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rGO composite for excellent visible light photocatalytic activity
Applied Surface Science 458 (2018) 586–596
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Full Length Article
Construction of 3D porous g-C3N4/AgBr/rGO composite for excellent visible light photocatalytic activity ⁎
Yaju Zhoua, Jinze Lia, Chongyang Liua, Pengwei Huoa, , Huiqin Wangb, a b
T
⁎
Institute of Green Chemistry and Chemical Technology, School of Chemistry & Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: g-C3N4 AgBr Heterojunction rGO Photocatalyst
A multiple transmission channels, heterojunction and 3D PCN/AgBr/rGO photocatalyst was successfully synthetized by introducing the reduced graphene oxide (rGO) covering AgBr onto the surface of 3D porous g-C3N4 (PCN). In PCN/AgBr/rGO, PCN as a semiconductor photocatalyst provides the 3D framework, the heterojunction was formed with AgBr particles and PCN, rGO promotes the electron transfer simultaneously and introduces a multiple transmission channels system to increase the active species. Thanks to the heterojunction formed between PCN and AgBr, resulting rapid separation of photo-generated e−/h+ at interface. The 3D PCN/AgBr/rGO nanocomposite exhibits excellent photocatalytic efficiencies for tetracycline (TC) up to 78.4% within 90 min and 2,4-dichlorophenol (2,4-DCP) up to 68.2% within 6 h. The photocatalytic rate of PCN/AgBr/rGO was much higher than 3D PCN (21%), AgBr particles (31%) and most of g-C3N4-based photocatalysts in TC degradation. Moreover, the PCN/AgBr/rGO shows high photocatalytic stability due to the significantly photocatalytic stability after four photo-degradation cycles. This study highlights the potential application of PCN/AgBr/rGO highly efficient waste water purification.
1. Introduction The increasingly problem of global environmental pollution poses a serious threat to the sustainable development of human society [1]. Researchers are working hard to find some green technologies to solve these problems. Among most of the solutions, semiconductor-based photocatalytic technology has exhibited outstanding Strong oxidation capacity and sewage treatment capability [2] due to its economic, clean, renewable and safe [3], which uses infinite sunlight as a driving force to perform catalytic reactions for various applications [4]. Such as using semiconductor photocatalysts hydrogen production [5,6], reduction of sterilization [7] and carbon dioxide [8], especially, using semiconductor photocatalysts to remove organic pollutants in wastewater [9,10]. In recent years, graphitic carbon nitride (g-C3N4) has obtained broad notices as a non-metal photocatalyst [11–13], which is favored by many researchers for its high chemical and thermal stability, simple preparation method and suitable band gap [14–17]. However, the conventional bulk g-C3N4 has a low photocatalytic activity due to the high recombination rate of g-C3N4 photo-generated e−/h+ [18–20]. Researchers have investigated different strategies to enhance the performances of g-C3N4, for instance, designing different morphologies, ⁎
forming heterojunction with metal semiconductors [21,22], doping modifications [23,24], etc. Through morphological modification, a tubular, flake, spherical and 3D porous photocatalysts are mainly prepared by the template methods, which contain the hard template methods and the soft template methods [25,26]. It has been proved that the 3D porous structure has an attractive and effective structure duo to its agglomerated and large specific surface area [27,28]. It is significant to explore the novel and simple ways for preparation the PCN. The 3D porous structure could provide high-performance photocatalytic activity. In this work, the PCN was prepared by an ordinary method. However the performance of a single g-C3N4 catalyst cannot be fully reflected, which needs to couple with metal semiconductor and form heterojunction to improve the performance of photocatalyst. Among most of semiconductor catalysts, AgBr as photosensitive material could form a little Ag layers on the surface under illumination, which could protect AgBr from decomposition and improve the stability of AgBr. More importantly, AgBr, with a small band gap could increase the absorption range of visible light and perfectly form heterojunction with gC3N4 to accelerate the separation of the photo-generated e−/h+ [29]. The heterojunction can improve the visible light absorption range of gC3N4 [30,31]. In photocatalytic processes, the photo-carrier transport efficiency plays a vital role in photocatalytic properties. Therefore,
Corresponding authors. E-mail address: [email protected] (P. Huo).
https://doi.org/10.1016/j.apsusc.2018.07.121 Received 1 June 2018; Received in revised form 13 July 2018; Accepted 18 July 2018 Available online 19 July 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic diagrams of preparation process of the PCN/AgBr/rGO nanocomposite.
composite functional materials can effectively improve the overall performance of the catalyst. The rGO has the advantages of large specific surface area, good conductivity, ductility, etc. which can improve the electron transfers rate [32–36] and introduces a multiple transmission channels system to increase the active species. Therefore, AgBr was covered by rGO supporting on g-C3N4 seems a good way to degrade organic pollution. In this work, we obtained the bulk g-C3N4 by simple calcination, grinded the bulk g-C3N4 and NaHCO3 in a certain ratio and fully calcined to obtain PCN. The PCN/AgBr composite photocatalyst was prepared by chemical bath precipitation method. Finally, the PCN/AgBr/ rGO composite photocatalyst was synthesized by hydrothermal reaction. Fig. 1 shows the preparation process of the PCN/AgBr/rGO nanocomposite. The photocatalytic properties of bulk g-C3N4, PCN, AgBr, PCN/AgBr and PCN/AgBr/rGO were evaluated by the degradation of 2,4-DCP and TC under visible light irradiation. In addition, PCN/AgBr/ rGO photocatalysts has a multiple transmission channels system to increase the active species. The stability of PCN/AgBr/rGO was studied by four successive cycles. Finally, based on experimental testing, the possible mechanism was studied with the photocatalytic activity.
mixture was stirred magnetically for 6 h, which according to previous reports [39]. AgNO3 (0.12 mol/L) was added into the solutions. When AgNO3 was completely added, the solutions were stirred magnetically for 18 h. The production was cleansed with ethanol and deionized water for several times. Conclusively, the sample was dried at 60 °C for 18 h under vacuum and PCN/AgBr would be obtained.
2. Experimental section
X-ray powder diffraction (XRD) patterns were recorded to characterize the crystal structure by X-ray diffractometer (model MAC Science, Japan). Structure, constituent components and morphology were investigated in scanning electron microscopy (SEM, JSM-7001F, Japan), transmission electron microscopy (JEM-2010, Japan) and the elemental mapping (ESI). X-ray Photoelectron Spectroscopy (XPS) of nanomaterial was measured by Monochrome MGKα Source PerkinElmer φ1600 Instrument. Fourier transform infrared (FT-IR) spectra were tested by a FT-IR spectrometer. UV–vis diffuse reflectance spectra (DRS) were investigated at room temperature on a Hitachi U-3010 spectrometer. Solid state photoluminescence (PL) spectra were recorded on photoluminescence detector (F-4500, Japan Hitachi). Electron spin resonance (ESR) was performed on the ESR spectrometer (Brucker, A300). The photo-electrochemical properties of the selected samples were measured by testing electrochemical impedance response (EIS) and photocurrent, which was implemented on a Versa-STAT electrochemical workstation (Versa-STAT 3, Beijing, China).
