The effect of different variables (precursor reagents, temperature, irradiation time, microwave radiation power, and additives addition) on the final morphology of nano-ZnO obtained through the microwave assisted technique has been investigated. The characterization of the samples has been carried out by field emission scanning electron microscopy (FE-SEM) in transmission mode, infrared (FTIR), UV-Vis spectroscopy, and powder X-ray diffraction (XRD). The results showed that all the above-mentioned variables influenced to some extent the shape and/or size of the synthetized nanoparticles. In particular, the addition of an anionic surfactant (sodium di-2-ethylhexyl-sulfosuccinate (AOT)) to the reaction mixture allowed the synthesis of smaller hexagonal prismatic particles (100 nm), which show a significant increase in UV absorption.
ZnO powder has been widely used into numerous materials and products including paints, plastics, ceramics, and adhesives. It is a semiconductor of the II–VI semiconductor group with several favorable properties such as high electron mobility, wideband gap, and strong room temperature luminescence. These properties make ZnO an attractive compound for different emerging applications.
In the last two decades many methods ranging from gas-phase processes to solution routes have been investigated for the synthesis of ZnO nanoparticles including solution precipitation [
In cases where the synthesis has been carried out through a conventional thermostatic system, the walls of the reactor are heated by convection or conduction, the core of the sample needs longer time to achieve the target temperature, and this may result in inhomogeneous temperature profiles. One possible solution to this problem is the use of microwave heating, which has become a very promising method of synthesis for both organic [
The microwave heating is based on two conversion mechanisms of the electromagnetic radiation into heat energy, namely, dipolar rotation and ionic conduction, which are directly related to the chemistry composition of the reaction mixture. So that, different compounds have different microwave absorbing properties, and this behavior allows a selective heating of compounds in the reaction mixture.
The general advantages of microwave mediated synthesis over conventional ones are
Since 2007, the number of publications dealing with the microwave assisted synthesis of ZnO has increased significantly. In this sense there have been many published results concerned with aqueous and nonaqueous solution microwave assisted synthesis where the effect of different experimental variables over ZnO size, shape, and physicochemical behavior have been analyzed [
Some authors [
Other studies [
On the other hand, other authors [
However, most synthetic studies did not show a complete study about different parameters involved during the microwave assisted technique. In this sense, this work deals with a complete analysis of different variables (precursor reagents, temperature, irradiation time, microwave radiation power, and the addition of additives) and their effect on the final morphology of nano-ZnO obtained through the microwave assisted technique.
In a typical synthesis process, 15 mL (1.6 mol L−1) of a precursor salt (Zn(NO3)2
Experimental conditions employed during microwave assisted synthesis of ZnO.
Synthesis code | Experimental conditions (irradiation timea, temperature, and microwave power) | Zn precursor, 15 mL |
Base, 4 mL 3.2 mol L−1 | Yield, % | Average crystallite size, nm | Average particle sizeh, |
---|---|---|---|---|---|---|
MW-1 | 20 min, 80°C, 600 W | Zn(NO3)2 6H2O | NaOH | N.A | N.A | N.A |
MW-2 | 20 min, 100°C, 600 W | Zn(NO3)2 6H2O | NaOH | N.A | N.A | 0.413 (Wi) |
MW-3 | 20 min, 120°C, 600 W | Zn(NO3)2 6H2O | NaOH | 93.51 | 8.05 | 1.116 (W) |
MW-4 | 20 min, 140°C, 600 W | Zn(NO3)2 6H2O | NaOH | 89.71 | 8.25 | 1.927 (W) |
MW-5b | 20 min, 140°C, 600 W | Zn(NO3)2 6H2O | NaOH | 99.31 | 6.41 | 1.534 (W) |
MW-6c | 20 min, 140°C, 600 W | Zn(NO3)2 6H2O | NaOH | 87.02 | 6.41 | 0.128 (W) |
MW-7d | 20 min, 140°C, 600 W | Zn(NO3)2 6H2O | NaOH | 92.30 | 6.31 | 0.123 (W) |
MW-8e | 20 min, 140°C, 600 W | Zn(NO3)2 6H2O | NaOH | 82.72 | 7.53 | 0.394 (W) |
MW-9f | 20 min, 140°C, 600 W | Zn(NO3)2 6H2O | NaOH | 89,73 | 8.22 | 1.308 (W) |
MW-10 | 0 min, 140°C, 600 W | Zn(NO3)2 6H2O | NaOH | N.A | 8.25 | N.A |
