J. Phys. Chem. C 2009, 113, 10747–10750
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Immobilization of RuO2 on Carbon Nanotube: An X-ray Absorption Near-Edge Structure Study J. G. Zhou,† H. T. Fang,‡ Y. F. Hu,† T. K. Sham,*,§ C. X. Wu,| M. Liu,⊥ and F. Li⊥ Canadian Light Source Inc, Saskatoon, Canada, School of Materials Science and Engineering, Harbin Institute of Technology, Harbin, People’s Republic of China, Department of Chemistry, UniVersity of Western Ontario, London, Canada, School of Chemical Engineering and Technology, Harbin Institute of Technology, Harbin, People’s Republic of China, and Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, China ReceiVed: March 30, 2009; ReVised Manuscript ReceiVed: April 28, 2009
The electronic structures of carbon nanotube/RuO2 core/shell nanocomposite (RuO2 thin layer coated multiwalled carbon nanotubes (MWNTs)) have been studied by X-ray absorption near-edge structures (XANES) at C K-edge, O K-edge, and Ru M5,4- and L3-edges. The variation in white-line features of the XANES at these edges supports strongly that RuO2 interacts with MWNTs through Ru-O-C bonding, which also results in charge redistribution between C 2p-derived states in MWNT and the conduction band in RuO2. Such chemical bonding is necessary to immobilize RuO2 on MWNT and ensures good conductivity of MWNT/ RuO2 core/shell nanocomposite. Introduction Ruthenium oxide (RuO2)-coated carbon nanotubes are useful functional nanocomposites in many applications, including supercapacitors, fuel cells, catalysts, biosensors, and field emitters.1-8 In particular, this nanocomposite has exhibited excellent performance in supercapacitors, which are the essential device in electric vehicles. The information of its electronic behavior and bonding is crucial in understanding and predicting its properties. These properties strongly depend on the interaction between RuO2 and carbon nanotubes. As has been recognized in conventional solid catalyst-support9 and nanoparticles (Pt or SnO2)-carbon nanotube composites,10-12 the interaction between particles and the support is important in achieving the desired functionality. Despite the significance to the understanding of its functionality, studies of nanoparticlesubstrate interaction are still lacking, partly because of the complexity of the problem and partly because of relatively limited analysis methods. X-ray absorption near-edge structures (XANES) spectroscopy involves the measurement and interpretation of the photoexcitation cross-section across a particular core level (absorption edge) of an atom in a chemical environment up to ∼50 eV above the threshold. The absorption features in this region track bound to bound and bound to quasi-bound (multiple scattering) transitions. Thus this technique is element specific and it is very sensitive to the local chemistry of the absorbing atom. XANES has been successfully applied to investigate the chemical bonding, electronic structure, and surface chemistry of many nanomaterials.13-17 Specifically, the XANES of C K-edge and Pt M3-edge, and C K-edge and Sn M5,4 edge, have been applied * To whom correspondence should be addressed. Fax: +1-519-661-3022. E-mail:
[email protected]. † Canadian Light Source Inc. ‡ School of Materials Science and Engineering, Harbin Institute of Technology. § University of Western Ontario. | School of Chemical Engineering and Technology, Harbin Institute of Technology. ⊥ Chinese Academy of Sciences.