2.3. Synthesis of PCN/AgBr/rGO Graphene oxide graphite (GO) was synthesized by a modified Hummers method [40]. GO (0.005 g) was put in 15 ml deionized water while sonication for 1 h, then put 0.1 g PCN/AgBr into 40 ml polytetrafluoroethylene container and add 15 ml deionized water. Finally, the solutions were treated by sonication for 30 min and mixed in 40 ml polytetrafluoroethylene container, then heated the solutions to 180 °C for 6 h. The sample was washed by centrifugation and collected after being naturally cooled to room temperature, and then dried at 60 °C for 24 h under vacuum to obtain PCN/AgBr/rGO. 2.4. Materials characterization
2.1. Preparation of PCN Urea (CH4N2O), ethanol (C2H5OH) and silver nitrate (AgNO3) were purchased from Sinopharm Chemical Reagent Co, Ltd. Hexadecyl trimethyl ammonium bromide (C16H33(CH3)3NBr), tetracycline (TC) and 2,4-Dichlorophenol were bought from Shanghai Aladdin Bio-Chem Technology Co., Ltd. The urea (10 g) was put into the crucible in the muffle furnace for the condensation reaction [37], which was heated to 500 °C for 4 h (2.5 °C/min). The resultant g-C3N4 was collected and milled into the powder [38]. Mechanical mixing the bulk g-C3N4 (0.4 g) and different ratios of NaHCO3, then put them in a tube furnace calcination reaction, which was heated to 350 °C for 2 h (5 °C/min). The PCN will be obtained. 2.2. Synthesis of PCN/AgBr
2.5. Photocatalytic activity test
The PCN (0.2 g) was dissolved into 25 ml ethanol solutions and the suspensions were treated by sonication for 1 h, then added 50 ml cetyltrimethyl ammonium bromide (CTAB 0.12 mol/L) solutions. The
The photo-degradation rates of g-C3N4, PCN, AgBr, PCN/AgBr and 587
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AgBr/rGO. The highest point 284.6 eV is geared to an indeterminate carbon or sp2 CeC bond from rGO [44,45]. The PCN/AgBr/rGO shows peaks at 286.2 eV and 289.3 eV, which certify the contains of CeOH and OHeC]O bonds [46]. The above results are good agreement with the O 1s spectrum from PCN/AgBr/rGO, where the folding peaks centrality of 531.4 eV and 533.2 eV in accordance with HOeC]O and CeOH in Fig. 3c, respectively [47]. Fig. 3d shows the N 1s spectra, which demonstrate three types of N bonding in PCN/AgBr/rGO. The peaks at 398.4 eV and 400 eV were feature of the sp2 bonded aromatic N bound to C atoms in the triazine units (CeN]C) and the bridging N atoms in Ne(C)3 [48–50]. During the thermal polymerization, the peak of 401.2 eV was confirmed to be as a terminal amino group (CeNeH) due to incomplete condensation [51]. Furthermore, the spectrum of Br 3d in Fig. 3e indicates that the binding energies of Br 3d5/2 and Br 3d3/2 were presented at 67.9 eV and 69 eV. Fig. 3f is indicative of the highresolution spectrum that the peaks of Ag 3d5/2 and Ag 3d3/2 were presented about 368 eV and 374 eV [23]. Infer small changes in binding energy shows that an electronic interaction was appeared between PCN and AgBr, which indicate the heterojunction in the PCN/AgBr/rGO has been formed. According to XRD and XPS analyses, in PCN/AgBr/rGO composites, there are strong coupling and hybridization among PCN, rGO and AgBr.
Fig. 2. XRD patterns of g-C3N4, PCN, AgBr, rGO, PCN/AgBr and PCN/AgBr/ rGO.
PCN/AgBr/rGO photocatalysts were tested by using TC (20 mg/L) and 2,4-DCP (50 mg/L) solutions as goal contaminants under visible radiation. The visible-light source used A 250 W Xenon lamp. Briefly speaking, the reactor system was made up with 0.05 g samples and 100 ml of TC or 2,4-DCP solutions while open recycling water, respectively. To ensure adsorption–desorption equilibrium was reached, the mixture would stir 30 min under dark conditions. The specimen was taken out from the suspension with 15 min intervals in TC solutions and 60 min in 2,4-DCP solutions for centrifuging to extract samples, respectively. The concentration of TC and 2,4-DCP were tested at a wavelength of 357 nm and 284 nm in UV–vis spectrophotometer.
3.3. SEM and EDS analysis The morphology of the sample was studied by SEM and the composition was studied by energy dispersive spectroscopy (EDS). The bulk g-C3N4, PCN, PCN/AgBr and PCN/AgBr/rGO were studied by the SEM and EDS in Fig. 4. Fig. 4(a1) shows the bulk g-C3N4 is a block structure. Fig. 4(a2) shows the PCN has a 3D porous structure. Fig. 4(a3) shows the EDS of PCN consisting with the elements of N, C. Fig. 4(b1, b2) shows the low and high magnification SEM images of PCN/AgBr, it could be observed the surface of PCN become rougher after combined with AgBr. Fig. 4(b3) shows the EDS of PCN/AgBr consisting with the elements of C, N, Ag and Br. Fig. 4(c1, c2) shows the low and high magnification SEM images of PCN/AgBr/rGO, it can be seen that AgBr nanoparticles is covered by rGO loaded on the inner and outer surfaces of the PCN. Fig. 4(c3) shows the EDS of PCN/AgBr consisting with the elements of C, N, O, Ag and Br, respectively. On the basis of the above XPS, EDS, SEM and XRD prove that the 3D porous composite photocatalyst has been prepared.
3. Results and discussion 3.1. XRD analysis Fig. 2 shows the crystalline structure and phase purity of bulk gC3N4, AgBr, PCN, rGO, PCN/AgBr and PCN/AgBr/rGO, which were studied by XRD measurements. For bulk g-C3N4, the different peaks at 27.4° can be pointed to the (0 0 2) plane of the graphite material and this is a well-known C-N network structure. After calcination with NaHCO3, the diffraction peaks at 27.4° of the obtained PCN were obviously enhanced, which indicate that inter planar stacking structure and the aromatic structure inside the crystal surface of the treated gC3N4 aromatic ring were well preserved [41]. For AgBr, the diffraction peaks appear at 31°, 44.5°, 55.1°, 64.6° and 73.5°, which are considered to be homologous with the (2 0 0), (2 2 0), (2 2 2), (4 0 0) and (4 2 0) AgBr of the cubic phase in plane (JCPDS No. 06-0438), respectively [42]. The PCN/AgBr presented concomitance of both AgBr and PCN phases, then the main diffraction peaks from AgBr and the weak peak at 27.4° corresponds to PCN. Compared with AgBr, the diffraction peak of PCN is so weak that it is difficult to show PCN diffraction peaks in PCN/ AgBr composites [39]. At the same time, there was no significant rGO diffraction peaks were found in PCN/AgBr/rGO composites, which may due to the low content of rGO in the composite [43]. From the above results, which prove that the prepared composite photocatalyst is PCN/ AgBr/rGO.
3.4. TEM and elemental mapping analysis The TEM images of g-C3N4, PCN, PCN/AgBr, PCN/AgBr/rGO and the elemental mapping (ESI) of the PCN/AgBr/rGO were shown in Fig. 5, respectively. Fig. 5a shows that g-C3N4 processed by ultrasound is thick structure before TEM test. Fig. 5b shows PCN sample has a 3D porous morphology and the aperture is within the range of about 40–300 nm. In Fig. 5c, the AgBr nanoparticles are evenly scattered on the surface of PCN without aggregation. Fig. 5d shows the PCN/AgBr/ rGO composite that AgBr nanoparticles are well covered by rGO and dispersed on the surface of PCN. The above results show a strong effect of PCN on AgBr nanocrystals and rGO, which leads to increase the surface areas of PCN/AgBr/rGO contact with pollutants. These results prove that rGO and AgBr were successfully composed with PCN. The ESI of the PCN/AgBr/rGO was shown in Fig. S1 indicates that the elements C, N, O, Ag and Br are uniformly distributed on the surface. The above characterization corresponds to the XRD, EDS and TEM results, indicating that the 3D porous composite photocatalyst has been prepared. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.07.121.
3.2. XPS analysis To study the valence of various elements and chemical composition in PCN/AgBr/rGO samples, which were detected by XPS. Fig. 3a describes the C, N, O, Br and Ag elements are expressed on the PCN/AgBr/ rGO composites. Fig. 3b describes the XPS spectrum of C 1s in PCN/ 588
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Fig. 3. XPS spectra of PCN/AgBr /rGO: (a) survey, (b) C 1s, (c) O 1s, (d) N 1s, (e) Br 3d and (f) Ag 3d.
rGO. A strong and wide peak from PCN is detected at around 1552 cm−1, which is different from usually observed in bulk g-C3N4, indicating that the PCN was well stripped into nanostructures [56,57]. The Raman spectrum of PCN/AgBr/rGO also displays a broad band at 1500 cm−1, indicating the successful combination of PCN and rGO in the composite [58].
