Pillaring of Layered Perovskites,K_(1-x)La_xCa_(2-x)Nb_3O_(10') with Nanosized Fe_2O_3 Particles

摘要

Nanosized Fe_2O_3 clusters are pillared in the interlayer spaces of layered perovskites,H_(1-x)La_xCa_(2-x)Nb_3O_(10)(0 <- x <- 0.75) by a guest-exchange reaction using the trinuclear acetato-hydroxo iron cation,[Fe_3(OCOCH_3)_7 OH centre dot 2H_2O]~+.The interlayer spaces of niobate layers are pre-expanded with n-butylam-monium cations (n-C_4H_9NH_3~+),which are subsequently replaced by bulky iron pillaring species to form Fe(III) complex intercalated layer niobates.Upon heating at 380 deg C,the interlayered acetato-hydroxo iron complexes are converted into Fe_2O_3 nanoc-lusters with a thickness of ca.3.5A irrespective of the interlayer charge density (x).The band-gap energy of the Fe_2O_3 pillars (E_g approx 2.25eV) is slightly larger than that of bulk Fe_2O_3 (E_g approx 2.20eV) but is smaller than that expected for such a small-sized semiconductor,which can be assigned to the pancake-shaped Fe_2O_3 pillars of 3.5A 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 Fe_2O_3 pillars stabilized between the niobate layers.XANES analysis reveals that the interlayer FeO_6 oc-tahedra have coordination environments similar to that of bulk #alpha#-Fe_2O_3,but noncentrosymmetric distorition of interlayered FeO_6 is enhanced due to the asymmetric electric potential exerted by the negatively charged niboate layers.Scanning elec-tron microscopic observation and nitrogen adsorption-desorp-tion isotherm measurement suggest that the pillared derivatives are nanoporous materials with the highest BET specific surface area of ca.116 m~2/g.