Composition, Electronic and Magnetic Investigation of the Encapsulated ZnFe2O4 Nanoparticles in Multiwall Carbon Nanotubes Containing Ni Residuals
© Al Khabouri et al. 2015
Received: 16 April 2015
Accepted: 2 June 2015
Published: 11 June 2015
We report investigation on properties of multiwall carbon nanotubes (mCNTs) containing Ni residuals before and after encapsulation of zinc ferrite nanoparticles. The pristine tubes exhibit metallic character with a 0.3 eV reduction in the work function along with ferromagnetic behavior which is attributed to the Ni residuals incorporated during the preparation of tubes. Upon encapsulation of zinc ferrite nanoparticles, 0.5 eV shift in Fermi level position and a reduction in both the π band density of state along with a change in the hybridized sp2/sp3 ratio of the tubes from 2.04 to 1.39 are observed. As a result of the encapsulation, enhancement in the σ bands density of state and coating of the zinc ferrite nanoparticles by the internal layers of the CNTs in the direction along the tube axis is observed. Furthermore, Ni impurities inside the tubes are attracted to the encapsulated zinc ferrite nanoparticles, suggesting the possibility of using these particles as purifying agents for CNTs upon being synthesized using magnetic catalyst particles. Charge transfer from Ni/mCNTs to the ZnFe2O4 nanoparticles is evident via reduction of the density of states near the Fermi level and a 0.3 eV shift in the binding energy of C 1 s core level ionization. Furthermore, it is demonstrated that encapsulated zinc ferrite nanoparticles in mCNTs resulted in two interacting sub-systems featured by distinct blocking temperatures and enhanced magnetic properties; i.e., large coercivity of 501 Oe and saturation magnetization of 2.5 emu/g at 4 K.
Since the discovery of carbon nanotubes (CNTs), there has been great interest in the synthesis and characterization of CNTs composites . For instance, encapsulation of magnetic materials in CNTs offers great potential since for particle applications carbon shielding of magnetic materials provides a stable coating against oxidation and degradation. Although the intrinsic properties of the CNTs such as nanometric cross section, high aspect ratio, good thermal, and electrical conductivity suggest high application potential , the magnetic properties of the composites are effected by residual magnetic impurities originating from the catalyst used to prepare the CNTs. Depending on the catalyst used to prepare the CNTs, the catalyst particles are left as residuals in the tubes and accordingly the magnetic properties of the CNTs are influenced .
Zinc ferrite is a promising microwave absorber; however, it is quite heavy due to high density, which restricts its utilization in applications requiring lightweight materials . ZnFe2O4/CNTs composites may provide immediate advantage over ZnFe2O4 nanoparticles because of the relatively low density of the composites. In addition, it has been demonstrated that encapsulation of paramagnetic particles into CNTs leads to paramagnetic needles whose movements can be controlled . Despite these advantages, encapsulating ferrites inside the nanotubes lead to effects which are not yet fully understood and explored. For example, in addition to ambiguous electronic and magnetic effects of the catalyst used to prepare the CNTs, it is unclear if there will be electron transfer from the host CNTs to the encapsulated ferrites or vice versa. Also, it is anticipated that stress in CNTs with small inner diameters (IDs) causes deformation of the encapsulated particles, consequently affecting their magnetic properties. In addition to that, the nanoparticles can adopt the internal shape of the CNTs and accommodate themselves along the tubes axis leading to enhancement of the magnetic anisotropy, which affects the blocking temperature of the composite system. Furthermore, the tendency of nanoparticles to agglomerate inside the tubes can influence the magnetic properties rendering them similar properties of large particles.
Wet chemistry has been used to grow metal oxides such as NiO and Nd2O3 inside the inner cavity of the CNTs . This study is the first to use the aforementioned method successfully to synthesise zinc ferrite multiwall CNTs composites in order to investigate their structural, compositional, electronic, and magnetic properties. The use of multiwall CNTs (mCNTs) containing residual Ni impurities reflects the reality that catalysts are needed for the production of most of the CNTs. Details of electron transfer, quality of CNTs, oxidation states of zinc and iron inside the composite, and magnetic properties at various temperatures are discussed.
