Science Bulletin 62 (2017) 546–553

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Article

Surface Na2CO3 etching induced activity enhancement of 2D BiOIphotocatalyst working under visible light

Lei Liang a, Jing Cao a,b,⇑, Haili Lin a,b, Shifu Chen a,⇑aCollege of Chemistry and Materials Science/Information College, Huaibei Normal University, Huaibei 235000, ChinabAnhui Key Laboratory of Energetic Materials, Huaibei 235000, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 13 February 2017Received in revised form 5 March 2017Accepted 7 March 2017Available online 18 March 2017

Keywords:2D BiOIIn situ conversionPhotocatalysisSurface charge separation

http://dx.doi.org/10.1016/j.scib.2017.03.0122095-9273/� 2017 Science China Press. Published by

⇑ Corresponding authors.E-mail addresses: caojing@mail.ipc.ac.cn (J. Cao), ch

The visible light photocatalytic activity of two-dimensional (2D) BiOI microplates was intensivelyenhanced through simply dipping in Na2CO3 solutions at room temperature. The X-ray powder diffrac-tion (XRD) and scanning electron microscopy (SEM) investigations suggest that little amount of(BiO)2CO3 phase was formed on the surface of 2D BiOI via the in situ chemical conversion. The concen-tration of Na2CO3 solutions affected the structure, morphology, light absorption and surface elementcomponent of 2D BiOI. The surface loaded (BiO)2CO3 mainly trapped the photoinduced electrons ofBiOI, improved the separation efficiency of photocharges and finally raised the photocatalytic activityof BiOI under visible light (k > 420 nm). Furthermore, the product of the as-prepared (BiO)2CO3/BiOIdisplayed excellent stability in the repeated experiment. This study provides a facile way to improvethe photocatalytic activity of BiOX (X = Cl, Br, I) by means of surface treatment with Na2CO3 solutions.

� 2017 Science China Press. Published by Elsevier B.V. and Science China Press. All rights reserved.

1. Introduction

In recent years, semiconductor photocatalysis has been widelyapplied for contaminant removal, hydrogen production and solarenergy conversion [1–3]. Outstanding photocatalytic activity isconsidered as one of the basic characteristics of a photocatalyst.Due to the unavoidably fast recombination of photocharges, how-ever, weak photocatalytic activity of photocatalyst must be largelyenhanced via various strategies.

In general, many factors remarkably affect the photocatalyticactivity of photocatalysts, such as crystal phase [4], morphology[5], size [6], crystal face [7,8], energy band structure [9], architec-tures [10,11] etc. Because the photocatalytic reaction mainly hap-pens on the surface of semiconductor photocatalysts, the surfaceproperty was undoubtedly considered as the crucial factor to thephotocatalytic activity of photocatalysts [12]. For example, whena stable photocatalyst was treated with acidic or basic solutions,the changed surface charges and OH contents will influence theactivity ultimately [13]. However, as for an unstable photocatalyst,it will react with the acid/base to generate product. Especially, ifcontrolling the reaction degree incomplete, the newly formed solidproduct may assemble with the residual matrix to construct acomposite photocatalyst. This can be regarded as a special kind

Elsevier B.V. and Science China Pr

shifu@chnu.edu.cn (S. Chen).

among the ion-exchange strategy used for the composite prepara-tion [14]. As for the extensive Bi-based semiconductors family[15–18], many of them could transform to each other in acid/basesolutions. For example, Bi2O3 and (BiO)2CO3 could be converted toBiOX (X = Cl, Br, I) in haloid acids [19,20]. In return, BiOI could bechanged to Bi5O7I in NaOH solution [21]. In summary, the incom-plete reaction during surface treatment of the photocatalysts withacid/base provides potential way to enhance the activity of thematrix via constructing highly efficient composite photocatalysts.

In BiOX (X = Cl, Br, I) system, the narrow band gap endows BiOIexcellent visible light absorption but results in fast photochargerecombination [22,23]. Considering the abovementioned intensivesurface effect, it is expected to enhance the activity of BiOI throughdipping it in basic solution. As reported, flower-like BiOI could besuccessfully transform to flower-like (BiO)2CO3 in basic urea solu-tion, as a result, the treated BiOI displayed enhanced activity forrhodamine B and bisphenol A degradation [24]. However, theactivity improvement mechanism was still not clear enough. Thus,it is vital important to clarify the photocharge separation pathwayfor the alkali treated BiOI system.

