Constructing an Asymmetric Covalent Triazine Framework to Boost the Efficiency and Selectivity of Visible‐Light‐Driven CO2 Photoreduction

Abstract The photocatalytic reduction of CO2 represents an environmentally friendly and sustainable approach for generating valuable chemicals. In this study, a thiophene‐modified highly conjugated asymmetric covalent triazine framework (As‐CTF‐S) is developed for this purpose. Significantly, single‐component intramolecular energy transfer can enhance the photogenerated charge separation, leading to the efficient conversion of CO2 to CO during photocatalysis. As a result, without the need for additional photosensitizers or organic sacrificial agents, As‐CTF‐S demonstrates the highest photocatalytic ability of 353.2 µmol g−1 and achieves a selectivity of ≈99.95% within a 4 h period under visible light irradiation. This study provides molecular insights into the rational control of charge transfer pathways for high‐efficiency CO2 photoreduction using single‐component organic semiconductor catalysts.


Introduction
In recent years, fossil fuels have served as the primary energy source to meet the demands of global economic and social development.[3][4] Consequently, there is an urgent need to develop effective strategies or technologies to reduce atmospheric carbon dioxide levels and rebalance the carbon cycle.The photocatalytic reduction of carbon dioxide has emerged as a promising solution. [5,6]9][10] Researchers have explored transition metal complexes, [11] inorganic or hybrid semiconductors, [12,13] and metal-modified zeolites. [14]A notable example includes binary graphite carbon nitride (g-C 3 N 4 ). [15,16]owever, the inefficiency and the poor selectivity in reducing and converting inert CO 2 into CO remain challenges.Therefore, developing more effective and highly selective photocatalysts for CO 2 conversion is highly desirable.Metal-free catalysts exhibit enhanced stability due to their covalent bonding, and they are environmentally benign and cost-effective.Carrier separation commonly impedes photocatalyst efficiency.As a novel type of non-metallic materials, covalent organic frameworks, [19] especially covalent triazine frameworks (CTFs), have garnered significant attention in catalytic supports, [20] gas capture and storage, [21][22][23] energy conversion, [24] and catalysis. [25,26]However, in photocatalytic reduction, the presence of simple conjugated structures leads to rapid recombination of photoexcited carriers, thereby hindering the generation of excitons and subsequently reducing both selectivity and product yield.To overcome this challenge, researchers are focusing on constructing D-A systems to improve CTFs performance.Wang et al. devised a photosensitizing system incorporating electron D-A units using heptathiophene-based cucurbituril, thereby enhancing photocatalytic activity through enhanced separation and transfer of photoexcited carriers. [27]Likewise, Fan et al. documented benzodithiophene-based covalent triazine framework materials (BDT-CTFs), employing donor-acceptor motifs to adjust photocatalytic activity. [28]Tan et al. demonstrated that N-doped fluorene (carbazole), possessing the strongest electron-donating ability, achieved the highest photocatalytic performance among POPs.This highest performance can be attributed to the enhanced charge transfer efficiency and reduced charge recombination within these heteroatom-doped donoracceptor CTFs. [29]Jin et al. presented a photocatalytic electron transfer system, fabricating D-A1-A2 conjugated polymers for photocatalytic hydrogen evolution. [30]Zhang et al. developed asymmetric covalent triazine frameworks as highly efficient single-component semiconductor photocatalysts for organic transformations.The integration of four distinct molecular donor-acceptor modules enables intramolecular energy transfer cascades, leading to exceptional photocatalytic performance in organic transformations. [25]Nonetheless, significant advancements have been made in the prompt recombination of photoexcited carriers in D-A type covalent triazine frameworks.However, the utilization of single-component intramolecular energy transfer in D-A CTFs for carbon dioxide reduction remains unexplored.Thus, a thiophene-modified highly-conjugated asymmetric covalent triazine framework (As-CTF-S) was developed for this purpose.In our approach (Figure 1), 4-iodobenzonitrile and 4bromobenzeneboronic acid were coupled to synthesize 4-bromo-4-cyanobenzene.This intermediate was further coupled with 2cyanothiophene to create As-CTF-S.Under visible light exposure without photosensitizers or organic sacrificial agents, As-CTF-S showed exceptional photo-catalytic activity, achieving a rate of 353.2 μmol g −1 and a selectivity of ≈99.95% within 4 h.This study highlights the significance of asymmetry in enhancing charge separation efficiency in D-A CTFs and offers a strategic guideline for designing efficient photocatalysts.

