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Materials Science in Semiconductor Processing

Materials Science in Semiconductor Processing 23 (2014) 151–158

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Visible light photocatalytic performance of hierarchical BiOBrmicrospheres synthesized via a reactable ionic liquid

Bo Chai n, Huan Zhou, Fen Zhang, Xiang Liao, Meixia RenSchool of Chemical and Environmental Engineering, Wuhan Polytechnic University, Wuhan 430023, PR China

a r t i c l e i n f o

Available online 16 March 2014

Keywords:BiOBr microspheresIonic liquidSolvothermalPhotocatalytic degradation

x.doi.org/10.1016/j.mssp.2014.02.02101 & 2014 Elsevier Ltd. All rights reserved.

esponding author. Tel./Fax: þ86 27 839439ail address: willycb@163.com (B. Chai).

a b s t r a c t

Hierarchical BiOBr microspheres were synthesized via a one-pot solvothermal process in thepresence of ethylene glycol and 1-butyl-3-methylimidazolium bromide ([BMIM]Br) as areactable ionic liquid. The products were characterized by X-ray diffraction, field-emissionscanning electron microscopy, transmission electron microscopy, X-ray photoelectron spec-troscopy, UV-Vis diffuse reflectance absorption spectra, nitrogen adsorption–desorptionmeasurements, and photoluminescence spectroscopy. The photocatalytic activity of BiOBrmicrospheres was evaluated in terms of the degradation of Rhodamine B (RhB), methylorange (MO), and 4-chlorophenol (4-CP) under visible light irradiation. We found that thesolvothermal temperature had important effects on the crystallinity, crystallite size, opticalproperty, adsorptive performance, and photocatalytic activity of BiOBr microspheres. BiOBrmicrospheres with a specific surface area of 15.7 m2 g�1 prepared at 160 1C exhibited the bestadsorption and photocatalytic performance for RhB degradation in aqueous solution.However, this sample showed hardly any activity for photodegradation of 4-CP. Tests usingradical scavengers confirmed that hþ 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.

1. Introduction

Semiconductor photocatalysis is a promising advan-ced oxidation technology for environmental remediation.Among semiconductor photocatalysts, TiO2 has beenextensively studied because of its high photosensitivityand nontoxicity [1,2]. However, TiO2 can only be activatedby UV light (λ o 400 nm), which represents only 4% of thesolar energy available. Visible light accounts for 43% of thesolar spectrum, so the development of efficient photoca-talysts with a visible light response is the focus of muchphotocatalytic research [3,4].

Bismuth oxyhalides (BiOX, X¼Cl, Br, I) have beeninvestigated owing to their excellent photocatalytic

56.

performance for the degradation of organic compounds[5–7]. In particular, BiOBr has attracted much attentionbecause it is responsive to visible light and is a relativelystable photocatalyst. Several methods have been reportedfor the preparation of BiOBr micro/nanostructures forphotocatalytic degradation of organic contaminants. Liet al. synthesized uniform, well-defined, 3D flowerlikeBiOBr nanostructures using microwave irradiation withcetyltrimethylammonium bromide (CTAB) as the brominesource 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 sourceand template using a hydrothermal method [10]. TheirBiOBr lamellar structures showed high photocatalytic activ-ity and stability for MO degradation under visible light.Feng et al. synthesized mesoporous 3D BiOBr microspheresusing a facile solvothermal method with absolute ethanol as

B. Chai et al. / Materials Science in Semiconductor Processing 23 (2014) 151–158152

the solvent and studied their activity for photodecomposi-tion of toluene [11]. Jiang et al. fabricated flake-like BiOBrusing a HAc-assisted hydrothermal route for photocatalyticdegradation of MO [12]. Two different research groupsprepared BiOBr microspheres using ethylene glycol (EG)and NaBr as the solvent and Br source, respectively [13,14].

