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Preparation and characterization of porous sponge-like Pd@Pt nanotubes with high catalytic activity for ethanol oxidation
Materials Letters 173 (2016) 43–46
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation and characterization of porous sponge-like Pd@Pt nanotubes with high catalytic activity for ethanol oxidation Wucheng Luo a,b, Haihui Zhou a,b,n, Chaopeng Fu a,b, Zhongyuan Huang a,b, Na Gao a,b, Yafei Kuang a,b,n a b
State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, PR China College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 14 December 2015 Received in revised form 29 February 2016 Accepted 4 March 2016 Available online 5 March 2016
Porous sponge-like Pd@Pt bimetallic nanotubes (PS Pd@Pt NTs) were prepared by a two-step method for the first time. The as-prepared PS Pd@Pt have an outer diameter of 45 nm and a thickness of 5 nm, numerous Pt nanoparticles distribute discontinuously around the surface of Pd NTs. The electro-catalytic behavior of the PS Pd@Pt NTs for ethanol oxidation was studied by using cyclic voltammetry and chronoamperometry. The results indicated that PS Pd@Pt NTs exhibit higher catalytic activity and better stability than Pt nanowires and Pt on Pd nanotubes. & 2016 Published by Elsevier B.V.
Keywords: Porous sponge-like Pd@Pt nanotubes Nanocomposites Electrical properties
1. Introduction Platinum (Pt) is an essential component of numerous catalysts on account of its special and versatile capability of accelerating redox reactions of small molecules, especially for electro-oxidation of ethanol in acid media [1]. However, the high cost and limited reserve of Pt hinder its wide application [2,3]. Moreover, monometallic Pt tends to be poisoned by intermediate, such as CO [4]. It is therefore necessary to prepare a catalyst with high catalytic performance and small amount of Pt. It has demonstrated that the properties of the catalysts are strongly dependent on their composition and shape [5,6]. Hence partial substitution of Pt by a secondary metal and controlling of the structure of catalysts are effective strategies which can not only reduce the consumption of Pt but also exhibit superior performance due to a possible synergistic effect between Pt and nonPt elements [7,8]. Accordingly, Pt and Pd bimetallic nanostructures with various morphologies, including nanoparticles, nanocubes, nanodendrites and nanowires, have been prepared and shown improved electro-catalytic activity and durability [5,9,10]. Generally, Pt and Pd combine tightly [11]. In this case, the synergistic effect between Pt and Pd is hard to display fully, and electrolyte is hard to penetrate to the inner Pt and hence lead to the uncompleted usage of Pt. Therefore, more and more interests are n Corresponding authors at: College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China. E-mail addresses: [email protected] (H. Zhou), [email protected] (Y. Kuang).
http://dx.doi.org/10.1016/j.matlet.2016.03.012 0167-577X/& 2016 Published by Elsevier B.V.
focused on preparing Pt and Pd bimetallic composites which not only can display the synergistic effect between Pt and Pd, but also can make full use of Pt [12]. In this work, PS Pd@Pt NTs with a larger number of Pt nanoparticles around Pd NTs are fabricated for the first time. Interestingly, Pt NPs are discontinuously arrayed on Pd NTs and interweaved with each other to form a porous structure [13]. The asprepared composites possess high electrochemical active surface areas (ECSA) and the substrate Pd of the composites can directly contact with electrolyte, which will enhance electro-catalytic activity toward ethanol oxidation [14]. The electrochemical performance of the synthesized PS Pd@Pt NTs was evaluated by ethanol oxidation in acid media.
2. Material and methods The synthesis of Te nanowires (NWs) was following the literature with little modified [15]. Namely, polyvinylpyrrolidone (PVP K30), Na2TeO3, hydrazine hydrate (80%, w/w%) and aqueous ammonia solution (25%, w/w%) were mixed together and then transferred into Teflon-lined stainless-steel autoclave and maintained at 180 °C for 4 h. Finally, the product was washed several times by ethanol and water. The preparation of Pd NTs was as follows: The Te NWs were uniformly dispersed in water, and (NH4)2PdCl6 solution was added into the above suspension under stirring for 3 h. To prepare porous sponge-like Pd@Pt NTs, PVP (K30) and ascorbic acid were added into the pre-prepared Pd NTs suspension.
