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BiOBr ternary heterojunction with Z-scheme system for efficient visible-light photocatalytic performance
Journal of Alloys and Compounds 798 (2019) 741e749
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Engineering design of hierarchical g-C3N4@Bi/BiOBr ternary heterojunction with Z-scheme system for efficient visible-light photocatalytic performance Hong Liu, Hualei Zhou*, Xintong Liu, Hongda Li, Chaojun Ren, Xinyang Li, Wenjun Li, Zhongqin Lian, Mai Zhang Beijing Key Laboratory for Science and Application of Functional Molecular and Crystalline Materials, University of Science and Technology Beijing, Xueyuan Road No. 30, Haidian District, Beijing, 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2019 Received in revised form 19 April 2019 Accepted 27 May 2019 Available online 29 May 2019
Novel g-C3N4@Bi/BiOBr ternary heterostructured composites were successfully fabricated by a facile solvothermal method, in which ethylene glycol functioned as both a solvent and a reductant. As observed by scanning electron microscopy (SEM), Bi/BiOBr nanoplates were embedded on the surface of layered gC3N4 to form three-dimensional hierarchical structure. Under visible-light irradiation, the ternary gC3N4@Bi/BiOBr composites displayed notably boosted photocatalytic performance for photodegrading tetracycline and rhodamine B. As revealed via the UVevisible diffuse reflection spectra (UVevis DRS) and photocurrent experiments, the three-dimensional hierarchical structure obviously enhanced the visiblelight photoresponse and the ternary heterojunction greatly promoted the separation of photo-induced carriers, thus leading to the high photocatalytic performance. Based on the results of trapping radical experiments and the energy band potentials, an indirect Z-scheme system was proposed to illuminate the possible photodegradation mechanism. This work provides a novel strategy to design the multiple heterostructured photocatalysts with high efficiency for environmental purification and energy conversion. © 2019 Elsevier B.V. All rights reserved.
Keywords: Photocatalysis Visible light g-C3N4@Bi/BiOBr Ternary heterojunction Z-scheme system
1. Introduction Semiconductor photocatalysis has attracted increasing attention as an eco-friendly technology with great promise for degrading organic contaminants, reducing carbon dioxide, splitting water and removing poisonous gas [1e5]. High-efficiency and visible lightdriven photocatalysts are vital to the pragmatic application of photocatalytic technology [6]. Until now, various visible-lightresponsive photocatalysts have been widely synthesized for degrading organic contaminants such as ZnFe2O4 [7], Bi2WO6 [8], Ag2V4O11 [9] and BiOBr [10]. Among them, BiOBr is recognized as a promising photocatalyst by reason of its appropriate band gap (2.7 eV), characteristic layered structure and high chemical stability [11]. Nevertheless, the rapid recombination of photoinduced charge carrier and the narrow photoresponse range limit the photocatalytic application of BiOBr [12].
* Corresponding author. E-mail address: [email protected] (H. Zhou). https://doi.org/10.1016/j.jallcom.2019.05.303 0925-8388/© 2019 Elsevier B.V. All rights reserved.
The construction of semiconductor heterojunction has been widely applied in improving the charge separation efficiency [13e15]. Especially, Z-scheme heterojunction could not only accelerate the separation of photoexcited electrons and holes but also maintain their strong redox ability [16,17]. With the matched energy band levels and similar layered structure of g-C3N4 and BiOBr, the Z-scheme heterojunction between BiOBr and g-C3N4 were constructed and presented the improved photocatalytic activity [18,19]. Nevertheless, the direct Z-scheme between two semiconductors is not distinctly efficient enough in improving the charge separation because of the poor electron transport ability. Many indirect Z-scheme heterojunctions with noble metals like Ag and Au as Z-scheme bridges were consequently fabricated to further accelerate the charge transfer and separation [20,21]. In recent years, low-cost metal bismuth (Bi) has attracted much attention as a substitute for the noble metals because of its good conductivity and appropriate work function [22e25]. Furthermore, Bi can be attained from the in-situ reduction of BiOBr, which is beneficial for the formation of Bi/BiOBr close heterojunction. Given
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this, the ternary Z-scheme heterojunction of g-C3N4, Bi and BiOBr with Bi as the bridge could be expected to greatly make for the charge separation, leading to the high photocatalytic activity. However, it has not been reported yet to our knowledge. Morphology majorization is also a common method to boost the photocatalytic performance of photocatalysts [26]. In the recent years, various hierarchical-structured nanocomposites with large surface area have been constructed and exhibited the enhanced photocatalytic activity [27,28]. It was recognized that the structure was beneficial for not only shortening the transport distance of photogenerated carriers but also enhancing the light absorption. The solvent is often vital in constructing the hierarchical structure. For instance, ethylene glycol (EG) was used as the solvent and the chelating agent to restrict the growth of BiOI sheet-like crystals and assemble them on other semiconductor components like BiOBr [29] and CeO2 [30] plates to form the hierarchical structure. More interestingly, EG can also serve as a reductant for the reduction of Bi(III) as previously reported [23,31]. Given these facts, the intimate Bi/BiOBr heterojunction could form in-situ and at the same time, be assembled on the layered g-C3N4 to form a novel hierarchical structure with ethylene glycol as a solvent, which would notably improve the visible-light absorption. In this study, novel hierarchical g-C3N4@Bi/BiOBr ternary heterojunction were successfully fabricated via a facile solvothermal method, in which EG functioned as both a solvent and a reductant. Benefiting from the enhanced visible-light response and valid separation of electron-hole pairs, g-C3N4@Bi/BiOBr ternary composites presented notably enhanced photocatalytic performance compared to the single BiOBr and binary Bi/BiOBr. Taking all the results of trapping experiment, photocurrent responses and the calculated band potentials into consideration, Z-scheme system with metal Bi acting as an electron-conduction bridge was put forward to illuminate the photodegradation process. This work provides a novel strategy to design the heterostructured photocatalysts with high efficiency for environmental purification and energy conversion. 2. Experimental 2.1. Preparation of g-C3N4 All analytical grade chemicals in the process of experiment were used without further purification. The metal-free powders of gC3N4 were synthesized via heating melamine in alumina crucible with a cover. Typically, the alumina crucible with 6 g melamine was heated up to 520 C at the rate of 10 C min1 and maintained at 520 C for 2 h. Till being entirely cooled to room temperature, the obtained products were ground for next-step use.
