Nanostructural and Morphological Control of Ruthenium Compounds

Oct 30, 2009 - sodium dodecylsulfate and sodium dodecylbenzenesulfonate were used ... On the other hand, when a mixture of sodium hexadecylsulfate and...
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DOI: 10.1021/cg900261s

Nanostructural and Morphological Control of Ruthenium Compounds Templated by Surfactant Assemblies

2009, Vol. 9 5092–5100

Yuko Inoue, Seiji Ohtsuka, Toshio Torikai, Takanori Watari, and Mitsunori Yada* Saga University, Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, 1 Honjo, Saga 840-8502, Japan Received March 3, 2009; Revised Manuscript Received October 7, 2009

ABSTRACT: This paper presents the synthesis of nanospherical, hollow spherical, stringlike, and nanotubular mesostructured ruthenium compounds templated by anionic surfactant assemblies by the homogeneous precipitation method using urea. When sodium dodecylsulfate and sodium dodecylbenzenesulfonate were used as a template, nanospherical ruthenium compounds were formed. On the other hand, when a mixture of sodium hexadecylsulfate and octadecylsulfate was used as a template, hollow spherical ruthenium compounds were obtained. Furthermore, when sodium bis(2-ethylhexyl)sulfosuccinate was used as a template, nanospherical, stringlike, and nanotubular ruthenium compounds were synthesized. For example, the nanotube framework comprised a less-ordered mesostructure phase composed of the cationic ruthenium compound with a thickness of 1.4 nm and the anionic bis(2-ethylhexyl)sulfosuccinate phase with a thickness of 1.0 nm. When the nanotubes were calcined in air, they were first transformed into amorphous ruthenium compound nanotubes at 300 °C and subsequently into conductive mesoporous crystalline RuO2 nanotubes at 500 °C. On the other hand, when they were calcined at 700 °C in vacuum, they were transformed into metallic ruthenium nanotubes.

Introduction Since the discovery of MCM-411 templated by organic molecules, inorganic/organic nanocomposites have been studied extensively. In synthesis using organic molecules as a template or a structure directing agent, various novel inorganic nanostructures and morphologies are expected to be obtained due to the diversity of organic molecules and their assemblies.2 Therefore, new catalytic, electric, and electrochemical characteristics may be obtained due to specific nanostructures and morphologies. Because silica easily forms an amorphous structure, silica/organic molecule nanocomposites with various morphologies, which reflect various morphologies of organic molecular assemblies, can be synthesized. Conversely, because many inorganic compounds other than silica are crystalline, the diversity in the morphologies of the inorganic compound/organic molecule nanocomposites drastically decreases, compared to the silica system. Therefore, in order to apply the morphological control method using an organic molecular assembly as a template to various inorganic compounds, it is important for inorganic compounds other than silica to accumulate the example of morphological control, using an organic molecular assembly as a template, and to explicate the morphology formation mechanism. Ruthenium compounds such as ruthenium oxide (RuO2) and ruthenium oxide hydrate (RuO2 3 nH2O) have been investigated for applications such as electrocatalysts,3 materials for electrochemical supercapacitors,4 catalysts for hydrogen production,5 or CO oxidizing catalysts.6 Since the surface, crystal structure, and nanostructure of ruthenium compounds are closely involved in these applications, they can be improved considerably by increasing the specific surface area and formation of particular surface structures through nanostructuring. Recently, ruthenium oxide nanotubes were formed *To whom correspondence should be addressed. E-mail: [email protected]. ac.jp. Phone: þ81-952-28-8682. Fax: þ81-952-28-8548. pubs.acs.org/crystal

Published on Web 10/30/2009

using porous anodized alumina,7-9 ruthenic acid nanosheets were formed by exfoliation of layered ruthenic acid,10 and ruthenium oxide-based nanocomposites were formed by depositing ruthenium oxide on carbon nanotubes.11 The characteristics of these nanostructured ruthenium compounds as capacitors were reported to be better than that of ruthenium oxide synthesized by conventional methods.8,10,11 Novel fluorescent RuO2 nanotubes templated by porous anodized alumina have also been reported.9 Recently, the strategy of using organic molecules as a template for the synthesis of nanostructured inorganic compounds was first applied by us for synthesizing a helical ruthenium compound nanotube templated by 1-dodecanesulfonate (CH3(CH2)11SO3-) assemblies.12 In this paper, we describe the synthesis of nanotubular, stringlike, nanospherical, and hollow spherical mesostructured ruthenium compounds templated by surfactant assemblies and the conversion of these compounds, by calcination in air or vacuum, into amorphous ruthenium compounds, mesoporous crystalline RuO2, and metallic ruthenium, while maintaining their morphology. Experimental Section Synthesis. Nanostructured ruthenium compounds were synthesized by the homogeneous precipitation method using urea (Wako Pure Chemical Industries, Ltd.). RuCl3 3 nH2O (n = 1-3) (Wako Pure Chemical Industries, Ltd.) was used as the ruthenium source. Sodium bis(2-ethylhexyl)sulfosuccinate (AOT, C20H37O7SNa, SIGMAAldrich), sodium dodecylsulfate (SDS, CH3(CH2)11OSO3Na, SIGMA-Aldrich), sodium dodecylbenzenesulfonate (SDBS, CH3(CH2)11C6H4SO3Na, Wako Pure Chemical Industries, Ltd.), CH3(CH2)11(OCH2CH2)4OSO3Na (Nikko Chemicals Co., Ltd.), and a mixture of sodium hexadecylsulfate and sodium octadecylsulfate (CH3(CH2)15OSO3Na:CH3(CH2)15OSO3Na = 3:2, Tokyo Chemical Industry Co., Ltd.) were used as the template. Ruthenium chloride, surfactant, urea, and water were mixed at a molar ratio of 1:2:10:x (x = 50-5000) and stirred at 40 °C for 2 h to obtain a homogeneously mixed solution. Urea was used to gradually raise the pH of the reaction mixture, because it is hydrolyzed on heating r 2009 American Chemical Society

