Synthesis of visible-light-driven BiOBr x I 1-x solid solution nanoplates by ultrasound-assisted hydrolysis method with tunable bandgap and superior photocatalytic activity

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Synthesis of visible-light-driven BiOBr _x I _1-x solid solution nanoplates by ultrasound-assisted hydrolysis method with tunable bandgap and superior photocatalytic activity

Junlin Lu

^a

, Qingguo Meng

^b

, Haiqin Lv

^b

, Lingling Shui

^a

, Mingliang Jin

^a^,^c

,

Zhang Zhang

^c^,^d

, Zhihong Chen

^b^,^c^,^*

, Mingzhe Yuan

^b

, Xin Wang

^a^,^c^,^**

, Jun-Ming Liu

^d^,^e

, Guofu Zhou

^a

aInstitute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, Guangdong Province, China

bShenyang Institute of Automation, Guangzhou, Chinese Academy of Sciences, Guangzhou 511458, China

cInternational Academy of Optoelectronics at Zhaoqing, South China Normal University, Guangdong Province, China

dInstitute of Advanced Materials, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, Guangdong Province, China

eLaboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China

a r t i c l e i n f o

Article history:

Received 11 July 2017 Received in revised form 9 October 2017 Accepted 21 October 2017 Available online 25 October 2017

Keywords:

BiOBrxI1-xsolid solution Visible-light-driven photocatalysts Nanoplates

a b s t r a c t

In this study, a series of visible-light-driven BiOBrxI1-xsolid solution nanoplates photocatalysts are successfully prepared by an ultrasound-assisted hydrolysis method, which does not use organic reagents, with advantages of cost-effectiveness and non-toxicity. Under visible-light irradiation, all of the as- prepared BiOBrxI1-x nanoplates exhibit superior photocatalytic activities compared to those of pure BiOBr and BiOI for the degradation of methyl orange (MO). BiOBr0.3I0.7exhibits the highest photocatalytic activity, corresponding to the degradation of 92% MO in 40 min under visible-light irradiation. The structures and elemental composition of the as-prepared BiOBrxI1-xnanoplates samples are character- ized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, and high-resolution transmission electron microscopy. From the results obtained from X-ray photoelectron spectroscopy, UVevis diffuse reﬂectance spectroscopy, and transient time-resolved luminescence decay, it is suggested that the enhanced photocatalytic activity of BiOBrxI1-xis possibly related to the narrowing of the band gap and high separation of the photo-generated electronehole pairs. Electron paramagnetic resonance and mechanistic experiments indicated thatO2and h^þare active radicals for photocatalytic degradation. In conclusion, an ultrasound-assisted hydrolysis method which is free of organic reagents is developed for synthesizing BiOBrxI1-xnanoplates photocatalysts with tunable bandgap and enhanced photocatalytic activity.

1. Introduction

In recent years, photocatalysis has attracted increasing attention

because of its considerable advantages of cost-effectiveness, outstanding degradation ability, mild reaction conditions, and environmental friendliness for applications in water treatment, air purification, and water splitting[1e13]. Traditional photocatalysts such as metal oxides and metal sulfides exhibit some drawbacks, which in turn restrict their applications. Metal oxide photocatalysts have wide band gap, leading to the weak absorption of the solar spectrum, while sulfide photocatalysts are not stable under light irradiation, which are oxidized by the photo-generated holes[14].

Hence, it is imperative to develop a novel photocatalyst exhibiting high visible-light absorption as well as high efﬁciency and stability.

*Corresponding author. Shenyang Institute of Automation, Guangzhou, Chinese Academy of Sciences, Guangzhou 511458, China.

**Corresponding author. Institute of Electronic Paper Displays, South China Academy of Advanced Optoelectronics, South China Normal University, Guangzhou, Guangdong Province, China.

E-mail addresses:chenzhihong1227@sina.com(Z. Chen),wangxin@scnu.edu.cn (X. Wang).

