Alloying and dealloying of CuPt bimetallic nanocrystals - Core

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Progress in Natural Science: Materials International 2013;23(3):331–337 Chinese Materials Research Society

Progress in Natural Science: Materials International www.elsevier.com/locate/pnsmi www.sciencedirect.com

ORIGINAL RESEARCH

Alloying and dealloying of CuPt bimetallic nanocrystals Fengjiao Yu, Wuzong Zhoun School of Chemistry, University of St Andrews, St Andrews KY16 9ST, United Kingdom Received 5 August 2012; accepted 26 March 2013 Available online 17 May 2013

KEYWORDS CuPt alloy; Dealloying; Nanocrystal; Electron microscopy; Crystal growth

Abstract Bimetallic CuPt nanocrystals with size ranging from 3 to 30 nm were synthesized in the presence of either hexadecylamine or poly(vinylpyrrolidone) as a capping agent. Different growth stages of CuPt nanoparticles prepared with hexadecylamine have been investigated and a non-classic mechanism governing the formation of the metal alloy was revealed. It was found that the precursor molecules aggregate into amorphous spheres at a very early stage, followed by surface multiple nucleation, formation and combination of crystalline islands to produce a core–shell structure with surface-to-core extension of the crystallization to achieve single crystals. CuPt nanocrystals synthesized with poly (vinylpyrrolidone) grew via the classic route. Dealloying treatment was applied on these CuPt nanoalloys to selectively remove Cu. Large particles (30 nm) with Cu-rich cores exhibited hollow structures after dealloying while 3 nm particles remained solid, demonstrating that particle size and composition have a great influence on the final morphology of dealloyed particles. & 2013 Chinese Materials Research Society. Production and hosting by Elsevier B.V. All rights reserved.

1.

Introduction

Bimetallic alloy nanoparticles have raised considerable interest recently due to their unique properties and applications in catalysis, optical devices, biodiagnostics and so on [1]. In particular, Pt and Ptbased nanoparticles are important industrial catalysts and have wide practical applications [2–5]. Considering the high cost and easily n

Corresponding author. E-mail address: [email protected] (W.Z. Zhou). Peer review under responsibility of Chinese Materials Research Society.

poisoned property of Pt, bimetallic Pt-containing nanomaterials have great potential to be both active and durable in catalysis. Specifically, copper–platinum nanoalloys have received increasing attention as they show fascinating applications and enhanced catalytic activities in methanol oxidation [6], heterogeneous NOx reduction [7], preferential oxidation of CO in excess hydrogen [8], oxygen reduction reaction (ORR) in polymer electrolyte membrane fuel cells (PEMFCs) [9] and many other reactions. The controllable synthesis of nanocrystals has been an intriguing realm of research. Understanding fundamental growth mechanisms is essential to design and engineer nanomaterials. Ostwald ripening is a crystal growth mechanism widely used to explain how larger particles grow at the expense of consuming smaller ones. The driving force is to reduce the total energy, as the chemical potential of a particle decreases with increased size,

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described by the Gibbs-Thompson equation [10]. This classic mechanism, however, cannot explain the formation and growth processes of crystals in many systems in which a homogeneous growth environment is disturbed by the aggregation of small particles. According to previous investigations into the growth of zeolite analcime and zeolite A [11–13], crystallization could begin on the surface of disordered spherical aggregates and then extend from surface to core, which means the crystal growth can follow a reversed route. A similar phenomenon was also observed in the crystal growth of perovskites with the presence of polymers [14,15]. Growth of metallic crystals may also follow this nonclassic crystal growth route [16]. In more recent work, some organic crystals have also been found to develop via a reversed crystal growth route [17]. Reports on non-classic growth of bimetallic nanoparticles, however, still remain rare. Dealloying, which refers to the selective removal of the more active metallic component of an alloy, has a history of more than 80 years since the development of the Raney nickel [18] catalyst and has recently been receiving renewed attention due to the intriguing properties of dealloyed materials [19,20]. Though corrosion was the main research field for dealloying previously, dealloying has started to show its potential as an effective method to prepare nanoparticles with both large surface area and with structural properties such as core–shell structures with a noble metal rich shell [21] and nanoporous structures [22,23]. Understanding the relationship between the particle size and microstructure of uniform bimetallic nanoparticles is important for the rational design and controllable synthesis of these nanomaterials. Herein, we present our systematical investigation of the growth process of CuPt bimetallic alloy nanoparticles step-by-step using high resolution transmission electron microscopy (HRTEM) as the principal characterization technique. We demonstrate a reversed crystal growth mechanism in this bimetallic nanomaterial system. The dealloying processes of these nanocrystals are also examined by selective dissolution of less-noble Cu metal, and different morphologies of nanoparticles are achieved depending on their sizes. Our work gives practical guidelines for the preparation of CuPt alloy nanomaterials with various structures, which are considered as potential candidates for catalysts.