prepared via the same process without adding g-C3N4, and single BiOBr was synthesized by hydrothermal treatment using Bi(NO3)3$5H2O as precursor and deionized water as a solvent. 2.3. Materials characterization The crystalline structures of all the as-prepared materials were obtained by X-ray diffraction (XRD, D/MAX-RB, Rigaku, Japan). The diffraction peaks in a range from 10 to 90 of crystalline structures were collected with Cu Ka radiation. The morphology and microstructure of the samples were detected on scanning electron microscopy (SEM, SU8010, Hitachi, Japan) and high-resolution transmission electron microscopy (HRTEM, F-20, FEI, USA). The specific surface areas of photocatalysts were collected using Brunauer-Emmett-Teller (BET) in an analytical instrument of Quantchrome NOVA 4200e (USA). X-ray photoelectron spectra (XPS, ESCALAB 250Xi, Thermo Fisher, USA) accompanying radiation with Al Ka were performed to obtain the elemental composition and surface chemical state of the as-prepared photocatalysts. The optical absorption performance of the samples was studied by UVevisible diffuse reflectance spectra (UVevis DRS) on a Persee T9s spectrophotometer (China). Photocurrent responses were conducted by an electrochemical workstation (5060F, RST, China). 2.4. Evaluation of visible-light photocatalytic performance The photocatalytic performance of these photocatalysts was assessed by decomposing rhodamine B (RhB, 20 mg L1) and tetracycline (TC, 12 mg L1) under simulated sunlight irradiation (l > 420 nm). At the beginning of each experiment, 30 mg of the photocatalyst were distributed well in 30 mL of organic pollutant aqueous solution. To establish an adsorption-desorption equilibrium between the organics and the solid photocatalyst, the suspension was continuously stirred in the dark for 1 h. Then, it was exposed to visible-light irradiation for photodegradation. During the process of photoreaction, 3 mL supernatant were collected and centrifuged at each constant time interval (20 min for RhB; 1 h for TC). The concentration of organics in the supernatant were determined using a UVevis spectrophotometer (T9s, Persee, China) at 554 nm for RhB and 359 nm for TC, respectively [32]. The ratios (C/C0) were used to assess the photodegradation efficiency, in which C0 is the initial concentration of organic pollutes and C corresponds to the lessened concentration after photodegrading at a given time. 3. Results and discussion 3.1. Crystalline phase structure
2.2. Preparation of g-C3N4@Bi/BiOBr A facile solvothermal method was performed to obtain the photocatalyst of g-C3N4@Bi/BiOBr. Firstly, 1.455 g Bi(NO3)3$5H2O was dissolved completely in 30 mL of EG. Then, a proportionate content of g-C3N4 (a mass ratio of 10%, 20% or 30% for g-C3N4: BiOBr, separately) was added and distributed well by strong magnetic stirring to acquire homogeneous suspension. Subsequently, another 30 mL of EG containing 0.357 g KBr was brought slowly into the previous homogeneous suspension under continuously stirring. The ultima suspension was moved into a 100 mL autoclave, heated at 180 C for 18 h and then naturally cooled in air. Finally, the precipitate was collected by centrifuge, washed several times with deionized water and ethanol, and dried at 80 C overnight. The obtained product were labeled as x-g-C3N4@Bi/BiOBr (x: the mass ratio of g-C3N4 to BiOBr). In contrast, the sample of Bi/BiOBr was
The crystal phase structure of the prepared samples was determined by XRD spectra (Fig. 1). Single BiOBr presents the major characteristic diffraction peaks at 10.9 , 25.2 , 31.6 , 32.2 , 46.2 and 57.1 which are indexed to the lattice planes of (001), (101), (102), (110), (200) and (212), respectively, of tetragonal BiOBr (JCPDS card No. 09-0393) [33]. The diffraction patterns of Bi/BiOBr and g-C3N4@Bi/BiOBr composites are similar to those of single BiOBr except that the peak intensity at 10.9 clearly presents a decrease, which may be attributed to the restricted growth of (001) lattice planes of BiOBr by EG during the forming process of the composites [34]. Additionally, a new weak peak appears at 27.2 , which can be indexed to (012) lattice planes of metallic Bi (JCPDS card 44-1246) [35]. However, no distinct peak at 27.4 belonging to g-C3N4 was observed for all the ternary heterojunction, which may be attributed to low crystallinity of the prepared g-C3N4 and the
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Table 1 The surface areas of the samples.
Fig. 1. XRD patterns of the BiOBr, g-C3N4, Bi/BiOBr and g-C3N4@Bi/BiOBr composites.
shadowing effect under the peak at 27.2 assigned to the Bi phase [36]. The existence of g-C3N4 and metal Bi was further determined by the following SEM, TEM and XPS analysis. 3.2. Morphology observations The morphology and micro-size of those as-synthesized products were observed from SEM images. As shown in Fig. 2A and B, pure BiOBr presents a sheet-like shape with ~5 mm length and gC3N4 shows the micron-sized layered structure. Compared with pure BiOBr, Bi/BiOBr nanoplates (100e200 nm length) displays an obvious decrease in the size (Fig. 2C), which could be caused by the slower reaction between the Bi(OCH2CH2OH)2þ and Br ions after the substitution of EG for water as a solvent during the solvothermal process [29]. The ternary g-C3N4@Bi/BiOBr composites present three-dimensional fluffy and hierarchical structure where
Photocatalysts
BET surface areas (m2/g)
BiOBr g-C3N4 Bi/BiOBr 10%-g-C3N4@Bi/BiOBr 20%-g-C3N4@Bi/BiOBr 30%-g-C3N4@Bi/BiOBr
2.09 9.24 13.40 17.08 21.75 19.19
Bi/BiOBr nanoplates are embedded on the surface of the layered gC3N4 (Fig. 2E and F). Nevertheless, the structure doesn't form well for 10%-g-C3N4@Bi/BiOBr possibly due to the small quantity of gC3N4 (Fig. 2D). Correspondingly, the binary Bi/BiOBr has a large specific surface areas compared with pure BiOBr (Table 1), which is well matched with its relative small size. Furthermore, the ternary g-C3N4@Bi/BiOBr composites display notably larger specific surface areas than pure BiOBr, g-C3N4 and binary Bi/BiOBr (Table 1), among which 20%-g-C3N4@Bi/BiOBr has the largest surface area. It is expected that the three-dimensional hierarchical structure with large specific area would notably facilitate the adsorption of organic pollutes on the surface of photocatalysts and enhance the light absorption from the multi-level light diffusion. The images of TEM and HRTEM display the in-depth information about micro-structure of the composites. As shown in Fig. 3A and B, it is clear that g-C3N4 is covered with the nanoplates of Bi/BiOBr. The lattice spacings of 0.282, 0.277 and 0.193 nm correspond to the (102), (110) and (200) lattice planes of pure BiOBr, respectively [37]. And the lattice spacing of 0.328 nm corresponds to the (012) lattice planes of metallic Bi [24]. The amorphous region should be assigned to g-C3N4, which agrees with the previous XRD analysis. These results demonstrate that g-C3N4@Bi/BiOBr ternary heterojunction with three-dimensional fluffy and hierarchical structure was successfully fabricated.
Fig. 2. SEM images of (A) BiOBr; (B) g-C3N4; (C) Bi/BiOBr; (D) 10%-g-C3N4@Bi/BiOBr; (E) 20%-g-C3N4@Bi/BiOBr; (F) 30%-g-C3N4@Bi/BiOBr.
