Article pubs.acs.org/JPCC
Photodriven Oxidation of Surface-Bound Iridium-Based Molecular Water-Oxidation Catalysts on Perylene-3,4-dicarboximide-Sensitized TiO2 Electrodes Protected by an Al2O3 Layer Rebecca J. Kamire,† Kelly L. Materna,‡,§ William L. Hoffeditz,† Brian T. Phelan,† Julianne M. Thomsen,‡,§ Omar K. Farha,†,∥ Joseph T. Hupp,†,⊥ Gary W. Brudvig,‡,§ and Michael R. Wasielewski*,† †
Department of Chemistry and Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208-3113, United States ‡ Department of Chemistry and ANSER Center, Yale University, 225 Prospect Street, New Haven, Connecticut 06520, United States § Yale Energy Sciences Institute, Yale University, West Haven, Connecticut 06516, United States ∥ Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah 23218, Saudi Arabia ⊥ Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 South Cass Avenue, Argonne, Illinois 60439, United States S Supporting Information *
ABSTRACT: Improving stability and slowing charge recombination are some of the greatest challenges in the development of dye-sensitized photoelectrochemical cells (DSPECs) for solar fuels production. We have investigated the effect of encasing dye molecules in varying thicknesses of Al2O3 deposited by atomic layer deposition (ALD) before catalyst loading on both the stability and the charge transfer dynamics in organic dye-sensitized TiO2 photoanodes containing iridium-based molecular water-oxidation catalysts. In the TiO2|dye|Al2O3|catalyst electrodes, a sufficiently thick ALD layer protects the perylene-3,4-dicarboximide (PMI) chromophores from degradation over several weeks of exposure to light. The insulating capacity of the layer allows a higher photocurrent in the presence of ALD while initial charge injection is slowed by only 1.6 times, as observed by femtosecond transient absorption spectroscopy. Rapid picosecond-scale catalyst oxidation is observed in the presence of a dinuclear catalyst, IrIr, but is slowed to tens of picoseconds for a mononuclear catalyst, IrSil, that incorporates a long linker. Photoelectrochemical experiments demonstrate higher photocurrents with IrSil compared to IrIr, which show that recombination is slower for IrSil, while higher photocurrents with IrIr upon addition of ALD layers confirm that ALD successfully slows charge recombination. These findings demonstrate that, beyond stability improvements, ALD can contribute to tuning charge transfer dynamics in photoanodes for solar fuels production and may be particularly useful for slowing charge recombination and accounting for varying charge transfer rates based on the molecular structures of incorporated catalysts.
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INTRODUCTION Within the search for energy technologies that limit carbon emissions and store energy for future use, methods to produce hydrogen and other small-molecule fuels from sunlight, water, and carbon dioxide have become an increasingly prominent research focus, but their efficient generation remains elusive.1−6 One approach to production of solar fuels uses chromophoreand catalyst-functionalized photoelectrodes as part of dyesensitized photoelectrochemical cells (DSPECs).7−9 DSPECs transform sunlight directly into chemical energy by providing protons and electrons from water oxidation at a photoanode to a photocathode for reductive fuel-forming reactions. Photodriven water oxidation is particularly challenging because four © 2017 American Chemical Society
oxidative equivalents must accumulate at a single catalytic site before one molecule of oxygen is formed without charge recombination occurring first.10,11 In addition, catalyst and chromophore molecules tend to desorb over time into the electrolyte, and the harsh oxidative environment may degrade the organic components.12 These stringent requirements and stability challenges necessitate an exploration of morphologies that might improve DSPEC efficiency. A detailed mechanistic approach, as pursued in this work, can not only recognize Received: November 19, 2016 Revised: January 26, 2017 Published: January 27, 2017 3752
DOI: 10.1021/acs.jpcc.6b11672 J. Phys. Chem. C 2017, 121, 3752−3764
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Figure 1. Structures of TiO2|PMI, TiO2|IrIr, and TiO2|IrSil and their precursors. The binding modes for IrIr on the semiconductor surface are not known. See ref 74 for the identity of the chelating ligand.
