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Synthesis of nitrogen-doped graphene–BiOBr nanocomposites with enhanced visible light photocatalytic activity
Materials Letters 164 (2016) 502–504
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of nitrogen-doped graphene–BiOBr nanocomposites with enhanced visible light photocatalytic activity Xiao Li a, Chao Dong a, Kong-Lin Wu a, Shan-Hui Xia a, Yu Hu a, Min Ling a, Kun Liu a, Xiao-Ling Lu b, Yin Ye a, Xian-Wen Wei a,n a College of Chemistry and Materials Science, Key Laboratory of Functional Molecular Solids, The Ministry of Education, Anhui Laboratory of Molecular-Based Materials, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal University, Wuhu 241000, China b Hospital of Anhui Normal University, Wuhu 241000, China
art ic l e i nf o
a b s t r a c t
Article history: Received 13 August 2015 Received in revised form 9 October 2015 Accepted 26 October 2015 Available online 26 October 2015
A series of nitrogen-doped graphene–BiOBr (NG–BiOBr) nanocomposites with different weight addition ratios of nitrogen-doped graphene were firstly prepared by a facile solvothermal method, and found to possess a higher photocatalytic activity than pure BiOBr toward degradation of methyl orange in water under visible light irradiation. The NG–BiOBr composite with 1.76 wt% NG content exhibited the highest photodegradation efficiency of methyl orange, its degradation rate was about 50, 4.6 and 3.8 times of P25, BiOBr microsphere and 1.76 wt% RGO–BiOBr composite, respectively. The enhanced photocatalytic performance could be ascribed to more visible light harvest and more effective separation of photogenerated electron–hole pairs. & 2015 Elsevier B.V. All rights reserved.
Keywords: Nitrogen-doped graphene BiOBr Nanocomposites Photocatalytic activity Optical materials and properties
1. Introduction
2. Experimental
In order to solve the problem of the fast recombination of electron–hole species in visible light photocatalyst bismuth oxybromide (BiOBr) [1–3], efforts have been done by combination of BiOBr with other semiconductors (e.g. CdS [4], Bi2S3 [5], BiOI [6], BiPO4 [7], Bi2WO6[8], AgBr [9], Ag3PO4 [10] and Ag2CO3 [11], etc.) or two-dimensional support materials (e.g. MoS2[12], g-C3N4 [13] and graphene [14–16]). Recently, the utilization of nitrogen-doped graphene (NG), a new analog of graphene, in the field of photocatalysis has attracted particular interest. Therefore, it is expected that NG–BiOBr composites will have good performance for the photocatalytic degradation of organic dyes, since NG–Ag2CO3[17], NG–AgX@Ag (X ¼Br, Cl) [18], NG–ZnO [19], NG–CdS [20,21] and NG–ZnSe [22] composites possessed enhanced photocatalytic performances. Herein, NG–BiOBr composites were prepared for the first time by a simple solvothermal approach, and showed higher photocatalytic performance than pure BiOBr toward degradation of methyl orange (MO) in water under visible light irradiation.
All the chemicals were of analytical grade and used as received without further purification. Detailed synthesis process, characterization means and photocatalytic evaluation were given in the Supplementary information (SI).
n
Corresponding author. Fax: þ86 553 3869303. E-mail address: [email protected] (X.-W. Wei).
http://dx.doi.org/10.1016/j.matlet.2015.10.128 0167-577X/& 2015 Elsevier B.V. All rights reserved.
