Abstract
Nanoparticles are characterized by their small and finite-size. Indeed they are made of tens to thousands of atoms, corresponding to a size of 1 to tens nanometers. Nanoparticles’ finite-size entails three significant implications: internal transitional symmetry breaking occurs, electronic confinement effects are relevant, and nanoparticle’s surface to volume ratio is not neglegible. In turn, at the nanoscale, we witness: the occurrence of many peculiar structures, the arising of unique optical, magnetic, and catalytic properties (different from the one of the single atom and of its bulk counterpart) and characterized by peculiar structureproperty relationships, the failure of bulk thermodynamics modeling in the prediction of the nanoparticle structural stability, also for the case of relatively large systems. Indeed size- and shape-effects are known to non-trivially affect the (meta)stability of a nanoparticle against melting and pre-melting.Borrowing the words spelt by Richard Feynman during his famous talk ’There is plenty of Room at the bottom’: "When we get to the very, very small world . . . we have a lot of new things that would happen that represent completely new opportunities for design." Tailoring the nanoparticles architecture - i.e. size, shape, chemical composition and ordering - at will, one could harness their full potential in technological devices which will trigger a revolution in many fields, ranging from biomedicine to catalysis and optics. The identification of optimal nanoparticle architectures for target purposes is, from here on, mentioned under the name of rational design. The complexity inherent to this practice represents a longstanding high-reward challenge as it hinges on the understanding of how each structural feature contributes towards the nanoparticle global chemophysical property.
Moreover, the rational design of nanoparticles for target application should encompass the in depth study of the (meta)stability of the chosen bespoke nanoarchitecture. Indeed, coming back again to Feynman’s notorious contribution to nanoscience, he mentioned that "... [he was] not afraid to consider the final question as to whether, ultimately – in the great future – we can arrange the atoms the way we want; the very atoms, all the way down! ... (within reason, of course; you can’t put them so that they are chemically unstable, for example)". A degree of atomistic control of the nanoparticle components is currently at reach during both colloidal and physical growth methods. Thus, it is of fundamental importance to assess whether the so synthesized nanoparticles present a satisfactory structural stability, or if their inherent (meta)stability manifests and determines too dramatic changes in the nanoparticles chemophysical properties, and thus in the performance of the device which exploits them.
This thesis will contain a discussion of the application of state-of-the-art sampling techniques to probe the complexity of the conformational and energetic landscape of noble and quasi-noble metal nanoparticles of 100-1000 atoms. Conversely, a thorough characterization of the mechanisms driving melting and pre-melting will be sought and rationalized in terms of size, shape, and composition effects. As a final case study, the structural properties of Pt nanoparticles will be carefully assessed to predict their performance for Oxygen reduction reaction, identifying design criteria towards the synthesis of nanoarchitectures with enhanced catalytic properties.
Chapter I will introduce the state-of-the-art of metallic nanoparticles characterization, synthesis, theoretical modelling, and application as nanocatalyst. Chapter II will present the numerical techniques employed to study the melting and pre-melting of metallic (group X and XI) nanoparticles and the prediction of their catalytic properties. Chapter III and IV will follow with a thorough investigation of solid-solid transitions in mono- and bimetallic nanoparticles. Structural rearrangements will be discussed with a focus on size and composition effects determining whether they are concerted or diffusion driven. Chapter V will tackle a discussion on how to disentangle univocally kinetic and thermodynamic contributions determining phase changes in metallic nanoparticles as well as to discriminate faithfully solid and liquid structures. Finally, Chapter VI will detail the application of a method based upon a geometrical descriptor to determine design criteria towards the synthesis of Pt nanoparticles highly active towards oxygen reduction reaction.
| Date of Award | 1 Jun 2019 |
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| Original language | English |
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| Supervisor | Francesca Baletto (Supervisor) & Carla Molteni (Supervisor) |