80378; fax: +86 532 859 55529. ∗∗ Corresponding author. Tel.: +86 532 837 80378; fax: +86 532 859 55529. E-mail addresses: [email protected], [email protected] (P. Guo), [email protected] (C. Lu). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.12.008
As a relatively abundant resource, palladium has been intensively studied as one of the important candidates for the Pt-alternative electrocatalysis due to its desirable activity toward the catalysis of small organic molecules, such as alcohol, formic acid and methanol [8–11]. Catalytic and electrocatalytic activities strongly depend on the size and shape of the metal nanoparticles [12–18]. Much effort has been devoted to synthesizing various palladium nanoparticles with specific shapes in aqueous or nonhydrolytic media [19–27]. For example, concave tetrahedral/trigonal bipyramidal palladium nanocrystals are synthesized through a hydrothermal method using formaldehyde solution mixed together with benzyl alcohol as solvent [23]. Palladium icosahedra can be prepared controllably in ethylene glycol systems through control over experimental parameters [24]. Self-standing Pa nanowires are obtained via galvanic displacement deposition using anodic alumina membranes as the template [25]. Electrochemical method has been successfully used to synthesize Pd nanorods [26] and tetrahexahedral Pd nanocrystals [27] with highindex facets, which show high electrocatalytic activity for ethanol electrooxidation. However, it still is a great challenge to synthesize Pd nanocrystals with high electrocatalytic activity and stability for electrooxidation of small molecules. In this paper, two different palladium nanocrystals are prepared controllably using a simple hydrothermal method through control over the addition of acetone into the synthesis systems.
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Pd nanocrystals synthesized from aqueous systems (Pd–A) are ∼14 nm while Pd nanocrystals prepared from the system containing acetone (Pd–M) are ∼9 nm. Both of Pd nanocrystals show electrocatalytic activity toward alcohol and formic acid, however, the electrocatalytic activity of Pd–M is 8–12 times as high as that of Pd–A. The formation mechanisms of Pd nanocrystals and the structure–property relationship have been discussed based on the experimental results.
2. Experiments 2.1. Materials Palladium(II) chloride, hydrazine hydrate, sulfuric acid, potassium hydroxide, formic acid, ethanol and acetone were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyvinyl pyrrolidone (PVP) (molar weight = 58 000) was purchased from Acros. All chemicals were analytical grade and used without further purification. Double distilled water was used in the experiments.
Fig. 1. XRD patterns of samples Pd–A (a) and Pd–M (b).
3. Results and discussion 2.2. Synthesis of Pd nanocrystals In a typical synthesis, aqueous PdCl2 solution (3 mM, 20 mL) was dropped into aqueous PVP solution (0.2 mM, 10 mL) under stirring at room temperature and then 30 mg hydrazine hydrate was added into the mixture slowly. The mixture was then transferred to a 40 mL teflon-lined autoclave. Hydrothermal synthesis was carried out in an oven at 150 ◦ C for 8 h. The collected products were washed thoroughly with distilled water and ethanol, and finally dried in an oven at 60 ◦ C for 6 h. The sample obtained in this condition was referred as Pd–A. Sample Pd–M was made in the same condition as that of Pd–A, which used the mixture of water and acetone with the volume ratio of 1:1 as the solvent.
3.1. Crystal structure Fig. 1 shows the XRD patterns of samples Pd–A and Pd–M. Three strong diffraction peaks at 2 degrees of 40.3, 46.7 and 68.3 are observed for these two samples, which can be well ascribed to the (1 1 1), (2 0 0) and (2 2 0) peaks of pure Pd (PDF No. 65-6174) with cubic phase, respectively. The broadening of the diffraction peaks indicates the formation of small Pd nanocrystals. The average sizes of samples Pd–A and Pd–M calculated from XRD pattern according to the Scherrer equation [24] are 14.1 nm and 9.0 nm from the (1 1 1) peak, respectively.
