Single Nanoparticle Collisions Studied By Correlated Optical Imaging And Electrochemical Recording

Nanomaterials have excellent electrochemical properties,so they are widely used in various fields such as electrocatalysis,sensing and energy.Traditional nanoelectrochemistry often studies the macroscopic electrode interface containing millions or even billions of nanoparticles.The results reflect the average performance of all individuals.However,due to the inherent heterogeneity of nanoparticles,this average effect will blur the structure-activity relationship,which hampers the clarification of the interfacial electron transfer law and mechanism and the improvement of their performance.With the development of electroanalytical chemistry and nano-fabrication in recent years,a novel research method based on random collisions between single electroactive nanoparticles and the inert ultramicroelectrode(UME)has emerged in the field of nanoelectrochemistry,referred to as single nanoparticle collisions.The technique adopts a chronoamperometry method to detect current spikes generated by nanoparticles colliding randomly on an inert UME.By analyzing the current spikes,one could obtain the properties of individual nanoparticles.Although this technique can detect electrochemical or electrocatalytic current signals of individual nanoparticles,there are still some bottlenecks.First,the traditional single nanoparticle collisions technology lacks spatial resolution.Although one could use statistical principles to "make sure" that a certain current peak comes from a single nanoparticle,it is difficult to identify and characterize a specific nanoparticle.Therefore,only the activity information can be obtained in the structure-activity relationship,and the structure information is completely lost.Second,although statistical theory confirms that most of electrochemical signals come from the collisions of single nanoparticles,it is impossible to confirm that an unusual current signal corresponds to a single nanoparticle with excellent performance,or is caused by the simultaneous collisions of multiple nanoparticles on the electrode.At the same time,several optical microscopy imaging techniques have fully met the sensitivity and resolution required to perform single nanoparticle optical imaging.Based on the abo,ve understanding,this thesis introduces surface plasmon resonance microscopy(SPRM),an emerging optical microscopy imaging technique,into the study of collision electrochemistry of single nanoparticles.Combining the high spatial resolution of optical microscopy with the high sensitivity of electrochemical measurement technology,the electrochemical reaction kinetics of single nanoparticles has been studied to elucidate the electron transfer behavior and law at the nano-scale.It provides theoretical basis and technical support for the structure-activity relationship of electroactive materials at the level of single nanoparticles.The main contents of this paper include:1.Integrated Apparatus for Simultaneous Surface Plasmon Resonance Imaging and Electrochemical RecordingIn order to make up for the lack of spatial information in the single nanoparticle collisions,this work built an integrated apparatus for SPRM and electrochemical instruments.In the previous work of our group,the reason why SPRM was not combined with single nanoparticle collisions was that the background current of the electrode was too large to directly detect the electrochemical signal of a single nanoparticle.Therefore,this work has made three efforts in technology and data processing.First,transparent microelectrodes were fabricated by the nano-fabrication technique to reduce the background current and ensure that all electron transfer occurs in the optical field of view,which is beneficial to the simultaneous analysis of optical images and electrochemical signals.Second,the micro-current amplifier was used as an electrochemical system to meet the need of detecting pA-level current signal.The synchronization of optical and electrochemical recording was realized by using a data acquisition card to connect the optical microscopy and electrochemical instruments.Thirdly,the data analysis code based on Matlab software was written by ourselves,and the graphical user interface processing experimental data was designed to extract the optical and electrochemical signal of the nanoparticle collision moment.2.Simultaneous Optical and Electrochemical Recording of Single Silver Nanoparticle ElectrochemistryIn this work,the electrochemical dissolution process of silver nanoparticles was studied.The optical and electrochemical signals of the collision-dissolution process of single silver nanoparticles were detected,and the feasibility of optical and electrochemical synchronization devices was tested.The optical signal provides information on the size and position of the nanoparticle,and the electrochemical signal reflects the activity of the nanoparticle,which establishes a new method for the study of structure-activity relationship at the level of single nanoparticle.On this basis,the electron transfer rate of a single silver nanoparticle was calculated from the optical signal by further utilizing the quantitative relationship between the volume of individual silver nanoparticle and the optical signal intensity.The results show that the"opticar" current calculated by an optical-electrochemical signal conversion model is consistent with the directly recorded electrochemical current.For