PNCs are three-dimensional (3D)-like arrangements of constituent atoms/ions formed in solution prior to nucleation, whose structural arrangement is related to the final crystal structure 15, 30, 31. One theoretically and experimentally predicted pathway is through intermediate prenucleation clusters (PNCs) 14, 15, 19, 20, 27, 28, 29. Oriented attachment and colloidal assembly growth have also been reported for many systems 22, 23, 24, 25, 26, 27.
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Recent work has shown deviations from classical nucleation-growth models towards non-classical series of events involving ‘stable’, metastable or transient intermediates 18, 19, 20, 21. Studies on real systems have also been reported 13, 14, 15, 16, 17. Thus, there has been significant amount of experimental and modelling studies to understand these mechanisms in model systems such as atomic liquids, Lennard–Jones liquids, water, colloids, solutions, binary systems and natural gas hydrates 9, 10, 11, 12. Nucleation and crystal growth mechanisms attract much interest because of their importance in science, technology and in many natural and biological processes 1, 2, 3, 4, 5, 6, 7, 8. We anticipate that the work may be a platform for future experimental and theoretical investigation promoting deeper understanding of the nucleation-growth phenomena of a variety of practical systems.
We suggest that this nucleation process is generic to most closed systems that go through precipitation, and, therefore, important for the crystallisation of a variety of metal oxides, composites and minerals. We show that the nucleation-growth processes result from temporal and dynamic activity of constituent ions and gaseous molecules in solution and by the irreversible expulsion of the dissolved gaseous molecules. A prenucleation clusters pathway is observed for nucleation, followed by crystallite assembly-growth. Defying all previous reports, we show that the nucleation commences only when the heating is terminated.
Combining coherent experimental data, we here demonstrate a fundamental nucleation-growth mechanism that occurs in a model zinc oxide system when particles are formed under continuous, rapid heating under closed reaction conditions. Specifically, crystallisation occurring in closed systems are largely unproven. However, the microscopic phenomena of particle formation-nucleation and growth-in real systems are still not fully understood. These features clearly dictate the properties of materials and their applicability in technologies. Atomic and molecular level interactions in solutions dictate the structural and functional attributes of crystals.
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