Fabrication of Li-doped ZnO nanoparticles in aqueous solution and their ferroelectricity-like behavior

Li-doped ZnO nanoparticles were fabricated in aqueous solution of zinc nitrate and hexamethylenetetramine (HMTA) to which lithium chloride was added. SEM, XRD and TEM results revealed that the as-synthesized nanoparticles were crystalline wurzite ZnO with hexagonal flake morphology. ICP analysis confirmed the existence of Li element in these ZnO nanoparticles. For the measurement on their dielectric properties, the nanoparticles were further annealed in air under 900°C for 1 hour and then pressed into tablets under a pressure of 5×10³ kg/cm². When voltage cycles with varying magnitude were applied to these tablets of the Li-doped ZnO nanoparticles, the dependence of the polarization versus the applied field turned out to be hysteresis loops. The remanent polarizations and coercive fields were measured to be 10^-5 to 10^-4 Cm^-2 and 10⁴ to 10⁵ Vm^-1 order of magnitude, respectively. Since a variety of phenomena other than ferroelectricity would also lead to hysteresis loops, similar measurements for comparison were further performed with a view to probing the origin of the observed loops. The dielectric behaviors of ZnO nanoparticles fabricated in the Li-free solutions and the commercial ZnO particles were also measured and the polarization-field plots both appeared to be straight lines instead of loops.