Iodine ion doped bromo bismuth oxide modified bismuth germanate: A direct Z-scheme photocatalyst with enhanced visible-light photocatalytic performance

Journal of Colloid and Interface Science 553 (2019) 186–196

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Regular Article

Iodine ion doped bromo bismuth oxide modified bismuth germanate: A direct Z-scheme photocatalyst with enhanced visible-light photocatalytic performance Xiaowen Ruan a, Hao Hu a, Guangbo Che b, Pengjie Zhou c, Chunbo Liu a,⇑, Hongjun Dong a,⇑ a b c

Institute of Green Chemistry and Chemical Technology, School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China Key Laboratory of Preparation and Applications of Environmental Friendly Materials, Jilin Normal University, Ministry of Education, Changchun 130103, PR China School of Materials and Engineering, Jiangsu University of Science and Technology, 2 Mengxi Road, Zhenjiang 212003, PR China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 The successful preparation of Z-

scheme I-BiOBr/Bi12GeO20 heterostructures.  The enhanced photocatalytic performance by forming Z-scheme structure.  The degradation of various organic pollutants as well as aqueous Cr(VI).

a r t i c l e

i n f o

Article history: Received 10 April 2019 Revised 1 June 2019 Accepted 3 June 2019 Available online 4 June 2019 Keywords: Z-scheme I-BiOBr/Bi12GeO20 heterostructure Photocatalytic degradation and reduction Doping I ions Degradation pathways

a b s t r a c t A series of Z-scheme I-BiOBr/Bi12GeO20 heterostructures were successfully obtained by a simple method. The Z-scheme I-BiOBr/Bi12GeO20 heterostructures show outstanding photocatalytic performance for degrading the various organic pollutants of the waste water. For degradation of Tetracycline (TC), the Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure exhibits the superior rate constant, which is about 7.73 times, 3.52 times and 1.66 times higher than that of the pure Bi12GeO20, BiOBr and I-BiOBr, respectively. Meanwhile, as we expected, the Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure also displays the enhanced photocatalytic perfomance for degradation of Ciprofloxacin (CIP), 2-Mercaptobenzothiazole (MBT) and reduction of aqueous Cr(VI). The enhancement of photocatalytic performance is attributed to the high redox capacity and the strong interfacial interaction between I-BiOBr and Bi12GeO20, which can effectively improve the separation of photo-induced electron-hole pairs. Additionally, the photocatalytic mechanism over the Z-scheme I-BiOBr/Bi12GeO20 heterostructure is provided. The research work may provide a promising approach to fabricate other Z-scheme heterostructures with efficient photocatalytic performance. Ó 2019 Published by Elsevier Inc.

⇑ Corresponding authors. E-mail addresses: [email protected] (C. Liu), [email protected] (H. Dong). https://doi.org/10.1016/j.jcis.2019.06.007 0021-9797/Ó 2019 Published by Elsevier Inc.

