Hierarchical Nanostructures Self-Assembled by Polyoxometalate and

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Hierarchical Nanostructures Self-Assemblied by Polyoxometalate and Alkylamine for Photocatalytic Degradation of Dye Congxin Xia, Zhi Wang, Di Sun, Baolai Jiang, and Xia Xin Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03495 • Publication Date (Web): 30 Oct 2017 Downloaded from http://pubs.acs.org on November 3, 2017

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Hierarchical Nanostructures Self-Assemblied by Polyoxometalate and Alkylamine for Photocatalytic Degradation of Dye Congxin Xia a, Zhi Wang a, Di Sun a *, Baolai Jiang b, Xia Xin a, b * a

Key Lab for Colloid and Interface Chemistry of Education Ministry, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P. R. China.

b

National Engineering Technology Research Center for Colloidal Materials, Shandong University, Jinan, 250100, P. R. China.

*Author to whom correspondence should be addressed, E-mail: [email protected]. Phone: +86-531-88364218. Fax: +86-531-88564750 *Author to whom correspondence should be addressed, E-mail: [email protected]. Phone: +86-531-88363597. Fax: +86-531-88361008

ACS Paragon Plus Environment

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ABSTRACT A novel simple strategy for alkylamine-directed self-assembly of weakley-type polyoxometalate (POM, Na9[EuW10O36]·32H2O, abbreviate to EuW10) to form three dimensional (3D) nanoflowers have been successfully developed through ionic self-assembly (ISA) method. For comparison, different

molecular

weights

of

alkylamines

including

diethylenetriamine

(DETA),

triethylenetetramine (TETA) and tetraethylenepentamine (TEPA) were selected to construct hierarchical nanostructures. Our results revealed that the morphologies and sizes of the nanostructures could be simply controlled by varying the molecular weights and concentrations of alkylamines. The fluorescent colour of EuW10/TEPA nanoflowers changed compared with that of EuW10 owning to the symmetry degree of europium coordination in EuW10/TEPA nanoflowers was varied. It is demonstrated that this effective self-assembly occurs mainly though hydrogen bond and electrostatic interaction between EuW10 and TEPA. What’s more, the EuW10/TEPA nanoflowers after calcining showed excellent decomposition efficiency towards methylene blue (MB) dyes. Our results further confirmed that ISA method between small molecules and POM can provide a unique “bottom-up” strategy to construct novel structures with functional properties.


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INTRODUCTION Supramolecular self-assembly is recognized as a valid, “bottom-up” approach for the fabrication of advanced functional nanostructures.1 The key to obtain different ordered nanostructures which can possess special functions that beyond any compositional subunits is controlling the spatial arrangement of molecules via balance of non-covalent interactions such as hydrogen bonding, electrostatic interaction, hydrophobic interaction, van der Waals force, steric effect and π–π stacking.2-4 And those self-assembled, well-defined nanomaterials such as micelles,5-9 vesicles,10-12 multilayer films,13,14 microspheres15 and gels,16 display widespread potential applications in electro-optical materials, drug or gene delivery, catalysis science, smart microcarrier and microrector.17-20 Polyoxometalates (POMs) are a versatile clusters of nanoscale discrete metal-oxide consisted of some heteroatoms (for example, silicon or phosphorus) and early transition metals (generally include molybdenum, vanadium or tungsten) in the high oxidation states, which draw a great deal of attention for their significant functionalities in catalysis, optics, magnetics, electronics and gene delivery.21-24 The rich architectures, uniform morphologies and multiple negative charges make POMs outstanding candidates for self-assembly. Particularly, lanthanide-containing POMs elements, possessing excellent photoluminescent properties with narrow emission bands, large Stokes shift, long lifetime and tunable emission,25 can be regarded as connection node and the electrostatic interaction between POM and cationic linker is applied as binding force to build the construction of ionic organic–inorganic frameworks. For example, Gong et al. adopted ISA method to fabricate hybrid materials consisted of Eu-POM and ionic liquid-type gemini surfactant, which shows aggregation-induced emission (AIE).26 Wei et al. demonstrated a DyW10/PEO-bPDMAEMA hybrid


