Structural defects working as active oxygen-reduction sites in partially oxidized Ta-carbonitride core-shell particles probed by using surface-sensitive conversion-electron-yield x-ray absorption spectroscopyHideto Imai, Masashi Matsumoto, Takashi Miyazaki, Shinji Fujieda, Akimitsu Ishihara, Motoko Tamura, andKen-ichiro Ota Citation: Applied Physics Letters 96, 191905 (2010); doi: 10.1063/1.3430543 View online: http://dx.doi.org/10.1063/1.3430543 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/96/19?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Role of oxygen defects on the magnetic properties of ultra-small Sn1−xFexO2 nanoparticles J. Appl. Phys. 113, 17B504 (2013); 10.1063/1.4794140 Optimizing core-shell nanoparticle catalysts with a genetic algorithm J. Chem. Phys. 131, 234103 (2009); 10.1063/1.3272274 Charge redistribution in core-shell nanoparticles to promote oxygen reduction J. Chem. Phys. 130, 194504 (2009); 10.1063/1.3134684 Enhanced photoluminescence from CdS:Mn/ZnS core/shell quantum dots Appl. Phys. Lett. 82, 1965 (2003); 10.1063/1.1563305 Surface reduction of Cr–V 2 O 3 by CO J. Vac. Sci. Technol. A 18, 1906 (2000); 10.1116/1.582444

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Structural defects working as active oxygen-reduction sites in partiallyoxidized Ta-carbonitride core-shell particles probed by usingsurface-sensitive conversion-electron-yield x-ray absorption spectroscopy

Hideto Imai,1,a� Masashi Matsumoto,1 Takashi Miyazaki,1 Shinji Fujieda,1

Akimitsu Ishihara,2 Motoko Tamura,2 and Ken-ichiro Ota2

1Nano Electronics Research Laboratories, NEC Corporation, 34 Miyukigaoka, Tsukuba,Ibaraki 305-8501, Japan2Chemical Energy Laboratory, Yokohama National University, 79-5 Tokiwadai, Hodogaya-ku, Yokohama,Kanagawa 240-8501, Japan

�Received 24 March 2010; accepted 26 April 2010; published online 12 May 2010�

We analyzed the local structure of the surface Ta-oxide phase of TaCN /Ta2O5 core-shell particlesthat have a high oxygen reduction activity by using surface-sensitive conversion-electron-yieldx-ray absorption spectroscopy, suppressing the contribution from the TaCN cores. The radialstructure analysis revealed that the catalytically-active Ta2O5 phase in the TaCN /Ta2O5 particlesurface contains oxygen-vacancy defects with shorter Ta–O bonds leading to the slight expansion ofthe first Ta–O shell. Such oxygen defects are likely responsible for the oxygen reduction capabilityby creating electronically favorable oxygen adsorption sites and electron conduction pathways.© 2010 American Institute of Physics. �doi:10.1063/1.3430543�

Partially oxidized tantalum carbonitrides �TaCN /Ta2O5

or Ta–CNO� have been attracting a lot of attention as a classof nonprecious-metal oxygen reduction electrocatalysts thatpotentially improve the performance of air-breezing energydevices such as fuel cells and lithium-air batteries for wide-spread use.1 While Ta–CNO has a high oxygen reductionreaction �ORR� onset potential �EORR� that is comparable tothat of platinum and even a higher endurance than platinum,2

the ORR current remains insufficient, limiting the overalloutput power of batteries. The primary reason seems to bethe rather low density of the catalytically active sites on Ta–CNO, although the details of the active sites remain un-known. Thus, identifying ORR active sites and modifyingthe surface structure to increase the density of these activesites are the primary issues for developing a high-power bat-tery using Ta–CNO.

The ORR active Ta–CNO particles are synthesized byslightly oxidizing the TaCN �approximately 1000 nm� undera low oxygen pressure of 10−5 Pa at 1273 K, which resultsin a core-shell structure consisting of a TaCN core and a�-Ta2O5 shell �see the transmission electron micrograph inthe inset of Fig. 1�b��.3 Since TaCN does not show ORRresponses, the active sites that adsorb oxygen molecules andcatalyze the water production should be in the near-surfaceTa2O5 phases. We performed x-ray absorption spectroscopy�XAS� measurements on the Ta L3 absorption edge in theconversion-electron-yield �CEY� mode to elucidate the struc-tural characteristics of the active sites present near the sur-face.

