Design of 3D flowerlike BiOClxBr1-x nanostructure with high surface area for visible light photocatalytic activities

Journal of Alloys and Compounds 725 (2017) 1144e1157

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Design of 3D flowerlike BiOClxBr1-x nanostructure with high surface area for visible light photocatalytic activities Jian Yang a, b, Yujun Liang a, b, *, Kai Li a, b, Yingli Zhu a, b, Shiqi Liu a, b, Rui Xu a, b, Wei Zhou a, b a b

Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan, 430074, China Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, 430074, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 12 March 2017 Received in revised form 21 July 2017 Accepted 22 July 2017 Available online 26 July 2017

A three-dimensional (3D) BiOClxBr1-x (x ¼ 0e1) solid solutions nanostructures with compositional tunability were successfully synthesized via a one-step glycol-assisted hydrothermal process in the presence of cationic polyacrylamide (C-PAM) and cetyltrimethylammonium bromide (CTAB). The prepared samples displayed hierarchical flower-like microspheres with rich in specific surface area and appropriate pore size, providing suitable pathways for transportation and separation of photogenerated electrons and holes. The controllable ratio of Cl and Br led to the decrease in the optical bandgap, thus increasing the absorption of visible light range. The solid solutions delivered remarkable adsorption capacity and photocatalytic activity in comparison with pure BiOCl, BiOBr and P25 (TiO2 nanoparticles) in the photodegradation of methyl orange (MO) aqueous solution under visible light irradiation, and the degradation rate reached the maximum at x ¼ 0.5 and decreased gradually with x moving away from 0.5. The BiOCl0.5Br0.5 sample displayed high photostability under repeated visible light irradiation, which is especially important for its practical application. Based on the above findings, the reasonable formation mechanism and the main active species of the samples were also discussed in detail. © 2017 Elsevier B.V. All rights reserved.

Keywords: BiOClxBr1-x Solid solution High surface area Visible light photocatalytic

1. Introduction The energy crisis and environmental problems become increasingly serious with the rapid development of science and technology on a global level, it is necessary to explore a novel approach to address these urgent problems. Since the report that photocatalytic splitting of water on TiO2 electrode was published in 1972 by Fujishima and Honda [1], the research and development of the photocatalytic technology have gone to a new era. The studies on pollutant decomposition, water splitting and CO2 reduction have far reaching practical significance [2]. Nevertheless, most of the intensively investigated semiconductor photocatalysts, such as TiO2 and ZnO [3,4], suffering from the problems of low quantum yields, weak adsorptive performance and wide band gap, thus hindering their widespread applications. In order to overcome these disadvantages, the development of novel alternative semiconductor photocatalysts which can utilize visible light with high

* Corresponding author. Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan, 430074, China. E-mail address: [email protected] (Y. Liang). http://dx.doi.org/10.1016/j.jallcom.2017.07.213 0925-8388/© 2017 Elsevier B.V. All rights reserved.

efficiency is highly anticipated and indispensable. Bismuth based compounds are attractive because of the special layer structure and the appropriate band gap. In earlier research, Bi3þ was doped as a modifier in TiO2 [5]. Later, it was discovered that many bismuth based compounds had photocatalytic properties, such as Bi2O3, BiOX (X ¼ F, Cl, Br, I), Bi2WO6, BiVO4, Bi2MoO6 and some of the more complex Bi compounds [6e12]. The most representative one is the BiOX which are beneficial from their safety, non-toxicity, high stability and efficient photocatalytic activity. Moreover, they have the special layered structure feature in which [Bi2O2]2þ slabs interleaved by double slabs of halogen atoms and formed the [eXeBieOeOeBieXe] layers. The structure could induce the internal electric field which is propitious for the effective separation of photogenerated electrons and holes in the process of photocatalytic reaction [13]. However, corresponding to the wide band gap (3.20e3.50 eV) [14], BiOCl is only responsive to ultraviolet light which accounts for 5% of the solar spectrum. Although BiOBr and BiOI are able to absorb the visible sunlight, their photocatalytic activity is not enough to deal with the practical environment pollution owing to the high recombination rate of photogenerated electron-hole pairs and the high level of the

J. Yang et al. / Journal of Alloys and Compounds 725 (2017) 1144e1157

valence band [15,16]. As a result, many attempts have been made to enhance the photocatalytic efficiency of BiOX system, which include building the heterostructures (like BiOCl/Bi2O3, BiOBr/gC3N4, CdS/BiOI, Ag/AgX/BiOX) [17e20], doping with elements (Mn, Fe, Ti, C etc.) [14,21e23] and fabricating solid solution [24]. Among these strategies, the solid solutions have attracted considerable attention because they can achieve the continuous adjustment of the band structure and reach the best balance between light absorption and redox capability [25]. Base on the similar crystal structures of BiOCl, BiOBr and BiOI, to date there were many studies aimed at the preparation of bismuth oxyhalides solid solutions, such as BiOClxBr1-x [26e31], BiOBrxI1-x [32,33] and BiOClxI1-x [34e36]. Gnayem et al. [26] first reported the synthesis of hierarchical nanostructured 3D flowerlike BiOClxBr1-x semiconductors with exceptional visible light photocatalytic activity. Next moment their group developed the bismuth doped BiOClxBr1-x solid solutions [27] and BiOClxBr1-x-embedded alumina films [28], and the samples all exhibited remarkably efficient visible light photocatalytic activity. After that, some previous reports also indicated that BiOX solid solutions possess superior photocatalytic activity than the individual BiOX systems. Wu et al. [29] prepared the flower-like BiOClxBr1-x solid solutions through a facile temple free hydrothermal method, and the obtained BiOCl0.5Br0.5 sample displayed enhanced photocatalytic activity and stability for the oxidation of NO under visible light. Qin groups [30] reported that the hierarchical microspheres BiOCl1-xBrx solid solutions exhibited superior photocatalytic capability to the corresponding 2D nanosheets, and they found the balance between the suitable band gap and adequate potential of the valence band in samples dominated their photocatalytic activity. More recently, Xu et al. [33] prepared BiOBrxI1-x nanoplate solid solutions with a high exposure of {001} crystal facets via a facile alcoholysis method at room temperature and atmospheric pressure. Xie et al. [36] synthesized BiOClxI1-x solid solutions with dahlia-shaped hierarchitectures by a rapid and cheap solid-state chemical process, which had not only excellent adsorption ability but also good photocatalytic ability for RhB. Therefore the bismuth oxyhalides solid solutions have become an important family of visible light photocatalysts, but it is still necessary to further improve their photocatalytic efficiency for practical applications. Another strategy to improve the visible-light photocatalytic performance is to synthesize catalyst with large specific surface area which can provide more active sites and numbers of substrates adsorbed. Studies have shown that the photocatalysts with high surface to volume ratio would be beneficial to increase the content of surface oxygen defects, which prevented the recombination of electrons and holes [37]. For instance, Qi et al. [38] synthesized the high-surface area mesoporous Pt/TiO2 hollow chains by using a simple microwave-hydrothermal route, and the samples exhibited high adsorption capacity and photocatalytic performance for HCHO, fast diffusion and transport of gas molecules, and good contact between gases and active sites at ambient temperature. Although the relationship between large specific surface area and visible-light photocatalytic activity of catalyst nanostructures was widely investigated, little or no work addressed the surface area effects in BiOX systems. The surface areas of the BiOX nanostructures and their solid solutions used in above studies were generally small (<35 m2/g), or they were not reported. Thus, the fabrication of BiOX solid-solution photocatalysts with modulating band gaps and high surface areas has emerged as a requirement. Herein, we have successfully prepared a number of BiOCl0.5Br0.5 samples firstly with various reactants and solvents through a tunable solvothermal process, and the results proved that the cationic polyacrylamide (C-PAM), cetyltrimethylammonium bromide (CTAB) and glycol were the most appropriate reactants and

