Abstract
Computational modeling of titanium dioxide nanoparticles of realistic size is extremely relevant for the direct comparison with experiments but it is also a rather demanding task. We have recently worked on a multistep/scale procedure to obtain global optimized minimum structures for chemically stable spherical titania nanoparticles of increasing size, with diameter from 1.5 nm (~300 atoms) to 4.4 nm (~4000 atoms). We use first self-consistent-charge density functional tight-binding (SCC-DFTB) methodology to perform thermal annealing simulations to obtain globally optimized structures and then hybrid density functional theory (DFT) to refine them and to achieve high accuracy in the description of structural and electronic properties. This allows also to assess SCC-DFTB performance in comparison with DFT(B3LYP) results. As a further step, we investigate photoexcitation and photoemission processes involving electron/hole pair formation, separation, trapping and recombination in the nanosphere of medium size by hybrid DFT. Finally, we show how a recently defined new set of parameters for SCC-DFTB allows for a proper description of titania/water multilayers interface, which paves the way for modeling large realistic nanoparticles in aqueous environment.
Highlights
TiO2 nanoparticles are fundamental building blocks in photocatalysis [1,2,3,4]
We will present a critical review of our work, relative to the topics highlighted and discussed above: in Section 2, we present the Computational methodology; in Section 3, we describe how to obtain realistic spherical nanoparticles models; in Section 4, we discuss the description of the photoexcitation processes; and, in Section 5, we analyze how the water environment can be modeled with sufficient accuracy
We present a comparison of the density of states (DOS) for the nanospheres of different size, as shown in Figure 3, that have been obtained with density functional theory (DFT)(B3LYP) and DFT-based method (DFTB) on the corresponding fully relaxed structures
Summary
TiO2 nanoparticles are fundamental building blocks in photocatalysis [1,2,3,4]. Their theoretical description is relevant and requires the size of the model to be as realistic as possible, for direct comparison with experimental samples. Catalysts 2017, 7, 357 accurate method based on density functional theory (DFT), which is the self-consistent-charge density functional tight-binding (SCC-DFTB) [19], to perform a global structure optimization search of the nanoparticles; to run a further DFT relaxation to determine structural and electronic properties with first-principles level accuracy. For the latter, we use hybrid functionals since they are known to better describe electronic structure details of TiO2 materials [20,21,22]. We will present a critical review of our work, relative to the topics highlighted and discussed above: in Section 2, we present the Computational methodology; in Section 3, we describe how to obtain realistic spherical nanoparticles models; in Section 4, we discuss the description of the photoexcitation processes; and, in Section 5, we analyze how the water environment can be modeled with sufficient accuracy
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