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Synthesis, characterization and magnetic properties of highly monodispersed PtNi nanoparticles
Materials Chemistry and Physics 155 (2015) 47e51
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Synthesis, characterization and magnetic properties of highly monodispersed PtNi nanoparticles Juan-Juan Du, Yi Yang, Rong-Hua Zhang*, Xin-Wen Zhou* College of Biological and Pharmaceutical Science, China Three Gorges University, Daxue Road, No. 8, Yichang 443002, China
h i g h l i g h t s Highly monodispersed PtNi nanoparticles were synthesized by galvanic displacement reaction. The formation of Pt nanocrystals was the foremost step because of its self-catalysis effect. The PtNi nanoparticles show a superparamagnetic behavior with a TB about 8.0 K.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2014 Received in revised form 10 January 2015 Accepted 31 January 2015 Available online 26 February 2015
In this paper, we report the controlled-synthesis of PtNi nanoparticles through galvanic displacement reaction and chemical reduction. The size, composition and morphology of the products are characterized by transmission electron microscopy (TEM), powder X-ray diffraction (XRD), energy dispersed X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analyses. The structure and composition of the PtNi nanoparticles can be controlled by adjusting the synthetic conditions. The possible formation mechanism is obtained from the academic analysis and experimental studies. The results of the magnetic measurement illustrate that the PtNi nanoparticles show a superparamagnetic behavior with a blocking temperature (TB) about 8.0 K. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Chemical synthesis Magnetic properties Electron microscopy
1. Introduction Magnetic platinumenickel (PtNi) alloy nanoparticles have widely potential applications in high density data storage [1], catalysts used in fuel cells [2,3], and different sensors [4,5]. For example, PtNi nanoparticles are frequently used in direct methanol fuel cell (DMFC) as anode catalysts for methanol oxidation [6,7], and as cathode catalysts for oxygen reduction reaction [8,9]. These catalytic properties are highly dependent on the size, shape and composition of the nanoparticles. At the same time, geometry of a nanomagnet has great impact on its magnetic properties resulting from the interplay among different types of magnetic energies [10]. Until now, rod-shaped superparamagnetic PtNi nanoalloy and PtNi/ C nanoparticles have been synthesized through different methods [11,12]. H Weller et al. [13] have reported the synthesis of Pt1-xNix nanoparticles with tunable composition and size by a hot
* Corresponding authors. E-mail address: [email protected] (X.-W. Zhou). http://dx.doi.org/10.1016/j.matchemphys.2015.01.063 0254-0584/© 2015 Elsevier B.V. All rights reserved.
organometallic route, which needed a high temperature and expensive poisonous reactants. PtNi hollow spheres prepared by a template-replacement route as catalysts for hydrogen generation from ammonia borane have been investigated by Chen et al. [14]. This template-replacement method consisted of the removal of the template after synthesis, which may destroy the structure of the products. Then, synthesis of PtNi nanoparticles with controllable morphology and composition is crucial for progress in this field. Galvanic displacement reaction is a special template method, in which one substance is served as a suitable sacrificial template and reacts with other appropriate metal ions according to their different standard reduction potentials, resulting in the controlled formation of nanomaterials [15e18]. This method has been extended for the synthesis of PtNi nanoparticles with a hollow structure using Ni nanoparticles as a sacrificial template [19,20]. Here, we report the synthesis of highly monodispersed PtNi nanoparticles by galvanic displacement reaction. The size, composition and morphology of the PtNi nanoparticles can be controlled by varying the synthesis conditions. Based on a series of additional experiments, a probable mechanism of the formation of the PtNi
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nanoparticles is proposed. The magnetic properties are studied through superconducting quantum interference device (SQUID) magnetometer.
2. Experimental section 2.1. Synthesis of PtNi nanoparticles All chemicals are analytical grade and are used without further purification. The growth of the PtNi nanoparticles was carried out in a solution-phase system using the galvanic displacement
reaction. In brief, 9.5 mg NiCl2, 60 mL H2O and 100 mg poly(vinylpyrrolidone) (PVP) (K30) were mixed in a three-necked flask equipped with a heat controller. The mixture was kept at 75 C with vigorous stirring. Then, 25 mL of 0.01 M freshly prepared NaBH4 solution was added dropwise. The solution turned dark, indicating the formation of Ni nanoparticles. Once the NaBH4 was completely dropped into the solution, 20 mL of 4 mM K2PtCl6 was added dropwise. To avoid oxidation of the Ni nanoparticles, high-purity N2 was bubbled into the solution during the whole procedure. The obtained black solution was stirred for 2 h at 75 C and cooled to room temperature. Then the nanoparticles were separated, washed thoroughly and desiccated in vacuum for the following studies.
