X-ray absorption fine structure study of amorphous metal oxide thin films prepared by photochemical metalorganic deposition

The oxidation state and local geometry of the metal centers in amorphous thin films of Fe2O3 (Fe3+ oxidation state), CoFe2O4 (Co2+/Fe3+ oxidation states), and Cr2O3 (Cr3+ oxidation state) are determined using K edge X-ray absorption near-edge structure (XANES) spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. The metal oxide thin films were prepared by the solid-state photochemical decomposition of the relevant metal 2-ethylhexanoates, spin cast as thin films. No peaks are observed in the X-ray diffraction patterns, indicating the metal oxides are X-ray amorphous. The oxidation state of the metals is determined from the edge position of the K absorption edges, and in the case of iron-containing samples, an analysis of the pre-edge peaks. In all cases, the EXAFS analysis indicates the first coordination shell consists of oxygen atoms in an octahedral geometry, with a second shell consisting of metals. No higher shells are observed beyond 3.5 Å for all samples, indicating the metal oxides are truly amorphous, consistent with X-ray diffraction results.

Graphical abstractUsing X-ray absorption fine structure spectroscopy, the oxidation state and local order of metal oxides produced from the solid-state photochemical decomposition of metal 2-ethylhexanoates (metal=Co, Fe, Cr) is presented. Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Photochemical metalorganic deposition leads to amorphous metal oxides. ► Complete absence of long-range order beyond second nearest neighbors. ► XAFS provides speciation and characterization of local structure in PMOD-prepared thin films. ► New polymorphs of Fe2O3, CoFe2O4, and Cr2O3 characterized.