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Polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF achieving distinctly enhanced photocatalytic activity
Journal Pre-proofs Polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF achieving distinctly enhanced photocatalytic activity Yuan Guan, Shaomang Wang, Zhongyu Li, Xun Ding, Mingfei Wu, Mingmin Zhang, Weifeng Yu PII: DOI: Reference:
S0167-577X(19)31829-4 https://doi.org/10.1016/j.matlet.2019.127197 MLBLUE 127197
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
25 October 2019 3 December 2019 17 December 2019
Please cite this article as: Y. Guan, S. Wang, Z. Li, X. Ding, M. Wu, M. Zhang, W. Yu, Polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF achieving distinctly enhanced photocatalytic activity, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet.2019.127197
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF achieving distinctly enhanced photocatalytic activity Yuan Guana, Shaomang Wangb,*, Zhongyu Lia, Xun Dinga, Mingfei Wua, Mingmin Zhanga, Weifeng Yua a School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, P. R. China. b School of Environment and Safety Engineering, Changzhou University, Changzhou, Jiangsu 213164, P. R. China. E-mail address: [email protected] Abstract: Polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF was successfully synthesized by solvothermal and subsequent calcination at 375 ℃ in air for the first time. The photocatalytic degradation rate of methyl orange over BiO0.51F1.98 with self-doped BiOF was 4.9 times that over pure BiO0.51F1.98. Significantly enhanced photocatalytic activity of BiO0.51F1.98 with self-doped BiOF was basically ascribed to broadened visible-light absorption, and lower recombination rate of carries. Keywods: Photocatalysis; Semiconductors; Solar energy materials; BiO0.51F1.98; BiOF 1. Introduction Over the past decade, bismuth oxyhalide compounds (BiOX, X = Cl, Br, I) have been studied widely because of remarkable chemical stability, unique layer structure and excellent photocatalytic performance under UV or visible-light illumination [1, 2]. 1
As a member of bismuth oxyhalide, BiOF has also gained a lot of attention on its photocatalytic activity recently. BiOF possesses a direct band-gap value of about 3.5 eV, which makes it sensitive to UV region. The latest research indicates that BiOF can degrade effectively many organic pollutants under UV light irradiation [1, 3]. Unfortunately, the wide band gap of BiOF greatly limits its real application. Although the light absorption of BiOF was broadened and its photocatalytic activity was enhanced by adding metal or non-metal impurities [4-6], the photocatalytic performance of modified BiOF is still not ideal. Until now, fluorine-rich BiO0.51F1.98, which is different from the crystal structure of BiOF is rarely reported as a photocatalyst. Li et al found that when the photocatalytic materials with same elements and different crystal structures such as anatase-rutile of TiO2 and a–β phases of Ga2O3 formed a heterophase junction, the photocatalytic activity of the heterophase material was greatly improved [5, 7]. Based on this design idea, we successfully prepared a polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF via a facile solvothermal approach coupled with thermal decomposition for the first time. The results of photocatalytic degradation of methyl orange (MO) demonstrated that the photocatalytic activity of the polycrystalline bismuth oxyfluoride was significantly enhanced by forming a heterophase junction between BiO0.51F1.98 and BiOF. 2. Experimental 2.1. Preparation of samples BiO0.51F1.98 was synthesized using simple solvothermal method. In a typical 2
procedure, 4.0 g of Bi(NO3)3·5H2O and 0.34 g of NaF were dissolved in 30 mL of ethylene glycol, respectively. The NaF solution was added into the Bi(NO3)3·5H2O solution dropwise with vigorous stirring. After stirring for 60 min, the mixed solution was transferred into an autoclave and maintained at 180 ℃ for 10 h. The resulting product was washed three times with distilled water and absolute ethanol, and finally dried at 80 ℃ for 4 h to obtain single BiO0.51F1.98 crystal. The polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF was prepared through thermal decomposition method. 2.0 g of as-prepared BiO0.51F1.98 was placed into a ceramic crucible, which was calcined at 375 ℃ for 6 h in a muffle furnace. After the furnace was naturally cooled to room temperature, the polycrystalline bismuth oxyfluoride of BiO0.51F1.98 and BiOF (calcined BiO0.51F1.98) was obtained. 2.2. Characterization Detailed characterization section was presented in supporting information. 2.3. Photocatalytic activity evaluation of samples In a typical test of photocatalysis, a 500 W xenon lamp was used as the light source. 0.5 g L-1 of photocatalyst was added into MO aqueous solution (10 mg L-1). At specified intervals, 4 mL of the suspensions were extracted and centrifuged to remove the catalysts. According to the maximum absorption wavelength of MO at 464 nm, the concentration of the sample was analyzed via a UV–vis spectrophotometer. To estimate the overall MO degradation process, the rate constant k was calculated by pseudo–second-order model( Eq. (1)). 3
1 𝐶
1
(1)
― 𝐶0 = 𝑘𝑡
Where C0 is the initial concentration, and C is the concentration at certain reaction time t. 3. Results and discussion 3.1. Characterization of samples
Fig. 1. (a) XRD patterns of BiO0.51F1.98 and calcined BiO0.51F1.98, (b) SEM images of BiO0.51F1.98, (c) and (d) SEM images of calcined BiO0.51F1.98.
