A new perspective of inorganic crystallization: Non-classical nucleation and growth

Inorganic crystals surround us and play important roles in our daily life. They form integral parts of our body (e.g., bones), make up all the ground we stand on (ours and any other rocky planet consist of crystals), as well as show significant necessity for industrial processes and technologies (from table salt over concrete to biomedical nanoparticles). Accordingly, the formation mechanisms and properties of crystals have been extensively studied over the past decades. While there are longstanding theories on both the birth of crystals (nucleation) and their subsequent evolution (growth, recrystallization, and/or transformation), a vast amount of evidence suggests that the classical view on crystallization is over-simplified. In addition to the monomer-by-monomer (i.e., atoms, ions, or molecules) addition described in the classical theories, the particle attachment has been recognized as one of the most important pathways to inorganic crystallization. These particles range from the ion pairs to well-crystallized nanoparticles, such as amorphous precursors, magic-size clusters, nanocrystals, etc. The complexity of both the free-energy landscapes and reaction kinetics leads to diverse crystallization pathways. While experimental observations clearly demonstrate the non-classical crystallization pathways, many fundamental aspects remain unknown-Particularly the interaction of solution structure, interfacial forces, and particle motion. Thus, a predictive description linking molecular details to ensemble behavior is lacking. As such description develops, long-term interpretations of inorganic crystal formation should be revisited, which is important to broaden the scope of research across various disciplines such as geological events, biomineralization mechanisms, environmental remediation, and the development of environmentally functional materials. Based on several exemplary spotlights, the current state of the art in non-classical nucleation has been firstly illustrated in our review. Specifically, two non-classical nucleation pathways (i.e., the prenucleation clusters (PNCs) and aggregation pathway) in the current crystal research field are summarized. According to the PNCs pathway, PNCs were considered as the stable solute species with chainlike structural that are precursors to nucleation. Once system reaches the equilibrium ion activity product, the change in the chemistry of linkages within PNCs causes PNCs aggregation, as a consequence of nucleus formation. Another nonclassical view (aggregation pathway) pointed that the fine crystalline nuclei first form in solution and then aggregate into a larger and more stable nucleus. Besides, the non-classical oriented attachment (OA) and random attachment (RA) growth pathways are also elucidated. OA and RA growth are similar in involving the self-assembly of primary nanocrystals. However, their main difference is the orientation of the crystal lattice at the grain boundary. For RA growth pathway, there is no particular preference for the attachment whereas for the OA growth, there is a common crystallographic alignment of the attachment to occur, which is allowing for continuous crystallographic planes. Finally, the implications and perspective of non-classical crystallization research are proposed. The aim of this review is to guide the readers through the complex world of crystallizing systems, highlighting a new perspective on non-classical crystallization of inorganic materials, which drives a true renaissance in the field and provides completely new perspectives on the underlying crystallization mechanisms. Additionally, through a mechanism-based understanding, we believed that non-classical crystallization processes can be used to produce innovative structures that retain the dimensional properties of their nanoscale building blocks and create materials with enhanced or novel physical and chemical properties.