Journal of Colloid and Interface Science 431 (2014) 187–193
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
One-pot synthesis of Bismuth Oxyhalide/Oxygen-rich bismuth oxyhalide Heterojunction and its photocatalytic activity ZhangSheng Liu ⇑, HuaShen Ran, JiNan Niu, PeiZhong Feng, YaBo Zhu School of Material Science and Engineering, China University of Mining and Technology, XuZhou 221116, China
a r t i c l e
i n f o
Article history: Received 29 March 2014 Accepted 8 June 2014 Available online 18 June 2014 Keywords: Photocatalytic activity BiOBr/Bi24O31Br10 Heterojunction Solvothermal
a b s t r a c t BiOBr/Bi24O31Br10 heterojunction photocatalysts were prepared by a facile solvothermal method for the first time. The X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), N2 sorption, UV–vis diffuse reflectance spectroscopy (UV–vis DRS) and photoluminescence (PL) were applied to investigate the structures, morphologies, surface areas and photocatalytic properties of as-prepared samples. The photocatalytic activity of the samples was evaluated by the photocatalytic degradation of Rhodamine B under the visible-light irradiation. The results showed that BiOBr/Bi24O31Br10 heterojunctions with the different Bi24O31Br10 contents could be obtained by simply adjusting the amount of NaOH solution, all of which exhibited enhanced photocatalytic activity compared with bare BiOBr or Bi24O31Br10. Among them, the BiOBr/Bi24O31Br10 heterojunction prepared with 1.5 ml of NaOH solution possessed the highest photocatalytic activity. The photogenerated holes and O 2 radicals were confirmed to be the main active species responsible for the photodegradation of RhB. The mechanism of enhanced photocatalytic activity was discussed and the transfer process of the photogenerated charges carrier between BiOBr and Bi24O31Br10 was proposed on the basis of the estimated energy band positions. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Semiconductor photocatalysis, as a kind of ‘‘green’’ technology, has become one of the most pivotal topics in contemporary photocatalysis research [1,2]. TiO2 is a mostly studied semiconductor photocatalyst owing to its cheapness, strong oxidizing power and nontoxic nature, whose effectiveness for degrading pollutants has been well-documented [3]. However, TiO2 can only be activated by UV irradiation (k < 390 nm) due to the relatively wide band gap (3.2 eV), which comprises less than 5% of the solar spectrum [4]. In order to use the solar energy more effectively, great efforts have been made to develop visible-light-induced photocatalysts. Recently, BiOBr has drawn considerable attention for its potential application in environmental remediation due to excellent photocatalytic activity and high stability [5,6]. BiOBr has an indirect band gap and crystallizes in a tetragonal layered structure, which is characterized by [Bi2O2] slabs interleaved by double slabs of bromine atoms. The indirect band gap requires the photogenerated electrons to travel a certain k-space distance from the conduction band (CB) to the valence band (VB) before their recombination,
⇑ Corresponding author. Fax: +86 51683591870. E-mail address: [email protected] (Z. Liu). http://dx.doi.org/10.1016/j.jcis.2014.06.020 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
while the intrinsic layered structure creates a strong internal electric field between the Br negative layer and [Bi2O2] positive layer, both of which reduce the recombination probability of the photo-generated electrons and holes, favoring the enhancement of the photocatalytic activity [7–9]. However, the photocatalytic activity of BiOBr is still far from efficient for the practical applications, and it needs some further improvements. A variety of strategies, such as impurity doping [10] and engineering BiOX (X = Cl, Br, I) nanostructures [11], have been developed to improve its photocatalytic performance. In recent years, construction of semiconductor heterostructures has been recognized as a promising method to develop highly efficient photocatalysts. Up to now, many BiOBr-based heterostructures such as Graphene/BiOBr [12], g-C3N4/BiOBr [13], Ag/BiOBr [14], Bi2WO6/BiOBr [15] and TiO2/ BiOBr [16] have been developed. The results indicate that the photocatalytic activity of BiOBr is indeed improved by coupling other semiconductors to some extent. Nevertheless, BiOBr-based heterostructures with high stability and excellent photocatalytic performance are still rare. Very recently, Bi24O31Br10, as a Br-poor bismuth oxybromide, has been known to researcher, and it has been proven to possess excellent photocatalytic activity under the visible light irradiation [17,18]. However, to the best of our knowledge, the heterostructure photocatalyst containing the same elements, BiOBr/Bi24O31Br10, has not been reported.
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Herein, the novel BiOBr/Bi24O31Br10 heterostructures were prepared by a facile one-pot solvothermal method for the first time. The structural and optical properties of the resultant products were characterized by XRD, SEM, TEM, BET, FT-IR, UV–Vis DRS and PL. The photocatalytic activity of as-prepared samples was evaluated by the photocatalytic degradation of RhB under the visible light irradiation. The results show that BiOBr/Bi24O31Br10 heterostructures possess higher photocatalytic performance compared with individual BiOBr or Bi24O31Br10, whose photocatalytic mechanism is investigated and discussed in detail.
2. Experimental 2.1. Preparation of photocatalysts All the reagents were provided by Sinopharm Chemical Reagent Co., Ltd (China), and they were used as received without further purification. BiOBr/Bi24O31Br10 heterostructures were synthesized via a facile one-pot solvothermal method. In a typical process, 1 mmol of Bi(NO3)35H2O and 1 mmol of cetyltriethylammonium bromide (CTAB) were orderly dissolved in 40 ml of ethylene glycol (EG) under magnetic stirring, and then certain volume of 2 M NaOH solution was added dropwise. The resultant precursor solution was transferred into a 50 ml Teflon-lined stainless steel autoclave after 30 min of stirring. The autoclave was sealed and maintained at 160 °C for 16 h and allowed to cool down to room temperature naturally. The precipitate was washed with absolute ethanol and distilled water for three times, respectively, and then dried at 80 °C in air. The volumes of the NaOH solution used were 0, 1, 1.5, 2 and 3 ml, and the as-prepared samples were denoted as S0, S1, S1.5, S2 and S3 accordingly.
2.2. Characterization Crystal structures of as-prepared samples were identified by an X-ray diffractometer (XRD, Bruker D8 Advance with Cu Ka1 radiation at 40 kV and 30 mA). The morphology and structure were performed by field emission scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, Philips, Tecnai 12). The Brunauer–Emmett–Teller (BET) surface area was determined by N2 adsorption–desorption isotherm measurement at 77 K on a Quantachrome NOVA-4200E system. FT-IR spectra were recorded by using Vertex70 FTIR spectrometer. The optical property was analyzed by both UV–vis diffuse reflectance spectra (DRS, Varian Cary 300) and photoluminescence spectra (PL, Varian Cary-Eclipse 500).
