Investigation of Interface Characteristics and Physisorption Mechanism in Quantum Dots/TiO 2 Composite for Efficient and Sustainable Photoinduced Interfacial Electron Transfer.

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)/TiO 2 composites are being actively investigated as photosensitizing components for diverse solar energy conversion systems. However, the conversion efficiencies and durabilities of QD/TiO 2 -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 TiO 2 surface, resulting in insufficient interfacial interactions between the two. The mechanism underlying QD adsorption on the TiO 2 surface and its relationship to the photosensitization process remain unclear. In this study, we established that the surface characteristics of the TiO 2 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 TiO 2 and the interfacial interactions between the QDs and TiO 2 , which relates to the photosensitization process from the photoexcited QDs to TiO 2 (QD* → TiO 2 ). The interfacial interaction between the QDs and TiO 2 is maximized when the shape/thickness-modulated triangular QDs are composited with defect-rich anatase TiO 2 . 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 TiO 2 defect concentration can synergistically amplify the interfacial interaction between the QDs and TiO 2 . This augmentation markedly improved the efficiency of photoinduced electron transfer from the photoexcited QDs to TiO 2 , resulting in significantly increased photocatalytic activity of the QD/TiO 2 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/TiO 2 composite exhibits exceptional adsorption stability, resisting QD detachment from the TiO 2 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.