In this paper, two-dimensional (2D) BiOI microplate wasselected as substrate and simply treated with common Na2CO3

solution. The phase, morphology, light absorption, surface elementchanges of 2D BiOI with altering the concentration of Na2CO3 solu-tion were investigated. The enhanced photocatalytic activity formethyl orange (MO) degradation over the treated 2D BiOI was

ess. All rights reserved.

L. Liang et al. / Science Bulletin 62 (2017) 546–553 547

ensured by the transient photocurrent, surface photovoltage andphotoluminescence spectra. This is a facile way to improve theactivity of BiOX (X = Cl, Br, I) family via simple surface treatmentwith Na2CO3 solutions.

2. Experimental

2.1. Preparation of samples

All chemical reagents used in this experiment were of analyticalgrade without further purification. Deionized water was appliedthroughout this study.

The 2D BiOI microplate was synthesized by a room directhydrolysis method [25]. A 1.358 g Bi(NO3)3�5H2O solid powderwas added into 20 mL (0.140 mol/L) KI solution with magnetic stir-ring. After sonicating 2 min and further magnetically stirring 5 h atroom temperature, the BiOI precipitate was filtrated, washed withdeionized water and dried at 60 �C for 8 h.

Surface treatment of 2D BiOI with Na2CO3 solutions was alsocarried out at room temperature. A 1.0 g as-prepared 2D BiOImicroplate was homogeneously dispersed in 70 mL Na2CO3 solu-tion (1.9 mmol/L). After magnetically stirring 5 h, the resultingsolid product was filtrated, washed with deionized water and driedat 60 �C for 8 h. The product was named as S1, in which the theo-retical exchange ratio of CO3

2� to I� was 5%. Through adjusting theconcentrations of Na2CO3 solutions (1.9–7.5 mmol/L), the 2D BiOItreated with Na2CO3 solutions were expressed as S2, S3 and S4(the theoretical exchange ratio of CO3

2� to I� was 10%, 15% and20%). In addition, the complete reaction product S5 was also fabri-cated by dipping 2D BiOI microplate in excess of Na2CO3 solution.

2.2. Characterization

The X-ray powder diffraction (XRD) were carried out using aBRUKER D8 ADVANCE X-ray powder diffractometer with Cu Karadiation (k = 1.5406 Å) at 40 kV and 40 mA. Energy dispersive X-ray spectroscopy (EDS) was obtained on a FEI Sirion200 scanningelectron microscope equipped with Oxford instruments INCA X-act detector. The morphologies were observed by scanning elec-tron microscopy (SEM) using a FEI Sirion200 scanning electronmicroscope. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were used to further analyzethe morphology and crystallinity of the products on a JEOL-2011transmission electron microscope with an accelerating voltage of200 kV. X-ray photoelectron spectroscopy (XPS) measurementwas performed on a Thermo ESCALAB 250 with Al Ka(1,486.6 eV) line at 150 W. UV–vis diffuse reflectance spectra

Fig. 1. (Color online) (a) XRD patterns and (b)

(DRS) were recorded with a TU-1901 UV–vis spectrophotometerequipped with an integrating sphere attachment. The photolumi-nescence (PL) spectra were measured on a JASCO FP-8300 spec-trofluorometer with 365 nm excitation wavelength.

2.3. Surface charge transfer properties investigation

The surface photovoltage (SPV) spectroscopy was measured onTLS-SPV530 spectrometer (Zolix instruments Co., Beijing, China).Monochromatic light was obtained by passing light from a500W Xe lamp through a double prism monochromator. The slitwidth of entrance and exit is 1 mm. A lock-in amplifier, synchro-nized with a light chopper was employed to amplify the photovolt-age signal. The range of modulating frequency is from 20 to 70 Hz.The spectral resolution is 1 nm.

The photocurrent properties of the samples weremeasuredwithan electrochemical workstation (CHI660E, Chenhua InstrumentsCo. Shanghai, China) in a standard three-electrode configurationwith a working electrode, a counter electrode (a platinum wire)and a reference electrode (Ag/AgCl, 3.0 mol/L KCl). Phosphate buf-fered saline aqueous solution constituted by 0.1 mol/L Na2HPO4

and 0.1 mol/L NaH2PO4 was employed as the electrolyte. A 500WXe lamp with a 420 nm cut-off glass filter was applied as lightsource and a low-temperature water bath was used to provide cir-culated cooling water.

