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black-BiOCl heterojunction
Chemical Physics Letters 674 (2017) 130–135
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Enhancement of photocatalytic activity over Bi2O3/black-BiOCl heterojunction Dahye Kim, Dongwoon Jung ⇑ Department of Chemistry, Wonkwang University, Iksan, Jeonbuk 570-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 3 January 2017 In final form 12 February 2017 Available online 21 February 2017 Keywords: BiOCl Bi2O3 Black-BiOCl Heterojunction Photocatalytic activity
a b s t r a c t Several Bi2O3/BiOCl heterojunction compounds with different Bi2O3/BiOCl ratios were prepared by treating Bi2O3 with HCl. Within the Bi2O3/BiOCl heterojunction, white BiOCl was turned into black by thermal treatment. Upon the result, Bi2O3/black-BiOCl heterojunction could be prepared. The photocatalytic activities of samples were tested depending upon the Bi2O3/BiOCl ratio. Basically, Bi2O3/black-BiOCl samples showed advanced photocatalytic activity compared with the original Bi2O3/white-BiOCl. The highest photocatalytic efficiency was found in the Bi2O3/black-BiOCl when Bi2O3/BiOCl ratio was 15/85. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Bismuth oxychloride (BiOCl) has attracted a lot of attention because of its superior photocatalytic activity compared with P25 to degrade organic compounds under UV irradiation [1–10]. The reduced recombination probability of the excited electron and the hole, originated from the indirect band gap of this material, is known to be an important factor for the excellent photocatalytic activity. The relatively large band gap of 3.4 eV for BiOCl, however, limits the wide application of this compound as a photocatalyst since it is activated only under UV irradiation. Similarly with TiO2, many ideas to improve the photocatalytic activity of BiOCl under visible light have been proposed and tested [11–13]. BiOCl/Bi2O3 heterojunction has been one of the most attractive combinations as a BiOCl containing photocatalyst working under visible light [12]. The photocatalytic activity of this heterojunction exceeded more than 10 times that of P-25 under visible light. In this heterojunction, BiOCl acts as a main photocatalyst while Bi2O3 as a sensitizer to absorb visible light. A BiOCl/Bi2O3 heterojunction could be prepared from Bi2O3 by treating this compound with HCl, and the Bi2O3/BiOCl ratio could be easily controlled by changing the amount of HCl. NaBiO3/BiOCl composite photocatalysts with heterostructure have been also prepared by treating NaBiO3 with HCl [13]. The composite have shown enhanced photocatalytic activity compared with single compound.
⇑ Corresponding author. E-mail address: [email protected] (D. Jung). http://dx.doi.org/10.1016/j.cplett.2017.02.034 0009-2614/Ó 2017 Elsevier B.V. All rights reserved.
Recently, Ye et al. reported the preparation and physical properties of black-BiOCl whose photocatalytic efficiency to degrade RhB under visible light was 20 times higher than that of original white BiOCl [14,15]. Black-BiOCl was prepared from white sample by UV light irradiation with Ar blowing. The color change from white to black is originated from the decrease of the band gap of this compound. Consequently, the resulting black compound could absorb visible light and became a photocatalyst working under visible light. The EPR experiment showed that the black color of the sample was originated from the oxygen vacancies and the amount of oxygen vacancies could be controlled by changing the UV irradiation time. The intrinsic semiconductors with oxygen vacancy can absorb visible light and are known to exhibit advanced photocatalytic activity under visible light [16–18]. After the first report to prepare black-BiOCl, a visible light irradiation on carboxylic acid treated BiOBr [19], in-situ combustion method to construct surface oxygen vacancy on BiOX [20,21], and several other results to prepare and to characterize black-BiOX by different methods have been proposed [22,23]. Our preliminary experiments showed that thermal energy also could cause the removal of oxygen from white BiOCl. A black-BiOCl, therefore, could be generated by a simple heating instead of UV irradiation. According to the previous results, consequently, the new heterojunction Bi2O3/black-BiOCl can be prepared from Bi2O3 very easily in two steps; first, treat Bi2O3 with certain amount of HCl and second, heat the sample for appropriate time. This paper reports on the preparation of a Bi2O3/black-BiOCl heterojunction material from Bi2O3. The photocatalytic activities of various Bi2O3/black-BiOCl heterojunction samples are
D. Kim, D. Jung / Chemical Physics Letters 674 (2017) 130–135
