Engineering Covalent Organic Frameworks for Light-Driven Hydrogen

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Engineering Covalent Organic Frameworks for Light-Driven Hydrogen Production from Water Ting He,† Keyu Geng,† and Donglin Jiang*,†,‡ †

Department of Chemistry, Faculty of Science, National University of Singapore, 3 Science Drive 3, 117543, Singapore Joint School of National University of Singapore and Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou 350207, P. R. China

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‡

ABSTRACT: Production of sustainable and renewable energy is a subject of global importance and is a challenging goal in the fields of chemistry and materials science. This Perspective focuses on the use of two sustainable resources on this planet, that is, water and sunlight, for the production of green chemical energyhydrogen. Special interest is directed to the recent progress in engineering covalent organic framework systems that allow the construction of tailor-made semiconducting structures to produce hydrogen from water upon irradiation. We scrutinized the structure design concept and synthetic strategy with an aim to disclose a full picture of photo-to-chemical energy conversion.

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ater and sunlight are two irreplaceable and sustainable resources on this planet. Using these two resources to produce green energy, such as hydrogen, is the essence of and of fundamental importance in addressing the ever increasing world energy demand and global warming issues.1,2 However, simple irradiation of water cannot generate hydrogen. How to exploit photoenergy to split water is key to the energy conversion. This process involves a series of elementary photochemical processes, including light harvesting, photoinduced electron transfer, and redox reaction, which must be merged into one molecular system, so that the system can absorb photons to form exciton, the generated excitons can be split into charges, and the charges can be transported to catalytic centers where redox reactions occur. This artificial photosynthesis resembles natural photosynthesis since both use sunlight and water resources and involve similar photochemical processes. Increasing the efficiency of each step and creating a regime that can reduce the energy loss between steps are crucial for high-throughput conversion. In this context, covalent organic frameworks (COFs) offer a unique platform for designing photo-to-chemical energy conversion as they enable the construction of tailor-made systems to merge the processes in a seamless way (Figure 1).

Figure 1. Using water and sunlight as resources to produce hydrogen in the presence of covalent organic frameworks (red, O; white, H; yellow, COFs; blue arrow, exciton (red, Ex) migration and split; dotted black arrow, electrons from COFs accepted by the reaction center (orange, RC) for producing hydrogen in the half reaction. A sacrificial donor (omitted for clarity) is used to accept holes to produce hydrogen).

COFs are a class of crystalline porous polymer that can integrate organic units through covalent bonds into extended

Water and sunlight are two irreplaceable and sustainable resources on this planet © XXXX American Chemical Society

Received: May 8, 2019 Accepted: June 11, 2019 Published: June 11, 2019 203

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two- or three-dimensional (2D or 3D) structures with periodically ordered skeletons and built in pores.3−6 Especially, the finding of semiconducting COFs is one of the greatest breakthroughs in both fields of COFs and semiconductors, as they offer an unprecedented chemical strategy so that not only primary-order structure but also high-order structure of semiconductors can be predesigned, bring us to an era of functional design.7,8 Indeed, by using different π building blocks, linkages, and topologies, COFs can be predesigned into p-, n-, and ambipolar semiconductors.9−13 The linkage and topology control the status of π conjugation and the density of π system; nonconjugated, partially conjugated, and fully πconjugated COFs have been explored with prominent lightharvesting capability and tunable band gap structures.14−17 The key feature is that the ordered π columns in the framework form preorganized pathways that enable high-rate exciton migration and transport through overlapped π clouds.12,13 The inherent polygon channels offer a well-defined nanospace for docking catalytic species, while the pore walls constitute interface that enables a seamless connection of photofunctional modules with reaction centers. With their high degree of freedom in the design of the structure and function of semiconductors, COFs have been explored as a platform for engineering molecular systems to achieve light-driving hydrogen production from water. This Perspective scrutinizes stateof-the-art strategies and analyzes issues and future directions.

