The Role of the Surface Coverage on the Structural and Electronic Properties of TiO 2 Nanocrystals
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The Role of the Surface Coverage on the Structural and Electronic Properties of TiO2 Nanocrystals Amilcare Iacomino1, Giovanni Cantele2, Fabio Trani2, Domenico Ninno2, Ivan Marri3 and Stefano Ossicini3 1 Dipartimento di Fisica E. Amaldi, Università degli Studi Roma Tre, Via della Vasca Navale 84, I-00146 Roma, Italy and CNISM, U. di R. Università degli Studi di Napoli Federico II. 2 CNR-INFM-Coherentia and Università degli Studi di Napoli Federico II, Dipartimento di Scienze Fisiche, Complesso Universitario Monte S. Angelo, Via Cintia, I-80126 Napoli, Italy. 3 CNR-INFM-S3 and Università di Modena e Reggio Emilia, Dipartimento di Scienze e Metodi dell'Ingegneria, Via Amendola 2 Pad. Morselli, I-42100 Reggio Emilia, Italy.
ABSTRACT We present here a characterization of TiO2 0D nanoclusters and 1D nanowires in the framework of ab initio density functional theory (DFT) calculations. We analyze the effect of the surface coverage by functionalizing dangling bonds with simple adsorbates modeling the basical interactions of TiO2 nanosystems with the hydration sphere. We thus address the electronic reorganization and the surface role in determining the overall properties of the nanostructures. The structural reconstruction is found to depend on the surface coverage and the experimental evidences on the structural variations can be explained by a topological analysis of the Ti-O bonds. Q-size effects are observed through the bandgap widening, but the surface competes to determine the energy distribution of the electronic levels. The hydrogenated nanocrystals do show occupied levels at the bottom of the conduction bands, which can enhance the conductive properties of the nanowires. In the hydrogenated cluster such levels present a localized charge distribution with strong similarities (orbital character, energy position) to the defect states arising after oxygens desorption. From the analysis of the electronic density of states we found that Ti-H bonds induce in-gap states above the valence bands, whereas hydration leads to occupied states that shift the valence bands to lower binding energies.
INTRODUCTION The possibility to enhance the sunlight conversion efficiency in the modern generation of metal oxides based nanodevices is a key issue for the nowadays research and of social relevance for a sustainable energy production. TiO2 based dye-sensitized solar cells (DSSC) are emerging for the contained costs, versatility, increasing stability through commercial standards and fairly good efficiencies [1,2]. Nanostructured TiO2 also shows higher photocatalytic efficiencies for several heterogeneous reactions such as pollutant compounds decomposition and water splitting [3]. All these applications are based on the possibility to enhance the photogenerated charge separation at the interface between the nanocrystal and the sensitizing molecule in DSSC and on the efficient charge transfer from/to the reactive species for the photocatalysis. In the present study we want to highlight the role played by the surface config
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