- Email: [email protected]
Fabrication of BiOX (X=Cl, Br, and I) nanosheeted films by anodization and their photocatalytic properties
Materials Letters 158 (2015) 40–44
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fabrication of BiOX (X ¼ Cl, Br, and I) nanosheeted films by anodization and their photocatalytic properties Jianling Zhao a, Xiaowei Lv a,b, Xixin Wang a,n, Jing Yang a, Xiaojing Yang a, Xiaobo Lu a a Key Lab for Micro- and Nano-Scale Boron Nitride Materials, School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, China b Institute of Nanostructured Functional Materials, Huanghe Science and Technology College, Zhengzhou 450063, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 March 2015 Received in revised form 8 May 2015 Accepted 11 May 2015 Available online 21 May 2015
BiOX (X ¼Cl, Br, and I) nanosheeted films were fabricated by anodization of bismusth foils in the electrolytes containing halide ions. The morphology, crystal structure, and photocatalytic activity of the samples were characterized by scanning electron microscope, X-ray diffractometer and UV–vis spectrophotometer, respectively. The influences of composition and concentration of the electrolyte, applied voltage, reaction time and reaction temperature on the films' morphology were studied in detail. Formation mechanism of the nanosheeted films was discussed as well. Results show that the preparation conditions of nanosheeted films differed markedly when X was Cl, Br and I, respectively. The obtained nanosheeted films were of tetragonal structure and possessed obvious photocatalytic activities. The BiOCl nanosheeted films had the best activity in degrading methyl orange solutions and the solutions became almost colorless after irradiation for 6 h under a Xe arc lamp. & 2015 Elsevier B.V. All rights reserved.
Keywords: Anodization BiOX Nanosheeted films Photocatalytic
1. Introduction It is universally acknowledged that environmental pollution becomes one of the most serious problems facing mankind today. Especially, the production and use of azo dyes bring about serious environmental pollution. Various methods such as biodegradation, adsorption, photocatalysis, ultrasonic irradiation and Fenton degradation can be used for the treatment of dyes [1,2]. In the last few years, bismuth oxyhalides have attracted increasing attention because of their good photocatalytic activities [3–5]. For example, some organic contaminants such as methyl orange [6,7], rhodamine [8–10] and neutral red [11] could be effectively degraded by BiOX under visible light illumination [12]. However, most of the previous investigations focused on the synthesis and characterization of BiOX nanoparticle samples. As it is well known that the nanoparticle photocatalysts have obvious disadvantages including secondary pollution and difficulty in recovering. Compared to the nanoparticles, BiOX films are easy to be separated and reused, hence various methods including temple-assisted sol-gel method [13], spray pyrolysis [14], hydrolysis [15] and hydrothermal method [16] have been studied to prepare BiOX films. Recently, Zhang et al. prepared BiOCl film through a two step process composed of a cathodic electrodeposition and an anodic
n
Corresponding author. Tel.: þ 86 22 60204525; fax: þ 86 22 60204129. E-mail address: [email protected] (X. Wang).
http://dx.doi.org/10.1016/j.matlet.2015.05.037 0167-577X/& 2015 Elsevier B.V. All rights reserved.
oxidation step [17]. However, to the best of our knowledge, there is still no report on the preparation of BiOX (X¼Cl, Br, and I) nanosheeted films through direct anodization method which is a more convenient and environment-friendly method. In this paper, BiOX (X¼ Cl, Br, I) nanosheeted films were fabricated through direct anodization of bismuth foils for the first time. The influences of experiment conditions on the samples' morphology and photocatalytic activity were investigated. Results show that the BiOCl nanosheeted film possessed the best activity in degrading methyl orange solution.
2. Experimental The bismuth foils (99.99% purity, 10 20 1 mm3) used in this study were obtained from Beijing TIANRY Science & Technology Developing Center (Beijing, China). They were polished by abrasive paper and ultrasonic washed in twice deionized (DI) water before use. Anodization was conducted using a programcontrolled DC source (Dahua Coop., Beijing, China). Bismuth foils were used as anodic electrode while platinum (20 20 0.1 mm3) was used as cathodic electrode. The distance between anodic and cathodic electrodes was 20 mm. All reagents were of analytical grade without further purification. All anodization experiments were conducted at presupposed temperature. After the anodization, the samples were rinsed in DI water, air dried
J. Zhao et al. / Materials Letters 158 (2015) 40–44
41
Fig. 1. SEM images of the samples anodized in electrolytes containing Cl , (a) 0.3 wt% NH4Cl, 30 V, 40 1C, 1 min, (b) 0.3 wt% NH4Cl, 30 V, 40 1C, 10 min, (c) 0.3 wt% NH4Cl, 30 V, 40 1C, 20 min, (d) 0.3 wt% NH4Cl, 30 V, 40 1C, 60 min, (e) 0.3 wt%NH4Cl, 30 V, 60 1C, 20 min; (f) 0.3 wt% NH4Cl, 40 V, 40 1C, 20 min; (g) 0.05 wt% NH4Cl, 30 V, 40 1C, 20 min; (h) EG þ2 wt% H2Oþ 0.3 wt% HCl, 20 V, 40 1C, 20 min.
