Article history: Received 16 August 2016 Received in revised form 21 September 2016 Accepted 21 September 2016 Available online 6 October 2016

A B S T R A C T 

BiVO4 as one of the promising visible-light-driven photocatalysts attracted considerable research on morphology and composition control. In this work, a modified carbonaceous spheres sacrificial template growth technique are developed to build up multi-shell hollow spheres of the heterostructured Bi–V–O. By treating the carbonaceous spheres with NaOH aqueous, the simultaneous adsorption of Bi3+ and VO3 are achieved successfully, and through the precisely controlled calcination, the nanoparticles of BiVO4 and Bi4V2O11 are crystallized and interconnected into the Bi–V–O heterostructured multi-shell hollow spheres. These Bi–V–O hollow spheres demonstrate a high visible-light-driven photocatalytic activity towards the decomposition of Methylene blue, and the double-shell one with the highest Bi4V2O11 content shows the best photocatalytic activity. The high photocatalytic activity may due to the effective utilization of visible light induced by multiple reflections of their special multi-shell hollow spheres. The heterostructure between BiVO4 and Bi4V2O11 may also make a contribution to the enhanced photocatalytic activity. ã 2016 Elsevier Ltd. All rights reserved.

1. Introduction Solar energy has been widely used in photocatalytic degradation of organic pollutants [1], photoelectrochemical water oxidation [2], dye sensitized solar cells [3]. In the past years, semiconductor photocatalysts have received signifificant attention because their broaden application including solar water splitting [4,5], photodegration of organics [6–8], CO2 conversion [9,10]. Among numerous photocatalysts, bismuth vanadate has drawn great attention because of its suitable band gap for visible light irradiation, nontoxicity, higher stability, and environment friend liness [11–13]. BiVO4 can crystallize as three phases: the monoclinic scheelite structure (ms-BiVO4),the tetragonal scheelite structure (ts-BiVO4), and the tetragonal zircon structure (zt-BiVO4) [14]. It is observed that the properties of BiVO4 depend strongly on their crystalline phase and morphology [15]. Tetragonal BiVO4 with a band gap of 2.9 eV can only absorb the UV radiation, while the monoclinic ms-BiVO4 with a band gap of 2.4–2.5 eV can absorb both the UV and the visible light. On the other hand, the low separation efficiency of photogenerated electrons and holes and the electrical conductivity hides the practical applications of BiVO4 [16]. Many attempts including the synthesis of heterojunction structures [17,18], doping [19] and controlling the morphologies [20] have been explored to overcome these disadvantages. Among them, morphology control attracted considerable attention, and ms-BiVO4 with morphologies such as nanorods [21], thin film [22], irregular polyhedron[23], nanosheets [24] have been achieved through a variety of approaches. Recent research revealed that the special multi-shell hollow structures can help the materials to get more active site, better performance and higher effective density [25–27]. However, report on the controlled synthesis of the msBiVO4 multi-shell hollow spheres is rather rare, due to the diffificulties in the synthesis [28]. In addition, it is also proved that building up heterojunction structure or mixed crystalline phases can further improve the photocatalytic activity of the BiVO4 remarkably [29]. Apparently, to build the multi-shell hollow spheres with heterostructure or mixed crystalline phases in Bi–V– O system may be of great signifificance and lead to much improved photocatalytic performance. To obtain multi-shell hollow spheres, hard template method [30–33], with uniform carbonaceous spheres as the sacrifificed templates are believed as the most common and effective method. However, the conventional carbonaceous spheres hard templates approach does not work well with the systems having large ions like Bi3+ and VO3
, due to the low adsorption capacity and diffificult diffusion of large ions in the carbonaceous spheres [34], as well as 

the facility to crystallize of the Bi3+-based compounds. So it is still a big challenge to use the carbonaceous spheres hard templates approach to synthesize the controlled multi-shell hollow spheres of ms-BiVO4. In this work, with the surface pretreated carbonaceous spheres as template, the successful absorption of Bi3+ and VO3
