An X-Ray Photoelectron Spectroscopy Study of Hydrous Ruthenium ...

Electrochemical and Solid-State Letters, 9 共6兲 A268-A272 共2006兲

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1099-0062/2006/9共6兲/A268/5/$20.00 © The Electrochemical Society

An X-Ray Photoelectron Spectroscopy Study of Hydrous Ruthenium Oxide Powders with Various Water Contents for Supercapacitors A. Foelske,z O. Barbieri, M. Hahn, and R. Kötz* Electrochemistry Laboratory, Paul Scherrer Institut, 5232 Villigen PSI, Switzerland Hydrous ruthenium oxide powders annealed at different temperatures were studied by X-ray photoelectron spectroscopy 共XPS兲 and X-ray diffraction. The transition from the amorphous to the crystalline state was followed by recording the Ru 3d5/2 XPspectra. The peak shape of the O 1s XP-spectra provides information about composition and morphology. Comparison of the water amount obtained from deconvolution of the O 1s signal with the one calculated from gravimetric analysis demonstrates that the physically adsorbed water in the amorphous material evaporates under ultrahigh vacuum conditions. The observed decrease of chemically bound water with increasing annealing temperature is attributed to an increase of particle size. © 2006 The Electrochemical Society. 关DOI: 10.1149/1.2188078兴 All rights reserved. Manuscript submitted November 30, 2005; revised manuscript received January 17, 2006. Available electronically April 3, 2006.

Hydrous ruthenium oxide 共RuO2·xH2O兲 is of great interest for electrochemical applications such as electrocatalysis and electrochemical capacitors.1-3 It was found that pure amorphous ruthenium oxides can provide specific capacitance values as high as 720 F/g under certain preparation conditions.4-6 Important parameters are the temperature and time of annealing of the prepared powder. Recently published investigations of composite materials with a low loading of RuO2·xH2O have shown that a specific capacitance of more than 1100 F/g can be achieved.7,8 However, the mixing with carbon led to a loss of the overall mass-specific capacitance. Several nonelectrochemical methods were applied to explain the dependence of the capacitance on the heat-treatment procedure. X-ray diffraction 共XRD兲 measurements revealed that the capacitance is greatest for a heat-treatment that produces a powder near the phase transition from amorphous to crystalline.5,6 An X-ray fine structure analysis 共EXAFS兲 by McKeown et al.9 explains the high capacitance by the ordering of the RuO6 octahedra in combination with the total water amount in the amorphous structure. According to Ref. 9 charge storage is maximized when the local structures retain facile transport pathways for both electrons and protons. A nuclear magnetic resonance 共NMR兲 study verified that the mobility of water molecules within the structure influenced the electrochemical capacitance.10 Transmission electron microscopy 共TEM兲 in combination with selected area electron diffraction 共SAED兲 investigations confirmed the results obtained from EXAFS and showed that insertion of protons into the bulk of the amorphous material occurs, whereas the crystalline RuO2 with a 3D network of octahedral chains showed no proton insertion.11 An in situ EXAFS 共X-ray absorption near-end structure, XANES兲 study of electroprecipitated ruthenium dioxide films proved that during potential cycling a change in oxidation state of Ru, i.e., Ru3+ /Ru4+ takes place.12 X-ray photoelectron spectroscopy 共XPS兲 is another powerful tool to investigate the chemical environment of elements in a compound at the surface. Shifts in binding energy 共BE兲 of the spectra can be used to determine oxidation states of the cations and anions. The peak shape of the main XP-signal of ruthenium, i.e., the Ru 3d core level, is complex. The XP-spectra overlap with the C 1s signal and can show two spin-orbit doublets located at BE = 280.8 and BE = 282.6 eV. For ruthenium oxides the peak at BE = 280.8 eV dominates the spectrum and the origin of the smaller peak at BE = 282.6 eV has been intensely discussed in the literature.10-19 One of the coauthors of this paper and others have attributed the second spin-orbit doublet to Ru共VI兲 located at the very top of the surface.13-15 Others have proposed that the splitting is due to final state screening effects.16,17 Density functional theory calculations suggest that the peak originates from a special surface termination of