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1. Introduction With the acceleration of industrialization, the continuous development of social economy and rapid growth of population, the demand for fossil energy increases day by day [1,2]. At present, people can use fossil energy mainly including coal, oil and natural gas, which are non-renewable energy sources [3]. The overexploitation of fossil energy has made the energy crisis one of the most urgent problems in human development. In addition, as human beings consume a large amount of fossil energy, they also directly or indirectly discharge a large number of waste water, waste gas, waste residue and other pollutants into the environment, resulting in serious environmental pollution [4–7]. Among the numerous pollutants, the toxic and harmful organics, which are difficult to be degraded, enter into the water environment and ecological system due to improper treatment and discharge. In the long-term accumulation and diffusion of these organic pollutants in the environment, their own toxicity is high, and it is difficult to be biodegraded and easy to accumulate in organisms, thus causing serious harm to the normal operation of the ecosystem and human health [8–11]. In recent years, semiconductor photocatalytic technology has attracted much attention because of its excellent application prospects in sewage treatment, air purification, cleaning and sterilization and solar energy conversion [12–16]. Photocatalytic technology represented by TiO2 has the characteristics of simple equipment, strong oxidation ability, no secondary pollution and wide application range, which is regarded as a promising waste water treatment method [17]. Unfortunately, the band gap of TiO2 is 3.2 eV, so it can only be excited by the ultraviolet light that only accounts for 4% in sunlight to produce active groups with strong oxidation [18]. Therefore, it is particularly important to develop new semiconductor materials with strong visible light response. Bismuth-based heterostructures have been generally considered as resultful photocatalytst such as BiTiO3 [19], BiOI [20], Bi2O2(OH)(NO3) [21] and Bi3O4Cl [7], etc. Amidst all of them, Bi12GeO20 that belongs to the Bi12MO20 (M = Ge, Si and Ti) has caused great concern because of its strong chemical oxidation ability [22]. Nevertheless, the practical adhibition is inhibited in the related field of degradation and purification of waste water due to its high recombination of photogenerated electron-hole pairs [22]. For the sake of overcoming the above mentioned issues, constructing a durable and efficient semiconductor heterostructure is caught by many researchers’ attention. Recently, Wan et al. constructed the Z-scheme Bi12GeO20/g-C3N4 semiconductor heterostructure, demonstrating the improved photocatalytic performance for organic pollutants than both pure Bi12GeO20 and g-C3N4 [23]. At the same time, Ruan et al. fabricated the Ag2O/ Bi12GeO20 semiconductor heterostructure, significantly enhancing the photocatalytic performance [22]. To sum up, it can be depicted that it is of great importance to construct other Bi12GeO20-based heterostructures with highly efficient and superior stability. BiOX (X = Cl, Br and I) which is regarded as a novel type of semiconductor photocatalysts is intensively applied in the degradation and purification of organic contaminants [24–26]. So far, different BiOX-based semiconductors are constructed to enhance the photocatalytic performance. For instance, the BiOCl/BiOCOOH [27], BiOI@Bi12O17Cl2 [28] and BiOBr/BiOI [29] semiconductor heterostructures which all have the well matched band gap show the superior photocatalytic performance. In this research work, we firstly raise the valence band position of pure BiOBr by using I ions which are worked as dopants. The valence band position of I-BiOBr succeeds in locating above that of pure BiOBr through doping I ions. Meanwhile, the Z-scheme I-BiOBr/Bi12GeO20 heterostructures are successfully synthesized.

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The as-prepared Z-scheme I-BiOBr/Bi12GeO20 heterostructures are involved in the photocatalytic degradation of 2Mercaptobenzothiazole (MBT), Ciprofloxacin (CIP), Tetracycline (TC) as well as the photocatalytic reduction of aqueous Cr(VI). The synthesized Z-scheme I-BiOBr/Bi12GeO20 heterostructures can evidently boost the efficiency of photocatalytic performance in comparison with the Bi12GeO20, BiOBr and I-BiOBr, respectively. In addition, the photoelectrochemistry tests are measured by the electrochemical workstation. Furthermore, the probable mechanism in detail is proposed. 2. Experimental 2.1. Synthesis of Bi12GeO20 Bi12GeO20 was synthesized by the method which is displayed in the Supporting information. 2.2. Preparation of I-BiOBr nanosheets I-BiOBr nanosheets were obtained through a chemical precipitation approach. Typically, 4.85 g Bi(NO3)3
5H2O was firstly added in a mixed solution which included 20 mL glacial acetic acid and 30 mL deionized water. After that, another 50 mL solution containing 1.95 g NaBr and 0.17 g KI (the determined amount of NaBr and KI is achieved by our continuous experiments) was added into the above mixed solution drop by drop. After continuous stirring for 180 min, the yellow precipitate was obtained, washed with deionized water and dried at 70 °C the whole night (Note: pure BiOBr was synthesized by the same approach except the addition of NaBr and KI). 2.3. Preparation of Z-scheme I-BiOBr/Bi12GeO20 heterostructure Z-scheme I-BiOBr/Bi12GeO20 heterostructure was fabricated by a simple water bath method. A certain amount of Bi12GeO20 was added into 50 mL deionized water and stirred for 30 min. Then, a given mass ratio amount of synthesized I-BiOBr was added into the above mentioned solution and vigorously stirred for 8 h in order to mix enough. After that, the mixed solution was centrifuged by adding distilled water and ethanol and finally dried at 60 °C the whole night. Following these steps, various proportions of Z-scheme I-BiOBr/Bi12GeO20 heterostructures are achieved. (Note: the amount of I-BiOBr is relative to Bi12GeO20, and tagged as 10I-BiOBr/Bi12GeO20, 20I-BiOBr/Bi12GeO20, 30I-BiOBr/Bi12GeO20, 40I-BiOBr/Bi12GeO20 and 50I-BiOBr/Bi12GeO20). 2.4. Characterization of the synthesized heterostructure The detailed characterization instrument of the as-prepared heterostructure as well as the specific photocatalytic activity measurement are displayed in the Supporting information. 3. Results and discussions 3.1. Structure and composition of synthesized heterostructure The structure and specific phase composition of as-prepared samples are tested by XRD analysis. As shown in Fig. 1a, the pure Bi12GeO20 exhibits high intensity peaks revealing the good crystallinity of the sample which is consistent with the standard card of pure Bi12GeO20 (JCPDS Card No: 77-0861) [22]. At the same time, the pure BiOBr also shows the good crystallinity (JCPDS Card No: 09-0393). Interestingly, all the peaks have slightly shifted with