Contents lists available atScienceDirect

Journal of Alloys and Compounds

j o u rn a l h o m e p a g e :h t t p : / / w w w . e l s e v i e r . c o m / l o c a t e / j a l c o m

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Recently, bismuth oxyhalides (BiOX, where X¼Cl, Br, and I) as novel photocatalysts are attracting immense attention in photocatalysis because of their unique electrical, optical, and catalytic properties, as well as the internal electricﬁeld effect[15e18]. Bi(III) containsﬁlled (d¹⁰) orbitals, and its 6s orbitals could hybridize with the O 2p orbitals, affording a new hybridized valence band (VB).

This hybridized VB narrows the band gap to permit visible-light absorption. With increasing atomic number, the band gap of BiOX becomes narrower: approximately 3.2 eV for BiOCl, 2.7 eV for BiOBr, and 1.7 eV for BiOI. The internal electricﬁeld between the [Bi₂O₂] layers and halogen atom layers could accelerate the separation of the photo-generated electronehole pairs[17]. However, pure BiOX still exhibits some drawbacks, which in turn restrict its future applications. For example, BiOCl can only absorb UV light, and BiOBr can only absorb a small fraction of visible light [18].

Meanwhile, although BiOI has a narrow band gap for absorbing a large range of visible light, it exhibits a weak redox capability because of the high conduction band (CB) level[19].

Several strategies have been reported to control the electronic structure of BiOX for the improvement in its photocatalytic activities, e.g., metal hybridization [20e26], heterojunction [27e36], and solid solution[37e43]. Among these strategies, solid solution has been regarded as a promising method to enhance the photocatalytic activity of BiOX, because the solid solutions exhibit a wide range of band gaps, and their electronic structures can be modu- lated by changing the components. Furthermore, they facilitate a facile, efﬁcient route to obtain a suitable VB and CB for a BiOX-based photocatalyst with enhanced photocatalytic activity and solve the contradiction between the light absorption and redox capability of BiOX. Some methods have been reported to synthesize BiOX-based solid solutions, e.g., hydrothermal[37], solvothermal[38e40]and precipitation methods [41e43]. However, the hydrothermal method requires the heating of reaction systems, which in turn leads to high energy consumption. The solvothermal method not only requires the heating of reaction systems but also uses organic reagents as the solvent; and the precipitation method uses organic reagents or requires the heating of reaction systems. These afore- mentioned routes fail to satisfy the requirement for a green, environmentally friendly method, which hinders the future applications of BiOX-based solid solutions. Thus, it is a challenge to develop a simple, green, and environmental friendly method to synthesize the BiOX-based solid solutions.

In this study, BiOBr_xI_1-x(x¼0, 0.1, 0.2, 0.3, 0.4, and 0.5) solid solution nanoplates with enhanced photocatalytic activity is successfully synthesized by an ultrasound-assisted hydrolysis method.

Compared to other methods utilized to synthesize BiOXxY1-x, the developed reaction system is free of organic reagents, and the reaction is carried out at room temperature, making it a green, environmentally friendly method. The BiOBrxI1-x solid solution nanoplates exhibits enhanced photocatalytic ability for the degradation of methyl orange (MO) under visible light (l>400 nm), and BiOBr0.3I0.7exhibits the highest photocatalytic activity. The theo- retical calculations for the electronic structure, as well as mechanistic experiments, were carried out to estimate the photocatalytic activities.

2. Experimental 2.1. Preparation

All chemical reagents used in this experiment were of analytical grade and used without further puriﬁcation. Deionized water was used throughout.

Fig. 1 shows the schematic for the preparation of BiOBrxI1-x

photocatalysts by the ultrasound-assisted hydrolysis method.

First, Bi(NO₃)3$5H2O was mixed with the halide solution, followed by the sonication and stirring of the resulting solution to afford BiOBrxI1-xnanoplates. In a typical experiment, KI and KBr in a total molar amount of 0.0028 mol were dissolved in deionized water of 80 mL under magnetic stirring. After KI and KBr were completely dissolved, 0.0028 mol of Bi(NO3)3$5H2O was added into the solution, and stirring was continued for 2 min. Next, the solution was subjected to sonication for 2 min and was magnetically stirred for another 5 h at room temperature. Finally, the product was collected by suctionﬁltration, washed three times with deionized water, and dried at 60C for 12 h. The different BiOBr_xI_1-xnanoplates samples were prepared by changing the molar ratios of KBr/KI.

