XPS measurements could provide the direct evidence of the
purity of fillers. Fig. 4A shows the XPS spectra of graphite, graphite
nanoflake and graphene sheet. The graphite exhibits two peaks at
284.8 and 532.2 eV, which are attributed to C1s and O1s, respectively.
O atoms may come from the atmosphere materials with
oxygen element, adsorbed onto the surface of graphite. The
graphite nanoflake and graphene sheet show the same XPS spectrums.
Fig. 4B shows the C1s spectra of graphite, graphite nanoflake
and graphene sheet. It is seen that the C1s spectrum of these fillers
exhibit a uniform sharp peak characteristic of single-phase. Due to
the preparation method of graphene sheets related to oleum and
dilute hydrogen peroxide solution treatments, graphene sheets
may be with a certain oxygen functional groups even after the
thermal reduction. The detailed elements content of three samples
are shown in Table 2, from which we can find the carbon content of
graphene sheets is about 96.39 wt %, which is higher than that of
graphite nanoflake, but lower than that of natural graphite, and the
result indicated the graphene sheets with less oxygen functional
groups and high quality.