- Email: [email protected]
Facile synthesis of PdAgTe nanowires with superior electrocatalytic activity
Journal of Power Sources 272 (2014) 940e945
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Facile synthesis of PdAgTe nanowires with superior electrocatalytic activity Wei Hong a, b, Jin Wang a, c, *, Erkang Wang a, b, * a
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China b University of Chinese Academy of Sciences, Beijing 100039, China c Department of Chemistry and Physics, State University of New York at Stony Brook, New York, NY 11794-3400, USA
h i g h l i g h t s PdAgTe nanowires have been synthesized by a simple method. Te nanowires was used as the sacrificial template. Cyclic voltammetry was used to obtain a highly active and stable surface. The as-prepared catalysts exhibit superior electrocatalytic activity.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 July 2014 Received in revised form 30 August 2014 Accepted 3 September 2014 Available online 16 September 2014
In this work, ultrathin Te nanowires (NWs) with high-aspect-ratio are prepared by a simple hydrothermal method. By using Te NWs as the sacrificial template, we demonstrate a facile and efficient method for the synthesis of PdAgTe NWs with high-quality through the partly galvanic replacement between Te NWs and the corresponding noble metal salts precursors in an aqueous solution. The compositions of PdAgTe NWs can be tuned by simply altering the concentration of the precursors. After cyclic voltammetry treatment, multi-component PdAgTe NW with a highly active and stable surface can be obtained. The structure and composition of the as-prepared nanomaterials are analyzed by transmission electron microscope, X-ray diffraction, energy dispersive X-ray spectroscopy, inductively coupled plasma-mass spectroscopy and X-ray photoelectron spectroscopy. Electrochemical catalytic measurement results prove that the as synthesized PdAgTe NWs present superior catalytic activity toward ethanol electrooxidation in alkaline solution than the commercial Pd/C catalyst, which making them can be used as effective catalysts for the direct ethanol fuel cells. © 2014 Published by Elsevier B.V.
Keywords: Palladium Silver Tellurium Nanowires Catalytic activity
1. Introduction As a potential viable alternative for environmentally friendly and highly efficient electric power generation device, direct fuel cells (DFCs) have been attracting increasing attention in recent years [1e3]. Among the different types of commonly used DFCs, direct ethanol fuel cells (DEFCs) are receiving special attention due to its obvious advantages, such as lower toxicity and higher energy density compared to their counterparts such as direct methanol
* Corresponding authors. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. Fax: þ86 43185689711. E-mail addresses: [email protected] (J. Wang), [email protected] (E. Wang). http://dx.doi.org/10.1016/j.jpowsour.2014.09.029 0378-7753/© 2014 Published by Elsevier B.V.
fuel cells and direct formic acid fuel cells [4e6]. Moreover, ethanol is a renewable biofuel which can be obtained from the fermentation of biomass in large quantities. To improve the utilization efficiency of ethanol and energy conversion efficiency, two general methods have been developed. One way is by operating DEFCs in alkaline solution, in which the kinetics of ethanol electrooxidation reaction can be greatly improved. Another efficient way is to by employing electrode catalysts with high catalytic performance [7e9]. However, there are still some problems exist in the development of DEFCs which will severely prevent their ultimate commercialization, such as high cost, slow kinetics and poor durability of the anode catalysts. Maximum reducing the usage of high-cost noble metals while without cutting down their catalytic activity of a catalyst is still highly desired and technologically important for the final practical commercialization of DEFCs [10,11].
W. Hong et al. / Journal of Power Sources 272 (2014) 940e945
Pd-based catalysts are emerging as a hot focus for ethanol oxidation owe to their relatively lower cost, higher abundance and greater resistance to CO as compared to Pt-based catalysts [12e14]. In the light of these considerations, considerable efforts have been devoted to developing simple and practical techniques to prepare Pd-based electrocatalysts with enhanced performance for the DEFCs [15e17]. To achieve this goal, one way is to effectively control the morphology of the nanostructures, of which can provide a great opportunity to improve its surface area and thus increase its activity on a mass or specific basis. In this regard, many Pd-based nanomaterials with different morphologies have been developed for DEFCs, such as nanocubes [18], hollow nanospheres [19], porous nanospheres [20,21]. Among these developed materials, onedimensional nanostructures are receiving special attention recently, owing to their various potential applications deriving from their unique optical, electronic, magnetic, catalytic, and sensing properties [22,23]. Another efficient way to improve the catalytic performance of a catalyst is the use of bimetallic/multimetallic surfaces. The incorporation of a secondly transition metals has been proved to be can greatly improve the catalytic performance of a catalyst due to the synergetic effect between the different components. Materials such as PdAu and PdPt nanocrystals have been synthesized for DEFCs, and enhanced catalytic performance has been achieved [24e26]. Take account of the above considerations, not surprisingly, onedimensional material such as nanowires (NWs) with multicomponents is highly desired for the DEFCs. In this regard, Dong's group reported the synthesis of PdPt and AuPdPt NWs with enhanced electrocatalytic activity for ethanol electrooxidation [27,28]. However, these materials are not economical, because Au and Pt are very expensive noble metals. What's more, in their experiment, the Pt or Au precursors used are 2e4 fold excess compared to Te in order to replace the Te NWs completely, and the excess precursors are almost all wasted. It is still of great significance to lower the cost by reducing the usage of the expensive noble metals such as Pt and Au. In this regard, Ag, as a much cheaper metal, their incorporation into Pd-based materials for the DEFCs has been studied relatively less. And as far as we know, there are still no reports about the well-defined one-dimensional PdAgTe nanowires. In this work, PdAgTe NWs were prepared through the partly galvanic replacement by employing Te NWs with high-quality as the sacrificial templates. The morphology and composition of the obtained PdAgTe NWs were characterized with transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), inductively coupled plasmamass spectroscopy (ICP-MS) and X-ray photoelectron spectroscopy (XPS). The catalytic activity and stability of the prepared catalysts toward ethanol electrooxidation were investigated by cyclic voltammetry (CV) and chronoamperometry methods in alkaline media. 2. Experimental 2.1. Materials Palladium nitrate, hydrazine hydrate (50% w/w%), ammonia solution (25 wt.%), polyvinylpyrrolidone (PVP K30, Mw 55000e58000), and sodium tellurite were obtained from Sinopharm (Shanghai, China). Nafion ethanol solution (5 wt.%) was obtained from Aldrich. Silver nitrate and commercial Pd/C (10 wt.%) were purchased from Aladdin Chemistry Co. Ltd (Shanghai, China). All chemicals used were of analytical grade and used as received without further purification. Milli-Q ultrapure water (Millipore, 18.2 MU cm) was used throughout the experiments.
