Microscopy of advanced materials for energy and environment

High Resolution TEM images are obtained from the interference phenomenon that arises when the incident electron wave goes through the specimen and is scattered by the different atomic planes of a crystal. The resulting interference fringes reproduce the spatial periodicity corresponding to those planes. Structure and defects in crystals can then be described at atomic level. These so-called phase-contrast images can be misleading in the interpretation, as this contrast is highly sensitive to experimental conditions such as defocus or sample thickness. Therefore, it is important to simulate HRTEM images from proposed atomic models under different conditions in order to check the structural features.

We have structurally characterized a wide range of materials for energy or sensing applications. Recent work has been done on a series of rare-earth niobates (RE3NbO7, RE = Er, Lu, Y, Yb) for applications as a proton conducting electrolyte in low operating temperature Solid Oxide Fuel Cells (SOFCs). Transport properties in these materials were explained by the formation of short-range ordered domains in the matrix fluorite crystal structure, revealed in HRTEM images and as diffuse scattering in the Selected Area Electron Diffraction (SAED) patterns. Such small domains are unreachable by any of the other techniques used in structure characterization, such as X-ray diffraction.

In the field of sensing materials, Nb2O5 nanorods and Nb2O5/SnO2 heterostructures as humidity sensors have been recently characterized (Fig.1). For the first system, the contrast fringes along the axial direction of the nanorods that could be attributed to a high density of defects were proved to be intrinsic of an unexpected low defective phase of Nb2O5, thanks to the comparison with simulated images. For the heterostructure, a variety of defects were found in the interface, and a highly strained growth of Nb2O5 islands on SnO2 nanorods was mapped using Geometric Phase Analysis (GPA) software.

a-d) HRTEM images of Nb2O5 nanorods. Characterization of the structure and defects along the axial direction was performed by the combination of HRTEM and the simulation of images from the proposed atomic model for the nanorods. e) Strain mapping of an Nb2O5 island growing on a SnO2 nanowire. Strain propagates into Nb2O5 from interface dislocations.




Universitat de Barcelona