Facile microwave-assisted ionic liquid synthesis of sphere-like BiOBr hollow and porous nanostructures with enhanced photocatalytic performance

Accepted Manuscript Facile microwave-assisted ionic liquid synthesis of sphere-like BiOBr hollow and porous nanostructures with enhanced photocatalytic performance Zhigang Chen, Jie Zeng, Jun Di, Dexiang Zhao, Mengxia Ji, Jiexiang Xia, Huaming Li PII:

S2468-0257(16)30108-X

DOI:

10.1016/j.gee.2017.01.005

Reference:

GEE 50

To appear in:

Green Energy and Environment

Received Date: 28 November 2016 Revised Date:

20 January 2017

Accepted Date: 22 January 2017

Please cite this article as: Z. Chen, J. Zeng, J. Di, D. Zhao, M. Ji, J. Xia, H. Li, Facile microwaveassisted ionic liquid synthesis of sphere-like BiOBr hollow and porous nanostructures with enhanced photocatalytic performance, Green Energy & Environment (2017), doi: 10.1016/j.gee.2017.01.005. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Facile microwave-assisted ionic liquid synthesis of sphere-like BiOBr hollow and porous nanostructures with

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enhanced photocatalytic performance

Zhigang Chen1,3*, Jie Zeng1, Jun Di2, Dexiang Zhao2, Mengxia Ji2, Jiexiang Xia2,*, Huaming Li2,*,

Key Laboratory of Modern Agriculture Equipment and Technology, Ministry of Education,

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School of Environment and Safety Engineering,2 School of Chemistry and Chemical Engineering,

Jiangsu University, Zhenjiang, 212013, P R China *Corresponding author: Tel: +86-511-8879108; Fax: +86-511-88791108;

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E-mail address: [email protected]; [email protected]

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Abstract In this work, two kinds of self-assembled hierarchical BiOBr microcrystals were

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rapidly synthesized through a simple microwave-assisted route in the presence of reactable ionic liquid 1-hexadecyl-3-methylimidazolium bromide ([C16mim]Br). These porous and hollow BiOBr microspheres were obtained via a facile solvothermal method with or without Polyvinyl Pyrrolidone (PVP), respectively. During the

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synthetic process, ionic liquid [C16mim]Br played as solvent, reactant and template during the synthetic process at the same time. Moreover, the BiOBr hollow and

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porous microspheres exhibited outstanding photocatalytic activities for the degradation of Rhodamine B (RhB) under visible light irradiation. A possible photocatalytic mechanism was also discussed in detail. It can be assumed that the higher photocatalytic activities of BiOBr porous microspheres materials could be ascribed to the novel structure, larger specific surface area, narrower band gap

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structure and smaller particle size.

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Keywords: BiOBr; Photocatalytic; Ionic liquid; Microwave

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1. Introduction Semiconductor photocatalysis has been regarded as an efficient, green and promising solution in solving global environment and energy problems[1-4]. As significant Bi III–VIA–VIIA ternary semiconductor compounds[5], bismuth oxyhalides

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BiOX (X = Cl, Br, I) belong to new types of prospective layer material for photocatalytic energy conversion and environmental decontamination considering their excellent physicochemical properties and low cost[6-8]. Among these

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considerable properties of BiOX material, the open crystalline structures, the indirect-transition band-gap and the layered structure have attracted increasing

morphologies

of

BiOX

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attention of researchers in the field of photocatalysis application[9-11]. So far, a various nano/micro-structures,

including

nanoplates[12],

[13]

,

nanobelts[14] and microspheres[15], [16], have been synthesized by numerous approaches to maximize their potency of photocatalytic degradation of pollutants. Among the three kinds of BiOX, it is well acknowledged that BiOCl has the

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largest band gap (Eg=3.4 eV), which makes its main light response range lie in the UV region. The smallest band gap (Eg=1.8 eV) of BiOI means high recombination rate of photo generated electrons and holes, which greatly limits their applications[17–19]. The bismuth oxybromide (BiOBr) semiconductor, shows the best

