Ionic Self-Assembly of Polyoxometalate ... - ACS Publications

Research Article pubs.acs.org/journal/ascecg

Ionic Self-Assembly of Polyoxometalate−Dopamine Hybrid Nanoflowers with Excellent Catalytic Activity for Dyes Han Zhang,† Ling-Yu Guo,† Jianmei Jiao,‡ Xia Xin,*,†,‡ Di Sun,*,† and Shiling Yuan*,† †

Key Lab for Colloid and Interface Chemistry of Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P. R. China ‡ National Engineering Technology Research Center for Colloidal Materials, Shandong University, Jinan 250100, P. R. China S Supporting Information *

ABSTRACT: In this paper, new inorganic−organic hybrid nanoflowers consisting of a Weakley-type polyoxometalate Na9[EuW10O36]·32H2O (denoted as EuW10) and biomolecule dopamine (DA) were fabricated through a simple ionic selfassembly (ISA) method. The hybrid nanoflowers were fully characterized by transmission electron microscopy (TEM), field emission scanning electron microscopy (FE-SEM), Fourier transform infrared (FT-IR) spectroscopy, Raman spectra, X-ray diffraction (XRD), and fluorescence spectra. We found that the electrostatic interaction and hydrogenbonding interaction between EuW10 and DA favored the formation of the hierarchical flowerlike structure with hundreds of nanopetals and their morphologies could be controlled simply by tuning the ratio and respective concentrations of the components. Once forming EuW10/DA vesicles or nanoflowers, the fluorescence of EuW10 was quenched due to the hydrogen bonding between the ammonium group of DA and the oxygen atom of EuW10 that blocked the hopping of the d1 electron in EuW10. Interestingly, the calcinated nanoflower showed excellent decomposition efficiency toward the organic pollutants such as the dyes of methyl orange (MO) and rhodamine B (RhB). Moreover, the catalyst for MO can be reused at least 6 cycles with only a slight dropping of catalytic efficiency, suggesting their promising applications in the treatment of wastewater. KEYWORDS: Eu-containing polyoxometalate, Dyes, Nanoflowers, Fluorescence, Catalysis

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INTRODUCTION As an advanced nanotechnology, self-assembly is emerging as a valid, bottom-up approach for the fabrication of novel functional nanomaterials through integrating different components together.1,2 Especially, self-assembly between cations and anions by the electrostatic interactions, named ionic selfassembly (ISA) is ubiquitous throughout the nature and plays a fundamental role in creating unique nanostructures.3−7 Due to the well-controlled shapes, well-defined sizes, and promising properties, hybrid nanomaterials fabricated have been widely explored and used in electro-optical materials, drug or gene delivery, nanoreactors and catalysis science.8−11 Among various self-assembled hybrid nanomaterials, the bioinspired materials in micro- and nanoscale have been proposed as a big breakthrough on the design of advanced functional materials and have attracted much attention in recent years.12,13 In particular, hybriding biomolecules and inorganic components is a robust scaffold to realize the integration of both functions.14 Especially, polyoxometalates (POMs), one kind of inorganic clusters, are a well-known class of unique polyanions with a rigid framework, monodispersed size, watersolubility, and versatile properties.15−17 With these superior structural and physicochemical properties, POMs are © 2016 American Chemical Society

deemed as promising building blocks for the construction of functional hybrid materials.18 It has been established that ISA strategy based on cationic biomolecules and anionic POMs is a convenient and effective way to produce three-dimensional (3D) hierarchical nanostructures,19−21 thus resulting in potential applications in catalysis, electronics, photo- and electrochromic devices, magnetic materials, gene delivery, and antibacterial therapy.22−24 For example, Li et al. reported the dopamine (DA) and phosphotungstic acid (PTA) constructed 3D flowerlike structures, and their application of drug loading.25 Moreover, they investigated the hybrid colloidal spheres through the coassembly of phosphotungstic acid (PTA) and cationic dipeptide (CDP) which enables adaptive encapsulation for a range of guest materials from small molecules to nanoscale materials.26 Wu et al. reported the polyoxometalate (H4SiW12O40) driven self-assembly of short peptides, giving nanofibers with concentrated positive charges, which effectively enhance the binding ability to bacterial cells, then improve their bioactivity.21 However, there has only Received: July 30, 2016 Revised: December 20, 2016 Published: December 30, 2016 1358

