Synthesis and photochemical properties of zinc-aluminum layered double hydroxide/organic UV ray absorbing molecule/silica nanocomposites

Organic ultraviolet (UV) ray absorbents have been used as sunscreen materials, but may pose a safety problem when used at high concentration. In order to prevent direct contact of organic UV ray absorbent to the human skin several organic UV absorbents such as 4-hydroxy-3-methoxybenzoic acid, 2-hydroxy-4-methoxybenzophenone-5-sulfonic acid, 4-hydroxy-3-methoxycinnamic acid, 4,4′-diaminostilbene-2,2′-disulfonic acid, p-aminobenzoic acid and urocanic acid were intercalated into Zn₂Al layered double hydroxides (Zn₂Al-LDHs) by anion-exchange and/or co-precipitation reaction. Thereafter, the obtained nanocomposites were coated with amorphous silica. Significant amounts (20-40mass%) of organic UV absorbers were intercalated as univalent and/or divalent anions. The UV ray absorption ability of the organic UV absorbents increased and their catalytic activity for the air oxidation of castor oil greatly decreased when they were intercalated into the interlayer spaces of the Zn₂Al-LDHs. The deintercalation of organic molecules from LDHs by the anion exchange reaction with carbonate ion could be greatly depressed by coating the nanocomposite powder with amorphous silica.