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photocatalytic oxidation and reduction activity under full light spectrum irradiation due to the befitting p-type indirect bandgap (Eg=2.8 eV)[20], which makes it a hotspot

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to a great deal of researchers in recent years. Besides the intrinsic crystal structure, the photocatalytic performance of a specific semiconductor is closely related to its particle size, morphology and dimensionality[4],

[21]

. While these parameters

significantly depend on their synthesis routes[22-24]. For example, Xiang et al. prepared microsphere structure Bi2WO6/BiOI heterojunction photocatalyst via a chemical etching method[25]. Zhang et al. synthesized BiOBr nanosheets via a hydrolysis process exhibiting a selective visible-light photocatalytic behavior as the activity over RhB[26]. Feng et al. synthesized mesoporous BiOBr 3-D microspheres with remarkably high photocatalytic activity in ethanol-mediated condition[27]. Guo et al. 3

ACCEPTED MANUSCRIPT employed a facile in situ crystallization approach at room temperature to synthesized core-satellites structured BiOBr-CdS highly efficient photocatalyst[28]. Huang et al. developed a facile room-temperature precipitation method to prepare multiple heterojunctions with tunable photocatalytic reactivity in full-range BiOBr−BiOI

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composites[5]. Though these BiOBr photocatalysts synthesized via above conventional routes have considerable photocatalytic activities, these synthesis methods are time-consuming for researchers especially in the field of synthetic chemistry where trial-and-error experiments take much of their energies. Encouragingly, microwave

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heating has pushed the limits of fast chemical reactions time in minutes since microwave heating process is able to heat target molecules efficiently without heating [30]

. Therefore, bottom-up microwave-assisted method offers

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the entire reactor[29],

many unique capabilities, for instance, relatively higher reaction rate, higher yield and energy saving [31], [32].

As the novel green media, ionic liquids have been frequently reported in literature recently[33-37]. The importance of ionic liquids in the field of inorganic

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materials synthesis has been realized in terms of their excellent properties such as high thermal stability, wide temperature range for liquid state, low interfacial tension and high ionic conductivity[38], [39]. Moreover, many ionic liquids, especially those

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based on imidazolium and quaternary ammonium salts, are chemically very similar to the types of organic cations that are commonly used as structure-directing agents or templates in the preparation of unique morphology of inorganic materials[40],

[41]

.

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Furthermore, ionic liquids are excellent solvents for absorbing microwaves irradiation because of their large numbers of positive ions with high polarizability and ionic conductivity. Therefore, the use of microwave heating in ionic liquids for the synthesis of ideal inorganic materials has apparent advantages over other solvents[42], [43]

. So far, many various morphologies of inorganic materials prepared by ionic liquid

assisted microwave synthesis route have been reported, including N–B–F-tri-doped TiO2[43], nanoparticle sizes Gd4F3[44], g-C3N4/BiOBr porous microspheres[45], hexagonal platelet-like Bi2Te3 crystals[46], CuS quantum dots[47] and so on. 4

ACCEPTED MANUSCRIPT Based on above studies, a novel ionic liquid assisted microwave synthesis route for the fast controlled synthesis of hollow and porous sphere-like BiOBr within 20 min has developed. It can be demonstrated that the ionic liquid [C16mim]Br had a significant influence on the morphology of BiOBr and played important roles as

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solvent, reactant, template and microwave-absorbing agent at the same time. The obtained hollow and porous BiOBr both have satisfactory photocatalytic activities in the degradation of RhB under visible light. This method has some obvious advantages: the process is fast, high yield and environmental friendly; the reaction can be

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performed under atmospheric pressure in a microwave oven; the morphology of BiOBr can be easily controlled. It is believed that this environmental friendly route

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can be developed into a general way to synthesize other nanomaterials.