3.2. Morphology and TEM analysis 2.3. Characterization X-ray diffraction experiments (XRD) were performed on a Bruker D8 Advance X-ray diffractometer equipped with graphite monochromatized Cu K␣ radiation ( = 0.15418 nm) from 10◦ to 80◦ (2) using a solid detector. Scanning electron microscopy (SEM) images were taken with a JSM-6390LV scanning electron microscope operated at 20 kV. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were obtained with a JEM-2000EX and JEOL 2100-FEG transmission electron microscope, respectively. All the electrochemical measurements were conducted on a CHI760c working station in a three-electrode cell at room temperature. A saturated calomel electrode (SCE) for acidic solutions, or an Hg/HgO electrode for alkaline solutions, was used as the reference electrode, and a platinum foil was used as the counter electrode. The working electrode was prepared using a glassy carbon (GC) disk as the substrate. During the electrochemical measurements, a mixture containing 1.0 mg of electrocatalyst and 1 mL of ethanol was pretreated for 1 h under ultrasonication to obtain a well-dispersed ink. A 15 L portion of the catalyst ink was then transferred onto the surface of the GC electrode and dried under in the air to obtain a catalyst thin film. Cyclic voltammetry (CV) of formic acid was performed in aqueous solutions of 0.5 M H2 SO4 containing 0.5 M HCOOH in certain potential windows. For cyclic voltammetry of ethanol oxidation, the electrolytic solution was 1 M CH3 CH2 OH in 1 M KOH.
Fig. 2 shows the TEM and HRTEM images of samples Pd–A and Pd–M. As depicted in the low-magnification TEM image (Fig. 2a), sample Pd–A shows conglutinated structures composed of Pd nanocrystals with the size scales less than 20 nm. Obvious diffraction circles can be observed from the selected-area electron diffraction (SAED) pattern of sample Pd–A in the inset of Fig. 2a, in which some diffraction spots also appear. This should be attributed to the structural nature of Pd–A, in which Pd nanocrystals are not inclined to form large well-crystalline aggregates. These results are further confirmed by the corresponding HRTEM results (Fig. 2b), in which the edges of nanocrystal can be observed obviously and poor crystalline parts are appeared probably. Possible edge and defect domains are also marked with black and white arrows, respectively, as shown in Fig. 2b. The lattice spacings of ∼0.235 and 0.208 nm are observed corresponding to (2 0 0) and (1 1 1) lattice planes of cubic phase palladium, respectively, indicating the crystalline nature of a single nanoparticle. As can be seen from Fig. 2c, the morphology of Pd–M is somewhat different with that of Pd–A. It can be seen that quasi-one dimensional string aggregates of Pd nanocrystals with the size of about 8–10 nm are formed. Weak diffraction cycles are observed from the SAED pattern of Pd–M (in the inset of Fig. 2c) compared with those of Pd–M, which is in accord with the XRD observations (Fig. 1). As shown in the HREM image of Pd–M (Fig. 1d), irregular nanocrystals are formed. The natural spacings of lattice planes are 0.089 and 0.087 nm ascribed to the (3 3 1) and (4 2 0) planes of cubic phase palladium, respectively, which exhibit an open structure exposed to electrolyte [26,28].
P. Guo et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 395 (2012) 75–81
Fig. 2. TEM (a,c) and HRTEM (b,d) images of Pd–A (a,b) and Pd–M (c,d). The insets in a and c are the SAED patterns of the corresponding samples.
Fig. 3. TEM images of intermediary products obtained from aqueous (a–c) and mixed (e–f) synthesis systems after the reaction time of 0.5 h (a, d), 1 h (b, e) and 4 h (c, f).
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Fig. 4. CV curves of Pd–A (a) and Pd–M (b) modified GC electrodes in 1 M H2 SO4 solutions at the scan rate of 50 mV s−1 .
3.3. Evolution of the intermediates To study the formation mechanism of Pd nanostructures, intermediary products are collected and characterized. Fig. 3 shows TEM images of the intermediates obtained after different reaction times. It can be seen that the shape and sizes of the products change gradually with the increase of synthesis time. For aqueous synthesis system, the product collected at 0.5 h show irregular granular nanostructures with the size scales of less than 10 nm (Fig. 3a) while smaller nanoparticles can be observed for the intermediate of another synthesis system (Fig. 3d) obtained at the same period. If the reaction time is elongated to 1 h, the sizes of the products (Fig. 3b and e) become somewhat larger which are changed slightly compared with those of the former mesophases. When the synthesis time for aqueous systems is further extended to 4 h, small nanoparticles are gradually grown with the size increased to 10–15 nm (Fig. 3c). Furthermore, boundaries among particles gradually disappear, and connect into aggregate-like structures. Aggregate-like nanostructures can also be observed for the mixed systems with the synthesis time of 4 h (Fig. 3f). It is proposed that the solubility of PVP in water and acetone should play the key role in the formation of Pd nanocrystals. As water is a good solvent of PVP, Pd nanocrystals grow gradually in the presence of PVP with the increase of synthesis time due to the Ostwald ripening in aqueous synthesis systems. With the addition of acetone, a bad solvent of PVP, into the synthesis system, the conformation of PVP in the solution may be altered and block the continuous grown of Pd nanocrystals [24]. It is suggested that the specific adsorption of functional groups of PVP on the crystalline facets of Pd promotes the formation of Pd–A and Pd–M under different synthesis environments [29].