acetylacetonate [Pt(acac)2 ], 0.11 mmol copper sulfate pentahydrate (CuSO4  5H2O), 3 mg PVP (Mw 55000), and 5 ml ethylene glycol. It was then slowly heated with strong stirring to dissolve the chemicals. The solution was further refluxed for 2 h at 200 1C to form a black colloid suspension. The nanoparticles were separated from the solvent by addition of acetone followed by centrifugation. In another synthesis, we changed the initial ratio of Cu and Pt precursors from 1:1 to 7:3 with all the other experiment conditions unchanged to obtain larger CuPt nanoparticles. For dealloying the specimens, the as-prepared Cu–Pt alloys were treated with 25 ml of 1 M H2SO4 at 80 1C for 42 h under stirring to selectively dissolve Cu. The products were then washed with de-ionized water several times. 2.2.

Selected area electron diffraction (SAED) patterns, transmission electron microscopy (TEM), and HRTEM images were obtained on a JEOL JEM-2011 electron microscope operating at 200 kV with a point resolution of 0.194 nm. For TEM experiments, samples were diluted in hexane or acetone and were drop-cast onto a holey carbon coated copper grid followed by solvent evaporation in air at room temperature. The images were recorded using a Gatan 794 CCD camera. Chemical compositions of the nanoparticles were examined by energy-dispersive X-ray spectroscopy (EDX) using an Oxford INCA system attached to a JEOL JSM-5600 scanning electron microscope operating at 20 kV. Average EDX results were made on at least ten different regions. Individual compositions of nanocrystals were determined by the measurement of the lattice d-spacings after the camera length of the microscope was carefully calibrated, assuming that the unit cell dimensions of the alloys obey Vegard's law. Powder X-ray diffraction (XRD) was performed on a PANalytical Empyrean diffractometer with Cu Kα radiation.

3. 3.1.

2. 2.1.

Experimental Synthesis of alloyed and dealloyed nanoparticles

CuPt nanoparticles capping with hexadecylamine for mechanism study were synthesized via a modified polyol process [7]. 22 mg of platinum acetylacetonate [Pt(acac)2], 14 mg of copper sulfate pentahydrate [CuSO4  5H2O] and 370 mg of hexadecylamine (HDA) were mixed together with 4.0 ml of ethylene glycol in a round-bottom flask equipped with a condenser. The mixture was heated to 200 1C at a heating rate of 20 1C/min. Intermediate products were collected at different reaction times during the temperature-rising process. The CuPt nanoparticles were isolated by precipitating the colloids from the reaction system using a sufficient amount of ethanol followed by centrifugation at 3600 rpm for 10 min. The product (precipitate) was re-dispersed in hexane for further characterization. CuPt nanoparticles for dealloying treatment were also synthesized in a similar method but using poly(vinylpyrrolidone) (PVP) as a new capping agent to replace HDA. A 25 ml three-neck round-bottom flask was charged with 0.11 mmol platinum

