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One-pot solvothermal synthesis of Ag nanoparticles decorated BiOCOOH microflowers with enhanced visible light activity
Materials Letters 196 (2017) 343–346
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
One-pot solvothermal synthesis of Ag nanoparticles decorated BiOCOOH microflowers with enhanced visible light activity Shijie Li a,b,⇑, Shiwei Hu a, Wei Jiang a, Kaibing Xu c a
Innovation & Application Institute, Zhejiang Ocean University, Zhoushan, Zhejiang Province 316022, China Zhejiang Provincial Key Laboratory of Health Risk Factors for Seafood, Zhoushan Municipal Center for Disease Control and Prevention, Zhoushan 316021, China c State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Research Center for Analysis and Measurement, Donghua University, Shanghai 201620, China b
a r t i c l e
i n f o
Article history: Received 3 January 2017 Received in revised form 20 February 2017 Accepted 18 March 2017 Available online 20 March 2017 Keywords: Ag/BiOCOOH Visible light Nanocomposites Photocatalysis Semiconductors
a b s t r a c t In this work, plasmonic Ag nanoparticles (NPs) decorated BiOCOOH microflowers as one kind of efficient visible-light-driven photocatalysts were prepared via a one-pot solvothermal method and systematically characterized. Compared with pristine BiOCOOH, Ag/BiOCOOH heterojunctions showed greatly enhanced visible light photocatalytic activities towards RhB degradation. Especially, the 5 wt% Ag/BiOCOOH heterojunction displayed the maximum activity, and the degradation rate is 10.9 times higher than that obtained by using pristine BiOCOOH. The exceptional photocatalytic activity of Ag/BiOCOOH is attributed to the surface plasmon resonance (SPR) effect induced by Ag NPs and formation of the heterojunction between Ag and BiOCOOH. Holes and superoxide radicals mainly dominate the RhB degradation. More importantly, Ag/BiOCOOH is highly stable in photocatalysis. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor photocatalysts, as the key of photocatalysis technique, have drawn stupendous attention [1]. Among them, Bi-based materials have been widely developed as efficient photocatalysts in virtue of their unique layer configuration [2–6]. BiOCOOH, as a new Bi-based compound, has emerged as a promising photocatalyst for pollutant removal [7,8]. However, its wide band gap and low quantum efficiency undermine its practical application. Constructing heterojunctions can effectively overcome these drawbacks [9,10]. However, there is little work that reports on the exploration of BiOCOOH-based heterojunctions, except for BiOI/BiOCOOH [11], Ag2O/BiOCOOH [12] and G/BiOCOOH [13]. These heterojunctions exhibited superior activity for pollutant removal. Thus, the further development of BiOCOOH-based heterojunctions with higher performance is highly desirable and anticipated. The combination of semiconductors with metallic Ag to form plasmonic photocatalysts is an effective strategy towards ameliorating the photocatalytic activity. Several Ag-based heterojunctions, such as Ag/Bi2O2CO3 [14], Ag/TiO2 [15], and Ag/SnWO4 [16]
⇑ Corresponding author at: Innovation & Application Institute, Zhejiang Ocean University, Zhoushan, Zhejiang Province 316022, China. E-mail address: [email protected] (S. Li). http://dx.doi.org/10.1016/j.matlet.2017.03.093 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
have been reported to present exceptional photocatalytic activity for pollutant degradation. However, to our knowledge, there is no report on the development of Ag/BiOCOOH heterojuctions. In this work, Ag nanoparticles (NPs) uniformly anchoring on BiOCOOH microflowers were prepared by a one-pot solvothermal method. The photocatalytic properties of Ag/BiOCOOH were assessed by the decomposition of RhB dye under visible light. All heterojunctions depicted better photocatalytic activity and stability for RhB degradation. The mechanism for the improved activity of Ag/BiOCOOH was studied.
