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Characterization of Carbon-Encapsulated Nickel and Iron Nanoparticles by Means of X-ray Absorption and Photoelectron Spectroscopy V. R. Galakhov,*,†,‡ A. S. Shkvarin,† A. S. Semenova,§ M. A. Uimin,† A. A. Mysik,† N. N. Shchegoleva,† A. Ye. Yermakov,† and E. Z. Kurmaev† Institute of Metal Physics, Russian Academy of Sciences s Ural DiVision, S. KoValeVskaya str. 18, 620990 Yekaterinburg, Russia, Ural State Mining UniVersity, Yekaterinburg, Russia, and Institute of Solid State Chemistry, Russian Academy of Sciences s Ural DiVision, PerVomayskaya str. 91, 620990 Yekaterinburg, Russia ReceiVed: July 16, 2010; ReVised Manuscript ReceiVed: NoVember 8, 2010
Nickel and iron nanoparticles encapsulated in carbon were synthesized by contactless levitation melting of metal drops and their evaporation in a flow of an inert gas containing a hydrocarbon. The products were characterized by means of transmission electron microscopy, photoelectron spectroscopy, and X-ray absorption spectroscopy. It was established that carbon coating protects the metallic nanoparticles from the environmental degradation by providing a barrier against oxidation and ensures stability of the ferromagnetic core metal inside that provides their extremely high catalytic activity, biocompatibility, and nontoxicity. Introduction Understanding the electronic structure of nanoscale systems will lead to progress in their application both in technical and biological fields (for instance, nanoparticles are utilized very actively in drug delivery,1 cancer cell diagnostics,2,3 etc.) Carbonencapsulated metallic nanoparticles with ferromagnetic core, which usually contain ferromagnetic nanoparticles inside, constitute a group of new functional materials.4 They are of especial importance due to their high chemical stability toward air oxidation and acid-dissolution as compared with bare ferromagnetic nanoparticles. Interesting magnetic behavior, that is, the soft ferromagnetic characteristic at room temperature, the relative lower saturation magnetization due to the carbon shell, and the large percentage of surface spin with disordered magnetization orientation for nanosized particles, together with their high oxidation resistance makes them significant for catalyst and bionanotechnology applications.4-10 An unusual magnetic behavior of these systems has been found dependent on the size of the particles as well as their degree of crystallinity and chemical state. Carbon-coated ferromagnetic metal nanoparticles are predominantly single domain and display the superparamagnetic behavior.10 Moreover, the carbon coatings can endow magnetic particles with biocompatibility and stability in many organic and inorganic media.4-8,11 The instability, large aggregation, and rapid biodegradation of the pure uncoated (naked) magnetic nanoparticles could be overcome by replacement of the naked magnetic nanoparticles by the encapsulated ones, since the intrinsic properties of the inner magnetic cores in this case are well protected by outer carbon shells. This in turn gives many possibilities for further modification, which provide their extremely high catalytic activity. In earlier work,12 the high catalytic activity and selectivity of carbon-containing nanocomposites based on 3d-metal [Ni@C, Pd@C, (Pd5%Ni)@C] in the reaction of hydrodechlorination of chlorobenzene were * To whom correspondence should be addressed. † Institute of Metal Physics, Russian Academy of Sciences. ‡ Ural State Mining University. § Institute of Solid State Chemistry, Russian Academy of Sciences.
