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Morphology and Structure of TiO2 Thin Films and Nanostructures Deposited on ZnO Nanorods for Photovoltaic Application

Nanostructured titanium dioxide is known as a material with excellent physical and chemical properties, such as high specific surface area, high photo-activity and environmental stability as well as low cost synthesis. TiO2 exists in three different crystalline phases: anatase, rutile and brookite, but it was found that anatase (Eg ~ 3.2 eV) is more photo-active than other modifications. It is widely used for photovoltaic applications, primarily as electron transporting layer in perovskite and dye sensitized solar cells due to suitable band gap for acceptance of electrons from active layer of solar cells. However, by combination of high reactivity of TiO2 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. With the aim to prepare high performance TiO2@ZnO core–shell nanostructure for photovoltaic application, in this work TiO2 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. TiO2 thin films were prepared on ZNR using PLD in Ar or vacuum and by reactive MS with the aim to select the preference deposition procedure. The structural phase of TiO2 were preliminary studied by confocal micro-Raman spectroscopy (mRS), while morphology and crystal structure on nanoscale were studied by high resolution electron microscopy ; JEOL-ARM300 microscope in HAADF-DF mode and FEG-SEM JEOL JSM-700F. The optical properties are evaluated by UV-vis spectroscopy, while for electrical characterization impedance spectroscopy was applied. Raman spectroscopy results indicated that TiO2 thin films prepared by MS and PLD were amorphous after deposition (Fig. 1). The SEM images show that heating of the samples after deposition did not influence the morphology of TiO2. 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 (Fig. 1). It was shown that morphology of the TiO2 layers obtained by PLD considerably depend on preparation atmosphere and the number of pulses (Fig. 2). 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, while it was shown that morphology of the MS deposited TiO2 depend on deposition duration (Fig. 3). The obtained phase composition and morphology of the TiO2 will be discussed in the frame of the preparation parameters, while optical and electrical properties of prepared TiO2@ZnO core–shell nanostructure will be discussed in the view of structure and morphology of the obtained TiO2 layers.

TiO2, ZnO, core-shell, photovoltaic application

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