Ostwald Ripening: A Synthetic Approach for Hollow Nanomaterials

Fabrication of nanomaterials with hollow interiors is an important research area in nanoresearch, owing to their potential applications in photonic devices, drug delivery, material encapsulation, ionic intercalation, surface functionalization, nanocatalysts, membrane nanoreactors, and many other technologies. The common preparative methods for this new class of materials can be broadly divided into hard and soft template-assisted syntheses. In recent years, furthermore, the interest in template-free techniques for these materials has also increased, as the new processes involved in these techniques are relatively simple and less demanding, compared to the template-assisted processes. In this short review, we will introduce the application of a well-known physical phenomenon of crystal growth - Ostwald ripening - in the fabrication of hollow nanomaterials. It has been demonstrated that formation of the interior spaces of nanostructures depends on the aggregative states of the primary crystallites during the synthesis. With this new development, many inorganic nanomaterials with interior spaces can now be fabricated in solution media together with the materials synthesis. Different types of Ostwald ripening observed in this synthetic approach have been reviewed. In particular, various geometric structures and configurations prepared with these methods have been discussed. The prepared hollow materials also allow further compositional and structural modifications under the similar process conditions. Future directions in this research area are also discussed. Keywords: Polyhedral Hollow Structures, transition metal oxide, nanocrystallites, nanospheres, Asymmetric Ostwald ripening