Microdroplets as ambient 'hydrothermal' reactors

Microdroplet reactors offer a unique platform for driving chemical transformations under ambient conditions, mimicking key features of hydrothermal environments. Here, we report a microdroplet-enabled strategy for the room-temperature synthesis of phase-pure, porous, flower-like tenorite (CuO) nanostructures from copper acetate monohydrate via a copper hydroxide acetate monohydrate intermediate. Ultrasonic nebulization of homogeneous ethanol-water solutions of copper acetate monohydrate creates confined microenvironments with rapid solvent evaporation and steep concentration gradients, enabling nucleation and growth pathways analogous to hydrothermal systems under ambient conditions, effectively resulting in an "ambient hydrothermal" reaction. The resulting CuO exhibits exceptional functional performance, achieving rapid U(VI) removal from 1000 ppb to below 10 ppb within 5 minutes; over 20-fold higher efficiency than commercial CuO, while retaining structural integrity. This gram-scale, energy-efficient synthesis eliminates high-temperature calcination and demonstrates the broader potential of microdroplets as "ambient hydrothermal" reactors. The method provides a scalable, surfactant-free route to complex oxide nanostructures, bridging the gap between solution-phase and solid-state approaches.