A hybrid of g-C3N4 and porphyrin-based covalent organic frameworks via liquid-assisted grinding for enhanced visible-light-driven photoactivity

Yuxia Hou *a, Cheng-Xing Cui a, Enhui Zhang a, Ji-Chao Wang *a, Ying Li a, Yuping Zhang a, Yuquan Zhang a, Qing Wang a and Jianzhuang Jiang *b
aDepartment of Chemistry and Chemical Engineering, Henan Institute of Science and Technology, Xinxiang, 453003, China. E-mail: yxhou@hist.edu.cn; wangjichao@hist.edu.cn
bBeijing Key Laboratory for Science and Application of Functional Molecular and Crystalline Materials, Department of Chemistry, University of Science and Technology Beijing, Beijing, 100083, China. E-mail: jianzhuang@ustb.edu; Fax: (+86) 10-6233-2592

Received 14th August 2019 , Accepted 31st August 2019

First published on 2nd September 2019


Designing photocatalysts with heterostructures is an effective way to promote visible-light-driven photocatalytic degradation. Herein, a series of 2D/2D heterojunction photocatalysts, denoted as CuPor-Ph-COF/g-C3N4 composites, were prepared through in situ synthesis on the surface of g-C3N4 by a facile liquid-assisted grinding method. The photocatalytic performance of the as-prepared CuPor-Ph-COF/g-C3N4 composites was evaluated by the degradation of a model pollutant rhodamine B. The CuPor-Ph-COF/g-C3N4 composites displayed superior photocatalytic performance to pure g-C3N4 or pure CuPor-Ph-COF because of the faster separation of photogenerated charges. This represents the first composite fabricated between a 2D porphyrin-based covalent organic framework (COF) and g-C3N4, demonstrating not only the possibility but also more importantly the affordability of the application of costly porphyrin-based COFs in catalysis.


1. Introduction

During the past few decades, graphitic carbon nitride (g-C3N4), a visible-light-responsive semiconductor photocatalyst,1–3 has attracted immense attention in the fields of H2 generation,4–7 chemical sensors, and photocatalytic degradation of environmental pollutants because of its excellent properties such as visible-light response, cost-effectiveness, ease of synthesis, non-toxicity, and chemical and thermal stabilities.8–11 However, the inherent disadvantages of g-C3N4 such as the low surface area, poor absorbance of visible light, and the fast recombination rate of photogenerated electrons and holes have precluded its wide range of applications.12–14 In the past several years, numerous methodologies have been employed to improve the performance of g-C3N4, such as dye sensitization, chemical doping with foreign elements, and hybridization with other materials for fabricating heterojunction composites.15,16 Among these methods, the construction of heterogeneous structures based on g-C3N4 is quite efficient in many cases to promote the separation of photogenerated electrons and holes. Thus far, various metallic oxide semiconductors including Fe2O3, TiO2, BiOI, ZnO, CdS, and MoS2 have been utilized to construct heterojunctions. Specifically, 2D/2D-type heterojunctions based on g-C3N4 were revealed to usually display a lower recombination rate of photogenerated electrons and holes as well as faster charge mobility than 0D/2D or 1D/2D heterojunctions due to the larger interface region in the 2D/2D heterojunction created through face-to-face contact. However, the noncovalent interaction between the above-listed 2D inorganic materials and g-C3N4 is weak, leading to inferior sustainable stability.17,18

2D covalent organic frameworks (COFs) formed usually by a Schiff-base condensation reaction represent a fascinating class of materials that possess large π-systems and are easily functionalized by various substituents.19–21 Recent investigations have demonstrated that the ordered eclipse-stacking structures in COFs are able to generate ordered columnar π-arrays and periodic 1D channels, which in turn afford an efficient pathway for the transport of photogenerated electrons and holes.22 However, the traditional synthesis of imine-based COFs usually involves harsh experimental conditions and long reaction times. Fortunately, in 2014 Banerjee and co-workers successfully fabricated a variety of imine-based COFs by means of the liquid-assisted grinding (LAG) approach at a fast rate with a high reaction yield,23 providing a novel environment-friendly mechanochemical synthesis method for the imine-based COFs. Since the amino groups functionalized onto the edges of g-C3N4 can form imine bonds with aldehyde groups easily, the corresponding 2D imine COF/2D g-C3N4 heterojunctions yielded in this way are therefore expected to have stronger interactions than 2D inorganic materials/2D g-C3N4 heterojunctions.

