Synthesis and Characterization of Zinc Oxide Obtained by Combining Zinc Nitrate with Sodium Hydroxide in Polyol Medium

2020. Polyols as solvents are widely and successfully used in various methodologies to obtain zinc oxide nanostructures. One of the main reasons of using polyols is related to its capacity to prevent the agglomeration of ZnO nanoparticles during the synthesis and to promote a more stable colloidal solution of it. In this work it is presented, for the first time, the obtaining of nanostructured ZnO by a polyol mediated solvothermal synthesis using zinc nitrate as precursor. The combination of the zinc source with sodium hydroxide in a polyol medium with PVA as dispersing agent provided a simple process for obtaining nanostructured ZnO. The zinc oxide nanostructures obtained were characterized by UV-visible, XRD, FESEM, TEM, DLS and BET. As result, was obtained a wurtzite phase with hexagonal structure with crystallite size of 12 nm and, according to TEM micrographs, an average size of 14 nm, which was corroborated to other


Introduction
The nanostructured ZnO is employed in several areas of industry and science due to its interesting properties and characteristics. It acts as antimicrobial agent 1,2 and protector of epidermis 3 in the health industry, as photocatalyst 4 , as pigments 5 in the textile industry and, because of its luminescence and its semiconductor properties, it's applied in advanced ceramics 6 and in solar cells 7 , respectively. The hexagonal wurtzite crystal structure (lattice parameters: a = 0.3254 nm, c = 0.5215 nm), a direct energy band gap of 3.37 eV, excitation binding energy of 60 meV and high electron mobility are some of the properties of ZnO 8 .
With the aim to obtain nanostructures with different sizes and morphologies, such as rod [9][10][11] , spherical 12 , nanowire 13 and nanosheets 14 , different physical 15,16 , chemical 17,18 and biological 19,20 methods have been reported. However, for the most of them, the nanoparticulate system obtained tends to agglomerate due to both large specific surface area and high surface energy. Moreover, the formation of the Zn-O-Zn bond between the nanoparticles, that occurs because of the existence of water molecules in the synthesis process (solvent most commonly used), it's also a hindrance in promoting the desired system of ZnO nanoparticles 21 . Therefore, the removal of water or the use of another solvent for the synthesis may be able to avoid agglomeration of ZnO nanoparticles and promote a more stable colloidal ZnO solution. The solvothermal method with the use of polyols, multivalent alcohols with high boiling points, allows the reactions to occur at high temperatures with no need for high pressures or autoclaves, that's why it is a versatile and interesting method. It enables the control of the size and morphology of ZnO nanostructures [22][23][24][25][26] and it's also used for obtaining doped ZnO nanoparticles 27,28 . The polyols more often used are ethylene glycol, diethylene glycol, tetraethylene glycol and glycerol 29,30 , whose boiling points are 197.3, 245, 285 and 290 °C 31 , respectively. Also, due to the elevate solubility capacity of the polyols, cheap and simple metal salts can be used as precursors [32][33][34] .
To comprehend how this method works we need to understand its four stages, Figure 1. In the first one, the solid precursor must be dissolved into a liquid polyol; in the second, the salt is reduced into small particles while the third step promotes a homogeneous nucleation process followed by the last stage, in which the nanostructures grow 35 .
In this paper, a polyol mediated solvothermal synthesis method was used to obtain ZnO nanostructured. The nanoparticulate system was characterized by different analyzes that proved that the simple method proposed can be successfully used to obtain ZnO particles with sizes in the range of 14 nm.
For the reactions, 25 mL of a solution of NaOH 1 mol L -1 in ethylene glycol was added dropwise to a stirred solution of Zn(NO 3 ) 2 ·6H 2 O solution (0.5 mol L -1 , 25 mL) in ethylene glycol as the stoichiometric ratio demands for the complete *e-mail: birondionisio@gmail.com formation of Zn(OH) 2 . 0.05 g of PVA was added over that solution to provide a stable environment. Finally, the solution was heated in reflux at 140 °C for 4 h.
The zinc oxide particulate system obtained was centrifuged at 8,000 rpm for 15 min and washed with ethyl alcohol. The solid material was dried for 24 h at 80 °C for further characterization.