designs that perform more efficiently than others but also identify the causes of these differences at the level of individual photodriven electron transfer steps. Several approaches have been demonstrated to improve the binding and molecular stability of chromophores on electrode surfaces.4 Historically, this work has focused on improving the strength of binding groups under operating conditions, such as our recent work on hydroxamates, silatranes, and other linkers for robust binding of organic dyes and inorganic complexes to metal oxide surfaces.13−15 Newer methods include surface modification after dye functionalization. For example, electropolymerization of ruthenium-based sensitizers improves the stability of the dye layer.16,17 Alternatively, thin layers of TiO2 or Al2O3 deposited by atomic layer deposition (ALD) over surface-bound molecules slow both desorption of molecules from semiconducting electrodes18−24 and dye degradation, as in recent work by us and others on electrodes for DSPECs.25,26 Modifications of semiconductor morphologies have also been used to favorably tune electron transfer dynamics. For example, ALD of insulating materials such as Al2O3 slows charge recombination between electrons in the semiconducting electrode and positive charges, either in solution by creating a tunneling barrier or in oxidized molecules on the semiconductor surface, through passivation of surface-localized states (coordinatively unsaturated trap sites) in TiO2 that are below the energy of the conduction band.19,27−36 Meyer and co-workers have pioneered work on core/shell SnO2/TiO237,38 and core/shell nanoITO/TiO224,39 morphologies that have significantly slowed back electron transfer compared to TiO2 alone. Similar behavior has been reported for coaxial core/ shell/shell (silica/ZnO/TiO2) aerogel photoelectrodes and can be understood in terms of favorable band-bending effects in the ALD-formed TiO2 layer.29
While most dye-sensitized photoanodes use ruthenium-based dyes,24,37,39−45 others have used high-potential porphyrins,46−51 organic donor-π-acceptor chromophores,38 and perylene3,4:9,10-bis(dicarboximide) (PDI) chromophores52−54 that have been developed for use in water oxidation photoanodes. However, many of these organic dyes are incapable of electron injection into TiO2, which is the semiconductor most commonly used for water-oxidation photoanodes, and are either rapidly desorbed from the surface or degraded under the harsh operating conditions. Perylene-3,4-dicarboximide (PMI) derivatives offer stronger reducing power than the PDIs and as a result have been investigated for electron injection into TiO2 with more success.55−68 We have reported a PMI derivative (PMI, Figure 1) that, when bound to nanostructured semiconductor films, can inject an electron into TiO2 or a hole into NiO upon photoexcitation at 495 nm.25,69 The resulting radical anion or cation is sufficiently high in energy to reduce or oxidize many molecular proton-reduction or wateroxidation catalysts, respectively. Further, when PMI is embedded in a layer of Al2O3 deposited by ALD on a NiO electrode, the dye is protected against desorption and degradation and the charge recombination processes are slowed, which contribute to a high Faradaic efficiency of photodriven hydrogen evolution by molecular catalysts in solution.25 We posited that recombination and degradation would be slowed by an Al2O3 layer for water-oxidation photoanodes as well and that rates of photoinduced electron transfer could be correlated with the thickness of the ALD layer and the properties of incorporated molecular catalysts. We have identified a highly active family of dimeric iridiumbased molecular water-oxidation electrocatalysts with bidentate organic ligands that are formed under chemically or electrochemically oxidative conditions from pentamethylcyclopenta3753
DOI: 10.1021/acs.jpcc.6b11672 J. Phys. Chem. C 2017, 121, 3752−3764