investigated by analyzing the photocatalytic efficiencies under visible light irradiation. 2. Experimental 2.1. Materials and apparatus Bi2O3 (99.99%, Aldrich, Seoul, Korea), HCl (35.0–37.0%, Samchun, Yeosu, Korea), ethanol (95%, Samchun, Yeosu, Korea) were used for the preparation of Bi2O3/black-BiOCl heterojunction. All chemicals were used as purchased without further purification. A UV–vis spectrophotometer (Shimadtzu, Kyoto, Japan) and an Xray diffractometer (Rigaku, Tokyo, Japan) were employed for structure identification and absorbance analysis of the samples, respectively. The morphology of the sample was monitored with a scanning electron microscopy (SEM, Hitachi, S-4800, Ibaraki, Japan). X-band continuous-wave EPR analysis was performed by using a Bruker EMX Plus 6/1 spectrometer equipped with a dualmode cavity (ER 4116DM, Bruker, Billerica, Massachusetts, U.S. A.). The photocatalytic activity of the sample under visible light was tested by irradiation with a Xenon lamp (300 W, Excelitas Tech., Fremont, U.S.A.) 2.2. The preparation and characterization of Bi2O3/black-BiOCl heterojunction 1 g of yellow Bi2O3 was dispersed into 10 mL ethanol with gentle stirring. To prepare the samples with different Bi2O3/BiOCl ratios, appropriate amount of concentrated HCl was added drop by drop into the solution with vigorous stirring. After the addition of HCl, the solution was kept at room temperature for 3 h with stirring. Then, the solution was sonicated for 1 h. The color of the powder changed from yellow to white, meaning that some amount of BiOCl was formed. The resulting Bi2O3/BiOCl heterojunction was centrifuged and dried in a drying oven at 60 °C for 1 day. Finally the pale yellow Bi2O3/BiOCl heterojunction was obtained. After thorough grinding, the powder was moved to an alumina crucible and allowed to stand for 1–5 h at a constant temperature of 300 °C. The pale yellow powder turned into grey since white-BiOCl was changed to black-BiOCl, resulting in the formation of the Bi2O3/
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black-BiOCl heterojunction. The photocatalytic activities of the samples with different Bi2O3/BiOCl ratio were investigated by changing the ratio as 95/5, 75/25, 45/55, and 15/85. The crystal structure of each heterojunction product was characterized with XRD within the two theta range of 20–80°. The morphologies of the samples were investigated with SEM. Reflectance UV spectroscopy was used to measure the absorbance of light by the Bi2O3/BiOCl heterojunction samples in the range of 200–800 nm. 2.3. Measurements of photocatalytic activity Photocatalytic activities of Bi2O3/black-BiOCl samples with different Bi2O3/BiOCl ratios and P-25(Degussa) were tested by measuring the decomposed amount of 1,4-dichlorobenzene under visible light. For the test, 1,4-dichlorobenzene was dissolved in ethylalcohol and this solution was added evenly into each flask which is filled with water. Powder samples were dispersed separately into each flask, and the solutions were stirred for 20 min. in the dark so that the particles could absorb the 1,4dichlorobenzene. The initial concentration of 1,4-dichlorobenzene was measured with a UV–vis spectrometer, after the solution was thoroughly mixed. The sample vessels were placed under the xenon lamp for 2 h with UV filter. During the irradiation process, the solutions were withdrawn with a syringe and centrifuged to remove the particles. The degradation process was monitored as a function of irradiation time by measuring the absorption spectra of the samples at 220 nm with UV–vis absorption spectroscopy. 3. Results and discussion The XRD pattern of Bi2O3 is shown in Fig. 1a. Sharp peaks at typical 2h positions of 27°, 33°, and 46° confirm that the compound is pure Bi2O3. Fig. 1b shows the XRD pattern of BiOCl prepared from Bi2O3 by treating with HCl. Two main peaks at 26° and 33° confirm that BiOCl is successfully synthesized from Bi2O3 without any second phase. The XRD pattern for black-BiOCl which was made from white-BiOCl by heat treatment is shown in Fig. 1c. The XRD pattern of black-BiOCl is identical with that of white-BiOCl, which means that the basic structures of both compounds are not changed during heat treatment. This result is consistent with the previous
Fig. 1. XRD patterns of (a) Bi2O3, (b) BiOCl and (c) black-BiOCl.
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report that structures of white and black BiOCl are same although some amount of oxygen was removed in black-BiOCl [14,15]. Fig. 2a and b shows the XRD patterns of Bi2O3/white-BiOCl and Bi2O3/black-BiOCl heterojunction samples, respectively, with the Bi2O3/BiOCl ratio of 45/55. All typical peaks representing Bi2O3 and BiOCl are shown at the special 2h positions. No peaks corresponding to a second phase were detected. This means that Bi2O3/BiOCl and Bi2O3/black-BiOCl heterojunction complexes have been successfully prepared.
Similarly, Bi2O3/white-BiOCl and Bi2O3/black-BiOCl heterojunction samples with the Bi2O3/BiOCl ratio of 15/85 are shown in Fig. 3a and b, respectively. The peaks representing Bi2O3 at 27° and 33° become weaker as the mole ratio of Bi2O3 is decreases, so that the peaks representing Bi2O3 are barely recognizable when the Bi2O3/BiOCl ratio is 15/85. The low intensity of the Bi2O3 peaks can be explained by the fact that the amount of BiOCl overwhelms that of Bi2O3 as the Bi2O3/BiOCl ratio decreases. The XRD patterns of Bi2O3/white-BiOCl and Bi2O3/black-BiOCl heterojunction sam-
Fig. 2. XRD patterns of (a) Bi2O3/white-BiOCl and (b) Bi2O3/black-BiOCl with the Bi2O3/BiOCl ratio of 45/55.
