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BiOBr composites with enhanced visible-light photocatalytic activities for the degradation of rhodamine B
Separation and Purification Technology 154 (2015) 211–216
Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Facile hydrothermal synthesis of Bi/BiOBr composites with enhanced visible-light photocatalytic activities for the degradation of rhodamine B Meichao Gao, Dafeng Zhang, Xipeng Pu ⇑, Hong Li, Dongdong Lv, Bingbing Zhang, Xin Shao School of Materials Science and Engineering, Liaocheng University, Liaocheng, Shandong 252000, PR China
a r t i c l e
i n f o
Article history: Received 15 July 2015 Received in revised form 23 September 2015 Accepted 24 September 2015
Keywords: Bi/BiOBr Photocatalyst Oxygen vacancy Visible light
a b s t r a c t Bi/BiOBr photocatalysts with improved photodegradation performances were synthesized by a facile onestep hydrothermal method using ethanol–water mixed solution. The samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy, UV–vis diffuse reflectance spectroscopy and photoluminescence spectroscopy. The addition of ethanol results in the partial reduction of Bi3+ ions to metallic Bi, different morphology from pure BiOBr, and the formation of oxygen vacancies. The dosage of ethanol plays a vital role in the morphologies and photodegradation performances of as-obtained Bi/BiOBr. The Bi/BiOBr photocatalysts show improved photodegradation performances, which is attributed to the high surface area, narrow band gap, and inhibited recombination of electron–hole pairs . Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor photocatalysts have long been used to mitigate the deterioration of environments created by toxic pollutants in water and air [1,2]. In the past few decades, semiconductor photocatalysts have got considerable attention due to their potential applications in degrading organic compounds for environmental remediation [3,4]. As is known, TiO2 has been mostly applied as photocatalysts due to its low cost, strong oxidizing power, and nontoxic nature [5]. Regrettably, TiO2 has a wide band gap (3.2 eV for anatase and 3.0 eV for rutile), which limits the absorption of visible light of the solar spectrum [6,7]. Some narrow band gap semiconductors display the ability to absorb visible-light, which should be good alternatives [8]. In recent years, bismuth oxybromide (BiOBr), with an indirect-transition band gap value of 2.92 eV [9], has received much attention due to its potential photocatalytic abilities under visible light. BiOBr is of tetragonal structure that consists of tetragonal [Bi2O2] positively charged slabs, which are interleaved by double slabs of bromine atoms to form [Br–Bi–O–Bi–Br] layers along the c-axis. The self-built electric field between [Bi2O2] and Br slabs would effectively separate the photoinduced electron–hole pairs, thus transfers the electrons and holes to the surface of BiOBr, which enhances the photocatalytic activity [10]. However, the photocatalytic activity of BiOBr is still far from efficient for practical applications. ⇑ Corresponding author. E-mail address: [email protected] (X. Pu). http://dx.doi.org/10.1016/j.seppur.2015.09.063 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
Moreover, varieties of strategies, such as impurity doping, formation of semiconductor heterojunctions and surface metallization, have been employed to improve the photocatalytic efficiency of BiOBr. At present, some noble metal/BiOBr composites have been proved as efficient photocatalysts under visible-light irradiation, such as Ag/BiOBr [11], Pd/BiOBr [12], and Pt/BiOBr [13]. Recently, some researchers found that metallic Bi modification could enhance the photocatalytic efficiency of semiconductor BiOCl and BiOI photocatalysts [14,15]. Moreover, Bi/BiOBr composites were synthesized by various methods and showed enhanced visible-light photocatalytic activity. Liu synthesized Bi/BiOBr composites by a facile one-step solvothermal method with the help of dimethyl sulfoxide, and the photocatalytic results showed that Bi/BiOBr composites exhibited enhanced visible-light photocatalytic performance, which was attributed to an increased amount of photo-induced charge carriers and efficient separation of photogenerated carriers [16]. Recently, Zhang and co-workers reported a one step solvothermal method to prepare Bi/BiOBr with oxygen vacancies in the presence of chloride dehydrate [17]. In this work, we prepared Bi/BiOBr composites via a hydrothermal method. Different from above mentioned two reports, partial Bi3+ cations were conveniently reduced to metallic Bi by ethanol under hydrothermal condition. Different from pure BiOBr, the asprepared Bi/BiOBr samples exhibit different morphologies; meanwhile, a great number of oxygen vacancies were generated in the as-obtained Bi/BiOBr composites. As-synthesized Bi/BiOBr composites exhibit significantly improved photocatalytic performance, whose mechanism was discussed in detail.