features of Pd–M and Pd–A nanocrystals should play an important role in determining the electrochemical responses of these modified electrodes. The real surface area of Pd–A and Pd–M modified electrodes are evaluated to be 0.010 and 0.041 cm2 , respectively, from the formula Q(C)/424 cm2 in which Q stands for the corresponding electrical quantity of inflection point according to the relationship between the maximum potential and the electrical quantity of reduction peak (Fig. 5) [7]. This method is a standard electrochemical method frequently used for determining the real surface area of palladium. If regard the particles of the samples as spherical structures approximately, the specific surface area of Pd–M is about three times as large as that of Pd–A. For comparison of the electrocatalytic performance of Pd–A and Pd–M, the measured current is normalized either with the surface area or with the mass to obtain the current density. 3.5. Electrocatalytic performance for formic acid To study the electrochemical activity of nanostructured Pd samples, Pd-modified GC electrodes are used as working electrode for electrocatalytic oxidation of formic acid and ethanol. It is found that Pd nanocrystals, especially for Pd–M nanocrystals show a high electrocatalytic activity no matter to ethanol or to formic acid [9,19,25]. As can be seen from Fig. 6A, Pd–M modified electrode displays much higher electrocatalytic activity toward formic acid than that of Pd–A modified electrode. The current densities at peak potentials of 0.14 and 0.13 V normalization to mass of Pd–M modified electrodes are
3.4. Cyclic voltammogram characterization Fig. 4 shows the cyclic voltammograms (CVs) for the Pdmodified electrodes in 1 M H2 SO4 solutions. The CV curves of the modified electrodes are rather different from the naked GC electrode, in which several new features can be observed compared with that of the naked electrode. Firstly, the sharp change of current at −0.18 V is attributed to the generation of H2 from the electrolysis of H2 O. Secondly, a couple of weak peaks between −0.05 and −0.1 V (vs. SCE) are attributed to the adsorption/desorption of hydrogen in the surface of Pd samples [30,31] Thirdly, the peak at 0.37 V (vs. SCE) is the reduction peak of produced Pd oxide [32]. It can also be observed from Fig. 4 that the current of the Pd–M modified electrode is much larger than that of the Pd–A modified electrode. Combined with the results of XRD and HRTEM, it is proposed that the electrochemical-active surface area and structural
Fig. 5. The relationship curve between the maximum potential and the electrical quantity of reduction peak under different voltage window ranges: Pd–A (a) and Pd–M (b).
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Fig. 6. CV (A and B) and (C) chronoamperometric curves of Pd–A (a) and Pd–M (b) modified GC electrodes after normalization in 0.5 M HCOOH + 0.5 M H2 SO4 solutions at room temperature.
270 and 210 mA mg−1 in an aqueous electrolytic solution (0.5 M HCOOH + 0.5 M H2 SO4 ) for the positive and negative scan, respectively, which are about 9 times as high as those of Pd–A modified electrodes. It can also be observed from Fig. 6A and B that the currents of positive peaks are stronger than those of negative ones. This may be caused by adsorption of intermediate product which produces in the HCOOH oxidation process and covers partial active points of Pd film on GC electrode [5,28]. In order to further disclose the electrocatalytic activity of Pd nanocrystals, the CV curves with the current density normalized to the active area are shown in Fig. 6B. Two features can be concluded. Firstly, the electrocatalytic activity of Pd–M is still about 3 times as high as that of Pd–A. Secondly, both positive and negative oxidized peaks for Pd–M in the CV curves are around 0.184 V at the scan rate of 50 mV s−1 . However, the oxidized peaks for Pd–A appear at about 0.26 V at the same scan rate in which the positive peak is also stronger than the negative one. Furthermore, the
oxidation current density of Pd–M is nearly tenfold of that of Pd–A in the experimental period in the transient current density curves of formic acid oxidation at 0.25 V (Fig. 6C). These results confirm better electrocatalytic performance of Pd–M compared with that of Pd–A, which may be ascribed to the higher electrochemical active area and smaller size of Pd–M than that of Pd–A [22,27]. 3.6. Electrocatalytic performance for ethanol The electrooxidation activity of Pd–A and Pd–M toward ethanol has also been investigated (Fig. 7). Both of Pd samples show higher electrocatalytic activity than that of the commercial Pd black catalyst reported obviously [26]. It can also be seen from Fig. 7A that the electrocatalytic currents per mass of Pd–M modified electrodes respectively are 400 and 380 mA mg−1 in 1 M CH3 CH2 OH + 1 M KOH solutions for the positive and negative scan, which are about 12 and 8 times as high as those of Pd–A modified electrodes,
Fig. 7. CV (A and C) and chronoamperometric (B and D) curves of Pd–A (a) and Pd–M (b) modified GC electrodes after normalization in 1 M CH3 CH2 OH + 1 M KOH solutions.
