Hierarchical BiF3–Bi2NbO5F Core–Shell Structure and Its Application

Dec 18, 2014 - Department of Cardiothoracic Surgery, The Second Affiliated Hospital of Nanchang University, Nanchang, Jiangxi 330006, People's. Republ...
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Hierarchical BiF3−Bi2NbO5F Core−Shell Structure and Its Application in the Photosensitized Degradation of Rhodamine B under Visible Light Irradiation Shuijin Lei,*,† Chuanning Wang,† Di Cheng,† Xijie Gao,† Lianfu Chen,† Yutao Yan,† Jianliang Zhou,‡ Yanhe Xiao,† and Baochang Cheng† †

School of Materials Science and Engineering, Nanchang University, Nanchang, Jiangxi 330031, People’s Republic of China Department of Cardiothoracic Surgery, The Second Affiliated Hospital of Nanchang University, Nanchang, Jiangxi 330006, People’s Republic of China

‡

S Supporting Information *

ABSTRACT: Catalytic photodegradation has been found to be a versatile, low-cost, and green technology for environmental decontamination. Bismuth-based compounds have attracted a lot of attention for their efficient photocatalytic properties. It is always of great importance to develop new catalysts in the photodegradation field. In this research, hierarchical porous BiF3−Bi2NbO5F core−shell structures have been successfully prepared via a simple solvothermal route. A possible growth mechanism for the core−shell structure was proposed based on time-dependent-evolution experiments. X-ray powder diffraction was used to determine the phase composition. Scanning electron microscopy and transmission electron microscopy were employed to characterize the morphologies of the as-prepared samples. Experiments demonstrated that the volume ratio of ethylene glycol to water played a determinative role in the final morphology of the products. The band gap of the as-prepared BiF3−Bi2NbO5F composite was estimated to about 3.47 eV. The novel hierarchical BiF3−Bi2NbO5F core−shell structures could serve as a catalyst for photosensitized degradation of Rhodamine B under visible light irradiation. Moreover, the photodegradation efficiency of the samples was greatly associated with their surface morphology.

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INTRODUCTION With decreasing fossil fuels and increasing greenhouse gases, human beings are facing serious challenges of energy shortage and environmental pollution. As a possible avenue for sustainable development, photocatalysis has been one of the most promising technologies due to its effective application in environmental purification and energy conversion, and it has extensively investigated over the past few decades.1−3 It is well recognized that photocatalysis can be divided into two kinds of processes, namely, direct semiconductor photoexcitation and indirect dye photosensitization.4 The former is derived from the photoexcitation above the band gap of the photocatalyst to form photogenerated electrons and holes, which will react with O2, H2O, or OH− to produce •OH radical to degrade dyes. The latter occurs when the energy of light is lower than the band gap of the catalyst, in which the dye molecule absorbs the light energy and the catalyst plays the roles of electron carriers and acceptors.5,6 Therefore, photosensitization has been proved to be a promising pathway for the visible photodegradation and mineralization of dye pollutants.7 To date, a vast variety of semiconductors, including metal oxides or sulfides, have been exploited as photocatalytic materials. However, it is still a great challenge to design new materials with efficient catalytic activity © 2014 American Chemical Society

in photodegradation. Recently, bismuth compounds have played a significant role in the area of catalytic photodegradation. Bismuth (Bi), a group V semimetal element, generally shows unusual electronic properties due to its highly anisotropic Fermi surface, large electron Fermi wavelength, low charge carrier density, small effective electron mass, and long mean free path of the carriers.8 In those Bi based compounds, the Bi3+ ion has the stereochemically active 6s2 lone electron pairs, from which the intrinsic polarization has been demonstrated to be advantageous in the separation of photoexcited electron−hole pairs and the fair mobility of these charge carriers.9,10 On the basis of this point, a host of bismuth compounds, including bismuth oxides Bi2O3;11 metal bismuthates such as NaBiO312 and CaBi2O4;13 bismuth oxometallates such as Bi2MoO6,14 Bi 2 WO 6 , 15 BiVO 4 , 16 BiNbO 4 , 17 BiTaO 4 , 18 BiSbO 4 , 19 Bi2Sn2O7,20 BiFeO3,21 and Bi2Fe4O9;22 bismuth nonmetal oxysalts such as BiPO4,23 Bi2O2CO3,24 Bi2SiO5,25 and Bi 2 GeO 5 ; 26 and bismuth oxyhalides such as BiOCl, 27 Received: October 30, 2014 Revised: November 24, 2014 Published: December 18, 2014 502