Ionic liquids (ILs), which are typically composed oforganic cations and large anions, have attracted muchinterest as functional materials for applications in cataly-sis, electrochemistry, and separation. In particular, ILs havereceived considerable attention as templates in the synth-esis of functional nanomaterials with unusual morphology[15–19]. For instance, Xia and co-workers synthesizedhollow BiOI microspheres in the presence of 1-butyl-3-methylimidazolium iodine ([BMIM]I) as an IL [15].The same group prepared flower-like hollow BiOBr micro-spheres in a one-pot EG-assisted solvothermal processin the presence of the reactable IL 1-hexadecyl-3-methylimidazolium bromide [16]. Nevertheless, the fullpotential of ILs as reagents in controllable synthesis ofbismuth oxyhalide nanostructures remains to be fullyexplored. In this study we developed a facile EG-assistedsolvothermal method for synthesis of BiOBr microspheresusing [BMIM]Br as a reactable IL. The effects of thesolvothermal 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) arediscussed in detail.

2. Experimental

2.1. Sample preparation

[BMIM]Br was obtained from Lanzhou Greenchem ILs(China). Other chemicals were analytical-grade reagentspurchased from Sinopharm Chemical Reagent Co. (China)and were used without further purification. In a typicalprocedure, 1 mmol of Bi(NO3)3 �5H2O was dissolved in70 mL of EG and stoichiometric amounts of [BMIM]Br wereadded under constant stirring at room temperature toensure good dispersion of the reactants. After stirring for30 min, the mixed solution was transferred to a 100-mLTeflon-lined stainless steel autoclave and kept at differenttemperature (120, 140, 160, and 180 1C) for 12 h. Afternatural cooling to room temperature, the products werecollected by centrifugation and washed with distilled waterand ethanol several times, then dried at 80 1C overnight.For comparison, hierarchical BiOBr microspheres weresynthesized as a control sample using NaBr as the Br sourceas previously described [13,14]. BiOBr microspheres pre-pared at 120, 140, 160, and 180 1C are denoted in the text asBiOBr120, 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 diffractometerwith Cu Kα irradiation (λ¼0.154178 nm) at 40 kV and40 mA. The morphology and structure of as-preparedsamples were analyzed by field-emission scanning

electron microscopy (FESEM; JSM-6700F) and transmis-sion electron microscopy (TEM; JEM-2100). X-Ray photo-electron spectroscopy (XPS) measurements wereperformed on a Kratos XSAM 800 instrument with a MgKα source operating at 200 W. UV-Vis diffuse reflectancespectra (DRS) were obtained on a Shimadzu UV-3600spectrophotometer equipped with an integrating sphereusing BaSO4 as the reference sample. Nitrogen adsorption–desorption measurements were conducted on a nitrogenadsorption system at 77 K (Micrometrics, ASAP 2020);samples were degassed at 120 1C for 5 h before measure-ment. Photoluminescence (PL) spectra were measured atroom temperature on a Varian Cary Eclipase fluorescencespectrophotometer with excitation at 315 nm.

2.3. Photocatalytic activity

The photocatalytic activity of as-prepared samples wasevaluated for degradation of RhB, MO, and 4-CP in aqueoussolutions under visible light irradiation. Samples of 50 mgof the photocatalysts were added to 100 mL of RhB, MO,and 4-CP solutions with initial concentrations of2.5�10�5 mol L�1, 10 mg L�1, and 10 mg L�1, respec-tively. A 500-W tungsten halogen lamp was positionedinside a cylindrical Pyrex vessel and surrounded by acirculating water jacket (Pyrex) to cool the lamp. A cutofffilter was placed outside the Pyrex jacket to completelyremove all radiation of λo420 nm to ensure irradiationwas with visible light only. Prior to irradiation, the suspen-sion was magnetically stirred in the dark for 60, 30, and30 min to reach adsorption–desorption equilibrium forRhB, MO, and 4-CP, respectively. After irradiation wasstarted, 4 mL of the suspension was collected time inter-vals and centrifuged (12,000 rpm, 15 min) to removephotocatalyst particles. The concentration of RhB, MO,and 4-CP was determined from the absorbance measuredat 554, 464, and 224 nm, respectively, TU-1810 spectro-photometer. To further determine the mineralization ofRhB, changes in total organic carbon (TOC) were measuredusing a total organic carbon analyzer (Multi N/C 2100).