3. Results and discussion 3.1. Characterization 3.1.1. XRD analysis The crystal structure of as-prepared samples was investigated by means of X-ray powder diffraction (XRD), as shown in Fig. 1. It can be seen that, in the absence of NaOH, the diffraction peaks of the control sample(S0) are in good agreement with the tetragonal phase of BiOBr (JCPDS card No. 09-0393), indicating the formation of pure BiOBr. When 3 ml of NaOH solution was used, the diffraction peaks of as-prepared S3 sample are readily indexed as a monoclinic phase Bi24O31Br10 (JCPDS card No. 75-0888). It seems that the alkaline condition is favorable for the formation of Bi24O31Br10. When 1–2 ml of NaOH solution was used, the diffraction peaks of as-prepared samples undergo a gradual change. With 1 ml of NaOH solution, S1 sample displays weak diffraction peaks of Bi24O31Br10, suggesting the initial formation of Bi24O31Br10 phase. With the increase of NaOH solution, the diffraction peaks of Bi24O31Br10 gradually increase in intensity, while the diffraction peaks of BiOBr correspondingly decrease in intensity, some of which even completely disappear. Therefore, it is reasonable to believe that Bi24O31Br10 phase is formed at the cost of BiOBr phase, which is in accordance with the previous report [17]. The corresponding reaction is based on Eq. (1):
24BiOBr þ 14OH ! Bi24 O31 Br10 þ 14Br þ 7H2 O
The relative mass fraction of Bi24O31Br10 in these three samples can be estimated according to Eq. (2) [19]:
W1 ¼
I1 I1 þ I2 ðRIR1 Þ=RIR2
ð2Þ
where W1 is the mass fraction of Bi24O31Br10 phase, I1 and I2 are the strongest integral intensities of Bi24O31Br10 and BiOBr phase, respectively. RIR1 and RIR2 value are obtained from MDI Jade 5.0. By calculation, the estimated contents of Bi24O31Br10 are 21.4%, 52.6% and 85.4% for S1, S1.5 and S2 sample, respectively. 3.1.2. SEM and TEM The morphologies of as-prepared samples were characterized by SEM, TEM and HRTEM. As shown in Fig. 2a, one can find that S0 sample consists of hierarchical microspheres with diameters about 3 lm, which agrees with the previous report [20]. These BiOBr microspheres are further assembled by radially grown nanosheets, which interweave with each other to create a large amount of nanopores in the surface of microspheres. After different amounts
2.3. Photocatalytic test The photocatalytic activity of the samples was determined by the degradation of Rhodamine B(RhB) under the visible light irradiation. The visible light source was a 150 W tungsten-halogen lamp (Beijing Institute of Opto-Electronic Technology, light intensity = 200 mW/cm2), whose short-wavelength components (k < 400 nm) of the light were cut off using a cutoff glass filter. Experiments were carried out at 20 ± 3 °C as follows: 0.1 g of photocatalyst was added into 100 ml of 15 mg/l RhB solution. Before irradiation, the suspension was stirred for 20 min in the dark in order to reach the adsorption–desorption equilibrium. At given irradiation time intervals, about 3 ml suspensions was taken and centrifuged to remove the catalyst particles. The concentration of remnant RhB was determined by the UV–vis spectroscopy at its characteristic wavelength of 554 nm.
ð1Þ
Fig. 1. XRD patterns of as-synthesized samples.
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Fig. 2. SEM images of as-prepared samples (a) S0, (b) S1, (c) S1.5, (d) S2, (e) S3 and (f) TEM and HRTEM image (inset) of S1.5.
of NaOH solution were introduced, the morphologies of as-prepared samples undergo a gradual change. When 1 ml of NaOH solution was used, although S1 is also composed of hierarchical microspheres with the similar size, these microspheres seem to be loosely assembled by nanosheets (Fig. 2b). Besides, the nanosheet building blocks are rugged in the edges, and some nanoparticles can be found on them. When the NaOH solution used was 1.5 ml, it is found that as-prepared S1.5 sample is composed of two different structures (Fig. 2c): degraded microspheres as the main component and delaminated individual nanosheets as a secondary component. The size of the degraded microsphere is about 1–2 lm, while the nanosheet is about 250 nm in diameter. With further increasing the amount of NaOH solution to 2 ml, as-prepared S2 sample displays an irregular hierarchical structure, in which nanosheets are randomly stacked together. It can be ascribed to the existence of a large amount of overlapping Bi24O31Br10 nanoclusters, which adhere to the nanosheet edges of BiOBr (inset of Fig. 2d), hindering the orderly assembling. Once the NaOH solution used was up to 3 ml, as-prepared S3 sample shows a special 3D hierarchical structure, which is composed of numerous nanoflakes. These nanoflakes are curved and significantly smaller than BiOBr nanosheets. The Bi24O31Br10/BiOBr heterojunctions were further characterized by TEM and S1.5 sample was taken as an example, as shown in Fig. 2f. It can be seen that the nanosheet building block is about 5–20 nm in thickness and displays a layered structure, which is further assembled by ultrathin nanosheets. To investigate the structure information, the HRTEM image of the nanosheet building block is shown in the inset of Fig. 2f. It is seen that there are two
sets of lattice fringes with spacings of 0.282 nm and 0.276 nm, corresponding to the (1 1 7) plane of the monoclinic Bi24O31Br10 and (1 1 0) plane of the tetragonal BiOBr, respectively. The result indicates that Bi24O31Br10 and BiOBr phase have been completely coupled together. 3.1.3. FT-IR analysis The FTIR spectra of as-prepared samples are shown in Fig. 3. The absorption band at 517 cm1 is ascribed to the Bi–O stretching mode, which is often believed to be the characteristic absorption peak of BiOBr [12,21]. Such a characteristic peak of BiOBr can also be found in S1.5 sample, indicating the existence of BiOBr in S1.5
Fig. 3. FT-IR transmittance spectra of the as-prepared samples.
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sample. Besides, it is worthy noting that there have been no reports involving the characteristic absorption peaks of Bi24O31Br10, and they are not found in our measurement. Nevertheless, considered that the peak shape and peak position of S1.5 sample are very similar to those of S3 sample within the wavenumber range of 600–1600 cm1, it may suggest that there is Bi24O31Br10 phase in S1.5 sample. Therefore, it is believed that S1.5 is the mixed phases of BiOBr and Bi24O31Br10, which is consistent with the XRD results. 3.1.4. N2 Adsorption–desorption isotherms The specific surface area and porosity of as-prepared samples were investigated by using nitrogen adsorption and desorption isotherms. The specific surface areas of S0, S1, S1.5, S2 and S3 samples were found to be 21.6, 27.6, 32.1, 28.4 and 29.8 m2/g, respectively. Obviously, BiOBr/Bi24O31Br10 heterojunctions possess higher specific surface area than BiOBr. To understand the mesoporous structures of as-prepared samples, the typical adsorption and desorption isotherms are shown in Fig. 4. It can be seen that both S0 and S1.5 sample show a type IV isotherm with a distinct hysteresis loops at relative pressures (P/P0) between 0.2 and 1.0, which is usually associated with capillary condensation in mesopores [22]. The high adsorption at P/P0 approaching 1.0 for S1.5 sample indicates the coexistence of mesopores (2–50 nm) and macropores (>50 nm) [23]. The detailed pore size distributions can be obtained by the BJH analysis, as shown in the inset in Fig. 4. Both S0 and S1.5 sample display bimodal pore size distributions. Pore diameters of S0 are 2.7 and 10.0 nm, while those of S1.5 are 3.0 and 17 nm, respectively, all of which are characteristic of mesopores. The occurrence of bimodal pore-size distributions can be attributed to their hierarchical structures [24]. 3.2. Optical property 3.2.1. DRS patterns The band gap structure of a semiconductor plays an important role in determining the photocatalytic activity. Fig. 5 shows the UV–vis DRS of as-prepared samples. It can be seen that all the samples display the photoabsorption from the UV light region to visible light until 440–500 nm, Compared with S0 sample, S1, S1.5 and S2 sample exhibit a systematical red shift and an obvious increased light absorption ability in the visible light region. This can also be concluded from a simple visual inspection of the samples, since their colors gradually turn from white to light yellow. The enhanced photo-absorption can be attributed to two factors. One
Fig. 4. N2 adsorption–desorption isotherms and pore size distribution of the asprepared samples.