Results and Discussion
The CTF was synthesized through a polymerization reaction using 5-(4′-cyano [1,1′-biphenyl]−4-yl) thiophen-2-cyano, named As-CTF-S (Detailed synthesis methods are described in the Experimental Section).The FT-IR spectra of the synthesized CTFs (shown in Figure 2a) revealed the disappearance of the C≡N stretching vibration at 2225 cm −1 from the monomers.Simultaneously, new stretching vibration bands for C═N at 1509 cm −1  and C─N at 1390 cm −1 emerged, indicating the successful formation of triazine units.These spectroscopic findings suggest that the poly-condensation of monomers led to the creation of the triazine framework structures. [31]Additionally, the identification of triazine units within CTFs was further supported by a chemical shift of 170 ppm observed in the solid-state 13 C-CPNMR spectrum (Figure 2b), corresponding to the triazine carbon.The other chemical shifts were assigned to the respective carbons in As-CTF-S.
The morphology of As-CTF-S was observed through SEM characterization, revealing sub-micron dimensions (Figure S1, Supporting Information).This feature is thought to stem from the relatively low degree of cross-linking in As-CTF-S.Thermogravimetric analysis (TGA), as shown in Figure S2 (Supporting Information), demonstrates that As-CTF-S maintains thermal stability up to ≈300 °C.The N 2 adsorption-desorption isotherms were conducted at 77 K.The isotherms of the As-CTF-S follow a complex type IV trend, exhibiting a steep increase at low P/P 0 values, followed by a continuous and monotonic increase showing an H3-type hysteresis loop.This complex trend exhibits a certain amount of micropores and mesopores and an external surface.As shown in Figure S3 (Supporting Information), the specific Brunauer-Emmett-Teller (BET) surface area of As-CTF-S is 17 m 2 g −1 .The NLDFT pore-size distribution profiles indicate that As-CTF-S possessed a micropore size of 2.0 nm and mesopore peaks centered at 16.0 nm.In addition, the CO 2 adsorption capacity of As-CTF-S was evaluated at 298 K.As shown in Figure S4 (Supporting Information), CO 2 uptake capacity for As-CTF-S reaches 8 cm 3 g −1 .The relatively low adsorption capac-ity may be due to the amorphous nature of the material.Powder X-ray diffraction (PXRD) patterns reveal the amorphous nature of As-CTF-S, marked by the lack of distinct crystalline peaks, as shown in Figure S5 (Supporting Information).This amorphous characteristic is corroborated by the typical diffraction patterns and is a contributing factor to its relatively low BET surface area.
The UV-vis diffuse reflectance spectra (DRS) were utilized to assess the absorption properties of As-CTF-S.Figure 3a shows that As-CTF-S captures a broad range of visible light, particularly in the 400-600 nm area.The optical bandgap (E g ) of As-CTF-S was determined using the Kubelka−Munk (K-M) method, as depicted in the inset of Figure 3a, revealing an E g of 1.9 eV.This relatively narrow bandgap enables more efficient photo-induced electron transitions.We conducted an in-depth analysis of the conduction band (CB) and valence band (VB) positions through Mott−Schottky plot measurements at 500, 800, and 1000 Hz frequencies, as shown in Figure 3b.These measurements determined that the CB of As-CTF-S is situated at −0.75 V versus NHE, while the VB of As-CTF-S lies at +1.15 V.This positioning indicates that the photogenerated electrons in As-CTF-S are suitable for the reduction of CO 2 to CO −0.53 V vs NHE), and the VB of As-CTF-S is adequately positive for oxidizing H 2 O to O 2 (+ 0.82 V vs NHE).The efficiency of charge separation in As-CTF-S is demonstrated by their photoelectric properties, where the photocurrent indicates the movement of generated charges under intermittent visible light exposure.As shown in Figure 3c, As-CTF-S produces a significant photo-current upon light exposure.Electrochemical impedance spectroscopy (EIS) analysis, presented in Figure 3d, was used to evaluate the electrical conductivity of CTFs.The Nyquist plot for As-CTF-S exhibits a diameter of only 60 Ω, indicating a high capacity for charge migration within the CTFs, which is advantageous for the photo-catalytic process.
The CO 2 photo-reduction activity was conducted under visible light irradiation ( > 420 nm) at room temperature using the Labsolar-6A All glass automatic online trace gas analysis System from Beijing PerfectLight.We employed the prepared sample as the photocatalyst in a gas-solid reaction mode with water vapor, without the use of sacrificial agents, organic solvents, or photosensitizers.Gas chromatography was utilized to identify the main reduction products.As-CTF-S demonstrated superior photocatalytic performance, achieving the highest activity of 353.2 μmol g −1 and a selectivity of ≈99.95% within a 4 h period (Figure 4a).The exceptional photocatalytic activity and selectivity of As-CTF-S can be attributed to the incorporation of the thiophene unit into the triazine polymerization network.This modification broadens the light absorption spectrum of the catalyst and promotes an asymmetric structure, enhancing photoinduced electron separation and transfer.To further investigate the mechanism behind photo-induced CO 2 reduction, control experiments were conducted using As-CTF-S as the photocatalyst.In these experiments, the absence of CO 2 , the catalyst, water vapor, and light led to no detectable products (Figure 4b), underscoring