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The Journal of Physical Chemistry C BiOBr,28 and BiOI,29 have been widely researched as photocatalysts. Bismuth trifluoride (BiF3) usually exists in two familiar polymorphs: cubic α-BiF3 and orthorhombic β-BiF3. The former is more common in the literature. Recently, Zhao et al. have prepared monodisperse β-BiF3 nanocrystals with various morphologies by a novel solvent extraction method.30 Due to its outstanding gravimetric and volumetric energy density, BiF3 has been utilized as a promising cathode material for lithium ion batteries.31−34 On the other hand, considering that the cationic radius of Bi3+ is comparable with that of lanthanide cations (Ln3+), BiF3 has also been used as an excellent host for doping with Ln3+ cations to achieve Ln-based luminescence.35,36 Bi2NbO5F, one of the Aurivillius family members, was first synthesized by Aurivillius via the solid state reaction of BiF3 and Nb2O5 in a molar ratio of 4:1 at 640 °C.37 A similar approach was used by McCabe et al. to prepare the Bi2NbO5F phase based on the solid state reaction of stoichiometric quantities of Bi 2 O 3 , BiF 3 , and Nb 2 O 5 at 640 °C. 38 Unfortunately, the products obtained by both methods are not pure. However, the pure Bi2NbO5F phase can be prepared by the hydrothermal route at 400 °C or above using Bi2O3, BiF3, and Nb2O5 as the starting materials in a stoichiometric molar ratio of 5:2:3.39,40 Actually, the Bi2NbO5F phase can be regarded as the oxygen substitution by fluorine from Bi2NbO6, which is shown to lead to a stabilization of the structure. It is generally recognized that Bi2NbO5F possesses the bodycentered tetragonal structure in the space group I4/ mmm.37,39,40 Recently, however, McCabe et al. have proposed a lower symmetry orthorhombic space group Pbca for the Bi2NbO5F phase.38 Aurivillius Bi2NbO5F has been demonstrated to be a ferroelectric material for potential information storage application. To the best of our knowledge, for both BiF3 and Bi2NbO5F phases, the photodegradation applications of them have never been reported. It should be mentioned that Bi4NbO8Cl41 and Bi2TiO4F242 have been investigated as efficient photocatalysts. It is well-known that the properties of materials are greatly affected by their components and morphologies. To improve the photodegradation performance of the catalysts, componentand morphology-controlled fabrication may be the regular strategy. One of the effective approaches is to couple two different compounds, for example, a core−shell structure.43 Compared to the single-component system, the core−shell structure consists of both inner and outer layer materials, between which the interaction can substantially increase the overall catalytic activity and even produce useful synergistic effects. On the other hand, morphology control is also essential for the catalytic activity. First, the charge migration requires a suitable concentration or potential gradient from the core to the surface, which is highly associated with morphology and surface features of materials.44 Second, to achieve high light absorption efficiency and more active sites, the morphology with a large surface area is always desired, such as porous structures. Therefore, combining the two aspects above, the hierarchical porous core−shell structure should be considered as an ideal morphological form for the development of high performance catalysts owing to its large specific surface area, high light absorption efficiency, and efficient charge separation and migration. In the present study, hierarchical porous BiF3−Bi2NbO5F core−shell structures have been successfully fabricated by a facile template-free solvothermal method for the first time.

Time-dependent evolution of the phase and morphology, and the effect of the solvent composition on the product morphology have been systematically studied. Interestingly, the as-prepared novel BiF3−Bi2NbO5F core−shell structures could be used as a catalyst in photosensitized degradation of Rhodamine B under visible light irradiation. Moreover, the morphology-controlled photodegradation activity was also investigated.

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EXPERIMENTAL SECTION Materials. All the chemicals were used as received without any further purification. Niobium pentoxide (Nb2O5, 99.99%), ethylene glycol (OH(CH2)2OH, analytical grade), and Rhodamine B (C28H31ClN2O3, analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). Pentahydrate bismuth nitrate (Bi(NO3)3·5H2O, analytical grade) and hydrofluoric acid (HF, 40%) were obtained from Xilong Chemical Co., Ltd. (Shantou, China). Synthesis. All samples were prepared by a facile solvothermal process. In a typical synthesis, 0.5 mmol of Nb2O5 was dissolved in HF at about 140 °C with an oil bath. To volatilize the excess HF, the solution was maintained at about 90 °C for evaporation. After dilution with water, the solution continued to evaporate for several hours to remove HF as far as possible, and was then diluted to 13 mL. On the other hand, 1 mmol of Bi(NO3)3 was dissolved in 26 mL of ethylene glycol (EG). Subsequently, the Nb-containing solution was added dropwise into the Bi(NO3)3 solution under continuous magnetic stirring. The resulting mixture was further stirred and incubated for a few minutes and then transferred into a stainless steel Teflon-lined autoclave of 50 mL capacity. The autoclave was sealed and maintained at 150 °C for 12 h and then cooled to room temperature naturally. The precipitates were filtered off and washed with absolute ethanol and distilled water several times to remove the organic residues and soluble impurities, and then dried at 60 °C for 5 h in air. Characterization. X-ray powder diffraction (XRD) patterns were recorded on a Philips X’pert PRO SUPER diffractometer with Cu Kα radiation (λ = 1.541 874 Å). Scanning electron microscopy (SEM) measurements were preformed on a Quanta 200F environmental scanning electron microscope (FEI, Netherlands). Transmission electron microscopy (TEM) images, high resolution transmission electron microscopy (HRTEM) images, selected area electron diffraction (SAED) patterns, and energy dispersive X-ray spectroscopy (EDS) spot scan spectra were taken from a JEM-2010 transmission electron microscope (JEOL, Japan) with an accelerating voltage of 200 kV. During the TEM tests, after ultrasonic agitation, one or more drops of the ethanol solution containing the assynthesized composites were deposited onto the amorphous carbon film supported on a copper grid and allowed to dry at room temperature in air. The elemental distributions were examined by EDS line-scan analyses on an S-4800 (Hitachi, Japan) field-emission scanning electron microscope at an accelerating voltage of 15 kV. The ultraviolet−visible (UV− vis) diffuse reflectance spectrum was obtained on a TU-1950 UV−vis spectrophotometer (Perkinje General, China) with an integrating sphere using barium sulfate as a reference material. The nitrogen adsorption−desorption isotherms were measured on a Micromeritics ASAP-2000 nitrogen adsorption apparatus. The surface area was calculated by the Brunauer−Emmett− Teller (BET) method, and the pore size distribution was calculated using the Barrett−Joyner−Halenda (BJH) model. 503