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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 20803037 and 21143006), the Natural Science Foundation of Shandong Province (No. ZR2009BM013), the Foundation of Qingdao Municipal Science and Technology Commission (11-2-4-2-(8)-jch) and the foundation of “Taishan Scholar” program of Shandong Province, China.
References
Fig. 8. Variation of current densities with cyclic number associated with the electrocatalytic oxidation in 1 M CH3 CH2 OH + 1 M KOH solutions at the scan rate of 50 mV s−1 : Pd–A (a) and Pd–M (b).
respectively. Transient current densities of ethanol oxidation at 0.3 V of these two samples are shown in Fig. 7B, which indicate that Pd–M has much higher catalytic activity toward ethanol electrooxidation that of Pd–A. Similarly, two features can also be concluded from Fig. 7C and D. Firstly, the current density normalized to the surface area of Pd–M is about 2 or 3 times as high as those of Pd–A for positive or negative scan, respectively. These values are slightly smaller than those of electrooxidation of formic acid, which may be attributed to the different sizes of ethanol and formic acid molecules [28]. Second, the potentials for positive and negative oxidized peaks of Pd–M in the CV curves are red-shifted about 37 mV and 52 mV, respectively, at the scan rate of 50 mV s−1 , compared with those of Pd–A, also indicating better electrocatalytic performance of Pd–M. Chronoamperometric curves shown in Fig. 7D show that Pd–M has better stability even in a short period (about 1000 s) than that of Pd–A. Fig. 8 shows the relationship curves between the current density and cycle number of Pd-modified electrodes toward ethanol electrocatalytic oxidation. It can be seen that Pd–M can reach the highest current density of electrooxidation and maintain a steady state in a short time (about 50th cycle) while Pd–A needs somewhat longer time to reach steady state (after 100th). This is consistent with the short period chronoamperometric curves (Fig. 7D). As depicted in Fig. 8, it is also clear that even though Pd–A has better stability in long term (600 cycle), the current density of Pd–M at 600th still is 10.6 times as high as that of Pd–A. These results should be contributed to the dual positively effect of real surface area and small size effect of Pd–M compared with those of Pd–A.
4. Conclusion Two palladium nanocrystal aggregates are synthesized using a hydrothermal method through controlling the addition of acetone into the synthesis systems. Pd nanocrystals synthesized from aqueous system (Pd–A) are 14 nm while Pd nanocrystals prepared from the system containing acetone (Pd–M) are 9 nm. Both of Pd samples show high electrocatalytic activity toward formic acid and ethanol. Especially, the electrocatalytic performance of Pd–M is much higher than that of Pd–A, which should be ascribed to both the large electroactive area and small size effect of Pd–M. The formation mechanisms of Pd–A and Pd–M as well as the relationship between their microstructures and electrocatalytic activity have been discussed based on the experimental results.
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Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
Preparation and characterization of nanostructured Pd with high electrocatalytic activity Peizhi Guo a,b,∗ , Zhongbin Wei a,b , Wanneng Ye a , Wei Qin a , Qinchao Wang a,b , Xianfen Guo a , Chaojing Lu a,∗∗ , X.S. Zhao a,b,c a
Laboratory of New Fiber Materials and Modern Textile, The Growing Base for State Key Laboratory, Qingdao University, Qingdao 266071, PR China School of Chemistry, Chemical Engineering and Environment, Qingdao University, Qingdao 266071, PR China c School of Chemical Engineering, The University of Queensland, St Lucia, QLD 4072, Australia b
a r t i c l e
i n f o
Article history: Received 19 October 2011 Received in revised form 6 December 2011 Accepted 7 December 2011 Available online 16 December 2011 Keywords: Pd Hydrothermal synthesis Formic acid Ethanol Electrocatalysis
a b s t r a c t Two kinds of Pd nanoparticle aggregates with different electrocatalytic activity are developed in a facile hydrothermal synthesis method. The average size of Pd nanoparticles synthesized from the mixed solvent containing water and acetone (Pd–M) is ∼9 nm smaller than that of Pd samples synthesized from aqueous systems (Pd–A) with the size of ∼14 nm. Nanoparticles of Pd–M and Pd–A show the crystalline nature based on the XRD and HRTEM results. It is found that Pd–M display a remarkably high electrooxidation activity per unit mass toward formic acid and ethanol, which is 8–12 times as high as that of Pd–A. The electrooxidation activity per unit area of Pd–M is also higher than that of Pd–A. The formation mechanisms of Pd–A and Pd–M as well as the relationship between their microstructures and electrocatalytic activity have been discussed based on the experimental results. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Fuel cells are considered as green and efficient power generation assembly due to their high efficiency and little or no pollution [1–4]. Among various types of fuel cells, the direct fuel cells which take small organic molecules as fuels have attracted much attention as power sources for portable electronic devices and fuel-cell vehicles owing to its high energy density and low operating temperature [5]. Challenging issues such as low catalytic activity of electrodes both for the oxygen reduction reaction and for small organic molecules oxidation reaction, high costs of the Pt-based electrocatalysis and susceptibility of the catalysts to be poisoned by the CO-like intermediates formed in the oxidation reaction are the main obstacles to the commercialization of fuel cells [6]. Fundamental research efforts are currently directed towards the development of anode electrocatalysis with high catalytic activity [7].
