Detailed structural features of theWO3 nanofiberswere observed by
TEM. Fig. 3A shows a TEM image taken for one nanofiber that was
formed by the axial agglomeration ofmany crystalswith individual sizes
of 10–30 nm. These sizes are well consistent with the average crystal
sizes calculated using the FWHMin XRD. The nanofiber appears to have
a rough surface morphology and wormhole-like porosity with a size of
several nanometers. The inset displays a typical high-resolution TEM
image of the WO3 nanofiber. The image clearly exhibits a crystal
structure of (001) planes thatwere observed as themost intensive peak
in the XRD. It demonstrates highly crystalline characteristics of theWO3
nanofiber. In addition, crystal structures with pore diameters of 0.7 and
1.0 nm were also observed locally in the circled regions, indicating the
existence of the microporosity within the grain.
Fig. 3B shows nitrogen adsorption and desorption isotherms of the
WO3 nanofibers, and the inset displays a corresponding pore size
distribution calculated using the BJH model. Two maximum peaks
were observed at around 4 and 7 nm, and the pore volume was
gradually decreased in the region of 10–100 nm. This result indicates
that our WO3 nanofibers were mesoporous. The small-sized porosity
(less than 10 nm) could be attributed to the inter-gain space within
the nanofibers as mentioned above in TEM. On the other hand, the
large-sized porosity could have resulted from the void among the
nanofibers. The BET surface area and specific pore volume of the
sample were 39.2 m2/g and 0.05 cm3/g, respectively.