Expedient alkyne semi-hydrogenation by using a bimetallic AgCu-C3N4 single atom catalyst

Metal-catalyzed semi-hydrogenation of alkynes is an important step in organic synthesis to produce diverse chemical compounds. However, conventional noble metal catalysts often suffer from poor selectivity owing to over-hydrogenation. Here, we demonstrate a high-loading bimetallic AgCu-C3N4 single-atom catalyst (SAC) for alkyne semi-hydrogenation. The AgCu-C3N4 SACs exhibit higher activity and selectivity (99%) than their low-loading variants due to the synergistic interaction of heteronuclear Ag-Cu sites at small inter-site distances. Using a combination of techniques such as phenylacetylene-DRIFTS, H2-temperature programmed desorption and DFT calculations, we showed that the cooperative bimetallic interaction during alkyne semi-hydrogenation was achieved by isolated Ag centers as hydrogen activation sites and isolated Cu centers as alkyne activation sites. Our work highlights the importance of achieving high catalyst loading to reduce the inter-site distance in bimetallic SACs for cooperative interactions, which can potentially open new catalytic pathways for synthesizing fine chemicals and pharmaceuticals.

Development of graphitic carbon nitride quantum dots-based oxygen self-sufficient platforms for enhanced corneal crosslinking

Keratoconus, a disorder characterized by corneal thinning and weakening, results in vision loss. Corneal crosslinking (CXL) can halt the progression of keratoconus. The development of accelerated corneal crosslinking (A-CXL) protocols to shorten the treatment time has been hampered by the rapid depletion of stromal oxygen when higher UVA intensities are used, resulting in a reduced cross-linking effect. It is therefore imperative to develop better methods to increase the oxygen concentration within the corneal stroma during the A-CXL process. Photocatalytic oxygen-generating nanomaterials are promising candidates to solve the hypoxia problem during A-CXL. Biocompatible graphitic carbon nitride (g-C3N4) quantum dots (QDs)-based oxygen self-sufficient platforms including g-C3N4 QDs and riboflavin/g-C3N4 QDs composites (RF@g-C3N4 QDs) have been developed in this study. Both display excellent photocatalytic oxygen generation ability, high reactive oxygen species (ROS) yield, and excellent biosafety. More importantly, the A-CXL effect of the g-C3N4 QDs or RF@g-C3N4 QDs composite on male New Zealand white rabbits is better than that of the riboflavin 5'-phosphate sodium (RF) A-CXL protocol under the same conditions, indicating excellent strengthening of the cornea after A-CXL treatments. These lead us to suggest the potential application of g-C3N4 QDs in A-CXL for corneal ectasias and other corneal diseases.

Construction of a CoNiHHTP MOF/PHI Z-Scheme Heterojunction for ppb Level NO2 Photoelectric Sensing with 405 nm Irradiation at RT

Ultrasensitive photoelectric detection of nitrogen dioxide (NO2) with PHI under visible light irradiation at room temperature (RT) remains an ongoing challenge due to the low charge separation and scarce adsorption sites. In this work, a dimensionally matched ultrathin CoNiHHTP MOF/PHI Z-scheme heterojunction is successfully constructed by taking advantage of the π-π interactions existing between the CoNiHHTP MOF and PHI. The amount-optimized heterojunction possesses a record detection limit of 1 ppb (response = 15.6%) for NO2 under 405 nm irradiation at RT, with reduced responsive (3.6 min) and recovery (2.7 min) times, good selectivity and reversibility, and long-time stability (150 days) compared with PHI, even superior to others reported at RT. Based on the time-resolved photoluminescence spectra, in situ X-ray photoelectron spectra, and diffuse reflectance infrared Fourier transform spectroscopy results, the resulting sensing performance is attributed to the favorable Z-scheme charge transfer and separation. Moreover, the Ni nodes favorably present in adjacent metal sites between the lamellae contribute to charge transfer and redistribution, whereas Co nodes could act as selective centers for promoted adsorption of NO2. Interestingly, it is confirmed that the CoNiHHTP MOF/PHI heterojunction could effectively reduce the influence of O2 in the gas-sensitive reaction due to their unique bimetallic (Co and Ni) nodes, which is also favorable for the improved sensing performances for NO2. This work provides a feasible strategy to develop promising PHI-based optoelectronic gas sensors at RT.

In-situ one-step construction of poly(heptazine imide)/poly(triazine imide) heterojunctions for photocatalytic hydrogen evolution

The construction of heterojunctions is challenging, requiring atomic-level contact and interface matching. Here, we have achieved atomic-level interfacial matching by constructing poly(heptazine imide)/poly(triazine imide) crystalline carbon nitride heterojunctions in an in-situ one-step method. The content of poly(triazine imide) in heterojunctions is positively related to the proportion of lithium chloride in potassium chloride and lithium chloride mixed-salts. The optimized heterojunction achieves an apparent quantum efficiency of 48.34 % for photocatalytic hydrogen production at 420 nm, which is at a good level in polymeric carbon nitride photocatalysts. The proposed ion-thermal assisted heterojunction construction strategy contributes to the development of polymeric carbon nitride photocatalysts with high crystallization and high charge separation efficiency.

Metal Poly(heptazine imides) as Multifunctional Photocatalysts for Solar Fuel Production

Solar-driven photocatalysis employing particulate semiconductors represents a promising approach for sustainable production of valuable chemical feedstock. Metal poly(heptazine imide) (MPHI), a novel 2D ionic carbon nitride, has been recognized as an emerging photocatalyst with distinctive properties. In this minireview, we first delineate the forefront innovations of MPHI photocatalysts, spanning from synthetic strategies and solving structures to the exploration of novel properties. We place special emphasis on the structural design principles aimed at developing high-performance MPHI systems toward photocatalytic solar fuel production such as H2 evolution, H2O oxidation, H2O2 production and CO2 reduction. Finally, we discuss crucial insights and challenges in leveraging highly active MPHIs for efficient solar-to-chemical energy conversion.