∗ Corresponding author at: School of Chemistry, Chemical Engineering and Environment, Qingdao University, Qingdao 266071, PR China. Tel.: +86 532 837 80378; fax: +86 532 859 55529. ∗∗ Corresponding author. Tel.: +86 532 837 80378; fax: +86 532 859 55529. E-mail addresses: [email protected], [email protected] (P. Guo), [email protected] (C. Lu). 0927-7757/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2011.12.008
As a relatively abundant resource, palladium has been intensively studied as one of the important candidates for the Pt-alternative electrocatalysis due to its desirable activity toward the catalysis of small organic molecules, such as alcohol, formic acid and methanol [8–11]. Catalytic and electrocatalytic activities strongly depend on the size and shape of the metal nanoparticles [12–18]. Much effort has been devoted to synthesizing various palladium nanoparticles with specific shapes in aqueous or nonhydrolytic media [19–27]. For example, concave tetrahedral/trigonal bipyramidal palladium nanocrystals are synthesized through a hydrothermal method using formaldehyde solution mixed together with benzyl alcohol as solvent [23]. Palladium icosahedra can be prepared controllably in ethylene glycol systems through control over experimental parameters [24]. Self-standing Pa nanowires are obtained via galvanic displacement deposition using anodic alumina membranes as the template [25]. Electrochemical method has been successfully used to synthesize Pd nanorods [26] and tetrahexahedral Pd nanocrystals [27] with highindex facets, which show high electrocatalytic activity for ethanol electrooxidation. However, it still is a great challenge to synthesize Pd nanocrystals with high electrocatalytic activity and stability for electrooxidation of small molecules. In this paper, two different palladium nanocrystals are prepared controllably using a simple hydrothermal method through control over the addition of acetone into the synthesis systems.
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Pd nanocrystals synthesized from aqueous systems (Pd–A) are ∼14 nm while Pd nanocrystals prepared from the system containing acetone (Pd–M) are ∼9 nm. Both of Pd nanocrystals show electrocatalytic activity toward alcohol and formic acid, however, the electrocatalytic activity of Pd–M is 8–12 times as high as that of Pd–A. The formation mechanisms of Pd nanocrystals and the structure–property relationship have been discussed based on the experimental results.
2. Experiments 2.1. Materials Palladium(II) chloride, hydrazine hydrate, sulfuric acid, potassium hydroxide, formic acid, ethanol and acetone were purchased from Sinopharm Chemical Reagent Co., Ltd. Polyvinyl pyrrolidone (PVP) (molar weight = 58 000) was purchased from Acros. All chemicals were analytical grade and used without further purification. Double distilled water was used in the experiments.
Fig. 1. XRD patterns of samples Pd–A (a) and Pd–M (b).
3. Results and discussion 2.2. Synthesis of Pd nanocrystals In a typical synthesis, aqueous PdCl2 solution (3 mM, 20 mL) was dropped into aqueous PVP solution (0.2 mM, 10 mL) under stirring at room temperature and then 30 mg hydrazine hydrate was added into the mixture slowly. The mixture was then transferred to a 40 mL teflon-lined autoclave. Hydrothermal synthesis was carried out in an oven at 150 ◦ C for 8 h. The collected products were washed thoroughly with distilled water and ethanol, and finally dried in an oven at 60 ◦ C for 6 h. The sample obtained in this condition was referred as Pd–A. Sample Pd–M was made in the same condition as that of Pd–A, which used the mixture of water and acetone with the volume ratio of 1:1 as the solvent.