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The Journal of Physical Chemistry C Photocatalytic Activity Test. The photocatalytic performance of the as-synthesized samples was evaluated through photodegradation of Rhodamine B (RhB) under visible light irradiation at room temperature. A 300 W xenon lamp (HSXF/UV300, Beijing NBeT Technology Co., Ltd.) was used as the light source with an ultraviolet cutoff filter (λ ≥ 400 nm) to provide visible light irradiation. The catalytic reaction was carried out in a closed quartz cell with a capacity of 500 mL, and the system temperature was controlled with a circulating water cooling system on the reaction cell. A photograph of the photocatalytic test equipment is shown in the Supporting Information (Figure S1). In each test, 0.2 g of the as-prepared powder catalyst was dispersed into 100 mL of RhB aqueous solution (10 mg·L−1). Prior to illumination, the suspension was vigorously stirred in the dark for 30 min to ensure the adsorption−desorption equilibrium between catalyst and dye. The suspension was then stirred and exposed to the light irradiation. At every 15 min interval, approximately 3 mL of the suspension was collected and centrifuged to remove the catalyst. Experiments showed that the solution temperature can reach about 40 °C after 90 min of irradiation. The concentration of RhB was analyzed using a TU-1810 UV−vis spectrophotometer (Purkinje General, China) at λ = 554 nm.

the level of impurities in the sample is lower than the resolution limit of the XRD instrument. Therefore, the product should consist of cubic BiF3 and tetragonal Bi2NbO5F. Morphology Analysis. The morphology of the products was examined by electron microscopy. Figure 2 presents the

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RESULTS AND DISCUSSION Phase Analysis. The phase composition of the as-prepared sample is studied by XRD. Figure 1 presents the XRD patterns

Figure 2. (a, b) SEM and (c, d) TEM images of the as-prepared BiF3− Bi2NbO5F composites via the solvothermal route with a volume ratio of EG:H2O = 2:1 at 150 °C for 12 h.

SEM and TEM images of the solvothermal products with a volume ratio of EG:H2O = 2:1 at 150 °C. From the lowmagnification SEM image displayed in Figure 2a, it can be found that the sample consists of spherelike architectures with an average diameter of about 3 μm. Actually, these microspheres have a very rough surface. The high-magnification SEM image (inset of Figure 2a) clearly demonstrates the presence of hierarchical porous superstructures on the surface of these microspheres, which are composed of two-dimensional thin nanoplates with an average thickness of about 15 nm. These nanoplates are close-packed and interconnected with each other perpendicularly to the spherical surface, forming a threedimensional multipore structure. Those pores are irregular with diameters from several nanometers to several tens of nanometers. More impressively, the core−shell structure can be recognized from the broken spheres which are often the intuitionistic evidence for a hollow or core−shell structure. Because the outer shell of some spheres is partially destroyed, the entire spherical inner core with a broken outer shell distinctly implies the formation of core−shell structure. Figure 2b shows the SEM close-up view of a half-destroyed sphere, which indicates that the inner core is a coarse sphere and the whole outer shell is assembled by nanoplates mentioned above. The diameters of the inner core and outer shell are about 2 μm and 500 nm, respectively. The TEM images shown in Figure 2c,d further confirm the construction of the hierarchical core− shell structure. An evident boundary between the outer shell and inner core can be distinguished. From the TEM images, it also can be seen that the inner core is actually a solid sphere, and the outer shell is really made up with nanoplates. Structural Component Analysis. Figure 3a gives the TEM image of the Bi2NbO5F nanoplates exfoliated from the

