Graphene Oxide Colloidal Particles Swarming And“Breathing”under Light Stimulus

Light-driven self-propelled particles are micro-or nano-particles that drive their own motion through light stimulation.Light-driven the movement of self-propelled particles simultaneously has the advantages of clean energy and easy remote control.Due to the low Reynolds number and Brown's random thermal motion,the study of self-propelled particles and the control of self-propelled particle motion direction are all challenging issues.In addition,the clustering movements of living materials in nature tend to occur in a synergistic movement mode,for example,the orderly state that is far from the equilibrium state,and the latter is more the current concern of scientists,but it has rarely been reported that the synthetic self-propelled particles has such cluster synergistic motion properties.In this paper,the self-propulsion of graphene oxide colloidal particles under light stimulation and the special nonlinear motion phenomenon of multilayer graphene oxide particles are reported and studied for the first time,mainly complete the following work:1.Study on the self-propelled movement law of graphene oxide colloidal particles and the oscillation phenomenon of multilayer graphene oxide colloidal particles under light stimulation.The graphene oxide colloidal particles undergo self-propelled clustering motion toward the center of the light source under light stimulation,and collect at the center of the light source.This phototaxis self-propelled movement of graphene oxide colloidal particles has photoresponsivity,the speed of which is related to the light source and its intensity,and the direction of motion is unitary towards the center of the light source.Specifically,the light source that can be used to drive the graphene oxide colloidal particles has an energy threshold,and only the blue-green light or higher-energy light source can cause self-propellation,and the motion speed of self-propelled movement decreases as the intensity of the light stimulus becomes weaker,and when the light stimulus is too weak,the graphene oxide colloidal particles will stop moving.In addition,we found that multi-layer graphene oxide particles can undergo a very special "stop and breathe" motion phenomenon under light stimulation,under steady and continuous light stimulation,the self-propelled movement speed of graphene oxide particle clusters will appear periodic pause phenomenon,showing the "stop and go" oscillation process.When the graphene oxide particles have a"breathe" phenomenon,the diameter of the circular aggregation region of the graphene oxide particles in the center of illumination will "expand" and "shrink" as the graphene oxide particle clusters "stop" and "go" process.The monolayergraphene oxide particles with no oscillation phenomenon were intercrosslinked and fixed,and we successfully synthesized the multi-layered graphene oxide particle with oscillation phenomenon,indicating that the multilayer structure is very important for the "breathe" phenomenon.2.Study on the mechanism of the self-propelled motion of graphene oxide colloidal particles and the oscillation phenomenon of multilayer graphene oxide particles under light stimulation.We have demonstrated that the light induced locomotion is caused by the photochemical processes occurred in the surface of graphene oxide colloidal particles by Fourier infrared spectroscopy(FT-IR),UV-visible absorption spectrum(UV-vis),Zeta potential and electrochemical tests,etc.The graphene oxide particles will generate hydrogen ions with the action of light stimulation,and a concentration gradient of hydrogen ions will be generated near the graphene oxide particles to form a electroosmotic flow,then the graphene oxide particles move toward the center of the light source,which is opposite to the electroosmotic flow.Therefore,the phototaxis motion of graphene oxide particles is in accordance with self-electrophoresis self-propulsion mechanism.It was found that only multilayer graphene oxide has oscillations after repeated experiments.We characterized the difference in thickness of oscillating and non-oscillating graphene oxide particles by atomic force microscopy(AFM)and X-ray diffraction(XRD).According to the difference in the thickness of these two kinds of graphene oxide particles,we have given the possible mechanism of oscillation movement phenomenon of graphene oxide under light stimulation.Graphene oxide contains semiconducting ?-? conjugated micro domains,which can generate electron transitions under light excitation.Multi-layered graphene oxide particles can be regarded as a miniature graphene oxide capacitor,multi-layer graphene oxide particles undergo electron accumulation and collapse under light stimulation,just as the charging and discharging processes of the capacitor,this may lead to "stop and go" phenomenon of multilayer graphene oxide particles.The simultaneous in situ stopping and going of a group of graphene oxide particles may be related to the synergy effect of hydrodynamic coupling forces.This work may help to better understand the surface or interface properties of graphene oxide or graphene,which determines many important applications of graphene oxide or graphene,such as catalysts,electrodes,and supercapacitors.On the other hand,this work is also helpful for the development of smart micro devices,adaptive catalytic systems,etc.,which contribute to the study of the synergy movement phenomenon of living material groups and so on.