Supramolecular assembly-derived carbon-nitrogen-based functional materials for photo/electrochemical applications: progress and challenges

: Supramolecular chemistry during the synthesis of carbon-nitrogen-based materials has recently experienced a renaissance in the arena of photocatalysis and electrocatalysis. In this review, we start with the discussion of supramolecular assemblies-derived carbon-nitrogen-based materials’ regulation from the aspect of morphology, chemical composition, and micro/nanostructural control. Afterwards the recent advances of these materials in energy and environment related applications, including degradation of pollutants, water splitting, oxygen reduction reactions, CO 2 reduction reactions along with organic synthesis are summarized. The correlations between the structural features and physicochemical properties of the carbon-nitrogen-based materials and the specific catalytic activity are discussed in depth. By highlighting the opportunities and challenges of supramolecular assembly strategies, we attempt an outlook on possible future developments for highly efficient carbon-based photo/electrocatalysts.


Construction of supramolecular derivatives
Supramolecular chemistry greatly depends on the reaction conditions, including the adopted solvents, pH values, temperatures, and the starting monomers, which will affect the strength and number of noncovalent bonds between molecules, and thus affecting the morphology, composition and structure of resulting supramolecular assemblies.The bonding modes and the arrangement of the component monomers play important roles in the properties of assembled supramolecules, and further in the final carbon-nitrogen-based materials.In this section, the morphology, composition, and structure control of the supramolecular derivatives will be discussed to highlight avenues of physicochemical properties regulations that seek to guide the reasonable design of highly efficient photo/electrocatalysts for sustainable energy conversion techniques.

The role of solvent
The solvent is one of the significant parameters to control the structure and morphology of supramolecular since it is the reaction media for the assembling process.The polarity, molecule size, and viscosity of solvent have a great effect on the final structure of supramolecules [43,56].
The polarity of the solvent can affect the assembling process of the monomers and works on their arrangement, thus resulting in the diversity of the structure and morphology of the supramolecule and further the derived carbon-nitrogen-based materials.Jiang et al. [57] reported the effects of two different polar solvents on the self-assembly of melamine.It is found that the composition and chemical structure of the assynthesized C 3 N 4 were significantly distinct under two solvents with different polarities.The trithiocyanuric acid-melamine supramolecular precursor obtained in chloroform (TCM-C) possesses a relatively rough surface, and a well-organized morphology (Figure 2A).The resulting C 3 N 4 exhibits a bilayer structure that possesses a rough outer layer and an irregular porous inner layer after calcination (Figure 2B).By comparison, the supramolecular precursor using ethanol as the solvent (TCM-E) shows the morphology of large flakes with a much smoother surface (Figure 2C), the as-obtained C 3 N 4 reveals a uniform, lamellar surface morphology (Figure 2D).Evidenced by the X-ray diffraction (XRD) analysis, the C 3 N 4 obtained in chloroform manifests higher defect rates and smaller interlayer distance, which further proves the effect of the polarity on the local structure of C 3 N 4 , leading to a district morphology.
Sun et al. [39] studies the influence of different solvents on the assembling behaviors of the MCA supramolecule, and proves that the morphology change is attributed to the interaction of hydrogen bonds in different solvents.Typically, the obtained supramolecular assemblies showed a spherical morphology in (F-H) SEM images of MCA aggregates after treatment with less polar solvent, (F) methanol, (G) ethanol, and (H) acetone.Adapted with permission from [39].Copyright 2016, Elsevier.SEM images of C 3 N 4 layer stripped by (I) methanol, and (J) ethanol.SEM images of C 3 N 4 inserted by (K) glycol, and (L) glycerol.Adapted with permission from [49].Copyright 2019, American Chemical Society.
dimethyl sulfoxide (DMSO), a rod-like morphology in water, a flower-like morphology in ethanol and irregular shapes in methanol and acetone.In order to study the strength of hydrogen bonding, they introduce various polar solvents in DMSO to adjust the sphere morphology.After adding water to the DMSO system, scanning electron microscopy (SEM) results unveil the process of the spheres gradually collapsing into rods and crystals over time.When adding the lower polar solvents of methanol, ethanol, and acetone, it is found that the structures are unchanged, however, the morphologies become sheets or stick-like after 20 h (Figures 2E-2H).This is attributed to the polarity of the solvents and the solubility of the monomers that regulates the hydrogen bonding arrangement, which in turn leads to the morphology change.The water molecules can easily enter the spherical supramolecules to form strong hydrogen bonds with cyanuric acid and melamine.Thus, the sphere swelled into rod crystals.In contrast, the other solvents, due to their low polar and solubility of the monomer, cannot penetrate the CM assembling, and reacted only with their surface, thus altering to sheet or stick-like morphology.
The molecular viscosity and size of the solvents also play important roles on the self-assembling process of the supramolecules.Xiao et al. [49] studied the effects of various alcohols, including methanol, ethanol, ethylene glycol, and propylene glycol on the structures of the resulting C 3 N 4 .XRD results demonstrate that the (002) peak of the precursor obtained in methanol and ethanol shift to a lower angle due to the insertion of the methanol and ethanol molecules.Moreover, this signal in polyols is much higher, which indicates that it is difficult for large molecules and polyols with high viscosity to insert into the layered precursor.Meanwhile, SEM results evidence a morphology of aggregated nanosheets of the obtained C 3 N 4 due to the insertion of methanol and ethanol (Figures 2I, 2J), whereas a morphology of striped aggregates in polyol solvents can be noticed (Figures 2K, 2L).This lies in that methanol, ethanol and most of polyols are easily evaporated, causing incomplete flaking phenomenon.

The role of pH
The pH value of the media is closely associated with the hydrolysis of the supramolecules, thereby leading to the morphology and structure change.Wang et al. [58] regulated pH by nitric acid and synthesized a series of carbon nitrides (CN-x, x represents the pH value) with different structures and morphologies.As shown in (Figures 3A-3C), SEM images revealed that CN-5 presents loose fiber laminated morphology and CN-10 shows tubular morphology.However, CN-15 delivers a sea urchin-like morphology which quite differs from the one of CN-5 and CN-10.This is speculated as the occurrence of reunion during the self-assembly process (Figure 3D).Further increasing the amount of nitric acid found that CN-20 possesses a morphology of staggered nanosheet (Figure 3E).The large variety of change in morphology is presumed because the decrease of the pH values accelerates melamine hydrolysis, hence restructuring the intermolecular van der Waals forces and the flatness of the linking groups.In addition, excessive acidity may also lead to immoderate agglomeration and instability [59].

The role of temperature
Considering the kinetics of the solute molecules are significantly related to the temperature, it is also a vital parameter of the temperature on the supramolecular self-assembly behavior.For example, Jun et al. [38] synthesized various MCA using DMSO as the solvent (MCA-DMSO) by modulating the temperatures from 30 to 150°C (Figures 3F-3J).The increasing temperature manipulates the morphology from hexagonal nanoplates to hexagonal nanorods.This may be assigned to the enhanced solvation at high temperature, resulting in the reduction of the strength of the hydrogen bond, and the increase of the quantity of the hydrogen bonds in the hexameric networks, thus altering the morphology.Furthermore, the solubility of the monomers can be strengthened with increasing temperature, which caused the size of aggregates varying from 2 to 10 μm.