Fig. 3. XRD patterns of (a) Bi2O3/white-BiOCl and (b) Bi2O3/black-BiOCl with the Bi2O3/BiOCl ratio of 15/85.
Fig. 4. The color of each sample; from left to right (a) Bi2O3/white-BiOCl with the Bi2O3/BiOCl ratio of 100/0, 95/5, 75/25, 45/55, 15/85 and 0/100 (b) Bi2O3/black-BiOCl with the Bi2O3/BiOCl ratio of 100/0, 95/5, 75/25, 45/55, 15/85 and 0/100. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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with the Bi2O3/BiOCl ratios of 95/5, 75/25, 45/55, and 15/85 shown in Fig. 7a represent the stepwise pattern corresponding to the ratio, as expected. The spectra for black-BiOCl, and Bi2O3/black-BiOCl heterojunction samples with different Bi2O3/BiOCl ratios are shown in Fig. 7b. The absorption spectrum of black-BiOCl is somewhat different from that of white-BiOCl, in that the absorption occurs in the whole visible range. This means that the band gap of BiOCl is reduced when the oxygen vacancy is occurred by the heat treatment. Consequently, Bi2O3/black-BiOCl heterojunction samples also absorb visible light more easily. In the Bi2O3/BiOCl heterojunction, it is considered that BiOCl behaves as a main photocatalyst, while Bi2O3 as a sensitizer absorbing visible light. The reduced band gap of black-BiOCl can accelerate the photocatalytic reaction of Bi2O3/black-BiOCl heterojunction samples by pumping up the electrons from the valence band to the conduction band more easily. In the Bi2O3/BiOCl heterojunction, the BiOCl and Bi2O3 are tightly bound each other in nanosize level, since Bi2O3
g = 1.999
Intensity
ples with the Bi2O3/BiOCl ratio of 25/75 and 95/5 are not shown here. The color of each sample having different Bi2O3/BiOCl ratio as 100/0, 95/5, 75/25, 45/55, 15/85, and 0/100 is shown in Fig. 4a. It is yellow when the sample is pure Bi2O3, and it is turned into white gradually depending upon the amount of BiOCl formed by treating with HCl. After the heat treatment at 300 °C for 5 h, all samples except pure Bi2O3 become darker because of the black-BiOCl formation, as shown in Fig. 4b. The color change illustrates that black-BiOCl can be formed by just heat treatment and a Bi2O3/ black-BiOCl heterojunction can be successfully prepared from a Bi2O3/BiOCl complex. SEM images of Bi2O3 and Bi2O3/BiOCl heterojunction are shown in Fig. 5. Large Bi2O3 particle is partially changed to BiOCl and forms Bi2O3/BiOCl heterojunction after the treatment with HCl. The size of the particle in heterojunction is getting smaller and smaller as Bi2O3 turned into BiOCl. Fig. 6 shows the EPR spectrum of black-BiOCl which is prepared by thermal treatment of BiOCl. It was previously reported that the EPR spectrum shows remarkable signal at g = 2.001 when BiOCl becomes black by irradiation with UV light. This signal at g = 2.001 is originated from the oxygen vacancy in black-BiOCl [14]. The EPR result of our samples is exactly same that of previously reported black-BiOCl in that it has signal at g = 1.999. This EPR spectrum result proves that the oxygen vacancy in BiOCl can be occurred not only by UV irradiation but also by heat treatment. The UV–visible diffuse reflectance spectra for Bi2O3, whiteBiOCl, and Bi2O3/white-BiOCl heterojunction samples with different Bi2O3/BiOCl ratios are shown in Fig. 7a. In the case of pure Bi2O3, the absorption edge appearing near 480 nm, which is corresponding to the band gap of 2.6 eV, is consistent with literature. Likewise for white-BiOCl, the absorption edge appearing near 380 nm represents the band gap of 3.4 eV, which is also consistent with the previous data. The spectra for heterojunction samples
Fig. 6. The EPR spectrum of black-BiOCl.
Fig. 5. SEM images of (a) Bi2O3, (b) Bi2O3/BiOCl heterojunction with the Bi2O3/BiOCl ratio of 45/55, (c) Bi2O3/BiOCl heterojunction with the Bi2O3/BiOCl ratio of 15/85 and (d) BiOCl.
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100
White-BiOCl 15/85 45/55 75/25 95/5 Bi2O3
0
200
400
600
800
(b)
Reflectance (%)
(a)
Reflectance (%)
100
Black-BiOCl 15/85 45/55 75/25 95/5 Bi2O3
0
200
400
Wavelength (nm)
600
800
Wavelength (nm)
Fig. 7. UV–vis spectra of (a) Bi2O3/white-BiOCl and (b) Bi2O3/black-BiOCl. Table 1 The degradation of 1,4-dichlorobenzene in the solution with different catalyst sample after irradiation of visible light.
