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Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion
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Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2017) 1–6
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Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion Chenghua Ding, Zhaoyu Ma, Chunqiu Han, Xinxin Liu, Zhuoya Jia, Haiquan Xie, Keyan Bao∗, Liqun Ye∗ Engineering Technology Research Center of Henan Province for Solar Catalysis, Collaborative Innovation Center of Water Security for Water Source Region of Mid-line of South-to-North Diversion Project, College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China
a r t i c l e
i n f o
Article history: Received 21 May 2017 Revised 20 June 2017 Accepted 21 June 2017 Available online xxx Keywords: BiOX Photocatalytic activities Large scale preparation CO2 conversion
a b s t r a c t BiOX (X = Br and Cl) have been widely used for photocatalytic environmental remediation and solar fuels generation. Here, we applied a new method for large scale preparation of BiOX ultrathin nanosheets. Samples were characterized by X-ray diffraction patterns (XRD), transmission electron microscopy (TEM), BET surface area (BET), photoluminescence spectra (PL), UV–vis diffuse reflectance spectra (DRS) and X-ray photoelectron spectroscopy (XPS). The BiOCl ultrathin nanosheets (BOC-U) and BiOBr ultrathin nanosheets (BOB-U) showed enhanced photocatalytic activity for CO2 conversion than bulk BiOCl and BiOBr. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction In recent decades, with rapid increase in population and development of modern industry, the content of carbon dioxide in atmosphere, produced by combustion of fossil fuels–petrol or solid fuels–from cars, industry and home fires, has risen dramatically and is far more than the level of the past. The greenhouse effect, caused by increasing carbon dioxide in the air, changes the earth’s temperature. Finding an efficient strategy to convert CO2 into available energy has been regarded as an ideal method to deal with above crisis. Photocatalytic technology is a new trend in environmental treatment in recent years. For example, anatase titanium dioxide have been widely used to solve them, water splitting into H2 and CO2 conversion to energy bearing carbon fuel sources [1–6]. However, the practical application of traditional photocatalysts is still limited because they cannot respond to the visible light and their low quantum. So, great efforts have been done to enhance the activities of traditional photocatalysts and find new visible-light responsible photocatalysts [7,8]. In recent years, bismuth-based oxyhalides, BiOX (X = I, Br, Cl), has caught researchers’ sight [9–14]. BiOX, has been confirmed to perform favorable photocatalytic activities under UV or visible light. BiOX compounds all crystallize in the tetragonal matlockite structure [15–18]. The structural
∗
Correspondence authors. E-mail addresses: [email protected] (K. Bao), [email protected] (L. Ye).
feature of BiOX comprises a layer of [Bi2 O2 ] slabs interleaved by double slabs of halogen atoms. The efficient separation of photogenerated electron-hole pairs can be induced by its internal static electric fields, which is between [Bi2 O2 ]2+ and halogen anionic layers. But BiOX is still limited in practical use because of its low photocatalytic efficiency [19–21]. So how to enhance the performances of BiOX has become a hotspot in photocatalytic field. In this study, the synthesis methods of large-scale preparation of BiOX ultrathin nanosheets were applied. Here, hydroxypropyl guar gum was used to BiOX synthesis and as prepared samples were used for CO2 conversion, through the characterizations and photocatalytic activities experiments we showed CO2 conversion study of BiOX. Our studies showed a deeper understanding of photocatalytic CO2 conversion performances, and provide a reference for the design of higher efficiency photocatalysts. 2. Experimental 2.1. Synthesis 1.00 g of hydroxypropyl guar gum and 20 mmol of Bi(NO3 )3 ·5H2 O were ground together for 30 min in an agate mortar. Then the mixtures were added slowly into 10 mL of an aqueous solution containing 20 mmol KCl or KBr. And it was ground into a paste and transferred to a 500 ml flask, added 190 mL water and stirred at 65 °C for 4 h. The white product was washed with water, and dried at 80 °C, and called them as BOC-U or BOB-U, respectively. More importantly, the scale-up experiment
http://dx.doi.org/10.1016/j.jtice.2017.06.044 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: C. Ding et al., Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.044
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Fig. 1. XRD patterns of BOB, BOB-U, BOC and BOC-U.