MW-11 | 5 min, 140°C, 600 W | Zn(NO3)2 6H2O | NaOH | 91.17 | 8.22 | 1.467 (W) |
MW-12 | 10 min, 140°C, 600 W | Zn(NO3)2 6H2O | NaOH | 92.23 | 8.23 | 1.006 (W) |
MW-13 | 20 min, 140°C, 300 W | Zn(NO3)2 6H2O | NaOH | 81.40 | 8.26 | 1.487 (W) |
MW-14 | 20 min, 140°C, 1200 W | Zn(NO3)2 6H2O | NaOH | 90.35 | 8.40 | 1.351 (W) |
MW-15 | 20 min, 140°C, 600 W | Zn(NO3)2 6H2O | KOH | 87.00 | 7.77 | 0.870 (W) |
MW-16 | 20 min, 140°C, 600 W | Zn(NO3)2 6H2O | KOHg | N.A | 7.81 | 0.220 (W) |
MW-17 | 20 min, 140°C, 600 W | Zn(NO3)2 6H2O | NH4OH | N.A | 7.54 | 1.689 (W) |
MW-18 | 20 min, 100°C, 600 W | Zn(CH3COO)2 2H2O | NaOH | N.A | 8.05 | 3.716 (W) |
MW-19 | 20 min, 120°C, 600 W | Zn(CH3COO)2 2H2O | NaOH | N.A | 7.97 | 3.128 (W) |
MW-20 | 20 min, 140°C, 600 W | Zn(CH3COO)2 2H2O | NaOH | N.A | 8.15 | 1.656 (W) |
MW-21 | 20 min, 140°C, 600 W | ZnCl2 | NaOH | N.A | N.A | N.A |
aAn initial 10 min period required to reach the target temperature must be considered in all cases.
bSolid product was isolated by centrifugation. The product was not washed.
cChange in the addition order.
d0.24 g of sodium di-2-ethylhexyl-sulfosuccinate dissolved in water were added before base addition.
e0.50 g of sodium di-2-ethylhexyl-sulfosuccinate dissolved in water were added before base addition.
f1 g of ethylene glycol dissolved in water was added before base addition.
g13 mL of base were added instead of 4 mL.
hCalculated from SEM images employing ImageJ Software.
iAverage width of the rod-like particles.
jAverage length of the rod-like particles.
N.A: not available.
To investigate the structural properties of ZnO nanoparticles, field emission scanning electron microscopy (FE-SEM), infrared (FTIR), UV-Vis spectroscopy, and powder X-ray diffraction (XRD) studies were carried out. ZnO powders synthetized were characterized by X-ray diffraction with a Siemens D500 diffractometer. Diffraction patterns were recorded from 10 to 80° 2
On the other hand, an aliquot of solid state material was placed in a carbon label for analysis by FE-SEM (JSM-7401F). Samples were analyzed using a secondary electron detector. The infrared spectra of ZnO nanoparticles were taken in a Nicolet spectrophotometer model Nexus 470 Nicolet brand in transmittance mode. The sample preparation was made in tablet way by mixing nano-ZnO and KBr in an agate mortar. The solid samples were dispersed in deionized water and ultrasonicated for 10 min. Then UV-Vis spectra of colloidal systems were obtained through a Shimadzu double beam spectrophotometer model UV-2401PC.
Characterization data of all ZnO synthesis carried out is described in Electronic Supplementary Material (ESM) that accompanies this paper (see Supplemetary Material available online at
The microwave system employed to synthetize the ZnO particles allows to set a constant reaction temperature. In order to analyze the temperature effect on the final morphology of the product obtained, the synthetic reactions were carried out in the temperature interval 80–140°C using Zn(NO3)2 as the precursor salt (from MW-1 to MW-4, Table
Temperature effect during microwave synthesis of ZnO on its morphological development.
On the other hand, FTIR studies (Figure
Three different precursors were employed to synthetize ZnO (temperature 140°C; power 600 W): Zn(NO3)2
Effect of precursor salt during microwave synthesis of ZnO on its morphological development, at 140°C and 600 W.
As it can be observed from Figure
On the contrary, when ZnCl2 is used as the precursor salt, the corresponding diffractogram shows many signals attributed to precursor reagents and unknown impurities, and a heterogeneous and non-well-defined morphology and a broad particle size distribution can be observed.
Taking into account that the growth of large crystals depends on the counteranion as it was previously described [
This behavior can also be confirmed by the fact that nitrate anion is a noncoordinative ligand, so that the hexagonal wurtzite prefers growing in the
On the other hand and for the case of Zn(NO3)2, the order of reagents addition during the synthesis of nano-ZnO was analyzed; when a solution of Zn(NO3)2 precursor was added dropwise over NaOH solution (basic route), different morphologies (short rows, etc.) were obtained in the final product (MW-6) with a wide particle size distribution and arranged in the form of agglomerates (Figure
Isolation and precursors order addition effects on ZnO final morphology.
