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Enhanced Photocatalytic Activity of Bi24O31Br10: Constructing Heterojunction with BiOI
ARTICLE IN PRESS Journal of Materials Science & Technology ■■ (2016) ■■–■■
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Journal of Materials Science & Technology j o u r n a l h o m e p a g e : w w w. j m s t . o r g
Enhanced Photocatalytic Activity of Bi24O31Br10: Constructing Heterojunction with BiOI Xi Lou 1, Jun Shang 2, Liang Wang 1, Haifeng Feng 3, Weichang Hao 1,3,*, Tianmin Wang 1, Yi Du 3 1
Center of Material Physics and Chemistry, Beihang University, Beijing 100191, China College of Physics and Electronic Engineering, Henan Normal University, Xinxiang 453007, China 3 Institute for Superconducting and Electronic Materials (ISEM), University of Wollongong, Wollongong, NSW 2500, Australia 2
A R T I C L E
I N F O
Article history: Received 21 October 2015 Received in revised form 14 January 2016 Accepted 26 January 2016 Available online Key words: Bi24O31Br10 Hydroiodic acid BiOI Heterojunction Photocatalyst
Bismuth-based compounds have been regarded as an important class of visible-light photocatalysts due to their special electronic structures. In this paper, iodide ions are introduced to modify bismuth-based compound, Bi24O31Br10, forming a Bi24O31Br10/BiOI heterojunction structure. A significant enhancement of photocatalytic activity compared to the parent compounds is observed in de-coloration of rhodamine B (Rh.B) solution. The improved photocatalytic property of Bi24O31Br10/BiOI heterojunction is ascribed to the unique electronic structure consisting of complementary band structures of BiOI and Bi24O31Br10. Iodide ions are regarded as an effective reagent to construct bismuth-based photocatalytic heterojunctions with improved photocatalytic activity. Copyright © 2016, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited.
1. Introduction Photocatalysis is one of the most promising methods for environmental purification and energy conservation[1,2], which possesses advantages such as being environment friendly and low cost. Thus, photocatalyst with high activity attracts growing attention and gains immense research interest. Many strategies, such as doping, heterojunction construction, dye sensitization, facet control, nanoassembly, plasmon enhancement, and graphene-based photocatalysis, have been developed to improve the activity and efficiency of photocatalysts[3–9]. Constructing a heterojunction interface between the semiconductors with matching band potentials has been extensively adopted to effectively enhance the photocatalytic activity by facilitating the separation of photo-induced electron–hole pairs. Recently, bismuth-based compounds are identified as a significant class of photocatalyst due to their unique electronic structures that benefit photocatalytic process. Bi 6s and O 2p levels in bismuthbased compounds can form a preferable hybridized valence band (VB), which meets the requirement of organic oxidation[10]. In addition, the hybridization of the Bi 6s and O 2p levels induces a highly
* Corresponding author. Prof.; Tel.: +86 10 82339306; Fax: +86 10 82317931. E-mail address: [email protected] (W. Hao).
dispersed VB that increases the mobility of photo-induced holes in VB[11] and hence favors oxidation reaction in photocatalysis. Many bismuth-based compounds including BiOX (X = Cl, Br, I)[12,13], BiVO4[10], NaBiO3[14], and bismuth titanate[15] have been reported to demonstrate excellent photocatalytic and photoelectron chemical performances. Very recently, Bi24O31X10 (X = Cl, Br)[16–19] were reported to be novel bismuth oxyhalides with narrower band gap and higher photocatalytic activity compared with BiOX. However, very few studies on Bi24O31X10 heterojunction photocatalytic properties are reported so far. In this work, a novel strategy is demonstrated to enhance the photocatalytic activity of Bi24O31Br10, in which heterojunctions of Bi24O31X10 and BiOI are fabricated by a chemical etching with hydroiodic acid (HI). Iodide ions in HI tend to substitute the bromide ions or oxygen ions in Bi24O31Br10 and bring out BiOI. BiOI, a bismuthbased photocatalyst with a band gap of 1.9 eV, exhibits great ability in separation of the photo-induced electrons and holes under visible light[16]. Photocatalytic activity of such Bi-based photocatalytic heterojunction composites is significantly improved in contrast to Bi24O31Br10. The enhanced photocatalytic activity is attributed to the increased absorption of visible light and the improved separation of photo-induced charge carriers induced by BiOI in the heterojunction, which is evident in a heterojunction structure analysis and photoelectric measurements. HI possibly acts as a reagent
http://dx.doi.org/10.1016/j.jmst.2016.05.002 1005-0302/Copyright © 2016, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited.