941
2.2. Synthesis of Te nanowires The process was according to the reported literature with small modifications [29]. Briefly, 0.092 g sodium tellurite (Na2TeO3) and 1 g PVP were dissolved in 30 mL of water. Then, 1.65 mL of hydrazine hydrate (50% w/w) and 3.35 mL of ammonia solution (25% w/w) were added under vigorous magnetic stirring to form a homogeneous solution at room temperature. Then, the obtained solution was transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 180 C for 4 h. Finally, the product was cooled to room temperature and centrifuged with acetone, washed with water, and finally dissolved in 35 mL of water. 2.3. Synthesis of PdAgTe nanowires To synthesize PdAgTe nanowires, 1 mL of the above Te nanowires was added into 15 mL of water under stirring and maintained at 40 C in a water bath. Then a mixture containing 1 mL 5 mg mL1 Pd(NO3)2 and 0.2 mL 0.1 mol L1AgNO3 was added quickly. After react for 20 min, the product was centrifuged and washed several times with ultrapure water and finally collected for further characterizations. For comparison, another types of PdAgTe nanowires with different compositions were also prepared just by increased the volume of AgNO3 to 0.4 mL and 0.6 mL respectively. 2.4. Material structure and composition characterizations The morphology and structure of the as prepared catalysts were analyzed with a HITACHI H-600 Analytical TEM with an accelerating voltage of 100 kV. Energy dispersive X-ray spectrum (EDS) was made on a JEM-2100F high-resolution transmission electron microscope operating at 200 kV. The exact composition of PdAgTe NWs was determined by ICP-MS (X Series 2, Thermo Scientific USA). XRD pattern of PdAgTe NWs was performed on a D8 ADVANCE (BRUKER, Germany) diffractometer using CuKa radiation with a Ni filter (l ¼ 0.154059 nm at 30 kV and 15 mA). XPS measurement was performed on an ESCALAB-MKII spectrometer (VG Co., United Kingdom) with Al Ka X-ray radiation as the X-ray source for excitation. 2.5. Electrochemical catalytic experiment toward ethanol electrooxidation For electrochemical measurements, a glassy carbon electrode (GCE) was employed as working electrode, while a platinum wire was used as the counter electrode and KCl saturated Ag/AgCl electrode as the reference electrode respectively. GCE was sequentially polished carefully with 1.0, 0.3, and 0.05 mm alumina slurry and then rinsed with deionized water followed by sonication in ethanol and Milli-Q ultrapure water. For electrooxidation test, 5 mL of commercial Pd/C or PdAgTe catalysts solution was dropped on the surface of the GCE and dried with an infrared lamp carefully. The Pd loading mass of all the prepared PdAgTe NWs and commercial Pd/C catalyst was 35.3 mg cm2. After that, 5 mL of Nafion (0.5% wt) was coated on the surface of the above material modified GCE and dried before electrochemical experiments. All the electrochemical measurements were carried out on a CHI 832B electrochemical workstation, Chenhua Instruments corp (Shanghai, China). 3. Results and discussion 3.1. TEM, EDS, ICP-OES, XRD and XPS characterizations of the PdAgTe nanowires Figs. 1(A and B) shows the typical TEM images of the prepared Te NWs. The as-synthesized Te NWs present an average diameter of
942
W. Hong et al. / Journal of Power Sources 272 (2014) 940e945
about 10 nm, while with a length of several micron meters. By using Te NWs as the template, PdAgTe NWs can be obtained. The asprepared PdAgTe NWs present a high-aspect-ratio (Fig. 1C and D), demonstrating that the developed method is effective. EDS of the product demonstrates that the prepared catalyst is purely composed of Pd, Ag and Te elements (Fig. 2B), the peaks of Cu and C element were originated from the carbon coated copper grid. The exact composition of as-prepared catalyst was determined by ICPMS results and calculated to be Pd93Ag24Te55. The loading weight of Pd on the electrode can also be calculated exactly and further used to compare with that of commercial Pd/C catalyst during the electrochemical measurements. The X-ray diffraction (XRD) pattern of the as-prepared catalyst is shown in Fig. 2A. The intense peak at around 30 (2theta) demonstrates presence of pure Te phases which shows that the ternary component catalyst is not well alloyed. Compared with those of the pure Pd, Ag, or Te phases, all the diffraction peaks of the PdAgTe NWs did not present any obvious shift of diffraction angles, such a consistency could also be an evidence for the not alloyed formation among Pd, Ag, and Te [30,31]. The surface chemistry state of as-prepared multicomponent PdAgTe NWs was characterized by XPS. Fig. 2(CeE) shows the Te 3d, Pd 3d and Ag 3d core level spectrum of the PdAgTe NWs. There are two intense peaks at 573.11 and 583.53 eV which assigned to Te 3d5/2 and Te 3d3/2, respectively (Fig. 2C). Along with the two main tellurium peaks, two relative weak features were observed at 575.55 and 586.11 eV, which can be indexed to Te4þ, due to presence of TeO2 [32,33]. According to Te/Te4þ intensity ratio, it can be known that both Te and Te4þ species are dominantly exist in the NWs, which can be further an evidence to prove the mix-phase rather than alloy state of the as-prepared NWs. Similarly, the peaks at the binding energy values of 335.52 and 340.97 eV are corresponding to the Pd 3d5/2 and Pd 3d3/2, respectively. Two more peaks at 337.01 and 342.48 eV indicating the presence of Pd2þ species [34]. Similarly, based on the Pd/Pd2þ intensity ratio, it can be known that the zero-valent state Pd species
are predominant in the prepared NWs. The presence of Pd2þ species may be originated from the using of Pd(NO3)2 aqueous solution, because the solubility of Pd(NO3)2 in neutral water is not very well, which may be lead to the dissociation for the formation of Pd(OH)2. Similar observations can also be found in the reported literature [34]. As for the Ag, the peaks at the binding energy values of 368.0 and 373.9 eV correspond to Ag 3d5/2 and Ag 3d3/2, respectively, indicating that Ag is mainly present in the metallic state. 3.2. Electrocatalytic measurements for ethanol electrooxidation Inspired by their special nanostructure, we investigated the catalytic performance of the as-prepared catalyst for the ethanol electrooxidation. The catalytic activity of the as-prepared PdAgTe NWs toward ethanol electrooxidation in alkaline condition was investigated by an electrochemical measurement system and further compared with a commercial Pd/C catalyst. Mass activity was used to evaluate the catalytic activity of the catalysts. Electrooxidation of ethanol was carried out in an aqueous solution containing 0.5 mol L1 sodium hydroxide and 1 mol L1 ethanol, using CV measurement technique, sweeping from 0.8 V to 0.4 V at a scan rate of 50 mV s1. Considering the presence of the unstable Te atoms in the catalyst, we treated the catalyst in 0.5 mol L1 H2SO4 (aqueous solution) by using CV technique with a scan rate at 50 mV s1 before the catalytic measurements. The profiles of the treatment process can be found in Fig. 3. The peak between 0.6 and 0.9 V in the cycles can be attributed to the dissolution of Te atoms from the surface of the PdAgTe NWs in the acidic environment [35,36], thus leaving a more stable and Pd enriched surface which can further improving their catalytic performance. The TEM images of the as-prepared nanowires after CV treatment are presented in Fig. S3.,Fig. 4 shows the mass catalytic activity for the ethanol oxidation of the PdAgTe NWs with/without CV treatment in H2SO4 solution. Without treatment, the mass activity of the as-prepared
Fig. 1. (A, B) Typical TEM images of the Te NWs with different magnifications (C, D) TEM images of the as-prepared PdAgTe NWs with different magnifications.
