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Photocathode Chromophore-Catalyst Assembly via Layer-By-Layer Deposition of a Low Band-Gap Isoindigo Conjugated Polyelectrolyte Gyu Leem, Hayden Thompson Black, Bing Shan, Jose P. Bantang, Thomas J. Meyer, John R. Reynolds, and Kirk S. Schanze ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00223 • Publication Date (Web): 03 Jan 2018 Downloaded from http://pubs.acs.org on January 5, 2018
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ACS Applied Energy Materials
Photocathode Chromophore-Catalyst Assembly via Layer-ByLayer Deposition of a Low Band-Gap Isoindigo Conjugated Polyelectrolyte Gyu Leem,† Hayden T. Black,§ Bing Shan,≠ Jose P. O. Bantang, †,⊥ Thomas J. Meyer,≠ John R. Reynolds,§,* and Kirk S. Schanze†,* †
Department of Chemistry, University of Texas at San Antonio, San Antonio, Texas 78249, United States School of Chemistry and Biochemistry, School of Materials Science and Engineering, Center for Organic Photonics and Electronics, Georgia Tech Polymer Network, Georgia Institute of Technology, Atlanta, Georgia 30332, United States ≠ Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States ⊥Department of Chemistry, De La Salle University, Taft Avenue, Manila 1004, Philippines Supporting Information §
ABSTRACT: Low band-gap conjugated polyelectrolytes (CPEs) can serve as efficient chromophores for use on photoelectrodes for dye-sensitized photoelectrochemical cells. Herein is reported a novel CPE based on poly(isoindigo-cothiophene) with pendant sodium butylsulfonate groups (PiIT) and its use in construction of layer-by-layer (LbL) + chromophore-catalyst assemblies with a Pt-based H reduction catalyst (PAA-Pt) for water reduction. A novel Stille polymerization/ post-polymerization ion-exchange strategy was used to convert an organic soluble CPE to the watersoluble poly(isoindigo-co-thiophene). The anionic PiIT polyelectrolyte and polyacrylate stabilized Pt-nanoparticles (PAA-Pt) were co-deposited with cationic poly(diallyldimethylammonium) chloride (PDDA) onto inverse opal (IO), nanostructured indium tin oxide film (nITO) (IO nITO) atop fluorine doped tin oxide (FTO) by using LbL self-assembly. To evaluate the performance of novel conjugated PiIT//PAA-Pt chromphore-catalyst assemblies, interassembly hole transfer was investigated by photocurrent density measurements on FTO//IO nITO electrodes. Enhanced cathodic photocurrent is observed for the polychromophore-catalyst assemblies, compared to electrodes modified with only PiIT, pointing towards photoinduced hole transfer from the excited PilT to the IO ITO. Prolonged photoelectrolysis experiments reveal H2 production with a Faradaic yield of approximately 45%. This work provides new routes to carry out visible-light-driven water reduction using photocathode assemblies based on low band-gap CPEs.
The intermittent nature of solar energy creates challenges for the efficient distribution of photovoltaic power, making energy storage an essential component of large scale solar technologies. Artificial photosynthesis provides a route for solar energy storage via direct conversion of sunlight into chemical fuels via photoelectrochemical water splitting. In this process water is oxidized in one half-cell to produce O2 + and hydronium (H3O ) is reduced at the other half-cell to 1 produce hydrogen fuel. Alternatively, CO2 can be reduced in the presence of protons to form carbon based fuels such as 2 syngas or methanol.
