Study Of In-situ Liquid Cell TEM In Crystal Nucleation And Electrocatalytic Reduction Of CO₂

Classical nucleation theory and two-step nucleation theory are the theoretical basis employed to explain the nucleation of crystals,and have been widely concerned by researchers.The nucleation and growth mechanism of MOF has been controversial since its birth.In-situ liquid phase transmission electron microscopy(in-situ TEM)provides a visual technology for crystal nucleation.The development of in-situ TEM is not limited to the study on crystal nucleation and growth.With the development of micromachining technology,in-situ liquid phase electrochemical system has attracted more and more attention.In this paper,the nucleation of metal nanoparticles and MOF was studied via insitu TEM,and the in-situ liquid phase electrochemical system was further expanded.The CO2 reduction reaction(CO2RR)and the synthesis process of CO2RR electrocatalyst were studied.The main research of this paper includes the following aspects:1.The nucleation of metal nanoparticles from solution was directly observed by in-situ liquid cell TEM:the metastable variable valence ion-water complex was separated from the initial homogeneous solution because of redox reaction;As the metal ions continue to be reduced,the ion-water complex continues to shrink.Finally,nanocrystals are crystallized directly from the comlpex.Combined with SAED and EELS,it is proved that the valence of indium ions reduces gradually during nucleation.It is further proved by DFT-MD that the valence of indium ions decreases gradually during the nucleation of indium nanoparticles,which is accompanied by the shrinkage process of the composite.In-situ results provide direct evidence for the two-step nucleation theory,and further reveal the important role of valence change of metal ions in the nucleation process.2.The nucleation of MOF-2 and MOF-5 was directly observed by in-situ liquid phase TEM.The nucleation of MOF-2 is to form SBU monomers in solution,and these SBU monomers gather together to form dimers,and then the dimers combine to form tetramers.The nucleation process of MOF-5 was observed after heating for 2 h:there were a large number of quaternary rings in the solution,which aggregated to form octamer,and cubic octamer was the first crystal nucleus formed in the early nucleation stage of MOF-5.The phase transformation process of MOF growth was observed by XRD.The evolution of metal oxygen clusters during nucleation was monitored by ESIMS.In-situ results support that MOF follows SBU nucleation theory during nucleation and growth.3.The indium nanobundles synthesized by surfactants shown excellent CO2RR performance.At-1.2 V potential,the current density reached-30 mA·cm-2,the selectivity of formate reached over 90%,and it kept good stability in the reaction for 24 hours.In-situ liquid-phase electrochemical system was developed by micromachining technology.In-situ liquid-phase electrochemical liquid cells were engaged to directly observe the reaction process of CO2RR on indium nanobundles.Hydrogen bubbles produced by HER side reaction were observed,and formate was produced around indium nanobundles due to CO2RR,which provided a new idea for in-situ study of CO2RR process and mechanism.4.Taking the transformation of ZIF-67 into cobalt hydroxide as a model reaction,the transformation mechanism from ZIF-67 into porous cobalt hydroxide or layer cobalt hydroxide was studied by in-situ liquid phase TEM.The original periodic structure of ZIF-67 was etched because 2-methylimidazole was replaced by hydroxide,and nanobubbles were formed on the surface.Under the slow diffusion condition,the bubbles are relatively stable and move slowly,which can aggregate and merge,forming cobalt hydroxide clusters around them,and the clusters are aggregated and connected with each other to form internal cobalt hydroxide nanocages,which constitute porous cobalt hydroxide.Under the faster diffusion condition,bubbles move violently and gather rapidly,and the surrounding cobalt hydroxide clusters gather rapidly,and nucleate and crystallize to form nanosheets.Combined with in-situ experimental results and diffusion theory,porous cobalt hydroxide and hollow cobalt hydroxide were synthesized by adjusting diffusion coefficient at-80℃ and room temperature,respectively.In-situ experiments show that the porous cobalt hydroxide synthesized after pretreatment has excellent CO2RR performance.At the potential of-1.2 V,the current density reaches-38 mA·cm-2.The Faraday efficiency of formate is above 90%ranging from-0.7 V to-1.0 V,and it keeps good stability after 28 h reaction.In-situ liquid phase TEM guided the preparation of high performance CO2RR electrocatalysts,which provides a new direction for the design of CO2RR electrocatalysts.