NMR study of magnetic nanoparticles Ni@C

The 61Ni, 13C NMR spectra of carbon encapsulated nickel nanoparticles have been obtained. It has been shown that the cores of the particles consist of metallic nickel with face-centered cubic structure, nickel carbide Ni3C and carbon-nickel solid solution. The carbon shell of nanoparticles is a highly defective structure and close to an amorphous glassy-like carbon.


Introduction
Magnetic nanoparticles are of considerable interest both from the fundamental point of view, and with the possibility of their practical application in medicine, spintronics, sensor devices, supercapacitors, catalytic processes, etc. [1][2][3][4][5][6]. As for medical purposes, it is the most convenient to use nanoparticles in a carbon graphite-like shell, as it is extremely stable under the influence of chemicals and temperature factor [7,8]. On the other hand, the carbon shell is compatible with biological tissues [9].
The properties of these particles depend on many factors: size, phase composition, thickness of the carbon shell, method of synthesis, and so on. In our research the nanocomposites synthesized by the gas-phase method were studied [10].
Experimental methods used to study nanoparticles and nanocomposites usually include both a set of traditional methods (magnetization measurement, x-ray diffraction, neutron diffraction), and local methods (electron microscopy, photoemission spectroscopy, optical methods, EPR, NMR). It should be noted that diffraction methods of structure investigation (x-rays and neutrons) are ineffective for nanoparticles smaller than 10 nm, and the analysis of photoelectron spectra provides only qualitative information [10]. In this case the advantage of local methods is significant [11].

Experimental details
Nanoparticles Ni@C were prepared by the gas-phase synthesis. Levitating droplet of liquid nickel was blown around a stream of inert gas (argon) containing hydrocarbons. Nanopowder accumulated on a special filter. Details of the synthesis are available in [12,13].
The X-ray diffraction patterns of the nanoparticles were measured using a high resolution X-ray diffractometer Empyrean 2 with the Cu Kα radiation. Initial processing, calculation of lattice parameters, and determination of the size of coherent scattering blocks were performed using the HighScore Plus software. The magnetization has been measured at room temperature using a vibrating sample magnetometer in a magnetic field up to 3 T.
The 13 C NMR spectra have been obtained on a Bruker AVANCE 500 pulsed NMR spectrometer in an external magnetic field, H0 = 11.747 T. The 61 Ni NMR signals were detected in H0 = 0 at a temperature of 4.2 K. A few MHz wide NMR spectrum has been obtained by measuring an integrated intensity of the 61 Ni spin echo signals at equidistant (Δν = 500 kHz) operating frequencies.

Results and discussion
According to X-ray diffraction (XRD) data, the average size of nanoparticles is 6 nm. The diffraction peaks correspond only to the face centered cubic (fcc) structure (space group Fm-3m), which refers to the core of the nanoparticles. The unit cell parameter, a = 0.3538(8) nm, is close to a = 0.3531 nm, the lattice constant of a bulk fcc-Ni.
The magnetization reversal curve (figure 1) implies ferromagnetic state of the particles under study. Its sigmoid shape is qualitatively similar to those of nanoparticles with an iron or cobalt core (figure 1). The coercive force is absent (HС ≈ 0 Oe). A negligible value of HС is typical for small particles close to the transition to the superparamagnetic state. However, the magnetization curve cannot be described by the Langevin function (or a superposition of these functions). It is worth noting that the saturation magnetization Msat(Ni@C) = 17.5 emu/g is significantly less than that in pure bulk metallic nickel Msat(Ni) = 55 emu/g [14]. Let us assume that the core with average size 6 nm (see XRD) consists only of the pure nickel and the average thickness of the carbon shell is 1 nm. Then this Msat value corresponds to mVCore = 42.2 at. % (mCore = 77.5 wt. %) of nickel magnetization. Then we should expect the magnetization of the nanoparticles Msat(Ni@C)calc1 = 42.6 emu/g which is much higher than the measured value Msat(Ni@C) = 17.5 emu/g. This means that fraction of pure nickel should be much smaller than it follows from the two phase core-shell model. One of the reasonable explanations for such significant reduction of Msat(Ni@C) is probably due to the formation of Ni:C solid solution and/or the formation of nickel-based metastable carbide phases. Such values of the corresponding induced fields (Table 1) are qualitatively agreed with the values obtained from Mössbauer spectroscopy data [12]. In the case of the formation of a Ni:C solid solution with a different amount of carbon and nickel in the environment and with a different magnetization value and the Curie point, a continuous pedestal would be observed in the spectrum (figure 2) demonstrating the continuous distribution of hyperfine fields. Analysis of the integral intensities of the NMR lines in the spectrum allowed us to determine the concentration of each of the ferromagnetic phases in the core of nanoparticles (table 1).  It drastically differs from the narrow lines of graphene or graphite [16]. A similar spectrum was observed in carbon encapsulated cobalt nanoparticles Co@C [13] prepared following the same procedure. Thus, we may conclude that the carbon shell of the investigated nanoparticles consists of amorphous glass-like carbon.

Summary
The 61 Ni, 13 C NMR spectra of nanoparticles Ni@C have been firstly obtained and analyzed. On the basis of the joint analysis of the 61 Ni NMR data, XRD, magnetization and electron microscopy, the phase composition and average size of nanoparticles have been determined. It has been shown that the NMR method has found phases which may not be detected by the XRD method in small nanoparticles. According to 13 С NMR data, the carbon shell of the studied nanoparticles is most likely to consist of amorphous glassy-like carbon.