Conjugated Microporous Polymer Nanosheets for Overall Water Splitting Using Visible Light

Enhanced Light-Driven Hydrogen-Production Activity Induced by Accelerated Interfacial Charge Transfer in Donor-Acceptor Conjugated Polymers/TiO2 Hybrid

Donor-acceptor (D-A) conjugated polymers have proved to be desired candidates to couple with inorganic semiconductors for enhanced photocatalytic activity. Herein, the matched energy levels between polymer BFB and TiO₂ make them form BFB-TiO₂ composites with moderate photocatalytic H₂ evolution rate (HER). To further enhance the interfacial interaction, BFB was modified with a carboxylic acid end group, which reacted with surface OH of TiO₂ to form an ester bond. As a result, the functionalized BFBA-TiO₂ composites exhibited superior photocatalytic activity. Especially, HER of 4 % BFBA-TiO₂ can reach up to 228.2 μmol h^-1 under visible light irradiation (λ>420 nm), which is about 2.02 times higher than that of BFB-TiO₂ . The enhanced photocatalytic activity originated from the formed ester bond between polymer and TiO₂ , and photogenerated electrons injection from lowest unoccupied molecular orbital (LUMO) of the exited polymer to conduction band of TiO₂ were accelerated. Therefore, based on an intermolecular interaction mechanism, more suitable D-A conjugated polymers with anchoring groups could be designed to couple with other semiconductors for enhancing photocatalytic activity.

In situ Stabilization of Au and Co Nanoparticles in a Redox-Active Conjugated Microporous Polymer Matrix: Facile Heterogeneous Catalysis and Electrocatalytic Oxygen Reduction Reaction Activity

We report a novel in situ method for synthesis of metal nanoparticles (NPs)-CMP (conjugated microporous polymer) composites based on a redox-active, donor-acceptor CMP, tris-(4-aminophenyl)amine (TPA)-perylenediimide (PDI). The TPA-PDI CMP, comprising triphenylamine as an electron donor and PDI as an acceptor, showed stable charge-separated state and semiconducting behavior. Further, TPA-PDI CMP has been exploited for in situ stabilization of metal (Au and Co) NPs, and two novel nanocomposites (Au@TPA-PDI and Co@TPA-PDI) were prepared. The catalytic reduction of nitro aryls to amino aryls was studied using Au@TPA-PDI, which showed excellent yields and fast kinetics. The CMP itself was found to show good activity as a metal-free oxygen reduction reaction (ORR) electrocatalyst with an onset potential of 0.82 V. Stabilizing merely 2.56 wt % Co nanoparticles in the CMP matrix improved the electrochemical ORR activity of as-synthesized TPA-PDI immensely and showed an onset potential of 0.91 V, which has also been supported by density functional theory (DFT) calculations.

Recent developments in heterogeneous photocatalysts for solar-driven overall water splitting

Overall water splitting based on particulate photocatalysts is an easily constructed and cost-effective technology for the conversion of abundant solar energy into clean and renewable hydrogen energy on a large scale.

Wettability control of conjugated polymer films by electric-field polarization technique

The wettability of conjugated polymer poly(3-hexylthiophene) (P₃HT) films was accurately controlled by an electric field polarization technique, and transition of the films from being hydrophobic to hydrophilic was successfully achieved.

Acetylenic carbon-rich frameworks on copper foam as conjugated polymer photocathodes for efficient and stable water reduction

A poly(1,3,5-triethynylbenzene) (PTEB) nanofiber is synthesized on a copper foam surface and presents a 100 times increased record-high photocathodic current density for efficient water reduction.

Promoting water dissociation performance by borinic acid for the strong-acid/base-free hydrogen evolution reaction

Borinic acid is reported as a new proton donor with promoted water dissociation performance for the strong-acid/base-free hydrogen evolution reaction.

