The Construction and Photocatalytic Application of Covalent Triazine Framework (CTF)-Based Composites: A Brief Review

Abstract

Covalent triazine frameworks (CTFs) are a class of porous organic semiconductors containing a large number of triazine units, which gives them many properties suitable for photocatalysis, such as high porosity, good tunability, and excellent chemical stability. However, it is difficult to achieve high activity, stability, and selectivity at the same time using a single CTF in a specific catalytic reaction. Therefore, it is necessary to find ways to combine CTFs with other materials to improve their photocatalysis activity. From this perspective, some construction methods and the latest progress of CTF-based composites are presented, and their applications in the field of photocatalysis are introduced. Finally, the future of CTF materials in catalytic applications is proposed, which provides some insights into the research and exploration of CTF-based composites.

1. Introduction

The excessive reliance on fossil fuels has become a major driver of global environmental and climate change, leading to a sharp increase in greenhouse gas emissions, a rise in global temperatures, frequent extreme weather events, and damage to ecosystems. Over-reliance on fossil fuels not only exacerbates atmospheric pollution and the greenhouse effect but also poses a potential risk of energy depletion, threatening energy security and sustainable development [1]. In this context, the application of photocatalytic technology is of significant importance. By utilizing photocatalytic reactions to convert inexhaustible solar energy into chemical energy, it not only helps avoid an energy crisis but also reduces carbon emissions, making it a highly promising approach.

Since Fujishima and Honda discovered that TiO₂ could serve as a photocatalyst for water splitting [2], numerous researchers have dedicated efforts to exploring semiconductor photocatalysts. Various effective photocatalysts have been discovered, including ZnO [3], CdS [4], BiVO₄ [5], C₃N₄ [6], COF [7], MOF [8], and others. In recent years, novel materials known as covalent triazine frameworks (CTFs) have gradually gained attention.

As porous organic polymers containing abundant triazine structures, CTF materials possess a large specific surface area, rich nitrogen sites, and π-conjugated structures [9]. These characteristics enable CTF materials to have advantages in both the thermodynamics and kinetics of photocatalytic reactions. However, pure CTF materials still face issues such as fast charge carrier recombination and low visible light utilization efficiency [10]. Therefore, the combination of CTFs with other materials to optimize photocatalytic performance has gradually become a promising research direction.

This brief review primarily focuses on CTF-based composite photocatalytic materials. It introduces several widely studied methods for constructing CTF-based composites, including chemical doping, and the combination of CTFs with non-metals, single atoms, metal oxides, and metal sulfides. This brief review also showcases recent applications of CTF-based composite materials in several major photocatalytic reactions, including photocatalytic hydrogen evolution, photocatalytic H₂O₂ production, photocatalytic CO₂ reduction, and other photocatalytic processes. Finally, the future research directions of CTF-based composite materials in the field of photocatalysis are discussed, with the hope of providing insights for researchers in this field.

2. Composite Design

2.1. Structure and Synthesis of CTFs

In 2008, Thomas et al. [11] used a series of aromatic nitrile monomers to react with molten zinc chloride under high-temperature conditions, preparing a series of materials containing triazine units. These materials exhibit large specific surface areas and porosities, demonstrating excellent gas adsorption properties and stability. These represent the earliest reported CTF materials. The structure of CTF materials can be summarized as follows: a large number of triazine units, which serve as linking groups, connect functionalized monomers to form an ordered conjugated structure (Figure 1). By altering the raw materials and synthesis methods, CTF materials containing different monomers can be prepared. For example, Thomas et al. synthesized CTF-1 [11] and CTF-0 [12] using 1,4-dicyanobenzene and 1,3,5-tricyanobenzene as monomers, respectively. The flexibility and adjustability of the monomers ultimately lead to the CTF materials exhibiting a rich variety of structures, different band gaps, and light-responsive properties, highlighting their extraordinary potential in photocatalysis.

Several methods for the synthesis of CTFs have been developed. For instance, the ionothermal polymerization method initially used by Thomas et al. [11] involves the use of molten ZnCl₂ as a solvent and catalyst at 400 °C, allowing for the trimerization of 1,4-dicyanobenzene to yield black CTF-1 powder. The disappearance of the cyano group peak at 2218 cm⁻¹ and the significant triazine group signal at 1352 cm⁻¹ in the infrared spectrum confirmed the successful synthesis of CTFs. Baek et al. [13] innovatively proposed the use of P₂O₅ as a catalyst for the polymerization of terephthalamide to form pCTF-1. pCTF-1 synthesized using this phosphorus-based catalytic method exhibits a larger specific surface area than CTF-1 obtained via ionothermal polymerization; however, the process still requires high temperatures. Additionally, the superacid catalytic method proposed by Cooper et al. [14] is an effective approach for synthesizing CTFs. The team synthesized various CTF materials under both room-temperature and microwave-assisted conditions using trifluoromethanesulfonic acid as the catalyst, naming the materials P1-P6 and P1M-P6M. It was ultimately discovered that the materials synthesized by this method exhibited color changes; for example, P6M was light yellow, which is distinct from the black CTF obtained via traditional ZnCl₂ ionothermal polymerization, and demonstrated excellent light-responsive properties. Another synthesis method based on amidine–aldehyde condensation was first proposed by Tan et al. [15]. Using dimethyl sulfoxide as a solvent and Cs₂CO₃ as a base, a series of CTF materials (CTF-HUST-1~4) were synthesized at 120 °C. This method offers milder reaction conditions, avoiding the use of high temperatures and superacid, but the multi-step process presents challenges in terms of synthesis cost and efficiency. In addition to the synthesis of triazine units followed by polymerization into CTFs, CTF materials can also be synthesized by directly using monomers containing triazine units. For example, the Yamamoto coupling reaction and Suzuki coupling reaction have been applied in CTF synthesis [16,17].