X-ray diffraction (XRD) measurements were carried out in a Philips PW 1700 diffractometer with CuKα source (λ = 0.154060 nm). The high resolution transmission electron microscopy (HR-TEM) and energy dispersive x-ray spectroscopy (EDS) elemental mapping were performed on a (JEOL JEM-ARM200F) instrument. The images were acquired at bright field and dark field scanning transmission electron microscopy (STEM) at 80 kV capabilities. The magnetization was measured with a DMS 1660 vibrating sample magnetometer (VSM) in a magnetic field up to 13 kOe. Superconducting Quantum Interference Device (SQUID) (Quantum Design) is used to measure the magnetic properties at 77 and 4 K and field and zero field cooling curves. Mossbauer spectra were recorded on the powder sample using a constant-acceleration spectrometer with 50 mCi 57Co in Rh source. The X-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission (UPS) measurements were carried out using an Omicron Nanotechnology system (Omicron Nanotechnology Gmbhm Taunusstein, Germany). The XPS radiation was a monochromatic Al Kα radiation of hν = 1486.6 eV . The chemical composition was extracted from the wide scan using CASA XPS software (Fairly, N. CASA XPS, version 2.0; CASA Software Ltd., Devon, UK). The fitting of the spectrum was done by Gaussian-Lorentzian functions with a Shirley background subtraction. In order to avoid charging effects during the XPS scans, electron gun flooding was used for charge compensation. A He lamp with 21.2 eV (He Ι) excitation energy was used for the UPS analysis. Indium tin oxide (ITO) was used as a standard sample to check the validity of the work function value estimated following the procedure reported in ref .
Results and Discussion
Multi Wall Carbon Nanotubes
No absorption of gamma rays is detected in the spectrum indicating the absence of Fe content in the mCNTs.
Figure 1d presents the valence band data obtained by UPS for mCNTs and for a sample of few layers of graphene acquired by peeling layers from highly oriented pyrolytic graphite (HOPG ZYA) (i.e., the few layers of graphene data are included for comparison and clarity in explanations). The spectrum of the graphene reveals five C 2p and C 2 s band features associated with the crystalline state of the material: (1) 2p π at ~2.6 eV, (2) crossing of 2p π and 2p σ bands at 5.9 eV, (3) 2p σ broad band feature around 7.9 eV, (4) 2 s-2p hybridized state at 10.4 eV, and (5) 2 s σ band at 13.4 eV . The observed π band feature for the mCNTs is small, and the σ band appears broad. The CNT UPS spectrum can be understood as angle integrated spectra of graphene. Since the nanotube is a rolled graphene sheet, photoelectrons ejected from both normal and tangential directions of the nanotube/graphene surface are simultaneously detected . The work function was determined by the intersection of the high binding energy cut-off of the accelerated electrons with the base line of the spectrum (as shown in Fig. 1d and labeled by a circle. The work function of the mCNTs used in this study was found to be (4.0 ± 0.1 eV) suggesting metallic character of CNTs. This value is 0.3 eV lower than that of the reported value of 4.3 eV for the purified mCNTs and attributed to the effect of Ni impurities in the mCNTs. Similar reduction in the work function has been reported by Giusca et al.  upon filling CNTs with GeTe. Theoretically , the work function will reduce due to charge transfer from metal to CNT, which shifts the Fermi level of conduction band towards the vacuum. Experimental realization of this shift and charge transfer is revealed in the inset of Fig. 1d. The charge transfer is reflected in the increase of the density of state intensity near the Fermi level (i.e., from 0 to 3 eV) of the mCNTs in comparison to that of graphene layers and provides a strong indication of the metallic character of the tubes.
Figure 3b shows the magnetization curve at room temperature for mCNTs and ZnFe2O4/mCNTs. The magnetization loop shows larger hysteresis for mCNTs than ZnFe2O4/mCNTs. The saturation magnetization of ZnFe2O4/mCNTs is 1.17 emu/g lower than that of the mCNTs. As shown in the inset of Fig. 3b, ZnFe2O4/mCNTs is found have a remanence of 0.104 emu/g and coercivity 70 Oe. The magnetization curve of ZnFe2O4 nanoparticles prepared by co-precipitation method shows super-paramagnetic behavior . CNTs decorated with ZnFe2O4 nanoparticles  show an incremental value of magnetization compared to pure ZnFe2O4 nanoparticles. Therefore, coercivity shown by ZnFe2O4/mCNTs can be attributed to Ni impurities. The decrease in magnetic parameters of ZnFe2O4/mCNTs compared to mCNTs could be because less content of Ni impurities is present in ZnFe2O4/mCNTs sample than the amount in the same mass of mCNTs.
