Synthesis of Anatase (Core)/Rutile (Shell) Nanostructured TiO₂ Thin Films by Magnetron Sputtering Methods for Dye-Sensitized Solar Cell Applications

Currently, anatase/rutile core/shell structures are accepted as highly efficient building blocks for TiO₂-based catalysts or photoelectrodes used in dye-sensitized solar cells (DSSCs). It is understood that a thin layer of rutile covering the core anatase pillar would improve the performance of DSSCs by retarding the charge recombination at the semiconductor/sensitizer/electrolyte interfaces. In this work, we report on the synthesis of core/shell nanostructured TiO₂ thin films using reactive magnetron sputtering at a glancing angle with different power applying modes: well-separated pillars of pure anatase were synthesized using the DC mode, and then high-pulse peak power was applied to the Ti target (high-power impulse magnetron sputtering (HiPIMS)), resulting in the covering of the anatase columns with a thin layer of rutile. The latter technique is well-known to increase the energy load during the growth of the film, which is a key parameter to successfully obtain the TiO₂ phase normally only achieved at high temperature, i.e., rutile. The peak current, the frequency, and the pulse width were optimized to obtain the desired crystalline structure and thickness of the rutile top layer. Scanning electron microscopy (SEM) cross-section views of the synthesized films clearly show that the pillar-like structures are not affected by the energetic species striking the surface during the HiPIMS process. Grazing incidence X-ray diffraction (GIXRD) suggests the presence of both anatase and rutile phases in the films. Further characterization of the anatase/rutile core/shell interface by electron transmission techniques such as transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) mapping confirms the hypothesis and reveals that the anatase pillars are partly covered by a rutile crust.