Pillaring of Layered Perovskites, K1−xLaxCa2−xNb3O10, with Nanosized Fe2O3 Particles

Nanosized Fe2O3 clusters are pillared in the interlayer spaces of layered perovskites, H1−xLaxCa2−xNb3O10 (0≤x≤0.75) by a guest-exchange reaction using the trinuclear acetato-hydroxo iron cation, [Fe3(OCOCH3)7 OH·2H2O]+. The interlayer spaces of niobate layers are pre-expanded with n-butylammonium cations (n-C4H9NH+3), which are subsequently replaced by bulky iron pillaring species to form Fe(III) complex intercalated layer niobates. Upon heating at 380°C, the interlayered acetato-hydroxo iron complexes are converted into Fe2O3 nanoclusters with a thickness of ca. 3.5 Å irrespective of the interlayer charge density (x). The band-gap energy of the Fe2O3 pillars (Eg∼2.25 eV) is slightly larger than that of bulk Fe2O3 (Eg∼2.20 eV) but is smaller than that expected for such a small-sized semiconductor, which can be assigned to the pancake-shaped Fe2O3 pillars of 3.5 Å in height with comparatively large lateral dimension. X-ray absorption spectroscopic measurements at the Fe K-edge are carried out in order to obtain structural information on the Fe2O3 pillars stabilized between the niobate layers. XANES analysis reveals that the interlayer FeO6 octahedra have coordination environments similar to that of bulk α-Fe2O3, but noncentrosymmetric distortion of interlayered FeO6 is enhanced due to the asymmetric electric potential exerted by the negatively charged niobate layers. Scanning electron microscopic observation and nitrogen adsorption–desorption isotherm measurement suggest that the pillared derivatives are nanoporous materials with the highest BET specific surface area of ca. 116 m2/g.