W. Hong et al. / Journal of Power Sources 272 (2014) 940e945
943
Fig. 2. XRD pattern (A) and EDS (B) of the as-prepared Pd93Ag24Te55 NWs. XPS spectrum of as-prepared Pd93Ag24Te55 NWs, Te 3d (C), Pd 3d (D) and Ag 3d (E).
catalyst is significantly lower than the treated one, and many miscellaneous peaks can be observed. In sharp contrast, there are only two well separated peaks presented in the CV curve of the ethanol oxidation of the treat one. The results strongly suggest that after CV treatment, a highly active and stable surface can be obtained and thus lead to enhanced catalytic activity. Considering the fact that composition is a very important role that governing the catalytic activity of a catalyst, another two types of PdAgTe NWs with different compositions have also successfully prepared. The compositions were determined to be Pd72Ag23Te62 and Pd53Ag28Te53 respectively by the ICP-MS results. Some of the NW tended to break at some extent with Ag content increased (Fig. S2, Supplementary data). It can be noted that the atomic ratio of Ag in the three types of PdAgTe NWs with different compositions did not increased obviously along with the increased amount of AgNO3 precursors added. The result may be attributed to the slow reaction rate of the Ag. Similarly, another two types of the prepared NWs with different compositions were also treated by CV in H2SO4 solution before the catalytic measurements. Fig. 5 shows the mass
Fig. 3. CV profiles of PdAgTe NWs recorded in 0.5 mol L1 H2SO4 solution at a sweep rate of 50 mV se1. The anodic peak of the cycles between 0.6 and 0.9 V is attributed to the dissolusion of Te from Pd93Ag24Te55 NWs.
activity of the prepared catalysts and commercial Pd/C toward ethanol electrooxidation. It can be noted clearly that Pd93Ag24Te55 NWs possess the highest mass activity, suggesting that appropriate composition is favorable for the best catalytic activity. The forward anodic peak of Pd93Ag24Te55 is 1.43 A mg1Pd, about 11 times of commercial Pd/C of 0.13 A mg1Pd as illustrated in Fig. 5, indicating that the as prepared Pd93Ag24Te55 possess an excellent catalytic activity. The value is also much higher than 0.6 A mg1Pd of the free-standing Pd nanomembranes [37], 0.55 A mg1Pd of Pd/GO/ CFP [38], 0.9 A mg1Pd of Pd/rGO/CFP [38], 0.9 A mg1Pd of Pd NWs [27]. Learned from Fig. 5, it can also be found that the peak value in backward scan of Pd93Ag24Te55 NWs is 1.18 A mg1Pd, which is also much higher than that of the commercial Pd/C. Moreover, all of the prepared PdAgTe NW present a much higher mass activity value than the commercial Pd/C catalyst, which prove that all the asprepared catalysts possess superior catalytic activity for the ethanol oxidation. The stability of PdAgTe NWs and Pd/C toward ethanol electrooxidation was investigated by chronoamperometry technique at 0.2 V as presented in Fig. 6. Pd93Ag24Te55 NWs show a distinct slower current decay than another two types of PdAgTe NWs and commercial Pd/C over the whole 4000 s test range, suggesting that Pd93Ag24Te55 NWs possess the best long-term durability. Apart from this, it can also be found that all the prepared NWs show a much slower current decay than the commercial Pd/C catalyst. The results from both activity and stability studies indicate that the asprepared PdAgTe NWs possess superior catalytic performance for ethanol electrooxidation in alkaline media. The superior electrocatalytic activity of PdAgTe NWs for the ethanol electrooxidation should be attributed to the following reasons. Firstly, the contribution of the one-dimensional nanostructure. As revealed by the literature, one-dimensional structure can maintain improved electron transport characteristics during the electro-catalysis as a result of the path-directing effects of the structural anisotropy [22,23]. Moreover, the NWs as the selfsupported catalyst, are less vulnerable to dissolution, Ostwald ripening, and aggregation during electrocatalytic processes due to the micrometresized length [39,40]. In addition, the small diameter of the NWs can offer more opportunity to provide more exposed
944
W. Hong et al. / Journal of Power Sources 272 (2014) 940e945
Fig. 4. The mass activity of Pd93Ag24Te55 NWs toward ethanol electrooxidation in a solution containing 1 mol L1 ethanol þ 0.5 mol L1 sodium hydroxide, without (A) and with (B) CV electrochemical treatment in 0.5 mol L1 H2SO4 solution before the electrocatalytic measurements.
surface atoms thus lead to more active sites. Secondly, the incorporation of Ag and Te. It is generally believed that a CH3COads intermediate is formed on the Pd catalyst in the alkaline medium, and its reaction with OHads is the rate-determining step [41]. The cocomponents Ag and small amount of Te on the Pd can provide their respective opportunities for activating the surface with the formation of OHads through synergistic effect. And the incorporation of Ag into Pd can improve the activity for ethanol oxidation has
been well studied by the previous studies [6,21]. The above mentioned features of the as-prepared catalysts may be finally leaded to the superior electrocatalytic performance. 4. Conclusions In conclusion, we have successfully developed a facile and effective strategy for the high-yield synthesis of uniform PdAgTe nanowires with high-quality in aqueous solution. The prepared catalysts exhibit superior electrocatalytic activity for ethanol electrooxidation in alkaline condition, demonstrating that they can be used as promising anodic electrocatalysts for the direct alkaline ethanol fuel cells and also possible for other direct fuel cell applications such as methanol oxidation etc. Acknowledgments The authors acknowledge the financial support of National Natural Science Foundation of China with Grant Nos. 21190040, 11174105 and 91227114. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2014.09.029.