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2. Experimental procedure 2.1. Photocatalyst preparation All the chemical reagents were of analytical grade and purchased from Aladdin Industrial Corporation without further purification before use. Deionized water was used throughout the work. In a typical synthesis procedure, 0.02 mol bismuth nitrate pentahydrate (Bi(NO3)35H2O) was dissolved in 20 mL of 10% (w/w) nitric acid under vigorous stirring. Meanwhile, 0.06 mol ammonium bromide (NH4Br) was dissolved in 20 mL deionized water, and the obtained solution was added quickly into the above mixed solution, and milky suspension was obtained. Then x mL (x = 0, 2, 4, 6, 8, and 10) ethanol was added into the above mixed solution under stirring. These mixtures were transferred into an 80 mL Teflon-lined stainless steel autoclave to perform hydrothermal process at 180 °C for 6 h. After completion of the hydrothermal reaction, the precipitate was washed using absolute ethanol and deionized water several times, then dried at 80 °C in air for 12 h. The as-obtained Bi/BiOBr were denoted as Bi/BiOBr-x (the volume of ethanol). 2.2. Characterizations X-ray diffraction (XRD) patterns were recorded on a diffractometer (D8 Advanced, Bruker Co., Germany) with Cu Karadiation. Microstructures of the products were obtained by a field emission scanning electron microscope (FE-SEM, S4800, Hitachi Ltd., Japan). Transmission electron microscopy (TEM) was performed with a JEM-2100F microscope (JEOL Ltd., Japan). The Brunauer–Emmett–Teller (BET) specific surface areas of the samples were investigated by a Quantachrome Autosorb IQ-C nitrogen adsorption apparatus. X-ray photoelectron spectroscopy (XPS) was recorded using an X-ray photoelectron spectrometer (Kratos Axis Ultra) which uses Al Ka (1486.6 eV) X-ray source. The curve fitting was done using Casa XPS software by means of least square peak fitting procedure using a Gaussian–Lorentzian function. Electron paramagnetic resonance (EPR) measurements were performed with a Bruker ESP-300 EPR spectrometer. The settings for the EPR spectrometer were center field 3488 G, modulation frequency 100 kHz, microwave frequency 9.5 GHz, and power 12.9 mW. Ultraviolet–visible (UV–vis) diffuse reflectance spectra (DRS) of samples and the absorption spectra of rhodamine B (RhB) were recorded on a UV-3600 spectrophotometer (Shimadzu, Japan). Photoluminescence (PL) spectra of samples were recorded with an FLS 920 Fluorescence and Phosphorescence spectrometer (Edinburgh Instruments Ltd.) with kex = 320 nm. All these measurements were carried out at room temperature.
ize the stability of the as-obtained BiOBr, 6-cycle photodegradation experiment for RhB was carried out. Each cycle lasts for 120 min. After each cycle, the sample was allowed to settle down naturally (which takes less than 30 min), and then re-dispersed in fresh RhB solution for the next cycle after washing three time with deionized water. 3. Results and discussion 3.1. Structures and morphologies Fig. 1 shows the XRD patterns of the as-synthesized BiOBr and Bi/BiOBr. In the absence of alcohol, all the diffraction peaks of BiOBr samples can be indexed to tetragonal phase BiOBr (JCPDS Card No. 78-0348), and the strong and sharp diffraction peaks reveal a high degree of crystallization. When alcohol was added, apart from the diffraction peaks of BiOBr phase, the characteristic peaks of metallic Bi at 2h = 27.16°, 37.95° and 48.71° (JCPDS Card No. 85-1329) can be observed, suggesting the formation of Bi/BiOBr composites. Fig. S1 shows the XRD pattern of Bi/BiOBr-8 and the standard reference in order to see the position of Bi diffraction peak clearly. In addition, the intensity of peaks decreases gradually with the increase of the volume of ethanol, indicating that smaller crystal size of BiOBr can be obtained. The crystallite sizes of the samples were estimated using the Scherrer formula A = 0.94 k/b Cos h, where A is average crystalline size, b is the full-widths at half-maximum of the diffraction peak, k is the wavelength (1.54 Å) of X-ray radiation, and h is the angle of diffraction. We calculated the crystal sizes of BiOBr, Bi/BiOBr-2, Bi/BiOBr-4, Bi/BiOBr-6, Bi/BiOBr-8 and Bi/BiOBr-10, which are about 331.3, 93.2, 77.4, 70.7, 65.0 and 64.5 nm, respectively. The morphologies of pure BiOBr and Bi/BiOBr-8 were investigated by SEM. Fig. 2(a) shows the SEM image of pure BiOBr. It is observed that the sample is composed of a large quantity of uniform sheets with an average diameter of about 5 lm and an average thickness of 250 nm. When alcohol were used, as shown in Fig. 2(b), different from pure BiOBr sample, Bi/BiOBr-8 exhibits sheet-like morphology with coarse surface, and some nanoparticles and flower-like nanostructures are observed on the surface of sheets, as shown in Fig. 2(c). The coarse morphology of Bi/BiOBr-8 endows it higher BET specific area of Bi/BiOBr-8 (53.697 m2/g), which is higher than that of BiOBr (22.025 m2/g). The better adsorption performance is advantageous for the diffusion of RhB to the surface of photocatalyst. The as-synthesized Bi/BiOBr-8 was further characterized by HRTEM, as shown in Fig. 2(d). Tow kinds of lattice fringes with d spacings of about 0.278 and 0.328 nm were observed which can be indexed as the
2.3. Photocatalytic activity tests Photocatalysis experiment was carried out in a home-made photocatalytic reaction box. A 300 W Xenon lamp with a UV cutoff filter (JB450) was positioned about 10 cm over a cylindrical container with a circulating water jacket for cooling. 100 mg of our samples was dispersed in 100 mL aqueous solution of RhB (10 mg/L). The solution was stirred in dark for 30 min to ensure the establishment of the adsorption–desorption equilibrium. The concentration of RhB was analyzed by recording the absorption band maximum (554 nm) in the absorption spectra and taken as the initial concentration (C0). During the irradiation, 5 mL solution was extracted at an interval of 15 min, then absorbance of RhB solution was measured after centrifugation at 1000 rpm for 5 min to remove the catalysts. The normalized temporal concentration changes (C/C0) of RhB were obtained. In order to character-
Fig. 1. XRD patterns of BiOBr and Bi/BiOBr samples.
