Synthesis of dumbbell-like ZnO microcrystals via a simple solution route
- Zhenqing Hou†1,
- Yixiao Wang†1,
- Lihua Shen1,
- Hao Guo1,
- Gongxin Wang1,
- Yang Li1,
- Shuifan Zhou1,
- Qiqing Zhang2Email author and
- Qian Jiang3Email author
© Hou et al.; licensee Springer. 2012
Received: 24 July 2012
Accepted: 22 August 2012
Published: 11 September 2012
Uniform dumbbell-like ZnO microcrystals had been successfully fabricated on a large scale via a facile solution technique under mild conditions. Obtained ZnO, with length of 1.2 to 1.6 μm and diameters of 350 to 600 nm, exhibited well-defined dumbbell-like morphology and hexagonal wurtzite structure and grew along the  direction. Effects of the reactant concentration on the sizes and morphologies of the ZnO products had been investigated, indicating that the reactant concentration played a crucial role in determining final sizes and shapes of the samples. In addition, the growth process of the dumbbell-like ZnO microcrystals was studied, and a possible formation mechanism was proposed. Furthermore, the optical properties of ZnO samples obtained at various reaction times were also investigated by photoluminescence (PL) spectroscopy. The PL spectra of the as-prepared dumbbell-like ZnO microcrystals showed a strong UV emission peak.
As a wide bandgap (3.37 eV) and large exciton binding energy (60 meV) at room temperature, ZnO is recognized as one of the most important photonic materials for applications in electronic devices such as piezoelectric transducers, blue light-emitting diodes[2–4], solar cells[5–7], and gas sensors[8, 9]. Substantial efforts have been employed in controlling the morphology, size, and dimension of ZnO crystals because these parameters represent key elements that largely determine the electronic and optical performances of ZnO. In particular, the synthesis of one-dimensional ZnO nano- or microstructures in shapes including rods, tubes, needles, dumbbells, and wires has attracted immense attentions from the research communities due to their potential applications in the optoelectronic devices and functional materials. Different techniques, such as the reaction of zinc salts with base[10–12], the wet chemical bath deposition[13–15], solvothermal, chemical vapor deposition, template methods[18–21], hydrothermal[22–26], etc., have been exploited to prepare one-dimensional ZnO nano- or microstructure materials with various morphologies. Among these methods, vapor-phase processes are expensive and energy-consuming, which are not suitable for large scale production. The solution-based methods, however, demonstrate obvious advantages of low-cost, low-temperature operation to prepare large-scale and well-crystallized ZnO materials.
Recently, Riley and co-workers had synthesized ZnO one-dimensional nanostructures on Si wafers coated with a thin film of ZnO via a hydrothermal method. However, the synthesis process was comparatively complicated, which was not appropriate for large-scale production. Hu and co-workers had reported the synthesis of ZnO nanowires and nanobelts on a large scale using ZnCl2 as zinc source, Na2CO3 as mineralizer, and sodium dodecyl sulfonate as morphology controller agent via a low-temperature one-pot hydrothermal technique. However, the reaction time was too long. Zhang et al. had prepared dumbbell-like ZnO microcrystals of hexagonal phase using poly (vinyl alcohol) as the capping molecules at 200°C for 12 h. However, the morphologies of the ZnO crystals were controlled by additives and still required a high temperature. Although progresses have been made in the synthesis of one-dimensional ZnO, a simple and fast approach had remained a great challenge.
In this paper, a conventional technique was utilized to simply prepare one-dimensional ZnO. The highly uniformed dumbbell-like ZnO could be easily fabricated via a simple low-temperature solution route. This synthetic approach also allowed further reducing the growth temperature to 95°C and shortening the reaction time to 3 h, leading to the development of an effective and low-cost fabrication process for high-quality ZnO. Moreover, we reported the low-temperature synthesis of dumbbell-like ZnO, which synthesized at ambient pressure without any substrates or additives. The morphology, structure, and properties of the obtained ZnO were examined; the effects of the reactant concentration and reaction time on the size and shapes of the ZnO products were analyzed, and the possible growth mechanism of the ZnO microstructure was discussed. Furthermore, the room temperature UV–vis absorption and photoluminescence (PL) spectrum of the obtained products were also investigated.