Many studies on methods for the synthesis of CTFs have been conducted, and numerous reviews have discussed this research direction. This brief review primarily focuses on CTF-based composite materials, so further elaboration on this topic will not be provided.

2.2. Chemical Doping

To improve the light absorption and charge separation properties of CTFs, it is essential to modify factors such as the electronic band structure, Fermi level, and carrier density. A common and effective method is chemical doping [18,19]. Research has demonstrated that introducing elements like halogens (F, Cl, Br) [20,21], N [22,23], P [24], and S [25] via doping can notably influence the photocatalytic performance of a CTF by altering its electronic configuration. For instance, Cheng et al. [20] synthesized CTF-1 using trifluoromethanesulfonic acid as a catalyst and subsequently mixed it with ammonium halides for thermal treatment to prepare CTF materials doped with different halogen atoms (CTFF, CTFCl, CTFBr). They observed that all three doped materials exhibited narrower band gaps and enhanced charge separation capabilities, ultimately demonstrating superior activity in the photocatalytic hydrogen evolution reaction. Among them, the hydrogen production activity of CTFCl was increased most significantly, and the hydrogen evolution rate was 7.1 times higher than that of undoped materials. The variations in performance due to different doping elements might be linked to their electronegativity and atomic size. Compared to the other two elements (F and Br), chlorine’s more suitable electronegativity and atomic size may provide important insights into this behavior (C: 2.55, 0.67 Å; N: 3.04, 0.56 Å; F: 3.98, 0.42 Å; Cl: 3.16, 0.79 Å; Br: 2.96, 0.94 Å). Moreover, Cheng et al. [24] investigated the effects of phosphorus doping on a CTF. In this case, the composite material was prepared by mixing a CTF with red phosphorus and subjecting it to simple thermal treatment at 250 °C for one hour. The phosphorus-doped CTF material demonstrated superior photocatalytic hydrogen absorption due to enhanced visible light absorption and photocatalytic electron reduction capabilities, improving the separation and transfer of photogenerated carriers.

A nitrogen-doped CTF (NCTF-1) [22] was synthesized by hydrothermally treating CTF-1 with hydrazine hydrate. Compared to unmodified CTFs, NCTF-1 has a higher nitrogen content. This provides more basic sites for the material, improving its CO₂ adsorption ability. Consequently, it achieved an activity of 11.48 μmol·g⁻¹·h⁻¹ in photocatalytic CO₂ reduction to CH₄, which was nine times higher than that of bare CTF-1. In another study, Han et al. [23] created a different nitrogen-doped CTF (T3N-CTF) by the Schiff base reaction. The additional three pyridine nitrogen atoms around the triazine unit provide more active sites for hydrogen production, reduce the recombination of electrons and holes, and improve charge transport at the catalyst surface.

S-doped CTF [25] materials also exhibit enhanced photocatalytic performance and stability. When compared to the original CTF-T1, the composite material showed a faster separation and transfer of photogenerated electron–hole pairs (Figure 2a–d). Theoretical models [26] suggest that this improvement is due to sulfur impurity atoms, which introduce asymmetric impurity energy levels near the Fermi level, promoting electron spin polarization and creating an internal electric field (IEF). This aids the migration of photogenerated carriers in a specific direction, preventing recombination (Figure 2e,f).

Doping with other elements in a CTF that is an organic semiconductor is more challenging than in inorganic semiconductors because doping typically disrupts its original covalent bonds. This implies that the conjugated structure of the CTF is disrupted, leading to a decrease in crystallinity. Moreover, achieving the precise insertion of dopants at specific positions within CTF materials is also highly challenging. Compared to inorganic semiconductors, these difficulties make the doping process in CTF materials more complex and harder to control.

