Since the ZnO NW/MWNT nanocomposites exhibit much
higher photocatalytic activities than pure ZnO, the incorporation
of MWNTs plays an important role in enhancing the
photocatalytic efficiency. According to the photocatalytic
mechanism based on the excitation of semiconductor [24],
Fig. 5 shows the enhanced photocatalytic degradation mechanism
of the ZnO NW/MWNT nanocomposites, which includes
the generation of reactive oxygen species (ROSs) by exciting
semiconductor using UV irradiation and the oxidation of dye
molecules by these ROSs [24]. Under UV irradiation, the
photo-induced electrons (e) will transit from valence band
(VB) to conduction band (CB) and leave positive holes (hþ) in
VB, forming the electron–hole pairs in ZnO NWs. Considering
the potential of the conduction band (4.05 eV) and the
valence band (7.25 eV) of ZnO [22] and MWNTs (4.5–
5.0 eV) [25,26], direct electron transfer from ZnO NWs to
MWNT surface is thermodynamically favorable, which will
result in low recombination rate of the photo-induced e/hþ
pairs [17,27]. These photo-generated electrons on MWNT
surface react easily with the dissolved oxygen (O2) adsorbed
on nanocomposite surface to form superoxide radical (dO2
),
and the hydroxyl ions (OH) will be oxidized into hydroxyl
radicals (dOH) by photo-induced holes. The continuous
generation of these ROSs results in the degradation process
that the dye molecules decomposed into simple organics and
further converted into CO2 and H2O. Therefore, the enhanced
photocatalytic degradation of the ZnO NW/MWNT nanocomposites
should be attributed to the electron transfer between
ZnO NWs and MWNTs under UV irradiation.