Benzothiadiazole functionalized D–A type covalent organic frameworks for effective photocatalytic reduction of aqueous chromium(VI)

Weiben Chen a, Zongfan Yang a, Zhen Xie a, Yusen Li a, Xiang Yu b, Fanli Lu a and Long Chen *a
aDepartment of Chemistry, Tianjin Key Laboratory of Molecular Optoelectronic Science, Tianjin University, Tianjin 300072, China. E-mail:
bAnalytical and Testing Center, Jinan University, Guangzhou 510632, China

Received 18th October 2018 , Accepted 16th November 2018

First published on 20th November 2018

Covalent organic frameworks (COFs) have received increasing research interest as an emerging class of crystalline and porous polymers. Herein, we prepared two new benzothiadiazole (BT) functionalized COFs (i.e. TPB-BT-COF and TAPT-BT-COF), which exhibit good crystallinity, high porosity, and excellent stability in harsh conditions. Their applications for photoreduction of Cr(VI) species under visible light irradiation were investigated. Over 99% Cr(VI) was reduced by utilizing TPB-BT-COF as catalyst without any sacrificial agents or additional pH adjustment. The photocatalytic rate of TPB-BT-COF is faster than that of TAPT-BT-COF, which can be attributed to more negative conduction band and narrower bandgap. Our results indicate that co-condensation of electron deficient units into the COF skeleton is conducive to efficient separation of photoexcited electrons and holes, which in turn leads to superior photocatalytic activities.


Hexavalent chromium (Cr(VI)) is known as a toxic heavy metal, and is notorious for its mutagenic and carcinogenic effect on the environment because it is largely utilized in electroplating, leather tanning, chemical fertilizer, printing, and pigment industries.1,2 It is well known that trivalent chromium (Cr(III)) is 500–1000 folds less toxic compared to Cr(VI) and is indispensable to human nutrition.3 Therefore, the efficient removal of Cr(VI) or reduction of Cr(VI) from wastewater is a prior research topic. Compared to conventional strategies, such as chemical sedimentation,4 physical adsorption,5 membrane separation,6 chemical reduction7 or electroreduction process,8 photocatalytic reduction9–11 directly utilizing solar energy is more effective, less costly and does not produce any secondary pollutants. Although many inorganic photocatalytic semiconductors have been used for photocatalytic detoxification of Cr(VI),12–16 it still suffer from severe drawbacks such as low reduction efficiency, slow reduction rate, poor cyclic stability etc.16 Also, those inorganic semiconductor based photocatalysts have potential risks for secondary heavy metal pollutions. Thus, it is essential to develop efficient and nonmetallic photocatalysts for Cr(VI) degradation.

Covalent organic frameworks (COFs) represent a burgeoning class of crystalline and porous polymers featuring designable topologies, permanent porosity and tailor-made functionalities.17,18 Among them, two-dimensional (2D) COFs serve as a special platform for developing new photoactive and conductive materials due to their well-defined alignment of conjugated building blocks and segregated arrays of π-columns.19–24 For instance, when n-type organic semiconductors (i.e. electron acceptor units) are incorporated into the 2D COF periodic lattice, a photoconductive material with strong light absorption could be developed with tunable bandgaps and variable charge separation and transfer properties.25 Due to the above mentioned merits, increasing research interest has been focused on wastewater treatment using 2D COFs as absorbents. For example, Hg(II) could be efficiently removed from wastewater by thioether post-functionalized 2D COFs.26–28 In addition, a salicylideneanilines-based 2D COF was reported as chemoselective molecular sieves for removing organic dyes.29 However, COFs as photocatalysts for the photoreduction of toxic heavy metals have only been rarely explored.

Herein, we have introduced a strong electron deficient benzothiadiazole (BT) unit to construct two highly porous and crystalline BT-COFs through acid-catalyzed Schiff base reaction (Scheme 1a). Benzothiadiazole, which is known as one of the state-of-the-art electron acceptor, could facilely tune the bandgaps and improve the charge separation and transfer properties of conjugated polymers. It has been widely used in organic photovoltaics (OPVs),30,31 organic field effect transistors (OFETs)32 and photocatalysts.33,34 The incorporation of electron donor (D) and acceptor (A) units into crystalline COF skeletons would offer a topologically ordered D–A heterojunction structure with independent pathways for ambipolar electron and hole transport leading to enhanced photoconductivity and photocatalytic acitivity.10a,23 Tris(4-aminophenyl)benzene (TPB) and tris-(4-aminophenyl)triazine (TAPT) were selected as donor building blocks to build the D–A alternative COFs scaffold.22–24 The BT-COFs exhibit high crystallinity, large porosity and excellent stability in harsh conditions. Moreover, both BT-COFs can effectively degrade Cr(VI) upon visible light illumination.

