Synthesis and Characterization of ZnONanorods Based on a New Gel Pyrolysis Method

ZnO nanorods were fabricated by a template-free gel pyrrolysis method based on polyvinyl alcohol (PVA) polymeric network. In the present method, zinc salt precursor is trapped in the homogenized gel network to control the mechanism and kinetics of zinc salt calcinations process. By controlling the gel structure and gel pyrrolysis rate, zinc salt precursor can be calcinated to zinc oxide nanorods. The morphology and particle size of the synthesized sample depend on some parameters including amount of zinc salt and PVA in the initial solution, type and composition of the solvent, type and amount of the additives, solution pH, pyrrolysis temperature, and the time of pyrrolysis, which were optimized by the “one at a time” method. The prepared zinc oxide nanorods were carefully characterized using SEM, TEM, XRD, BET, and UV-visible spectrophotometer. The obtained result showed that the present method can be used to synthesize pure and uniform zinc oxide nanorods with energy band gap 3.31 eV, effective surface area of 19 m2·g−1, average diameters of 60 nm, and length of 1000 nm.

ZnO nanostructures were synthesized by different physical and chemical methods.Chemical methods have more performance than physical methods in controlling particle size and morphology.Several chemical methods for the synthesis of zinc oxide and mixed metal oxides have been reported, for example, preparation of fine zinc oxide by means of spray pyrrolysis [13], sol-gel technique [14][15][16], and thermal decomposition [17].The synthesis of zinc oxide from organic solutions has also been reported, for example, precipitation from alcohols and amines [18][19][20].In most of these studies, the control of particle morphology and the rate of particle growth have been considered in order to avoid the formation of large particles.
Some of nanostructures such as nanorods can be synthesized by the template methods.The template-based methods are time consuming and difficult.
In this work, a new gel network was used to control template-free synthesizing of ZnO nanorods.The advantages of gel pyrrolysis method for preparing of nanostructured materials comparative with conventional methods are as follows: (i) the small average particle size and narrow size distribution, (ii) homogeneity at molecular level, (iii) purity and increased reactivity, and (iv) possibility to obtain nanorods.
For practical applications, it appears that the advantages of the new method far exceed the disadvantages, such as increasing cost and also processing time.The synthesized samples were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Bronouer-Emmet-Teller (BET), and UVvisible spectroscopy.

Reagents and Instrumentals.
All materials and reagents used in this work were of reagent grade and were produced in Loba Chemie Co. India.Double-distilled water was used in all the experiments.A scanning electron microscope with EDX instrument from Philips Co. (XL30) was used for the studying of morphology, particle size, and surface analysis of the prepared iron nanopowders.X-ray diffraction (XRD) studies were performed by a Decker D8 instrument.A JEOL-200 CX transmission electron microscope was used to obtain TEM images.Shimadzu Double-beam UV-Visible spectrometer was used to determine bad gap energy.

Procedures.
In this procedure, 91 g of a mixed ethanol : water (70 : 30) solvent was used to dissolve 6 g zinc  acetate and 3 g PVA.The mixture was heated to 70 • C to from a homogeneous sol solution.The obtained sol was heated to 120 • C to evaporate the solvent and form a hard homogenous gel.The final gel was pyrolyzed at 400 • C for 4 hours.During the paralysis process, the PVA polymeric network was slowly burnt through the outer surface, and zinc acetate salt was calcinated and converted into ZnO nanorods.The obtained samples were crushed to prepare a fine powder.Morphology, particles sizes, and chemical composition of sample were analyzed by the SEM, TEM, XRD, BET, and UV-visible.In this procedure, there are some parameters such as zinc salt, PVA, solvent composition, pyrrolysis temperature, pyrrolysis time, solution pH, and additives which can affect the morphology and particle sizes of the final powder.The effect of each parameter was studied by the "one at a time" method [21,22].

