Investigation of Interface Characteristics and Physisorption Mechanism in Quantum Dots/TiO₂ Composite for Efficient...

Owing to their superior stability compared to those of conventional molecular dyes, as well as their high UV-visible absorption capacity, which can be tuned to cover the majority of the solar spectrum through size adjustment, quantum dot (QD)/TiO₂ composites are being actively investigated as photosensitizing components for diverse solar energy conversion systems. However, the conversion efficiencies and durabilities of QD/TiO₂-based solar cells and photocatalytic systems are still inferior to those of conventional systems that employ organic/inorganic components as photosensitizers. This is because of the poor adsorption of QDs onto the TiO₂ surface, resulting in insufficient interfacial interactions between the two. The mechanism underlying QD adsorption on the TiO₂ surface and its relationship to the photosensitization process remain unclear. In this study, we established that the surface characteristics of the TiO₂ semiconductor and the QDs (i.e., surface defects of the metal oxide and the surface structure of the QD core) directly affect the QD adsorption capacity by TiO₂ and the interfacial interactions between the QDs and TiO₂, which relates to the photosensitization process from the photoexcited QDs to TiO₂ (QD* → TiO₂). The interfacial interaction between the QDs and TiO₂ is maximized when the shape/thickness-modulated triangular QDs are composited with defect-rich anatase TiO₂. Comprehensive investigations through photodynamic analyses and surface evaluation using X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and photocatalysis experiments collectively validate that tuning the surface properties of QDs and modulating the TiO₂ defect concentration can synergistically amplify the interfacial interaction between the QDs and TiO₂. This augmentation markedly improved the efficiency of photoinduced electron transfer from the photoexcited QDs to TiO₂, resulting in significantly increased photocatalytic activity of the QD/TiO₂ composite. This study provides the first in-depth characterization of the physical adhesion of QDs dispersed on a heterogeneous metal-oxide surface. Furthermore, the prepared QD/TiO₂ composite exhibits exceptional adsorption stability, resisting QD detachment from the TiO₂ surface over a wide pH range (pH = 2-12) in aqueous media as well as in nonaqueous solvents during two months of immersion. These findings can aid the development of practical QD-sensitized solar energy conversion systems that require the long-term stability of the photosensitizing unit.