Adsorption reactions of alkali-halide nanocrystals: Identification of an important surface defect

The adsorption reactions of alkali-halide clusters have been investigated on a size-selected basis using flow-reactor methods. The reactivity of larger sodium-fluoride clusters [Na_nF_n-1]⁺ toward polar molecules NH₃ and H₂O shows a distinctive pattern as a function of n at ambient temperature. Comparison with computed structures shows that aparticular kind of defect greatly facilitates the initial adsorption process. This defect can be formed by removal of an ion-pair from the face of an otherwise perfect nanocrystal, to create a basket-like opening for the adsorbed molecule. It is shown that this kind of defect occurs as a most stable low-temperature structure only to a certain size, after which a less reactive defect takes its place. The implications of these findings for the adsorption reactivity of the surfaces of ionic solids are briefly discussed.