2.2. Instruments The samples were characterized by X-ray diffraction (XRD) using a Panalytical X'pert PRO diffractormeter. Transmission electron microscopy (TEM) and high-magnification transmission electron microscopy patterns were obtained on instruments of JEM-100CXII and FEI Tecnai-F30 electron microscopy, respectively. Samples for TEM analysis were prepared by adding a drop of the solution of PtNi nanoparticles onto a carbon-coated copper grid. The energy dispersed X-ray spectroscopy (EDS) was attached to the TEM system. X-ray photoelectron spectroscopy (XPS) spectrum was recorded by a PHI Quantum 2000 XPS system (Physical Electronics, Inc.) using AI Ka radiation at a base pressure below 5 108 Torr. Magnetic properties were measured by a superconducting quantum interference device (SQUID) magnetometer.
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Fig. 1. (a) TEM and (b) high-magnification TEM images of PtNi nanoparticles. The insets to (a) and (b) shown the size distribution and the selected area electron diffraction (SAED) pattern of the PtNi nanoparticles, respectively.
Fig. 2. (a) XRD and (b) EDS patterns of PtNi nanoparticles.
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3. Results and discussion 3.1. Characterization of PtNi nanoparticles TEM images in Fig. 1a illustrate that large-scale PtNi nanomaterials have been synthesized. It can be seen that the PtNi nanomaterials are solid, spherical and highly monodispersed. The size distribution of the sample in the inset of Fig. 1a shows that a number average mean diameter is about 52.6 nm obtained from the literature [21]. The structural details are revealed in highmagnification TEM (Fig. 1b). The individual PtNi nanoparticle has a non-uniform surface and high monodispersity. The dissolution (oxidation) of Ni and the co-reduction of Ni2þ and PtCl2 6 were done at the same time during the synthetic process. The diffusion of the oxidized Ni2þ through the surface may destroy the formation of the products. On the other side, the surface of the PtNi nanoparticles may be oxidized partly. So the surface of the PtNi nanoparticles is rough [16]. The selected area electron diffraction (SAED) pattern (inset in Fig. 1b) indicates that the PtNi nanoparticles have a facecentered cubic (fcc) structure. The concentric rings can be assigned as (111), (200), (220), (311) and (222), which can be confirmed by the following XRD results [22]. Fig. 2a shows the XRD pattern of the PtNi nanoparticles in the 2q region between 20 and 90 . Reflection peaks are found at 40.10 , 46.32 , 67.78 and 81.85 corresponding to the (111), (200), (220), and (311) fundamental peaks of the chemically disordered fcc Pt. The reflection peak of (222) is not clear. There are no peaks for phase separated structures such as pure Ni or its oxides in the XRD pattern, which could be the result of the alloy between Pt and Ni since platinum is known to alloy well with Ni [11]. Fig. 2b shows the
Fig. 3. XPS spectra of (a) Pt4f and (b) Ni2p regions of PtNi nanoparticles.
49
energy dispersed X-ray spectroscopy (EDS) analysis taken from many PtNi nanoparticles. The results demonstrate the coexistence of Pt and Ni with an average stoichiometry of Pt68Ni32, which is consistent with the theoretical ratio (Pt66.7Ni33.3). The element Cu in the EDS pattern is attributed to the TEM grid. Fig. 3a shows the region of Pt4f in the XPS spectrum of the PtNi nanoparticles. The spectrum can be fit by two pairs of overlapping Lorentzian curves [23]. The Pt4f7/2 and Pt4f5/2 lines appearing at 71.45 eV (peak 1) and 74.75 eV (peak 3) are attributed to metallic Pt0. The other two peaks appearing at 72.62 eV (peak 2) and 75.82 eV (peak 4) can be assigned to Pt2þ species in PtO and Pt(OH)2 [11,23]. A comparison of the relative intensities of the peaks shows that Pt in the PtNi nanoparticles is predominately present in the zerovalent metallic state (Pt0). The XPS spectrum of Ni2p is shown in Fig. 3b. The Ni states consist of NiO, NiO, Ni(OH)2 and NiOOH. In general, the Ni2p3/2 spectrum is made complicated by the presence of satellite signals of high binding energy adjacent to the main
Fig. 4. TEM images of (a) Ni nanoparticles obtained 75 C, and (b) PtNi nanoparticles obtained when NiCl2 and K2PtCl6 were mixed together first.
