Electron Trap States and Photopotential of Nanocrystalline Titanium Dioxide Electrodes Filled with Single-Walled Carbon Nanotubes

Hybrid film electrodes were made of nanocrystalline titanium dioxide (TiO2) and various percentages of single-walled carbon nanotube (SWCNT) fillers up to 5.5 wt%. A complete photoelectrochemical study was performed in both alkaline and acidic conditions by using cyclic voltammetry, potentiostatic photocurrent measurements, and open-circuit-potential measurements under UV/Vis irradiation. Dark voltammograms show a transition from a capacitive to a resistive behavior in the TiO2 accumulation region upon increasing SWCNT percentages. In addition, the energy levels of deep electron traps change inside the TiO2 bandgap. The new peak positions correspond to the reduction potentials of SWCNTs, and their associated charge capacities increase with the SWCNT percentage. The modifications observed in dark experiments directly impact on the shape profile of cyclic voltammograms under irradiation. As a consequence, reduction potentials of SWCNTs appear near the cathodic peaks of certain water oxidation intermediates, and may be interacting with them. On the other hand, open-circuit photopotentials reach maximum values for small SWCNTs percentages around 0.02 wt%. An increase in potentiodynamic and potentiostatic photocurrents is observed for the electrode containing 0.02 wt% SWCNTs, compared to reference TiO2 in acidic conditions.