3. Results and discussion The formation of BiOBr and NG–BiOBr composites was proven by XRD patterns as shown in Fig. S1. All of the diffraction peaks could be indexed to the tetragonal BiOBr (JCPDS No. 73–2061). The XRD patterns of NG–BiOBr composites were similar to pure BiOBr, and no obvious peaks assigned to NG were detected, probably due to its low contents and the disappearance of the layer-stacking regularity of NG sheets after solvothermal treatment [23]. The chemical composition of the as-prepared samples was determined by the EDX spectrometry as shown in Fig. S2. The only detectable elements are C, Bi, O, and Br peaks (C comes from the conductive tape for pure BiOBr), and the atomic ratios of Bi:O:Br are closed to 1:1:1. The morphology of the products was investigated by SEM and TEM. The pure BiOBr was comprised of microspheres with 2–5 μm in diameter, and individual microsphere consisted of plate-like
X. Li et al. / Materials Letters 164 (2016) 502–504
503
Fig. 1. SEM images of (a) BiOBr (inset is an individual microsphere), (b) 1.76 wt% NG–BiOBr. TEM image (c) and HRTEM image (d) of 1.76 wt% NG–BiOBr.
crystals (Fig. 1(a)). However, when 1.26% NG was added, hierarchical BiOBr microspheres in size of 1–2 μm were loaded on NG sheets (Fig. S3(a)). When the NG weight addition ratio increased to 1.76%, BiOBr plates in side length of several hundred nanometers and thickness of about 10–15 nm were attached to NG sheets (Fig. 1(b and c)). Further increasing the NG weight amount to 2.27%, BiOBr nanoplates with smaller size were attached to and even enwrapped by NG sheets (Fig. S3(b)). Therefore, it is noted that NG played an important role in controlling the morphology of BiOBr crystals, namely, the NG sheets served as special agent to tailor the size and morphology of the BiOBr crystals and effectively prevented their agglomeration. In addition, the pure plate-like BiOBr was in size of 80–300 nm and in thickness of 8–15 nm (Fig. S3(c)). Furthermore, HRTEM image of 1.76 wt% NG–BiOBr displayed in Fig. 1(d) indicated that BiOBr crystals were dispersed on the surface of NG. The optical properties of the samples were measured by UV–vis diffuse reflectance spectroscopy (Fig. S4(a)). It could be seen that
light on
in dark
1.2
1.0
1.0
0.8
0.8
Ct /C0
Ct /C0
1.2
an enhanced absorption in the visible light region appeared with addition of NG. The indirect band gap values of BiOBr microsphere, 1.26 wt% NG–BiOBr, 1.76 wt% NG–BiOBr and 2.27 wt% NG–BiOBr composite were estimated to be about 2.71, 2.31, 2.29 and 2.25 eV, respectively (Fig. S4(b)). This indicated a band gap narrowing of the BiOBr appeared due to the introduction of NG. The narrower band gap is beneficial to harvest more light for NG–BiOBr composites, leading to their higher photocatalytic performances. The photocatalytic activities of the NG–BiOBr composites were evaluated by photocatalytic degradation of MO under visible light irradiation, and the results were shown in Fig. 2(a). The photocatalytic degradation efficiency of MO and the first-order reaction rate constant (Table S1) follows the order 1.76 wt% NG– BiOBr 41.26 wt% NG–BiOBr 4 2.27 wt% NG–BiOBr 4Plate-like BiOBr 4BiOBr microsphere. Obviously, the addition of an appropriate amount of NG could enhance the photocatalytic activity effectively, while with the excess amount addition of the NG content, the photocatalytic performance was decreased. It could be
0.6 0.4
BiOBr microsphere Plate-like BiOBr 1.26 wt% NG-BiOBr 1.76 wt% NG-BiOBr 2.27 wt% NG-BiOBr
0.2 0.0 -50
0
50
Time (min)
0.6 P25 1.76 wt% RGO-BiOBr 1.76 wt% NG-BiOBr BiOBr microsphere Plate-like BiOBr 1.76 wt% NG-BiOBr microsphere mixing
0.4 0.2 0.0 100
150
light on
in dark
-50
0
50
100
150
Time (min)
Fig. 2. Photocatalytic degradation of MO over (a) the as-obtained samples, (b) P25, 1.76 wt% RGO–BiOBr, 1.76 wt% NG–BiOBr, 1.76 wt% NG–BiOBr microsphere mixing, pure BiOBr under visible light irradiation.
X. Li et al. / Materials Letters 164 (2016) 502–504
PL intensity (a.u.)