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sensitized solar cells. The development of interfacial electron transfer at the photocathode represents an important challenge to artificial photosynthesis, and has become a topic of growing interest. Recent strategies for assembly of chromophore/catalyst structures at a photocathode include cova4 lently linking chromophore and catalyst, non-covalent as5 sembly via metal-N interactions, polyelectrolyte LbL with the use of a covalently linked Ru chromophore-catalyst pol6,7 ymeric assembly, and co-sensitization by co-deposition of 8 chromophore and catalyst. Self-assembly of chromophore/catalyst systems for efficient light-driven H2 production on metal oxide photocathodes remains as a challenge. π-Conjugated polyelectrolytes (CPEs) are an interesting class of chromophores with potential in dye-sensitized photocathodes/photoanodes, where the ionic side chains of CPEs could be utilized for non-covalent self-assembly of chromo9,10 phore/catalyst. CPEs have become important electronic 11,12 components in organic light-emitting devices and photo13,14 voltaic cells (OPVs), where they have been successfully employed as charge injection/extraction interlayers. Moder-4 2 -1 -1 ate charge mobility on the order of 10 cm V s has also been demonstrated in thin film transistors based on a low band15 gap CPE. These recent advancements have sparked a growing interest in low band-gap CPEs and encourage further developments in the synthesis of donor-acceptor CPEs. The synthesis of low band-gap donor-acceptor CPEs has exclusively relied on Suzuki polymerization, regardless of whether a polymer precursor route or a direct polymeriza16 tion route has been used. For example, direct Suzuki polymerization was used to prepare a series of low band-gap CPEs incorporating isoindigo repeat units with pendant zwit17 terionic sulfobetaine substitutents. In addition, Suzuki coupling proceeds efficiently in aqueous conditions allowing for 18 the direct polymerization of water soluble ionic monomers. On the other hand, Stille coupling is a ubiquitous technique within the field of low band-gap conjugated polymers but until now Stille coupling has not been utilized for the preparation of CPEs.
There has been significant development of dyesensitized photoanodes for water oxidation, leveraging early groundwork on TiO2 based photoanodes used in dye-
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O S
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Figure 1. (a) Synthetic scheme for PiIT. (b) UV-visible absorption of il2T and PiIT electrolytes acquired in H2O. Inset: solution color of il2T and PiIT. Here we report the synthesis of a low band-gap poly(isoindigo-co-thiophene) with pendant sodium butylsulfonate groups (PiIT) via Stille coupling polymerization. The synthesis is unique in that it involves direct polymerization of an anionic monomer in which the ionic groups are + “protected” as PPh4 salts, which render the monomer and resulting polymer soluble in organic solution. Subsequent + ion exchange to the Na form releases the water soluble CPE. The anionic PiIT CPE enables LbL deposition of thin films of the poly(chromophore) on a metal oxide surface to afford photocathodes with controlled structure. Previously, we reported the use of photoanodes containing LbL chromophorecatalyst assemblies atop the mesoporous SnO2/TiO2 elec6 trodes for light-driven water oxidation. Building on this earlier success at a photoanode, here we demonstrate the + LbL self-assembly of a chromophore-catalyst assembly for H reduction at a photocathode. The assembly uses the light harvesting chromophore, PiIT and a proton reduction catalyst comprised of polyacrylate stabilized Pt nanoparaticles (PAA-Pt). Photocathodes were assembled using the LbL process on an inverse opal, nanostructured indium tin oxide film (IO nITO) atop fluorine doped tin oxide (FTO) surface.
