Zinc oxide nanocrystals stabilized by alkylammonium alkylcarbamates

Nearly monodispersed, spherical ZnO nanocrystals were synthesized from the reaction of an amide precursor, [Zn(N¹Bu₂) ₂]₂, with hexylamine followed by reactions of the as-formed solution in a moist air flow. Extensive experiments were conducted to optimize the synthesis and to characterize the nanocrystals. The room temperature reactions led to 3.3-5.3 nm nanocrystals with the sizes increasing in direct proportion to the relative humidity. Purification afforded high yields of free-flowing nanocrystals that were dispersible in nonpolar solvents. The overall synthesis requires several days, but it results in multigram quantities of stable, redispersible nanocrystals. The nanocrystals were characterized using elemental analysis, X-ray diffraction (XRD), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), solution and solid-state NMR, IR, UV-vis absorption, and photoluminescence spectroscopies. In addition to providing H₂O to serve as the source of oxygen in the ZnO, the air flow adds CO₂ that converts the alkylamine into an alkylammonium alkylcarbamate, which serves as the surfactant. Elemental analysis, TGA, and XPS results established that the total number of N-hexyl fragments on a 3.7 nm nanocrystal was 200, where they exist as an equal number of anionic carbamates and cationic ammonium ions. The addition of pure hexylammonium hexylcarbamate to ZnO nanocrystals prepared by literature methods resulted in the formation of a product that was similar to the ZnO formed using [Zn(N¹Bu₂)₂]₂. Larger nanocrystals up to 7.3 nm were also obtained by heating smaller nanocrystals in a mixture of hexylamine and toluene at 119 °C.