Fig. 5. UV–vis DRS spectra of as-prepared samples and their band gap energies (inset).
is that Bi24O31Br10 phase in BiOBr/Bi24O31Br10 heterojunction possesses narrower band gap energy (Eg) than BiOBr [17]. The other can be related with the heterojunction effect, which results in a possible charge-transfer transition on the interface between BiOBr and Bi24O31Br10. The band gap energies of as-prepared samples can be estimated by extrapolating the straight portion of (ahm)1/2 against hm plot to a = 0 based on Eq. (3) [25], as shown in the inset of Fig. 5. 1=2
ðahmÞ
¼ Aðhm Eg Þ
ð3Þ
where a, h, m, Eg, and A are absorption coefficient, Planck constant, light frequency, band gap, and a constant, respectively. The band gap energies are established as about 2.64 eV, 2.59 eV, 2.51 eV, 2.39 eV and 2.23 eV for S0, S1, S1.5, S2 and S3 sample, respectively. Among them, the Eg values of Bi24O31Br10 and BiOBr are very close to those reported in the literature [17,26]. 3.2.2. PL patterns In order to investigate the recombination of photoinduced electrons and holes, the PL spectra of as-prepared samples were measured with an excitation wavelength of 250 nm, as shown in Fig. 6. It can be seen that all the BiOBr/Bi24O31Br10 heterojunctions display lower emission band intensities of the spectra than bare BiOBr and Bi24O31Br10 photocatalyst, suggesting that the recombination of photogenerated charge carriers is greatly restrained by the coupling of BiOBr and Bi24O31Br10. In addition, the emission band
Fig. 6. PL spectra of as-prepared samples.
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intensities of the spectra are different with the content of Bi24O31Br10. S1.5 sample with 52.6% of Bi24O31Br10 displays the lowest PL intensity. Therefore, it can be inferred that the sample prepared with 1.5 ml of NaOH solution will possess the highest photocatalytic activity.
3.3. Photocatalytic performance The photocatalytic performances of as-prepared samples were evaluated by the photocatalytic removal of RhB as a function of irradiation time under visible light irradiation. Fig. 7 shows the temporal UV–vis absorption spectra variation of RhB over the S1.5 sample. It is found that the characteristic absorption band of RhB at 554 nm diminishes quickly under the visible light irradiation, accompanied by a slight blue shift of the maximum absorption from 554 to 496 nm. The color of RhB solution also changes correspondingly from red to orange red, light green and finally colorless, as shown in the inset of Fig. 7. These changes indicate that the degradation of RhB over the S1.5 sample undergoes two processes, i.e. the de-ethylation process firstly and then the destruction of a conjugated structure under sunlight irradiation, which have been reported in the previous BiOBr-based system [13,15]. After 2 h of the visible light irradiation, the characteristic absorption band of RhB disappears and no new absorption bands appear, suggesting that RhB has been completely mineralized by using BiOBr/Bi24O31Br10 heterojunction. The photocatalytic performances of as-prepared samples were evaluated by the degradation of RhB under visible light irradiation. The results are shown in Fig. 8a. It can be seen that no evident photolysis of RhB is detected in the absence of catalyst, suggesting the stability of RhB under the visible light. After 2 h of visible light irradiation, RhB with different photocatalysts is degraded in varying degrees. The degradation rates are 89.6%, 98.1%, 99.7%, 95.8%, and 76.6 for S0, S1, S1.5, S2 and S3 sample, respectively. All the BiOBr/Bi24O31Br10 heterojunctions display higher photocatalytic activity than individual BiOBr and Bi24O31Br10. To quantify the photocatalytic ability of these samples and understand the reaction kinetics of RhB degradation, the apparent pseudo-first-order model is applied [27], as shown in Fig. 6b. ln(C/C0) exhibits a well linear relationship with the irradiation time, indicating that the photocatalytic reaction belongs to the pseudo-first-order reaction. Their reaction rate constants are also concluded in Fig. 8b. Clearly, the reaction rate constants of all the BiOBr/Bi24O31Br10 heterojunctions are higher than bare BiOBr and Bi24O31Br10. Among them, the reaction rate constant of S1.5 sample is as about 2.6 and 4.8 times as
that of BiOBr and Bi24O31Br10, respectively. The enhanced photocatalytic activity can be attributed to the heterojunction effect resulting from the coupling of BiOBr and Bi24O31Br10, which can effectively restrain the recombination of photogenerated electrons and holes. The Bi24O31Br10 content seems to be a key point for producing BiOBr/Bi24O31Br10 heterojunction with high activity. It is believed to be related with the amount of BiOBr/Bi24O31Br10 heterojunction. Generally speaking, the higher the Bi24O31Br10 content, the more BiOBr/Bi24O31Br10 heterojunction we can obtain. However, excess Bi24O31Br10 tends to form overlapping nanoclusters, which only physically adhere to the edges of BiOBr nanosheets (Fig. 2d). These nanoclusters not only are disadvantageous to the formation of BiOBr/Bi24O31Br10 heterojunction but also may become recombination centers of photoinduced charge carriers, reducing their separation efficiency. Therefore, there is an optimum Bi24O31Br10 content for BiOBr/Bi24O31Br10 heterojunction with high photocatalytic activity. In this study, the suitable content of Bi24O31Br10 is obtained by S1.5 sample, which is about 52.6%. 3.4. Photocatalytic mechanism 3.4.1. Mechanism of the enhanced photocatalytic activity According to the above results, BiOBr/Bi24O31Br10 heterojunctions display higher activity than individual BiOBr and Bi24O31Br10. The enhanced photocatalytic activity can be explained as follows. Firstly, the photocatalytic reaction rate is often proportional to the number of photons absorbed by photocatalyst. Based on Fig. 5, BiOBr/Bi24O31Br10 heterojunctions expand the visible light response range to some extent and improve the light absorpting ability, which has to provide more available photons, favoring the enhancement of the photocatalytic activity. Secondly, the catalytic process is often related to the adsorption and desorption of pollution molecules on the surface of the catalyst. The high surface areas can lead to strong adsorption towards the target molecules and offer more active sites [27,28]. Based on the above morphology observation and BET surface area measurement, BiOBr/Bi24O31Br10 heterojunctions tends to form loose 3D hierarchical structures and possesses higher specific surface areas, which has to be beneficial to the photocatalytic reaction. Finally, the efficient separation of photoinduced electron–hole pairs was believed to be a key factor for the photocatalytic decomposition of organic pollutants [29]. The flowchart of photogenerated electrons and holes in the heterojunction depends on the band edge positions of semiconductors. The valence band potentials of the semiconductors at the point of zero charge can be calculated by Mulliken electronegativity theory [30].