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Polymer synthesis was carried out by direct polymerization of the organic soluble, tetra-phenylphosphonium sulfonate substituted dibromoiso-indigo monomer in DMF solution. The resulting conjugated polymer was converted to a water-soluble form by ion exchange. This process can be considered as a hybrid of the direct polymerization method 16 and the polymer precursor method (Figure 1a). The tetraphenylphosphonium counterion was chosen as an ideal precursor to the sodium sulfonate functionalized CPE since it is a bulky organic counterion that was previously used to solubilize a sulfonate functionalized quinacridone in organic solvents, and was shown to undergo quantitative conversion 19 to the sodium sulfonate salt upon addition of NaI. The tetra-phenylphosphonium sulfonate (TPPS) functionalized dibromoisoindigo monomer (iI-Br2-TPPS) was synthesized via deprotonation of dibromoisoindigo, followed by ring opening of 1,4-butane sultone; it was subsequently purified by reverse phase HPLC. In order to test the efficiency of the Stille coupling reaction using iI-Br2-TPPS, a model compound (iI2T, Figure 1a) was prepared by reacting iI-Br2-TPPS with 2-(tributylstannyl)thiophene under typical Stille coupling conditions (5 mol% Pd(PPh3)4 at 90 °C in DMF). After reacting overnight, the product was directly converted to the sodium sulfonate derivative by addition of the crude reaction mixture to a solution of NaI in acetone, which precipitated the pure iI2T product as a red/black solid in almost quantita1 13 tive yield. The H and C NMR spectra showed that the Stille coupling proceeded cleanly, with < 3% side products identi1 fied by H NMR shown Figure S1. The synthesis of the TPPS substituted polymer (PiITTPPS) was carried out under the same conditions as that for the model compound except that the reaction was allowed to proceed for 3 days. The reaction mixture was precipitated into ethyl acetate to isolate the organic soluble precursor 1 31 polymer, which was characterized by H and P NMR in 1 DMSO and by GPC in DMF (Figure S2). The H NMR spectrum revealed the expected aromatic signals arising from the polymer backbone and tetra-phenylphosphonium side chains, as well as some solvent impurities and a peak corresponding to a trimethyl tin byproduct. The water soluble sodium sulfonate functionalized polymer PiIT was isolated upon addition of PiIT-TPPS in DMF to a solution of NaI in 1 acetone, which precipitated PiIT as a blue/black solid. The H NMR spectrum of PiIT in D2O showed the complete removal of trimethyl tin byproduct. Trace amounts of acetone and DMF were removed from the PiIT sample by using dialysis. The absorption spectra of iI2T and PiIT were measured in water at room temperature, as presented in Fig. 1b. The absorption of PiIT in aqueous solution features a strong band with a maximum at λ ~ 700 nm and an absorption onset ~ 830 nm. Thermochromism is observed in water, where cooling leads to more structured spectra containing two peaks ~ 700 nm and 750 nm arising from vibrational coupling due to the planarization of the polymer backbone, which is attributed to formation of an aggregated state (Figure S3). This behavior is common for conjugated polyelectrolytes that tend to undergo strong π-π interactions in the aqueous environment. The slight shoulder of the absorption spectrum at 95 °C indicates there is some level of aggregation even at the higher temperatures. This is consistent with the NMR spec-
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trum of PiIT at 90 °C which shows very broad resonances (Fig. S1a). SO3-Na+ n COO- Na
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ing in FTO//IO nITO//(PDDA/PiIT)m multilayers (m = bilayer number). Similarly, PAA-Pt and PDDA were deposited atop FTO//IO nITO// (PDDA/PilT)m films by the LbL process affording chromophore-catalyst photocathode, (FTO//IO nITO//(PDDA/PiIT)m//(PDDA/PAA-Pt)n).
