Catalytic Nanomotors:  Autonomous Movement of Striped Nanorods

• Published in: Journal of the American Chemical Society Vol. 126; no. 41; pp. 13424 - 13431
• Main Authors: Paxton, Walter F, Kistler, Kevin C, Olmeda, Christine C, Sen, Ayusman, St. Angelo, Sarah K, Cao, Yanyan, Mallouk, Thomas E, Lammert, Paul E, Crespi, Vincent H
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 20.10.2004
• Subjects: Catalysis; Catalysts: preparations and properties; Chemistry; Cross-disciplinary physics: materials science; rheology; Exact sciences and technology; General and physical chemistry; Materials science; Nanoscale materials and structures: fabrication and characterization; Nanotubes; Physics; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Mixed catalyst; Atomic force microscopy; Gold; Interface properties; Aqueous solutions; Platinum; Transition elements; Interfacial tension; Particle suspension; Metal particle; Catalyst activity; Nanostructured materials
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja047697z

Rod-shaped particles, 370 nm in diameter and consisting of 1 μm long Pt and Au segments, move autonomously in aqueous hydrogen peroxide solutions by catalyzing the formation of oxygen at the Pt end. In 2−3% hydrogen peroxide solution, these rods move predominantly along their axis in the direction of the Pt end at speeds of up to 10 body lengths per second. The dimensions of the rods and their speeds are similar to those of multiflagellar bacteria. The force along the rod axis, which is on the order of 10-14 N, is generated by the oxygen concentration gradient, which in turn produces an interfacial tension force that balances the drag force at steady state. By solving the convection-diffusion equation in the frame of the moving rod, it was found that the interfacial tension force scales approximately as SR 2 γ/μDL, where S is the area-normalized oxygen evolution rate, γ is the liquid−vapor interfacial tension, R is the rod radius, μ is the viscosity, D is the diffusion coefficient of oxygen, and L is the length of the rod. Experiments in ethanol−water solutions confirmed that the velocity depends linearly with the product Sγ, and scaling experiments showed a strong dependence of the velocity on R and L. The direction of motion implies that the gold surface is hydrophobic under the conditions of the experiment. Tapping-mode AFM images of rods in air-saturated water show soft features that are not apparent in images acquired in air. These features are postulated to be nanobubbles, which if present in hydrogen peroxide solutions, would account for the observed direction of motion.