ЖУРНАЛ ФИЗИЧЕСКОЙ ХИМИИ, 2012, том 86, № 5, с. 981-983
КРАТКИЕ СООБЩЕНИЯ =
УДК 548.56
SOLVENT EFFECT ON THE SIZE OF PLATINUM NANOPARTICLE SYNTHESIZED IN MICROEMULSION SYSTEMS
© 2012 Alireza Salabat, Mina Rahmati Far
Department of Chemistry, Arak University, Arak, 38156-8-8349, Iran E-mail: a-salabat@araku.ac.ir Received May 01, 2011
Abstract — In this research work, the efect of solvent on the paltinume nanoparticle size which synthesised by microemulsion method was investigated. Platinum nanoparticles have been prepared by the reduction of H2PtCl6 with hydrazine in water-in-oil (w/o) microemlsions consisting of sodium bis (2-ethylhexyl) sulpho-succinate (AOT) and solvents n-hexane, cyclohexane and n-nonane. The size of the platinum nanoparticles was measured using transmission electron microscope (TEM). It was verified that, reducing of H2PtCl6 by hydrazine in microemulsion system with different organic solvents, turn to arrangement nanoparticle size as follows: n-nonane> cyclohexane> n-hexane.
Keywords: nanoparticles, platinum nanoparticles, microemulsion, AOT, hexachloroplatinic acid.
INTRODUCTION
Nanoparticle synthesis within the aqueous cores of water-in-oil microemulsion systems is a viable method, which allows control over the size and shape of the particles. Microemulsion systems are isotropic, mac-roscopically homogeneous, and thermodynamically stable solutions containing at least three components, namely a polar phase (usually water), a non-polar phase (usually oil), and a surfactant. On a microscopic level the surfactant molecules form an interfacial film separating the polar and non-polar domains. This interfacial layer forms different microstructures ranging from droplets of oil dispersed in a continuous water phase (o/w-microemulsion) over a bicontinuous "sponge" phase to water droplets dispersed in a continuous oil phase (w/o-microemulsion). The latter can be used as nano-reactors for the synthesis of metal nanoparticles leading to nanoparticles with a low polydispersity [1—5]. The size of the aqueous droplets in a water-in-oil microemulsion can be varied by the choice of the solvent [6], and so affecting the size of the nanoparticles obtained, particularly in anionic microemulsion systems.
Considerable research has been done in order to better understand the factors controlling the synthesis of nanomaterials through aqueous phase reduction reactions within the reverse micelle core, particularly stability and the intermicellar exchange of the contents [7—9]. The intermicellar exchange rate may be affected by the bulk solvent type, the contents dissolved within the core, and the size of the reverse micelle or the water content, referred to as the lvalue, where Wis the molar ratio of the water to surfactant concentrations. Fletcher and co-workers [10] presented a comprehensive study on the water/alkane/AOT
system where the rate of micellar exchange kex was measured as a function of W, the AOT concentration, temperature, and the bulk solvent type. The results from their study reveal that kex decreases with increasing water content for W = 10, 15, 20, and 30; kex is independent of AOT and reactant concentration; kex increases with an increase in temperature; and kex increases slightly with carbon number for pentane through dodecane in addition to a significant decrease in kex for the corresponding cyclic compounds, particularly cyclohexane. The dependency of kex on the bulk solvent can be explained by the solvent interactions with the AOT tails where a more favorable interaction will allow for the solvent to insert itself within the surfactant tails, thus creating a more rigid micelle. Binks et al. [11] discuss the extent of oil penetration into the surfactant chain region, showing an increase in micelle rigidity with decreasing solvent chain length from C14 to C7.
In the present work we have synthesized platinum nanoparticles in w/o microemulsion system. The effect of solvent («-hexane, cyclohexane and «-nonane) on the size of the synthesized platinum nanoparticles was investigated. These nanoparticles are very important as a catalist, so catalyses a range of reactions, including the evolution of hydrogen, reduction of oxygen, oxidation of hydrogen and methanol and hydrogenation reactions [12—14].
EXPERIMENTAL
Materials. The anionic surfactant sodium bis(2-ethylhexyl) sulphosuccinate (AOT) was purchased from Acros organic co. and used without further puri-
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ALIREZA SALABAT, MINA RAHMATI FAR
Fig. 1. Model for the micelles served as a nanoreactor for fabricating of platinum nanoparticles; (1) collision and exchange, (2) reaction , (3) aggregation, (4) growth.
fication. Hexachloroplatinic acid hexahydrate (H2PtCl6 • 6H2O), hydrazine hydrate (N2H4 • H2O), n-hexane, cyclohexane and n-nonane were purchased from Merck.