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complex which is very sensitive to CO2 content and shows rapid responsiveness in luminescence.27 Zhang et al. constructed a monodisperse dendrimer complex consisting of a gadolinium polyoxometalate core and dendritic shell through an electrostatic self-assembly strategy which plays a role as fluorescence and magnetic resonance imaging (MRI) contrast agent.28 Moreover, monoamine- or polyamine-based template techniques have broad application in the synthesis of various nanostructures because the amine group could strongly bond to a particular surface facet of clusters and accelerate the formation of precipitation on the more weakly passivated surfaces, forming some nanostructures with novel morphology.29,30 For example, Xi et al. used a new ligand (TEPA)-assisted solid–solid growth mechanism to obtain ZnSe materials from 1D to 3D nanostructures, including nanobelts, nanowires, and hierarchically hollow spheres by nanobelt (or nanorod) energyminimizing-driven self-assembly.31 Nissen et al. attach nitrogen (N) groups from tetraethylenepentamine to the surface of a polyethylene terephthalate film, discovering a more simpler alternative for the introduction of nitrogenous groups on a PET surface than plasma treatment.32 Nissen et al. synthesized a branched polyethyleneimine-based hydrogel containing polyoxometalate and a n-octylamine-epichlorohydrin cross-linking reagent which can selective catalytic oxidation 2-alkanols to 2-alkanones.33 Thus, it is expected that the combination of alkylamine and POMs will also bring rich structure evolution and favourable functionality. Herein, in this article, a Weakley-type (an europium ion sandwiched by two Lindqvist-type) POM (Na9(EuW10O36)·32H2O, abbreviate to EuW10) was employed as a polyoxoanion to assemble with different molecular weights of cationic alkylamine into well-ordered three dimensional (3D) nanoflowers through ionic self-assembly (ISA) strategy. The properties of these nanostructures have been systematically characterized by various techniques such as field-emission scanning electron


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microscopy (FE-SEM), transmission electron microscopy (TEM), high-resolution TEM (HR-TEM), X-ray diffraction (XRD) and fluorescence spectroscopy. Moreover, the mechanism of the self-assembly process of these hierarchical nanostructures was studied in detail, and the nanoflowers after calcining can be applied to photocatalytic degradation of methylene blue dye. Our research is another instance for the construction of organic–inorganic hybrid materials through self-assembly. EXPERIMENTAL SECTION Chemicals and Materials EuW10 was obtained as described by Sugesta and Yamase.34 Diethylenetriamine (DETA), triethylenetetramine (TETA) and tetraethylenepentamine (TEPA) were purchased from Aladdin Chemistry Co., Ltd., of China and used as received. The structures of EuW10, DETA, TETA and TEPA were shown in Figure 1. Hydrogen peroxide was purchased from Sinopharm Chemical Reagent Co., Ltd. Methylene blue (MB) was purchased from Alfa Aesarand. Water with a resistivity of 18.25 MΩ cm used in this experiments was obtained using a UPH-IV ultrapure water purifier (China).

Figure 1. The structures of (a) EuW10, (b) DETA, (c) TETA and (d) TEPA. Methods and Characteristic


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Transmission electron microscopy (TEM) observation was carried out on JEM-1011 (JEOL) operating at 100 kV. A small amount of sample was placed on carbon-coated copper grid and the excess solution was removed with filter paper, then copper grids were put under infrared lamp for 30 min. Field-emission scanning electron microscopy (FE-SEM) image and element mapping analysis were observed on a Hitachi SU8010 at 5.0 or 8.0 kV. Silica wafer was used to carry an amount of sample, excess sample was suck by filter paper. The silica wafers need to freeze-drying in a vacuum extractor at −60 °C for 1 day. High-resolution transmission electron microscopy (HR-TEM) images were recorded on a HRTEM JEOL 2100 system operating at 200 kV. Fourier transform infrared (FT-IR) spectrum was measured on an AlPHA-T spectrometer (Bruker Optics, Germany) within 7800 to 370 cm⁻¹ region. The room temperature X-ray powder diffraction patterns (XRD) were taken on a D8 ADVANCE (Germany Bruker) diffractometer equipped with CuKα radiation and a graphite monochromator. The model of fluorescence spectrometer is Thermo Scientific Lumina. Polarized optical microscopy (POM) images came from an Axio Scope.A1 (Germany) microscope. Confocal laser scanning microscope (CLSM) (Panasonic Super Dynamic II WV-CP460) observations were performed with excitation wavelength at 488 nm. X-ray photoelectron spectroscopy (XPS) dates were collected by an X-ray photoelectron spectrometer (ESCALAB250) with a monochromatized Al Ka X-ray source (1486.71 eV). Thermogravimetric analyses (TGA) were measured on a TA SDT Q600 thermal analyzer from room temperature to 800 °C with a heating rate of 10 °C min-1 under a nitrogen atmosphere. Raman spectra were obtained from a LabRAM HR800 module (HORLBA JY) and the samples were excited by a laser source with a wavelength of 633 nm. Sample Preparation of EuW10/TEPA hybrid nanostructures In a typical experiment process, sample solutions were prepared by dissolving 0.25 mL TEPA