2.2. Characterization

The crystal structures of the BiOBrxI1-xnanoplates were char- acterized on a PANalytical X'Pert PRO X-ray diffraction (XRD) instrument. Scanning electron microscopy (SEM) images were recorded on a Zeiss Ultra 55 thermal FESEM system. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were recorded on a JEM-2100 instrument. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo ESCALAB 250Xi instrument with a monochromatized Al Ka line source (150 W). Electron paramagnetic resonance (EPR) measurements were carried out on a Bruker ER 200-SRC spectrometer.

UVevisible diffuse reﬂectance spectroscopy (UVeVis DRS) measurements were carried out on a U-41000 HITACHI spectropho- tometer (Tokyo, Japan) using BaSO4as the reference. Fluorescence decay spectra measurements were carried out on an FLS-980 Fluorescence Lifetime and Steady State Spectrometer.

2.3. Photocatalytic activity tests

Photocatalytic activity was determined by the degradation of MO and phenol under visible-light irradiation. In a typical experiment, a 300-W Xe lamp (AM 1.5, output light current is 15 A) with a 400-nm cut-offﬁlter was used as the visible-light source, and the overall system was cooled by circulating water. First, 100 mg of the as-prepared photocatalyst was added into 150 mL of an aqueous solution containing 10 mg/L MO or phenol. Second, the suspension was stirred for 30 min in the dark to attain the adsorp- tionedesorption equilibrium. Third, the suspension was irradiated using the Xe lamp. Next, 7 mL of the solution was sampled and centrifuged to remove the catalysts at 10-min time intervals. The concentration of MO in the degraded solution was detected by UVevis spectroscopy at 465 nm. The concentration of phenol in the degraded solution was detected by UVevis spectroscopy at 510 nm after chromogenic reaction.

2.4. Chromogenic method

Solutions of 4-Aminoantipyrine (20 g/L), K3[Fe(CN)6] (80 g/L) and ammonia buffer (PH¼10.7), each wasﬁrst individually prepared in 100 mL volumetricﬂask. Two milliliters (2 mL) of the degraded solution and 8 mL of deionized water were poured into a 25 mL test tube with a glass stopper. After that, 0.5 mL of ammonia buffer, 1 mL of 4-Aminoantipyrine and 1 mL of K₃[Fe(CN)₆] solutions were sequentially added into the test tube. After the mixture solution was thoroughly mixed by shaking until a uniformly mixed solution was obtained, it was let stand for 10 min to allow the chromogenic reaction to take place. Within 30 min, the absorbance of the solution was measured.

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3. Results and discussion

3.1. Characterization of phase structures and morphology

XRD was utilized to characterize the crystal phase of the BiO- BrxI1-x solid solution. Fig. 2 shows the XRD patterns. All of the diffraction peaks for as-prepared BiOBr and BiOI were in good agreement with tetragonal BiOBr (JCPDS No. 73e2061) and tetragonal BiOI (JCPDS No. 73e2062). With increasing Br/I ratios, the diffraction peak of BiOBr_xI_1-xclearly shifted toward large angles, indicating that the Br content gradually increases in the BiOBrxI1-x

solid solution. Meanwhile, the diffraction peak intensity of BiOI gradually decreased with increasing Br content. The successive shift in the diffraction peak revealed that the obtained sample is a BiO- BrxI1-xsolid solution, not a mixture of BiOBr and BiOI.

Morphology and microstructure of the samples were examined by SEM and TEM. SEM elemental mapping analysis was carried out to study the distribution of Bi and other elements. The structure of BiOBrxI1-x nanoplates of sizes 200 nme600 nm was observed (Fig. 3AeF). The structure of non-sonicated BiOBr_0.3I_0.7nanoplates was found to be asymmetric (Fig. 3G), indicating that BiOBr0.3I0.7