42
J. Zhao et al. / Materials Letters 158 (2015) 40–44
Fig. 2. SEM images of the samples anodized in electrolytes containing Br and I , (a) 0.4 wt% HBr, 20 V, 40 1C, 20 min; (b) 0.3 wt% HBr, 20 V, 50 1C, 20 min; (c) EG þ 10 wt% H2Oþ0.3% NH4I, 30 V, 50 1C, 20 min; (d) EG þ10 wt% H2Oþ0.3% NH4I, 30 V, 30 1C, 20 min.
and characterized. The microstructures were characterized on a field emission scanning electron microscope (S-4800, Hitachi, Japan) and a X-ray diffractometer (D/maxRB, Rigaku, Japan). Photocatalytic experiments were carried out to degrade methyl orange (MO) solution in a concentration of 20 mg/L with a Xe arc lamp (CHF-XM-500 W, Trusttech, China) as the light source. The absorbance of MO was measured on a UV–vis spectrophotometer (U-3900H, Hitachi, Japan).
3. Results and discussion Fig. 1a–d shows the SEM images of bismuth after anodization in 0.3 wt% NH4Cl at 30 V, 40 1C for different reaction time. When anodization time was 1 min, some sheets appeared at the surface of bismuth foil (Fig. 1a) and the sheets became bigger when extending anodization time to 10 min (Fig. 1b). After anodization for 20 min, the nanosheets became more compact due to mutual extrusion (Fig. 1c) and the morphology at anodization of 60 min was similar to that in Fig. 1a but in a bigger thickness which might be due to the peeling off of surface nanosheets (Fig. 1d). Fig. 1e–h give the surface SEM images of the samples prepared in electrolytes containing Cl at different conditions, indicating that the temperature, voltage, and electrolyte composition and concentration affected the surface morphologies obviously. In comparison with the sample in Fig. 1c (the reaction temperature was 40 1C with other conditions being the same), nanosheets obtained at 60 1C covered the surface more loosely (Fig. 1e). When the voltage increased to 40 V, nanosheets were also obtained but the surface film was uneven (Fig. 1f). The obtained nanosheets became thicker and more irregular when the concentration of NH4Cl was 0.05 wt% (Fig. 1g). Nanosheets were hardly obtained in
Fig. 3. XRD patterns of the samples prepared in electrolytes containing Cl (a), Br (b) and I (c).
ethylene glycol (EG) solution containing NH4Cl and nanosheet clusters appeared when anodization was conducted in ethylene glycol solution containing 2 wt% H2O and 0.3 wt% HCl (Fig. 1h). Nanosheets could also be obtained when anodization was conducted in the electrolytes containing Br or I and Fig. 2 shows the samples with better morphologies. The nanosheets obtained in 0.4 wt% HBr at 40 1C were in a smaller size (Fig. 2a) and the nanosheets obtained in 0.3 wt% HBr at 50 1C had smoother surface (Fig. 2b). Densely packed nanosheets were obtained at 50 1C (Fig. 2c) and relatively loose nanosheets were obtained at 30 1C (Fig. 2d) in the ethylene glycerol solution containing NH4I.
J. Zhao et al. / Materials Letters 158 (2015) 40–44
43
Fig. 4. UV–vis absorption spectra of MO solutions with BiOCl as photocatalyst (A) and photocatalytic properties of BiOCl, BiOBr and BiOI nanosheeted films (B).