that As-CTF-S effectively facilitates CO 2 conversion under mild conditions with high selectivity for converting gaseous CO 2 to CO.The produced CO was confirmed to result from CO 2 reduction rather than from the catalyst or reaction equipment.In addition, the photocatalytic cycling stability was evaluated by cyclic experiments.After six continuous cycles, the conversion yield for CO was still maintained up 95%, suggesting the good stability of As-CTF-S.Furthermore, to investigate the long-term stability of the catalyst's performance, we carried out a continuous catalytic test for 40 h (Figure S13, Supporting Information).The results indicated that the catalytic performance remained stable without significant decay.The results indicated that the catalytic performance remained relatively stable, with no significant decay detected.FT-IR spectra analysis before and after the reaction revealed no significant changes, indicating that As-CTF-S maintains a stable chemical structure within the conjugate network (Figure 4d).Furthermore, As-CTF-S was compared with similar chemical structure CTF photocatalysts for CO 2 reduction, as outlined in Table S1 (Supporting Information).As-CTF-S not only surpasses similar CTF photocatalysts in CO 2 reduction in terms of both performance and selectivity but also outperforms most comparable catalysts currently reported in the literature.
X-ray Photoelectron Spectroscopy (XPS) results (Figures 5a,b) provided insight into the XPS survey spectrum for As-CTF-S, revealing the presence of C, N, and S elements.The S 2p spectrum of As-CTF-S showed binding energies at 163.4 and 164.4 eV, corresponding to the S 2p3/2 and S 2p1/2 states of sulfur within the thiophene units (Figure 5c).The high-resolution N1s spectra of these polymers exhibited a binding energy of 397.7 eV, indicating the presence of triazine rings (C═N─C) in CTFs (Figure 5d).To delve deeper into the photoelectron transfer process in As-CTF-S, XPS analyses under illumination were performed to observe changes in the binding energies of different elements with and without light exposure.An observed increase in binding energy under light suggests a reduction in electron cloud density around specific atoms or groups, indicative of electron transfer.Notably, the binding energy of the S 2p peak increased under visible light, while the N1s peak's binding energy decreased, suggesting electron donation by sulfur atoms and acceptance by nitrogen atoms due to this structural effect. [25,31,32]hotoluminescence (PL) spectra, with an excitation wavelength of 470 nm, were used to monitor electronic recombination.The results indicated weak fluorescence emission, suggesting minimal photo-electron recombination (Figure 6a).Timeresolved fluorescence spectra at steady-state emission peaks yielded an average irradiation lifetime of 2.59 ns (Figure 6b), implying that an extended fluorescence lifetime is beneficial for photocatalytic CO 2 reduction.
The photo-oxidation and reduction capabilities of CTF are significantly affected by its electronic structure.In a donor-acceptor (D-A) system, the electron-donating unit influences the highest occupied molecular orbital (HOMO) energy level, while the acceptor impacts the lowest unoccupied molecular orbital (LUMO) energy level.Density functional theory (DFT) analysis showed that M4 has the highest HOMO level, while M1 has the highest LUMO level.A decrease in HOMO levels was observed in other D-A models as the presence of the thiophene ring decreased, making M4 the most potential electron donor (Figures 7a,b).Furthermore, calculations revealed that M4 has the narrowest HOMO-LUMO energy gap (E g ), indicating that electron transitions within it are more readily facilitated.Kinetic competition experiments with methylbenzonitrile and thiophene-2-carbonitrile in a 1:1 ratio led to the formation of four D-A model combinations.These experiments ranked the likelihood of forming these combinations as M'2 > M'3 > M'1 > M'4.Combining insights from the literature, theoretical analyses, and experimental results, it is clear that M'2 demonstrates a substantial driving force for the photocatalytic reduction of CO 2 , promoting quick charge separation within the material (Figure S6, Supporting Information).
The asymmetric structure, which incorporates four donoracceptor (D-A) sub-units, promotes the efficient separation and transfer of charges.This mechanism ensures that the photogenerated electrons are directed toward the triazine ring.Due to its inherent electronic structure that absorbs well, the triazine ring facilitates the conversion of CO 2 to CO through electron transfer at the active site of a nitrogen atom. [33]Additionally, the nitrogen atom within the triazine ring interacts with polarizable CO 2 molecules via dipole-quadrupole interactions, [34] effectively capturing and concentrating negative electrons.The photocatalytic reduction of CO 2 to CO occurs in two primary stages.First, the abundant holes on the thiophene and benzene rings oxidize H 2 O to O 2 , producing protons (H 2 O → 1/2 O 2 + 2H + + 2e − ).Then, the generated protons combine with CO 2, and the photo-generated electrons collected at the active site of the nitrogen atom aid in CO formation (CO 2 + 2H + + 2e − → CO + H 2 O) [35,36] (Figure 8).Because of the amorphous porous As-CTF-S, the photocatalytic reaction may take place in both internal pore space and external surfaces.

