Metal-free organic sensitizers for use in water-splitting dye-sensitized photoelectrochemical cells.

Solar fuel generation requires the efficient capture and conversion of visible light. In both natural and artificial systems, molecular sensitizers can be tuned to capture, convert, and transfer visible light energy. We demonstrate that a series of metal-free porphyrins can drive photoelectrochemical water splitting under broadband and red light (λ > 590 nm) illumination in a dye-sensitized TiO2 solar cell. We report the synthesis, spectral, and electrochemical properties of the sensitizers. Despite slow recombination of photoinjected electrons with oxidized porphyrins, photocurrents are low because of low injection yields and slow electron self-exchange between oxidized porphyrins. The free-base porphyrins are stable under conditions of water photoelectrolysis and in some cases photovoltages in excess of 1 V are observed.

Understanding the Effect of Monomeric Iridium(III/IV) Aquo Complexes on the Photoelectrochemistry of IrO(x)·nH2O-Catalyzed Water-Splitting Systems.

Soluble, monomeric Ir(III/IV) complexes strongly affect the photoelectrochemical performance of IrO(x)·nH2O-catalyzed photoanodes for the oxygen evolution reaction (OER). The synthesis of IrO(x)·nH2O colloids by alkaline hydrolysis of Ir(III) or Ir(IV) salts proceeds through monomeric intermediates that were characterized using electrochemical and spectroscopic methods and modeled in TDDFT calculations. In air-saturated solutions, the monomers exist in a mixture of Ir(III) and Ir(IV) oxidation states, where the most likely formulations at pH 13 are [Ir(OH)5(H2O)](2-) and [Ir(OH)6](2-), respectively. These monomeric anions strongly adsorb onto IrO(x)·nH2O colloids but can be removed by precipitation of the colloids with isopropanol. The monomeric anions strongly adsorb onto TiO2, and they promote the adsorption of ligand-free IrO(x)·nH2O colloids onto mesoporous titania photoanodes. However, the reversible adsorption/desorption of electroactive monomers effectively short-circuits the photoanode redox cycle and thus dramatically degrades the photoelectrochemical performance of the cell. The growth of a dense TiO2 barrier layer prevents access of soluble monomeric anions to the interface between the oxide semiconductor and the electrode back contact (a fluorinated tin oxide transparent conductor) and leads to improved photoanode performance. Purified IrO(x)·nH2O colloids, which contain no adsorbed monomer, give improved performance at the same electrodes. These results explain earlier observations that IrO(x)·nH2O catalysts can dramatically degrade the performance of metal oxide photoanodes for the OER reaction.

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.

What determines catalyst functionality in molecular water oxidation? Dependence on ligands and metal nuclearity in cobalt clusters.

The metal-oxo M4O4 "cubane" topology is of special significance to the field of water oxidation as it represents the merging of bioinspired structural principles derived from natural photosynthesis with successful artificial catalysts known to date. Herein, we directly compare the rates of water oxidation/O2 evolution catalyzed by six cobalt-oxo clusters including the Co4O4 cubanes, Co4O4(OAc)4(py)4 and [Co4O4(OAc)2(bpy)4](2+), using the common Ru(bpy)3(2+)/S2O8(2-) photo-oxidant assay. At pH 8, the first-order rate constants for these cubanes differ by 2-fold, 0.030 and 0.015 s(-1), respectively, reflecting the number of labile carboxylate sites that allow substrate water binding in a pre-equilibrium step before O2 release. Kinetic results reveal a deprotonation step occurs on this pathway and that two electrons are removed before O2 evolution occurs. The Co4O4 cubane core is shown to be the smallest catalytic unit for the intramolecular water oxidation pathway, as neither "incomplete cubane" trimers [Co3O(OH)3(OAc)2(bpy)3](2+) and [Co3O(OH)2(OAc)3(py)5](2+) nor "half cubane" dimers [Co2(OH)2(OAc)3(bpy)2](+) and [Co2(OH)2(OAc)3(py)4](+) were found capable of evolving O2, despite having the same ligand sets as their cubane counterparts. Electrochemical studies reveal that oxidation of both cubanes to formally Co4(3III,IV) (0.7 V vs Ag/AgCl) occurs readily, while neither dimers nor trimers are oxidized below 1.5 V, pointing to appreciably greater charge delocalization in the [Co4O4](5+) core. The origin of catalytic activity by Co4O4 cubanes illustrates three key features for water oxidation: (1) four one-electron redox metals, (2) efficient charge delocalization of the first oxidation step across the Co4O4 cluster, allowing for stabilization of higher oxidizing equivalents, and (3) terminal coordination site for substrate aquo/oxo formation.

A Co4O4 "cubane" water oxidation catalyst inspired by photosynthesis.

Herein we describe the molecular Co(4)O(4) cubane complex Co(4)O(4)(OAc)(4)(py)(4) (1), which catalyzes efficient water oxidizing activity when powered by a standard photochemical oxidation source or electrochemical oxidation. The pH dependence of catalysis, the turnover frequency, and in situ monitoring of catalytic species have revealed the intrinsic capabilities of this core type. The catalytic activity of complex 1 and analogous Mn(4)O(4) cubane complexes is attributed to the cubical core topology, which is analogous to that of nature's water oxidation catalyst, a cubical CaMn(4)O(5) cluster.

Ultrafast proton-assisted tunneling through ZrO2 in dye-sensitized SnO2-core/ZrO2-shell films.

Core-shell architectures are used to modulate injection and recombination in dye-sensitized photoelectrochemical cells. Here, we demonstrate that exposing SnO2-core/ZrO2-shell films to acid permits photoinduced electron transfer through ZrO2-shells at least 4 nm thick. A novel mechanism of charge transfer is proposed where protonic defects permit ultrafast trap-assisted tunneling of electrons.

Correction: Visible-light controlled catalytic Cu2O-Au micromotors.

Correction for 'Visible-light controlled catalytic Cu2O-Au micromotors' by Dekai Zhou, et al., Nanoscale, 2017, DOI: 10.1039/c6nr08088j.

Visible-light controlled catalytic Cu2O-Au micromotors.

Visible light driven Cu2O-Au micromotors exhibit rapid on/off switching and speed control. Electrochemical measurements confirm that the light-induced movement of the Cu2O-Au micromotors involves a self-electrophoresis mechanism modulated by the photoconductivity of Cu2O. This study extends the utilization of the electromagnetic spectrum for micro/nanomotors into the visible range.

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.

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.

Two-dimensional protein crystals for solar energy conversion.

Two-dimensional photosynthetic protein crystals provide a high density of aligned reaction centers. We reconstitute the robust light harvesting protein Photosystem I into a 2D crystal with lipids and integrate the crystals into a photo-electrochemical device. A 4-fold photocurrent enhancement is measured by incorporating conjugated oligoelectrolytes to form a supporting conductive bilayer in the device which produces a high photocurrent of ∼600 µA per mg PSI deposited.