Donor–Acceptor Porous Conjugated Polymers for Photocatalytic

Article pubs.acs.org/Macromolecules

Donor−Acceptor Porous Conjugated Polymers for Photocatalytic Hydrogen Production: The Importance of Acceptor Comonomer Lianwei Li, Wai-yip Lo, Zhengxu Cai, Na Zhang, and Luping Yu* Department of Chemistry and the James Franck Institute, The University of Chicago, 929 E 57th Street, Chicago, Illinois 60637, United States S Supporting Information *

ABSTRACT: Porous conjugated polymer (PCP) is a new kind of photocatalyst for photocatalytic hydrogen production (PHP). Here, we report the importance of the electronic properties of acceptor comonomer in determining the reactivity of 4,8-di(thiophen-2-yl)benzo[1,2-b:4,5b′]dithiophene (DBD)-based PCP photocatalyst for PHP application. It was found that the incorporation of nitrogen-containing ligand acceptor monomers into PCP network is an effective strategy to enhance the PHP activity. These moderately electron-deficient comonomers enhanced the dipole polarization effect. These PCPs exhibit appropriate solid-state morphology for charge transport. Powder X-ray diffraction (XRD) studies demonstrate that these PCP materials are semicrystalline materials. A strong correlation between the crystalline property and PHP activity is observed. The replacement of nitrogen-containing ligand acceptors with ligand-free strong acceptors is proved to be detrimental to the PHP process, indicating the proper choice in the electronic properties of monomer pair is important for achieving high photoactivity.

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INTRODUCTION This paper investigates the effect of chemical structure of porous conjugated polymers (PCPs) on photocatalytic hydrogen production (PHP) from water (half-reaction of water splitting) and searches for structure−property relationship in this new photocatalytic system. Although the focus of exploring high efficient photocatalysts for PHP was mainly on inorganic semiconductors over the past 30 years,1−3 organic semiconductors are gradually gaining attention for this purpose. The most widely studied organic semiconductor for PHP application is graphitic carbon nitride (g-C3N4) and its derivatives.4−9 There are a few other organic systems were explored, including poly(azomethines),10 poly(triazine),11 poly(heptazine),12 poly(hydrazone),13 poly(triphenylazine),14 and poly(pyrene) based polymer networks.15 Conjugated linear polymers have also been studied for their PHP photocatalytic activity.16−18 Recently, we have designed and synthesized several porous conjugated polymer (PCP) photocatalyts for PHP application, in which the PCP made of fully conjugated donor chromophore 4,8-di(thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (DBD, Scheme 1) and bipyridyl (bpy) unit showed the best photocatalytic performance.19 The results indicated the importance of internal polarization for defraying excitons binding energy and assisting exciton dissociation, consistent with our past experience on designing highly efficient organic photovoltaic (OPV) polymers for solar cells.20 The result also revealed that the residual palladium in the networks plays a key role for the catalysis. However, to claim firmly that © XXXX American Chemical Society

internal polarization is important in enhancing PHP reactivity, a broader scope of monomers must be investigated. This paper further explores the universality of donor−acceptor strategy for constructing efficient PCP photocatalysts by copolymerizing DBD with other monodentate ligand monomers and ligandfree acceptor monomers. Though the donor−acceptor strategy has been previously adopted by us19 and other groups21,22 to design photocatalysts for PHP application, systematic studies were lacking. This work provides the first detailed study on the importance of choice of acceptor comonomer on the PHP performance of PCP photocatalyts.

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RESULTS AND DISCUSSION These new PCP photocatalysts were synthesized through palladium-mediated Suzuki polycondensation (Scheme 1, also see Experimental Section). The preliminary screen of reaction solvents (THF, toluene, dioxane, and DMF) showed that DMF furnished PCP0 with the highest degree of polycondensation (Figures S1 and S2). Thus, all the polycondensation reactions were carried out in DMF/K2CO3(aq) at 130 °C. The PCP structures were confirmed by FTIR characterization (Figure S3). TGA results indicate that most of these PCPs are thermally stable up to 300 °C (Figure S4). The PHP activities for these polymers were evaluated under full-arc irradiation in water/triethylamine mixture (19.2 mL/4.8 Received: August 12, 2016 Revised: September 5, 2016