were realized. Uniform Bi–V–O multi-shell hollow spheres with heterostructure (BiVO4 and Bi4V2O11) could be successfully obtained through controlled calcination. With this premium structure, the as-synthesized materials express excellent visiblelight-driven photocatalytic activity. 2. Experimental section 2.1. Materials and preparation 2.1.1. Reagents The Bi(NO3)3 5H2O, NH4VO3, NaOH and ethylene glycol were purchased from the Sinopharm Chemical Reagent Co., Ltd. All chemicals were analytical-grade reagents and used as purchased without further purifification. 2.1.2. The preparation of the carbonaceous microspheres templates In a typical synthesis of carbonaceous microspheres, sucrose (130 g) was dissolved in deionized water (250 mL) to form a clear solution. The solution was then added into a 500 mL Teflflon-lined autoclave and then the autoclave was heated at 200  C for 2 h. Subsequently, when the autoclave was cooled to room temperature, the precipitates were collected and washed with deionized water and absolute ethanol for several times, then dried at 80  C for 6 h 2.1.3. The synthesis of Bi–V–O multi-shell hollow spheres In a water bath at 60  C, 1.455 g Bi(NO3)3 5H2O was added to 30 mL of ethylene glycol. The mixture was stirred vigorously for several minutes to form a clarifying solution. Then followed by adding 0.35 g NH4VO3. The as-prepared carbonaceous spheres templates (0.4 g) were added and well dispersed into the above solution as hard templates. Subsequently, the mixture was adequately stirred for 6–8 h at 60  C. At last the suspension was collected and washed alternately with deionized water and ethanol to obtain single-shell precursors. Bi–V–O single-shell hollow spheres were obtained by calcination of the single-shell precursors in air at the heating rate of R = 10  C/min and the fifinal temperature of 550  C. In order to increase the surface functional groups, we modifified the carbonaceous spheres by immersing them in 0.1 M NaOH for 24 h. After that, the modifified carbonaceous spheres were centrifuged for one time. Finally, they were dried at 80  C for 12 h. Instead of the carbonaceous spheres, the surface pretreated carbonaceous spheres (0.4 g) were used as templates to increase the adsorption quantity of Bi3+ and VO3
. Correspondingly precursors with higher amount of Bi3+ and VO3
were obtained, which are further calcined at a controlled heating rates to obtain the fifinal Bi–V–O double-shell and triple-shell hollow spheres. When the precursor was treated under the fast heating rate of 10  C/min and the fifinal temperature of 550  C, the double-shell hollow spheres were formed; while the sample was treated under the heating rate of 2  C/min fifirst and then at 10  C/min as well as the fifinal temperature of 550  C, the triple-shell hollow spheres were obtained. 2.2. Characterization The crystal structures of the samples were characterized by means of x-ray diffraction (XRD, X’pert PRO, PANalytical) with Cu Karadiation. In order to determine the composition of the precursors and the calcination temperature of the precursors, thermogravimetric (TG) and differential scanning calorimetry (DSC) data of the precursor were recorded on a thermal analysis instrument (TG/DTA6300) with a heating rate of 10  C/min under the air flflow. The morphologies and microstructures of the asprepared samples were inspected by the scanning electron microscope (FE-SEM, ZEISS SUPRATM 55) and transmission electron microscopy (TEM, JEM-2100, accelerating voltage 200 kV). The UV–vis diffuse reflflectance spectra were obtained on a TU-1901 spectrophotometer in the 250–800 nm wavelength range. Brunauer-Emmett-Teller (BET) surface areas of the samples were analyzed by nitrogen adsorption-desorption measurement on a Quantachrome Autosorb-1MP sorption analyzer with prior degassing under vacuum at 200  C. A 300W Xe lamp (CEL-HXF300) was used as a light source with a cutoff fifilter (Kenko L-42) was employed for the visible-light irradiation (l> 420 nm). 2.3. Measurement of photocatalytic activity Methylene blue (MB) is difficult to be degraded directly under light irradiation. In our work, MB is selected as the model pollutant to evaluate the photocatalytic activity of the Bi–V–O multi-shell hollow spheres under visible light irradiation using a 300W Xe lamp (CEL-HXF300). 