2.3. Composite with Non-Metallic Materials

Non-metallic materials such as carbon nitride (C₃N₄) and black phosphorus (BP) nanosheets, with their remarkable optical characteristics and electrical conductivity, have been widely explored to form heterojunctions with CTFs, which improves the visible light absorption and charge transfer capabilities of CTF-based photocatalysts [27,28]. For instance, He et al. [27] were the first to combine a CTF with g-C₃N₄ to create a 2D/2D heterostructure aimed at photocatalytic CO₂ reduction. By using Co(bpy)₃Cl₂ as a co-catalyst, the composite material achieved 99% efficiency in the selective photocatalytic reduction of CO₂ to CO. Regarding its stability, the CO production rate remained largely unaffected after three cycles over 30 h. Zhang et al. [28] proposed a method for synthesizing the CTF/BP composite, where a CTF was applied onto BP nanosheets via sonication. Under light exposure, the photogenerated electrons efficiently transferred from the CTF to the BP surface through the P-C bond, and the accumulated electrons protected the BP nanosheets from surface oxidation. This composite exhibited excellent photocatalytic hydrogen production, reaching an activity of 17.1 mmol·g⁻¹·h⁻¹ when formaldehyde was used as a sacrificial agent.

As a carbon-based material, carbon quantum dots (CQDs) not only demonstrate high electrical conductivity but also show outstanding biocompatibility and photoluminescence (PL) due to their size being under 10 nm [29]. Yang et al. [30] proposed loading CQDs onto a CTF to boost electron transfer efficiency and photocatalytic performance (Figure 3a). The results indicated that CQDs in the composite acted as electron transfer mediators, significantly enhancing conductivity, and exhibited photocatalytic H₂O₂ production of 1036 μmol·g⁻¹·h⁻¹, which was 4.6 times greater than the activity of a pure CTF (Figure 3b,c).

In contrast, Chen et al. [31] developed the AA-stacked CTF-1@rGO composite, which exhibits exceptional solvent stability, using a two-step synthesis approach (first, monomers are pre-assembled on the rGO surface, followed by gas–solid interface catalytic synthesis) (Figure 3d). The addition of rGO improved the crystallinity of the composite material and enhanced its structural stability during stacking. The CTF-1@15mgrGO catalyst demonstrated consistent photocatalytic activity over 50 cycles, indicating superior durability when compared to the CTF-1-AA alone (Figure 3e).

2.4. Single Atoms

Single-atom catalysts (SACs) offer numerous active sites and achieve nearly perfect atomic utilization (close to 100%). Additionally, they can be immobilized on photocatalyst surfaces, facilitating the high-efficiency dissociation of photogenerated carriers, which makes them a key area of study in photocatalysis [32,33,34]. However, SACs have high surface free energy, causing them to aggregate, which reduces their catalytic performance. To counteract this challenge, nitrogen-enriched CTFs have emerged as promising materials to improve the dispersion and stability of SACs. In recent years, various single atoms, such as Pt [35], Fe [36], Ni [37,38], Pd [39], Ir [40], Cu [41], Co [42], Ti [43], and others, have been successfully anchored onto CTFs for various photocatalytic tasks.

Zhu et al. [44] pioneered the synthesis of Py-CTF, a material rich in pyridine nitrogen sites, using a ligand engineering approach. Subsequently, under the combined influence of two nitrogen species and a confined channel structure, Co atoms were anchored as single atoms onto Py-CTF nanosheets (Co_SA/Py-CTF). This composite showed remarkable photocatalytic activity for H₂O₂ production (2898.3 μmol·g⁻¹·h⁻¹). Furthermore, the apparent quantum yield (AQY) reached its maximum value of 13.2% at 420 nm (Figure 4a,b). Significantly, in situ XAFS experiments tracked the dynamic changes in the coordination structure of Co sites during photocatalysis. Upon exposure to light and O₂ adsorption, the Co-N₃ sites in Co_SA/Py-CTF were altered, with the Co-N bonds stretching and the coordination number decreasing. As a result, Co single atoms were released and formed transient atomic pairs with neighboring Co atoms, promoting the conversion of the O₂ adsorption configuration from the Pauling type to the Yeager type, thereby triggering a one-step, two-electron oxygen reduction reaction. Following the reaction, the Co atom pairs returned to their original single-atom form, restoring the catalyst (Figure 4c–g).

Meanwhile, Liu et al. [45] modified the synthesis of a triazine-deficient CTF by substituting part of 1,4-dicyanobenzene with 4-cyanopyridine. They subsequently incorporated Ni single atoms, creating a covalent triazine framework (d-CTF-Ni) with both Ni and pyridine N acting as dual active sites (Figure 4h). In this structure, Ni single atoms function as centers for electron capture, aiding the O₂ reduction process, while the pyridine N, owing to its electronic deficiency, serves as hole acceptors to enhance the water oxidation reaction. The combined effect of these two processes greatly boosts the photocatalytic generation of H₂O₂, leading to superior activity of the composite material (Figure 4i–k).