showed that 500 g product can be obtained. The synthesis of bulk BOC and BOB is similar to the above procedure except that no hydroxypropyl guar gum was added. 2.2. Characterization The crystalline phase of the samples was characterized by Xray diffraction (XRD) by a Bruker D8 advance X-ray diffractometer at room temperature with Cu–Ka radiation. Diffraction patterns were taken over the range 8–70 °. X-ray photoelectron spectroscopy (XPS) measurements were carried out by Thermo ESCALAB 250XI X-ray photoelectron spectrometer (Al Ka, 150 W, C1s 284.8 eV). Morphology of the samples was analyzed using the Sigma Zeiss Field emission scanning electron microscopy (FESEM) with the accelerating voltage 20 KV. The high-resolution transmission electron microscopy (HRTEM) images and element mapping were obtained by a JEOL JEM-2100F (UHR) field emission transmission electron microscopy. UV–vis diffuse reflectance spectroscopy (DRS) of samples were determined by a UV–vis spectrometer (Perkin Elmer, Lambda 850, BaSO4 as a reference) and record within the scope of 20 0–80 0 nm. A Quantachrome Autosorb-IQ automated gas sorption system was utilized to assessing the Brunauer–Emmett–Teller (BET) surface areas at 77 K. PL spectra recorded by a FLS980 multifunction steady state and transient state fluorescence spectrometer (Edinburgh Instruments, room temperature). 2.3. Photocatalytic reduction of CO2 The photocatalytic reduction activities for CO2 conversion were done in Labsolar-III AG (Beijing Perfect light Technology Co., Ltd., China) closed gas system. The volume of the reaction system was 350 mL 1.3 g NaHCO3 was added first. Then 0.05 g photocatalyst in the ultrasonic cleaning apparatus dissolved in an appropriate amount of water, then the resulting suspension transfer on a watch-glass with an area of 28.26 cm2 paving, the watch-glass is then placed in a vacuum drying oven at a temperature of 60 °C and then the watch-glass was put in mid-air of the reaction cell. Prior to the light irradiation, the above system was thoroughly vacuumtreated to remove the air completely, and then 10 mL 4 M H2 SO4 was injected into the reactor to react with NaHCO3 . Then, 1 atm CO2 gas was achieved. After that, the reactor was irradiated from the top by a 300 W high pressure xenon lamp (PLS-SXE300, Beijing Perfect light Technology Co., Ltd., China), and the photoreaction temperature was kept at 20 °C by DC-0506 low-temperature
Fig. 2. XPS spectra of BOB, BOB-U, BOC and BOC-U: (a) survey, (b) Cl 2p, (c) Br 3d.
thermostat bath (Shanghai Sunny Hengping Scientific Instrument Co., Ltd., China). During the irradiation, 1 mL of gas was taken from the reaction cell for subsequent qualitative analysis by GC9790II gas chromatography (GC, Zhejiang Fuli Analytical Instrument Co., Ltd., China) equipped with a flame ionization detector (FID, GDX01 columns). The quantification of the production yield was based on a calibration curve. The outlet gases were determined to be CO, CH4 and CO2 .
Please cite this article as: C. Ding et al., Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.044
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Fig. 3. SEM image of BOC-U (a and c) and BOB-U (b and d).