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W. Luo et al. / Materials Letters 173 (2016) 43–46
Subsequently, H2PtCl6 was added under stirring at 95 °C for 4 h. For comparison, Pt on Pd nanotubes were synthesized by a similar process in the absence of PVP. Morphologies and crystal structure of catalysts were characterized respectively by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). The working electrode for electrochemical measurements was fabricated as follows: 5 mg of theobtained product was uniformly dispersed in 5 mL deionized water. Then, 10 μL of the well-dispersed suspension was dropped onto a pre-polished glassy carbon electrode and dried at room temperature. After that, 5 μL of Nafion solution (0.5 wt%) was dropped onto the modified glassy carbon electrode and dried in air.
3. Results and discussion 3.1. Characterization of the PS Pd@Pt NTs Fig. 1a shows a large amount of the Pd nanotubes was successfully prepared. Fig. 1b shows the typical TEM image of the Pd NTs with an average outside diameter of 15 nm and inner diameter of 7 nm. Fig.1c shows the SEM image of the PS Pd@Pt NTs, which displays similar length with the Pd NTs but larger diameter of 45 nm. Fig. 1d shows the TEM image of the PS Pd@Pt NTs at a high magnification. It is observed that the numerous Pt particles connected together with some gaps and formed the sponge-like morphology with a thickness of 10 nm surrounded the Pd core. Fig. 1e shows a high resolution TEM (HRTEM) of the PS Pd@Pt NTs, and Pt particles with a mean diameter of 3 nm linked and intercrossed together to form the sponge-like Pt. The d spacing of 0.21 nm is assigned to the (111) plane of face-centered cubic Pt. Fig.1f shows the X-ray diffraction (XRD) pattern of the PS Pd@Pt NTs. The diffraction peak at 2θ ¼ 45.1° is assigned to the (200) of Pd, and the diffraction peaks at 2θ ¼39.8°, 67.8° and 81.8° are assigned to the(111), (220) and (311) of Pt respectively, confirming that the PS Pd@Pt NTs is composed of Pt and Pd. The domain size of the catalyst was also estimated by Scherrer equation using XRD [16].
Fig. 2. CV curves of Pt NWs, Pd NTs, Pt on Pd NTs and PS Pd@Pt NTs at a scan rate of 50 mV s 1 in N2 purged 0.5 M H2SO4 solution.
B=
Kγ βcosθ
Where B is the mean size of the ordered domains, which may be smaller or equal to the grain size; K is a dimensionless shape factor with a value of 0.9 here. λ is the X-ray wavelength of 0.154056 nm, β is the line broadening at half the maximum intensity, after subtracting the instrumental line broadening in radians, and θ is the Bragg angle. The domain size of the Pt was estimated to be 2.6 nm, which was very close to the value estimated from TEM. The domain size of the Pt was estimated to be 2.6 nm, which was very close to the value estimated from TEM. The contents of the Pt and Pd in the Pd@Pt nanotubes quantitatively determined by ICPAES were 53.3% and 46.7% respectively. 3.2. Electrochemical properties of the PS Pd@Pt NTs Fig. 2 shows the cyclic voltammetry (CV) curves of the Pt NWs, Pd NTs, Pt on Pd NTs and PS Pd@Pt NTs in N2 purged 0.5 M H2SO4 solution at a scan rate of 50 mV s 1. The Pt NWs, Pt on Pd NTs and
Fig. 1. (a) SEM and (b) high-resolution TEM images of Pd NTs, (c) SEM, (d) TEM and (e) high-resolution TEM images of PS Pd@Pt NTs, (f) XRD pattern of PS Pd@Pt NTs.
