Hong-Gang Liao

Real-time imaging of Pt3Fe nanorod growth in solution.

Growing in Liquid The ability to control the growth of materials at the nanometer scale is key to nanotechnology. Materials grown in liquids, however, are difficult to track on a particle-by-particle basis during growth. Two studies used an in situ liquid cell to follow the formation of larger nanoparticles or nanorods grown in solvents using high-resolution transmission electron microscopes. Liao et al. (p. 1011) watched platinum iron nanorods form from kinked chains of connected nanoparticles that gradually reoriented and straightened to form rigid rods. Li et al. (p. 1014) observed the coalescence of iron oxyhydroxide nanoparticles through an oriented attachment mechanism, whereby two similar particles rotated until their corresponding crystal lattices aligned. An in situ liquid stage is used to study the formation of nanowires from solution in a transmission electron microscope. The growth of colloidal nanocrystal architectures by nanoparticle attachment is frequently reported as an alternative to the conventional growth by monomer attachment. However, the mechanism whereby nanoparticle attachment proceeds microscopically remains unclear. We report real-time transmission electron microscopy (TEM) imaging of the solution growth of Pt3Fe nanorods from nanoparticle building blocks. Observations revealed growth of winding polycrystalline nanoparticle chains by shape-directed nanoparticle attachment followed by straightening and orientation and shape corrections to yield single-crystal nanorods. Tracking nanoparticle growth trajectories allowed us to distinguish the force fields exerted by single nanoparticles and nanoparticle chains. Such quantification of nanoparticle interaction and understanding the growth pathways are important for the design of hierarchical nanomaterials and controlling nanocrystal self-assembly for functional devices.

Polyelectrolyte-Induced Reduction of Exfoliated Graphite Oxide: A Facile Route to Synthesis of Soluble Graphene Nanosheets

Here we report that poly(diallyldimethylammonium chloride) (PDDA) acts as both a reducing agent and a stabilizer to prepare soluble graphene nanosheets from graphite oxide. The results of transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, and Fourier transform infrared indicated that graphite oxide was successfully reduced to graphene nanosheets which exhibited single-layer structure and high dispersion in various solvents. The reaction mechanism for PDDA-induced reduction of exfoliated graphite oxide was proposed. Furthermore, PDDA facilitated the in situ growth of highly dispersed Pt nanoparticles on the surface of graphene nanosheets to form Pt/graphene nanocomposites, which exhibited excellent catalytic activity toward formic acid oxidation. This work presents a facile and environmentally friendly approach to the synthesis of graphene nanosheets and opens up a new possibility for preparing graphene and graphene-based nanomaterials for large-scale applications.

Facet development during platinum nanocube growth

Watching platinum nanocube growth Size and shape drive the properties of metal nanoparticles. Understanding the factors that affect their growth is central to making use of the particles in a range of applications. Liao et al. tracked the growth of platinum nanoparticle shapes at high resolution using state-of-the-art liquid cells for in situ monitoring inside an electron microscope. The authors tracked changes in the growth rates at different crystal facets caused by differences in the mobility of the capping ligand. Science, this issue p. 916 Observation of atomic facet development during platinum nanocube growth reveals shape control. An understanding of how facets of a nanocrystal develop is critical for controlling nanocrystal shape and designing novel functional materials. However, the atomic pathways of nanocrystal facet development are mostly unknown because of the lack of direct observation. We report the imaging of platinum nanocube growth in a liquid cell using transmission electron microscopy with high spatial and temporal resolution. The growth rates of all low index facets are similar until the {100} facets stop growth. The continuous growth of the rest facets leads to a nanocube. Our calculation shows that the much lower ligand mobility on the {100} facets is responsible for the arresting of {100} growing facets. These findings shed light on nanocrystal shape-control mechanisms and future design of nanomaterials.

Visualization of Electrode–Electrolyte Interfaces in LiPF6/EC/DEC Electrolyte for Lithium Ion Batteries via in Situ TEM

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.

Observation of growth of metal nanoparticles

An understanding of nanocrystal growth mechanisms is of significant importance for the design of novel materials. The development of liquid cells for transmission electron microscopy (TEM) has enabled direct observation of nanoparticle growth in a liquid phase. By tracking single particle growth trajectories with high spatial resolution, novel growth mechanisms have been revealed. In recent years, there has been an increasing interest in liquid cell TEM and its applications include real time imaging of nanoparticles, biological materials, liquids, and so on. This paper reviews the development of liquid cell TEM and the progress made in using such a wonderful tool to study the growth of nanoparticles (mostly metal nanoparticles). Achievements in the understanding of coalescence, shape control mechanisms, surfactant effects, etc. are highlighted. Other studies relevant to metal precipitation in liquids, such as electrochemical deposition, nanoparticle motion and electron beam effects, are also included. At the end, our perspectives on future challenges and opportunities in liquid cell TEM are provided.