Figure 1. Typical XRD patterns of the as-synthesized sample by the solvothermal method with a volume ratio of EG:H2O = 2:1 at 150 °C for 12 h.

of the sample prepared by the solvothermal treatment with a volume ratio of EG:H2O = 2:1 at 150 °C. It can be seen that the sample contains two phases. The diffraction peaks located at 26.3, 30.5, 43.6, and 51.6° (marked by triangles) can be indexed to the cubic BiF3 phase with a lattice constant of a = 5.842 Å, which is in good agreement with the reported value (JCPDS Card File No. 74-0144, a = 5.865 Å). The residual diffraction peaks (marked by asterisks) can be readily indexed to the tetragonal structure of Bi2NbO5F with lattice constants of a = 3.839 Å and c = 16.643 Å, which are perfectly consistent with the reported data (JCPDS Card File No. 84-1616, a = 3.835 Å and c = 16.630 Å). No characteristic reflection peaks originating from other contaminants such as niobium oxide, bismuth oxide, or niobates can be detected, which indicates that 504

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Figure 4. (a) EDS line-scan profile across the diameter of a broken sphere, and corresponding elemental distribution for (b) Bi, (c) F, (d) Nb, and (e) O elements.

located at the edge of the sphere, which strengthens the evidence that the product is composed of BiF3−Bi2NbO5F core−shell structure. Time-Dependent Evolution. In order to reveal the formation process of the BiF3−Bi2NbO5F core−shell structure in more detail, time-dependent experiments were carried out and the resultant products synthesized with different reaction times from 60 to 120 min were analyzed by SEM images and XRD patterns as displayed in Figure 5. When the reaction time is only 60 min, as shown in Figure 5a, it is apparent that the sample consists of large-scale nanoparticles with a diameter of

Figure 3. (a) TEM image, (b) HRTEM image, (c) SAED patterns, and (d) EDS spectrum of the Bi2NbO5F nanoplates from the outer shell.

outer shell. Generally, these nanoplates have an approximately rectangular shape. To explore the microstructure of the Bi2NbO5F nanoplates, the HRTEM and SAED analyses were undertaken from a single constituent nanoplate. As shown in Figure 3b, the HRTEM image shows clearly resolved twodimensional atomic lattice fringes, suggesting a good crystallinity of these nanoplates. A square lattice can be detected, in which the two interplanar spacings can be measured to be 0.19 nm with a separation angle of 90°, which match well with the (200) and (020) planes of the tetragonal structure of Bi2NbO5F phase. It is easily known that this image should be typically taken along the [001] zone axis. In other words, the exposed flat rectangular surface of these Bi2NbO5F nanoplates should be characterized as {001} planes. Figure 3c presents the corresponding SAED patterns recorded along the [001] zone axis, which also can be indexed as the tetragonal structure of Bi2NbO5F phase and support the singlecrystalline feature of these nanoplates. The EDS spot scan spectrum taken from the nanoplates is displayed in Figure 3d, which shows the presence of Bi, Nb, O, F, Cu, C, and Cr. The elements Cu and C are derived from the copper grid and carbon film, respectively, while the element Cr should be originated from the TEM sample holder. The EDS spectrum shows that the atomic ratio of Bi:Nb is about 2.1:1, which is close to the stoichiometry of Bi2NbO5F. Therefore, it also confirms the formation of Bi2NbO5F phase for the outer shell. Unfortunately, however, due to the large size of the inner core, the HRTEM and SAED analyses could not be performed on it. To further investigate the structural composition of the assynthesized BiF3−Bi2NbO5F core−shell architecture, the EDS line-scan analyses were conducted across the diameter of a broken sphere to examine the elemental distributions. Figure 4a is the overview of the SEM image and compositional scan profile. According to the elemental concentration distribution, as shown in Figure 4b−e, the Bi and F signals are distributed over the sphere, while the Nb and O signals are dominantly

Figure 5. SEM images and XRD patterns of the morphology and phase evolution of the samples prepared with different reaction times: (a, b) 60, (c, d) 80, and (e, f) 120 min. 505

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growing at the expense of the amorphous contents to form the larger-sized BiF3 spheres arising from the Ostwald ripening process. Then, with further prolonging of the reaction time, the obtained BiF3 will be reacted with HNbOF4 in the solution. As a result, the Bi2NbO5F nanoplates are formed on the surface of BiF3 microspheres. On the other hand, the reaction between BiONO3 and HNbOF4 also can support the formation and growth of Bi2NbO5F nanoplates. Therefore, as the growth time goes on, the BiF3 sphere should be entirely enwrapped by Bi2NbO5F nanoplates, and the hierarchical BiF3−Bi2NbO5F core−shell structure is formed eventually. It is certainly acceptable that the Bi2NbO5F shell is getting thicker and thicker along with the growth time until the reaction ceases. Effect of Solvent Composition Ratio. Experiments showed that the volume ratio of EG to H2O has a critical effect on the morphology of the products. Figure 6 presents the

