A Terahertz-Transparent Electrochemical Cell for In Situ Terahertz Spectroelectrochemistry

Terahertz spectroscopy is broadly applicable for the study of a wide variety of materials, but spectroelectrochemistry has not been performed in the THz range because of the lack of a THz-transparent electrochemical cell. While THz-transparent electrodes do exist, they have never been utilized in a complete three-electrode cell, which is the configuration required for accurate potential control in aqueous media. We have designed and constructed a THz-transparent three-electrode electrochemical cell and have performed THz spectroelectrochemistry of a SnO₂ thin film. The cell utilizes a custom-made reference electrode and tubing which allows the composition of electrolyte to be changed during an experiment. THz spectroelectrochemical measurements show a decrease in THz transmission at potentials where SnO₂ conduction band states are potentiostatically filled. We also describe a simple method for measuring the uncompensated resistance and RC time constant

Dynamics of Electron Recombination and Transport in Water-Splitting Dye-Sensitized Photoanodes

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) use visible light to split water using molecular sensitizers and water oxidation catalysts codeposited onto mesoporous TiO₂ electrodes. Despite a high quantum yield of charge injection and low requirement for the catalytic turnover rate, the quantum yield of water splitting in WS-DSPECs is typically low (<1%). Here we examine the charge separation and recombination processes in WS-DSPECs photoanodes functionalized with varying amounts of IrO₂ nanoparticle catalyst. Charge extraction and transient open-circuit voltage decay measurements provide insight into the relationship between light intensity, conduction band electron density, open-circuit photovoltage, and recombination time scale. We correlate these results with electrochemical impedance spectroscopy and present the first complete equivalent circuit model for a WS-DSPEC. The data show quantitatively that recombination of photoinjected electrons with oxidized sensitizer molecules and scavenging by the water oxidation catalyst limit the concentration of conduction band electrons and by extension the photocurrent of WS-DSPECs

Frequency-Dependent Terahertz Transient Photoconductivity of Mesoporous SnO₂ Films

The transient photoconductive properties of tin­(IV) oxide (SnO₂) mesoporous films have been studied by time-resolved terahertz (THz) spectroscopy. We gain insight into carrier dynamics by measuring overall injection and trapping lifetimes using optical pump–THz probe spectroscopy, as well as the frequency-dependent complex conductivity at various pump–probe delay times. It is found that the method of charge generation, either direct above band gap excitation (at 267 nm) or dye-sensitized electron injection (at 400 nm), has a dramatic effect on the overall injection and trapping dynamics of mobile charge carriers on the picosecond to nanosecond time scale. In the presence of aqueous electrolyte, direct band gap excitation of nonsensitized SnO₂ films results in instrument response limited subpicosecond injection lifetimes, while dye-sensitized films require tens of picoseconds for interfacial electron transfer to complete. On the other hand, the rate for trapping of mobile charges is at least 2 orders of magnitude faster in the nonsensitized films compared to the dye-sensitized films, which is likely due to photoinduced charges being more highly concentrated in the nonsensitized films. Finally, we find that the transient photoconductivity deviates from the behavior described by standard conductivity models such as the Drude and Drude–Smith models. This is due to the contribution from a photoinduced change in the permittivity of the SnO₂ films

Rutile TiO₂ as an Anode Material for Water-Splitting Dye-Sensitized Photoelectrochemical Cells

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) use a wide bandgap metal oxide semiconductor functionalized with a light-absorbing dye and water-oxidation catalyst to harvest light and drive water oxidation, respectively. We demonstrate here that the rutile polymorph of TiO₂ (r-TiO₂) is a promising anode material for WS-DSPECs. Recombination between the injected electron and oxidized sensitizer with r-TiO₂ is an order of magnitude slower than with anatase TiO₂ (a-TiO₂), with injection yields approaching 100%. Studies with a reductive quencher demonstrate that r-TiO₂ is significantly more efficient than a-TiO₂, while exhibiting greater dye stability. Furthermore, comparison of direct band gap excitation photocurrent generation for bare and sensitized r-TiO₂ suggests that the sensitizer functions as a light harvester and redox mediator