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Fig. 3. TEM image (A) and HRTEM image (B) of 20%-g-C3N4@Bi/BiOBr sample.
3.3. Chemical state and composition characterization The elemental composition and surface chemical state of the 20%-g-C3N4@Bi/BiOBr composites were characterized by XPS spectra. The characteristic peaks of C, N, Bi, O and Br elements are observed in Fig. 4A, suggesting the coexistence of these elements in ternary composites. More detailed C 1s peak of this product is consecutively fitted by two peaks (Fig. 4B). The peak at 284.8 eV can be assigned to sp2 CeC bonds of graphitic or amorphous carbons adsorbed on the composites surface [38]. The peak at 288.0 eV can be identified as sp2-bonded carbon in N-containing aromatic rings (NeC]N) [39]. Four peaks of N 1s spectrum at 398.7, 399.7, 400.5 and 404.3 eV (Fig. 4C) correspond to sp2-hybridized nitrogen involved in triazine rings (C]NeC), tertiary N bonded in N-(C)3 groups, NeH groups and p excitations, respectively [40,41]. Both the high-resolution spectra of C 1s and N1s further confirm the existence of g-C3N4 in the ternary heterojunction. On the high resolution spectrum of Bi 4f (Fig. 4D), two strong peaks at 159.3 and 164.6 eV can be designated to the Bi 4f7/2 and Bi 4f5/2 by reason of forming the BieO bond in BiOBr lattice, respectively [42] while two tiny peaks located at 157.2 and 162.5 eV are attributed to metallic Bi in the ternary heterojunction [41]. The peaks at 529.6 and 530.9 eV (Fig. 4E) correspond to the O 1s in the BieO bond and the hydroxyl groups (OeH), respectively [43]. The peak of Br 3d located at 68.4 and 69.2 eV (Fig. 4F) are in agreement with Br 3d5/2 and Br 3d3/2, respectively [44]. Obviously, the XPS analyses are well consistent with the results of XRD, SEM and TEM, further confirming the formation of g-C3N4@Bi/BiOBr ternary heterostructure. 3.4. Optical absorption analysis To investigate the optical response range of the obtained photocatalysts, UVeVis DRS measurements were performed and shown in Fig. 5. Pure BiOBr and g-C3N4 exhibit obvious absorption with fundamental absorption edge at 450 nm and 466 nm, respectively. The absorption edge of Bi/BiOBr presents a little redshift (483 nm) relative to BiOBr, suggesting that the loaded metal Bi could improve the ability of visible light absorption to some degree [22]. Different from Bi/BiOBr and g-C3N4, 20%-g-C3N4@Bi/ BiOBr and 30%-g-C3N4@Bi/BiOBr present obviously enhanced photoresponse in the whole visible-light range, which is just corresponding to their fluffy and hierarchical ternary
heterostructure. Therefore, it can be reasoned that the enhanced visible-light photoresponse is ascribed to the hierarchical structure of the composites as expected. The band gap of those assynthesized photocatalysts could be estimated via the equation (Ahv) ¼ a(hv - Eg)n/2, in which A, h, v, a, and Eg represent the absorption coefficient, Planck constant, light frequency, constant and band gap energy, respectively. In addition, the numerical value of n (n ¼ 1 or 4) is determined by the optical transition type of the photocatalyst [45e47]. For this system, the value of n should be 4 due to their indirect transition character of BiOBr and g-C3N4 [13,48,49]. As presented in insert of Fig. 5, the band gaps of pure BiOBr, g-C3N4, Bi/BiOBr and 20%-g-C3N4@Bi/BiOBr were calculated to be about 2.76, 2.66, 2.57 and 2.43 eV, respectively. It is obvious that the ternary g-C3N4@Bi/BiOBr photocatalyst with a hierarchical structure have a relative narrow bandgap and enhanced visible-light response, which would notably boost the photocatalytic performance for degrading organic pollutants under visible-light illumination. 3.5. Photocatalytic performance Photocatalytic activity of the prepared samples was evaluated by decomposing RhB and TC under visible-light illumination, respectively. Besides, the binary g-C3N4/BiOBr (a mass ratio of 20% for g-C3N4: BiOBr) heterojunction synthesized by hydrothermal treatment (at 180 C for 18 h) was employed as a reference. Before the photoreaction, an adsorption-desorption equilibrium between the photocatalysts and organic pollutants was obtained. As shown in Fig. 6A and B, compared with the single components and binary composites, the ternary g-C3N4@Bi/BiOBr photocatalysts all present stronger ability to absorb organic pollutants, which might be caused by their larger surface areas. After being exposed to visiblelight irradiation for 80 min, only 52% and 26% of RhB were degraded by pure BiOBr and g-C3N4, respectively (Fig. 6A). The binary Bi/ BiOBr and g-C3N4/BiOBr present a higher degradation rates (65% and 79%, respectively). In contrast, all the ternary composites display a notable high photodegradation performance, among which 20%-g-C3N4@Bi/BiOBr exhibits the highest efficiency (98%). 30%-g-C3N4@Bi/BiOBr presents slightly low photodegradation efficiency (94%) though its visible-light photoresponse is similar to 20%-g-C3N4@Bi/BiOBr, which could be caused by its relative small specific surface area. Moreover, compared with other g-C3N4/BiOBr
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Fig. 4. (A) Overall X-ray photoelectron spectra of 20%-g-C3N4@Bi/BiOBr; and high-resolution spectra of (B) C 1s; (C) N 1s; (D) Bi 4f; (E) O 1s and (F) Br 3d.
composites reported in related literatures [19,50], the ternary gC3N4@Bi/BiOBr heterojunction still present a higher photocatalytic performance in degrading RhB. Accordingly, It could be concluded that the presence of metal Bi has notable contribution to enhance the photocatalytic performance. The photodegradation performance of the obtained samples was further evaluated by decomposing colorless TC for excluding the indirect dye photosensitization in photodegradation RhB system. As shown in Fig. 6B, the binary Bi/BiOBr and g-C3N4/BiOBr
present slightly higher photocatalytic efficiency (51% and 58%) than pure BiOBr (44%) and g-C3N4 (19%) after being irradiated for 4 h under visible light. Furthermore, the ternary composites exhibit notably higher photocatalytic performance than the binary composites. Among them, 20%-g-C3N4@Bi/BiOBr displays the highest photodegradation performance (78%). These results are well congruent to those discussed about RhB degradation, suggesting that the fabricated ternary composites have the excellent photocatalytic performance for degrading organic pollutants. Also, it is
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indicated that the ternary heterojunction play a key role in enhancing the photocatalytic performance. The cycling experiments for degrading RhB and TC over 20%-gC3N4@Bi/BiOBr were performed to assess the stability of the ternary composites during the photodegradation process. As shown in Fig. 6C and D, the photocatalytic efficiency displays a negligible loss after each cycle, indicating the ternary composites have an outstanding stability in the practical application. In addition, the structural stability of ternary g-C3N4@Bi/BiOBr composites was further determined by comparing the XRD patterns of 20%-g-C3N4@Bi/BiOBr before and after the four cycling experiments (Fig. 7). It is obvious that the structure of the ternary heterojunction has almost no change, further confirming the stability of the obtained photocatalysts. 3.6. Possible photocatalytic mechanism
Fig. 5. UVevis diffuse reflectance spectra of the as-obtained samples; the insert displays the band gaps of the corresponding samples.