2. Experimental section 2.1. Preparation of Ag/BiOCOOH 2 mmol Bi(NO3)5H2O was ultrasonically dissolved in the mixture (25 mL glycerol + 5 mL H2O + 10 mL DMF). Then, a certain amount of AgNO3 was dissolved in the solution under vigorously stirring. The resulting solution was transferred to a 100 mL autoclave and reacted at 160 °C for 15 h. After reaction, the system was cooled down naturally. Finally, the obtained samples with 1 wt%, 3 wt%, 5 wt% and 10 wt% Ag (named as Ag/BiOCOOH-1%, Ag/BiOCOOH-3%, Ag/BiOCOOH-5% and Ag/BiOCOOH-10%) were washed with water-ethanol for several times, and then dried at 70 °C overnight. For comparison, pristine BiOCOOH was prepared under same condition without adding AgNO3.
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2.2. Characterization
3. Results and discussion
The catalysts were studied by a X-ray diffractometer (XRD; Bruker), a scanning electron microscopy (SEM; Hitachis-4800), a transmission electron microscopy (TEM; JEM-2100F), energy dispersive X-ray spectrometer (EDX), and optical diffuse reflectance spectra were acquired on a spectrophotometer (Shimadzu, UV-2600).
3.1. Characterization of photocatalysts
2.3. Photocatalytic tests The photocatalytic activity of all samples were evaluated by degrading RhB (50 mL, 5 mg L 1) under visible light (300 W xenon lamp, k > 400 nm). Specifically, 20 mg of the sample was ultrasonically dispersed in RhB aqueous solution for 2 min. Prior to illumination, the suspension was continuously stirred in the dark for 1 h. After that, the suspension was irradiated by visible light. 4 mL of the solution was taken from the reactor at certain intervals. The RhB concentrations were determined by measuring the characteristic absorption peak at 554 nm on a UV-2600 spectrophotometer. Trapping experiments were carried out by adding different sacrificial agents into the RhB solution (50 mL, 5 mg L 1), including 6 mM AgNO3, 1 mM ammonium oxalate (AO), 1 mM p-benzoquinone (BQ) or 1 mM iso-propanol (IPA).
The morphology of BiOCOOH and Ag/BiOCOOH (Ag/BiOCOOH5%) was examined by SEM (Fig. 1a-d). Pure BiOCOOH exhibits a flower-like nanostructure (diameters: 2–4 lm) assembled by nanosheets with smooth surface (Fig. 1a, b). As for Ag/BiOCOOH5%, it maintains the flower-like morphology, and these microflowers are constructed by BiOCOOH nanosheets and Ag NPs (Fig. 1c, d). Apparently, Ag NPs (size:21 nm) are uniformly distributed on the surface of BiOCOOH microflowers (Fig. 1d). TEM images (Fig. 1e, f) clearly show that Ag/BOCH-5% is composed of BiOCOOH microflower (diameter:3.5 lm) and Ag NPs (size:21 nm). Ag NPs are well embedded in the BiOCOOH substrate, forming tight contact between BiOCOOH and Ag NPs. Fig. 2a shows the XRD patterns of all samples. The XRD pattern of pristine BiOCOOH can be indexed to tetragonal BiOCOOH (JCPDS card No. 35-0939). The XRD patterns of Ag/BiOCOOH heterojunctions can be assigned to the combination of Ag and BiOCOOH phases. With the amount of Ag increasing, the intensity of Ag peaks at 2h = 38° and 44° was gradually strengthened, whereas those of BiOCOOH were weakened.
Fig. 1. SEM images of BiOCOOH (a, b) and Ag/BiOCOOH-5% (c, d); TEM images of Ag/BiOCOOH-5% (e, f).
S. Li et al. / Materials Letters 196 (2017) 343–346
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Fig. 2. (a) XRD patterns of Ag/BiOCOOH heterojunctions and BiOCOOH; (b) EDS pattern of Ag/BiOCOOH-5%; (c) UV-vis DRS spectra of Ag/BiOCOOH heterojunctions and BiOCOOH.
Fig. 3. (a) The absorption spectra of RhB over Ag/BiOCOOH-5%; (b) Photocatalytic degradation of RhB over different catalysts (20 mg); (c) The photocatalytic degradation rate constants of RhB over different catalysts (d) Recycling runs of Ag/BiOCOOH-5%; (e) Photocatalytic degradation of RhB in the presence of different scavengers.