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Figure 2. (a) Molecular structures of Pilt, poly(diallyldimethylammonium chloride) (PDDA), and polyacrylate stabilized Pt nanoparticles (PAA-Pt). (b) Schematic illustration of fabrication of FTO//IOnITO//(PDDA/PilT)10//(PDDA/PAA-Pt)10. To enable application of polychromophore-catalyst assembly at a photocathode, LbL multilayer films were prepared and deposited onto semiconductor electrodes for lightdriven proton reduction. As illustrated schematically in Figure 2, polychromophore-catalyst assemblies were prepared by the LbL process to construct multilayers of light harvesting PiIT chromophore/proton reduction catalyst on IO nITO modified FTO electrodes (FTO//IO nITO) (See Supporting Information for details). IO nITO was selected as the electrode material because the use of photocathodes consisting of IO nITO films promotes high surface coverage of chromophore-catalyst assembly, resulting in easily measurable pho20,21 tocurrent. The polychromophore-catalyst assemblies utilize PiIT as a light harvesting CPE and the PAA-Pt proton reduction catalyst with PDDA as an oppositely charged polyelectrolyte (structures of PiIT, PAA-Pt and PDDA are shown in Figure 2a). Previously, colloidal PAA-Pt nanoparticles in aqueous solution were studied in the presence of colloidal 22 quantum dots for hydrogen photosynthesis. On the basis of these prior successful studies, the PAA-Pt reduction catalyst was used in the present system. The particles were prepared as described in the SI. The size and morphology of the PAA-stabilized Pt nanoparticles were characterized by transmission electron microscopy (TEM, Figure S4) which revealed that the PAA-Pt nanoparticles were irregularly shaped with an average diameter of 10 nm. Films of the PiIT polyanion and PDDA polycation (PiIT/PDDA) were prepared by alternately immersing an FTO//IO nITO substrate into aqueous solutions of 0.1 mM PiIT and PDDA with intermediate washing in water, result-
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Figure 3. (a) UV−visible absorption of PiIT chromophore only films, FTO//IO nITO//(PDDA/PiIT)m (m=1 (black line), 5 (red line), 10 (green line), 20 (blue)) and (b) PAA-Pt catalyst only films (black line), PilT only films (blue line) and PilT/PAA-Pt chromophore-catalyst assembly films (red line). (c) SEM images of unmodified films of macro-mesoporous IO nITO (left), PilT films (center), and PilT/PAA-Pt chromophore-catalyst assembly films (right). Inset: PAA-Pt nanoparticles with the size of approximately 10 nm (red dot circle) onto PilT films (SEM images are same films used in part b). UV-visible absorption and cyclic voltammetry (CV) were used to characterize the light harvesting and electrochemical characteristics of the (PDDA/PiIT)m multilayer films. Absorption was carried out on FTO//IO nITO substrates, and glassy carbon substrates were used for CV. Figure 3a shows that the visible absorption of PiIT increases monotonically as the number of bilayers increases, i.e., (PDDA/PilT)m where m = 1, 5, 10, and 20. This behavior is consistent many previ14,23,24 ous studies on polyelectrolyte LbL assemblies. Additionally, a blue shift of ~ 50 nm was observed in the absorption maximum of the multilayer films (λmax ~ 650 nm) compared to that of the aqueous PiIT solution ( λmax ~ 700 nm).
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Next, we turn to the characterization and response of the full photocathode assemblies, consisting of polychromophore and catalyst layers atop the mesoporous ITO substrates. Figure 3b compares visible absorption spectra for the films containing PAA-Pt, PiIT, and both PiIT and PAA-Pt units. The spectrum of the multilayer PDDA/PAA-Pt film is nearly the same as that for the colloidal PAA-Pt nanoparti22 cles in aqueous solution. After deposition of PAA-Pt atop PiIT coated-IO nITO films, the visible spectrum of the film containing PiIT is similar to that containing both PiIT