Alkali halide nanocrystals: Molecular adsorption reactions and structural calculations

Molecular adsorption reactions on positively and negatively charged alkali halide clusters and nanocrystals have been measured using fast-flow-reactor methods. Distinct reactivity patterns are seen as a function of cluster size for different adsorption reactions. Computed structures for the sodium fluoride clusters are presented, and the reactivity patterns for sodium and potassium fluoride nanocrystals are correlated to recurring defects in the nanocrystalline structures. The reactivity patterns of lithium fluoride clusters suggest that they have different structures from the other alkali halide nanocrystals. The correlation between reactivity and structure for alkali-halide nanocrystals shows that the size evolution of nanocrystal structure generates repeating structures of perfect (non-reactive) crystals and defective (reactive) crystals. These mimic small regions of a bulk surface and the experiments can be used to understand molecular adsorption on bulk surface.; The adsorption of ammonia and water vapors on positively charged alkali fluoride nanocrystals (alkali metal = sodium, potassium, and lithium) was studied at ambient temperatures on a size selective basis. Comparison between the reactivity pattern of sodium fluoride nanocrystals and their structures shows that a special type of defect in the nanocrystal structure facilitates adsorption. This defect is formed by removing an alkali-fluoride ion-pair from adjoining face and internal sites of a perfect crystal, creating a basket-like opening. Potassium fluoride nanocrystals show very similar reactivity patterns, and their structures therefore, are believed to be the same as the sodium fluoride nanocrystals. The reactivity pattern of lithium fluoride nanocrystals towards ammonia, however, are very different from the other alkali-fluorides.; By varying the flow-reactor length it was determined that the adsorption reactions of sodium fluoride nanocrystals with ammonia were taking place under equilibrium conditions. Studies of the adsorption reaction at different temperatures further showed that the extent of reaction (or equilibrium constant) increased with decreasing temperature. Studies of the equilibrium constants at different temperatures yielded heats-of-adsorption for ammonia on some of the more reactive nanocrystals.; The adsorption of carbon dioxide on positively and negatively charged sodium-fluoride nanocrystals was studied over a range of temperatures on a size selective basis. The reactivity patterns for the positively and negatively charged nanocrystals show that although reactivity can be correlated with the structure of the nanocrystals, the adsorption of carbon dioxide is facilitated by different structural features than the adsorption of ammonia.