Moreover, the EDS pattern (Fig. 2b) indicates that Ag/BiOCOOH consists of Bi, O, C and Ag elements. Fig. 2c shows the UV–Vis DRS spectra of all samples. The DRS spectra of BiOCOOH presents an intense absorption in UV region,
and the absorption edge is about 365 nm (Eg: 3.4 eV), with no absorption in visible light (VL) region [12]. The DRS spectra of Ag/BiOCOOH heterojunctions show broad absorption in VL region due to the surface plasmon resonance (SPR) effect of metallic Ag
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mation of Ag/BiOCOOH heterojunction [9]. The Ag/semiconductor heterostructure can largely enhance the visible light absorption due to the SPR effect of Ag (Fig. 2c), which is beneficial for photocatalysis [15]. Under visible light, BiOCOOH can absorb visible light to produce holes on the VB and electrons on the CB. And the CB electrons on BiOCOOH may transfer to Ag. The electrons on Ag NPs will react with oxygen to produce superoxide radicals to attend the photocatalytic reactions. Meanwhile, the holes on BiOCOOH are feasible to degrade RhB directly. As a consequence, the interfacial charge transfer from BiOCOOH to Ag NPs retards the recombination of electron-hole pairs, resulting in the superior photocatalytic performance of Ag/BiOCOOH. 4. Conclusion
Fig. 4. The possible mechanism for RhB degradation over Ag/BiOCOOH.
[17]. Moreover, the VL absorption ability was gradually enhanced with the increase of Ag. 3.2. Photocatalytic activity The photocatalytic activity of Ag/BiOCOOH was tested by degrading RhB under visible light (Fig. 3a, b). Fig. 3a shows the temporal absorption spectra of RhB over Ag/BiOCOOH-5%. The maximum absorption peak of RhB at 554 nm rapidly diminishes with the reaction time increasing, and the degradation efficiency is up to 97.9% within 60 min of reaction. For comparison, 26.4%, 68.7%, 85.7%, % or 57.9% of RhB within 60 min was degraded over BiOCOOH, Ag/BiOCOOH-1%, Ag/BiOCOOH-3% or Ag/BiOCOOH-10%. Apparently, the Ag/BiOCOOH heterojunctions are much more active than the pristine BiOCOOH. Among these heterojunctions, Ag/BiOCOOH-5% obtains the optimum activity. Moreover, the photocatalytic degradation of RhB over these catalysts matches with pseudo-first-order kinetics model: -ln(C/C0) = kt, where k is the reaction rate constant (Fig. 3c). Clearly, the k of Ag/BiOCOOH-5% (0.0606 min 1) is greatly higher than those of BiOCOOH (0.0051 min 1), Ag/BiOCOOH-1% (0.0192 min 1), Ag/ BiOCOOH-3% (0.0310 min 1) and Ag/BiOCOOH-10% (0.0144 min 1). Notably, Ag/BiOCOOH-5% achieves the highest photocatalytic activity, which is 10.9 times higher than that of BiOCOOH. These facts demonstrate that the introduction of a suitable amount of metallic Ag markedly improves the photocatalytic activity of BiOCOOH [14]. The recyclability of Ag/BiOCOOH-5% was studied via the successive degradation of RhB under visible light. There was no obvious decay in the photocatalytic activity and the RhB degradation efficiency of 91.1% was achieved after 4 runs (Fig. 3d). In addition, no phase change of Ag/BiOCOOH-5% was observed (Fig. S1), demonstrating a good stability of Ag/BiOCOOH-5%. To understand the photocatalytic mechanism, trapping experiments were carried out (Fig. 3e). Here, AgNO3, ammonium oxalate (AO), benzoquinone (BQ) and isopropyl alcohol (IPA) serve as scavengers to quench photogenerated electrons (e–), holes (h+), superoxide radicals (O–2) and hydroxyl free radicals (OH), respectively. The introduction of BQ or AO greatly suppresses the degradation efficiency of RhB from 97.9% to 31.8% or 13.7% in 60 min. By contrast, the addition of IPA only slightly inhibits RhB degradation. These facts suggest that O2 and h+ play a vital role in RhB degradation (Fig. 3e). On the basis of the above results and band edge analysis (Fig. 4), the enhancement in photocatalytic activity can be due to the for-
