A binary polymer composite of graphitic carbon nitride and poly(diphenylbutadiyne) with enhanced visible light photocatalytic activity

Abstract

New polymer composites consisting of poly(diphenylbutadiyne) (PDPB) and g-C₃N₄ have been successfully prepared. UV-vis diffuse reflectance spectra show that the existence of PDPB in composites can clearly increase the visible light absorption of the catalysts. Photoluminescence spectroscopy and photoelectrochemical measurements reveal that PDPB can effectively facilitate the charge carrier separation in the composites. Compared with pure g-C₃N₄ or pure PDPB, the composite catalysts exhibit observably enhanced visible-light photocatalytic activity for degradation of RhB and phenol. A possible mechanism for the charge separation and transfer in the composite catalysts is proposed. In addition, the composite catalysts show stable catalytic performance after five successive runs, displaying potential for applications in various fields of photocatalysis.

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Introduction

Using semiconductor photocatalysts to degrade organic pollutants has attracted much attention due to the global energy crisis and environment pollution.^1–3 To date, a variety of environmentally responsible photocatalysts have been developed, including TiO₂,^4–7 ZnO,^8,9 CdS^10,11 and so on. In particular, graphitic carbon nitride (g-C₃N₄), a novel metal-free material with a special layered structure, has unique properties, such that it is considered to be an ideal material for photocatalysis.^12,13 In 2009, Wang et al. first reported that g-C₃N₄ has excellent photocatalytic ability.¹⁴ Since then, the catalytic performance of g-C₃N₄ for the photocatalytic degradation of organic pollutants in water has been extensively studied.

However, the g-C₃N₄ has many weak points such as the small surface area, high recombination rate of photogenerated electron–hole pairs and low visible light utilization efficiency.¹⁵ Hence, various methods have been used to solve these problems, for example, synthesizing different structures,^16,17 doping with metal or nonmetal elements,^18–21 coupling with other semiconductors,^22–26 and modifying by conjugated polymers.^27,28 Among these methods, the conjugated polymer modification has attracted many attentions because the conjugated polymer not only can enhance the absorption of the visible light but also can act as a semiconductor. Yan et al. synthesized carbon nitride–poly(3-hexylthiophene) (g-C₃N₄–P₃HT) composite photocatalyst.²⁴ They found that the novel photocatalyst exhibited significantly enhanced photocatalytic activity. Thakare et al. reported on the synthesis of a ternary polymer composite of graphene, carbon nitride, and poly(3-hexylthiophene) (G–g-C₃N₄–P₃HT) which had enhanced photocatalytic activity.²⁹ Ge et al. prepared a novel polyaniline–graphitic carbon nitride (PANI/g-C₃N₄) composite photocatalyst which had intensive visible light photocatalytic activity.³⁰ Wang et al. prepared an efficient visible-light photocatalyst using g-C₃N₄ and ordinary polyvinyl chloride (PVC) as main precursors.²⁸ However, to the best of our knowledge, the report about using polymer modified g-C₃N₄ is still scarce.

Poly(diphenylbutadiyne) (PDPB) is a newly developed polymer semiconductor which was reported by Hynd Remita in 2015.³¹ The PDPB obtained by π-stacking of oligomers presents a high photocatalytic activity and long-term stability. Recent research has also shown that the composite of PDPB and semiconductor ZnO exhibited high visible light photocatalytic activity,³² due to the visible light harvesting of PDPB. Inspired by this, we speculate that if we combine PDPB with g-C₃N₄, based on the good visible light absorption of g-C₃N₄ itself, the combination of g-C₃N₄ and PDPB will bring a better absorption of visible light, leading to a better visible light activity.

