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.

Insulating and Semiconducing Oxides, Metal Carbides for Energy and Catalysis: Surfaces and Interfaces

The understanding of the structure-properties relationship is of fundamental importance for the design of new materials for energy and heterogeneous catalysis. Inorganic, ceramic, and carbon-based materials can be used in a variety of applications in the field of energy. Under this research line we investigate semiconducting oxides of interest in photocatalysis or in new generation of solar cells, their interfaces and mixed phases, oxo-carbides and carbides of interest in catalysis, oxide nanostructures and nanoparticles for catalysts. The focus is on band gap engineering, levels alignment, interface properties, surface chemistry, chemisorption, active sites etc.

Relevant literature:

  1. C. Di Valentin, F. Wang, G. Pacchioni, “Tungsten oxide in catalysis and photocatalysis: hints from DFT”, Topics in catalysis, 56, 1404-1419 (2013).
  2. G. Pacchioni, “Ketonization of carboxylic acids in biomass conversion over TiO2 and ZrO2 surfaces: a DFT perspective”, ACS Catalysis, 4, 2874-2888 (2014).
  3. A. Ruiz Puigdollers, S. Tosoni, G. Pacchioni, “Turning a Nonreducible into a Reducible Oxide via Nanostructuring: Opposite Behavior of Bulk ZrO2 and ZrO2 Nanoparticles Toward H2 Adsorption”, J. Phys. Chem. C, 120, 15329-15337 (2016).

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).