Making the Most of a Scarce Platinum-Group Metal: Conductive

May 13, 2009 - The physical properties of the resultant self-wired nanoscale ruthenia significantly differ depending on the nature of the porous suppo...
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NANO LETTERS

Making the Most of a Scarce Platinum-Group Metal: Conductive Ruthenia Nanoskins on Insulating Silica Paper

2009 Vol. 9, No. 6 2316-2321

Christopher N. Chervin,† Alia M. Lubers,† Katherine A. Pettigrew,†,‡ Jeffrey W. Long,† Mark A. Westgate,§ John J. Fontanella,§ and Debra R. Rolison*,† Surface Chemistry Branch, Code 6170, NaVal Research Laboratory, Washington, D.C. 20375, and Physics Department, Stop 9c, 572c Holloway Road, U.S. NaVal Academy, Annapolis, Maryland 21402 Received February 18, 2009; Revised Manuscript Received April 30, 2009

ABSTRACT Subambient thermal decomposition of ruthenium tetroxide from nonaqueous solution onto porous SiO2 substrates creates 2-3 nm thick coatings of RuO2 that cover the convex silica walls comprising the open, porous structure. The physical properties of the resultant self-wired nanoscale ruthenia significantly differ depending on the nature of the porous support. Previously reported RuO2-modified SiO2 aerogels display electron conductivity of 5 × 10-4 S cm-1 (as normalized to the geometric factor of the insulating substrate, not the conducting ruthenia phase), whereas RuO2-modified silica filter paper at ∼5 wt % RuO2 exhibits ∼0.5 S cm-1. Electron conduction through the ruthenia phase as examined from -160 to 260 °C requires minimal activation energy, only 8 meV, from 20 to 260 °C. The RuO2(SiO2) fiber membranes are electrically addressable, capable of supporting fast electron-transfer reactions, express an electrochemical surface area of ∼90 m2 g-1 RuO2, and exhibit energy storage in which 90% of the total electron-proton charge is stored at the outer surface of the ruthenia phase. The electrochemical capacitive response indicates that the nanocrystalline RuO2 coating can be considered to be a single-unit-thick layer of the conductive oxide, as physically stabilized by the supporting silica fiber.

Platinum-group metals are not only precious because of their scarcity in the Earth’s crustsRu, Rh, and Ir at 1 part per billion (ppb) apiece; 5 ppb of Os; 10 ppb of Pt, and 15 ppb of Pd1sbut because this subset of the periodic table is so critical to high performance in a host of applications. The structure-dependent electrical and electrochemical properties of ruthenium dioxide (RuO2) make it technologically desirable for electrocatalysis (especially for production of chlorine gas from brine), electrolysis, photovoltaic devices, capacitors, and thick and thin film resistors.2 In its single crystal and polycrystalline form, RuO2 is a metallic electronic conductor (σ > 104 and ∼102 S cm-1, respectively, at 25 °C), with excellent diffusion barrier properties and chemical and thermal stability.3 As the surfaces and bulk of ruthenia are made defective and hydrous, the material retains metallic conductivity through a nanocrystalline network of rutile RuO2,4 while adding proton conduction at hydrous inter* To whom correspondence should be addressed, [email protected]. † Surface Chemistry Branch, Code 6170, Naval Research Laboratory. ‡ Current address: Department of Chemistry & Biochemistry, George Mason University, 4400 University Dr., Fairfax, VA 22030. § Physics Department, U.S. Naval Academy. 10.1021/nl900528q CCC: $40.75 Published on Web 05/13/2009