Liquid Cell Transmission Electron Microscopy Study of Platinum Iron Nanocrystal Growth and Shape Evolution

We study solution growth of platinum iron nanocrystals in situ in a liquid cell by using transmission electron microscopy. By varying the oleylamine concentration, we observed that platinum iron nanoparticle growth follows different trajectories with diverse shape evolution. With 20% oleylamine, three stages of growth were observed: (i) nucleation and growth of platinum iron nanoparticles in the precursor solution; (ii) nanowire formation by shape-directed nanoparticle attachment; and (iii) breakdown or shrinkage of the nanowires into individual nanoparticles with large size distribution. With 30% oleylamine, formation of platinum iron nanowires similar to that with 20% oleylamine was observed. However, those nanowires do not break down or shrink, which suggests that nanowires are stabilized by oleylamine as surfactant binding on the surface. With 50% oleylamine, after the individual nanoparticles are formed, they do not merge into nanowires. The shape of the nanoparticle is strongly influenced by the neighboring nanoparticles due to stereo-hindrance effects. Real-time observation of the dynamic growth process sheds light on the controllable synthesis of nanomaterials.

Structural and Morphological Evolution of Lead Dendrites during Electrochemical Migration

The electrochemical deposition and dissolution of lead on gold electrodes immersed in an aqueous solution of lead nitrate were studied in situ using a biasing liquid cell by transmission electron microscopy (TEM). We investigate in real time the growth mechanisms of lead dendrites as deposited on the electrodes under an applied potential. TEM images reveal that lead dendrites are developed by the fast protrusion of lead branches in the electrolyte and tip splitting. And, the fast growing tip of the dendritic branch is composed of polycrystalline nanograins and it develops into a single crystalline branch eventually. This study demonstrated unique electrochemical growth of single crystal dendrites through nucleation, aggregation, alignment and attachment of randomly oriented small grains. Additionally, we found the lead concentration in the electrolyte drastically influences the morphology of dendritic formation.

Liquid Cell Transmission Electron Microscopy.

Liquid cell transmission electron microscopy (TEM) has attracted significant interest in recent years. With nanofabricated liquid cells, it has been possible to image through liquids using TEM with subnanometer resolution, and many previously unseen materials dynamics have been revealed. Liquid cell TEM has been applied to many areas of research, ranging from chemistry to physics, materials science, and biology. So far, topics of study include nanoparticle growth and assembly, electrochemical deposition and lithiation for batteries, tracking and manipulation of nanoparticles, catalysis, and imaging of biological materials. In this article, we first review the development of liquid cell TEM and then highlight progress in various areas of research. In the study of nanoparticle growth, the electron beam can serve both as the illumination source for imaging and as the input energy for reactions. However, many other research topics require the control of electron beam effects to minimize electron beam damage. We discuss efforts to understand electron beam-liquid matter interactions. Finally, we provide a perspective on future challenges and opportunities in liquid cell TEM.

Tracking Nanoparticle Diffusion and Interaction during Self-Assembly in a Liquid Cell

Nanoparticle self-assembly has been well studied theoretically, but it remains challenging to directly observe and quantify individual nanoparticle interactions. With our custom image analysis method, we track the trajectories of nanoparticle movement with high precision from a stack of relatively noisy images obtained using liquid cell transmission electron microscopy. In a time frame of minutes, Pt-Fe nanoparticles self-assembled into a loosely packed hcp lattice. The energetics and stability of the dynamic assembly were studied quantitatively. From velocity and diffusion measurements, we experimentally determined the magnitude of forces between single particles and the related physical properties. The results illustrate that long-range anisotropic forces drive the formation of chains, which then clump and fold to maximize close range van der Waals interactions.

Visualization of the coalescence of bismuth nanoparticles.

Coalescence is a significant pathway for the growth of nanostructures. Here we studied the coalescence of Bi nanoparticles in situ by liquid cell transmission electron microscopy (TEM). The growth of Bi nanoparticles was initiated from a bismuth neodecanoate precursor solution by electron beam irradiation inside a liquid cell under the TEM. A significant number of coalescence events occurred from the as-grown Bi nanodots. Both symmetric coalescence of two equal-sized nanoparticles and asymmetric coalescence of two or more unequal-sized nanoparticles were analyzed along their growth trajectories. Our observation suggests that two mass transport mechanisms, i.e., surface diffusion and grain boundary diffusion, are responsible for the shape evolution of nanoparticles after a coalescence event.