Conjugated Polymers with Oligoethylene Glycol Side Chains for Improved Photocatalytic Hydrogen Evolution

摘要

class="head no_bottom_margin" id="sec1title">IntroductionPhotocatalytic hydrogen evolution from water is a promising technology to transfer solar energy into hydrogen energy with high-energy capacity and zero-emission features. Since the pioneering research of Honda and Fujishima, there has been much work on the development of semiconductors that enable efficient photocatalytic water splitting (, , , ). Organic photocatalysts for photocatalytic hydrogen evolution have received tremendous attention in the past several years (, , , , , , , , ) because of their unique feature of tunable electronic properties via molecular engineering (). The energy levels, absorption spectrum, and carrier mobility/type of organic photocatalysts can be easily tuned to realize efficient water reduction/oxidation. Consequently, various polymers with tailor-made chemical structures have been extensively studied. For example, g-C3N4 (, href="#bib23" rid="bib23" class=" bibr popnode">Martin et al., 2014, href="#bib16" rid="bib16" class=" bibr popnode">Lau et al., 2016), porous conjugated polymers(href="#bib32" rid="bib32" class=" bibr popnode">Sprick et al., 2015, href="#bib17" rid="bib17" class=" bibr popnode">Li et al., 2016a, href="#bib45" rid="bib45" class=" bibr popnode">Yang et al., 2016, href="#bib39" rid="bib39" class=" bibr popnode">Wang et al., 2017), covalent conjugated polymers (href="#bib36" rid="bib36" class=" bibr popnode">Vyas et al., 2015, href="#bib26" rid="bib26" class=" bibr popnode">Pachfule et al., 2018), and linear conjugated polymers (href="#bib33" rid="bib33" class=" bibr popnode">Sprick et al., 2016, href="#bib18" rid="bib18" class=" bibr popnode">Li et al., 2016b, href="#bib22" rid="bib22" class=" bibr popnode">Lu et al., 2017, href="#bib43" rid="bib43" class=" bibr popnode">Woods et al., 2017, href="#bib27" rid="bib27" class=" bibr popnode">Pati et al., 2017, href="#bib35" rid="bib35" class=" bibr popnode">Tseng et al., 2018, href="#bib14" rid="bib14" class=" bibr popnode">Kosco et al., 2018) have been widely developed as organic photocatalysts for hydrogen evolution and have shown promising photocatalytic activity over the past few years. Moreover, through multiple modification strategies, including doping (href="#bib21" rid="bib21" class=" bibr popnode">Liu et al., 2010, href="#bib31" rid="bib31" class=" bibr popnode">Ran et al., 2015), hybridization (href="#bib7" rid="bib7" class=" bibr popnode">Du et al., 2012, href="#bib3" rid="bib3" class=" bibr popnode">Chen et al., 2017b, href="#bib47" rid="bib47" class=" bibr popnode">Yu et al., 2018), and copolymerization (href="#bib49" rid="bib49" class=" bibr popnode">Zhang et al., 2010) on organic photocatalysts, highly efficient hydrogen evolution can be realized. In addition, suitable metal co-catalysts have been used to lower the redox overpotential and improve charge transfer and separation, which has greatly improved the photocatalytic performance of organic photocatalysts (href="#bib44" rid="bib44" class=" bibr popnode">Wu et al., 2018).To achieve high-performance hydrogen evolution, it is required that organic photocatalysts with strong light-harvesting capabilities and suitable energy levels be designed (href="#bib24" rid="bib24" class=" bibr popnode">Ong et al., 2016, href="#bib37" rid="bib37" class=" bibr popnode">Vyas et al., 2016, href="#bib50" rid="bib50" class=" bibr popnode">Zhang et al., 2016, href="#bib10" rid="bib10" class=" bibr popnode">Fu et al., 2018, href="#bib42" rid="bib42" class=" bibr popnode">Wang et al., 2018c, href="#bib46" rid="bib46" class=" bibr popnode">Yang et al., 2018). Furthermore, because of the short exciton diffusion length (href="#bib29" rid="bib29" class=" bibr popnode">Peumans et al., 2004) and low mobility of organic materials, the powder size of organic photocatalysts dispersed in water must be small to provide shorter distances for the separated charges emigrating to the edges of the organic photocatalysts, thus reducing the recombination inside the organic photocatalysts. Moreover, strong interactions between organic photocatalysts and metal co-catalysts are also encouraged to improve charge separation (href="#bib16" rid="bib16" class=" bibr popnode">Lau et al., 2016). However, it is challenging to realize all these characteristics because most reported