The synthesis of novel ZnO microcrystals through a simple solvothermal method and their optical properties
Graphical abstract
ZnO micronut have been synthesized by a solvothermal method that is facile, low-cost and environmentally friendly. There is no need of any surface surfactant, and Zn(NO3)2·6H2O is the only precursor and absolute ethanol the solvent. Futhermore, this work will not only inspire rational preparation and assembly of other novel single crystalline nanostructures but also open up new opportunities for the development of environment-benign for prepare methods.
Research highlights
▶ In this paper, ZnO microcrystals have been synthesized in high selectivity by a solvothermal method. The ZnO micronuts (length ∼8 μm and width ∼5 μm) are uniform in morphology, displaying an opening gap on the surface that divides the body into two. ▶ The solvothermal method used in this paper is facile, low-cost, environmentally friendly, and Zn(NO3)2·6H2O is non-toxic and relatively cheap. There is no need of using any surface surfactant, and Zn(NO3)2·6H2O is the only precursor and absolute ethanol the solvent. The alcohol system is biosafe and environmentally benign. ▶ We find that the ZnO microcrystals are similar in excitonic absorption edge and PL properties. ▶ This work will not only inspire rational preparation and assembly of other novel single crystalline nanostructures but also open up new opportunities for the development of environment-benign for prepare methods. It is envisaged that the approach can be used to synthesize nanostructures of other oxides such as MgO and FeO.
Introduction
With the ever decreasing dimensions of devices used in catalysis, optical storage, light emitting display, energy conversion and storage, chemical and biological sensing and diagnosis, the growth of micro- and nano-structure materials in a controlled manner is getting more and more important [1]. In the academic sector, the systems are studied because the properties of nanomaterials can be significantly different from those of the bulk counterparts (a result of quantum-sized effects). As for the synthesis of nanomaterials, it is meaningful to adopt methods that are simple, efficient, and environment-friendly. From a commercial view point, low cost, high throughput, and ease of production (e.g., template-free) are favored factors. In our previous work, we prepared single-crystalline CdO and Cd(OH)2 nanowires by a simple hydrothermal method using aqueous Cd(NO3)2 as the only precursor [2].
In the past decade, zinc oxide (ZnO) has been studied widely. The material exhibits wide band gap (3.37 eV), large exciton binding energy (60 meV), large electron mass (0.3 me; me denotes bare electron mass) and excellent chemical and thermal stability. With such properties, ZnO is used in areas such as optical absorption and emission [3], piezoelectricity [4], photocatalysis [5] and gas sensing [6]. Researches on the generation of nano- and micro-structure ZnO of various shapes have been conducted [7]. The popular routes for the fabrication of nano- and micro-structure ZnO are thermal evaporation [8], chemical vapor deposition (CVD) [9], metal–organic chemical vapor deposition (MOCVD) [10], pulsed laser deposition (PLD) [11] and template-based growth [12]. Looking into these approaches, one can realize that stringent requirements, e.g. vacuum techniques, high temperatures, complicated controlling processes and the use of catalysts and noxious compounds are often involved. In other words, the methods are not suitable for large-scale production in low cost.
Recently, solution-based solvothermal and hydrothermal methods were adopted for the synthesis of ZnO nano- and micro-materials. Overall, the approaches appear to be facile and promising. For example, Zeng et al. [13] generated nutlike ZnO microcrystals in large quantity through a low-temperature hydrothermal route that involved pH adjustment of the aqueous solution of zinc acetate dihydrate and triethanolamine (TEA). Through a surfactant-directed process using zinc nitrate, sodium dodecyl benzene sulfonate (SDBS) and sodium hydroxide as ingredients, Zhang et al. [14] produced hierarchical nanostructure of ZnO. Nonetheless, most of the reported solvothermal and hydrothermal methods for preparing ZnO involved the use of various reagents in specific amounts and the procedures required close monitoring.
Herein, we report a simple solvothermal method for the fabrication of ZnO microcrystals. In this approach, Zn(NO3)2·6H2O is the precursor and absolute ethanol is the solvent, and there is no need to use a surfactant. By varying the reaction time, temperature and concentration of zinc nitrate, the growth of ZnO microcrystals can be selectively controlled. To the best of our knowledge, the fabrication of ZnO microcrystals in such a manner has never been reported before.
Section snippets
Experimental
All the reagents were of analytical grade (purchased from NanJing Chemical Industrial Co.) and used without further purification. In a typical procedure [15], various amount of Zn(NO3)2·6H2O were dissolved in absolute ethanol to form 40.0 mL solutions of different zinc concentrations. The solvothermal synthesis was conducted in an electric oven at 150 °C for a designated period of time. The optimal conditions for the generation of ZnO microcrystals are depicted in Table 1. After cooling naturally
Results and discussion
Fig. 1 shows the XRD patterns of the materials fabricated under different conditions. Shown in Fig. 1a are the patterns of samples collected after different synthesis times with the amount of Zn(NO3)2·6H2O fixed at 0.1 M (40 mL) and temperature at 150 °C. The sample of 0.5 h shows poor crystallinity. When the time reaches 1 h, Zn(OH)2, ZnO and a kind of zinc compound in the form of Zn5(OH)8(NO3)2(H2O)2 (JCPDS no. 720627) can be detected. For the samples collected after 3, 24 and 48 h, the XRD
Conclusion
By means of the described solvothermal approach that is environment-friendly, facile and low-cost, ZnO microcrystals can be synthesized from Zn(NO3)2·6H2O. By adjusting parameters such as reaction time, temperature and reactant concentration, ZnO microcrystals that are different in shape can be fabricated in high selectivity. They can be spherical or in the form of flowers, nuts, hexagon-pillars, popcorns and brushes. The FE-SEM results show that the nutlike ZnO microcrystals are composed of
Acknowledgments
We would like to thank the Foundation of National Laboratory of Solid State Microstructures, Nanjing University (grant no. 2010ZZ18), and the National Key Project for Basic Research (grant nos. 2011CB922102 and 2010CB923402), People's Republic of China, for financial support.
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