1145

solvent in our work. Subsequently, a series of BiOClxBr1-x solid solutions were synthesized using the C-PAM, CTAB and glycol. As expected, they exhibited the continuously modulated band gaps from 3.21 to 2.72 eV. It was interesting that the products exhibited the high surface area and 3D hierarchical flower-like microspheres, they displayed excellent adsorption performance and efficient visible light photocatalytic for degrading methyl orange (MO) and the photocatalytic activity reached the maximum at x ¼ 0.5 and decreased gradually with x moving away from 0.5. Furthermore, the formation mechanism, stability and possible visible light responsive mechanism of BiOClxBr1-x solid solutions have also been investigated in detail. 2. Experimental 2.1. Materials All the chemical reagents were analytical grade and used without further purification. Glycol and bismuth nitrate (Bi(NO3)3$5H2O) were purchased from Tianjin Fuchen Chemical Reagent Factory. CTAB was purchased from Shanghai source poly Biological Technology Co., Ltd. Citric acid, C-PAM, MO and P25 (TiO2 nanoparticles) were purchased from Sinopharm Chemical Reagent Co., Ltd, China. 2.2. The preparation of BiOClxBr1-x photocatalysts The BiOClxBr1-x(x ¼ 0.00, 0.25, 0.50, 0.75, 1.00) samples were all synthesized using a hydrothermal synthesis process by different raw materials and solvents to reveal the effects of them. In a typical synthesis, while the water as the reaction medium, 2.4 mmol citric acid was dissolved into a HNO3 (15 mL, 2 mol L1) solution containing 1.2 mmol of Bi(NO3)3$5H2O, the mixture was stirred and till be soluble to form the solution ①. Then, stoichiometric amounts of reactants (C-PAM, NH4Cl as the chlorine source, and CTAB, KBr as the bromine source, respectively) were added into 15 mL NaOH aqueous solution (20 g L1) under magnetic stirring to obtain the solution ②. The HNO3 and NaOH solutions would be replaced by glycol when treated with glycol as an assisting solvent. Subsequently, the solution ① was added into the solution ② dropwisely under further stirring for 1 h at room temperature. The suspension was transferred into 50 mL Telfon-lined oxidation-resisting steel autoclave for the hydrothermal treatment, and kept at 150  C for 8 h, then cooled down to room temperature naturally. The resulting products were separated by filtration, washed thoroughly with distilled water and absolute alcohol for three times respectively to remove the other ionic species, and then dried at 80  C overnight. 2.3. Material characterization and analysis The crystallinity phase of the as-prepared samples were characterized by X-ray powder diffraction (XRD), the measurements were carried on an X-ray diffractometer (D8-FOCUS, Bruker, Germany) and scanned over the range of 10 to 70 (2q) with a step size 0.01, and Cu Ka radiation (l ¼ 1.5418 Å) was used as the radiation source. The micro morphology and particle size of the samples were observed by a field emission scanning electron microscope (FE-SEM, SU8010, Hitachi, Japan) and transmission electron microscopy (TEM, Tecnai G2 T20, FEI, America). EDS was employed to measure the composition of the product which was attached to the SEM. The surface area and pore volume of samples were analyzed by N2 adsorption-desorption method using automatic surface area analyzer (MicroActive for ASAP 2460, America). Zeta-potentials of the samples in water were measured by a Malvern zeta analyzer (Nano-ZS 90, MalvernInstrument, UK). The UVevis absorption

1146

J. Yang et al. / Journal of Alloys and Compounds 725 (2017) 1144e1157

spectra over a range of 200e700 nm were recorded on an UV2550PC spectrophotometer using BaSO4 as a reference (Shimadzu Corporation, Japan). Photoluminescence emission spectra were recorded on a Horiba Jobin Yvon FLUOROMAX-4P fluorescence spectrometer equipped with a 150 W xenon lamp. All the measurements were carried out at room temperature. 2.4. Photocatalytic activity measurement The photocatalytic performances of the BiOClxBr1-x powders were evaluated by degrading MO aqueous solution under visible light illumination. Photocatalytic reactor was irradiated by an overhead 300 W Xenon lamp equipped with a 400 nm cutoff filter which can guarantee the visible light irradiation only. What's more, the temperature of the reaction system was controlled by a circulating water system. In a typical run, a certain amount of the asprepared samples were dispersed into MO aqueous solution (50 mL). Prior to illumination, the mixed solution was treated with ultrasonic washer for 5 min in the dark to achieve a very good dispersion. Subsequently, the suspension was magnetically stirred in the dark for 30 min to establish adsorption-desorption equilibrium of MO on the photocatalyst surface. During irradiation, about 5 mL solution was removed from the reaction suspension at the interval of 10 min. The photocatalyst powders and the MO solution were separated by centrifuge, and the concentration of remnant MO was analyzed through a UVevis spectrophotometer by measuring the absorption peak strength at its characteristic wavelength to determine the extent of MO degradation. In addition, the reactions kinetics of MO degradation was pseudo-first-order kinetics in our experiments which reported as following equation:

Fig. 1. The XRD patterns of BiOCl0.5Br0.5 samples. (a) NH4Cl and KBr as the raw materials, water as the solvent, (b) C-PAM and KBr as the raw materials, water as the solvent, (c) C-PAM and CTAB as the raw materials, water as the solvent, (d) C-PAM and CTAB as the raw materials, glycol as the solvent.

3. Results and discussion

peaks of BiOCl and BiOBr samples were in good agreement with their standard XRD patterns and no peaks of other impurities were observed on the patterns, indicating the high purity and single phase of the samples. In addition, the XRD patterns of BiOClxBr1-x displayed the corresponding diffraction peaks for BiOCl and BiOBr. The enlarged image of Fig. 2 illustrated the change of (101) and (110) peaks, the diffraction peaks gradually shifted to a smaller angle with the decrease of Cl content (view the larger version), this would attribute to the larger ionic radius of Br (re Br ¼ 0.196 nm, re Cl ¼ 0.181 nm). Furthermore, the broader peaks of BiOClxBr1x solid solutions indicated that the samples would have a larger specific surface area and a smaller crystallite size. The average crystallite size could estimate based on broadening of (110) peak using the Scherrer's equation: [44]