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Fig. 1. (a) XRD and (b, c) enlarged XRD patterns of Bi12GeO20, BiOBr, I-BiOBr and Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure. (d) EDX spectrum of Z-scheme 30I-BiOBr/ Bi12GeO20 heterostructure.

doping little amounts of I ions, which is presented in Fig. 1b. Moreover, the characteristic peaks, especially (1 0 2) and (1 1 0) whose enlarged XRD spectra is shown in Fig. 1c, shift slightly to lower angle area with regard to I-BiOBr, demonstrating that iodine ions succeed in incorporating into the BiOBr lattice by taking the place of bromide ions due to its larger ionic radius [30,31]. It can be noted that compared with pure Bi12GeO20 and I-BiOBr, all the disparate diffraction peaks are found in as-prepared 30I-BiOBr/ Bi12GeO20 heterostructure suggesting that the 30I-BiOBr/ Bi12GeO20 heterostructure succeeds in being prepared. The similar peak shift phenomenon is displayed in as-prepared Z-scheme I-BiOBr/Bi12GeO20 heterostructure as well. Additionally, the characteristic peak intensities of I-BiOBr in as-prepared Z-scheme I-BiOBr/Bi12GeO20 heterostructure are not obvious which is due to the low loading contents. Furthermore, the elemental analysis of Z-scheme I-BiOBr/Bi12GeO20 heterostructure is measured by EDX. The elements of Bi, Ge, O, I and Br are detected in asprepared Z-scheme I-BiOBr/Bi12GeO20 heterostructure which is shown in Fig. 1d. Considering all the above discussion, it can draw the conclusion that Z-scheme I-BiOBr/Bi12GeO20 heterostructures are successfully prepared. To study the specific chemical compositions, the as-prepared pure BiOBr, Bi12GeO20, I-BiOBr and the Z-scheme 30I-BiOBr/ Bi12GeO20 heterostructure are examined by XPS. Fig. 2a shows the survey spectra of all the as-prepared samples, it can be depicted that the as-prepared Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure consists of Bi, Ge, Br, I and O elements. Meanwhile, the high-resolution XPS spectra are demonstrated in Fig. 2b–f, respectively. It can be noted in Fig. 2b that two obvious peaks in pure Bi12GeO20 which are located in the binding energies of 158.5 eV and 164 eV are corresponding to the Bi 4f7/2 and Bi