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Fig. 2. SEM images of (a) BiOBr; (b and c) Bi/BiOBr-8 and (d) TEM image of Bi/BiOBr-8.
(1 1 0) plane of BiOBr and the (0 1 2) plane of metallic Bi, respectively. Fig. 3 shows the XPS spectra of pure BiOBr and Bi/BiOBr-8. Two peaks with binding energies of 165.1 and 159.8 eV are observed in the spectrum of the BiOBr, which can be ascribed to the Bi3+ 4f5/2 and Bi3+ 4f7/2 binding energies, respectively [18]. The splitting between these bands was 5.3 eV, referring to the presence of the normal state of BiOBr [18]. In comparison to pure BiOBr sample, the Bi3+ 4f7/2 and 4f5/2 peaks of Bi/BiOBr-8 exhibit a shift of about 0.7 eV toward lower binding energy, which is indicative of the formation of lower charge Bi ions in Bi/BiOBr-8 [15]. In addition, the O 1s peak can be divided into two peaks at 530.9 and 529.8 eV (Fig. 3b). The peak at 529.8 eV is attributed to the lattice oxygen in BiOBr, while the other peak with a higher energy of 530.9 eV can ascribed not only to the surface hydroxyl oxygen [19], but also to the oxygen vacancies in the surface of BiOBr. The presence of oxygen vacancies was further confirmed by EPR spectroscopy, as shown in Fig. 4. The EPR spectrum of Bi/BiOBr-8 shows a remarkable signal at g = 2.001, typical character of oxygen vacancy, while no signal is found from pure BiOBr [20].
2.64 eV, while the values of Bi/BiOBr-2, -4, -6, -8, and -10 are approximately 2.53, 2.42, 1.92, 1.75 and 1.90 eV, respectively, which are smaller than that of pure BiOBr due to the formation of metallic Bi and oxygen vacancies in the Bi/BiOBr sheet. These results indicate that all these Bi/BiOBr samples have suitable band gap energies for photodegradation of RhB under visible light irradiation. Photoluminescence emission spectrum was usually used to research the efficiency of charge trapping, immigration, transfer, and to understand the behavior of electron–hole pairs [22]. Fig. 6 shows the PL spectra of pure BiOBr and Bi/BiOBr composites. It can be observed that pure BiOBr shows a strong emission peak in the range of 400–600 nm, deriving from the direct electron–hole recombination of band transition. Though shapes and peaks position of Bi/BiOBr composites are similar to the pure BiOBr, the emission intensities of the as-obtained Bi/BiOBr composites decrease significantly. This implies that the electron – hole recombination were effectively prohibited, which can achieve a better photocatalytic activity. 3.3. Photocatalytic properties
3.2. Optical properties UV–vis diffuse reflectance spectra (DRS) of the BiOBr and Bi/BiOBr samples are given in Fig. 5(a). Compared to pure BiOBr, Bi/BiOBr samples reveal a more intense continuous absorption in UV and visible light region. Moreover, as shown in inset of Fig. 5 (a), after the addition of ethanol, the samples became darker. This can be attributed to the formation of metallic Bi, which makes Bi/BiOBr ready for the efficient utilization of photon energy in both UV and visible light range. The energy of the band gap (Eg) was calculated following the equation: (ahm)1/2 = B(hm Eg), where a, hm, Eg, and B are the absorption coefficient, photon energy, band gap, and a constant, respectively [21]. The Eg values of as-synthesized samples could be estimated from a Tauc plots of (ahm)1/2 versus the photon energy (hm), as shown in Fig. 4(b). The intercepts of the tangent to the x-axis will give a good approximation of the band gap energies for the BiOBr and Bi/BiOBr samples. The estimated band gap energy of pure BiOBr is approximately
RhB is selected as a model pollutant to evaluate the photocatalytic activities of the as-prepared samples. As shown in Fig. 7(a), compared with neat BiOBr, Bi/BiOBr composites exhibit higher absorption ability for RhB due to the high BET specific surface, and about 50% of RhB were adsorbed by the Bi/BiOBr photocatalyst at the beginning of the photodegradation. The adsorption properties of BiOBr and Bi/BiOBr for RhB were characterized in dark, as shown in Fig. S2. Obviously, the adsorption/desorption equilibrium was almost established in 30 min, and thus the adsorption has no contribution in the following decoloration. As shown in Fig. 7(b), TiO2 (P25) shows poor photoactivity under visible light irradiation. In the case of pure BiOBr, the photolytic fade is only 25% after 2 h of visible light irradiation. In contrast, Bi/BiOBr samples exhibit significantly improved photodegradation performance. Bi/BiOBr-8 has the best photodegradation performance. After 30 min of visible light irradiation, 95.5% of the initial RhB was photodegraded in the case of Bi/BiOBr-8, whereas only 10% of the initial dye was
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Fig. 3. XPS spectra of (a) Bi 4f of BiOBr and Bi/BiOBr-8, and (b) O 1s of Bi/BiOBr-8.
Fig. 4. EPR spectra of BiOBr and Bi/BiOBr-8.
Fig. 6. PL spectra of pure BiOBr and Bi/BiOBr.
decomposed by neat BiOBr. Fig. 7(c) shows the RhB photodegradation curves when the Bi/BiOBr-8 photocatalyst was reused 6 times. Obviously, the result demonstrated no deactivation during the 6-cycle, indicating the excellent recyclability and stability of Bi/ BiOBr under visible light irradiation. To investigate the active species during the photocatalytic reaction, a series of trapping experiments were carried out.
