High-Yield Synthesis of Zinc Oxide Nanoparticles from Bicontinuous Microemulsions

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.

Different techniques for preparing ZnO nanoparticles have been investigated such as sol-gel process [9,10], chemical coprecipitation [11,12], chemical vapor deposition [13], thermal decomposition [14,15], hydrothermal synthesis [16], solid-state reaction [17], spray pyrolysis [18], and microemulsion precipitation [19][20][21][22][23].These techniques have allowed obtaining various ZnO structures such as nanospheres, nanorods, nanowires, nanotubes, and flowerlike nanostructures.Among these, precipitation in reverse microemulsions is a well-established technique for preparing particles with average diameters smaller than 10 nm and low particle size polydispersity [24][25][26].However, the preparation of ZnO nanoparticles by this method has been scarcely investigated [19][20][21][22][23].According to these reports, ZnO nanoparticles with average diameters around 10 nm [19][20][21]23] up to ca. ∼ =150 nm [22] have been obtained.Hingorani et al. reported the first synthesis of ZnO nanoparticles via reverse microemulsions in the early 1990s [19,20].These authors mixed two reverse microemulsions, one containing a zinc nitrate aqueous solution and the other an ammonium carbonate aqueous solution, to obtain zinc carbonate nanoparticles; further calcination of this compound leaded to spherical nanoparticles of ZnO with 14 nm in average diameter [19].Later, the same group used reverse microemulsions stabilized with zinc (bis ethylhexyl)sulfosuccinate to precipitate zinc oxalate by adding oxalic acid.ZnO nanoparticles of about 5-13 nm in diameter were obtained by zinc oxalate calcination [21].Lim et al. reported the preparation of ZnO particles via bicontinuous microemulsions [22].A solution of sodium hydroxide was added to a microemulsion containing a zinc nitrate aqueous solution to precipitate zinc hydroxide, which was then calcined to obtain spherical-shaped ZnO particles.However, these particles were very large (ca.150 nm in diameter).This unusually large particle size obtained via microemulsion precipitation was ascribed by the authors to grain growth and particle agglomeration during the calcination process.More recently, Inoguchi et al. [23] reported the preparation of spherical ZnO nanoparticles as small as 5 nm in average diameter by direct precipitation in a reverse microemulsion containing an aqueous solution of sodium hydroxide to which an alcoholic solution of zinc diethoxide was added.
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 [22,[27][28][29].
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.

Phase Diagram and Electrical Conductivity Determinations.
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(NO 3 ) 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 1 were determined with a 3173 R JENCO conductivity meter at 70 • C.

Bicontinuous Microemulsion Precipitation.
The hydrolysis of Zn(NO 3 ) 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(NO 3 ) 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.

Nanoparticles Characterization.
The resulting product, a fine powder, was characterized in a Siemens D-5000 Xray 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.

Phase Diagram.
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(NO 3 ) 2 aqueous solutions of different concentration.These experiments revealed that it was possible to use Zn(NO 3 ) 2 aqueous solutions with concentrations up to 0.9 M. Figure 1 shows the microemulsion region for the system containing a mixture of AOT/SDS (2/1 by weight), toluene, and a 0.9 M aqueous solution of Zn(NO 3 ) 2 .Three ratios of the surfactants/toluene mixture, which allowed a high percent of aqueous solution inside the microemulsion region, were selected.To find the bicontinuous microemulsion zone, the content of the 0.9 M aqueous Zn(NO 3 ) 2 solution was increased while maintaining the ratio of surfactants/toluene constant (lines A, B, and C, in Figure 1) and the conductivity of the different resulting mixtures was measured.

Electrical Conductivities.
Figure 2 depicts the electrical conductivity as a function of the amount of aqueous solution of Zn(NO 3 ) 2 for the three surfactants/toluene ratios selected (50/50, 55/45, and 60/40 w/w).In all cases, a bell-shaped curve was obtained.By increasing the aqueous solution content, the microemulsion conductivity increased until a maximum value was reached, followed by a drop in conductivity.According to the percolation theory, bicontinuous microemulsions show a high conductivity as a result of having a continuous (or connected) aqueous phase domains [30][31][32][33].When the ratios of surfactants/toluene were 50/50 and 55/45, the conductivities were very low, which indicated that w/o microemulsions, instead of bicontinuous microemulsions, formed.However, a big increase in  conductivity was detected when the surfactants/toluene ratio of 60/40 (w/w) with a percent of aqueous solution larger than 15% was used, reaching a maximum of 1575 μS/cm around 27% of aqueous solution, indicating bicontinuous structure.Then it decreased to low values, due to the formation of o/w microemulsions.Similar behavior was reported with a 0.5 M Zn(NO 3 ) 2 aqueous solution in a microemulsion containing 57.4 wt.% petroleum spirit as the oil phase and 24.6 wt.% poly(oxyethylene) 5 nonylphenol ether as the nonionic surfactant, that is, a conductivity rise as the amount of aqueous solution was increased up to a maximum (600 μS/cm), followed by a large decrease.This was attributed to the transition from inverse to bicontinuous and then to inverse microemulsions [22].Transitions from inverse to bicontinuous microemulsions or from bicontinuous to inverse microemulsions are well documented in the literature [30][31][32][33].

