Enhanced Photocatalytic Hydrogen Evolution by Star-Shaped Viologen-Sensitized TiO2 Nanoparticles

Viologens are well-suited for serving as electron-transfer mediators in redox systems due to their low reduction potential and ability to form stable radical cations. Because of this feature, viologens can play a key role in modifying semiconductors toward enhanced photocatalytic performance. Herein, a series of hybrid photocatalysts composed of TiO2 nanoparticles and star-shaped viologen derivatives with different alkyl chains [TPCBP-X_TiO2; X: ethyl (E), butyl (B), hexyl (H) and octyl (O)] were prepared for the photocatalytic hydrogen evolution from water under visible-light irradiation. The TPCBP-X molecules not only provide photosensitization of TiO2 nanoparticles in the visible-light region but also act as an efficient electron-transfer mediator for the transfer of photoinduced electrons to TiO2 and Pt. Among these photocatalysts, TPCBP-E_TiO2 exhibited a 1.013 mmol g-1 h-1 H2 evolution rate with an apparent quantum yield (AQY) of 20.15% (470 nm), which dramatically improved hydrogen evolution activity among the other structures [TPCBP-X_TiO2 (X; B, H, O)] due to the more porous and uniform surface, resulting in its low barrier effect for electron transfer. In addition, in the presence of a Pt cocatalyst, TPCBP-E_TiO2 yielded a H2 evolution rate of 17.7 mmol g-1, which is about 2.2 times higher than that of pure TPCBP-E_TiO2 (8.1 mmol g-1) after 8 h of visible-light illumination.