3.5. FT-IR and Raman analysis The FT-IR spectra of g-C3N4, PCN, rGO, AgBr, PCN/AgBr and PCN/ AgBr/rGO are shown in Fig. S2a. The spectra of g-C3N4 and PCN display three characteristic absorption bands at > 3000, 1200–1800 and < 1000 cm−1, which are owing to the NeH bonds [52] and the stretching contraction modes of the heterocycle [53]. Because the optical absorbability of AgBr is relatively weak, PCN/AgBr has similar absorption band properties with PCN [54]. Similarly, due to the low content of rGO that the absorption peaks from rGO are not observed in PCN/AgBr/ rGO. It is noteworthy that the above characteristic peaks appear in the spectrum of PCN/AgBr/rGO sample, which not only prove the successful hybridization of PCN with AgBr and rGO, but also prove the forming of a hybrid heterojunction in the PCN/AgBr/rGO and an electronic interaction was developed between PCN and AgBr [55]. Fig. S2b shows that the Raman spectroscopy was further used to identify the degree of disorder and the number of defects in PCN/AgBr/
3.6. UV–vis diffuse reflection spectra Fig. 6a shows the light absorption efficiency is highly correlated with photocatalytic property by UV–vis DRS. The absorption wavelength of bulk g-C3N4 is 446 nm. After calcination with NaHCO3, the absorption wavelength of PCN shifts from 446 nm blue to 440 nm. With the addition of AgBr, the red shift from the response wavelength of the composite system to visible light increases the utilization of visible light. When AgBr and PCN are combined, the absorption edge of PCN/ AgBr is between 440 nm (PCN) and 495 nm (AgBr), which proves that 589
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Fig. 4. SEM images of (a1, a2, a3) initial g-C3N4, PCN and EDS of PCN, (b1, b2, b3) different magnification and EDS of PCN/AgBr, (c1, c2, c3) different magnification and EDS of PCN/AgBr/rGO.
3.8. Photoluminescence spectra
the heterojunction between PCN and AgBr is higher in visible light absorption and leads to more photo-generated e−/h+ pairs and better photocatalytic performance [45]. After rGO is composited with PCN/ AgBr, the PCN/AgBr/rGO sample has an increased absorption range, which proves that rGO is synergistic with PCN/AgBr [24]. Fig. 6b shows the band gaps of pure AgBr and PCN are calculated to be 2.39 eV and 2.63 eV.
Photoluminescence (PL) spectra could be extensively used to study the photoelectron-cavity pair migration, transfer and composite processes in nanocomposites. Therefore, to further confirm the introduction of AgBr and rGO promotes the uncoupling of photo-generated e−/ h+ pairs, the PL spectra of g-C3N4, PCN, PCN/AgBr and PCN/AgBr/rGO are shown in Fig. 8. Photoluminescence intensity represents the composite degree of radiation charges, which is used as an indirect evidence of charge separation [61]. After calcination with NaHCO3, the fluorescence intensity of the PCN is weaker than bulk g-C3N4, which may due to the 3D porous structure can promotes the uncoupling of photogenerated e−/h+ pairs compared with bulk g-C3N4. By adding rGO and AgBr, the intensity of the forceful emission peak at 440 nm is momentous reduced, indicating the recombination of the photo-generated e−/h+ pair is restrained [62].
3.7. BET analysis The specific surface areas of g-C3N4, PCN, PCN/AgBr and PCN/ AgBr/rGO composite are assured by NOVA3000E (Quantachrome Instruments U.S.). Fig. 7 shows the isotherms and pore sizes distributions. Compared with g-C3N4, the specific surface areas of PCN increased about 2 m2/g. The BET specific surface area of PCN/AgBr is lower than PCN, which may be due to the partial pores of PCN covered by AgBr nanoparticles, so the surface area of PCN/AgBr is less than PCN after the composite material is formed [59]. Moreover, the BET surface areas of PCN/AgBr/rGO are increased than PCN/AgBr, it may be because the AgBr nanoparticles are covered by rGO. The surface areas of PCN/AgBr/rGO are smaller than PCN after forming composite. In the degradation of wastewater, the specific surface area of the PCN/AgBr/ rGO does not play a master role. The pure g-C3N4, PCN, PCN/AgBr and PCN/AgBr/rGO displayed type IV and H3 hysteresis loops, which certified the existence of macropores [60]. After calcining with NaHCO3, the pore size distribution is obvious, and the pore structure of PCN is about 40 nm. Furthermore, the BET surface areas and pore parameters were shown in table 1.
3.9. Electrochemical properties of the prepared photocatalysts The charge transfer capacity is a significant factor to study the photocatalytic properties [63], which can be further researched by photochemical method. Fig. 9a displays the experimental Nyquist impedance plots of g-C3N4, PCN, AgBr, rGO, PCN/AgBr and PCN/AgBr/ rGO. Compared with g-C3N4, the EIS of PCN is weaker than g-C3N4, which may due to the 3D porous structure promotes the separation of photo-generated e−/h+ pairs. Compare with PCN, due to AgBr and PCN formed heterojunction that the radius of PCN/AgBr Nyquist arc is obviously smaller [64]. Compared with PCN/AgBr, the radius of Nyquist arc of PCN/AgBr/rGO is significantly smaller, which exhibits that 590
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Fig. 5. TEM images of (a) g-C3N4, (b) PCN, (c) PCN/AgBr and (d) PCN/AgBr/rGO.
which coincident with the DRS and PL results. The enhanced photocurrent may be due to the enhanced electromagnetic field of AgBr and the increased electron transfer rate by the rGO. The results show the AgBr and rGO vectors can enhance the separation of photo-generated e−/h+ pairs and prolong the photo-generated charge [65]. The results further confirmed that PCN/AgBr/rGO has relatively high transient photocurrent response.
load rGO could improve the charge transfer efficiency [43]. Fig. 9b shows the transient photocurrent response of g-C3N4, PCN, AgBr, PCN/ AgBr and PCN/AgBr/rGO. The photocurrent intensity was studied by the optical switching process (pulse 30 s). The PCN/AgBr/rGO has the strongest photocurrent response to all of the g-C3N4, PCN, AgBr and PCN/AgBr. The photocurrent enhancement of the obtained PCN/AgBr/ rGO composite shows the increased light-induced carrier transport rate and the improved separation of photo-generated e−/h+ pairs [5],
Fig. 6. (a) UV–vis diffuse reflectance spectra of g-C3N4, PCN, AgBr, rGO, PCN/AgBr and PCN/AgBr/rGO. (b) Band gaps of pure AgBr and PCN. 591
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core, which can strength to the plasma effect of Ag+ [3] and prevent the successive recombination of photoelectrons and holes provocative from PCN[66]. However, a large number of AgBr nanoparticles may produce enhanced local electromagnetic field of AgBr nanoparticles and delay the acceptance of electron excitation from the PCN [67]. At the same time, PCN is not enough to mediate the formation of AgBr core due to the increase of AgBr nanoparticles. Therefore, The increase of light absorption leads to the photo-thermal conversion to AgBr nanoparticles and the decrease of plasmon resonance effects [68]. More importantly, only so small amount of effective photoelectrons and holes can be added to the reaction that the photocatalytic efficiency cannot be maximized. As is shown in Fig. 10b, the contents of rGO also play a decisive role the photocatalytic activity of the PCN/AgBr/rGO. With the increased content of rGO, the photocatalytic activity of PCN/AgBr/rGO increased gradually, but the photocatalytic activity of PCN/AgBr/rGO began to decrease when rGO content was more than 5%. Therefore, when rGO content was 5%, CN/AgBr/rGO exhibited the highest activity. Fig. 10c reveals the photocatalytic activity of g-C3N4, PCN, PCN/ AgBr and PCN/AgBr/rGO photodegrading 2,4-DCP. The results show that PCN/AgBr/rGO has higher photocatalytic capability than other catalysts, which could point out that the correct introduction of AgBr nucleus and rGO into PCN would enhance the effective utilization of visible light [10]. The enhanced photocatalytic properties of CN/AgBr/ rGO may be attributed to the 3D porous core-shell structure of PCN/ AgBr/rGO and the plasma effect of the Ag+ to promote the separation of photo-generated e−/h+ pairs [69]. The recycling is of great significance to the practical application of photocatalysts. Fig. 10d is the result of the cyclic degradation of TC by PCN/AgBr/rGO composites. It shows the diagram that the degradation rate of PCN/AgBr/rGO to TC during the second cycle catalytic process is 75.5%, which is lower than the first degradation rate (79.80%). However, during the next cycle of catalysis, the activity of PCN/AgBr/rGO to TC was not significantly reduced. This may be part of the reduction of AgBr in the first and second cycles of the catalytic process, while reducing the formation of Ag coated on the surface of the AgBr particles, which can inhibit further reduction, so in the subsequent cycle PCN/AgBr/rGO. The degradation activity of TC indicates that the catalytic effect of the sample is unchanged, and it can be deduced that the sample has good stability. The XRD diffraction diagrams of PCN/AgBr/rGO before and after reaction indicate that the crystal structure of PCN/AgBr/rGO keeps balance in Fig. S3. Consider the above results, it can be authenticated that the PCN/AgBr/rGO compound material is stable during photocatalysis process. Through XRD analysis of fourth cycles after the catalyst photocatalytic, a small amount of silver does appear in the complex system. More importantly, after fourth cycles, the samples still have high photocatalytic activity, indicating that the PCN/AgBr/RGO system has practical significance.
