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Synthesis and characterization of Pt–Cu bimetallic alloy nanoparticles by reverse micelles method
Colloids and Surfaces A: Physicochem. Eng. Aspects 273 (2006) 35–42
Synthesis and characterization of Pt–Cu bimetallic alloy nanoparticles by reverse micelles method Wang Weihua, Tian Xuelin, Chen Kai, Cao Gengyu ∗ Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, Anhui, China Received 11 May 2005; received in revised form 23 July 2005; accepted 29 July 2005 Available online 8 September 2005
Abstract Platinum–copper bimetallic alloy nanoparticles were synthesized in water-in-oil (w/o) microemulsions of water/cetyltrimethyammonium bromide (CTAB)/isooctane/n-butanol by the co-reduction of H2 PtCl6 and CuCl2 with hydrazine at room temperature. The samples were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectrum (XPS). XRD results show that there is only one peak in the pattern of bimetallic nanoparticles, corresponding to the (1 1 1) plane of the PtCu3 bulk alloy. XPS illustrates that both elements in the nanoparticles are in zero-valence and possess the characteristic metallic binding energy. HRTEM analyses confirm the formation of the PtCu3 alloy nanoparticles with a mean diameter of about 1.6 nm, where the corresponding ˚ is consistent with that of the (2 0 0) plane of PtCu3 bulk alloy. Moreover, the factors influencing the size of PtCu3 lattice spacing of 1.87 A nanoparticles had also been investigated and discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: Pt/Cu; CTAB; Water-in-oil; Bimetallic nanoparticles
1. Introduction Recently, considerable efforts have been devoted to bimetallic nanoparticles since they are of great interest from both scientific and technological perspective for the modification of physical and chemical properties of metal nanoparticles [1–4]. Bimetallic colloids, in which two kinds of metals are assembled in one entity, have well different catalytic, electronic and optical properties distinct from those of the corresponding monometallic nanoparticles [1,5,6]. Bimetallic colloids can be prepared by simultaneous co-reduction of two kinds of metal ions with or without protective agent (usually polymer or surfactant) or by successive reduction of one metal over the nuclei of another. Many methods for the preparation of bimetallic nanoparticles have been reported, such as alcohol reduction [7], citrate reduction [8], hydrothermal [9,10], sonochemical method [11–13], co-precipitation
∗
Corresponding author. Tel.: +86 551 3603418; fax: +86 551 3602969. E-mail address: [email protected] (C. Gengyu).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.07.029
[14,15] and reverse micelles [16–29]. In general, their stability and sizes are controlled by the addition of protective agents, such as soluble polymers, surfactants, organic ligands. Moreover, the size distribution, structure and composition of the bimetallic nanoparticles have also been affected by the preparation conditions. It is well known that the Pt–X (X = Cu, Au, or Ag, etc.) alloys are of particular importance due to their many applications in catalytic fields, such as hydrogenation [30], isomerization [31], dehydrogenation [32] and hydrocracking [33]. Especially, for the Pt–Cu alloy, the polymer-protected colloidal Pt–Cu particles were prepared and showed their extensive utilities in the catalytic hydrogenation in solution, where the bimetallic clusters exhibit excellent prosperities as the active and selective for the hydration of acrylonitrile to acrylamide as well as the hydrogenation of 1,3-cyclooctadiene to cyclooctene [34]. At the same time, the supported Pt-Cu bimetallic catalyst may also be effective in NOx reduction and systematic investigation of the synthesis and study of bimetallic PtCu nanoparticles have been done to evaluate their heterogeneous catalytic activities for reduction of gas
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W. Weihua et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 273 (2006) 35–42
phase NO with H2 as the reducing agent [35,36]. Additionally, in previous investigations, Pt–Cu alloy had been prepared by co-precipitation method supported on the silica and alumina oxide. However, the investigations on the Pt–Cu bimetallic nanoparticles prepared by reverse micelles have seldom been reported to date to our best knowledge, although many samples of platinum or its alloy, such as Pt–Pd [8,20], Pt–Au [37] and Pt–Ag [38], prepared by this method have been well researched. Reverse micelles are water-in-oil (w/o) microemulsions, which are thermodynamically stable, transparent and isotropic liquid–liquid media. In reverse micelles, nanometersized water cores can be varied by changing the molar ratio of water to surfactant, where these water pools not only offer nanoreactors for the formation of nanoparticles but also prevent the aggregation of nanoparticles [39,40]. Thus, reverse micelles are appropriate microscopic reactors for the synthesis of monodisperse nanoparticles with narrow size distributions. Many kinds of nanoparticles have been synthesized by this method, such as metals [16–19], bimetals [20–23], metal oxide and sulfides [24–26], metal borides [27], metal carbonates [28] and organic polymers [29]. In the present paper, the Pt–Cu bimetallic nanoparticles were prepared by reverse micelle method in the cetyltrimethyammonium bromide (CTAB)/isooctane/nbutanol/water system. Subsequently, the morphology, size, structure and composition of the resultant nanoparticles were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectrum (XPS), respectively.