3.1. Crystal structure Fig. 1 shows the XRD patterns of samples Pd–A and Pd–M. Three strong diffraction peaks at 2 degrees of 40.3, 46.7 and 68.3 are observed for these two samples, which can be well ascribed to the (1 1 1), (2 0 0) and (2 2 0) peaks of pure Pd (PDF No. 65-6174) with cubic phase, respectively. The broadening of the diffraction peaks indicates the formation of small Pd nanocrystals. The average sizes of samples Pd–A and Pd–M calculated from XRD pattern according to the Scherrer equation [24] are 14.1 nm and 9.0 nm from the (1 1 1) peak, respectively.
3.2. Morphology and TEM analysis 2.3. Characterization X-ray diffraction experiments (XRD) were performed on a Bruker D8 Advance X-ray diffractometer equipped with graphite monochromatized Cu K␣ radiation ( = 0.15418 nm) from 10◦ to 80◦ (2) using a solid detector. Scanning electron microscopy (SEM) images were taken with a JSM-6390LV scanning electron microscope operated at 20 kV. Transmission electron microscopy (TEM) and high resolution TEM (HRTEM) images were obtained with a JEM-2000EX and JEOL 2100-FEG transmission electron microscope, respectively. All the electrochemical measurements were conducted on a CHI760c working station in a three-electrode cell at room temperature. A saturated calomel electrode (SCE) for acidic solutions, or an Hg/HgO electrode for alkaline solutions, was used as the reference electrode, and a platinum foil was used as the counter electrode. The working electrode was prepared using a glassy carbon (GC) disk as the substrate. During the electrochemical measurements, a mixture containing 1.0 mg of electrocatalyst and 1 mL of ethanol was pretreated for 1 h under ultrasonication to obtain a well-dispersed ink. A 15 L portion of the catalyst ink was then transferred onto the surface of the GC electrode and dried under in the air to obtain a catalyst thin film. Cyclic voltammetry (CV) of formic acid was performed in aqueous solutions of 0.5 M H2 SO4 containing 0.5 M HCOOH in certain potential windows. For cyclic voltammetry of ethanol oxidation, the electrolytic solution was 1 M CH3 CH2 OH in 1 M KOH.
Fig. 2 shows the TEM and HRTEM images of samples Pd–A and Pd–M. As depicted in the low-magnification TEM image (Fig. 2a), sample Pd–A shows conglutinated structures composed of Pd nanocrystals with the size scales less than 20 nm. Obvious diffraction circles can be observed from the selected-area electron diffraction (SAED) pattern of sample Pd–A in the inset of Fig. 2a, in which some diffraction spots also appear. This should be attributed to the structural nature of Pd–A, in which Pd nanocrystals are not inclined to form large well-crystalline aggregates. These results are further confirmed by the corresponding HRTEM results (Fig. 2b), in which the edges of nanocrystal can be observed obviously and poor crystalline parts are appeared probably. Possible edge and defect domains are also marked with black and white arrows, respectively, as shown in Fig. 2b. The lattice spacings of ∼0.235 and 0.208 nm are observed corresponding to (2 0 0) and (1 1 1) lattice planes of cubic phase palladium, respectively, indicating the crystalline nature of a single nanoparticle. As can be seen from Fig. 2c, the morphology of Pd–M is somewhat different with that of Pd–A. It can be seen that quasi-one dimensional string aggregates of Pd nanocrystals with the size of about 8–10 nm are formed. Weak diffraction cycles are observed from the SAED pattern of Pd–M (in the inset of Fig. 2c) compared with those of Pd–M, which is in accord with the XRD observations (Fig. 1). As shown in the HREM image of Pd–M (Fig. 1d), irregular nanocrystals are formed. The natural spacings of lattice planes are 0.089 and 0.087 nm ascribed to the (3 3 1) and (4 2 0) planes of cubic phase palladium, respectively, which exhibit an open structure exposed to electrolyte [26,28].
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Fig. 2. TEM (a,c) and HRTEM (b,d) images of Pd–A (a,b) and Pd–M (c,d). The insets in a and c are the SAED patterns of the corresponding samples.
Fig. 3. TEM images of intermediary products obtained from aqueous (a–c) and mixed (e–f) synthesis systems after the reaction time of 0.5 h (a, d), 1 h (b, e) and 4 h (c, f).
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Fig. 4. CV curves of Pd–A (a) and Pd–M (b) modified GC electrodes in 1 M H2 SO4 solutions at the scan rate of 50 mV s−1 .