to elucidate the synergic-bonding interactions in a Pt NPs-carbon nanotubes composite system5 and in a SnO2 NPs-carbon nanotubes composite,12 respectively. Ru K-edge X-ray absorption spectroscopy has also been applied to study the structure of zeolite-confined nano-RuO2,21 especially the role of RuO2 · xH2O. In this work, we report a study of RuO2-CNT interactions in CNT/RuO2 core/shell nanocomposite using XANES. Experimental Section RuO2-coated multiwalled carbon nanotubes (MWNTs) and RuO2 nanoparticles were prepared by a sol-gel method.8 The as-received MWNTs with diameters of 60-100 nm (purchased from Shenzhen Nanotech Port Co. Ltd.) were first treated by refluxing in nitric acid (40%) at 110 °C for 2 h to generate oxygen-containing functional groups on the surface of MWNTs and are henceforth denoted MWNTs. MWNTs (100 mg) were then dispersed in a 0.1 M RuCl3 (30 mL) solution by ultrasonication for 5 min. A 0.3 M NaHCO3 aqueous solution was added slowly to the above mixture under stirring until the pH of the solution reached 7. After another 15 h of stirring, the sediments were washed several times with distilled water, and then dried in air flow at room temperature for 10 h. The black powder thus obtained was annealed at 150 °C for 19 h in air and the final product is henceforth denoted MWNT/RuO2. RuO2 nanoparticles were prepared following a similar sol-gel process in 0.1 M RuCl3 in the absence of MWNTs. The black powder was also annealed at 150 °C for 19 h in air and is henceforth denoted RuO2 NPs. The SEM image of MWNT/RuO2 in Figure 1a exhibits that MWNTs are coated by RuO2 to form a core/shell structure. The TEM image of CNT/RuO2 (inset) at the upper corner of Figure 1a shows the RuO2 coating on MWNTs. The SEM image of RuO2 NPs in Figure 1b shows that the diameter is around 100 nm. The XANES at the C K-edge, Ru M5,4-edge, and O K-edge were obtained on the spherical grating monochromator (SGM) beamline (∆E/E: ∼10-4) and Ru L3-edge on the Soft X-ray Microcharacterization Beamline (SXRMB, ∆E/ E: ∼10-4) at the Canadian Light Source (CLS), a 2.9 GeV third
10.1021/jp902871b CCC: $40.75 2009 American Chemical Society Published on Web 05/19/2009
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Figure 1. (a) SEM image of MWNT/RuO2 and TEM of MWNT/RuO2 (inset) and (b) SEM image of RuO2 NPs.
generation synchrotron source, using a Si(111) double crystal monochromator. XANES were recorded in the surface sensitive total electron yield (TEY) with use of specimen current and the bulk sensitive total fluorescence yield (FLY) with a channel plate detector. In soft X-ray XANES, FLY often suffers from thickness effect and exhibits broadening except in very thin specimens. The FLY, however, complements TEY in tracking major surface contamination if it is present. Data were first normalized to the incident photon flux I0 measured with a refreshed gold mesh at SGM prior to the measurement and with an ion chamber at SXRMB. After background correction, the XANES are then normalized to the edge jump, the difference in absorption coefficient just below and at a flat region above the edge (297, 550, and 2920 eV for C, O, and Ru, respectively). Results and Discussions Figure 2a shows the C K-edge XANES of MWNTs and MWNTs/RuO2 overlapping with the Ru M5,4-edge XANES of RuO2 NPs in TEY. The photon energy is calibrated to the C 1s to π* transition of CNTs at 285 eV.12The FLY (Figure 2b) is consistent with that of the TEY (Figure 2a) in terms of peak positions except for some broadening and variation in relative intensities but clearly shows no noticeable surface contamination. Ru M5,4-edge in RuO2 occurs in the same energy range as the C K-edge as seen in Figure 2. Since XANES follows the dipole transition selection rule,18 the Ru M5,4-edge and C K-edge are dominated by the electronic transitions from Ru 3d5/2,3/2 to Ru 4p and C 1s to C 2p, respectively. The area under the resonance in the vicinity of the threshold in XANES (also called whiteline) is proportional to the unoccupied density of states (DOS) for a randomly oriented sample (angular dependence averages out). The resonances beyond whiteline involve transitions to the higher energy band/multiple scattering states which are very sensitive to the local environment. Let us first focus on the XANES of MWNTs collected at TEY in Figure 2 a, where two main peaks are clearly displayed at ∼285 and ∼291 eV attributable to C 1s to the graphitic C-C π* and C-C σ* transitions,19 respectively. The transitions at ∼288 eV are characteristic of chemical defects and can be attributed to carboxylic-type CdO groups resulting from the oxidation of MWNTs.14 A weak pre-edge transition at 283.5 eV is also observed, which can be associated with disordered carbon (e.g., amorphous carbon). The carbon-oxygen species and disordered carbon in MWNTs result from the oxidation of pristine MWNTs in HNO3. Introducing CdO function groups
Figure 2. C K-edge XANES of CNTs and CNT/RuO2 nanocomposite along with Ru M5,4-edge XANES of RuO2 NPs recorded in (a) TEY and (b) FLY.
