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Energy, Environmental, and Catalysis Applications
Photocatalytic Hydrogen Production with Conjugated Polymers as Photosensitizers Wen-Wen Yong, Huan Lu, Han Li, Shu Wang, and Ming-Tian Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18917 • Publication Date (Web): 12 Mar 2018 Downloaded from http://pubs.acs.org on March 14, 2018
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ACS Applied Materials & Interfaces
Photocatalytic Hydrogen Production with Conjugated Polymers as Photosensitizers Wen-Wen Yonga, Huan Lub, Han Lia, Shu Wang*b and Ming-Tian Zhang*a a
Center of Basic Molecular Science (CBMS), Department of Chemistry, Tsinghua University,
Beijing, 100084, China b
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids,
Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China Keywords: artificial photosynthehsis; hydrogen evolution; conjugated polymer; photosensitizer; photocatalysis
ABSTRACT: Artificial photosynthesis is a chemical process that seeks 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+, 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 (PFP, 429 mmolH2•gCP-1•h-1), exhibit steady and high reactivity towards hydrogen evolution; this performance can rival that of a phosphonated [Ru(bpy)3]2+ (RuP) under the same conditions, indicating that CPs are promising metal-free photosensitizers for future application in photocatalysis.
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Introduction Solar energy is a promising carbon-neutral energy alternative to fossil fuels to meet rising global energy demand.1 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 photosensitizer, be both effective and robust.6 Despite light-driven hydrogen evolution having been extensively investigated, the available selection of photosensitizers is small and typically limited to nanoparticles,7-14 organic dyes15,16 and [Ru(bpy)3]2+ derivatives.17-20 Moreover, none of these available photosensitizers fully meet current demands, due to 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 conductivity and rate of energy migration provide amplified feasibility for its application in artificial photosynthesis. Due to their excellent photonic and electronic properties, CPs have been extensively used in organic LEDs,25,26 field-effect transistors,27 sensors28 and photovoltaic cells29,30, but their application in artificial photosynthesis31,32 requires in-depth investigation.
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ACS Applied Materials & Interfaces
Herein, we establish a light-driven 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 (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 [Ru(bpy)3]2+ under the same conditions,19 indicating that CPs are promising photosensitizer for future applications in artificial photosynthesis. Scheme 1. Structures of photosensitizer investigated in this work and previous work CPs: O N
Br
N
5
5
Br
N 6
O
Br N
n
m
O
6
O
n
Br PPV
PFP
O O Na
O N 3
S
O N 6
Cl
S
n
PT1
n
N O Na
Br n
S
O
PT3
PT2
Previous used photosensitizers: 2+
N N
N Ru N RuP
PO(OH) 2 N CQDs
N PO(OH) 2
CQDs
Results and Discussions Structure and Photophysical Properties of CPs. Five CPs (Scheme 1) containing fluorene or thiophene core structures were selected due to their excellent performance 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
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solution according to literature methods. They were synthesized and characterized as reported: poly (fluorene-co-phenylene) derivative (PFP)36,37, poly(phenylene vinylene) derivative (PPV)38, polythiophene derivatives (PT139, PT240, PT341).These water-soluble 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 SI. These selected CPs displayed a wide absorption band ranging from 380 nm to 450 nm, that can be assigned to the π-π* transition of the conjugated backbone. Poly(fluorene-co-phenylene) (PFP) exhibits a high molar absorption coefficient (ε) over 50000 M-1 cm-1 indicating its excellent lightabsorption ability. The lifetime of singlet state was measured by time correlated single photon counting (TCSPC) and the singlet states of all 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 triplet states displayed long lifetime on the microsecond timescale, from 20 μs to 125 μs, measured from laser flash spectroscopy (Table 1). According to the information discussed above, these CPs features 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. Table 1. Light driven hydrogen evolution using CPs as photosensitizers λmax (ε) a
E(CP•+/0)
E(CP0/•-)
λabs-fluod
E00d
E(CP•+/*)e
E(CP*/•-)e
τ(triplet) f
H2 evolution Activity g
/nm (M-1.cm-1)
/V
/V
/nm
/V
/V
/V
/μs
/mmolH2.gCP-1.h-1
PFP
380 (52609)
0.99 b
−2 b
402
3.08
−2.09
1.08
124
122
PPV
449 (19385)
0.75 b
−1.41 b
496
2.50
−1.75
1.09
94
8
PT1
400 (5078)
0.94 c
c