3.3. Evolution of the intermediates To study the formation mechanism of Pd nanostructures, intermediary products are collected and characterized. Fig. 3 shows TEM images of the intermediates obtained after different reaction times. It can be seen that the shape and sizes of the products change gradually with the increase of synthesis time. For aqueous synthesis system, the product collected at 0.5 h show irregular granular nanostructures with the size scales of less than 10 nm (Fig. 3a) while smaller nanoparticles can be observed for the intermediate of another synthesis system (Fig. 3d) obtained at the same period. If the reaction time is elongated to 1 h, the sizes of the products (Fig. 3b and e) become somewhat larger which are changed slightly compared with those of the former mesophases. When the synthesis time for aqueous systems is further extended to 4 h, small nanoparticles are gradually grown with the size increased to 10–15 nm (Fig. 3c). Furthermore, boundaries among particles gradually disappear, and connect into aggregate-like structures. Aggregate-like nanostructures can also be observed for the mixed systems with the synthesis time of 4 h (Fig. 3f). It is proposed that the solubility of PVP in water and acetone should play the key role in the formation of Pd nanocrystals. As water is a good solvent of PVP, Pd nanocrystals grow gradually in the presence of PVP with the increase of synthesis time due to the Ostwald ripening in aqueous synthesis systems. With the addition of acetone, a bad solvent of PVP, into the synthesis system, the conformation of PVP in the solution may be altered and block the continuous grown of Pd nanocrystals [24]. It is suggested that the specific adsorption of functional groups of PVP on the crystalline facets of Pd promotes the formation of Pd–A and Pd–M under different synthesis environments [29].
features of Pd–M and Pd–A nanocrystals should play an important role in determining the electrochemical responses of these modified electrodes. The real surface area of Pd–A and Pd–M modified electrodes are evaluated to be 0.010 and 0.041 cm2 , respectively, from the formula Q(C)/424 cm2 in which Q stands for the corresponding electrical quantity of inflection point according to the relationship between the maximum potential and the electrical quantity of reduction peak (Fig. 5) [7]. This method is a standard electrochemical method frequently used for determining the real surface area of palladium. If regard the particles of the samples as spherical structures approximately, the specific surface area of Pd–M is about three times as large as that of Pd–A. For comparison of the electrocatalytic performance of Pd–A and Pd–M, the measured current is normalized either with the surface area or with the mass to obtain the current density. 3.5. Electrocatalytic performance for formic acid To study the electrochemical activity of nanostructured Pd samples, Pd-modified GC electrodes are used as working electrode for electrocatalytic oxidation of formic acid and ethanol. It is found that Pd nanocrystals, especially for Pd–M nanocrystals show a high electrocatalytic activity no matter to ethanol or to formic acid [9,19,25]. As can be seen from Fig. 6A, Pd–M modified electrode displays much higher electrocatalytic activity toward formic acid than that of Pd–A modified electrode. The current densities at peak potentials of 0.14 and 0.13 V normalization to mass of Pd–M modified electrodes are
3.4. Cyclic voltammogram characterization Fig. 4 shows the cyclic voltammograms (CVs) for the Pdmodified electrodes in 1 M H2 SO4 solutions. The CV curves of the modified electrodes are rather different from the naked GC electrode, in which several new features can be observed compared with that of the naked electrode. Firstly, the sharp change of current at −0.18 V is attributed to the generation of H2 from the electrolysis of H2 O. Secondly, a couple of weak peaks between −0.05 and −0.1 V (vs. SCE) are attributed to the adsorption/desorption of hydrogen in the surface of Pd samples [30,31] Thirdly, the peak at 0.37 V (vs. SCE) is the reduction peak of produced Pd oxide [32]. It can also be observed from Fig. 4 that the current of the Pd–M modified electrode is much larger than that of the Pd–A modified electrode. Combined with the results of XRD and HRTEM, it is proposed that the electrochemical-active surface area and structural
Fig. 5. The relationship curve between the maximum potential and the electrical quantity of reduction peak under different voltage window ranges: Pd–A (a) and Pd–M (b).
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Fig. 6. CV (A and B) and (C) chronoamperometric curves of Pd–A (a) and Pd–M (b) modified GC electrodes after normalization in 0.5 M HCOOH + 0.5 M H2 SO4 solutions at room temperature.