in MWNTs has been identified as a crucial step for the immobilization of RuO2 NP, forming a uniform coating on carbon nanotubes.20 We next look at the spectrum of the RuO2 NP in Figure 2a. The peaks at 283.5 and ∼288 eV correspond to the Ru M5and M4-edge (3d5/2, 3/2-p transitions), respectively, probing the unoccupied 4p densities of state (DOS) in RuO2. The shoulder just above the M5-edge and the weak feature at 286 eV are either part of the RuO2 DOS or carbonaceous species on the RuO2 surface. There is no noticeable feature present at 285 eV indicating that there is no detectable unsaturated carbon on the RuO2 surface. The feature at 290.5 eV, which is also seen in FLY, is a possible sign of surface impurity, but this feature is also reproduced in band structure calculations of RuO2 rutile
Immobilization of RuO2 on Carbon Nanotube
Figure 3. O K-edge XANES of CNT/RuO2 nanocomposite and RuO2 NPs.
(to be published elsewhere). Fortunately, regardless of its origin, it is weak and is not in the region of interest (286-289.5 eV) where significant CNT-RuO2 interaction is observed. We can now interpret the XANES of MWNT/RuO2. From Figure 2a, we can clearly identify the Ru M5,4-edges from RuO2 NP and the MWNT π* and σ* transitions. The intensity of the RuO2 features (Ru M5,4) is greatly reduced in the FLY in which the underlying MWNT features are more apparent as expected from a more bulk sensitive technique. Several interesting features are noted from the TEY. First, π* transition around 285 eV in the nanocomposites is shifted by ∼0.3 eV to lower photon energy from that of MWNTs. The presence of π* transition in MWNT/RuO2 shows that the graphitic framework of the MWNT remains intact upon the coating of RuO2. Thus good electric conductivity in this nanocomposite can still be expected. The energy shift infers interaction of RuO2 with MWNT at the interface, which is required for the immobilization of RuO2 on MWNT. Second, the π* transition intensity, which reflects unoccupied DOS of π* character, is reduced in MWNT/ RuO2 compared to MWNTs. This provides direct evidence that charge transfer from RuO2 to C 2p-derived π* states in MWNTs has occurred at the interface. In the FLY XANES (Figure 2b), which is more bulk sensitive, the Ru M5 is suppressed and the π* intensity is nearly normal. It is conceivable that RuO2, which is slightly metallic, donates electrons to MWNT resulting in the reduction of unoccupied π* DOS in MWNT at the interface. The third and perhaps the most interesting feature in MWNT/ RuO2 is the dramatic intensity increase of the 288 eV transitions, indicating that there is a strongly oxidized C environment at the interface, which withdraws a significant amount of charge from C producing a localized, high density of unoccupied states of C p character. It should be noted that Ru M4 transition (close to 288 eV) from RuO2 coating alone cannot make such a sharp increase because its contribution is proportional to Ru M54 transition, which is already weak. The presence of this sharp peak at 288 eV, together with observation of the behavior of the π* transition, must be interpreted as the result of strong interaction of RuO2 with MWNT through a Ru-O-C bonding, conceivably the carboxylic oxygens are bonded to the Ru. It should be noted that this interaction is confined to the interface since the more bulk-like FLY exhibits a normal MWNT XANES and a considerably weaker 288 eV peak, as seen in Figure 2b. The O K-edge XANES of the MWNT/RuO2 and RuO2 NPs are shown in Figure 3. Those spectra again reflect the dipole electron transitions from the core level O 1s into the unoccupied O 2p projected states above the Fermi level in RuO2. Due to hybridization between O 2p and Ru 4d and Ru 5sp,22 the XANES features represent (1) transitions into the O 2p-Ru 4d
J. Phys. Chem. C, Vol. 113, No. 24, 2009 10749
Figure 4. Ru L3-edge XANES of CNT/RuO2 nanocomposite, RuO2 NPs, and RuO2 powder (Aldrich).