Preparation and Characterization of ZnO Nanoparticles.
To obtain the ZnO nanoparticles, two compositions in the bicontinuous region with high conductivity along the line A (see Figure 1) were chosen (22% and 27% of 0.9 M aqueous Zn(NO 3 ) 2 solution).Table 1 shows the Zn(NO 3 ) 2 concentrations and the dosing times used in this work.When the NaOH aqueous solution was added, a precipitate formed.The X-ray diffraction (XRD) pattern of the precipitate of the 125-2 sample (Figure 3) before calcination shows the characteristic peaks of the ZnO with a hexagonal wurtzite crystal structure and also small peaks which are attributed   to (Zn 5 (OH) 8 (NO3) 2 •2H 2 O. Similar XRD patterns were obtained for the other precipitates.Gusatti et al. [34] reported the production of ZnO particles with sizes between 25 and 28.5 nm with high purity by reacting an aqueous Zn(NO 3 ) 2 •6H 2 O solution with an aqueous NaOH solution.All the XRD patterns of the calcined samples showed only peaks that correspond to ZnO with a hexagonal wurtzite crystal structure indicating that ZnO nanoparticles with high purity were obtained.The XRD pattern of sample 125-2 is shown in Figure 4   that the purity of the nanoparticles is around 98%, which confirms the results of XRD.
Micrographs of the precipitate before calcination (Figure 5(a)) and of the ZnO nanoparticles (Figure 5(b)) of the sample 125-2 show that primary particles with diameters between 3 and 9 nm were obtained.The ZnO nanoparticles number-average diameter (D n ) and the polydispersity index, PDI (D w /D n ), where D w is the weight-average diameter, were calculated by measuring ca.540 particles.These results are included in Table 1.
Table 1 discloses that by decreasing the addition rate of the NaOH solution, smaller particles were obtained, which is more obvious with the sample containing 22% of aqueous solution of Zn(NO 3 ) 2 .This can be explained by the smaller amount of primary nanoparticles formed at a given time, which causes a lower probability for particle coalescence and agglomeration by interparticle collision.Table 1 also shows that by increasing the content of the Zn(NO 3 ) 2 aqueous solution, smaller particles were obtained because the water channels become wider, and hence there are less primary particles per unit volume to promote particle growth or agglomeration.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(NO 3 ) 2 is converted to ZnO [18]; if the total amount of mixture was taken into account (adding the precipitating agent solution), its productivity would be much smaller.

Conclusions
Here we report the synthesis of ZnO nanoparticles by hydrolysis and precipitation with NaOH aqueous solution from bicontinuous microemulsions containing Zn(NO 3 ) 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(NO 3 ) 2 solution or decreasing the NaOH addition rate, smaller particles were produced.

Figure 1 :
Figure 1: Partial phase diagram obtained at 70 • C for mixtures of toluene, AOT/SDS (2/1 w/w), and 0.9 M Zn(NO 3 ) 2 aqueous solution.The one-phase microemulsion region (1φ) is located between the surfactants mixture/toluene axis and the solid line; here, lines A, B, and C indicate where conductivity measurements were performed to examine the structure of the microemulsions.Solid triangles indicate the compositions where the precipitation reactions were made.

Figure 2 :
Figure2: Electrical conductivity of the one-phase microemulsions versus concentration of a 0.9 M Zn(NO 3 ) 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.

Figure 4 :
Figure 4: X-ray diffraction pattern of sample 125-2 after calcination (a); the standard X-ray diffraction pattern for ZnO from the literature [34] was included (b).

Table 1 :
ZnO nanoparticles D n and PDI determined by TEM, purity, productivity, and yield of the reaction.
a Calculated from atomic absorption determinations.b (Experimental weight of ZnO/theoretical weight of ZnO) × 100.