Fig. 5. (A) Mass activity of the as-prepared PdAgTe NWs with different compositions and commercial Pd/C toward ethanol electrooxidation.
References
Fig. 6. (B) Currentetime curve of PdAgTe NWs with different compositions and commercial Pd/C recorded at 0.2 V in solution containing 1 mol L1 ethanol þ 0.5 mol L1 sodium hydroxide.
[1] A.L. Wang, H. Xu, J.X. Feng, L.X. Ding, Y.X. Tong, G.R. Li, J. Am. Chem. Soc. 135 (2013) 10703e10709. [2] R. Ganesan, J.S. Lee, Angew. Chem. Int. Ed. 44 (2005) 6557e6560. [3] L. Wang, Y. Nemoto, Y. Yamauchi, J. Am. Chem. Soc. 133 (2011) 9674e9677. [4] C. Bianchini, P.K. Shen, Chem. Rev. 109 (2009) 4183e4206. [5] X. Wang, J. Yang, H. Yin, R. Song, Z. Tang, Adv. Mater. 25 (2013) 2728e2732. [6] S.T. Nguyen, H.M. Law, H.T. Nguyen, N. Kristian, S. Wang, S.H. Chan, X. Wang, Appl. Catal. B 91 (2009) 507e515. [7] Z.Y. Zhou, Z.Z. Huang, D.J. Chen, Q. Wang, N. Tian, S.G. Sun, Angew. Chem. Int. Ed. 49 (2010) 411e414. [8] Y.-H. Qin, H.-H. Yang, R.-L. Lv, W.-G. Wang, C.-W. Wang, Electrochim. Acta 106 (2013) 372e377. € [9] S.C.S. Lai, S.E.F. Kleijn, F.T.Z. Oztürk, V.C. van Rees Vellinga, J. Koning, P. Rodriguez, M.T.M. Koper, Catal. Today 154 (2010) 92e104. [10] Y. Wang, F.-F. Shi, Y.-Y. Yang, W.-B. Cai, J. Power Sources 243 (2013) 369e373. [11] S.Y. Shen, T.S. Zhao, J.B. Xu, Y.S. Li, J. Power Sources 195 (2010) 1001e1006. [12] E. Antolini, Energy Environ. Sci. 2 (2009) 915e931. [13] S. Sun, Z. Jusys, R.J. Behm, J. Power Sources 231 (2013) 122e133. [14] X. Yang, Q. Yang, J. Xu, C.-S. Lee, J. Mater. Chem. 22 (2012) 8057e8062. [15] W. Hong, Y. Liu, J. Wang, E. Wang, J. Power Sources 241 (2013) 751e755. [16] W. Hong, J. Wang, E. Wang, Int. J. Hydrogen Energy 39 (2014) 3226e3230. [17] R.N. Singh, R. Awasthi, Catal. Sci. Technol. 1 (2011) 778e783. [18] J. Zhang, L. Zhang, S. Xie, Q. Kuang, X. Han, Z. Xie, L. Zheng, Chem. Eur. J. 17 (2011) 9915e9919.
W. Hong et al. / Journal of Power Sources 272 (2014) 940e945 [19] Z. Liu, B. Zhao, C. Guo, Y. Sun, Y. Shi, H. Yang, Z. Li, J. Colloid Interface Sci. 351 (2010) 233e238. [20] W. Hong, Y. Fang, J. Wang, E. Wang, J. Power Sources 248 (2014) 553e559. [21] W. Hong, J. Wang, E. Wang, Electrochem. Commun. 40 (2014) 63e66. [22] L. Zhang, N. Li, F. Gao, L. Hou, Z. Xu, J. Am. Chem. Soc. 134 (2012) 11326e11329. [23] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353e389. [24] D. Kim, Y.W. Lee, S.B. Lee, S.W. Han, Angew. Chem. Int. Ed. 51 (2012) 159e163. [25] L.F. Zhang, S.L. Zhong, A.W. Xu, Angew. Chem. Int. Ed. 52 (2013) 645e649. [26] J. Datta, A. Dutta, M. Biswas, Electrochem. Commun. 20 (2012) 56e59. [27] C. Zhu, S. Guo, S. Dong, Adv. Mater. 24 (2012) 2326e2331. [28] C. Zhu, S. Guo, S. Dong, J. Mater. Chem. 22 (2012) 14851e14855. [29] H.-W. Liang, S. Liu, J.-Y. Gong, S.-B. Wang, L. Wang, S.-H. Yu, Adv. Mater. 21 (2009) 1850e1854. [30] B.N. Wanjala, B. Fang, J. Luo, Y. Chen, J. Yin, M.H. Engelhard, R. Loukrakpam, C.J. Zhong, J. Am. Chem. Soc. 133 (2011) 12714e12727.
