Molten Salt Assisted Carbon Nitride Overcomes Inherent Photocatalytic Limitations: Unique Characteristics for High‐Efficiency Light‐Driven Energy Production

ABSTRACT

Under the dual challenges of global energy transition and environmental governance, the development of novel photocatalytic materials to realize the efficient conversion of solar to clean energy has become an urgent need. Despite extensive research having demonstrated the impressive photocatalysis performance of molten salt‐based carbon nitride (MSCN), the precise structure regulation mechanism of MSCN and its conformational relationship with photocatalytic performance are still crucial issues that need to be addressed. Based on this, this review first systematically analyzes the structure characteristics of MSCN, with a focus on the precise regulation mechanisms of molten salt‐assisted synthesis (MSAS). Thereafter, the intrinsic mechanism of its performance enhancement caused by the structure characteristics is discussed in depth from various aspects. Then, innovative achievements of advanced MSCN in the fields of photocatalytic energy production are summarized, which reveals the intrinsic correlation between the structure characteristics, photogenerated charge carrier behaviors, and surface reaction kinetics. Finally, the major challenges and prospects of MSCN toward real‐world implementations are outlined. Undoubtedly, this study provides insights into the potential challenges associated with rationally designing high‐performance photocatalysts to reinforce photocatalysis, which will guide the generation of ideal catalysts.

This review highlights the rational structural design of molten salt‐based carbon nitride (MSCN) for advanced photocatalytic energy conversion. By tailoring its structural characteristics, MSCN exhibits enhanced photoexcitation ability, improved charge carrier behavior, and optimized target reactions, thereby significantly boosting photocatalytic energy production. Critical assessments of current challenges and future prospects are provided to guide the future development of MSCN.

1. Introduction

Against the backdrop of the intensifying global energy and environmental crises, the development of renewable and clean energy has become an urgent necessity to replace fossil fuels [1, 2, 3]. Solar energy, as the most abundant natural energy source, has been widely utilized, while its diffuse and intermittent characteristics limit its direct utilization efficiency [4, 5]. Therefore, researchers have focused on the central role of photocatalytic systems in energy conversion and environmental governance, providing innovative solutions for building a sustainable energy system [6, 7, 8]. Currently, constructing a catalyst with high stability, selectivity and activity is the core issue in enhancing various photocatalytic reactions and plays a decisive role in photocatalytic systems [9, 10, 11, 12].

Graphitic carbon nitride (CN), which is composed of triazine and tri‐s‐triazine/heptazine rings, has attracted extensive attention due to its great visible‐light‐responsive, low cost and environmentally friendly nature [13, 14, 15]. Compared to conventional semiconductor photocatalysts, CN with appealing electronic structure and narrow bandgap can satisfy the visible‐light application requirement of photocatalysts [16]. Solid‐ and wet‐phase chemical methods are the main methods used to synthesize CN. The kinetically unstable intermediates in solid‐phase strategy and the narrow reaction window in wet‐phase method, however, strongly limit the degree of polymerization and lead to poor photocatalytic performance [17]. Therefore, it is urgently needed to develop a new strategy for synthesizing CN to overcome these problems. Based on this, the molten‐salt assisted synthesis (MSAS) method, which simultaneously combines both the solid and wet phase characteristics and endows the products with numerous advantages, has gradually attracted the attention of researchers [18].

The MSAS method synthesizes catalysts in an ionic liquid environment formed by one or more molten salts, offering advantages such as a wide reaction window, environmental friendliness, and easy product separation [19, 20, 21]. Compared to other CN‐based photocatalysts, the molten salt ‐ based CN (MSCN) exhibits better physicochemical properties, thereby simplifying complex modification steps. First, this method optimizes the kinetics and thermodynamics of the synthesis process, promoting reactant diffusion and facilitating the elimination of structural defects to produce structurally uniform products [16]. Second, salt ions with distinct electronic potential can affect the electronic structure of CN, thereby altering its physicochemical properties and catalytic activity [22]. Third, the robust polar forces within the ionic liquid phase environment significantly suppress atomic aggregation and disrupt the stability of intrinsic bonds, providing the prerequisites for the formation of new bonds within MSCN [23]. Accordingly, MSCN with superior macroscopic and microscopic structures exhibits exceptional photocatalytic performance.

The past few years have witnessed the wide application of MSAS strategies in the field of preparing MSCN to achieve efficient photocatalytic energy conversion (Figure 1). Although previous reviews have reported the application of MSAS in catalyst construction, they have mainly focused on the function of molten salts, the insight into the MSCN itself with unique characteristics has not yet received core attention [18, 24]. Meanwhile, the impact of synthesis parameters on the structure and properties of catalyst, as well as the intrinsic relationship among synthesis parameters, structure parameters, and photocatalytic mechanisms, remains to be clarified. To fill these gaps, a comprehensive overview of this subject is urgently needed. This review aims to systematically elucidate the “synthesis‐structure‐performance” relationship of materials from a multidimensional perspective. The article first explores the complex effects of actively controlling synthesis parameters on the evolution of structure features from a unique perspective. It then highlights the advanced performance enhancement of this material caused by its structure characteristics. Subsequently, the latest progress of MSCN in various photocatalysis systems is summarized, providing guidance for solving the hindrance faced within photocatalysis energy production from a reaction mechanism perspective and paving the way for the practical application of MSCN. Future prospects mainly involve developing role‐shaping technologies, monitoring the dynamic synthesis process, exploring deeper reaction mechanisms, and overcoming the practical application limitations.

FIGURE 1.

Milestone timeline of MSCN‐based photocatalytic energy production. Reproduced with permission [25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36]. Copyright 2008, 2021, 2020, 2021, 2022, 2023, 2025, Wiley‐VCH. Copyright 2011, 2014, 2017, 2019, 2024, American Chemical Society. Copyright 2018, Elsevier.

2. Rational Design of MSCN With Desired Characteristics

The MSAS technique has demonstrated significant advantages in the preparation of CN material, and the deepening of the mechanistic studies has revealed the unique properties that this method confers to MSCN [37, 38, 39]. Four types of structure modulations, from macro‐ to micro, can be achieved through the MSAS strategy, including crystal and morphology optimization, functional group/chemical bond construction, microstructure modulation and energy band engineering implementation (Figure 2). These properties together constitute the core design elements to enhance the photocatalytic activity and profoundly affect the stability and reaction kinetics of the materials. Based on the breakthrough advantages of MSCN over CN synthesized by traditional methods, this section systematically analyzes its structure‐property correlation mechanism. In particular, the influence of external and internal synthesis parameters on the micro‐ and nanostructures of the materials is explored to provide theoretical support for the oriented design of efficient catalysts for different photocatalytic systems.

FIGURE 2.

Structure characteristics within MSCN imparted by MSCN.

2.1. Crystal and Morphological Structure Control

Generally speaking, the crystalline state and morphology of the catalyst can greatly affect its catalytic activity. Optimizing the catalytic performance of catalysts can be started with the dual adjustment of external and internal structures [40, 41, 42]. For MSCN, the use of the MSAS method is expected to achieve favorable changes in the crystalline state and morphology of CN, such as desired crystallinity, optimal interlayer spacing and ideal framework.

2.1.1. Ideal Crystallinity Modulation

Crystallinity is a crucial parameter for assessing the perfection of the crystal structure of photocatalytic materials [43, 44]. Calcination parameters, especially heating rate and holding time, are the key factors to regulate the crystallinity of MSCN. Wang et al. investigated the effects of different heating rates (1°C/min, 2°C/min, 10°C/min) on changes of crystallinity in poly(triazine imide) (PTI) samples within the KCl‐LiCl molten salt system [45]. The crystallinity of PTI was found to be regulated by the heating rate and reached best at 2°C/min, as a faster heating rate (10°C/min) caused insufficient diffusion and dissolution of precursors in the molten salt, whereas a slower heating rate (1°C/min) led to the production of impurities. It was pointed out that variation in the holding time (2, 4, 6, 8, and 12 h) could affect the amount of unpolymerized amino groups, which was inversely correlated with the level of crystallinity, thereby causing corresponding changes in crystallinity [46]. The amount of the unpolymerized amino groups was first reduced and then increased along with the heating time prolonged from 2 to 12 h, allowing the best crystallinity to be achieved at 6 h.

Modulating the composition of the molten salt system is a central internal factor in optimizing material crystallinity, which guides the orderly crystal growth through domain‐limiting effects and templating [47]. By varying the type and proportion of molten salt, the formation of highly crystalline materials is expected. For example, K⁺ doped poly(heptazine) imide (KPHI) synthesized with potassium thiocyanate (KSCN) exhibited higher crystallinity than Ca²⁺ doped PHI (CaPHI) synthesized with calcium chloride (CaCl₂) [48]. This might be due to the difference in charge and radius between the metal ions (monovalent for K⁺ vs divalent for Ca²⁺; 152 pm for K⁺ vs 114 pm for Ca²⁺). K⁺ engaged in a gentle interaction with the monolayer frames, while Ca²⁺ simultaneously interacted with multiple frames. The difference in the mode of action led to the formation of different layered structures, thereby altering the confined space in which the material formed, which in turn affected the crystallinity of the materials. The obtained PHI/Na_xLi_y with different levels of crystallinity through changing the NaCl/LiCl ratio (x/y = 1/10, 0/10, 1/9, 1/3, 7/3, and 9/1) further confirmed the role of the molten salt system in the formation of high‐crystallinity products [49]. It was found that the function of verified molten salt proportion lay in the different crystal growth modes. The single LiCl salt lacked nucleation sites and clear interfaces, while an excessive Na salt content (x/y > 1/9) disrupted the planar structure and changed the stacking sequence, which was not effective in inducing rapid growth of crystalline materials, thus making the PHI/Na₁Li₉ exhibit the best crystallinity.

