The formation of nanorods, driven by the physico-chemical phenomena during the freezing of ceria nanoparticle suspension is reported. During freezing a dilute solution of CeO2 nanocrystals, some nuclei remain in solution while others are trapped inside the voids formed within the growing ice front. Over time the particles trapped within the constrained geometries combined by an oriented attachment process to form ceria nanorods. The experimental observations are further supported through Molecular Dynamics (MD) simulations. These observations suggest a new possible strategy for the templated formation of nanostructures through self assembly by exploiting natural phenomena such as freezing of water. Nature epitomizes ‘self assembly’ –living cells arranged in a myriad of complex forms and structures act as one particularly complex example. The challenge of synthesizing hierarchically ordered and complex structures with molecular level recognition is another example of a natural process that cannot yet be duplicated in the laboratory. Fundamental research in understanding such self assembly of molecules has led to new strategies for synthesizing functional nanomaterials(1, 2) Several nanomaterials structures, facilitated via self assembled nanoparticles, have already been demonstrated by various researchers (3-13). Similarities between self assembly of nanoparticles and simple forms of life may lead to additional efforts to unravel and mimic nature(1). Among multitudes of morphologies and configurations, one dimensional nanostructures (ODNS) have received a significant attention from the researchers owing to their potential use in new generation electronics, photo catalysis, sensors and biomedical applications (7). Here, we demonstrate the self assembly and time dependant evolution of ceria nanoparticles into ultra-long polycrystalline ceria nanorods (length ca. 3 – 3.5 micron, diameter ca. 30nm) using a simple water based synthesis, freezing and subsequent aging in ice. The mechanism we propose is effectively a natural form of templated self assembly. Over time in the frozen condition particles gradually evolve into polycrystalline nanorods. Our understanding of the process involves structure templating associated with voids known to form in ice(14-16) and localised oriented attachment process. Oriented attachment and anisotropic growth of nanoparticles in solution(17, 18) are vital to the self assembly of polycrystalline nanoparticles in solution. Advances in molecular dynamics (MD) based simulation have enabled interesting and exciting observations in self assembly(3, 19-21) and we use MD calculations to understand the observed aggregation process. We have recently reported the synthesis of polycrystalline ceria octahedral superstructures by self-assembly through oriented agglomeration in both water and Poly (ethylene glycol)(22). These contrast with the spherical (high index planes stabilized), single crystal ceria nanoparticles synthesized by Feng et al(23) in a high temperature gas phase reaction. Clearly, agglomeration kinetics and behaviour of ceria nanoparticles change with ambient conditions. Motivated by this, we have studied their aging characteristics under different conditions including altering the solution environment and temperature. We have observed the evolution of ceria nanorods supported by the ice templates under freezing conditions. In addition to the formation of nanorods we also observe the formation of polyhedral superstructures which appear during the freezing process and are unaffected by templating effect of ice. In present work a seeding solution of ceria nanoparticles was synthesized using a simple water based method by oxidizing the Ce(III) precursor salt to Ce(IV) (SI-1). Several 35nm crystallites nucleate and start to agglomerate within a short time frame (Figure 1a). The solution containing ceria crystallites was subjected to sub-zero temperatures (-18C) and aged for different time scales. Crystalline ceria nanoparticles agglomerate in an oriented fashion within 1 day of aging and result in rod type structures together with large faceted agglomerates (octahedral super structures) (Figure1b, SI 2). Both the superstructures and the nanorods were constituted of truncated octahedral building blocks of ceria nanoparticles(19). The observation of octahedral superstructure within one day of aging in ice directly contrasts the time dependent agglomeration of nanoparticles into polycrystalline superstructures(22). Subsequent aging for one week, facilitated long polycrystalline nanorods (Figure 1c, 1d, SI 3). Retention of the initial 3-5 nm particle size, with predominant (111) surfaces, is observed even in the highly dense and polycrystalline ceria nanorods about 40nm in diameter (SI 3). Further aging of the nanoparticles in ice (2 weeks) resulted in the formation of ultra long ceria nanorods (Figure 1e, 1f, SI 4a,). We propose simultaneous occurrence of two different processes during