Toward understanding of the propulsion mechanism of rod-shaped nanoparticles that catalyze gas-generating reactions

There is no consensus in the literature on the propulsion mechanism of rod-shaped nanomotors that catalyze hydrogen peroxide decomposition to oxygen and water. Historically, the directional motion was discovered with bimetallic nanorods,^1,2 and the related asymmetry of catalytic rod surface has been taken as a necessary condition of self-propulsion in all of the mechanisms proposed to date. This study for the first time demonstrates that hydrodynamic behavior of monocomponent catalytic nanorods in a H₂O₂ solution is similar to that of the bicomponent ones, and hence, the surface chemical asymmetry is unlikely to be a governing factor in the rods self-propulsion. The experimental results have shown that oxygen bubbles' evolution from the rod surface and related gravitational forces alone can drive directional motion of the rods. A new model is proposed that is based on the gravitational forces and considers rod motion in the 3D space. Nonparallel rods orientation relative to the horizontal plane has been found to play a major role in rods propulsion. Momentum exchange between such a "tilted" rod and water flux, caused by the oxygen bubbles' departure, drives the rod propulsion. The proposed mechanism provides a reasonable explanation for all behavior patterns of the nanorod motors. Motility of Pt and Rh nanorods was studied in the rod length range of 2-7 μm and the H₂O₂ concentration range of 3-30 wt %. Confocal microscopy was used to estimate nanorods orientation relative to the horizontal plane.