Shape-controlled synthesis of 3D copper nicotinate hollow microstructures and their catalytic properties

Abstract

A series of three-dimensional (3D) copper nicotinate hollow microstructures were fabricated by a one step method. The shape of the copper nicotinate hollow microstructures evolved from a hexagon, to an octagon and to a tetragonum, depending on the setting time and concentrations of the reactants. The 3D copper nicotinate hollow microstructures were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), electron paramagnetic resonance (EPR) and scanning electron microscopy (SEM). The mechanism of the growth of the copper nicotinate hollow microstructures has also been investigated, which indicated that 3D copper nicotinate hollow microstructures were assembled by ultrathin copper nicotinate nano-sheets. The catalytic activities of the as-obtained copper nicotinate hollow microstructures for the reduction of 4-nitrophenol by sodium borohydride were examined. The catalytic activity of copper nicotinate hollow microstructures was greatly improved due to the hollow structure.

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Introduction

Noble metal materials have attracted much attention in recent years due to their promising potential in biological, chemical sensing, energy conversion and storage, cancer therapy, sensing, etc., especially for catalysis.^1–3 The typical pseudo-homogeneous catalysis of noble nanoparticles has advantages of efficient activity and high selectivity under mild reaction conditions, and is of particular interest in reactions including water splitting, hydrogenation, and coupling reactions, etc.^4–7 However, the cost of noble metals is one of the key limitations for their practical application. One possible solution is to use transition-metal to replace the usage of noble metal. To improve the efficiency and save resources, there has been an increasing trend toward the use of transition-metal such as copper and its chemical compound.^8–10 For instance, Li et al. reported the controlled synthesis of copper telluride nanostructures for long-cycling anodes in lithium ion batteries.⁹ Li and Yamauchi reported the preparation of ternary PtPdCu spheres with a 3D nanoporous structure and used as superior electrocatalysts.¹¹ Pal and co-workers reported CuO anisotropic nanoparticles supported on mesoporous SBA-15 and use as catalyst displayed the highest catalytic activity for the reduction of m-chloronitrobenzene and m-nitrotoluene.¹² These heterogeneous catalysts have received much attention due to their notable increase in reactivity.^10,13

It is accepted that catalytic activity is strongly affected by particle size and shape, and consequently, the synthesis of catalyst which have well-controlled size and shape has become very important. For instance, Qiu and co-workers reported Cu₂O with octahedrons shape on boron nitride nanosheets, which exhibit high catalytic activity in the reduction of 4-nitrophenol (4-NP) into 4-aminophenol (4-AP).¹⁴ Otherwise, compared with the solid metal nanostructures, the hollow ones are quite distinguishable, because of their large specific surface area, unique optical properties, high reactivity and atom economy.^15–17 Hollow-structured materials have high specific surface areas, high capsulation capacity and good permeation, and show widespread potential applications in areas such as catalysis, photocatalysts, gas sensors, and electrode materials.^18,19 Zhang and Wang reported Cu₇S₄ hierarchical hollow cubic cages and used as catalyst, the investigation of photocatalytic performances proved superior activity due to hollow structure.²⁰ Thus, it is scientifically significant and unquestionably challenging to design metal structure materials with controlled shape and size. Although significant efforts have been made to develop effective strategies to control the shape,^14,21 size,²² a slight improvement has been achieved.

Herein, a series of 3D copper nicotinate hollow microstructures with different shapes were fabricated by one step method in mild condition. By changing the reaction times, molar ratio of reactants, copper nicotinate hollow microstructures are synthesized with morphology tunable from hexagon, to octagon and to tetragonum. The scheme of the growth of the copper nicotinate hollow microstructures was evaluated by controlling the reaction time. And the role of nicotinic acid and the growth mechanism of copper nicotinate hollow microstructures were discussed. The fabrication process of copper nicotinate hollow microstructures is illustrated in Scheme 1. The catalytic activities of the copper nicotinate were also examined. The copper nicotinate hollow microstructures exhibit excellent catalytic activity for the reduction of 4-nitrophenol using NaBH₄ in aqueous solution.

Scheme 1 View of the process of preparation and catalysis of copper nicotinate hollow microstructures.

