Thiazolo[5,4-d]thiazole-Based Covalent Organic Frameworks for the Rapid Removal of RhB

Abstract

Two thiazolo[5,4-d]thiazole(TzTz)-based donor(D)-acceptor(A) COFs (TPTZ-COF and TBTZ-COF) are synthesized for the photocatalytic degradation of RhB in water. The D-A structure COFs with the weak deficient electron TzTz as an acceptor promotes the separation of photo-generated charge carriers. The rigid structure of TzTz can enhance the π–π and dipole–dipole interactions, which improve the mobility of charge carriers and result in the enhancement of the photocatalytic performance of COFs. Optical and electrical tests show that TBTZ-COF has more efficient electron–hole separation and transfer performance and degrades 99.76% of RhB (10 mg/L) in 60 min under visible light irradiation, while the hydroxyl groups on the TPTZ-COF surface enable the formation of a large number of hydrogen bonds with RhB, so TPTZ-COF exhibits excellent adsorption ability for RhB. Furthermore, TBTZ-COF maintains high photocatalytic activity after five consecutive cycles, making it a promising photocatalyst for the rapid removal of RhB.

1. Introduction

Organic pollutants in water bodies, particularly persistent organic pollutants [1], have significant consequences for the ecosystem and human survival due to their high solubility, high toxicity, difficulties in natural degradation, and long-term existence. RhB is a typical organic pollutant, widely used as a colorant in many industries such as dyeing, paper, cosmetics, leather, etc. [2]. Removing it from contaminated waters through green catalysis is a current issue that needs to be addressed [3]. The application of photocatalysis as an economical, efficient, and environmentally friendly method for the degradation of contaminants in water has long been reported in much of the literature [4,5]. Various photocatalysts have been developed, e.g., conventional inorganic semiconductors, two-dimensional materials, polymer network semiconductors, and metal–organic frameworks [6]. Covalent organic frameworks (COFs) are innovative polymers formed from designable organic building blocks by creating strong covalent bonds [7] and have the advantage of large specific surface area, permanent pore structure, high porosity, and chemical and thermal stability [8]. The periodic π-conjugated aromatic units in COFs provide pathways for electron (e⁻) -hole (h⁺) transport, making them up-and-coming candidates for photocatalytic activities. So far, many reports using COFs as photocatalysts for RhB degradation have surfaced [9,10]. Regrettably, high exciton binding energy and low carrier mobility are two drawbacks of using COFs as photocatalysts, thus limiting their efficacy in dye degradation. Nonetheless, these impediments can be effectively alleviated by constructing D-A structure COFs [11].

The ingenious combinations and ordered arrangements of D and A units can form spatially separated D-A columns of intrinsic heterostructures in COF [12], promoting charge carrier separation upon photoexcitation and providing a favorable pathway to improve photocatalytic activity [13]. Therefore, it is necessary to search for more novel building blocks to provide greater possibilities for the design of D-A structural COFs with excellent photocatalytic performance. In particular, doping heterocyclic rings into COFs is an effective strategy for modulating electronic properties and extending the visible absorption range [14,15]. Hou and co-workers found that oxazole/thiazole-linked COFs are able to efficiently regulate the dissociation behavior of excitons (e⁻-h⁺ pairs) on COFs by modulating the π conjugation and local charge polarization of the skeleton. Meanwhile, the rigid thiazole linkage of COF-S effectively increases the activity of neighboring benzene units to modulate the O₂⁻ adsorption energy barrier, leading to the extraordinary degradation performance of the pollutants under visible light irradiation [16]. Our research group is committed to the research of electrochromic devices and discovered that materials containing thiazolo[5,4-d]thiazole (TzTz) groups have good optoelectronic properties. It has been demonstrated that this type of TzTz fraction has excellent visible light-trapping ability and high oxidative stability [17]. Furthermore, the electron-deficient thiazolo[5,4-d]thiazole thickened heterocyclic ring has a stable planar π-conjugated structure, which enables efficient intermolecular π–π stacking and facilitates conjugated electron transport [18,19].