DOI: 10.1021/acssuschemeng.6b01805 ACS Sustainable Chem. Eng. 2017, 5, 1358−1367

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Figure 1. Detailed characterizations of the sample of 2 mg mL−1 EuW10/2 mg mL−1 DA. SEM images of (a) hybrid nanoflowers, (b) single nanoflower (a local enlarged image of panel a) and (c) nanopetals (a local enlarged image of panel b). TEM image of (d) a single nanoflower, (e) a local enlarged image of panel d. (f) HR-TEM image of nanopetals (the yellow circles denote individual EuW10 clusters). (g) CLSM image, (h) POM image and (i) EDS spectrum of the hybrid nanoflowers.

After aging for 5 days, the yellow precipitation was collected by removing upper-phase, washing with water for three times and freeze-drying in a vacuum extractor at −60 °C for 1 day. Catalytic Applications. The MO and RhB dyes were chosen as the degrading pollutions to test the photocatalytic activities of the as-prepared samples. The light irradiation was performed using a solar simulator 300 W Xe lamp (Japan) with a super cold filter (YSC0750), which provides visible light ranging from 400 to 700 nm. The light intensity was around 95 mW cm−2 and the distance is kept at 10 cm. For a typical sample of calcinated nanoflower/MO/H2O2, the catalyst of 5 mg was dispersed in 4 mL 10 mg L−1 MO/135 mmol L−1 H2O2 solution and then left undisturbed at 20 °C. During the degradation, about 3 mL of suspension was continually taken from the tube at given time intervals for UV−vis measurement to obtain the absorption value of a maximum peak of MO and RhB solution for further concentration analysis. The concentration of the target dye is calculated by a calibration curve. The degradation efficiency (%) can be calculated as

limited reports on POM-based 3D hierarchical nanostructures based on ISA strategy that could exhibit the remarkable catalytic function. Biomolecule dopamine (DA) is a neurotransmitter ubiquitous in the mammalian central nervous system, and has been extensively employed in various fields such as surface modifications, biomedicine, and Li-ion battery. Herein, a Weakley-type POM (an europium ion sandwiched by two Lindqvist-type POM) EuW10 was employed as a polyoxoanion to assemble with the cationic DA into well-ordered nanoflowers structure through simple ISA strategy. The sizes and morphologies of the 3D nanostructures were dependent on the concentration and ratio of the anionic and cationic components and the assembled morphologies, fluorescence quenching mechanisms and catalytic performance were studied in details. The approach presented here may open a novel access for the fabrication of multifunctional 3D hierarchical nanostructures. Reagents and Materials. Na9(EuW10O36)·32H2O was prepared as described by Sugesta and Yamase.27 Dopamine hydrochloride (DA), rhodamine B (RhB) and methyl orange (MO) were purchased from Alfa Aesarand used as received. Hydrogen peroxide was purchased from Sinopharm Chemical Reagent Co., Ltd. The structures of Na9(EuW10O36)·32H2O and dopamine were shown in Figure S1. Ultrapure water with a resistivity of 18.25 MΩ cm was obtained using a UPH-IV ultrapure water purifier (China). Fabrication of the Hybrid Nanoflower Structures. In a typical experiment, the sample was prepared by adding 0.2 mL EuW10 aqueous solution (10 mg mL−1) to 0.8 mL DA aqueous solution (2.5 mg mL−1) with stirring. The transparent solution became yellow immediately and showed turbid within 2 min.

