Structural and electronic properties of TiO2 nanostructures from first principles

International Conference on Hybrid and Organic Photovoltaics
Proceedings of International Conference on Hybrid and Organic Photovoltaics 2015 (HOPV15)

Poster, Francesca Nunzi, 371
Publication date: 5th February 2015

Among the devices based on mesoporous TiO₂ materials, Dye-Sensitized Solar Cells and the related devices based on organohalide perovskites represent a promising approach for photovoltaic conversion of solar radiation at low cost and high efficiency. Mesoporous anatase TiO₂ films are generally constituted by nanometric particles (grains) below 100 nm in size and typically around 10 - 20 nm, with a porosity  in the range 50 - 65 %. The nanoparticulated morphology of TiO₂ films strongly increases the available surface area and consequently the number of reactive centers at the interface between the semiconductor and the interacting media. However, the conductivity and the electron mobility within the nanocrystalline oxide films are strongly reduced compared to the bulk, since the 3D packed oxide particles network provides a tortuous pathway for electron transport, thus increasing the possibility of charge recombination at grain boundaries and limiting the charge collection efficiency. Theoretical investigations based on quantum mechanics methods have been carried out to characterize the effects of size, shape and surface properties of the individual particles on the distribution of the electronic (trap and extended) states in the conduction band in order to shed light on the parameters controlling the electron transport through the NPs oxide network.^.For instance, we investigated the nature of electronic trap states in realistic individual and sintered anatase TiO₂ NPs of ca. 3 nm diameter, finding unoccupied electronic states of lowest energy localized within the central part of the NPs and originated from under-coordinated surface Ti atoms at the (100) facets.¹ Afterwards, we analyzed the nature and distribution of electronic trap states in shape-tailored anatase TiO₂ nanostructures.² We considered linear nanocrystal model up to 6 nm in length with various morphologies, reproducing both flattened and elongated rod-shaped TiO₂ NPs, to clarify the effect of the crystal facet percentage on the electronic structure, with particular reference to the energetics and distribution of trap states. We also investigated the structure and energetics of excitons and individual electron and hole polarons  in anatase TiO₂ NPs by Density Functional Theory (DFT)  and Time-Dependent DFT calculations, showing that the nanoscale size of the TiO₂ NP give rise to some significant differences with respect to anatase bulk and extended surfaces.³
Schematic drawing of a single NP in the equlibrium truncated bypiramidal shape (middle), a flattened NP with rich -(001) facets (left) and an elongated NP with rich-(100) facets (right).
1. Nunzi, F.; Mosconi, E.; Storchi, L.; Ronca, E.; Selloni, A.; Gräetzel, M.; De Angelis, F. Inherent electronic trap states in TiO2 nanocrystals: effet of saturation and sintering. Energy Environ. Sci. 2013, 6, 1221-1229. 2. Nunzi, F.; Storchi, L.; Manca, M.; Giannuzzi, R.; Gigli, G.; De Angelis, F. Shape and morphology effects on the electronic structure of TiO2 nanostructurels: from nanocrystals to nanorods. ACS Appl. Mater. Interfaces 2014, 6, 2468-2475. 3. Nunzi, F.; Agrawal, S.; Selloni, A.; De Angelis, F. Structural and electronic properties of photoexcited TiO2 nanoparticles from first principles. J. Chem. Theory Comput., 2015, DOI: 10.1021/ct500815x.

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