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aqueous solution (2 mg mL⁻¹) in 0.55 mL water, and then 0.2 mL EuW10 aqueous solution (10 mg mL⁻¹) was added with stirring. The transparent solution turns to turbid within 1 min. Before characterization, the samples were incubated at 20.0 ± 0.1 °C in a thermostat for 1 week to react completely. The final product as white powder was collected by centrifugation and removing upper-phase, washing with deionized water for three times and freeze-drying in a vacuum extractor at −60 °C for 1 day. Catalytic applications For the sample of calcinated nanoflower/MB/H2O2, 5 mg catalyst was put into 4 mL 10 mg L-1 MB/140 mmol L-1 H2O2 solution at 25℃. During the process of degradation, 2 mL of suspension after centrifuged was taken out at a certain time gradient to obtain the absorption value of a maximum peak of MB solution by UV-vis measurement for further concentration analysis. The concentration of the dye is calculated by a calibration curve, and the degradation efficiency (%) can be obtained though equation (1) Efficiency (%) =

–

× 100%

(1)

Where C0 is the initial concentration of dye and C is the revised concentration using the UV-vis spectra.

RESULTS AND DISCUSSION Synthesis and Characterization of EuW10/TEPA Nanostructures Firstly, take the samples of 2 mg mL⁻¹ EuW10 and 0.5 mg mL⁻¹ TEPA (at mole ratio of 1:4.5) for example, it is interesting to find that the morphologies and sizes of the hierarchical nanostructures have an evolution with the aging time. In regard to the samples of aging for 1 day, Figure 2a-c


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clearly reveal that the aggregates are adhesive nanospheres, mainly diameter is about 260 nm, which presenting rough surface observed though the enlarged SEM image (Figure 2c). The analysis of SEM- energy-dispersive X-ray (EDX) mapping (Figure S1 a-f, Supporting Informatin) and EDX proved the successfully hybridization of EuW10 and TEPA. As the cultivation time prolonged from 1 day to 1 week, the nanospheres gradually transformed to nanoflowers with smooth surfaces that are similar to daisy sakura with a mean size of 4.6 µm. These results clearly revealed the transformation process of the morphologies of 2 mg mL⁻¹ EuW10/0.5 mg mL⁻¹ TEPA from the spheres to nearly monodisperse nanoflowers with different incubation time. One of the possible reasons may be that the electrostatic attraction between inorganic cluster and organic component enhanced with time goes by, and the repulsion between polyanions increases the flexibility of the molecular arrangement, which offers the possibility of forming into flower architectures.35


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Figure 2. Characterization of the sample of 2 mg mL⁻¹ EuW10/0.5 mg mL⁻¹ TEPA with different cultivation time. TEM images (a) and SEM images (b, c) of nanospheres with aging time for 1 day, c is local enlarged image of b; TEM images (d) and SEM images (e, f) of nanoflowers with aging time for 1 week, f is local enlarged image of e; (g) HR-TEM (inset is SAED pattern of nanoflower petal), (h) POM and (j) CLSM images of nanoflowers. Then, the white precipitate of 2 mg mL⁻¹ EuW10/0.5 mg mL⁻¹ TEPA aging for 1 week was collected to carry out farther characterizations. For the HR-TEM results of nanoflower petals (Figure 2g), lots of dark spots which assume to be the clusters of EuW10 due to its high content of heavy element tungsten and many basic structural units can be observed. The selected area electron diffraction (SAED) pattern of petals (inset of Figure 2g) exhibited a set of dispersive diffraction spots in a ring distribution, demonstrating the nanoflowers have polycrystal structures and the oriented alignment of the nanocrystals induced the quasi-single-crystalline structure.36 SEM-EDX mapping (Figure 3) and EDX analysis (Figure S1h) also proved the successful hybridization of EuW10 and TEPA by showing the element Eu, W, O of EuW10 as well as C, N of TEPA distributed in nanoflowers structure. POM image (Figure 2h) illustrated the nanoflowers structure is anisotropic growth that always driven by using capping ligands to bind selectively onto particular facets of the nano-particles. Due to EuW10 exist in the nanoflowers, red fluorescence can be observed in CLSM (Figure 2i). According to the above results, the evolution process of morphology can be summarized. At an early stage, effective self-assembly is induced by the linear molecular structure of TEPA molecules and form primary rough nanoparticles complexes with EuW10, predominant driving force is the coordination effect of amide groups in the TEPA. Subsequently, large agglomerates of primary