Efficiency (%) =

C0 − C × 100% C0

(1)

where C0 is the initial concentration of dye and C is the revised concentration using the UV−vis spectra. Methods and Characterizations. Transmission electron microscopy (TEM) observation was manipulated on a JEM100CX II (JEOL) operating at 80 kV with a Gatan multiscan CCD recording images. A small amount of suspension was placed on carbon-coated copper grid for 2 min before removal of excess solution with filter paper. Field-emission scanning electron microscopy (FE-SEM) image and element mapping analysis were characterized by a JEOL JSM-6700F at 5.0 kV. 1359


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Figure 2. (a) SEM image of hybrid nanoflowers, SEM element mapping analyses of nanoflowers: (b) W, (c) O, (d) C, (e) Na, (f) N.

Figure 3. Formation process of EuW10-DA nanoflowers with different incubation time. SEM images at (a, e) 1 min, (b, f) 0.5 h, (c, g) 1 h and (d, h) 2 h. Panels e, f, g and h are the high-resolution images of the samples of panels a, b, c and d, respectively.

The specific surface area measurements of the HCSs samples were monitored with an automated Surface Area & Pore Size Analyzer (Qudrasorb SI, America, Quantachrome Instruments).

Samples were applied to silica wafers, which were subjected to freeze-drying in a vacuum extractor at −60 °C for 1 day. The fluorescent images were acquired by confocal laser scanning microscope (CLSM) (Panasonic Super Dynamic II WVCP460) with excitation wavelength at 488 nm. FT-IR spectra were recorded on a VERTEX-70/70v spectrometer (Bruker Optics, Germany). Raman spectra were measured on an NXR FT-Raman module (Nexus 670, Nicolet Co.) equipped with a Ge detector. The samples were excited by a laser source with a wavelength of 633 nm and a power of 0.103 W. The XRD patterns were taken on a DMAX-2500PC diffractometer with Cu Kα radiation (λ = 0.154 18 nm) and a graphite monochromator. The fluorescence spectra were performed on a LS-55 spectrofluorometer (PerkinElmer, Waltham, MA, USA) with a quartz cell (1 × 1 cm). Ultraviolet and visible (UV−vis) measurements were operated on a Cary 60 UV−vis spectrometer (Agilent, America). The product was calcinated on a high-temperature tube furnace energy (HY1200 °C). X-ray photoelectron spectroscopy (XPS) was performed on the Thermo Scientic ESCALab 250Xi using 200 W monochromated Al Kα radiation. The binding energies were calibrated based on the graphite C 1s peak at 284.8 eV.

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RESULTS AND DISCUSSION Synthesis and Characterization of EuW10/DA Flowerlike Nanostructures. First, supramolecular nanostructures were construted by the addition of EuW10 solution to DA solution (at mass ratio of 1:1) at room temperature. A yellow cloudy suspension was formed once mixing, indicating the assembly occurs. The sample of 2 mg mL−1 EuW10/2 mg mL−1 DA (at charge ratio of 15:9) as a representative was mainly investigated. TEM and SEM observations were employed to image the separated precipitates. Figure 1a−c reveals that the samples consist of large quantities of flowerlike nanostructures that closely resemble the shape of peony in nature with an average diameter about 7.9 μm. Figure 1d,e showsTEM images of the single nanoflower and a local partial enlarged image of the nanoflower. The high-resolution TEM (HR-TEM) study demonstrated that there were many basic structural units in the petals composed of EuW10 clusters and lower-electron-contrast 1360


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Figure 4. TEM, SEM and CLSM images of vesicles observed when the ratio of EuW10 and DA at 1:9.

Figure 5. Morphological changes with different charge ratio of DA to EuW10: (a−c) SEM and (d) TEM images at the ratio of 5:9, (e−g) SEM and (h) TEM images at the ratio of 9:9, (i−k) SEM and (l) TEM images at the ratio of 25:9. The concentration of EuW10 was fixed at 1 mg mL−1 and the concentration of DA was varied.

