From Layered Double Hydroxides to ZnO-based Mixed Metal Oxides by Thermal Decomposition: Transformation Mechanism and UV-Blocking Properties of the Product

Abstract

Thermal decomposition of layered double hydroxides (LDHs) is a way of fabricating mixed metal oxide (MMO) nanocomposite materials composed of metal oxide and spinel phases. A detailed understanding of the mechanism of the transformation of the LDH precursor to the MMO should allow the properties of the resulting MMO nanocomposites to be tailored to specific applications. Here we report a systematic investigation of the structure, composition, and morphology evolution from ZnAl-LDHs to ZnO-based MMO nanocomposites composed of ZnO and ZnAl 2 O 4 on calcination at different temperatures. The nucleation and oriented growth of ZnO crystallites and the formation of ZnAl 2 O 4 were monitored by high resolution transmission electron microscopy (HRTEM) combined with selected-area electron diffraction (SAED), in situ X-ray diffraction (XRD), solid-state 27 Al magic-angle spinning nuclear magnetic resonance ( 27 Al MAS NMR), and thermogravimetric and differential thermal analyses (TG−DTA). The layered structure of the LDH precursor was maintained as the temperature was increased from room temperature to 180 °C. Upon further heating from 200 to 400 °C, ZnO nuclei doped with Al 3+ were first formed as an amorphous phase and then underwent oriented growth along the ⟨101̅0⟩ direction. The high aspect ratio of the LDH platelets is responsible for the oriented growth of the resulting ZnO crystallites. On further increasing the calcination temperature, Zn 2+ ions were continuously released from the amorphous phase resulting in the formation of crystalline ZnO nanoparticles doped with Al 3+, which are homogeneously dispersed throughout the amorphous phase. When the calcination temperature reached 500 °C, Al 3+ ions were released from the ZnO-like structure resulting in the formation of ZnAl 2 O 4 spinel and the crystallinity of the spinel increased gradually with increasing temperature. Sintering of ZnO and ZnAl 2 O 4, with concomitant loss of the platelet-like morphology, occurred below 800 °C. UV−visible spectroscopy showed that the ZnO/ZnAl 2 O 4 nanocomposite prepared by calcination of the ZnAl-LDH precursor at 800 °C has superior UV-blocking properties to both commercial ZnO and a physical mixture of ZnO and ZnAl 2 O 4 .