Structure of reverse microemulsion-templated metal hexacyanoferrate nanoparticles
- Alberto Gutiérrez-Becerra†1,
- Maximiliano Barcena-Soto†1,
- Víctor Soto†1,
- Jesús Arellano-Ceja†1,
- Norberto Casillas†1,
- Sylvain Prévost†2,
- Laurence Noirez†3,
- Michael Gradzielski†2 and
- José I Escalante1Email author
© Gutiérrez-Becerra et al; licensee Springer. 2012
Received: 3 October 2011
Accepted: 20 January 2012
Published: 20 January 2012
The droplet phase of a reverse microemulsion formed by the surfactant cetyltrimethylammonium ferrocyanide was used as a matrix to synthesize nanoparticles of nickel hexacyanoferrate by adding just a solution of NiCl2 to the microemulsion media. Dynamic light scattering and small-angle neutron scattering measurements show that the reverse microemulsion droplets employed have a globular structure, with sizes that depend on water content. Transmission electron microscopy and electron diffraction are used to obtain information about the structure of the synthesized nanoparticles. The results show that the size and shape of the coordination compound nanoparticles correspond with the size and shape of the droplets, suggesting that the presented system constitutes an alternative method of the synthesis of metal hexacyanoferrate nanoparticles.
Keywordsreverse micelles template method nanoparticles nickel hexacyanoferrate
Microemulsions represent thermodynamically stable liquid dispersions containing surfactant aggregates, which can often be found in a large region of the phase diagram of two- or multicomponent surfactant systems [1, 2]. They exhibit a well-defined structure that is characterized by a typical correlation length in the nanometer scale. Such microemulsions are of special interest because a variety of reactants can be introduced into the nanometer-sized aqueous domains, leading to materials with controlled size and shape [3–9]. Such characteristics play pivotal roles in controlling the physical, chemical, optical, and electronic properties of these nanomaterials. In the past few years, significant research has been conducted in the reverse microemulsion-mediated synthesis of inorganic (metal halides, selenides, or sulfides) and organic (cholesterol, rhodiarome, rhovanil, nimesulide, etc.) nanoparticles [3–13], and there has been substantial progress in the characterization of microemulsions of various types .
The synthesis of nanoparticles by reverse microemulsions is viable and attractive because it does not only produce nanoparticles that have a narrow size distribution, but also the particle size can be controlled by varying the microemulsion composition . The reaction in a microemulsion may be conducted in two modes: (1) a multiple microemulsion method, where two or more microemulsions, each containing one reactant, are mixed together . Upon mixing, the droplets collide with one another as a result of the Brownian motion. These collisions lead to the formation of product monomers [7, 17, 18]. Nucleation takes place in a given droplet when the number of product monomers exceeds the critical nucleation number [19–21]. Further collisions between a droplet carrying a nucleus and another one carrying the product monomers cause the growth of the nucleus [19, 22]; (2) in the simple addition type, the reducing or precipitating reagent is directly added to the microemulsion containing the other reactant [23, 24], i.e., this mode promotes intramicellar nucleation and growth [22, 25]. When particles are formed in single microemulsions, their size and polydispersity are controlled by one or more of the following mechanisms: reaction kinetics, intramicellar nucleation and growth, intermicellar nucleation and growth, and particle aggregation [26, 27]. A variation of this synthetic path could proceed by replacing the counterion of the surfactant, and only then the addition of a salt to this reverse microemulsion media. This last method has been successfully used to synthesize nanoparticles using the anionic surfactant AOT , for instance, for the case of cobalt ferrocyanide salt nanoparticles . However, to the best of our knowledge, there is no report on the modification of cationic surfactants with ionic coordination compounds such as the cetyltrimethylammonium ferrocyanide [CTAFeII].
Some advantages of this novel cationic surfactant are readily apparent; for instance, inverse microemulsion formed with this surfactant will allow synthesizing different transition metal hexacyanoferrates [Mhcf] by simply adding different salts to the microemulsion media, i.e., with the same surfactant, it is possible to produce different nanoparticles of coordination compounds (MIIhcf or MIIIhcf). Such compounds and other Prussian blue analogues have been a subject of several studies because of their promising characteristics which include electrochromism, the ability to mediate (electrocatalyze) redox reactions, ionic and electronic (mixed valence electron hopping) conductivities, capability for storage of countercations, and molecular magnetism [30–32].
According to this motivation, in this paper, we studied the formation of a novel type of ferrocyanide-containing cationic surfactant and its ability to form reverse microemulsions. In this work, we use as surfactant a mixture of cetyltrimethylammonium bromide [CTAB] (95 wt.%) and CTAFeII (5 wt.%). The latter was prepared by replacing the bromide (Br-) ions of the cationic surfactant CTAB with ferrocyanide ([Fe(CN)6]4-) ions following a direct metathesis reaction in an aqueous phase . This new surfactant, CTAFeII, presents a very limited area for a microemulsion phase, so when the mixture of surfactants was used, we reach a more extended region of the microemulsion. This can be explained taking into account the interfacial stiffness caused by the bulky molecules of CTAFeII (a huge counterion and four aliphatic chains). However, by adding CTAB molecules, it was possible to obtain an improvement in the interface flexibility. In addition, by changing the surfactant ratio of the mixture, it was found that the system offers better results to the nanoparticle synthesis when a low concentration of CTAFeII (5 wt.%) was used in the surfactant mixture. Furthermore, López-Quintela established that smaller nanoparticles can be obtained in microemulsions when there is a significant difference in the concentrations of the reactants .