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The Journal of Physical Chemistry C dienyl (Cp*) Ir precursors in solution.70−75 The proposed structure for one of these dinuclear catalysts, IrIr, derived from the Cp*Ir(pyalc)OH precursor, where pyalc = 2-(2′-pyridyl)-2propanolate, is shown in Figure 1.75 The formation process includes oxidative degradation of the Cp* ligand into acetic acid and other products. When IrIr is heterogenized on a metal oxide semiconductor, the molecular catalyst not only retains its structure and electrocatalytic performance but it displays a lower overpotential for water oxidation, which demonstrates great promise for incorporation into PECs.76 Subsequently heterogenized on a hematite photoanode, it improves upon the water oxidation of hematite alone by enhancing the favorable charge transfer direction, though fast recombination of charges at the surface occurs because the underlying hematite is exposed to solution.77,78 Photocatalysis by a dye-sensitized electrode, however, has not yet been demonstrated for the species. The mononuclear iridium precursors have shown apparent catalytic activity without dimerization when they are heterogenized, and such derivatives have facilitated mechanistic study. One of these, Cp*Ir(ppy)Cl, where ppy = 2-phenylpyridine, when functionalized with a carboxylate binding group and bound to SnO2 electrodes can be photooxidized by codeposited porphyrin photosensitizers.50,79 Similarly, if PMI is covalently bound to both TiO2 and Cp*Ir(ppy)Cl, photoexcitation of PMI is followed by both electron injection into TiO2 and the one-electron oxidation of Ir III to Ir IV . 69 Others have demonstrated that related complexes are capable of water oxidation when heterogenized within metal−organic frameworks80 or mesoporous organosilica.81 We have recently synthesized and studied a new Cp*Ir(pyalc)OH wateroxidation catalyst containing a silatrane surface anchoring group, IrSil (Figure 1).82 Silatranes are favorable alternatives to conventional surface anchoring groups (carboxylates, phosphonates, hydroxamates, etc.) because they form strong and stable siloxane surface bonds at pH 2−11 in water and do not chelate with metal centers during synthesis of molecular catalysts, which aids in the isolation of the desired product.13 The silatrane-containing Cp*Ir catalyst IrSil has been bound to nanoporous ITO (nanoITO) and found to perform electrochemically driven water-oxidation catalysis at 1.35 V vs NHE at pH 5.8.82 While water oxidation is slower than for IrIr, the IrSil catalyst likely remains monomeric on the surface. The current work incorporates the dimeric catalyst IrIr, which binds directly to the metal oxide surface, and IrSil, which contains an extended phenylene spacer, into dye-sensitized photoanodes with varying amounts of Al2O3 applied by ALD after dye sensitization. A comparison of the full TiO2|PMI| Al2O3|catalyst systems with 0−30 cycles of Al2O3 to those without catalyst allows us to probe the effects of each of the components of the electrode architecture on the dynamics of charge transfer processes.
2 h in a solution of 1 mM [Cp*Ir(pyalc)OH] and 100 mM NaIO4 that had first been stirred for 30 min to form the active catalyst73 or with IrSil by soaking overnight in a ∼0.02−0.05 mM solution of IrSil in acetonitrile at 70 °C under nitrogen in the dark. Films for femtosecond transient absorption (fsTA) spectroscopy were sealed under nitrogen with glass slides using UV-curable epoxy (Epoxies, Etc., 60-7180) in a glovebox. The films for photoelectrochemical experiments were prepared by using sand paper to remove residual Al2O3 at the top of the electrode to make electrical contact with the FTO-coated glass, and clear nail polish was coated over the FTO nonactive area that would be exposed to the electrolyte during the experiments. The films for dual electrode experiments were attached to stranded conductive wire using conductive silver epoxy (CircuitWorks Chemtronics), which was then covered with nonconductive epoxy (LOCTITE 9340 Hysol) and cured at 110 °C for 10 min. Optical Spectroscopy. UV−vis transmission spectra of the films were acquired using a PerkinElmer LAMBDA 1050 spectrophotometer equipped with a 150 mm integrating sphere. The FTO|TiO2 background was subtracted to yield the reported spectra. An aging study was performed by