In summary, we developed a one-pot solvothermal method to construct Ag/BiOCOOH microflowers. The optimal composite with 5 wt% of Ag to BiOCOOH presented the highest activity and excellent stability. The superior photocatalytic activity is mainly ascribed to SPR effect of metallic Ag and the formation of heterojunction between the counterparts, facilitating the charge separation. This study provides a facile avenue for the construction of new noble metal/Bi-based plasmonic photocatalysts with remarkable activity for water purification. Acknowledgements This work was financially supported by the Natural Science Foundation of Zhejiang Province (Grant No. LQ15E090006), and the Research Startup Foundation of Zhejiang Ocean University (Q1447). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.matlet.2017.03. 093. References [1] S.J. Li, L.S. Zhang, H.L. Wang, Z.G. Chen, J.Q. Hu, K.B. Xu, J.S. Liu, Sci. Rep. 4 (2014) 3978. [2] Y. Hao, X. Dong, S. Zhai, X. Wang, H. Ma, X. Zhang, Chem. Commun. 52 (2016) 6525–6528. [3] Y. Zhou, Y. Zhang, M. Lin, J. Long, Z. Zhang, H. Lin, J.C.S. Wu, X. Wang, Nat. Commun. 6 (2015) 8340. [4] F. Dong, T. Xiong, Y.J. Sun, Y.X. Zhang, Y. Zhou, Chem. Commun. 51 (2015) 8249–8252. [5] W. Chen, G.-R. Duan, T.-Y. Liu, S.-M. Chen, X.-H. Liu, Mater. Sci. Semicond. Process. 35 (2015) 45–54. [6] W. Chen, T.-Y. Liu, T. Huang, X.-H. Liu, J.-W. Zhu, G.-R. Duan, X.-J. Yang, Appl. Surf. Sci. 355 (2015) 379–387. [7] J.Y. Xiong, G. Cheng, Z. Lu, J.L. Tang, X.L. Yu, R. Chen, CrystEngComm 13 (2011) 2381–2390. [8] C. Wei, L. Wang, L. Dang, Q. Chen, Q. Lu, F. Gao, Sci. Rep. 5 (2015) 10599. [9] X. Liang, J.C. Zhao, X.D. Chen, Chem. Soc. Rev. 45 (2016) 3026–3038. [10] S.J. Li, J.L. Zhang, S.W. Hu, K.B. Xu, W. Jiang, J.S. Liu, J. Alloys Compd. 695 (2017) 1137–1144. [11] B. Chai, X. Wang, RSC Adv. 5 (2015) 7589–7596. [12] S.J. Li, K.B. Xu, S.W. Hu, W. Jiang, J.L. Zhang, J.S. Liu, L.S. Zhang, Appl. Surf. Sci. 397 (2017) 95–103. [13] S.-H. Xia, C. Dong, X.-W. Wei, J. Wang, K.-L. Wu, Y. Hu, Y. Ye, Mater. Lett. 156 (2015) 36–38. [14] F. Dong, Q.Y. Li, Y. Zhou, Y.J. Sun, H.D. Zhang, Z.B. Wu, Dalton Trans. 43 (2014) 9468–9480. [15] L. Gomathi Devi, R. Kavitha, Appl. Surf. Sci. 360 (2016) 601–622. [16] X.W. Liu, B. Liang, M. Zhang, Y.M. Long, W.F. Li, J. Colloid Interface Sci. 490 (2017) 46–52. [17] J. Augustynski, K. Bienkowski, R. Solarska, Coordin. Chem. Rev. 325 (2016) 116–124.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
One-pot solvothermal synthesis of Ag nanoparticles decorated BiOCOOH microflowers with enhanced visible light activity Shijie Li a,b,⇑, Shiwei Hu a, Wei Jiang a, Kaibing Xu c a
Innovation & Application Institute, Zhejiang Ocean University, Zhoushan, Zhejiang Province 316022, China Zhejiang Provincial Key Laboratory of Health Risk Factors for Seafood, Zhoushan Municipal Center for Disease Control and Prevention, Zhoushan 316021, China c State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, Research Center for Analysis and Measurement, Donghua University, Shanghai 201620, China b
a r t i c l e
i n f o
Article history: Received 3 January 2017 Received in revised form 20 February 2017 Accepted 18 March 2017 Available online 20 March 2017 Keywords: Ag/BiOCOOH Visible light Nanocomposites Photocatalysis Semiconductors