The essence of crystallinity regulation lies in the synergistic mechanism between exogenous calcination parameters and endogenous molten salt system. It has been shown that increasing the calcination temperature can significantly enhance the ion diffusion kinetics of the molten salt and optimize the crystallinity by promoting lattice reconstruction, but the threshold of this temperature effect is not yet clear [50]. Currently, the focus of research is shifting to analyzing the coupling mechanism of internal and external parameters, such as exploring the dynamic matching relationship between the molten salt components and thermodynamic conditions in a specific temperature interval. This will provide theoretical support for the construction of a precise model for the regulation of crystallinity.

2.1.2. Golden Zone of Interlayer Spacing Exploration

The crux of the interlayer spacing control strategy lies in balancing the interlayer forces with the reaction space requirements, in which MSAS technology realizes the precise control of the interlayer structure through copolymerization and doping effects [51]. The copolymerization effect of the MSAS refers to “cooking” suitable raw material in the liquid environment provided by the molten salt, thus enhancing chemical reaction and promoting the catalyst formation [52]. Following this property, Wang et al. successfully compressed the interlayer spacing of tri‐s‐triazine‐based CN from 3.26 to 3.2 Å compared to pristine CN without MSAS [53]. This reduction in layer spacing originated from the enhancement of copolymerization by the molten salt system, which accelerated the deamination process of melamine, and the optimized synthesis parameters could further enhance the modulation effect. The group of Antonietti introduced oxalamide additives in combination with molten salt post‐processing to induce stronger van der Waals interactions and covalent cross‐linking between the heptazine layers, resulting in an ultra‐dense stacked structure with a spacing as low as 2.92 Å [52]. This study revealed the physicochemical mechanism by which component design and reaction kinetics synergize to regulate the interlayer architecture in the molten salt system.

Doping, as another effective method, has the core principle of changing the layer spacing by embedding external atoms into the material layers or letting them replace the C/N atoms in the original CN units. In the process of MSAS, once the molten salt ions entered the interlayer, the adjustment of layer spacing could be realized through the enhancement of interlayer interaction and size effect. Zhu et al. successfully achieved the regulation of the interlayer spacing of indium (In) doped‐CN (ICN‐x) by embedding varying amounts of In atoms between the layers through altering the dosage of molten salt [51]. The regulation of the interlayer spacing (from 3.2546 to 3.2185 Å) was attributed to the gradual increasing ratio of In atoms (from 0 wt.% to 0.326 wt.%), which led to the enhancement of interlayer electrostatic attraction (Figure 3a).

FIGURE 3.

The regulation role of MSAS in crystal and morphological structure control. (a) Decreased d(002) along with the increased In atoms. Reproduced with permission [51]. Copyright 2022, Wiley‐VCH. (b) Synthesis process of PTI‐M catalyst within different molten salt systems (LiCl/KCl/NaCl). (c) Gas evolution rate of PTI/Li⁺Cl⁻ with diverse molten salt ratios. Reproduced with permission [58]. Copyright 2023, Wiley‐VCH.

It should be noted that the regulation of the interlayer spacing is also closely associated with the size effect of molten salt atoms, and the varying size effect among different atoms may result in diverse interlayer spacing. V. Lotsch et al. achieved active control of the interlayer spacing of M‐PHI (M = Li, Na, K) through regulating the type of salt system [34]. The increase in interlayer distance from K‐PHI (3.15 ± 0.01 Å) to Na‐PHI (3.17 ± 0.01 Å) to Li‐PHI (3.18 ± 0.02 Å) was attributed to the changes in interlayer stacking mode and pore channels caused by differences in interatomic geometric radius and hydration ability. Specifically, the higher hydration ability of Li⁺ (two hydrated shells) than that of K⁺ and Na⁺ (one hydrated shell) weakened interlayer interactions, shortened interlayer slipping distances, and enlarged pore channels, which resulted in larger interlayer spacing in Li‐PHI than Na‐PHI and K‐PHI. Whereas, the difference in interlayer spacing between Na‐PHI and K‐PHI was mainly due to the slightly larger atomic radius of Na (3.6 Å) than K (3.3 Å).

2.1.3. Favorable Framework Construction

Constructing an ideal framework, which aims to break the reaction inertia of original CN, can not only be used to tailor MSCN but is also a powerful means of ensuring that the photocatalytic reaction achieves superior performance [54, 55]. The framework adjustment of CN endowed by MSAS can be divided into two parts: the apparent structure and the internal unit structure.

The regulation of the apparent morphology and structure to find the well‐matched golden zone between reactive area and excellent photocatalytic performance can be achieved by promoting the adjustment of synthesis factors [56]. With the addition of NH₄Cl to the raw material, the group of Zhao successfully decomposed a large amount of C₃N₄ into smaller fragments (CN‐NH₄) and created a CN materials with multiple order‐disorder interfaces (CN‐NH₄‐NaK) [57]. This order‐disorder interface connection facilitated the formation of more spacious and orderly separated redox centers, exhibiting excellent photocatalytic capacity. Considering the effects of various salts on the formation of the ideal framework, Wang et al. further revealed the synergistic effects among molten salts on the formation of ideal framework through synthesizing PTI/Li⁺Cl⁻ within the binary/ternary molten salt systems (LiCl‐KCl‐NaCl) (Figure 3b) [58]. Different from the binary system, in which the PTI/Li⁺Cl⁻ tended to grow along the c‐axis to form hexagonal prism frameworks. The ternary system with a lower LiCl content was more conducive to reducing the initial carbon and nitrogen polymerization of PTI/Li⁺Cl⁻ nuclei, thus facilitating the extension of PTI/Li⁺Cl⁻ along the 2D plane to form a hexagonal sheet framework. This two‐dimensional π‐conjugated system provided an additional way for charge transfer and improved the reaction efficiency. Therefore, the best golden balance in the dynamic crystal‐growth polymerization process of PTI/Li⁺Cl⁻ was obtained when the mass ratio of LiCl in the salt system reached 0.1 (Figure 3c).

In light of the slight powerlessness of the modified apparent structure to fight against the obstacles within the body of MSCN, a special revolution of the internal framework needs to be carried out to obtain MSCN with satisfactory photocatalytic performance. For instance, a successful internal structural transformation from a fully ionic structure (AA′ structure) to a AA′ stacking with open channels structure and non‐ionic structure (AB structure) was achieved through employing hydrochloric acid (HCl) to deplete the molten salt ions that were maintained in the PTI structure [59]. Thanks to the HCl treatment, the molten salt ions that occupied the pores and were not conducive to the transport of reaction molecules were significantly exchanged with H⁺, thus promoting the formation of open channels. These open channels greatly facilitated the rapid transport of water molecules and protons in PTI, which contributed to the subsequent reactions.

2.2. Beneficial Functional Groups/Bonds Formation

The core of introducing beneficial functional groups/bonds is to set up the link associated with the molecular‐level structure and the excellent reaction performance [60, 61]. Through conversion of the original structure or brought by reaction substances, beneficial groups/bonds can be successfully introduced into MSCN.

In the realm of introduced foreign functional groups, the cyanide group (‐C≡N) and the hydroxyl group (‐OH) have emerged as the two most common groups [29, 62]. To investigate the effect of different molten salt systems on the introduction of favorable functional groups, Zhang et al. successfully realized the conversion of amine group (‐NH₂) to ‐C≡N by reacting KOH with the ‐NH₂ on the triazine ring in the feedstock [63]. The ‐C≡N significantly enhanced the adsorption of the reactants, thus facilitating the photocatalytic reactions. Indeed, some molten salts can act as reaction terminators, which leads to the formation of beneficial groups. A suitable case was terminating polymerization PCN (TP‐PCN), which was successfully prepared by using NaI molten salt as the terminator of the polymerization reaction [64]. The introduced iodine did not destroy the internal structure of PCN but terminated the polymerization by destroying the strong π–π interaction in bulk PCN, and then formed a fragment structure, thus exposing abundant edge active sites (AEASs) (‐C≡N and ‐OH), which indirectly enhanced the activity of TP‐PCN.

By the way, the selection of specific raw materials to introduce other favorable groups is also an excellent choice to achieve good catalytic performance. Zhang et al. successfully synthesized Na, K co‐doped PHI (NaK‐PHI‐A) within NaCl/KCl system through using electron‐deficient monomers 5‐amino‐tetrazole and adenine as raw materials [65]. The imide group formed a local donor‐acceptor (D‐A) structure with the nucleoside base, accelerated charge transfer while increasing the surface area, thus giving the entire reaction chain better photochemical properties.

For the foreign chemical bonds, the salt components play a crucial role in their formation. Yang et al. used KCl and KOH molten salt systems to promote the formation of plane‐rich CNs through the “stitching” effect of KCl and the “cutting” effect of KOH [66]. In that process, OH⁻ reacted with C‐NH_X to form an imine bond, which terminated polymerization of heptazine and weakened the van der Waals forces between the chains. This mechanism exposed numerous CNs planes, made the CNs have sufficient active sites and reduced the resistance of electron transfer between layers, which was conducive to the catalytic reaction.

Considering the crucial role of various salts in the formation of beneficial groups/bonds, it is conceivable that the synergistic effect between salts may also exert force for the formation of beneficial groups/bonds. Inspired by this, Wang et al. compared binary/ternary salt systems (KCl‐LiCl‐KSCN) to explore the link between the specific effects of salt synergies and the formation of favorable chemical bonds [67]. The introduction of KSCN promoted the formation of K‐N‐C structures compared with that in the binary salt systems (KCl‐LiCl). It was worth noting that the formation of K‐N‐C structure not only reduced the exciton binding energy but also provided sufficient protons for H₂ precipitation by forming interfacial K•H₂O clusters.

