Immobilization of doped TiO2 nanostructures with Cu or In inside of macroporous silicon using the solvothermal method: Morphological, structural, optical and functional properties

Copper (Cu) and Indium (In) doped titanium dioxide (TiO2) obtained by a solvothermal method was confined in Porous Silicon (PS) prepared by Metal-Assisted Chemical Etching -MACE-, a low-cost and versatile method to investigate the morphological, structural, optical, and electrical properties of the TiO2:X/PS/Si structure (X = In, Cu, O). According to Scanning Electron Microscopy, different morphologies were found, such as small particles (without dopant) or well-defined geometries such as cubes (indium-doped) and micro-flowers (copper-doped) that covered the PS surface depending on the kind of dopant elements. Cross-section images showed that the TiO2 nanoparticles decorated and filled the macroporous silicon and that they can even form conformal deposits. Raman spectroscopy and x-ray diffraction patterns revealed the crystalline phase structure of the In and Cu-doped TiO2 materials, which correspond to anatase and rutile, respectively. Furthermore, the lattice parameters corroborated the incorporation of the dopant elements. The photoluminescence (PL) spectra show broad-bands around 2.8 eV, which can be related to oxygen vacancies. Additionally, a blue shift was observed in the PL spectra when the dopant changed from Cu to In. Diffuse reflectance and the Kubelka-Munk plots were used to determine the optical bandgap of the TiO2 structures with and without dopant. Bandgap reduction was observed due to the dopant changes. Current-voltage curves showed a rectifying behavior for negative voltages; however, a negative differential resistance effect was observed for positive voltages in all the samples. Finally, current-voltage curves showed a rectifying behavior for negative voltages; however, it was observed a negative differential resistance effect for positive voltages in all the samples. Besides, these current voltages curves were carried out under dark and illumination conditions, which revealed a photocurrent effect.