Gelatin-assisted hydrothermal synthesis of single crystalline zinc oxide nanostars and their photocatalytic properties
Graphical abstract
Introduction
As one of the semiconducting materials, zinc oxide (ZnO) is widely used for its non-toxicity and low-cost. ZnO has a wide 3.37 eV bandgap and a large excitation binding energy of 60 meV, which shows distinguished optic, electrical, and chemical properties. Thus, it has wide applications in sensors [1], dye-sensitized solar cells [2], [3], catalysts [4], [5], [6], optical [7], [8], and piezoelectric [9] devices. However, the size, shape, and orientation of the ZnO nanocrystals can affect their above-mentioned applications [1], [4].
A lot of approaches have been developed for control synthesis of ZnO hierarchical structures, including thermal evaporation [9], hydrothermal synthesis [5], and chemical vapor deposition [10]. And many morphologies have been prepared, such as flowers [11], wires [1], [6], [12], hollow spheres [13], [14], belts [1], sheets [15], tubes [2], and dendritic arrays [16]. The hydrothermal method is a more facile, cost-efficient, and large scale one among the above methods. Its main advantage is that the final product owns a small size distribution, as well as high purity and crystallinity, without further treatment at high heat temperature [5].
Recently, much interest has been attracted in the synthesis of inorganic materials, with the help of biominerals such as nacre-like, echinoderms, and egg-shells in natural [17], [18], [19], [20]. The inspired biominerals-assisted synthesis demands the addition of surfactants or polymers. These additions are always served as structure-directing agents, which will make the nucleation and alignment of the crystals be stringently controlled and then obtain desired structures [20], [21]. Using this biomimetic method, many ZnO nanostructures have been prepared recently. For example, Yang et al fabricated ZnO quantum dots directed by poly(acrylic acid) brushes [22], [23]. In another example, ZnO twinned platelets of hexagonal-shaped crystals can be obtained in the presence of gelatin [24].
In these biominerals, gelatin is the denaturation of collagen, which is the major structural protein connecting the tissue of animal skin and bone [24]. It has a single chain with series of repeating amino acids, along with a good many of carboxylate groups [20], [24]. These polar groups can coordinate with metal ions, and thereby, the dissolved gelatin can act as a structure-directing agent for control synthesis of ZnO superstructures. Tseng and his coworkers constructed well-defined hexagonal twin plates of ZnO using gelatin as a structure-directing agent [18]. Similarly, Bauermann group fabricated a ZnO hexagonal plate with the help of gelatin [24]. In our study, we also choose gelatin as a structure-directing agent, but nanorods-assembled ZnO stars are obtained, instead of ZnO hexagonal plates in the literature [18], [24]. Thus, prepared products show high photocatalytic ability in the photodegradation of methyl orange (MO) under UV irradiation.
Section snippets
Chemical and reagents
Zinc acetate dihydrate (Zn(AC)2⋅2H2O), gelatin, methyl orange, and ammonia solution (28%, w/V) were purchased from Aladdin Industrial Corporation (Shanghai, China). All the reagents were analytical grade and used without further purification. All aqueous solutions were prepared with twice-distilled water.
Preparation of ZnO nanostars
In a typical procedure, 0.036 g gelatin was put into 20 mL of Zn(AC)2⋅2H2O (0.1 M) under stirring, followed by the addition of 1 mL ammonia. The mixture was vigorously stirred to form a homogeneous
Structural characterization
Scanning electron microscopy (SEM) measurements were performed to investigate the morphology and microstructure of the typical ZnO product. The sample contains a large quantity of well-dispersed star-like nanostructures (Fig. 1A). Furthermore, high magnification SEM images also tell us that the nanostars are assembled by a few of short ZnO nanorods. The corresponding nanorods radiate from center to edge. The rods are about 1–2 μm in length, with a diameter ranging from 100 to 300 nm (Fig. 1B).
Conclusions
In summary, ZnO nanostars have been synthesized with the assistance of gelatin under hydrothermal conditions. Gelatin not only serves as a soft biotemplate to confine the growth of ZnO nanorods, but also acts as a directing agent of assembly to transform nanorods into star-like nanostructures. It is found that the amount of gelatin and ammonia and the reaction temperature are the key factors during the preparation process. The associated growth mechanism of the ZnO nanostars is discussed in
Acknowledgments
This work has been supported by the NSFC (Nos. 20905021, 21175118, and 21275130).
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