Studies of Buried Layers and Interfaces of Tungsten Carbide Coatings on the MWCNT Surface by XPS and NEXAFS Spectroscopy

Abstract

<jats:p>Currently, X-ray photoelectron spectroscopy (XPS) is widely used to characterize the nanostructured material surface. The ability to determine the atom distribution and chemical state with depth without the sample destruction is important for studying the internal structure of the coating layer several nanometers thick, and makes XPS the preferable tool for the non-destructive testing of nanostructured systems. In this work, ultra-soft X-ray spectroscopy methods are used to study hidden layers and interfaces of pyrolytic tungsten carbide nanoscale coatings on the multi-walled carbon nanotube (MWCNT) surfaces. XPS measurements were performed using laboratory spectrometers with sample charge compensation, and Near Edge X-ray Absorption Fine Structure (NEXAFS) studies using the Russian–German dipole beamline (RGBL) synchrotron radiation at BESSY-II. The studied samples were tested by scanning and transmission electron microscopy, X-ray diffractometry, Raman scattering and NEXAFS spectroscopy. It was shown that the interface between MWCNT and the pyrolytic coating of tungsten carbide has a three-layer structure: (i) an interface layer consisting of the outer graphene layer carbon atoms, forming bonds with oxygen atoms from the oxides adsorbed on the MWCNT surface, and tungsten atoms from the coating layer; (ii) a non-stoichiometric tungsten carbide WC1-x nanoscale particles layer; (iii) a 3.3 nm thick non-stoichiometric tungsten oxide WO3-x layer on the WC1-x/MWCNT nanocomposite outer surface, formed in air. The tungsten carbide nanosized particle’s adhesion to the nanotube outer surface is ensured by the formation of a chemical bond between the carbon atoms from the MWCNT upper layer and the tungsten atoms from the coating layer.</jats:p>

The Identification of Cu–O–C Bond in Cu/MWCNTs Hybrid Nanocomposite by XPS and NEXAFS Spectroscopy

“…At the same time, there are significant differences in the spectrum of the nanocomposite: a 2.1-fold decrease in the area under the spectral curve, the appearance of an additional structure with three low-intensity peaks, A, B, C, and a shoulder D in the region between the π* and σ* resonances. The energy positions of the additional peaks are in good agreement with the energies of the C1s → π* transitions in the C-O (A, 286.9 eV), C-O-C (B, 287.7 eV), (C, 288.6 eV, as part of the carboxy and carbonyl group), and CO 3 (D, 290.4 eV) groups, which is confirmed by the numerous studies of carbon oxides [12,14,33,42] and various oxygen-carbon containing molecular groups [34,[43][44][45][46]. However, in [47], it is noted that the C 1s → π* electron transition in the carboxy-COOH group C=O coupling lies in the 290.0-290.4 eV range, although other authors associate this transition with the absorption band at a photon energy of 288.6 eV [34,42,44,45].…”

“…It is also suggested [20] that the shoulder D at 290.4 eV in the C 1s NEXAFS spectrum of CuO/graphene reflects the presence of the strong bonding between the Cu 2 O and graphene sheet by forming a Cu-O-C bond. In our previous studies [12,14], the D band was also observed in the C 1s NEXAFS spectra of the Cr/MWCNTs, Fe/MWCNTs, and WC/MWCNTs nanocomposites. Therefore, the appearance of the D band should be associated with the formation of compounds containing anion CO 3 2− during the nanocomposite synthesis.…”

“…The C 1s NEXAFS spectra of the initial MWCNTs and the Cu/MWCNTs hybrid nanocomposite were obtained using the technique described earlier in [12] and successfully approved for nanocomposites based on MWCNTs coated with chromium and iron [12], and tungsten carbide [14]. Figure 7 shows the C 1s NEXAFS partial spectra of the MWC-NTs and Cu/MWCNTs derived from the above spectra by subtracting the contributions from the overlying shells, obtained by the power law extrapolation of the absorption cross section dependence in the long-wave region before the C 1s absorption edge.…”

“…Due to a variety of organometallic compounds, relatively low pyrolysis temperatures, and easy removal of volatile reaction products from the solid-state phase deposition zone, the method makes it possible to regulate the coating composition and the rate of the synthesis process. MOCVD was successfully used to form metal-containing nanostructured coatings of pyrolytic iron [9,10], chromium [11,12], titanium carbide [13], tungsten carbide [14], rhenium [15], aluminum [16], and bimetallic rhenium-tungsten nanodendrites (Re-W/MWCNTs) [17].…”

The Formation of Nanoscale Closed Graphene Surfaces during Fullerite C60 Hot Isostatic Pressing

“…The near edge X-ray absorption fine structure (NEXAFS) spectroscopy [26] and X-ray photoelectron spectroscopy (XPS) [27,28] are effective experimental methods that allow us to directly obtain detailed information about the nanoscale atomic-electron structure of the polyatomic system under study. Their combination with traditional Raman spectroscopy and X-ray diffractometry (XRD) provides new reliable knowledge about the atomic and electronic structure of nanostructured carbon materials [29,30]. In this work, the successive modification of fullerite C 60, as a result of HIP at a pressure of 0.1 GPa in an argon atmosphere in the temperature range of 920-1270 K, was characterized by XRD, XPS, NEXAFS and Raman spectroscopy.…”