2. Experimental section

2.1. Material and sample preparation

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Bismuth nitrate [Bi(NO3)3·5H2O], polyvinylpyrrolidone(PVP K30), ethylene glycol (EG) and absolute ethanol were of analytical grade and used without further purification.

The

ionic

liquid

1-hexadecyl-3-methylimidazolium

bromide

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([C16mim]Br, 99%) was purchased from Shanghai Chengjie Chemical Co., Ltd. 2.2. Preparation of hollow BiOBr microspheres

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Microwave oven (SINEOMAS-I) was used to perform synthesis of BiOBr

hollow microspheres, the details were as follows: 1 mmol of Bi(NO3)3·5H2O was dissolved into 20 ml EG solution containing stoichiometric amounts of ionic liquid [C16mim]Br. After stirring for 30 min, the mixed homogeneous solution was transferred to a 100 mL microwave reaction bottle in the microwave reactor and heated for 20 min at 160 °C and then cooled down to room temperature. Temperature was monitored by an infrared temperature sensor. The final product was separated by centrifugation, washed with distilled water and absolute ethanol for several times respectively, and dried under vacuum at 50 °C for 12 h before further 5

ACCEPTED MANUSCRIPT characterizations. 2.3. Preparation of porous BiOBr nanospheres architectures The porous BiOBr nanospheres were prepared similarly via Microwave-assisted

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synthesis procedure, the details were as follows: 1 mmol of Bi(NO3)3·5H2O was dissolved into 20 mL EG solution containing stoichiometric amounts of ionic liquid [C16mim]Br and 0.1 g polyvinylpyrrolidone (PVP K30). After stirring for 30 min, the mixed homogeneous solution was transferred to a 100 mL microwave reaction bottle

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in the microwave reactor and heated for 20 min at 160 °C and then cooled down to room temperature. The final product was separated by centrifugation, washed with

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distilled water and absolute ethanol for several times respectively, and dried under vacuum at 50 °C for 12 h before further characterizations. 2.4. Characterization

X-ray powder diffraction (XRD) analysis was carried out on a Shimadzu

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XRD-600 X-ray diffractometer with high-intensity Cu-Kα (λ = 1.54 Å) radiation. Structural information for the samples was obtained via a Fourier transform spectrophotometer (FT-IR, Nexus 470, Thermo Electron Corporation) by using the

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standard KBr disk method. The field emission scanning electron microscopy (SEM) measurements were carried out with a field-emission scanning electron microscope (JEOL JSM-7001F) equipped with an energy-dispersive X-ray spectroscope (EDS)

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operating at an acceleration voltage of 10 kV. Transmission electron microscopy (TEM) micrographs were obtained using a JEOL-JEM-2010 microscope (JEOL, Japan) operating at 200 kV. The nitrogen adsorption–desorption isotherms at 77 K were investigated using a TriStar II 3020 Surface Area and Porosity Analyzer (Micromeritics Instrument Corporation, USA). Diffuse reflectance spectra (DRS) was measured in the range of 200 to 800 nm by using a UV-vis spectrophotometer (Shimadzu UV-2450, Japan). BaSO4 was used as the reflectance standard material. Photocurrent measurements were performed on an electrochemical workstation (CHI 660B, Chenhua Instrument Company, Shanghai, China). 6

ACCEPTED MANUSCRIPT 2.5. Photocatalytic activity measurement Photocatalytic activity of hollow BiOBr and porous BiOBr samples was evaluated by the degradation of RhB under visible light irradiation. Experiments were carried out in a cylindrical Pyrex vessel (100 mL) by means of a 300 W Xe lamp with

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a UV cutoff filter (λ > 400 nm) as the visible light source. Aeration was performed using an air pump to ensure a constant supply of oxygen and full mixing of the solution. In a typical run, 0.02 g of BiOBr powder was dispersed into 100 mL of RhB (10 mg L−1) solution. Whereafter, the suspensions were magnetically stirred for 30

