Synthesis of Ruthenium Dioxide Nanoparticles by a Two-Phase Route

Oct 1, 2008 - are immobilized by simple dip-coating on a current collector, and the cyclic ... oxidative synthesis,21 and sol-gel route, etc.22,23 As ...
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J. Phys. Chem. C 2008, 112, 16219–16224

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Synthesis of Ruthenium Dioxide Nanoparticles by a Two-Phase Route and Their Electrochemical Properties Yuhan Lin, Nana Zhao, Wei Nie, and Xiangling Ji* State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, People’s Republic of China ReceiVed: January 17, 2008; ReVised Manuscript ReceiVed: August 12, 2008

Dissolvable, size- and shape-controlled ruthenium dioxide nanoparticles are successfully achieved through a two-phase route. The influence of reaction time, temperature, and monomer concentration and the nature of capping agents on the morphologies of nanoparticles are studied through transmission electron microscopy (TEM). A possible mechanism for the formation and growth of nanoparticles is also involved. X-ray powder diffraction (XRD) confirms the amorphous structure for as-prepared ruthenium dioxide nanoparticles. Samples are immobilized by simple dip-coating on a current collector, and the cyclic voltammetry measurement is utilized to investigate their electrochemical properties. The specific capacitance of one sample can reach as high as 840 F g-1, which reveals the promising application potential to electrochemical capacitors. Introduction It is well-recognized that anhydrous rutenium dioxide with tetragonal rutile polymorphs exhibits remarkable chemical properties and thermal stability, as well as high metallic electronic conductivity. The hydrate compound denoted as RuO2 · nH2O is a mixed proton-electronic conductor which can lead to an ultrahigh pseudocapacitance.1-4 These unique features motivate many researchers to study RuO2 applications in integrated circuits,5 catalysts,6,7 solar selective coatings, and energy storage/conversion materials.8-11 Up to now research on the preparation of nanosized RuO2 has mostly focused on metal-organic chemical vapor deposition (MOCVD),12-14 pulse laser deposition,15 electrostatic spray deposition,16 and reactive sputtering, etc.17,18 Such methods, which can synthesize some size-/shape-controlled nanoparticles, however, are generally expensive and take a long time to produce final products. RuO2 nanoparticles can also be obtained via wet chemistry routes, such as electrochemical deposition,19,20 oxidative synthesis,21 and sol-gel route, etc.22,23 As for the electrochemical deposition, it is restrained to synthesize RuO2 on the substrates, and the products cannot easily to be separated. Subramanian and co-workers produced a mesoporous anhydrous RuO2 via sol-gel route,22 which serves as an active electrode material for an electrochemical supercapacitor. However, it is necessary to remove the surfactant at 400 °C, which easily results in nanoparticle agglomeration finally. Besides, the poly(vinylidene fluroide-co-hexafluoropropene) (PVDF-HFP) binder will bring a negative effect on the electrochemical properties. Recently, Hu and his co-workers concentrated on RuO2 nanoparticles from RuCl3 · xH2O and its aqueous solution.10,21,24-28 RuCl3 · iH2O served as a precursor; thus, oxidative synthesis and hydrate reaction followed by electrochemical deposition25,26,28 were employed and studied systematically. Moreover, hydrous RuO2 nanocrystals with independent controlled crystal size and water content were synthesized through a mild hydrothermal process. The crystallite size of RuO2 · nH2O * To whom correspondence should be addressed. Tel.: 86-431-8526 2876. Fax: 86-431-8568 5653. E-mail: [email protected].