Conclusion
In summary, As-CTF-S has emerged as an innovative photocatalyst, designed specifically for the reduction of CO 2 to CO under visible light exposure.Impressively, As-CTF-S delivers exceptional photo-catalytic performance, achieving a production rate of 353.2 μmol g −1 within a 4 h timeframe and showcasing a remarkable selectivity of ≈99.95%.This high level of efficiency is attained without reliance on photosensitizers or organic sacrificial agents.The development of a single-component system utilizes donor-acceptor (D-A) models to enhance the combination of energy levels and improve charge separation efficiency.This study provides a promising path for the creation of new D-A type CTFs as highly effective heterogeneous catalysts for the selective photoreduction of CO 2 under the gaseous phase with no toxic sacrificial agent.Further investigation of novel D-A CTFs with various donor-acceptor configurations for photocatalytic application is in progress.

Synthesis of 5-(4′-cyano[1,1′-biphenyl]−4-yl) Thiophen-2-Cyano (TC):
Under argon atmosphere, 2-Cyanothiophene (1.09 g, 10 mmol) and BC (1.29 g, 5 mmol) were reacted at 150 °C for 24 h in a flask containing N, N dimethylacetamide (12 mL), potassium acetate (1.47 g, 15 mmol) and Palladium acetate (0.002 g, 0.01 mmol).After the reaction was over, the resulting mixture was cooled to room temperature.The resulting solid was filtered to a given yellow-green crude solid, which was washed with deionized water 3-6 times to obtain TC (Gray-green powder, 1.5 g, 88.2%). 1  Polymerization Reaction of Monomer: Using TC (1.50 g, 3.5 mmol) as the polymerization monomer and trifluoromethanesulfonic acid as the catalyst, the synthesis of As-CTF-S was conducted under a 100 °C temperature and argon atmosphere.After the reaction was completed, cool it to room temperature.Subsequently, wash it with deionized water first, followed by washing with ethanol and ethyl acetate three -six times.The obtained samples should be soaked in a diluted ammonia aqueous solution overnight.The obtained sample was dried at room temperature under vacuum conditions to obtain As-CTF-S as the final product (Brown powder, 1.41 g, 94%).Elemental analysis, calculated values (%): C, 75.50;N, 9.78; S, 11.20; experimental values (%): C, 73.98; N, 9.83; S, 11.55.
Characterization: Powder X-ray diffraction (XRD) was collected by Rigaku Ultima IV diffractometer (CuK radiation,  = 1.5406Å).Thermogravimetric (TG) curves were performed on a NETZSCH 449C thermal analyzer with a heating rate of 10 °C min −1 under air atmosphere.The UVvis diffuse reflection spectroscopy (DRS) was taken using Shimadzu UVvis spectrophotometer (2550, Japan).Nitrogen adsorption/desorption isotherms were performed on Microtrac BEL BELSORP-max.The specific surface areas were evaluated by the Brunauer-Emmett-Teller (BET) method and pore size distributions were tested using the density function theory (DFT) method.X-ray photoelectron spectroscopic (XPS) was investigated by an ESCALAB 250 with a monochromatic Al K X-ray source.Transient photocurrent measurements, electrochemical impedance spectroscopy (EIS), and Mott-Schottky analyses were performed using an electrochemical workstation (CHI660E).
Photoelectrochemical Measurements: The photocatalytic reaction of CO 2 reduction was performed in a 120 mL reactor cell with 1 atm CO 2 partial pressure.The reaction temperature was kept at 30 °C by a circulating water bath.During the experiments, 5 mg of the catalyst was added to 1 mL of methanol and was sonicated for 10 min.The solution was dispersed on a 35 mm Petri dish and then dried under a vacuum.The deionized water (1 mL) was added to the bottom of the reactor and the Petri dish was fixed above the water.As a low boiling-point liquid, methanol can easily generate a film of the sample on the petri dish.Before illumination, the atmosphere in the reactor was exchanged with high-purity CO 2 gas several times and then the reactor was tightly closed.A 300 W Xe lamp with a 420 nm cutoff optical filter was used as the light source and irradiated from the front of the reactor to start the CO 2 reduction reaction.After the reaction, the amount of CO was analyzed and quantified by gas chromatography with a detector of Shimadzu GC-2014 FID, TCD, with argon as the carrier gas.All photoelectrochemical experiments were performed on an electrochemical workstation (CHI660E) containing a three-electrode system with a 0.2 m Na 2 SO 4 aqueous solution as an electrolyte.The samples were coated on fluorine-doped tin oxide (FTO) glass (1 cm 2 ) and were employed as the working electrode and Ag/AgCl and platinum wire as the reference electrode and the counter electrode, respectively.

