Mar 12, 2018 - ABSTRACT: Artificial photosynthesis is a chemical process that aims to capture energy ... KEYWORDS: artificial photosynthesis, hydrogen evolution, conjugated polymer, photosensitizer, photocatalysis ..... in the paper (PDF).
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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 10828−10834
Photocatalytic Hydrogen Production with Conjugated Polymers as Photosensitizers Wen-Wen Yong,† Huan Lu,‡ Han Li,† Shu Wang,*,‡ and Ming-Tian Zhang*,† †
Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University, Beijing 100084, China Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
‡
S Supporting Information *
ABSTRACT: Artificial photosynthesis is a chemical process that aims to capture energy from sunlight to produce solar fuels. Light absorption by a robust and efficient photosensitizer is one of the key steps in solar energy conversion. However, common photosensitizers, including [Ru(bpy)3]2+ (RuP), remain far from the ideal. In this work, we exploited the performance of conjugated polymers (CPs) as photosensitizers in photodriven hydrogen evolution in aqueous solution (pH 6). Interestingly, CPs, such as poly(fluorene-co-phenylene) derivative (429 mmolH2·gCP−1·h−1), exhibit steady and high reactivity toward hydrogen evolution; this performance can rival that of a phosphonated RuP under the same conditions, indicating that CPs are promising metalfree photosensitizers for future applications in photocatalysis. KEYWORDS: artificial photosynthesis, hydrogen evolution, conjugated polymer, photosensitizer, photocatalysis
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INTRODUCTION Solar energy is a promising carbon-neutral energy alternative to fossil fuels to meet the rising global energy demand.1 An artificial photosynthetic system aims to capture and then to convert solar energy into the chemical bonds of what are known as solar fuels, such as hydrogen, methane, or methanol.2,3 The primary steps of artificial photosynthesis involve the absorption of sunlight and its conversion into charge-separated states (or electron/hole pairs).4,5 To this extent, the first requirement for solar energy capture is that the light absorber, also referred as a photosensitizer, be both effective and robust.6 Despite the light-driven hydrogen evolution has been extensively investigated, the available selection of photosensitizers is small and typically limited to nanoparticles,7−14 organic dyes,15,16 and [Ru(bpy)3]2+ (RuP) derivatives.17−20 Moreover, none of these available photosensitizers fully meet the current demands because of their intrinsic shortcomings such as high cost when containing precious metals,19 toxicity,21 or poor aqueous solubility and dispersibility.22 Consequently, the development of new photosensitizers that can meet these demands is critical in building both effective and robust artificial solar energy conversion systems. Organic conjugated polymers (CPs) offer myriad opportunities to couple efficient light harvesting, as well as effective hole and electron transfer, into energy conversion.23,24 A key advantage of CP-based sensitizers over small-molecule organic dyes is the potential of CPs to exhibit collective properties toward energy conversion. In particular, CPs’ electrical © 2018 American Chemical Society
conductivity and rate of energy migration provide amplified feasibility for their application in artificial photosynthesis. Because of their excellent photonic and electronic properties, CPs have been extensively used in organic light-emitting diodes,25,26 field-effect transistors,27 sensors,28 and photovoltaic cells29,30 but their application in artificial photosynthesis31,32 requires in-depth investigation. Herein, we establish a lightdriven hydrogen production system, in which CPs as excellent photosensitizers in combination with a molecular catalyst. Five CPs (Scheme 1) were investigated,33 and these CPs produced reducing equivalents under irradiation in a homogeneous photocatalytic system with both a water-soluble DuBois-type NiP catalyst19,34,35 and a sacrificial electron donor [ethylenediaminetetraacetic acid (EDTA)] (Scheme 1). Poly(fluorene-co-phenylene) (PFP) exhibits an activity of 429 mmolH2·gCP−1·h−1 and a “per Ni catalyst” turnover frequency (TOF) of 215 h−1. This performance rivals that of RuP under the same conditions,19 indicating that CPs are promising photosensitizer for future applications in artificial photosynthesis.