Please cite this article in press as: Xi Lou, et al., Enhanced Photocatalytic Activity of Bi24O31Br10: Constructing Heterojunction with BiOI, Journal of Materials Science & Technology (2016), doi: 10.1016/j.jmst.2016.05.002
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to introduce BiOI to bismuth-based compounds and to enhance their photocatalytic activity. 2. Experimental 2.1. Preparation of Bi24O31Br10 All reagents were analytical reagents and used as received without further purification. In 50 mL 1.6 mol/L dilute nitric acid, 2.42 g Bi(NO3)3·5H2O was dissolved, and 0.76 g cetyltrimethyl ammonium bromide (CTAB) was dissolved in 200 mL 0.14 mol/L NaOH aqueous solution. The alkali solution was slowly dropped into the acid solution with vigorously magnetic stirring, and a white precipitate formed at the same time. The precipitate was purified by filtration, washed with distilled water several times, and then heated at 500 °C for 2 h. 2.2. Preparation of Bi24O31Br10/BiOI One gram of Bi24O31Br10 was put into 50 mL ethanol and stirred vigorously. Simultaneously, a certain amount of HI was added to the above suspension drop by drop. After 1 h, the precipitate was purified by filtration, washed with distilled water several times, and dried at 80 °C. Samples with 0, 10, 30, 50 and 70 μL HI additions were marked as Bi24O31Br10, Bi24O31Br10/BiOI-a, Bi24O31Br10/BiOI-b, Bi24O31Br10/BiOI-c, and Bi24O31Br10/BiOI-d. TiO2–xNx reference sample was prepared by annealing TiO2 (Degussa P25) under NH3 atmosphere at 500 °C for 8 h. 2.3. Characterization Powder X-ray diffraction (XRD) patterns were collected under a PANalytical X’ Pert Pro X-ray diffractometer using CuKα radiation (0.154 nm), and the working voltage was 40 kV. Ultraviolet visible (UV-Vis) diffuse reflectance spectra were collected on a Cintra10e spectrometer using BaSO4 as the reference sample. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) observations were performed with JEM2100 high-resolution transmission electron microscope, using an accelerating voltage of 200 kV. The surface photovoltage (SPV)
spectroscopy apparatus is composed of a source of monochromatic light, a lock-in amplifier (SR830-DSP) with a light chopper (SR540), and a photovoltaic cell. A 500 W xenon lamp (CHFXQ500W, Global xenon lamp power) and a grating monochromator (Omni-5007, No.09010, Zolix) provided monochromatic light. The construction of the photovoltaic cell was a sandwich-like structure of indium tin oxide (ITO)-sample-ITO. In photocurrent-time response system, a 300W Xe lamp with a monochromator and a cutoff filter (λ > 420 nm) was used as the light source, and KEITHLEY 2400 source meter was used to collect the electrical signal. 2.4. De-coloration of rhodamine B dye The catalyst (50 mg) was added into an aqueous solution of rhodamine B (Rh.B, 0.02 mmol/L, 100 mL) in a 150 mL quartz reactor. The photocatalytic experiments were carried out under irradiation of a 300 W mercury lamp with a filter glass (λ > 420 nm) to remove UV. Prior to irradiation, the suspensions were stirred in the dark for 2 h to reach adsorption–desorption equilibrium. The absorption spectra of Rh.B were collected on a HITACHI U3010 UV/ Vis spectrophotometer. 3. Results and Discussion 3.1. Characterization Fig. 1(a) shows the XRD patterns of the as-prepared Bi24O31Br10/ BiOI samples (labeled as Bi24O31Br10/BiOI-a, -b, -c and -d with the increase of HI amounts). With increasing HI amounts, a diffraction peak of BiOI (JCPDS card number 10-0445) appears and its intensity increases gradually in the Bi24O31Br10/BiOI, whereas the intensity of Bi24O31Br10 (JCPDS card number 75-0888) peak decreases simultaneously. TEM and high-resolution transmission electron microscopy HRTEM images of Bi24O31Br10/BiOI-b sample are shown in Fig. 1(b). The samples are nanoparticles with a plate-like morphology. It can be seen from HRTEM image that there are two sets of different lattices with d-spaces of 0.228 nm and 0.18 nm, respectively. They corresponds to (004) plane of BiOI and (511) plane of Bi24O31Br10, which are in a good agreement with the XRD results shown in Fig. 1(a).