3. Results and discussion As shown in Fig. 1. The XRD pattern of BOB, BOB-U, BOC and BOC-U displays a series of diffraction peaks. BOB and BOB-U displayed the same shape of peaks and were in agreement with the standard XRD pattern of JCPDS no. 01-009-0393 and BOC and BOCU were in accordance with JCPDS no. 00-06-0249 [22,23]. All the characteristic peaks of BOB, BOB-U, BOC and BOC-U were corresponding to the previous work. The XRD results also demonstrated that the BOB-U and BOC-U were just having a change in morphology, the original nature of BiOBr and BiOCl were retained. It can be seen that the crystallinity of BOB-U and BOC-U are lower than BOB and BOC, which is due to the thin thickness of nanosheets. The X-ray photoelectron spectroscopy (XPS) was used to examine the composition and the chemical status of these samples. As shown in Fig. 2, the survey spectrum of main binding energy peaks, Bi, O, Br/Cl were detected, indicating the high purity of prepared samples. The Bi 4f7/2 and Bi 4f5/2 peaks of BOB samples were at 158.54 eV and 164.90 eV, and the Bi 4f7/2 and Bi 4f5/2 peaks of BOB-U samples were at 159.38 eV and 164.65 eV As for BOC and BOC-U, the Bi 4f7/2 and Bi 4f5/2 peaks were at 158.98 eV and 164.31 eV, 159.08 eV and 164.37 eV, respectively. It was suggesting that Bi3+ exists in these samples. The band energies of 197.67 eV and 199.64 eV were corresponding to Cl 2p3/2 and Cl 2p1/2 of BOC and BOC-U associated with Cl− in these two samples, and the band energies of 68.50 eV of BOB-U and 68.78 eV of BOB were corresponding to Br 3d, which associated with Br− in these two samples, respectively. A deeper insight into the detailed structures of BOB-U and BOCU samples was acquired by SEM. As shown in Fig. 3, all BiOX samples were nanosheets. For bulk BiOCl and BiOBr, the thickness was about 50 nm. However, for BOB-U and BOC-U samples, the
thickness was only about 10 nm. Fig. 4 showed the TEM and HRTEM images. For BOB-U samples, as shown in Fig. 4a and b, it was proved that BOB-U were nanosheets and the thickness was very small. The thickness of the BOC-U and BOB-U nanosheets were 11 and 9 nm, respectively. The clear lattice fringes with interplanar lattice spacing was of 0.76 nm. And the same phenomenon occurred in BOC-U sample, the clear lattice fringes with interplanar lattice spacing of BOC-U nanosheets was of 0.87 nm (Fig. 4c, d). The experimental results implied that the nanosheets of BOB-U and BOC-U were successfully synthesized. The specific surface area and porosity of the as-prepared films were investigated by nitrogen adsorption and desorption isotherms. In the Fig. 5a, the hysteresis loop of these four samples were matched well with a type IV isotherm with an indistinct H3 hysteresis loop. It can be observed when the relative pressure (P/P0 ) increases, there were more adsorbed volumes of these four samples, which showed there were many heterogeneous pores on the surface of these four samples. The corresponding BET surface areas of BOB, BOB-U, BOC and BOC-U were of 5.5 m2 /g, 19.8 m2 /g, 6.4 m2 /g and 27.4 m2 /g, respectively. In addition, the Barret–Joyner–Halenda (BJH) method was used to measure the pore diameter of the samples. The results showed that the main pore diameter of BOB-U and BOC-U were of 3.9 nm and 8.9 nm. The results implied BOC-U may show better photocatalytic activities. Fig. 6 showed the UV–vis DRS spectra of BOB, BOC, BOB-U and BOC-U. It can be seen that BOB and BOC had fundamental adsorption edges at 445 nm and 365 nm, respectively. As for modified samples, BOB-U showed adsorption edges at 436 nm and BOCU was of 365 nm. It can be observed from the spectra that all the samples could response the visible light. Based on energy gap
Please cite this article as: C. Ding et al., Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.044
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Fig. 4. TEM image (a), and HRTEM image (b) of BOB-U; TEM image (c), and HRTEM image (d) of BOC-U.
Fig. 6. DRS spectra of BOB, BOB-U, BOC, BOC-U.
Fig. 5. Nitrogen adsorption–desorption isotherm and pore size distribution of BOB, BOB-U, BOC and BOC-U.