2.5. Compounded with Metal Oxides

TiO₂, a prominent member of the metal oxide group in semiconductor photocatalysis, has attracted significant attention due to its exceptional chemical stability, non-toxic nature, resistance to corrosion, and cost-effectiveness. The semiconductor characteristics of TiO₂ allow it to capture light energy within a particular wavelength range, facilitating the excitation of electrons from the valence band to the conduction band, which in turn drives surface redox reactions. However, photocatalytic processes involving TiO₂ are hindered by several issues, such as its large bandgap, slow charge carrier transfer rate, and limited photocatalytic efficiency. To address these challenges, researchers have explored combining TiO₂ with covalent triazine frameworks as a support material, resulting in composites that have been successfully used in a variety of photocatalytic applications [46,47,48]. For instance, a bifunctional heterojunction was created by incorporating TiO₂ particles onto CTF sheets using a simple impregnation method, with the goal of improving both the adsorption and photocatalytic degradation of ciprofloxacin [49]. In contrast to the broad bandgap (3.20 eV) of pure TiO₂, the bandgap of this composite material was significantly reduced to 2.86 eV, leading to enhanced photocatalytic performance. The CT-2 composite (with a CTF/TiO₂ mass ratio of 1:2) achieved a 77.5% photodegradation rate for ciprofloxacin within 120 min, while the CT-2/H₂O₂ system boosted the rate to 90.7% in just 40 min (Figure 5a,b). An S-scheme heterojunction was formed between a CTF and TiO₂, based on their respective energy band structures. Under visible light exposure, electrons and holes accumulated on the CTF and TiO₂, respectively, causing band bending, which, in combination with H₂O₂, accelerated ciprofloxacin degradation (Figure 5c). In another investigation, an AA-stacked CTF-TiO₂ heterojunction (CTF-AA/TiO₂) was fabricated for photocatalytic CO₂ reduction [50]. Compared to AB-stacked materials, the CTF-AA/TiO₂ showed a stronger photocurrent response, a faster charge transfer rate at the interface, and more efficient electron–hole separation, ultimately demonstrating superior CO₂ reduction capability.

ZnO is another well-known metal oxide. Despite its inherently large bandgap, which restricts its activation to ultraviolet light, it not only acts as an oxidizing photocatalyst (OP) but also serves as an excellent electron-donating photocatalyst [52,53]. Building on this, Xia et al. [51] endeavored to create an S-scheme heterojunction made of ZnO and a CTF to enhance photocatalytic H₂O₂ production (Figure 5d). Their study showed that in situ XPS measurements indicated a reversal of the peak shift direction before and after exposure to light. Under dark conditions, ZnO absorbs electrons, but when exposed to light, it releases them, providing solid proof of the formation of the S-scheme heterojunction (Figure 5e–g). The resulting composite demonstrated remarkable photocatalytic H₂O₂ generation, achieving 12,000 μmol·g⁻¹·h⁻¹, a performance 10.3 times superior to that of ZnO material and 164 times greater than that of the CTF (Figure 5h).

2.6. Compounded with Metal Sulfides

A widely studied composite material based on a CTF includes its combination with metal sulfides [54,55,56]. By incorporating MoS₂ quantum dots into CTFs, the photocatalytic hydrogen production rate can be enhanced, reaching a value up to 8 times higher than that of the unmodified material [57]. However, increasing the MoS₂ concentration beyond a certain point leads to the obstruction of active sites, thus reducing photocatalytic efficiency. Huang et al. [58] employed a straightforward photoreduction technique to integrate CdS quantum dots with CTF-1. They observed that the favorable band alignment between the two materials facilitated the formation of a type-II heterojunction, which significantly improved the separation and migration of photogenerated charge carriers. Under visible light exposure, the CdS/CTF-1 composite showed a photocatalytic hydrogen evolution rate that was 55 times greater than that of the bare CTF material. Zhang et al. [59] further developed a photocatalyst with a core–shell structure, where CdS nanospheres formed the core, enveloped by a CTF layer (Figure 6a). The electron transfer mechanism facilitated rapid recombination of conduction band electrons from the CTF component and valence band holes from the CdS, generating carriers with superior redox potential and resulting in the formation of an S-scheme heterojunction. Additionally, the porous CTF shell significantly enhanced the composite’s CO₂ adsorption capacity. As a result, the photocatalytic CO₂ reduction reaction achieved an R_electron value of 389.22 μmol·g⁻¹·h⁻¹, which was 2.57 times and 4.74 times greater than those of the individual CdS and CTF components, respectively (Figure 6b–e).