504
respectively, being added (Fig. S9).
BiOBr microsphere 1.26 wt% NG-BiOBr 1.76 wt% NG-BiOBr 2.27 wt% NG-BiOBr Plate-like BiOBr 1.76 wt% RGO-BiOBr
360
420
4. Conclusions
480
540
Wavelength (nm) Fig. 3. The photoluminescence spectra of BiOBr, NG–BiOBr composites and 1.76 wt% RGO–BiOBr composite with an excitation wavelength of 290 nm.
concluded that the superfluous NG should block light absorption and shield the active site on the catalyst surface [17,19]. Compared to commercial TiO2 (P25), pure BiOBr, 1.76 wt% RGO (reduced graphene oxide)–BiOBr composite and the 1.76 wt% NG–BiOBr microsphere-mixing prepared by a simple mechanical mixing of NG and BiOBr microsphere, the 1.76 wt% NG–BiOBr composite exhibited the highest degradation efficiency of MO (Fig. 2(b)), its degradation rate was about 50, 4.6 and 3.8 times of P25, BiOBr microsphere and 1.76 wt% RGO–BiOBr composite (Table S1), respectively. Its photocatalytic activity is also better than those of Bi2S3–BiOBr [5] and BiOI–BiOBr [6], and it is suitable for the degradation of rhodamine B (RhB) (Fig. S5) and phenol (Fig. S6). The improved charge carrier separation and the prolonged lifetime of photogenerated electron–hole pairs could be confirmed by the photoluminescence (PL) spectra and the electrochemical impedance spectra (EIS). As shown in Fig. 3, under excitation wavelength of 290 nm, for BiOBr, a broad band at 370–550 nm with a peak at 474 nm was detected, for 1.76% RGO–BiOBr composite and NG–BiOBr composites, the obviously quenched emission intensity represented an effective interfacial charge-transfer process. Additionally, the PL quenching efficiency follows the order 1.76 wt% NG–BiOBr 42.27 wt% NG–BiOBr 41.26 wt% NG–BiOBr 41.76 wt% RGO–BiOBr, which is in consistence with the result of rate constant. As displayed in Fig. S7, compared with BiOBr electrode, the electrode modified by 1.76 wt% NG–BiOBr or 1.76 wt% RGO–BiOBr composite exhibited smaller semicircle over the high frequency range, which indicated that the photo-generated electron–hole pairs in the 1.76 wt% NG–BiOBr or 1.76 wt% RGO–BiOBr composite electrode were separated more efficiently through an interfacial interaction between NG (RGO) and BiOBr. The surface area also influences the photocatalytic activity but the relationship between Brunauer–Emmett–Teller surface area (Table S2) and the photocatalytic activity is weak [24,25]. The as-prepared 1.76 wt% NG–BiOBr composite was a relatively stable photocatalyst since its photocatalytic activity decreased moderately (Fig. S8) after the successive 4 cycling runs. The decrease in photocatalytic activity might be partly caused by the inevitable loss of the photocatalyst during washing and centrifugation process, which also leading to the gradually decrease of the corresponding adsorption efficiencies. The degradation of MO over 1.76 wt% NG–BiOBr composite was mainly attributed to ·O−2 and h þ , since there were obviously changes on the degradation efficiency of MO after benzoquinone (BQ) and ethylene diamine tetraacetic acid (EDTA), used as scavengers for ·O−2 and holes,
In summary, NG–BiOBr composites with different weight addition ratios of NG were first prepared by a facile solvothermal route. It was demonstrated that the NG sheets could play an important role in controlling the morphology of the final BiOBr crystals. It was found that the 1.76 wt% NG–BiOBr composite exhibited the highest photocatalytic activity, which could be ascribed to the enhanced visible light harvest, narrower band gap and more effective separation of photo-generated electron–hole pairs.
Acknowledgments This work was supported by the National Natural Science Foundation of PR China (Nos. 21271006 and 21071005).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2015.10. 128.