Eg = 240/λ, the Eg of BOB, BOC, BOB-U and BOC-U were 2.79, 3.40, 2.83, and 3.40 eV, respectively. Recently, our group developed photocatalytic CO2 conversion as a new application for BiOX [24–28]. Fig. 7 showed the photocatalytic CO2 conversion activities of BOB, BOB-U, BOC and BOC-U, and the main products were CO and CH4 . We can clearly see that the BOC-U performed best in these four samples with 57 μmol/g CO and 4.5 μmol/g CH4 after 4 h reaction, about two times than BOBU and five times than BOB and BOC (Fig. 7a and b). The experimental results demonstrated that the modified BOC-U and BOB-U showed better CO2 conversion ability than bulk samples. On the other hand, BOC-U and BOB-U also showed higher activity than TiO2 (Fig. S1). The cycle experiment (Fig. 7c) of BOC-U and BOB-U indicated that BiOX ultrathin nanosheets were stable photocatalytic material. The previous studies showed conduction band (CB) position affected the photocatalytic reduction activity. In order to obtain the CB position, the valence bands (VB) position of BOB, BOB-U, BOC and BOC-U were tested and the results were shown in valenceband XPS spectra (Fig. 8). XPS VB patterns revealed VB positions of BOC, BOC-U, BOB and BOB-U were 2.00 V, 2.20 V, 2.37 V and 2.23 V
Please cite this article as: C. Ding et al., Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.044
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Fig. 8. (a) Valence band XPS of BOC and BOC-U; (b) valence band XPS of BOB and BOB-U; (c) band structure of the BOC-U and BOB-U.
Fig. 7. Photocatalytic CO2 conversion activities of BOB, BOB-U, BOC and BOC-U: (a) CO generation, (b) CH4 generation, and (c) cycle experiment.
respectively. Based on the above energy gap of BOC-U and BOB-U. The CB positions of BOC-U and BOB-U can be calculated as shown in Fig. 8c. They were −1.20 and −0.60 V, which are higher than that of redox potentials of CO2 /CO at −0.53 V. So, BOC-U showed the higher activity than BOB-U for CO2 conversion into CO due to the higher CB position. On the other hand, PL spectra were performed to illustrate the electron-hole separation efficiency in Fig. 9. The lower intensity of BOC-U and BOB-U indicated the ultrathin structure enhanced the electron-hole separation efficiency.
Please cite this article as: C. Ding et al., Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.044
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Fig. 9. PL spectra of BOC, BOB, BOC-U and BOB-U.
4. Conclusions In this paper, we reported the large scale preparation of BiOX ultrathin nanosheets and their favorable CO2 conversion abilities. The characterization of XRD, XPS, DRS, TEM, BET and HRTEM showed the BOB-U and BOC-U maintained original natures of bulk BiOX and could response visible light. The photocatalytical CO2 conversion results showed their enhanced photocatalytic abilities. Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 51502146, U1404506, 21671113, U1404505) the Program for Innovative Talent in University of Henan Province (No. 16HASTIT010), and Student Innovation Training Program (No. 201710481010Z). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.06.044. References [1] Shang M, Wang W, Zhang L. Preparation of BiOBr lamellar structure with high photocatalytic activity by CTAB as Br source and template. J Hazard Mater 2009;167:803–9. [2] Rong H, Xin X, Xiaoxi Z, Junmin N, Weide Z. Efficient adsorption and visible-light photocatalytic degradation of tetracycline hydrochloride using mesoporous BiOI microspheres. J Hazard Mater 2012;209–210:137. [3] Zhang KL, Liu CM, Huang FQ, Zheng C, Wang WD. Study of the electronic structure and photocatalytic activity of the BiOCl photocatalyst. Appl Catal B 2006;68:125–9. [4] Chai SY, Yong JK, Jung MH, Chakraborty AK, Jung D, Wan IL. Heterojunctioned BiOCl/Bi2 O3 , a new visible light photocatalyst. J Catal 2009;262:144–9. [5] Chang X, Huang J, Cheng C, Sui Q, Sha W, Ji G, et al. BiOX (X = Cl, Br, I) photocatalysts prepared using NaBiO3 as the Bi source: characterization and catalytic performance. Catal Commun 2010;11:460–4.