is approximately 2.6 nm, but the particle agglomeration is apparent and the size distribution is broad.24 Hu’s papers start an application using as-prepared RuO2 nanoparticles on an electrochemical supercapacitor with aqueous electrolyte. They found that prolonging the hydrothermal time, called “hydrothermal annealing”, can enlarge the crystal size but maintain the water content of the primary nanocrystals. Ultimately, this process can improve the electron pathways so that promotes the electrochemical supercapacitor properties. And it is great significance for the successful design and tailoring of the 3D, arrayed, electrodes based on the hydrothermal derived RuO2 · xH2O nanocrystallites, using the anodic deposition technique which can result in the perfect performance for the next generation supercapacitors.27 Interestingly, a narrower size distribution of RuO2 particles can be obtained through the reaction of NaBH4 with RuCl3 in ionic liquids, however the Schlenk manipulation has become an obstacle in wide applications.29 On the other hand, nanosized noble metal and their oxide materials have been successfully synthesized through wet chemistry in the presence of protective agents to avoid particle agglomeration.30 In our previous work, some soluble and accurately tailored nanoparticles have been prepared through controlling the nucleation and growth at the interface of two phases.31-34 This technique is convenient and reproducible in order to manufacture II-VI semiconductor nanocrystals and some metal oxides. However, no attempt has been tried on RuO2 · xH2O nanoparticles using this method. In this paper, we report a convenient procedure to synthesize dissolvable RuO2 · xH2O nanoparticles on the basis of the two-phase thermal route. The possible mechanism for the formation and growth of the nanoparticles is proposed. As-prepared samples can be readily dispersed in organic solvents with different polarities through surface ligand exchange. They also can be easily immobilized on the substrate, by dip-coating or spin-coating, without or with surface ligand elimination through heating. Besides, cyclic voltammorgram (CV) results exhibit the unique merits of the samples as electrode materials for electrochemical capacitor applications.

10.1021/jp803782u CCC: $40.75  2008 American Chemical Society Published on Web 10/01/2008

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Figure 1. Time-dependent shape evolution of RuO2 nanoparticles: (a) 2 h, (b) size distribution histogram of a, (c) 3 h, (d) size distribution histogram of c, (e) 4 h, (f) 6 h, (g) 10 h, and (h) 18 h. Sample prepared at 180 °C with [Ru] ) 0.01 M, 0.05 mL of tert-butylamine, and toluene/water ) 1:2 (in volume).

Experimental Section Preparation of Ruthenium Dioxide Nanoparticles. Dodecylamine (DA), tert-butylamine (98.5%), RuCl3 · xH2O, oleic acid (OA, tech, 90%), myristic acid (MA, 99.5%), di-noctylamine (DOA), trioctylphosphine (TOP), oleylamine (OLA, tech, 70%), and sodium stearate were used directly without further purification. Sodium stearate was reacted with RuCl3 · xH2O to get ruthenium stearate as a precursor. A typical synthesis is described in the following. Ruthenium stearate (0.05 mmol) and OLA (1.0 mL) were dissolved in 5 mL of toluene inside a 30 mL Teflon-lined stainless-steel autoclave. Subsequently, 10 mL of aqueous solution containing 50 µL of tert-butylamine was added to the organic phase without any stirring. The autoclave was then sealed and maintained at 180 °C for a fixed time and cooled to room temperature using tap water. Particularly, lowtemperature reaction (less than 80 °C) was carried out in an oil bath under magnetic stirring in the two-phase system. Finally, the vessel was immediately cooled using tap water. The ruthenium oxide nanoparticles were purified by precipitating the crude nanoparticles with excess methanol and further isolating it by centrifugation and decantation. The nanoparticles can be re-dispersed in nonpolar organic solvents for structural characterization without any size sorting. Surface Ligands Exchange. About 10 mg of the above products were mixed with 5 mL of pyridine. The solution was then heated at 50 °C for over 10 h to obtain the pyridine-capped nanoparticles, noted as Ru-Py, which can be re-dispersed in polar solvents for further study. Characterization. The products were characterized by transmission electron microscopy (TEM, JEOL-1011), X-ray powder diffraction (XRD, Phillips PW1700 instrument), Raman spectrum (Raman, Confocal Raman Microscope, Renishaw inVia), X-ray photoelectron spectroscopy spectrum (XPS, VG ESCALAB MKII), and thermal gravimetric analysis (TG-DTA, PerkinElmer TGA-7). The cyclic voltammetry was performed on CHI 660B (CH Instruments) at room temperature using a three-electrode cell, with the Pt as counter electrode and Ag/ AgCl (saturated KCl) as reference electrode. A polished surface of glassy carbon (GC) is chosen as the current collector. Asprepared ruthenium dioxide nanoparticles were dissolved in CH3OH and immobilized on a current collector without additional binders and through a mild heating with an infrared lamp to remove CH3OH and surface ligands. The amount of Ru-Py was controlled by the concentration of solution. The cyclic voltammetry (CV) data were normalized to the geometric electrode area (0.07 cm2), and the loading of the ruthenium oxide