about 100 nm. The corresponding XRD patterns are presented in Figure 5b. Only two broad diffuse peaks centered at around 28 and 49° can be observed, which is suggestive of the amorphous nature of the sample. With the reaction time increasing to 80 min, the SEM image in Figure 5c reveals that the spherical particles exhibit an obviously larger size of about 400 nm. From the XRD patterns presented in Figure 5d, although a small amorphous background is still present, the strong diffraction peaks can be easily indexed to the cubic BiF3 phase. With further prolonging the reaction time to 120 min, microspheres with a core−shell structure are formed (Figure 5e). As discussed above, the outer shell is composed of nanoplates. The difference is that the shell herein is significantly thinner than that in Figure 2. The XRD patterns of the sample in Figure 5f reveal the coexistence of cubic BiF3 and tetragonal Bi2NbO5F, demonstrating the formation of BiF3−Bi2NbO5F core−shell structure. It can be deduced that the formation of BiF3−Bi2NbO5F composite phase should include four steps: (i) the acid heptafluorniobic (H2NbF7) complex solution can be formed by dissolution of Nb2O5 with hydrofluoric acid; (ii) the bismuth precursor Bi(NO3)3 will be quickly hydrolyzed to bismuth oxynitrate (BiONO3); (iii) BiF3 phase is first formed through the reaction of H2NbF7 and BiONO3 together with the generation of HNbOF4; (iv) the produced BiF3 and unreacted BiONO3 can be sequentially reacted with HNbOF4 to form Bi2NbO5F phase. The reaction equations can be proposed as follows: Nb2 O5 + 14HF = 2H 2NbF7 + 5H 2O

Bi(NO3)3 + H 2O = BiONO3 + 2HNO3

H 2NbF7 + BiONO3 = BiF3 + HNbOF4 + HNO3 2BiF3 + HNbOF4 + 4H 2O = Bi 2NbO5F + 9HF 2BiONO3 + HNbOF4 + 2H 2O = Bi 2NbO5F + 3HF + 2HNO3

Figure 6. SEM images of the as-prepared samples when the volume ratio of EG:H2O is (a) 1:2, (b) 1:1, (c) 2:1, and (d) 3:1, respectively.

Based on the experimental results above, as illustrated in Scheme 1, a possible growth mechanism for the hierarchical

SEM images of the obtained samples when the volume ratio of EG:H2O is 1:2, 1:1, 2:1, and 3:1, respectively. It can be noted that all four samples are composed of spherelike structures with hierarchical nanoplates on the surface. Additionally, they have similar sizes of about 3−4 μm. However, between them, a greatly obvious distinction is their surface morphology. These hierarchical nanoplates are assembled in different manners. In detail, at the volume ratio of EG:H2O = 1:2, flowerlike products are obtained, in which those constituent nanoplates are loosely connected creating large-size pores (Figure 6a). In contrast, when the volume ratio of EG:H2O is increased to 1:1, as displayed in Figure 6b, these “petals” are more compact. That is, there are more nanoplates on the surface. However, as the volume proportion of EG is further increased to the ratio of EG:H2O = 2:1 or even 3:1, perfectly rounded spheres can be obtained. Accordingly, the close-packed nanoplates make for a mass of nanopores (Figure 6c,d). It is worth mentioning that the nanoplates formed at the volume ratio of EG:H2O = 3:1 (Figure 6d) are observably thicker than those prepared at EG:H2O = 2:1 (Figure 6c). According to the results discussed above, it can be deduced that the large volume proportion of EG should be good for the

Scheme 1. Schematic Illustration of the Morphological Evolution for the Hierarchical BiF3−Bi2NbO5F Core−Shell Structure at Various Stages

porous BiF3−Bi2NbO5F core−shell structure is speculated. The obtained BiF3−Bi2NbO5F core−shell structure may be formed through the following processes. Initially, with the mixing of niobium and bismuth precursors, the primary nucleation reaction occurs to form amorphous nanoparticles. In general, to minimize the surface energy of the system, these nanoparticles tend to aggregate. With the reaction time increasing, the resulted nanoparticles keep crystallizing and 506

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The Journal of Physical Chemistry C formation of spherical morphology with compact nanoplates. As a polyol, EG (HOCH2CH2OH) possesses two hydroxy groups in each molecule. As a result, it usually exhibits a strong coordination effect as a bidentate ligand. On one hand, the EG molecules can remain tightly coordinated with the formed BiF3 microspheres. On the other hand, HNbOF 4 may be coordinated with EG based on a similar esterification. At this point, the EG molecules presumably serve as the link to adsorb HNbOF4 on BiF3 spheres. Therefore, it should be believed that the coordination effect of EG would play a vital role in morphological control of the products. In this research, the larger volume proportion of EG is used, the higher concentration of hydroxy groups and the stronger coordination effect thus would be obtained, and then the more HNbOF4 sources can be adsorbed on BiF3 spheres. Consequently, the much denser nanoplates should emerge. As the volume ratio of EG:H2O reaches 2:1, the nanoplates are compact enough to form well-rounded spheres. If the EG:H2O ratio is further increased (for example, 3:1), owing to the high concentration of HNbOF4 adsorbed on BiF3 spheres, the nanoplates then grow thicker. Nitrogen Adsorption−Desorption. In consideration of the special hierarchical porous structure and the different surface morphologies of the products prepared at different volume ratios of EG to H2O, the nitrogen adsorption− desorption isotherms were measured to determine the specific surface area, pore volume, and pore size distribution, and the corresponding results are presented in Figure 7. It can be found

