Morphological and chemical characterization of nanoparticulate matter by SEM/EDX analytical techniques

Nanoparticles (NPs) are particles with at least one dimension smaller than 100 nm but can reach molecular length scales. As a matter of fact, nanoparticulate matter should be considered a distinct state of matter owing to its peculiar properties, that induce very different behaviors compared with that of bulk materials. NPs have a huge specific surface area which affects their chemical reactivity as well as their mechanical, optical, electric, thermal and magnetic properties. The ultrafine size of NPs itself is a workable feature; for example, their surfaces can act as carriers for liquid, gaseous or solid material adsorbed on them. The real state of the NPs is highly influenced by the surroundings. In particular NPs are rarely used as isolated objects but are commonly dispersed in other materials or combined with them to obtain the so-called nanomaterials (NMs). The performance of the final product is strongly affected by the degree of dispersion and by the interactions between the NPs and the neighboring materials. Synthetically produced NPs play a key role in nanotechnology and nowadays find use in a wide variety of applications, ranging from engineering to life sciences. Many NPs are also created by natural phenomena. An enormous number of NPs are unintentionally released in the environment by combustion processes. Since the early 1990s the awareness of possible health and safety concerns for some nanoparticles has increased progressively. Human skin, lungs, and the gastro-intestinal tract are the biological membranes protecting the interior of organism from foreign substances. Due to their small size, NPs can cross these barriers and entry into the circulatory and lymphatic systems and ultimately into body tissues and organs. Some NPs, depending on their composition, size and morphology, can produce irreversible damage to cells. The most critical physical properties of particulate matter are by far size and shape, because they directly influence properties and behavior. So, size and shape are considered the most valuable indicators in two important contexts: assessment of quality and performance of engineered NMs; evaluation of potential toxicity of nano-structured materials. Usually, particles of various sizes are present in a sample and therefore it is necessary to determine not only the mean particle size but also the particle size distribution. For a complete understanding of NM structure a thorough study on morphology and chemical composition is required. This task can be undertook with the aid of different characterization methods, but the ideal and most complete technique is electron microscopy. In particular Field Emission Scanning Electron Microscopy (FE-SEM) coupled with Energy- Dispersive X-ray analysis (EDX) is a very powerful tool providing simultaneously information on size and size distribution, shape, morphology and composition of particles as small as a few nanometers. In electron microscopy observation, sample preparation can be of great importance, especially for nanocomposite materials. Common preparation methods are: dispersion, in which the sample is dispersed in a suitable liquid, deposited on a holder and dried; crushing, in which the sample is crushed with mortar prior to observation; embedding, in which the sample is embedded into a resin and polished; ultramicrotomy, in which an ultrathin layer of the sample is created; ion milling, which forms the observation side of the sample using a ion beam; etching, in which the observation side is formed by etching of bulk sample. In order to apply the correct procedure, the operator must possess great expertise and skill and a good knowledge of the sample. FE-SEM analysis can take advantage of different signals such as SE (secondary electrons), BSE (backscattered electrons), characteristic X-rays and can reach very high magnifications of about 1.000.000 to observe nanosized structures. The objectives of my PhD project were: - to develop appropriate laboratory procedures for sample handling and sample preparation - to select the most suitable signal for the analysis depending on the sample type and the required information - to determine the effect of different important operation parameters on the analysis outcome - to identify possible artifacts and sources of errors in the sample preparation step as well as in the sample analysis step and in the subsequent image processing step - to establish reliable protocols for successful manual and automated analyses - to study issues relating to data interpretation, data quality and data treatment - to explore the usefulness of some popular image processing software. The final aim was to provide the key elements that may help to fully deploy the potential of Field Emission Scanning Electron Microscopy in the study of the nanoparticulate matter.