The efficient separation of photogenerated charges in polymer photocatalysts remains challenging due to their high exciton binding energies. Herein, a novel metal-free 3D/2D S-scheme heterojunction is constructed by anchoring a three-dimensional flower-like covalent organic framework (Tpttpa-COF) onto two-dimensional polymeric carbon nitride (PCN) nanosheets. The resulting 3D/2D Tpttpa-COF/PCN heterojunction possesses a unique optically controlled tuning capability, in which light illumination can significantly strengthen the internal built-in electric field (IEF) through photoinduced interfacial charge transfer, thereby further enhancing the separation and migration efficiency of photogenerated carriers. Density functional theory (DFT) calculations reveal the formation of an IEF directed from Tpttpa-COF to PCN at the interface and the optimized hydrogen adsorption free energy. Femtosecond transient absorption (fs-TA) spectroscopy further verifies an ultrafast (∼0.24 ps) S-scheme charge transfer at the interface, yielding long-lived spatially separated states. Consequently, the optimized 0.25%-Tpttpa-COF/PCN demonstrates remarkable photocatalytic hydrogen evolution performance, surpassing most reported COF-based heterojunctions. It achieves an average hydrogen evolution rate of 2.02 mmol g−1 h−1, which is 5.05 times that of pristine PCN, with a high apparent quantum efficiency of 33.14% at 400 nm. Excellent stability is retained over 64 h of cyclic operation. This work presents a new design concept to further enhance the charge separation efficiency of polymer-based S‑scheme heterojunctions, thereby offering a novel approach for developing efficient photocatalysts.

It is highly difficult to modify the high-density quantum dots (QDs) on the photocatalyst, which also suffers from the issues of uneven dispersion and low stability caused by weak bonding forces. Here, a new "in situ evolution of alloy" strategy is proposed to fabricate a hybrid photocatalyst of high-density and well-dispersed SnTe3O8 QDs modified honeycomb-like carbon nitride (HCN) nanotubes. In the resulting SnTe3O8/HCN hybrid, SnTe3O8 QDs act as charge transport bridges that effectively extract photogenerated electrons produced from HCN nanotubes to improve the separation efficiency of photogenerated carriers, thus enhancing the photocatalytic hydrogen evolution performance. As a result, the hydrogen production rate of the optimal SnTe3O8/HCN-3 % sample is 1716.5 mu mol g- 1 h- 1 under visible light, which is 3.3 times that of HCN nanotubes (513.3 mu mol g- 1 h- 1). Meanwhile, the optimal SnTe3O8/HCN-3 % sample achieves an apparent quantum yield of 7.8 % at 420 nm and demonstrates outstanding cyclic stability.

It is an arduous issue to significantly improve the charge separation efficiency of polymer photocatalysts due to their inherently high exciton binding energy. Herein, based on an interfacial coupling and atom diffusion strategy, a metal-free 3D/2D van der Waals (VdW) heterojunction is fabricated through the modification of rich-vacancy wrinkle-like S8 (Vs-S8) microspheres on the surface of S-doped polymeric carbon nitride (S-PCN) nanosheets. The insight into the mechanism reveals that the interfacial coupling effect induces a strong built-in electric field from S-PCN to Vs-S8, and the carrier transfer behavior abides by the type-II charge transfer pathway, thereby dramatically improving the separation efficiency and transport kinetics of photogenerated carriers. As a result, the as-prepared metal-free 3D/2D Vs-S8/S-PCN VdW heterojunction is endowed with more superior photocatalytic hydrogen evolution (PHE) performance than PCN. The highest average PHE rate is about 11.3 times higher than that of PCN, and the apparent quantum efficiency reaches 30.1% at 400 nm on the optimal 3%-Vs-S8/S-PCN sample. This work contributes a new design strategy for a metal-free VdW heterojunction and provides a feasible avenue for improving the carrier separation efficiency of polymer photocatalysts.

It is a challenging issue for the creation of photogenerated carrier collectors on the photocatalyst to drive charge separation and promote reaction kinetics in the photocatalytic reaction. Herein, based on one-step dual-modulation strategy, IrO2 nanodots are modified at the edge of polymeric carbon nitride (PCN) nanosheets and atomically dispersed Ir atoms are implanted in the skeleton of PCN to obtain a unique Ir-PCN/IrO2 photocatalyst. IrO2 nanodots and atomically dispersed Ir atoms act as hole and electron collectors to synergistically promote the carrier separation and reaction kinetics, respectively, thereby greatly improving the photocatalytic hydrogen evolution (PHE) performance. As a result, without adding additional cocatalyst, the PHE rate over the optimal Ir-PCN/IrO2-2% sample reaches up to 1564.4 μmol h-1 g-1 under the visible light irradiation, with achieving an apparent quantum yield (AQY) of 15.7% at 420 nm.