Materials Advances in Photocatalytic Solar Hydrogen Production: Integrating Systems and Economics for a Sustainable Future

Photocatalytic solar hydrogen generation, encompassing both overall water splitting and organic reforming, presents a promising avenue for green hydrogen production. This technology holds the potential for reduced capital costs in comparison to competing methods like photovoltaic-electrocatalysis and photoelectrocatalysis, owing to its simplicity and fewer auxiliary components. However, the current solar-to-hydrogen efficiency of photocatalytic solar hydrogen production has predominantly remained low at ≈1-2% or lower, mainly due to curtailed access to the entire solar spectrum, thus impeding practical application of photocatalytic solar hydrogen production. This review offers an integrated, multidisciplinary perspective on photocatalytic solar hydrogen production. Specifically, the review presents the existing approaches in photocatalyst and system designs aimed at significantly boosting the solar-to-hydrogen efficiency, while also considering factors of cost and scalability of each approach. In-depth discussions extending beyond the efficacy of material and system design strategies are particularly vital to identify potential hurdles in translating photocatalysis research to large-scale applications. Ultimately, this review aims to provide understanding and perspective of feasible pathways for commercializing photocatalytic solar hydrogen production technology, considering both engineering and economic standpoints.

Harnessing the Power of Light: The Synergistic Effects of Crystalline Carbon Nitride and Ti3C2Tx MXene in Photocatalytic Hydrogen Production

Photocatalytic hydrogen evolution is an environmentally friendly means of energy generation. Although g-C3N4 possesses fascinating features, its inherent shortcomings limit its photocatalytic applications. Therefore, modifying the intrinsic properties of g-C3N4 and introducing cocatalysts are essential to ameliorate the photocatalytic efficiency. To achieve this, metal-like Ti3C2Tx is integrated with crystalline g-C3N4 via a combined salt-assisted and freeze-drying approach to form crystalline g-C3N4/Ti3C2Tx (CCN/TCT) hybrids with different Ti3C2Tx loading amounts (0, 0.2, 0.3, 0.4, 0.5, 1, 5, 10 wt.%). Benefiting from the crystallization of CN, as evidenced by the XRD graph, and the marvelous conductivity of Ti3C2Tx supported by EIS plots, CCN/TCT/Pt loaded with 0.5 wt.% Ti3C2Tx displays an elevated H2 (2) should be subscripted evolution rate of 2651.93 µmol g-1 h-1 and a high apparent quantum efficiency of 7.26% (420 nm), outperforming CN/Pt, CCN/Pt, and other CCN/TCT/Pt hybrids. The enhanced performance is attributed to the synergistic effect of the highly crystalline structure of CCN that enables fleet charge transport and the efficient dual cocatalysts, Ti3C2Tx and Pt, that foster charge separation and provide plentiful active sites. This work demonstrates the potential of CCN/TCT as a promising material for hydrogen production, suggesting a significant advancement in the design of CCN heterostructures for effective photocatalytic systems.

Increasing Profitability of Ethanol Photoreforming by Simultaneous Production of H2 and Acetal

Often, H2 is produced photocatalytically at the expense of sacrificial agents. When a sacrificial agent is selectively oxidized, this allows coupling of H2 production with synthesis of value-added organic compounds. Herein, it is argued that the conversion of bioethanol into 1,1-diethoxyethane with simultaneous H2 production increases the economic viability of photocatalysis and suggests a semiconductor material that is the most relevant for this purpose.

Unveiling the Role of Water in Heterogeneous Photocatalysis of Methanol Conversion for Efficient Hydrogen Production

Water molecules, which act as both solvent and reactant, play critical roles in photocatalytic reactions for methanol conversion. However, the influence of water on the adsorption of methanol and desorption of liquid products, which are two essential steps that control the performance in photocatalysis, has been well under-explored. Herein, we reveal the role of water in heterogeneous photocatalytic processes of methanol conversion on the platinized carbon nitride (Pt/C3N4) model photocatalyst. In situ spectroscopy techniques, isotope effects, and computational calculations demonstrate that water shows adverse effects on the adsorption of methanol molecules and desorption processes of methanol oxidation products on the surface of Pt/C3N4, significantly altering the reaction pathways in photocatalytic methanol conversion process. Guided by these discoveries, a photothermal-assisted photocatalytic system is designed to achieve a high solar-to-hydrogen (STH) conversion efficiency of 2.3 %, which is among the highest values reported. This work highlights the important roles of solvents in controlling the adsorption/desorption behaviours of liquid-phase heterogeneous catalysis.

Solvated Electrons Generated on the Surface of Na-SPHI for Boosting Visible Light Photocatalytic Hydrogen Evolution with Ultra-High AQE

Enhancing the utilization of visible-light-active semiconductors with an excellent apparent quantum efficiency (AQE) remains a significant and challenging goal in the realm of photocatalytic water splitting. In this study, a fully condensed sulfur-doped poly(heptazine imide) metalized with Na (Na-SPHI) is synthesized by an ionothermal method by using eutectic NaCl/LiCl mixture as the ionic solvent. Comprehensive characterizations of the obtained Na-SPHI reveal several advantageous features, including heightened light absorption, facilitated exciton dissociation, and expedited charge transfer. More importantly, solvated electron, powerful reducing agents, can be generated on the surface of Na-SPHI upon irradiation with visible light. Benefiting from above advantage, the Na-SPHI exhibits an excellent H2 evolution rate of 571.8 µmol·h-1 under visible light illumination and a super-high AQE of 61.7% at 420 nm. This research emphasizes the significance of the solvated electron on the surface of photocatalyst in overcoming the challenges associated with visible light-driven photocatalysis, showcasing its potential application in photocatalytic water splitting.

Templated 2D Polymer Heterojunctions for Improved Photocatalytic Hydrogen Production

2D polymers have emerged as one of the most promising classes of organic photocatalysts for solar fuel production due to their tunability, charge-transport properties, and robustness. They are however difficult to process and so there are limited studies into the formation of heterojunction materials incorporating these components. In this work, a novel templating approach is used to combine an imine-based donor polymer and an acceptor polymer formed through Knoevenagel condensation. Heterojunction formation is shown to be highly dependent on the topological match of the donor and acceptor polymers with the most active templated material found to be between three and nine times more active for photocatalysis than its constituent components. Transient absorption spectroscopy reveals that this improvement is due to faster charge separation and more efficient charge extraction in the templated heterojunction. The templated material shows a very high hydrogen evolution rate of >20 mmol h-1 m-2 with an ascorbic acid hole scavenger but also produces hydrogen in the presence of only water and a cobalt-based redox mediator. This suggests the improved charge-separation interface and reduced trapping accessed through this approach could be suitable for Z-scheme formation.

