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Preparation of BiOBr-Bi heterojunction composites with enhanced photocatalytic properties on BiOBr surface by in-situ reduction
Materials Science in Semiconductor Processing 108 (2020) 104882
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Preparation of BiOBr-Bi heterojunction composites with enhanced photocatalytic properties on BiOBr surface by in-situ reduction Zhanyao Gao a, b, Binghua Yao a, *, Fan Yang a, Tiantian Xu c, Yangqing He a a
Department of Material Physics and Chemistry, Xi’an University of Technology, Xi’an, 710048, China Department of Environmental Engineering, Xi’an Polytechnic University, Xi’an, 710048, China c Key Laboratory of Thermo-Fluid Science and Engineering of MOE, Xi’an Jiaotong University, Xi’an, 710049, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: BiOBr nanosheets Electron–hole pairs Antibiotic Kinetic analysis Photocatalysis mechanism
A new kind of BiOBr-Bi composite with improved photocatalytic capabilities was successfully prepared by in-situ reduction. The X-ray diffraction (XRD), Scanning electron microscopy (SEM), Ultraviolet–visible spectroscopy (UV–VisDRS), X-ray photoelectron spectroscopy (XPS), Brunner-Emmet-Teller measurements (BET), Fourier transform infrared spectroscopy (FT-IR), photoluminescence (PL) and the electron paramagnetic resonance (EPR) were used to characterize the morphological structures, physical properties and surface element compo sition of the photocatalysts. In addition, photocatalytic activity was evaluated by degrading the antibiotic nor floxacin (NOR). The results showed that the BiOBr-Bi (40 mmol/L NaBH4) had the highest activity. The degradation rate reached 97.2%. The half-life of NOR was shortened to 25 min. Moreover, the results of capture experiments indicated that the main active groups involved in the photocatalytic process under visible light conditions were ⋅O-2 and hþ. The reason why the activity of the BiOBr-Bi material photocatalyst increased is that the heterojunction formed by BiOBr with wider bandgap and Bi metal with a lower Fermi level effectively in hibits the recombination of holes and photogenerated electrons, which leads to the enhancement of oxidation capacity.
1. Introduction Inappropriate use of antibiotics is a significant public health prob lem. At present, humans are more prominent in the application of an tibiotics, but they also seriously affect the ecological environment [1]. According to relevant data, Sulfonamide resistance gene (sul1), Tetra cycline resistance gene (tet-A), β-lactamase resistance gene (bla TEM) and Streptomycin resistance gene (strA) can be detected in domestic water treatment plants and reservoirs, indicating several resistance ge notypes of antibiotics such as sulfonamides, streptomycin, tetracycline and ampicillin [2]. There has been a data display that China uses more than 160 thousand tons of antibiotics each year, about 52% for veteri nary use, about 48% for medical treatment, and more than 50 thousand tons of antibiotics are discharged into the soil and water environment [3, 4]. This causes serious pollution to the water environment. Containment of antibiotic pollution is imminent. So far, photocatalytic technology has been identified as one of the fastest, most effective and concise methods for degrading organic wastewater [5,6]. TiO2 is widely studied because of its excellent photocatalytic activity and stability [7–9]. However, due
to the poor response to visible light, great efforts have been made in the development of widening the range of visible light response in order to effectively use solar energy, and a large number of photocatalytic ma terials have been studied [10]. It is reported that the Bismuth series semiconductors have suitable visible light response band gap, and the valence bands composed of the O 2p and Bi 6s hybrid orbitals are uni formly distributed. Therefore, they have obvious catalysis and good application prospects [11]. However, the pure single material has the disadvantage of higher hþ-e- recombination rate, further research is needed to improve its photocatalytic activity. There are two main ways to ameliorate the photocatalytic ability of BiOBr. One is to change the morphology of BiOBr by changing the hydrothermal reaction conditions to improve its photocatalytic ability. Lu [12] has prepared a series of BiOBr nano/microstructures such as nanosheets, wafers, nanorods, and microspheres by hydrothermal method and adding different additives. It is confirmed that the morphology and size of the samples have a very large influence on the catalytic effect. Taking RhB as the substrate for degradation, and the nanosheets show stronger photocatalytic ability than the other three morphologies. On the other hand, in order to break
* Corresponding author. E-mail address: [email protected] (B. Yao). https://doi.org/10.1016/j.mssp.2019.104882 Received 19 April 2019; Received in revised form 14 October 2019; Accepted 10 December 2019 Available online 12 December 2019 1369-8001/© 2019 Published by Elsevier Ltd.
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this limitation, the coupling of the semiconductor precursor with the appropriate band gap is improved through synthesizing the photo catalytic composite. Thereby, the separation effect of the photocatalytic carriers is achieved, and the photocatalytic efficiency is greatly improved [13–15]. Cui et al. have synthesized CdS/BiOBr photocatalytic composites and compared them with a pure material [16]. It is found that the degradation of malachite green (MG) by CdS/BiOBr photo catalytic composites under visible light irradiation is much higher than that of single catalytic materials, which is mainly due to the fact that the formation of heterostructures promotes the separation of optical car riers. The photogenerated electrons generated by CdS and the hole pairs generated by BiOBr are easily transferred to the corresponding positions of the other part. Their effective separation can significantly improve the catalytic performance of the composite photocatalyst. According to re ports, Zhang et al. synthesized Bi2WO6-Bi composite materials by an in-situ photoreduction method. Taking RhB as degradation target, the photocatalytic degradation rate of Bi2WO6-Bi under visible light irra diation was 2.4 times that of the pure Bi2WO6 [17]. Dong et al. reported the preparation of BiOCl-Bi composite photocatalyst and the photo catalytic performance of removing nitric oxide. The results show that BiOCl-Bi exhibits higher photocatalytic performance than pure BiOCl and BiOCl-Ag in terms of photocatalytic removal of NO [18]. Comparing the two approaches, the latter is simpler and more efficient and its pa rameters are easier to control. Nowadays, a large number of theoretical studies and experimental results show that it will always be one of the most effective methods through the creation of band gap coupling and lattice heterostructure to improve the separation efficiency of hole electron pairs or extend the energy range of optical excitation [19,20]. This research used Bi(NO3)3 and NaBr as raw materials, and prepared BiOBr nanosheets via hydrolysis method in water at room temperature. On this basis, BiOBr-Bi was synthesisd by a simple chemical reduction reaction. A part of Bi3þ on the surface of the BiOBr nanosheet was reduced in situ to a simple substance of Bi, and Bi nanowires were formed on the surface of BiOBr to form a BiOBr-Bi composite photo catalyst. The XRD, SEM, UV–VisDRS, XPS, BET, FT-IR, PL and EPR were used to characterize the morphological structures, physical properties and surface element composition of the photocatalysts. The photo catalytic activity of the samples prepared under visible light irradiation was evaluated by degradation of NOR at a concentration of 20 mg/L. In addition, the photocatalytic mechanism of BiOBr-Bi composites was discussed by free radical trapping experiments and EPR test experiments.