Fig. 1. (a) XRD patterns of different catalysts labeled as Bi24O31Br10, Bi24O31Br10/BiOI-a, -b, -c and -d with increasing HI amounts; and (b) TEM and HRTEM images of the plate-like Bi24O31Br10/BiOI-b sample.
Please cite this article in press as: Xi Lou, et al., Enhanced Photocatalytic Activity of Bi24O31Br10: Constructing Heterojunction with BiOI, Journal of Materials Science & Technology (2016), doi: 10.1016/j.jmst.2016.05.002
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Fig. 2. De-coloration of Rh.B over different samples: (a) C/C0–time curve; (b) apparent rate constant of different samples; and (c) stability of Bi24O31Br10/BiOI-b in the photocatalytic reaction with five repetitions.
3.2. Evolution of photocatalytic activity The photocatalytic performances of Bi24O31Br10/BiOI samples in de-coloration of Rh.B solution are shown in Fig. 2(a). The apparent rate constant (k) is deduced[20] and shown in Fig. 2(b). The apparent rate constants for Bi24O31Br10, P25, Bi24O31Br10/BiOI-a, -b, -c, and -d are 0.0643, 0.0271, 0.0689, 0.1091, 0.0772 and 0.0195 min−1, respectively. With increasing BiOI amount, the decoloration rate increases first, and then decreases. Bi24O31Br10/BiOI-a, -b, and -c have higher activity than Bi24O31Br10, while Bi24O31Br10/BiOI-d has the lowest photocatalytic efficiency, even lower than P25-N powders. Bi24O31Br10/ BiOI-b shows the highest photocatalytic activity which can decolorize 90% of Rh.B within 30 min. The apparent rate constant of Bi24O31Br10 increases by 70%. However, excessive HI during chemical etching is harmful to photocatalytic reaction. In this case, BiOI will be wrapped around Bi24O31Br10 and hinders Bi24O31Br10 involving photocatalytic reaction. The effective photocatalytic activity of Bi24O31Br10/BiOI-d thus is only contributed by BiOI, which is much weaker than the other samples. It is shown in Fig. 2(c) that the photocatalytic activity of Bi24O31Br10/BiOI-b sample is slightly changed after five runs of photocatalytic reactions, but still remains high de-coloration capacity for Rh.B. This observation indicates that Bi24O31Br10/BiOI-b catalyst is stable under repeated photocatalytic runs.