Progress on enhancing the charge separation efficiency of carbon nitride for robust photocatalytic H2 production

Solar-driven H2 production from water splitting with efficient photocatalysts is a sustainable strategy to meet the clean energy demand and alleviate the approaching environmental issues caused by fossil fuel consumption. Among various semiconductor-based photocatalysts, graphitic carbon nitride (g-C3N4) has attracted much attention due to its advantages of long term-stability, visible light response, low cost, and easy preparation. However, the intrinsic Coulombic attraction between charge carriers and the interlayer electrostatic barrier of bulk g-C3N4 result in severe charge recombination and low charge separation efficiency. This perspective summarizes the recent progress in the development of g-C3N4 photocatalytic systems, and focuses on three main modification strategies for promoting charge transfer and minimizing charge recombination, including structural modulation, heterojunction construction, and cocatalyst loading. Based on this progress, we provide conclusions regarding the current challenges of further improving photocatalytic efficiency to fulfill commercial requirements, and propose some recommendations for the design of novel and satisfactory g-C3N4 photocatalysts, which is expected to progress the solar-to-hydrogen conversion.

Rejoint of Carbon Nitride Fragments into Multi-Interfacial Order-Disorder Homojunction for Robust Photo-Driven Generation of H2 O2

Photocatalytic technology based on carbon nitride (C3 N4 ) offers a sustainable and clean approach for hydrogen peroxide (H2 O2 ) production, but the yield is severely limited by the sluggish hot carriers due to the weak internal electric field. In this study, a novel approach is devised by fragmenting bulk C3 N4  into smaller pieces (CN-NH4 ) and then subjecting it to a directed healing process to create multiple order-disorder interfaces (CN-NH4 -NaK). The resulting junctions in CN-NH4 -NaK significantly boost charge dynamics and facilitate more spatially and orderly separated redox centers. As a result, CN-NH4 -NaK demonstrates outstanding photosynthesis of H2 O2 via both two-step single-electron and one-step double-electron oxygen reduction pathways, achieving a remarkable yield of 16675 µmol h-1  g-1 , excellent selectivity (> 91%), and a prominent solar-to-chemical conversion efficiency exceeding 2.3%. These remarkable results surpass pristine C3 N4 by 158 times and outperform previously reported C3 N4 -based photocatalysts. This work represents a significant advancement in catalyst design and modification technology, inspiring the development of more efficient metal-free photocatalysts for the synthesis of highly valued fuels.

Organic Photovoltaic Materials for Solar Fuel Applications: A Perfect Match?

This work discusses the use of donor and acceptor materials from organic photovoltaics in solar fuel applications. These two routes to solar energy conversion have many shared materials design parameters, and in recent years there has been increasing overlap of the molecules and polymers used in each. Here, we examine whether this is a good approach, where knowledge can be translated, and where further consideration to molecular design is required.

Polymer carbon nitride nanosheet-based lamellar membranes inspired by "couple hardness with softness" for ultrafast molecular separation in organic solvents

Membranes with ultrafast molecular separation ability in organic solvents can offer unprecedented opportunities for efficient and low-cost solvent recovery in industry. Herein, a graphene-like polymer carbon nitride nanosheet (PCNN) with a low-friction surface was applied as the main membrane building block to boost the ultrafast transport of the solvent. Meanwhile, inspired by the concept of "couple hardness with softness", soft and flexible graphene oxide (GO) was chosen to fix the random stack of the rigid PCNN and tailor the lamellar structure of the PCNN membrane. The optimal PCNN/GO lamellar membrane shows a remarkable methanol permeance of 435.5 L m-2 h-1 bar-1 (four times higher than that of the GO membrane) while maintaining a high rejection for reactive black (RB, 98.9% in ethanol). Molecular dynamics simulations were conducted to elucidate the ultrafast transport mechanism of the PCNN/GO membrane. This study reveals that PCNN is a promising building block for lamellar membranes and may open up new avenues for high-performance molecular separation membranes.

Order-Disorder Engineering of Carbon Nitride for Photocatalytic H2O2 Generation Coupled with Pollutant Removal

Highly crystalline carbon nitride (CCN), benefiting from the reduced structural imperfections, enables improved electron-hole separation. Yet, the crystalline phase with insufficient inherent defects suffers from a poor performance toward the reaction intermediate adsorption with respect to the amorphous phase. Herein, a crystalline-amorphous carbon nitride (CACN) with an isotype structure was constructed via a two-step adjacent calcination strategy. Through specific oxygen etching and crystallization, the formation of a built-in electric field at the interface could drive charge transfer and separation, thus promoting photoredox reaction. As expected, the optimized CACN exhibited a H2O2 generation efficiency as high as 2.15 mM gcat-1 h-1, paired with a promoted pollutant degradation efficiency, which outperform its crystalline (CCN) and amorphous [amorphous carbon nitride (ACN)] counterparts. The detailed electron/hole transportation via a built-in electronic field and free radical formation based on the enhanced adsorption of oxygen were considered, and the synchronous reaction pathway was carried out. This work paves a novel pathway for the synthesis of carbon nitride with an isotype structure from the perspective of interfacial engineering.

Flexo-/Piezoelectric Polarization Boosting Exciton Dissociation in Curved Two-Dimensional Carbon Nitride Photocatalyst

Two-dimensional (2D) carbon nitride (CN) materials have received tremendous attention as photocatalysts for clean energy and environmental treatment. However, the photocatalytic efficiency of CN is constrained by the high exciton binding energy and sluggish charge kinetics due to weak dielectric screening, impeding the overall process. Herein, localized flexo-/piezoelectric polarization is introduced via strain engineering, boosting exciton dissociation and promoting charge separation to enhance the multielectron photocatalytic process. Consequently, the exciton binding energy of polarized CN is reduced from 52 to 34 meV, and the hydrogen evolution yield increased by 2.9 times compared to that of the pristine CN. For other photocatalytic reactions (e.g., H2O2 production), the polarized CN also maintained a 2.1-fold increase compared to the pristine CN. This strategy of inducing localized polarization via strain engineering provides new insights for boosting photocatalytic reactions involving electrons.