Experimental section

Chemicals

All chemicals used were of analytical grade or of the high purity available. Nicotinic acid (C₆H₅NO₂, 99.5–100.5%) and cupric nitrate trihydrate (Cu(NO₃)₂·3H₂O, 99.0%) were obtained from Sinopharm Chemical Reagent. Sodium borohydride (NaBH₄, 98%) was obtained from Aladin. 4-Nitrophenol (4-NP) was supplied by Shanghai Chemical Corp. All glass ware was thoroughly cleaned with freshly prepared 3 : 1 HCl/HNO₃ (aqua regia) and rinsed thoroughly with Mill-Q (18.2 MΩ cm⁻¹ resistance) water prior to use.

Characterization

The morphology and size of the copper nicotinate hollow microstructures were characterized by TEM using a JEOLFETEM-2100 transmission electron microscope operated at an accelerating voltage of 200 kV. The XPS was recorded on an Axis Ultra DLD (SHIMADZU, Japan), and the C 1s line at 284.6 eV was used as the binding energy reference. SEM image was obtained by using a Hitachi Su-70 electron microscope. XRD pattern was carried out by using a Rigaku DMax-2600 PC diffraction meter using monochromatic Cu Kα radiation. In the experiment, a Bruker ESRA-300 spectrometer operating at 9.438 GHz (X band) 100 K was used to take electron paramagnetic resonance (EPR) data of the samples. Absorption spectra were recorded on a UV-vis spectroscopy was performed with a UV-2550 spectrophotometer (SHIMADZU, Japan) at room temperature.

Synthesis of copper nicotinate hollow microstructures

Nicotinic acid was used as both reducing and capping agents to prepare the copper nicotinate hollow microstructures through a facile one-pot aqueous approach. Briefly, a 30 mL aqueous solution of 15 mM Cu(NO₃)₂ was added into 100 mL round bottom flask and heated to boiling. Then, a 30 mL solution of nicotinic acid with a certain concentration (0.10 M, 0.15 M, 0.20 M, 0.30 M) was added and mixed with the solution of Cu(NO₃)₂ under magnetic stirring. The mixed solution was kept boiling, after the color of the solution changed from light blue to dark blue, the copper nicotinate hollow microstructures were prepared. Then, the sample was centrifuged for 10 min (at 7000 rpm) to remove the redundant nicotinic acid. The final products were washed three times with boiling deionized water and dried at 50 °C.

Catalytic reduction of 4-nitrophenol (4-NP)

For catalytic reduction of 4-NP, aqueous solutions of 4-NP (0.01 M, 0.03 mL) and freshly prepared aqueous NaBH₄ solution (0.5 M, 0.2 mL) were mixed with water (2.5 mL) in a quartz cuvette without stirring. Then the copper nicotinate hollow microstructures aqueous suspension (25 μL, 1 mg mL⁻¹) was injected without any stirring. The reaction was monitored by taking absorption spectra. To further test the reusability of the copper nicotinate as catalysts, the used copper nicotinate hollow microstructures were separated from the solution by pipetting the solution and adding the equal amount fresh reactant solution in next cycles. Similar to the above mentioned procedure would be repeated 20 times. In order to ensure the quantity of catalyst was enough in the process of recycles, the 2.5 mg of copper nicotinate was used.

Results and conclusion

The synthetic procedure in Scheme 1 demonstrates the synthesis and application of 3D copper nicotinate hollow microstructures. The shape of copper nicotinate hollow microstructures was readily controlled by modification of reaction time and molar ratio of reactants as well as the type of host materials. The different concentrations of nicotinic acid determined the final shape of the copper nicotinate hollow microstructures. Accompanied the addition of concentration of the nicotinic acid from 0.1 M to 0.3 M, the shapes of copper nicotinate hollow microstructures were turned from hexagon, to octagon, and to tetragonum gradually. The obtained copper nicotinate hollow microstructures can be used as catalyst in the reduction of 4-NP by NaBH₄ in water. With regard to this, the catalytic properties of the different shapes of copper nicotinate hollow microstructures were investigated in this study, where the overall reaction is presented in Scheme 1.