hybridized bands which split into peaks a1 and a2 and (2) transitions into O 2p-Ru 5sp hybridized bands (a broad peak b). Since the transition maps the O 2p projected electronic states, area under the whiteline is proportional to unoccupied DOS of O 2p character. A close examination of the absorption whitelines shown in the Figure 3 reveals an increase in intensity in the MWNT/RuO2 XANES relative to that of RuO2 NPs. Pending no countervailing symmetry (texture effect) and inhomogeneity arguments, the increase in unoccupied DOS indicates charge redistribution (depletion of O p electrons) between RuO2 and MWNT, which has been observed by C K-edge XANES (C π* states (transition at 285 eV) gains e- charge). Another interesting feature is the lack of well-defined O 1s to σ* O-H transitions in water. Active RuO2 catalysts often contain hydrated water. Thus it is likely that water is present in a small amount and its O K-edge resonance suffers from solid state broadening. We will return to this in the following Ru L3,2-edge XANES discussion. In Figure 4, we show the Ru L3-edge XANES of MWNT/ RuO2, RuO2 NPs, and RuO2 powder (Aldrich). The similarity between these spectra confirms the same valence state in these substances. The weak peak at ∼2820 eV in sol-gel produced MWNT/RuO2 and RuO2 NPs is the Cl K-edge from Cl-, which is from the remaining precursor, RuCl3. The intense Ru L3 whiteline at ∼2841 eV arises from Ru 2p3/2 to 3d5/2,3/2 transition in a tetragonally distorted Oh environment (rutile) and the broad resonance at ∼50 eV above the whiteline is from the shape resonance transition due to the multiple scattering.23,24 Close examination of the Ru L3-edge whiteline in MWNT/RuO2, RuO2 NPs, and the standard, rutile RuO2 powder (microcrystals), reveals that the whiteline is a doublet resulting from crystal field splitting. Analysis of the doublet (second derivative) shows a significant difference in the apparent crystal field splitting. Rutile RuO2 microcrystal has the largest value of 2.64 eV compared to the values of 2.22 and 2.03 eV for RuO2 NP and MWNT/ RuO2, respectively This difference is apparent from the inset of Figure 4 where the 2p-t2 g transition is clearly visible for rutile RuO2 but less obvious for RuO2 NP and MWNT/RuO2. We attribute this reduction in crystal field splitting in the RuO2 NP and MWNT/RuO2 to a reduction in tetragonal distortion and repulsion due to the presence of RuO2 · xH2O and RuO2-MWNT interaction, respectively. This, to our knowledge, is the first direct observation of a change in local environment in these materials. More detailed discussion on the interplay of theory and spectroscopy of this system will be described elsewhere.
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Conclusions In summary, XANES at the C K-edge (along with Ru M5,4), O K-edges, and Ru L3-edge has been used to characterize the electronic structure of RuO2-coated MWNTs. We found strong evidence from C K-edge XANES, especially the sharp feature at 288 eV, that RuO2 interacts with MWNT through Ru-O-C bonding at the interface, which facilitates the immobilization of RuO2 on MWNT and ensures good conductivity. We also found noticeable changes in the local environment between microcrystals of rutile RuO2, RuO2 NP, and MWNT/ RuO2. Such interaction also alters the unoccupied electronic states in MWNT and RuO2 and potentially can be used to track the performance of these materials. The results also demonstrate the advantage of XANES in clarifying the interaction in this complex nanocomposite. Acknowledgment. We thank Dr. R. Blyth and T. Regier of the Canadian Light Source for their technical assistance on the beamline. The work at the University of Western Ontario was supported by NSERC, OIT, and CRC (T.K.S.). Research at HIT and IMR was supported by the National Science Foundation of China (Grant Nos. 50872026 and 5060211). Research at CLS is supported by NSERC, NRC, CIHR, and the University of Saskatchewan. References and Notes (1) Hu, H.; Zeng, K.; Zhang, Y.; Peng, F.; Wang, H. J.; Yang, J. J. Phys. Chem. C 2008, 112, 11875–11880. (2) Leela Mohana Reddy, A.; Ramaprabhu, S. J. Phys. Chem. C 2007, 111, 7727–7734. (3) Cao, L.; Scheiba, F.; Roth, C.; Schweiger, F.; Cremers, C.; Stimming, U.; Fuess, H.; Chen, L.; Zhu, W.; Wiu, X. Angew. Chem. 2006, 118, 5441–5445. (4) Sun, Z.; Liu, Z.; Han, B.; Miao, S.; Du, J.; Miao, Z. Carbon 2006, 44, 888–893. (5) Ye, J.; Cui, H.; Liu, X.; Lim, T.; Zhang, W.; Sheu, F. Small 2005, 1, 560–565.