a b s t r a c t In this work, plasmonic Ag nanoparticles (NPs) decorated BiOCOOH microflowers as one kind of efficient visible-light-driven photocatalysts were prepared via a one-pot solvothermal method and systematically characterized. Compared with pristine BiOCOOH, Ag/BiOCOOH heterojunctions showed greatly enhanced visible light photocatalytic activities towards RhB degradation. Especially, the 5 wt% Ag/BiOCOOH heterojunction displayed the maximum activity, and the degradation rate is 10.9 times higher than that obtained by using pristine BiOCOOH. The exceptional photocatalytic activity of Ag/BiOCOOH is attributed to the surface plasmon resonance (SPR) effect induced by Ag NPs and formation of the heterojunction between Ag and BiOCOOH. Holes and superoxide radicals mainly dominate the RhB degradation. More importantly, Ag/BiOCOOH is highly stable in photocatalysis. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor photocatalysts, as the key of photocatalysis technique, have drawn stupendous attention [1]. Among them, Bi-based materials have been widely developed as efficient photocatalysts in virtue of their unique layer configuration [2–6]. BiOCOOH, as a new Bi-based compound, has emerged as a promising photocatalyst for pollutant removal [7,8]. However, its wide band gap and low quantum efficiency undermine its practical application. Constructing heterojunctions can effectively overcome these drawbacks [9,10]. However, there is little work that reports on the exploration of BiOCOOH-based heterojunctions, except for BiOI/BiOCOOH [11], Ag2O/BiOCOOH [12] and G/BiOCOOH [13]. These heterojunctions exhibited superior activity for pollutant removal. Thus, the further development of BiOCOOH-based heterojunctions with higher performance is highly desirable and anticipated. The combination of semiconductors with metallic Ag to form plasmonic photocatalysts is an effective strategy towards ameliorating the photocatalytic activity. Several Ag-based heterojunctions, such as Ag/Bi2O2CO3 [14], Ag/TiO2 [15], and Ag/SnWO4 [16]
⇑ Corresponding author at: Innovation & Application Institute, Zhejiang Ocean University, Zhoushan, Zhejiang Province 316022, China. E-mail address: [email protected] (S. Li). http://dx.doi.org/10.1016/j.matlet.2017.03.093 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
have been reported to present exceptional photocatalytic activity for pollutant degradation. However, to our knowledge, there is no report on the development of Ag/BiOCOOH heterojuctions. In this work, Ag nanoparticles (NPs) uniformly anchoring on BiOCOOH microflowers were prepared by a one-pot solvothermal method. The photocatalytic properties of Ag/BiOCOOH were assessed by the decomposition of RhB dye under visible light. All heterojunctions depicted better photocatalytic activity and stability for RhB degradation. The mechanism for the improved activity of Ag/BiOCOOH was studied.
2. Experimental section 2.1. Preparation of Ag/BiOCOOH 2 mmol Bi(NO3)5H2O was ultrasonically dissolved in the mixture (25 mL glycerol + 5 mL H2O + 10 mL DMF). Then, a certain amount of AgNO3 was dissolved in the solution under vigorously stirring. The resulting solution was transferred to a 100 mL autoclave and reacted at 160 °C for 15 h. After reaction, the system was cooled down naturally. Finally, the obtained samples with 1 wt%, 3 wt%, 5 wt% and 10 wt% Ag (named as Ag/BiOCOOH-1%, Ag/BiOCOOH-3%, Ag/BiOCOOH-5% and Ag/BiOCOOH-10%) were washed with water-ethanol for several times, and then dried at 70 °C overnight. For comparison, pristine BiOCOOH was prepared under same condition without adding AgNO3.
