describes the overall synthesis process of PACS. During
the heat-treatment process, petroleum asphalt melts and
uniformly envelops ZnO templates with thin carbon layer before
condensation. Afterwards, the mixture is carbonized at 1073 K
for 1 h. With the assistance of nitric acid solution, the metal oxide
templates are easily and thoroughly removed, leaving PACS with
size-controllable holes and ultrathin carbon shell, and the yield
of PACS in a typical run is about 22%. Fig. 1(b) shows the XRD patterns
of ZnO@PACS, PACS and graphite, respectively. The sharp
peaks of ZnO@PACS can be indexed to the wurtzite-type hexagonal
ZnO (JCPDS, No. 36-1451). Besides, only two diffraction peaks
around 24 and 43 corresponding to (002) and (100) planes of
graphitic carbon can be observed from the XRD pattern of PACS,
indicating that all ZnO nanotemplates have been removed by the
nitric acid solution. Notably, peaks of PACS are much broader compared
with the sharp ones of graphite, demonstrating an apparent
amorphous carbon structure of PACS with lower crystallinity.
Fig. 2 shows the morphology and microstructure of ZnO@PACS.
The cobweb-like material in the yellow circle (Fig. 2(a) and (b))
might be the ultrathin carbon layer coating the ZnO templates.
Fig. 2(c) further demonstrates that there is a relatively uniform
coating of petroleum asphalt based carbon on ZnO.
To further examine the porous structure of PACS and
ZnO@PACS, nitrogen adsorption–desorption analysis was
measured. As shown in Fig. 3(a), according to IUPAC classification
method, the isotherm of PACS has a typical type IV shape
with a type H3 hysteresis located at a relative pressure of
0.41–0.95, confirming the existence of mesoporous structure [17].
Besides the mesopores, PACS also possesses some micropores
and abundant macropores as shown in Fig. 3(b). Nevertheless,
there is seldom porous structure in ZnO@PACS and the specific
surface area is only 10.4 m2 g1. Mesopores and macropores
formed in PACS could be ascribed to the removal of ZnO
template via acid wash, while micropores might arise from the
vaporization of volatile materials in petroleum pitch. Such a
hierarchical porous microstructure with specific surface area
around 188 m2 g1 can not only provide abundant active sites
for the insertion and storage of Li+ but also improve the
accessibility of the active sites to Li+ [18].
Fig.