Result and Discussion
At the proposed method, the gel network rigidity controls the morphology and particle size of the synthesized zinc oxide nanostructures.To make uniform ZnO nanorods, zinc acetate molecules were homogeneously dispersed among polymeric network of the gel.Because of get net work rigidity, the dispersed molecules in the gel network cannot alter their positions [23].Therefore, during the pyrrolysis of the gel, outer layers of gel were burnt, and zinc acetate contents of the burnt layer calcinated into ZnO.The following reaction was performed during pyrrolysis (1) and calcination (2) processes: In this method, the amount of Zn(CH 3 CCO) 2 , PVA, the type and composition of solvent, pH, temperature and time of pyrrolysis, and the type and amount of additives have an effect on the synthesis ability.The degree of these effects was optimized by the "one at a time" method.Several synthesizes were performed to obtain optimum conditions for synthesizing uniform zinc oxide nanorods.Table 1 shows the experimental conditions of performed synthesizes for optimizing set.Each sample was characterized by SEM and XRD.Based on SEM images and XRD patterns, the amount of each parameter was changed to obtain uniform ZnO nanorods.initial sol.Figure 1 shows SEM images of the synthesized samples in these experiments.As it can be seen in Figure 1, salt percentage can change ZnO morphology from simple spherical nanoparticles (for 1 g zinc acetate) to uniform nanorods (for 6 and 8 g zinc acetate).Zinc salt amount can control nucleation rate and the particle growth mechanism.Based on the SEM images, the sample synthesized at zinc acetate of 6% wt has narrower nanorods, so this value was selected as optimum degree.

Effect of Gel Making
Agent.Experiments 7 to 10 were an optimization set for the PVA as gel making agent.SEM images of the samples were shown in Figure 2. The nucleation and nuclear growth rates depend on the gel viscosity and rigidity.The gel specifications were changed by varying PVA, and, as an optimum value, 3% wt of PVA was used to synthesize ZnO nanorods.

Solvent Composition.
Figure 3 shows the effect of solvent composition on the ZnO morphology by SEM images (Experiments 11 to 22 in Table 1).As it can be seen in Figure 3, solvent composition is an important factor which can change the morphology of the synthesized samples.The polarity of mixed solvent depends on its composition.In the present method, the solvent polarity controls the solubility of sol ingredients to determine final morphology of product.
For evaluation of solvent effect, Snyder polarity coefficients (P ) for all used solvents were calculated [24].Among  these solvents are cyclohexane, cyclohexene, and acetonitrile which are different from others in polarity and dielectric constant.Based on Figure 3, the last solvents are not suitable for ZnO synthesis.ZnO was synthesized in nanorod form by all compositions of ethanol/water and isopropanol/water (only 30/70) mixed solvents.Therefore, we selected all these solvents and investigated the effect of solvent polarity on the nanorods lengths (Figure 4).
As it can be seen in Figure 4, the Snyder polarity coefficient of 6 is suitable to synthesize long nanorods.It should be mentioned that the diameter of all nanorods in these experiment was approximately constant.Therefore, the mixed solvent of ethanol-water (70-30) is a suitable solvent to synthesize ZnO in uniform nanorods.

Temperature Studies.
In each chemical and electrochemical synthesizes, temperature is an important agent that can affect the morphology, particle size, and phase composition of the final product [21,22].Therefore, the effect of pyrrolysis temperature on the morphology, particle size, and phase composition of ZnO was studied.Experiments 23-26 were performed to optimize the degree of the pyrrolysis temperature.Figure 5 shows the pyrrolysis temperature effect by SEM images and XRD patterns.
As Figure 5 shows (XRD patterns), the proposed method produces pure ZnO.The pyrrolysis temperature has no effect on the phase composition of ZnO, but it can change the morphology and particle size of ZnO nanopowder.Based on the obtained SEM images, 400 • C is a suitable temperature to pyrrolize the gel and calcination of zinc acetate into ZnO.At higher temperatures (higher than 400 • C), burning rate of gel is high so that the gel cannot control calcination rate.Thus, at higher temperatures, there are no uniform nanorods (Figures 5(c) and 5(d)).We also checked the lower temperature (350 • C).At lower temperatures, PVA gel cannot be completely pyrrolized, so the final power contains some PVA impurity.