The XRD patterns of pure and calcined BiO0.51F1.98 are shown in Fig. 1a. For pure BiO0.51F1.98, the characteristic peaks locating at 2θ values of 26.4◦, 30.6◦, 43.7◦, 51.8◦, and 54.2◦ were indexed to (111), (200), (220), (311) and (222) crystal planes of cubic BiO0.51F1.98, respectively (PDF No. 24–0147). No other peaks were observed, implying high purity of the sample. The pattern of BiO0.51F1.98 calcined at 375 ℃ 4
corresponded to a polycrystalline bismuth oxyfluoride of cubic BiO0.51F1.98 with tetragonal BiOF (PDF No. 22–0114). The results of XRD indicated that the polycrystalline bismuth oxyfluoride containing cubic BiO0.51F1.98 and tetragonal BiOF was successfully fabricated by calcining pure BiO0.51F1.98 at appropriate temperature in air. As presented in Fig. 1b, the morphology of BiO0.51F1.98 contained plate-like primary particles with thickness of less than 20 nm, which were interrelated and interacted on each other to form agglomerated secondary particles. The SEM images of calcined BiO0.51F1.98 are shown in Fig. 1c-d. The sample was composed of uniform microspheres with average diameter of 10 μm. The microspheres were consisted of irregular small-thin nanosheet, and large pores were formed. Compared with pure BiO0.51F1.98, the nanosheets of calcined BiO0.51F1.98 became much thinner, which suggests that heat-treatment process can lead to different crystal growth direction.
Fig. 2. XPS spectrum of (a) survey scan, (b) C 1s, (c) Bi 4f, (d) O 1s, (e) F 1s and (f) valence band 5
of BiO0.51F1.98 and calcined BiO0.51F1.98.
XPS was used to analyze the surface chemical composition and states of pure and calcined BiO0.51F1.98. As depicted in Fig. 2a, the emergence of the C 1s, Bi 4f, O 1s and F 1s peaks in the survey spectra revealed that the composites were constituted by the elements of C, Bi, O and F. Fig. 2b displays C 1s XPS measurements of pure and calcined BiO0.51F1.98 in the energy range in order to detect possible doped carbon in BiO0.51F1.98 lattice [8]. The first C1s peak at 284.83 eV belonged to carbon tape with C0, and the second at 287.88 eV was identified as doped carbon atoms (C=O or C-F). Compared with pure BiO0.51F1.98, the intensity of doped carbon peak in calcined BiO0.51F1.98 decreased obviously, which illustrates that the heat-treatment procedure can effectively remove a number of carbon impurity. From Fig. 2c, in BiO0.51F1.98, two peaks at 159.13 and 164.43 eV were assigned to Bi 4f7/2 and Bi 4f5/2, both of which had a shift of 0.3 eV to low energy for calcined BiO0.51F1.98, respectively. The O 1s XPS spectra of BiO0.51F1.98 was fitted into two peaks locating at 529.83 and 531.83 eV, which is attributed to lattice oxygen and surface adsorbed oxygen species [9]. Similarly, the two typical peaks of calcined BiO0.51F1.98 shifted to lower binding energies as well. According to Fig. 2e, the F 1s peaks at 682.93 eV are ascribable to F element with monovalent oxidation state. The binding energy value of F 1s was identical for BiO0.51F1.98 and calcined BiO0.51F1.98. Besides, The valence bands of BiO0.51F1.98 and calcined BiO0.51F1.98 were measured by XPS (Fig. 2f). The pure BiO0.51F1.98 showed valence-band edge of about 1.52 eV. For calcined BiO0.51F1.98, the valence-band edge shifted to 1.18 eV, indicating that the Femi level of calcined 6
BiO0.51F1.98 shifts upward of 0.34 eV with respect to that of BiO0.51F1.98 [6, 8].
