The Structure and Chemical Composition of the Cr and Fe Pyrolytic Coatings on the MWCNTs’ Surface According to NEXAFS and XPS Spectroscopy

The paper is devoted to the structure and properties of the composite material based on multi-walled carbon nanotubes (MWCNTs) covered with pyrolytic iron and chromium. Fe/MWCNTs and Cr/MWCNTs nanocomposites have been prepared by the metal organic chemical vapor deposition (MOCVD) growth technique using iron pentacarbonyl and bis(arene)chromium compounds, respectively. Composites structures and morphologies preliminary study were performed using X-ray diffraction, scanning and transmission electron microscopy and Raman scattering. The atomic and chemical composition of the MWCNTs’ surface, Fe-coating and Cr-coating and interface—(MWCNTs surface)/(metal coating) were studied by total electron yield method in the region of near-edge X-ray absorption fine structure (NEXAFS) C1s, Fe2p and Cr2p absorption edges using synchrotron radiation of the Russian-German dipole beamline (RGBL) at BESSY-II and the X-ray photoelectron spectroscopy (XPS) method using the ESCALAB 250 Xi spectrometer and charge compensation system. The absorption cross sections in the NEXAFS C1s edge of the nanocomposites and MWCNTs were measured using the developed approach of suppressing and estimating the contributions of the non-monochromatic background and multiple reflection orders radiation from the diffraction grating. The efficiency of the method was demonstrated by the example of the Cr/MWCNT nanocomposite, since its Cr2p NEXAFS spectra contain additional C1s NEXAFS in the second diffraction order. The study has shown that the MWCNTs’ top layers in composite have no significant destruction; the MWCNTs’ metal coatings are continuous and consist of Fe3O4 and Cr2O3. It is shown that the interface between the MWCNTs and pyrolytic Fe and Cr coatings has a multilayer structure: a layer in which carbon atoms along with epoxy –C–O–C– bonds form bonds with oxygen and metal atoms from the coating layer is formed on the outer surface of the MWCNT, a monolayer of metal carbide above it and an oxide layer on top. The iron oxide and chromium oxide adhesion is provided by single, double and epoxy chemical binding formation between carbon atoms of the MWCNT top layer and the oxygen atoms of the coating, as well as the formation of bonds with metal atoms.

Novel Nanohybrids Derived from the Attachment of FePt Nanoparticles on Carbon Nanotubes

Multiwalled carbon nanotubes (MWCNTs) were used as nanotemplates for the dispersion and stabilization of FePt nanoparticles (NPs). Pre-formed capped FePt NPs were connected to the MWCNTs external surface via covalent binding through organic linkers. Free FePt NPs and MWCNTs-FePt hybrids were annealed in vacuum at 700 °C in order to achieve the L10 ordering of the FePt phase. Both as prepared and annealed samples were characterized and studied using a combination of experimental techniques, such as Raman and Mössbauer spectroscopies, powder X-ray Diffraction (XRD), magnetization and transmittion electron microscopy (TEM) measurements. TEM measurements of the hybrid sample before annealing show that a fine dispersion of NPs along the MWCNTs surface is achieved, while a certain amount of free particles attached to each other in well connected dense assemblies of periodical or non-periodical particle arrangements is also observed. XRD measurements reveal that the FePt phase has the face-centered cubic (fcc) disordered crystal structure in the as prepared samples, which is transformed to the face-centered tetragonal (fct) L10 ordered crystal structure after annealing. An increase in the average particle size is observed after annealing, which is higher for the free NPs sample. Superparamagnetic phenomena due to the small FePt particle size are observed in the Mössbauer spectra of the as prepared samples. Mössbauer and magnetization measurements of the MWCNTs-FePt hybrids sample reveal that the part of the FePt particles attached to the MWCNTs surface shows superparamagnetic phenomena at RT even after the annealing process. The hard magnetic L10 phase characteristics are evident in the magnetization measurements of both samples after annealing, with the coercivity of the hybrid sample over-scaling that of the free NPs sample by a factor of 1.25.