illumination. Therefore, it can be utilized to investigate the photophysics of excited states generated by photon absorption[21,22]. As shown in Fig. 4(c), a broad peak (in a range of 300–515 nm) in SPV spectrum of Bi24O31Br10 indicates that the surface potential barriers of the Bi24O31Br10 changes before and after irradiation[21–23]. It demonstrates that visible-light absorption of Bi24O31Br10 induces a charge generation and separation. For Bi24O31Br10/BiOI sample, the intensity of surface photovoltage is greatly increased. The photoresponse region is broadened to 550 nm. It indicates the photoinduced electron–hole pairs are separated more effectively compared to that of Bi24O31Br10 under visible light irradiation. 3.4. Mechanism of charge transfer According to previous work[17], conduction band (CB) bottom of BiOI (0.48 eV) lies below that of Bi24O31Br10 (0.15 eV), while valence band (VB) top of BiOI (2.39 eV) is higher than that of Bi24O31Br10 (2.93 eV). The band structure diagram is illustrated in Fig. 5. Under a visible-light illumination with photon energy less than 2.95 eV (λ > 420 nm), electrons in the VB of BiOI could be excited up to a higher potential edge (−0.56 eV), however only up to −0.02 eV in Bi24O31Br10. The reformed CB edge potential of BiOI (−0.56 eV) is more negative than that of Bi24O31Br10 (−0.02 eV) in Bi24O31Br10/BiOI heterojunctions. Hence, photo-induced electrons on the BiOI surface
3.3. Origin of photocatalytic activity Here, we search the origin of the improved photocatalytic activity based on visible light absorption and the separation of photoinduced carriers. Fig. 3 is an UV-Vis diffuse reflection spectrum (DRS) carried out on the samples. The absorption edge of Bi24O31Br10/ BiOI composite shows an obvious red shift from 450 nm to 700 nm when the amount of HI is increased. It attributes to a strong visible light response of BiOI. Accordingly, the color of Bi24O31Br10/BiOI samples darkens gradually. DRS results demonstrate that, with the introduction of BiOI, photo-absorption in visible light region is enhanced greatly over the samples. Fig. 4(b) and (c) shows the current density transient with light on/off for Bi24O31Br10 (Fig. 4(a)) and Bi24O31Br10/BiOI-b (Fig. 4(b)) powders. The photocurrent value of Bi24O31Br10 is 8 × 10−6 mA. After BiOI is incorporated, photocurrent value is enhanced by two orders of magnitude reaching 3 × 10−4 mA. The variation of photocurrent also indicates that photo-induced charge carriers are separated much more effectively due to the introduction of BiOI. The surface photovoltage (SPV) spectroscopy method is a wellestablished contactless and nondestructive technique for semiconductor characterization. The photovoltage signal results from the changes in the surface potential barrier coming from effective separation of photo-induced electron–hole pairs before and after
Fig. 3. UV–Vis diffuse reflectance spectra of different catalysts. The bottom color spots are the photographs of Bi24O31Br10, Bi24O31Br10/BiOI-a, -b, -c and -d samples from left to right.
Please cite this article in press as: Xi Lou, et al., Enhanced Photocatalytic Activity of Bi24O31Br10: Constructing Heterojunction with BiOI, Journal of Materials Science & Technology (2016), doi: 10.1016/j.jmst.2016.05.002
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Fig. 4. Current density transient with light on/off for Bi24O31Br10 (a) and Bi24O31Br10/BiOI-b (b); and (c) surface photovoltage spectroscopies of Bi24O31Br10 and Bi24O31Br10/BiOI-b.
to separate photo-induced charge carriers. This work provides an effective and feasible approach to improve the photocatalytic performance of bismuth-based photocatalysts. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51072012 and 51272015), partially supported by the Ph.D. Research Startup Foundation of Henan Normal University (No. 5101029170290) and the Australian Research Council through a Discovery Project (DP140102581). References
Fig. 5. Band structure diagram of Bi24O31Br10/BiOI heterostructure and charge separation as well as the possible pathway for photodegradation of Rh.B.