Therefore, two COFs with TzTz as the linking unit were designed through a catalyst-free condensation reaction in this study. C3 symmetric molecules 2,4,6-triformylphloroglucinol and 1,3,5-benzenetricarboxaldehyde were reacted with C2 symmetric molecules of dithiooxamide under solvothermal conditions to synthesize two kinds of COFs, named TPTZ-COF and TBTZ-COF, respectively. The structure and morphology of the prepared materials were thoroughly characterized by various techniques. Their degradation performance for RhB under visible light was assessed. The degradation mechanism was investigated by free radical-trapping experiments. The reusability was examined in successive recycling cycles.

2. Results and Discussion

2.1. Synthesis and Characterization of TzTz-Linked COFs

The TPTZ-COF was synthesized via a condensation reaction of 2,4,6-triformylphloroglucinol and dithiooxamide, while the TBTZ-COF was synthesized between 1,3,5-benzenetricarboxaldehyde and dithiooxamide (Scheme 1). Both of them were stable in water and insoluble in common organic solvents. TGA revealed their good thermal stability, and the dramatic decomposition occurred at approximately 400 °C (Figure S1). The weight loss below 300 °C can be attributed to the removal of water or other solvent molecules adsorbed on the sample [20]. Notably, the weight loss rate of TBTZ-COF was lower than that of TPTZ-COF in the temperature range of 300 °C to 800 °C.

The successful synthesis of the TzTz-linked COFs was validated by ¹³C NMR spectra combined with FT-IR spectra. As shown in Figure 1a,b, the peaks at 165 ppm and 167 ppm were attributed to the carbon signals of N=C in the TzTz moieties of TPTZ-COF and TBTZ-COF, respectively. Meanwhile, the peak at 151 ppm was assigned to the sp² carbon of the C=C bond in the TzTz unit [21]. The above two peaks indicated the presence of a thiazole ring in COFs [22]. From the FT-IR spectra (Figure 1c,d), it could be seen that the -HC=O absorption at 1695 cm⁻¹ attenuated after condensation. Moreover, the vibrational bands of -NH₂ at 3300 cm⁻¹ nearly disappeared, further proving the polymerization [23]. In addition, the signals at 1600 cm⁻¹ derived from the C=N double bond in the TzTz units provided further evidence for forming thiazole rings. PXRD patterns (Figure 1e) of TPTZ-COF and TBTZ-COF revealed broad 2θ peaks, which indicated their amorphous nature [24]. The broad set of reflections at about 20 and 26° were consistent with the π–π stacking interlayers of COFs. The orderly structures of COFs might originate from the effective π–π stacking interactions promoted by planar TzTz moieties [25]. Nitrogen adsorption–desorption isotherms at 77 K were used to evaluate the porosities of the prepared COFs (Figure 1f). The adsorption curves bore characteristic features of type II isotherms with the Brunauer–Emmett–Teller (BET) surface area of 81.7 m²·g⁻¹ for TPTZ-COF and 185.61 m²·g⁻¹ for TBTZ-COF. TBTZ-COF showed a far faster uptake of N₂ compared to TPTZ-COF under low pressure (P/P₀ < 0.001), indicating a larger content of the microporous structure of TBTZ-COF. According to the non-local density functional theory model calculation, the pore size distribution was around 1.36 nm for TPTZ-COF and 1.27 nm for TBTZ-COF (Figure S2). The total pore volume of TPTZ-COF at P/P₀ = 0.99 was calculated to be 0.25 cm³·g⁻¹, while that of TBTZ-COF was estimated to be 0.31 cm³·g⁻¹.

X-ray photoelectron spectroscopy (XPS) was used to determine the surface chemical state of the samples as well as the bonding configuration (Figure 2). Two similar element compositions of TPTZ-COF and TBTZ-COF were observed in the XPS survey spectra, showing that all the samples mainly contained C, N, O, and S elements. Subsequently, the C 1s spectra of each sample were analyzed, with the C 1s spectrum of TPTZ-COF (Figure 2b) fitting into five peaks corresponding to C-C (284.8 eV), C=C (285.8 eV), C-N (286.5 eV), C=N (287.4 eV), and C-O-H (288.8 eV). Similarly, TBTZ-COF (Figure 2f) exhibited four peaks attributed to C-C, C=C, C-N, and C=N at 284.8, 285.2, 286.1, and 288.8 eV, respectively. The N 1s spectra were shown in Figure 2c,g, where the strongest peak with binding energy around 399 eV may be attributed to C=N [26].