3. Results and discussion

Fig. 1 shows XRD patterns for BiOBr samples preparedat different temperatures. All the diffraction peaks can beindexed 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 litera-ture [13,14,16]. No other crystal-phase diffraction peakswere detected, indicating that the samples were pure. It isworth noting that the diffraction peak intensity increasedwith TST, suggesting that the crystallinity and crystallitesize of BiOBr microspheres improved with increasing TST.

Representative SEM and TEM images of BiOBr samplesprepared at different solvothermal temperatures areshown in Fig. 2. It is evident that the BiOBr crystallitesself-organized into hierarchical microspheres (averagediameter 1–4 mm) composed of massive interleavingnanosheets. The BiOBr microsphere diameter graduallyincreased with TST. The BiOBr120 sample diameter rangedfrom 1 to 2 mm (Fig. 2a). BiOBr140 had a particle size

Fig. 1. XRD patterns for BiOBr samples prepared at different solvother-mal temperatures.

Fig. 2. SEM and TEM images of BiOBr products: (a) 120 1C, (b) 140 1C, (c) 160 1Chigh-magnification TEM images of BiOBr microspheres obtained at 160 1C.

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distribution of 2–3 mm (Fig. 2b). For BiOBr160 and BiOBr180,the microsphere diameter and particle size distributionfurther increased to 2–4 mm (Fig. 2c,d). A nanosheetthickness of approximately 10–20 nm can be observedfor an individual BiOBr microsphere (inset in Fig. 2c).Further investigation was carried out by TEM to revealthe microstructure of the samples. The TEM image ofBiOBr microspheres in Fig. 2e reveals a zigzag circularexterior, in accordance with the SEM images. The magni-fication of the microsphere edge Fig. 2f shows that it iscomposed of numerous nanosheets.

XPS was performed to determine the chemical compo-sition and valence state of various species. The peakpositions in all the XPS spectra were calibrated using theC 1s peak at 284.6 eV. Fig. 3a shows the XPS surveyspectrum for BiOBr160. As expected, the sample contains

(an individual BiOBr microsphere, inset), (d) 180 1C, and (e) low- and (f)

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 3dspectrum, and (d) high-resolution O 1s spectrum.

Fig. 4. UV-Vis diffuse reflectance absorption spectra for BiOBr samplesprepared at different solvothermal temperatures. The inset shows thebandgap value estimated from a plot of (αhν)1/2 versus photon energy.

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Bi, O, Br, and C as elements. A high-resolution Bi 4fspectrum is shown in Fig. 3b. The peaks at binding energyof 159.3 and 164.6 eV can be attributed to Bi 4f7/2 and Bi4f5/2, which are characteristic of Bi3þ in BiOBr materials[16]. In Fig. 3c, the Br 3d peak is associated with bindingenergy of 68.6 eV, which is characteristic of Br� [16]. The O1s XPS spectrum in Fig. 3d can be deconvoluted into twopeaks. The peak at 530.2 eV can be ascribed to oxygenanions in the BiOBr lattice, while the peak centered at531.6 eV is assigned to chemisorbed oxygen of surfacehydroxyl groups.

Fig. 4 compares UV-Vis DRS spectra for BiOBr samplesprepared at different temperatures. BiOBr microspheres

prepared at 120, 140 and 160 1C show almost the samesteep absorption edge at 425 nm. For BiOBr180, the absorp-tion edge is slightly shifted to a longer wavelength, whichcan be attributed to the increase in grain size, as observedin SEM images [20]. The steep gradient in the visibleregion is ascribed to the intrinsic bandgap transitionbetween the valence band (VB) and the conduction band(CB), rather than a transition from impurity levels. Notably,the absorption intensity for the BiOBr microspheresincreases with TST over the whole visible light range. Thisis because the surface of BiOBr microspheres prepared athigher TST contains carbon species, as revealed by the graycolor of the samples. UV-Vis DRS data for the BiOBrsamples were used to determine the absorption coefficientα according to the Kubelka–Munk function [21]

α¼ ð1�RÞ2R

; ð1Þ

where R is reflectance (R¼10�A, where A is the opticalabsorbance). The bandgap energy can be estimated from aplot of (αhν)1/2 versus photon energy hν, where h isPlanck’s constant and ν is frequency (inset in Fig. 4). Thebandgap decreased from 2.96 to 2.95 to 2.92 to 2.75 eV asTST increased from 120 to 180 1C.