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above 60 °C to release ammonia. The mixed solution was heated at 80 °C and then kept at that temperature with vigorous stirring. The pH of the reaction mixture increased, which caused precipitation of products. After a reaction time of 20 h, the resulting mixture was immediately cooled to room temperature to prevent further hydrolysis of urea. After centrifugation, the resulting solid was washed several times with ethanol and water, and then dried in air. Characterization. The product was examined with transmission electron microscopy (TEM; H-800MU, Hitachi, and JEM-1210, JEOL), scanning electron microscopy (SEM; S-3000N, Hitachi), X-ray microanalysis (XMA; EMAX-5770, Horiba, and Genesis 2000, EDAX), powder X-ray diffraction (XRD; XRD-6100, Shimadzu, Cu KR), thermogravimetric and differential thermal analysis (TG-DTA; TG/DTA320U, Seiko), and Fourier transform infrared absorption spectroscopy (FT-IR; FT/IR-300, Nippon Bunko).

Results and Discussion Synthesis of Ruthenium Compound Nanotube. After investigation of various factors that influence the formation of nanostructured ruthenium compounds, such as reaction temperature, reaction time, and raw materials ratio, it became clear that the particle morphology varies significantly with the molar ratio of water (x) in the raw materials. When the synthesis was performed under the condition of x=1000 using AOT as a template, nanotubes were observed. Stringlike morphologies of nanotubes and open-tips of nanotubes were shown in parts a and b, respectively, of Figure 1. In the TEM observation, both sides along the long axis of the stringlike particles appeared dark (Figure 1c), being characteristic of the tubelike structure. The outer diameter distribution of these nanotubes was as broad as approximately 200-350 nm (Supporting Information); the average diameter was 290 nm, and the inner diameter and length were tens of nanometers and several micrometers, respectively. The IR spectrum of the x = 1000 product showed two sharp peaks at 2932 and 2874 cm-1 due to the -CH2- group and a peak at 2960 cm-1 due to the -CH3 group, along with broad bands at around 1715 cm-1 due to the -CdO group and at around 1227 cm-1 due to the -SO3- group (Supporting Information). This implies that bis(2-ethylhexyl)sulfosuccinate anions are incorporated in the nanotube. Furthermore, the magnified image of the tip of one of these nanotubes showed a less ordered or a worm-hole nanostructure (Figure 1d, Supporting Information). The black portion with a thickness of 1.4 nm represents the ruthenium compound phase, and the white portion with a thickness of 1.0 nm represents the bis(2-ethylhexyl)sulfosuccinate phase. The ruthenium compound/bis(2-ethylhexyl)sulfosuccinate molecule nanocomposite is considered to be formed by concerted self-assembly between the cationic ruthenium compound and the anionic bis(2-ethylhexyl)sulfosuccinate molecules, and the nanotubular morphology is considered to be formed as a result of using the nanocomposite as a framework, as shown in a schematic representation in Figure 2. Moreover, although most of the nanotubes were straight, some of them had constrictions (Figure 1e), an open- and a closed-tip (Figure 1f), or a bottle gourd-shaped (Figure 1g) morphology. Our group has already reported helical ruthenium compound/1-dodecanesulfonate nanocomposites with a hexagonal mesostructure.12 Among these helical nanocomposites, hollow helical nanocomposites, particularly helical nanotubes, were included. The hollow helical nanocomposite was synthesized as follows: (1) a nanocomposite with a hexagonal mesostructure grew helically, (2) a hollow portion was formed, and (3) consequently a nanotube was formed.

Figure 1. SEM (a, b) and TEM (c-i) images of mesostructured ruthenium compounds templated by bis(2-ethylhexyl)sulfosuccinate molecules synthesized at x=1000: (a) entire image of the obtained product; (b) high-magnification image of tips of nanotubes; (c) nanotubes; (d) high-magnification image of a tip of a nanotube; (e) nanotube with constrictions and open tips; (f) nanotube with an open and a closed tip; (g) bottle gourd-shaped particle and nanotube with a constriction; (h) dishlike particles; and (i) high-magnification image of a dishlike particle.