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min in the dark to establish an adsorption/desorption equilibrium. Then the Pyrex photocatalytic reactor was exposed to visible light irradiation with maximum

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illumination time up to 105 min. Furthermore, all experiments were performed at 30 °C via a circulating water system to prevent thermal catalytic effects. During every irradiation time interval (15 min), 3 mL suspension was sampled from the reactor cell, the photocatalyst powders were separated by centrifuge to obtain RhB supernatant liquid, which was analyzed with a UV-Vis spectrophotometer (UV-2450, Shimadzu)

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at the maximal absorption wavelength (553 nm) of RhB. 2.6. Photoelectrochemical measurements

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To investigate the transition of photogenerated electrons in hollow BiOBr and porous BiOBr materials, the photocurrents were measured with an electrochemical

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analyzer (CHI660B, Chen Hua Instruments, Shanghai, China) in a standard three-electrode system, which employed a platinum wire as counter electrode, a saturated Ag/AgCl electrode as the reference electrode, an indium tin oxide (ITO) glass as working electrode, respectively. Hollow BiOBr and porous BiOBr modified electrode were prepared by a simple casting method as follows: 5 mg of the as-prepared sample was dispersed in 0.5 mL ethanol and 0.5 mL EG to produce a suspension, in which 20 µL of the resulting colloidal dispersion was then dip-coated onto a fixed area (0.5 × 1 cm2) of ITO glass electrode and dried under oven at 55 °C for 8 h. A 500 W Xe arc lamp was utilized as the photosource. The electrolyte solution 7

ACCEPTED MANUSCRIPT for the photocurrent measurements was phosphate buffered saline (0.1 mol L−1, pH = 7.0).

3 Results and discussion

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3.1 XRD analysis

The crystal structure of the hollow BiOBr and porous BiOBr microspheres were ascertained by powder XRD instrument and showed in Fig. 1. All the diffraction

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peaks in the patterns can distinctly indicate that two kinds of samples both possess pure phase and tetragonal structure of BiOBr with lattice parameter a = b = 3.915 Å, c

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= 8.076 Å, which were consistent with the reported values (JCPDS Card No.73-2061) .The good crystallinities of the BiOBr samples were directly proved with the intense and narrow diffraction peaks. No clear differences of XRD patterns were found between the two BiOBr samples, excepting the diffraction peaks of porous BiOBr samples synthesized with [C16mim]Br and PVP were slightly broader than the

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hollow BiOBr synthesized with [C16mim]Br, which was consistent with the reported data[48]. Moreover, the pseudo-average crystal size of the BiOBr samples were calculated based on Scherrer’s Equation D=Kλ/(βcosθ), where K is constant, λ is

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X-ray wavelength, β is a half high width of the diffraction peak and θ is diffraction angle. Based on the Scherrer’s Equation, the average grain sizes of porous and hollow BiOBr materials were determined to be ca. 12.8 and 16.0 nm, respectively. It can

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conservatively concluded that hollow and porous BiOBr samples consist of nano-sized crystal particles. It agreed well with the results observed with SEM as described later.

3.2 FT-IR analysis The FT-IR spectra of samples in the range of 480-4000 cm-1 were showed in Fig. 2. These spectra were taken so as to identify the functional groups in the synthesized sample. Generally, tetragonal BiOBr contains numerous alternating [Bi2O2]2+ layers and Br- layers[49], [50]. In the FT-IR spectra of as-prepared samples, the characteristic 8

ACCEPTED MANUSCRIPT peaks at 517 cm−1 and 720 cm-1 were attributed to the vibrations of Bi–O bonds in BiOBr, which were accordance with the stretching vibration of the bonds in tetragonal BiOX (X = Cl, Br and I)[51]. In addition, the peak at 1600 cm−1 was assigned to the bending vibrations of the free water molecules, and the broad absorption peak at about

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3300 cm−1 was associated with the O-H stretch of the intermolecular hydrogen bonds or molecular water[52]. It could be attributed to the absorbed H2O on the surface of the BiOBr materials. No characteristic peaks of imidazolium C–H stretching of the ionic liquid were observed in FT-IR spectra. It can be assumed that the ionic liquid can be

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removed completely from the surface of the material by washing with deionized water

3.3 SEM ,TEM and EDS analysis

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and alcohol.

