The high-yield synthesis of zinc oxide (ZnO) primary nanoparticles with high purity and with diameters between 6 and 22 nm using bicontinuous microemulsions is reported in this work. The ZnO nanoparticles were made by hydrolysis of Zn(NO3)2 with NaOH aqueous solution and precipitation, followed by calcination of the precipitate. Higher yields and productivities of ZnO nanoparticles were obtained compared to values produced with w/o micremulsions reported in the literature. Particles were characterized by transmission electronic microscopy (TEM), X-ray diffraction, and atomic absorption spectroscopy.
ZnO is attracting considerable attention because its wide band energy gap (3.37 eV) and its high exciting emission efficiency, which makes it suitable for UV emission devices [
Different techniques for preparing ZnO nanoparticles have been investigated such as sol-gel process [
In reverse microemulsion precipitation, the hydrolysis reactions take place inside the water-microemulsion droplets suspended in a hydrophobic medium; however, this technique has the drawback that the productivity of nanoparticles is low. Some authors have employed bicontinuous microemulsions rather than w/o microemulsions because the ratio of water-to-oil is larger, which increases the productivity of metal oxide nanoparticles, keeping the typical size of nanoparticles obtained in reverse microemulsions [
In this work, we report the synthesis of ZnO nanoparticles employing bicontinuous microemulsions. This method allows obtaining higher yields and productivities because bicontinuous microemulsions contain larger aqueous phase concentrations, where the ZnO precursor is located, than w/o microemulsions. Particles were characterized by transmission electronic microscopy (TEM), X-ray diffraction, and atomic absorption spectroscopy.
Sodium dodecyl sulfate (SDS), sodium bis-2-ethylhexyl sulfosuccinate (AOT), and Zn(NO3) · 6H2O were all 98% pure from Sigma-Aldrich. NaOH, 98.2% pure (Golden Bell), and toluene, 99% pure (Golden Bell), were used as received. De-ionized and triple-distilled water with conductivity smaller than 6
The one-phase microemulsion region at the reaction temperature (70°C) was determined by titrating solutions of AOT/SDS (2/1 by weight) in toluene at different surfactant concentrations (in the range of 5 to 70 surfactant wt.%) with 0.9 M Zn(NO3)2 aqueous solution under continuous agitation. The phase boundaries were detected as those compositions where samples became turbid. Samples were also examined with cross-polarizers to rule out liquid crystalline phases that might form. Phase boundaries were determined more accurately by weighing the components and by observing the samples in the neighborhood of the titration-determined compositions.
To determine the mixture compositions where bicontinuous microemulsion formed, conductivities of samples along lines A, B, and C in Figure
Partial phase diagram obtained at 70°C for mixtures of toluene, AOT/SDS (2/1 w/w), and 0.9 M Zn(NO3)2 aqueous solution. The one-phase microemulsion region (1
The hydrolysis of Zn(NO3)2 and precipitation of the nanoparticles were carried out by duplicate at 70°C by dosing an NaOH aqueous solution at different feeding rates to give total feeding times of 100, 125, and 150 min. The total added amount of NaOH was 1.43 times the stoichiometric ratio (NaOH/Zn(NO3)2). After the addition period, the reacting system was left to stand for 30 min at the reaction temperature. The precipitate was recovered by filtration and mixed with an aqueous acetone solution (81/19 w/w) in a sonic bath for 15 minutes to remove the surfactants and nonreacted material and centrifuged to recover the wet solids. This procedure was repeated 10 times. The residual wet solid was then dried in an oven at 60°C and then calcined at 400°C for two hours in an oven.
The resulting product, a fine powder, was characterized in a Siemens D-5000 X-ray diffractometer (XRD). Particle size was determined by transmission electronic microscopy (TEM) in a JEOL JEM-1010; for this, the resulting powder was dispersed in acetone with an ultrasonicator, and then a drop of the dispersion was deposited on a copper grid, where the solvent was allowed to evaporate. The purity of the final product was determined by atomic absorption spectrometry with a Varian Spectra 250 AA equipment.
Because it is desirable to obtain the largest amount possible of ZnO nanoparticles during the synthesis, preliminary experiments in bicontinuous microemulsions were carried out using Zn(NO3)2 aqueous solutions of different concentration. These experiments revealed that it was possible to use Zn(NO3)2 aqueous solutions with concentrations up to 0.9 M. Figure
Figure
Electrical conductivity of the one-phase microemulsions versus concentration of a 0.9 M Zn(NO3)2 aqueous solutions for three surfactant mixture/toluene weight ratios. Inset: enlargement of conductivity data for the 50/50 w/w surfactant mixture/toluene ratio.
To obtain the ZnO nanoparticles, two compositions in the bicontinuous region with high conductivity along the line A (see Figure
ZnO nanoparticles
Sample | Zn(NO3)2 concentration (wt.%) in microemulsion | Dosing time (min) | PDI | ZnOa purity (%) | Productivity (g ZnO/100 g reaction mixture) | Yieldb (%) | |
---|---|---|---|---|---|---|---|
100-1 | 22 | 100 | 1.10 | 98 | 1.25 | 98.27 | |
125-1 | 22 | 125 | 1.17 | 99 | 1.13 | 88.83 | |
150-1 | 22 | 150 | 1.38 | 95 | 1.11 | 87.26 | |
100-2 | 27 | 100 | 1.19 | 99 | 1.30 | 85.30 | |
125-2 | 27 | 125 | 1.21 | 98 | 1.34 | 87.93 | |
150-2 | 27 | 125 | 1.3 | 96 | 1.36 | 89.24 |
aCalculated from atomic absorption determinations.
b(Experimental weight of ZnO/theoretical weight of ZnO) × 100.
The X-ray diffraction (XRD) pattern of the precipitate of the 125-2 sample (Figure
X-ray diffraction pattern of sample 125-2 before calcination (a). For comparison, the standard X-ray diffraction patterns of Zn(NO3)2·6H2O (b) (JCPDS card 24-1460) and ZnO (c) [
X-ray diffraction pattern of sample 125-2 after calcination (a); the standard X-ray diffraction pattern for ZnO from the literature [
Micrographs of the precipitate before calcination (Figure
TEM micrographs of (a) sample 125-2 before calcination and (b) the calcinated sample (ZnO nanoparticles).
Table
High yield (87–98%), high purity (96–99%), and high productivity of ZnO nanoparticles were obtained (between 1.13 and 1.36 g ZnO/100 g of reaction mixture). This productivity is larger than the largest value calculated from the data in the literature, which is 0.73 g of ZnO/100 g of microemulsion considering that all Zn(NO3)2 is converted to ZnO [
Here we report the synthesis of ZnO nanoparticles by hydrolysis and precipitation with NaOH aqueous solution from bicontinuous microemulsions containing Zn(NO3)2 as the precursor, followed by calcination of the precipitate. Primary nanoparticles of ZnO with average particle size from 6.0 to 22 nm with high productivity (1.13 and 1.36 g ZnO/g of reaction mixture), high yield (87–98%), and high purity (96–99%) were obtained. As far as we know, this is the highest productivity of pure ZnO nanoparticles employing microemulsion media. By increasing the amount of the aqueous Zn(NO3)2 solution or decreasing the NaOH addition rate, smaller particles were produced.
This work was supported by FOMIXJAL (Grant no. 2009-05-124211) and CONACYT (Grants nos. CB-2007.82437 and CB-2007.84009). One of us (S. López-Cuenca) acknowledges the scholarship from CONACYT. The authors are grateful to Alejandro Espinoza and Blanca Huerta for their technical assistance.