945
[31] J.H. Kim, S.M. Choi, S.H. Nam, M.H. Seo, S.H. Choi, W.B. Kim, Appl. Catal. B 82 (2008) 89e102. [32] A.K. Samal, T. Pradeep, Langmuir 26 (2010) 19136e19141. [33] J.-M. Song, Y.-Z. Lin, Y.-J. Zhan, Y.-C. Tian, G. Liu, S.-H. Yu, Cryst. Growth Des. 8 (2008) 1902e1908. [34] M. Liu, Y. Lu, W. Chen, Adv. Funct. Mater. 23 (2013) 1289e1296. [35] H.H. Li, S. Zhao, M. Gong, C.H. Cui, D. He, H.W. Liang, L. Wu, S.H. Yu, Angew. Chem. Int. Ed. 52 (2013) 7472e7476. [36] X. Li, L. Liu, J.-W. Lee, B.N. Popov, J. Power Sources 182 (2008) 18e23. [37] H. Wu, H. Li, Y. Zhai, X. Xu, Y. Jin, Adv. Mater. 24 (2012) 1594e1597. [38] M. Sawangphruk, A. Krittayavathananon, N. Chinwipas, J. Mater. Chem. A 1 (2013) 1030e1034. [39] S. Guo, S. Dong, E. Wang, Energy Environ. Sci. 3 (2010) 1307e1310. [40] Y. Tan, J. Fan, G. Chen, N. Zheng, Q. Xie, Chem. Commun. 47 (2011) 11624e11626. [41] Z.X. Liang, T.S. Zhao, J.B. Xu, L.D. Zhu, Electrochim. Acta 54 (2009) 2203e2208.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Facile synthesis of PdAgTe nanowires with superior electrocatalytic activity Wei Hong a, b, Jin Wang a, c, *, Erkang Wang a, b, * a
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China b University of Chinese Academy of Sciences, Beijing 100039, China c Department of Chemistry and Physics, State University of New York at Stony Brook, New York, NY 11794-3400, USA
h i g h l i g h t s PdAgTe nanowires have been synthesized by a simple method. Te nanowires was used as the sacrificial template. Cyclic voltammetry was used to obtain a highly active and stable surface. The as-prepared catalysts exhibit superior electrocatalytic activity.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 July 2014 Received in revised form 30 August 2014 Accepted 3 September 2014 Available online 16 September 2014
In this work, ultrathin Te nanowires (NWs) with high-aspect-ratio are prepared by a simple hydrothermal method. By using Te NWs as the sacrificial template, we demonstrate a facile and efficient method for the synthesis of PdAgTe NWs with high-quality through the partly galvanic replacement between Te NWs and the corresponding noble metal salts precursors in an aqueous solution. The compositions of PdAgTe NWs can be tuned by simply altering the concentration of the precursors. After cyclic voltammetry treatment, multi-component PdAgTe NW with a highly active and stable surface can be obtained. The structure and composition of the as-prepared nanomaterials are analyzed by transmission electron microscope, X-ray diffraction, energy dispersive X-ray spectroscopy, inductively coupled plasma-mass spectroscopy and X-ray photoelectron spectroscopy. Electrochemical catalytic measurement results prove that the as synthesized PdAgTe NWs present superior catalytic activity toward ethanol electrooxidation in alkaline solution than the commercial Pd/C catalyst, which making them can be used as effective catalysts for the direct ethanol fuel cells. © 2014 Published by Elsevier B.V.
Keywords: Palladium Silver Tellurium Nanowires Catalytic activity
1. Introduction As a potential viable alternative for environmentally friendly and highly efficient electric power generation device, direct fuel cells (DFCs) have been attracting increasing attention in recent years [1e3]. Among the different types of commonly used DFCs, direct ethanol fuel cells (DEFCs) are receiving special attention due to its obvious advantages, such as lower toxicity and higher energy density compared to their counterparts such as direct methanol
* Corresponding authors. State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. Fax: þ86 43185689711. E-mail addresses: [email protected] (J. Wang), [email protected] (E. Wang). http://dx.doi.org/10.1016/j.jpowsour.2014.09.029 0378-7753/© 2014 Published by Elsevier B.V.
fuel cells and direct formic acid fuel cells [4e6]. Moreover, ethanol is a renewable biofuel which can be obtained from the fermentation of biomass in large quantities. To improve the utilization efficiency of ethanol and energy conversion efficiency, two general methods have been developed. One way is by operating DEFCs in alkaline solution, in which the kinetics of ethanol electrooxidation reaction can be greatly improved. Another efficient way is to by employing electrode catalysts with high catalytic performance [7e9]. However, there are still some problems exist in the development of DEFCs which will severely prevent their ultimate commercialization, such as high cost, slow kinetics and poor durability of the anode catalysts. Maximum reducing the usage of high-cost noble metals while without cutting down their catalytic activity of a catalyst is still highly desired and technologically important for the final practical commercialization of DEFCs [10,11].
W. Hong et al. / Journal of Power Sources 272 (2014) 940e945
Pd-based catalysts are emerging as a hot focus for ethanol oxidation owe to their relatively lower cost, higher abundance and greater resistance to CO as compared to Pt-based catalysts [12e14]. In the light of these considerations, considerable efforts have been devoted to developing simple and practical techniques to prepare Pd-based electrocatalysts with enhanced performance for the DEFCs [15e17]. To achieve this goal, one way is to effectively control the morphology of the nanostructures, of which can provide a great opportunity to improve its surface area and thus increase its activity on a mass or specific basis. In this regard, many Pd-based nanomaterials with different morphologies have been developed for DEFCs, such as nanocubes [18], hollow nanospheres [19], porous nanospheres [20,21]. Among these developed materials, onedimensional nanostructures are receiving special attention recently, owing to their various potential applications deriving from their unique optical, electronic, magnetic, catalytic, and sensing properties [22,23]. Another efficient way to improve the catalytic performance of a catalyst is the use of bimetallic/multimetallic surfaces. The incorporation of a secondly transition metals has been proved to be can greatly improve the catalytic performance of a catalyst due to the synergetic effect between the different components. Materials such as PdAu and PdPt nanocrystals have been synthesized for DEFCs, and enhanced catalytic performance has been achieved [24e26]. Take account of the above considerations, not surprisingly, onedimensional material such as nanowires (NWs) with multicomponents is highly desired for the DEFCs. In this regard, Dong's group reported the synthesis of PdPt and AuPdPt NWs with enhanced electrocatalytic activity for ethanol electrooxidation [27,28]. However, these materials are not economical, because Au and Pt are very expensive noble metals. What's more, in their experiment, the Pt or Au precursors used are 2e4 fold excess compared to Te in order to replace the Te NWs completely, and the excess precursors are almost all wasted. It is still of great significance to lower the cost by reducing the usage of the expensive noble metals such as Pt and Au. In this regard, Ag, as a much cheaper metal, their incorporation into Pd-based materials for the DEFCs has been studied relatively less. And as far as we know, there are still no reports about the well-defined one-dimensional PdAgTe nanowires. In this work, PdAgTe NWs were prepared through the partly galvanic replacement by employing Te NWs with high-quality as the sacrificial templates. The morphology and composition of the obtained PdAgTe NWs were characterized with transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive X-ray spectroscopy (EDS), inductively coupled plasmamass spectroscopy (ICP-MS) and X-ray photoelectron spectroscopy (XPS). The catalytic activity and stability of the prepared catalysts toward ethanol electrooxidation were investigated by cyclic voltammetry (CV) and chronoamperometry methods in alkaline media. 2. Experimental 2.1. Materials Palladium nitrate, hydrazine hydrate (50% w/w%), ammonia solution (25 wt.%), polyvinylpyrrolidone (PVP K30, Mw 55000e58000), and sodium tellurite were obtained from Sinopharm (Shanghai, China). Nafion ethanol solution (5 wt.%) was obtained from Aldrich. Silver nitrate and commercial Pd/C (10 wt.%) were purchased from Aladdin Chemistry Co. Ltd (Shanghai, China). All chemicals used were of analytical grade and used as received without further purification. Milli-Q ultrapure water (Millipore, 18.2 MU cm) was used throughout the experiments.
