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.

Construction of Li/K dopants and cyano defects in graphitic carbon nitride for highly efficient peroxymonosulfate activation towards organic contaminants degradation

As an emerging peroxymonosulfate (PMS) activation catalyst, graphitic carbon nitride (g-C₃N₄) is non-toxic and eco-friendly, while its poor catalytic performance hinders the application of pristine g-C₃N₄. Herein, a simple LiCl/KCl molten salts-assisted thermal polymerization method was adopted to promote the photocatalytic performance of g-C₃N₄. With the insertion of Li/K dopants and the introduction of surface cyano defects, the modified catalyst exhibited greatly enhanced ability on PMS activation towards acetaminophen removal, observing a 13 times higher rate constant than pristine g-C₃N₄ (k = 0.0435 min^-1 vs. 0.0033 min^-1). The main reactive oxygen species for pollutant degradation were identified as sulfate radicals and singlet oxygen. The wavefunction analysis at excited states based on density functional theory suggests that the introduction of cyano defects greatly promotes the separation of photo-generated electron-hole pairs, thereby achieving higher photocatalytic efficiency. In addition, the doping of Li/K significantly enhances the interaction between PMS and the catalyst surface, and orients the electron transfer from PMS to catalyst to generate non-radical species singlet oxygen, which improves the catalyst resistance to anions-containing water matrices.

Conjugated porphyrin materials for solar fuel generation

Abstract:

Conjugated materials have emerged as a new class of photocatalysts for solar fuel generation, thus allowing for the Sun’s energy to be converted into a storable fuel that can be used without further emissions at the point of use. Many different building blocks have been used to make conjugated materials that act as photocatalysts allowing for efficient light absorption and tuing of photophysical properties. The porphyrin moiety is a very interesting building block for photocatalysts as the large π-conjugated system allows efficient light absorption. Metalation of porphyrins allows for further tuning of the materials’ properties, thus further expanding the property space that these materials can cover. This allows to design and better control over the properties of the materials, which is discussed in this review together with the state-of-the-art in porphyrin photocatalysts and hybrid systems.

Ion-Induced Synthesis of Crystalline Carbon Nitride Ultrathin Nanosheets from Mesoporous Melon for Efficient Photocatalytic Hydrogen Evolution with Synchronous Highly Selective Oxidation of Benzyl Alcohol

Crystalline carbon nitride (CCN) with a poly(heptazine imide) structure is efficient in photocatalytic hydrogen evolution (PHE), but synthesis of CCN ultrathin nanosheets (CCNuns) and their use in PHE with selective organic oxidation are still rare. Herein, CCNuns with Na⁺ doping are prepared using NaCl as the ion-induction and templating agent and mesoporous melon as the feedstock, exhibiting efficient synchronous PHE and benzyl alcohol oxidation to benzaldehyde, with an apparent quantum yield of 10.5% at 420 nm and a visible light PHE rate that is 94.3 times that of bulk polymeric carbon nitride (PCN). The selectivity of benzaldehyde formation (90.5%) is also much higher than that of PCN (40.7%). Interestingly, this selectivity increases gradually with increasing light wavelengths. The high photoactivity of CCNuns originates from their ultrathinness and Na⁺ doping, which considerably enhance the photogenerated charge separation. This work opens up an avenue for the synthesis of CCNuns and extends their application.

