Anion effect on the shape evolution of gold nanoparticles during seed-induced growth in imidazolium-based ionic liquids.

The relevance of anion structure on the shape regulating effect of 3-ethyl-1-methylimidazolium-based ionic liquids during the seed-induced growth of gold nanocrystals is evaluated for the particular case of lactate, acetate, methylsulfate, ethylsulfate and tosylsulfonate anion systems. Carboxylate-based anions (lactate and acetate) are found to inhibit the reduction of the gold precursor salt presumably due to the deprotonation of the reducing agent ascorbic acid. The formation of non-uniform, 'head-tail'-type anisotropic particle structures is observed in both methyl- and ethylsulfate anion systems whereas rapid precipitation is observed in the case of tosylsulfonate anions. The particular efficiency of the ethylsulfate solvent system in promoting shape anisotropic growth is interpreted to be a consequence of both reduced anion/cation interactions that act to support the coordination of imidazolium to the metal surface and the enhanced capacity of anions to participate in the particle stabilization process.