Alloy nanocrystals

The ability to synthesize lattices of nanociystals have led to explorations of their collective physical properties. Thus, it is observed in the case, of Co nanociystals (5.8 nm) that, accompanying lattice formation, the blocking temperature increases. 421 FePt alloy nanocrystals yield ferromagnetic assemblies for which the coerrivity is tunable by controlling the parameters such as Fe Pt ratio and the particle size.1431 The evolution of collective electronic states in CdSe nanocrystals have been followed by optical spectroscopic methods. Compared with isolated nanocrystals, those in the lattice exhibited... 

An interesting approach to synthesize metal alloy nanocrystals is the use of simultaneous salt reduction and thermal decomposition processes. Sun et al. [18] reported on the synthesis of iron-platinum (FePt) nanoparticles through the reduction of platinum acetylacetonate by a diol, and decomposition of iron pentacarbonyl (Fe(CO)5) in the presence of a surfactant mixture (oleic acid and oleyl amine). On the basis of a similar approach, Chen and Nikles [217] synthesized ternary alloy nanoparticles (FC cCo3,Ptioo x-y), using a simultaneous reduction of acetylacetonate and platinum acetylacetonate and thermal decomposition of Fe(CO)5 and obtaining an average particle diameter of 3.5 nm and narrow particle size distribution. 

The tendency of monodisperse nanocrystals to arrange into ordered three-dimensional arrays extending to a few microns has been noticed [117]. Careful tuning of crystallization conditions have yielded crystallites of micrometer dimensions consisting of AU55 nanocrystals and Fe-Pt alloy nanocrystals (4.5 nm) [118, 119]. However, it was observed that the nanocrystal arrangement in all the above crystallites was polymorphous. It is believed that such crystallites, consisting of ordered nanocrystals, could prove to be the best candidates to study the collective properties of an ensemble of nanocrystals. 

Synthesis of CoPtj Magnetic Alloy Nanocrystals The synthetic approach developed for the preparation of elemental nanopartides can be further extended to intermetallic compounds. Thus, high-quality CoPt3 nanocrystals can be synthesized via the simultaneous reduction of platinum acetylacetonate and the thermal decomposition of cobalt carbonyl in the presence of 1-adamantanecarboxylic add (ACA) and hexadecylamine (HDA) as stabilizing agents [65]. 

Gurusinghe NP, Hewa-Kasakarage NN, Zamkov M (2008) Composition-tunable properties of CdSxTej X alloy nanocrystals. J Phys Chem C 112 12795... 

FIGURE 2.2 (a-c) TEM and (d-f) HR-TEM micrographs of octahedral Pt-Ni alloy nanocrystals (a,d) PtjNi, (h,e) PtNi, and (c,f) PtNi. Reprinted with permission from Ref [19]. American Chemical Society. 

Colloids of alloys have been made by the chemical reduction of the appropriate salt mixture in the solution phase. Thus, Ag-Pd and Cu-Pd colloids of varying composition have been prepared by alcohol reduction of mixtures of silver nitrate or copper oxide with palladium oxide (Vasan and Rao 1995). Fe-Pt alloy nanocrystals have been made by thermal decomposition of the Fe and Pt acetylac-etonates in high-boiling organic solvents (Sun et al. 2000). Au-Ag alloy nanocrystals have been made by co-reduction of silver nitrate and chloroauric acid with sodium borohydride (Sandhyarani et al. 2000 He et al. 2002). 

Harfenist SA, Wang ZL, Whetten RL, Vezmar 1, Alvarez MM (1997) Three-dimensional hexagonal close-packed superlattice of passivated Ag nanocrystals. Adv Mater 9 817-822 Haruta M (1997) Size- and support-dependency in the catalysis of gold. Catal Today 36 153-166 He ST, Xie SS, Yao IN, Gao HI, Pang SJ (2002) Self-assembled two-dimensional superlattice of Au-Ag alloy nanocrystals. Appl Phys Lett 81 150-152... 

ORR on Facet- and Shape-Controlled Pt-Alloy Nanocrystal Electrocatalysts... 

This selected brief review will be focused on the research and development progress on ORR kinetics. The origin of the problem related with the low ORR activity of platinum will be discussed, followed by a review of recent progress in making more active, more durable platinum-based ORR catalysts. These include platinum alloy catalysts, platinum monolayer catalysts, platinum nanowire and nanotube catalysts, and the more recent shape- and facet-controlled platinum-alloy nanocrystal catalysts. The progress in the mechanistic understanding on the correlation between the activity and the electronic and structural properties of surface platinum atoms will be reviewed as well. The future direction of the research on platinum-based catalysts for PEM fuel cell apphcation will be proposed. 

B. Neppolian, V. Saez, J.-G. Garcia, F. Grieser, R. Gomez, M. Ashokkumar, Sonochemical synthesis of graphene oxide supported Pt-Pd alloy nanocrystals as efficient electrocatalysts for methanol oxidation. J. Solid State Electrochem. 18, 3163-3171 (2014)... 

Figure 25 shows the variation in band gap of alloyed nanoparticles of various compositions with the size of the nanoparticles. Both Figs. 24 and 25 reveal that for any cluster size, the band gap values of any alloy are lower than those of pure cluster and it is also observed that for 50% (x = 0.5) substituted nanoalloys has the lowest band gap value compare to any other substituted alloys for all cluster size. The extent of charge transfer among the different components, i.e., Ga, Al, and As in the alloyed nanocrystals are responsible for this interesting behaviour. A close inspection to the values of the charge transfer of both Al cGai As and Ga cAli c As alloyed... 

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