aging films uncovered on a lab benchtop exposed to ambient air and light over 3 months, and UV−vis absorption spectra were acquired using a Cary 5000 UV−vis−NIR spectrophotometer. Catalyst loading was monitored using a Shimadzu UV-2600 spectrophotometer. The fsTA experiments were conducted using a regeneratively amplified Ti:sapphire laser system with samples translated in two dimensions and irradiated at 495 nm as previously described.69,83 All fsTA data were corrected for group delay dispersion (GDD, or “chirp”) and t0 prior to kinetic analysis. Single-wavelength kinetic analysis was performed using a nonlinear least-squares fit to a sum of exponentials convoluted with a Gaussian instrument response function, which was fit to about 0.3 ps. The three-dimensional fsTA data sets were analyzed by singular value decomposition and global fitting to obtain kinetic time constants and their decay-associated spectra (DAS) using Surface Xplorer software, version 4.2.0 (Ultrafast Systems LLC, Sarasota, FL).84 While we have previously reported fsTA results on TiO2|PMI|0ALD,69 data were reacquired here in order to best compare results. All the films were prepared in the same batch, and fsTA experiments were acquired within several days of each other in order to minimize variation in the TiO2 and the instrument’s white light probe, respectively. However, fsTA results acquired from different batches and at different times were almost identical to those reported here. Electrochemistry and Photoelectrochemistry. Samples of TiO2|PMI|0−30ALD|catalyst with IrIr were studied in 0.25 M Na2SO4 adjusted to pH 2.5 with H2SO4, while those with IrSil were studied in 0.1 M KNO3 (pH 5.8). Samples of TiO2| 0−5ALD, TiO2|catalyst, TiO2|5ALD|catalyst, and TiO2|PMI|0− 30ALD were also studied under both conditions. The film and Ag/AgCl (3 M NaCl) reference electrode were placed in the working compartment of an H-cell and a Pt mesh counter electrode was placed in the auxiliary compartment. Linear sweep voltammetry (LSV) was performed using a Princeton Applied Research VersaSTAT4 potentiostat with a scan rate of 5 mV/s. The samples were illuminated every 10 s (50 mV in the sweep) using a Xe lamp (∼100 mW/cm2) with a 420 nm long pass filter. Additional experimental details for the collector−generator experiments26,37,43 are provided in the SI.
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EXPERIMENTAL SECTION Synthesis and Film Preparation. The PMI dye,69 IrIr catalyst,73 and IrSil catalyst82 were prepared as previously reported. The preparation of the dye-loaded nanostructured TiO2 electrodes is reported in the Supporting Information (SI). The TiO2|PMI surfaces were treated with 0−30 ALD cycles of dimethylaluminum isopropoxide and water to yield TiO2|PMI| 0−30ALD films, where PMI molecules on films with 30 cycles of ALD are entirely encased in Al2O3.25 Further details are provided in the SI. Samples were loaded with IrIr by soaking for 3754
DOI: 10.1021/acs.jpcc.6b11672 J. Phys. Chem. C 2017, 121, 3752−3764
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months, while other films are bleached completely. This result demonstrates the robustness of the approach for protecting PMI on TiO2. Electrochemistry. Electrochemical experiments on each of the molecular components were used to estimate the driving force for the electron transfer processes that are necessary for photodriven water oxidation. The estimated potentials for the first oxidation and reduction of PMI are 1.61 and −0.52 V vs NHE, respectively, and PMI is known to inject electrons into TiO2 upon photoexcitation (E00 = 2.30 eV).69 The first oxidation of IrIr on nanoITO occurs at the IrIII/IV redox couple at 0.75 V vs NHE and is followed by the onset of catalysis at 1.3 V vs NHE at pH 2.6.76 The first oxidation of IrSil on nanoITO occurs at the IrIII/IV redox feature at 0.9 V vs NHE and is followed by the onset of catalysis at 1.35 V vs NHE at pH 5.8.82 Thus, photoexcited PMI should have a favorable driving force for each step of water oxidation using either catalyst. In the photoelectrochemical experiments at pH 2.5 and 5.8, the TiO2 conduction band