Chemical Science Research Frontiers 2016 72 Synchrotron radiation Mössbauer spectroscopy for 61 Ni nanoparticles Mössbauer spectroscopy is a well-established and effective method in physics, chemistry, biology and Earth science. It provides element-specific information on the electronic states of each constituent in complex materials, such as valence and magnetism. Most Mössbauer studies have been performed using 57 Fe and 119 Sn with γ -rays from radioactive isotope (RI) sources, although the Mössbauer effect has been observed in approximately 100 nuclides of nearly 50 elements (Fig. 1 ). One major difficulty in Mössbauer spectroscopy using nuclides other than 57 Fe and 119 Sn is the preparation of RI sources. With the exception of 57 Fe and 119 Sn (purchasable), we should synthesize the RI using a reactor or an accelerator, as is also the case with 61 Ni Mössbauer spectroscopy. This difficulty is avoided by using synchrotron radiation (SR); we can extract X-rays with an appropriate energy for Mössbauer spectroscopy from SR. Furthermore, the high brilliance of SR facilitates Mössbauer measurement for materials under extreme conditions, such as high pressures. SR-based Mössbauer absorption spectroscopy is one such method in the energy domain [1], and measurement using the many nuclides shown in Fig. 1 is promising. Recently, the measurement efficiency of this method was drastically improved [2]. Thus, although the natural abundance of 61 Ni is only 1.14%, it has become realistic to perform the SR-based Mössbauer absorption spectroscopy of Ni samples including nanoparticles without expensive isotope enrichment. The hexagonal close-packed (hcp) structure is known to be the metastable state for Ni metal. Recently, hcp-Ni nanoparticles have been synthesized by many groups using chemical reduction methods because hcp-Ni nanoparticles are expected to show novel functionality different from that of typical face-centered cubic (fcc) Ni, such as catalytic action. However, it has been suspected that the nanoparticles formed nickel carbide (Ni 3 C) in which the Ni arrangement is the same as that in hcp-Ni; hcp-Ni and Ni 3 C can hardly be distinguished by X-ray diffraction (XRD). Furthermore, the magnetism of hcp-Ni is still debated. Theoretical calculations predicted the ferromagnetism of hcp-Ni with an atomic magnetic moment similar to that of fcc- Ni of approximately 0.6 μ B ( μ B : Bohr magneton). In contrast, the experimentally observed magnetization of hcp-Ni nanoparticles is at most one-fifth of that of fcc- Ni. Since Ni 3 C is non-magnetic, this discrepancy also casts doubts. Still, other possible factors should be considered: surface oxidization, magnetic impurities, and so forth. To study hcp-Ni nanoparticles in detail, we performed measurements using 61 Ni SR-based Mössbauer absorption spectra. The experimental setup for 61 Ni SR-based Mössbauer absorption spectroscopy is shown in Fig. 2. The experiments were performed at SPring-8 BL09XU and BL11XU . The operating mode of the electron- storage ring was the “203 bunch” mode, where SR with the energy of 61 Ni nuclear resonance is transmitted by a sample in a cryostat. The samples were Ni metal (1 mm thickness at 20 K) or hcp-Ni nanoparticles (0.9 g (component Ni)/cm 2 at 4 K). The nanoparticles were synthesized by a chemical reduction method and coated with polyvinylpyrrolidone. Their crystallite size was around 40 nm as determined by XRD. Both samples were without 61 Ni enrichment. Then, the SR was scattered by 61 Ni 0.86 V 0.14 (enrichment: 86.2%) foil at typically 30 K in another vacuum cryostat. The nuclear resonance energy of the foil was scanned using the Doppler effect by controlling its velocity. An eight-element Si avalanche photodiode (APD) detector was built in the vacuum cryostat of the Ni-V foil, and Fig. 1. Table of elements exhibiting Mössbauer effect. H He Li Be B C N O F Ne Na Mg Al Si P S Cl Ar K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe Cs Ba * Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn Fr Ra ** 104~ *Lanthanide La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu **Actinide Ac Th Pa U Np Pu Am Cm Cf Es Fm Md No Lr Elements including nuclides where Mössbauer