Porphyrins possessing large π-conjugated systems and exhibiting excellent sunlight-absorbing capacities are an important class of light-harvesting materials in nature.24–26 In addition, the π-stacked aromatic subunits of 2D porphyrin-based COFs provide a good pathway for electronic transport between the COF layers.22 However, the very high cost of the starting materials and the harsh synthesis conditions would seriously restrict their large-scale industrial production and application. To overcome the drawbacks of these materials, constructing a heterojunction between 2D porphyrin-based COFs and cheap g-C3N4 appears to be worth trying. In particular, the 2D porphyrin COFs/g-C3N4 heterojunction created by this strategy would result in not only increased visible-light absorption but also faster charge carrier transfer, facilitating superior photocatalytic performance. Herein, novel 2D/2D heterojunction photocatalysts were fabricated through the in situ synthesis of a 2D CuPor-Ph-COF based on g-C3N4 by a benign LAG process for the first time. The as-synthesized photocatalytic 2D CuPor-Ph-COF/g-C3N4 composites exhibit higher photodegradation for the dye rhodamine B (RhB) than pure g-C3N4 and CuPor-Ph-COFs. This result not only represents the first composite between a 2D porphyrin COF and g-C3N4 but also more importantly opens a new economical pathway for the potential applications of costly porphyrin-based COFs.

2. Experimental

2.1 Photocatalyst preparation

5,10,15,20-Tetra(p-amino-phenyl)porphyrinato copper (CuTAPP)27 and graphitic carbon nitride (g-C3N4) were prepared according to the published procedures.13 All the other reagents were purchased from commercial sources and used directly without further purification. As shown in Scheme 1, the hybridized CuPor-Ph-COF/g-C3N4 composites were synthesized by a simple LAG method. Typically, 200 mg of g-C3N4 and 5.76 mg of terephthalaldehyde (based on CuTAPP) were placed in a mortar together with 1–2 drops of o-dichlorobenzene/n-butanol (1/1 by vol.) and a drop of 6 M aqueous acetic acid. The mixture was milled at room temperature for 30 minutes. Then 16 mg of CuTAPP was added into the mixture and ground for 60 minutes. According to the amount of CuTAPP added, the composites contained CuTAPP with weight percentages of 5, 6, 8, 10, and 20 wt%, respectively. The powder formed was collected and washed with trichloromethane and methanol, and dried under vacuum to give a red solid. The resulting composites are labeled as x wt% CuPor-Ph-COF/g-C3N4, where x stands for CuTAPP wt% in the samples. For comparison, a pure CuPor-Ph-COF was also prepared under the exactly same experimental conditions.
image file: c9dt03307f-s1.tif
Scheme 1 The preparation of CuPor-Ph-COF/g-C3N4 by the liquid-assisted grinding approach.

2.2 Photodegradation experiment

The activities of the as-prepared catalysts for the photodegradation of RhB under visible-light irradiation were determined with a 300 W Xe lamp containing an ultraviolet cutoff filter (>400 nm) as the visible-light source. The RhB photodegradation experiments were carried out with 20 mg of the as-prepared catalyst suspended in 50 mL of 10 mg L−1 RhB aqueous solution with constant magnetic stirring for 1 h in the dark to reach the adsorption equilibrium. Then the RhB mixture containing the photocatalysts was irradiated. At 15 min intervals, a 3.5 mL portion of the mixture was taken out, and the liquid was separated from this extract by centrifugation and analyzed directly for the concentration of residual RhB by UV-vis spectroscopy at a wavelength of 550 nm. The degradation efficiency of the photocatalysts was taken as the average results of three tests. Hydrochloric acid (HCl) was used to adjust the original pH values of the system.

3. Results and discussion

3.1. Characterization of the as-prepared materials

Fig. 1A shows the Fourier transform infrared (FT-IR) spectra of pure g-C3N4, pure CuPor-Ph-COF, and the series of CuPor-Ph-COF/g-C3N4 composites. In the FT-IR spectrum of g-C3N4, the band at 1648 cm−1 is assigned to the typical C[double bond, length as m-dash]N stretching vibration. The bands corresponding to the C–N stretching appear at 1243, 1319, and 1430 cm−1. The characteristic peak at 808 cm−1 is attributed to the triazine rings, suggesting the formation of the typical structure of g-C3N4. Moreover, these characteristic peaks appear in all the CuPor-Ph-COF/g-C3N4 composites, implying that the structure of g-C3N4 is maintained in the composites.28,29 The peak at 1604 cm−1, attributed to the C[double bond, length as m-dash]N stretching, is the characteristic absorption peak of the CuPor-Ph-COF.30,31 It can be seen that the peak at 1619 cm−1 of CuPor-Ph-COF/g-C3N4 becomes stronger than that of pure g-C3N4, confirming the incorporation of CuPor-Ph-COF species in CuPor-Ph-COF/g-C3N4.
image file: c9dt03307f-f1.tif
Fig. 1 (A) FT-IR spectra of g-C3N4, CuPor-Ph-COF, and a series of CuPor-Ph-COF/g-C3N4 composites. (B) Experimental PXRD patterns of g-C3N4 and a series of CuPor-Ph-COF/g-C3N4 composites.

