Nanocrystalline materials are of great interest and are being widely studied because many physical and chemical properties of these materials are found to be dissimilar to those of conventional polycrystalline, coarse-grained materials. Such difference in properties arise because of the large fraction of atoms (5-50%) located at the surfaces and/or interfaces. Quantum size effects are observed in metal and semiconductor fine particles if their size is small enough so that the spacing between the discrete levels in the electronic energy spectrum becomes comparable with some of the characteristic energy parameters like kT, mH, hw, etc., thereby giving rise to changes in the thermodynamic, magnetic and optical properties respectively. Nanocrystalline materials have been found to exhibit increased strength and hardness, enhanced diffusivity, improved ductility, reduced density, etc. compared to the bulk. Hence, these materials have found applications in microelectronic and optoelectonic devices, catalysis, sensing, drug delivery, etc. Metallic nanostructures with high aspect ratios are promising candidates for the development of sensors, catalysts and nanoscopic electrical connections.

Bimetallic nanoparticles are of greater interest than the monometallic ones, both technologically and scientifically. The selective behavior of such bimetallic catalysts can be controlled by changing their composition ratios. Earlier studies have mostly been done on inorganic oxide-supported bimetallic nanoparticles. Recently, novel techniques like microemulsion, decomposition of organometallic precursors, radiolysis, etc. have been developed for the synthesis of colloidal suspensions of these nanoparticle systems. Such systems are advantageous for study since the interference of substrate interaction is absent.  I have used X-ray absorption fine structure spectroscopy to investigate alloy formation in radiolytically synthesized Ag-Pt and Ag-Pd bimetallic nanoparticles in solution, possessing high aspect ratio.

Max Bertino at the University of Missouri, Rolla synthesized Ag-Pt and Ag-Pd nanowires by radiolysis. In the radiolysis method, aqueous solutions are exposed to g-rays to create solvated electrons, which in turn reduce the metal ions. The latter coalesce to form aggregates. The samples were irradiated with g radiation from the fission products of University of Missouri-Rolla’s nuclear reactor pool. samples were irradiated with g radiation from the fission products of University of Missouri-Rolla’s nuclear reactor pool.   The reactor was operated at 180 kW for 1 hour and the samples were positioned 1 h after the reactor shutdown to prevent neutron bombardment and activation of samples. The dose rate was decreased exponentially from a value of  0.5 kGy/h in the first hour to about 0.05 kGy/h 48 hours after shutdown.  Exposure to a total dose between 3 and 3.5 kGy typically required 36 to 48 hours. Total doses were measured with Thermoluminescent Dosimeters (TLD) placed in vials adjacent to the samples to be irradiated.

Since at the Advanced Photon Source, we have another high radiation source. We decided to try to form the nanoclusters within an aerogel matrix. Max prepared a saturated hydrogel that I patterned with the X-ray beam at MR-CAT. The collimated X-ray beam was used to spell out APS with nanocrystals in the hydrogel/aerogel substrate. This work is published in the Journal of Non-Crystalline Solids 333 (2004) 108.