is ca. 43 µg cm-2 (measured with quartz crystal microbalance, QCM) on each electrode. A potential window of 0.9 V was used in the testing. Results and Discussions Reaction time. The TEM images in Figure 1 show a temporal evolution of RuO2 · xH2O nanoparticles during the reaction. At the initial stage, we observed the arrowlike nanoparticles with an average length of 6.1 ( 0.7 nm along a relatively wide size distribution (SD, 11.4%), as shown in Figure 1a,b. A subsequent increase in reaction time not only decreased the size of nanoparticles but also led to the formation of spherical and rodlike morphologies. After 3 h, the as-prepared nanoparticles appeared to be roughly spherical particles with an average diameter of 1.6 ( 0.1 nm (SD, 6.3%) as shown in Figure 1c,d. A few monorods and multiarmed rods were recognized after 4 h (Figure 1e). The multiarmed rods became the dominant morphology until 18 h. For example, the rods have an average diameter of approximately 1.6 ( 0.2 nm (SD, 12.5%) for 10 h (Figure 1g) and 1.9 ( 0.1 nm (SD, 5.2%) for 18 h (Figure 1h). And the length of the rods is difficult to measure. Unlike the hydrothermal process, the oil-soluble RuOx · yH2O nanoparticles obtained in our two-phase route is achieved by precursor hydroxylation at the interface of the oil-water phase. This interface is for the only location where the nanoparticles form and grow (Figure S1 of the Supporting Information). At the initial stage, alkali (tert-butylamine) will generate lots of OH- in the aqueous phase, while the mono capped colloids of ruthenium formed by self-assembled capping agents will be generated in the oil phase. Thus, the colloids of ruthenium ions will combined with OH- at the oil-water interface as Ru(OH)x · yH2O and formed capped (Ru + O:) colloids, which will act as the reactive sites to form the RuO2 nanoparticles. This reaction will be ended when the capped nanoparticles enter the oil phase again, and the nanoparticles will continue growing once they diffuse into the interface of the two phases. And RuO2 · xH2O colloids were quickly generated. Definitely, the capped (Ru + O:) colloids obtained at the interface of the oil-water phase will either grow into RuO2 nanoparticles or be consumed through a solution process. Progressive decrease in Ru(OH)x · yH2O concentration followed the formation of colloidal RuO2 · xH2O, at a higher OH- concentration until most of the precursor and OH- is consumed. Thus, bigger colloids were formed (Figure 1a), and this formation process is fast. However, after the initial stage, part of the colloids may be dissolved again in the organic phase by stearic acid which was the side product.35 Then the system underwent a second

Size and Shape-Controlled RuO2 Nanoparticles

Figure 2. TEM images prepared at (a) 80, (b) 120, and (c) 180 °C. Sample prepared with toluene/water ) 1:1 for 6 h.