Stability and Crystallinity of Sodium Poly(Heptazine Imide) in Photocatalysis

Poly(heptazine imide) (PHI) salts, as crystalline carbon nitrides, exhibit high photocatalytic activity and are being extensively researched, but its photochemical instability has not drawn researchers' attention yet. Herein, sodium PHI (PHI-Na) ultrathin nanosheets with increased crystallinity, synthesized by enhancing contact of melamine with NaCl functioning as a structure-induction agent and hard template, exhibits improved photocatalytic hydrogen evolution activity, but low photochemical stability, owing to Na+ loss in the photocatalytic process, which, interestingly, can be enhanced by the common ion effect, e.g., addition of NaCl that is also able to remarkably increase the photoactivity with the apparent quantum yield at 420 nm reaching 41.5 %. This work aims at attracting research peers' attention to photochemical instability of PHI salts and provides a way to enhance their crystallinity.

Electrostatic Self-Assembled Synthesis of Amorphous/Crystalline g-C3N4 Homo-Junction for Efficient Photocatalytic H2 Production with Simultaneous Antibiotic Degradation

g-C3N4 has been regarded as a promising photocatalyst for photo-reforming antibiotics for H2 production but still suffers from its high charge recombination, which has been proven to be solvable by constructing a g-C3N4 homo-junction. However, those reported methods based on uncontrollable calcination for preparing a g-C3N4 homo-junction are difficult to reproduce. Herein, an amorphous/crystalline g-C3N4 homo-junction (ACN/CCN) was successfully synthesized via the electrostatic self-assembly attachment of negatively charged crystalline g-C3N4 nanorods (CCN) on positively charged amorphous g-C3N4 sheets (ACN). All the ACN/CCN samples displayed much higher photo-reforming of antibiotics for H2 production ability than that of pristine ACN and CCN. In particular, ACN/CCN-2 with the optimal ratio exhibited the best photocatalytic performance, with a H2 evolution rate of 162.5 μmol·g-1·h-1 and simultaneous consecutive ciprofloxacin (CIP) degradation under light irradiation for 4 h. The UV-vis diffuse reflectance spectra (DRS), photoluminescence (PL), and electrochemical results revealed that a homo-junction is formed in ACN/CCN due to the difference in the band arrangement of ACN and CCN, which effectively suppressed the charge recombination and then led to those above significantly enhanced photocatalytic activity. Moreover, H2 was generated from the water reduction reaction with a photogenerated electron (e-), and CIP was degraded via a photogenerated hole (h+). ACN/CCN exhibited adequate photostability and reusability for photocatalytic H2 production with simultaneous CIP degradation. This work provides a new idea for rationally designing and preparing homo-junction photocatalysts to achieve the dual purpose of chemical energy production and environmental treatment.

Copper-Loaded Nanoheterojunction Enables Superb Orthotopic Osteosarcoma Therapy via Oxidative Stress and Cell Cuproptosis

Catalytic tumor therapy based on two-dimensional (2D) nanomaterials is a burgeoning and promising tumor therapeutic modality. However, the inefficient utilization and conversion of exogenous stimulation, single catalytic modality, and unsatisfactory therapeutic efficiency in the tumor microenvironment (TME) have seriously restricted their further application in tumor therapy. Herein, the heterogeneous carbon nitride-based nanoagent named T-HCN@CuMS was successfully developed, which dramatically improved the efficiency of the tumor therapeutic modality. Benefiting from the donor-acceptor (triazine-heptazine) structure within the heterogeneous carbon nitride nanosheets (HCN) and the construction of interplanar heterostructure with copper loaded metallic molybdenum bisulfide nanosheets (CuMS), T-HCN@CuMS presented a favorable photo-induced catalytic property to generate abundant reactive oxygen species (ROS) under near-infrared (NIR) light irradiation. Besides, the choice of CuMS simultaneously enabled this nanoagent to efficiently catalyze the Fenton-like reaction and trigger cell cuproptosis, a recently recognized regulated cell death mode characterized by imbalanced intracellular copper homeostasis and aggregation of lipoylated mitochondrial proteins. Moreover, upon surface modification with cRGDfk-PEG2k-DSPE, T-HCN@CuMS was prepared and endowed with improved dispersibility and αvβ3 integrins targeting ability. In general, through the rational design, T-HCN@CuMS was facilely prepared and had achieved satisfactory antitumor and antimetastasis outcomes both in vitro and in a high-metastatic orthotopic osteosarcoma model. This strategy could offer an idea to treat malignant diseases based on 2D nanomaterials.

Construction of Hexameric Ru-Substitution POMs to Improve Photocatalytic H2 Evolution

Converting solar energy into storable hydrogen energy by employing green photocatalytic technology offers a reliable alternative for meeting the energy crisis. The polyoxometalates are a promising candidate for hydrogen production photocatalysts because of their unique electronic and structural properties and controllable design at the molecular level. Introducing noble metals was proven to be an effective method to greatly enhance the photocatalytic efficiency of polyoxometalates. Herein, two unprecedented compounds of hexameric Ru-POMs, Na4H10[As2RuIV2W11O18(OH)4(H2O)6{AsW8RuIVO31(OH)Cl}2(B-β-AsW9O33)4]·93H2O (1) and Na2H19[AsRuIII2W11O20(OH)2(H2O)6(RuIIICl3)(B-β-AsW9O33)6]·90H2O (2), were successfully self-assembled. The H2 evolution rates of 1 and 2 under optimal conditions were 3578.75 and 3027.69 μmol h-1 g-1 with TONs of 255 and 205, respectively. The stability of 1 was demonstrated by a series of characterizations. Besides, a possible photocatalytic mechanism was proposed.

Molecular Heptazine-Triazine Junction over Carbon Nitride Frameworks for Artificial Photosynthesis of Hydrogen Peroxide

Revealing the photocatalytic mechanism between various junctions and catalytic activities has become a hotspot in photocatalytic systems. Herein, an internal molecular heptazine/triazine (H/T) junction in crystalline carbon nitride (HTCN) is constructed and devoted to selective two-electron oxygen reduction reaction (2e- ORR) for efficient hydrogen peroxide (H2 O2 ) production. In-situ X-ray diffraction spectra under various temperatures authenticate the successful formation of molecular H/T junction in HTCN during the calcining process rather than physically mixing. The increased surface photovoltage and transient photovoltage signals, and the decreased exciton binding energy undoubtably elucidate that an obvious increasement of carrier density and diffusion capability of photogenerated electrons are realized over HTCN. Additionally, the analyses of in situ photoirradiated Kelvin probe force microscopy and femto-second transient absorption spectra reveal the successful construction of the strong internal built-in-electric field and the existence of the majority of long-lived shallow trapped electrons associated with molecular H/T junction over HTCN, respectively. Benefiting from these, the photocatalytic results exhibit an incredible improvement (96.5-fold) for H2 O2 production. This novel work provides a comprehensive understanding of the long-lived reactive charges in molecular H/T junctions for strengthening the driving-force for photocatalytic H2 O2 production, which opens potential applications for enhancing PCN-based photocatalytic redox reactions.

