g-C3N4 organic hybrid material

Materials Letters 255 (2019) 126546

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Interfacial charge-transfer transitions enhanced photocatalytic activity of TCNAQ/g-C3N4 organic hybrid material Hairui Huang, Zhijie Zhang ⇑, Shaoke Guo, Kun Cheng, Jiayue Xu, Na Zhang School of Materials Science and Engineering, Shanghai Institute of Technology, 100 Haiquan Road, Shanghai 201418, PR China

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Article history: Received 23 April 2019 Received in revised form 29 July 2019 Accepted 14 August 2019 Available online 16 August 2019 Keywords: Nanocomposites Semiconductors Photocatalysis TCNAQ/g-C3N4 Charge transfer

a b s t r a c t Graphitic carbon nitride (g-C3N4) photocatalyst has a promising application prospect in solving environment problems and energy crisis. In order to improve the photocatalytic activity of g-C3N4, a novel TCNAQ/g-C3N4 organic hybrid photocatalyst was designed by introducing 11,11,12,12-tetracyanonaph tho-1,4-quinodimethane (TCNAQ) as an electron acceptor. Due to the interfacial charge-transfer transitions between g-C3N4 donor and TCNAQ acceptor, separation and migration of photo-induced charge carriers were promoted, while their recombination was prohibited, which led to enhanced photocatalytic performance in pollutant degradation. This work provides a strategy to produce efficient hybrid photocatalysts with high light-to-current conversions due to the interfacial charge-transfer transitions, which have potential applications in light-to-energy conversions. Ó 2019 Elsevier B.V. All rights reserved.

1. Introduction In recent years, graphitic carbon nitride (g-C3N4) has attracted extensive attention as a promising metal-free polymeric photocatalyst. It is considered to possess desirable thermal and chemical stability, as well as relatively small band gap (2.7 eV), proper conjugated band structure, etc. [1–3] Even so, there still remains some drawbacks in pristine g-C3N4, which seriously hinders the practical application of this material. The most obvious one is the low charge mobility, which leads to high recombination rate of photogenerated electron-hole pairs [4,5]. Up to now, many strategies have been applied to overcome this limitation, such as metal or nonmetal doping [6,7], heterojunctions construction [8,9], dye sensitization [10,11] and so on. Nevertheless, developing novel g-C3N4-based photocatalysts with high quantum efficiency and photocatalytic activity remains a great challenge. It has been reported that there exist interfacial charge transfer transitions in organic-inorganic hybrid materials, which enable direct photo-induced charge separation and can be applied in photoenergy conversion processes such as photovoltaics [12] and photocatalysis [13]. 11,11,12,12-tetracyanonaphtho-1,4-quino dimethane (TCNAQ), with highly conjugated structure and abundant p electrons, exhibits strong electron-accepting property and can act as an excellent organic electron acceptor. Fujisawa et al.

⇑ Corresponding author. E-mail address: [email protected] (Z. Zhang). https://doi.org/10.1016/j.matlet.2019.126546 0167-577X/Ó 2019 Elsevier B.V. All rights reserved.

reported a TCNAQ/TiO2 charge-transfer complex, which showed strong visible-light absorption due to the interfacial electronic transition between TCNAQ and TiO2 [14]. Different from TiO2, g-C3N4 also has a conjugated structure, and an ideal combination between g-C3N4 and TCNAQ can be achieved by their strong p–p stacking interactions. Inspired by this, we intend to design a hybrid photocatalyst by combining g-C3N4 with TCNAQ. Applying this hybrid material to photocatalysis, we demonstrated efficient charge transfer in the complex and consequent enhanced photocatalytic activity in pollutant degradation.

2. Experimental TCNAQ and melamine were purchased from J&K Chemical and used without any purification. g-C3N4 was prepared via a typical pyrolysis method as follows: 5 g of melamine was placed into a corundum crucible and heated from room temperature to 550 °C at the rate of 2 °C/min. After that, the system was kept for 2 h in muffle furnace. Finally, yellow g-C3N4 was collected by grounding the product into powder for further use. The TCNAQ/g-C3N4 hybrid photocatalyst was prepared by the following method: appropriate amount of TCNAQ was dissolved in 20 mL of N,N-Dimethylformamide solution, then 0.3 g of the as-prepared g-C3N4 was added into the above solution. Subsequently, the mixture was put in an ultrasonic bath for one hour to disperse g-C3N4. Then the system was placed in a fume hood with constant stirring until the solvent was completely evaporated.