XAS offers both structural and electronic structuralinformation from the extended x-ray absorption fine-structure �EXAFS� and x-ray absorption near-edge structure�XANES�, respectively, and thus, it is a suitable method foranalyzing the catalytic properties of materials. However, formultiphase materials that include x-ray-absorbing atoms in at

least two or more phases, the XAS analysis has difficultyseparating the structural and electronic structural informationin the target phase from those in the other phases. For thepresent Ta L3 XAS analysis of TaCN /Ta2O5 core-shells, theabsorption of Ta in the surface Ta2O5 shell is superimposedby that of the Ta in the TaCN cores in the XAS spectra whentaken in the conventional transmission and fluorescencemodes.

To overcome this difficulty, we adopted a CEY-XASmethod. The CEY-XAS, which detects the flux of He+ ionsproduced by the electrons emitted from the sample surfacedue to an Auger process, has a high surface sensitivity, re-stricting the probing depth within the escape depth of the

a�Electronic mail: h-imai@ce.jp.nec.com.

FIG. 1. �Color online� �a� DOO variation in catalytic activity, EORR �theonset potential of ORR�, with respect to reversible hydrogen electrode�RHE�. The degree of the oxidation parameter DOO is defined by the inte-grated peak intensity for the �1 11 0� diffraction of �-Ta2O5 and the �1 1 0�diffraction of TaCN. �b� Atomic fraction of Ta in Ta2O5 phase in partiallyoxidized Ta–CNO particles, estimated by using XRD intensity simulation.The transmission electron micrograph shows a cross-section image of theTa–CNO particles. The surface layer consists of �-Ta2O5. The inset shows atypical XRD pattern of the TaCN /Ta2O5 core-shell particles.

APPLIED PHYSICS LETTERS 96, 191905 �2010�

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Auger electrons.4 The probing depth of CEY-XAS for aTa L3 edge �corresponding Ta-LMM Auger process� is esti-mated to be 28.5 nm for �-Ta2O5 �density: 8.73 g /cm3� byusing the Schroeder formula.5 This indicates that for a sur-face oxide with a thickness greater than approximately 30nm, we can selectively analyze the local structure around theTa atoms for the surface oxides within the probe depth with-out the unwanted contribution from the Ta atoms in theTaCN cores.

Figure 1 shows the relationship between the degree ofsurface oxidation �DOO� and EORR for several Ta–CNOcore-shell particles. We define DOO by using a ratio of theprimal XRD peak intensities for TaCN and �-Ta2O5:DOO= ITa2O5

�1 11 0� / �ITa2O5�1 11 0�+ ITaCN�1 0 0��. As shown

in Fig. 1�b�, DOO is related to the atomic fraction of Ta inthe surface Ta-oxides that are relative to the total amount ofTa in TaCN /Ta2O5. In Fig. 1�a�, the EORR monotonicallyincreases up to DOO=0.15 �atomic fraction of the oxide isapproximately 30%�, and then saturated at EORR=0.86 V.We note that the stoichiometric �-Ta2O5 �DOO=1� does notshow any remarkable ORR activity. �The observed ORR ac-tivities for DOO=0 and DOO=1 are attributed to the contri-bution from carbon black that is mixed with the catalysts tomaintain electrical contact between the catalysts andelectrodes1� These behaviors indicate that once the surface ofthe TaCN core is covered with a �-Ta2O5-like phase, thecatalytic properties of the particles depends on the nature ofthe surface Ta2O5 phase, irrespective of DOO, viz., the thick-ness or volume of the oxide. Moreover, the surfaceTa2O5-like phase coexisting with the TaCN cores �in an in-termediate DOO range of 0.15–0.96� should have differentphysical and chemical properties from those of stoichio-metric �-Ta2O5, and the differences are presumably closelyrelated to the emergence of ORR activity.