image file: c8ta10046b-s1.tif
Scheme 1 (a) Schematic diagram for the synthesis of BT-COFs; (b) top and (c) side views of TPB-BT-COF; (d) top and (e) side views of TAPT-BT-COF; (f) photographs of TPB-BT-COF, TAPT-BT-COF and TPB-TP-COF.

Results and discussion

Synthesis and characterization

Tris(4-aminophenyl)benzene (TPB), tris(4-aminophenyl)triazine (TAPT) and benzo[c][1,2,5]thiadiazole-4,7-dicarbaldehyde (BT) were synthesized according to the previous literature.35 TPB and TAPT were adopted as the donors, while BT served as the acceptor for preparation of the hexagonal 2D D–A COFs (TPB-BT-COF and TAPT-BT-COF, Scheme 1a). The condensation reactions were performed in o-dicholorobenzene (o-DCB), n-butanol (n-BuOH) and acetic acid (n-BuOH/o-DCB/AcOH = 1/1/0.2, v/v/v) under 120 °C for 72 h. The colors of the two COFs are rubine red and orange for TPB-BT-COF and TAPT-BT-COF, respectively (Scheme 1f). The as-synthesized BT-COFs were fully characterized using various methods (ESI). As displayed in Fig. S4, FT-IR spectra showed dramatic attenuation of N–H (∼3300 cm−1) and C[double bond, length as m-dash]O (1690 cm−1) stretching bands of the free amine and formyl groups. The appearance of new stretching vibration bands at 1618 cm−1 and 1589 cm−1 confirmed the formation of imine linkages for TPB-BT-COF and TAPT-BT-COF, respectively.36 Compared to the reported non-BT version analogue TPB-TP-COF37 constructed by TPB and terephthalaldehyde (TP), these new BT-COFs exhibit much deeper color with significant red shifts (>100 nm) in the solid-state UV-Vis reflectance spectra (Fig. S5). As displayed in Fig. S3, the 13C cross polarization magic angle spinning (CP-MAS) NMR spectra of the two BT-COFs further validated the existence of C[double bond, length as m-dash]N bonds in imine linkages, which exhibited intensive peaks around 151–152 ppm. However, the signal of the C[double bond, length as m-dash]N linkages in triazine segments appeared at ca. 169 ppm. Other signals could be properly assigned to the rest of aromatic carbons (Fig. S3). Elemental analysis of BT-COFs indicated that the experimental data of C, H and N contents agreed well with the theoretically calculated ones (Table S1). Scanning electron microscopy (SEM) images illustrated that the two BT-COFs displayed uniform nanowires with several tens of micrometers in length (Fig. S13). High-resolution transmission electron microscopy (HR-TEM) images further proved the high crystallinity and the mesoporous structure (Fig. 1, S14 and S15).
image file: c8ta10046b-f1.tif
Fig. 1 HR-TEM images of (a) TPB-BT-COF and (b) TAPT-BT-COF. The inset represents the enlarged view of a single COF nanowire. The white frame indicates the magnified area.