The XRD patterns of samples with different reactant were carried out to characterize the phase and purity of the as-synthesized final product (Fig. 1). As can be seen, the samples of copper nicotinate hollow microstructures exhibited relatively characteristic diffraction peaks at 12.8, 15.32, 16.76, 20.02, 24.64, 25.88, 29.18 and 37.72°, which was corresponded to the standard PDF card of copper nicotinate (JCPDS no. 21-1601). To investigate the process, the XRD of the samples with the reaction time from 1 min to 40 min were provided in the Fig. S1.† From the data, the copper nicotinate was generated after the reaction proceeded 1 min, and accompanied the process of reaction, the XRD data of samples were similar without obvious change.

Fig. 1 XRD pattern of copper nicotinate hollow microstructures with different concentrations of nicotinic acid: (a) 0.10 M, (b) 0.15 M, (c) 0.20 M, (d) 0.30 M.

The XPS analysis was carried out to determine the electronic state of copper in the samples. A full-scan XPS spectrum for the as-prepared copper nicotinate hollow microstructures is illustrated in Fig. 2a. Peaks corresponding to C, N, O and Cu can be clearly observed. As shown in Fig. 2b, the high-resolution XPS spectrum displays binding energies Cu 2p. Three dominant peaks are observed at 284.5, 399.15 and 530.9 eV, corresponding to C 1s, N 1s and O 1s, respectively. As shown in Fig. 2b, the high-resolution XPS spectrum displays binding energies Cu 2p. The Cu 2p XPS spectrum of copper nicotinate showed shakeup satellite peaks of the Cu 2p_3/2 at 942.4 and Cu 2p_1/2 at 962.6 eV, which confirmed the presence of Cu(II) species. The main Cu 2p XPS peaks were observed at 932.4 eV (Cu 2p_3/2) and 952.5 eV (Cu 2p_1/2) with a spin–orbit splitting of ∼20 eV.²³ Generally, the characteristic shakeup satellite is peculiar to the Cu(II) species, which relates to d⁹ configuration of Cu.²³ The nicotinic acid would be coordinated with Cu²⁺ by NH in process of synthesis. It is considered that the Cu possess unoccupied orbitals to act as electron acceptors.²⁴ The binding energy of N 1s is shown in Fig. 2c, which showed a shift of the N 1s peak to higher binding energy compared with 398.4 eV (N 1s), indicate that there does exist an electron transfer from the NH groups to the Cu²⁺.^25–27

Fig. 2 (a) XPS spectrum of copper nicotinate hollow microstructures (0.20 M nicotinic acid), (b) Cu 2p high-resolution XPS spectrum, (c) N 1s peak and (d) O 1s peak of the copper nicotinate hollow microstructures.

In order to investigate the oxidation state of Cu in copper nicotinate, the EPR of the sample has been performed and provided in the ESI (Fig. S2†). The EPR spectrum showed similar fingerprints in the main region of the EPR spectra at g ≈ 2.^28,29 The broad resonance signal at g = 2.094 is a characteristic of Cu²⁺ ion. The EPR results are in good agreement with the XRD and XPS analyses.

The SEM and TEM measurements were carried out to characterize the morphology and size distribution of the copper nicotinate hollow microstructures prepared at different conditions. As shown in the SEM images (Fig. 3), the copper nicotinate hollow microstructures were displayed different shapes with the different ratio of reactant. The prepared copper nicotinate hollow microstructures emerged hexagon, octagon, irregular octagon and tetragonum, when the reactant concentration of nicotinic acid was 0.10, 0.15, 0.20 and 0.30 M respectively. From the Fig. 3a, the hexagon shape of copper nicotinate hollow microstructures were dispersed uniformly with the average size of the length of a side 6 μm, and a rectangular pyramid shape of hollow hole in it with the bottom margin about 3 μm, which can be observed clearly in the TEM image (inset of the Fig. 3a). The thickness of the hexagon shape of copper nicotinate hollow microstructures was about 1.5 μm. Accompanied by the addition of nicotinic acid, the shape of copper nicotinate hollow microstructures was changed. The irregular hexagon was changed to octagon, when the concentration of nicotinic acid was increased to 0.15 M, and the average size of the octagon was about 6 μm in diameter, and thickness was also about 1.5 μm, which was similar to the hexagon shape. From the TEM (inset of the Fig. 3b) of the octagon, the hollow structure shrank obviously, the diameter of the hole was about 2 μm. Some irregular thin section around the marginal of the octagon can be observed, which proved the process of growth of the octagon. When the concentration of nicotinic acid was increased to 0.2 M, the four sides of octagon have grown and the other sides have turned to lessen, the octagon shape has turned to the tetragonum which was cut off the four corners. The thickness of the structure was increased to about 2 μm, and the middle hollow hole was still remained rectangular pyramid shape, just the thickness of the hole has been increased, which can also be detected in the TEM (inset of Fig. 3c). With the increase of the nicotinic acid, the growth of the copper nicotinate hollow microstructures was continued. The copper nicotinate hollow microstructures turned to the tetragonum with the average diameter at 5–6 μm, thickness at 2–3 μm. The all kinds of shapes of copper nicotinate hollow microstructures were uniform distribution. The shape of the hollow part looked like a handstand tetragonal pyramid, the hollow space was seemed like a funnel. When the concentration of nicotinic acid was up to 0.3 M, the hollow hole of 3D structure shrank almost with the growth of the microstructure (Fig. 3d and TEM inset of Fig. 3d). The shape of copper nicotinate was turned to cube with quadrilateral sides in average diameter of 7–9 μm. Therefore, we can deduce that the shape of copper nicotinate hollow microstructures could be well controlled by tuning the ratio of reactant in the reaction system.