nanoplates without ultrasound tends to form a cluster as opposed to a disperse form. The elements Bi (Fig. 4B), Br (Fig. 4C) and I (Fig. 4D) were uniformly distributed in BiOBr0.3I0.7solid solution. To further investigate the morphology of the BiOBrxI1-xsolid solution, TEM and HRTEM images were recorded, as well as Fourier trans- form infrared (FFT) images. The nanoplate structures were observed again (Fig. 5), indicating that the BiOBrxI1-xnanoplates solid solution is obtained by the ultrasound-assisted hydrolysis

method. The HRTEM image (Fig. 5) indicated that the BiOBr_xI_1-x nanoplates solid solution exhibits a high degree of crystallization and clear lattice fringes. The interplanar lattice spacing values for BiOI, BiOBr_0.3I_0.7, and BiOBr were 0.274 nm, 0.295 nm, and 0.305 nm, respectively. The interplanar lattice spacing of BiO- Br0.3I0.7 was between those of pure BiOBr and BiOI, which is a characteristic of solid solutions; this result further conﬁrms the successful formation of a BiOBrxI1-x solid solutions rather than heterojunction. The FFT images (Fig. 5) indicated that the BiOBrxI1-x

nanoplates solid solution has a single-crystalline structure.

3.2. Chemical composition and chemical state characterization The chemical composition and chemical state of BiOBrxI1-xwere analyzed by XPS spectroscopy. From the XPS survey spectrum (Fig. 6A), Bi, Br, I, and O were observed for the BiOBr_xI_1-xsolid solution. The highest-intensity peaks with binding energy values of 159 eV and 164 eV, corresponding to the Bi 4f7/2 and Bi 4f5/2, respectively, were observed (Fig. 6B). Binding energy values of 619 eV and 630 eV, corresponding to I 3d5/2and I 3d3/2, respectively, were observed (Fig. 6C). The peak of Br was resolved into two componentsdBr 3d5/2and Br 3d3/2with binding energy values of 68 eV and 69 eVdrespectively (Fig. 6D). From the results obtained from XPS, the BiOBrxI1-xsolid solution is successfully synthesized.

3.3. UVevisible diffuse-reﬂectance spectroscopy

Fig. 7shows the DRS spectrum of the BiOBr_xI_1-xsolid solution.

The BiOBr_xI_1-x solid solution absorbed visible light. Compared to other samples, BiOBr only absorbed a narrow range of the visible light, while the remaining samples absorbed visible light in the 400 nme650 nm range. The DRS spectrum indicated that the BiOBrxI1-x solid solution exhibits photocatalytic activity under visible light.

The bandgap of the BiOBr_xI_1-xsolid solution was calculated by the KubelkaeMunk method based on the UVevis DRS spectra. As shown inTable 1, the band gaps potential of BiOI and BiOBr were 1.63 eV and 2.62 eV, respectively, while the band gap of the BiO- BrxI1-xsolid solution ranged from 1.51 to 2.62 eV. The VB and CB potentials were calculated by the following empirical formulas:

E_vb¼cE_eþ0.5E_g (1)

Ecb¼EvEg (2)

where Egis the band gap potential, Evbis the valence band potential, Ecbis the conduction band potential, Eeis the energy of free electrons on the hydrogen scale, which is approximately 4.5 eV, and cis the absolute electronegativity of the semiconductor, expressed as the geometric mean of the absolute electronegativity of the Fig. 1.Schematic of the preparation of the BiOBrxI1-xphotocatalyst.

Fig. 2.XRD patterns of BiOBrxI1-x(x¼0, 0.1, 0.2, 0.3, 0.4, 0.5).

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constituent atoms. As shown inTable 1, BiOBr0.3I0.7exhibited the lowest CB potential (1.02 eV), indicating that the hole on the CB of BiOBr0.3I0.7possess the best oxide capability. The result obtained from electronic studies indicated that the fabricated BiOBrxI1-xsolid solution optimizes the electronic structure of BiOX.

3.4. EPR characterization

EPR measurements were carried out to investigate the electronic band structures of the BiOBr0.3I0.7solid solution. BiOBr0.3I0.7

exhibited only one single Lorentzian line centered at 3145.8 G, with a g-value of 1.9995 (Fig. 8) under dark or visible light. The BiO- Br0.3I0.7 EPR signal intensity under light was approximately 4.5

times greater than that in the dark, indicating that the BiOBr0.3I0.7

solid solution forms photo-generated electronehole pairs under visible-light irradiation.