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

Effects of Electron Trapping and Protonation on the Efficiency of Water-Splitting Dye-Sensitized Solar Cells

Water-splitting dye-sensitized photoelectrochemical (WS-DSPECs) cells employ molecular sensitizers to absorb light and transport holes across the TiO₂ surface to colloidal or molecular water oxidation catalysts. As hole diffusion occurs along the surface, electrons are transported through the mesoporous TiO₂ film. In this paper we report the effects of electron trapping and protonation in the TiO₂ film on the dynamics of electron and hole transport in WS-DSPECs. When the sensitizer bis­(2,2′-bipyridine)­(4,4′-diphosphonato-2,2′-bipyridine)­ruthenium­(II) is adsorbed from aqueous acid instead of from ethanol, there is more rapid hole transfer between photo-oxidized sensitizer molecules that are adsorbed from strong acid. However, the photocurrent and open-circuit photovoltage are dramatically lower with sensitizers adsorbed from acid because intercalated protons charge-compensate electron traps in the TiO₂ film. Kinetic modeling of the photocurrent shows that electron trapping is responsible for the rapid electrode polarization that is observed in all WS-DSPECs. Electrochemical impedance spectroscopy suggests that proton intercalation also plays an important role in the slow degradation of WS-DSPECs, which generate protons at the anode as water is oxidized to oxygen

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

Photovoltage Effects of Sintered IrO₂ Nanoparticle Catalysts in Water-Splitting Dye-Sensitized Photoelectrochemical Cells

Water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) utilize high surface area TiO₂ electrodes functionalized with light absorbing sensitizers and water oxidation catalysts. Because water splitting requires vectorial electron transfer from the catalyst to the sensitizer to the TiO₂ surface, attaching both sensitizer and catalyst to TiO₂ in the correct sequence and stabilizing them under photoelectrochemical conditions has been a challenging problem. Rutile-phase IrO₂ nanoparticles can be deposited directly on the TiO₂ electrode by adsorbing citrate-capped amorphous IrO_<i>x</i> and then sintering at 450 °C. Electrodes functionalized with these nanocrystalline particles show higher activity than those made from ligand-capped amorphous IrO_<i>x</i> without sintering. In the WS-DSPEC, the Coulombic efficiency for oxygen evolution from the sintered nanoparticle photoelectrodes was near unity. The loading of colloidal IrO_<i>x</i> and IrO₂ particles onto the porous TiO₂ electrodes was quantified by neutron activation analysis. Photovoltage measurements suggest that at high catalyst loading the dominant charge recombination pathway is from photoinjected electrons to IrO₂

High-Potential Porphyrins Supported on SnO₂ and TiO₂ Surfaces for Photoelectrochemical Applications

We report CF₃-substituted porphyrins and evaluate their use as photosensitizers in water-splitting dye-sensitized photoelectrochemical cells (WS-DSPECs) by characterizing interfacial electron transfer on metal oxide surfaces. By using (CF₃)₂C₆H₃ instead of C₆F₅ substituents at the meso positions, we obtain the desired high potentials while avoiding the sensitivity of C₆F₅ substituents to nucleophilic substitution, a process that limits the types of synthetic reactions that can be used. Both the number of CF₃ groups and the central metal tune the ground and excited-state potentials. A pair of porphyrins bearing carboxylic acids as anchoring groups were deposited on SnO₂ and TiO₂ surfaces, and the interfacial charge-injection and charge-recombination kinetics were characterized by using a combination of computational modeling, terahertz measurements, and transient absorption spectroscopy. We find that both free-base and metalated porphyrins inject into SnO₂ and that recombination is slower for the latter case. These findings demonstrate that (CF₃)₂C₆H₃-substituted porphyrins are promising photosensitizers for use in WS-DSPECs