formation and growth of the colloids. In this stage, the very lower remaining monomer concentrations less than the solubility of the colloids restrained the nanoparticle growth, and size distribution refocusing occurs during the growth stage. As above-mentioned, the formation and growth of the colloids can only occur at the interface of the oil-water phase, where they must have diffused to prior to the further growth. Since the diffusion resistance is size-dependent, the smaller nanoparticles grow faster and will in the end catch up with the larger one, due to their higher energy and lower stability.36 With an enough long time for growth, their size distribution becomes narrow. However, individual spherical particles were scarcely found at the final stage of the reaction. Meanwhile, the elongated nanoparticles get thicker in diameter with increasing reaction time. This is probably because the adhesions of nanoparticles by some of the remaining monomers partly act as an adhesive.31 Reaction Temperature. The reaction temperature is also a key factor in controlling the size and shape of the nanoparticles. Figure 2 presents TEM images of the nanoparticles prepared at different temperatures but with a fixed reaction time of 6 h. The samples prepared at 80 °C show the larger rods with an average aspect ratio of 2.1 ( 0.1 and the smaller spherical particles with an average diameter of 2.3 ( 0.3 nm (Figure 2a). However, some larger rods and smaller spherical particles were found synchronously in the samples prepared at 120 °C (Figure 2b), while the 180 °C sample had spherical shapes with a diameter of 2.4 ( 0.2 nm, in Figure 2c. These phenomena were similar to “Ostwald ripening” as the early studies on the synthesis of nanocrystal have proved.37 In fact, higher temperature speeds up the diffusion. It should be mentioned the reaction in oil bath at 80 °C, mixing between the aqueous and organic phases at lower temperature, is incomplete and the monomer diffusion is difficult. At the growth stage, some small particles are unable to grow into large ones due to a lower probability of their arrival at the interface from the oil phase. So for the lower temperature preparation at 80 °C, the shape and size are not easy to control. We believe that the rod mainly formed under interface reaction control and the smaller spherical ones came from the growth of residual monomer with further increased reaction time under diffusion. On the other hand, at higher temperature of 180 °C, the shape and size can be tuned easily through varying the reaction time. Monomer Concentration. The monomer concentration also influences the shapes of the nanoparticles. We could obtain rough spherical particles with an average diameter of 1.6 nm at a reaction time of 3 h at [Ru] ) 0.01 M (Figure 3a). A relatively high [Ru] monomer concentration favors the formation of elongated shapes. The arrowlike and rice-shaped, as well as the spherical, particles can be found in Figure 3b at [Ru] ) 0.013 M. The rice-shaped nanoparticles at [Ru] ) 0.02 M in Figure 3c possess an average length of 6.7 ( 0.6 nm (SD, 8.9%) and an aspect ratio of 2.2 ( 0.3. The anisotropic shapes are due to the difference of the growth rate on different facets. However, at a high [Ru] concentration of 0.1 M, we cannot obtain well-

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Figure 3. TEM images of nanoparticles prepared at 180 °C for 3 h with [Ru] monomer concentrations of (a) 0.01, (b) 0.013, and (c) 0.02 M.

Figure 4. Sample synthesized at 180 °C for 12 h with [Ru] ) 0.01 M: TEM images in the presence of different capping agents, (a) MA, (b) OA, (c) OLA, (d) DA, (e) TOP, (f) DOA, and (g) sample prepared at 180 °C for 4 h and further ligands exchange with pyridine.

developed nanoparticles. Meanwhile, a large amount of precipitates in the aqueous phase can be found, which is probably due to the relatively low concentration of capping agent in the system. Capping Agents. The nature of the capping agent plays an important role in the shape control. Parts a-f of Figure 4 present the various structures, such as leaf, spherical, monorod, teardrop, multiarmed rods, and wormlike nanoparticles, using different capping agents. The shapes of the nanoparticles evolve from elongated branchy structures to small spherical particles with an increase in the length of the hydrocarbon chain (MA < OA and DA < OLA), and carboxyl-capped nanoparticles appear more round-shaped due to a stronger capping ability. And this process is under the control of the diffusion. The direction of growth and the diffusion flux are determined by the monomer concentration gradient between the bulk solution and the stagnant solution as well as the diffusion coefficient of the monomers.38 The nanoparticles tend toward nearly spherical shapes at a slow growth rate driven by the minimization of interfacial energy. Shorter-chain capping agents cannot hinder the nanoparticles effectively to form attachments with each other as much as the long ones. Thus, the samples capped with TOP and DOA produce elongated shapes. The steric hindrance can be applied to explain the formation of wormlike nanoparticles with DOA being the capping agent (Figure 4f) in comparison with shorter multiarmed rods formed with TOP (Figure 4e). Asprepared colloidal RuO2 nanoparticles are proven to be very stable in a solution for at least more than 1 year, because of the capping agent. And they can be spin-coated on many substrates,

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Figure 5. XRD patterns (left) and Raman spectrum (right) of the RuO2 · xH2O sample treated at 25 (a), 220 (b), and 280 °C (c).

