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Morphology dependence of electrochemical properties on palladium nanocrystals
Journal of Colloid and Interface Science 490 (2017) 190–196
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Morphology dependence of electrochemical properties on palladium nanocrystals Jinchang Fan a, Kun Qi a, Hong Chen a,b, Weitao Zheng a, Xiaoqiang Cui a,⇑ a State Key Laboratory of Automotive Simulation and Control, Department of Materials Science, and Key Laboratory of Automobile Materials of MOE, Jilin University, Changchun 130012, People’s Republic of China b Department of Control Science & Engineering, Jilin University, Changchun 130012, People’s Republic of China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 20 October 2016 Revised 14 November 2016 Accepted 16 November 2016 Available online 17 November 2016 Keywords: Shape control synthesis Pd nanostructures Two-dimensional material Electrochemistry Fuel cells
a b s t r a c t In recent years, shape control has received the most attention in the exploration of Pd nanocrystals (NCs). However, exploring an efficient approach for the systematic production of Pd NCs under similar reaction conditions still presents a significant challenge, which is significantly important to clearly explain the effectiveness of morphology on the catalytic activity of Pd NCs. We designed and accomplished a facile strategy for the morphology transformation between Pd nanosheets and Pd nanotetrahedra by simply controlling the reaction temperature. A growth mechanism was proposed based on TEM images of the time-dependent morphology evolution. The Pd nanosheets and Pd nanotetrahedra exhibit higher activity for the methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) compared with the benchmark Pd/C catalysts, and their activities are dependent on the morphology. In particular, Pd nanosheets show an increased activity by 3.81 (MOR) and 2.86 (ORR) times due to their large specific surface area and exposed facets. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction Palladium nanocrystals (NCs) have attracted a great deal of attention in the area of electrocatalysis, including oxygen reduc⇑ Corresponding author. E-mail address: [email protected] (X. Cui). http://dx.doi.org/10.1016/j.jcis.2016.11.061 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
tion reaction [1–3], formic acid oxidation [4–6], and direct alcohol fuel cells [7–9]. Over the past few decades, various morphologies of Pd NCs have been prepared by carefully controlling the reaction conditions [10–15]. For instance, Pd truncated octahedrons were formed through a fast reduction process by ascorbic acid [10], while Pd octahedrons and Pd icosahedrons were prepared through a moderate reduction process by citric acid [11,12]. The adsorption
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of halides on Pd (1 0 0) facilitates the formation of Pd nanocubes enclosed with (1 0 0) facets [13,14], while Fe3+ is used as an oxidative etchant to prepare complex Pd morphologies [15]. Notably, these syntheses were conducted under different conditions, which are not suitable for the study of the shape-dependent properties. Moreover, Pd nanocrystals with systematically tuneable structures are important for the rational design of high-performance catalysts with a wide range of potential practical applications [16–18]. Recent studies have shown that CO is an efficient reducing and capping agent for the synthesis of Pd NCs [19–25]. The strong and preferential chemisorption of CO on Pd (1 1 1) leads to the formation of Pd nanosheets by preventing the deposition of fresh Pd atoms on the (1 1 1) basal plane [19]. Other morphologies of Pd NCs can be obtained only when the strong chemisorption of CO on Pd (1 1 1) is overcome. Pd tetrapods and tetrahedra were prepared by mixing H2 with CO to weaken the chemisorption of CO [26], and concave tetrahedra were formed at a slow reducing rate by bubbling CO at a lower flow rate [27]. The possible dangers of mixing H2 and CO and the complexity of controlling the CO flow rate may inhibit large-scale synthesis and the potential for further applications. Therefore, a more facile and effective method is still required. The reaction temperature determines the final products by affecting the energy of the reaction system [28,29]. The products are kinetically controlled at low temperature, while thermodynamically dominates the reaction at high temperature because of the need to surpass the energy barrier of the reaction system [30,31]. Pd nanosheets are formed as the typical kinetically controlled products because of the high energy barrier from the chemisorption of CO on Pd (1 1 1) [19]. Taking the aforementioned points into consideration, it provides a possibility of synthesizing different morphological Pd NCs by controlling the reaction temperature. In this work, we have developed a simple and efficient method for the morphology transformation between Pd nanosheets and Pd nanotetrahedra by controlling the reaction temperature. The Pd nanosheets are the primary products at low temperature because of the strong chemisorption of CO on Pd (1 1 1). On the other hand, the Pd nanotetrahedra are the dominant products at high temperature, which provides enough energy to overcome the chemisorption of CO on Pd (1 1 1). Moreover, under similar synthesis conditions, the as-prepared Pd NCs are an ideal model for the investigation of the relationship between morphology and catalytic performance. The Pd nanosheets show better methanol oxidation and oxygen reduction activity than Pd nanotetrahedra and commercial Pd/C because of the large specific surface area and exposed facets.
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and water (volume ratio = 5:1) under magnetic stirring. The resulting homogeneous yellow solution was transferred into a 50 mL Schlenk flask and then charged with CO gas for 4 min at a flow rate of 500 mL min1. The flask was immediately placed into an oil bath at pre-heated temperature of 40 °C, 60 °C, 80 °C, or 100 °C for 3.0 h. The resulting products were collected by centrifugation (13,000 rpm, 10 min) and washed three times with ethanol solution. The products were redispersed in ethanol for further characterization. PVP serves as an effective coating to stabilize the Pd NCs against agglomeration [32], and CTAB is a structure-directing agent [19,23,33]. To demonstrate the importance of CTAB, two control experiments without CTAB were carried out (Fig. S1). At 40 °C, the products are sheet-like Pd nanocrystals and Pd nanoparticles, while the products are concave Pd nanotetrahedra and irregular Pd nanostructures at 100 °C. 2.3. Characterization Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were conducted using a JEM-2100F transmission electron microscope (JEOL Co., Japan). UV–Vis spectroscopy was performed using a CHEMUSB4000-UV/ vis spectrophotometer (Ocean Optics Inc., USA). X-ray diffraction (XRD) patterns were obtained using a Bragg–Brentano diffractometer (D8-tools, Germany) with the source from a Cu-Ka line at 0.15418 nm. Inductively coupled plasma mass spectrometry (ICP-MS) data were obtained with an ELAN 9000/DRC ICP-MS system. Fast Fourier transform (FFT) contrast-refined HRTEM images were obtained using the Gatan digital micrograph software. Optical photographs were all acquired using an iPhone 6. 2.4. Electrochemical measurements The electrochemical measurements were conducted on a CHI760D electrochemical workstation (Shanghai, Chenhua Co., China). Typically, a three-electrode cell was used with a glassycarbon rotating disk electrode (RDE) (diameter: 5 mm, area: 0.196 cm2) as the working electrode, Ag/AgCl (in a 3 M KCl solution) as the reference electrode, and a platinum foil electrode (1 cm 1 cm) as the counter electrode. For the preparation of the working electrode (WE), 12 lL of the Pd nanosheets, the Pd nanotetrahedra, or the Pd/C ethanol dispersion were dispersed on the RDE, and then 6 lL of Nafion-ethanol solution (0.05% wt.) was dispersed on the surface of the RDE electrode. The Pd loading of these catalysts was measured by ICP-MS. All of the electrochemical tests were performed at room temperature.
