Lactam-mediated synthesis of inorganic nanoparticles and size/surface-dependent properties: a case study on zinc oxide

Abstract

Lactams have strong polarity, high boiling points, and strong coordination capacity. However they are yet to be well accepted as solvents for the synthesis of nanomaterials. This work aims at developing a facile and general lactam method to synthesize inorganic nanoparticles such as ZnO and Fe₃O₄ using three homologous lactams, γ-butyrolactam (BL), δ-valerolactam (VL) and ε-caprolactam (CL), as solvents. The resulting nanoparticles (NPs) are uniform and spherical in shape and have good dispersion. The ZnO NPs synthesized in BL, VL and CL have average diameters of ca. 30.0, 7.3, and 8.5 nm, respectively. The average diameters of Fe₃O₄ NPs synthesized in BL, VL and CL are ca. 6.3, 4.0 and 4.8 nm, respectively. Moreover, their dispersion tends to deteriorate with increasing number of methylene groups in the lactam. Some explanations for the influence of lactam on the morphology of the synthesized NPs are proposed. Optical properties of ZnO NPs are investigated and are correlated with their morphologies and surface states. 6-Aminohexanoic acid (AHA), a biocompatible amino acid, is used to displace lactam capped on the surfaces of ZnO NPs.