established. However, the mechanism and the nature of such high catalytic efficiency of nanocatalysts based on 3d metals encapsulated in carbon in the conversion of chlorobenzene of hydrogenation is still a subject of discussion. Two variants for the mechanism of catalysis are discussed. One of them is suggested to be caused by zero-valence state of unoxidized metal core, for example, Ni. According to this model, a hydrogen molecule penetrates through a defective layer of carbon coating, reaches the nickel-carbon interface, and dissociates on unoxidized nickel. After that, hydrogen atoms going back to the carbon surface react with chlorbenzene. The second variant suggests the dissociation of hydrogen molecules directly on the carbon coatings containing various defects (such as Stone-Wales topological defects; see details in refs 3 and 4), which can serve as the sites on which the dissociation of hydrogen with low activation energy can occur. There are literature data confirming the existence of a small activation barrier for the dissociation of hydrogen on the topological defects of carbon.13-15 However, up to now, the nature of the catalitic activity of the mentioned nanoparticles is the open question. In the present study, we expect to get an answer to the question about the oxidation state of metal cores, covered with a carbon shell. It is also desirable to obtain information about the electronic state of the carbon coating and its features. The answer to this question is of more general significance, given the nanoscale nature of the carbon coating and its structural features (the presence of topological defects, curvature, etc.) and its comparison with multiwalled carbon nanotubes and graphene. The characterization of carbon-coated nanoparticles is of crucial importance to get better control of their size and morphology, production, and protection against oxidation, which significantly influence their magnetic properties. Many attempts have been made to characterize ferromagnetic metal nanoparticles encapsulated in carbon.4,16-19 The characterization of carbon-coated metal nanoparticles is not trivial. It is very complicated to obtain information about the chemical state of the metal core using common surface-sensitive methods, such
10.1021/jp106612b 2010 American Chemical Society Published on Web 12/02/2010
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as ultraviolet and X-ray photoelectron spectroscopy, because of the protective carbon layer. In the present paper. we have carried out photoelectron and X-ray absorption measurements of carbon-encapsulated Fe@C and Ni@C nanoparticles. We have found that carbon coating protects the metallic nanoparticles from the environmental degradation by providing a barrier against oxidation and ensures stability of the ferromagnetic core metal inside, which provides their extremely high catalytic activity. Experimental Section Nanocomposites on the base of Fe@C, Ni@C were produced in Institute of Metal Physics, Ural Division of the RAS. The piece of Fe or Ni (weight was about 1 g) was heated by induction levitation melting inside of two oppositely directed turns of inductive coil in closed system filled with hydrocarbon containing inert gas (Ar). To melt and suspend the metallic drop, a high frequency (f ∼ 250 kHZ) generator was used. The amplitude of the alternative magnetic field in the volume of drop was about 750 Oe. The drop of liquid metal is permanently fed by the wire of the same material in the process of nanoparticle preparation. The strongly overheated (∼2000 °C) and evaporating drop of liquid metal was blown around by Ar gas containing butane. The expense of the flowing Ar gas in the process of nanopowder synthesis was 130 L/hour at the pressure of 70 Torr. Some amount of butane gas (3.6 L/hour) was injected in the process. The metal vapors were taken away by flow of argon in the colder part of reactor in which the condensation of nanoparticles occurred. Simultaneously, the nucleation of particles is accompanied by hydrocarbon decomposition (or pyrolysis) on the surface of metal nanoparticles and it leads to the formation of the layered carbon coating and capsulation of particles. The coating thickness consists of few or tens of layers of carbon and can vary depending on the hydrocarbon content in argon. Hereinafter the carbon-encapsulated nanoparticles are collected using a special bag house. The average particle size including a carbon coating is controlled by metal drop temperature, argon pressure in the system, and the rate of its passing nearby of melted drop. The morphological characteristics and the structure of the composites were estimated from the high-resolution transmission electron microscopy (TEM) data. The average particle size of nanocomposites can vary in the range of few nanometers up to about 20 nm or more as a function of the mentioned parameters and type of metal. The measurements of the X-ray absorption spectra (XAS) and photoelectron spectra (XPS) of Ni@C and Fe@C were performed using the Multi User Stage for Angular Resolved Photoemission (MUSTANG) experimental station at the Russian-German beamline at BESSY equipped with a PHOIBOS 150 SPECS GmbH electron energy analyzer. The absorption spectra are registered in the total electron yield mode (TEY). The Fe 2p XPS spectrum of Fe@C was measured at the excitation energy of 900 eV. The Ni 2p XPS spectra of Ni@C and C 1s spectra for both Ni@C and Fe@C were obtained at the photon energy of 950 eV. The valence band photoelectron spectra of the nanostructures were measured at 200 eV. The choice of the excitation energy was determined by the production of the highest intensity of photoelectron spectra. The TEM, XAS, and XPS investigations were perfomed in 2 days after the synthesis of the Ni@C and Fe@C samples. We do have have the possibility to carry out measurements of reference samples (pure metals and metal oxides) using synchrotron radiation. X-ray photoelectron spectra of a single
Figure 1. TEM picture of the Ni@C nanoparticles.