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peaks [11,24]. Taking the satellite peaks into account, the Ni2p3/2 peaks at the binding energies of 852.56, 853.73, 855.87, and 857.51 eV correspond to NiO, NiO, Ni(OH)2 and NiOOH, respectively. But in the XRD studies, we could not find the peaks of the hydroxides or oxides of Ni, which is possibly because of their amorphous nature [11]. 3.2. Formation mechanism of PtNi nanoparticles Several formation mechanisms of PtNi nanoparticles have been discussed. Lee et al. [11] reported the preparation of PtNi nanoparticles using hydrazine as reducer. They thought Pt nanoparticles were initially formed and then seeded the growth of the PtNi nanoparticles. Later, T Pal et al. [12] and H Weller et al. [13] obtained PtNi nanoparticles through different methods using the similar formation mechanism. In order to illuminate the formation
mechanism of the PtNi nanoparticles in our experiment, a series of control experiments were carried out. Firstly, monodispersed Ni nanoparticles could not be produced under the same condition (Fig. 4a). NiCl2 reacted with NaBH4 to form a darkish bulk nickel, which means PVP was not able to stabilize Ni nanoparticles [11]. Secondly, if NiCl2 and K2PtCl6 were mixed together first, the product was formless and agglomerated (Fig. 4b). These observations indicated that the presence of the platinum and the sequence of the addition of the NiCl2 and K2PtCl6 were crucial to the formation of the PtNi nanoparticles. Then, it was postulated that the basic formation mechanism of the PtNi nanoparticles proceeded by the following steps. NiCl2 was reduced to bulk nickel. Since the standard reduction potential of the PtCl2 6 /Pt [0.735 V vs standard hydrogen electrode (SHE)] is much higher than that of the Ni2þ/Ni (0.25 V vs SHE), PtCl2 6 was reduced to Pt as soon as K2PtCl6 was added to the bulk nickel solution. PVP has been proved to be a good stabilizer to protect Pt nanoparticle [25]. With the action of vigorous stirring and the protection of PVP, the reaction of bulk Ni and PtCl2 6 would result the dissolution of bulk Ni and form monodispersed nanoparticles. At the same time, bulk nickel was oxidized to Ni2þ ions. Then, the Ni2þ and PtCl2 6 will be co-reduced by the excess NaBH4 in the solution to form bimetallic PtNi nanoparticles. The influence of other reaction conditions on the size, shape, and composition of PtNi nanoparticles were investigated. The same results could be obtained when the ration of platinum and nickel precursors was varied from 1:2 to 1:1 (Fig. 5a). The influence of the reaction temperature to the nanoparticle morphology was also studied. Fig. 5b shown the TEM images of the PtNi nanoparticles synthesized at 30 C, in which we could see that the center portions of the nanoparticls are lighter than their wall edge, indicating a hollow structure [16,17]. This kind of hollow PtNi nanoparticles is a potential catalyst for DMFC [19]. Another important observation was that the particle size and atomic ration of Pt/Ni in the final products varied with the temperature. The particle size of PtNi nanoparticles increased along with rising reaction temperature (from 32.0 nm at 30 C to 52.6 nm at 75 C), and the nickel-toplatinum ratio increased concomitantly (from Pt75Ni25 at 30 C to Pt68N32 at 75 C). Actually, H Weller et al. [13] has observed the same phenomena of PtNi nanoparticles obtained by a hot organometallic synthesis. These results corresponded to the fact that Ni
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Fig. 6. FC/ZFC specific magnetization of PtNi nanoparticles in a field of 1000 Oe. Open circle: FC, solid circle: ZFC.
J.-J. Du et al. / Materials Chemistry and Physics 155 (2015) 47e51
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300 K. For a superparamagnet, ferromagnetic behavior is observed when the temperature is below TB. When the temperature is above TB, there is a superparamagnetic relaxation. Therefore, we did not observe any hysteresis and coerciety at room temperature. 4. Conclusions
Fig. 7. Hysteresis loops measured at 5 K and 300 K of NiPt nanoparticles. The inset is an open-hysiteresis loop in the low-field region (<4500 Oe).