2.4. Photocatalytic activity evaluation

The photocatalytic activities of the samples were investigatedby degrading methyl orange (MO) aqueous solution under visiblelight (k > 420 nm). Typically, 0.10 g photocatalyst was dispersedinto 50 mL of MO solution (10 mg/L) and stirred magnetically for30 min in the dark to acquire adsorption–desorption equilibrium.At intervals of 1 h, about 2 mL of suspension was taken out, cen-trifuged and analyzed by 722 s spectrophotometer (ShanghaiMetash Instruments Co., Ltd, China) with deionized water as a ref-erence sample. The testing wavelength was set as 464 nm.

3. Results and discussion

3.1. XRD analysis

Fig. 1a shows the XRD patterns of the as-synthesized samples.Sharp diffraction peaks indexing as (001), (002), (102), (110),(004), (200), (114) and (212) were found in pure 2D BiOI plate,corresponding to the tetragonal phase BiOI (JCPDS No. 10-0445).After dipping in Na2CO3 solution, the characteristic peaks of BiOI

EDS spectra of BiOI and S1�S5 samples.

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plate gradually decreased meanwhile those of newly-formedtetragonal phase (BiO)2CO3 (JCPDS No. 25-1464) in S1–S4 samples,such as (101), (103) and (110) peaks, strengthened with increas-ing the concentrations of Na2CO3 solutions. That is, partial BiOIreacted with Na2CO3 and transformed to be (BiO)2CO3 phase. Dueto the low concentrations of Na2CO3 solutions, only a little amountof (BiO)2CO3 was formed on the surface of 2D BiOI plates. In partic-ular, excess of Na2CO3 solution completely changed 2D BiOI into(BiO)2CO3 (S5 sample) via replacing I� by CO3

2�. In S1�S4 samples,the surface loaded (BiO)2CO3 will affect the activity enhancementof 2D BiOI.

In the EDS spectra (Fig. 1b) of 2D BiOI sample, Bi, O and I ele-ments were found. Differently, extra C element was discovered inS1�S4 samples, suggesting that (BiO)2CO3 was generated in theetching process. But only Bi, O and C elements existed in the S5sample, indicating 2D BiOI completely reacted with enough Na2-CO3. The EDS result was consistent with the XRD characterization.Based on the EDS spectra, the actual contents of (BiO)2CO3 on thesurface of BiOI were calculated to be 3.0%, 6.3%, 13.1% and 17.6%for S1, S2, S3 and S4 samples.

3.2. SEM and TEM analysis

Fig. 2 presents the morphology evolution of 2D BiOI withincreasing the concentration of Na2CO3 solutions. The pure BiOI(Fig. 2a) was of typical 2D microplate structure with smooth andclean surface. Differently, when BiOI was dipped in Na2CO3 solu-tions, the surface of 2D BiOI became rough owing to the in situ for-mation of little amount of (BiO)2CO3 nanoparticles (Fig. 2b�d). Asincreasing the concentrations of Na2CO3 solutions, a loose (BiO)2-CO3 layer was generated and covered on the surface of BiOI plate(Fig. 2e). If excess of Na2CO3 solution was used, the pure (BiO)2CO3

microplate was obtained (Fig. 2f) at last. To illustrate the formationof (BiO)2CO3/BiOI heterojuction interface in S1�S4 samples, S3sample was further measured by TEM and HRTEM. Fig. 2g clearlyshows that many (BiO)2CO3 nanoparticles were closely depositedon the surface of 2D BiOI nanosheets. The HRTEM image (Fig. 2h)

Fig. 2. (Color online) SEM images of BiOI (a), S1 (b), S2 (c), S3 (d), S4 (e) and S5 (f). TEM (g)BiOI dipped in Na2CO3 solutions.

displays that, there are two sets of different lattice images with dspacing of 0.150 and 0.193 nm, corresponding to (204) and(200) faces of BiOI and (BiO)2CO3, respectively. The above resultsconfirm that the surface formed (BiO)2CO3 and the residual 2D BiOIconstructed (BiO)2CO3/BiOI heterojuction interface via simple Na2-CO3 etching process.

