In situ growth of ZnO nanoparticles in precursor-insensitive water-in-oil microemulsion as soft nanoreactors
© Bumajdad and Madkour; licensee Springer. 2015
Received: 21 November 2014
Accepted: 3 January 2015
Published: 28 January 2015
Zinc oxide (ZnO) nanostructures of uniform shapes and sizes (spherical, needle-like, and acicular) were directly synthesized using a relatively precursor-insensitive water-in-n-heptane microemulsion system stabilized by a mixture of cationic and non-ionic surfactants. With this colloidal system, the synthesized ZnO possesses the highest reported surface area (76 m2 g−1) among the published reports utilizing other microemulsion systems. Such precursor insensitivity allowed studying the effect of Zn precursor:precipitating agent molar ratio (as high as 1:8) on the particle size, specific surface area, porosity, and morphology of the synthesized nanoparticles. The interaction of the cationic surfactant head groups and their Br− counter ions with Zn2+ and OH− ions is believed to play a major role in controlling the ZnO characteristics. Due to such interactions, it is believed that the nucleation processes are retarded while the growth is more dominating if compared with other microemulsion systems.
Although it has been demonstrated that water-in-oil microemulsion (W/O μ) method is an efficient medium for the preparation of monodispersed functional oxide nanoparticles (NPs), only few studies employed such medium for the synthesis of ZnO NPs, all of which were using anionic or nonionic surfactants [1-4]. Due to the microemulsion destabilization, the maximum Zn precursor:precipitating agent molar ratio studied before was 1:2 or 1:4 for NaOH and NH4OH, respectively [1-4]. For example, it was found that the microemulsion destabilized upon using NaOH at 0.525 M , whereas in this study, a value of 0.8 M was tested with no obvious destabilization.
In this communication, the preparation of ZnO NPs of different shapes inside the dispersed nano-water droplets was investigated using the W/O μ system of composition (DDAB + Brij®35/n-heptane/water). Such microemulsion system was not employed before for the synthesis of ZnO NPs. Using small-angle neutron scattering technique and phase behavior studies, such surfactant mixture was found to produce a minimum droplet interaction and result in a much lower sensitivity toward precursor addition [5,6]. The minimum interaction insured spherical shape of the droplets and low microemulsion viscosity, which would enhance the dynamic nature of the microemulsion droplets. The low precursor insensitivity, however, enabled us to study the effect of varying the Zn precursor:precipitating agent molar ratio without destabilization of the microemulsion. Such effect was studied in details in an aqueous system , and this work aims to fill the literature gap for W/O μ system.
In a typical synthesis, two microemulsion (μ1 and μ2) systems were prepared from a mixture of water, n-heptane, and surfactants (90:10% molar ratio of the double-tailed cationic DDAB (didodecyldimethylammonium bromide):the single-tailed non-ionic Brij®35 (Sigma-Aldrich, St. Louis, MO, USA) at water-to-surfactant molar ratio of W = 18. The total surfactant concentration was 0.2 M. For μ1, the aqueous phase contains the precipitating agent, NaOH, with concentrations 0.1, 0.2, 0.4, and 0.8 M. For μ2, the aqueous phase contains the precursor, Zn(NO3)2, with a fixed concentration of 0.1 M. In a typical procedure, equal volumes of the two microemulsion systems μ1 and μ2 were mixed, giving a transparent microemulsion. Afterwards, the mixture was refluxed 16 h at T = 60°C which results in a turbid solution. The mixture then is centrifuged, and the precipitate (designated hereafter as Z1, Z2, Z4, and Z8 for 0.1, 0.2, 0.4, and 0.8 M NaOH, respectively) was washed several times with a mixture of acetone and water and then dried at 110°C overnight.
