Characterization of Pt-Pd bimetallic nanoparticles by Cs-corrected

Materials Science Forum Vol. 755 (2013) pp 69-74 © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.755.69

Characterization of Pt-Pd bimetallic nanoparticles by Cs-corrected STEM. O. Téllez-Vázquez1,a, R. Esparza1,b, G. Rodríguez-Ortiz1,c, Amado F. García-Ruiz2,d, R. Pérez1,e and M. José-Yacamán3,f 1

Centro de Física Aplicada y Tecnología Avanzada, Universidad Nacional Autónoma de México, Boulevard Juriquilla 3001, Santiago de Querétaro, Qro., 76230, México. 2 UPIICSA-COFAA, Instituto Politécnico Nacional, Te 950, Col. Granjas-México, Iztacalco, C. P. 08400 México, D. F. México. 3 Department of Physics and Astronomy, The University of Texas at San Antonio, One UTSA Circle, San Antonio, TX 78249, USA. a

[email protected], [email protected], [email protected] d [email protected], [email protected], [email protected]

Keywords: Microwave-assisted process, nanostructure, Pt-Pd, Electron microscopy, Aberration corrected, Molecular dynamics.

Abstract. Pt-Pd bimetallic nanoparticles were characterized using aberration (Cs) corrected scanning transmission electron microscopy (STEM) along with molecular dynamics simulations. The nanoparticles were synthesized through a microwave-assisted process. This technique has been applied to synthesize metallic nanoparticles at relatively short times, allowing a good control of size distribution. The structure of the bimetallic nanoparticles is fcc-like with an average size of 5 nm. To understand the properties of the bimetallic nanoparticles, it is necessary to know the positions of all the atoms in the nanostructure. We have used a recent quantitative method to analyze HAADF STEM images which allowed us to measure the total intensity of the scattered electrons for each atomic column. Beside with the characterization of the nanoparticles, we have performed classical molecular dynamics simulation for the structural and dynamical analysis of the cuboctahedral Pt-Pd bimetallic nanoparticles. Introduction In recent years, metallic nanoparticles with specific size, morphology and composition are of considerable interest because of their importance in many technological fields [1-3]. Bimetallic nanoparticles (NPs) composed of two different metal elements, are of greater interest than monometallic counterparts because of their superior and interesting electrical, chemical, catalytic, and optical properties [4]. They improve the catalytic activity of the materials, sometimes creating new unknown catalysts. Besides, structural changes can be created in small bimetallic NPs as a result of alloying of the component metals [5,6]. Bimetallic NPs have been synthesized with a variety of structures, getting systems as bimetallic alloys, random alloyed nanoparticles and also core-shell structures [7]. Bimetallic NPs with cluster-in-cluster structures, one element forms nanoclusters and the other element surrounds the nanoclusters and acts as a binder. Similar to bulk metals, two kinds of metal elements often provide an alloy structure. If the atomic sizes of two elements are similar to each other, then it will be a random alloy. In a core-shell structure, one metal element forms an inner core and the other element surrounds the core to form a shell. Platinum (Pt) and Palladium (Pd) are of great importance because of their extraordinary physical and chemical properties [8-11]. These metals have been studied as high-efficiency catalysts for electrocatalytic reactions [12]. However, the distribution of the elements in the bimetallic Pt-Pd nanoparticles is a challenge now days. Some of the most important characteristics about bimetallic NPs are concerned with aggregation state, size, morphology and distribution of the elements. Many techniques have been used to reveal the size and homogeneity of bimetallic NPs obtained by the synthesis methods. As distinct from All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 132.248.179.216, Universidad Nacional Autónoma de México (UNAM), Mexico, Mexico-12/03/13,18:11:33)

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Structural and Chemical Characterization of Metals, Alloys and Compounds

