Formation of Ruthenium Nanoparticles by the Mixing of Two Reactive

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Formation of Ruthenium Nanoparticles by the Mixing of Two Reactive Microemulsions Sachin U. Nandanwar, Mousumi Chakraborty,* and Z. V. P. Murthy Department of Chemical Engineering, S.V. National Institute of Technology, Surat-395 007, Gujarat, India ABSTRACT: In this study, two reactants (ruthenium chloride and sodium borohydrate) were premicellized in two separate microemulsions and brought into contact through intermicellar exchange to conduct the reaction. As a result, ruthenium nanoparticles were formed. The overall reaction rate was governed by the intermicellar exchange rate. Particle size was controlled by varying surfactant concentration, water-to-surfactant molar ratio (ω), precursor (ruthenium chloride) concentration, and molar ratio of reducing agent-to-reagent (R). Dynamic light scattering and transmission electron microscopy were used to determine the size, size distribution, and structure of the synthesized ruthenium nanoparticles. The molar ratio ω was varied from 3 to 7; sizes of the particles were found to be in the range of 17.08 25.09 nm. The precursor (ruthenium chloride) concentration was varied in the range of 0.1 0.3 M; particle size was observed to decrease up to 0.2 M then increase due to particle agglomeration at higher precursor concentrations. Smaller nanoparticles were obtained at higher R values due to faster intramicellar nucleation and growth rate. Dispersion destabilization of colloidal ruthenium nanoparticles was detected by Turbiscan.

’ INTRODUCTION Transition metal nanoparticles synthesis has been extensively investigated in recent years because of its many unique characteristics in physical (electronic, magnetic, mechanical, and optical) and chemical properties. Ruthenium (Ru) has various valencies (0 8 valence) and is not such an expensive transition metal; therefore, various useful catalytic reactions for organic synthesis have been explored. In recent years, the synthesis of Ru nanoparticles have become of great interest from a scientific as well as an industrial point of view, due to its huge application in catalysis. Ruthenium has prospects as a catalyst for many applications in such areas as fuel cell,1,2 methanol oxidation,3 and electrochemical capacitor,4 etc., which are highly size specific. Ru nanoparticles were synthesized by a chemical reduction method and stabilized using protective agents,5 9 organometallic synthesis,10 refluxing a polyol solution,11 sonochemical reduction,12 thermal decomposition of Ru3(CO)12,13 solvothermal14 and microwave assisted reduction,15,16 and also by microemulsion techniques.1,2 Many methods have been exploited for synthesis of ruthenium nanoparticles in the past few years, wherein a chemical reaction in a microemulsion is one of the most important methods for synthesis of monodisperse nanoparticles.17 The microemulsion technique was used to produce nanoparticles of several metals like iron, platinum, cadmium, palladium, silver, copper, nickel, and gold (I. Capek).18 However, the synthesis of ruthenium nanoparticles is scarcely reported, despite the important technological role of ruthenium. Xiong et al. in 20051 prepared Pt Ru/C catalysts by a reverse microemulsion (reverse micellar) method using sodium bis(2-ethylhexyl) sulfosuccinate (AOT) as the surfactant and heptane as the oil phase. They investigated the effect of different parameters like pH values of the NaBH4 solution, water to AOT molar ratio on particle size and its distribution, crystallinity, and microstructure of nanoparticles. Rojas et al.2 prepared r 2011 American Chemical Society

carbon-supported platinum and platinum ruthenium electrocatalyst by the microemulsion technique. They studied the influence of parameters such as the preparation route, the metal loading, and the PtRu stoichiometry on the morphology of the final nanoparticles. Microemulsions are isotropic, macroscopically homogeneous, and thermodynamically stable solutions containing at least three components, namely a polar phase (usually water), a nonpolar phase (usually oil), and a surfactant. The two basic types of microemulsions are direct oil-in-water (oil dispersed in water, o/w) and reversed water-in-oil (water dispersed in oil, w/o), where the second solution is the dispersion medium or solvent.19 To synthesize nanoparticles, two microemulsions of equal structure need to be mixed. One of these microemulsions contains the metal precursor while the other contains the reducing agent. The micelles undergo numerous collisions, and thereby the reactants are exchanged, mixed, and react to form the product. The synthesis of nanoparticles by reverse micelles is viable and attractive because not only does it produce nanoparticles that have a narrow size distribution but also the particle sizes can be easily controlled by varying the microemulsion composition.20 The aim of the present article was to synthesize highly monodisperse ruthenium nanoparticles in a w/o microemulsion system at room temperature. The size and morphological properties of the ruthenium nanoparticles were controlled by surfactant concentration, water-to-surfactant molar ratio (ω), concentration of the precursor, and reducing agent-to-ruthenium trichloride molar ratio (R). The prepared nanoparticles were characterized by dynamic light scattering (DLS) and transmission electron microscope (TEM) images. Turbiscan was used to Received: May 16, 2011 Accepted: August 20, 2011 Revised: June 27, 2011 Published: August 20, 2011 11445