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The powder was obtained by drying in an oven at 60° C for 12 h. In this experiment, the mass percentage of TCNAQ were 0.1 wt%, 0.5 wt% and 1.0 wt%, respectively. The structure, morphology and UV–vis diffuse reflectance spectra of the products were characterized by X-ray diffractometer (Rigaku Co. Ltd., Tokyo, Japan), transmission electron microscopy (TEM, FEI tecnaiG2F30) and UV–vis spectrophotometer (PE Lambda 900), respectively. Photoluminescence spectra (PL) of the products were measured using a FluoroMax-4 fluorescence spectrometer at room temperature. The photocurrents were obtained on an electrochemical system (CHI 650E, Shanghai Chenhua). The photocatalytic activity of the TCNAQ/g-C3N4 photocatalysts were evaluated by the degradation of Rhodamine B (RhB) under simulated solar light irradiation. In this experiment, the irradiation source was a 500 W xenon lamp. A suspension was obtained by dispersing 0.05 g of photocatalysts into 50 mL of RhB solution (10-5 mol/L), which was stirred under simulated solar light irradiation. During the experiment, 3 mL of suspension was taken at regular intervals as a test sample, which was centrifuged to obtain a clear liquid for testing. The concentration of RhB was obtained by recording the variations of the absorption band maximum (552 nm) on a UV–Vis spectrophotometer (PE Lambda 900). 3. Results and discussion Fig. 1(a) shows the XRD spectra of pristine g-C3N4 and the TCNAQ/g-C3N4 hybrid photocatalysts with different contents of TCNAQ. Two obvious diffraction peaks can be seen in the patterns, which are well indexed to g-C3N4 according to JCPDS card No. 87–1526. The weaker peak at 13.2° represents the (1 0 0) crystal plane and the repeated tri-s-triazine unit of g-C3N4, while the stronger one at 27.7° represents the (0 0 2) crystal plane. It is worthwhile to mention that after TCNAQ hybridization, there is no obvious change in the crystal phase of g-C3N4 and no crystalline TCNAQ can be observed, which may be due to the low content of TCNAQ. Moreover, the (0 0 2) peak of g-C3N4 shifts to lower angle with adding TCNAQ, which can be due to the lattice distortion caused by the sp2 hybrid between the N atom of g-C3N4 and C atom of TCNAQ. The morphology and microstructure of the 0.5 wt% TCNAQ/g-C3N4 hybrid are observed by TEM, as shown in Fig. 1 (b). It can be seen that g-C3N4 exhibits a sheet structure, with TCNAQ layer coating on the surface of g-C3N4. The highresolution TEM (HRTEM) image shown in Fig. 1(c) revealed that the thickness of the TCNAQ layer is about 15–30 nm. This result suggests that the TCNAQ/g-C3N4 hybrid is formed with TCNAQ adhering to g-C3N4, rather than a mechanical mixture. The formation of this ideal combination can be due to the strongp–pstacking interactions between g-C3N4 and TCNAQ, which is favorable to the

charge transfer between the two components. TCNAQ is wellknown to form charge-transfer complexes with unique optical and electrochemical properties owing to the characteristicpstacking of the TCNAQ into the complexes [15]. The rapid photoinduced charge separation arising from thep–pstacking interactions leads to enhanced photocatalytic activity of the conjugativepstructure material hybridized photocatalysts [16,17]. The photocatalytic properties of the TCNAQ/g-C3N4 hybrid photocatalysts are characterized by the degradation of RhB under a 500 W xenon lamp to simulate the sunlight. As shown in Fig. 2 (a), all the TCNAQ/g-C3N4 hybrid materials show enhanced photocatalytic degradation activity than pristine g-C3N4. As a contrast, the photocatalytic property of pure TCNAQ is tested, which shows that pure TCNAQ has negligible photocatalytic activity. This result indicated that there is a synergetic effect between TCNAQ and g-C3N4. Moreover, the influence of the content of TCNAQ on the photocatalytic activity is investigated, which shows that with the increase of TCNAQ, the photocatalytic activity enhances first, and reaches the highest when the mass percentage of TCNAQ is 0.5 wt%. However, when the content of TCNAQ further increases, the photocatalytic activity decreases. The change of the photocatalytic performance is due to the balance between the light harvesting and charge separation. When the mass percentage of TCNAQ exceeds the optimal amount, it will shade g-C3N4 for light harvesting. In order to quantificationally compare the photocatalytic efficiency, the apparent rate constant k values for RhB degradation by different samples are calculated and exhibited in Fig. 2(b). It can be obviously seen that the k value for sample 0.5 wt% TCNAQ/g-C3N4 is the highest, which is 2.2 times that of pristine g-C3N4. In addition, the stability of the TCNAQ/g-C3N4 hybrid is tested through circulating runs. As shown in Fig. 2(c), after repeated use for five recycles, no obvious decrease of the photocatalytic activity of the TCNAQ/g-C3N4 hybrid is observed, implying the desirable stability of the TCNAQ/g-C3N4 hybrid. In order to reveal the photocatalytic mechanism, the optical absorption properties of the samples are investigated by UV–vis diffuse reflection spectra (DRS), as shown in Fig. 3(a). Compared to pristine g-C3N4, a double band structure can be observed after introducing TCNAQ, which is due to the intra-molecular absorption of TCNAQ. Moreover, the TCNAQ/g-C3N4 hybrid materials exhibit an absorption band located at ca. 561 nm, which can be ascribed to interfacial charge-transfer transitions between TCNAQ and g-C3N4 [14]. It is suggested that the interfacial charge-transfer transitions may originate from the high electron affinity of TCNAQ, which accepts the lone electron pair of nitrogen atom in the g-C3N4 molecule. The charge-transfer absorption indicates the broadened absorption of TCNAQ/g-C3N4 in the visible region and superior band structure for charge separation and transport [18].