EVB ¼ X E0 þ 0:5Eg
Fig. 7. The temporal evolution of the spectra during the photodegradation of RhB over the S1.5 sample.
191
ð4Þ
where EVB is the VB edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, E0 is the energy of free electrons on the hydrogen scale (about 4.5 eV), Eg is the band gap energy of the semiconductor. The conduction band (CB) position can be obtained by ECB = EVB Eg. Based on Eq. (4), the valence band tops of BiOBr and Bi24O31Br10 are calculated as 3.14 and 2.93 eV, and correspondingly the conduction band bottoms of BiOBr and Bi24O31Br10 are 0.50 and 0.56 eV, respectively. Once the BiOBr/Bi24O31Br10 heterostructure is exposed to the visible light irradiation, both BiOBr and Bi24O31Br10 can be activated due to their narrow band gap energies. The electrons on the VB of BiOBr are excited to a potential edge (0.50 eV). However, the electrons on the VB of Bi24O31Br10 can be excited up to much higher potential edge than 0.56 eV, since there is a big uplift of the conduction band for Bi24O31Br10 due to the orbital hybridization of Br4s and Bi6p [31]. As a result, the photogenerated electrons on the CB of Bi24O31Br10 can migrate to the CB
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Fig. 8. (a) Photocatalytic degradation of RhB over as-prepared samples under visible light irradiation and (b) degradation kinetics of RhB.
Fig. 9. Photocatalytic degradation of RhB over S1.5 sample in the presence of various radicals scavengers.
of BiOBr. On the other hand, the photogenerated holes on the VB of BiOBr tend to migrate to that of Bi24O31Br10, since the VB potential of BiOBr (3.14 eV) is more positive than that of Bi24O31Br10 (2.93 eV). The transference of the photogenerated charge carriers results in their prolonged lifetimes due to the extended transfer path, hindering the undesired recombination of electron–hole pairs, which is in accordance with the PL result (Fig. 6). In view of the above discussion, the enhanced photocatalytic activity for BiOBr/ Bi24O31Br10 heterostructure can be attributed to strong visible light response, high BET specific surface area and efficient separation of photogenerated electrons and holes.
3.4.2. Main active species In order to better understand the photocatalytic mechanism, it is necessary to detect main active oxidative species in the photocatalytic reaction. To do so, a series of experiments were carried out by dissolving different trapping agents in the reaction solution before light irradiations. As shown in Fig. 9, there are no obvious changes in the degradation rate of RhB with the addition of isopropanol (IPA), which is a well-known scavenger of OH radicals [32]. However, the photocatalytic degradation efficiencies of RhB are significantly suppressed by the addition of benzoquinone (BQ) + and triethanolamine (TEOA), which can trap O 2 and holes (h ), respectively [8]. The results indicate that the photocatalytic degradation of RhB over BiOBr/Bi24O31Br10 can be mainly attributed to the photoinduced holes (h+) and O 2 radicals, instead of OH radicals. In fact, the results can also make sense from the theoretical viewpoint. Firstly, it has been accepted that OH radicals are difficultly produced in the Bi-based photocatalysts, since the reduction potential of BiV/BiIII(+1.59 eV) is more negative than the standard redox potentials of both OH/H2O(+2.27 eV) and OH/OH(+1.99 eV) [33,34]. Therefore, the role of OH radicals can be neglected for the photocatalytic degradation of RhB over BiOBr/Bi24O31Br10. Secondly, although the calculated CB potentials of both BiOBr and Bi24O31Br10 are less negative than the standard redox potential of O2/O 2 (0.284 eV), the big uplift of the CB of Bi24O31Br10 resulting from the orbital hybridization of Br4s and Bi6p makes the CB potential more negative than E(O2/O 2 )(0.284 eV). Therefore, the photogenerated electrons on the new CB of Bi24O31Br10 with the potential higher than 0.284 eV can react with O2 to form O 2 radicals. Meanwhile, some of them can also easily migrate to the CB of BiOBr. However, it is worthy noting that the electrons on the new CB of Bi24O31Br10 with the potential lower than 0.284 eV can not
Fig. 10. (a) Cycling runs in photocatalytic degradation of RhB over S1.5 sample and (b) XRD pattern for S1.5 sample before and after photocatalytic reaction.
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react with O2, and most of them tend to be directly transferred to the CB of BiOBr, whose migration only play a role in restraining the undesired recombination of photogenerated holes and electrons. Finally, the photogenerated holes possess strong oxidizing ability. It has been reported that the photogenerated holes can directly participate in the photodegradation of RhB [35]. Based on the above discussion, it can be concluded that photogenerated holes and O 2 radicals are the main active species responsible for the photocatalytic degradation of RhB over the BiOBr/Bi24O31Br10 heterostructure. 3.5. Reusability and stability To investigate the reusability and stability of BiOBr/Bi24O31Br10 heterostructure, the recycled experiment was carried out on S1.5 sample, whose result is shown in Fig. 10a. For each cycle, the used catalyst is collected by centrifugation and reused in the photocatalytic reaction under the same conditions. As shown in Fig. 10a, there is no obvious loss of photocatalytic activity, and the photocatalytic efficiency of BiOBr/Bi24O31Br10 heterostructure is still as high as 96.7% at the forth run, suggesting good reusability. In addition, XRD patterns of S1.5 sample before and after photocatalytic reaction, as shown in Fig. 10b, reveal that there is no detectable difference between the as-prepared and recollected S1.5 sample, indicating good photocorrosion resistance and excellent stability. 4. Conclusion BiOBr/Bi24O31Br10 heterostructures have been successfully synthesized by a facile solvothermal method. The amount of NaOH solution used has great effect on the morphologies of as-prepared samples and the relative mass fraction of Bi24O31Br10. BiOBr/Bi24O31Br10 heterostructures exhibit enhanced photocatalytic activity for RhB degradation under visible light irradiation compared with bare BiOBr or Bi24O31Br10. Among them, the heterostructure prepared with 1.5 ml of NaOH solution displays the highest degradation efficiency, whose reaction rate constant is as about 2.6 and 4.8 times as that of BiOBr and Bi24O31Br10, respectively. Based on the UV–vis DRS, BET and PL results, the enhanced photocatalytic activity for BiOBr/Bi24O31Br10 heterostructures is attributed to strong visible light response, high BET specific surface area and efficient separation of photogenerated electrons and holes. In addition, the transfer process of the photogenerated charges carrier between BiOBr and Bi24O31Br10 is proposed, and photogenerated holes and O 2 radicals are believed to be the main active species responsible for the photocatalytic degradation. Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (2013XK07). We would like to thank
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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis
One-pot synthesis of Bismuth Oxyhalide/Oxygen-rich bismuth oxyhalide Heterojunction and its photocatalytic activity ZhangSheng Liu ⇑, HuaShen Ran, JiNan Niu, PeiZhong Feng, YaBo Zhu School of Material Science and Engineering, China University of Mining and Technology, XuZhou 221116, China
a r t i c l e
i n f o
Article history: Received 29 March 2014 Accepted 8 June 2014 Available online 18 June 2014 Keywords: Photocatalytic activity BiOBr/Bi24O31Br10 Heterojunction Solvothermal
a b s t r a c t BiOBr/Bi24O31Br10 heterojunction photocatalysts were prepared by a facile solvothermal method for the first time. The X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FT-IR), N2 sorption, UV–vis diffuse reflectance spectroscopy (UV–vis DRS) and photoluminescence (PL) were applied to investigate the structures, morphologies, surface areas and photocatalytic properties of as-prepared samples. The photocatalytic activity of the samples was evaluated by the photocatalytic degradation of Rhodamine B under the visible-light irradiation. The results showed that BiOBr/Bi24O31Br10 heterojunctions with the different Bi24O31Br10 contents could be obtained by simply adjusting the amount of NaOH solution, all of which exhibited enhanced photocatalytic activity compared with bare BiOBr or Bi24O31Br10. Among them, the BiOBr/Bi24O31Br10 heterojunction prepared with 1.5 ml of NaOH solution possessed the highest photocatalytic activity. The photogenerated holes and O 2 radicals were confirmed to be the main active species responsible for the photodegradation of RhB. The mechanism of enhanced photocatalytic activity was discussed and the transfer process of the photogenerated charges carrier between BiOBr and Bi24O31Br10 was proposed on the basis of the estimated energy band positions. Ó 2014 Elsevier Inc. All rights reserved.