dx.doi.org/10.1021/ie201043v | Ind. Eng. Chem. Res. 2011, 50, 11445–11451

Industrial & Engineering Chemistry Research monitor back scattering profiles of colloidal ruthenium nanoparticles and dispersions in the kinetic studies of their stability.

Figure 1. DLS size distribution histograms of water droplets for the pure microemulsion [micoemulsion-I, RuCl3 water/cyclohexane/ Triton X-100; micoemulsion-II, NaBH4 water/cyclohexane/Triton X-100 at ω = 5; and microemulsion-III, mixture of microemulsion-I + microemulsion-II].

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’ EXPERIMENTAL PROCEDURE Materials. Ruthenium trichloride (RuCl3.nH2O, Ru content g 37%) and cyclohexane (continuous oil phase) were purchased from Finar chemicals, India. The nonionic surfactant polyoxyethylene octyl phenyl ether (Triton X-100) and reducing agent sodium borohydride (NaBH4, 95%) were purchased from S.D. Fine Chemicals, India. All the chemicals used were of analytical grade without further purification. Distilled water of pH 5.9 ( 0.2, conductivity 1.0 μS/cm (Millipore, Elix, India), was used throughout the experiments for preparing all the aqueous solutions. Synthesis of Ruthenium Nanoparticles. High purity oil phase, cyclohexane, and nonionic surfactant, Triton X-100, were used for the preparation of water-in-oil (w/o) microemulsion. Microemulsion-I was prepared by mixing an aqueous solution of RuCl3 in cyclohexane-Triton X-100 mixtures. Uniform stirring was maintained with ultraturax T25 high-speed mechanical stirrer (Ultraturax IKA WERKE, GmBH & Co. KG) at 6500 rpm for 5 min at room temperature for proper mixing. Both metal ions and surfactant concentration in the microemulsion were initially maintained at 0.2 M. The volume content of the aqueous phase was varied to achieve a water-to-surfactant (ω) value equal to 3. Higher ω was achieved by adding an

Figure 2. BS profile of the microemulsion ω = 5. 11446

dx.doi.org/10.1021/ie201043v |Ind. Eng. Chem. Res. 2011, 50, 11445–11451

Industrial & Engineering Chemistry Research

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Figure 3. Effect of surfactant concentration on average particle size.

additional amount of aqueous solution as required. Similarly, microemulsion-II of the same ω value was prepared simply by replacing the solution of RuCl3 by that of NaBH4 (0.5 M) solution. The two microemulsions were mixed to produce microemulsion-III containing Ru nanoparticles and were used for further characterization. Characterization. The sizes and size distribution of microemulsions and nanoparticles were measured by dynamic light scattering (DLS; Malvern Zetasizer, Nano ZS 90, U.K.). Optical properties of nanoparticles will provide a convenient way to detect particle size. TEM images were obtained with a Philips Tecnai-20, which at 200 kV provides 0.27 nm point resolution. TEM was prepared by placing one droplet of nanoparticle sample on a carbon film of a copper grid having a mesh size of 300 and a diameter of 3 nm, covered with perforated carbon, followed by evaporation of the solvent at atmosphere. The measurements were repeated using freshly prepared sample to obtain reproducible results. Nanoparticle stability was analyzed using transmission and back scattering profiles, scanning the colloidal sample by light rays of 880 nm wavelength using Turbiscan classic MA 2000 (Formulaction, France).