A cutoff filter was placed outside the Pyrex jacket to remove the wavelengths under 420 nm completely. In each experiment, 0.1 g photocatalyst was added into 100 mL MB solution (10 5 mol L 1 ). Before irradiation, the suspensions were magnetically stirred for half an hour in the dark to ensure the establishment of an adsorption-desorption equilibrium between the photocatalyst and MB. Then, the solution was exposed to visible light irradiation with constant stirring. At given time intervals (30 min), 4 mL suspension was sampled and centrifuged to remove the remnant photocatalyst. The fifiltrates were analyzed by checking the alteration of intensity of the absorption-band maximum (553 nm) of MB in the UV–vis spectrum. 3. Results and discussion The detailed synthesis procedures for the controlled Bi–V–O multi-shell hollow spheres are described in the experimental section. Successful synthesis of the Bi–V–O multi-shell hollow spheres are confifirmed by the SEM and TEM images. The corresponding morphologies are given in Fig. 1. The phase identifification of the hollow spheres was carried out by means of X-ray diffraction (XRD). The powder XRD patterns of Bi–V–O multishell hollow spheres are collected in Fig. S1. All the peaks of the XRD patterns of the products are in good agreement with the monoclinic scheelite BiVO4 and orthorhombic Bi4V2O11. No impurity peaks are observed in all of the XRD patterns, indicating that all hollow spheres are heterostructure of monoclinic scheelite BiVO4 and orthorhombic Bi4V2O11. The Bi–V–O (Fig. 1a and b) hollow spheres with single and double-shells have uniformly spherical morphology with the average diameter of about 800 nm. The Bi–V–O with single-shell hollow structure has rough surface while the others are smooth relatively. This phenomenon can be further demonstrated by the corresponding TEM images (Fig. 1c and d). In comparison with the simple single-shell hollow spheres, double-shell hollow spheres possess more subunits which divide their inner space into several nanoscale parts. The Bi–V–O single-shell hollow spheres are obtained by controlling the low content of Bi3+ and VO3
in the carbonaceous spheres templates and the proper calcination condition. While the concentric internal double-shell hollow spheres are achieved by increasing the Bi3+ and VO3
concentration in the surface pretreated carbonaceous spheres and using

different calcination conditions respectively. These Bi–V–O double-shell hollow spheres have an average shell thickness of 50 nm and contain irregularly shaped nanoparticles. The TEM image (Fig. 1e) of a randomly selected sphere indicates that multi-shell hollow spheres are composed of interconnected grains which is further certifified by the enlarged view of the double-shell structured hollow spheres (Fig. 1f). The HRTEM image of the double-shell hollow spheres exhibited clear lattice fringes, corresponding to the (121), (040) and (113) planes of the monoclinic BiVO4 and orthorhombic Bi4V2O11 particles respectively. In addition, an interface (highlighted by the white line) between the BiVO4 and Bi4V2O11 particles can be observed in the HRTEM image shown in Fig. 1f. The obvious interface between the Bi4V2O11 and the BiVO4 particle implies the formation of the heterostructure. By further adjusting the heating program, even triple-shell hollow spheres could be obtained, however the yield of triple-shelled spheres is low and the synthesis control is relatively diffificult (Fig. S2). Both the shell number controlling and the heterostructured Bi– V–O hollow spheres are achieved by decoration of carbonaceous spheres and tuning the Bi3+ and VO3 concentration in the carbonaceous spheres templates as well as the appropriate heating rate. Scheme 1 illustrates the formation processes of the Bi–V–O multi-shell hollow spheres via a facile templates approach. When the carbonaceous spheres with the low amount of Bi3+ and VO3 was treated under the heating rate of R = 10  C/min, Bi3+ and VO3