3.1. Crystal structure

D¼

The XRD patterns of BiOCl0.5Br0.5 samples which were synthesized with different reactants and solvents are presented in Fig. 1. It was seen that the 4 samples showed the characteristic diffraction peaks at the same angles. Although the XRD patterns were not consistent with the standard cards of BiOCl (JCPDS card No. 06e0249) and BiOBr (JCPDS card No. 85e0862), the position of each diffraction peaks of samples were located in middle of the diffraction peaks of two crystal planes corresponding to the BiOCl and BiOBr respectively. Obviously, as-prepared BiOCl0.5Br0.5 crystals were not mixtures of BiOCl and BiOBr phases, and they were formed the solid solution by lattice substitution of Cl and Br atoms each other. Moreover, the preferred orientation existed in the crystal growth, and the growth direction of samples strongly depended on the halogen sources and solvents, it would anisotropic growth along the (001) crystal face with the addition of water (Fig. 1aec). In Fig. 1d, it was clear that the relative intensity ratio of the (110)/(001) was larger than the standard values in the JCPDS cards no. 06e0249 and no. 85e0862, and the diffraction peak of (110) face was stronger compared with the other peaks, indicating that the sample had a preferred orientation alone the (110) plane when the glycol as the reaction medium. Fig. 2 shows the XRD patterns acquired from as-synthesized BiOClxBr1-x (0 < x < 1) samples. All the characteristic diffraction

where l is the wavelength of Cu Ka source used, b is full width at half maximum (FWHM) of (110) diffraction plane, k is a shape factor (0.89) and q is angle of diffraction. We also calculated the cell parameter of all compounds by the least square extrapolation method, and the calculated values of crystallite size, lattice parameters and cell volume for BiOClxBr1-x samples are listed in Table 1. The crystallite sizes of BiOClxBr1-x phases were practically the same (10e15 nm) for all the tested family members. Moreover, a significant increase was observed in the a and c values with x values decreasing, suggesting that the introduction of Br ions into the solid solutions indeed resulting in an obvious lattice dilatation. All in all, it was confirmed that the as-prepared BiOClxBr1-x samples had the same tetragonal crystal structure and were a series of continuous solid solutions.

ln

C0 ¼ kt C

(1)

where k (min1) is the apparent first-order rate constant, C0 (mg$L1) is the initial concentration of MO, and C (mg$L1) is the remaining concentration of MO after irradiation time t (min).

kl b cosq

(2)

3.2. Morphology and microstructure The morphologies of BiOClxBr1-x samples are displayed in Fig. 3. It was revealed that the BiOCl0.5Br0.5 samples were just a simple piece of stacking with microplates when water as the reaction medium, and the agglomeration phenomenon for powder would increase with the addition of KBr (Fig. 3a and b). However, the

J. Yang et al. / Journal of Alloys and Compounds 725 (2017) 1144e1157

1147

Fig. 2. The XRD patterns of BiOClxBr1-x samples synthesized using C-PAM and CTAB as the raw materials and glycol as the solvent.

Table 1 The cell parameters and crystallite size of BiOClxBr1-x (x ¼ 0.00, 0.25, 0.50, 0.75, 1.00). Samples

a ¼ b, Å

c, Å

V, Å3

Crystallite size (nm)

BiOCl BiOCl0.75Br0.25 BiOCl0.5Br0.5 BiOCl0.25Br0.75 BiOBr

3.8834(12) 3.8926(1) 3.9013(9) 3.9157(3) 3.9219(4)

7.3408(8) 7.5654(6) 7.7305(4) 7.9377(1) 8.0954(2)

110.71(3) 114.64(1) 117.66(7) 121.70(6) 124.52(2)

11.78 15.05 14.19 13.43 10.93

sample showed the three dimensional structure, well dispersed and the thinner plates which synthesized with C-PAM and CTAB (Fig. 3c). This could be ascribed to the fact that C-PAM and CTAB

were not only surfactants but also reactants in the formation of BiOClxBr1-x. Meanwhile, it could be clearly observed that the BiOCl0.5Br0.5 displayed 3D hierarchical flower-like microspheres architectures self-assembled with ultrathin nanosheets when synthesized using glycol, C-PAM and CTAB, with an average diameter of 0.5e1 mm (Fig. 3d). Those changes of morphology would influence the photocatalytic activity of the photocatalyst. The formation of the different morphologies of BiOCl0.5Br0.5 samples could be expressed as a mechanism which was a process of dissolution and recrystallization from the kinetically controlled [39]. That is, the precursors dissolved quickly and a slow process of recrystallization and self-assembly to the new products under hydrothermal condition [40]. The formation of BiOClxBr1-x could be

Fig. 3. SEM images of BiOCl0.5Br0.5 samples. (a) NH4Cl and KBr as the raw materials, water as the solvent, (b) C-PAM and KBr as the raw materials, water as the solvent, (c) C-PAM and CTAB as the raw materials, water as the solvent, (d) C-PAM and CTAB as the raw materials, glycol as the solvent.

1148

J. Yang et al. / Journal of Alloys and Compounds 725 (2017) 1144e1157

proposed with the follow equations:

BiðNO3 Þ3 $5H2 O/Bi3þ þ 3NO 3 þ 5H2 O

(3)

Bi3þ þ C6 H8 O7 /½C6 H5 O7 3 Bi3þ þ 3Hþ

(4)

½C6 H5 O7 3 Bi3þ þ H2 O þ Hþ þ x Cl Hydrothermal

þ ð1  xÞBr 





! BiOClx Br1x þ 3Hþ þ ½C6 H5 O7 3 (5) 3þ

Firstly, Bi dissolved by citric acid in solvent in advance(Eqn. (3)), and the addition of citric acid could control the reaction rate of system by chelating with Bi3þ to produce the [C6H5O7]3-Bi3þ complex (Eqn. (4)). Then, the solution of [C6H5O7]3-Bi3þ was added dropwise into the halogen sources in solvent. Subsequently, Bi3þ would be released and reacted with H2O, Cl and Br to form BiOClxBr1-x via the hydrothermal processes (Eqn. (5)). A large number of organic functional groups on the sample surface may form defects and increase surface energy during the reaction. Therefore, the surface with high energy begins to shrink, while the low surface energy of the crystal face began to generate. Combining with the test results of XRD and SEM, it could be seen that the BiOCl0.5Br0.5 samples were favor to grow along the (001) crystal facet into nanosheets when water was used as the solvent. Square nanocrystals were formed firstly under hydrothermal conditions. When NH4Cl and KBr were used as the resources, the molecular weight and volume of [NH4]þ and Kþ were smaller, which had no obvious effect on the crystal growth, and the BiOCl0.5Br0.5 still exhibited a foursquare sheet-like morphology with uniform size and smooth surface (Fig. 3a). However, when NH4Cl was replaced by C-PAM, the size of the polymer group was bigger and the

viscosity of the reaction would also increase, the square structure would be destroyed during the crystal growth, and the microstructure of the samples showed irregular lamellar structure (Fig. 3b). The total molecular weight and size of groups increased again when C-PAM and CTAB as the raw materials, nanocrystals would grow into the curved thin sheets according to the asymmetry of force, and self-assembled into three-dimensional structures by surfactants (Fig. 3c). Glycol had the high viscosity so that it could hinder the mass transfer of ions and control the whole rates indirectly, and it also possessed the special physical and chemical properties because of the special structure for chain or network which was formed by the intermolecular hydrogen bond and hydroxyl, acting as the template agent in hydrothermal process. Gauzy-nanosheets were formed under the media of glycol, and then the curving thinner nanosheets were guided to self-assembly into a 3D hierarchical flower-like microspheres structure, which were growth along the (110) direction (Fig. 3d). Meanwhile, C-PAM and CTAB acted as not only the surfactants but also the reactants in the formation of the samples. In a word, glycol, C-PAM and CTAB played the crucial roles for controlling the crystal nucleation and growth. The possible growth and assembly mechanisms for the 3D hierarchical flower-like microsphere of BiOCl0.5Br0.5 are depicted in Fig. 4. In order to conclude the existence and proportion of Bi, O, Cl and Br clusters in an individual BiOCl0.5Br0.5 (Fig. 5a), EDS spectrum was acquired using SEM with an energy dispersive X-ray (EDX) on the same field of view at 15 kV accelerating voltage (Fig. 5b). It had become distinctly that the BiOCl0.5Br0.5 solid solution only contained the elements of Bi, O, Cl and Br, and the proportion of them was almost closing to 2:2:1:1. This result proved once again that the as-prepared sample was BiOCl0.5Br0.5. The elemental mappings were conducted and the results are showed in Fig. 5cef. It was clearly indicated that four elements were evenly distributed in the

Fig. 4. Schematic illustration of the possible growth and assembly mechanisms for the 3D hierarchical flower-like microsphere of BiOCl0.5Br0.5 in glycol.