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Please cite this article as: C. Ding et al., Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.044
JID: JTICE
[m5G;July 8, 2017;21:17]
Journal of the Taiwan Institute of Chemical Engineers 0 0 0 (2017) 1–6
Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion Chenghua Ding, Zhaoyu Ma, Chunqiu Han, Xinxin Liu, Zhuoya Jia, Haiquan Xie, Keyan Bao∗, Liqun Ye∗ Engineering Technology Research Center of Henan Province for Solar Catalysis, Collaborative Innovation Center of Water Security for Water Source Region of Mid-line of South-to-North Diversion Project, College of Chemistry and Pharmaceutical Engineering, Nanyang Normal University, Nanyang 473061, China
a r t i c l e
i n f o
Article history: Received 21 May 2017 Revised 20 June 2017 Accepted 21 June 2017 Available online xxx Keywords: BiOX Photocatalytic activities Large scale preparation CO2 conversion
a b s t r a c t BiOX (X = Br and Cl) have been widely used for photocatalytic environmental remediation and solar fuels generation. Here, we applied a new method for large scale preparation of BiOX ultrathin nanosheets. Samples were characterized by X-ray diffraction patterns (XRD), transmission electron microscopy (TEM), BET surface area (BET), photoluminescence spectra (PL), UV–vis diffuse reflectance spectra (DRS) and X-ray photoelectron spectroscopy (XPS). The BiOCl ultrathin nanosheets (BOC-U) and BiOBr ultrathin nanosheets (BOB-U) showed enhanced photocatalytic activity for CO2 conversion than bulk BiOCl and BiOBr. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction In recent decades, with rapid increase in population and development of modern industry, the content of carbon dioxide in atmosphere, produced by combustion of fossil fuels–petrol or solid fuels–from cars, industry and home fires, has risen dramatically and is far more than the level of the past. The greenhouse effect, caused by increasing carbon dioxide in the air, changes the earth’s temperature. Finding an efficient strategy to convert CO2 into available energy has been regarded as an ideal method to deal with above crisis. Photocatalytic technology is a new trend in environmental treatment in recent years. For example, anatase titanium dioxide have been widely used to solve them, water splitting into H2 and CO2 conversion to energy bearing carbon fuel sources [1–6]. However, the practical application of traditional photocatalysts is still limited because they cannot respond to the visible light and their low quantum. So, great efforts have been done to enhance the activities of traditional photocatalysts and find new visible-light responsible photocatalysts [7,8]. In recent years, bismuth-based oxyhalides, BiOX (X = I, Br, Cl), has caught researchers’ sight [9–14]. BiOX, has been confirmed to perform favorable photocatalytic activities under UV or visible light. BiOX compounds all crystallize in the tetragonal matlockite structure [15–18]. The structural
∗
Correspondence authors. E-mail addresses: [email protected] (K. Bao), [email protected] (L. Ye).
feature of BiOX comprises a layer of [Bi2 O2 ] slabs interleaved by double slabs of halogen atoms. The efficient separation of photogenerated electron-hole pairs can be induced by its internal static electric fields, which is between [Bi2 O2 ]2+ and halogen anionic layers. But BiOX is still limited in practical use because of its low photocatalytic efficiency [19–21]. So how to enhance the performances of BiOX has become a hotspot in photocatalytic field. In this study, the synthesis methods of large-scale preparation of BiOX ultrathin nanosheets were applied. Here, hydroxypropyl guar gum was used to BiOX synthesis and as prepared samples were used for CO2 conversion, through the characterizations and photocatalytic activities experiments we showed CO2 conversion study of BiOX. Our studies showed a deeper understanding of photocatalytic CO2 conversion performances, and provide a reference for the design of higher efficiency photocatalysts. 2. Experimental 2.1. Synthesis 1.00 g of hydroxypropyl guar gum and 20 mmol of Bi(NO3 )3 ·5H2 O were ground together for 30 min in an agate mortar. Then the mixtures were added slowly into 10 mL of an aqueous solution containing 20 mmol KCl or KBr. And it was ground into a paste and transferred to a 500 ml flask, added 190 mL water and stirred at 65 °C for 4 h. The white product was washed with water, and dried at 80 °C, and called them as BOC-U or BOB-U, respectively. More importantly, the scale-up experiment
http://dx.doi.org/10.1016/j.jtice.2017.06.044 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: C. Ding et al., Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.044
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Fig. 1. XRD patterns of BOB, BOB-U, BOC and BOC-U.
