Visible-Light-Induced Water Splitting Based on Two-Step Photoexcitation between Dye-Sensitized Layered Niobate and Tungsten Oxide Photocatalysts in the Presence of a Triiodide/Iodide Shuttle Redox Mediator

Abstract

Water splitting into H2 and O2 under visible light was achieved using simple organic dyes such as coumarin and carbazole as photosensitizers on an n-type semiconductor for H2 evolution, a tungsten(VI) oxide (WO3) photocatalyst for O2 evolution, and a triiodide/iodide (I3(-)/I(-)) redox couple as a shuttle electron mediator between them. The results on electrochemical measurements revealed that the oxidized states of the dye molecules having an oligothiophene moiety (two or more thiophene rings) in their structures are relatively stable even in water and possess sufficiently long lifetimes to exhibit reversible oxidation-reduction cycles, while the carbazole system required more thiophene rings than the coumarin one to be substantially stabilized. The long lifetimes of the oxidized states enabled these dye molecules to be regenerated to the original states by accepting an electron from the I(-) electron donor even in an aqueous solution, achieving sustained H2 and I3(-) production from an aqueous KI solution under visible light irradiation when they were combined with an appropriate n-type semiconductor, ion-exchangeable layered niobate H4Nb6O17. The use of H4Nb6O17 loaded with Pt cocatalyst inside the interlayer allowed the water reduction to proceed preferentially with a steady rate even in the presence of a considerable amount of I3(-) in the solution, due to the inhibited access of I3(-) to the reduction site, Pt particles inside, by the electrostatic repulsion between the I3(-) anions and the negatively charged (Nb6O17)(4-) layers. It was also revealed that the WO3 particles coloaded with Pt and IrO2 catalysts exhibited higher rates of O2 evolution than the WO3 particles loaded only with Pt in aqueous solutions containing a considerable amount of I(-), which competitively consumes the holes and lowers the rate of O2 evolution on WO3 photocatalysts. The enhanced O2 evolution is certainly due to the improved selectivity of holes toward water oxidation on IrO2 cocatalyst, instead of undesirable oxidation of I(-). Simultaneous evolution of H2 and O2 under visible light was then achieved by combining the Pt/H4Nb6O17 semiconductor sensitized with the dye molecules having an oligothiophene moiety, which can stably generate H2 and I3(-) from an aqueous KI solution, with the IrO2-Pt-loaded WO3 photocatalyst that can reduce the I3(-) back to I(-) and oxidize water to O2.

Dye-Sensitized Photocatalytic Water Splitting and Sacrificial Hydrogen Generation: Current Status and Future Prospects

“…The oxidation/reduction activities and the stability of dyes were studied by cyclic voltammetry (CV) in both aqueous and organic solutions [38]. Dye-adsorbed Pt/H 4 Nb 6 O 17 photocatalyst was used for hydrogen evolution while IrO 2 and Pt co-loaded WO 3 photocatalyst were applied to achieve oxygen evolution in the presence of I 3 − /I − redox couple as a shuttle redox mediator ( Figure 9) [38]. The photocatalytic reaction continued up to 2 days without any deactivation.…”

“…However, the reduction of I 3 − to I − occurred on the Pt co-catalysts on TiO 2 suppressing the H 2 evolution consequently. Abe et al [38] demonstrated that selective loading of Pt nanoparticles in the interlayer spaces of H 4 Nb 6 O 17 suppressed the reduction of I 3 − to I − on Pt nanoparticles ( Figure 10) [38]. There are few other parameters which are equally important in the dye-sensitization process: (i) the energy level compatibility between excited sensitizer and the conduction band of the semiconductor; (ii) redox potential of the dye/dye + couple, and stability of the dye to undergo about 10 8 turnover cycles [36].…”

“…The oxidized forms of the dye molecules possess two or more thiophene rings in their structures and enjoy long lifespans to undergo reversible oxidation-reduction cycles. The oxidation/reduction activities and the stability of dyes were studied by cyclic voltammetry (CV) in both aqueous and organic solutions [38]. Dye-adsorbed Pt/H4Nb6O17 photocatalyst was used for hydrogen evolution while IrO2 and Pt coloaded WO3 photocatalyst were applied to achieve oxygen evolution in the presence of I3 − /I − redox couple as a shuttle redox mediator ( Figure 9) [38].…”

“…However, the reduction of I3 − to I − occurred on the Pt co-catalysts on TiO2 suppressing the H2 evolution consequently. Abe et al [38] demonstrated that selective loading of Pt nanoparticles in the interlayer spaces of H4Nb6O17 suppressed the reduction of I3 − to I − on Pt nanoparticles ( Figure 10) [38]. Le et al [39] synthesized Co-doped TiO2 nanomaterials by impregnation method and sensitized with Rhodamine B dye for visible-light-assisted photocatalytic water splitting.…”

Photochromic Intercalation Compounds

“…Thus, the efficiency of the reduction and the sability of the formed charge separated states has been designed through the nanostructural design and the inter-particle level contolled spatial distribution of donor and acceptors in and on layered solids. Since the roles of viologens in photocataytic applications have long been recognized, the organization (and the characterization of them) of viologens achieved so far will bring useful information and hint to understand and modify the artificail photosynthteses based on layered solids [213].…”