Direct evidence of catalyst reduction on dye and catalyst co-sensitized NiO photocathodes by mid-infrared transient absorption spectroscopy

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.

Fig. 1. Chemical structures, reduction potentials and investigated charge transfer processes between the NiO semiconductor, the molecular dye and catalyst. (A) Molecular structures of the dyes E2, coumarin 343 (C343) and the catalyst [1]. (B) The potentials for reductive quenching (arrow 1) of the dye C343 by hole transfer to the NiO valence band, and subsequent oxidation of the C343^– anion (arrow 2) by the catalyst [1], as well as for oxidative quenching of C343 by the catalyst (red digits); the proposed mechanism (arrows) for reduction of the catalyst [1] as shown in our previous work using UV-vis transient absorption spectroscopy. (C) Potential diagram corresponding to B, but with the E2 dye, and investigated charge transfer processes (arrows 1a and 1b) investigated in this study.

Fig. 2. FTIR absorption spectra of the co-sensitized NiO films, [1]|E2|NiO (black) and [1]|C343|NiO (red). The reference sensitized films, [1]|NiO (blue), E2|NiO (gray), C343|NiO films (orange), are also shown. The spectra have been offset by 0.1 absorbance units. Inset in panel B shows the region of the ν_CN stretch of the E2 dye.

Fig. 3. UV-vis absorption spectra of the sensitized NiO films: (A) the [1]|E2|NiO co-sensitized NiO film (blue), E2|NiO reference film (red), [1]|NiO reference film (black); (B) the [1]|C343|NiO co-sensitized NiO film (blue), C343|NiO reference film (red), [1]|NiO reference film (black). The arrows indicate the excitation wavelength used in the femtosecond transient absorption experiments. All the spectra have been corrected for the absorption of NiO. The UV-vis absorption spectra before subtraction of the NiO absorption are shown in the Fig. S1 in the ESI.

Fig. 4. (Right panel): infrared TA spectra showing the reduction of the catalyst [1]via the carbonyl bands (gray areas) in the co-sensitized [1]|E2|NiO and [1]|C343|NiO films. (Left panel): infrared TA spectra of the sensitized E2|NiO and C343|NiO films without catalyst [1]. The solvent was propylene carbonate. The E2|NiO and [1]|E2|NiO films were excited at λ_exc 532 nm with 200 nJ pulse intensity. The C343|NiO and [1]|C343|NiO films were excited at 440 nm with 530 nJ pulse intensity.

Fig. 5. Comparison of the IR transient absorption spectrum of the [1]|E2|NiO film 1 ps after excitation at 532 nm (black), the [1]|C343|NiO film 1 ps after excitation at 440 nm (blue), and the IR transient absorption spectrum of [Fe₂(bdt)(CO)₆]^––[Fe₂(bdt)(CO)₆] obtained by reduction of [Fe₂(bdt)(CO)₆] with flash-quench generated [Ru(dmb)₃]⁺ (ref. 18) (red).

Fig. 6. Kinetics of reduction of [1] upon photoexcitation of the co-sensitized dye in (A) [1]|E2|NiO and (B) [1]|C343|NiO films. For [1]|E2|NiO, the kinetics of reduction of [1] is obtained from the difference between the transient absorption signals of 1^– and [1] taken at 2024 cm^–1 and 2006 cm^–1,respectively. In a similar way, the kinetics of reduction of [1] in [1]|C343|NiO is generated from the difference of the sum of transient absorption signals of 1^– taken at 2072 cm^–1 and 2034 cm^–1 and the sum of the transient signals of [1] taken at 2053 cm^–1 and 2010 cm^–1. Two peaks difference, instead of one peak difference was used for [1]|C343|NiO to improve the signal to noise ratio. The red lines are fits with three exponentials to the experimental data (Table 1).

Scheme 1. (A) Competitive photoinduced charge transfer processes in co-sensitized [1]|E2|NiO films; (B) sequence of photoinduced electron transfer processes leading to catalyst reduction in co-sensitized [1]|C343|NiO films, in agreement with the data in ref. 17.