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Influence of preparation parameters on morphology and structure of TiO 2 deposited at ZnO nanorods for photovoltaic application

Nanostructured titanium dioxide is widely used for photovoltaic applications, primarily as electron transporting layer in perovskite and dye sensitized solar cells, basically due to suitable band gap for acceptance of electrons from active layer of solar cells. However, by combination of high reactivity of TiO 2 together with the large binding energy of ZnO, the process of electron transfer between the corresponding conduction and valence bands can be facilitated in the composite system. Different techniques have been reported for the growth of ZnO/TiO 2 nanocomposites in various forms. With the aim to prepare high performance TiO2@ZnO core–shell nanostructure for photovoltaic application, in this work TiO 2 was deposited on ZnO nanorods by pulsed laser deposition (PLD) and magnetron sputtering (MS). We studied influence of the preparation parameters to crystal structure and morphology of nanocomposite and correlate them with the optical and electrical properties. ZnO nanorods (ZNR) arrays were prepared by sol-gel procedure followed by annealing of nanorods to obtain crystallinity. TiO 2 thin films were prepared on ZNR using PLD in Ar or vakuum and by reactive MS with the aim to select the preference deposition procedure. The structural phase of TiO 2 were primary studied by confocal micro-Raman spectroscopy (mRS), while morphology and crystal structure on nanoscale were studied by high resolution transmission and scanning electron microscopy (HRTEM and HRSEM). The optical properties are evaluated by UV-vis spectroscopy, while for electrical characterization impedance spectroscopy was applied. Raman spectroscopy results indicated that TiO 2 thin films prepared by MS and PLD were amorphous after deposition. The annealing temperature for crystallization were optimised to obtain anatase structure, since mRS study showed the formation of rutile already at temperature between 450 and 500 °C. It was shown that morphology of the TiO 2 layers obtained by PLD considerably depend on preparation atmosphere and the number of pulses. In the case of preparation by MS the attention should be focused on reactive magnetron atmosphere as well as on the pre-treatment of the ZNR substrate. The obtained phase composition and morphology of the TiO 2 will be discussed in the frame of the preparation parameters, while optical and electrical properties of prepared TiO 2 @ZnO core–shell nanostructure will be discussed in the view of structure and morphology of the obtained TiO 2 layers. Acknowledgements: This work has been supported by Croatian Science Foundation under the project IP-2014-09-9419 and by Croatian-Germany DAAD bilateral project.

nanostructures ; TiO2 ; ZnO nanorods ; nanocomposites ; photovoltaics

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