It can be seen from Figs. 1 and 2 that nanosheets could be obtained in whichever of the electrolytes containing Cl , Br or I , however, the preparation conditions and the product morphology differed markedly due to the different features of Cl , Br and I . The morphologies in Fig. 1c, Fig. 2a and c were optimal for BiOCl, BiOBr and BiOI, respectively. X-ray diffraction (XRD) patterns of BiOX nanosheets prepared under the conditions in Fig. 1c, Fig. 2a and c are shown in Fig. 3. As indicated in Fig. 3, the nanosheets prepared in the electrolytes containing Cl , Br and I were of tetragonal BiOCl (PDF Card no.73-2060), tetragonal BiOBr (PDF Card no.09-0393) and tetragonal BiOI (PDF Card no.10-0445) respectively. BiOX (X¼Cl, Br, and I) compounds all crystallize in the tetragonal matlockite structure, a layer structure characterized by [Bi2O2] slabs interleaved by double slabs of halogen atoms which are combined through weak bonding force. The specific crystal structures of BiOX result in the easily forming of sheet-like shapes [18]. There may exist the following reactions during the anodization process: Anode : 3þ
Bi
Bi–3e-Bi
3þ
þ H2 O þ X -BiOX þ 2H þ
Cathode :
þ
2H þ 2e-H2
ð1Þ ð2Þ ð3Þ
When anodization was conducted in electrolytes containing halide ions, sheet-like BiOX would form at the surface of bismuth foil (Eqs. (1) and (2)). The newly formed BiOX nanosheets would tilt up from the surface due to volume expansion and mass transfer. Small tilted nanosheets would form again at the exposed metal surface and then the surface of bismuth foil would be full of BiOX nanosheets. Because the inducing effect of the existing BiOX, the newly formed BiOX would extend outside along with the early formed BiOX nanosheets and the nanosheets would grow to a larger size. The mutual squeeze of nanosheets in larger size would lead to the fabrication of densely packed nanosheeted film (Fig. 1c) and excessive squeeze between the nanosheets would result in the peeling off of the nanosheeted film. In addition, the densely packed nanosheeted film of BiOX at the surface would obstruct the growth of underneath nanosheets and result in different morphology of the underneath nanosheets (Fig. 1d). Using BiOCl nanosheeted film as photocatalyst, the UV–vis absorption spectra of methyl orange solution after different irradiation time were given in Fig. 4A. The strong absorption peak of MO solution at 508 nm decreased steadily with the increasing
light irradiation time, and the orange color of the solution turned gradually to colorless. Fig. 4B gives the photocatalytic properties of BiOCl, BiOBr and BiOI nanosheeted films. Results show that the properties of BiOCl and BiOBr nanosheeted films are superior to that of BiOI nanosheeted film. The BiOCl nanosheet film shows the best activity compared with that of BiOBr and BiOI. The photocatalytic degradation rate of MO solution with BiOCl nanosheet film was 96.1% after irradiation for 4 h and the methyl orange solution is almost colorless after 6 h while the MO solution was hardly degraded without catalysts under the same conditions. Moreover, photocatalytic activity of the BiOCl nanosheet film did not decrease obviously after being reused for four times, indicating that the BiOCl nanosheet film could be recycled after cleaning. The generally accepted photocatalytic mechanism is that the photocatalysts absorb light energy and generate electron–hole pairs whose redox reactions would result in the degradation of dyes [19]. Accordingly, the better photocatalytic activity of BiOCl nanosheet film might be ascribed to the wider band gap and the stronger redox ability of photogenerated electrons-holes than those of BiOBr and BiOI films [20].
4. Conclusions BiOX (X ¼Cl, Br, I) nanosheeted films with tetragonal structure were obtained by anodization of bismuth foils in the electrolytes containing halide ions. The temperature, voltage, reaction time and electrolytes compositions and concentration had significant influence on the surface morphologies. The optimal preparation conditions for BiOCl, BiOBr and BiOI nanosheets were 0.3 wt% NH4Cl, 30 V, 40 1C, 20 min; 0.4 wt% HBr, 20 V, 40 1C, 20 min; and EGþ10 wt% H2O þ0.3% NH4I, 30 V, 50 1C, respectively. The experimental results show that the photocatalytic properties of BiOCl, BiOBr and BiOI nanosheeted films decreased in turn. BiOCl nanosheeted film shows high activity in degrading methyl orange. The decolorization rate of BiOCl nanosheeted film was 96.1% after 4 h irradiation and the methyl orange solution was almost colorless after 6 h irradiation.
Acknowledgments This work is supported by National Natural Science Foundation of China (51272064, 51301057), Key Basic Research Program of Hebei Province of China (No. 12965135D), Natural Science Foundation of Hebei Province of China (E2013202032, B2013202211), the Talent
44
J. Zhao et al. / Materials Letters 158 (2015) 40–44
Training Project of Hebei Province (2013) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13060).