Characterization of nanostructured zinc oxide
For characterization in UV-Visible, the sample was firstly dispersed in ultrapure water (100 ppm) and analyzed in a Genesys ultraviolet spectrophotometer, 10 mV, Thermo Spectronic. In addition, the optical band gap energy was determined by the Tauc plot, using the relationship proposed by Tauc, Davis, and Mott.
The X-ray diffraction patterns (XRD) of the nanostructured zinc oxide powder sample was obtained on a Shimadzu XRD-6000 X-ray diffractometer with CuKα radiation and λ = 1.5406 Å. The crystallite sizes were determined by using the Scherrer equation: where d is the size of the crystallite, k is the form factor (0.9), λ is the wavelength of the radiation and β is broadening of the Bragg's peaks (β 2 = β sample 2 -β instrumental 2 ). The silicon standard (MERCK, 99.99%) was used to determine the instrumental broadening, Figure 2. The refinement of the diffractogram obtained was performed by the Rietveld method with the aid of Fullprof software, July 2017 version. The corresponding fitting equation (polynomial regression) is ³ 2 y a bx cx dx = + + + , where y stands for the broadening in FWHM and x is the angle in 2θ. a, b, c and d are the constants with values 0.13154, 2.84904 × 10 4 , -7.55556×10 -6 and 9.91083 × 10 -8 , respectively. In this study was neglected the strain broadening.
The morphology, particle size and particle size distribution (both analyzed with the aid of ImageJ software, 1.52a version) were observed in a field emission scanning electron microscope (FESEM) of the brand Tescan, equipment model MIRA3, and in a transmission electron microscope (TEM) JEM-1200 EX II, Jeol.
The powdered ZnO sample was suspended in absolute ethylene glycol anhydrous and exposed in ultrasound for 10 min at concentration of 100 ppm. Then, particle size  The surface area and pore volume of the ZnO powder sample, determined by the Brunauer, Emmett and Teller (BET) technique, was performed on the Quantachrome Instruments, Nova 1200e model. The sample was degassed at 80 °C for 20 h before analysis. Additionally, the particle size was estimated by the equation: where As is the surface area of the ZnO powder sample e ρ is the specific mass of ZnO (5.67 g cm -3 ).

Synthesis process of ZnO
The results show that the use of Zn(NO 3 ) 2 combined with NaOH ensures a simple process for ZnO synthesis in nanoscale and with uniformity of its structure. By removing the NaOH addition from the steps, it was verified that ZnO formation does not occur. Thus, NaOH has the function of facilitating the obtaining of nanostructures and to ensure the formation, in an earlier stage, of Zn(OH) 2 . Figure 3 shows some steps of the synthesis as well as the final nanoparticulate product.
In this synthesis, the first equation concerns the double exchange that occurred between Zn(NO 3 ) 2 and NaOH (at 23 °C), forming Zn(OH) 2 . During the reflux process, the solution remained heated for 4 h, when the thermal degradation of ethylene glycol may have occurred to a small extent 35 . The second equation concerns the formation of ZnO particles, which occurred due to the heating of the solution that decomposes zinc hydroxide into nanoparticulate zinc oxide.