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RESULTS AND DISCUSSION Structure and Photophysical Properties of CPs. Five CPs (Scheme 1) containing fluorene or thiophene core structures were selected because of their excellent performance Received: December 12, 2017 Accepted: March 12, 2018 Published: March 12, 2018 10828
DOI: 10.1021/acsami.7b18917 ACS Appl. Mater. Interfaces 2018, 10, 10828−10834
Research Article
ACS Applied Materials & Interfaces
triplet states displayed a long lifetime on the microsecond timescale, from 20 to 125 μs, measured from laser flash spectroscopy (Table 1). According to the information discussed above, these CPs feature an excited-state lifetime sufficient for efficient photoinduced charge separation, which is the first step for photocatalytic reactions such as light-driven hydrogen evolution in aqueous solution. Photoinduced Charge Separation between the Donor and Acceptor Using CPs as Photosensitizers. A photosensitizer is a key part of artificial synthesis; its main role is to absorb a broad range of solar light and to convert solar energy into charge separation states, which will initiate the following chemical process. The ability to capture photons and transduce them into electron flows was thus first evaluated by a threecomponent system in which methyl viologen [MV2+ , E(MV2+/•+) = −0.42 V vs normal hydrogen electrode (NHE)] was selected as an electron acceptor44,45 and EDTA was used as the sacrificial electron donor (Figure 1a).22,46 A three-component aqueous sacrificial electron donor buffer solution (pH = 6) including PFP (0.01 mM), EDTA (0.1 M), and methyl viologen (0.2 mM) was irradiated at 100 mW/ cm2 under argon atmosphere, and the solution was monitored by UV−vis spectroscopy. After several seconds of irradiation, the absorption corresponding to the reduced viologen species (MV•+) at both λ = 395 and 603 nm gradually increased and the solution turned blue, which accords with the color of the MV•+ solution (Figures 1 and S23−25).22,45 Similarly, for the other CPs listed in Scheme 1(PPV, PT1, PT2, and PT3), threecomponent irradiation experiments showed that the absorptions at λ = 395 and 603 nm corresponding to MV•+ also increased and the solution changed from yellow to green (Figure S4). This indicates that the investigated CPs satisfy at least the basic photosensitizer requirement of transporting electrons from donors to acceptors with the help of captured photons. Photoexcitation-formed 3CP* can be quenched via either a reductive quenching pathway or an oxidative quenching pathway, as shown in Figure S3. In the reductive quenching pathway, 3CP* is first reduced to CP•− by an electron donor reagent and then, the catalyst is reduced to complete the catalytic cycle; in the oxidative quenching pathway, 3CP* is first oxidized to CP•+ by the catalyst and then reduced by the electron donor reagent. The CP quenching pathway was identified by controlled experiments under irradiation. Under the same irradiation conditions, the UV−vis spectra of CP solutions with MV2+ displayed that the added MV2+ does not
Scheme 1. Structures of Photosensitizers Investigated in This Work and Previous Work
in organic photovoltaic cells.30 Charged side chains, such as cationic quaternary ammonium groups and anionic carboxyl groups, were introduced to improve the solubility of these CPs in aqueous solution according to literature methods. They were synthesized and characterized as reported: PFP derivative,36,37 poly(phenylene vinylene) (PPV) derivative,38 and polythiophene derivatives (PT1,39 PT2,40 and PT341). These watersoluble CPs have been widely used in imaging, diagnosis, and therapy because of their unique light-harvesting ability.42,43 Their electronic absorption and fluorescence spectra were recorded (Table 1) and are listed in the Supporting Information. These selected CPs displayed a wide absorption band ranging from 380 to 450 nm that can be assigned to the π−π* transition of the conjugated backbone. PFP exhibits a high molar absorption coefficient (ε) over 50 000 M−1 cm−1, indicating its excellent light-absorption ability. The lifetime of a singlet state was measured by time-correlated single photon counting, and the singlet states of all of the CPs have short lifetimes less than 1 ns (Table S2). These singlet states could convert quickly to the triplet state via intersystem crossing. All
Table 1. Light-Driven Hydrogen Evolution Using CPs as Photosensitizers CP
λmax (ε)a/nm (M−1 cm−1)
PFP PPV PT1 PT2 PT3
380 449 400 404 435
(52 609) (19 385) (5078) (4462) (6255)
E(CP•+/0)/V
E(CP0/•−)/V
λabs‑fluod /nm
E00d /V
E(CP•+/*)e /V
E(CP*/•−)e /V
τ(triplet)f /μs
H2 evolution activityg/mmolH2·gCP−1·h−1
0.99b 0.75b 0.94c 1.26c 0.93c
−2b −1.41b c
