Browsing by Author "Zheng, Haimei"

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Dynamics of Nanoscale Dendrite Formation in Solution Growth Revealed Through in Situ Liquid Cell Electron Microscopy
(2018-10-01)
Hauwiller, Matthew R.
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Zhang, Xiaowei
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Liang, Wen-I
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Chiu, Chung-Hua
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Zhang, Qian
Formation mechanisms of dendrite structures have been extensively explored theoretically, and many theoretical predictions have been validated for micro- or macroscale dendrites. However, it is challenging to determine whether classical dendrite growth theories are applicable at the nanoscale due to the lack of detailed information on the nanodendrite growth dynamics. Here, we study iron oxide nanodendrite formation using liquid cell transmission electron microscopy (TEM). We observe "seaweed"-like iron oxide nanodendrites growing predominantly in two dimensions on the membrane of a liquid cell. By tracking the trajectories of their morphology development with high spatial and temporal resolution, it is possible to explore the relationship between the tip curvature and growth rate, tip splitting mechanisms, and the effects of precursor diffusion and depletion on the morphology evolution. We show that the growth of iron oxide nanodendrites is remarkably consistent with the existing theoretical predictions on dendritic morphology evolution during growth, despite occurring at the nanoscale.
In Situ Study of Fe3Pt-Fe2O3 Core-Shell Nanoparticle Formation
(2015-12-02)
Liang, Wen-I
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Zhang, Xiaowei
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Zan, Yunlong
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Pan, Ming
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Czarnik, Cory
We report an in situ study of Fe3Pt-Fe2O3 coreshell nanoparticle growth using liquid cell transmission electron microscopy. By controlling the Fe-to-Pt ratio in the precursor solution, we achieved the growth of nanoparticles with the formation of an ironplatinum alloy core followed by an iron oxide shell in the electron beam-induced reactions. There was no substantial change in the growth kinetics of the iron oxide shell after the FePt alloy core stopped growing. The core growth was arrested by depletion of the Pt precursor. Heteroepitaxy of Fe3Pt [101] (core)||alpha-Fe2O3 [111] (shell) was observed in most of the nanoparticles, while a polycrystalline iron oxide shell is developed eventually for strain relaxation. Our studies suggest that Pt atoms catalyze the reduction of Fe ions to form the Fe3Pt alloy core, and when Pt is depleted, a direct precipitation of iron oxide results in the core-shell nanostructure formation.
In Situ Study of Spinel Ferrite Nanocrystal Growth Using Liquid Cell Transmission Electron Microscopy
(2015-12-08)
Liang, Wen-I
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Zhang, Xiaowei
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Bustillo, Karen
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Chiu, Chung-Hua
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Wu, Wen-Wei
We report transition metal oxide nanocrystal formation in a liquid cell using. transmission electron microscopy (TEM). The growth of M-Fe-oxide (M = Ni, Mn, Co, or Zn) nanoparticles from a growth solution of metal acetylacetonates dissolved in oleylamine, oleic acid, and benzyl ether was studied. Nickel iron oxide nanocrystals with spinel structure were obtained under electron beam irradiation of the Ni-Fe growth solution, whereas iron oxide nanocrystals were achieved with Mn remaining in the Mn-Fe growth solution. Similarly, we achieved cobalt iron oxide nanocrystals in the Co-Fe precursor solution, while iron oxide nanoparticles were obtained in the Zn-Fe solution. By tracking nanoparticle size evolution as a function of time along the Ni-Fe oxide nanoparticle growth trajectories, we found the growth kinetics follow a Lifshitz-Slyozov-Wagner (LSW) model suggesting surface reaction-limited growth. Ex situ characterization shows elemental distribution and structural and valence state of the different nanoparticles. The trend of nanoparticle growth in a liquid cell shares many similarities with those in "one-pot" flask synthesis by thermal heating. We compare reduction potentials (E-r) of the metal ions and thermal decomposition temperatures (T-d) of the precursors and correlate them with nanoparticle growth in a liquid cell under TEM. We found a tendency to form mixed metal ion oxide nanoparticles instead of single metal ion (iron) oxides when the two precursors have similar values of T-d and metal ion reduction potential. The higher T-d and smaller E-r values of Mn and Zn precursors than those of Fe precursor, as well as Ni and Co precursors, may have resulted in the single metal ion (iron) oxide formation in M-Fe (M = Mn and Zn) precursor systems. This study sheds light on nanoparticle growth mechanisms by liquid cell TEM. In situ study of oxide nanocrystal growth using liquid cell TEM provides the opportunity to explore solution chemistry during nanocrystal growth beyond the nanoparticle growth that occurs in a TEM cell.
In situ TEM study of the Li-Au reaction in an electrochemical liquid cell
(2014-01-01)
Zeng, Zhiyuan
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Liang, Wen-I.