2. Experimental section 2.1. Materials and reagents Palladium acetylacetonate (Pd(acac)2, 99%) and Pd/C catalyst (30 wt.%) were obtained from Sigma Aldrich (USA). Cetyltrimethyl ammonium bromide (CTAB, AR), polyvinylpyrrolidone (PVP MW = 40,000, AR), N,N-dimethylformamide (DMF, AR), acetone (AR), ethanol (AR), potassium hydroxide (KOH, AR), and methanol (CH3OH, AR) were all purchased from Sinopharm Chemical Reagent (China). Carbon oxide (ultra-purity) was purchased from Xinguang Gas Co. (Changchun, China). Nafion-ethanol solution was obtained from Adamas-beta Chemical Co. (Switzerland). Milli-Q deionized water (DI water, 18.2 MX cm1) was used in all experiments. All reagents were used as received without further purification. 2.2. Synthesis of Pd nanocrystals with different structures In a typical synthesis, 50 mg of Pd(acac)2, 160 mg of PVP, and 185 mg of CTAB were mixed with 12 mL of a mixture of DMF
2.4.1. Electrochemical methanol oxidation reaction (MOR) measurements Before the data collection, all working electrodes were cleaned using a steady-state potential sweep from 0.9 V to 0.5 V at 50 mV s1 in N2-saturated 1 M KOH solution. The cyclic voltammograms (CV) were recorded at 50 mV s1 after the curve was stable. The electrochemically active surface area (ECSA) of the catalyst was determined by integrating the area surrounded by the reduction peak of the palladium oxide and the CV baseline, according to the following equation [9,34]:
ECSA ¼ Q 0 =q0 ; where Q0 is the surface charge obtained from the area under the cyclic voltammetry scan for oxygen desorption (0.2 V to 0.6 V), and q0 is the charge required for the desorption of a monolayer of oxygen on the Pd surface (424 lC cm2). The catalytic activity and cycling stability measurements were all carried out in N2saturated 1 M KOH + 1 M CH3OH solution by cyclic voltammetry scanning between 0.8 V and 0.3 V with a scan rate of 50 mV s1.
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The long-term stability test was recorded by chronoamperometric scanning at a constant potential of 0.22 V.
3. Results and discussion 3.1. Characterization of Pd NCs
2.4.2. Electrochemical oxygen reduction reaction (ORR) measurements The potential measurements were converted to the reversible hydrogen electrode (RHE) based on the following formula: E vs. h RHE = E vs. Ag/AgCl + E Ag/AgCl + 0.059 pH. The working electrode was cleaned by a steady-state potential sweep from 0.2 V to 1.0 V at 50 mV s1 in N2-saturated 0.1 M KOH solution before data collection. ORR measurements were conducted in a 0.1 M KOH solution purged with oxygen. The scan rate and rotation rate for the ORR measurements were 5 mV s1 and 1600 rpm, respectively. The ORR electron transfer number (n) per O2 molecule in ORR and kinetic current density (Jk) were determined according to the Koutecky-Levich (K-L) equation [35,36]:
1 1 1 ¼ þ J J K 0:2nFD2=3 m1=6 C O2 x1=2 where J is the measured current density, F is the Faraday constant (96,485 C mol1), D is the O2 diffusion coefficient (1.9 105 cm2 s1), m is the kinetic viscosity (0.01 cm2 s1), C O2 is the O2 concentration (1.2 106 mol cm3), and x is the rotation rate.
The morphology of the Pd NCs was determined by electron microscopy. TEM images of the as-prepared Pd NCs are shown in Fig. 1a–d, which indicate that different Pd morphologies are formed at different temperature. Uniform Pd nanosheets with a yield approaching 100% are obtained at 40 °C (Fig. 1a). Most of these Pd nanosheets are face-to-face self-assembled, and they form column-like superstructures. The average diameter and thickness of the Pd nanosheets are of 20.3 ± 1.7 nm and 1.2 ± 0.2 nm, respectively (Fig. S2a). Both the diameter and thickness of as-prepared Pd nanosheets are smaller than those of the Pd nanosheets previously reported by Zheng’s group [19]. The HRTEM images of the Pd nanosheets laid flat and vertically stood on the copper grid were investigated (Fig. 1a1–a4). The Pd nanosheets laid flat on the copper grid show a weak TEM contrast due to their ultrathin nature (Fig. 1a1). The lattice fringe spacing along the flat plane is measured as 0.23 nm, which corresponds to the (1 1 1) plane of the Pd NCs (Fig. 1a2) [37]. The lattice fringe spacing from the vertical plane is of 0.20 nm corresponding to the (2 0 0) plane (Fig. 1a3 and a4) [9]. The Pd nanotetrahedra appear with a percentage of 19.8% when the reaction temperature is increased to 60 °C (Fig. 1b). There is an even proportion of Pd nanosheets and Pd
Fig. 1. Typical TEM images of as-prepared Pd NCs at different reaction temperature of: (a) 40 °C, (b) 60 °C, (c) 80 °C, and (d) 100 °C. HRTEM images and FFT-refined HRTEM images of Pd nanosheets (a1, a2) laid flat and (a3, a4) standing vertically on a TEM grid. (e) HRTEM images of Pd nanotetrahedra. (f) The histogram of the percentage of Pd nanosheets and Pd nanotetrahedra at different reaction temperature. (g) The UV–vis spectra of as-prepared Pd NCs, the inset shows their photographs.
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Fig. 2. Time-dependent morphology evolution of the Pd NCs prepared at (a–d) 40 °C and (e–h) 100 °C. TEM images of Pd NCs under 40 °C collected at (a) 20 min, (b) 30 min, and (c) 120 min, and (d) statistics of the diameter distribution of Pd nanosheets at different reaction time; TEM images of Pd NCs under 100 °C collected at (e) 5 min, (f) 10 min, and (g) 20 min, (h) statistics of the diameter distribution of Pd nanotetrahedra at different reaction time. The inset blue arrows in (a) and (e) represent the Pd nanosheet seeds, while the red arrows indicate the Pd nanotetrahedron seeds. (The scale bars are 50 nm.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
nanotetrahedra at 80 °C (Fig. 1c). Pure Pd nanotetrahedra are obtained at 100 °C (Fig. 1d) with an average edge length of 9.8 ± 0.8 nm (Fig. S2b). The stereoscopic nature of Pd nanotetrahedra is demonstrated by collecting TEM images at different tilt angles (Fig. S3). The HRTEM image of Pd nanotetrahedra shows a lattice spacing of 0.23 nm, corresponding to the (1 1 1) plane (Fig. 1e). The relationship between the percentage of the Pd nanosheets and Pd nanotetrahedra at different temperature is summarized in Fig. 1f. Apparently, the morphology of the products can be simply controlled by the reaction temperature. The Pd nanosheets prepared at 40 °C exhibit a broad localized surface plasmon resonance (LSPR) absorption peak at approximately 750 nm (Fig. 1g), which is assigned to the in-plane dipole resonance of the Pd nanosheets [20,38]. The colour of the product solution changes from blue1 to dark-brown (the inset in Fig. 1g) as the reaction temperature increases, and the absorption peak decreases gradually because of the morphological changes. The XRD patterns of the Pd nanosheets and Pd nanotetrahedra show a face-centered cubic (fcc) structure, and the peak positions match well with the metallic Pd diffractions (Fig. S4), which confirms the formation of only Pd NCs and dominated by (1 1 1) facets. 3.2. Growth mechanism of Pd NCs The time-dependent morphological evolution of the Pd NCs prepared at 40 °C and 100 °C was explored by analysing a series of TEM images during the formation process (Fig. 2). The TEM snapshots started from the time point of 0 min when the Schlenk flask was placed into the oil bath. A mass of Pd nanosheet seeds and a few Pd nanotetrahedron seeds are formed at the initial growth stage when the reaction temperature is at 40 °C (Fig. 2a). The 1 For interpretation of colour in Fig. 1, the reader is referred to the web version of this article.