Roles of alkali metal dopants and surface defects on polymeric carbon nitride in photocatalytic peroxymonosulfate activation towards water decontamination

Polymeric carbon nitride (PCN) has been extensively employed in peroxymonosulfate (PMS) activation for water decontamination. However, limited photocatalytic efficiency can be achieved by pristine PCN due to its intrinsic deficiencies like high electron-hole recombination rate and resistance to charge transfer. Herein, in a two-stage thermal treatment process, the nontoxic and stable Na and K were successfully anchored among the PCN skeleton with surface defects created, leading to an elevated photocatalytic activity for PMS activation. The SO₄·^- and ¹O₂ were identified as the dominant reactive oxygen species, which were generated from electron transfer processes between PMS and catalyst. Experimental and theoretical analyses suggested that the defective structures and metal dopants improved the optical properties of catalyst, endowing it a wider light absorption range and a lower energy barrier for electron transitions. The modified structures were also beneficial to electron transfer processes due to the weaker electron confinement effect, accelerating the production of SO₄·^- on the defective sites and ¹O₂ on the metal sites. The synergy of radical and non-radical species weakened the influence of side reactions between radicals from PMS and coexisting inorganic anions in practical water, hence to promote the resistance of modified catalysts in complex water matrices.

Semiconducting Polymers for Oxygen Evolution Reaction under Light Illumination

Sunlight-driven water splitting to produce hydrogen fuel has stimulated intensive scientific interest, as this technology has the potential to revolutionize fossil fuel-based energy systems in modern society. The oxygen evolution reaction (OER) determines the performance of overall water splitting owing to its sluggish kinetics with multielectron transfer processing. Polymeric photocatalysts have recently been developed for the OER, and substantial progress has been realized in this emerging research field. In this Review, the focus is on the photocatalytic technologies and materials of polymeric photocatalysts for the OER. Two practical systems, namely, particle suspension systems and film-based photoelectrochemical systems, form two main sections. The concept is reviewed in terms of thermodynamics and kinetics, and polymeric photocatalysts are discussed based on three key characteristics, namely, light absorption, charge separation and transfer, and surface oxidation reactions. A satisfactory OER performance by polymeric photocatalysts will eventually offer a platform to achieve overall water splitting and other advanced applications in a cost-effective, sustainable, and renewable manner using solar energy.

Synthesis of an iso-type graphitic carbon nitride heterojunction derived from oxamide and urea in molten salt for high-performance visible-light driven photocatalysis

Thrice-modified g-C3N4 with cyano groups and an asymmetric planar heptazine/triazine-based iso-type heterojunction structure (MOCN) exhibits significantly higher photocatalytic activity.

Dynamics of photoconversion processes: the energetic cost of lifetime gain in photosynthetic and photovoltaic systems

The continued development of solar energy conversion technologies relies on an improved understanding of their limitations. In this review, we focus on a comparison of the charge carrier dynamics underlying the function of photovoltaic devices with those of both natural and artificial photosynthetic systems. The solar energy conversion efficiency is determined by the product of the rate of generation of high energy species (charges for solar cells, chemical fuels for photosynthesis) and the energy contained in these species. It is known that the underlying kinetics of the photophysical and charge transfer processes affect the production yield of high energy species. Comparatively little attention has been paid to how these kinetics are linked to the energy contained in the high energy species or the energy lost in driving the forward reactions. Here we review the operational parameters of both photovoltaic and photosynthetic systems to highlight the energy cost of extending the lifetime of charge carriers to levels that enable function. We show a strong correlation between the energy lost within the device and the necessary lifetime gain, even when considering natural photosynthesis alongside artificial systems. From consideration of experimental data across all these systems, the emprical energetic cost of each 10-fold increase in lifetime is 87 meV. This energetic cost of lifetime gain is approx. 50% greater than the 59 meV predicted from a simple kinetic model. Broadly speaking, photovoltaic devices show smaller energy losses compared to photosynthetic devices due to the smaller lifetime gains needed. This is because of faster charge extraction processes in photovoltaic devices compared to the complex multi-electron, multi-proton redox reactions that produce fuels in photosynthetic devices. The result is that in photosynthetic systems, larger energetic costs are paid to overcome unfavorable kinetic competition between the excited state lifetime and the rate of interfacial reactions. We apply this framework to leading examples of photovoltaic and photosynthetic devices to identify kinetic sources of energy loss and identify possible strategies to reduce this energy loss. The kinetic and energetic analyses undertaken are applicable to both photovoltaic and photosynthetic systems allowing for a holistic comparison of both types of solar energy conversion approaches.