Because the helical morphology was not observed in the ruthenium compound/bis(2-ethylhexyl)sulfosuccinate nanocomposites, ruthenium compound/bis(2-ethylhexyl)sulfosuccinate nanocomposite nanotubes were considered to be synthesized by a different mechanism than that of ruthenium

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Figure 2. Schematic representations of the nanotubular ruthenium compound/bis(2-ethylhexyl)sulfosuccinate nanocomposites with a spongelike or less-ordered structure.

compound/1-dodecanesulfonate nanocomposite nanotubes. We previously reported cerium compound nanowires13 and nanorings13 with a layered nanostructure templated by bis(2-ethylhexyl)sulfosuccinate molecules, suggesting that a bis(2-ethylhexyl)sulfosuccinate-based assembly can form a concentric, one-dimensional structure. Furthermore, AOT is an especially powerful tool for synthesizing one-dimensional morphologies of materials. It is known that bis(2-ethylhexyl)sulfosuccinate molecules are likely to have a reverse micellar or a bilayer geometry due to their molecular structure. CaSO4,14 BaSO4,15 Cu,16 BaCrO4,17 BaWO4,18 and CdS19 nanowires have been synthesized in reverse micelles composed of bis(2-ethylhexyl)sulfosuccinate molecules. By the electrostatic adsorption of bis(2-ethylhexyl)sulfosuccinate molecules on inorganic compound particle surfaces, inorganic compound nanowires were formed.14-19 SiO2 nanotubes20 have also been templated by the reverse micelles. However, the framework of the SiO2 nanotubes is composed of only SiO2 and is quite different from that of the ruthenium compound nanotubes, whose framework is composed of inorganic/organic nanocomposites. Therefore, in this study, nanotubes were considered to be formed by composite functions of two phenomena: formations of a ringlike shape and a one-dimensional structure. Moreover, the only known hexagonal mesostructured oxide prepared using AOT as a template is the less-ordered hexagonal mesostructured tin oxide,21 although it is believed that hexagonal mesostructured oxides are difficult to prepare using bis(2-ethylhexyl)sulfosuccinate molecules. The nanostructure of the ruthenium compound nanotubes may be analogous to that of mesostructured tin oxide. Furthermore, particles having dishlike morphology were also observed (Figure 1h). The particles were found to have a nanostructure similar to that of a nanotube, since the same pattern as that of a nanotube was observed in Figure 1i, and the thicknesses of the ruthenium compound and the bis(2-ethylhexyl)sulfosuccinate phases were 1.3 and 1.1 nm, respectively. Therefore, the above-mentioned bottle gourd-shaped particles and nanotubes with constrictions (Figure 1e-g) are considered to be a mixed form of the dishlike and straight nanotubular morphologies. To raise the proportion of nanotubes formed, synthetic conditions are currently being investigated. The XRD pattern of the x = 1000 product was characterized by a single peak at 2θ = 3.5° or d = 2.5 nm, as shown in Figure 3d, indicating that it has a periodic structure with a periodicity of 2.5 nm, and the ruthenium compound framework is composed of an amorphous phase. The d spacing of 2.5 nm is consistent with the sum of the thicknesses of the bis(2-ethylhexyl)sulfosuccinate and ruthenium compound phases measured by TEM. When the synthesis was carried out using an NaOH aqueous solution instead of urea, no

Figure 3. XRD patterns of the mesostructured ruthenium compounds templated by bis(2-ethylhexyl)sulfosuccinate molecules synthesized at various molar ratios of water: x = 50 (a), 100 (b), 500 (c), 1000 (d), 1500 (e), 2000 (f), 3000 (g), and 5000 (h).

nanotubes were observed, although in the XRD pattern of the NaOH-based solid, a diffraction peak at d = 2.5 nm, similar to that of the urea-based product, was observed. The homogeneous precipitation method using urea is thus essential for the formation of nanotubular morphology. EDX analysis of the as-grown x = 1000 product detected Ru, S, O, C, and N in the ratio of Ru:S:O:C:N = 4.69:2.17:27.66:50.32: 15.15. The detected S originated from bis(2-ethylhexyl)sulfosuccinate. The S/Ru molar ratio for the as-grown x = 1000 product was 0.463, indicating that bis(2-ethylhexyl)sulfosuccinate molecules are incorporated in the product at the bis(2-ethylhexyl)sulfosuccinate/Ru molar ratio of 0.463. Furthermore, Porta et al.22 had synthesized a spherical ruthenium compound by the homogeneous precipitation method using RuCl3, K2SO4, urea, and water as raw materials. They investigated the composition of this ruthenium compound in detail. They reported that although small amounts of SO42- and Cl- were contained in the compound, the composition was mainly (NH3)2RuIIIO(NO) 3 2RuNOCO3 3 5H2O.22 In the solution, Ru4þ seemed to be generated by oxidation of Ru3þ in the raw material.22 They also stated that RuIII is very reactive with both π-acid and σ-donor ligands, such as ammine, nitrosyl, and carbonyl, and that relatively few RuIV complexes are formed such as RuIV sulfate, carboxylate, and related species.22 Moreover, since the purity of RuCl3, a raw material, was low at that time and the raw material contained nitrosyl (NO), nitrosyl was reported to be incorporated in the product ((NH3)2RuIIIO(NO) 3 2RuNOCO3 3 5H2O).22 The differences between our experiment and that conducted by Porta et al.22 are the anion source (AOT was used in our experiment, and K2SO4 was used in the experiment conducted by Porta et al.), the

