Topotactic synthesis and photocatalytic performance of one-dimensional ZnNb₂O₆ nanostructures and one-dimensional ZnNb₂O₆/KNbO₃ hetero-nanostructures

Abstract

This paper introduces one-dimensional ZnNb₂O₆/KNbO₃ hetero-nanostructures and one-dimensional ZnNb₂O₆ nanostructures. These nanostructures are synthesized via in situ topotactic structural transformation reaction using the tunnel structure K₂Nb₂O₆ filiform crystal as precursor. Firstly, Zn²⁺ ions intercalate into K₂Nb₂O₆ crystal by exchanging K⁺ ions from the K₂Nb₂O₆ crystal with Zn²⁺ from Zn(NO₃)₂ or Zn(CH₃COO)₂ aqueous solution, to form two different Zn²⁺-exchanged samples, and then these Zn²⁺-exchanged samples topotacticly transform into one-dimensional ZnNb₂O₆/KNbO₃ hetero-nanostructures and ZnNb₂O₆ nanostructures during heat-treatment. The formation reaction and structure of these samples were characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and energy-dispersive spectroscopy (EDS). Photocatalytic experiments showed that one-dimensional ZnNb₂O₆/KNbO₃ hetero-nanostructures and ZnNb₂O₆ nanostructures have excellent photocatalytic performance for the degradation of methylene blue (MB), rhodamine B (RhB), and methyl orange (MO).

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Introduction

The soft chemical process is a useful and unique method for preparation and design of function inorganic materials.^1–3 This process typically comprises two steps: the first step is the preparation of a framework precursor with layered or tunnel structure and insertion of template ions or molecules into the interlayer space by ion-exchange reaction, and the second step is the transformation of the template-inserted precursor into a desired structure by the soft chemical method, such as solvothermal treatment, ion-exchange treatment and low temperature heat-treatment. The crystal structure of the product can be controlled by the used template, and the product particle morphology is dependent on the morphology of the used precursor. This method has been utilized for the synthesis and design of metal oxides and nanocomposites with controlled structure, morphology, and chemical composition.^4–6

Chemical composition, structure, morphology, and grain size of semiconductor materials as photocatalysts are some key issues that influence the photocatalytic capability.^7–9 Because photocatalytic reactions are typically surface-based processes and the photocatalytic efficiency is closely related to the morphology and micro-structure of the material. One-dimensional (1D) nanostructures and hetero-nanostructures such as wires, belts and tubes have attracted considerable attention for photocatalytic applications due to their distinct electronic, optical and chemical properties, which differ from their bulk counterparts.^10–12

Among all the semiconductor photocatalysts reported, some niobates showed certain photocatalytic performance, such as KNbO₃,¹³ NaNbO₃,¹⁴ K₄Nb₆O₁₇,¹⁵ CaNb₂O₆,¹⁶ pyrochlore type K₂Nb₂O₆,¹⁷ Sn₂Nb₂O₇,¹⁸ Nb⁴⁺ self-doped K₄Nb₆O₁₇,¹⁹ SnNb₂O₆–graphene nanocomposite²⁰ and so on. ZnNb₂O₆, with a columbite structure, has received a great deal of attention due to its microwave dielectric, photoluminescence and photocatalytic properties.^21,22 Most studies have found that ZnNb₂O₆ show photocatalytic activity for water splitting and degradation of diverse organic pollutants.^22–24 Currently, its main synthesis approaches includes traditional solid-state,²¹ sol–gel,²³ hydrothermal²⁴ and molten-salt method.²⁵ Until now, however, 1D nanostructures and hetero-nanostructures ZnNb₂O₆ have not been reported because these methods mentioned above are difficult to simultaneously control chemical composite, grain size, morphology and nanostructure.

In our previous study of controlling morphology and chemical composition, we used the layered structure titanate H_1.07T_1.73O₄·nH₂O particles with plate-like morphology as precursor to prepare some plate-like particles (BaTiO₃, Ba_0.9Ca_0.1TiO₃, Ba_0.5(Bi_0.5K_0.5)_0.5TiO₃) by the soft chemical process.^26,27 In the soft chemical process, the layered structure of HTO precursor is transformed into the perovskite structure by an in situ topotactic transition reaction, and the precursor morphology is remained after the reaction. In this paper, therefore, an in situ topotactic structural transformation synthesis of 1D nanostructures ZnNb₂O₆ and hetero-nanostructures ZnNb₂O₆/KNbO₃ is described in detail and their formation reaction mechanism is clarified. In addition, the photocatalytic activities of these materials are discussed.