Crystalline structure

The crystallinity of the BT-COFs was confirmed by powder X-ray diffraction (PXRD) measurements (Fig. 2). As displayed in Fig. 2a, the PXRD patterns of TPB-BT-COF showcased a prominent peak at 2.75°, and other minor peaks around 4.80°, 5.50°, 7.35°, 9.60° and 25.60°, which could be perfectly assigned to the (100), (110), (200), (210), (220), and (001) diffractions, respectively. Similar results are also obtained for TAPT-BT-COF (Fig. 2d). A series of distinctive diffraction peaks at 2.82°, 4.88°, 5.66°, 7.50°, 9.68° and 26.24° were detected, which corresponded to the (100), (110), (200), (210), (220), and (001) planes, respectively. The lattice simulation and Pawley refinement using Materials Studio software (ver. 7.0 Tables S4–S7) were performed to acquire optimized parameters. The space groups of both BT-COFs are determined to be P6. Both simulated AA stacking modes (Fig. 2b, e) matched well with the observed PXRD patterns on the peak position and relative intensities (Fig. 2a, d, cyan lines). By comparison, the staggered AB stacking modes (Fig. 2c, f; S29, S30) cannot reproduce the experimental results (Fig. 2a, d, magenta lines). The Pawley refinement is also in line with the experimentally observed patterns. Their negligible deviation further confirmed the PXRD simulation results (Fig. 2a and d, black and orange lines). The Pawley refinement afforded the hexagonal unit cell with parameters of a = b = 3.71 ± 0.08 nm, c = 0.35 ± 0.01 nm, α = β = 90°, and γ = 120° for TPB-BT-COF with Rwp = 3.71% and Rp = 2.82%. While the lattice parameters for TAPT-BT-COF are a = b = 3.65 ± 0.04 nm, c = 0.36 ± 0.03 nm, α = β = 90°, and γ = 120°, with Rwp = 3.80% and Rp = 3.08%.
image file: c8ta10046b-f2.tif
Fig. 2 Experimental PXRD profiles of (a) TPB-BT-COF (red) and (d) TAPT-BT-COF (blue), and their corresponding Pawley refined (black) and difference (orange), calculated profiles by the AA (cyan) and AB (magenta) stacking modes. (b) and (e) represent unit cells for the AA stacking fashion; (c) and (f) represent the unit cells for the AB stacking modes.

Porosity of BT-COFs

The inherent porosity of BT-COFs was evaluated by N2 sorption experiments at 77 K. The sorption profiles of TPB-BT-COF and TAPT-BT-COF are typical type IV isotherms, which indicates the characteristics of mesopores (Fig. 3a). The BET surface area was determined to be 1376 m2 g−1 for TPB-BT-COF and 1035 m2 g−1 for TAPT-BT-COF. While the pore volumes of TPB-BT-COF and TAPT-BT-COF were calculated as 1.21 and 1.00 cm3 g−1 at P/P0 = 0.99, respectively. The poresize simulated by the nonlocal density functional (NLDFT) theory suggested a uniform poresize of 3.26 nm for both BT-COFs (Fig. 3b, c), which is consistent with the values of d-spacing obtained from PXRD profiles at 2θ = 2.75° (3.18 nm for TPB-BT-COF) and 2θ = 2.82° (3.13 nm for TAPT-BT-COF).
image file: c8ta10046b-f3.tif
Fig. 3 (a) N2 adsorption/desorption isotherms and pore size distribution profiles of (b) TPB-BT-COF (red) and (c) TAPT-BT-COF (blue).

Stability of BT-COFs

Both BT-COFs are not soluble in ordinary solvents such as MeOH, DMF, ethyl acetate THF, hexane, etc. Furthermore, their chemical stabilities were checked by separately stirring in boiling water, aqueous HCl (pH = 1) or NaOH (pH = 13) solutions at 298 K for 24 h. To our delight, all of the PXRD patterns for treated samples were maintained in these harsh conditions (Fig. S8 and S10). In addition, the characteristic vibration peaks of imine C[double bond, length as m-dash]N bond in FT-IR spectra also remained, which indicated that the two BT-COF frameworks were retained under these harsh conditions (Fig. S7 and S9). Excellent photostabilities of the BT-COFs were confirmed by continuous light irradiation (>400 nm) for 12 h (Fig. S11 and S12). The chemical robustness of BT-COFs might originate from the extended 2D network scaffold, π–π stacking arrangements, and high crystallinity.38,39 Thermogravimetric analysis (TGA) indicated that both BT-COFs exhibit excellent thermal stability below ca. 450 °C (Fig. S6). The high chemical and thermal stabilities are propitious to such applications in sorption, catalysis, energy storage, etc.

