Introduction
Increasing fossil fuel consumption alongside the related ecological and environmental issues have attracted continuous attention and stimulated enormous investigations [1–10]. Energy storage and conversion technologies based on the utilization of sustainable and green energy sources, such as solar energy, tidal energy, and wind energy, are intensively studied and meanwhile are considered the most promising strategies to address the above issues [11–14]. For example, solar energy is able to accelerate the degradation of pollutants and can further drive their conversion into value-added chemicals through photochemical catalysis [15,16]. Tidal and wind energy are the clean and sustainable energy sources for electricity generation [17,18]. All of the related energy conversion technologies involve a series of redox reactions, such as hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) [19–22], oxygen reduction reaction (ORR) [23–25], CO2 reduction reaction [26–29], degradation of pollutants [30–32], and organic synthesis [33–35]. At the heart of such redox reaction systems, the photo/electrocatalysts play vital roles in the activity, selectivity, stability and the kinetics. To this end, it is crucial to explore low-cost, ecofriendly, and highly active photo/electrocatalysts.
Supramolecular assemblies are constructed via noncovalent interactions, including hydrogen bonds, van der Waals force, π-π interaction, and electrostatic force [36,37]. Thus, they possess well-organized micro/nanostructures and versatile composition. The controllable modulation of morphology and chemical composition of supramolecular aggregates can be easily achieved via adjusting the reaction conditions (e.g., solvents, temperatures, and pH values) and the starting monomers [38–42]. Specially, thanks to the strong reversibility, specificity, and directionality nature of hydrogen bonds, the hydrogen-bonded supramolecular assemblies can realize a certain arrangement of the small organic molecules thereby building a stable aggregate with desired framework [43–45]. A variety of organic molecules with specific structures can act as hydrogen-bond donors or acceptors, which form hydrogen-bonded framework with counterparts and subsequently generate supramolecular aggregates with inherited chemical structures [46–48]. Typically, melamine and cyanuric acid can form a classic supramolecular complex (MCA), since they can serve as both hydrogen-bond donors and acceptors and the connected hexamers will arrange in planar sheets and further stacked [49–52].
Fabrication of carbon-nitrogen-based functional materials derived from supramolecular aggregates is an efficient and effective strategy to improve their physicochemical properties, which has attracted a tremendous amount of attention from researchers in widespread areas. Benefiting from the diversity of starting monomers and the facile structural approach, it is feasible to design and regulate the morphology, chemical compositions, micro/nanostructure, and electronic structure of the supramolecular assembly-derived carbon-nitrogen-based materials (Figure 1 ). As a typical example, MCA-derived polymeric carbon nitride (C3N4) has been widely studied that the heteroatom doping, defects engineering, three-dimensional (3D) porous architecture construction, and insertion of functional groups were successfully achieved at the molecular level for strengthening the photocatalytic performance.
| [Image omitted. See PDF.] | Figure 1 Schematic of supramolecular assembly-derived materials for photo/electrochemical applications. |
Such a supramolecular chemistry strategy demonstrates great potential for preparing low-cost and highly efficient carbon-nitrogen-based photo/electrocatalysts toward energy storage and conversion. A number of emerging intrinsic properties of these carbon-nitrogen materials are derived that are certainly interesting, but less well covered [53–55]. In addition, the relationships between the structure regulation of supramolecular assembly and the catalytic performance of final carbon-nitrogen-based materials are still ambiguous. From the perspective of materials science, all these aspects are related and essentially deal with the modification of photo/electrocatalysts for renewable energy conversion applications. Therefore, it is highly desirable to collect and summarize recent advances in this research field, aiming to reasonably design and regulate highly efficient carbon-nitrogen-based functional materials. Here, we aim to give an overview of this field and discuss the significant breakthroughs that supramolecular assembly-derived carbon-nitrogen-based functional materials have brought for photo/electrochemical energy conversion. First, we will discuss how different functionalities of supramolecular assembly-derived carbon-nitrogen-based materials are realized from the aspect of morphology and chemical composition control of supramolecular aggregates. Then, the energy and environmental-related applications of the supramolecular assembly-derived carbon-nitrogen-based materials will be discussed in-depth, including water splitting, CO2 reduction, ORR, degradation of pollutants, and organic synthesis. Finally, we expect that with the new properties and engineering of the supramolecular assembly-derived carbon-nitrogen-based functional materials being continually discovered, scientists with various research backgrounds will inspire each other, thus enabling faster, further progress of such carbon-nitrogen materials and other related research studies.
