One-step electrochemical synthesis of ZnO/Ru(dcbpy) 2(NCS) 2 hybrid thin films and their photoelectrochemical properties

Abstract

Hybrid thin films of crystalline zinc oxide (ZnO) and Ru(dcbpy) 2(NCS) 2 (dcbpy=4,4′-dicarboxy-2,2′-bipyridine) have been synthesized by one-step cathodic electrodeposition from oxygen-saturated aqueous solutions containing zinc chloride and the Ru complex. While the pure ZnO thin films are oriented with the c-axis perpendicular to the substrate, the ZnO crystallites in the hybrid thin films are oriented with the c-axis parallel to the substrate as a consequence of the adsorption of the Ru complex onto the growing surface of ZnO. Thin hybrid films (up to about 0.5 μm) consist of grains with approximately 300 nm diameter, which are composed of ZnO crystallites of approximately 20 nm. For thicker films, the formation of an overlayer in a totally different morphology has been observed. The hybrid thin films have a composition of ZnO:Ru(dcbpy) 2(NCS) 2=30:1 in molar ratio, corresponding to a volume ratio of ZnO:Ru(dcbpy) 2(NCS) 2=1:1.7, indicating a very high amount of the Ru complex in the film. The loaded Ru complex can be mostly extracted by dipping the film in a dilute alkaline solution, indicating the porous nature of the film and the presence of the dye mainly on the surface of ZnO. The hybrid thin film electrodes exhibit sensitized photoanodic currents upon white light illumination in organic electrolyte solutions containing iodide which are analyzed in terms of wavelength dependence, I–V characteristics and by time-resolved photocurrent measurements. The thinner films performed considerably better than the thicker films in which dye aggregation seemed to decrease the electron injection efficiency.