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2.2. Characterization
3. Results and discussion
The catalysts were studied by a X-ray diffractometer (XRD; Bruker), a scanning electron microscopy (SEM; Hitachis-4800), a transmission electron microscopy (TEM; JEM-2100F), energy dispersive X-ray spectrometer (EDX), and optical diffuse reflectance spectra were acquired on a spectrophotometer (Shimadzu, UV-2600).
3.1. Characterization of photocatalysts
2.3. Photocatalytic tests The photocatalytic activity of all samples were evaluated by degrading RhB (50 mL, 5 mg L 1) under visible light (300 W xenon lamp, k > 400 nm). Specifically, 20 mg of the sample was ultrasonically dispersed in RhB aqueous solution for 2 min. Prior to illumination, the suspension was continuously stirred in the dark for 1 h. After that, the suspension was irradiated by visible light. 4 mL of the solution was taken from the reactor at certain intervals. The RhB concentrations were determined by measuring the characteristic absorption peak at 554 nm on a UV-2600 spectrophotometer. Trapping experiments were carried out by adding different sacrificial agents into the RhB solution (50 mL, 5 mg L 1), including 6 mM AgNO3, 1 mM ammonium oxalate (AO), 1 mM p-benzoquinone (BQ) or 1 mM iso-propanol (IPA).
The morphology of BiOCOOH and Ag/BiOCOOH (Ag/BiOCOOH5%) was examined by SEM (Fig. 1a-d). Pure BiOCOOH exhibits a flower-like nanostructure (diameters: 2–4 lm) assembled by nanosheets with smooth surface (Fig. 1a, b). As for Ag/BiOCOOH5%, it maintains the flower-like morphology, and these microflowers are constructed by BiOCOOH nanosheets and Ag NPs (Fig. 1c, d). Apparently, Ag NPs (size:21 nm) are uniformly distributed on the surface of BiOCOOH microflowers (Fig. 1d). TEM images (Fig. 1e, f) clearly show that Ag/BOCH-5% is composed of BiOCOOH microflower (diameter:3.5 lm) and Ag NPs (size:21 nm). Ag NPs are well embedded in the BiOCOOH substrate, forming tight contact between BiOCOOH and Ag NPs. Fig. 2a shows the XRD patterns of all samples. The XRD pattern of pristine BiOCOOH can be indexed to tetragonal BiOCOOH (JCPDS card No. 35-0939). The XRD patterns of Ag/BiOCOOH heterojunctions can be assigned to the combination of Ag and BiOCOOH phases. With the amount of Ag increasing, the intensity of Ag peaks at 2h = 38° and 44° was gradually strengthened, whereas those of BiOCOOH were weakened.
Fig. 1. SEM images of BiOCOOH (a, b) and Ag/BiOCOOH-5% (c, d); TEM images of Ag/BiOCOOH-5% (e, f).
S. Li et al. / Materials Letters 196 (2017) 343–346
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Fig. 2. (a) XRD patterns of Ag/BiOCOOH heterojunctions and BiOCOOH; (b) EDS pattern of Ag/BiOCOOH-5%; (c) UV-vis DRS spectra of Ag/BiOCOOH heterojunctions and BiOCOOH.
Fig. 3. (a) The absorption spectra of RhB over Ag/BiOCOOH-5%; (b) Photocatalytic degradation of RhB over different catalysts (20 mg); (c) The photocatalytic degradation rate constants of RhB over different catalysts (d) Recycling runs of Ag/BiOCOOH-5%; (e) Photocatalytic degradation of RhB in the presence of different scavengers.