941
2.2. Synthesis of Te nanowires The process was according to the reported literature with small modifications [29]. Briefly, 0.092 g sodium tellurite (Na2TeO3) and 1 g PVP were dissolved in 30 mL of water. Then, 1.65 mL of hydrazine hydrate (50% w/w) and 3.35 mL of ammonia solution (25% w/w) were added under vigorous magnetic stirring to form a homogeneous solution at room temperature. Then, the obtained solution was transferred into a Teflon-lined stainless steel autoclave, sealed and maintained at 180 C for 4 h. Finally, the product was cooled to room temperature and centrifuged with acetone, washed with water, and finally dissolved in 35 mL of water. 2.3. Synthesis of PdAgTe nanowires To synthesize PdAgTe nanowires, 1 mL of the above Te nanowires was added into 15 mL of water under stirring and maintained at 40 C in a water bath. Then a mixture containing 1 mL 5 mg mL1 Pd(NO3)2 and 0.2 mL 0.1 mol L1AgNO3 was added quickly. After react for 20 min, the product was centrifuged and washed several times with ultrapure water and finally collected for further characterizations. For comparison, another types of PdAgTe nanowires with different compositions were also prepared just by increased the volume of AgNO3 to 0.4 mL and 0.6 mL respectively. 2.4. Material structure and composition characterizations The morphology and structure of the as prepared catalysts were analyzed with a HITACHI H-600 Analytical TEM with an accelerating voltage of 100 kV. Energy dispersive X-ray spectrum (EDS) was made on a JEM-2100F high-resolution transmission electron microscope operating at 200 kV. The exact composition of PdAgTe NWs was determined by ICP-MS (X Series 2, Thermo Scientific USA). XRD pattern of PdAgTe NWs was performed on a D8 ADVANCE (BRUKER, Germany) diffractometer using CuKa radiation with a Ni filter (l ¼ 0.154059 nm at 30 kV and 15 mA). XPS measurement was performed on an ESCALAB-MKII spectrometer (VG Co., United Kingdom) with Al Ka X-ray radiation as the X-ray source for excitation. 2.5. Electrochemical catalytic experiment toward ethanol electrooxidation For electrochemical measurements, a glassy carbon electrode (GCE) was employed as working electrode, while a platinum wire was used as the counter electrode and KCl saturated Ag/AgCl electrode as the reference electrode respectively. GCE was sequentially polished carefully with 1.0, 0.3, and 0.05 mm alumina slurry and then rinsed with deionized water followed by sonication in ethanol and Milli-Q ultrapure water. For electrooxidation test, 5 mL of commercial Pd/C or PdAgTe catalysts solution was dropped on the surface of the GCE and dried with an infrared lamp carefully. The Pd loading mass of all the prepared PdAgTe NWs and commercial Pd/C catalyst was 35.3 mg cm2. After that, 5 mL of Nafion (0.5% wt) was coated on the surface of the above material modified GCE and dried before electrochemical experiments. All the electrochemical measurements were carried out on a CHI 832B electrochemical workstation, Chenhua Instruments corp (Shanghai, China). 3. Results and discussion 3.1. TEM, EDS, ICP-OES, XRD and XPS characterizations of the PdAgTe nanowires Figs. 1(A and B) shows the typical TEM images of the prepared Te NWs. The as-synthesized Te NWs present an average diameter of
942
W. Hong et al. / Journal of Power Sources 272 (2014) 940e945
about 10 nm, while with a length of several micron meters. By using Te NWs as the template, PdAgTe NWs can be obtained. The asprepared PdAgTe NWs present a high-aspect-ratio (Fig. 1C and D), demonstrating that the developed method is effective. EDS of the product demonstrates that the prepared catalyst is purely composed of Pd, Ag and Te elements (Fig. 2B), the peaks of Cu and C element were originated from the carbon coated copper grid. The exact composition of as-prepared catalyst was determined by ICPMS results and calculated to be Pd93Ag24Te55. The loading weight of Pd on the electrode can also be calculated exactly and further used to compare with that of commercial Pd/C catalyst during the electrochemical measurements. The X-ray diffraction (XRD) pattern of the as-prepared catalyst is shown in Fig. 2A. The intense peak at around 30 (2theta) demonstrates presence of pure Te phases which shows that the ternary component catalyst is not well alloyed. Compared with those of the pure Pd, Ag, or Te phases, all the diffraction peaks of the PdAgTe NWs did not present any obvious shift of diffraction angles, such a consistency could also be an evidence for the not alloyed formation among Pd, Ag, and Te [30,31]. The surface chemistry state of as-prepared multicomponent PdAgTe NWs was characterized by XPS. Fig. 2(CeE) shows the Te 3d, Pd 3d and Ag 3d core level spectrum of the PdAgTe NWs. There are two intense peaks at 573.11 and 583.53 eV which assigned to Te 3d5/2 and Te 3d3/2, respectively (Fig. 2C). Along with the two main tellurium peaks, two relative weak features were observed at 575.55 and 586.11 eV, which can be indexed to Te4þ, due to presence of TeO2 [32,33]. According to Te/Te4þ intensity ratio, it can be known that both Te and Te4þ species are dominantly exist in the NWs, which can be further an evidence to prove the mix-phase rather than alloy state of the as-prepared NWs. Similarly, the peaks at the binding energy values of 335.52 and 340.97 eV are corresponding to the Pd 3d5/2 and Pd 3d3/2, respectively. Two more peaks at 337.01 and 342.48 eV indicating the presence of Pd2þ species [34]. Similarly, based on the Pd/Pd2þ intensity ratio, it can be known that the zero-valent state Pd species
are predominant in the prepared NWs. The presence of Pd2þ species may be originated from the using of Pd(NO3)2 aqueous solution, because the solubility of Pd(NO3)2 in neutral water is not very well, which may be lead to the dissociation for the formation of Pd(OH)2. Similar observations can also be found in the reported literature [34]. As for the Ag, the peaks at the binding energy values of 368.0 and 373.9 eV correspond to Ag 3d5/2 and Ag 3d3/2, respectively, indicating that Ag is mainly present in the metallic state. 3.2. Electrocatalytic measurements for ethanol electrooxidation Inspired by their special nanostructure, we investigated the catalytic performance of the as-prepared catalyst for the ethanol electrooxidation. The catalytic activity of the as-prepared PdAgTe NWs toward ethanol electrooxidation in alkaline condition was investigated by an electrochemical measurement system and further compared with a commercial Pd/C catalyst. Mass activity was used to evaluate the catalytic activity of the catalysts. Electrooxidation of ethanol was carried out in an aqueous solution containing 0.5 mol L1 sodium hydroxide and 1 mol L1 ethanol, using CV measurement technique, sweeping from 0.8 V to 0.4 V at a scan rate of 50 mV s1. Considering the presence of the unstable Te atoms in the catalyst, we treated the catalyst in 0.5 mol L1 H2SO4 (aqueous solution) by using CV technique with a scan rate at 50 mV s1 before the catalytic measurements. The profiles of the treatment process can be found in Fig. 3. The peak between 0.6 and 0.9 V in the cycles can be attributed to the dissolution of Te atoms from the surface of the PdAgTe NWs in the acidic environment [35,36], thus leaving a more stable and Pd enriched surface which can further improving their catalytic performance. The TEM images of the as-prepared nanowires after CV treatment are presented in Fig. S3.,Fig. 4 shows the mass catalytic activity for the ethanol oxidation of the PdAgTe NWs with/without CV treatment in H2SO4 solution. Without treatment, the mass activity of the as-prepared
Fig. 1. (A, B) Typical TEM images of the Te NWs with different magnifications (C, D) TEM images of the as-prepared PdAgTe NWs with different magnifications.