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.
Morphology control
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 as-synthesized C3N4 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 2 A). The resulting C3N4 exhibits a bilayer structure that possesses a rough outer layer and an irregular porous inner layer after calcination (Figure 2 B). By comparison, the supramolecular precursor using ethanol as the solvent (TCM-E) shows the morphology of large flakes with a much smoother surface (Figure 2 C), the as-obtained C3N4 reveals a uniform, lamellar surface morphology (Figure 2 D). Evidenced by the X-ray diffraction (XRD) analysis, the C3N4 obtained in chloroform manifests higher defect rates and smaller interlayer distance, which further proves the effect of the polarity on the local structure of C3N4, leading to a district morphology.
| [Image omitted. See PDF.] | Figure 2 SEM images of different precursors and calcined C3N4 samples: (A) TCN-C, (B) TCM-C, (C) TCN-E, (D) TCN-E. Adapted with permission from [57]. Copyright 2019, Elsevier. (E) Schematic representation of MCA transformation from DMSO to less polar solvent. (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 C3N4 layer stripped by (I) methanol, and (J) ethanol. SEM images of C3N4 inserted by (K) glycol, and (L) glycerol. Adapted with permission from [49]. Copyright 2019, American Chemical Society. |
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 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 C3N4. 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 C3N4 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 3 D). Further increasing the amount of nitric acid found that CN-20 possesses a morphology of staggered nanosheet (Figure 3 E). 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].
| [Image omitted. See PDF.] | Figure 3 Typical SEM images of (A) CN, (B) CN-5, (C) CN-10, (D) CN-15, and (E) CN-20. Adapted with permission from [58]. Copyright 2018, American Chemical Society. SEM images of as-synthesized MCA-DMSO precipitated at (F) 30, (G) 60, (H) 90, (I) 120, and (J) 150°C. Adapted with permission from [38]. Copyright 2013, Wiley-VCH. |
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
C3N4 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 [62–68], and other molecules [69–75]. It is found the alignment of the structural units conspicuously alters the morphology and porosity of C3N4, 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 C3N4, hence strengthening their catalytic activities. The supramolecular assemblies can be polymerized into C3N4 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 C3N4 via cyanuric acid-2,4-diamino-6-phenyl-1,3,5-triazine supramolecules [77], and porous crimped C3N4 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 C3N4 (C3N4-HCl) presented an enhanced light harvesting ability, charge separation efficiency and hydrogen production. Furthermore, Barrio et al. [81] prepared a unique halogen decorated C3N4 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 4 A). Additionally, other protonic acids and alkalis (such as HNO3, H2SO4, H3PO4 and (NH2OH)·H2SO4) are adopted not only to tailor the dissolution and hydrolysis rate of melamine [82–84], but also to optimize its self-assembly behavior [58,85]. The protonated C3N4 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 4 B). Such a C3N4 exhibits excellent electrochemical performance as electrodes for supercapacitors and photocatalytic activity toward photodegradation of rhodamine B.
| [Image omitted. See PDF.] | Figure 4 (A) Schematic synthetic route for MCBt supramolecular assemblies. Adapted with permission from [81]. Copyright 2018, Elsevier. (B) Schematic representation of chemical reaction for the synthesis of GCNNF. Adapted with permission from [86]. Copyright 2014, American Chemical Society. (C) Formative process of oxygen-doped C3N4 nanotubes. Adapted with permission from [42]. Copyright 2021, Elsevier. (D) Schematic representation of the synthesized process of MTCN. Adapted with permission from [90]. Copyright 2018, Elsevier. (E) The formation of C3N4 nanostructures (CNWs and CNFs). Adapted with permission from [93]. Copyright 2016, Elsevier. (F) Synthetic route towards C-rich C3N4 with a carbon layer gradient. Adapted with permission from [97]. Copyright 2018, Royal Society of Chemistry. |
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 C3N4. Oxygen doping as one of effective strategies to optimize the electronic and optical properties of C3N4 has attracted extensive attention [87–89]. Zhang et al. [42] synthesized oxygen-doped C3N4 (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 4 C). Highly crystalline S-doped C3N4 (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 as-obtained MTCN exhibits a superb visible-light H2-generation performance compared with the pristine C3N4 (Figure 4 D) [90]. S-doped C3N4 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 4 E) [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 C3N4 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 dynamic gas template and enhancing thermal oxidation etching during thermal decomposition. Carbon-rich C3N4 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 C3N4 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 C3N4 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 C3N4 is identified originating from the enlarged surface area, shortened charge-to-surface migration length, and involved nitrogen vacancies (Figure 4 F).