organic materials/conjugated polymer-based photocatalysts sharing hydrophobic alkyl side chains show poor water dispersion (href="#bib18" rid="bib18" class=" bibr popnode">Li et al., 2016b) and lack binding points with metal co-catalysts.Hydrophilic conjugated polymers share both semiconductive conjugated backbones and hydrophilic side chains (href="#bib8" rid="bib8" class=" bibr popnode">Duan et al., 2013, href="#bib4" rid="bib4" class=" bibr popnode">Chueh et al., 2015, href="#bib5" rid="bib5" class=" bibr popnode">Cui and Bazan, 2018). The hydrophilic side chains impart polymers with excellent dispersity/solubility in polar solvents and water, enabling the application of such polymers in biosensing and imaging applications (href="#bib34" rid="bib34" class=" bibr popnode">Traina et al., 2011). Moreover, hydrophilic side chains can robustly interact with metal substrates, resulting in well-modified metal surfaces that promote optoelectronic device performance (href="#bib8" rid="bib8" class=" bibr popnode">Duan et al., 2013, href="#bib4" rid="bib4" class=" bibr popnode">Chueh et al., 2015, href="#bib5" rid="bib5" class=" bibr popnode">Cui and Bazan, 2018). The advantages of hydrophilic conjugated polymers are identical to the requirements of organic photocatalysts for hydrogen evolution. However, despite the above-mentioned multiple potential advantages, hydrophilic conjugated polymers for highly efficient hydrogen evolution have rarely been reported.Herein, we demonstrate a highly efficient strategy to boost the photocatalytic hydrogen evolution of conjugated polymers by functionalizing conjugated backbones with hydrophilic oligo (ethylene glycol) monomethyl ether (OEG) side chains. With rational chemical design, benzodithiophene and difluorobenzothiadiazole moieties were copolymerized to yield conjugated polymeric backbones with a wide absorption spectrum of 300–720 nm. Moreover, hydrophilic tetra- and hepta-(ethylene glycol) monomethyl ether side chains were employed to modify the conjugated polymer backbones (PBDTBT-4EO/PBDTBT-7EO, href="/pmc/articles/PMC6393733/figure/fig1/" target="figure" class="fig-table-link figpopup" rid-figpopup="fig1" rid-ob="ob-fig1" co-legend-rid="lgnd_fig1">Figure 1A), resulting in outstanding dispersion of conjugated polymers in water and high photocatalytic activity for hydrogen evolution. Compared with an alkyl-functionalized polymer (PBDTBT-C6C10, href="/pmc/articles/PMC6393733/figure/fig1/" target="figure" class="fig-table-link figpopup" rid-figpopup="fig1" rid-ob="ob-fig1" co-legend-rid="lgnd_fig1">Figure 1A), the OEG side-chain-functionalized conjugated polymers exhibited a 90-fold improvement in hydrogen evolution rate, reaching to 40 μmol h⁻¹. The OEG side chains interact robustly with Pt co-catalysts, resulting in better charge transfer from the conjugated polymers to the co-catalysts. The photocurrent response and electrochemical impedance spectroscopy results showed that OEG side chains improved the charge separation efficiency of the conjugated polymers when in contact with water. The Mott–Schottky plots and density functional theory (DFT) calculations revealed that the OEG side chains in conjugated polymers can adsorb H⁺ in water, resulting in lower energy bands of PBDTBT-4EO/-7EO film on the surface when in contact with water. This is the first report of conjugated polymers with hydrophilic side chains strongly interacting with water and improving charge separation, which paves the way for the development of hydrophilic conjugated polymers for highly efficient hydrogen evolution from water.href="/pmc/articles/PMC6393733/figure/fig1/" target="figure" rid-figpopup="fig1" rid-ob="ob-fig1">class="inline_block ts_canvas" href="/core/lw/2.0/html/tileshop_pmc/tileshop_pmc_inline.html?title=Click%20on%20image%20to%20zoom&p=PMC3&id=6393733_gr1.jpg" target="tileshopwindow">target="object" href="/pmc/articles/PMC6393733/figure/fig1/?report=objectonly">Open in a separate windowclass="figpopup" href="/pmc/articles/PMC6393733/figure/fig1/" target="figure" rid-figpopup="fig1" rid-ob="ob-fig1">Figure 1Chemical Structures and Basic Properties of Conjugated Polymers(A) Chemical structures of PBDTBT-C6C10, PBDTBT-4EO, and PBDTBT-7EO.(B) UV-vis-NIR absorption spectra of PBDTBT-C6C10, PBDTBT-4EO, and PBDTBT-7EO.(C) Energy levels of conjugated polymers and the energy transfers among photocatalysts, co-catalysts, scarifying agent, and water.