showed that 500 g product can be obtained. The synthesis of bulk BOC and BOB is similar to the above procedure except that no hydroxypropyl guar gum was added. 2.2. Characterization The crystalline phase of the samples was characterized by Xray diffraction (XRD) by a Bruker D8 advance X-ray diffractometer at room temperature with Cu–Ka radiation. Diffraction patterns were taken over the range 8–70 °. X-ray photoelectron spectroscopy (XPS) measurements were carried out by Thermo ESCALAB 250XI X-ray photoelectron spectrometer (Al Ka, 150 W, C1s 284.8 eV). Morphology of the samples was analyzed using the Sigma Zeiss Field emission scanning electron microscopy (FESEM) with the accelerating voltage 20 KV. The high-resolution transmission electron microscopy (HRTEM) images and element mapping were obtained by a JEOL JEM-2100F (UHR) field emission transmission electron microscopy. UV–vis diffuse reflectance spectroscopy (DRS) of samples were determined by a UV–vis spectrometer (Perkin Elmer, Lambda 850, BaSO4 as a reference) and record within the scope of 20 0–80 0 nm. A Quantachrome Autosorb-IQ automated gas sorption system was utilized to assessing the Brunauer–Emmett–Teller (BET) surface areas at 77 K. PL spectra recorded by a FLS980 multifunction steady state and transient state fluorescence spectrometer (Edinburgh Instruments, room temperature). 2.3. Photocatalytic reduction of CO2 The photocatalytic reduction activities for CO2 conversion were done in Labsolar-III AG (Beijing Perfect light Technology Co., Ltd., China) closed gas system. The volume of the reaction system was 350 mL 1.3 g NaHCO3 was added first. Then 0.05 g photocatalyst in the ultrasonic cleaning apparatus dissolved in an appropriate amount of water, then the resulting suspension transfer on a watch-glass with an area of 28.26 cm2 paving, the watch-glass is then placed in a vacuum drying oven at a temperature of 60 °C and then the watch-glass was put in mid-air of the reaction cell. Prior to the light irradiation, the above system was thoroughly vacuumtreated to remove the air completely, and then 10 mL 4 M H2 SO4 was injected into the reactor to react with NaHCO3 . Then, 1 atm CO2 gas was achieved. After that, the reactor was irradiated from the top by a 300 W high pressure xenon lamp (PLS-SXE300, Beijing Perfect light Technology Co., Ltd., China), and the photoreaction temperature was kept at 20 °C by DC-0506 low-temperature
Fig. 2. XPS spectra of BOB, BOB-U, BOC and BOC-U: (a) survey, (b) Cl 2p, (c) Br 3d.
thermostat bath (Shanghai Sunny Hengping Scientific Instrument Co., Ltd., China). During the irradiation, 1 mL of gas was taken from the reaction cell for subsequent qualitative analysis by GC9790II gas chromatography (GC, Zhejiang Fuli Analytical Instrument Co., Ltd., China) equipped with a flame ionization detector (FID, GDX01 columns). The quantification of the production yield was based on a calibration curve. The outlet gases were determined to be CO, CH4 and CO2 .
Please cite this article as: C. Ding et al., Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.044
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Fig. 3. SEM image of BOC-U (a and c) and BOB-U (b and d).