Donor Bandgap Engineering without Sacrificing the Reduction Ability of Photogenerated Electrons in Crystalline Carbon Nitride

Crystalline carbon nitride (CCN) with a light response up to 700 nm has been seldom reported but is significant for the artificial photocatalysis. In this study, it is proposed that, unlike acceptors, introducing donors can effectively narrow the bandgap without sacrificing the reduction ability of photogenerated electrons, which is more advantageous to photocatalytic reduction reactions. Hence, a series of heptazine-based K⁺ -implanted CCN (KCN) with a narrow bandgap (2.87-1.86 eV) are constructed by copolymerization of pyrimidine donors. The optimized photocatalysts can extend the light response to 700 nm and account for approximately 122- and 33-fold enhancements in H₂ production (λ>500 nm) in comparison to CN and KCN, respectively. The apparent quantum efficiency (AQE) can reach 8.2 % at 500 nm and is comparable to the top-level CN- and CCN- based materials. Its photoactive wavelength has significant advantages over previously reported CCN-based photocatalysts. This method offers a universal donor bandgap engineering strategy towards photocatalytic reduction reactions.

Donor-Acceptor Conjugated Heptazine Polymers with Highly Efficient Photocatalytic Degradations towards Tetracyclines

Photocatalysis is an efficient and green technology in the environmental protection. Due to the high charge separation and transfer, donor-acceptor (D-A) conjugated polymers attract much attention for their photocatalytic degradations towards organic pollutants. Herein, the authors reported three novel D-A conjugated polymers, named as HPBP, HPTP, and HPF, with heptazine moieties as electron acceptors, while biphenyl, terphenyl, or fluorene moieties as electron donors, respectively, which indeed exhibit a highly efficient photocatalytic degradation towards tetracyclines upon the visible-light irradiation. Among them, the photocatalytic performance of HPF is especially noticeable with the degradation rate up to 87% within 30 min, almost 11 times in comparison to those of pristine g-C₃ N₄ , which is mainly attributed to its high crystallinity and conjugation. For their photocatalytic mechanism, the •O₂ ^- radical anions are regarded as the active species.

Constructing a WC/NCN Schottky Junction for Rapid Electron Transfer and Enrichment for Highly Efficient Photocatalytic Hydrogen Evolution

The low charge-transfer efficiency and slow surface reaction kinetics are the main factors affecting the performance of carbon nitride photocatalysts. Here, a Schottky heterostructure (WCN) was constructed by combining WC with porous carbon nitride nanosheets with a cyanide group (NCN). The Schottky junction provides a convenient way for photoinduced electrons to transfer and promotes the effective separation of photoinduced carriers. Furthermore, due to the good conductivity of WC and an electronic structure similar to Pt, the W atom in WC as the active site of hydrogen production can realize efficient reaction kinetics. In this way, the WCN Schottky heterostructure showed a 2.0- and 5.0-fold enhancement in photocatalytic H₂ evolution as compared to the single NCN component under visible-light and near-infrared light irradiation. By combining with theoretical simulations, as an electron acceptor in the WCN heterostructure, WC can effectively improve the charge-transfer efficiency and also act as an active site for hydrogen production.

Synergy of intermolecular Donor-Acceptor and ultrathin structures in crystalline carbon nitride for efficient photocatalytic hydrogen evolution