Isopropyl alcohol (IPA) [14], p-benzoquinone (BZQ) [23], and disodium ethylenediaminetetraacetate (Na2-EDTA) [23] were used as hydroxyl radical (OH), superoxide radical (O2 ), and hole (h+) specie quenchers, respectively. As shown in Fig. 8, quenching results showed that IPA did not influence the catalysis process, while BZQ and Na2-EDTA significantly slowed the degradation rate of RhB, indicating that the O2 and h+ are the main active species of Bi/BiOBr in aqueous solution under visible light irradiation.
Fig. 5. (a) UV–vis DRS spectra and (b) corresponding Tauc plots of (ahm)1/2 versus (hm) of BiOBr and Bi/BiOBr composites. Insets are the digital photos of as-synthesized BiOBr and Bi/BiOBr-8.
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Fig. 7. The variations of (a) absorbance, (b) normalized C/C0 of RhB solution with different visible light irradiation time, and (c) RhB degradation curves of Bi/BiOBr-8 when reused 6 times.
Fig. 9. Photocatalytic mechanism scheme of Bi/BiOBr under visible light irradiation. Fig. 8. Photocatalytic performances of Bi/BiOBr-8 with different specie quenchers.
Based on the previous discussion, the photocatalytic mechanism scheme of Bi/BiOBr under visible light irradiation is given in Fig. 9. First, the high BET specific area of Bi/BiOBr can accelerate the diffusion of RhB molecules from solution to the active sites of photocatalyst, which has a beneficial effect on the photocatalytic reaction. Second, it has been reported that oxygen vacancies can draw an impurity level between the valence band (VB) and the conduction band (CB) to narrow the band gap of photocatalysts [14,17,24]. Consequently, as-obtained Bi/BiOBr samples exhibit narrow band gap and are much more sensitive to visible light than pure BiOBr. Third, when the Bi/BiOBr is excited by visible light with an energy higher than the band gap, the transition of electrons
from VB to CB of BiOBr or oxygen vacancies states forms photogenerated electron–hole pairs. As active electron traps, oxygen vacancies promote the separation efficiency of photo-induced electron–hole pairs, resulting in a high photoactivity [20,25]. Furthermore, the metallic Bi can be the center to capture photoinduced electrons during the process of photoreactions. So the photo-induced electrons preferably transfer to metallic Bi rather than recombine with the photo-induced holes, leading to inhibited charge-pair recombination. Then, based on the results of quenching experiments, a larger amount of O2 active oxidants are formed over the catalyst surfaces through the reaction of e and absorbed O2, which can degrade RhB and enhances the photocatalytic efficiencies. Meanwhile, the holes can degrade organic compounds on the BiOBr surface.
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4. Conclusions A simple hydrothermal method for the synthesis of Bi/BiOBr composite photocatalysts was developed. During the hydrothermal synthesis, partial Bi3+ were reduced to metallic Bi. Different uniform sheet-like morphology of pure BiOBr, Bi/BiOBr was consisted of sheets of different size with nanoparticles and flower-like nanostructures attached on surface. The as-synthesized Bi/BiOBr photocatalysts show a more intense continuous absorption in UV and visible light region, accompanied with decreased energy band gap. The metallic Bi and oxygen vacancies were generated in Bi/ BiOBr photocatalysts. Compared with pure BiOBr, Bi/BiOBr composites exhibit significantly improved photodegradation performances in the photodegradation of RhB under visible light irradiation. The improved photocatalytic properties can be attributed to higher BET specific area, narrow energy band gap, and inhibited charge-pair recombination. Moreover, the reactive species trapping experiments results show that O2 radicals and h+ contribute greatly to photocatalytic performance and OH radicals do not. We believe that this novel Bi/BiOBr photocatalyst will be a promising material for degrading organic pollutants and other applications. Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51172102), a project of Shandong Province Higher Educational Science and Technology Program (J15LA10), and Scientific Research Foundation of Liaocheng University. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2015.09. 063. References [1] Y. Gao, X. Pu, D. Zhang, G. Ding, X. Shao, J. Ma, Combustion synthesis of graphene oxide–TiO2 hybrid materials for photodegradation of methyl orange, Carbon 50 (2012) 4093–4101. [2] W.L. Ong, S. Natarajan, B. Kloostra, G.W. Ho, Metal nanoparticle-loaded hierarchically assembled ZnO nanoflakes for enhanced photocatalytic performance, Nanoscale 5 (2013) 5568–5575. [3] A.O. Ibhadon, P. Fitzpatrick, Heterogeneous photocatalysis: recent advances and applications, Catalysts 3 (2013) 189–218. [4] D. Zhao, G. Sheng, C. Chen, X. Wang, Enhanced photocatalytic degradation of methylene blue under visible irradiation on graphene@TiO2 dyade structure, Appl. Catal. B-Environ. 111–112 (2012) 303–308. [5] S. Zhang, J. Li, M. Zeng, G. Zhao, J. Xu, W. Hu, X. Wang, In situ synthesis of water-soluble magnetic graphitic carbon nitride photocatalyst and its synergistic catalytic performance, ACS Appl. Mater. Int. 5 (2013) 12735–12743.