References [1] Esquivel K, García MG, Rodríguez FJ, Ortiz-Frade LA, Godínez LA. Study of the photo-electrochemical activity of cobalt-and nickel-doped TiO2 photo-anodes for the treatment of a dye-contaminated aqueous solution. J Appl Electrochem 2013;43:433–40. [2] Zhao J, Wang X, Zhang L, Hou X, Li Y, Tang C. Degradation of methyl orange through synergistic effect of zirconia nanotubes and ultrasonic wave. J Hazard Mater 2011;188:231–4. [3] Mills A, Hunte SL. An overview of semiconductor photocatalysis. J Photochem Photobiol A: Chem 1997;108:1–36. [4] Wolfrum EJ, Huang J, Blake DM, Maness PC, Huang Z, Fiest J, et al. Photocatalytic oxidation of bacteria, bacterial and fungal spores, and model biofilm components to carbon dioxide on titanium dioxide-coated surfaces. Environ Sci Technol 2002;36:3412–9. [5] Zang Y, Farnood R. Photocatalytic decomposition of methyl tert-butyl ether in aqueous slurry of titanium dioxide. Appl Catal B Environ 2005;57:275–82. [6] Liu Z, Wu B, Xiang D, Zhu Y. Effect of solvents on morphology and photocatalytic activity of BiOBr synthesized by solvothermal method. Mater Res Bull 2012;47:3753–7. [7] Xia J, Yin S, Li H, Xu H, Xu L, Zhang Q. Enhanced photocatalytic activity of bismuth oxyiodine (BiOI) porous microspheres synthesized via reactable ionic liquid-assisted solvothermal method. Colloids Surf A:Physicochem Eng Asp 2011;387:23–8. [8] Chang X, Gondal M, Al-Saadi A, Ali M, Shen H, Zhou Q, et al. Photodegradation of Rhodamine B over unexcited semiconductor compounds of BiOCl and BiOBr. J Colloid Interf Sci 2012;377:291–8.
[9] Gondal MA, Chang X, Ali MA, Yamani ZH, Zhou Q, Ji G. Adsorption and degradation performance of Rhodamine B over BiOBr under monochromatic 532 nm pulsed laser exposure. Appl Catal A: Gen 2011;397:192–200. [10] Shenawi-Khalil S, Uvarov V, Kritsman Y, Menes E, Popov I, Sasson Y. A new family of BiO (ClxBr 1 x) visible light sensitive photocatalysts. Catal Commun 2011;12:1136–41. [11] Sarwan B, Acharya A, Pare B, Jonnalagadda S. Mineralization and toxicity reduction of textile dye neutral red in aqueous phase using BiOCl photocatalysis. J Photochem Photobiol B 2012;116:48–55. [12] Lei Y, Wang G, Guo P, Song H. The Ag–BiOBrxI1 x composite photocatalyst: preparation, characteration and their novel pollutants removal property. Appl Surf Sci 2013;279:374–9. [13] Wu S, Wang C, Cui Y, Hao W, Wang T, Brault P. BiOCl nano/microstructures on substrates: synthesis and photocatalytic properties. Mater Lett 2011;65: 1344–1347. [14] Hahn NT, Hoang S, Self JL, Mullins CB. Spray pyrolysis deposition and photoelectrochemical properties of n-type BiOI nanoplatelet thin films. ACS Nano 2012;6:7712–22. [15] Wang Y, Shi Z, Fan C, Wang X, Hao X, Chi Y. Synthesis, characterization, and photocatalytic properties of BiOBr catalyst. J Solid State Chem 2013;199:224–9. [16] Jiang Z, Yang F, Yang G, Kong L, Jones MO, Xiao T, et al. The hydrothermal synthesis of BiOBr flakes for visible-light-responsive photocatalytic degradation of methyl orange. J Photochem Photobio A: Chem 2010;212:8–13. [17] Zhang X, Liu X, Fan C, Wang Y, Wang Y, Liang Z. A novel BiOCl thin film prepared by electrochemical method and its application in photocatalysis. Appl Catal B: Environ 2012;132–133:332–41. [18] Kijima N, Matano K, Saito M, Oikawa T, Konishi T, Yasuda H, et al. Oxidative catalytic cracking of n-butane to lower alkenes over layered BiOCl catalyst. Appl Catal A: Gen 2001;206:237–44. [19] Sivakumar A, Murugesan B, Loganathan A, Sivakumar P. A review on decolourisation of dyes by photodegradation using various bismuth catalysts. J Taiwan Inst Chem Eng 2014;45:2300–6. [20] Huang W, Zhu Q. DFT Calculations on the electronic structures of BiOX (X 5F, Cl, Br, I) photocatalysts with and without semicore Bi 5d states. J Comput Chem 2008;30:183–90.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Fabrication of BiOX (X ¼ Cl, Br, and I) nanosheeted films by anodization and their photocatalytic properties Jianling Zhao a, Xiaowei Lv a,b, Xixin Wang a,n, Jing Yang a, Xiaojing Yang a, Xiaobo Lu a a Key Lab for Micro- and Nano-Scale Boron Nitride Materials, School of Material Science and Engineering, Hebei University of Technology, Tianjin 300130, China b Institute of Nanostructured Functional Materials, Huanghe Science and Technology College, Zhengzhou 450063, China
art ic l e i nf o
a b s t r a c t
Article history: Received 7 March 2015 Received in revised form 8 May 2015 Accepted 11 May 2015 Available online 21 May 2015
