Annabella Selloni
Research Focus
Research in the Selloni group is focused on theoretical/computational studies of materials properties via first principles electronic structure and molecular dynamics simulations. Our goal is to understand and predict the properties and behavior of materials at a fundamental level, using computational tools based on first principles (or “ab-initio”) quantum mechanical methods.
Our research include bulk materials (mainly semiconductors and insulators), surfaces and interfaces (solid-liquid and organic-inorganic interfaces), nanostructures. We are interested in electronic, magnetic and optical properties; defect states; surface structure and reactivity (structure-function relations); adsorption of molecules and molecular monolayers on metal and semiconductor surfaces, surface functionalization; electron and proton transfer reactions, particularly at the solid/water interface, reaction pathways and free energy barriers; electrochemical cells and dye sensitized solar cells.
Some highlights on recent research projects are given below.
Theoretical design of a bio-inspired catalyst for hydrogen production. We have used First Principles Molecular Dynamics simulations to learn how the active heart of an enzyme of hydrogen-producing bacteria could be modified so as to remain stably active in an artificial production system. The coupled catalyst-electrode system consists of a modification of the active site of the di-iron hydrogenase enzyme forming a stable, tridentate link to a pyrite electrode. There is a low-activation-energy pathway for H2 production from water, a mild bottleneck being the first protonation of the catalyst. The second protonation is much faster, and there is no further barrier for H2 desorption.
Adsorption of water on the anatase (101) surface. In collaboration with experimental colleagues (U. Diebold and coworkers), we have obtained a detailed atomic-scale picture of water molecules on the anatase TiO2(101) surface. Water adsorbs as an intact monomer with a computed binding energy of 730 meV. The charge rearrangement at the molecule-anatase interface affects the adsorption of further water molecules, resulting in short-range repulsive and attractive interactions along [010] and [-111]/[11-1] directions, respectively, and a locally-ordered (2x2) superstructure of molecular water.
Simulated STM images of a water molecule on anatase (101) and comparison with experiment. (a) Simulated constant density image; (b) optimized geometry (top view) and (c) isosurface of the charge density difference resulting from adsorption of a water molecule. Positive (electron excess) and negative (electron deficit) lobes are shown in blue and yellow, respectively. (d) Height profile along the line indicated in (c). On the right is the STM experiment. Each water molecule appears as a ‘bright-dark-bright’ feature.
Structure and Energetics of subsurface defects at the anatase (101) surface. Using DFT calculations at both the GGA and GGA+U levels, we predicted that oxygen vacancies at anatase TiO2(101) are energetically more stable at bulk and subsurface sites than on the surface. Moreover, the energy barriers to diffuse from the surface to the bulk are rather low, while the opposite is true for the barriers to diffuse from the bulk to the surface. Subsequent STM experiments have confirmed this unusual result: the (101) surfaces of anatase single crystals show no evidence of surface oxygen vacancies, which are instead quite numerous on other TiO2 surfaces.
Electronic structure of the hydroxylated and reduced rutile TiO2(110) surface Defects on oxide surfaces have long been identified as key to understanding the chemical reactivity of these materials. In particular, surface oxygen vacancies not only change the surface geometric structure, but also the surface electronic structure and are thus highly-active sites for adsorption and, possibly, the trapping of photo-excited charge carriers. Such oxygen vacancies on rutile (110) have become the most-studied types of defects on any oxide surface. The electronic structure that is related to these defects is firmly-established experimentally; missing O atoms introduce an energetically localized gap state, located ~0.8 eV below the Fermi energy. This gap state has long evaded a reliable theoretical description, however. Due to the self-interaction error, DFT has well-known limitations in the description of localized states. Our study showed for the first time that a hybrid DFT approach, where the exact Fock exchange is partially mixed with the DFT exchange, significantly improves the description of trapped electron states. Electronic densities of states for the hydroxylated and reduced surfaces show two well-defined peaks are present in the band gap, about 1 eV below the edge of the conduction band. These peaks correspond to singly occupied states localized on two Ti ions which are reduced to Ti3+. The electron trapping in Ti 3d states is accompanied by
Dye-sensitized TiO2 sensitizer absorbs the solar radiation and transfers the photoexcited electron to a nanostructured TiO2 electrode. We performed electronic structure calculations for different Ru(II)-polypyridyl molecular dyes adsorbed onto a model anatase TiO2 nanoparticle in solution (modeled as a dielectric continuum). The results suggest that two different electron injection mechanisms (adiabatic and non-adiabatic) may be present in DSSCs employing dyes carrying a different number of protons. In addition, sensitizers with inequivalent bipyridine ligands were found to exert strong Dye sensitized solar cells (DSSCs) represent a promising approach to the direct conversion of light into electrical energy at low cost and with high efficiency. In these devices, a dye dipolar fields at the TiO2 surface, causing a conduction band down-shift and a reduction of the cell open circuit potential, thus resulting in a reduced DSSC efficiency.
Selected Recent Publications
- F. Zipoli, R. Car, M.H. Cohen, A. Selloni, Simulation of Electrocatalytic Hydrogen production by a bio-inspired catalyst anchored to a pyrite electrode, J. Am. Chem. Soc. 2010, 132, 8593.
- H. Cheng and A. Selloni, Hydroxide ions at the water/anatase(101) interface: structure and electronic states from first principles molecular dynamics, Langmuir 2010, 26, 11518
- C. Di Valentin, G. Pacchioni, A. Selloni, Reduced and n-type doped TiO2: Nature of Ti3+ species, J. Phys .Chem. C 2009, 113, 20543 (Feature Article)
- Y. He, A. Tilocca, O. Dulub, A. Selloni, U. Diebold, Submonolayer water on TiO2 anatase (101): structural, dynamical, and electronic signatures, Nature Materials 2009, 8, 585-589.
- 5. X. Wu, A. Selloni, R. Car, Order-N implementation of exact-exchange in extended systems, Phys Rev. B 2009, 79, 085103 (Editors Suggestion)
- Li, Shao-Chun; Wang, Jian-guo; Jacobson, Peter; Gong, Xue-Qing; Diebold, Ulrike; Selloni, Annabella, Correlation between bonding geometry and band gap states at organic-inorganic interfaces: catechol on rutile, J. Am. Chem. Soc., 2009, 131, 980-984.
- F. De Angelis, S. Fantacci, A. Selloni, M.K. Nazeeruddin, M. Graetzel, Time Dependent Density Functional Theory Investigations on the Excited States of Ru(II)-Dye-Sensitized TiO2 Nanoparticles: The Role of Sensitizer Protonation, J. Am. Chem. Soc. 2007, 129, 14156 (Commun.)
- J. Wang, A. Selloni, The c(4×2) structure of short- and intermediate-chain length alkanethiolate monolayers on Au(111): a DFT study, J.Phys. Chem. C, 2007, 111, 12149-12151.
- C. Di Valentin, G. Pacchioni, A. Selloni, Electronic structure of defect states in hydroxylated and reduced rutile TiO2 (110) surfaces, Phys. Rev. Lett., 2006, 97, 166803.
- Y. Kanai, A. Selloni, Competing Mechanisms in the Optically Activated Functionalization of the Hydrogen Terminated Si(111) Surface, J. Am. Chem. Soc. 2006, 128, 3892-3893.