Fig. 7. N2 adsorption-desorption isotherms and the inset is corresponding poresize distribution curves of bulk g-C3N4, PCN, PCN/AgBr and PCN/AgBr/rGO composites. Table 1 BET surface area, pore size and pore volume of g-C3N4, PCN, PCN/AgBr and PCN/AgBr/rGO samples. Sample
SBET (m2/g)
Pore size (nm)
Pore volume (mg3/g)
g-C3N4 PCN PCN/AgBr PCN/AgBr/rGO
33.652 35.747 3.526 10.783
38.344 37.842 37.808 37.842
0.055 0.063 0.013 0.021
3.11. Active species analysis Fig. 8. Solid PL emission spectrums of g-C3N4, PCN, PCN/AgBr and PCN/AgBr/ rGO.
To determine what is the major role in the photocatalytic activity of PCN/AgBr/rGO materials, a series of capture and quantitation experiments of active species were investigated by adding different capture scavengers. Hence, we used isopropanol (IPA), ascorbic acid (VC) and EDTA-2Na as the active particle quenchers for %OH, %O2− and holes. When IPA was added, the photo-degradation effect of TC is less shown in Fig. 11, which indicates that %OH was not the main active ingredient. In contrast, when VC and EDTA-2Na were added, the photocatalytic activity was greatly suppressed, which show that %O2− and h+ play a key role in photo-degradation of TC. To research the major active species among the photocatalysis degradation process, ESR was used to characterize the performance of %OH and %O2− radicals in PCN/AgBr/ rGO photocatalysts under visible light [70,71]. There were four characteristic peaks of DMPO-%O2− appear in the spectra of Fig. 12a, which revealed the photo-generated electrons in the conduction band of semiconductors could be converted into %O2− radicals during the
3.10. Photocatalytic activity and stability analysis The photo-activity of bulk g-C3N4, PCN, AgBr, PCN/AgBr and PCN/ AgBr/rGO photocatalysts was studied by degradation of 20 mg/L TC and 50 mg/L 2,4-DCP under visible light. As a comparison, TC and 2,4DCP degradation was performed with bulk g-C3N4, PCN, AgBr, PCN/ AgBr and PCN/AgBr/rGO under the same conditions. Fig. 10a shows the TC degradation profile of the bulk g-C3N4, PCN, AgBr and different AgBr content of PCN/AgBr. The degradation of g-C3N4, PCN and AgBr were 14.3%, 31.6% and 41.3% under visible light source, and when add 0.12 mol/L of AgBr, the PCN/AgBr has high activity than other samples and the visible light degradation rat is about 69.3% within 90 min. The photosensitivity enhancement of PCN/AgBr may be due to the AgBr 592
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Fig. 9. (a) EIS spectra of bulk g-C3N4, PCN, AgBr, PCN/AgBr and PCN/AgBr/rGO in light and in the dark. (b) Photocurrent of g-C3N4, PCN, AgBr, PCN/AgBr and PCN/AgBr/rGO under visible light.
EVB = ECB + Eg
photo-degradation system [72,73]. As is shown in Fig. 12b, compared with DMPO-%O2−, DMPO-%OH shows no obvious signal appeared, intimating %OH could be generated small amounts in PCN/AgBr/rGO systems. According to the above analysis of capture and quantitation experiments, we could conclude that the active species of %O2− and h+ have higher activity than %OH in visible light photocatalytic degradation.
(1)
where EVB was VB edge potential and ECB was CB edge potential. AgBr and PCN are estimated to −1.11 eV and 1.52 eV, −0.75 eV and 1.64 eV in CB and VB. Based on the above analysis, two possible mechanisms for the PCN/AgBr/rGO photocatalysis are illustrated in Fig. 13. Obviously, the PCN/AgBr/rGO has a multiple transmission channels system to increase the number of active species. A possible mechanism is shown in Fig. 13a that the PCN and AgBr as oxidation and reduction semiconductors in the indirect PCN/AgBr/rGO heterojunctions, which could be expressed under simulated solar light irradiation [48]. Then the photo-generated electrons would move to the AgBr side. Furthermore, rGO could cooperate well with the VB level of PCN and competing with the recombination process of e−/h+ pairs. The reasonable spatial distribution enables ultrafast charge separation.
3.12. Mechanism of PCN/AgBr/rGO photocatalysts. Typical, the VB of XPS has been tested to calculate the band construction of AgBr and PCN in Fig. S4. The conduction band (CB) and the valence Band (VB) of AgBr and PCN could be calculated by the equation:
Fig. 10. The TC photo-degradation efficiency of bulk g-C3N4, PCN, AgBr and as-prepared PCN/AgBr with different concentration of AgBr (a), and the different contents of introduced rGO (b), the 2,4-DCP irradiation time for the bulk g-C3N4, PCN, AgBr, PCN/AgBr and PCN/AgBr/rGO (c), cycling runs in photo-degradation 20 mg/L TC of PCN/AgBr/rGO (d). 593
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of AgBr by rGO, leaving the e−/h+ in the CB/VB of PCN/AgBr for further reduction/oxidation reactions. Therefore, the strengthen photocatalytic activity of PCN/AgBr/rGO is attributed to form heterojunction to improve the absorption of visible light and enhance the charge separation. From the PL and photocurrent results can show that PCN/AgBr/rGO exhibit high separation efficiency of photo-generated charge carriers. Based on the result and discussion, the prefer mechanism is shown in Fig. 13b that the PCN and AgBr as oxidation and reduction semiconductors in the Z-scheme PCN/AgBr/rGO heterojunctions. 4. Conclusions In summary, a novel ternary visible light-driven PCN/AgBr/rGO heterojunction photocatalyst has been prepared by introducing reduced graphene covering AgBr onto the surface of PCN. Thanks to the formation of Z-scheme heterojunction between PCN and AgBr, the efficient use of e−/h+ are achieved and resulting rapid separation of charge
Fig. 11. Effect of different scavengers on the TC degradation of PCN/AgBr/ rGO.
Fig. 12. DMPO spin-trapping ESR spectra of PCN/AgBr/rGO in aqueous dispersion (a), and in methanol dispersion (b).
Subsequently, the electrons on CB of the PCN have stronger reduction ability could reacted with O2 to form %O2− and the holes on VB of the PCN have stronger oxidation ability. Finally, the pollutants were oxidized by %O2− and h+. Furthermore, rGO can be used as an electron transport channel due to its excellent electron transport capability and increase the electron transport rate, which could raise the number of active species. Simultaneously, the photo-generated holes on the VB of PCN are straight participated in the oxidation of pollutants. The photoinduced electrons could easily jump from the conduction band (CB) of PCN to that of AgBr or react with adsorbed O2 to yield %O2−and partially form %OH. Another possible mechanism is shown in Fig. 13b that the PCN and AgBr as oxidation and reduction semiconductors in the Zscheme PCN/AgBr/rGO heterojunctions [74–76]. The photo-generated electrons in the CB of PCN can be rapidly transferred to the holes in VB
carriers at the interface. Moreover, due to the introduction of rGO, electrons in AgBr can be transported at high speed, which further effectively inhibits the reunion of photo-generated e−/h+ pairs and introduces a multiple transmission channel system to increase the number of active species. Under visible light, the PCN/AgBr/rGO has great photo-degradation performance for TC and 2,4-DCP. Therefore, this study provides a novel and simple method for synthesizing high-efficiency and multiple components composite photocatalysts to purify the environment. Acknowledgments We are very grateful for the financial support of the National Natural Science Foundation of China (Grant No. 21576125, 21776117,
Fig. 13. Schematic of two possible mechanisms of PCN/AgBr/rGO hybrids upon visible light irradiation. 594
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U1510126, 21676127), the Natural Science Foundation of Jiangsu Province (Grant No. BK20151349), China Postdoctoral Science Foundation (2017M611716 and 2017M611734), Six talent peaks project in Jiangsu Province (Grant No. XCL-014), Zhenjiang Science & Technology Program (Grant No. SH2016012).