W. Luo et al. / Materials Letters 173 (2016) 43–46
45
Fig. 3. (a) CV curves at a scan rate of 50 mV s 1 and (b) chronoamperometric curves at 0.6 V of the Pt NWs, Pt on Pd NTs and PS Pd@Pt NTs in 0.5 M H2SO4 þ1 M CH3CH2OH solution.
PS Pd@Pt NTs electrodes display the typical adsorption and desorption characteristics in the potential range of 0.2–0.1 V, while there is no such a characteristic for the Pd NTs electrode. Additionally, the PS Pd@Pt NTs electrode shows the largest adsorption and desorption current densities as well as hydrogen adsorption peak area, indicating the largest electrochemical active surface area of the PS Pd@Pt NTs. Moreover, all electrodes display a pair of redox peaks in the potential range of 0.2–1.0 V due to the redox reaction of Pt and Pd. The PS Pd@Pt NTs and Pt on Pd NTs show better performances than pure Pt NWs, indicating that the addition of less active Pd catalyst into Pt not only didn't sacrifice the electrochemical activity, but also enhanced the activity. Also it is observed that PS Pd@Pt NTs have the typical a pair of redox peaks as similar as Pd NTs, indicating that PS Pd@Pt NTs have porous structure. The electrochemical surface area (ECSA) of the catalyst was also determined from the CV curves [17]. The ECSA of the PS Pd@Pt NTs was 188.1 m2 g 1, which was much larger than the ECSA values of the Pt NWs (44.0 m2 g 1) and Pt on Pd (153.3 m2 g 1). Fig. 3a shows the CV curves of the Pt NWs, Pt on Pd NTs and PS Pd@Pt NTs in 0.5 M H2SO4 þ1 M CH3CH2OH solution at a scan rate of 50 mV s 1. Each curve shows show an oxidation peak at 0.7 V during the forward scan and another oxidation peak during the reverse scan, which are the typical characteristics peak for ethanol electrochemical oxidation. The onset potential of the PS Pd@Pt NTs shifts shift negatively compared to the Pt NWs and Pt on Pd NTs. The peak current density of the PS Pd@Pt NTs electrode (164.3 A g 1) in the forward scan is almost 3 times as high as that of the Pt NWs (57.5 A g 1) and 2 times as high as that of the Pt on Pd NTs (75 A g 1). The enhanced performance is ascribed to the porous sponge core-shell structure of the PS Pd@Pt NTs and the synergistic effect between Pd and Pt. Fig. 3b shows the chronoamperometric curves of Pt NWs, Pt on Pd NTs and PS Pd@Pt NTs at 0.6 V in 0.5 M H2SO4 þ 1 M CH3CH2OH. All the three curves show similar trend, the current decreases first, and then reaches to a stable state. The initial rapid current decay is caused by the poisoning of catalysts. However, the current decrease on the PS Pd@Pt NTs electrode is much slower than on Pt NWs or Pt on Pd NTs. The initial current density of the PS Pd@Pt NTs is almost two times higher than that of Pt NWs or Pt on Pd NTs, and the stable current density of the PS Pd@Pt NTs electrode (11.89 A g 1) is 7 times as high as that of Pt NWs (1.69 A g 1) and almost 10 times as high as that of Pt on Pd NTs (1.27 A g 1), which might be explained that the porous sponge core-shell structure facilitated the exchange of intermediate species. The enhanced performance of the PS Pd@Pt NTs indicates that the addition of less active metal
into Pt is an effective way to not only reduce the usage of Pt, but also improve the materials' performance.
4. Conclusion In summary, the PS Pd@Pt NTs have been successfully prepared by a facile way. Numerous Pt NPs with a diameter of 2 nm discontinuously disperse onto the surface of Pd NTs, which formed the porous sponge-like structure of the PS Pd@Pt NTs. The asprepared PS Pd@Pt NTs exhibits significantly enhanced activity and stability towards ethanol electro-oxidation in acid medium, demonstrating an effective way to enhance electrochemical performance of Pt-based catalysts with a small amount of Pt.
