Because of their interesting properties for electronic and light emission devices applications, the ZnO structures constitute an interesting topic of research [1,2]. In particular, the Zinc Oxide (ZnO) nanostructures with different shapes, such as wires [3,4], spheres [5–7] and belts [8,9], are useful for applications in nanophotonics [10]. The objective of this study is the growth the particles of ZnO and its relationship with the deposition zone.

ZnO can also be used in UV light emitters [11], spin functional devices, gas sensors [12], transparent electronics and surface acoustic wave devices [13]. The three most common methods for the synthesis of bulk ZnO crystals are hydrothermal [14,15], vapor phase [16–18], and growth from the melt state [19]. It has also been observed that the tetrapod contain legs of single crystals with hexagonal structure [3,10].

The Zinc Oxide generality grows with a wurtzita crystalline hexagonal structure [20-26]. The related lattice parameters are a=0.325 nm and c=0.512 [10,27,28]. The Zn atoms are tetrahedrally coordinate with four oxygens atoms [27–29], where the electrons of the d zinc layer hybridizes with the oxygen p layer, building the Zn-O alternating layers [13].

The ZnO particles synthetized were analyzed by means of X Ray Diffraction (XRD), Transmission Electron Microscopy (TEM), and Scanning Electron Microscopy (SEM). The XRD characterization indicated these particles have wurtzite structure and the optical properties have been studied with cathodeluminescence (CL). For TEM analysis a Tecnai G2 Spirit Twin instrument was used operated at 120 kV, mainly under diffraction contrast mode.

The synthesis of ZnO is thermodynamically viable because its reaction has a large negative ΔG°, and the vapor pressure of Zn is about 1 atm at 900°C. The synthesis of ZnO is carried out for homogeneous reaction, gas-gas. In agreement with the Ellinghan diagram the growth of ZnO from Zn(g) and O₂(g) has favorable conditions when the furnace temperature is over 900°C. In such conditions, the production of Zn(g) is higher and there will be more molecules for reacting with O₂, and the reaction occurred in few minutes after beginning.

In this work, it is reported a synthesis of ZnO in a furnace tube in static air conditions. Zinc pieces with a size of 3-8 mm (99.98% purity, Pb 0.01%, Fe 0.01%. As 0.01 ppm) where placed in an alumina boat located in a quartz tube with a diameter of 5 cm. The two ends of the tube were open during the growth process. Then, the horizontal tube furnace was heated from 250°C up to 940°C using a rate of 10°C/min, maintaining the work temperature for 15 minutes, and then cooled down to room temperature whit a ramp of 120°C/h. This experimental procedure is similar to the procedure reported in Ronning et al. [10]. However, authors of that research work growth ZnO on Si (100) and set up an alumina tube with a diameter of 5cm with one side of the tube sealed and the other opened along through the growth process. These factors determine differences in a reaction temperature and deposition temperature zones.

During the heating process a ZnO layer forms on the surface of the original Zn particles, creating a crust or capsule that separates liquid Zn and the surrounding air. When the internal pressure of the capsule is larger than the pressure in the chamber, some cracks are produced in the crust, and Zn(g) is liberated. The Zn(g) reacts with O₂ while it is transported towards the end of the alumina tube, where it is deposited in regions under different temperatures. Abduev et al. [30], reported the chamber formation and the cracks presence when the system exceeds 920°C, obtaining white fluffy and bulk products.

The structural analysis of the ZnO particles was performed by means of XRD using Cu-Kα 1.54184 Ǻ radiation. The optical properties of the ZnO particles were studied and analyzed by CL measurements in a SEM equipped with a CL detector with spatial resolution. The CL analysis was performed at room temperature. The electron beam voltage was kept constant at 4.5 kV throughout the whole experiment.

The ZnO deposited on the ends of tube has a tetrapod-like homogeneous morphology. This structure is present in all the experiments. The Figure 1 shows a relation between deposition zone and the ZnO morphologies. There were also observed tetrapods and whiskers out of the furnace on the alumina rod, on the reactor wall and plates. In addition, it was observed dendritic structures on the furnace center.

Figure 1: SEM -SE images of ZnO synthesized at 920°C. A. ZnO particles deposited out of furnace it observe whiskers, B. Particles deposited on the wall reactor can see wires and tetrapod, C. Particles deposited on the alumina rod inside the furnace in can observe tetrapods, D. ZnO deposited in the center of furnace present plates, wires, dendritic structures.

The high purity of ZnO synthesized by this method, in agreement with the spectrum of the wurtzite structure and the absence the impurity peaks, indicates that the Zinc Oxide have a well-defined crystalline structure [16,31].

Figures 2 and 3 show the SEM and TEM images. At low magnification it can be clearly observed multiple-twined on the core. The diameter of the tetrapods’ legs decreases from the center up to de tip. The same results were reported by Ye Zhang et al. [31]. Figures 2A and 2B show some tetrapods whose legs’ length are around 2 um and 750 nm. Figure 3 shows a tetrapod with legs of approximately 1 um of length.

Figure 2: SEM-SE imagens composition ZnO particles synthetized at 920°C and collected out of the center of reactor, A. a cluster of tetrapods, B. Image SEM-SE High resolution of tetrapod show legs of 1um of length.

Figure 3: TEM Images composition of ZnO particles synthetized. A. TEM multi-beam image of tetrapods, B centered-dark-field images of individual nanorod belonging to the tetrapods (leg) it insert the corresponding selected area diffraction pattern; the arrow indicated the reflection responsible for the image.

In Figure 4A it can be observed that the length of the tetrapods legs is in the order of 1.5 to 3 um. In Figure 4B it can be seen the CL qualitative characterization image at room temperature. This results is in agreement with results obtained by Roy et al. [32]. The intensity contrast in the legs is due to the incidence angle between the surface and the electron beam.

Figure 4: SEM–CL images composition of ZnO Tetrapods. A SEMSE image show tetrapods and whiskers, B. SEM-CL image show magnification of A can observe the qualitative CL emission.

Synthesis: This experiment is interesting because: (a) A very close to thermodynamics equilibrium, with little or no temperature gradient. It can be affirmed because the synthetized particles have no defects in these crystalline structure. It is evidenced in the symmetry of the particles observed in Figure 2 and in the fact that the tetrapod’s legs are a monocrystal as confirmed in Figure 3. (b) The evaporation occurs in an oxygen deficient region. Therefore, the ZnO crystals are deposited at the ends of tube and the reaction forms a tetrahedron core. This is related with the luminescence properties of ZnO particles and with the oxygen superficial vacancies [33]. The tetrahedron core formation is reported on some recent investigations [10]. Based on the results obtained in this research work we can affirm that if the core is deposited at the ends of reactor, it grows as tetrapods-like structure. Instead, if the core stays around the reactors’ center, it can observe the formation of nanostructured particles, as shown in Figure 1. (c) The crystals that grew on the synthetization process have high quality crystallite, and little defects. Figure 3B confirms that every tetrapod leg is a monocrystal. Such effect can be seen in the selected area diffraction pattern (SDAP) on Figure 3 that showing the electron beam that form the dark field image.

Optical properties: The ZnO particles synthesized presented a CL emission, it is intense in bulk of particles and decreases on the surface, also is less intense at the wires.

The ZnO synthesized by simple evaporation and direct oxidation of Zinc with ambient air grows in different shapes, but reports the presence of tetrapods in all runs. The length of legs varies from 0.75 to 20 um for different particles, while the diameter varies from 0.2 to 2 um. The crystal shapes produced have hexagonal, wires, plates and tetrapod morphologies in all runs. The shape of the nanostructure depends on the deposition zone.