Fig. 1B shows the XRD patterns of the pure g-C3N4, pure CuPor-Ph-COF, and the as-prepared CuPor-Ph-COF/g-C3N4. Fig. S1 (ESI) shows the XRD pattern of the CuPor-Ph-COF prepared by the LAG process. As can be seen, the XRD pattern of the CuPor-Ph-COF displays characteristic diffraction peaks,30 confirming the successful preparation of this material by the LAG method. As shown in Fig. 1B, the diffraction peak of g-C3N4 at 27.8° can be assigned to the (002) planes.32–34 Moreover, the crystal phase of g-C3N4 was not strongly affected by increasing the CuPor-Ph-COF content. However, it is noticeable that the characteristic peak of g-C3N4 in the CuPor-Ph-COF/g-C3N4 composite samples shifts from 27.8° to 28.1°, accompanied by the inter-planar distance changing from 0.320 to 0.317 nm. Furthermore, the characteristic diffraction peaks of CuPor-Ph-COF were not observed in CuPor-Ph-COF/g-C3N4 possibly because of the low content of the CuPor-Ph-COF.

To further confirm the incorporation of the CuPor-Ph-COF into the CuPor-Ph-COF/g-C3N4 composites, X-ray photoelectron spectroscopy (XPS) was employed to detect the chemical states of Cu and N. The survey spectrum of CuPor-Ph-COF/g-C3N4 shows that the material contains Cu, N, C, and O (Fig. 2A). The high-resolution Cu 2p spectrum displays two characteristic peaks at 954.6 (Cu 2p1/2) and 934.4 (Cu 2p3/2) eV (Fig. 2C), implying the existence of metallic Cu in CuPor-Ph-COF/g-C3N4.35,36Fig. 2B displays the N 1s spectrum of g-C3N4. Four peaks at 398.6, 400.1, 400.9 and 405.3 eV are ascribed to sp2 hybridized aromatic N bonded to C atoms (C[double bond, length as m-dash]N–C), tertiary N bonded to C atoms ((C)3–N), N–H side groups, and π-excitation, respectively.37–39 Moreover, the N 1s spectrum of the CuPor-Ph-COF/g-C3N4 composite also displays typical peaks of g-C3N4, with slightly shifted peaks compared with those of pure g-C3N4, suggesting some interaction, such as the π–π interaction, between the CuPor-Ph-COF and g-C3N4.40 The above XPS results further confirm the successful combination of the CuPor-Ph-COF and g-C3N4 in the CuPor-Ph-COF/g-C3N4 composite, consistent with the FT-IR analysis.


image file: c9dt03307f-f2.tif
Fig. 2 (A) XPS survey spectrum of CuPor-Ph-COF/g-C3N4. (B) N 1s XPS spectrum of g-C3N4. (C) Cu 2p XPS spectrum of CuPor-Ph-COF/g-C3N4 (C). (D) N 1s XPS spectrum of CuPor-Ph-COF/g-C3N4 (D).

The morphology of CuPor-Ph-COF/g-C3N4, pure CuPor-Ph-COF, and g-C3N4 was investigated by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The representative images are shown in Fig. 3. In comparison with a smooth surface of g-C3N4, the CuPor-Ph-COF is composed of numerous aggregates of amorphous nanoparticles (Fig. S2, ESI). After the incorporation of the CuPor-Ph-COF into the as-obtained g-C3N4, numerous small nanoparticles were observed on the g-C3N4 sheets, which show the obvious tendency toward aggregation (Fig. 3B). Compared with pure g-C3N4, the surface of the CuPor-Ph-COF/g-C3N4 becomes rougher and coated by small nanoparticles, suggesting the successful loading of the CuPor-Ph-COF onto the surface of g-C3N4 with good interfacial contact. The TEM image of g-C3N4 in Fig. 3C displays a 2D layered sheet structure. As shown in Fig. 3D, the CuPor-Ph-COF/g-C3N4 composite retains a similar layered structure. Furthermore, some smaller 2D layered sheets resembling the CuPor-Ph-COF are also uniformly distributed across the CuPor-Ph-COF/g-C3N4 material.


image file: c9dt03307f-f3.tif
Fig. 3 SEM images of g-C3N4 (A) and CuPor-Ph-COF/g-C3N4 (B). TEM images of g-C3N4 (C) and CuPor-Ph-COF/g-C3N4 (D).

