Research Article | Open Access
Microwave Assisted Synthesis of ZnO Nanoparticles: Effect of Precursor Reagents, Temperature, Irradiation Time, and Additives on Nano-ZnO Morphology Development
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 [1, 2], spray pyrolysis [3, 4], hydrothermal synthesis [5–7], sol-gel processes [8–11], and microemulsion synthesis .
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 [13, 14], and inorganic  chemistry. This technique enables the rapid and homogenous heating of the reaction mixture to the desire temperature, which saves time and energy.
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 reaction rate acceleration as a consequence of high heating rates, wide range of reaction conditions, that is, mild conditions or autoclave conditions, high reaction yields, reaction selectivity due to different microwave absorbing properties, excellent control over reaction conditions, and simple handling, allowing simple and fast optimization of experimental parameters.
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 [16–18]. This irradiation technique has been used to obtain nanostructures with various morphologies including spherical particles, rods, hexagonal rings, hexagonal columns, Flowers, wires, hexagonal tubes, and sheets.
Some authors  demonstrated the possibility of ZnO synthesis with a novel three-dimensional morphology, where microwave radiation as heating source plays an important role in the formation of complex nanostructures. In addition, the absence of metal catalyst, template, or surfactant in this method avoids the subsequent effort for the removal of residual additives.
Other studies  dealt with the effect of power of microwave radiation on the shape and size of the synthetized ZnO nanostructures, where the average particle size decreases with decreasing microwave power being the optical properties sensitive to the variation of this parameter.
On the other hand, other authors  analyzed the effect of solvent (water, ethanol, and isopropanol) on the synthetized ZnO nanostructures, and an important change in the size and shape of nanoparticles was reported. When distilled water was used as a solvent, nanoparticles presented ellipse shape and larger size (larger axis was about 100 nm and size of the other axis was about 40 nm). With ethanol, the obtained nanostructures had rod form with smaller sizes than those obtained in water, and when isopropanol was used as the reaction medium, spherical particles with radius of 10–12 nm were obtained.
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.
2.1. Synthesis of ZnO
In a typical synthesis process, 15 mL (1.6 mol L−1) of a precursor salt (Zn(NO3)26H2O, Zn(CH3COO)22H2O or ZnCl2) were diluted in 32 mL of deionized water (or a sodium di-2-ethylhexyl-sulfosuccinate aqueous solution) to obtain a Zn+2 solution. Afterwards, 4 mL of a base (3.2 mol L−1) (NaOH, KOH, or NH4OH) was added dropwise (2 min) into the above-described solution with magnetic stirring at room temperature to get a colloid system, which was maintained under stirring for 10 min. Then, the reaction mixture was transferred into a Teflon autoclave and treated at selected temperature (80, 100, 120, or 140°C) for specific time (5, 10 or 20 min) under temperature-controlled mode in a microwave accelerated reaction system (Mars-X) operating at different powers (300, 600, or 1200 W). It must be mentioned that the system was programmed with a first temperature ramp where the target temperature was reached after 10 min in all cases (Table 1). When the reaction was finished and cooled to room temperature, the white precipitate was collected by filtration and washed with deionized water and ethyl alcohol several times. Finally, the product was dried at 65°C in a vacuum oven for 3 h.
| 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 with a step size of 0.06° at 35 kV and 25 mA. The average crystallite sizes (Table 1) of the ZnO nanostructures were calculated using Scherrer’s formula where is the X-ray wavelength of Cu-K radiation source, is the full width at half maximum intensity of the diffraction peak located at 2, and is the Bragg angle.
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.
3. Results and Discussion
3.1. Temperature Effect
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 1). XRD studies (Figure 1) show that, when the reaction is carried out for 20 minutes at 80 or 100°C (MW-1 and MW-2, resp.), the signals are not clear, and some peaks attributable to precursor reagents and unknown impurities are observed. On the contrary, at 120 and 140°C (MW-3 and MW-4, resp.) all the diffraction peaks match those of the wurtzite ZnO with lattice constants of Å and Å. The strong diffraction peaks appear at , and 36.5°, which correspond to the (100), (002), and (101) planes of wurtzite ZnO, respectively. FE-SEM images show an almost homogeneous morphological distribution at a set reaction temperature of 140°C. In this case most ZnO particles were formed as bihexagonal rods which were probably stacked from sheets with a starting growth from both sides. Additionally, a low population of ZnO particles with a more complex three-dimensional structure can be observed (MW-4, Figure 1) where a radial growth from a center seems to be favored yielding nanodandelions-like morphologies . On the other hand, when the precursors are irradiated at 120°C (MW-3, Figure 1), the FE-SEM images show that, while some particles are formed, there is still a heterogeneous background. A possible explanation for this behavior can be linked to the existence of a greater irradiation to achieve a higher temperature, which increases the polarization of the system by producing a more orderly system that favors the nucleation and growth of the ZnO nanoparticles as was also described by other authors .
