Manganese Oxide Based Catalytic Micromotors: Effect of Polymorphism on Motion

Manganese oxide (MnO₂) has recently emerged as a promising alternate material for the fabrication of self-propelled micromotors. Platinum (Pt) has been traditionally used as a catalytic material for this purpose. However, the high cost associated with Pt restricts its widespread use toward practical applications where large amounts of material are required. MnO₂ exists in different crystalline forms (polymorphs), which govern its catalytic behavior. In spite of this, the recent reports on MnO₂ based micromotors have seldom reported on the polymorphic form involved. In the present work, we synthesized six different types of MnO₂ based micromotors, which represent different geometrical designs (i.e., spherical, rod-like, and tube-like microparticles) and polymorphs, and characterized their motion behavior in different chemical environments. Out of all micromotors tested, the hollow spherical MnO₂ microparticles reached the maximum velocity of ∼1600 μm s^-1, which represents the fastest MnO₂ based catalytic micromotor reported until date. The findings of this study will have a profound impact on the design and application of the next-generation synthetic micro- and nanomotors based on MnO₂ as a low-cost and environment friendly material.