In summary, well monodispersed PtNi nanoparticles were synthesized by galvanic displacement reaction and chemical reduction. The size, composition and morphology could be controlled by varying the reaction conditions. A probable mechanism for the formation of the PtNi nanoparticles was proposed based on a series of assistant experiments. The reaction temperature was a crucial factor to determine the structures of the PtNi nanoparticles. The formation of Pt nanocrystal in the reaction process was the foremost step because of its self-catalysis effect. The results of magnetic measurements illustrated that the PtNi nanoparticles show a superparamagnetic behavior with a blocking temperature (TB) about 8.0 K. Acknowledgments
could be reduced easily at higher temperature [11,13]. So, the reaction temperature is a crucial factor to determine the size, composition, and morphology of the PtNi nanoparticles.
Financial supports from National Natural Science Foundation of China (21403126) and the help from Prof. Shi-Gang Sun in Xiamen University are highly acknowledged.
3.3. Magnetic properties of PtNi nanoparticles References The temperature-dependent magnetization of PtNi nanomaterials was measured by superconducting quantum interference device (SQUID) magnetometer between 5 and 300 K using zerofield-cooling (ZFC) and field-cooling (FC) procedures in an applied field of 1000 Oe. Superparamagnetic behavior was observed for the PtNi nanoparticles as shown in Fig. 6. For single-domain particles, the temperature of the maximum in the ZFC curve is regarded as the average blocking temperature (TB), below which the sample is ferromagnetic and above which is superparamagnetic [23]. The sharp maximum at 8.0 K in the ZFC curve indicated a clear blocking behavior of the PtNi nanoparticles. Accordingly, the TB of the PtNi nanoparticles was 8.0 K. It is well known that TB varies with the variation of particle size, shape and the atomic ration of Pt to Ni. For example, the TB obtained by H Weller [13] was 7.4 K and 65 K for rod-like PtNi particles reported by T Pal [12]. The well overlap of the ZFC curve and FC curve when the temperature was higher than TB indicated that the PtNi nanoparticles had a narrow size distribution, which has been confirmed by the TEM studies. Fig. 7 shows the hysteresis loops of the PtNi nanomaterials measured at 5 K and 300 K. The inset was an open-hysiteresis loop in the low-field region (<4500 Oe). For 5 K, saturation magnetization (Ms), remanent magnetization (Mr), coercivity (Hc), and squareness (Sr ¼ Mr/Ms) were 10.4 emu g1, 1.1 emu g1, 517 Oe and 0.1, respectively (emu: electromagnetic unit) between 30 kOe and 30 kOe. When temperatures was higher than TB, irreversible magnetization, that is, hysteretic behavior and coerciety, disappeared, exemplified by the magnetization measurement at
[1] A. Cheng, P. Holt-Hindle, Chem. Rev. 110 (2010) 3767. [2] H. You, S. Yang, B. Ding, H. Yang, Chem. Soc. Rev. 42 (2013) 2880. [3] X.W. Zhou, Y.L. Gan, J.J. Du, D.N. Tian, R.H. Zhang, C.Y. Yang, Z.X. Dai, J. Power Sources 232 (2013) 310. [4] C. Chen, Q. Xie, D. Yang, H. Xiao, Y. Fu, Y. Tan, S. Yao, RSC Adv. 3 (2013) 4473. [5] C. Xu, J. Wang, J. Zhou, Sensors Actuat. B-Chem. 182 (2013) 408. [6] F. Liu, J.Y. Lee, J. Zhou, Small 2 (2006) 121. [7] B. Luo, S. Xu, X. Yan, Q. Xue, J. Electrochem. Soc. 160 (2013) F262. [8] H.M. Wu, D. Wexler, H.K. Liu, Mater. Chem. Phys. 136 (2012) 845. [9] C. Cui, L. Gan, H.H. Li, S.H. Yu, M. Heggen, P. Strasser, Nano Lett. 12 (2012) 5885. [10] F. Zhao, C. Liu, P. Wang, S. Huang, H. Tian, J. Alloy Compd. 577 (2013) 669. [11] T.C. Deivaraj, W. Chen, J.Y. Lee, J. Mater. Chem. 13 (2003) 2555. [12] M. Mandal, S. Kundu, T.K. Sau, S.M. Yusuf, T. Pal, Chem. Mater. 