On the other hand, FTIR studies (Figure 1) show the characteristic vibrations of ZnO in the product obtained at 140°C. In all the cases the FTIR spectrums show absorption bands centered at about 3430 and 2344 cm−1, which can be assigned to the O–H stretching vibrations, and bands centered at 1630 and 1384 cm−1 that correspond to the asymmetric and symmetric C=O stretching modes (CO2 modes). At low wavenumbers there can be observed two peaks located at approximately 420 and 512 cm−1, corresponding to the ZnO rod like-nanostructures [20, 21].
3.2. Precursors Effect
Three different precursors were employed to synthetize ZnO (temperature 140°C; power 600 W): Zn(NO3)26H2O (MW-4), Zn(CH3COO)22H2O (MW-20), and ZnCl2 (MW-21). Figure 2 shows the corresponding morphologies as well as XRD analysis for these samples.
As it can be observed from Figure 2, XRD analysis shows that when Zn(NO3)2 and Zn(CH3COO)2 are the precursor salts, all the diffraction peaks match those of the wurtzite ZnO as it was described above. The morphological distribution is homogeneous in both cases, being in the first case particles with a bihexagonal rod-like structure of about 6 µm long and 2 µm wide; meanwhile in the second case the morphology is of the hexagonal prismatic type, and the average particle size is of about 3-4 µm long and 1 µ wide. In these both systems the larger particles observed could be grown from small primary particles through an oriented attachment process in which the adjacent nanoparticles are self-assembled by sharing a common crystallographic orientation and docking of these particles at a planar interface.
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 , the difference in the morphology of the three analyzed systems can be attributed to the fact that, in the case of the halide anion, it adsorbs more strongly on surfaces than acetate ions and nitrate ions which exhibits very weak surface interaction. As the adsorption on surfaces increases, the growth process decreases conducting particles with lower sizes as it can be observed in Figure 2 where the size of the synthesized ZnO particles decreases in the following order: > CH3COO− > Cl−.
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 c axis (0001 plane); meanwhile in the case of acetate ion it is a chelating ligand that allows a better control on the ZnO particle development yielding particles with lower particle size.
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 3). This behavior can be explained attending to the following reaction mechanism. The first step in the synthetic reaction, irrespective of the route employed (acidic or basic), corresponds to the formation of a colloidal system:
The zinc cations are known to react with hydroxide anions to form stable complexes, which could act as the growing unit of ZnO nanostructures [23, 24]. Therefore, the growth mechanism can be considered as follows when the synthesis is carried out through the acidic route: When the addition is made in such a way that OH− is aggregated over the Zn+2 solution, reaction symbolized by (2) takes place, and the formation of growing units during the irradiation stage is favored as it is symbolized in (3). In this sense, species are generated under microwave effect, and it is well known that growing species can be polarized  allowing a controlled and directed growth of the particles.
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 species formed have no polarization by microwave radiation, and the growth of the particles is in a random way.
3.3. Washing Solvent Effect
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 , and it was evidenced that they can affect the morphological characteristics of the final product damaging the surface of ZnO, depending on the solvent used and on its polarity and vapor pressure. Attending to these results it was decided to carry out a synthesis where the isolation of nano-ZnO was made by centrifugation (10 min at 10000 rpm; MW-5) without previous washes; instead water is a preferable solvent to be used for ZnO . XRD analysis for the former case, it means centrifugation without further purification, shows the wurtzite hexagonal ZnO characteristics (Figure 3). From the images obtained by FE-SEM a more dispersed system from both, morphological and size distribution aspects (Figure 3), can be observed with a less resolution on the lattices of the bihexagonal rod-like structure.
3.4. Precursor Base Effect
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 4(a)); one of them is similar to that obtained when NaOH is used as the precursor base (Figure 1, MW-4), in the form of bihexagonal rods, and there is another population of particles in the form of agglomerates which are constituted by small particles. When NH4OH is used as the base, two morphologies can be clearly observed; the characteristic hexagonal rods previously described in the case of NaOH and KOH and other one where two- or three-dimensional growths are favored (Figure 4(c)). In this last case the growth is favored radially and in different directions yielding three-dimensional complex particles, similar to the dandelions particles obtained by Huang et al. . The changes in morphology are attributed to the different degrees of electrostatic effects of the hydrated ion (Na+, K+, or ) which can act on the surface of the growing crystals causing different growth on their surfaces. Therefore, the size of hydrated ions ( Na+) and their charge are important factors in this approach.
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. .