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Scheme 1. Structures of Comonomers (M0−M11) Used for the Preparation of PCP Photocatalysts PCP0−PCP11, Where the Number Refers to the Number in Comonomer Used) by Suzuki Coupling

Figure 1. (a) Photocatalytic hydrogen production rates of PCP0−11 under full-arc irradiation for 2 h. (b) Retention ratios of photocatalytic hydrogen production rates of PCP0−11 under visible light (H2,visible) and full-arc irradiation (H2,full). (c) PHP reaction time dependence of PHP rate using PCP10 as photocatalyst under visible light irradiation, where the system was degassed every 2 h. (d) Wavelength-dependent apparent quantum yields (AQY) for PCP10 with and without loading 2 wt % Pt cocatalyst.

of ortho- or meta-substituted component into system inevitably break the conjugation of backbones either due to twisted structures (ortho mode) or cross-conjugation (meta mode), which are detrimental for the charge transport. The observed trend of PHP activities for PCP4−8 indicates that effective charge transport is important. Third, based on the above two points, PCP9−11 containing stronger acceptor diazines as the building units (M9−11) were further synthesized and tested as photocatalysts. PCP10 and PCP11 show greatly enhanced PHP activities (103.6 and 106.9 μmol/h) compared to PCP1−8. We ascribe this improvement partially to the enhanced internal polarization feature. PCP9 only shows moderate PHP rate ∼30.4 μmol/h, which can be explained by its different internal dipole orientation. Fourth, the visible-light activities of these PCP photocatalysts were investigated by using a long-pass filter (λ > 400 nm, Figure S5). Their retention ratios (H2,visible/H2,full) vary from 0.09 to 0.30 (Figure 1b and Figure S6), and most of them maintain ∼20% of their PHP activities under visible light irradiation. Interestingly, PCP3 shows the lowest visible-light activity with a retention ratio of ∼0.09, while high retention

mL) with 12.0 mg of polymer photocatalyst (Figure 1a). Several observations are noteworthy from these studies. First, we observed low photocatalytic activity of hydrogen production (1.9−10.1 μmol/h) for donor−donor based PCPs (PCP1−3), and the PHP rate decreases as the chain length of oligo(phenylene) linker increases from 1 to 3. Again, the poor PHP activities of PCP1−3 demonstrate that the full donor based PCPs are not ideal photocatalysts for PHP application due to the lack of internal polarization for effective charge separation process and hydrophobicity of the PCPs. Second, photocatalysts (PCP4−8) based on a set of pyridine (monodentate ligand) exhibit the same chemical component, but different bonding connectivities and dipole orientations for the networks. PHP tests (Figure 1a) demonstrate that their photocatalytic activities are significantly structure dependent. A relatively high PHP rate (59.8 μmol/h) was achieved with PCP6 containing para-substituted pyridine (M6), while it decreases to 18.2−34.9 μmol/h when meta-substituted pyridines are introduced into networks (PCP5, PCP7, and PCP8). The incorporation of ortho-substituted M4 dramatically reduces the PHP rate to 7.8 μmol/h (PCP4). The introduction B

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Figure 2. (a−c) Diffuse reflectance UV−vis absorption spectra of PCP0−3, PCP4−8, and PCP9−11. (d) Plots of PHP rates of PCP0−11 under fullarc irradiation versus their optical bandgaps (Eg,opt).

Figure 3. (a) Experimental optical bandgaps (Eg,opt) of PCP0−11. (b) Calculated energy bandgaps (Eg,cal) of PCP0−11. (c) Calculated HOMO energy levels (EHOMO,cal) of PCP0−11. (d) Calculated LUMO energy levels (ELUMO,cal) of PCP0−11.

