## 4 Discussion  
The present research has a dual aim of synthesizing nanofibers of Co-Si LDHs for the first time and developing their optical properties to be suitable for designing optical nanodevices. In this trend, nanofibers and nanoneedles of Co-Si LDH were prepared and modified by the inclusion of zirconium and titanium inside the nanolayers of the LDH structure. SEM images showed individual nanofibers of Co-Si LDHs. In case of the inclusion of zirconium inside the nanolayers of Co-Si LDHs, the nanolayered structure was confirmed by X-ray diffraction, and the nanofibers morphology became clearer. In addition, the band gap energy of Co-Si LDHs shifted to be 2.95 eV after inserting zirconium, and became more effective in the visible light region. Through titanium inclusion beside zirconium inside Co-Si LDHs, the nanofiber morphology converted to become plate-like morphology, and the band gap energy decreased to a lower value to become 2.8 eV.  
These effective changes of Co-Si LDHs could be explained by inducing new energy levels into the main band gap because of the new bands of Co(II)-O-Zr(IV) and Co(II)-O-Ti(IV) that were introduced into the LDH structure. Meanwhile, the formation mechanism of the nanofibers could be clarified through a dual anions intercalation process. By intercalating cyanate and nitrate anions in the same interlayered region of the Co-Si LDH structure, strong repulsion and steric hindrance occurred among their cyanide groups, leading to curling for the nanolayers of LDH. This curling converted the shape of plates to nanofibers or nanoneedles.  
It has been known that the performance of optical materials is controlled by main two factors. The first factor is the band gap energy of the material. The second factor depends on the electron-hole recombination process [37]. To maximize these two factors, the separation of the photogenerated electron-hole pairs should be enhanced, and the light absorption must shift toward the visible light region.  
Comparing with the most familiar optical materials, TiO\({}_{2}\) and ZnO, the band gap energies of Co-Si LDHs after inserting zirconium and titanium are lower than those of TiO\({}_{2}\) and ZnO. In addition, the nanofibers' morphology decreases the rate of electron-hole recombination by the mobility of electrons through the nanofibers.  
Finally, based on the above considerations, we can conclude that Co-Si LDHs after inserting zirconium and titanium are considered as promising candidates for optical applications such as photocatalysis. In addition, the nanofibers and nanoneedles are suitable components for designing optical nanodevices.  
Conceptualization, O.S. and A.A.; methodology, O.S.; software, O.S.; validation, O.S., N.S. and A.O.; formal analysis, A.A.; investigation, N.S.; resources, A.O.; data curation, N.S.; writing--original draft preparation, A.A.; writing--review and editing, O.S.; visualization, O.S.; supervision, O.S.; project administration, O.S.; funding acquisition, O.S. This research was funded by the Deanship of Scientific Research in King Faisal University (Saudi Arabia), grant number 186289, and the APC was funded by the same grant number, 186289.  
The authors acknowledge the Deanship of Scientific Research at King Faisal University (Saudi Arabia) for the financial support under Nasher Track (Grant No. 186289).  
The authors declare no conflict of interest.