2. Experimental 2.1. Materials Hydrogen hexachloroplatinate(IV), hydrate (A.R.) and nbutanol (A.R.) were purchased from the First Reagent Plant of Shanghai. Copper(II) chloride dihydrate (A.R.), hydrazine hydrate (A.R.), isooctane (A.R.) and cetyltrimethyammonium bromide (A.R.) were obtained from Shanghai Chemical Company. All these reagents were used without further purification. The water used in this work was distilled water. The aqueous solution of H2 PtCl6 (0.01, 0.02, 0.08 M), CuCl2 (0.01, 0.02, 0.08 M) and hydrazine hydrate (0.5, 2 M) were prepared by dissolving them in de-ionized water, respectively. According to the molar ratio of Pt/Cu (3:1, 1:1, 1:3), the Pt and Cu salts were mixed together for use. The reverse micelle solutions containing hydrazine or salt solution were prepared by injecting them into the isooctane and n-butanol solution of CTAB. 2.2. Preparation of nanoparticles The reverse micelle solutions were prepared using CTAB as the surfactant, isooctane as the oil phase and n-butanol as
the cosurfactant, where the n-butanol can increase the polarity of the surfactant and the stability of the reverse micelle solution. The reverse micelle solutions have the different molar ratios of water to CTAB (w) at 6, 8, 10, 20 and 30, where the molar ratio n-butanol to CTAB (p) and the CTAB concentration are fixed at 5 and 0.3 M according to overall volume of n-butanol and isooctane, respectively. The Pt–Cu alloy nanoparticles were obtained by mixing and stirring the equal volume of two w/o microemulsion solutions in a bottle, one containing an aqueous solution of metal salts and the other containing an aqueous solution of hydrazine. The reduction reaction can be expressed as H2 PtCl6 + N2 H5 OH → Pt + 6HCl + N2 + H2 O 2CuCl2 + N2 H5 OH → 2Cu + 4HCl + N2 + H2 O Obviously, the solution became dark immediately after mixing. The reaction time was hold for 5 h with rapidly stirring at room temperature. Then ethanol was added to the bottle to make phase separation. The final mixture was centrifuged to get sample. Finally, the obtained products were washed with ethanol and dried at 303 K for further characterization. 2.3. Characterization The morphology and size of the obtained nanoparticles were determined by high-resolution transmission electron microscopy on a JEOL 2010 transmission electron microscope at an acceleration voltage of 200 kV. First, those samples were redispersed into alcohol solution under ultrasound to get colloidal solution for HRTEM analysis. Then, place a droplet of the colloidal solution onto a carbon-coated copper grid and evaporate it in air at room temperature. The sizes of nanoparticles are obtained based on a count of about 300 nanoparticles. X-ray diffraction measurements were carried out on a Philips X’Pert Pro Super X-ray diffractometer using Cu K␣ ˚ Continuous X-ray scans were carradiation (λ = 1.54178 A). ◦ ried out from 10 to 70◦ of 2θ. The X-ray photoelectron spectrum measurements were performed on a VG ESCALAB MKII spectrometer equipped with Al K␣ X-ray source.
3. Results and discussion 3.1. Particle size (HRTEM) The size and morphology of the bimetallic nanoparticles were characterized by HRTEM. As displayed in Fig. 1, the PtCu3 nanoparticles (w = 8, [metal salts] = 0.02 M) were very fine and monodisperse with narrow size distribution obviously, where the dark particles in the image were PtCu3 nanoparticles. Further analyses suggest that those particles
W. Weihua et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 273 (2006) 35–42
37
Fig. 3. Schematic structure for PtCu3 .