Figure 2. Chemical structure of Nx-COFs.

exhibits a prominent change in the HOMO level from −6.34 to −6.77 eV and a decrease in the LUMO level from −2.50 to −3.08 eV, as the number of the nitrogen atom is increased from 0 to 3. More explicitly, as the stack number is increased from 1 to 3, the macrocycle with aldehyde terminal increases the HOMO level from −6.77 to −6.25 eV, while the LUMO level decreases from −3.08 to −3.63 eV. For the macrocycle with hydrazine terminal, the HOMO level increases from −5.94 to −5.19 eV, and the LUMO level changes from −2.66 to −2.80 eV, as the layer number is increased from 1 to 3. These computational studies reveal a general tendency that the HOMO and LUMO levels decrease with the increment of the number of the nitrogen atom. Increasing layer number raises the HOMO level and decreases the LUMO level, leading to a decreased band gap. These simulations imply the real COF materials would have a similar trend as those of the model compounds. A lowered HOMO level will increase the oxidation capability of the hole in Nx-COFs. The enhanced oxidation ability would facilitate hole removal from Nx-COFs by sacrificial donor. Pt nanoparticles (5 mL, 8 wt % hexachloroplatinic acid aqueous solution) as catalyst and triethanolamine (100 mL; 0.738 mmol) as sacrificial donor were used to test Nx-COFs (5 mg) for hydrogen production in a phosphate buffer solution at pH 7 and 25 °C under a 300-W Xenon light source (λ ⩾ 420 nm). The Nx-COFs exhibit a steady evolution of hydrogen without any induction period, while its amount is in a linear proportion with the irradiation time. Interestingly, the

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ENGINEERING NITROGEN-RICH COF SYSTEMS Azine linkage offers a skeleton with a partially π-conjugated structure by topology-directed condensation of aldehyde as the knot and hydrazine as the linker.18 The photoelectric

The photoelectric properties are highly dependent on the geometry and structure of the knot properties are highly dependent on the geometry and structure of the knot; a tetragonal structure enables an extended conjugation over the 2D sheet, while the hexagonal topology yields a limited extension of π system, providing a platform for designing organic semiconductor with tunable π-conjugation state.19 Moreover, the resulting COFs are chemically stable and maintain crystallinity and porosity in organic solvents and aqueous solutions. Lotsch and co-workers have reported a series of azine-linked COFs (Nx-COF, x = 0, 1, 2, and 3) that are synthesized by condensation of hydrazine and triphenylarylaldehydes under solvothermal conditions (Figure 2).19 Aryl rings with different numbers of nitrogen atoms (0−3) as the focal unit of the knot are used to prepare Nx-COFs with the same topology but different knot structures. The optical band is broad, and the COFs absorb photons from ultraviolet to visible regions that extend to 475 nm. Notably, as demonstrated by using virtual macrocyclic models with aldehyde or hydrazine terminals, the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels are highly dependent on the focal aryl units, that is, the number of nitrogen atoms. In the case of the terminal aldehyde, the macrocycle changes the HOMO level changes from −5.84 to −5.94 eV, while the LUMO level decreases from −2.21 to −2.66 eV, as the number of the nitrogen atom is increased from 0 to 3. Similarly, the macrocycle with terminal hydrazine

Interestingly, the hydrogen evolution rate is highly dependent on the number of nitrogen atoms at the focal aryl unit hydrogen evolution rate is highly dependent on the number of nitrogen atoms at the focal aryl unit. Indeed, Nx-COFs greatly enhance the capability of hydrogen evolution to yield a rate constant of 23, 90, 438, and 1703 μmol h−1 g−1 as the number of nitrogen atom is increased from 0 to 1, 2, and 3, respectively. The N3-COF is superior to the state-of-the-art amorphous melon (720 μmol h−1 g−1),20 g-C3N4 (840 μmol h−1 g−1),21 and crystalline poly(triazine imide) (864 μmol h−1 g−1).20 The N3-COF robustly produces hydrogen under 204

