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Visible light photocatalytic performance of hierarchical BiOBr microspheres synthesized via a reactable ionic liquid
Materials Science in Semiconductor Processing 23 (2014) 151–158
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Visible light photocatalytic performance of hierarchical BiOBr microspheres synthesized via a reactable ionic liquid Bo Chai n, Huan Zhou, Fen Zhang, Xiang Liao, Meixia Ren School of Chemical and Environmental Engineering, Wuhan Polytechnic University, Wuhan 430023, PR China
a r t i c l e i n f o
abstract
Available online 16 March 2014
Hierarchical BiOBr microspheres were synthesized via a one-pot solvothermal process in the presence of ethylene glycol and 1-butyl-3-methylimidazolium bromide ([BMIM]Br) as a reactable ionic liquid. The products were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-Vis diffuse reflectance absorption spectra, nitrogen adsorption–desorption measurements, and photoluminescence spectroscopy. The photocatalytic activity of BiOBr microspheres was evaluated in terms of the degradation of Rhodamine B (RhB), methyl orange (MO), and 4-chlorophenol (4-CP) under visible light irradiation. We found that the solvothermal temperature had important effects on the crystallinity, crystallite size, optical property, adsorptive performance, and photocatalytic activity of BiOBr microspheres. BiOBr microspheres with a specific surface area of 15.7 m2 g 1 prepared at 160 1C exhibited the best adsorption and photocatalytic performance for RhB degradation in aqueous solution. However, this sample showed hardly any activity for photodegradation of 4-CP. Tests using radical scavengers confirmed that h þ and dO2 were the main reactive species during RhB degradation. A possible mechanism for photocatalysis by BiOBr microspheres is proposed. & 2014 Elsevier Ltd. All rights reserved.
Keywords: BiOBr microspheres Ionic liquid Solvothermal Photocatalytic degradation
1. Introduction Semiconductor photocatalysis is a promising advanced oxidation technology for environmental remediation. Among semiconductor photocatalysts, TiO2 has been extensively studied because of its high photosensitivity and nontoxicity [1,2]. However, TiO2 can only be activated by UV light (λ o 400 nm), which represents only 4% of the solar energy available. Visible light accounts for 43% of the solar spectrum, so the development of efficient photocatalysts with a visible light response is the focus of much photocatalytic research [3,4]. Bismuth oxyhalides (BiOX, X¼Cl, Br, I) have been investigated owing to their excellent photocatalytic
n
Corresponding author. Tel./Fax: þ86 27 83943956. E-mail address: [email protected] (B. Chai).
http://dx.doi.org/10.1016/j.mssp.2014.02.021 1369-8001 & 2014 Elsevier Ltd. All rights reserved.
performance for the degradation of organic compounds [5–7]. In particular, BiOBr has attracted much attention because it is responsive to visible light and is a relatively stable photocatalyst. Several methods have been reported for the preparation of BiOBr micro/nanostructures for photocatalytic degradation of organic contaminants. Li et al. synthesized uniform, well-defined, 3D flowerlike BiOBr nanostructures using microwave irradiation with cetyltrimethylammonium bromide (CTAB) as the bromine source and soft template for Cr(VI) removal [8]. Henle et al. prepared nanosized bismuth oxyhalide (BiOX, X¼Cl, Br, I) particles via a reverse micro-emulsion [9]. Shang et al. prepared BiOBr lamellar structures with CTAB as a Br source and template using a hydrothermal method [10]. Their BiOBr lamellar structures showed high photocatalytic activity and stability for MO degradation under visible light. Feng et al. synthesized mesoporous 3D BiOBr microspheres using a facile solvothermal method with absolute ethanol as
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the solvent and studied their activity for photodecomposition of toluene [11]. Jiang et al. fabricated flake-like BiOBr using a HAc-assisted hydrothermal route for photocatalytic degradation of MO [12]. Two different research groups prepared BiOBr microspheres using ethylene glycol (EG) and NaBr as the solvent and Br source, respectively [13,14]. Ionic liquids (ILs), which are typically composed of organic cations and large anions, have attracted much interest as functional materials for applications in catalysis, electrochemistry, and separation. In particular, ILs have received considerable attention as templates in the synthesis of functional nanomaterials with unusual morphology [15–19]. For instance, Xia and co-workers synthesized hollow BiOI microspheres in the presence of 1-butyl-3methylimidazolium iodine ([BMIM]I) as an IL [15]. The same group prepared flower-like hollow BiOBr microspheres in a one-pot EG-assisted solvothermal process in the presence of the reactable IL 1-hexadecyl-3methylimidazolium bromide [16]. Nevertheless, the full potential of ILs as reagents in controllable synthesis of bismuth oxyhalide nanostructures remains to be fully explored. In this study we developed a facile EG-assisted solvothermal method for synthesis of BiOBr microspheres using [BMIM]Br as a reactable IL. The effects of the solvothermal temperature (TST) on the crystal structure, morphology, optical properties, adsorption performance, and photocatalytic activity for degradation of Rhodamine B (RhB), methyl orange (MO), and 4-chlorophenol (4-CP) are discussed in detail. 2. Experimental 2.1. Sample preparation [BMIM]Br was obtained from Lanzhou Greenchem ILs (China). Other chemicals were analytical-grade reagents purchased from Sinopharm Chemical Reagent Co. (China) and were used without further purification. In a typical procedure, 1 mmol of Bi(NO3)3 5H2O was dissolved in 70 mL of EG and stoichiometric amounts of [BMIM]Br were added under constant stirring at room temperature to ensure good dispersion of the reactants. After stirring for 30 min, the mixed solution was transferred to a 100-mL Teflon-lined stainless steel autoclave and kept at different temperature (120, 140, 160, and 180 1C) for 12 h. After natural cooling to room temperature, the products were collected by centrifugation and washed with distilled water and ethanol several times, then dried at 80 1C overnight. For comparison, hierarchical BiOBr microspheres were synthesized as a control sample using NaBr as the Br source as previously described [13,14]. BiOBr microspheres prepared at 120, 140, 160, and 180 1C are denoted in the text as BiOBr120, BiOBr140, BiOBr160, and BiOBr180, respectively. 2.2. Sample characterization The products were characterized by X-ray diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer with Cu Kα irradiation (λ ¼0.154178 nm) at 40 kV and 40 mA. The morphology and structure of as-prepared samples were analyzed by field-emission scanning
electron microscopy (FESEM; JSM-6700F) and transmission electron microscopy (TEM; JEM-2100). X-Ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos XSAM 800 instrument with a Mg Kα source operating at 200 W. UV-Vis diffuse reflectance spectra (DRS) were obtained on a Shimadzu UV-3600 spectrophotometer equipped with an integrating sphere using BaSO4 as the reference sample. Nitrogen adsorption– desorption measurements were conducted on a nitrogen adsorption system at 77 K (Micrometrics, ASAP 2020); samples were degassed at 120 1C for 5 h before measurement. Photoluminescence (PL) spectra were measured at room temperature on a Varian Cary Eclipase fluorescence spectrophotometer with excitation at 315 nm. 2.3. Photocatalytic activity The photocatalytic activity of as-prepared samples was evaluated for degradation of RhB, MO, and 4-CP in aqueous solutions under visible light irradiation. Samples of 50 mg of the photocatalysts were added to 100 mL of RhB, MO, and 4-CP solutions with initial concentrations of 2.5 10 5 mol L 1, 10 mg L 1, and 10 mg L 1, respectively. A 500-W tungsten halogen lamp was positioned inside a cylindrical Pyrex vessel and surrounded by a circulating water jacket (Pyrex) to cool the lamp. A cutoff filter was placed outside the Pyrex jacket to completely remove all radiation of λo420 nm to ensure irradiation was with visible light only. Prior to irradiation, the suspension was magnetically stirred in the dark for 60, 30, and 30 min to reach adsorption–desorption equilibrium for RhB, MO, and 4-CP, respectively. After irradiation was started, 4 mL of the suspension was collected time intervals and centrifuged (12,000 rpm, 15 min) to remove photocatalyst particles. The concentration of RhB, MO, and 4-CP was determined from the absorbance measured at 554, 464, and 224 nm, respectively, TU-1810 spectrophotometer. To further determine the mineralization of RhB, changes in total organic carbon (TOC) were measured using a total organic carbon analyzer (Multi N/C 2100). 3. Results and discussion Fig. 1 shows XRD patterns for BiOBr samples prepared at different temperatures. All the diffraction peaks can be indexed to the tetragonal phase of BiOBr (JCPDS No. 73-2061) with well-resolved (011), (012), (110), (112), (020), and (212) reflections, in agreement with the literature [13,14,16]. No other crystal-phase diffraction peaks were detected, indicating that the samples were pure. It is worth noting that the diffraction peak intensity increased with TST, suggesting that the crystallinity and crystallite size of BiOBr microspheres improved with increasing TST. Representative SEM and TEM images of BiOBr samples prepared at different solvothermal temperatures are shown in Fig. 2. It is evident that the BiOBr crystallites self-organized into hierarchical microspheres (average diameter 1–4 mm) composed of massive interleaving nanosheets. The BiOBr microsphere diameter gradually increased with TST. The BiOBr120 sample diameter ranged from 1 to 2 mm (Fig. 2a). BiOBr140 had a particle size
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Fig. 1. XRD patterns for BiOBr samples prepared at different solvothermal temperatures.