P-25 Bi2O3 BiOCl 15/85 Bi2O3/BiOCl 45/55 Bi2O3/BiOCl 75/25 Bi2O3/BiOCl
white black white black white black white black
30 min
60
90
120
5.27 10.12 5.03 11.71 12.00 20.12 12.07 13.82 10.99 9.16
6.86 20.24 6.41 24.17 22.67 37.31 21.95 27.76 21.14 25.27
8.51 28.91 8.64 32.82 33.07 52.32 31.46 40.44 31.29 39.07
9.53 38.37 9.72 43.76 43.47 64.55 43.52 51.03 41.44 51.28
has been partially converted to BiOCl by HCl. Therefore, the electron transfer through the junction will be greatly efficient. The photocatalytic efficiencies of Bi2O3, white-BiOCl, blackBiOCl, Bi2O3/white-BiOCl, and Bi2O3/black-BiOCl heterojunction samples to remove 1,4-dichlorobenzene in the solution under visible light are summarized in Table 1. The photocatalytic efficiency of commercial P-25 is also measured as a reference for comparison. Black-BiOCl shows much higher activity under visible light than white-BiOCl, which is consistent with the previous results. Generally, the photocatalytic activities of Bi2O3/BiOCl heterojunction samples are higher than those of singly used Bi2O3, black-BiOCl. As shown in Table 1, Bi2O3/black-BiOCl heterojunction samples exhibit higher photocatalytic activity than Bi2O3/white-BiOCl heterojunction samples. The highest activity was detected when Bi2O3/black-BiOCl heterojunction was used as a catalyst with the Bi2O3/BiOCl ratio of 15/85. To verify the stability of the sample, we recovered the sample after the photocatalytic experiment. The XRD and photocatalytic efficiency data of the recovered sample showed same results compared with the original sample. There are two types of photocatalysts; first is a reductive photocatalyst that can release electrons from the conduction band. A sensitizer/TiO2 heterojunction is a typical example of this type. The excited electrons in the conduction band of a sensitizer are transferred to that of TiO2, and these electrons are released to reduce organic compounds. Second is an oxidative photocatalyst that can produce holes in the valence band. In Bi2O3/BiOCl heterojunction, BiOCl is known to be the main catalyst. Under visible light, electrons are excited from the valence band to conduction band of Bi2O3. Then, electrons in the valence band of BiOCl are transferred to that of Bi2O3, thereby forming holes in the valence band of BiOCl. These holes oxidize organic molecules. The bandgap of black BiOCl is reduced so that black-BiOCl itself can generate holes in the valence band under visible light. In blackBiOCl/Bi2O3 heterojunction, therefore, the number of holes might be doubled in the valence band of BiOCl because of the self-
generation by excitation and the electron transfer to Bi2O3. In addition, the electrons in the conduction band of BiOCl may play an important role to decompose organic compounds. The outstanding efficiency of black-BiOCl/Bi2O3 sample can be explained upon the explanation. The detailed mechanism should be searched further. 4. Conclusion Bi2O3/BiOCl heterojunction compounds with different Bi2O3/ BiOCl ratios were prepared by treating Bi2O3 with HCl, as reported earlier. Within the Bi2O3/BiOCl heterojunction, white BiOCl was turned into black by simple thermal treatment. Upon the result, Bi2O3/black-BiOCl heterojunction was successfully prepared. The EPR spectrum showed clear proof of the formation of oxygen vacant black-BiOCl after the heat treatment. UV–vis spectrum of black-BiOCl revealed that the absorption occurred in the visible range. This result was originated from the oxygen vacancy in the black-BiOCl. The photocatalytic activities of heterojunction samples were higher than that of single compound. In addition, Bi2O3/black-BiOCl samples showed advanced photocatalytic activity compared with the original Bi2O3/white-BiOCl. The highest photocatalytic efficiency was found in the Bi2O3/black-BiOCl when Bi2O3/BiOCl ratio was 15/85. Acknowledgement This work was supported by Wonkwang University Research Grant in 2015. References [1] R. Ghosh, S. Maiti, A. Chakraborty, Tetrahedron Lett. 45 (2004) 6775. [2] W.D. Wang, F.Q. Huang, X.P. Lin, Scr. Mater. 56 (2007) 669. [3] K.L. Zhang, C.M. Liu, F.Q. Huang, C. Zheng, W.D. Wang, Appl. Catal. B 68 (2006) 125. [4] Z.Q. Shi, Y. Wang, C.M. Fan, Y.F. Wang, G.Y. Ding, Trans. Nonferrous Met. Soc. China 21 (2011) 2254.