4f5/2, respectively [22,23]. At the same time, the two strong peaks in pure BiOBr and I-BiOBr are located in the binding energies of 159.5 eV and 164.8 eV [32,33]. Interestingly, the two peaks in asprepared Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure have a slight shift which is different from them in pure Bi12GeO20, BiOBr and I-BiOBr, respectively. The peak located at 25.7 eV and 28.7 eV can be attributed to Bi 5d as well as Ge 3d, which is shown in Fig. 2c [23]. What’ more, the strong peaks of Br 3d which is displayed in Fig. 2d are associated with binding energy 70.2 and 69.2 eV referring to Br 3d3/2 and Br 3d5/2 [34,35]. It is not difficult to observe that both the peaks of Br 2d and Ge 3d have little difference with the pure sample which is possibly because of the little quantity of iodine ions doping. Additionally, it can be depicted in Fig. 2e that the I 3d peak is separated into two different peaks which are at approximately 630.8 and 619.3 eV that is corresponding to I 3d3/2 and I 3d5/2, suggesting the valence state of I is 1 [36]. The O 1s spectrum (Fig. 2f) in as-prepared Z-scheme 30I-BiOBr/ Bi12GeO20 heterostructure consists of two peaks as well. One peak located at about 530.3 eV is belonging to crystal O of the asprepared heterostructure, while the other peak which is located at 531.8 eV can be imputed to mixed contributions from the hydroxyl groups attached to the surface of the as-prepared semiconductors [37]. As a result, considering all the above discussion, it can be concluded that the I-BiOBr/Bi12GeO20 heterostructure with the Zscheme system succeeds in being synthesized. 3.2. Morphology of as-prepared Z-scheme I-BiOBr/Bi12GeO20 photocatalyst The morphology and microstructure of as-prepared Bi12GeO20, I-BiOBr and Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure are

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Fig. 2. XPS spectra of Bi12GeO20, BiOBr, I-BiOBr and Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure: (a) survey spectra, (b) Bi 4f, (c) Ge 3d, (d) Br 3d, (e) I 3d and (f) O 1s.

discussed by FESEM and TEM analyses. From Fig. 3a, it is easily observed that the pure Bi12GeO20 shows a spherical microstructure whose average size is about 10 lm. Fig. 3b demonstrates that pure I-BiOBr has the hierarchical microflower morphology which consists of nanosheets. Compared to pure Bi12GeO20, it can be seen in Fig. 3c that a number of I-BiOBr nanosheets are coated on the surface of the Bi12GeO20 spheres, which contributes to the generation of Z-scheme I-BiOBr/Bi12GeO20 heterostructure. The TEM image is further to prove that the I-BiOBr/Bi12GeO20 heterostructure succeeds in being prepared. It can be simply observed in Fig. 3e that I-BiOBr is in close contact with Bi12GeO20, forming a clear interface which can contribute to be favorable for improving the transport and separation of photo-induced carriers. Addition-

ally, a lattice fringe spacing of 0.144 nm is clearly obtained in Fig. 3d (the inset), which is corresponding to the (2 0 4) plane of I-BiOBr. Moreover, it is noteworthy in Fig. 3f that the HRTEM image of Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure is used to further prove the strong interface between I-BiOBr and Bi12GeO20, where the distinct lattice spacing of 0.29 nm is consistent with the (2 2 2) plane of Bi12GeO20, and the lattice fringe spacing of 0.144 nm is ascribed to the (2 0 4) plane of I-BiOBr, respectively. Comparing the lattice spacing of I-BiOBr in Fig. 3d and f, it can be discovered that the lattice spacing of pure I-BiOBr is the same as it in Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure, indicating that the introduction of Bi12GeO20 will not influence the basic structure of I-BiOBr. Furthermore, in Fig. 4, EDS mapping of

Fig. 3. FESEM images of (a) Bi12GeO20 (b) I-BiOBr (c) Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure. TEM images of (d) I-BiOBr (The inset: the high resolution TEM image of I-BiOBr) (e) Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure and (f) high resolution TEM image of Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure.

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Fig. 4. EDS mapping of Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure: (a) SEM image, (b) Bi, (c) Ge, (d) Br, (e) O and (f) I.

Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure reveals the presence of Bi, Ge, I, Br and O in the whole selected region, suggesting the uniform dispersion of five elements, thus inferring that the Zscheme 30I-BiOBr/Bi12GeO20 heterostructure succeeds in being synthesized.

transition). According to the prevenient works [22,23,42], all the value of n for pure Bi12GeO20, BiOBr and I-BiOBr is 1. Therefore, the settled Eg of the Bi12GeO20 is 2.85 eV, the settled Eg of the BiOBr is 2.81 eV and the settled Eg of the I-BiOBr is 2.24 eV, which is shown in Fig. 5b.

