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.