Irradiation of a semiconductor quantum dot (QD) with photons of energy equal to or greater than the bandgap of the material results in electron-hole charge separation. While much attention has been given to the utilization of these free carriers, an in-depth analysis of the complex nature of the transfer and relaxation of these carriers, and of the incremental electronic events that ultimately lead to their utilization, was lacking.
Using various methods of transient spectroscopy, photoinduced events at the surface of 3.4 nm CdSe QDs stabilized in reverse micelles were probed. Reverse micelles were employed both as templates for particle growth and as nano-sized reaction chambers in order to produce monodisperse, transparent colloidal solutions of CdSe without the use of surface capping agents. Ultrafast events such as electron-hole recombination, as well as electron and hole transfer were monitored by femtosecond pump-probe spectroscopy. An electrochromic probe molecule, methyl viologen (MV⁺·) was employed to provide insight into the events occurring at the QD surface at a given time. The slower trap-hole reactions were investigated using nanosecond flash photolysis. By monitoring the decay of the MV⁺· radical, it was determined that the photogenerated product is stabilized following a series of fast and slow oxidation steps. Steady state experiments were carried out to observe the accumulation of the stabilized MV⁺· over long periods of irradiation to give a complete time-resolved study (ps-to-sec), and the results were found to be in agreement across all time domains.
The study was expounded upon by coupling the CdSe QDs with TiO₂ (d<5 nm) and with Pt (2.8 nm) to compare the roles of the respective species. It was determined that TiO₂ acts both an electron shuttle and as a suppressor of the back reaction, which results in two-fold enhancement of the MV⁺· yield. Contrastingly, Pt acts exclusively as an electron sink and demonstrates the capability to not only oxidize MV⁺· rapidly and efficiently, but also discharge electrons into solution.
The information gathered from these “test” systems was used to design a hybrid Nafion/CdS/Pd-Pt/MV⁺· photocatalyst for HC evolution. H₂ was obtained at a maximum rate of 60μL/cm²/hr.