It is a challenging issue to achieve efficient charge transfer between layers for the layered polymer photocatalysts, because the photogenerated carriers would prefer random migration within layers and thus have high reverse recombination rate due to their inherent anisotropy and high exciton binding energy. Herein, an innovative surface grafting strategy is developed to modify the layered polymeric carbon nitride (PCN) making use of phosphotungstic (PWO) polyanions. The surface coupling effect of PWO on PCN effectively improves reduction ability and utility rate of luminous energy through shifting the conduction band (CB) upward and broadening light absorption range to cover near infrared region up to 1000 nm, respectively. In especial, the strong electronwithdrawing effect of surface PWO moieties can reduce layer spacing to shorten electron transfer path and attract the photogenerated electrons transfer from PCN matrix to surface, thus conquering the charge transfer restriction between layers to improve the carrier separation efficiency. As a result, the photocatalytic hydrogen evolution (PHE) performance is dramatically enhanced. The optimal PWO-PCN-1% sample achieves an average PHE rate of 9.6 times that of PCN, stable operation for a total of 28 h on 7 cycles, and an AQY up to 22.3% at 400 nm. This contribution affords a viable modification avenue to overcome the limitation of charge transfer between layers for layered polymer photocatalysts.

In order to achieve the sustainable production of hydrogen fuel, the heterogeneous photocatalysts with inimitable structure are always desired for the high-efficiency and stable H2 production reaction from water splitting owing to the perfect structure-activity relationship. In this study, a novel metal-free one-dimensional/twodimensional (1D/2D) heterojunction is constructed by a series of dissolution and diffusion, recrystallization and in-situ growth, and self-assembly process of 1-pyrene carboxaldehyde (PCHO) nanoribbons on the surface of polymeric carbon nitride (PCN) nanosheets. The obtained 1D/2D heterojunction can realize fast carrier transport along 1D nanoribbons to prevent the recombination of photogenerated carriers at the interface. Furthermore, the intrinsic Z-scheme reaction mechanism within the heterostructure also effectively inhibits electron-hole recombination and isolates the reduction and oxidation sites of the photocatalytic reaction. As a result, the dramatically enhanced photocatalytic hydrogen evolution (PHE) activity is achieved over the 1%-PCHO/PCN sample, the optimal PHE rate of which is approximately 5.4 times that of pure PCN. This work provides the deep insight into the design and exploitation of metal-free heterojunction with unique structure applied in the photocatalytic energy conversion field.

Hydrogen extraction from the water splitting driven by solar energy is considered to be a sustainable technology, but it is still a challenging issue to develop high-efficient and stable photocatalysts. Herein, a new metal-free polymeric carbon nitride (PCN)-based Van der Waals (VdW) heterojunction photocatalyst is exploited by con-trolling the micro-morphologies of another structural unit, the unique 2D/1D microstructure of which is fabri-cated by decorating polythiocyanogen (SCN)x nanoribbons on PCN nanosheets. The coupling effect of (SCN)x on PCN effectively propels interplanar charge separation through Z-scheme charge transfer path formed by the compact face-to-face it-it electronic interaction between them. Moreover, the 1D nanoribbon morphology of (SCN)x also can shorten one direction of charge migration distance in the plane to inhibit the intramolecular carrier recombination compared with traditional 2D structure. The resultant photocatalytic hydrogen evolution (PHE) rate on the optimal PCN/(SCN)x-3 % sample reaches about 4 times as large as pure PCN, and a high apparent quantum efficiency (AQE) of 18.5 % is also achieved at 420 nm. This work develops a new design approach based on spatial dimension regulation strategy of structure unit for developing PCN-based VdW het-erojunction photocatalyst applied to hydrogen production from the water splitting.