We determined the above differences by using a CEY-XAS method. Figures 2�a�, 2�c�, 2�b�, and 2�d� show the

Ta L3 XANES spectra and radial structure functions obtainedfrom the Ta L3 EXAFS in a CEY and in a transmissionmode, respectively. In the transmission XAS, both white lineintensities in the XANES spectra and the radial structurefunctions continuously change from DOO=0 �TaCN� toDOO=1 �stoichiometric Ta2O5�, reflecting the increase inoxide-phase volume, since the transmission XAS probes theaverage characteristic of whole particles. In the CEY mode,on the other hand, the XANES spectra and radial structurefunctions have discontinuous behaviors that are similar to theDOO variation in EORR �Figs. 2�c� and 2�d��; the white lineintensities and the peak in the radial structure function near1.6 Šslightly increases up to DOO=0.15, and then remainsat almost the same level up to 0.96, and finally increasesagain at DOO=1.0.

These results indicate that, below DOO=0.15, the TaCNphase remains within approximately 30 nm �probing depth�below the surface, and thus, the CEY-XAS spectra includes acontribution from the TaCN cores. Above DOO=0.15, how-ever, the surface oxide phase grows to a thickness that islarger than the probing depth, and consequently, CEY-XAScan selectively detect the surface information with the exclu-sion of any contribution from the cores. Since both theXANES spectra and the radial structure functions �thus, thestructural and electronic properties� are identical in the inter-mediate ORR range �0.15�DOO�0.96� regardless ofDOO, the electronic and structural differences between suchintermediate DOOs �from 0.15 to 0.96� and DOO=1 shouldbe responsible for the emergence of the ORR activity.

Accordingly, we examined the structural difference be-tween the stoichiometric Ta2O5 �DOO=1� and the ORR-active Ta2O5 �DOO=0.96�, focusing on the FT amplitudenear the scattering length �R� of 1.6 Å �Fig. 3�a��. The crystalstructure of �-Ta2O5 �orthorhombic, P2mm �25�, a=6.198 Å, b=40.290 Å, and c=3.888 Å� consists of pen-tagonal TaO7 bipyramids and TaO6 octahedra set in corner-and edge-sharing arrangements �the lower part of Fig. 3�b��.6

The bond lengths for the first Ta–O shells are 1.91–1.97 Åfor TaO6 and 1.91–2.36 Å for TaO7. The broad peak around1.6 Å in the radial structure function resulted from thesebonds. �Note that the peak maximums in the radial structurefunctions are shorter than the actual bond lengths due to thedifference in phase shifts in Ta and O.7� Thus, the decrease inFT amplitude with R near 1.6 Å is reasonably ascribed to theformation of oxygen vacancies in the Ta–O first shells.

To get further information, we simulated the Ta–O localstructure by using a reduced �-Ta2O5 model, although acomplete determination of the local structure is difficult toobtain due to the Nyquist’s theorem restriction.8 We used a

FIG. 2. �Color online� XAS Spectra for TaCN /Ta2O5 particles. �a� CEY-XAS XANES spectra. �b� XANES spectra taken in transmission mode. �c�Fourier transforms of Ta L3 EXAFS �CEY mode�. �d� Fourier transforms ofTa L3 EXAFS �transmission mode�. The FT amplitude for CEY-XAS forbelow DOO=0.15 is rather small due to the presence of the Ta–C or Ta–Nbonds, while in the DOO range from 0.15 to 0.96, the radial structure func-tions have a Ta2O5-like characteristic with a peak near 1.6 Å correspondingto the Ta–O bonds. However, the maximum amplitude is slightly smallerthan those of DOO=1 �stoichiometric Ta2O5�.

FIG. 3. �Color online� Fourier transforms of Ta-L3 EXAFS for Ta–CNOparticles with DOO=1.0 and DOO=0.96. The solid lines are the simulatedradial structure functions assuming a simplified �-Ta2O5 model.