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Figure 4. SEM (a, d, f) and TEM (b, c, e, g) images for the mesostructured ruthenium compounds templated by bis(2-ethylhexyl)sulfosuccinate molecules synthesized at x = 500 (a-c), 100 (d, e), and 5000 (f, g).

purity of the raw materials, the molar ratio of the raw materials, the synthetic temperature, and the reaction time. However, the reaction in our experiment is considered to be similar to that in the experiment conducted by Porta et al.22 It is also presumed that the basic composition of the ruthenium compound phase in the as-grown x = 1000 product is also similar to that obtained in the experiment conducted by Porta et al.22 Moreover, it is known that NH3 is coordinated with Ru in compounds such as [(NH3)5Ru-O-Ru(NH3)4-O-Ru(NH3)5]Cl6 3 4H2O and [Ru(NH3)6]Cl3. A broad band at 1500-1600 cm-1 in the FT-IR spectrum shown in Supporting Information also indicates the existence of CO32- in the product, which is similar to mesostructured rare earth compounds.23 Therefore, the N detected in our experiment can be attributed to NH3 coordinated with Ru, and C is attributed to bis(2-ethylhexyl)sulfosuccinate (C20H37O7S-) and CO32-. NH3 and CO32- are generated by the hydrolysis of urea. However, since the amount of N in our composition is clearly larger than that shown by Porta et al.,22 there is a possibility that NH3 as well as unreacted urea coordinated or adsorbed on the ruthenium compound phase with a large surface area. The ruthenium compound synthesized by Porta et al.22 consisted of spherical particles with several hundreds of nanometers in size, whereas the as-grown x = 1000 product is composed of the cationic

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ruthenium compound phase of approximately 1.4 nm thickness and the anionic bis(2-ethylhexyl)sulfosuccinate to form a particle. Therefore, the specific surface area (the area of the interface between the ruthenium compound phase and bis(2-ethylhexyl)sulfosuccinate and that of the interface between the ruthenium compound phase and air) of the ruthenium compound phase in the as-grown x = 1000 product was considered to be larger than that of the ruthenium compound reported by Porta et al.22 At the same time, we cannot determine the average valence of ruthenium ion, and it is difficult to distinguish N in NH3 from N in (NH2)2CO. It is also difficult to distinguish C in CO32and C in (NH2)2CO. Based on the above descriptions, the composition of the as-grown x = 1000 product is estimated to be “Ru(III,IV)OR(OH)β(CO3)γ(C20H37O7S)0.463(NH3)δ((NH2)2CO)ε”. Effect of x (Molar Ratio of Water) on the Morphology of the Ruthenium Compound. In the x = 500 product, both string- and dishlike particles, similar to those in the x=1000 product, were observed (Figure 4a). Some of these stringlike particles were nanotubes, but the majority were solid particles without internal cavities (Figure 4b). However, in the magnified image (Figure 4c), the solid stringlike particles were also confirmed to consist of the ruthenium compound/ bis(2-ethylhexyl)sulfosuccinate nanocomposite similar to the nanotubes. The bis(2-ethylhexyl)sulfosuccinate/Ru molar ratio for the x = 500 product was 0.43, which was similar to that of the x = 1000 product. In addition, the average width and length for the stringlike ruthenium compound were 366 nm and ones of micrometers, respectively. The outer diameter of the x = 500 product was larger by approximately 100 nm than that of the x=1000 product, as shown in the Supporting Information. Moreover, the dishlike particles were larger and rounder compared to the x = 1000 product. However, when x was reduced to 100, neither stringlike nor dishlike particles were observed, and only irregular morphologies were observed (Figure 4d). However, the nanostructure for the x = 100 product observed in the TEM image of Figure 4e was similar to that of the x = 1000 product, and the bis(2-ethylhexyl)sulfosuccinate/Ru molar ratio for the x = 100 product was 0.39, which was similar to that of the x=1000 product. On the other hand, when x was increased to 2000, solid spherical particles as well as spherically developed dish- or bowl-like particles were observed. The proportion of the spherical particles further increased at x = 3000. Moreover, when the synthesis was performed at x = 5000, uniform solid spherical particles with an average diameter of 265 nm were observed (Figure 4f). The diameters of the particles mainly ranged from 150 to 250 nm (Supporting Information). A high-magnification image for the x = 5000 product also showed a similar nanostructure to that of the x=1000 product (Figure 4g). The bis(2-ethylhexyl)sulfosuccinate/Ru molar ratio was 0.30, being slightly smaller than that of the x=1000 product. In the XRD patterns of Figure 3, when x was smaller than 1000, the d spacing increased with a decrease in x, and the d spacing became as large as 3.8 nm at x=50, indicating that the nanostructure changed with a decrease in x. However, when x was larger than 1000, the d spacing did not change much from 2.5 nm, and the nanostructure did not change significantly with an increase in x. The difference in the nanostructure and nanomorphology is considered to be a result of the following mechanism of formation. At small molar ratios of water (x= 50 or 100), the concentrations of bis(2-ethylhexyl)sulfosuccinate,