J. Yang et al. / Journal of Alloys and Compounds 725 (2017) 1144e1157

exceptional structure. The transmission electron microscopy (TEM) image of BiOCl0.5Br0.5 (Fig. 5g) further demonstrated the obtained sample had hierarchical flower-like microspheres architecture selfassembled with Gauzy-nanosheets, the result was consistent with the SEM observation. The high-resolution TEM (HRTEM) image is illustrated in Fig. 5h. The spacing of the lattice fringe was measured to be 0.276 nm, which was slightly larger than the spacing of the (110) crystal plane of BiOCl (0.275 nm) and slightly smaller than the spacing of the (110) crystal plane of BiOBr (0.277 nm) in the BiOCl0.5Br0.5 photocatalyst, suggesting that the sample was grown

1149

along (110) direction and it was solid solution rather than the mixtures of BiOCl and BiOBr phases, which was consistent with the XRD result. Furthermore, based on the Bragg equation, the spacing of the solid solution could be calculated according to XRD result. The spacing of the (110) crystal plane of BiOCl0.5Br0.5 was calculated to be 0.27594 nm, which was close to the HRTEM result (0.276 nm). It was reported that the ratio of Cl-Br in BiOClxBr1-x could influence the oxidizing capacity and the light utilization of the photocatalysts. We have successfully synthesized the BiOClxBr1-x solid solutions with C-PAM and CTAB as the materials, glycol as an

Fig. 5. SEM image (a), EDS spectrum (b), element mapping images (cef), TEM image (g) and HRTEM image (f) of the individual BiOCl0.5Br0.5.

1150

J. Yang et al. / Journal of Alloys and Compounds 725 (2017) 1144e1157

Fig. 6. SEM images of BiOClxBr1-x samples synthesized using C-PAM and CTAB as raw materials and glycol as the solvent. BiOCl (a and a’), BiOCl0.75Br0.25 (b and b’), BiOCl0.5Br0.5 (c and c’), BiOCl0.25Br0.75 (d and d’), BiOBr (e and e’).

J. Yang et al. / Journal of Alloys and Compounds 725 (2017) 1144e1157

assisting solvent, respectively. The morphologies of BiOClxBr1-x samples are showed in Fig. 6. It could expressly observe that the samples showed the flower-like microspheres structure with an average diameter of 0.5e2 mm, and the samples were grown along the three-dimensional direction of self-assemble which constructed by numerous interlaced two-dimensional nanosheets with the smooth surface. The number of nanosheets had risen and the microspheres structure was more obvious with the increase of Br content. The results were different from the others' works [26e31], they concluded that all the solid solutions were microspheres which consisted of microplates with no obvious difference in the thickness and size, or the samples were in the form of nanosheets. Moreover, the staggered arrangement of BiOClxBr1-x nanosheets resulted in the existence of a large number of slits and holes between the sheets, this could lead to a very large specific surface area of BiOClxBr1-x. 3.3. Optical properties The UVevis absorption spectra of BiOClxBr1-x samples are showed in Fig. 7a. Pure BiOCl sample absorbed only ultraviolet light, originating from the charge transfer response from the valence band (VB) (hybrid orbitals of O 2p and Cl 3p) to conduction band (CB) (Bi 6p orbitals), while the pure BiOBr showed a stronger absorption in the visible range, contributing from the transition from VB (hybridized O 2p and Br 4p orbitals) to the CB (Bi 6p orbitals). It could be found that the absorption edges of BiOClxBr1-x solid solutions displayed a continuous redshift from 360 nm to 430 nm with decreasing of x, attributing to a band gap transition from VB (hybrid orbitals of O 2p, Cl 3p and Br 4p) to CB (Bi 6p orbitals) [41e43]. The optical properties of the catalysts were studied using the Kubelka-Munk function F(R), and the band gap energy (Eg) of catalyst could be estimated with Tauc's law, which were expressed as follows: [44]

FðRÞ ¼

ð1  RÞ2 2R

(7)

R ¼ 10A

(8)

1240

l

Table 2 Values of the band gap energy (Eg) and corresponding CB (ECB) and VB (EVB) edge positions calculated from diffuse reflectance spectra for BiOClxBr1-x photocatalysts. Value of x

X (eV)

Eg (eV)

ECB (eV)

EVB (eV)

0.00 0.25 0.50 0.75 1.00

6.18 6.22 6.27 6.32 6.36

2.72 2.85 2.95 3.04 3.21

0.320 0.295 0.295 0.300 0.255

3.040 3.145 3.245 3.340 3.465

Planck constant, light frequency, absorption wavelength and constant respectively. The value of n was determined by the type of optical transition of a semiconductor (n ¼ 1 for direct transition and n ¼ 4 for indirect transition). Taking into account the pure compounds, BiOX are indirect transition, so the value of n is 4. Fig. 7b reveals the [F(R)hn]1/2 versus hn plots of BiOClxBr1-x, and the Eg were estimated from the intercept of the tangents to the xaxis. It is helpful for researching the degradation mechanism of BiOClxBr1-x photocatalysts to determine the edge potentials of the CB and VB. At the zero point potential, the CB and VB edge position of the semiconductor could be predicted by the following empirical equations: [44]

(6)

 n FðRÞhn ¼ C hn  Eg 2

hn ¼

1151

(9)

Fig. 8. Nitrogen adsorption-desorption isotherm and corresponding pore-size distribution (insert) of the as-prepared BiOClxBr1-x.

where R, A, h, n, l and C are the reflection coefficient, absorbance,

Fig. 7. (a) UVevis diffuse reflectance spectra of the BiOClxBr1-x samples, (b) (ahn)1/2 versus hn plots of BiOClxBr1-x.

1152

J. Yang et al. / Journal of Alloys and Compounds 725 (2017) 1144e1157

Table 3 BET surface areas (SBET), Pore Volume and Pore Size of various BiOClxBr1-x microspheres as a function of x. x value

SBET (m2$g1)

Pore Volume (cm3$g1)

Pore Size (nm)

0.00 0.25 0.50 0.75 1.00

40.57 50.23 51.36 54.46 71.65

0.05 0.17 0.20 0.11 0.19

8.68 12.56 14.59 8.39 10.51

ECB ¼ X e EC e 0.5Eg

(10)

EVB ¼ ECB þ Eg

(11)

where X is the electronegativity of the semiconductor, EC is the energy of free electrons on the hydrogen scale (about 4.5 eV). The semiconductor electronegativity is defined as geometric mean of electronegativities of the constituent atoms is explained as follows: [44]

X¼

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi N cn1 cs2 ……cpm1 cqm

(12)

where c, n, m and N are the electronegativity of the constituent atom, the number of atoms for an element, the total number of elements and total number of atoms in the compound, respectively. Meanwhile, values of Eg and corresponding CB and VB edge positions of BiOClxBr1-x solid solutions are summarized in Table 2. The results indicated that the band gaps of BiOClxBr1-x solid solutions were narrowing gradually as the x value decreased, thus increasing the absorption of visible light range. 3.4. N2 adsorption - desorption characterization It has been widely recognized that the photocatalysis capability and reaction efficiency are closely related to the adsorption capability of powders, and the adsorption capability could be influenced by materials morphology, surface area and electrostatic interactions [45]. Generally, the greater of the catalyst specific surface area, the larger the effective contact area acquires between organic pollutant and photocatalyst, and thus increases the contact chance of reactant with the active groups of catalyst, and eventually leads to higher photocatalytic activity. The porosity and specific surface area of the BiOClxBr1-x microspheres were measured by using