Fig. 1. (a) XRD patterns of the products; (b) and (c) TEM images of the 0.5 wt% TCNAQ/g-C3N4 hybrid.

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Fig. 2. (a) Photocatalytic RhB degradation performances of pure TCNAQ, pristine g-C3N4 and the TCNAQ/g-C3N4 hybrid materials; (b) Comparison of the apparent rate constants k over pristine g-C3N4 and TCNAQ/g-C3N4 with different amounts of TCNAQ; (c) circulating runs of the TCNAQ/g-C3N4 hybrid.

Fig. 3. (a) UV–vis diffuse reflectance absorption spectra of the samples; (b) Transient photocurrents of pristine g-C3N4 and TCNAQ/g-C3N4 hybrids; (c) PL spectra of pristine g-C3N4 and TCNAQ/g-C3N4 hybrids; (d) Illustration of charge-transfer transitions between TCNAQ and g-C3N4.

In order to prove that the interfacial charge-transfer transitions between TCNAQ and g-C3N4 can facilitate the charge separation and migration, the photocurrents of TCNAQ/g-C3N4 composite (0.5 wt%) and pristine g-C3N4 electrodes are measured. As can be seen from Fig. 3(b), the photocurrents of the TCNAQ/g-C3N4 electrodes are obviously enhanced than pristine g-C3N4, which suggests that via the formation of charge-transfer complex between TCNAQ and g-C3N4, the separation and migration efficiency of photo-generated charge carriers is improved. Furthermore, photoluminescence (PL) test is carried out in order to investigate the electron-hole recombination efficiency of the samples. As shown in Fig. 3(c), the PL intensities of TCNAQ/g-C3N4 are lower than that of pristine g-C3N4, which implies lower recombination rate of charge carriers for the TCNAQ/g-C3N4 composites. The decreased PL intensity of TCNAQ/g-C3N4 can be attributed to the interfacial chargetransfer transitions between TCNAQ and g-C3N4 without excessive energy loss, which is favorable to efficient photoenergy conversion [19].

According to the reference, the redox potential of TCNAQ is 0.07 eV [14], while the flat band potential of g-C3N4 is 1.09 eV [18]. The corresponding energy band schematic diagram is drawn in Fig. 3(d), which indicates that TCNAQ can act as an electron acceptor and form a charge-transfer complex with g-C3N4. The interfacial charge-transfer transitions between TCNAQ and g-C3N4 can benefit the separation and migration of photogenerated charge carriers while inhibiting the recombination rate of electron-hole pairs, which leads to an enhanced photocatalytic activity of the TCNAQ/g-C3N4 hybrid material. 4. Conclusions In summary, a novel TCNAQ/g-C3N4 hybrid material with enhanced photocatalytic performance was synthesized. DRS measurement showed that there are interfacial charge-transfer transitions between TCNAQ and g-C3N4. Photocurrent measurement proved that the interfacial charge-transfer transitions can facilitate the separation and migration of photo-generated charge carriers.

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In addition, PL test further confirmed that recombination rate of electron-hole pairs can be suppressed by the formation of charge-transfer complex between TCNAQ and g-C3N4. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51402194) and Young and Middle-aged Teachers Development Fund of Shanghai Institute of Technology (Grant No. 39120 K199048-A06-ZQ2019-19). References [1] Y. Wang, X.C. Wang, M. Antonietti, Angew. Chem. Int. Ed. 51 (2012) 68–89. [2] Q. Liu, T.X. Chen, Y.R. Guo, Z.G. Zhang, X.M. Fang, Appl. Catal. B: Environ. 193 (2016) 248–258. [3] Y. Zheng, Y. Jiao, Y. Zhu, L.H. Li, Y. Han, Y. Chen, A. Du, M. Jaroniec, S.Z. Qiao, Nat. Commun. 5 (2014) 3783.

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