Figure 6. Cyclic voltammograms at scan rates of 500, 400, 300, 200, 100, 50, and 20 mV s-1: (A) Ru-Py-a with multiarmed rods; (B) Ru-Py-b with rice-shaped nanoparticles.

such as silicon wafer, glass slide, and mica, without additional binders. On the one hand, the existence of a capping agent prevents the aggregation of nanoparticles on the substrate. On the other hand, the capping agent with a long-chain probably makes the electronic migration difficult in the electrochemical devices. Thus, the ligand exchange on the surface is necessary to the device application. The TEM image in Figure 4g shows a representative observation for one pyridine-capped sample. Pyridine was chosen as a suitable capping agent due to its higher conductivity (5.3 × 10-8 S cm-1, 18 °C). This exchange enables the RuO2 nanoparticles well-dispersed in polar solvents such as CH3OH and also makes charge transfer easy. XRD Patterns and Raman Spectra. RuO2 · xH2O is a mixed proton-electronic conductor, as its unique structure. The framework of RuO2 offers electronic conductivity and the crystal water offers proton conductivity, which is essential to exhibit pseudocapacitance. We use XRD and Raman spectra to study the structure of the as-prepared sample. The XPS and TG-DTA are also preformed, and the results are shown in the Supporting Information (Figures S2-S4). Figure 5 (left) shows the effect of the temperature on the crystalline behavior of the as-prepared sample, and the XRD pattern at 25 °C confirms an amorphous phase. However, crystal growth is significant when the temperature is elevated to 220 °C. At 280 °C, a considerable increase in intensity and narrowing of the characteristic peaks can be observed. Evidently, annealing leads to particle coarsing and irreversible particle agglomeration, as well as the formation of tetragonal rutile-type RuO2. No Raman peaks at 25 °C can be found in Figure 5 (left) and it confirmed the amorphous structure of as-prepared nanoparticles further. After heating the samples at 220 or 280 °C for 2 h, local RuO6 rutile crystalline structure is formed as indicated by the three peaks of the spectrum.3,10 Besides, small blue shifts in peak position can be found when the temperature increases due to crystal growth. We noticed that no distinct peak can be observed in the XRD pattern measured at room temperature.

Electrochemical Properties. As we mentioned above, a long alkane capping agent will give a negative effect on the dielectric property of nanocolloids/electrolyte interface. This can be proved by water contact angle measurement and cyclic voltammograms, as shown in Figure 6 and Figure S5 (Supporting Information). It indicates that the RuO2 samples with long alkane ligands (such as OLA, marked as Ru-OLA) is hydrophobic since the contact angle is 118.2° and the Ru-Py sample capped with pyridine is hydrophilic with the contact angle of 46.3°. And Ru-OLA only show a high resistance effect in aqueous electrolytes (i.e., a straight line in the CV), while the pyridine-capped samples show some electrochemical response. Thus, long alkane chains are bad conductors that retard the access of aqueous electrolytes, and Ru-Py sample ensures the electrochemical access for ions in aqueous solution. To study the shape effect on the electrochemical behavior, the pyridine-capped multiarmed rod sample in Figure 1f named Ru-Py-a (1.6 ( 0.1 nm; SD, 6.3%) and the pyridine-capped rice-shaped sample in Figure 3c named Ru-Py-b (6.7 ( 0.6 nm; SD, 8.9%; aspect ratio of 2.2 ( 0.3) are chosen. The steadystate cyclic voltammograms in Figure 6 for both Ru-Py-a and Ru-Py-b in 0.5 M H2SO4 at low scan rates (below 300 mV s-1) show a typical capacitive behavior with two very weak redox peaks similar to the RuO2 film electrode. Meanwhile, the current magnitude has no significant change during either the cathodic or anodic scan in the potential window (see Figure S1 in the Supporting Information). However, at a higher scan rate above 300 mV s-1, both of the Ru-Py/GC electrodes show a response different from the ideal pseudocapacitive behavior. This feature becomes more pronounced with increasing scan rate. The amorphous Ru-Py nanoparticles at the surface of GC have a low electrochemical series resistance. The hydrous region allows a facile proton transfer into the bulk material for efficient charge storage by dilation of the oxide lattice,39 while the interconnected network accounts for the electronic conduction. So the dramatic symmetry of the curves obtained at low scan rates is due to the reversibility feature of surface redox couples.