The zinc cations are known to react with hydroxide anions to form stable
On the other hand, if Zn+2 is added slowly over the OH- solution as in the case of MW-6, the growing units can be formed rapidly, and the following reaction can occur before irradiation:
The
There has been described in the literature the effect of different solvents (ethanol, water, or both) used during the washing step on the morphological parameters of the nano-ZnO [
The results presented up to this point have been obtained using NaOH as the precursor base. However, KOH (MW-15) and NH4OH (MW-17) have also been used under the same experimental conditions in order to analyze their effect on the morphological characteristics of the nanoparticles produced. In the case of using KOH two populations of well-defined morphologies can be observed (Figure
Base effect in microwave assisted ZnO synthesis. (a) KOH, (b) KOH 13 mL, and (c) NH4OH.
In this sense, when the more bulky Na+ is present, the Zn+2 is preferably adsorbed on the 0001+/− planes so that the crystals show oriented growth along these directions. On the contrary, when K+ is the counterion, there is a competition between K+ and Zn+2, and K+ inhibited the adsorption of Zn+2 on the 0001+/− planes, so Zn+2 must be adsorbed not only on the 0001+/− planes but also onto the other six planes yielding particles with shorter length and higher diameter, in a similar way as the behavior described by Nejati et al. [
The stoichiometry relation between Zn+2 and OH- was also studied, and instead of adding the amount of base described in the synthesis (Zn+2/OH− = 1.87/1) (Section
If an excess of OH− is added (like in MW-16 synthesis) to the aqueous solution, the precipitated
In order to analyze the microwave irradiation system controllable variables, such as power and irradiation time on the produced ZnO morphology, some experiments were carried out under the same experimental conditions but with different irradiation times: only initial ramp (10 min) (MW-10) to reach the target temperature (140°C), and 5, 10, and 20 min after reaching the desirable temperature (Table
FE-SEM images of ZnO obtained at different irradiation times: (a) only initial temperature ramp, (b) 5 min at 140°C, and (c) 10 min at 140°C.
On the other hand, when the effect of power microwave irradiation was analyzed (samples MW-13, MW-4, and MW-14 for 300, 600, and 1200 W, resp., Table
FE-SEM images of ZnO obtained at different irradiation powers at 140°C: (a) 300 W, (b) 600 W, and (c) 1200 W.
It is well known that the use of additives leads to changes in ZnO crystal growth [
As it can be inferred from Figure
XRD, FE-SEM, and FTIR characterization of ZnO synthetized with different additives: (a) 1 g ethylene glycol, (b) 0.24 g AOT, and (c) 0.5 g AOT (experimental conditions: 140°C, 20 min, and 600 W).
On the contrary, when AOT surfactant was added, important changes could be observed. With 0.24 g of AOT (0.5 mmol) (MW-7) the particles show a different morphology (Figure
The previous results can be attributed to the fact that AOT can be acting as an external factor that can control the growth rate of various faces of the crystals, due to the adsorption of the counterions on the negatively charged surfaces (precursor
Having in mind that the absorption of colloidal system depends on the particle size and shape [
UV-Vis spectra of ZnO nanoparticles dispersed in water. (a) MW-17, (b) MW-15, (c) MW-20, (d) MW-4, and (e) MW-7.
A simple and effective microwave assisted aqueous solution method has been used to synthetize ZnO with high crystallinity and purity when Zn(NO3)2 is used as precursor reagent. In addition, several parameters have been evaluated in order to analyze their effects on the ZnO nanoparticles obtained. An increase from 80°C to 140°C in set reaction temperature allowed to obtain a system with high purity and homogeneous in size and shape. On the other hand, three different Zn+2 precursors were employed to synthetize the nanoparticles: Zn(NO3)2
When different OH− precursors were employed (NaOH, KOH, and NH4OH), some changes were observed in the morphological characteristics of the synthesized ZnO. If NH4OH is used as the base, two morphologies were found: the hexagonal rods described in the case of NaOH and KOH and other ones where two- or three-dimensional growths are favored. The final reaction pH can be mentioned as another important variable that causes significant changes on the morphology of the final product.
An increase in irradiation time generates more defined structures and a final system with a more homogeneous particle size and morphology distribution as a consequence of more time for the
The addition of an anionic surfactant (AOT) to the reaction media allowed the synthesis of smaller particles and a significant increase in UV absorption, and a well-defined maximum at 374 nm was observed.
As final conclusions it can be enounced that to obtain a very pure phase of ZnO of high density, employing microwave as energy source, it is recommended to use Zn(NO3)2 as precursor.
The authors would like to thank Miriam Lozano and Jesús Angel Cepeda Garza for their contribution on the characterization of the ZnO nanoparticles by FE-SEM. Also, the authors wish to thank the Mexican National Council for Science and Technology (CONACYT) for its financial support for the realization of this study through a postdoctoral fellowship for Gastón P. Barreto, Ph.D. (CB-2008-01 Project no. 101934).