3.3. UV–vis light adsorption of synthesized photocatalysts

3.4. Photocatalytic performance tests

In order to investigate the detailed band gap energies and light properties of the as-prepared samples, the UV–vis reflectance absorption spectra is displayed in Fig. 5. Seen from Fig. 5a, both pure Bi12GeO20 and BiOBr show poor visible light response. However, after doping the I ions, the light absorption range is greatly extended to 520 nm. The band-gap energy is narrowed due to I ions form impurity levels in as-prepared I-BiOBr. Obviously, after Bi12GeO20 is assembled with small quantity of I-BiOBr, the light absorption ranges of Z-scheme I-BiOBr/Bi12GeO20 are improved, ensuring the superior visible light absorption of Z-scheme IBiOBr/Bi12GeO20 heterostructure. Based on the above mentioned, the band gap energies of all the as-prepared samples are estimated by the equation: Aht = A(ht Eg)n/2, where A represents the light frequency, Eg represent the band gap energy, a represents the absorption coefficient and ht represents the Plank constant [38– 41]. Moreover, the value of n is determined by the type of optical transition (i. e. n = 1 for direct transition and n = 4 for indirect

To investigate the detailed properties of the as-prepared photocatalysts, Tetracycline, as the model organic pollutant, is used to evaluate the photocatalytic performance. In Fig. 6a, it can be noted that both pure Bi12GeO20 and BiOBr show poor photocatalytic performance, which is only 17% and 36%, respectively. However, when doping quantity of iodine ions, TC can be degraded about 57%, which ensures that the doping influence of iodine ions is significant in BiOBr. More interestingly, the construction of I-BiOBr and Bi12GeO20 can significantly enhance the photocatalytic performance towards TC. For instance, Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure demonstrates superior photocatalytic performance than pure Bi12GeO20, BiOBr and I-BiOBr, which can degrade nearly 80% within 120 min. Meanwhile, Fig. 6b shows the absorbance variation of TC in the presence of Z-scheme 30I-BiOBr/ Bi12GeO20 heterostructure. Additionally, the other as-prepared different mass ratios of heterostructures also show the better photocatalytic performance compared with pure Bi12GeO20. The

Fig. 5. (a) UV–vis DRS of as-prepared samples. (b) Plots of the (Aht)1/2 versus ht for pure Bi12GeO20, pure BiOBr and pure I-BiOBr.

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Fig. 6. (a) The degradation of pure Bi12GeO20, BiOBr, I-BiOBr and different ratios of Z-scheme I-BiOBr/Bi12GeO20 heterostructures towards Tetracycline (TC). (b) Absorption spectra of TC over the Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure. (c) The pseudo-first-order reaction kinetics and (d) Values of reaction rate constants over the asprepared samples.

enhanced photocatalytic performance can be attributed to the strong heterostructure interface effect of Z-scheme I-BiOBr/ Bi12GeO20. Moreover, the loading quantity of I-BiOBr on Bi12GeO20 can remarkably influence the photocatalytic performance of Zscheme I-BiOBr/Bi12GeO20 heterostructure. Scilicet, the photocatalytic performance rises firstly and decreases with the increasing addition of I-BiOBr, which may be due to the abundant amount of I-BiOBr will cover the active sites causing the imperfect photocatalytic performance. In order to understand the reaction kinetics of TC, the kinetic equation of first-order reaction is carried out. The rate constants are obtained by the means of the equation of ln(C0/C) = kt (Fig. 6c). It can be clearly seen from Fig. 6d that the rate constant of I-BiOBr/Bi12GeO20 is 0.0116 min 1, which is nearly 7.73 times, 3.52 times and 1.66 times than that of pure Bi12GeO20 (0.0015 min 1), BiOBr (0.0033 min 1) and I-BiOBr (0.007 min 1), respectively. Considering the above discussed issues, it can be deduced that the loading of I-BiOBr on the surface of Bi12GeO20 can improve the photocatalytic performance and the as-prepared Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure can be targeted as the highly efficient photocatalyst. Encouraged by the above discussions and results, MBT and CIP degradation over the as-prepared photocatalysts are also tested (Fig. 7a and d). It is not difficult to find that the as-prepared Zscheme I-BiOBr/Bi12GeO20 heterostructures show the enhanced photocatalytic performance in comparison with pure BiOBr and Bi12GeO20, respectively. More importantly, the as-prepared Zscheme 30I-BiOBr/Bi12GeO20 heterostructure shows the superior photocatalytic performance towards the degradation of MBT and CIP as we expected. Additionally, the absorption spectra of (b) MBT and (e) CIP are also displayed. At the same time, the kinetic

