Hydrophilicity Control of Visible-Light Hydrogen Evolution and Dynamics of the Charge-Separated State in Dye/TiO2/Pt Hybrid Systems

Visible-light-driven H2 evolution based on Dye/TiO2/Pt hybrid photocatalysts was investigated for a series of (E)-3-(5'-{4-[bis(4-R1-phenyl)amino]phenyl}-4,4'-(R2)2-2,2'-bithiophen-5-yl)-2-cyanoacrylic acid dyes. Efficiencies of hydrogen evolution from aqueous suspensions in the presence of ethylenediaminetetraacetic acid as electron donor under illumination at lambda>420 nm were found to considerably depend on the hydrophilic character of R1, varying in the order MOD (R1=CH3OCH2, R2=H)approximate to MO4D (R1=R2=CH3OCH2)>HD (R1=R2=H)>PD (R1=C3H7, R2=H). In the case of MOD/TiO2/Pt, the apparent quantum yield for photocatalyzed H2 generation at 436 nm was 0.27 +/- 0.03. Transient absorption measurements for MOD- or PD-grafted transparent films of TiO2 nanoparticles dipped into water at pH 3 commonly revealed ultrafast formation (<100 fs) of the dye radical cation (Dye.+) followed by multicomponent decays, which involve minor fast decays (<5 ps) almost independent of R1 and major slower decays with significant differences between the two samples: 1) the early decay of the major components for MOD is about 2.5 times slower than that for PD and 2) a redshift of the spectrum occurred for MOD with a time constant of 17 ps, but not for PD. The substituent effects on H2 generation as well as on transient behavior have been discussed in terms substituent-dependent charge recombination (CR) of Dye.+ with electrons in bulk, inner-trap, and/or interstitial-trap states, arising from different solvent reorganization.