Developing the high-performance semiconductor photocatalytic materials is an eternal topic under the background of the current energy and environment requirements. In recent years, single-atom photocatalysts (SAPCs) have been brought a lot of attention in energy conversion and environmental purification because of their unique characteristics and properties, including the unique coordination patterns, outstanding atomic utilization, quantum confinement effects, high catalytic activity, etc. Hence, this critical review focuses on the summarized various synthetic methods and the recent important applications of SAPCs, including photocatalytic H-2 evolution (PHE) from water splitting, photocatalytic CO2 reduction, photodegradation of organic pollutants, etc. The prospects and challenges for future research topics of SAPCs with excellent activity and stability for various photocatalytic applications are prospected at the end of this review. We sincerely expect that this critical review can promote deep-level insight into the SAPCs subject for the future significant applications in other fields.

Micro/nanostructure control of heterostructures is still a challenge for achieving high efficiency and selectivity of photocatalytic CO2 conversion. In this work, a new three-dimensiona/two-dimensional (3D/2D) heterostructure is fabricated by encapsulating RuS2 nanospheres in the interlayer of mesoporous polymeric carbon nitride (PCN) nanosheets based on an in situ growth and polymerization strategy. The unique microstructure of the obtained 3D/2D RuS2/PCN heterojunction can effectively improve the transfer and separation efficiency of photogenerated charge carriers, reduce the mass transfer resistance of CO2 toward active sites, and provide a confined reaction space, thus propelling the photocatalytic CO2 reduction to CO with high selectivity. The CO yield over the optimal 5%-RuS2/PCN sample reaches 4.2 and 2.8 times as high as that of single PCN and RuS2 within 4 h, respectively. Furthermore, the plausible charge transfer mechanism and CO2 reduction path are revealed by time-dependent in situ Fourier transform infrared (FT-IR) spectra combined with photophysical, electrochemical, and photoelectrochemical techniques and density functional theory (DFT) calculations. This work develops the microstructural engineering design strategy of PCN-based heterojunctions for selective photocatalytic CO2 fuel conversion.

It is a challenge to fabricate atomically dispersed metal clusters in polymeric carbon nitride (PCN) for durable photocatalytic reactions owing to the thermodynamic stability limitation. Herein, atomically dispersed Ru clusters are implanted into the PCN skeleton matrix based on an ionic diffusion and coordination (IDC) strategy, the stability of which is improved owing to the robust Ru-N bonds in the formed RuN4 and RuN3 configurations. Additionally, RuN4 and RuN3 as charge transport bridges between two adjacent melon strands efficaciously conquer hydrogen bond restriction in the skeleton to facilitate the in-plane mobility and separation of charge carriers. Moreover, the synergistic effect of adjacent Ru atoms is triggered on the assembled RuN3-RuN4 and RuN3-RuN3 in the atomically dispersed Ru clusters to significantly decrease hydrogen adsorption energy. As a result, the optimal PCN-Ru photocatalyst achieves nearly 6 times higher than the photocatalytic hydrogen evolution (PHE) rate of the Pt/PCN benchmark and maintains the long-term stable running for 104 h of 26 cycles; its overall PHE performance is far superior to the most of single atoms supported on g-C3N4 photocatalysts reported. The findings here gain new insight into the preparation strategy, structure configuration, and reaction mechanism for atomically dispersed metal clusters supported on PCN, which further stimulates the intensive investigations toward developing more efficient and stable PCN-like photocatalytic materials.

The interfacial charge transfer control is a key and arduous issue for propelling the migration/separation of photogenerated carriers for heterojunction photocatalysts. Here, a new 2D/2D C3N4/C3N5 nonmetal van der Waals (VdW) heterojunction is fabricated by the simple self-assembly technique in acidic medium, whose charge separation efficiency is promoted dramatically, thus being endowed with the high-efficiency photocatalytic hydrogen evolution (PHE) performance. The PHE rate reaches up to 3.33 mmol h-1 g-1 under the visible light and the apparent quantum efficiency (AQE) of 20.6% is achieved at 420 nm on the optimal 2D/2D C3N4/C3N5-5% sample. Furthermore, the 2D/2D C3N4/C3N5 nonmetal VdW heterojunction also exhibits the desired stability because there was no significant decrease after PHE reaction of 10 cycles with total 40 h. Such outstanding PHE activity and stability originate from the impelled separation of photoinduced charge car-riers and the powerful interfacial interaction through forming Z-Scheme charge transfer path and TC-TC coupling effect between C3N4 and C3N5 nanosheets. This work takes a sig-nificant guiding and demonstration for designing and exploiting other novel nonmetallic polymer-based VdW heterojunctions in the photocatalytic application field.(c) 2022 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.