15 (2003) 3710. €rlitz, H. Weller, Small [13] K. Ahrenstorf, O. Albrecht, H. Heller, A. Kornowski, D. Go 3 (2007) 271. [14] F. Cheng, H. Ma, J. Chen, Inorg. Chem. 46 (2007) 788. [15] Y. Sun, B. Wiley, Z. Li, Y. Xia, J. Am, Chem. Soc. 126 (2004) 9399. [16] H.P. Liang, H.M. Zhang, J.S. Hu, Y.G. Guo, L.J. Wan, C.L. Bai Angew, Chem. Int. 43 (2004) 1540. [17] X.W. Zhou, Q.S. Chen, Z.Y. Zhou, S.G. Sun, J. Nanosci, Nanothch 9 (2009) 2392. [18] X.W. Zhou, R.H. Zhang, D.M. Zeng, S.G. Sun, J. Solid State Chem. 183 (2010) 1340. [19] J.X. Wang, C. Ma, Y.M. Choi, D. Su, Y.M. Zhu, P. Liu, R. Si, M.B. Vukmirovic, Y. Zhang, R.R. Adzic, J. Am. Chem. Soc. 133 (2011) 13551. [20] X.W. Zhou, R.H. Zhang, Z.Y. Zhou, S.G. Sun, J. Power Sources 196 (2011) 5844. [21] I.V. Beketov, A.P. Safronov, A.I. Medvedv, J. Alonso, G.V. Kurlyandskaya, S.M. Bhagat, AIP Adv. 2 (2012) 022154. [22] G.V. Kurlyandskaya, S.M. Bhagat, S.E. Jacobo, J.C. Aphesteguy, N.N. Schegoleva, J. Phys, Chem. Solids 72 (2011) 276. [23] X. Zhang, K.Y. Chan, Chem. Mater. 15 (2003) 451. [24] P.F. Luo, T. Kuwana, D.K. Paul, P.M.A. Sherwood, Anal. Chem. 68 (1996) 3330. [25] H. Song, F. Kim, S. Connor, G.A. Somorjai, P. Yang, J. Phys. Chem. B 109 (2005) 188.
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Synthesis, characterization and magnetic properties of highly monodispersed PtNi nanoparticles Juan-Juan Du, Yi Yang, Rong-Hua Zhang*, Xin-Wen Zhou* College of Biological and Pharmaceutical Science, China Three Gorges University, Daxue Road, No. 8, Yichang 443002, China
h i g h l i g h t s Highly monodispersed PtNi nanoparticles were synthesized by galvanic displacement reaction. The formation of Pt nanocrystals was the foremost step because of its self-catalysis effect. The PtNi nanoparticles show a superparamagnetic behavior with a TB about 8.0 K.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 9 May 2014 Received in revised form 10 January 2015 Accepted 31 January 2015 Available online 26 February 2015
In this paper, we report the controlled-synthesis of PtNi nanoparticles through galvanic displacement reaction and chemical reduction. The size, composition and morphology of the products are characterized by transmission electron microscopy (TEM), powder X-ray diffraction (XRD), energy dispersed X-ray spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analyses. The structure and composition of the PtNi nanoparticles can be controlled by adjusting the synthetic conditions. The possible formation mechanism is obtained from the academic analysis and experimental studies. The results of the magnetic measurement illustrate that the PtNi nanoparticles show a superparamagnetic behavior with a blocking temperature (TB) about 8.0 K. © 2015 Elsevier B.V. All rights reserved.
Keywords: Nanostructures Chemical synthesis Magnetic properties Electron microscopy
1. Introduction Magnetic platinumenickel (PtNi) alloy nanoparticles have widely potential applications in high density data storage [1], catalysts used in fuel cells [2,3], and different sensors [4,5]. For example, PtNi nanoparticles are frequently used in direct methanol fuel cell (DMFC) as anode catalysts for methanol oxidation [6,7], and as cathode catalysts for oxygen reduction reaction [8,9]. These catalytic properties are highly dependent on the size, shape and composition of the nanoparticles. At the same time, geometry of a nanomagnet has great impact on its magnetic properties resulting from the interplay among different types of magnetic energies [10]. Until now, rod-shaped superparamagnetic PtNi nanoalloy and PtNi/ C nanoparticles have been synthesized through different methods [11,12]. H Weller et al. [13] have reported the synthesis of Pt1-xNix nanoparticles with tunable composition and size by a hot
* Corresponding authors. E-mail address: [email protected] (X.-W. Zhou). http://dx.doi.org/10.1016/j.matchemphys.2015.01.063 0254-0584/© 2015 Elsevier B.V. All rights reserved.