The morphology and microstructures of the BiOBr obtained by using ionic liquid [C16mim]Br microwave-assisted synthesis at 160 °C for 20 min were investigated by SEM and TEM observation. As shown in Figure 3a, these uniform and compact

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BiOBr microspheres with diameters ranging from ca. 1.0 to 1.5 µm could be observed. Higher magnification microscopy image (Fig. 3b) showed that each delicate sphere-like BiOBr was constructed of numerous agminated nanosheets with tens of

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nanometers in thickness. These nanosheets were highly directed to grow from one center (crystal nucleus) to all directions, developing into a self-assembled BiOBr spheroidal architectures. It can suggest that the individual nanosheets formed and simultaneously

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grown

and

were

connected

together

by

ionic

liquid

[C16mim]Br ,which had aggregation behaviors and constitution of micelles in solution[53], [54]. A representative TEM image (Fig. 3c, d) further confirmed that the obtained BiOBr product is a fluff spheroidal morphology. At the same time, it was interesting to find that there were dark periphery and relatively bright center of BiOBr microspheres in TEM image, confirming that as-prepared BiOBr samples were hollow microspheres structures. Similar findings were also reported in the previous literature[49], which concluded that ionic liquid [C16mim]Br played an essential part in the construction of hollow BiOBr microspheres since the BiOBr microspheres 9

ACCEPTED MANUSCRIPT synthesized with NaBr were not hollow structures[53]. Therefore, the ionic liquid [C16mim]Br was not solely the Br source for BiOBr microspheres but also acted as solvent and template, orienting the growth of BiOBr nanosheets into fluff sphere-like hollow architectures. Elemental dispersive spectrum (EDS) analysis of the hollow

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BiOBr microspheres was presented in Fig. 3e, and the characteristic peaks of Bi, O, Br could be observed and no other elements could be observed, in which the peak of the element Si is generated by the Si substrate. The atom ratio of Bi and Br on the surface of hollow BiOBr was 41.18 : 38.51, which was nearly around 1 : 1.

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The surface morphology of the porous BiOBr obtained by using [C16mim]Br and PVP (K30) microwave-assisted synthesis at 160 oC for 20 min was further

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investigated by SEM. Fig. 4a is the typical SEM image of the as-prepared BiOBr samples. It can be clearly seen that incompact flower-like BiOBr microspheres structures are formed, with an average diameter of ca. 1µm. As shown in the high-magnification SEM image (Fig. 4b), numerous BiOBr nanosheets with a thickness of ca. 5 nm aggregated and interweaved together to form the entire

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flower-like BiOBr microspheres .In addition, these interlaced nanosheets joined together to form plenty of irregular cavities on the surface of the BiOBr microspheres. It is well known that the introduction of surfactant, which can form micelles with

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ionic liquid, plays an important role in the formation of BiOBr porous structure. In this work, the PVP surfactant had successfully improved the control ability of the system by combining ionic liquid [C16mim]Br. It can conservatively speculate that the

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incompact porous structure of BiOBr nanospheres could increase the contact area with pollutants, the enhancement in the photocatalytic activities of porous BiOBr nanospheres may ensue. These conjectures agreed well with the results observed with photocatalytic degradation of RhB under the visible light irradiation as described later. The EDS pattern in Fig. 4c showed that the main elements in the porous BiOBr were Bi, O, Br. The atom ratio of Bi and Br on the surface of porous BiOBr was 40.44 : 35.63, it revealed that the atomic ratio of Bi : Br in the porous BiOBr sample was also approximately to 1 : 1, the result was similar with the hollow BiOBr, which were consistent with the XRD results. 10