3. Results and discussion As shown in Fig. 1. The XRD pattern of BOB, BOB-U, BOC and BOC-U displays a series of diffraction peaks. BOB and BOB-U displayed the same shape of peaks and were in agreement with the standard XRD pattern of JCPDS no. 01-009-0393 and BOC and BOCU were in accordance with JCPDS no. 00-06-0249 [22,23]. All the characteristic peaks of BOB, BOB-U, BOC and BOC-U were corresponding to the previous work. The XRD results also demonstrated that the BOB-U and BOC-U were just having a change in morphology, the original nature of BiOBr and BiOCl were retained. It can be seen that the crystallinity of BOB-U and BOC-U are lower than BOB and BOC, which is due to the thin thickness of nanosheets. The X-ray photoelectron spectroscopy (XPS) was used to examine the composition and the chemical status of these samples. As shown in Fig. 2, the survey spectrum of main binding energy peaks, Bi, O, Br/Cl were detected, indicating the high purity of prepared samples. The Bi 4f7/2 and Bi 4f5/2 peaks of BOB samples were at 158.54 eV and 164.90 eV, and the Bi 4f7/2 and Bi 4f5/2 peaks of BOB-U samples were at 159.38 eV and 164.65 eV As for BOC and BOC-U, the Bi 4f7/2 and Bi 4f5/2 peaks were at 158.98 eV and 164.31 eV, 159.08 eV and 164.37 eV, respectively. It was suggesting that Bi3+ exists in these samples. The band energies of 197.67 eV and 199.64 eV were corresponding to Cl 2p3/2 and Cl 2p1/2 of BOC and BOC-U associated with Cl− in these two samples, and the band energies of 68.50 eV of BOB-U and 68.78 eV of BOB were corresponding to Br 3d, which associated with Br− in these two samples, respectively. A deeper insight into the detailed structures of BOB-U and BOCU samples was acquired by SEM. As shown in Fig. 3, all BiOX samples were nanosheets. For bulk BiOCl and BiOBr, the thickness was about 50 nm. However, for BOB-U and BOC-U samples, the
thickness was only about 10 nm. Fig. 4 showed the TEM and HRTEM images. For BOB-U samples, as shown in Fig. 4a and b, it was proved that BOB-U were nanosheets and the thickness was very small. The thickness of the BOC-U and BOB-U nanosheets were 11 and 9 nm, respectively. The clear lattice fringes with interplanar lattice spacing was of 0.76 nm. And the same phenomenon occurred in BOC-U sample, the clear lattice fringes with interplanar lattice spacing of BOC-U nanosheets was of 0.87 nm (Fig. 4c, d). The experimental results implied that the nanosheets of BOB-U and BOC-U were successfully synthesized. The specific surface area and porosity of the as-prepared films were investigated by nitrogen adsorption and desorption isotherms. In the Fig. 5a, the hysteresis loop of these four samples were matched well with a type IV isotherm with an indistinct H3 hysteresis loop. It can be observed when the relative pressure (P/P0 ) increases, there were more adsorbed volumes of these four samples, which showed there were many heterogeneous pores on the surface of these four samples. The corresponding BET surface areas of BOB, BOB-U, BOC and BOC-U were of 5.5 m2 /g, 19.8 m2 /g, 6.4 m2 /g and 27.4 m2 /g, respectively. In addition, the Barret–Joyner–Halenda (BJH) method was used to measure the pore diameter of the samples. The results showed that the main pore diameter of BOB-U and BOC-U were of 3.9 nm and 8.9 nm. The results implied BOC-U may show better photocatalytic activities. Fig. 6 showed the UV–vis DRS spectra of BOB, BOC, BOB-U and BOC-U. It can be seen that BOB and BOC had fundamental adsorption edges at 445 nm and 365 nm, respectively. As for modified samples, BOB-U showed adsorption edges at 436 nm and BOCU was of 365 nm. It can be observed from the spectra that all the samples could response the visible light. Based on energy gap
Please cite this article as: C. Ding et al., Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.044
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Fig. 4. TEM image (a), and HRTEM image (b) of BOB-U; TEM image (c), and HRTEM image (d) of BOC-U.
Fig. 6. DRS spectra of BOB, BOB-U, BOC, BOC-U.
Fig. 5. Nitrogen adsorption–desorption isotherm and pore size distribution of BOB, BOB-U, BOC and BOC-U.