stirred for 1 h and then aged for an additional hour. The resulting pre cipitate was collected, rinsed four times with ethanol and deionized water, and dried at 80 � C in air. The prepared samples were classified from low to high with the concentration of adding NaBH4, respectively named as BiOBr, 10 mmol/L NaBH4/BiOBr-Bi, 20 mmol/L NaBH4/ BiOBr-Bi, 30 mmol/L NaBH4/BiOBr-Bi, 40 mmol/L NaBH4/BiOBr-Bi, 50 mmol/L NaBH4/BiOBr-Bi. 2.4. Characterization The crystal structures of the sample were characterized by XRD-6100 ray diffractometer (Shimadzu, Japan). The Cu K� a target was operated at the voltage of 40 kV, the current of 30 mA, and € e of 0.15418 nm. The 2�e range is 10� –80� . The scanning speed was 10� min 1, and the data was collected by continuous scanning. The sample was analyzed by scanning electron microscope with high resolution (FEI, UK). X-ray photoelectron spectroscopy (T Escalab, America) was used to study surface properties. The UV–visible absorption spectroscopy was measured by TU-1901 dual-beam UV–visible diffuse reflectance spectrophotometer (Beijing Puji General Instrument Co., Ltd.), using BaSO4 as a reference, and the diffuse reflectance was converted to absorbance by the ulblka-Munk method. The structural information for samples was measured by Fourier transform spectrophotometer (FT-IR, Avatar470, Thermo Nico let) using the standard KBr disk method. The pore structure of the ob tained sample was characterized by N2 adsorption using an adsorption apparatus (Quantachrome autosorb-iQ-2MP gas sorption analyzer). Specific surface area of the samples was determined from the BrunauerEmmett-Teller (BET) equation, and pore volume was determined from the total amount adsorbed at relative pressures near unity. The photo luminescence spectra were obtained by an F-280 spectrometer (Tianjin Keqi, China). The photoluminescence (PL) spectra were obtained using an Edinburgh Analytical Instrument FL/FSTCSPC920 Spectrophotom eter. The electron paramagnetic resonance (EPR) spectra were recorded on a Bruker (A200–9.5/12) spectrometer with the X-band frequency of 9.8 GHz under room temperature (the magnetic field was tuned at 100 kHz). The scavengers of holes, Superoxide anion and hydroxyl radicals used in capture experiment were ethylenediamine tetraacetic acid (EDTA), benzoquinone (BQ) and isopropanol (IPA), respectively. 2.5. Photocatalytic activity measurement Usually, 50 mg of the photocatalyst was weighed and added to a quartz tube containing 50 mL of NOR aqueous solution (20 mg/L). The quartz tube was placed in the photocatalytic reactor for dark reaction for 30 min, and then the light source (500 W xenon lamp) was turned on for photocatalytic degradation. 4 mL of the suspension was taken every 30 min, centrifuged, and the absorbance of the supernatant in the 278 nm band was measured (The maximum absorbance of NOR solution at 278nm was measured by UV–Vis spectrometer). The photocatalytic ac tivity is proportional to the rate of degradation, and the degradation rate is calculated by the following formula.
2. Experimental 2.1. Materials All chemicals and reagents mentioned in this article are of analytical grade and can be used directly. 2.2. Preparation of BiOBr nanosheets 30 mL of ultrapure water was prepared, and 0.97 g of Bi(NO3)3⋅5H2O (2 mmol) and 0.206 g of NaBr (2 mmol) were separately added and stirred for 30 min. The mixture was transferred to a reaction kettle and calcined at 180 � C for 24 h. After cooling, the precipitate was collected and washed three times with ethanol and ultrapure water. The product is dried and ready for use.
η¼
А0
Аt А0
� 100%
(1)
where η is the degradation rate, A0 is the initial absorbance of the so lution, At is the absorbance of the solution after t min reaction. 3. Results and discussion
2.3. Preparation of BiOBr-Bi nanosheets
3.1. Materials characterization
BiOBr-Bi was synthesisd by a simple chemical reduction reaction. Usually, 2 mmol of BiOBr was added to 100 mL of distilled water con taining 1.0 g of polyvinyl pyrrolidone (PVP) and stirred for 20 min. Then a certain concentration of NaBH4 (0, 10, 20, 30, 40 and 50 mmol/L) was added dropwise to the suspension. Next, the resulting suspension was
The XRD pattern of the BiOBr-Bi photocatalyst is shown in Fig. 1. The XRD peaks of BiOBr-Bi are highly fitted with the tetragonal phase of BiOBr (JCPDS 09–0393) [21]. In the XRD patterns of BiOBr-Bi, besides the diffraction peaks of the Quartet phase BiOBr, the diffraction peaks 2
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Fig. 1. XRD patterns of the samples. (a) 10 mmol/L NaBH4/BiOBr-Bi, (b) 20 mmol/L NaBH4/BiOBr-Bi, (c) 30 mmol/L NaBH4/BiOBr-Bi, (d) 40 mmol/L NaBH4/ BiOBr-Bi, (e) 50 mmol/L NaBH4/BiOBr-Bi.
appeared at 2θ ¼ 27.2� , 37.9� , 39.6� and 62.2� are in accordance with the standard cards of the single crystal Bi (JCPDS 05–0519) [22], which correspond to (012), (104), (110) and (122) planes, respectively. In addition, no other impurity peaks appear, indicating that the BiOBr-Bi samples are of high purity. As shown in Fig. 1, the position of the BiOBr characteristic peaks do not shifted, indicating that the Bi nano wire do not damage the crystal structure of BiOBr or enter the crystal lattice. With the concentration of NaBH4 increasing from 10 mmol/L to 50 mmol/L, the diffraction characteristic peak of Bi metal in the diffraction spectrum of BiOBr-Bi composite photocatalyst appeared and enhanced continuously, means that the content of Bi metal is increasing.