2.3. Microstructure Control

2.3.1. Sites Regulation

Lewis acid‐base sites/pairs are generally formed in MSCN with the help of molten salt atoms, either by self‐doping or by interacting with the unit structure [68, 69, 70]. Long et al. synthesized K‐doped triazine or tri‐triazine CN (KTTCN) and used it for photocatalytic H₂O₂ production [71]. The doped K⁺ finely regulated the reaction process: K⁺ (Lewis acid site) and N₂ (Lewis base site) (Figure 4a) synergistically promoted isopropanol (IPA) adsorption and deprotonation, so that KTTCN could efficiently generate H₂O₂ even at a low concentration of IPA (only 0.5 vol%). Indeed, frustrated Lewis pairs (FLPs) composed of Lewis acid and base sites are more attractive than Lewis acid or base sites alone, since the push‐pull effect of FLPs, in which electrons are transferred between FLPs and reaction molecules, can significantly improve molecular activation and reaction kinetics [72]. Inspired by this, Xiong et al. took advantage of the molecules evaporated from NH₄Cl to prepare PHI nanosheets with hydrogen bond‐modified FLPs (HM‐FLPs) (Figure 4b,c) [73]. Experimental and theoretical results confirmed the existence of unquenching Lewis acid‐base sites and the hydrogen bond between N and H atoms within PHI. The hydrogen bond acted as a steric hindrance to create suitable spatial spacing between Lewis acid and base sites, which was further conducive to the push‐pull effect of FLPs. As a result, the HM‐FLPs with enhanced push‐pull effect played an advanced role in reducing the adsorption energy of reaction molecules, contributing to the capture, bending and polarization of reaction molecules.

FIGURE 4.

The Microstructure control role of MSAS in MSCN. (a) Charge density and structural model of KTTCN. Reproduced with permission [71]. Copyright 2020, Elsevier. (b) Structure model and electron density iso‐surface of PHI‐FLP. (c) Illustration of HM‐FLPs in PHI. Reproduced with permission [73]. Copyright 2024, Wiley‐VCH. (d) The formation of carbon‐rich phase within PTI‐LiBr under different calcination temperatures and times. Reproduced with permission [83]. Copyright 2024, Wiley‐VCH. (e) The structure model of PTI/Li⁺Cl⁻ without defects (left) and with defects (right). Reproduced with permission [77]. Copyright 2021, Wiley‐VCH.

Active substances, which help improve the catalytic activity of catalysts, can also be easily embedded into the CN unit with the assistance of MSAS [74, 75]. Xiang et al. successfully prepared a copper single atoms (Cu SAs)/crystal CN photocatalyst (Cu‐CCN) by adding copper dichloride (CuCl₂) to the synthetic raw material [76]. The results showed that with the help of KCl‐LiCl molten salt system, the Cu SAs were stably and uniformly anchored in the structure of CCN. The introduced Cu SAs acted as adsorption sites, which endowed the Cu‐CCN with strengthened capacity to absorb reaction molecules, thus giving the Cu‐CCN highly selective and efficient photocatalytic.

2.3.2. Defect Control

Construction defects, as a strategy to adjust the structure of CN without the introduction of foreign atoms, have been recognized as an effective tool to overcome the barriers that impede the performance of catalysts [77, 78]. For example, the CN synthesized within an alkali metal molten‐salt system formed more nitrogen vacancies (N_v) compared to bulk CN, and the formation of N_v inhibited the charge recombination and significantly improved the catalytic performance [79]. Notably, the controllability of defect engineering by adjusting the synthesis parameters has been successfully confirmed [80]. Mi et al. revealed that the defects concentration in the lithium ion embedded crystalline CN (LCCN) could be controlled by the molten salt ratio (LiBr/LiCl = 10/0, 7/3, 5/5, 3/7, 2/8, 1/9, and 0/10) [22]. It was pointed out that the defect level increased with the decreasing LiBr/LiCl ratio, which might be due to the higher point temperature of LiCl than that of LiBr weakened the mass transfer process during thermal polymerization, thus contributing to defect level control.

The defect repair strategy can significantly inhibit the non‐radiative polymerization caused by low energy defect level. Since most photocatalysts inevitably introduce structural defects during the synthesis process, this strategy can be universally applied to improve catalytic activity [81]. For example, Li et al. successfully synthesized U₂‐PHI with low defect content by using NH₃ produced from urea decomposition [82]. The NH₃ generated locally in the closed system provided a high partial pressure, which made the conversion of melamine to melon maintained a balance in a certain period of time. Consequently, the degree of polymerization of melon was increased, and the in‐surface N_v caused by high temperature oxidation was effectively filled, thus significantly improving the reaction transfer kinetics.

It is worth noting that the carbon‐rich phase within MSCN, which is often overlooked or considered to contribute to the reaction, actually produces low‐energy defect states that disrupt charge transfer pathways and absorb photoluminescence. Therefore, an effective strategy for promoting the repair of defects is located in considering how to curb the excessive formation of the carbon‐rich phase. Bojdys et al. explored the changes of the carbon‐rich phase of PTI‐LiBr by finely regulating the polymerization temperature and reaction time during molten‐salts treatment [83]. It was found that the color of the materials synthesized under harsh reaction (600°C, 12 h) conditions was brown, indicating the introduction of carbon pollutants. The content of carbon pollutants would further increase when the calcination temperature was prolonged to 72 h, as reflected by the black color of PTI. In contrast, under relatively mild conditions (550°C, 48 h), almost colorless products were obtained, and the C: N ratio was closest to the theoretical value of PTI, indicating that the product had the least defects (Figure 4d).

As for internal parameters, successful defect repair is mainly achieved through the function of the molten salt system. Mi et al. employed LiCl‐NaCl/KCl molten salt systems for defect repair to activate the near‐infrared activity of red PCN (RCN) [84]. The defect repair process was achieved through passivation of the valence electron pair of Na⁺ or K⁺ and substantial elimination of structural defects such as carbon vacancies (C_v) and N_v, which led to improved activity of RCN. Interestingly, the proximity between the melting point and polymerization temperature, as another factor, can enable the structure repair strategy through adjusting the polymerization process. A case in point was PTI/Li⁺Cl⁻ crystals synthesized within different molten salt systems (LiCl/MCl, M = Na, K, Rb, Cs) at a certain polymerization temperature (550°C) [77]. The closest match between melting point and polymerization temperature was achieved in LiCl/NaCl, which provided mild reaction conditions and inhibited depolymerization of the triazine unit, thus producing PTI/Li⁺Cl⁻ crystals with fewer structural defects (Figure 4e).

It is worth noting that the structure characteristics discussed above are not isolated but often coexist and operate synergistically or present specific trade‐offs that define the final material performance. For instance, the ideal framework construction may coexist with the interlayer space regulation, as both rely on the molten salt environment and salt ions. Interestingly, the salt ions contained in the interlayer spacing act as chemical anchors to fine‐tune atomic stacking through size effect or ionic interaction forces, while serving as template and structure direct agent to guide structure variation in the framework construction. Moreover, the high crystallinity acts as a highway to ensure rapid electron delivery, which provides a stable matrix for the growth of specific defects or functional groups. However, the degree of defect concentration needs to be further considered, as excessive defects will disrupt the long‐range order. The key to maximizing material potential lies not in isolated manipulation but in the comprehensive integration of these interdependent structural characteristics.

2.4. Electronic and Band Structure Control

Studies have shown that the use of alkali metal salt systems (e.g., KCl) can induce the electronic rearrangement within the carbon and nitrogen skeleton and effectively enhance the conjugated structure properties of the materials [85, 86, 87]. This process not only reduces the surface electrostatic potential of the material but also narrows the bandgap by reconfiguring the energy band structure, thus giving the catalyst good photocatalytic performance [88]. However, the study failed to reveal the core mechanism of the alkaline molten salt system in the electronic regulation. Unlike conventional non‐metal doping (e.g., B, P, and S), which primarily modulate electronic structures through band bending, electronic structure modulated by alkaline ions incorporated via MSAS leverages a dynamic ionic environment [89, 90]. The uniform liquid‐phase nucleation allows for rapid ion mobility, thereby accelerating reaction kinetics and lowering growth barriers. More importantly, the incorporated salt ions can actively modulate crystal plane features, thereby enabling precise electronic structure regulation. The strong polarity forces generated by the ionized cations and anions within MSCN also provide an additional driving force for electron release and transfer, which further facilitates the generation of built‐in electric field (BIEF). Numerous recent studies have confirmed the effectiveness of this mechanism, which collectively demonstrates the capability of the MSAS in modulating electronic architectures. For example, the core‐shell S‐scheme heterojunctions formed in the NaCl system triggered interfacial charge rearrangement, thereby resulting in the formation of a BIEF that optimized the charge transfer pathways by utilizing the synergistic effects of the alkali metal ions and the unique heterojunction microstructure [91]. Based on this, the research team further constructed a composite molten salt system (KCl‐MCl₂, M = Mg^{2 +}, Ca^{2 +}, Sr^{2 +}, Ba^{2 +}) to carry out the mechanism exploration [92]. They found that doping of alkali metal ions prompted the conversion of extra valence electrons into free electrons, and this significant increase in electron density not only activated the n → π* electron‐leaping process. What was more important, the controllable regulation of the bandgap width was realized by filling the material conduction band (CB) through electron injection of K⁺, which favored the intrinsic reaction activity of M, K‐doped CN (MKCN). This finding not only revealed the key role of alkali metal ions in energy band engineering but also corrected the theoretical limitation of the traditional knowledge about the weak influence of alkaline‐earth metal ions on the energy band structure.