Zhou et al. [60] investigated the use of small NiS nanoparticles and successfully developed a photocatalyst featuring a p-n heterojunction (NiS/CTF-ES₂₀₀) via a simple and environmentally friendly photodeposition technique. This method involved embedding NiS, a material with a narrow band gap and high electrical conductivity, onto CTF-ES₂₀₀ (Figure 6f). The creation of the p-n heterojunction triggered the buildup of photo-generated electrons in the conduction band of CTF-ES₂₀₀, which in turn preserved a strong reduction potential for the hydrogen evolution reaction and markedly improved the efficiency of photoelectron usage. Furthermore, EXAFS analysis confirmed the formation of Ni-N bonds within the composite, which promoted more efficient transmission of interfacial charge carriers (Figure 6g,h). These synergistic effects resulted in the NiS/CTF-ES₂₀₀ composite demonstrating remarkable photocatalytic hydrogen production activity, with a rate of 22.98 mmol·g⁻¹·h⁻¹, greatly surpassing that of the pure CTF material (Figure 6i,j).

3. Photocatalytic Application of CTF-Based Composites

CTFs have emerged as prominent materials in semiconductor photocatalysis due to their distinctive and stable structure, high nitrogen content, and adaptable functional groups. In recent times, CTF-based composites have found extensive use in various photocatalytic applications. The purpose of this section is to review the advancements in the development and utilization of CTF-based composites in several photocatalytic fields, including hydrogen production, oxygen evolution, H₂O₂ synthesis, CO₂ reduction, and pollutant remediation.

3.1. Photocatalytic Hydrogen Evolution

Hydrogen is acknowledged as one of the most high-potential clean and sustainable energy solutions, significantly contributing to alleviating energy crises and environmental pollution. Under the action of a catalyst, utilizing solar energy to complete the reduction reaction of hydrogen ions to hydrogen gas in water is considered a promising method for hydrogen production. C₃N₄ has been extensively studied as a photocatalyst for hydrogen generation, and CTF materials, with their abundant triazine units, share similar properties to C₃N₄ [61,62]. A substantial body of research has explored the use of CTF-based composites for photocatalytic hydrogen production [63,64,65,66,67,68,69].

Table 1. Performance comparison of CTF-based composites for photocatalytic hydrogen production.

Photocatalyst	Cocatalyst	Sacrificial Reagent	Light Source	HER (μmol g−1·h−1)	AQE (%)	Ref.
TAPT-COF-CQDs	Pt (3 wt%)	TEOA	300 W Xe lamp (λ > 420 nm)	69,570	9.5 (440 nm)	[63]
PtSA@S-TFPT	/	TEOA	300 W Xe lamp (λ > 420 nm)	11,400	4.65 (420 nm)	[64]
CTF-Cu2O@NC	Pt (3 wt%)	TEOA + MeOH	300 W Xe lamp (λ > 420 nm)	15,645	1.67 (420 nm)	[65]
Co1-PCTF	/	TEOA	300 W Xe lamp (λ > 420 nm)	2562.4	10.22 (365 nm)	[67]
TMT-BO-COF	Pt (5 wt%)	AA	300 W Xe lamp	23,700	0.11 (420 nm)	[68]
Pt@CTF-Py-1	/	TEA	AM 1.5 G	14,960	4.51 (420 nm)	[69]

shows the comparison of different CTF-based composites in photocatalytic hydrogen evolution reactions.

For example, Chen et al. [64] developed a photocatalyst called PtSA@S-TFPT, incorporating low-valent platinum single atoms into a sulfur-containing COF. During synthesis, Pt was reduced to its low-valent state using methanol, forming four stable asymmetric coordination bonds with sulfur/oxygen sites, which prevented particle aggregation. Under the illumination of a 300 W Xenon lamp, with triethanolamine as a sacrificial agent, PtSA@S-TFPT demonstrated excellent stability during three 3 h cyclic hydrogen evolution reaction tests, achieving an average hydrogen production rate of 11.44 mmol·g⁻¹·h⁻¹. At 420 nm, the material exhibited a maximum apparent quantum yield of 4.65%. Electrochemical analysis showed that PtSA@S-TFPT exhibited a higher photocurrent and lower impedance and reduced photoluminescence intensity, suggesting that the presence of Pt single atoms significantly enhanced the migration and separation of photo-generated carriers, boosting catalytic activity.