 2009 American Chemical Society

faces,5 creating the ability to store energy at ∼720 F g-1 proton-1.6,7 When anhydrous but nanoscopic, RuO2 exhibits high values of Li-ion uptake (∼260 mA h g-1) as a function of its degree of disorder.8 Because of the high cost of ruthenium precursors, it is desirable to minimize the amount of RuO2 required for a particular function, for example by preparing RuO2 in nanoscale, high-surface-area forms to maximize the number of reaction sites for catalytic applications or energy storage. We report a multifunctional and inexpensive electrode produced by converting silica fiber paper into a conductive platform via low-temperature deposition of nanoscale RuO2 coatings. The RuO2-coated filter paper, RuO2(SiO2), exhibits high electronic conductivity and desirable electrochemical properties, but does so at low mass loadings (∼300 µg cm-2), thereby maximizing the utilization of this expensive platinumgroup metal. The conductive nanoscopic coating is air- and water-stable and readily functionalized to modify its surface character and catalytic activity. The macroscopic properties of the lightweight SiO2 substrate, i.e., flexibility, compressibility, and robustness, are retained while the nanoskin of ruthenia imparts electronic, electrochemical, and electro-

catalytic functions. This new assembly has potential for use as an electrode structure in important technological applications, including gas-diffusion electrodes for fuel cells (when modified with an electrocatalyst, such as Pt), conductive membranes, and multifunctional platforms for electrocatalysis/photocatalysis. This inexpensive, practical manifestation on the macroscale of a conductor on the nanoscale is an adaptation of our method for wiring the interior of silica aerogels, based on the subambient thermal decomposition of RuO4 in nonaqueous media to RuO2.9 High weight loadings (>25 wt %) are necessary to form the conducting network through aerogellike nanoarchitectures, and the resultant monolithic objects have only modest electronic conductivity (5 × 10-4 S cm-1) as normalized to the geometric factor of the silica monolith. Furthermore, the cost of producing aerogel monolithic substrates and their fragile nature present additional challenges to commercialization and application. In the pursuit of a more robust and practical substrate, we chose commercial SiO2 filter paper as a fibrous scaffold for the subambient deposition of RuO2. In addition to achieving macroscopic mechanical properties associated with the SiO2 membrane, the nanoscopic RuO2 coating that forms on the SiO2 fibers yields dramatic improvements in electrical and electrochemical properties relative to nanometric RuO2 deposited on 10-nm-wide substrates such as the silica particulate network in aerogels. The electronic conductivity of the RuO2(SiO2) paper is 3 orders of magnitude higher than that of the RuO2-wired aerogel and is achieved with a significant reduction in the weight loading of RuO2 (5% vs ∼40%). The compressible nature of the silica paper allows for excellent electrical contact with rough and curved surfaces, a feature not possible with rigid substrates such as an aerogel. The RuO2(SiO2) papers also tolerate immersion into (and removal from) liquid phases, including water, allowing for solution-based modification and extended operation (and recovery) when used as an electrode structure. Synthesis of the RuO2(SiO2) paper is far simpler than that developed for aerogel substrates,9 namely, the time for equilibration and RuO2 deposition is 3 orders of magnitude is consistent with conversion of the poorly crystalline, as-deposited RuO2 into the metallically conductive, rutile form.8,9 The fiber-supported, 200°C-calcined nanoskin remains amorphous to X-rays; however, lattice fringes are observed by high-resolution transmission electron 2317

Figure 2. Transmission electron micrographs of a 200°C-calcined RuO2(SiO2) paper (imaged on a carbon-supported Cu grid). (a) STEM image in the vicinity of an island of uncoated SiO2 (seen in lower right quadrant) with accompanying elemental map (b) verifying that the Ru distribution (red) corresponds to the observed nanoskin on the fiber. (c) Bright-field TEM of a smaller fiber in which the RuO2 is observed to form a filigreed network over the surface of the silica fiber and fill into the crevice formed where two wires intersect. On wider fibers, the RuO2 network fills in forming the contiguous nanoskin observed by SEM (Figure 1c).