diameter of the Pd nanosheets increases when the reaction time is prolonged (Fig. 2b and c). The preferential growth of Pd nanosheet seeds eventually leads to the larger size of the Pd nanosheets compared to the Pd nanotetrahedra. Meanwhile, the Pd nanotetrahedra disappear gradually due to the Ostwald ripening phenomenon [39]. Pd nanosheets are face-to-face assembled to minimize the surface energy at 120 min (Fig. 2c) [40]. The average diameter of the Pd nanosheets at different reaction times is summarized in Fig. 2d. The results suggest that the growth process of the Pd NCs under 40 °C includes two steps: (a) the initial formation of two types of Pd seeds at the early stage, and (b) the subsequent growth of the Pd nanosheets along with the disappearance of the Pd nanotetrahedra. The growth process of the Pd NCs under 100 °C is similar, as shown in Fig. 2e–h, and the amount of Pd nanotetrahedron seeds is more than that of Pd nanosheet seeds at the initial stage. The Pd nanotetrahedra grow, and the Pd nanosheets disappear gradually as the reaction proceeds. The proposed growth mechanism is shown in Scheme 1 based on the aforementioned observations. The Pd precursor is first reduced to metallic atoms (Pd0) by CO, and then the Pd nanosheet seeds and Pd nanotetrahedron seeds are rapidly formed at the initial stage. CO, as a strong and specific adsorbate, occupies the atom sites on the Pd (1 1 1) surface, which prevents the continuous deposition of the freshly reduced Pd0 [22,41]. The overcome energy of the Pd0 deposited on Pd (1 1 1) is much higher than that on Pd (1 0 0), as illustrated by the curves in Scheme 1. The Pd0 is preferentially deposited on Pd (1 0 0) at low temperature because the energy of the reaction system is lower than that of Pd (1 1 1). Therefore, the kinetically controlled Pd nanosheets become the main products at low temperature. The Pd0 can be deposited on the (1 1 1) plane at high temperature because enough energy is provided to overcome the energy barrier of Pd (1 1 1). The thermodynamically controlled Pd nanotetrahedra are the dominant products at high temperature.
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Scheme 1. The proposed growth mechanism of Pd NCs at low and high temperature.
3.3. Electrocatalytic measurements of the catalysts The methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) were selected to research the effect of the Pd morphology on the catalytic activities of the NCs. The electrocatalytic performances for MOR from the Pd nanosheets and Pd nanotetrahedra were investigated by cyclic voltammetry and chronoamperometric techniques. The electrochemical surface area (ECSA) was calculated as 34.88 m2 g1 for Pd/C, 85.5 m2 g1 for Pd nanosheets, and 47.66 m2 g1 for Pd nanotetrahedra, according to the CV curves (Fig. 3a and Fig. S5a). The current density was normalized by the ECSA to give the specific activity (Fig. S5b), or by the Pd loading to give the mass activity (Fig. 3b). Pd nanosheets exhibit the highest catalytic activity compared to the other structures. The mass activity of Pd nanosheets was measured to be 1.9 A mg1 Pd , which
is 1.48 times and 3.81 times higher than those of Pd nanotetrahedra and Pd/C catalysts, respectively. The Pd nanosheets and Pd nanotetrahedra prepared under similar synthesis conditions are ideal models for investigating the effect of the morphology on the catalytic performance [18]. The specific surface areas were also calculated using geometric models of the Pd nanosheets and nanotetrahedra to better understand the morphological effect on the catalytic performance (Figs. S6 and S7). The higher specific surface area of asprepared Pd nanostructures provide more catalytic active sites than Pd/C [42,43]. Furthermore, the ratio between mass activity and specific surface area of as-prepared Pd nanostructures is higher than that of Pd/C, which may due to the high atoms utilization. The specific surface area of Pd nanosheets is 1.26 times larger than that of Pd nanotetrahedra, which is slightly smaller than the enhanced effect of 1.48 times for MOR, as observed in Fig. 3c. Therefore, in addition to the large specific surface area, the side (1 0 0) facets also contribute to the enhanced catalytic properties of Pd nanosheets. The cycling stability curves in Fig. 3d reveal that Pd nanosheets have a better stability, with 76.39% of the current density retained after 200 cycles scanning, which is higher than 73.69% and 68.57% for Pd nanotetrahedra and Pd/C catalysts, respectively. Pd nanosheets retain a higher current density than the other Pd-based catalysts after scanning is performed for 3600 s, which implies better long-term stability (Fig. 3e). The oxygen reduction reaction (ORR) activities of the Pd-based catalysts were also compared. Fig. 4a presents positive-going linear sweep voltammograms (LSV) for the Pd-based catalysts, and the current density was normalized by the geometric area of the electrode (0.196 cm2). The Pd nanosheets show better ORR catalytic activity, with a positive shift on half-peak potential (E1/2) of 11 mV, 29 mV relative to the Pd nanotetrahedra and Pd/C catalysts. The mass activity of Pd nanosheets is 2.86 times
Fig. 3. (a) CV curves of three kinds of Pd-based catalysts in 1 M KOH. (b) Mass activity for the methanol oxidation of three kinds of Pd-based catalysts in 1 M KOH + 1 M CH3OH. (c) The histogram of the mass activity and calculated specific surface area of Pd-based catalysts. (d) MOR cycling stability of the catalysts with a cycle number of 200. (e) Chronoamperometric curves of three Pd-based catalysts at an applied potential of 0.22 V. The inset is the partially enlarged image between 3000 s and 3600 s.
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Fig. 4. (a) ORR polarization curves of the commercial Pd/C catalysts, Pd nanosheets, and Pd nanotetrahedra. Curves were obtained using a rotating disk electrode in oxygensaturated 0.1 M KOH at a rotation rate of 1600 rpm. (b) ORR mass activities of three Pd-based catalysts at 0.875 V and 0.9 V.
Table 1 MOR and ORR parameters of the Pd-based catalysts used in this work. Catalyst
MOR
ORR 2
ECSA (m g Pd/C Pd nanosheets Pd nanotetrahedra
34.88 85.5 47.66
1
)
Mass Activity (A/mg)
Half peak potential (V)
Mass activity (A/mg) at 0.875 V
Mass activity (A/mg) at 0.9 V
0.499 1.9 1.283
0.805 0.834 0.823
0.028 0.08 0.042
0.013 0.032 0.016
higher than that of Pd/C at 0.875 V and 2.46 times higher than that of Pd/C at 0.9 V (Fig. 4b). The mass activities of ORR follow the order of Pd nanosheets > Pd nanotetrahedra > Pd/C, which corresponds to the order for MOR. The rotating-disk measurements of Pd-based catalysts show that the limiting current density increases with increasing rotation rate, and the corresponding K–L plots at different potentials indicate the activity of Pd-based catalysts with a 4e transfer pathway for ORR (Fig. S8). The electrocatalytic performance of the Pd/C catalysts, Pd nanosheets, and Pd nanotetrahedra for MOR and ORR are summarized in Table 1.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.11.061.