organometallic route, which needed a high temperature and expensive poisonous reactants. PtNi hollow spheres prepared by a template-replacement route as catalysts for hydrogen generation from ammonia borane have been investigated by Chen et al. [14]. This template-replacement method consisted of the removal of the template after synthesis, which may destroy the structure of the products. Then, synthesis of PtNi nanoparticles with controllable morphology and composition is crucial for progress in this field. Galvanic displacement reaction is a special template method, in which one substance is served as a suitable sacrificial template and reacts with other appropriate metal ions according to their different standard reduction potentials, resulting in the controlled formation of nanomaterials [15e18]. This method has been extended for the synthesis of PtNi nanoparticles with a hollow structure using Ni nanoparticles as a sacrificial template [19,20]. Here, we report the synthesis of highly monodispersed PtNi nanoparticles by galvanic displacement reaction. The size, composition and morphology of the PtNi nanoparticles can be controlled by varying the synthesis conditions. Based on a series of additional experiments, a probable mechanism of the formation of the PtNi
48
J.-J. Du et al. / Materials Chemistry and Physics 155 (2015) 47e51
nanoparticles is proposed. The magnetic properties are studied through superconducting quantum interference device (SQUID) magnetometer.
2. Experimental section 2.1. Synthesis of PtNi nanoparticles All chemicals are analytical grade and are used without further purification. The growth of the PtNi nanoparticles was carried out in a solution-phase system using the galvanic displacement
reaction. In brief, 9.5 mg NiCl2, 60 mL H2O and 100 mg poly(vinylpyrrolidone) (PVP) (K30) were mixed in a three-necked flask equipped with a heat controller. The mixture was kept at 75 C with vigorous stirring. Then, 25 mL of 0.01 M freshly prepared NaBH4 solution was added dropwise. The solution turned dark, indicating the formation of Ni nanoparticles. Once the NaBH4 was completely dropped into the solution, 20 mL of 4 mM K2PtCl6 was added dropwise. To avoid oxidation of the Ni nanoparticles, high-purity N2 was bubbled into the solution during the whole procedure. The obtained black solution was stirred for 2 h at 75 C and cooled to room temperature. Then the nanoparticles were separated, washed thoroughly and desiccated in vacuum for the following studies.
2.2. Instruments The samples were characterized by X-ray diffraction (XRD) using a Panalytical X'pert PRO diffractormeter. Transmission electron microscopy (TEM) and high-magnification transmission electron microscopy patterns were obtained on instruments of JEM-100CXII and FEI Tecnai-F30 electron microscopy, respectively. Samples for TEM analysis were prepared by adding a drop of the solution of PtNi nanoparticles onto a carbon-coated copper grid. The energy dispersed X-ray spectroscopy (EDS) was attached to the TEM system. X-ray photoelectron spectroscopy (XPS) spectrum was recorded by a PHI Quantum 2000 XPS system (Physical Electronics, Inc.) using AI Ka radiation at a base pressure below 5 108 Torr. Magnetic properties were measured by a superconducting quantum interference device (SQUID) magnetometer.
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Fig. 1. (a) TEM and (b) high-magnification TEM images of PtNi nanoparticles. The insets to (a) and (b) shown the size distribution and the selected area electron diffraction (SAED) pattern of the PtNi nanoparticles, respectively.
Fig. 2. (a) XRD and (b) EDS patterns of PtNi nanoparticles.
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3. Results and discussion 3.1. Characterization of PtNi nanoparticles TEM images in Fig. 1a illustrate that large-scale PtNi nanomaterials have been synthesized. It can be seen that the PtNi nanomaterials are solid, spherical and highly monodispersed. The size distribution of the sample in the inset of Fig. 1a shows that a number average mean diameter is about 52.6 nm obtained from the literature [21]. The structural details are revealed in highmagnification TEM (Fig. 1b). The individual PtNi nanoparticle has a non-uniform surface and high monodispersity. The dissolution (oxidation) of Ni and the co-reduction of Ni2þ and PtCl2 6 were done at the same time during the synthetic process. The diffusion of the oxidized Ni2þ through the surface may destroy the formation of the products. On the other side, the surface of the PtNi nanoparticles may be oxidized partly. So the surface of the PtNi nanoparticles is rough [16]. The selected area electron diffraction (SAED) pattern (inset in Fig. 1b) indicates that the PtNi nanoparticles have a facecentered cubic (fcc) structure. The concentric rings can be assigned as (111), (200), (220), (311) and (222), which can be confirmed by the following XRD results [22]. Fig. 2a shows the XRD pattern of the PtNi nanoparticles in the 2q region between 20 and 90 . Reflection peaks are found at 40.10 , 46.32 , 67.78 and 81.85 corresponding to the (111), (200), (220), and (311) fundamental peaks of the chemically disordered fcc Pt. The reflection peak of (222) is not clear. There are no peaks for phase separated structures such as pure Ni or its oxides in the XRD pattern, which could be the result of the alloy between Pt and Ni since platinum is known to alloy well with Ni [11]. Fig. 2b shows the
Fig. 3. XPS spectra of (a) Pt4f and (b) Ni2p regions of PtNi nanoparticles.
