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Tracking the Adsorption and Electron Injection Rates of CdSe Quantum Dots on TiO2: Linked versus Direct Attachment

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journal contribution
posted on 09.08.2021, 18:34 by Douglas R. Pernik, Kevin Tvrdy, Emmy J. Radich, Prashant V. Kamat
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 Kad for submonolayer coverage was (6.7 ± 2.7) × 103 M–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 (kET = 7.2 × 109 s–1) compared with MPA-linked QDs (kET = 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.