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[15] Liqun Ye, Kejian Deng, Xu Feng, Lihong Tian, Tianyou Peng, Ling Zan, Phys. Chem. Chem. Phys. 14 (2012) 82–85. [16] I. Nakamura, N. Negishi, S. Kutsuna, T. Ihara, S. Sugihara, K. Takeuchi, J. Mol. Catal. A: Chem. 161 (2000) 205. [17] T. Ihara, M. Miyoshi, Y. Iriyama, O. Matsumoto, S. Sugihara, Appl. Catal., B 42 (2003) 403. [18] X. Chen, L. Liu, P.Y. Yu, S.S. Mao, Science 331 (2011) 746. [19] X.J. Wang, Y. Zhao, F.T. Li, L.J. Dou, Y.P. Li, J. Zhao, Y.J. Hao, Sci. Rep. 6 (2016) 24918. [20] F.T. Li, Y.L. Li, M.J. Chai, B. Li, Y.J. Hao, X.J. Wang, R.H. Liu, Catal. Sci. Technol. 6 (2016) 7985. [21] F.T. Li, Q. Wang, J. Ran, Y.J. Hao, X.J. Wang, D. Zhao, S.Z. Qiao, Nanoscale 7 (2015) 1116. [22] J. Cheng, C. Wang, Y. Cui, Y. Sun, Y. Zuo, T. Wang, Mater. Lett. 127 (2014) 28. [23] Y. Li, C. Li, X. Sun, Z. Zhang, Z. Peng, Mater. Lett. 116 (2014) 98.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Enhancement of photocatalytic activity over Bi2O3/black-BiOCl heterojunction Dahye Kim, Dongwoon Jung ⇑ Department of Chemistry, Wonkwang University, Iksan, Jeonbuk 570-749, Republic of Korea
a r t i c l e
i n f o
Article history: Received 3 January 2017 In final form 12 February 2017 Available online 21 February 2017 Keywords: BiOCl Bi2O3 Black-BiOCl Heterojunction Photocatalytic activity
a b s t r a c t Several Bi2O3/BiOCl heterojunction compounds with different Bi2O3/BiOCl ratios were prepared by treating Bi2O3 with HCl. Within the Bi2O3/BiOCl heterojunction, white BiOCl was turned into black by thermal treatment. Upon the result, Bi2O3/black-BiOCl heterojunction could be prepared. The photocatalytic activities of samples were tested depending upon the Bi2O3/BiOCl ratio. Basically, Bi2O3/black-BiOCl samples showed advanced photocatalytic activity compared with the original Bi2O3/white-BiOCl. The highest photocatalytic efficiency was found in the Bi2O3/black-BiOCl when Bi2O3/BiOCl ratio was 15/85. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Bismuth oxychloride (BiOCl) has attracted a lot of attention because of its superior photocatalytic activity compared with P25 to degrade organic compounds under UV irradiation [1–10]. The reduced recombination probability of the excited electron and the hole, originated from the indirect band gap of this material, is known to be an important factor for the excellent photocatalytic activity. The relatively large band gap of 3.4 eV for BiOCl, however, limits the wide application of this compound as a photocatalyst since it is activated only under UV irradiation. Similarly with TiO2, many ideas to improve the photocatalytic activity of BiOCl under visible light have been proposed and tested [11–13]. BiOCl/Bi2O3 heterojunction has been one of the most attractive combinations as a BiOCl containing photocatalyst working under visible light [12]. The photocatalytic activity of this heterojunction exceeded more than 10 times that of P-25 under visible light. In this heterojunction, BiOCl acts as a main photocatalyst while Bi2O3 as a sensitizer to absorb visible light. A BiOCl/Bi2O3 heterojunction could be prepared from Bi2O3 by treating this compound with HCl, and the Bi2O3/BiOCl ratio could be easily controlled by changing the amount of HCl. NaBiO3/BiOCl composite photocatalysts with heterostructure have been also prepared by treating NaBiO3 with HCl [13]. The composite have shown enhanced photocatalytic activity compared with single compound.
⇑ Corresponding author. E-mail address: [email protected] (D. Jung). http://dx.doi.org/10.1016/j.cplett.2017.02.034 0009-2614/Ó 2017 Elsevier B.V. All rights reserved.
Recently, Ye et al. reported the preparation and physical properties of black-BiOCl whose photocatalytic efficiency to degrade RhB under visible light was 20 times higher than that of original white BiOCl [14,15]. Black-BiOCl was prepared from white sample by UV light irradiation with Ar blowing. The color change from white to black is originated from the decrease of the band gap of this compound. Consequently, the resulting black compound could absorb visible light and became a photocatalyst working under visible light. The EPR experiment showed that the black color of the sample was originated from the oxygen vacancies and the amount of oxygen vacancies could be controlled by changing the UV irradiation time. The intrinsic semiconductors with oxygen vacancy can absorb visible light and are known to exhibit advanced photocatalytic activity under visible light [16–18]. After the first report to prepare black-BiOCl, a visible light irradiation on carboxylic acid treated BiOBr [19], in-situ combustion method to construct surface oxygen vacancy on BiOX [20,21], and several other results to prepare and to characterize black-BiOX by different methods have been proposed [22,23]. Our preliminary experiments showed that thermal energy also could cause the removal of oxygen from white BiOCl. A black-BiOCl, therefore, could be generated by a simple heating instead of UV irradiation. According to the previous results, consequently, the new heterojunction Bi2O3/black-BiOCl can be prepared from Bi2O3 very easily in two steps; first, treat Bi2O3 with certain amount of HCl and second, heat the sample for appropriate time. This paper reports on the preparation of a Bi2O3/black-BiOCl heterojunction material from Bi2O3. The photocatalytic activities of various Bi2O3/black-BiOCl heterojunction samples are
