Research

The understanding of the structure-properties relationship is of fundamental importance for the design of new materials. In our group various models are employed to study the electronic structure of inorganic and ceramic materials in combination with highly accurate quantum-mechanical techniques. Particularly important is the role of theory in the study of point defects, impurities in solids, active sites or functional groups on surfaces, phenomena like atomic and molecular chemisorption, ultrathin films, supported clusters, light-matter interactions, and for the interpretation of various spectroscopies, IR and Raman, X-ray absorption and photoemission, EPR and NMR, optical transitions, STM etc.

Defects and Dopants in Oxides

Point defects in oxide materials are of paramount importance as they determine the behaviour of these systems in photocatalysis, photoelectrochemistry, microelectronics, fiber optics, catalysis, etc. The activity is directed towards the determination of stability, structure, and spectral properties of intrinsic and extrinsic point defects (vacancies, metal and non-metal dopants, OH groups, trapped electrons, etc.) and their interplay through charge transfer processes. Particular attention is devoted to the study of optical absorption for activation in the visible region and of EPR spectra for identification of paramagnetic centres.

Relevant literature:

  1. C. Di Valentin, G. Pacchioni, “Spectroscopic properties of doped and defective semiconducting oxides from hybrid density functional calculations”, Accounts of Chemical Research, 47, 3233-3241 (2014).

  2. C. Gionco, S. Livraghi, S. Maurelli, E. Giamello, S. Tosoni, C. Di Valentin, G. Pacchioni, “Al-and Ga-Doped TiO2, ZrO2, and HfO2: The Nature of O 2p Trapped Holes from a Combined Electron Paramagnetic Resonance (EPR) and Density Functional Theory (DFT) Study”, Chem. Mater., 27, 3936-3945 (2015).

  3. E. Albanese, G. Pacchioni, “Ferromagnetism in nitrogen-doped BaO: a self-interaction corrected DFT study”, Phys. Chem. Chem. Phys., 19, 3279-3286 (2017).

Supported Metal Clusters on Inorganic Surfaces

Metal nanoclusters as models of supported catalysts represent a wide and important subject of research. We study the interaction and stabilization of the metal clusters at specific sites of the support like oxygen vacancies, reduced or exposed ions, hydroxyl groups and other defects. We investigate the possible electronic modification of metal clusters on oxide surfaces via charge transfers induced, for instance, by dopings and defects, or by nanostructuring of the support. We also study the reactivity of supported clusters in elementary steps of catalytic reactions.

Relevant literature:

  1. G. Pacchioni, “Electronic interactions and charge transfers of metal atoms and clusters on oxide surfaces”, Physical Chemistry Chemical Physics, 15, 1737-1757 (2013).

  2. G. Pacchioni, H. J. Freund, “Electron transfer at oxide surfaces. The MgO paradigm: from defects to ultrathin films”, Chemical Reviews, 113, 4035-4072 (2013).

  3. A. Ruiz Puigdollers, P. Schlexer, S. Tosoni, G. Pacchioni, “Increasing Oxide Reducibility: The Role of Metal/Oxide Interfaces in the Formation of Oxygen Vacancies”, ACS Catal., 7, 6493-6513 (2017).

Two-dimensional Oxides (Ultrathin Films)

Ultrathin oxide films grown on metal supports represent a new class of materials with novel and unprecedented properties. At film thicknesses below 1-2 nanometers these systems may exhibit uncommon properties that depend on a number of factors: film stoichiometry and composition, metal support, film thickness, nature of the interface, surface termination. Our activity is directed towards the determination of the electronic, chemical and structural properties of two-dimensional oxides: work function changes, presence of nanoholes or regular arrays of adsorption and reactive sites, charge transfer from or to the adsorbed species, tunnelling effects, etc.

Relevant literature:

  1. H. J. Freund, G. Pacchioni, “Oxide ultra-thin films on metals: new materials for the design of supported metal catalysts”, Chemical Society Reviews, 37, 2224-2242 (2008).

  2. L. Giordano, G. Pacchioni, “Oxide films at the nanoscale: new structures, new functions, and new materials”, Accounts of Chemical Research, 44, 1244-1252 (2011).

  3. G. Pacchioni, S. Valeri (Editors),"Oxide Ultrathin Films: Science and Technology”, Wiley-VCH, Weinheim 2012, pp. XVI, 352, ISBN 978-3-527-33016-4.

Electronic and Structural Properties of Oxide Heterojunctions

Devices based on oxides heterojunctions are interesting for photocatalytic purposes. The aim is the design of new materials where the band gap and the band edges are optimized for tasks such as water splitting, hydrogen evolution or decomposition of pollutants. Moreover, the electrostatic potential at the interface can, in principle, split the photogenerated charge carriers preventing recombination. The study of the band alignment and charge carriers flow at the junction, as well as the effect of point defects, require accurate electronic structure calculations on large and complex model structures. Also for this research line, a close cooperation with experimental groups performing electronic spectroscopies and photocalytic activity measurements is crucial.

Relevant literature:

  1. E. Cerrato, C. Gionco, M.C. Paganini, E. Giamello, E. Albanese, G. Pacchioni, "Origin of Visible Light Photoactivity of the CeO2/ZnO Heterojunction", ACS APPLIED ENERGY MATERIALS, 1, 4247-4260 (2018).

  2. G. Di Liberto, S. Tosoni, G. Pacchioni, "Role of Heterojunction in Charge Carrier Separation in Coexposed Anatase (001)-(101) Surfaces", Journal of Physical Chemistry Letters, 10, 2372-2377 (2019).

New Materials for Batteries

Batteries are key-devices in the ongoing effort toward cleaner and more sustainable technologies for energy storage and conversion. Layered materials, either oxides or carbides, are promising candidates as electrodes, due to the relatively easy intercalation and deintercalation of alkali metal cations. Lithium, on the one hand, provides excellent electrochemical performances. On the other hand, the well-known issues in terms of lithium availability are driving the research toward new devices based on sodium or other metals. Computational modelling is a key-tool for rationally designing innovative materials with a high potential for applications in batteries. We are currently engaged in studying metal intercalation both in layered oxides such as MoO3 or V2O5, as well as in the classes of layered titanium carbides (MXenes).

Relevant literature:

  1. A. Gentile, C. Ferrara, S. Tosoni, M. Balordi, S. Marchionna, F. Cernuschi, M.‐H. Kim, H.‐W. Lee, R. Ruffo, "Enhanced Functional Properties of Ti3C2Tx MXenes as Negative Electrodes in Sodium‐Ion Batteries by Chemical Tuning", Small Methods, 4, 2000314 (2020).

  2. T. Das, S. Tosoni, G. Pacchioni, "Structural and electronic properties of bulk and ultrathin layers of V2O5 and MoO3", Computational Materials Science, 163, 230-240 (2019).