Selective preparation and enhanced microwave electromagnetic characteristics of polymorphous ZnO architectures made from a facile one-step ethanediamine-assisted hydrothermal approach

Abstract

The current study describes a facile one-step ethanediamine (en)-assisted hydrothermal approach for the selective synthesis of ZnO architectures with morphologies that evolve from nanocones, to twinned nanoroses, to dispersed microneedles, and even to complex, flower-shaped architectures. Kinetic factors, such as time, temperature, en-to-Zn(NO₃)₂ molar ratio (δ), [Zn²⁺], and Zn sources can be easily utilized to control the oriented attachment growth of [Zn(OH)₄]²⁻ on the (0001) polar surface, thereby regulating the morphology and growth direction of the ZnO architectures. Time lengthening as well as increases in temperature, δ, and [Zn²⁺] can promote the morphological evolution from needle-like to flower-shaped and can change the structurally oriented growth from along the c-axis to along the a-axis. The flower-shaped ZnO–wax composites exhibit enhanced permittivity and microwave-absorbing properties as mass fraction increases. However, this distinct morphology is prone to high dielectric loss. Thus, the flower-shaped ZnO showed stronger microwave absorption performances than the needle-like ZnO, with a minimum reflection loss (R_L) of −21.85 dB at 8.4 GHz, corresponding to a matching thickness of 3.0 mm. In particular, interesting nesting microwave absorption peaks can be observed in the reflection loss plots of the flower-shaped ZnO. The current work provides insights into the absorption mechanism of flower-shaped ZnO absorption materials.