equation of first-order reaction is also carried out (Fig. 7c and f). It can be noted in Fig. 7c that the rate constant of 30I-BiOBr/ Bi12GeO20 heterostructure towards the degradation of MBT is 0.0479 min 1, while the rate constant of 30I-BiOBr/Bi12GeO20 heterostructure towards the degradation of CIP is 0.006 min 1 (Fig. 7f). Both of the rate constant of as-prepared heterostructure not only towards the degradation of MBT but also the degradation of CIP shows the highest, indicating that the as-prepared 30IBiOBr/Bi12GeO20 heterostructure with Z-scheme system is indeed an efficient and durable photocatalyst. To further investigate the synthesized photocatalyst which shows the enhanced photocatalytic performance, the aqueous Cr (VI) photocatalytic reduction towards the synthesized Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure is tested. From Fig. 8a, it can be simply discovered that the synthesized Z-scheme 30I-BiOBr/ Bi12GeO20 heterostructure photocatalyst shows the better photocatalytic reduction than any other single photocatalysts. Meantime, Fig. 8b demonstrates the absorption spectra of aqueous Cr(VI) over the Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure. Additionally, it can be depicted in Fig. 8c that the as-prepared 30I-BiOBr/Bi12GeO20 heterostructure shows the highest value of reaction rate constant than any other samples. Considering all the above mentioned, it can be exactly concluded that the as-prepared heterostructure is a a durable and efficient photocatalyst. For the purpose of exploring the separation and transfer of photocarriers and the transient photocurrent intensity, the photocurrent responses and electrochemical impedance spectroscopy of the as-prepared pure Bi12GeO20, BiOBr, I-BiOBr and Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure are measured by using electrochemical workstation. As shown in Fig. 9a, the as-prepared

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Fig. 7. The degradation of pure Bi12GeO20, BiOBr, I-BiOBr and a series of different ratios of Z-scheme I-BiOBr/Bi12GeO20 heterostructures towards (a) MBT, (d) CIP. Absorption spectra of (b-MBT), (e-CIP) over the Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure. The pseudo-first-order reaction kinetics (c-MBT), (f-CIP) and values of reaction rate constants over the as-prepared samples.

Fig. 8. (a) Photocatalytic reduction curves of aqueous Cr(VI) over the pure Bi12GeO20, BiOBr, I-BiOBr and Z-scheme 30I-BiOBr/Bi12GeO20 under visible-light irradiation. (b) Absorption spectra of aqueous Cr(VI) over the Z-scheme 30I-BiOBr/Bi12GeO20. (c) The pseudo-first-order reaction kinetics and values of reaction rate constants over the asprepared samples.

Fig. 9. (a) Transient photocurrent responses and (b) electrochemical impedance spectroscopy of different as-prepared pure Bi12GeO20, BiOBr, I-BiOBr and Z-scheme 30IBiOBr/Bi12GeO20 heterostructure.

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Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure is of strong photocurrent response, which is higher than it in pure Bi12GeO20, BiOBr and I-BiOBr, suggesting the efficient separation of photocarriers. Meanwhile, it can be noticed in Fig. 9b that the as-prepared Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure reveals the smaller radius than it of pure Bi12GeO20, BiOBr and I-BiOBr, indicating that introducing I-BiOBr can intensively strengthen the separation efficiency of photocarriers, thus enhancing the photocatalytic performance. 3.5. Stability tests As we all know, the stability of the synthesized photocatalyst is of great importance to be applied to the practical application. Thence, a series of cycling photocatalytic experiments towards the Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure for the degradation of TC are tested. It can be observed in Fig. 10a that the photocatalytic efficiency maintains almost unchangeable while suffering from four cycling experiments. Meanwhile, Fig. 10b