may transfer to Bi24O31Br10, and the holes can also move in the opposite direction from the VB of Bi24O31Br10 to VB of BiOI. In such a way, the photo-induced electron–hole pairs could be effectively separated at the interface of Bi24O31Br10/BiOI. Therefore, Bi24O31Br10/ BiOI heterostructures exhibit much better photocatalytic properties than Bi24O31Br10 and BiOI. 4. Conclusion An enhanced photocatalytic activity of Bi24O31Br10 through chemical etching with HI has been reported. Heterostruction between Bi24O31Br10 and BiOI is constructed which favors photo-induced electrons–holes separation. As a result, the apparent rate constant of Bi24O31Br10/BiOI in Rh.B de-coloration test increases by 70% in contrast to Bi24O31Br10. By using HI, it is found that BiOI can be easily formed on bismuth-based compounds. This grown BiOI increased visible-light absorption of bismuth-based compounds and may help
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Please cite this article in press as: Xi Lou, et al., Enhanced Photocatalytic Activity of Bi24O31Br10: Constructing Heterojunction with BiOI, Journal of Materials Science & Technology (2016), doi: 10.1016/j.jmst.2016.05.002
Contents lists available at ScienceDirect
Journal of Materials Science & Technology j o u r n a l h o m e p a g e : w w w. j m s t . o r g
Enhanced Photocatalytic Activity of Bi24O31Br10: Constructing Heterojunction with BiOI Xi Lou 1, Jun Shang 2, Liang Wang 1, Haifeng Feng 3, Weichang Hao 1,3,*, Tianmin Wang 1, Yi Du 3 1
Center of Material Physics and Chemistry, Beihang University, Beijing 100191, China College of Physics and Electronic Engineering, Henan Normal University, Xinxiang 453007, China 3 Institute for Superconducting and Electronic Materials (ISEM), University of Wollongong, Wollongong, NSW 2500, Australia 2
A R T I C L E
I N F O
Article history: Received 21 October 2015 Received in revised form 14 January 2016 Accepted 26 January 2016 Available online Key words: Bi24O31Br10 Hydroiodic acid BiOI Heterojunction Photocatalyst
Bismuth-based compounds have been regarded as an important class of visible-light photocatalysts due to their special electronic structures. In this paper, iodide ions are introduced to modify bismuth-based compound, Bi24O31Br10, forming a Bi24O31Br10/BiOI heterojunction structure. A significant enhancement of photocatalytic activity compared to the parent compounds is observed in de-coloration of rhodamine B (Rh.B) solution. The improved photocatalytic property of Bi24O31Br10/BiOI heterojunction is ascribed to the unique electronic structure consisting of complementary band structures of BiOI and Bi24O31Br10. Iodide ions are regarded as an effective reagent to construct bismuth-based photocatalytic heterojunctions with improved photocatalytic activity. Copyright © 2016, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited.
1. Introduction Photocatalysis is one of the most promising methods for environmental purification and energy conservation[1,2], which possesses advantages such as being environment friendly and low cost. Thus, photocatalyst with high activity attracts growing attention and gains immense research interest. Many strategies, such as doping, heterojunction construction, dye sensitization, facet control, nanoassembly, plasmon enhancement, and graphene-based photocatalysis, have been developed to improve the activity and efficiency of photocatalysts[3–9]. Constructing a heterojunction interface between the semiconductors with matching band potentials has been extensively adopted to effectively enhance the photocatalytic activity by facilitating the separation of photo-induced electron–hole pairs. Recently, bismuth-based compounds are identified as a significant class of photocatalyst due to their unique electronic structures that benefit photocatalytic process. Bi 6s and O 2p levels in bismuthbased compounds can form a preferable hybridized valence band (VB), which meets the requirement of organic oxidation[10]. In addition, the hybridization of the Bi 6s and O 2p levels induces a highly
* Corresponding author. Prof.; Tel.: +86 10 82339306; Fax: +86 10 82317931. E-mail address: [email protected] (W. Hao).
