  Herein, we report on the use of vertically aligned multiwall carbon nanotubes
(CNTs) films as support for ZnO/ZnS photocatalytic active nanostructures. The
CNTs were synthetized via a hot-filament chemical vapor deposition (HfCVD),
using Fe catalyst on top of Al$_2$O$_3$ buffer layer. Controlled point defects
in the CNTs outer walls were created by exposure to a low pressure nonthermal
water vapors diffusive plasma and acted as seeds for subsequent
pulsed-electrodeposition of Zn nanoparticles. This was to achieve a direct and
improved contact between the nanoparticles and CNTs. To obtain ZnO, ZnS and mix
phase of ZnO/ZnS spread on CNTs, the oxidation, sulfurization and 2 steps
subsequent annealing in oxygen and sulfur rich atmospheres were applied. High
resolution transmission electron microscopy (HRTEM) with energy dispersive
X-rays spectroscopy (EDS) in scanning mode, provided the chemical mapping of
the structures. X-ray diffraction (XRD) analyses proved the hexagonal phase of
ZnO and ZnS, obtained after oxidation in H$_2$O and S vapors, respectively. In
the case of the samples obtained by the 2 steps subsequent annealing, XRD
showed mainly the presence of ZnO and a small amount of ZnS. The benefit of the
secondary annealing in S vapor was seen as absorption enhancement of the
ZnO$_{1-x}$S$_x$@CNTs sample having the absorption edge at 417 nm, whereas the
absorption edge of ZnO@CNTs was 408 nm and of ZnS@CNTs 360 nm. For all the
samples, compared to the bare ZnO and ZnS, the absorption red shift was
observed which is attributed to the CNTs involvement. Therefore, this study
showed the double sides benefit to induce the absorption of ZnO of the visible
light, one from S doping and second of CNTs involvement. The absorption
enhancement had a positive impact on photocatalytic degradation of methyl blue
dye, showing that ZnO$_{1-x}$S$_x$@CNTs heteronanostructure was the best
photocatalyst among the studied samples.