ratios (0.25−0.30) are maintained for PCP0, PCP6, PCP10, and PCP11, which also show the best performances under fullarc irradiation. The observed trend of retention ratios agrees well with the trend of internal polarizations for these PCPs. Spectroscopic characterizations indicated no obvious photodegradation for these PCP photocatalysts before and after photoreaction. (Figure S7). Long-term PHP test under visible light using PCP10 as photocatalyst shows the continuous hydrogen production within 14 h (Figure 1c and Figure S8), with very mild loss in activity (from 31.7 to 25.8 μmol/h). The apparent quantum yields (AQY) of the most efficient PCP10 photocatalyst without and with platinum (Pt) cocatalyst were also measured. It was shown that AQY decreases as wavelength increases in the visible light range, and the wavelength for efficient hydrogen production can be extended to 550 nm (Figure 1d). Similar to our previous observation,19

by loading 2 wt % Pt cocatalyst, the AQY at 400 nm can be further enhanced from 1.05% to 1.93%, indicating the important role of metal cocatalyst in the catalysis. Though the overall efficiency is still far away from that of the best g-C3N4 materials (AQY > 50% at 400 nm),9 the above results have clearly revealed that the introduction of appropriate ligand acceptor monomers into network can help to achieve high PHP performance by tuning the internal dipole polarization. However, in addition to internal polarization, multiple factors can affect the efficiency of PHP system, including light absorption, thermodynamic driving force, charge transport property, and surface chemical reactivity. These factors are related to various structural parameters of PCP photocatalyst. Namely, the light absorption property is correlated with the gap between LUMO−HOMO energy level; the thermodynamic C

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Macromolecules driving force is correlated with the energy level offset between the LUMO level and H2O/H2 redox potential; the charge transport property is greatly affected by the solid-state morphology and structure defects of PCP photocatalyst; the surface chemical reaction rate is related with the loading of metal cocatalyst and interfacial properties of PCP photocatalyst. In this DBD-based PCP photocatalyst system, these factors were evaluated to decipher their contribution to the PHP activity. The optical bandgaps (Eg,opt) were extracted from the absorption onsets in their diffuse reflectance UV−vis absorption spectra (Figure 2a−c). The Eg,opt values vary from 1.88 to 2.31 eV, corresponding to the absorption onset of 660 nm (PCP11) to 537 nm (PCP3), respectively. For PCP1−3, Eg,opt increases as the oligo(phenylene) linker length increases. For PCP4−8, PCP6 presents the lowest bandgap due to its full conjugation feature. Pyrazine-based PCP11 shows the lowest bandgap due to its strongest push−pull effect in the conjugated backbone and less steric hindrance. When we plot the PHP rates of different samples with their band gaps, a sharp increases in reaction rate can be observed when Eg,opt is close to ∼2.1 eV (Figure 2d), indicating that broad and strong light absorption and full conjugation feature are essential for efficient PHP process. To gain information about the trends of LUMO and HOMO energy levels (ELUMO and EHOMO), DFT calculations of the fragmental structures (pentamer) of PCP networks were carried out (Figures S9). The calculated bandgap (Eg,cal) values of PCP0−11 follow the same trend as experimentally measured ones (Figures 3a,b). In principle, thermodynamic driving force is needed for water reduction half-reaction; considering the H2O/H2 redox potential at pH ∼ 0 is 4.5 eV, the driving force for PCP0−11 was estimated to range from ∼1.9 to ∼2.5 eV, but no positive correlation between the LUMO energy level and PHP activity was found (Figure 3d). The previous result already showed that PCP10 and PCP11 presented the best PHP activities while they hold the smallest driving forces (∼1.9 eV). Not like oxygen evolution half-reaction, the hydrogen evolution normally needs small overpotential μg,PCP6 (1.65 D) > μg,PCP10 (1.48 D) > μg,PCP11 (1.10 D) > μg,PCP1 (0.45 D). The introduction of polar nitrogen atom significantly increases the dipole moment for these PCPs. But most importantly, these calculations indicate that appropriate internal dipole polarization (1.10 D < μg < 1.65 D) is crucial for enhancing PHP activity of PCP photocatalyst. The residual metal Pd contents of PCP0−11 were measured by ICP-MS and found to vary from 0.73% to 2.13% (Figure 4a). Similar to our previous observation,19 no apparent correlation between Pd content and PHP activity was found (Figure 4b), which was ascribed to the saturation effect of metal cocatalyst.19,23−25 The present result demonstrates that the amounts of residual Pd of PCP networks during the polymerization process are normally significant and enough for catalyzing the surface chemical reaction. Another factor potentially affecting the surface chemical reaction is the interfacial wettability. Our previous work showed that the bpy-containing PCP0 photocatalyst can be better