A carbon-rich nanofiber framework based on a conjugated arylacetylene polymer for photocathodic enzymatic bioanalysis

The metal-free photocathode fabricated by porous carbon-rich nanofiber framework of PTEB film realized “signal-off” photocathodic bioanalysis of glucose.

Hydrophilicity-Controlled Conjugated Microporous Polymers for Enhanced Visible-Light-Driven Photocatalytic H2 Evolution

To take advantage of high surface area of network conjugated microporous polymers, four linear or network conjugated polymers L-PDBT, L-PDBT-O, N-PDBT, and N-PDBT-O are designed in terms of water-compatibility, and it turned out that microporous network N-PDBT-O exhibited the highest hydrogen evolution rate (HER) at 366 µmol h^-1 under visible light irradiation (λ > 420 nm, one of best reported pristine polymer-based photocatalysts), which is three times higher than the corresponding linear L-PDBT-O. Water contact angle measurements revealed that benzothiophene-sulfone-based conjugated polymers display better water compatibility and adsorption, and the synergic effect of better hydrophilic surface and higher surface area of N-PDBT-O might eventually lead to more exposed active sites in comparison to linear L-PDBT-O in the H₂ evolution suspension system. The hydrophilicity-controlled strategy could be applied to design of other network conjugated microporous polymer photocatalysts in an attempt to improve the activity.

Porous Organic Polymers: An Emerged Platform for Photocatalytic Water Splitting

Porous organic polymers (POPs), known for its high surface area and abundant porosity, can be easily designed and constructed at the molecular level. The POPs offer confined molecular spaces for the interplay of photons, excitons, electrons and holes, therefore featuring great potential in catalysis. In this review, a brief summary on the recent development of some current state-of-the-art POPs for photocatalytic water splitting and their design principles and synthetic strategies as well as relationship between structure and photocatalytic hydrogen or oxygen evolution performance are presented. Future prospects including research directions are also proposed, which may provide insights for developing POPs for photocatalytic water splitting with our expectations.

Understanding structure-activity relationships in linear polymer photocatalysts for hydrogen evolution

Conjugated polymers have sparked much interest as photocatalysts for hydrogen production. However, beyond basic considerations such as spectral absorption, the factors that dictate their photocatalytic activity are poorly understood. Here we investigate a series of linear conjugated polymers with external quantum efficiencies for hydrogen production between 0.4 and 11.6%. We monitor the generation of the photoactive species from femtoseconds to seconds after light absorption using transient spectroscopy and correlate their yield with the measured photocatalytic activity. Experiments coupled with modeling suggest that the localization of water around the polymer chain due to the incorporation of sulfone groups into an otherwise hydrophobic backbone is crucial for charge generation. Calculations of solution redox potentials and charge transfer free energies demonstrate that electron transfer from the sacrificial donor becomes thermodynamically favored as a result of the more polar local environment, leading to the production of long-lived electrons in these amphiphilic polymers.

While inorganic semiconductors are well-studied for their solar-to-fuel energy conversion abilities, organic materials receive far less attention. Here, authors prepare linear conjugated polymers as H₂ evolution photocatalysts and rationalize photocatalytic activities with fundamental properties.

2D Polymers as Emerging Materials for Photocatalytic Overall Water Splitting

Converting solar energy into storable and transportable chemical fuels using artificial photosynthetic systems can provide an alternative route to the current unsustainable use of fossil fuels, addressing the worldwide energy crisis and environmental issues. Recently, semiconducting polymers have emerged as a very promising class of photocatalysts for water splitting as their electronic and structural properties can be conveniently controlled and systematically designed at a molecular level. Among the various polymer photocatalysts that are reported so far, 2D polymer nanosheets are particularly interesting and gaining more attention. The 2D planar structure offers unique features such as high surface area, abundant surface active sites, efficient charge separation, and facile formation of heterostructures. The design and synthesis of 2D polymer nanosheets have greatly advanced the research in photocatalytic overall water splitting. Here, recent advances in developing photocatalysts based on 2D polymer nanosheets for photocatalytic overall water splitting are highlighted. Specifically, the existing approaches to tune their electronic structures and surface active sites for photocatalysis are discussed. Future opportunities and challenges for developing 2D polymers for photocatalytic overall water splitting are also included.

