Probing Ultrafast Photoinduced Electron Transfer to TiO₂ from CdS Nanocrystals of Varying Crystallographic Phase Content

The photoinduced electron transfer processes in a nanoheterostructured semiconductor assembly are complex and depend on various parameters of the constituents of the assembly. We present here the ultrafast electron transfer characteristics of an assembly comprised of a wide band semiconductor, titanium dioxide (TiO2), attached to light-harvesting cadmium sulfide (CdS) nanocrystals of varying crystallographic phase content. Quantitative analysis of synchrotron high-resolution X-ray diffraction data of CdS nanocrystals precisely reveals the presence of both wurtzite and zinc blende phases in varying amounts. The estimated content of crystal phases is observed to be strongly dependent on an important synthesis parameter, viz., the ratio of the two solvents. The biphasic nature of CdS influences directly the shape of the nanocrystal at long reaction times as well as the transfer of the photoexcited electrons from the CdS to TiO2 as obtained from transient absorption spectroscopy. A higher amount of zinc blende phase is observed to be beneficial for fast electron transfer across the CdS–TiO2 interface. The electron transfer rate constant differs by one order of magnitude between the CdS nanocrystals and varies linearly with the fraction of the phases.