the first time,this work directly and quantitatively verified the reliability of the optical-electrochemical signal conversion model.3.Multi-Peak Behavior of Single Silver Nanoparticle CollisionsAt lower temporal resolution,the electrochemical dissolution process of silver nanoparticles is a continuous electron transfer process.Further studies have found that when the temporal resolution of electrochemical recording is increased to the microsecond level,the current generated by a single nanoparticle collision is no longer continuous,but appears as multiple current peaks separated from each other.Several nanoelectrochemical research groups have reported this phenomenon and attributed it to the physical separation process between nanoparticles and electrodes caused by electrostatic repulsion or Brownian motion,but lacks direct evidence.Because SPRM can measure the distance between silver nanoparticles and the electrode interface with high sensitivity,this work improves the temporal resolution of SPRM correspondingly,and traces the three-dimensional motion trajectory in the process of single silver nanoparticle collision and electrochemical dissolution.Simultaneous recording of optical and electrochemical signals can correlate the physical motion of the nanoparticle(optical signal)with the electron transfer process(electrochemical signal).Optical imaging results show that multiple current peaks do not necessarily accompany the change of spatial position of single nanoparticles.The nanoparticles could maintain an apparent physical contact with the electrode even during the interim period in which electron transfer does not occur.This indicates that the dynamic characteristics of the electrical contact between single nanoparticles and the electrode interface at the nanometer or even atomic scale,rather than a simple space migration of a single nanoparticle,may be the cause of the multiple peak phenomenon of the electrochemical current.4.Collision and Oxidation of Single LiCoO2 NanoparticlesIn addition to the direct electrochemical dissolution of silver nanoparticles,the application of this technology in other electroactive nanomaterials was further explored.Lithium cobaltate(LiCoO2)is not only a classic model material for studying ion-embedded electrochemical reactions,but also one of the most successful cathode materials in commercial lithium-ion batteries,which has important practical significance.Previous studies by our group found that the dielectric constant of LiCoO2 nanomaterials showed reversible and subtle changes during the electrochemical oxidation-reduction cycle.The present work further carried out SPRM imaging studies of the collision process of single LiCoO2 nanoparticles.Among them,the electrochemical signal provides information on the electron transfer rate and quantity of a single LiCoO2 nanoparticle colliding on the electrode.The SPRM records the size and collision location of the same nanoparticle for morphological characterization in combination with scanning electron microscopy.For the first time,this work correlates the electrochemical activity with morphology of single lithium ion storage materials,which not only enables the single nanoparticle collisions to be used to study different types of electrode nanomaterials,but also explores the structure-activity relationship of lithium ion storage nanomaterials.5.Electron Transfer of Single Prussian Blue Nanoparticles during Collision ProcessPrussian blue has become a well-known potassium/sodium ion battery cathode material in recent years due to its good electrochemical cycle life and rate performance in aqueous and organic electrolytes.This work combined SPRM with single nanoparticle collisions to monitor the electron transfer during the collision and potassium insertion process of a single Prussian blue nanoparticle simultaneously.The optical imaging results showed that when a single Prussian blue nanoparticle collided with the microelectrode,the nanoparticle first stayed on the electrode surface for a period of time before SPRM intensity began to change.By comparing the moment at which the SPRM intensity of the nanoparticles changed and the electron transfer process occurred,the asynchronism of the two processes at the single nanoparticle level was found.In this paper,by combining SPRM with single nanoparticle collisions,we construct a synchronizing device for SPRM and electrochemical instruments to record the optical and electrochemical signals in the electrochemical reaction of single nanoparticles simultaneously.The reliability of the optical-electrochemical signal conversion model is verified directly and quantitatively for the first time.By improving the temporal resolution of optical and electrochemical recording,the multiple peak phenomenon of the electrochemical current is further revealed.This paper not only studies the basic electrochemical reaction process,but also extends this technology to the energy storage material system,which correlates the activity with morphology of a single lithium ion energy storage material for the first time.In the future,more efforts should be made to improve the temporal and spatial resolution of optical recording and study faster electron transfer processes,to further reveal the mechanism of electron transfer at the nano-scale.It is also possible to extend this technology to the biological field,such as studying the kinetics of metabolic reactions in individual organelles(mitochondria,chloroplasts,etc.).