Visible light induced hydrogen production over thiophenothiazine-based dye sensitized TiO2 photocatalyst in neutral water

In this study, we report the design and synthesis of a series of organic sensitizers, namely, (E)-3-(10-butyl-8-(methylthio)-10H-phenothiazin-3-yl)-2-cyanoacrylic acid (UP1), (E)-3-(10-pentyl-8-(methylthio)-10H-phenothiazin-3-yl)-2-cyanoacrylic acid (UP2) and (E)-3-(10-hexyl-8-(methylthio)-10H-phenothiazin-3-yl)-2-cyanoacrylic acid (UP3), and their application for photocatalytic hydrogen production. The as-prepared sensitizers show pronounced light harvesting capability under a wide visible region (400-650 nm) and exhibit relatively strong fluorescence at 625-628 nm with lifetimes of 0.59-0.62 ns. The extinction coefficient of the dyes increases by increasing the different substituents of methine chain on the nitrogen of the thiophenothiazine ring. Alkyl substitution in the sensitizers led to desirable electrochemical behaviour, which facilitates the effective electron injection from the dye to TiO2. The resulting thiophenothiazine@TiO2-Pt composites exhibited high hydrogen production efficiency from water splitting at neutral conditions. The UP3 sensitized Pt-TiO2 (UP3@PT) photocatalyst showed hydrogen evolution up to 1048 mu mol (TON 1397) with an excellent apparent quantum yield, similar to 50%, which is much higher than the other photocatalysts such as UP1@PT, UP2@ PT and pure TiO2. Operational parameters, such as the effects of the substitution in dye molecules, dye concentration, activation temperature of TiO2 and pH of the reaction medium, were explored. Theoretical studies and experimental measurements corroborated that, the addition of methine unit in the sensitizer enhanced the efficiency in dye@TiO2 composite via reduced charge recombination and increased light capture.