Towards the Continuous Hydrothermal Synthesis of ZnO@Mg₂Al-CO₃ Core-Shell Composite Nanomaterials

Core-shell Zinc Oxide/Layered Double Hydroxide (ZnO@LDH) composite nanomaterials have been produced by a one-step continuous hydrothermal synthesis process, in an attempt to further enhance the application potential of layered double hydroxide (LDH) nanomaterials. The synthesis involves two hydrothermal reactors in series with the first producing a ZnO core and the second producing the Mg₂Al-CO₃ shell. Crystal domain length of single phase ZnO and composite ZnO was 25 nm and 42 nm, respectively. The ZnO@LDH composite had a specific surface area of 76 m² g^-1, which was larger than ZnO or Mg₂Al-CO₃ when produced separately (53 m² g^-1 and 58 m² g^-1, respectively). The increased specific surface area is attributed to the structural arrangement of the Mg₂Al-CO₃ in the composite. Platelets are envisaged to nucleate on the core and grow outwards, thus reducing the face-face stacking that occurs in conventional Mg₂Al-CO₃ synthesis. The Mg/Al ratio in the single phase LDH was close to the theoretical ratio of 2, but the Mg/Al ratio in the composite was 1.27 due to the formation of Zn₂Al-CO₃ LDH from residual Zn²⁺ ions. NaOH concentration was also found to influence Mg/Al ratio, with lower NaOH resulting in a lower Mg/Al ratio. NaOH concentration also affected morphology and specific surface area, with reduced NaOH content in the second reaction stage causing a dramatic increase in specific surface area (> 250 m² g^-1). The formation of a core-shell composite material was achieved through continuous synthesis; however, the final product was not entirely ZnO@Mg₂Al-CO₃. The product contained a mixture of ZnO, Mg₂Al-CO₃, Zn₂Al-CO₃, and the composite material. Whilst further optimisation is required in order to remove other crystalline impurities from the synthesis, this research acts as a stepping stone towards the formation of composite materials via a one-step continuous synthesis.