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Figure 5. XRD patterns of the as-grown and calcined mesostructured ruthenium compounds templated by bis(2-ethylhexyl)sulfosuccinate molecules synthesized at x=1000: (a) as-grown; (b) 300 °C; (c) 500 °C; and (d) 700 °C. Peak assignments: b, RuO2.

carbonate ion, NH3, and (NH2)2CO are large, and the viscosity of the solution is also large. When these components self-assemble to form the mesostructured ruthenium compound, since free movement of each component is inhibited by the large viscosity, particles with irregular morphologies are formed. On the other hand, at moderate molar ratios of water (x = 500 or 1000), the tubular and stringlike morphologies, which reflect the specific morphology of the assembly generated by the combination of the ruthenium compound species and bis(2-ethylhexyl)sulfosuccinate molecules, are observed, depending on the synthetic conditions. Furthermore, at greater molar ratios of water such as x = 5000, since the concentration of the components and viscosity of the solution are small, the formation and growth of the mesostructured ruthenium compound is governed by the diffusion of the components in the solution, and thus, spherical particles are formed. Calcination of Ruthenium Compound Nanotube. Next, as a typical case of the calcination behaviors of the as-grown ruthenium compounds, the x=1000 product was examined. A TG-DTA curve for the x=1000 product measured in air is shown in Supporting Information. The exothermic weight loss at ca. 200-320 °C is mainly due to partial desorption of degradable bis(2-ethylhexyl)sulfosuccinate molecule moieties as well as desorption of carbonate species, NH3, (NH2)2CO and water formed by condensation of hydroxyl groups in the ruthenium compound phase, because the S/Ru, N/Ru, and C/Ru molar ratios decreased to 0.20, 0, and 0 at the calcination temperature (300 °C), respectively. The weight loss at ca. 350-450 °C is mainly due to complete desorption of the remaining surfactant moieties or sulfonate groups, since the S/Ru molar ratio decreased with an increase in the calcination temperature, from 0.20 at 300 °C to 0.085 at 500 °C, and then to 0 at 600 °C. From the weight loss at 600 °C, the RuO2 content in the as-grown x = 1000 product was thus determined to be 38.2 wt %. The as-grown x = 1000 product was then calcined at a heating rate of 1 °C min-1 up to 300, 500, and 700 °C in air and then kept at those temperatures for 3 h to study the change in nanostructure, morphology, and crystal structure with temperature. XRD patterns of the calcined products are shown in Figure 5b-d. The single peak at d = 2.5 nm that was observed for the

Figure 6. TEM images for the x=1000 (a-e) and 5000 (f-h) mesostructured ruthenium compounds templated by bis(2-ethylhexyl)sulfosuccinate molecules calcined at 300 (a, b, f), 500 (c, d, g), and 700 °C (e, h).

as-grown x = 1000 product completely disappeared at 300 °C, and no diffraction peaks were observed at 300 °C, indicating that the nanostructure for the as-grown product collapsed into an amorphous structure by the 300 °C calcination. Weak peaks attributable to RuO2 appeared at 500 °C, and those peaks increased in intensity with the increase in calcination temperature, indicating that the crystallinity of RuO2 increased with the increase in calcination temperature. TEM images, shown in Figure 6a-d, indicate that the nanotubular morphologies are maintained up to 500 °C. We also confirmed that the nanotubes collapsed and transformed into aggregates of nanoparticles at 700 °C (Figure 6e). In a magnified image of the product at 300 °C, the less-ordered nanostructure observed for the as-grown product was not observed (Figure 6b). However, a high-magnification image for the product at 500 °C showed that the nanotubes were composed of aggregates of nanoparticles with an average diameter of 8 nm (Figure 6d). From the results of these TEM images, XRD patterns, and TG-DTA curve, it can be concluded that the nanotubular ruthenium compound/bis(2-ethylhexyl)sulfosuccinate nanocomposites were transformed into amorphous ruthenium compound nanotubes at 300 °C and subsequently to RuO2 nanotubes at 500 °C, as a result of the collapse of nanostructure caused by the