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 5 A) [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 5 B) [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 photo-generated carriers, hence accelerating the separation efficiency (Figure 5 C) [100]. Besides, other aromatic compounds such as 2,4-diamino-6-phenoxy-1,3,5-triazine (Figure 5 D) [101], 2,4-diamino-6-phenyl-1,3,5-triazine (Figure 5 E) [102], and polycyclic aromatic hydrocarbons (PAH)-substituted 1,3,5-triazine (Figure 5 F) [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.
| [Image omitted. See PDF.] | Figure 5 (A) Reaction path for caffeine-modified carbon nitride materials from cyanuric acid-melamine supramolecular complexes. Adapted with permission from [98]. Copyright 2015, Wiley-VCH. (B) The formation process of C-doped hollow spherical C3N4. Adapted with permission from [96]. Copyright 2017, American Chemical Society. (C) Graphic representation of the CMB-x% supramolecular complex. Adapted with permission from [100]. Copyright 2014, American Chemical Society. (D) Reaction path for the modified-C3N4 materials from MCA and MA-phO supramolecular complexes. Adapted with permission from [101]. Copyright 2020, Elsevier. (E) Hydrogen-bonded supramolecular C-M-Mp complex and the proposed carbon nitride structure after calcination. Adapted with permission from [102]. Copyright 2014, American Chemical Society. (F) Carbon nitride (CM and CMR-CN) synthesis at 550°C from hydrogen-bonded supramolecular complexes as starting reactants. Adapted with permission from [103]. Copyright 2020, Wiley-VCH. |
Supramolecular assembly-derived carbon materials
Apart from the typical C3N4 materials, the supramolecular assemblies can be further carbonized into heteroatom doped carbon materials above 600°C [104–106]. 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 Pt-based catalysts for energy storage and conversion [108–112]. 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 6 A). 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 6 B) [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 6 C), demonstrating heteroatom-doped carbon catalysts are a promising alternative for precious metal based catalysts.
| [Image omitted. See PDF.] | Figure 6 The synthetic procedure of the supramolecular-derived carbon-based materials. (A) Nitrogen-doped three-dimensional porous graphene frameworks (NGA) materials. Adapted with permission from [116]. Copyright 2018, Elsevier. (B) Melamine, phytic acid, and graphene oxide (MPSA/GO-1000) via cooperative assembly and pyrolysis. Adapted with permission from [119]. Copyright 2013, Wiley-VCH. (C) Dual heteroatoms doped N, S−C/800. Adapted with permission from [120]. Copyright 2016, Royal Society of Chemistry. (D) N,P,S tri-doped carbon nanorods. Adapted with permission from [121]. Copyright 2018, Wiley-VCH. |
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 6 D).
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.
Applications in photo/electrocatalysis
Photocatalysis applications
The supramolecular assembly strategy is a persuasive pathway for the synthesis of C3N4-based functional materials with diversified structures [122–124]. 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 C3N4-based functional materials for the solar-energy-conversion techniques, e.g., photocatalytic water splitting [125,126], photocatalytic CO2 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 C3N4-based functional materials for photocatalysis applications have been summarized.
Photocatalytic water splitting
Simultaneous production of H2 and O2 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 C3N4-based photocatalysts, motivated by its relatively low energy band gap (Eg) 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 C3N4-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 broom-like oxygen-doped carbon nitride (O-CN-NTs) which was prepared through the “calcination-hydrothermal-calcination” method based on the supramolecular assembly process (Figure 7 A). 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 7 C). 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 H2 production rate of 13.6 mmol g−1 h−1 (Figure 7 B).
| [Image omitted. See PDF.] | Figure 7 (A) Schematic illustration of the synthesis process, (B) SEM image, (C) photocatalytic hydrogen generation performance, and (D) simulated electric field distribution of O-CN-NTs. Adapted with permission from [133]. Copyright 2022, Elsevier. (E) Conceptual photocatalysis scheme on core-shell like carbon nitride, and (F) photocatalytic hydrogen evolution performance. Adapted with permission from [39]. Copyright 2017, Elsevier. (G) Schematic of the amidation reaction assisted supramolecular assemble process. (H) SEM image of actiniae-like ACN-14. (I) Photocatalytic hydrogen evolution performance. Adapted with permission from [134]. Copyright 2021, Wiley-VCH. (J) Schematic synthesis routes, and (K) SEM image of CT1 supramolecular aggregate. (L) Photocatalytic performance. Adapted with permission from [135]. Copyright 2020, American Chemical Society. |
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 7 E). As a result, the carbon nitride heterojunction will be constructed which can further improve its 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 7 F), which is much higher than that of pristine C3N4.