Characterization methods

Results and discussion Reversed growth of CuPt nanoparticles

Bimetallic CuPt nanoalloys were prepared with Pt(acac)2 and CuSO4  5H2O in ethylene glycol at 200 1C with hexadecylamine (HDA) as a capping agent. Fig. 1(a) shows a TEM image of a representative sample of CuPt nanoparticles after a 10 h synthesis with a 1:1 ratio of Pt and Cu precursors. Most of the nanoparticles appeared to be single crystals. The HRTEM image in Fig. 1(b) demonstrates a single crystalline particle with lattice spacing of 2.18 Å, which matches well with the expected spacing of (111) planes of the CuPt alloy (2.191 Å). EDX indicates a Cu/Pt ratio of 1:1.0470.13, consistent with the 1:1 CuPt alloy. It is noted that the {111} facets are more apparent than other surface terminations. The XRD peaks (Fig. S1a in supporting information) are broad, indicating small crystallite size in the nanoscale. According to the classic crystal growth theory, a crystal is developed from a single nucleus and the building units are deposited on the crystal surface layer by layer. Consequently, we can expect a single crystal state at any time during the early stage growth process, with only a gradual increase in the size of the crystallite. When we investigated the early stage crystal growth of CuPt nanoalloy, however, we found a totally different growth route.

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Fig. 1 Direct observation of CuPt nanoparticles after a 10 h reaction, (a) low magnification TEM image and (b) HRTEM image showing lattice fringes of the CuPt (111) planes.

In order to explore the growth mechanism, we systematically analyzed the intermediate specimens at early stages. During the temperature-rise period to 200 1C in the synthesis, the solution underwent a quick color change from deep green to slightly yellow, then to brown and finally to dark brown, indicating that reduction of metals occurred. Fig. 2(a) gives a representative HRTEM image of a typical particle from the specimen collected when the solution color just changed to yellow after heating for 6 min. It is an amorphous spherical particle with a diameter of about 20 nm, which gave neither diffraction spots in the SAED patterns nor lattice fringes in the HRTEM images. The low density of the spherical particles, as indicated by a significant shrinkage of the particles with elongated reaction time (Fig. 2c and d), implies that they consist of the precursor and HDA molecules. The original green color of the solution was from a mixture of yellow from [Pt(acac)2] and blue from [CuSO4  5H2O]. When the temperature was increased to over 110 1C, CuSO4  5H2O underwent a dehydration process, losing the blue color. The yellow color of the solution implies that no metal was reduced at this stage and Pt(acac)2 precursor molecules were intact. A HRTEM image of a particle from the sample collected when the solution color changed from yellow to brown after heating for 8 min is shown in Fig. 2(b). Reduction of metals started at this stage and some small crystalline islands appear on the particle. Lattice fringes (as indicated by the circle and arrows) can be observed, indicating a multiple nucleation process likely taking place on the particle surface [24]. It is believed that, in the synthetic system, the principal reductant is the solvent ethylene glycol and the surface of the aggregated spheres is the site to have the best contact between the precursor and the reductant. The inset of Fig. 2(b) is the low magnification TEM image with a non-uniform contrast of the same particle, in which the dark parts caused by diffraction contrast correspond to areas where crystallization occurs. EDX spectra from the particles in these early stages of growth show the principal elements of C, O, S and Pt with only a very small amount of Cu (see the supporting information, Fig. S2a). On the other hand, the EDX spectrum from the 10 h sample shows strong peaks of Pt and Cu with an atomic ratio of about 1:1 (Fig. S2b). It is, therefore, concluded that the amorphous spheres mainly consist of Pt(acac)2, HDA and SO2– 4 anions, presumably due to their much stronger interaction [25] than that between HDA and Cu cations. A Pt-blank experiment was conducted with all the other parameters identical but in the absence of a Pt precursor.

