Contraction and expansion of nanocomposites during ion exchange reactions

The next generation of advanced functional materials can greatly benefit from methods for realizing the
right chemical composition at the right place. Nanocomposites of amorphous silica and metal carbonate nanocrystals
(BaCO3/SiO2) form an attractive starting point as they can straightforwardly be assembled in different controllable
three-dimensional (3D) shapes, while the chemical composition of the nanocrystals can be completely converted via
ion-exchange. Nevertheless, it is still unknown—let alone predictable—how nanoscopic changes in the lattice volume
of the nanocrystals translate to changes in the microscopic dimensions of the 3D BaCO3/SiO2 structures during
ion-exchange. Here we demonstrate that the microscopic shape adapts to contraction and expansion of the atomic
spacing of nanocrystals. Starting from BaCO3/SiO2, we systematically decrease and increase lattice volumes by converting
the BaCO3 nanocrystals into a range of chalcogenides and perovskites. Based on geometrical analysis, we
obtain a precise prediction for how the microscopic nanocomposite volume follows the change in nanoscopic crystal
volume. The silica matrix facilitates mechanical flexibility to adapt to nanoscopic volume changes, while preserving
the 3D morphology and fine details of the original composite with high fidelity. The versatility and predictability of
shape-preserving conversion reactions open up exciting opportunities for using nanocomposites as functional components.
Introduction