The Electronic Structure and Photoinduced Electron Transfer Rate of CdSe Quantum Dots on Single Crystal Rutile TiO2: Dependence on the Crystal Orientation of the Substrate

Semiconductor quantum dots (QDs) have many desirable characteristics for use as sensitizers, such as enabling tuning of the band gap on the basis of the quantum confinement effect, a higher extinction coefficient, and facilitating charge injection as a result of the large dipole moment. Despite these potential advantages, no major advance in the efficiency of quantum-dot-sensitized solar cells (QDSCs) has yet been reported. The poor efficiency can be attributed to electron-transfer (ET) reactions that compete with the ideal energy generation cycle in QDSCs. Despite the great technological significance, the interfacial ET between QDs and inorganic species remains poorly understood. In this paper, we describe the electronic structure and the interactions between multiple sized CdSe QDs and single crystal rutile TiO2 with (001), (110), and (111) orientations. Single crystal TiO2 is well characterized and is not only ideal for comparing the amount and the structure of the QDs but is also useful for studying ET reactions. The rate of adsorption of CdSe QDs depends on the crystal orientation, although the average increase in diameter of the QDs is independent of the crystal orientation. The highest occupied molecular orbital (HOMO) level is independent of the adsorption time. On the other hand, the value of the HOMO level depends on the crystal orientation of the R-TiO2 substrate. The ET rate constant increases as the change in free energy increases and depends on the crystal orientation. This suggests that the mixing of the wave functions between the conduction band in the R-TiO2 and the lowest unoccupied molecular orbital (LUMO) level in the CdSe QDs depends on the crystal orientation.