In Situ Studies of Photoluminescence Quenching and Photocurrent Yield in Quantum Dot Sensitized Single Crystal TiO₂ and ZnO Electrodes

Utilizing time-correlated single photon counting and photocurrent spectroscopy we have studied the fluorescence intensity, fluorescence decay time, and sensitized photocurrents of a liquid junction photoelectrochemical cell consisting of CdSe quantum dots coupled to glass, single crystal TiO₂, and single crystal ZnO substrates through a variety of capping ligands. This system is ideal to compare electron transfer rates obtained from optical techniques with external current flow obtained from electrical techniques. We find that for all configurations of capping ligands and substrate the photoluminescence decay rate is quenched compared to free quantum dots in solution; whereas only the quantum dots capped with short chain 3-mercaptopropionic acid ligands coupled to the single crystal TiO₂ or ZnO produce photocurrents. The longer chain capping groups, oleic acid/tri-<i>n</i>-octylphosphine and 11-mercaptoundecanoic acid, inhibit electron injection and can promote clustering or aggregation of the quantum dots on the substrate surface as observed by atomic force microscope imaging. These findings illustrate the crucial role of capping ligands on the electron transfer properties and morphology of quantum dot interfaces. A key finding of this research is the fact that optical measurements alone may be insufficient to infer electron injection in a quantum dot sensitized photoelectrode system, and a combination of optical and photocurrent measurements are needed to fully characterize the photoresponse of such systems