Figure 1 .
Figure 1.Synthesis routes of the As-CTF-S: a) The synthesis of two monomers.b) Polymerization reaction of monomer.

Figure 2 .
Figure 2. a) FTIR spectra of As-CTF-S and monomer.b) Solid state 13 C CPNMR spectra of As-CTF-S.

Figure 3 .
Figure 3. a) UV-vis spectra of As-CTF-S.b) Mott-Schottky plots of As-CTF-S in 0.2 M Na 2 SO 4 aqueous solution.(Inset: the energy diagram of the CB and VB levels of As-CTF-S).c) Photocurrent transient response of As-CTF-S.d) Electrochemical impedance spectroscopies of As-CTF-S.

Figure 4 .
Figure 4. a) Time-course plots of products with As-CTF-S as the photocatalyst.b) Control experiments of CO 2 photoreduction.c) Recycling performance of As-CTF-S.d) The FTIR spectra of As-CTF-S before and after the photoreduction.

Figure 5 .
Figure 5. a) The XPS survey spectra of As-CTF-S, b) High-resolution C 1s XPS spectrum.c) High-resolution S 2p XPS spectrum.and d) High-resolution N1s XPS spectrum.

Figure 6 .
Figure 6.a) The photoluminescence (PL) emission of As-CTF-S excited at 470 nm.b) Time-resolved PL spectra of As-CTF-S.

Figure 7 .
Figure 7. a) The frontier orbital distribution of model molecules.b) The HOMO/LUMO values calculated at the B3LYP/6-31G (d, p) level.

Figure 8 .
Figure 8.The proposed mechanism of photocatalytic CO 2 reduction for As-CTF-S.