References [1] [2] [3] [4] [5] [6]
4. Conclusion In summary, we have developed a simple and effective strategy for the fabrication of Pd nanosheets and Pd nanotetrahedra by controlling the reaction temperature. The growth mechanism of asprepared Pd NCs was investigated by studying the timedependent morphology evolution using TEM images. Both the Pd nanosheets and Pd nanotetrahedra present enhanced electrocatalytic activity in the MOR and ORR compared to commercial Pd/C catalysts, and their activities are dependent on the morphology. Especially, the Pd nanosheets show the highest activity due to their larger specific surface area and exposed facets. More importantly, the fabrication method provides a new pathway for controlling the synthesis of Pd NCs and studying their shape-dependent properties.
Acknowledgments This work was financially supported by the National Key Research and Development Program of China (2016YFA0200400), the Project 2016075 Support by Graduate Innovation Fund of Jilin University, the Program for New Century Excellent Talents in University (NCET-10-0433), the National Natural Science Foundation of China (Nos. 21275064, 51372095), and the Specialized Research Fund for the Doctoral Program of Higher Education (20130061110035).
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
L. Xiao, L. Zhuang, Y. Liu, J. Lu, H.D. Abruña, J. Am. Chem. Soc. 131 (2009) 602. Y. Lu, Y. Jiang, X. Gao, X. Wang, W. Chen, J. Am. Chem. Soc. 136 (2014) 11687. M. Shao, T. Yu, J.H. Odell, M. Jin, Y. Xia, Chem. Commun. 47 (2011) 6566. X. Wang, J. Yang, H. Yin, R. Song, Z. Tang, Adv. Mater. 25 (2013) 2728. H. Zhao, J. Yang, L. Wang, C. Tian, B. Jiang, H. Fu, Chem. Commun. 47 (2011) 2014. X. Zhang, H. Yin, J. Wang, L. Chang, Y. Gao, W. Liu, Z. Tang, Nanoscale 5 (2013) 8392. C. Bianchini, P.K. Shen, Chem. Rev. 109 (2009) 4183. H. Wu, H. Li, Y. Zhai, X. Xu, Y. Jin, Adv. Mater. 24 (2012) 1594. K. Qi, Q. Wang, W. Zheng, W. Zhang, X. Cui, Nanoscale 6 (2014) 15090. B. Lim, M.J. Jiang, P.H.C. Camargo, E.C. Cho, J. Tao, X.M. Lu, Y.M. Zhu, Y.N. Xia, Science 324 (2009) 1302. B. Lim, Y. Xiong, Y. Xia, Angew. Chem. 119 (2007) 9439. Y. Xiong, J.M. McLellan, Y. Yin, Y. Xia, Angew. Chem. 119 (2007) 804. X. Huang, H. Zhang, C. Guo, Z. Zhou, N. Zheng, Angew. Chem. 121 (2009) 4902. Y. Xiong, H. Cai, B.J. Wiley, J. Wang, M.J. Kim, Y. Xia, J. Am. Chem. Soc. 129 (2007) 3665. X. Huang, S. Tang, J. Yang, Y. Tan, N. Zheng, J. Am. Chem. Soc. 133 (2011) 15946. Z. Niu, Q. Peng, M. Gong, H. Rong, Y. Li, Angew. Chem. 123 (2011) 6439. Y. Wang, H.-C. Peng, J. Liu, C.Z. Huang, Y. Xia, Nano Lett. 15 (2015) 1445. Y. Zhang, M. Wang, E. Zhu, Y. Zheng, Y. Huang, X. Huang, Nano Lett. 15 (2015) 7519. X. Huang, S. Tang, X. Mu, Y. Dai, G. Chen, Z. Zhou, F. Ruan, Z. Yang, N. Zheng, Nat. Nano 6 (2011) 28. C. Hu, K. Lin, X. Wang, S. Liu, J. Yi, Y. Tian, B. Wu, G. Chen, H. Yang, Y. Dai, H. Li, N. Zheng, J. Am. Chem. Soc. 136 (2014) 12856. Y.-T. Pan, X. Yin, K.S. Kwok, H. Yang, Nano Lett. 14 (2014) 5953. X. Yin, X. Liu, Y.-T. Pan, K.A. Walsh, H. Yang, Nano Lett. 14 (2014) 7188. Y. Li, Y. Yan, Y. Li, H. Zhang, D. Li, D. Yang, CrystEngComm 17 (2015) 1833. Y. Li, W. Wang, K. Xia, W. Zhang, Y. Jiang, Y. Zeng, H. Zhang, C. Jin, Z. Zhang, D. Yang, Small 11 (2015) 4745. J.W. Hong, Y. Kim, D.H. Wi, S. Lee, S.U. Lee, Y.W. Lee, S.I. Choi, S.W. Han, Angew. Chem. Int. Ed. 55 (2016) 2753. Y. Dai, X. Mu, Y. Tan, K. Lin, Z. Yang, N. Zheng, G. Fu, J. Am. Chem. Soc. 134 (2012) 7073. H. Zhu, Q. Chi, Y. Zhao, C. Li, H. Tang, J. Li, T. Huang, H. Liu, Mater. Res. Bull. 47 (2012) 3637. P. Atkins, J. De Paula, Physical Chemistry, New York, 2006. A. Cooksy, Physical Chemistry: Thermodynamics, Statistical Mechanics, and Kinetics, Pearson Higher Ed, 2013. Y. Xia, X. Xia, H.-C. Peng, J. Am. Chem. Soc. 137 (2015) 7947.
196
J. Fan et al. / Journal of Colloid and Interface Science 490 (2017) 190–196
[31] Y. Wang, J. He, C. Liu, W.H. Chong, H. Chen, Angew. Chem. Int. Ed. 54 (2015) 2022. [32] K.M. Koczkur, S. Mourdikoudis, L. Polavarapu, S.E. Skrabalak, Dalton Trans. 44 (2015) 17883. [33] K. Qi, S. Yu, Q. Wang, W. Zhang, J. Fan, W. Zheng, X. Cui, J. Mater. Chem. A 4 (2016) 4025. [34] L. Shi, A. Wang, T. Zhang, B. Zhang, D. Su, H. Li, Y. Song, J. Phys. Chem. C 117 (2013) 12526. [35] S. Guo, S. Zhang, L. Wu, S. Sun, Angew. Chem. 124 (2012) 11940. [36] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Nat. Mater. 10 (2011) 780.
[37] Q. Wang, W. Zheng, H. Chen, B. Zhang, D. Su, X. Cui, J. Power Sources 316 (2016) 29. [38] E.S. Kooij, W. Ahmed, H.J. Zandvliet, B. Poelsema, J. Phys. Chem. C 115 (2011) 10321. [39] C.L. Kuo, K.C. Hwang, Chem. Mater. 25 (2013) 365. [40] Y. Pi, N. Zhang, S. Guo, J. Guo, X. Huang, Nano Lett. 16 (2016) 4424. [41] S.E. Mason, I. Grinberg, A.M. Rappe, Phys. Rev. B 69 (2004) 161401. [42] R.K. Roy, J.I. Njagi, B. Farrell, I. Halaciuga, M. Lopez, D.V. Goia, J. Colloid Interface Sci. 369 (2012) 91. [43] D.V. Goia, J. Mater. Chem. 14 (2004) 451.
