Reactions of silicon cluster ions, Sin+ (n=10-65), with water

Abstract

The chemical reactions of size selected Sin+ (n = 10-65) with D2O have been studied using injected ion drift tube techniques between temperatures of 258 and 404 K. The only products detected were a series of Sin (D2O)m+ adducts. Large variations in reactivity were observed for the smaller clusters (n < 40) that diminish with increasing cluster size. Si11+, Si13+, Si14+, Si 19+, and Si23+ are particularly unreactive compared to their neighbors. At room temperature the larger clusters (n > 40) are a factor of ∼ 10-1000 (depending on the bulk surface) less reactive towards water than bulk silicon. The reaction rates for all clusters exhibit an unusually strong negative temperature dependence but are independent of the buffer gas pressure. These results suggest that the reaction mechanism probably involves two steps. In the first step, a weakly bound molecularly adsorbed Sin+⋯D2O adduct is produced. The second step involves rearrangement to give a more strongly bound (and probably dissociatively adsorbed) Sin D2O+ product. It appears that the reaction rates for some of the smaller clusters show a faster than linear dependence on D2O pressure. One possible explanation for this unusual observation is that a second D2O molecule solvates the transition state and significantly lowers the activation barrier for dissociative adsorption. © 1991 American Institute of Physics.