3.5. Fluorescence decay and lifetime characterization

The ns-level time-resolved ﬂuorescence decay spectra were recorded to investigate the recombination and transfer behavior of the photoexcited charge carriers. As shown inFig. 9, the radiative lifetimes of charge carriers for pure BiOBr, BiOI, BiOBr0.3I0.7, and BiOBr_0.4I_0.6 solid solutions were determined as 0.4486, 0.5448, 0.5911, and 0.5598 ns, respectively, where the longest lifetime was observed for BiOBr0.3I0.7. The radiative lifetimes of the charge Fig. 3.SEM images of BiOI (A,B), BiOBr0.3I0.7(C,D), BiOBr (E,F), non-sonication BiOBr0.3I0.7(G) and BiOBr0.3I0.7after recycling reactions (H).

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Fig. 4.SEM elemental mapping analysis of BiOBr0.3I0.7.

Fig. 5.TEM, HRTEM, and FFT images of BiOI (A,B), BiOBr0.3I0.7(C,D), and BiOBr (E,F).

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carriers increased with the Br content because the addition of Br in the BiOBr_xI_1-xsolid solution modulates the electronic structure to narrow the band gap and facilitates the separation of photo- generated electronehole pairs for the improvement of photocatalytic activity. With increase in the Br content to greater than 0.3 (x is greater than 0.3), the BiOBrxI1-x shows a decrease radiative

lifetimes, suggesting that the recombination of photo-generated electron-hole pairs increase as the Br content increase in the BiO- Br_xI_1-xsolid solution, which may be attribute to that the excess Br form the recombination site in the BiOBrxI1-xsolid solution.

3.6. Photocatalytic activity experiment

The photocatalytic activity of the BiOBr_xI_1-xsolid solution was investigated by the degradation of MO in an aqueous solution under visible irradiation. Before the photocatalytic experiment, all the samples had reached the adsorption-desorption equilibrium during 30 min dark reaction (Fig. 10C). Based on the blank (in the absence of any catalyst) experiment, the self-photolysis of MO under visible light irradiation can be ignored. The BiOBr_xI_1-xphotocatalysts exhibited superior photocatalytic activity compared with those of pure BiOI and BiOBr (Fig. 10A), indicating that the Fig. 6.XPS survey spectra of BiOI, BiOBr0.3I0.7, and BiOBr (A) XPS spectra of Bi 4f (D), I 3d (C), and Br 3d (D).

Fig. 7.UVevis DRS spectra of BiOBrxI1-x.

Table 1

Band gap, valence band, and conduction band potentials of the BiOBrxI1-xsolid solution.

Samples Band gap/eV Valence band/eV Conduction band/eV

BiOI 1.63 2.52 0.89

BiOBr0.1I0.9 1.59 2.52 0.93

BiOBr0.2I0.8 1.56 2.53 0.97

BiOBr0.3I0.7 1.51 2.53 1.02

BiOBr0.4I0.6 1.58 2.59 1.01

BiOBr0.5I0.5 1.67 2.655 0.985

BiOBr 2.62 3.26 0.64

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formation of the BiOBrxI1-xsolid solution facilitates the separation of the photo-generated carriers, which in turn enhances photocatalytic activity. With increasing Br content from 0 to 0.3, the photocatalytic activity of BiOBrxI1-xgradually increased. BiOBr0.3I0.7

exhibited the highest photocatalytic activity, which degraded approximately 92% MO within 40 min under visible-light

irradiation. With the increase in the Br content to greater than 0.3, the photocatalytic activity of BiOBr_xI_1-x decreased possibly because the increased proportion of Br cannot narrow the band gap of the BiOBrxI1-xsolid solution further, which may lead to a high recombination of photogenerated electronehole pairs.