Materials and methods
All the reactants used in this report were of analytical grade. CTAB was purchased from Sigma-Aldrich Corporation (99%; St. Louis, MO, USA), ferrocyanide salt (K4[Fe(CN)6]3H2O), from J.T. Baker (99%; Deventer, The Netherlands), n-hexane (C6H14) and NiCl26H2O, from Caledon Laboratories Ltd. (98% and 99%, respectively; Halton Hills, Canada), n-butanol (C4H9OH), from Productos Químicos Monterrey (99%; Monterrey, Nuevo León, Mexico), and double distilled water, from Selectropura S.A. de C.V. (σ = 1.5 to 3 μS/cm; Guadalajara, Jalisco, Mexico). For neutron scattering experiments, D2O (99.9% D; Euriso-Top, Gif-sur-Yvette, France) was used instead of H2O to increase the contrast and lower the background.
Synthesis of modified surfactant
The surfactant CTAFeII was prepared by a direct metathesis reaction in an aqueous phase. The detailed procedure is described in the study of Gutierrez-Bercerra et al. . Functional groups of CTAB and CTAFeII were identified by a Fourier transform infrared [FTIR] spectrometer (Spectrum One, PerkinElmer, Waltham, MA, USA). Infrared spectra were recorded in the 400- to 4,000-cm-1 region, with a resolution of 4.00 cm-1.
The pseudo-ternary phase diagram for the CTAB + CTAFeII + n-butanol/n-hexane/water system was constructed, considering as surfactant a mixture of CTAB, CTAFeII, and n-butanol, using W CTAB/W CTAFeII ratios of 0.95:0.05 and the (W CTAB + W CTAFeII)/W but ratio of 1, where W CTAB, W CTAFeII, and W but are the weights of CTAB, CTAFeII, and n- butanol, respectively. A simple titration technique was used to construct the diagram. Microemulsions were prepared by mixing weighed appropriate amounts of the individual components. The amount of n-hexane (W hex) in the surfactant mixture determines the H value (H = [W CTAB + W CTAFeII + W but]/[W hex + W CTAB + W CTAFeII + W but]), while W w = W water /(W water + W hex + W CTAB + W CTAFeII + W but) represents the weight fraction of water used as the titration component. Water was added in small volumes under permanent stirring in a tightly closed vial to avoid evaporation. Then, the vials were placed in a thermostatic bath (25°C) until a homogeneous media is reached.
Dynamic light scattering
Dynamic light scattering [DLS] measurements were performed using an ALV/CGS-3 goniometer with an ALV/LSE-5004 multiple tau digital correlator (ALV-Laser Vertriebsgesellschaft m-b.H., Langen, Germany). The light source was an He-Ne laser operating at a wavelength of 633 nm. The homodyne intensity autocorrelation function g (2)(τ) was measured at 90°. Data analysis was performed with the normalized intensity autocorrelation function using a third-order cumulant fit  that yielded as key parameter the effective collective diffusion coefficient.
Small-angle neutron scattering
Small-angle neutron scattering [SANS] measurements were done on the instrument PAXY at Laboratoire Léon Brillouin, Gif-sur-Yvette, France. A wavelength of 0.5 nm (FWHM 10%) was selected, and two configurations were used with sample-to-detector distances of 1.25 and 5.05 m.
Synthesis of nickel hexacyanoferrate
The synthesis of nickel hexacyanoferrate [Nihcf] nanoparticles was carried out at H = 0.4 and W w = 0.09. Appropriated amounts of CTAFeII, CTAB, n-butanol, and hexane were mixed until an H value of 0.4 was reached, and then it was maintained under stirring. After that, as an aqueous phase, a solution of 5 mM NiCl2 was added to the mixture to reach W w = 0.09. The microemulsion formed was stable for several days and at the same time maintaining a transparent state. Nihcf nanoparticles were separated from the microemulsion media by centrifugation at 9,000 rpm for 10 min. The precipitate was then washed several times with acetone. Despite the washing process, a small quantity of CTAB remained mixed with the nanoparticles. To obtain transmission electron microscopy [TEM] micrographs (JEM-1010, JEOL de Mexico S.A. de C.V., Mexico City, Mexico), a drop of the nanoparticles dispersed in acetone was placed directly on a carbon-coated copper grid. X-ray diffraction [XRD] patterns were recorded with a STOE Theta/theta X-ray diffractometer (STOE & Cie GmbH, Darmstadt, Germany) using a CuKα (λ = 0.15406 nm) at room temperature. FTIR spectra of the Nihcf were carried out in a PerkinElmer Spectrum One spectrometer.