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Full Length Article
Construction of 3D porous g-C3N4/AgBr/rGO composite for excellent visible light photocatalytic activity ⁎
Yaju Zhoua, Jinze Lia, Chongyang Liua, Pengwei Huoa, , Huiqin Wangb, a b
T
⁎
Institute of Green Chemistry and Chemical Technology, School of Chemistry & Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China School of Energy and Power Engineering, Jiangsu University, Zhenjiang 212013, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: g-C3N4 AgBr Heterojunction rGO Photocatalyst
A multiple transmission channels, heterojunction and 3D PCN/AgBr/rGO photocatalyst was successfully synthetized by introducing the reduced graphene oxide (rGO) covering AgBr onto the surface of 3D porous g-C3N4 (PCN). In PCN/AgBr/rGO, PCN as a semiconductor photocatalyst provides the 3D framework, the heterojunction was formed with AgBr particles and PCN, rGO promotes the electron transfer simultaneously and introduces a multiple transmission channels system to increase the active species. Thanks to the heterojunction formed between PCN and AgBr, resulting rapid separation of photo-generated e−/h+ at interface. The 3D PCN/AgBr/rGO nanocomposite exhibits excellent photocatalytic efficiencies for tetracycline (TC) up to 78.4% within 90 min and 2,4-dichlorophenol (2,4-DCP) up to 68.2% within 6 h. The photocatalytic rate of PCN/AgBr/rGO was much higher than 3D PCN (21%), AgBr particles (31%) and most of g-C3N4-based photocatalysts in TC degradation. Moreover, the PCN/AgBr/rGO shows high photocatalytic stability due to the significantly photocatalytic stability after four photo-degradation cycles. This study highlights the potential application of PCN/AgBr/rGO highly efficient waste water purification.
1. Introduction The increasingly problem of global environmental pollution poses a serious threat to the sustainable development of human society [1]. Researchers are working hard to find some green technologies to solve these problems. Among most of the solutions, semiconductor-based photocatalytic technology has exhibited outstanding Strong oxidation capacity and sewage treatment capability [2] due to its economic, clean, renewable and safe [3], which uses infinite sunlight as a driving force to perform catalytic reactions for various applications [4]. Such as using semiconductor photocatalysts hydrogen production [5,6], reduction of sterilization [7] and carbon dioxide [8], especially, using semiconductor photocatalysts to remove organic pollutants in wastewater [9,10]. In recent years, graphitic carbon nitride (g-C3N4) has obtained broad notices as a non-metal photocatalyst [11–13], which is favored by many researchers for its high chemical and thermal stability, simple preparation method and suitable band gap [14–17]. However, the conventional bulk g-C3N4 has a low photocatalytic activity due to the high recombination rate of g-C3N4 photo-generated e−/h+ [18–20]. Researchers have investigated different strategies to enhance the performances of g-C3N4, for instance, designing different morphologies, ⁎
forming heterojunction with metal semiconductors [21,22], doping modifications [23,24], etc. Through morphological modification, a tubular, flake, spherical and 3D porous photocatalysts are mainly prepared by the template methods, which contain the hard template methods and the soft template methods [25,26]. It has been proved that the 3D porous structure has an attractive and effective structure duo to its agglomerated and large specific surface area [27,28]. It is significant to explore the novel and simple ways for preparation the PCN. The 3D porous structure could provide high-performance photocatalytic activity. In this work, the PCN was prepared by an ordinary method. However the performance of a single g-C3N4 catalyst cannot be fully reflected, which needs to couple with metal semiconductor and form heterojunction to improve the performance of photocatalyst. Among most of semiconductor catalysts, AgBr as photosensitive material could form a little Ag layers on the surface under illumination, which could protect AgBr from decomposition and improve the stability of AgBr. More importantly, AgBr, with a small band gap could increase the absorption range of visible light and perfectly form heterojunction with gC3N4 to accelerate the separation of the photo-generated e−/h+ [29]. The heterojunction can improve the visible light absorption range of gC3N4 [30,31]. In photocatalytic processes, the photo-carrier transport efficiency plays a vital role in photocatalytic properties. Therefore,
Corresponding authors. E-mail address: [email protected] (P. Huo).
https://doi.org/10.1016/j.apsusc.2018.07.121 Received 1 June 2018; Received in revised form 13 July 2018; Accepted 18 July 2018 Available online 19 July 2018 0169-4332/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic diagrams of preparation process of the PCN/AgBr/rGO nanocomposite.
composite functional materials can effectively improve the overall performance of the catalyst. The rGO has the advantages of large specific surface area, good conductivity, ductility, etc. which can improve the electron transfers rate [32–36] and introduces a multiple transmission channels system to increase the active species. Therefore, AgBr was covered by rGO supporting on g-C3N4 seems a good way to degrade organic pollution. In this work, we obtained the bulk g-C3N4 by simple calcination, grinded the bulk g-C3N4 and NaHCO3 in a certain ratio and fully calcined to obtain PCN. The PCN/AgBr composite photocatalyst was prepared by chemical bath precipitation method. Finally, the PCN/AgBr/ rGO composite photocatalyst was synthesized by hydrothermal reaction. Fig. 1 shows the preparation process of the PCN/AgBr/rGO nanocomposite. The photocatalytic properties of bulk g-C3N4, PCN, AgBr, PCN/AgBr and PCN/AgBr/rGO were evaluated by the degradation of 2,4-DCP and TC under visible light irradiation. In addition, PCN/AgBr/ rGO photocatalysts has a multiple transmission channels system to increase the active species. The stability of PCN/AgBr/rGO was studied by four successive cycles. Finally, based on experimental testing, the possible mechanism was studied with the photocatalytic activity.
mixture was stirred magnetically for 6 h, which according to previous reports [39]. AgNO3 (0.12 mol/L) was added into the solutions. When AgNO3 was completely added, the solutions were stirred magnetically for 18 h. The production was cleansed with ethanol and deionized water for several times. Conclusively, the sample was dried at 60 °C for 18 h under vacuum and PCN/AgBr would be obtained.
2. Experimental section
X-ray powder diffraction (XRD) patterns were recorded to characterize the crystal structure by X-ray diffractometer (model MAC Science, Japan). Structure, constituent components and morphology were investigated in scanning electron microscopy (SEM, JSM-7001F, Japan), transmission electron microscopy (JEM-2010, Japan) and the elemental mapping (ESI). X-ray Photoelectron Spectroscopy (XPS) of nanomaterial was measured by Monochrome MGKα Source PerkinElmer φ1600 Instrument. Fourier transform infrared (FT-IR) spectra were tested by a FT-IR spectrometer. UV–vis diffuse reflectance spectra (DRS) were investigated at room temperature on a Hitachi U-3010 spectrometer. Solid state photoluminescence (PL) spectra were recorded on photoluminescence detector (F-4500, Japan Hitachi). Electron spin resonance (ESR) was performed on the ESR spectrometer (Brucker, A300). The photo-electrochemical properties of the selected samples were measured by testing electrochemical impedance response (EIS) and photocurrent, which was implemented on a Versa-STAT electrochemical workstation (Versa-STAT 3, Beijing, China).