References [1] X. Zhang, Z.H. Ai, F.L. Jia, L.Z. Zhang, J. Phys. Chem. C 112 (2008) 747–753. [2] J. Zhang, F.J. Shi, J. Lin, D.F. Chen, J.M. Gao, Z.X. Huang, et al., Chem. Mater. 20 (2008) 2937–2941. [3] L. Chen, R. Huang, M. Xiong, Q. Yuan, J. He, J. Jia, et al., Inorg. Chem. 52 (2013) 11118–11125. [4] W.Q. Cui, W.J. An, L. Liu, J.S. Hu, Y.H. Liang, Appl. Surf. Sci. 319 (2014) 298–305. [5] H.-P. Jiao, X. Yu, Z.-Q. Liu, P.-Y. Kuang, Y.-M. Zhang, RSC Adv. 5 (2015) 16239–16249. [6] J. Cao, B.Y. Xu, B.D. Luo, H.L. Lin, S.F. Chen, Catal. Commun. 13 (2011) 63–68. [7] Z.S. Liu, B.T. Wu, J.N. Niu, P.Z. Feng, Y.B. Zhu, Mater. Res. Bull. 63 (2015) 187–193. [8] J.X. Xia, J. Di, S. Yin, H. Xu, J. Zhang, Y.G. Xu, et al., RSC Adv. 4 (2014) 82–90. [9] L. Kong, Z. Jiang, H.H. Lai, R.J. Nicholls, T.C. Xiao, M.O. Jones, et al., J. Catal. 293 (2012) 116–125. [10] O. Mehraj, N.A. Mir, B.M. Pirzada, S. Sabir, Appl. Surf. Sci. 332 (2015) 419–429. [11] J. Wang, C. Dong, B.-B. Jiang, K.-L. Wu, J. Sun, X.-Z. Li, et al., Mater. Lett. 131 (2014) 108–111. [12] J. Di, J.X. Xia, Y.P. Ge, L. Xu, H. Xu, J. Chen, et al., Dalton Trans. 43 (2014) 15429–15438. [13] L.Q. Ye, J.Y. Liu, Z. Jiang, T.Y. Peng, L. Zan, Appl. Catal. B: Environ. 142–413 (2013) 1–7. [14] Z.H. Ai, W.K. Ho, S.C. Lee, J. Phys. Chem. C 115 (2011) 25330–25337. [15] S.Y. Song, W. Gao, X. Wang, X.Y. Li, D.P. Liu, Y. Xing, H.J. Zhang, Dalton Trans. 41 (2012) 10472–10476. [16] X.M. Tu, S.L. Luo, G.X. Chen, J.H. Li, Chem. Eur. J. 18 (2012) 14359–14366. [17] C. Dong, B.-B. Jiang, K.-L. Wu, Y. Hu, S.-H. Xia, X.-W. Wei, Mater. Lett. 146 (2015) 37–39. [18] C. Dong, K.-L. Wu, X.-W. Wei, J. Wang, L. Liu, B.-B. Jiang, Appl. Catal. A: Gen. 488 (2014) 11–18. [19] L. Liu, C. Dong, K.-L. Wu, Y. Ye, X.-W. Wei, Mater. Lett. 129 (2014) 170–173. [20] L. Jia, D.H. Wang, Y.X. Huang, A.W. Xu, H.Q. Yu, J. Phys. Chem. C 115 (2011) 11466–11473. [21] Q. Mi, D.Q. Chen, J.C. Hu, Z.X. Huang, J.L. Li, Chin. J. Catal. 34 (2013) 2138–2145. [22] P. Chen, T.Y. Xiao, H.H. Li, J.J. Yang, Z. Wang, H.B. Yao, et al., ACS Nano 6 (2012) 712–719. [23] C. Dong, K.-L. Wu, X.-W. Wei, X.-Z. Li, L. Liu, T.-H. Ding, et al., CrystEngComm 16 (2014) 730–736. [24] S. Bae, S. Kim, S. Lee, W. Choi, Catal. Today 224 (2014) 21–28. [25] J. Ryu, W. Choi, Environ. Sci. Technol. 42 (2008) 294–300.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis of nitrogen-doped graphene–BiOBr nanocomposites with enhanced visible light photocatalytic activity Xiao Li a, Chao Dong a, Kong-Lin Wu a, Shan-Hui Xia a, Yu Hu a, Min Ling a, Kun Liu a, Xiao-Ling Lu b, Yin Ye a, Xian-Wen Wei a,n a College of Chemistry and Materials Science, Key Laboratory of Functional Molecular Solids, The Ministry of Education, Anhui Laboratory of Molecular-Based Materials, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal University, Wuhu 241000, China b Hospital of Anhui Normal University, Wuhu 241000, China
art ic l e i nf o
a b s t r a c t
Article history: Received 13 August 2015 Received in revised form 9 October 2015 Accepted 26 October 2015 Available online 26 October 2015