Composition and structure control
The recent intensive studies prosper the generation of self-assembled supramolecular family with abundant morphologies and micro/nanostructures, which are continually extended every day.The starting monomers, assemble environments and conditions are important factors to determine the obtained chemical composition and structure of the supramolecule and further the derived carbon-nitrogen-based materials.The additional introduction of new components can controllably modulate their physicochemical properties to adapt the specific applications [43,44].In this section, we focus on the most recent representative research that deals with the composition and micro/nanostructural control of supramolecular derived carbon-nitrogen materials including carbon nitride and heteroatoms doped carbons.

Supramolecular assembly-derived carbon nitride materials
C 3 N 4 has attracted significant interest due to their unique structural features, and it can be synthesized via condensation of N-containing organics such as cyanamide, dicyandiamide [60,61], melamine [62][63][64][65][66][67][68], and other molecules [69][70][71][72][73][74][75].It is found the alignment of the structural units conspicuously alters the morphology and porosity of C 3 N 4 , which is strongly related to its optical properties such as the absorbance in the visible region and the bandgap [76].Supramolecular pre-organization emerges as an effective approach to allowing an ordered arrangement of the monomers prior the calcination.Meanwhile, the composition control of specific supramolecular further achieves modulating the micro/nanostructures of C 3 N 4 , hence strengthening their catalytic activities.The supramolecular assemblies can be polymerized into C 3 N 4 at 500 to 600°C.The yields along with the electronic and structural configures highly depend on the annealing temperatures.We recently synthesized the hierarchically porous C 3 N 4 via cyanuric acid-2,4-diamino-6-phenyl-1,3,5-triazine supramolecules [77], and porous crimped C 3 N 4 via dicyandiamide-dicyandiamidine nitrate supramolecules [78,79], the yields are around 15%-30%.
Upon maintaining the structural integrity, it is necessary to modify the self-assembly behavior and introduce other hydrogen bond donors for a supramolecular system.Barrio et al. [80] introduced hydrochloric acid into melamine solution to create extra hydrogen-halogen bonds.The hydrochloric acid optimizes the arrangement of melamine units and the release of Cl atoms upon condensation induces defects sites and increases the specific surface areas.Combining the theoretical prediction and experimental analysis, it is found that these defect sites are favorable for charge separation during the catalytic processes.The hydrochloric acid modified C 3 N 4 (C 3 N 4 -HCl) presented an enhanced light harvesting ability, charge separation efficiency and hydrogen production.Furthermore, Barrio et al. [81] prepared a unique halogen decorated C 3 N 4 through thermal condensation melamine-halogen complexes.The modification of Cl-Br dual-halogen creates more defects, resulting in significantly strengthened light-harvesting properties, high charge separation efficiency and large specific surface area (Figure 4A).Additionally, other protonic acids and alkalis (such as HNO 3 , H 2 SO 4 , H 3 PO 4 and (NH 2 OH)•H 2 SO 4 ) are adopted not only to tailor the dissolution and hydrolysis rate of melamine [82][83][84], but also to optimize its self-assembly behavior [58,85].The protonated C 3 N 4 exhibits intense luminescent emission, fast photo response, and reproducible photoconductivity.Tahir et al. [86] reported that the nitric acid can facilitate the melamine or cyamelurine to form heptazine through the polyaddition and polycondensation reactions, which can limit the sublimation of melamine at relatively high temperatures owing to the strongly bonding effect, leading to higher specific surface area, suitable band gap, and less texture defects (Figure 4B).Such a C 3 N 4 exhibits excellent electrochemical performance as electrodes for supercapacitors and photocatalytic activity toward photodegradation of rhodamine B.
Considering the hydrolysis of melamine is difficult to control, the structural analogs of melamine such as trithiocyanuaric acid, cyanuric chloride, cyanuric fluoride, 2,4,6-triaminopyrimidine and chloranilic acid are chosen as secondary monomer to control the assembly of supramolecules.The introducing of these monomers will involve structural defect sites and doped-heteroatoms, such as O, S, Cl and F, to the frameworks of the resulting C 3 N 4 .Oxygen doping as one of effective strategies to optimize the electronic and optical properties of C 3 N 4 has attracted extensive attention [87][88][89].Zhang et al. [42] synthesized oxygendoped C 3 N 4 (OCN) nanotubes via a hydrothermal calcination process.The introducing of citric acid promotes the hydrolysis of melamine, and induces oxygen doping in carbon nitride via the following thermal polycondensation process.Due to the synergistic effect of nanotube structure and oxygen doping, the OCN shows large specific surface area, wide photo response range and improved overall quantum efficiency (Figure 4C).Highly crystalline S-doped C 3 N 4 (MTCN) was synthesized by calcinating the supramolecular assembly composed of melamine and trithiocyanuric acid.The insertion S atom induces electronic modulation process, thereby tunes the bandgap and alters optoelectronic properties of MTCN.As a result, the asobtained MTCN exhibits a superb visible-light H 2 -generation performance compared with the pristine C 3 N 4 (Figure 4D) [90].S-doped C 3 N 4 was also reported through the calcination of melamine-bismuthiol assembled supramolecule in different solvents [91].The Cl atoms can be introduced by chloranilic acid and cyanuric chloride during supramolecular assembly [92].The density functional theory (DFT) calculation reveals that the Cl atom can interact with the amino group of melamine weakly via halogen-hydrogen bonding after the introduction of halogen substitution.Thanks to the limited lateral growth of supramolecular crystals, the converted 1D carbon nitride shows distinguished photocatalytic performance and excellent rhodamine B degradation efficiency (Figure 4E) [93].
Besides the structural analogs of melamine mentioned above, the exploration of other monomers for supramolecular assembly through multidentate hydrogen bonding interactions has gained extensive attention.Urea, as a common raw material to prepare C 3 N 4 by solid-phase sintering can also be adopted to assemble with melamine in solution through intermolecular hydrogen bonding, further inducing various tubular structures with controlling the concentration of urea [94].Han et al. [95] adopted oxalic acid and melamine as precursors of the supramolecular framework to synthesize nanosheets like carbon nitride.The function of the oxalic acid is not only modifying the thermal condensation model, but also acting as the Natl Sci Open, 2023, Vol.2, 20220032 dynamic gas template and enhancing thermal oxidation etching during thermal decomposition.Carbon-rich C 3 N 4 nanosheets could be obtained from the supramolecular precursors constructed from melamine and glucose, which are connected by the hydroxyl and aldehyde groups of glucose with the amino group of melamine.By sequentially three stages annealing treatment, in-plane C 3 N 4 and graphitic carbon ring heterostructure was retained.The unique heterostructure shortens the electron and ion diffusion length and enhances the lifetime of photo-induced carriers [96].Chen et al. [97] realized the assembly of melamine and methyl cyclodextrin by hydrothermal method and porous carbon-rich C 3 N 4 is prepared by subsequent annealing.The introduction of methyl cyclodextrin brings about the "gradual doping" of carbon formed at the interface of carbon nitride, which could improve the charge mobility.The enhanced visible-light absorption ability of C-rich C 3 N 4 is identified originating from the enlarged surface area, shortened charge-to-surface migration length, and involved nitrogen vacancies (Figure 4F).
The MCA framework can be manipulated through hydrogen bonding and π-π interactions balance by adjusting monomers or other molecules, which leads to new spatial arrangements.Jordan et al. found that caffeine can act as "lining agent" for the assembly of supramolecular precursor, resulting in greatly enhanced photo-activity of the final carbon nitride (Figure 5A) [98].The glucose can not only interact with melamine to form a stable supramolecule as discussed above, but also incorporate into the assembled MCA framework (Figure 5B) [96].Creatine can act as linkers to bond melamine and cyanuric acid at the interface of two immiscible solutions, leading to robust grafting and interlink of MCA framework [99].The introduction of barbituric acid into the MCA framework results in the insertion of C-C bonds after calcination.The involved C-C bonds are reported, enhancing the visible light absorption, strengthening the mobility rate of photogenerated carriers, hence accelerating the separation efficiency (Figure 5C) [100].Besides, other aromatic compounds such as 2,4-diamino-6-phenoxy-1,3,5-triazine (Figure 5D) [101], 2,4-diamino-6-phenyl-1,3,5triazine (Figure 5E) [102], and polycyclic aromatic hydrocarbons (PAH)-substituted 1,3,5-triazine (Figure 5F) [103], are intensely studied to introduce aromatic groups into MCA.The introducing of phenyl groups improves the light absorption ability, introduces abundant surface defects, and creates Z-scheme homojunctions, which works in concert for the enhancement of photocatalytic activity.