Pyridine can be considered a structure where one carbon atom in the benzene ring is replaced by nitrogen. Due to the higher electronegativity of nitrogen, electrons accumulate at the nitrogen atom. This gives pyridine a strong coordination ability, making it an effective carrier for metal atoms to anchor. Based on this idea, Yao et al. [67] developed a pyridine-enriched CTF modified with Co single atoms (Co₁-PCTF), which served as an efficient catalyst for photocatalytic hydrogen production. For this, 2,6-pyridine dicarbonitrile (2,6-DCPY) was chosen as the monomer for polymerizing PCTF, providing abundant nitrogen sites to anchor Co single atoms (Figure 7a). XAFS analysis was employed to examine the valence state and coordination environment of Co atoms in the Co-CTF composite material. XANES results showed that the Co absorption edge in Co₁-PCTF was positioned higher than that of Co foil, but lower than that of CoO, indicating an oxidation state of 0 to +2 (Figure 7b). EXAFS data revealed that no Co-Co peaks were observed in Co₁-PCTF, but a distinct peak at 1.68 Å was attributed to the Co-N bond. Fitting analysis indicated that Co atoms in the material formed coordination bonds with three surrounding nitrogen atoms (Figure 7c,d). Under visible light, using TEOA as a sacrificial agent for photocatalytic hydrogen evolution, the hydrogen evolution rate of Co₁-PCTF was 2562.4 μmol·g⁻¹·h⁻¹, several times higher than that of other pure and composite materials (Figure 7e). The material’s AQY peaked at 10.22% at 365 nm, displaying significant wavelength dependence (Figure 7f). In terms of stability, the material maintained excellent hydrogen evolution performance after six consecutive 4 h cycles, with only a 4.86% decrease in efficiency (Figure 7g). Furthermore, the authors synthesized a series of transition metal single-atom-modified M₁-PCTFs (Figure 7h). Among them, Co₁-PCTF demonstrated the best photocatalytic hydrogen production rate, attributed to the unsaturated d orbitals of Co, efficient electron capture, and its unique coordination bond with pyridine nitrogen.

3.2. Photocatalytic Production of Hydrogen Peroxide

The conventional method for producing hydrogen peroxide (H₂O₂) in industry is the anthraquinone oxidation process. However, this technique often comes with significant risks and generates numerous by-products that negatively impact energy consumption, the environment, and other factors. As an alternative, using light energy to drive redox or water oxidation reactions has gained attention as a promising approach for H₂O₂ production. Given the outstanding performance of CTF materials in photocatalytic hydrogen production, some researchers are now exploring their potential for photocatalytic H₂O₂ production [70,71,72,73,74].

Table 2. Performance comparison of CTF-based composites for photocatalytic H₂O₂ production.

Photocatalyst	Cocatalyst	Sacrificial Reagent	Light Source	H2O2 Yields (μmol g−1·h−1)	AQE (%)	Ref.
F-ol-COF	/	HMF	300 W Xe lamp (λ > 420 nm)	12,558	13.2 (420 nm)	[70]
TAPT-TFPA COFs@Pd IC	/	EtOH	300 W Xe lamp (λ > 400 nm)	2143	6.5 (400 nm)	[71]
CDs@CTFs CsCl	/	NaIO3	AM 1.5 G	2464	13.0 (500 nm)	[72]
CNT@COF-H	/	IPA	AM 1.5 G	1581	34.0 (500 nm)	[73]
TF50-COF	/	EtOH	300 W Xe lamp (λ > 400 nm)	1739	5.1 (400 nm)	[74]

shows a comparison of different CTF-based composites in photocatalytic H₂O₂ production reactions.

Zhang et al. [70] developed CTF materials with varying degrees of F substitution for use in photocatalytic H₂O₂ production and the oxidation of 5-hydroxymethylfurfural (HMF) to create functionalized furans. The introduction of F atoms in the material can form a p-π conjugated structure with the olefin bond, which improves the charge separation. H-ol-CTF, used as a control, was synthesized using 2,4,6-trimethyl-1,3,5-triazine (TMTA) and terephthalaldehyde (TA). The introduction of the F atom was performed by substituting TA with 2,3,5,6-tetrafluoroterephthalaldehyde (TFTA), resulting in F-ol-CTF. The synthesis process involved reacting TA and TFTA in a 1:1 molar ratio with TMTA to create partially fluorinated HF-ol-CTF (Figure 8a). F-ol-CTF exhibited the lowest photoluminescence intensity and the longest charge carrier lifetime among the three materials (Figure 8b,c), demonstrating its superior charge separation and transfer properties. These characteristics contributed to its highest photocatalytic H₂O₂ production activity of 12,558 µmol·g⁻¹·h⁻¹, surpassing that of HF-ol-COF (9382 µmol·g⁻¹·h⁻¹) and H-ol-CTF (6373 µmol·g⁻¹·h⁻¹), along with excellent apparent quantum yield and stability. Furthermore, the conversion rate of HMF to functionalized furan was significantly higher with this material than with H-ol-COF and HF-ol-COF, reaching 95% after 12 h (Figure 8d–g).

In a similar approach, Liu et al. [71] introduced a fluorinated CTF material for H₂O₂ production, which also featured Pd metal clusters (TAPT-TFPA COFs@Pd ICs) embedded within the material’s pores (Figure 9a). TEM and HRTEM analysis revealed that the Pd ICs had an average size of 2.4 nm, which closely aligned with the pore dimensions of the CTF material (Figure 9b,c). The photocatalytic H₂O₂ production activity of the material was tested under visible light irradiation, and the results showed that the material with 3% Pd content provided the highest H₂O₂ generation rate (2143 μmol·g⁻¹·h⁻¹). Compared with unfluorinated materials, the fluorinated CTF had better catalytic activity. Notably, the photocatalytic efficiency of the composite material was maintained for over 100 h, significantly outperforming other reported photocatalysts (Figure 9d–g). The results of the density of states (DOS) calculations explained the role of Pd in photocatalytic reactions. In the fluorinated COF material, the Pd d-band centers were lower than those observed in the non-fluorinated COF material (Figure 9h,i). These lower d-band centers were linked to weaker binding energies between the Pd IC and intermediate oxygen, which facilitated the easier desorption of the products (Figure 9j).