Herein, we prepare a polymer composite photocatalyst consisting of g-C₃N₄ and PDPB (CN-PDPB). The photocatalytic activity is investigated by evaluating the photocatalytic degradation of Rhodamine B (RhB) and phenol. The results exhibit that the CN–PDPB has higher photocatalytic activity than pure g-C₃N₄ and PDPB. It is maybe because the existence of PDPB can obviously improve the absorption of g-C₃N₄ in the visible light range. What's more, the interaction between g-C₃N₄ and PDPB improves the separation efficiency of the photo-generated charge carriers. In addition, the composites catalysts show stable catalytic performance after five successive runs.

Experimental

The preparation of photocatalysts

g-C₃N₄ was synthesized by directly heating melamine powder. Melamine powder was put into a covered alumina crucible and heated in a muffle furnace. Firstly, the powder was heated to 500 °C with a heating temperature rate of 2 °C min⁻¹ and kept at this temperature for 2 h, then heated to 520 °C within 10 min and kept at this temperature for 2 h. Finally, we obtained a light yellow g-C₃N₄. The product was ground to powder for further usage.

PDPB was synthesized following the previously method published by Hynd Remita.³¹

The composite catalysts of g-C₃N₄ and PDPB were prepared by impregnating g-C₃N₄ (0.5 g) with an ethanol solution of PDPB for a night, and then the solvent was vapored in oil bath at 363 K. The resultant samples were labeled as 50 : x CN–PDPB, where 50 : x was the mass ratio of g-C₃N₄ and PDPB in the above-mentioned process for preparation of CN–PDPB photocatalysts.

Photocatalytic activity test

The photocatalytic activity of the samples was tested by degradation of RhB dyes and phenol at room temperature using a 300 W xenon lamp with AM 1.5 or 420 nm filter as light source. 10 mg of catalyst powder was added to the 50 mL of 10 mg L⁻¹ RhB (or phenol) aqueous solution. Before irradiation, the suspensions were continuously stirred for 30 min in the dark in order to reach an adsorption–desorption equilibrium. At given time intervals, aliquots of the irradiated suspension were collected and centrifuged. The RhB was analyzed by the UV-vis spectra at 553 nm and the phenol was analyzed by a HPLC system.

Results and discussion

Morphology and structural properties

To confirm the structure and morphology of the polymer composites, the g-C₃N₄ and the polymer composite 50 : 2 CN–PDPB were observed by TEM. As shown in Fig. 1, both pure g-C₃N₄ and 50 : 2 CN–PDPB display layered structure. Pure g-C₃N₄ (Fig. 1a) exhibits a flat and smooth surface and it became rough after the modification of PDPB (Fig. 1b). It can be seen clearly that the PDPB distributes on the surface of g-C₃N₄. In addition to TEM, other characterizations were also carried out to prove the existence of PDPB in the composite CN–PPDB.

Fig. 1 TEM images of (a) g-C₃N₄ and (b) 50 : 2 CN–PDPB.

Fig. 2 shows the wide-angle XRD patterns of pure PDPB, pure g-C₃N₄ and CN–PDPB photocatalysts. The peak at 27.7° in the XRD pattern of g-C₃N₄ coming from the interlayer stacking of aromatic segments can be indexed as the (002) planes. Another pattern at 13.1° indexed as (100) planes is due to the in-plane structural packing motif.^5,33,34 From the XRD patterns of CN–PDPB photocatalysts, we can see that the two characteristic peaks of g-C₃N₄ are clearly observed, indicating that PDPB does not change the crystalline structure of g-C₃N₄. However, the characteristic patterns of PDPB are not observed at first because of its low content in the composites but with the increase of content of PDPB, the XRD of 50 : 4 CN–PDPB displays patterns at 14°, 20.4°, 23.6°, 24.7°, which are ascribed to the characteristic patterns of PDPB.³¹ This results further confirm the existence of PDPB in the CN–PDPB composites.

Fig. 2 Wide-angle XRD patterns of (a) PDPB, (b) g-C₃N₄ and CN–PDPB.