Fig. 3 SEM images of the copper nicotinate hollow microstructures prepared at different concentrations of nicotinic acid: (a) 0.10 M, (b) 0.15 M, (c) 0.20 M, (d) 0.30 M (inset is the TEM image, scale bar, 1 μm).

The EDX microanalysis of the copper nicotinate hollow microstructures confirmed the presence of peaks characteristic of pure Cu (Fig. S3†). The presence of small amounts of carbon, nitrogen and oxygen can be ascribed to the nicotinic acid. Another part of Si peak was due to the substrate used to perform the measurement, which is unavoidable.

In order to understand the shape evolution of the copper nicotinate hollow microstructures, the identical reactions are conducted at a concentration of the nicotinic acid with different reaction time. The concentration of nicotinic acid was chosen at 0.2 M. The SEM images of copper nicotinate hollow microstructures with different reactant time are shown in Fig. 4. From Fig. 4a, a faster reaction rate was observed, when reaction time is 1 min, a half-baked quadrilateral structure with a hollow ring composed of ultrathin nano-sheets of copper nicotinate can be observed, the average length of side of the structure was about 5 μm, and the thickness was about 1 μm. The basic shape of the product formed in the first 1 min. As the process of reaction, accompanying the nano-sheets of copper nicotinate hollow microstructures assembled constantly, the structure of copper nicotinate hollow microstructures looked like tetragonum, and the hollow hole shrank continuously. The copper nicotinate ultrathin nano-sheets pasted on the surface of the quadrilateral unit can be observed in all stages of the growth (Fig. 4b–e). As the reaction time reached 15 min, a crowd of tetragonum which was cut off the four corners are formed (Fig. 4f). The middle hollow ring has been reduced to a concave-tetragonal pyramid morphology, which can be detected in the magnified image (inset of Fig. 4f). When reaction time reached 40 min, copper nicotinate hollow microstructures were still growing, the edge and middle areas of microstructures were developed by the growth of the slices (Fig. 4h). The length of side of the microstructure has run up to 6 μm. Further, when the reaction time was up to 60 min, uniform copper nicotinate hollow microstructures were obtained. When reaction time extended to 70 min, the microstructure has been turned to concave shape. And accompanying the process of the growth, the middle hollow hole was narrowed almost to vanishing point (inset of Fig. 4i). The investigation of growth mechanism of copper nicotinate hollow microstructures process of growth with reaction time indicated that the reaction of preparation was very fast, and the basic shape can be confirmed by the concentration of nicotinic acid. On the basis of the above experimental facts, the shape of copper nicotinate hollow microstructures was assembled with thin slices and readily controlled by changing the concentration of nicotinic acid. The formation was governed by an assemble-selective epitaxial growth synergistic process.

Fig. 4 SEM images of the process of growth of copper nicotinate hollow microstructures (0.2 M nicotinic acid) at different time: (a) 1 min, (b) 3 min, (c) 5 min, (d) 8 min, (e) 10 min, (f) 15 min, (g) 20 min, (h) 40 min, (i) 70 min (inset is magnified images, scalebar: 1 μm).