The electronic rearrangement optimizes the electronic structure, creating a potential difference that is a prerequisite for forming an internal electric field [93]. Wang et al. pointed out that the semi‐coherent interface with an electrostatic potential difference in PHI/PTI allowed the generation of a BIEF [94]. This generated BIEF facilitated the electron transport, which significantly enhanced the catalytic activity of PHI/PTI. Notably, the intensity of BIEF can be effectively controlled through the variation in the doping gradient of molten salt. Zhang et al. clarified the consistency between K⁺ ion gradient and BIEF by controlling the diffusion of K⁺ from the surface to the body within K⁺ implanted CCN (KCCN) [95]. The variation of the K⁺ concentration gradient both guided the generation and the intensity of the BIEF. On the one hand, the presence of a K⁺ concentration gradient caused a potential difference, which induced the generation of the BIEF. On the other hand, the increased diffusion time (from 2 to 4 h) caused a relatively uniform concentration of K⁺ on the surface and bulk, thus reducing K⁺ gradient and weakening the strength of the BIEF. Asymmetric elemental doping is another effective method for regulating the intensity of the BIEF. Zhu et al. confirmed that the constructed target catalysts with an asymmetric framework exhibited a higher crystal dipole moment compared with bulk catalysts. This enhanced dipole moment significantly boosted the intensity of BIEF, thus significantly promoting directional carrier migration [96].

The bottlenecks in regulating the electronic and band structure lies in achieving effective electron reconstruction. Upon incorporating the salt ions, the primary mechanism by which these ions regulate the electronic and band structure derives from their ability to vary the electron distribution and activation. The variation of raw materials also induces uneven electron distribution, just as the addition of methyl violet (MV) constructs an external electric field (EEF) for fostering the transmission of electrons [22]. Undoubtedly, the MSAS with controllable synthesis parameters presents an alternative approach to constructing an effective MSCN with the desired electronic and band structure.

Overall, the formation of unique structure characteristics of MSCN is essentially highly related to the synthesis parameters (external and internal). The external parameters focus on improving the copolymerization process, thereby enabling the formation of MSCN with high crystallinity and low impurity content. As for the internal parameters, the variation of raw materials can exert a driving force on the introduction of active centers, which in turn affects the directional synthesis process. Molten salt system, as another vital internal parameter, can tailor the polymerization through tuning the reaction process or affecting the properties of bulk CN. Figure 5 summarizes the exact effects of the synthetic parameters within the MSAS process on the formation of the four unique structure characteristics of MSCN, aiming to provide clear and general guidance for the rational design and synthesis of MSCN. The enhancement of the unique properties of CN by the above‐mentioned unique structures may further clarify the structure‐property correlation mechanism, which will be discussed in depth later.

FIGURE 5.

The key function mechanism and relationship regarding the structure characteristic formation and synthesis parameters variation within MSCN.

3. Unique Advantages of MSCN in Photocatalytic

The performance optimization of photocatalytic materials needs to be based on an in‐depth understanding of the constitutive relationship of the catalyst, which is of great significance for guiding the directed design of efficient catalytic systems. The performance of an ideal photocatalyst is affected by the synergistic effect of multiple key parameters, including material stability, light absorption ability, charge carrier separation efficiency and surface reaction kinetics [97, 98]. Based on the previous systematic analysis of the synthesis factors and structural properties of MSCN, the unique performance advantages conferred on this material by the MSAS modification strategy have been deeply revealed. This chapter focuses on the “collision sparks” between the structural properties of MSCN and the current well‐established photocatalytic mechanisms. The research in this chapter will establish a systematic model of the constitutive relationship between the intrinsic properties of MSCN and practical application requirements, which will provide a theoretical framework for the design and regulation of subsequent photocatalytic system.

3.1. Photocatalytic Stability Enhancement

Enhancing the stability of CN photocatalytic materials is the core challenge for large‐scale applications, and MSAS technology provides an innovative solution to this challenge through a multi‐dimensional regulatory mechanism. For example, the cyamelurate groups formed during the MSAS process modulated the hydrophilicity of CN, thus endowing K and Na co‐doped PHI (K, Na‐PHI) with strengthened stability [99]. The cyamelurate groups were formed by replacing the ‐NH_x group with cyamelurate moieties, and these negatively charged groups preferentially interacted with the water molecules of the Na⁺ compact hydration shell to form a hydration layer on the surface of the material. The existence hydration layer excluded the hydrogen bond between heptazine units, which prevented the occurrence of solidification and precipitation. Interestingly, the dissolved K, Na‐PHI could be separated from the reaction medium without affecting on its size (100 ± 30 Å) and any noticeable loss in activity and selectivity (>80%).

Beyond the introduction of beneficial groups, the bonds formed within the structure can also support the realization of high stability of MSCN in a new light. BCN nanostructures (BCNO) with B‐O bonds synthesized within LiCl/KCl achieved stable dispersion over wide pH adaptation [26]. This was mainly due to the negative state on the surface of the BCNO caused by B‐O bonds, which contributed to preventing the aggregation of BCNO particles, thus endowing the BCNO with strong stability over a wide pH range (pH ≥ 4). This kind of interfacial self‐regulation without additional modification enabled BCNO to maintain a high dispersion state in an aqueous environment for a long period, providing a new paradigm for designing the structural stability of photocatalytic materials under complex working conditions.

Considering the challenge that the presence of numerous cations in the reaction solution would impair the stability of the catalyst, Zhang et al. constructed a lattice oxygen modified CN (CCNO) with a unique three‐dimensional configuration using MSAS [100]. The introduced lattice oxygen and N_v during the MSAS process destroyed the in‐plane symmetry of CCNO and acted as strong FLPs, thus markedly enhancing the adsorption barriers of CCNO for the abundant ions Na⁺, K⁺, Mg²⁺, and Ca²⁺ in seawater compared with bulk CN. The enhanced adsorption barriers of bulk cations reduced the competitive adsorption between these ions and H₃O⁺ in seawater, thus greatly enhancing resistance to ionic interference. This construction of the distinctive molecule configuration of CCNO demonstrated strong resistance to bulk cation contained within reaction medium, highlighting the exceptional applicability and stability of the unique characteristics in utilizing the sunlight.

It should be noted that the molten salt ions contained in MSCN will gradually be lost along with the reaction process, which is not conducive to the subsequent catalytic. Fortunately, Hou et al. verified the repair of the photochemical instability of PHI‐Na through the common ion effects [101]. The common ion effects was induced through the addition of NaCl, which significantly reduced the loss of Na⁺ in PHI‐Na and stabilized the band and electronic structure of PHI. The stabilized band and electronic structure inhibited the decrease in the delocalized electron density in heptazine units along with the loss of Na⁺, thus endowing the PHI‐Na with NaCl a strengthened stability compared to the PHI‐Na system without NaCl.

3.2. Photoexcitation Ability Reinforcement

Broadening the photo‐responsive range of photocatalytic materials is the basis for achieving efficient photoexcitation, which is realized by increasing the absorption range and efficiency of optical radiation [65, 102]. At present, one of the main research hotspots for improving the light absorption range is the development of MSCN with an excellent electronic structure. Taking the K⁺‐containing crystalline PCN (CPCN) synthesized by the KCl molten salt system as an example, the embedding of K⁺ induced an inhomogeneous distribution of the electrons in CPCN, although it was not directly involved in the formation of the electronic states [47]. This inhomogeneous distribution caused CPCN to exhibit a broad and sharp light absorption threshold at 460 nm, which was significantly shifted compared with that of PCN.

The absorption properties of semiconductor materials are determined by their bandgap in some extent, allowing for easily manipulating the light absorption properties by adjusting the bandgap of CN. Chu et al. produced Na and B co‐doped CN (NaB‐CN) and found the modified material exhibited significantly enhanced light absorption intensity in the full wavelength range of 200‐800 nm, accompanied by a pronounced redshift of the absorption edge [92]. This could be attributed to the introduction of Na and B, which significantly reduced bandgap and made electrons become more excited in the valence band (VB), thus improving the absorption range of visible light.

As for the efficiency of optical radiation, which refers to enhancing the total amount of radiant light used in the light reaction, MSCN with desired framework provides an opportunity to significantly improve it. A suitable case was the P and K co‐doped CN (PKCN) with a three‐dimensional flower‐like structure [103]. The three‐dimensional flower shape structure expanded the specific surface area of PKCN and exposed more active sites, thus showing the excellent light absorption performance. By the way, the modified morphological structure not only exerts force to form more active sites, but it also reduces the mass transfer resistance and creates a tight contact interface, all of which are conducive to enhancing light harvest efficiency [104].

Beneficial foreign functional groups/bonds with multiple functions can also provide a feasible way to enhance light absorption efficiency. Huang et al. revealed the crucial mechanism of ‐C≡N and P─N bonds in enhancing the light absorption [105]. The grafted ‐C≡N blocked the in‐plane growth of heptazine ring structural units, cleaved CN into small nanosheets and exposed more active sites, thereby creating more chances to absorb the light. Meanwhile, the P─N bond, combined with ‐C≡N, increased the light absorption level through the rearrangement of photogenerated electrons and holes.

Defect levels, which act as electron trap sites, can effectively improve the light trapping ability. The crucial role of N_v in g‐C₃N_x‐M confirmed the improved light absorption efficiency mechanism of defect levels [106]. It was found that the absorption edge of g‐C₃N_x‐M was redshifted by the introduction of N_v, and the redshift increased gradually aligned with the increase of defects. The defect level generated in the bandgap had a very low reduction driving force and could capture part of photogenerated electrons, which further improved the light absorption capability of g‐C₃N_x‐M. However, the relationship between the number of defect levels and the enhancement of light trapping capacity needs to be viewed with caution, as the presence of excessive defect levels may lead to a reduction in the crystallinity of MSCN, thereby indirectly reducing its light‐trapping capability [84, 106].