Material Design for Photocatalytic Water Splitting from a Theoretical Perspective

Currently, problems associated with energy and environment have become increasingly serious. Producing hydrogen, a clean and renewable resource, through photocatalytic water splitting using solar energy is a feasible and efficient route for resolving these problems, and great efforts have been devoted to improve the solar-to-hydrogen efficiency. Light harvesting and electron-hole separation are key in enhancing the efficiency of solar energy utilization, which stimulates the development of new photocatalytic materials. Here, recent advances in material design for photocatalytic water splitting are presented from a theoretical perspective. Specifically, aiming to enhance the photocatalytic performance, general strategies of materials design are discussed, including codoping and introducing a built-in electric field to improve the light harvesting of materials, reducing the dimension of materials to shorten the migration pathway of carriers to inhibit electron-hole recombination, and constructing heterojunctions to enhance light harvesting and electron-hole separation. Future opportunities and challenges in the theoretical design of photocatalytic materials toward water splitting are also included.

Sulfone-containing covalent organic frameworks for photocatalytic hydrogen evolution from water

Nature uses organic molecules for light harvesting and photosynthesis, but most man-made water splitting catalysts are inorganic semiconductors. Organic photocatalysts, while attractive because of their synthetic tunability, tend to have low quantum efficiencies for water splitting. Here we present a crystalline covalent organic framework (COF) based on a benzo-bis(benzothiophene sulfone) moiety that shows a much higher activity for photochemical hydrogen evolution than its amorphous or semicrystalline counterparts. The COF is stable under long-term visible irradiation and shows steady photochemical hydrogen evolution with a sacrificial electron donor for at least 50 hours. We attribute the high quantum efficiency of fused-sulfone-COF to its crystallinity, its strong visible light absorption, and its wettable, hydrophilic 3.2 nm mesopores. These pores allow the framework to be dye-sensitized, leading to a further 61% enhancement in the hydrogen evolution rate up to 16.3 mmol g^-1 h^-1. The COF also retained its photocatalytic activity when cast as a thin film onto a support.

Ultrastable and Efficient Visible-Light-Driven Hydrogen Production Based on Donor-Acceptor Copolymerized Covalent Organic Polymer

Developing stable and efficient photocatalysts for H₂ production under visible light is still a big challenge. In this work, a novel covalent organic polymer (COP)-based photocatalyst with trace ending groups was prepared by the efficient irreversible kinetic coupling reaction, i.e., nickel(0)-catalyzed Yamamoto-type Ullmann cross-coupling, using pyrene as electron donor and countpart, e.g., phenanthrolene, benzene, pyrazine, as electron acceptor. The newly developed optimal photocatalyst (termed as COP-TP_3:1) has a 14-fold improvement in the H₂ evolution rate from 3 to 42 μmol h^-1 under visible light compared with the sample without donor-acceptor structure. Moreover, COP-TP_3:1 also performs excellent photocatalytic activity under different water quality (deionized water, municipal water, commercial mineral water, and simulated seawater (NaCl 3 wt %)). Significantly, ignored decrease in H₂ evolution can be observed after 20 hours cycling H₂ production, and the performance is only reduced by about 7% even after discontinuous cycles of photocatalysis and storage for a month. The donor-acceptor units with trace ending groups contribute to suppress electron-holes recombination kinetics and the N coordination sites in electron-acceptors conduce to anchor Pt (as the cocatalyst) onto the surface of photocatalyst, both of which are conducive to the outstanding photocatalytic activity and stability. Accordingly, this work can provide guidance to design a stable and efficient photocatalyst by copolymerization for visible-light-driven H₂ production.