Figure 2. TEM picture of the Fe@C nanoparticles.
crystal of FeO and of Ni metal were measured with an ESCA spectrometer from Physical Electronics (PHI 5600 ci) using monochromatic Al KR radiation (hν ) 1486.6 eV). To get a surface free of contamination, the samples were fractured in situ. Results and Discussion Structure of the Fe@C and Ni@C Nanoparticles. The TEM images of Ni@C and Fe@C microstructrues are shown in Figures 1 and 2, respectively. One can see a carbonencapsulated core-shell structure, where the carbon layers with well-ordered arrangement are closely compacted to form the quasi onion nanostructure and surround the dark nickel or iron nanoparticles. Outside the carbon onions, the carbon layers form the disordered structure. The size of Ni nanoparticles ranges from of 2 to 10 nm. The size of Fe nanoparticles is about 10 nm. Many nanocapsules contain a metal particle encapsulated in the cavity and the outer shell is composed of several carbon layers of a few nanometers in thickness. Note in Figure 2 the relatively large particle size was chosen to enhance the visibility of carbon layers. Our X-ray-diffraction analysis for carbon-coated-nickel and iron nanoparticles shows that the positions of all diffraction lines exactly correspond to those of bulk nickel and iron in the facecentered cubic and body-centered cubic lattices, correspondingly. Significant broadening of the diffraction lines is due to the relatively small size of Ni and Fe metal particles (encapsulated with carbon). The average size of coherent scattering regions estimated from the broadening lines of the XRD was less than 8 nm. This value is in agreement with the size of the nanoparticles estimated from TEM measurements (2-10 nm). The carbon shells have a low graphitic crystalline degree, small
Carbon-Encapsulated Nickel and Iron Nanoparticles
Figure 3. Core-level Fe 2p, Ni 2p, and C 1s X-ray photoelectron spectra of Fe@C and Ni@C. The Fe 2p spectrum of Fe@C is measured at the excitation energy of 900 eV. The Ni 2p and C 1s spectra are measured at the excitation energy of 950 eV. For comparison, the spectra of pure metals and oxides are shown. The spectra of metals and oxides are measured at the excitation energy of 1486.6 eV. The spectrum of the Fe metal is reproduced from the paper of Wett et al.21 (Microchimica Acta 156, p. 57-60; D. Wett, A. Demund, and R. Szargan; “Reactions at the interface of ultrathin Fe films deposited on ZnO(0001) and ZnO (000 21) single crystal substrates”, 2007) with permission from Springer-Verlag.
thickness, and even amorphous carbon structure, resulting in an undetectable graphite signal in the X-ray diffraction patterns. The spacing between two neighboring lattice fringes 0.34 nm, as reported in the literature for similar carbon structure.3,19,20 Core-Level X-ray Photoelectron Spectra. Core-level X-ray photoelectron spectra (XPS) could provide information about the chemical states of transition elements in Fe@C and Ni@C nanoparticles. Figure 3 shows the Fe 2p, Ni 2p, and C 1s X-ray photoelectron spectra of Fe@C and Ni@C. For comparison, the spectra of pure metals and oxides are shown. The spectrum of Fe metal is reproduced from the paper of Wett et al.21 One can see that the Fe 2p and Ni 2p spectra are similar to those of pure metals. Therefore the Fe and Ni nanoparticles are characterized by the metallic state. The C 1s peaks at a binding energy of 284.4 eV correspond to the carbon shells of metal-C nanocapsules related to the X-ray photoelectron studies on carbon nanotubes.19,22,23 Note that the low-intensive shoulder of the C 1s spectrum of Fe@C at about 283 eV should be assigned to the iron carbide Fe3C. No carbide signal in the C 1s spectrum of Ni@C was detected. X-ray Absorption Spectra. In transition metal oxides, cation 2p X-ray absorption spectra are dominated by intra-atomic and short-range effects. In view of this, metal 2p X-ray absorption spectra correspond to the metal 2p f metal valence band (3d)
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Figure 4. Fe 2p, Ni 2p, and C 1s X-ray absorption spectra of Fe@C and Ni@C. For comparison, Ni 2p spectrum of pure Ni metal and C 1s spectrum of carbon nanotubes are shown. The XAS spectrum of FeO is reproduced from the work by Prince et al.24 (Phys. ReV. B 71, 085102; K. C. Prince, M. Matteucci, K. Kuepper. S. G. Chiuzbaian, S. Bartkowski, M. Neumann; “Core-level spectroscopic study of FeO and FeS2”; Copyright 2005 by The American Physical Society). Reproduced with permission of authors.