Acknowledgements This work was supported by National Natural Science Foundation of China (Grant no.51071067, 21271069, J1210040, 51238002, J1103312), Science and Technology Program of Hunan Province (No.2015JC3049) and Hunan Provincial Innovation Foundation for Postgraduate (No.CX2015B083).
References [1] S. Guo, S. Dong, E. Wang, Three-dimensional Pt-on-Pd bimetallic nanodendrites supported on graphene nanosheet: facile synthesis and used as an advanced nanoelectrocatalyst for methanol oxidation, ACS Nano. 4 (2009) 547–555. [2] Y. Hao, Y. Yang, L. Hong, J. Yuan, L. Niu, Y. Gui, Facile preparation of Ultralong dendritic PtIrTe nanotubes and their high electrocatalytic activity on methanol oxidation, ACS Appl. Mater. Interfaces 6 (2014) 21986–21994. [3] R. Wang, D.C. Higgins, M.A. Hoque, D. Lee, F. Hassan, Z. Chen, Controlled growth of platinum nanowire arrays on sulfur doped graphene as high performance electrocatalyst, Sci. Rep. (2013) 3. [4] Z. Peng, H. Yang, Synthesis and oxygen reduction electrocatalytic property of Pt-on-Pd bimetallic heteronanostructures, J. Am. Chem. Soc. 131 (2009) 7542–7543. [5] X. Huang, Y. Li, Y. Li, H. Zhou, X. Duan, Y. Huang, Synthesis of PtPd bimetal nanocrystals with controllable shape, composition, and their tunable catalytic properties, Nano Lett. 12 (2012) 4265–4270. [6] J. Yang, J. Yang, J.Y. Ying, Morphology and lateral strain control of Pt nanoparticles via core–shell construction using alloy AgPd core toward oxygen reduction reaction, ACS Nano. 6 (2012) 9373–9382. [7] G. Gupta, D.A. Slanac, P. Kumar, J.D. Wiggins-Camacho, X. Wang, S. Swinnea, et al., Highly stable and active Pt–Cu oxygen reduction electrocatalysts based on mesoporous graphitic carbon supports, Chem. Mater. 21 (2009) 4515–4526. [8] H. Zhang, M. Jin, J. Wang, W. Li, P.H. Camargo, M.J. Kim, et al., Synthesis of Pd– Pt bimetallic nanocrystals with a concave structure through a bromide-induced galvanic replacement reaction, J. Am. Chem. Soc. 133 (2011) 6078–6089.
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[9] S. Xie, S.-I. Choi, N. Lu, L.T. Roling, J.A. Herron, L. Zhang, et al., Atomic layer-bylayer deposition of Pt on Pd nanocubes for catalysts with enhanced activity and durability toward oxygen reduction, Nano Lett. 14 (2014) 3570–3576. [10] T. Yang, G. Cao, Q. Huang, Y. Ma, S. Wan, H. Zhao, et al., Surface-limited synthesis of Pt nanocluster decorated Pd hierarchical structures with enhanced electrocatalytic activity toward oxygen reduction reaction, ACS Appl. Mater. Interfaces 7 (2015) 17162–17170. [11] L. Wang, Y. Yamauchi, Metallic nanocages: synthesis of bimetallic Pt–Pd hollow nanoparticles with dendritic shells by selective chemical etching, J. Am. Chem. Soc. 135 (2013) 16762–16765. [12] Z. Huang, H. Zhou, F. Sun, C. Fu, F. Zeng, T. Li, et al., Facile self-assembly synthesis of PdPt bimetallic nanotubes with good performance for ethanol oxidation in an alkaline medium, Chem.-A Eur. J. 19 (2013) 13720–13725. [13] L. Guo, Q.J. Huang, X.Y. Li, S. Yang, PVP-coated iron nanocrystals: anhydrous synthesis, characterization, and electrocatalysis for two species, Langmuir 22
