Gianluca Fazio1 Lara Ferrighi1 Daniele Selli1 Cristiana Di Valentin1

1, University of Milan-Bicocca, Milano, , Italy

Nanoparticles of very small size (below 10 nm) of TiO2 material are nowadays the functional building blocks for many applications in photocatalysis and photovoltaics. [1] In order to improve the photocatalytic performances of this nanomaterial, it is of great importance to understand how the size confinement, the morphology and the interaction with water influence charge carriers separation and migration to the surface.
In the first part of the talk, we present a hybrid density functional theory (DFT) investigation [2,3] of the size and shape effects on the life path of energy (excitons) and charge (electrons and holes) carriers in real-size anatase TiO2 nanoparticle models with different size (2-3 nm) and shape (faceted vs spherical). We focus our attention on the exciton/charge carriers formation, separation, recombination, self-trapping processes, which are analyzed in terms of structural distortions, energy gain or cost, charge localization/delocalization and electronic transitions of the trapped charges. The migration of photoinduced charges from the bulk towards the surface is always computed to be a downhill process, although differences are observed for spherical vs faceted nanoparticles because of the higher disorder and larger diversity of surface sites. The computational models are corroborated by an extensive comparison with available experimental data from photoluminescence measurements, electron paramagnetic resonance and transient absorption spectroscopies.
In the last part of the talk, we briefly discuss a recent combined experimental and theoretical study [4] about the effect of a water environment on the hole trapping mechanism for TiO2 nanoparticles with different morphologies. Comparing the results from steady and transient infrared spectroscopy with DFT calculations, we clarify why water enhances hole trapping at the surface of spherical TiO2 nanoparticles, but not of well-faceted ones.
The project has received funding from the European Research Council (ERC) under the European Union's HORIZON2020 research and innovation programme (ERC Grant Agreement No [647020]) and from the network QM-FORMa: Designing New Materials with Quantum Mechanics.

[1] Sang, L.; Zhao, Y.; Burda, C. Chem. Rev. 2014, 114, 9283–9318.
[2] Fazio, G.; Ferrighi, L.; Di Valentin, C. Nano Energy 2016, 27, 673 – 689.
[3] Selli, D.; Fazio, G.; Di Valentin, C. J. Chem. Phys. 2017, 147, 164701.
[4] Shirai, K.; Fazio, G.; Sugimoto, T.; Selli, D.; Ferraro, L.; Watanabe, K.; Haruta, M.; Ohtani, B.; Kurata, H.; Di Valentin, C.; Matsumoto, Y., J. Am. Chem. Soc. 2017, under review.