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 co-workers [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 H2 generation rate of 95.3 mmol h–1 (Figure 7 I). 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 7 J). 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 7 K, the cyanuric acid-T1 supramolecular precursor (CT1) possessed a novel 3D morphology consisting of micro-stick 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 C3N4 (Figure 7 L).
Photocatalytic CO2 reduction
Besides water splitting, photocatalytic CO2 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 CO2 has attracted great attention, since the strong interaction between N atoms in heptazine rings and the polarized CO2 molecules, as well as the Lewis basic NH and NH2 could facilitate the adsorption of CO2 molecules [6]. Most importantly, the conduction band (CB) position of carbon nitride is enough negative to reduce the CO2. The products of photocatalytic CO2 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 CO2 reduction products can be significantly improved by reasonably adjusting the energy band structure of carbon nitrides.
Increasing the NH and/or NH2 group contents of C3N4-based photocatalysts is an effective strategy to boost their CO2 reduction performance, which can engineer its electronic structure and facilitate the sorption of CO2 molecules [6]. Recently, Mo et al. [137] fabricated a porous nitrogen rich C3N4 (TCN(NH3)) 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 CO2 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 8 C). The rich amino groups in the TCN(NH3) 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(NH3) will be conducive to catch CO2 through the hydrogen bonding interaction. As shown in Figure 8 D, the CO2 adsorption energy of TCN(NH3) was determined to be −0.358 eV, which was much higher than that of the unmodified C3N4. In addition, the N–H–O distance between TCN(NH3) and CO2 molecule was 2.445 Å, very similar to that of hydrogen bonds. The adsorption model of CO on TCN(NH3) was C-terminated adsorption (N–H–C). The CO desorption energy of TCN(NH3) and unmodified C3N4 was 0.032 and 0.148 eV, respectively. The higher CO desorption energy of unmodified C3N4 was due to the stronger N–H–O interaction. These results suggested that Lewis basic amino groups in TCN(NH3) can simultaneously increase the CO2 adsorption energy and decrease the CO desorption energy. Both of them are conducive to improve the photocatalytic CO2 reduction performance.
| [Image omitted. See PDF.] | Figure 8 (A) Schematic illustration of the synthesis process, and (B) SEM image of TCN(NH3). (C) Photocatalytic CO2 reduction performance. (D) Adsorption and desorption energies based on DFT calculations. Adapted with permission from [137]. Copyright 2019, Elsevier. (E) HAADF-STEM of Cu/CN. (F) FT-EXAFS, and XANES of the Cu K-edge. (H) Free energy diagram of CO2 reduction. (I) Photocatalytic CO2 reduction activity of Cu/CN. Adapted with permission from [138]. Copyright 2020, American Chemical Society. |
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 photo-induced carriers’ separation and transfer, as well as facilitate the CO2 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 8 E) and Fourier transform extended X-ray absorption fine structure (FT-EXAFS, Figure 8 F) tests. As shown in Figure 8 G, the coordination environment of Cu single atom in Cu/CN was identified as C–Cu–N2 by the XANES spectrum of Cu K-edge. These active centers can improve the charge transfer capacity and reduce the energy barrier of photocatalytic CO2 reduction (Figure 8 H). As a result, the optimized Cu/CN-0.25 showed the most efficient photocatalytic CO2 reduction activity, and the generation of CO was as high as 11.21 μmol g−1 h−1 (Figure 8 I).
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 C3N4 as initiator (Figure 9 A). The well-dispersed C3N4 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 C3N4 generates abundant surface radicals which could induce polymerization of the DMA and the cross linker (N,N′-methylenebis(acrylamide)) to get cross-linked CNB-G hydrogel network. As shown in Figure 9 B, 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 C3N4 will be directly injected into methylene blue (Figures 9C, 9D).
| [Image omitted. See PDF.] | Figure 9 (A) Schematic process of hydrogelation. (B) Adsorption ability for dyes of CNB-G hydrogel. (C, D) Photodegradation of various dyes. Adapted with permission from [139]. Copyright 2017, American Chemical Society. (E) Synthesis routes of Cu-pCN. (F) UV-vis absorption, and (G) PL spectra. Adapted with permission from [140]. Copyright 2022, Elsevier. (H) Schematic presentation of the preparation of CM-CN-Th A. (I, J) Photodegradation activity. Adapted with permission from [141]. Copyright 2022, Elsevier. |
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 C3N4 (Cu-pCN), which was calcined from the Cu-melamine-cyanuric acid supramolecule (Figure 9 E). The Cu doping endowed Cu-pCN with enhanced light absorption, good photo-generated 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 C3N4 nanosheet (CM-CN-Th A), which was synthesized through the poly-condensation of supramolecular precursor and 3-thiophenecarboxylic acid (Figure 9 H). 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 9 I, 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 9 J).