ions on the surface of the carbonaceous spheres were changed into the crystallographic Bi–V–O. Finally all of the crystallized Bi–V–O nanoparticles cross-linked to form a single-shell hollow spheres. Treating the carbonaceous spheres templates in the alkali solution prior to the adsorption is one of the keys to prepare the hollow spheres with double- and triple-shell hollow structures. The pretreatment of carbonaceous spheres templates in the alkali solution could increase the concentration of negatively charged hydroxyl surface groups [35], which can increase the Bi3+ and VO3 absorption in the carbonaceous spheres templates (Fig. S3). While the carbonaceous spheres with higher amount of Bi3+ and VO3 were treated under the heating rate of R = 10  C/min, Bi–V–O double-shell hollow spheres were obtained. The detailed formation mechanism of double-shell hollow spheres is revealed below. The key to generate double-shell hollow spheres was the splitting of the outer functional layer from the inner carbonaceous spheres core derived from the difference of rate between the nucleation of Bi–V–O and the carbonaceous spheres combustion. At the initial stage of heat treatment, the reaction leads to the formation of the fifirst shell of Bi–V–O at the surface of the Bi3+ and VO3 ions penetrated carbonaceous spheres. As the carbonaceous spheres combustion rate is faster than the Bi–V–O shell formation, the core (Bi3+ and VO3
ions penetrated carbonaceous spheres)-shell (Bi– V–O shell) structure are formed. The difference of rate between nucleation of Bi3+ and VO3
ions and carbonaceous spheres combustion lead to another separation process of the inside Bi3+ and VO3 ions penetrated carbonaceous spheres, and then the second Bi–V–O shell is emerged as well. To obtain more shells like the triple shell, the sample with higher amount of Bi3+ and VO3
was treated fifirst under the low heating rate (e.g. R = 2  C/min) and then under the fast heating treatment (e.g. R = 10  C/min). Under the lower heating rate, Bi3+ and VO3
ions start to redistribute to the inner part of the carbonaceous spheres template, and benefifit the formation of multi-shell hollow structures under following higher heating rate. However, the synthetic control is not easy to realize, and the yield of Bi–V–O triple-shell hollow spheres is low. In summary, by controlling the amount of Bi3+ and VO3
ions in the carbonaceous spheres templates and manipulating the heating rate, we can easily synthesis large quantities of Bi–V–O multi-shell hollow spheres with controlled inner structures. The carbonaceous spheres with low content of Bi3+ and VO3
ions favors the formation of the Bi–V–O single-shell hollow spheres. The doubleshell hollow spheres are formed when the carbonaceous spheres with high content of Bi3+ and VO3
ions were treated under the heating rate of R = 10  C/min. The cooperation of low heating rate and high heating rate favors the formation of the triple-shell hollow spheres. The chemical states and surface compositions of the heterostructured Bi–V–O multi-shell hollow spheres were investigated by X-ray photoelectron spectroscopy (XPS),the results of which are shown in Fig. 2. According to the XPS extended spectrum (Fig. 2a), only Bi, O, V, and C were detected in the sample. The C1 s peak at around 284.1 eV can be attributed to the signal from carbon contained in the instrument and was used for calibration. Two strong peaks in the high-resolution XPS spectra (Fig. 2b), at 164.4 and 159.1 eV, are assigned to Bi4f5/2 and Bi4f7/2, respectively, which confifirms that the bismuth species in the Bi–V–O multi-shell hollow structures is Bi3+ cation[36]. The characteristic peaks of V2p1/2 and V2p3/2 (Fig. 2c) is observed at approximately 524.4 and 516.7 eV respectively, corresponding to the V5+ cations in this Bi– V–O multi-shell hollow spheres [37]. In addition, the XPS signals for O1 s (Fig. 2d) at the binding energy of 530.1 eV and 532 eV correspond to the O2 anions in the Bi–V–O multi-shell hollow spheres [38]. The XPS results are consistent with the XRD patterns, which further demonstrate the coexistence of BiVO4 and Bi4V2O11 in the multi-shell hollow spheres. Methylene blue, a widely used dye, was selected as the model pollutant to evaluate the photocatalytic performance of the prepared heterostructured Bi–V–O multi-shell hollow spheres under visible light irradiation (l> 420 nm). Owing to the poor repeatability and low yield of Bi–V–O triple-shell hollow spheres, the single- and double-shell samples were selected to test their photocatalytic activity. Fig. 3 displayed the temporal evolution of the degradation effificiency during the photodegradation of MB solution over Bi–V–O hollow spheres under visible light irradiation. From the spectra, it could be seen that under the irradiation of visible light, the decomposition of MB dye over the Bi–V–O doubleshell hollow spheres is close to 100% after 80 min, which is much higher than that of Bi–V–O single-shell hollow spheres (75%) and BiVO4 (65%) (Fig. S4). Notably, although both the single- and Fig. 2. XPS analyses of as-prepared Bi–V–O multi-shell hollow spheres (a) typical XPS survey, (b) Bi4f, (c) V2p and (d) O1s spectra. Fig. 3. Kinetics of photodegradation of MB using Bi–V–O multi-shell hollow spheres under the irradiation of visible