Dynamics of Electron Injection in SnO₂/TiO₂ Core/Shell Electrodes for Water-Splitting Dye-Sensitized Photoelectrochemical Cells

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) rely on photoinduced charge separation at a dye/semiconductor interface to supply electrons and holes for water splitting. To improve the efficiency of charge separation and reduce charge recombination in these devices, it is possible to use core/shell structures in which photoinduced electron transfer occurs stepwise through a series of progressively more positive acceptor states. Here, we use steady-state emission studies and time-resolved terahertz spectroscopy to follow the dynamics of electron injection from a photoexcited ruthenium polypyridyl dye as a function of the TiO₂ shell thickness on SnO₂ nanoparticles. Electron injection proceeds directly into the SnO₂ core when the thickness of the TiO₂ shell is less than 5 Å. For thicker shells, electrons are injected into the TiO₂ shell and trapped, and are then released into the SnO₂ core on a time scale of hundreds of picoseconds. As the TiO₂ shell increases in thickness, the probability of electron trapping in nonmobile states within the shell increases. Conduction band electrons in the TiO₂ shell and the SnO₂ core can be differentiated on the basis of their mobility. These observations help explain the observation of an optimum shell thickness for core/shell water-splitting electrodes

Ultrafast Electron Injection Dynamics of Photoanodes for Water-Splitting Dye-Sensitized Photoelectrochemical Cells

Efficient conversion of solar energy into useful chemical fuels is a major scientific challenge. Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) utilize mesoporous oxide supports sensitized with molecular dyes and catalysts to drive the water-splitting reaction. Despite a growing body of work, the overall efficiencies of WS-DSPECs remain low, in large part because of poor electron injection into the conduction band of the oxide support. In this study, we characterize the ultrafast injection dynamics of several proposed oxide supports (TiO₂, TiO₂/Al₂O₃, SnO₂, SnO₂/TiO₂) under identical conditions using time-resolved terahertz spectroscopy. In the absence of an Al₂O₃ overlayer, we observe a two-step injection from the dye singlet state into nonmobile surface traps, which then relax into the oxide conduction band. We also find that, in SnO₂-core/TiO₂-shell configurations, electron injection into TiO₂ trap states occurs rapidly, followed by trapped electrons being released into SnO₂ on the hundreds of picoseconds time scale

Proton-Induced Trap States, Injection and Recombination Dynamics in Water-Splitting Dye-Sensitized Photoelectrochemical Cells

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) utilize a sensitized metal oxide and a water oxidation catalyst in order to generate hydrogen and oxygen from water. Although the Faradaic efficiency of water splitting is close to unity, the recombination of photogenerated electrons with oxidized dye molecules causes the quantum efficiency of these devices to be low. It is therefore important to understand recombination mechanisms in order to develop strategies to minimize them. In this paper, we discuss the role of proton intercalation in the formation of recombination centers. Proton intercalation forms nonmobile surface trap states that persist on time scales that are orders of magnitude longer than the electron lifetime in TiO₂. As a result of electron trapping, recombination with surface-bound oxidized dye molecules occurs. We report a method for effectively removing the surface trap states by mildly heating the electrodes under vacuum, which appears to primarily improve the injection kinetics without affecting bulk trapping dynamics, further stressing the importance of proton control in WS-DSPECs

Highly Active NiO Photocathodes for H₂O₂ Production Enabled via Outer-Sphere Electron Transfer

Tandem dye-sensitized photoelectrosynthesis cells are promising architectures for the production of solar fuels and commodity chemicals. A key bottleneck in the development of these architectures is the low efficiency of the photocathodes, leading to small current densities. Herein, we report a new design principle for highly active photocathodes that relies on the outer-sphere reduction of a substrate from the dye, generating an unstable radical that proceeds to the desired product. We show that the direct reduction of dioxygen from dye-sensitized nickel oxide (NiO) leads to the production of H₂O₂. In the presence of oxygen and visible light, NiO photocathodes sensitized with commercially available porphyrin, coumarin, and ruthenium dyes exhibit large photocurrents (up to 400 μA/cm²) near the thermodynamic potential for O₂/H₂O₂ in near-neutral water. Bulk photoelectrolysis of porphyrin-sensitized NiO over 24 h results in millimolar concentrations of H₂O₂ with essentially 100% faradaic efficiency. To our knowledge, these are among the most active NiO photocathodes reported for multiproton/multielectron transformations. The photoelectrosynthesis proceeds by initial formation of superoxide, which disproportionates to H₂O₂. This disproportionation-driven charge separation circumvents the inherent challenges in separating electron–hole pairs for photocathodes tethered to inner sphere electrocatalysts and enables new applications for photoelectrosynthesis cells