Molecular Adsorption on ZnO(10-10) Single Crystal Surfaces: Morphology and Charge Transfer

TitleMolecular Adsorption on ZnO(10-10) Single Crystal Surfaces: Morphology and Charge Transfer
Publication TypeJournal Article
Year of Publication2012
AuthorsChen, J, Ruther, RE, Tan, Y, Bishop, LM, Hamers, RJ
Date Published06/2012

While ZnO has excellent electrical properties, it has not been widely used for dye-sensitized solar cells, in part because ZnO is chemically less stable than the widely used TiO2. The chemical linkages typically used for surface passivation and for attaching dye molecules either bind weakly or etch the ZnO surface. We have compared the formation of molecular layers from alkane molecules with terminal carboxylic acid, alcohol, amine, phosphonic acid, or thiol functional groups on single-crystal zinc oxide (10-10) surfaces. Atomic force microscopy (AFM) images show that alkyl carboxylic acids etch the surface, while alkyl amine and alkyl alcohols bind only weakly on the ZnO(10-10) surface. Phosphonic acid-terminated molecules were found to bind to the surface in a heterogeneous manner, forming clusters of molecules. Alkanethiols were found to bind to the surface forming highly uniform monolayers with some etching detected after long immersion times in an alkanethiol solution. Monolayers of hexadecylphosphonic acid and octadecanethiol were further analyzed by Fourier transform infrared spectroscopy (FTIR), x-ray photoelectron spectroscopy (XPS) and electrochemical measurements. AFM scratching shows that thiols were bound strongly to the ZnO surface, suggesting formation of strong Zn-S covalent bonds. Surprisingly, the tridentate phosphonic acids adhered much more weakly than the monodentate thiol. The influence of organic grafting on the charge transfer to ZnO was studied by time-resolved surface photovoltage measurements and electrochemical impedance measurements. Smaller band-bending near the ZnO surface after organic grafting was observed in these measurements.