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distribution of 2–3 mm (Fig. 2b). For BiOBr160 and BiOBr180, the microsphere diameter and particle size distribution further increased to 2–4 mm (Fig. 2c,d). A nanosheet thickness of approximately 10–20 nm can be observed for an individual BiOBr microsphere (inset in Fig. 2c). Further investigation was carried out by TEM to reveal the microstructure of the samples. The TEM image of BiOBr microspheres in Fig. 2e reveals a zigzag circular exterior, in accordance with the SEM images. The magnification of the microsphere edge Fig. 2f shows that it is composed of numerous nanosheets. XPS was performed to determine the chemical composition and valence state of various species. The peak positions in all the XPS spectra were calibrated using the C 1s peak at 284.6 eV. Fig. 3a shows the XPS survey spectrum for BiOBr160. As expected, the sample contains
Fig. 2. SEM and TEM images of BiOBr products: (a) 120 1C, (b) 140 1C, (c) 160 1C (an individual BiOBr microsphere, inset), (d) 180 1C, and (e) low- and (f) high-magnification TEM images of BiOBr microspheres obtained at 160 1C.
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Fig. 3. XPS spectra for BiOBr microspheres obtained at 160 1C: (a) XPS survey spectrum, (b) high-resolution Bi 4f spectrum, (c) high-resolution Br 3d spectrum, and (d) high-resolution O 1s spectrum.
Fig. 4. UV-Vis diffuse reflectance absorption spectra for BiOBr samples prepared at different solvothermal temperatures. The inset shows the bandgap value estimated from a plot of (αhν)1/2 versus photon energy.
prepared at 120, 140 and 160 1C show almost the same steep absorption edge at 425 nm. For BiOBr180, the absorption edge is slightly shifted to a longer wavelength, which can be attributed to the increase in grain size, as observed in SEM images [20]. The steep gradient in the visible region is ascribed to the intrinsic bandgap transition between the valence band (VB) and the conduction band (CB), rather than a transition from impurity levels. Notably, the absorption intensity for the BiOBr microspheres increases with TST over the whole visible light range. This is because the surface of BiOBr microspheres prepared at higher TST contains carbon species, as revealed by the gray color of the samples. UV-Vis DRS data for the BiOBr samples were used to determine the absorption coefficient α according to the Kubelka–Munk function [21] α¼
Bi, O, Br, and C as elements. A high-resolution Bi 4f spectrum is shown in Fig. 3b. The peaks at binding energy of 159.3 and 164.6 eV can be attributed to Bi 4f7/2 and Bi 4f5/2, which are characteristic of Bi3 þ in BiOBr materials [16]. In Fig. 3c, the Br 3d peak is associated with binding energy of 68.6 eV, which is characteristic of Br [16]. The O 1s XPS spectrum in Fig. 3d can be deconvoluted into two peaks. The peak at 530.2 eV can be ascribed to oxygen anions in the BiOBr lattice, while the peak centered at 531.6 eV is assigned to chemisorbed oxygen of surface hydroxyl groups. Fig. 4 compares UV-Vis DRS spectra for BiOBr samples prepared at different temperatures. BiOBr microspheres
ð1 RÞ ; 2R
ð1Þ
where R is reflectance (R¼10 A, where A is the optical absorbance). The bandgap energy can be estimated from a plot of (αhν)1/2 versus photon energy hν, where h is Planck’s constant and ν is frequency (inset in Fig. 4). The bandgap decreased from 2.96 to 2.95 to 2.92 to 2.75 eV as TST increased from 120 to 180 1C. Fig. 5 shows nitrogen adsorption–desorption isotherms and the pore size distribution for BiOBr microspheres prepared at different temperatures. The isotherms can be categorized as type IV with a H3 hysteresis loop, which is characteristic of mesoporous materials. The BET specific surface area for BiOBr120, BiOBr140, BiOBr160, and BiOBr180 was calculated as 16.3, 16.0, 15.7, and 14.8 m2 g 1,
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Fig. 5. Nitrogen adsorption–desorption isotherm and pore size distribution (inset) for BiOBr microspheres prepared at different solvothermal temperatures.
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respectively. It is evident that the specific surface area slightly decreased with increasing TST. BiOBr120, BiOBr140, and BiOBr160 samples contained small mesopores with a diameter of 10 nm, as determined by the Barrett– Joyner–Halenda (BJH) method (inset in Fig. 5). These mesopores can be attributed to the space between nanosheets in the hierarchical microspheres, which is consistent with the SEM and TEM observations. The photocatalytic activity of as-prepared samples was evaluated for RhB degradation under visible light irradiation. For comparison, a blank and a control sample were also tested under the same reaction conditions. Fig. 6a–e shows temporal changes in the UV-Vis spectrum of the RhB solution during adsorption and photocatalytic degradation over BiOBr microspheres and the control sample. For BiOBr160, the wavelength corresponding to maximum absorbance (λmax) gradually shifted from 554 nm towards
Fig. 6. Temporal spectral evolution during photocatalytic degradation of RhB over samples prepared at (a) 120 1C, (b) 140 1C, (c) 160 1C, and (d) 180 1C, and (e) a control sample; “origin” denotes the concentration of RhB before absorption equilibrium. (f) Comparison of the photocatalytic activity of the samples for degradation of RhB in solution.
