Biomedical Engineering Reference
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(A)
(B)
Rim atoms
FIGURE 13.3
Schematic presentation of (A) graphite (the C-atoms in purple) and (B) MoS 2 with the Mo atoms in purple
and sulfur atoms in yellow.
of their physical and chemical properties. The basal (van der Waals) surfaces of the crystal,
which are perpendicular to the c-axis, consist of sulfur atoms that form bonds to three underlying
W/Mo atoms. These sulfur atoms are chemically inert. However, rim W (or Mo) and S atoms
( Figure 13.3 ), that is, atoms on edge of the layer, which are abundant in the nanostructure, are only
four- and twofold bonded, respectively, making the planar form unstable and forcing it to fold and
close on itself. Therefore, by folding the molecular sheet and stitching the rim atoms together,
seamless and stable nanotubular (one dimensional) and spherical (zero-dimensional fullerene-like)
structures with all W/Mo and S atoms being six- and threefold bonded, respectively, are produced
[14
17] . Initially, only the transition metal chalcogenides of WS 2 and MoS 2 were known in the
form of closed-cage structures and nanotubes. However, over the years this family has been
expanded considerably and it now encompasses a large number of other compounds like oxides,
hydroxides, nitrides, chlorides, sulfides, selenides, and even pure elements like bismuth, arsine, and
phosphorus.
13.3.2 Synthesis
The most useful method for the synthesis of IF-WS 2 NP and WS 2 nanotubes (INT-WS 2 ) is the
sulfidization of WO 3 NP at elevated temperatures (850 C) [22
25] . Thereafter the initial route for
large-scale synthesis of IF NP and INT of WS 2 involved the use of oxides as the starting materials.
After a clear understanding of the growth mechanism was established, large-scale synthesis of
IF-WS 2 NP and INT-WS 2 was realized by using the fluidized-bed reactor (FBR) [23
25] , which
has significant advantages over the previous synthetic approaches. It resulted in IF NP with a more
perfect crystalline structure compared with the product of the previous synthetic systems (which
used horizontal reactors). This observation can be attributed to the fact that much of the reaction
takes place in the gas phase, where an isotropic environment for the reaction prevails. The vertical
posture of the oven allows continuous addition of oxide powder into the chamber during the
reaction, thereby increasing the output of the reactor. Moreover, the fluidized-bed concept lends
itself to further scale-up and to produce larger amounts of a pure IF/INT phases with little or no
compromise on the quality of the synthesized NP.
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