Table 1. Textural Properties of the Samples Prepared at Different EG:H2O Volume Ratios BET surf. area (m2·g−1) BJH desorption av pore diam (nm) BJH desorption cumul vol (cm3·g−1)

1:2

1:1

2:1

3:1

47.6 24.7 0.43

49.5 21.8 0.34

64.0 12.7 0.23

59.2 13.0 0.23

prepared at 1:2 and 1:1 another broad peak in the pore size region of about 20−50 nm also can be observed. It is particularly worth noting that the peak at 3−4 nm is gradually weakened with decreasing the volume proportion of EG, and it nearly fades away at EG:H2O = 1:2. The results are in good agreement with the SEM images in Figure 6. The average pore diameter and the cumulative volume of pores between 1.7 and 300 nm diameter based on the BJH desorption of the samples are also listed in Table 1. Obviously, the smaller pore size and volume signify a larger surface area. UV−Vis Diffuse Reflectance Spectrum. In order to characterize the optical absorption property and determine the optical band gap of the obtained BiF3−Bi2NbO5F core−shell structures, UV−vis diffuse reflectance spectroscopy has been examined at room temperature as shown in Figure 8. An

Figure 8. UV−vis diffuse reflectance spectrum and the corresponding plot of (αhυ)1/2 vs photon energy (hυ) (inset) of the as-synthesized hierarchical porous BiF3−Bi2NbO5F core−shell structures at a volume ratio of EG:H2O = 2:1. Figure 7. Nitrogen adsorption−desorption isotherms of the different hierarchical porous spheres when the volume ratio of EG:H2O is 1:2, 1:1, 2:1, and 3:1, respectively. The inset shows the corresponding pore size distribution curves (deduced from the desorption branch) of these samples.

intense absorption edge at about 350 nm in the near-UV region can be clearly observed. Additionally, the steep absorption edge generally indicates that the light absorption is due to the intrinsic interband transition of the material.45 As we know, for a crystalline semiconductor, the optical absorption near the band edge follows the equation αhυ = A(hυ − Eg)n/2, where α, hυ, A, and Eg are the absorption coefficient, the photonic energy, a proportionality constant, and the band gap energy, respectively, while the value of n depends on the transition type of the semiconductor (n = 4 and 1 for indirect and direct transitions, respectively). To measure the optical band gap (Eg) value of the as-prepared composites, as shown in the inset of Figure 8, the plot of (αhυ)1/2 versus hυ in the absorption edge region is nearly linear, which suggests that the absorption edge of the sample may be ascribed to an indirect transition (n = 4). Therefore, it should be acceptable to determine the band gap by extrapolating a straight line to the (αhυ)1/2 = 0 axis and

the adsorption−desorption isotherms of all the four samples exhibit type IV curves with characteristic H3-shaped hysteresis loops of mesoporous structures according to the IUPAC classification. The textural property data of these samples are summarized in Table 1. The BET specific surface areas of the samples prepared at the EG:H2O volume ratios of 1:2, 1:1, 2:1, and 3:1 are 47.6, 49.5, 64.0, and 59.2 m2·g−1, respectively. The inset panel of Figure 7 shows the pore size distribution curves derived from the desorption branch using the BJH model. For the samples obtained at EG:H2O = 2:1 and 3:1, only one sharp peak located at 3−4 nm can be observed, while for those 507

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The Journal of Physical Chemistry C intercepting the hυ axis to give it. In this way, the band gap of the prepared hierarchical porous BiF3−Bi2NbO5F core−shell composites is estimated to be 3.47 eV. Photocatalytic Activity and Mechanism. The photocatalytic activity of the hierarchical porous BiF3−Bi2NbO5F core−shell structures synthesized at a volume ratio of EG:H2O = 2:1 was evaluated by the photodegradation of RhB under visible light irradiation (λ ≥ 400 nm) after the adsorption− desorption equilibrium between the sample and RhB. Figure 9