Figure 3. (a) log conductance vs temperature measured in situ by a two-point probe technique upon heating and cooling an as-prepared RuO2(SiO2) paper. Resistance measurements were made 15 min after the sample reached each temperature, except for the measurement at 200 °C (2 h). (b) log conductivity vs 1/T measured in situ by a four-terminal technique for a 200°C-calcined RuO2(SiO2) paper. Resistance was measured from high to low temperature, initially from 200 to -160 °C (b) and then from 260 to -160 °C (O). (c, d) Steady-state cyclic voltammograms of 200°C-calcined RuO2(SiO2) membranes in aqueous media: (c) 0.5 M H2SO4 at a scan rate of 2 mV s-1; (d) 5 mM K3Fe(CN)6 in 1 M KNO3 electrolyte at 25 mV s-1.

microscopy (HRTEM) analysis correlated to 2-3 nm ordered domains. Prolonged heating of RuO2(SiO2) papers at temperatures g300 °C sharply decreases conductivity, which arises from dewiring of the electronic network as the close-packed pebbly nanoskin of RuO2 (Figure 1c) coarsens into disconnected particles (see Figure S2b in Supporting Information). The conductivity measured at a given calcination temperature is 2318

maintained upon cooling from that temperature to room temperature. We limit thermal processing to e260 °C in order to maintain high conductivity, remove organic residuals, and minimize dewiring. Despite the modest weight loading of RuO2 on the low surface area, macroporous SiO2 paper, we observe high macroscopic electronic conductivity for the object. The conductivity of the 200°C-calcined papers measured by fourNano Lett., Vol. 9, No. 6, 2009

Table 1. Density-Normalized Electronic Conductivity of Rutile RuO2 as a Function of Form form of RuO2 RuO2(SiO2 paper, calcined at 200 °C RuO2 powder harvested from the subambient synthesis, calcined at 200 °C, pressed into a pellet RuO2, polycrystalline

density-normalized electronic conductivity S cm2 mg-1 50 14 12

point probe ranges from 0.3 to 0.8 S cm-1 at room temperature in air (n ) 14, where each of the 14 samples derives from independent synthetic batches). The magnitude of this conductivity, as normalized using the geometric dimensions of the insulating silica paper, does not completely capture the remarkable behavior observed here because the RuO2 occupies 250 °C, the RuO2(SiO2) papers bleach to a pale green, indicating sufficient particle growth to isolate particles, thereby quantum confining the conduction electrons Nano Lett., Vol. 9, No. 6, 2009

in the localized particles (see Figures S2 and S3 in Supporting Information). The enhanced volume-normalized conductivity of the ruthenia nanoskin relative to dense forms of the oxide (Table 1), an electron mean free path longer than the size of the primary particle of the deposited oxide, and the closed-shell formulation of the conductive oxide all hint that in this particular conductor(dielectric) core-shell arrangement the conduction electrons of the RuO2 are aware of the dimension of the object they coat, just as are the conduction electrons in Au nanoshells that coat micrometersized silica spheres and rods. To probe the mechanism of conduction, we examined the temperature dependence of the conductivity of 200°Ccalcined RuO2(SiO2) papers using in situ four-terminal measurements of resistance (see Figure 3b). When the data are fit to an Arrhenius model, the electrical transport is consistent with an activated conduction process, although one with an atypically low activation energy for semiconductors: 8 meV from 20 to 260 °C. At temperatures below 20 °C, the temperature dependence of the conductivity follows a power law, which may arise from the disjointed character of the electron connection between domains in the ruthenia nanoskin and between ruthenia domains across fibers. The growth of the nanoscale RuO2 phase starts as a filigreed, networked coating on the individual fibers (verified by TEM imaging of the thinner silica fibers in the paper, Figure 2c). The ruthenia then fills in on the wider fibers, consistent with the fact that these curved surfaces appear planar on the length scale of the ruthenia nanoparticles (Figure 1c), but junctions will still exist between fibers. The 200°C-calcined RuO2(SiO2) papers exhibit a negative temperature coefficient of resistivity (TCR), unlike the positive TCR expected of a metallic conductor and obtained for compacted RuO2 powder.20 The change in resistivity with temperature for 200°C-calcined RuO2(SiO2) paper is negative over the entire measured range (-160 to 260 °C) with a TCR of -0.7 mΩ cm K-1 derived from the linear portion from 20 to 180 °C. Negative TCRs from 20 to 150 °C were previously observed for cracked, 1-2 µm thick films of ruthenia and for compact films calcined at temperatures 90% of the deposited RuO2 is electrically addressable and contributes to the observed electrochemical response. The specific capacitance of polycrystalline, anhydrous RuO2 is at least five times lower than that of hydrous ruthenium oxide,4,6,22 so the magnitude of the specific capacitance expressed by the RuO2 nanoparticles in the nanoskin that coats the SiO2 paper is atypically high for an anhydrous, bulk oxide. The dichotomy arises from the nanoscale texture of the film. Electrochemical evidence for the ultrathin nature of the ruthenia nanoskin comes from the dependence of the capacitance on potential scan rate (ν). A linear dependence with ν-1/2 was obtained (from 2 to 100 mV s-1; see Supporting Information). Extrapolation of the capacitance to ν-1/2 ) 0 (i.e., infinite scan rate) provides the fraction of the total capacitance that is derived from the “outer” surface of the electrode, i.e., the fraction that is readily accessible for rapid proton exchange.27 At 2 mV s-1, which is a scan rate sufficiently slow to access all protonassociating sites, 90% of the charge stored in the ruthenia phase is surface-confined. The pseudocapacitance results establish that the oxide is sufficiently nanoscopic to be almost all surface. Two nanometers is the size typically observed for growth of RuO2 nanoparticles nucleated at a surface via decomposition of RuO4 in nonaqueous solution at subambient temperature.9,28 More than 90% of the RuO2 units are surface-sited in a 2 nm particle, dropping to 63% for a 3 nm particle.29 The pseudocapacitance of RuO2 in RuO2(SiO2) papers indicates the oxide is all outer-surface sited, in agreement with the finely textured coating seen in Figure 4. In essence and in function, as with graphene,30 the conductive nanoskin can be considered to be “exfoliated”sa unit slice of polycrys2320