(2006) 7867–7872. [14] B. Lim, M. Jiang, T. Yu, P.H. Camargo, Y. Xia, Nucleation and growth mechanisms for Pd-Pt bimetallic nanodendrites and their electrocatalytic properties, Nano Res. 3 (2010) 69–80. [15] H.W. Liang, S. Liu, J.Y. Gong, S.B. Wang, L. Wang, S.H. Yu, Ultrathin Te nanowires: an excellent platform for controlled synthesis of ultrathin platinum and palladium nanowires/nanotubes with very high aspect ratio, Adv. Mater. 21 (2009) 1850–1854. [16] F. Afsahi, H. Vinh-Thang, S. Mikhailenko, S. Kaliaguine, Electrocatalyst synthesized from metal organic frameworks, J. Power Sources 239 (2013) 415–423. [17] Z. Huang, H. Zhou, Z. Chen, F. Zeng, L. Chen, W. Luo, et al., Facile synthesis of porous Pt botryoidal nanowires and their electrochemical properties, Electrochim. Acta 147 (2014) 643–649.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Preparation and characterization of porous sponge-like Pd@Pt nanotubes with high catalytic activity for ethanol oxidation Wucheng Luo a,b, Haihui Zhou a,b,n, Chaopeng Fu a,b, Zhongyuan Huang a,b, Na Gao a,b, Yafei Kuang a,b,n a b
State Key Laboratory for Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, PR China College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 14 December 2015 Received in revised form 29 February 2016 Accepted 4 March 2016 Available online 5 March 2016
Porous sponge-like Pd@Pt bimetallic nanotubes (PS Pd@Pt NTs) were prepared by a two-step method for the first time. The as-prepared PS Pd@Pt have an outer diameter of 45 nm and a thickness of 5 nm, numerous Pt nanoparticles distribute discontinuously around the surface of Pd NTs. The electro-catalytic behavior of the PS Pd@Pt NTs for ethanol oxidation was studied by using cyclic voltammetry and chronoamperometry. The results indicated that PS Pd@Pt NTs exhibit higher catalytic activity and better stability than Pt nanowires and Pt on Pd nanotubes. & 2016 Published by Elsevier B.V.
Keywords: Porous sponge-like Pd@Pt nanotubes Nanocomposites Electrical properties
1. Introduction Platinum (Pt) is an essential component of numerous catalysts on account of its special and versatile capability of accelerating redox reactions of small molecules, especially for electro-oxidation of ethanol in acid media [1]. However, the high cost and limited reserve of Pt hinder its wide application [2,3]. Moreover, monometallic Pt tends to be poisoned by intermediate, such as CO [4]. It is therefore necessary to prepare a catalyst with high catalytic performance and small amount of Pt. It has demonstrated that the properties of the catalysts are strongly dependent on their composition and shape [5,6]. Hence partial substitution of Pt by a secondary metal and controlling of the structure of catalysts are effective strategies which can not only reduce the consumption of Pt but also exhibit superior performance due to a possible synergistic effect between Pt and nonPt elements [7,8]. Accordingly, Pt and Pd bimetallic nanostructures with various morphologies, including nanoparticles, nanocubes, nanodendrites and nanowires, have been prepared and shown improved electro-catalytic activity and durability [5,9,10]. Generally, Pt and Pd combine tightly [11]. In this case, the synergistic effect between Pt and Pd is hard to display fully, and electrolyte is hard to penetrate to the inner Pt and hence lead to the uncompleted usage of Pt. Therefore, more and more interests are n Corresponding authors at: College of Chemistry and Chemical Engineering, Hunan University, Changsha 410082, PR China. E-mail addresses: [email protected] (H. Zhou), [email protected] (Y. Kuang).