1. Introduction Semiconductor photocatalysis, as a kind of ‘‘green’’ technology, has become one of the most pivotal topics in contemporary photocatalysis research [1,2]. TiO2 is a mostly studied semiconductor photocatalyst owing to its cheapness, strong oxidizing power and nontoxic nature, whose effectiveness for degrading pollutants has been well-documented [3]. However, TiO2 can only be activated by UV irradiation (k < 390 nm) due to the relatively wide band gap (3.2 eV), which comprises less than 5% of the solar spectrum [4]. In order to use the solar energy more effectively, great efforts have been made to develop visible-light-induced photocatalysts. Recently, BiOBr has drawn considerable attention for its potential application in environmental remediation due to excellent photocatalytic activity and high stability [5,6]. BiOBr has an indirect band gap and crystallizes in a tetragonal layered structure, which is characterized by [Bi2O2] slabs interleaved by double slabs of bromine atoms. The indirect band gap requires the photogenerated electrons to travel a certain k-space distance from the conduction band (CB) to the valence band (VB) before their recombination,
⇑ Corresponding author. Fax: +86 51683591870. E-mail address: [email protected] (Z. Liu). http://dx.doi.org/10.1016/j.jcis.2014.06.020 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.
while the intrinsic layered structure creates a strong internal electric field between the Br negative layer and [Bi2O2] positive layer, both of which reduce the recombination probability of the photo-generated electrons and holes, favoring the enhancement of the photocatalytic activity [7–9]. However, the photocatalytic activity of BiOBr is still far from efficient for the practical applications, and it needs some further improvements. A variety of strategies, such as impurity doping [10] and engineering BiOX (X = Cl, Br, I) nanostructures [11], have been developed to improve its photocatalytic performance. In recent years, construction of semiconductor heterostructures has been recognized as a promising method to develop highly efficient photocatalysts. Up to now, many BiOBr-based heterostructures such as Graphene/BiOBr [12], g-C3N4/BiOBr [13], Ag/BiOBr [14], Bi2WO6/BiOBr [15] and TiO2/ BiOBr [16] have been developed. The results indicate that the photocatalytic activity of BiOBr is indeed improved by coupling other semiconductors to some extent. Nevertheless, BiOBr-based heterostructures with high stability and excellent photocatalytic performance are still rare. Very recently, Bi24O31Br10, as a Br-poor bismuth oxybromide, has been known to researcher, and it has been proven to possess excellent photocatalytic activity under the visible light irradiation [17,18]. However, to the best of our knowledge, the heterostructure photocatalyst containing the same elements, BiOBr/Bi24O31Br10, has not been reported.
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Herein, the novel BiOBr/Bi24O31Br10 heterostructures were prepared by a facile one-pot solvothermal method for the first time. The structural and optical properties of the resultant products were characterized by XRD, SEM, TEM, BET, FT-IR, UV–Vis DRS and PL. The photocatalytic activity of as-prepared samples was evaluated by the photocatalytic degradation of RhB under the visible light irradiation. The results show that BiOBr/Bi24O31Br10 heterostructures possess higher photocatalytic performance compared with individual BiOBr or Bi24O31Br10, whose photocatalytic mechanism is investigated and discussed in detail.
2. Experimental 2.1. Preparation of photocatalysts All the reagents were provided by Sinopharm Chemical Reagent Co., Ltd (China), and they were used as received without further purification. BiOBr/Bi24O31Br10 heterostructures were synthesized via a facile one-pot solvothermal method. In a typical process, 1 mmol of Bi(NO3)35H2O and 1 mmol of cetyltriethylammonium bromide (CTAB) were orderly dissolved in 40 ml of ethylene glycol (EG) under magnetic stirring, and then certain volume of 2 M NaOH solution was added dropwise. The resultant precursor solution was transferred into a 50 ml Teflon-lined stainless steel autoclave after 30 min of stirring. The autoclave was sealed and maintained at 160 °C for 16 h and allowed to cool down to room temperature naturally. The precipitate was washed with absolute ethanol and distilled water for three times, respectively, and then dried at 80 °C in air. The volumes of the NaOH solution used were 0, 1, 1.5, 2 and 3 ml, and the as-prepared samples were denoted as S0, S1, S1.5, S2 and S3 accordingly.
2.2. Characterization Crystal structures of as-prepared samples were identified by an X-ray diffractometer (XRD, Bruker D8 Advance with Cu Ka1 radiation at 40 kV and 30 mA). The morphology and structure were performed by field emission scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, Philips, Tecnai 12). The Brunauer–Emmett–Teller (BET) surface area was determined by N2 adsorption–desorption isotherm measurement at 77 K on a Quantachrome NOVA-4200E system. FT-IR spectra were recorded by using Vertex70 FTIR spectrometer. The optical property was analyzed by both UV–vis diffuse reflectance spectra (DRS, Varian Cary 300) and photoluminescence spectra (PL, Varian Cary-Eclipse 500).