Figure 5. Scanning electron micrograph of a 200°C-calcined RuO2(SiO2) composite after electroless deposition of Pt from a 2 mM H2PtCl6 solution in 0.5 M H2SO4. Platinum ions are reduced at the electrified RuO2 coating, which creates nanoparticles at the contiguous nanoskin without forming electrically unaddressable Pt on bare SiO2.

talline RuO2 in which the ultrathin material is physically stabilized by the supporting silica fiber. Taking advantage of the effective wiring of the RuO2 network, we can calculate the surface area from the doublelayer capacitance measured in an aprotic solvent (acetonitrile) with a bulky electrolyte cation (tetrabutylammonium). The current response is featureless and capacitive in nature (Supporting Information) and assuming a typical double-layer capacitance of 10 µF cm-2, the surface area of the RuO2 is determined to be ∼90 m2 g-1. For comparison, the surface area of 200°C-calcined RuO2 powder (harvested from the subambient thermal decomposition of RuO4 in nonaqueous media) is 30 m2 g-1 as measured by N2 physisorption. The larger surface area achieved for the nanoscale RuO2 coating is due to the significantly smaller domain size of RuO2 on the SiO2 fibers as compared with the precipitated, agglomerated, but still nanoscopic powder. From the electrochemically determined surface area and the mass loading of RuO2, we calculate the thickness of the nanoskin to be ∼3 nm,31 consistent with that estimated by transmission microscopy and the size necessary to establish the oxide as predominantly “outer surface” as indicated by the pseudocapacitance scan rate data. The electrochemically addressable high-surface-area RuO2 phase also functions as a fast electron-transfer electrode material. The voltammetric response of ferricyanide (Fe(CN)63-) at the RuO2(SiO2) paper shows a reversible redox process with a peak separation of 76 mV even in the absence of chloride ion (Figure 3d). We can also electrochemically and selectively modify the RuO2 in the RuO2(SiO2) paper with catalytic metal nanoparticles by charging the ruthenia in aqueous electrolyte and then immersing it in a bath of Pt ions whereby Pt electrolessly nucleates and grows as Pt ions are discharged by electrons only at the electrified ruthenia (Figure 5). The capacitive and faradaic electrochemical results together demonstrate that extremely low weight loadings of RuO2 deposited as a shell around a low surface area, insulating substrate yield a multifunctional electrode with Nano Lett., Vol. 9, No. 6, 2009

