Synthesis, coating, intercalation and ToF-SIMS study of MgAl Layered Double Hydroxide (LDH)

Abstract

Layered Double Hydroxides (LDHs) are novel clay-like materials with metal octahedral layers bearing a positive charge and compensating anions in the interlayer space. They can be used as catalysts, absorbents, electrodes, and in controlled release systems. 2:1 MgAl LDH powders were prepared by both coprecipitation (C-type) and urea (U-type) methods. The 003 (dspacing= 7.6 Å), 006 and 009 XRD reflections were indicative of the interlayer spacing of the layered structure, while 012, 015 and 018 were more prominent in crystals of random orientation. LDH crystals were observed as hexagonal platelets under SEM and the U-type LDH had higher crystallinity and bigger crystals than C-type material (15-25 μm, aspect ratio 20-33 vs. 100-400 nm, aspect ratio 5-18). In the FT-IR spectrum, the broad band around 3400-3600 cm-1 (stretching mode of OH) and three strong peaks at 963 cm-1, 747 cm-1, 658 cm-1 (M-O vibration and M-OH bending) confirmed the existence of LDHs. A very thin film of C-type LDH was successfully coated on Si wafers by ultrasonification with 1-propanol, the most successful medium in dispersing LDH crystals. The LDH crystals in the thin film were highly oriented, supported by the absence of the 012, 015, and 018 reflections in the diffraction pattern. The dicarboxylic acids dodecanedioic acid (C12), pimelic acid (C7) and suberic acid (C8) were intercalated into LDHs with resulting increased d-spacing ranging from 14.7-21.1 Å with the highest d-spacing found in C12 intercalation. These dicarboxylic acid molecules form oriented layers in the interlayer gallery, with or without additional water layers, and were found in either vertical orientation or at a 34-38̊ slant which allowed maximum hydrogen bonding. The 2864 cm-1 and 2930 cm-1 (C-H stretching) FT-IR peaks observed for these samples were characteristic of aliphatic dicarboxylates and confirmed intercalation. The intercalated LDHs were examined by ToF-SIMS to determine the effects of intercalation on the mass spectral fragmentation pattern of the layered structure. Fragments, especially in the high mass range, were interpreted in terms of the number of Al2O3 and MgO units. The patterns of intercalated LDHs showed disruption due to the impacts of intercalation on interlayer hydrogen bonding. This suggests that ToF-SIMS samples beyond the surface octahedral layer and is sensitive to the interlayer bonding.