ACCEPTED MANUSCRIPT In order to highlight the influence of microwave heating methods on the synthesis of BiOBr, the previous works[53] of hydrothermal-synthesized hollow and porous structures of BiOBr materials prepared at 140 °C for 24 h in conventional heating instrument (electric furnace) were used to make a contrast. It can be learned

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from this article’s time-dependent experiments results that BiOBr nanoplates were the only product after heating at 140 °C for 1 h reaction and hollow BiOBr microspheres were finally produced after 24 h reaction. In contrast, hollow BiOBr and porous BiOBr were produced by microwave-heating at 160 °C for only 20 min. The heating

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time for synthesis of hollow BiOBr and porous BiOBr can be shortened by more than two orders of magnitude by using microwave heating. Therefore, ionic liquid assisted

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microwave synthesis route had definite advantages, including high synthesis efficiency, significant energy and time savings. 3.4 Optical absorption properties

The optical properties of the hollow and porous BiOBr microcrystals were

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verified by UV-vis diffuse reflectance spectra (Fig. 5a), which revealed that the absorption edge of the samples extended to the visible-light region. This results indicated the possibility of high photocatalytic activity of these materials under visible-light irradiation. The band gaps of the BiOBr microcrystals can be determined

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by the following equation: α Ephoton = K (Ephoton - Eg)n/2, in which K is a constant, α is the absorption coefficient, Ephoton is the discrete photo energy , Eg is the band gap

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energy. Especially, n depends on the characteristics of the transition in a semiconductor (direct transition n = 1 and indirect transition n =4). It's easy to conclude that n value is 4 since the intrinsic feature of BiOBr is indirect transition[55]. The Eg value is obtained by the energy intercept of a plot of (αEphoton)1/2 vs. Ephoton. From the above the absorption onset of the hollow and porous BiOBr microcrystals were both at ca. 420 nm, corresponding to band-gap energy (Fig. 5b) about 2.3 eV and 2.25 eV, respectively. Moreover, the colours of both the hollow and porous BiOBr materials were white, and the corresponding picture was inset in DRS spectra. These results supported that both two kinds of BiOBr samples had suitable band gaps to be 11

ACCEPTED MANUSCRIPT activated by visible-light for photocatalytic decomposition of organic contaminants. It is well known that the narrow band gap could facilitate electronic transitions and excite easier production of active species[1], thus it can make the photocatalytic process more efficient. Wrap these all together, it can prudently speculate that porous

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BiOBr microcrystals could provide a more effective approach to enhance its photocatalytic reactivity. 3.5 BET analysis

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In order to investigate the surface properties of the porous and hollow BiOBr, the Brunauer-Emmett-Teller (BET) specific surface areas of the two samples were by

nitrogen

adsorption-desorption

isotherms.

The

nitrogen

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investigated

absorption-desorption isotherms of porous and hollow BiOBr were presented in Fig. 6, both of which were of type IV (BDDT classification). A hysteresis loop exhibited in this figure suggests the presence of mesopores on materials surfaces[56]. The BET specific surface areas of porous and hollow BiOBr were calculated to be 41.04 and

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23.81 m2/g, respectively. These results indicated that BiOBr with porous structure possessed larger surface area, which could contribute BiOBr to absorb more visible light and supply more adsorption sites and more active sites to fully contact with pollutants to achieve the enhanced photocatalytic activity. The existence of porous

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structure is benefical for BiOBr to obtain larger surface area to expose more absorption and active sites during the photocatalytic reaction, and then the higher