Particles as large as 200 nm were obtained (see the supporting information, Fig. S3), indicating weak protection of HDA on the Cu surface. The HRTEM images of products after reaction at 200 1C for 0.5 h and 1.5 h are shown in Fig. 2(c) and (d). After 0.5 h, the crystalline islands grow to larger pieces, while the whole particle is still polycrystalline. The d-spacing of the marked lattice fringes is 2.21 Å, which corresponds to the (111) planes of Pt-rich alloy (d111 ¼2.19 Å for 1:1 CuPt alloy). After 1.5 h nanoparticles with a core–shell structure were observed, as shown in Fig. 2(d). Polycrystalline pieces fused together and self-orientated to form a single crystalline shell covering a disordered core. Clear boundaries between core and shell can be seen. It is noted that the particles in the 0.5 h and 1.5 h samples are much smaller (10 nm) in comparison with the previous ones. It is believed that this shrinkage is associated with the increase in density and the loss of (acac) anions and HDA molecules. Based on the above results, we are now able to propose a new mechanism for the growth of CuPt bimetallic nanocrystals under the present conditions, as illustrated in Scheme 1. The Pt precursor molecules and capping agent HDA aggregate into amorphous spherical particles at a very early stage. This process suppresses individual nucleation in the solution and multiple nucleation takes place on the surface of the amorphous spheres instead. At the same time, Cu gradually migrates into the particles to enhance the multiple nucleation on the surface of the spheres, which further develop into Pt-rich crystalline islands. These crystalline islands grow to connect each other and then self-adjust their orientations to form a monocrystalline shell on a disordered core. Finally, the crystallization extends from the shell to the core via an Ostwald ripening process to achieve a single crystal state with a long reaction time, e.g. 10 h as shown in Fig. 1. To our best knowledge, this is the first time a reversed crystal growth route in nanometer scale metal crystallites has been shown with solid evidence. Uncovering the growth mechanism of CuPt nanoparticles provides us with the opportunity to fine-tune the morphology and size by simply controlling the reaction time. For example, intermediates with crystalline islands at early growth stages could be taken as extremely small crystallites loaded on support, which could probably exhibit promising properties, coupled with the possibility to further optimize the application of this strategy in catalysis.

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Fig. 2 HRTEM images from specimens of CuPt nanoparticles collected after different reaction times: (a) 6 min when the solution color changed to yellow. (b) 8 min when the solution color changed to brown. The arrows indicate some separated nanocrystallites. The inset is the low magnification TEM image of this particle. (c) and (d) HRTEM images of nanoparticles from the specimens after heating for 0.5 h and 1.5 h, respectively.

Scheme 1 Proposed growth mechanism of CuPt nanoparticles via a surface to core route.

3.2. Size and composition dependent morphology of dealloyed particles CuPt nanoalloys were treated with 1M H2SO4 to selectively remove Cu under relatively mild conditions. Chemically dealloyed nanoparticles exhibit different morphologies depending on size and composition.

3.2.1. Dealloying of CuPt nanospheres CuPt nanospheres with an average size of 3.870.3 nm were synthesized using PVP as a stabilizing agent following the previously reported procedures [7]. No amorphous aggregation was observed in this system. Consequently, the crystal growth followed the classic route and homogenous crystallites with high crystallinity were produced. This is because PVP does not enhance

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aggregation with Pt(acac)2 at early stages. Development of individual crystallites dominated in the crystal growth all the time. Dealloyed CuPt nanospheres maintained the spherical shape of the as-prepared samples (Fig. 3a and c) with a slightly decreased size of 3.570.3 nm in diameter. Whereas, enlarged lattice dspacings were observed from the dealloyed particles with the (111) d-spacing of 2.23 Å shown in Fig. 3(d) compared to 2.18 Å of CuPt (Fig. 3b). It is obvious that the Cu content decreased in the chemically dealloyed nanoparticles, leading to an increase of lattice d-spacing. This is further confirmed by EDX analysis which shows a Cu/Pt ratio of 17:83. Surprisingly, after the dissolution of Cu, the lattice fringes seem to remain perfect with no detectable defects or lattice distortion. One assumption might be that the remaining Cu and Pt atoms re-arranged when the Cu left due to acid etching. In this case, volume shrinkage could not be avoided. We did not observe the reported structure of Cu-rich core surrounded by Pt-rich shell in particles [21] dealloyed by electrochemical method. We hypothesize that our mild dealloying condition left the particles enough time to reorganize their structures.