BiOX (X ¼Cl, Br, and I) nanosheeted films were fabricated by anodization of bismusth foils in the electrolytes containing halide ions. The morphology, crystal structure, and photocatalytic activity of the samples were characterized by scanning electron microscope, X-ray diffractometer and UV–vis spectrophotometer, respectively. The influences of composition and concentration of the electrolyte, applied voltage, reaction time and reaction temperature on the films' morphology were studied in detail. Formation mechanism of the nanosheeted films was discussed as well. Results show that the preparation conditions of nanosheeted films differed markedly when X was Cl, Br and I, respectively. The obtained nanosheeted films were of tetragonal structure and possessed obvious photocatalytic activities. The BiOCl nanosheeted films had the best activity in degrading methyl orange solutions and the solutions became almost colorless after irradiation for 6 h under a Xe arc lamp. & 2015 Elsevier B.V. All rights reserved.
Keywords: Anodization BiOX Nanosheeted films Photocatalytic
1. Introduction It is universally acknowledged that environmental pollution becomes one of the most serious problems facing mankind today. Especially, the production and use of azo dyes bring about serious environmental pollution. Various methods such as biodegradation, adsorption, photocatalysis, ultrasonic irradiation and Fenton degradation can be used for the treatment of dyes [1,2]. In the last few years, bismuth oxyhalides have attracted increasing attention because of their good photocatalytic activities [3–5]. For example, some organic contaminants such as methyl orange [6,7], rhodamine [8–10] and neutral red [11] could be effectively degraded by BiOX under visible light illumination [12]. However, most of the previous investigations focused on the synthesis and characterization of BiOX nanoparticle samples. As it is well known that the nanoparticle photocatalysts have obvious disadvantages including secondary pollution and difficulty in recovering. Compared to the nanoparticles, BiOX films are easy to be separated and reused, hence various methods including temple-assisted sol-gel method [13], spray pyrolysis [14], hydrolysis [15] and hydrothermal method [16] have been studied to prepare BiOX films. Recently, Zhang et al. prepared BiOCl film through a two step process composed of a cathodic electrodeposition and an anodic
n
Corresponding author. Tel.: þ 86 22 60204525; fax: þ 86 22 60204129. E-mail address: [email protected] (X. Wang).
http://dx.doi.org/10.1016/j.matlet.2015.05.037 0167-577X/& 2015 Elsevier B.V. All rights reserved.
oxidation step [17]. However, to the best of our knowledge, there is still no report on the preparation of BiOX (X¼Cl, Br, and I) nanosheeted films through direct anodization method which is a more convenient and environment-friendly method. In this paper, BiOX (X¼ Cl, Br, I) nanosheeted films were fabricated through direct anodization of bismuth foils for the first time. The influences of experiment conditions on the samples' morphology and photocatalytic activity were investigated. Results show that the BiOCl nanosheeted film possessed the best activity in degrading methyl orange solution.
2. Experimental The bismuth foils (99.99% purity, 10 20 1 mm3) used in this study were obtained from Beijing TIANRY Science & Technology Developing Center (Beijing, China). They were polished by abrasive paper and ultrasonic washed in twice deionized (DI) water before use. Anodization was conducted using a programcontrolled DC source (Dahua Coop., Beijing, China). Bismuth foils were used as anodic electrode while platinum (20 20 0.1 mm3) was used as cathodic electrode. The distance between anodic and cathodic electrodes was 20 mm. All reagents were of analytical grade without further purification. All anodization experiments were conducted at presupposed temperature. After the anodization, the samples were rinsed in DI water, air dried
J. Zhao et al. / Materials Letters 158 (2015) 40–44
41
Fig. 1. SEM images of the samples anodized in electrolytes containing Cl , (a) 0.3 wt% NH4Cl, 30 V, 40 1C, 1 min, (b) 0.3 wt% NH4Cl, 30 V, 40 1C, 10 min, (c) 0.3 wt% NH4Cl, 30 V, 40 1C, 20 min, (d) 0.3 wt% NH4Cl, 30 V, 40 1C, 60 min, (e) 0.3 wt%NH4Cl, 30 V, 60 1C, 20 min; (f) 0.3 wt% NH4Cl, 40 V, 40 1C, 20 min; (g) 0.05 wt% NH4Cl, 30 V, 40 1C, 20 min; (h) EG þ2 wt% H2Oþ 0.3 wt% HCl, 20 V, 40 1C, 20 min.
