**4.3 Lack of efficient and greener synthesis methods to make ultrathin anisotropic nanostructures**

The controlled synthesis of ultrathin anisotropic nanostructures is limited to few materials, and the more efficient, scalable, greener, facile, and reproducible synthesis techniques are rarely introduced. The attempts of developing new strategies are needed to enhance the prospects for the preparation of controlled 1D nanostructures. Although there is a rapid progress on OA-based 1D metal/metal oxide nanostructures without any organic additives, we are still far away from the full understanding of OA crystal growth. The new efficient and greener synthesis methods will certainly open doorways toward rationally designing different ultrathin nanostructures. Although most common method to produce 1D noble metal nanostructures is seed-mediated growth methods, they are ineffective methods to make long nanostructures such as nanowires. Therefore, enough attempts are required to change the aspect ratio of 1D noble metal nanostructures with in-depth understanding of the controlling factors. The new solution-based wet chemical strategies are required to develop unrestrictedly size and shape-controlled 1D nanostructures to get the advantages of costeffectiveness, energy effectiveness, and scalability of the process.

#### **4.4 The insufficient theoretical investigation to improve fundamental understanding**

The theoretical investigations further advance our fundamental understanding on OA crystal dynamic growth processes. Molecular dynamics, DFT simulations, molecular mechanics, ab initio methods are a few tools that can be used to develop theories and strengthen the understanding of the OA mechanism. The necessity of these theoretical tools combined with experimental techniques is increasing demand for further insight into the OA growth at different levels including molecular, atomic, and crystal lattice scale. If we combine real-time crystal growth dynamics with simulations of interparticle forces and interactions, it will provide the explanation of guiding factors quantitatively. However, the utilization of theoretical models to develop more accurate experimental OA growth models is limited, thus necessitating collaborative experimental work for further advancement.

## **5. Conclusions**

In conclusion, the in-depth discussion of potential of "oriented attachment" mechanism has been overviewed in size and shape-controlled inorganic metal/metal oxide 1D nanostructures to use in different applications. The understanding of the interplay between OA crystal growth mechanism, phase transformation, and kinetics is the key to produce controlled 1D nanostructures, but the establishment of their relationship remains a tremendous challenge. The recent advances on in-situ, ex-situ techniques and theoretical simulations provide important insights into OA growth. However, the applicability of general kinetic models to interpret the OA mechanism was already questioned recently, as the growing interest of 1D nanostructures based on OA crystal growth. Therefore, the development of OA-based elongation kinetics of 1D nanostructures represents a key knowledge gap and a challenging task. Our recent time-dependent HR-TEM study provides new insights into crystal growth of Cu(OH)2 nanowire formation from primary nanocrystals via OA mechanism in a sol-gel system. We introduced novel multistep sigmoidal kinetic models for the OA-based Cu(OH)2 nanowire formation. This study significantly contributes to the advancement on the fabrication of ultrathin nanowires using OA attachment without using any surfactants by correlating OA-based elongation growth kinetics. We anticipate that the next decade will be an exciting time for both materials scientists and computational scientists to study crystal growth dynamics with state-of-the-art advanced instruments.