2.3. Synthesis of PCN/AgBr/rGO Graphene oxide graphite (GO) was synthesized by a modified Hummers method [40]. GO (0.005 g) was put in 15 ml deionized water while sonication for 1 h, then put 0.1 g PCN/AgBr into 40 ml polytetrafluoroethylene container and add 15 ml deionized water. Finally, the solutions were treated by sonication for 30 min and mixed in 40 ml polytetrafluoroethylene container, then heated the solutions to 180 °C for 6 h. The sample was washed by centrifugation and collected after being naturally cooled to room temperature, and then dried at 60 °C for 24 h under vacuum to obtain PCN/AgBr/rGO. 2.4. Materials characterization
2.1. Preparation of PCN Urea (CH4N2O), ethanol (C2H5OH) and silver nitrate (AgNO3) were purchased from Sinopharm Chemical Reagent Co, Ltd. Hexadecyl trimethyl ammonium bromide (C16H33(CH3)3NBr), tetracycline (TC) and 2,4-Dichlorophenol were bought from Shanghai Aladdin Bio-Chem Technology Co., Ltd. The urea (10 g) was put into the crucible in the muffle furnace for the condensation reaction [37], which was heated to 500 °C for 4 h (2.5 °C/min). The resultant g-C3N4 was collected and milled into the powder [38]. Mechanical mixing the bulk g-C3N4 (0.4 g) and different ratios of NaHCO3, then put them in a tube furnace calcination reaction, which was heated to 350 °C for 2 h (5 °C/min). The PCN will be obtained. 2.2. Synthesis of PCN/AgBr
2.5. Photocatalytic activity test
The PCN (0.2 g) was dissolved into 25 ml ethanol solutions and the suspensions were treated by sonication for 1 h, then added 50 ml cetyltrimethyl ammonium bromide (CTAB 0.12 mol/L) solutions. The
The photo-degradation rates of g-C3N4, PCN, AgBr, PCN/AgBr and 587
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AgBr/rGO. The highest point 284.6 eV is geared to an indeterminate carbon or sp2 CeC bond from rGO [44,45]. The PCN/AgBr/rGO shows peaks at 286.2 eV and 289.3 eV, which certify the contains of CeOH and OHeC]O bonds [46]. The above results are good agreement with the O 1s spectrum from PCN/AgBr/rGO, where the folding peaks centrality of 531.4 eV and 533.2 eV in accordance with HOeC]O and CeOH in Fig. 3c, respectively [47]. Fig. 3d shows the N 1s spectra, which demonstrate three types of N bonding in PCN/AgBr/rGO. The peaks at 398.4 eV and 400 eV were feature of the sp2 bonded aromatic N bound to C atoms in the triazine units (CeN]C) and the bridging N atoms in Ne(C)3 [48–50]. During the thermal polymerization, the peak of 401.2 eV was confirmed to be as a terminal amino group (CeNeH) due to incomplete condensation [51]. Furthermore, the spectrum of Br 3d in Fig. 3e indicates that the binding energies of Br 3d5/2 and Br 3d3/2 were presented at 67.9 eV and 69 eV. Fig. 3f is indicative of the highresolution spectrum that the peaks of Ag 3d5/2 and Ag 3d3/2 were presented about 368 eV and 374 eV [23]. Infer small changes in binding energy shows that an electronic interaction was appeared between PCN and AgBr, which indicate the heterojunction in the PCN/AgBr/rGO has been formed. According to XRD and XPS analyses, in PCN/AgBr/rGO composites, there are strong coupling and hybridization among PCN, rGO and AgBr.
Fig. 2. XRD patterns of g-C3N4, PCN, AgBr, rGO, PCN/AgBr and PCN/AgBr/ rGO.
PCN/AgBr/rGO photocatalysts were tested by using TC (20 mg/L) and 2,4-DCP (50 mg/L) solutions as goal contaminants under visible radiation. The visible-light source used A 250 W Xenon lamp. Briefly speaking, the reactor system was made up with 0.05 g samples and 100 ml of TC or 2,4-DCP solutions while open recycling water, respectively. To ensure adsorption–desorption equilibrium was reached, the mixture would stir 30 min under dark conditions. The specimen was taken out from the suspension with 15 min intervals in TC solutions and 60 min in 2,4-DCP solutions for centrifuging to extract samples, respectively. The concentration of TC and 2,4-DCP were tested at a wavelength of 357 nm and 284 nm in UV–vis spectrophotometer.
3.3. SEM and EDS analysis The morphology of the sample was studied by SEM and the composition was studied by energy dispersive spectroscopy (EDS). The bulk g-C3N4, PCN, PCN/AgBr and PCN/AgBr/rGO were studied by the SEM and EDS in Fig. 4. Fig. 4(a1) shows the bulk g-C3N4 is a block structure. Fig. 4(a2) shows the PCN has a 3D porous structure. Fig. 4(a3) shows the EDS of PCN consisting with the elements of N, C. Fig. 4(b1, b2) shows the low and high magnification SEM images of PCN/AgBr, it could be observed the surface of PCN become rougher after combined with AgBr. Fig. 4(b3) shows the EDS of PCN/AgBr consisting with the elements of C, N, Ag and Br. Fig. 4(c1, c2) shows the low and high magnification SEM images of PCN/AgBr/rGO, it can be seen that AgBr nanoparticles is covered by rGO loaded on the inner and outer surfaces of the PCN. Fig. 4(c3) shows the EDS of PCN/AgBr consisting with the elements of C, N, O, Ag and Br, respectively. On the basis of the above XPS, EDS, SEM and XRD prove that the 3D porous composite photocatalyst has been prepared.
3. Results and discussion 3.1. XRD analysis Fig. 2 shows the crystalline structure and phase purity of bulk gC3N4, AgBr, PCN, rGO, PCN/AgBr and PCN/AgBr/rGO, which were studied by XRD measurements. For bulk g-C3N4, the different peaks at 27.4° can be pointed to the (0 0 2) plane of the graphite material and this is a well-known C-N network structure. After calcination with NaHCO3, the diffraction peaks at 27.4° of the obtained PCN were obviously enhanced, which indicate that inter planar stacking structure and the aromatic structure inside the crystal surface of the treated gC3N4 aromatic ring were well preserved [41]. For AgBr, the diffraction peaks appear at 31°, 44.5°, 55.1°, 64.6° and 73.5°, which are considered to be homologous with the (2 0 0), (2 2 0), (2 2 2), (4 0 0) and (4 2 0) AgBr of the cubic phase in plane (JCPDS No. 06-0438), respectively [42]. The PCN/AgBr presented concomitance of both AgBr and PCN phases, then the main diffraction peaks from AgBr and the weak peak at 27.4° corresponds to PCN. Compared with AgBr, the diffraction peak of PCN is so weak that it is difficult to show PCN diffraction peaks in PCN/ AgBr composites [39]. At the same time, there was no significant rGO diffraction peaks were found in PCN/AgBr/rGO composites, which may due to the low content of rGO in the composite [43]. From the above results, which prove that the prepared composite photocatalyst is PCN/ AgBr/rGO.
3.4. TEM and elemental mapping analysis The TEM images of g-C3N4, PCN, PCN/AgBr, PCN/AgBr/rGO and the elemental mapping (ESI) of the PCN/AgBr/rGO were shown in Fig. 5, respectively. Fig. 5a shows that g-C3N4 processed by ultrasound is thick structure before TEM test. Fig. 5b shows PCN sample has a 3D porous morphology and the aperture is within the range of about 40–300 nm. In Fig. 5c, the AgBr nanoparticles are evenly scattered on the surface of PCN without aggregation. Fig. 5d shows the PCN/AgBr/ rGO composite that AgBr nanoparticles are well covered by rGO and dispersed on the surface of PCN. The above results show a strong effect of PCN on AgBr nanocrystals and rGO, which leads to increase the surface areas of PCN/AgBr/rGO contact with pollutants. These results prove that rGO and AgBr were successfully composed with PCN. The ESI of the PCN/AgBr/rGO was shown in Fig. S1 indicates that the elements C, N, O, Ag and Br are uniformly distributed on the surface. The above characterization corresponds to the XRD, EDS and TEM results, indicating that the 3D porous composite photocatalyst has been prepared. Supplementary data associated with this article can be found, in the online version, at https://doi.org/10.1016/j.apsusc.2018.07.121.
3.2. XPS analysis To study the valence of various elements and chemical composition in PCN/AgBr/rGO samples, which were detected by XPS. Fig. 3a describes the C, N, O, Br and Ag elements are expressed on the PCN/AgBr/ rGO composites. Fig. 3b describes the XPS spectrum of C 1s in PCN/ 588
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Fig. 3. XPS spectra of PCN/AgBr /rGO: (a) survey, (b) C 1s, (c) O 1s, (d) N 1s, (e) Br 3d and (f) Ag 3d.
rGO. A strong and wide peak from PCN is detected at around 1552 cm−1, which is different from usually observed in bulk g-C3N4, indicating that the PCN was well stripped into nanostructures [56,57]. The Raman spectrum of PCN/AgBr/rGO also displays a broad band at 1500 cm−1, indicating the successful combination of PCN and rGO in the composite [58].