3.5. Stoichiometry Effect
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 2.1), that allows carrying out the reaction at pH of 5.8, 13 mL of KOH (Zn+2/OH− = 1/1.73) (MW-16) were added, and a pH of 10.8 was reached. This modification in OH− concentration has been directly reflected in a morphological change of the particles obtained (Figure 4(b)). This effect is attributed to a different reaction mechanism in the generation of ZnO particles.
If an excess of OH− is added (like in MW-16 synthesis) to the aqueous solution, the precipitated species formed in (2) would dissolve back into the alkali solution to form the initial ions , and, after certain reaction time during the stirring period, the reaction symbolized by (4) can occur before irradiation with the formation of complexes. As KOH concentration increases, the complexes are surrounded by a large amount of OH− so that the differences in the observed morphologies (Figures 4(a) and 4(b)) can be attributed to the different stages where growth unit can be formed and to the initial zinc species and their environment in the reactor as it was demonstrated by Xu et al. in the case of ZnO synthesis via a hydrothermal method .
3.6. Power and Irradiation Time Effect
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 1; MW-11, MW-12, and MW-4, resp.). By XRD and IR techniques it could be observed that only in the case of using the initial ramp (MW-10) there is evidence of signals indicating the presence of impurities in the final product. In the other three cases (MW-11, MW-12, and MW-4) only wurtzite hexagonal signals and ZnO characteristics vibrations are present. In Figure 5 the morphology evolution of ZnO nanoparticles with irradiation time can be observed where an increase in this parameter 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 0001 plane to growth.
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 1) (Figure 6), it was observed that the formation of aggregates is favored as the microwave power increases. As microwave power increases, there is an increase in the mixture aqueous solution temperature enhancing the tendency of the ZnO nuclei to aggregate, in a similar way to that described by Al-Gaashani et al. . Moreover, when the power is set to 1200 W (Figure 6(c)), the generation of agglomerates makes the system not homogeneous and not well dispersed like the one obtained at 600 W (Figure 6(b)).
3.7. Additives Effect
It is well known that the use of additives leads to changes in ZnO crystal growth . In this work the effect of the addition of an anionic surfactant (sodium di-2-ethylhexyl-sulfosuccinate (AOT)) and a dispersant (ethylene glycol) on ZnO particles obtained by microwave synthetic method was evaluated. In these experiments the syntheses were carried out in the same way as it was previously described by dissolving Zn+2 in an additive (AOT or ethylene glycol) aqueous solution.
As it can be inferred from Figure 7(a), when 1 g (16 mmol) of ethylene glycol was added, (MW-9) to the reaction media, there were no significant changes neither in the morphology nor in the size of the obtained ZnO particles in comparison with the product obtained in the absence of any additive (MW-4).
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 7(b)) in the sense that there is a lower growth in c axis ( direction), so smaller hexagonal prismatic particles are formed with sizes in the interval of 60–80 nm in diameter and length between 90 and 110 nm when compared with MW-4. Similar results were obtained when 0.5 g of AOT (1.1 mmol) were added (MW-8). In this case characteristic vibrations of surfactant could be observed by FTIR due to surfactant traces are still present after washes made with ethanol and deionized water.
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 ) reducing the surface energy of the specific crystal faces. This behavior can lead to the control of the crystal growth while retaining in some way the morphology.
3.8. Optical Properties
Having in mind that the absorption of colloidal system depends on the particle size and shape , absorption spectra of some synthetized ZnO nanoparticles were obtained. As it can be observed from Figure 8, particles with size higher than 500 nm do not show important absorption band, and no maximum can be detected in the absorbance curves (MW-4, MW-15, MW-17, and MW-20). However, when surfactant AOT is added to the reaction media and smaller particles are produced (particles size < 100 nm), a significant increase in UV absorption and a well-defined maximum at 374 nm can be observed. This absorption is caused by two factors; the first one is due to the excitation of the electrons from the valence band to the conduction band when they are irradiated with UV light, which causes the absorption of UV radiation. The second is light scatter; the particle size of a colloidal solution within the wavelength of UV light results in light scattering, which also causes the absorption.
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)26H2O, Zn(CH3COO)22H2O, and ZnCl2. It was observed that, as the adsorption of counteranions on surfaces increases, the synthesized ZnO particles sizes decrease in the following order: > CH3COO− > Cl−.
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 direction to growth. Furthermore, it was observed that the formation of aggregates is favored as the microwave power increases.
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).
In the attached file called Electronic Supplementary Material (ESM) are present all experimental data corresponding to the characterizations of synthesized materials. In this sense are shown the images obtained by scanning electron microscopy, X-ray diffractograms and infrared spectra corresponding to the products obtained from each synthesis made.
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