Fig. 1. TEM images of the PtCu3 alloy nanoparticles: the bar represents 20 nm ([metal salts] = 0.02 M; [N2 H5 OH] = 0.5 M; [CTAB] = 0.3 M; w = 8; p = 5).
have a mean size of 1.6 nm, which is smaller than those prepared by Toshima and Wang [34]. 3.2. Particle structure 3.2.1. XRD analyses To gain more insights into the alloy structure of the PtCu3 bimetallic nanoparticles, XRD and HRTEM were carried out. Fig. 2 illustrates the XRD patterns of Pt–Cu nanoparticles
Fig. 2. XRD patterns of (a) Pt nanoparticles, (b) Pt3 Cu nanoparticles, (c) PtCu nanoparticles and (d) PtCu3 nanoparticles ([metal salts] = 0.02 M; [N2 H5 OH] = 0.5 M; [CTAB] = 0.3 M; w = 8; p = 5).
Fig. 4. XPS peaks of Cu 2p (a) and Pt 4f (b) for the bimetallic PtCu3 nanoparticles (w = 8; [metal salts] = 0.02 M).
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in various compositions, which have been prepared under the same condition as that of the PtCu3 sample shown in Fig. 1. In Fig. 2, the lines of a–d indicate the XRD patterns of Pt, Pt3 Cu, PtCu and PtCu3 nanoparticles, respectively, where the XRD pattern of Cu nanoparticles is not shown because of the oxidation of Cu nanoparticles in air. Obviously, the characteristic peaks of the (1 1 1) and (2 0 0)
planes for Pt (2θ = 39.77◦ , 46.25◦ ) in line a revealed that Pt nanoparticles essentially were face-centered cubic (fcc) structure similar to that of bulk metallic Pt reported previously [41]. Moreover, the diffraction angles of (1 1 1) plane for Pt/Cu (3:1, 1:1 and 1:3) nanoparticles were located between the (1 1 1) planes of metallic Pt and Cu, and also shifted toward higher value with the decreasing of the Pt/Cu ratios,
Fig. 5. (a–g) TEM images of the PtCu3 alloy nanoparticles.
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indicating that the interplanar spacing changes with composition. Compared with the XRD patterns of Pt and Cu [34], no diffraction peaks for metallic Cu, copper oxide and Pt are detected in the XRD pattern of PtCu3 (line d), implying that the copper and platinum ions have been reduced to the
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zero-valence state under the present conditions. At the same time, the broad peak at the 2θ = 42.21◦ in the diffraction patten of PtCu3 , corresponding to the (1 1 1) plane of PtCu3 bulk alloy, is in accordance with that of standard PtCu3 alloy phase. On the basis of these analyses above, it is further established that PtCu3 bimetallic nanoparticles are
Fig. 5. (Continued)
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W. Weihua et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 273 (2006) 35–42
Fig. 5. (Continued).
composed of a PtCu3 alloy phase rather than a mixed phase of monometallic copper and platinum nanoparticles. Moreover, the XRD pattern for PtCu3 bimetallic nanoparticles exhibited broader characteristic peaks owing to the small particle sizes as usually observed for nanoparticles reported previously [34]. 3.2.2. HRTEM analyses To further characterize the bimetallic nanoclusters, their lattice images were obtained by HRTEM. As shown in Fig. 1, the inset represents the HRTEM micrographs of the PtCu3 bimetallic nanoparticles. The lattice fingers were mostly ˚ So (2 0 0) plane and the lattice spacing was found to be 1.87 A. the lattice constant of (2 0 0) plane of the bimetallic nanoparticles of PtCu3 agreed with the value of bulk PtCu3 , implying that the PtCu3 alloy nanoparticles were formed and no other impurities existed, such as Pt, Cu or copper oxide. 3.2.3. Schematic structure of PtCu3 Analyses of XRD and HRTEM data show that the obtained PtCu3 nanoparticles have face-centered cubic structure. The value of plane distance is located at between that of Pt and Cu, which proves the formation of the alloy nanoparticles. It may be presumed that Cu atoms incorporated into the lattices of Pt and replaced the Pt atoms located at the face-centered plane. The schematic structure for PtCu3 nanoparticles are showed in Fig. 3, where the Pt and Cu atoms arrange according to the ratio of 1 to 3. 3.2.4. Composition analyses (XPS) The surface atomic ratio of the bimetallic nanoparticles was measured by XPS. XPS spectra of the Pt 4f and Cu 2p regions in the sample of PtCu3 are displayed in Fig. 4. Corresponding binding energies (BE) have been presented in Table 1. The characteristic peak of Pt 4f7/2 BE for the metallic Pt appears at 71.1 eV [42]. For the sample of PtCu3 (w = 8, [metal salts] = 0.02 M), the Pt 4f7/2 lines become broader by 0.2 eV with a shift of the BE by