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displays both the XRD spectra of Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure before and after the photocatalytic reaction which shows that the spectra also has no obvious change, further indicating that the synthesized Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure is a durable and efficient photocatalyst. Generally speaking, the reactive species which are generated on the surface of photocatalyst play a vital role in the photocatalytic degradation of organic pollutants [43–46]. Thence, Fig. 11a displays the photocatalytic degradation of TC when adding different scavengers. For pure Bi12GeO20, the photocatalytic degradation towards TC is significantly decreased with the addition of EDTA2Na, declaring that holes (h+) can be considered as main reactive species. However, the photocatalytic degradation towards TC maintain unchangeable with the addition of BQ and isopropyl alcohol (IPA). With regard to the as-prepared Z-scheme 30I-BiOBr/ Bi12GeO20 heterostructure, it can be seen that the degradation rate is obviously inhibited when adding 1,4-ben-zoquinone (BQ) and EDTA-2Na, suggesting that the reactive species holes (h+) and O2

Fig. 10. (a) Cycling photocatalytic experiments of Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure for degradation of TC (b) XRD of Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure before and after the photocatalytic reaction.

Fig. 11. (a) The photocatalytic degradation of TC with different scavengers over pure Bi12GeO20, I-BiOBr and as-prepared Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure. The ESR spectra of as-prepared Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure (b) in methanol dispersion for 5,5-Dimethyl-1-pyrroline N-oxide (DMPO-O2 ) and (c) in aqueous dispersion for 5,5-Dimethyl-1-pyrroline N-oxide (DMPO-OH) under visible light for 5 min, respectively. (d) VB-XPS spectra of pure BiOBr and I-BiOBr. Mott-Schottky plot of the as-prepared samples (e) Bi12GeO20 and (f) I-BiOBr in 0.5 M Na2SO4 solution.

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radicals make an important contribution to the photocatalytic degradation. Meantime, adding IPA scavenger almost has no obvious change on the degradation rate towards TC, manifesting that  OH radical is not the major reactive species. Interestingly, the effects of trapping experiments for pure I-BiOBr are similar to that of as-prepared Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure, which both the holes (h+) and O2 radicals are the important reactive species for the degradation of TC. Furthermore, to further investigate the active radicals for the degradation reaction, the electron spin resonance (ESR) spin-trap technique is tested and explained whose results are shown in Fig. 11b–c. It can be clearly observed that six DMPO-O2 characteristic peaks towards the asprepared Z-scheme 30I-BiOBr/Bi12GeO20 heterostructure are detected under light irradiation while no any other obvious DMPO-O2 characteristic peaks can be discovered under dark condition, designating that the as-prepared Z-scheme 30I-BiOBr/ Bi12GeO20 heterostructure can generate O2 radicals towards the photocatalytic degradation of organic pollutants. On account of the above analysis, there are two major reasons for the enhanced visible-light photocatalytic performance of Z-scheme I-BiOBr/ Bi12GeO20 heterostructure. First, self-doping of I ions can lower the conduction band position and introduce the intermediate levels into the forbidden band to narrow the bandgap, thus expanding the visible response range of BiOBr, which can improve the visible light absorption as well. Second, coupling the doping I ions BiOBr with Bi12GeO20 forming the Z-scheme I-BiOBr/Bi12GeO20 heterostructure can contribute to the charge transfer and reduce the recombination of charge carriers. Due to the intimate interfacial interaction and Fermi energy equilibration, the band bending forces the surviving photoexcited charge carriers on the surface of the photocatalytic component to the opposite directions, which can effectively promote the photocatalytic performance. The synergistic effects of the above mentioned two factors give rise to the high performance of I-BiOBr/Bi12GeO20 heterostructure under visible light irradiation. To ulteriorly explore the detailed influence of doped iodine ions, the band edge positions of BiOBr and I-BiOBr are determined by VB-XPS. As shown in Fig. 11d, the valence band of the pure BiOBr is 2.21 eV while the valence band of I-BiOBr is 1.64 eV. At the same time, the Mott-Schottky curves are tested whose results are displayed in Fig. 11e–f. As is known to all, the conduction band potential (ECB) is on the verge of the Vfb for n-type semiconductors [47,48], we can reach a decision that the ECB values of Bi12GeO20 and I-BiOBr are 0.25, 0.48 V vs NHE. Moreover, we are intensively curious about the valence band potential (EVB) of the pure Bi12GeO20 and I-BiOBr. Since the ECB values and the Eg values are complete and accurate, the EVB values can be calculated according to the formula: ECB = EVB Eg [49–52]. Thence, the EVB values of pure Bi12GeO20 and I-BiOBr are estimated to be 2.60 V and 1.76 V, respectively. Considering the above discussed issues, a possible Z-scheme reaction mechanism is proposed here. As shown in Fig. 12, both Bi12GeO20 and I-BiOBr can produce the photoexcited carriers under visible light irradiation. After that, the photoexcited electrons in the conduction band of Bi12GeO20 will transfer and have the fast recombination with the holes on the valence band of I-BiOBr forming a direct Z-scheme charge transfer path. Meanwhile, the O2 radicals are generated on the conduction band of I-BiOBr by the photogenerated electrons reducing the O2 because of its more negative potential (ECB, 0.48 eV vs. NHE) [53]. The organic pollutant molecules can be oxidized into small non-toxic molecules by the generated O2 radicals [56,57]. Additionally, the photogenerated holes on the valence band of Bi12GeO20 can directly oxidize the TC molecules owing to its more positive potential (EVB: 2.60 eV vs. NHE) [54,55]. Considering the complete photocatalytic reaction system, the photogenerated electrons which are separated and transferred by the