Experimental section

Preparation of K₂Nb₂O₆ precursor

The starting material of the tunnel structure potassium niobate K₂Nb₂O₆·nH₂O fibrous crystal was prepared using the method described in literature.²⁸ 5 g of Nb₂O₅ and 30 mL of 3 mol L⁻¹-KOH water solution were placed in a Teflon-lined, sealed stainless-steel vessel with an inner volume of 85 mL, and then hydrothermally treated at 200 °C for 2 h under stirring conditions. After the hydrothermal treatment, a potassium hexaniobate clear solution was obtained. Potassium hexaniobate was precipitated out from this solution by adding the ethanol into the solution. The precipitate was filtered, washed with ethanol, and then dried at room temperature. 0.5 g of potassium hexaniobate and 30 mL of 0.5 mol L⁻¹-KOH water–ethanol (volume ratio = 1/1) solution were added into a Teflon-lined, sealed stainless-steel vessel with an inner volume of 85 mL, and then solvothermally treated at 230 °C for 12 h under stirring conditions. After the solvothermal treatment, the tunnel structure K₂Nb₂O₆·nH₂O fibrous crystal sample was filtered, washed with distilled water, and then dried at room temperature.

Preparation of ZnNb₂O₆ and ZnNb₂O₆/KNbO₃

0.5 g of K₂Nb₂O₆·nH₂O samples were respectively put into 200 mL of 0.5 mol L⁻¹-Zn(CH₃COO)₂ and 0.5 mol L⁻¹-Zn(NO₃)₂ aqueous solution, then stirred and ion-exchanged for 24 h at room temperature. The ion-exchange treatment was performed twice for the complete exchange. After ion-exchange, the products were filtered, washed with distilled water, and dried at room temperature. The Zn²⁺-exchanged samples were obtained. Finally, the Zn²⁺-exchanged samples were heat-treated at 570 °C for 2 h, to respectively obtain ZnNb₂O₆ and ZnNb₂O₆/KNbO₃ samples.

Physical analysis

The crystal structure of the sample was investigated using a powder X-ray diffractometer (Shimadzu, Model XRD-6100) with Cu Kα (λ = 0.15418 nm) radiation. The size and morphology of the particles were observed using scanning electron microscopy (SEM) (Hitachi, Model S-900). Transmission electron microscopy (TEM) observation and selected-area electron diffraction (SAED) were performed on a JEOL Model JEM-3010 system at 300 kV, and the powder sample was supported on a micro grid. Chemical composition analysis was carried out with energy dispersive spectroscopy (EDS) (JEOL Model JED-2300T) on the TEM system. Thermo gravimetric and differential thermal analyses (TG-DTA) were conducted on a DTG-60Hthermogravimetric analyzer (SHIMADZU). UV-vis absorption spectra were recorded on a UV/vis/NIR Spectrophotometer (LAMBDA950, PerkinElmer).

The photocatalytic performance of samples were evaluated by degradation of methylene blue (MB), rhodamine B (RhB), and methyl orange (MO) under xenon lamp irradiation. In each experiment, 50 mg of samples were added into the solution (50 mL, 10 mg L⁻¹). The suspensions were magnetically stirred in dark for 40 min to ensure the establishment of an adsorption–desorption equilibrium. Then, the solution was exposed to the lamp irradiation under magnetic stirring. At different irradiation time intervals, 6 mL of the solution was collected with centrifugation. The concentration of the remnant dye in the collected solution was monitored by UV-vis spectroscopy (Unico UV-2600) each 5 or 10 min.

Result and discussion

Zn²⁺-exchange of tunnel structure K₂Nb₂O₆

In the previous study,²⁸ we have reported a tunnel structure K₂Nb₂O₆, which is composed of NbO₆ double chains and KO₆ octahedron layers with the K⁺ occupying the tunnel sites, and a single crystal with filiform shape. Similar to the most metal oxides with tunnel structure, e.g. zeolites, tunnel structure manganese oxides,^29,30 layered niobates of KNb₃O₈ and K₄Nb₆O₁₇,^31,32 K⁺ ions in the tunnel structure K₂Nb₂O₆ can be exchanged with other cation by ion-exchange treatment.