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shorter wavelength and finally reached 498 nm during visible light illumination; this was accompanied by a gradual decrease in maximum absorbance (Fig. 6c). A blue shift of λmax is frequently observed during oxidation of RhB over multimetal oxide photocatalysts, and is associated with stepwise removal of N-ethyl groups during degradation of RhB (N,N,N',N'-tetraethyl rhodamine) [16,20,22]. The characteristic λmax for RhB and de-ethylated rhodamine species are 554 nm (RhB), 539 nm (N,N,N'-triethyl rhodamine), 522 nm (N,N'-diethyl rhodamine), 510 nm (Nethyl rhodamine), and 498 nm (rhodamine). For BiOBr120, λmax was 498 nm after 60 min of visible light irradiation, indicating that rhodamine was present among the deethylated products (Fig. 6a). For the control sample and BiOBr140 and BiOBr180, λmax after irradiation for 60 min was observed at 522 nm, indicating the presence of N,N'diethyl rhodamine (Fig. 6b,d,e). Fig. 6f compares the photocatalytic activity of the samples for degradation of RhB. The results indicate that BiOBr160 exhibited the best adsorption capacity and photocatalytic performance. The ability of BiOBr160 to mineralize RhB was evaluated by monitoring changes in TOC (Fig. 7). The results suggest that the rate of TOC removal is much slower than the rate of the photocatalytic decolorization for RhB. After 60 min of irradiation, the solution TOC decreased by approximately 42%, implying that RhB molecules were degraded but not completely mineralized to inorganic molecules. The effect of solution pH on the photocatalytic degradation of RhB over BiOBr140 was evaluated for an initial pH range of 2.5–7.5 by adjusting the initial pH with 0.1 mol L 1 HNO3 and NaOH solutions. As shown in Fig. S1, the photocatalytic activity increased with decreasing pH. This suggests that an acidic environment is more beneficial for RhB degradation, in agreement with the literature [10]. Photocatalytic degradation of MO and 4-CP under visible light in the presence of different BiOBr samples was also investigated. The results shown in Figs. S2 and S3a–e indicate that the BiOBr catalysts cannot totally degrade MO in 3 h. BiOBr160 showed the best adsorption ability but little degradation activity for MO. For 4-CP, BiOBr samples prepared at different temperatures showed extremely weak adsorption and degradation performance during photocatalysis, while the control sample exhibited
slight degradation activity. Thus, the BiOBr microspheres exhibit selective photocatalytic behavior, as their activity was much higher for RhB than for MO or 4-CP photodegradation under visible light. According to DRS analysis, the lowest bandgap energy was observed for BiOBr180, indicating that this sample can absorb more visible light. However, its photocatalytic activity was weaker than that of the other samples (Fig. 6f). Thus, the variation in optical properties is not a dominant factor affecting the photocatalytic activity of asprepared BiOBr microspheres. It is well known that photocatalytic activity is closely related to the adsorption ability of catalysts [16]. Although the BiOBr microspheres had similar specific surface areas, BiOBr160 showed the best adsorption capacity for the organic dyes. This may be because it has more negative charges on its surface [23]. RhB molecules form positively charged cations in aqueous solution and thus can be adsorbed by BiOBr via electrostatic interaction. In contrast, MO molecules form anions in solution so they cannot be adsorbed by BiOBr. Sufficient dye adsorption has a positive effect on the photocatalysis efficiency. The crystallinity, crystallite size, and surface defects of catalysts also affect their photocatalytic activity. According to the XRD and SEM results, the crystallinity and crystallite size of BiOBr microspheres improved with increasing TST. In general, better photocatalyst crystallinity and fewer surface defects lead to higher photocatalytic activity, whereas an increase in particle size can have a negative effect on photocatalytic performance. We can deduce that BiOBr160 had appropriate crystallinity, crystallite size, and surface defects, which increased the number of photogenerated electrons and holes available for participating in the photocatalytic degradation of contaminants. In part, photocatalytic activity is a function of the lifetime and trapping of photogenerated electrons and holes in the semiconductor. PL emission spectra are often used to study surface structures and excited states. Fig. 8 shows PL spectra over the wavelength range 325–550 nm for BiOBr microspheres prepared at different temperatures. Two emission peaks are evident at approximately 410 and 490 nm. The first (main peak) is ascribed to emission for bandgap transition and the second to surface oxygen vacancies and defects [24]. The highest-intensity
Fig. 7. Changes in TOC in the reaction system during photocatalytic degradation of RhB over BiOBr microspheres prepared at 160 1C.
Fig. 8. PL spectra for BiOBr samples prepared at different solvothermal temperatures.
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Fig. 9. Effects of reactive species on the photocatalytic degradation of RhB over BiOBr microspheres prepared at 160 1C.
main peak was observed for BiOBr120. A moderate decrease in PL intensity with increasing TST is evident. PL emission mainly results from recombination of excited electrons and holes, and a lower PL intensity indicates a decrease in the recombination rate. However, the least intense main PL peak was observed for BiOBr180, which can be attributed to PL quenching by surface carbon species [25]. Interestingly, the lowest intensity for the emission peak ascribed to surface defects was observed for BiOBr160, indicating that these microspheres had the fewest surface defects. Hence, surface recombination of photogenerated electrons and holes is greatly inhibited over BiOBr160, which enhances its photocatalytic activity. The PL results confirm that BiOBr160 can effectively separate photogenerated electron–hole pairs. To clarify the reaction mechanism, isopropanol (IPA), triethanolamine (TEOA), and p-benzoquinone (BQ) were introduced as scavengers of hydroxyl radicals (dOH), holes (h þ ), and superoxide radicals (dO2 ), respectively, to determine the effects of reactive species on photocatalytic degradation of RhB [26]. The concentration of IPA, TEOA, and BQ in the RhB photocatalytic reaction system was 10, 10, and 2 mmol L 1, respectively. Fig. 9 shows that TEOA significantly suppressed the RhB degradation rate; BQ had a weaker inhibitory, and IPA had hardly any effect on RhB degradation. The results confirm that h þ and dO2 play a d more important role than that of OH in photocatalytic degradation of RhB. The CB and VB edge potentials of BiOBr are important factors in understanding the degradation of contaminants over these photocatalysts. According to theoretical speculation, the CB and VB potentials over BiOBr are 0.27 and 3.19 eV, respectively [12]. Thus, the VB edge potential of BiOBr is more positive than the standard redox potential of dOH/OH (1.99 eV), suggesting that photogenerated holes could oxidize OH to dOH. However, production of dOH in the present system is almost impossible because the standard redox potential of Bi(V)/Bi(III) (1.59 eV) is more negative than that of dOH/OH [27]. Thus, degradation of RhB over BiOBr should involve direct reaction with photogenerated holes. Therefore, photocatalytic degradation of RhB over BiOBr is little effect in the presence of the dOH radical scavenger (IPA). In addition, the CB edge potential of BiOBr (0.27 eV) is not negative enough to reduce O2 to dO2 because the single-electron
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Fig. 10. Stability study for photocatalytic degradation of RhB over BiOBr microspheres prepared at 160 1C.