A series of nitrogen-doped graphene–BiOBr (NG–BiOBr) nanocomposites with different weight addition ratios of nitrogen-doped graphene were firstly prepared by a facile solvothermal method, and found to possess a higher photocatalytic activity than pure BiOBr toward degradation of methyl orange in water under visible light irradiation. The NG–BiOBr composite with 1.76 wt% NG content exhibited the highest photodegradation efficiency of methyl orange, its degradation rate was about 50, 4.6 and 3.8 times of P25, BiOBr microsphere and 1.76 wt% RGO–BiOBr composite, respectively. The enhanced photocatalytic performance could be ascribed to more visible light harvest and more effective separation of photogenerated electron–hole pairs. & 2015 Elsevier B.V. All rights reserved.
Keywords: Nitrogen-doped graphene BiOBr Nanocomposites Photocatalytic activity Optical materials and properties
1. Introduction
2. Experimental
In order to solve the problem of the fast recombination of electron–hole species in visible light photocatalyst bismuth oxybromide (BiOBr) [1–3], efforts have been done by combination of BiOBr with other semiconductors (e.g. CdS [4], Bi2S3 [5], BiOI [6], BiPO4 [7], Bi2WO6[8], AgBr [9], Ag3PO4 [10] and Ag2CO3 [11], etc.) or two-dimensional support materials (e.g. MoS2[12], g-C3N4 [13] and graphene [14–16]). Recently, the utilization of nitrogen-doped graphene (NG), a new analog of graphene, in the field of photocatalysis has attracted particular interest. Therefore, it is expected that NG–BiOBr composites will have good performance for the photocatalytic degradation of organic dyes, since NG–Ag2CO3[17], NG–AgX@Ag (X ¼Br, Cl) [18], NG–ZnO [19], NG–CdS [20,21] and NG–ZnSe [22] composites possessed enhanced photocatalytic performances. Herein, NG–BiOBr composites were prepared for the first time by a simple solvothermal approach, and showed higher photocatalytic performance than pure BiOBr toward degradation of methyl orange (MO) in water under visible light irradiation.
All the chemicals were of analytical grade and used as received without further purification. Detailed synthesis process, characterization means and photocatalytic evaluation were given in the Supplementary information (SI).
n
Corresponding author. Fax: þ86 553 3869303. E-mail address: [email protected] (X.-W. Wei).
http://dx.doi.org/10.1016/j.matlet.2015.10.128 0167-577X/& 2015 Elsevier B.V. All rights reserved.
3. Results and discussion The formation of BiOBr and NG–BiOBr composites was proven by XRD patterns as shown in Fig. S1. All of the diffraction peaks could be indexed to the tetragonal BiOBr (JCPDS No. 73–2061). The XRD patterns of NG–BiOBr composites were similar to pure BiOBr, and no obvious peaks assigned to NG were detected, probably due to its low contents and the disappearance of the layer-stacking regularity of NG sheets after solvothermal treatment [23]. The chemical composition of the as-prepared samples was determined by the EDX spectrometry as shown in Fig. S2. The only detectable elements are C, Bi, O, and Br peaks (C comes from the conductive tape for pure BiOBr), and the atomic ratios of Bi:O:Br are closed to 1:1:1. The morphology of the products was investigated by SEM and TEM. The pure BiOBr was comprised of microspheres with 2–5 μm in diameter, and individual microsphere consisted of plate-like
X. Li et al. / Materials Letters 164 (2016) 502–504
503
Fig. 1. SEM images of (a) BiOBr (inset is an individual microsphere), (b) 1.76 wt% NG–BiOBr. TEM image (c) and HRTEM image (d) of 1.76 wt% NG–BiOBr.
