Abstract
We reported the synthesis of vertically aligned dense ZnO nanowires using Zn powder as the source material by a hydrothermal method and a postannealing process at 200°C. The as-synthesized ZnO nanowires are 100–200 nm in diameter and several micrometers in length and each nanowire has a tapered tip. The morphologies of the products remain after post-annealing treatment. Structural analysis indicates the ZnO nanowire is single crystalline and grows along the [0001] direction. The possible growth mechanism for ZnO nanowire bundles is proposed.
1. Introduction
Nanostructured materials have been a wide research focus due to their versatile morphologies and excellent physical and chemical properties superior to the corresponding bulk counterparts [1–3]. One-dimensional (1D) nanostructures are investigated not only for their fundamental scientific significance but also for their diverse technological applications in electrical, optical, and sensing fields [4–7]. In general, it is believed that 1D nanomaterials can show quantum size effect when the sizes are less than the exciton Bohr radius. In view of applications, single nanostructure like nanowire and nanorod should be assembled into devices, in succession integrated into a system. Therefore, recent studies have been inverted into constructing hierarchical and well-aligned 1D nanostructures [8–10]. A lot of efforts have been devoted to synthesize ordered nanostructures, for example, vertically or horizontally aligned nanowire arrays [11, 12]. The vertically aligned nanowires arrays are in the spotlight owing their conveniences and attractive properties in using as nanodevices, including the nanogenerator [13–15]. 1D nanostructures can be fabricated in various methods, such as vapor deposition [16], laser ablation [17], AAO template plating [18], solution growth [19–21]. Much progress has been made in the growth of 1D nanostructures, it is still a challenge to assemble well-aligned nanostructures in a rational pathway.
ZnO is a direct bandgap semiconductor with the band gap energy of 3.37 eV at room temperature and large exciton binding energy of 60 meV [2]. Research of 1D ZnO nanostructures can be ascended to the two papers published in the magazine of Science in 2001 [22, 23], from then on the synthesis of ZnO nanostructures has been a hot issue, splendid morphologies of ZnO have been synthesized, such as nanonails [24, 25], nanorings [26], nanohelices [27], nanowire arrays [28], superlattice structures [29], even hierarchical [30], and hetero-structures [31]. Herein, we reported the synthesis of highly oriented ZnO nanowire bundles by a two-step process. The growth mechanism of ZnO nanowire bundles is proposed based on the experimental conditions. The aligned ZnO bundled nanostructures presented here can be applicable in micro- and nano-optoelectronic devices.
2. Experimental Details
Two grams of zinc powder (Alfa Aesar, 99.99% purity) was put into 10 ml deionized water and stirred at the room temperature ceaselessly, and then transferred to an autoclave together with a preblended solution, which is obtained by mixing 5 ml Zn(NO3)2 solution with 0.5 M concentration and 15 ml NaOH solution with 5 M concentration. Silicon substrates were also put into the autoclave to collect products. The zinc powder and mixed solution were heated at C for 2 hours and then cooled down. The product was washed using deionized water and ethanol and dried at C. As-synthesized products were characterized by X-ray powder diffraction using Cu K radiation (XRD, Rigaku Dmax-rB, .1542 nm), scanning electron microscope (SEM, Hitachi S-4800), and transmission electron microscope (TEM, JEOL 2010EX) affiliated with energy dispersive X-ray spectrometer (EDS).
3. Results and Discussion
As-synthesized ZnO nanostructures are shown in Figure 1. The nanowire arrays are found to be vertically aligned in the surfaces of the precursors, similar to the result reported by Gu et al. [32], which is realized by a vapor phase process. Each ZnO nanowires have a tapered tip, a diameter around 200 nm and a length-diameter ratio of over 10. The XRD spectrum (Figure 2) shows that the as-grown sample is made up of ZnO and a trace of Zn, without any other phase. This result is consistence with the EDS spectrum, which shows only Zn and O (Figure 3). In order to remove the residual Zn, the as-grown samples were annealed at C for 2 h. The morphologies still remain (shown in Figure 4) after annealing because ZnO has a very high melting point of C. EDS spectrum of as-obtained product is also presented in Figure 5. TEM is used to study the microstructure of as-grown product. Figure 6(a) is a bright field TEM image of a single nanowire, and the corresponding selected area electron diffraction (SAED) pattern is shown in the inset in Figure 6(a), which indicates that the nanowire are crystalline and grown along the [] crystal direction, a common fast growth direction of ZnO. HRTEM image of the ZnO nanowire showed in Figure 6(b) is consistent with SAED pattern.
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Li et al. synthesized hierarchical ZnO nanowire arrays at elevated temperatures, and found that ZnO nanowire arrays were formed by multistep route. Hexagonal disks of ZnO were firstly formed as basic floor, whose surface may be roughened. ZnO nanowires were then assembled into a highly oriented array on the top/bottom surfaces of disks in the presence of dense ammonia [33]. Yang et al. synthesized ZnO nanorod arrays utilizing Zn foil as Zn source and water and aqueous ammonia as the solvent at C, respectively. The as-synthesized nanorods own a smooth tip [34]. Zhang et al. synthesized double tower-tip-like hierarchical Cu2O nanostructure. It is thought that the microemulsion system is a prerequisite for the formation of Cu2O nanostructures [35]. In this work, ZnO nanowires array is synthesized at C without using ammonia or other organic solution, and the as-synthesized ZnO nanowires present tapered tips. It means that morphology of as-grown product is relative with not only types and concentration of solvent but also the reaction temperature. Concentration and pH value of solvent probably also play an important role. A possible growth mechanism of the nanowire bundles we synthesized can be initially proposed as follows: the formation of nanostructure can divided into two step, that is nucleation and growth. ZnO nucleation occurs at first, Zn reacts with and forms in alkali sodium hydroxide solution. Once the concentration of ions reaches saturation shall react with H2O and induces the nucleation, and subsequent growth of ZnO nanowire bundles. There is a tapered tip at the top of each nanowire, and it is suggested that sudden perturbation of temperature may lead to formation of tapered tip. Actually reduction of the concentration of reactant is also a factor for growth of nanostructure with tapered tip [36].
4. Conclusion
ZnO nanowire bundles have been synthesized on large scale by a two-step process using Zn powder as the source. The experimental results indicate the as-synthesized product owns bundle-like morphology in a well-aligned manner. Each nanowire forming bundle is 100–200 nm in diameter and several micrometers in length, and owns a tapered tip. Formation mechanism for nanowire bundles is elucidated based on the reaction conditions. As-grown ZnO nanowire bundles are expected as ideal functional units for micro-/nanodevices. Furthermore, this growth approach can also be used to fabricate nanostructures of other oxide nanostructures.
Acknowledgment
This paper was supported by the Science Technology and Research Project of Education Bureau, Heilongjiang Province (nos.11551117,11531236)