SEM can be used to observe the microscopic morphology and dimension of the prepared samples. As seen from Fig. 2(a), the morphology of the pure BiOBr sample is a lamellar structure with irregular size, irregular shape, and relatively dispersed. In Fig. 2(b-f), it is obvious that an irregular sheet structure with sizes ranging between 2 μm and 4 μm and uneven size is seen, to a large extent, maintains the original morphology of the BiOBr. In addition, the formation of the Bi nanowire is obviously seen on the sheet structure, which is due to the addition of NaBH4 make Bi3þ on the surface of BiOBr be reduced to Bi metal. The results are in accordance with the results of XRD analysis. The EDS spectrum of 40 mmol/L NaBH4/BiOBr-Bi was analyzed and
Fig. 2. Typical SEM images of the obtained products. (a) BiOBr, (b) 10 mmol/L NaBH4/BiOBr-Bi, (c) 20 mmol/L NaBH4/BiOBr-Bi, (d) 30 mmol/L NaBH4/BiOBr-Bi, (e) 40 mmol/L NaBH4/BiOBr-Bi, (f) 50 mmol/L NaBH4/BiOBr-Bi. 3
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the result shown in Fig. 3. The results show that the BiOBr-Bi photo catalyst contains C, O, Br and Bi elements. It is confirmed that the pre pared sample is BiOBr-Bi composite. This result is in good agreement with XRD, and SEM analysis. The UV–visible diffuse reflectance spectrum of the samples is shown in Fig. 4(a). The absorption edges of the BiOBr nanosheets, 10 mmol/L NaBH4/BiOBr-Bi, 20 mmol/L NaBH4/BiOBr-Bi, 30 mmol/L NaBH4/ BiOBr-Bi, 40 mmol/L NaBH4/BiOBr-Bi, and 50 mmol/L NaBH4/BiOBrBi are 423 nm, 434 nm, 466 nm, 475 nm, 502 nm, and 438 nm, respectively. This indicates that both the BiOBr nanosheet and the BiOBr-Bi composite photocatalyst can degrade the NOR under visible light. Compared with pure BiOBr, the absorption edge of BiOBr-Bi composites have a blue-shift. Typically, the blue-shift of 40 mmol/L NaBH4/BiOBr-Bi is the strongest. The results demonstrate that the 40 mmol/L NaBH4/BiOBr-Bi composite is a promising candidate in degrading pollutant. By means of the Kubelka-Munk function, the relationship between the band gap Eg and the absorption coefficient α is explored by using the graph of (αhν) 2 and photoenergy (hν). The curve equation is as follows [23]: Еg
�n2 =
ahv ¼ А hv
changes the location of the conduction band and is more conducive to the production of active substances. These may be the reasons for the enhancement of visible light absorption and the improvement of pho tocatalytic activity. However, when large amount of Bi element pro duced by the surface reduction of BiOBr, the main composition of the composite surface changed and the light transmittance was also low ered, thereby limiting the photocatalytic activity of the catalyst. It is also supported by the above SEM analysis. X-ray photoelectron spectroscopy (XPS) can further study the surface element composition and chemical state of the prepared samples. The constituent elements of BiOBr and BiOBr-Bi shown in Fig. 5(a) are Bi, Br, O and C. The appearance of the C element is generated by the XPS equipment. The other peaks were produced by photocatalyst elements, which proves that the sample prepared is of higher purity. The XPS peak of Bi 4f is observed in Fig. 5(b). The peak positions of Bi 4f7/2 and Bi 4f5/2 of BiOBr and BiOBr-Bi are both 159.68 eV and 164.98 eV, indicating that the Bi element mainly exists in the trivalent form [26]. In addition, BiOBr-Bi also appears two peaks at 160.18 eV and 165.28 eV, which is due to the formation of Bi nanowire space [17]. The results of UV–visible diffuse reflection are further confirmed. Fig. 5(c) shows the XPS peak of Br 3d [27–29], where the peak position of BiOBr is at 68.78 eV and the peak position of BiOBr-Bi is moved to 69.38 eV. This is possibly also related to the formation of the Bi nanowire space. In Fig. 5(d), BiOBr and BiOBr-Bi respectively exhibit binding energies of 530.78 eV and 531.58 eV which are attributed to O 1s [30–33]. It is reported that the oxygen of BiOBr is mainly present in the form of –OH [34], while the oxygen of BiOBr-Bi is mainly present in the form of molecular oxygen (⋅O-2, etc.) [35]. The FT-IR spectra of the pure BiOBr and BiOBr-Bi composites are shown in Fig. 6. The absorption band at about 3500 cm 1 is attributed to O-H stretching and bending vibrations in free water molecules [36]. The peak at about 1407 cm 1 region was assigned to the Bi-Br band in BiOBr [37]. In particular, pure BiOBr has an absorption band in the 400-750 cm 1 region [38,39]. However, it can be observed that as the content of Bi in the metal increases, the absorption vibration band undergoes a blue shift, which may be due to the fact that the metal Bi has a certain in fluence on the vibration of BiOBr. This result is consistent with the re sults of the XRD and XPS experiments. It is well known that larger BET and pore volume materials corre spond to more surface active sites and easier transport paths, which effectively absorb light energy and promote separation of electron-hole pairs, thereby improving photocatalytic performance [40,41]. The N2 adsorption-desorption isotherms of BiOBr and 40 mmol/L NaBH4/ BiOBr-Bi are shown in Fig. 7. According to the IUPAC classification standard, the adsorption hysteresis loop of the sample belongs to type IV pattern with H3-type (typical mesoporous material). The specific surface areas of BiOBr and 40 mmol/L NaBH4/BiOBr-Bi shown in Fig. 7(a) were 15.72 m2/g and 38.25 m2/g, respectively. Obviously, specific surface area of the composite is larger than that of pure BiOBr, providing more active sites for photocatalytic reactions. In addition, it can be seen from Fig. 7(b) that the pore volume of the composite is significantly larger than that of pure BiOBr, which can provide an efficient reactant trans port path. Photoluminescence (PL) emission is a common technique for detecting the separation efficiency of photogenerated electron-hole pairs [42]. As shown in Fig. 8, the emission peaks of all samples were concentrated at around 468 nm. Lower PL intensities usually indicate lower photogenerated e--hþ recombination rates [26]. Obviously, 40 mmol/L NaBH4/BiOBr-Bi showed the lowest PL intensity. Compared with the pure BiOBr, it shows that the charge recombination decreases significantly and has higher photogenic charge separation efficiency.
(2)
where α is the absorption coefficient, ν is the frequency of the light, and h is Planck’s constant [24]. Fig. 4(b) shows the relationship between (αhν) 2 and hν. The band gap energy (Eg) of samples are known from Fig. 4(b) and Table 1. The band gap energys of the pure BiOBr nano sheets, 10 mmol/L NaBH4/BiOBr-Bi, 20 mmol/L NaBH4/BiOBr-Bi, 30 mmol/L NaBH4/BiOBr-Bi, 40 mmol/L NaBH4/BiOBr-Bi, 50 mmol/L NaBH4/BiOBr-Bi are 2.93 eV, 2.86 eV, 2.66 eV, 2.61 eV, 2.47 eV, and 2.83 eV, respectively. Among them, compared with the pure BiOBr nanosheets, the band gap energy of the BiOBr-Bi composite photo catalyst decreased from 2.78 eV to 2.45 eV and then rose to 2.83 eV, indicating that the internal electronic structure changed after being recombined with Bi. The interval between the conduction band and the valence band is reduced, resulting in the electrons being more prone to transition and improving the photocatalytic activity of the sample [25]. Due to the wide band gap of the BiOBr nanosheet, which seriously af fects the visible light absorption and the visible light photocatalytic performance. The increasing of the NaBH4 reducing agent will cause the BiOBr surface to produce more Bi nanowires, forming a number of pore structure. The light can be reflected in the hole, which improves the utilization of light. Meanwhile, Bi nanowires are modified on the surface of BiOBr to form a new composite photocatalytic material, which
3.2. Photocatalytic activity analysis The catalytic activity of BiOBr and BiOBr-Bi was evaluated by degrading NOR. The photocatalytic degradation of NOR by different
Fig. 3. EDS of the 40mmol/LNaBH4/BiOBr-Bi nanosheets. 4
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Fig. 4. UV–vis diffuse reflectance spectra (DRS) of the samples. (a) The UV–vis absorption spectra, (b) (αhυ)2 vs. hυ.
reduction. This result confirms the objective response of UV–visible diffuse reflection.