3. Results and discussion 3.1. Characterization 3.1.1. XRD analysis The crystal structure of as-prepared samples was investigated by means of X-ray powder diffraction (XRD), as shown in Fig. 1. It can be seen that, in the absence of NaOH, the diffraction peaks of the control sample(S0) are in good agreement with the tetragonal phase of BiOBr (JCPDS card No. 09-0393), indicating the formation of pure BiOBr. When 3 ml of NaOH solution was used, the diffraction peaks of as-prepared S3 sample are readily indexed as a monoclinic phase Bi24O31Br10 (JCPDS card No. 75-0888). It seems that the alkaline condition is favorable for the formation of Bi24O31Br10. When 1–2 ml of NaOH solution was used, the diffraction peaks of as-prepared samples undergo a gradual change. With 1 ml of NaOH solution, S1 sample displays weak diffraction peaks of Bi24O31Br10, suggesting the initial formation of Bi24O31Br10 phase. With the increase of NaOH solution, the diffraction peaks of Bi24O31Br10 gradually increase in intensity, while the diffraction peaks of BiOBr correspondingly decrease in intensity, some of which even completely disappear. Therefore, it is reasonable to believe that Bi24O31Br10 phase is formed at the cost of BiOBr phase, which is in accordance with the previous report [17]. The corresponding reaction is based on Eq. (1):
24BiOBr þ 14OH ! Bi24 O31 Br10 þ 14Br þ 7H2 O
The relative mass fraction of Bi24O31Br10 in these three samples can be estimated according to Eq. (2) [19]:
W1 ¼
I1 I1 þ I2 ðRIR1 Þ=RIR2
ð2Þ
where W1 is the mass fraction of Bi24O31Br10 phase, I1 and I2 are the strongest integral intensities of Bi24O31Br10 and BiOBr phase, respectively. RIR1 and RIR2 value are obtained from MDI Jade 5.0. By calculation, the estimated contents of Bi24O31Br10 are 21.4%, 52.6% and 85.4% for S1, S1.5 and S2 sample, respectively. 3.1.2. SEM and TEM The morphologies of as-prepared samples were characterized by SEM, TEM and HRTEM. As shown in Fig. 2a, one can find that S0 sample consists of hierarchical microspheres with diameters about 3 lm, which agrees with the previous report [20]. These BiOBr microspheres are further assembled by radially grown nanosheets, which interweave with each other to create a large amount of nanopores in the surface of microspheres. After different amounts
2.3. Photocatalytic test The photocatalytic activity of the samples was determined by the degradation of Rhodamine B(RhB) under the visible light irradiation. The visible light source was a 150 W tungsten-halogen lamp (Beijing Institute of Opto-Electronic Technology, light intensity = 200 mW/cm2), whose short-wavelength components (k < 400 nm) of the light were cut off using a cutoff glass filter. Experiments were carried out at 20 ± 3 °C as follows: 0.1 g of photocatalyst was added into 100 ml of 15 mg/l RhB solution. Before irradiation, the suspension was stirred for 20 min in the dark in order to reach the adsorption–desorption equilibrium. At given irradiation time intervals, about 3 ml suspensions was taken and centrifuged to remove the catalyst particles. The concentration of remnant RhB was determined by the UV–vis spectroscopy at its characteristic wavelength of 554 nm.
ð1Þ
Fig. 1. XRD patterns of as-synthesized samples.
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Fig. 2. SEM images of as-prepared samples (a) S0, (b) S1, (c) S1.5, (d) S2, (e) S3 and (f) TEM and HRTEM image (inset) of S1.5.
of NaOH solution were introduced, the morphologies of as-prepared samples undergo a gradual change. When 1 ml of NaOH solution was used, although S1 is also composed of hierarchical microspheres with the similar size, these microspheres seem to be loosely assembled by nanosheets (Fig. 2b). Besides, the nanosheet building blocks are rugged in the edges, and some nanoparticles can be found on them. When the NaOH solution used was 1.5 ml, it is found that as-prepared S1.5 sample is composed of two different structures (Fig. 2c): degraded microspheres as the main component and delaminated individual nanosheets as a secondary component. The size of the degraded microsphere is about 1–2 lm, while the nanosheet is about 250 nm in diameter. With further increasing the amount of NaOH solution to 2 ml, as-prepared S2 sample displays an irregular hierarchical structure, in which nanosheets are randomly stacked together. It can be ascribed to the existence of a large amount of overlapping Bi24O31Br10 nanoclusters, which adhere to the nanosheet edges of BiOBr (inset of Fig. 2d), hindering the orderly assembling. Once the NaOH solution used was up to 3 ml, as-prepared S3 sample shows a special 3D hierarchical structure, which is composed of numerous nanoflakes. These nanoflakes are curved and significantly smaller than BiOBr nanosheets. The Bi24O31Br10/BiOBr heterojunctions were further characterized by TEM and S1.5 sample was taken as an example, as shown in Fig. 2f. It can be seen that the nanosheet building block is about 5–20 nm in thickness and displays a layered structure, which is further assembled by ultrathin nanosheets. To investigate the structure information, the HRTEM image of the nanosheet building block is shown in the inset of Fig. 2f. It is seen that there are two
sets of lattice fringes with spacings of 0.282 nm and 0.276 nm, corresponding to the (1 1 7) plane of the monoclinic Bi24O31Br10 and (1 1 0) plane of the tetragonal BiOBr, respectively. The result indicates that Bi24O31Br10 and BiOBr phase have been completely coupled together. 3.1.3. FT-IR analysis The FTIR spectra of as-prepared samples are shown in Fig. 3. The absorption band at 517 cm1 is ascribed to the Bi–O stretching mode, which is often believed to be the characteristic absorption peak of BiOBr [12,21]. Such a characteristic peak of BiOBr can also be found in S1.5 sample, indicating the existence of BiOBr in S1.5
Fig. 3. FT-IR transmittance spectra of the as-prepared samples.
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sample. Besides, it is worthy noting that there have been no reports involving the characteristic absorption peaks of Bi24O31Br10, and they are not found in our measurement. Nevertheless, considered that the peak shape and peak position of S1.5 sample are very similar to those of S3 sample within the wavenumber range of 600–1600 cm1, it may suggest that there is Bi24O31Br10 phase in S1.5 sample. Therefore, it is believed that S1.5 is the mixed phases of BiOBr and Bi24O31Br10, which is consistent with the XRD results. 3.1.4. N2 Adsorption–desorption isotherms The specific surface area and porosity of as-prepared samples were investigated by using nitrogen adsorption and desorption isotherms. The specific surface areas of S0, S1, S1.5, S2 and S3 samples were found to be 21.6, 27.6, 32.1, 28.4 and 29.8 m2/g, respectively. Obviously, BiOBr/Bi24O31Br10 heterojunctions possess higher specific surface area than BiOBr. To understand the mesoporous structures of as-prepared samples, the typical adsorption and desorption isotherms are shown in Fig. 4. It can be seen that both S0 and S1.5 sample show a type IV isotherm with a distinct hysteresis loops at relative pressures (P/P0) between 0.2 and 1.0, which is usually associated with capillary condensation in mesopores [22]. The high adsorption at P/P0 approaching 1.0 for S1.5 sample indicates the coexistence of mesopores (2–50 nm) and macropores (>50 nm) [23]. The detailed pore size distributions can be obtained by the BJH analysis, as shown in the inset in Fig. 4. Both S0 and S1.5 sample display bimodal pore size distributions. Pore diameters of S0 are 2.7 and 10.0 nm, while those of S1.5 are 3.0 and 17 nm, respectively, all of which are characteristic of mesopores. The occurrence of bimodal pore-size distributions can be attributed to their hierarchical structures [24]. 3.2. Optical property 3.2.1. DRS patterns The band gap structure of a semiconductor plays an important role in determining the photocatalytic activity. Fig. 5 shows the UV–vis DRS of as-prepared samples. It can be seen that all the samples display the photoabsorption from the UV light region to visible light until 440–500 nm, Compared with S0 sample, S1, S1.5 and S2 sample exhibit a systematical red shift and an obvious increased light absorption ability in the visible light region. This can also be concluded from a simple visual inspection of the samples, since their colors gradually turn from white to light yellow. The enhanced photo-absorption can be attributed to two factors. One
Fig. 4. N2 adsorption–desorption isotherms and pore size distribution of the asprepared samples.