The morphologies of the two COFs were investigated by SEM and TEM. The SEM images showed that TBTZ-COF was composed of small particles (Figure 3a,b), while a uniform fibrous structure was observed in TPTZ-COF (Figure S3). Elemental mapping of SEM confirmed the existence of C, O, N, and S in the two polymers. The morphologies of both materials were further confirmed by TEM images (Figure 3c,d and Figure S4). Obvious layered structures were observed for both COFs in HRTEM, which was consistent with the SEM images.

2.2. Photoelectric Property Studies

The ultraviolet–visible diffuse reflectance spectrum (UV-vis DRS) showed wide photo-absorption from the UV region to the visible light (Figure 4a). In addition, the bandgap energies of TPTZ-COF and TBTZ-COF were calculated to be 1.62 eV and 2.36 eV (Figure S5) according to the equation [27]:

$$\alpha h\upsilon ={A(h\upsilon -E_{\mathrm{g}})}^{\mathrm{n}}$$

(1)

where α, hυ, A, and E_g represent the absorption coefficient, the photon energy, the absorption constant, and the band gap energy, respectively. The E_g of the samples could be evaluated through the intercept of the tangent in the plots of (αhυ)² vs. photon energy (hυ) because TPTZ-COF and TBTZ-COF were direct bandgap semiconductors [28]. As demonstrated in Figure S6, the slopes of the curves for both samples were positive, indicating the characteristic n-type semiconductors of TPTZ-COF and TBTZ-COF. The Mott–Schottky plots revealed that the values of flat-band potential (E_fb) were estimated to be −0.73 V for TPTZ-COF and −1.27 V for TBTZ-COF versus Ag/AgCl (−0.53 V and −1.07 V vs. NHE). Because the bottom of the conduction band (CB) of n-type semiconductors was more negative 0.1 V than the flat-band potential [29], the conduction band of TPTZ-COF and TBTZ-COF were estimated to be −0.63 V and −1.17 V (vs. NHE), respectively. As a result, the relative band gap positions were obtained (Figure 4b). The resistance of the charge transfer performance in TzTz-COFs was probed using EIS. As shown in Figure 4c, TBTZ-COF exhibited a smaller Nyquist semicircle diameter in comparison with TPTZ-COF, suggesting that TBTZ-COF had lower electrochemical impendence and higher electron (e⁻)/hole (h⁺) separation efficiency. Furthermore, the visible light response photocurrent of TBTZ-COF was higher than that of TPTZ-COF (Figure 4d), displaying the more effective carrier separation efficiency in TBTZ-COF.

In order to better understand the influence of the building blocks on the optoelectronic properties of COFs, the front molecular orbitals of two model fragments belonging to COFs were analyzed using density functional theory (DFT) calculations (Figure 5). Simulation results showed that the highest occupied molecular orbital (HOMO) of TPTZ-COF was found to be distributed across the entire skeleton, and the lowest unoccupied molecular orbital (LUMO) was mainly concentrated on the acceptor unit of the TzTz ring. However, the HOMO and LUMO orbitals of TBTZ-COF were uniformly distributed over the simplified fragment structure. In addition, surface electrostatic potential (ESP) maps for the two COFs were calculated using the predicted densities. The results demonstrated that the negative polarization of TPTZ-COF was primarily focused on the O atoms in the hydroxyl group, whereas the negative polarization of TBTZ-COF was mainly concentrated on the N atoms of the TzTz ring. Compared to TBTZ-COF, the extent of this charge density heterogenization was comparatively low in TPTZ-COF, suggesting that TPTZ-COF may perform relatively poorly under photocatalytic conditions.

