Conjugated polymers, comprising fully π‐conjugated systems, present a new generation of heterogeneous photocatalysts for solar‐energy utilization. They have three key features, namely robustness, ...nontoxicity, and visible‐light activity, for photocatalytic processes, thus making them appealing candidates for scale‐up. Presented in this Minireview, is a brief summary on the recent development of various promising polymer photocatalysts for hydrogen evolution from aqueous solutions, including linear polymers, planarized polymers, triazine/heptazine polymers, and other related organic conjugated semiconductors, with a particular focus on the rational manipulation in the composition, architectures, and optical and electronic properties that are relevant to photophysical and photochemical properties. Some future trends and prospects for organic conjugated photocatalysts in artificial photosynthesis, by water splitting, are also envisaged.
Split up: Organic conjugated photocatalysts provide an emerging platform for sustainable H2 production, through water splitting, by artificial photosynthesis. The catalysts have unique properties, including light weight, low cost, accessibility, and fine‐tunability of chemical composition, electronic structure, surface properties, and texture.
Exciton binding energy has been regarded as a crucial parameter for mediating charge separation in polymeric photocatalysts. Minimizing the exciton binding energy of the polymers can increase the ...yield of charge‐carrier generation and thus improve the photocatalytic activities, but the realization of this approach remains a great challenge. Herein, a series of linear donor–acceptor conjugated polymers has been developed to minimize the exciton binding energy by modulating the charge‐transfer pathway. The results reveal that the reduced energy loss of the charge‐transfer state can facilitate the electron transfer from donor to acceptor, and thus, more electrons are ready for subsequent reduction reactions. The optimized polymer, FSO‐FS, exhibits a remarkable photochemical performance under visible light irradiation.
Modulating the charge‐transfer pathway in a series of linear donor–acceptor conjugated polymers controls their ability to minimize the exciton binding energy. A low exciton binding energy promotes the photoreaction for artificial photosynthesis
Conjugated polymers (CPs) are emerging and appealing light harvesters for photocatalytic water splitting owing to their adjustable band gap and facile processing. Herein, we report an advanced mild ...synthesis of three conjugated triazine‐based polymers (CTPs) with different chain lengths by increasing the quantity of electron‐donating benzyl units in the backbone. Varying the chain length of the CTPs modulates their electronic, optical, and redox properties, resulting in an enhanced performance for photocatalytic oxygen evolution, which is the more challenging half‐reaction of water splitting owing to the sluggish reaction kinetics. Our results could stimulate interest in these functional polymers where a molecular engineering strategy enables the production of suitable semiconductor redox energetics for oxygenic photosynthesis.
Chain length optimization: Conjugated triazine‐based polymers with finely tunable band structures and optical properties have been used as organic semiconductors for photocatalytic O2 evolution. The performance of the polymers can be optimized by altering the linkage structure, demonstrating an emerging and appealing application of polymeric materials in oxygenic photosynthesis.
Covalent triazine‐based frameworks (CTFs) are typically produced by the salt‐melt polycondensation of aromatic nitriles in the presence of ZnCl2. In this reaction, molten ZnCl2 salt acts as both a ...solvent and Lewis acid catalyst. However, when cyclotrimerization takes place at temperatures above 300 °C, undesired carbonization occurs. In this study, an ionothermal synthesis method for CTF‐based photocatalysts was developed using a ternary NaCl‐KCl‐ZnCl2 eutectic salt (ES) mixture with a melting point of approximately 200 °C. This temperature is lower than the melting point of pure ZnCl2 (318 °C), thus providing milder salt‐melt conditions. These conditions facilitated the polycondensation process, while avoiding carbonization of the polymeric backbone. The resulting CTF‐ES200 exhibited enhanced optical and electronic properties, and displayed remarkable photocatalytic performance in the hydrogen evolution reaction.
Covalent triazine‐based frameworks with ordered molecular structures were synthesized in a ternary NaCl‐KCl‐ZnCl2 eutectic salt system at 200 °C to produce photocatalysts for hydrogen production.
Charge generation and separation are regarded as the major constraints limiting the photocatalytic activity of polymeric photocatalysts. Herein, two new linear polyarylether‐based polymers (PAE–CPs) ...with distinct linking patterns between their donor and acceptor motifs were tailor‐made to investigate the influence of different linking patterns on the charge generation and separation process. Theoretical and experimental results revealed that compared to the traditional single‐stranded linker, the double‐stranded linking pattern strengthens donor–acceptor interactions in PAE–CPs and generates a coplanar structure, facilitating charge generation and separation, and enabling red‐shifted light absorption. With these prominent advantages, the PAE–CP interlinked with a double‐stranded linker exhibits markedly enhanced photocatalytic activity compared to that of its single‐strand‐linked analogue. Such findings can facilitate the rational design and modification of organic semiconductors for charge‐induced reactions.
Two polyarylether‐based polymers with single/double linking patterns were investigated to establish the effect of the linking patterns on the charge generation and separation process in D–A conjugated polymer photocatalysts, thus achieving remarkable photocatalytic HER and HPER performances.