* Electrochemical Society Active Member. z

E-mail: [email protected]

RuO2共110兲 planes by oxygen.18 Over et al.19 disproved this theory and suggested that Plasmon excitation is the source of the second spin-orbit doublet. To the best of the authors’ knowledge, except for a recently published article by Chang and Hu,20 a systematic XPS study of hydrous ruthenium oxides is still missing. These authors investigated the less intense and less structured Ru 2p3/2 XP-signal from amorphous materials with a specific capacitance of 500 F/g only. Herein, we present an XPS study of hydrous ruthenium oxide powders with a specific capacitance of 100-750 F/g. We show that the transition from amorphous to crystalline ruthenium oxide can be followed by XPS and that the occurrence of the second spin-orbit doublet at 282.6 eV in the Ru 3d5/2 XP-spectra is strongly related to the morphology of these materials. Additionally, we show that the peak shape of the O 1s XP-spectra can provide valuable information about the ruthenium oxide composition and morphology. Experimental Preparation.— The RuO2 powders were prepared according to the method of Zheng and co-workers.4 First, an aqueous solution was prepared dissolving solid RuCl3·xH2O in distilled water. The concentration was 0.1 M. Then a 0.3 M aqueous solution of NaOH in distilled water was slowly added to the RuCl3 solution while stirring with a magnetic rod. At a pH value around 7.0, hydrous RuO2 starts to precipitate. After a rest period of 15 h the black powder was separated from the solution by filtration. After filtration the same volume of fresh distilled water was added to the filtrate and the solution was mixed for 30 min before another filtration. The product was filtrated in this way five times. The black RuO2·xH2O paste obtained was then dried in ambient air over night and finally annealed at an appropriate temperature for 17 h in air. From the resultant weight loss the water content of these materials was calculated. The product powders were characterized by X-ray diffraction spectrometry 共Philips: XⱊPert MPD/DY636, Fe K␣, ␭ = 1.94 Å兲. Electrochemistry.— Cyclic voltammograms were carried out to determine the gravimetric capacitance 共Cg兲 of the different powders. The capacitance was calculated according to the following equation Cg =

⌬Q ⌬E * m

关1兴

⌬Q is the charge obtained by current integration during the forward scan, ⌬E is the voltage range, and m is the mass of the hydrous ruthenium oxide. The voltammograms were recorded with a sweep rate of 2 mV/s in the range −0.2 and +0.8 V vs a carbon quasireference electrode. The potential of the reference was determined to be +0.4 V vs standard hydrogen electrode 共SHE兲. The cyclic voltammetry measurements were carried out in a three-electrodes-cell using 3 M H2SO4 as the electrolyte. The counter and reference elec-


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Figure 1. Gravimetric capacitance of hydrous ruthenium oxide powders as a function of annealing temperature; black circles: This study; white squares: values taken from Ref. 4.