3.3. Charge Carrier Separation and Transportation Improvement

The effective utilization of charge depends on the efficient separation and transport of charge carrier [33, 107, 108]. To address the problem of severe carrier complexity in CN materials due to intrinsic structural defects, the charge separation efficiency can be significantly improved by modulating the material morphology and structure [3]. For example, in K‐doped PCN (KPCN), the denser and more ordered interlayer structure induced by embedded K⁺ significantly reduced structure defects and improved the crystallinity [109]. This kind of structural optimization endowed KPCN with the capacity to highly promote the separation of electron‐hole pairs (e⁻–h⁺). Meanwhile, reconfiguring the intrinsic framework of the material also provides a new way to break through the charge complex limitation [110]. Tian et al. crushed the CN symmetric framework with the assistance of an N‐rich triazole group and constructed a molten‐salt based asymmetric CN (MACN), revealing the crucial role of reconstructing the original framework in inhibiting charge carrier recombination [111]. The formed asymmetric framework enhanced the molecular dipole moment (from 1.5 D to 10.8 D) and reduced the surface potential, which provided a strong driving force to improve the separation and the directional transfer of charge carrier, resulting in MACN with high separation efficiency (Figure 6a,b). The S‐scheme heterojunction structure constructed by the group of Jin further supports the key role of an ideal framework in optimizing charge carrier behavior [112]. The results showed that the formation of the S‐scheme heterojunction inhibited the charge carrier recombination and facilitated the directed migration of charge carrier, achieving the high transfer efficiency of charge carrier.

FIGURE 6.

The charge carrier regulation role of structure characteristics in MSCN. (a) Different surface potential between BCN, ACN, and MACN. (b) ESP and dipole moment comparison of BCN, ACN, and MACN. Reproduced with permission [111]. Copyright 2025, the Royal Society of Chemistry. (c) Surface potential and charge density of SCN and SCN_V. Reproduced with permission [113]. Copyright 2025, Elsevier. (d) Structural merits differences between bulk PHI and highly ordered crystalline PHI. Reproduced with permission [35]. Copyright 2024, American Chemical Society.

In fact, beneficial groups and defects are believed to inhibit the recombination of charge carrier through trapping or activating the generated electrons, which indirectly promotes the charge carrier separation. By introducing N_v and ‐C≡N into sulfur self‐doped CN (SCN_V), the charge separation could be effectively realized [113]. On the one hand, N_v promoted n → π* electron transition. On the other hand, ‐C≡N trapped photo‐excitons and promoted the directional transfer of photogenerated electrons, all of which were conducive to charge carrier separation. The overlap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) was also significantly reduced in SCN_V compared with SCN, which further confirmed the high charge carrier separation efficiency of N_v and ‐C≡N (Figure 6c).

After achieving efficient charge‐carrier separation, enhancing directional migration becomes a key challenge. The construction of an ordered structure within PHI could improve crystallinity and optimize charge storage kinetics, thus showing excellent charge carrier transmission performance [35]. The analysis of PHI showed that the increased crystallinity reduced the electrons energy barrier, increased the active sites for electrons and generated a transfer channel at the heterogeneous interface between carbon and PHI, which facilitated the transport of charge carrier (Figure 6d).

It is noteworthy that the generated BIEF can act as a diode, which significantly reduces transmission barriers, thus promoting the directional transmission of charge carrier [22, 114]. Xiang et al. revealed the crucial role of BIEF on the charge carrier transmission within a highly crystalline CN (HCCN) with an internal ternary homogeneous junction [115]. The analysis of HCCN showed that highly enhanced charge transfer was attributed to the generated BIEF: (i) the BIEF promoted the directed migration of electrons and reduced the charge transmission barriers; And (ii) the electron accumulation within the molecular orbital/unoccupied orbital energy levels was facilitated by BIEF, which significantly facilitated the charge transmission kinetics in HCCN.

The MSCN materials constructed through the multiscale structure modulation strategy show significant advantages in photocatalytic performance and at reaction mechanism level. The material stability, as the basis for practical applications, can be synergistically enhanced by introducing functional exogenous groups, constructing moderate defects, and optimizing structural integrity. In terms of light trapping capability, electronic structure modulation can effectively broaden the spectral response range, while morphology optimization can enhance energy utilization by enhancing the light scattering effect. For the charge carrier separation and transport challenges, the directional driving effect of the BIEF and the defect modulation mechanism of the strong interactions jointly construct an efficient transport channel. By systematically comparing the effects of different modification processes on the stability, photoexcitation efficiency, and carrier mobility of the materials, the research data fully confirm the comprehensive performance advantages of the MSCN system over traditional materials in the photocatalytic field (Figure 7).

FIGURE 7.

The enhanced photocatalytic properties and unique advantages of MSCN. (a) The closed loop of “structure characteristics‐functions‐properties”; (b) Unique catalysis properties advantages of MSCN compared to bulk CN.

4. The Photocatalytic Applications of MSCN

MSCN materials prepared by multiscale structure modulation strategy have shown promising applications in photocatalysis. The unique structural advantages of MSCN not only enhance the light trapping ability and carrier separation efficiency, but also generate a deep synergistic effect with the intrinsic optical properties of carbon nitride, which shows good adaptability. For the third stage of the photocatalytic reaction, it is necessary to analyze the structure‐activity relationship of MSCN with the kinetic characteristics of the specific reaction. In this paper, the representative results of MSCN in various photocatalytic systems are systematically sorted out to further focus on the activation of the redox reaction pathway by its surface interface regulatory mechanism. By revealing the dynamic correlation between the distribution of functional sites, the evolution of electronic states, and the adsorption of reactive intermediates, the paper focuses on analyzing the intrinsic mechanism of its performance enhancement, which provides theoretical support and practical guidance for the subsequent development of high‐performance photocatalytic systems. Next, it will be shown how the comprehensive performance advantages imparted by MSAS, as elaborated in Chapter 3, collectively address the specific challenges of different photocatalytic energy reactions (such as H₂O₂ selectivity, HER kinetics, CO₂RR adsorption/activation).

4.1. H₂O₂ Evolution Reactions

As an environmentally friendly oxidizer, H₂O₂ is touted as one of the most promising emerging clean energy sources. In the photocatalytic production of H₂O₂, two primary reaction pathways are involved: oxygen reduction reaction (ORR) (Equations ((1), (2), (3), (4))) and water oxidation reaction (WOR) (Equations ((5), (6), (7), (8), (9))) [116]. The main challenges of H₂O₂ production currently faced are twofold [117]. (i) the insufficient reaction molecules utilization due to the low redox potential makes the competitive reaction exist in both ORR and WOR; And (ii) the dependence of sacrificial agent (SA) to reduce high reaction barriers and strengthen reaction kinetics. Reassuringly, the MSCN with unique characteristics provides a new pathway to overcome these obstacles and meet the different requirements of H₂O₂ redox reactions. To more intuitively illustrate the advantages of MSCN in H₂O₂ production applications, some recently reported representative catalysts for H₂O₂ production are summarized (Table 1).

TABLE 1.

Summary of some recently H₂O₂ evolution reaction catalysts.

Methods	Catalysts	H2O2 yield	Molten‐salt systems	Key structural parameters	Experiment parameters	Photocatalytic efficiency a	Reactive species	Refs.
MSAS	Ox‐KPHI	16.9 mM h−1	KCl/LiCl	‐C≡N, Nv and n→π∗ electronic transition	600°C, 4 h	AQY = 41%	OOH∗	[118]
	CN‐I/I3	62.52 mM g−1 h−1	KI	High crystallinity Electronic structure	80°C, 4 h	AQY = 34.61%	•O2 −	[119]
	g‐C3N5‐K, K	151.14 µM h−1	KCl/KOH	‐C≡N and ‐OH groups	550°C, 4 h	—	•O2 −	[120]
	UiO/IKCN	72.6 mM g−1 h−1	KI/KCl	Interlayer spacing Electronic structure	550°C,4 h	AQY = 10.28%	•O2 −	[121]
Thermally treated	CoOx‐NvCN	244.8 µM L−1 h−1	—	Nv and CoOx clusters	550°C, 3 h	AQY = 5.73%	OOH∗	[123]
	C3N4‐nv	75.66 µM g−1 h−1	—	Nv	520‐650°C, 2‐10 h	AQE = 3.85%	1,4 ‐ endoperoxide	[124]
Deposition	HCP	116.25 µM g−1 h−1	—	Hollow structure, PDA overlayer	550°C, 4 h	AQY = 3.60%	•O2 −, •OH	[125]
Solvothermal	Ph‐MCN	195.4 µM h−1	—	Fragmented D‐A polymers	450°C, 4 h	—	•O2 −, •OH	[126]

^a Apparent Quantum Yield (AQY) or apparent quantum efficiency (AQE).