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photocatalytic activity could be obtained. It can be suggested that the porous BiOBr with larger surface area may achieve the improved degradation ability. 3.6. Photoelectrochemical properties To give further evidence to confirm and gain insight into the separation efficiency of photo-induced electrons and holes of hollow and porous BiOBr samples, a photocurrent measurement study was performed. The photocurrent versus time curves recorded from the photoanodes during repeated on/off illumination cycles at 0.3 V were shown in Fig.7. Both the hollow and porous BiOBr microcrystals 12

ACCEPTED MANUSCRIPT exhibited a prompt, reasonably reversible and stable photocurrent produced under each illumination, and the photocurrent quick disappeared in the dark. The photocurrent response is mainly generated by the fast separation and transportation of the photoinduced charges on the surface of the working electrodes[57]. In comparison

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with hollow BiOBr, porous BiOBr exhibited an increased current density than that of the hollow BiOBr, which meant that porous structure BiOBr can effectively reduce the recombination of photogenerated electrons and holes .It was also observed that slightly decay of the photocurrent of porous and hollow BiOBr microcrystals was

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trivial in duration of the experiment, indicating reasonable photostability of the BiOBr samples. As a result, compared with hollow BiOBr microcrystals, the inherent visible

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light activity in combination with enhanced conductivity makes porous BiOBr microcrystals more appropriate for a promising visible light driven photocatalyst. 3.7 Photocatalytic activity of BiOBr samples

As shown in Fig. 8, the photocatalytic degradation performance of various

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BiOBr samples were evaluated by the degradation of RhB under the visible light irradiation. After irradiation for 105 min, the blank test confirmed that the self-degradation of RhB can be neglected. Moreover, during the same irradiation time,

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hollow structure BiOBr showed the ability to partly degrade RhB. After irradiation for 15 min, 40% of RhB was photodegraded by hollow BiOBr material, then 95% of RhB was degraded by hollow BiOBr material after 105 min photocatalytic reaction.

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Furthermore, compared with hollow BiOBr, the porous BiOBr presented a significant improvement in RhB photocatalytic degradation under same reaction condition. The degradation efficiency of RhB has smoothly reached to 80% after photodegraded by porous BiOBr for 15 min, and then RhB was almost completely degraded in 60 min. These results suggested that the porous and hollow structures BiOBr microspheres synthesized with ionic liquid [C16mim]Br showed outstanding photocatalytic activity on the degradation of RhB. The porous BiOBr microspheres synthesized with ionic liquid [C16mim]Br and PVP showed higher photoactivities for its smaller band gap and larger contact area. These results consist with the reports published formerly[48], 13

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. Moreover, compared the activity of the prepared materials with different

conventional methods (such as ionic liquids-assisted hydrothermal method and hydrolysis process) in these literatures[13,53,59], the porous BiOBr synthesized via microwave-assisted route presented superior photocatalytic activity. In summary,

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BiOBr material prepared by the microwave-assisted method in this work possessed the higher photocatalytic activity under visible light irradiation. 3.8 The proposed photocatalytic mechanism

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In order to explore photocatalytic mechanism of porous BiOBr catalysts during the degradation of RhB process, radicals trapping experiments were carried out to

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confirm the main active species by adding scavengers. As shown in Fig. 9, disodium ethylene diamine tetraacetate (EDTA) was added to the solution to scavenge photogenerated h+ [57]. Remarkable diminishing for degradation efficiency of RhB was observed for only 10% of RhB was photodegraded eventually. The tert butyl alcohol (TBA) was added to the solution as hydroxyl radicals scavenger simultaneously[60],

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the photocatalytic degradation of RhB over porous BiOBr was no noticeably inhibited. In conclusion, the photocatalytic degradation process of RhB over the porous BiOBr photocatalyst under visible light irradiation may be dominated by hole oxidation,

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which may explain the high degradation efficiency of RhB in the porous BiOBr photocatalytic degradation system. Combined with the aboved analysis results, a possible reaction mechanism for