According to the abovementioned results, the chemical reactionprocess from BiOI to (BiO)2CO3 was proposed in Fig. 2i. Firstly, 2DBiOI microplate was prepared via Ostwald ripening pathway in theprocess of direct hydrolysis. After adding Na2CO3 solution, the baresurface of BiOI was slowly corroded by Na2CO3 solution (2BiOI+ Na2CO3 ? (BiO)2CO3 + 2NaI). In other words, I� ions in BiOI wasreplaced by CO3

2� ions. Through controlling the concentration ofNa2CO3 solution, only a part of I� ions were substituted by CO3

2�

ions (S1�S4 samples). As a result, the newly-formed (BiO)2CO3

nanoparticles were tightly adhered to residual BiOI plates, whichled to the in situ construction of (BiO)2CO3/BiOI heterostructure.However, excess of Na2CO3 solution totally turned BiOI to (BiO)2-CO3 (S5 sample) instead.

3.3. DRS analysis

The light absorption spectra of 2D BiOI treated with differentamount of Na2CO3 solutions were presented in Fig. 3a. The absorp-tion edge of pure BiOI located at about 673 nm while that of S1�S4samples blue-shifted gradually due to the formation of littleamount of (BiO)2CO3 nanoparticles. As for the S5 sample, theabsorption edge was merely situated at 400 nm. Little amount of(BiO)2CO3 on the surface of BiOI ensures the excellent visible lightabsorption of S1�S4 samples.

According to Eq. (1) [26,27], the corresponding band gap ener-gies (Eg) of the samples were estimated as following:

ahv ¼ Aðhv � EgÞn=2; ð1Þwhere a, h, m and A stand for the absorption coefficient, Planck con-stant, the light frequency and a constant, respectively. In addition, nis determined by the type of optical transition of a semiconductor.

and HRTEM (h) images of S3. (i) Schematic diagram of the morphology change of 2D

Fig. 3. (Color online) (a) UV–vis diffuse reflectance spectra and (b) the band gapenergies of BiOI and S1�S5 samples.

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As previous literatures reported, the n values of both BiOI and(BiO)2CO3 were 4 [28,29]. Therefore, the corresponding Eg valuesof BiOI and (BiO)2CO3 (S5 sample) were determined to be 1.72and 2.92 eV from plots of (ahv)2/n versus energy (hv), respectively.

Furthermore, the band edge positions of BiOI and (BiO)2CO3

were calculated using the following empirical formula [30,31]:

Fig. 4. (Color online) XPS survey spectra of S3 sample:

EVB ¼ X � Ee þ 0:5Eg; ð2Þ

ECB ¼ EVB � Eg; ð3Þwhere EVB is the valence band (VB) edge potential, ECB is the con-duction band (CB) edge potential, X is the electronegativity of thesemiconductor and Ee is the energy of free electrons on the hydro-gen scale (about 4.5 eV). As a result, the EVB and ECB of BiOI were cal-culated to be 2.31 and 0.59 eV, meanwhile those of (BiO)2CO3 were3.30 and 0.38 eV, separately.

3.4. XPS analysis

To investigate the element variety of the as-prepared S1�S4samples, the XPS investigation of the S3 sample was carried out.Fig. 4a displays that S3 sample had Bi, O, I and C peaks, suggestingthat both BiOI and (BiO)2CO3 phases coexisted. Furthermore, thehigh-resolution XPS spectra of every element were also displayedas follows. As shown in Fig. 4b, two peaks at 159.3 and 164.6 eVcould be assigned to Bi 4f7/2 and Bi 4f5/2 [32,33], respectively, whichis the characteristic of Bi3+. Besides, the I 3d peaks (Fig. 4c) at 619.6and 630.8 eV were attributed to I 3d5/2 and I 3d3/2 of I� ion in S3sample [34]. As can be seen from the O 1 s peak in Fig. 4d, the strongpeak at 530.9 eV was assigned to the Bi�O bonds in (Bi2O2)2+ slabsof S3 sample meanwhile the weak peak located at about 533.6 eVcould be originated from the hydroxyl groups adhered on the sur-face of S3 sample [35]. In addition, the C 1 s peak was split intotwo parts, as shown in Fig. 4e. The peak of 284.8 eV correspondedto CO3

2� ions in S3 sample, whereas the peak at 288.8 eV belongedto the adventitious hydrocarbon from the XPS instrument [36].