reaction conditions (the absorption wavelength of λ = 271 nm for phenol) to those of RhB to estimate the degradation mechanism of prepared BiF3−Bi2NbO5F sample. No degradation can be detected (the cyan pentagon curve in Figure 9). This confirms that the visible light degradation of RhB over the hierarchical BiF3−Bi2NbO5F composites is an indirect dye photosensitization process. To determine the main active species responsible for the degradation of RhB, a comparison experiment under N2-bubbled condition was performed. In brief, before illumination, the suspension was bubbled with N2 for 2 h to remove dissolved O2, which can serve as an efficient electron scavenger. N2 purging was continued until the degradation reaction was terminated. The result shows that only a very slight degradation can be achieved (the orange hexagon curve in Figure 9) when O2 is excluded by N2-purging. For this reason, it can be concluded that the electron transfer via O2 plays a crucial role in this photocatalytic reaction. Namely, based on the familiar photosensitization degradation mechanism, the RhB dye adsorbed on catalyst is first excited to produce cationic RhB radicals (•RhB+) and the photogenerated electrons which are immediately injected into the conduction band of catalyst. These photoelectrons are then captured by O2 to form reactive oxygen radical species such as •O2−, •OH, and • OOH, which can ultimately react with •RhB+ to induce the degradation of RhB.4−7 The corresponding photodegradation mechanism can be illustrated as Scheme 2.

Figure 9. Photodegradation of RhB under visible light irradiation as a function of the irradiation time without catalyst (black open square), and over the hierarchical porous BiF3−Bi2NbO5F core−shell structures prepared at a volume ratio of EG:H2O = 2:1 in air (red star) and N2-bubbled (orange hexagon) conditions, and the visible light photodegradation of phenol in air for comparison (cyan pentagon).

Scheme 2. Schematic Illustration of the Proposed Photodegradation Mechanism of RhB under Visible Light Irradiation with the Hierarchical BiF3−Bi2NbO5F Core− Shell Structures

presents the variation of RhB concentrations (C/C0) with irradiation time over the as-prepared sample, where C0 is the initial concentration of RhB and C is the concentration of RhB at a given time. For comparison, the blank test without photocatalyst was also performed under identical conditions. It can be clearly observed that the degradation of RhB is almost negligible in the absence of catalyst (the black open square curve), while RhB can be significantly degraded after use of the photocatalysts. The photodegradation efficiency of RhB can reach 99% after 90 min of irradiation (the red star curve). This indicates that the BiF3−Bi2NbO5F composites can exhibit a good catalytic activity for the photodegradation of RhB under visible light irradiation. However, as discussed above, the obtained sample has a wide band gap of 3.47 eV, whose absorption typically occurs in the ultraviolet region. Therefore, it cannot be directly excited by visible light, and accordingly, the degradation of RhB does not originate from the photoexcitation. It is known that RhB dye can absorb visible light in the range of 460−600 nm. In this research, the employed light source (λ ≥ 400 nm) is able to excite the RhB dye rather than the catalyst. At this point, the degradation of RhB under visible light should be governed by a photosensitization process. Because colorless phenol can only absorb ultraviolet light with an absorption range from 240 to 280 nm, which cannot be photolyzed by visible light irradiation, the indirect photosensitization would not happen in the degradation process. Therefore, the visible light degradation experiment of phenol solution was conducted under parallel

In order to study the photostability of the prepared hierarchical BiF3−Bi2NbO5F core−shell structures, the recycling experiments for the photodegradation of RhB under visible light irradiation were performed. After each run (90 min) of photoreaction, the catalyst was centrifuged out, washed with distilled water, and then redispersed into 100 mL of the new RhB solution with the initial concentration (10 mg·L−1). As shown in Figure 10, after five cycling runs of photodegradation of RhB, only about a 1% decrease in photodegradation ratio can be found, which reveals that the hierarchical porous BiF3−Bi2NbO5F core−shell structures are stable during the photodegradation of RhB dye. To explore the synergistic effect of the prepared BiF3− Bi2NbO5F composite, the photocatalytic activity of the individual BiF3 and Bi2NbO5F phases should be studied. Unfortunately, however, the pure Bi2NbO5F phase is very hard to obtain. In this research, it cannot be prepared. As for the BiF3 phase, it can be formed by dissolving the BiF3−Bi2NbO5F sample in hydrofluoric acid. The XRD patterns (Supporting Information, Figure S2) prove that the product is pure BiF3, 508

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Figure 10. Cycling photodegradation of RhB by the hierarchical porous BiF3−Bi2NbO5F core−shell structures prepared at a volume ratio of EG:H2O = 2:1 under visible light irradiation.