0.3 to 71.4 eV. The Cu 2p3/2 peak in the PtCu3 bimetallic nanoparticles can be observed at the BE of 932.3 eV, which is lower by 1.7 eV than that of CuO, suggesting the existence of the metallic copper in the PtCu3 . Additionally, the surface atomic ratios and concentrations of elements in the sample could be obtained from the intensity of XPS peaks. The results show that the ratio of Pt to Cu is about 1:2.9, which is similar to the feed ratio of Pt to Cu. 3.3. The influences of w and concentration of the metal salts on the nanoparticle size To investigate the influences of w and concentration of PtCl6 2− and Cu2+ on the size of PtCu3 nanoparticle, we have prepared two series of PtCu3 samples at different w (6, 8, 10, 20 and 30, 0.02 M) and concentrations of PtCl6 2− and Cu2+ (0.01, 0.02 and 0.08 M, w = 8), respectively. Correspondingly, Fig. 5 shows the observed TEM images and size distributions of the bimetallic nanoparticles. 3.3.1. The influence of w The samples with different values of w were prepared through keeping the total concentration of PtCl6 2− and Cu2+ at 0.02 M. The TEM images and size distributions of the samples (w = 6, 8, 10, 20 and 30) are illustrated in Fig. 5(a–e). As displayed in Fig. 5, the mean sizes of PtCu3 nanoparticles Table 1 XPS data of Pt, Cu and PtCu3 (w = 8; [metal salts] = 0.02 M) Samples
BE (eV) Cu 2p3/2
Cua CuOa Pta PtCu3 a
See ref. [42].
Pt 4f7/2
933.1 934.0 932.3
71.1 71.4
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were 1.5, 1.6, 1.8, 1.9 and 2.0 nm, corresponding to the w of 6, 8, 10, 20 and 30, respectively. Thus, it is concluded that the value of w has a relatively small influence on the size of nanoparticles. Here, this phenomenon is slightly different from those in the reports previously [43–45], where there was a strong increase in size at high w values. Probably, the difference may be due to the fact that the total concentration of metal ions under the present situation is so small that the variation of w has no great influence on the size of nanoparticles. Additionally, those TEM images also suggested that the sizes of nanoparticles are very fine and monodisperse at the present conditions and also prove the formation of Pt–Cu alloy. This point is also consistent with the fact that if alloy particles were not formed, there would be a broad or bimodal size distribution due to the different growth rates for the two metal colloids [40]. 3.3.2. The influence of metal salts concentration Similarly, the different PtCu3 nanoparticle samples with the w of 8 were prepared through changing the total concentration of the metal salts. As displayed in Fig. 5(f and g), the PtCu3 nanoparticles have mean sizes of about 1.2 and 1.8 nm, corresponding to the samples with the total concentration of 0.01 and 0.08 M, respectively. Thus, it can be seen that the total concentration of metal salts has a greater influence on the size of nanoparticles than that of the value of w, which is also in accordance with the results reported previously [40].
4. Conclusions In the present study, platinum–copper alloy nanoparticles were synthesized in water-in-oil microemulsions of water/CTAB/isooctane/n-butanol by the co-reduction of H2 PtCl6 and CuCl2 with hydrazine at room temperature. Compared with that of glycol refluxing, the method of reverse micelles can produce the well monodisperse PtCu3 alloy nanoparticles with very small sizes at room temperature. Additionally, the factors influencing the size of PtCu3 nanoparticles have also been investigated by changing the value of w and the concentration of metal salts, where the latter has a greater influence than the former in the present study. Experimental results show that there is only one diffraction peak in the XRD pattern of PtCu3 alloy nanoparticles, corresponding to the (1 1 1) plane of the PtCu3 bulk alloy. XPS illustrates that the Pt and Cu in PtCu3 nanoparticles are in zero-valence and possess the characteristic metallic binding energy. Further, HRTEM analyses confirm the formation of the PtCu3 alloy nanoparticles with a mean diameter of about ˚ is 1.6 nm, where the corresponding lattice spacing of 1.87 A consistent with that of the (2 0 0) plane of PtCu3 bulk alloy. Thus, the HRTEM, XRD and XPS confirm the formation of PtCu3 alloy nanoparticles.
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Acknowledgements This work is supported by The Chinese Academy of Sciences through the “Overseas Outstanding Talents” program and National Natural Science Foundation of China under Grant No. 20473076. We are also grateful to the referees for their suggestions to improve the presentation of the results.