crystals (Fig. 1(a)). However, when 1.26% NG was added, hierarchical BiOBr microspheres in size of 1–2 μm were loaded on NG sheets (Fig. S3(a)). When the NG weight addition ratio increased to 1.76%, BiOBr plates in side length of several hundred nanometers and thickness of about 10–15 nm were attached to NG sheets (Fig. 1(b and c)). Further increasing the NG weight amount to 2.27%, BiOBr nanoplates with smaller size were attached to and even enwrapped by NG sheets (Fig. S3(b)). Therefore, it is noted that NG played an important role in controlling the morphology of BiOBr crystals, namely, the NG sheets served as special agent to tailor the size and morphology of the BiOBr crystals and effectively prevented their agglomeration. In addition, the pure plate-like BiOBr was in size of 80–300 nm and in thickness of 8–15 nm (Fig. S3(c)). Furthermore, HRTEM image of 1.76 wt% NG–BiOBr displayed in Fig. 1(d) indicated that BiOBr crystals were dispersed on the surface of NG. The optical properties of the samples were measured by UV–vis diffuse reflectance spectroscopy (Fig. S4(a)). It could be seen that
light on
in dark
1.2
1.0
1.0
0.8
0.8
Ct /C0
Ct /C0
1.2
an enhanced absorption in the visible light region appeared with addition of NG. The indirect band gap values of BiOBr microsphere, 1.26 wt% NG–BiOBr, 1.76 wt% NG–BiOBr and 2.27 wt% NG–BiOBr composite were estimated to be about 2.71, 2.31, 2.29 and 2.25 eV, respectively (Fig. S4(b)). This indicated a band gap narrowing of the BiOBr appeared due to the introduction of NG. The narrower band gap is beneficial to harvest more light for NG–BiOBr composites, leading to their higher photocatalytic performances. The photocatalytic activities of the NG–BiOBr composites were evaluated by photocatalytic degradation of MO under visible light irradiation, and the results were shown in Fig. 2(a). The photocatalytic degradation efficiency of MO and the first-order reaction rate constant (Table S1) follows the order 1.76 wt% NG– BiOBr 41.26 wt% NG–BiOBr 4 2.27 wt% NG–BiOBr 4Plate-like BiOBr 4BiOBr microsphere. Obviously, the addition of an appropriate amount of NG could enhance the photocatalytic activity effectively, while with the excess amount addition of the NG content, the photocatalytic performance was decreased. It could be
0.6 0.4
BiOBr microsphere Plate-like BiOBr 1.26 wt% NG-BiOBr 1.76 wt% NG-BiOBr 2.27 wt% NG-BiOBr
0.2 0.0 -50
0
50
Time (min)
0.6 P25 1.76 wt% RGO-BiOBr 1.76 wt% NG-BiOBr BiOBr microsphere Plate-like BiOBr 1.76 wt% NG-BiOBr microsphere mixing
0.4 0.2 0.0 100
150
light on
in dark
-50
0
50
100
150
Time (min)
Fig. 2. Photocatalytic degradation of MO over (a) the as-obtained samples, (b) P25, 1.76 wt% RGO–BiOBr, 1.76 wt% NG–BiOBr, 1.76 wt% NG–BiOBr microsphere mixing, pure BiOBr under visible light irradiation.
X. Li et al. / Materials Letters 164 (2016) 502–504
PL intensity (a.u.)
504
respectively, being added (Fig. S9).