Optical study
UV-vis spectra provides the information about the excitonic and intertransition of nanomaterials. Figure 4a shows the UV-visible molecular absorption spectrum of the obtained nanostructured ZnO. It was detected an absorption band in the spectrum at 344 nm, which is a characteristic band for ZnO confirmed by literature 29,[36][37][38] . The absence of any other band in the spectrum confirms that the synthesized product is formed only by zinc oxide. Due to the nanodimensions, ZnO sample exhibit blue shifted absorbance peak when compared to its bulk that has the absorbance band at 386 nm at room temperature 11 .
Based on the UV-visible spectrum, it was also possible to determine the optical band gap energy by applying the Tauc plot method, Figure 4b. The result found was 3.23 eV, similar to the ones already reported 37,39 . Figure 5a presents the diffractogram from X-ray diffraction of the zinc oxide powder synthesized. All diffraction peaks are related to the hexagonal wurtzite phase of the zinc oxide, with space group P6 3 mc, according to the standard diffractogram ICSD 180050. No other peaks of any other phases were observed, what proves that monophasic wurtzite zinc oxide nanoparticles were obtained. Figure 5b shows the XRD refined data by the Rietveld method. As result, the size of the crystallite was approximately 12 nm. This result is close to the average particle size found in DLS, FESEM and TEM analyses, leading us to believe that the characterization in the different methods was performed successfully.

Crystallinity and phases
In the study of Nithiya et al. 40 the crystallite size of ZnO sample, also obtained by the polyol method, in a synthesis that lasted 5 h at a temperature of 100 °C, was estimated in 35 nm. In another study, the size of the crystallite was estimated between 21 and 46 nm using zinc acetate as precursor salt, with synthesis temperature set at 150 °C and reaction time up to 5 h 41 .   by BET technique, have an acute stage of capillary condensation at high relative pressure, which belong to the type IV isotherm, according to the IUPAC classification, Figure 8. The surface area of 54.2 m 2 g -1 and pore volume of 0.31 cm 3 g -1 were determined. The surface area value allowed to determine the average particle size, which was 20 nm corroborating to the other techniques applied in this study. The particles are considered agglomerated if the particle size determined by BET is greater than the size of the crystallite so, the larger the difference the higher degree of agglomeration of the particles. Similar results of surface area were found in the paper presented by Efa and Imae 39 , 51.5 and 55.7 m 2 g -1 .

Morphology and particles size
The ethylene glycol, with its structure and more voluminous functional groups (OH), proved to act as a limiting agent of   the growth of the crystal planes. Consequently, more uniform dimensional shapes, such as spheres, were produced 38 .
In the DLS analysis, Figure 9a, an average particle size value of 15 nm was found, which corresponded to 91.7% of the total volume of the sample analyzed. Table 1 shows a summary of the ZnO particles size obtained by the different characterization techniques used in this study. As can be seen, the results from the different techniques were similar, indicating that the proposed method is effective in obtaining nanoparticles with a size in the range of 20 nm. The Figure 9b shows the white-colored colloidal solution of the nanostructured ZnO obtained.
In the work presented by Chieng and Loo 29 , by using zinc acetate dehydrate as the salt precursor, they obtained spherical ZnO nanostructures with an average size of 19.62 nm, but by applying a higher temperature (160 °C) and for a longer time, 12 h. So, herein, we present a brander and faster synthesis for the same aim.

Conclusion
Zinc oxide nanostructures were prepared by the combination of zinc nitrate with sodium hydroxide in polyol medium. Ethylene glycol was used as solvent and, by increasing the temperature (140 °C), promoted the decomposition of Zn(OH) 2 into ZnO nanoparticulate.
The UV-visible spectrum showed absorption band at 344 nm with a band gap of 3.23 eV, both characteristics of the nanostructured ZnO. The particles presented an average size of 14 nm and a crystallite size of 12 nm. FESEM, DLS and BET results presented similar sizes, 20, 15 and 20 nm, respectively. BET analysis showed surface area of 54.2 m 2 g -1 with a pore volume of 0.31 cm 3 g -1 .
The results presented in this study reveal a new alternative for ZnO nanoparticulate synthesis that, due to its particle size quite competitive, leads to the possibility of several applications in nanotechnology, such as advanced ceramics, solar cells and antimicrobial agent.

Acknowledgments
National Council for Scientific and Technological Development (CNPq) of Brazil. Microscopy and Microanalysis Center (CMM) of UFRGS.