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Synthesis and characterization of Pt–Cu bimetallic alloy nanoparticles by reverse micelles method Wang Weihua, Tian Xuelin, Chen Kai, Cao Gengyu ∗ Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemical Physics, University of Science and Technology of China, Hefei 230026, Anhui, China Received 11 May 2005; received in revised form 23 July 2005; accepted 29 July 2005 Available online 8 September 2005
Abstract Platinum–copper bimetallic alloy nanoparticles were synthesized in water-in-oil (w/o) microemulsions of water/cetyltrimethyammonium bromide (CTAB)/isooctane/n-butanol by the co-reduction of H2 PtCl6 and CuCl2 with hydrazine at room temperature. The samples were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectrum (XPS). XRD results show that there is only one peak in the pattern of bimetallic nanoparticles, corresponding to the (1 1 1) plane of the PtCu3 bulk alloy. XPS illustrates that both elements in the nanoparticles are in zero-valence and possess the characteristic metallic binding energy. HRTEM analyses confirm the formation of the PtCu3 alloy nanoparticles with a mean diameter of about 1.6 nm, where the corresponding ˚ is consistent with that of the (2 0 0) plane of PtCu3 bulk alloy. Moreover, the factors influencing the size of PtCu3 lattice spacing of 1.87 A nanoparticles had also been investigated and discussed. © 2005 Elsevier B.V. All rights reserved. Keywords: Pt/Cu; CTAB; Water-in-oil; Bimetallic nanoparticles
1. Introduction Recently, considerable efforts have been devoted to bimetallic nanoparticles since they are of great interest from both scientific and technological perspective for the modification of physical and chemical properties of metal nanoparticles [1–4]. Bimetallic colloids, in which two kinds of metals are assembled in one entity, have well different catalytic, electronic and optical properties distinct from those of the corresponding monometallic nanoparticles [1,5,6]. Bimetallic colloids can be prepared by simultaneous co-reduction of two kinds of metal ions with or without protective agent (usually polymer or surfactant) or by successive reduction of one metal over the nuclei of another. Many methods for the preparation of bimetallic nanoparticles have been reported, such as alcohol reduction [7], citrate reduction [8], hydrothermal [9,10], sonochemical method [11–13], co-precipitation
∗
Corresponding author. Tel.: +86 551 3603418; fax: +86 551 3602969. E-mail address: [email protected] (C. Gengyu).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.07.029
[14,15] and reverse micelles [16–29]. In general, their stability and sizes are controlled by the addition of protective agents, such as soluble polymers, surfactants, organic ligands. Moreover, the size distribution, structure and composition of the bimetallic nanoparticles have also been affected by the preparation conditions. It is well known that the Pt–X (X = Cu, Au, or Ag, etc.) alloys are of particular importance due to their many applications in catalytic fields, such as hydrogenation [30], isomerization [31], dehydrogenation [32] and hydrocracking [33]. Especially, for the Pt–Cu alloy, the polymer-protected colloidal Pt–Cu particles were prepared and showed their extensive utilities in the catalytic hydrogenation in solution, where the bimetallic clusters exhibit excellent prosperities as the active and selective for the hydration of acrylonitrile to acrylamide as well as the hydrogenation of 1,3-cyclooctadiene to cyclooctene [34]. At the same time, the supported Pt-Cu bimetallic catalyst may also be effective in NOx reduction and systematic investigation of the synthesis and study of bimetallic PtCu nanoparticles have been done to evaluate their heterogeneous catalytic activities for reduction of gas
36
W. Weihua et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 273 (2006) 35–42
phase NO with H2 as the reducing agent [35,36]. Additionally, in previous investigations, Pt–Cu alloy had been prepared by co-precipitation method supported on the silica and alumina oxide. However, the investigations on the Pt–Cu bimetallic nanoparticles prepared by reverse micelles have seldom been reported to date to our best knowledge, although many samples of platinum or its alloy, such as Pt–Pd [8,20], Pt–Au [37] and Pt–Ag [38], prepared by this method have been well researched. Reverse micelles are water-in-oil (w/o) microemulsions, which are thermodynamically stable, transparent and isotropic liquid–liquid media. In reverse micelles, nanometersized water cores can be varied by changing the molar ratio of water to surfactant, where these water pools not only offer nanoreactors for the formation of nanoparticles but also prevent the aggregation of nanoparticles [39,40]. Thus, reverse micelles are appropriate microscopic reactors for the synthesis of monodisperse nanoparticles with narrow size distributions. Many kinds of nanoparticles have been synthesized by this method, such as metals [16–19], bimetals [20–23], metal oxide and sulfides [24–26], metal borides [27], metal carbonates [28] and organic polymers [29]. In the present paper, the Pt–Cu bimetallic nanoparticles were prepared by reverse micelle method in the cetyltrimethyammonium bromide (CTAB)/isooctane/nbutanol/water system. Subsequently, the morphology, size, structure and composition of the resultant nanoparticles were characterized by high-resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectrum (XPS), respectively.