Figure 4. (a) Residual Pd contents for PCP0−11. (b) Plot of PHP rate vs residual Pd content for PCP0−11.

wetted by miscible diethylamine/water homogeneous solution.19 Here we found that diazine-containing PCP9−11 can even be better wetted by the immiscible triethylamine/water mixed solution. The PCP9−11 can form stable suspension in the water-rich aqueous phase while most of other PCPs can only suspend in the trimethylamine-rich organic phase (Figure S11). This is attributed to the enhanced hydrogen-bonding interaction of diazine species with water molecules, which is beneficial for the interfacial charge transfer and mass transport processes and may also help decrease the activation energy for the catalysis process.26 The solid-state morphologies of these PCPs were investigated by scanning electron microscopy (SEM, Figure 5 and Figures S12−S14). The PCP morphology strongly depends on the structure of acceptor monomer. The PCP0, PCP6, PCP10, and PCP11 present bricklike large block morphology with dimensions of ∼40 μm, which are also the top-four best photocatalysts. The other PCPs are mainly spatially discrete globular particles with sizes on the nanoscale. The overall size follows the order as PCP10 ≃ PCP11 ≃ PCP0 ≃ PCP6 (∼40 μm) > PCP5 (5−10 μm) > PCP4 (2−5 μm) > PCP7−9 (∼1 μm) > PCP1−3 (100−300 nm). It is known that the existence of long-range transport pathway on the surfaces of photocatalyst is beneficial for efficient photoinduced charge separation and migration processes.27 The observed trend of particle sizes of PCPs is consistent with the trend of their PHP activities, implying the importance of long-range transport pathway for highly efficient PHP process. To gain a deeper insight into the correlation between morphology and PHP activity of PCP photocatalysts, the crystalline properties of these PCP photocatalysts were investigated by powder X-ray diffraction (XRD) measurement. These DBD based PCP materials were shown to be semicrystalline (Figure 6), reflected in the presence of sharp diffraction peaks located at 18° (2θ) accompanied by humps ranging from 10° to 25° due to the amorphous phase. Multiple sharp crystalline peaks (15° < 2θ < 25°) were observed in XRD D

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Figure 5. SEM images of PCP0−11 prepared in DMF.

crystalline phase to the charge transport property and PHP activity. Besides monodentate ligands, a few ligand-free acceptor monomers were also copolymerized with donor DBD to furnish the corresponding PCP12−15 photocatalysts (Figure S15). The initial motivation is to further improve PHP activities by introducing stronger internal polarization. Unfortunately, these PCPs present PHP activities 1−3 orders of magnitude lower than that of PCP0 (Figure S16). The highest PHP rate of ∼2.8 μmol/h was achieved by using perylenediimide-based PCP14 as photocatalyst. All of these PCPs present dark color, and their absorption onsets range from 800 to 900 nm (Figure S17), corresponding to the optical bandgap of 1.38−1.55 eV. These control experiments clearly demonstrate the detrimental effect of incorporating ligand-free strong acceptors into networks. Though the enhanced internal polarization can improve the light absorption, the strong electron-withdrawing acceptor may also act as charge carrier trap for the photoexcited electron, which can hinder the electron transfer and separation processes. In addition, the big loss of driving force (low LUMO energy level) and lack of hydrogen-bonding interaction with water molecules may also contribute to their poor photocatalytic activities. Further spectroscopic studies are necessary to elucidate the true mechanism.

Figure 6. Powder X-ray diffraction patterns of all the PCP photocatalysts in this study.

patterns of PCP1 and PCP2. There exists only one significant crystalline peak for other PCPs (2θ = 18°). The diffraction peak at 18° corresponds to a d-spacing ∼0.48 nm, which is due to the ordered stacking of local two-dimensional segments. As we know, only few PCP systems prepared through Schiff-base reaction were reported to be crystalline materials,28,29 while PCP materials prepared by Pd-catalyzed cross-coupling reaction are normally amorphous in nature. Thus, these PCPs are rather unique materials. For PCP1−3, the crystallization feature decreases as the linker length increases. Though PCP1 and PCP2 are semicrystalline materials, the lack of internal polarization for charge dissociation accounts for their poor PHP activities. For PCP0 and PCP4−11, their crystalline diffraction peak intensities are varied, and the relative intensities agree well with their PHP activities, demonstrating the important contribution of intrinsic