270 and 210 mA mg−1 in an aqueous electrolytic solution (0.5 M HCOOH + 0.5 M H2 SO4 ) for the positive and negative scan, respectively, which are about 9 times as high as those of Pd–A modified electrodes. It can also be observed from Fig. 6A and B that the currents of positive peaks are stronger than those of negative ones. This may be caused by adsorption of intermediate product which produces in the HCOOH oxidation process and covers partial active points of Pd film on GC electrode [5,28]. In order to further disclose the electrocatalytic activity of Pd nanocrystals, the CV curves with the current density normalized to the active area are shown in Fig. 6B. Two features can be concluded. Firstly, the electrocatalytic activity of Pd–M is still about 3 times as high as that of Pd–A. Secondly, both positive and negative oxidized peaks for Pd–M in the CV curves are around 0.184 V at the scan rate of 50 mV s−1 . However, the oxidized peaks for Pd–A appear at about 0.26 V at the same scan rate in which the positive peak is also stronger than the negative one. Furthermore, the
oxidation current density of Pd–M is nearly tenfold of that of Pd–A in the experimental period in the transient current density curves of formic acid oxidation at 0.25 V (Fig. 6C). These results confirm better electrocatalytic performance of Pd–M compared with that of Pd–A, which may be ascribed to the higher electrochemical active area and smaller size of Pd–M than that of Pd–A [22,27]. 3.6. Electrocatalytic performance for ethanol The electrooxidation activity of Pd–A and Pd–M toward ethanol has also been investigated (Fig. 7). Both of Pd samples show higher electrocatalytic activity than that of the commercial Pd black catalyst reported obviously [26]. It can also be seen from Fig. 7A that the electrocatalytic currents per mass of Pd–M modified electrodes respectively are 400 and 380 mA mg−1 in 1 M CH3 CH2 OH + 1 M KOH solutions for the positive and negative scan, which are about 12 and 8 times as high as those of Pd–A modified electrodes,
Fig. 7. CV (A and C) and chronoamperometric (B and D) curves of Pd–A (a) and Pd–M (b) modified GC electrodes after normalization in 1 M CH3 CH2 OH + 1 M KOH solutions.
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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 20803037 and 21143006), the Natural Science Foundation of Shandong Province (No. ZR2009BM013), the Foundation of Qingdao Municipal Science and Technology Commission (11-2-4-2-(8)-jch) and the foundation of “Taishan Scholar” program of Shandong Province, China.
References
Fig. 8. Variation of current densities with cyclic number associated with the electrocatalytic oxidation in 1 M CH3 CH2 OH + 1 M KOH solutions at the scan rate of 50 mV s−1 : Pd–A (a) and Pd–M (b).
respectively. Transient current densities of ethanol oxidation at 0.3 V of these two samples are shown in Fig. 7B, which indicate that Pd–M has much higher catalytic activity toward ethanol electrooxidation that of Pd–A. Similarly, two features can also be concluded from Fig. 7C and D. Firstly, the current density normalized to the surface area of Pd–M is about 2 or 3 times as high as those of Pd–A for positive or negative scan, respectively. These values are slightly smaller than those of electrooxidation of formic acid, which may be attributed to the different sizes of ethanol and formic acid molecules [28]. Second, the potentials for positive and negative oxidized peaks of Pd–M in the CV curves are red-shifted about 37 mV and 52 mV, respectively, at the scan rate of 50 mV s−1 , compared with those of Pd–A, also indicating better electrocatalytic performance of Pd–M. Chronoamperometric curves shown in Fig. 7D show that Pd–M has better stability even in a short period (about 1000 s) than that of Pd–A. Fig. 8 shows the relationship curves between the current density and cycle number of Pd-modified electrodes toward ethanol electrocatalytic oxidation. It can be seen that Pd–M can reach the highest current density of electrooxidation and maintain a steady state in a short time (about 50th cycle) while Pd–A needs somewhat longer time to reach steady state (after 100th). This is consistent with the short period chronoamperometric curves (Fig. 7D). As depicted in Fig. 8, it is also clear that even though Pd–A has better stability in long term (600 cycle), the current density of Pd–M at 600th still is 10.6 times as high as that of Pd–A. These results should be contributed to the dual positively effect of real surface area and small size effect of Pd–M compared with those of Pd–A.
4. Conclusion Two palladium nanocrystal aggregates are synthesized using a hydrothermal method through controlling the addition of acetone into the synthesis systems. Pd nanocrystals synthesized from aqueous system (Pd–A) are 14 nm while Pd nanocrystals prepared from the system containing acetone (Pd–M) are 9 nm. Both of Pd samples show high electrocatalytic activity toward formic acid and ethanol. Especially, the electrocatalytic performance of Pd–M is much higher than that of Pd–A, which should be ascribed to both the large electroactive area and small size effect of Pd–M. The formation mechanisms of Pd–A and Pd–M as well as the relationship between their microstructures and electrocatalytic activity have been discussed based on the experimental results.
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