reduction potential of O2 is 0.046 eV [28]. However, this does not exclude the formation of dO2 by photogenerated electrons via other photochemical reactions. We postulate that dye photosensitization leads to the formation of dO2 radicals. RhB molecules adsorbed on BiOBr are regarded as sensitizers and that visible light irradiation stimulates the generation of electrons that subsequently transfer to the CB of BiOBr and then react with O2 to form dO2 radicals. The results for photocatalytic degradation of RhB, MO, and 4-CP confirm the dye photosensitization mechanism for degradation over BiOBr microspheres. For RhB, the BiOBr microspheres show excellent adsorption and photocatalytic activity. In comparison, the microspheres display weaker adsorption capacity for MO, and thus their photocatalytic activity is lower for this dye. For colorless 4-CP, they exhibit hardly any degradation activity. Consequently, the following plausible reaction scheme can be proposed: DyeðadsÞ þhv-DyeðadsÞ n
ð2Þ
DyeðadsÞ n þ BiOBr-BiOBrðe Þ þDyeðadsÞ þ
ð3Þ
BiOBrðe Þ þ O2 -BiOBr þ d O2
ð4Þ
BiOBr þ hv-e þ h þ
þ
Dye þ h ðd O2 Þ-products
ð5Þ ð6Þ
To test the stability and reusability of the BiOBr microspheres for photocatalytic degradation of RhB, the catalysts were reused four times for photocatalysis under the same reaction conditions. The results are shown in Fig. 10. The decolorization efficiency of BiOBr microspheres decreased by only 6% after four cycles. Comparison of XRD patterns reveals a high degree of similarity for the BiOBr catalyst recycled four times and the original catalyst (Fig. 11). There is no obvious deviation in peak location and the slight decrease in peak intensity indicates a slight decline in crystallinity after visible light irradiation. These observations suggest that only slight photocorrosion occurred during the photocatalytic reaction and that BiOBr microspheres have good photostability.
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References
Fig. 11. XRD patterns for BiOBr prepared at 160 1C before and after photocatalytic recycling experiments.
4. Conclusions Hierarchical BiOBr microspheres with an average diameter of 1–4 mm were prepared via a one-pot EG-assisted solvothermal process in the presence of a reactable IL. The experimental results indicate that the solvothermal temperature has important effects on the crystallite size, optical properties, adsorption capacity, and photocatalytic activity of BiOBr microspheres. Among all the samples, BiOBr160 showed the best adsorption capacity and photocatalytic activity for RhB degradation under visible light illumination. This result can be attributed to appropriate crystallinity, crystallite size, and surface defects. Investigation of the photocatalytic mechanism demonstrated that h þ and dO2 species play a key role. Moreover, results for photocatalytic degradation of RhB, MO, and 4-CP confirmed a photosensitization mechanism for degradation over BiOBr microspheres. These BiOBr hierarchical microspheres hold promise as efficient photocatalysts for the degradation of organic dyes and in other applications. Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 51302200). 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. mssp.2014.02.021.
[1] X.B. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891–2959. [2] M.D. Hernández-Alonso, F. Fresno, S. Suárez, J.M. Coronado, Energy Environ. Sci. 2 (2009) 1231–1257. [3] H.J. Zhang, G.H. Chen, D.W. Bahnemann, J. Mater. Chem. 19 (2009) 5089–5121. [4] S. Rehman, R. Ullah, A.M. Butt, N.D. Gohar, J. Hazard. Mater. 170 (2009) 560–569. [5] F. Chen, H.Q. Liu, S. Bagwasi, X.X. Shen, J.L. Zhang, J. Photochem. Photobiol. A 215 (2010) 76–80. [6] X.F. Chang, J. Huang, C. Cheng, Q. Sui, W. Sha, G.B. Ji, S.B. Deng, G. Yu, Catal. Commun. 11 (2010) 460–464. [7] X. Xiao, W.D. Zhang, J. Mater. Chem. 20 (2010) 5866–5870. [8] G.F. Li, F. Qin, H. Yang, Z. Lu, H.Z. Sun, R. Chen, Eur. J. Inorg. Chem. 15 (2012) 2508–2513. [9] J. Henle, P. Simon, A. Frenzel, S. Scholz, S. Kaskel, Chem. Mater. 19 (2007) 366–373. [10] M. Shang, W.Z. Wang, L. Zhang, J. Hazard. Mater. 167 (2009) 803–809. [11] Y.C. Feng, L. Li, J.W. Li, J.F. Wang, L. Liu, J. Hazard. Mater. 192 (2011) 538–544. [12] Z. Jiang, F. Yang, G.D. Yang, L. Kong, M.O. Jones, T.C. Xiao, P. P. Edwards, J. Photochem. Photobiol. A 212 (2010) 8–13. [13] X. Zhang, Z.H. Ai, F.L. Jia, L.Z. Zhang, J. Phys. Chem. C 112 (2008) 747–753. [14] J. Zhang, F.J. Shi, J. Lin, D.F. Chen, J.M. Gao, Z.X. Huang, X.X. Ding, C. C. Tang, Chem. Mater. 20 (2008) 2937–2941. [15] J.X. Xia, S. Yin, H.M. Li, H. Xu, Y.S. Yan, Q. Zhang, Langmuir 27 (2011) 1200–1206. [16] J.X. Xia, S. Yin, H.M. Li, H. Xu, L. Xu, Y.G. Xu, Dalton Trans. 40 (2011) 5249–5258. [17] D.Q. Zhang, M.C. Wen, B. Jiang, G.S. Li, J.C. Yu, J. Hazard. Mater. 211 (2012) 104–111. [18] J.M. Ma, X.D. Liu, J.B. Lian, X.C. Duan, W.J. Zhang, Cryst. Growth Des. 10 (2010) 2522–2527. [19] D.Q. Zhang, G.S. Li, X.F. Yang, J.C. Yu, Chem. Commun. 29 (2009) 4381–4383. [20] Y.Y. Li, J.P. Liu, X.T. Huang, J.G. Yu, Dalton Trans. 39 (2010) 3420–3425. [21] P. Gao, D.D. Sun, Appl. Catal. B Environ. 147 (2014) 888–896. [22] S.W. Liu, K. Yin, W.S. Ren, B. Cheng, J.G. Yu, J. Mater. Chem. 22 (2012) 17759–17767. [23] D. Zhang, J. Li, Q.G. Wang, Q.S. Wu, J. Mater. Chem. A 1 (2013) 8622–8629. [24] L.Q. Ye, L. Zan, L.H. Tian, T.Y. Peng, J.J. Zhang, Chem. Commun. 47 (2011) 6951–6953. [25] K.L. Lv, J.C. Hu, X.H. Li, M. Li, J. Mol. Catal. A 356 (2012) 78–84. [26] J. Cao, B.Y. Xu, H.L. Lin, B.D. Luo, S.F. Chen, Dalton Trans. 41 (2012) 11482–11490. [27] H.B. Fu, C.S. Pan, W.Q. Yao, Y.F. Zhu, J. Phys. Chem. B 109 (2005) 22432–22439. [28] J. Cao, B.Y. Xu, B.D. Luo, H.L. Lin, S.F. Chen, Catal. Commun. 13 (2011) 63–68.
