## 3 Results
### Conversion Mechanism of Nanolayers to Nanofibers by Dual Anions Intercalation  
It is known that the most familiar morphology of LDHs is plate-like morphology. The experimental results demonstrated that there is a relation between the morphology of LDH and the insertion of silicon, zirconium, and titanium inside the nanolayers of LDH. When silicon was inserted in the sheets of the LDHs, the conventional morphology of the LDHs transformed from plates to nanofibers. In the same trend, the insertion of zirconium beside silicon converted the plate-like morphology of LDHs to nanofibers and nanoneedles. However, the insertion of titanium beside both silicon and zirconium prevented this transformation from plates to nanofibers and showed plate-like morphology. According to the thermal analyses, this relation appears between the decomposition rate of the interlayered anions of each LDH and its morphology.  
In the case of Co-Si LDHs before and after the insertion of zirconium, the decomposition of the interlayered anions happened through two stages. It means that their nanolayered structures have formed through a dual anions intercalation process. This finding was confirmed by infrared results where these results showed clear bands for nitrate and cyanate anions in the structure of Co-Si LDHs after the insertion of zirconium.  
The cyanate anions were formed during the urea hydrolysis, as shown in the following Equation (1):  
\[(\text{NH}_{2})_{2}\text{CO}+\text{H}_{2}\text{O}\ \rightarrow\ \text{NH}_{4} ^{+}+\text{CNO}^{-}+\text{H}^{+}+\text{OH}^{-} \tag{1}\]  
Many researchers [33] reported that ammonia and cyanate anions were produced from urea hydrolysis. In addition to cyanate anions, the nitrate anions are accompanied with the beginning salts.  
According to the dual anions intercalation process, the nanofibers and nanoneedles morphologies of Co-Si LDHs could be explained. The intercalated cyanate anions (-1) have two routes or directions to neutralize the positive charges (+2) of the nanolayers of the LDHs. The first route is attacking the positive charge by two anions of cyanate from only one side of the nanolayer. The second route is attacking the positive charge by one anion of cyanate to each side of the nanolayers. If the intercalated cyanate anions followed the first route, the cyanate anions would have strong repulsion, and the steric hindrance among their cyanide groups would lead to curling for the nanolayers of LDHs. This curling converted the shape of plates to nanofibers or nanoneedles. In the presence of nitrate as secondary anions in the LDH structure, the intercalated cyanate anions follow the first route, avoiding the repulsion with nitrate anions to build nanofibers or nanoneedles. By this explanation, we can suggest a model for the nanofibers and nanoneedles of Co-Si LDHs, as shown in Figure 9.  
By using both the XRD results and the known thickness of the brucite-like layer, 0.48 nm, the interlayer spacing available for the anion was calculated as 0.31 nm. The size of the NCO\({}^{-}\) anion is 0.34 nm, and the thickness of a flat-lying NO3\({}^{-}\) is 0.28 nm. Therefore, it was considered that cyanate anions connect with the positive charge of the layers through one side in order to neutralize the positive charge of tetravalent cation and build the nanofibers' morphology, as shown in Figure 9. The presence of nitrate anions among the layers may direct the cyanate anions to attack the positive charge from only one side to build nanofibers.  
In the case of Co-Si LDH, after inserting both zirconium and titanium, the main interlayered anions are made out of isocyanate, in addition to traces of nitrate. Therefore, the intercalation of cyanate anions followed the second route, forming plate-like morphology, as shown in Figure 10.