small organometallic molecules, the composition of metal nanoparticles cannot be so exactly controlled. However, the homogeneity of particle size or shape on the atomic scale is quite important to reveal the physical properties of nanosized materials. Advanced electron microscopy techniques, especially scanning transmission electron microscopy (STEM), are indispensable for the characterization of the bimetallic NPs. The most important feature of a STEM instrument is its versatility: atomic resolution images, nano-beam diffraction patterns and spectroscopy data can be obtained either simultaneously or sequentially from the same region of the specimen [13]. Since the development of spherical aberration (Cs) correctors [14], the resolution of transmission electron microscopy (TEM) and STEM has been greatly improved to less than 0.1 nm. Using Cs-correction in a STEM instrument, it is possible to obtain atomic resolution of high-angle annular dark field (HAADF) STEM images with high quality that can be used in the quantitative analysis. HAADFSTEM images essentially produce at high angles an incoherent signal and are dominated by Rutherford scattering where the scattered intensity scale is associated with the atomic number Z of the elements in the sample. When the atomic numbers of the sample have large differences, Zcontrast can be interpreted directly from the image. In this work, Pt-Pd bimetallic NPs with alloy structure were synthesized by microwave-assisted process. The structural characterization was carried out by aberration-corrected scanning transmission electron microscopy. Simulations of molecular dynamics for the structural and dynamic analysis of the cuboctahedral Pt-Pd bimetallic nanoparticles have been performed. Experimental Procedure Samples were prepared by means of a simple two steps process in ETHOS EZ Digestion System MicroWave (Milestone, 2.5 GHz, sensor ATC400) equipment. The used chemicals came from Sigma-Aldrich and all of them were utilized without any further treatment. The analytical reagents were chloroplatinic acid hexahydrate (H2PtCl6.6H2O, 99.99%), palladium chloride (PdCl2, 99.99%), poly(N-vinyl-2-pyrrolidone) (PVP, MW = 55,000), and ethylene glycol (C2H6O2, 99.95%). Ethanol and acetone were used for washing and cleaning samples after the reactive process. In the first step of this synthesis of nanoparticles process, 22 ml of a solution obtained by mixing 0.16 g of PVP diluted in 20 ml of ethylene glycol and 0.0177 g of PdCl2 diluted in 2 ml of water. This yellow solution was heated by microwaves to 800 W from room temperature up to 110 ºC in 6 min, remaining in this temperature during 15 min, and then cooled down to room temperature during 20 min. The solution changed from its yellow color to a dark brown color, this supposes that a reaction occurred in which seeds of Pd were formed in the stage. Then, in a second step, 1 ml of aqueous solution of H2PtCl6, 0.1 M, was added to the system mixing it thoroughly by vibration and sonication. The resultant solution was heated again in microwaves to 1000 W carrying the system from room temperature up to 140 ºC in 6 min, remaining it at this temperature for 2 h to get a good reaction between the reagents, and then cooled down to room temperature during 20 min. The result was a dark solution containing the bimetallic nanoparticles Pd-Pt; this colloidal solution was washed two or three times using ethanol and acetone and the nanoparticles were separated and dispersed by adding ethanol and by successive centrifugation treatments at 5500 rpm for 20 minutes. To characterize the Pt-Pd bimetallic nanoparticles, copper grids with holey carbon film were prepared with a drop of the solution. The samples were analyzed using aberration-corrected scanning transmission electron microscopy (Cs-STEM) with a Jeol ARM200F (200 kV) FEGSTEM/TEM microscope equipped with a CEOS Cs-corrector on the illumination system. Highangle annular dark field scanning transmission electron microscopy (HAADF-STEM) images were obtained by Cs-STEM. The probe current used in STEM mode was 23.2 pA (spot size 9C) using a condenser lens aperture size of 40 microns. The HAADF images were registered using a camera length of 80 mm and a collection angle of 67 - 250 mrad.

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The molecular dynamics simulations were carried out in the canonical ensemble (NVT) with the application of the Andersen thermostat for maintaining the constant temperature condition. Simulations were performed in a series of temperature conditions ranging from 300 to 1200 1750 K with a heating rate of 1K/ps. in increments of 50 K. The simulated models include solid solution clusters with 561 atoms and a cuboctahedral structure to pure Pt, Pt75Pd25, Pt50Pd50, Pt25Pd75 and pure Pd. Results and Discussion The properties of the bimetallic nanoparticles are determined not only by their size, morphology and structure, also, by the distribution of the elements [4]. One of the most important techniques used to characterize the bimetallic nanoparticles is the scanning transmission electron microscopy (STEM) [17]. Images with Z-contrast and high-resolution can be acquire using high-angle annular dark field (HAADF) STEM with Cs-corrected. Fig. 1 shows a typical HAADF-STEM image of the Pt-Pd bimetallic nanoparticles synthesized by microwave-assisted process. The average particle size is about 4.35 nm. The bimetallic nanoparticles were found with a homogeneous distribution, it is interesting to observe that the majority of the nanoparticles produced by this synthesis process (more than 90%) did not present the agglomeration phenomenon. The contrast of the image shows that the nanoparticles have an alloy structure, where the atoms of Pd and Pt have a random distribution. From the image, it can observe that the main crystallographic structure of the bimetallic nanoparticles is fcc-like, however, at this magnification is not possible determine the distribution of the elements. To determine the distribution of the elements is necessary acquire HAADF-STEM images with high-resolution.