Fig. 3. (a) UV–vis DRS, (b) the band-gap energies, (c) PL spectra, (d) photocurrent density, (e) impedance of BiO0.51F1.98 and calcined BiO0.51F1.98, and (f) MO removal profiles of BiO0.51F1.98, BiOCl and calcined BiO0.51F1.98.
The UV–vis DRS and band gap of pure and calcined BiO0.51F1.98 are illustrated in Fig. 3a-b. It was found that the absorption edge of pure and calcined BiO0.51F1.98 located at 360 nm and 400 nm in the spectrum. According to the formula obtained from the literature [10], the corresponding band-gap values of the samples were 3.31 eV and 3.06 eV, respectively. For calcined BiO0.51F1.98, a red-shift was observed for the absorption edge. To investigate the recombination behavior of photo-generated carriers, Fig. 3c displays the PL spectra of pure and calcined BiO0.51F1.98 with the excitation wavelength at 320 nm. It was observed that the PL intensity of calcined BiO0.51F1.98 was much lower than pure BiO0.51F1.98, suggesting that calcined BiO0.51F1.98 owns a 7
lower recombination rate of photo-induced electrons and holes [11]. The separation efficiency of photo-generated carriers was further confirmed by photocurrent density (Fig. 3d) and EIS (Fig. 3e). It was seen that the calcined BiO0.51F1.98 possessed much higher photocurrent and lower electrical resistance. 3. 2. Photocatalytic activity evaluation Fig. 3f shows the photocatalytic activity of BiO0.51F1.98, BiOCl and calcined BiO0.51F1.98 for MO degradation under light irradiation. The blank experiment displayed that the self-photodegradation of MO was negligible. Prior to the irradiation, the suspension was magnetically stirred in dark for 30 min to establish an adsorption/desorption equilibrium between the MO and the catalyst surface. It was found that the polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF exhibited much better photocatalytic activity than pure BiO0.51F1.98 and BiOCl. In addition, as shown in Fig. S1, the rate constants (k) were 7.05×10-4, 1.47×10-3 and 0.0141 min-1 for BiO0.51F1.98, BiOCl and calcined BiO0.51F1.98, respectively. The calcined BiO0.51F1.98 owned a maximum rate constant, which was 9.6 and 20 times higher than that of BiOCl and BiO0.51F1.98, respectively. 4. Conclusions Compared with pure BiO0.51F1.98, the polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF indeed could exhibit significantly enhanced photocatalytic activity. This novel polycrystalline bismuth oxyfluoride will increase research interest in the use of bismuth oxyfluoride as a photocatalyst to decompose organic pollutants. 8
Conflict of interest The authors declare no conflict of interest. Acknowledgments This work was supported primarily by the National Natural Science Foundation of China (21876015), the Natural Science Foundation of Jiangsu Province (BK20161277 and BK20190934), the Natural Science Foundation of Jiangsu Education Department (16KJB610002), and the Postdoctoral Science Foundation of China (2017M611784). References [1] M. Yang, Q. Yang, J. Zhong, J. Li, S. Huang, X. Li, Mater. Lett. 201 (2017) 35-38. [2] X. Meng, Z. Zhang, Mater. Lett. 225 (2018) 152-156. [3] A. Zhang, F. Teng, Y. Zhai, Z. Liu, W. Hao, Z. Liu, Z. Yang, W. Gu, J. Alloy. Compd. 794 (2019) 127-136. [4] S. Vadivel, V. P. Kamalakannan, N. P. Kavitha, T. S. Priya, N. Balasubramanian, Mat. Sci. Semicon. Proc. 41 (2016) 59-66. [5] C. C. Chen, J. Y. Fu, J. L. Chang, S. T. Huang, T. W. Yeh, J. T. Hung, P. H. Huang, F. Y. Liu, L. W. Chen, J. Colloid Interf. Sci. 532 (2018) 375-386. [6] T. Jiang, J. Li, Y. Gao, L. Li, T. Lu, L. Pan, J. Colloid Interf. Sci. 490 (2017) 812-818. [7] X. Wang, Q. Xu, M. Li, S. Shen, X. Wang, Y. Wang, Z. Feng, J. Shi, H. Han, C. Li, Angew. Chem. Int. Edit. 51 (2012) 13089-13092. [8] X. Chen, L. Liu, P. Y. Yu, S.S. Mao, Science 331 (2011) 746-750. 9
