An Improved Z-Scheme for Overall Water Splitting Using Dye-Sensitized Calcium Niobate Nanosheets Synthesized by a Flux Method

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 PtOx/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.