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Visible light photocatalytic performance of hierarchical BiOBr microspheres synthesized via a reactable ionic liquid Bo Chai n, Huan Zhou, Fen Zhang, Xiang Liao, Meixia Ren School of Chemical and Environmental Engineering, Wuhan Polytechnic University, Wuhan 430023, PR China
a r t i c l e i n f o
abstract
Available online 16 March 2014
Hierarchical BiOBr microspheres were synthesized via a one-pot solvothermal process in the presence of ethylene glycol and 1-butyl-3-methylimidazolium bromide ([BMIM]Br) as a reactable ionic liquid. The products were characterized by X-ray diffraction, field-emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, UV-Vis diffuse reflectance absorption spectra, nitrogen adsorption–desorption measurements, and photoluminescence spectroscopy. The photocatalytic activity of BiOBr microspheres was evaluated in terms of the degradation of Rhodamine B (RhB), methyl orange (MO), and 4-chlorophenol (4-CP) under visible light irradiation. We found that the solvothermal temperature had important effects on the crystallinity, crystallite size, optical property, adsorptive performance, and photocatalytic activity of BiOBr microspheres. BiOBr microspheres with a specific surface area of 15.7 m2 g 1 prepared at 160 1C exhibited the best adsorption and photocatalytic performance for RhB degradation in aqueous solution. However, this sample showed hardly any activity for photodegradation of 4-CP. Tests using radical scavengers confirmed that h þ and dO2 were the main reactive species during RhB degradation. A possible mechanism for photocatalysis by BiOBr microspheres is proposed. & 2014 Elsevier Ltd. All rights reserved.
Keywords: BiOBr microspheres Ionic liquid Solvothermal Photocatalytic degradation
1. Introduction Semiconductor photocatalysis is a promising advanced oxidation technology for environmental remediation. Among semiconductor photocatalysts, TiO2 has been extensively studied because of its high photosensitivity and nontoxicity [1,2]. However, TiO2 can only be activated by UV light (λ o 400 nm), which represents only 4% of the solar energy available. Visible light accounts for 43% of the solar spectrum, so the development of efficient photocatalysts with a visible light response is the focus of much photocatalytic research [3,4]. Bismuth oxyhalides (BiOX, X¼Cl, Br, I) have been investigated owing to their excellent photocatalytic
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http://dx.doi.org/10.1016/j.mssp.2014.02.021 1369-8001 & 2014 Elsevier Ltd. All rights reserved.
performance for the degradation of organic compounds [5–7]. In particular, BiOBr has attracted much attention because it is responsive to visible light and is a relatively stable photocatalyst. Several methods have been reported for the preparation of BiOBr micro/nanostructures for photocatalytic degradation of organic contaminants. Li et al. synthesized uniform, well-defined, 3D flowerlike BiOBr nanostructures using microwave irradiation with cetyltrimethylammonium bromide (CTAB) as the bromine source and soft template for Cr(VI) removal [8]. Henle et al. prepared nanosized bismuth oxyhalide (BiOX, X¼Cl, Br, I) particles via a reverse micro-emulsion [9]. Shang et al. prepared BiOBr lamellar structures with CTAB as a Br source and template using a hydrothermal method [10]. Their BiOBr lamellar structures showed high photocatalytic activity and stability for MO degradation under visible light. Feng et al. synthesized mesoporous 3D BiOBr microspheres using a facile solvothermal method with absolute ethanol as
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the solvent and studied their activity for photodecomposition of toluene [11]. Jiang et al. fabricated flake-like BiOBr using a HAc-assisted hydrothermal route for photocatalytic degradation of MO [12]. Two different research groups prepared BiOBr microspheres using ethylene glycol (EG) and NaBr as the solvent and Br source, respectively [13,14]. Ionic liquids (ILs), which are typically composed of organic cations and large anions, have attracted much interest as functional materials for applications in catalysis, electrochemistry, and separation. In particular, ILs have received considerable attention as templates in the synthesis of functional nanomaterials with unusual morphology [15–19]. For instance, Xia and co-workers synthesized hollow BiOI microspheres in the presence of 1-butyl-3methylimidazolium iodine ([BMIM]I) as an IL [15]. The same group prepared flower-like hollow BiOBr microspheres in a one-pot EG-assisted solvothermal process in the presence of the reactable IL 1-hexadecyl-3methylimidazolium bromide [16]. Nevertheless, the full potential of ILs as reagents in controllable synthesis of bismuth oxyhalide nanostructures remains to be fully explored. In this study we developed a facile EG-assisted solvothermal method for synthesis of BiOBr microspheres using [BMIM]Br as a reactable IL. The effects of the solvothermal temperature (TST) on the crystal structure, morphology, optical properties, adsorption performance, and photocatalytic activity for degradation of Rhodamine B (RhB), methyl orange (MO), and 4-chlorophenol (4-CP) are discussed in detail. 2. Experimental 2.1. Sample preparation [BMIM]Br was obtained from Lanzhou Greenchem ILs (China). Other chemicals were analytical-grade reagents purchased from Sinopharm Chemical Reagent Co. (China) and were used without further purification. In a typical procedure, 1 mmol of Bi(NO3)3 5H2O was dissolved in 70 mL of EG and stoichiometric amounts of [BMIM]Br were added under constant stirring at room temperature to ensure good dispersion of the reactants. After stirring for 30 min, the mixed solution was transferred to a 100-mL Teflon-lined stainless steel autoclave and kept at different temperature (120, 140, 160, and 180 1C) for 12 h. After natural cooling to room temperature, the products were collected by centrifugation and washed with distilled water and ethanol several times, then dried at 80 1C overnight. For comparison, hierarchical BiOBr microspheres were synthesized as a control sample using NaBr as the Br source as previously described [13,14]. BiOBr microspheres prepared at 120, 140, 160, and 180 1C are denoted in the text as BiOBr120, BiOBr140, BiOBr160, and BiOBr180, respectively. 2.2. Sample characterization The products were characterized by X-ray diffraction (XRD) using a Bruker D8 Advance X-ray diffractometer with Cu Kα irradiation (λ ¼0.154178 nm) at 40 kV and 40 mA. The morphology and structure of as-prepared samples were analyzed by field-emission scanning
electron microscopy (FESEM; JSM-6700F) and transmission electron microscopy (TEM; JEM-2100). X-Ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos XSAM 800 instrument with a Mg Kα source operating at 200 W. UV-Vis diffuse reflectance spectra (DRS) were obtained on a Shimadzu UV-3600 spectrophotometer equipped with an integrating sphere using BaSO4 as the reference sample. Nitrogen adsorption– desorption measurements were conducted on a nitrogen adsorption system at 77 K (Micrometrics, ASAP 2020); samples were degassed at 120 1C for 5 h before measurement. Photoluminescence (PL) spectra were measured at room temperature on a Varian Cary Eclipase fluorescence spectrophotometer with excitation at 315 nm. 2.3. Photocatalytic activity The photocatalytic activity of as-prepared samples was evaluated for degradation of RhB, MO, and 4-CP in aqueous solutions under visible light irradiation. Samples of 50 mg of the photocatalysts were added to 100 mL of RhB, MO, and 4-CP solutions with initial concentrations of 2.5 10 5 mol L 1, 10 mg L 1, and 10 mg L 1, respectively. A 500-W tungsten halogen lamp was positioned inside a cylindrical Pyrex vessel and surrounded by a circulating water jacket (Pyrex) to cool the lamp. A cutoff filter was placed outside the Pyrex jacket to completely remove all radiation of λo420 nm to ensure irradiation was with visible light only. Prior to irradiation, the suspension was magnetically stirred in the dark for 60, 30, and 30 min to reach adsorption–desorption equilibrium for RhB, MO, and 4-CP, respectively. After irradiation was started, 4 mL of the suspension was collected time intervals and centrifuged (12,000 rpm, 15 min) to remove photocatalyst particles. The concentration of RhB, MO, and 4-CP was determined from the absorbance measured at 554, 464, and 224 nm, respectively, TU-1810 spectrophotometer. To further determine the mineralization of RhB, changes in total organic carbon (TOC) were measured using a total organic carbon analyzer (Multi N/C 2100). 3. Results and discussion Fig. 1 shows XRD patterns for BiOBr samples prepared at different temperatures. All the diffraction peaks can be indexed to the tetragonal phase of BiOBr (JCPDS No. 73-2061) with well-resolved (011), (012), (110), (112), (020), and (212) reflections, in agreement with the literature [13,14,16]. No other crystal-phase diffraction peaks were detected, indicating that the samples were pure. It is worth noting that the diffraction peak intensity increased with TST, suggesting that the crystallinity and crystallite size of BiOBr microspheres improved with increasing TST. Representative SEM and TEM images of BiOBr samples prepared at different solvothermal temperatures are shown in Fig. 2. It is evident that the BiOBr crystallites self-organized into hierarchical microspheres (average diameter 1–4 mm) composed of massive interleaving nanosheets. The BiOBr microsphere diameter gradually increased with TST. The BiOBr120 sample diameter ranged from 1 to 2 mm (Fig. 2a). BiOBr140 had a particle size
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Fig. 1. XRD patterns for BiOBr samples prepared at different solvothermal temperatures.