[9] Y. Guan, H. Qian, J. Guo, S. Yang, X. Wang, S. Wang, Y. Fu, Appl. Clay Sci. 114 (2015) 124-132. [10]Y. Guan, S. Wang, X. Wang, C. Sun, Y. Huang, C. Liu, H. Zhao, Appl. Catal. B-Environ. 209 (2017) 329-338. [11]L. Hu, S. Dong, Q. Li, J. Feng, Y. Pi, M. Liu, J. Sun, J. Sun, J. Alloy. Compd. 633 (2015) 256-264.
Author contributions section 1. Yuan Guan was responsible for writing manuscript and material preparation.
2. Shaomang Wang was responsible for overall experimental design and manuscript revision. 3. Zhongyu Li was responsible for manuscript revision. 4. Xun Ding and Mingfei Wu were responsible for MO degradation using as-prepared
samples. 5. Mingmin Zhang and Weifeng Yu were responsible for test characterization.
Highlights 1. Polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with BiOF was first prepared. 2. The bismuth oxyfluoride exhibited distinctly enhanced photocatalytic activity. 3. A heterophase junction between BiO0.51F1.98 and BiOF greatly promoted its activity.
Conflict of interest The authors declare no conflict of interest.
10
Declaration of Interest Statement We have thanked all the fund-funded institutions, and all the authors who contributed to the paper have been added. No conflict of interest.
Polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF achieving distinctly enhanced photocatalytic activity Yuan Guana, Shaomang Wangb,*, Zhongyu Lia, Xun Dinga, Mingfei Wua, Mingmin Zhanga, Weifeng Yua
a School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, P. R. China. b School of Environment and Safety Engineering, Changzhou University, Changzhou, Jiangsu 213164, P. R. China. E-mail address: [email protected] 2.2. characterization The crystal structures of samples were determined by a ARL X’TRA X-ray diffractometer (XRD) using Cu Kα radiation (λ=0.154 nm) in the 2theta range of 5– 80◦. Morphologies of as-obtained materials were observed through a scanning electron microscope (SEM) at an accelerating voltage of 2.0 kV (HITACHI SU8020). The surface electronic state was identified through X-ray photo-electron spectroscopy (XPS) performed on a PHI 5000 Versa Probe electron spectrometer using Al Kα radiation. The UV–vis diffuse reflectance spectra (DRS) were analyzed by a Shimadzu UV2550 spectrophotometer with BaSO4-coated integrating sphere in the wavelength range of 300–800 nm. The photoluminescence (PL) spectra were collected by a HORIBA Fluoromax-4 with an excitation wavelength at 320 nm. The photocurrent measurement was determined in an electrochemical workstation (CHI660D) with a standard three electrode system. 0.5 M of Na2SO4 aqueous solution 11
was used as the electrolyte. A
xenon lamp
was employed as the light source.
Electrochemical impedance spectroscopy (EIS) was tested from 0.1 Hz to 100 kHz at an open circuit potential of 0.3 V and an alternating current (AC) voltage amplitude of 5 mV. 0.025 M of K3[Fe(CN)6] aqueous solution was used as the electrolyte.
Fig.S1 The pseudo-second-order kinetic plots of MO over BiO0.51F1.98, BiOCl and calcined.