Photocurrent techniques were used to evaluate the separation efficiency and transfer rate of charge carriers. As displayed in Fig. 8, the photocurrent responses intensity of Bi/BiOBr is far higher than that of single BiOBr, suggesting that the loading of metal Bi can tremendously facilitate the charge transfer and inhibit the recombination of photoexcited charge carriers as demonstrated in previous reports [24,25]. Compared with binary Bi/BiOBr and single
Fig. 6. Photocatalytic performance of obtained samples under visible-light illumination for (A) degrading RhB and (B) TC; cycling experiments for the photodegradation of (C) RhB and (D) TC.
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Fig. 7. XRD of 20%-g-C3N4@Bi/BiOBr before and after the four cycling experiments.
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Fig. 9. Photocatalytic efficiency for degrading RhB with the addition of different scavengers over the 20%-g-C3N4@Bi/BiOBr composites.
band positions of the corresponding components were investigated to profoundly illuminate the photodegradation mechanism. The conduction band (ECB) and valence band (EVB) potentials of BiOBr and g-C3N4 were calculated via the following equations [51e53]:
Fig. 8. Photocurrent responses of BiOBr, g-C3N4, Bi/BiOBr and 20%-g-C3N4@Bi/BiOBr samples.
g-C3N4, 20%-g-C3N4@Bi/BiOBr ternary composites present the clearly high photocurrent intensity and strong residual current, indicating that the constructed ternary heterojunction among Bi, BiOBr and g-C3N4 further accelerate the charge separation and transfer of photoinduced carriers. To further explore the main active species during the photodegradation process over 20%-g-C3N4@Bi/BiOBr composites, trapping experiments were systematically performed via adding different scavengers of active radicals in the system of RhB photodegradation and the results are shown in Fig. 9. Isopropanol (IPA, 10 mM), benzoquinone (BQ, 1 mM) and sodium oxalate (Na2C2O4, þ 10 mM) were used as scavenger of ∙OH, ∙O radicals, 2 and h respectively. Obviously, the photodegradation efficiency slightly declines with the addition of IPA, while it drastically decreases when Na2C2O4 or BQ was added. These results demonstrate that þ both ∙O 2 and h radicals play a decisive role during the photodegrading process, and the photo-generated electrons and holes have strong reducibility and oxidizability, respectively. The transfer direction of carriers on the interface of heterojunction was thermodynamically determined by the band-edge positions of two components forming the heterojunction. So, the
EVB ¼ X Ee þ 0:5Eg
(1)
ECB ¼ EVB Eg
(2)
in which X and Ee respectively corresponds to the electronegativity of semiconductor and the energy for free electrons (4.5 eV). According to the results of Uvevis analysis, the Eg values of BiOBr and g-C3N4 are respectively measured about 2.76 and 2.66 eV. Subsequently, the ECB values of BiOBr and g-C3N4 are calculated to be 0.30 and 1.12 eV, respectively, and the values of BiOBr and g-C3N4 EVB are respectively 3.06 and 1.54 eV [54,55]. As discussed above, the ternary heterojunction among BiOBr, Bi and g-C3N4 greatly accelerates the charge transfer and separation of charge carriers. Considering the dominant active species during the photodegradation process and the relative band levels of BiOBr and g-C3N4, an indirect Z-scheme system with Bi served as an electronconduction bridge (Fig. 10) was proposed to expound the possible photodegradation mechanism of the ternary g-C3N4@Bi/BiOBr heterojunction for degrading organic pollutants. After being exposed to visible-light illumination, both BiOBr and g-C3N4 absorb photons to produce electron-hole pairs. In this process, the light absorption is notably enhanced by the three-dimensional hierarchical structure of composites (Fig. 5). The photo-induced electrons in the BiOBr conduction band tend to transfer to the interface of the heterojunction and combine with the holes in the valence band of g-C3N4 due to their different band level. The Bi bridge as a medium between BiOBr and g-C3N4 greatly accelerates their combination rate due to its good charge mobility. As a result, the bulk recombination of charge carriers inside BiOBr and g-C3N4 is significantly suppressed, leading to the accumulation of the electrons in the conduction band of g-C3N4 (1.12 eV) and holes in the valence band of BiOBr (3.06 eV). Eventually, the accumulated electrons with strong reducibility can reduce the absorbed O2 to ∙O 2 (E(O2/ ∙O 2 ) ¼ -0.33 eV) [56] which would efficiently degrade adsorbed organic pollutants together with the accumulated holes with strong oxidability. Benefiting from the extended visible-light absorption range and the high-efficiency separation of charge carriers, the ternary g-C3N4@Bi/BiOBr composites presented notably
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Fig. 10. Schematic illustration of the possible mechanism of the g-C3N4@Bi/BiOBr photocatalyst for degrading organic pollutants under visible light irradiation.
enhanced photocatalytic performance for degrading organic pollutants.
4. Conclusions To sum up, the novelty ternary g-C3N4@Bi/BiOBr heterojunction were successfully fabricated through in-situ reduction and selfassembly processes with ethylene glycol as both a solvent and a reductant. The ternary composites present the three-dimensional fluffy and hierarchical structure in which Bi/BiOBr nanoplates were embedded on the surface of layered g-C3N4. Under the visible light irradiation, the g-C3N4@Bi/BiOBr composites exhibited notably high photocatalytic activity for degrading RhB and TC comparing with the binary g-C3N4/BiOBr and Bi/BiOBr composites as well as the single semiconductor components. The threedimensional hierarchical structure obviously improves the visible-light photoresponse, as suggested from the UVeVis DRS and the ternary heterojunction with Bi loading greatly facilitate the separation of photo-induced carriers, as revealed by the photocurrent experiments. Their combined action results in the excellent photodegradation performance of the ternary g-C3N4@Bi/BiOBr composites. According to the acquired results of trapping experiments and the energy band potentials, an indirect Z-scheme system with Bi as an electron-conduction bridge was proposed to illuminate the possible photodegradation mechanism. This work provides a novel strategy combining with loading metal and morphology majorization to design the multiple heterostructured photocatalysts for environmental purification and energy conversion.
Acknowledgments The authors sincerely acknowledge the financial support provided by the Project of the National Natural Science Foundation of China (Grant No. 21271022) and the Fundamental Research Funds for the Central Universities (FRF-BR-18-002A).