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Contents lists available at ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Facile hydrothermal synthesis of Bi/BiOBr composites with enhanced visible-light photocatalytic activities for the degradation of rhodamine B Meichao Gao, Dafeng Zhang, Xipeng Pu ⇑, Hong Li, Dongdong Lv, Bingbing Zhang, Xin Shao School of Materials Science and Engineering, Liaocheng University, Liaocheng, Shandong 252000, PR China
a r t i c l e
i n f o
Article history: Received 15 July 2015 Received in revised form 23 September 2015 Accepted 24 September 2015
Keywords: Bi/BiOBr Photocatalyst Oxygen vacancy Visible light
a b s t r a c t Bi/BiOBr photocatalysts with improved photodegradation performances were synthesized by a facile onestep hydrothermal method using ethanol–water mixed solution. The samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, electron paramagnetic resonance spectroscopy, UV–vis diffuse reflectance spectroscopy and photoluminescence spectroscopy. The addition of ethanol results in the partial reduction of Bi3+ ions to metallic Bi, different morphology from pure BiOBr, and the formation of oxygen vacancies. The dosage of ethanol plays a vital role in the morphologies and photodegradation performances of as-obtained Bi/BiOBr. The Bi/BiOBr photocatalysts show improved photodegradation performances, which is attributed to the high surface area, narrow band gap, and inhibited recombination of electron–hole pairs . Ó 2015 Elsevier B.V. All rights reserved.
1. Introduction Semiconductor photocatalysts have long been used to mitigate the deterioration of environments created by toxic pollutants in water and air [1,2]. In the past few decades, semiconductor photocatalysts have got considerable attention due to their potential applications in degrading organic compounds for environmental remediation [3,4]. As is known, TiO2 has been mostly applied as photocatalysts due to its low cost, strong oxidizing power, and nontoxic nature [5]. Regrettably, TiO2 has a wide band gap (3.2 eV for anatase and 3.0 eV for rutile), which limits the absorption of visible light of the solar spectrum [6,7]. Some narrow band gap semiconductors display the ability to absorb visible-light, which should be good alternatives [8]. In recent years, bismuth oxybromide (BiOBr), with an indirect-transition band gap value of 2.92 eV [9], has received much attention due to its potential photocatalytic abilities under visible light. BiOBr is of tetragonal structure that consists of tetragonal [Bi2O2] positively charged slabs, which are interleaved by double slabs of bromine atoms to form [Br–Bi–O–Bi–Br] layers along the c-axis. The self-built electric field between [Bi2O2] and Br slabs would effectively separate the photoinduced electron–hole pairs, thus transfers the electrons and holes to the surface of BiOBr, which enhances the photocatalytic activity [10]. However, the photocatalytic activity of BiOBr is still far from efficient for practical applications. ⇑ Corresponding author. E-mail address: [email protected] (X. Pu). http://dx.doi.org/10.1016/j.seppur.2015.09.063 1383-5866/Ó 2015 Elsevier B.V. All rights reserved.
Moreover, varieties of strategies, such as impurity doping, formation of semiconductor heterojunctions and surface metallization, have been employed to improve the photocatalytic efficiency of BiOBr. At present, some noble metal/BiOBr composites have been proved as efficient photocatalysts under visible-light irradiation, such as Ag/BiOBr [11], Pd/BiOBr [12], and Pt/BiOBr [13]. Recently, some researchers found that metallic Bi modification could enhance the photocatalytic efficiency of semiconductor BiOCl and BiOI photocatalysts [14,15]. Moreover, Bi/BiOBr composites were synthesized by various methods and showed enhanced visible-light photocatalytic activity. Liu synthesized Bi/BiOBr composites by a facile one-step solvothermal method with the help of dimethyl sulfoxide, and the photocatalytic results showed that Bi/BiOBr composites exhibited enhanced visible-light photocatalytic performance, which was attributed to an increased amount of photo-induced charge carriers and efficient separation of photogenerated carriers [16]. Recently, Zhang and co-workers reported a one step solvothermal method to prepare Bi/BiOBr with oxygen vacancies in the presence of chloride dehydrate [17]. In this work, we prepared Bi/BiOBr composites via a hydrothermal method. Different from above mentioned two reports, partial Bi3+ cations were conveniently reduced to metallic Bi by ethanol under hydrothermal condition. Different from pure BiOBr, the asprepared Bi/BiOBr samples exhibit different morphologies; meanwhile, a great number of oxygen vacancies were generated in the as-obtained Bi/BiOBr composites. As-synthesized Bi/BiOBr composites exhibit significantly improved photocatalytic performance, whose mechanism was discussed in detail.