ORR:

$${\mathrm{O}}_2+{\mathrm{e}}^{-}\to ·{\mathrm{O}}_2^{-}\left(-0.33\mathrm{V}\mathrm{vs}.\mathrm{NHE}\right)$$ (1)

$$·{\mathrm{O}}_2^{-}+{\mathrm{e}}^{-}+2{\mathrm{H}}^{+}\to {\mathrm{H}}_2{\mathrm{O}}_2\left(1.44\mathrm{V}\mathrm{vs}.\mathrm{NHE}\right)$$ (2)

$$·{\mathrm{O}}_2^{-}+{\mathrm{H}}^{+}\to \ast \mathrm{OO}\mathrm{H},\ast \mathrm{OO}\mathrm{H}+{\mathrm{e}}^{-}+{\mathrm{H}}^{+}\to {\mathrm{H}}_2{\mathrm{O}}_2$$ (3)

$$\begin{array}{ccc}\hfill {\mathrm{O}}_2+2{\mathrm{e}}^{-}+2{\mathrm{H}}^{+}& & \to 1,4-\mathrm{endoperoxide}\mathrm{species}\hfill \\ & & \to {\mathrm{H}}_2{\mathrm{O}}_2\left(0.68\mathrm{V}\mathrm{vs}.\mathrm{NHE}\right)\hfill \end{array}$$ (4)

WOR:

$$2{\mathrm{H}}_2\mathrm{O}+4{\mathrm{h}}^{+}\to {\mathrm{O}}_2+4{\mathrm{H}}^{+}\left(1.23\mathrm{V}\mathrm{vs}.\mathrm{NHE}\right)$$ (5)

$$2{\mathrm{H}}_2\mathrm{O}+2{\mathrm{h}}^{+}\to {\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{H}}^{+}\left(1.76\mathrm{V}\mathrm{vs}.\mathrm{NHE}\right)$$ (6)

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{h}}^{+}\to ·\mathrm{OH}+{\mathrm{H}}^{+}\left(2.73\mathrm{V}\mathrm{vs}.\mathrm{NHE}\right)\left(\mathrm{acidic}\mathrm{condition}\right)$$ (7)

$$\mathrm{O}{\mathrm{H}}^{-}+{\mathrm{h}}^{+}\to ·\mathrm{OH}\left(\mathrm{alkaline}\mathrm{condition}\right)$$ (8)

$$·\mathrm{OH}+·\mathrm{OH}\to {\mathrm{H}}_2{\mathrm{O}}_2\left(1.14\mathrm{V}\mathrm{vs}.\mathrm{NHE}\right)$$ (9)

As for the insufficient reaction molecules utilization, MSCN can overcome it through regulating reactive centers to effectively trap reaction molecules with the help of modified morphological structures, beneficial groups and defect levels. Pelicano et al. successfully confirmed the key mechanism by which structural distortion, ‐C≡N and N_v improve the O₂ molecules utilization within the oxide amide‐doped KPHI (O_x‐KPHI) framework [118]. Specifically, the structural distortion increased the distance between N atoms from 7.4 to 7.6 Å, which led to an uneven distribution of electron density, thus causing the O₂ adsorption and convention sites located on N_v and ‐C≡N. This reactive site conversion lowered the O₂ adsorption energy while generating more reactive sites in O_x‐KPHI, which significantly promoted the efficient trapping of O₂ (Figure 8a). Since the efficient reaction molecules utilization depends not only on the adsorption of reaction molecules but also on the inhibition of competitive reaction, the modified crystal and electronic structures that can achieve the selection of effective reaction pathways are definitely feasible means. Cite highly semi‐crystalline CN embedded with an I⁻/I₃ ⁻ redox medium (CN‐I/I₃) as an example, the enhanced crystallinity and the inserted I⁻/I₃ ⁻ synergistically achieved high O₂ utilization from two aspects: adsorption and selective reaction pathway [119]. The O₂ adsorption was enhanced through the van der Waals force between the material fixed dipole and the O₂ dipole. Meanwhile, the lower O₂ adsorption energy and the increased O‐O bond length of CN‐I/I₃ compared to CN implied the enhanced activation affinity of O₂, which enabled CN‐I/I₃ to energetically favor the *OOH formation reaction pathway and the subsequent hydrogenation process (Figure 8b).

FIGURE 8.

(a) Relative formation energy comparison between KPHI and O_x‐KPHI within H₂O₂ production. Reproduced with permission [118]. Copyright 2024, Wiley‐VCH. (b) Energy comparison of H₂O₂ production between CN, CN‐I and CN‐I/I₃. Reproduced with permission [119]. Copyright 2024, Nature Publishing Group. (c) Comparison of O₂ adsorption energy of g‐C₃N₅, g‐C₃N₅‐K, and g‐C₃N₅‐K, K. (d) Dispersed as‐prepared catalysts in water for 14 days. Reproduced with permission [120]. Copyright 2024, Wiley‐VCH. (e) Verified Fermi level of IKCN and NH₂‐UiO‐66 before and after contact [121]. Copyright 2024, Wiley‐VCH.

SA dependence will cause additional costs and difficulties in subsequent product collection. The key to addressing this issue lies in lowering the reaction energy barrier and improving reaction kinetics to meet the requirements of redox potentials. The introduction of the abundant edge active sites (AEASs) (‐C≡N and ‐OH) was believed to contribute to achieving the de‐dependence of SA through kinetic optimization [120]. On the one hand, AEASs significantly provided more surface‐active sites for the adsorption and activation of O₂, which significantly enhanced the O₂ adsorption energy (Figure 8c). On the other hand, AEASs made the g‐C₃N₅‐K, K have good hydrophilicity and dispersion, forming a quasi‐homogeneous photocatalytic system, thus ensuring that the reaction could be stably carried out in a pure water system (Figure 8d).

Actively regulating the electronic structure to enhance the redox capacity is another feasible strategy to improve slow kinetic processes and achieve SA independence. Ma et al. clarified the role of MSAS in achieving the SA‐independence through electronic structure modulation from both thermodynamic and kinetic aspects within I⁻/K⁺ co‐doped CN with NH₂‐UiO‐66 (UiO‐IKCN) [121]. In thermodynamic aspect, the matched band structure between NH₂‐UiO‐66 and IKCN caused the formation of the BIEF, which in turn endowed the band structure with strong reduction and oxidation potentials, thus enhancing the redox capacity of UiO‐IKCN. This strengthened redox capacity endowed UiO‐IKCN with a lower barrier from *O₂ to *OOH compared to CN, suggesting a kinetic advantage in the 2e⁻ ORR pathway (Figure 8e). It should be noted that the finding of this research supports the crucial role of thermodynamic and kinetic optimization in enhancing the efficiency of MSCN under SA‐free or low‐SA conditions, which provides new insight into the design and comprehensive view of a SA‐independent photocatalytic H₂O₂ production system. However, the specific contribution values of thermodynamics and kinetics to the SA‐free performance remain ambiguous. Future studies could employ ultrafast spectroscopy and other means to directly visualize charge carrier behavior under SA‐free conditions, thereby quantitatively deconvoluting the contributions of each factor [122].

4.2. H₂ Evolution Reactions (HER)

As one of the key reactions in photocatalytic energy production technology, HER is considered a promising solution to the current energy crisis. Currently, photocatalytic H₂ production via overall water splitting (OWS) involves two half‐reactions: proton reduction in CB (Equation (10)) and oxygen precipitation in VB (Equation (11)) [127, 128]. The efficiency of OWS is limited by the inherent problems of high activation energies, limited active sites, and severe competitive reactions, which must be overcome. Previous studies have already confirmed that photo‐thermal synergy is a pivotal strategy for enhancing the reaction kinetics, offering an effective pathway to boost HER performance [129]. Moreover, since most semiconductors do not have the ability to overcome these challenges and the application of cocatalysts will cause secondary problems that need to be addressed, further modified catalysts are needed to achieve effective reaction efficiency [130, 131]. In this context, MSCN demonstrates unique advantages, effectively addressing the inherent limitations of the OWS reaction and avoiding the additional issues associated with cocatalysts, thereby providing an ideal platform for achieving efficient HER [132]. Recent reports on representative HER catalyst studies further highlight the superiority of MSCN in HER applications (Table 2).

$$\mathrm{HER}:2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2$$ (10)

$$\mathrm{OER}:2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}_2+4{\mathrm{H}}^{+}+4{\mathrm{e}}^{-}$$ (11)

TABLE 2.

Summary of some recently H₂ evolution reaction catalysts.

Methods	Catalysts	H2 yield	Molten‐salt systems	Key structural parameters	Experiment parameters	Photocatalytic efficiency a	Refs.
MSAS	PTI/HCN	478 µM h−1	KCl/LiCl	Desired framework	550°C,4 h	—	[36]
	H‐PHI	18.2 mM g−1 h−1	KCl/LiCl	Open channel structure	550°C,4 h	AQY = 73%	[133]
	CNx|Ni2P	microfibers: 104 ± 10 µmolH2 gsub−1 PET:22.0 ± 1.3 µmolH2 gsub−1	KSCN	Band structure	400°C, 1 h and 500°C, 0.5 h	PET: 24.5 ± 3.3% PLA: 6.7 ± 0.8%	[30]
	Pt/C3N4	5.8 mL h−1 cm−2	KCl/LiCl	—	525°C, 2 h	AQE=40%	[135]
Thermally treated	SS‐CNP	136.2 µM h−1 mg−1	—	Break symmetry structure	550°C, 4 h	AQY = 76.4%	[136]
	g‐C3N4‐Cx	22.45 µM h−1	—	electronic polarization area	550°C, 4 h	AQE = 4.7%	[137]
	CNND/CNUN	580 µM h−1	—	Regulation of band structure and electronic structure	550°C, 4 h	—	[138]
Solvothermal	C‐Mo3S9@CN	559 µM h−1	—	Mo3S9 clusters	125°C, 1 h	AQY = 88.6%	[139]

^a Apparent Quantum Yield (AQY).

Highly modified structural parameters of MSCN allow for significant potential for overcoming the limitations of HER. V. Lotsch et al. pointed out that the nanosheets could break the sluggish HER kinetics limitation of bulk PTI [27]. The analysis of the excellent HER performance showed that the nanostructure made the PTI more feasible to contain water due to the matched surface energy between nanosheets and water molecules, which endowed the PTI nanosheets with strong resistance to environmental parameters and strengthened capacity to trap more reaction molecules. More importantly, more active sites and larger reactive areas brought by the nanosheets all favored photocatalysis. These nanostructure characteristics made the PTI nanosheets show stronger stability and better HER performance than bulk PTI under both acidic/basic and long time (130 h) conditions.

