Different manganese oxide phases were prepared as thin films to elucidate their structure–function relationship with respect to oxygen evolution in the process of water splitting. For this purpose, amorphous MnOx films anodically deposited on F:SnO2/glass and annealed at different temperatures (to improve film adherence and crystallinity) were tested in neutral and alkaline electrolytes. Differential electrochemical mass spectroscopy showed that the anodic current correlated well with the onset of the expected oxygen evolution, where in 1 M KOH, the anodic current of crystalline α-Mn2O3 films was determined to onset at an overpotential (η) of 170 mVRHE (at J = 0.1 mA/cm2) with current densities of ca. 20 mA/cm2 at η = 570 mVRHE. Amorphous MnOx films heated at 573 K (MnOx-573 K) were found to improve their adherence to F:SnO2/glass substrate after heat treatment with a slight crystallization detected by Raman spectroscopy. The onset of water oxidation of MnOx-573 K films was identified at η = 230 mVRHE (at...

http://pubs.acs.org/doi/pdf/10.1021/jp500939d

Evaluation of MnOx, Mn2O3, and Mn3O4 Electrodeposited Films for the Oxygen Evolution Reaction of Water

Mn $K$ edge ab initio FEFF8.2 calculations of the pre-edge features of the x-ray-absorption near-edge structure (XANES) region were undertaken for a series of Mn-bearing oxide-type compounds. The aim of the study is to provide a reliable method for determining quantitative and accurate redox and symmetry information for manganese. In agreement with multiplet calculations by Glatzel and co-workers, FEFF8.2 predicts a doublet and a triplet for Mn(II) and Mn(III) in octahedral symmetry, respectively, in agreement with high-resolution XANES experiments. Site distortion increases notably the contribution from dipolar transitions and, consequently, the pre-edge feature integrated area. An even more intense pre-edge feature is calculated and measured for the ${T}_{d}$ symmetry (singletlike). For Mn(IV), a triplet is predicted and measured for the ${O}_{h}$ symmetry. However, additional transitions are found in Mn(IV)-rich compounds, that are related to metal-metal transitions. These transitions overlap strongly with the ``true pre-edge,'' making extraction of redox and symmetry information for Mn(IV) more challenging. However, a model of the pre-edge with pseudo-Voigt functions of fixed calculated width (based on core-hole lifetime and experimental resolution) helps to separate the contributions related to first-neighbor symmetry from those of the metal-metal pairs. Application to multivalent defective manganese oxide materials suggests that the pre-edge information varies linearly as a function of Mn redox state or symmetry but varies nonlinearly as a function of both parameters. Finally, the polymerization of the manganese networks can be estimated from the metal-metal transitions found in the pre-edge region.

Ab initio and experimental pre-edge investigations of the Mn K -edge XANES in oxide-type materials

High-resolution manganese K-edge X-ray absorption near edge structure spectra were collected on a set of 40 Mn-bearing minerals. The pre-edge feature information (position, area) was investigated to extract as much as possible quantitative valence and symmetry information for manganese in various “test” and “unknown” minerals and glasses. The samples present a range of manganese symmetry environments (tetrahedral, square planar, octahedral, and cubic) and valences (II to VII). The extraction of the pre-edge information is based on a previous multiple scattering and multiplet calculations for model compounds. Using the method described in this study, a robust estimation of the manganese valence could be obtained from the pre-edge region at 5% accuracy level. This method applied to 20 “test” compounds (such as hausmannite and rancieite) and to 15 “unknown” compounds (such as axinite and birnessite) provides a quantitative estimate of the average valence of manganese in complex minerals and silicate glasses.

A pre-edge analysis of Mn K-edge XANES spectra to help determine the speciation of manganese in minerals and glasses

Using a combination of HAADF-STEM imaging and atomically resolved EELS in an aberration-corrected TEM we demonstrate the possibility of 2D atom by atom valence mapping in the mixed valence compound . The ELNES from and cation sites are similar to those of and references. Comparison with simulations shows that even though a local interpretation is valid here, intermixing of the inelastic signal plays a significant role. This type of experiments should be applicable to challenging topics in materials science, like the investigation of charge ordering or single atom column oxidation states in e.g. dislocations.

/pdf/2d-atomic-mapping-of-oxidation-states-in-transition-metal-1d9nyy6byr.pdf

2D atomic mapping of oxidation states in transition metal oxides by scanning transmission electron microscopy and electron energy-loss spectroscopy.

In situ x-ray diffraction and Raman spectroscopy have been carried out to pressures of 93.6 and 63.2 GPa, respectively, to explore the pressure-induced phase transformation of ${\mathrm{CoFe}}_{2}{\mathrm{O}}_{4}$ spinel. ${\mathrm{CoFe}}_{2}{\mathrm{O}}_{4}$ adopts a distorted tetragonal spinel structure at one atmosphere. At a pressure of \ensuremath{\sim}32.5 GPa, both x-ray diffraction and Raman spectroscopy indicate that ${\mathrm{CoFe}}_{2}{\mathrm{O}}_{4}$ transforms to the orthorhombic ${\mathrm{CaFe}}_{2}{\mathrm{O}}_{4}$ structure, which remains stable to at least 93.6 GPa. The bulk modulus ${(K}_{0})$ of the tetragonal and the high-pressure polymorphs were calculated to be 94(12) and 145(16) GPa, respectively, with ${K}^{\ensuremath{'}}\ensuremath{\equiv}4.$ Upon release of pressure the orthorhombic phase persists and appears to be structurally metastable. At zero pressure, laser induced heating leads to a significant transformation back to the tetragonal phase. The high-pressure orthorhombic phase at one atmosphere is 14.7% denser than the tetragonal phase.

/pdf/high-pressure-x-ray-diffraction-and-raman-spectroscopic-26tpgm8gvc.pdf

High-pressure x-ray diffraction and Raman spectroscopic studies of the tetragonal spinel CoFe 2 O 4

The occurrence of hausmannite with an apparent Curie temperature close to 750 K, instead of 41.8 K was recently described from hydrothermally altered manganese ore from the Kalahari manganese field, South Africa. The unusual magnetic properties were relat

R. Tellgren

Papers

The influence of iron substitution on the magnetic properties of hausmannite, Mn2+(Fe,Mn)(2)(3+)O-4