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broad absorption band extended to 700 nm. FS-COF is hydrophilic because of the dense sulfone units on the pore walls to show a contact angle of 23.6°, which improves the affinity of the COF to water. By using a Pt nanoparticle as a cocatalyst (hexachloroplatinic acid, 5 μL, 8 wt % aqueous solution) and ascorbic acid (aqueous solution, 0.1 M, 25 mL) as electron donor, the solution of FS-COF (5 mg) was irradiated with a 300 W xenon lamp (λ > 420 nm) to investigate the hydrogen evolution from water. The hydrogen evolution rate of FS-COF is 10.1 mmol h−1 g−1. Notably, this system stably produces hydrogen even after a 50 h continuous run. This rate is 22-fold higher than that of N3-COF (0.47 mmol h−1 g−1) under identical conditions over 5 h (with ascorbic acid) and nearly six-fold higher than the rate reported for N3-COF using triethanolamine as a sacrificial donor.19 Interestingly, compared to that of an amorphous analogue with the same chemical composition, FS-COF has much higher photocatalytic activity; the hydrogen evolution rate is nine-fold higher than that of amorphous counterpart (1.12 mmol h−1 g−1). The greatly enhanced activity of FS-COF originates from its high structural ordering, narrow band gap, high wettability and small Pt nanoparticle. These results clearly demonstrate that the control of COF structures, as well as a hydrophilic interface between COF and water and Pt nanoparticle, should be considered in designing systems for photocatalytic hydrogen production from water.

continuous irradiation and retains crystallinity and porosity after 48 h. Simulations with DFTB+/mio-1-0 level of theory using periodic single-point calculations reveal that the HOMO is localized solely on the azine linker unit, while the LUMO is distributed over the π system of the framework. Further calculations confirm that the anion radical species of Nx-COFs have different stabilization energies; as the x value is increased from 0 to 1, 2, and 3, the energy decreases from 0.00 to −0.19, −0.39, and −0.45 eV. This result indicates that Nx-COFs at the photoexcited state can be reduced by the sacrificial donor to form anion radicals that flow electrons to the Pt catalyst where hydrogen evolution occurs. A high stabilization energy endows the COF with an enhanced efficiency of electron flow, leading to a high hydrogen evolution rate. This study showcases the key role of the knot in enhancing the electron-accepting ability of the COF material, which absorbs visible light and pumps electrons from sacrificial donor to reaction center. Although the band gap does not change significantly, introducing a nitrogen atom to the focal aryl unit probably offers an efficient approach to tune the LUMO level, which needs to be proved further by a direct measurement of the band gap structure.

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ENGINEERING HYDROPHILIC COF SYSTEMS In hydrogen production, COFs serve as light-harvesting antennae and electron relay stations to promote electron flow. This means not only the band gap structure but also the way to manage various interfaces that are involved in the electron pass will definitely affect the overall performance. Cooper and co-workers have developed a strategy for demonstrating these concepts by synthesizing benzothiophene sulfone-based FS-COF (Figure 3).22 The FS-COF is a crystalline material with a BET surface area of 1288 m2 g−1 and a pore size of 2.76 nm. It forms eclipsed AA stacking so that the skeleton self-assembles to form a layered structure; computational studies on the model compound reveal the HOMO and LUMO levels are suitable for water reduction. The optical band gap of FS-COF is 1.85 eV and exhibits a

Control of COF structures should be considered in designing systems for photocatalytic hydrogen production from water