The UV-vis diffuse reflectance spectra (DRS) of g-C3N4, CuPor-Ph-COF/g-C3N4, and CuPor-Ph-COF were recorded and are shown in Fig. 4A and B. It can be seen that the CuPor-Ph-COF/g-C3N4 composite exhibits a stronger and wider visible-light absorption from 450 to 700 nm than g-C3N4, which is likely responsible for strongly enhanced photocatalytic ability. Furthermore, the corresponding band gaps are 2.74 eV and 2.56 eV for g-C3N4 and CuPor-Ph-COF, respectively. Based on the valence band (VB) XPS spectra of g-C3N4 and CuPor-Ph-COF shown in Fig. 4C and D, the VB of CuPor-Ph-COF is 1.08 eV. As a result, the conduction band (CB) of the CuPor-Ph-COF is −1.48 eV which is more negative about 0.61 eV than that of g-C3N4 (−0.87 eV). Therefore, the photoinduced electrons of the CuPor-Ph-COF will transfer to g-C3N4 through the interface. At the same time, the photogenerated holes migrate from g-C3N4 to the CuPor-Ph-COF which results in an efficient charge separation and obviously enhanced photocatalytic ability.


image file: c9dt03307f-f4.tif
Fig. 4 (A) UV-vis diffuse reflectance spectra of g-C3N4, CuPor-Ph-COF and CuPor-Ph-COF/g-C3N4. (B) Band gap of g-C3N4 and CuPor-Ph-COF. VB XPS spectra of g-C3N4 (C) and CuPor-Ph-COF (D).

3.2. Enhancement of photocatalytic activity

RhB, a common hazardous model pollutant, was selected to evaluate the photocatalytic performance. The pseudo-first-order kinetics model was used to simulate the photocatalytic degradation process, in which the value of the rate constant k is equal to the corresponding slope of the fitting line. The results of the photodegradation of RhB are shown in Fig. 5. As shown in Fig. 5A and B, the self-photolysis of RhB is very weak and can be neglected. The pure g-C3N4 and pure CuPor-Ph-COP also display weak photocatalytic activity. Only 23 and 36% of RhB were removed by g-C3N4 and the CuPor-Ph-COF, respectively, after irradiation for 90 min. The rate constants k of the pure CuPor-Ph-COF and g-C3N4 were 0.0049 and 0.0027 min−1, suggesting the only slight visible-light-driven activity.
image file: c9dt03307f-f5.tif
Fig. 5 Photocatalytic degradation of RhB in the presence of g-C3N4, CuPor-Ph-COF, and a series of CuPor-Ph-COF/g-C3N4 composites under visible-light irradiation (A) together with the corresponding kinetic fitting curves (B). Photocatalytic degradation of RhB at a series of pH values by 8 wt% CuPor-Ph-COF/g-C3N4 (C) and the corresponding kinetic fitting curves (D).

In comparison with the CuPor-Ph-COF and g-C3N4, all the CuPor-Ph-COF/g-C3N4 composites exhibited enhanced photocatalytic ability for the degradation of RhB. Specifically, the photocatalytic activity was enhanced when the CuPor-Ph-COF content was increased from 5 to 8 wt%, with the corresponding degradation efficiency increasing from 74 to 86%. However, further increasing the loading amount of CuPor-Ph-COF from 8 to 20 wt% does not lead to notable improvement in the degradation efficiency. Considering the high cost of the starting materials for the CuPor-Ph-COF, the optimum loading amount of CuPor-Ph-COF over g-C3N4 is 8 wt%. The apparent k of the 8 wt% CuPor-Ph-COF/g-C3N4 composite was 0.021 min−1, which is 7.78 times higher than that of pure g-C3N4 (0.0027 min−1). Furthermore, the CuPor-Ph-COF/g-C3N4 composite displays good photodegradation performance for dye methylene and methyl orange (Fig. S3, ESI).