Thermogravimetric analysis (TGA) was performed on 10- to 15-mg portion of test materials using a Shimadzu TGA-50 thermogravimetric analyzer (Shimadzu Scientific Instruments, Kyoto, Japan) under nitrogen atmosphere in the temperature range 20°C to 800°C with a heating rate of 10°C min−1. X-ray diffraction (XRD) measurements were conducted by using Siemens D-5000 (Siemens AG, Munich, Germany) with copper target and nickel filter with CuKα radiation (λ = 0.154056 nm). The morphology of the ZnO NPs was obtained by transmission electron microscopy (TEM) using a JEOL JEM 1230 (JEOL Ltd., Tokyo, Japan) operating at 120 KV. The powders were dispersed by ultrasonication in suitable solvent for 3 min before deposition on the TEM grid. Brunauer-Emmett-Teller (BET) surface area was calculated using a model ASAP 2010 automatic Micromeritics sorptiometer (Micromeritics Instrument Corporation, Norcross, GA, USA) equipped with an outgassing platform. X-ray photoelectron spectroscopy (XPS) was conducted using a model Thermo ESCA Lab 250Xi (Thermo Fisher Scientific Inc., MA, USA) equipped with MgKα radiation (1,253 eV) and operated at 23 kV and 13 mA.
Results and discussion
The process of nucleation and growth of nanoparticles inside the W/O μ droplets starts with droplet collision, coalescence, and then exchange of their contents . This exchange is too rapid and precipitation reaction occurs inside the nanodroplets, which is followed by nucleation, growth, and coagulation of the primary particles, resulting in the formation of the final nanoparticles. The cationic head group of surfactant is believed to play a role in the formation and aggregation behavior of the nanoparticles. The cationic head group attracts the hydroxyl ions and forces the Zn+2 to stay at the droplet center. Also, the relatively large Br− ions are expected to have higher binding tendency to Zn2+ over that of the small OH− ions (see, for example,  for micellar systems and  for microemulsion system). Since the ZnBr2 solubility is very high (447 g per 100 ml of H2O at 20°C ) and the Br− ion concentration in the studied system was calculated to be 2.9 g per 100 g of H2O which is much less than the solubility limit, hence, ZnBr2 precipitation is not expected. As a proof of this conclusion, neither the bulk XRD nor the surface-sensitive XPS results (see later) show any indication of the formation of ZnBr2 crystal or the presence of Br− ions, which means that the high solubility and the washing procedure resulted in only ZnO/Zn(OH)2. There is also the possibility of forming soluble complexes between Zn2+ and Br− , but our results show no indication of ZnO NP contamination with such complexes. Upon increasing the number of OH− ions, the aggregation behavior orients itself from the spherical shape to the elongated shape. At high pH, ZnO carries negative charges, and hence, the cationic surfactant will preferentially adsorb on the nanoparticles and present constrain on the growth direction and hence on the shape. Such constrain is absent when anionic or non-ionic surfactants are used [1-4]. It is worth mentioning here that beside the phase behavior change (minimum droplet interaction and lower precursor sensitivity), the presence of small amount of non-ionic surfactants (10 mol%) with the majority of cationic surfactants (90 mol%) is expected to lower the polarity of the surfactant films, and hence, a slightly milder interaction with the ions is expected.
Specific surface area, S BET , BJH average pore diameter and volume, D p and V p , crystallite size, l , and particle size, D
S BET (m 2 g −1 )
D p (nm)
V p (cm 3 g −1 )
L (nm) a
In summary, well-organized monophasic and monodispersed ZnO NPs with high surface area and porosity (Table 1) and different morphologies (Figure 3) were synthesized using the W/O μ system of relative insensitivity toward precursor addition. This insensitivity is established by replacing 10 mol% of the cationic DDAB surfactant films by the non-ionic Brij®35 surfactants [5,6]. In this short communication, particular focus was given to the role of different microemulsion constituents. Polarizability and electron charge density (which is a function of the ion size) are believed to control the initial interaction between the Zn+2 and Br− and/or OH− ions, and hence, they control also the nucleation process. In spite of the retardation to the nucleation, the surface area of the ZnO NPs was found to be high and this is not due to the NP sizes but due to the relatively high porosity.
The authors gratefully acknowledge the support of the research administration of Kuwait University project nos. SC08/09, GS03/01, GS02/08, and GS01/01 and the Nanoscopy Science Center.
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