and PAA-Pt units. To confirm the presence of PiIT polyelectrolyte and PAA-Pt nanoparticles on the FTO//IO nITO, the films were characterized by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX). The SEM images in Figure 3c show that the presence of (PDDA/PiIT) multilayers affords a smoother morphology on the IO nITO compared to the uncoated IO nITO. Moreover, the SEM image of the chromophore-catalyst coated substrates verify the existence of the PAA-Pt nanoparticles on the surface (Figure 3c, right). As shown in Figure S6a-c, the film surfaces were analyzed for the S, Pt, C elements by EDX. The S species from the sulfonate and thiophene units in PiIT were detected by EDX for both FTO//IO nITO//(PDDA/PiIT)10 and FTO//IO nITO//(PDDA/PiIT)10//(PDDA/PAA-Pt)10 films. As shown in Figure S6c, EDX of the FTO//IO nITO//(PDDA/PiIT)10//(PDDA/PAA-Pt)10 film verifies the presence of both PiIT and Pt on the IO nITO. Figure 4 shows photocurrent-time traces at FTO//IO nITO//(PDDA/PiIT)10 and FTO//IO ITO//(PDDA/PiIT)10//(PDDA/PAA-Pt)10 electrodes. Measurements were carried out with substrates immersed in 0.1 M acetate buffer with 0.4 M NaClO4 supporting electrolyte. As shown by Figure 4a, during 300 s irradiation periods, enhanced photocurrent is observed for FTO//IO nITO//(PDDA/PiIT)10//(PDDA/PAA-Pt)10 electrodes compared to an FTO//IO ITO//(PDDA/PilT)10 electrode (the latter lacks the PAA-Pt catalyst). The photocurrent measurements confirm that electron transfer from PiIT to PAA-Pt is occurring along with activation of the PAA-Pt proton reduction catalysts in the LbL multilayer assembly. Photocatalytic H2 production was confirmed using FTO//IO nITO//(PDDA/PiIT)10//(PDDA/PAA-Pt)10 photocathode electrode in pH 4.5 acetate buffer with NaClO4 under visiblelight irradiation and mild pH condition in Figure 4b. The photocurrent density for the photocathode was also carried out under continuous illumination for 60 min. (Figure S7, −2 Eappl = - 0.4 V, 100 mW-cm , λ > 400 nm). A H2 microsensor
was utilized to monitor the Faradic efficiency of the photocathode electrode for the production of H2 during prolonged illumination. Performing this measurement revealed 0.11 µmol of H2, which was produced with 0.047 C of cathodic charge, corresponding to ~45 % Faradaic efficiency. (Note that H2 was also detected by illumination of an FTO//IO ITO//(PDDA/PilT)10 electrode, Figure S8; however, the amount was lower in accord with the difference in photocurrents seen in Figure 4a).
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The blue shift is probably due to reduced planarity and disorder of the PiIT backbone in the LbL films resulting from disorder induced by interactions with the cationic PDDA. Figure S5 shows the cyclic voltammetry of PiIT deposited on a glassy carbon electrode substrate, i.e., GC//(PDDA/PilT)10. The LbL film modified electrode was immersed in a 0.1 M TBAPF6 acetonitrile solution under an inert atmosphere and probed with a scan rate of 100 mV/s. The reductive CV of the films shows a single reversible wave with cathodic and anodic peaks at -1.24 and -1.03 V, respectively, characteristic of isoindigo reduction. The half-wave potential at E1/2 ~ -1.13 V vs Ag/AgCl, and the peak-to-peak potential difference was 210 mV.
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Figure 4. (a) Current–time traces with illumination on FTO//IO nITO//(PDDA/PiIT)10 (black), and FTO//IO nITO//(PDDA/PiIT)10//(PDDA/PAA-Pt)10 (red) in 0.1 M acetate buffer, 0.4 M NaClO4 at pH 4.5 with an applied bias of -0.4 V versus Ag/AgCl. Light on-off cycles of 360 s were used. (b) H2 evolution from an FTO//IO nITO//(PDDA/PiIT)10//(PDDA/PAA-Pt)10 film in pH 4.5, 0.1 M acetate buffer with 0.4 M NaClO4 supporting electrolyte at an applied bias of -0.4 V vs. Ag/AgCl. The films were -2 irradiated by an AM 1.5 solar simulator (1 sun, 100 mW cm ; 400 nm cut-off filter). (c) Proposed mechanism for charge generation/separation in PiIT/PAA-Pt films.