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Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Morphology dependence of electrochemical properties on palladium nanocrystals Jinchang Fan a, Kun Qi a, Hong Chen a,b, Weitao Zheng a, Xiaoqiang Cui a,⇑ a State Key Laboratory of Automotive Simulation and Control, Department of Materials Science, and Key Laboratory of Automobile Materials of MOE, Jilin University, Changchun 130012, People’s Republic of China b Department of Control Science & Engineering, Jilin University, Changchun 130012, People’s Republic of China
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 20 October 2016 Revised 14 November 2016 Accepted 16 November 2016 Available online 17 November 2016 Keywords: Shape control synthesis Pd nanostructures Two-dimensional material Electrochemistry Fuel cells
a b s t r a c t In recent years, shape control has received the most attention in the exploration of Pd nanocrystals (NCs). However, exploring an efficient approach for the systematic production of Pd NCs under similar reaction conditions still presents a significant challenge, which is significantly important to clearly explain the effectiveness of morphology on the catalytic activity of Pd NCs. We designed and accomplished a facile strategy for the morphology transformation between Pd nanosheets and Pd nanotetrahedra by simply controlling the reaction temperature. A growth mechanism was proposed based on TEM images of the time-dependent morphology evolution. The Pd nanosheets and Pd nanotetrahedra exhibit higher activity for the methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) compared with the benchmark Pd/C catalysts, and their activities are dependent on the morphology. In particular, Pd nanosheets show an increased activity by 3.81 (MOR) and 2.86 (ORR) times due to their large specific surface area and exposed facets. Ó 2016 Elsevier Inc. All rights reserved.
1. Introduction Palladium nanocrystals (NCs) have attracted a great deal of attention in the area of electrocatalysis, including oxygen reduc⇑ Corresponding author. E-mail address: [email protected] (X. Cui). http://dx.doi.org/10.1016/j.jcis.2016.11.061 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
tion reaction [1–3], formic acid oxidation [4–6], and direct alcohol fuel cells [7–9]. Over the past few decades, various morphologies of Pd NCs have been prepared by carefully controlling the reaction conditions [10–15]. For instance, Pd truncated octahedrons were formed through a fast reduction process by ascorbic acid [10], while Pd octahedrons and Pd icosahedrons were prepared through a moderate reduction process by citric acid [11,12]. The adsorption
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of halides on Pd (1 0 0) facilitates the formation of Pd nanocubes enclosed with (1 0 0) facets [13,14], while Fe3+ is used as an oxidative etchant to prepare complex Pd morphologies [15]. Notably, these syntheses were conducted under different conditions, which are not suitable for the study of the shape-dependent properties. Moreover, Pd nanocrystals with systematically tuneable structures are important for the rational design of high-performance catalysts with a wide range of potential practical applications [16–18]. Recent studies have shown that CO is an efficient reducing and capping agent for the synthesis of Pd NCs [19–25]. The strong and preferential chemisorption of CO on Pd (1 1 1) leads to the formation of Pd nanosheets by preventing the deposition of fresh Pd atoms on the (1 1 1) basal plane [19]. Other morphologies of Pd NCs can be obtained only when the strong chemisorption of CO on Pd (1 1 1) is overcome. Pd tetrapods and tetrahedra were prepared by mixing H2 with CO to weaken the chemisorption of CO [26], and concave tetrahedra were formed at a slow reducing rate by bubbling CO at a lower flow rate [27]. The possible dangers of mixing H2 and CO and the complexity of controlling the CO flow rate may inhibit large-scale synthesis and the potential for further applications. Therefore, a more facile and effective method is still required. The reaction temperature determines the final products by affecting the energy of the reaction system [28,29]. The products are kinetically controlled at low temperature, while thermodynamically dominates the reaction at high temperature because of the need to surpass the energy barrier of the reaction system [30,31]. Pd nanosheets are formed as the typical kinetically controlled products because of the high energy barrier from the chemisorption of CO on Pd (1 1 1) [19]. Taking the aforementioned points into consideration, it provides a possibility of synthesizing different morphological Pd NCs by controlling the reaction temperature. In this work, we have developed a simple and efficient method for the morphology transformation between Pd nanosheets and Pd nanotetrahedra by controlling the reaction temperature. The Pd nanosheets are the primary products at low temperature because of the strong chemisorption of CO on Pd (1 1 1). On the other hand, the Pd nanotetrahedra are the dominant products at high temperature, which provides enough energy to overcome the chemisorption of CO on Pd (1 1 1). Moreover, under similar synthesis conditions, the as-prepared Pd NCs are an ideal model for the investigation of the relationship between morphology and catalytic performance. The Pd nanosheets show better methanol oxidation and oxygen reduction activity than Pd nanotetrahedra and commercial Pd/C because of the large specific surface area and exposed facets.
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and water (volume ratio = 5:1) under magnetic stirring. The resulting homogeneous yellow solution was transferred into a 50 mL Schlenk flask and then charged with CO gas for 4 min at a flow rate of 500 mL min1. The flask was immediately placed into an oil bath at pre-heated temperature of 40 °C, 60 °C, 80 °C, or 100 °C for 3.0 h. The resulting products were collected by centrifugation (13,000 rpm, 10 min) and washed three times with ethanol solution. The products were redispersed in ethanol for further characterization. PVP serves as an effective coating to stabilize the Pd NCs against agglomeration [32], and CTAB is a structure-directing agent [19,23,33]. To demonstrate the importance of CTAB, two control experiments without CTAB were carried out (Fig. S1). At 40 °C, the products are sheet-like Pd nanocrystals and Pd nanoparticles, while the products are concave Pd nanotetrahedra and irregular Pd nanostructures at 100 °C. 2.3. Characterization Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) were conducted using a JEM-2100F transmission electron microscope (JEOL Co., Japan). UV–Vis spectroscopy was performed using a CHEMUSB4000-UV/ vis spectrophotometer (Ocean Optics Inc., USA). X-ray diffraction (XRD) patterns were obtained using a Bragg–Brentano diffractometer (D8-tools, Germany) with the source from a Cu-Ka line at 0.15418 nm. Inductively coupled plasma mass spectrometry (ICP-MS) data were obtained with an ELAN 9000/DRC ICP-MS system. Fast Fourier transform (FFT) contrast-refined HRTEM images were obtained using the Gatan digital micrograph software. Optical photographs were all acquired using an iPhone 6. 2.4. Electrochemical measurements The electrochemical measurements were conducted on a CHI760D electrochemical workstation (Shanghai, Chenhua Co., China). Typically, a three-electrode cell was used with a glassycarbon rotating disk electrode (RDE) (diameter: 5 mm, area: 0.196 cm2) as the working electrode, Ag/AgCl (in a 3 M KCl solution) as the reference electrode, and a platinum foil electrode (1 cm 1 cm) as the counter electrode. For the preparation of the working electrode (WE), 12 lL of the Pd nanosheets, the Pd nanotetrahedra, or the Pd/C ethanol dispersion were dispersed on the RDE, and then 6 lL of Nafion-ethanol solution (0.05% wt.) was dispersed on the surface of the RDE electrode. The Pd loading of these catalysts was measured by ICP-MS. All of the electrochemical tests were performed at room temperature.