BiOBr0.3I0.7 exhibited higher photocatalytic activity compared with that of its non-sonicated counterpart (Fig. 10D). This difference may be due to the clustered structure of the non-sonicated nanoplates, which can reduce contact surface/area of the photocatalyst, and MO may lead to the decrease of reaction site. These showed that the ultrasound treatment was able to disperse the nanoplates so that it could provide more reaction sites, and led to improved photocatalytic activity of BiOBrxI1-x.

Total organic carbon (TOC) analysis was also conducted to investigate the mineralization ratio of MO in presence of BiOBr0.3I0.7

photocatalyst under visible light irradiation. The removal of TOC (by BiOBr0.3I0.7) increased with increasing irradiation time and reached 77% MO after 40 min (Fig. 10B). This demonstrated that most MO was thoroughly photocatalytically degraded by BiO- Br0.3I0.7, rather than only being decolorized into other organic intermediates.

To further examine the photocatalytic activity between the different photocatalysts, the reaction kinetics was investigated. The photodegradation of MO was ﬁtted using the pseudo-ﬁrst-order kinetics model.

ln (C/C₀)¼Kt (3)

Fig. 8.EPR spectra in dark and under visible light (l>420 nm) for BiOBr0.3I0.7.

Fig. 9.Nanosecond-level time-resolvedﬂuorescence decay curves of BiOI(A), BiOBr0.3I0.7(B), BiOBr0.4I0.6(C), and BiOBr(D).

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Here, C is the MO concentration at time t, C₀is the initial concentration of the MO solution, and the slope K is the apparent reaction rate constant. With increasing Br content from 0 to 0.3, the reaction rate constant gradually increased (Fig. 11A and B). BiOBr_0.3I_0.7

clearly exhibited the largest K value. It is approximately 4 and 11 times greater than those of BiOI and BiOBr, respectively. The reaction rate constant decreased with the increase in the Br content to greater than 0.3. The improvement in the photocatalytic activity of Fig. 10.Photocatalytic activities of BiOBrxI1-xby degradation of MO (A), TOC removal of MO over BiOBr0.3I0.7(B), MO degradation experiments in absence of light (C) and Pho- tocatalytic activities of BiOBrxI1-xand non-sonication BiOBr0.3I0.7(D).

Fig. 11.Kinetic studies for MO degradation (A) and the bar graph (B) for the apparent rate constants of BiOBrxI1-x.

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BiOBr_0.3I_0.7is probably related to the narrowing of the band gap and the high separation of the photo-generated electronehole pairs.

The results obtained from the photocatalytic activity experiments are in agreement with those obtained from the electronic studies of the BiOBrxI1-xsolid solutions. The optimal photocatalytic activity for BiOBr0.3I0.7is related to the most suitable VB and CB potentials and best redox capability. From the above discussion, the fabricated BiOBrxI1-xsolid solution can modulate the electronic structure and improve the redox capability for the photocatalytic activity enhancement of BiOX.

To further evaluate the photocatalytic activity of the samples, the photocatalytic experiment using phenol was carried out.

Colorless phenol was used as the substance in the reactions which BiOBr, BiOBr0.3I0.7, and BiOI were used as the photocatalysts. The results showed that BiOBr0.3I0.7 exhibited the highest photocatalytic activity (Fig. 12), in which approximately 55% phenol was degraded within 8 h under visible light irradiation. The photocatalytic activity of BiOBr0.3I0.7towards phenol was lower than that towards MO because of the stable structure of phenol, however, this data still showed its potential ability in degradation of small molecular organic pollutant under visible light.

Fig. 12.Photocatalytic activities of BiOBr, BiOBr0.3I0.7 and BiOI by degradation of phenol.

Fig. 13.Cycling experiments in the presence of BiOBr0.3I0.7.

Fig. 14.Results from the radical trapping experiment in the presence of BiOBr0.3I0.7(A) and the EPR spectrum of theO2radical (B).

Fig. 15. Schematic of the mechanism for the photodegradation of MO by BiOBr0.3I0.7.