Fig. 5 shows nitrogen adsorption–desorption isothermsand the pore size distribution for BiOBr microspheresprepared at different temperatures. The isotherms can becategorized as type IV with a H3 hysteresis loop, which ischaracteristic of mesoporous materials. The BET specificsurface area for BiOBr120, BiOBr140, BiOBr160, and BiOBr180was calculated as 16.3, 16.0, 15.7, and 14.8 m2 g�1,

Fig. 5. Nitrogen adsorption–desorption isotherm and pore size distribu-tion (inset) for BiOBr microspheres prepared at different solvothermaltemperatures.

Fig. 6. Temporal spectral evolution during photocatalytic degradation of RhB ove(e) a control sample; “origin” denotes the concentration of RhB before absorptiofor degradation of RhB in solution.

B. Chai et al. / Materials Science in Semiconductor Processing 23 (2014) 151–158 155

respectively. It is evident that the specific surface areaslightly decreased with increasing TST. BiOBr120, BiOBr140,and BiOBr160 samples contained small mesopores with adiameter of �10 nm, as determined by the Barrett–Joyner–Halenda (BJH) method (inset in Fig. 5). Thesemesopores can be attributed to the space betweennanosheets in the hierarchical microspheres, which isconsistent with the SEM and TEM observations.

The photocatalytic activity of as-prepared samples wasevaluated for RhB degradation under visible light irradia-tion. For comparison, a blank and a control sample werealso tested under the same reaction conditions. Fig. 6a–eshows temporal changes in the UV-Vis spectrum of theRhB solution during adsorption and photocatalytic degra-dation over BiOBr microspheres and the control sample.For BiOBr160, the wavelength corresponding to maximumabsorbance (λmax) gradually shifted from 554 nm towards

r samples prepared at (a) 120 1C, (b) 140 1C, (c) 160 1C, and (d) 180 1C, andn equilibrium. (f) Comparison of the photocatalytic activity of the samples

B. Chai et al. / Materials Science in Semiconductor Processing 23 (2014) 151–158156

shorter wavelength and finally reached 498 nm duringvisible light illumination; this was accompanied by agradual decrease in maximum absorbance (Fig. 6c). A blueshift of λmax is frequently observed during oxidation of RhBover multimetal oxide photocatalysts, and is associatedwith stepwise removal of N-ethyl groups during degrada-tion of RhB (N,N,N',N'-tetraethyl rhodamine) [16,20,22].The characteristic λmax for RhB and de-ethylated rhoda-mine species are 554 nm (RhB), 539 nm (N,N,N'-triethylrhodamine), 522 nm (N,N'-diethyl rhodamine), 510 nm (N-ethyl 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 de-ethylated products (Fig. 6a). For the control sample andBiOBr140 and BiOBr180, λmax after irradiation for 60 minwasobserved at �522 nm, indicating the presence of N,N'-diethyl rhodamine (Fig. 6b,d,e). Fig. 6f compares thephotocatalytic activity of the samples for degradation ofRhB. The results indicate that BiOBr160 exhibited the bestadsorption capacity and photocatalytic performance.

The ability of BiOBr160 to mineralize RhB was evaluatedby monitoring changes in TOC (Fig. 7). The results suggestthat the rate of TOC removal is much slower than the rateof the photocatalytic decolorization for RhB. After 60 minof irradiation, the solution TOC decreased by approxi-mately 42%, implying that RhB molecules were degradedbut not completely mineralized to inorganic molecules.

The effect of solution pH on the photocatalytic degra-dation of RhB over BiOBr140 was evaluated for an initial pHrange of 2.5–7.5 by adjusting the initial pH with0.1 mol L�1 HNO3 and NaOH solutions. As shown inFig. S1, the photocatalytic activity increased with decreas-ing pH. This suggests that an acidic environment is morebeneficial for RhB degradation, in agreement with theliterature [10].