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distribution of 2–3 mm (Fig. 2b). For BiOBr160 and BiOBr180, the microsphere diameter and particle size distribution further increased to 2–4 mm (Fig. 2c,d). A nanosheet thickness of approximately 10–20 nm can be observed for an individual BiOBr microsphere (inset in Fig. 2c). Further investigation was carried out by TEM to reveal the microstructure of the samples. The TEM image of BiOBr microspheres in Fig. 2e reveals a zigzag circular exterior, in accordance with the SEM images. The magnification of the microsphere edge Fig. 2f shows that it is composed of numerous nanosheets. XPS was performed to determine the chemical composition and valence state of various species. The peak positions in all the XPS spectra were calibrated using the C 1s peak at 284.6 eV. Fig. 3a shows the XPS survey spectrum for BiOBr160. As expected, the sample contains
Fig. 2. SEM and TEM images of BiOBr products: (a) 120 1C, (b) 140 1C, (c) 160 1C (an individual BiOBr microsphere, inset), (d) 180 1C, and (e) low- and (f) high-magnification TEM images of BiOBr microspheres obtained at 160 1C.
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Fig. 3. XPS spectra for BiOBr microspheres obtained at 160 1C: (a) XPS survey spectrum, (b) high-resolution Bi 4f spectrum, (c) high-resolution Br 3d spectrum, and (d) high-resolution O 1s spectrum.
Fig. 4. UV-Vis diffuse reflectance absorption spectra for BiOBr samples prepared at different solvothermal temperatures. The inset shows the bandgap value estimated from a plot of (αhν)1/2 versus photon energy.
prepared at 120, 140 and 160 1C show almost the same steep absorption edge at 425 nm. For BiOBr180, the absorption edge is slightly shifted to a longer wavelength, which can be attributed to the increase in grain size, as observed in SEM images [20]. The steep gradient in the visible region is ascribed to the intrinsic bandgap transition between the valence band (VB) and the conduction band (CB), rather than a transition from impurity levels. Notably, the absorption intensity for the BiOBr microspheres increases with TST over the whole visible light range. This is because the surface of BiOBr microspheres prepared at higher TST contains carbon species, as revealed by the gray color of the samples. UV-Vis DRS data for the BiOBr samples were used to determine the absorption coefficient α according to the Kubelka–Munk function [21] α¼
Bi, O, Br, and C as elements. A high-resolution Bi 4f spectrum is shown in Fig. 3b. The peaks at binding energy of 159.3 and 164.6 eV can be attributed to Bi 4f7/2 and Bi 4f5/2, which are characteristic of Bi3 þ in BiOBr materials [16]. In Fig. 3c, the Br 3d peak is associated with binding energy of 68.6 eV, which is characteristic of Br [16]. The O 1s XPS spectrum in Fig. 3d can be deconvoluted into two peaks. The peak at 530.2 eV can be ascribed to oxygen anions in the BiOBr lattice, while the peak centered at 531.6 eV is assigned to chemisorbed oxygen of surface hydroxyl groups. Fig. 4 compares UV-Vis DRS spectra for BiOBr samples prepared at different temperatures. BiOBr microspheres
ð1 RÞ ; 2R
ð1Þ
where R is reflectance (R¼10 A, where A is the optical absorbance). The bandgap energy can be estimated from a plot of (αhν)1/2 versus photon energy hν, where h is Planck’s constant and ν is frequency (inset in Fig. 4). The bandgap decreased from 2.96 to 2.95 to 2.92 to 2.75 eV as TST increased from 120 to 180 1C. Fig. 5 shows nitrogen adsorption–desorption isotherms and the pore size distribution for BiOBr microspheres prepared at different temperatures. The isotherms can be categorized as type IV with a H3 hysteresis loop, which is characteristic of mesoporous materials. The BET specific surface area for BiOBr120, BiOBr140, BiOBr160, and BiOBr180 was calculated as 16.3, 16.0, 15.7, and 14.8 m2 g 1,
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Fig. 5. Nitrogen adsorption–desorption isotherm and pore size distribution (inset) for BiOBr microspheres prepared at different solvothermal temperatures.
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respectively. It is evident that the specific surface area slightly decreased with increasing TST. BiOBr120, BiOBr140, and BiOBr160 samples contained small mesopores with a diameter of 10 nm, as determined by the Barrett– Joyner–Halenda (BJH) method (inset in Fig. 5). These mesopores can be attributed to the space between nanosheets in the hierarchical microspheres, which is consistent with the SEM and TEM observations. The photocatalytic activity of as-prepared samples was evaluated for RhB degradation under visible light irradiation. For comparison, a blank and a control sample were also tested under the same reaction conditions. Fig. 6a–e shows temporal changes in the UV-Vis spectrum of the RhB solution during adsorption and photocatalytic degradation over BiOBr microspheres and the control sample. For BiOBr160, the wavelength corresponding to maximum absorbance (λmax) gradually shifted from 554 nm towards
Fig. 6. Temporal spectral evolution during photocatalytic degradation of RhB over samples prepared at (a) 120 1C, (b) 140 1C, (c) 160 1C, and (d) 180 1C, and (e) a control sample; “origin” denotes the concentration of RhB before absorption equilibrium. (f) Comparison of the photocatalytic activity of the samples for degradation of RhB in solution.