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particles as building blocks are formed around parent cores as the scaffold for the nanopetals, causing the appearance of separate petals by combining with molecules expelled from other primary particles. At last, TEPA molecules play an important role as a ‘glue’ to bind the petals together, then anisotropic growth results in complete formation of a multilayered nanoflowers structure.37 The key step during the formation of nanoflowers is aggregation and growth originates at [EuW10O36]9-binding sites which maybe because of the electrostatic interaction principally derived from amine groups coming from TEPA and oxygen atom origining from EuW10. We detected the pH of EuW10 and TEPA aqueous solution with the results that the former is 6.0 while the later is 8.5. And the solution pH during the preparation of nanoflowers is about 6.7, lower than the pKa of TEPA (10.1), indicating TEPA exists in a protonate state and the electrostatic interaction dominates the complexation.

Figure 3. (a) SEM image of 2 mg mL⁻¹ EuW10/0.5 mg mL⁻¹ TEPA hybrid nanoflowers, SEM element mapping analyses of nanoflowers: (b) Eu, (c) W, (d) C, (e) N, (f) O. FT-IR, XRD and TGA Analysis


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In order to investigate the variation of the properties of EuW10 before and after TEPA assembly, FT-IR spectra of TEPA, EuW10 and 2 mg mL⁻¹ EuW10/0.5 mg mL⁻¹ TEPA were performed (Figure 4). It can be seen that the bands at 2949 and 2846 cm-1 can be reckoned as the asymmetric and symmetric stretching vibrations of CH2 in the TEPA alkyl chains (Figure 4A, a).38 After assembly with EuW10, these stretching vibrations shift to 2923 and 2856 cm-1 for EuW10/TEPA system (Figure 4A, c). What’s more, the scissoring vibration (1574 cm-1) and stretching vibration (1476 cm-1) are representative of the NH2 group and C-N of TEPA, respectively. After assembly with EuW10, the C-N stretching vibration still presence, but the NH2 group’s scissoring vibration disappeared as a result of electrostatic interaction and hydrogen bond interaction (Figure 4A, c). Moreover, for EuW10 (Figure 4B, a), the characteristic vibration bands were listed as below: ν (W=Od, 942 cm-1), ν (W-Ob-W, 843 cm-1), ν (W-Oc-W, 784/705 cm-1), and Ob means the bridged oxygen of two octahedra sharing a corner, Oc represents the bridged oxygen of two octahedra sharing an edge while Od is the terminal oxygen.39 Because of the interaction between TEPA and EuW10, these peaks moved to 945, 839, 753 and 622 cm-1 for EuW10/TEPA composites (Figure 4B, b), respectively, demonstrating the metal-oxygen framework of EuW10 cluster exists in the EuW10/TEPA nanoflowers as well as the powerful driving force in the system may be electrostatic interaction, hydrogen bonding. What’s more, the UV-visible absorption of the inorganic cluster in EuW10/TEPA complexes enhanced in comparison to that of the free cluster alone (Figure S2) which means the electrostatic interaction contributes to the combination of EuW10 and TEPA. The crystalline phases of as-prepared EuW10/TEPA nanoflowers were characterized by XRD method which is shown in Figure 4C, revealing EuW10 have well-defined structure with sharp Bragg reflections. In the TEPA spectra, only one peak was observed which means it doesn’t have ordered


