Probing Ultrafast Photoinduced Electron Transfer to\nTiO₂ from CdS Nanocrystals of Varying Crystallographic\nPhase Content

Abstract

The\nphotoinduced electron transfer processes in a nanoheterostructured\nsemiconductor assembly are complex and depend on various parameters\nof the constituents of the assembly. We present here the ultrafast\nelectron transfer characteristics of an assembly comprised of a wide\nband semiconductor, titanium dioxide (TiO₂), attached to\nlight-harvesting cadmium sulfide (CdS) nanocrystals of varying crystallographic\nphase content. Quantitative analysis of synchrotron high-resolution\nX-ray diffraction data of CdS nanocrystals precisely reveals the presence\nof both wurtzite and zinc blende phases in varying amounts. The estimated\ncontent of crystal phases is observed to be strongly dependent on\nan important synthesis parameter, viz., the ratio of the two solvents.\nThe biphasic nature of CdS influences directly the shape of the nanocrystal\nat long reaction times as well as the transfer of the photoexcited\nelectrons from the CdS to TiO₂ as obtained from transient\nabsorption spectroscopy. A higher amount of zinc blende phase is observed\nto be beneficial for fast electron transfer across the CdS–TiO₂ interface. The electron transfer rate constant differs by\none order of magnitude between the CdS nanocrystals and varies linearly\nwith the fraction of the phases.