Synthesis and Structural Control of Bimetallic Pt‐Ni nanoparticles

Abstract

Noble metals nanoparticles (e.g. platinum, palladium) are commonly used as catalysts in alcohol oxidation reactions [1]. Catalytic efficiency of the nanoparticles may be greatly increased by a partial replacement of one of the precious metals with d‐block transition metals (e.g. iron, cobalt, nickel) [2]. One of many examples of such combinations are bimetallic Pt‐Ni nanoparticles, which have a higher catalytic performance in the oxidation of methanol than pure platinum nanoparticles [3]. However, the design and synthesis of nanoparticles, which will have the appropriate size, shape and composition, is challenging. Due to the rapid course of the synthesis reaction and the possibility of changing a variety of parameters such as temperature, reactant concentration, reaction time or even the rapidity of reactants addition, it is possible obtaining nanoparticles significantly different from each other in shape and size. Appropriate selection of the reaction conditions allows to obtain nanoparticles of various shapes, ranging from simple shapes (e.g. circular, cubic, polyhedral) [4] to more complicated 3D structures (e.g. stars [2] or dendritic structures [5]). Changing the reaction conditions affects on the structure of the nanoparticles, thus it is possible to achieve bimetallic nanoalloys, in which the atoms of two metals are randomly mixed. It is also possible to obtain an ordered structure of core‐shell type, in which the atoms of one metal form the core of nanoparticle and the atoms of the second surround this core [6]. The aim of this study was to synthesize 3D nanoparticles having a rhombic dodecahedron shape, composed of a Pt frame around a Ni core. For this purpose, a number of syntheses was performed in order to investigate the influence of various reaction parameters on the obtained bimetallic nanoparticles. The following parameters were changed: concentration of metal precursors, temperature in which the metal precursors were added to the solution and the duration of the reaction. The obtained nanoparticles were characterized using transmission electron microscopy (TEM) technique. The morphology of the nanoparticles and their size distribution was imaged by HAADF STEM. Energy‐dispersive X‐ray spectroscopy (EDX) was used to examine the distribution of chemical elements in the sample. The HAADF STEM structural analysis showed that in all syntheses bimetallic nanoparticles in different shape were obtained. Dendritic rhombic dodecahedron shapes, regular rhombic dodecahedron and approximately spherical shapes were observed, HAADF images in Fig. 1. All samples had a crystalline structure, which was confirmed by HRTEM images. The size of the nanoparticles varied from 20 to 50 nm, depending on the synthesis method. EDS analysis of all samples confirmed the presence of platinum in the frame and nickel in the core of the nanoparticles (Fig. 1). The obtained results allow to conclude that even a small change of a single parameter during the synthesis procedure, leads to a different structure of the 3D nanoparticles.