The recent decades have shown that nanocrystals (NCs) can play an important role in a lot of different fields such as biology, catalysis, and magnetism. From a sustainable point of view it is necessary to limit the use of rare metal (platinum, palladium etc..) as catalyst. Due to their low cost and low toxicity, cobalt NCs and their derivatives are appealing materials. However, to gain valuable electronic and chemical properties, a careful design of the nano-object is required, i.e. its shape, size, phase and composition. In several industrial domains, the production of nanomaterials used as catalysts in fuel cells, batteries, etc.. needs to be improved. This calls for a better understanding of their structure at the atomic level in relation with their properties, not only in vacuum, but also in real conditions. Nevertheless, despite important research efforts in the field of colloid chemistry, the understanding of the growth mechanisms of nanomaterials is still incomplete, because of i) the impossibility of directly visualizing dynamical processes at the nanoscale in liquids, ii) the complexity of the chemicals reaction itself due to the number of components and of the role of the chemical byproducts. Therefore, the influence of the physical and chemical parameters (concentration of precursors, time of reaction, temperature, role of organic ligands…) are still in debate. At the same time, modern studies of nanoscale materials are being revolutionized by in-situ and operando characterization. Indeed, it is now possible to follow in real-time the reactivity and evolution of nanomaterials in response to chemical, thermal, mechanical, or electrical stimuli i.e, in operando conditions, but also the growth of NCs in solution or the atomic structure of NCs in liquids. These cutting-edge techniques (E-TEM, NAP-XPS, in situ STM) should lead to advanced understanding of the mechanisms of nanomaterials synthesis with functional properties, e.g. nanoalloys or core-shell nanoparticles. Thus the aim of this project is to study in situ the nucleation and growth process of metallic and bimetallic NCs, and their further reactivity in paradigmatic reactions, starting from a very simple one-pot synthesis of metallic hcp cobalt NCs recently patented.