Sun Litao

The in situ TEM observation of rapid lithium encapsulation and release in LiCl nanoshells and nanotubes

In this work, we performed detailed in situ TEM observations of the rapid encapsulation and release of lithium in nanometric structures under e-beam irradiation. With fast kinetics, the e-beam irradiation induced the formation of a LiCl shell due to the surface reaction between lithium and the surrounding chloride. In parallel, a rapid lithium release from the LiCl surface shell underwent as a competitive process. The detailed dynamic encapsulation and releasing processes were revealed with enhanced time and spatial resolution. Lithium nanoparticles, nanorods and Li/LiCl core–shell nanoparticles and nanotubes were observed during these processes. Due to the important application of lithium materials in the energy storage field, where lithium encapsulation and releasing processes play the fundamentally important roles, our findings may trigger new application methods for lithium based energy storage, in which e-beam irradiation may be used to enhance the ultra-rapid energy harvesting or releasing process.

Preparation of graphene-MoS 2 hybrid aerogels as multifunctional sorbents for water remediation

The increasing demand of clean water and effective way to recycle industrial wastewater has offered a new application for carbon-based three-dimensional (3D) porous networks as sorbents due to their superior sorption abilities. Through the surface modification and hybridization with functional materials, the physical and chemical properties of the 3D carbon-based materials can be engineered. In this work, graphene-MoS2 aerogels (GMAs) with bulky shape are synthesized via a one-pot hydrothermal method. The obtained GMAs show quick sorption rate and high sorption capacity towards a wide variety of contaminants. The sorption covers not only organic solvents or organic dyes, but also toxic heavy metals ions such as Hg2+ and Pb2+. More importantly, the sorption capacity towards metal ions can be optimized by simply changing the loading amount of MoS2.摘要三维碳基多孔材料因其独特的结掏和超高的吸附性能, 已成为最有水污染处理应用前景的吸附材料之一. 本文通过有效的调控, 合成了多种具有不同孔道结掏和成分组成的石墨烯-二硫化钼复合气凝胶材料. 这种材料在吸附重金属离子, 有机染料, 油及有机溶剂方面都有很多优异的表现. 通过调控二硫化钼的比例, 可以有效改善材料的吸附性能. 得益于此, 其吸附重金属汞离子的效率可以达到 1245mg g−1.

Graphene-based gas sensor.

Graphene, a honeycomb structure with single layer sp2 hybridization carbon atom, has extraordinary mechanical and electrical properties, exhibiting great potential in sensing area. The ultrahigh electron mobility in room temperature as well as the ultrahigh specific surface area, endows graphene a promising candidate for ultrasensitive gas sensor. As a typical two dimensional material, every atom in graphene can be regarded as surface atom. Therefore, every atom will be able to interact with gas molecule, which provides an ultrahigh sensitivity and the ultralow detection limit (as low as for single molecule detection). The current researches for improvement of gas sensing mainly focus on two aspects: (1) Design of different working principle devices; (2) surface modification on graphene and composite with other materials (i.e . metal, metal oxide and organic polymer). The specific adsorption sites can be achieved, resulting the improvement of selectivity. This review will present and summarize recent achievements of graphene-based gas sensor in both aspects and also predict the potential research direction in the future. According to different working mechanism, graphene-based gas sensor can be classified as resistive type, field-effect-transistor (FET) type, mass sensitive type, and micro-electromechanical system (MEMS) type. Each kind has its advantages. Thanks to the simple manufacture process, resistive gas sensor is investigated broadly. Compared to passive resistive sensor, the active FET sensor exhibits better performance in sensitivity and stability generally. As for mass sensitive sensor, the unique mechanism can provide a new device structure different from the other three types. However, concerning the compatibility with integrated circuits (IC) manufacture, MEMS sensor can be a good choice. Materials with high specific surface area are able to provide more adsorption sites. Consequently, sensor performance can be improved sharply. Therefore, surface modification on graphene and composite with other materials attracts broad attention recently. By changing the density and the type of functional groups on graphene surface, specific adsorption sites can be achieved. Hence, the ability for selective gas detection is enhanced. Besides, graphene-based composite with other materials (i.e. metal, metal oxide and organic polymer) is another effective strategy to optimize sensor performance. In conclusion, lots of achievements and big breakthroughs for graphene-based gas sensor have been made in recent years. Especially the obvious enhancement for sensitivity (as low as for single molecule detection) and gas selective detection, graphene-based gas sensor exhibits great advantage compared with traditional sensors. However, the time-consuming process of gas adsorption and desorption hampers the real-time measurements due to the long response and recover time. Therefore, improvement in response and recover performance will be a potential research direction in the future. In addition, the integration of different kinds of gas sensors even other types of sensors will be a trend. Also, it will be a desirable fundamental research to give explanations for the sensing mechanism as well as to investigate the dynamic interaction between gas molecules and graphene. Recently, it is believed that with the help of special in - situ holder, researchers can launch experiments in transmission electron microscope (TEM) to explore this fundamental study.