DA shell (Figure 1f). CLSM showed red fluorescence attributed to EuW10, suggesting EuW10 was incorporated in the nanoflowers (Figure 1g). Energy-dispersive X-ray (EDX) analysis and SEM-EDX mapping (Figure 1i and Figure 2) gave the elements of W, C, O and N in the samples, confirming the successful hybridation of EuW10 and DA. From SEM-EDX mapping observation, the element W and majority O coming from EuW10 mostly distributed in nanoflower structure, C and N elements representative of DA distributed in both nanoflowers and solution, and Na element is absent in the nanoflower structure, thus, it can be concluded that the electrostatic interaction primarily derived from oxygen atom of EuW10 and the protonated amine groups of DA and DA is excess due to the charge ration of DA to EuW10 is about 5:3 in this case. Formation of EuW10/DA Nanoflowers with Different Incubation Time. The evolution of the morphologies over incubation time was monitored by SEM to understand further

the formation mechanism of the EuW10/DA nanoflower (Figure 3). At an early stage (1 min and 0.5 h, Figure 3a−f), the cores of the flowerlike nanostructures were initially formed. In this case, DA with protonated amino group as a cationic surfactant formed hydrid with EuW10 through electrostatic and hydrogen bonding interactions. Subsequently, the hybrid as the basic building block further stacked into the surface of parent cores (∼240 nm) forming a rough surface. Then, more building blocks aggregate around the parent cores to form the resulted nanoflower (Figure 1a). In the second growth step (0.5 h, Figure 3b,f), nanopetals began to appear that may be rationalized by considering that the branched nanopetals grow out from the solid cores by combining the free molecules undergo a anisotropic growth and led to ellipsoidal multilayered nanoflowers with a size about 7.9 μm (1 h, Figure 3c,g).25 Finally, the hierarchical nanoflowers grew more compact and the surfaces of the nanopetals became very smooth due to Ostwald ripening in the further growth stage (2 h, Figure 1361


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ACS Sustainable Chemistry & Engineering Scheme 1. Schematic Illustration of the Assembly Process

Figure 6. (A) FT-IR spectra of (a) pure DA, (b) pure EuW10, (c) EuW10/DA hybrid nanoflowers. (B) Partial enlarged (500−1000 cm−1) spectra of (a) pure EuW10, (b) EuW10/DA hybrid nanoflowers. (C) Raman spectra of (a) pure DA, (b) pure EuW10, (c) EuW10/DA hybrid nanoflowers. (D) XRD spectra of (a) pure DA, (b) pure EuW10, (c) EuW10/DA hybrid nanoflowers.

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Figure 7. (A) Fluorescence spectra of (a) 2 mg mL−1 EuW10, (b) 0.13 mg mL−1 DA/2 mg mL−1 EuW10. (B) (a) EuW10, (b) EuW10/DA hybrid nanoflowers. The insets are sample photographs under daylight (upper) and UV-light (down): (A) left, 2 mg mL−1 EuW10; right, 0.13 mg mL−1 DA/ 2 mg mL−1 EuW10 and (B) left, the powder of EuW10; right, the powder of EuW10/DA hybrid nanoflowers.