2. Experimental section 2.1. Materials and reagents Palladium acetylacetonate (Pd(acac)2, 99%) and Pd/C catalyst (30 wt.%) were obtained from Sigma Aldrich (USA). Cetyltrimethyl ammonium bromide (CTAB, AR), polyvinylpyrrolidone (PVP MW = 40,000, AR), N,N-dimethylformamide (DMF, AR), acetone (AR), ethanol (AR), potassium hydroxide (KOH, AR), and methanol (CH3OH, AR) were all purchased from Sinopharm Chemical Reagent (China). Carbon oxide (ultra-purity) was purchased from Xinguang Gas Co. (Changchun, China). Nafion-ethanol solution was obtained from Adamas-beta Chemical Co. (Switzerland). Milli-Q deionized water (DI water, 18.2 MX cm1) was used in all experiments. All reagents were used as received without further purification. 2.2. Synthesis of Pd nanocrystals with different structures In a typical synthesis, 50 mg of Pd(acac)2, 160 mg of PVP, and 185 mg of CTAB were mixed with 12 mL of a mixture of DMF
2.4.1. Electrochemical methanol oxidation reaction (MOR) measurements Before the data collection, all working electrodes were cleaned using a steady-state potential sweep from 0.9 V to 0.5 V at 50 mV s1 in N2-saturated 1 M KOH solution. The cyclic voltammograms (CV) were recorded at 50 mV s1 after the curve was stable. The electrochemically active surface area (ECSA) of the catalyst was determined by integrating the area surrounded by the reduction peak of the palladium oxide and the CV baseline, according to the following equation [9,34]:
ECSA ¼ Q 0 =q0 ; where Q0 is the surface charge obtained from the area under the cyclic voltammetry scan for oxygen desorption (0.2 V to 0.6 V), and q0 is the charge required for the desorption of a monolayer of oxygen on the Pd surface (424 lC cm2). The catalytic activity and cycling stability measurements were all carried out in N2saturated 1 M KOH + 1 M CH3OH solution by cyclic voltammetry scanning between 0.8 V and 0.3 V with a scan rate of 50 mV s1.
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The long-term stability test was recorded by chronoamperometric scanning at a constant potential of 0.22 V.
3. Results and discussion 3.1. Characterization of Pd NCs
2.4.2. Electrochemical oxygen reduction reaction (ORR) measurements The potential measurements were converted to the reversible hydrogen electrode (RHE) based on the following formula: E vs. h RHE = E vs. Ag/AgCl + E Ag/AgCl + 0.059 pH. The working electrode was cleaned by a steady-state potential sweep from 0.2 V to 1.0 V at 50 mV s1 in N2-saturated 0.1 M KOH solution before data collection. ORR measurements were conducted in a 0.1 M KOH solution purged with oxygen. The scan rate and rotation rate for the ORR measurements were 5 mV s1 and 1600 rpm, respectively. The ORR electron transfer number (n) per O2 molecule in ORR and kinetic current density (Jk) were determined according to the Koutecky-Levich (K-L) equation [35,36]:
1 1 1 ¼ þ J J K 0:2nFD2=3 m1=6 C O2 x1=2 where J is the measured current density, F is the Faraday constant (96,485 C mol1), D is the O2 diffusion coefficient (1.9 105 cm2 s1), m is the kinetic viscosity (0.01 cm2 s1), C O2 is the O2 concentration (1.2 106 mol cm3), and x is the rotation rate.
The morphology of the Pd NCs was determined by electron microscopy. TEM images of the as-prepared Pd NCs are shown in Fig. 1a–d, which indicate that different Pd morphologies are formed at different temperature. Uniform Pd nanosheets with a yield approaching 100% are obtained at 40 °C (Fig. 1a). Most of these Pd nanosheets are face-to-face self-assembled, and they form column-like superstructures. The average diameter and thickness of the Pd nanosheets are of 20.3 ± 1.7 nm and 1.2 ± 0.2 nm, respectively (Fig. S2a). Both the diameter and thickness of as-prepared Pd nanosheets are smaller than those of the Pd nanosheets previously reported by Zheng’s group [19]. The HRTEM images of the Pd nanosheets laid flat and vertically stood on the copper grid were investigated (Fig. 1a1–a4). The Pd nanosheets laid flat on the copper grid show a weak TEM contrast due to their ultrathin nature (Fig. 1a1). The lattice fringe spacing along the flat plane is measured as 0.23 nm, which corresponds to the (1 1 1) plane of the Pd NCs (Fig. 1a2) [37]. The lattice fringe spacing from the vertical plane is of 0.20 nm corresponding to the (2 0 0) plane (Fig. 1a3 and a4) [9]. The Pd nanotetrahedra appear with a percentage of 19.8% when the reaction temperature is increased to 60 °C (Fig. 1b). There is an even proportion of Pd nanosheets and Pd
Fig. 1. Typical TEM images of as-prepared Pd NCs at different reaction temperature of: (a) 40 °C, (b) 60 °C, (c) 80 °C, and (d) 100 °C. HRTEM images and FFT-refined HRTEM images of Pd nanosheets (a1, a2) laid flat and (a3, a4) standing vertically on a TEM grid. (e) HRTEM images of Pd nanotetrahedra. (f) The histogram of the percentage of Pd nanosheets and Pd nanotetrahedra at different reaction temperature. (g) The UV–vis spectra of as-prepared Pd NCs, the inset shows their photographs.
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Fig. 2. Time-dependent morphology evolution of the Pd NCs prepared at (a–d) 40 °C and (e–h) 100 °C. TEM images of Pd NCs under 40 °C collected at (a) 20 min, (b) 30 min, and (c) 120 min, and (d) statistics of the diameter distribution of Pd nanosheets at different reaction time; TEM images of Pd NCs under 100 °C collected at (e) 5 min, (f) 10 min, and (g) 20 min, (h) statistics of the diameter distribution of Pd nanotetrahedra at different reaction time. The inset blue arrows in (a) and (e) represent the Pd nanosheet seeds, while the red arrows indicate the Pd nanotetrahedron seeds. (The scale bars are 50 nm.) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
nanotetrahedra at 80 °C (Fig. 1c). Pure Pd nanotetrahedra are obtained at 100 °C (Fig. 1d) with an average edge length of 9.8 ± 0.8 nm (Fig. S2b). The stereoscopic nature of Pd nanotetrahedra is demonstrated by collecting TEM images at different tilt angles (Fig. S3). The HRTEM image of Pd nanotetrahedra shows a lattice spacing of 0.23 nm, corresponding to the (1 1 1) plane (Fig. 1e). The relationship between the percentage of the Pd nanosheets and Pd nanotetrahedra at different temperature is summarized in Fig. 1f. Apparently, the morphology of the products can be simply controlled by the reaction temperature. The Pd nanosheets prepared at 40 °C exhibit a broad localized surface plasmon resonance (LSPR) absorption peak at approximately 750 nm (Fig. 1g), which is assigned to the in-plane dipole resonance of the Pd nanosheets [20,38]. The colour of the product solution changes from blue1 to dark-brown (the inset in Fig. 1g) as the reaction temperature increases, and the absorption peak decreases gradually because of the morphological changes. The XRD patterns of the Pd nanosheets and Pd nanotetrahedra show a face-centered cubic (fcc) structure, and the peak positions match well with the metallic Pd diffractions (Fig. S4), which confirms the formation of only Pd NCs and dominated by (1 1 1) facets. 3.2. Growth mechanism of Pd NCs The time-dependent morphological evolution of the Pd NCs prepared at 40 °C and 100 °C was explored by analysing a series of TEM images during the formation process (Fig. 2). The TEM snapshots started from the time point of 0 min when the Schlenk flask was placed into the oil bath. A mass of Pd nanosheet seeds and a few Pd nanotetrahedron seeds are formed at the initial growth stage when the reaction temperature is at 40 °C (Fig. 2a). The 1 For interpretation of colour in Fig. 1, the reader is referred to the web version of this article.