and the SEM image (Supporting Information, Figure S3) shows that it is composed of rough spheres similar to the core of BiF3−Bi2NbO5F composite. Figure S4 in the Supporting Information presents the photodegradation of RhB under visible light irradiation over the BiF3 sample. It can be seen that the degradation efficiency is very low. Only about 10% can be reached even after 3 h irradiation. Considering that the band gap of the BiF3−Bi2NbO5F sample lies in the ultraviolet region, the photocatalytic activity of the hierarchical BiF3−Bi2NbO5F core−shell structures was also investigated by the photodegradation of RhB under UV light irradiation (λ = 200−400 nm). As shown in the Supporting Information (Figure S5), 99% of RhB can be degraded after 75 min of UV light irradiation. Therefore, the synthesized hierarchical porous BiF3−Bi2NbO5F core−shell structures also can be used as the UV photocatalyst. Morphology-Dependent Photocatalytic Performance. To investigate the effects of morphology on the photocatalytic performance, the photodegradation of RhB under visible light over the samples prepared at different volume ratios of EG:H2O was compared in Figure 11a. It is obvious that RhB can be adsorbed and then degraded over all four products. In the dark equilibration, only the adsorption process occurs. The sample prepared with the EG:H2O volume ratio of 2:1 shows higher dye adsorption (14.8%) than those prepared with the EG:H2O volume ratios of 3:1 (14.0%), 1:1 (12.5%), and 1:2 (11.9%). Under visible light irradiation, the sample prepared with the EG:H2O volume ratio of 2:1 also exhibits the highest catalytic activity, and the total degradation efficiency of RhB reaches 99% after 90 min of irradiation (the red star curve in Figure 11a). The photodegradation performance of the catalyst with EG:H2O = 3:1 is the second best, where the RhB removal still can reach 89% (the green square curve in Figure 11a). However, those samples synthesized at the volume ratios of EG:H2O = 1:1 and 1:2 have relatively weaker catalytic activities, and only 72% (the blue circle curve in Figure 11a) and 49% (the magenta triangle curve in Figure 11a) of RhB can be bleached, respectively. Combined with the nitrogen adsorption−desorption results, it is very apparent that the catalyst with larger surface area exhibits better adsorption ability and photodegradation activity. According to the indirect photosensitization mechanism, the valence band of the catalyst is not involved in the dye degradation process. As a result, the direct interaction between the dye molecules and the surface of

Figure 11. (a) Comparison of visible light photodegradation of RhB with irradiation time over different samples prepared at the EG:H2O volume ratios of 1:2, 1:1, 2:1, and 3:1, respectively. (b) Calculation of the reaction rate constants of the samples synthesized at different EG:H2O volume ratios.

catalyst is prerequisite for an efficient electron injection, and then the dye adsorption amount and strength are crucial for the electron transfer from the excited dye to catalyst.6,27 Therefore, the degradation performance of the BiF3−Bi2NbO5F composites during the photosensitization process strongly depends on the surface-related interface properties between the sample and dye molecules. The sample with a larger surface area can adsorb more RhB molecules and provide more active sites, which contribute to more efficient electron injection from excited dye to catalyst, leading to more RhB degradation through the indirect dye photosensitization process.27,46 Obviously, the photodegradation performance is greatly associated with the surface morphology of the products. The different surface morphologies result in different surface areas, pore sizes, and active sites, and accordingly, the degradation efficiency of RhB is also different. The reaction kinetics of RhB photodegradation over the samples synthesized at different EG:H2O volume ratios can be described by the pseudo-first-order kinetic model ln(C0/C) = kt, where C0 is the initial concentration of RhB after adsorption−desorption equilibrium, C is the actual concentration of RhB at irradiation time t, and k is the reaction rate constant. Figure 11b displays the plots of ln(C0/C) vs t, and then the k values can be obtained by linear fitting. It should be noted that the last data point (90 min) for the sample with EG:H2O = 2:1 is far deviated from the linear relationship, so it 509

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CONCLUSION In summary, the BiF3−Bi2NbO5F core−shell structures have been successfully prepared by a convenient solvothermal method. The outer shell was composed of hierarchical porous Bi 2 NbO 5 F nanoplates, and the inner core was a BiF 3 microsphere. The phase evolution and growth process of the BiF3−Bi2NbO5F core−shell structure could be deduced based on the time-dependent investigation. Experiments proved that the volume ratio of EG to H2O exerted a significant effect on the morphology of the products. The UV−vis diffuse reflectance spectrum exhibited a steep absorption edge at about 350 nm and showed an indirect band gap of 3.47 eV. The photodegradation tests demonstrated that the prepared hierarchical porous BiF3−Bi2NbO5F core−shell structures could be used as a catalyst for the photodegradation of RhB under visible light irradiation via the photosensitization process. Combining the photocatalytic performance with the surface texture of the products synthesized at different EG:H2O volume ratios, it could be concluded that the smaller the pore size, the larger the surface area, the more active sites, and then the higher the photocatalytic activity would be achieved.

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ASSOCIATED CONTENT

S Supporting Information *

Photograph of the photocatalytic test equipment; XRD patterns, SEM image, and visible light photodegradation of RhB of the HF-treated sample; UV light photodegradation of RhB over the hierarchical porous BiF3−Bi2NbO5F core−shell structures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support by the National Natural Science Foundation of China (21461014, 21001062), the Natural Science Foundation of Jiangxi Province (20132BAB216016), and The National High Technology Research and Development Program of China (863 Program, 2014AA020539) are gratefully acknowledged.

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REFERENCES

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