Bottom-up Synthesis of Single-Crystalline Poly (Triazine Imide) Nanosheets for Photocatalytic Overall Water Splitting

Poly (triazine imide) (PTI/Li+ Cl- ), one of the crystalline versions of polymeric carbon nitrides, holds great promise for photocatalytic overall water splitting. In principle, the photocatalytic activity of PTI/Li+ Cl- is closely related to the morphology, which could be reasonably tailored by the modulation of the polycondensation process. Herein, we demonstrate that the hexagonal prisms of PTI/Li+ Cl- could be converted to hexagonal nanosheets by adjusting the binary eutectic salts from LiCl/KCl or NaCl/LiCl to ternary LiCl/KCl/NaCl. Results reveal that the extension of in-plane conjugation is preferred, when the polymerisation was performed in the presence of ternary eutectic salts. The hexagonal nanosheets bears longer lifetimes of charge carriers than that of hexagonal prisms due to lower intensity of structure defects and shorter hopping distance of charge carriers along the stacking direction of triazine nanosheets. The optimized hexagonal nanosheets exhibits a record apparent quantum yield value of 25 % (λ=365 nm) for solar hydrogen production by one-step excitation overall water splitting.

Effect of molten-salt modulation on the composition and structure of g-C3N4-based photocatalysts

Graphitic carbon nitride (g-C3N4), as an attractive metal-free polymer photocatalyst, has attracted extensive attention in energy and environmental fields in recent years. The photoactivity of bulk g-C3N4 is moderate on account of solid-phase thermal-condensation synthesis. This leads to inadequate light absorption, limited surface area, and easy recombination of charge carriers. The composition and nanostructure of g-C3N4 have been studied extensively. Molten-salt modulation is fascinating because of its "green" credentials and the properties of liquid-phase reaction systems. The review focuses mainly on molten-salt modulation of the composition and structure of g-C3N4 based-photocatalysts. We focus on elemental doping, molecular doping, and defect engineering, as well as control of the crystal structure, multi-dimensional structure, hom/heterostructures for photocatalytic applications. This review provides new insights to develop g-C3N4-based photocatalysts with control of composition and structure by facile molten-salt modulation in energy-conversion and environmental fields.

Direct Growth of Polymeric Carbon Nitride Nanosheet Photoanode for Greatly Efficient Photoelectrochemical Water-Splitting

A general method for the direct synthesis of highly homogeneous and dense polymerized carbon nitride (PCN) nanosheet films on F: SnO2 (FTO) is developed. Detailed photoelectrochemical (PEC) water-splitting studies reveal that the as-synthesized PCN films exhibit outstanding performance as photoanode for PEC water-splitting. The optimal PCN photoanode exhibits excellent photocurrent density of 650 µA cm-2 , and monochromatic incident photon-to-electron conversion efficiency (IPCE) value up to 30.55% (λ = 400 nm) and 25.97% (λ = 420 nm) at 1.23 VRHE in 0.1 m KOH electrolyte. More importantly, the PCN photoanode has an excellent hole extraction efficiency of up to 70 ± 3% due to the abundance of active sites provided by the PCN photoanode nanosheet, which promotes the transport rates of OER-relevant species. These PCN films provide a new benchmark for PCN photoanode materials.

Understanding the unique S-scheme charge migration in triazine/heptazine crystalline carbon nitride homojunction

Understanding charge transfer dynamics and carrier separation pathway is challenging due to the lack of appropriate characterization strategies. In this work, a crystalline triazine/heptazine carbon nitride homojunction is selected as a model system to demonstrate the interfacial electron-transfer mechanism. Surface bimetallic cocatalysts are used as sensitive probes during in situ photoemission for tracing the S-scheme transfer of interfacial photogenerated electrons from triazine phase to the heptazine phase. Variation of the sample surface potential under light on/off confirms dynamic S-scheme charge transfer. Further theoretical calculations demonstrate an interesting reversal of interfacial electron-transfer path under light/dark conditions, which also supports the experimental evidence of S-scheme transport. Benefiting from the unique merit of S-scheme electron transfer, homojunction shows significantly enhanced activity for CO₂ photoreduction. Our work thus provides a strategy to probe dynamic electron transfer mechanisms and to design delicate material structures towards efficient CO₂ photoreduction.

Photochemical Synthesis of Porous Triazine-/Heptazine-Based Carbon Nitride Homojunction for Efficient Overall Water Splitting

Porous triazine-/heptazine-based carbon nitride (THCN) homojunction with chloride (Cl) doping was synthesized by a simple, one-step photochemical synthesis route for efficient visible-light-driven overall water splitting. The phase ratio of triazine-based carbon nitride (TCN) and heptazine-based carbon nitride (HCN), texture and morphology of the THCN isotype junction were finely tuned by varying ultraviolet irradiation time and washing solvents. After washing with acetonitrile, the resulting porous THCN nanosheets with 48 h irradiation contain 21 wt % TCN and 79 wt % HCN units and reveal a significantly improved photocatalytic performance with H₂ and O₂ production rates up to 7.9 and 4.2 μmol h^-1 , respectively, about 3.8 times higher than that of THCN prepared by 36 h illumination. The dual-phase interaction, holey structure, and Cl dopants favor the exposure of active sites, extended visible-light harvesting, accelerated charge transfer, and enhanced photoreduction ability, thereby improving photocatalytic activity.

