Abstract
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.
Highlights
Layered double hydroxides (LDHs) are a type of anionic clay
The results obtained from this research suggest that, for the first time, a one-step continuous synthesis method of a core-shell ZnO-LDH composite material has been achieved
A pure two-phase composite material was not fully synthesised, as there was an indication of the presence of Zn2Al-CO3, as well as single phase ZnO and Mg2Al-CO3 mixtures
Summary
Layered double hydroxides (LDHs) are a type of anionic clay. LDHs conform to a general formula: MI1I−xMIxII(OH2) (An−)x/n.zH2O [1], where MII and MIII represent bivalent (Mg2+, Ca2+, Co2+, Zn2+, Cu2+, Ni2+) and trivalent metal ions (Al3+, Co3+, Fe3+, Cr3+, Ni3+ Co3+) are distributed in brucite-like sheets [2]. Magnetic core-shell LDHs have been used as drug delivery vehicles, produced via a two-step synthesis process whereby the magnetic ferrite core was produced in the first step, and the LDH shell was grown via a co-precipitation reaction step [15,16]. Zeolites have been employed as core materials for adsorbent composites for water treatment as a way to combine the high specific surface area of zeolites with the ion exchange functionality of LDHs [19,20]. By altering the LDH in composites from MgAl-CO3 to ZnCr-CO3 type, magnetic core-shell composites have been applied to adsorption and photocatalysis of methylene blue [31]. Two-step synthesis methods for core-shell composite materials can be time consuming and to the knowledge of the authors, there have been no attempts to incorporate both reactions into a single process
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