The relatively low overall photocurrent observed under white light illumination suggests that the quantum efficiency of the photoreduction is low (~10-3 - 10-4). One reason for the low quantum efficiency is the large size of the Pt
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ACS Applied Energy Materials nanoparticles which lead to a comparatively low surface area for catalysis. The shape and size of Pt nanoparticles play an important role in hydrogen production.25 Previously, Bhoware et. al reported that smaller Pt nanoparticles (< 5 nm) increased the photocatalytic activity compared to larger sized (10 nm) Pt nanoparticles.26 A key question concerns the mechanism of charge separation in the multilayer assemblies. Clearly the primary step is photoexcitation of the PiIT chromophore; but the subsequent step whereby charge separation occurs is not clear. As shown in the diagram in Figure 4c, one possibility is that charge separation occurs within the PiIT multilayer assembly. Specifically, 1PiIT* could directly undergo charge separation, producing the separated ion pairs, (PiIT+.) and (PiIT-.). The resulting cations (PiIT+.) diffuse to and react with the IO nITO electrode, while the anions (PiIT-.) transfer charge to the PAA-Pt catalyst layer where H2 production occurs. Previous work on CPE films of donor-acceptor type polymers suggests that charge separation can occur rapidly following photoexcitation.27 In these systems, one polymer chain acts as a donor, while the other acts as acceptor; this donor-acceptor interaction facilitates dissociation of the singlet exciton into an ion pair state.27 We also note that if the proposed mechanism is correct, it provides an additional reason for the relatively low photocurrent. Specifically, since ITO is highly doped, and therefore a conductor, it is possible that (PiIT-.) anions that diffuse to the electrode could be oxidized because it is biased at -0.4 V, which is positive of the PiIT(0/-) potential (-1.13 V). Future experiments using a p-type semiconductor electrode such as macro-mesoporous NiO may address this point. In summary, a new type of donor-acceptor conjugated polyelectrolyte featuring anionic sulfonate side groups has been prepared via a unique method employing Stille coupling polymerization followed by post-polymerization ionexchange. The post-polymerization ion-exchange route allows improved characterization of the organic-soluble polyelectrolyte precursor and facile conversion to the watersoluble form while avoiding hydrolysis of the backbone. The LbL polyelectrolyte self-assembly approach has been used to assemble PiIT and PAA-Pt along with oppositely charged PDDA onto FTO//IO nITO electrodes. The polychromophore-catalyst modified PilT/PAA-Pt photocathode showed enhanced photocurrent relative to the PilT only modified photocathode. Our concept utilizes a low band gap CPE deposited onto a macro-mesoporous (IO nITO) electrode and demonstrates a novel method for developing unique chromophore/catalyst assemblies for light-driven hydrogen evolution at a photocathode in the photoelectrochemical systems (e.g. DSPECs). Future work will explore controlling the morphology of the PAA-Pt and optimizing the LbL assembly process to improve the photoelectrocatalytic properties of polymeric chromophore-catalyst assemblies for hydrogen evolution. ASSOCIATED CONTENT
Materials and methods, experimental description, additional data analysis. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION
Corresponding Author *E-mail:
[email protected] [email protected];
Author Contributions Gyu Leem and Hayden Black contributed equally to the experimental work. Gyu Leem led the writing of the manuscript. Bing Shen, Soojin Kim and Jose Bantang carried out experimental work. Thomas Meyer, John Reynolds and Kirk Schanze directed the research and contributed to the writing of the manuscript.
Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This material is based on work supported as part of the UNC EFRC: Center for Solar Fuels, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences under Award Number DE-SC0001011. We also acknowledge the Welch Foundation (Grant No. AX-0045-20110629), USAID, and the University of Texas at San Antonio. We thank Josefina ArellanoJimenez in the Kleberg Advanced Microscopy Center Interdisciplinary at UTSA for help with the SEM and EDX. We acknowledge Soojin Kim (Ewha Womans University) for her assistance with the H2 evolution measurements.
