Atomic-scale observation of pressure-dependent reduction dynamics of W18O49 nanowires using environmental TEM.

The real-time observation of structural evolution of materials can provide critical information for understanding their reduction mechanisms under different environments. Herein, we report the atomic-scale observation of the reduction dynamics of W18O49 nanowires (NWs) using environmental transmission electron microscopy. Intriguingly, the reduction pathway is found to be affected by oxygen pressure. Under high oxygen pressure (∼0.095 Pa), a W18O49 NW epitaxially transforms into a WO2 NW via mass transport across the interface between (010)W18O49 and (101)WO2. While under low oxygen pressure (∼0.0004 Pa), the transformation follows the sequence of W18O49(NW) → WO2(NW) → ß-W(nanoparticles), which is identified as a new reduction pathway. These findings reveal the pressure-dependent reduction and a new transformation pathway, and extend our current understanding of the reduction dynamics of metal oxides.

Atomic scale observation of a defect-mediated first-order phase transition in VO2(A).

The study of first-order structural transformations has attracted extensive attention due to their significant scientific and industrial importance. However, it remains challenging to exactly determine the nucleation sites at the very beginning of the transformation. Here, we report the atomic scale real-time observation of a unique defect-mediated reversible phase transition between the low temperature phase (LTP) and the high temperature phase (HTP) of VO2(A). In situ Cs-corrected scanning transmission electron microscopy (STEM) images clearly indicate that both phase transitions (from the HTP to the LTP and from the LTP to the HTP) start at the defect sites in parent phases. Intriguingly, the structure of the defects within the LTP is demonstrated to be the HTP of VO2 (A), and the defect in the HTP of VO2(A) is determined to be the LTP structure of VO2(A). These findings are expected to broaden our current understanding of the first-order phase transition and shed light on controlling materials' structure-property phase transition by "engineering" defects in applications.

Atomic-Scale Observation of Vapor-Solid Nanowire Growth via Oscillatory Mass Transport.

In situ atomic-scale transmission electron microscopy (TEM) can provide critical information regarding growth dynamics and kinetics of nanowires. A catalyst-aided nanowire growth mechanism has been well-demonstrated by this method. By contrast, the growth mechanism of nanowires without catalyst remains elusive because of a lack of crucial information on related growth dynamics at the atomic level. Herein, we present a real-time atomic-scale observation of the growth of tungsten oxide nanowires through an environmental TEM. Our results unambiguously demonstrate that the vapor-solid mechanism dominates the nanowire growth, and the oscillatory mass transport on the nanowire tip maintains the noncatalytic growth. Autocorrelation analysis indicates that adjacent nucleation events in the nanowire growth are independent of each other. These findings may improve the understanding of the vapor-solid growth mechanism of nanowires.

In situ observation of facet-dependent oxidation of graphene on platinum in an environmental TEM.

We performed a direct observation of a crystal facet-dependent oxidation of graphene layers on platinum nanocrystals at atomic resolution in an environmental transmission electron microscope. Combined with density functional theory calculations, our work provides a novel approach for the dynamical exploration of the facet-dependent reactions at the atomic level.

Layer-by-layer assembly of nacre-like nanostructured composites with antimicrobial properties.

In a recent report, we have presented the layer-by-layer (LBL) assembly of a biomimetic nanostructured composite from Na(+)-montmorillonite clay nanosheets and poly(diallylmethylammonium chloride) (Tang, Z.; Kotov, N.; Magonov, S.; Ozturk, B. Nat. Mater. 2003, 2, 413). The structure, deformation mechanism, and mechanical properties of the material are very similar to those of natural nacre and lamellar bones. This fact prompts further investigation of these composites as potential bone implants. LBL assembly affords preparation of multifunctional composites, and here we demonstrate that not only mechanical strength, but also antibacterial activity, can be introduced in these implantable materials by alternating clay layers with starch-stabilized silver nanoparticles. The resulting composite showed excellent structural stability with no detectable levels of silver lost over a 1 month period. Evaluation of the antibacterial properties showed almost complete growth inhibition of E. coli over an 18 h period. The amount of silver eluted from the LBL composite over a 1 month period was determined to be only 0.5-3.0 microg/L. This concentration of silver did not prevent the growth of the mammalian tissue cultures. The LBL composite has shown biocompatibility with the human osteoblast cell line.