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desorption of bis(2-ethylhexyl)sulfosuccinate moieties followed by crystallization into crystalline RuO2 and grain growth. The existence of mesopores of diameter e10 nm for the product calcined at 500 °C is due to the interparticle space in the aggregates of RuO2 nanoparticles, with an average diameter of 8 nm constituting the nanotubes, as shown in Figure 6d, indicating that the product calcined at 500 °C contained novel mesoporous RuO2 nanotubes different from the previously reported RuO2 nanotubes.7-9 The resistivities of the as-grown and the calcined products were measured. A pellet (diameter 1.5 cm and thickness ca. 1 mm) was produced from the powdered product by uniaxial molding at 500 kg/cm2, and its resistivity was measured by a four-point probe method. Since the resistivity of the as-grown product was as large as 1.07  109 Ω 3 m, it was determined to be an insulator. However, the resistivity of the product calcined at 300 °C decreased remarkably to 4.68  10-4 Ω 3 m. Therefore, the insulating as-grown product was transformed into an electrical conductor after calcination. Moreover, the resistivity of the product calcined at 500 °C decreased only slightly to 7.6610-5 Ω 3 m, which is close to a measured value obtained from commercially available RuO2. These results suggest that because the components of the bis(2-ethylhexyl)sulfosuccinate, carbonate ion, NH3, and (NH2)2CO were apparently removed from the as-grown product by calcination, the composition of the calcined product was almost the same as RuO2, which is known as an electrical conductor. Because nanotubes of various oxide ceramics have been reported as either semiconductors or insulators, the conductive nanotubes of the ruthenium oxide obtained in this study can be considered as unusual inorganic nanotubes. Similar calcination behaviors for nanostructural and electrical changes would be observed for the other mesostructured ruthenium compounds synthesized at various x values in this study. In fact, the nanospherical ruthenium compound, the x = 5000 product, also indicated the nanostructural changes shown in Figures 6f-h, similar to those of the x = 1000 product by calcination in air. In their XRD patterns, the single peak at d=2.5 nm observed for the as-grown x=5000 product almost disappeared at 300 °C and weak peaks attributable to RuO2 appeared at 500 °C, and those peaks increased in intensity with the increase in calcination temperature, which is similar to those for the x=1000 product. The resistivity of the x = 5000 product decreased from 7.17  107 Ω 3 m for the as-grown product to 2.25  10-5 Ω 3 m for the product calcined at 300 °C, and then to 6.70  10-5 Ω 3 m for the product calcined at 500 °C. In addition, the as-grown x=1000 product was calcined in vacuum at a heating rate of 1 °C min-1 up to 700 °C and then kept at that temperature for 3 h. In the XRD pattern of the calcined product, peaks attributable to metallic ruthenium appeared as shown in Figure 7a. TEM images of the product calcined at 700 °C shown in Figure 7b-c indicate that the nanotubular morphology is barely maintained (Figure 7b), and the nanotubes were composed of aggregates of nanoparticles with an average diameter of 8.4 nm (Figure 7c). These TEM images and XRD patterns suggest that the asgrown nanotubular ruthenium compounds were transformed into metallic ruthenium nanotubes at 700 °C in vacuum, as a result of the collapse of mesostructure and the subsequent reduction into crystalline metallic ruthenium caused by the combustion desorption of bis(2-ethylhexyl)sulfosuccinate moieties. This procedure for conversion of the mesostructured ruthenium compound into

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Figure 7. XRD pattern (a) and TEM images (b, c) of the x = 1000 mesostructured ruthenium compound calcined in vacuum at 700 °C. Peak assignments: O, Ru.

metallic ruthenium is simpler than the previously reported procedure,12 in which the mesostructured ruthenium compound was calcined at 300 °C in air and was subsequently treated with H2 gas at 4.8 MPa at room temperature. Effect of Surfactant on Nanostructure and Morphology. Moreover, using various surfactants, the syntheses were performed at a molar ratio of RuCl3:surfactant:urea:H2O = 1:2:10:1000, as summarized in Table 1. Parts a-c of Figure 8 show the XRD patterns of the products synthesized in the presence of three organic molecules having different hydrophilic groups that were surfactants in which there were 12 carbon atoms in the alkyl chain. When SDBS and SDS were used as the surfactant, diffraction peaks of 001, 002, and 003 appeared, indicating the syntheses of layered nanocomposites having an interlayer spacing of 3.2 and 3.4 nm,

0.81 3.4

0.82 4.8

0.65 0.24 1.39 3.4 4.3 3.3

3.8 3.2 2.7 2.5 2.4 2.2 2.3 2.5 3.2

Inoue et al.

helicoid, irregular shape 500 10 *

Sulfur/ruthenium molar ratio determined by X-ray microanalysis.