dispersed VB that increases the mobility of photo-induced holes in VB[11] and hence favors oxidation reaction in photocatalysis. Many bismuth-based compounds including BiOX (X = Cl, Br, I)[12,13], BiVO4[10], NaBiO3[14], and bismuth titanate[15] have been reported to demonstrate excellent photocatalytic and photoelectron chemical performances. Very recently, Bi24O31X10 (X = Cl, Br)[16–19] were reported to be novel bismuth oxyhalides with narrower band gap and higher photocatalytic activity compared with BiOX. However, very few studies on Bi24O31X10 heterojunction photocatalytic properties are reported so far. In this work, a novel strategy is demonstrated to enhance the photocatalytic activity of Bi24O31Br10, in which heterojunctions of Bi24O31X10 and BiOI are fabricated by a chemical etching with hydroiodic acid (HI). Iodide ions in HI tend to substitute the bromide ions or oxygen ions in Bi24O31Br10 and bring out BiOI. BiOI, a bismuthbased photocatalyst with a band gap of 1.9 eV, exhibits great ability in separation of the photo-induced electrons and holes under visible light[16]. Photocatalytic activity of such Bi-based photocatalytic heterojunction composites is significantly improved in contrast to Bi24O31Br10. The enhanced photocatalytic activity is attributed to the increased absorption of visible light and the improved separation of photo-induced charge carriers induced by BiOI in the heterojunction, which is evident in a heterojunction structure analysis and photoelectric measurements. HI possibly acts as a reagent
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Please cite this article in press as: Xi Lou, et al., Enhanced Photocatalytic Activity of Bi24O31Br10: Constructing Heterojunction with BiOI, Journal of Materials Science & Technology (2016), doi: 10.1016/j.jmst.2016.05.002
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to introduce BiOI to bismuth-based compounds and to enhance their photocatalytic activity. 2. Experimental 2.1. Preparation of Bi24O31Br10 All reagents were analytical reagents and used as received without further purification. In 50 mL 1.6 mol/L dilute nitric acid, 2.42 g Bi(NO3)3·5H2O was dissolved, and 0.76 g cetyltrimethyl ammonium bromide (CTAB) was dissolved in 200 mL 0.14 mol/L NaOH aqueous solution. The alkali solution was slowly dropped into the acid solution with vigorously magnetic stirring, and a white precipitate formed at the same time. The precipitate was purified by filtration, washed with distilled water several times, and then heated at 500 °C for 2 h. 2.2. Preparation of Bi24O31Br10/BiOI One gram of Bi24O31Br10 was put into 50 mL ethanol and stirred vigorously. Simultaneously, a certain amount of HI was added to the above suspension drop by drop. After 1 h, the precipitate was purified by filtration, washed with distilled water several times, and dried at 80 °C. Samples with 0, 10, 30, 50 and 70 μL HI additions were marked as Bi24O31Br10, Bi24O31Br10/BiOI-a, Bi24O31Br10/BiOI-b, Bi24O31Br10/BiOI-c, and Bi24O31Br10/BiOI-d. TiO2–xNx reference sample was prepared by annealing TiO2 (Degussa P25) under NH3 atmosphere at 500 °C for 8 h. 2.3. Characterization Powder X-ray diffraction (XRD) patterns were collected under a PANalytical X’ Pert Pro X-ray diffractometer using CuKα radiation (0.154 nm), and the working voltage was 40 kV. Ultraviolet visible (UV-Vis) diffuse reflectance spectra were collected on a Cintra10e spectrometer using BaSO4 as the reference sample. Transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) observations were performed with JEM2100 high-resolution transmission electron microscope, using an accelerating voltage of 200 kV. The surface photovoltage (SPV)
spectroscopy apparatus is composed of a source of monochromatic light, a lock-in amplifier (SR830-DSP) with a light chopper (SR540), and a photovoltaic cell. A 500 W xenon lamp (CHFXQ500W, Global xenon lamp power) and a grating monochromator (Omni-5007, No.09010, Zolix) provided monochromatic light. The construction of the photovoltaic cell was a sandwich-like structure of indium tin oxide (ITO)-sample-ITO. In photocurrent-time response system, a 300W Xe lamp with a monochromator and a cutoff filter (λ > 420 nm) was used as the light source, and KEITHLEY 2400 source meter was used to collect the electrical signal. 2.4. De-coloration of rhodamine B dye The catalyst (50 mg) was added into an aqueous solution of rhodamine B (Rh.B, 0.02 mmol/L, 100 mL) in a 150 mL quartz reactor. The photocatalytic experiments were carried out under irradiation of a 300 W mercury lamp with a filter glass (λ > 420 nm) to remove UV. Prior to irradiation, the suspensions were stirred in the dark for 2 h to reach adsorption–desorption equilibrium. The absorption spectra of Rh.B were collected on a HITACHI U3010 UV/ Vis spectrophotometer. 3. Results and Discussion 3.1. Characterization Fig. 1(a) shows the XRD patterns of the as-prepared Bi24O31Br10/ BiOI samples (labeled as Bi24O31Br10/BiOI-a, -b, -c and -d with the increase of HI amounts). With increasing HI amounts, a diffraction peak of BiOI (JCPDS card number 10-0445) appears and its intensity increases gradually in the Bi24O31Br10/BiOI, whereas the intensity of Bi24O31Br10 (JCPDS card number 75-0888) peak decreases simultaneously. TEM and high-resolution transmission electron microscopy HRTEM images of Bi24O31Br10/BiOI-b sample are shown in Fig. 1(b). The samples are nanoparticles with a plate-like morphology. It can be seen from HRTEM image that there are two sets of different lattices with d-spaces of 0.228 nm and 0.18 nm, respectively. They corresponds to (004) plane of BiOI and (511) plane of Bi24O31Br10, which are in a good agreement with the XRD results shown in Fig. 1(a).
