Yilong Zhou

Low Temperature Casting of Graphene with High Compressive Strength

has attracted attention due to its fascinating properties such as high carrier mobility, [ 6–8 ] high thermal conductivity, [ 9 , 10 ] extraordinary elasticity and stiffness [ 11 ] and other properties. While mechanical exfoliation, [ 6 ] liquid exfoliation, [ 12 ] and epitaxial growth [ 13 ] can produce pristine graphene, graphene yields are currently too low for large-scale production of macrostructures. In contrast, chemical reduction of graphene oxide provides ‘graphene’ sheets in large scale for graphene macrostructures. [ 14–16 ] Graphene-based macrostructures prepared to date have been relatively weak mechanically, given their fl exible and often relatively porous or open structures, [ 17–26 ] particularly with respect to compressive strength when compared with commercial graphite products. [ 27–29 ] Achieving highly compacted and thus “fully dense” macrostructures based on graphene and measuring the physical properties of such material(s) is thus an important goal. Here, we report a pH-mediated hydrothermal reduction which is combined with moulding methods and allows controllable fabrication of compact high density graphene macrostructures with various shapes. The compact graphene (CG) product that is fabricated in this study shows great advantages over hitherto reported 3-D graphene products, [ 17–26 ] e.g. , a solid microstructure and a high density ( ∼ 1.6 g cm − 3 ) which is comparable to conventional graphite products [ 27–29 ] and an ultrahigh compressive strength ( ∼ 361 Mpa) which is 6 times higher than

Large-range Control of the Microstructures and Properties of Three-dimensional Porous Graphene

Graphene-based three-dimensional porous macrostructures are believed of great importance in various applications, e.g. supercapacitors, photovoltaic cells, sensors and high-efficiency sorbents. However, to precisely control the microstructures and properties of this material to meet different application requirements in industrial practice remains challenging. We herein propose a facile and highly effective strategy for large-range tailoring the porous architecture and its properties by a modified freeze casting process. The pore sizes and wall thicknesses of the porous graphene can be gradually tuned by 80 times (from 10 to 800 μm) and 4000 times (from 20 nm to 80 μm), respectively. The property experiences the changing from hydrophilic to hydrophobic, with the Youngs Modulus varying by 15 times. The fundamental principle of the porous microstructure evolution is discussed in detail. Our results demonstrate a very convenient and general protocol to finely tailor the structure and further benefit the various applications of porous graphene.

Highly enhanced performance of spongy graphene as an oil sorbent

This work demonstrates a brand-new spongy graphene with a highly enhanced performance as an oil sorbent. The absorption capacity of the new spongy graphene to chloroform reaches 616 times of its own weight, which is approximately 8 times higher than that in previous reports. The absorption capacity towards other organic chemicals is also greatly improved.

Microscopic bimetallic actuator based on a bilayer of graphene and graphene oxide

We present an actuator, consisting of a bilayer of graphene and graphene oxide, which allows us to exert forces in micromechanical systems that are at least 50 times higher than reported for other actuators of comparable size. The durability of such a device and stability during many cycles are demonstrated, and the related mechanism is discussed in detail.

Unsupported single-atom-thick copper oxide monolayers

Oxide monolayers may present unique opportunities because of the great diversity of properties of these materials in bulk form. However, reports on oxide monolayers are still limited. Here we report the formation of single-atom-thick copper oxide layers with a square lattice both in graphene pores and on graphene substrates using aberration-corrected scanning transmission electron microscopy. First-principles calculations find that CuO is energetically stable and its calculated lattice spacing matches well with the measured value. Furthermore, free-standing copper oxide monolayers are predicted to be semiconductors with band gaps ~3 eV. The new wide-bandgap single-atom-thick copper oxide monolayers usher a new frontier to study the highly diverse family of two-dimensional oxides and explore their properties and their potential for new applications.

Electrically driven cation exchange for in situ fabrication of individual nanostructures

Cation exchange (CE) has been recognized as a particularly powerful tool for the synthesis of heterogeneous nanocrystals. At present, CE can be divided into two categories, namely ion solvation-driven CE reaction and thermally activated CE reaction. Here we report an electrically driven CE reaction to prepare individual nanostructures inside a transmission electron microscope. During the process, Cd is eliminated due to Ohmic heating, whereas Cu+ migrates into the crystal driven by the electrical field force. Contrast experiments reveal that the feasibility of electrically driven CE is determined by the structural similarity of the sulfur sublattices between the initial and final phases, and the standard electrode potentials of the active electrodes. Our experimental results demonstrate a strategy for the selective growth of individual nanocrystals and provide crucial insights into understanding of the microscopic pathways leading to the formation of heterogeneous structures.

Electron Beam Etching of CaO Crystals Observed Atom by Atom

With the rapid development of nanoscale structuring technology, the precision in the etching reaches the sub-10 nm scale today. However, with the ongoing development of nanofabrication the etching mechanisms with atomic precision still have to be understood in detail and improved. Here we observe, atom by atom, how preferential facets form in CaO crystals that are etched by an electron beam in an in situ high-resolution transmission electron microscope (HRTEM). An etching mechanism under electron beam irradiation is observed that is surprisingly similar to chemical etching and results in the formation of nanofacets. The observations also explain the dynamics of surface roughening. Our findings show how electron beam etching technology can be developed to ultimately realize tailoring of the facets of various crystalline materials with atomic precision.

Surface Energy and Surface Stability of Ag Nanocrystals at Elevated Temperatures and Their Dominance in Sublimation-Induced Shape Evolution

The surface energy and surface stability of Ag nanocrystals (NCs) are under debate because the measurable values of the surface energy are very inconsistent, and the indices of the observed thermally stable surfaces are apparently in conflict. To clarify this issue, a transmission electron microscope is used to investigate these problems in situ with elaborately designed carbon-shell-capsulated Ag NCs. It is demonstrated that the {111} surfaces are still thermally stable at elevated temperatures, and the victory of the formation of {110} surfaces over {111} surfaces on the Ag NCs during sublimation is due to the special crystal geometry. It is found that the Ag NCs behave as quasiliquids during sublimation, and the cubic NCs represent a featured shape evolution, which is codetermined by both the wetting equilibrium at the Ag-C interface and the relaxation of the system surface energy. Small Ag NCs (≈10 nm) no longer maintain the wetting equilibrium observed in larger Ag NCs, and the crystal orientations of ultrafine Ag NCs (≈6 nm) can rotate to achieve further shape relaxation. Using sublimation kinetics, the mean surface energy of Ag NCs at 1073 K is calculated to be 1.1-1.3 J m-2 .

Formation of Single-atom-thick Copper Oxide Monolayers

1. SEU-FEI Nano-Pico Center, Key Lab of MEMS of Ministry of Education, Southeast University, Nanjing 210096, China. 2. Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA. 3. Department of Physics and Astronomy and Department of Electrical Engineering and Computer Science, Vanderbilt University, Nashville, TN 37235, USA. 4. Center for Advanced Materials and Manufacture, Joint Research Institute of Southeast University and Monash University, Suzhou 215123, China

Fabrication of graphene based electrothermal cantilever actuator

A novel actuator has been fabricated using graphene and graphene oxide composite paper as building blocks. The two materials have nearly equal Youngs modulus, however a large displacement was observed during the electromechanically driven operation, which was attributed to great difference in coefficient of thermal expansion of graphene paper and graphene oxide paper.