3d,h).28 It can be concluded that the whole process for the formation of EuW10/DA nanoflowers with different incubation time in our system mainly mimics the growth process of flowers in nature, from small buds to blooming flowers. Influences of Concentration and Ratio on the Morphologies of EuW10/DA Nanostructures. In the supramolecular assembly, the sizes and morphologies of the assemblies can be easily controlled by manipulating the fabrication conditions.29−31 Thus, the modulation of the sizes and morphologies of EuW10/DA nanostructures by variation of either their ratios or concentrations were studied. With the increase of the concentrations of DA (CDA) while keeping the concentration of EuW10 (CEuW10) constant, we can realize the ratios of DA to EuW10 varying from 1:9 to 5:9 to 9:9 (1:1) to 15:9 (5:3) to 25:9, respectively. Vesicles were observed first at 1:9 (Figure 4a−c), and then nanoflowers (average size, 10.6 μm) with high-density petals began to appear at a ratio of 5:9 (Figure 5a−d). The average size of the nanoflowers decreased to 8.8 μm and the petals became broader and looser when the ratio changed to 9:9 (1:1) (Figure 5e−h). At a ratio of 15:9 (5:3), the size of the aggregates continued to reduce to about 7.9 μm and the petals were more separated and flexible which may endow it a high specific surface area (Figure 1a−d). When the ratio is 25:9, smaller structures (3.8 μm) were formed and the petals of which were thin and less uniform (Figure 5i−l). As we see from the evolution process, the optimal morphology was achieved at the charge ratio of 5:3 where the numbers of positive charges exceeded that of negative charges, it is speculated that there were other interactions, such as hydrogen bonding interactions and van der Waals forces between the two components besides electrostatic interactions. It also can be observed that the obtained nanoflowers became smaller in the process and this result may be caused by the fact that increasing CDA leads to a corresponding increase of the nucleation sites, resulting in the size of nanoflowers decreasing.32,33 Moreover, the morphologies of EuW10/DA hierarchical nanostructures were also varied by changing the concentration with constant ratio of EuW10 and DA. The vesicles were preferentially formed when the concentrations of EuW10 and DA fixed at 0.1 mg mL−1 (Figure S2). After the concentrations of both the components increased from 0.5, 1, 2 to 5 mg mL−1, the increased nucleation sites caused the average sizes of the nanoflowers decreased from 23.7, 18.8, 7.9 to 2.8 μm, respectively (Figure S3, Figure 1) which is also consistent

with the observations in other assembly system such as bovine serum albumin (BSA) and copper phosphate (Cu3(PO4)2· 3H2O) formed hybrid.33 The schematic illustration of the assembly mechanism and morphology changes of EuW10/DA nanostructures with increasing CDA and incubation time was shown in Scheme 1. FT-IR and Raman Analysis. Vibrational spectroscopies, such as FTIR, UV−vis and Raman, are valid tools to characterize the structural alteration and the interactions between EuW10 and DA. The formation of the hybrid nanoflowers was first proved by the FT-IR spectroscopy in Figure 6A,B. The four main characteristic vibration peaks of EuW10 were assigned as follows: 944 cm−1 ascribed to ν(W Od), 843 cm−1 attributed to ν(WObW), and 787 and 705 cm−1 assigned to ν(WOcW), where Ob is the bridged oxygen of two octahedra sharing a corner, Oc is the bridged oxygen of two octahedra sharing an edge and Od is the terminal oxygen.34 After the complexation of DA with EuW10, the peaks shifted to 947, 893, 817 and 647 cm−1, respectively. These results demonstrated that there exist hydrogen bonding or electrostatic interaction between the two components and EuW10 clusters have been incorporated into the flowerlike nanostructures. Meanwhile, the primary peaks of pure DA were observed in the absorption bands of the flowerlike nanostructures.35 Then, the Raman and XRD spectra provided additional assembly information on DA and EuW10. In Raman spectra (Figure 6C), the symmetric and asymmetric stretching vibration of the WO terminal group located at 965 and 944 cm−1, shifted to 962 and 940 cm−1, respectively, when EuW10 incorporated into the nanoflowers. The band at 888 cm−1 denotes W2O corner-sharing moved to 893 cm−1, and 554 cm−1 denotes W3O stretching vibration band changed to 596 cm−1. Moreover, several Raman peaks of DA were observed in Figure 6a. These variations indicated that the strong hydrogen bonding or electrostatic interaction exists between EuW10 molecules and DA and the successful coassembly.34 In powder XRD spectra (Figure 6D), EuW10 exhibited sharp Bragg reflections, indicating itself had welldefined structure. However, two new peaks appeared at 2θ = 13.9° and 16.8° for the nanoflowers, suggesting novel crystalline structure formation which needs further identification.25 1363


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Figure 8. (a−c) SEM images of calcinated porous EuW10 with different magnifications.