3.5. FT-IR and Raman analysis The FT-IR spectra of g-C3N4, PCN, rGO, AgBr, PCN/AgBr and PCN/ AgBr/rGO are shown in Fig. S2a. The spectra of g-C3N4 and PCN display three characteristic absorption bands at > 3000, 1200–1800 and < 1000 cm−1, which are owing to the NeH bonds [52] and the stretching contraction modes of the heterocycle [53]. Because the optical absorbability of AgBr is relatively weak, PCN/AgBr has similar absorption band properties with PCN [54]. Similarly, due to the low content of rGO that the absorption peaks from rGO are not observed in PCN/AgBr/ rGO. It is noteworthy that the above characteristic peaks appear in the spectrum of PCN/AgBr/rGO sample, which not only prove the successful hybridization of PCN with AgBr and rGO, but also prove the forming of a hybrid heterojunction in the PCN/AgBr/rGO and an electronic interaction was developed between PCN and AgBr [55]. Fig. S2b shows that the Raman spectroscopy was further used to identify the degree of disorder and the number of defects in PCN/AgBr/
3.6. UV–vis diffuse reflection spectra Fig. 6a shows the light absorption efficiency is highly correlated with photocatalytic property by UV–vis DRS. The absorption wavelength of bulk g-C3N4 is 446 nm. After calcination with NaHCO3, the absorption wavelength of PCN shifts from 446 nm blue to 440 nm. With the addition of AgBr, the red shift from the response wavelength of the composite system to visible light increases the utilization of visible light. When AgBr and PCN are combined, the absorption edge of PCN/ AgBr is between 440 nm (PCN) and 495 nm (AgBr), which proves that 589
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Fig. 4. SEM images of (a1, a2, a3) initial g-C3N4, PCN and EDS of PCN, (b1, b2, b3) different magnification and EDS of PCN/AgBr, (c1, c2, c3) different magnification and EDS of PCN/AgBr/rGO.
3.8. Photoluminescence spectra
the heterojunction between PCN and AgBr is higher in visible light absorption and leads to more photo-generated e−/h+ pairs and better photocatalytic performance [45]. After rGO is composited with PCN/ AgBr, the PCN/AgBr/rGO sample has an increased absorption range, which proves that rGO is synergistic with PCN/AgBr [24]. Fig. 6b shows the band gaps of pure AgBr and PCN are calculated to be 2.39 eV and 2.63 eV.
Photoluminescence (PL) spectra could be extensively used to study the photoelectron-cavity pair migration, transfer and composite processes in nanocomposites. Therefore, to further confirm the introduction of AgBr and rGO promotes the uncoupling of photo-generated e−/ h+ pairs, the PL spectra of g-C3N4, PCN, PCN/AgBr and PCN/AgBr/rGO are shown in Fig. 8. Photoluminescence intensity represents the composite degree of radiation charges, which is used as an indirect evidence of charge separation [61]. After calcination with NaHCO3, the fluorescence intensity of the PCN is weaker than bulk g-C3N4, which may due to the 3D porous structure can promotes the uncoupling of photogenerated e−/h+ pairs compared with bulk g-C3N4. By adding rGO and AgBr, the intensity of the forceful emission peak at 440 nm is momentous reduced, indicating the recombination of the photo-generated e−/h+ pair is restrained [62].
3.7. BET analysis The specific surface areas of g-C3N4, PCN, PCN/AgBr and PCN/ AgBr/rGO composite are assured by NOVA3000E (Quantachrome Instruments U.S.). Fig. 7 shows the isotherms and pore sizes distributions. Compared with g-C3N4, the specific surface areas of PCN increased about 2 m2/g. The BET specific surface area of PCN/AgBr is lower than PCN, which may be due to the partial pores of PCN covered by AgBr nanoparticles, so the surface area of PCN/AgBr is less than PCN after the composite material is formed [59]. Moreover, the BET surface areas of PCN/AgBr/rGO are increased than PCN/AgBr, it may be because the AgBr nanoparticles are covered by rGO. The surface areas of PCN/AgBr/rGO are smaller than PCN after forming composite. In the degradation of wastewater, the specific surface area of the PCN/AgBr/ rGO does not play a master role. The pure g-C3N4, PCN, PCN/AgBr and PCN/AgBr/rGO displayed type IV and H3 hysteresis loops, which certified the existence of macropores [60]. After calcining with NaHCO3, the pore size distribution is obvious, and the pore structure of PCN is about 40 nm. Furthermore, the BET surface areas and pore parameters were shown in table 1.
3.9. Electrochemical properties of the prepared photocatalysts The charge transfer capacity is a significant factor to study the photocatalytic properties [63], which can be further researched by photochemical method. Fig. 9a displays the experimental Nyquist impedance plots of g-C3N4, PCN, AgBr, rGO, PCN/AgBr and PCN/AgBr/ rGO. Compared with g-C3N4, the EIS of PCN is weaker than g-C3N4, which may due to the 3D porous structure promotes the separation of photo-generated e−/h+ pairs. Compare with PCN, due to AgBr and PCN formed heterojunction that the radius of PCN/AgBr Nyquist arc is obviously smaller [64]. Compared with PCN/AgBr, the radius of Nyquist arc of PCN/AgBr/rGO is significantly smaller, which exhibits that 590
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Fig. 5. TEM images of (a) g-C3N4, (b) PCN, (c) PCN/AgBr and (d) PCN/AgBr/rGO.
which coincident with the DRS and PL results. The enhanced photocurrent may be due to the enhanced electromagnetic field of AgBr and the increased electron transfer rate by the rGO. The results show the AgBr and rGO vectors can enhance the separation of photo-generated e−/h+ pairs and prolong the photo-generated charge [65]. The results further confirmed that PCN/AgBr/rGO has relatively high transient photocurrent response.
load rGO could improve the charge transfer efficiency [43]. Fig. 9b shows the transient photocurrent response of g-C3N4, PCN, AgBr, PCN/ AgBr and PCN/AgBr/rGO. The photocurrent intensity was studied by the optical switching process (pulse 30 s). The PCN/AgBr/rGO has the strongest photocurrent response to all of the g-C3N4, PCN, AgBr and PCN/AgBr. The photocurrent enhancement of the obtained PCN/AgBr/ rGO composite shows the increased light-induced carrier transport rate and the improved separation of photo-generated e−/h+ pairs [5],
Fig. 6. (a) UV–vis diffuse reflectance spectra of g-C3N4, PCN, AgBr, rGO, PCN/AgBr and PCN/AgBr/rGO. (b) Band gaps of pure AgBr and PCN. 591
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core, which can strength to the plasma effect of Ag+ [3] and prevent the successive recombination of photoelectrons and holes provocative from PCN[66]. However, a large number of AgBr nanoparticles may produce enhanced local electromagnetic field of AgBr nanoparticles and delay the acceptance of electron excitation from the PCN [67]. At the same time, PCN is not enough to mediate the formation of AgBr core due to the increase of AgBr nanoparticles. Therefore, The increase of light absorption leads to the photo-thermal conversion to AgBr nanoparticles and the decrease of plasmon resonance effects [68]. More importantly, only so small amount of effective photoelectrons and holes can be added to the reaction that the photocatalytic efficiency cannot be maximized. As is shown in Fig. 10b, the contents of rGO also play a decisive role the photocatalytic activity of the PCN/AgBr/rGO. With the increased content of rGO, the photocatalytic activity of PCN/AgBr/rGO increased gradually, but the photocatalytic activity of PCN/AgBr/rGO began to decrease when rGO content was more than 5%. Therefore, when rGO content was 5%, CN/AgBr/rGO exhibited the highest activity. Fig. 10c reveals the photocatalytic activity of g-C3N4, PCN, PCN/ AgBr and PCN/AgBr/rGO photodegrading 2,4-DCP. The results show that PCN/AgBr/rGO has higher photocatalytic capability than other catalysts, which could point out that the correct introduction of AgBr nucleus and rGO into PCN would enhance the effective utilization of visible light [10]. The enhanced photocatalytic properties of CN/AgBr/ rGO may be attributed to the 3D porous core-shell structure of PCN/ AgBr/rGO and the plasma effect of the Ag+ to promote the separation of photo-generated e−/h+ pairs [69]. The recycling is of great significance to the practical application of photocatalysts. Fig. 10d is the result of the cyclic degradation of TC by PCN/AgBr/rGO composites. It shows the diagram that the degradation rate of PCN/AgBr/rGO to TC during the second cycle catalytic process is 75.5%, which is lower than the first degradation rate (79.80%). However, during the next cycle of catalysis, the activity of PCN/AgBr/rGO to TC was not significantly reduced. This may be part of the reduction of AgBr in the first and second cycles of the catalytic process, while reducing the formation of Ag coated on the surface of the AgBr particles, which can inhibit further reduction, so in the subsequent cycle PCN/AgBr/rGO. The degradation activity of TC indicates that the catalytic effect of the sample is unchanged, and it can be deduced that the sample has good stability. The XRD diffraction diagrams of PCN/AgBr/rGO before and after reaction indicate that the crystal structure of PCN/AgBr/rGO keeps balance in Fig. S3. Consider the above results, it can be authenticated that the PCN/AgBr/rGO compound material is stable during photocatalysis process. Through XRD analysis of fourth cycles after the catalyst photocatalytic, a small amount of silver does appear in the complex system. More importantly, after fourth cycles, the samples still have high photocatalytic activity, indicating that the PCN/AgBr/RGO system has practical significance.