Results and discussion
D eff and R h depend on the H values
Deff × 10-10 (m2/s)
Deff × 10-10 (m2/s)
In addition, a correlation peak is visible that becomes much more prominent with increasing water content in the reverse microemulsion, and at the same time, its maximum moves from 1.4 to 0.85 nm-1 (for a fixed H of 0.5). This, together with the intensity increase, shows that the aggregates grow substantially in size with increasing water content, where, however, it should be noted that in SANS basically, only the D2O core is visible as an aggregate due to the strong contrast between the two isotopes H and D.
The pronounced correlation peak has to be due to steric interactions between the reverse aggregates as electrostatic interactions in the oil-continuous medium should be negligible, but of course, at the concentrations employed, the volume fractions of the amphiphilic material (CTAB, CTAFeII, n-butanol) plus D2O are in the range of 34% to 59% v/v and therefore high enough to explain effective repulsion already on the basis of purely steric interactions. In addition, it is well known that in reverse microemulsions, the solvent oil molecules are to a certain extent bound to the reverse microemulsion aggregates [42, 43], thereby enhancing the effective volume fraction further.
The first analysis of the SANS data can be performed using the peak position q Peak to determine the number density 1 N of aggregates assuming simple cubic packing: 2π /q peak = 1N-1/3. Then, it can be recalled that 1 N can be expressed with the volume fraction Φ of the aggregated material and the volume of one aggregate V: N = Φ/V. Assuming spheres, a radius can be deduced. Depending on the choice of material to consider, either only D2O or the whole aggregated material (D2O, n-butanol, surfactant), two values are found that can be considered as values for the core (neglecting dissolved butanol) and for the entire droplet (neglecting the contribution from oil swelling the aliphatic chains of the surfactant); the core radius varies from 0.8 to 3.1 nm and is proportional to the water content of the microemulsion. The difference between the core and the overall radius is in all cases around 1.1 nm (1.03 to 1.25 nm), a reasonable value for the surfactant acting as a shell; taking into account the solvation of this shell by hexane, a higher value would be reached; using Tanford's length, the stretched C15 chain is 2.05 nm; the typically retained value of 75% to 80% of this elongation corresponds to lengths of 1.54 to 1.64 nm; the radius of the tetramethylammonium head group is 0.285 nm; the overall thickness expected for the swollen shell would then be 2.11 to 2.21 nm. However, notice that the R h, obtained by DLS and SANS, increases roughly linear with the water content as typically observed for reverse microemulsion droplets [44–46].
Two-dimensional data were reduced using BerSANS accounting for dead time, transmission, and background scattering assimilated to the empty cuvette (which means that the incoherent scattering in the spectra still contains contributions from all compounds in the samples including the solvent), and the scattering from H2O in a 1-mm cuvette was used to account for the detector pixel efficiency and solid angle variations. Absolute scale was deduced from the evaluation of the direct beam flux. As all corrected scattering patterns are isotropic, they were finally radial-averaged, and data from two configurations were merged.
Fit parameters for scattering curves
SLD × 10-9 (cm-2)
Radius of the microemulsion droplets
Synthesis of nanoparticles
An advantage identified in this work for the synthesis of coordination compound nanoparticles is that with this system, different transition Mhcf can be obtained only by varying the transition metal (copper(II), cobalt(II), iron(III), etc.) in the aqueous phase. This constitutes an alternative method using cationic, modified surfactants in reverse microemulsion for the synthesis of this type of nanoparticles.
In this work, the preparation of nanoparticles of transition Mhcf with a homogeneous size was performed using a simple process in which a droplet is regarded as a nanoreactor. Such soft technique provides good crystallinity in the absence of high temperature and pressure requirements, which favors the formation of small nanoparticles with controlled size and size distribution. Furthermore, it was found that the nanostructure of the particles obtained seems to be related to the structure of the template involved, namely the spherical water pool, at the conditions mentioned in this work.
The ratio of water to surfactant concentration plays an important role in determining the interaction of the water pool with the surfactant or bulk water. Hence, the size of the reverse microemulsion droplets increases as the water pool increases and vice versa. By varying the amount of water content, change in the size of the droplet formed is possible.
Furthermore, using a modified form of the surfactant CTAB (CTAFeII), it was possible to introduce a metal complex ion directly into a reverse microemulsion system without adding a salt as a further component. This procedure allows synthesizing, in a simple way, nanoparticles that correspond in size and shape to the microemulsion droplet morphology. In summary, these experiments demonstrate the feasibility of producing Nihcf nanoparticles using the surfactant CTAFeII.
This research was supported by the Mexican National Council of Science and Technology (CONACyT) and the Deutscher Akademischer Austauschdienst (DAAD, project: D/07/10253) through PROALMEX 2008-2010. The authors thank the CCMC-UNAM for performing the TEM experiments. The Laboratoire Léon Brillouin (LLB) of Saclay, France, is gratefully acknowledged for providing the SANS beamtime.
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