## 4 Results and Discussion  
### Transmission Electron Microscopy (TEM)  
Three catalysts with the same initial composition (nickel nitrate and glycine), but with different quantities of water added in the initial solution, were firstly preheated at 70 \({}^{\circ}\)C and then placed in the furnace at the same temperature (500 \({}^{\circ}\)C), where the water evaporated and the reaction occurred in the gel. After SCS, it was found that the microstructures of all the catalysts were different, and this was why they were studied by TEM. Figure 1 exhibits the typical TEM micrographs of sample S_29 (100 mL of water as solvent). It can be noticed by comparing Figure 1a,b that there were sample regions where the crystallites obtained a size ranging between 15 and 50 nm, while in other regions, the crystallite size was much smaller, ranging between 5 and 12 nm. The latter areas constituted the majority of the sample mass. Another interesting feature was that the fine crystallites formed the shells of the empty spheres as shown in the low-magnification bright field image of Figure 1c. Since these spheres formed a three-dimensional solid, their shells were projected as dark rings of 50-170 nm diameter due to the higher absorption of the electron beam along the sphere edges. A typical selected area electron diffraction pattern is presented in Figure 1d. Figure 1d shows the selected area electron diffraction (SAED) pattern of the same sample. The polycrystalline diffraction rings, indicated by the arrows, corresponded to 0.241 nm (NiO), 0.209 nm (NiO), 0.203 nm (Ni), 0.176 nm (Ni), 0.148 nm (NiO), 0.126 nm (NiO), 0.124 nm (Ni), 0.106 nm (Ni), and 0.104 nm (NiO), being in agreement with our previous work [40]. Figure 2a,b exhibits high-resolution TEM images of the large and small crystallite regions, respectively. Since in both regions (101) NiO as well as (111) Ni lattice fringes can be seen with d-spacings of 0.241 nm and 0.203 nm, respectively, it can be concluded that both regions consisted of Ni and NiO nano-crystallites. Figure 3a-c demonstrates typical TEM bright field images of sample S27 (75 mL H\({}_{2}\)O). The microstructure of this sample was very similar to that of S29 (100 mL H\({}_{2}\)O), consisting of  
Figure 1: S_29 (100 mL of H\({}_{2}\)O). TEM bright field images of areas exhibiting 15–50 nm crystallites (**a**) and 5–12 nm crystallites (**b**). (**c**) A low-magnification TEM image of area (**b**). A typical selected area electron diffraction (SAED) pattern (**d**). Figure 2: HRTEM images of larger and smaller Ni and NiO nanocrystals. regions with larger Ni and NiO 20-40 nm crystallites, as shown in Figure 3a, and regions of smaller 5-12 nm nanocrystals as shown in Figure 3b. As in the case of sample S29, the smaller nanocrystals formed the shell of empty spheres, as shown in Figure 3c, with the main difference with S_29 being their larger size ranging between 100 and 300 nm. It is also worth noting that in sample S27, the areas consisted of small nanocrystals constituting the minority of the sample's mass in contrast to sample S29 where they constituted the majority. This trend was further enhanced in sample S25 (50 mL H\({}_{2}\)O), where only larger Ni and NiO crystallites were observed as shown in Figure 3d, i.e., no areas of small nanocrystals forming shells of empty spheres were observed. As can be seen from Figure 4, the combustion in the gel went through the dendrites. When the precursors' concentration decreased (i.e., 100 mL, 75 mL, 50 mL H\({}_{2}\)O), the combustion temperature increased correspondingly (i.e., 760 \({}^{\circ}\)C, 880 \({}^{\circ}\)C, 900 \({}^{\circ}\)C). This could be associated to the fact that dendrites growing in a larger volume may have a more distributed structure and that synthesis heat loss is more intensive during combustion, resulting in a decrease in the combustion temperature. It appears that the shape and amount of dendrite growth affects all kinds of material properties.