Fig. 7. N2 adsorption-desorption isotherms and the inset is corresponding poresize distribution curves of bulk g-C3N4, PCN, PCN/AgBr and PCN/AgBr/rGO composites. Table 1 BET surface area, pore size and pore volume of g-C3N4, PCN, PCN/AgBr and PCN/AgBr/rGO samples. Sample
SBET (m2/g)
Pore size (nm)
Pore volume (mg3/g)
g-C3N4 PCN PCN/AgBr PCN/AgBr/rGO
33.652 35.747 3.526 10.783
38.344 37.842 37.808 37.842
0.055 0.063 0.013 0.021
3.11. Active species analysis Fig. 8. Solid PL emission spectrums of g-C3N4, PCN, PCN/AgBr and PCN/AgBr/ rGO.
To determine what is the major role in the photocatalytic activity of PCN/AgBr/rGO materials, a series of capture and quantitation experiments of active species were investigated by adding different capture scavengers. Hence, we used isopropanol (IPA), ascorbic acid (VC) and EDTA-2Na as the active particle quenchers for %OH, %O2− and holes. When IPA was added, the photo-degradation effect of TC is less shown in Fig. 11, which indicates that %OH was not the main active ingredient. In contrast, when VC and EDTA-2Na were added, the photocatalytic activity was greatly suppressed, which show that %O2− and h+ play a key role in photo-degradation of TC. To research the major active species among the photocatalysis degradation process, ESR was used to characterize the performance of %OH and %O2− radicals in PCN/AgBr/ rGO photocatalysts under visible light [70,71]. There were four characteristic peaks of DMPO-%O2− appear in the spectra of Fig. 12a, which revealed the photo-generated electrons in the conduction band of semiconductors could be converted into %O2− radicals during the
3.10. Photocatalytic activity and stability analysis The photo-activity of bulk g-C3N4, PCN, AgBr, PCN/AgBr and PCN/ AgBr/rGO photocatalysts was studied by degradation of 20 mg/L TC and 50 mg/L 2,4-DCP under visible light. As a comparison, TC and 2,4DCP degradation was performed with bulk g-C3N4, PCN, AgBr, PCN/ AgBr and PCN/AgBr/rGO under the same conditions. Fig. 10a shows the TC degradation profile of the bulk g-C3N4, PCN, AgBr and different AgBr content of PCN/AgBr. The degradation of g-C3N4, PCN and AgBr were 14.3%, 31.6% and 41.3% under visible light source, and when add 0.12 mol/L of AgBr, the PCN/AgBr has high activity than other samples and the visible light degradation rat is about 69.3% within 90 min. The photosensitivity enhancement of PCN/AgBr may be due to the AgBr 592
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Fig. 9. (a) EIS spectra of bulk g-C3N4, PCN, AgBr, PCN/AgBr and PCN/AgBr/rGO in light and in the dark. (b) Photocurrent of g-C3N4, PCN, AgBr, PCN/AgBr and PCN/AgBr/rGO under visible light.
EVB = ECB + Eg
photo-degradation system [72,73]. As is shown in Fig. 12b, compared with DMPO-%O2−, DMPO-%OH shows no obvious signal appeared, intimating %OH could be generated small amounts in PCN/AgBr/rGO systems. According to the above analysis of capture and quantitation experiments, we could conclude that the active species of %O2− and h+ have higher activity than %OH in visible light photocatalytic degradation.
(1)
where EVB was VB edge potential and ECB was CB edge potential. AgBr and PCN are estimated to −1.11 eV and 1.52 eV, −0.75 eV and 1.64 eV in CB and VB. Based on the above analysis, two possible mechanisms for the PCN/AgBr/rGO photocatalysis are illustrated in Fig. 13. Obviously, the PCN/AgBr/rGO has a multiple transmission channels system to increase the number of active species. A possible mechanism is shown in Fig. 13a that the PCN and AgBr as oxidation and reduction semiconductors in the indirect PCN/AgBr/rGO heterojunctions, which could be expressed under simulated solar light irradiation [48]. Then the photo-generated electrons would move to the AgBr side. Furthermore, rGO could cooperate well with the VB level of PCN and competing with the recombination process of e−/h+ pairs. The reasonable spatial distribution enables ultrafast charge separation.
3.12. Mechanism of PCN/AgBr/rGO photocatalysts. Typical, the VB of XPS has been tested to calculate the band construction of AgBr and PCN in Fig. S4. The conduction band (CB) and the valence Band (VB) of AgBr and PCN could be calculated by the equation:
Fig. 10. The TC photo-degradation efficiency of bulk g-C3N4, PCN, AgBr and as-prepared PCN/AgBr with different concentration of AgBr (a), and the different contents of introduced rGO (b), the 2,4-DCP irradiation time for the bulk g-C3N4, PCN, AgBr, PCN/AgBr and PCN/AgBr/rGO (c), cycling runs in photo-degradation 20 mg/L TC of PCN/AgBr/rGO (d). 593
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of AgBr by rGO, leaving the e−/h+ in the CB/VB of PCN/AgBr for further reduction/oxidation reactions. Therefore, the strengthen photocatalytic activity of PCN/AgBr/rGO is attributed to form heterojunction to improve the absorption of visible light and enhance the charge separation. From the PL and photocurrent results can show that PCN/AgBr/rGO exhibit high separation efficiency of photo-generated charge carriers. Based on the result and discussion, the prefer mechanism is shown in Fig. 13b that the PCN and AgBr as oxidation and reduction semiconductors in the Z-scheme PCN/AgBr/rGO heterojunctions. 4. Conclusions In summary, a novel ternary visible light-driven PCN/AgBr/rGO heterojunction photocatalyst has been prepared by introducing reduced graphene covering AgBr onto the surface of PCN. Thanks to the formation of Z-scheme heterojunction between PCN and AgBr, the efficient use of e−/h+ are achieved and resulting rapid separation of charge
Fig. 11. Effect of different scavengers on the TC degradation of PCN/AgBr/ rGO.
Fig. 12. DMPO spin-trapping ESR spectra of PCN/AgBr/rGO in aqueous dispersion (a), and in methanol dispersion (b).
Subsequently, the electrons on CB of the PCN have stronger reduction ability could reacted with O2 to form %O2− and the holes on VB of the PCN have stronger oxidation ability. Finally, the pollutants were oxidized by %O2− and h+. Furthermore, rGO can be used as an electron transport channel due to its excellent electron transport capability and increase the electron transport rate, which could raise the number of active species. Simultaneously, the photo-generated holes on the VB of PCN are straight participated in the oxidation of pollutants. The photoinduced electrons could easily jump from the conduction band (CB) of PCN to that of AgBr or react with adsorbed O2 to yield %O2−and partially form %OH. Another possible mechanism is shown in Fig. 13b that the PCN and AgBr as oxidation and reduction semiconductors in the Zscheme PCN/AgBr/rGO heterojunctions [74–76]. The photo-generated electrons in the CB of PCN can be rapidly transferred to the holes in VB
carriers at the interface. Moreover, due to the introduction of rGO, electrons in AgBr can be transported at high speed, which further effectively inhibits the reunion of photo-generated e−/h+ pairs and introduces a multiple transmission channel system to increase the number of active species. Under visible light, the PCN/AgBr/rGO has great photo-degradation performance for TC and 2,4-DCP. Therefore, this study provides a novel and simple method for synthesizing high-efficiency and multiple components composite photocatalysts to purify the environment. Acknowledgments We are very grateful for the financial support of the National Natural Science Foundation of China (Grant No. 21576125, 21776117,
Fig. 13. Schematic of two possible mechanisms of PCN/AgBr/rGO hybrids upon visible light irradiation. 594
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U1510126, 21676127), the Natural Science Foundation of Jiangsu Province (Grant No. BK20151349), China Postdoctoral Science Foundation (2017M611716 and 2017M611734), Six talent peaks project in Jiangsu Province (Grant No. XCL-014), Zhenjiang Science & Technology Program (Grant No. SH2016012).
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