Investigation of Interface Characteristics and Physisorption Mechanism in Quantum Dots/TiO₂ Composite for Efficient and Sustainable Photoinduced Interfacial Electron Transfer

Abstract

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.

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