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

Abstract

Visible-light-driven H(2) evolution based on Dye/TiO(2)/Pt hybrid photocatalysts was investigated for a series of (E)-3-(5'-{4-[bis(4-R(1)-phenyl)amino]phenyl}-4,4'-(R(2))(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 λ>420 nm were found to considerably depend on the hydrophilic character of R(1), varying in the order MOD (R(1)=CH(3)OCH(2), R(2)=H)≈MO4D (R(1)=R(2)=CH(3)OCH(2))>HD (R(1)=R(2)=H)>PD (R(1)=C(3)H(7), R(2)=H). In the case of MOD/TiO(2)/Pt, the apparent quantum yield for photocatalyzed H(2) generation at 436 nm was 0.27±0.03. Transient absorption measurements for MOD- or PD-grafted transparent films of TiO(2) 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 R(1) 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 H(2) 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.

An aqueous, organic dye derivatized SnO2/TiO2 core/shell photoanode

“…They show that oxidation is accompanied by loss of the neutral dye absorption at ~450 nm and the appearance of new absorption features at 610 nm and 720 nm for both cations but with different molar absorptivities. 23 Figure 1. (a) Scan rate dependent CV traces for a FTO|TiO 2 |-[P-A-p-D] film (~20 nm particle diameter, ~ 4 μm thick) as a function of scan rate with visual snapshots of the film illustrating the color change upon oxidation in acetonitrile 0.1 M in TBAP.…”

“…[19][20][21] The electronic properties, stabilities, and photoinduced electron transfer properties of these dyes on TiO 2 are well known in organic solvents but only limited information is available in aqueous solutions. 22,23 We report here, that when surface-bound on SnO 2 /TiO 2 core-shell electrodes, with added hydroquinone (H 2 Q) as a reductive electron transfer scavenger, the surface-bound dye gives rise to impressively high, sustained photocurrents. When co-loaded with a phosphonic acid-derivatized water oxidation catalyst, the dye has also been utilized in a DSPEC for visible light driven water splitting.…”

Highly Robust Hybrid Photocatalyst for Carbon Dioxide Reduction: Tuning and Optimization of Catalytic Activities of Dye/TiO2/Re(I) Organic–Inorganic Ternary Systems

“…Probable electron donors for the second electron transfer in homogeneous-solution photocatalysis are such long-lived reactive species as LReY •-40,43 and a neutral radical generated from the electron donor radical cation (SD •+ ). In the present hybrid system, on the other hand, the second-reduction step does not necessarily require the direct participation of such reactive species, but can occur with the electrons "pooled" in TiO2, which might be long-lived 34,90,91 enough to be transferred to the RePH site ready for the secondreduction step following the above mentioned chemical processes. The on-demand electron transfer from TiO2 to the RePH site is a likely major origin for the persistent photocatalytic behavior of the hybrid system.…”

“…[71][72][73][74][75][76] In a previous paper, we reported that excited-singlet MOD ( 1 MOD*) can inject an electron into TiO2 within 1 ps in MOD-anchored TiO2 nanoparticle films dipped into water at pH 3. 34 In this case, the flat-band potential (Vfb) of TiO2 is estimated to be -(0.53 ~ 0.62) V vs SCE 54-59 which is much more positive than Eox* of MOD (-1.86 V), a situation that allows the occurrence of the ultrafast electron injection. In DMF, on the other hand, the Vfb value observed for the "dry" nanocrystalline film is -2.25 V, more negative by 0.39 eV than Eox* of MOD.…”

Towards Sustainable H2 Production: Rational Design of Hydrophobic Triphenylamine‐based Dyes for Sensitized Ethanol Photoreforming

“…For instance, it hasbeen recently shown [18,19] that hydrophilic substituents placedo nt he terminal portion of organic dyes can enhance their affinity towards the aqueous environment used in the experiments, thusi mproving catalytic performances by favoring the interaction with reactants in solution. On the other hand, positioningh ydrophobic side chains on dye molecules coulda lso have ap ositive impact on H 2 production as they shouldp revent adsorption of water on the TiO 2 surface, reducing the possibility of dye deactivation/desorption from the semiconductor.F urthermore, in analogy to what happens in dye-sensitized solar cells, [11] hydrophobic side chains should also be able to suppressu ndesirable dye aggregation and increase the stability of oxidized dyes by preventing fast charger ecombination between them and electrons in the conductionb and of TiO 2 as well as protecting them from the attack of nucleophilic species presenti nt he reactione nvironment.…”