Photoreduction of Cr(VI)

To evaluate the photocatalytic activity of BT-COFs, the solid-state UV-Vis reflectance spectra of the two BT-COFs were measured (Fig. S5a). The spectra suggested broad absorption bands of both BT-COFs ranged from 400 nm to 800 nm, which covered almost the whole visible light region. In addition, TPB-BT-COF possesses much better light harvesting efficiency40 (LHE) than that for TAPT-BT-COF and TPB-TP-COF, including a plateau of 400–600 nm with a value more than 91% (Fig. S31). The bandgaps of both BT-COFs were estimated by Kubelka–Munk-transformed reflectance spectra (Fig. S5b). The optical bandgap of TPB-BT-COF is 2.05 eV, which is narrower than that of TAPT-BT-COF (2.13 eV). The redox properties of both BT-COFs were further evaluated by cyclic voltammetry (CV, Fig. S16).41,42 The calculated HOMO and LUMO are in accordance with the experimental data from the CV results (Table S2). The simulated bandgap of TPB-BT-COF is also smaller than that of TAPT-BT-COF. The conduction band of TPB-BT-COF is −0.44 V versus the normal hydrogen electrode (NHE) (ENHE = EFc/Fc+ − 0.64, Table S2),10,43 which is lower than the TAPT-BT-COF (−0.30 V). Both conduction bands of BT-COFs are much more negative than the redox reaction potential of Cr(VI) to Cr(III) (1.33 V vs. NHE),10 thus the energy levels of BT-COFs are sufficient for the reduction of Cr(VI).

Photoreduction of Cr(VI) (K2Cr2O7 10 mg L−1) was investigated using the BT-COFs as a catalyst, using a xenon lamp with a 400 nm cutoff filter as the light source.44 The Cr(VI) concentration after light illumination was determined by the intensity of light absorption at 540 nm by addition of 1,5-diphenylcarbazide (DPC) and sulfuric acid.45,46 The disappearance of the pink color demonstrated the degradation of Cr(VI) by BT-COFs upon visible light irradiation (Fig. 4d and e). The Cr(VI) concentrations decreased with the elongation of irradiation time as shown in Fig. 4a and b, and over 99% of Cr(VI) could be reduced by TPB-BT-COF upon irradiation for 75 min. The photocatalytic rate of TPB-BT-COF is faster than that of TAPT-BT-COF (Fig. 4c). This could be attributed to the more negative conduction band and narrower bandgap of TPB-BT-COF, which promote the visible light harvesting efficiency and generate more photoinduced charge carriers under light illumination.47 To demonstrate the photocatalytic activities of these BT-COFs, we performed control experiments under same light irradiation conditions but without adding BT-COFs (Fig. S17). The results showed that concentration of Cr(VI) was almost unchanged when no BT-COFs were added, which demonstrated the essential photocatalytic performance of BT-COFs. Furthermore, additional control experiments of physical sorption of Cr(VI) onto the two BT-COFs in the dark were carried out (Fig. S18). The results further confirmed that the removal of Cr(VI) was mainly due to the photocatalytic effect. The photoreduction rate of the reported TPB-TP-COF (without benzothiadiazole units)37 is slower than that of TPB-BT-COF (Fig. 4c, S19 and S20). This illustrates that the electron deficient BT unit could effectively facilitate the separation and migration of the charges. It is noteworthy that the porosity exhibit negligible effect on the photocatalytic performance (Fig. S21) probably due to the inappreciable contribution of adsorption of Cr(VI). BT-COFs maintained both their photocatalytic activities (Fig. S24) and scaffolds for at least five recycles, as both evidenced in the FTIR spectra (Fig. S25) and PXRD pattern of the COF samples collected after the photocatalysis experiments (Fig. S26). Compared to the recently reported metal-free photocatalysts like g-C3N4 (ref. 16 and 48) P-FL-BT-3 polyelectrolyte,10 MOFs49–53 and conjugated polymer based photocatalysts47,54 (Table S3), BT-COFs can efficiently reduce Cr(VI) under mild conditions without adding any hole scavenger and no additional pH adjustment55–57 is needed.

image file: c8ta10046b-f4.tif
Fig. 4 Photoreduction of Cr(VI) using (a) TPB-BT-COF and (b) TAPT-BT-COF upon visible light irradiation; (c) the photocatalytic rate comparison of TPB-TP-COF, TAPT-BT-COF and TPB-BT-COF (C denotes the concentration of Cr(VI) while C0 represents the initial concentration after sorption equilibrium); photograph of photocatalytic reduction using (d) TPB-BT-COF and (e) TAPT-BT-COF.

