Wall-Induced Symmetry Breaking and Directional Stability in Single-Component Light-Driven Micromotors

Light-driven micromotors offer a versatile platform for studying nonequilibrium systems and enabling applications; however, most existing designs rely on material asymmetry or multicomponent architectures for self-propulsion and additionally require external fields or geometric confinement for directional control, which complicates design and limits autonomy. Here, we report a simple class of single-component, cube-shaped hematite (α-Fe₂O₃) micromotors that exhibit autonomous propulsion with exceptional directional stability under visible-light illumination. The micromotors are synthesized by using a scalable, single-step hydrothermal method, yielding uniform cubes in large quantities. Under visible-light illumination in dilute aqueous hydrogen peroxide, the hematite cubes self-propel efficiently, achieving measurable speeds even at ultralow fuel concentrations (0.01% (v/v)). Strikingly, once activated, the micromotors maintain persistent, nearly perfectly linear trajectories over extended distances and times despite the absence of intrinsic material asymmetric or external alignment fields. We rationalize these observations using a theoretical framework that incorporates chemiosmotic slip at the underlying planar surface and hydrodynamic interactions. The model demonstrates that wall-induced spontaneous symmetry breaking can generate a stable sliding state with an enhanced orientational persistence. Together, these results establish cube-shaped hematite micromotors as a minimal and intrinsically stable active colloid system, offering new insights into geometry-controlled propulsion near a solid surface and providing a robust platform for studying active matter with high directional fidelity.