Nanotubular TiO2 films sensitized with CdTe quantum dots: Stability and adsorption distribution

Abstract

Quantum dots (QDs) are zero-dimensional nanostructured semiconductors that are widely studied for their size and shape dependent optical behavior, making them attractive for numerous applications, including photovoltaics. Within this group of nanomaterials, CdTe QDs represent an interesting material as a sensitizer for TiO2 (Nt-TiO2) nanotubular matrices due to their bulk band gap of 1.45 eV and their high extinction coefficient. However, its functionality may be limited, being unstable in the presence of some redox probes due to photocorrosion processes. In this work, the composition and internal distribution of CdTe QDs adsorbed on Nt-TiO2 from toluene and toluene/acetone (70:30) and (50:50) mixtures were studied by X-ray photoelectron spectroscopy (XPS), before and after photocurrent measurements. XPS was used to characterize the coverage and distribution of QDs along the Nt-TiO2. It is shown that the increased polarity of the solvent causes a higher coverage and an extended distribution of QDs toward the interior of Nt-TiO2, which causes the photogenerated electrons to travel shorter distances to reach the underneath electrical contact, thus achieving a higher photocurrent. Additionally, the photoanode shows a progressive decrease in the Te signals inside the nanotubes after the photocurrent recording, presumably due to a photocorrosion process in the presence of Na2S and illumination.