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ENGINEERING FULLY π-CONJUGATED SP2 CARBON COF SYSTEMS COFs, upon design of appropriate π systems, are expected to combine different functions into one material, including strong light-harvesting capability, narrow band gap with suitable orbital levels, and ability to facilitate exciton migration and electron transfer and transport. However, such a structural design remains a challenge. To create such a structure, COFs with enhanced π conjugation may provide an approach to merge these functions in one material. The azine and imine linkages allow for a partial π conjugation between the knot and linker, while the hexagonal frameworks greatly break the extension of π delocalization over the skeleton as the 1,3,5trisubstituted aryl core at the knot position cannot transmit the π conjugation. A fully π conjugated COF, that is, sp2 c-COF, has been synthesized by the topology directed polycondensation of 1,4phenylenediacetonitrile (PDAN) and 1,3,6,8-tetrakis(4formylphenyl)pyrene (TFPPy) in mesitylene/1,4-dioxane (1 mL, 1/5 by vol.) with NaOH solution (0.1 mL, 4 M) at 90 °C for 3 days.23−25 The sp2 c-COF is unique in that it constitutes an all sp2 carbon-based tetragonal skeleton in which the polygon backbones are linked by CC bond and fully π conjugated along both x and y directions. The conjugated 2D sheets stack to form an ordered layered structure. The extended π conjugation yields a narrow band gap of 1.9 eV with HOMO and LUMO levels suitable for water reduction.

Figure 3. Chemical structure of FS-COF. 205

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h−1 g−1 for sp2 c-COF, 140 μmol h−1 g−1 for sp2 c-CMP, and a negligible activity for CN linked pyrene-COF, respectively. Therefore, the activity of sp2 c-COFERDN is 1.6- and 15-fold higher than those of sp2 c-COF and sp2 c-CMP, respectively. The greatly enhanced activity is correlated with the unique structure of sp2 c-COFERDN. Interestingly, the HOMO level of sp2c-COF is suitably positioned for oxidizing water into oxygen. Indeed, the sp2 c-COF (50 mg) dispersed in an aqueous solution (100 mL) containing AgNO3 as an electron acceptor (0.01 M), Co(NO3)2 as a cocatalyst (0.6 mg), and La2O3 as a pH buffer agent (0.2 g) enables oxygen evolution at a rate of 22 μmol h−1 g−1.25 These results suggest that sp2 cCOF with suitable reaction centers can promote both hydrogen and oxygen evolution from water upon irradiation. The sp2 c-COFERDN is designed to be fully π conjugated along both x and y directions so that the framework can harvest a broad range of visible to near-infrared photons while the built-in donor−acceptor heterojunction offers interface for exciton split (Figure 5). The frameworks consist of dense yet

The sp2 c-COF is exceptionally stable and retains its structure in organic solvents, concentrated HCl solution, and aqueous NaOH solution (14 M) for 7 days. Notably, it retains crystallinity and porosity upon exposure to air under room light for over 1 year. This stability suggests that sp2 c-COF is robust enough as a photocatalyst to promote hydrogen evolution from water. Development of a three-component polycondensation system with TFPPy as knot, PDAN as linker, and an electron-deficient 3-ethylrhodanine (ERDN) as a monofunctional terminal allowed the synthesis of sp2 c-COFERDN (Figure 4), which possesses the same lattice structure as sp2 c-COF but

Figure 4. Chemical structure of sp2 c-COFERDN.

which is decorated with ERDN units at the periphery of the lattice.25 The effect of these ERDN terminals is prominent because it further enhances π conjugation through electron donor−acceptor push−pull effect (Figure 4, inset), decreases the band gap to 1.85 eV, enables the harvesting of near infrared light extended to 800 nm, and lowers reduction and oxidation potentials. The extended π conjugation and stacking enable exciton migration over the framework and split into charges at the donor−acceptor interface as demonstrated by photocurrent measurements. The channels with a pore size of 2 nm can trap Pt nanoparticles of small size, offering a proximate interface between pore wall and reaction center. The π columns offer a preorganized pathway for charge delocalization, which prevents backward electron transfer.12 Therefore, three molecular mechanisms for light harvesting, exciton migration and split, electron transfer, and charge transport have been built into one sp2 c-COFERDN framework. The photocatalytic activity of sp2 c-COFERDN (50 mg) was investigated in an aqueous solution (100 mL) in the presence of in situ generated Pt nanoparticles (3 wt %, 1−3 nm) and triethanolamine (10 vol %) under a 300 W Xenon light source (λ ⩾ 420 nm). The sp2 c-COFERDN exhibits a hydrogen evolution rate of 2120 μmol h−1 g−1. The robust stability enables sp2 c-COFERDN to achieve the same hydrogen production rate upon 20 h of continuous irradiation, without showing any decrease in photocatalytic activity. Notably, the sp2 c-COFERDN retains its crystallinity, porosity, and chemical integrity. The sp2 c-COFERDN is superior to sp2 c-COF, sp2 c-CMP (amorphous conjugated microporous polymer), and CN linked pyrene-COF in terms of photocatalysis under otherwise same conditions.25 The hydrogen evolution rate is 1360 μmol

