Quantum Mechanical Modeling of Physical and Chemical Properties of Fe3O4 Nanoparticles and Surfaces for Nanomedicine and Energy Conversion

The aim of my PhD thesis is the computational investigation of structural, electronic, and magnetic properties of magnetite (Fe3O4) nanostructures and their interaction with various chemical species for technological applications. On one hand, we have considered a cubic Fe3O4 nanoparticle (NP) enclosed by six (001) facets to investigate the effects of surface functionalization for nanomedical applications. On the other hand, we have considered a Fe3O4(001) surface model to study the interaction and immobilization of technetium and as a support material for single atom catalysts (SACs) for the oxygen evolution reaction (OER). Most of the calculations are based on the density functional theory (DFT), using HSE06 hybrid functional and localized basis sets, which is a well-suited approach for the description of Fe3O4 structural, electronic, and magnetic properties. An approximated DFT-based method, namely the Hubbard-corrected self-consistent charge density-functional tight-binding (DFTB+U), has been considered for the structural properties and dynamical behavior of the largest systems. In chapter 3, we studied the effect of surface functionalization on the magnetization of Fe3O4 NPs. We explored the effects of the adsorption of various ligands (containing hydroxyl, carboxylic, phosphonic, catechol, and silanetriol groups) onto the spin and structural disorder, which contribute to the lowering of the NP magnetization. We simulated the spin disorder through a spin-flip process at octahedral Fe ions and correlated it with the energy separation between O 2p and Fe 3d states. We found that only multidentate bridging ligands hamper the spin disorder by establishing additional electronic channels between octahedral Fe ions for an enhanced ferromagnetic superexchange interaction. We also observed that the presence of anchoring organic acids interferes with structural disorder, by disfavoring surface reconstruction. In chapter 4, we studied the interaction between the Fe3O4(001) surface and the pertechnetate ion (TcVIIO4−), that is a nuclear fission product whose major issue is the high mobility in the environment. Experimentally, it is clear that Fe3O4 can reduce TcVIIO4− to TcIV species and retain such products quickly and completely, but the exact nature of the process and redox products is not completely defined. Therefore, we studied a possible initiation step of the TcVIIO4− reduction and model structures for the immobilized final products. The interaction of TcVIIO4− with Fe3O4 surface leads to the formation of a reduced TcVI species through an electron transfer that is favored by the Fe3O4 surfaces with a higher FeII content. Regarding the final products of the TcVII reduction, TcIV can be incorporated into a subsurface octahedral site or adsorbed on the surface in the form of TcO2·xH2O chains. In chapter 5, we investigated Fe3O4(001) surface as support material for SACs for the OER. We trapped inexpensive and abundant transition metals, such as Ti, Co, Ni, and Cu, in various configuration in the Fe3O4(001) surface. The structural, electronic, and magnetic properties of so-formed SACs were studied. As a second step, we investigated the performance of these model electrocatalysts towards the OER via two different mechanisms, by means of the computational hydrogen electrode (CHE) model developed by Nørskov. Co-doped systems were found to be the most promising model electrocatalysts among those investigated. In chapter 6, a set of CLASS2 force field parameters is optimized for the description of the Fe−OWater cross-interaction to catch the main features of iron oxides/water interfaces. We used as references HSE06 calculations of the potential energy function for a single water molecule adsorbed on the Fe3O4(001) surface and DFTB+U molecular dynamics simulations for a water trilayer on the same surface. We assessed the transferability of the new parameters for the water adsorption on a spherical Fe3O4 nanoparticle.