Analysis of radiation-induced small Cu particle cluster formation in aqueous CuCl2

Synchrotron x-ray radiation is being used extensively as a structure probe to investigate the coordination environment and thus gain insight into the ion-water and ion-ion interactions in aqueous solutions. However, under fav orable conditions, there may be instances where the incident x-ray beam can induce oxidation and/or reduction in the solution, thus altering its chemi stry. Successive x-ray absorption fine structure spectra, measured in the f luorescence mode from a 55 ppm Cu in CuCl2 aqueous solution, show the forma tion of copper clusters and their growth as a function of time of irradiati on. Initially, the clusters have a nearest neighbor distance of 2.48 +/-0.0 2 Angstrom which, with increase in time of irradiation, increases to 2.55 /-0.01 Angstrom, indicating that the clusters approach the lattice dimensio ns of bulk copper. Similarly, the Debye-Waller factor of the copper cluster s is found to increase by similar to 50%-55% over the range of time of irra diation. Analysis of spectra measured in the intermediate time period shows signal contributions from a mixture of clusters that can be represented by a mixture of a small cluster (5-10 Angstrom across) and bulk copper. The n earest neighbor coordination number is found to increase in a manner consis tent with the decrease in the surface to volume ratio as the average cluste r size approaches its bulk dimensions. The initiation of cluster growth occ urs through agglomeration of copper atoms that possibly react to form dimer s upon reaction. The copper ions in the solution are reduced to the metalli c state by reacting with hydrated electrons produced as a result of radioly sis of water by the incident x-ray beam. (C) 2001 American Institute of Phy sics.