BiOBr microsphere 1.26 wt% NG-BiOBr 1.76 wt% NG-BiOBr 2.27 wt% NG-BiOBr Plate-like BiOBr 1.76 wt% RGO-BiOBr
360
420
4. Conclusions
480
540
Wavelength (nm) Fig. 3. The photoluminescence spectra of BiOBr, NG–BiOBr composites and 1.76 wt% RGO–BiOBr composite with an excitation wavelength of 290 nm.
concluded that the superfluous NG should block light absorption and shield the active site on the catalyst surface [17,19]. Compared to commercial TiO2 (P25), pure BiOBr, 1.76 wt% RGO (reduced graphene oxide)–BiOBr composite and the 1.76 wt% NG–BiOBr microsphere-mixing prepared by a simple mechanical mixing of NG and BiOBr microsphere, the 1.76 wt% NG–BiOBr composite exhibited the highest degradation efficiency of MO (Fig. 2(b)), its degradation rate was about 50, 4.6 and 3.8 times of P25, BiOBr microsphere and 1.76 wt% RGO–BiOBr composite (Table S1), respectively. Its photocatalytic activity is also better than those of Bi2S3–BiOBr [5] and BiOI–BiOBr [6], and it is suitable for the degradation of rhodamine B (RhB) (Fig. S5) and phenol (Fig. S6). The improved charge carrier separation and the prolonged lifetime of photogenerated electron–hole pairs could be confirmed by the photoluminescence (PL) spectra and the electrochemical impedance spectra (EIS). As shown in Fig. 3, under excitation wavelength of 290 nm, for BiOBr, a broad band at 370–550 nm with a peak at 474 nm was detected, for 1.76% RGO–BiOBr composite and NG–BiOBr composites, the obviously quenched emission intensity represented an effective interfacial charge-transfer process. Additionally, the PL quenching efficiency follows the order 1.76 wt% NG–BiOBr 42.27 wt% NG–BiOBr 41.26 wt% NG–BiOBr 41.76 wt% RGO–BiOBr, which is in consistence with the result of rate constant. As displayed in Fig. S7, compared with BiOBr electrode, the electrode modified by 1.76 wt% NG–BiOBr or 1.76 wt% RGO–BiOBr composite exhibited smaller semicircle over the high frequency range, which indicated that the photo-generated electron–hole pairs in the 1.76 wt% NG–BiOBr or 1.76 wt% RGO–BiOBr composite electrode were separated more efficiently through an interfacial interaction between NG (RGO) and BiOBr. The surface area also influences the photocatalytic activity but the relationship between Brunauer–Emmett–Teller surface area (Table S2) and the photocatalytic activity is weak [24,25]. The as-prepared 1.76 wt% NG–BiOBr composite was a relatively stable photocatalyst since its photocatalytic activity decreased moderately (Fig. S8) after the successive 4 cycling runs. The decrease in photocatalytic activity might be partly caused by the inevitable loss of the photocatalyst during washing and centrifugation process, which also leading to the gradually decrease of the corresponding adsorption efficiencies. The degradation of MO over 1.76 wt% NG–BiOBr composite was mainly attributed to ·O−2 and h þ , since there were obviously changes on the degradation efficiency of MO after benzoquinone (BQ) and ethylene diamine tetraacetic acid (EDTA), used as scavengers for ·O−2 and holes,
In summary, NG–BiOBr composites with different weight addition ratios of NG were first prepared by a facile solvothermal route. It was demonstrated that the NG sheets could play an important role in controlling the morphology of the final BiOBr crystals. It was found that the 1.76 wt% NG–BiOBr composite exhibited the highest photocatalytic activity, which could be ascribed to the enhanced visible light harvest, narrower band gap and more effective separation of photo-generated electron–hole pairs.
Acknowledgments This work was supported by the National Natural Science Foundation of PR China (Nos. 21271006 and 21071005).
Appendix A. Supplementary material Supplementary data associated with this article can be found in the online version at http://dx.doi.org/10.1016/j.matlet.2015.10. 128.
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