The solution pH is an important factor that can strongly affect catalytic reactions. As a result, we evaluated the effects of pH on the photodegradation of RhB by 8 wt% CuPor-Ph-COF/g-C3N4. As shown in Fig. 5C and D, the composite worked effectively over a wide pH range from 1.0 to 7.0. The degradation of RhB by this composite was faster in acid solutions than in neutral solutions. The degradation of RhB was markedly faster at the pH value of 3 than at any other acidic pH, with the largest apparent rate constant. Fig. 6A shows the time-dependent UV-vis absorption spectra of RhB solution irradiated for 20 min in the presence of 8 wt% CuPor-Ph-COF/g-C3N4 composite at pH 3. Evidently, the intensity of the main RhB absorption band at 550 nm is significantly decreased together with a shift from 550 to 502 nm, which can be ascribed to the step-by-step de-ethylation process.41


image file: c9dt03307f-f6.tif
Fig. 6 (A) UV-Vis spectra of the RhB degraded by 8 wt% CuPor-Ph-COF/g-C3N4 in pH = 3 solution. (B) Reusability of 8 wt% CuPor-Ph-COF/g-C3N4 for RhB degradation.

The sustainability and reusability of the photocatalysts were evaluated by a cycling experiment. The 8 wt% CuPor-Ph-COP/g-C3N4 composite, which achieved the highest photodegradation capacity at pH 3, was employed to represent the photocatalysts. As 8 wt% CuPor-Ph-COF/g-C3N4 was insoluble in water, it could be separated from the RhB solution by centrifugation. The recovered catalyst was then reused to degrade the RhB solution without further treatment. The results of RhB removal are shown in Fig. 6B. It is clear that there was no obvious decrease in the degradation efficiency after the composite was reused four times. Obviously, the photocatalyst is reusable and photostable, which is a very significant criterion towards practical applications.

3.3. Photoluminescence spectroscopic analysis

Photoluminescence (PL) spectroscopy is widely applied to investigate the separation efficiency of the photoinduced electron–hole pairs in semiconductors.42 Therefore, PL was employed here to elucidate the reason that the photocatalytic ability of the CuPor-Ph-COF/g-C3N4 composite exceeds that of pure g-C3N4. The PL spectra of g-C3N4, CuPor-Ph-COF, and the 8 wt% CuPor-Ph-COF/g-C3N4 composite sample were recorded upon excitation at 360 nm. As can be seen in Fig. S4 (ESI), g-C3N4 displayed a strong peak at 455 nm, while the CuPor-Ph-COF had only a weak peak at 422 nm at the same excitation wavelength. After coupling g-C3N4 with the CuPor-Ph-COF, the peak intensity of the 8 wt% CuPor-Ph-COF/g-C3N4 composite was remarkably weaker than that of pure g-C3N4, indicating the significant suppression of the recombination of photogenerated electrons and holes. Moreover, the peak of the 8 wt% CuPor-Ph-COF/g-C3N4 composite was obviously blue shifted from 455 to 443 nm, further suggesting the strong possibility of interactions between the CuPor-Ph-COF and g-C3N4. It therefore can be concluded that the CuPor-Ph-COF was successfully combined with g-C3N4 and the faster separation of photogenerated charges was the main reason for the enhanced photocatalytic ability of the 8 wt% CuPor-Ph-COF/g-C3N4 composite.

3.4. Photoelectrochemical measurement

To further confirm the faster separation and transfer of photogenerated charges in the 8 wt% CuPor-Ph-COF/g-C3N4 composite than in pure g-C3N4, both materials together with the CuPor-Ph-COF were subjected to photocurrent measurement. The results indicate that the photocurrent density of 8 wt% CuPor-Ph-COF/g-C3N4 was significantly increased compared with those of pure g-C3N4 and CuPor-Ph-COF (Fig. 7A). This is attributed to the formation of the 2D/2D heterojunction, which was beneficial for the separation and transfer of photoelectron–hole pairs in g-C3N4. The almost constant photocurrent response during repeated light on–off cycles clearly reveals the good stability of the CuPor-Ph-COF/g-C3N4 photocatalysts.
image file: c9dt03307f-f7.tif
Fig. 7 (A) Transient photocurrent responses of CuPor-Ph-COF/g-C3N4, CuPor-Ph-COF, and g-C3N4. (B) Effects of radical trapping by various scavengers and N2 on the photodegradation of RhB by CuPor-Ph-COF/g-C3N4 under visible-light irradiation.