42
J. Zhao et al. / Materials Letters 158 (2015) 40–44
Fig. 2. SEM images of the samples anodized in electrolytes containing Br and I , (a) 0.4 wt% HBr, 20 V, 40 1C, 20 min; (b) 0.3 wt% HBr, 20 V, 50 1C, 20 min; (c) EG þ 10 wt% H2Oþ0.3% NH4I, 30 V, 50 1C, 20 min; (d) EG þ10 wt% H2Oþ0.3% NH4I, 30 V, 30 1C, 20 min.
and characterized. The microstructures were characterized on a field emission scanning electron microscope (S-4800, Hitachi, Japan) and a X-ray diffractometer (D/maxRB, Rigaku, Japan). Photocatalytic experiments were carried out to degrade methyl orange (MO) solution in a concentration of 20 mg/L with a Xe arc lamp (CHF-XM-500 W, Trusttech, China) as the light source. The absorbance of MO was measured on a UV–vis spectrophotometer (U-3900H, Hitachi, Japan).
3. Results and discussion Fig. 1a–d shows the SEM images of bismuth after anodization in 0.3 wt% NH4Cl at 30 V, 40 1C for different reaction time. When anodization time was 1 min, some sheets appeared at the surface of bismuth foil (Fig. 1a) and the sheets became bigger when extending anodization time to 10 min (Fig. 1b). After anodization for 20 min, the nanosheets became more compact due to mutual extrusion (Fig. 1c) and the morphology at anodization of 60 min was similar to that in Fig. 1a but in a bigger thickness which might be due to the peeling off of surface nanosheets (Fig. 1d). Fig. 1e–h give the surface SEM images of the samples prepared in electrolytes containing Cl at different conditions, indicating that the temperature, voltage, and electrolyte composition and concentration affected the surface morphologies obviously. In comparison with the sample in Fig. 1c (the reaction temperature was 40 1C with other conditions being the same), nanosheets obtained at 60 1C covered the surface more loosely (Fig. 1e). When the voltage increased to 40 V, nanosheets were also obtained but the surface film was uneven (Fig. 1f). The obtained nanosheets became thicker and more irregular when the concentration of NH4Cl was 0.05 wt% (Fig. 1g). Nanosheets were hardly obtained in
Fig. 3. XRD patterns of the samples prepared in electrolytes containing Cl (a), Br (b) and I (c).
ethylene glycol (EG) solution containing NH4Cl and nanosheet clusters appeared when anodization was conducted in ethylene glycol solution containing 2 wt% H2O and 0.3 wt% HCl (Fig. 1h). Nanosheets could also be obtained when anodization was conducted in the electrolytes containing Br or I and Fig. 2 shows the samples with better morphologies. The nanosheets obtained in 0.4 wt% HBr at 40 1C were in a smaller size (Fig. 2a) and the nanosheets obtained in 0.3 wt% HBr at 50 1C had smoother surface (Fig. 2b). Densely packed nanosheets were obtained at 50 1C (Fig. 2c) and relatively loose nanosheets were obtained at 30 1C (Fig. 2d) in the ethylene glycerol solution containing NH4I.
J. Zhao et al. / Materials Letters 158 (2015) 40–44
43
Fig. 4. UV–vis absorption spectra of MO solutions with BiOCl as photocatalyst (A) and photocatalytic properties of BiOCl, BiOBr and BiOI nanosheeted films (B).
