Novel Synthesis Pathway of ZnO Nanoparticles from the Spontaneous Hydrolysis of Zinc Carboxylate Salts

Abstract

A novel and easy synthesis pathway to synthesize small ZnO nanoparticles with a narrow size distribution is reported. The synthesis implies the simple dissolution of a zinc carboxylate hydrated salt (cyclohexanebutyrate or acetate) in a polar basic aprotic solvent as dimethyl sulfoxide (DMSO) or N,N‘-dimethylformamide (DMF) at room temperature. It is necessary to control the water content and temperature to ensure the reproducibility. The hydrolysis of zinc carboxylates allows the formation of ZnO nanoparticles of different sizes, depending on reaction conditions. Solvent basicity and the interaction of DMSO−H2O play crucial roles on the hydrolysis mechanism. The stability and the optical properties of the ZnO colloids were monitored by UV−visible electronic absorption and emission spectroscopies. From an HR-TEM study it was established that low concentration (2 × 10-4 M) of zinc cyclohexanebutyrate and zinc acetate afforded ZnO nanocrystallites of (2.12 nm, SD = 0.76) and (3.0 nm, SD = 0.5), average size, respectively. ZnO nanocrystals with rock salt structure coexist with wurtzite structure when zinc cyclohexanebutyrate is used as the starting salt. Dynamic light backscattering size measurements of ZnO nanoparticles were accomplished in DMSO colloid dispersions, resulting in the detection of small individual nanoparticles and assemblies of nanoparticles. Powder X-ray diffraction spectroscopy was used to accomplish the nanoparticle characterization, of DMF dispersions. Experimental results show that cyclohexanebutyrate acts as a more effective capping agent than acetate. Low concentration (2 × 10-4 M) colloidal ZnO dispersions in DMSO did not show any flocculation or red shift in two months, probably due to the concatenated dynamic stabilizing action of carboxylate ions and solvent molecules. The ZnO colloids in DMF are not stable and readily precipitate; moreover, nanoparticles in this solvent tend to adhere to glass walls, which allows production of ZnO films.