Synthesis Additives.
In this work, the effect of polyvinyl pyrrolidone (PVP), sodium dodecyl sulfate (SDS), and cetyl trimethyl ammonium bromide (CTAB) as synthesis additives was studied at different weight percentages.Figure 6 shows the SEM images of ZnO samples which were synthesized by experiments 27 to 38 (Table 1).CTAB is a cationic surfactant so it cannot form a complex with zinc ions.SDS is an anionic surfactant which can make an interaction with zinc ions.In addition, PVP is a polymer which has a long chain including donor sites (nitrogen atoms) to trap zinc ions.On the other hands, PVP has more solubility in water/ethanol mixed solvent rather than SDS.As Figure 6 shows, PVP with an initial amount of 5% wt acts as a suitable additive to synthesize uniform ZnO nanorods.As it can be expected, PVP is a suitable ligand to form a stable complex with zinc ions to control ZnO kinetics and mechanism.
3.6.Effect of Pyrrolysis Time. Figure 7 shows the SEM images of the synthesized sample at different pyrrolysis times (Experiment 39 to 41 at Table 1).In a short time (less than 4 h), the gel cannot completely pyrrolize so that, the obtained gel has some PVA impurity.In a long time (more than 4 h), not only the diameter of ZnO nanorods is increased but also the length of nanorods is decreased.At longer pyrrolysis times, solid-state reaction is probable to rearrange ZnO nanorods.

pH Studies.
The solution pH as last factor was studied (Experiments 42 to 46).SEM studies showed that ZnO nanorods can only be synthesized at pH 7.5 to 8 (Figure 8).At higher pH, Zn(OH) 2 was precipitated, and, at lower than 7.5, hydroxyl groups in PVA structure can be protonated.By the decreasing pH, degree of protonation is increased so that PVA cannot make a rigid gel network.

Optimum Sample.
Experiments 1 to 46 showed that the optimum conditions to synthesize uniform ZnO nanorods are zinc acetate 6% wt, PVA 3% wt, mixed solvent ethanolwater (70 : 30), pyrrolysis temperature 400 • C, PVP 5% wt as additive, pyrrolysis time 4 h, and pH solution 8. The optimum ZnO nanorods in nanopowder form were ultrasonicated in ethanol before TEM studies.Figure 9 shows SEM images of the ZnO sample synthesized at the optimized conditions before (a) and after ultrasonic irradiation (b).Ultrasonication of ZnO nanopowder in ethanol isolates the nanorods.Figures 9(c) and 9(d) show TEM images of the optimized ZnO sample.Using the present method, ZnO can be synthesized in uniform nanorods with average diameter and length of 60 and 1000 nm, respectively.3.9.Band Gap Determination.ZnO as a semiconductive metal oxide is widely used in different applications.As it has been previously reported, the ZnO nanostructures has blue shift in UV-visible spectrum with respect to micrometersized ZnO [25].The direct band gap of ZnO was determined by UV-visible spectrum (Figure 10).
Based on UV-visible data, the sample has the maximum absorbance at 360 nm wavelength.Therefore, for the optimum sample, band gap was calculated 3.31 eV.

Conclusion
It is concluded that the presented method is a new selective and template-free method based on PVA gel pyrrolysis which can be used as a useful and selective method to synthesize pure and uniform ZnO nanorods.In this method, zinc salt concentration, gel making agent content, solvent composition, synthesis additive, pyrrolysis temperature, and pyrrolysis time are important parameters which can change the morphology and particles sizes of ZnO samples.In content to the major previous methods, at this method, temperature cannot affect the composition of ZnO samples.

Figure 9 :Figure 10 :
Figure 9: SEM images of the optimized ZnO nanorods before (a) and after ultrasonic irradiation (b), and TEM images of the optimized ZnO nanorods, (c) and (d).

Table 1 :
Experimental conditions of the synthesized samples in optimization set.