Table 1 The absorption band edge and band gap energy of the as-prepared samples. NaBH4/BiOBr-Bi (mmol/L)
0
10
20
30
40
50
absorption band edge(nm) band gap energy (Eg)(eV)
423 2.93
434 2.86
466 2.66
475 2.61
502 2.47
438 2.83
3.4. Possible photocatalytic mechanism The principle of the free radical capture experiment is that the photocatalytic degradation rate of NOR decreases after adding a capture agent of an active group. The catalyzed reaction was inhibited and proved to be the main reactive group involved in the reaction in the system. In order to distinguish the functions of these different active species, BQ, EDTA and IPA with the concentration of 1.0 mmolL 1 are usually used as the capture agents for ⋅O-2, hþ and ⋅OH, respectively [43, 44]. As shown in Fig. 10(a), the addition of IPA has a weak effect on the effect of BiOBr-Bi on degrading NOR under visible light irradiation. However, the degradation rates of NOR were both significantly reduced after adding EDTA and BQ, respectively. It is indicated that hþ and ⋅O-2 are the main active substances of the BiOBr-Bi photocatalyst. The EPR test can further confirm the active species during the pho tocatalytic reaction [40,45]. As shown in Fig. 10b, no characteristic peaks of DMPO ⋅OH were observed under dark and light conditions, indicating that there may be no ⋅OH in this photocatalytic process [46]. However, we can clearly observe the characteristic peak of ⋅O-2 under light conditions. It is indicated that ⋅O-2 belongs to an active material, and ⋅OH is not an active material. This result is basically consistent with the reported studies on the active substances of BiO(Cl, Br, I) [45,47]. It is well known that the activity of heterojunction photocatalysts relies on the transition and transfer of photogenerated electrons to form vacancy defects under illumination (Eq. (3)) [48,49]. As shown in Fig. 11, the conduction band position of BiOBr is 0.29 eV, and the po sition of the valence band VB is 3.06 eV [29]. Although the conduction band of BiOBr is more positive than the O2/⋅O-2 superoxide radical reduction potential ( 0.046 eV) [50], the conduction band of BiOBr-Bi photocatalyst is more negative than O2/⋅O-2 superoxide radical reduction potential since the Fermi level of Bi is 0.17 eV [18,51], making elec trons easily trapped by O2 to produce ⋅O-2 (Eq. (4)), which can effectively degrade NOR (Eq. (5)). At the same time, the photogenerated holes in the BiOBr-Bi valence band can directly react and degrade the NOR (Eq. (5)). The photoelectron transfer between the heterojunctions reduces the recombination rate of the hole electron pairs, resulting in more holes in the valence band. The continuous consumption of holes further in hibits the recombination of hole electron pairs. The entire system has good photoelectron and hole transport and separation capabilities.
catalysts under visible light irradiation is shown in Fig. 9(a). The degradation rates of NOR by BiOBr, 10 mmol/L NaBH4/BiOBr-Bi, 20 mmol/L NaBH4/BiOBr-Bi, 30 mmol/L NaBH4/BiOBr-Bi, 40 mmol/L NaBH4/BiOBr-Bi, and 50 mmol/L NaBH4/BiOBr-Bi were 34.7%, 69.2%, 91.9%, 94.9%, 97.2%, and 92.6%, respectively. The degradation rate of the BiOBr-Bi photocatalyst to the contaminant gradually becomes larger as the concentration of NaBH4 increases. However, the degradation rate decreases rapidly when the concentration of NaBH4 exceeds 40 mmol/L. This is attributed to the fact that the coverage of the Bi nanowires severely hinders the absorption of visible light by the catalyst, so that holes and photogenerated electrons are not effectively separated. Thereby the photocatalytic performance is lowered. 3.3. NOR degradation kinetic analysis As shown in Fig. 9(b), according to the results of photocatalytic experiment, ln(c0/ct) was taken as the ordinate and reaction time t as the abscess to draw a scatter plot and perform linear fitting, obtaining the kinetic fitting lines of BiOBr, 10 mmol/L NaBH4/BiOBr-Bi, 20 mmol/L NaBH4/BiOBr-Bi, 30 mmol/L NaBH4/BiOBr-Bi, 40 mmol/L NaBH4/ BiOBr-Bi, 50 mmol/L NaBH4/BiOBr-Bi photocatalyst degradation of NOR. The results show that the degradation reaction is closer to the firstorder reaction kinetics model. The photocatalytic performance is eval uated by calculating the slope (k) of the straight line. The larger the slope, the faster the degradation is and the higher the degradation rate is [23]. Table 2 shows the reaction kinetics and parameter values of the photocatalytic degradation of NOR. The apparent rate constants for BiOBr, 10 mmol/L NaBH4/BiOBr-Bi, 20 mmol/L NaBH4/BiOBr-Bi, 30 mmol/L NaBH4/BiOBr-Bi, 40 mmol/L NaBH4/BiOBr-Bi, 50 mmol/L NaBH4/BiOBr-Bi were determined as 0.00225 min 1, 0.00638 min 1, 0.01419 min 1, 0.01616 min 1, 0.01881 min 1, 0.01454 min 1, respectively. Among them, 40 mmol/L NaBH4/BiOBr-Bi catalyst has the shortest half-life degradation time of NOR for 25 min, and the degra dation reaction rate is about 8 times that of BiOBr, indicating that the photocatalytic activity of BiOBr is significantly improved after in-situ
BiOBr 5
Bi þ hν→BiOBr
Bi þ hν→BiOBr
Biðhþ þ e Þ
(3)
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Fig. 5. XPS spectra of the samples. (a) Survey of the sample, (b) Bi 4f, (c) Br 3d, (d) O 1s.
Fig. 6. FT-IR of the samples.
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Fig. 7. (a) N2 adsorption-desorption isotherm curves, (b) Pore size distribution curves of the samples.
Ο2 þ e →⋅Ο2
(4)
� NOR þ reactive species ⋅Ο2 ; hþ →Degraded products
(5)
Table 2 Kinetics equation and parameter value of NOR degradation reaction. Sample
kinetic equation
R2
t1/2/ min
BiOBr
ln(C0/C) ¼ 0.00225tþ0.03929 ln(C0/C) ¼ 0.00638tþ0.15471 ln(C0/C) ¼ 0.01419tþ0.02657 ln(C0/C) ¼ 0.01616tþ0.04777 ln(C0/C) ¼ 0.01881tþ0.21422 ln(C0/C) ¼ 0.01454tþ0.05638
0.97635
291
0.93345
84
0.99465
47
0.99281
40
0.98745
25
0.99493
44
10mmol/LNaBH4/BiOBrBi 20mmol/LNaBH4/BiOBrBi 30mmol/LNaBH4/BiOBrBi 40mmol/LNaBH4/BiOBrBi 50mmol/LNaBH4/BiOBrBi
Fig. 8. Photoluminescence (PL) spectra of the samples.
Fig. 9. (a) Photocatalytic degradation of NOR, (b) Relationship curves between ln(c0/ct) and time t. 7
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Fig. 10. (a) Degradation of Norfloxacin by photocatalyst under different capture agents, (b) DMPO spin trapping EPR spectra for ⋅O-2and ⋅OH with 40mmol/LNaBH4/ BiOBr-Bi nanosheets.
Fig. 11. Photocatalytic mechanism scheme of NOR degradation under Visible Light Irradiation.
Fermi level Bi metal element effectively inhibits the recombination of hole electron pairs. When reacting with pollutants under visible light, it can effectively promote the transfer of photoelectrons and holes, thereby enhancing the catalytic activity of the photocatalyst and the removal rate of contaminants. These results are favorable for practical applica tions in high performance BiOBr photocatalytic material.
4. Conclusion To sum up, BiOBr-Bi composite material with Bi nanowire on the surface of BiOBr nanosheets were successfully synthesized through a hydrothermal and chemical reduction method. The experiment proves that the concentration of NaBH4 is significant for the photocatalytic effect of BiOBr.When the concentration of NaBH4 is 40 mmol/L,BiOBr-Bi showed the best photocatalytic activity. The degradation rate reached 97.2%, and the half-life of NOR was shortened to 25 min. The reason why the activity of the BiOBr-Bi material photocatalyst increased is that the heterojunction formed by the wider bandgap BiOBr and the lower
Declaration of competing interest We declare that we have no conflict of interest.
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Acknowledgments
[23] [24] [25] [26]
This research was funded by the National Natural Science Founda tion of China (No.21276208).
[27] [28] [29]
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2019.104882.