Surface energy guided sub-10 nm hierarchy structures fabrication by direct e-beam etching

It is found that surface energy plays the dominant role in direct e-beam etching of single crystal ZnO and ITO nanostructures through a surface etching (sputtering) process, especially for electron energies smaller than the characteristic energy for bulk atoms knock-off. This mechanism can be used for the fabrication of sub-10 nm hierarchical structures on single crystal nanostructure precursors. We performed detailed in situ HRTEM observations of the surface energy guided etching (SEGE) process on ZnO nanowires. The etching of (0001) crystal planes, that have the lowest surface energy, was found to dominate the etching process. It is also shown that the SEGE mechanisms can be extended to single crystal ITO nanowires for sub-10 nm hierarchical structure fabrication. The hierarchical structures fabricated by the SEGE process show increased specific surface areas (SSA), which may be useful in fields such as sensors, catalysts and solar cells.

In-situ study of electron irradiation on two-dimensional layered materials

High precision fabrication of nanostructure is a primary limiting factor to devices miniaturization. Electron beam is expected to promote the process on the accuracy of fabrication. For instance, electron beam lithography has been developed to replace optical lithography to transfer patterns and has produced line width on the order of 10 nm or smaller. Furthermore, electrons can directly interact with the sample to cause changes in structure and properties, which is expected to be used for fabrication of specialized nanostructures with nanometer even sub-nanometer precision. Transmission electron microscopy (TEM) has become an indispensable tool for the study of the interaction between energetic electrons and sample. A large number of experimental studies have been carried out inside TEM where electron beam can not only be used for atomic resolution imaging but also for irradiation. These studies are conductive to understanding phenomena induced by irradiation in essence, which are the theoretical and experimental basis for controllable manipulation via electron irradiation. However, TEM images are parallel projections of three-dimensional (3D) structures onto the image plane, which tremendously increases the difficulty of atomic-structure analysis. Two-dimensional (2D) layered materials (graphene, hexagon boron nitride, transition metal dichalcogenides and so on) emerged and boomed in the past decade, which provide ideal systems to study electron irradiation effects on matter. As mono- or bi-atomic sheets, their atomic structures are accessible to image directly with atomic resolution in the advent of spherical aberration corrected TEM. The lateral resolution in TEM image, which is smaller than the bond length, leaves almost no room for uncertain interpretation. Meanwhile, the reconstruction of the lattice occurs in the 2D plane can be seen without any projection artifacts. Besides, structural changes, as they are generated under irradiation with energetic electrons, can be monitored in real time. On this basis, novel mechanisms of lattice reconstruction, which did not appear in other materials, have been discovered, such as transformations between hexagons and pentagon-heptagon pairs by in-plane 90˚ rotation of C–C bonds in graphene sheet. In addition to a detailed understanding of radiation-induced structure evolution and their influence on the properties of 2D sheet, the design of new structures has become feasible. Especially, electron beams inside modern TEM can be focused onto spots less than 1 A, which allows use of the beam as tweezers to manipulate materials even at the single atom level. In this way, sub-nanometer quasi one dimensional structures, such as nanowires, nanoribbons, nanotubes and atomic chains, have been fabricated in 2D nanosystems. Sub-5 nm nanopores can also be sculpted in 2D sheets, which is particularly suitable for chemical and biological molecule detection. Besides, 2D materials can serve as substrates to study radiation-induced structural evolution of substance on their surface. This review gives a brief summary of our current knowledge about electron irradiation in the aspect of elastic scattering and inelastic scattering, and a summary of the most recent experimental results on in situ irradiation in 2D layered material systems, including atomic structural evolution under electron irradiation and controllable fabrication of nanostructures by TEM.