Supramolecular assembly-derived carbon materials
Apart from the typical C 3 N 4 materials, the supramolecular assemblies can be further carbonized into heteroatom doped carbon materials above 600°C [104][105][106].The yield of these carbons, as a typical example, starting from pyromellitic acid-melamine supramolecular assembly is about 20% [107].Heteroatoms doping can improve the electrical conductivity of these carbons.In addition, with the increase of carbonization temperature, the electrical conductivity will be strengthened and the yield will be reduced.Compared with the carbon-nitrogen-based materials derived from metal-organic frameworks, biomass, macromolecular precursor, and chemical vapor deposition (CVD) growth, the atomic composition and structure of supramolecular assembly-derived carbon materials can be precisely regulated, because of the diversity of starting monomers and assembling approaches.The controllable heteroatom doping, functional group modification, and transition metal atoms loading make the supramolecular assembly-derived carbon materials an ideal model and research platform to study the structure-activity relationship of electrochemical reactions.
To date, heteroatom-doped (such as B, N, S, and P) carbon materials are considered potential alternatives to Natl Sci Open, 2023, Vol.2, 20220032 Pt-based catalysts for energy storage and conversion [108][109][110][111][112]. The introduction of nitrogen atoms influences the spin density and charge distribution of the neighboring carbon atoms, and endows fast electron transport ability, so that such carbon materials possess specific physicochemical properties in catalytic performances [113,114].Han et al. [115] obtained nitrogen-doped carbon nano-net (g-N-MM-Cnet) by heating block copolymer P123 supramolecular assemblies, in which nitrogen atoms are inserted into the carbon lattice precisely all at the graphitic sites.The graphitic nitrogen atoms in g-N-MM-Cnet activate adjacent carbon atoms while promoting electron transfer.Accompanying with the unique microporous structure, the g-N-MM-Cnet verifies an excellent bi-functional electrocatalyst.The supramolecular assembly method can simultaneously attain providing nitrogen source and self-sacrificial pore former.Then, Zhao et al. [116] used MCA to fabricate nitrogen-doped porous graphene frameworks, which further serve as the building blocks to load Pt particles with graphene oxide (GO), indicative of an excellent methanol electro oxidation performance (Figure 6A).Furthermore, nitrogen and phosphorus dual-doped carbon materials are widely investigated and are found to be effectively synthesized by pyrolysis of the supramolecular aggregate composed by chitosan and phytic acid [117], gelatin and phytic acid [118], as well as melamine and phytic acid (Figure 6B) [119].The N, P codoped carbons synthesized by phytic acid as phosphorus source and chitosan, gelatin, or melamine as nitrogen source show a remarkably synergistic effect to enhance the electrochemical activities.Yang et al. [120] chose nitrogen and sulfur-rich supramolecular assembly (melamine and trithiocyanuric acid) for in situ synthesis of N,S-codoped carbon nanosheets.Accompanied with the unique structure features with high content of N and S and the mismatch of the outermost orbitals of sulfur and carbon, the nitrogen and sulfur dual doped carbon (N,S-C/800) catalysts show excellent electrocatalytic ORR performance with typical four-electron pathways (Figure 6C), demonstrating heteroatomdoped carbon catalysts are a promising alternative for precious metal based catalysts.Inspired by the above hetero-doping strategy, tri-heteroatoms doping upon carbons was also demonstrated to increase the effective synergistic effect for promoting the electrochemical performance.Ren et al. [121] obtained the nitrogen, phosphorus and sulfur tri-doped carbon materials via polymerization of aniline and diphenylamine-4-sulfonic acid induced by the organophosphonic acid.The excellent electrocatalytic ability can be attributed to the concerted effect of the individual components, large surface area and porous nanostructure of the as-prepared catalysts (Figure 6D).
In summary, the morphology and composition of supramolecular derivatives can be reasonably designed and regulated by adjusting the employed solvents, pH values, temperatures, and the starting monomers.Therefore, heteroatom doping, functional group modification, transition metal atoms loading, and other modifications can be achieved.The obtained supramolecular-derived carbon-nitrogen-based materials possess high porosity, suitable electronic structure, good electronic conductivity, and excellent stability, which makes it widely used in the field of energy conversion related photo/electrocatalysis.

Photocatalysis applications
The supramolecular assembly strategy is a persuasive pathway for the synthesis of C 3 N 4 -based functional materials with diversified structures [122][123][124].Thomas and his colleagues [44] firstly reported a methodology to synthesize the layered spherical supramolecular MCA from melamine and cyanuric acid in DMSO.The mesoporous hollow sphere-like carbon nitride was obtained after the thermal polymerization of MCA.Through the poly-condensation of supramolecular complexes, novel structures and morphology can be generated in micro-/nano scales.The unique optical and electronic adjustable properties endow an accessible approach to design the structures of the resulting carbon nitride materials for a certain application.Recently, motivated by these significant advantages, many researchers have focused on the utilization of supramolecular assembly-derived C 3 N 4 -based functional materials for the solar-energy-conversion techniques, e.g., photocatalytic water splitting [125,126], photocatalytic CO 2 reduction [127], degradation of pollutants [128,129], organic synthesis [130], as well as photocatalytic ORR [123,131].In this section, recent advances of supramolecular assembly-derived C 3 N 4 -based functional materials for photocatalysis applications have been summarized.

