Nanomachines & Nanomanipulation Bimetallic/Multimetallic Nanoparticles and Nanowires
Bimetallic and multimetallic core-shell nanoparticle/nanowire catalysts of noble metal alloys possesses superior activity, high selectivity and stability at low temperature compared to traditional bulk metals. This makes them ideal candidates for carrying out investigations on their structure, nature of alloy formation and thus understand the way the catalytic activity is enhanced. Bimetallic nanoparticles have two fundamental structures: a) alloys and b) core shell structures. There are several routes to the preparation of alloy nanoparticles both bimetallic and core-shell. Factors that will play a role in determining which of the two alternatives is more likely to occur include: the solubility of the metals, the relative surface energy, the kinetics of the synthesis and several others. Direct microscopic studies of various bimetallic nanoparticles have been carried out using high-resolution transmission electron microscopy (HRTEM). However, this method is limited because of the small difference in the lattice constant involved. Conventional transmission electron microscopy (CTEM) can produce chemical contrast due to due to different electron beam extinction distances. This has been applied successfully to image bimetallic nanoparticles down to 10 nm in diameter. Since the particles need to be in a low index zone orientation, this method is not easy to use in practice. As an alternative method of nanostructure determination high angle angular dark field (HAADF) imaging technique has been used successfully. The HAADF method has been used to detect variation in chemical composition down to single atom level in suitable samples, so it is ideally suited for the compositional analysis of the internal structures of nanosized particles. STEM-HAADF imaging carried out in an aberration corrected microscope reveals the atomistic structure and the alloying of bimetallic nanoparticles and nanowires. In combination with high resolution spectral and chemical analysis this techniques provides unprecedented information never obtained till date. Understanding the nature and structure of such bimetallic/multimetallic nanocatalysts is important to enable modification of their structure/morphology/composition and enhance their catalytic performance for fuel cells and other industrial applications.