Crystalline carbon nitride is regarded as the new generation of emerging metal-free photocatalysts as opposed to polymeric carbon nitride (g-C₃N₄) because of its high crystalline structure and ultrahigh photocatalytic water splitting performance. However, further advances in crystalline g-C₃N₄ are significantly restricted by the sluggish separation of charge carriers and limited active sites. In this study, we demonstrate the successful synthesis of heptazine-triazine donor-acceptor-based ultrathin crystalline g-C₃N₄ nanosheets (UCCN) using a combined hot air exfoliation and molten salt (NaCl/KCl) copolymerization approach. The synergy of the donor-acceptor heterojunction and the ultrathin structure greatly accelerated the separation of the charge carriers and enriched the active sites. Accordingly, the superior hydrogen evolution activity and an ultrahigh apparent quantum efficiency of 73.6% at 420 nm under a natural photosynthetic environment were achieved by UCCN, positioning this material at the top among reported conjugated g-C₃N₄ materials. This study provides a novel paradigm for the development of donor-acceptor-based ultrathin crystalline layered materials.

Homogeneous Carbon/Potassium-Incorporation Strategy for Synthesizing Red Polymeric Carbon Nitride Capable of Near-Infrared Photocatalytic H2 Production

The efficient utilization of near-infrared (NIR) light for photocatalytic hydrogen generation is vitally important to both solar hydrogen energy and hydrogen medicine, but remains a challenge at present, owing to the strict requirement of the semiconductor for high NIR responsiveness, narrow bandgap, and suitable redox potentials. Here, an NIR-active carbon/potassium-doped red polymeric carbon nitride (RPCN) is achieved for by using a similar-structure dopant as the melamine (C₃ H₆ N₆ ) precursor with the solid KCl. The homogeneous and high incorporation of carbon and potassium remarkably narrows the bandgap of carbon nitride (1.7 eV) and endows RPCN with a high NIR-photocatalytic activity for H₂ evolution from water at the rate of 140 µmol h^-1 g^-1 under NIR irradiation (700 nm ≤ λ ≤ 780 nm), and the apparent quantum efficiency is high as 0.84% at 700 ± 10 nm (and 13% at 500 ± 10 nm). A proof-of-concept experiment on a tumor-bearing mouse model verifies RPCN as being capable of intratumoral NIR-photocatalytic hydrogen generation and simultaneous glutathione deprivation for safe and high-efficacy drug-free cancer therapy. The results shed light on designing efficient photocatalysts to capture the full spectrum of solar energy, and also pioneer a new pathway to develop NIR photocatalysts for hydrogen therapy of major diseases.

On-Surface Polymerization of In-Plane Highly Ordered Carbon Nitride Nanosheets toward Photocatalytic Mineralization of Mercaptan Gas

2D carbon nitride nanosheets have attracted ever-increasing interest in photocatalysis due to their unique structural advantages. However, the nanosheets synthesized by the traditional methods, such as post oxidation and liquid exfoliation, have suffered from in-plane disorder with abundant structural defects, which seriously counteracts their structural benefits for photocatalysis. Herein, it is demonstrated that polymer carbon nitride nanosheets with in-plane highly ordered structure (PCNNs-IHO) can be successfully prepared by on-surface polymerization of melamine on NaCl crystal surface at elevated temperatures. The NaCl crystals with relative high surface energy not only facilitate the adsorption and activation of melamine to undergo condensation reaction, but also function as unique substrates to orientate the assembly of 2D nanosheet structure. In addition, NaCl also acts as a reactant to provide Na⁺ doping into carbon nitride matrix, affording PCNNs-IHO with robust structural base sites. Benefiting from this structural basicity, PCNNs-IHO exhibits superior photocatalytic performance toward CH₃ SH mineralization under visible light irradiation.