Eg = 240/λ, the Eg of BOB, BOC, BOB-U and BOC-U were 2.79, 3.40, 2.83, and 3.40 eV, respectively. Recently, our group developed photocatalytic CO2 conversion as a new application for BiOX [24–28]. Fig. 7 showed the photocatalytic CO2 conversion activities of BOB, BOB-U, BOC and BOC-U, and the main products were CO and CH4 . We can clearly see that the BOC-U performed best in these four samples with 57 μmol/g CO and 4.5 μmol/g CH4 after 4 h reaction, about two times than BOBU and five times than BOB and BOC (Fig. 7a and b). The experimental results demonstrated that the modified BOC-U and BOB-U showed better CO2 conversion ability than bulk samples. On the other hand, BOC-U and BOB-U also showed higher activity than TiO2 (Fig. S1). The cycle experiment (Fig. 7c) of BOC-U and BOB-U indicated that BiOX ultrathin nanosheets were stable photocatalytic material. The previous studies showed conduction band (CB) position affected the photocatalytic reduction activity. In order to obtain the CB position, the valence bands (VB) position of BOB, BOB-U, BOC and BOC-U were tested and the results were shown in valenceband XPS spectra (Fig. 8). XPS VB patterns revealed VB positions of BOC, BOC-U, BOB and BOB-U were 2.00 V, 2.20 V, 2.37 V and 2.23 V
Please cite this article as: C. Ding et al., Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.044
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Fig. 8. (a) Valence band XPS of BOC and BOC-U; (b) valence band XPS of BOB and BOB-U; (c) band structure of the BOC-U and BOB-U.
Fig. 7. Photocatalytic CO2 conversion activities of BOB, BOB-U, BOC and BOC-U: (a) CO generation, (b) CH4 generation, and (c) cycle experiment.
respectively. Based on the above energy gap of BOC-U and BOB-U. The CB positions of BOC-U and BOB-U can be calculated as shown in Fig. 8c. They were −1.20 and −0.60 V, which are higher than that of redox potentials of CO2 /CO at −0.53 V. So, BOC-U showed the higher activity than BOB-U for CO2 conversion into CO due to the higher CB position. On the other hand, PL spectra were performed to illustrate the electron-hole separation efficiency in Fig. 9. The lower intensity of BOC-U and BOB-U indicated the ultrathin structure enhanced the electron-hole separation efficiency.
Please cite this article as: C. Ding et al., Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.044
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Fig. 9. PL spectra of BOC, BOB, BOC-U and BOB-U.
4. Conclusions In this paper, we reported the large scale preparation of BiOX ultrathin nanosheets and their favorable CO2 conversion abilities. The characterization of XRD, XPS, DRS, TEM, BET and HRTEM showed the BOB-U and BOC-U maintained original natures of bulk BiOX and could response visible light. The photocatalytical CO2 conversion results showed their enhanced photocatalytic abilities. Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 51502146, U1404506, 21671113, U1404505) the Program for Innovative Talent in University of Henan Province (No. 16HASTIT010), and Student Innovation Training Program (No. 201710481010Z). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.06.044. References [1] Shang M, Wang W, Zhang L. Preparation of BiOBr lamellar structure with high photocatalytic activity by CTAB as Br source and template. J Hazard Mater 2009;167:803–9. [2] Rong H, Xin X, Xiaoxi Z, Junmin N, Weide Z. Efficient adsorption and visible-light photocatalytic degradation of tetracycline hydrochloride using mesoporous BiOI microspheres. J Hazard Mater 2012;209–210:137. [3] Zhang KL, Liu CM, Huang FQ, Zheng C, Wang WD. Study of the electronic structure and photocatalytic activity of the BiOCl photocatalyst. Appl Catal B 2006;68:125–9. [4] Chai SY, Yong JK, Jung MH, Chakraborty AK, Jung D, Wan IL. Heterojunctioned BiOCl/Bi2 O3 , a new visible light photocatalyst. J Catal 2009;262:144–9. [5] Chang X, Huang J, Cheng C, Sui Q, Sha W, Ji G, et al. BiOX (X = Cl, Br, I) photocatalysts prepared using NaBiO3 as the Bi source: characterization and catalytic performance. Catal Commun 2010;11:460–4.
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Please cite this article as: C. Ding et al., Large-scale preparation of BiOX (X = Cl, Br) ultrathin nanosheets for efficient photocatalytic CO2 conversion, Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.044