49
energy dispersed X-ray spectroscopy (EDS) analysis taken from many PtNi nanoparticles. The results demonstrate the coexistence of Pt and Ni with an average stoichiometry of Pt68Ni32, which is consistent with the theoretical ratio (Pt66.7Ni33.3). The element Cu in the EDS pattern is attributed to the TEM grid. Fig. 3a shows the region of Pt4f in the XPS spectrum of the PtNi nanoparticles. The spectrum can be fit by two pairs of overlapping Lorentzian curves [23]. The Pt4f7/2 and Pt4f5/2 lines appearing at 71.45 eV (peak 1) and 74.75 eV (peak 3) are attributed to metallic Pt0. The other two peaks appearing at 72.62 eV (peak 2) and 75.82 eV (peak 4) can be assigned to Pt2þ species in PtO and Pt(OH)2 [11,23]. A comparison of the relative intensities of the peaks shows that Pt in the PtNi nanoparticles is predominately present in the zerovalent metallic state (Pt0). The XPS spectrum of Ni2p is shown in Fig. 3b. The Ni states consist of NiO, NiO, Ni(OH)2 and NiOOH. In general, the Ni2p3/2 spectrum is made complicated by the presence of satellite signals of high binding energy adjacent to the main
Fig. 4. TEM images of (a) Ni nanoparticles obtained 75 C, and (b) PtNi nanoparticles obtained when NiCl2 and K2PtCl6 were mixed together first.
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peaks [11,24]. Taking the satellite peaks into account, the Ni2p3/2 peaks at the binding energies of 852.56, 853.73, 855.87, and 857.51 eV correspond to NiO, NiO, Ni(OH)2 and NiOOH, respectively. But in the XRD studies, we could not find the peaks of the hydroxides or oxides of Ni, which is possibly because of their amorphous nature [11]. 3.2. Formation mechanism of PtNi nanoparticles Several formation mechanisms of PtNi nanoparticles have been discussed. Lee et al. [11] reported the preparation of PtNi nanoparticles using hydrazine as reducer. They thought Pt nanoparticles were initially formed and then seeded the growth of the PtNi nanoparticles. Later, T Pal et al. [12] and H Weller et al. [13] obtained PtNi nanoparticles through different methods using the similar formation mechanism. In order to illuminate the formation
mechanism of the PtNi nanoparticles in our experiment, a series of control experiments were carried out. Firstly, monodispersed Ni nanoparticles could not be produced under the same condition (Fig. 4a). NiCl2 reacted with NaBH4 to form a darkish bulk nickel, which means PVP was not able to stabilize Ni nanoparticles [11]. Secondly, if NiCl2 and K2PtCl6 were mixed together first, the product was formless and agglomerated (Fig. 4b). These observations indicated that the presence of the platinum and the sequence of the addition of the NiCl2 and K2PtCl6 were crucial to the formation of the PtNi nanoparticles. Then, it was postulated that the basic formation mechanism of the PtNi nanoparticles proceeded by the following steps. NiCl2 was reduced to bulk nickel. Since the standard reduction potential of the PtCl2 6 /Pt [0.735 V vs standard hydrogen electrode (SHE)] is much higher than that of the Ni2þ/Ni (0.25 V vs SHE), PtCl2 6 was reduced to Pt as soon as K2PtCl6 was added to the bulk nickel solution. PVP has been proved to be a good stabilizer to protect Pt nanoparticle [25]. With the action of vigorous stirring and the protection of PVP, the reaction of bulk Ni and PtCl2 6 would result the dissolution of bulk Ni and form monodispersed nanoparticles. At the same time, bulk nickel was oxidized to Ni2þ ions. Then, the Ni2þ and PtCl2 6 will be co-reduced by the excess NaBH4 in the solution to form bimetallic PtNi nanoparticles. The influence of other reaction conditions on the size, shape, and composition of PtNi nanoparticles were investigated. The same results could be obtained when the ration of platinum and nickel precursors was varied from 1:2 to 1:1 (Fig. 5a). The influence of the reaction temperature to the nanoparticle morphology was also studied. Fig. 5b shown the TEM images of the PtNi nanoparticles synthesized at 30 C, in which we could see that the center portions of the nanoparticls are lighter than their wall edge, indicating a hollow structure [16,17]. This kind of hollow PtNi nanoparticles is a potential catalyst for DMFC [19]. Another important observation was that the particle size and atomic ration of Pt/Ni in the final products varied with the temperature. The particle size of PtNi nanoparticles increased along with rising reaction temperature (from 32.0 nm at 30 C to 52.6 nm at 75 C), and the nickel-toplatinum ratio increased concomitantly (from Pt75Ni25 at 30 C to Pt68N32 at 75 C). Actually, H Weller et al. [13] has observed the same phenomena of PtNi nanoparticles obtained by a hot organometallic synthesis. These results corresponded to the fact that Ni
2.5
Magnetization (emu/g)
2.0
1.5
1.0
0.5
0.0 0
50
100
150
200
250
300
Temperature (K) Fig. 5. TEM images of PtNi nanoparticles obtained at (a) m (NiCl2): m (K2PtCl6) ¼ 1:1, and (b) 30 C.