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arrangement. However, after the assembly of TEPA and EuW10, two new peaks displayed for the nanoflowers at 2θ =17.1° and 27.5° with the decrease of EuW10 reflection, confirming that the two components were successfully co-assembled and novel crystalline structure were formed.40 A

B c

b -1

-1

2856 cm

2923 cm

b

Absorbance

Absorbance

a

a

705

942 784

843

-1

2949 cm

945 753

-1

2846 cm

4000

3500

3000

2500

622

839

2000

1500

1000

500

1000

800

Wavenumber(cm-1)

600 -1

Wavenumber(cm ) D 100

C

3.8% 170℃

95

c

b

Weight/%

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8.6%

90 400℃ 5.7%

85

a 80 10

20

30

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60

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0

200

2θ (degree)

400

600

800

Temperature/°C

Figure 4. (A) FT-IR spectra (a) TEPA, (b) EuW10, (c) 2.0 mg mL-1 EuW10/0.5 mg mL-1 TEPA and (B) partial enlarged (500−1000 cm−1) spectra (a) EuW10, (b) 2.0 mg mL-1 EuW10/0.5 mg mL-1 TEPA, (C) XRD spectra (a) TEPA, (b) EuW10, (c) 2.0 mg mL-1 EuW10/0.5 mg mL-1 TEPA, (D) TGA curves of 2 mg mL⁻¹ EuW10/0.5 mg mL⁻¹ TEPA. To study the thermal stabilities of the hybrid materials, TGA were measured. First of all, it can be verified that TEPA molecules can fully decompose at the temperature of 270℃ (Fig. S3A). Then, three main distinct thermal weight loss processes can be observed in the TGA curve of EuW10/TEPA nanoflowers as shown in Figure 4D. The first weight loss of 3.8% from 30 to 170℃ is mainly owned


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to the evaporation of crystal water in POMs and a small number of physically adsorbed water. The second weight loss about 8.6% during 170 and 400℃ is caused by the decomposition of residual TEPA components while the last mass change of 5.7% above 400℃ ascribed to the burning of remaining organic groups with the destruction of chemical bonds. From the TGA analysis, it can be concluded that the EuW10/TEPA hybrid materials has good thermal stability with the great mass of organic part decomposed and metal-oxide nanostructures remained.41,42 Therefore, according to the above analysis, the formation mechanism and morphology evolution of EuW10/TEPA nanoflower can be drawn in Scheme 1.

Scheme 1. The formation mechanism and morphology evolution of EuW10/TEPA nanoflowers. Fluorescence Properties of EuW10/TEPA Hybrid Nanoflowers Compared to other inorganic fluorescent nanomaterials, rare earth element doped polyoxometalates possess more excellent photoluminescent properties.25 Thereby, it is necessary to study the fluorescence performance of EuW10/TEPA hybrid materials. Although the pure TEPA can


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emit blue fluorescence (Figure S3B), the fluorescent intensity is much lower (Figure 5A, c) than that of EuW10 which appears orange color. In the emission spectra of the pure EuW10 powder (Figure 5A, a), four main sharp emission bands from 550 to 750 nm were shown as follow: 5D0→ 7F1 at 594 nm, 5

D0 → 7F2 at 623 nm, 5D0 → 7F3 at 651 nm and 5D0 → 7F4 at 699 nm, respectively. The fluorescence

of EuW10, exhibiting the characteristic transitions of Eu3+ ions, can be emitted and the high quantum yield emission can be obtained by the energy transition. That is, photoexcitation of the ligand-to-metal charge-transfer (O → W LMCT) bands make the hopping of the d1 electron achievable, then the intramolecular energy transfer from the O → W LMCT states to the 5D0 emitting state of Eu3+ ions, finally the emission originated from 5D0 excited states relaxes to the 7Fj (j=1, 2, 3, 4) ground state.43 In our work, after the assembly of TEPA and EuW10, three main sharp emission bands moved to 617 nm (5D0 → 7F2), 654 nm (5D0 → 7F3) and 704 nm (5D0 → 7F4) for EuW10/TEPA hybrid materials (Figure 5A, b), respectively. It is well-known the orange luminescence of 5D0 → 7F1 transition belongs to magneticdipole type that has no concern with the surrounding coordination environment; on the other hand, the red luminescence of 5D0 → 7F2 is attributed to the electric-dipole transition which is hypersensitive to chemical bonds of Eu3+ ions in the micro-environment.44 Based on the red /orange luminescence intensity ratio (R/O intensity) will increase in the wake of the symmetry of Eu3+ decrease, the relative intensity ratio of 5D0 → 7F2 transition to 5D0 → 7F1 transition is often used as the standard to estimate the symmetry variation degree of europium coordination environment in different system.45 As for the emission spectra of EuW10, the fluorescence intensity of 5D0 → 7F1 transition is stronger than that of 5D0 → 7F2 transition, suggesting Eu3+ is located at a higher symmetry, thus, the orange luminescence is the dominated color. However, the larger value of R/O


