In situ electron microscopy of nanomaterials dynamics in heterogeneous phase environments
Engineered nanomaterials with desired properties and structures are indispensable to catalyze the processes of reliable energy conversion and storage systems for a sustainable future. These functional nanomaterials often experience dynamic physical and chemical changes during the operating cycles. I...
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| Format: | Thesis |
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Massachusetts Institute of Technology
2024
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| Online Access: | https://hdl.handle.net/1721.1/155355 https://orcid.org/0000-0002-3672-5076 |
| Summary: | Engineered nanomaterials with desired properties and structures are indispensable to catalyze the processes of reliable energy conversion and storage systems for a sustainable future. These functional nanomaterials often experience dynamic physical and chemical changes during the operating cycles. Investigating the dynamics in real-time allows establishing structure-property-performance relationships to optimize the materials design. In situ transmission electron microscopy (TEM) is one of the most powerful tools to capture dynamic changes with high spatial and temporal resolution. More importantly, the reaction or operating conditions of the materials can be mimicked by controlling external stimuli of in situ TEM experiments, including control of temperature, electrochemical biasing, and exposure to liquid and gas. In this thesis, nanomaterials dynamics are investigated using in situ TEM coupled under the control of external stimuli in a heterogeneous phase consisting of solids exposed to a liquid or gas environment.
First, I developed a temperature-dependent radiolysis model to explain the effect of temperature on the electron beam-induced radiolysis in liquid cell TEM. Radiolysis leads to the nucleation and growth of metal nanocrystals by reacting with the radicals, and I used the model to address the temperature-dependent chemical environment and corresponding kinetics of nanocrystal growth. The results demonstrated that the combination of microscopy and temperature-dependent modeling of the chemical environment can guide the analysis of the thermally controlled liquid cell TEM experiments. Moreover, the approach can be expanded to engineering the nanocrystal structure in lab-scale synthesis while acknowledging the differences compared with TEM experiments. Next, nanoscale electrochemistry under a controlled environment was discussed. In particular, the effect of temperature and substrate on electrochemical deposition is explained. In situ TEM results and modeling of the temporal evolution of the ion concentration demonstrated that the temperature accelerates the growth rate while it also controls the transition of growth modes. When 2D material graphene was used as a substrate for deposition, along with the classical nucleation and growth during the pulse on stage, transient growth and coarsening occurred, which could be attributed to the intrinsic properties of graphene to hold the charges. The results suggested that in situ TEM enables addressing the effect of electrochemical parameters and controlling nanoscale electrochemical phenomena. Finally, I applied the simultaneous acquisition of 2D projection and 3D topographic imaging in environmental TEM (ETEM) setup to analyze the structure dynamics of supported catalytic nanoparticles during heating and gas exposure. Particle migration and coalescence dominated above an onset temperature that depends on the gas. 3D topography captured that particles migrate through the bulk support and across the support surface. The degradation of the support during particle migration was also observed in certain gas environments. In some gas environments, the particle coalescence via oriented attachment took place. These results showed that the combination of imaging modes can provide information to explain the catalyst degradation during operation.
This thesis demonstrates that in situ TEM coupled with an understanding of the physical and chemical environments can provide insight into the nanostructure dynamics, which could contribute to revealing the degradation mechanisms of functional materials. |
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