all the features of the bulk metallic oxide and the bulk hydrous oxide. This one structure can serve as a conductive platform to design low-cost, high-performance electrode structures for a range of energy storage and conversion devices. The cost and strategic importance of the platinum group metals have always tempered their practical adoption in the vast array of technologies in which they would otherwise be used. We can now attain the vaunted electronic and electrochemical properties of ruthenium oxideshigh electronic conductivity, high capacitive charge storage, and fast electron transfersby distributing the material at modest amounts on dirt cheap, curved, insulating substrates and where aspect ratios of the conductive phase are >104. The design strategy of forming closed nanoscale shells of chargetransport materials around curved substrates should be applicable as well to other technologically desirable but expensive materials. Acknowledgment. This work was supported by the Office of Naval Research. C.N.C. (2006-2009) and K.A.P. (2005-2007) are NRC-NRL postdoctoral associates. A.M.L. thanks Carnegie Mellon University for permission to extend her undergraduate internship at the NRL for one year (2008). Supporting Information Available: Description of synthetic protocols and details on the various methods for assessing electronic character. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) Emsley, J. The Elements; Clarendon Press: Oxford, 1989. (2) Adams, D.; Brus, L.; Chidsey, C. E. D.; Creager, S.; Creutz, C.; Kagan, C. R.; Kamat, P.; Lieberman, M.; Lindsay, S.; Marcus, R. A.; Metzger, R. M.; Michel-Beyerle, M. E.; Milller, J. R.; Newton, M. D.; Rolison, D. R.; Sankey, O.; Schanze, K. S.; Yardley, J.; Zhu, X. J. Phys. Chem. B 2003, 107, 6668. (3) Schafer, H.; Schneidereit, G.; Gerhardt, W. Z. Anorg. Allg. Chem. 1963, 319, 372. (4) Dmowski, W.; Egami, T.; Swider-Lyons, K. E.; Love, C. T.; Rolison, D. R. J. Phys. Chem. B 2002, 106, 12677. (5) Rolison, D. R.; Hagans, P. L.; Swider, K. E.; Long, J. W. Langmuir 1999, 15, 774. (6) Zheng, J. P.; Cygan, P. J.; Jow, T. R. J. Electrochem. Soc. 1995, 142, 2699. (7) Jow, T. R.; Zheng, J. P. J. Electrochem. Soc. 1998, 145, 49. (8) Lytle, J. C.; Rhodes, C. P.; Long, J. W.; Pettigrew, K. A.; Stroud, R. M.; Rolison, D. R. J. Mater. Chem. 2007, 17, 1292. (9) Ryan, J. V.; Berry, A. D.; Anderson, M. L.; Long, J. W.; Stroud, R. M.; Cepak, V. M.; Browning, V. M.; Merzbacher, C. I.; Rolison, D. R. Nature 2000, 406, 169. (10) The SiO2 fiber membranes are equilibrated at either -78 °C (dry ice/ acetone) or 0 °C (aqueous ice bath) with a chilled petroleum ether solution of RuO4 previously extracted from aqueous RuO4 (Strem Chemicals). Upon warming to room temperature, RuO4 decomposes to O2 and RuO2, the latter of which deposits as ∼2 nm particles onto the SiO2 surfaces, turning the initially white SiO2 membrane black (Figure 1a). A fine black powder also forms in the reaction vessel as disordered ruthenia aggregates to ∼30 nm in size and precipitates. This byproduct can be reclaimed and ultimately recycled back to RuO4

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(11) (12) (13)

(14)

(15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29)