diameter of the Pd nanosheets increases when the reaction time is prolonged (Fig. 2b and c). The preferential growth of Pd nanosheet seeds eventually leads to the larger size of the Pd nanosheets compared to the Pd nanotetrahedra. Meanwhile, the Pd nanotetrahedra disappear gradually due to the Ostwald ripening phenomenon [39]. Pd nanosheets are face-to-face assembled to minimize the surface energy at 120 min (Fig. 2c) [40]. The average diameter of the Pd nanosheets at different reaction times is summarized in Fig. 2d. The results suggest that the growth process of the Pd NCs under 40 °C includes two steps: (a) the initial formation of two types of Pd seeds at the early stage, and (b) the subsequent growth of the Pd nanosheets along with the disappearance of the Pd nanotetrahedra. The growth process of the Pd NCs under 100 °C is similar, as shown in Fig. 2e–h, and the amount of Pd nanotetrahedron seeds is more than that of Pd nanosheet seeds at the initial stage. The Pd nanotetrahedra grow, and the Pd nanosheets disappear gradually as the reaction proceeds. The proposed growth mechanism is shown in Scheme 1 based on the aforementioned observations. The Pd precursor is first reduced to metallic atoms (Pd0) by CO, and then the Pd nanosheet seeds and Pd nanotetrahedron seeds are rapidly formed at the initial stage. CO, as a strong and specific adsorbate, occupies the atom sites on the Pd (1 1 1) surface, which prevents the continuous deposition of the freshly reduced Pd0 [22,41]. The overcome energy of the Pd0 deposited on Pd (1 1 1) is much higher than that on Pd (1 0 0), as illustrated by the curves in Scheme 1. The Pd0 is preferentially deposited on Pd (1 0 0) at low temperature because the energy of the reaction system is lower than that of Pd (1 1 1). Therefore, the kinetically controlled Pd nanosheets become the main products at low temperature. The Pd0 can be deposited on the (1 1 1) plane at high temperature because enough energy is provided to overcome the energy barrier of Pd (1 1 1). The thermodynamically controlled Pd nanotetrahedra are the dominant products at high temperature.
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Scheme 1. The proposed growth mechanism of Pd NCs at low and high temperature.
3.3. Electrocatalytic measurements of the catalysts The methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR) were selected to research the effect of the Pd morphology on the catalytic activities of the NCs. The electrocatalytic performances for MOR from the Pd nanosheets and Pd nanotetrahedra were investigated by cyclic voltammetry and chronoamperometric techniques. The electrochemical surface area (ECSA) was calculated as 34.88 m2 g1 for Pd/C, 85.5 m2 g1 for Pd nanosheets, and 47.66 m2 g1 for Pd nanotetrahedra, according to the CV curves (Fig. 3a and Fig. S5a). The current density was normalized by the ECSA to give the specific activity (Fig. S5b), or by the Pd loading to give the mass activity (Fig. 3b). Pd nanosheets exhibit the highest catalytic activity compared to the other structures. The mass activity of Pd nanosheets was measured to be 1.9 A mg1 Pd , which
is 1.48 times and 3.81 times higher than those of Pd nanotetrahedra and Pd/C catalysts, respectively. The Pd nanosheets and Pd nanotetrahedra prepared under similar synthesis conditions are ideal models for investigating the effect of the morphology on the catalytic performance [18]. The specific surface areas were also calculated using geometric models of the Pd nanosheets and nanotetrahedra to better understand the morphological effect on the catalytic performance (Figs. S6 and S7). The higher specific surface area of asprepared Pd nanostructures provide more catalytic active sites than Pd/C [42,43]. Furthermore, the ratio between mass activity and specific surface area of as-prepared Pd nanostructures is higher than that of Pd/C, which may due to the high atoms utilization. The specific surface area of Pd nanosheets is 1.26 times larger than that of Pd nanotetrahedra, which is slightly smaller than the enhanced effect of 1.48 times for MOR, as observed in Fig. 3c. Therefore, in addition to the large specific surface area, the side (1 0 0) facets also contribute to the enhanced catalytic properties of Pd nanosheets. The cycling stability curves in Fig. 3d reveal that Pd nanosheets have a better stability, with 76.39% of the current density retained after 200 cycles scanning, which is higher than 73.69% and 68.57% for Pd nanotetrahedra and Pd/C catalysts, respectively. Pd nanosheets retain a higher current density than the other Pd-based catalysts after scanning is performed for 3600 s, which implies better long-term stability (Fig. 3e). The oxygen reduction reaction (ORR) activities of the Pd-based catalysts were also compared. Fig. 4a presents positive-going linear sweep voltammograms (LSV) for the Pd-based catalysts, and the current density was normalized by the geometric area of the electrode (0.196 cm2). The Pd nanosheets show better ORR catalytic activity, with a positive shift on half-peak potential (E1/2) of 11 mV, 29 mV relative to the Pd nanotetrahedra and Pd/C catalysts. The mass activity of Pd nanosheets is 2.86 times
Fig. 3. (a) CV curves of three kinds of Pd-based catalysts in 1 M KOH. (b) Mass activity for the methanol oxidation of three kinds of Pd-based catalysts in 1 M KOH + 1 M CH3OH. (c) The histogram of the mass activity and calculated specific surface area of Pd-based catalysts. (d) MOR cycling stability of the catalysts with a cycle number of 200. (e) Chronoamperometric curves of three Pd-based catalysts at an applied potential of 0.22 V. The inset is the partially enlarged image between 3000 s and 3600 s.
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Fig. 4. (a) ORR polarization curves of the commercial Pd/C catalysts, Pd nanosheets, and Pd nanotetrahedra. Curves were obtained using a rotating disk electrode in oxygensaturated 0.1 M KOH at a rotation rate of 1600 rpm. (b) ORR mass activities of three Pd-based catalysts at 0.875 V and 0.9 V.
Table 1 MOR and ORR parameters of the Pd-based catalysts used in this work. Catalyst
MOR
ORR 2
ECSA (m g Pd/C Pd nanosheets Pd nanotetrahedra
34.88 85.5 47.66
1
)
Mass Activity (A/mg)
Half peak potential (V)
Mass activity (A/mg) at 0.875 V
Mass activity (A/mg) at 0.9 V
0.499 1.9 1.283
0.805 0.834 0.823
0.028 0.08 0.042
0.013 0.032 0.016
higher than that of Pd/C at 0.875 V and 2.46 times higher than that of Pd/C at 0.9 V (Fig. 4b). The mass activities of ORR follow the order of Pd nanosheets > Pd nanotetrahedra > Pd/C, which corresponds to the order for MOR. The rotating-disk measurements of Pd-based catalysts show that the limiting current density increases with increasing rotation rate, and the corresponding K–L plots at different potentials indicate the activity of Pd-based catalysts with a 4e transfer pathway for ORR (Fig. S8). The electrocatalytic performance of the Pd/C catalysts, Pd nanosheets, and Pd nanotetrahedra for MOR and ORR are summarized in Table 1.
Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.jcis.2016.11.061.