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AS = 4.35 nm

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Fig. 1. a) and b) HAADF-STEM image of the Pt-Pd bimetallic nanoparticles with its distribution of particle size, respectively. Fig. 2 shows a HAADF-STEM image of Pt-Pd bimetallic nanoparticles with a homogeneous distribution. High-resolution HAADF-STEM image of a Pt-Pd bimetallic nanoparticle is shown in Fig. 2b. From the HAADF-STEM image, d-spacings 0.225, 0.193 and 0.146 nm were obtained. Such d-spacings correspond to (111), (200) and (022) crystalline planes respectively of the Pd structure with a0 = 3.8898 Å (JCPDS-ICDD 05-0681). Fast Fourier transform (FFT) shows the main reflections confirming the [0-11] zone axis (Fig. 2c). From the image, it is interesting to observe the difference in contrast obtained from the Pt and Pd atoms. The contrast in the HAADFSTEM images is associated with the atomic number Z of the elements in the sample, therefore, the strong brightness contrast corresponds to the heaviest element (Pt) and the low brightness contrast corresponds to the lightest element (Pd). The distribution of the Pt and Pd elements is clearly observable in this figure, which will be discussed later in more detail.

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Fig. 2. a) HAADF-STEM image shows Pt-Pd bimetallic nanoparticles. b) High-resolution HAADF-STEM image of a Pt-Pd bimetallic nanoparticle. c) FFT shows the [0-11] zone axis. To visualize a better contrast in the above image, Apply_CLUT (color look-up table) script was used [18]. The value of the contrast intensity of the image was calculated subtracting of the original image, the contribution of the background (the contrast intensity of the C support), this with the aim of obtain only the contribution of the elements. The values of the intensity are shown in the Fig. 3 as a color scale. This image shows clearly differences in contrast intensity. The brightness contrast intensity of the image is associated with the atomic number (Z) of the element by I α cZ1.8Nt, where c is a constant, N the atomic density and t the thickness. We will consider mainly the contribution of the atomic number (Z1.8), and the other variables will be considered as a constant. In the case of the Pt (Z=78), the intensity is 2.5x103, and in the case of Pd (Z=46), the intensity is 0.98x103. From the image, the color scale is associated with the atomic number of the Pt and Pd. The values of the color scale has a factor 10 with respect to the values obtained from the intensity of the components, this factor can be related with the c, N and t variables. Therefore, the distribution of the Pt and Pd in the nanoparticle clearly is observed, where the Pt is located mainly in the surface of the nanoparticle, this due to the process of synthesis. x103 26

13

0.0

Fig. 3. CLUT image of the HAADF-STEM image of Pt-Pd bimetallic nanoparticle with its color intensity scale. To study the stability of the Pt-Pd bimetallic nanoparticles, simulations of dynamical molecular were performed. The results of the simulations are shown in Fig. 4, where the variation of the energy with respect to the temperature is represented in the graph. In the case of Pt25Pd75, a transformation of the structure can be identified by a simple jump in the total energy curve around

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1020 K, this transformation is not observed in the particles with higher content of Pt, and it shows that the structures with higher Pt content are more stable. However, in all cases, the amorphous state is reached at 1750 K or above, which can be observed in the structure. On the other hand, the structures obtained at the end of simulations contain more atoms of Pd in their surface; this is because the Pd has a lower stability and is segregated by the Pt which forms the body of the particle. As can be seen in the graphs, the melting temperature increases as the platinum content is increased, even Pt25Pd75 particle undergoes two transformations before the 1750 K, while the particle transitions Pt75Pd25 are much more less pronounced.

Fig. 4. The dependence of energy per atom on temperature during heating process for: (a) Pt25Pd75, (b) Pt50Pd50 and (c) Pt75Pd25 (Pt atoms color blue and Pd atoms color green). Conclusions The method for the calculation of HAADF-STEM images proposed is dependent of atomic number and thickness. It is shown that HAADF-STEM technique is a sensitive method for the identification of the atoms in bimetallic nanoparticles. Simulations of HAADF-STEM images demonstrated the atomic positions and intensities of the Pt and Pd elements. The increase in platinum content increases the thermal stability of the particle, so that the palladium is segregated and forms the shell. Moreover, the recovery of the initial morphology is more evident by increasing the content of platinum.

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