Unravelling the doping effect of potassium ions on structural modulation and photocatalytic activity of graphitic carbon nitride

Graphitic carbon nitride (GCN), as a promising photocatalyst, has been intensely investigated in the photocatalytic fields, but its performance is still unsatisfactory. To date, metal ion doping has been proven to be an effective modification method to improve the photocatalytic activity of GCN. More importantly, comprehensive understanding of the doping mechanism will be of benefit to synthesize efficient GCN based photocatalysts. In this work, K⁺-doped GCN samples were prepared via heating the mixture of the preheated melamine and a certain amount of KCl at different synthetic temperatures. XRD and Raman characterization studies indicated that the introduction of K⁺ could improve its crystallinity at higher temperature but reduce its crystallinity at lower temperature. Moreover, FTIR and SEM-EDS measurements implied that K⁺ are found dominantly in the surface of the ion-doped sample prepared at lower temperature, while they are found both in the surface and bulk of the ion-doped sample prepared at higher temperature. These observations revealed that K⁺ distributed in the surface of the ion-doped GCN could inhibit its crystal growth, while K⁺ distributed inside of the ion-doped GCN could promote its crystallinity. Owing to the greater inducing effect of the bulk K⁺ than the disturbing effect of the surface K⁺, the improvement of the crystallinity for K⁺-doped GCN was achieved. As a result, the K⁺-doped GCN with higher crystallinity yielded an obviously higher H₂ evolution rate than that with lower crystallinity under visible light irradiation (>420 nm). Besides, it was observed that the K⁺-doped GCN prepared at higher temperature exhibits significantly greater adsorption capacity for methylene blue than the K⁺-doped GCN prepared at lower temperature. This work would provide an insight into optimizing metal ion doped GCN with high photocatalytic activity.

Partially cross-linked carbon nitride with unimpeded charge transfer between different chains for boosting photocatalytic hydrogen production

A heptazine-based, partially cross-linked carbon nitride (PC-CN) was successfully prepared via a solid-chloride-salt-assisted polycondensation method. The cross-linking favors the charge transport between different chains, thus dramatically boosting the photocatalytic hydrogen evolution activity of PC-CN, up to 29.3 times that of traditional 1D-CN.

New Strategy for the Persistent Photocatalytic Reduction of U(VI): Utilization and Storage of Solar Energy in K+ and Cyano Co-Decorated Poly(Heptazine Imide)

The photocatalytic conversion of soluble U(VI) into insoluble U(IV) is a robust strategy to harvest aqueous uranium, but remains challenging owing to the intermittent availability of solar influx and reoxidation of U(IV) without illumination. Herein, a dual platform based on K⁺ and cyano group co-decorated poly(heptazine imide) (K-CN-PHI) is reported that can drive persistent U(VI) extraction upon/beyond light. K-CN-PHI achieves the photocatalytic reduction of U(VI) with a reaction rate of 0.89 min^-1 , being 47 times greater than that over pristine carbon nitride (PCN). This system can further be triggered by light to form long-living radicals, driving the reduction of U(VI) in the dark for over 3 d. The flexible structural K⁺ as counterions stabilize the electrons trapped by cyanamide groups, enabling the long lifetime of the generated radicals. The results collectively prove K-CN-PHI to be a novel and efficient photocatalyst enabling persistent U(VI) extraction around the clock, and broadening the practical applications of the photocatalytic extraction of U(VI).

Optical and quantitative detection of cobalt ion using graphitic carbon nitride-based chemosensor for hydrometallurgy of waste lithium-ion batteries

A hydrometallurgy is one of the most important techniques for recycling waste LIBs, where identifying the exact composition of the metal-leached solution is critical in controlling the metal extraction efficiency and the stoichiometry of the regenerated product. In this study, we report a simple and selective optical detection of high-concentrated Co²⁺ using a graphitic carbon nitride (g-CN)-based fluorescent chemosensor. g-CN is prepared by molten salt synthesis using dicyandiamide (DCDA) and LiI/KI. The mass ratio of LiI/KI to DCDA modifies the resulting g-CN (CNI) in terms of in-plane molecular distances of base sites including cyano functional groups (─CN) and fluorescent emission wavelength via nucleophilic substitution. The fluorescent sensing performance of CNIs is evaluated through photoluminescence (PL) emission spectroscopy in a broad Co²⁺ concentration range (10^-4-10⁰ M). The correlation between the surface exposure of hidden nitrogen pots (base sites) and PL intensity change is achieved where the linear relationship between the PL quenching and the logarithm of Co²⁺ concentration in the analyte solution is well established with the regression of 0.9959. This study will provide the design principle of the chemosensor suitable for the fast and accurate optical detection of Co²⁺ present in a broad concentration range for hydrometallurgy for the recycling of waste LIBs.

Excited-state and charge-carrier dynamics in binary conjugated polymer dots towards efficient photocatalytic hydrogen evolution

Aqueous dispersed conjugated polymer dots (Pdots) have shown promising application in photocatalytic hydrogen evolution. To efficiently extract photogenerated charges from type-II heterojunction Pdots for hydrogen evolution, the mechanistic study of photophysical processes is essential for Pdot optimization. Within this work, we use a PFODTBT donor (D) polymer and an ITIC small molecule acceptor (A) as a donor/acceptor (D/A) model system to study their excited states and charge/energy transfer dynamics via steady-state and time-resolved photoluminescence spectroscopy, respectively. Charge-carrier generation and the recombination dynamics of binary Pdots with different D/A ratios were followed using femtosecond transient absorption spectroscopy. A significant spectral relaxation of photoluminescence was observed for individual D Pdots, implying an energetic disorder by nature. However, this was not seen for charge carriers in binary Pdots, probably due to the ultrafast charge generation process at an early time (<200 fs). The results showed slower charge recombination upon increasing the ratio of ITIC in binary Pdots, which further resulted in an enhanced photocatalytic hydrogen evolution, twice that as compared to individual D Pdots. Although binary Pdots prepared via the nanoprecipitation method exhibit a large interfacial area that allows high charge generation efficiencies, it also provides a high possibility for charge recombination and limits the further utilization of free charges. Therefore, for the future design of type-II heterojunction Pdots, suppressing the charge carrier recombination via increasing the crystallinity and proper phase segregation is necessary for enhanced photocatalytic hydrogen evolution.

Mesoporous Carbon Nitride with π-Electron-Rich Domains and Polarizable Hydroxyls Fabricated via Solution Thermal Shock for Visible-Light Photocatalysis

Carbon nitride semiconductors are competitive candidates for visible-light-responsive photocatalysts, but encounter weakened exciton dissociation arising from the elevated Coulomb force of singlet Frenkel excitons with narrowing bandgaps. We overcome this contradiction by co-infusing π-electron-rich domains and polarizable hydroxyl units into mesoporous carbon nitride, realized by solution thermal shock. The embedded delocalized π-conjugated aromatic domains derived from nonconjugated macromolecules downshift the conduction band edge and contribute to spatial separation of photogenerated electrons in the lowest unoccupied molecular orbital and holes in the highest occupied molecular orbital. Meanwhile, polarizable hydroxyls induce distinct electron flow from heptazine-based skeletons to periphery sites and enhance water adsorption as well as proton reduction capacity. Consequently, the polymeric carbon nitride delivers an enhanced hydrogen evolution rate that is 17.5 times larger than thermally treated counterparts derived from urea fabricated via conventional strategies. These results show that our strategy can infuse different functional motifs into carbon nitride and thus improve photocatalytic activity.