REFERENCES (1) Brennaman, M. K.; Dillon, R. J.; Alibabaei, L.; Gish, M. K.; Dares, C. J.; Ashford, D. L.; House, R. L.; Meyer, G. J.; Papanikolas, J. M.; Meyer, T. J. Finding the Way to Solar Fuels with Dye-Sensitized Photoelectrosynthesis Cells. J. Am. Chem. Soc. 2016, 138, 13085-13102. (2) Álvarez, A.; Bansode, A.; Urakawa, A.; Bavykina, A. V.; Wezendonk, T. A.; Makkee, M.; Gascon, J.; Kapteijn, F. Challenges in the Greener Production of Formates/Formic Acid, Methanol, and DME by Heterogeneously Catalyzed CO2 Hydrogenation Processes. Chem. Rev. 2017, 117, 9804-9838. (3) McCool, N. S.; Swierk, J. R.; Nemes, C. T.; Schmuttenmaer, C. A.; Mallouk, T. E. Dynamics of Electron Injection in SnO2/TiO2 Core/Shell Electrodes for Water-Splitting Dye-Sensitized Photoelectrochemical Cells. J. Phys. Chem. Lett. 2016, 7, 2930-2934. (4) Kaeffer, N.; Massin, J.; Lebrun, C.; Renault, O.; Chavarot-Kerlidou, M.; Artero, V. Covalent Design for Dye-Sensitized H2-Evolving Photocathodes Based on a Cobalt Diimine–Dioxime Catalyst. J. Am. Chem. Soc. 2016, 138, 12308-12311. (5) Ji, Z.; He, M.; Huang, Z.; Ozkan, U.; Wu, Y. Photostable p-Type DyeSensitized Photoelectrochemical Cells for Water Reduction. J. Am. Chem. Soc. 2013, 135, 11696-11699. (6) Leem, G.; Sherman, B. D.; Burnett, A. J.; Morseth, Z. A.; Wee, K.-R.; Papanikolas, J. M.; Meyer, T. J.; Schanze, K. S. Light-Driven Water Oxidation Using Polyelectrolyte Layer-by-Layer Chromophore–Catalyst Assemblies. ACS Energy Lett. 2016, 1, 339-343. (7) Jiang, J.; Sherman, B. D.; Zhao, Y.; He, R.; Ghiviriga, I.; Alibabaei, L.; Meyer, T. J.; Leem, G.; Schanze, K. S. Polymer Chromophore-Catalyst Assembly for Solar Fuel Generation. ACS Appl. Mater. Interfaces 2017, 9, 19529-19534.
Supporting Information
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(8) Brown, A. M.; Antila, L. J.; Mirmohades, M.; Pullen, S.; Ott, S.; Hammarström, L. Ultrafast Electron Transfer between Dye and Catalyst on a Mesoporous NiO Surface. J. Am. Chem. Soc. 2016, 138, 8060-8063. (9) Taranekar, P.; Qiao, Q.; Jiang, H.; Ghiviriga, I.; Schanze, K. S.; Reynolds, J. R. Hyperbranched Conjugated Polyelectrolyte Bilayers for Solar-Cell Applications. J. Am. Chem. Soc. 2007, 129, 8958-8959. (10) Mwaura, J. K.; Zhao, X.; Jiang, H.; Schanze, K. S.; Reynolds, J. R. Spectral Broadening in Nanocrystalline TiO2 Solar Cells Based on Poly(pphenylene ethynylene) and Polythiophene Sensitizers. Chem. Mater. 2006, 18, 6109-6111. (11) Huang, F.; Wu, H.; Wang, D.; Yang, W.; Cao, Y. Novel Electroluminescent Conjugated Polyelectrolytes Based on Polyfluorene. Chem. Mater. 2004, 16, 708-716. (12) Cutler, C. A.; Bouguettaya, M.; Kang, T.-S.; Reynolds, J. R. Alkoxysulfonate-Functionalized PEDOT Polyelectrolyte Multilayer Films: Electrochromic and Hole Transport Materials. Macromolecules 2005, 38, 3068-3074. (13) Duan, C.; Zhang, K.; Zhong, C.; Huang, F.; Cao, Y. Recent Advances in Water/Alcohol-Soluble π-Conjugated Materials: New Materials and Growing Applications in Solar Cells. Chem. Soc. Rev. 2013, 42, 90719104. (14) Page, Z. A.; Liu, F.; Russell, T. P.; Emrick, T. Tuning the Energy Gap of Conjugated Polymer Zwitterions for Efficient Interlayers and Solar Cells. J. Polym. Sci. A 2015, 53, 327-336. (15) Henson, Z. B.; Zhang, Y.; Nguyen, T.