W. Hong et al. / Journal of Power Sources 272 (2014) 940e945
943
Fig. 2. XRD pattern (A) and EDS (B) of the as-prepared Pd93Ag24Te55 NWs. XPS spectrum of as-prepared Pd93Ag24Te55 NWs, Te 3d (C), Pd 3d (D) and Ag 3d (E).
catalyst is significantly lower than the treated one, and many miscellaneous peaks can be observed. In sharp contrast, there are only two well separated peaks presented in the CV curve of the ethanol oxidation of the treat one. The results strongly suggest that after CV treatment, a highly active and stable surface can be obtained and thus lead to enhanced catalytic activity. Considering the fact that composition is a very important role that governing the catalytic activity of a catalyst, another two types of PdAgTe NWs with different compositions have also successfully prepared. The compositions were determined to be Pd72Ag23Te62 and Pd53Ag28Te53 respectively by the ICP-MS results. Some of the NW tended to break at some extent with Ag content increased (Fig. S2, Supplementary data). It can be noted that the atomic ratio of Ag in the three types of PdAgTe NWs with different compositions did not increased obviously along with the increased amount of AgNO3 precursors added. The result may be attributed to the slow reaction rate of the Ag. Similarly, another two types of the prepared NWs with different compositions were also treated by CV in H2SO4 solution before the catalytic measurements. Fig. 5 shows the mass
Fig. 3. CV profiles of PdAgTe NWs recorded in 0.5 mol L1 H2SO4 solution at a sweep rate of 50 mV se1. The anodic peak of the cycles between 0.6 and 0.9 V is attributed to the dissolusion of Te from Pd93Ag24Te55 NWs.
activity of the prepared catalysts and commercial Pd/C toward ethanol electrooxidation. It can be noted clearly that Pd93Ag24Te55 NWs possess the highest mass activity, suggesting that appropriate composition is favorable for the best catalytic activity. The forward anodic peak of Pd93Ag24Te55 is 1.43 A mg1Pd, about 11 times of commercial Pd/C of 0.13 A mg1Pd as illustrated in Fig. 5, indicating that the as prepared Pd93Ag24Te55 possess an excellent catalytic activity. The value is also much higher than 0.6 A mg1Pd of the free-standing Pd nanomembranes [37], 0.55 A mg1Pd of Pd/GO/ CFP [38], 0.9 A mg1Pd of Pd/rGO/CFP [38], 0.9 A mg1Pd of Pd NWs [27]. Learned from Fig. 5, it can also be found that the peak value in backward scan of Pd93Ag24Te55 NWs is 1.18 A mg1Pd, which is also much higher than that of the commercial Pd/C. Moreover, all of the prepared PdAgTe NW present a much higher mass activity value than the commercial Pd/C catalyst, which prove that all the asprepared catalysts possess superior catalytic activity for the ethanol oxidation. The stability of PdAgTe NWs and Pd/C toward ethanol electrooxidation was investigated by chronoamperometry technique at 0.2 V as presented in Fig. 6. Pd93Ag24Te55 NWs show a distinct slower current decay than another two types of PdAgTe NWs and commercial Pd/C over the whole 4000 s test range, suggesting that Pd93Ag24Te55 NWs possess the best long-term durability. Apart from this, it can also be found that all the prepared NWs show a much slower current decay than the commercial Pd/C catalyst. The results from both activity and stability studies indicate that the asprepared PdAgTe NWs possess superior catalytic performance for ethanol electrooxidation in alkaline media. The superior electrocatalytic activity of PdAgTe NWs for the ethanol electrooxidation should be attributed to the following reasons. Firstly, the contribution of the one-dimensional nanostructure. As revealed by the literature, one-dimensional structure can maintain improved electron transport characteristics during the electro-catalysis as a result of the path-directing effects of the structural anisotropy [22,23]. Moreover, the NWs as the selfsupported catalyst, are less vulnerable to dissolution, Ostwald ripening, and aggregation during electrocatalytic processes due to the micrometresized length [39,40]. In addition, the small diameter of the NWs can offer more opportunity to provide more exposed
944
W. Hong et al. / Journal of Power Sources 272 (2014) 940e945
Fig. 4. The mass activity of Pd93Ag24Te55 NWs toward ethanol electrooxidation in a solution containing 1 mol L1 ethanol þ 0.5 mol L1 sodium hydroxide, without (A) and with (B) CV electrochemical treatment in 0.5 mol L1 H2SO4 solution before the electrocatalytic measurements.
surface atoms thus lead to more active sites. Secondly, the incorporation of Ag and Te. It is generally believed that a CH3COads intermediate is formed on the Pd catalyst in the alkaline medium, and its reaction with OHads is the rate-determining step [41]. The cocomponents Ag and small amount of Te on the Pd can provide their respective opportunities for activating the surface with the formation of OHads through synergistic effect. And the incorporation of Ag into Pd can improve the activity for ethanol oxidation has
been well studied by the previous studies [6,21]. The above mentioned features of the as-prepared catalysts may be finally leaded to the superior electrocatalytic performance. 4. Conclusions In conclusion, we have successfully developed a facile and effective strategy for the high-yield synthesis of uniform PdAgTe nanowires with high-quality in aqueous solution. The prepared catalysts exhibit superior electrocatalytic activity for ethanol electrooxidation in alkaline condition, demonstrating that they can be used as promising anodic electrocatalysts for the direct alkaline ethanol fuel cells and also possible for other direct fuel cell applications such as methanol oxidation etc. Acknowledgments The authors acknowledge the financial support of National Natural Science Foundation of China with Grant Nos. 21190040, 11174105 and 91227114. Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2014.09.029.