Photocatalytic organic synthesis
The photocatalytic oxidation and hydrogenation conversion of organics or wastes can produce value-added chemical products. C3N4-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 10 A). 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 light in the hollow structure will excite a lot of electrons. The optimized LCN/Pt exhibited the highest photocatalytic H2 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 10 D).
| [Image omitted. See PDF.] | Figure 10 (A) TEM image, (B) electronic band structure, and (C) density of states of LCN. (D) Photocatalytic hydrogenation of 4-nitrophenol. Adapted with permission from [142]. Copyright 2022, Elsevier. (E) Density of states, and (F) charge density distribution of SCN-HMS. Adapted with permission from [143]. Copyright 2020, Elsevier. (G) HAADF-STEM image of SA-Cu-TCN. (H) X-ray absorption spectra at Cu K-edge. (I) Ultrafast transient absorption spectra containing fitting results. Adapted with permission from [149]. Copyright 2020, Wiley-VCH. |
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 sulfur-doped 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 10 E, 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 10 F). Due to the synergistic effects between nanostructure construction and electronic structure regulation, the SCN-HMS could act as a bi-functional photocatalyst for efficient H2 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–N3 group (one Cu atom coordinated with three N atoms) and Cu–N4 group (one Cu atom bonded with four N atoms in the neighboring C3N4 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 10 I). 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 C3N4 (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 (H2O2) 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 H2O2 through the two electron (2e−) ORR with alcohol as the hydrogen source and O2 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 H2O2 production performance. The introduction of exposed edge active groups can facilitate the O2 adsorption and activation capacity of C3N4-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 H2O2 production. The PCN-NaCA was synthesized through the poly-condensation reaction of the melamine-cyanuric acid supramolecular precursor and the subsequent thermal treatment with sodium thiocyanate (NaSCN) molten salt (Figure 11 A). 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 11 B, 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 O2 adsorption capacity, which can promote the interfacial electrons transfer to the adsorbed O2 (Figure 11 C). The DFT calculations for the free energy diagrams of ORR steps further confirmed the selective 2e− ORR pathway on PCN-NaCA (Figure 11 D). The optimized PCN-NaCA sample showed a photocatalytic H2O2 production rate of 18.7 μmol h−1 mg−1 and an apparent quantum yield of 27.6% at 380 nm (Figure 11 E).
| [Image omitted. See PDF.] | Figure 11 (A) Schematic synthesis process of PCN-NaCA. (B) fs-TAS of PCN-NaCA. (C) Temperature programmed oxygen desorption profiles. (D) Free energy diagrams of ORR steps. (E) Solar-driven H2O2 generation activity. Adapted with permission from [123]. Copyright 2021, Springer Nature. (F) Schematic synthesis procedure of O-CNC. (G) UV-vis light absorption spectra, and (H) O 1s XPS spectra of O-CNC. (I) Photocatalytic H2O2 generation performance under simulated solar irradiation for 2 h. Adapted with permission from [151]. Copyright 2021, American Chemical Society. |
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 H2O2 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 11 F). 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 H2O2. Further accompanied with the mesoporous structure with large specific surface area and optimized electronic band structure, the O-CNC exhibited a good photocatalytic H2O2 production rate of 2008.4 μmol h−1 g−1 under simulated sunlight irradiation (Figure 11 I).
