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Cite This: ACS Appl. Energy Mater. 2018, 1, 62−67
Photocathode Chromophore−Catalyst Assembly via Layer-By-Layer 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 ‡
S 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-co-thiophene) 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/postpolymerization ion-exchange strategy was used to convert an organic-soluble CPE to the water-soluble poly(isoindigo-co-thiophene). The anionic PiIT polyelectrolyte- and polyacrylate-stabilized Pt-nanoparticles (PAA-Pt) were codeposited 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 toward photoinduced hole transfer from the excited PilT to the IO nITO. 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. KEYWORDS: conjugated polyelectrolyte, Stille polymerization, layer-by-layer polyelectrolyte assembly, solar fuels, hydrogen evolution, photocathode
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structures at a photocathode include covalently linking chromophore and catalyst,4 noncovalent assembly via metal− N interactions,5 polyelectrolyte LbL with the use of a covalently linked Ru chromophore−catalyst polymeric assembly,6,7 and cosensitization by codeposition of chromophore and catalyst.8 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 noncovalent self-assembly of chromophore/catalyst.9,10 CPEs have become important electronic components in organic light-emitting devices11,12 and photo-
he 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 produce hydrogen fuel.1 Alternatively, CO2 can be reduced in the presence of protons to form carbon-based fuels such as syngas or methanol.2 There has been significant development of dye-sensitized photoanodes for water oxidation, leveraging early groundwork on TiO2-based photoanodes used in dye-sensitized solar cells.3 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 © 2018 American Chemical Society
Received: December 4, 2017 Accepted: January 3, 2018 Published: January 3, 2018 62
DOI: 10.1021/acsaem.7b00223 ACS Appl. Energy Mater. 2018, 1, 62−67
Letter
ACS Applied Energy Materials voltaic cells,13,14 where they have been successfully employed as charge injection/extraction interlayers. Moderate charge mobility on the order of 10−4 cm2 V−1 s−1 has also been demonstrated in thin film transistors based on a low band-gap CPE.15 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 polymerization route has been used.16 For example, direct Suzuki polymerization was used to prepare a series of low band-gap CPEs incorporating isoindigo repeat units with pendant zwitterionic sulfobetaine substitutents.17 In addition, Suzuki coupling proceeds efficiently in aqueous conditions allowing for the direct polymerization of water-soluble ionic monomers.18 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. 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 chromophore−catalyst assemblies atop the mesoporous SnO2/TiO2 electrodes for light-driven water oxidation.6 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 composed of polyacrylate-stabilized Pt nanoparticles (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. Polymer synthesis was carried out by direct polymerization of the organic-soluble, tetra-phenylphosphonium sulfonate substituted dibromoisoindigo 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 and the polymer precursor method (Figure 1a).16 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 to the sodium sulfonate salt upon addition of NaI.19 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. For a test of the efficiency of the Stille coupling reaction using iI-Br2-TPPS, a model compound (iI2T, Figure 1a) was prepared by reacting iIBr2-TPPS with 2-(tributylstannyl)thiophene under typical Stille coupling conditions (5 mol % Pd(PPh3)4 at 90 °C in DMF).
Figure 1. (a) Synthetic scheme for PiIT. (b) UV−vis absorption of il2T and PiIT electrolytes acquired in H2O. Inset: solution color of il2T and PiIT.
After overnight reaction, 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 quantitative yield. The 1H and 13C NMR spectra showed that the Stille coupling proceeded cleanly, with 400 nm). A H2 microsensor was utilized to monitor the Faradaic 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 nITO// (PDDA/PilT)10 electrode, Figure S8; however, the amount was lower in accord with the difference in photocurrents seen in Figure 4a.) The relatively low overall photocurrent observed under white light illumination suggests that the quantum efficiency of the photoreduction is low (∼10−3 to 10−4). One reason for the low quantum efficiency is the large size of the Pt nanoparticles which leads 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 65
DOI: 10.1021/acsaem.7b00223 ACS Appl. Energy Mater. 2018, 1, 62−67
Letter
ACS Applied Energy Materials ORCID
smaller Pt nanoparticles (