Fig. 10. The zeta potentials of BiOCl0.5Br0.5 flower-like microspheres at different pH.

nitrogen adsorption-desorption and desorption isotherms analysis (Fig. 8). It could be concluded from these plots that the N2 adsorption desorption isotherm curves with hysteresis loops belonged to the IV type, which was the typical adsorption isotherm curve of mesoporous materials (2e50 nm) [23]. When the relative pressure P/P0 was 0.5e0.95, the adsorption-desorption capacity of N2 grown stronger rapidly with the increasing of relative pressure, which indicated that the pore size distribution was concentrated. The pore size distribution curve of BiOCl0.5Br0.5 microsphere was relatively wide, that was the coexistence of large holes and little holes in the samples. The little holes were distributed in the inner part of the ultrathin nanosheets, meanwhile the large holes might be caused by overlapping of BiOCl0.5Br0.5 gauzy-nanosheets. Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area of conventional BiOClxBr1-x solid solutions, and the BET specific surface areas, Pore Volume and Pore Size of various BiOClxBr1-x microspheres are listed in Table 3. In comparison to the BiOBr (40.57 m2 g1), the SBET of the others BiOClxBr1x samples were extremely bigger, and SBET were increasing as the value of x rose. In this series of samples, the flower-like microspheres of BiOCl0.5Br0.5 possessed the biggest Pore Volume (0.20 cm3 g1) and Pore Size (14.59 nm). This large SBET and porous structure were more favorable for the adsorption of organic molecules on the catalyst surface as well as the transport of reactants and products.

Fig. 9. Adsorption and photocatalytic activities of MO under visible light illumination, C0 ¼ 20 mg L1, volume 50 mL, catalyst 50 mg. (a) BiOCl0.5Br0.5 synthesized with various halogen sources and solvents, (b) BiOClxBr1-x synthesized with glycol as solvent.

J. Yang et al. / Journal of Alloys and Compounds 725 (2017) 1144e1157

1153

Fig. 11. Experimental data of MO degradation on BiOClxBr1-x with glycol as an assisting solvent under visible light irradiation, C0 ¼ 50 mg L1, volume 50 mL, catalyst 20 mg. (a) Degradation curves, (b) Time variation of UVevis spectra in visible light photocatalytic degradation of MO towards BiOCl0.5Br0.5 microsphere, (c) Degradation rates of MO, (d) Comparison of apparent reaction rate constants.

Afterwards, we could find that all the samples displayed enormous surface areas compared with those of other researchers' works in BiOX based photocatalysts [26e31]. Accordingly, this would result in higher photocatalytic activity and efficiency.

3.5. Photocatalytic properties The photocatalytic activity of BiOClxBr1-x samples was investigated by the degradation of methyl orange in water under visible illumination. Fig. 9a reveals the adsorption and photocatalytic degradation of MO aqueous solution over BiOCl0.5Br0.5 samples which were synthesized with various halogen sources and solvents. All adsorption-desorption equilibria between powders and MO molecule were reached in 30 min under the dark with strong agitation. The results suggested that the BiOCl0.5Br0.5 nanoplates acted the worst performance and lower adsorption when the NH4Cl Table 4 The pseudo-first order rate constants k for MO photocomposition over BiOClxBr1-x solid solutions. Sample

Fitted equation

P25 BiOCl BiOCl0.75Br0.25 BiOCl0.5Br0.5 BiOCl0.25Br0.75 BiOBr

y y y y y y

¼ ¼ ¼ ¼ ¼ ¼

0.0005xþ0.0021 0.0012xþ0.0129 0.0256x-0.1551 0.0446x-0.2646 0.0180x-0.0227 0.0072xþ0.0072

k (min1)

Correlation coefficient R

0.0005 0.0012 0.0256 0.0446 0.0180 0.0072

0.9897 0.9690 0.9718 0.9792 0.9972 0.9963

and KBr as the raw materials, while the sample synthesized using C-PAM and CTAB as halogen source had the better adsorption performance and visible light photocatalytic activity. When the water was changed into glycol, the adsorption rate reached to 83.6% after 30 min, most of the MO molecules were adsorbed onto the surface of the catalyst powders, and the dye was completely degraded after about 50 min under visible light illumination. It was found that the BiOCl0.5Br0.5 nanosheets exhibited obviously lower activity compared with the spherical ones. The great adsorption performance was accordance with the previous SEM results (Fig. 3aed). From the above discussion we could conclude that the C-PAM and CTAB were the optimum halide sources, and glycol was the optimum solvent in this series of experiments. Whereafter, the photocatalytic properties of BiOClxBr1-x solid solutions which were synthesized using optimum halide sources and solvent were investigated. For comparison, P25 was also tested under the same conditions. Initially, we used the general amount of catalyst (50 mg) and the concentration of MO (20 mg L1) in the degradation system, and the results are showed in Fig. 9b. The sample of Blank suggested that MO didn't exhibit significant degradation without the addition of photocatalyst, so the self-degradation of MO under visible light irradiation was negligible. The adsorption capacity of BiOCl0.75Br0.25 was the largest, followed by BiOCl0.5Br0.5. Nevertheless, the visible light photocatalytic activity of BiOCl0.5Br0.5 was better than that of BiOCl0.75Br0.25. It was clearly that the BiOClxBr1-x (0 < x < 1) solid solutions exhibited powerful adsorption ability and catalytic performance than the pure BiOCl, BiOBr and P25, and the excellent absorption capacity was related to the unique hierarchical

1154

J. Yang et al. / Journal of Alloys and Compounds 725 (2017) 1144e1157

Fig. 12. (a) Recycling experiments of visible light photocatalytic degradation of MO over the BiOCl0.5Br0.5 microspheres. Catalyst 20 mg, C0 ¼ 50 mg L1; volume 50 mL, (b) XRD patterns of BiOCl0.5Br0.5 microspheres sample before and after reaction.

flower-like microspheres structure and lager surface area. Although adsorption was an essential step in the photocatalytic process, sometimes the large adsorption quantity would hinder the photocatalytic, and resulted in a less obvious degradation process. First of all, too much dye molecule adsorbed on the surface might prevent the light from reaching the surface of the catalyst. Besides, the adsorption of reactants, products or the formation of carbon deposition might cover on the catalyst surface and eventually lead to low catalytic activity. The Zeta potential (ZP) could help to obtain information about the surface charge of solid powder samples, which was helpful for further identification of the functional groups on the surface. However, the ZP had a strong dependence for the value of pH [28]. Fig. 10 depicts the plot of ZP of BiOCl0.5Br0.5 flower-like microspheres with pH values ranging from 1 to 13. The results indicated that the as-prepared BiOCl0.5Br0.5 possessed surface negative charges (35.3 mV) at neutral condition and the isoelectric point (IEP) of sample was approximately 1.78. Theoretically, it was apparent that negatively charged BiOClxBr1-x microspheres were not favorable for the adsorption of anionic dye MO (with a sulphuric group) [27]. Thus the surface electrostatic effect was not dominant in the adsorption process of BiOClxBr1-x solid solutions. But they exhibited relatively excellent adsorption capability in the 20 mg L1 MO aqueous solution (Fig. 9b), this phenomenon would attribute to the superior SBET (Table 3). In order to reduce the influences of adsorption, we added the initial concentration of MO aqueous solution (50 mg L1) and reduced the amount of the photocatalyst (20 mg). In Fig. 11a, the BiOClxBr1-x samples still carried strong adsorption capacity. Over a period of 70 min visible-light irradiation, the photodegradation efficiencies of MO were 3.3%, 9.0%, 57.0%, 72.1%, 58.4% and 36.0% with P25, BiOCl, BiOCl0.75Br0.25, BiOCl0.5Br0.5, BiOCl0.25Br0.75 and BiOBr, respectively. The results showed the excellent absorption property and photocatalytic activity of BiOCl0.5Br0.5 compared to Qin's work [30]. The BiOCl0.5Br0.5 microspheres still displayed the highest visible-light photocatalytic activity among six tested catalysts. At the same time, P25 and BiOCl exhibited considerably poor visible-light photocatalytic activity because of the big band gaps (3.20 eV and 3.21 eV, respectively). Although BiOBr had a lower band gap (2.72 eV) and could absorb the visible light, its photocatalytic activity was still lower, mainly because of the high recombination of photogenerated electrons and holes. Afterwards, time variation of UVevis spectrum in visible light photocatalytic degradation of MO towards BiOCl0.5Br0.5 microsphere was