References [1] [2] [3] [4] [5] [6]
4. Conclusion In summary, we have developed a simple and effective strategy for the fabrication of Pd nanosheets and Pd nanotetrahedra by controlling the reaction temperature. The growth mechanism of asprepared Pd NCs was investigated by studying the timedependent morphology evolution using TEM images. Both the Pd nanosheets and Pd nanotetrahedra present enhanced electrocatalytic activity in the MOR and ORR compared to commercial Pd/C catalysts, and their activities are dependent on the morphology. Especially, the Pd nanosheets show the highest activity due to their larger specific surface area and exposed facets. More importantly, the fabrication method provides a new pathway for controlling the synthesis of Pd NCs and studying their shape-dependent properties.
Acknowledgments This work was financially supported by the National Key Research and Development Program of China (2016YFA0200400), the Project 2016075 Support by Graduate Innovation Fund of Jilin University, the Program for New Century Excellent Talents in University (NCET-10-0433), the National Natural Science Foundation of China (Nos. 21275064, 51372095), and the Specialized Research Fund for the Doctoral Program of Higher Education (20130061110035).
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30]
L. Xiao, L. Zhuang, Y. Liu, J. Lu, H.D. Abruña, J. Am. Chem. Soc. 131 (2009) 602. Y. Lu, Y. Jiang, X. Gao, X. Wang, W. Chen, J. Am. Chem. Soc. 136 (2014) 11687. M. Shao, T. Yu, J.H. Odell, M. Jin, Y. Xia, Chem. Commun. 47 (2011) 6566. X. Wang, J. Yang, H. Yin, R. Song, Z. Tang, Adv. Mater. 25 (2013) 2728. H. Zhao, J. Yang, L. Wang, C. Tian, B. Jiang, H. Fu, Chem. Commun. 47 (2011) 2014. X. Zhang, H. Yin, J. Wang, L. Chang, Y. Gao, W. Liu, Z. Tang, Nanoscale 5 (2013) 8392. C. Bianchini, P.K. Shen, Chem. Rev. 109 (2009) 4183. H. Wu, H. Li, Y. Zhai, X. Xu, Y. Jin, Adv. Mater. 24 (2012) 1594. K. Qi, Q. Wang, W. Zheng, W. Zhang, X. Cui, Nanoscale 6 (2014) 15090. B. Lim, M.J. Jiang, P.H.C. Camargo, E.C. Cho, J. Tao, X.M. Lu, Y.M. Zhu, Y.N. Xia, Science 324 (2009) 1302. B. Lim, Y. Xiong, Y. Xia, Angew. Chem. 119 (2007) 9439. Y. Xiong, J.M. McLellan, Y. Yin, Y. Xia, Angew. Chem. 119 (2007) 804. X. Huang, H. Zhang, C. Guo, Z. Zhou, N. Zheng, Angew. Chem. 121 (2009) 4902. Y. Xiong, H. Cai, B.J. Wiley, J. Wang, M.J. Kim, Y. Xia, J. Am. Chem. Soc. 129 (2007) 3665. X. Huang, S. Tang, J. Yang, Y. Tan, N. Zheng, J. Am. Chem. Soc. 133 (2011) 15946. Z. Niu, Q. Peng, M. Gong, H. Rong, Y. Li, Angew. Chem. 123 (2011) 6439. Y. Wang, H.-C. Peng, J. Liu, C.Z. Huang, Y. Xia, Nano Lett. 15 (2015) 1445. Y. Zhang, M. Wang, E. Zhu, Y. Zheng, Y. Huang, X. Huang, Nano Lett. 15 (2015) 7519. X. Huang, S. Tang, X. Mu, Y. Dai, G. Chen, Z. Zhou, F. Ruan, Z. Yang, N. Zheng, Nat. Nano 6 (2011) 28. C. Hu, K. Lin, X. Wang, S. Liu, J. Yi, Y. Tian, B. Wu, G. Chen, H. Yang, Y. Dai, H. Li, N. Zheng, J. Am. Chem. Soc. 136 (2014) 12856. Y.-T. Pan, X. Yin, K.S. Kwok, H. Yang, Nano Lett. 14 (2014) 5953. X. Yin, X. Liu, Y.-T. Pan, K.A. Walsh, H. Yang, Nano Lett. 14 (2014) 7188. Y. Li, Y. Yan, Y. Li, H. Zhang, D. Li, D. Yang, CrystEngComm 17 (2015) 1833. Y. Li, W. Wang, K. Xia, W. Zhang, Y. Jiang, Y. Zeng, H. Zhang, C. Jin, Z. Zhang, D. Yang, Small 11 (2015) 4745. J.W. Hong, Y. Kim, D.H. Wi, S. Lee, S.U. Lee, Y.W. Lee, S.I. Choi, S.W. Han, Angew. Chem. Int. Ed. 55 (2016) 2753. Y. Dai, X. Mu, Y. Tan, K. Lin, Z. Yang, N. Zheng, G. Fu, J. Am. Chem. Soc. 134 (2012) 7073. H. Zhu, Q. Chi, Y. Zhao, C. Li, H. Tang, J. Li, T. Huang, H. Liu, Mater. Res. Bull. 47 (2012) 3637. P. Atkins, J. De Paula, Physical Chemistry, New York, 2006. A. Cooksy, Physical Chemistry: Thermodynamics, Statistical Mechanics, and Kinetics, Pearson Higher Ed, 2013. Y. Xia, X. Xia, H.-C. Peng, J. Am. Chem. Soc. 137 (2015) 7947.
196
J. Fan et al. / Journal of Colloid and Interface Science 490 (2017) 190–196
[31] Y. Wang, J. He, C. Liu, W.H. Chong, H. Chen, Angew. Chem. Int. Ed. 54 (2015) 2022. [32] K.M. Koczkur, S. Mourdikoudis, L. Polavarapu, S.E. Skrabalak, Dalton Trans. 44 (2015) 17883. [33] K. Qi, S. Yu, Q. Wang, W. Zhang, J. Fan, W. Zheng, X. Cui, J. Mater. Chem. A 4 (2016) 4025. [34] L. Shi, A. Wang, T. Zhang, B. Zhang, D. Su, H. Li, Y. Song, J. Phys. Chem. C 117 (2013) 12526. [35] S. Guo, S. Zhang, L. Wu, S. Sun, Angew. Chem. 124 (2012) 11940. [36] Y. Liang, Y. Li, H. Wang, J. Zhou, J. Wang, T. Regier, H. Dai, Nat. Mater. 10 (2011) 780.
[37] Q. Wang, W. Zheng, H. Chen, B. Zhang, D. Su, X. Cui, J. Power Sources 316 (2016) 29. [38] E.S. Kooij, W. Ahmed, H.J. Zandvliet, B. Poelsema, J. Phys. Chem. C 115 (2011) 10321. [39] C.L. Kuo, K.C. Hwang, Chem. Mater. 25 (2013) 365. [40] Y. Pi, N. Zhang, S. Guo, J. Guo, X. Huang, Nano Lett. 16 (2016) 4424. [41] S.E. Mason, I. Grinberg, A.M. Rappe, Phys. Rev. B 69 (2004) 161401. [42] R.K. Roy, J.I. Njagi, B. Farrell, I. Halaciuga, M. Lopez, D.V. Goia, J. Colloid Interface Sci. 369 (2012) 91. [43] D.V. Goia, J. Mater. Chem. 14 (2004) 451.