J. S. Wu

Porous cobalt oxides with tunable hierarchical morphologies for supercapacitor electrodes

We synthesized porous Co3O4 with different hierarchical morphologies and studied the effects on their capacitor properties when they were used as electrode materials. By employing ammonia as the source of the hydroxide anion (OH−), an effective and robust precipitation route, free of surfactants, was developed to synthesize cobalt hydroxides with different hierarchical structures. When ammonia was used, the insufficient OH− supply could tune the formation of cobalt hydroxide to have different hierarchical structures. The (0001) cobalt hydroxide nanosheet is found to be the first product in the nucleation stage, and the basic building block for the hierarchical structures. Three types of anions were tested: the chloride (Cl−) anion prompted the formation of large plates with a similar aspect ratio to the basic building block; the nitrate (NO3−) anion formed large plates at the beginning, but this soon led to the formation of nanocolumn structures with a high aspect ratio; and the acetate (C2H3OO−) anion led to the formation of a flower-shaped hierarchical morphology with stacking of curved (0001) cobalt hydroxide nanosheets. Subsequent calcination transformed the cobalt hydroxides into porous cobalt oxide while the hierarchical morphology remained the same. Cobalt oxides with such complex hierarchical structures have a better capacitor performance, with higher specific capacitance, than Co3O4 nanoparticles. Among them, the Co3O4 hierarchical structure made with the nitrate anion shows the highest capacitance. The mechanism for forming the different hierarchical structures is discussed based on an electron microscopy investigation.

Facile biosynthesis of phosphate capped gold nanoparticles by a bacterial isolate Stenotrophomonas maltophilia

We report intracellular biosynthesis of gold nanoparticles (GNPs) by a strain Stenotrophomonas maltophilia (AuRed02) isolated from the soil samples of Singhbhum gold mines, India. An aqueous solution of gold chloride was reduced to metallic gold in a suspension of disrupted cell mass of AuRed02, which progressively turns into cherry red within 8 h of incubation at 25 °C. The optical spectrum showed the plasmon resonance at 530 nm and analysis by transmission electron microscopy and dynamic light scattering confirmed the formation of around 40 nm GNPs. Zeta potential and Fourier transform infrared measurements confirmed GNPs are capped by negatively charged phosphate groups of NADP.

Imaging and elemental mapping of biological specimens with a dual-EDS dedicated scanning transmission electron microscope

A dedicated analytical scanning transmission electron microscope (STEM) with dual energy dispersive spectroscopy (EDS) detectors has been designed for complementary high performance imaging as well as high sensitivity elemental analysis and mapping of biological structures. The performance of this new design, based on a Hitachi HD-2300A model, was evaluated using a variety of biological specimens. With three imaging detectors, both the surface and internal structure of cells can be examined simultaneously. The whole-cell elemental mapping, especially of heavier metal species that have low cross-section for electron energy loss spectroscopy (EELS), can be faithfully obtained. Optimization of STEM imaging conditions is applied to thick sections as well as thin sections of biological cells under low-dose conditions at room and cryogenic temperatures. Such multimodal capabilities applied to soft/biological structures usher a new era for analytical studies in biological systems.

Evolution of cobalt hydroxide from 2D microplatelets to a 3D hierarchical structure mediated by precursor concentration

We presented a facile and surfactant free approach to prepare 3-dimensional (3D) hierarchical Co(OH)2 with enhanced specific capacitance. The hierarchical Co(OH)2 was synthesized by a simple reaction of cobalt nitrate and ammonia solution with an appropriate concentration of the precursor. The effects of cobalt nitrate concentration on the crystal morphology were studied. When the concentration of cobalt nitrate was as low as 0.05 mol L−1, 2-dimensional (2D) hexagonally shaped (001) Co(OH)2 microplatelets were formed. As the concentration of the cobalt precursor was increased to 0.1 mol L−1, 3D hierarchical Co(OH)2 structures with nano-sized nanorods vertically assembled on the hexagonal microplatelets formed. Electrochemical tests showed that the hierarchical Co(OH)2 had a much higher specific capacitance than that of the Co(OH)2 microplatelets. This was mainly due to the increased number of pores in the hierarchical Co(OH)2 structure.

Green synthesis and characterization of size tunable silica-capped gold core–shell nanoparticles

Silica-coated gold nanoparticles (Au@SiO2) with controlled silica-shell thickness were prepared by a modified Stober’s method using 10-nm gold nanoparticles (AuNPs) as seeds. The AuNPs were silica-coated with a sol–gel reaction using tetraethylorthosilicate (TEOS) as a silica source and ammonia as a catalyst. An increase in TEOS concentration resulted in an increase in shell thickness. The NPs were characterized by transmission electron microscopy, selected area electron diffraction, energy-dispersive X-ray spectroscopy, scanning near-field ultrasound holography and scanning transmission electron microscopy. The method required no surface modification and the synthesized core shell nanoparticles can be used for various types of biological applications.

3D Complete-Tilt Electron Tomography of Semiconductor Nanowires

Electron tomography approach is rapidly gaining prominence for solving 3-D morphology and structure of physical and biological materials [1]. The spatial resolution of electron tomography achieved in image-based 3D reconstructions is about 1 nm [2]. However, one of the well recognized challenges that prevents further improvement of resolution and spatial correlation is the “missing wedge” problem; conventional copper grids preclude TEM image recording at very high tilt angles. This missing wedge problem can be solved by using cylindrical specimens without any supporting grid, such that the thickness along the electron incident direction remains unchanged (or heightadjusted) upon rotation about the cylindrical axis [3]. Here, we report 3-D electron tomography of Ge nanowires by an innovative specimen mounting approach and collecting images with a full-space tilting holder. Fig.1 shows a TEM image and diffraction pattern of a Ge nanowire, synthesized by chemical vapor deposition in a Au-catalyzed vapor-liquid-solid growth process. The usual approach for attaching single nanowires to the tip of full-space holder with a micromanipulator coupled to electron beam based welding system in the FIB is often difficult due to severe damage and contamination. We have developed an alternative sample preparation method to mount nanowires for tomographic analysis. The Ge nanowire synthesis was carried out on silicon micro-posts 50 μm in length and 5 μm in diameter. Fig. 2(a) shows an SEM image of one such micro-post. The micro-post was then transferred, as a whole, to the tip of the full-space tilting holder. The end of the micro-post, which supports many nanowires, was then welded to the end of the micromanipulator (Fig. 2b), for subsequent S/TEM. Three of the tilted STEM images are shown in Fig.3. The reconstructed 3D structure of the Ge nanowire is shown in Fig.3 (d). The satisfactory quality of specimen preparation protocol is evidenced by the visibility of clear {112} facets in the reconstructed volume, and resolution of secondary particles on the nanowire surface. The remainder artifacts in the reconstruction are attributed primarily to electron induced contamination and evaporation during data collection during the image acquisition process. Only three {112} planes can be distinguished in Fig. 3(e) due to the accumulation of contamination (amorphous carbon) during imaging. The presentation will highlight the integration of innovative specimen preparation with fulltilt tomography that will be invaluable to advancing the 3-D analysis of nanostructures. [4]