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2. Experimental procedure 2.1. Photocatalyst preparation All the chemical reagents were of analytical grade and purchased from Aladdin Industrial Corporation without further purification before use. Deionized water was used throughout the work. In a typical synthesis procedure, 0.02 mol bismuth nitrate pentahydrate (Bi(NO3)35H2O) was dissolved in 20 mL of 10% (w/w) nitric acid under vigorous stirring. Meanwhile, 0.06 mol ammonium bromide (NH4Br) was dissolved in 20 mL deionized water, and the obtained solution was added quickly into the above mixed solution, and milky suspension was obtained. Then x mL (x = 0, 2, 4, 6, 8, and 10) ethanol was added into the above mixed solution under stirring. These mixtures were transferred into an 80 mL Teflon-lined stainless steel autoclave to perform hydrothermal process at 180 °C for 6 h. After completion of the hydrothermal reaction, the precipitate was washed using absolute ethanol and deionized water several times, then dried at 80 °C in air for 12 h. The as-obtained Bi/BiOBr were denoted as Bi/BiOBr-x (the volume of ethanol). 2.2. Characterizations X-ray diffraction (XRD) patterns were recorded on a diffractometer (D8 Advanced, Bruker Co., Germany) with Cu Karadiation. Microstructures of the products were obtained by a field emission scanning electron microscope (FE-SEM, S4800, Hitachi Ltd., Japan). Transmission electron microscopy (TEM) was performed with a JEM-2100F microscope (JEOL Ltd., Japan). The Brunauer–Emmett–Teller (BET) specific surface areas of the samples were investigated by a Quantachrome Autosorb IQ-C nitrogen adsorption apparatus. X-ray photoelectron spectroscopy (XPS) was recorded using an X-ray photoelectron spectrometer (Kratos Axis Ultra) which uses Al Ka (1486.6 eV) X-ray source. The curve fitting was done using Casa XPS software by means of least square peak fitting procedure using a Gaussian–Lorentzian function. Electron paramagnetic resonance (EPR) measurements were performed with a Bruker ESP-300 EPR spectrometer. The settings for the EPR spectrometer were center field 3488 G, modulation frequency 100 kHz, microwave frequency 9.5 GHz, and power 12.9 mW. Ultraviolet–visible (UV–vis) diffuse reflectance spectra (DRS) of samples and the absorption spectra of rhodamine B (RhB) were recorded on a UV-3600 spectrophotometer (Shimadzu, Japan). Photoluminescence (PL) spectra of samples were recorded with an FLS 920 Fluorescence and Phosphorescence spectrometer (Edinburgh Instruments Ltd.) with kex = 320 nm. All these measurements were carried out at room temperature.
ize the stability of the as-obtained BiOBr, 6-cycle photodegradation experiment for RhB was carried out. Each cycle lasts for 120 min. After each cycle, the sample was allowed to settle down naturally (which takes less than 30 min), and then re-dispersed in fresh RhB solution for the next cycle after washing three time with deionized water. 3. Results and discussion 3.1. Structures and morphologies Fig. 1 shows the XRD patterns of the as-synthesized BiOBr and Bi/BiOBr. In the absence of alcohol, all the diffraction peaks of BiOBr samples can be indexed to tetragonal phase BiOBr (JCPDS Card No. 78-0348), and the strong and sharp diffraction peaks reveal a high degree of crystallization. When alcohol was added, apart from the diffraction peaks of BiOBr phase, the characteristic peaks of metallic Bi at 2h = 27.16°, 37.95° and 48.71° (JCPDS Card No. 85-1329) can be observed, suggesting the formation of Bi/BiOBr composites. Fig. S1 shows the XRD pattern of Bi/BiOBr-8 and the standard reference in order to see the position of Bi diffraction peak clearly. In addition, the intensity of peaks decreases gradually with the increase of the volume of ethanol, indicating that smaller crystal size of BiOBr can be obtained. The crystallite sizes of the samples were estimated using the Scherrer formula A = 0.94 k/b Cos h, where A is average crystalline size, b is the full-widths at half-maximum of the diffraction peak, k is the wavelength (1.54 Å) of X-ray radiation, and h is the angle of diffraction. We calculated the crystal sizes of BiOBr, Bi/BiOBr-2, Bi/BiOBr-4, Bi/BiOBr-6, Bi/BiOBr-8 and Bi/BiOBr-10, which are about 331.3, 93.2, 77.4, 70.7, 65.0 and 64.5 nm, respectively. The morphologies of pure BiOBr and Bi/BiOBr-8 were investigated by SEM. Fig. 2(a) shows the SEM image of pure BiOBr. It is observed that the sample is composed of a large quantity of uniform sheets with an average diameter of about 5 lm and an average thickness of 250 nm. When alcohol were used, as shown in Fig. 2(b), different from pure BiOBr sample, Bi/BiOBr-8 exhibits sheet-like morphology with coarse surface, and some nanoparticles and flower-like nanostructures are observed on the surface of sheets, as shown in Fig. 2(c). The coarse morphology of Bi/BiOBr-8 endows it higher BET specific area of Bi/BiOBr-8 (53.697 m2/g), which is higher than that of BiOBr (22.025 m2/g). The better adsorption performance is advantageous for the diffusion of RhB to the surface of photocatalyst. The as-synthesized Bi/BiOBr-8 was further characterized by HRTEM, as shown in Fig. 2(d). Tow kinds of lattice fringes with d spacings of about 0.278 and 0.328 nm were observed which can be indexed as the
2.3. Photocatalytic activity tests Photocatalysis experiment was carried out in a home-made photocatalytic reaction box. A 300 W Xenon lamp with a UV cutoff filter (JB450) was positioned about 10 cm over a cylindrical container with a circulating water jacket for cooling. 100 mg of our samples was dispersed in 100 mL aqueous solution of RhB (10 mg/L). The solution was stirred in dark for 30 min to ensure the establishment of the adsorption–desorption equilibrium. The concentration of RhB was analyzed by recording the absorption band maximum (554 nm) in the absorption spectra and taken as the initial concentration (C0). During the irradiation, 5 mL solution was extracted at an interval of 15 min, then absorbance of RhB solution was measured after centrifugation at 1000 rpm for 5 min to remove the catalysts. The normalized temporal concentration changes (C/C0) of RhB were obtained. In order to character-
Fig. 1. XRD patterns of BiOBr and Bi/BiOBr samples.