’ RESULTS AND DISCUSSION Formation Mechanism of Ruthenium Nanoparticles in Microemulsion. The nonionic surfactant polyoxyethylene octyl

phenyl ether (Triton X-100) has a particular structure which favors an interface curved on the water core to form reverse micelles. Figure 1 shows the DLS size distribution for water droplets of the microemulsion-I system (RuCl3-water/cyclohexane/Triton X-100) at ω = 5. It was found that droplets were narrow in size distribution, and the average size was found to be 18.55 nm. Figure 1 also shows a size distribution of water droplets of the microemulsion-II system (NaBH4-water/cyclohexane/Triton X-100) at the same water-to-surfactant ratio (ω = 5). It was observed that microemulsion-II had a slightly broader droplets size distribution than microemulsion-I, and the average size was found to be 33.33 nm. Two microemulsions (I and II) were gradually mixed. The similarity of droplet sizes of both the microemulsions was favorable from the point of view of synthesis, as synthesis was

Figure 4. DLS size distribution histograms of ruthenium nanoparticles obtained with different water-to-surfactant molar ratio (ω).

preferable with two microemulsions of the same water droplet size.21 Brownian motion of the micelles leads to intermicellar collisions and sufficiently energetic collisions lead to a mixing of micelle contents. Reaction occurs when micelles undergo such fusion, which is followed by fission of the fused mass back to two micelles. Microemulsion-III represented the DLS size distribution of ruthenium nanoparticles which resulted after the reaction between two micelles. The hydrodynamic diameter of the colloidal ruthenium nanoparticles was found to be 22.46 nm, which is much less than the expected average water droplet diameters of microemulsion-III. The polydispersity index values were all close to 0.1 indicating that the synthesized ruthenium nanoparticles are fairly monodisperse.22 Similar results were observed by Solanki and Murthy23 during the synthesis of nanosilver colloidal particles using a w/o microemulsion. Actually when a growing nanoparticle approaches the diameter of a microemulsion drop, the headgroup of surfactant molecules surrounding the drop can physically adsorb on to the nanoparticle surface, and thus the nanoparticle can become strongly encapsulated in the drop in comparison with the particle diameter, which was much less than the drop diameter.24 Before mixing, microemulsion-I and microemulsion-II were scanned in Turbiscan. It was observed that the (%) backscattering (BS) value of microemulsion-I (Figure 2a) was very low because of the smaller size of water droplets formed, which was 11447



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Figure 5. TEM images of Ru nanoparticles prepared with different water-to-surfactant molar ratio: (a) ω = 3; (b) ω = 5; (c) ω = 7.

confirmed by DLS (Figure 1). On the hand, microemulsion-II showed higher % BS as it contained larger water droplets than the inner phase (Figure 2b). Natarajan et al.25 suggested fusion fission mechanisms for the intermicellar exchange process, which leads to nuclei formation in micelles having atoms greater than the critical nucleation number and also subsequent growth of the nucleus. Reaction, nucleation, and growth were assumed to be instantaneous processes. Thus an amount of ruthenium atoms reduced by the reducing agent in the later exchange process would be available for the growth of the nucleated particles. Owing to the fast exchange between the water cores, the initially formed ruthenium nuclei grow to reach a certain size, which corresponds to the thermodynamically best species in the presence of the microemulsion. Rapid change in % BS value during the mixing of two microemulsions indicated the growth of nucleated particles. % BS data indicated that the structure and average sizes of the nanoparticles would change with progression of the reaction time. Once formed, the surfactant casing impeded the growth of the clusters, which then remained small. % BS data was superimposed after completion of the reaction.

Effect of Surfactant Concentration. It was found from the literature that critical micelle concentration (CMC) of Triton X-100 in cyclohexane was 43.5 10 4 mole fraction, that is ∼0.04 (M).26 With increasing surfactant concentration, interfacial film strength will increase and enhance resistance to coalescence. The surfactant concentration was varied keeping other parameters constant. It was observed that the microemulsions containing low surfactant concentrations (0.05 - 0.15 M) were not very stable and resulted in phase separation within a short period of time. At 0.2 M surfactant concentration, stable microemulsions were obtained to synthesize ruthenium nanoparticles. It was observed that with increasing surfactant concentration from 0.2 to 0.5 M, the average size of the nanoparticle was slightly decreased (from 22.46 to 22.19 nm) due to the lower rate of particle aggregation but polydispersity index was increased from 0.1 to 0.38 (Figure 3), resulting in broader particle size distribution. Therefore, in the remainder of the experiments the surfactant concentration was kept at 0.2 M. Effect of the Water to Surfactant Molar Ratio. In microemulsions, the volume of water was proportional to the cubic 11448