Phosphorus doped and defect modified graphitic carbon nitride for boosting photocatalytic hydrogen production

The enhancement of photogenerated carrier separation efficiency is a significant factor in the improvement of photocatalyst performance in photocatalytic hydrogen evolution. Heteroatom doping and defect construction have been considered valid methods to boost the photocatalytic activity of graphitic carbon nitride. Herein, we report graphitic carbon nitride modified with P doping and N defects (PCN_x), and the effects of doping and defects were investigated in photocatalytic H₂ evolution. Its hydrogen evolution rate can reach up to about 59.1 μmol h^-1, which is more than 123.1 times higher than pristine graphitic carbon nitride under visible light irradiation. Importantly, the apparent quantum efficiency reaches 8.73% at 420 nm. The excellent performance of the PCN_x photocatalyst was attributed to the following aspects: (I) the large BET surface area of PCN_x affords more active sites for H₂ production and (II) the introduction of P and N defects can accelerate the charge carrier separation and transfer efficiency, leading to more efficient photocatalytic hydrogen production. The photocatalyst showed obviously enhanced activities.

Flux-assisted synthesis of bismuth nanoparticle decorated carbon nitride for efficient photocatalytic degradation of endocrine disrupting compound

Traditional approaches to synthesizing bismuth nanoparticle decorated carbon nitride (C₃N₄) materials suffer from the complex synthesis process and the addition of a surfactant, which is not conducive to environmental protection. To address these problems, we adopted a simple and green flux-assisted approach for the first time to fabricate metallic bismuth nanoparticle decorated C₃N₄ (BiCCN). Electron microscopy results suggested that bismuth vanadate was converted into small bismuth nanoparticles via the flux-assisted approach. Highly dispersed Bi nanoparticles dramatically intensify light absorption, facilitate spatial charge separation as electron acceptors, shorten the charge diffusion length, and reserve more active sites for generating reactive species via surface photo-redox reactions. Consequently, the derived optimized photocatalyst BiCCN-15 rendered around 26 times higher photocatalytic degradation efficiency toward an endocrine disrupting compound (bisphenol A) than C₃N₄. This work provides a novel approach for developing non-precious metal decorated photocatalytic materials for sustainable water decontamination.

Soluble carbon nitride nanosheets as an alternate precursor for hard-templated morphological control

Carbon nitride (CN_x) is an organic semiconductor with promising applications in solar energy conversion via photocatalytic water splitting. Current efforts are being devoted to improve the photocatalytic activity of CN_x as it is limited by charge recombination which may be exacerbated by its low surface area. Hard-templated synthesis of CN_x by filling mesoporous silica with small molecule precursors is one of the conventional approaches to improving the efficiency of CN_x. However the relationship between activity and surface area has yet to be fully established. Herein we develop a new approach using soluble acidified CN_x nanosheets as the precursor in an effort to decrease the potential for unintended chemical modifications and better understand the complex relationship between morphology and activity. Deprotonation of acidified CN_x occurs at moderate temperatures to restore the π-π interactions. We prepared three modified morphologies with this synthetic route and completed a thorough study of the structural, optical, and photocatalytic properties. Supported by charge carrier dynamics studies, we found that the silica-templated CN_x with modified morphologies suffered from higher trap state densities and resulted in lower photocatalytic activity compared to CN_x prepared without a template. Characterization techniques showed that the chemical structure of the templated CN_x obtained is not sensitive to changes in the silica template shape. Our observations highlight the complex relationship between structure, photophysics, and activity, and demonstrate that hard templating can modify more than the intended surface area.

Boosting Photosynthetic H2O2 of Polymeric Carbon Nitride by Layer Configuration Regulation and Fluoride-Potassium Double-Site Modification

Photocatalytic hydrogen peroxide (H₂O₂) production will become a burgeoning strategy for solar energy utilization by selective oxygen reduction reaction (ORR). Polymeric carbon nitride (PCN) shows relatively high two-electron ORR selectivity for H₂O₂ production but still limited low H₂O₂ production efficiency due to slow exciton dissociation. Herein, we constructed a heptazine/triazine layer stacked carbon nitride heterojunction with fluorine/potassium (F/K) dual sites (FKHTCN). The introduction of F/K not only can regulate layer components to enhance the charge separation efficiency but, more importantly, also optimize the adsorption of surface oxygen molecules and intermediate *OOH during H₂O₂ production. Consequently, FKHTCN efficiently improves the photocatalytic H₂O₂ production rate up to 3380.9 μmol h^-1 g^-1, nearly 15 times higher than that of traditional PCN. Moreover, a production-utilization cascade system was designed to explore their practical application in environmental remediation. This work lays out the importance of engineering a layer-stacked configuration and active sites for enhancing photocatalysis.

Promoting Dinuclear-Type Catalysis in Cu1 -C3 N4 Single-Atom Catalysts

Reducing particle size in supported metal catalysts to single-atom level isolates the active metal sites and maximizes the atomic utilization efficiency. However, the large inter-atom distance, particularly in low-loading single-atom catalyst (SAC), is not favorable for a complex reaction where two (or more) reactants have to be activated. A key question is how to control the inter-atom distances to promote dinuclear-type coactivation at the adjacent metal sites. Here, it is reported that reducing the average inter-atom distance of copper SACs supported on carbon nitride (C₃ N₄ ) to 0.74 ± 0.13 nm allows these catalysts to exhibit a dinuclear-type catalytic mechanism in the nitrile-azide cycloaddition. Operando X-ray absorption fine structure study reveals a dynamic ligand exchange process between nitrile and azide, followed by their coactivation on dinuclear Cu SAC sites to form the tetrazole product. This work highlights that reducing the nearest-neighbor distance of SAC allows the mechanistic pathway to diversify from single-site to multisite catalysis.