Fig. 1. (a) XRD patterns of different catalysts labeled as Bi24O31Br10, Bi24O31Br10/BiOI-a, -b, -c and -d with increasing HI amounts; and (b) TEM and HRTEM images of the plate-like Bi24O31Br10/BiOI-b sample.
Please cite this article in press as: Xi Lou, et al., Enhanced Photocatalytic Activity of Bi24O31Br10: Constructing Heterojunction with BiOI, Journal of Materials Science & Technology (2016), doi: 10.1016/j.jmst.2016.05.002
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Fig. 2. De-coloration of Rh.B over different samples: (a) C/C0–time curve; (b) apparent rate constant of different samples; and (c) stability of Bi24O31Br10/BiOI-b in the photocatalytic reaction with five repetitions.
3.2. Evolution of photocatalytic activity The photocatalytic performances of Bi24O31Br10/BiOI samples in de-coloration of Rh.B solution are shown in Fig. 2(a). The apparent rate constant (k) is deduced[20] and shown in Fig. 2(b). The apparent rate constants for Bi24O31Br10, P25, Bi24O31Br10/BiOI-a, -b, -c, and -d are 0.0643, 0.0271, 0.0689, 0.1091, 0.0772 and 0.0195 min−1, respectively. With increasing BiOI amount, the decoloration rate increases first, and then decreases. Bi24O31Br10/BiOI-a, -b, and -c have higher activity than Bi24O31Br10, while Bi24O31Br10/BiOI-d has the lowest photocatalytic efficiency, even lower than P25-N powders. Bi24O31Br10/ BiOI-b shows the highest photocatalytic activity which can decolorize 90% of Rh.B within 30 min. The apparent rate constant of Bi24O31Br10 increases by 70%. However, excessive HI during chemical etching is harmful to photocatalytic reaction. In this case, BiOI will be wrapped around Bi24O31Br10 and hinders Bi24O31Br10 involving photocatalytic reaction. The effective photocatalytic activity of Bi24O31Br10/BiOI-d thus is only contributed by BiOI, which is much weaker than the other samples. It is shown in Fig. 2(c) that the photocatalytic activity of Bi24O31Br10/BiOI-b sample is slightly changed after five runs of photocatalytic reactions, but still remains high de-coloration capacity for Rh.B. This observation indicates that Bi24O31Br10/BiOI-b catalyst is stable under repeated photocatalytic runs.