Zhou et al. (6) Fu, X.; Yu, H.; Peng, F.; Wang, H.; Qian, Y. Appl. Catal., A 2007, 321, 190–197. (7) Wang, J.; Tangkuaram, T.; Loyprasert, S.; Vazquez-Alvarez, T.; Veerasai, W.; Kanatharana, P.; Thavarungkul, P. Anal. Chim. Acta 2007, 581, 1–6. (8) Liu, H.; Noguchi, T.; Kato, S. J. Vac. Sci. Technol. B 2007, 25 (6), 1814–1818. (9) Baker, R. T. K.; Tauster, S. J.; Dumesic, J. A.; Strong Metal Support Interactions; American Chemical Scociety: Washington, D.C., 1986. (10) Zhou, J. G.; Zhou, X. T.; Sun, X. H.; Li, R. Y.; Murphy, M.; Ding, Z. F.; Sun, X. L.; Sham, T. K. Chem. Phys. Lett. 2007, 437, 229–232. (11) Hull, R. V.; Li, L.; Xing, Y. C.; Chusuci, C. C. Chem. Mater. 2006, 18, 1780–1788. (12) Zhou, J. G.; Fang, H. T.; Maley, J. M.; Ko, J. Y. P.; Murphy, M.; Chu, Y.; Sammynaiken, R.; Sham, T. K. J. Phys. Chem. C 2009, 113, 6114– 6117. (13) Tang, Y. H.; Sham, T. K.; Hu, Y. F.; Lee, C. S.; Lee, S. T. Chem. Phys. Lett. 2002, 366, 636–641. (14) Kuznetsova, A.; Popova, I.; Yates, J. T.; Bronikowski, M.; Huffman, C.; Liu, J.; Smalley, R. E.; Hwu, H. H.; Chen, J. G. J. Am. Chem. Soc. 2001, 123, 10699–10704. (15) Zhou, X. T.; Heigl, F.; Murphy, M. W.; Sham, T. K.; Regier, T.; Coulthard, I.; Blyth, R. I. R. Appl. Phys. Lett. 2006, 89, 213109(1)213109(3) . (16) Zhou, J. G.; Zhou, X. T.; Sun, X. H.; Murphy, M.; Heigl, F.; Sham, T. K.; Ding, Z. F. Can. J. Chem. 2007, 85, 756–760. (17) Zhou, X. T.; Zhou, J. G.; Murphy, M. W.; Ko, J. Y. P.; Heigl, F.; Rehier, T.; Blyth, R. I. R.; Sham, T. K. J. Chem. Phys. 2008, 128, 144703(1)-144703(5) . (18) Sham, T. K.; Naftel, S. J.; Coulthard, I. J. Appl. Phys. 1996, 79, 7134–7138. (19) Banerjee, S.; Hemraj-Benny, T.; Balasubramanian, M.; Fischer, D. A.; Misewich, J. A.; Wong, S. S. Chem. Commun. 2004, 7, 772–773. (20) Kim, Y. T.; Mitani, T. Appl. Phys. Lett. 2006, 89, 033107(1)033107(3) . (21) Zhan, B. Z.; White, M. A.; Sham, T. K.; Pincock, J. A.; Doucet, R. J.; Ramana Rao, K. V.; Robertson, K. N.; Cameron, T. S. J. Am. Chem. Soc. 2003, 125, 2195–2199. (22) Tsai, H. M.; Babu, P. D.; Pao, C. W.; Chiou, J. W.; Jan, J. C.; Krishna Kumar, K. P.; Chien, F. Z.; Pong, W. F.; Tsai, M. H.; Chen, C. H.; Jang, L. Y.; Lee, J. F.; Chen, R. S.; Huang, Y. S.; Tsai, D. S. Appl. Phys. Lett. 2007, 90, 042108(1)-042108(3) . (23) Sham, T. K. J. Am. Chem. Soc. 1983, 105, 2269–2270. (24) Kim, P. S.; Sham, T. K.; Zhang, P.; Fung, M. K.; Lee, S. T.; Hu, Y. F.; Yates, B. W. J. Am. Chem. Soc. 2001, 123, 8870–8871.
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