edge is at about −0.31 and −0.51 V vs NHE, respectively,86 though a distribution of trap states below the band edge is also available for electron injection from PMI,67,87 which is especially important to consider under the solvent-free conditions investigated by fsTA, where the band edge is more negative. In addition to the pH effects, which favor faster electron injection at pH 2.5 than at pH 5.8 based on the larger driving force and expected Marcus normal region behavior, electron injection is favored more in the Na2SO4 solution (pH 2.5) than in the KNO3 solution (pH 5.8) by the smaller size of the Na+ ion and the favorable shift in the conduction band edge that results.87 Recombination is also expected to be more rapid for the pH 2.5 conditions due to Marcus inverted region behavior for recombination.87,88 However, when Al2O3 is deposited over the dye, it passivates the surface states of TiO2, decreases the availability of these lower-energy intra-band-gap states for electron injection, and thus decreases the rates and/or yields of electron injection.31,32,66 These values were used to construct the energy level diagrams shown in Figure 3 in order to follow the various decay pathways that are possible following excitation of PMI. There is an estimated favorable driving force for both electron injection into TiO2 and catalyst oxidation following photoexcitation of PMI under either of the electrolyte and catalyst conditions studied. Furthermore, by continued excitation and injection
RESULTS AND DISCUSSION Steady-State Spectroscopy. The UV−vis absorption spectrum of TiO2|PMI|0ALD displays a maximum at 511 nm and a higher-energy vibronic band at 485 nm (Figure 2). A
Figure 2. Normalized UV−vis absorption spectra of TiO2|PMI|ALD with 0, 5, 20, and 30 cycles of Al2O3 ALD. The absorption of the TiO2 background has been subtracted.
blue-shift of the maximum to 505 nm after the deposition of five cycles of Al2O3 by ALD results from disaggregation of PMI aggregates,85 and a red-shift as more Al2O3 is deposited results from the increasingly rigid environment experienced by PMI, as we have previously reported for Al2O3-encased PMI on NiO electrodes.25 Films with both PMI and IrIr or IrSil are almost unchanged compared to films with PMI alone (Figure S1, SI), as expected based on the low extinction coefficients of the catalysts in the visible region.73,82 Catalyst loadings were determined by monitoring the UV−vis spectrum of the sensitization solutions of the films and found to be ∼56 and ∼10 nmol/cm2 for IrIr and IrSil, respectively. The ability of the Al2O3 layer to stabilize PMI against degradation was investigated for TiO2|PMI|Al2O3 films with 0− 30 cycles of ALD exposed to ambient light and air over 3 months. Significant dye degradation occurred over the first 2 weeks for films not protected by ALD. Following a slight loss in absorption that occurs during the ALD process, films that are protected with more cycles of ALD degrade less over the 2 weeks following ALD, until by 30 cycles there is no apparent change over that time (Figures S2 and S3, SI). Films with 30 cycles have a significant amount of PMI remaining after even 3
Figure 3. Energy level diagrams for (A) TiO2|PMI|0ALD|IrIr in pH 2.5 aqueous conditions and (B) TiO2|PMI|0ALD|IrSil in pH 5.8 aqueous conditions, showing the photophysical pathways accessible from the PMI excited state. 3755
DOI: 10.1021/acs.jpcc.6b11672 J. Phys. Chem. C 2017, 121, 3752−3764
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rise in a 600 nm absorption band. Using literature precedent,66,89,90 this new band was identified as a Stark shift of the PMI ground state absorption caused by the local electric field induced by electron injection into TiO2. The Stark shift and a broad band from 600 to 800 nm that results from PMI•+ 91 decay multiexponentially from ∼10 ps to beyond the time scale of the experiment as charge recombination (τ3) occurs. Each photodriven process is observed over several time constants in the multiexponential fits because of the heterogeneity of the surface environment.87 Similarly, the new charge transfer processes reported in