The Propulsion Mechanism And Motion Control Of Platinum-based Micro/nano-motors

As an indispensable component of micromachines,chemically powered micromotors that can move autonomously and independently by harvesting energy from their surrounding environment have great application prospects in drug delivery,environmental remediation,and minimally invasive surgery because of their wide adaptability and do not require equipment for activation.One archetypical micromotor,Pt Janus motor,has received mounting interest among materials scientists and soft matter physicists since its appearance in 2007,which not only has great potential application,but also is suitable for the fundamental research as an ideal physical model due to the simplicity in its design,fabrication,and motion dynamics.However,the propulsion mechanism of Pt Janus motor remains unsettled and the motion control techniques are also limited,hindering its further research and application.Therefore,starting from the influence of the morphology of the catalytic end on the motor motion behavior,this thesis focuses on the propulsion mechanism of the Pt Janus motor and its new motion control technique,laying the foundation for its practical application.We first studied the motion behavior of Si O₂-Pt_film and Si O₂-Pt_nanoparticle micromotors that were prepared by annealing Si O₂-Pt_film in argon atmosphere.We discovered that the speed of Si O₂-Pt_nanoparticle micromotors was much lower than that of the Si O₂-Pt_filmmicromotors.However,by measuring the oxygen evolution rate,Pt with different morphology showed the same catalytic activity toward the catalytic decomposition of H₂O₂.Based on the numerical simulations of ionic self-diffusiophoresis and self-electrophoresis for Si O₂-Pt_film and Si O₂-Pt_nanoparticle micromotors,our observation supported that the self-electrophoretic driving force was dominant during the movement of Pt Janus micromotors,arising from the electrochemically catalytic decomposition of H₂O₂ because of the uneven thickness of Pt shell.On the other hand,the self-diffusiophoretic driving force arising from the asymmetric chemical catalytic decomposition of H₂O₂ on the surface of Pt Janus micromotor could be ignored.Then,based on the above research,we developed a new simple and chemical synthesis to controllably grow Ag nanowires(Ag NWs)on Pt Janus spheres.An electrochemical growth mechanism of Ag NWs based on electron transfer was proposed by measuring the current between Pt electrodes with different thickness,and the mechanism was further verified by the growth of Ag NWs on Ti O₂-Pt Janus microspheres and Au-Rh bimetallic rods.Finally,we investigated the motion of tadpole-shaped PS-Pt Janus-Ag NWs microrotors.Since the presence of Ag NWs broke the shape symmetry of micromotors and introduced hydrodynamic torque,micromotor's motion modes transformed from linear motion to circular motion.Moreover,the angular velocity and the curvature of micromotor's trajectory showed the same trend of first increasing and then decreasing with a gradual increase in the length of Ag NWs,which was related to the degree of its shape symmetry.In addition,based on numerical simulations,we confirmed that the PS-Pt Janus-Ag NWs micromotor was also driven by self-electrophoresis in H₂O₂.The results from this thesis have improved our understanding of the motion dynamics of Pt Janus motor at the micro-scale,and developed a new strategy to precisely control its motion behavior,laying the foundation for the practical application of micromotors in the future.