Photocatalytic water splitting
Simultaneous production of H 2 and O 2 from water splitting is a promising pathway for the conversion of sustainable solar energy.Since Antonietti and coworkers firstly used carbon nitride as a metal free photocatalyst for water splitting in 2009 [132], many research groups attempted to explore highly active C 3 N 4based photocatalysts, motivated by its relatively low energy band gap (E g ) of 2.70 eV, suitable conduction (−1.4 eV versus NHE, pH 7) and valence band (1.3 eV versus NHE, pH 7) positions, and excellent thermal and chemical stability [6].Nevertheless, the photocatalytic activities of raw carbon nitrides are insufficient due to the bulky structure, weak visible light absorption and severe recombination of charge carriers.Thus, using supramolecular aggregates as the precursors for the synthesis of C 3 N 4 -based materials with specific properties for photocatalytic water splitting have been widely investigated during the last decade, due to the flexible selection of monomers and the adjustable topology and morphology from the molecular level.
The morphology and chemical structure of carbon nitride have a significant influence on its photocatalytic water splitting performance.These features can be easily constructed through tuning the corresponding supramolecular precursors.3D porous nanostructure is benefited to improve the specific surface area with abundant exposed reactive sites, as well as enhanced light absorption.Shang et al. [133] reported a broomlike oxygen-doped carbon nitride (O-CN-NTs) which was prepared through the "calcination-hydrothermalcalcination" method based on the supramolecular assembly process (Figure 7A).The novel tube-in-tube hollow nanostructure allows the incident light to undergo multiple scattering in the hollow interior and produces resonance, which can induce a powerful electric field on the edge and interior of the hollow nanostructure (Figure 7C).Therefore, the charge transfer and separation efficiency of O-CN-NTs can be improved through the local electric fields.Simultaneously, the light absorption ability and mass transfer distance can also be optimized.As a result, O-CN-NTs demonstrated outstanding photocatalytic water splitting performance with a H 2 production rate of 13.6 mmol g −1 h −1 (Figure 7B).
In addition, the adopted solvents also portray significant effects to modify the morphology of supramolecular aggregates.Recently, Sun et al. [39] reported a sequential solvent treatment strategy to induce the novel rearrangement of the self-assembled supramolecular frameworks.Through adjusting the polarity of solvents, the surface morphology of supramolecular aggregate can be accurately modulated and the bulk structure can be preserved.In this case, the resulting carbon nitride materials own gradient energy levels, since the outer carbon nitride layer possesses slightly downshift energy bands compared with the inner layer (Figure 7E).As a result, the carbon nitride heterojunction will be constructed which can further improve its Natl Sci Open, 2023, Vol.2, 20220032 charge separation ability.Benefitting from the advanced features, the carbon nitride prepared from DMSO and acetone (CN-DA) possesses enhanced photocatalytic hydrogen evolution rate (Figure 7F), which is much higher than that of pristine C 3 N 4 .
Using other triazine-based monomers with variable carbon/nitrogen ratios to prepare the supramolecular aggregates can also adjust the electronic property of the resulting carbon nitrides.Recently, Fu and coworkers [134] used L-arginine and melamine as the starting monomers to synthesize an asymmetric supramolecule (L-ArgM14), which was assembled of bundles with tentacle like structure (Figures 7G, 7H).During the self-assemble process, the primary amino group on α-carbon and the guanidine group of L-ArgM14 make the aqueous solution to be alkaline, then induce the amidation reaction between the L-arginine and melamine-cyanuric acid.After thermal polymerization of L-ArgM14, the obtained hierarchically anemones-like carbon nitride (ACN-14) possesses adjustable C/N ratio and bandgap structure (2.75 to 2.25 eV).The optimized properties, e.g., enriched reactive sites, improved visible light absorption, as well as enhanced separation efficiency, enable ACN-14 an excellent water splitting performance with a H 2 generation rate of 95.3 mmol h -1 (Figure 7I).Sun et al. [135] also reported a triazine-based monomer (6-substituted-2,4-diamino1,3,5-triazine, labeled as T1) with implanted sodium and cyano groups through the cycloaddition reaction (Figure 7J).The hydrogen bonds driven supramolecular assembly between T1 and cyanuric acid was carried out in the mixed solvents of water and dimethyl sulfoxide with equal volume.As shown in Figure 7K, the cyanuric acid-T1 supramolecular precursor (CT1) possessed a novel 3D morphology consisting of microstick cluster.After thermal polymerization, the obtained 3D carbon nitride (3D CN1) exhibited highly ordered porous micro-tube structure with large surface area, efficient charge separation, and enhanced visible-light absorption.Finally, the optimized 3D CN1 photocatalyst exhibited efficient hydrogen generation activity, much higher than that of the pristine C 3 N 4 (Figure 7L).

Photocatalytic CO 2 reduction
Besides water splitting, photocatalytic CO 2 reduction into valuable hydrocarbon products and fuels is also a feasible technique to utilize the solar energy and alleviate the greenhouse gas effect [136].Recently, the application of carbon nitrides-based semiconductor photocatalysts for the reduction of CO 2 has attracted great attention, since the strong interaction between N atoms in heptazine rings and the polarized CO 2 molecules, as well as the Lewis basic NH and NH 2 could facilitate the adsorption of CO 2 molecules [6].Most importantly, the conduction band (CB) position of carbon nitride is enough negative to reduce the CO 2 .The products of photocatalytic CO 2 reduction technique, such as formic acid, carbon monoxide, methanal, methanol and methane, are associated with the multiple electron transfer process and appropriate reduction potential.The selectivity of photocatalytic CO 2 reduction products can be significantly improved by reasonably adjusting the energy band structure of carbon nitrides.
Increasing the NH and/or NH 2 group contents of C 3 N 4 -based photocatalysts is an effective strategy to boost their CO 2 reduction performance, which can engineer its electronic structure and facilitate the sorption of CO 2 molecules [6].Recently, Mo et al. [137] fabricated a porous nitrogen rich C 3 N 4 (TCN(NH 3 )) with Lewis basicity via the poly-condensation of melamine and hydroxylammonium chloride supramolecular precursor (Figures 8A, 8B).The optimized sample exhibited a high visible light photocatalytic CO 2 to CO conversion activity of 103.6 μmol g −1 h −1 and an apparent quantum yield (AQY) of 0.43% at 400 nm (Figure 8C).The Natl Sci Open, 2023, Vol.2, 20220032 rich amino groups in the TCN(NH 3 ) framework were confirmed by the fluorescein isothiocyanate isomer (FTIR), elemental analysis (EA), X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) analysis.The DFT calculation results validated that the amino group of TCN(NH 3 ) will be conducive to catch CO 2 through the hydrogen bonding interaction.As shown in Figure 8D, the CO 2 adsorption energy of TCN(NH 3 ) was determined to be −0.358eV, which was much higher than that of the unmodified C 3 N 4 .In addition, the N-H-O distance between TCN(NH 3 ) and CO 2 molecule was 2.445 Å, very similar to that of hydrogen bonds.The adsorption model of CO on TCN(NH 3 ) was C-terminated adsorption (N-H-C).The CO desorption energy of TCN(NH 3 ) and unmodified C 3 N 4 was 0.032 and 0.148 eV, respectively.The higher CO desorption energy of unmodified C 3 N 4 was due to the stronger N-H-O interaction.These results suggested that Lewis basic amino groups in TCN(NH 3 ) can simultaneously increase the CO 2 adsorption energy and decrease the CO desorption energy.Both of them are conducive to improve the photocatalytic CO 2 reduction performance.
In addition, the supramolecular assembly-derived 3D porous carbon nitride can serve as an ideal substrate to anchor transition metal single atoms.The introduced transition metal atoms can improve the photoinduced carriers' separation and transfer, as well as facilitate the CO 2 molecules adsorption and activation capacity.In a recent study reported by Cao and coworkers [138], a single-atom copper-modified CN (Cu/CN) was prepared through the supramolecular preorganization with subsequent condensation method.The uniform dispersion of Cu single atoms was evidenced through the high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM, Figure 8E) and Fourier transform extended X-ray absorption fine structure (FT-EXAFS, Figure 8F) tests.As shown in Figure 8G, the coordination environment of Cu single atom in Cu/CN was identified as C-Cu-N 2 by the XANES spectrum of Cu K-edge.These active centers can improve the charge transfer capacity and reduce the energy barrier of photocatalytic CO 2 reduction (Figure 8H).As a result, the optimized Cu/CN-0.25 showed the most efficient photocatalytic CO 2 reduction activity, and the generation of CO was as high as 11.21 μmol g −1 h −1 (Figure 8I).