Molecular-functionalized engineering of porous carbon nitride nanosheets for wide-spectrum responsive solar fuel generation

Carbon nitride (C₃N₄) is a promising metal-free photocatalyst for solar-to-energy conversion, but bulk carbon nitride (BCN) shows insufficient light absorption, sluggish photocarrier transfer and moderate activity for photocatalysis. Herein, a facile strategy to significantly increase solar spectrum absorption of the functionalized porous carbon nitride nanosheets (MFPCN) via molecule self-assembly engineering coupled thermal polymerization is reported. This strategy can greatly enhance the wide-solar-spectrum absorption of MFPCN up to 1000 nm than most reported carbon nitride-based photocatalysts. Experimental characterizations and theoretical calculations together display that this strategy could introduce hydroxyl groups into the structure of MFPCN as well as the rich pores and active sites at the edges of framework, which can narrow the bandgap and accelerate the transfer and separation of photoinduced carries. As a result, the optimal MFPCN photocatalyst exhibit the excellent photocatalytic hydrogen evolution rate of 7.745 mmol g^-1h^-1 under simulated solar irradiation, which is ≈13 times that of BCN with remarkable durable CO₂ reduction activities. New findings in this work will provide an approach to extend solar spectrum absorption of metal-free catalysts for solar fuel cascades.

Photochemistry of carbon nitrides and heptazine derivatives

We explore the photochemistry of polymeric carbon nitride (C₃N₄), an archetypal organic photocatalyst, and derivatives of its structural monomer unit, heptazine (Hz). Through spectroscopic studies and computational analysis, we have observed that Hz derivatives can engage in non-innocent hydrogen bonding interactions with hydroxylic species. The photochemistry of these complexes is influenced by intermolecular nπ*/ππ* mixing of non-bonding orbitals of each component and the relative energy of intermolecular charge-transfer (CT) states. Coupling of the former to the latter appears to facilitate proton-coupled electron transfer (PCET), resulting in biradical products. We have also observed that Hz derivatives exhibit an extremely rare inverted singlet/triplet energy splitting (ΔE_ST). In violation of Hund's multiplicity rules, the lowest energy singlet (S₁) is stabilized relative to the lowest triplet (T₁) electronic excited state. Exploiting this unique inverted ΔE_ST character has obvious implications for transformational discoveries in solid-state OLED lighting and photovoltaics. Harnessing this inverted ΔE_ST, paired with light-driven intermolecular PCET reactions, may enable molecular transformations relevant for applications ranging from solar energy storage to new classes of non-triplet photoredox catalysts for pharmaceutical development. To this end, we have explored the possibility of optically controlling the photochemistry of Hz derivatives using ultrafast pump-push-probe spectroscopy. In this case, the excited state branching ratios among locally excited states of the chromophore and the reactive intermolecular CT state can be manipulated with an appropriate secondary "push" excitation pulse. These results indicate that we can predictively redirect chemical reactivity with light in this system, which is an avidly sought achievement in the field of photochemistry. Looking forward, we anticipate future opportunities for controlling heptazine photochemistry, including manipulating PCET reactivity with a diverse array of substrates and optically delivering reducing equivalents with, for example, water as a partial source of electrons and protons. Furthermore, we wholly expect that, over the next decade, materials such as Hz derivatives, that exhibit inverted ΔE_ST character, will spawn a significant new research effort in the field of thin-film optoelectronics, where controlling recombination via triplet excitonic states can play a critical role in determining device performance.

Porous g-C3N4 with defects for the efficient dye photodegradation under visible light

Porous graphitic carbon nitride (p-C₃N₄) was fabricated via simple pyrolyzing treatment of graphitic carbon nitride (g-C₃N₄). The defects could be introduced into the structure of g-C₃N₄ by breakage of some bonds, which was beneficial for the generation of electron-hole pairs and inhibiting their recombination. Compared with g-C₃N₄, p-C₃N₄ showed a narrow band gap to promote the utilization of visible light. Furthermore, the porous structure also increased the specific surface area to maximize the exposure of active sites and promote mass transfer during photodegradation. As a result, the as-reported p-C₃N₄ exhibited considerably higher degradation efficiency for Rhodamine B (RhB) and Methyl Orange (MO) than that of the original g-C₃N₄. Moreover, the photocatalyst showed high durability and stability in recycling experiments.