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Figure 6. Stability of ruthenium nanoparticles at different water-to-surfactant molar ratios.

radius of the water core, and the amount of surfactant present as a film around the water core was proportional to its surface area.27 Thus radii of the water cores, that is, the size of the water droplet in the w/o emulsion, changes linearly with the molar ratio of water to surfactant (ω) which influences the average diameters of ruthenium nanoparticles; they increase with an increase in the molar ratio. The ratio of water-to-surfactant (ω) was varied from 3 to 7 keeping both surfactant and ruthenium chloride concentration constant at 0.2 M and R = 5. It was observed that at ω = 3, the smallest ruthenium nanoparticles size (Z-average = 17.08 nm) was obtained. Particle size increased with an increase in water to surfactant molar ratio. On the contrary, there would be a decrease in particle size if the water content decreased. The deduction is confirmed by DLS and TEM images (Figures 4 and 5). It was also observed that there was a decrease in the size distribution at low water content compared to that of the particles obtained at higher ω values. Because of low water content, the water solubilized in the polar core was bound by the surfactant molecules, which increased the boundary strength and decreased the intermicellar exchange rate among the reverse micelles which controlled micellar sizes as well as sizes of the nanoparticles.27 Hence lower water content induces formation of smaller reverse micelles, and the decrease in the micellar size induces formation of smaller and more monodisperse particles.

Zhang et al.28 also found that a decrease in the water content in the w/o microemulsion induced formation of monodisperse silver nanoparticles with a smaller diameter. Stability of Colloidal Ruthenium Nanoparticle. For synthesis of highly monodisperse stable nanoparticles, there needs to be rapid nucleation that brings the solution below saturation and then slow controlled growth until all precursors are consumed. The rate of aggregation and growth control needs to stabilize at the period of their formation. Solutions of Triton X-100-stabilized ruthenium nanoparticles obtained using different water-to-surfactant ratios (ω = 3, 5, and 7) were scanned from the bottom (0 mm) to the top of the vial (∼60 mm) for a period of 20 min. Scanning was performed at different time intervals up to 24 h (Figure 6a,b,c). It was observed that BS profiles at different times were not superimposing. It indicated that the structure and average sizes of the nanoparticles would change with the progression of time, but since the nanoparticles were stabilized by the surfactant, no abrupt change in BS data were observed. It was found that after 24 h, BS profiles of all samples at different times were superimposing which suggested that particles remained stable after 24 h. % BS data at 24 h of three different samples were compared (Figure 6d). It was found that ruthenium nanoparticle obtained at ω = 7 showed the lowest % BS value due to a larger particle size 11449



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Figure 7. DLS size distribution histograms of ruthenium nanoparticles synthesized at different concentrations of reactant molar ratio (a) 0.1; (b) 0.15; (c) 0.2; (d) 0.25; and (e) 0.3 M.

(25.09 nm) of lower population density which was confirmed by the TEM (Figure 5c). At ω = 3, smaller nanoparticles (17.08 nm) were produced with higher population density (Figure 5a). At ω = 5 comparatively larger diameter (22.46 nm) nanoparticles were produced and also particle population density was higher because of the large number of water-pooled nanoparticles generated at higher ω value; therefore this ratio showed the highest % BS value. Effect of Ruthenium Chloride Concentration. Ruthenium chloride concentration was varied from 0.1 to 0.3 M keeping surfactant concentration fixed at 0.2 M, ω at 5, and R = 5 for all reactions. At lower concentrations of ruthenium (0.1 M), average particle size was found to be 19.26 nm. With an increase in concentration of ruthenium from 0.1 to 0.2 M, particle size decreased (16.61 nm) (Figure 7). With a further increase in concentration, the average particle size increased again. Actually intermicellar exchange rate plays an important role in the synthesis of nanoparticles in the microemulsion. The exchange rate among the microemulsion water droplets is slow when nanoparticles are synthesized in low ruthenium chloride concentration. Slow exchange of materials will lead to formation of a relatively lower number of ruthenium nuclei. After formation of nuclei, if large numbers of ruthenium atoms are available for the growth of the nucleated particles, it will form larger nanoparticles. At slightly higher concentration of ruthenium chloride (up to 0.2 M), large number of particles with relatively smaller diameter would be formed due rapid exchange rate of the micelles and sufficient amount of surfactant was also available to protect from further agglomeration, so particles size decreased up to 0.2 M concentration. With further increase in