It can be seen from Figs. 1 and 2 that nanosheets could be obtained in whichever of the electrolytes containing Cl , Br or I , however, the preparation conditions and the product morphology differed markedly due to the different features of Cl , Br and I . The morphologies in Fig. 1c, Fig. 2a and c were optimal for BiOCl, BiOBr and BiOI, respectively. X-ray diffraction (XRD) patterns of BiOX nanosheets prepared under the conditions in Fig. 1c, Fig. 2a and c are shown in Fig. 3. As indicated in Fig. 3, the nanosheets prepared in the electrolytes containing Cl , Br and I were of tetragonal BiOCl (PDF Card no.73-2060), tetragonal BiOBr (PDF Card no.09-0393) and tetragonal BiOI (PDF Card no.10-0445) respectively. BiOX (X¼Cl, Br, and I) compounds all crystallize in the tetragonal matlockite structure, a layer structure characterized by [Bi2O2] slabs interleaved by double slabs of halogen atoms which are combined through weak bonding force. The specific crystal structures of BiOX result in the easily forming of sheet-like shapes [18]. There may exist the following reactions during the anodization process: Anode : 3þ
Bi
Bi–3e-Bi
3þ
þ H2 O þ X -BiOX þ 2H þ
Cathode :
þ
2H þ 2e-H2
ð1Þ ð2Þ ð3Þ
When anodization was conducted in electrolytes containing halide ions, sheet-like BiOX would form at the surface of bismuth foil (Eqs. (1) and (2)). The newly formed BiOX nanosheets would tilt up from the surface due to volume expansion and mass transfer. Small tilted nanosheets would form again at the exposed metal surface and then the surface of bismuth foil would be full of BiOX nanosheets. Because the inducing effect of the existing BiOX, the newly formed BiOX would extend outside along with the early formed BiOX nanosheets and the nanosheets would grow to a larger size. The mutual squeeze of nanosheets in larger size would lead to the fabrication of densely packed nanosheeted film (Fig. 1c) and excessive squeeze between the nanosheets would result in the peeling off of the nanosheeted film. In addition, the densely packed nanosheeted film of BiOX at the surface would obstruct the growth of underneath nanosheets and result in different morphology of the underneath nanosheets (Fig. 1d). Using BiOCl nanosheeted film as photocatalyst, the UV–vis absorption spectra of methyl orange solution after different irradiation time were given in Fig. 4A. The strong absorption peak of MO solution at 508 nm decreased steadily with the increasing
light irradiation time, and the orange color of the solution turned gradually to colorless. Fig. 4B gives the photocatalytic properties of BiOCl, BiOBr and BiOI nanosheeted films. Results show that the properties of BiOCl and BiOBr nanosheeted films are superior to that of BiOI nanosheeted film. The BiOCl nanosheet film shows the best activity compared with that of BiOBr and BiOI. The photocatalytic degradation rate of MO solution with BiOCl nanosheet film was 96.1% after irradiation for 4 h and the methyl orange solution is almost colorless after 6 h while the MO solution was hardly degraded without catalysts under the same conditions. Moreover, photocatalytic activity of the BiOCl nanosheet film did not decrease obviously after being reused for four times, indicating that the BiOCl nanosheet film could be recycled after cleaning. The generally accepted photocatalytic mechanism is that the photocatalysts absorb light energy and generate electron–hole pairs whose redox reactions would result in the degradation of dyes [19]. Accordingly, the better photocatalytic activity of BiOCl nanosheet film might be ascribed to the wider band gap and the stronger redox ability of photogenerated electrons-holes than those of BiOBr and BiOI films [20].
4. Conclusions BiOX (X ¼Cl, Br, I) nanosheeted films with tetragonal structure were obtained by anodization of bismuth foils in the electrolytes containing halide ions. The temperature, voltage, reaction time and electrolytes compositions and concentration had significant influence on the surface morphologies. The optimal preparation conditions for BiOCl, BiOBr and BiOI nanosheets were 0.3 wt% NH4Cl, 30 V, 40 1C, 20 min; 0.4 wt% HBr, 20 V, 40 1C, 20 min; and EGþ10 wt% H2O þ0.3% NH4I, 30 V, 50 1C, respectively. The experimental results show that the photocatalytic properties of BiOCl, BiOBr and BiOI nanosheeted films decreased in turn. BiOCl nanosheeted film shows high activity in degrading methyl orange. The decolorization rate of BiOCl nanosheeted film was 96.1% after 4 h irradiation and the methyl orange solution was almost colorless after 6 h irradiation.
Acknowledgments This work is supported by National Natural Science Foundation of China (51272064, 51301057), Key Basic Research Program of Hebei Province of China (No. 12965135D), Natural Science Foundation of Hebei Province of China (E2013202032, B2013202211), the Talent
44
J. Zhao et al. / Materials Letters 158 (2015) 40–44
Training Project of Hebei Province (2013) and the Program for Changjiang Scholars and Innovative Research Team in University (IRT13060).
