Tracking the adsorption and electron injection rates of CdSe quantum dots on TiO₂: Linked versus direct attachment

Understanding CdSe quantum dot (QD) adsorption phenomena on mesoscopic TiO₂ films is important for improving the performance of quantum dot sensitized solar cells (QDSSCs). A kinetic adsorption model has been developed to elucidate both Langmuir-like submonolayer adsorption and QD aggregation processes. Removal of surface-bound trioctylphosphine oxide as well as the use of 3-mercaptopropionic acid (MPA) as a molecular linker improved the adsorption of toluene-suspended QDs onto TiO₂ films. The adsorption constant K_ad for submonolayer coverage was (6.7±2.7)×10 ³M^-1 for direct adsorption and (4.2 ± 2.0) ± 10⁴ M^-1 for MPA-linked assemblies. Prolonged exposure of a TiO₂ film to a CdSe QD suspension resulted in the assembly of aggregated particles regardless of the method of adsorption. A greater coverage of TiO₂ was achieved with smaller QDs due to reduced size constraints. Ultrafast transient absorption spectroscopy demonstrated faster electron injection into TiO₂ from directly adsorbed QDs (k _ET = 7.2×10⁹ s^-1) compared with MPA-linked QDs (k_ET = 2.3×10⁹ s^-1). The adsorption kinetic details presented in this study are useful for controlling CdSe QD adsorption on TiO₂ and designing efficient photoanodes for QDSSCs.