# Fabrication of Novel CeO2/GO/CNTs Ternary Nanocomposites with Enhanced Tribological Performance
## 3 Results and Discussion  
The flow diagram of the preparation of CeO\({}_{2}\)/GO/CNTs nanocomposites is shown in Figure 1. The CNTs and GO nanosheets can combine together by covalent bonds due to the existence of aplurality of oxygen-containing groups on the surface of both CNTs and GO. In addition, Ce\({}^{3+}\) can also be adsorbed on the surface of GO and CNTs by covalent bonding. Subsequently, NaOH solution was added to the mixed solution to provide a large amount of free OH\({}^{-}\) which provided hydrolysis conditions for Ce\({}^{3+}\). The hydrothermal reaction is carried out to form Ce(OH)\({}^{2+}\), Ce(OH)\({}_{2}\)\({}^{+}\) or Ce(OH)\({}_{3}\) compound, which were converted into CeO\({}_{2}\) via a hydrothermal reaction under high temperature and pressure. Then, in the reaction vessel, these products are converted into more stable CeO\({}_{2}\) with the high temperature and pressure hydrothermal reaction progresses, and finally the CeO\({}_{2}\)/GO/CNTs nanocomposite is obtained. The morphologies of CeO\({}_{2}\)/CNTs and CeO\({}_{2}\)/GO/CNTs nanocomposites were characterized by TEM as shown in Figure 2. In Figure 2a, numerous CeO\({}_{2}\) nanoparticles were unevenly deposited on CNTs which were intertwined with each other accompanied by agglomeration. Figure 2b-f shows the microstructure of CeO\({}_{2}\)/GO/CNTs with different weight ratios of CeO\({}_{2}\), GO, and CNTs. From Figure 2b-f, it can be seen that GO exists as a carrier where CNTs combines and CeO\({}_{2}\) nanoparticles deposits. Specifically, via TEM observation of CeO\({}_{2}\)/GO/CNTs-d nanocomposites (Figure 2e), CNTs and CeO\({}_{2}\) nanoparticles displayed excellent uniformity compared with other nanocomposites. It can be speculated that CeO\({}_{2}\)/GO/CNTs-d nanocomposite is more suitable as lubricant additives in terms of the micromorphology of nanocomposites. Raman spectroscopy is an efficient approach to prove the electronic structure of CeO\({}_{2}\)/GO/CNTs nanocomposites, especially for the changes of the C=C, C-C, and defects. Figure 3 shows the Raman spectra of GO, CeO\({}_{2}\)/CNTs, and CeO\({}_{2}\)/GO/CNTs nanocomposites. From the Raman spectra of GO, there are characteristic bands related to the D (\(\approx\)1333 cm\({}^{-1}\)) and G (1583 cm\({}^{-1}\)) band in both GO and CNTs [33; 34]. Apart from the D and G bands, the distinct strong peak at 456.13 cm\({}^{-1}\) corresponds to the characteristic peak of CeO\({}_{2}\) nanoparticles [35]. However, in the Raman spectra of the CeO\({}_{2}\)/GO/CNTs nanocomposite, there were only one set of characteristic peaks, which appeared at 1333.54 cm\({}^{-1}\) and 1583.53 cm\({}^{-1}\) due to the coincidence of the D and G peaks of GO and CNTs. Ordinarily, the degree of the defects of the composites is characterized by the ratio of the D peak and G peak (I\({}_{\rm D}\)/I\({}_{\rm G}\)) [36; 37; 38]. In comparison with the I\({}_{\rm D}\)/I\({}_{\rm G}\) of GO and CeO\({}_{2}\)/CNTs (0.94, 0.99), the I\({}_{\rm D}\)/I\({}_{\rm G}\) of CeO\({}_{2}\)/GO/CNTs-d nanocomposite (1.05) increased. These results implied the changes in the structure of surface functional groups of nanocomposites and the formation of the CeO\({}_{2}\)/GO/CNTs nanocomposite. Specific results can also be observed in the XPS survey spectrum (Figure 4). Figure 1: The formation processes of CeO\({}_{2}\)/GO/CNTs nanocomposites. The elemental composition and chemical state of nanocomposites can be analyzed using XPS. The C1s XPS spectrum of CeO\({}_{2}\)/CNTs nanocomposites shown in Figure 4a was deconvoluted into four peaks located at 289.2 eV (O=C-O), 286.5 eV (C=O), 285.6 eV (C-OH) and 284.8 eV (C-C), which were similar to the C1s XPS spectrum of CeO\({}_{2}\)/GO/CNTs nanocomposites in Figure 4b [39]. From Figure 4c, the presence of the Ce-O bond can demonstrate the successful preparation of CeO\({}_{2}\)/GO/CNTs nanocomposites via hydrothermal reaction.