Interplay between Ruthenium Sensitizer and Ruthenium Catalyst in Photoelectrochemical Cells with Different Water-Based Electrolytes

Abstract

Popular ruthenium sensitizer (RuP) and ruthenium catalyst (RuOEC) are studied in the cosensitized configurations of a dye-sensitized photoelectrochemical cell with several frequently used electrolytes. Using transient absorption techniques, we have observed that the initial steps of electron transfer from the catalyst to the oxidized sensitizer occur on a very fast time scale (below 100 ps in most of the electrolytes), comparable to that of previously reported supramolecular systems. Electron injection from RuP to TiO 2 occurs over a broad span of time scales ranging from tens of picoseconds to hundreds of nanoseconds. Its average rate constant strongly depends on the TiO 2 conduction band potential, which varies in different electrolytes and does not follow a simple pH dependence. On the contrary, the excited state of the catalyst deactivates very fast, mostly within 1 ps in all electrolytes. Also, the ratios of initial to stable photocurrents and dark to stable photocurrents, recorded in the operating photoelectrochemical cell, have been found to depend on the conduction band potential. Both sensitizer and catalyst show different resistances to desorption in various environments, e.g., RuP is the most stable in Na 2 SO 4 as well as HCl and RuOEC in phosphate buffer and ethylphosphonic acid. Thus, finding the optimum conditions ensuring the best stability of both compounds is required. The studies prove the potential of cosensitized configurations for efficient water-splitting systems and give more insight into further ways of optimization of such systems.

Molecular Water Oxidation Catalysis: Characterization of Subnanosecond Processes and Ruthenium “Green Dimer” Formation

“…The steady photocurrents are similar to those recently reported by us for RuP samples cosensitized with RuOEC in an analogous configuration. 28 Initial peaks reveal a clear synergistic effect of RuP and RuCAT that can be observed in the mixed sample. In the first cycle, the peak of RuP is 20 μA/cm 2 and that of RuCAT is 14 μA/cm 2 .…”

“…Exemplary transient absorption data of RuP, RuCAT, and the mixture of both (RuP + RuCAT), all titania-bound, are presented in Figure 3 Transient absorption spectra and their changes recorded for RuP on TiO 2 have been discussed by us before in detail. 28,31 The negative signal (bleach) below 530 nm (Figure 3A) is due to the ground state depopulation (and corresponds to the stationary absorption of RuPcompare with Figure 1A), while the positive signal above 550 nm with maximum around 700 nm (Figure 3A) represents the combined spectra of the triplet state and the oxidized dye. The initial ultrafast evolution of the transient signal (comparable or below the temporal IRF of our setup) originates from the rapid decay of singlet excited state.…”

“…The changes in transient spectra on longer time scales (tens to hundreds of ps, Figure 3A) are mainly due to electron injection from the triplet state. 28,31 Upon RuCAT cosensitization (Figure 3B), a significant quenching of the positive and negative transient signal of RuP occurs. This can be seen by comparing Figure 3A (RuP only) with Figure 3B (RuP + RuCAT); in particular, the relatively strong residual signal after 2−3 ns for RuP (Figure 3A) significantly decreases for the cosensitized sample (Figure 3B).…”

“…This can be seen by comparing Figure 3A (RuP only) with Figure 3B (RuP + RuCAT); in particular, the relatively strong residual signal after 2−3 ns for RuP (Figure 3A) significantly decreases for the cosensitized sample (Figure 3B). In accordance to our recent studies, 28 we ascribe this quenching to the fast component of RuP regeneration by RuCAT (electron transfer from RuCAT to the oxidized RuP, accompanied by a conversion of RuCAT to higher oxidation state).…”

“…This finding is based on indentation of the amplitude spectra toward a negative signal at the spectral range of phosphorescence ∼650 nm and the dependence of this time constant on the titania conduction band potential. 28,31 The constant offset component reflects the residual spectrum (at ∼3 ns), and both oxidized RuP and triplet state of RuP contribute to this signal (the electron injection from the triplet state partially takes place also on longer, ns time scale, so its population is still present at ∼3 ns 28,34 ). Along with the contribution from the oxidized dye, the signals from the trapped electrons in the conduction band of titania also cannot be excluded in the investigated spectral range (up to 800 nm).…”