## 4 Results and Discussion
### X-ray Photoelectron Spectroscopy (XPS)  
In our previous study [40], it was concluded that catalytic activity increases by decreasing the precursor's concentration. However, it is of high importance to investigate the influence of the precursor's concentration in the initial mixture on the composition of the catalytic surface. To begin with, the XRD analysis revealed the presence of two resulting phases in the composition of the final products: nickel oxide and metallic cubic nickel. The presence of vacancies is very common in the stoichiometry of NiO. Hence, XPS would be a handy tool to detect any changes in the surface's chemical or electronic properties. The percentage concentration of Ni and O at the surface of nano-nickel catalysts produced by SCS is presented in Table 3. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Water Volume, mL** & **Ni, \%** & **O, \%** \\ \hline
25 & 19.70 & 80.30 \\
50 & 21.10 & 78.90 \\
75 & 22.50 & 77.50 \\
100 & 23.30 & 76.70 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Influence of water concentration in the initial mixture on the concentration of nickel and oxygen on the surface of each sample (taken from the survey scan). Figure 4: The influence of precursor concentration in water on combustion temperature during SCS: (a) 100 mL, (b) 75 mL, and (c) 50 mL (preheating temperature 300 \({}^{\circ}\)C). The data presented in Table 3 were acquired from the survey spectrum and shows a consistent decrease in oxygen content at the surface. This finding could be of interest, as the decreased concentration of oxygen may lead to bridging of the oxygen structures in the NiO lattice. Table 4 shows the binding energy values of the Ni\({}^{2+}\) and Ni\({}^{3+}\) states in the Ni \(2p\) spectra of the XPS analysis. As can be seen from Table 4, in all cases (from 25 mL to 100 mL), NiO was oxidized to Ni\({}_{2}\)O\({}_{3}\). The two Ni\({}^{2+}\) ions lost an extra electron each to form Ni\({}^{3+}\) to maintain charge neutrality, generating quasi-localized holes for each Ni vacancy. The NiO structure is more Ni deficient and can result in the bridging of the oxygen structure. These bridging structures can cause problems in the adsorption of hydrogen on the catalyst surface and, as consequence, reduce its activity. The size dependence of the conventional exchange bias and anisotropic energy between the Ni-core and the NiO-shell nanoparticles was studied. The shell grew at the expense of the Ni-core, forming a passivating layer of NiO as confirmed and estimated by the XPS measurements. Figure 5 demonstrates the Ni \(2p\) core level spectra for all four samples. These spectra were fitted to find the influence of the water concentration in defining the oxidation state of Ni. The intense Ni \(2p_{3/2}\) peak could be fitted with two components: one at \(854.2\pm 0.2\) eV and the other at \(856.2\pm 0.2\) eV. The lower energy component corresponded to the Ni\({}^{2+}\) state in the standard Ni-O octahedral bonding configuration in NiO, while the one at the higher energy corresponded to the Ni\({}^{2+}\) vacancy-induced Ni\({}^{3+}\) states. The changes in the area of these two components can be directly  
Figure 5: Ni\(2p\) core level spectra. \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Water Volume, mL**} & \multicolumn{2}{c}{**Ni \(2p_{3/2}\)**} & \multicolumn{2}{c}{**Ni \(2p_{1/2}\)**} \\ \cline{2-5}  & **Ni\({}^{2+}\)** & **Ni\({}^{3+}\)** & **Ni\({}^{2+}\)** & **Ni\({}^{3+}\)** \\ \hline
25 & \(854.2\pm 0.2\) eV & \(856.2\pm 0.2\) eV & \(872.1\pm 0.2\) eV & \(874.5\pm 0.2\) eV \\
50 & \(854.1\pm 0.2\) eV & \(856.1\pm 0.2\) eV & \(872.1\pm 0.2\) eV & \(874.4\pm 0.2\) eV \\
75 & \(854.3\pm 0.2\) eV & \(856.2\pm 0.2\) eV & \(872.2\pm 0.2\) eV & \(874.2\pm 0.2\) eV \\
100 & \(854.3\pm 0.2\) eV & \(856.3\pm 0.2\) eV & \(872.1\pm 0.2\) eV & \(874.2\pm 0.2\) eV \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Binding energy values of the Ni\({}^{2+}\) and Ni\({}^{3+}\) states in the Ni \(2p\) spectra.