Abstract
Co-sensitization of molecular dyes and catalysts on semiconductor surfaces is a promising strategy to build photoelectrodes for solar fuel production. In such a photoelectrode, understanding the charge transfer reactions between the molecular dye, catalyst and semiconductor material is key to guide further improvement of their photocatalytic performance. Herein, femtosecond mid-infrared transient absorption spectroscopy is used, for the first time, to probe charge transfer reactions leading to catalyst reduction on co-sensitized nickel oxide (NiO) photocathodes. The NiO films were co-sensitized with a molecular dye and a proton reducing catalyst from the family of [FeFe](bdt)(CO)6 (bdt = benzene-1,2-dithiolate) complexes. Two dyes were used: an organic push-pull dye denoted E2 with a triarylamine-oligothiophene-dicyanovinyl structure and a coumarin 343 dye. Upon photo-excitation of the dye, a clear spectroscopic signature of the reduced catalyst is observed a few picoseconds after excitation in all co-sensitized NiO films. However, kinetic analysis of the transient absorption signals of the dye and reduced catalyst reveal important mechanistic differences in the first reduction of the catalyst depending on the co-sensitized molecular dye (E2 or C343). While catalyst reduction is preceded by hole injection in NiO in C343-sensitized NiO films, the singly reduced catalyst is formed by direct electron transfer from the excited dye E2* to the catalyst in E2-sensitized NiO films. This change in mechanism also impacts the lifetime of the reduced catalyst, which is only ca. 50 ps in E2-sensitized NiO films but is >5 ns in C343-sensitized NiO films. Finally, the implication of this mechanistic study for the development of better co-sensitized photocathodes is discussed.
Highlights
Dye sensitized solar fuel devices (DSSFDs) for conversion of water to hydrogen (H2) and oxygen (O2) gases suggest a practical solution for storage of solar energy.1 Feasibility of such a technology has already been demonstrated with a few reported devices, with photoanodes for water oxidation and photocathodes for hydrogen reduction.2–12 Most of these photoelectrodes consist of photosensitizers and/or molecular catalysts anchored on transparent semiconductor materials
While for [1]|coumarin 343 (C343)|nickel oxide (NiO), our results con rm the multi-step mechanism proposed by Antila et al, we show that switching the dye C343 to E2 impacts the reaction path for the formation of the reduced catalyst and its lifetime
We have employed femtosecond mid-infrared transient absorption spectroscopy to follow charge transfer reactions in NiO photocathodes co-sensitized with molecular dyes and catalyst [1]
Summary
Dye sensitized solar fuel devices (DSSFDs) for conversion of water to hydrogen (H2) and oxygen (O2) gases suggest a practical solution for storage of solar energy. Feasibility of such a technology has already been demonstrated with a few reported devices, with photoanodes for water oxidation and photocathodes for hydrogen reduction. Most of these photoelectrodes consist of photosensitizers and/or molecular catalysts anchored on transparent semiconductor materials. Dye sensitized solar fuel devices (DSSFDs) for conversion of water to hydrogen (H2) and oxygen (O2) gases suggest a practical solution for storage of solar energy.. Dye sensitized solar fuel devices (DSSFDs) for conversion of water to hydrogen (H2) and oxygen (O2) gases suggest a practical solution for storage of solar energy.1 Feasibility of such a technology has already been demonstrated with a few reported devices, with photoanodes for water oxidation and photocathodes for hydrogen reduction.. Feasibility of such a technology has already been demonstrated with a few reported devices, with photoanodes for water oxidation and photocathodes for hydrogen reduction.2–12 Most of these photoelectrodes consist of photosensitizers and/or molecular catalysts anchored on transparent semiconductor materials. Less is understood on the charge transfer dynamics between the photosensitizer, the molecular catalyst and the NiO semiconductor material
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