http://dx.doi.org/10.1016/j.matlet.2016.03.012 0167-577X/& 2016 Published by Elsevier B.V.
focused on preparing Pt and Pd bimetallic composites which not only can display the synergistic effect between Pt and Pd, but also can make full use of Pt [12]. In this work, PS Pd@Pt NTs with a larger number of Pt nanoparticles around Pd NTs are fabricated for the first time. Interestingly, Pt NPs are discontinuously arrayed on Pd NTs and interweaved with each other to form a porous structure [13]. The asprepared composites possess high electrochemical active surface areas (ECSA) and the substrate Pd of the composites can directly contact with electrolyte, which will enhance electro-catalytic activity toward ethanol oxidation [14]. The electrochemical performance of the synthesized PS Pd@Pt NTs was evaluated by ethanol oxidation in acid media.
2. Material and methods The synthesis of Te nanowires (NWs) was following the literature with little modified [15]. Namely, polyvinylpyrrolidone (PVP K30), Na2TeO3, hydrazine hydrate (80%, w/w%) and aqueous ammonia solution (25%, w/w%) were mixed together and then transferred into Teflon-lined stainless-steel autoclave and maintained at 180 °C for 4 h. Finally, the product was washed several times by ethanol and water. The preparation of Pd NTs was as follows: The Te NWs were uniformly dispersed in water, and (NH4)2PdCl6 solution was added into the above suspension under stirring for 3 h. To prepare porous sponge-like Pd@Pt NTs, PVP (K30) and ascorbic acid were added into the pre-prepared Pd NTs suspension.
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W. Luo et al. / Materials Letters 173 (2016) 43–46
Subsequently, H2PtCl6 was added under stirring at 95 °C for 4 h. For comparison, Pt on Pd nanotubes were synthesized by a similar process in the absence of PVP. Morphologies and crystal structure of catalysts were characterized respectively by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). The working electrode for electrochemical measurements was fabricated as follows: 5 mg of theobtained product was uniformly dispersed in 5 mL deionized water. Then, 10 μL of the well-dispersed suspension was dropped onto a pre-polished glassy carbon electrode and dried at room temperature. After that, 5 μL of Nafion solution (0.5 wt%) was dropped onto the modified glassy carbon electrode and dried in air.
3. Results and discussion 3.1. Characterization of the PS Pd@Pt NTs Fig. 1a shows a large amount of the Pd nanotubes was successfully prepared. Fig. 1b shows the typical TEM image of the Pd NTs with an average outside diameter of 15 nm and inner diameter of 7 nm. Fig.1c shows the SEM image of the PS Pd@Pt NTs, which displays similar length with the Pd NTs but larger diameter of 45 nm. Fig. 1d shows the TEM image of the PS Pd@Pt NTs at a high magnification. It is observed that the numerous Pt particles connected together with some gaps and formed the sponge-like morphology with a thickness of 10 nm surrounded the Pd core. Fig. 1e shows a high resolution TEM (HRTEM) of the PS Pd@Pt NTs, and Pt particles with a mean diameter of 3 nm linked and intercrossed together to form the sponge-like Pt. The d spacing of 0.21 nm is assigned to the (111) plane of face-centered cubic Pt. Fig.1f shows the X-ray diffraction (XRD) pattern of the PS Pd@Pt NTs. The diffraction peak at 2θ ¼ 45.1° is assigned to the (200) of Pd, and the diffraction peaks at 2θ ¼39.8°, 67.8° and 81.8° are assigned to the(111), (220) and (311) of Pt respectively, confirming that the PS Pd@Pt NTs is composed of Pt and Pd. The domain size of the catalyst was also estimated by Scherrer equation using XRD [16].
Fig. 2. CV curves of Pt NWs, Pd NTs, Pt on Pd NTs and PS Pd@Pt NTs at a scan rate of 50 mV s 1 in N2 purged 0.5 M H2SO4 solution.
