Layered double hydroxides containing intercalated zinc sulfide nanoparticles: Synthesis and characterization

Nanosized ZnS has been synthesized in the interlayer galleries of Mg-Al layered double hydroxides (LDHs) by a process involving ion exchange of a Mg2Al-NO3 LDH precursor with a zinc-citrate complex {Na-2[Zn(C6H4O7)]center dot 3H(2)O} followed by reaction between the intercalated [Zn(C6H4O7)](2-) anions and H2S. The materials have been characterized by elemental analysis, powder X-ray diffraction (XRD), transmission electron microscopy, FTIR spectroscopy, MAS C-13 NMR spectroscopy, and UV/Vis diffuse reflectance spectroscopy, and structural models have been proposed. The XRD diffraction patterns indicate that the layered structure is maintained and that the basal spacing in the intercalated materials depends on the orientation of the citrate moiety. The results confirm that cubic ZnS (sphalerite) is formed in the interlamellar domain rather than on the external surfaces and is co-intercalated with citrate dianions. The growth of ZnS particles is constrained by the layers of the LDH resulting in a large blue shift in the bandgap compared with the bulk material. The thermal decomposition process of the hybrid material has been characterized by in situ high-temperature powder XRD and thermogravimetry-differential thermal analysis (TG/ DTA) coupled with mass spectrometry. The thermal stability of the zinc-citrate complex anions intercalated in LDHs is lower than that in the sodium salt. Thermal treatment below 270 degrees C leads to a reorientation of the citrate anions in the interlayer galleries associated with a significant interlayer contraction. No obvious changes in the XRD peaks corresponding to ZnS are apparent below 400 degrees C, indicating that sintering to form larger particles is successfully inhibited by the layered host; at higher temperatures the ZnS is oxidized with evolution of SO2.