Layer-by-layer deposition of open-pore mesoporous TiO2 - Nafion® film electrodes

The formation of variable thickness TiO2 nanoparticle – Nafion® composite films with open pores is demonstrated via a layer-by-layer deposition process. Films of ca. 6 nm diameter TiO2 nanoparticles grow in the presence of Nafion® by “clustering” of nanoparticles into bigger aggregates and the resulting hierarchical structure thickens with ca. 25 nm per deposition cycle. Film growth is characterized by electron microscopy, AFM, and quartz crystal microbalance techniques. SAXS/WAXS measurements for films before and after calcination demonstrate the effect of Nafion® binder causing aggregation. Electrochemical methods are employed to characterize the electrical conductivity and diffusivity of charge through the TiO2-Nafion® composite films. Characteristic electrochemical responses are observed for cationic redox systems (diheptylviologen2+/+, Ru(NH3)63+/2+, and ferrocenylmethyl-trimethylammonium2+/+) immobilized into the TiO2–Nafion® nanocomposite material. Charge conduction is dependent on the type of redox system and is proposed to occur either via direct conduction through the TiO2 backbone (at sufficiently negative potentials) or via redox center based diffusion / electron hopping (at more positive potentials).