the freezing of ice lead to the formation of nanorods along with observed octahedral superstructures. While the formation of octahedral superstructures is inherent to the process of freezing, the formation of nanorods occurs due to ice templating effect. To explain it further, it is proven through both experimental and modeling studies that the “ice structure can not accommodate foreign molecules other than water”, this results in the well known “solute rejection” phenomenon(24). This process is very well acknowledged by the metallurgical engineering literature for alloy solidification and zone refining kind of phenomenon, where the progress of solidification front results in the rejection of impurity particles into the liquid phase. In case, of dilute ceria suspensions, as the freezing front proceeds from top to bottom, the ceria nanoparticles will be expelled into the water phase. This leads to a significant increase in concentration of nanoparticles in unfrozen sol, followed by aggregation and kinetically driven octahedral morphology evolution. The higher concentration of solute ceria particles and the increasing pressure exerted by the ice front on the water phase results in the chemico-physical process (physical process driving assisting chemical bonding) mentioned above. It is well known, the ice front will be porous with some capillaries present at the ice – water interface. The nanoparticle suspension will end up absorbed into these channels due to pressure differences, however, significantly higher concentration of nanoparticles results in their entrapment in these channels as the solidification continues. It was earlier shown that there will be high concentration, unfrozen brine sols left out with in channels formed in ice(25). The formation of nanorods is assisted by the physical templating of ice. At -18C under atmospheric pressure, ice conforms to the “Ice I” structural polymorph(26) and microstructural features, including nano and micro channels, evolve to accommodate the stress and expansion during solidification(27). These channels, with nanometer diameters and a few microns in length, serve as nano-capillaries (hard templates) and the water, supersaturated with ceria nanoparticles, is forced into the capillary, driven by capillary forces emanating from the freezing front(28, 29). The initially oriented particles are locked in the capillary under the ice pressure and undergo uni-dimensional oriented agglomeration to form ceria nanorods(30). It should be noted from this study that a very dilute solution and nanometer size of the particles are central to the formation of channels or capillaries. The perturbation, caused by nanoparticles at the ice-supercooled liquid interface causes channel formation as depicted in figure 2 (and SI-5). Ceria nanoparticles trapped in such nanochannels transform into nanorods with predominant {111} terminated surfaces. Specifically, particles trapped in long nanochannels form the nanorods while those trapped in voids or wide capillaries or in the last portion of the frozen water will retain the octahedral superstructures. A schematic, depicting such morphological evolution, is presented in figure 3a and 3b. Once trapped inside the channels and surrounded by ice, the mobility of ceria nanoparticles is restricted to one dimension. However, the very high surface energy of the nanoaparticles drives the intra – agglomerate rotation to achieve partial oriented attachment at two, three particle interfaces. Such orientation proceeds by incorporation of defects such as dislocations along the interfaces of nanoparticles as shown in figure 3c. The white circles indicate the interfacial dislocations formed during the oriented agglomeration. Both, the formation of octahedral morphologies and nanorods involve rotation and orientation of the nanoparticles to facilitate low energy configurations and coherent/semi-coherent interfaces. This can be visualized as an imperfect attachment as described in an earlier observation with titania nanoparticls(17, 18). Higher activation energies and brownian motion etc. are required for a full crystal rotation or recrystallisation to facilitate complete coherence and formation of single crystals. However, in a time limited aggregation under freezing conditions, complete rotation is not achieved and screw/edge dislocations form at the interface between three or more orienting particles (SI-6). As three or more particles align, elastic deformation results in partial coherence together with the evolution of screw dislocations(18). The formation of screw dislocations is also predicted by the theoretical investigation and is discussed below and shown in SI-7. Upon aging, the misorientation and associated dislocations are finally annihilated resulting in energetically stable interfaces. A comparison of oriented attachment was made using the particle encounter complex (Kos)(31) model, following equation 1. ) / ) ( exp( 3 4000 3 kT h V h N K T av o