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manifests that the symmetry of Eu3+ is lower in our EuW10/TEPA nanocomposites, accounts for the stronger intensity of red fluorescence in the hybrid material.44, 46

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0

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Figure 5. Fluorescence spectra of (A) EuW10 powder (a), hybrid nanoflowers (b) of 2 mg mL⁻¹ EuW10/0.5 mg mL⁻¹ TEPA, TEPA (c) with the excitation wavelength at 378 nm, the insets of (A) are sample photographs under daylight (upper) and UV-light (down): left is EuW10 powder, right is EuW10/TEPA nanoflowers powder; (B) EuW10/TEPA nanoflowers (a) and calcinated nanoflowers (b), the insets of (B) are sample photographs under daylight (right) and UV-light (left) of calcinated nanoflowers powder. In order to further investigate the fluorescent properties, the EuW10/TEPA nanoflowers were calcinated at 350 °C for 2 h. It can be observed that the calcinated nanoflowers is fluorescence quenched and the morphology of calcinated nanoflowers has changed to some content compared with the nanoflowers before calcinations (figure S4). However, the specific surface areas and pore volumes

before

and

after calcinations

are

nearly

unchanged

according

the

nitrogen

adsorption–desorption isotherms (figure S5, table1). For studying the reason of quenching after calcinations, XPS was used to characterize the existing state of chemical composition. In Figure S6


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and Figure S7, it can be seen that Eu atoms remained its +3 valence and W atoms kept hexavalent state in hybrid as well as calcinated nanoflowers.47-49 It is apparent that two resolved peaks of the N1s signals for nanoflowers (Figure S7C) emerges at 398.3 and 400.4eV. The peak centered at 398.3eV can be contributed from alkylamines,50 while the other peak at 400.4 eV can be ascribed to protonation of the amine groups in TEPA, providing evidence for the existence of electrostatic or hydrogen-bonding interactions between EuW10 and TEPA.51 However, the two resolved peaks of calcinated nanoflowers (Figure S7F) are less clearly which can be attributed to the decomposition of TEPA after calcinations. In Raman spectra (Figure S8), calcination gave rise to carbonization of EuW10/TEPA nanoflowers which shielded the fluorescence of Eu element and then induced fluorescence quenching. Influences of concentration and different molecular weights of alkylamines on the morphologies of nanostructures To study how the phase transition influenced the self-assembly behavior of EuW10/TEPA system, we keep the concentration of EuW10 at 2 mg mL⁻¹ and change the concentration of TEPA to investigate its phase behavior. As shown in Figure S9, the samples experienced various phase behavior including solution, precipitate, two phase and hydrogel phase with the amount increasing of TEPA, which may be caused by the different interaction between EuW10 and TEPA. Then, we choose 0.078 and 1.0 mg mL⁻¹ TEPA as contrast to discuss the morphologies changes of EuW10/TEPA. When the concentration of TEPA fixed at 0.078 mg mL⁻¹ (at mole ratio of 1:0.7) where the system owns more numbers of negative charges, smooth nanospheres about 350nm were obtained (Figure 6, a-c) no matter with the cultivation time. We think the loss of the flower structure is extremely likely caused by removal of amine groups from the core. For the sample of 2 mg mL⁻¹ EuW10/0.5 mg mL⁻¹


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TEPA (at mole ratio of 1:4.5), well-ordered 3D nanoflowers (Figure 6, d-f) can be formed which was charactered above. However, further increasing the concentration of TEPA, only rough nanospheres with seriously ununiform in size were observed (Figure 6, g-i) for 1.0 mg mL⁻¹ TEPA (at mole ratio of 1:9), which can be due to high concentrations of TEPA in system suppress the bonding of primary particles and restrains the formation of petals in turn. Based on the analyses, we further confirmed that in the EuW10/TEPA nanocomposites, the nine protons of each EuW10 ion can quantificationally protonate amine groups of TEPA and facilitate the formation of nanoflowers, driving by electrostatic cross-linking effect on the TEPA chains.