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Fig. 2. SEM images of (a) BiOBr; (b and c) Bi/BiOBr-8 and (d) TEM image of Bi/BiOBr-8.
(1 1 0) plane of BiOBr and the (0 1 2) plane of metallic Bi, respectively. Fig. 3 shows the XPS spectra of pure BiOBr and Bi/BiOBr-8. Two peaks with binding energies of 165.1 and 159.8 eV are observed in the spectrum of the BiOBr, which can be ascribed to the Bi3+ 4f5/2 and Bi3+ 4f7/2 binding energies, respectively [18]. The splitting between these bands was 5.3 eV, referring to the presence of the normal state of BiOBr [18]. In comparison to pure BiOBr sample, the Bi3+ 4f7/2 and 4f5/2 peaks of Bi/BiOBr-8 exhibit a shift of about 0.7 eV toward lower binding energy, which is indicative of the formation of lower charge Bi ions in Bi/BiOBr-8 [15]. In addition, the O 1s peak can be divided into two peaks at 530.9 and 529.8 eV (Fig. 3b). The peak at 529.8 eV is attributed to the lattice oxygen in BiOBr, while the other peak with a higher energy of 530.9 eV can ascribed not only to the surface hydroxyl oxygen [19], but also to the oxygen vacancies in the surface of BiOBr. The presence of oxygen vacancies was further confirmed by EPR spectroscopy, as shown in Fig. 4. The EPR spectrum of Bi/BiOBr-8 shows a remarkable signal at g = 2.001, typical character of oxygen vacancy, while no signal is found from pure BiOBr [20].
2.64 eV, while the values of Bi/BiOBr-2, -4, -6, -8, and -10 are approximately 2.53, 2.42, 1.92, 1.75 and 1.90 eV, respectively, which are smaller than that of pure BiOBr due to the formation of metallic Bi and oxygen vacancies in the Bi/BiOBr sheet. These results indicate that all these Bi/BiOBr samples have suitable band gap energies for photodegradation of RhB under visible light irradiation. Photoluminescence emission spectrum was usually used to research the efficiency of charge trapping, immigration, transfer, and to understand the behavior of electron–hole pairs [22]. Fig. 6 shows the PL spectra of pure BiOBr and Bi/BiOBr composites. It can be observed that pure BiOBr shows a strong emission peak in the range of 400–600 nm, deriving from the direct electron–hole recombination of band transition. Though shapes and peaks position of Bi/BiOBr composites are similar to the pure BiOBr, the emission intensities of the as-obtained Bi/BiOBr composites decrease significantly. This implies that the electron – hole recombination were effectively prohibited, which can achieve a better photocatalytic activity. 3.3. Photocatalytic properties
3.2. Optical properties UV–vis diffuse reflectance spectra (DRS) of the BiOBr and Bi/BiOBr samples are given in Fig. 5(a). Compared to pure BiOBr, Bi/BiOBr samples reveal a more intense continuous absorption in UV and visible light region. Moreover, as shown in inset of Fig. 5 (a), after the addition of ethanol, the samples became darker. This can be attributed to the formation of metallic Bi, which makes Bi/BiOBr ready for the efficient utilization of photon energy in both UV and visible light range. The energy of the band gap (Eg) was calculated following the equation: (ahm)1/2 = B(hm Eg), where a, hm, Eg, and B are the absorption coefficient, photon energy, band gap, and a constant, respectively [21]. The Eg values of as-synthesized samples could be estimated from a Tauc plots of (ahm)1/2 versus the photon energy (hm), as shown in Fig. 4(b). The intercepts of the tangent to the x-axis will give a good approximation of the band gap energies for the BiOBr and Bi/BiOBr samples. The estimated band gap energy of pure BiOBr is approximately
RhB is selected as a model pollutant to evaluate the photocatalytic activities of the as-prepared samples. As shown in Fig. 7(a), compared with neat BiOBr, Bi/BiOBr composites exhibit higher absorption ability for RhB due to the high BET specific surface, and about 50% of RhB were adsorbed by the Bi/BiOBr photocatalyst at the beginning of the photodegradation. The adsorption properties of BiOBr and Bi/BiOBr for RhB were characterized in dark, as shown in Fig. S2. Obviously, the adsorption/desorption equilibrium was almost established in 30 min, and thus the adsorption has no contribution in the following decoloration. As shown in Fig. 7(b), TiO2 (P25) shows poor photoactivity under visible light irradiation. In the case of pure BiOBr, the photolytic fade is only 25% after 2 h of visible light irradiation. In contrast, Bi/BiOBr samples exhibit significantly improved photodegradation performance. Bi/BiOBr-8 has the best photodegradation performance. After 30 min of visible light irradiation, 95.5% of the initial RhB was photodegraded in the case of Bi/BiOBr-8, whereas only 10% of the initial dye was
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Fig. 3. XPS spectra of (a) Bi 4f of BiOBr and Bi/BiOBr-8, and (b) O 1s of Bi/BiOBr-8.
Fig. 4. EPR spectra of BiOBr and Bi/BiOBr-8.
Fig. 6. PL spectra of pure BiOBr and Bi/BiOBr.
decomposed by neat BiOBr. Fig. 7(c) shows the RhB photodegradation curves when the Bi/BiOBr-8 photocatalyst was reused 6 times. Obviously, the result demonstrated no deactivation during the 6-cycle, indicating the excellent recyclability and stability of Bi/ BiOBr under visible light irradiation. To investigate the active species during the photocatalytic reaction, a series of trapping experiments were carried out.