hexavalent state W6+.44 Two clearly resolved peaks for the N1s band of nanoflowers can be seen in Figure S4E. The signal at 399.4 eV can be ascribed to alkylamines,45 and the higher energy signal can be assigned to protonated the amine groups of DA, which proves the existence of electrostatic interaction or hydrogen-bonding interaction between DA and EuW10. The N1s band in the calcinated sample is not well resolved, it is maybe that DA decomposed after calcination. Next, UV−vis spectra were collected from EuW10, EuW10/ DA nanoflowers, calcinated nanoflowers (Figure S5). The UV− visible absorption of EuW10/DA nanoflowers became broaden from 200 to 370 nm, and the absorption spectrum of the EuW10 exhibited the expected EuW10 peaks at 205 and 255 nm coexist with a new absorption bands centered at 335 nm This phenomenon is well-known signatures for oxygen → tungsten charge transfer (CT) and also indicate that the formation of EuW10/DA nanoflowers is essential to the charge transfer between EuW10 and DA.46 Calcinated porous EuW10 exhibit obvious visible light absorption compared to that of calcinated before, and the absorption edge occurs at a wavelength longer than 570 nm. The light response range of calcinated porous EuW10 extends to the visible region and the photogenerated electrons and holes increase also, which improves the optical absorption of materials.47 Furthermore, Figure S6 shows nitrogen adsorption− desorption isotherms and the corresponding pore size distribution curves (inset) for EuW10/DA nanoflowers and calcinated nanoflowers. For the nitrogen adsorption−desorption measurements, the type IV isotherms with hysteresis loops for the two samples are indentified according to the IUPAC classification,48 indicating there are mesopores in the samples. Moreover, the rapid increased branches of the isotherms at a higher relative pressures resemble to type II isotherms, demonstrating the existence of macropores also. Associated with slitlike mesopores, the hysteresis loops can be ascribed to the type H3, which is in accordance with the feature of our hierarchical nanoarchitectures. The pore size distribution curves (the inset in Figure S6) indicated that both samples possess mesopores and macropores in a wide size range of 2 to over 60 nm. Such porous structures are facilated for the transport of the reactant molecules and products. For the calcinated sample, the lower pressure and the increased areas of the hysteresis loops demonstrated the increasement of the specific surface areas and pore volume.49 Then, to explore the potential applications of calcinated porous EuW10, herein we applied the porous EuW10 to catalyze the decomposition of organic pollutants (MO and RhB were chosen as typical organic wastes). For comparison, the catalytic performances of EuW10-DA hybrid nanoflowers/H2O2, EuW10/ H2O2, DA/H2O2, H2O2 and calcinated porous EuW10 on the