Degradation of pollutants
The continuous progress of human society is inseparable from industrialization.However, industrial production will endlessly discharge a wide range of contaminants into the environment, including methyl orange, rhodamine B, methylene blue, aromatic compounds, and aldehydes.Photocatalytic degradation of pollutants is an attractive technology to restore the ecological environment and realize the sustainable and green development.In this respect, many researches used carbon nitride as a visible-light absorption and metal-free photocatalyst for the degradation of pollutants.
In a recent work, Sun et al. [139] reported a series of carbon nitride hydrogel (CNB-G) with tunable shapes, such as cylindrical and tubelike, based on the photo-polymerization of N,N-dimethylacrylamide (DMA) using C 3 N 4 as initiator (Figure 9A).The well-dispersed C 3 N 4 was obtained through the polymerization of a melamine-cyanuric acid-barbituric acid supramolecular aggregate.Under light-emitting diode (LED) light irradiation, the aqueous colloidal dispersion of C 3 N 4 generates abundant surface radicals which could induce Natl Sci Open, 2023, Vol.2, 20220032 polymerization of the DMA and the cross linker (N,N′-methylenebis(acrylamide)) to get cross-linked CNB-G hydrogel network.As shown in Figure 9B, the self-standing CNB-G hydrogel exhibited selective adsorption properties for methylene blue, rhodamine B and methyl orange, which can be found intuitively via the color changes in hydrogels.Under white LED irradiation, CNB-G showed superior abilities for the photocatalytic degradation of cationic dyes, which was due to that the photo-induced electron of C 3 N 4 will be directly injected into methylene blue (Figures 9C, 9D).
The supramolecular pre-organization strategy could introduce non-metal and metal heteroatoms into the resulting carbon nitrides materials, including O, S, P, F, and Cu.For example, Liu and coworkers [140] reported a Cu-doped porous C 3 N 4 (Cu-pCN), which was calcined from the Cu-melamine-cyanuric acid supramolecule (Figure 9E).The Cu doping endowed Cu-pCN with enhanced light absorption, good photogenerated carriers' separation and transfer ability (Figures 9F, 9G).As a result, the Cu-pCN photocatalyst with 12 wt% Cu contents showed an efficient photocatalytic tetracycline degradation activity and excellent recycling stability.After light irradiation for 120 min, 98% of tetracycline can be degraded.Xu et al. [141] reported a porous thiophene grafted C 3 N 4 nanosheet (CM-CN-Th A), which was synthesized through the poly-condensation of supramolecular precursor and 3-thiophenecarboxylic acid (Figure 9H).The novel grafted thiophene group can improve the visible light absorption ability of CM-CN-Th A, as well as accelerate the charge carriers' separation and transport efficiency.As shown in Figure 9I, CM-CN-Th A showed favorable photocatalytic degradation activity of rhodamine B with a removal efficiency of 89.22%.It also exhibited excellent photocatalytic removal of methyl orange, methylene blue, and crystalline violet, indicating its versatility to treat dye wastewater (Figure 9J).

Photocatalytic organic synthesis
The photocatalytic oxidation and hydrogenation conversion of organics or wastes can produce value-added chemical products.C 3 N 4 -based semiconductor photocatalysts have attracted wide attention for photocatalytic organic synthesis, since its cost-efficient and ecofriendly features, affluent surface amino groups and strong Lewis basic sites, as well as its flexible electronic band structure and good physicochemical stability.In terms of photocatalytic organic synthesis, 3D hierarchical porous structure could provide fast mass transport, ample exposed reactive sites, and favorable adsorption site of reaction substrates.In addition, carbon nitride with hollow and porous nanostructure possesses favorable light capture ability, since the incident light will undergo multiple scattering and reflection in the hollow nanostructure.
Recently, Yao et al. [142] reported a cylindrical supramolecular precursor (CT), which was obtained through the hydrogen bonding driven supramolecular assemble strategy.The specific synthesized 6-substituted-2,4-diamino-1,3,5-triazine and cyanuric acid (CA) were used as the starting monomers.Then, the loofah like carbon nitride (LCN) was obtained via a two-step thermal treatment process, including the polymerization of supramolecular precursor and the sequential oxidation etching process (Figure 10A).The supramolecular assembly technique endows the carbon nitride with 3D porous morphology, which could efficiently facilitate the separation and transport of photo-induced charge carriers.Then, the thermal oxidation etching process created abundant mesopores on the surface of 3D architecture.In addition, O heteroatoms and N vacancy defects will be introduced into the carbon nitride framework, which can tune the electronic band structure and introduce defect states (Figures 10B, 10C).The multiple reflection of incident Natl Sci Open, 2023, Vol.2, 20220032 light in the hollow structure will excite a lot of electrons.The optimized LCN/Pt exhibited the highest photocatalytic H 2 generation performance of 4812 μmol h −1 g −1 .In addition, it showed a favorable photocatalytic activity toward transfer hydrogenation of 4-nitrophenol to 4-aminophenol, with a conversion rate of 96.5%, a yield of 95.4% and a selectivity of 98.9% (Figure 10D).
It is also feasible to create reactive structural defects and introduce heteroatoms into the carbon nitride framework through the supramolecular preorganization strategy.Zhang et al. [143] synthesized a sulfurdoped carbon nitride hierarchical mesoporous spheres (SCN-HMS) via a supramolecular assembly method.The hierarchical mesoporous spherical structure and S doping were obtained through the polymerization of melamine-cyanuric acid-trithiocyanuric acid supramolecule, which was driven by the hydrogen bonding assembly and the acid base cross linking effect.The nanostructure engineering can expose abundant reactive sites, promote the mass transport process, and decrease the charge carriers' transfer distance.As shown in Figure 10E, after S heteroatoms were introduced into the carbon nitride framework, the bandgap decreased to 2.40 eV.The doped S atoms can induce electronic structure modulation and create defect state between CB and VB, which will act as donors for lower energy photoexcitation.In addition, S doping can concentrate electrons around the nitrogen atom nearby the doped S sites (Figure 10F).Due to the synergistic effects between nanostructure construction and electronic structure regulation, the SCN-HMS could act as a bifunctional photocatalyst for efficient H 2 production (3.76 μmol h −1 ) and benzyl alcohol oxidation to benzaldehyde (3.87 μmol h −1 ) under visible-light irradiation.
The incorporation of metal atoms (e.g., Pt [33], Fe [144,145], Ni [146], Cu [147,148], Sb [23], and Ru [27]) into the carbon nitride framework is beneficial to improving the photo-excited carriers' separation and transfer and adjusting the adsorption energies of intermediates.Fu and coworkers [149] reported a porous carbon nitride supported Cu single atom photocatalyst (SA-Cu-TCN), which owned both Cu-N 3 group (one Cu atom coordinated with three N atoms) and Cu-N 4 group (one Cu atom bonded with four N atoms in the neighboring C 3 N 4 layers) (Figures 10G, 10H).The Cu-Nx groups were formed by the thermal polymerization of a chlorophyll sodium copper salt inserted melamine-derived supramolecular complex.The resulting SA-Cu-TCN exhibited excellent performance for the conversion of benzene to phenol under visible light irradiation.The conversion and selectivity were 92.3% and 99.9%, respectively.The significant effect of Cu atom for facilitating the charge carriers transfer was demonstrated by ultrafast transient absorption spectroscopy (Figure 10I).Based on the biexponential global fitting results, the average time constants of SA-Cu-TCN were determined as 78±2 ps, which was decreased obviously compared with C 3 N 4 (109±6 ps).The shorter lifetime can be attributed to that the incorporated Cu atoms could provide an additional channel for charge carriers transfer.