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Materials Science in Semiconductor Processing journal homepage: http://www.elsevier.com/locate/mssp
Preparation of BiOBr-Bi heterojunction composites with enhanced photocatalytic properties on BiOBr surface by in-situ reduction Zhanyao Gao a, b, Binghua Yao a, *, Fan Yang a, Tiantian Xu c, Yangqing He a a
Department of Material Physics and Chemistry, Xi’an University of Technology, Xi’an, 710048, China Department of Environmental Engineering, Xi’an Polytechnic University, Xi’an, 710048, China c Key Laboratory of Thermo-Fluid Science and Engineering of MOE, Xi’an Jiaotong University, Xi’an, 710049, China b
A R T I C L E I N F O
A B S T R A C T
Keywords: BiOBr nanosheets Electron–hole pairs Antibiotic Kinetic analysis Photocatalysis mechanism
A new kind of BiOBr-Bi composite with improved photocatalytic capabilities was successfully prepared by in-situ reduction. The X-ray diffraction (XRD), Scanning electron microscopy (SEM), Ultraviolet–visible spectroscopy (UV–VisDRS), X-ray photoelectron spectroscopy (XPS), Brunner-Emmet-Teller measurements (BET), Fourier transform infrared spectroscopy (FT-IR), photoluminescence (PL) and the electron paramagnetic resonance (EPR) were used to characterize the morphological structures, physical properties and surface element compo sition of the photocatalysts. In addition, photocatalytic activity was evaluated by degrading the antibiotic nor floxacin (NOR). The results showed that the BiOBr-Bi (40 mmol/L NaBH4) had the highest activity. The degradation rate reached 97.2%. The half-life of NOR was shortened to 25 min. Moreover, the results of capture experiments indicated that the main active groups involved in the photocatalytic process under visible light conditions were ⋅O-2 and hþ. The reason why the activity of the BiOBr-Bi material photocatalyst increased is that the heterojunction formed by BiOBr with wider bandgap and Bi metal with a lower Fermi level effectively in hibits the recombination of holes and photogenerated electrons, which leads to the enhancement of oxidation capacity.
1. Introduction Inappropriate use of antibiotics is a significant public health prob lem. At present, humans are more prominent in the application of an tibiotics, but they also seriously affect the ecological environment [1]. According to relevant data, Sulfonamide resistance gene (sul1), Tetra cycline resistance gene (tet-A), β-lactamase resistance gene (bla TEM) and Streptomycin resistance gene (strA) can be detected in domestic water treatment plants and reservoirs, indicating several resistance ge notypes of antibiotics such as sulfonamides, streptomycin, tetracycline and ampicillin [2]. There has been a data display that China uses more than 160 thousand tons of antibiotics each year, about 52% for veteri nary use, about 48% for medical treatment, and more than 50 thousand tons of antibiotics are discharged into the soil and water environment [3, 4]. This causes serious pollution to the water environment. Containment of antibiotic pollution is imminent. So far, photocatalytic technology has been identified as one of the fastest, most effective and concise methods for degrading organic wastewater [5,6]. TiO2 is widely studied because of its excellent photocatalytic activity and stability [7–9]. However, due
to the poor response to visible light, great efforts have been made in the development of widening the range of visible light response in order to effectively use solar energy, and a large number of photocatalytic ma terials have been studied [10]. It is reported that the Bismuth series semiconductors have suitable visible light response band gap, and the valence bands composed of the O 2p and Bi 6s hybrid orbitals are uni formly distributed. Therefore, they have obvious catalysis and good application prospects [11]. However, the pure single material has the disadvantage of higher hþ-e- recombination rate, further research is needed to improve its photocatalytic activity. There are two main ways to ameliorate the photocatalytic ability of BiOBr. One is to change the morphology of BiOBr by changing the hydrothermal reaction conditions to improve its photocatalytic ability. Lu [12] has prepared a series of BiOBr nano/microstructures such as nanosheets, wafers, nanorods, and microspheres by hydrothermal method and adding different additives. It is confirmed that the morphology and size of the samples have a very large influence on the catalytic effect. Taking RhB as the substrate for degradation, and the nanosheets show stronger photocatalytic ability than the other three morphologies. On the other hand, in order to break
* Corresponding author. E-mail address: [email protected] (B. Yao). https://doi.org/10.1016/j.mssp.2019.104882 Received 19 April 2019; Received in revised form 14 October 2019; Accepted 10 December 2019 Available online 12 December 2019 1369-8001/© 2019 Published by Elsevier Ltd.
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this limitation, the coupling of the semiconductor precursor with the appropriate band gap is improved through synthesizing the photo catalytic composite. Thereby, the separation effect of the photocatalytic carriers is achieved, and the photocatalytic efficiency is greatly improved [13–15]. Cui et al. have synthesized CdS/BiOBr photocatalytic composites and compared them with a pure material [16]. It is found that the degradation of malachite green (MG) by CdS/BiOBr photo catalytic composites under visible light irradiation is much higher than that of single catalytic materials, which is mainly due to the fact that the formation of heterostructures promotes the separation of optical car riers. The photogenerated electrons generated by CdS and the hole pairs generated by BiOBr are easily transferred to the corresponding positions of the other part. Their effective separation can significantly improve the catalytic performance of the composite photocatalyst. According to re ports, Zhang et al. synthesized Bi2WO6-Bi composite materials by an in-situ photoreduction method. Taking RhB as degradation target, the photocatalytic degradation rate of Bi2WO6-Bi under visible light irra diation was 2.4 times that of the pure Bi2WO6 [17]. Dong et al. reported the preparation of BiOCl-Bi composite photocatalyst and the photo catalytic performance of removing nitric oxide. The results show that BiOCl-Bi exhibits higher photocatalytic performance than pure BiOCl and BiOCl-Ag in terms of photocatalytic removal of NO [18]. Comparing the two approaches, the latter is simpler and more efficient and its pa rameters are easier to control. Nowadays, a large number of theoretical studies and experimental results show that it will always be one of the most effective methods through the creation of band gap coupling and lattice heterostructure to improve the separation efficiency of hole electron pairs or extend the energy range of optical excitation [19,20]. This research used Bi(NO3)3 and NaBr as raw materials, and prepared BiOBr nanosheets via hydrolysis method in water at room temperature. On this basis, BiOBr-Bi was synthesisd by a simple chemical reduction reaction. A part of Bi3þ on the surface of the BiOBr nanosheet was reduced in situ to a simple substance of Bi, and Bi nanowires were formed on the surface of BiOBr to form a BiOBr-Bi composite photo catalyst. The XRD, SEM, UV–VisDRS, XPS, BET, FT-IR, PL and EPR were used to characterize the morphological structures, physical properties and surface element composition of the photocatalysts. The photo catalytic activity of the samples prepared under visible light irradiation was evaluated by degradation of NOR at a concentration of 20 mg/L. In addition, the photocatalytic mechanism of BiOBr-Bi composites was discussed by free radical trapping experiments and EPR test experiments.
