High Stability of Immobilized Polyoxometalates on TiO2 Nanoparticles and Nanoporous Films for Robust, Light-Induced Water Oxidation

Solutions to two general limitations in the immobilization of molecular water oxidation catalysts (WOCs) on nanoparticle or photo-electrode surfaces have been investigated: (a) a straightforward electrostatic method to bind charged WOCs more effectively to these surfaces and (b) a method to increase the concentration of the semiconductor- and/or electrode-immobilized WOCs so they can be spectroscopically characterized in this form. Polyoxometalate (POM) WOCs, known to be fast, selective, and oxidatively stable under homogeneous conditions, and their high negative charges are a good test case to assess the viability of electrostatic surface immobilization. The POM WOCs, [(Ru4O5)-O-Iv(OH)(H2O)(4)(gamma-PW10O36)(2)](9-) (Ru4P2) and [{Ru-4(IV)(OH)(2)(H2O)(4)}(gamma-SiW10O34)(2)](10-) (Ru4Si2), have been immobilized by silanization on TiO2 nanoparticles and nanoporous electrodes and have been found to retain catalytic water oxidation activity. In photoelectrochemical systems, increased current density of WOC-TiO2/FTO electrodes is consistent with water oxidation occurring on the derivatized, modified semiconductor surface. The simple use of semiconductor nanoparticles (versus conventional larger particles or surfaces) provides sufficient concentrations of immobilized POM WOCs to enable their spectroscopic and electron microscopic characterization after photoelectrocatalytic use. Multiple techniques have been used to observe the effectiveness of silanization for POM WOC immobilization on nanoparticle surfaces as well as TiO2/FTO electrodes before and after photoelectrocatalysis.