2. Experimental 2.1. Materials Hydrogen hexachloroplatinate(IV), hydrate (A.R.) and nbutanol (A.R.) were purchased from the First Reagent Plant of Shanghai. Copper(II) chloride dihydrate (A.R.), hydrazine hydrate (A.R.), isooctane (A.R.) and cetyltrimethyammonium bromide (A.R.) were obtained from Shanghai Chemical Company. All these reagents were used without further purification. The water used in this work was distilled water. The aqueous solution of H2 PtCl6 (0.01, 0.02, 0.08 M), CuCl2 (0.01, 0.02, 0.08 M) and hydrazine hydrate (0.5, 2 M) were prepared by dissolving them in de-ionized water, respectively. According to the molar ratio of Pt/Cu (3:1, 1:1, 1:3), the Pt and Cu salts were mixed together for use. The reverse micelle solutions containing hydrazine or salt solution were prepared by injecting them into the isooctane and n-butanol solution of CTAB. 2.2. Preparation of nanoparticles The reverse micelle solutions were prepared using CTAB as the surfactant, isooctane as the oil phase and n-butanol as
the cosurfactant, where the n-butanol can increase the polarity of the surfactant and the stability of the reverse micelle solution. The reverse micelle solutions have the different molar ratios of water to CTAB (w) at 6, 8, 10, 20 and 30, where the molar ratio n-butanol to CTAB (p) and the CTAB concentration are fixed at 5 and 0.3 M according to overall volume of n-butanol and isooctane, respectively. The Pt–Cu alloy nanoparticles were obtained by mixing and stirring the equal volume of two w/o microemulsion solutions in a bottle, one containing an aqueous solution of metal salts and the other containing an aqueous solution of hydrazine. The reduction reaction can be expressed as H2 PtCl6 + N2 H5 OH → Pt + 6HCl + N2 + H2 O 2CuCl2 + N2 H5 OH → 2Cu + 4HCl + N2 + H2 O Obviously, the solution became dark immediately after mixing. The reaction time was hold for 5 h with rapidly stirring at room temperature. Then ethanol was added to the bottle to make phase separation. The final mixture was centrifuged to get sample. Finally, the obtained products were washed with ethanol and dried at 303 K for further characterization. 2.3. Characterization The morphology and size of the obtained nanoparticles were determined by high-resolution transmission electron microscopy on a JEOL 2010 transmission electron microscope at an acceleration voltage of 200 kV. First, those samples were redispersed into alcohol solution under ultrasound to get colloidal solution for HRTEM analysis. Then, place a droplet of the colloidal solution onto a carbon-coated copper grid and evaporate it in air at room temperature. The sizes of nanoparticles are obtained based on a count of about 300 nanoparticles. X-ray diffraction measurements were carried out on a Philips X’Pert Pro Super X-ray diffractometer using Cu K␣ ˚ Continuous X-ray scans were carradiation (λ = 1.54178 A). ◦ ried out from 10 to 70◦ of 2θ. The X-ray photoelectron spectrum measurements were performed on a VG ESCALAB MKII spectrometer equipped with Al K␣ X-ray source.
3. Results and discussion 3.1. Particle size (HRTEM) The size and morphology of the bimetallic nanoparticles were characterized by HRTEM. As displayed in Fig. 1, the PtCu3 nanoparticles (w = 8, [metal salts] = 0.02 M) were very fine and monodisperse with narrow size distribution obviously, where the dark particles in the image were PtCu3 nanoparticles. Further analyses suggest that those particles
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Fig. 3. Schematic structure for PtCu3 .