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Engineering design of hierarchical g-C3N4@Bi/BiOBr ternary heterojunction with Z-scheme system for efficient visible-light photocatalytic performance Hong Liu, Hualei Zhou*, Xintong Liu, Hongda Li, Chaojun Ren, Xinyang Li, Wenjun Li, Zhongqin Lian, Mai Zhang Beijing Key Laboratory for Science and Application of Functional Molecular and Crystalline Materials, University of Science and Technology Beijing, Xueyuan Road No. 30, Haidian District, Beijing, 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 February 2019 Received in revised form 19 April 2019 Accepted 27 May 2019 Available online 29 May 2019
Novel g-C3N4@Bi/BiOBr ternary heterostructured composites were successfully fabricated by a facile solvothermal method, in which ethylene glycol functioned as both a solvent and a reductant. As observed by scanning electron microscopy (SEM), Bi/BiOBr nanoplates were embedded on the surface of layered gC3N4 to form three-dimensional hierarchical structure. Under visible-light irradiation, the ternary gC3N4@Bi/BiOBr composites displayed notably boosted photocatalytic performance for photodegrading tetracycline and rhodamine B. As revealed via the UVevisible diffuse reflection spectra (UVevis DRS) and photocurrent experiments, the three-dimensional hierarchical structure obviously enhanced the visiblelight photoresponse and the ternary heterojunction greatly promoted the separation of photo-induced carriers, thus leading to the high photocatalytic performance. Based on the results of trapping radical experiments and the energy band potentials, an indirect Z-scheme system was proposed to illuminate the possible photodegradation mechanism. This work provides a novel strategy to design the multiple heterostructured photocatalysts with high efficiency for environmental purification and energy conversion. © 2019 Elsevier B.V. All rights reserved.
Keywords: Photocatalysis Visible light g-C3N4@Bi/BiOBr Ternary heterojunction Z-scheme system
1. Introduction Semiconductor photocatalysis has attracted increasing attention as an eco-friendly technology with great promise for degrading organic contaminants, reducing carbon dioxide, splitting water and removing poisonous gas [1e5]. High-efficiency and visible lightdriven photocatalysts are vital to the pragmatic application of photocatalytic technology [6]. Until now, various visible-lightresponsive photocatalysts have been widely synthesized for degrading organic contaminants such as ZnFe2O4 [7], Bi2WO6 [8], Ag2V4O11 [9] and BiOBr [10]. Among them, BiOBr is recognized as a promising photocatalyst by reason of its appropriate band gap (2.7 eV), characteristic layered structure and high chemical stability [11]. Nevertheless, the rapid recombination of photoinduced charge carrier and the narrow photoresponse range limit the photocatalytic application of BiOBr [12].
* Corresponding author. E-mail address: [email protected] (H. Zhou). https://doi.org/10.1016/j.jallcom.2019.05.303 0925-8388/© 2019 Elsevier B.V. All rights reserved.
The construction of semiconductor heterojunction has been widely applied in improving the charge separation efficiency [13e15]. Especially, Z-scheme heterojunction could not only accelerate the separation of photoexcited electrons and holes but also maintain their strong redox ability [16,17]. With the matched energy band levels and similar layered structure of g-C3N4 and BiOBr, the Z-scheme heterojunction between BiOBr and g-C3N4 were constructed and presented the improved photocatalytic activity [18,19]. Nevertheless, the direct Z-scheme between two semiconductors is not distinctly efficient enough in improving the charge separation because of the poor electron transport ability. Many indirect Z-scheme heterojunctions with noble metals like Ag and Au as Z-scheme bridges were consequently fabricated to further accelerate the charge transfer and separation [20,21]. In recent years, low-cost metal bismuth (Bi) has attracted much attention as a substitute for the noble metals because of its good conductivity and appropriate work function [22e25]. Furthermore, Bi can be attained from the in-situ reduction of BiOBr, which is beneficial for the formation of Bi/BiOBr close heterojunction. Given
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this, the ternary Z-scheme heterojunction of g-C3N4, Bi and BiOBr with Bi as the bridge could be expected to greatly make for the charge separation, leading to the high photocatalytic activity. However, it has not been reported yet to our knowledge. Morphology majorization is also a common method to boost the photocatalytic performance of photocatalysts [26]. In the recent years, various hierarchical-structured nanocomposites with large surface area have been constructed and exhibited the enhanced photocatalytic activity [27,28]. It was recognized that the structure was beneficial for not only shortening the transport distance of photogenerated carriers but also enhancing the light absorption. The solvent is often vital in constructing the hierarchical structure. For instance, ethylene glycol (EG) was used as the solvent and the chelating agent to restrict the growth of BiOI sheet-like crystals and assemble them on other semiconductor components like BiOBr [29] and CeO2 [30] plates to form the hierarchical structure. More interestingly, EG can also serve as a reductant for the reduction of Bi(III) as previously reported [23,31]. Given these facts, the intimate Bi/BiOBr heterojunction could form in-situ and at the same time, be assembled on the layered g-C3N4 to form a novel hierarchical structure with ethylene glycol as a solvent, which would notably improve the visible-light absorption. In this study, novel hierarchical g-C3N4@Bi/BiOBr ternary heterojunction were successfully fabricated via a facile solvothermal method, in which EG functioned as both a solvent and a reductant. Benefiting from the enhanced visible-light response and valid separation of electron-hole pairs, g-C3N4@Bi/BiOBr ternary composites presented notably enhanced photocatalytic performance compared to the single BiOBr and binary Bi/BiOBr. Taking all the results of trapping experiment, photocurrent responses and the calculated band potentials into consideration, Z-scheme system with metal Bi acting as an electron-conduction bridge was put forward to illuminate the photodegradation process. This work provides a novel strategy to design the heterostructured photocatalysts with high efficiency for environmental purification and energy conversion. 2. Experimental 2.1. Preparation of g-C3N4 All analytical grade chemicals in the process of experiment were used without further purification. The metal-free powders of gC3N4 were synthesized via heating melamine in alumina crucible with a cover. Typically, the alumina crucible with 6 g melamine was heated up to 520 C at the rate of 10 C min1 and maintained at 520 C for 2 h. Till being entirely cooled to room temperature, the obtained products were ground for next-step use.
