These interpretations could be supported by the analysis of Au
4f peaks in XPS data as shown in Fig. 3. Compared to binding energies
of Au 4f peaks measured in GNPs, the others shifted toward
the lower energies; (Au 4f5/2 and Au 4f7/2) 87.2 eV and 83.5 eV for
GNPs, 86.8 eV and 83.2 eV for Au-MWNT(SDS), 86.6 eV and 82.9 eV
for Au-MWNT(PSS), 86.3 eV and 82.6 eV for Au-MWNT(PVP). The
previous reports [18,33] demonstrated that the analysis of binding
energy shifts was useful to investigate the electrical interaction
between metal nanoparticles and CNTs. The lower shift of binding
energy is caused by strengthening of the electrical contact, i.e., by
the redistribution of the electron density with respect to the contact
of GNPs and MWNTs. Poor electric interaction induces charging of
the GNPs during the photoionization process. A series of XPS spectra
in Fig. 3 showed that Au 4f peaks shifted toward lower binding
energy with changing dispersion agent for MWNT from SDS to PVP
via PSS. As a consequence, PVP as a dispersion agent of MWNTs was
most efficient to prepare Au-MWNT nanocomposite. The results of
XPS analysis agree with the morphological changes obtained from
TEM images of Fig. 2.