References [1] Esquivel K, García MG, Rodríguez FJ, Ortiz-Frade LA, Godínez LA. Study of the photo-electrochemical activity of cobalt-and nickel-doped TiO2 photo-anodes for the treatment of a dye-contaminated aqueous solution. J Appl Electrochem 2013;43:433–40. [2] Zhao J, Wang X, Zhang L, Hou X, Li Y, Tang C. Degradation of methyl orange through synergistic effect of zirconia nanotubes and ultrasonic wave. J Hazard Mater 2011;188:231–4. [3] Mills A, Hunte SL. An overview of semiconductor photocatalysis. J Photochem Photobiol A: Chem 1997;108:1–36. [4] Wolfrum EJ, Huang J, Blake DM, Maness PC, Huang Z, Fiest J, et al. Photocatalytic oxidation of bacteria, bacterial and fungal spores, and model biofilm components to carbon dioxide on titanium dioxide-coated surfaces. Environ Sci Technol 2002;36:3412–9. [5] Zang Y, Farnood R. Photocatalytic decomposition of methyl tert-butyl ether in aqueous slurry of titanium dioxide. Appl Catal B Environ 2005;57:275–82. [6] Liu Z, Wu B, Xiang D, Zhu Y. Effect of solvents on morphology and photocatalytic activity of BiOBr synthesized by solvothermal method. Mater Res Bull 2012;47:3753–7. [7] Xia J, Yin S, Li H, Xu H, Xu L, Zhang Q. Enhanced photocatalytic activity of bismuth oxyiodine (BiOI) porous microspheres synthesized via reactable ionic liquid-assisted solvothermal method. Colloids Surf A:Physicochem Eng Asp 2011;387:23–8. [8] Chang X, Gondal M, Al-Saadi A, Ali M, Shen H, Zhou Q, et al. Photodegradation of Rhodamine B over unexcited semiconductor compounds of BiOCl and BiOBr. J Colloid Interf Sci 2012;377:291–8.
[9] Gondal MA, Chang X, Ali MA, Yamani ZH, Zhou Q, Ji G. Adsorption and degradation performance of Rhodamine B over BiOBr under monochromatic 532 nm pulsed laser exposure. Appl Catal A: Gen 2011;397:192–200. [10] Shenawi-Khalil S, Uvarov V, Kritsman Y, Menes E, Popov I, Sasson Y. A new family of BiO (ClxBr 1 x) visible light sensitive photocatalysts. Catal Commun 2011;12:1136–41. [11] Sarwan B, Acharya A, Pare B, Jonnalagadda S. Mineralization and toxicity reduction of textile dye neutral red in aqueous phase using BiOCl photocatalysis. J Photochem Photobiol B 2012;116:48–55. [12] Lei Y, Wang G, Guo P, Song H. The Ag–BiOBrxI1 x composite photocatalyst: preparation, characteration and their novel pollutants removal property. Appl Surf Sci 2013;279:374–9. [13] Wu S, Wang C, Cui Y, Hao W, Wang T, Brault P. BiOCl nano/microstructures on substrates: synthesis and photocatalytic properties. Mater Lett 2011;65: 1344–1347. [14] Hahn NT, Hoang S, Self JL, Mullins CB. Spray pyrolysis deposition and photoelectrochemical properties of n-type BiOI nanoplatelet thin films. ACS Nano 2012;6:7712–22. [15] Wang Y, Shi Z, Fan C, Wang X, Hao X, Chi Y. Synthesis, characterization, and photocatalytic properties of BiOBr catalyst. J Solid State Chem 2013;199:224–9. [16] Jiang Z, Yang F, Yang G, Kong L, Jones MO, Xiao T, et al. The hydrothermal synthesis of BiOBr flakes for visible-light-responsive photocatalytic degradation of methyl orange. J Photochem Photobio A: Chem 2010;212:8–13. [17] Zhang X, Liu X, Fan C, Wang Y, Wang Y, Liang Z. A novel BiOCl thin film prepared by electrochemical method and its application in photocatalysis. Appl Catal B: Environ 2012;132–133:332–41. [18] Kijima N, Matano K, Saito M, Oikawa T, Konishi T, Yasuda H, et al. Oxidative catalytic cracking of n-butane to lower alkenes over layered BiOCl catalyst. Appl Catal A: Gen 2001;206:237–44. [19] Sivakumar A, Murugesan B, Loganathan A, Sivakumar P. A review on decolourisation of dyes by photodegradation using various bismuth catalysts. J Taiwan Inst Chem Eng 2014;45:2300–6. [20] Huang W, Zhu Q. DFT Calculations on the electronic structures of BiOX (X 5F, Cl, Br, I) photocatalysts with and without semicore Bi 5d states. J Comput Chem 2008;30:183–90.