stirred for 1 h and then aged for an additional hour. The resulting pre cipitate was collected, rinsed four times with ethanol and deionized water, and dried at 80 � C in air. The prepared samples were classified from low to high with the concentration of adding NaBH4, respectively named as BiOBr, 10 mmol/L NaBH4/BiOBr-Bi, 20 mmol/L NaBH4/ BiOBr-Bi, 30 mmol/L NaBH4/BiOBr-Bi, 40 mmol/L NaBH4/BiOBr-Bi, 50 mmol/L NaBH4/BiOBr-Bi. 2.4. Characterization The crystal structures of the sample were characterized by XRD-6100 ray diffractometer (Shimadzu, Japan). The Cu K� a target was operated at the voltage of 40 kV, the current of 30 mA, and € e of 0.15418 nm. The 2�e range is 10� –80� . The scanning speed was 10� min 1, and the data was collected by continuous scanning. The sample was analyzed by scanning electron microscope with high resolution (FEI, UK). X-ray photoelectron spectroscopy (T Escalab, America) was used to study surface properties. The UV–visible absorption spectroscopy was measured by TU-1901 dual-beam UV–visible diffuse reflectance spectrophotometer (Beijing Puji General Instrument Co., Ltd.), using BaSO4 as a reference, and the diffuse reflectance was converted to absorbance by the ulblka-Munk method. The structural information for samples was measured by Fourier transform spectrophotometer (FT-IR, Avatar470, Thermo Nico let) using the standard KBr disk method. The pore structure of the ob tained sample was characterized by N2 adsorption using an adsorption apparatus (Quantachrome autosorb-iQ-2MP gas sorption analyzer). Specific surface area of the samples was determined from the BrunauerEmmett-Teller (BET) equation, and pore volume was determined from the total amount adsorbed at relative pressures near unity. The photo luminescence spectra were obtained by an F-280 spectrometer (Tianjin Keqi, China). The photoluminescence (PL) spectra were obtained using an Edinburgh Analytical Instrument FL/FSTCSPC920 Spectrophotom eter. The electron paramagnetic resonance (EPR) spectra were recorded on a Bruker (A200–9.5/12) spectrometer with the X-band frequency of 9.8 GHz under room temperature (the magnetic field was tuned at 100 kHz). The scavengers of holes, Superoxide anion and hydroxyl radicals used in capture experiment were ethylenediamine tetraacetic acid (EDTA), benzoquinone (BQ) and isopropanol (IPA), respectively. 2.5. Photocatalytic activity measurement Usually, 50 mg of the photocatalyst was weighed and added to a quartz tube containing 50 mL of NOR aqueous solution (20 mg/L). The quartz tube was placed in the photocatalytic reactor for dark reaction for 30 min, and then the light source (500 W xenon lamp) was turned on for photocatalytic degradation. 4 mL of the suspension was taken every 30 min, centrifuged, and the absorbance of the supernatant in the 278 nm band was measured (The maximum absorbance of NOR solution at 278nm was measured by UV–Vis spectrometer). The photocatalytic ac tivity is proportional to the rate of degradation, and the degradation rate is calculated by the following formula.
2. Experimental 2.1. Materials All chemicals and reagents mentioned in this article are of analytical grade and can be used directly. 2.2. Preparation of BiOBr nanosheets 30 mL of ultrapure water was prepared, and 0.97 g of Bi(NO3)3⋅5H2O (2 mmol) and 0.206 g of NaBr (2 mmol) were separately added and stirred for 30 min. The mixture was transferred to a reaction kettle and calcined at 180 � C for 24 h. After cooling, the precipitate was collected and washed three times with ethanol and ultrapure water. The product is dried and ready for use.
η¼
А0
Аt А0
� 100%
(1)
where η is the degradation rate, A0 is the initial absorbance of the so lution, At is the absorbance of the solution after t min reaction. 3. Results and discussion
2.3. Preparation of BiOBr-Bi nanosheets
3.1. Materials characterization
BiOBr-Bi was synthesisd by a simple chemical reduction reaction. Usually, 2 mmol of BiOBr was added to 100 mL of distilled water con taining 1.0 g of polyvinyl pyrrolidone (PVP) and stirred for 20 min. Then a certain concentration of NaBH4 (0, 10, 20, 30, 40 and 50 mmol/L) was added dropwise to the suspension. Next, the resulting suspension was
The XRD pattern of the BiOBr-Bi photocatalyst is shown in Fig. 1. The XRD peaks of BiOBr-Bi are highly fitted with the tetragonal phase of BiOBr (JCPDS 09–0393) [21]. In the XRD patterns of BiOBr-Bi, besides the diffraction peaks of the Quartet phase BiOBr, the diffraction peaks 2
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Fig. 1. XRD patterns of the samples. (a) 10 mmol/L NaBH4/BiOBr-Bi, (b) 20 mmol/L NaBH4/BiOBr-Bi, (c) 30 mmol/L NaBH4/BiOBr-Bi, (d) 40 mmol/L NaBH4/ BiOBr-Bi, (e) 50 mmol/L NaBH4/BiOBr-Bi.
appeared at 2θ ¼ 27.2� , 37.9� , 39.6� and 62.2� are in accordance with the standard cards of the single crystal Bi (JCPDS 05–0519) [22], which correspond to (012), (104), (110) and (122) planes, respectively. In addition, no other impurity peaks appear, indicating that the BiOBr-Bi samples are of high purity. As shown in Fig. 1, the position of the BiOBr characteristic peaks do not shifted, indicating that the Bi nano wire do not damage the crystal structure of BiOBr or enter the crystal lattice. With the concentration of NaBH4 increasing from 10 mmol/L to 50 mmol/L, the diffraction characteristic peak of Bi metal in the diffraction spectrum of BiOBr-Bi composite photocatalyst appeared and enhanced continuously, means that the content of Bi metal is increasing.