this work should also be interpreted as multiexponential and thus are identified by changes in the multiexponential fits rather than by unique time constants. The fsTA spectra for TiO2|PMI|Al2O3 films with 5, 20, and 30 cycles of ALD display similar spectral features as those without ALD [Figures 4 and S4 (SI)]. The 1*PMI signal decays and the Stark shift at 600 nm rises as electron injection occurs. However, it is apparent that charge injection is slower in the presence of Al2O3, and the Stark shift band is weaker. A comparison of the amplitudes of the 1*PMI and SE signals with respect to the GSB signals at 0.5 and 5.0 ps for the four films and for Al2O3|PMI, where no electron injection occurs, shows close agreement between the spectra for all the films except TiO2|PMI|0ALD [Figures 5A and S5A (SI)]. Thus, while some electron injection occurs during the instrument response time for TiO2|PMI|0ALD, much less occurs even within 5 ps with an Al2O3 layer. Indeed, the slower rise of the Stark shift at 600 nm, the more rapid decays at 530 and 680 nm, and the first time constants in the global fits show that the injection (τ2) that is outside of the instrument response time occurs about 1.6 times more slowly with ALD than without [Figures 6 and S6−S8 (SI) and Tables 1 and S1 (SI)].69 Between the films with 5, 20, and 30 cycles of ALD, there is much less variation in electron injection rates, which indicates that the first few cycles at the interface have the largest impact on the rate of the process. This decrease in the rate of charge injection in the presence of the ALD layer likely results from passivation of the surface states in TiO2 to make fewer acceptor sites available.31,32,66 Thus, less electron injection occurs overall, and those electrons that do inject are injected deeper into the bulk of TiO2. The resulting charge distribution decreases the local electric field experienced by the PMI molecules and may explain the decrease in intensity of the Stark shift. Films with ALD can therefore be expected to have higher-energy TiO2−-containing states than those shown in Figure 3, even in the presence of electrolyte; however, electron injection still occurs even with ALD and in the absence of electrolyte. After the first few cycles of ALD, the accessible surface states are passivated, so additional ALD cycles do not affect the remaining bulk acceptor sites or slow electron injection rates further. This observation also suggests that the ALD coating is conformal and covers the entire surface of the electrode, as expected on the basis of our previous work.33 A favorable effect of ALD on recombination rates between charges on the catalysts and in TiO2 (vide infra) is expected to more than negate these deleterious effects on injection rates, especially in the presence of the acidic electrolyte under operating conditions. Meyer et al. have recently observed symmetry-breaking charge separation (SB-CS) to form [PDI•−PDI•+] in less than 0.8 ps when PDI is adsorbed onto nanostructured TiO2 electrodes.92 Under these conditions, the TiO2 conduction band is apparently inaccessible for electron injection, so the
into TiO2, there should be a driving force for each subsequent oxidation of either catalyst until turnover occurs. In the absence of aqueous electrolyte or when ALD covers the TiO2 surface, these energy levels will differ, but the estimates are provided here for reference. Transient Absorption Spectroscopy of Films without a Catalyst. Electron injection from PMI into TiO2 and charge recombination processes following excitation of PMI at 495 nm were investigated by fsTA spectroscopy on dry films and compared for films with varied thicknesses of Al2O3. As previously described,69 excitation of PMI in TiO2|PMI displays a ground-state bleach (GSB) at 430−575 nm, a stimulated emission (SE) band at 575 nm, and a singlet excited state absorption (1*PMI) band with a maximum at about 680 nm (Figure 4). There is a small population of PMI that does not
Figure 4. fsTA spectra of TiO2|PMI|0ALD, TiO2|PMI|30ALD, TiO2| PMI|30ALD|IrIr, and TiO2|PMI|30ALD|IrSil.
inject electrons into TiO2 upon photoexcitation (