transitions and are determined by the valence state of metal atoms. Figure 4 displays the measured Fe and Ni 2p X-ray absorption spectra the carbon-encapsulated Ni and Fe nanoparticles. For comparison, the XAS spectra of pure metals and oxides are presented. The XAS spectrum of FeO is reproduced from the work by Prince et al.24 The Fe and Ni 2p X-ray absorption spectra of the carbon-encapsulated Ni and Fe nanoparticles are similar to those of metallic Fe and Ni, respectively. This means that Fe and Ni nanoparticles are in the metallic state. Note that the Fe 2p3/2 XAS spectrum of the Fe@C nanoparticles show a feature at about 709 eV corresponded to the main maximum of the Fe 2p XAS spectrum of FeO. It indicates a small oxidiation effect of the Fe@C nanoparticles. C 1s X-ray absorption spectra of Fe@C and Ni@C show the features at 285.6, 288.8, and 292.0 eV which are very similar to those of hydrogenated carbon nanotubes and can be ascribed to transitions of C 1s electron to π*, C-H σ* and σ* states.25 The additional features at 284.5 and 290.5 eV should be attributed to the rolling (helicity) of the carbon layers and defects of nanotubes with corresponding transitions to π′* and σ′* states.26 Valence-Band Photoelectron Spectra. To study the electronic structure, photoemission is ideal for providing information about the occupied states. Figure 5 shows valence band photoemission spectra using photon energy of 200 eV for
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Galakhov et al. the ferromagnetic core metal inside which provides their extremely high catalytic activity. Acknowledgment. We thank Professor M. Neumann (University of Osnabru¨ck, Germany) for the possibility to carry out measurements of photoelectron spectra at the ESCA spectrometer. We thank Mr. D. E. Marchenko and Dr. D. V. Vyalikh for technical support at BESSY. This work is partly supported by the Russian Foundation for Basic Research (Grants 08-0200148 and 10-02-00323), by the Program for Basic Research of the Russian Academy of Sciences “Physics of New Materials and Structures” (Project No 22), and by the bilateral Program “Russian-German Laboratory at BESSY”. References and Notes
Figure 5. Valence-band photoelectron spectra of Fe@C and Ni@C. The spectra are measured at the excitation energy of hν ) 200 eV. For comparison, the spectra of pure metals and oxides are shown. The spectra of Ni metal, NiO, and FeO are measured at the excitations energy of 1486.6 eV. The spectrum of the Fe metal is reproduced from the paper of Gao et al.27 (measured at the excitations energy of 500 eV). (J. Electron Spectrosc. Relat. Phenom. 151, p. 199-203; Xingyu Gao, Dongchen Qi, Swee Ching Tan, A.T.S. Wee, Xiaojiang Yu, Herbert O. Moser; “Thickness dependence of X-ray absorption and photoemission in Fe thin films on Si (001)”, 2006) with permission from Elsevier.
carbon-encapsulated Fe@C and Ni@C nanoparticles. For comparison, the spectra of pure metals and oxides are shown. The XPS spectra of Ni metal, NiO, and FeO are measured at the excitation energy of 1486.6 eV (Al KR excitation line). The spectrum of Fe metal is reproduced from the paper of Gao et al.27 (measured at the excitations energy of 500 eV). It is significant that the spectra of the Fe@C and Ni@C nanoparticles are similar of those of pure metals and are different from those of oxides. According to data of Yeh and Lindau,28 for Fe@C the electron photoionization cross-section ratio for the excitation energy of 200 eV is equal to σ(Fe 3d)/σ(C 2p) ) 63. For Ni@C, the crosssection ratio corresponds to to σ(Ni 3d)/σ(C 2p) ) 110. Consequently, the main contribution to the XPS valence-band spectrum in the presented energy region stems from the metal 3d states. As a rule, X-ray photoelectron valence-band spectra of Fe and Ni metals free of oxidation can be measured on single crystals (after cleaning in high vacuum before the measurements) or on freshly prepared in situ thin films. In our case, it is shown that the comparable high-quality spectra can be obtained in Me@C nanocomposites, which demonstrates high efficiency of carbon-protecting layers. Conclusions In conclusion, we have carried out photoelectron and X-ray absorption measurements of carbon-encapsulated Fe@C and Ni@C nanoparticles. We have found that carbon coating protects the metallic nanoparticles from the environmental degradation by providing a barrier against oxidation, and ensure stability of
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