Desired framework of MSCN is believed to contribute to inhibiting the side and back reactions, which is another indirect pathway to enhance HER performance. Taking the PTI/heptazine‐based CN (PTI/HCN) as an example, the construction of the PTI and HCN framework contributed to inhibiting the reverse reaction in HER [36]. Theoretical calculations confirmed the generation of BIEF due to the different potential of PTI and HCN, which in turn enabled the spatial separation of redox‐active sites at the interface between PTI and HCN. The spatially separated redox active sites inhibited the occurrence of reverse reactions, thus allowing the PTI/HCN to achieve better HER compared to HCN (12 times) and PTI (2.4 times) (Figure 9,b).

FIGURE 9.

(a) Charge carrier separation mechanism of PTI and PTI/HCN. (b) OWS activity of PTI/HCN with different phase ratios. Reproduced with permission [36]. Copyright 2025, Wiley‐VCH. (c) Comparison of H₂ generation rate between different final concentrations of HCl on the content of Pt. Reproduced with permission [133]. Copyright 2024, American Chemical Society. (d) Polymer photoreforming mechanism and practical application of CN_x|Ni₂P. Reproduced with permission [30]. Copyright 2019, American Chemical Society. (e) Practical panel reactors for methanol‐to‐H₂ conversion. Reproduced with permission [135]. Copyright 2024, Wiley‐VCH.

Widely using cocatalysts will lead to secondary problems about the amount selection and the subsequent problems of stability and selectivity that significantly suppress efficient HER. To overcome these challenges brought by cocatalyst, the construction of MSCN is oriented to break the HER limitations while gradually realizing the de‐dependence of cocatalysts. The guided framework transition from K‐PHI to H‐PHI had exhibited great potential in breaking the excessive use limitation of cocatalysts [133]. This framework transition transformed the ionic structure into a covalent structure, which facilitated the formation of the “locked charges”, thus forming more photocatalytic deposition centers for platinum (Pt). The increased deposition centers promoted the homogenization of Pt on the catalyst surface, which significantly reduced the requirement of Pt. With these results, the H‐PHI exhibited a higher HER activity even with a sparse platinum (Pt) cocatalyst (only 0.05%, two orders of magnitude lower than previous systems) compared to K‐PHI (Figure 9c).

In addition to OWS, MSCN also provides other pathways that contribute to excellent HER [134]. A suitable case was the successful conversion from plastics to H₂ through employing the cyanamide‐functionalized CN/nickel phosphide (CNx|Ni₂P) (Figure 9d) [30]. The analysis of the plastic‐to‐H₂ process showed that the cyanamide groups tuned the VB and CB to +2.2 eV and ‐0.5 eV, respectively, which made the CNx|Ni₂P conducive to the plastic‐to‐H₂ reaction. This verified band structure made CNx|Ni₂P exhibit efficient plastics‐to‐H₂ conversion while producing more high value organic chemicals (acetate and formate). In practical application, the optimal H₂ yield reached 53.5 µmolH₂ g_sub ⁻¹ over the course of 5 days within a 60 cm² irradiation area reactor and no loss in system efficiency was observed as the volume of pollutants increased from 2 to 120 mL. The pure organic products‐to‐H₂ conversion also supported the great potential of MSCN in achieving excellent HER, just as the Pt/C₃N₄ exhibited excellent HER performance in both experiments and reactors. Although pure methanol had been confirmed with the advantages of inhibiting competitive reactions and promoting desorption of CH₂O, the “structure‐mechanism” relationship of Pt/C₃N₄ in promoting methanol‐to‐H₂ conversion was not yet clear, which needed to be further explored (Figure 9e) [135].

4.3. CO₂ Reduction Reactions (CO₂RR)

Photocatalytic CO₂RR is an environmentally friendly emission reduction technology that employs clean solar energy to convert CO₂ into high value‐added C₁ (e.g., CO, CH₄) or C₂₊ products (e.g., C₂H₄, C₂H₅OH) [10, 140]. The process commences with the adsorption of CO₂ molecules, followed by activation and desorption steps involving electrons and holes to generate high value‐added products. Moreover, the coupling of reduction half‐reactions with the selective oxidation of organic compounds represents a promising direction for improving the atomic economy and added value of this process. Although various strategies have been taken to solve the challenges of adsorption, activation, and desorption in CO₂RR caused by the intrinsic limitations of active sites and the sluggish kinetics of CN coupling reactions, two major challenges remain [11]. The first is the extreme difficulty in activating highly stable CO₂ molecules, which stems from their inherent thermodynamic stability (due to their high symmetry and linear structure). The second is the similar thermodynamic energy barriers of CO₂ reduction products, which limit reaction efficiency and selectivity. MSCN with broadened photoexcitation, heightened charge carrier separation/transportation and strengthened redox capacity definitely provides a new insight to solve the abovementioned challenges. Meanwhile, the enhanced activity/selectivity of CO₂RR in turn provides insight into the unique structure‐CO₂RR activity relationship in MSCN. Recent advancements in MSCN applications for CO₂RR further highlight its competitive advantages in the field of CO₂RR (Table 3).

TABLE 3.

Summary of some recently CO₂ evolution reaction catalysts.

Methods	Catalysts	Products yield	Molten‐salt systems	Key structural parameters	Experiment parameters	Active species	Selectivity a	Refs.
MSAS	TCN/Ni(OH)2	CO: 23.32 µM g−1 h−1 CH4: 12.47 µM g−1 h−1	LiCl/KCl	Desired framework	550°C,2 h	*COOH	—	[141]
	PTI	CO: 44 µM h−1	KCl/LiCl	surface pyridine N sites	400°C, 6 h and 550°C,24 h	*COOH	93%	[142]
	CeCo‐PTI	CH4: 181.7 µM g−1	KCl/LiCl	Co SAs and CeO2 nanoparticles; BIEF	550°C,4 h	CH3O*	88.3%	[146]
	CoPc/PHI	CO: 116 µM g−1 h−1	KCl/LiCl	Co SAs	550°C,4 h	*COOH	97%	[145]
Thermally treated	NS‐P‐g‐C3N4‐Ni	HCOOH: 72.3 µM h−1	—	Ni metal cluster	400°C, 2 h	*CO2•−, *COOH	60%	[147]
	VN‐CN‐HNSs	CO: 35.2 µM h−1	—	VNs, delocalized π electron clouds	500°C, 2 h	*COOH *CO2	100%	[148]
Solvothermal	CCN	Hydrocarbon fuels:12.07 µM h−1	—	‐C≡N and carboxyl groups highly ordered crystallinity	550°C, 2 h	—	91.5%	[149]

^a The selectivity of the products.

Efficient CO₂ activation typically requires high energy input to break its stable structure. The tailored MSCN with modified morphological structures and specific structural functional components provides an opportunity to solve this high energy barrier limitation. The tubular triazine‐based CN with Ni(OH)₂ nanosheets (TCN/Ni(OH)₂) was employed to clarify the crucial role of modified morphological structure and introduced external substance in enhancing CO₂RR [141]. On the one hand, the tubular structure gave TCN more adsorption sites and reduced the formation energy barriers of ∗OOH, which effectively strengthened the activation ability of CO₂ (Figure 10a). On the other hand, the modified tubular structure also ensured sufficient protons transferred to the Ni(OH)₂ nanosheets, thus matching the reaction kinetics through the enhanced proton‐coupled electron transfer reaction process induced by “3CO₂ + 2H₂O = CH₄ + 2CO + 3O₂”. This tandem effect of modified morphological structure and foreign substance provided a new perspective in designing a MSCN with excellent CO₂RR ability.

FIGURE 10.

(a) Energy profile comparison for CO₂RR between HCN, TCN, and TCN/Ni(OH)₂. Reproduced with permission [141]. Copyright 2024, Wiley‐VCH. (b) Photocatalytic CO₂RR activity of PTI, PTI/melon and melon. Reproduced with permission [142]. Copyright 2025, American Chemical Society. (c) Free energy and reduction way for CO₂RR of PHI and CoPc/PHI. Reproduced with permission [145]. Copyright 2025, Wiley‐VCH. (d) Illustration of the well‐designed photocatalytic platform by Co‐doped CeO₂ with PTI. (e) 3D charge density of Co core and 2D valence charge density of Co‐doped CeO₂. (f) Comparison of CH₄ and CO selectivity of various prepared catalysts. Reproduced with permission [146]. Copyright 2024, Wiley‐VCH.

With the capacity to effectively reduce the activation barrier of CO₂RR, site regulation engineering has also been proven to be an effective means to promote the activation and conversion of CO₂. Based on this concept, abundant active pyridine N sites were embedded into the hexagonal prism PTI crystals to endow PTI with superior CO₂ reduction capacity [142]. The introduced surface pyridine N sites not only acted as reactive sites to trap more CO₂ molecules but also reduced the formation energy barrier of *COOH species, which played a key role in the capture, activation and conversion of CO₂, thus giving the PTI manifest superiority in photoreduction CO₂ (Figure 10b).