3.5. Photocatalytic activity and mechanism

The commonly used MO was selected to evaluate the activitychanges of 2D BiOI treated by Na2CO3 solution under visible light(k > 420 nm). Fig. 5a displays that pure 2D BiOI had weak activityowing to the fast recombination of photocarriers [37] meanwhilethe (BiO)2CO3 (S5 sample) revealed low activity due to its wideband gap energy [38]. Interestingly, the activities of all the 2D BiOIsamples treated by Na2CO3 solutionswere significantly enhanced incomparison with that of 2D BiOI. S3 sample degraded 90% of MOafter 5 h irradiation and possessed the best activity among the S1,S2, S3 and S4 composites. That is, the surface formed little amount

(a) survey, (c) Bi 4f, (c) I 3d, (d) O 1s, and (e) C 1s.

Fig. 5. (Color online) (a) Photocatalytic activity of 2D BiOI treated by Na2CO3 solution under visible light (k > 420 nm). (b) Degradation rate constant kapp values of the as-prepared samples. (c) The kapp values of S3 sample changed with scavengers and (d) separation pathway of photocharges over 2D BiOI after Na2CO3 solution etching.

550 L. Liang et al. / Science Bulletin 62 (2017) 546–553

of (BiO)2CO3 exhibited significant positive effect on the activity of2D BiOI. According to the Langmuir-Hinshelwood kinetics model[39], the apparent pseudo-first-order rate constants (kapp, h�1) ofMO degradation were calculated and shown in Fig. 5b. The kapp val-ues first increased and then decreased, in which S3 sample had thefastest degrading speed of 0.396 h�1. The concentrations of Na2CO3

solutions influenced the yield of (BiO)2CO3, which further deter-mined the activity of (BiO)2CO3/BiOI composites.

In addition, the probable reactive species generated for MOdegradation in the reaction system were also investigated by add-ing various radical scavengers. Benzoquinone (BQ) [40], ammo-nium oxalate (AO) [41] and tert-butyl alcohol (TBA) [42] wereapplied for trapping �O2

�, h+ and �OH radicals, respectively. In Fig. 5c,AO and BQ remarkably decreased the kapp value of S3 sample com-pared to the reference condition, illustrating that h+ and �O2

� radi-cals as the main reactive species effectively degraded the MOmolecules in the photocatalytic process.

Fig. 6. (Color online) Photocatalytic activity of (a) BiOCl and (b) BiOBr treated by Na2CO3

the samples.

In order to explain the activity enhancement of 2D BiOI afterNa2CO3 solution etching, a schematic diagram of separation path-way of photocharges was proposed here, as shown in Fig. 5d.Under visible light, only 2D BiOI substrate was motivated to gener-ate photoinduced electrons and holes. Due to the energy bandstructure, the photoinduced electrons in the CB of BiOI werequickly trapped and accumulated on the surface of (BiO)2CO3

nanoparticles. Then, the electrons were captured by the surfaceadsorbed O2 to form reactive �O2

� working for eliminating MOmolecules. Simultaneously, the holes located on the VB of BiOIdegraded MO directly. In such a way, the photocarriers producedby BiOI were efficiently separated through the (BiO)2CO3/BiOIinterface.

Moreover, the effect of Na2CO3 etching on BiOCl and BiOBr wasalso measured. For instance, Fig. 6a displays that (BiO)2CO3/BiOCl(15%) (The theoretical exchange ratio of CO3

2� to Cl� was 15%)achieved activity enhancement in comparison with the corre-

solution under visible light (k > 420 nm). Inset is the corresponding XRD patterns of

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sponding BiOCl. The XRD patterns of product (BiO)2CO3/BiOCl(15%) obtained through etching BiOCl with Na2CO3 solutionensured that little amount of (BiO)2CO3 was produced in the treat-ment process. The similar result was also obtained for BiOBr trea-ted with Na2CO3 solution (Fig. 6b). The above results suggest thatsurface Na2CO3 etching is a universal approach to improve theactivity of BiOX (X = Cl, Br, I).

Fig. 7. (Color online) (a) SPV responses and (b) visible light (k > 420 nm) inducedtransient photocurrent change of BiOI, S1�S5 samples. (c) The photoluminescencespectra of BiOI and S3 irradiated by 365 nm light.