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porous BiOBr catalysts in the degradation of RhB was proposed in Fig. 10. Before visible light irradiation, abundant RhB molecule adsorbed on the surface of the porous BiOBr catalysts in the dark. After visible light irradiation, the easier transition of numerous excited electrons from the VB to the CB started on the huge surface of porous BiOBr microspheres, leaving numerous holes to oxidize and degrade the adsorbed RhB molecules to small molecules. With the process of photocatalytic reaction, the concentration of RhB dye gradually decreased. Finally, RhB and small molecule compounds could thoroughly become degradation productions in an hour. Based on the above analysis, it can conclude that the small band gap is the primary 14

ACCEPTED MANUSCRIPT advantage of porous BiOBr catalysts, which could increase the quantity of excited active species (h+) bringing about more efficiently photocatalytic process. Moreover, the prorous structure of the BiOBr photocatalysts could initiate multiple scattering of visible light, which means that the improvement of light utilization and increase of

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photogenerated e- and h+ hole density. Last but no the least, the layer structure of prorous BiOBr could provide large space to separate and transfer the hole-electron pair efficiently. Accordingly, the enhanced photocatalytic decomposition ability of the

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contaminants was owing to the synergistic effect of above multiple factors.

4. Conclusions

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In summary, BiOBr hollow microsphere and porous nanosphere structures have been fast controlled and prepared via a new ionic liquid assisted microwave synthesis route,

which

is

a

rapid,

reliable,

high-yielding,

template-free

and

environment-friendly route. Experiments results indicated that both the ionic liquid and the microwave heating method play important roles in the formation of BiOBr

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hollow microsphere and porous nanosphere structures. The average diameter of flower-like hollow microspheres and BiOBr porous nanospheres was 1.0–1.5 µm and 1 µm, respectively. Furthermore, both samples exhibited excellent photocatalytic

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degradation of RhB. The higher photocatalytic activities of BiOBr porous nanospheres might be ascribed to its novel structures, large surface area, smaller

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particle size and band gap. As demonstrated by this successful example, the ionic liquid assisted microwave synthesis method may also be a promising route for a fast and large scale synthesis of other elemental and compound nanostructures. This will open up a new feasibility for fast controlled production of a variety of 3D nanostructures in high yields.

Acknowledgment This work was financially supported by the National Natural Science Foundation 15

ACCEPTED MANUSCRIPT of China (No. 21476098, 21471069 and 21576123), the Doctoral Innovation Fund of Jiangsu Province (KYZZ16_0340), the Science and Technology support program of Zhenjiang (SH2014018) and the Natural Science Foundation of Jiangsu Province

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(BK2012717).

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Figure caption: Fig. 1 XRD patterns of BiOBr samples of: (a) hollow BiOBr; (b) porous BiOBr. Fig. 2 FT-IR spectra of BiOBr samples: (a) hollow BiOBr,; (b) porous BiOBr. Fig. 3 BiOBr microspheres structures: (a, b) the high magnification SEM images; (c,

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d) TEM image of the hollow BiOBr microspheres; (e) EDS images of hollow BiOBr. Fig. 4 (a, b) SEM (c) EDS images of the porous BiOBr microspheres structures.

Fig. 5 (a) Hollow BiOBr and Porous BiOBr samples of UV-vis diffuse reflectance spectra (DRS); (b) direct band gap of hollow BiOBr and porous BiOBr nanospheres.

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Fig. 6 Nitrogen absorption–desorption isotherms of porous and hollow BiOBr samples.

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Fig. 7 Transient photocurrent response for the porous BiOBr and hollow BiOBr microspheres with and without irradiation in 0.1mol L-1 PBS solution (pH =7) under visible light irradiation.

Fig. 8 Photodegradation of RhB with different structure BiOBr under visible-light irradiation.

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Fig. 9 Comparison of photocatalytic activities of the microwave porous BiOBr catalysts for the degradation of RhB with or without adding EDTA-2Na and TBA under visible light irradiation.

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Fig. 10 A schematic illustration of RhB degradation over microwave porous BiOBr

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