Fig. 5. UV–vis DRS spectra of as-prepared samples and their band gap energies (inset).
is that Bi24O31Br10 phase in BiOBr/Bi24O31Br10 heterojunction possesses narrower band gap energy (Eg) than BiOBr [17]. The other can be related with the heterojunction effect, which results in a possible charge-transfer transition on the interface between BiOBr and Bi24O31Br10. The band gap energies of as-prepared samples can be estimated by extrapolating the straight portion of (ahm)1/2 against hm plot to a = 0 based on Eq. (3) [25], as shown in the inset of Fig. 5. 1=2
ðahmÞ
¼ Aðhm Eg Þ
ð3Þ
where a, h, m, Eg, and A are absorption coefficient, Planck constant, light frequency, band gap, and a constant, respectively. The band gap energies are established as about 2.64 eV, 2.59 eV, 2.51 eV, 2.39 eV and 2.23 eV for S0, S1, S1.5, S2 and S3 sample, respectively. Among them, the Eg values of Bi24O31Br10 and BiOBr are very close to those reported in the literature [17,26]. 3.2.2. PL patterns In order to investigate the recombination of photoinduced electrons and holes, the PL spectra of as-prepared samples were measured with an excitation wavelength of 250 nm, as shown in Fig. 6. It can be seen that all the BiOBr/Bi24O31Br10 heterojunctions display lower emission band intensities of the spectra than bare BiOBr and Bi24O31Br10 photocatalyst, suggesting that the recombination of photogenerated charge carriers is greatly restrained by the coupling of BiOBr and Bi24O31Br10. In addition, the emission band
Fig. 6. PL spectra of as-prepared samples.
Z. Liu et al. / Journal of Colloid and Interface Science 431 (2014) 187–193
intensities of the spectra are different with the content of Bi24O31Br10. S1.5 sample with 52.6% of Bi24O31Br10 displays the lowest PL intensity. Therefore, it can be inferred that the sample prepared with 1.5 ml of NaOH solution will possess the highest photocatalytic activity.
3.3. Photocatalytic performance The photocatalytic performances of as-prepared samples were evaluated by the photocatalytic removal of RhB as a function of irradiation time under visible light irradiation. Fig. 7 shows the temporal UV–vis absorption spectra variation of RhB over the S1.5 sample. It is found that the characteristic absorption band of RhB at 554 nm diminishes quickly under the visible light irradiation, accompanied by a slight blue shift of the maximum absorption from 554 to 496 nm. The color of RhB solution also changes correspondingly from red to orange red, light green and finally colorless, as shown in the inset of Fig. 7. These changes indicate that the degradation of RhB over the S1.5 sample undergoes two processes, i.e. the de-ethylation process firstly and then the destruction of a conjugated structure under sunlight irradiation, which have been reported in the previous BiOBr-based system [13,15]. After 2 h of the visible light irradiation, the characteristic absorption band of RhB disappears and no new absorption bands appear, suggesting that RhB has been completely mineralized by using BiOBr/Bi24O31Br10 heterojunction. The photocatalytic performances of as-prepared samples were evaluated by the degradation of RhB under visible light irradiation. The results are shown in Fig. 8a. It can be seen that no evident photolysis of RhB is detected in the absence of catalyst, suggesting the stability of RhB under the visible light. After 2 h of visible light irradiation, RhB with different photocatalysts is degraded in varying degrees. The degradation rates are 89.6%, 98.1%, 99.7%, 95.8%, and 76.6 for S0, S1, S1.5, S2 and S3 sample, respectively. All the BiOBr/Bi24O31Br10 heterojunctions display higher photocatalytic activity than individual BiOBr and Bi24O31Br10. To quantify the photocatalytic ability of these samples and understand the reaction kinetics of RhB degradation, the apparent pseudo-first-order model is applied [27], as shown in Fig. 6b. ln(C/C0) exhibits a well linear relationship with the irradiation time, indicating that the photocatalytic reaction belongs to the pseudo-first-order reaction. Their reaction rate constants are also concluded in Fig. 8b. Clearly, the reaction rate constants of all the BiOBr/Bi24O31Br10 heterojunctions are higher than bare BiOBr and Bi24O31Br10. Among them, the reaction rate constant of S1.5 sample is as about 2.6 and 4.8 times as
that of BiOBr and Bi24O31Br10, respectively. The enhanced photocatalytic activity can be attributed to the heterojunction effect resulting from the coupling of BiOBr and Bi24O31Br10, which can effectively restrain the recombination of photogenerated electrons and holes. The Bi24O31Br10 content seems to be a key point for producing BiOBr/Bi24O31Br10 heterojunction with high activity. It is believed to be related with the amount of BiOBr/Bi24O31Br10 heterojunction. Generally speaking, the higher the Bi24O31Br10 content, the more BiOBr/Bi24O31Br10 heterojunction we can obtain. However, excess Bi24O31Br10 tends to form overlapping nanoclusters, which only physically adhere to the edges of BiOBr nanosheets (Fig. 2d). These nanoclusters not only are disadvantageous to the formation of BiOBr/Bi24O31Br10 heterojunction but also may become recombination centers of photoinduced charge carriers, reducing their separation efficiency. Therefore, there is an optimum Bi24O31Br10 content for BiOBr/Bi24O31Br10 heterojunction with high photocatalytic activity. In this study, the suitable content of Bi24O31Br10 is obtained by S1.5 sample, which is about 52.6%. 3.4. Photocatalytic mechanism 3.4.1. Mechanism of the enhanced photocatalytic activity According to the above results, BiOBr/Bi24O31Br10 heterojunctions display higher activity than individual BiOBr and Bi24O31Br10. The enhanced photocatalytic activity can be explained as follows. Firstly, the photocatalytic reaction rate is often proportional to the number of photons absorbed by photocatalyst. Based on Fig. 5, BiOBr/Bi24O31Br10 heterojunctions expand the visible light response range to some extent and improve the light absorpting ability, which has to provide more available photons, favoring the enhancement of the photocatalytic activity. Secondly, the catalytic process is often related to the adsorption and desorption of pollution molecules on the surface of the catalyst. The high surface areas can lead to strong adsorption towards the target molecules and offer more active sites [27,28]. Based on the above morphology observation and BET surface area measurement, BiOBr/Bi24O31Br10 heterojunctions tends to form loose 3D hierarchical structures and possesses higher specific surface areas, which has to be beneficial to the photocatalytic reaction. Finally, the efficient separation of photoinduced electron–hole pairs was believed to be a key factor for the photocatalytic decomposition of organic pollutants [29]. The flowchart of photogenerated electrons and holes in the heterojunction depends on the band edge positions of semiconductors. The valence band potentials of the semiconductors at the point of zero charge can be calculated by Mulliken electronegativity theory [30].