2 1 CH3(CH2)11SO3Na12

hollow sphere 1000 2 1

10

2 2 2 1 1 1

sodium dodecylbenzenesulfonate

CH3(CH2)11OSO3Na CH3(CH2)11(OCH2CH2)4OSO3Na mixture of CH3(CH2)15OSO3Na and CH3(CH2)17OSO3Na

10 10 10

1000 1000 100

solid sphere, irregular shape irregular shape hollow sphere, solid sphere

less-ordered structure less-ordered structure less-ordered structure less-ordered structure less-ordered structure less-ordered structure less-ordered structure less-ordered structure multilamellar vesicle-like layered structure, layered structure layered structure less-ordered structure multilamellar vesicle-like layered structure multilamellar vesicle-like layered structure hexagonal structure irregular shape irregular shape string, dish nanotube, dish dish, solid sphere solid sphere, bowl solid sphere, irregular shape solid sphere solid sphere, irregular shape 50 100 500 1000 1500 2000 3000 5000 1000 10 10 10 10 10 10 10 10 10 2 2 2 2 2 2 2 2 2 1 1 1 1 1 1 1 1 1 sodium bis(2-ethylhexyl)sulfosuccinate

d100/nm mesostructure morphology H2O urea molar ratio

surfactant RuCI3 sufactant

Table 1. Properties of the As-Grown Mesostructured Ruthenium Compounds Templated by Surfactant Assemblies

0.33 0.39 0.43 0.46 0.38 0.34 0.32 0.30 0.71

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respectively. On the other hand, when CH3(CH2)11(OCH2CH2)4OSO3Na was used as the surfactant, a single diffraction peak appeared at d = 4.3 nm; thereby, a nanostructure with poor regularity was formed. In EDX analysis, Ru, N, S, O, and C were also observed, and values of dodecylbenzenesulfonate/Ru =0.71, dodecylsulfate/Ru=0.65, and CH3(CH2)11(OCH2CH2)4OSO3-/Ru = 0.24 were obtained. In the product obtained using SDBS, spherical particles with a diameter of hundreds of nanometers were observed, as shown in the SEM image in Figure 9a. By TEM, a concentric stripe image was observed (Figure 9b). The white portion represents the surfactant phase, and the black portion represents the ruthenium compound phase in this figure; therefore, nanocomposites having a multilamellar vesicle-like layered structure were formed. When SDS was used, spherical particles with a diameter of ones of micrometers were observed, and multiple swollen, convex regions on these surfaces were observed (Figure 9c). Using TEM, a concentric striped image was not observed, unlike the case for the product obtained using SDBS, but a striped image (a layered nanostructure) was observed in every convex region (Figure 9d). In the SEM image obtained using CH3(CH2)11(OCH2CH2)4OSO3Na, irregular morphologies were observed (Figure 9e). In the TEM image, synthesis of nanocomposites with a less-ordered or a worm-hole nanostructure was confirmed (Figure 9f). Generally, when organic molecules have the same hydrophobic group type, an organic molecule with a smaller hydrophilic group volume easily forms a layered structure, i.e., bilayer phases, and an organic molecule with a larger hydrophilic group volume easily forms a rodlike micelle. Therefore, a less ordered nanostructure or a wormhole-like nanostructure was considered to be formed when CH3(CH2)11(OCH2CH2)4OSO3Na, the hydrophilic group volume of which was the largest among three surfactants, was used. Moreover, a layered structure was formed when surfactants with smaller hydrophilic group volumes (SDBS and SDS) were used. In the SEM image, when a mixture of sodium hexadecylsulfate and sodium octadecylsulfate, which have carbon chains longer than those of SDS, was used as a template, spherical particles with diameters between hundreds of nanometers and tens of micrometers were synthesized. The TEM image shown in Figure 9g verified that these particles were hollow spherical particles. In the TEM image (Figure 9h) of the wall regions composing these hollow spherical particles, striped images at intervals of approximately 5 nm were observed. The value of surfactant/Ru = 0.82 was obtained in the EDX analysis. Thus, synthesis of multilamellar vesicle-like layered nanocomposites consisting of the ruthenium compound phase and the surfactant phase was confirmed. In the XRD pattern (Figure 8d), diffraction peaks were observed at d = 4.8, 2.7, and 1.8 nm. These diffraction peaks were slightly broader than those observed when SDS or SDBS was used as a template. Although these diffraction peaks could not be clearly attributed to a layered structure, they were presumed to be a layered structure. The reason for obtaining the XRD pattern was attributed to the fact that two types of surfactants were mixed to compose a template, causing the regularity of the layered structure to decrease slightly; therefore, the mixing state of each surfactant was not uniform in the nanocomposites. Furthermore, the diameter of this hollow particle was controllable. When the synthesis was performed with a molar ratio of RuCl3: surfactant:urea:H2O=1:2:10:100, i.e., a lesser proportion of

