Attachment of CdSe Nanoparticles to TiO2 via Aqueous Linker-Assisted Assembly: Influence of Molecular Linkers on Electronic Properties and Interfacial Electron Transfer

CdSe nanoparticles (NPs) capped with cysteinate (Cys), 3-mercaptopropionate (MP), and mercaptosuccinate (MS) were adsorbed to TiO2 from basic aqueous dispersions. Native capping groups served as molecular linkers to TiO2. Thus, the materials-assembly chemistry was simplified and made more reproducible and environmentally benign. The electronic properties of CdSe and the electron-transfer reactivity at CdSe-linker-TiO2 interfaces varied with the structure and functionality of the capping groups. Cys-capped CdSe NPs exhibited a narrow and intense first excitonic absorption band centered at 422 nm, suggesting that they were magic-sized nanocrystals (MSCs) with diameters less than 2 rim. MP- and MS-capped CdSe NPs had broader and lower-energy absorption bands, which are typical of regular quantum dots. Photocurrent action spectra of nanocrystalline TiO2 films functionalized with Cys-CdSe, MP-CdSe, and MS-CdSe overlaid closely with absorption spectra, indicating that excitation of CdSe gave rise to the injection of electrons into TiO2. Under white-light illumination, the global energy-conversion efficiency for Cys-capped CdSe ((0.45 +/- 0.11)%) was 1.2-to-6-fold greater than for MP- and MS-capped CdSe. Similarly, the absorbed photon-to-current efficiency was 1.3-to-3.3-fold greater. These differences arose from linker-dependent variations of electron-injection and charge-recombination reactivity. Transient absorption measurements indicated that electron injection from Cys-capped CdSe was more efficient than from MS-capped CdSe. In addition, charge recombination at CdSe-MS-TiO2 interfaces was complete within hundreds of nanoseconds, whereas the charge-separated-state lifetime at CdSe-Cys-TiO2 interfaces was on the order of several microseconds. Thus, Cys-capped CdSe MSCs are readily attached to TiO2 and exhibit unusual electronic properties and desirable electron-transfer reactivity.