Estudo de nanoclusters de metais de transição com 13 átomos baseados em Ti, Zr e Hf via Teoria do Funcional da Densidade

Abstract

In this master thesis, the theoretical study of bimetallic (transition metals - TM) 13-atom nanoclusters (NCs) in icosahedral geometry (ICO) was carried out via Density Functional Theory (DFT). We focus on systems based on Ti, Zr, and Hf combined with the 30 TMs of the Periodic Table (3d series: Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Zn; 4d series: Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, and Cd; and 5d series: Lu, Hf, Ta, W, Re, Os, Ir, Pt, Au, and Hg), forming the Ti12TM, Zr12TM, and Hf12TM systems. We sought to understand the energetic, structural, electronic, and vibrational properties of the systems in two configurations, i.e., a single atom was replaced in the internal or external region of the ICO, forming the core-shell configurations (in, Ti12TMin, Zr12TMin, and Hf12TMin) and the out configurations (Ti12TMout, Zr12TMout, and Hf12TMout). The results were compared with the literature, in addition to the verification between bimetallic and monometallic 13-atom systems in ICO structure, to obtain the effects caused by the replacement of a MT. Firstly, the study of the vibrational, energetic, and structural properties of the hetero- and homo-dimers was carried out, as a lower coordination limit for the NCs. This result served to establish a relationship between the TM interactions, with the aim of understanding the bonds strength through vibrational frequencies, binding energies, and bond lengths. After that, we studied the bulk properties to establish a direct comparison with the literature and, thus, confirm the methodology used, in addition to comparing the energetic, structural, and electronic properties of crystalline systems, with the objective of helping in the understanding of the bimetallic NCs formation. Finally, the study of bimetallic NCs was performed, where we sought to understand the energy stability of the studied systems, the energy gains and losses related to the MT interaction and to the distortion suffered due to this mixture. We also sought to understand the structural properties of the systems, such as effective coordination number and average bond length for the in and out configurations, where the distortion results are analyzed. Also, we present an estimate of the NC reactivity through the d-band model, calculated by the center of gravity of the occupied d states, verifying that most of the NCs are reactive according to this model. Finally, we propose an explanation for the preference of the in or out structures formation through the total relative energy, cohesion energy (from bulks), atomic radius, electronegativity, and formation of homo- and hetero-dimers, where we can establish the atomic radius as the main property responsible for the MT position inside or outside Ti, Zr, and Hf based NCs.