Graphitic carbon nitride based polymers, being metal-free, accessible, environmentally benign and sustainable, have been widely investigated for artificial photosynthesis in recent years for the ...photocatalytic splitting of water to produce hydrogen fuel. However, the photocatalytic stoichiometric splitting of pure water into H
2
and O
2
with a molecular ratio of 2 : 1 is far from easy, and is usually hindered by the huge activation energy barrier and sluggish surface redox reaction kinetics. Herein, we provide a concise overview of cocatalyst modified graphitic carbon nitride based photocatalysts, with our main focus on the modulation of the water splitting redox reaction kinetics. We believe that a timely and concise review on this promising but challenging research topic will certainly be beneficial for general readers and researchers in order to better understand the property-activity relationship towards overall water splitting, which could also trigger the development of new organic architectures for photocatalytic overall water splitting through the rational control of surface chemistry.
Overall water splitting for the stoichiometric generation of H
2
and O
2
has been achieved by rational cocatalyst modification of g-C
3
N
4
polymers to modulate the surface redox reaction kinetics.
Covalent triazine frameworks (CTFs) represent promising polymeric photocatalysts for photocatalytic hydrogen production with visible light. However, the separation and transfer of charges in CTFs are ...isotropic because of the uniform distribution of donor–acceptor motifs in the skeleton. Herein, to achieve the anisotropic charge carrier separation and migration, thiophene (Th) or benzothiadiazole (BT) unit is selected as the dopant to modify the molecular structure of CTF‐based photocatalysts. Both theoretical and experimental studies reveal that the incorporation of Th or BT units induces the anisotropic charge carrier separation and migration at the interface of CTFs. The optimized polymer manifests a much enhanced photocatalytic activity for photocatalytic hydrogen production with visible light, and thus this study provides a useful tool to design conjugated polymer photocatalysts at the molecular level for solar energy conversion.
Anisotropic charge carrier separation and migration are obtained by the rational design of the molecular structure of covalent triazine frameworks (CTFs). Both theoretical and experimental results prove the enhanced optical absorption, fast charge carrier transfer, and promoted photocatalytic hydrogen production performance of the modified CTFs in comparison with the pristine sample.
Polymeric or organic semiconductors are promising candidates for photocatalysis but mostly only show moderate activity owing to strongly bound excitons and insufficient optical absorption. Herein, we ...report a facile bottom‐up strategy to improve the activity of a carbon nitride to a level in which a majority of photons are really used to drive photoredox chemistry. Co‐condensation of urea and oxamide followed by post‐calcination in molten salt is shown to result in highly crystalline species with a maximum π–π layer stacking distance of heptazine units of 0.292 nm, which improves lateral charge transport and interlayer exciton dissociation. The addition of oxamide decreases the optical band gap from 2.74 to 2.56 eV, which enables efficient photochemistry also with green light. The apparent quantum yield (AQY) for H2 evolution of optimal samples reaches 57 % and 10 % at 420 nm and 525 nm, respectively, which is significantly higher than in most previous experiments.
Free to dissociate: The simultaneous modification of the electronic band structures, layer‐stacking geometry, and crystallinity of carbon nitride polymers enables the liberation of Frenkel exciton dissociation and magnification of the hot‐charges yield, which dramatically enhances the visible‐light photocatalytic activities.
Conjugated polymers (CPs) represent a promising platform for photocatalytic CO2 fixation owing to their suitable band structures that meet the requirements of the reduction potential of CO2 to ...value‐added fuels. However, the photocatalytic performance of CPs is rather restrained by the low charge transfer efficiency. Herein, we rationally designed three CPs with a more delocalized electronic transmission channel and planar molecular structure, which are regarded to evidently reduce the exciton binding energy (Eb) and accelerate the internal charge transfer process. Besides, the assembly of suitable electron‐output “tentacles” and cocatalysts on the surface of CPs could effectively facilitate interfacial electron delivery. Accordingly, the optimal P‐2CN exhibits an apparent quantum yield of 4.6 % at 420 nm for photocatalytic CO2 to CO. Further adjusting the amounts of cyano groups and cocatalysts, the CO selectivity could be obtained in the range of 0–80.5 %.
An accelerated electronic transmission channel with electron‐output “tentacles” was designed in conjugated polymers. This channel was found to promote charge separation and photocatalytic CO2‐to‐CO conversion.
Pretreatment is widely used before drying of agro-products to inactivate enzymes, enhance drying process and improve quality of dried products. In current work, the influence of various pretreatments ...on drying characteristics and quality attributes of fruits and vegetables is summarized. They include chemical solution (hyperosmotic, alkali, sulfite and acid, etc.) and gas (sulfur dioxide, carbon dioxide and ozone) treatments, thermal blanching (hot water, steam, super heated steam impingement, ohmic and microwave heating, etc), and non-thermal process (ultrasound, freezing, pulsed electric field, and high hydrostatic pressure, etc). Chemical pretreatments effectively enhance drying kinetics, meanwhile, it causes soluble nutrients losing, trigger food safety issues by chemical residual. Conventional hot water blanching has significant effect on inactivating various undesirable enzymatic reactions, destroying microorganisms, and softening the texture, as well as facilitating drying rate. However, it induces undesirable quality of products, e.g., loss of texture, soluble nutrients, pigment and aroma. Novel blanching treatments, such as high-humidity hot air impingement blanching, microwave and ohmic heat blanching can reduce the nutrition loss and are more efficient. Non-thermal technologies can be a better alternative to thermal blanching to overcome these drawbacks, and more fundamental researches are needed for better design and scale up.
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