Photocatalytic ORR
Photocatalytic ORR to produce hydrogen peroxide (H 2 O 2 ) production is an attractive alternative to the traditional anthraquinone method, since its remarkable low-energy consumption, safety and ecofriendly merits.In 2014, Shiraishi et al. [150] reported the ability of carbon nitride to produce H 2 O 2 through the two electron (2e − ) ORR with alcohol as the hydrogen source and O 2 as the oxygen source under visible light irradiation.It is generally agreed that improving the selectivity of 2e − ORR process and developing new 2e − reduction-based photocatalysts are two main pathways to enhance the photocatalytic H 2 O 2 production performance.The introduction of exposed edge active groups can facilitate the O 2 adsorption and activation capacity of C 3 N 4 -based photocatalysts, consequently improve the selectivity of 2e − ORR.Recently, Zhao et al. [123] reported a carbon nitride with sodium cyanaminate moiety (PCN-NaCA) for photocatalytic H 2 O 2 production.The PCN-NaCA was synthesized through the poly-condensation reaction of the melaminecyanuric acid supramolecular precursor and the subsequent thermal treatment with sodium thiocyanate (NaSCN) molten salt (Figure 11A).The conversion of the amino group to sodium cyanaminate moiety by the thermal treatment in molten salt can create the electron-withdrawing cyanamino-goup and induce coordinative interaction between sodium and pyridinic nitrogen.As shown in Figure 11B, the femtosecond transient absorption spectra (fs-TAS) confirmed that the photo-generated electron will be accumulated in PCN-NaCA-2, and its life time can be prolonged by the surface dioxygen adsorption.In addition, PCN-NaCA-2 exhibited higher O 2 adsorption capacity, which can promote the interfacial electrons transfer to the Natl Sci Open, 2023, Vol.2, 20220032 adsorbed O 2 (Figure 11C).The DFT calculations for the free energy diagrams of ORR steps further confirmed the selective 2e − ORR pathway on PCN-NaCA (Figure 11D).The optimized PCN-NaCA sample showed a photocatalytic H 2 O 2 production rate of 18.7 μmol h −1 mg −1 and an apparent quantum yield of 27.6% at 380 nm (Figure 11E).
Furthermore, introducing structural defects into the carbon nitride framework, e.g., nitrogen vacancy, carbon vacancy, and heteroatoms, have been demonstrated to be beneficial to improving the photocatalytic H 2 O 2 production performance.In a recent research by Xie et al. [151], an O-doped graphitic carbon nitride with carbon vacancies (O-CNC) was prepared through the thermal treatment of the mixture of MCA and oxalic acid using microwave method (Figure 11F).Oxygen doping and carbon vacancies in O-CNC will help to extend the light absorption edge and enhance the separation efficiency of photo-generated charge carriers (Figures 11G, 11H).In addition, the oxygen doping sites have a high selectivity for 2e − oxygen reduction to H 2 O 2 .Further accompanied with the mesoporous structure with large specific surface area and optimized electronic band structure, the O-CNC exhibited a good photocatalytic H 2 O 2 production rate of 2008.4 μmol h −1 g −1 under simulated sunlight irradiation (Figure 11I).

Electrocatalysis applications
Carbon-nitrogen-based materials have recently attracted numerous attention for their potential applications in electrocatalysis, including the electrochemical conversion processes in hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction [152][153][154].The following factors offer the supramolecular-derivatives attractive electrocatalytic activities in electrochemical energy conversion reactions.(i) High specific surface area and ample porous structure provide accessible active sites for electrocatalytic reactions as well as excellent electron and mass transfer pathways [155].(ii) Differing from the metal organic Natl Sci Open, 2023, Vol.2, 20220032 framework (MOF)/covalent organic framework (COF) precursors with precise crystal texture [156][157][158], the component adjustable ability accomplishes the effective manipulation of the allocation of different elements doping in supramolecular-derivatives, including dopant proportion, content, and even configuration [159,160].Consequently, multiple active sites with controllable adsorption energy towards H and O species may be generated in supramolecular-derivatives, hence accelerating the whole electrochemical energy conversion reactions.(iii) The pore structure and functional groups in the supramolecular assemblies provide abundant and uniform adsorption sites for metal ions [161,162], the strong adsorbing environment makes the metal ions homogeneously dispersed, efficiently preventing the agglomeration and facilitating the construction of the single metal atom or subnanometer metal catalyst [163,164].