prepared via the same process without adding g-C3N4, and single BiOBr was synthesized by hydrothermal treatment using Bi(NO3)3$5H2O as precursor and deionized water as a solvent. 2.3. Materials characterization The crystalline structures of all the as-prepared materials were obtained by X-ray diffraction (XRD, D/MAX-RB, Rigaku, Japan). The diffraction peaks in a range from 10 to 90 of crystalline structures were collected with Cu Ka radiation. The morphology and microstructure of the samples were detected on scanning electron microscopy (SEM, SU8010, Hitachi, Japan) and high-resolution transmission electron microscopy (HRTEM, F-20, FEI, USA). The specific surface areas of photocatalysts were collected using Brunauer-Emmett-Teller (BET) in an analytical instrument of Quantchrome NOVA 4200e (USA). X-ray photoelectron spectra (XPS, ESCALAB 250Xi, Thermo Fisher, USA) accompanying radiation with Al Ka were performed to obtain the elemental composition and surface chemical state of the as-prepared photocatalysts. The optical absorption performance of the samples was studied by UVevisible diffuse reflectance spectra (UVevis DRS) on a Persee T9s spectrophotometer (China). Photocurrent responses were conducted by an electrochemical workstation (5060F, RST, China). 2.4. Evaluation of visible-light photocatalytic performance The photocatalytic performance of these photocatalysts was assessed by decomposing rhodamine B (RhB, 20 mg L1) and tetracycline (TC, 12 mg L1) under simulated sunlight irradiation (l > 420 nm). At the beginning of each experiment, 30 mg of the photocatalyst were distributed well in 30 mL of organic pollutant aqueous solution. To establish an adsorption-desorption equilibrium between the organics and the solid photocatalyst, the suspension was continuously stirred in the dark for 1 h. Then, it was exposed to visible-light irradiation for photodegradation. During the process of photoreaction, 3 mL supernatant were collected and centrifuged at each constant time interval (20 min for RhB; 1 h for TC). The concentration of organics in the supernatant were determined using a UVevis spectrophotometer (T9s, Persee, China) at 554 nm for RhB and 359 nm for TC, respectively [32]. The ratios (C/C0) were used to assess the photodegradation efficiency, in which C0 is the initial concentration of organic pollutes and C corresponds to the lessened concentration after photodegrading at a given time. 3. Results and discussion 3.1. Crystalline phase structure
2.2. Preparation of g-C3N4@Bi/BiOBr A facile solvothermal method was performed to obtain the photocatalyst of g-C3N4@Bi/BiOBr. Firstly, 1.455 g Bi(NO3)3$5H2O was dissolved completely in 30 mL of EG. Then, a proportionate content of g-C3N4 (a mass ratio of 10%, 20% or 30% for g-C3N4: BiOBr, separately) was added and distributed well by strong magnetic stirring to acquire homogeneous suspension. Subsequently, another 30 mL of EG containing 0.357 g KBr was brought slowly into the previous homogeneous suspension under continuously stirring. The ultima suspension was moved into a 100 mL autoclave, heated at 180 C for 18 h and then naturally cooled in air. Finally, the precipitate was collected by centrifuge, washed several times with deionized water and ethanol, and dried at 80 C overnight. The obtained product were labeled as x-g-C3N4@Bi/BiOBr (x: the mass ratio of g-C3N4 to BiOBr). In contrast, the sample of Bi/BiOBr was
The crystal phase structure of the prepared samples was determined by XRD spectra (Fig. 1). Single BiOBr presents the major characteristic diffraction peaks at 10.9 , 25.2 , 31.6 , 32.2 , 46.2 and 57.1 which are indexed to the lattice planes of (001), (101), (102), (110), (200) and (212), respectively, of tetragonal BiOBr (JCPDS card No. 09-0393) [33]. The diffraction patterns of Bi/BiOBr and g-C3N4@Bi/BiOBr composites are similar to those of single BiOBr except that the peak intensity at 10.9 clearly presents a decrease, which may be attributed to the restricted growth of (001) lattice planes of BiOBr by EG during the forming process of the composites [34]. Additionally, a new weak peak appears at 27.2 , which can be indexed to (012) lattice planes of metallic Bi (JCPDS card 44-1246) [35]. However, no distinct peak at 27.4 belonging to g-C3N4 was observed for all the ternary heterojunction, which may be attributed to low crystallinity of the prepared g-C3N4 and the
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Table 1 The surface areas of the samples.
Fig. 1. XRD patterns of the BiOBr, g-C3N4, Bi/BiOBr and g-C3N4@Bi/BiOBr composites.
shadowing effect under the peak at 27.2 assigned to the Bi phase [36]. The existence of g-C3N4 and metal Bi was further determined by the following SEM, TEM and XPS analysis. 3.2. Morphology observations The morphology and micro-size of those as-synthesized products were observed from SEM images. As shown in Fig. 2A and B, pure BiOBr presents a sheet-like shape with ~5 mm length and gC3N4 shows the micron-sized layered structure. Compared with pure BiOBr, Bi/BiOBr nanoplates (100e200 nm length) displays an obvious decrease in the size (Fig. 2C), which could be caused by the slower reaction between the Bi(OCH2CH2OH)2þ and Br ions after the substitution of EG for water as a solvent during the solvothermal process [29]. The ternary g-C3N4@Bi/BiOBr composites present three-dimensional fluffy and hierarchical structure where
Photocatalysts
BET surface areas (m2/g)
BiOBr g-C3N4 Bi/BiOBr 10%-g-C3N4@Bi/BiOBr 20%-g-C3N4@Bi/BiOBr 30%-g-C3N4@Bi/BiOBr
2.09 9.24 13.40 17.08 21.75 19.19
Bi/BiOBr nanoplates are embedded on the surface of the layered gC3N4 (Fig. 2E and F). Nevertheless, the structure doesn't form well for 10%-g-C3N4@Bi/BiOBr possibly due to the small quantity of gC3N4 (Fig. 2D). Correspondingly, the binary Bi/BiOBr has a large specific surface areas compared with pure BiOBr (Table 1), which is well matched with its relative small size. Furthermore, the ternary g-C3N4@Bi/BiOBr composites display notably larger specific surface areas than pure BiOBr, g-C3N4 and binary Bi/BiOBr (Table 1), among which 20%-g-C3N4@Bi/BiOBr has the largest surface area. It is expected that the three-dimensional hierarchical structure with large specific area would notably facilitate the adsorption of organic pollutes on the surface of photocatalysts and enhance the light absorption from the multi-level light diffusion. The images of TEM and HRTEM display the in-depth information about micro-structure of the composites. As shown in Fig. 3A and B, it is clear that g-C3N4 is covered with the nanoplates of Bi/BiOBr. The lattice spacings of 0.282, 0.277 and 0.193 nm correspond to the (102), (110) and (200) lattice planes of pure BiOBr, respectively [37]. And the lattice spacing of 0.328 nm corresponds to the (012) lattice planes of metallic Bi [24]. The amorphous region should be assigned to g-C3N4, which agrees with the previous XRD analysis. These results demonstrate that g-C3N4@Bi/BiOBr ternary heterojunction with three-dimensional fluffy and hierarchical structure was successfully fabricated.
Fig. 2. SEM images of (A) BiOBr; (B) g-C3N4; (C) Bi/BiOBr; (D) 10%-g-C3N4@Bi/BiOBr; (E) 20%-g-C3N4@Bi/BiOBr; (F) 30%-g-C3N4@Bi/BiOBr.