trodes were made from polytetrafluoroethylene 共PTFE兲-bound activated carbon 共YP17, Kuraray Chemical Co., Japan兲, while the working electrode was RuO2 powder supported by a 10 mm diam platinum foil. The working and counter electrodes were sandwiched with a 0.3 mm thick glass fiber separator. The tip of the reference electrode was placed at the border of the separator. X-ray photoelectron spectroscopy (XPS).— For the XPS studies, ruthenium oxide powders were pressed at 1 ton for 10 min between two cylindrical graphite pieces 共Sigraflex graphite, SGL carbon, thickness 0.2 cm兲 with a diameter of 0.6 cm. The graphite pieces could be removed easily, and the powder pills were carefully attached with conductive silver and ultrahigh vacuum 共UHV兲 compatible glue onto the sample holder. This procedure led to well-resolved XP-spectra without any charging effects and almost no contamination of the surface with carbon was observed. Analysis of the surface has been carried out using an ESCALAB 220i XL 共Thermo VG Scientific兲 spectrometer. The monochromatic Al K␣ X-ray source used was operated at a power of 200 W 共10 kV, 20 mA兲. After background subtraction according to the method of Shirley21 the XP-spectra of the different species were corrected by the corresponding photoionization cross sections of Scofield 共Ru 3d, O 1s兲22 to calculate the atomic ratios. The O 1s signal was separated by a fitting procedure into the contributions of the different species which are described in more detail below. Results and Discussion Figure 1 depicts the gravimetric capacitance of the ruthenium oxide materials as a function of the applied annealing temperature and compares our results with the data reported by Zheng et al.4 The measured values are in good agreement with the published data. The capacitance increases from 300 F/g for the untreated material 共THT = 25°C兲 up to 700 F/g for the oxides treated at temperatures between 75 and 175ⴰC. After this plateau, the capacitance decreases to 400 F/g and for the materials annealed at THT ⬎ 200°C even lower values than those measured for the material dried at THT = 25°C are observed. From the literature it is well known that this sudden decay is related to a phase transition from amorphous to crystalline RuO2.5,6,23,24 To confirm this and to characterize the morphology of the materials prepared in our study, XRD spectra have been recorded. The XRD patterns are shown in Fig. 2. For THT ⬍ 160°C the spectra do not show any evidence of crystallinity. For the sample treated at THT = 160°C the diffraction lines start to evolve. The sharpness of the diffraction peaks increases with temperature, which corresponds to an ordering of the structure through

Figure 2. XRD spectra of hydrous ruthenium oxide powders as a function of annealing temperature.

annealing. These results are consistent with the results obtained by Zheng and Jow5 for hydrous RuO2 obtained with the same sol-gel method, but also with the experiments made by Kim and Popov6 on hydrous RuO2 obtained with a different sol-gel process. Dmowski et al.23 and Sugimoto et al.24 who characterized the evolution of crystalline structure of commercial hydrous RuO2 with annealing temperature also showed that the material becomes more and more crystalline with increasing annealing temperature. Both these workers used an annealing procedure very close to ours as they treated their sample between 18 and 24 h in air and pointed out that the crystalline structure appears around THT = 200°C. XPS investigations.— Figure 3a depicts high resolution Ru 3d spectra recorded from the ruthenium oxide powders with increasing annealing temperature from the bottom 共THT = 25°C兲 to the top 共THT = 300°C兲. Notably, only the spectrum of the untreated sample 共THT = 25°C兲 shows a shoulder at BE = 284.5 eV, resulting from surface contamination with carbon. No such contamination can be observed for the other samples. The samples annealed at temperatures below 200°C display one spin-orbit doublet at BE = 281.1 eV 共Ru 3d5/2兲 and at BE = 285.4 eV 共Ru 3d3/2兲, respectively. Further increase of the annealing temperature has three effects on the appearance of these spectra: 共i兲 a shift of binding energy to lower values 关BE 共Ru 3d5/2兲 ⫽ 280.7 eV; BE 共Ru 3d3/2兲 ⫽ 285.0 eV兴; 共ii兲 the Ru 3d core level spectra appear sharper 共decrease of half width兲; and 共iii兲 a second spin-orbit doublet at BE = 282.6 eV evolves in the Ru 3d5/2 spectral line 共see marker in Fig. 3a兲. From the XRD patterns 共Fig. 2兲 it is evident that the occurrence of the second doublet is strongly related to the morphology of the material. In agreement with the XRD data the second spin-orbit doublet at BE = 282.6 eV occurs parallel to the transition from amorphous to crystalline structure of the powders and gets more pronounced the higher the annealing temperature is. As mentioned in the introduction, the appearance of this second spin-orbit doublet is strongly discussed in the literature. The samples investigated in the present work are polycrystalline. Accordingly, a special O termination of RuO2共110兲 planes, as suggested from density functional theory calculations for a single RuO2 crystal surface,18 can hardly account for this signal. More likely, this doublet is due to final state

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Electrochemical and Solid-State Letters, 9 共6兲 A268-A272 共2006兲 Table I. Parameters used for deconvolution of the O 1s XP-spectra. Species 2−

O OH− H 2O

BE 共eV兲

HW 共eV兲

G 共L兲

TM

CT

ET

529.4 530.8 532.4

0.8 1.6 1.6

0 0.111 0.355

0.124 0.6 0.664

0.013 0.009 0.010

0.029 0.058 0.076

BE: binding energy; HW: half width; G/L: Gauss-Lorentz ratio; TM: tail mix; CT: tail height; ET: tail exponential.