B=
Kγ βcosθ
Where B is the mean size of the ordered domains, which may be smaller or equal to the grain size; K is a dimensionless shape factor with a value of 0.9 here. λ is the X-ray wavelength of 0.154056 nm, β is the line broadening at half the maximum intensity, after subtracting the instrumental line broadening in radians, and θ is the Bragg angle. The domain size of the Pt was estimated to be 2.6 nm, which was very close to the value estimated from TEM. The domain size of the Pt was estimated to be 2.6 nm, which was very close to the value estimated from TEM. The contents of the Pt and Pd in the Pd@Pt nanotubes quantitatively determined by ICPAES were 53.3% and 46.7% respectively. 3.2. Electrochemical properties of the PS Pd@Pt NTs Fig. 2 shows the cyclic voltammetry (CV) curves of the Pt NWs, Pd NTs, Pt on Pd NTs and PS Pd@Pt NTs in N2 purged 0.5 M H2SO4 solution at a scan rate of 50 mV s 1. The Pt NWs, Pt on Pd NTs and
Fig. 1. (a) SEM and (b) high-resolution TEM images of Pd NTs, (c) SEM, (d) TEM and (e) high-resolution TEM images of PS Pd@Pt NTs, (f) XRD pattern of PS Pd@Pt NTs.
W. Luo et al. / Materials Letters 173 (2016) 43–46
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Fig. 3. (a) CV curves at a scan rate of 50 mV s 1 and (b) chronoamperometric curves at 0.6 V of the Pt NWs, Pt on Pd NTs and PS Pd@Pt NTs in 0.5 M H2SO4 þ1 M CH3CH2OH solution.
PS Pd@Pt NTs electrodes display the typical adsorption and desorption characteristics in the potential range of 0.2–0.1 V, while there is no such a characteristic for the Pd NTs electrode. Additionally, the PS Pd@Pt NTs electrode shows the largest adsorption and desorption current densities as well as hydrogen adsorption peak area, indicating the largest electrochemical active surface area of the PS Pd@Pt NTs. Moreover, all electrodes display a pair of redox peaks in the potential range of 0.2–1.0 V due to the redox reaction of Pt and Pd. The PS Pd@Pt NTs and Pt on Pd NTs show better performances than pure Pt NWs, indicating that the addition of less active Pd catalyst into Pt not only didn't sacrifice the electrochemical activity, but also enhanced the activity. Also it is observed that PS Pd@Pt NTs have the typical a pair of redox peaks as similar as Pd NTs, indicating that PS Pd@Pt NTs have porous structure. The electrochemical surface area (ECSA) of the catalyst was also determined from the CV curves [17]. The ECSA of the PS Pd@Pt NTs was 188.1 m2 g 1, which was much larger than the ECSA values of the Pt NWs (44.0 m2 g 1) and Pt on Pd (153.3 m2 g 1). Fig. 3a shows the CV curves of the Pt NWs, Pt on Pd NTs and PS Pd@Pt NTs in 0.5 M H2SO4 þ1 M CH3CH2OH solution at a scan rate of 50 mV s 1. Each curve shows show an oxidation peak at 0.7 V during the forward scan and another oxidation peak during the reverse scan, which are the typical characteristics peak for ethanol electrochemical oxidation. The onset potential of the PS Pd@Pt NTs shifts shift negatively compared to the Pt NWs and Pt on Pd NTs. The peak current density of the PS Pd@Pt NTs electrode (164.3 A g 1) in the forward scan is almost 3 times as high as that of the Pt NWs (57.5 A g 1) and 2 times as high as that of the Pt on Pd NTs (75 A g 1). The enhanced performance is ascribed to the porous sponge core-shell structure of the PS Pd@Pt NTs and the synergistic effect between Pd and Pt. Fig. 3b shows the chronoamperometric curves of Pt NWs, Pt on Pd NTs and PS Pd@Pt NTs at 0.6 V in 0.5 M H2SO4 þ 1 M CH3CH2OH. All the three curves show similar trend, the current decreases first, and then reaches to a stable state. The initial rapid current decay is caused by the poisoning of catalysts. However, the current decrease on the PS Pd@Pt NTs electrode is much slower than on Pt NWs or Pt on Pd NTs. The initial current density of the PS Pd@Pt NTs is almost two times higher than that of Pt NWs or Pt on Pd NTs, and the stable current density of the PS Pd@Pt NTs electrode (11.89 A g 1) is 7 times as high as that of Pt NWs (1.69 A g 1) and almost 10 times as high as that of Pt on Pd NTs (1.27 A g 1), which might be explained that the porous sponge core-shell structure facilitated the exchange of intermediate species. The enhanced performance of the PS Pd@Pt NTs indicates that the addition of less active metal
into Pt is an effective way to not only reduce the usage of Pt, but also improve the materials' performance.
