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Water freezing is ubiquitous and affects areas as diverse as climate, the chemical industry, cryobiology and materials science. Ice nucleation is the controlling step in water freezing1-5 and has, for nearly a century, been assumed to require the formation of a critical ice nucleus6-10. But there has been no direct experimental evidence for the existence of such a nucleus, owing to its transient and nanoscale nature6,7. Here we report ice nucleation in water droplets containing graphene oxide nanosheets of controlled sizes and show that they have a notable impact on ice nucleation only above a certain size that varies with the degree of supercooling ofthe droplets. We infer from our experimental data and theoretical calculations that the critical size of the graphene oxide reflects the size ofthe critical ice nucleus, which in the case of sufficiently large graphene oxides sits on their surface and gives rise to ice formation behaviour consistent with classical nucleation theory. By contrast, when the graphene oxide size is smaller than that of the critical ice nucleus, pinning at the periphery of the graphene oxide deforms the ice nucleus as it grows. This gives rise to a much higher free-energy barrier for nucleation and suppresses the promoting effect of the graphene oxide11. The results provide experimental information on the existence and temperature-dependent size ofthe critical ice nucleus, which has previously only been explored theoretically and through simulations12-16. As pinning of a pre-critical nucleus at a nanoparticle edge is not specific to the ice nucleus on graphene oxides, we expect that our approach could be extended to probe the critical nuclei in other nucleation processes.
Theory17 and experiment18 have shown that for radii ranging from around 10 Å to 1,000 Å, size profoundly influences a particle's ability to induce ice nucleation. Such a size effect is evident when we consider that antifreeze proteins (AFPs) suppress ice formation, whereas structurally similar but larger ice nucleation proteins (INPs) promote it (Fig. 1a, b)19-21. Because graphene oxide (GO) nanosheets influence ice nucleation22-25 and can be prepared in a wide range of sizes, we used them to systematically explore the effect of nanoparticle size on ice nucleation.
GOs with different sizes were prepared by fractionating commercial GO aqueous dispersions by consecutively filtering through ultrafiltration membranes...