D. Kim, D. Jung / Chemical Physics Letters 674 (2017) 130–135
investigated by analyzing the photocatalytic efficiencies under visible light irradiation. 2. Experimental 2.1. Materials and apparatus Bi2O3 (99.99%, Aldrich, Seoul, Korea), HCl (35.0–37.0%, Samchun, Yeosu, Korea), ethanol (95%, Samchun, Yeosu, Korea) were used for the preparation of Bi2O3/black-BiOCl heterojunction. All chemicals were used as purchased without further purification. A UV–vis spectrophotometer (Shimadtzu, Kyoto, Japan) and an Xray diffractometer (Rigaku, Tokyo, Japan) were employed for structure identification and absorbance analysis of the samples, respectively. The morphology of the sample was monitored with a scanning electron microscopy (SEM, Hitachi, S-4800, Ibaraki, Japan). X-band continuous-wave EPR analysis was performed by using a Bruker EMX Plus 6/1 spectrometer equipped with a dualmode cavity (ER 4116DM, Bruker, Billerica, Massachusetts, U.S. A.). The photocatalytic activity of the sample under visible light was tested by irradiation with a Xenon lamp (300 W, Excelitas Tech., Fremont, U.S.A.) 2.2. The preparation and characterization of Bi2O3/black-BiOCl heterojunction 1 g of yellow Bi2O3 was dispersed into 10 mL ethanol with gentle stirring. To prepare the samples with different Bi2O3/BiOCl ratios, appropriate amount of concentrated HCl was added drop by drop into the solution with vigorous stirring. After the addition of HCl, the solution was kept at room temperature for 3 h with stirring. Then, the solution was sonicated for 1 h. The color of the powder changed from yellow to white, meaning that some amount of BiOCl was formed. The resulting Bi2O3/BiOCl heterojunction was centrifuged and dried in a drying oven at 60 °C for 1 day. Finally the pale yellow Bi2O3/BiOCl heterojunction was obtained. After thorough grinding, the powder was moved to an alumina crucible and allowed to stand for 1–5 h at a constant temperature of 300 °C. The pale yellow powder turned into grey since white-BiOCl was changed to black-BiOCl, resulting in the formation of the Bi2O3/
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black-BiOCl heterojunction. The photocatalytic activities of the samples with different Bi2O3/BiOCl ratio were investigated by changing the ratio as 95/5, 75/25, 45/55, and 15/85. The crystal structure of each heterojunction product was characterized with XRD within the two theta range of 20–80°. The morphologies of the samples were investigated with SEM. Reflectance UV spectroscopy was used to measure the absorbance of light by the Bi2O3/BiOCl heterojunction samples in the range of 200–800 nm. 2.3. Measurements of photocatalytic activity Photocatalytic activities of Bi2O3/black-BiOCl samples with different Bi2O3/BiOCl ratios and P-25(Degussa) were tested by measuring the decomposed amount of 1,4-dichlorobenzene under visible light. For the test, 1,4-dichlorobenzene was dissolved in ethylalcohol and this solution was added evenly into each flask which is filled with water. Powder samples were dispersed separately into each flask, and the solutions were stirred for 20 min. in the dark so that the particles could absorb the 1,4dichlorobenzene. The initial concentration of 1,4-dichlorobenzene was measured with a UV–vis spectrometer, after the solution was thoroughly mixed. The sample vessels were placed under the xenon lamp for 2 h with UV filter. During the irradiation process, the solutions were withdrawn with a syringe and centrifuged to remove the particles. The degradation process was monitored as a function of irradiation time by measuring the absorption spectra of the samples at 220 nm with UV–vis absorption spectroscopy. 3. Results and discussion The XRD pattern of Bi2O3 is shown in Fig. 1a. Sharp peaks at typical 2h positions of 27°, 33°, and 46° confirm that the compound is pure Bi2O3. Fig. 1b shows the XRD pattern of BiOCl prepared from Bi2O3 by treating with HCl. Two main peaks at 26° and 33° confirm that BiOCl is successfully synthesized from Bi2O3 without any second phase. The XRD pattern for black-BiOCl which was made from white-BiOCl by heat treatment is shown in Fig. 1c. The XRD pattern of black-BiOCl is identical with that of white-BiOCl, which means that the basic structures of both compounds are not changed during heat treatment. This result is consistent with the previous
Fig. 1. XRD patterns of (a) Bi2O3, (b) BiOCl and (c) black-BiOCl.
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report that structures of white and black BiOCl are same although some amount of oxygen was removed in black-BiOCl [14,15]. Fig. 2a and b shows the XRD patterns of Bi2O3/white-BiOCl and Bi2O3/black-BiOCl heterojunction samples, respectively, with the Bi2O3/BiOCl ratio of 45/55. All typical peaks representing Bi2O3 and BiOCl are shown at the special 2h positions. No peaks corresponding to a second phase were detected. This means that Bi2O3/BiOCl and Bi2O3/black-BiOCl heterojunction complexes have been successfully prepared.