Size and Shape-Controlled RuO2 Nanoparticles

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Figure 7. (A) Charging current dependence upon the scan rate. The solid line shows the linear fit over the whole range of scan rates of Ru-Py-b; two dotted lines show the fit over two regions of Ru-Py-a at slow and fast scan rates. (B) Specific capacitance as a function of potential scan rate. GC modified with Ru-Py-a and Ru-Py-b.

In view of the selected potential range, the proton intercalation on the surface of the oxide in the presence of water molecules covered by a carpet of OH groups is fast and reversible, as shown in (1). The pseudocapacitance is derived mainly from the following transitions:

Ru(III) f Ru(IV) between 0.2 and 0.8 V Ru(IV) f Ru(VI) between 0.8 and 0.9 V RuOx(OH)y + δe- + δH+ f RuOx-δ(OH)y+δ

(1)

However, at high scan rate, both of the Ru-Py/GC electrodes undergo some polarization, resulting from a few dissolved oxygen in aqueous solution during the test, which leads to a reduction on the surface of the electrode.40 In spite of the complicated mechanism of polarization, the charge (or discharge) of the Ru-Py/GC electrodes can be fully achieved until a time scale of about 10 s, as calculated from the CV data. This time scale allows the use of the Ru-Py/GC-based electrochemical capacitors in many applications where the high-power property is required.41 The current dependence upon the voltage scan rate for these two samples is shown in Figure 7A. Both of the two samples exhibit the almost linear behavior, but Ru-Py-b has a lower capacitance compared with Ru-Py-a from the slopes of the two curves. Obviously, we can see that the capacitance decreases with potential scan rates in Figure 7B, except for Ru-Py-b at a scan rate of 50 mV s-1 compared with 20 mV s-1.The capaticances of the supercapacitors are calculated through the formula C ) I ν-1, where I is the current and ν is the sweep rate in V s-1. Overall, the value of specific capacitance is extremely large. For example, Ru-Py-a, Carea is normalized to the electrode geometric area and can be determined over the whole range of scan rates, reaching 25.2 ( 1.4 mF cm-2, higher than the latest reported results based on RuO2 modified carbon nanotube electrodes of 16.94 mF cm-2.42,43 Cm is determined by the integration of the CV curve. Here, it can reach as high as ca. 840 F g-1, which exceeds most of the reported results based on pure nanostructured RuO2 electrodes, such as sol-gel prepared RuO2,44 760 F g-1 except for the latest work by Hu and co-workers, RuO2 nanotube with 1300 F g-1 after annealing.27 Unquestionably, the capacitance is related to the nature of the materials and the nanoparticles/electrolyte interface. When the samples were immobilized on the surface of the current collector, the randomly packed particles with a narrow distribution overlapped with the neighboring particles and electrically interconnected with each other. And Ru-Py-a (1.6 ( 0.1 nm; SD, 6.3%) would probably form a more tangled network structure with a larger surface area since it was prepared with smaller and branchy nanoparticles (Figure 1f). Such thin multilayers and porous structure facilitate the electrolyte to

effectively infiltrate into a three-dimensional network. So it has a higher capacitance (2.22 mF cm-2 at 50 mV s-1). On the other hand, the rice-shaped Ru-Py-b (6.7 ( 0.6 nm; SD, 8.9%; an aspect ratio of 2.2 ( 0.3) had a smaller surface than Ru-Pya, and the rice shape of the nanoparticles was unfavorable to the formation of a much tangled network. Therefore, the resistance of Ru-Py-b electrode could be higher. So it has a lower capacitance (1.82 mF cm-2 at 50 mV s-1). In addition, Figure 7 implies that deep penetration of the potential perturbation into the porous structure has been achieved at a lower scan rate; hence, the fast faradic reaction on the interface of pseudocapacitive behavior is mainly the key point for the large capacitance. At a higher scan rate, the diffusion effect on both Ru-Py-a and Ru-Py-b could not be neglected, and it results in a drop in the capacitance of the electrode. We believe 50 mV s-1 is an appropriate rate for our testing system, since both samples exhibit relatively high capacitance at the time of 20 s. The specific power of the Ru-Py-a electrode is calculated to be 400 W at 50 mV s-1. Those above results originated from a dependence on the near surface proton quantity.22,45 It is, however, difficult to measure the net charge from an adsorption of proton. It should be pointed out that no species were deposited in the electrolyte after 1000 consecutive cycles. This reveals excellent interface stability between the GC and nanoparticles, which is also important for the application in electrochemical capacitors. Conclusions We have developed a facile two-phase approach to successfully synthesize ruthenium dioxide nanoparticles with controllable size, shape, and surface ligands. Here, rich morphologies include leaf, spherical, arrow, monorod, multiarmed rod, teardrop, and wormlike nanoparticles. Surface ligands exchange makes the application of as-prepared amorphous nanoparticles convenient for device fabrication. Two pyridine-capped samples were coated on the GC as a stable electrode of electrochemical capacitors. One sample has a specific capacitance as high as ca. 840 F g-1, higher than most of the reported values based on RuO2 nanoparticle. We believe that this is due to the intrinsically high electrolyte accessible surface and low resistance of the amorphous ruthenium dioxide nanoparticles that formed the 3D framework. However, further study on how the morphology of nanoparticles influences the electrochemical properties is still in progress. Acknowledgment. We are grateful for the support for this study provided by the National Natural Science Foundation of China (Creative Research Group, Grant 50621302), “863”