EVB ¼ X E0 þ 0:5Eg
Fig. 7. The temporal evolution of the spectra during the photodegradation of RhB over the S1.5 sample.
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ð4Þ
where EVB is the VB edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, E0 is the energy of free electrons on the hydrogen scale (about 4.5 eV), Eg is the band gap energy of the semiconductor. The conduction band (CB) position can be obtained by ECB = EVB Eg. Based on Eq. (4), the valence band tops of BiOBr and Bi24O31Br10 are calculated as 3.14 and 2.93 eV, and correspondingly the conduction band bottoms of BiOBr and Bi24O31Br10 are 0.50 and 0.56 eV, respectively. Once the BiOBr/Bi24O31Br10 heterostructure is exposed to the visible light irradiation, both BiOBr and Bi24O31Br10 can be activated due to their narrow band gap energies. The electrons on the VB of BiOBr are excited to a potential edge (0.50 eV). However, the electrons on the VB of Bi24O31Br10 can be excited up to much higher potential edge than 0.56 eV, since there is a big uplift of the conduction band for Bi24O31Br10 due to the orbital hybridization of Br4s and Bi6p [31]. As a result, the photogenerated electrons on the CB of Bi24O31Br10 can migrate to the CB
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Fig. 8. (a) Photocatalytic degradation of RhB over as-prepared samples under visible light irradiation and (b) degradation kinetics of RhB.
Fig. 9. Photocatalytic degradation of RhB over S1.5 sample in the presence of various radicals scavengers.
of BiOBr. On the other hand, the photogenerated holes on the VB of BiOBr tend to migrate to that of Bi24O31Br10, since the VB potential of BiOBr (3.14 eV) is more positive than that of Bi24O31Br10 (2.93 eV). The transference of the photogenerated charge carriers results in their prolonged lifetimes due to the extended transfer path, hindering the undesired recombination of electron–hole pairs, which is in accordance with the PL result (Fig. 6). In view of the above discussion, the enhanced photocatalytic activity for BiOBr/ Bi24O31Br10 heterostructure can be attributed to strong visible light response, high BET specific surface area and efficient separation of photogenerated electrons and holes.
3.4.2. Main active species In order to better understand the photocatalytic mechanism, it is necessary to detect main active oxidative species in the photocatalytic reaction. To do so, a series of experiments were carried out by dissolving different trapping agents in the reaction solution before light irradiations. As shown in Fig. 9, there are no obvious changes in the degradation rate of RhB with the addition of isopropanol (IPA), which is a well-known scavenger of OH radicals [32]. However, the photocatalytic degradation efficiencies of RhB are significantly suppressed by the addition of benzoquinone (BQ) + and triethanolamine (TEOA), which can trap O 2 and holes (h ), respectively [8]. The results indicate that the photocatalytic degradation of RhB over BiOBr/Bi24O31Br10 can be mainly attributed to the photoinduced holes (h+) and O 2 radicals, instead of OH radicals. In fact, the results can also make sense from the theoretical viewpoint. Firstly, it has been accepted that OH radicals are difficultly produced in the Bi-based photocatalysts, since the reduction potential of BiV/BiIII(+1.59 eV) is more negative than the standard redox potentials of both OH/H2O(+2.27 eV) and OH/OH(+1.99 eV) [33,34]. Therefore, the role of OH radicals can be neglected for the photocatalytic degradation of RhB over BiOBr/Bi24O31Br10. Secondly, although the calculated CB potentials of both BiOBr and Bi24O31Br10 are less negative than the standard redox potential of O2/O 2 (0.284 eV), the big uplift of the CB of Bi24O31Br10 resulting from the orbital hybridization of Br4s and Bi6p makes the CB potential more negative than E(O2/O 2 )(0.284 eV). Therefore, the photogenerated electrons on the new CB of Bi24O31Br10 with the potential higher than 0.284 eV can react with O2 to form O 2 radicals. Meanwhile, some of them can also easily migrate to the CB of BiOBr. However, it is worthy noting that the electrons on the new CB of Bi24O31Br10 with the potential lower than 0.284 eV can not
Fig. 10. (a) Cycling runs in photocatalytic degradation of RhB over S1.5 sample and (b) XRD pattern for S1.5 sample before and after photocatalytic reaction.
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react with O2, and most of them tend to be directly transferred to the CB of BiOBr, whose migration only play a role in restraining the undesired recombination of photogenerated holes and electrons. Finally, the photogenerated holes possess strong oxidizing ability. It has been reported that the photogenerated holes can directly participate in the photodegradation of RhB [35]. Based on the above discussion, it can be concluded that photogenerated holes and O 2 radicals are the main active species responsible for the photocatalytic degradation of RhB over the BiOBr/Bi24O31Br10 heterostructure. 3.5. Reusability and stability To investigate the reusability and stability of BiOBr/Bi24O31Br10 heterostructure, the recycled experiment was carried out on S1.5 sample, whose result is shown in Fig. 10a. For each cycle, the used catalyst is collected by centrifugation and reused in the photocatalytic reaction under the same conditions. As shown in Fig. 10a, there is no obvious loss of photocatalytic activity, and the photocatalytic efficiency of BiOBr/Bi24O31Br10 heterostructure is still as high as 96.7% at the forth run, suggesting good reusability. In addition, XRD patterns of S1.5 sample before and after photocatalytic reaction, as shown in Fig. 10b, reveal that there is no detectable difference between the as-prepared and recollected S1.5 sample, indicating good photocorrosion resistance and excellent stability. 4. Conclusion BiOBr/Bi24O31Br10 heterostructures have been successfully synthesized by a facile solvothermal method. The amount of NaOH solution used has great effect on the morphologies of as-prepared samples and the relative mass fraction of Bi24O31Br10. BiOBr/Bi24O31Br10 heterostructures exhibit enhanced photocatalytic activity for RhB degradation under visible light irradiation compared with bare BiOBr or Bi24O31Br10. Among them, the heterostructure prepared with 1.5 ml of NaOH solution displays the highest degradation efficiency, whose reaction rate constant is as about 2.6 and 4.8 times as that of BiOBr and Bi24O31Br10, respectively. Based on the UV–vis DRS, BET and PL results, the enhanced photocatalytic activity for BiOBr/Bi24O31Br10 heterostructures is attributed to strong visible light response, high BET specific surface area and efficient separation of photogenerated electrons and holes. In addition, the transfer process of the photogenerated charges carrier between BiOBr and Bi24O31Br10 is proposed, and photogenerated holes and O 2 radicals are believed to be the main active species responsible for the photocatalytic degradation. Acknowledgments This work was supported by the Fundamental Research Funds for the Central Universities (2013XK07). We would like to thank
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