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Fig. 3. TEM image (A) and HRTEM image (B) of 20%-g-C3N4@Bi/BiOBr sample.
3.3. Chemical state and composition characterization The elemental composition and surface chemical state of the 20%-g-C3N4@Bi/BiOBr composites were characterized by XPS spectra. The characteristic peaks of C, N, Bi, O and Br elements are observed in Fig. 4A, suggesting the coexistence of these elements in ternary composites. More detailed C 1s peak of this product is consecutively fitted by two peaks (Fig. 4B). The peak at 284.8 eV can be assigned to sp2 CeC bonds of graphitic or amorphous carbons adsorbed on the composites surface [38]. The peak at 288.0 eV can be identified as sp2-bonded carbon in N-containing aromatic rings (NeC]N) [39]. Four peaks of N 1s spectrum at 398.7, 399.7, 400.5 and 404.3 eV (Fig. 4C) correspond to sp2-hybridized nitrogen involved in triazine rings (C]NeC), tertiary N bonded in N-(C)3 groups, NeH groups and p excitations, respectively [40,41]. Both the high-resolution spectra of C 1s and N1s further confirm the existence of g-C3N4 in the ternary heterojunction. On the high resolution spectrum of Bi 4f (Fig. 4D), two strong peaks at 159.3 and 164.6 eV can be designated to the Bi 4f7/2 and Bi 4f5/2 by reason of forming the BieO bond in BiOBr lattice, respectively [42] while two tiny peaks located at 157.2 and 162.5 eV are attributed to metallic Bi in the ternary heterojunction [41]. The peaks at 529.6 and 530.9 eV (Fig. 4E) correspond to the O 1s in the BieO bond and the hydroxyl groups (OeH), respectively [43]. The peak of Br 3d located at 68.4 and 69.2 eV (Fig. 4F) are in agreement with Br 3d5/2 and Br 3d3/2, respectively [44]. Obviously, the XPS analyses are well consistent with the results of XRD, SEM and TEM, further confirming the formation of g-C3N4@Bi/BiOBr ternary heterostructure. 3.4. Optical absorption analysis To investigate the optical response range of the obtained photocatalysts, UVeVis DRS measurements were performed and shown in Fig. 5. Pure BiOBr and g-C3N4 exhibit obvious absorption with fundamental absorption edge at 450 nm and 466 nm, respectively. The absorption edge of Bi/BiOBr presents a little redshift (483 nm) relative to BiOBr, suggesting that the loaded metal Bi could improve the ability of visible light absorption to some degree [22]. Different from Bi/BiOBr and g-C3N4, 20%-g-C3N4@Bi/ BiOBr and 30%-g-C3N4@Bi/BiOBr present obviously enhanced photoresponse in the whole visible-light range, which is just corresponding to their fluffy and hierarchical ternary
heterostructure. Therefore, it can be reasoned that the enhanced visible-light photoresponse is ascribed to the hierarchical structure of the composites as expected. The band gap of those assynthesized photocatalysts could be estimated via the equation (Ahv) ¼ a(hv - Eg)n/2, in which A, h, v, a, and Eg represent the absorption coefficient, Planck constant, light frequency, constant and band gap energy, respectively. In addition, the numerical value of n (n ¼ 1 or 4) is determined by the optical transition type of the photocatalyst [45e47]. For this system, the value of n should be 4 due to their indirect transition character of BiOBr and g-C3N4 [13,48,49]. As presented in insert of Fig. 5, the band gaps of pure BiOBr, g-C3N4, Bi/BiOBr and 20%-g-C3N4@Bi/BiOBr were calculated to be about 2.76, 2.66, 2.57 and 2.43 eV, respectively. It is obvious that the ternary g-C3N4@Bi/BiOBr photocatalyst with a hierarchical structure have a relative narrow bandgap and enhanced visible-light response, which would notably boost the photocatalytic performance for degrading organic pollutants under visible-light illumination. 3.5. Photocatalytic performance Photocatalytic activity of the prepared samples was evaluated by decomposing RhB and TC under visible-light illumination, respectively. Besides, the binary g-C3N4/BiOBr (a mass ratio of 20% for g-C3N4: BiOBr) heterojunction synthesized by hydrothermal treatment (at 180 C for 18 h) was employed as a reference. Before the photoreaction, an adsorption-desorption equilibrium between the photocatalysts and organic pollutants was obtained. As shown in Fig. 6A and B, compared with the single components and binary composites, the ternary g-C3N4@Bi/BiOBr photocatalysts all present stronger ability to absorb organic pollutants, which might be caused by their larger surface areas. After being exposed to visiblelight irradiation for 80 min, only 52% and 26% of RhB were degraded by pure BiOBr and g-C3N4, respectively (Fig. 6A). The binary Bi/ BiOBr and g-C3N4/BiOBr present a higher degradation rates (65% and 79%, respectively). In contrast, all the ternary composites display a notable high photodegradation performance, among which 20%-g-C3N4@Bi/BiOBr exhibits the highest efficiency (98%). 30%-g-C3N4@Bi/BiOBr presents slightly low photodegradation efficiency (94%) though its visible-light photoresponse is similar to 20%-g-C3N4@Bi/BiOBr, which could be caused by its relative small specific surface area. Moreover, compared with other g-C3N4/BiOBr
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Fig. 4. (A) Overall X-ray photoelectron spectra of 20%-g-C3N4@Bi/BiOBr; and high-resolution spectra of (B) C 1s; (C) N 1s; (D) Bi 4f; (E) O 1s and (F) Br 3d.
composites reported in related literatures [19,50], the ternary gC3N4@Bi/BiOBr heterojunction still present a higher photocatalytic performance in degrading RhB. Accordingly, It could be concluded that the presence of metal Bi has notable contribution to enhance the photocatalytic performance. The photodegradation performance of the obtained samples was further evaluated by decomposing colorless TC for excluding the indirect dye photosensitization in photodegradation RhB system. As shown in Fig. 6B, the binary Bi/BiOBr and g-C3N4/BiOBr
present slightly higher photocatalytic efficiency (51% and 58%) than pure BiOBr (44%) and g-C3N4 (19%) after being irradiated for 4 h under visible light. Furthermore, the ternary composites exhibit notably higher photocatalytic performance than the binary composites. Among them, 20%-g-C3N4@Bi/BiOBr displays the highest photodegradation performance (78%). These results are well congruent to those discussed about RhB degradation, suggesting that the fabricated ternary composites have the excellent photocatalytic performance for degrading organic pollutants. Also, it is
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indicated that the ternary heterojunction play a key role in enhancing the photocatalytic performance. The cycling experiments for degrading RhB and TC over 20%-gC3N4@Bi/BiOBr were performed to assess the stability of the ternary composites during the photodegradation process. As shown in Fig. 6C and D, the photocatalytic efficiency displays a negligible loss after each cycle, indicating the ternary composites have an outstanding stability in the practical application. In addition, the structural stability of ternary g-C3N4@Bi/BiOBr composites was further determined by comparing the XRD patterns of 20%-g-C3N4@Bi/BiOBr before and after the four cycling experiments (Fig. 7). It is obvious that the structure of the ternary heterojunction has almost no change, further confirming the stability of the obtained photocatalysts. 3.6. Possible photocatalytic mechanism
Fig. 5. UVevis diffuse reflectance spectra of the as-obtained samples; the insert displays the band gaps of the corresponding samples.
