Sensitization of a wide-gap oxide semiconductor with a visible-light-absorbing dye has been studied for decades as a means of producing H2 from water. However, efficient overall water splitting using a dye-sensitized oxide photocatalyst has remained an unmet challenge. Here we demonstrate visible-light-driven overall water splitting into H2 and O2 using HCa2Nb3O10 nanosheets sensitized by a Ru­(II) tris-diimine type photosensitizer, in combination with a WO3-based water oxidation photocatalyst and a triiodide/iodide redox couple. With the use of Pt-intercalated HCa2Nb3O10 nanosheets further modified with amorphous Al2O3 clusters as the H2 evolution component, the dye-based turnover number and frequency for H2 evolution reached 4580 and 1960 h–1, respectively. The apparent quantum yield for overall water splitting using 420 nm light was 2.4%, by far the highest among dye-sensitized overall water splitting systems reported to date. The present work clearly shows that a carefully designed dye/oxide hybrid has great potential for photocatalytic H2 production, and represents a significant leap forward in the development of solar-driven water splitting systems.

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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.

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