Fig. 5. (A) Mass activity of the as-prepared PdAgTe NWs with different compositions and commercial Pd/C toward ethanol electrooxidation.
References
Fig. 6. (B) Currentetime curve of PdAgTe NWs with different compositions and commercial Pd/C recorded at 0.2 V in solution containing 1 mol L1 ethanol þ 0.5 mol L1 sodium hydroxide.
[1] A.L. Wang, H. Xu, J.X. Feng, L.X. Ding, Y.X. Tong, G.R. Li, J. Am. Chem. Soc. 135 (2013) 10703e10709. [2] R. Ganesan, J.S. Lee, Angew. Chem. Int. Ed. 44 (2005) 6557e6560. [3] L. Wang, Y. Nemoto, Y. Yamauchi, J. Am. Chem. Soc. 133 (2011) 9674e9677. [4] C. Bianchini, P.K. Shen, Chem. Rev. 109 (2009) 4183e4206. [5] X. Wang, J. Yang, H. Yin, R. Song, Z. Tang, Adv. Mater. 25 (2013) 2728e2732. [6] S.T. Nguyen, H.M. Law, H.T. Nguyen, N. Kristian, S. Wang, S.H. Chan, X. Wang, Appl. Catal. B 91 (2009) 507e515. [7] Z.Y. Zhou, Z.Z. Huang, D.J. Chen, Q. Wang, N. Tian, S.G. Sun, Angew. Chem. Int. Ed. 49 (2010) 411e414. [8] Y.-H. Qin, H.-H. Yang, R.-L. Lv, W.-G. Wang, C.-W. Wang, Electrochim. Acta 106 (2013) 372e377. € [9] S.C.S. Lai, S.E.F. Kleijn, F.T.Z. Oztürk, V.C. van Rees Vellinga, J. Koning, P. Rodriguez, M.T.M. Koper, Catal. Today 154 (2010) 92e104. [10] Y. Wang, F.-F. Shi, Y.-Y. Yang, W.-B. Cai, J. Power Sources 243 (2013) 369e373. [11] S.Y. Shen, T.S. Zhao, J.B. Xu, Y.S. Li, J. Power Sources 195 (2010) 1001e1006. [12] E. Antolini, Energy Environ. Sci. 2 (2009) 915e931. [13] S. Sun, Z. Jusys, R.J. Behm, J. Power Sources 231 (2013) 122e133. [14] X. Yang, Q. Yang, J. Xu, C.-S. Lee, J. Mater. Chem. 22 (2012) 8057e8062. [15] W. Hong, Y. Liu, J. Wang, E. Wang, J. Power Sources 241 (2013) 751e755. [16] W. Hong, J. Wang, E. Wang, Int. J. Hydrogen Energy 39 (2014) 3226e3230. [17] R.N. Singh, R. Awasthi, Catal. Sci. Technol. 1 (2011) 778e783. [18] J. Zhang, L. Zhang, S. Xie, Q. Kuang, X. Han, Z. Xie, L. Zheng, Chem. Eur. J. 17 (2011) 9915e9919.
W. Hong et al. / Journal of Power Sources 272 (2014) 940e945 [19] Z. Liu, B. Zhao, C. Guo, Y. Sun, Y. Shi, H. Yang, Z. Li, J. Colloid Interface Sci. 351 (2010) 233e238. [20] W. Hong, Y. Fang, J. Wang, E. Wang, J. Power Sources 248 (2014) 553e559. [21] W. Hong, J. Wang, E. Wang, Electrochem. Commun. 40 (2014) 63e66. [22] L. Zhang, N. Li, F. Gao, L. Hou, Z. Xu, J. Am. Chem. Soc. 134 (2012) 11326e11329. [23] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353e389. [24] D. Kim, Y.W. Lee, S.B. Lee, S.W. Han, Angew. Chem. Int. Ed. 51 (2012) 159e163. [25] L.F. Zhang, S.L. Zhong, A.W. Xu, Angew. Chem. Int. Ed. 52 (2013) 645e649. [26] J. Datta, A. Dutta, M. Biswas, Electrochem. Commun. 20 (2012) 56e59. [27] C. Zhu, S. Guo, S. Dong, Adv. Mater. 24 (2012) 2326e2331. [28] C. Zhu, S. Guo, S. Dong, J. Mater. Chem. 22 (2012) 14851e14855. [29] H.-W. Liang, S. Liu, J.-Y. Gong, S.-B. Wang, L. Wang, S.-H. Yu, Adv. Mater. 21 (2009) 1850e1854. [30] B.N. Wanjala, B. Fang, J. Luo, Y. Chen, J. Yin, M.H. Engelhard, R. Loukrakpam, C.J. Zhong, J. Am. Chem. Soc. 133 (2011) 12714e12727.
945
[31] J.H. Kim, S.M. Choi, S.H. Nam, M.H. Seo, S.H. Choi, W.B. Kim, Appl. Catal. B 82 (2008) 89e102. [32] A.K. Samal, T. Pradeep, Langmuir 26 (2010) 19136e19141. [33] J.-M. Song, Y.-Z. Lin, Y.-J. Zhan, Y.-C. Tian, G. Liu, S.-H. Yu, Cryst. Growth Des. 8 (2008) 1902e1908. [34] M. Liu, Y. Lu, W. Chen, Adv. Funct. Mater. 23 (2013) 1289e1296. [35] H.H. Li, S. Zhao, M. Gong, C.H. Cui, D. He, H.W. Liang, L. Wu, S.H. Yu, Angew. Chem. Int. Ed. 52 (2013) 7472e7476. [36] X. Li, L. Liu, J.-W. Lee, B.N. Popov, J. Power Sources 182 (2008) 18e23. [37] H. Wu, H. Li, Y. Zhai, X. Xu, Y. Jin, Adv. Mater. 24 (2012) 1594e1597. [38] M. Sawangphruk, A. Krittayavathananon, N. Chinwipas, J. Mater. Chem. A 1 (2013) 1030e1034. [39] S. Guo, S. Dong, E. Wang, Energy Environ. Sci. 3 (2010) 1307e1310. [40] Y. Tan, J. Fan, G. Chen, N. Zheng, Q. Xie, Chem. Commun. 47 (2011) 11624e11626. [41] Z.X. Liang, T.S. Zhao, J.B. Xu, L.D. Zhu, Electrochim. Acta 54 (2009) 2203e2208.