Core-shell metal nanoparticles research

Recently, however, the development of nanotechnology may provide the changes on the research and development of practical catalysts. As mentioned in the previous section we can now design and synthesize a metal nanoparticle with not only various sizes and shapes, but also with various combinations of elements and their locations. Thus, we can now design the synergetic effect of two elements. In the case of core/shell structured bimetallic nanoparticles, the shell element can provide a catalytic site and the core element can give an electronic effect (a ligand effect) on the shell element. Since only the atoms on the surface can be attached by substrates, the thickness of the shell should be an important factor to control the catalytic performance. 

Considerable research effort was focused on systems of colloidal gold of which a broad variety of synthetic procedures were reported [140 b, fj. While native colloidal gold solutions are only stable for a restricted time, Brust et al. [141] were able to overcome this problem by developing a simple method for the in situ preparation of alkyl thiol-stabihzed gold nanoparticles. This synthetic route yields air-stable and easy to handle passivated nanoparticles of moderate polydispersity, and is now commonly employed for the preparation of inorganic-organic core-shell composites. Such composites are used as catalytic systems with principally two different functions of the protective 3D-SAM layer. Either the metal nanoparticle core can be used as the catalytically active center and the thiol layer is only used to stabihze the system [142], or the 3D-SAM is used as a Hnker system to chemically attach further catalytic functions [143]. 

Smaller catalyst libraries have been investigated by several other research groups. For example, Chen et al. electrodeposited a small library of noble metals (Pt, Ru, Rh, and Au) by means of a capillary-based droplet cell onto carbon nanotubes predeposited on glassy carbon by electrophoretic accumulation, and screened their ORR activity in a 0.1 M phosphate solution using the oxygen competition mode (pH = 6.7) [68, 82]. Our group employed RC-SECM to map the ORR activity of Pt, and Pt-based alloy nanoparticles, including core-shell structures... 

As can be seen in Fig. 7.10, the larger Ru nanoparticles are more reactive than the smaller Ru nanoparticles. These results are consistent with the reports of several research groups, as described earlier, that larger Ru nanoparticles show higher catalytic activity due to enhanced stability of the core-shell-type surface oxide layer on the Ru metallic core [36, 48, 50]. We also see that the TOF of Ru nanoparticles decreases after UV-ozone treatment. The activation energy of the 2.8 nm Ru nanoparticles remained the same, within the error of measurement, while that of the 6 nm Ru nanoparticles increased from 33.2 to 38.7 kcal/mol after UV-ozone... 

The nanoscale coating of colloid particles with materials of different compositions has been an active area of research in nanoscience and nanotechnology [2]. Deposition of metal nanoparticles on different colloid particles to form core-shell particles has been one of the most effective tools for achieving such composite nanostructures [172]. In particular, a number of studies on such composite structures were concentrated on the fabrication of metal coated latex particles, because of their potential applications in the fields of surface-enhanced I man scattering (SERS), catalysis, biochemistry, and so forth [173]. Conventionally, silver shells on polymer latex were prepared via wet-chemistry methods, which involve the activation of a latex surface by seeds of a different metal, followed by the deposition of the desired metal [174], or the modification of the latex with groups capable of interacting with the metal precursor ions on the latex surface via complex or ion pairs, and subsequent reduction [175]. 

---