Fluorescence Properties of EuW10/DA Hybrid Nanoflowers. The polyoxometalates containing Eu can emit red fluorescence, thus, the fluorescence properties of EuW10/DA hybrid nanoflowers is studied. For the pure EuW10 solution, it displays the characteristic 5D0 → 7F1 at 590 nm and 5D0 → 7F2 at 618 nm (Figure 7A)36 while for the pure EuW10 powder, it shows 5D0 → 7F1 at 590 and 596 nm and 5D0 → 7F2 at 616 and 622 nm (Figure 7B).22 It is considered that coupling with OH oscillators can influence the 5D0 state in the presence of water molecules. Thus, in aqueous solutions, the emmission of the Eu3+ can be quenched by the surrounding OH oscillators.22 The energy transfer from the heteropolyoxotung state to Eu3+ endows the Eu-containing polyoxometalates a red fluorescence.37,38 Two steps are included: first, photoexcitation of the O → W ligand-to-metal charge transfer (LMCT) bands would cause the hopping of the d1 electron. Next, the energy transfer from the O → W LMCT states to the 5D0 emitting state of Eu3+. The emission originated from 5D0 excited states relaxes to the 7Fj ground state, which emits the fluorescence ultimately.34 However, in our study, it can be observed that the fluorescences of the EuW10/DA vesicles and nanoflowers were partly or totally quenched both in visual and spectra features, respectively (Figure 7A,B). We speculated the fluorescence quenching of EuW10 may be due to the hydrogen bond existing between the ammonium group of DA and the oxygen atom of EuW10 acting as a bridge that blocked the hopping of the d1 electron in EuW10 in the first step. Therefore, the photoluminescence was silent in EuW10/DA nanoflowers due to the block of the electron transfer from heteropolyoxotungstate to Eu3+ and it may be induced new charge transfer between EuW10 and DA.39 Photocatalytic Behaviors for the Degradation of MO and RhB Dyes. POMs play an indispensable role in the catalysis science and are capable of oxidating various compounds such as alkenes, sulfides and dyes,40−42 thus, in our work, we intend to investigate the photocatalytic behaviors for the degradation of dyes under visible-light irradiation. We first calcinated the EuW10/DA nanoflower at 350 °C for 2 h to get a pure EuW10 framework (Figure 8a−c). From SEM images, it can be seen that after the calcinations, although the morphology has changed to some content compared with the EuW10/DA nanoflower, the porous structures still maintained. Then, XPS was used to indentify optical properties and chemical composition. The binding energy of Eu3d in nanoflowers (Figure S4A) is 1135.7 eV (Figure S4) similar to that of in calcinated nanoflowers of 1135.8 eV (Figure S4B), which indicates the Eu atoms remained its +3 valence.43 The W4f doublet (W4f7/2 and W4f5/2) can be observed at 35.2 and 37.5 eV for nanoflowers (Figure S4C), 35.3 and 37.3 eV for calcinated sample (Figure S4D), which corresponds to the 1364


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Figure 9. (A) UV−vis curves of the degradation of MO by calcinated nanoflower. (B) MO degradation over time with different substances: (a) calcinated porous EuW10/H2O2, (b) EuW10-DA hybrid nanoflowers/H2O2,(c) EuW10/H2O2, (d) DA/H2O2, (e) H2O2 and (f) calcinated porous EuW10. (C) MO removal of calcinated porous EuW10/H2O2 and ln(Ct/C0) as functions of reaction time. (D) Recycling experiment for the degradation of MO using calcinated porous EuW10/H2O2 system.

accepting electron from DA easily facilitated by the reduced molecular distance under the effect of hydrogen-bonding and electrostatic interaction. The W−O bonds acted as active species interacted with H2O2 and made H2O2 absorb on the catalyst surface. Then, H2O2 was activated to ·OH by capturing electron from LUMO of EuW10, dyes were degraded by the · OH as a consequence.2 That is the main reason to explain the catalytic efficiency of EuW10/DA hybrid nanoflowers/H2O2 is higher than EuW10/H2O2. Besides, 8% of RhB and 6% of MO can be degraded with POM/H2O2 in our work, transformed by photosensitized action. Thus, the photocatalysis by the photosensitization work concurrently in the process. The porous EuW10/H2O2 also showed the high photocatalytic efficiency for the degradation of RhB; however, it needs longer reaction time and the catalyst cannot be reused (Figure S7). To explain the phenomenon, the ζ-potential was tested. The surface potential of nanoflowers changed from −17 to −21 mV after calcination, it is easy for the cationic RB approach the negatively charged framework in fact. However, the cationic RB will absorb on the anionic surface, preventing H2O2 activated by the active sites at the surface, which called catalyst poisoning. On the contrary, the H2O2 can be easily activated by capturing electron from EuW10, ·OH exist in the bulk solution will sustainably degradate MO rather than RhB. Thus, the lower catalytic efficiency for RhB is due to the catalyst poisoning. The reason for the calcinated porous EuW10 possesses better decomposition efficiency than EuW10/DA hybrid nanoflowers