Figure 8. DLS size distribution of ruthenium nanoparticles synthesized at different molar ratios (continuous oil phase = cyclohexane; ω = 5; CRuCl3 = 0.1 M).

concentration the collision frequency of the formed particles increased and protection obtained by adsorption of surfactant molecules on the particles surface was weakened due to lower surfactant concentration compared to precursor concentration. Thus the tiny particles agglomerated to form larger particles. Effect of Molar Ratio of Reducing Agent to Reagent (R). It was found from the DLS histograms of ruthenium nanoparticles 11450


Industrial & Engineering Chemistry Research obtained at molar ratio of 3 (not shown in the figure), that the average size of ruthenium nanoparticles obtained was 30.03 nm. On increasing molar ratio to 5 (Figure 8), the average size was found to 22.46 nm. On increasing the molar ratio to 10 and 20, the average size of ruthenium nanoparticle obtained was 21.25 and 17.38 nm, respectively. At a lower molar ratio larger nanoparticles were obtained, which indicated incomplete reduction of ruthenium ions. At a lower concentration of NaBH4 (lower molar ratio), a large amount of ruthenium nanoclusters and a few nanoparticles were formed. At higher molar ratio intramicelles nucleation and growth would be promoted. If the reduction takes place at a faster rate, the size of the particles can be controlled in a better way. Once formed, the available sufficient concentration of surfactant (0.2 M for this study) would be helpful attached on the surface of particles, to stabilize and protect them against further growth and to remain small.

’ CONCLUSIONS Highly monodisperse ruthenium nanoparticles were synthesized successfully using water-in-oil (w/o) microemulsion by proper selection of different parameters such as surfactant concentration (0.2 M), water to surfactant molar ratio (ω = 5), ruthenium chloride concentration (0.2 M), and molar ratio of reducing agent to reagent (R = 5). Varying ω ratio, the size of particles also quantified on the basis of TEM images, which gave a size range from 16.32 28.89 nm. Turbiscan results showed that synthesized ruthenium remained stable after 24 h. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: +91 261 2201641. Fax: +91 261 2227334. E-mail: [email protected].

’ ACKNOWLEDGMENT The authors wish to acknowledge the Department of Science & Technology (DST), New Delhi, India, for financial support through DST R&D Project No. SR/S3/CE/091/2009. ’ REFERENCES (1) Xiong, L.; Manthiram, A. Catalytic activity of Pt Ru alloys synthesized by a microemulsion method in direct methanol fuel cells. Solid State Ionics 2005, 176, 385. (2) Rojas, S.; Garcia-Garcia, F. J.; Jaras, S.; Martinez-Huerta, M. V.; Garcia Fierro, J. L.; Boutonnet, M. Preparation of carbon supported Pt and PtRu nanoparticles from microemulsion electrocatalysts for fuel cell applications. Appl. Catal., A 2005, 285, 24. (3) Rabelo de Moraes, I.; Jose da Silva, W.; Tronto, S.; Rosolen, J. M. Carbon fibers with cup-stacked-type structure: An advantageous support for Pt Ru catalyst in methanol oxidation. J. Power Sources 2006, 160, 997. (4) Yan, S.; Qu, P.; Wang, H.; Tian, T.; Xiao, Z. Synthesis of Ru/ multiwalled carbon nanotubes by microemulsion for electrochemical supercapacitor. Mater. Res. Bull. 2008, 43, 2818. (5) Bonet, F.; Delmas, V.; Grugeon, S.; Herrera Urbina, R.; Silvert, P.-Y.; Tekaia-Elhsissen, K. Synthesis of monodisperse Au, Pt, Pd, Ru and Ir nanoparticles in ethylene glycol. Nanostruct. Mater. 1999, 11, 1277. (6) Yang, J.; Lee, J. Y.; Deivaraj, T. C.; Too, H.-P. Preparation and characterization of positively charged ruthenium nanoparticles. J. Colloid Interface Sci. 2004, 271, 308. (7) Takashi, T.; Hisashi, F. New method for facile synthesis of amphiphilic thiol-stabilized ruthenium nanoparticles and their redoxactive ruthenium nanocomposite. Langmuir 2005, 21, 12093.

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Formation of Ruthenium Nanoparticles by the Mixing of Two Reactive

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