4. Conclusion In summary, the PS Pd@Pt NTs have been successfully prepared by a facile way. Numerous Pt NPs with a diameter of 2 nm discontinuously disperse onto the surface of Pd NTs, which formed the porous sponge-like structure of the PS Pd@Pt NTs. The asprepared PS Pd@Pt NTs exhibits significantly enhanced activity and stability towards ethanol electro-oxidation in acid medium, demonstrating an effective way to enhance electrochemical performance of Pt-based catalysts with a small amount of Pt.
Acknowledgements This work was supported by National Natural Science Foundation of China (Grant no.51071067, 21271069, J1210040, 51238002, J1103312), Science and Technology Program of Hunan Province (No.2015JC3049) and Hunan Provincial Innovation Foundation for Postgraduate (No.CX2015B083).
References [1] S. Guo, S. Dong, E. Wang, Three-dimensional Pt-on-Pd bimetallic nanodendrites supported on graphene nanosheet: facile synthesis and used as an advanced nanoelectrocatalyst for methanol oxidation, ACS Nano. 4 (2009) 547–555. [2] Y. Hao, Y. Yang, L. Hong, J. Yuan, L. Niu, Y. Gui, Facile preparation of Ultralong dendritic PtIrTe nanotubes and their high electrocatalytic activity on methanol oxidation, ACS Appl. Mater. Interfaces 6 (2014) 21986–21994. [3] R. Wang, D.C. Higgins, M.A. Hoque, D. Lee, F. Hassan, Z. Chen, Controlled growth of platinum nanowire arrays on sulfur doped graphene as high performance electrocatalyst, Sci. Rep. (2013) 3. [4] Z. Peng, H. Yang, Synthesis and oxygen reduction electrocatalytic property of Pt-on-Pd bimetallic heteronanostructures, J. Am. Chem. Soc. 131 (2009) 7542–7543. [5] X. Huang, Y. Li, Y. Li, H. Zhou, X. Duan, Y. Huang, Synthesis of PtPd bimetal nanocrystals with controllable shape, composition, and their tunable catalytic properties, Nano Lett. 12 (2012) 4265–4270. [6] J. Yang, J. Yang, J.Y. Ying, Morphology and lateral strain control of Pt nanoparticles via core–shell construction using alloy AgPd core toward oxygen reduction reaction, ACS Nano. 6 (2012) 9373–9382. [7] G. Gupta, D.A. Slanac, P. Kumar, J.D. Wiggins-Camacho, X. Wang, S. Swinnea, et al., Highly stable and active Pt–Cu oxygen reduction electrocatalysts based on mesoporous graphitic carbon supports, Chem. Mater. 21 (2009) 4515–4526. [8] H. Zhang, M. Jin, J. Wang, W. Li, P.H. Camargo, M.J. Kim, et al., Synthesis of Pd– Pt bimetallic nanocrystals with a concave structure through a bromide-induced galvanic replacement reaction, J. Am. Chem. Soc. 133 (2011) 6078–6089.
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