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Chu, Ying-Hao
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Zheng, Haimei
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材料科學與工程學系
We study the lithiation of a Au electrode in an electrochemical liquid cell using transmission electron microscopy (TEM). The commercial liquid electrolyte for lithium ion batteries (1 M lithium hexafluorophosphate LiPF6 dissolved in 1 : 1 (v/v) ethylene carbonate (EC) and diethyl carbonate (DEC)) was used. Three distinct types of morphology change during the reaction, including gradual dissolution, explosive reaction and local expansion/shrinkage, are observed. It is expected that significant stress is generated from lattice expansion during lithium-gold alloy formation. There is vigorous bubble formation from electrolyte decomposition, likely due to the catalytic effect of Au, while the bubble generation is less severe with titanium electrodes. There is an increase of current in response to electron beam irradiation, and electron beam effects on the observed electrochemical reaction are discussed.
In-situ Multimodal Imaging and Spectroscopy of Mg Electrodeposition at Electrode-Electrolyte Interfaces
(2017-02-10)
Wu, Yimin A.
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Yin, Zuwei
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Farmand, Maryam
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Yu, Young-Sang
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Shapiro, David A.
We report the study of Mg cathodic electrochemical deposition on Ti and Au electrode using a multimodal approach by examining the sample area in-situ using liquid cell transmission electron microscopy (TEM), scanning transmission X-ray microscopy (STXM) and X-ray absorption spectroscopy (XAS). Magnesium Aluminum Chloride Complex was synthesized and utilized as electrolyte, where non-reversible features during in situ charging-discharging cycles were observed. During charging, a uniform Mg film was deposited on the electrode, which is consistent with the intrinsic non-dendritic nature of Mg deposition in Mg ion batteries. The Mg thin film was not dissolvable during the following discharge process. We found that such Mg thin film is hexacoordinated Mg compounds by in-situ STXM and XAS. This study provides insights on the non-reversibility issue and failure mechanism of Mg ion batteries. Also, our method provides a novel generic method to understand the in situ battery chemistry without any further sample processing, which can preserve the original nature of battery materials or electrodeposited materials. This multimodal in situ imaging and spectroscopy provides many opportunities to attack complex problems that span orders of magnitude in length and time scale, which can be applied to a broad range of the energy storage systems.
Real time imaging of two-dimensional iron oxide spherulite nanostructure formation
(2019-11-01)
Zheng, Wenjing
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Hauwiller, Matthew R.
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Liang, Wen-, I
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Ophus, Colin
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Ercius, Peter
The formation of complex hierarchical nanostructures has attracted a lot of attention from both the fundamental science and potential applications point of view. Spherulite structures with radial fibrillar branches have been found in various solids; however, their growth mechanisms remain poorly understood. Here, we report real time imaging of the formation of two-dimensional (2D) iron oxide spherulite nanostructures in a liquid cell using transmission electron microscopy (TEM). By tracking the growth trajectories, we show the characteristics of the reaction front and growth kinetics. Our observations reveal that the tip of a growing branch splits as the width exceeds certain sizes (5.5-8.5 nm). The radius of a spherulite nanostructure increases linearly with time at the early stage, transitioning to nonlinear growth at the later stage. Furthermore, a thin layer of solid is accumulated at the tip and nanoparticles from secondary nucleation also appear at the growing front which later develop into fibrillar branches. The spherulite nanostructure is polycrystalline with the co-existence of ferrihydrite and Fe3O4 through-out the growth. A growth model is further established, which provides rational explanations on the linear growth at the early stage and the nonlinearity at the later stage of growth.
Self-assembled vertical heteroepitaxial nanostructures: from growth to functionalities
(2014-06-01)
Liu, Heng-Jui
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Liang, Wen-I
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Chu, Ying-Hao
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Zheng, Haimei
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Ramesh, Ramamoorthy
Self-assembled vertical heteroepitaxial nanostructures (VHN) in the complex oxide field have fascinated scientists for decades because they provide degrees of freedom to explore in condensed matter physics and design-coupled multifunctionlities. Recently, of particular interest is the perovskite-spinel-based VHN, covering a wide spectrum of promising applications. In this review, fabrication of VHN, their growth mechanism, control, and resulting novel multifunctionalities are discussed thoroughly, providing researchers a comprehensive blueprint to construct promising VHN. Following the fabrication section, the state-of-the-art design concepts for multifunctionalities are proposed and reviewed by suitable examples. By summarizing the outlook of this field, we are excitedly expecting this field to rise with significant contributions ranging from scientific value to practical applications in the foreseeable future.
Visualization of Electrode-Electrolyte Interfaces in LiPF6/EC/DEC Electrolyte for Lithium Ion Batteries via in Situ TEM
(2014-04-01)
Zeng, Zhiyuan
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Liang, Wen-I
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Liao, Hong-Gang
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Xin, Huolin L.
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Chu, Yin-Hao
We report direct visualization of electrochemical lithiation and delithiation of Au anodes in a commercial LiPF6/EC/DEC electrolyte for lithium ion batteries using transmission electron microscopy (TEM). The inhomogeneous lithiation, lithium metal dendritic growth, electrolyte decomposition, and solid-electrolyte interface (SEI) formation are observed in situ. These results shed lights on strategies of improving electrode design for reducing short-circuit failure and improving the performance of lithium ion batteries.