Figure 6. TEM images (a) and SEM images (b, c) of the nanospheres of 2 mg mL⁻¹ EuW10/0.078 mg mL⁻¹ TEPA; TEM images (d) and SEM images (e, f) of the nanoflowers of 2 mg mL⁻¹ EuW10/0.5 mg mL⁻¹ TEPA; TEM images (g) and SEM images (h, i) of the nanospheres of 2 mg mL⁻¹ EuW10/1.0 mg mL⁻¹ TEPA. In order to testify whether the formation of nanoflower structures is strictly limited to the TEPA


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system, alkylamines with different molecular weights were used for comparation. DETA as well as TETA are used instead of TEPA and keep the mole ratio of EuW10 and alkylamines at 1:4.5. In Figure 7a, it can be seen that 2 mg mL⁻¹ EuW10/0.393 mg mL⁻¹ TETA constructed adhesive and nonuniform nanospheres with major diameter about 120 nm after aging for 1 day. Few nanoflowers (Figure 7b-c) transformed from nanospheres until aging for 3 weeks with the thickness of petals bigger than that of EuW10/TEPA. However, only nanospheres with mean diameter approximately 70 nm which has no connection with cultivate time for 2 mg mL⁻¹ EuW10/0.277 mg mL⁻¹ DETA can be observed (Figure 7d-f). In summary, the initial size of alkylamines/EuW10 nanospheres is gradually larger as the increasing of the number of amine groups. And the morphology of the final products was determined by the characteristic of the inherent chain type structure of alkylamines, which speculates the TEPA molecules maybe form interparticle bilayers leading EuW10 to mix together faster in the process of self-assembly and finally form regular EuW10/TEPA nanoflowers.31 This illustrated that the role of amine groups is essential, but the formation of nanoflowers can not generally achieve for all common structural alkylamines.

Figure 7. TEM image (a) of nanospheres with aging for 1 day and SEM image (b, c) of nanoflowers


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with aging for 3 weeks for the sample of 2 mg mL⁻¹ EuW10/0.393 mg mL⁻¹ TETA; TEM image (d) of nanospheres with aging for 1 day and SEM image (e, f) of nanospheres with aging for 3 weeks for the sample of 2 mg mL⁻¹ EuW10/0.277 mg mL⁻¹ DETA. Photocatalytic Behaviors for the Degradation of MB Dyes. POMs have been widely applied in the catalysis science and oxidate different materials such as alkenes, sulfides and dyes. Methylene blue (MB) belongs to the commonly used dyes in textile industry, although EuW10 with the presence of H2O2 can exhibit 99% MB degradation, on the one hand, EuW10 cannot be recycled and reused for it has great solubility in water; on the other hand, EuW10 itself has structure stability as well as can cause secondary pollution, disobeying the view of sustainability development.52 Thus, the photocatalytic behaviors of EuW10/TEPA nanoflowers for the degradation of MB dyes in the presence of H2O2 under visible-light irradiation were investigated. The photocatalytic experiments showed that EuW10/TEPA nanoflowers displayed 99 % (based on Equation 1) MB degradation efficiency after 140 min (Figure 8A), but the degradation time was cut by half of calcinated nanoflowers (Figure 8B), which convincingly demonstrates the necessity of the calcination process for the catalytic oxidation and this sample was chosen as further study sample. The linear fit of the ln (Ct/C0) data (Figure 8C) reveals that the catalytic reaction exhibits pseudo-first-order kinetics for the desulfurization of MB (R2 = 0.917) with the degradation rate constant k = 0.047 min-1 based on Equation 2 and Equation 3. − dCt /dt = kCt

(2)

ln(C0/Ct) = kt

(3)

The good photocatalytic properties on MB degradation drove us to study the degradation mechanism. We can speculate that hierarchical nanoflowers/H2O2 revealed photocatalytic active