Enhanced visible light photocatalytic hydrogen production over poly(dibenzothiophene-S,S-dioxide)-based heterostructures decorated by Earth-abundant layered double hydroxides

Layered double hydroxides (LDHs) have emerged as one of the promising catalyst substitutes to noble metals in photocatalytic water splitting due to their unique optoelectronic properties. Herein, a series of novel PSO@NiFeLDH composites have been designed and synthesized to investigate photocatalytic performance. Various physicochemical techniques characterized their structural, nanomorphological, and optical properties. These results demonstrated the existence of NiFeLDH particles on the surface of PSO and the strong interaction between NiFeLDH and PSO. The photocatalytic performance was much increased in the case of PSO@NiFeLDH as compared to that of Pt-modified PSO because of the synergistic effect between PSO and NiFeLDH. Remarkably, PSO@NiFeLDH-15 exhibits the highest photocatalytic activity with a rate of 52.8 mmol h^-1 g^-1 at an optimal content without a Pt cocatalyst under visible light irradiation.

Recent Progress on Photoelectrochemical Water Splitting of Graphitic Carbon Nitride (g−CN) Electrodes

Graphitic carbon nitride (g−CN), a promising visible-light-responsive semiconductor material, is regarded as a fascinating photocatalyst and heterogeneous catalyst for various reactions due to its non-toxicity, high thermal durability and chemical durability, and “earth-abundant” nature. However, practical applications of g−CN in photoelectrochemical (PEC) and photoelectronic devices are still in the early stages of development due to the difficulties in fabricating high-quality g−CN layers on substrates, wide band gaps, high charge-recombination rates, and low electronic conductivity. Various fabrication and modification strategies of g−CN-based films have been reported. This review summarizes the latest progress related to the growth and modification of high-quality g−CN-based films. Furthermore, (1) the classification of synthetic pathways for the preparation of g−CN films, (2) functionalization of g−CN films at an atomic level (elemental doping) and molecular level (copolymerization), (3) modification of g−CN films with a co-catalyst, and (4) composite films fabricating, will be discussed in detail. Last but not least, this review will conclude with a summary and some invigorating viewpoints on the key challenges and future developments.

An Efficient Electron Donor for Conjugated Microporous Polymer Photocatalysts with High Photocatalytic Hydrogen Evolution Activity

Conjugated microporous polymers (CMPs) with donor-acceptor (D-A) molecular structure show high photocatalytic activity for hydrogen evolution due to the efficient light-induced electron/hole separation, which is mostly determined by the nature of electron donor and acceptor units. Therefore, the selection of electron donor and acceptor holds the key point to construct high performance polymer photocatalysts. Herein, two dibenzo[b,d]thiophene-S,S-dioxide (BTDO) containing CMP photocatalysts using tetraphenylethylene (TPE) or dibenzo[g,p]chrysene (DBC) as the electron donor to investigate the influence of the geometry of electron donor on the photocatalytic activity are design and synthesized. Compared with the twisted TPE donor, DBC has a planar molecular structure with extended π-conjugation, which promotes the charges transmission and light-induced electron/hole separation. As a result, the polymer DBC-BTDO produced from DBC donor shows a remarkable photocatalytic hydrogen evolution rate (HER) of 104.86 mmol h^-1  g^-1 under full arc light (λ > 300 nm), which is much higher than that of the polymer TPE-BTDO (1.80 mmol h^-1  g^-1 ), demonstrating that DBC can be an efficient electron donor for constructing D-A polymer photocatalysts with high photocatalytic activity for hydrogen evolution.

Oligoethylene Glycol Side Chains Increase Charge Generation in Organic Semiconductor Nanoparticles for Enhanced Photocatalytic Hydrogen Evolution

Organic semiconductor nanoparticles (NPs) composed of an electron donor/acceptor (D/A) semiconductor blend have recently emerged as an efficient class of hydrogen-evolution photocatalysts. It is demonstrated that using conjugated polymers functionalized with (oligo)ethylene glycol side chains in NP photocatalysts can greatly enhance their H₂ -evolution efficiency compared to their nonglycolated analogues. The strategy is broadly applicable to a range of structurally diverse conjugated polymers. Transient spectroscopic studies show that glycolation facilitates charge generation even in the absence of a D/A heterojunction, and further suppresses both geminate and nongeminate charge recombination in D/A NPs. This results in a high yield of photogenerated charges with lifetimes long enough to efficiently drive ascorbic acid oxidation, which is correlated with greatly enhanced H₂ -evolution rates in the glycolated NPs. Glycolation increases the relative permittivity of the semiconductors and facilitates water uptake. Together, these effects may increase the high-frequency relative permittivity inside the NPs sufficiently, to cause the observed suppression of exciton and charge recombination responsible for the high photocatalytic activities of the glycolated NPs.

Geoinspired syntheses of materials and nanomaterials

The search for new materials is intimately linked to the development of synthesis methods. In the current urge for the sustainable synthesis of materials, taking inspiration from Nature's ways to process matter appears as a virtuous approach. In this review, we address the concept of geoinspiration for the design of new materials and the exploration of new synthesis pathways. In geoinspiration, materials scientists take inspiration from the key features of various geological systems and processes occurring in nature, to trigger the formation of artificial materials and nanomaterials. We discuss several case studies of materials and nanomaterials to highlight the basic geoinspiration concepts underlying some synthesis methods: syntheses in water and supercritical water, thermal shock syntheses, molten salt synthesis and high pressure synthesis. We show that the materials emerging from geoinspiration exhibit properties differing from materials obtained by other pathways, thus demonstrating that the field opens up avenues to new families of materials and nanomaterials. This review focuses on synthesis methodologies, by drawing connections between geosciences and materials chemistry, nanosciences, green chemistry, and environmental sciences.

Photocatalytic hydrogen evolution based on carbon nitride and organic semiconductors

Photocatalytic hydrogen evolution (PHE) presents a promising way to solve the global energy crisis. Metal-free carbon nitride (CN) and organic semiconductors photocatalysts have drawn intense interests due to their fascinating properties such as tunable molecular structure, electronic states, strong visible-light absorption, low-cost etc. In this paper, the recent progresses of photocatalytic hydrogen production based on organic photocatalysts, including CN, linear polymers, conjugated porous polymers and small molecules, are reviewed, with emphasis on the various strategies to improve PHE efficiency. Finally, the possible future research trends in the organic photocatalysts are prospected.