Fig. 1 is the XRD patterns of samples obtained by the ion-exchange of K₂Nb₂O₆ with Zn²⁺ ion. The precursor K₂Nb₂O₆ possessed a characteristic peak of tunnel structure at 2θ = 10° nearby and the fine crystallinity (Fig. 1a). When the K₂Nb₂O₆ precursor was respectively treated by ion-exchange in 0.5 mol L⁻¹-Zn(NO₃)₂ and 0.5 mol L⁻¹-Zn(CH₃COO)₂ aqueous solution, the Zn²⁺-exchanged samples were formed (Fig. 1b and c). Although the crystallinity of Zn²⁺-exchanged samples decreased, they retained the tunnel structure because of existing the characteristic peak at 2θ = 10° nearby in their XRD patterns (Fig. 1b and c). The FE-SEM result reveals that the Zn²⁺-exchanged samples all retained the filiform morphology of precursor after the Zn²⁺-exchange process (Fig. 2).

Fig. 1 XRD patterns of (a) the tunnel structure K₂Nb₂O₆ and the Zn²⁺-exchanged samples obtained in (b) Zn(NO₃)₂ and (c) Zn(CH₃COO)₂ solution, respectively.

Fig. 2 FE-SEM images of (a) the tunnel structure K₂Nb₂O₆ niobate and the Zn²⁺-exchanged samples obtained in (b) Zn(NO₃)₂ and (c) Zn(CH₃COO)₂ solution, respectively.

The chemical composite analysis result revealed that the mole ratio of K, Zn and Nb was 0.4/0.6/1 in the Zn²⁺-exchanged sample obtained in Zn(CH₃COO)₂ aqueous solution. This indicated that K⁺ ions in the K₂Nb₂O₆ precursor cannot be completely exchanged with Zn²⁺ ions in Zn(CH₃COO)₂ aqueous solution. This is similar to the our previous report that K⁺ ions in the K₂Nb₂O₆ precursor cannot be completely exchanged by Li⁺ ions in LiCl aqueous solution.²⁸ In the Zn²⁺-exchanged sample obtained from Zn(NO₃)₂ aqueous solution, however, no K component is detected, and the mole ratio of Zn and Nb is 0.99/1. This reason is that the 0.5 mol L⁻¹-Zn(CH₃COO)₂ aqueous solution close to neutral (pH = 6.7 by actual measurement), whereas the Zn(NO₃)₂ aqueous solution is acidity (pH = 4.1 by actual measurement), meanwhile, the KO₆ octahedral layers in the tunnel structural framework is not steady and easily reacted with H⁺ in the acid environment,²⁸ resulting in the KO₆ octahedral layers dissolved out. These results suggested that the acid-base property in the ion-exchange system have an effect on the K⁺ exchange process of K₂Nb₂O₆. But Zn²⁺ ion can react with OH⁻, forming Zn(OH)₂ precipitate at alkaline condition, we did not systematically research the pH effect over Zn²⁺-exchange reaction system.

Structure transition reaction

The tunnel structure K₂Nb₂O₆ filiform crystal can topotacticly transform into the pure perovskite KNbO₃ (noted as KNbO₃-HT) filiform crystal via heat-treatment (Fig. 3a).²⁸ Therefore, we heat-treated the Zn²⁺-exchanged samples according to their TG-DTA curves (Fig. 4). Two types of Zn²⁺-exchanged samples obtained respectively in the Zn(CH₃COO)₂ and Zn(NO₃)₂ solution showed the different exothermic peaks without weight loss around 528 and 560 °C, which correspond to the phase transformations. Thus, the heat-treatments were processed at 570 °C.

Fig. 3 XRD patterns of (a) the KNbO₃ sample obtained by heat-treating the tunnel structure K₂Nb₂O₆ sample, the samples obtained by heat-treating the Zn²⁺-exchanged samples from (b) Zn(CH₃COO)₂ and (c) Zn(NO₃)₂ solution, and (d) the ZnNb₂O₆ sample obtained by solid-state method.

Fig. 4 TG-DTA curves of the Zn²⁺-exchanged samples from (a) Zn(CH₃COO)₂ and (b) Zn(NO₃)₂ solution, respectively.