Pathways towards Boosting Solar-Driven Hydrogen Evolution of Conjugated Polymers

Photocatalytic H₂ evolution under solar illumination has been considered to be a promising technology for green energy resources. Developing highly efficient photocatalysts for photocatalytic water splitting is long-term desired but still challenging. Conjugated polymers (CPs) have attracted ongoing attention and have been considered to be promising alternatives for solar-driven H₂ production due to the excellent merits of the large π-conjugated system, versatile structures, tunable photoelectric properties, and well-defined chemical composites. The excellent merits have offered numerous methods for boosting photocatalytic hydrogen evolution (PHE) of initial CP-based photocatalysts, whose apparent quantum yield is dramatically increased from <1 to >20% in recent five years. According to the photocatalytic mechanism, this review herein systematically summarizes three major strategies for boosting photocatalytic H₂ production of CPs: 1) enhancing visible light absorption, 2) suppressing recombination of electron-hole pairs, and 3) boosting surface catalytic reaction, mainly involving eleven methods, that is, copolymerization, modifying cross-linker, constructing a donor-acceptor structure, functionalization, fabricating organic heterojunction, loading cocatalyst, and surface modification. Finally, the perspectives towards the future development of PHE are proposed.

Insight into the influence of donor-acceptor system on graphitic carbon nitride nanosheets for transport of photoinduced charge carriers and photocatalytic H2 generation

The rapid recombination of photogenerated charges is one of the main restriction for promoting the photocatalytic H₂ generation of graphitic carbon nitride (CN) material. Herein, donor-acceptor (D-A) system was introduced into CN nanosheets by oxygen and/or phenyl doping (DA-CN) strategy to facilitate the transport of photoinduced charge carriers and H₂ generation. Experimental and theoretical results revealed that the nanosheet structure of DA-CN shortened the photoexcited charges transport length to the surface, and the D-A system embedded in DA-CN provided the dipole-induced internal electric field for charges transport. As a consequence, compared with pristine CN, DA-CN samples performed the improved transport of photogenerated charges and photocatalytic H₂ evolution. Notably, DA-CN-OP (oxygen and phenyl co-doping) with the strongest dipole-induced internal electric field originated from D-A system displayed the highest photocatalytic H₂ evolution rate at 7.394 mmol g^-1h^-1, which was 7.67 times as that of pristine CN (0.964 mmol g^-1h^-1). This work not only provides a simple strategy to construct highly efficient CN nanosheet photocatalyst with D-A system, but also promote the deep insight into the effect of molecular dipole originated from D-A system on the transport of photoinduced charge carriers and photocatalytic activity for CN material.

Sol-Gel Processing of Water-Soluble Carbon Nitride Enables High-Performance Photoanodes*

In spite of the enormous promise that polymeric carbon nitride (PCN) materials hold for various applications, the fabrication of high-quality, binder-free PCN films and electrodes has been a largely elusive goal to date. Here, we tackle this challenge by devising, for the first time, a water-based sol-gel approach that enables facile preparation of thin films based on poly(heptazine imide) (PHI), a polymer belonging to the PCN family. The sol-gel process capitalizes on the use of a water-soluble PHI precursor that allows formation of a non-covalent hydrogel. The hydrogel can be deposited on conductive substrates, resulting in formation of mechanically stable polymeric thin layers. The resulting photoanodes exhibit unprecedented photoelectrochemical (PEC) performance in alcohol reforming and highly selective (∼100 %) conversions with very high photocurrents (>0.25 mA cm-2 under 2 sun) down to <0 V vs. RHE. This enables even effective PEC operation under zero-bias conditions and represents the very first example of a 'soft matter'-based PEC system capable of bias-free photoreforming. The robust binder-free films derived from sol-gel processing of water-soluble PCN thus constitute a new paradigm for high-performance 'soft matter' photoelectrocatalytic systems and pave the way for further applications in which high-quality PCN films are required.