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 [152–154]. 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 framework (MOF)/covalent organic framework (COF) precursors with precise crystal texture [156–158], 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 [165–167]. 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 H2 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 (ΔGH). If ΔGH is too weak, the H proton is difficult to be activated, which may limit the Volmer step. Whereas, if the ΔGH is too strong, the desorption step of produced H2 (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.
| [Image omitted. See PDF.] | 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. |
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 macrocycle cucurbituril (CB[6]) as carrier to anchor the Ir ions (Figure 12 C). 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 12 D). 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 portray more active sites for HER due to the lower shift of d orbital, which decrease the desorption energy of produced H2. For example, Liu and coworkers [176] proposed a unique supramolecular-confinement pyrolysis method to prepare ultrafine Ni-Mo2C catalyst (Figures 13A, 13B). Since the metallic Ni and Mo atom was tightly anchored in the cavity of supramolecule composed of cetyltrimethylammonium bromide-melamine-phytic acid, the Ni species can be atomically dispersed between Mo2C lattice and P atoms after pyrolysis at high temperature. DFT calculations and electrochemical tests revealed that the generated Mo(C)-Ni-P active sites can optimize the hydrogen adsorption free energies, and precisely activate and regulate the neighboring C atom. This unique structure indeed enhanced the intrinsic catalytic activity of Mo2C toward the HER process. Similarly, Tang and coworkers [177] developed a melamine-benzene tricarboxylic acid supramolecule network via hydrogen-bonding self-assemble method (Figures 13C, 13D). The 1D porous channel with adequate ligand, such as amido, carboxyl, and pyridine N group, enable the uniform distribution of molybdates, and consequently prevent the aggregation of metal species during the annealing treatment. Simultaneously, the in-situ released N and C atoms will dissolve into the lattice of metallic Mo, generating the N doping Mo2C embedded in porous carbon. With appropriately adjusting the N content, the resulting N-Mo2C/PC electrocatalyst exhibited low overpotential of 109 mV to achieve the current density of 10 mA cm−2 in 0.5 M H2SO4, as well as 100 mV in 1.0 M KOH. The excellent HER activity strikingly affirmed the superiority of the supramolecular confinement pyrolysis strategy.
| [Image omitted. See PDF.] | Figure 13 (A) Schematic illustration and XAS analysis of the Ni-Mo2C@NPC. (B) Electrocatalysis test. Adapted with permission from [176]. Copyright 2021, Elsevier. (C) Schematic illustration of the synthesis and structure of N-doped Mo2C/PC. (D) The DFT calculations of N-doped Mo2C/PC towards HER. Adapted with permission from [177]. Copyright 2022, Elsevier. |
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 assembly-derived 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 non-metallic 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 14 D). 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.
| [Image omitted. See PDF.] | 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. |
Apart from non-metallic elements doped carbon materials, dispersed transition metal atoms supported by carbon materials are another kind of prospective catalysts for ORR [179–181]. 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 [182–185]. Li and his colleagues [186] prepared a novel Co and N dual-doped trumpet flower-like 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 O2 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]. As described in Li et al.’s study [189] (Figures 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-N2 site, indicating that O2 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 P-doped carbon (PC) supported on P-doped graphene (PG) ((NixFe1−x)2P@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 (NixFe1−x)2P 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 (NixFe1−x)2P@PC/PG is optimized under the induction of phase transition mechanism, which effectively raises the adsorption and activation of H2O, and thus enhancing the catalytic activity of OER. Similarly, the mixed-phase CoP-Co2P@PC/PG possesses favorable activation of H2O molecules on account of the redistribution of surface electronic structure induced by PC/PG [196].
| [Image omitted. See PDF.] | Figure 15 (A) Schematic illustration of the synthesis procedure of TMP@RGO. (B) Mo, and (C) Co K-edge extended XAFS oscillation function k3[χ(k)] (inset) and their corresponding Fourier transform. Average density of electronic states of different atoms in (D) O-doped MoP, and (E) O-doped CoP. Adapted with permission from [193]. Copyright 2016, American Chemical Society. (F) Schematic illustration of the synthesis procedure of (NixFe1−x)2P@PC/PG. (G) OER polarization curves and (NixFe1−x)2P@PC/PG. Adapted with permission from [195]. Copyright 2019, Willy-VCH. |
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 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.
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 metal-organic 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.
Funding
This work was supported by the National Natural Science Foundation of China (52125202, 21908110, U2004209), the Natural Science Foundation of Jiangsu Province (BK20190479), and the Fundamental Research Funds for the Central Universities (30922010707).
Conflict of interest
The authors declare no conflict of interest.
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Fanglei Yao,#, Chenchen Fang,#, Junjie Cui,#, Liming Dai, Xiaoyuan Zhang, Liang Xue, Pan Xiong, Yongsheng Fu, Wenyao Zhang, Jingwen Sun and Junwu Zhu
Key Laboratory for Soft Chemistry and Functional Materials, Ministry of Education, Nanjing University of Science and Technology, Nanjing 210094, China
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Abstract
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, CO2 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.
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