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shorter wavelength and finally reached 498 nm during visible light illumination; this was accompanied by a gradual decrease in maximum absorbance (Fig. 6c). A blue shift of λmax is frequently observed during oxidation of RhB over multimetal oxide photocatalysts, and is associated with stepwise removal of N-ethyl groups during degradation of RhB (N,N,N',N'-tetraethyl rhodamine) [16,20,22]. The characteristic λmax for RhB and de-ethylated rhodamine species are 554 nm (RhB), 539 nm (N,N,N'-triethyl rhodamine), 522 nm (N,N'-diethyl rhodamine), 510 nm (Nethyl rhodamine), and 498 nm (rhodamine). For BiOBr120, λmax was 498 nm after 60 min of visible light irradiation, indicating that rhodamine was present among the deethylated products (Fig. 6a). For the control sample and BiOBr140 and BiOBr180, λmax after irradiation for 60 min was observed at 522 nm, indicating the presence of N,N'diethyl rhodamine (Fig. 6b,d,e). Fig. 6f compares the photocatalytic activity of the samples for degradation of RhB. The results indicate that BiOBr160 exhibited the best adsorption capacity and photocatalytic performance. The ability of BiOBr160 to mineralize RhB was evaluated by monitoring changes in TOC (Fig. 7). The results suggest that the rate of TOC removal is much slower than the rate of the photocatalytic decolorization for RhB. After 60 min of irradiation, the solution TOC decreased by approximately 42%, implying that RhB molecules were degraded but not completely mineralized to inorganic molecules. The effect of solution pH on the photocatalytic degradation of RhB over BiOBr140 was evaluated for an initial pH range of 2.5–7.5 by adjusting the initial pH with 0.1 mol L 1 HNO3 and NaOH solutions. As shown in Fig. S1, the photocatalytic activity increased with decreasing pH. This suggests that an acidic environment is more beneficial for RhB degradation, in agreement with the literature [10]. Photocatalytic degradation of MO and 4-CP under visible light in the presence of different BiOBr samples was also investigated. The results shown in Figs. S2 and S3a–e indicate that the BiOBr catalysts cannot totally degrade MO in 3 h. BiOBr160 showed the best adsorption ability but little degradation activity for MO. For 4-CP, BiOBr samples prepared at different temperatures showed extremely weak adsorption and degradation performance during photocatalysis, while the control sample exhibited
slight degradation activity. Thus, the BiOBr microspheres exhibit selective photocatalytic behavior, as their activity was much higher for RhB than for MO or 4-CP photodegradation under visible light. According to DRS analysis, the lowest bandgap energy was observed for BiOBr180, indicating that this sample can absorb more visible light. However, its photocatalytic activity was weaker than that of the other samples (Fig. 6f). Thus, the variation in optical properties is not a dominant factor affecting the photocatalytic activity of asprepared BiOBr microspheres. It is well known that photocatalytic activity is closely related to the adsorption ability of catalysts [16]. Although the BiOBr microspheres had similar specific surface areas, BiOBr160 showed the best adsorption capacity for the organic dyes. This may be because it has more negative charges on its surface [23]. RhB molecules form positively charged cations in aqueous solution and thus can be adsorbed by BiOBr via electrostatic interaction. In contrast, MO molecules form anions in solution so they cannot be adsorbed by BiOBr. Sufficient dye adsorption has a positive effect on the photocatalysis efficiency. The crystallinity, crystallite size, and surface defects of catalysts also affect their photocatalytic activity. According to the XRD and SEM results, the crystallinity and crystallite size of BiOBr microspheres improved with increasing TST. In general, better photocatalyst crystallinity and fewer surface defects lead to higher photocatalytic activity, whereas an increase in particle size can have a negative effect on photocatalytic performance. We can deduce that BiOBr160 had appropriate crystallinity, crystallite size, and surface defects, which increased the number of photogenerated electrons and holes available for participating in the photocatalytic degradation of contaminants. In part, photocatalytic activity is a function of the lifetime and trapping of photogenerated electrons and holes in the semiconductor. PL emission spectra are often used to study surface structures and excited states. Fig. 8 shows PL spectra over the wavelength range 325–550 nm for BiOBr microspheres prepared at different temperatures. Two emission peaks are evident at approximately 410 and 490 nm. The first (main peak) is ascribed to emission for bandgap transition and the second to surface oxygen vacancies and defects [24]. The highest-intensity
Fig. 7. Changes in TOC in the reaction system during photocatalytic degradation of RhB over BiOBr microspheres prepared at 160 1C.
Fig. 8. PL spectra for BiOBr samples prepared at different solvothermal temperatures.
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Fig. 9. Effects of reactive species on the photocatalytic degradation of RhB over BiOBr microspheres prepared at 160 1C.
main peak was observed for BiOBr120. A moderate decrease in PL intensity with increasing TST is evident. PL emission mainly results from recombination of excited electrons and holes, and a lower PL intensity indicates a decrease in the recombination rate. However, the least intense main PL peak was observed for BiOBr180, which can be attributed to PL quenching by surface carbon species [25]. Interestingly, the lowest intensity for the emission peak ascribed to surface defects was observed for BiOBr160, indicating that these microspheres had the fewest surface defects. Hence, surface recombination of photogenerated electrons and holes is greatly inhibited over BiOBr160, which enhances its photocatalytic activity. The PL results confirm that BiOBr160 can effectively separate photogenerated electron–hole pairs. To clarify the reaction mechanism, isopropanol (IPA), triethanolamine (TEOA), and p-benzoquinone (BQ) were introduced as scavengers of hydroxyl radicals (dOH), holes (h þ ), and superoxide radicals (dO2 ), respectively, to determine the effects of reactive species on photocatalytic degradation of RhB [26]. The concentration of IPA, TEOA, and BQ in the RhB photocatalytic reaction system was 10, 10, and 2 mmol L 1, respectively. Fig. 9 shows that TEOA significantly suppressed the RhB degradation rate; BQ had a weaker inhibitory, and IPA had hardly any effect on RhB degradation. The results confirm that h þ and dO2 play a d more important role than that of OH in photocatalytic degradation of RhB. The CB and VB edge potentials of BiOBr are important factors in understanding the degradation of contaminants over these photocatalysts. According to theoretical speculation, the CB and VB potentials over BiOBr are 0.27 and 3.19 eV, respectively [12]. Thus, the VB edge potential of BiOBr is more positive than the standard redox potential of dOH/OH (1.99 eV), suggesting that photogenerated holes could oxidize OH to dOH. However, production of dOH in the present system is almost impossible because the standard redox potential of Bi(V)/Bi(III) (1.59 eV) is more negative than that of dOH/OH [27]. Thus, degradation of RhB over BiOBr should involve direct reaction with photogenerated holes. Therefore, photocatalytic degradation of RhB over BiOBr is little effect in the presence of the dOH radical scavenger (IPA). In addition, the CB edge potential of BiOBr (0.27 eV) is not negative enough to reduce O2 to dO2 because the single-electron
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Fig. 10. Stability study for photocatalytic degradation of RhB over BiOBr microspheres prepared at 160 1C.
