## 2 Results and Discussion
### Surface Characterization of the Carbon Composites Containing Nano-ZrO\({}_{2}\) and Nano-ZrO  
We characterized the surface of the carbonized samples containing carbon balls decorated with nano-ZrO\({}_{2}\) polymorphs, nano-ZrC or their mixture by N\({}_{2}\) BET surface area analysis (Tables 2 and 3), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The surface area of nano-ZrO\({}_{2}\)@C composites prepared at 1000 \({}^{\circ}\)C for 2 h was 140, 278 and 33 m\({}^{2}\)/g for the samples with X=Cl, N and S, respectively. We found the same tendency for the samples containing ZrC and ZrO\({}_{2}\) + ZrC prepared in a tubular furnace as the ZrO\({}_{2}\)@C samples. The highest values were found for X=N and the lowest values for X=S. Increasing the temperature from 1200 to 1400 \({}^{\circ}\)C had no critical effect on the surface area, whereas heating time (8 h instead of 2 h at 1400 \({}^{\circ}\)C) only had a considerable influence in the case of the X=Cl resin. The plasma treatment brought about significant sintering, resulting in a decreased surface area (Table 3). The surface composition of some nano-ZrO\({}_{2}\)@C, nano-(ZrC\({}_{0.58}\), ZrO\({}_{2}\))@C and nano-ZrC\({}_{0.94}\)@C composites were determined by XPS, and the Zr\({}_{3d}\), C\({}_{1s}\) and O\({}_{1s}\) spectra are given in Figures 6-9 and Supplementary Materials, Figures S12 and S13. The calculated surface concentrations are given in Supplementary Materials, Tables S2-S5. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & \multicolumn{2}{c}{**ZrC,**} & \multirow{2}{*}{**Carbon (wt\%)**} & **BET Surface Area** & **Total Pore Volume** \\  & **(\%**)** & & **(m\({}^{2}\)/g)** & **(cm\({}^{3}\)/g)** \\ \hline L-Cl-1000-2-P-He & 95 (49) & 5 & 50 & 0.21 \\ L-Cl-1000-2-P-H\({}_{2}\) & 90 (40) & 10 & 62 & 0.26 \\ L-N-1000-2-P-He & 95 (62) & 5 & 52 & 0.22 \\ L-N-1000-2-P-H\({}_{2}\) & 90 (35) & 15 & 82 & 0.22 \\ L-S-1000-2-P-He & 80 (40) & 20 & 52 & 0.20 \\ L-S-1000-2-P-H\({}_{2}\) & 75 (42) & 20 & 58 & 0.22 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Composition and properties of ZrC\({}_{0.94}\)@C composites prepared in RF plasma. The surface of the L-Zr-X-T-t samples with X=Cl and S contains approximately 85-95% graphite. Zr\({}_{\rm 0.58}\)C (Zr\({}_{\rm 3d5}\) BE \(\approx\) 179.9 \(\pm\) 0.5 eV) was found only in samples heated above 1200 \({}^{\circ}\)C in low concentration, but its presence was undoubtedly detected based on the zirconium spectra (Figure 6). Increasing the temperature to 1400 \({}^{\circ}\)C or applying prolonged heating had no significant effect on the concentration of ZrC on the surface. The surface concentration of ZrC in the L-Zr-Cl samples is approximately double that of the L-Zr-S samples. Zirconium was present in the samples in two other forms: ZrO\({}_{2}\) (Zr\({}_{\rm 3d5}\) BE \(\approx\) 183.8 \(\pm\) 0.5 eV) and presumably some kind of zirconium oxycarbide (Zr\({}_{\rm 3d5}\) BE \(\approx\) 182.5 \(\pm\) 0.5 eV) (Tables S2 and S3, Figures 6-9, Supplementary Materials, Figures S12 and S13). Oxidized carbon derivatives are also detectable on the surface in a few percentages. The line positions of the corresponding oxygen, O\({}_{\rm 1s}\) 531.2 eV and carbon, C\({}_{\rm 1s}\) 182.0 eV agreed with the literature data [39]. The appearance of the ZrO\({}_{2}\) minor phase can be attributed to the high affinity between Zr and O [37; 38]. Some of the oxygen is chemically bound to carbon (O\({}_{\rm 1s}\) 533.0 \(\pm\) 0.2 eV, C\({}_{\rm 1s}\) 286.7 \(\pm\) 0.2 eV). In the carbon  
Figure 6: X-ray photoelectron spectra of the Zr\({}_{\rm 3d}\) lines of the composites containing nano-ZrO\({}_{\rm 2}\)@C and nano-(Zr\({}_{\rm 0.58}\),C,ZrO\({}_{\rm 2}\)@C.  
spectra, the quantity of the oxidized carbon species can only be determined with an error due to the presence of the high graphite peak. The graphite peak was fitted with an asymmetric peak shape with AS(40;0.6) parameters.