measured by ultravioletevisible spectrophotometer to clearly observe the degradation process of MO (Fig. 11b). It could be seen that the intensity of characteristic absorption peak of MO at 462 nm became weak gradually under the continuous irradiation of visible light, and disappeared after 70 min, proving that MO molecule was degraded totally. Fig. 11c shows the plot of ln(C/C0) of MO concentration vs time and the fitting curves for the experiment data of BiOClxBr1-x photocatalysts. The determined rate constants k and correlation coefficient R are listed in Table 4. All the values of R were closely to 1, suggesting that the photodegradation of MO was pseudo-first-order reaction kinetic. It was seen that the k values of BiOClxBr1-x solid solutions were considerably higher than P25, pure BiOCl and BiOBr. Especially when x ¼ 0.5, the reaction rate constant was run up to 0.0446, which showed that the BiOClxBr1-x (0 < x < 1) microspheres could be used as effective photocatalysts in visible light region (Fig. 11d). It was well known that the stability of photocatalyst was the key to its practical application. The results of recycling experiments of visible light photocatalytic degradation of MO over the BiOCl0.5Br0.5 microspheres are showed in Fig. 12a. It was seen that the degradation percentage of MO decreased from 98.27% to 94.71%, the visible light photocatalytic activity was slightly reduced under the same experimental conditions after six runs, attributing to the loss of some nanoparticles during the centrifugation and washing

Fig. 13. Schematic illustration of the band gap structures of BiOCl, BiOCl0.5Br0.5 and BiOBr.

J. Yang et al. / Journal of Alloys and Compounds 725 (2017) 1144e1157

1155

process. The XRD patterns of BiOCl0.5Br0.5 before and after reaction are also displayed in Fig. 12b. The intensity and position of the diffraction peak did not obvious change after 6 cycles. These results demonstrated that the sample had high stability in the process of cycling photocatalysis. Therefore, the as-prepared BiOClxBr1-x photocatalysts exhibited excellent photocatalytic stability for the degradation of MO aqueous solution.

3.6. Photocatalytic reaction mechanism In theory, the potentials of VB and CB were the important factors for the degradation of organic. Classically, the more positive the VB edge potential was, the stronger the oxidation ability was. Similarly, the CB edge potential was more negative, the reduction ability was more powerful. In addition, redox ability and the visible light energy utilization rate were influenced by the composition of the photocatalysts. On the basis of the results of Table 2, the schematic illustration of the band gap structures of BiOCl, BiOCl0.5Br0.5 and BiOBr is drawn in Fig. 13. The band gap energy declined obviously with quantity of Cl content decrease, although this would lead to the reduction of oxidation ability of hole and reduction ability of electron, the visible light utilization rate of the catalyst was improved. Due to the competition between two sides, BiOCl0.5Br0.5 exhibited the most excellent visible light photocatalytic activity. Besides that, BiOCl0.5Br0.5 had a special hierarchical flower-like microspheres structure and superhigh surface area which could promote diffusion of reactants and products, and the light could be reflected between the layers to increase the utilization ratio of light energy. This structure could also provide enough space for the polarization orbit to enhance the separation and migration efficiency of photogenerated electron-hole pairs. Due to the peak intensity strongly depended on the recombination between photogenerated electrons and holes, photoluminescence (PL) spectrum is usually used to consider the efficiency of charge transfer and carrier trapping of the photocatalysts [46]. Fig. 14 shows the PL spectra of BiOClxBr1-x samples which were excited under the light of 320 nm. A strong emission band centered at 430 nm in spectra, and another emission peak of slightly weaker was displayed at about 468 nm. Clearly, the PL emission intensity of BiOClxBr1-x photocatalysts decreased initially and then increased as the content decreased of Cl element, and the BiOCl0.5Br0.5 had the lowest emission intensity. It could be attributed to the carries transfer in the 3D hierarchical flower-like

Fig. 14. Photoluminescence (PL) spectra of as-prepared BiOClxBr1-x.

Fig. 15. Influence of various scavengers on the visible light photocatalytic activity of BiOCl, BiOCl0.5Br0.5 and BiOBr microspheres towards the degradation of an aqueous MO. C0 ¼ 50 mg L1, volume 50 mL, catalyst 20 mg.

microsphere of sample [47], revealing the BiOClxBr1-x solid solutions could effectively reduce the recombination rate of the electron-hole pairs. In order to clarify the mechanism of photocatalysis, we studied the effect of various active species on the degradation process by adding active species trapping agent into the reaction system. EDTA-2Na, isopropanol (IPA) as well as 1, 4-benzoquinone (BQ) were used as the hole (hþ ) scavVB) scavenger, hydroxyl radical ( enger as well as superoxide radical ( ) scavenger, respectively. As shown in Fig. 15. It was clear that radical was the predominant active species generated in the BiOClxBr1-x since the degradation rate of MO decreased significantly upon addition of BQ. It seemed that the CB potentials of BiOClxBr1-x and BiOBr were not negative enough to reduce O2 to radicals because the CB potential of BiOClxBr1-x was more positive than the standard redox potential of O2/ (0.046 eV) (Fig. 13), yet, the higher energy part of visible light could induce the photoexcited electrons exciting up to a reformed higher potential position more negatively than standard redox potential of O2/ . As a result, still could generate by the reduction of O2 with e CB [30]. The influence of IPA was hardly able to observe, suggesting that few radicals could be generated in reaction system. This is because in the valence band of BiOClxBr1-x (Bi3þ), the photogenerated holes could regarded as Bi5þ, and the standard redox potential of BiV/BiIII (þ1.59 eV) was too negative than that of /OH (þ1.99 eV), so the hþ on the surface of BiOClxBr1-x and BiOBr cannot oxidate OH into [48]. In the presence of EDTA-2Na, the degradation of MO was restrained and hþ was one of the active species in BiOCl and BiOCl0.5Br0.5 system, but the degradation efficiency of MO was somewhat speeded in BiOBr system because EDTA-2Na could capture hþ and prevent the recombination of hþ and e. Based on the above discussion, the photocatalytic degradation of MO could be described as the following equations: þ BiOClxBr1-x þ hv / BiOClxBr1-x (e CB þ hVB)

(13)

e CB þ O2 /

(14)

