Tadpole-Shaped Catalytic Janus Microrotors Enabled by Facile and Controllable Growth of Silver Nanotails

Microrotors are an indispensable component in micromachines, yet their usefulness has been limited by a lack of simple, inexpensive, and controlled fabrication technique that yields microrotors of controlled shapes in large quantities. To address this challenge, the chemical synthesis, characterization, and activation of tadpole-shaped catalytic microrotors that consist of a spherical, platinum (Pt)-coated head and a silver (Ag) nano-tail of tunable lengths are reported herein. Importantly, this tail spontaneously grows on Pt in an aqueous solution of Ag⁺ and hydrogen peroxide (H₂O₂), at a speed of ≈100 nm s⁻¹, preferably along the Ag (111) plane. The growth of Ag nanowires is attributed to an electrochemical reaction occurring on a tapered Pt cap, a mechanism corroborated by control experiments with photo-active titania microspheres, which introduce the additional advantage of light-controlled growth. The presence of a long Ag nano-tail on a tadpole-shaped microrotor breaks its symmetry and induces rotation in H₂O₂, and its structure-dependent dynamics is quantitatively studied and supported by numerical simulation. The chemical synthesis of microrotors with Ag nano-tails will introduce new designs of micromachines of controlled dynamics, as well as functional materials and devices where mild, controllable, and facile growth of Ag nanostructures is desired.