12
S0167-577X(19)31829-4 https://doi.org/10.1016/j.matlet.2019.127197 MLBLUE 127197
To appear in:
Materials Letters
Received Date: Revised Date: Accepted Date:
25 October 2019 3 December 2019 17 December 2019
Please cite this article as: Y. Guan, S. Wang, Z. Li, X. Ding, M. Wu, M. Zhang, W. Yu, Polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF achieving distinctly enhanced photocatalytic activity, Materials Letters (2019), doi: https://doi.org/10.1016/j.matlet.2019.127197
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2019 Published by Elsevier B.V.
Polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF achieving distinctly enhanced photocatalytic activity Yuan Guana, Shaomang Wangb,*, Zhongyu Lia, Xun Dinga, Mingfei Wua, Mingmin Zhanga, Weifeng Yua a School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, P. R. China. b School of Environment and Safety Engineering, Changzhou University, Changzhou, Jiangsu 213164, P. R. China. E-mail address: [email protected] Abstract: Polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF was successfully synthesized by solvothermal and subsequent calcination at 375 ℃ in air for the first time. The photocatalytic degradation rate of methyl orange over BiO0.51F1.98 with self-doped BiOF was 4.9 times that over pure BiO0.51F1.98. Significantly enhanced photocatalytic activity of BiO0.51F1.98 with self-doped BiOF was basically ascribed to broadened visible-light absorption, and lower recombination rate of carries. Keywods: Photocatalysis; Semiconductors; Solar energy materials; BiO0.51F1.98; BiOF 1. Introduction Over the past decade, bismuth oxyhalide compounds (BiOX, X = Cl, Br, I) have been studied widely because of remarkable chemical stability, unique layer structure and excellent photocatalytic performance under UV or visible-light illumination [1, 2]. 1
As a member of bismuth oxyhalide, BiOF has also gained a lot of attention on its photocatalytic activity recently. BiOF possesses a direct band-gap value of about 3.5 eV, which makes it sensitive to UV region. The latest research indicates that BiOF can degrade effectively many organic pollutants under UV light irradiation [1, 3]. Unfortunately, the wide band gap of BiOF greatly limits its real application. Although the light absorption of BiOF was broadened and its photocatalytic activity was enhanced by adding metal or non-metal impurities [4-6], the photocatalytic performance of modified BiOF is still not ideal. Until now, fluorine-rich BiO0.51F1.98, which is different from the crystal structure of BiOF is rarely reported as a photocatalyst. Li et al found that when the photocatalytic materials with same elements and different crystal structures such as anatase-rutile of TiO2 and a–β phases of Ga2O3 formed a heterophase junction, the photocatalytic activity of the heterophase material was greatly improved [5, 7]. Based on this design idea, we successfully prepared a polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF via a facile solvothermal approach coupled with thermal decomposition for the first time. The results of photocatalytic degradation of methyl orange (MO) demonstrated that the photocatalytic activity of the polycrystalline bismuth oxyfluoride was significantly enhanced by forming a heterophase junction between BiO0.51F1.98 and BiOF. 2. Experimental 2.1. Preparation of samples BiO0.51F1.98 was synthesized using simple solvothermal method. In a typical 2
procedure, 4.0 g of Bi(NO3)3·5H2O and 0.34 g of NaF were dissolved in 30 mL of ethylene glycol, respectively. The NaF solution was added into the Bi(NO3)3·5H2O solution dropwise with vigorous stirring. After stirring for 60 min, the mixed solution was transferred into an autoclave and maintained at 180 ℃ for 10 h. The resulting product was washed three times with distilled water and absolute ethanol, and finally dried at 80 ℃ for 4 h to obtain single BiO0.51F1.98 crystal. The polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF was prepared through thermal decomposition method. 2.0 g of as-prepared BiO0.51F1.98 was placed into a ceramic crucible, which was calcined at 375 ℃ for 6 h in a muffle furnace. After the furnace was naturally cooled to room temperature, the polycrystalline bismuth oxyfluoride of BiO0.51F1.98 and BiOF (calcined BiO0.51F1.98) was obtained. 2.2. Characterization Detailed characterization section was presented in supporting information. 2.3. Photocatalytic activity evaluation of samples In a typical test of photocatalysis, a 500 W xenon lamp was used as the light source. 0.5 g L-1 of photocatalyst was added into MO aqueous solution (10 mg L-1). At specified intervals, 4 mL of the suspensions were extracted and centrifuged to remove the catalysts. According to the maximum absorption wavelength of MO at 464 nm, the concentration of the sample was analyzed via a UV–vis spectrophotometer. To estimate the overall MO degradation process, the rate constant k was calculated by pseudo–second-order model( Eq. (1)). 3
1 𝐶
1
(1)
― 𝐶0 = 𝑘𝑡
Where C0 is the initial concentration, and C is the concentration at certain reaction time t. 3. Results and discussion 3.1. Characterization of samples
Fig. 1. (a) XRD patterns of BiO0.51F1.98 and calcined BiO0.51F1.98, (b) SEM images of BiO0.51F1.98, (c) and (d) SEM images of calcined BiO0.51F1.98.