Mechanism analysis

To gain more insight into the mechanism of photoreduction and unravel the plausible reason for better performance of TPB-BT-COF over others, the photoelectrochemical experiments were performed in aq. Na2SO4 (0.5 M) under visible light irradiation (λ ≥ 400 nm). Fig. 5a showcases the transient photocurrent intensities of the three COFs upon several time intervals of on-off switching of visible light. TPB-BT-COF exhibited the highest photocurrent density compared to the others, implying the most efficient light harvesting and separation of the photogenerated charges.47 The arc diameter of electrochemical impedance spectra (EIS) for TPB-BT-COF was smaller than those of TAPT-BT-COF and TPB-TP-COF, which further confirmed the lower charge transfer impedance in TPB-BT-COF sample (Fig. S32).58,59 Moreover, the capability of light-excited 2D COF samples to form photogenerated holes was also investigated.60 It is well known that the photoexcited holes could oxidize the –OH groups and H2O molecules adsorbed on the photocatalyst's surface to generate ˙OH radicals, which further reacted with terephthalic acid (TA) to afford 2-hydroxy-terephthalic acid (TAOH) with characteristic emission at 426 nm.11,44 Thus, the COF samples were suspended in a TA solution and illuminated under visible light for 20 minutes. As shown in Fig. 5b, the characteristic fluorescence of TAOH can be clearly detected. In addition, the number of formed ˙OH radicals follow the trend of TPB-BT-COF > TAPT-BT-COF > TPB-TP-COF, as judged from the fluorescence intensity order (Fig. 5b). This result further confirmed that TPB-BT-COF featured better separation and migration of the photogenerated charges to participate in the overall photocatalytic reaction. Furthermore, significant improvement on the photocatalytic performance was observed when phenol (the h+ scavenger) was added to the TPB-BT-COF system (Fig. S22). This could be explained by the promoted separation between the electrons and holes upon quenching of the photo-generated h+, thus further increasing the generation efficiency of electrons that directly contributed to the reduction of Cr(VI). The Cr(VI) species is CrO42− in the treated Cr(VI) solution under initial pH value of 6.1 determined by the pH meter.53 Therefore, the generation and precipitation of Cr(OH)3 on the COF surface was further verified by the XPS measurement, where two characteristic peaks at 576.7 eV (2p3/2) and 586.2 eV (2p1/2) assignable to the Cr(III) species were observed (Fig. S23).46,61 As illustrated in Fig. 6, a probable mechanism for the photoreduction of Cr(VI) for 2D BT-COFs was proposed. Firstly, the electron/hole pairs were generated by visible light excitation followed by further separation and migration to the surface of the photocatalyst. The photoinduced electrons directly reduced adsorbed Cr(VI) to Cr(III), while the photoexcited holes oxidized H2O and hydroxyl groups to form the desirable ˙OH radical species. The photogenerated holes tend to accumulate at the COF skeletons, leading to a positively charged COF skeleton, which is facilitated for the adsorption of the negatively charged CrO42− and also beneficial for the reduction of CrO42−. The quenching of ˙OH radicals with the COF scaffold cause the partial oxidation of the polymer skeleton (Fig. S26) or self-combination leading to the formation of water and oxygen.62 Ultimately, the photo-generated electrons directly lead to the photoreduction of Cr(VI).11,63–65
image file: c8ta10046b-f5.tif
Fig. 5 (a) Photocurrent intensity comparisons of BT-COFs in aq. Na2SO4 (0.5 M) upon visible light irradiation; (b) fluorescence spectra of TAOH.

image file: c8ta10046b-f6.tif
Fig. 6 The probable mechanism of photoreduction of Cr(VI) by BT-COFs.


In summary, two novel BT-COFs with electron deficient BT units were synthesized under solvothermal conditions. Both BT-COFs exhibited good crystallinity, high porosity, and excellent stability in acidic, basic and visible light irradiation conditions. TPB-BT-COF showed extraordinary activity on photoreduction of Cr(VI) over 99% efficiency without any sacrificial agent or pH regulation. The rate of photoreduction for TPB-BT-COF is faster than that of TAPT-BT-COF, which might be ascribed to the more negative conduction band, narrower bandgap and more favorable separation and migration of the photogenerated electron/hole pairs. Our results demonstrate that integrating alternative donor–acceptor units in COF scaffold provides a new strategy for exploration of novel photocatalysts.

Conflicts of interest

The authors declare no conflict of interest.


This study was financially supported by the National Natural Science Foundation of China (51522303, 21602154), National Key Research and Development Program of China (2017YFA0207500), the Natural Science Foundation of Tianjin (17JCJQJC44600).

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Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ta10046b

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