illumination. Therefore, it can be utilized to investigate the photophysics of excited states generated by photon absorption[21,22]. As shown in Fig. 4(c), a broad peak (in a range of 300–515 nm) in SPV spectrum of Bi24O31Br10 indicates that the surface potential barriers of the Bi24O31Br10 changes before and after irradiation[21–23]. It demonstrates that visible-light absorption of Bi24O31Br10 induces a charge generation and separation. For Bi24O31Br10/BiOI sample, the intensity of surface photovoltage is greatly increased. The photoresponse region is broadened to 550 nm. It indicates the photoinduced electron–hole pairs are separated more effectively compared to that of Bi24O31Br10 under visible light irradiation. 3.4. Mechanism of charge transfer According to previous work[17], conduction band (CB) bottom of BiOI (0.48 eV) lies below that of Bi24O31Br10 (0.15 eV), while valence band (VB) top of BiOI (2.39 eV) is higher than that of Bi24O31Br10 (2.93 eV). The band structure diagram is illustrated in Fig. 5. Under a visible-light illumination with photon energy less than 2.95 eV (λ > 420 nm), electrons in the VB of BiOI could be excited up to a higher potential edge (−0.56 eV), however only up to −0.02 eV in Bi24O31Br10. The reformed CB edge potential of BiOI (−0.56 eV) is more negative than that of Bi24O31Br10 (−0.02 eV) in Bi24O31Br10/BiOI heterojunctions. Hence, photo-induced electrons on the BiOI surface
3.3. Origin of photocatalytic activity Here, we search the origin of the improved photocatalytic activity based on visible light absorption and the separation of photoinduced carriers. Fig. 3 is an UV-Vis diffuse reflection spectrum (DRS) carried out on the samples. The absorption edge of Bi24O31Br10/ BiOI composite shows an obvious red shift from 450 nm to 700 nm when the amount of HI is increased. It attributes to a strong visible light response of BiOI. Accordingly, the color of Bi24O31Br10/BiOI samples darkens gradually. DRS results demonstrate that, with the introduction of BiOI, photo-absorption in visible light region is enhanced greatly over the samples. Fig. 4(b) and (c) shows the current density transient with light on/off for Bi24O31Br10 (Fig. 4(a)) and Bi24O31Br10/BiOI-b (Fig. 4(b)) powders. The photocurrent value of Bi24O31Br10 is 8 × 10−6 mA. After BiOI is incorporated, photocurrent value is enhanced by two orders of magnitude reaching 3 × 10−4 mA. The variation of photocurrent also indicates that photo-induced charge carriers are separated much more effectively due to the introduction of BiOI. The surface photovoltage (SPV) spectroscopy method is a wellestablished contactless and nondestructive technique for semiconductor characterization. The photovoltage signal results from the changes in the surface potential barrier coming from effective separation of photo-induced electron–hole pairs before and after
Fig. 3. UV–Vis diffuse reflectance spectra of different catalysts. The bottom color spots are the photographs of Bi24O31Br10, Bi24O31Br10/BiOI-a, -b, -c and -d samples from left to right.
Please cite this article in press as: Xi Lou, et al., Enhanced Photocatalytic Activity of Bi24O31Br10: Constructing Heterojunction with BiOI, Journal of Materials Science & Technology (2016), doi: 10.1016/j.jmst.2016.05.002
ARTICLE IN PRESS X. Lou et al. / Journal of Materials Science & Technology ■■ (2016) ■■–■■
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Fig. 4. Current density transient with light on/off for Bi24O31Br10 (a) and Bi24O31Br10/BiOI-b (b); and (c) surface photovoltage spectroscopies of Bi24O31Br10 and Bi24O31Br10/BiOI-b.
to separate photo-induced charge carriers. This work provides an effective and feasible approach to improve the photocatalytic performance of bismuth-based photocatalysts. Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 51072012 and 51272015), partially supported by the Ph.D. Research Startup Foundation of Henan Normal University (No. 5101029170290) and the Australian Research Council through a Discovery Project (DP140102581). References
Fig. 5. Band structure diagram of Bi24O31Br10/BiOI heterostructure and charge separation as well as the possible pathway for photodegradation of Rh.B.
may transfer to Bi24O31Br10, and the holes can also move in the opposite direction from the VB of Bi24O31Br10 to VB of BiOI. In such a way, the photo-induced electron–hole pairs could be effectively separated at the interface of Bi24O31Br10/BiOI. Therefore, Bi24O31Br10/ BiOI heterostructures exhibit much better photocatalytic properties than Bi24O31Br10 and BiOI. 4. Conclusion An enhanced photocatalytic activity of Bi24O31Br10 through chemical etching with HI has been reported. Heterostruction between Bi24O31Br10 and BiOI is constructed which favors photo-induced electrons–holes separation. As a result, the apparent rate constant of Bi24O31Br10/BiOI in Rh.B de-coloration test increases by 70% in contrast to Bi24O31Br10. By using HI, it is found that BiOI can be easily formed on bismuth-based compounds. This grown BiOI increased visible-light absorption of bismuth-based compounds and may help
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Please cite this article in press as: Xi Lou, et al., Enhanced Photocatalytic Activity of Bi24O31Br10: Constructing Heterojunction with BiOI, Journal of Materials Science & Technology (2016), doi: 10.1016/j.jmst.2016.05.002