Direct Imaging of the Recombination/Reduction Sites in Porous TiO2 Electrodes

In photoelectrochemical cells, one major recombination pathway involves a reaction between the photogenerated electrons that diffuse inside the semiconductor electrode and holes, in the form of oxidized ions, which travel in the electrolyte to the counter electrode. Here we present direct imaging of the recombination/reduction sites in two types of porous TiO2 electrodes, P25 and submicrometer particles, chosen for studying the influence of the TiO2 particles' sizes and shapes on the recombination sites. The sites were labeled with 2-5 nm silver particles, electrodeposited on the TiO2 surface using chronoamperometry. The model assumes that reduction and recombination are similar with respect to the electron transfer from the TiO2 surface to an ionic electron acceptor in the electrolyte redox mediator/Ag+ ion. Consequently the metal deposit marks the reaction locations. This first high-resolution view clearly identifies the connecting points between TiO2 particles and then the {101} facets as the sites of recombination.