Elisa Albanese

Quantum mechanical predictions to elucidate the anisotropic elastic properties of zeolitic imidazolate frameworks: ZIF-4 vs. ZIF-zni

We use ab initio density functional theory (DFT) to elucidate the mechanical properties of two topologically distinct zeolitic imidazolate framework (ZIF) materials: ZIF-4 and ZIF-zni, both of which have the same chemical composition of Zn(Im)2 [Im = C3H3N2−] and are constructed from an identical Zn—Im—Zn basic building block. The CRYSTAL code was used to compute the single-crystal elastic constants Cij of the (orthorhombic) ZIF-4 and (tetragonal) ZIF-zni structures at the PBE level of theory. Through tensorial analysis of the Cij, we reveal the three-dimensional representation surfaces of the Youngs modulus, shear modulus, Poissons ratio and linear compressibility, which enable us to describe the detailed elasticity behaviour and to pinpoint basic crystal structure–property correlations. Notably, we discover that ZIF-4 can potentially exhibit a negative Poissons ratio, thereby representing the first example of an ‘auxetic-ZIF’ to be identified to date. Furthermore, we show that our DFT predictions are consistent with recently reported experimental measurements of the Youngs and bulk moduli of such complex ZIF structures.

Carbon Dioxide Adsorption in Amine-Functionalized Mixed-Ligand Metal-Organic Frameworks of UiO-66 Topology

A series of mixed-ligand [1,4-benzenedicarboxylic acid (BDC)/2-amino-1,4-benzenedicarboxylic acid (ABDC)] UiO-66 metal-organic frameworks (MOFs) synthesized through two different methods (low (LT) and high temperature (HT)) have been investigated for their carbon dioxide adsorption properties from 0 to 1 bar to clarify the role of amino loading on carbon dioxide uptake. Volumetric CO2 isotherms show that the CO2 capacity (normalized to the Langmuir surface area) increases with a degree of functionalization of about 46%; for similar NH2 contents, the same values are found for both synthetic procedures. Microcalorimetric isotherms reveal that amino-functionalized materials have a larger differential heat of adsorption (q(diff) ) towards CO2 ; reaching 27(25) and 20(22) kJ mol(-1) on HT(LT)-UiO-66-NH2 and UiO-66, respectively, at the lowest equilibrium pressures used in this study. All experimental results are supported by values obtained through quantum mechanical calculations.

Magnetic properties of nitrogen-doped ZrO2: Theoretical evidence of absence of room temperature ferromagnetism

N-dopants in bulk monoclinic ZrO2 and their magnetic interactions have been investigated by DFT calculations, using the B3LYP hybrid functional. The electronic and magnetic properties of the paramagnetic N species, substitutionals and interstitials, are discussed. Their thermodynamic stability has been estimated as a function of the oxygen partial pressure. At 300 K, N prefers interstitial sites at any range of oxygen pressure, while at higher temperatures (700–1000 K), oxygen poor-conditions facilitate substitutional dopants. We have considered the interaction of two N defects in various positions in order to investigate the possible occurrence of ferromagnetic ordering. A very small magnetic coupling constant has been calculated for several 2N-ZrO2 configurations, thus demonstrating that magnetic ordering can be achieved only at very low temperatures, well below liquid nitrogen. Furthermore, when N atoms replace O at different sites, resulting in slightly different positions of the corresponding N 2p levels, a direct charge transfer can occur between the two dopants with consequent quenching of the magnetic moment. Another mechanism that contributes to the quenching of the N magnetic moments is the interplay with oxygen vacancies. These effects contribute to reduce the concentration of magnetic impurities, thus limiting the possibility to establish magnetic ordering.

Theoretical and experimental characterization of pyrazolato-based Ni(II) metal-organic frameworks

The structural, electronic, dielectric and vibrational features of two metal–organic frameworks with bispyrazolate ligands are investigated by combining ab initio periodic calculations and experimental techniques. UV-Vis spectroscopy has been used to study the electronic structure, while FTIR spectroscopy to obtain information about the vibrational features of the samples and their adsorptive properties. The examined MOFs contain square-planar Ni(II) nodes bridged either by the 4,4′-bispyrazolato (bpz) or by the 1,4-bis(4-pyrazolato)benzene (bpb) spacers. As expected, they are diamagnetic solids, with Ni(II) in low spin state. This has been confirmed through computations, other magnetic phases being predicted as remarkably less stable. Both materials possess a band gap of ∼3.4 eV and show interesting dielectric properties due to the microporous structure and the low dielectric constant. Although Ni(II) is coordinatively unsaturated, it does not show any specific interaction with the probe molecules tested, as evidenced by FTIR measurements. This unexpected behavior is explained on the basis of computed results. The high stability of the low spin state of Ni(II) in square-planar coordination and a reduced accessibility of the metal sites due to steric effects are considered to be the reasons for the absence of preferential interactions between the metal centers and the probe molecules adopted.

H2O Adsorption on WO3 and WO3-x (001) Surfaces

The nature of the interaction of water with the WO3 surface is of crucial importance for the use of this semiconductor oxide in photocatalysis. In this work, we investigate water adsorption and dissociation on both clean and O-deficient (001) WO3 surfaces by means of an accurate DFT approach. The O vacancy formation energy (computed with respect to O2) has been evaluated for all possible surface configurations, and the removal of the terminal O atom along the c axis is found to be preferred, costing about half the corresponding energy in the bulk. The presence of oxygen vacancies leads to a semiconductor to metal transition, confirming the experimental evidence of n-type conductivity in defective WO3 films. H2O preferably adsorbs on WO3 in a molecular undissociated form, due to the presence of W ions at the surface that act as Lewis acid sites. This interaction, about -1 eV per H2O molecule, is not very strong. Contrary to what is usually expected, the presence of oxygen vacancies does not significantly affect H2O adsorption. Finally, we investigated the H2O desorption from a hydroxylated surface. This suggests that the exposure of WO3 to H2 directly results in a hydroxylated surface and the corresponding H2O desorption turns out to be a very efficient mechanism to generate a reduced oxide surface, with important consequences on the electronic structure of this oxide.

The photoactive nitrogen impurity in nitrogen-doped zirconium titanate (N-ZrTiO4): A combined electron paramagnetic resonance and density functional theory study

Nitrogen doping represents an important strategy to modulate the optical, magnetic and photochemical properties of oxides for applications spanning from photocatalysis to optoelectronics and spintronics. In this work for the first time zirconium titanate, a material that exhibits many attractive properties, including an excellent dielectric constant, high corrosion resistance, high permittivity at microwave frequencies and excellent temperature stability, has been doped with nitrogen via a wet chemistry method. A detailed description of the geometrical and the electronic structure of the dopant centre has been obtained coupling Electron Paramagnetic Resonance (EPR) spectroscopy and density functional theory (DFT) calculations. Insertion of nitrogen impurities into the ZrTiO4 lattice modifies the optical properties causing an appreciable absorption in the visible range. The joint analysis of the EPR evidence and the DFT elaboration indicates that the nitrogen impurities preferentially occupy an interstitial position of the lattice generating intra-band gap states about 1 eV above the valence band edge that are responsible for the visible light absorption. The majority of the intra-band gap states are diamagnetic (N−) while a minor fraction is paramagnetic (N˙). These centres are photosensitive in that the ratio between N− and N˙ is modified, upon irradiation, because of electron excitation from the intra-band gap states to the conduction band.