Isopropyl alcohol (IPA) [14], p-benzoquinone (BZQ) [23], and disodium ethylenediaminetetraacetate (Na2-EDTA) [23] were used as hydroxyl radical (OH), superoxide radical (O2 ), and hole (h+) specie quenchers, respectively. As shown in Fig. 8, quenching results showed that IPA did not influence the catalysis process, while BZQ and Na2-EDTA significantly slowed the degradation rate of RhB, indicating that the O2 and h+ are the main active species of Bi/BiOBr in aqueous solution under visible light irradiation.
Fig. 5. (a) UV–vis DRS spectra and (b) corresponding Tauc plots of (ahm)1/2 versus (hm) of BiOBr and Bi/BiOBr composites. Insets are the digital photos of as-synthesized BiOBr and Bi/BiOBr-8.
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Fig. 7. The variations of (a) absorbance, (b) normalized C/C0 of RhB solution with different visible light irradiation time, and (c) RhB degradation curves of Bi/BiOBr-8 when reused 6 times.
Fig. 9. Photocatalytic mechanism scheme of Bi/BiOBr under visible light irradiation. Fig. 8. Photocatalytic performances of Bi/BiOBr-8 with different specie quenchers.
Based on the previous discussion, the photocatalytic mechanism scheme of Bi/BiOBr under visible light irradiation is given in Fig. 9. First, the high BET specific area of Bi/BiOBr can accelerate the diffusion of RhB molecules from solution to the active sites of photocatalyst, which has a beneficial effect on the photocatalytic reaction. Second, it has been reported that oxygen vacancies can draw an impurity level between the valence band (VB) and the conduction band (CB) to narrow the band gap of photocatalysts [14,17,24]. Consequently, as-obtained Bi/BiOBr samples exhibit narrow band gap and are much more sensitive to visible light than pure BiOBr. Third, when the Bi/BiOBr is excited by visible light with an energy higher than the band gap, the transition of electrons
from VB to CB of BiOBr or oxygen vacancies states forms photogenerated electron–hole pairs. As active electron traps, oxygen vacancies promote the separation efficiency of photo-induced electron–hole pairs, resulting in a high photoactivity [20,25]. Furthermore, the metallic Bi can be the center to capture photoinduced electrons during the process of photoreactions. So the photo-induced electrons preferably transfer to metallic Bi rather than recombine with the photo-induced holes, leading to inhibited charge-pair recombination. Then, based on the results of quenching experiments, a larger amount of O2 active oxidants are formed over the catalyst surfaces through the reaction of e and absorbed O2, which can degrade RhB and enhances the photocatalytic efficiencies. Meanwhile, the holes can degrade organic compounds on the BiOBr surface.
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4. Conclusions A simple hydrothermal method for the synthesis of Bi/BiOBr composite photocatalysts was developed. During the hydrothermal synthesis, partial Bi3+ were reduced to metallic Bi. Different uniform sheet-like morphology of pure BiOBr, Bi/BiOBr was consisted of sheets of different size with nanoparticles and flower-like nanostructures attached on surface. The as-synthesized Bi/BiOBr photocatalysts show a more intense continuous absorption in UV and visible light region, accompanied with decreased energy band gap. The metallic Bi and oxygen vacancies were generated in Bi/ BiOBr photocatalysts. Compared with pure BiOBr, Bi/BiOBr composites exhibit significantly improved photodegradation performances in the photodegradation of RhB under visible light irradiation. The improved photocatalytic properties can be attributed to higher BET specific area, narrow energy band gap, and inhibited charge-pair recombination. Moreover, the reactive species trapping experiments results show that O2 radicals and h+ contribute greatly to photocatalytic performance and OH radicals do not. We believe that this novel Bi/BiOBr photocatalyst will be a promising material for degrading organic pollutants and other applications. Acknowledgements This work was supported by National Natural Science Foundation of China (No. 51172102), a project of Shandong Province Higher Educational Science and Technology Program (J15LA10), and Scientific Research Foundation of Liaocheng University. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2015.09. 063. References [1] Y. Gao, X. Pu, D. Zhang, G. Ding, X. Shao, J. Ma, Combustion synthesis of graphene oxide–TiO2 hybrid materials for photodegradation of methyl orange, Carbon 50 (2012) 4093–4101. [2] W.L. Ong, S. Natarajan, B. Kloostra, G.W. Ho, Metal nanoparticle-loaded hierarchically assembled ZnO nanoflakes for enhanced photocatalytic performance, Nanoscale 5 (2013) 5568–5575. [3] A.O. Ibhadon, P. Fitzpatrick, Heterogeneous photocatalysis: recent advances and applications, Catalysts 3 (2013) 189–218. [4] D. Zhao, G. Sheng, C. Chen, X. Wang, Enhanced photocatalytic degradation of methylene blue under visible irradiation on graphene@TiO2 dyade structure, Appl. Catal. B-Environ. 111–112 (2012) 303–308. [5] S. Zhang, J. Li, M. Zeng, G. Zhao, J. Xu, W. Hu, X. Wang, In situ synthesis of water-soluble magnetic graphitic carbon nitride photocatalyst and its synergistic catalytic performance, ACS Appl. Mater. Int. 5 (2013) 12735–12743.
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