SEM can be used to observe the microscopic morphology and dimension of the prepared samples. As seen from Fig. 2(a), the morphology of the pure BiOBr sample is a lamellar structure with irregular size, irregular shape, and relatively dispersed. In Fig. 2(b-f), it is obvious that an irregular sheet structure with sizes ranging between 2 μm and 4 μm and uneven size is seen, to a large extent, maintains the original morphology of the BiOBr. In addition, the formation of the Bi nanowire is obviously seen on the sheet structure, which is due to the addition of NaBH4 make Bi3þ on the surface of BiOBr be reduced to Bi metal. The results are in accordance with the results of XRD analysis. The EDS spectrum of 40 mmol/L NaBH4/BiOBr-Bi was analyzed and
Fig. 2. Typical SEM images of the obtained products. (a) BiOBr, (b) 10 mmol/L NaBH4/BiOBr-Bi, (c) 20 mmol/L NaBH4/BiOBr-Bi, (d) 30 mmol/L NaBH4/BiOBr-Bi, (e) 40 mmol/L NaBH4/BiOBr-Bi, (f) 50 mmol/L NaBH4/BiOBr-Bi. 3
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the result shown in Fig. 3. The results show that the BiOBr-Bi photo catalyst contains C, O, Br and Bi elements. It is confirmed that the pre pared sample is BiOBr-Bi composite. This result is in good agreement with XRD, and SEM analysis. The UV–visible diffuse reflectance spectrum of the samples is shown in Fig. 4(a). The absorption edges of the BiOBr nanosheets, 10 mmol/L NaBH4/BiOBr-Bi, 20 mmol/L NaBH4/BiOBr-Bi, 30 mmol/L NaBH4/ BiOBr-Bi, 40 mmol/L NaBH4/BiOBr-Bi, and 50 mmol/L NaBH4/BiOBrBi are 423 nm, 434 nm, 466 nm, 475 nm, 502 nm, and 438 nm, respectively. This indicates that both the BiOBr nanosheet and the BiOBr-Bi composite photocatalyst can degrade the NOR under visible light. Compared with pure BiOBr, the absorption edge of BiOBr-Bi composites have a blue-shift. Typically, the blue-shift of 40 mmol/L NaBH4/BiOBr-Bi is the strongest. The results demonstrate that the 40 mmol/L NaBH4/BiOBr-Bi composite is a promising candidate in degrading pollutant. By means of the Kubelka-Munk function, the relationship between the band gap Eg and the absorption coefficient α is explored by using the graph of (αhν) 2 and photoenergy (hν). The curve equation is as follows [23]: Еg
�n2 =
ahv ¼ А hv
changes the location of the conduction band and is more conducive to the production of active substances. These may be the reasons for the enhancement of visible light absorption and the improvement of pho tocatalytic activity. However, when large amount of Bi element pro duced by the surface reduction of BiOBr, the main composition of the composite surface changed and the light transmittance was also low ered, thereby limiting the photocatalytic activity of the catalyst. It is also supported by the above SEM analysis. X-ray photoelectron spectroscopy (XPS) can further study the surface element composition and chemical state of the prepared samples. The constituent elements of BiOBr and BiOBr-Bi shown in Fig. 5(a) are Bi, Br, O and C. The appearance of the C element is generated by the XPS equipment. The other peaks were produced by photocatalyst elements, which proves that the sample prepared is of higher purity. The XPS peak of Bi 4f is observed in Fig. 5(b). The peak positions of Bi 4f7/2 and Bi 4f5/2 of BiOBr and BiOBr-Bi are both 159.68 eV and 164.98 eV, indicating that the Bi element mainly exists in the trivalent form [26]. In addition, BiOBr-Bi also appears two peaks at 160.18 eV and 165.28 eV, which is due to the formation of Bi nanowire space [17]. The results of UV–visible diffuse reflection are further confirmed. Fig. 5(c) shows the XPS peak of Br 3d [27–29], where the peak position of BiOBr is at 68.78 eV and the peak position of BiOBr-Bi is moved to 69.38 eV. This is possibly also related to the formation of the Bi nanowire space. In Fig. 5(d), BiOBr and BiOBr-Bi respectively exhibit binding energies of 530.78 eV and 531.58 eV which are attributed to O 1s [30–33]. It is reported that the oxygen of BiOBr is mainly present in the form of –OH [34], while the oxygen of BiOBr-Bi is mainly present in the form of molecular oxygen (⋅O-2, etc.) [35]. The FT-IR spectra of the pure BiOBr and BiOBr-Bi composites are shown in Fig. 6. The absorption band at about 3500 cm 1 is attributed to O-H stretching and bending vibrations in free water molecules [36]. The peak at about 1407 cm 1 region was assigned to the Bi-Br band in BiOBr [37]. In particular, pure BiOBr has an absorption band in the 400-750 cm 1 region [38,39]. However, it can be observed that as the content of Bi in the metal increases, the absorption vibration band undergoes a blue shift, which may be due to the fact that the metal Bi has a certain in fluence on the vibration of BiOBr. This result is consistent with the re sults of the XRD and XPS experiments. It is well known that larger BET and pore volume materials corre spond to more surface active sites and easier transport paths, which effectively absorb light energy and promote separation of electron-hole pairs, thereby improving photocatalytic performance [40,41]. The N2 adsorption-desorption isotherms of BiOBr and 40 mmol/L NaBH4/ BiOBr-Bi are shown in Fig. 7. According to the IUPAC classification standard, the adsorption hysteresis loop of the sample belongs to type IV pattern with H3-type (typical mesoporous material). The specific surface areas of BiOBr and 40 mmol/L NaBH4/BiOBr-Bi shown in Fig. 7(a) were 15.72 m2/g and 38.25 m2/g, respectively. Obviously, specific surface area of the composite is larger than that of pure BiOBr, providing more active sites for photocatalytic reactions. In addition, it can be seen from Fig. 7(b) that the pore volume of the composite is significantly larger than that of pure BiOBr, which can provide an efficient reactant trans port path. Photoluminescence (PL) emission is a common technique for detecting the separation efficiency of photogenerated electron-hole pairs [42]. As shown in Fig. 8, the emission peaks of all samples were concentrated at around 468 nm. Lower PL intensities usually indicate lower photogenerated e--hþ recombination rates [26]. Obviously, 40 mmol/L NaBH4/BiOBr-Bi showed the lowest PL intensity. Compared with the pure BiOBr, it shows that the charge recombination decreases significantly and has higher photogenic charge separation efficiency.
(2)
where α is the absorption coefficient, ν is the frequency of the light, and h is Planck’s constant [24]. Fig. 4(b) shows the relationship between (αhν) 2 and hν. The band gap energy (Eg) of samples are known from Fig. 4(b) and Table 1. The band gap energys of the pure BiOBr nano sheets, 10 mmol/L NaBH4/BiOBr-Bi, 20 mmol/L NaBH4/BiOBr-Bi, 30 mmol/L NaBH4/BiOBr-Bi, 40 mmol/L NaBH4/BiOBr-Bi, 50 mmol/L NaBH4/BiOBr-Bi are 2.93 eV, 2.86 eV, 2.66 eV, 2.61 eV, 2.47 eV, and 2.83 eV, respectively. Among them, compared with the pure BiOBr nanosheets, the band gap energy of the BiOBr-Bi composite photo catalyst decreased from 2.78 eV to 2.45 eV and then rose to 2.83 eV, indicating that the internal electronic structure changed after being recombined with Bi. The interval between the conduction band and the valence band is reduced, resulting in the electrons being more prone to transition and improving the photocatalytic activity of the sample [25]. Due to the wide band gap of the BiOBr nanosheet, which seriously af fects the visible light absorption and the visible light photocatalytic performance. The increasing of the NaBH4 reducing agent will cause the BiOBr surface to produce more Bi nanowires, forming a number of pore structure. The light can be reflected in the hole, which improves the utilization of light. Meanwhile, Bi nanowires are modified on the surface of BiOBr to form a new composite photocatalytic material, which
3.2. Photocatalytic activity analysis The catalytic activity of BiOBr and BiOBr-Bi was evaluated by degrading NOR. The photocatalytic degradation of NOR by different
Fig. 3. EDS of the 40mmol/LNaBH4/BiOBr-Bi nanosheets. 4
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Fig. 4. UV–vis diffuse reflectance spectra (DRS) of the samples. (a) The UV–vis absorption spectra, (b) (αhυ)2 vs. hυ.
reduction. This result confirms the objective response of UV–visible diffuse reflection.