The surface chemical environment of the catalysts is analyzed by XPS. Fig. 3a and b show the C 1s XPS spectra of g-C₃N₄ and 50 : 2 CN–PDPB, respectively. In Fig. 3b, the C 1s has two peaks at 284.8 and 288.1 eV. The C peak at 284.8 eV corresponds to graphitic carbon, which is usually observed in the XPS spectrum of carbon nitrides.³⁵ The peak at 288.1 eV is ascribed to the sp²-bonded carbon (N–C=N) from carbon nitride and carbon atoms in conjugated structure (–C=C–) from PDPB.^28,36–39 Compared with Fig. 3a, the intensity ratio of the peaks at 284.8 eV and 288.1 eV in Fig. 3b reduces, indicating the existence of –C=C– on the surface of 50 : 2 CN–PDPB. This further confirms the existence of PDPB in the composite CN–PDPB.

Fig. 3 The XPS spectra of (a) C 1s of g-C₃N₄ and (b) C 1s of 50 : 2 CN–PDPB.

Fig. 4 shows the FTIR spectra of PDPB, g-C₃N₄ and 50 : 2 CN–PDPB. For g-C₃N₄, the strong bands at 1241, 1322, 1406, 1571, and 1631 cm⁻¹ are corresponding to the typical stretching modes of CN heterocycles and the band at 806 cm⁻¹ is attributed to the characteristic breathing mode of triazine units. Compared with the FTIR of PDPB and g-C₃N₄, the FTIR band at 685 cm⁻¹ and 787 cm⁻¹ can be found in 50 : 2 CN–PDPB which is consistence with the previous report³¹ but the intensities of the peaks are weak. That is because the low content of PDPB in 50 : 2 CN–PDPB. All of these prove the existence of PDPB in 50 : 2 CN–PDPB.

Fig. 4 FTIR spectra of PDPB, g-C₃N₄ and 50 : 2 CN–PDPB.

The UV-vis diffuse reflectance spectrum for the catalysts in Fig. 5 shows that the absorption of the CN–PDPB composites is higher than that of pure g-C₃N₄ in the range of 360–800 nm due to the existence of PDPB. Furthermore, the absorption intensity of the CN–PDPB increases with the increase of PDPB content. This improved absorption is helpful to the improvement of the photocatalytic activity of g-C₃N₄.

Fig. 5 UV-vis diffuse reflectance spectra of pure g-C₃N₄ and CN–PDPB composites.

Photocatalytic performances

In order to evaluate the photocatalytic activity of the as-synthesized samples, a test reaction is carried out for the degradation of 10 mg L⁻¹ of RhB with 10 mg of the catalyst under simulated sunlight or visible light and the results are shown in Fig. 6. In addition, Fig. 7 is the UV-vis spectra of the degraded RhB using the 50 : 2 CN–PDPB under simulated sunlight (with AM 1.5). As can be seen from Fig. 6, no matter under irradiation of simulated sunlight or visible light, the photocatalytic degradation of RhB with the CN–PDPB composites is clearly higher than that of the pure g-C₃N₄ and the pure PDPB. The reason may be ascribed to that the combination of g-C₃N₄ and PDPB can improve the separation efficiency of the photo-generated electrons and holes, meaning the recombination of electrons and holes is inhibited. However, from Fig. 6 we can also find that with the content of PDPB increased, the photocatalytic activity of CN–PDPB composites increases at first and then decreases, and the 50 : 2 CN–PDPB shows the highest photocatalytic activity. This is because with the contents of PDPB increase, extra PDPB will hinder the light exposed to g-C₃N₄ and less g-C₃N₄ can be excited to generate electron–hole pairs. As a result, the catalytic activity is reduced. Therefore, there is an optimal ratio to achieve the best photocatalytic activity.

Fig. 6 Photocatalytic degradation of RhB over different catalysts (a) under simulated sunlight (with AM 1.5), (b) under visible light irradiation (λ > 420 nm).