Method. Microemulsions were prepared by mixing the same volume of aqueous solution of H2PtCl6 (0.01 M) and hydrazine hydrate (0.05M) to the 0.12 M AOT/n-hexane, AOT/n-nonane or AOT/cyclohexane solution. The water-to-AOT molar ratio, W, was kept the same in all cases and was equal to 5. The microemulsion containing hydrazine hydrate was added into the microemulsion containing H2PtCl6 drop by drop. After the microemulsion was added, the vigorous magnetic stirring was maintained for 2 h. Morphology and size of the platinum nanoparticles was determined by transmission electron microscopy (Philips CM 200) operating at 200 kV. Samples for the electron microscopy were prepared by placing a drop of the microemulsion containing the platinum nanoparticles onto a carbon-copper grid and allowing the evaporation of the organic solvent at room temperature.
RESULTS AND DISCUSSION
After mixing of two microemulsions, the interchange of the reactants takes place during the collisions of the water droplets. For the formation of platinum nanoparticles, a model can be given to illustrate the template role of the reverse micelles in the AOT microemulsions for fabricating nanoparticles. This model is illestrated in Fig. 1 schematically. At the beginning of the mechanisem, the attractive van der Waals forces and the repulsive osmotic and elastic forces between reverse micelles lead to collisions of micelles and result in exchange the compounds (H2PtCl6 and N2H4) solubilized within the water cores in two different reverse micelles [15]. In result, the initial monomeric platinum nuclei will form and further grow. Owing to the fast exchange between the water cores, the initially formed platinum nuclei grow to reach a certain size, which corresponds to the thermo-dynamically best stabilized species in the presence of microemulsion.
The process of the exchange of solubilizates and subsequent reaction can be followed by the five main steps [16]: (1) Brownian diffusion of reverse micelles leading to collisions; (2) surfactant layer opening through effective collisions and a coalescence of the micelles happening; (3) diffusion of solubilizate molecules resulting in exchange of materials between micelles; (4) reaction among solubilizate molecules resulting in formation of metallic nuclei; (5) the aggregates splitting to return as reverse micelles. The aggregation step is essential for grwing the nanoparticles and depend on interaction between microemulsion droplets [16].
Figure 1 shows TEM micrographs of platinum nanoparticles in microemulsion systems containing H2PtCl6 as platinum source and n-hexane, cyclohexane and n-nonane solvents. As can be seen from the micrographs, the nanoparticles size with n-hexane as solvent is less than cyclohexane and it is less than n-nonane. The reason for this phenomenon can be related to the interaction of the microemulsion droplets in different solvents, which is related to the solvent interactions with the AOT tails. The extent of solvent penetration into the surfactant chain region creates more regid micelle and then decreasing the kex. Obviously, the solvent penetration decrease with increasing the chain length of the solvents from n-hexane to n-nonane. Then, the repulsive interaction in the systems with n-hexane or cyclohexane solvents, which have rigid microemulsion droplets, is more than systems containing n-nonane. It means that the attractive interaction or aggregation step for systems containing nonane is domain, and then produces greater nano-particle size. On the othe hand, comparing the n-hexane and "cyclohexane solvents, it can be said that n-hexane has more penetration into the surfactant chain region, because of its shape and volume. It causes more rigidity of microemulsion droplets and then more repulsive interaction between microemulsion droplets and decreasing of particle size.
Previous investigations by other researchers on the effects of the bulk alkane solvent on the AOT reverse micelle system have produced results that support
SOLVENT EFFECT ON THE SIZE OF PLATINUM NANOPARTICLE SYNTHESIZED
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Fig. 2. TEM micrograph of platinum nanoparticles synthesized in microemulsion systems containing (a) «-hexane, (b) cyclohex-ane, and (c) «-nonane solvents.
these findings [10, 11, 16]. Considerable work has been done to gain a better understanding of CdS nanocrystal growth where Towey states that particle formation is controlled to an extent by the nature of the continuous phase, where the growth rate increases from cyclohexane to «-heptane to «-decane [17]. Bag-we demonstrated with the production of silver and silver chloride particles that an increased intermicellar exchange rate has the effect to increase the growth rate and more importantly decrease the final particle size [16]. These results are explained by the solvent—tail interactions or more generally, a decrease in the rigidity of the micelle.
CONCLUSION
This research demonstrates that, for the synthesis of platinum nanoparticles within the AOT reverse micelle system, the bulck organic phase influences the particle size. It was found that, reducing of H2PtCl6 by hydrazine in microem
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