Ultrafine particulate matter and nanostructured materials: A comprehensive transmission electron microscopy approach

The twenty-first century may be termed the “nano-century,” as nano-structured materials have become deep-rooted in our everyday life. Cosmetics, pharmaceuticals, textiles, electronics, food packaging, automobile, and many more industries directly or indirectly consume or produce nano-crystalline powders, or in general nanomaterials, in large quantities. The incredible growth in their production has challenged the conventional characterization tools available to researchers. Characterization of nanoparticles is a prerequisite before their possible usage in any field, however, it becomes indispensable for nanostructured materials of environmental concerns and unlike for chemical toxicants, the characterization of such materials is not limited only to purity and chemical composition. A transmission electron microscope (TEM) is a robust characterization tool providing imaging, diffraction, and spectroscopic techniques possibly at the atomic resolution in one single instrument. This PhD thesis aims to combine the three methodologies to yield the best possible route for quantitative characterization of the nanostructured assemblies. In this context, the Rietveld refinement of selected area electron diffraction (SAED) patterns, which encodes structural information in the form of the diffracted electron intensities, has capabilities to yield phase composition and microstructure from material volumes many orders of magnitude lesser than in complementary techniques such as X-ray diffraction (XRD) and neutron diffraction (ND). This unique superiority of TEM is crucial in the study of nanostructured materials as local quantitative characterization may not be adequately performed by XRD and ND. Moreover, TEM readily permits the attainment of elemental composition from the same region of interest using energy dispersive x-ray spectroscopy (EDXS) or electron energy loss spectroscopy (EELS) to further support the diffraction-based studies. This thesis has been divided into four parts, as shown in the schematic below. After introductory topics, Part 1 (Chapter III) focuses on the development of a sample preparation technique for particulate matter collected using environmental sampling systems to be analyzed using electron microscopy tools, i.e., scanning electron microscopy (SEM) and TEM. The method based on extraction replica, has been applied to wear debris collected on aluminium substrates during dynamometer studies of disc brakes. Especially concerning the TEM analysis, the technique is useful since the collected particles cannot be directly observed on the substrates. As for SEM, the accurate quantification of Al, which is usually present in the brake wear debris, is difficult if the analysis is performed directly on the Al filter. Moreover, the development of the technique was crucial since it gave the opportunity to extend the methodology of electron powder diffraction (EPD) based microstructure characterization also to complex samples, like those related to environmental research. The first step forward to the proposed methodology to study nanostructured materials concerns establishing a calibration method to separate the instrumental effects from the physical broadening caused by the microstructure of the analyzed samples. To fulfil this very important criterion, in Part 2 (Chapter IV), a standard nanocrystalline CeO2 powder has been used and calibration for camera lengths and instrumental broadening function has been performed in correlation with the microstructural data obtained from XRD results of the same material. In this approach, a total of twelve diffraction patterns were collected using three SAED apertures- 800, 200, and 40 μm, and four camera lengths- 1360, 1080, 844, and 658 mm. The instrumental broadening function determined through Rietveld refinement-based characterization of these diffraction patterns yielded an interesting trend, wherein for a particular SAED aperture, the instrumental broadening function decreased with the increase in camera length. A dedicated procedure for the Rietveld refinement of SAED patterns of nanostructured materials has been developed using MAUD (Material Analysis Using Diffraction) software. In the field of nanostructured materials, new compounds/phases are being synthesized at a rapid pace, with high-output techniques that can yield many variants within a single experiment. Such techniques find usage in a variety of fields, such as drug delivery, catalysis, and semiconductors. The physical and chemical properties of these newly designed materials are not a function only of their structural form, but in the case of multiphase materials, also depend on the relative amounts of the components. In line with the crucial role of phase identification and consequently determination, Part 3 (Chapter V) demonstrates the capabilities and limits of EPD to study the microstructure of homogeneous and complex multi-phase systems, also for the sampled particulate matter discussed in Part 1 (Chapter III). These include nanocrystalline silicon, yttrium oxide, titanium dioxide, wear debris from disc brake wear, and hematite obtained from the heat treatment of natural goethite, each displaying microstructural parameters that needed to be tackled with the proposed methodology. These case studies with different complexities were appropriately selected and studied. In Part 4 (Chapter VI), the limitations of relying only on electron diffraction, wherein significant changes in the structural information of a particular phase could be easily overlooked, have been demonstrated for a mixed oxide and countered by combining SAED with EDXS in one Rietveld fitting. For this analysis, cobalt iron oxide featuring impurities with a chemical composition of Na, Mg, Ca, Ti, and Mn was studied, with its modelled EDX spectrum coupled with the diffractogram. The developed technique, although at a nascent stage with scope for further development, allows determining in addition to the microstructure, the degree of substitution of the parent elements- Fe and Co, by the impurity atoms. Thus, although the mathematics of electron diffraction is critical from the view of electron-matter interaction, the current work rather delves more into the development of suitable methodologies and their application to fill the existing gap in the literature by extending the approach of the Rietveld refinement to SAED patterns for studying mainly the microstructure, also combined with other characterization techniques such as XRD and EDXS, for solving problems of scientific interest in the area of nanostructured materials.