2.3. Photodegradation of RhB

The photocatalytic efficiency of TzTz-COFs was evaluated for the degradation of RhB under visible light irradiation in aqueous solution. In the preliminary experiment, the RhB aqueous solution containing the photocatalyst was stirred continuously for 30 min in a dark environment to achieve adsorption–desorption equilibrium before visible light irradiation (Figure S7). Although the BET surface area of TPTZ-COF (81.7 m²·g⁻¹) was smaller than that of TBTZ-COF (185.61 m²·g⁻¹), the adsorption efficiency of TPTZ-COF was higher than that of TBTZ-COF. A previous study also found that decreasing the BET surface did not affect the adsorption of RhB by the material [30]. When TPTZ-COF was used as the photocatalyst, no matter how the amount of photocatalyst was adjusted, the concentration of RhB decreased only slightly throughout the experiment with visible light irradiation (Figure 6a). This experimental result confirmed that TPTZ-COF was not an efficient photocatalyst for RhB degradation. On the contrary, the TBTZ-COF demonstrated superior photodegradation capabilities compared to the TPTZ-COF. As shown in Figure 6b, the photodegradation efficiency of RhB by TBTZ-COF (0.1 g/L) was 99.76% after 60 min of visible light irradiation. We could observe that the color of the RhB solution changed from pink to colorless during the reaction. Equation (2) was applied to evaluate the degradation efficiency of RhB.

$$\mathrm{l}\mathrm{n}(c_0/c_t)=\mathrm{k}\mathrm{t}$$

(2)

where c₀ represents the initial concentration of RhB when adsorption–desorption equilibrium was established, c_t represents the residual concentration of RhB at time t (min), k refers to the reaction rate constant, and t represents the duration (min) of UV-vis light irradiation. The reaction rate constant calculated with the Langmuir–Hinshelwood pseudo-first-level kinetic model [31] was 9.63 × 10⁻² min⁻¹ for TBTZ-COF, which was 11.3 times greater than that of the TPTZ-COF (0.85 × 10⁻² min⁻¹) (Figure S8), proving the higher photocatalytic performance of the TBTZ-COF. As demonstrated in Table S1, the photocatalytic efficiency of TBTZ-COF was superior to most COF-based materials reported so far.

2.4. Degradation Mechanism

Photocatalytic degradation of organic pollutants is known to produce a wide range of reactive species, such as superoxide radicals (^.O₂⁻), singlet oxygen (¹O₂), hydroxyl radicals (^.OH), photogenerated holes (h⁺), and electrons (e⁻).[32] To further explore the catalytic mechanism of TBTZ-COF, TEMPO, L-his, IPA, EDTA-2Na, and AgNO₃ were added to the RhB solution as scavengers to capture the ^.O₂⁻, ¹O₂, ^.OH, h⁺, and e⁻, respectively. Figure 6c illustrates that the presence of TEMPO and L-his markedly suppressed the degradation of RhB, suggesting that ^.O₂⁻and ¹O₂ radicals serve as key reactive species in the photocatalytic degradation process. When comparing the effects of adding IPA, EDTA-2Na, or AgNO₃ to the RhB solutions, it was observed that these additions resulted in the least reduction in degradation efficiency. The present study provided empirical evidence that TBTZ-COF exhibited significant photocatalytic properties, particularly in producing ^.O₂⁻ and ¹O₂. The electrons on the CB also played an important role in the photocatalytic process because the dissolved oxygen in the aqueous environment acted as an effective electron trap, producing ^.O₂⁻ and preventing the complexation of electrons and holes [33]. In addition, it can be seen from the energy band structure diagram that the calculated CB potential of the TBTZ-COF (−1.17 V vs. NHE) was more negative than the reduction potential of O₂ to ^.O₂⁻ (−0.33 V vs. NHE). This result provided the feasibility of generating superoxide species and enhancing the photodegradation of dyes [34]. A possible mechanism for the photocatalytic degradation of RhB by TBTZ-COF was postulated based on the above analysis. Under visible light irradiation, the TBTZ-COF catalyst absorbed energy to generate electronic transitions, forming photo-generated electron–hole pairs. During this process, electrons (e⁻) in VB were excited into CB, resulting in many positively charged holes (h⁺) in VB, and more photogenerated holes were exposed on the catalyst surface. In the CB, the accumulated electrons reacted with dissolved O₂ to form the active species ^.O₂⁻, which was primarily in charge of RhB degradation. Likewise, the produced photogenerated holes participated actively in the oxidation of RhB. The ^.O₂⁻ acquired in CB could react with the h⁺, leading to the production of ¹O₂. At the same time, the h⁺ effectively oxidized H₂O to O₂ and H⁺ and directly oxidized and destroyed the RhB molecules. Furthermore, since the calculated VB potential of TBTZ-COF (1.19 V vs. NHE) was lower than the oxidation potential of H₂O to ^.OH (2.27 V vs. NHE), ^.OH could not be generated by direct water oxidation of h⁺. This further suggests that reducing H₂O₂ was the only possible way to produce ^.OH. The ^.O₂⁻ was involved in the indirect two-electron (2e⁻) pathway, leading to the production of H₂O₂. The subsequently formed H₂O₂ was further reduced, leading to the production of ^.OH, which played a minor role in RhB degradation. It is worth noting that the existence of ^.O₂⁻ played a crucial role in the entire photocatalytic degradation process, since it was the precursor for forming secondary free radicals ¹O₂ and ^.OH.