-Q.; Seo, J. H.; Bazan, G. C. Synthesis and Properties of Two Cationic Narrow Band Gap Conjugated Polyelectrolytes. J. Am. Chem. Soc. 2013, 135, 4163-4166. (16) Pinto, M. R.; Schanze, K. S. Conjugated Polyelectrolytes: Synthesis and Applications. Synthesis 2002, 2002, 1293-1309. (17) Liu, Y.; Page, Z. A.; Russell, T. P.; Emrick, T. Finely Tuned Polymer Interlayers Enhance Solar Cell Efficiency. Angew. Chem., Int. Ed. 2015, 54, 11485-11489. (18) Child, A. D.; Reynolds, J. R. Water-Soluble Rigid-Rod Polyelectrolytes: A New Self-Doped, Electroactive SulfonatoalkoxySubstituted Poly(p-phenylene). Macromolecules 1994, 27, 1975-1977. (19) Pho, T. V.; Zalar, P.; Garcia, A.; Nguyen, T.-Q.; Wudl, F. Electron Injection Barrier Reduction for Organic Light-Emitting Devices by Quinacridone Derivatives. Chem. Commun. 2010, 46, 8210-8212. (20) Shan, B.; Das, A. K.; Marquard, S.; Farnum, B. H.; Wang, D.; Bullock, R. M.; Meyer, T. J. Photogeneration of Hydrogen from Water by a Robust Dye-Sensitized Photocathode. Energy Environ. Sci. 2016, 9, 3693-3697. (21) Preliminary experiments attempts were caried out to deposit PDDA/PilT multilayer films on mesoporous NiO films. However, the surface coverage was nearly three-fold less than could be achieved with IO nITO films. The difference is believed to be due to the inability of the polymers to access nanopores in the NiO films. The IO nITO provides an effectively larger surface area that is accessible to the polymers. (22) Li, X.-B.; Gao, Y.-J.; Wang, Y.; Zhan, F.; Zhang, X.-Y.; Kong, Q.Y.; Zhao, N.-J.; Guo, Q.; Wu, H.-L.; Li, Z.-J.; Tao, Y.; Zhang, J.-P.; Chen, B.; Tung, C.-H.; Wu, L.-Z. Self-Assembled Framework Enhances Electronic Communication of Ultrasmall-Sized Nanoparticles for Exceptional Solar Hydrogen Evolution. J. Am. Chem. Soc. 2017, 139, 4789-4796. (23) Decher, G. Polyelectrolyte Multilayers, an Overview. In Multilayer Thin Films; Wiley-VCH Verlag GmbH & Co. KGaA: 2003, 1-46. (24) Schlenoff, J. B.; Dubas, S. T. Mechanism of Polyelectrolyte Multilayer Growth: Charge Overcompensation and Distribution. Macromolecules 2001, 34, 592-598. (25) Luo, M.; Yao, W.; Huang, C.; Wu, Q.; Xu, Q. Shape Effects of Pt Nanoparticles on Hydrogen Production via Pt/CdS Photocatalysts under Visible Light. J. Mater. Chem. A 2015, 3, 13884-13891. (26) Bhoware, S. S.; Kim, K. Y.; Kim, J. A.; Wu, Q.; Kim, J. Photocatalytic Activity of Pt Nanoparticles for Visible Light-Driven Production of NADH. J. Phys. Chem. C 2011, 115, 2553-2557. (27) Hodgkiss, J. M.; Tu, G.; Albert-Seifried, S.; Huck, W. T. S.; Friend, R. H. Ion-Induced Formation of Charge-Transfer States in Conjugated Polyelectrolytes. J. Am. Chem. Soc. 2009, 131, 8913-8921.
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