see from the d-spacings in the HRTEM image in Fig. 4(b), the elemental distribution is not uniform in these particles. Most nanocrystallites on the surface are Cu7Pt3 alloy, but the crystallites in the core are mainly Cu. The corresponding (111) d-spacings measured from these two areas are 2.14 Å and 2.08 Å respectively. Consequently, without dealloying, a core–shell structure has been produced with a Cu rich core and a polycrystalline shell containing Cu7Pt3 nanocrystallites. After dealloying, all particles showed a hollow structure as seen in Fig. 4(c). This is because the original cores of the particles were Cu rich and most Cu atoms in the cores have been removed. The surface alloy nanocrystallites underwent a re-crystallization process into higher crystallinity shells as shown in a typical HRTEM image in Fig. 4(d). The single crystal area extended in the shell with an observed d-spacing of 1.90 Å, which can be indexed to the (200) planes of alloy, consistent with the approximate Cu to Pt ratio of 4:6 as detected by EDX. As the size of these particles is big in comparison with the 3 nm ones, the large loss of Cu in core area leads to the formation of hollow particles.

3.2.2. Dealloying of Cu7Pt3 nanoparticles Polycrystalline Cu7Pt3 nanoparticles, 30 nm in size (Fig. 4a), were produced with a nominal Cu:Pt precursor ratio of 7:3 after 2 h synthesis as described in the Experimental Section. As we can

4.

Conclusions

Bimetallic CuPt nanocrystals of different sizes and crystallinities can be prepared by varying the capping agents as well as the initial

Fig. 3 CuPt nanospheres observed by TEM and HRTEM before and after dealloying. (a) TEM and (b) HRTEM images of CuPt nanospheres. (c) TEM and (d) HRTEM images of dealloyed nanospheres.

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Fig. 4 Cu7Pt3 nanospheres observed by TEM and HRTEM before and after dealloying. (a) TEM and (b) HRTEM images of Cu7Pt3 polycrystalline core–shell particles. (c) TEM and (d) HRTEM images of hollow nanoparticles after dealloying.

Cu:Pt ratio of metal precursors. From the investigation into the growth process of CuPt nanoparticles in the presence of hexadecylamine, a reversed crystal growth mechanism has been revealed. Hexadecylamine enhances aggregation with the metal precursor molecules, producing amorphous spherical particles. In this case, the crystal growth will not follow the classic theory since the most active site for nucleation and crystal growth is at the surface of the aggregates, or at the solid–liquid interface, instead of the center of these particles. Multiple nucleation then occurs on the surface of the spheres, leading to a single crystal shell, followed by extension of the crystallization towards the core. This is a good example to demonstrate that the reversed crystal growth route can take place in nanometer scale alloy particles. In the synthetic system using poly(vinylpyrrolidone) as the capping agent instead of HDA, no reversed crystal growth was observed indicating that PVP cannot enhance aggregation in the ethylene glycol solution. We also demonstrate that the size and precursor ratio affect the morphology of the dealloyed nanoparticles. When the particle size is small, a volume shrinkage is induced by dealloying, resulting in smaller solid particles. When a high nominal ratio of Cu:Pt was used, large core–shell particles can be produced with a Cu rich core coated by a polycrystalline shell consisting of Cu7Pt3 nanocrystallites. Dealloying these particles leads to hollow particles.

Acknowledgments The authors would like to acknowledge University of St Andrews for an SORS award and Sasol Technology UK Ltd. for financial support. They also thank Robert P. Tooze and Pascal Lignier for helpful discussions, and Katherine Self for her assistance in paper preparation.

Appendix A.

Supporting information

Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.pnsc.2013.04.009.

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