3.3. Photocatalytic Carbon Dioxide Reduction

Excessive carbon dioxide emissions have emerged as a key contributor to global climate change, exacerbating the greenhouse effect, accelerating global warming, frequent extreme weather events, and ecosystem imbalance, all of which pose serious threats to both human society and the natural environment. Photocatalytic CO₂ reduction presents a promising solution, as it effectively lowers atmospheric CO₂ levels, mitigates the greenhouse effect, and generates alternative fuels, facilitating the shift towards a more sustainable energy system. Covalent triazine frameworks are characterized by a large specific surface area and abundant nitrogen sites, which are advantageous for CO₂ adsorption. However, as organic polymers, CTFs alone often show limited photocatalytic CO₂ reduction efficiency due to their relatively high exciton binding energy [75]. Therefore, researchers have constructed CTF-based composite materials to improve charge transfer and photocatalytic performance [76,77,78,79,80,81,82,83,84,85].

Table 3. Performance comparison of CTF-based composites for photocatalytic CO₂ reduction.

Photocatalyst	Photosensitizer	Sacrificial Reagent	Light Source	CO2RR Activity	Selectivity (%)	Ref.
CTF-240	/	/	300 W Xe lamp (λ > 420 nm)	118.69 μmol·g−1·h−1 (CO)	97.25	[76]
CTFB10	/	TEA	300 W Xe lamp (λ > 420 nm)	27.4 μmol·L−1 (CH4)	90.3	[77]
Br-COFs@BiOCl	/	/	300 W Xe lamp (λ > 320 nm)	27.4 μmol·g−1·h−1 (CO)	≈100	[78]
CTF/Bi19S27Br3	/	/	AM 1.5 G	572.2 μmol·g−1·h−1 (CO)	99.9	[79]
Fe-COF	/	/	300 W Xe lamp (320 < λ < 780 nm)	992 μmol·g−1·h−1 (CO)	≈100	[80]
SCTF/ZnIn2S4	/	FFA	AM 1.5 G	207.8 μmol·g−1·h−1 (CO)	≈100	[81]
DA-CTF@DPT-Co	/	TEOA	300 W Xe lamp (λ > 420 nm)	724 μmol·g−1·h−1 (CO) 695 μmol·g−1·h−1 (H2)	/	[82]
Ni-PT-CTF	/	TEOA	300 W Xe lamp (λ > 420 nm)	784.5 μmol·g−1·h−1 (CO)	96.6	[83]
TPy-COF-Co	[Ru(bpy)3]Cl2	TEOA	300 W Xe lamp (λ > 420 nm)	426,000 μmol·g−1·h−1 (CO) 343,000 μmol·g−1·h−1 (H2)	/	[84]
Cu2O/SnO2/CTF	/	AA	250 W high pressure Hg discharge lamp	40.33 μmol·g−1·h−1 (CO)	≈100	[85]

shows a comparison of different CTF-based composites in photocatalytic CO₂ reduction reactions.

Li et al. [81] fabricated spherical SCTF cores by reacting cyanuric chloride and thiocyanuric acid, then coating these cores with ZnIn₂S₄ nanosheets using a low-temperature hydrothermal process, resulting in a core–shell SCTF/ZnIn₂S₄-x structure (Figure 10a). SEM and TEM images reveal that SCTF has a spherical shape with a diameter of around 1 μm, while ZnIn₂S₄ tends to form aggregates. In the composite, SCTF serves as a substrate for the growth of ZnIn₂S₄, with the ZnIn₂S₄ nanosheets forming a layer on the SCTF surface (Figure 9b–d). This composite material creates an S-scheme heterojunction due to its well-matched band structure. When exposed to light, electrons accumulate in the SCTF core, and holes accumulate at the ZnIn₂S₄ shell, with each component participating in CO₂ reduction and furfuryl alcohol oxidation, respectively (Figure 10e). The photocatalytic activity of the composite material was evaluated under simulated sunlight (AM 1.5G filter). As shown in the star diagram, the composite significantly outperformed both SCTF and ZnIn₂S₄ in various aspects of the photocatalytic reaction (Figure 10f). The results demonstrate that the composite material achieved a CO yield of 263.5 µmol·g⁻¹ over a 6 h period in CO₂ reduction, which was substantially higher than the yields from the individual SCTF (37.1 µmol·g⁻¹) and ZnIn₂S₄ (49.6 µmol·g⁻¹) (Figure 10g). Moreover, the composite material showed a 95% conversion rate and nearly 100% selectivity in the oxidation of furfuryl alcohol to furfural (Figure 9h,i). Further analysis of reaction intermediates, pathways, and adsorption energies in both CO₂ reduction and furfuryl alcohol oxidation reactions led to the development of a comprehensive mechanism, as depicted in Figure 10j.