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CONCLUSION In summary, this work reveals the importance of electronic match between comonomers in determining the photocatalytic activity of porous conjugated polymer (PCP) for photocatalytic hydrogen production (PHP). It was found that besides bidentate ligand, the incorporation of monodentate ligand E

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Macromolecules monomers containing nitrogen atoms, such as pyridine or diazine, into PCP network could also improve the PHP activity. The observed enhancement can mainly be attributed to enhanced internal dipole polarization effect. Meanwhile, the light absorption, interfacial wettability, solid-state morphology, and crystalline properties were synergistically improved by the introduction of diazine monomer. This study also demonstrated the detrimental effect of introducing ligand-free strong acceptors into PCP networks for PHP application.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b01764. Characterization techniques and Figures S1−S17 (PDF)

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N=

V × 6.02 × 1023 22.4t

AQY =

2N N0

AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (L.Y.). Notes

The authors declare no competing financial interest.

General Procedure for the Synthesis of PCP0−15 by Suzuki Polycondensation. All polymerization reactions were carried out with a constant monomer concentration in DMF at 130 °C for 24 h. The mole ratio of monomers DBD and M0−11 was fixed to be 1/2. A representative experimental procedure for PCP0 is given as an example: Monomer DBD (171.6 mg, 0.200 mmol), monomer M0 (124 mg, 0.400 mmol), and tetrakis(triphenylphosphine)palladium(0) (46.4 mg, 0.040 mmol) were added into a 25 mL round bottle. The reaction bottle was vacuumed and backfilled with nitrogen for three times. Predegassed anhydrous DMF (16 mL) and K2CO3 solution (4 mL, 2 M) were added. The reaction mixture was heated at 130 °C and stirred for 24 h under a nitrogen atmosphere. The mixture was cooled to room temperature, precipitated into methanol, filtered, and further washed with water, methanol, water, and acetone. Further purification was carried out by Soxhlet extraction with tetrahydrofuran for 24 h. The product was dried in vacuum for 12 h at 60 °C to give PCP0. Because of the insolubility of M14 in DMF, the preparation of PCP12− 15 was conducted in toluene/K2CO3(aq) for meaningful comparison. Photocatalytic Hydrogen Production. In order to carry out the photocatalytic reaction in triethylamine/water solution, 12.0 mg of the PCP photocatalyst was suspended in a triethylamine/water mixture (4.0 mL, 2/8, v/v), where both the triethylamine and water were predegassed. The vial was sealed and stirred vigorously in the dark for 12 h to furnish a uniform suspension. Then, the suspension was transferred into a homemade quartz cell (48 mL free volume). After adding 20 mL of triethylamine/water mixture (2/8, v/v), the cell was sealed with a gastight cap. Nitrogen was bubbled for 5 min in the solution phase and 5 min in the gas phase at a fixed flow rate inside the reactor to remove the residual oxygen. The reaction mixture was illuminated with a Newport solar simulator (150 W Xe light source). Two powerful cooling fans were used to maintain the reaction temperature at room temperature. Gas samples were taken and analyzed by GC. Hydrogen was detected with a thermal conductivity detector referencing against standard gas with a known concentration of hydrogen. Apparent Quantum Yield (AQY) Calculation. The energy (E) of incident light at a given wavelength was determined by a calibrated power meter. The number of incident photons (N0) is calculated by eq 1, where T% represents the transmittance of the homemade quartz cell (∼90%), λ represents the wavelength, and c represents the velocity of light. The volume (V) of H2 molecules generated within a fixed time interval (t) was determined by GC. The number of collected H2 (N) is calculated by eq 2. The apparent quantum yield (AQY) can be calculated from eq 3: E λT % hc

ASSOCIATED CONTENT

S Supporting Information *

EXPERIMENTAL SECTION

N0 =

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ACKNOWLEDGMENTS This work was supported by National Science Foundation (DMR-1263006 and NSF-SEP-1229089, LPY) and by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, through Argonne National Laboratory under Contract DE-AC02-06CH11357. This work also benefited from NSF MRSEC at the University of Chicago.

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