Crafting Hollow Spheres via Bulk Ice Melting with ppb-Level Gas Sensing Performance.

Ice melting, a common yet complex phenomenon, remains incompletely understood. While theoretical studies suggest that preexisting defects in ice generate "off-lattice" water molecules, triggering bulk ice melting, direct experimental evidence of their form has been lacking as the transparent and transient nature of ice poses significant challenges for observation with current techniques. Here, we introduce an ice-melting-induced lyophilization (IMIL) technique that employs graphene-based nanoprobes to replicate and track liquid evolution within melting bulk ice. Our experimental data and theoretical calculations indicate that "off-lattice" water molecules form spherical droplets that enlarge and coalesce as the melting progresses. Notably, the IMIL technique represents a novel nanotechnology for crafting high-quality hollow spheres by leveraging naturally occurring droplets as templates, offering advantages in simplicity, environmental friendliness, scalability, and size adjustability over traditional methods. Additionally, platinum-loaded graphene-based hollow spheres fabricated via the IMIL technique demonstrate ultrasensitive formaldehyde detection with a 5 parts per billion detection limit, rapid response and recovery times (∼4.9 s), and room-temperature operation without auxiliary technology, outperforming WHO standards and current detection methods. These findings highlight the potential of the IMIL technique for creating versatile hollow spheres for diverse applications.