þ MO / intermediates / CO2 þ H2O

(15)

hþ VB þ MO / intermediates / CO2 þ H2O

(16)

1156

J. Yang et al. / Journal of Alloys and Compounds 725 (2017) 1144e1157

4. Conclusions In summary, a series of BiOCl0.5Br0.5 photocatalysts were successfully synthesized using various halogen sources and solvents through a tunable solvothermal process, and the photocatalytic performances were investigated in room temperature. The results proved that C-PAM and CTAB were the optimum halide sources, and glycol was the optimum solvent in our work. Furthermore, the morphology and band gap could be controlled by changing of the ratio of Cl and Br in BiOClxBr1-x solid solutions, which was an interesting result that the 3D hierarchical flower-like BiOClxBr1-x microspheres had comparatively superhigh surface area, this resulted in the excellent adsorption performance and highly efficient visible light photocatalytic. In addition, the features of these morphology and greater specific surface area highly improved light harvesting, boosted catalytic active sites, charge separation and migration of BiOClxBr1-x solid solutions, and meanwhile the recombination of photogenerated electrons and holes was largely suppressed. The photocatalytic activity of BiOClxBr1-x reached the maximum at x ¼ 0.5 and decreased gradually with x moving away from 0.5. were the dominating active species, with the secondary and minor factors of hþ and under visible light irradiation by radicals trapping experiments. The catalyst has also shown excellent stability and efficiency even after 6 runs in the photodegradation process. This work could open new possibilities to provide some insight into the synthesis of photocatalysts with controllable structures and high surface area with bandgap engineering, for degrading organic pollutants and other applications. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 21571162 and 11305145), the Guangdong Province Enterprise-University-Academy Collaborative Project (No. 2012B091100474). References [1] A. Fujishima, K. Honda, Electrochemical photocatalysis of water at semiconductor electrode, Nature 238 (1972) 37e38. [2] Y. Ma, X. Wang, Y. Jia, X. Chen, H. Han, C. Li, Titanium dioxide-based nanomaterials for photocatalytic fuel generations, Chem. Rev. 114 (2014) 9987e10043. [3] L. Zheng, S. Han, H. Liu, P. Yu, X. Fang, Hierarchical MoS2 nanosheet@TiO2 nanotube array composites with enhanced photocatalytic and photocurrent performances, Small 12 (2016) 1527e1536. [4] W. He, H.K. Kim, W.G. Wamer, D. Melka, J.H. Callahan, J. Yin, Photogenerated charge carriers and reactive oxygen species in ZnO/Au hybrid nanostructures with enhanced photocatalytic and antibacterial activity, J. Am. Chem. Soc. 136 (2014) 750e757. [5] S. Rengaraj, X.Z. Li, P.A. Tanner, Z.F. Pan, G.K.H. Pang, Photocatalytic degradation of methylparathion-an endocrine disruptor by Bi3þ-doped TiO2, J. Mol. Catal. A Chem. 247 (2006) 36e43. [6] H.Y. Jiang, P. Li, G. Liu, J. Ye, J. Lin, Synthesis and photocatalytic properties of metastable b-Bi2O3 stabilized by surface-coordination effects, J. Mater. Chem. A 3 (2015) 5119e5125. [7] K. Li, Y. Liang, J. Yang, Q. Gao, Y. Zhu, S. Liu, R. Xu, X. Wu, Controllable synthesis of {001} facet dependent foursquare BiOCl nanosheets: a high efficiency photocatalyst for degradation of methyl orange, J. Alloys Compd. 695 (2017) 238e249. [8] L. Ye, X. Jin, C. Liu, C. Ding, H. Xie, K.H. Chu, P.K. Wong, Thickness-ultrathin and bismuth-rich strategies for BiOBr to enhance photoreduction of CO2 into solar fuels, Appl. Catal. B Environ. 187 (2016) 281e290. [9] K. Ren, K. Zhang, J. Liu, H. Luo, Y. Huang, X. Yu, Controllable synthesis of hollow/flower-like BiOI microspheres and highly efficient adsorption and photocatalytic activity, CrystEngComm 14 (2012) 4384e4390. [10] Y. Guo, G. Zhang, J. Liu, Y. Zhang, Hierarchically structured a-Fe2O3/Bi2WO6 composite for photocatalytic degradation of organic contaminants under visible light irradiation, RSC Adv. 3 (2013) 2963e2970. [11] Y. Zhang, W. Li, Z. Sun, Q. Zhang, L. Wang, Z. Chen, In-situ synthesis of heterostructured BiVO4/BiOBr core-shell hierarchical mesoporous spindles with highly enhanced visible-light photocatalytic performance, J. Alloys Compd. 713 (2017) 78e86.