The key to slowing the poor selectivity and activity limitations of CO₂RR lies in regulating the reaction pathways toward target products. In this context, various SAs have been anchored and integrated within MSCN to enhance CO₂RR due to their strong ability to enhance the adsorption capacity and mediate the reaction pathways [90, 143, 144]. For example, cobalt phthalocyanine (CoPc) assembled PHI (CoPc/PHI) that contained Co SAs exhibited excellent CO₂RR performance [145]. Experimental and theoretical results showed that Co SAs not only acted as reactive sites to enhance the adsorption of CO₂ molecules, but also reduced both the formation energy barriers of *COOH and the desorption energy barriers of CO, thus facilitating CO₂‐to‐CO conversion and the rapid desorption of CO (Figure 10c). The synergistic effect between SAs and nanoparticles that can reduce the energy barrier of CO₂RR reactive intermediates is another feasible pathway to break the selectivity and activity limitations of CO₂RR. Crystalline CN ligands that contained the synergistic effect between Co SAs and CeO₂ nanoparticles revealed the contribution of the synergistic effect in CO₂RR (Figure 10d) [146]. The electric potential difference between Co SAs‐doped CeO₂ and PTI caused the generation of BIEF, which created a strong interconnect for subsequent directed dynamic charge transfer, thus facilitating reaction kinetics toward multiple electron‐involving reduction pathways (Figure 10e). Meanwhile, the synergistic effects between CeO₂ and Co SAs caused the directional charge control, which made the CeCo‐PTI more inclined to protonate CO* into hydrogenation species rather than dissociate it from surface, thus enhancing the capacity for the ideal protonation pathway (Figure 10f).

5. Summary and Outlook

Designing efficient and stable photocatalysts is a key approach to significantly improving solar energy conversion efficiency and addressing environmental crisis and energy depletion issues. As an emerging CN material, MSCN has garnered significant attention for its ability to overcome the inherent limitations of traditional materials. This review first explores the effect of extra‐ and internal synthesis parameters on the formation of the unique structure characteristics, involving crystal and morphological structures, beneficial functional groups/bonds, microstructures and electronic and band structures. Subsequently, building on the clarification of structural regulation, the paper analyzes how these unique characteristics enhance material stability, photoexcitation capability, and the separation and transport efficiency of photogenerated carrier. Then, the energy applications of MSCN clearly demonstrate its great potential in breaking through the energy barriers existing in the H₂O₂ evolution, HER and CO₂RR, and achieving efficient production of energy substances. Although MSCN with ideal photocatalytic performance can be successfully synthesized by regulating the MSAS process, numerous challenges remain to be overcome (Figure 11).

• (I)Detecting the details of the dynamic reaction process. Although there has been considerable discussion on the influence of adjusting MSAS process parameters on the final MSCN structure parameters, the dynamic process, especially the growth process of reaction products in the molten salt system, and the specific mechanisms guiding the selection of reaction parameters still need to be explored in depth. More importantly, the current understanding of the influencing parameters is incomplete, and research on how environmental factors significantly affect MSCN remains insufficient. Current studies have only addressed the impact of environmental humidity on material properties, and the mechanism by which it affects interlayer spacing has not been thoroughly elucidated. Considering these challenges, the advanced exploration techniques, such as in situ transmission electron microscopy (in situ TEM), in situ X‐ray absorption fine spectroscopy (in situ XAFS), Artificial intelligence (AI), and density functional theory (DFT), need to be further developed to detect the dynamic formation process of key intermediates. Additionally, more attention can be paid to how the design principles of related porous materials inform MSCN engineering. For instance, covalent organic frameworks (COFs), with their well‐established structure‐property relationships, can serve as highly tailorable platforms to guide the precise design of MSCN [150]. Such comparative analysis is crucial for tailoring the porosity and electronic structure to enhance photocatalytic performance. Developing these advanced techniques and leveraging insights from analogous materials help to establish the specific equation of reaction process‐reaction parameters‐reaction activity, which will guide the design of more efficient MSCN.
• (II)Exploring the deeper light on reaction mechanism. The catalytic mechanism of MSCN in specific production reactions needs to be further studied. A key challenge lies in better understanding the synergy between thermodynamic feasibility and kinetic dominance of MSCN under SA‐free or cocatalyst‐free conditions. While previous research has confirmed the crucial role of kinetic optimization in enhancing the energy production efficiency of MSCN under SA‐free or cocatalyst‐free conditions, the thermodynamic feasibility also provides the essential framework that enables these kinetic advantages. Future work should further quantify the individual and combined contributions of thermodynamics and kinetics, enabling a more precise elucidation of the underlying reaction mechanism. Another challenge lies in exploring the multiple‐dimensional mechanism. Nowadays, only the role of a single response mechanism has been revealed in most MSCN research. Since the dual‐channel mechanism has been proven to have better potential for energy production than a single response mechanism, it is of great potential to explore the various dimension mechanisms in energy production. Previous studies have also confirmed that coupling the half reduction and the selective oxidation of organic compounds exhibits significant potential in enhancing the atom economy and value of the process, as they can facilitate energy production while generating some value‐added substances [143]. For instance, Li et al. constructed a COF (TFPD)/CdS catalyst, which broke the slow e⁻–h⁺ separation and uncontrolled 5‐hydroxymethylfurfural conversion limitations, achieving highly selective oxidation of 5‐hydroxymethylfurfural or C–C coupling in conjunction with H₂ evolution [128]. Future research should prioritize exploring the reaction path and couple reactions to rationally design MSCN with multi‐pathway capacity, thus improving energy efficiency and economic value, achieving a win‐win situation, and broadening application prospects of MSCN. More importantly, there still remains a system theory vacancy in determining the optimal ratio between various structures and the synergistic effect between various structure characteristics. With the deepening of the research, the synergistic roles between various structure characteristics have been gradually noted. Li et al. confirmed that the synergistic interaction between morphology regulation and S‐scheme heterojunction significantly enhanced the loading capacity and improved charge carrier transportation, thereby enhancing photocatalytic efficiency [151]. However, the relationship between the optimal proportion of CN structures endowed by MSAS is still unclear, which should be deeply revealed to find the golden zone between the optimal proportion of structure and various catalytic capacity systems.

FIGURE 11.

The major challenges and prospects of MSCN toward real‐word implementations.

• (III)Practical application barriers. There remain great challenges in achieving the application of MSCN from lab to field. The first is economic effectiveness. Although MSCN has demonstrated the potential to enhance the reaction kinetics and reduce reliance on SA and cocatalysts, the challenges of the SA/cocatalyst dependence still remain. Moreover, due to the high cost of the molten salts and the significant obstacles remain in the recovery procedure, the cost of MSAS is still higher than that of other methods. Further efforts should focus on developing low‐cost and recoverable molten salts to achieve higher cost effectiveness. Safety is the second point that needs to be considered. While the MSAS process can achieve efficient synthesis under high‐temperature conditions, the inherent properties of molten salt media, such as high corrosiveness and vapor pressure, pose significant safety challenges to equipment integrity. Additionally, the possible release of toxic gases during the reaction requires stringent safety measures. To tackle these challenges, developing molten salts with lower melting points and reduced toxicity has emerged as a key strategy to enhance process safety. Thirdly, the recovery efficiency of the molten salt system needs to be enhanced. Previous studies have confirmed the feasibility of reusing recovered molten salts [152, 153]. For example, compared with crystalline polymer CN (CPCN) synthesized using fresh KCl, CPCN synthesized using recovered KCl exhibited the same crystal structure, with no observable loss of activity [47]. However, the recovery of molten salt in MSCN systems is usually achieved by water washing, which makes it difficult to achieve high‐efficiency molten salt recovery. Based on this, numerous advanced recovery technologies for the large‐scale synthesis of MSCN can be developed. For instance, the molten salt hydropyrolysis used in the biomass field can be employed in MSCN systems, as it can achieve a salt recovery efficiency of 95–100 wt.% with minimal impact on the thermal and catalytic properties of the salts [153]. Multi‐dimensional consideration is also an important direction for achieving the large‐scale application. Since the ideal MSCN currently carried out within the laboratory scale still requires demanding experimental conditions and the potential problems within more complex situations will be amplified by the large‐scale production of MSCN, the impact of MSCN on the environment should be further considered. Furthermore, the practical application of MSCN needs to address the complexity of real‐world wastewater, which often involves coexisting pollutants [154]. Future research should focus on the performance and synergistic removal mechanisms of MSCN when treating multi‐pollutant systems, such as systems containing both antibiotics and heavy metal ions. Understanding these interactions is crucial for advancing the technology from controlled laboratory settings to field applications. In this context, utilizing advanced tools such as machine learning to intelligently screen molten salts and predict MSCN performance represents a cutting‐edge direction [155]. These approaches can significantly accelerate the practical application of this technology by overcoming the limitations of traditional trial‐and‐error methods.
• (IV)Life Cycle Assessments (LCA). Although the application of MSCN to photocatalytic productivity reactions has many unique advantages, more factors need to be considered for expanding its practical engineering applications. From an environmental perspective, LCA can minimize the environmental impact of MSCN catalyzed processes by defining the target and scope of material use, identifying and calculating input energy, material use, waste disposal, and estimating the environmental and human health impacts of new generation [156]. In this context, it is necessary to use LCA to define the target range of MSCN in the subsequent research. The LCA of MSCN mainly commences with the goal and scope, including the definition of MSCN system boundaries, life‐cycle inventory analysis (LCI), and life‐cycle impact assessment (LCIA), which collect all energy inputs and outputs and translate them into environmental categories. In the interpretation phase, the results are summarized and discussed to analyze the reliability and put forward improvement suggestions. LCA is an important decision‐making tool that needs to be carefully considered to achieve practical applications of MSCN photocatalytic capacity on an industrial scale.

Funding

This study was financially supported by the National Natural Science Foundation of China (52470188), the Science and Technology Innovation Program of Hunan Province, China (2023RC3107), the Program for the Natural Science Foundation of Changsha, China (kg2402075).

Conflicts of Interest

The authors declare no conflicts of interest.

Data Availability Statement

No primary research results, software or code have been included and no new data were generated or analyzed as part of this review.