Electrocatalytic HER
The HER has been supposed as a highly efficient and clean method for producing hydrogen as it provides a new opportunity for coupling with other renewable energy [165][166][167].Nevertheless, the industrialization is still limited by the lack of highly efficient catalysts.As reported by previous studies, no matter how the consecutive reaction will proceed, both the H proton adsorption (Volmer step) and the produced H 2 release (Heyrovsky or Tafel step) are necessary in HER process [168].Therefore, the overall kinetics of HER are highly determined by the hydrogen adsorption free energy (ΔG H ). If ΔG H is too weak, the H proton is difficult to be activated, which may limit the Volmer step.Whereas, if the ΔG H is too strong, the desorption step of produced H 2 (via either the Heyrovsky or Tafel step) will be prevented [169,170].Both of them may cause a poor apparent HER activity.Furthermore, high electronic conductivity, nano-porous structure with large surface area, and ample electrocatalytically active sites should be also required when designing the electrocatalysts for HER [171].
Carbon materials synthesized by pyrolysis of supramolecular precursors typically exhibit nano-porous feature and high specific surface area.Further benefiting from self-doping one or more elements into the carbon ring with different electronegativity, the supramolecular assembly-derived carbon material will show excellent HER performance.For example, Ren and coworkers [172] reported a supramolecular-controlled synthetic strategy to precisely regulate the proportion and content of N and P in graphene via assembling melamine and phosphoric acid on the graphene (Figures 12A, 12B).By tuning the proportion and content of melamine and phosphoric, the author showed that the C atoms between meta-type N and P in the heterocycles portrayed the most optimal H adsorption free energy, thus improving the electrochemical HER activity.Wu's group [173] further constructed a N, P, and O tri-doped defect-rich porous carbon electrocatalyst by annealing the hybrid supramolecular of assembled graphene oxide (GO), phytic acid (PA), and polyethyleneimine (PEI).Profiting from the tri-doped strategy and multi-porous structure with ample exposed active sites, the resulting electrocatalyst generated excellent HER performance with low overpotential of 179 mV and small Tafel value of 93 mV dec −1 .
Since the supramolecular aggregates usually contain many functional entities, such as carbonyl, amide, pyridine group, conjugated system, and tunable cavity, the metal ions can be uniformly or even atomically anchored in the supramolecular structure.After pyrolysis under high temperature, these metal ions should be in-situ converted to metal nanoclusters or single atoms embedded in mesoporous carbon which will serve as highly HER active sites.For example, Yang's group [174] used pumpkin-shaped rigid supramolecular Natl Sci Open, 2023, Vol.2, 20220032 macrocycle cucurbituril (CB [6]) as carrier to anchor the Ir ions (Figure 12C).Compared with other carbon sources, such as graphene, carbon nanotube, and metal-organic frameworks, the supramolecular CB [6] possesses a unique 1D porous channel with numerous carbonyl and amide groups, which can efficiently prevent the metal agglomeration at high temperature.Furthermore, the N atom can be in-situ doped into the carbon framework without any additional N sources due to the rich content of N atoms in the original structure.Meanwhile, the ample in-plane mesoporous structure can also be inherited from the porous supramolecule after the pyrolysis process.Taking advantage of the distinctive structure, the resulting electrocatalyst owned abundant exposed active sites, and hence portrayed highly efficient HER performance with low overpotential, high Faradaic efficiency, and turnover frequency (TOF) values.Lei et al. [175] further reported a hybrid electrocatalyst of Ni single atom and Ni particle on N-doping carbon catalyst via assembling of dicyandiamide and Ni ions, followed by pyrolysis and an acid etching process (Figure 12D).Both the large specific surface area and the strong interaction between Ni single atom and Ni particle endow the hybrid electrocatalyst a high HER catalytic activity with low overpotential of 147 mV.
Other than the metallic and non-metallic atom doped carbon materials, various transition metal compounds  Electrocatalytic ORR Supramolecular assembly is appropriate for in situ heteroatoms doping and transition metals confining owing to their adjustable monomers and abundant binding sites.However, supramolecular assembly are greatly restrained by high electron transport resistance and poor chemical/electrochemical stability, which makes them very unfavorable to be directly used in electrocatalytic reactions.Recently, novel carbon materials have aroused extensive attention owing to their enormous potential as substitution to commercial Pt/C catalyst for ORR.Among numerous methods of synthesizing carbon materials, supramolecular assembly strategy stands out since they can be applied as the structure-inducing reagent to generate hierarchical and loose porous 3D structure, which leads to more exposed active sites [115,116].The application of supramolecular assemblyderived carbon materials in ORR mainly includes two types: (1) non-metallic-doped carbon materials with unique pore structure, and (2) metallic and non-metallic elements co-doped carbon materials.Non-metallic atom-doped carbon is one of the most commonly used catalysts for ORR.The electronegativity differences between elements realize the alkalization of the residual carbon atoms when nonmetallic atoms replace the carbon atoms in the carbon ring.The electron-rich carbon is conducive to the adsorption of oxygen atoms and promotes the ORR [114].For example, Zhao et al. [113] synthesized a sort of 1D carbon nanotubes (h-NCT-900) by annealing glucose coated MCA (Figures 14A-14C).It was revealed that the in-situ doped N brought a wealth of holes and defects to the carbon substrate and exposed more active sites.Furthermore, the 3D network of interlaced 1D nanotubes not only endued the adequate contact between intermediates and active sites, but also worked as conductive network to regulate the electrons transmission.As a result, the obtained h-NCT-900 possessed a brilliant onset potential of 0.97 V and superior half potential of 0.863 V. Apart from single element doping, multi-element co-doped carbon materials were also devised to elevate the ORR performance [117,118,121].The introduction of diverse elements in porous structure adjusts the charge redistribution and provides more active sites, thus accelerating the ORR process [119,120].Wu et al. [178] prepared N,S dual-doped porous graphene networks (N,S-PGN) nanosheets by supramolecular induction, which demonstrated excellent ORR catalytic performance and durability (Figure 14D).The formation of defects and construction of hierarchical pores will strengthen the transmission of electron and electrolyte.Meanwhile, the synergy of dual-doped N and S atoms regulated the local electronic structure, which further promoted the electrocatalytic property by enhancing the adsorption of oxygen molecules.
Apart from non-metallic elements doped carbon materials, dispersed transition metal atoms supported by carbon materials are another kind of prospective catalysts for ORR [179][180][181].Compared with typical covalently bonded nitrogen-containing precursors, the cross-linking of nitrogen-containing groups is more likely to occur in supramolecular aggregates during pyrolysis, which largely restricts the agglomeration of metal particles [182][183][184][185]. Li and his colleagues [186] prepared a novel Co and N dual-doped trumpet flowerlike porous carbon (CoNC) (Figures 14E, 14F), which was demonstrated to be an efficient ORR catalyst.On the one hand, CoNC not only possessed excellent electronic transmission performance which profited from the hierarchical porous structure, but also modulated the adsorption of O 2 during ORR process due to the surface charge redistribution induced by embedded heteroatoms on carbon matrix.Furthermore, the thermal stability of the original volatile monomer is improved after supramolecular self-assembly, such that the structure of material can be preserved after carbonization, thus the homogeneous atomic scale distribution of metal active sites can be obtained in this manner [187,188] 14G, 14H), a variety of single Cu atoms anchored by supramolecules exhibited high atomic efficiency up to 95.6%.It indicated that the synergy between the substrate and metal is of vital importance for the consistency.Additionally, DFT calculation revealed that the O-O bond is more likely to break at the Cu-N 2 site, indicating that O 2 is more likely to be activated.