Fig. 6. FC/ZFC specific magnetization of PtNi nanoparticles in a field of 1000 Oe. Open circle: FC, solid circle: ZFC.
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51
300 K. For a superparamagnet, ferromagnetic behavior is observed when the temperature is below TB. When the temperature is above TB, there is a superparamagnetic relaxation. Therefore, we did not observe any hysteresis and coerciety at room temperature. 4. Conclusions
Fig. 7. Hysteresis loops measured at 5 K and 300 K of NiPt nanoparticles. The inset is an open-hysiteresis loop in the low-field region (<4500 Oe).
In summary, well monodispersed PtNi nanoparticles were synthesized by galvanic displacement reaction and chemical reduction. The size, composition and morphology could be controlled by varying the reaction conditions. A probable mechanism for the formation of the PtNi nanoparticles was proposed based on a series of assistant experiments. The reaction temperature was a crucial factor to determine the structures of the PtNi nanoparticles. The formation of Pt nanocrystal in the reaction process was the foremost step because of its self-catalysis effect. The results of magnetic measurements illustrated that the PtNi nanoparticles show a superparamagnetic behavior with a blocking temperature (TB) about 8.0 K. Acknowledgments
could be reduced easily at higher temperature [11,13]. So, the reaction temperature is a crucial factor to determine the size, composition, and morphology of the PtNi nanoparticles.
Financial supports from National Natural Science Foundation of China (21403126) and the help from Prof. Shi-Gang Sun in Xiamen University are highly acknowledged.
3.3. Magnetic properties of PtNi nanoparticles References The temperature-dependent magnetization of PtNi nanomaterials was measured by superconducting quantum interference device (SQUID) magnetometer between 5 and 300 K using zerofield-cooling (ZFC) and field-cooling (FC) procedures in an applied field of 1000 Oe. Superparamagnetic behavior was observed for the PtNi nanoparticles as shown in Fig. 6. For single-domain particles, the temperature of the maximum in the ZFC curve is regarded as the average blocking temperature (TB), below which the sample is ferromagnetic and above which is superparamagnetic [23]. The sharp maximum at 8.0 K in the ZFC curve indicated a clear blocking behavior of the PtNi nanoparticles. Accordingly, the TB of the PtNi nanoparticles was 8.0 K. It is well known that TB varies with the variation of particle size, shape and the atomic ration of Pt to Ni. For example, the TB obtained by H Weller [13] was 7.4 K and 65 K for rod-like PtNi particles reported by T Pal [12]. The well overlap of the ZFC curve and FC curve when the temperature was higher than TB indicated that the PtNi nanoparticles had a narrow size distribution, which has been confirmed by the TEM studies. Fig. 7 shows the hysteresis loops of the PtNi nanomaterials measured at 5 K and 300 K. The inset was an open-hysiteresis loop in the low-field region (<4500 Oe). For 5 K, saturation magnetization (Ms), remanent magnetization (Mr), coercivity (Hc), and squareness (Sr ¼ Mr/Ms) were 10.4 emu g1, 1.1 emu g1, 517 Oe and 0.1, respectively (emu: electromagnetic unit) between 30 kOe and 30 kOe. When temperatures was higher than TB, irreversible magnetization, that is, hysteretic behavior and coerciety, disappeared, exemplified by the magnetization measurement at
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