All of the reactants and solvents were of analytical grade and used as received without any further purification. In a typical synthesis process, 50 mL of aqueous solution of Zinc nitrate hexahydrate (Zn(NO3)2·6H2O) and 50 mL of hexamethylenetetramine ((CH2)6 N4, HMT) aqueous solution of equal concentration (0.25 M) were mixed together. Then, the mixture was heated at 95°C for 3 h under mild magnetic stirring with refluxing, followed by cooling to room temperature naturally. Subsequently, the resulting white products were washed with deionized water, and dried at 60°C in the air for further characterization.
Transmission electron microscope (TEM) images and selected area electron diffraction (SAED) patterns were obtained on a JEOL JEM-2100 (Tokyo, Japan) operated at an accelerating voltage of 200 kV. Samples for TEM and high-resolution transmission electron microscope (HR-TEM) analyses were prepared by spreading a drop of as-prepared magnetite nanoparticle dilute dispersion on copper grids coated with a carbon film followed by evaporation under ambient conditions. The scanning electron microscopy (SEM) images were obtained on a LEO 1530 microscope (LEO, Germany). Atomic force microscope (AFM) characterization was carried out using a Scan Asyst-Air (Bruker Multimode 8, Bruker ASX Inc., Madison, WI, USA). Measurements were carried out in air using non-contact AFM mode. The X-ray diffraction (XRD) patterns were collected between 20° and 80° (2θ) on an X-ray diffraction system (X’Pert PRO, PANalytical Co., Almelo, The Netherlands) with a graphite monochromator and Cu Kα radiation (λ = 0.15406 nm). X-ray photoelectron spectrum (XPS) was obtained using a PHI Quantum-2000 electron spectrometer (Physical Electronics, Inc., Chanhassen, MN, USA) with 150-W monochromatized Al Kα radiation (1,486.6 eV). The PL emission spectra were recorded using an Edinburgh FLS 920 fluoresence spectrophotometer (Xe 900 lamp) (Edinburgh Photonics, Livingston, UK) at room temperature. The excitation wavenumber was 290 nm. The optical absorption was measured with an ultraviolet–visible (UV–vis) spectrophotometer (UV, BECKMAN COULTER DU 800, Conquer Scientific, San Diego, CA, USA) at room temperature.
Results and discussion
Structure and morphology
Effect of the reaction conditions on the ZnO microstructures
Effect of the reaction concentration
Effect of the reaction time
In the synthesis systems, HMT acted as alkaline source to release OH−, which subsequently reacted with Zn2+ to form ZnO22−. At the last period of the reaction, ZnO was largely formed via homogeneous precipitation under mild conditions.
The PL from ZnO were always composed of two emission bands at room temperature: a near-band-edge (UV) emission and a broad, deep-level (visible) emission. The visible emission was usually ascribed to various intrinsic defects produced during the synthesis of ZnO, such as zinc vacancy and oxygen vacancy[33, 34]. Figure8b showed the room temperature PL spectra of the ZnO samples collected at different times. The PL spectra of the samples collected at reaction times of 10, 60, 120, and 180 min revealed similar features. There appeared a strong broad UV emission at approximately 392 nm, which was the band-edge emission resulting from the recombination of free excitons. No other peaks such as green emission (approximately 520 nm) were found. It should be mentioned that the green emission resulted from the radiative recombination of a photogenerated hole with electron occupying the oxygen vacancy. It was commonly accepted that high-quality ZnO crystals would only emit UV light. Therefore, it was reasonably believed that the as-prepared dumbbell-like ZnO microcrystals were of good quality and may have a wide application in the photocatalytic field.
In summary, single crystalline ZnO with hexagonal dumbbell-like microstructure had been successfully synthesized via a facile solution method under mild conditions without any additives, templates, or substrates. The effects of the reactant concentration on the size and shapes of the ZnO samples were studied, and the results indicated that the ZnO preferential growth difference between  and other directions would decrease with increasing the reaction concentration. The growth mechanism of the dumbbell-like ZnO microcrystals was discussed in the point of nucleation and morphology view. Moreover, the dumbbell-like ZnO microcrystals had a very strong UV emission at approximately 392 nm, which might be very interesting for further application to microscale optoelectronic devices due to their excellent UV emission properties.
ZH and QJ are Ph.Ds. and associate professors. YW, LS, HG, GW, YL, and SZ are M.D. students in the Research Center of Biomedical Engineering. QZ is a Ph.D. and a professor.
This work was financially supported by the National Natural Science Foundation of China (81000660), major research plan of the National Nature Science Foundation of China (90923042), and Xiamen Science and Technology project (3502Z20123001).
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