Reversible Magnetization Modulation in Ordered Mesoporous Mixed Metal Oxide Thin Films By Double Layer Charging and Topotactic Lithium Insertion

Abstract

Mesostructured metal oxide thin films have received much attention in recent years. The primary reason for this is the fact that many such materials have been shown to outperform their bulk counterparts. Energy storage and conversion is one of the fields where this is very evident. Advances in polymer templating over the past 10 years have enabled the preparation of various thin film materials with cubic and 2D-hexagonal pore structures and different ordering lenghts.[1] Their formation relies in principle on the coassembly of either sol-gel precursors or preformed building blocks with an amphiphilic polymer structure-directing agent. Despite the progress made so far, complex metal oxides (mixed metal oxides) often exhibit an ill-defined mesoporous morphology after removal of the polymer template and thermally-induced crystallization because of their limited stability. Here, we describe the evaporation-induced self-assembly (EISA)[2] synthesis of cubic mesoporous mixed metal oxide thin films with >15 nm diameter pores and nanocrystalline walls by using a large polyisobutylene-block-poly(ethylene oxide) diblock copolymer. We focus specifically on perovskite-type La1–x Sr x MnO3 (LSMO) and La1–y Ca y MnO3 (LCMO) as well as spinel-type LiFe5O8. These materials were characterized by various state-of-the-art techniques, including electron microscopy, GISAXS, RBS, XRD and XPS, and were found to be of high quality after calcination at elevated temperatures. For LSMO and LCMO, we show that the charge carrier density, and therefore the magnetization, can be modulated through double layer charging (i.e., electrostatic hole- or electron-doping).[3] Magnetization modulation values of up to 10% have been achieved, the highest thus far reported for electrolyte-gated mixed-valence manganese oxides. Apart from that, we show that the room temperature magnetization of LiFe5O8 can be tuned by topotactic insertion/extraction of Li-ions into/from the spinel lattice and provide insights into the mechanism. Both processes allow for control of magnetism, are highly reversible, and strongly benefit from the high surface-to-volume ratio of the films. [1] C. Sanchez, C. Boissiere, D. Grosso, C. Laberty, and L. Nicole, Chem. Mater.20, 682 (2008). [2] C. J. Brinker, Y. F. Lu, A. Sellinger, and H. Y. Fan, Adv. Mater. 11, 579 (1999). [3] C. Reitz, P. M. Leufke, R. Schneider, H. Hahn, and T. Brezesinski, Chem. Mater. 26, 5745 (2014).