Fig. 3 presented the XRD patterns of the heat-treated samples. It is found that the mixed phase of columbite ZnNb₂O₆ (JCPDS file no. 37-1371) and perovskite KNbO₃ (JCPDS file no. 32-0822) were formed after the heat-treatment of the Zn²⁺-exchanged sample obtained in Zn(CH₃COO)₂ solution (Fig. 3b). While the Zn²⁺-exchanged sample obtained in Zn(NO₃)₂ solution transformed into a pure columbite phase ZnNb₂O₆ (noted as ZnNb₂O₆-HT) (JCPDS file no. 37-1371) after the heat-treatment (Fig. 3c). The chemical composite analysis indicated that the mole ratios of K, Zn and Nb are the same in samples before and after the heat-treatment. Above results suggested that the Zn²⁺-exchanged niobate samples can transform into the columbite ZnNb₂O₆ by soft chemical heat-treatment process.

The FE-SEM images revealed that the mixed phase sample and the ZnNb₂O₆-HT sample still keep the filamentous morphology of precursor after the heat-treatment (Fig. 5). It can be clearly seen that the mixed phase filamentous particle was constructed from nanocube with a size of ∼20 nm (Fig. 5a), meaning a polycrystalline. And the ZnNb₂O₆-HT filamentous particle was also a polycrystalline which consists of the needle-like nanocrystals with a size of ∼10 nm (Fig. 5b). Nevertheless, the solid-state method can only synthesized ZnNb₂O₆ sample (ZnNb₂O₆-solid) with sphere morphology and a size of 500 nm (Fig. 5c). These results suggested that the filamentous morphology of products depends on the filiform morphology of the tunnel structure K₂Nb₂O₆ precursor. Therefore, the transformation reactions from tunnel structure K₂Nb₂O₆ to columbite structure ZnNb₂O₆ and perovskite structure KNbO₃ were topotactic structural transition reactions.

Fig. 5 FE-SEM images of the samples obtained by heat-treating the Zn²⁺-exchanged samples from (a) Zn(NO₃)₂ and (b) Zn(CH₃COO)₂ solution, and (c) the ZnNb₂O₆ sample obtained by solid-state method.

Nanostructure study

Similar to the SEM results, the TEM images also showed that the mixed phase sample and the ZnNb₂O₆-HT sample have filamentous shapes (Fig. 6). In HRTEM image of the mixed phase sample (Fig. 6b), it can be observed the lattice image of ZnNb₂O₆ structure with lattice spacing of d₍₀₀₂₎ = 0.368 and the lattice image of KNbO₃ structure with lattice spacing of d₍₁₁₀₎ = 0.286, in addition a grain boundary existing between the (002) plane of the ZnNb₂O₆ phase and the (110) plane of the KNbO₃ phase. It was demonstrated that the columbite ZnNb₂O₆ and the perovskite KNbO₃ phases coexisted in one filamentous particle. Therefore, it can be confirmed that the mixed phase polycrystalline fiber was a ZnNb₂O₆/KNbO₃ nanocomposite (noted as ZnNb₂O₆/KNbO₃-HT), meaning a 1-D hetero-nanostructures.

Fig. 6 TEM (a) and HRTEM image (b) of the samples obtained by heat-treating the Zn²⁺-exchanged samples from Zn(CH₃COO)₂ solution, and TEM image(c) and SAED pattern (d) of the samples obtained by heat-treating the Zn²⁺-exchanged samples from Zn(NO₃)₂ solution.

It was interesting that although the ZnNb₂O₆-HT sample was a polycrystalline fiber, it showed a SAED pattern similar to a ZnNb₂O₆ single crystal (Fig. 6c). In SAED pattern of ZnNb₂O₆-HT sample (Fig. 6d), the d-values of diffraction spots were 0.254 and 0.369 nm, which corresponded to the (440) and (002) planes of the ZnNb₂O₆ structure, respectively. This indicated that all the needle-like nanocrystals in each fiber were arranged in the same crystal-axis orientation, meaning perfectly oriented polycrystalline fiber, and the fiber-axis direction directs to the [110]-axis direction of the ZnNb₂O₆ structure. In our previous report,²⁸ the fiber axis direction of precursor was [010]-axis direction of the K₂Nb₂O₆ tunnel structure. It implied that the [010]-axis direction of K₂Nb₂O₆ tunnel structure transformed into the [110]-axis direction of ZnNb₂O₆ columbite structure and there was a corresponding relation between them. Therefore, the polycrystalline ZnNb₂O₆-HT fiber was a 1-D nanostructures with [110]-crystal-axis orientation.