Dibenzothiophene-S,S-Dioxide-Based Conjugated Polymers: Highly Efficient Photocatalyts for Hydrogen Production from Water under Visible Light

Three dibenzothiophene-S,S-dioxide-based alternating copolymers were synthesized by facile Suzuki polymerization for visible light-responsive hydrogen production from water (> 420 nm). Without addition of any cocatalyst, FluPh2-SO showed a photocatalytic efficiency of 3.48 mmol h^-1 g-¹ , while a larger hydrogen evolution rate (HER) of 4.74 mmol h^-1 g^-1 was achieved for Py-SO, which was ascribed to the improved coplanarity of the polymer that facilitated both intermolecular packing and charge transport. To minimize the possible steric hindrance of FluPh2-SO by replacing 9,9'-diphenylfluorene with fluorene, Flu-SO exhibited a more red-shifted absorption than FluPh2-SO and yielded the highest HER of 5.04 mmol h^-1 g^-1 . This work highlights the potential of dibenzothiophene-S,S-dioxide as a versatile building block and the rational design strategy for achieving high photocatalytic efficiency.

Benzoxazole-Linked Ultrastable Covalent Organic Frameworks for Photocatalysis

The structural uniqueness of covalent organic frameworks (COFs) has brought these new materials great potential for advanced applications. One of the key aspects yet to be developed is how to improve the robustness of covalently linked reticular frameworks. In order to make the best use of π-conjugated structures, we develop herein a "killing two birds with one stone" strategy and construct a series of ultrastable benzoxazole-based COFs (denoted as LZU-190, LZU-191, and LZU-192) as metal-free photocatalysts. Benefiting from the formation of benzoxazole rings through reversible/irreversible cascade reactions, the synthesized COFs exhibit permanent stability in the presence of strong acid (9 M HCl), strong base (9 M NaOH), and sunlight. Meanwhile, reticulation of the benzoxazole moiety into the π-conjugated COF frameworks decreases the optical band gap and therefore increases the capability for visible-light absorption. As a result, the excellent photoactivity and unprecedented recyclability of LZU-190 (for at least 20 catalytic runs, each with a product yield of 99%) have been illustrated in the visible-light-driven oxidative hydroxylation of arylboronic acids to phenols. This contribution represents the first report on the photocatalytic application of benzoxazole-based structures, which not only sheds new light on the exploration of robust organophotocatalysts from small molecules to extended frameworks but also offers in-depth understanding of the structure-activity relationship toward practical applications of COF materials.

Surface and interface design for photocatalytic water splitting

Surface and interface structures are considered as the critical parameters which can be engineered to improve the performance of catalysts. This Frontier article highlights our recent advances in surface and interface design toward photocatalytic water splitting.

Superfine palladium nanocrystals on a polyphenylene framework for photocatalysis

To enable the recycling and recovery of these nanosized noble metals, various carriers are adopted for catalyst design.

The effect of molecular structure and fluorination on the properties of pyrene-benzothiadiazole-based conjugated polymers for visible-light-driven hydrogen evolution

The linear non-fluorinated polymer L-PyBT exhibited an impressive hydrogen evolution rate up to 83.7 μmol h⁻¹ under visible light irradiation.

Porous Organic Polymers: An Emerged Platform for Photocatalytic Water Splitting

Porous organic polymers (POPs), known for its high surface area and abundant porosity, can be easily designed and constructed at the molecular level. The POPs offer confined molecular spaces for the interplay of photons, excitons, electrons and holes, therefore featuring great potential in catalysis. In this review, a brief summary on the recent development of some current state-of-the-art POPs for photocatalytic water splitting and their design principles and synthetic strategies as well as relationship between structure and photocatalytic hydrogen or oxygen evolution performance are presented. Future prospects including research directions are also proposed, which may provide insights for developing POPs for photocatalytic water splitting with our expectations.

Synergistic design for enhancing solar-to-hydrogen conversion over a TiO2-based ternary hybrid

Sensitization of a conjugated polymer and SPR of Au are synergistically designed on TiO₂ for photocatalytic H₂ production.