Excited Carrier Dynamics in a Dye-Sensitized Niobate Nanosheet Photocatalyst for Visible-Light Hydrogen Evolution

Abstract

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 RuP2+ and RuCP2+; 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 RuP2+ was increased by modification of the nanosheets with Al2O3. On the other hand, Al2O3 had a negative impact on the H2 evolution rate when RuCP2+ 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 RuP2+, 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.