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Figure 8. XRD patterns of the mesostructured ruthenium compounds synthesized using SDBS (a), SDS (b), CH3(CH2)11(OCH2CH2)4OSO3Na (c), a mixture of sodium hexadecylsulfate and sodium octadecylsulfate (d, e) synthesized at x = 1000 (a-d) and x=100 (e), and the ruthenium compound synthesized without using a surfactant at x = 1000 (f).

water, the formation of solid and hollow spherical particles was observed (Figure 9i). The average particle diameters decreased remarkably to hundreds of nanometers. In the TEM image of these particles, multilamellar vesicle-like images were observed (Figure 9j). In the XRD pattern (Figure 8e), diffraction peaks were observed at d = 3.3, 1.6, and 1.1 nm, indicating the formation of a layered nanostructure. Such multilamellar vesicle-like nanostructures were also reported to be formed in silica/surfactant systems.24 Because similar hollow spherical particles were partially observed when C16H33SO3Na was used as a template molecule, it was considered that stabilization of the layered structure achieved by using a surfactant with a long chain as a template was considered to play an important role for forming a hollow spherical shape. It is rare that hollow spherical particles of inorganic materials other than silica are synthesized using a commercially available, inexpensive, surfactant as a template.25 The nanocomposites described herein also transformed into porous particles composed of aggregates of RuO2 nanoparticles, when calcination was performed, similarly to nanocomposites synthesized using AOT as a template. It was confirmed in the XRD patterns and the TEM images (Supporting Information) that, when calcination at 500 °C or higher was employed, the nanocomposites were transformed into ruthenium oxide while retaining their morphologies. As shown in our previous paper,12 when the synthesis was performed under conditions similar to those of this study

Figure 9. SEM (a, e) and TEM (b-d, f-j) images of the mesostructure ruthenium compounds synthesized using SDBS (a, b), SDS (c, d), CH3(CH2)11(OCH2CH2)4OSO3Na (e, f), and a mixture of sodium hexadecylsulfate and sodium octadecylsulfate (g-j) synthesized at x = 1000 (a-h) and x = 100 (i, j).

without using a surfactant, amorphous-like ruthenium compounds with poor crystallinity were synthesized (Figure 8f).

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As described in the Introduction section, it was presumed from the reports on silica/surfactant nanocomposites that the inorganic compound phase must be amorphous in order to form various morphologies using a surfactant as a template. Because the phase of the ruthenium compounds synthesized in this study was almost amorphous, it is considered that ruthenium compound/surfactant nanocomposites having various morphologies, which reflected various morphologies of surfactant assemblies, were synthesized. Conclusion This study introduced various nanostructural and morphological controls of ruthenium compound/surfactant nanocomposites. The ruthenium compound/surfactant nanocomposites were transformed into aggregates consisting of ruthenium oxide nanoparticles with diameters between a few nanometers and tens of nanometers while retaining their various morphologies. Regarding the synthesis and application of ruthenium oxide having a special shape, it is known that Hu et al.8 synthesized a hydrous RuO2 (RuO2 3 xH2O) nanotubular array using anodic porous alumina as a template for next generation supercapacitors. The wall thickness of the 200 °C annealed RuO2 3 xH2O nanotubes is ∼40 ( 5 nm, and the outer diameter of the nanotubes is about 200 ( 20 nm. RuO2 3 xH2O grains are adherently stacked to form the tubular structure. The mesoporous architecture, hydrous nature, and metallic conductivity are considered to provide the proton and electron “superhighway” for the extremely rapid charge/discharge processes. Tan et al.9 synthesized RuO2 nanotubes using anodic porous alumina as a template. The reported nanotubes’ outer diameters were 25-40 nm, inner diameters were 15-20 nm, and lengths were 200 nm-3 μm. When the excitation wavelength was set at 200-220 nm, the luminescence was observed in the ultraviolet-visible range. In both studies, unique physical properties of particles with diameters of a few micrometers, consisting of ruthenium oxide nanoparticles with diameters of tens of nanometers, were reported. As we reported this time, the walls of RuO2 nanotubes consisted of nanoparticles with ones of nanometers sizes, and those of metallic Ru nanotubes consisted of nanoparticles with ones of nanometers sizes. Therefore, the characteristics peculiar to nanoparticles are expected to be revealed, and detailed evaluation will find interesting physical properties. In specific nanostructures such as nanotubes, specific electrochemical reactions or sensor characteristics associated with electrons caused by the nanotubular structure are known to occur.26 Therefore, the appearance of specific physical and chemical properties in various nanostructures and morphologies obtained in this study will be anticipated, and the ruthenium compounds obtained in this study are expected to be applied in many fields. By investigating the physical properties of the ruthenium compounds with various

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morphologies, the relationship between the morphologies and physical properties will be clarified in the future. Acknowledgment. This research was partially supported by KAKENHI (16685021, 19750172) and Nippon Sheet Glass Foundation for Materials Science and Engineering. Supporting Information Available: Particle size distributions, FTIR spectrum, TG-DTA curve, and TEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

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