Effect of Phosphonate-Functionalized Surface Modification on Nanocrystalline TiO2Film Electrode

Abstract

Since the pioneering works of Gratzel’s group for dyesensitized solar cells (DSSC), nanocrystalline semiconductor film electrodes have been intensively investigated. In DSSC, the interfacial charge transfer between the surface of the film and the solution electrolyte is very important. Especially, the effect of surface complexation on the rate of interfacial electron transfer reaction is known to be crucial. In the Gratzel’s DSSC, the light-absorbing ruthenium complexes were adsorbed on the surface of TiO2 through the carboxylate chelation. The strong electronic coupling between π orbital of the ligand and 3d orbital of the conduction band of TiO2 rendered the very rapid electron transfer from the metal-to-ligand charge transfer state to the conduction band feasible and the rate of recombination between the conduction band and Ru(III) was retarded by the high driving force in the Marcus inverted region and the spacer ligand. In the result, about 80% incident photon-to-current conversion efficiency in a certain wavelength region was achieved in those cells. Recent studies showed that a phosphonate group instead of a carboxylate group was also strongly adsorbed on the surface of nanocrystalline TiO2 film and an efficient electron transfer between them occurred. Using the metal bisphosphonate multiplayer technique, we have also demonstrated that the redox couple multiplayers of bisphosphonate derivatives were constructed on the surface of a nanocrystalline TiO2 film. Whereas a carboxylate compound was sensitive to pH of an electrolyte solution and was desorbed from the surface of TiO2 at the pH higher than 4, a phosphonate compound was less sensitive to the solution pH and remains on the surface until pH 9. To elucidate the surface sites and the surface complexation effect of nanocrystalline TiO2 (NT) film electrode, here we briefly report the effect of surface chelation of two different types of phosphonate-functionalized compounds on the interfacial electron transfer of NT film electrode for water oxidation: one is electron-donating and the other is electron-accepting. VP and DBPA were both adsorbed directly on the surface of NT electrode by a simple immersion in aqueous and ethanolic solutions, respectively. After the immersion for an hour, the electrodes were thoroughly washed with copious amounts of water and ethanol. The adsorption of DBPA on the surface of TiO2 film could be confirmed from CHstretching mode (at 2919 and 2851 cm−1) in the IR spectra of DBPA-treated TiO2 that is not shown here, although the amount of the adsorbed DBPA could not be estimated quantitatively. The cyclic voltammograms of NT electrodes prepared here are shown in Figure 1. For the bare NT electrode, the cathodic current appeared at potentials negative than flat band potential (Efb) and increased as the applied potential increased. Since there was no appreciable faraday component except solvent water, the current can be assigned to the capacitive current that can be due to the complex capacitance such as Helmholtz layer, double layer and space charge layer capacitance, if space charge layer exists, etc.