Carbon Allotrope Nanomaterials Based Catalytic Micromotors

Carbon allotropes nanomaterials are explored here for the preparation of highly efficient tubular micromotors: 0D (C₆₀ fullerene), 1D (carbon nanotubes), 2D (graphene), and 3D (carbon black, CB). The micromotors are prepared by direct electrochemical reduction or deposition of the nanomaterial into the pores of a membrane template. Subsequent electrodeposition of diverse inner catalytic layers (Pt, Pd, Ag, Au, or MnO₂) allows for efficient bubble-propulsion in different media (seawater, human serum, and juice samples). Atomic-force microscopy (AFM) and scanning electron microscopy characterization reveals that the micromotors exhibit a highly rough outer surface and highly microporous inner catalytic structures. A key aspect derived from the AFM characterization is the demonstration that the rough outer surface of the micromotors can greatly affect their overall speed. To date, the literature has only focused on studying the effect of the inner catalytic layer upon their speed and performance and has underestimated the effect of the outer surface layer. The speed of carbon-based micromotors is a compromise between two opposite forces: the increased catalytic activity because of improved fuel decomposition in the inner catalytic layer, which propels their advance, and the friction of the rough outer surface with the fluid, which is opposed to it. The largest outer surface area associated with the highest surface roughness of C₆₀ fullerene and carbon black-Pt micromotors leads to a large friction force, which results in a reduced speed of ∼180 μm/s (1% H₂O₂). In contrast, for carbon-nanotube-Pt based micromotors, the dominant force is the high catalytic activity of the micromotor, which allows them to reach ultrafast speeds up to 440 μm/s (1% H₂O₂). The new protocol opens new avenues for the universal preparation of carbon based multifunctional micromotors for a myriad of practical applications exploiting the features of carbon allotropes.