3.6. Surface charge transfer properties

The surface Na2CO3 etching induced activity enhancement of2D BiOI pushed us to illustrate the inner enhancement mechanismof photocharge separation efficiency. Thus, the surface photovolt-age (SPV) spectra, transient photocurrent spectra and photolumi-nescence spectra were investigated respectively.

Since the separation of photocharges happened in space, theSPV signal was produced correspondingly [43]. As shown in Fig. 7a,all the samples had distinct positive SPV responses, revealing thatthe photogenerated electrons transferred to bulk of photocatalystwhile holes to surface oppositely [44]. The 2D BiOI substrate dis-played strong SPV signals in the visible light region due to itsextensive visible light absorption. In addition, the peak locationcorresponded to band-to-band transition of BiOI. Differently, thefinal product (BiO)2CO3 (S5 sample) only exhibited weakSPV response owing to its large band gap energy. After in situ gen-erating (BiO)2CO3 nanoparticles on the surface of 2D BiOI after Na2-CO3 etching, the SPV response significantly reduced. The probablereason is that the (BiO)2CO3 nanoparticles acting as electron accep-tor offset a part of positive photoelectric signal and finally led tothe reduction of SPV signal for (BiO)2CO3/BiOI. In this process,the separation efficiency of (BiO)2CO3/BiOI interface mainlydepended on the loading amount of (BiO)2CO3 nanoparticles. Whenthe loading amount of (BiO)2CO3 was low, the size of (BiO)2CO3

was very small. The weak capture ability led to little decrease inSPV signal. However, too much amount of (BiO)2CO3 in S4 samplewould result in the generation of large sized (BiO)2CO3, which willreduce the capture ability of (BiO)2CO3 instead and then displayedhigher SPV signal than that of S3 sample. Therefore, S3 sampledisplayed the lowest SPV intensity as well as the highestphotocatalytic activity.

In general, higher photocurrent intensity suggests higher sepa-ration efficiency of photocarriers as well as higher photocatalyticactivity [45]. The photocurrent signals (Fig. 7b) of S1�S4 sampleswere higher than that of BiOI substrate, indicating that the separa-tion efficiency of photocarriers was significantly improved. Espe-cially, S3 sample displayed the maximal photocurrent intensity.Namely, the surface Na2CO3 etching induced activity enhancementof 2D BiOI can be exactly explained by the inner enhancement ofphotocharge separation efficiency owing to the formation of(BiO)2CO3/BiOI interface. Furthermore, the lower photolumines-cence intensity of S3 sample than that of BiOI (Fig. 7c) alsoreflected the enhanced photocharge separation efficiency [46].

3.7. Stability

The stability of photocatalyst is vital for the practical applica-tion, so the cycling experiment of S3 sample was carried out viadegrading MO for 5 times under visible light (k > 420 nm). Fig. 8ashows that S3 sample still kept excellent activity for MO degrada-tion in the repeated experiment. Furthermore, the crystal structureof the used S3 sample had no obvious change compared to that offresh one (Fig. 8b). The above result indicates that the 2D BiOI sam-ple etched by Na2CO3 solution possessed outstanding stability and

could be considered as a potential candidate in the practicalapplication.

4. Conclusion

Through simple Na2CO3 solution treatment, the 2D BiOI micro-plates largely enhanced its visible light activity for methyl orangedegradation. The surface generated (BiO)2CO3 nanoparticles onthe 2D BiOI microplates acted as electron trapper to efficiently sep-arate the photocharges on 2D BiOI. The concentrations of Na2CO3

solutions controlled the extent of reaction from BiOI to (BiO)2CO3

and finally determined the activity of the products. This studyhad shied light on increasing the activity of BiOX (X = Cl, Br, I)via facile surface basic treatment.

Fig. 8. (Color online) (a) Cycling runs of S3 sample for the degradation of MO under visible light (k > 420 nm) and (b) corresponding XRD patterns of the fresh and used S3sample.

552 L. Liang et al. / Science Bulletin 62 (2017) 546–553

Conflict of interest

The authors declare that they have no conflict of interest.

Acknowledgments

This work was supported by the National Natural Science Foun-dation of China (51472005, 51272081), the Natural Science Founda-tion of Educational Committee of Anhui Province (gxyqZD2016413,gxyqZD2016414, and KJ2015A027), the Natural Science Foundationof Anhui Province (1708085MB32) and Innovation Team of Designand Application of Advanced Energetic Materials.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.scib.2017.03.012.

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