Table 1 The absorption band edge and band gap energy of the as-prepared samples. NaBH4/BiOBr-Bi (mmol/L)
0
10
20
30
40
50
absorption band edge(nm) band gap energy (Eg)(eV)
423 2.93
434 2.86
466 2.66
475 2.61
502 2.47
438 2.83
3.4. Possible photocatalytic mechanism The principle of the free radical capture experiment is that the photocatalytic degradation rate of NOR decreases after adding a capture agent of an active group. The catalyzed reaction was inhibited and proved to be the main reactive group involved in the reaction in the system. In order to distinguish the functions of these different active species, BQ, EDTA and IPA with the concentration of 1.0 mmolL 1 are usually used as the capture agents for ⋅O-2, hþ and ⋅OH, respectively [43, 44]. As shown in Fig. 10(a), the addition of IPA has a weak effect on the effect of BiOBr-Bi on degrading NOR under visible light irradiation. However, the degradation rates of NOR were both significantly reduced after adding EDTA and BQ, respectively. It is indicated that hþ and ⋅O-2 are the main active substances of the BiOBr-Bi photocatalyst. The EPR test can further confirm the active species during the pho tocatalytic reaction [40,45]. As shown in Fig. 10b, no characteristic peaks of DMPO ⋅OH were observed under dark and light conditions, indicating that there may be no ⋅OH in this photocatalytic process [46]. However, we can clearly observe the characteristic peak of ⋅O-2 under light conditions. It is indicated that ⋅O-2 belongs to an active material, and ⋅OH is not an active material. This result is basically consistent with the reported studies on the active substances of BiO(Cl, Br, I) [45,47]. It is well known that the activity of heterojunction photocatalysts relies on the transition and transfer of photogenerated electrons to form vacancy defects under illumination (Eq. (3)) [48,49]. As shown in Fig. 11, the conduction band position of BiOBr is 0.29 eV, and the po sition of the valence band VB is 3.06 eV [29]. Although the conduction band of BiOBr is more positive than the O2/⋅O-2 superoxide radical reduction potential ( 0.046 eV) [50], the conduction band of BiOBr-Bi photocatalyst is more negative than O2/⋅O-2 superoxide radical reduction potential since the Fermi level of Bi is 0.17 eV [18,51], making elec trons easily trapped by O2 to produce ⋅O-2 (Eq. (4)), which can effectively degrade NOR (Eq. (5)). At the same time, the photogenerated holes in the BiOBr-Bi valence band can directly react and degrade the NOR (Eq. (5)). The photoelectron transfer between the heterojunctions reduces the recombination rate of the hole electron pairs, resulting in more holes in the valence band. The continuous consumption of holes further in hibits the recombination of hole electron pairs. The entire system has good photoelectron and hole transport and separation capabilities.
catalysts under visible light irradiation is shown in Fig. 9(a). The degradation rates of NOR by BiOBr, 10 mmol/L NaBH4/BiOBr-Bi, 20 mmol/L NaBH4/BiOBr-Bi, 30 mmol/L NaBH4/BiOBr-Bi, 40 mmol/L NaBH4/BiOBr-Bi, and 50 mmol/L NaBH4/BiOBr-Bi were 34.7%, 69.2%, 91.9%, 94.9%, 97.2%, and 92.6%, respectively. The degradation rate of the BiOBr-Bi photocatalyst to the contaminant gradually becomes larger as the concentration of NaBH4 increases. However, the degradation rate decreases rapidly when the concentration of NaBH4 exceeds 40 mmol/L. This is attributed to the fact that the coverage of the Bi nanowires severely hinders the absorption of visible light by the catalyst, so that holes and photogenerated electrons are not effectively separated. Thereby the photocatalytic performance is lowered. 3.3. NOR degradation kinetic analysis As shown in Fig. 9(b), according to the results of photocatalytic experiment, ln(c0/ct) was taken as the ordinate and reaction time t as the abscess to draw a scatter plot and perform linear fitting, obtaining the kinetic fitting lines of BiOBr, 10 mmol/L NaBH4/BiOBr-Bi, 20 mmol/L NaBH4/BiOBr-Bi, 30 mmol/L NaBH4/BiOBr-Bi, 40 mmol/L NaBH4/ BiOBr-Bi, 50 mmol/L NaBH4/BiOBr-Bi photocatalyst degradation of NOR. The results show that the degradation reaction is closer to the firstorder reaction kinetics model. The photocatalytic performance is eval uated by calculating the slope (k) of the straight line. The larger the slope, the faster the degradation is and the higher the degradation rate is [23]. Table 2 shows the reaction kinetics and parameter values of the photocatalytic degradation of NOR. The apparent rate constants for BiOBr, 10 mmol/L NaBH4/BiOBr-Bi, 20 mmol/L NaBH4/BiOBr-Bi, 30 mmol/L NaBH4/BiOBr-Bi, 40 mmol/L NaBH4/BiOBr-Bi, 50 mmol/L NaBH4/BiOBr-Bi were determined as 0.00225 min 1, 0.00638 min 1, 0.01419 min 1, 0.01616 min 1, 0.01881 min 1, 0.01454 min 1, respectively. Among them, 40 mmol/L NaBH4/BiOBr-Bi catalyst has the shortest half-life degradation time of NOR for 25 min, and the degra dation reaction rate is about 8 times that of BiOBr, indicating that the photocatalytic activity of BiOBr is significantly improved after in-situ
BiOBr 5
Bi þ hν→BiOBr
Bi þ hν→BiOBr
Biðhþ þ e Þ
(3)
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Materials Science in Semiconductor Processing 108 (2020) 104882
Fig. 5. XPS spectra of the samples. (a) Survey of the sample, (b) Bi 4f, (c) Br 3d, (d) O 1s.
Fig. 6. FT-IR of the samples.
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Materials Science in Semiconductor Processing 108 (2020) 104882
Fig. 7. (a) N2 adsorption-desorption isotherm curves, (b) Pore size distribution curves of the samples.
Ο2 þ e →⋅Ο2
(4)
� NOR þ reactive species ⋅Ο2 ; hþ →Degraded products
(5)
Table 2 Kinetics equation and parameter value of NOR degradation reaction. Sample
kinetic equation
R2
t1/2/ min
BiOBr
ln(C0/C) ¼ 0.00225tþ0.03929 ln(C0/C) ¼ 0.00638tþ0.15471 ln(C0/C) ¼ 0.01419tþ0.02657 ln(C0/C) ¼ 0.01616tþ0.04777 ln(C0/C) ¼ 0.01881tþ0.21422 ln(C0/C) ¼ 0.01454tþ0.05638
0.97635
291
0.93345
84
0.99465
47
0.99281
40
0.98745
25
0.99493
44
10mmol/LNaBH4/BiOBrBi 20mmol/LNaBH4/BiOBrBi 30mmol/LNaBH4/BiOBrBi 40mmol/LNaBH4/BiOBrBi 50mmol/LNaBH4/BiOBrBi
Fig. 8. Photoluminescence (PL) spectra of the samples.
Fig. 9. (a) Photocatalytic degradation of NOR, (b) Relationship curves between ln(c0/ct) and time t. 7
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Fig. 10. (a) Degradation of Norfloxacin by photocatalyst under different capture agents, (b) DMPO spin trapping EPR spectra for ⋅O-2and ⋅OH with 40mmol/LNaBH4/ BiOBr-Bi nanosheets.
Fig. 11. Photocatalytic mechanism scheme of NOR degradation under Visible Light Irradiation.
Fermi level Bi metal element effectively inhibits the recombination of hole electron pairs. When reacting with pollutants under visible light, it can effectively promote the transfer of photoelectrons and holes, thereby enhancing the catalytic activity of the photocatalyst and the removal rate of contaminants. These results are favorable for practical applica tions in high performance BiOBr photocatalytic material.
4. Conclusion To sum up, BiOBr-Bi composite material with Bi nanowire on the surface of BiOBr nanosheets were successfully synthesized through a hydrothermal and chemical reduction method. The experiment proves that the concentration of NaBH4 is significant for the photocatalytic effect of BiOBr.When the concentration of NaBH4 is 40 mmol/L,BiOBr-Bi showed the best photocatalytic activity. The degradation rate reached 97.2%, and the half-life of NOR was shortened to 25 min. The reason why the activity of the BiOBr-Bi material photocatalyst increased is that the heterojunction formed by the wider bandgap BiOBr and the lower
Declaration of competing interest We declare that we have no conflict of interest.
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Acknowledgments
[23] [24] [25] [26]
This research was funded by the National Natural Science Founda tion of China (No.21276208).
[27] [28] [29]
Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.mssp.2019.104882.
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