## 3 Results and Discussions  
### Growth Mechanism of TiO\({}_{2}\) Nanocrystals  
Titanium dioxide nanocrystals (NCs) were synthesized following a slightly modified synthesis method as described by [PERSON] et al. [34] where oleic acid (OLAC) was used as the solvent, reagent, and ligand [40]. Our modifications were mainly related to the reduced precursor amounts and lowering the reaction temperature from 270 \({}^{\circ}\)C to 250 \({}^{\circ}\)C to control the reaction temperature fluctuations and foam expansions. Figure 1 shows the as-synthesized TiO\({}_{2}\) NCs characterized by TEM and high-resolution TEM (HRTEM) images. As can be seen in Figure 1, as-synthesized TiO\({}_{2}\) NCs mostly consisted of nanorods (NRs) with various lengths as well as some semispherical NCs. To effectively separate the TiO\({}_{2}\) NRs specifically in terms of their length to obtain the desired polydispersity index (standard deviation divided by mean length), size-selective precipitation, or more correctly aggregation, was performed on the TiO\({}_{2}\) mixture. This well-known purification technique is mainly based on the magnitude of the van der Waals attraction forces between particles which increase with particle size [41]. In the case of NRs, the van der Waals attraction forces induced the aggregation between NRs with larger lengths [42]. We found that to achieve a higher yield of monodisperse NRs (yield of \(\sim\)50% or almost 1 g of NRs), it was crucial to separate first the larger NCs (here TiO\({}_{2}\) NRs) from the reaction mixture _just through centrifugation_ prior to the addition of antisolvent in the size-selective precipitation process. After centrifugation of the reaction mixture, separation of the TiO\({}_{2}\) NRs by length was highly efficient by the dropwise addition of acetone as an antisolvent to the dispersion of the NRs in toluene. Due to the hydrophobicity of the ligand layer around TiO\({}_{2}\) NRs, the dispersion of the NRs was destabilized by the addition of the polar acetone which resulted in the aggregation and fast sedimentation of the NRs leaving many of the synthetic by-products and shorter rods in solution. Finally, both the TiO\({}_{2}\) NRs as main products and the semispherical NCs left in the supernatant were redesversible in a variety of nonpolar solvents such as toluene as shown in Figure 1d,e and resulted in the isolation of relatively monodisperse TiO\({}_{2}\) NRs (\(\sigma_{L}\) = 14%). The dimension of the NRs was on average 3.6 \(\pm\) 0.4 nm (thickness) and 24.6 \(\pm\) 3.4 nm (length), while the semispherical NCs had an average diameter of 5.7 nm. Figure 1 shows the selected area electron diffraction (SAED) pattern of these NRs revealing the highly crystalline anatase phase. The reflected rings were clearly indexed to (101), (103), (004), (200), and (105) planes of anatase indicating the high crystallinity of these particles which is consistent with the HRTEM images and X-ray diffraction (XRD) results in Figure 4. In Figure 1 where a (010) zone axis is perpendicular to the imaging plane, a single nanorod can be seen to have grown along its \(c\) axis in the (001) direction. The reason is that (001) surfaces in the anatase phase have a higher surface energy in comparison to (101) surfaces [26; 43]. Thus, the typical fast growth in anisotropic anatase NRs is observed with a preferential growth in the (001) direction as was also the case in our synthesis. The lattice fringes of approximately 0.35 nm are depicted with yellow parallel lines corresponding to the (101) planes of the anatase phase. In the following, we will discuss the mechanism of the titania NRs formation in our experiment based on the literature on the nonhydrolytic sol-gel routes to synthesize titanium oxide NPs [27; 31; 34; 44; 45; 46; 47] and on what we observed by following our synthesis in time by taking samples of reaction mixtures at 10-min intervals. In general, nonhydrolytic sol-gel processes can be divided into two main categories: ligand-assisted and solvent-controlled routes.