Heterogeneous photocatalytic cyanomethylarylation of alkenes with acetonitrile: synthesis of diverse nitrogenous heterocyclic compounds

A visible light-mediated heterogeneous photocatalytic cyanomethylarylation of alkenes with acetonitrile has been established using K-modified carbon nitride (CN-K) as a recyclable semiconductor photocatalyst. This protocol, employing readily accessible alkyl N-hydroxyphthalimide (NHPI) ester as a radical initiator, allows the efficient construction of a broad array of structural diverse nitrogenous heterocyclic compounds including indolines, oxindoles, isoquinolinones, and isoquinolinediones.

Promising Technological and Industrial Applications of Deep Eutectic Systems

Deep Eutectic Systems (DESs) are obtained by combining Hydrogen Bond Acceptors (HBAs) and Hydrogen Bond Donors (HBDs) in specific molar ratios. Since their first appearance in the literature in 2003, they have shown a wide range of applications, ranging from the selective extraction of biomass or metals to medicine, as well as from pollution control systems to catalytic active solvents and co-solvents. The very peculiar physical properties of DESs, such as the elevated density and viscosity, reduced conductivity, improved solvent ability and a peculiar optical behavior, can be exploited for engineering modular systems which cannot be obtained with other non-eutectic mixtures. In the present review, selected DESs research fields, as their use in materials synthesis, as solvents for volatile organic compounds, as ingredients in pharmaceutical formulations and as active solvents and cosolvents in organic synthesis, are reported and discussed in terms of application and future perspectives.

Graphitic C2N3: An Allotrope of g-C3N4 Containing Active Azide Pentagons as Metal-Free Photocatalyst for Abundant H2 Bubble Evolution

A g-C₃N₄ allotrope, a curved leaf-like graphitic C₂N₃ (g-C₂N₃) with an intrinsic spontaneous polarization electric field (ISPEF), has been constructed for efficient solar energy conversion into H₂ energy via photocatalytic H₂O splitting. The curved leaf-like π-delocalization g-C₂N₃ was composed of aromatic azide pentagons and normal triazine hexagons obtained by cycloaddition between -C≡N groups from dicyandiamide polymerization and azide from the heat-treated polypyrrole fibers. Under light irradiation (λ > 420 nm), photo-generated charges are driven to separate efficiently and transfer from bulk to active sites of the surface under ISPEF that is opposite to the Coulomb field. Consequently, without any cocatalyst, g-C₃N₄ allotrope demonstrates a very high H₂-production activity of 14.9 mmol g^-1 h^-1 accompanied by a lot of H₂ bubbles, which is 2.6 times of g-C₃N₄ loading with Pt. In comparison with the reported metal-free photocatalysts or those supported with noble metals, g-C₃N₄ allotrope (i.e., leaf-like g-C₂N₃) is confirmed to be the best metal-free photocatalyst for H₂O splitting into H₂ fuel so far. The contructed leaf-like g-C₂N₃ with SPEF supplies a suitable platform for solar energy conversion into H₂ fuel, which actively contributes to clean energy production.

Experimental determination of charge carrier dynamics in carbon nitride heterojunctions

Carbon nitride (CN_x) is an emerging photocatalyst with the potential to efficiently produce solar fuels. CN_x heterojunctions often show significant photocatalytic activity improvements. We review the charge carrier dynamics in a range of CN_x heterojunctions including carbon-based material, black phosphorus, Ru complexes, molybdenum sulphide and metal phosphides. Time resolved photoluminescence (TRPL) and transient absorption spectroscopy (TAS) were the most common techniques employed for experimental charge carrier dynamics measurements. The low photoluminescence quantum yield of CN_x appeared to limit the depth of conclusions from TRPL, with both lengthening and shortening of the TRPL lifetimes observed and attributed to enhanced charge separation. Overall, the charge carrier dynamics studies often showed a relative lifetime change of ∼2-fold and an activity improvement of >10-fold. We highlight the need for the use of a wider range of techniques to monitor the charge carrier dynamics for conclusive determination of photophysics-activity relationships and elucidation of improvement mechanisms.