Photocurrent techniques were used to evaluate the separation efficiency and transfer rate of charge carriers. As displayed in Fig. 8, the photocurrent responses intensity of Bi/BiOBr is far higher than that of single BiOBr, suggesting that the loading of metal Bi can tremendously facilitate the charge transfer and inhibit the recombination of photoexcited charge carriers as demonstrated in previous reports [24,25]. Compared with binary Bi/BiOBr and single
Fig. 6. Photocatalytic performance of obtained samples under visible-light illumination for (A) degrading RhB and (B) TC; cycling experiments for the photodegradation of (C) RhB and (D) TC.
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Fig. 7. XRD of 20%-g-C3N4@Bi/BiOBr before and after the four cycling experiments.
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Fig. 9. Photocatalytic efficiency for degrading RhB with the addition of different scavengers over the 20%-g-C3N4@Bi/BiOBr composites.
band positions of the corresponding components were investigated to profoundly illuminate the photodegradation mechanism. The conduction band (ECB) and valence band (EVB) potentials of BiOBr and g-C3N4 were calculated via the following equations [51e53]:
Fig. 8. Photocurrent responses of BiOBr, g-C3N4, Bi/BiOBr and 20%-g-C3N4@Bi/BiOBr samples.
g-C3N4, 20%-g-C3N4@Bi/BiOBr ternary composites present the clearly high photocurrent intensity and strong residual current, indicating that the constructed ternary heterojunction among Bi, BiOBr and g-C3N4 further accelerate the charge separation and transfer of photoinduced carriers. To further explore the main active species during the photodegradation process over 20%-g-C3N4@Bi/BiOBr composites, trapping experiments were systematically performed via adding different scavengers of active radicals in the system of RhB photodegradation and the results are shown in Fig. 9. Isopropanol (IPA, 10 mM), benzoquinone (BQ, 1 mM) and sodium oxalate (Na2C2O4, þ 10 mM) were used as scavenger of ∙OH, ∙O radicals, 2 and h respectively. Obviously, the photodegradation efficiency slightly declines with the addition of IPA, while it drastically decreases when Na2C2O4 or BQ was added. These results demonstrate that þ both ∙O 2 and h radicals play a decisive role during the photodegrading process, and the photo-generated electrons and holes have strong reducibility and oxidizability, respectively. The transfer direction of carriers on the interface of heterojunction was thermodynamically determined by the band-edge positions of two components forming the heterojunction. So, the
EVB ¼ X Ee þ 0:5Eg
(1)
ECB ¼ EVB Eg
(2)
in which X and Ee respectively corresponds to the electronegativity of semiconductor and the energy for free electrons (4.5 eV). According to the results of Uvevis analysis, the Eg values of BiOBr and g-C3N4 are respectively measured about 2.76 and 2.66 eV. Subsequently, the ECB values of BiOBr and g-C3N4 are calculated to be 0.30 and 1.12 eV, respectively, and the values of BiOBr and g-C3N4 EVB are respectively 3.06 and 1.54 eV [54,55]. As discussed above, the ternary heterojunction among BiOBr, Bi and g-C3N4 greatly accelerates the charge transfer and separation of charge carriers. Considering the dominant active species during the photodegradation process and the relative band levels of BiOBr and g-C3N4, an indirect Z-scheme system with Bi served as an electronconduction bridge (Fig. 10) was proposed to expound the possible photodegradation mechanism of the ternary g-C3N4@Bi/BiOBr heterojunction for degrading organic pollutants. After being exposed to visible-light illumination, both BiOBr and g-C3N4 absorb photons to produce electron-hole pairs. In this process, the light absorption is notably enhanced by the three-dimensional hierarchical structure of composites (Fig. 5). The photo-induced electrons in the BiOBr conduction band tend to transfer to the interface of the heterojunction and combine with the holes in the valence band of g-C3N4 due to their different band level. The Bi bridge as a medium between BiOBr and g-C3N4 greatly accelerates their combination rate due to its good charge mobility. As a result, the bulk recombination of charge carriers inside BiOBr and g-C3N4 is significantly suppressed, leading to the accumulation of the electrons in the conduction band of g-C3N4 (1.12 eV) and holes in the valence band of BiOBr (3.06 eV). Eventually, the accumulated electrons with strong reducibility can reduce the absorbed O2 to ∙O 2 (E(O2/ ∙O 2 ) ¼ -0.33 eV) [56] which would efficiently degrade adsorbed organic pollutants together with the accumulated holes with strong oxidability. Benefiting from the extended visible-light absorption range and the high-efficiency separation of charge carriers, the ternary g-C3N4@Bi/BiOBr composites presented notably
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Fig. 10. Schematic illustration of the possible mechanism of the g-C3N4@Bi/BiOBr photocatalyst for degrading organic pollutants under visible light irradiation.
enhanced photocatalytic performance for degrading organic pollutants.
4. Conclusions To sum up, the novelty ternary g-C3N4@Bi/BiOBr heterojunction were successfully fabricated through in-situ reduction and selfassembly processes with ethylene glycol as both a solvent and a reductant. The ternary composites present the three-dimensional fluffy and hierarchical structure in which Bi/BiOBr nanoplates were embedded on the surface of layered g-C3N4. Under the visible light irradiation, the g-C3N4@Bi/BiOBr composites exhibited notably high photocatalytic activity for degrading RhB and TC comparing with the binary g-C3N4/BiOBr and Bi/BiOBr composites as well as the single semiconductor components. The threedimensional hierarchical structure obviously improves the visible-light photoresponse, as suggested from the UVeVis DRS and the ternary heterojunction with Bi loading greatly facilitate the separation of photo-induced carriers, as revealed by the photocurrent experiments. Their combined action results in the excellent photodegradation performance of the ternary g-C3N4@Bi/BiOBr composites. According to the acquired results of trapping experiments and the energy band potentials, an indirect Z-scheme system with Bi as an electron-conduction bridge was proposed to illuminate the possible photodegradation mechanism. This work provides a novel strategy combining with loading metal and morphology majorization to design the multiple heterostructured photocatalysts for environmental purification and energy conversion.
Acknowledgments The authors sincerely acknowledge the financial support provided by the Project of the National Natural Science Foundation of China (Grant No. 21271022) and the Fundamental Research Funds for the Central Universities (FRF-BR-18-002A).
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