Photocatalytic degradation of MO and 4-CP undervisible light in the presence of different BiOBr sampleswas also investigated. The results shown in Figs. S2 andS3a–e indicate that the BiOBr catalysts cannot totallydegrade MO in 3 h. BiOBr160 showed the best adsorptionability but little degradation activity for MO. For 4-CP,BiOBr samples prepared at different temperatures showedextremely weak adsorption and degradation performanceduring photocatalysis, while the control sample exhibited

Fig. 7. Changes in TOC in the reaction system during photocatalyticdegradation of RhB over BiOBr microspheres prepared at 160 1C.

slight degradation activity. Thus, the BiOBr microspheresexhibit selective photocatalytic behavior, as their activitywas much higher for RhB than for MO or 4-CP photode-gradation under visible light.

According to DRS analysis, the lowest bandgap energywas observed for BiOBr180, indicating that this sample canabsorb more visible light. However, its photocatalyticactivity was weaker than that of the other samples(Fig. 6f). Thus, the variation in optical properties is not adominant factor affecting the photocatalytic activity of as-prepared BiOBr microspheres. It is well known that photo-catalytic activity is closely related to the adsorption abilityof catalysts [16]. Although the BiOBr microspheres hadsimilar specific surface areas, BiOBr160 showed the bestadsorption capacity for the organic dyes. This may bebecause it has more negative charges on its surface [23].RhB molecules form positively charged cations in aqueoussolution and thus can be adsorbed by BiOBr via electro-static interaction. In contrast, MO molecules form anionsin solution so they cannot be adsorbed by BiOBr. Sufficientdye adsorption has a positive effect on the photocatalysisefficiency.

The crystallinity, crystallite size, and surface defects ofcatalysts also affect their photocatalytic activity. Accordingto the XRD and SEM results, the crystallinity and crystallitesize of BiOBr microspheres improved with increasing TST.In general, better photocatalyst crystallinity and fewersurface defects lead to higher photocatalytic activity,whereas an increase in particle size can have a negativeeffect on photocatalytic performance. We can deduce thatBiOBr160 had appropriate crystallinity, crystallite size, andsurface defects, which increased the number of photo-generated electrons and holes available for participating inthe photocatalytic degradation of contaminants.

In part, photocatalytic activity is a function of thelifetime and trapping of photogenerated electrons andholes in the semiconductor. PL emission spectra are oftenused to study surface structures and excited states. Fig. 8shows PL spectra over the wavelength range 325–550 nmfor BiOBr microspheres prepared at different tempera-tures. Two emission peaks are evident at approximately410 and 490 nm. The first (main peak) is ascribed toemission for bandgap transition and the second to surfaceoxygen vacancies and defects [24]. The highest-intensity

Fig. 8. PL spectra for BiOBr samples prepared at different solvothermaltemperatures.

Fig. 9. Effects of reactive species on the photocatalytic degradation ofRhB over BiOBr microspheres prepared at 160 1C.

Fig. 10. Stability study for photocatalytic degradation of RhB over BiOBrmicrospheres prepared at 160 1C.

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main peak was observed for BiOBr120. A moderate decreasein PL intensity with increasing TST is evident. PL emissionmainly results from recombination of excited electronsand holes, and a lower PL intensity indicates a decrease inthe recombination rate. However, the least intense main PLpeak was observed for BiOBr180, which can be attributed toPL quenching by surface carbon species [25]. Interestingly,the lowest intensity for the emission peak ascribed tosurface defects was observed for BiOBr160, indicating thatthese microspheres had the fewest surface defects. Hence,surface recombination of photogenerated electrons andholes is greatly inhibited over BiOBr160, which enhances itsphotocatalytic activity. The PL results confirm thatBiOBr160 can effectively separate photogenerated elec-tron–hole pairs.