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facets, the EuW10 plays a role of electron acceptor from MB accelerated by decreasing distance of molecules via hydrogen-bonding and electrostatic interaction. Besides, H2O2 interacted with active species, the W-O bonds, making H2O2 stay at the nanoflowers surface. Then, H2O2 was activated to

⋅OH though getting electron from LUMO of EuW10, and the ⋅OH can degrade dyes. However, 14.8% and 52.7% of MB was decreased with EuW10/TEPA nanoflowers and calcinated nanoflowers in dark treatment with the absence of H2O2, caused by adsorption action. The reason why calcinated nanoflowers possess better adsorption efficiency is that the EuW10/TEPA compounds after calcinatation remained metal-oxide nanostructures and carbon element. Above the discussed, our results indicated that the photocatalysis and the adsorption process concurrently work in our study. Recycling degradation experiments was also investigated to test the high photocatalytic activity for long-term use in practical applications. As shown in Figure 8D, the calcinated EuW10/TEPA nanoflowers still remained high photocatalytic activity after six recycle, and the reaction time of first three recycles is 90 min, and the last three recycle extended to 180 min mainly caused by the adsorption equilibrium of carbon element. The results implied that calcinated porous hybrid materials possessed excellent cycling photocatalyst property of MB dyes.

B Initial MB Dark treatment 0 min 30 min 60 min 90 min 150 min 180 min

1.2

0.8

Initial MB Dark treatment 0 min 30 min 60 min 90 min

1.2

Abs.(a.u.)

A

Abs.(a.u.)

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0.8

0.4

0.4

0.0

0.0 500

600

700

Wavelength (nm)

800

500


600

700

Wavelength (nm)

800

100

-1

80

-2

60

-3

40

-4 2

20

R = 0.917 -1 k = -0.047 min

-5

D

100

Dye degradation(%)

C

MB Removel(%)

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80 60 40 20

0 0

20

40

60

Reaction Time/min

80

-6 100

0 1

2

3

4

Cycle

5

6

Figure 8 The UV-vis curves of the degradation of MB by (A) hybrid nanoflowers/H2O2 and (B) calcinated nanoflower/H2O2. (C) MB removal of calcinated nanoflowers/H2O2 and ln(Ct/C0) as functions of reaction time. (D) The recycling experiment for the degradation of MB using calcinated nanoflowers/H2O2 system, the reaction time of 1-3 is 90 min and the 4-6 is 180 min. CONCLUSION In summary, we have demonstrated the spontaneous formation of 3D hierarchical nanoflowers formed by a cationic component (TEPA) and a Weakley-type POM (EuW10) through simple and versatile ISA strategy. Nanostructures with various sizes and morphologies were associated with the ratios of EuW10/TEPA, cultivation time or different molecular weights of alkylamines. Moreover, the as-prepared calcinated nanoflowers showed effective properties to catalysis degrade dyes which still remained high photocatalytic activity even after six recycles. The supramolecular chemical complementarities between the alkylamines and POM open a wide scope for the design of hybrid materials, and thus it is promising to be a new class of hybrid materials with accumulates synergistic functionalities. ASSOCIATED CONTENT


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: ××××××. SEM element mapping analyses of smooth nanospheres, TGA curves and fluorescence spectra of TEPA, XRD spectra, XPS spectra, TEM and SEM images of different alkylamines, adsorption−desorption isotherms, pore size distribution and specific surface area of the prepared samples (PDF) AUTHOR INFORMATION Corresponding Authors *

E-mail: [email protected]. Phone: +86-531-88363597. Fax: +86-531-88361008.

*

E-mail: [email protected]. Phone: +86-531-88364218. Fax: +86-531-88564750.

Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT We gratefully acknowledge the financial support from the National Natural Science Foundation of China (21201110, 21571115) and Young Scholars Program of Shandong University (2016WLJH20). REFERENCES (1) Huang, Y.; Ran, X.; Lin, Y.; Ren, J.; Qu, X. Self-assembly of an organic−inorganic hybrid nanoflower as an efficient biomimetic catalyst for self-activated tandem reactions. Chem. Commun. 2015, 51, 4386−4389.


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Hydroxide Composites as Efficient Oxidative Catalysts. Chem. Eur. J. 2015, 21, 10812–10820.


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Hierarchical Nanostructures Self-Assembled by Polyoxometalate and

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