Photoelectrochemical Behavior of Phthalocyanine-Sensitized TiO 2 in the Presence of Electron-Shuttling Mediators.

Dye-sensitized TiO 2 has found many applications for dye-sensitized solar cells (DSSC), solar-to-chemical energy conversion, water/air purification systems, and (electro)chemical sensors. We report an electrochemical system for testing dye-sensitized materials that can be utilized in photoelectrochemical (PEC) sensors and energy conversion. Unlike related systems, the reported system does not require a direct electron transfer from semiconductors to electrodes. Rather, it relies on electron shuttling by redox mediators. A range of model photocatalytic materials were prepared using three different TiO 2 materials (P25, P90, and PC500) and three sterically hindered phthalocyanines (Pcs) with electron-rich tert -butyl substituents ( t- Bu 4 PcZn, t- Bu 4 PcAlCl, and t- Bu 4 PcH 2 ). The materials were compared with previously developed TiO 2 modified by electron-deficient, also sterically hindered fluorinated phthalocyanine F 64 PcZn, a singlet oxygen ( 1 O 2 ) producer, as well as its metal-free derivative, F 64 PcH 2 . The PEC activity depended on the redox mediator, as well as the type of TiO 2 and Pc. By comparing the responses of one-electron shuttles, such as K 4 Fe(CN) 4 , and 1 O 2 -reactive electron shuttles, such as phenol, it is possible to reveal the action mechanism of the supported photosensitizers, while the overall activity can be assessed using hydroquinone. t -Bu 4 PcAlCl showed significantly lower blank responses and higher specific responses toward chlorophenols compared to t -Bu 4 PcZn due to the electron-withdrawing effect of the Al 3+ metal center. The combination of reactivity insights and the need for only microgram amounts of sensing materials renders the reported system advantageous for practical applications.