Structural studies on colloidal tetraalkylammonium manganese oxides.

Abstract

t. Introduction Mixed valent manganese oxides are of interest due to their diverse range of electrical, magnetic, and catalytic properties. Recently, novel colloids of manganese oxide have been prepared using tetraalkylammonium cations (allcyl = methyl, ethyl, propyl, and butyl) to prevent agglomerization (Brock et al., 1998). These materials are of interest due to unique optical properties associated with quantum confinement and their potential as starting materials for novel manganese oxide materials. In this work Mn K-edge XAFS studies were performed on colloidal MnOx samples prior and upon sol to gel transition and after different heat treatment procedures to reveal structural and electronic changes in the colloidal particles. Theoretical XAFS calculations known to yield reliable backscattering phases and amplitudes are applied in combination with suitable fitting routines to the analysis of colloidal manganese oxide systems. 2. Experimental section 2.1 Preparation of colloidal manganese oxides A detailed description of the synthesis of tetraalkylammonium (TAA) manganese oxide colloids appears elsewhere (Brock et al., 1999). The materials are prepared via reduction of tetraalkylanunonium permanganate salts (alkyl = ethyl, prowl) with 2-butanol in aqueous solutions. XAFS measurements have been obtained on the colloids as-prepared (TAA-A), with the residual 2-butanol removed (TAA-H), and on the gels (TAA-G). Measurements have been made on both tetraethylammonium (TEA) and tetrapropylammonium (TPA) manganese oxide colloids and gels with manganese concentrations from 0.1-0.4 M.