Figure 5. Functions of sp2 c-COFERDN in hydrogen evolution.

ordered π columns that serve as pathways to promote exciton migration and charge transport. Moreover, docking catalytic sites in pores or on surface decreases the electron-transfer distance, facilitating electron flow to the reaction centers. These three molecular regimes are seamlessly incorporated in one framework and endow the system with efficient hydrogen production that can be driven by low-energy photons. Using sunlight to split water into hydrogen and oxygen could be a sustainable path toward sustainable green energy production. However, this process itself is energy demanding and cannot occur spontaneously. The development of energysaving and high-throughput photocatalysts that can promote the splitting of water into both hydrogen and oxygen is the key toward meeting this challenging goal. The reduction of water into hydrogen requires the smooth passage of electrons to accumulate at the reaction center, while the oxidation of water to generate oxygen requires extraction of electrons from water. These reduction and oxidation processes are on the opposite sides of water redox reactions and constitute two different electron flow diagrams. An ideal photocatalyst should create a 206

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Notes

regime that merges these two sides into one working package to enable an unstopped electron flow from oxidation to reduction side. At present, the oxidation and reduction sides are studied using individual systems in which the metal-based reaction centers are required to be further explored to deploy non-noble metal species. Therefore, one strategy for merging these two reactions is to develop a scalable photocatalyst that can efficiently cover broad solar spectrum to collect visible and near infrared photons, split excitons into electrons and holes, and transport electrons to the water reduction sites to generate hydrogen and pass holes to the oxidation sites to produce oxygen. If this becomes possible, additional sacrificial electron donor or acceptor will be unnecessary, which should be a big step toward a simple and clean energy production scheme. Despite the fact that great progress has been made in recent years, nevertheless, current studies are still far away from an ideal system and synthetic efforts on these issues could be further directed. As for the reaction centers for splitting water into hydrogen and oxygen, currently metal complexes or particles, including Pt and Ir, are used. From the viewpoint of large-scale application, developing non-noble metal-based reaction centers is necessary. Because the reduction of water requires two electrons and oxidation involves four electrons, the reaction centers are expected to have the capability of “stocking” electrons. In this context, developing COFs as reaction centers to pool electrons is highly interesting, and this direction would challenge pure organic systems for water splitting methods. Covalent organic frameworks offer an ideal platform for organizing organic units into tailor-made light-harvesting

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS D.J. acknowledges MOE tier 1 grant (R-143-000-A71-114).

Covalent organic frameworks offer an ideal platform for organizing organic units into tailor-made light-harvesting antennae and semiconductors antennae and semiconductors. As demonstrated by the systems engineered for light-driven hydrogen production from water, COFs are far superior to other state-of-the-art synthetic antennae and semiconductors in many aspects. Especially, the high designability of structure and the overall synthetic control over the long-range ordering, density, and composition are inaccessible to any other organic materials. Because of these unique structural features, COFs are highly possible for the exploration of skeletons and pores for merging both oxidation and reduction sides into one material or for combining semiconducting COFs of different redox potentials into one system, whereby the photoenergy can be truly transformed into chemical energy through a simple yet seamless electron flow diagram. We anticipate such a dream will come true through a full understanding of interplay of COFs with photons, excitons, electrons, and holes.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Donglin Jiang: 0000-0002-3785-1330 Author Contributions

D.J., T.H., and K.G. wrote the manuscript. All authors have given approval to the final version of the manuscript. 207

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