Fig. 1. TEM images of the PtCu3 alloy nanoparticles: the bar represents 20 nm ([metal salts] = 0.02 M; [N2 H5 OH] = 0.5 M; [CTAB] = 0.3 M; w = 8; p = 5).
have a mean size of 1.6 nm, which is smaller than those prepared by Toshima and Wang [34]. 3.2. Particle structure 3.2.1. XRD analyses To gain more insights into the alloy structure of the PtCu3 bimetallic nanoparticles, XRD and HRTEM were carried out. Fig. 2 illustrates the XRD patterns of Pt–Cu nanoparticles
Fig. 2. XRD patterns of (a) Pt nanoparticles, (b) Pt3 Cu nanoparticles, (c) PtCu nanoparticles and (d) PtCu3 nanoparticles ([metal salts] = 0.02 M; [N2 H5 OH] = 0.5 M; [CTAB] = 0.3 M; w = 8; p = 5).
Fig. 4. XPS peaks of Cu 2p (a) and Pt 4f (b) for the bimetallic PtCu3 nanoparticles (w = 8; [metal salts] = 0.02 M).
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in various compositions, which have been prepared under the same condition as that of the PtCu3 sample shown in Fig. 1. In Fig. 2, the lines of a–d indicate the XRD patterns of Pt, Pt3 Cu, PtCu and PtCu3 nanoparticles, respectively, where the XRD pattern of Cu nanoparticles is not shown because of the oxidation of Cu nanoparticles in air. Obviously, the characteristic peaks of the (1 1 1) and (2 0 0)
planes for Pt (2θ = 39.77◦ , 46.25◦ ) in line a revealed that Pt nanoparticles essentially were face-centered cubic (fcc) structure similar to that of bulk metallic Pt reported previously [41]. Moreover, the diffraction angles of (1 1 1) plane for Pt/Cu (3:1, 1:1 and 1:3) nanoparticles were located between the (1 1 1) planes of metallic Pt and Cu, and also shifted toward higher value with the decreasing of the Pt/Cu ratios,
Fig. 5. (a–g) TEM images of the PtCu3 alloy nanoparticles.
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indicating that the interplanar spacing changes with composition. Compared with the XRD patterns of Pt and Cu [34], no diffraction peaks for metallic Cu, copper oxide and Pt are detected in the XRD pattern of PtCu3 (line d), implying that the copper and platinum ions have been reduced to the
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zero-valence state under the present conditions. At the same time, the broad peak at the 2θ = 42.21◦ in the diffraction patten of PtCu3 , corresponding to the (1 1 1) plane of PtCu3 bulk alloy, is in accordance with that of standard PtCu3 alloy phase. On the basis of these analyses above, it is further established that PtCu3 bimetallic nanoparticles are
Fig. 5. (Continued)
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Fig. 5. (Continued).
composed of a PtCu3 alloy phase rather than a mixed phase of monometallic copper and platinum nanoparticles. Moreover, the XRD pattern for PtCu3 bimetallic nanoparticles exhibited broader characteristic peaks owing to the small particle sizes as usually observed for nanoparticles reported previously [34]. 3.2.2. HRTEM analyses To further characterize the bimetallic nanoclusters, their lattice images were obtained by HRTEM. As shown in Fig. 1, the inset represents the HRTEM micrographs of the PtCu3 bimetallic nanoparticles. The lattice fingers were mostly ˚ So (2 0 0) plane and the lattice spacing was found to be 1.87 A. the lattice constant of (2 0 0) plane of the bimetallic nanoparticles of PtCu3 agreed with the value of bulk PtCu3 , implying that the PtCu3 alloy nanoparticles were formed and no other impurities existed, such as Pt, Cu or copper oxide. 3.2.3. Schematic structure of PtCu3 Analyses of XRD and HRTEM data show that the obtained PtCu3 nanoparticles have face-centered cubic structure. The value of plane distance is located at between that of Pt and Cu, which proves the formation of the alloy nanoparticles. It may be presumed that Cu atoms incorporated into the lattices of Pt and replaced the Pt atoms located at the face-centered plane. The schematic structure for PtCu3 nanoparticles are showed in Fig. 3, where the Pt and Cu atoms arrange according to the ratio of 1 to 3. 3.2.4. Composition analyses (XPS) The surface atomic ratio of the bimetallic nanoparticles was measured by XPS. XPS spectra of the Pt 4f and Cu 2p regions in the sample of PtCu3 are displayed in Fig. 4. Corresponding binding energies (BE) have been presented in Table 1. The characteristic peak of Pt 4f7/2 BE for the metallic Pt appears at 71.1 eV [42]. For the sample of PtCu3 (w = 8, [metal salts] = 0.02 M), the Pt 4f7/2 lines become broader by 0.2 eV with a shift of the BE by
