## 3 Results and Discussions
### Growth Mechanism of TiO\({}_{2}\) Nanocrystals
We also noticed that during the growth steps, some NRs became unequally thicker such that the thickness was somewhat larger in either one tip of the NRs (matchstick-like NR shown in Figure 2b-inset) or in both tips (dumbbell-like NR shown in Figure 2d-inset). For titania NRs, a process of shape evolution after initial crystallization has been described in several previous reports, as well [26; 27; 43; 49; 52; 53]. In some cases, anatase NCs that were formed in acidic conditions nucleated almost exclusively as truncated tetragonal bipyramidal NCs with (101), (001), and (010) facets [22; 54]. Then, these primary NCs were kinetically promoted to grow via the oriented attachment mechanism [55; 56]. We note that oriented attachment in one, two, three dimensions as well as mesocrystal formation have been reported frequently for surface-functionalized titania NPs synthesized in both hydrolytic and nonhydrolytic media [26; 55; 57]. Elaborate investigations by [PERSON] and [PERSON] [47] on anatase NRs, synthesized by thermal decomposition of titanium(IV) butoxide in an OLAC mixture revealed that the oriented attachment of these truncated tetragonal bipyramidal NCs occurs via sharing the (001) plane with a preferential growth in the (001) direction. In our results, we also observe typical morphological evolution as a function of time, for instance, an increase in the thickness of the NRs while the length reduced in the NRs and the emergence of relatively smaller NRs and NCs. [PERSON] and [PERSON] attributed these shape alterations to the fragmentation of the titania NRs into smaller NCs induced by a Rayleigh instability-like phenomenon [36; 47]. Furthermore, they stated that the mass transportation to the tips of the NRs via a surface diffusion process could kinetically advance the detachment process. In the Rayleigh instability phenomenon, an unstable liquid cylinder breaks into droplets driven by surface tension fluctuations coupled with a lowering of the total energy. It has been proposed that under certain circumstances, this concept can be extended to solid NPs and therefore NRs could also be vulnerable to fragmentation into smaller NPs [58; 59; 60; 61]. Although we cannot rule out the occurrence of directed attachment during the growth steps, we believe that due to the existence of ligands in the solution, the growth mechanism in Figure 2 is mainly attributed to the fast out-of-equilibrium growth of the NRs from the solution. In Figure 3, HRTEM images of several synthesized anatase NRs have been presented. As can be observed, the structural features depicted by yellow arrows and lines on the surface of the NRs such as steps, corrugated sides, zigzag patterns, and sharp edges give indications of the critical pinch points formed during the surface diffusion step that are vulnerable to the detachment. Considering TEM images of ultrathin NRs formed in the early stages of the reaction (Figure 2) and HRTEM images of final NRs in Figure 3, the Rayleigh instability-like detachment phenomena could also be the reason in our experiment that led to shorter titania NRs and pinched off semispherical titania NCs. However, further in-depth investigations are needed to precisely confirm the exact mechanism behind these TiO\({}_{2}\) NRs growth and their shape evolution during this experiment. Figure 2: **(a–d)** TEM micrographs of TiO\({}_{2}\) NCs depicting progressive evolution of NCs at different time intervals (10 min, 20 min, 60 min, and 120 min, respectively) from ultrathin NRs to the final mixture of NRs of various lengths and semispherical NCs. The length distributions of TiO\({}_{2}\) are shown in histograms as insets with black columns. The average thickness of the NRs are 1.8 \(\pm\) 0.3 nm in (**a**), 2.6 \(\pm\) 0.5 nm in (**b**), 2.9 \(\pm\) 0.7 nm in (**c**), and 3.6 \(\pm\) 0.4 nm in (**d**). Inset in (**b**) shows the occurrence of irregularities at the tip of the NRs. Inset in (**d**) shows an HRTEM image of a single NR exhibiting a dumbbell shape at higher resolutions. Scale bar in b-inset is 25 nm and in d-inset is 5 nm.