The following equations can be adapted to represent the RhB degrading process:

TBTZ-COF + hυ →TBTZ-COF + e⁻ + h⁺

e⁻ + O₂→^.O₂⁻

^.O₂⁻ + h⁺→¹O₂

2 H₂O + 4 h⁺→O₂ + 4 H⁺

^.O₂⁻ + e⁻ + 2H⁺→H₂O₂

H₂O₂ + e⁻ + 2H⁺→^.OH + H₂O

^.O₂⁻/¹O₂/h⁺/e⁻/^.OH + RhB→ degradation products (CO₂, H₂O, etc.)

The recyclability of photocatalytic materials as a key indicator for practical applications also needed to be assessed [35]. For this purpose, the reusability and stability of TBTZ-COF were evaluated by five cycles of photocatalytic degradation targeting RhB. Each photocatalytic experiment was also carried out after RhB reached adsorption equilibrium. After each photocatalytic experiment, the TBTZ-COF was thoroughly washed with water and ethanol and dried before being reused in the next photocatalytic experiment, which realized the recycling of the photocatalyst. The results showed that the TBTZ-COF could photodegrade more than 99% of RhB even after five cycles, implying that TBTZ-COF is an excellent recyclable photocatalyst (Figure 6d).

3. Experimental Section

3.1. Materials

Dithiooxamide, 2,4,6-triformylphloroglucinol, benzimidazole, 1,3,5-benzenetricarboxaldehyde, N,N-dimethylformamide (DMF), mesitylene, N-methyl-2-pyrrolidinone, 2,2,6,6-tetramethylpiperidine oxide (TEMPO), L-histidine (L-his), isopropyl alcohol (IPA), ethylenediaminetetraacetic acid disodium salt (EDTA-2Na), silver nitrate (AgNO₃), and other common solvents were purchased from commercial suppliers (Adamas, Shanghai, China and Leyan, Shanghai, China) without further purification.

3.2. Synthesis of TPTZ-COF

2,4,6-Triformylphloroglucinol (250 mg, 1.19 mmol), dithiooxamide (214.5 mg, 1.78 mmol), and benzimidazole (281 mg, 2.38 mmol) were heated at 180 °C in a 24 mL mixed solution (mesitylene: N-methyl-2-pyrrolidinone = 1:1) under N₂ protection for 4 days. The precipitates were filtrated after the reaction mixture was cooled to room temperature. Then the crude polymer was purified with H₂O, EtOH, and acetone several times. After that, the polymer was thoroughly washed with CHCl₃ in a Soxhlet extractor for 24 h. Finally, the product was dried under vacuum for 24 h at 80 °C to yield a black powder (339.1 mg, 73% yield).

3.3. Synthesis of TBTZ-COF

1,3,5-Benzenetricarboxaldehyde (0.5 g, 3.08 mmol) and dithiooxamide (0.556 g, 4.63 mmol) were heated at 160 °C in DMF (15 mL) under N₂ protection for 1 day. The precipitates formed were filtrated after the reaction mixture was cooled to room temperature. Then the crude polymer was purified with H₂O and acetone several times. After that, the polymer was thoroughly washed with CHCl₃ in a Soxhlet extractor for 24 h. Finally, the product was dried under vacuum for 24 h at 100 °C, resulting in a yellow powder (739.2 mg, 70% yield).