Figure 3. XPS spectra of hydrous ruthenium oxide powders as a function of annealing temperature; 共a兲 Ru 3d region, evolution of a second spin orbit doublet as indicated; 共b兲 O 1s region, binding energies used for deconvolution as indicated.

screening effects caused by the strong Coulomb interaction between d-band electrons and the core hole produced by photoionization.17 The effect can only occur if the RuO2 is crystalline so that hybridization of the orbitals is achieved. The amorphous powders annealed at temperatures below 200°C therefore do not show the second doublet in the XP-spectra. Figure 3b depicts the corresponding O 1s spectra recorded from these samples. The peak shape suggests that one can distinguish three oxide species that can be attributed to H2O, OH−, and O2−.25 It is evident that with increasing annealing temperature that the amount of O2− increases, whereas the amount of H2O and OH− decreases. In order to determine the quantitative evolution of the anionic fraction in dependence of the annealing temperature, deconvolution of the O 1s spectra has been performed. Peak shape and binding energy of all three oxygen species used for deconvolution have been kept constant. The corresponding parameters are listed in Table I. Binding energy of the O2− line has been taken from the sample annealed at 300°C and is determined to be 529.4 eV. This value fits quite well to already published data.17,26 In Ref. 17 a well-defined RuO2共110兲 surface has been measured and an asymmetric line shape of the O2− line in the O 1s spectra has been observed. This tailing has been considered in our analysis. The binding energy from the OH− species has been taken from the maximum of the O 1s signal of the sample dried at 25°C and is 530.8 eV. A closer look on the peak shape of the sample annealed at 300°C strongly suggests the presence of a third species having a maximum at a binding energy of 532.5 eV. This peak position has been reported by OⱊGrady et al.,26,27 interpreted by Chambers et al.17 to be adsorbed H2O and/or CO and CO2 species and is attributed to be H2O in this work. Figure 4 depicts two examples of the fitted O1s spectra. The chosen examples differ strongly in their composition, i.e., Fig. 4a presents the spectrum recorded from the untreated sample 共THT = 25°C兲, while Fig. 4b shows the deconvolution of the powder heated at T = 250°C for 17 h. The sum of the various components fits very well with the measured signal, independent of the materials composition, indicating the quality of the peak evaluation. The quantitative results of this analysis are presented in Fig. 5. Starting with the untreated material in which OH− and H2O dominate the structure, the O2− content increases with increasing annealing temperature from 35% to ⬎ 85% at THT = 300°C. For the ruthenium oxide with the highest capacitance, i.e., annealed at THT = 175°C, 30% of the O 1s signal can be attributed to OH−; 10% to water and about 60% to O2−. In other words, the O2− content determined from the O 1s signal of ruthenium oxides with high capacitance should not exceed a limit of about 60%. In Fig. 6 the water amount determined by XPS is compared to the water amount determined by measurement of the weight loss after the heat-treatment. The starting value for the calculation of water from the gravimetric analysis has been taken from Ref. 4 and the measured weight loss has been totally attributed to the evaporation of water from the hydrous ruthenium oxide. According to Ref. 4 the material calcinated at THT = 300°C for 17 h has 0.11 mol of water in the structure 共RuO2·0.11H2O兲. Comparison of the water amount from gravimetric calculation with the XPS data 共Fig. 6, black and white triangles兲 reveals that for the materials annealed at lower temperatures less water is detected in the O 1s XP-signal than


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Figure 6. Comparison of the Ru/O ratio and water amount determined by gravimetric and XPS analysis; circles: ratio of ruthenium and oxygen 共open symbols: gravimetric analysis; filled symbols: XPS analysis兲; triangles: x共H2O兲 in RuO2·xH2O 共open symbols: gravimetric analysis; filled symbols: XPS analysis兲.