Fig. 12. Schematic illustration of band structure diagram and photo-induced carriers transfer of Z-scheme I-BiOBr/Bi12GeO20 heterostructure under visible light irradiation.

Z-scheme can also react with aqueous Cr(VI), thus resulting in the better photocatalytic reduction. According to such a way, the asprepared Z-scheme I-BiOBr/Bi12GeO20 heterostructure exhibits enhanced photocatalytic degradation of organic pollutants and reduction of aqueous Cr(VI) caused by the Z-scheme photocatalytic reaction path. In order to prove the Z-scheme mechanism, the normal impossible mechanism is also displayed in Fig. S1. This work affords a promising method to construct any other efficient and durable semiconductor heterostructures. 4. Conclusion To sum up, the Z-scheme I-BiOBr/Bi12GeO20 heterostructures succeed in being synthesized. The obtained Z-scheme 30I-BiOBr/ Bi12GeO20 semiconductor heterostructure demonstrates the enhanced photocatalytic performance for degrading TC organic pollutants, whose rate constant is nearly 7.73 and 1.66 times higher than that of the pure Bi12GeO20 (kapp = 0.0015 min 1) and I-BiOBr (kapp = 0.007 min 1), respectively. Meantime, the Z-scheme 30IBiOBr/Bi12GeO20 semiconductor heterostructure also shows the comparatively higher photocatalytic performance for degrading other organic contaminants, such as MBT and CIP and reduction of aqueous Cr(VI). The enhanced photocatalytic performance originates from the improved separation and transfer of photogenerated carriers of the as-prepared Z-scheme I-BiOBr/Bi12GeO20 heterostructures. Additionally, the as-prepared Z-scheme I-BiOBr/ Bi12GeO20 heterostructures may provide the methods of designing other Z-scheme heterostructures with better prospects. Acknowledgements We would like to acknowledge the National Natural Science Foundation of China (21576112, 21805115 and 21606114), the NSFC-Shanxi Coal Based Low Carbon Joint Fund (U1810117), the Postdoctoral Science Foundation of China (2017M611712, 2017M611717) and Jiangsu Planned Projects for postdoctoral Research Funds (1701025A). Appendix A. Supplementary material Supplementary data to this article can be found online at https://doi.org/10.1016/j.jcis.2019.06.007. References [1] C. Li, G. Chen, J. Sun, J. Rao, Z. Han, Y. Hu, W. Xing, C. Zhang, Doping effect of phosphate in Bi2WO6 and universal improved photocatalytic activity for

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