0.3 to 71.4 eV. The Cu 2p3/2 peak in the PtCu3 bimetallic nanoparticles can be observed at the BE of 932.3 eV, which is lower by 1.7 eV than that of CuO, suggesting the existence of the metallic copper in the PtCu3 . Additionally, the surface atomic ratios and concentrations of elements in the sample could be obtained from the intensity of XPS peaks. The results show that the ratio of Pt to Cu is about 1:2.9, which is similar to the feed ratio of Pt to Cu. 3.3. The influences of w and concentration of the metal salts on the nanoparticle size To investigate the influences of w and concentration of PtCl6 2− and Cu2+ on the size of PtCu3 nanoparticle, we have prepared two series of PtCu3 samples at different w (6, 8, 10, 20 and 30, 0.02 M) and concentrations of PtCl6 2− and Cu2+ (0.01, 0.02 and 0.08 M, w = 8), respectively. Correspondingly, Fig. 5 shows the observed TEM images and size distributions of the bimetallic nanoparticles. 3.3.1. The influence of w The samples with different values of w were prepared through keeping the total concentration of PtCl6 2− and Cu2+ at 0.02 M. The TEM images and size distributions of the samples (w = 6, 8, 10, 20 and 30) are illustrated in Fig. 5(a–e). As displayed in Fig. 5, the mean sizes of PtCu3 nanoparticles Table 1 XPS data of Pt, Cu and PtCu3 (w = 8; [metal salts] = 0.02 M) Samples
BE (eV) Cu 2p3/2
Cua CuOa Pta PtCu3 a
See ref. [42].
Pt 4f7/2
933.1 934.0 932.3
71.1 71.4
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were 1.5, 1.6, 1.8, 1.9 and 2.0 nm, corresponding to the w of 6, 8, 10, 20 and 30, respectively. Thus, it is concluded that the value of w has a relatively small influence on the size of nanoparticles. Here, this phenomenon is slightly different from those in the reports previously [43–45], where there was a strong increase in size at high w values. Probably, the difference may be due to the fact that the total concentration of metal ions under the present situation is so small that the variation of w has no great influence on the size of nanoparticles. Additionally, those TEM images also suggested that the sizes of nanoparticles are very fine and monodisperse at the present conditions and also prove the formation of Pt–Cu alloy. This point is also consistent with the fact that if alloy particles were not formed, there would be a broad or bimodal size distribution due to the different growth rates for the two metal colloids [40]. 3.3.2. The influence of metal salts concentration Similarly, the different PtCu3 nanoparticle samples with the w of 8 were prepared through changing the total concentration of the metal salts. As displayed in Fig. 5(f and g), the PtCu3 nanoparticles have mean sizes of about 1.2 and 1.8 nm, corresponding to the samples with the total concentration of 0.01 and 0.08 M, respectively. Thus, it can be seen that the total concentration of metal salts has a greater influence on the size of nanoparticles than that of the value of w, which is also in accordance with the results reported previously [40].
4. Conclusions In the present study, platinum–copper alloy nanoparticles were synthesized in water-in-oil microemulsions of water/CTAB/isooctane/n-butanol by the co-reduction of H2 PtCl6 and CuCl2 with hydrazine at room temperature. Compared with that of glycol refluxing, the method of reverse micelles can produce the well monodisperse PtCu3 alloy nanoparticles with very small sizes at room temperature. Additionally, the factors influencing the size of PtCu3 nanoparticles have also been investigated by changing the value of w and the concentration of metal salts, where the latter has a greater influence than the former in the present study. Experimental results show that there is only one diffraction peak in the XRD pattern of PtCu3 alloy nanoparticles, corresponding to the (1 1 1) plane of the PtCu3 bulk alloy. XPS illustrates that the Pt and Cu in PtCu3 nanoparticles are in zero-valence and possess the characteristic metallic binding energy. Further, HRTEM analyses confirm the formation of the PtCu3 alloy nanoparticles with a mean diameter of about ˚ is 1.6 nm, where the corresponding lattice spacing of 1.87 A consistent with that of the (2 0 0) plane of PtCu3 bulk alloy. Thus, the HRTEM, XRD and XPS confirm the formation of PtCu3 alloy nanoparticles.
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Acknowledgements This work is supported by The Chinese Academy of Sciences through the “Overseas Outstanding Talents” program and National Natural Science Foundation of China under Grant No. 20473076. We are also grateful to the referees for their suggestions to improve the presentation of the results.
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