Fig. 7 The UV-vis spectra of the degraded RhB using the 50 : 2 CN–PDPB under simulated sunlight (with AM 1.5).

In addition, we use TOC measurement to further confirm the degree of mineralization of RhB solution. The RhB solution is almost decolorized after 60 min irradiation (in Fig. 6a) and about 30% removal of TOC has been seen in Fig. 8, manifesting that RhB has been not only decolorized but also mineralized partly.

Fig. 8 TOC removal of RhB solution in the presence of 50 : 2 CN–PDPB.

Phenol was chosen as another model pollutant to eliminate the sensitization effect of RhB.⁵ Fig. 9 presents the results of photocatalytic degradation of phenol catalyzed by pure g-C₃N₄ and 50 : 2 CN–PDPB under visible light irradiation. We can see that the photocatalytic degradation efficiency of phenol with 50 : 2 CN–PDPB is obviously higher than that of pure g-C₃N₄, further illustrating that the composite has increscent photocatalytic activity. In addition, we use TOC measurement to further confirm the degree of mineralization of phenol solution. From Fig. 10, it can be seen that the TOC removal values are increased. The degradation of phenol takes time until being finally mineralized.

Fig. 9 Photocatalytic degradation of phenol over different catalysts under visible light irradiation.

Fig. 10 TOC removal of phenol solution in the presence of 50 : 2 CN–PDPB.

Mechanism for the enhanced photocatalytic activity

To investigate the mechanism for the enhancement of the photocatalytic activity by the modification of g-C₃N₄ with PDPB, photoluminescence (PL) spectra were measured. Fig. 11 shows the results of PL spectra of pure g-C₃N₄ and 50 : 2 CN–PDPB excited at 360 nm. As can be seen from the figure, the pure g-C₃N₄ exhibits a PL emission band at 460 nm. In comparison, the PL intensity of 50 : 2 CN–PDPB composites is lower than that of g-C₃N₄ due to the easy transfer of carriers between g-C₃N₄ and PDPB, indicating the increase in the separation efficiency of photo-generated electron–hole pairs.^40–42

Fig. 11 Photoluminescence spectra of g-C₃N₄ and 50 : 2 CN–PDPB.

Besides the photoluminescence measurement, electrochemical impedance spectroscopy (EIS) is also applied to investigate the charge transfer resistance of the catalysts.^43–45 The EIS results of prepared samples are presented in Fig. 12 in the form of Nyquist plots. It can be clearly found that the g-C₃N₄ has the highest charge transport resistance for its biggest arc radius and the arc radius of CN–PDPB is significantly smaller than that of the pure g-C₃N₄. In addition, the 50 : 2 CN–PDPB has the smallest arc radius among the prepared materials, revealing an effective separation of photogenerated electron–hole pairs and faster charge transfer occurred on 50 : 2 CN–PDPB.

Fig. 12 Electrochemical impedance spectroscopy of g-C₃N₄ and CN–PDPB photocatalysts.

To further investigate the photocatalytic mechanism of the CN–PDPB, the transient photocurrent responses of pure g-C₃N₄ and 50 : 2 CN–PDPB were tested by on–off cycles of irradiation.

Transient photocurrent analysis is also a widely used typical method to investigate the electron–hole separation effect for a material.⁴⁶ In Fig. 13, it can be seen that the photocurrent of 50 : 2 CN–PDPB is much larger than the photocurrent of pure g-C₃N₄, indicating higher separation and transfer efficiency of photo-generated electrons and holes due to the synergetic effect of g-C₃N₄ and PDPB.

Fig. 13 Transient photocurrent response for PDPB, g-C₃N₄ and 50 : 2 CN–PDPB photocatalysts.