The XRD patterns of pure and calcined BiO0.51F1.98 are shown in Fig. 1a. For pure BiO0.51F1.98, the characteristic peaks locating at 2θ values of 26.4◦, 30.6◦, 43.7◦, 51.8◦, and 54.2◦ were indexed to (111), (200), (220), (311) and (222) crystal planes of cubic BiO0.51F1.98, respectively (PDF No. 24–0147). No other peaks were observed, implying high purity of the sample. The pattern of BiO0.51F1.98 calcined at 375 ℃ 4
corresponded to a polycrystalline bismuth oxyfluoride of cubic BiO0.51F1.98 with tetragonal BiOF (PDF No. 22–0114). The results of XRD indicated that the polycrystalline bismuth oxyfluoride containing cubic BiO0.51F1.98 and tetragonal BiOF was successfully fabricated by calcining pure BiO0.51F1.98 at appropriate temperature in air. As presented in Fig. 1b, the morphology of BiO0.51F1.98 contained plate-like primary particles with thickness of less than 20 nm, which were interrelated and interacted on each other to form agglomerated secondary particles. The SEM images of calcined BiO0.51F1.98 are shown in Fig. 1c-d. The sample was composed of uniform microspheres with average diameter of 10 μm. The microspheres were consisted of irregular small-thin nanosheet, and large pores were formed. Compared with pure BiO0.51F1.98, the nanosheets of calcined BiO0.51F1.98 became much thinner, which suggests that heat-treatment process can lead to different crystal growth direction.
Fig. 2. XPS spectrum of (a) survey scan, (b) C 1s, (c) Bi 4f, (d) O 1s, (e) F 1s and (f) valence band 5
of BiO0.51F1.98 and calcined BiO0.51F1.98.
XPS was used to analyze the surface chemical composition and states of pure and calcined BiO0.51F1.98. As depicted in Fig. 2a, the emergence of the C 1s, Bi 4f, O 1s and F 1s peaks in the survey spectra revealed that the composites were constituted by the elements of C, Bi, O and F. Fig. 2b displays C 1s XPS measurements of pure and calcined BiO0.51F1.98 in the energy range in order to detect possible doped carbon in BiO0.51F1.98 lattice [8]. The first C1s peak at 284.83 eV belonged to carbon tape with C0, and the second at 287.88 eV was identified as doped carbon atoms (C=O or C-F). Compared with pure BiO0.51F1.98, the intensity of doped carbon peak in calcined BiO0.51F1.98 decreased obviously, which illustrates that the heat-treatment procedure can effectively remove a number of carbon impurity. From Fig. 2c, in BiO0.51F1.98, two peaks at 159.13 and 164.43 eV were assigned to Bi 4f7/2 and Bi 4f5/2, both of which had a shift of 0.3 eV to low energy for calcined BiO0.51F1.98, respectively. The O 1s XPS spectra of BiO0.51F1.98 was fitted into two peaks locating at 529.83 and 531.83 eV, which is attributed to lattice oxygen and surface adsorbed oxygen species [9]. Similarly, the two typical peaks of calcined BiO0.51F1.98 shifted to lower binding energies as well. According to Fig. 2e, the F 1s peaks at 682.93 eV are ascribable to F element with monovalent oxidation state. The binding energy value of F 1s was identical for BiO0.51F1.98 and calcined BiO0.51F1.98. Besides, The valence bands of BiO0.51F1.98 and calcined BiO0.51F1.98 were measured by XPS (Fig. 2f). The pure BiO0.51F1.98 showed valence-band edge of about 1.52 eV. For calcined BiO0.51F1.98, the valence-band edge shifted to 1.18 eV, indicating that the Femi level of calcined 6
BiO0.51F1.98 shifts upward of 0.34 eV with respect to that of BiO0.51F1.98 [6, 8].
