The Structures and Compositions of Colloidal Metal Particles

What have we learned about the structure and composition of colloidal metal particles in the 1-10 nm size range from the above techniques There is a consensus in those reports in which structure measurements were made that above a certain size, the colloid particles adopt the structure of the bulk phase. Thus palladium colloids with a 6.5 nm mean diameter were found to have the fee structure found in bulk palladium. [113] Similar bulk-like structures were found for platinum colloids of varying sizes, [185, 200] and for colloidal platinum, palladium, rhodium, and iridium produced in reverse micelles. [151] In fact it seems that regardless of whether the colloids are prepared in aqueous or nonaqueous media, by chemical or physical means, a crystalline elemental structure can usually be identified in particles above ca. 2.5 nm. Below this size, at which the use of diffraction methods becomes problematic, there is evidence from reports 

EXAFS analysis of colloidal metals has begun to shed light on this complex state of affairs. In the few examples reported so far for bimetallic colloids, a nonuniform distribution of metals has been observed. Ibshima [164, 165] has studied a series of bimetallic PtPd catalysts, and concluded that the distribution of the two metals in the particles is nonuniform on the basis of differences in their respective mean coordination numbers. In our laboratory, colloidal 4.0 nm PdCu/PVP has been analyzed by this method and the distribution of the metals in the alloy particles shown to be nonuniform. However the component which is enriched at the surface appears to be palladium, in contrast to the segregation of copper to the surface of bulk copper-palladium alloys. [203] In addition, the ability of surface deposited copper to dissolve in a preformed palladium particle has been clearly demonstrated on the basis of the analysis of the coordination sphere of the copper. [204] 

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