To examine the scheme of growth of copper nicotinate hollow microstructures further, such as the growth of other shapes, the shape evolution copper nicotinate hollow microstructures prepared other have been performed in the same conditions. Fig. S4 in the ESI† gives a SEM image of the comparison of the four kinds of 3D copper nicotinate hollow microstructures with reaction time at 30 min. Compared with the Fig. 3, the terminal shapes of all samples have been formed, the results showed that the concentrations of nicotinic acid were crucial factor in the shape evolution. It if interesting that in the growth of the copper nicotinate hollow microstructures at 0.3 M of the nicotinic acid, the star shape emerged on the surface of the structure of tetragonum (Fig. S4d†). So, during the reduction and self-assemble of copper nicotinate nano-sheets, the nicotinic acid played an important role. We can synthesize the different shapes of the 3D copper nicotinate hollow microstructures by regulating the amount of reactant.

Property of catalysis

Catalytic applications have been envisaged to evaluate the activity of the copper nicotinate hollow microstructures as catalyst. Generally, the direct reduction of 4-NP over noble metal particles is considered as a green process for the production of 4-AP.^30,31 Due to their high cost and scarcity of the noble metal, the development of alternative catalysts for the conversion of 4-NP to 4-AP has been actively pursued, including noble-metal-free catalysts.³² Here, the catalytic activity of copper nicotinate hollow microstructures was assessed by the reduction of 4-NP in aqueous solution as a model reaction. The advantage of this system is that the whole reaction can be monitored spectroscopically, consequently, the catalytic effects of different metals can be readily compared.³³

To evaluate the catalytic effect of copper nicotinate hollow microstructures prepared at different conditions, the identical catalytic reductions of 4-NP were performed at same condition. As usual, the light yellow aqueous 4-NP solution shows a maximum absorption at 317 nm. The addition of NaBH₄ deprotonates the OH group of 4-NP, the absorption peak shifts to 400 nm immediately (Fig. S5†), which is due to the formation of 4-nitrophenolate ion. No change in the absorption was determined even after standing for 10 h, indicating that there reduction does not proceed without catalyst. After addition of a small amount (25 μL of copper nicotinate hollow microstructures solution with 1 mg dispersed in 1 mL) of the copper nicotinate hollow microstructures, the color of the 4-nitrophenolate ions diminished after 150–330 s without stirring or ultrasonic treatment (Fig. 5). The catalytic activity of the copper nicotinate hollow microstructures prepared at different conditions was investigated respectively. In Fig. 5a–d, the characteristic absorption peak of 4-nitrophenolate ion at 400 nm significantly decreased, while a new absorption peak centered at 300 nm and gradually increases, revealing there reduction of 4-NP to form 4-AP. Moreover, two isosbestic points are observed at 280 and 314 nm, indicating the clean conversion without producing any byproducts.³⁰ According to the report by Qiu et al.,¹⁴ the mechanism of the reduction of 4-NP with NaBH₄ catalyzed by copper compounds is as follows: borohydride ions react with the copper compounds and transfer a surface hydrogen species and electrons to them, thereby resulting in the efficient reduction of the –NO₂ group of p-nitrophenol to the –NH₂ group. The catalytic performance of the copper nicotinate hollow microstructure was quantitatively evaluated in the liquid-phase reduction of 4-NP by NaBH₄. In order to estimate the efficiency of the catalyst, the predetermined calibration curve has been confirmed in Fig. S6.† The reduction kinetics could be followed by UV-vis absorption spectroscopy of the reaction mixture after the addition of the catalyst. The reduction of adsorbed 4-NP to 4-AP is the rate-determining step. Considering the reductant concentration was much higher than that of 4-NP, there action should be of pseudo-first order with regard to the reactant,³⁴ and a good linear relationship between ln(C_t/C₀) and reaction time can be obtained (where C_t and C₀ were the concentrations of 4-NP at time t and 0, respectively). Fig. 5 showed the linear relationships between ln(C_t/C₀) and reaction time, which converted from the peak of absorbance at 400 nm according to the predetermined calibration curve (Fig. S6†). The rate constant (k) of the catalytic reaction was determined from the slope of the linear plot. According to the linear plot (Fig. 5a–d), the reaction rate constant k was determined to be 1.344 × 10⁻² s⁻¹, 1.332 × 10⁻² s⁻¹, 2.999 × 10⁻² s⁻¹ and 1.848 × 10⁻² s⁻¹, respectively. The highest catalytic sample was the copper nicotinate hollow microstructures prepared at 0.2 M. Compared with the similar catalysis condition (Table S1†), the copper nicotinate hollow microstructures showed a higher activity than some noble metal nanocatalysts.^35–38 Remarkably, the k value is also higher than those of the noble metal nanocatalysts nanoparticles (Table S1†). Therefore, the catalytic performance may be ascribed to the hollow structures. More chemicals (ions) can be stored in the hollow structures of the sample with a greater chance of participating in the reactions, thus greatly improving the catalytic efficiency of copper nicotinate hollow microstructures. The copper nicotinate hollow microstructures exhibited good catalytic activity.