Figure 4. Deconvolution of the O1s signal; 共a兲 hydrous ruthenium oxide as prepared; 共b兲 hydrous ruthenium oxide annealed at T = 250°C for t = 17 h.

Figure 5. Anionic fraction as a function of annealing temperature determined by deconvolution of the O 1s spectra; circles: water; triangles: hydroxide; squares: oxide.

expected. However, at THT ⬎ 160°C, which represents the transition from amorphous to crystalline material, the XPS data is in very good agreement with the calculation from the weight loss measurements. Figure 6 also depicts the changes of the ratio of ruthenium and oxide in dependence of the annealing temperature. The white circles show the ratio calculated from weight loss measurements and the black circles present the data obtained from quantitative XPanalysis, i.e., the ratio of the Ru 3d and O 1s signals. The expected increase of the curve caused by an increase of water losses from the annealing procedure can be nicely followed by gravimetric analysis. The ratio increases continuously from 0.27 共THT = 25°C兲 to 0.47 共THT = 300°C兲. In contrast, for the XPS analysis only a slight increase of the ratio can be observed. Again at the transition point from amorphous to crystalline material at THT = 175°C both methods lead to similar results. This observation fits quite well to accepted models explaining the differences of capacitance in dependence on the morphology and composition of hydrous and anhydrous RuO2.4,7,23,24 According to the model of Sugimoto et al.24 hydrous and anhydrous RuO2 is formed of small primary particles which agglomerate into larger secondary particles. The size and the packing of the primary particles and the water content in the material depend on the annealing temperature. Anhydrous RuO2 has large 共20 nm兲 and densely packed primary particles and hydrous RuO2· 0.5H2O has small 共2 nm兲 and loosely packed particles. According to this study24 anhydrous RuO2 has almost no micropores while hydrous RuO2 has appreciable hydrated micropores. The water is bound to the RuO2 surface in two ways, a chemically bound layer as postulated by Hu et al.7 and a second one, supposed to be physically adsorbed in the micropore’s volume. The physically adsorbed water is expected to evaporate under ultrahigh vacuum 共UHV兲 conditions, and, therefore is not detected in the O 1s XP-signal. In contrast the chemically adsorbed water is detected. Parallel to an increase of particle size the number of adsorbtion sites at the surface decreases, which leads to the observed decreased contribution of water in the O 1s signal. An XPS study by Kim et al.17 with a well-defined RuO2共110兲 surface shows that RuO2 has a strong affinity to adsorb water at the surface, even in the UHV. Kim et al. observed a broadening of the O 1s signal after several hours in their XPS chamber which disappeared after heating to 425°C at an oxygen pressure of 5 ⫻ 10−6 mbar. They assigned this feature to the presence of adsorbed water.

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Electrochemical and Solid-State Letters, 9 共6兲 A268-A272 共2006兲 Conclusions

The transition from amorphous to crystalline ruthenium oxide can be followed by recording the Ru 3d5/2 signal as a second spinorbit doublet evolves at 282.6 eV in the spectra indicating crystallinity. The peak shape of the O 1s XP-spectra provides valuable information about the ruthenium oxide composition and for identification of the material providing the highest electrochemical capacitance values, i.e., 700 F/g. For the best material a formula of RuO2·0.3H2O was determined. Comparison of the water amount obtained from deconvolution of the O 1s signal with the one calculated from gravimetric analysis shows that the water in the micropores of the amorphous material evaporates under UHV conditions whereas the chemically adsorbed water is detected. The decrease of adsorbed water with increasing annealing temperature is related to changes of morphology and to the increase of nanoparticle size. Acknowledgments Financial support by the Swiss Federal Office of Energy 共BfE project no. 100260/150333兲 is gratefully acknowledged. We thank Alwin Frei for the measurement of the XRD spectra. Paul Scherrer Institut assisted in meeting the publication costs of this article.

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