effect has been observed Elements without nuclides where Mössbauer effect has been observed Research Frontiers 2016 73 thus there was no X-ray window between the foil and detector, whereas such a window was used in the previous measurement system reported in Ref. 1. This X-ray windowless system [2] drastically improves the detection rate of signals, because both X-rays and electrons can be detected as the emission from the foil, and the distance between the foil and detector can be short, giving a large solid angle for the detector subtended at the foil. Since nuclear resonant scattering (NRS) by 61 Ni is emitted with a delay (half-life 5.34 ns), the NRS was separated from the strong immediate electronic scattering by setting a time window of 3 ns to 20 ns after the SR incidence. The detail of the measurement system is given in Ref. 3. The 61 Ni Mössbauer spectra of bulk fcc-Ni metal and hcp-Ni nanoparticles are shown in Fig. 3. We successfully obtained the spectra within 20 h. The spectrum of the bulk shows a magnetic hyperfine field of 7.7 ± 0.2 T, which agrees well with previous results. The spectrum of the hcp-Ni nanoparticles was reasonably analyzed by assuming one component with a magnetic hyperfine field of 3.4 ± 0.9 T. It does not show a secondary component, such as a 7.7 T (fcc-Ni) or 10 T (nickel oxide NiO) component, and thus the contamination of fcc-Ni and NiO due to surface oxidization of the nanoparticles was below the statistical error of the spectrum. The 3.4 T field also disagrees with that of non-magnetic Ni 3 C. One likely substance for this magnetic hyperfine field is the presence of nickel carbide with less carbon than Ni 3 C. In fact, carbon might be supplied during the synthesis. Fang et al. [4] calculated the decrease in the atomic magnetic moment of Ni in NiC x as x increases in the framework of ferromagnetic order. In the framework, the 3.4 T field corresponds to an atomic magnetic moment of 0.3 μ B under a proportionality assumption similar to that for fcc-Ni, where 7.7 T corresponds to 0.6 μ B . From the value of 0.3 μ B , the chemical composition of the nanoparticles is NiC 0.1 using the calculation in Ref. 4. In summary, we observed 61 Ni SR-based Mössbauer absorption spectra of hcp-Ni nanoparticles using an X-ray windowless system. Although the nanoparticles were not enriched, we successfully obtained the spectrum within 20 h. The Mössbauer spectrum of hcp-Ni revealed that the impurity levels of fcc-Ni and NiO were below the detectable limit, and the nanoparticles consisted of a single component. The spectrum shows the magnetic hyperfine field of 3.4 ± 0.9 T at 4 K, corresponding to an atomic magnetic moment of 0.3 μ B . This can be understood as corresponding to nickel carbide (NiC 0.1 ). Note that this method is also effective for the detailed study of many materials containing Ni. In fact, 61 Ni Mössbauer spectroscopy of the cathode material of a Li-ion battery and a Ni-substituted protein has already been measured using this system [5]. References [1] M. Seto et al. : Phys. Rev. Lett. 102 (2009) 217602. [2] R. Masuda et al. : Appl. Phys. Lett. 104 (2014) 082411. [3] R. Masuda, Y. Kobayashi, S. Kitao, M. Kurokuzu, M. Saito, Y. Yoda, T. Mitsui, K. Hosoi, H. Kobayashi, H. Kitagawa and M. Seto: Sci. Rep . 6 (2016) 20861 . [4] C.M. Fang et al. : Phys. Rev. B 86 (2012) 134114. [5] T. Segi et al .: Hyperfine Interact. 237 (2016) 7; L.B. Gee et al. : Inorg. Chem. 55 (2016) 6866. Ryo Masuda and Makoto Seto* Research Reactor Institute of Kyoto University *Email: seto@rri.kyoto-u.ac.jp Fig. 2. Schematic drawing of the developed X-ray windowless system for 61 Ni SR-based Mössbauer absorption spectroscopy. Fig. 3. 61 Ni SR-based Mössbauer absorption spectra of (a) Ni metal and (b) hcp-Ni nanoparticles. The open circles and squares are experimental data and the lines were fitted by the formulas discussed in Ref. 1. Transmitter Ni-metal or hcp-Ni nanoparticles Scatter: Energy standard 61 Ni 86 V 14 foil 8-element APD Velocity Transducer The same chamber Synchrotron radiation 30.5 × 10 3 30.0 29.5 29.0 28.5 6 4 2 0 –2 –4 –6 33.0 × 10 3 32.0 31.0 30.0 Velocity (mm/s) Counts (a) (b)
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