To clarify the reaction mechanism, isopropanol (IPA),triethanolamine (TEOA), and p-benzoquinone (BQ) wereintroduced as scavengers of hydroxyl radicals (dOH), holes(hþ), and superoxide radicals (dO2

�), respectively, to deter-

mine the effects of reactive species on photocatalyticdegradation 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 TEOAsignificantly suppressed the RhB degradation rate; BQ hada weaker inhibitory, and IPA had hardly any effect on RhBdegradation. The results confirm that hþ and dO2

�play a

more important role than that of dOH in photocatalyticdegradation of RhB. The CB and VB edge potentials ofBiOBr are important factors in understanding the degrada-tion of contaminants over these photocatalysts. Accordingto theoretical speculation, the CB and VB potentials overBiOBr are 0.27 and 3.19 eV, respectively [12]. Thus, the VBedge potential of BiOBr is more positive than the standardredox potential of dOH/OH� (1.99 eV), suggesting thatphotogenerated holes could oxidize OH� to dOH. However,production of dOH in the present system is almost impos-sible 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 directreaction with photogenerated holes. Therefore, photocata-lytic degradation of RhB over BiOBr is little effect in thepresence of the dOH radical scavenger (IPA). In addition,the CB edge potential of BiOBr (0.27 eV) is not negativeenough to reduce O2 to dO2

�because the single-electron

reduction potential of O2 is �0.046 eV [28]. However, thisdoes not exclude the formation of dO2

�by photogenerated

electrons via other photochemical reactions. We postulatethat dye photosensitization leads to the formation of dO2

�

radicals. RhB molecules adsorbed on BiOBr are regarded assensitizers and that visible light irradiation stimulates thegeneration of electrons that subsequently transfer to theCB of BiOBr and then react with O2 to form dO2

�radicals.

The results for photocatalytic degradation of RhB, MO, and4-CP confirm the dye photosensitization mechanism fordegradation over BiOBr microspheres. For RhB, the BiOBrmicrospheres show excellent adsorption and photocataly-tic activity. In comparison, the microspheres displayweaker adsorption capacity for MO, and thus their photo-catalytic 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-DyeðadsÞn ð2Þ

DyeðadsÞnþBiOBr-BiOBrðe�ÞþDyeðadsÞþ ð3Þ

BiOBrðe�ÞþO2-BiOBrþdO2� ð4Þ

BiOBrþhv-e�þhþ ð5Þ

Dyeþhþ ðdO2� Þ-products ð6Þ

To test the stability and reusability of the BiOBr micro-spheres for photocatalytic degradation of RhB, the cata-lysts were reused four times for photocatalysis under thesame reaction conditions. The results are shown in Fig. 10.The decolorization efficiency of BiOBr microspheresdecreased by only 6% after four cycles. Comparison ofXRD patterns reveals a high degree of similarity for theBiOBr catalyst recycled four times and the original catalyst(Fig. 11). There is no obvious deviation in peak location andthe slight decrease in peak intensity indicates a slightdecline in crystallinity after visible light irradiation. Theseobservations suggest that only slight photocorrosionoccurred during the photocatalytic reaction and that BiOBrmicrospheres have good photostability.

Fig. 11. XRD patterns for BiOBr prepared at 160 1C before and afterphotocatalytic recycling experiments.

B. Chai et al. / Materials Science in Semiconductor Processing 23 (2014) 151–158158

4. Conclusions

Hierarchical BiOBr microspheres with an average dia-meter of 1–4 mm were prepared via a one-pot EG-assistedsolvothermal process in the presence of a reactable IL. Theexperimental results indicate that the solvothermal tem-perature has important effects on the crystallite size,optical properties, adsorption capacity, and photocatalyticactivity of BiOBr microspheres. Among all the samples,BiOBr160 showed the best adsorption capacity and photo-catalytic activity for RhB degradation under visible lightillumination. This result can be attributed to appropriatecrystallinity, crystallite size, and surface defects. Investiga-tion of the photocatalytic mechanism demonstrated thathþ and dO2

�species play a key role. Moreover, results for

photocatalytic degradation of RhB, MO, and 4-CP con-firmed a photosensitization mechanism for degradationover BiOBr microspheres. These BiOBr hierarchical micro-spheres hold promise as efficient photocatalysts for thedegradation of organic dyes and in other applications.

Acknowledgment

This work was supported by the National NaturalScience Foundation of China (No. 51302200).

Appendix A. Supplementary Material

Supplementary data associated with this article can befound in the online version at http://dx.doi.org/10.1016/j.mssp.2014.02.021.

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