Conjugated nanomaterials for solar fuel production

Photocatalytic hydrogen production from water has the potential to fulfil future energy needs by producing a clean and storable fuel. In recent years polymer photocatalysts have attracted significant interest in an attempt to address these challenges. One reason organic photocatalysts have been considered an attractive target is their synthetic modularity, therefore, the ability to tune their opto-electronic properties by incorporating different building blocks. A wide range of factors has been investigated and in particular nano-sized particles have found to be highly efficient due to the size effect resulting from the ability of these to increase the number of charges reaching catalytic sites.

Efficient synthesis of vinylene-linked conjugated porous networks via the Horner-Wadsworth-Emmons reaction for photocatalytic hydrogen evolution

A simple, yet efficient synthetic approach for the construction of vinylene-linked conjugated porous networks was developed. Based on the Horner-Wadsworth-Emmons reaction, the condensation polymerization for the formation of an sp2 carbon-linkage can be achieved at room temperature. The resulting vinylene-linked frameworks exhibit a promising porous nature with the best surface area of up to 1373 m2 g-1. Their intrinsic conjugated architectures and semiconducting properties lead to photocatalytic evolution of hydrogen from water under visible light irradiation. This new synthesis method provides a facile means to prepare attractive sp2 carbon linked porous organic frameworks.

Intermolecular Hydrogen Bonding Tunes Vibronic Coupling in Heptazine Complexes

To better understand how hydrogen bonding influences the excited-state landscapes of aza-aromatic materials, we studied hydrogen-bonded complexes of 2,5,8-tris (4-methoxyphenyl)-1,3,4,6,7,9,9b-heptaazaphenalene (TAHz), a molecular photocatalyst related to graphitic carbon nitride, with a variety of phenol derivatives (R-PhOH). By varying the electron-withdrawing character of the para-substituent on the phenol, we can modulate the strength of the hydrogen bond. Using time-resolved photoluminescence, we extract a spectral component associated with the R-PhOH-TAHz hydrogen-bonded complex. Surprisingly, we noticed a striking change in the relative amplitude of vibronic peaks in the TAHz-centered emission as a function of R-group on phenol. To gain a physical understanding of these spectral changes, we employed a displaced-oscillator model of molecular emission to fit these spectra. This fit assumes that two vibrational modes are dominantly coupled to the emissive electronic transition and extracts their frequencies and relative nuclear displacements (related to the Huang-Rhys factor). With the aid of quantum chemical calculations, we found that heptazine ring-breathing and ring-puckering modes are likely responsible for the observed vibronic progression, and both modes indicate decreasing molecular distortion in the excited state with increasing hydrogen bond strength. This finding offers new insights into intermolecular excited-state hydrogen bonding, which is a crucial step toward controlling excited-state proton-coupled electron transfer and proton transfer reactions.

Emerging Concepts in Carbon Nitride Organic Photocatalysis

Carbon nitrides encompass a class of transition-metal-free materials possessing numerous advantages such as low cost (few Euros per gram), high chemical stability, broad tunability of redox potentials and optical bandgap, recyclability, and a high absorption coefficient (>10⁵  cm^-1 ), which make them highly attractive for application in photoredox catalysis. In this Review, we classify carbon nitrides based on their unique properties, structure, and redox potentials. We summarize recently emerging concepts in heterogeneous carbon nitride photocatalysis, with an emphasis on the synthesis of organic compounds: 1) Illumination-Driven Electron Accumulation in Semiconductors and Exploitation (IDEASE); 2) singlet-triplet intersystem crossing in carbon nitride excited states and related energy transfer; 3) architectures of flow photoreactors; and 4) dual metal/carbon nitride photocatalysis. The objective of this Review is to provide a detailed overview regarding innovative research in carbon nitride photocatalysis focusing on these topics.