Synthesis of Novel Flower-Like Zn(OH)F via a Microwave-Assisted Ionic Liquid Route and Transformation into Nanoporous ZnO by Heat Treatment
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Zinc hydroxide fluoride (Zn(OH)F) with novel flower-like morphology has been prepared via a microwave-assisted ionic liquid route. The flower-like Zn(OH)F particle has six petals and every petal is composed of lots of acicular nano-structure. Nanoporous ZnO is obtained by thermal decomposition of as-prepared Zn(OH)F in air, and the flower-like morphology is well retained. In the process of synthesis, ionic liquid 1-Butyl-3-methylimidazolium tetrafluoroborate is used as both the reactant and the template.
KeywordsIonic liquid Microwave Zn(OH)F Nanoporous ZnO Flower-like
The conventional inorganic synthetic procedures usually demand long reaction time, high temperature, and toxic solvents. Different from the traditional solvents, ionic liquids are potential green solvents with many advantages, such as negligible vapor pressure, low interface tension, supramolecular solvents, and microwave absorbing ability [1, 2]. Because of these excellent performances, ionic liquids can be the new “all in one” solvents, which are combination of solvent, template, and reactant . Compared with the traditional heating methods, microwave irradiation can obviously shorten the heating time. Moreover, microwave irradiation also has many other advantages, including volumetric heating, selectivity, fast kinetics, homogeneity, and energy saving [4–9]. Microwave-assisted ionic liquid reaction systems have been studied for the synthesis of inorganic materials, such as ZnO frameworks , high quality TiO2 nanocrystals , Bi2Se3 nanosheets , indium tin oxide nanocrystals , metal fluorides , cuboid-like crystallites , tellurium nanorods and nanowires , manganese oxide , CdF2 nanoflakes , and ZnO nanosheet aggregates .
Zn(OH)F has been demonstrated to be an important catalyst for the formation of pyridine from tetrahydrofurfuryl alcohol and ammonia, and it has also been used as the precursor for preparing ZnO [19, 20]. Huang et al.  have presented a simple hydrothermal route toward Zn(OH)F, which was then used as the precursor to prepare ZnO by calcination. As we know, ZnO has a wide range of applications in gas sensors, piezoelectric transducers, optical waveguides, acoustic–optical devices, catalysis, and solar cells, mainly due to its unique catalytic, electronical, and optoelectric properties [21–23]. Recently, porous ZnO with large specific surface area has generated considerable interest because of its potential applications in photocatalysis, environmental engineering to chemical, and gas sensors [24, 25]. Porous ZnO with various nanostructures have been reported, including hollow ZnO mesocrystals , porous ZnO nanoparticles , porous ZnO architectures , porous ZnO nanodisks , and porous ZnO nanowires .
Herein, for the first time, ionic liquid 1-Butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF4) is used as the reactant and the template to synthesize novel flower-like Zn(OH)F via an easy and fast microwave-assisted route. Nanoporous ZnO is obtained by thermal decomposition of the Zn(OH)F.
All reagents are of analytical grade and used without further purification. In a typical synthesis, 0.22 g (1 mmol) of Zn(CH3COO)2·2H2O powder was dissolved in 8 mL of distilled water under stirring at room temperature for 10 min, then 2 mL of [Bmim]BF4was added slowly while the stirring continued. Afterward, the mixture was transferred to a 30 mL Teflon-liner tube and kept inside a domestic microwave oven for 5 min (800 W, 40% of maximum power). The obtained white precipitate was washed with distilled water and pure ethanol and collected by centrifugation. The collected Zn(OH)F sample was dried at 60 °C in vacuum for 12 h. Then, the Zn(OH)F sample was calcined at 400 °C for 2 h in air atmosphere, to yield final product ZnO.
The obtained samples were characterized by X-ray diffraction (XRD) (Bruker D8 advance), field-emission scanning electron microscopy (FESEM) (LEO 1530), transmission electron microscopy (TEM) (JEOL JEM-2100), and nitrogen adsorption–desorption analysis (Micromeritics ASAP2010).
Figure 2d–f shows the SEM images of ZnO sample. The images reveal that the ZnO sample retains the flower-like morphology after the heat treatment at 400 °C. The size of the ZnO particles is similar to that of the precursor Zn(OH)F. The magnified image (Fig. 2f) indicates that ZnO sample possesses nanoporous structure.
The experiment results show that ionic liquid [Bmim]BF4 plays a critical role in the formation of flower-like structure. In the reaction, supramolecular effects and solvent self-structuration of ionic liquid are important when it reacts with high concentration of zinc acetate . The extended hydrogen-bonding and π–π stack interaction of the neighboring imidazolium rings make ionic liquids molecular recognition and self-assembly . As “supramolecular” solvent, the self-assembled ability of ionic liquid ([Bmim]BF4) has an important influence on the structural orientation in the reaction [31, 32]. Therefore, the flower-like morphology of Zn(OH)F is formed. The detailed formation mechanism needs to be further investigated.
ZnO is obtained after the calcination of Zn(OH)F at 400 °C, the chemical reaction takes place as follows: Zn(OH)F → ZnO + HF. During the thermal decomposition process, molecule-size pores generated when the HF molecules released from the flower-like particles. Meanwhile, small ZnO units formed during this process. With the temperature increasing, the small ZnO units assembled into nanoparticles and the molecule-size pores became nanopores. Because of the existence of nanopores, flower-like ZnO sample possesses a large BET surface area.
Novel flower-like Zn(OH)F has been successfully synthesized via microwave-assisted ionic liquid route. In addition, nanoporous ZnO is obtained after the heat treatment of Zn(OH)F, and the morphology is well retained. This method is fast and simple without using complex template. The ionic liquid [Bmim]BF4is used as the reactant and the template in the synthesis. It is expected that this method may be extended to the preparation of other inorganic materials.
This work was supported by Natural Science Foundation of Jiangsu Province (BK2006195), Doctor Innovation Funds of Jiangsu Province (BCXJ06-13), and National Natural Science Foundation of China (50502020, 50701024).