(30) (31)

for subsequent depositions. Materials and methods are available as Supporting Information. The weight loading of RuO2 in the calcined composite was determined for all samples using a microbalance and verified for selected samples by commercial elemental analysis by ICP-AES. Because SEM analyzes only a small portion of the sample, we imaged numerous RuO2(SiO2) composites prepared under nominally similar conditions, confirming the reproducibility of the synthesis. To determine the volume fraction of RuO2 that coats the silica paper, we divide the mass of RuO2 in the sample (determined after calcination to 200 °C) by the bulk density of rutile RuO2 (6.97 g cm-3) and divide by the geometric volume of the RuO2(SiO2) paper. The primary measurement uncertainty lies in determining the thickness of the silica paper. We use a micrometer to determine thickness, which can compress the paper to yield a lower calculated object volume; this uncertainty would yield an underestimation of actual volume by no more than 15%. The nanoscopic 200 °C calcined RuO2 was pressed into a 1 cm × 0.16 cm × 0.17 cm pellet with a mass of 70.3 mg; the conductivity (four-terminal measurement) was divided by the geometric pellet density to obtain the density-normalized conductivity. For bulk RuO2, we divide the known intrinsic conductivity of polycrystalline RuO2 (∼100 S cm-1) by the theoretical density. For the RuO2(SiO2) composite, we divide the measured conductivity (four-terminal measurement) by the density of RuO2 in the composite. Sze, S. M. Semiconductor DeVices: Physics and Technology; Wiley: New York, 2000; p 53. Wang, H.; Brandl, D. W.; Nordlander, P.; Halas, N. J. Acc. Chem. Res. 2007, 40, 53. Cox, P. A.; Goodenough, P. B.; Tavener, P. J.; Telles, D.; Egdell, R. G. J. Solid State Chem. 1986, 62, 360–370. Hones, P.; Gerfin, T.; Gra¨tzel, M. Appl. Phys. Lett. 1995, 67, 3078– 3080. Glassford, K. M.; Chelikowsky, J. R. Phys. ReV. B 2004, 49, 7107. Sahul, R.; Tasovski, V.; Sudarshan, T. S. Sens. Actuators, A 2006, 125, 358. Lodi, G.; De Asmundis, C.; Ardizzone, S.; Sivieri, E.; Trasatti, S. Surf. Technol. 1981, 14, 335. Iles, G. S.; Collier, O. N. Platinum Met. ReV. 1967, 11, 126. Trasatti, S. Electrochim. Acta 1991, 36, 225, and references therein. Zheng, J. P.; Jow, T. R. J. Electrochem. Soc. 1995, 142, L6. Rolison, D. R.; Kuo, K.; Uman˜a, M.; Brundage, D.; Murray, R. W. J. Electrochem. Soc. 1979, 126, 407. Conway, B. E. Electrochemical Supercapacitors: Scientific Fundamentals and Technological Applications; Kluwer Academic: New York, 1999. Ardizzone, S.; Gregonara, G.; Trasatti, S. Electrochim. Acta 1990, 35, 263. Long, J. W.; Rhodes, C. P.; Lytle, J. C.; Pettigrew, K. A.; Stroud, R. M.; Rolison, D. R. Prepr. Pap.sAm. Chem. Soc., DiV. Fuel Chem. 2006, 51 (1), 41. The percentage of RuO2 formula units that are surface-sited for a given particle size was calculated assuming the RuO2 particles have the theoretical density of 6.97 g cm-3 and the RuO2 unit cell parameters are a ) b ) 0.451 nm and c ) 0.311 nm. For a 3 nm spherical particle the volume and surface area are 14.14 nm3 and 28.27 nm2, respectively. This volume equates to 9.8 × 10-20 g of RuO2 or 446 RuO2 formula units per particle. If one assumes that the larger unit cell faces (a × b ) 0.203 nm2) are expressed on the surfaces of the particle, then 140 unit cells will be surface-sited on the 3 nm particle. Each unit cell contains 2 RuO2 formula units equating to 280 RuO2 units at the surface, which is 63% of the total RuO2 units in the 3 nm particle. Yang, Q.-H.; Lu, W.; Yang, Y.-G.; Wang, M.-Z. New Carbon Mater. 2008, 23, 97. The thickness was calculated by dividing the bulk density of rutile RuO2 (6.97 g cm-3) by the electrochemically measured specific surface area (90 × 104cm2 g-1) and multiplied by 2 to compensate for the half of the RuO2 shell hidden by the silica substrate.

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