The 57Fe Mössbauer spectrum recorded for ZnFe2O4/mCNTs is shown in Fig. 3c. The main characteristic of the spectrum is the presence of a central paramagnetic doublet with isomer shift (δ) and quadruple splitting (ΔEQ) of (0.34 ± 0.02 mm/s) and (0.37 ± 0.02 mm/s), respectively, and (0.35 ± 0.02 mm/s) line width (Γ). The value of δ suggests the presence of only Fe3+ . The δ, ΔEQ, and Γ values of bulk spinal zinc ferrite are 0.350, 0.333, and 0.258 mm/s , respectively. The line width of ZnFe2O4/mCNTs is higher than that of the bulk and that could be due to a distribution of the isomer shift and/or the quadrupole splitting as a consequence of random strain and surface effects and/or random size distribution. No change is observed in Mossbauer parameters when measurement was conducted at liquid nitrogen temperature (not shown).
Lattice strain could result in non-cubic symmetry and increase of the quadrupole splitting . Figure 3d shows the UPS spectra of ZnFe2O4/mCNTs and mCNTs. Energy shift of the π and σ bands of ZnFe2O4/mCNTs with respect to mCNTs is observed. The DOS near the Fermi level of ZnFe2O4/mCNTs is slightly lower than that for mCNTs (see inset), indicating charge transfer between the Ni/mCNTs and the ZnFe2O4. Consequently, the work function of ZnFe2O4/mCNTs increased up to 4.6 eV which might lead to the increase of the electrical resistance of ZnFe2O4/mCNTs composite.
Figure 5c shows a Zn 2p narrow scan XPS spectrum. The binding energies of Zn 2p3/2 and Zn 2p1/2 were 1021.8 and 1044.95 eV, respectively. These binding energies agree with the reported values of the binding energies of Zn2+ [27, 28]. The photoelectron peak positions and the sharp peaks indicate tetrahedral coordination of zinc in the ZnFe2O4 . Asymmetric O 1 s spectrum is observed in Fig. 5d. Simulation results show that the spectrum is consisted of two components centered at 530.5 and 533.4 eV. The first is attributed to the oxygen in the ZnFe2O4 . The peak at higher binding energy 533.4 eV is assigned to (C–O) oxygen singly bonded to carbon groups . Deconvolution of C1s peak of ZnFe2O4/mCNTs is shown in Fig. 5e. As a result of preparing ZnFe2O4 inside the mCNTs, the proportion of sp3 hybridization of the carbon atoms increases by 9 % with respect to that in mCNTs alone and therefore the value of sp2/sp3 ratio decreases to 1.39. This change is associated with the damage observed from the STEM images in the internal walls of the mCNTs as a consequence of the growth of zinc ferrite particles inside the internal cavity of the mCNTs. It should be noted that there is a shift in the binding energy of C1s core level by 0.3 eV due to the presence of ZnFe2O4 nanoparticles indicating the existence of charge transfer between the ZnFe2O4 and the host mCNTs. The presence of Ni impurities in the ZnFe2O4/mCNTs is confirmed by the small Ni 2p3/2 and Ni 2p1/2 peaks observed in the narrow scan spectrum presented in Fig. 5f. After 100 short scans, Ni was detected in ZnFe2O4/mCNTs, which was not the case when the narrow scan was done for mCNTs alone which suggests that there is a tendency of agglomeration of Ni with the presence of ZnFe2O4.
Zinc ferrite nanoparticles have been encapsulated in mCNTs using wet chemistry technique. The composite was characterized using XRD, HRTEM, XPS, UPS, VSM, SQUID, and Mossbauer spectroscopy. The internal walls of CNTs were observed to partly coat the encapsulated zinc ferrite nanoparticles. The observed Ni impurities in the mCNTs are attracted to the zinc ferrite nanoparticles. This attraction provides a possibility of using encapsulated zinc ferrite nanoparticles to purify CNTs prepared using magnetic catalyst. In addition, Ni impurities are observed to be corresponding to the changes in electronic and magnetic properties. Decrease in the DOS of the ZnFe2O4/mCNTs indicates charge transfer from Ni impurities in the mCNTs to zinc ferrite nanoparticles in addition to the 0.3 eV shift of C1s core level. The ZnFe2O4/mCNTs composite exhibits two blocking temperatures and enhancement of magnetic properties, i.e., coercivity and magnetization remanence; resulting from its interacted ZnFe2O4 and Ni/mCNTs components.
S. Al Khabouri would like to express special appreciation to professor Dénes Lajos Nagy, the Chair of the International Board on the Applications of the Mössbauer Effect for valid discussions. Thanks to H. H. Kyaw for the support with XPS and UPS measurements.
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