Studies on the Growth Control of ZnO Nanostructures synthesized by the Chemical Method

ZnO nanostructures were synthesized through a chemical method using different Zn precursors and hexamethylenetetramine (HMTA) at 90 °C. The effects of the reactants on the morphological evolution of ZnO nanorods were investigated. The samples were characterized by using XRD, SEM, EDX and BET. The hexagonal wurtzite phase of ZnO was confirmed by X-ray diffraction (XRD). The performed analysis indicated that different morphologies were obtained by changing the reactants.


INTRODUCTION
Recently, many preparation techniques of ZnO nanostructures have been developed, which are classified as physical and chemical methods. Among the physical methods, pulsed laser deposition, magnetron sputtering, electrodeposition, microwave-assisted technique, electron beam evaporation, are found [1-6]. Hydrothermal process, sol-gel, solochemical, chemical vapor deposition, spray pyrolysis and precipitation methods are examples of chemical techniques [7][8][9][10][11]. Physical techniques usually need relatively expensive equipment with high temperatures and high vacuums. Chemical syntheses have some advantages, since they do not need sophisticated equipment with high vacuum level, and therefore they are less expensive. Microstructure of chemically synthesized nanostructures can be precisely controlled by changing the starting materials, temperature, additives, and their concentrations [12][13][14][15][16]. A significant problem of chemical synthesis is the appearance of pollutants and toxic materials that are created as a by-product of the reaction, which hinder the material´s practical and technological applications. Many times the starting materials are toxic and hazardous. There is a great need to develop newer methods and technologies that would make interesting products available through reactions in water or other aqueous media. Chemical techniques with non-toxic materials and at low temperature processing are of great preference [17][18][19]. In the present work, we synthesized ZnO nanorods by a simple chemistry synthesis from an aqueous solution containing different Zn precursors and HMTA under controlled temperature and without producing pollution waste. The results show that ZnO morphology and the size of the nanorods can be controlled by changing the precursor Zn solution.

Synthesis
Three Zn precursors were used: Zn(CH 3 COO) 2 ⋅2H 2 O; ZnCl 2 and Zn(NO 3 ) 2 ⋅6H 2 O. The reactants were first placed in a beaker and mixed during 5 minutes. Then the mixture was placed in a flask attached to a vertical open Liebig condenser in which 100 mL of the Zn precursor and 100 mL of HMTA of equal concentration (0.01M) were heated at 87 °C for 6 h under magnetic stirring. After that, the precipitate was cooled at room temperature, then filtered and washed several times with deionized water and ethanol and finally dried in an oven at 80 °C for 7 h. The ZnO samples obtained from zinc acetate dihydrate, zinc chloride and zinc nitrate hexahydrate were designated ZnO-Ace, ZnO-Chl and ZnO-Nit respectively.

Characterization
The morphology of as-synthesized ZnO nanostructures was characterized by scanning electron microscopy (SEM) on a FEI Model Quanta 200, 25 kV microscope combined with an energy dispersive X-ray analyzer (EDX) attached to the SEM. ImageJ software was used to determine the diameter and length of the ZnO nanostructures.
The crystalline structure of the powder was assessed by X-ray diffraction (XRD). The XRD patterns were recorded at room temperature with Cu Kα radiation (λ= 0.15418 nm) in a diffractometer (PANalytical model Empyrean) having theta-theta configuration and a graphite secondary-beam monochromator, using a generator voltage of 40 kV and current of 40 mA. The data were collected for scattering angles (2θ) ranging from 20° to 70° with a step of 0.026° for 2 s per point.
The surface area was calculated using the Brunauer-Emmett-Teller (BET) method based on the adsorption data. The BET specific surface area and pore distribution of the samples were carried out on a Micromeritics Accelerated Surface Area and Porosimetry System ASAP 2020 v 3.01 instrument.

XRD analysis
The X-ray diffraction pattern of ZnO powders are shown in Fig.1

ZnO-Chl
The interplanar spacing (dhkl) calculated from XRD is compared with JCPDS data card, and the percentage of variation of d are summarized in Table 1. The lattice constants calculated from the XRD data for (100) and (002) planes are shown in Table 2 [20]. The patterns are in accordance with the wurtzite hexagonal structure in the reference data (01-079-0207) and the lattice parameters are similar to the reference data a=b=3.2501Å and c=5.2071Å. No other diffraction peaks are presented, indicating the high purity of the synthesized ZnO products. The Scherrer method for calculating particle size gives an average value of the entire particle responsible for diffraction. The crystallite size is taken to be the size of a coherent diffraction domain and it is not necessarily equal to the particle size. The orientation degree of ZnO nanostructure can be calculated by the relative texture coefficient Tc, using the following equation, [21]: (1) where I(hkl) and I 0 (hkl) are the relative intensity from measured samples and standard sample (01-079-0207), respectively and n is the total number of diffraction peaks considered in the calculation.    The surface area and pore volume of the samples were investigated using nitrogen gas adsorptiondesorption method. Nitrogen adsorption-desorption isothermal and the corresponding Barrett-Joyner-Halenda (BJH) pore size distribution desorption are presented in Fig. 4. The pore volume distribution curves of ZnO are given as an inset in Figs. 4a, 4b and 4c, respectively. The profiles can be related to type III isotherm with a type III hysteresis loop [22].  Table 3 shows the results of the measured BET specific surface areas and BJH pore size distributions of the ZnO nanostructures. As it can be seen in Table 3, the ZnO-Chl has a higher specific surface area than the other samples probably because of its smaller particle size.

The role of HMTA during ZnO crystals growth
The whole reactions most often cited in the literature for white ZnO precipitation are [23]:  (7)]. A similar mechanism has been suggested by Govender et al. [24,25].
According to the present study, the use of different Zn precursors determines not only the morphology but also the particle size of the final product. This is probably related with the kinetic of the decomposition of the zinc salts in solution. Further systematic studies are required to obtain a quantitative information on the mechanism.

CONCLUSIONS
The use of different Zn precursors in the preparation of nanostructured ZnO is an important factor in determining the product morphology. All ZnO samples obtained show rod-like shape, being the only difference the size of the nanorods. When we use Zn Acetate dihydrate as precursor we get short rods and with the use of chloride longer nanorods can be obtained.

ACKNOWLEDGEMENT
The authors would like to acknowledge Daniel Vega from Condensed Matter Division, GAYANN, CNEA for XRD analysis.