The above results revealed that ZnNb₂O₆/KNbO₃ 1-D hetero-nanostructures and ZnNb₂O₆ 1-D nanostructures with crystal-axis orientation can be fabricated by the topotactic structural transformation of the tunnel structure K₂Nb₂O₆ fiber.

Formation reaction mechanism

A schematic representation of the formation mechanism of one-dimensional ZnNb₂O₆ nanostructures and ZnNb₂O₆/KNbO₃ hetero-nanostructures in the two-step soft chemical process was given in Fig. 7. When the ion-exchange treatment of tunnel structure K₂Nb₂O₆ filiform crystal was operated in near neutral Zn(CH₃COO)₂ solution, firstly Zn²⁺ ions intercalate into K₂Nb₂O₆ crystal by the exchange K⁺ in tunnel pathway of K₂Nb₂O₆ crystal with Zn²⁺ ions, and then these Zn²⁺ ions and KO₆ layer react with NbO₆ double chains to form the ZnNb₂O₆/KNbO₃ composite phase by an in situ topotactic structural transformation reaction during heat-treatment.^27,29 Nevertheless, in Zn(NO₃)₂ solution, not only K⁺ ions in tunnel but also in KO₆ layer of K₂Nb₂O₆ crystal were exchanged out by Zn²⁺ ions. It is well known that the Zn(NO₃)₂ aqueous solution presents acidity and the KO₆ layer of K₂Nb₂O₆ crystal is not steady in this acidic system and easily reacted with H⁺ ions and dissolved, leading that the Zn²⁺ ions filled among the NbO₆ double chains. In the heat-treatment process, the Zn²⁺ ions reacted with the NbO₆ double chains and formed the pure ZnNb₂O₆ phase by in situ topotactic structural transformation reaction. Since the topotactic structural transformation reaction occurs in ion-exchanged sample, ZnNb₂O₆/KNbO₃ hetero-nanostructures and ZnNb₂O₆ nanostructures, as shown in Fig. 5, were respectively formed and all remained the one-dimensional fibrous shape of precursor. The mechanism described above suggests that the acid–base property of the system during ion-exchange can determine the selective formation of 1-D ZnNb₂O₆ nanostructures and 1-D ZnNb₂O₆/KNbO₃ hetero-nanostructures.

Fig. 7 Formation mechanism of one-dimensional ZnNb₂O₆ nanostructures and ZnNb₂O₆/KNbO₃ hetero-nanostructures from K₂Nb₂O₆.

UV-vis absorption spectra analysis

Fig. 8a showed the UV-vis diffuse reflectance spectra of samples. It can be noticed that all of the samples exhibited absorption bands in the UV light region. The sphere ZnNb₂O₆-solid sample showed strong absorption in the UV-light regions (250−320 nm). The filiform ZnNb₂O₆-HT sample presented slight red-shift of absorption edge (270−350 nm) relative to ZnNb₂O₆-solid sample, and its color is the light orange, but ZnNb₂O₆-solid sample presents the white color (Fig. 8b and c). The reason is that the nanocrystal size of ZnNb₂O₆-HT sample is smaller than that of ZnNb₂O₆-solid sample (Fig. 5b and c) and the nanocrystal in ZnNb₂O₆-HT sample possess a higher internal stress of the nanocrystals relative to that of ZnNb₂O₆-solid sample, resulting in a red shift of the absorption edge.³³ Theoretically, although a generally received opinion is that the ubiquitous blue shift of the absorption edge of nanocrystals is the result of the quantum size effect owing to the downsized crystals. The absorption of KNbO₃-HT filamentous crystal sample was in the UV-light regions of 280–380 nm, but the filiform ZnNb₂O₆/KNbO₃-HT sample showed the step-like absorption curve within wavelength regions of 250–400 nm, which was related to its nanocomposite structure.³⁴ And its color was also the light orange (Fig. 8d). The differences in optical absorption of the different samples may be ascribed to their filiform shape and nanostructure.^7–9

Fig. 8 UV-vis diffuse reflectance spectra (a) and photos of (b) ZnNb₂O₆-solid sample, (c) ZnNb₂O₆-HT sample, and (d) ZnNb₂O₆/KNbO₃-HT sample.