3.5. Determination of reactive species

The reactive oxidizing species in the degradation process were identified by scavenging experiments. A series of radical scavengers were introduced into the systems, including tertiary butanol (t-BuOH) as a scavenger of ˙OH, KBrO3 as a scavenger of e, and benzoquinone (BQ) as a scavenger of ˙O2.11 As shown in Fig. 7B, the degradation efficiency of RhB with 8% CuPor-Ph-COF/g-C3N4 was remarkably inhibited upon the introduction of BQ, t-BuOH, and KBrO3 or purging the system with N2 gas, implying that photogenerated e, ˙OH, and ˙O2 all play major roles during the RhB photodegradation in the 8% CuPor-Ph-COF/g-C3N4 system.

3.6. A possible photocatalytic mechanism of the CuPor-Ph-COF/g-C3N4 composite

In our synthesis process, the CuPor-Ph-COF was formed and deposited on the surface of g-C3N4. The contact between the two phases forms a heterojunction, which was expected to promote the separation of photoelectrons between the phases. As shown in Fig. 8, a computational model of the two-layer g-C3N4-based system was optimized to its local geometry minimum on the potential energy surface at the theoretical level of M06-2X/6-31G (d, p). The structure was verified as a true minimum by the corresponding frequency calculation at the same theoretical level as the optimization. The calculated mean distance between adjacent g-C3N4 layers was found to decrease from 0.352 to 0.346 nm when the heterostructure was formed (Fig. 8A and B), which is qualitatively in line with the experimental findings, 0.320 to 0.317 nm. Moreover, the energies and distributions of the frontier orbitals of the CuPor-Ph-COF heterostructure were theoretically calculated (Fig. 8C and D). As can be found, the highest occupied molecular orbital (HOMO) is mainly distributed on the CuPor-Ph-COF moiety with an energy of −8.57 eV. The orbitals near the lowest unoccupied molecular orbital (LUMO) are nearly degenerate and correspond to the conduction band (CB) of the photocatalyst system. The LUMO* (LUMO+3 molecular orbital) is mainly distributed on the g-C3N4 moiety with an energy of −1.25 eV.
image file: c9dt03307f-f8.tif
Fig. 8 (A and B) The optimized geometries of g-C3N4 and the CuPor-Ph-COF at the theoretical level of M06-2X/6-31G(d, p). (C and D) The calculated energies and distributions of the frontier orbitals of the CuPor-Ph-COF at the theoretical level of M06-2X/6-31G(d, p). The LUMO* is the LUMO+3 molecular orbital.

According to all the experimental and theoretical investigation results of the CuPor-Ph-COF/g-C3N4 heterostructures, the enhanced efficiency of photocatalytic degradation can be mechanistically explained as follows: the electrons of the valence band (VB) are mainly distributed in the CuPor-Ph-COF orbitals before the visible-light irradiation of the photocatalyst. Then, the electrons in the VB are excited by the irradiation and react with the dissolved oxygen to form superoxide free radicals (O2˙). Furthermore, the photogenerated holes oxidize H2O molecules to H2O2, and H2O2 further decomposes into ˙OH. Meanwhile, ˙OH and O2˙ can efficiently degrade RhB molecules. In terms of the chemical reactions, the above proposed mechanism for RhB degradation by the CuPor-Ph-COF/g-C3N4 composite is described as Fig. 9.


image file: c9dt03307f-f9.tif
Fig. 9 Possible mechanism over the CuPor-Ph-COF/g-C3N4 photocatalyst under visible light irradiation.

4. Conclusions

In summary, a series of 2D/2D heterojunction photocatalysts, termed CuPor-Ph-COF/g-C3N4 composites, were prepared through easy in situ synthesis on the surface of g-C3N4 by a facile LAG method. The recombination of photogenerated electrons and holes in the composites was hindered by the heterostructure formed between g-C3N4 and the 2D CuPor-Ph-COF, endowing the as-prepared 2D/2D CuPor-Ph-COF/g-C3N4 composites with better photocatalytic performance for the degradation of rhodamine B dye than pure g-C3N4. This present result not only represents the first composite of a 2D porphyrin-based COF and g-C3N4 but also more importantly opens a new economical strategy for the potential applications of costly porphyrin-based COFs.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support from the Natural Science Foundation of China (No. 51802082), the Key Scientific and Technological Project of Henan Province (No. 192102310231), and the Key Scientific Research Project of Henan Provincial Education Department (No. 18A150027 and 19B150006). We also acknowledge the financial support from the Henan Institute of Science and Technology (2017040 and 2018T02).

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Footnote

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9dt03307f

This journal is © The Royal Society of Chemistry 2019