The effects of benzoquinone (BQ, a typical ˙O₂⁻ scavenger) and disodium oxalate (Na₂C₂O₄, a typical hole scavenger) on the photodegradation of RhB in the presence of 50 : 2 CN–PDPB are investigated. From the Fig. 14, we can see that the BQ clearly inhibits the photodegradation of RhB, but the Na₂C₂O₄ lightly decreases the photodegradation of RhB, indicating that the ˙O₂⁻ and the holes are all effective for the degradation of RhB but the ˙O₂⁻ is the main active species for the RhB degradation.

Fig. 14 Effects of BQ and Na₂C₂O₄ on RhB photodegradation catalyzed by 50 : 2 CN–PDPB photocatalysts.

On the basis of the above experiments and results, the mechanism is explained in Fig. 15. With irradiation of light, both g-C₃N₄ and PDPB can be excited to produce photo-generated electrons and holes. The conduction band (CB) position of PDPB (−1.78 eV) is more negative than that of g-C₃N₄ (−1.13 eV), and the valence band (VB) position of g-C₃N₄ (+1.57 eV) is more positive than that of PDPB (+0.18 eV).³¹ Hence, the photo-generated electrons of PDPB can easily transfer to the conduction band (CB) of g-C₃N₄, while the holes of g-C₃N₄ can transfer to the valence band (VB) of PDPB. Thus, the recombination of photo-generated electrons and holes can be inhibited and an effective charge separation is achieved, resulting in a remarkable enhancement of the photocatalytic activity. Moreover, the major possible reactions can be displayed as follows:

CN–PDPB + hv → g-C₃N₄*–PDPB*

g-C₃N₄*–PDPB* → g-C₃N₄ (e⁻e⁻/h⁺h⁺)–PDPB (e⁻e⁻/h⁺h⁺)

g-C₃N₄ (e⁻e⁻/h⁺h⁺)–PDPB (e⁻e⁻/h⁺h⁺) → g-C₃N₄ (e⁻e⁻e⁻/h⁺)–PDPB (e⁻/h⁺h⁺h⁺)

e⁻ + O₂ → ˙O₂⁻

˙O₂⁻ + organic pollutants → degradation products

h⁺ + organic pollutants → degradation products

Fig. 15 The proposed photocatalytic mechanism of CN–PDPB composites.

The stability of photocatalyst is another important issue for a photocatalyst. The recycling runs in the photodegradation of RhB in the presence of 50 : 2 CN–PDPB are carried out. Fig. 16 shows that the photocatalytic degradation degree of RhB still remains high after 5 cycles, manifesting the catalyst has good photocatalytic stability.

Fig. 16 Effect of cycling runs on RhB degradation in the presence of 50 : 2 CN–PDPB under visible-light irradiation.

Conclusions

In summary, a new photocatalyst has been prepared by compounding g-C₃N₄ and the polymer PDPB via a facile method. The photocatalytic activity of CN–PDPB composites is much higher than that of the pure catalysts (g-C₃N₄ or PDPB), which is confirmed by the photocatalytic degradation of RhB and phenol under simulated solar light or visible light. With the increase of PDPB content, the photocatalytic activity increases at first and then decreases, and 50 : 2 CN–PDPB has the highest photocatalytic activity. The enhanced photocatalytic activity may be caused by the increased charge separation which can be attributed to the synergistic effect between g-C₃N₄ and PDPB. In addition, the photocatalytic stability of the catalyst is good. This study is hopeful to provide some experience for the synthesis of other polymer composited g-C₃N₄ or other semiconductor composited PDPB in the application of photocatalysis and other fields.

Acknowledgements

The authors wish to express their deepest and sincerest recognition of Prof. András Dombi a key figure in the topic of photocatalytic materials for the degradation of contaminants of environmental concern. This work was financially supported by National Nature Science Foundation of China (21407049, 21377038, 21237003 and 21677048), China Postdoctoral Science Foundation (2015T80409), Shanghai Pujiang Program (14PJ1402100), the Science and Technology Commission of Shanghai (16JC1401400) and the Science and Technology Commission of Jiangsu Province (BC2015135).

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