Photocatalytic performance

Fig. 9 displayed the temporal evolution of the spectral changes during the photodegradation of RhB over ZnNb₂O₆-solid sample under UV-light illumination. It can be seen that the absorbance of RhB at the maximum absorption wavelength (551 nm) was gradually decreased with the prolongation of the irradiation time, which was because the chromophoric structure of the dye was destroyed. After 100 min, the major absorption band displayed blue-shift, indicating the de-ethylation of RhB molecules.³⁵

Fig. 9 Time-dependent UV-vis absorption spectra of the RhB solution in the presence of the ZnNb₂O₆-solid sample.

Fig. 10 showed the photocatalytic performance (C/C₀) versus UV-light irradiation time of samples for the degradation of methylene blue (MB), rhodamine B (RhB), and methyl orange (MO). The KNbO₃-HT sample and the ZnNb₂O₆-solid sample displayed the degradation efficiency of about 80% for MB after UV-light irradiation for 40 min. Nevertheless, the ZnNb₂O₆-HT sample took about 10 min to reach the degradation efficiency of about 88% for MB. It is surprise that the ZnNb₂O₆/KNbO₃-HT sample need only 5 min to reach to the degradation efficiency of about 90% for MB. It was found that the photocatalytic activities of both ZnNb₂O₆-HT and ZnNb₂O₆/KNbO₃-HT sample were superior to that of KNbO₃-HT and ZnNb₂O₆-solid samples for the degradation of MB. The similar results were also observed in the UV-light degradation process of RhB and MO (Fig. 9b and c). The reason is that the smaller the particle size, the higher the photocatalytic efficiency.^7,8 These small nanocrystals in ZnNb₂O₆-HT and ZnNb₂O₆/KNbO₃-HT samples are generally beneficial for surface-based photocatalysis. More importantly, the hetero-nanostructure of ZnNb₂O₆/KNbO₃-HT samples may be responsible for its highest photocatalytic activity because the synergetic effect on photocatalytic performance may occur in the grain boundary between ZnNb₂O₆ nanocrystal and KNbO₃ nanocrystal in 1D hetero-nanostructures.^9,36,37 Herein, we cannot give its band gap structures for discussing the detailed photocatalytic mechanism, because our experimental conditions are restricted.

Fig. 10 Photocatalytic degradation of (a) MB, (b) RhB, and (c) MO under UV-light irradiation using the KNbO₃-HT sample, the ZnNb₂O₆-solid sample, the ZnNb₂O₆-HT, and the ZnNb₂O₆/KNbO₃-HT sample.

Conclusion

The 1-D ZnNb₂O₆/KNbO₃ hetero-nanostructures and 1-D ZnNb₂O₆ nanostructures were successfully synthesized by in situ structural transformation reaction. Firstly Zn²⁺ ions intercalate into K₂Nb₂O₆ crystal by the exchange K⁺ ions in of K₂Nb₂O₆ crystal with Zn²⁺ in Zn(CH₃COO)₂ or Zn(NO₃)₂ aqueous solution, respectively forming two types of Zn²⁺-exchanged samples. And then these Zn²⁺-exchanged samples topotacticly transformed into 1-D ZnNb₂O₆/KNbO₃ hetero-nanostructures and 1-D ZnNb₂O₆ nanostructures during heat-treatment, respectively. These Zn²⁺ ions and KO₆ layer react with NbO₆ double chains to form the ZnNb₂O₆ and KNbO₃ phase by an in situ topotactic structural transformation reaction during heat-treatment. 1-D ZnNb₂O₆/KNbO₃ hetero-nanostructures and 1-D ZnNb₂O₆ nanostructures display excellent photocatalytic performance for the degradation MB, RhB and MO under ultraviolet irradiation. It is believed that the topotactic structural transformation process is an efficient method for designing and preparing other new types of nanostructures or hetero-nanostructures functional materials.

Acknowledgements

This work was financially supported by the Natural Science Basic Research Plan in Shaanxi Province of China (Program no. 2013JQ6012), the China Postdoctoral Science Foundation (Program no. 2013M542314), the research starting foundation from Shaanxi University of Science and Technology (BJ12-22), the Natural Science Foundation of China (no. 21173003& no. 51472153), and Innovation Team Assistance Foundation of Shaanxi Province (Grant no. 2013KCT-06).

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