3.4. Other Photocatalytic Reactions

Researchers have also explored the use of CTF-based composite materials in additional photocatalytic processes, such as the oxygen evolution reaction (OER) [86,87,88,89] and pollutant degradation [90,91,92,93,94,95].

The OER, a key half-reaction in water splitting, is more complex than the hydrogen evolution reaction, as it simultaneously transfers four electrons, breaks O-H bonds, and forms O-O bonds. Given the excellent performance of ruthenium catalysts in water oxidation reactions and their extensive research, Salati et al. [86] proposed the combination of Ru-tda water oxidation catalysts with CTFs to form a binary composite material for photocatalytic oxygen production. The photoluminescence and time-resolved photoluminescence measurements showed that a pure CTF had weak electron transfer properties, whether or not electron acceptors were added (Figure 11a). In contrast, the Ru-CTF composite significantly reduced carrier recombination, with PL decay occurring faster than the instrument’s response time in the TRPL tests (Figure 11b). Photocatalytic OER tests were performed in a buffered aqueous solution with a pH of 7 with sacrificial agent added. The results revealed that the composite efficiently catalyzed water oxidation (Figure 11c), achieving maximum turnover frequencies of 17 h⁻¹ and turnover numbers around 220. It surpassed most of the currently reported CTF-based water oxidation catalysts in activity.

Wang et al. [94] focused on two key pollutants in wastewater, perfluorooctanoic acid and 2,4,6-trichlorophenol, by combining Ga₂O₃-Bi₄O₇ heterojunctions with a fluorinated CTF (F-CTF). The addition of F-CTF created numerous micropores, significantly improving the composite’s adsorption capacity. Electrochemical tests revealed that the excellent electron transport efficiency of F-CTF enhanced charge separation in the composite (Figure 11d,e). The composite’s ability to degrade mixed pollutants perfluorooctanoic acid and 2,4,6-trichlorophenol was then investigated, as shown in Figure 11f,g. The results demonstrated that GaBi/CTF5 (with 5% CTF by mass) achieved the highest activity, successfully degrading 93% of perfluorooctanoic acid and 100% of 2,4,6-trichlorophenol in just 90 min. Further experiments, including quenching and electron spin resonance, helped identify the reactive species involved in the degradation process, ultimately revealing the reaction mechanism shown in Figure 11h. The composite, with a conduction band potential of −1.41 eV (lower than −0.33 eV, which is the redox potential of O₂/·O₂⁻), can reduce O₂ to generate ·O₂⁻, which then produces ·OH. Meanwhile, the oxidation of pollutants is mediated by the holes present in the valence band.

4. Conclusions and Outlook

CTFs, porous organic polymers, offer distinct advantages in photocatalysis, owing to their triazine structure, stable π-conjugated system, large surface area, and excellent chemical stability. However, their standalone performance has limitations, making it necessary to combine CTFs with other materials to enhance their photocatalytic activity. This review outlines several methods for developing CTF-based composites and discusses their recent applications in photocatalysis. The progress in CTF-based composite materials is advancing quickly, and this paper provides a glimpse into future research directions in this area.

4.1. Superior Composite Materials

For further advancement in this field, it is essential to develop catalysts with enhanced performance. According to the requirements of photocatalytic reactions, optimization can be carried out from perspectives such as the light absorption range, charge carrier separation efficiency, and band structure, in order to design superior CTF-based composite materials.

4.2. Deeper Understanding of Reaction Mechanisms

Currently, research on the reaction mechanisms of CTF-based composite materials in photocatalysis remains insufficient, which is largely related to the complexity of the materials themselves. Various in situ characterization techniques, such as in situ XPS and in situ infrared spectroscopy, have been developed, greatly facilitating our understanding of the reaction process. Additionally, theoretical calculations can simulate the reaction process from a non-experimental perspective. The integration of multiple techniques helps to enhance our understanding of the reaction mechanisms of composite materials, thereby contributing to the development of more efficient catalysts.

4.3. Large-Scale Applications

At present, the application of CTF-based composite materials in the field of photocatalysis remains confined to laboratory stages and has not been implemented in large-scale industrial applications. The underlying reasons may involve the synthesis cost of the materials. However, for the development of this field, practicality is a crucial issue that must be addressed. Therefore, current research on CTF-based composite materials should take into account economic feasibility, aiming for large-scale application.

In conclusion, the research on CTF-based composite materials in the field of photocatalysis still holds significant potential, and it requires researchers to invest more time and effort. Based on the existing foundation, gradual improvements in this area of research are expected, and it is believed that more options will be provided for solar energy utilization in the future.