Similarly, Bi2O3/white-BiOCl and Bi2O3/black-BiOCl heterojunction samples with the Bi2O3/BiOCl ratio of 15/85 are shown in Fig. 3a and b, respectively. The peaks representing Bi2O3 at 27° and 33° become weaker as the mole ratio of Bi2O3 is decreases, so that the peaks representing Bi2O3 are barely recognizable when the Bi2O3/BiOCl ratio is 15/85. The low intensity of the Bi2O3 peaks can be explained by the fact that the amount of BiOCl overwhelms that of Bi2O3 as the Bi2O3/BiOCl ratio decreases. The XRD patterns of Bi2O3/white-BiOCl and Bi2O3/black-BiOCl heterojunction sam-
Fig. 2. XRD patterns of (a) Bi2O3/white-BiOCl and (b) Bi2O3/black-BiOCl with the Bi2O3/BiOCl ratio of 45/55.
Fig. 3. XRD patterns of (a) Bi2O3/white-BiOCl and (b) Bi2O3/black-BiOCl with the Bi2O3/BiOCl ratio of 15/85.
Fig. 4. The color of each sample; from left to right (a) Bi2O3/white-BiOCl with the Bi2O3/BiOCl ratio of 100/0, 95/5, 75/25, 45/55, 15/85 and 0/100 (b) Bi2O3/black-BiOCl with the Bi2O3/BiOCl ratio of 100/0, 95/5, 75/25, 45/55, 15/85 and 0/100. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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with the Bi2O3/BiOCl ratios of 95/5, 75/25, 45/55, and 15/85 shown in Fig. 7a represent the stepwise pattern corresponding to the ratio, as expected. The spectra for black-BiOCl, and Bi2O3/black-BiOCl heterojunction samples with different Bi2O3/BiOCl ratios are shown in Fig. 7b. The absorption spectrum of black-BiOCl is somewhat different from that of white-BiOCl, in that the absorption occurs in the whole visible range. This means that the band gap of BiOCl is reduced when the oxygen vacancy is occurred by the heat treatment. Consequently, Bi2O3/black-BiOCl heterojunction samples also absorb visible light more easily. In the Bi2O3/BiOCl heterojunction, it is considered that BiOCl behaves as a main photocatalyst, while Bi2O3 as a sensitizer absorbing visible light. The reduced band gap of black-BiOCl can accelerate the photocatalytic reaction of Bi2O3/black-BiOCl heterojunction samples by pumping up the electrons from the valence band to the conduction band more easily. In the Bi2O3/BiOCl heterojunction, the BiOCl and Bi2O3 are tightly bound each other in nanosize level, since Bi2O3
g = 1.999
Intensity
ples with the Bi2O3/BiOCl ratio of 25/75 and 95/5 are not shown here. The color of each sample having different Bi2O3/BiOCl ratio as 100/0, 95/5, 75/25, 45/55, 15/85, and 0/100 is shown in Fig. 4a. It is yellow when the sample is pure Bi2O3, and it is turned into white gradually depending upon the amount of BiOCl formed by treating with HCl. After the heat treatment at 300 °C for 5 h, all samples except pure Bi2O3 become darker because of the black-BiOCl formation, as shown in Fig. 4b. The color change illustrates that black-BiOCl can be formed by just heat treatment and a Bi2O3/ black-BiOCl heterojunction can be successfully prepared from a Bi2O3/BiOCl complex. SEM images of Bi2O3 and Bi2O3/BiOCl heterojunction are shown in Fig. 5. Large Bi2O3 particle is partially changed to BiOCl and forms Bi2O3/BiOCl heterojunction after the treatment with HCl. The size of the particle in heterojunction is getting smaller and smaller as Bi2O3 turned into BiOCl. Fig. 6 shows the EPR spectrum of black-BiOCl which is prepared by thermal treatment of BiOCl. It was previously reported that the EPR spectrum shows remarkable signal at g = 2.001 when BiOCl becomes black by irradiation with UV light. This signal at g = 2.001 is originated from the oxygen vacancy in black-BiOCl [14]. The EPR result of our samples is exactly same that of previously reported black-BiOCl in that it has signal at g = 1.999. This EPR spectrum result proves that the oxygen vacancy in BiOCl can be occurred not only by UV irradiation but also by heat treatment. The UV–visible diffuse reflectance spectra for Bi2O3, whiteBiOCl, and Bi2O3/white-BiOCl heterojunction samples with different Bi2O3/BiOCl ratios are shown in Fig. 7a. In the case of pure Bi2O3, the absorption edge appearing near 480 nm, which is corresponding to the band gap of 2.6 eV, is consistent with literature. Likewise for white-BiOCl, the absorption edge appearing near 380 nm represents the band gap of 3.4 eV, which is also consistent with the previous data. The spectra for heterojunction samples
Fig. 6. The EPR spectrum of black-BiOCl.
Fig. 5. SEM images of (a) Bi2O3, (b) Bi2O3/BiOCl heterojunction with the Bi2O3/BiOCl ratio of 45/55, (c) Bi2O3/BiOCl heterojunction with the Bi2O3/BiOCl ratio of 15/85 and (d) BiOCl.