Fig. 5 UV-vis absorption spectra of reduction of 4-NP by NaBH₄ under the catalysis of copper nicotinate hollow microstructures reaction of 60 min with various concentrations nicotinic acid: (a) 0.10 M, (b) 0.15 M, (c) 0.20 M, (d) 0.30 M (insets: the corresponding ln(C_(t)/C₍₀₎) versus reaction time for reduction of 4-NP).

In order to evaluate the optimized condition in the process of growth, the catalytic properties of different stages of growth of copper nicotinate hollow microstructures (prepared at 0.2 M nicotinic acid) were also studied, and corresponding results were provided in Fig. S7.† From the monitored UV-vis absorption of all samples, the samples of copper nicotinate hollow microstructures with different stages of growth exhibited great catalytic activity, the optimized catalyst was the sample with reaction time at 60 min in comparison. The results indicated that copper nicotinate hollow microstructure was a good catalyst with high efficiency, and the optimized condition was prepared at concentration of nicotinic acid 0.2 M and reaction time at 60 min.

Because the reusability of the catalyst was an important issue for practical applications, the reusability of copper nicotinate hollow microstructures was tested in detail (Fig. 6). To further test the reusability of the catalysts, 20 successive cycles of catalytic reduction were carried out with the copper nicotinate hollow microstructures. From the absorbance spectra monitored using UV-vis spectroscopy (Fig. S8 and S9†), the 20 successive cycles of the reaction were completed by the same copper nicotinate hollow microstructures as catalyst. The corresponding reduction kinetics (inset of Fig. S8 and S9†) was monitored, and the reaction rate constant k was determined respectively. Compared with others, the copper nicotinate hollow microstructure prepared at 0.2 M of nicotinic acid was the most stable and highly active catalyst in recycles. In the first five cycles, reduction of 100% 4-NP was completed within 3 min and the first 15 cycles completed within 12 min (Fig. S8 and S9a–e†). From the sixteenth to the twentieth turns, more than 99% 4-NP reacted within 35 min (Fig. S9f–j†). From Fig. S10,† the other catalysts can be recycled 15 cycles with reduction of 100%. Remarkably, the k value was higher than those of catalyst prepared at other concentrations. The catalysts can be successfully recycled and reused for at least 15 successive cycles of reaction with a stable conversion efficiency of around 100%.

Fig. 6 (a) Conversion (%) of 4-NP with time by copper nicotinate hollow microstructures as catalyst (0.2 M nicotinic acid), (b) the reusability of copper nicotinate hollow microstructures as a catalyst for the reduction of 4-NP with NaBH₄.

Conclusions

In summary, we have demonstrated a size and shape control method of simply coordinating the amount of nicotinic acid into the reacting solution to obtain 3D copper nicotinate hollow microstructures from hexagon, to octagon and to tetragonum. The 3D copper nicotinate hollow microstructures can be used as highly active catalyst exhibited an excellent activity for catalytic reduction of 4-NP by NaBH₄. The preparation of copper nicotinate hollow microstructures was economical compared to noble metals materials such as Au, Ag and Pd.

Acknowledgements

The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (21205024), the National Science Foundation for Post-doctoral Scientists of China (2012M520659, 2013T60307), the National Hi-Technology Research and Development Program (863 Program) (No. 2013AA032204), the Natural Science Foundation of Heilongjiang Province (B201305), the project of Harbin Science and Technology bureau (2014RFQXJ151).

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Footnote

† Electronic supplementary information (ESI) available: SEM images. See DOI: 10.1039/c5ra25556b

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