[12] L. Zhang, T. Xu, X. Zhao, Y. Zhu, Controllable synthesis of Bi2MoO6 and effect of morphology and variation in local structure on photocatalytic activities, Appl. Catal. B Environ. 98 (2010) 138e146. [13] G. Wang, X. Luo, Y. Huang, A. Kuang, H. Yuan, H. Chen, BiOX/BiOY (X, Y¼ F, Cl, Br, I) superlattices for visible light photocatalysis applications, RSC Adv. 6 (2016) 91508e91516. [14] B. Pare, B. Sarwan, S.B. Jonnalagadda, Photocatalytic mineralization study of malachite green on the surface of Mn-doped BiOCl activated by visible light under ambient condition, Appl. Surf. Sci. 258 (2011) 247e253. [15] J. Fu, Y. Tian, B. Chang, F. Xi, X. Dong, BiOBr-carbon nitride heterojunctions: synthesis, enhanced activity and photocatalytic mechanism, J. Mater. Chem. 22 (2012) 21159e21166. [16] X. Xiao, W.D. Zhang, Facile synthesis of nanostructured BiOI microspheres with high visible light-induced photocatalytic activity, J. Mater. Chem. 20 (2010) 5866e5870. [17] X. Zhang, G. Xu, J. Hu, J. Lv, J. Wang, Y. Wu, Fabrication and photocatalytic performances of BiOCl nanosheets modified with ultrafine Bi2O3 nanocrystals, RSC Adv. 6 (2016) 63241e63249. [18] C. Liu, Q. Wu, M. Ji, H. Zhu, H. Hou, Q. Yang, C. Jiang, J. Wang, L. Tian, J. Chen, W. Hou, Constructing Z-scheme charge separation in 2D layered porous BiOBr/graphitic C3N4 nanosheets nanojunction with enhanced photocatalytic activity, J. Alloys. Compd. 723 (2017) 1121e1131. [19] C. Song, Y. Feng, W. Shi, C. Liu, Fabrication and mechanism of a novel direct solid-state Z-scheme photocatalyst CdS/BiOI under the visible light, CrystEngComm 18 (2016) 7796e7804. [20] L. Ye, J. Liu, C. Gong, L. Tian, T. Peng, L. Zan, Two different roles of metallic Ag on Ag/AgX/BiOX (X ¼ Cl, Br) visible light photocatalysts: surface plasmon resonance and Z-scheme bridge, Acs Catal. 2 (2012) 1677e1683. [21] G. Jiang, X. Wang, Z. Wei, X. Li, X. Xi, R. Hu, B. Tang, R. Wang, S. Wang, T. Wang, W. Chen, Photocatalytic properties of hierarchical structures based on Fedoped BiOBr hollow microspheres, J. Mater. Chem. A 1 (2013) 2406e2410. [22] R. Wang, G. Jiang, X. Wang, R. Hu, X. Xi, S. Bao, Y. Zhou, T. Tong, S. Wang, T. Wang, W. Chen, Efficient visible-light-induced photocatalytic activity over the novel Ti-doped BiOBr microspheres, Powder Technol. 228 (2012) 258e263. [23] J. Di, J.X. Xia, M.X. Ji, B. Wang, S. Yin, Q. Zhang, Z.G. Chen, H.M. Li, Carbon quantum dots modified BiOCl ultrathin nanosheets with enhanced molecular oxygen activation ability for broad spectrum photocatalytic properties and mechanism insight, ACS Appl. Mater. Interfaces 7 (2015) 20111e20123. [24] K.X. Ren, J. Liu, J. Liang, K. Zhang, X. Zheng, H.D. Luo, Y.B. Huang, P.J. Liu, X.B. Yu, Synthesis of the bismuth oxyhalide solid solutions with tunable band gap and photocatalytic activities, Dalton Trans. 42 (2013) 9706e9712. [25] S.X. Ouyang, J.H. Ye, b-AgAl1-xGaxO2 solid-solution photocatalysts: continuous modulation of electronic structure toward high-performance visiblelight photoactivity, J. Am. Chem. Soc. 133 (2011) 7757e7763. [26] H. Gnayem, Y. Sasson, Hierarchical nanostructured 3D flowerlike BiOClxBr1ex semiconductors with exceptional visible light photocatalytic activity, ACS Catal. 3 (2013) 186e191. [27] H. Gnayem, Y. Sasson, Nanostructured 3D sunflower-like bismuth doped BiOClxBr1-x solid solutions with enhanced visible light photocatalytic activity as a remarkably efficient technology for water purification, J. Phys. Chem. C 119 (2015) 19201e19209. [28] H. Gnayem, A. Dandapat, Y. Sasson, Development of hybrid BiOClxBr1-xembedded alumina films and their application as highly efficient visible-lightdriven photocatalytic reactors, Chem. Eur. J. 22 (2016) 370e375. [29] T. Wu, X. Li, D. Zhang, F. Dong, S. Chen, Efficient visible light photocatalytic oxidation of NO with hierarchical nanostructured 3D flower-like BiOClxBr1x solid solutions, J. Alloys Compd. 671 (2016) 318e327. [30] Q. Qin, Y. Guo, D. Zhou, Y. Yang, Y. Guo, Facile growth and compositiondependent photocatalytic activity of flowerlike BiOCl1-xBrx hierarchical microspheres, Appl. Surf. Sci. 390 (2016) 765e777. [31] D. Du, W. Li, S. Chen, T. Yan, J. You, D. Kong, Synergistic degradation of rhodamine B on BiOClxBr1-x sheets by combined photosensitization and photocatalysis under visible light irradiation, New J. Chem. 39 (2015) 3129e3136. [32] X. Zhang, C.Y. Wang, L.W. Wang, G.X. Huang, W.K. Wang, H.Q. Yu, Fabrication of BiOBrxI1x photocatalysts with tunable visible light catalytic activity by modulating band structures, Sci. Rep. 6 (2016) 22800. [33] H.Y. Xu, X. Han, Q. Tan, X.L. He, S.Y. Qi, Structure-Dependent photocatalytic performance of BiOBrxI1x nanoplate solid solutions, Catalysts 7 (2017) 153. [34] W.J. Kim, D. Pradhan, B.K. Min, Y. Sohn, Adsorption/photocatalytic activity and fundamental natures of BiOCl and BiOClxI1-x prepared in water and ethylene glycol environments, and Ag and Au-doping effects, Appl. Catal. B Environ. 147 (2014) 711e725. [35] Y. Zhang, X. Sun, G. Yang, Y. Zhu, H. Si, J. Zhang, Y. Li, Preparation and characterization of bifunctional BiOClxIy solid solutions with excellent adsorption and photocatalytic abilities for removal of organic dyes, Mat. Sci. Semicon. Proc. 41 (2016) 193e199. [36] J. Xie, Y. Cao, D. Jia, Dahlia-shaped BiOClxI1x structures prepared by a facile solid-state method: evidence and mechanism of improved photocatalytic degradation of rhodamine B dye, J. Colloid Interface Sci. 503 (2017) 115e123. [37] J. Wang, Y. Xia, Y. Dong, R. Chen, L. Xiang, S. Komarneni, Defect-rich ZnO nanosheets of high surface area as an efficient visible-light photocatalyst, Appl. Catal. B Environ. 192 (2016) 8e16. [38] L. Qi, B. Cheng, J. Yu, W. Ho, High-surface area mesoporous Pt/TiO2 hollow

J. Yang et al. / Journal of Alloys and Compounds 725 (2017) 1144e1157

[39]

[40]

[41]

[42]

[43]

chains for efficient formaldehyde decomposition at ambient temperature, J. Hazard. Mater 301 (2016) 522e530. L. Xu, X. Ji, J.G. Jiang, L. Han, S. Che, P. Wu, Intergrown zeolite MWW polymorphs prepared by the rapid dissolution-recrystallization route, Chem. Mater 27 (2015) 7852e7860. Y. Zhu, Y. Liang, S. Liu, K. Li, X. Wu, R. Xu, Morphology evolution of Gd0.99Eu0.01BO3 phosphors synthesized by hydrothermal method with Gd0.99Eu0.01(OH)3 nanorods as sacrificial precursors, J. Alloys Compd. 685 (2016) 848e859. B. Cao, P. Dong, S. Cao, Y. Wang, BiOCl/Ag3PO4 composites with highly enhanced ultraviolet and visible light photocatalytic performances, J. Am. Ceram. Soc. 96 (2013) 544e548. K. Rajeshwar, N.R. de Tacconi, C.R. Chenthamarakshan, Semiconductor-based composite materials: preparation, properties, and performance, Chem. Mater 13 (2001) 2765e2782. G.G. Liu, T. Wang, S.X. Ouyang, L.Q. Liu, H.Y. Jiang, Q. Yu, T. Kako, J.H. Ye, Bandstructure-controlled BiO(ClBr)(1-x)/2Ix solid solutions for visible-light photocatalysis, J. Mater. Chem. A 3 (2015) 8123e8132.

1157

[44] R. Kavitha, L.G. Devi, Synergistic effect between carbon dopant in titania lattice and surface carbonaceous species for enhancing the visible light photocatalysis, J. Environ. Chem. Eng. 2 (2014) 857e867. [45] A. Meng, J. Xing, Z. Li, Q. Li, Cr-doped ZnO nanoparticles: synthesis, characterization, adsorption property, and recyclability, ACS Appl. Mater. Interfaces 7 (2015) 27449e27457. [46] X. Xu, L. Hu, N. Gao, S. Liu, S. Wageh, A.A. Al-Ghamdi, A. Alshahrie, X. Fang, Controlled growth from ZnS nanoparticles to ZnS-CdS nanoparticle hybrids with enhanced photoactivity, Adv. Funct. Mater 25 (2015) 445e454. [47] S. Wang, D. Li, C. Sun, S. Yang, Y. Guan, H. He, Synthesis and characterization of g-C3N4/Ag3VO4 composites with significantly enhanced visible-light photocatalytic activity for triphenylmethane dye degradation, Appl. Catal. B Environ. 144 (2014) 885e892. [48] F.T. Li, Q. Wang, X.J. Wang, B. Li, Y.J. Hao, R.H. Liu, D.S. Zhao, In-situ one-step synthesis of novel BiOCl/Bi24O31Cl10 heterojunctions via self-combustion of ionic liquid with enhanced visible-light photocatalytic activities, Appl. Catal. B Environ. 150 (2014) 574e584.