SYNTHESIS OF POLYPYRROLE/TIO NANOPARTICLES IN WATER BY CHEMICAL OXIDATIVE POLYMERIZATION - Biomaterial Applications: Macro to Nanoscales - page 88

of decomposition of PPy to larger temperatures. At lower temperatures
the decomposition is less but at higher temperatures (after the cross linked
network is broken) the decomposition is faster, however, this decomposi-
tion is limited to a certain mass that includes TiO 2 and small oligomers
of pyrrole stabilized by TiO 2 , and the content of TiO 2 in the composite
(expected to be only 2.5%) (Fig. 4.10).
FIGURE 4.10 TGA traces of (a) pure PPy (red curves), (b) PPy/TiO 2 (green curves)
composite (3% TiO 2 ). The derivative plot is shown in the onset.
4.4.5 UV-VIS SPECTRA
We studied the UV-vis absorption spectra of PPy/TiO 2 composites (Fig.
4.11). The spectrum shows three bands at 275 nm, 372 nm, and 620 nm.
These three bands can be attributed to pyrrole oligomers and PPy. p-elec-
trons absorb ultraviolet energy to excite these electrons to higher anti-
bonding molecular orbitals, so the more easily excited the electrons (i.e. ,
the lower the energy gap between the HOMO and the LUMO), the longer
the wavelength. The relation between energy and wavelength can be il-
lustrated by the equation: E = hc /l, so the energy of the band gap of PPy
is 2.0 eV, which is lower than the band gap energy of rutile TiO 2 (3.02 eV)
[60]. Therefore the PPy/TiO 2 composites have potential applications as
photoelectric materials, like photovoltaic cells.