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100
White-BiOCl 15/85 45/55 75/25 95/5 Bi2O3
0
200
400
600
800
(b)
Reflectance (%)
(a)
Reflectance (%)
100
Black-BiOCl 15/85 45/55 75/25 95/5 Bi2O3
0
200
400
Wavelength (nm)
600
800
Wavelength (nm)
Fig. 7. UV–vis spectra of (a) Bi2O3/white-BiOCl and (b) Bi2O3/black-BiOCl. Table 1 The degradation of 1,4-dichlorobenzene in the solution with different catalyst sample after irradiation of visible light.
P-25 Bi2O3 BiOCl 15/85 Bi2O3/BiOCl 45/55 Bi2O3/BiOCl 75/25 Bi2O3/BiOCl
white black white black white black white black
30 min
60
90
120
5.27 10.12 5.03 11.71 12.00 20.12 12.07 13.82 10.99 9.16
6.86 20.24 6.41 24.17 22.67 37.31 21.95 27.76 21.14 25.27
8.51 28.91 8.64 32.82 33.07 52.32 31.46 40.44 31.29 39.07
9.53 38.37 9.72 43.76 43.47 64.55 43.52 51.03 41.44 51.28
has been partially converted to BiOCl by HCl. Therefore, the electron transfer through the junction will be greatly efficient. The photocatalytic efficiencies of Bi2O3, white-BiOCl, blackBiOCl, Bi2O3/white-BiOCl, and Bi2O3/black-BiOCl heterojunction samples to remove 1,4-dichlorobenzene in the solution under visible light are summarized in Table 1. The photocatalytic efficiency of commercial P-25 is also measured as a reference for comparison. Black-BiOCl shows much higher activity under visible light than white-BiOCl, which is consistent with the previous results. Generally, the photocatalytic activities of Bi2O3/BiOCl heterojunction samples are higher than those of singly used Bi2O3, black-BiOCl. As shown in Table 1, Bi2O3/black-BiOCl heterojunction samples exhibit higher photocatalytic activity than Bi2O3/white-BiOCl heterojunction samples. The highest activity was detected when Bi2O3/black-BiOCl heterojunction was used as a catalyst with the Bi2O3/BiOCl ratio of 15/85. To verify the stability of the sample, we recovered the sample after the photocatalytic experiment. The XRD and photocatalytic efficiency data of the recovered sample showed same results compared with the original sample. There are two types of photocatalysts; first is a reductive photocatalyst that can release electrons from the conduction band. A sensitizer/TiO2 heterojunction is a typical example of this type. The excited electrons in the conduction band of a sensitizer are transferred to that of TiO2, and these electrons are released to reduce organic compounds. Second is an oxidative photocatalyst that can produce holes in the valence band. In Bi2O3/BiOCl heterojunction, BiOCl is known to be the main catalyst. Under visible light, electrons are excited from the valence band to conduction band of Bi2O3. Then, electrons in the valence band of BiOCl are transferred to that of Bi2O3, thereby forming holes in the valence band of BiOCl. These holes oxidize organic molecules. The bandgap of black BiOCl is reduced so that black-BiOCl itself can generate holes in the valence band under visible light. In blackBiOCl/Bi2O3 heterojunction, therefore, the number of holes might be doubled in the valence band of BiOCl because of the self-
generation by excitation and the electron transfer to Bi2O3. In addition, the electrons in the conduction band of BiOCl may play an important role to decompose organic compounds. The outstanding efficiency of black-BiOCl/Bi2O3 sample can be explained upon the explanation. The detailed mechanism should be searched further. 4. Conclusion Bi2O3/BiOCl heterojunction compounds with different Bi2O3/ BiOCl ratios were prepared by treating Bi2O3 with HCl, as reported earlier. Within the Bi2O3/BiOCl heterojunction, white BiOCl was turned into black by simple thermal treatment. Upon the result, Bi2O3/black-BiOCl heterojunction was successfully prepared. The EPR spectrum showed clear proof of the formation of oxygen vacant black-BiOCl after the heat treatment. UV–vis spectrum of black-BiOCl revealed that the absorption occurred in the visible range. This result was originated from the oxygen vacancy in the black-BiOCl. The photocatalytic activities of heterojunction samples were higher than that of single compound. In addition, Bi2O3/black-BiOCl samples showed advanced photocatalytic activity compared with the original Bi2O3/white-BiOCl. The highest photocatalytic efficiency was found in the Bi2O3/black-BiOCl when Bi2O3/BiOCl ratio was 15/85. Acknowledgement This work was supported by Wonkwang University Research Grant in 2015. References [1] R. Ghosh, S. Maiti, A. Chakraborty, Tetrahedron Lett. 45 (2004) 6775. [2] W.D. Wang, F.Q. Huang, X.P. Lin, Scr. Mater. 56 (2007) 669. [3] K.L. Zhang, C.M. Liu, F.Q. Huang, C. Zheng, W.D. Wang, Appl. Catal. B 68 (2006) 125. [4] Z.Q. Shi, Y. Wang, C.M. Fan, Y.F. Wang, G.Y. Ding, Trans. Nonferrous Met. Soc. China 21 (2011) 2254.
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