reduction potential of O2 is 0.046 eV [28]. However, this does not exclude the formation of dO2 by photogenerated electrons via other photochemical reactions. We postulate that dye photosensitization leads to the formation of dO2 radicals. RhB molecules adsorbed on BiOBr are regarded as sensitizers and that visible light irradiation stimulates the generation of electrons that subsequently transfer to the CB of BiOBr and then react with O2 to form dO2 radicals. The results for photocatalytic degradation of RhB, MO, and 4-CP confirm the dye photosensitization mechanism for degradation over BiOBr microspheres. For RhB, the BiOBr microspheres show excellent adsorption and photocatalytic activity. In comparison, the microspheres display weaker adsorption capacity for MO, and thus their photocatalytic activity is lower for this dye. For colorless 4-CP, they exhibit hardly any degradation activity. Consequently, the following plausible reaction scheme can be proposed: DyeðadsÞ þhv-DyeðadsÞ n
ð2Þ
DyeðadsÞ n þ BiOBr-BiOBrðe Þ þDyeðadsÞ þ
ð3Þ
BiOBrðe Þ þ O2 -BiOBr þ d O2
ð4Þ
BiOBr þ hv-e þ h þ
þ
Dye þ h ðd O2 Þ-products
ð5Þ ð6Þ
To test the stability and reusability of the BiOBr microspheres for photocatalytic degradation of RhB, the catalysts were reused four times for photocatalysis under the same reaction conditions. The results are shown in Fig. 10. The decolorization efficiency of BiOBr microspheres decreased by only 6% after four cycles. Comparison of XRD patterns reveals a high degree of similarity for the BiOBr catalyst recycled four times and the original catalyst (Fig. 11). There is no obvious deviation in peak location and the slight decrease in peak intensity indicates a slight decline in crystallinity after visible light irradiation. These observations suggest that only slight photocorrosion occurred during the photocatalytic reaction and that BiOBr microspheres have good photostability.
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References
Fig. 11. XRD patterns for BiOBr prepared at 160 1C before and after photocatalytic recycling experiments.
4. Conclusions Hierarchical BiOBr microspheres with an average diameter of 1–4 mm were prepared via a one-pot EG-assisted solvothermal process in the presence of a reactable IL. The experimental results indicate that the solvothermal temperature has important effects on the crystallite size, optical properties, adsorption capacity, and photocatalytic activity of BiOBr microspheres. Among all the samples, BiOBr160 showed the best adsorption capacity and photocatalytic activity for RhB degradation under visible light illumination. This result can be attributed to appropriate crystallinity, crystallite size, and surface defects. Investigation of the photocatalytic mechanism demonstrated that h þ and dO2 species play a key role. Moreover, results for photocatalytic degradation of RhB, MO, and 4-CP confirmed a photosensitization mechanism for degradation over BiOBr microspheres. These BiOBr hierarchical microspheres hold promise as efficient photocatalysts for the degradation of organic dyes and in other applications. Acknowledgment This work was supported by the National Natural Science Foundation of China (No. 51302200). 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. mssp.2014.02.021.
[1] X.B. Chen, S.S. Mao, Chem. Rev. 107 (2007) 2891–2959. [2] M.D. Hernández-Alonso, F. Fresno, S. Suárez, J.M. Coronado, Energy Environ. Sci. 2 (2009) 1231–1257. [3] H.J. Zhang, G.H. Chen, D.W. Bahnemann, J. Mater. Chem. 19 (2009) 5089–5121. [4] S. Rehman, R. Ullah, A.M. Butt, N.D. Gohar, J. Hazard. Mater. 170 (2009) 560–569. [5] F. Chen, H.Q. Liu, S. Bagwasi, X.X. Shen, J.L. Zhang, J. Photochem. Photobiol. A 215 (2010) 76–80. [6] X.F. Chang, J. Huang, C. Cheng, Q. Sui, W. Sha, G.B. Ji, S.B. Deng, G. Yu, Catal. Commun. 11 (2010) 460–464. [7] X. Xiao, W.D. Zhang, J. Mater. Chem. 20 (2010) 5866–5870. [8] G.F. Li, F. Qin, H. Yang, Z. Lu, H.Z. Sun, R. Chen, Eur. J. Inorg. Chem. 15 (2012) 2508–2513. [9] J. Henle, P. Simon, A. Frenzel, S. Scholz, S. Kaskel, Chem. Mater. 19 (2007) 366–373. [10] M. Shang, W.Z. Wang, L. Zhang, J. Hazard. Mater. 167 (2009) 803–809. [11] Y.C. Feng, L. Li, J.W. Li, J.F. Wang, L. Liu, J. Hazard. Mater. 192 (2011) 538–544. [12] Z. Jiang, F. Yang, G.D. Yang, L. Kong, M.O. Jones, T.C. Xiao, P. P. Edwards, J. Photochem. Photobiol. A 212 (2010) 8–13. [13] X. Zhang, Z.H. Ai, F.L. Jia, L.Z. Zhang, J. Phys. Chem. C 112 (2008) 747–753. [14] J. Zhang, F.J. Shi, J. Lin, D.F. Chen, J.M. Gao, Z.X. Huang, X.X. Ding, C. C. Tang, Chem. Mater. 20 (2008) 2937–2941. [15] J.X. Xia, S. Yin, H.M. Li, H. Xu, Y.S. Yan, Q. Zhang, Langmuir 27 (2011) 1200–1206. [16] J.X. Xia, S. Yin, H.M. Li, H. Xu, L. Xu, Y.G. Xu, Dalton Trans. 40 (2011) 5249–5258. [17] D.Q. Zhang, M.C. Wen, B. Jiang, G.S. Li, J.C. Yu, J. Hazard. Mater. 211 (2012) 104–111. [18] J.M. Ma, X.D. Liu, J.B. Lian, X.C. Duan, W.J. Zhang, Cryst. Growth Des. 10 (2010) 2522–2527. [19] D.Q. Zhang, G.S. Li, X.F. Yang, J.C. Yu, Chem. Commun. 29 (2009) 4381–4383. [20] Y.Y. Li, J.P. Liu, X.T. Huang, J.G. Yu, Dalton Trans. 39 (2010) 3420–3425. [21] P. Gao, D.D. Sun, Appl. Catal. B Environ. 147 (2014) 888–896. [22] S.W. Liu, K. Yin, W.S. Ren, B. Cheng, J.G. Yu, J. Mater. Chem. 22 (2012) 17759–17767. [23] D. Zhang, J. Li, Q.G. Wang, Q.S. Wu, J. Mater. Chem. A 1 (2013) 8622–8629. [24] L.Q. Ye, L. Zan, L.H. Tian, T.Y. Peng, J.J. Zhang, Chem. Commun. 47 (2011) 6951–6953. [25] K.L. Lv, J.C. Hu, X.H. Li, M. Li, J. Mol. Catal. A 356 (2012) 78–84. [26] J. Cao, B.Y. Xu, H.L. Lin, B.D. Luo, S.F. Chen, Dalton Trans. 41 (2012) 11482–11490. [27] H.B. Fu, C.S. Pan, W.Q. Yao, Y.F. Zhu, J. Phys. Chem. B 109 (2005) 22432–22439. [28] J. Cao, B.Y. Xu, B.D. Luo, H.L. Lin, S.F. Chen, Catal. Commun. 13 (2011) 63–68.