Fig. 3. (a) UV–vis DRS, (b) the band-gap energies, (c) PL spectra, (d) photocurrent density, (e) impedance of BiO0.51F1.98 and calcined BiO0.51F1.98, and (f) MO removal profiles of BiO0.51F1.98, BiOCl and calcined BiO0.51F1.98.
The UV–vis DRS and band gap of pure and calcined BiO0.51F1.98 are illustrated in Fig. 3a-b. It was found that the absorption edge of pure and calcined BiO0.51F1.98 located at 360 nm and 400 nm in the spectrum. According to the formula obtained from the literature [10], the corresponding band-gap values of the samples were 3.31 eV and 3.06 eV, respectively. For calcined BiO0.51F1.98, a red-shift was observed for the absorption edge. To investigate the recombination behavior of photo-generated carriers, Fig. 3c displays the PL spectra of pure and calcined BiO0.51F1.98 with the excitation wavelength at 320 nm. It was observed that the PL intensity of calcined BiO0.51F1.98 was much lower than pure BiO0.51F1.98, suggesting that calcined BiO0.51F1.98 owns a 7
lower recombination rate of photo-induced electrons and holes [11]. The separation efficiency of photo-generated carriers was further confirmed by photocurrent density (Fig. 3d) and EIS (Fig. 3e). It was seen that the calcined BiO0.51F1.98 possessed much higher photocurrent and lower electrical resistance. 3. 2. Photocatalytic activity evaluation Fig. 3f shows the photocatalytic activity of BiO0.51F1.98, BiOCl and calcined BiO0.51F1.98 for MO degradation under light irradiation. The blank experiment displayed that the self-photodegradation of MO was negligible. Prior to the irradiation, the suspension was magnetically stirred in dark for 30 min to establish an adsorption/desorption equilibrium between the MO and the catalyst surface. It was found that the polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF exhibited much better photocatalytic activity than pure BiO0.51F1.98 and BiOCl. In addition, as shown in Fig. S1, the rate constants (k) were 7.05×10-4, 1.47×10-3 and 0.0141 min-1 for BiO0.51F1.98, BiOCl and calcined BiO0.51F1.98, respectively. The calcined BiO0.51F1.98 owned a maximum rate constant, which was 9.6 and 20 times higher than that of BiOCl and BiO0.51F1.98, respectively. 4. Conclusions Compared with pure BiO0.51F1.98, the polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF indeed could exhibit significantly enhanced photocatalytic activity. This novel polycrystalline bismuth oxyfluoride will increase research interest in the use of bismuth oxyfluoride as a photocatalyst to decompose organic pollutants. 8
Conflict of interest The authors declare no conflict of interest. Acknowledgments This work was supported primarily by the National Natural Science Foundation of China (21876015), the Natural Science Foundation of Jiangsu Province (BK20161277 and BK20190934), the Natural Science Foundation of Jiangsu Education Department (16KJB610002), and the Postdoctoral Science Foundation of China (2017M611784). References [1] M. Yang, Q. Yang, J. Zhong, J. Li, S. Huang, X. Li, Mater. Lett. 201 (2017) 35-38. [2] X. Meng, Z. Zhang, Mater. Lett. 225 (2018) 152-156. [3] A. Zhang, F. Teng, Y. Zhai, Z. Liu, W. Hao, Z. Liu, Z. Yang, W. Gu, J. Alloy. Compd. 794 (2019) 127-136. [4] S. Vadivel, V. P. Kamalakannan, N. P. Kavitha, T. S. Priya, N. Balasubramanian, Mat. Sci. Semicon. Proc. 41 (2016) 59-66. [5] C. C. Chen, J. Y. Fu, J. L. Chang, S. T. Huang, T. W. Yeh, J. T. Hung, P. H. Huang, F. Y. Liu, L. W. Chen, J. Colloid Interf. Sci. 532 (2018) 375-386. [6] T. Jiang, J. Li, Y. Gao, L. Li, T. Lu, L. Pan, J. Colloid Interf. Sci. 490 (2017) 812-818. [7] X. Wang, Q. Xu, M. Li, S. Shen, X. Wang, Y. Wang, Z. Feng, J. Shi, H. Han, C. Li, Angew. Chem. Int. Edit. 51 (2012) 13089-13092. [8] X. Chen, L. Liu, P. Y. Yu, S.S. Mao, Science 331 (2011) 746-750. 9