191905-2 Imai et al. Appl. Phys. Lett. 96, 191905 �2010�

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simplified model structure for �-Ta2O5 �Ta4O20� proposedby Ramprasad,9 which is cut out from a prefect �-Ta2O5

crystal and well explains the electronic properties of�-Ta2O5.8 As demonstrated in Fig. 3�a�, the simulation re-produced the experimental FT data by setting the appropriateparameters for DOO=1 listed in Table I. In the simulation,the representative R values were taken from Fig. 3�b� and thecoordination numbers for the Ta–O bonds were fixed to rep-resent the stoichiometry. Then, the fitting of the radial struc-ture function for DOO=0.96 was carried out and reasonablycompleted by reducing the coordination number for theshortest Ta–O bonds �1.912 Å� from 2.8 to 2.4 with the otherparameters left unchanged. This suggests that oxygen vacan-cies are introduced at an O site at a Ta–O length of approxi-mately 1.912 Å, causing slight lattice expansions �a slightshift in radial structure functions�. When looking at Fig. 3�b�,the in-plain O sites in a TaO6 octahedra, and the atop sites ofboth the pentagonal TaO7 bipyramids and TaO6 octahedraare the candidates for these vacancy sites. Since, generally,the formation energy of oxygen vacancies is lower for thethreefold in-plain sites than for the twofold interplain sites,oxygen vacancies would be introduced more easily in thein-plane sites.8,9 According to Ramprasad’s density-functional-theory calculation, such oxygen defects possiblycreate midgap states with deep occupied and shallow unoc-cupied states that potentially hybridize with oxygen mol-ecules. In another work, the defects states were found topossibly delocalize at a bandwidth of 1 eV.10 These states,therefore, could provide oxygen adsorption sites and alsocould produce electron conduction paths, both of which are

necessary for electrochemical ORR to effectively occur innearly insulating material.

In conclusion, we investigated the local structure aroundTa atoms in the surface oxide phase of TaCN /Ta2O5 core-shell particles to obtain information on the ORR mechanism,especially for the catalytically active sites. By using a CEY-XAS method, the local structure of Ta in the oxide shellscould be selectively analyzed by excluding the influence ofthe TaCN cores. Our CEY-XAS analysis revealed that theORR active Ta-oxide contains oxygen-vacancy sites, mostlikely at the in-plane threefold sites, providing both oxygenadsorption sites and electron conduction paths. It is interest-ing that nearly insulating materials, which are usually con-sidered unsuitable for electrocatalysts, could work if theyhave defects on their surfaces. We hope that such informa-tion may open up the possibility of other oxide-based ORRcatalysts, expanding the range of alternative choices forplatinum-based ORR catalysts.

This work was performed under the “Nonprecious metaloxide-based cathode for PEFC Project” supported by theNew Energy and Industrial Technology Development Orga-nization �NEDO�. The synchrotron experiments, the trans-mission XAS, and CEY-XAS measurements, were carriedout on the BL16B2 and BL14B2 beamlines at SPring-8 un-der approval from the Japan Synchrotron Radiation ResearchInstitute �JASRI� �Proposal Nos. 2008B5392, 2009A5391,2009B5390, 2008A1892, 2008B1850, 2009A1803, and2009B1821�.

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5S. L. Schroeder, Solid State Commun. 98, 405 �1996�.6N. C. Stephenson and R. S. Roht, Acta Crystallogr. B 27, 1037 �1971�.7D. C. Koningsberger and R. Prins, X-Ray Absorption �Wiley, New York,1988�.

8N. A. Young and A. J. J. Dent, J. Synchrotron Radiat. 6, 799 �1999�.9R. Ramprasad, J. Appl. Phys. 94, 5609 �2003�.

10H. Sawada and K. Kawakami, J. Appl. Phys. 86, 956 �1999�.

TABLE I. Coordination numbers and bond lengths for Ta–O first shells.Some of the Ta–O bonds were further merged for the simulation as listed.

R N �DOO=1� N �DOO=0.96�

1.912 2.8 2.41.923 0.5 0.51.944 1.0 1.01.976 0.5 0.52.008 0.5 0.52.359 1.0 1.0

191905-3 Imai et al. Appl. Phys. Lett. 96, 191905 �2010�

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