Electrocatalytic OER
Supramolecular assembly-derived carbon-based materials in OER mainly consist of a series of metal compounds and carbon composite catalysts.Similar to ORR, the possibility of designing supramolecules with tunable monomers presents the opportunity to anchor transition metal atoms in the supramolecular backbone.Meanwhile, porous structure after annealing provides large extended reaction interfaces to achieve apparently high anodic current densities.Based on these contexts, supramolecular aggregates were considered as practical and universal precursors for generating OER active phases.For instance, Wu et al. [190] reported a kind of metal embedded nitrogen doped carbon nanotubes (MM@NCNT, M=Fe, Co, Ni), which were used as efficient OER electrocatalyst with the low Tafel slope of 68 mV dec −1 .The micro-/meso-pores structure obtained by annealing supramolecular template of MM@NCNT, M=Fe, Co, Ni endow increasing specific surface area and excellent electronic transmission, further leading to a fast dynamic process.Meanwhile, the synergistic effect between the bimetallic particles also boosts the OER process.
From the perspective of designing materials, supramolecules can function as brackets to accommodate metal oxide/nitride/phosphide compounds as well, so that the dispersed metallic compounds can be in-situ obtained by simply annealing treatment.These composite structures are conducive to prevent the aggregation of metal compounds nanoparticles [191].On the other hand, conductivity is promoted as the consequence of the adjustment of the surface charges induced by doped non-metallic elements, thereby improving the overall OER activity [192].For example, Zhang et al. [193] reported a kind of O-doped transition metal phosphides (TMPs) through pyrolysis of phytic acid complexes (Figures 15A-15E).In addition to compensating the deficiency of intrinsic conductivity of TMPs, the existence of O lengthens the M-P bonds and regulates the electrons transportation from substrate to active sites.Apart from this, the presence of carbon substrate also plays a protective role in maintaining stable metallic compounds.It has been demonstrated that the coated P, N co-doped carbon matrix greatly stabilizes CoP during OER process [194].What is more, the stabilization of phase by carbon matrix was also discovered in nanohybrids which Ni/Fe phosphides wrapped with Pdoped carbon (PC) supported on P-doped graphene (PG) ((Ni x Fe 1−x ) 2 P@PC/PG) by Wang et al. (Figures 15F,15G) [195].PC/PG matrix can not only fix and protect the phosphides, but also promote the generation of hexagonal phase (Ni x Fe 1−x ) 2 P and suppress its transition to orthorhombic phase FeP at the same time.Their high activity originates from the unique electronic structure caused by the strong interaction between carbon matrix and metal compounds, which further impacts the adsorption of reactant/intermediate.Furthermore, carbon matrix plays a vital role in protecting and stabilizing metal compounds for enlarging the number of exposed active sites as well.In this regard, the surface electron distribution of (Ni x Fe 1−x ) 2 P@PC/PG is optimized under the induction of phase transition mechanism, which effectively raises the adsorption and activation of H 2 O, and thus enhancing the catalytic activity of OER.Similarly, the mixed-phase CoP-Co 2 P@PC/PG possesses favorable activation of H 2 O molecules on account of the redistribution of surface electronic structure induced by PC/PG [196].
The supramolecular assembly-derived carbon-nitrogen-based materials have had exciting progress in the field of energy and environmental-related photo/electrocatalysis applications.The reasonable design of supramolecular assembly greatly affects the morphology, chemical and electronic structure of its derivatives.As a result, the optimized optical and electronic properties for a given photo/electrocatalysis application can be realized based on the heteroatom doping, functional group modification, transition metal atoms loading, and other modifications.

Conclusions and outlook
This review shows that the supramolecular assembly route is a promising pathway for the rational design and Natl Sci Open, 2023, Vol.2, 20220032 regulation of morphology, chemical composition and micro/nanostructure of the carbon-nitrogen-based functional materials.The promising physicochemical properties of the supramolecular assembly-derived carbon-nitrogen materials, including high porosity, suitable electronic structure, good electronic conductivity, and excellent stability, attract numerous interdisciplinary studies and make it increasingly significant in the field of energy conversion related photo/electrocatalysis.
Although the potential applications of the supramolecular assembly-derived carbon-nitrogen-based materials have been obviously extended, the correlations between the structure and their photo/electrocatalytic activity are still not very clear.Further in-situ analyses, such as in-situ infrared spectroscopy, in-situ XPS, and in-situ XRD analysis, are highly desirable to understand the mechanism of the supramolecular assembly and following pyrolysis processes to elucidate the nature of the physicochemical properties.The combination of experimental studies and theoretical verification is needed to reveal the catalytic kinetics, mutative activity, selectivity and stability, alongside to illuminate their relationship with the micro/nanostructure.[193].Copyright 2016, American Chemical Society.(F) Schematic illustration of the synthesis procedure of (Ni x Fe 1−x ) 2 P@PC/PG.(G) OER polarization curves and (Ni x Fe 1−x ) 2 P@PC/PG.Adapted with permission from [195].Copyright 2019, Willy-VCH.
Natl Sci Open, 2023, Vol.2,20220032 Additionally, exploring pristine supramolecules with stable hydrogen-bonded framework and unique optical and electrical properties remains an intractable challenge, which might endow the supramolecules the straightforward applications as the catalysts or active components.Considering the feasibility to realize controllable design and fabrication, such supramolecules might possess distinct tunable photo-electric properties, thereby is speculated to be upcoming research hotspot, as a supplementary module to the metalorganic frameworks and covalent-organic frameworks.Further defect regulation, functionalization and fine structure characterization of these supramolecules require more intensive investigations.Finally, 2D supramolecules with intrinsic pore structure and large lateral size are highly desired since they deliver not only the above intrinsic physicochemical properties but transcendental 2D features.One of the expected applications for the 2D supramolecules is to serve as separators to induce the ion sieving effect for rechargeable batteries.On the other hand, in view of the practical application, simplifying the synthetic procedures and reducing the costs of the supramolecular are more appealing.
Overall, the supramolecular assemblies and their-derived carbon-nitrogen-based functional materials have experienced a renaissance recently.The boundaries have been possibly pushed far beyond what people could imagine in the beginning.Increasing photo/electrocatalysis applications are desired to be investigated owing to the corresponding texture features, such as abundant pores for polysulfide adsorption and conversion in Li-S batteries, ion-sieving abilities for the Zn anode protection in Zn-ion batteries, unique spin density and charge distribution of the carbon or nitrogen atoms for the nitrogen reduction reactions (NRR), where opportunities and challenges coexist.In addition to fundamental research, both the feasible structural design and the simple synthetic procedures provide the possibility for the large-scale and commercial production of the supramolecular assemblies along with the derived carbon-nitrogen-based materials.With continuous research progress, it holds great potential to promote the revolution of renewable energy utilization.

Figure 1
Figure 1 Schematic of supramolecular assembly-derived materials for photo/electrochemical applications.

Figure 12 (
Figure 12 (A) Schematic illustration and SEM images of the G-NP.(B) DFT calculations of H adsorption free energy and electronic property of different N,P co-doped graphene.Adapted with permission from [172].Copyright 2019, Royal Society of Chemistry.(C) The structure of CB[6]-Ir.Adapted with permission from [174].Copyright 2021, Willy-VCH.(D) The DFT calculations and HER catalytic activity of Ni NP|Ni-N-C/EG towards HER.Adapted with permission from [175].Copyright 2019, Royal Society of Chemistry.

Figure 13 (
Figure 13 (A) Schematic illustration and XAS analysis of the Ni-Mo 2 C@NPC.(B) Electrocatalysis test.Adapted with permission from [176].Copyright 2021, Elsevier.(C) Schematic illustration of the synthesis and structure of N-doped Mo 2 C/PC.(D) The DFT calculations of N-doped Mo 2 C/PC towards HER.Adapted with permission from [177].Copyright 2022, Elsevier.

Figure 14 (
Figure 14 (A) Schematic illustration of the synthesis procedure of hollow nitrogen-doped carbon tubes (h-NCTs).(B) SEM image of h-NCTs.(C) LSV curves of all h-NCTs samples and Pt/C.Adapted with permission from [113].Copyright 2018, Royal Society of Chemistry.(D) TEM image of N,S-PGN-800.Adapted with permission from [178].Copyright 2017, Royal Society of Chemistry.(E) SEM image of CoNC-800.(F) LSV curves of CoNC.Adapted with permission from [186].Copyright 2017, American Chemical Society.(G) Fourier transformed EXAFS after phase-corrected spectra and (H) corresponding WT plots of Cu-K edge at R space of PPyCuPcTs.Adapted with permission from [189].Copyright 2020, Elsevier.