[9] Y. Guan, H. Qian, J. Guo, S. Yang, X. Wang, S. Wang, Y. Fu, Appl. Clay Sci. 114 (2015) 124-132. [10]Y. Guan, S. Wang, X. Wang, C. Sun, Y. Huang, C. Liu, H. Zhao, Appl. Catal. B-Environ. 209 (2017) 329-338. [11]L. Hu, S. Dong, Q. Li, J. Feng, Y. Pi, M. Liu, J. Sun, J. Sun, J. Alloy. Compd. 633 (2015) 256-264.
Author contributions section 1. Yuan Guan was responsible for writing manuscript and material preparation.
2. Shaomang Wang was responsible for overall experimental design and manuscript revision. 3. Zhongyu Li was responsible for manuscript revision. 4. Xun Ding and Mingfei Wu were responsible for MO degradation using as-prepared
samples. 5. Mingmin Zhang and Weifeng Yu were responsible for test characterization.
Highlights 1. Polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with BiOF was first prepared. 2. The bismuth oxyfluoride exhibited distinctly enhanced photocatalytic activity. 3. A heterophase junction between BiO0.51F1.98 and BiOF greatly promoted its activity.
Conflict of interest The authors declare no conflict of interest.
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Declaration of Interest Statement We have thanked all the fund-funded institutions, and all the authors who contributed to the paper have been added. No conflict of interest.
Polycrystalline bismuth oxyfluoride of BiO0.51F1.98 with self-doped BiOF achieving distinctly enhanced photocatalytic activity Yuan Guana, Shaomang Wangb,*, Zhongyu Lia, Xun Dinga, Mingfei Wua, Mingmin Zhanga, Weifeng Yua
a School of Petrochemical Engineering, Changzhou University, Changzhou, Jiangsu 213164, P. R. China. b School of Environment and Safety Engineering, Changzhou University, Changzhou, Jiangsu 213164, P. R. China. E-mail address: [email protected] 2.2. characterization The crystal structures of samples were determined by a ARL X’TRA X-ray diffractometer (XRD) using Cu Kα radiation (λ=0.154 nm) in the 2theta range of 5– 80◦. Morphologies of as-obtained materials were observed through a scanning electron microscope (SEM) at an accelerating voltage of 2.0 kV (HITACHI SU8020). The surface electronic state was identified through X-ray photo-electron spectroscopy (XPS) performed on a PHI 5000 Versa Probe electron spectrometer using Al Kα radiation. The UV–vis diffuse reflectance spectra (DRS) were analyzed by a Shimadzu UV2550 spectrophotometer with BaSO4-coated integrating sphere in the wavelength range of 300–800 nm. The photoluminescence (PL) spectra were collected by a HORIBA Fluoromax-4 with an excitation wavelength at 320 nm. The photocurrent measurement was determined in an electrochemical workstation (CHI660D) with a standard three electrode system. 0.5 M of Na2SO4 aqueous solution 11
was used as the electrolyte. A
xenon lamp
was employed as the light source.
Electrochemical impedance spectroscopy (EIS) was tested from 0.1 Hz to 100 kHz at an open circuit potential of 0.3 V and an alternating current (AC) voltage amplitude of 5 mV. 0.025 M of K3[Fe(CN)6] aqueous solution was used as the electrolyte.
Fig.S1 The pseudo-second-order kinetic plots of MO over BiO0.51F1.98, BiOCl and calcined.
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