16224 J. Phys. Chem. C, Vol. 112, No. 42, 2008 Project (Grant 2006AA03Z224), the Distinguished Young Fund of Jilin Province (Grant 20050104), and the International Collaboration Project (Grant 20050702-2) from Jilin Province, China. Supporting Information Available: Figures showing a probable reaction mechanism of the two-phase route, XPS spectra of sample prepared at 180 °C, XPS, TGA, and DTA spectra of the as-prepared RuO2 · xH2O sample, and water contact angles and cyclic voltammograms of various samples. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Campbell, C. T. Phys. ReV. Lett. 2006, 96. (2) Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications [M]; Plenum: New York, 1999. (3) Ryan, J. V.; Berry, A. D.; Anderson, M. L.; Long, J. W.; Stroud, R. M.; Cepak, V. M.; Browning, V. M.; Rolison, D. R.; Merzbacher, C. I. Nature 2000, 406, 169. (4) Trasatti, S.; Buzzanca, G. J. Electroanal. Chem. 1971, 29, A1. (5) Sahul, R.; Tasovski, V.; Sudarshan, T. S. Sens. Actuators, A 2006, 125, 358. (6) Over, H. Appl. Phys. A: Mater. Sci. Eng. 2002, 75, 37. (7) Zhan, B. Z.; White, M. A.; Sham, T. K.; Pincock, J. A.; Doucet, R. J.; Rao, K. V. R.; Robertson, K. N.; Cameron, T. S. J. Am. Chem. Soc. 2003, 125, 2195. (8) Rochefort, D.; Dabo, P.; Guay, D.; Sherwood, P. M. A. Electrochim. Acta 2003, 48, 4245. (9) Ahn, Y. R.; Park, C. R.; Jo, S. M.; Kim, D. Y. Appl. Phys. Lett. 2007, 90. (10) Chang, K. H.; Hu, C. C.; Chou, C. Y. Chem. Mater. 2007, 19, 2112. (11) Morales-Ortiz, U.; Avila-Garcia, A.; Lara, V. H. Sol. Energy Mater. Sol. Cells 2006, 90, 832. (12) Chou, T. Y.; Lai, Y. H.; Chen, Y. L.; Chi, Y.; Prasad, K. R.; Carty, A. J.; Peng, S. M.; Lee, G. H. Chem. Vap. Deposition 2004, 10, 149. (13) Tan, H.; Ye, E. Y.; Fan, W. Y. AdV. Mater. 2006, 18, 619. (14) Tian, H. Y.; Chan, H. L. W.; Choy, C. L.; Choi, J. W.; No, K. S. Mater. Chem. Phys. 2005, 93, 142. (15) Iembo, A.; Fuso, F.; Arimondo, E.; Ciofi, C.; Pennelli, G.; Curro, G. M.; Neri, F.; Allegrini, M. J. Mater. Res. 1997, 12, 1433. (16) Sugimoto, W.; Iwata, H.; Murakami, Y.; Takasu, Y. J. Electrochem. Soc. 2004, 151, A1181.

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