Moreover, the EDS pattern (Fig. 2b) indicates that Ag/BiOCOOH consists of Bi, O, C and Ag elements. Fig. 2c shows the UV–Vis DRS spectra of all samples. The DRS spectra of BiOCOOH presents an intense absorption in UV region,
and the absorption edge is about 365 nm (Eg: 3.4 eV), with no absorption in visible light (VL) region [12]. The DRS spectra of Ag/BiOCOOH heterojunctions show broad absorption in VL region due to the surface plasmon resonance (SPR) effect of metallic Ag
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mation of Ag/BiOCOOH heterojunction [9]. The Ag/semiconductor heterostructure can largely enhance the visible light absorption due to the SPR effect of Ag (Fig. 2c), which is beneficial for photocatalysis [15]. Under visible light, BiOCOOH can absorb visible light to produce holes on the VB and electrons on the CB. And the CB electrons on BiOCOOH may transfer to Ag. The electrons on Ag NPs will react with oxygen to produce superoxide radicals to attend the photocatalytic reactions. Meanwhile, the holes on BiOCOOH are feasible to degrade RhB directly. As a consequence, the interfacial charge transfer from BiOCOOH to Ag NPs retards the recombination of electron-hole pairs, resulting in the superior photocatalytic performance of Ag/BiOCOOH. 4. Conclusion
Fig. 4. The possible mechanism for RhB degradation over Ag/BiOCOOH.
[17]. Moreover, the VL absorption ability was gradually enhanced with the increase of Ag. 3.2. Photocatalytic activity The photocatalytic activity of Ag/BiOCOOH was tested by degrading RhB under visible light (Fig. 3a, b). Fig. 3a shows the temporal absorption spectra of RhB over Ag/BiOCOOH-5%. The maximum absorption peak of RhB at 554 nm rapidly diminishes with the reaction time increasing, and the degradation efficiency is up to 97.9% within 60 min of reaction. For comparison, 26.4%, 68.7%, 85.7%, % or 57.9% of RhB within 60 min was degraded over BiOCOOH, Ag/BiOCOOH-1%, Ag/BiOCOOH-3% or Ag/BiOCOOH-10%. Apparently, the Ag/BiOCOOH heterojunctions are much more active than the pristine BiOCOOH. Among these heterojunctions, Ag/BiOCOOH-5% obtains the optimum activity. Moreover, the photocatalytic degradation of RhB over these catalysts matches with pseudo-first-order kinetics model: -ln(C/C0) = kt, where k is the reaction rate constant (Fig. 3c). Clearly, the k of Ag/BiOCOOH-5% (0.0606 min 1) is greatly higher than those of BiOCOOH (0.0051 min 1), Ag/BiOCOOH-1% (0.0192 min 1), Ag/ BiOCOOH-3% (0.0310 min 1) and Ag/BiOCOOH-10% (0.0144 min 1). Notably, Ag/BiOCOOH-5% achieves the highest photocatalytic activity, which is 10.9 times higher than that of BiOCOOH. These facts demonstrate that the introduction of a suitable amount of metallic Ag markedly improves the photocatalytic activity of BiOCOOH [14]. The recyclability of Ag/BiOCOOH-5% was studied via the successive degradation of RhB under visible light. There was no obvious decay in the photocatalytic activity and the RhB degradation efficiency of 91.1% was achieved after 4 runs (Fig. 3d). In addition, no phase change of Ag/BiOCOOH-5% was observed (Fig. S1), demonstrating a good stability of Ag/BiOCOOH-5%. To understand the photocatalytic mechanism, trapping experiments were carried out (Fig. 3e). Here, AgNO3, ammonium oxalate (AO), benzoquinone (BQ) and isopropyl alcohol (IPA) serve as scavengers to quench photogenerated electrons (e–), holes (h+), superoxide radicals (O–2) and hydroxyl free radicals (OH), respectively. The introduction of BQ or AO greatly suppresses the degradation efficiency of RhB from 97.9% to 31.8% or 13.7% in 60 min. By contrast, the addition of IPA only slightly inhibits RhB degradation. These facts suggest that O2 and h+ play a vital role in RhB degradation (Fig. 3e). On the basis of the above results and band edge analysis (Fig. 4), the enhancement in photocatalytic activity can be due to the for-
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