3.4. Characterizations

Fourier-transformed infrared (FT-IR) spectra were recorded on dried KBr disks in transmission mode using Bruker VERTEX 70 FTIR spectrometer. Solid-state cross-polarization magic angle spinning (CP/MAS) ¹³C NMR spectra were obtained on a 400 MHz Bruker-500 NMR spectrometer. Ultraviolet–visible spectra were measured on a UV-VIS-NIR spectrophotometer (Cary-100, Agilent Technologies Inc., Santa Clara, CA, USA) at room temperature. Surface area, N₂ adsorption isotherms (77 K), and pore size distributions were measured using the micromeritics ASAP 2460 surface area and porosity analyzer. The samples were degassed in a vacuum (10⁻⁵ bar) at 150 °C for 6 h before measurement. Thermo-gravimetric analysis (TGA) was conducted on NETZSCH 209 F3. The samples were heated at 10 °C/min under a nitrogen atmosphere up to 800 °C. Powder X-ray diffraction (PXRD) patterns were performed on an X-ray diffraction spectrometer (Bruker AXS GmbH, Germany). Polymer morphologies were investigated with a Zeiss SIGMA 500 field emission scanning electron microscope (SEM). Additionally, elemental mapping was performed using a BRUKER XFlash 6130. Before measurement, the samples were sputter-coated with gold. Detailed morphological and structural characterizations were performed using a high-resolution transmission electron microscope (HR-TEM,JEOL, Tokyo, Japan).

3.5. Electrochemical Measurement

Electrochemical measurements were measured on a CHI760E electrochemical workstation in a three-electrode system using photocatalyst-coated fluorine–tin oxide (FTO) glass as the working electrode, an Ag/AgCl electrode as the reference electrode, and a platinum flake as the counter electrode. To prepare working electrodes, 5 mg samples were dispersed in 0.1 mL of ethanol, followed by the addition of 20 μL 5 wt% Nafion solution. After sonication for 2 h, the working electrodes were prepared by dropping the suspension onto a piece of fluorine–tin oxide (FTO) glass substrate with a cover area of 1 cm². Electrochemical impedance spectroscopy (EIS), transient photocurrent measurements, and Mott–Schottky analyses were conducted in a 0.2 M Na₂SO₄ aqueous solution.

3.6. Photocatalytic Measurement

The photocatalytic activity of COFs was characterized by degradation of RhB under visible light illumination (λ ≥ 420 nm). For the degradation of RhB, COFs were dispersed in an aqueous solution of RhB (50 mL, 10 mg/L), and the mixture was stirred for 30 min in the darkness to obtain adsorption–desorption equilibrium. After that, the stirring solution was irradiated with a 300 W Xenon lamp. At defined time intervals, an appropriate amount of suspension was filtered through a filter membrane to remove solid particles and collect the filtrate for further analysis. The UV-vis absorbance of the filtrate at 554 nm of RhB was obtained by a UV–visible spectrophotometer. Different scavengers were added to the reaction suspension while keeping other reaction conditions the same to study the function of the active ingredients. The RhB removal efficiency was calculated using the following equation:

$$\mathrm{Removal}\mathrm{efficiency}(\%)={\displaystyle \frac{C_0-C}{C_0}}\times 100$$

(3)

where c₀ denotes the initial concentration of RhB and c denotes the concentration of RhB at each irradiated time interval.

3.7. Theoretical Calculations

All calculations were implemented in the DMol3 module of the Materials Studio 2020 and within the framework of density functional theory (DFT). In order to understand the electronic properties of the studied COF materials, the geometric structures of the simplified models were optimized using the BLYP functional level in the generalized gradient approximation (GGA). The optimized geometries, frontier molecular orbitals and electrostatic potentials (ESP) on the electron-dense surfaces were subsequently presented visually.

4. Conclusions

In summary, two TzTz-bridged COFs were synthesized by the solvothermal method. TBTZ-COF exhibited a stronger photocatalytic activity for the degradation of RhB compared to TPTZ-COF. Specifically, TBTZ-COF (0.1 g/L) effectively degraded 99.76% of RhB (10 mg/L) in 60 min under visible light irradiation, while TPTZ-COF (0.1 g/L) only degraded 21.77% of RhB. Scavenger experiments showed that ^.O₂⁻ and ¹O₂ were the main active substances in the photocatalytic process. Furthermore, TBTZ-COF demonstrated outstanding stability and high photocatalytic performance after 5 repeated recycling. The above achievements were accomplished by introducing TzTz as a linking group and electron acceptor in these two COFs, thereby activating O₂ to ROS. This work provided valuable insights into the strategic design of COFs as efficient photocatalysts and contributed to the effective and sustainable treatment of complex water pollution.