Dye-sensitized photocatalysts that consist of a light-absorbing dye and a wide-gap oxide semiconductor have been studied extensively as components of solar energy conversion systems. Although surface modification by a metal and/or metal oxide has a significant impact on the photocatalytic efficiency, the mechanism by which these modifications increase the activity has not been fully understood. Here, a dye-sensitized H2 evolution system was constructed by using Pt-intercalated HCa2Nb3O10 nanosheets, Ru­(II) complex photosensitizers ([Ru­(4,4′-(CH3)2-bpy)2(4,4′-(PO3H2)2-bpy)]2+ and [Ru­(4,4′-(CH3)2-bpy)2(4,4′-(CH2PO3H2)2-bpy)]2+, abbreviated as RuP 2+ and RuCP 2+ ; bpy = 2,2′-bipyridine), and amorphous Al2O3 as building blocks. In the presence of iodide as the electron donor, the H2 evolution rate from Pt/HCa2Nb3O10 nanosheets sensitized by RuP 2+ was increased by modification of the nanosheets with Al2O3. On the other hand, Al2O3 had a negative impact on the H2 evolution rate when RuCP 2+ was employed. These hybrid materials were studied by transient diffuse reflectance spectroscopy and steady-state emission spectroscopy. A detailed analysis of the transient absorption profiles of the adsorbed Ru­(II) complexes revealed that there are at least three states of the complexes on the nanosheet surface. The transient bleaching of the ground-state absorbance had different lifetime components ranging from a few μs to several hundred μs, which mainly reflect back electron-transfer rates from HCa2Nb3O10 to the oxidized Ru­(II) complexes. The Al2O3 modifier could inhibit not only the back electron-transfer events but also electron injection from the excited-state photosensitizer. Interestingly, the negative effect of Al2O3 on the electron injection rate was negligible in the case of RuP 2+, which also had a higher H2 evolution rate. This work highlights that suppressing fast back electron transfer from Pt/HCa2Nb3O10 to the oxidized Ru­(II) complex, which occurs on a time scale of a few μs, and maximizing the electron injection efficiency are both necessary for improving dye-sensitized H2 evolution.

While dye-sensitized metal oxides are good candidates as H 2 evolution photocatalysts for solar-driven Z-scheme water splitting, their solar-to-hydrogen (STH) energy conversion efficiencies remain low because of uncontrolled charge recombination reactions. Here, we show that modification of Ru dye–sensitized, Pt-intercalated HCa 2 Nb 3 O 10 nanosheets ( Ru /Pt/HCa 2 Nb 3 O 10 ) with both amorphous Al 2 O 3 and poly(styrenesulfonate) (PSS) improves the STH efficiency of Z-scheme overall water splitting by a factor of ~100, when the nanosheets are used in combination with a WO 3 -based O 2 evolution photocatalyst and an I 3 − /I − redox mediator, relative to an analogous system that uses unmodified Ru /Pt/HCa 2 Nb 3 O 10 . By using the optimized photocatalyst, PSS/ Ru /Al 2 O 3 /Pt/HCa 2 Nb 3 O 10 , a maximum STH of 0.12% and an apparent quantum yield of 4.1% at 420 nm were obtained, by far the highest among dye-sensitized water splitting systems and comparable to conventional semiconductor-based suspended particulate photocatalyst systems.

A Dion–Jacobson phase calcium niobate (KCa2Nb3O10) was synthesized both by a flux method and by solid-state reaction (SSR). Chemical exfoliation of the proton-exchanged niobate (HCa2Nb3O10) was conducted using tetra­(n-butyl)­ammonium hydroxide (TBA+OH–) to obtain colloidal Ca2Nb3O10 – nanosheets, which were examined as building blocks for dye-sensitized H2 evolution with the aid of a Pt cocatalyst and a ruthenium­(II) photosensitizer, [Ru­(4,4′-(CH3)2-bpy)2(4,4′-(PO3H2)2-bpy)]2+ (abbreviated as RuP2+ ; bpy = 2,2′-bipyridine). HCa2Nb3O10 nanosheets, prepared by the flux method, gave ∼3 times higher activity for half-cell H2 evolution from aqueous solutions containing NaI or ethylenediaminetetraacetic acid as an electron donor under visible light (λ > 400 nm), as compared to those synthesized by the SSR. The flux-based photocatalyst also worked better as the H2 evolution component in Z-scheme overall water splitting in the presence of PtO x /H–Cs–WO3 and I3 –/I–, which worked as the O2 evolution photocatalyst and redox mediator, giving 3–5 times enhancement of activity. Transient absorption spectroscopy measurements showed that in the flux-derived nanosheets, electrons injected from the excited state RuP2+ could move to the Pt cocatalyst through the nanosheets more efficiently than in the SSR sample. This could explain the improved H2 evolution activity provided by the flux-derived nanosheets.

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