degradation of MO and RhB dyes were also investigated. It can be observed that when the calcinated porous EuW10 and H2O2 were incubated in the MO solution, the MO was completely removed within 20 h (Figure 9A); the corresponding sample photographs are shown in the inset of Figure 9A. Moreover, as shown in Figure 9B, it can be clearly seen that the decomposition efficience of the porous EuW10/H2O2, EuW10DA hybrid nanoflowers/H2O2 and EuW10/H2O2 was 100%, 65% and 13% (based on Equation 1), respectively. The linear fit of the ln (Ct/C0) data reveals that the catalytic reaction exhibits pseudo-first-order kinetics for the desulfurization of MO (R2 = 0.949) (Figure 9C). The rate constant k of the oxidation reaction was determined to be 0.115 h−1, based on Equation 2 and Equation 3. −dCt /dt = kCt

(2)

ln(C0/Ct ) = kt

(3)

However, neither DA/H2O2 nor H2O2 could decompose the organic dye in the absence of EuW10. Furthermore, without H2O2, the calcinated porous EuW10 is also unable to degradate MO, suggesting H2O2 is indispensable for the catalysis.50 After six recycles for the degradation of MO, the catalyst only exhibit a little loss of photocatalytic activity, as shown in Figure 9D, which implied that the calcinated porous EuW10 can be used as an excellent photocatalyst for multiple cycles to catalyze MO. From our results, it can be speculated that for EuW10/DA hybrid nanoflowers/H2O2, it allows to expose photocatalytic active facets and the EuW10 can act as electron acceptor 1365


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can be ascribed to two aspects: First, calcinated porous EuW10 possess bigger surface area and larger pore volume than EuW10/DA hybrid nanoflowers proved by BET analyze (Table S1). Exposing more active sites endowed the accelerated reaction rate and desirable catalytic effect; second, calcinated porous EuW10 exhibit obvious visible light absorption compared to that of calcinated before, and the absorption edge occurs at a wavelength longer than 570 nm in UV−vis spetra. The extended light response range and increased number of photogenerated electrons and holes facilated the photocatalytic reaction, which improves the optical absorption of materials. Therefore, these demonstrations indicated that not only the surface structure but also the electronic structure could influence the photocatalytic properties of EuW10.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21573130, 21173128, 21201110 and 21571115) and Young Scholars Program of Shandong University (2016WLJH20).

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CONCLUSION In summary, we have fabricated 3D hierarchical nanoflowers by self-assembly of a cationic bioactive component (DA) and a Weakley-type POM (EuW10) through ISA strategy. It can be concluded that the sizes and morphologies of EuW10/DA hybrid nanostructures were dependent on the ratios of EuW10/ DA and the concentrations of EuW10 and DA. Moreover, the growth process of EuW10/DA hybrid nanoflowers was also investigated on the basis of time evolution. The electrostatic interactions and hydrogen bonds play a decisive role in the coassembly process. Moreover, our research manifests that the morphological control of POM endows exposure of photocatalytically active facets preferentially, which is applicable in the highly efficient visible-light-sensitive photocatalysts and the calcinated porous EuW10 could be employed to decompose organic pollutants (MO and RhB) with high efficiency. Especially for MO, it can be easily reused at least 6 times without an obvious decrease of activity. The organic−inorganic hybrid materials with novel functions will extend the research fields of both material and biomedical sciences.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b01805. Structures of Na9(EuW10O36)·32H2O, DA, MO and RhB, TEM and SEM images of vesicles, morphological changes of EuW10/DA, XPS spectra, UV−vis absorption spectra, adsorption−desorption isotherms, pore size distribution and specific surface area of the prepared samples, photocatalytic degradation RhB by calcinated nanoflower (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*X.X. E-mail: [email protected]. Phone: +86-531-88363597. Fax: +86-531-88361008. *D.S. E-mail: [email protected]. Phone: +86-531-88364218. Fax: +86-531-88564750. *S.Y. E-mail: [email protected]. Phone: +86-53188365896. Fax: +86-531-88564750. ORCID

Xia Xin: 0000-0002-4886-6028 Di Sun: 0000-0001-5966-1207 1366


Research Article

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