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3.7. Photocatalyst cycling experiments

Cycling experiments were carried out to demonstrate the stability of the BiOBrxI1-x solid solution (Fig. 13). After the photocatalytic experiments, the photocatalyst powders were collected, washed three times using deionized water, dried in a drying oven at 60C, and reused in subsequent photocatalysis experiments. After performing cycling experiments 5 times, BiOBr_0.3I_0.7 degraded approximately 78% MO within 40 min under visible-light irradiation. Compared to theﬁrst cycling run, the photocatalytic activity slightly decreased, but it still exhibited high degradation ability for MO. The SEM image of BiOBr0.3I0.7after cycling experiment shows that the BiOBr0.3I0.7 nanoplates structure was observed again (Fig. 3H), suggesting that the BiOBr_0.3I_0.7 nanoplates structure is stable in the photocatalytic reaction. From cycling experiments, the BiOBrxI1-xsolid solution exhibited stable photocatalytic activity and structure under visible light.

3.8. Photocatalytic mechanism

During photocatalysis, the photo-generated electronehole pairs react with H2O and O2, affording OH,O2, and h^þ as the active species. To examine which of the radical species play the main role in photocatalysis, radical trapping experiments and EPR measurements were carried out. In radical trapping experiments, tertiary butanol (t-BuOH), N2, and potassium iodide (KI) were added as scavengers for hydroxyl radicals (OH), superoxide radicals (O₂), and holes (h^þ), respectively[31,44]. N2and KI negatively affected the photocatalytic degradation of MO (Fig. 14A), indicating thatO2

and h^þare the active radicals, and h^þis the main active species in photocatalytic degradation. On the contrary, t-BuOH did not affect the photocatalytic degradation of MO. Meanwhile, from EPR (Fig. 14B), the intensity of theO2signals for the BiOBr0.3I0.7solid solution increased under visible-light irradiation, indicating that the BiOBr0.3I0.7 solid solution generates O2 under visible-light irradiation. According to the band structure study of BiOBr0.3I0.7, CB of BiOBr_0.3I_0.7unexpectedly was positive 1.02 eV, in contrast to the negative value required for the reduction of O2toO2to take place (O2/O2¼ 0.046 eV, vs. SHE)[45]. This could be explained based on the previously reported data[31,46e48]as well as the experimental result shown inFig. 14that under visible light irradiation, the electron in VB can be excited to a higher potential edge and form a new CB. As a result, the new CB of BiOBr_0.3I_0.7became more negative than E(O₂/O₂¼ 0.046 eV), and thus BiOBr_0.3I_0.7 was able to reduce O2toO2under visible light irradiation. From the discussion above, the possible photocatalytic degradation mechanism is shown inFig. 15. Under visible-light irradiation, the electrons in the VB are excited to the CB, affording the photo- generated electronehole pairs. The photo-generated hole left in the VB can directly degrade MO, and the photo-generated electrons react with O2, affordingO2, which can further undergo degradation with MO.

4. Conclusion

In summary, the BiOBr_xI_1-x nanoplates solid solutions were successfully synthesized by ultrasound-assisted hydrolysis method.

This method provided a low-cost, green route, which is free of organic reagents, to synthesize BiOBrxI1-xphotocatalysts. BiOBrxI1-x

exhibited enhanced photocatalytic activity for the degradation of MO compared to BiOBr and BiOI under visible-light irradiation.

Enhancement of the photocatalytic activity is probably related to the narrowing of the band gap by the formation of the BiOBr_xI_1-x solid solution, which facilitates the separation of the photo- generated electronehole pairs and suppresses their

recombination. BiOBr_0.3I0.7 exhibited the best photocatalytic activity, related to its suitable VB and CB potentials. Mechanistic experiments indicated that O² and h^þ are the active radicals in photocatalytic degradation. Therefore, a simple, facile, and green route to form BiOBr_xI_1-x nanoplates solid solutions is reported, which exhibit enhanced photocatalytic activity under visible light.

Acknowledgment

The authors acknowledge theﬁnancial support from the NSFC (Grant No. 51602111), Xijiang R&D Team (X. Wang), Guangdong Provincial Grant (2015A030310196, 2014B090915005), the Pearl River S&T Nova Program of Guangzhou (201506040045), PCSIRT Project No. IRT13064, the Hundred Talent Program of Chinese Academy Sciences (QG Meng), Guangzhou Post-doctoral Initial Funding and the National 111 Project.

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